Introduction to Wastewater Treatment Processes R. S. Rama/ho LAVAL UNIVERSITY QUEBEC, CANADA
ACADEMIC PRESS New York San Francisco London A Subsidiary of Harcourt Brace J o v a n o v i c h , Publishers
1977
COPYRIGHT © 1 9 7 7 , BY A C A D E M I C PRESS, I N C . ALL RIGHTS RESERVED. N O PART O F T H I S P U B L I C A T I O N M A Y B E R E P R O D U C E D OR T R A N S M I T T E D I N A N Y F O R M OR BY ANY M E A N S , E L E C T R O N I C OR M E C H A N I C A L , I N C L U D I N G P H O T O C O P Y , RECORDING, OR A N Y I N F O R M A T I O N STORAGE A N D RETRIEVAL S Y S T E M , W I T H O U T P E R M I S S I O N IN W R I T I N G F R O M T H E P U B L I S H E R .
A C A D E M I C PRESS, INC. I l l Fifth A v e n u e , N e w Y o r k , N e w Y o r k 10003
United
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Library of Congress Cataloging in Publication Data Ramalho, Rubens Sette. I n t r o d u c t i o n t o wastewater t r e a t m e n t Bibliography: p . Includes index. 1. Sewage-Purification. TD745.R36 628.3 ISBN 0 - 1 2 - 5 7 6 5 5 0 - 9
I.
processes.
Title. 76-26185
P R I N T E D I N T H E U N I T E D S T A T E S OF AMERICA 81 82
9 8 7 6 5 4 3
Preface This book is an introductory presentation meant for both students and practicing engineers interested in the field of wastewater treatment. Most of the earlier books discuss the subject industry by industry, providing solutions to specific treatment problems. More recently, a scientific ap proach to the basic principles of unit operations and processes has been utilized. I have used this approach to evaluate all types of wastewater problems and to properly select the mode of treatment and the design of the equipment required. In most cases, the design of specific wastewater treatment processes, e.g., the activated sludge process, is discussed following (1) a summary of the theory involved in the specific process, e.g., chemical kinetics, pertinent material and energy balances, discussion of physical and chemi cal principles; (2) definition of the important design parameters involved in the process and the determination of such parameters using laboratoryscale or pilot-plant equipment; and (3) development of a systematic design procedure for the treatment plant. Numerical applications are presented which illustrate the treatment of laboratory data, and subsequent design calculations are given for the wastewater processing plant. The approach followed, particularly in the mathematical modeling of biological treat ment processes, is based largely on the work of Eckenfelder and as sociates. Clarity of presentation has been of fundamental concern. The text should be easily understood by undergraduate students and practicing engineers. The book stems from a revision of lecture notes which I used for an introductory course on wastewater treatment. N o t only engineering students of diverse backgrounds but also practicing engineers from various fields have utilized these notes at the different times this course was offered at Laval University and COPPE/UFRJ (Rio de Janeiro, Brazil). Favorable acceptance of the notes and the encouragement of many of their users led me to edit them for inclusion in this work. I wish to express my appreciation to the secretarial staff of the Chemi cal Engineering Department of Laval University, Mrs. Michel, Mrs. Gagne, and Mrs. McLean, and to Miss Enidete Souza (COPPE/UFRJ) for typing the manuscript. I o w e sincere thanks to Mr. Alex Legare for the artwork, to Dr. and Mrs. Adrien Favre for proofreading the manu script, and to Mr. Roger Theriault for his assistance in the correction of the galleys. The valuable suggestions made by Dr. M. Pelletier (Laval University) and Dr. C. Russo (COPPE/UFRJ) are gratefully acknowledged. R. S. Ramalho ix
1 Introduction 1. Introduction 2. The Role of the Engineer in Water Pollution Abatement 2.1. The Necessity of a Multidisciplinary Approach to the Water Pollution Abatement Problem 2.2. A Survey of the Contribution of Engineers to Water Pollution Abatement 2.3. A Case History of Industrial Wastewater Treatment 2.4. The Chemical Engineering Curriculum as a Preparation for the Field of Wastewater Treatment 2.5. "Inplant" and " E n d - o f - P i p e " Wastewater Treatment 2.6. A N e w Concept in Process D e s i g n : The Flowsheet of the Future 3. Degrees of Wastewater Treatment and Water Quality Standards . . 4. Sources of Wastewaters 5. Economics of Wastewater Treatment and Economic Balance for Water Reuse 6. Effect of Water Pollution on Environment and Biota 6.1. Oxygen S a g Curve 6.2. Effect of Light 6.3. Decomposition of Carbonaceous and Nitrogenous Organic Matter 6.4. Sludge Deposits and Aquatic Plants 6.5. Bacteria and Ciliates 6.6. Higher Forms of Animal Species 7. Eutrophication 8. Types of Water Supply and Classification of Water Contaminants . References
1 2 2 2 3 4 4 7 8 9 10 14 14 16 17 18 19 20 22 22 25
1. Introduction I t w a s o n l y d u r i n g t h e d e c a d e o f t h e 1960's t h a t t e r m s s u c h a s " w a t e r a n d air p o l l u t i o n , " " p r o t e c t i o n of the e n v i r o n m e n t , " a n d " e c o l o g y " b e c a m e household words. Prior to that time, these terms would either pass un r e c o g n i z e d b y t h e a v e r a g e citizen, o r a t m o s t , w o u l d c o n v e y h a z y i d e a s t o h i s m i n d . Since t h e n m a n k i n d h a s b e e n b o m b a r d e d b y t h e m e d i a ( n e w s p a p e r s , r a d i o , television), w i t h t h e d r e a d f u l i d e a t h a t h u m a n i t y is effectively w o r k i n g for its s e l f - d e s t r u c t i o n t h r o u g h t h e s y s t e m a t i c p r o c e s s o f p o l l u t i o n o f t h e e n v i r o n m e n t , for t h e s a k e o f a c h i e v i n g m a t e r i a l p r o g r e s s . I n s o m e c a s e s , people have been a r o u s e d nearly to a state of m a s s hysteria. A l t h o u g h pollu t i o n is a s e r i o u s p r o b l e m , a n d it is, o f c o u r s e , d e s i r a b l e t h a t t h e c i t i z e n r y b e c o n c e r n e d a b o u t it, it is q u e s t i o n a b l e t h a t " m a s s h y s t e r i a " is i n a n y w a y justifiable. T h e i n s t i n c t o f p r e s e r v a t i o n o f t h e species is a v e r y b a s i c d r i v i n g 1
1.
2
Introduction
force of humanity, and man is equipped to correct the deterioration of his environment before it is too late. In fact, pollution control is not an exceedingly difficult technical problem as compared to more complex ones which have been successfully solved in this decade, such as the manned exploration of the moon. Essentially, the basic technical knowledge required to cope with pollution is already available to man, and as long as he is willing to pay a relatively reasonable price tag, the nightmare of self-destruction via pollution will never become a reality. Indeed, much higher price tags are being paid by humanity for development and maintenance of the war-making machinery. This book is primarily concerned with the engineering design of process plants for treatment of wastewaters of either domestic or industrial origin. It is only in the last few years that the design approach for these plants has changed from empiricism to a sound engineering basis. Also, fundamental research in new wastewater treatment processes, such as reverse osmosis and electrodialysis, has only recently been greatly emphasized.
2. The Role of t h e Engineer in Water Pollution Abatement 2.1. T H E N E C E S S I T Y O F A M U L T I D I S C I P L I N A R Y A P P R O A C H TO THE WATER POLLUTION ABATEMENT PROBLEM Although it has been stated previously that water pollution control is not an exceedingly difficult technical problem, the field is a broad one, and of sufficient complexity to justify several different disciplines being brought together for achieving optimal results at a minimum cost. A systems approach to water pollution abatement involves the participation of many disciplines: (1) engineering and exact sciences [sanitary engineering (civil engineering), chemical engineering, other fields of engineering such as mechanical and electrical, chemistry, physics]; (2) life sciences [biology (aquatic biology), microbiology, bacteriology]; (3) earth sciences (geology, hydrology, oceanog raphy); and (4) social and economic sciences (sociology, law, political sciences, public relations, economics, administration). 2.2. A S U R V E Y O F T H E C O N T R I B U T I O N O F E N G I N E E R S TO WATER POLLUTION ABATEMENT The sanitary engineer, with mainly a civil engineering background, has historically carried the brunt of responsibility for engineering activities in water pollution control. This situation goes back to the days when the bulk of wastewaters were of domestic origin. Composition of domestic wastewaters does not vary greatly. Therefore, prescribed methods of treatment are rela tively standard, with a limited number of unit processes and operations
2.
Engineer's Role in Water Pollution Abatement
3
involved in the treatment sequence. Traditional methods of treatment in volved large concrete basins, where either sedimentation or aeration were performed, operation of trickling filters, chlorination, screening, and occa sionally a few other operations. The fundamental concern of the engineer was centered around problems of structure and hydraulics, and quite naturally, the civil engineering background was an indispensable prerequisite for the sanitary engineer. This situation has changed, at first gradually, and more recently at an accelerated rate with the advent of industrialization. As a result of a new large variety of industrial processes, highly diversified wastewaters requiring more complex treatment processes have appeared on the scene. Wastewater treatment today involves so many different pieces of equipment, so many unit processes and unit operations, that it became evident that the chemical engineer had to be called to play a major role in water pollution abatement. The concept of unit operations, developed largely by chemical engineers in the past fifty years, constitutes the key to the scientific approach to design problems encountered in the field of wastewater treatment. In fact, even the municipal wastewaters of today are no longer the "domestic wastewaters" of yesterday. Practically all municipalities in industrialized areas must handle a combination of domestic and industrial wastewaters. Economic and technical problems involved in such treatment make it very often desirable to perform separate treatment (segregation) of industrial waste waters, prior to their discharge into municipal sewers. Even the nature of truly domestic wastewaters has changed with the advent of a whole series of new products now available to the average household, such as synthetic detergents and others. Thus, to treat domestic wastewaters in an optimum way requires modifications of the traditional approach. In summary, for treatment of both domestic and industrial wastewaters, new technology, new processes, and new approaches, as well as modifications of old approaches, are the order of the day. The image today is no longer that of the "large concrete basins," but one of a series of closely integrated unit operations. These operations, both physical and chemical in nature, must be tailored for each individual wastewater. The chemical engineer's skill in integrating these unit operations into effective processes makes him admirably qualified to design wastewater treatment facilities. 2.3. A C A S E H I S T O R Y O F I N D U S T R I A L WASTEWATER TREATMENT An interesting case history, emphasizing the role of the chemical engineer in the design of a wastewater treatment plant for a sulfite pulp and paper mill, is discussed by Byrd [ 2 ] . This pulp and paper plant was to discharge its waste waters into a river of prime recreational value, with a well-balanced fish population. For this reason, considerable care was taken in the planning and
4
1.
Introduction
d e t a i l e d d e s i g n o f t h e w a s t e w a t e r t r e a t m e n t facilities. A s t u d y o f a s s i m i l a t i v e c a p a c i t y o f t h e river w a s u n d e r t a k e n a n d m a t h e m a t i c a l m o d e l s w e r e d e v e l o p e d . Design of the t r e a t m e n t p l a n t involved a study to determine which waste w a t e r effluents s h o u l d b e s e g r e g a t e d f o r t r e a t m e n t , a n d w h i c h o n e s s h o u l d b e c o m b i n e d . F o r t h e t r e a t m e n t p r o c e s s e s a selection o f a l t e r n a t i v e s is d i s c u s s e d [ 2 ] . S o m e o f t h e u n i t o p e r a t i o n s a n d p r o c e s s e s i n v o l v e d in t h e t r e a t m e n t p l a n t , o r c o n s i d e r e d a t first b u t after f u r t h e r s t u d y r e p l a c e d b y o t h e r a l t e r natives, were the following: sedimentation, dissolved air
flotation,
equaliza
t i o n , n e u t r a l i z a t i o n , filtration ( r o t a r y filters), c e n t r i f u g a t i o n , r e v e r s e o s m o s i s , flash
drying,
fluidized
bed
oxidation,
multiple hearth
incineration,
wet
oxidation, adsorption in activated c a r b o n , activated sludge process, aerated lagoons,
flocculation
w i t h p o l y e l e c t r o l y t e s , c h l o r i n a t i o n , landfill, a n d s p r a y
irrigation. I n t e g r a t i o n of all t h e s e u n i t o p e r a t i o n s a n d p r o c e s s e s i n t o a n o p t i m a l l y d e s i g n e d t r e a t m e n t facility c o n s t i t u t e d a v e r y c h a l l e n g i n g p r o b l e m .
The
t r e a t m e n t p l a n t involved a capital cost of over $10 million a n d a n o p e r a t i n g c o s t i n excess o f $1 m i l l i o n p e r y e a r .
2.4. T H E C H E M I C A L E N G I N E E R I N G C U R R I C U L U M A S A P R E P A R A T I O N F O R T H E FIELD O F W A S T E W A T E R T R E A T M E N T [5] C h e m i c a l e n g i n e e r s h a v e c o n s i d e r a b l e b a c k g r o u n d t h a t is a p p l i c a b l e t o water pollution problems. Their knowledge
of m a s s transfer,
chemical
k i n e t i c s , a n d s y s t e m s a n a l y s i s is specially v a l u a b l e in w a s t e w a t e r t r e a t m e n t a n d c o n t r o l . T h u s , t r a i n i n g in c h e m i c a l e n g i n e e r i n g r e p r e s e n t s g o o d p r e p a r a t i o n f o r e n t e r i n g t h i s t y p e of a c t i v i t y . I n t h e p a s t , t h e m a j o r i t y o f e n g i n e e r s w o r k i n g in t h i s field h a v e b e e n s a n i t a r y e n g i n e e r s w i t h a civil e n g i n e e r i n g background. T h e m u l t i d i s c i p l i n a r y n a t u r e of t h e field s h o u l d b e r e c o g n i z e d . C h e m i c a l e n g i n e e r i n g g r a d u a t e s e n v i s i o n i n g m a j o r a c t i v i t y i n t h e field o f w a s t e w a t e r treatment are advised to complement their b a c k g r o u n d by studying micro biology, owing t o the great i m p o r t a n c e of biological wastewater t r e a t m e n t p r o c e s s e s , a n d a l s o h y d r a u l i c s [ s i n c e t o p i c s s u c h a s o p e n c h a n n e l a n d stratified flow, m a t h e m a t i c a l m o d e l i n g o f b o d i e s of w a t e r ( r i v e r s , e s t u a r i e s , l a k e s , i n l e t s , etc.) a r e n o t e m p h a s i z e d in fluid m e c h a n i c s c o u r s e s n o r m a l l y offered t o chemical engineering students].
2.5. " I N P L A N T " A N D E N D - O F - P I P E " W A S T E W A T E R T R E A T M E N T [6] 2.5.1. Introduction F r e q u e n t l y o n e m a y b e t e m p t e d t o t h i n k of i n d u s t r i a l w a s t e w a t e r t r e a t m e n t in t e r m s o f a n " e n d - o f - p i p e " a p p r o a c h . T h i s w o u l d i n v o l v e d e s i g n i n g a p l a n t
2.
Engineer's Role in Water Pollution Abatement
5
without m u c h regard to water pollution abatement, and then considering s e p a r a t e l y t h e d e s i g n o f w a s t e w a t e r t r e a t m e n t facilities. S u c h a n a p p r o a c h s h o u l d n o t b e p u r s u e d since it is, in g e n e r a l , h i g h l y u n e c o n o m i c a l . T h e right a p p r o a c h for a n industrial wastewater pollution
abatement
p r o g r a m is o n e w h i c h u n c o v e r s all o p p o r t u n i t i e s for i n p l a n t
wastewater
treatment. This m a y seem a m o r e complicated a p p r o a c h t h a n handling waste w a t e r s a t t h e final o u t f a l l . H o w e v e r , s u c h a n a p p r o a c h c a n b e v e r y p r o f i t a b l e .
2.5.2. W h a t Is Involved in Inplant W a s t e w a t e r Control Essentially, inplant wastewater control involves the three following steps: Step 1. P e r f o r m a d e t a i l e d s u r v e y o f all effluents i n t h e p l a n t . A l l p o l l u t i o n sources m u s t be a c c o u n t e d for a n d cataloged. T h i s involves, for each polluting s t r e a m , t h e d e t e r m i n a t i o n o f (a) flow r a t e a n d (b) s t r e n g t h o f t h e p o l l u t i n g streams. (a) Flow rate. F o r c o n t i n u o u s s t r e a m s , d e t e r m i n e flow r a t e s (e.g., g a l / m i n ) . F o r intermittent discharges, estimate total daily (or hourly) outflow. (b) Strength of the polluting streams. T h e " s t r e n g t h " of the polluting s t r e a m s ( c o n c e n t r a t i o n o f p o l l u t i n g s u b s t a n c e s p r e s e n t i n t h e s t r e a m s ) is e x p r e s s e d i n a v a r i e t y o f w a y s , w h i c h a r e d i s c u s s e d in l a t e r c h a p t e r s . F o r organic c o m p o u n d s which are subject to biochemical oxidation, the bio c h e m i c a l o x y g e n d e m a n d , B O D ( w h i c h is defined in C h a p t e r 2, S e c t i o n 2.3) is c o m m o n l y e m p l o y e d . I n t h e c a s e h i s t o r y s u m m a r i z e d in S e c t i o n 2 . 5 . 3 o f t h i s c h a p t e r , B O D is u s e d t o m e a s u r e c o n c e n t r a t i o n o f o r g a n i c s . Step 2. R e v i e w d a t a o b t a i n e d in S t e p 1 t o find all p o s s i b l e i n p l a n t a b a t e m e n t t a r g e t s . S o m e o f t h e s e a r e (1) i n c r e a s e d r e c y c l i n g in c o o l i n g w a t e r s y s t e m s ; (2) e l i m i n a t i o n of c o n t a c t c o o l i n g f o r off v a p o r s , e.g., r e p l a c e m e n t o f barometric condensers by shell-and-tube exchangers or air-cooling systems; (3) r e c o v e r y o f p o l l u t i n g c h e m i c a l s : Profit m a y o f t e n b e r e a l i z e d b y r e c o v e r i n g such chemicals, which are otherwise discharged into the plant sewers. A by p r o d u c t s p l a n t m a y b e d e s i g n e d t o r e c o v e r t h e s e c h e m i c a l s ; (4) r e u s e o f w a t e r from overhead accumulator drums, vacuum condensers, and p u m p glands. D e v i s e m o r e c o n s e c u t i v e o r m u l t i p l e w a t e r u s e s ; (5) d e s i g n a h e a t r e c o v e r y u n i t t o e l i m i n a t e q u e n c h i n g s t r e a m s ; a n d (6) e l i m i n a t e l e a k s a n d i m p r o v e housekeeping practices. A u t o m a t i c monitoring a n d additional personnel training might be profitable. Step 3. E v a l u a t e p o t e n t i a l s a v i n g s i n t e r m s o f c a p i t a l a n d o p e r a t i n g c o s t s for a p r o p o s e d " e n d - o f - p i p e " t r e a t m e n t , if e a c h o f t h e s t r e a m s c o n s i d e r e d in S t e p s 1 a n d 2 a r e e i t h e r e l i m i n a t e d o r r e d u c e d ( r e d u c t i o n i n flow r a t e s o r in terms of strength of polluting streams). T h e n design the " e n d - o f - p i p e " treat m e n t facilities t o h a n d l e t h i s r e d u c e d l o a d . C o m p a r e c a p i t a l a n d o p e r a t i n g c o s t s o f s u c h t r e a t m e n t facilities w i t h t h a t o f a n " e n d - o f - p i p e " facility d e s i g n e d
1.
6
Introduction
t o h a n d l e t h e o r i g i n a l full l o a d , i.e., t h e p o l l u t a n t s t r e a m s f r o m a p l a n t w h e r e i n p l a n t w a s t e w a t e r c o n t r o l is n o t p r a c t i c e d . T h e t w o c a s e h i s t o r i e s d e s c r i b e d in Ref. [ 6 ] a r e q u i t e r e v e a l i n g i n t h i s r e s p e c t . F o r practicing i n p l a n t wastewater control, a d e e p k n o w l e d g e of the process a n d a b i l i t y t o m o d i f y it, if n e c e s s a r y , a r e r e q u i r e d . T h e c h e m i c a l e n g i n e e r is a d m i r a b l y well s u i t e d t o h a n d l e t h i s j o b .
2.5.3. C a s e Histories of Inplant W a s t e w a t e r Control T w o interesting case histories are discussed by M c G o v e r n [ 6 ] . O n e of these, p e r t a i n i n g t o a p e t r o c h e m i c a l p l a n t , is s u m m a r i z e d n e x t . A p e t r o c h e m i c a l p l a n t a l r e a d y in o p e r a t i o n c o n d u c t e d a n effluent a n d inplant survey while evaluating a t r e a t m e n t p l a n t t o be designed a n d built, which w o u l d h a n d l e 20 million gal/day of wastewater with a B O D load of 52,000 lb/day. T h e plan called for a n activated sludge unit t o r e m o v e over 9 0 % of t h e B O D l o a d . T h i s i n c l u d e d v a c u u m filtration a n d i n c i n e r a t i o n o f t h e s l u d g e , a n d c h l o r i n a t i o n of t h e t o t a l effluent. C a p i t a l c o s t o f t h e t r e a t m e n t facility w a s e s t i m a t e d a t $ 1 0 m i l l i o n . O p e r a t i n g a n d maintenance costs were also estimated. All cost d a t a were converted t o a n a n n u a l b a s i s , u s i n g a 2 0 - y e a r p r o j e c t life a n d 1 5 % i n t e r e s t r a t e . T h e n a s t u d y of t h e p o s s i b i l i t y o f r e d u c i n g b o t h t h e flow a n d t h e s t r e n g t h o f the wastewaters was u n d e r t a k e n . This study followed the steps outlined u n d e r S e c t i o n 2.5.2, w i t h a n u m b e r o f c h a n g e s b e i n g p r o p o s e d f o r t h e p r o c e s s flow sheet. T h e r e d u c t i o n a c c o m p l i s h e d in flow r a t e a n d s t r e n g t h r e s u l t e d in s u b s t a n t i a l s a v i n g s in t h e t o t a l c o s t o f t h e p r o p o s e d t r e a t m e n t p l a n t . F i g u r e 1.1 s h o w s a g r a p h , p r e p a r e d for t h i s c a s e h i s t o r y , i l l u s t r a t i n g t h e effect o f r e d u c t i o n 100
8
1
Flow
80
o
-
8
BOD
60
ο
-
-£ 4 0 . ο §
1.1. Effect [6]. (Excerpted
Copyright
-
0
Fig.
Vc lid ran je
20
Q.
plant
1
of
1
20 40 60 80 100 Percent reduction in BOD or flow
waste
load
reductions
by special permission
by McGraw-Hill,
Inc., New York,
from 10020.)
on capital Chemical
cost
Engineering,
of
treatment
May
14,
1973.
2.
Engineer's Role in Water Pollution Abatement
7
T A B L E 1.1 Savings from Inplant Wastewater Reductions 8
Inplant savings F l o w reduction (1424 gal/min) B O D reduction (2000 lb/day) Water use reduction Treated water (0.24 M G D ) River water (1.37 M G D ) Product recovery
$/year $410,000 302,000
Total inplant saving
$774,000
Cost o f inplant control Engineering Capital investment Operating and maintenance
$/year $ 15,000 150,000 33,000
Total cost o f inplant control
$ 198,000
34,000 14,000 14,000
N e t savings: $ 7 7 4 , 0 0 0 - $ 1 9 8 , 0 0 0 = $576,000/year Excerpted by special permission from Chemical Engineering, May 14, 1 9 7 3 ; Copyright by M c G r a w Hill, Inc., N e w York, 10020. a
in B O D o r flow r a t e u p o n t h e c a p i t a l c o s t o f t h e t r e a t m e n t facilities. T h i s g r a p h is v a l i d t o a p p r o x i m a t e l y 6 0 % r e d u c t i o n in flow o r B O D . A n y f u r t h e r r e d u c t i o n p r o b a b l y r e q u i r e s a significantly different t r e a t m e n t s y s t e m . S a v i n g s f r o m i n p l a n t w a s t e w a t e r c o n t r o l a r e t a b u l a t e d in T a b l e 1.1. W a s t e w a t e r flow w a s c u t t o 8 5 % o f its v a l u e p r i o r t o i n p l a n t c o n t r o l a n d B O D l o a d was cut to 50%. Moreover, the cost of these inplant controls was m o r e t h a n offset b y e c o n o m i e s in t h e t r e a t m e n t p l a n t . A s s h o w n in T a b l e 1.1 t h e p r o g r a m realized a n e t s a v i n g o f $ 5 7 6 , 0 0 0 / y e a r .
2.6. A N E W C O N C E P T I N P R O C E S S D E S I G N : THE FLOWSHEET OF THE FUTURE T h e c o n s i d e r a t i o n s in S e c t i o n 2.5 a r e l e a d i n g e n g i n e e r s t o a n e w c o n c e p t i n p r o c e s s d e s i g n . T h e flowsheet o f t h e f u t u r e will n o l o n g e r s h o w a line w i t h a n a r r o w h e a d s t a t i n g " t o w a s t e . " E s s e n t i a l l y e v e r y t h i n g will b e r e c y c l e d , b y p r o d u c t s will b e r e c o v e r e d , a n d w a t e r will b e r e u s e d . F u n d a m e n t a l l y t h e o n l y s t r e a m s in a n d o u t o f t h e p l a n t will b e r a w m a t e r i a l s a n d p r o d u c t s . T h e o n l y p e r m i s s i b l e w a s t a g e s will b e c l e a n o n e s : n i t r o g e n , o x y g e n , c a r b o n d i o x i d e , w a t e r , a n d s o m e ( b u t n o t t o o m u c h ! ) h e a t . I n t h i s c o n n e c t i o n , it is a p p r o p r i a t e t o recall t h e g u i d e l i n e s o f t h e U n i t e d S t a t e s F e d e r a l W a t e r P o l l u t i o n C o n t r o l A c t o f 1 9 7 2 : (1) b e s t practical c o n t r o l t e c h n o l o g y , b y J u l y 1, 1 9 7 7 ; (2) b e s t available t e c h n o l o g y , b y J u l y 1, 1 9 8 3 ; a n d (3) z e r o d i s c h a r g e b y J u l y 1, 1985.
1.
8
Introduction
3. Degrees of W a s t e w a t e r Treatment and Water Quality Standards T h e d e g r e e of t r e a t m e n t r e q u i r e d for a w a s t e w a t e r d e p e n d s m a i n l y
on
d i s c h a r g e r e q u i r e m e n t s for t h e effluent. T a b l e 1.2 p r e s e n t s a c o n v e n t i o n a l classification
for
wastewater
treatment
processes.
e m p l o y e d for r e m o v a l o f s u s p e n d e d solids a n d
Primary
floating
treatment
is
materials, a n d also
T A B L E 1.2 Types of Wastewater Treatment Primary treatment Screening Sedimentation Flotation Oil separation Equalization Neutralization Secondary treatment Activated sludge process Extended aeration (or total oxidation) process Contact stabilization Other modifications of the conventional activated sludge process: tapered aeration, step aeration, and complete mix activated sludge processes Aerated lagoons Wastewater stabilization ponds Trickling filters Anaerobic treatment Tertiary treatment (or "advanced treatment") Microscreening Precipitation and coagulation Adsorption (activated carbon) Ion exchange Reverse osmosis Electrodialysis Nutrient removal processes Chlorination and ozonation Sonozone process
c o n d i t i o n i n g t h e w a s t e w a t e r for e i t h e r d i s c h a r g e t o a receiving b o d y o f w a t e r o r t o a s e c o n d a r y t r e a t m e n t facility t h r o u g h n e u t r a l i z a t i o n a n d / o r e q u a l i z a tion. Secondary treatment comprises conventional biological t r e a t m e n t p r o c e s s e s . T e r t i a r y t r e a t m e n t is i n t e n d e d p r i m a r i l y for e l i m i n a t i o n o f p o l l u t a n t s n o t removed by conventional biological treatment.
4.
Sources of Wastewaters
9
T h e s e t r e a t m e n t p r o c e s s e s a r e s t u d i e d in f o l l o w i n g c h a p t e r s . T h e a p p r o a c h utilized is b a s e d o n t h e c o n c e p t s o f u n i t p r o c e s s e s a n d o p e r a t i o n s . T h e
final
o b j e c t i v e is d e v e l o p m e n t of d e s i g n p r i n c i p l e s of g e n e r a l a p p l i c a b i l i t y t o
any
w a s t e w a t e r t r e a t m e n t p r o b l e m , l e a d i n g t o a p r o p e r selection o f p r o c e s s a n d the design of required equipment. Consequently, description of wastewater t r e a t m e n t s e q u e n c e s for specific i n d u s t r i e s , e.g., p e t r o l e u m refineries, steel mills, m e t a l - p l a t i n g p l a n t s , p u l p a n d p a p e r i n d u s t r i e s , b r e w e r i e s , a n d t a n n e r i e s , is n o t i n c l u d e d i n t h i s b o o k . F o r i n f o r m a t i o n o n specific w a s t e w a t e r treatment processes, the reader should consult Eckenfelder [3] a n d N e m e r o w [7]. W a t e r q u a l i t y s t a n d a r d s a r e u s u a l l y b a s e d o n o n e of t w o c r i t e r i a : s t r e a m s t a n d a r d s o r effluent s t a n d a r d s . Stream standards refer t o q u a l i t y o f r e c e i v i n g water d o w n s t r e a m from the origin of sewage discharge, whereas effluent standards p e r t a i n t o q u a l i t y o f t h e d i s c h a r g e d w a s t e w a t e r s t r e a m s t h e m s e l v e s . A d i s a d v a n t a g e o f effluent s t a n d a r d s is t h a t it p r o v i d e s n o c o n t r o l o v e r t o t a l a m o u n t o f c o n t a m i n a n t s d i s c h a r g e d in t h e r e c e i v i n g w a t e r . A l a r g e i n d u s t r y , for e x a m p l e , a l t h o u g h p r o v i d i n g t h e s a m e d e g r e e o f w a s t e w a t e r treatment as a small one, might cause considerably greater pollution of the receiving w a t e r . Effluent s t a n d a r d s a r e e a s i e r t o m o n i t o r t h a n s t r e a m s t a n d a r d s , w h i c h r e q u i r e d e t a i l e d s t r e a m a n a l y s i s . A d v o c a t e s o f effluent s t a n d a r d s a r g u e t h a t a l a r g e i n d u s t r y , d u e t o its e c o n o m i c v a l u e t o t h e c o m m u n i t y , should be allowed a larger share of the assimilative capacity of the receiving water. Q u a l i t y s t a n d a r d s selected d e p e n d o n i n t e n d e d u s e o f t h e w a t e r . S o m e o f these standards include: concentration of dissolved oxygen ( D O , mg/liter), p H , color, turbidity, hardness (mg/liter), total dissolved solids ( T D S , mg/liter), s u s p e n d e d solids ( S S , m g / l i t e r ) , c o n c e n t r a t i o n of t o x i c ( o r o t h e r w i s e o b j e c tionable) materials (mg/liter), odor, a n d temperature. Extensive tabulation of w a t e r q u a l i t y s t a n d a r d s f o r v a r i o u s u s e s a n d f o r s e v e r a l s t a t e s in t h e U n i t e d S t a t e s is p r e s e n t e d b y N e m e r o w [ 7 ] .
4. S o u r c e s of W a s t e w a t e r s F o u r m a i n s o u r c e s o f w a s t e w a t e r s a r e (1) d o m e s t i c s e w a g e , (2) i n d u s t r i a l w a s t e w a t e r s , (3) a g r i c u l t u r a l runoff, a n d (4) s t o r m w a t e r a n d u r b a n runoff. A l t h o u g h t h e p r i m a r y c o n s i d e r a t i o n i n t h i s b o o k is t h e s t u d y o f t r e a t m e n t o f domestic and industrial wastewaters, contamination due to agricultural a n d u r b a n runoffs is b e c o m i n g i n c r e a s i n g l y i m p o r t a n t . A g r i c u l t u r a l r u n o f f s c a r r y i n g fertilizers (e.g., p h o s p h a t e s ) a n d p e s t i c i d e s c o n s t i t u t e a m a j o r c a u s e of e u t r o p h i c a t i o n of l a k e s , a p h e n o m e n a w h i c h is d i s c u s s e d in S e c t i o n 7 o f t h i s c h a p t e r . S t o r m runoffs in h i g h l y u r b a n i z e d a r e a s m a y c a u s e significant
1.
10
Introduction
p o l l u t i o n effects. U s u a l l y w a s t e w a t e r s , t r e a t e d o r u n t r e a t e d , a r e d i s c h a r g e d i n t o a n a t u r a l b o d y o f w a t e r ( o c e a n , river, l a k e , etc.) w h i c h is r e f e r r e d t o a s the receiving water.
5. E c o n o m i c s of W a s t e w a t e r T r e a t m e n t a n d E c o n o m i c Balance for W a t e r R e u s e I n t h e U n i t e d S t a t e s a v e r a g e c o s t p e r t h o u s a n d g a l l o n s o f w a t e r is a p p r o x i m a t e l y $0.20, w h i c h c o r r e s p o n d s t o $ 0 . 0 5 / t o n . I t is a relatively c h e a p c o m m o d i t y , a n d a s a r e s u l t t h e e c o n o m i c s o f w a s t e w a t e r t r e a t m e n t is v e r y c r i t i c a l . I n p r i n c i p l e , b y utilizing s o p h i s t i c a t e d t r e a t m e n t p r o c e s s e s , o n e c a n o b t a i n potable water from sewage. E c o n o m i c considerations, however, prevent the practical application of m a n y available t r e a t m e n t m e t h o d s . I n countries w h e r e w a t e r is a t a p r e m i u m (e.g., I s r a e l , S a u d i A r a b i a ) s o m e s o p h i s t i c a t e d w a t e r t r e a t m e n t facilities, w h i c h a r e n o t e c o n o m i c a l l y justified i n N o r t h A m e r i c a , a r e n o w in o p e r a t i o n . I n e v a l u a t i n g a specific w a s t e w a t e r t r e a t m e n t p r o c e s s , it is i m p o r t a n t t o e s t i m a t e a cost-benefit ratio b e t w e e n t h e benefit d e r i v e d f r o m t h e t r e a t m e n t t o o b t a i n w a t e r o f a specified q u a l i t y , a n d t h e c o s t for a c c o m p l i s h i n g t h i s u p g r a d i n g o f q u a l i t y . R e u s e o f w a t e r b y recycling h a s b e e n m e n t i o n e d i n c o n n e c t i o n w i t h i n p l a n t w a s t e w a t e r c o n t r o l ( S e c t i o n 2.5). S e l e c t i o n of a n o p t i m u m recycle r a t i o f o r a specific a p p l i c a t i o n i n v o l v e s a n e c o n o m i c b a l a n c e in w h i c h t h r e e f a c t o r s m u s t b e c o n s i d e r e d [ 3 ] : (1) c o s t o f r a w w a t e r utilized in t h e p l a n t ; (2) c o s t o f w a s t e w a t e r t r e a t m e n t t o s u i t a b l e p r o c e s s q u a l i t y r e q u i r e m e n t s (in E x a m p l e 1.1, t h i s is t h e c o s t o f w a s t e w a t e r t r e a t m e n t p r e c e d i n g r e c y c l i n g t o t h e p l a n t for r e u s e ) ; a n d (3) c o s t o f w a s t e w a t e r t r e a t m e n t p r i o r t o d i s c h a r g e i n t o a r e c e i v i n g w a t e r , e.g., in a river. T h i s e c o n o m i c b a l a n c e is i l l u s t r a t e d b y E x a m p l e 1.1. E x a m p l e 1.1 [ 3 ] A p l a n t u s e s 10,000 g a l / h r o f p r o c e s s w a t e r w i t h a m a x i m u m c o n t a m i n a n t c o n c e n t r a t i o n o f 1 l b p e r 1000 g a l . T h e r a w w a t e r s u p p l y h a s a c o n t a m i n a n t c o n c e n t r a t i o n o f 0.5 l b / 1 0 0 0 g a l . O p t i m i z e a w a t e r r e u s e s y s t e m f o r t h i s p l a n t b a s e d o n r a w w a t e r c o s t o f $ 0 . 2 0 / 1 0 0 0 g a l . U t i l i z e d a t a in F i g . 1.2 t o e s t i m a t e c o s t s for t h e t w o w a t e r t r e a t m e n t p r o c e s s e s i n v o l v e d in t h e p l a n t . T h e c o n t a m i n a n t is n o n v o l a t i l e . T h e f o l l o w i n g c o n d i t i o n s a p p l y : (1) e v a p o r a t i o n a n d p r o d u c t loss ( s t r e a m Ε in F i g . 1.3): 1000 g a l / h r of w a t e r ; (2) c o n t a m i n a n t a d d i t i o n ( s t r e a m Y in F i g . 1.3): 100 l b / h r of c o n t a m i n a n t ; a n d (3) m a x i m u m d i s c h a r g e a l l o w e d t o receiving w a t e r : 20 lb/hr of c o n t a m i n a n t .
5.
Economics of Treatment and Reuse
11
50h
% Removal of contaminant Fig.
1.2. Relationship
SOLUTION
between
total
cost
and type of treatment
[3].
A b l o c k flow d i a g r a m for t h e p r o c e s s is p r e s e n t e d in F i g . 1.3.
V a l u e s e i t h e r a s s u m e d o r c a l c u l a t e d a r e u n d e r l i n e d in F i g . 1.3. V a l u e s n o t u n d e r l i n e d a r e b a s i c d a t a for t h e p r o b l e m . V o l u m e t r i c flow r a t e s o f s t r e a m s 9, 10, a n d 11 a r e negligible. T h e p r o c e d u r e for s o l u t i o n c o n s i s t s o f a s s u m i n g several v a l u e s for t h e w a t e r recycle R ( g a l / h r ) . F o r e a c h a s s u m e d v a l u e , t h e m a t e r i a l b a l a n c e is c o m p l e t e d a n d t h e e c o n o m i c e v a l u a t i o n is m a d e . Step 1. S t a r t a s s u m i n g a 7 0 % recycle, i.e., R/A = 0.7 (recycle r a t i o ) , w h e r e R is t h e recycle, i.e., s t r e a m 2 ( g a l / h r ) , a n d A is t h e c o m b i n e d feed, i.e., s t r e a m 3 (10,000 g a l / h r ) . T h e n , c a l c u l a t e t h e r e c y c l e : R = (0.7)(Λ) = (0.7)(10,000) = 7000 gal/hr
[stream 2]
T h u s , s t r e a m 5 in F i g . 1.3 a l s o c o r r e s p o n d s t o a flow r a t e o f 7 0 0 0 g a l / h r since t h e v o l u m e t r i c flow r a t e o f c o n t a m i n a n t r e m o v e d [ s t r e a m 11] is negligible. Step 2. F o r t h i s a s s u m e d recycle, t h e r a w w a t e r feed [ s t r e a m 1] is F = A - R = 10,000 - 7000 = 3000 gal/hr
1.
12
Introduction
Step 3. Effluent f r o m t h e p l a n t [ s t r e a m 4 ] is A - Ε = 10,000 - 1000 = 9000 gal/hr Step 4. F r o m t h e m a t e r i a l b a l a n c e it follows t h a t since s t r e a m 4 is s p l i t i n t o s t r e a m s 5 a n d 6, Stream 6: 9000 - 7000 = 2000 gal/hr Stream 7 : 2000 gal/hr T h u s f o r 7 0 % recycle, v o l u m e t r i c flow r a t e s f o r all s t r e a m s in F i g . 1.3 a r e n o w determined. Step 5. M a s s flow r a t e o f c o n t a m i n a n t in r a w w a t e r [ s t r e a m 1] is F x (0.5/1000) = 3000(0.5/1000) = 1.5 lb/hr Step 6. M a s s flow r a t e of c o n t a m i n a n t in s t r e a m 3 is (1/1000) χ 10,000 = 10 lb/hr Step
7. M a s s flow r a t e of c o n t a m i n a n t in t h e recycle [ s t r e a m 2 ] is 1 0 - 1 . 5 = 8.5 lb/hr
Θ
Loss-. lOOO g a l / h r of water
F- 3 0 0 0 gal/hr raw water / 0 . 5 lb contaminant per / 1000 gal; contaminant: \ \ 5 lb/hr
Contaminant addition 100 lb/hr -Plant effluent: 9 0 0 0 gal/hr 110 lb of contaminant
A = 10,000 gal/hr I lb contaminant per 1000 gal .·. 10 lb contaminant/hr
R, recycle R gal/hr 7 0 0 0 gal/hr contaminant: Treatment for reuse
8.5 lb/hr
Contaminant removed D lb/hr D=77 lb/hr
Fig. 1.3. Flow diagram for (Adaptedfrom Eckenfelder [3].)
Example
Contaminant removed Β lb/hr Β=4.5 lb/hr 7000 gal/hr contaminant; 85.5 lb/hr-7 2 0 0 0 gal/hr 24.5 lb/hr of contaminant
Treatment for discharge to receiving water
Water d i s c h a r g e d — η to river W gal/hr W = 2 0 0 0 gal/hr (20 lb/hr of contaminant) 1.1. Encircled
numbers
are
streams.
5.
Economics of Treatment and Reuse
13
Step 8. M a s s flow r a t e o f c o n t a m i n a n t in t h e p l a n t effluent [ s t r e a m 4 ] is 10 [from stream 3] + 100 [from stream 9] = 110 lb/hr Step 9. M a s s flow r a t e o f c o n t a m i n a n t in s t r e a m s 5 a n d 6 is Stream 5 : (7000/9000) χ 110 = 85.5 lb/hr Stream 6: 110 - 85.5 = 24.5 lb/hr Step
10. S i n c e t h e m a s s flow r a t e o f c o n t a m i n a n t i n s t r e a m 7 is 2 0 l b / h r ,
t h a t f o r c o n t a m i n a n t in s t r e a m 10 is 24.5 - 20.0 = 4.5 lb/hr Step
11. M a s s flow r a t e o f c o n t a m i n a n t r e m o v e d in t h e t r e a t m e n t f o r
r e u s e [ s t r e a m 11] is 85.5 - 8.5 = 77.0 lb/hr Step 12. T h e % r e m o v a l of c o n t a m i n a n t i n t h e t w o t r e a t m e n t s is T r e a t m e n t for reuse: (77/85.5) χ 100 = 90% T r e a t m e n t for discharge t o receiving w a t e r : (4.5/24.5) χ 100 = 18.4% Step
13. T h e t y p e o f t r e a t m e n t r e q u i r e d is e s s e n t i a l l y e s t a b l i s h e d f r o m
t h e s e % r e m o v a l s o f c o n t a m i n a n t ( F i g . 1.2). I n t h e t r e a t m e n t for r e u s e ( 9 0 % r e m o v a l ) , i o n e x c h a n g e is i n d i c a t e d . F o r d i s c h a r g e t o r e c e i v i n g w a t e r ( 1 8 . 4 % r e m o v a l ) , F i g . 1.2 i n d i c a t e s t h a t p r i m a r y t r e a t m e n t is sufficient. C o s t s f o r t h e s e t r e a t m e n t s a r e r e a d f r o m F i g . 1.2. T r e a t m e n t for reuse (90% r e m o v a l ) : $0.42/1000 gal T r e a t m e n t for discharge to receiving water (18.4% r e m o v a l ) : $0.05/1000 gal Step 14. D a i l y c o s t f o r 7 0 % r e c y c l e : Raw water:
gal $0.20 hr 3000^— χ — — — - χ 2 4 — = $14.40/day hr 1000 gal day
Cost Effluent treatment for discharge to river: gal $0.05 hr 2000f - χ — — χ 2 4 — = $ 2.40/day hr 1000 gal day gal $0.42 hr T r e a t m e n t for reuse: 7 0 0 0 — χ — — — · χ 2 4 — = $70.56/day hr 1000 gal day Total:
$87.36/day
Step 15. T h i s c o s t is p l o t t e d in F i g . 1.4 v s . 7 0 % reflux. A s i m i l a r series o f c a l c u l a t i o n s is m a d e for f r e s h w a t e r i n p u t s v a r y i n g f r o m 10,000 t o 2 0 0 0 g a l / h r ,
14
1.
Introduction
140
60
1
20
ι
ι
40
60
80
100
Recycle, % Fig. 1.4. Relationship recycle for reuse [3].
between
total
daily
water
cost
and
treated
waste
w i t h recycles v a r y i n g , r e s p e c t i v e l y , f r o m 0 - 8 0 % . F i g u r e 1.4 is o b t a i n e d , w h i c h i n d i c a t e s t h a t t h e o p t i m u m recycle is a p p r o x i m a t e l y 6 0 % f o r a c o s t o f a b o u t $83.00/day.
6. Effect of W a t e r Pollution on E n v i r o n m e n t a n d Biota B a r t s c h a n d I n g r a m [ 1 ] m a d e a n i n t e r e s t i n g s t u d y o f t h e effect o f w a t e r p o l l u t i o n o n e n v i r o n m e n t a n d b i o t a . T h e s e effects a r e i l l u s t r a t e d b y F i g s . 1.5-1.10, a n d a s u m m a r y o f t h e i r w o r k is p r e s e n t e d n e x t . T h e s o u r c e o f p o l lution considered w a s r a w domestic sewage for a c o m m u n i t y of 40,000 people, flowing t o a s t r e a m w i t h a v o l u m e flow o f 100 f t / s e c . L o w e r i n g o f t h e c o n c e n t r a t i o n o f d i s s o l v e d o x y g e n ( D O ) a n d f o r m a t i o n of s l u d g e d e p o s i t s a r e t h e most c o m m o n environmental disturbances which may damage aquatic biota. 3
6.1. O X Y G E N S A G C U R V E T h e c u r v e in F i g . 1.5, r e f e r r e d t o a s d i s s o l v e d o x y g e n c u r v e , is a p l o t o f d i s s o l v e d o x y g e n c o n c e n t r a t i o n ( m g / l i t e r ) for a s t r e a m . I t is r e f e r r e d t o h e n c e a s o x y g e n s a g c u r v e . S e w a g e is d i s c h a r g e d a t t h e p o i n t identified a s z e r o (0) o n t h e a b s c i s s a axis. T h e v a l u e s t o t h e r i g h t o f p o i n t z e r o r e p r e s e n t m i l e s d o w n s t r e a m o f t h e p o i n t o f s e w a g e d i s c h a r g e . C o m p l e t e m i x i n g is a s s u m e d , a n d t h e w a t e r t e m p e r a t u r e is 2 5 ° C . A n a l t e r n a t i v e scale f o r t h e a b s c i s s a , in t e r m s o f d a y s o f flow, is s h o w n in F i g . 1.5.
6.
Effect of Water Pollution on Environment and Biota
Fig.
1.5. DO and BOD
curves
for a stream
15
[1 ] .
O r d i n a t e o f t h e D O s a g c u r v e is in t e r m s o f m g / l i t e r o f d i s s o l v e d o x y g e n . T h e shape of the D O sag curve, d o w n s t r e a m of the p o i n t of sewage discharge, is u n d e r s t o o d f r o m e x a m i n a t i o n o f F i g . 1.6. T h e D O s a g c u r v e is t h e n e t resultant of t w o c u r v e s : o n e c o r r e s p o n d i n g to depletion of dissolved oxygen d u e t o its u t i l i z a t i o n for o x i d a t i o n o f o r g a n i c m a t e r i a l s f r o m t h e s e w a g e d i s charge, and the other corresponding to oxygen gain by natural reaeration. F i g u r e 1.5 s h o w s t h a t t h e D O s a g c u r v e r e a c h e s a l o w p o i n t a b o u t 2 7 m i l e s d o w n s t r e a m o f t h e p o i n t o f s e w a g e d i s c h a r g e , c o r r e s p o n d i n g t o 2\ d a y s o f flow a n d a D O of a b o u t 1.5 m g / l i t e r . Net oxygen-sag curve
Ε Ο Q
Miles (days) Fig.
1.6. Oxygen
sag
curve.
16
1.
Introduction
T h i s p r o c e s s o f d e o x y g e n a t i o n w o u l d r e d u c e t h e D O t o z e r o in a b o u t \ \ d a y s flow, if t h e r e w e r e n o f a c t o r s in o p e r a t i o n t h a t c o u l d r e s t o r e o x y g e n t o w a t e r . T h e river r e a c h w h e r e D O w o u l d b e c o m p l e t e l y g o n e w o u l d o c c u r a b o u t 18 m i l e s d o w n s t r e a m f r o m t h e d i s c h a r g e o f s e w a g e . A f t e r r e a c h i n g its m i n i m u m , D O level rises a g a i n t o w a r d a r e s t o r a t i o n , e v e n t u a l l y r e a c h i n g a v a l u e n e a r l y e q u a l t o t h a t f o r t h e u p s t r e a m u n p o l l u t e d w a t e r , i.e., a D O of approximately 7 mg/liter. If p o p u l a t i o n o f t h e city r e m a i n s fairly c o n s t a n t t h r o u g h o u t t h e y e a r , a n d flow r a t e is relatively c o n s t a n t , t h e l o w p o i n t o f t h e D O s a g c u r v e m o v e s u p o r d o w n t h e s t r e a m w i t h fluctuations i n t e m p e r a t u r e . D u r i n g t h e w i n t e r t h e r a t e o f o x i d a t i o n is l o w e r a n d g a i n o f o x y g e n b y r e a e r a t i o n is g r e a t e r , a s solubility of oxygen in water increases at lower t e m p e r a t u r e s . T h e s e t w o factors c o m b i n e d cause the low p o i n t of the oxygen sag curve t o m o v e farther d o w n s t r e a m . D u r i n g t h e s u m m e r , o n t h e o t h e r h a n d , t h e r a t e of o x i d a t i o n is h i g h e r a n d g a i n o f o x y g e n b y r e a e r a t i o n is less p r o n o u n c e d . T h e s e t w o f a c t o r s c o m b i n e d c a u s e t h e l o w p o i n t of t h e o x y g e n s a g c u r v e t o m o v e u p s t r e a m . T h e r e a c h o f a n y s t r e a m w h e r e t h e D O s a g c u r v e a t t a i n s its l o w p o i n t r e p r e s e n t s t h e s t r e a m e n v i r o n m e n t p o o r e s t in D O r e s o u r c e s . L i v i n g s p e c i m e n s t h a t n e e d a h i g h D O , s u c h a s c o l d w a t e r fish, suffocate a n d m o v e t o o t h e r stream areas where the D O resources are greater. T h e o t h e r c u r v e s h o w n i n F i g . 1.5 c o r r e s p o n d s t o t h e biochemical oxygen demand ( B O D ) . T h i s i m p o r t a n t p a r a m e t e r is d i s c u s s e d in C h a p t e r 2, S e c t i o n 2 . 3 . T h e b i o c h e m i c a l o x y g e n d e m a n d is u s e d a s a m e a s u r e o f t h e q u a n t i t y o f o x y g e n r e q u i r e d f o r o x i d a t i o n b y a e r o b i c b i o c h e m i c a l a c t i o n of t h e d e g r a d a b l e o r g a n i c m a t t e r p r e s e n t in a s a m p l e o f w a t e r . T h e B O D is l o w in t h e u p s t r e a m u n p o l l u t e d w a t e r ( a b o u t 2 m g / l i t e r ) , since t h e r e is n o t m u c h o r g a n i c m a t t e r present to c o n s u m e oxygen. T h e n B O D increases abruptly at point zero (sewage discharge), a n d gradually decreases d o w n s t r e a m from this point, as o r g a n i c m a t t e r d i s c h a r g e d is p r o g r e s s i v e l y o x i d i z e d , u n t i l r e a c h i n g e v e n t u a l l y a v a l u e o f a p p r o x i m a t e l y 2 m g / l i t e r , i n d i c a t i v e of u n p o l l u t e d w a t e r . A t t h i s p o i n t t h e r a w s e w a g e is stabilized. A s i n d i c a t e d in F i g . 1.5, s t a b i l i z a t i o n is a c h i e v e d a t a p p r o x i m a t e l y 100 m i l e s d o w n s t r e a m f r o m t h e s e w a g e d i s c h a r g e . B O D a n d D O a r e s o i n t e r r e l a t e d t h a t d i s s o l v e d o x y g e n c o n c e n t r a t i o n is l o w w h e r e B O D is h i g h , a n d t h e c o n v e r s e a l s o is t r u e . F o u r d i s t i n c t z o n e s a r e s h o w n in F i g . 1.5 u n d e r n e a t h t h e D O c u r v e : (1) c l e a n w a t e r z o n e ; (2) z o n e of d e g r a d a t i o n ; (3) z o n e of a c t i v e d e c o m p o s i t i o n ; a n d (4) z o n e o f r e c o v e r y .
6.2. E F F E C T O F L I G H T I n F i g . 1.6 t h e effects o f o x y g e n d e p l e t i o n b y o x i d a t i o n of o r g a n i c m a t e r i a l s a n d o x y g e n g a i n b y r e a e r a t i o n a r e t h e o n l y o n e s c o n s i d e r e d in e x p l a i n i n g t h e s h a p e of t h e o x y g e n s a g c u r v e . F o r a m o r e c o m p l e t e a n a l y s i s o f t h e p r o b l e m o n e n e e d s , in a d d i t i o n , t o c o n s i d e r t h e effect of light.
17
Effect of Water Pollution on Environment and Biota
6.
A t a n y selected p o i n t in t h e s t r e a m , t h e r e is a v a r i a t i o n i n c o n c e n t r a t i o n o f dissolved oxygen d e p e n d i n g o n the time of day. D u r i n g daylight h o u r s , algae a n d o t h e r p l a n t s give off o x y g e n i n t o t h e w a t e r t h r o u g h t h e p r o c e s s o f p h o t o synthesis. This a m o u n t of oxygen m a y be so considerable t h a t the w a t e r usually becomes supersaturated at some time during daylight h o u r s .
In
a d d i t i o n t o g i v i n g off o x y g e n , t h e p r o c e s s o f p h o t o s y n t h e s i s r e s u l t s in t h e m a n u f a c t u r e of s u g a r t o serve a s t h e b a s i s o f s u p p o r t f o r all s t r e a m life. T h i s c o r r e s p o n d s t o t h e c h e m i c a l r e a c t i o n s h o w n in E q . (1.1). 6C0 + 6H 0 2
C H
2
6
i 2
0
+ 60
6
(1.1)
2
W h i l e p h o t o s y n t h e s i s o c c u r s , s o d o e s r e s p i r a t i o n , w h i c h c o n t i n u e s for 2 4 h r a d a y , i r r e s p e c t i v e of i l l u m i n a t i o n . D u r i n g r e s p i r a t i o n 0
2
is t a k e n in a n d C 0
2
is g i v e n off. D u r i n g d a y l i g h t , a l g a e m a y yield o x y g e n in excess o f t h a t n e e d e d for r e s p i r a t i o n , a s well a s in excess of t h a t r e q u i r e d for r e s p i r a t i o n b y o t h e r a q u a t i c life, a n d for s a t i s f a c t i o n o f a n y b i o c h e m i c a l o x y g e n d e m a n d . T h i s c o u l d b e t r u e in t h e r e c o v e r y z o n e p a r t i c u l a r l y . U n d e r t h e s e c o n d i t i o n s , s u p e r s a t u r a t i o n of o x y g e n m a y o c c u r , a n d s u r p l u s o x y g e n m a y b e l o s t t o t h e atmosphere. D u r i n g t h e n i g h t , p h o t o s y n t h e s i s d o e s n o t o c c u r a n d t h e s u r p l u s D O is g r a d u a l l y u s e d u p b y r e s p i r a t i o n of all f o r m s o f a q u a t i c life, a s well a s for t h e satisfaction
of biochemical oxygen d e m a n d . Therefore, concentration
of
d i s s o l v e d o x y g e n is a t its m i n i m u m d u r i n g e a r l y m o r n i n g h o u r s . T o t a k e i n t o a c c o u n t s u c h D O v a r i a t i o n s , s a m p l i n g o f s t r e a m s for s a n i t a r y s u r v e y s is c o n ducted over a 24-hr period.
6.3.
D E C O M P O S I T I O N OF C A R B O N A C E O U S A N D NITROGENOUS ORGANIC MATTER
A c c e l e r a t e d b a c t e r i a l g r o w t h is a r e s p o n s e t o r i c h f o o d s u p p l i e s in t h e d o m e s t i c s e w a g e . D u r i n g r a p i d u t i l i z a t i o n o f f o o d , b a c t e r i a l r e p r o d u c t i o n is a t a n o p t i m u m , a n d u t i l i z a t i o n o f D O b e c o m e s fairly p r o p o r t i o n a l t o t h e r a t e o f f o o d u t i l i z a t i o n . F i g u r e 1.7 i l l u s t r a t e s t h e p r o g r e s s i v e d o w n s t r e a m c h a n g e s o f o r g a n i c n i t r o g e n t o a m m o n i a , n i t r i t e , a n d finally n i t r a t e . A h i g h initial c o n s u m p t i o n of oxygen by bacterial feeding o n proteinaceous c o m p o u n d s
^ O r g a n i c nitrogen 2I 0 L
2 24
1 0 0
I
2 24
3 48
4
5
6 72
7
8
9
96
Miles Fig.
1.7. Aerobic
decomposition
of nitrogenous
organic
matter
[1],
1.
18
Introduction
a v a i l a b l e in u p s t r e a m w a t e r s t a k e s p l a c e d u e t o t h e freshly d i s c h a r g e d d o m e s t i c s e w a g e . W i t h fewer a n d fewer o f t h e s e c o m p o u n d s left in d o w n s t r e a m w a t e r s , t h e D O c o n c e n t r a t i o n is p r o g r e s s i v e l y r e c o v e r e d , r e a c h i n g e v e n t u a l l y
its
initial value of approximately 7 mg/liter. A s i m i l a r p r o c e s s t a k e s p l a c e w i t h fat a n d c a r b o h y d r a t e foodstuffs.
The
final p r o d u c t s o f a e r o b i c a n d a n a e r o b i c d e c o m p o s i t i o n of n i t r o g e n o u s a n d carbonaceous matter are 1. D e c o m p o s i t i o n o f n i t r o g e n o u s o r g a n i c m a t t e r A e r o b i c (final p r o d u c t s ) : N 0 ~ , C 0 , H 0 , S O j " 3
2
2
A n a e r o b i c (final p r o d u c t s ) : m e r c a p t a n s , i n d o l e , s k a t o l e , H S , p l u s 2
miscellaneous products 2. D e c o m p o s i t i o n o f c a r b o n a c e o u s m a t t e r Aerobic: C 0 , H 0 2
Anaerobic:
2
acids,
alcohols,
C0 , 2
H , 2
CH , 4
plus
miscellaneous
products N i t r o g e n a n d p h o s p h o r u s i n s e w a g e p r o t e i n s c a u s e special p r o b l e m s in s o m e r e c e i v i n g w a t e r s . H i g h c o n c e n t r a t i o n s o f t h e s e e l e m e n t s in w a t e r c r e a t e c o n d i t i o n s especially f a v o r a b l e for g r o w i n g g r e e n p l a n t s . If t h e w a t e r is free flowing ( r i v e r s , b r o o k s ) , g r e e n velvety c o a t i n g s g r o w o n t h e s t o n e s a n d p o s s i b l y l e n g t h y s t r e a m e r s , p o p u l a r l y k n o w n a s m e r m a i d ' s tresses, w a v e in t h e c u r r e n t . T h e s e g r o w t h s a r e n o t u n a t t r a c t i v e a n d a l s o c o n s t i t u t e a m i n i a t u r e j u n g l e in w h i c h a n i m a l life of m a n y k i n d s p r e y o n e a c h o t h e r , w i t h t h e s u r v i v o r s g r o w i n g t o b e c o m e e v e n t u a l fish f o o d . If, h o w e v e r , t h e w a t e r is q u i e t (e.g., l a k e s ) , g r o w t h o f v e r y u n d e s i r a b l e t y p e s o f a l g a e is s t i m u l a t e d . T h e s e a l g a e m a k e t h e w a t e r p e a g r e e n , smelly, a n d u n a t t r a c t i v e . T h i s p h e n o m e n o n is d i s c u s s e d in Section 7 of this chapter. Sometimes, these blue-green algae develop poisons c a p a b l e of killing l i v e s t o c k , wildlife, a n d fish.
6.4. S L U D G E D E P O S I T S A N D A Q U A T I C
PLANTS
A profile s h o w i n g s l u d g e d e p t h vs. d i s t a n c e f r o m t h e outfall of t h e s e w a g e is s h o w n i n t h e b o t t o m p a r t o f F i g . 1.8. M a x i m u m d e p t h o c c u r s n e a r t h e o u t f a l l , a n d t h e n t h e s l u d g e is g r a d u a l l y r e d u c e d b y d e c o m p o s i t i o n t h r o u g h t h e a c t i o n o f b a c t e r i a a n d o t h e r o r g a n i s m s , u n t i l it b e c o m e s insignificant a b o u t 30 miles b e l o w t h e m u n i c i p a l i t y . A l s o a t t h e o u t f a l l t h e r e is g r e a t t u r b i d i t y d u e t o t h e p r e s e n c e o f fine s u s p e n d e d solids. A s t h e s e s o l i d s settle, t h e w a t e r b e c o m e s c l e a r a n d a p p r o a c h e s t h e t r a n s p a r e n c y o f u p s t r e a m w a t e r , a b o v e t h e p o i n t of s e w a g e d i s c h a r g e . D i s t r i b u t i o n o f a q u a t i c p l a n t s is i n d i c a t e d i n t h e u p p e r p a r t o f F i g . 1.8. S h o r t l y after t h e d i s c h a r g e , m o l d s a t t a i n m a x i m u m g r o w t h . T h e s e m o l d s a n d filamentous b a c t e r i a (Sphaerotilus) are associated with the sludge deposition
6.
Effect of Water Pollution on Environment and Biota
Fig.
1.8. Sludge
deposits
and aquatic
19
plants
[1 ] .
s h o w n in t h e l o w e r c u r v e . F r o m m i l e 0 t o m i l e 36, h i g h t u r b i d i t y is n o t c o n d u c i v e t o p r o d u c t i o n o f a l g a e , since t h e y n e e d s u n l i g h t in o r d e r t o g r o w a n d light c a n n o t p e n e t r a t e t h e w a t e r effectively. T h e o n l y t y p e o f a l g a e t h a t m a y grow are blue-green algae, characteristic of polluted waters. T h e y m a y cover m a r g i n a l r o c k s in s l i p p e r y layers a n d give off foul o d o r s u p o n s e a s o n a l decomposition. A l g a e b e g i n t o i n c r e a s e in n u m b e r a t a b o u t m i l e 36. P l a n k t o n o r freefloating f o r m s b e c o m e s t e a d i l y m o r e a b u n d a n t . T h e y c o n s t i t u t e a n e x c e l l e n t f o o d s u p p l y for a q u a t i c a n i m a l s a n d a l s o p r o v i d e s h e l t e r f o r t h e m . T h u s , a s p l a n t s r e s p o n d d o w n s t r e a m in d e v e l o p i n g a diversified p o p u l a t i o n i n t h e recovery a n d clean water zones, a n i m a l s follow a parallel d e v e l o p m e n t , p r o d u c i n g a g r e a t v a r i e t y o f species. 6.5.
BACTERIA A N D
CILIATES
F i g u r e 1.9 i l l u s t r a t e s t h e i n t e r r e l a t i o n b e t w e e n b a c t e r i a a n d o t h e r f o r m s o f a n i m a l p l a n k t o n s u c h a s ciliated p r o t o z o a n s , r o t i f e r s , a n d c r u s t a c e a n s . T w o die-off c u r v e s a r e s h o w n , o n e f o r t o t a l s e w a g e b a c t e r i a a n d t h e o t h e r f o r c o l i f o r m b a c t e r i a o n l y . T h e t w o b e l l - s h a p e d c u r v e s p e r t a i n t o ciliated p r o t o z o a n s a n d rotifers a n d c r u s t a c e a n s . After entering the stream with the sewage, bacteria r e p r o d u c e a n d b e c o m e a b u n d a n t , feeding o n t h e o r g a n i c m a t t e r o f s e w a g e . C i l i a t e d p r o t o z o a n s , initially few in n u m b e r , p r e y o n t h e b a c t e r i a . B a c t e r i a p o p u l a t i o n d e c r e a s e s g r a d u a l l y , b o t h b y a n a t u r a l p r o c e s s o f "die-off," a n d f r o m t h e p r e d a t o r y
20
Fig. 1.9. Bacteria thrive and finally become turn, are food for the rotifers and crustaceans
1.
prey [1 ] .
of the ciliates,
Introduction
which,
in
f e e d i n g b y p r o t o z o a n s . A f t e r a b o u t 2 d a y s flow, a p p r o x i m a t e l y 2 4 m i l e s d o w n s t r e a m o f p o i n t z e r o , t h e e n v i r o n m e n t b e c o m e s m o r e s u i t a b l e for ciliates, w h i c h f o r m t h e d o m i n a n t g r o u p o f a n i m a l p l a n k t o n . A f t e r a b o u t 7 d a y s , 84 m i l e s d o w n s t r e a m of p o i n t z e r o , ciliates fall v i c t i m t o r o t i f e r s a n d c r u s t a c e a n s , w h i c h b e c o m e t h e d o m i n a n t species. T h u s , t h i s s e w a g e - c o n s u m i n g b i o l o g i c a l p r o c e s s d e p e n d s o n a closely i n t e r r e l a t e d s u c c e s s i o n o f species o f a n i m a l p l a n k t o n , o n e k i n d o f o r g a n i s m c a p t u r i n g a n d e a t i n g a n o t h e r . T h i s r e l a t i o n s h i p b e t w e e n b a c t e r i a e a t e r s a n d t h e i r p r e y is f o u n d in t h e o p e r a t i o n o f a m o d e r n s e w a g e t r e a t m e n t p l a n t . I n fact, t h e s t r e a m c a n b e t h o u g h t of a s a n a t u r a l s e w a g e t r e a t m e n t p l a n t . S t a b i l i z a t i o n o f s e w a g e in a p l a n t is m o r e r a p i d w h e n f e r o c i o u s b a c t e r i a e a t i n g ciliates a r e p r e s e n t t o k e e p t h e b a c t e r i a p o p u l a t i o n a t a l o w b u t r a p i d l y g r o w i n g s t a t e . I n s o m e s e w a g e t r e a t m e n t p l a n t s , m i c r o s c o p i c e x a m i n a t i o n is m a d e r o u t i n e l y t o o b s e r v e t h e b a t t l e lines b e t w e e n b a c t e r i a e a t e r s a n d t h e i r prey.
6.6. H I G H E R F O R M S O F A N I M A L S P E C I E S F i g u r e 1.10 i l l u s t r a t e s t h e s e t y p e s o f o r g a n i s m s a n d t h e i r p o p u l a t i o n a l o n g t h e c o u r s e o f t h e s t r e a m . C u r v e (a) r e p r e s e n t s t h e v a r i e t y , i.e., t h e n u m b e r s o f species o f o r g a n i s m s f o u n d u n d e r v a r y i n g d e g r e e s o f p o l l u t i o n . C u r v e (b)
6.
21
Effect of Water Pollution on Environment and Biota
Fig. 1.10. Curve (a) shows the fluctuations variations in numbers of each species [1 ] .
in numbers
of species;
(b)
the
r e p r e s e n t s t h e p o p u l a t i o n in t h o u s a n d s o f i n d i v i d u a l s o f e a c h species p e r square foot. I n t h e c l e a n w a t e r , u p s t r e a m of p o i n t z e r o , a g r e a t v a r i e t y o f o r g a n i s m s is f o u n d w i t h v e r y few o f e a c h k i n d p r e s e n t . A t t h e p o i n t o f s e w a g e d i s c h a r g e , t h e n u m b e r o f different species is g r e a t l y r e d u c e d a n d t h e r e is a d r a s t i c c h a n g e in t h e species m a k e u p of t h e b i o t a . T h i s c h a n g e d b i o t a is r e p r e s e n t e d b y a few species, b u t t h e r e is a t r e m e n d o u s i n c r e a s e in t h e n u m b e r s o f i n d i v i d u a l s o f each kind as c o m p a r e d with the density of p o p u l a t i o n u p s t r e a m . I n c l e a n w a t e r u p s t r e a m t h e r e is a n a s s o c i a t i o n o f s p o r t s fish, v a r i o u s m i n n o w s , c a d d i s w o r m s , mayflies, stoneflies, h e l l g r a m m i t e s a n d g i l l - b r e a t h i n g snails, e a c h k i n d r e p r e s e n t e d b y a few i n d i v i d u a l s . I n b a d l y p o l l u t e d z o n e s t h i s b i o t a is r e p l a c e d b y a n a s s o c i a t i o n o f r a t t a i l e d m a g g o t s , s l u d g e w o r m s , b l o o d w o r m s , a n d a few o t h e r species, r e p r e s e n t e d b y a g r e a t n u m b e r o f individuals. W h e n d o w n s t r e a m conditions again resemble those of the u p stream clean water zone, the clean water a n i m a l association tends t o r e a p p e a r a n d the pollution-tolerant g r o u p of animals b e c o m e suppressed. P o l l u t i o n - t o l e r a n t a n i m a l s a r e especially well a d a p t e d t o life in t h i c k sludge deposits a n d to conditions of low dissolved oxygen. T h e rattailed m a g g o t , for e x a m p l e , p o s s e s s e s a " s n o r k l e l i k e " t e l e s c o p i c a i r t u b e w h i c h is p u s h e d t h r o u g h t h e surface film t o b r e a t h e a t m o s p h e r i c o x y g e n . T h u s , e v e n in t o t a l a b s e n c e o f d i s s o l v e d o x y g e n it s u r v i v e s . T h e s e t y p e s of a n i m a l s a r e found c o m m o n l y a r o u n d sewage treatment plants near the supernatant sludge beds.
22
1.
Introduction
The relationship between the number of species and the total population is expressed in terms of a species diversity index (SDI), which is defined in Eq. (1.2). SDI = ( 5 - l ) / l o g /
(1.2)
where 5, number of species; /, total number of individual organisms counted. From the preceding discussion it is clear that the SDI is an indication of the overall condition of the aquatic environment. The higher its value the more productive is the aquatic system. Its value decreases as pollution increases.
7. E u t r o p h i c a t i o n [4] Eutrophication is the natural process of lake aging. It progresses irrespective of man's activities. Pollution, however, hastens the natural rate of aging and shortens considerably the life expectancy of a body of water. The general sequence of lake eutrophication is summarized in Fig. 1.11. It consists of the gradual progression ("ecological succession") of one life stage to another, based on changes in the degree of nourishment or productivity. The youngest stage of the life cycle is characterized by low concentration of plant nutrients and little biological productivity. Such lakes are called oligotropic lakes (from the Greek oligo meaning "few" and trophein meaning "to nourish," thus oligotropic means few nutrients). At a later stage in the succes sion, the lake becomes mesotrophic (meso = intermediate); and as the life cycle continues the lake becomes eutrophic (eu = well) or highly productive. The final life stage before extinction is a pond, marsh, or swamp. Enrichment and sedimentation are the principal contributors to the aging process. Shore vegetation and higher aquatic plants utilize part of the in flowing nutrients, grow abundantly, and, in turn, trap the sediments. The lake gradually fills in, becoming shallower by accumulation of plants and sediments on the bottom, and smaller by the invasion of shore vegetation, and eventually becoming dry land. The extinction of a lake is, therefore, a result of enrich ment, productivity, decay, and sedimentation. The effect of nitrogen- and phosphorus-rich wastewater discharges on accelerating eutrophication has been discussed in Section 6 of this chapter.
8. T y p e s of W a t e r S u p p l y a n d Classification of W a t e r Contaminants According to their origin, water supplies are classified into three categories: (1) surface waters, (2) ground waters, and (3) meteorological waters. Surface waters comprise stream waters (e.g., rivers), oceans, lakes, and impoundment
24
1.
Introduction
waters. S t r e a m waters subject t o c o n t a m i n a t i o n exhibit a variable quality a l o n g t h e c o u r s e o f t h e s t r e a m , a s d i s c u s s e d in S e c t i o n 6. W a t e r s in l a k e s a n d i m p o u n d m e n t s , o n t h e o t h e r h a n d , a r e o f a relatively u n i f o r m q u a l i t y . G r o u n d w a t e r s s h o w , i n g e n e r a l , less t u r b i d i t y t h a n s u r f a c e w a t e r s . M e t e o r o l o g i c a l waters (rain) are of greater chemical a n d physical purity t h a n either surface or ground waters. W a t e r c o n t a m i n a n t s a r e classified
i n t o t h r e e c a t e g o r i e s : (1) c h e m i c a l ,
(2) p h y s i c a l , a n d (3) b i o l o g i c a l c o n t a m i n a n t s . C h e m i c a l c o n t a m i n a n t s c o m prise b o t h organic and inorganic chemicals. The main concern resulting from p o l l u t i o n b y o r g a n i c c o m p o u n d s is o x y g e n d e p l e t i o n r e s u l t i n g f r o m u t i l i z a t i o n o f D O in t h e p r o c e s s o f b i o l o g i c a l d e g r a d a t i o n o f t h e s e c o m p o u n d s . A s d i s c u s s e d i n S e c t i o n 6, t h i s d e p l e t i o n o f D O l e a d s t o u n d e s i r a b l e d i s t u r b a n c e s o f the environment a n d the biota. In the case of pollution resulting from t h e p r e s e n c e o f i n o r g a n i c c o m p o u n d s t h e m a i n c o n c e r n is t h e i r p o s s i b l e t o x i c effect, r a t h e r t h a n o x y g e n d e p l e t i o n . T h e r e a r e , h o w e v e r , c a s e s i n w h i c h i n o r g a n i c c o m p o u n d s e x e r t a n o x y g e n d e m a n d , so c o n t r i b u t i n g t o
oxygen
d e p l e t i o n . Sulfites a n d n i t r i t e s , for e x a m p l e , t a k e u p o x y g e n , b e i n g o x i d i z e d t o sulfates a n d n i t r a t e s , respectively [ E q s . (1.3) a n d ( 1 . 4 ) ] .
so|- + *o -> soj-
(1.3)
2
N0 - + ±0 2
2
-> N 0 -
(1.4)
3
H e a v y metal ions which are toxic to h u m a n s are i m p o r t a n t c o n t a m i n a n t s . T h e y o c c u r in i n d u s t r i a l w a s t e w a t e r s f r o m p l a t i n g p l a n t s a n d p a i n t a n d p i g m e n t industries. These include H g , A s , C u , Z n , N i , C r , P b , a n d C d . E v e n t h e i r p r e s e n c e in t r a c e q u a n t i t i e s (i.e., m i n i m u m d e t e c t a b l e concentrations) causes serious problems. 2 +
3 +
2 +
2 +
2 +
3 +
2 +
2 +
C o n s i d e r a b l e p r e s s c o v e r a g e h a s b e e n given t o c o n t a m i n a t i o n o f w a t e r b y mercury. Microorganisms convert the mercury ion to methylmercury ( C H H g ) or dimethylmercury [ ( C H ) H g ] . T h e dimethyl c o m p o u n d , being v o l a t i l e , is e v e n t u a l l y lost t o t h e a t m o s p h e r e . M e t h y l m e r c u r y , h o w e v e r , is a b s o r b e d b y fish tissue a n d m i g h t r e n d e r it u n s u i t a b l e f o r h u m a n c o n s u m p t i o n . M e r c u r y c o n t e n t in fish tissue is t o l e r a b l e u p t o a m a x i m u m o f 1 5 - 2 0 p p m . M e t h y l m e r c u r y p r e s e n t in fish is a b s o r b e d b y h u m a n tissues a n d e v e n t u a l l y c o n c e n t r a t e s in c e r t a i n vital o r g a n s s u c h a s t h e b r a i n a n d t h e liver. I n t h e c a s e o f p r e g n a n t w o m e n it c o n c e n t r a t e s in t h e fetus. R e c e n t l y in J a p a n , t h e r e w e r e s e v e r a l r e p o r t e d c a s e s of d e a t h s f r o m m e r c u r y p o i s o n i n g , d u e t o h u m a n c o n s u m p t i o n of m e r c u r y - c o n t a m i n a t e d fish. A n a l y s i s o f fish tissue r e v e a l e d mercury concentrations of a p p r o x i m a t e l y 110-130 p p m . These high m e r c u r y c o n c e n t r a t i o n s , c o u p l e d w i t h t h e l a r g e fish i n t a k e in t h e t y p i c a l J a p a n e s e diet, c a u s e d t h i s t r a g e d y . 3
3
2
25
References
C o n t a m i n a t i o n b y n i t r a t e s is a l s o d a n g e r o u s . F l u o r i d e s , o n t h e o t h e r h a n d , s e e m a c t u a l l y beneficial, t h e i r p r e s e n c e i n p o t a b l e w a t e r s b e i n g r e s p o n s i b l e f o r a p p r e c i a b l e r e d u c t i o n in t h e e x t e n t o f t o o t h d e c a y . T h e r e is, h o w e v e r , c o n siderable controversy concerning S o m e physical c o n t a m i n a n t s
fluoridization
of potable water.
i n c l u d e (1) t e m p e r a t u r e c h a n g e
(thermal
p o l l u t i o n ) . T h i s is t h e c a s e o f relatively w a r m w a t e r d i s c h a r g e d b y i n d u s t r i a l p l a n t s after u s e in h e a t e x c h a n g e r s ( c o o l e r s ) ; (2) c o l o r (e.g., c o o k i n g l i q u o r s d i s c h a r g e d b y c h e m i c a l p u l p i n g p l a n t s ) ; (3) t u r b i d i t y ( c a u s e d b y d i s c h a r g e s c o n t a i n i n g s u s p e n d e d s o l i d s ) ; (4) f o a m s [ d e t e r g e n t s s u c h a s a l k y l b e n z e n e s u l f o n a t e ( A B S ) c o n s t i t u t e i m p o r t a n t c a u s e o f f o a m i n g ] ; a n d (5) r a d i o a c t i v i t y . B i o l o g i c a l c o n t a m i n a n t s a r e r e s p o n s i b l e for t r a n s m i s s i o n o f d i s e a s e s b y w a t e r s u p p l i e s . S o m e o f t h e diseases t r a n s m i t t e d b y b i o l o g i c a l c o n t a m i n a t i o n of water are cholera, typhoid, paratyphoid, a n d shistosomiasis.
References 1. Bartsch, A . F., and Ingram, W. M , Public Works 9 0 , 104 (1959). 2. Byrd, J. P., AlChE Symp. Ser. 6 8 , 137 (1972). 3. Eckenfelder, W. W., Jr., "Water Quality Engineering for Practicing Engineers." Barnes & N o b l e , N e w York, 1970. 4. Greeson, P. E., Water Resour. BuH. 5 , 1 6 (1969). 5. Klei, W. E., and Sundstrom, D . W., AlChE Symp. Ser. 67, 1 (1971). 6. M c G o v e r n , J. G., Chem. Eng. (N.Y.) 8 0 , 137 (1973). 7. N e m e r o w , N . L., "Liquid Wastes o f Industry: Theories, Practice and Treatment." Addison-Wesley, Reading, Massachusetts, 1971.
2 Characterization of Domestic and Industrial Wastewaters 1. Measurement of Concentration of Contaminants in Wastewaters 2. Measurement of Organic Content: Group 1 — Oxygen Parameter Methods 2.1. Theoretical Oxygen Demand (ThOD) 2.2. Chemical Oxygen Demand ( C O D ) 2.3. Biochemical Oxygen Demand ( B O D ) 2.4. Total Oxygen Demand ( T O D ) 3. Measurement of Organic Content: Group 2—Carbon Parameter Methods 3.1. Wet Oxidation Method for T O C 3.2. Carbon Analyzer Determinations 3.3. Oxygen Demand-Organic Carbon Correlation 4. Mathematical Model for the B O D Curve 5. Determination of Parameters k and L 5.1. Log-Difference Method 5.2. Method of M o m e n t s 5.3. Thomas' Graphical Method 6. Relationship between k and Ratio B O D / B O D 7. Environmental Effects on the B O D Test 7.1. Effect of Temperature 7.2. Effect of pH 8. Nitrification 9. Evaluation of Feasibility of Biological Treatment for an Industrial Wastewater 9.1. Introduction 9.2. Warburg Respirometer 9.3. Batch Reactor Evaluation 10. Characteristics of Municipal S e w a g e 11. Industrial Wastewater Surveys 12. Statistical Correlation of Industrial Waste Survey Data Problems · References 0
5
u
26 27 27 28 33 39 44 44 44 46 47 48 48 51 56 58 58 58 59 59 61 61 61 65 65 66 66 68 69
1. M e a s u r e m e n t of C o n c e n t r a t i o n of C o n t a m i n a n t s in W a s t e w a t e r s C o n t a m i n a n t s in w a s t e w a t e r s a r e u s u a l l y a c o m p l e x m i x t u r e o f o r g a n i c a n d i n o r g a n i c c o m p o u n d s . I t is u s u a l l y i m p r a c t i c a l , if n o t n e a r l y i m p o s s i b l e , to obtain complete chemical analysis of m o s t wastewaters. 26
2.
Organic Content Measurement: Oxygen Parameter Methods
27
F o r this reason, a n u m b e r of empirical m e t h o d s for evaluation of c o n c e n t r a t i o n of c o n t a m i n a n t s in wastewaters h a v e been devised, t h e application of which does n o t require knowledge of the chemical c o m p o s i t i o n of the specific
wastewater
under
consideration.
The
most
important
standard
m e t h o d s for a n a l y s i s of o r g a n i c c o n t a m i n a n t s a r e d e s c r i b e d i n S e c t i o n s 2 a n d 3. F o r d i s c u s s i o n o f a n a l y t i c a l m e t h o d s f o r specific i n o r g a n i c c o n t a m i n a n t s in w a s t e w a t e r s , d e t e r m i n a t i o n o f p h y s i c a l p a r a m e t e r s ( t o t a l s o l i d s , c o l o r , o d o r ) , a n d b i o a s s a y tests ( c o l i f o r m s , t o x i c i t y t e s t s ) , t h e r e a d e r is r e f e r r e d t o Ref. [ 1 3 ] . S p e c i a l a t t e n t i o n is g i v e n i n t h i s c h a p t e r t o t h e b i o c h e m i c a l o x y g e n d e m a n d o f w a s t e w a t e r s ( B O D ) . A m a t h e m a t i c a l m o d e l f o r t y p i c a l B O D c u r v e s is d i s c u s s e d , a s well a s t h e e v a l u a t i o n o f feasibility o f b i o l o g i c a l t r e a t m e n t f o r a n industrial wastewater (Sections 4-9). Average characteristics of municipal sewage a n d the procedure followed in industrial wastewater surveys are d e s c r i b e d in S e c t i o n s 10 a n d 1 1 . S i n c e b o t h flow r a t e a n d s e w a g e s t r e n g t h m a y follow a n a l e a t o r y p a t t e r n o f v a r i a t i o n , it m a y b e d e s i r a b l e t o p e r f o r m a s t a t i s t i c a l c o r r e l a t i o n o f s u c h d a t a . T h i s s u b j e c t is d i s c u s s e d i n S e c t i o n 12. A n a l y t i c a l m e t h o d s for o r g a n i c c o n t a m i n a n t s a r e classified i n t o t w o g r o u p s : Group 1. O x y g e n p a r a m e t e r m e t h o d s 1. T h e o r e t i c a l o x y g e n d e m a n d ( T h O D ) 2. C h e m i c a l o x y g e n d e m a n d ( C O D ) [ s t a n d a r d d i c h r o m a t e o x i d a t i o n m e t h o d ; permanganate oxidation test; rapid C O D tests; instrumental C O D methods ("AquaRator")] 3. B i o c h e m i c a l o x y g e n d e m a n d ( B O D ) ( d i l u t i o n m e t h o d s ; m a n o m e t r i c methods) 4. T o t a l o x y g e n d e m a n d ( T O D ) Group 2. C a r b o n p a r a m e t e r m e t h o d s 1. T h e o r e t i c a l o r g a n i c c a r b o n ( T h O C ) 2. T o t a l
organic
carbon
(TOC)
(wet
oxidation
method;
carbon
analyzer determinations)
2. M e a s u r e m e n t of O r g a n i c C o n t e n t : G r o u p 1—Oxygen Parameter Methods 2.1. THEORETICAL OXYGEN D E M A N D
(ThOD)
Theoretical oxygen d e m a n d ( T h O D ) corresponds to the
stoichiometric
a m o u n t of oxygen required to oxidize completely a given c o m p o u n d . Usually e x p r e s s e d in m i l l i g r a m s o f o x y g e n r e q u i r e d p e r liter o f s o l u t i o n , it is a c a l c u l a t e d v a l u e a n d c a n o n l y b e e v a l u a t e d if a c o m p l e t e c h e m i c a l a n a l y s i s of t h e w a s t e w a t e r is a v a i l a b l e , w h i c h is v e r y r a r e l y t h e c a s e . T h e r e f o r e , its u t i l i z a t i o n is very l i m i t e d .
28
2.
Characterization of Domestic and Industrial Wastewaters
T o illustrate t h e calculation of T h O D , consider the simple case of a n a q u e o u s s o l u t i o n o f a p u r e s u b s t a n c e : a s o l u t i o n o f 1000 m g / l i t e r o f l a c t o s e . E q u a t i o n (2.1)* c o r r e s p o n d s t o t h e c o m p l e t e o x i d a t i o n o f l a c t o s e . (CH 0) + 0 30 2
Molecular weight:
2
-
C0
+ H 0 32
2
(2.1)
2
T h O D v a l u e is r e a d i l y o b t a i n e d f r o m a s t o i c h i o m e t r i c c a l c u l a t i o n , b a s e d on Eq. (2.1): 30 (wt. lactose) _ 32 (wt. 0 ) 2
Ϊ000
~
ThOD
.'. T h O D = (32/30)1000 = 1067 mg/liter
2.2. C H E M I C A L O X Y G E N D E M A N D ( C O D ) C h e m i c a l o x y g e n d e m a n d ( C O D ) c o r r e s p o n d s t o t h e a m o u n t of o x y g e n r e q u i r e d t o o x i d i z e t h e o r g a n i c f r a c t i o n of a s a m p l e w h i c h is s u s c e p t i b l e t o p e r m a n g a n a t e o r d i c h r o m a t e o x i d a t i o n in a n a c i d s o l u t i o n . Since o x i d a t i o n p e r f o r m e d i n a C O D l a b o r a t o r y test d o e s n o t n e c e s s a r i l y c o r r e s p o n d t o t h e s t o i c h i o m e t r i c E q . (2.1), C O D v a l u e is n o t e x p e c t e d t o e q u a l T h O D . S t a n d a r d C O D tests ( S e c t i o n s 2.2.1 a n d 2.2.2) yield v a l u e s w h i c h v a r y T A B L E 2.1 Average Values of Oxygen Parameters for Wastewaters as a Fraction of the Theoretical Oxygen Demand (Taken as 100)* ThOD TOD C O D (standard method) C O D (rapid tests) BOD With nitrification Nitrification suppressed
100 92 83 70
2 0
BOD With nitrification Nitrification suppressed
65 55
5
58 52
F o r carbon parameters the T O C represents an average o f about 95% o f the theoretical organic carbon ( T h O C ) . Relationships between T h O D and T h O C are discussed in Section 3 . a
* For simplicity in Eq. (2.1), lactose was represented
by o n e sugar unit
(CH 0). 2
Multiplying this unit by a factor o f 12 o n e obtains Q 2 H 2 4 O 1 2 , which is the molecular formula for lactose.
2.
29
Organic Content Measurement: Oxygen Parameter M e t h o d s
f r o m 8 0 - 8 5 % of t h e T h O D , d e p e n d i n g o n t h e c h e m i c a l c o m p o s i t i o n o f t h e w a s t e w a t e r b e i n g t e s t e d . R a p i d C O D tests, d i s c u s s e d in S e c t i o n 2 . 2 . 3 , yield v a l u e s e q u a l t o a p p r o x i m a t e l y 7 0 % of T h O D v a l u e . A p p r o x i m a t e relationships between the various oxygen a n d c a r b o n p a r a m eters a r e p r e s e n t e d in T a b l e 2 . 1 , a s e s t i m a t e d f r o m a g r a p h in E c k e n f e l d e r a n d F o r d [ 4 ] . V a l u e s i n d i c a t e d i n T a b l e 2.1 a r e t y p i c a l a v e r a g e v a l u e s ; c o r r e c t r e l a t i o n s h i p s s h o u l d b e d e t e r m i n e d for t h e w a s t e w a t e r in q u e s t i o n , a s t h e y a r e d e p e n d e n t u p o n its c h e m i c a l c o m p o s i t i o n . T h u s , v a l u e s in T a b l e 2.1 a r e o n l y u t i l i z e d f o r r o u g h e s t i m a t e s in t h e a b s e n c e o f a c t u a l d a t a . F o u r t y p e s o f C O D tests a r e d e s c r i b e d n e x t .
2.2.1. S t a n d a r d D i c h r o m a t e Oxidation M e t h o d [5, 8,13] T h e s t a n d a r d d i c h r o m a t e C O D test is w i d e l y u s e d f o r e s t i m a t i n g t h e c o n c e n t r a t i o n o f o r g a n i c m a t t e r in w a s t e w a t e r s . T h e t e s t is p e r f o r m e d b y h e a t i n g u n d e r t o t a l reflux c o n d i t i o n s a m e a s u r e d s a m p l e w i t h a k n o w n excess o f p o t a s s i u m d i c h r o m a t e ( K C r 0 ) , in t h e p r e s e n c e o f sulfuric a c i d ( H S 0 ) , 2
2
7
2
4
f o r a 2 - h r p e r i o d . O r g a n i c m a t t e r in t h e s a m p l e is o x i d i z e d a n d , a s a r e s u l t , y e l l o w d i c h r o m a t e is c o n s u m e d a n d r e p l a c e d b y g r e e n c h r o m i c [ E q . ( 2 . 2 ) ] . Silver sulfate ( A g S 0 ) is a d d e d a s c a t a l y s t . 2
4
C r 0 ? ~ + 14H+ + 6e ^ 2
2Cr
3 +
+ 7H 0
(2.2)
2
M e a s u r e m e n t is p e r f o r m e d b y t i t r a t i n g t h e r e m a i n i n g d i c h r o m a t e o r b y determining colorimetrically the green chromic produced. T h e titration m e t h o d is m o r e a c c u r a t e , b u t m o r e t e d i o u s . T h e c o l o r i m e t r i c m e t h o d , w h e n p e r f o r m e d w i t h a g o o d p h o t o e l e c t r i c c o l o r i m e t e r o r s p e c t r o p h o t o m e t e r , is m o r e r a p i d , easier, a n d sufficiently a c c u r a t e for all p r a c t i c a l p u r p o s e s . If c h l o r i d e s a r e p r e s e n t in t h e w a s t e w a t e r , t h e y i n t e r f e r e w i t h t h e C O D test since c h l o r i d e s a r e o x i d i z e d b y d i c h r o m a t e a c c o r d i n g t o E q . ( 2 . 3 ) . 6C1" + C r 0 ? ~ + 1 4 H 2
+
-
3 C l 4- 2 C r
+ 7H 0
3 +
2
(2.3)
2
T h i s i n t e r f e r e n c e is p r e v e n t e d b y a d d i t i o n o f m e r c u r i c sulfate ( H g S 0 ) t o t h e mixture, as H g combines with C I " to form mercuric chloride ( H g C l ) , w h i c h is essentially n o n i o n i z e d . A 10:1 r a t i o o f H g S 0 : C l " is r e c o m m e n d e d . This corresponds to the following chemical reaction [Eq. (2.4)]. 4
2 +
2
4
Hg
+ 2C1- -
2 +
HgCl j
(2.4)
2
T h e p r e s e n c e of t h e A g S 0 c a t a l y s t is r e q u i r e d for o x i d a t i o n o f s t r a i g h t c h a i n a l c o h o l s a n d a c i d s . If insufficient q u a n t i t y o f H g S 0 is a d d e d , t h e excess Cl~ precipitates the A g S 0 catalyst, thus leading to erroneously low values for t h e C O D test. T h i s c o r r e s p o n d s t o t h e f o l l o w i n g c h e m i c a l r e a c t i o n [Eq. (2.5)]. A g + C1" - A g C l i (2.5) 2
4
4
2
4
+
Characterization of Domestic and Industrial Wastewaters
2.
30
S t a n d a r d f e r r o u s a m m o n i u m sulfate [ F e ( N H ) ( S 0 ) - 6 H 0 ] 4
2
4
2
2
is u s e d
for t h e t i t r a t i o n m e t h o d . O r d i n a r i l y , s t a n d a r d f e r r o u s sulfate loses s t r e n g t h with age, due to air oxidation. Daily standardization and
mathematical
c o r r e c t i o n in t h e c a l c u l a t i o n o f C O D t o a c c o u n t for t h i s d e t e r i o r a t i o n a r e r e c o m m e n d e d [ 1 3 ] . C a d m i u m addition to the stock bottle of ferrous sulfate completely
prevents deterioration.
Ferrous
sulfate
available from
Hach
C h e m i c a l C o m p a n y for t h e C O D t e s t is p r e s e r v e d in t h i s m a n n e r , s o t h a t n o further standardization checks are required. T h e r e c o m m e n d e d p r o c e d u r e is t o c o o l t h e s a m p l e after t h e 2 - h r d i g e s t i o n w i t h K C r 0 , a d d five d r o p s of f e r r o i n i n d i c a t o r , a n d t i t r a t e w i t h t h e s t a n d a r d 2
2
7
f e r r o u s a m m o n i u m sulfate s o l u t i o n u n t i l a r e d - b r o w n c o l o r is o b t a i n e d . T h e e n d p o i n t is v e r y s h a r p . F e r r o i n i n d i c a t o r s o l u t i o n m a y b e p u r c h a s e d a l r e a d y p r e p a r e d (it is a n a q u e o u s s o l u t i o n of 1 , 1 0 - p h e n a n t h r o l i n e m o n o h y d r a t e a n d F e S 0 - 7 H 0 ) . T h e r e d - b r o w n c o l o r c o r r e s p o n d i n g t o t h e e n d p o i n t is d u e t o 4
2
f o r m a t i o n of a c o m p l e x o f f e r r o u s i o n w i t h p h e n a n t h r o l i n e . E q u a t i o n (2.6) c o r r e s p o n d s t o o x i d a t i o n o f f e r r o u s a m m o n i u m sulfate b y d i c h r o m a t e . C r 0 ? " + 14H + 6 F e +
^
2 +
2
2Cr
3 +
+ 6Fe
3 +
(2.6)
+ 7H 0 2
E q u a t i o n (2.7) c o r r e s p o n d s t o f o r m a t i o n o f t h e
ferrous-phenanthroline
c o m p l e x , w h i c h t a k e s p l a c e a s s o o n a s all d i c h r o m a t e is r e d u c e d t o C r
3 +
, and
t h e r e f o r e f u r t h e r a d d i t i o n of f e r r o u s a m m o n i u m sulfate r e s u l t s in a n excess of F e
2 +
(ferrous ion). Fe(C
1 2
H N )i 8
2
+
+ e ^
Fe(C H N )§ 1 2
phenanthroline-ferric (pale blue)
8
+
2
(2.7)
phenanthroline-ferrous (red-brown)
Details concerning preparation a n d standardization of reagents a n d cal c u l a t i o n p r o c e d u r e a r e given in R e f s . [ 5 ] , [ 8 ] , a n d [ 1 3 ] . R e p r o d u c i b i l i t y o f t h e C O D t e s t is affected b y t h e reflux t i m e . C O D v a l u e o b t a i n e d i n c r e a s e s w i t h reflux t i m e u p t o a b o u t 7 h r a n d t h e n r e m a i n s e s s e n t i a l l y c o n s t a n t [ 4 ] . I n s t e a d o f refluxing f o r 7 h r o r m o r e , a p r a c t i c a l reflux t i m e o f 2 h r is r e c o m m e n d e d in the standard procedure.
2.2.2. Permanganate Oxidation Test R e c o m m e n d e d a s t h e s t a n d a r d m e t h o d u n t i l 1965, t h i s test h a s b e e n r e p l a c e d b y t h e d i c h r o m a t e test j u s t d e s c r i b e d . T h i s t e s t utilizes p o t a s s i u m p e r m a n ganate ( K M n 0 ) instead of d i c h r o m a t e as the oxidizing agent. T h e w a s t e w a t e r s a m p l e is b o i l e d w i t h a m e a s u r e d excess o f p e r m a n g a n a t e in a c i d s o l u t i o n ( H S 0 ) f o r 3 0 m i n . T h e p i n k s o l u t i o n is c o o l e d a n d a k n o w n excess o f a m m o n i u m o x a l a t e [ ( N H ) C 0 ] is a d d e d , t h e s o l u t i o n b e c o m i n g c o l o r l e s s . E x c e s s o x a l a t e is t h e n t i t r a t e d w i t h K M n 0 s o l u t i o n u n t i l t h e p i n k 4
2
4
4
2
2
4
4
2.
Organic Content Measurement: Oxygen Parameter Methods
31
c o l o r r e t u r n s . O x a l a t e u s e d is c a l c u l a t e d b y difference, a n d
permanganate
utilized is c a l c u l a t e d f r o m s i m p l e s t o i c h i o m e t r y . E q u a t i o n (2.8) c o r r e s p o n d s t o o x i d a t i o n of t h e o x a l a t e . 5 C O i - + 2 M n 0 " + 16H 2
+
4
10CO + 2 M n
2 +
2
+ 8H 0
(2.8)
2
2.2.3. Rapid C O D Tests S e v e r a l r a p i d C O D tests h a v e b e e n p r o p o s e d i n v o l v i n g d i g e s t i o n
with
d i c h r o m a t e for p e r i o d s o f t i m e s h o r t e r t h a n t h e 2 h r p r e s c r i b e d i n t h e s t a n d a r d test. I n o n e of t h e s e t e c h n i q u e s , t h e w a s t e w a t e r is d i g e s t e d w i t h t h e K C r 0 2
H S0 -AgS0 2
4
4
2
7
s o l u t i o n a t 165°C for 15 m i n . T h e s o l u t i o n is d i l u t e d w i t h
distilled w a t e r a n d t i t r a t e d w i t h f e r r o u s a m m o n i u m sulfate, a s in t h e s t a n d a r d method. I n t h i s test, C O D yield for d o m e s t i c s l u d g e c o r r e s p o n d s t o a p p r o x i m a t e l y 6 5 % of the value obtained by the s t a n d a r d m e t h o d . F o r o t h e r wastewaters, C O D yield r a t i o b e t w e e n t h e r a p i d a n d t h e s t a n d a r d test v a r i e s d e p e n d i n g o n t h e n a t u r e of t h e w a s t e w a t e r .
2.2.4. Instrumental C O D M e t h o d s [ 1 1 , 1 4 , 1 5 ] I n s t r u m e n t a l C O D m e t h o d s a r e v e r y fast a n d yield r e p r o d u c i b l e r e s u l t s . I n this section, the Precision A q u a R a t o r
developed by the D o w
Chemical
C o m p a n y a n d licensed t o t h e P r e c i s i o n Scientific C o m p a n y is d e s c r i b e d . T h e C O D measurement requires only a b o u t 2 min and d a t a are reproducible to w i t h i n ± 3 % o r b e t t e r . R e s u l t s c o r r e l a t e well w i t h t h o s e of t h e s t a n d a r d C O D m e t h o d a n d are m u c h m o r e consistent t h a n B O D tests, which typically vary by ± 1 5 % . T h e A q u a R a t o r is d e s i g n e d t o m e a s u r e o x y g e n d e m a n d in t h e r a n g e o f 1 0 - 3 0 0 m g / l i t e r . S a m p l e s of h i g h e r c o n c e n t r a t i o n a r e h a n d l e d b y p r e l i m i n a r y d i l u t i o n of t h e s a m p l e . A flow d i a g r a m o f t h e P r e c i s i o n A q u a R a t o r is s h o w n in F i g . 2 . 1 . A 20-μ1 s a m p l e (20 χ 1 0 " liter » 0 . 0 2 c m ) , h o m o g e n i z e d if n e c e s s a r y , is injected b y a s y r i n g e i n t o t h e P r e c i s i o n A q u a R a t o r . (See s a m p l e i n j e c t i o n p o r t , S I P . ) T h e s a m p l e is s w e p t t h r o u g h a p l a t i n u m c a t a l y t i c c o m b u s t i o n furnace (SF) by a stream of dry C 0 , which oxidizes t h e c o n t a m i n a n t s t o C O a n d H 0 . W a t e r is s t r i p p e d o u t in a d r y i n g t u b e ( D T ) , a n d r e a c t i o n p r o d u c t s are then passed through a second platinum catalytic treatment. T h e C O con c e n t r a t i o n is m e a s u r e d b y a n i n t e g r a l n o n d i s p e r s i v e i n f r a r e d a n a l y z e r ( I A ) , sensitized for c a r b o n m o n o x i d e . T h e r e s u l t a n t r e a d i n g is d i r e c t l y c o n v e r t e d t o C O D b y u s e of a c a l i b r a t i o n c h a r t . C a r b o n d i o x i d e flow is set a t a p p r o x i m a t e l y 130 c m / m i n b y t h e flow c o n t r o l s y s t e m . A n y t r a c e of o x y g e n p r e s e n t in t h e feed g a s is r e d u c e d b y a "purifying" c a r b o n furnace ( P C F ) , yielding a b a c k g r o u n d gas s t r e a m of C O 6
3
2
2
3
32
Characterization of Domestic and Industrial Wastewaters
2.
PCF Gas "purifying" carbon furnace
Regulator set at 10 psig Control valve
SIP Sample injection port with purging manifold -Start
Flow meter Q
Check valve
F
ο , * Sample furnace Differential pressure regulator (preset) J
Bone dry C0
[Exhaust gas
DT Drying tube
9
Connection to external recorder Fig. 2.1. Flow diagram Scientific Company.)
and C 0
2
of Precision
AquaRator
[ 1 1 ] . (Courtesy
of
Precision
w h i c h is i n d i c a t e d a s a n o r m a l b a s e l i n e o f t h e r e c o r d e r . T h e s a m p l e
is injected i n t o t h e s a m p l e f u r n a c e ( S F ) , w h e r e c o n t a m i n a n t s a n d C 0
2
react
to form a typical mixture of C O , C 0 , a n d H 0 . T h e infrared analyzer (IA) 2
2
d e t e r m i n e s t h e i n c r e a s e o f C O c o n t e n t in t h e g a s s t r e a m , w h i c h is d i r e c t l y r e l a t e d t o C O D o f t h e s a m p l e . E x h a u s t g a s is t h e n d i s c h a r g e d t h r o u g h a s a m p l e inlet p u r g i n g m a n i f o l d . T h e A q u a R a t o r t h e o r y is d i s c u s s e d in S t e n g e r a n d V a n H a l l [ 1 4 , 1 5 ] . E q u a t i o n s (2.9) a n d (2.10) i n d i c a t e t h e t y p e s o f r e a c t i o n s t h a t t a k e p l a c e w h e n o r g a n i c m a t e r i a l is c o m b u s t e d in a t m o s p h e r e s o f o x y g e n a n d c a r b o n d i o x i d e , respectively. Ο,Η,,Ν,Ο, + (w/2)0 2
C H„N O e
c
d
+ mC0
2
-
aC0
2
+ (6/2) H 0 + ( c / 2 ) N 2
(2.9)
2
(m + a ) C O + ( 6 / 2 ) H 0 + ( c / 2 ) N 2
2
(2.10)
If o x y g e n r e q u i r e d i n E q . (2.9) c o u l d b e d e t e r m i n e d e x a c t l y , it w o u l d r e p r e s e n t t h e T h O D of t h e s a m p l e . I d e a l l y , t h e d i c h r o m a t e C O D
determination
a p p r o a c h e s t h i s v a l u e , b u t s o m e c o m p o u n d s a r e difficult t o o x i d i z e b y t h e d i c h r o m a t e t r e a t m e n t . O x i d a t i o n w h i c h t a k e s p l a c e in t h e A q u a R a t o r is m o r e vigorous than dichromate
oxidation, a n d t h u s results represent a
more
realistic level of o x y g e n d e m a n d of t h e c o n t a m i n a n t s p r e s e n t . T h e o r i g i n a t o r s o f t h e m e t h o d u s e d in t h e A q u a R a t o r [ 1 4 , 1 5 ] d e m o n s t r a t e d t h a t (m + a) in E q . (2.10) is e q u a l t o η in E q . ( 2 . 9 ) ; t h a t is, t h e n u m b e r of m o l e s o f c a r b o n m o n o x i d e p r o d u c e d is t h e s a m e a s t h e n u m b e r of o x y g e n a t o m s
2.
33
Organic Content Measurement: Oxygen Parameter M e t h o d s
required. Therefore, instrument readings of c a r b o n m o n o x i d e formed
are
d i r e c t l y r e l a t e d t o c h e m i c a l o x y g e n d e m a n d . C a l i b r a t i o n is c a r r i e d o u t b y injecting s t a n d a r d s o l u t i o n s of s o d i u m a c e t a t e t r i h y d r a t e , f o r w h i c h o x y g e n d e m a n d in m i l l i g r a m s p e r liter c a n b e c a l c u l a t e d . A g r a p h o f o x y g e n d e m a n d vs. r e c o r d e r o u t p u t ( c h a r t d i v i s i o n s ) is all t h a t is r e q u i r e d for d e t e r m i n i n g t h e unknown contaminant demand.
2.3. B I O C H E M I C A L O X Y G E N D E M A N D ( B O D ) B i o c h e m i c a l o x y g e n d e m a n d is u s e d a s a m e a s u r e o f t h e q u a n t i t y o f o x y g e n r e q u i r e d for o x i d a t i o n o f b i o d e g r a d a b l e o r g a n i c m a t t e r p r e s e n t in t h e w a t e r s a m p l e b y a e r o b i c b i o c h e m i c a l a c t i o n . O x y g e n d e m a n d o f w a s t e w a t e r s is e x e r t e d b y t h r e e classes o f m a t e r i a l s : (1) c a r b o n a c e o u s o r g a n i c m a t e r i a l s u s a b l e a s a s o u r c e o f f o o d b y a e r o b i c o r g a n i s m s ; (2) o x i d i z a b l e n i t r o g e n d e r i v e d f r o m n i t r i t e , a m m o n i a , a n d o r g a n i c n i t r o g e n c o m p o u n d s w h i c h serve a s f o o d for specific b a c t e r i a (e.g., Nitrosomonas
a n d Nitrobacter).
This type
o f o x i d a t i o n (nitrification) is d i s c u s s e d in S e c t i o n 8 ; a n d (3) c h e m i c a l r e d u c i n g c o m p o u n d s , e.g., f e r r o u s i o n ( F e
2 +
) , sulfites ( S O
2 -
) , a n d sulfide
(S ~), 2
which are oxidized by dissolved oxygen. F o r d o m e s t i c s e w a g e , n e a r l y all o x y g e n d e m a n d is d u e t o c a r b o n a c e o u s o r g a n i c m a t e r i a l s a n d is d e t e r m i n e d b y B O D tests d e s c r i b e d in
Sections
2.3.1 a n d 2.3.2. F o r effluents s u b j e c t e d t o b i o l o g i c a l t r e a t m e n t , a c o n s i d e r a b l e p a r t o f t h e o x y g e n d e m a n d m a y b e d u e t o n i t r i f i c a t i o n ( S e c t i o n 8 of t h i s chapter).
2.3.1. B O D Dilution Test D e t a i l e d d e s c r i p t i o n of t h e d i l u t i o n t e s t a s well a s p r e p a r a t i o n of r e a g e n t s is given in Ref. [ 1 3 ] . P r o c e d u r e is given b e l o w . 1. P r e p a r e several d i l u t i o n s o f t h e s a m p l e t o b e a n a l y z e d w i t h water of high purity. R e c o m m e n d e d dilutions depend on estimated t r a t i o n o f c o n t a m i n a n t s r e s p o n s i b l e for o x y g e n d e m a n d . F o r h i g h l y i n a t e d w a t e r s , d i l u t i o n r a t i o s ( m l of d i l u t e d s a m p l e / m l of o r i g i n a l m a y b e o f 1 0 0 : 1 . F o r river w a t e r s , t h e s a m p l e m a y b e t a k e n w i t h o u t for l o w p o l l u t i o n s t r e a m s , a n d in o t h e r c a s e s d i l u t i o n r a t i o s o f 4:1 utilized.
distilled concen contam sample) dilution m a y be
2. I n c u b a t i o n b o t t l e s (250- t o 3 0 0 - m l c a p a c i t y ) , w i t h g r o u n d - g l a s s s t o p p e r s a r e utilized. I n t h e B O D b o t t l e o n e p l a c e s (a) t h e d i l u t e d s a m p l e (i.e., t h e " s u b s t r a t e " ) , (b) a seed of m i c r o o r g a n i s m s ( u s u a l l y t h e s u p e r n a t a n t l i q u o r f r o m d o m e s t i c s e w a g e ) , a n d (c) n u t r i e n t s o l u t i o n for t h e m i c r o organisms. This solution contains sodium and potassium phosphates and a m m o n i u m chloride (nitrogen a n d p h o s p h o r u s are elements needed as n u t r i e n t s for m i c r o o r g a n i s m s ) . T h e p H of t h e s o l u t i o n in t h e B O D b o t t l e s h o u l d b e a b o u t 7.0 ( n e u t r a l ) .
34
2.
Characterization of Domestic and Industrial Wastewaters
P h o s p h a t e s o l u t i o n utilized is a buffer.
F o r samples containing
caustic
a l k a l i n i t y o r acidity, n e u t r a l i z a t i o n t o a b o u t p H 7 is m a d e w i t h d i l u t e H S 0 2
4
o r N a O H p r i o r t o t h e B O D test. F o r each B O D bottle a control bottle, which does not contain the substrate, is a l s o p r e p a r e d . 3. B o t t l e s a r e i n c u b a t e d a t 2 0 ° C . E a c h s u c c e e d i n g 2 4 - h r p e r i o d , a s a m p l e bottle and a corresponding control bottle are taken from the incubator, a n d d i s s o l v e d o x y g e n in b o t h is d e t e r m i n e d a s d e s c r i b e d a t t h e e n d of t h i s s e c t i o n . T h e difference
between concentrations of dissolved oxygen (mg/liter)
in
c o n t r o l b o t t l e a n d i n s a m p l e b o t t l e c o r r e s p o n d s t o t h e o x y g e n u t i l i z e d in biochemical oxidation of c o n t a m i n a n t s [ E q . (2.11)]. y (mg/liter) = D O (control bottle) - D O (sample bottle)
(2.11)
V a l u e s o f y ( B O D , m g / l i t e r ) a r e p l o t t e d vs. i n c u b a t i o n t i m e t ( d a y s ) . A t y p i c a l B O D c u r v e for o x i d a t i o n of c a r b o n a c e o u s m a t e r i a l s is s h o w n in F i g . 2 . 2 . C u r v e s for cases w h e r e nitrification t a k e s p l a c e a r e d i s c u s s e d in S e c t i o n 8.
b-BODy
t: Incubation time (days) Fig. 2.2.
Typical
BOD
curve
for oxidation
of carbonaceous
materials.
O x y g e n u t i l i z a t i o n in t h e B O D test is v e r y slow. A t y p i c a l c u r v e ( F i g . 2.2) o n l y r e a c h e s t h e l i m i t i n g B O D in a b o u t 2 0 d a y s o r m o r e . T h i s v a l u e is called ultimate BOD, d e n o t e d a s B O D . M
I t is i m p r a c t i c a l t o m o n i t o r c o n t i n u o u s l y b e c a u s e o f t h e t i m e f a c t o r i n v o l v e d in t h e i n t e r m s o f 5-day B O D , d e n o t e d a s B O D a p e r i o d of t i m e t o w a i t for t h e r e s u l t o f a 5
a process stream in terms of B O D test. I n p r a c t i c e , B O D is r e p o r t e d ( F i g . 2.2). E v e n 5 d a y s is t o o l o n g test.
I t is i m p o r t a n t t o n o t i c e t h a t t h e v a l u e o f B O D is n o t e q u a l t o T h O D , b e c a u s e in t h e B O D b o t t l e n o t all s u b s t r a t e is o x i d i z e d . R a t i o s o f v a l u e s o f B O D (or B O D ) to T h O D depend o n the chemical composition of the waste w a t e r . A v e r a g e v a l u e s a r e given in T a b l e 2 . 1 . M
M
5
T h e ratio of B O D
5
to B O D
M
also varies according t o the substrate. F o r
2.
35
Organic Content Measurement: Oxygen Parameter Methods
domestic sewage, this ratio is approximately 0.77 [Eq. (2.12)]. BOD /BOD 5
M
= 0.77
(2.12)
Considerable experience is required to obtain reliable results in the BOD dilution test. In general, reproducibility of results is not better than ±15%. Some of the difficulties involved in the BOD dilution test are discussed in the next sections. Because of these fluctuations it is recommended that several BOD bottles be taken from the incubator every 24 hr and that statistical averaging of results be performed. a. Ratio of COD
and
BOD
u
It has just been stated that values of BOD and ThOD are not equal. Similarly, the value of BOD is generally lower than that for COD obtained by the standard dichromate oxidation method, as indicated in Table 2.1. The reasons are that (1) many organic compounds which are oxidized by K C r 0 are not biochemically oxidizable and (2) certain inorganic ions such as sulfides ( S " ) , thiosulfates ( S 0 3 ~ ) , sulfites ( S O 3 " ) , nitrites ( N 0 " ) , and ferrous ion ( F e ) are oxidized by K C r 0 , thus accounting for inorganic COD, which is not detected by the BOD test. M
M
2
2
7
2
2
2
2 +
2
b. Effect of Seeding the BOD
2
7
and Acclimation
of Seed
on
Test
One of the most frequent reasons for unreliable BOD values is utilization of an insufficient amount of microorganism seed. Another serious problem for industrial wastes is acclimation of seed. For many industrial wastes, the presence of toxic materials interferes with growth of the microorganism population. BOD curves obtained exhibit a time lag period (Fig. 2.3). Low BOD values are obtained if adequate corrective action is not taken. It becomes necessary to acclimate the microorganism seed to the specific
t (days) Fig. 2.3. Lag period
in BOD
test.
2.
36
Characterization of Domestic and Industrial Wastewaters
waste. This is achieved by starting with a sample of settled domestic sewage which contains a large variety of microorganisms, and adding a small amount of industrial effluent. Air is bubbled through this mixture. The operation is performed in bench scale reactors of either continuous or batch type. These reactors are described in Chapter 5, Section 6.1. The process is repeated with gradual increase in the proportion of industrial waste to domestic sewage, until a microbial culture acclimated to the specific industrial waste is developed. This may be a long and difficult procedure for very toxic industrial wastewaters. When an acclimated culture has been developed, the BOD curve does not present a lag period, thus becoming a typical BOD curve of the general shape shown in Fig. 2.2. c. Effect of Presence
of Algae on the BOD
Test
Presence of algae in the wastewater being tested affects the BOD test. If the sample is incubated in the presence of light, low BOD values are obtained owing to production of oxygen by photosynthesis, which satisfies part of the oxygen demand. On the other hand, if incubation is performed in darkness, algae survive for a while. Thus, short-term BOD determinations show the effect of oxygen on them. After a period in the dark, algae die and algal cells contribute to the increase of total organic content of the sample, thus leading to high BOD values. Therefore, the effect of algae on the BOD test is difficult to evaluate. d. Glucose-Glutamic
Acid
Check
The quality of dilution water, which if contaminated leads to incorrect BOD values, the effectiveness of the seed, and the analytical technique are checked periodically by using pure organic compounds for which BOD is known or determinable. One of the most commonly used is a mixture of glucose ( C H 0 ) and glutamic acid [ H O O C C H C H C H ( N H ) C O O H ] . A mixture of 150 mg/liter of each is recommended. Pure glucose has an exceptionally high oxidation rate with relatively simple seeds. When used with glutamic acid, the oxidation rate is stabilized and is similar to that of most municipal wastewaters. BOD of the standard glucose-glutamic acid solution is 220 ± 1 1 mg/liter. Any appreciable divergence from these values raises questions concerning quality of the distilled water or viability of the seeding material. If a variation greater than ± 2 0 - 2 2 mg/liter occurs more frequently than 5% of the time, this indicates a faulty technique. 6
1 2
e. Determination
6
2
of Dissolved
Oxygen
2
2
(DO)
The BOD dilution method requires determinations of the amount of dis solved oxygen. These determinations are performed by either titration or instrumental methods. The basic titration method is that of Winkler. Waste-
2.
Organic Content Measurement: Oxygen Parameter M e t h o d s
37
w a t e r s m a y c o n t a i n several i o n s a n d c o m p o u n d s w h i c h i n t e r f e r e w i t h t h e original D O determination. T o eliminate these interferences, several m o d i fications
of the basic m e t h o d have been p r o p o s e d [ 1 3 ] . A brief description
follows of the azide modification of W i n k l e r ' s m e t h o d , which
effectively
r e m o v e s i n t e r f e r e n c e c a u s e d b y n i t r i t e s . T h i s is t h e m o s t c o m m o n i n t e r f e r e n c e f o u n d in p r a c t i c e . O t h e r m o d i f i c a t i o n s t o r e m o v e i n t e r f e r e n c e s a r e d e s c r i b e d in Ref. [ 1 3 ] . W i n k l e r ' s m e t h o d is b a s e d o n o x i d a t i o n o f i o d i d e i o n ( I " ) , w h i c h is c o n tained in the alkali-iodide-azide reagent, to iodine ( I ) by dissolved oxygen 2
of the sample, a n d titration of the iodine by s o d i u m thiosulfate ( N a S 0 ) , 2
2
3
utilizing s t a r c h a s i n d i c a t o r . O x i d a t i o n is p e r f o r m e d i n a c i d m e d i u m ( H S 0 ) 2
in t h e p r e s e n c e o f m a n g a n e s e sulfate ( M n S 0 ) . T h e r e a g e n t is a s o l u t i o n o f N a O H , N a l , a n d N a N
3
4
alkali-iodide-azide
4
(sodium azide).
E q u a t i o n (2.13) c o r r e s p o n d s t o t h e o x i d a t i o n of I " t o I . 2
2 1 - -> I + 2e
(2.13)
2
I n t e r f e r e n c e o f n i t r i t e s is d u e t o t h e i r o x i d a t i o n t o N O w i t h f o r m a t i o n o f I
2
[Eq. (2.14)]. 2 N 0 " + 21- + 4 H
+
2
Titration of I
2
-
2NO + I + 2 H 0 2
2
b y t h i o s u l f a t e c o r r e s p o n d s t o E q . (2.15) [ t h i o s u l f a t e
(2.14) (S 0|") 2
is o x i d i z e d t o t e t r a t h i o n a t e ( S 0 6 ) " ] . 4
2S 0§- + I 2
2
S Oi" + 214
(2.15)
S t a r c h yields a b l u e c o l o r in t h e p r e s e n c e o f i o d i n e . T i t r a t i o n w i t h s o d i u m t h i o s u l f a t e is c o n t i n u e d u n t i l t h e b l u e c o l o r d i s a p p e a r s . A v a r i a t i o n o f t h i s p r o c e d u r e utilizes a n e w r e a g e n t ( p h e n y l a r s i n e o x i d e , P A O ) instead of s o d i u m thiosulfate. This reagent h a s the a d v a n t a g e of being stable, whereas s o d i u m thiosulfate deteriorates rapidly a n d should be restandardized before each determination. A description of this i m p r o v e d p r o c e d u r e is f o u n d i n Ref. [ 8 ] . I n s t r u m e n t a l d e t e r m i n a t i o n of d i s s o l v e d o x y g e n is p e r f o r m e d b y D O a n a l y z e r s . A d i a g r a m o f a t y p i c a l m o d e l o f t h e i n s t r u m e n t is s h o w n i n F i g . 2 . 4 . T h e D O a n a l y z e r is a g a l v a n i c s y s t e m w h i c h utilizes a c y l i n d e r - s h a p e d l e a d a n o d e s u r r o u n d i n g a r o d - s h a p e d silver c a t h o d e . B o t h e l e c t r o d e s a r e c o v e r e d b y a l a y e r o f K O H e l e c t r o l y t e c o n t a i n e d in a t h i n e l e c t r o l y t i c p a d . A p l a s t i c m e m b r a n e c o v e r s t h e e l e c t r o d e s a n d e l e c t r o l y t e a n d serves a s a selective diffusion b a r r i e r w h i c h is p e r m e a b l e t o all g a s e s , i n c l u d i n g m o l e c u l a r o x y g e n , b u t is v i r t u a l l y i m p e r m e a b l e t o i o n i c species w h i c h m a y b e p r e s e n t in t h e w a s t e w a t e r s . T o m e a s u r e D O t h e p r o b e is d i p p e d i n t o t h e s a m p l e . A cell c u r r e n t w h i c h is p r o p o r t i o n a l t o t h e o x y g e n c o n c e n t r a t i o n i n t h e s a m p l e is m e a s u r e d directly in t e r m s o f m g / l i t e r o f d i s s o l v e d o x y g e n b y t h e n e e d l e in t h e o x y g e n
38
2.
Characterization of Domestic and Industrial Wastewaters
Fig. 2.4.
Dissolved
oxygen
analyzer.
m e t e r . T h e s a m p l e is c o n s t a n t l y s t i r r e d d u r i n g m e a s u r e m e n t , since o n l y u n d e r t h e s e c o n d i t i o n s is t h e c u r r e n t d i r e c t l y p r o p o r t i o n a l t o t h e o x y g e n c o n c e n t r a t i o n in t h e b u l k o f t h e t e s t s a m p l e . C a l i b r a t i o n o f t h e D O a n a l y z e r is p e r f o r m e d b y m e a s u r i n g t h e D O of a s a m p l e o f k n o w n o x y g e n c o n t e n t , w h i c h is d e t e r mined by standard analytical m e t h o d s (namely, the Winkler m e t h o d ) [ 1 3 ] .
2.3.2. B O D M a n o m e t r i c M e t h o d s T h e m a n o m e t r i c a p p a r a t u s d e s c r i b e d in t h i s s e c t i o n is t h e H a c h M o d e l 2173 [ 7 ] . T h e H a c h B O D a p p a r a t u s has been c o m p a r e d with the s t a n d a r d dilution m e t h o d u n d e r controlled l a b o r a t o r y conditions. In routine analysis it gives n e a r l y e q u i v a l e n t r e s u l t s a n d p r e c i s i o n . Since a p h y s i c a l c h a n g e is o b s e r v e d , c h e m i c a l l a b o r a t o r y a n a l y s i s is n o t r e q u i r e d . A d i a g r a m s h o w i n g o n l y o n e b o t t l e is d e p i c t e d in F i g . 2 . 5 . T h e p r i n c i p l e o f o p e r a t i o n is a s f o l l o w s : A m e a s u r e d s a m p l e of s e w a g e o r w a s t e w a t e r is p l a c e d in a b o t t l e o n t h e a p p a r a t u s , w h i c h is c o n n e c t e d t o a c l o s e d - e n d m e r c u r y m a n o m e t e r . A b o v e t h e s e w a g e o r w a t e r s a m p l e is a q u a n t i t y of air ( w h i c h c o n t a i n s a p p r o x i m a t e l y 2 1 % o x y g e n b y v o l u m e ) . O v e r a p e r i o d o f t i m e b a c t e r i a in t h e s e w a g e utilizes t h e o x y g e n t o o x i d i z e o r g a n i c m a t t e r p r e s e n t in t h e s a m p l e , a n d t h u s d i s s o l v e d o x y g e n is c o n s u m e d . A i r in t h e c l o s e d s a m p l e b o t t l e r e p l e n i s h e s t h e utilized o x y g e n , t h u s r e s u l t i n g in a d r o p of a i r p r e s s u r e in t h e s a m p l e b o t t l e . M e r c u r y in t h e leg o f t h e m a n o m e t e r c o n n e c t e d t o t h e b o t t l e m o v e s u p w a r d , a s i n d i c a t e d b y t h e a r r o w i n F i g . 2 . 5 . T h u s , t h e p r e s s u r e d r o p is r e g i s t e r e d o n t h e m e r c u r y
2.
Organic Content Measurement: Oxygen Parameter Methods
39
m a n o m e t e r a n d r e a d directly in m g / l i t e r B O D . P r i o r t o s t a r t i n g t h e test, set screws o n t h e m a n o m e t e r scale a r e l o o s e n e d a n d t h e z e r o m a r k is set a t t h e t o p of the mercury c o l u m n . D u r i n g t h e t e s t p e r i o d (5 d a y s for B O D ) , t h e s y s t e m is i n c u b a t e d a t 2 0 ° C a n d t h e s a m p l e c o n t i n u a l l y a g i t a t e d b y a m a g n e t i c s t i r r i n g b a r , w h i c h is r o t a t e d b y a p u l l e y s y s t e m c o n n e c t e d t o a m o t o r . C a r b o n d i o x i d e is p r o d u c e d b y o x i d a t i o n of o r g a n i c m a t t e r , a n d m u s t b e r e m o v e d f r o m t h e s y s t e m s o t h a t it d o e s n o t d e v e l o p a p o s i t i v e g a s p r e s s u r e w h i c h w o u l d r e s u l t in a n e r r o r . T h i s is a c c o m p l i s h e d b y a d d i t i o n o f a few d r o p s o f p o t a s s i u m h y d r o x i d e s o l u t i o n in t h e seal c u p o f e a c h s a m p l e b o t t l e . B O D r e a d i n g s a r e p e r i o d i c a l l y c h e c k e d b y utilizing t h e s t a n d a r d g l u c o s e - g l u t a m i c a c i d s o l u t i o n . 5
W h e n high oxygen d e m a n d s are encountered the sample m u s t be diluted. A c c u r a c y o f t h e m a n o m e t r i c test is c l a i m e d a s c o m p a r a b l e t o t h a t o f t h e d i l u t i o n test.
2.4. T O T A L O X Y G E N D E M A N D ( T O D ) [6, 9,17 ] U s e f u l n e s s of t h e s t a n d a r d C O D m e t h o d is d u e t o t h e fact t h a t r e s u l t s a r e o b t a i n e d in 2 h r , r a t h e r t h a n t h e 5 d a y s t a k e n f o r t h e c o m m o n B O D m e a s u r e m e n t . H o w e v e r , t h e C O D m e t h o d is k n o w n not t o o x i d i z e c o n t a m i n a n t s a s p y r i d i n e , b e n z e n e , a n d a m m o n i a , a l t h o u g h for m a n y o r g a n i c c o m p o u n d s oxidation h a s been reported as 9 5 - 1 0 0 % of the theoretical.
40
2.
Characterization of Domestic and Industrial Wastewaters
T h e r e f o r e , t h e s e a r c h for i m p r o v e d a n a l y t i c a l m e t h o d s for d e t e r m i n a t i o n o f o x y g e n d e m a n d h a s f o c u s e d o n t e c h n i q u e s [ 6 ] w h i c h a r e (1) m e a n i n g f u l a n d c o r r e l a t e w i t h t h e a c c e p t e d p a r a m e t e r s for c o n t r o l a n d
surveillance;
(2) r a p i d , s o r e s u l t s a r e k n o w n in m i n u t e s , n o t h o u r s o r d a y s ; a n d (3) t r u l y adaptable to automation and continuous monitoring. T h e Ionics m o d e l 225 T o t a l Oxygen D e m a n d ( T O D ) Analyzer determines t o t a l o x y g e n d e m a n d w i t h i n 3 m i n . F i g u r e 2.6 s h o w s t h e f u n c t i o n a l e l e m e n t s of the system which includes the injection system, the c o m b u s t i o n unit, the oxygen sensor assembly, a n d the recorder.
RECORDER
CATALYST SCRUBBERDETECTOR CELL ASSEMBLY COMBUSTION TUBE
Fig.
2.6.
Copyright
Flow
diagram
by The American
for
Chemical
the
TOD
analyzer
[ 6 ] . (Reprinted
with
permission.
Society.)
T h e w a s t e w a t e r s a m p l e is t r a n s m i t t e d b y a n a i r - o p e r a t e d a s p i r a t o r t o t h e l i q u i d i n j e c t i o n v a l v e . U p o n a c t u a t i o n , t h e v a l v e delivers a 20-μ1 (0.02 c m ) s a m p l e i n t o t h e c o m b u s t i o n c h a m b e r . T h e s a m p l i n g s y s t e m is c o n t r o l l e d b y a n adjustable p r o g r a m timer or by a m a n u a l pushbutton. A carrier gas (nitrogen) c o n t a i n i n g a s m a l l a m o u n t o f o x y g e n o f t h e o r d e r o f 2 0 0 p p m is i n t r o d u c e d simultaneously with the wastewater sample into the combustion chamber. T h e s a m p l e is v a p o r i z e d a n d t h e c o m b u s t i b l e c o m p o n e n t s a r e o x i d i z e d in a c o m b u s t i o n t u b e . T h e t u b e , c o n t a i n i n g a p l a t i n u m s c r e e n c a t a l y s t , is m o u n t e d i n a n electric f u r n a c e w h i c h is m a i n t a i n e d a t 9 0 0 ° C . A s a r e s u l t o f t h e o x y g e n u t i l i z a t i o n in t h e c o m b u s t i o n p r o c e s s , a m o m e n t a r y d e p l e t i o n o f o x y g e n o c c u r s in t h e i n e r t g a s s t r e a m . T h i s d e p l e t i o n is a c c u r a t e l y m e a s u r e d b y p a s s i n g t h e effluent t h r o u g h a p l a t i n u m - l e a d fuel cell. Before e n t e r i n g t h e cell, t h e g a s is s c r u b b e d a n d h u m i d i f i e d . S c r u b b i n g is d o n e b y p a s s i n g t h e g a s t h r o u g h a n a q u e o u s caustic solution which removes carrier gas impurities harmful t o t h e 3
2.
41
Organic Content Measurement: O x y g e n Parameter M e t h o d s
d e t e c t o r cell a n d h u m i d i f i e s t h e g a s e o u s s a m p l e . T h e fuel cell a n d s c r u b b e r a r e l o c a t e d in a t h e r m o s t a t i c a l l y c o n t r o l l e d a n d i n s u l a t e d c h a m b e r . F u e l cell c u r r e n t o u t p u t is a f u n c t i o n o f o x y g e n c o n c e n t r a t i o n . T h i s is graphically m o n i t o r e d on a potentiometer recorder, with changes in current t a k i n g the form of recorder peaks. T h e recorder system includes a n a u t o m a t i c zero circuit to m a i n t a i n a c o n s t a n t baseline. Peaks recorded are linearly p r o p o r t i o n a l t o t h e r e d u c e d o x y g e n c o n c e n t r a t i o n in t h e c a r r i e r g a s a n d t h e s a m p l e t o t a l o x y g e n d e m a n d . T O D m e a s u r e m e n t f o r u n k n o w n s a m p l e s is determined by c o m p a r i s o n of the recorded p e a k heights with a s t a n d a r d calibration curve. A typical calibration curve for s t a n d a r d solution analysis is s h o w n in F i g . 2 . 7 , w h i c h d e m o n s t r a t e s t h e l i n e a r i t y o f p e a k h e i g h t vs. T O D .
/
CHART DIVISIONS ·/
ΛΓ
— % Or ONE hIV
0
Fig.
2.7.
/
7
/
/
C A L I B R A T I O N CUR>IE T« 9 0 0 C N »20 cmVmii 02 200 ppm e
e
2
s
>
0
Typical
<
/
TOD-ppm 200
100 calibration
curve
for
TOD
300
analyzer
[9].
(Courtesy
of
Ionics
Incorporated.)
T h e T O D m e t h o d measures the a m o u n t of oxygen c o n s u m e d based o n the following chemical reactions for the catalytic c o m b u s t i o n process
[Eqs.
(2.16-2.18)]. C + 0 H
2
2
- C 0
+ K> -
Ν (combined) + ± 0
2
2
-
2
(2.16)
H 0
(2.17)
NO
(2.18)
2
S u l f u r o u s c o m p o u n d s a r e o x i d i z e d t o a s t a b l e c o n d i t i o n c o n s i s t i n g o f a fixed
42
2.
r a t i o of S 0
2
Characterization of Domestic and Industrial Wastewaters
to S 0 . Molecular nitrogen, normally used as the carrier gas, 3
does n o t react in the c o m b u s t i o n process. E q u a t i o n (2.19) c o r r e s p o n d s t o a t y p i c a l t h e o r e t i c a l o x i d a t i o n (for t h e c a s e of urea). 2NH CONH 2
2
+ 50
2C0
2
2
+ 4NO + 4 H 0
(2.19)
2
R e s u l t s o f T O D a n a l y s i s for a n u m b e r o f different c o m p o u n d s i n d i c a t e t h a t
1
2
3
4
5
6
7
8
9
IO «
12
13
14
15 16
17 18
WEEK Fig. 2.8. Copyright
Weekly
analyses
by The American
of a raw
Chemical
wastewater
Society.)
[ 1 7 ] . (Reprinted
with
permission.
2.
43
Organic Content Measurement: O x y g e n Parameter M e t h o d s
m e a s u r e d o x y g e n d e m a n d is u s u a l l y c l o s e r t o t h e t h e o r e t i c a l l y c a l c u l a t e d t h a n is t h e c a s e for c h e m i c a l m e t h o d s . T h e s e r e s u l t s a r e p r e s e n t e d in G o l d s t e i n et al. [ 6 ] . N o n e of t h e c o m m o n i o n s n o r m a l l y f o u n d in w a t e r a n d w a s t e w a t e r s causes serious interference with T O D analyses [ 6 ] . C o r r e l a t i o n of T O D a n a l y s e s w i t h C O D h a s b e e n c h e c k e d f o r a n u m b e r o f t y p i c a l w a s t e s t r e a m s [ 2 , 3 ] . F i g u r e 2.8 s h o w s c o r r e l a t i o n s o f T O D , C O D , a n d B O D for a r a w w a s t e w a t e r . V a l u e s o f C O D v s . T O D f r o m F i g . 2.8 a r e p l o t t e d 5
in F i g . 2 . 9 , w h i c h s h o w s a l i n e a r r e l a t i o n s h i p . T h e r e l a t i o n s h i p o f T O D t o C O D or B O D
5
depends entirely o n c o m p o s i t i o n of the wastewater. C o n
sequently, these ratios vary depending o n the degree of biological t r e a t m e n t t o w h i c h t h e w a s t e w a t e r is s u b j e c t e d .
υ
2,000
TOD(mg/| )
Fig. 2.9. The COD and TOD relationship of a raw wastewater with permission. Copyright by The American Chemical Society.)
[17].
(Reprinted
44
2.
Characterization of Domestic and Industrial Wastewaters
3. M e a s u r e m e n t of O r g a n i c C o n t e n t : G r o u p 2—Carbon P a r a m e t e r M e t h o d s [2, 3] T o t a l o r g a n i c c a r b o n ( T O C ) tests a r e b a s e d o n o x i d a t i o n o f t h e c a r b o n o f the organic m a t t e r t o c a r b o n dioxide, a n d d e t e r m i n a t i o n of C 0 absorption
in K O H
or instrumental
analysis (infrared
theoretical oxygen d e m a n d ( T h O D ) measures 0
either by
2
analyzer).
and theoretical
2
Since organic
c a r b o n ( T h O C ) m e a s u r e s c a r b o n , t h e r a t i o o f T h O D t o T h O C is r e a d i l y calculated from the stoichiometry of the oxidation equation. E q u a t i o n (2.20) c o r r e s p o n d s t o t o t a l o x i d a t i o n of s u c r o s e . C
H 0 (12x12) 1 2
2 2
1 1
+ 120 (12x32)
-» 1 2 C 0 + 1 1 H 0
2
2
(2.20)
2
Λ T h O D / T h O C = (12 χ 32)/(12 χ 12) = 2.67
(2.21)
T h e r a t i o of m o l e c u l a r w e i g h t s of o x y g e n t o c a r b o n is 2.67. T h u s , t h e t h e o r e t i c a l r a t i o of o x y g e n d e m a n d t o o r g a n i c c a r b o n c o r r e s p o n d s t o t h e s t o i c h i o m e t r i c r a t i o o f o x y g e n t o c a r b o n for t o t a l o x i d a t i o n o f t h e organic c o m p o u n d under consideration. The actual ratio obtained from C O D ( o r B O D ) tests a n d T O C
determinations varies considerably from
this
t h e o r e t i c a l r a t i o ( S e c t i o n 3.3). E x p e r i m e n t a l d e t e r m i n a t i o n o f T O D is p e r f o r m e d b y e i t h e r m a n u a l (wet o x i d a t i o n ) o r i n s t r u m e n t a l m e t h o d s .
3.1. W E T O X I D A T I O N M E T H O D F O R T O C T h e m a n u a l o r w e t o x i d a t i o n m e t h o d for T O C c o n s i s t s of o x i d a t i o n o f t h e s a m p l e in a s o l u t i o n o f p o t a s s i u m d i c h r o m a t e ( K C r 0 ) , f u m i n g 2
2
7
acid ( H S 0 ) , p o t a s s i u m iodate ( K I 0 ) , a n d p h o s p h o r i c acid 2
4
3
sulfuric (H P0 ). 3
4
Oxidation products are passed through a tube containing K O H , where the c a r b o n d i o x i d e c o l l e c t e d is d e t e r m i n e d b y w e i g h i n g t h e a b s o r p t i o n
tube
b e f o r e a n d after t h e e x p e r i m e n t .
3.2. C A R B O N A N A L Y Z E R D E T E R M I N A T I O N S [1] T h e f u n d a m e n t a l o p e r a t i n g p r i n c i p l e o f T O C a n a l y z e r s is c o m b u s t i o n o f organic m a t t e r t o c a r b o n dioxide a n d water. C o m b u s t i o n gases are t h e n p a s s e d t h r o u g h a n i n f r a r e d a n a l y z e r , sensitized f o r c a r b o n d i o x i d e , a n d t h e r e s p o n s e is r e c o r d e d o n a s t r i p c h a r t . A d i a g r a m of t h e B e c k m a n m o d e l 9 1 5 - A T o t a l O r g a n i c C a r b o n ( T O C ) A n a l y z e r is s h o w n in F i g . 2.10. T h i s i n s t r u m e n t permits separate m e a s u r e m e n t s for total c a r b o n a n d i n o r g a n i c c a r b o n . T o t a l c a r b o n includes the c a r b o n of organic materials a n d i n o r g a n i c c a r b o n in t h e f o r m of c a r b o n a t e s ( C 0 ~ ) , b i c a r b o n a t e s ( H C 0 ~ ) , a n d C 0 d i s s o l v e d in t h e s a m p l e . T h e r e a r e t w o s e p a r a t e r e a c t i o n t u b e s : o n e o p e r a t e d 3
3
2
46
2.
Characterization of Domestic and Industrial Wastewaters
a t h i g h t e m p e r a t u r e ( 9 5 0 ° C ) for m e a s u r e m e n t of t o t a l c a r b o n a n d a n o t h e r operated at low t e m p e r a t u r e (150°C) for m e a s u r e m e n t of inorganic c a r b o n . D e p e n d i n g o n r a n g e o f a n a l y s i s , a 2 0 - 2 0 0 μΐ w a t e r s a m p l e is s y r i n g e injected i n t o a flowing s t r e a m o f a i r a n d s w e p t i n t o a c a t a l y t i c c o m b u s t i o n t u b e c o n t a i n i n g a c o b a l t o x i d e - i m p r e g n a t e d p a c k i n g . T h e s o u r c e o f a i r w h i c h is u s e d as carrier/oxidizer should be a low h y d r o c a r b o n , low C 0 content cylinder. T h e c o m b u s t i o n t u b e ( h i g h t e m p e r a t u r e c o m b u s t i o n t u b e ) is e n c l o s e d in a n e l e c t r i c f u r n a c e t h e r m o s t a t e d a t 9 5 0 ° C . W a t e r is v a p o r i z e d a n d all c a r b o n a c e o u s m a t e r i a l is o x i d i z e d t o C 0 a n d s t e a m . A i r f l o w c a r r i e s t h i s c l o u d o u t o f t h e f u r n a c e w h e r e t h e s t e a m is c o n d e n s e d a n d r e m o v e d . T h e C 0 is s w e p t into the nondispersive infrared analyzer. 2
2
2
T r a n s i e n t C 0 is i n d i c a t e d a s a p e a k o n a s t r i p c h a r t r e c o r d e r . P e a k h e i g h t is a m e a s u r e of C 0 p r e s e n t , w h i c h is directly p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f t o t a l c a r b o n in t h e o r i g i n a l s a m p l e a n d i n c l u d e s o r g a n i c c a r b o n , inorganic carbon, and C 0 dissolved in the sample. By using s t a n d a r d s o l u t i o n s , t h e c h a r t is c a l i b r a t e d in m i l l i g r a m s t o t a l c a r b o n p e r liter o f s a m p l e . 2
2
2
I n a s e c o n d o p e r a t i o n , a s a m p l e o f s i m i l a r size is a l s o s y r i n g e injected i n t o a s t r e a m of a i r a n d s w e p t i n t o t h e s e c o n d r e a c t i o n t u b e (low t e m p e r a t u r e reaction tube), containing q u a r t z chips wetted with 8 5 % p h o s p h o r i c acid. T h i s t u b e is e n c l o s e d in a n electric h e a t e r t h e r m o s t a t e d a t 150°C, w h i c h is b e l o w t h e t e m p e r a t u r e a t w h i c h o r g a n i c m a t t e r is o x i d i z e d . T h e a c i d - t r e a t e d p a c k i n g c a u s e s release o f C 0 f r o m i n o r g a n i c c a r b o n a t e s , a n d t h e w a t e r is vaporized. Airflow carries the cloud of steam a n d C 0 o u t of the furnace, w h e r e s t e a m is c o n d e n s e d a n d r e m o v e d . B y p r e v i o u s r e p o s i t i o n i n g of a d u a l c h a n n e l selector v a l v e , t h e C 0 is s w e p t i n t o t h e i n f r a r e d a n a l y z e r . 2
2
2
T h i s q u a n t i t y of C 0 is a l s o i n d i c a t e d o n t h e s t r i p c h a r t r e c o r d e r a s a t r a n s i e n t p e a k . P e a k h e i g h t is a m e a s u r e of t h e C 0 p r e s e n t , w h i c h is p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f i n o r g a n i c c a r b o n a t e s p l u s C 0 d i s s o l v e d in t h e o r i g i n a l s a m p l e . B y u s i n g s t a n d a r d s o l u t i o n s , t h e c h a r t is c a l i b r a t e d in milli g r a m s i n o r g a n i c c a r b o n p e r liter of s a m p l e . S u b t r a c t i n g r e s u l t s o b t a i n e d in t h e s e c o n d o p e r a t i o n f r o m t h o s e in t h e first yields t o t a l o r g a n i c c a r b o n in m i l l i g r a m s T O C p e r liter of s a m p l e . 2
2
2
3.3. OXYGEN D E M A N D - O R G A N I C C A R B O N CORRELATION T h e r a t i o T h O D / T h O C , w h i c h t h e o r e t i c a l l y is e q u a l t o t h e s t o i c h i o m e t r i c ratio of oxygen t o c a r b o n for total oxidation of the organic c o m p o u n d u n d e r c o n s i d e r a t i o n , r a n g e s in p r a c t i c e f r o m n e a r l y z e r o , w h e n t h e o r g a n i c m a t t e r is r e s i s t a n t t o d i c h r o m a t e o x i d a t i o n (e.g., p y r i d i n e ) , t o v a l u e s o f t h e o r d e r o f 6.33 for m e t h a n e o r e v e n slightly h i g h e r w h e n i n o r g a n i c r e d u c i n g a g e n t s a r e p r e s e n t . T a b l e 2.2 p r e s e n t s r e l a t i o n s h i p s b e t w e e n o x y g e n d e m a n d a n d t o t a l c a r b o n for several o r g a n i c c o m p o u n d s .
4.
47
Mathematical Model for the B O D Curve
Table 2.2 Relationships between Oxygen Demand and Total Carbon for Organic Compounds [3] Substance
ThOD/ThOC (calculated)
COD/TOC (measured)
Acetone Ethanol Phenol Benzene Pyridine Salicylic acid Methanol Benzoic acid Sucrose
3.56 4.00 3.12 3.34 3.33 2.86 4.00 2.86 2.67
2.44 3.35 2.96 0.84
—
2.83 3.89 2.90 2.44
C o r r e l a t i o n o f B O D w i t h T O C for i n d u s t r i a l w a s t e w a t e r s is difficult b e c a u s e o f t h e i r c o n s i d e r a b l e v a r i a t i o n in c h e m i c a l c o m p o s i t i o n . F o r d o m e s t i c w a s t e w a t e r s a r e l a t i v e l y g o o d c o r r e l a t i o n h a s b e e n o b t a i n e d , w h i c h is r e p r e s e n t e d b y t h e s t r a i g h t line r e l a t i o n s h i p given b y E q . ( 2 . 2 2 ) . BOD
2
= 1 . 8 7 ( T O C ) - 17
(2.22)
4. M a t h e m a t i c a l Model for t h e BOD Curve It is d e s i r a b l e t o r e p r e s e n t t h e B O D c u r v e ( F i g . 2.2) b y a m a t h e m a t i c a l m o d e l . F r o m k i n e t i c c o n s i d e r a t i o n s ( C h a p t e r 5, S e c t i o n 3), t h e m a t h e m a t i c a l m o d e l utilized t o p o r t r a y t h e r a t e o f o x y g e n u t i l i z a t i o n is t h a t o f a first-order r e a c t i o n . F i g u r e 2.2 r e v e a l s t h a t t h e r a t e o f o x y g e n u t i l i z a t i o n , given b y t h e t a n g e n t t o t h e c u r v e a t a given i n c u b a t i o n t i m e , d e c r e a s e s a s c o n c e n t r a t i o n o f organic matter remaining unoxidized becomes gradually smaller. Since there is a p r o p o r t i o n a l i t y b e t w e e n t h e r a t e o f o x y g e n u t i l i z a t i o n a n d t h a t o f d e s t r u c t i o n of o r g a n i c m a t t e r b y b i o l o g i c a l o x i d a t i o n , r a t e e q u a t i o n [ E q . ( 2 . 2 3 ) ] is w r i t t e n in t e r m s o f o r g a n i c m a t t e r c o n c e n t r a t i o n ( L ; m g / l i t e r ) . dL/dt=-k L
(2.23)
1
w h e r e L is c o n c e n t r a t i o n of o r g a n i c m a t t e r ( m g / l i t e r ) a t t i m e t\ dL/dt r a t e o f d i s a p p e a r a n c e o f o r g a n i c m a t t e r b y a e r o b i c b i o l o g i c a l o x i d a t i o n (dL/dt < 0 ) ; r, t i m e o f i n c u b a t i o n ( d a y s ) ; a n d k r a t e c o n s t a n t ( d a y ) . 9
- 1
u
S e p a r a t i n g v a r i a b l e s L a n d /, a n d i n t e g r a t i n g f r o m t i m e z e r o c o r r e s p o n d i n g t o initial c o n c e n t r a t i o n of o r g a n i c m a t t e r , L , t o a t i m e t c o r r e s p o n d i n g t o concentration L [Eq. (2.24)]: 0
ln(L/L ) = - k t 0
1
(2.24)
48
2.
Characterization of Domestic and Industrial Wastewaters
C h a n g i n g t o d e c i m a l l o g a r i t h m s [ E q . (2.25)] l o g ( L / L ) = - * i //2.303
(2.25)
0
let fci/2.303 = k. T h e n [ E q . (2.26)] L/L
0
= 10"*'
(2.26)
· 10"
(2.27)
or L = L
0
k i
L e t j b e t h e o r g a n i c m a t t e r o x i d i z e d u p t o t i m e r, i.e., y = L
0
- L
(2.28)
Conversely, y also measures the oxygen c o n s u m p t i o n u p to time o r d i n a t e o f t h e B O D c u r v e in F i g . 2.2 a t t i m e
i.e., t h e
C o m b i n i n g E q s . (2.28) a n d
(2.27), y = LoO-10-*')
(2.29)
w h i c h is t h e m a t h e m a t i c a l m o d e l for t h e B O D c u r v e . F r o m E q . (2.29) it f o l l o w s t h a t f o r a v e r y l o n g o x i d a t i o n p e r i o d (i.e., t-> oo), y = L . 0
k and L
0
Therefore,
m e a s u r e , respectively, t h e r a t e of b i o c h e m i c a l s t a b i l i z a t i o n a n d t h e
total a m o u n t of putrescible m a t t e r present. F r o m E q . (2.27) l o g L = logLo - kt
(2.30)
E q u a t i o n (2.30) i n d i c a t e s t h a t c o n s t a n t s k a n d L
0
can be obtained from a
s e m i l o g a r i t h m i c p l o t o f L vs. f. T y p i c a l v a l u e s o f t h e r a t e c o n s t a n t k a r e p r e s e n t e d in C h a p t e r 5 for several t y p e s of w a s t e w a t e r s ( T a b l e 5.2, S e c t i o n 5).
5. D e t e r m i n a t i o n of P a r a m e t e r s k and L 0
I n a p p l i c a t i o n o f E q . (2.29) o n e u s u a l l y h a s a v a i l a b l e a series o f B O D m e a s u r e m e n t s (y) a t a s e q u e n c e (n= 1,2, 3 , . . . ,x) d a y s . It is d e s i r e d t o d e t e r m i n e t h e o p t i m u m v a l u e s o f p a r a m e t e r s k a n d L w h i c h satisfy E q . (2.29) for t h e set o f d a t a . T h u s , it is f u n d a m e n t a l l y a curve-fitting p r o b l e m . Several m e t h o d s for c a l c u l a t i n g p a r a m e t e r s k a n d L h a v e b e e n p r o p o s e d . T h r e e o f t h e s e , r e c o m m e n d e d b y E c k e n f e l d e r [ 3 ] , a r e (1) log-difference m e t h o d , (2) m e t h o d o f m o m e n t s [ 1 0 ] , a n d (3) T h o m a s ' g r a p h i c a l m e t h o d . 0
0
5.1. L O G - D I F F E R E N C E M E T H O D T h i s m e t h o d is b a s e d o n t h e f o l l o w i n g c o n s i d e r a t i o n s . D i f f e r e n t i a t i n g E q . (2.29) w i t h r e s p e c t t o / : dyjdt = r = L ( - 1 0 - ) ( l n 10)(-A:) ki
0
(2.31)
5.
49
D e t e r m i n a t i o n of P a r a m e t e r s k a n d L
0
or dy/dt = r = l.mLok
· IO"*'
(2.32)
w h e r e r is t h e r a t e o f o x y g e n u t i l i z a t i o n . T a k i n g d e c i m a l l o g a r i t h m s l o g r = log(2.303L A:) - kt o
E q u a t i o n (2.33) i n d i c a t e s t h a t k a n d L
(2.33)
c a n be obtained from a semilog plot
0
o f r vs. t. Step
1. P l o t y ( o x y g e n u t i l i z a t i o n ) v s . / o n c a r t e s i a n c o o r d i n a t e p a p e r .
D r a w a s m o o t h best-fit c u r v e t h r o u g h t h e p o i n t s , d i s c a r d i n g d a t a
which
seem t o be in error. Step 2. P l o t d a i l y differences, Ay/At
vs. time (on semilog g r a p h paper).
T i m e i n t e r v a l s a r e u s u a l l y t a k e n a s 0 , 1 , 2 , 3 , . . . d a y s , s o t h a t Δ ί = 1. V a l u e s of Ay's a r e c o n v e n t i o n a l l y p l o t t e d v s . t h e t i m e t c o r r e s p o n d i n g t o t h e m i d d l e o f e a c h i n t e r v a l (e.g., t h e v a l u e o f Ay c o r r e s p o n d i n g t o i n t e r v a l 0 - 1 is p l o t t e d vs. ? = 0 . 5 ) . D r a w t h e best-fit s t r a i g h t line t h r o u g h t h e s e p o i n t s . Step 3. C a l c u l a t i o n o f k a n d L . 0
F r o m t h e s t r a i g h t line d r a w n i n S t e p 2 ,
E q . (2.33) y i e l d s : k = -(slope)
(2.34)
Intercept = 2.303L A: 0
Λ L
0
= intercept/(2.303A:) = i n t e r c e p t / ( 2 . 3 0 3 ) ( - s l o p e )
Therefore, k a n d L
0
(2.35) (2.36)
a r e c a l c u l a t e d f r o m E q s . (2.34) a n d (2.36), respectively.
E x a m p l e 2.1 T h e d a t a in T a b l e 2.3 o n o x y g e n u t i l i z a t i o n a r e a v a i l a b l e f r o m B O D tests of a w a s t e w a t e r . O b t a i n t h e v a l u e s o f k a n d L in t h e B O D e q u a t i o n . 0
T A B L E 2.3 B O D Tests of Wastewater / (days)
>> ( m g / l i t e r of B O D )
0
0.0
1
9.2
2
15.9
3
20.9
4
24.4
5
27.2
6
29.1
7
30.6
50
2.
Characterization of Domestic and Industrial Wastewaters
SOLUTION Step 1. D a t a a r e p l o t t e d in F i g . 2 . 1 1 . T h e c u r v e is fairly s m o o t h a n d t h e r a w d a t a a r e u s e d i n S t e p 2. If n e e d e d , s m o o t h i n g is d o n e b y t h e b e s t s t r a i g h t line fit.
Ο
1
2
Fig. 2.11.
3 4 Time (days) Plot
γ vs. t (Example
5
6
7
2.1).
Step 2. C o n s t r u c t t h e difference t a b l e ( T a b l e 2 . 4 ) . V a l u e s in c o l u m n (3) a r e p l o t t e d vs. t h o s e in c o l u m n (4) o n s e m i l o g p a p e r . T h i s p l o t is s h o w n in F i g . 2.12. T A B L E 2.4 Log-Difference Values from Table 2.3 (4) Mid-interval values o f t
U)
t (days)
(2) y (mg/liter)
(J) Ay (mg/liter) = Ay/At; since Δ / = 1
0 1 2 3 4 5 6 7
0 9.2 15.9 20.9 24.4 27.2 29.1 30.6
—
—
9.2 6.7 5.0 3.5 2.8 1.9 1.5
0.5 1.5 2.5 3.5 4.5 5.5 6.5
5.
51
Determination of Parameters k and L
0
0
I
2
3
4
5
6
7
Time (days); [Column ® , table 2.4] Fig. 2.12.
Calculation
of k and L by the log-difference
method.
0
Step 3. C a l c u l a t e k a n d L . (a) Calculation ofk. B a s e c a l c u l a t i o n s o n t h e c o o r d i n a t e s o f t w o p o i n t s o n t h e s t r a i g h t line (7, 1.2; 0, 10.9) a n d E q . (2.34). 0
Slope = ( l o g l 0 . 9 - l o g l . 2 ) / ( 0 - 7 ) = - 0 . 1 3 7 .·. k = 0.137 d a y " (b) Calculation
of L .
L
0
0
1
F r o m E q . (2.36)
= 10.9/(2.303x0.137) = 34.5 mg/liter
5.2. M E T H O D O F M O M E N T S [10] T h i s m e t h o d is of s i m p l e a p p l i c a t i o n o n c e d i a g r a m s o f - ^ d Σ y/Σ ty vs. k a r e c o n s t r u c t e d f o r a n « - d a y s e q u e n c e o f B O D m e a s u r e m e n t s . E q u a t i o n s are derived next for c o n s t r u c t i o n of M o o r e ' s d i a g r a m s for a n «-day s e q u e n c e o f B O D m e a s u r e m e n t s . T h e s e e q u a t i o n s a r e a p p l i e d t o 7-, 5-, a n d 3-day sequences, yielding Figs. 2.13-2.15, respectively. Consider B O D measurements taken over a n η-day sequence, as indicated in T a b l e 2 . 5 . F i r s t , c a l c u l a t e r a t i o Σ y/^o- T h e s u m m a t i o n o f e n t r i e s in c o l u m n (2) of T a b l e 2.5 is [ E q . (2.37)] v s
= L [ ( l + 1 + 1 + ··· + l ) - ( 1 0 - * + 1 0 -
2 f c
0
+10-
3 k
a
n
+ ·· -MO" *)] 1
(2.37) or = L [/i-(10- +10f c
0
2 k
+10-
3 f c
+..+10-
n k
)]
(2.38)
2.
Characterization of Domestic and Industrial Wastewaters
T A B L E 2.5 B O D Measurements, /i-Day Sequence (2) Eq. (2.29); U) t (days)
^ = Lo(l-10-*0
(J) = (7)x(2) ty
1 2 3 4 5 6 7
Lo(l-10-*) Lo(l-10- ) LoO-lO" *) Lod-lO- *) LoO-lO" *) LoO-lO" *) Lo(l-10- *)
Lo(l-10-*) 2L (l-10- ) 3L (l-10- ) 4L (l-10- ) 5L (1-10- *) 6L (l-10- ) 7L (l-10- )
L (l-10-
nL (l-\0-" )
/ = 1,2,3,...,Λ;
2 f c
2 k
0
3
3 f c
0
4 k
4
o
5
5
0
6 f c
6
0
7
0
M f c
7 k
0
)
k
o
Zy/Zty|
0.240
0.235
0.230
0.225
k (day*) 1
Fig. 2.13.
Moore's
method
(7-day
sequence).
5.
53
Determination of Parameters k and L
0
Zy/Zty|
0.310
0.300
0.290
0.280 0.1
0.2 k (day' )
0.4'
0.3
1
Fig. 2.14. Moore's
method
(5-day
sequence).
T h e t e r m s w i t h i n p a r e n t h e s e s in E q . (2.38) f o r m a g e o m e t r i c a l p r o g r e s s i o n for w h i c h t h e s u m of t e r m s is [ E q . ( 2 . 3 9 ) ] S = [ ( 1 0 - * ) ( 1 0 - * - 1 ) ] / ( 1 0 - * - 1)
(2.39)
n
S u b s t i t u t i n g t h i s v a l u e in E q . (2.38) a n d s o l v i n g for t h e r a t i o Y^y/Lo
= η — [10
- f c
(10
- n k
— l)/(10~ — 1)] fe
Y,yjL : 0
(2.40)
F r o m E q . (2.40) it follows t h a t for a g i v e n s e q u e n c e of η d a y s , t h e r a t i o X y/L is o n l y a f u n c t i o n of k. T h u s for a fixed n, o n e a s s u m e s v a l u e s o f k a n d p l o t s a c u r v e of Σ y/L vs. k. 0
0
N o w c a l c u l a t e r a t i o Σ y/Έ 0>· Σ y is o b t a i n e d f r o m E q . (2.40), a n d Σ ty c o r r e s p o n d s t o s u m m a t i o n of e n t r i e s i n c o l u m n (3) o f T a b l e 2 . 5 . X ty = L [ ( l + 2 + 3 + · · +n) - (10"* + 2 χ 1 0 " 0
2fc
+ 3 χ 1 0 " + ··· + Λ Χ 10" )] (2.41) 3 Λ
nfc
or / i = n
i=n
\
(2.42)
54
2.
Characterization of Domestic and Industrial Wastewaters ^y/L
0
0.440
0.434
Fig. 2.15. Moore's
method
(3-day
sequence).
T h e r e f o r e , f r o m E q s . (2.40) a n d (2.42), r a t i o Σ ^ / Σ ^
is
Λ-[ι0-*(10-"*-1)/(10-*-1)]
Σ!=ΐ''-Σί=ΐ''χ ιο-
(2.43)
F r o m E q . (2.43) it f o l l o w s t h a t f o r a g i v e n s e q u e n c e o f η d a y s , r a t i o Σ y/Έ ty is o n l y a f u n c t i o n o f k. T h u s f o r a fixed n, o n e a s s u m e s v a l u e s o f k a n d p l o t s a c u r v e o f Σ y/Σ ty v s . * f r o m E q . (2.43). F o r specific c a s e s s u c h a s t h e 7 - d a y s e q u e n c e , E q s . (2.40) a n d (2.43) yield E q . (2.44). For
η=7 Σγ/Lo
= 7 - [10-*(10- *-l)/(10-*-l)]
(2.44)
7-[io- (io- -i)/(io- -i)] 2 8—- Σ^ ί r =: ϊ''* Σ y/Σ ty = ~ — 10''*
(2.45)
v
/v,
7
fc
7fc
fc
w h e r e , i n E q . (2.45) i=7
X ι = Σ1=1+2
+ 3 + 4 + 5 + 6 + 7 = 28
5.
55
Determination of Parameters k and L
0
and £
,· χ i o ~
ifc
= 10"* + 2 χ I O " * + 3 χ I O " * + 4 χ 1 0 " * 2
3
4
t= ι
+ 5 χ 10~
5fc
+ 6 x IO" * + 7 χ 10" 6
7fc
Figures 2.13-2.15 present g r a p h s of Σ^/^ο - & d Σ^/ΣΟ - & f ° 7-, 5-, a n d 3-day s e q u e n c e s , respectively. T h e s e figures a r e c o n s t r u c t e d f r o m E q s . (2.40) a n d (2.43), respectively, b y a s s u m i n g v a l u e s o f η ( 7 , 5, 3) a n d k a n d calculating the corresponding ratios. A p p l i c a t i o n of M o o r e ' s d i a g r a m for c a l c u l a t i o n o f p a r a m e t e r s k a n d L is i l l u s t r a t e d b y E x a m p l e 2.2. v s
a
7
n
v s
r
0
Example 2.2 D e t e r m i n e values of k a n d L
f r o m t h e set o f B O D d e t e r m i n a t i o n s o f
0
Example 2.1. SOLUTION Step L C o n s t r u c t T a b l e 2.6. T A B L E 2.6 Application of Moore's Method (Example 2.2) t (days)
y (mg/liter B O D )
ty
0.0 9.2 15.9 20.9 24.4 27.2 29.1 30.6
0 1 2 3 4 5 6 7
Ey=
Step 2. C a l c u l a t e r a t i o
0.0 9.2 31.8 62.7 97.6 136.0 174.6 214.2 Σ 0> = 726.1
157.3
Σ.ν/ΣΟ'·
YjfLty
=
(n =
Step 3. F r o m F i g . 2.13
k = 0.140 d a y "
157.3/726.1 = 0.217
7) r e a d for 1
ΣyflLty
= 0.217.
(Abscissa of lower curve)
F r o m the ordinate of u p p e r curve read X^/L •'· o = L W L
4
6 2
0
= 4.62
= 157.3/4.62 = 34.05 mg/liter
56
Characterization of Domestic and Industrial Wastewaters
2.
T h e s e v a l u e s of k a n d L
0
agree closely with those calculated by the log-
difference m e t h o d in E x a m p l e 2.1 (k = 0.137 d a y "
5.3. T H O M A S ' G R A P H I C A L M E T H O D
and L
1
0
= 34.5 m g / l i t e r ) .
[16]
T h i s is a n a p p r o x i m a t e m e t h o d w h i c h is justified since p r e c i s i o n o f t h e e x p e r i m e n t a l r e s u l t s is often l i m i t e d . T h e m e t h o d is b a s e d o n t h e s i m i l a r i t y o f the function (l-10" ) (2.46) f t t
w h i c h is a f a c t o r o f E q . (2.29), a n d t h e f u n c t i o n 2.3A:/[l + ( 2 . 3 / 6 ) t o ] -
(2.47)
3
T h i s s i m i l a r i t y is seen in t h e i r r e s p e c t i v e series e x p a n s i o n s , w h i c h a r e (l-10-
f c i
) = (2.3A:/)[l-(l/2)(2.3A:/) + ( l / 6 ) ( 2 . 3 A : 0 - ( l / 2 4 ) ( 2 . 3 / c r ) + ···] 2
3
(2.48) and 2.3A:/[l + ( 2 . 3 / 6 ) / c i ] - = (2.3kt)[l-(\/2)(23kt)
+
3
(\/6)(2.3kt)
2
-(l/21.6)(2.3A:r) + ···] 3
(2.49)
C o m p a r i s o n o f t h e r i g h t - h a n d m e m b e r s o f E q s . (2.48) a n d (2.49) r e v e a l s t h a t t h e first t h r e e t e r m s in t h e t w o series w i t h i n b r a c k e t s a r e i d e n t i c a l , a n d t h a t t h e difference b e t w e e n t h e f o u r t h t e r m s is s m a l l . R e p l a c i n g t h e f u n c t i o n b e t w e e n p a r e n t h e s e s in E q . (2.29) b y its a p p r o x i m a t i o n given b y E q . (2.47) yields E q . (2.50). y = L (2.3A:/) [ 1 + (2.3/6) kt]~
(2.50)
3
0
f r o m w h i c h , t a k i n g t h e inverse a n d r e a r r a n g i n g , t\y = [1 + (2.3/6) kty/23kL
(2.51)
0
T a k i n g t h e c u b e r o o t of b o t h m e m b e r s o f E q . (2.51) a n d r e a r r a n g i n g , (t/yyt*
=
l/(2.3A:Lo)
F r o m E q . (2.52), a p l o t o f (t/y)
1/3
1/3
+ [(2.3A:) /6L 2/3
1/3 0
] /
(2.52)
vs. t yields a s t r a i g h t line ( F i g . 2.16 for
E x a m p l e 2.3) f r o m w h i c h Slope = Β = ( 2 . 3 A : ) / 6 L 2/3
1/3 0
Intercept = A = 1/(2.3A:L )
1/3
0
(2.53) (2.54)
F r o m E q s . (2.53) a n d (2.54) o n e o b t a i n s E q s . (2.55) a n d (2.56). k = 6£/2.3Λ = 2.61 (£/Λ) L
= \/(2.3kA ) 3
0
(2.55) (2.56)
5.
57
Determination of Parameters k and L
0
0.4
0.3 0
2
4
6
8
t (days) Fig. 2.16. Application
of Thomas'
method
[14].
A p p l i c a t i o n o f t h i s m e t h o d is i l l u s t r a t e d b y E x a m p l e 2 . 3 . Example 2.3 [16] T h e B O D results tabulated below a r e observed o n a sample of r a w sewage at 23°C. Calculate parameters k a n d
L. 0
t (days)
y ( B O D , mg/liter)
0 1 2 4 6 8
0 32 57 84 106 111
SOLUTION Step
1. C o n s t r u c t T a b l e 2.7. T A B L E 2.7 Application of Thomas' Method (Example 2.3) (2) y
(5) = ( 7 ) - ( 2 ) r/y
(4)=[(5)]i/3
/
0 1 2 4 6 8
0 32 57 84 106 111
0.03125 0.03509 0.04762 0.05660 0.07207
0.315 0.327 0.362 0.384 0.416
58
2.
Step
2. P l o t (t/y)
Characterization of Domestic and Industrial Wastewaters
vs. u T h e p l o t is s h o w n in F i g . 2.16. F r o m F i g . 2.16
l/3
obtain A = 0.30
(intercept)
Β = ( 0 . 4 1 6 - 0 . 3 0 0 ) / ( 8 . 0 - 0 . 0 ) = 0.0145 Step
(slope)
3. F r o m E q s . (2.55) a n d (2.56), o b t a i n k a n d k = 2.61(0.0145/0.30) = 0.13 d a y " L
L. 0
1
= l / [ 2 . 3 x 0 . 1 3 ( 0 . 3 0 ) ] = 124 mg/liter 3
Q
6. R e l a t i o n s h i p b e t w e e n k a n d Ratio B O D / B O D „ 5
E q u a t i o n (2.29) is w r i t t e n a s in E q . (2.57) f o r / = 5 d a y s , l e t t i n g y = B O D and L = BOD . 0
5
M
BOD
5
= BOD (l - 1 0 "
)
(2.57)
= 1 - 1/10 *
(2.58)
5 f c
M
from which BOD /BOD 5
5
u
A s s u m i n g v a l u e s o f k, a c u r v e o f B O D / B O D vs. k is p l o t t e d f r o m E q . (2.58). T h i s c u r v e rises w i t h i n c r e a s i n g &'s, r e a c h i n g a p l a t e a u c o r r e s p o n d i n g t o a n o r d i n a t e B O D / B O D a p p r o a c h i n g u n i t y f o r v a l u e s o f k b e y o n d 0.3 [ 3 ] . 5
5
M
t t
F r o m E q . (2.58) f o r l a r g e v a l u e s o f k, B O D / B O D a p p r o a c h e s u n i t y . T h i s m e a n s t h a t for a g i v e n s u b s t r a t e , if t h e r a t e o f b i o c h e m i c a l o x i d a t i o n is v e r y h i g h , t h e v a l u e o f B O D is essentially e q u a l t o t h a t o f t h e u l t i m a t e B O D . 5
M
5
7. Environmental Effects on t h e BOD T e s t T h e B O D test is affected b y t e m p e r a t u r e a n d p H .
7.1. E F F E C T O F T E M P E R A T U R E T h e r e a c t i o n r a t e c o n s t a n t k is d i r e c t l y affected b y t e m p e r a t u r e . T h e t e m p e r a t u r e d e p e n d e n c e o f k is g i v e n b y t h e v a n ' t H o f f - A r r h e n i u s e q u a t i o n [Eq. (2.59)]. d\nkldT=
E/RT
(2.59)
2
w h e r e k is r e a c t i o n r a t e c o n s t a n t ; Γ , a b s o l u t e t e m p e r a t u r e ; R u n i v e r s a l g a s c o n s t a n t ; a n d E, a c t i v a t i o n e n e r g y f o r t h e r e a c t i o n ( c o m m o n v a l u e s f o r w a s t e w a t e r t r e a t m e n t p r o c e s s e s a r e in t h e r a n g e o f 2 0 0 0 - 2 0 , 0 0 0 c a l / g m o l e ) . I n t e g r a t i n g b e t w e e n limits [ E q . ( 2 . 6 0 ) ] : 9
InikJkJ
=
[EiK-TMKRT^)
(2.60)
8.
59
Nitrification
Since m o s t wastewater t r e a t m e n t processes t a k e place a t nearly r o o m t e m p e r a t u r e , t h e t e r m EIRT T is n e a r l y c o n s t a n t . L e t it b e d e n o t e d a s C . T h e n [ E q . (2.61)] ln(* /*,)= C(r -r ) (2.61) X
2
2
2
k /k 2
= e ^~ c
1
x
(2.62)
T l )
L e t e = θ = t e m p e r a t u r e coefficient. T h e n c
k /k 2
= Θ ~^ (Τ2
1
(2.63)
Τ
T h e m o s t usual application consists of estimation of c o n s t a n t k a t a tem p e r a t u r e Τ f r o m its v a l u e d e t e r m i n e d e x p e r i m e n t a l l y a t 20°C. F r o m E q . (2.63) w e o b t a i n E q . (2.64). k = £ 0 (2.64) ( r
T
2 O )
2 O
w h e r e k is r e a c t i o n r a t e a t T°C; k , r e a c t i o n r a t e a t 20°C; a n d Γ, t e m p e r a t u r e (°C). A l t h o u g h θ is a p p r o x i m a t e l y c o n s t a n t , it v a r i e s slightly w i t h t e m p e r a t u r e a n d its a p p r o p r i a t e v a l u e s h o u l d b e s e l e c t e d . V a l u e s g i v e n b e l o w a r e those r e c o m m e n d e d b y Schroepfer [12]. T
2 0
θ = 1.135 (4°-20°C) θ = 1.056 (20°-30°C) F r o m E q . (2.64) it follows t h a t f o r a 10° rise in t e m p e r a t u r e t h e r e a c t i o n r a t e nearly doubles.
7.2. E F F E C T O F p H T h e s t a n d a r d B O D test specifies a p H o f 7.2. If t h e p H is n o t 7.2, v a l u e s o f B O D o b t a i n e d a r e l o w e r . I t is r e c o m m e n d e d , t h e r e f o r e , t o a d j u s t t h e p H t o 7.2. A t y p i c a l c u r v e o f p e r c e n t a g e o f n o r m a l 5-day B O D v s . p H is p r e s e n t e d b y E c k e n f e l d e r a n d F o r d [ 4 ] ; its m a x i m u m (100%) c o r r e s p o n d i n g t o p H 7.2. 5
8.
Nitrification
E q u a t i o n (2.29) d e s c r i b e s t h e o x i d a t i o n o f c a r b o n a c e o u s m a t t e r . O x i d a t i o n o f n i t r o g e n o u s m a t e r i a l a l s o c o n t r i b u t e s t o o x y g e n d e m a n d if i n c u b a t i o n is c a r r i e d o u t f o r a sufficiently l o n g p e r i o d o f t i m e . T h i s o x i d a t i o n (referred t o a s nitrification) takes place in t w o steps: 1. A m m o n i u m i o n , N H , is o x i d i z e d t o n i t r i t e s in t h e p r e s e n c e o f Nitrosomonas m i c r o o r g a n i s m s [ E q . (2.65)]. +
4
2NH + + 3 0 4
W 2
' " T 2 N 0 " + 2 H 0 + 4H+ 2
2
2. N i t r i t e s a r e t h e n o x i d i z e d t o n i t r a t e s in t h e p r e s e n c e o f m i c r o o r g a n i s m s [ E q . (2.66)]. 2N0 - + 0 2
2
2N0 3
(2.65) Nitrobacter
(2.66)
60
2.
Characterization of Domestic and Industrial Wastewaters
R a t e c o n s t a n t s , k, for nitrification a r e m u c h l o w e r t h a n t h o s e for o x i d a t i o n of c a r b o n a c e o u s matter. A l t h o u g h oxidation of c a r b o n a c e o u s a n d n i t r o g e n o u s m a t t e r m a y o c c u r s i m u l t a n e o u s l y , nitrification n o r m a l l y d o e s n o t b e g i n u n t i l t h e c a r b o n a c e o u s o x y g e n d e m a n d is p a r t i a l l y satisfied. A t y p i c a l B O D c u r v e for a w a s t e w a t e r s h o w i n g c a r b o n a c e o u s o x i d a t i o n a n d nitrification p h a s e s is s h o w n in F i g . 2.17. N i t r i f i c a t i o n is s u p p r e s s e d b y
γ=Ι_ο(Ι-Ι0- ) j Μ
. > l
n
y
Ε
"
/
/
/
/^Combined demand curve-.(without suppression of nitrification)
f (Nitrification suppressed)
/^Carbonaceous! /oxygen demand j / curve ! f
ι
Fig. 2.17.
Carbonaceous
and nitrogenous
BOD.
a d d i t i o n o f c e r t a i n c h e m i c a l s (e.g., m e t h y l e n e b l u e , t h i o u r e a ) . If t h i s is d o n e , the B O D curve thus obtained approaches a limiting ordinate L (ultimate c a r b o n a c e o u s d e m a n d ) , a s i n d i c a t e d in F i g . 2.17. B e y o n d t i m e f , t h e c a r b o n a c e o u s o x y g e n d e m a n d is e s s e n t i a l l y satisfied, s o t h e o r d i n a t e v a l u e b e c o m e s c o n s t a n t a t L . If, o n t h e o t h e r h a n d , n i t r i f i c a t i o n is n o t s u p p r e s s e d b e y o n d t = t , t h e effect o f n i t r i f i c a t i o n is s u p e r i m p o s e d o n t h e c a r b o n a c e o u s o x y g e n d e m a n d t o yield t h e c o m b i n e d o x y g e n d e m a n d c u r v e ( c a r b o n a c e o u s + nitrification d e m a n d ) . 0
c
0
c
T h e c a r b o n a c e o u s o x y g e n d e m a n d c u r v e is d e s c r i b e d b y E q . (2.29). If a t r a n s l a t i o n of c o o r d i n a t e a x e s is p e r f o r m e d s o t h a t t h e o r i g i n of t h e n e w s y s t e m o f c o o r d i n a t e s c o i n c i d e s w i t h p o i n t C ( s y s t e m Ay v s . t'), t h e e q u a t i o n f o r t h e nitrification o x y g e n d e m a n d c u r v e [ E q . ( 2 . 6 7 ) ] is w r i t t e n a s Ay = L (\-\0- ) kNt
N
w h e r e t' = t - t . c
(t>t )
(2.67)
c
Thus
= L^l-lO-*""-^] where k
N
is t h e r a t e c o n s t a n t f o r n i t r o g e n o u s d e m a n d a n d L
(2.68) N
the ultimate
n i t r o g e n o u s d e m a n d . T h e a b s c i s s a axis of t h e c o o r d i n a t e s y s t e m Ay v s . t' essentially c o i n c i d e s w i t h t h e c a r b o n a c e o u s o x y g e n d e m a n d c u r v e b e y o n d t = t. c
9.
Evaluation of Biological Treatment
61
E q u a t i o n s (2.29) a n d (2.69) for c o m b i n e d o x y g e n d e m a n d c u r v e a r e F o r t < t (carbonaceous oxygen d e m a n d only) c
y = L (l-10-
f c t
0
)
(2.29)
For t > t
c
y =
L ( l - 1 0 - ) + L [1 - l()-*» -'<>]
> for t > t ,
fci
(t
0
c
L ( l — IO *') -
0
N
L
0
^ increment Ay
(2.69)
due to nitrogenous demand
Values of p a r a m e t e r s k a n d L are d e t e r m i n e d by a n y of the m e t h o d s pre v i o u s l y d i s c u s s e d w i t h r e f e r e n c e t o t h e n e w s y s t e m o f c o o r d i n a t e s [i.e., E q . (2.68)]. N
N
9. Evaluation of Feasibility of Biological T r e a t m e n t for an Industrial W a s t e w a t e r 9.1.
INTRODUCTION
F r e q u e n t l y , it is n e c e s s a r y t o c o n d u c t t r e a t a b i l i t y s t u d i e s f o r s t r e a m s o f i n d u s t r i a l w a s t e w a t e r s , since t h e y m a y c o n t a i n t o x i c s u b s t a n c e s w h i c h h a v e a n a d v e r s e effect o n b i o l o g i c a l s y s t e m s . T h e p r o b l e m o f a c c l i m a t i o n o f m i c r o o r g a n i s m seed t o t o x i c s u b s t a n c e s is d i s c u s s e d in S e c t i o n 2 . 3 . 1 . T w o t y p e s o f t e s t s t o e v a l u a t e t h e feasibility o f b i o l o g i c a l t r e a t m e n t for i n d u s t r i a l w a s t e w a t e r [ 4 ] a r e (1) m a n o m e t r i c t e c h n i q u e s (Warburg respirometer), a n d (2) batch reactor evaluation.
9.2. W A R B U R G R E S P I R O M E T E R A s c h e m a t i c d i a g r a m o f t h e W a r b u r g r e s p i r o m e t e r is s h o w n i n F i g . 2 . 1 8 . T h e p r i n c i p l e o f o p e r a t i o n , w h i c h c o n s i s t s in r e s p i r i n g a w a s t e w a t e r s a m p l e in a c l o s e d a i r a t m o s p h e r e a t c o n s t a n t t e m p e r a t u r e , is i d e n t i c a l t o t h a t o f t h e B O D m a n o m e t r i c a p p a r a t u s ( S e c t i o n 2.3.2). O x y g e n utilized is m e a s u r e d w i t h r e s p e c t t o t i m e b y n o t i n g t h e d e c r e a s e in p r e s s u r e o f t h e s y s t e m a t c o n s t a n t v o l u m e . T h e C 0 e v o l v e d is a b s o r b e d b y a s o l u t i o n o f K O H ; t h u s t h e d e c r e a s e in p r e s s u r e is a m e a s u r e o f o x y g e n c o n s u m p t i o n o n l y . 2
S t e p s in t h e o p e r a t i o n a l p r o c e d u r e a r e g i v e n b e l o w [ 4 ] . 1. T h e w a s t e w a t e r s a m p l e is p l a c e d in t h e s a m p l e flask w i t h t h e r e q u i r e d v o l u m e of b i o l o g i c a l seed. T h e s a m p l e flask is i m m e r s e d i n a c o n s t a n t t e m perature bath and agitated by a shaking mechanism. 2. A 2 0 % s o l u t i o n o f K O H is p l a c e d i n t h e c e n t e r well ( a b o u t o n e - q u a r t e r full). I n s e r t a s t r i p o f f o l d e d filter p a p e r i n s i d e t h e c e n t e r well t o e n h a n c e t h e alkali a b s o r p t i o n of c a r b o n dioxide. T h e p a p e r soaks u p K O H solution a n d in t h i s w a y a l a r g e r a l k a l i s u r f a c e b e c o m e s a v a i l a b l e f o r a b s o r p t i o n o f c a r b o n dioxide.
62
2.
Characterization of Domestic and Industrial Wastewaters
Final level (by adjustment with the screw clamp) Note: This is also the / I£ initial level of Brodie / ι fluid in the inner a r m ; / h thus volume of g a s e o u s / J system is the same at / -t the start of the / experiment as that / Brodie just before a / fluid reading is taken. /
^ \ Motion (shaking mechanism)
CO
2
evolved
Sample -Center well filled with KOH solution
Outer arm Initial level of Brodie fluid in both arms of manometer
Fig. 2.18.
Schematic
diagram
of Warburg
respirometer.
3. Set u p a r e f e r e n c e flask ( " t h e r m o b a r o m e t e r " ) b y a d d i n g t o a s a m p l e flask o n l y distilled w a t e r . T h e v o l u m e o f distilled w a t e r e q u a l s t h e t o t a l w a s t e seed v o l u m e in e a c h of t h e test flasks. T h i s r e f e r e n c e flask is u s e d for c o r r e c t i o n d u e t o changes of a t m o s p h e r i c pressure d u r i n g the time of the experiment, hence the n a m e " t h e r m o b a r o m e t e r . " 4 . S h a k e t h e s y s t e m * w i t h t h e g a s v e n t p o r t o p e n for a p p r o x i m a t e l y 5 m i n . T h e level o f t h e m a n o m e t r i c fluid ( B r o d i e ' s fluid) is t h e s a m e in b o t h a r m s o f t h e m a n o m e t e r w h e n e q u i l i b r i u m is r e a c h e d . A d j u s t t h e m a n o m e t e r fluid t o t h e r e f e r e n c e m a r k in t h e i n n e r a r m o f t h e m a n o m e t e r , w i t h t h e g a s v e n t p o r t o p e n . A d j u s t m e n t of t h e level of t h e m a n o m e t r i c fluid is m a d e b y m e a n s of a screw c l a m p , thus p e r m i t t i n g adjustment of the height of m a n o metric liquid within the t w o a r m s of the m a n o m e t e r . S t o p the shaking a n d c h e c k all fittings. 5. C l o s e t h e g a s v e n t p o r t , t u r n o n t h e s h a k i n g a s s e m b l y , a n d t a k e r e a d i n g s a t selected t i m e i n t e r v a l s . P r i o r t o a r e a d i n g , t u r n t h e s h a k e r off a n d a d j u s t level o f B r o d i e ' s fluid in t h e i n n e r a r m t o t h e r e f e r e n c e m a r k . T h u s , t h e v o l u m e o f t h e g a s e o u s s y s t e m is t h e s a m e a t t h e s t a r t o f t h e e x p e r i m e n t a s t h a t j u s t b e f o r e a r e a d i n g is t a k e n . 6. T h e reference flask r e a d i n g s serve t h e p u r p o s e o f c o r r e c t i n g f o r a t m o s p h e r i c p r e s s u r e c h a n g e s d u r i n g t h e test. If t h e fluid in t h e o u t e r a r m o f t h e * Shaking is necessary because a film with a depleted oxygen concentration forms at the interface between the gas phase and the liquid sample if there is n o agitation. This slows d o w n the rate o f oxygen utilization. Shaking provides for film renewal s o that the liquor is always in contact with a gas phase rich in oxygen.
63
Evaluation of Biological Treatment
9.
m a n o m e t e r a t t a c h e d t o t h e t h e r m o b a r o m e t e r flask rises, t h e r e h a s b e e n a d e c r e a s e in a t m o s p h e r i c p r e s s u r e , a n d t h e o b s e r v e d r e a d i n g m u s t b e a d d e d t o t h e test v a l u e . If, o n t h e o t h e r h a n d , t h e fluid in t h e o u t e r a r m o f t h e m a n o m e t e r falls, t h e r e h a s b e e n a n i n c r e a s e in a t m o s p h e r i c p r e s s u r e , a n d
the
o b s e r v e d r e a d i n g m u s t b e s u b t r a c t e d f r o m t h e test v a l u e . Sample reading (h) = P
at
-
(2.70)
P
si
w h e r e P is t h e v a l u e o f a t m o s p h e r i c p r e s s u r e a t s t a r t o f e x p e r i m e n t . D u r i n g a n e x p e r i m e n t if t h e a t m o s p h e r i c p r e s s u r e ( P ) rises, t h e c a l c u l a t e d h [ E q . (2.70)] w o u l d b e h i g h e r t h a n t h e t r u e v a l u e u n l e s s t h e a p p r o p r i a t e c o r r e c t i o n is s u b t r a c t e d . at
a t
7. O n c e t h e s u b s t r a t e h a s b e e n utilized, o x y g e n u p t a k e stabilizes a n d t h e test series is t e r m i n a t e d . T h e c u m u l a t i v e o x y g e n u p t a k e s ( m i l l i g r a m s o f o x y g e n p e r liter o f s o l u t i o n ) a r e t h e n p l o t t e d vs. t i m e ( h r ) . A t y p i c a l g r a p h o b t a i n e d for a t o x i c w a s t e w a t e r s t r e a m is s h o w n in F i g . 2 . 1 9 . T h i s s t r e a m is a d d e d t o d o m e s t i c s e w a g e ( i n d i c a t e d a s " s e e d " in F i g . 2.19) in i n c r e a s i n g l y l a r g e r p r o p o r t i o n s .
Seed" H O % waste "Seed"*5% waste Seed"*2% waste
Seed only Seed" • more than 10% waste Time (hours) Fig. 2.19.
Oxygen
uptakes
at different
wastewater
concentrations.
F i g u r e 2.19 i n d i c a t e s t h a t t h i s specific w a s t e w a t e r is t o x i c o r i n h i b i t o r y w h e n its c o n c e n t r a t i o n e x c e e d s 10% in v o l u m e , in w h i c h c a s e t h e o x y g e n u p t a k e suffers a l a r g e d r o p . Calculation h
of Oxygen
Uptake (mg Ο /Liter 2
of Sample)
from
the
Reading
(cm)
I n t h i s c a l c u l a t i o n p r o c e d u r e , it is a s s u m e d t h a t a t m o s p h e r i c p r e s s u r e h a s n o t c h a n g e d d u r i n g t h e t i m e of e x p e r i m e n t [if it d o e s , c o r r e c t a s i n d i c a t e d in S t e p 6, E q . ( 2 . 7 0 ) ] . A t t h e b e g i n n i n g o f t h e e x p e r i m e n t , t h e i d e a l g a s l a w is a p p l i e d t o t h e a i r in t h e c l o s e d s y s t e m , i.e., PiV
=
NiRT
(2.71)
2.
64
Characterization of Domestic and Industrial Wastewaters
w h e r e P is t h e a t m o s p h e r i c p r e s s u r e a t s t a r t o f e x p e r i m e n t ( c m o f B r o d i e ' s fluid)*; Κ t h e g a s v o l u m e in c l o s e d s y s t e m ( m l ) ; Γ t h e t e m p e r a t u r e o f c o n s t a n t t e m p e r a t u r e b a t h ( ° K ) ; N t h e g m o l e s of a i r a t t h e b e g i n n i n g o f e x p e r i m e n t in closed system; a n d R the universal gas c o n s t a n t . F r o m E q . (2.71) w e d e r i v e E q . (2.72). x
t
f
Ni = Λ VIRT
(2.72)
A t t h e t i m e a r e a d i n g (A) is t a k e n ( F i g . 2.18) P V=N RT 2
(2.73)
2
w h e r e P is t h e s y s t e m p r e s s u r e (P < Λ ) ί f t h e v o l u m e o f g a s p h a s e in s y s t e m (kept c o n s t a n t by adjustment with the screw c l a m p ) ; a n d N the g moles of g a s p h a s e in c l o s e d s y s t e m a t t i m e of r e a d i n g . F o r N < N d u e t o o x y g e n absorption, then N = Ν,χ (2.74) 2
2
2
2
l
2
w h e r e χ is g m o l e s of 0
2
adsorbed.
F r o m E q . (2.73) N
2
= P VIRT
(2.75)
2
E q u a t i n g E q s . (2.74) a n d (2.75) a n d s o l v i n g for x: χ = Νχ — (P VIRT)
(2.76)
2
S u b s t i t u t i n g in E q . (2.76) N
x
b y its v a l u e given b y E q . ( 2 . 7 2 ) : x = (V!RT)(P -P ) Y
w h e r e {Ρχ-Ρ )
(2.77)
2
e q u a l s t h e h e i g h t h of B r o d i e ' s fluid ( F i g . 2.18). T h e r e f o r e ,
2
x = (V/RT)h
(g moles 0 )
(2.78)
2
If V is t h e v o l u m e o f t h e w a s t e w a t e r s a m p l e in m l , o x y g e n u t i l i z a t i o n in s
m g / l i t e r is Oxygen utilization = (Vh/RT)
g moles Q
2
x
1
(V x s
3
2
3
x l O ^
g
gmole0
1 0 - ) liter
2
"
g
* Specific gravity of Brodie's fluid is 1.001 at 0°C (with respect to water at 4°C). There fore, normal atmospheric pressure is equivalent to a column of Brodie's fluid (at 0°C) of height equal to 76.0 cm Hg χ 13.6 cm water/cm Hg χ cm Brodie's fluid/1.001 cm water = (76.0 x 13.6)/(1.001) = 1032.6 cm Brodie's fluid at 0°C t R = PoVolT = 0
(1032.6 cm Brodie) (22,412 ml/g mole) 273.2°K
= 84,709 (cm Brodie) (ml)/(g mole)(°K)
10.
65
Characteristics of Municipal S e w a g e
or Oxygen utilization = 32 χ \0 (VIV )(hlRT)
(mg/liter)
6
s
(2.79)
w h e r e Κ is t h e g a s v o l u m e in c l o s e d s y s t e m ( m l ) ; V t h e v o l u m e o f w a s t e w a t e r s
s a m p l e ( m l ) ; h t h e r e a d i n g ( c m o f B r o d i e ' s fluid) ( F i g . 2 . 1 8 ) ; R t h e u n i v e r s a l gas c o n s t a n t [84,709 (cm Brodie)(ml)/(g m o l e ) ( ° K ) ] ; a n d Τ the t e m p e r a t u r e of b a t h (°K).
9.3. BATCH REACTOR EVALUATION A series o f b a t c h b i o l o g i c a l r e a c t o r s ( b e n c h scale) a r e u s e d t o a c c o m p l i s h essentially the s a m e objective as t h e W a r b u r g respirometer. A b a t t e r y of b a t c h r e a c t o r s r e c o m m e n d e d f o r t h i s t y p e o f w o r k is s h o w n i n F i g . 5.2 a n d d e s c r i b e d in S e c t i o n 3.1 o f C h a p t e r 5. A n a c c l i m a t e d seed is a d d e d t o t h e series o f r e a c t o r s . V a r i o u s c o n c e n t r a t i o n s of a wastewater are then a d d e d to each reactor. T h e mixed c o n t e n t s are aerated f o r 2 - 3 d a y s . T h e a p p a r e n t t o x i c i t y is e v a l u a t e d , s a m p l e s a r e w i t h d r a w n a t t h e e n d o f 1, 2, 4 , 8, 12, a n d 2 4 h r o f a e r a t i o n , a n d C O D o r B O D r e m o v a l t e s t s a r e p e r f o r m e d . T y p i c a l B O D c u r v e s o b t a i n e d in t h i s m a n n e r a r e s i m i l a r t o t h e o n e s s h o w n in F i g . 2 . 1 9 .
10. C h a r a c t e r i s t i c s of Municipal S e w a g e M u n i c i p a l s e w a g e is c o m p o s e d m a i n l y o f o r g a n i c m a t t e r , e i t h e r in s o l u b l e o r c o l l o i d a l f o r m o r a s s u s p e n d e d solids. E c k e n f e l d e r [ 3 ] r e p o r t s o f a n a l y s i s o f d a t a o n m u n i c i p a l s e w a g e for a s u r v e y w h i c h i n c l u d e d 73 cities in 2 7 s t a t e s of the United States. S o m e average per capita values from this survey are F l o w : 135 gal/(capita) (day) B O D : 0.2 lb/(capita)(day) = 90.7 g/(capita) (day) 5
Suspended solids: 0.23 lb/(capita) (day) = 104 g/(capita) (day) F o r a city of o n e m i l l i o n p e o p l e , t h e f o l l o w i n g v a l u e s a r e o b t a i n e d b y p r o r a t i n g this per capita data. F l o w : 135 Mgal/day χ 8.34 lb/gal = 1126 M l b / d a y B O D : 200,000 lb/day 5
o r in t e r m s o f m g / l i t e r B O D : 200,000 lb/day χ day/1126 M l b = 178 l b / M l b = 178 p p m * 178 mg/liter* 5
* Since most wastewaters contain small concentrations of soluble (and/or insoluble) matter, wastewater density is taken hence as equal to that for pure water, i.e., approximately 1 mg/liter. Consequently, mg/liter becomes essentially equivalent t o parts per million (ppm), since 1.0 mg/liter « 1.0 m g / 1 0 g = 1.0 g/10 g = 1.0 p p m . 3
6
66
2.
Characterization of Domestic and Industrial Wastewaters
P r e s e n c e o f i n d u s t r i a l w a s t e s in a m u n i c i p a l s e w a g e s y s t e m m a y c h a n g e t h e s e values considerably.
1 1 . Industrial W a s t e w a t e r S u r v e y s T h e p r o c e d u r e t o b e f o l l o w e d in i n d u s t r i a l w a s t e w a t e r s u r v e y s h a s b e e n d e s c r i b e d in C h a p t e r 1 ( S e c t i o n 2.5.2, S t e p 1). A c o m p l e t e s e w e r m a p o f t h e p l a n t is d e v e l o p e d . F o r a c c o m p l i s h i n g t h i s objective, s a m p l i n g a n d m e a s u r i n g s t a t i o n s a r e l o c a t e d in t h e p l a n t , i n c l u d i n g all significant s o u r c e s o f w a s t e w a t e r s . A n a l y s e s t o b e r u n a r e selected a n d s a m p l i n g a n d a n a l y s e s s c h e d u l e s carefully p l a n n e d . M a t e r i a l b a l a n c e s , i n c l u d i n g b o t h p r o c e s s a n d s e w e r lines, are written. S t a t i s t i c a l p l o t s f o r all significant c h a r a c t e r i s t i c s a r e p r e p a r e d . W h e n e v e r p o s s i b l e , t h e s e statistical p l o t s a r e r e l a t e d t o p r o d u c t i o n , t h a t is, g a l / t o n o f p r o d u c t or lb B O D / t o n of product. This permits extrapolation to other p r o d u c t i o n schedules. Sources for wastewater segregation, reuse, a n d re c i r c u l a t i o n a r e identified. F l o w measurements of wastewater streams are performed by a variety of methods, which are summarized by Eckenfelder [ 3 ] . 1. I n s t a l l a t i o n o f w e i r s for flow in o p e n c h a n n e l s a n d p a r t i a l l y filled s e w e r s 2. B u c k e t a n d s t o p w a t c h m e t h o d , s u i t a b l e f o r l o w flow r a t e s a n d / o r i n t e r m i t t e n t d i s c h a r g e s . I n t h e l a t t e r c a s e , flow r a t e a n d d u r a t i o n o f o p e r a t i o n are determined 3. P u m p i n g d u r a t i o n a n d r a t e . F l o w is e s t i m a t e d f r o m t h e c h a r a c t e r i s t i c c u r v e s of t h e p u m p 4. T i m i n g a floating o b j e c t b e t w e e n t w o fixed p o i n t s a l o n g t h e c o u r s e . T h i s m e t h o d is a p p l i e d t o p a r t i a l l y filled s e w e r s . D e p t h of flow in t h e s e w e r is a l s o m e a s u r e d . A v e r a g e velocity is e s t i m a t e d f r o m s u r f a c e velocity, w h i c h is t h e o n e directly m e a s u r e d . F o r l a m i n a r flow t h e a v e r a g e v e l o c i t y is a p p r o x i m a t e l y 0.8 t i m e s t h e s u r f a c e velocity. F l o w is t h e n e v a l u a t e d f r o m t h e k n o w l e d g e of t h i s a v e r a g e v e l o c i t y 5. E x a m i n a t i o n o f p l a n t w a t e r u s e r e c o r d s . T a k i n g i n t o a c c o u n t w a t e r losses in p r o d u c t o r d u e t o e v a p o r a t i o n , t h i s m e t h o d l e a d s t o a p p r o x i m a t e estimates 6. T i m i n g c h a n g e o f level in t a n k s o r r e a c t o r s , u s e d p r i m a r i l y for b a t c h operation discharges
12. S t a t i s t i c a l C o r r e l a t i o n of Industrial W a s t e S u r v e y Data I n d u s t r i a l w a s t e w a t e r d i s c h a r g e s a r e h i g h l y v a r i a b l e in v o l u m e a n d c o m p o s i t i o n a n d a r e a p p r o p r i a t e l y t r e a t e d b y statistical a n a l y s i s . P r o b a b i l i t y p l o t s a r e u s e d w h e n d e a l i n g w i t h statistics of e v e n t s w h i c h fall i n t o t h e b e l l shaped probability curve so familiar t o statisticians. A plot of d a t a o n p r o b -
12.
67
Correlation of Industrial Waste Survey Data
ability-type g r a p h p a p e r straightens o u t the probability curve, leading t o a s t r a i g h t line p l o t ( l i n e a r i z a t i o n o f t h e d a t a ) . T h e s t r a i g h t line t h u s o b t a i n e d is referred t o a s H e n r y ' s line. T h e r e f o r e , if a series o f e x p e r i m e n t a l d a t a is p l o t t e d o n t h i s p a p e r a n d t h e r e s u l t is a s t r a i g h t line, t h i s i n d i c a t e s a r a n d o m d i s t r i b u tion of experimental d a t a . P r o b a b i l i t y g r a p h p a p e r utilized in t h i s w o r k is i l l u s t r a t e d in F i g . 2 . 2 0 . T h e a b s c i s s a is a p r o b a b i l i t y scale a n d t h e o r d i n a t e is a l o g a r i t h m i c o n e . 20001
-
I0008 0 0
1
j
j—ι—|—|—ι
I
I
I
I
1
1
1
1
I
I
I
ι
1
Ξ
600-
£
400
Q Ο CD
1
Γ
200 [
1001
ι
1
1
I
2 5 10 30 50 70 90 95 98 % of time that BOD value is equal to or less than the one indicated at ordinate Fig. 2.20.
Probability
plot
for Example
2.4 (Method
f).
T w o m e t h o d s f o r statistical c o r r e l a t i o n o f i n d u s t r i a l w a s t e s u r v e y d a t a a r e r e c o m m e n d e d by Eckenfelder [ 3 ] . M e t h o d (1) is r e c o m m e n d e d for s m a l l a m o u n t s of d a t a (i.e., less t h a n 2 0 d a t u m points). Step 1. A r r a n g e d a t a in i n c r e a s i n g o r d e r o f m a g n i t u d e . Step 2. L e t η b e t h e t o t a l n u m b e r o f p o i n t s a n d m t h e a s s i g n e d serial n u m b e r f r o m 1 t o n. T a b u l a t e d a t a (in i n c r e a s i n g o r d e r o f m a g n i t u d e ) vs. m. Step 3. P l o t t i n g p o s i t i o n s ( a b s c i s s a s o f t h e p r o b a b i l i t y p l o t ) a r e d e t e r mined from Frequency = (100//*) ( w - 0 . 5 ) T h i s q u a n t i t y is e q u i v a l e n t t o t h e p e r c e n t o c c u r r e n c e o f t h e v a l u e p l o t t e d in t h e o r d i n a t e , i.e., p e r c e n t o f t i m e t h a t t h e v a l u e in q u e s t i o n is e q u a l t o o r less t h a n t h e r e a d i n g o f t h e o r d i n a t e . T h e p o s i t i o n o f t h e best-fit line is j u d g e d b y eye o r t h e l e a s t - s q u a r e s m e t h o d is u s e d . A p p l i c a t i o n o f m e t h o d (1) is i l l u s t r a t e d b y E x a m p l e 2.4.
Example 2.4 T h e f o l l o w i n g B O D d a t a ( m g / l i t e r ) a r r a n g e d in i n c r e a s i n g o r d e r o f m a g n i t u d e w a s o b t a i n e d for a n i n d u s t r i a l s t r e a m [ c o l u m n ( / ) o f T a b l e 2 . 8 ] . P l o t H e n r y ' s line b y t h e m e t h o d d e s c r i b e d .
68
2.
Characterization of D o m e s t i c and Industrial W a s t e w a t e r s
T A B L E 2.8 Calculations for Example 2.4
Step 1, B O D (mg/liter)
(2) Step 2 (/i = 8 , m = l , 2 , . . . , 8 ) , values o f m
(3) Frequency = ( 1 0 0 / / i ) ( m - 0 . 5 ) , % time equal to or less than
400 450 520 630 700 730 860 1100
1 2 3 4 5 6 7 8
6.25 18.75 31.25 43.75 56.25 68.75 81.25 93.75
U)
SOLUTION The procedure is indicated in Table 2.8 and Fig. 2.20. The probability of occurrence of any value is now estimated. For example, from Fig. 2.20 the BOD is equal to or less than 1000 mg/liter 90% of the time. A statistical analysis of the various waste characteristics provides a basis for choice of design values. For example, the hydraulic capacity of a plant is selected in excess of the 99% frequency (here the ordinate is flow rate). On the other hand, sludge-handling facilities are usually designed on the basis of the 50% frequency. Method (2) is employed when a large number of data (more than 20 datum points) have to be analyzed. Calculate the plotting position [column (5) of Table 2.8] from Frequency =
m/(n+\)
Otherwise, the procedure is the same as in method (1). Problems I. T h e following B O D data are given: t (days)
B O D (mg/liter)
1 2 3 4 5 6 7 8 9 10
6.5 11.0 15.0 18.0 20.0 22.0 23.0 24.0 25.0 26.0
69
References
1. Plot the B O D curve. 2. Calculate the values o f parameters k and L by the following m e t h o d s : (a) Log-difference method (b) Moore's method o f m o m e n t s , utilizing 3 - , 5-, and 7-day sequences (c) T h o m a s ' graphical method Tabulate values obtained under (a), (b), and (c). C o m p a r e values o f L with that obtained by visual extrapolation o f the curve. 0
0
II. F o r a wastewater, k = 0.1 (decimal log basis) and the 5-day B O D is 2 0 0 mg/liter. Estimate the 1-day B O D and the ultimate demand ( L ) . What is the 5-day B O D if the in cubation is at 30°C instead o f the conventional temperature o f 20°C? ΙΠ. Determine T h O D for alanine [ C H C H ( N H ) C O O H ] (in g 0 / g m o l e o f alanine) using the following assumptions: 0
3
2
2
1. Carbon atoms are oxidized t o C 0 while nitrogen is converted t o a m m o n i a . 2. A m m o n i a is then oxidized to H N 0 in the presence o f nitrite-forming bacteria. 3. Finally, H N 0 is oxidized t o H N 0 in the presence o f nitrate-forming bacteria. T h O D is the s u m o f the oxygen required for these three steps. 2
2
2
3
IV. N i n e determinations o f suspended solids ( p p m ) in a waste stream yield the following results, arranged in order o f increasing magnitude: 4 8 , 8 3 , 8 5 , 1 0 2 , 1 3 0 , 1 3 4 , 1 5 3 , 1 6 7 , and 180. 1. Linearize the distribution by a probability plot. 2. What is the probability o f occurrence o f a suspended solid (SS) value equal t o or less than 200 p p m ?
References 1. Beckman Instruments Inc., Process Instruments Bull. 4 0 8 2 - D for M o d e l 9 1 5 - A ' T o t a l Organic Carbon Analyzer." Beckman Instrum. Inc., Fullerton, California, 1975. 2. D o b b i n s , W . E., / . Sanit. Eng. Div., Am. Soc. Civ. Eng. 9 0 , S A S , 53 (1964). 3. Eckenfelder, W . W . , Jr., "Water Quality Engineering for Practicing Engineers." Barnes & N o b l e , N e w York, 1970. 4. Eckenfelder, W . W . , Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w York, 1970. 5. Gaudy, A . F . , and Ramanathan, M . , / . Water Pollut. Control Fed. 3 6 , 1470 (1964). 6. Goldstein, A . L., Katz, W . E . , Meller, F. H . , and Murdoch, D . M . , Pap., Div. Water, Air Waste Chem., Am. Chem. Soc, Atlantic City, N.J., 1968. 1. H a c h Chemical Company, Laboratory Instrumentation M a n . 1M-12-1-72 for M o d e l 2173 "Manometric B O D Apparatus." H a c h C h e m . C o . , A m e s , I o w a , 1973. 8. H a c h Chemical C o m p a n y , "Procedures for Water and Wastewater Analysis," Manual, 2nd ed., p. 288. H a c h Chem. C o . , A m e s , I o w a , 1975. 9. Ionics Incorporated, "Total Oxygen Analyzer," Brochure for Ionics M o d e l 225. Ionics Inc., Watertown, Massachusetts. 10. M o o r e , E . W . , T h o m a s , Η . Α . , Jr., and S n o w , W . B., Sewage Ind. Wastes 22,1343 (1950). 11. Precision Scientific Instruments, Precision A q u a R a t o r Bull. 644A. Precision Sci. Instrum., Chicago, Illinois. 12. Schroepfer, G. S. et al, "Advances in Water Pollution Control," Vol. 2. Pergamon, Oxford, 1964. 13. "Standard Methods for the Examination o f Water and Wastewater," 13th ed. A m . Public Health Assoc., Yearbook Publ., Chicago, Illinois, 1971. 14. Stenger, V. Α . , and V a n Hall, C. E . , 21st Ann. ISA Conf., 1966 Reprint 53-4-66 (1966). 15. Stenger, V. Α . , and V a n Hall, C . E . , Anal. Chem. 3 9 , 206 (1967). 16. T h o m a s , Η . Α . , Water Sewage Works 9 7 , 1 2 3 (1950). 17. W o o d , E. D . , Perry, A . E . , Hitchcock, M . C , and Sadlier, Μ . E., Pap. 159th Am. Chem. Soc. Meet., Houston, Texas, 1970.
3 Pretreatment and Primary Treatment 1. Introduction
70
2. Screening
71
3. Sedimentation 3.1. Introduction 3.2. Types of Settling 3.3. Theory of Discrete Settling 3.4. The Ideal Sedimentation Tank Concept 3.5. Flocculent Settling 3.6. Zone Settling 3.7. Types of Clarifiers
71 71 71 72 76 84 98 105
4. Flotation 4.1. Introduction 4.2. Evaluation of Flotation Variables for Process Design 4.3. Design Procedure for Flotation Units without and with Recycle
107 107 108 112
5. Neutralization (and Equalization) 5.1. Neutralization in the Field of Wastewater Treatment 5.2. Methods for Neutralization of Wastewaters 5.3. Equalization 5.4. Direct pH Control M e t h o d s : Neutralization of Acidic Wastes by Direct pH Control Methods 5.5. Limestone Beds 5.6. Slurried Lime Treatment 5.7. Neutralization of Alkaline Wastes
114 114 114 114 116 116 120 123
Problems References
123 125
1. I n t r o d u c t i o n Selection of a wastewater t r e a t m e n t process or sequence of processes d e p e n d s o n a n u m b e r o f f a c t o r s , i.e., (1) c h a r a c t e r i s t i c s o f t h e w a s t e w a t e r , e.g., B O D , % of s u s p e n d e d s o l i d s , p H , p r e s e n c e o f t o x i c m a t e r i a l s ; (2) r e q u i r e d effluent q u a l i t y ; (3) c o s t a n d a v a i l a b i l i t y o f l a n d , e.g., c e r t a i n b i o l o g i c a l p r o c e s s e s ( s t a b i l i z a t i o n p o n d s ) a r e o n l y e c o n o m i c a l l y feasible if l o w c o s t l a n d is a v a i l a b l e ; a n d (4) c o n s i d e r a t i o n o f a p o s s i b l e f u t u r e u p g r a d i n g o f w a t e r quality s t a n d a r d s , necessitating design of a m o r e sophisticated type of treat m e n t for future use. P r e t r e a t m e n t of w a s t e w a t e r i m p l i e s r e m o v a l o f s u s p e n d e d s o l i d s o r c o n 70
3.
71
Sedimentation
d i t i o n i n g o f w a s t e w a t e r for d i s c h a r g e i n t o e i t h e r a r e c e i v i n g b o d y o f w a t e r o r a s e c o n d a r y t r e a t m e n t facility t h r o u g h n e u t r a l i z a t i o n a n d / o r
equalization.
T y p e s o f p r i m a r y t r e a t m e n t d i s c u s s e d in t h i s c h a p t e r a r e (1) s c r e e n i n g , (2) s e d i m e n t a t i o n , (3) flotation, a n d (4) n e u t r a l i z a t i o n a n d e q u a l i z a t i o n .
2. S c r e e n i n g S c r e e n i n g is e m p l o y e d for r e m o v a l o f s u s p e n d e d s o l i d s o f v a r i o u s sizes. S c r e e n o p e n i n g s r a n g e in size d e p e n d i n g o n t h e i r p u r p o s e , a n d c l e a n i n g o f screens is d o n e e i t h e r m a n u a l l y o r m e c h a n i c a l l y . S c r e e n i n g s a r e d i s p o s e d o f b y b u r i a l , i n c i n e r a t i o n , o r a n a e r o b i c d i g e s t i o n . S c r e e n s a r e classified a s fine and coarse. F i n e s c r e e n s h a v e o p e n i n g s of 3/16 i n . o r s m a l l e r . T h e y a r e u s u a l l y m a d e o f steel m e s h o r p e r f o r a t e d steel p l a t e s a n d s o m e t i m e s u s e d i n s t e a d o f s e d i m e n t a tion tanks. However, whereas they r e m o v e from 5 to 2 5 % of suspended solids, 4 0 - 6 0 % is r e m o v e d b y s e d i m e n t a t i o n . F o r t h i s r e a s o n , a n d a l s o b e c a u s e c l o g g i n g is f r e q u e n t l y a p r o b l e m , u s e o f fine s c r e e n s is n o t v e r y c o m m o n . C o a r s e s c r e e n s h a v e o p e n i n g s r a n g i n g f r o m 1.5 t o 3.0 in. T h e y a r e u s e d a s p r o t e c t i n g devices s o t h a t l a r g e s u s p e n d e d s o l i d s d o n o t d a m a g e p u m p s a n d other equipment. S o m e t i m e s shredders are used instead of coarse screens. These devices tear d o w n suspended solids, which are then r e m o v e d by sedimentation.
3. S e d i m e n t a t i o n 3.1.
INTRODUCTION
S e d i m e n t a t i o n is u t i l i z e d in w a s t e w a t e r t r e a t m e n t t o s e p a r a t e s u s p e n d e d solids f r o m w a s t e w a t e r s . R e m o v a l b y s e d i m e n t a t i o n is b a s e d o n t h e difference in specific g r a v i t y b e t w e e n solid p a r t i c l e s a n d t h e b u l k o f t h e l i q u i d , w h i c h r e s u l t s in s e t t l i n g o f s u s p e n d e d solids. I n s o m e c a s e s , s e d i m e n t a t i o n is t h e o n l y t r e a t m e n t t o w h i c h t h e w a s t e w a t e r is s u b j e c t e d . S e d i m e n t a t i o n is a l s o utilized i n o n e o r m o r e s t e p s o f a t r e a t m e n t s e q u e n c e . I n a t y p i c a l a c t i v a t e d s l u d g e p l a n t , s e d i m e n t a t i o n is utilized in t h r e e of t h e t r e a t m e n t s t e p s : (1) in grit c h a m b e r s , in w h i c h i n o r g a n i c m a t t e r (e.g., s a n d ) is r e m o v e d f r o m t h e w a s t e w a t e r ; (2) i n t h e p r i m a r y clarifier, w h i c h p r e c e d e s t h e b i o l o g i c a l r e a c t o r , solids ( o r g a n i c a n d o t h e r s ) a r e s e p a r a t e d ; a n d (3) in t h e s e c o n d a r y clarifier, w h i c h f o l l o w s t h e b i o l o g i c a l r e a c t o r , t h e b i o l o g i c a l s l u d g e is s e p a r a t e d f r o m t h e t r e a t e d effluent.
3.2. T Y P E S O F S E T T L I N G T h r e e t y p e s o f settling a r e r e c o g n i z e d d e p e n d i n g o n t h e n a t u r e o f s o l i d s p r e s e n t in t h e s u s p e n s i o n .
72
3.
1. Discrete
settling.
Pretreatment and Primary Treatment
P a r t i c l e s b e i n g settled k e e p t h e i r i n d i v i d u a l i t y , i.e.,
they d o n o t coalesce with other particles. T h u s , the physical properties of the p a r t i c l e s (size, s h a p e , specific g r a v i t y ) a r e u n c h a n g e d d u r i n g t h e p r o c e s s . T h e s e t t l i n g of s a n d p a r t i c l e s i n g r i t c h a m b e r s is a t y p i c a l e x a m p l e o f d i s c r e t e settling. 2. Flocculent
settling.
A g g l o m e r a t i o n o f t h e s e t t l i n g p a r t i c l e s is a c c o m
p a n i e d b y c h a n g e s in d e n s i t y a n d s e t t l i n g velocity. T h e s e d i m e n t a t i o n o c c u r r i n g i n p r i m a r y clarifiers is a n e x a m p l e . 3. Zone
settling.
P a r t i c l e s f o r m a lattice ( o r b l a n k e t ) w h i c h settles a s a
mass exhibiting a distinct interface with the liquid phase. Examples include s e d i m e n t a t i o n o f a c t i v a t e d s l u d g e in s e c o n d a r y clarifiers a n d t h a t o f a l u m floes i n w a t e r t r e a t m e n t p r o c e s s e s .
3.3. T H E O R Y O F D I S C R E T E S E T T L I N G T h e f u n d a m e n t a l r e l a t i o n s h i p for s e t t l i n g of d i s c r e t e p a r t i c l e s is N e w t o n ' s l a w , w h i c h is b a s e d o n t h e a s s u m p t i o n t h a t p a r t i c l e s a r e s p h e r i c a l w i t h a u n i f o r m d i a m e t e r . W h e n a p a r t i c l e settles, it a c c e l e r a t e s u n t i l t h e
forces
p r o m o t i n g settling, i.e., t h e p a r t i c l e effective w e i g h t , a r e b a l a n c e d b y t h e d r a g o r f r i c t i o n a l r e s i s t a n c e o f t h e l i q u i d . W h e n t h i s e q u a l i t y is a c h i e v e d , t h e particle reaches a c o n s t a n t settling velocity called the terminal o r settling velocity of the particle.* C o n s i d e r t h e p a r t i c l e in F i g . 3 . 1 , w h i c h h a s r e a c h e d its t e r m i n a l v e l o c i t y , a n d write the a p p r o p r i a t e force balance. T h e force p r o m o t i n g sedimentation, i.e., t h e effective w e i g h t of t h e p a r t i c l e , is t h e difference b e t w e e n its a c t u a l w e i g h t a n d t h e h y d r o s t a t i c lift: F
S
where F
s
= vp g
- vp g
s
=
L
is t h e p a r t i c l e effective w e i g h t ; p
s
(3.1)
(ps-pdgv
the particle density; p
L
the liquid
d e n s i t y ; g t h e a c c e l e r a t i o n o f g r a v i t y ; a n d ν t h e p a r t i c l e v o l u m e , \nd , 3
where
d is t h e d i a m e t e r o f t h e s p h e r i c a l p a r t i c l e . T h e d r a g f o r c e i m p e d i n g s e d i m e n t a t i o n is FD
where F
D
is t h e d r a g f o r c e ; C
D
t h e p a r t i c l e , A = \nd ; 2
—
(3.2)
C A(p V /2) 2
D
L
t h e d r a g coefficient; A t h e p r o j e c t e d a r e a o f
a n d V t h e r e l a t i v e v e l o c i t y b e t w e e n p a r t i c l e a n d fluid.
F o r t h e c o n d i t i o n defining t h e t e r m i n a l v e l o c i t y , e q u a t e E q s . (3.1) a n d (3.2). (Ps-p )gv
=
L
C A(p Vs /2) 2
D
L
where V = v
s
= settling velocity
* This results from force = (mass) (acceleration). Thus zero acceleration corresponds t o a net force of zero, i.e., a perfect balance o f forces.
3.
73
Sedimentation
Fig.
3.1.
S u b s t i t u t i n g ν = \nd ,
Discrete
settling
A = bnd ,
3
of a
particle.
a n d solving for the t e r m i n a l velocity, V
2
s
[Eq.(3.3)]: 1/2
Vs =
(3.3)
PL
w h i c h is N e w t o n ' s l a w . F o r s p h e r i c a l p a r t i c l e s , t h e d r a g coefficient C n u m b e r N defined in E q . (3.4).
is r e l a t e d t o t h e R e y n o l d s
D
R
N
= dVsp /p
R
L
(3.4)
L
w h e r e d is t h e d i a m e t e r of s p h e r e , V t h e t e r m i n a l v e l o c i t y ( s e t t l i n g v e l o c i t y ) , s
and p
L
and p
L
t h e d e n s i t y a n d v i s c o s i t y of l i q u i d . T h i s r e l a t i o n s h i p is s h o w n
in F i g . 3.2.* 10'
io-
Sto ies* \ l a w i egion
Trc nsition 1 re^ |ion
Newtc n's regior
C =I8 D
iC =0 4 D
0.001
0.01
0.1
I 2 N
Fig.
3.2.
Correlation
IOOSOO^O
10
R
=
d
3
ΙΟ
4
ΙΟ
5
ΙΟ
6
W ^ L
for drag coefficient
for spherical
particles.
* For nonspherical particles Fig. 3.2 is plotted as a family o f curves, each curve corre sponding to specified value o f a parameter defined as sphericity [sphericity, φ = (surface area o f a sphere having same v o l u m e as particle)/(surface area o f particle)]. See Waddel [8].
74
3.
I n g e n e r a l , t h e d r a g coefficient C
Pretreatment and Primary Treatment
is a p p r o x i m a t e d b y
D
C
= b/N
D
(3.5)
n R
w h e r e coefficients b a n d η for t h e different r e g i o n s of F i g . 3.2 a r e i n d i c a t e d in Table 3.1. The approximate relationship between C
and N
D
given b y E q .
R
T A B L E 3.1 Drag Coefficient C
i
Region
b
η
C
=
24
1.0
C
18.5
0.6
C =\$.5/N
0.4
0.0
C
D
b/N
n R
Stokes' law N <2 R
D
=
24/N
R
Transition 2 < N
< 500
R
D
06 R
Newton's N> R
500
D
= 0.4
(3.5) is w r i t t e n in l o g a r i t h m i c f o r m for t h e t h r e e r e g i o n s i n d i c a t e d in F i g . 3.2. Stokes' region:
C
D
:. l o g C Transition region:
D
C
.·. l o g C
T h u s t h e c u r v e in F i g . s t r a i g h t line w i t h s l o p e s M a n y sedimentation region. Substituting C S t o k e s ' l a w is o b t a i n e d . D
D
C
D
.*. l o g C
24/N
R
= - l o g J V * + log 24 = 18.5/JV ' 0
D
N e w t o n ' s region:
=
D
6
K
= - 0 . 6 logiV + l o g 18.5 R
= 0.4 = log 0.4 = 0 . 0 1 o g ^ / + log 0.4 K
3.2 is a p p r o x i m a t e l y r e p l a c e d b y t h r e e s e g m e n t s o f of, respectively, — 1, —0.6, a n d 0.0. p r o b l e m s i n w a s t e w a t e r t r e a t m e n t o c c u r in S t o k e s ' = 24/N = 24p /dV p in E q . (3.3) a n d s i m p l i f y i n g , R
V
L
s
= 1/18- -±^-gd P
s
L
(3.6)
2
ML
F o r a specific p r o b l e m in S t o k e s ' r e g i o n ( p , p , a n d p s
L
L
fixed)
E q . (3.6) is
w r i t t e n a s [ E q . (3.7)] Vs = Kd
(3.7)
2
( w h e r e AT is a c o n s t a n t ) , w h i c h i n l o g a r i t h m i c f o r m b e c o m e s [ E q . ( 3 . 8 ) ] logKs = 2 log*/ + l o g # = 2 log*/ + C
( C is a constant)
(3.8)
3.
75
Sedimentation
T h u s , a l o g a r i t h m i c p l o t o f V vs. d yields a s t r a i g h t line o f s l o p e e q u a l t o 2.0 s
for Stokes' region. For (C
D
a
specific
problem
in N e w t o n ' s
region,
since
C
is a
D
constant
= 0.4), E q . (3.3) yields E q . (3.9). V
=
s
K'd
(3.9)
1/2
( w h e r e K' — a c o n s t a n t ) w h i c h in l o g a r i t h m i c f o r m b e c o m e s E q . (3.10). logKs = i l o g r f + l o g i T = i l o g < / + where C
(3.10)
C
is a c o n s t a n t . T h u s , a l o g a r i t h m i c p l o t of V v s . d y i e l d s a s t r a i g h t s
line o f s l o p e e q u a l t o \ for N e w t o n ' s r e g i o n . For
the
transition
region,
C
= 18.5/iV^ =
(\%^ - )l(d ' V° ' p - ).
6
D
0
6
0 6
6
L
0
s
6
L
S u b s t i t u t i n g t h i s v a l u e i n E q . (3.3) a n d s i m p l i f y i n g , o n e o b t a i n s E q . (3.11). Vs = [ ( 4 ^ / 5 5 . 5 ) ( / 7 - / / / - ) ( / - / ) / / > J ^ 0
6
L
0
6
L
1
? s
= K*d
1 1 4 3
l
? L
(3.11)
1 4 3
w h e r e K" is a c o n s t a n t . I n l o g a r i t h m i c f o r m t h i s b e c o m e s [ E q . ( 3 . 1 2 ) ] logKs = 1.143 \ogd+
l o g * " = 1.143 \ogd + C"
(3.12)
w h e r e C " is a c o n s t a n t . T h u s , a l o g a r i t h m i c p l o t o f V vs. d y i e l d s a s t r a i g h t s
line o f s l o p e 1.143 for t h e t r a n s i t i o n r e g i o n . T h e l o g a r i t h m i c p l o t o f V v s . d s
for t h e t h r e e r e g i o n s is s h o w n in F i g . 3 . 3 . E v e n in t h e c a s e o f g r i t c h a m b e r s , t h e t h e o r y j u s t o u t l i n e d suffers f r o m t w o s e r i o u s l i m i t a t i o n s : (1) g r i t p a r t i c l e s a r e s e l d o m s p h e r i c a l , a n d (2) grit p a r t i c l e s d o n o t h a v e u n i f o r m d e n s i t y . A g r a p h c o r r e s p o n d i n g t o E q . (3.3) is p l o t t e d in F i g . 3.4, g i v i n g t h e r e l a t i o n ship between particle d i a m e t e r a n d velocity V . s
P a r t i c l e s o f specific g r a v i t i e s
1.001, 1.01, a n d 2.65 a r e c o n s i d e r e d in p l o t t i n g F i g . 3.4. V a l u e 2.65 c o r r e s p o n d s t o t h e specific g r a v i t y of t y p i c a l s a n d . T h e l i q u i d u s e d is w a t e r a t t e m p e r a t u r e s indicated, c o r r e s p o n d i n g to respective values of p , μ . Values of C L
£
D
are
o b t a i n e d f r o m F i g . 3.2 b y a trial a n d e r r o r p r o c e d u r e : (1) f o r specified p a r t i c l e Stokes' region
iTransi,' tion
Newton's region
log d Fig. 3.3. Logarithmic
plot
of V
s
vs. d.
76
3.
10
10"'
Pretreatment and Primary Treatment
10
K)
Velocity of fall, c m / s e c Fig.
3.4.
Relation
between
settling
velocity
and particle
diameter
[4].
d i a m e t e r a n d t e m p e r a t u r e (p a n d p fixed) a s s u m e a s e t t l i n g v e l o c i t y V ; (2) C a l c u l a t e N b a s e d u p o n t h i s a s s u m e d v e l o c i t y ; (3) F r o m F i g . 3.2 r e a d C ; a n d (4) F r o m E q . 3.3 r e c a l c u l a t e V . If it a g r e e s w i t h t h e v a l u e a s s u m e d in (1) c a l c u l a t i o n s a r e c o n s i s t e n t . O t h e r w i s e , i t e r a t i o n is c o n t i n u e d u n t i l a g r e e m e n t is o b t a i n e d . L
L
s
R
D
s
F i g u r e 3.4 is c o n s t r u c t e d in t h i s m a n n e r . S i n c e c o n s t r u c t i o n is b a s e d u p o n t h e a c t u a l c u r v e o f C vs. N (i.e., F i g . 3.2), t h e lines in F i g . 3.4 e x h i b i t s o m e c u r v a t u r e , b y c o n t r a s t w i t h t h e t h r e e s t r a i g h t line s e g m e n t s in F i g . 3.3 [ c o n s t r u c t i o n o f w h i c h is b a s e d o n t h e a p p r o x i m a t e r e l a t i o n s h i p s given b y E q s . (3.8), (3.10), a n d ( 3 . 1 2 ) ] . A s a n a p p r o x i m a t i o n , h o w e v e r , c u r v e s in F i g . 3.4 a r e r e p l a c e a b l e b y t h r e e s t r a i g h t line s e g m e n t s . D
R
3.4. T H E I D E A L S E D I M E N T A T I O N T A N K C O N C E P T T h i s c o n c e p t , d e v e l o p e d b y H a z e n [ 5 ] a n d C a m p [ 1 ] , is t h e b a s i s f o r a r r i v i n g a t r e l a t i o n s h i p s utilized in t h e d e s i g n o f s e d i m e n t a t i o n t a n k s . T h e m o d e l c h o s e n f o r a s e d i m e n t a t i o n t a n k c o n s i s t s o f f o u r z o n e s ( F i g s . 3.5 a n d 3.6). 1. I n l e t z o n e . H e r e t h e flow b e c o m e s q u i e s c e n t . I t is a s s u m e d t h a t a t t h e l i m i t of t h i s z o n e (i.e., a l o n g v e r t i c a l line xt) p a r t i c l e s a r e u n i f o r m l y d i s t r i b u t e d a c r o s s t h e influent c r o s s s e c t i o n . 2. S e d i m e n t a t i o n z o n e . A p a r t i c l e is a s s u m e d t o b e r e m o v e d f r o m s u s p e n s i o n o n c e it h i t s t h e b o t t o m o f t h i s z o n e ( h o r i z o n t a l line ty).
3.
77
Sedimentation
t=0 x.
.
Inlet x_ __^r^_ zone ^ ^ - ^
V Case
Sedimentation z o n e ^ - i ^ ^
Outlet zone
π
Case^ *-^. 1
t_ Sludge zone _.
Η
Fig. 3.5. Model (Cases 1 and 2).
L
of a sedimentation
,
tank
with
discrete
settling
particles
|t=0 !x_
% /
Inlet j ^ ^ ^ - ^ ^ ^ ^ zone | ! !
_
'
sj · V, ^ C a s e 3 ^^"""""•"•••^ V
Sedimentation zone
^^^^^^
^Case 4
ΊI
^
Outlet zone
t
y"
It
Fig. 3.6. Model (Cases 3 and 4).
y_
!
Sludge zone
p-
L
of a sedimentation
*
tank
with
discrete
settling
particles
3. O u t l e t z o n e . W a s t e w a t e r is c o l l e c t e d h e r e p r i o r t o t r a n s f e r t o t h e n e x t treatment. 4. S l u d g e z o n e . T h i s z o n e is p r o v i d e d f o r s l u d g e r e m o v a l . S e t t l i n g p a t h s of p a r t i c l e s e n t e r i n g t h e s e d i m e n t a t i o n z o n e a t p o i n t s χ a n d x' f o r d i s c r e t e s e t t l i n g a r e i n d i c a t e d b y lines xy a n d x'y' i n F i g . 3.5 a n d lines xy" a n d x'y in F i g . 3.6. T h e s e settling p a t h s a r e t h e n e t r e s u l t o f t w o v e l o c i t y vector c o m p o n e n t s : 1. F l o w - t h r o u g h velocity V [ E q . (3.13)] V = Q/A'
(3.13)
= QIWH
w h e r e Κ is t h e flow-through velocity (ft/sec); Q t h e flow r a t e ( f t / s e c ) ; A' t h e v e r t i c a l c r o s s - s e c t i o n a l a r e a o f s e d i m e n t a t i o n z o n e , n a m e l y A' = WH ( f t ) (refer t o F i g . 3.7); W t h e w i d t h o f s e d i m e n t a t i o n z o n e (ft); a n d Η t h e d e p t h o f s e d i m e n t a t i o n z o n e (ft). 3
2
2. Settling velocity, i n d i c a t e d b y e i t h e r v e c t o r s V o r V in F i g s . 3.5 a n d 3.6, respectively. F o r d i s c r e t e s e t t l i n g t h e settling velocity is c o n s t a n t f o r a n y specific s e t t l i n g s
x
78
3.
Fig.
3.7.
Geometry
Pretreatment and Primary Treatment
of the sedimentation
zone.
path, i.e., V and V do not vary along their respective paths. This is due to the fact that a discrete particle is unhindered by neighboring ones (no coales cence), so it settles with a uniform velocity, read from Fig. 3.4, as a function of the particle diameter. For flocculent settling the situation is different. Figure 3.8 illustrates a typical sedimentation path of flocculent settling (Section 3.5). As coalescence s
t
Sludge zone Fig.
3.8.
Model
of a sedimentation
tank for flocculent
settling.
with neighboring particles takes place, the effective diameter of the particle increases, and thus its settling velocity V also increases. The net result is that settling paths are curved in contrast with straight line paths for discrete settling. Consider the following cases with reference to Figs. 3.5 and 3.6, keeping in mind that a particle is assumed to be removed from the suspension once it hits the bottom of the sedimentation zone. Case 1 (Fig. 3.5). A particle which at time zero (r = 0) is located at point χ and possesses a settling velocity V (and diameter d read from Fig. 3.4). This particle is removed, since it touches the bottom of the sedimentation zone at y (path xy). Case 2 (Fig. 3.5). A particle which at t = 0 is located at x on the same vertical line as χ but below x, and has a settling velocity V (or greater than V ). This particle is also removed since it hits the bottom of the sedimentation zone s
s
s
f
s
s
3.
79
Sedimentation
t o t h e left o f p o i n t y ( a t / ) - If t h e s e t t l i n g v e l o c i t y is g r e a t e r t h a n V t h e p a r t i c l e s
t o u c h e s t h e b o t t o m o f t h e s e d i m e n t a t i o n z o n e t o t h e left o f y'. p o r t r a y s t h e c a s e o f p a r t i c l e s w i t h a d i a m e t e r d (d x
velocity V (V < V ). ( S e t t l i n g velocity V x
t
s
x
F i g u r e 3.6
< d ), p o s s e s s i n g a s e t t l i n g s
is r e a d f r o m F i g . 3.4 f o r d =
l
d) v
T h e s e p a r t i c l e s a r e s h o w n s e p a r a t e l y in F i g . 3.6 s i m p l y t o a v o i d o v e r c r o w d i n g o f t h e d i a g r a m . I n fact t h e y a r e t o g e t h e r in t h e s l u r r y w i t h p a r t i c l e s o f s e t t l i n g velocity Case
V. s
3 ( F i g . 3.6). A p a r t i c l e w h i c h a t t = 0 is l o c a t e d a t χ a n d h a s a
s e t t l i n g v e l o c i t y V ( w h e r e V < V ). T h i s p a r t i c l e is n o t r e m o v e d since it d o e s x
t
s
n o t r e a c h t h e b o t t o m of t h e s e d i m e n t a t i o n z o n e (i.e., line ty) in t i m e f o r removal (sedimentation path
xy").
Case 4 ( F i g . 3.6). A p a r t i c l e w i t h s e t t l i n g v e l o c i t y V ( w h e r e V < V ) a n d x
t
s
s i t u a t e d a t x' a t t = 0. T h i s p a r t i c l e is r e m o v e d ( s e d i m e n t a t i o n p a t h
x'y).
C o n s i d e r n o w t h e settling velocity V . F r o m E q . (3.13) s
Q = VA' = VWH
(3.14)
F r o m c o n s i d e r a t i o n o f s i m i l a r t r i a n g l e s in F i g . 3.5 V=
Vs(L/H)
(3.15)
S u b s t i t u t i o n o f E q . (3.15) in E q . (3.14) a n d s i m p l i f i c a t i o n l e a d s t o Q = V LW S
w h e r e A = LW=
= VA S
(3.16)
horizontal cross-sectional area of the sedimentation z o n e
( f t ) ( F i g . 3.7). 2
F r o m E q . (3.16) Vs = QILW=
Q/A
(3.17)
F r o m E q . (3.17) it follows t h a t t h e s e t t l i n g efficiency is a f u n c t i o n o f t h e h o r i z o n t a l c r o s s - s e c t i o n a l a r e a , r a t h e r t h a n o f t h e d e p t h H. T h u s , in p r i n c i p l e , it is a d v i s a b l e t o utilize s e d i m e n t a t i o n t a n k s o f h i g h surface a r e a A a n d l o w d e p t h s . T h e o n l y r e a s o n s for u s i n g a r e a s o n a b l e d e p t h a r e (1) t o satisfy d e p t h r e q u i r e m e n t s in o r d e r t o p r o v i d e f o r m e c h a n i c a l r a k e s u t i l i z e d f o r r e m o v a l of settled s l u d g e , a n d (2) t h e h o r i z o n t a l c o m p o n e n t o f v e l o c i t y ( f l o w - t h r o u g h v e l o c i t y V) m u s t b e k e p t w i t h i n c e r t a i n l i m i t s t o p r e v e n t s c o u r i n g t h e p a r t i c l e s w h i c h h a v e settled. F r o m E q . (3.13) it follows t h a t Η s h o u l d n o t b e t o o l o w since Κ w o u l d rise a b o v e t h e s c o u r velocity. T h e s u b j e c t o f s c o u r v e l o c i t y is d i s c u s s e d in t h i s s e c t i o n . S c o u r o c c u r s w h e n flow-through v e l o c i t y V is sufficient t o s u s p e n d p r e v i o u s l y settled p a r t i c l e s . S c o u r is n o t u s u a l l y a p r o b l e m in l a r g e s e t t l i n g t a n k s , b u t it c a n b e a n i m p o r t a n t f a c t o r in g r i t c h a m b e r s a n d n a r r o w c h a n n e l s . T h e t w o f u n d a m e n t a l p r e m i s e s o f t h e i d e a l t a n k c o n c e p t a r e (1) u n i f o r m d i s t r i b u t i o n o f p a r t i c l e s a c r o s s t h e influent c r o s s s e c t i o n (i.e., a l o n g v e r t i c a l
80
3.
Pretreatment and Primary Treatment
line xt) a n d (2) a s s u m p t i o n t h a t a p a r t i c l e is c o n s i d e r e d r e m o v e d w h e n it r e a c h e s t h e b o t t o m o f t h e s e d i m e n t a t i o n z o n e (i.e., h o r i z o n t a l line ty). K e e p i n g in m i n d t h e s e t w o p r e m i s e s , t w o c o r o l l a r i e s f o l l o w : (1) A l l p a r t i c l e s w i t h a s e t t l i n g velocity e q u a l t o o r g r e a t e r t h a n V a r e r e m o v e d ; a n d (2) all p a r t i c l e s w i t h a settling v e l o c i t y less t h a n V ( s u c h a s V i n F i g . 3.6) a r e r e m o v e d i n a p r o p o r t i o n given b y t h e r a t i o VJV . s
s
x
S
F r o m g e o m e t r i c c o n s i d e r a t i o n o f t h e p a r t i c l e p a t h ( F i g s . 3.5 a n d 3.6) t o u c h i n g point y at the b o t t o m of the sedimentation t a n k (particles with s e t t l i n g v e l o c i t y V in F i g . 3.5 f o l l o w i n g p a t h xy, a n d p a r t i c l e s w i t h s e t t l i n g v e l o c i t y K i n F i g . 3.6 f o l l o w i n g p a t h x'y), o n e w r i t e s [ E q . ( 3 . 1 8 ) ] s
x
Vi/Vs = h/H
(3.18)
F o r e x a m p l e , if Η = 100 in. a n d h = 75 in., t h e n VJVs = 75/100 = 0.75 o r 7 5 % o f t h e p a r t i c l e s w i t h a s e t t l i n g velocity V
x
a r e r e m o v e d , i.e., t h o s e
w h i c h a t t i m e t = 0 a r e a t a h e i g h t x' o r b e l o w . T h e r e m a i n i n g p a r t i c l e s w i t h settling velocity V
i.e., t h o s e s i t u a t e d b e t w e e n χ a n d x' a t t i m e / = 0, a r e
u
not r e m o v e d . T h e overflow r a t e defined a s QjA = f t / ( f t ) ( h r ) = ft/hr 3
2
is defined a s t h e settling velocity V o f a p a r t i c l e t h a t settles t h r o u g h a d i s t a n c e s
e x a c t l y e q u a l t o t h e effective d e p t h of t h e t a n k d u r i n g t h e t h e o r e t i c a l d e t e n t i o n p e r i o d . T h i s r e s u l t s f r o m t h e definition o f d e t e n t i o n p e r i o d : / = detention period = (volume of t a n k ) / Q = HA/Q
(A = LW)
(3.19)
F r o m E q . (3.19) it follows t h a t a s e t t l i n g velocity V defined a s V = H/t s
s
is
e q u i v a l e n t t o t h e overflow r a t e , since Vs = Hit = H/(HA/Q) w h i c h is E q . (3.17). T h e scour velocity V is t h e v a l u e o f t h e c
V = V
c
=
Q/A
flow-through
velocity V [ E q . ( 3 . 2 0 ) ] ,
= Q/A' = QIWH
(3.20)
for which "previously settled" particles are scoured away.* * T h e words "previously settled" are placed in quotes because a particle which is scoured away never actually settles. Mentally o n e separates the processes of settling and scouring and imagines that a particle settles and subsequently is scoured away. This reasoning is compatible with the hypothetical resolution of the velocity trajectory into vectors V (flowthrough velocity) and V (settling velocity). s
3.
Sedimentation
81
T h e s c o u r v e l o c i t y is e s t i m a t e d b y t h e f o l l o w i n g e m p i r i c a l e q u a t i o n [ E q . (3.21)] [ 1 ] : V
c
= [8/fr
(3.21)
1 / 2
w h e r e V is t h e v e l o c i t y o f s c o u r ( m m / s e c ) ; i.e.,
flow-through
c
velocity required
t o s c o u r all p a r t i c l e s o f d i a m e t e r d o r s m a l l e r ; β t h e c o n s t a n t (0.04 f o r u n i g r a n u l a r s a n d , 0 . 0 6 for n o n u n i f o r m s t i c k y m a t e r i a l ) ; / t h e W e i s b a c h - D ' A r c y f r i c t i o n f a c t o r (0.03 f o r c o n c r e t e ) ; g t h e a c c e l e r a t i o n o f g r a v i t y ( m m / s e c ) 2
( n o r m a l : 9 8 0 0 m m / s e c ) ; dthe 2
particle diameter ( m m ) (particles with diameter
d o r less t h a n d a r e s c o u r e d a w a y ) ; a n d s t h e specific g r a v i t y o f p a r t i c l e .
Example 3 . 1 C o n s i d e r a s u s p e n s i o n of s a n d (s = 2.65) i n w a t e r a t 2 0 ° C w i t h a u n i f o r m p a r t i c l e size (d = 0.07 m m ) . F l o w is 1.0 M g a l / d a y . 1. C a l c u l a t e t h e s e t t l i n g t a n k s u r f a c e ( h o r i z o n t a l c r o s s s e c t i o n )
for
obtaining removal of 70% of the particles. 2. S u p p o s e t h a t i n s t e a d of a u n i f o r m p a r t i c l e d i a m e t e r , t h e r e is, b e s i d e s p a r t i c l e s ofd=
0.07 m m , a n o t h e r set w i t h a u n i f o r m l y l a r g e r d i a m e t e r , w h i c h
are completely r e m o v e d in the settling t a n k designed for 7 0 % r e m o v a l of the p a r t i c l e s w i t h d = 0.07 m m . D e t e r m i n e w h a t is t h e m i n i m u m p a r t i c l e d i a m e t e r for total removal. 3. F o r c a s e (2), d e t e r m i n e t h e
flow-though
v e l o c i t y V s o t h a t all p a r t i c l e s c
of lower settling velocity t h a n those completely r e m o v e d are scoured a w a y . W h a t c o m b i n a t i o n o f l e n g t h , w i d t h , a n d d e p t h for t h e s e t t l i n g t a n k m e e t s these requirements ? S O L U T I O N : Part 1 Step
1. F r o m F i g . 3.4 [ f o r d = 0.07 m m a n d s = 2.65 ( a t 2 0 ° C ) ] , r e a d V
s
= 0.45 cm/sec
or V
= 0.45 cm/sec χ ft/30.48 cm χ 3600 sec/hr
V
= 53.1 ft/hr = 53.1 f t / ( f t ) ( h r )
s
3
s
2
T h e overflow r a t e i n g a l / ( d a y ) ( f t ) is 2
53.1 f t / ( f t ) ( h r ) χ 7.48 gal/ft χ 2 4 h r / d a y 3
2
3
Λ V = 9533 gal/(day)(ft ) 2
s
Step 2. T h e h o r i z o n t a l c r o s s - s e c t i o n a l a r e a is (for 1 0 0 % r e m o v a l ) 1,000,000 gal/day A
= Q' * V
= 9533 gal/(day)(ft ) 2
=
1
0
5
*
82
3.
Pretreatment and Primary Treatment
W i t h t h i s a r e a , 1 0 0 % r e m o v a l is o b t a i n e d . F o r 7 0 % r e m o v a l t h e r e s i d e n c e t i m e ( a n d t h u s t h e c r o s s - s e c t i o n a l a r e a ) is r e d u c e d b y 3 0 % . T h e c r o s s - s e c t i o n a l a r e a is A = 105 χ 0.7 = 73.5 ft
2
U n d e r t h e s e c i r c u m s t a n c e s , t h e settling v e l o c i t y for 1 0 0 % r e m o v a l is 1,000,000/73.5 = 13,605 gal/(day)(ft ) 2
Note:
C h e c k o n p e r c e n t r e m o v a l . S i n c e t h e s e t t l i n g v e l o c i t y is 9 5 3 3
g a l / ( d a y ) ( f t ) , fixed b y t h e p a r t i c l e d i a m e t e r a s d e t e r m i n e d f r o m F i g . 3.4, 2
p e r c e n t a g e r e m o v a l is 9 5 3 3 / 1 3 , 6 0 5 = 0.70 ( 7 0 % ) . R e f e r r i n g t o F i g . 3.6 t h e p a r t i c l e s r e m o v e d a r e t h o s e w h i c h a t t h e e n d o f inlet z o n e , a l o n g v e r t i c a l line xt, a r e a l r e a d y a t d i s t a n c e h ( o r less t h a n h) f r o m t h e b o t t o m o f t h e s e d i m e n tation zone, where [from E q . (3.18)] V
= 13,605 gal/(day)(ft )
(100% removal)
2
s
Vi = 9533 gal/(day)(ft ) 2
(70% removal)
.·. h/H = 0.70 S O L U T I O N : Part 2
If t h e r e is a d i s t r i b u t i o n of p a r t i c l e d i a m e t e r s i n s t e a d
o f u n i f o r m d i a m e t e r d = 0.07 m m , o n e r e a d s f r o m F i g . 3.4 t h e d i a m e t e r , w h i c h is l a r g e r t h a n 0.07 m m , for w h i c h t h e s e t t l i n g v e l o c i t y c o r r e s p o n d s t o 13,605 g a l / ( d a y ) ( f t ) . T h e r e f o r e , t h e a b s c i s s a i n F i g . 3.4 is 2
(13,605/9533) χ 0.45 cm/sec = 0.45/0.7 = 0.642 cm/sec F r o m F i g . 3.4 [ f o r V = 0.642 c m / s e c a n d s = 2.65 (t = 2 0 ° Q ] , r e a d d = 0.085 s
m m ( 1 0 0 % r e m o v a l ) . If d i s t r i b u t i o n o f p a r t i c l e d i a m e t e r in t h e i n f l u e n t is k n o w n , one can calculate the % removal corresponding to each g r o u p of p a r t i c l e s for a g i v e n d i a m e t e r ( E x a m p l e 3.2). S O L U T I O N : Part 3
T h e s c o u r v e l o c i t y t o s w e e p all p a r t i c l e s o f l o w e r
s e t t l i n g v e l o c i t y t h a n t h o s e t o b e c o m p l e t e l y r e m o v e d is c a l c u l a t e d f r o m E q . (3.21). V
c
= [8 χ 0.04 x 9800 χ 0.07(2.65 - 1 ) / 0 . 0 3 ]
1/2
= 110 mm/sec
A s s u m i n g t h a t t h e s a n d c o n t a i n s o n l y t w o p a r t i c l e sizes, e.g., 0.07 a n d 0.085 m m , t h e s c o u r v e l o c i t y V = 110.0 m m / s e c s w e e p s a w a y all p a r t i c l e s
of
d = 0.07 m m , l e a v i n g b e h i n d t h o s e o f d = 0.085 m m . T h e v a l u e o f V
in
c
c
p r a c t i c a l u n i t s is V
c
= 110 mm/sec χ ft/304.8 m m = 0.36 ft/sec
T h e v e r t i c a l c r o s s - s e c t i o n a l a r e a is c a l c u l a t e d f r o m E q . (3.13). __ 1,000,000 gal/day χ day/86,400 sec χ ft /7.48 gal 3
0.36 ft/sec
=
4
3
ft2
3.
83
Sedimentation
A n y p r a c t i c a l c o m b i n a t i o n o f l e n g t h , w i d t h , a n d d e p t h is u s e d t o satisfy t h e r e q u i r e m e n t s A = 73.5 f t
2
= LW
a n d A' = 4 . 3 f t
2
=
WH.
Example 3.2 S u p p o s e t h a t for E x a m p l e 3.1 i n s t e a d o f a u n i f o r m p a r t i c l e size, t h e r e is a d i s t r i b u t i o n o f d i a m e t e r s . A s s u m e t h e s a m e specific g r a v i t y a n d t e m p e r a t u r e a s in E x a m p l e 3 . 1 , i.e., s = 2.65 a n d t = 2 0 ° C . A s s u m e t h a t f o r e a c h 100 l b o f g r i t t h e f o l l o w i n g d i s t r i b u t i o n o f p a r t i c l e sizes a p p l i e s (see t a b u l a t i o n b e l o w )
Group no.
(2) lb o f each particle size
(3) Particle size, d (mm)
1 2 3 4
50 20 20 10
0.085 0.070 0.060 0.050
TOO
T h e s e t t l i n g velocities for e a c h g r o u p o f p a r t i c l e s a r e r e a d f r o m F i g . 3.4. T h i s is i n d i c a t e d in c o l u m n (4) o f T a b l e 3.2. P e r c e n t r e m o v a l s a r e t h e n c a l c u l a t e d [ c o l u m n ( 5 ) ] a n d e x p r e s s e d a s f r a c t i o n s o f u n i t y in c o l u m n (r5). T h e T A B L E 3.2 Calculations for Example 3.2 (2) lb o f (3) (/) each (4) Particle G r o u p particle size, d V ( m m ) (Fig. 3.4) / no. size
(5)
s
1 2 3 4
50 20 20 10
0.085 0.070 0.060 0.050
0.642 0.450 0.350 0.220
0
removal-
0 6 4 2
(0.642/0.642)100 (0.45/0.642)100 (0.35/0.642)100 (0.22/0.642)100
100
= = = =
(6) Fraction (7) removed = lb r e m o v e d x 100 (7) = (2) χ (6) (5)-100 100" 70 54.5 34.3 fl
1.00 0.70 0.545 0.343
50.0 14.0 10.9 3.43 78.3
" Already calculated in Example 3.1.
w e i g h t of s a n d r e m o v e d b y settling for e a c h g r o u p o f p a r t i c l e s is c o m p u t e d in c o l u m n ( 7 ) . T h e r e f o r e 7 8 . 3 % o f t h e w e i g h t o f t h e o r i g i n a l p a r t i c l e s is r e m o v e d b y settling.
84
3.
Pretreatment and Primary Treatment
I n t h i s e x a m p l e , if t h e v e r t i c a l c r o s s s e c t i o n A' is t a k e n e q u a l t o 4.3 f t ( v a l u e c a l c u l a t e d in E x a m p l e 3.1), all p a r t i c l e s of < / = 0 . 0 7 m m a n d s m a l l e r a r e s c o u r e d a w a y ( g r o u p s 2, 3, a n d 4). T h e r e f o r e , t h e n e t r e m o v a l is o f o n l y 50 lb p e r 100 lb of t o t a l grit, i.e., t h e p a r t i c l e s w i t h d = 0.085 ( g r o u p 1). T h i s i n d i c a t e s a net removal by weight of 50%. 2
If A' is t a k e n l a r g e r t h a n 4.3 f t t h e n e t r e m o v a l is g r e a t e r , since t h e r e is less s c o u r i n g . E x a m i n i n g c a l c u l a t i o n s for E x a m p l e 3 . 1 , it follows t h a t if A' is t a k e n a s 8.6 f t ( t w i c e 4.3 f t ) t h e v a l u e of V is 0.18 ft/sec ( i n s t e a d o f 0.36 ft/sec). This corresponds to 2
2
2
c
V
c
= 55 mm/sec
(instead of 110.0 mm/sec)
S i n c e f r o m E q . (3.21), V is p r o p o r t i o n a l t o d
l / 2
c
,
it f o l l o w s t h a t d is 0 . 0 7 / 4 =
0.0175 m m , s o a s t o yield V = 55 m m / s e c ( h a l f o f 110.0 m m / s e c ) . T h e r e f o r e , c
o n l y p a r t i c l e s w i t h d = 0.0175 m m o r s m a l l e r a r e r e m o v e d b y s c o u r i n g . S i n c e for t h e given d i s t r i b u t i o n t h e s m a l l e s t p a r t i c l e d i a m e t e r is 0.05 m m , t h e r e is n o r e m o v a l by s c o u r i n g . C o n s e q u e n t l y , t h e n e t r e m o v a l is 78.3 lb f r o m e v e r y 100 lb of s a n d , o r 7 8 . 3 % b y w e i g h t . If t h i s v a l u e is a d o p t e d , A = 73.5 ft A' =
8.6 ft
2
=
LW
2
=
WH
S e l e c t i n g Η = 4 ft, t h e n W = 8.6/4 = 2.15 ft L = 73.5/2.15 = 34.2 ft T h i s i n d i c a t e s specification o f a n a r r o w settling c h a n n e l 34.2 ft l o n g , 2.15 ft w i d e , a n d 4 ft d e e p .
3.5. F L O C C U L E N T S E T T L I N G F l o c c u l e n t s e t t l i n g t a k e s p l a c e w h e n s e t t l i n g v e l o c i t y of t h e p a r t i c l e s in c r e a s e s d u e t o c o a l e s c e n c e w i t h o t h e r p a r t i c l e s . A d i a g r a m o f flocculent s e d i m e n t a t i o n profiles is s h o w n in F i g . 3.8. T h e s e t t l i n g p a t h s o f t h e p a r t i c l e s a r e c u r v e s , r a t h e r t h a n s t r a i g h t lines a s for d i s c r e t e settling. D e s i g n c r i t e r i a for s y s t e m s e x h i b i t i n g flocculent settling a r e e s t a b l i s h e d b y a l a b o r a t o r y settling a n a l y s i s . A t y p i c a l l a b o r a t o r y settling c o l u m n is s h o w n in F i g . 3.9. C o n c e n t r a t i o n of s u s p e n d e d solids is k e p t u n i f o r m t h r o u g h o u t t h e c o l u m n a t t h e b e g i n n i n g o f t h e t e s t b y m e a n s of a p o r t a b l e stirrer. T h e d e p t h o f t h e c o l u m n is a p p r o x i m a t e l y t h e s a m e a s t h a t o f t h e settling t a n k t o b e d e s i g n e d . T e m p e r a t u r e is k e p t c o n s t a n t d u r i n g t h e test. A p r a c t i c a l d e s i g n o f a s e t t l i n g c o l u m n ( F i g . 3.9) is 8 ft d e e p , w i t h s a m p l i n g p o r t s a t d e p t h s o f 2, 4, 6, a n d 8 ft.
3.
Sedimentation
85
r—ID—{
ι
! 5-1/2" j Τ
2'-0"
Depths counted in this direction
2'-0"
2'-0"
Testing cylinder (plexiglass)
..ajjap3
Fig.
3.9. Laboratory
settling
column.
D a t a t a k e n a t 2-, 4-, a n d 6-ft d e p t h s a r e u t i l i z e d t o d e t e r m i n e s e t t l i n g v e l o c i t y a n d d e t e n t i o n t i m e r e l a t i o n s h i p s . D a t a f r o m t h e 8-ft p o r t a r e u s e d f o r s l u d g e concentration and compaction determinations. Step
1. Fill t h e c o l u m n w i t h w a s t e w a t e r , m a i n t a i n i n g a u n i f o r m c o n
c e n t r a t i o n of s u s p e n d e d s o l i d s t h r o u g h o u t . A p o r t a b l e s t i r r e r is u s e d f o r t h i s purpose. Step 2. R e m o v e t h e s t i r r e r f r o m t h e c y l i n d e r . A t e a c h s a m p l i n g p o r t , t h e c o n c e n t r a t i o n s of s u s p e n d e d solids a r e m e a s u r e d a t p r e d e t e r m i n e d t i m e s . E x a m p l e s 3.3 a n d 3.4 i l l u s t r a t e t h e p r o c e d u r e f r o m d a t a t r e a t m e n t t o t h e d e s i g n o f a clarifier.
Example 3.3 T h e s u s p e n s i o n b e i n g t e s t e d h a s a n initial s u s p e n d e d s o l i d s c o n c e n t r a t i o n o f 4 3 0 m g / l i t e r ( S S ) . T h e s u s p e n d e d s o l i d s ( h e n c e a b b r e v i a t e d a s SS) c o n 0
c e n t r a t i o n s in T a b l e 3.3 a r e m e a s u r e d a t t h e i n d i c a t e d t i m e s a t t h e 2-, 4 - , a n d 6-ft s a m p l i n g p o r t s . P e r f o r m t r e a t m e n t o f t h e d a t a a r r i v i n g a t c u r v e s for (a) % SS r e m o v a l vs. d e t e n t i o n t i m e ( m i n ) , (b) % SS r e m o v a l vs. o v e r f l o w r a t e [ g a l / ( d a y ) ( f t ) ] , a n d (c) % S S r e m a i n i n g ( f r a c t i o n o f p a r t i c l e s w i t h less t h a n 2
s t a t e d velocity) vs. s e t t l i n g v e l o c i t y ( f t / h r ) . Step
1. C a l c u l a t e f r a c t i o n o f solids r e m a i n i n g in s u s p e n s i o n for e a c h
sample [Eq. (3.22)]. χ = SS/SSo orin% y = SS/SSo x 100
(3.22)
86
3.
Pretreatment and Primary Treatment
T A B L E 3.3 Laboratory Sedimentation Data (Example 3.3) SS concentrations at indicated depths Time (min)
2 ft (Tap 1)
4 ft (Tap 2)
6 ft (Tap 3)
5 10 20 30 40 50 60 75
356.9 309.6 251.6 197.8 163.4 144.1 116.1 107.5
387.0 346.2 298.9 253.7 230.1 195.7 178.5 143.2
395.6 365.5 316.1 288.1 251.6 232.2 204.3 180.6
T h e n calculate for each s a m p l e the fraction of solids removed 1 - x o r in
% ζ = 100 - y
(3.23)
A s a m p l e of t h e s e c a l c u l a t i o n s (for a 2-ft d e p t h ) is s h o w n in T a b l e 3.4. S i m i l a r c a l c u l a t i o n s a r e p e r f o r m e d f o r 4 - a n d 6-ft d e p t h s . T A B L E 3.4 Calculation of Fraction of Solids Remaining and Removed for a 2-ft Depth
ω
(3)
(4)
Time (min)
(2) SS remaining (mg/liter) (Table 3.3)
Solids remaining (%) y = (SS/SSo) x 100
Solids removed (%) ζ = 100->>
5 10 20 30 40 50 60 75
356.9 309.6 251.6 197.8 163.4 144.1 116.1 107.5
83.0 72.0 58.5 46.0 38.0 33.5 27.0 25.0
17.0 28.0 41.5 54.0 62.0 66.5 73.0 75.0
Step 2. I n o r d e r t o s m o o t h t h e e x p e r i m e n t a l d a t a c o n s t r u c t a g r a p h o f % SS r e m o v e d v s . t i m e . T h i s p l o t is s h o w n in F i g . 3.10 for t h e 2-, 4 - , a n d 6-ft depths.
3.
87
Sedimentation 90 80 ο
11*
70
I ο
60!
Ε
;so ) ϊ
30 20 10
0
10
20
30
40
50
60
70
80
90
t (min) Fig. 3.10.
Suspended
solids
(%SS)
removed
vs.
time.
Step 3. F r o m F i g . 3.10 c o n s t r u c t a s e t t l i n g profile g r a p h ( F i g . 3.11). T h i s is d o n e b y r e a d i n g f r o m t h e s m o o t h e d c u r v e s o f F i g . 3.10 t h e a b s c i s s a s (/, m i n ) c o r r e s p o n d i n g t o selected v a l u e s o f % S S r e m o v e d (e.g., 5, 10, 2 0 , 7 0 , 7 5 % ) f o r e a c h o n e o f t h e t h r e e d e p t h s . T h e s e v a l u e s a r e t a b u l a t e d ( T a b l e 3.5) a n d utilized f o r c o n s t r u c t i o n o f F i g . 3 . 1 1 . Step 4. C a l c u l a t e % r e m o v a l o f SS a n d o v e r f l o w r a t e [ g a l / ( d a y ) ( f t ) ] . Before t h e p r o c e d u r e d e s c r i b e d h e r e is fully u n d e r s t o o d , s o m e p r e l i m i n a r y c o n s i d e r a t i o n s m u s t b e m a d e . A n effective s e t t l i n g v e l o c i t y V is defined a s t h e effective d e p t h (6 ft in t h i s e x a m p l e ) d i v i d e d b y t h e t i m e ( d e t e n t i o n t i m e , t) r e q u i r e d for a given p a r t i c l e t o t r a v e l t h i s d i s t a n c e , i.e. [ E q . ( 3 . 2 4 ) ] , 2
s
V
s
= Hit
(3.24)
If a s u s p e n s i o n c o n t a i n s p a r t i c l e s w i t h different s e t t l i n g velocities, t h e efficiency of r e m o v a l b y s e d i m e n t a t i o n is o b t a i n e d b y p e r f o r m i n g a s e t t l i n g c o l u m n t e s t a s j u s t d e s c r i b e d . L e t SS b e t h e c o n c e n t r a t i o n o f s o l i d s r e m a i n i n g for o n e specific s a m p l e a n d t i m e . T h u s x
0
= SS/SSo = fraction of solids remaining
and 1 — *
0
= 1 - SS/SSo = yo = fraction of solids removed
88
3.
Pretreatment and Primary Treatment
40
60
80
t (minutes) Fig. 3.11.
Settling
profile.
Encircled
numbers
are % SS
removed.
T A B L E 3.5 Values for Plotting Fig. 3.11 / (min) %SS removed
2 ft
4 ft
6ft
5 10 20 30 40 50 60 70 75
1.2 2.5 6.7 11.7 18.0 27.0 38.5 55.0 75.0
2.5 5.0 11.0 19.0 30.0 44.0 61.5 87.5
3.7 6.5 14.5 25.0 39.0 56.5 77.5
—
— —
P a r t i c l e s w i t h a s e t t l i n g velocity V o r h i g h e r ( w h e r e V = H/t) a r e c o m p l e t e l y r e m o v e d . P a r t i c l e s w i t h a l o w e r settling velocity V (V < V ) a r e r e m o v e d a t a r a t i o g i v e n b y E q . (3.18). s
s
1
t
s
A t y p i c a l g r a p h like t h e o n e in F i g . 3.12 is p l o t t e d b y a n a l y s i s o f d a t a o b t a i n e d w i t h t h e s e d i m e n t a t i o n c o l u m n . T h e d e t a i l s for c o n s t r u c t i o n o f s u c h
3.
89
Sedimentation
Fig. 3.12.
Determination
of overall
removal.
a g r a p h from experimental d a t a are discussed later in Step 4(d). However, for u n d e r s t a n d i n g t h e c a l c u l a t i o n p r o c e d u r e d e s c r i b e d n e x t , it is c o n v e n i e n t t o a s s u m e t h a t t h i s g r a p h is a l r e a d y a v a i l a b l e . F i g u r e 3.12 is a p l o t o f t h e f r a c t i o n o f p a r t i c l e s w i t h less t h a n t h e s t a t e d v e l o c i t y v s . t h e settling velocity in q u e s t i o n . N o t i c e t h a t t h e f r a c t i o n o f p a r ticles w i t h less t h a n t h e s t a t e d v e l o c i t y (if V = Hjt) c o r r e s p o n d s t o t h e f r a c t i o n o f p a r t i c l e s not c o m p l e t e l y r e m o v e d . F o r e x a m p l e , if 4 0 % o f t h e p a r t i c l e s in a specific c a s e a r e c o m p l e t e l y r e m o v e d , t h e n x = 0.6 is t h e o r d i n a t e c o r r e s p o n d i n g t o t h e s e t t l i n g velocity V = Hjt. s
0
s
F o r p a r t i c l e s w i t h settling velocities b e t w e e n V a n d V +dV (where V < V ), t h e f r a c t i o n r e m o v e d is VJV . S i m i l a r l y , for p a r t i c l e s w i t h s e t t l i n g velocities b e t w e e n V + dVand V +2 dV, t h e f r a c t i o n r e m o v e d is (Vi+dV)/V . T h e o v e r a l l r e m o v a l o f s u s p e n d e d s o l i d s is 1
1
s
x
S
x
x
s
Overall removal = (1 - x ) + 0
J ' V i / P s ) dx
(3.25)
I n E q . (3.25) V is a v a r i a b l e (0 < V < V ) w i t h V = / ( * ) p o r t r a y e d b y t h e c u r v e in F i g . 3.12. T e r m (1 — x ) is t h e f r a c t i o n c o m p l e t e l y r e m o v e d , c o r r e s p o n d i n g t o p a r t i c l e s w i t h velocities ^ V . T h e s e c o n d t e r m in E q . (3.25), i.e., x
x
s
x
0
s
ο
(KIVs)dx=i/Vs\
Jo
Kdx
90
3.
Pretreatment and Primary Treatment
w h i c h is t h e f r a c t i o n o f r e m o v a l c o r r e s p o n d i n g t o p a r t i c l e s w i t h velocities less t h a n V (calculated by graphical integration as indicated by the hatched a r e a s
i n F i g . 3.12). T h e differential a r e a of w i d t h dx, i n d i c a t e d in F i g . 3.12, c o r r e s p o n d s t o p a r t i c l e s w i t h s e t t l i n g velocities b e t w e e n V a n d t
V -\-dV. 1
C o m b i n i n g E q s . (3.18) a n d (3.25) t h e final e x p r e s s i o n for t h e o v e r a l l r e m o v a l is o b t a i n e d [ E q . ( 3 . 2 6 ) ] .
J
xo
fxo
m
(Vi/Vs) dx = (1 - jc ) + (l/Vs) 0
0
= (l-*o) +
Jo
J \h/H)dx X
Vi dx
(3.26)
A f t e r t h e s e c o n s i d e r a t i o n s r e t u r n t o d i s c u s s i o n o f S t e p 4. Step
4(a).
F r o m F i g . 3.11 f o r a d e p t h o f 6 ft r e a d t h e v a l u e s t ( m i n )
c o r r e s p o n d i n g t o 5, 10, 2 0 , 30, 4 0 , 50, a n d 6 0 % r e m o v a l , a n d c a l c u l a t e t h e c o r r e s p o n d i n g s e t t l i n g velocities V
s
(ft/hr). T h e s e v a l u e s a r e t a b u l a t e d in
T a b l e 3.6. T A B L E 3.6 Settling Velocities (H = 6 ft) Constant
% removal
t (min) i / = 6 ft
Settling velocity (ft/hr) V = Hjt = 6/(//60) = 3 6 0 / /
5 10 20 30 40 50 60
3.7 6.5 14.5 25.0 39.0 56.5 77.5
97.2 55.2 24.8 14.4 9.2 6.35 4.64
s
Step 4(b). C a l c u l a t e % r e m o v a l o f S S . C a l c u l a t i o n s f o r % r e m o v a l o f S S a n d overflow r a t e for a 2 5 - m i n s e t t l i n g t i m e ( f o u r t h e n t r i e s i n T a b l e 3.6) a r e i l l u s t r a t e d n e x t . S i m i l a r c a l c u l a t i o n s a r e a l s o p e r f o r m e d for t h e o t h e r s e t t l i n g t i m e s listed in t h e s e c o n d c o l u m n o f T a b l e 3.6. F o r t = 2 5 m i n f o r t h e s e t t l i n g d e p t h Η = 6 ft, 3 0 % o f t h e s u s p e n d e d solids a r e c o m p l e t e l y r e m o v e d . C o n s i d e r n e x t t h e p a r t i c l e s i n e a c h a d d i t i o n a l 1 0 % r a n g e . S t a r t w i t h t h o s e in t h e r a n g e 3 0 - 4 0 % r e m o v a l i n F i g . 3 . 1 1 . P a r t i c l e s i n t h i s r a n g e a r e r e m o v e d in t h e p r o p o r t i o n VJV o r in t h e p r o p o r t i o n o f a v e r a g e settled d e p t h ( A ) t o t h e t o t a l settling d e p t h (H). T h e a v e r a g e settled d e p t h (h ) is e s t i m a t e d b y d r a w i n g ( b y i n t e r p o l a t i o n ) a c u r v e c o r r e s p o n d i n g t o 3 5 % c o n s t a n t r e m o v a l in F i g . 3 . 1 1 , a n d r e a d i n g f r o m it t h e d e p t h h c o r r e s p o n d i n g t o t = 2 5 m i n . T h e r e f o r e f o r t h i s first i n t e r v a l , t h e % solids r e m o v a l is (hJH) χ 10 = (4.2/6) χ 10 = 7 . 0 % . S
t
t
l
3.
Sedimentation
91
In a similar m a n n e r for succeeding 10% intervals, t h e curves for c o n s t a n t % r e m o v a l o f 4 5 , 5 5 , 6 5 , a n d 7 5 % a r e d r a w n a n d t h e a v e r a g e settled d e p t h s o f 2.4, 1.4, 0.84, a n d 0.28 ft a r e r e a d f o r t = 2 5 m i n . T h e c a l c u l a t i o n s f o r t = 2 5 min are indicated below. Settling velocity: V = Hjt = 6.0/(25/60) = 14.4ft/hr Percent solids removal (for t — 25 min) 100% removal @ 30% 30.00% 1st interval (35%): (4.2/6.0) χ 10 = 7.00% 2nd interval (45%): (2.4/6.0) χ 10 = 4.00% 3rd interval (55%): (1.4/6.0) χ 10 = 2 . 3 3 % 4th interval (65%): (0.84/6.0) χ 10 = 1.40% 5th interval (75%): (0.28/6.0) χ 10 = 0.46% s
45.19% 45.2%
Total removed after 25 min
B e y o n d t h e fifth i n t e r v a l t h e % r e m o v a l s a r e n e g l i g i b l e , s o c a l c u l a t i o n s a r e s t o p p e d a t t h a t p o i n t . I n g e n e r a l , if 1 0 % i n t e r v a l s a r e selected, t h e t o t a l % r e m o v a l is given b y Total % removal = X
χ 10 + (h /H)
χ 10 + · · (3.27)
E q u a t i o n (3.27) is s i m p l y a n a p p r o x i m a t i o n o f E q . (3.26). (\—x )
corresponds
total
+ (hJH)
χ 10 + (h /H) 2
3
0
to X
tolal
a n d t h e i n t e g r a l Jg° (h/H) dx is r e p l a c e d b y a finite s u m m a t i o n o f
terms, £ (h JH) a
where A
a v e
χ Ax
is t h e a v e r a g e settled d e p t h for e a c h selected i n t e r v a l . T h e Ax's i n
t h i s e x a m p l e a r e selected a r b i t r a r i l y a s a 1 0 % r a n g e . T h e s m a l l e r t h e Ax selected, t h e c l o s e r t h e a p p r o x i m a t i o n b e t w e e n t h e finite s u m m a t i o n a n d t h e integral. S i m i l a r c a l c u l a t i o n s a r e p e r f o r m e d f o r t h e o t h e r r e s i d e n c e t i m e s listed i n t h e s e c o n d c o l u m n o f T a b l e 3.6. T h e final r e s u l t s a r e s u m m a r i z e d i n T a b l e 3.7. T A B L E 3.7 S S (%) Removed vs. Detention Time (/) t (min) 3.7 6.5 14.5 25.0 39.0 56.5 577.5
(2) % SS r e m o v a l 13.4 20.1 33.9 45.2 55.0 64.3 71.1
92
Pretreatment and Primary Treatment
3.
F r o m T a b l e 3.7 a g r a p h of % SS r e m o v e d vs. d e t e n t i o n t i m e is p r e p a r e d ( F i g . 3.13). Step 4(c). P r e p a r e a p l o t of % SS r e m o v e d v s . overflow r a t e . C a l c u l a t i o n s n e e d e d t o p r e p a r e t h i s p l o t a r e p r e s e n t e d in T a b l e 3.8. 80 70 ro
60
O) .Ω Ο
50
θ
C
Ε 40
oval
Ο ,Ο,
30
ε
α>
20
CO CO
J
δ*
10
0
Fig. 3.13.
/\f
τ- H.5 mir
20 10 30 40 50 60 70 Detention time (min) [Column®,table 3.7]
Suspended
solids
removal
(% SS)
vs. detention
80
time.
T A B L E 3.8 S S {%) Removed vs. Overflow Rate (3) Overflow rate
t (min)
(2) Settling velocity, V (ft/hr) (Table 3.6)
3.7 6.5 14.5 25.0 39.0 56.5 77.5
97.2 55.2 24.8 14.4 9.2 6.35 4.64
4
F
V
s
h
2
24 hr
_, Γ ft s
gal (day)(ft )
r
X
day
*
7.48 gal X
ft
3
x 24 χ 7.48 = 179.5 V
s
17,450 9,908 4,452 2,585 1,651 1,140 833
{4) % SS removal [column (2) o f Table 3.7]
13.4 20.1 33.9 45.2 55.0 64.3 71.1
3.
93
Sedimentation
90
ο
ile 3.8]
80
-Ο O
"-60
©
ι \V
| 50 ο
8
Ε £ 30
0
2000 6000 10000 14000 Overflow rate, gal/(dayKft ) [Column @ , t a b l e 3.8] 2
Fig. 3.14. Suspended
solids removal
(% SS)
vs. overt low
rate.
t (min) Fig. 3.15. Suspended solids removal initial SS concentrations.
(% SS)
vs. detention
time for
different
94
3.
Pretreatment and Primary Treatment
T h e p l o t of % SS r e m o v e d vs. overflow r a t e is p r e s e n t e d in F i g . 3.14. All c a l c u l a t i o n s a r e p e r f o r m e d f o r a n initial s u s p e n d e d solids c o n c e n t r a t i o n S S
0
o f 4 3 0 m g / l i t e r . If s i m i l a r c a l c u l a t i o n s a r e p e r f o r m e d for o t h e r v a l u e s o f t h e s e c o n c e n t r a t i o n s (C
C , C , . . . ) , t h e d a t a p l o t t e d in F i g s . 3.13 a n d 3.14 yield
l9
2
3
families of c u r v e s , a s i n d i c a t e d in F i g s . 3.15 a n d 3.16.
remo>
σ
en
c,
«^1 3
Overflow rate;gal/(day)(ft ) 2
Fig. initial
Fig.
3.16. SS
Suspended
3.17. Percentage
velocity.
solids
removal
(% SS)
vs. overflow
rate for
different
concentrations.
of particles
with less than stated
velocity
vs.
settling
3.
95
Sedimentation
Step 4(d).
P r e p a r e a p l o t o f p e r c e n t a g e o f p a r t i c l e s w i t h less t h a n s t a t e d
velocity (percentage n o t r e m o v e d ) vs. settling velocity (ft/hr). Calculations n e e d e d t o p r e p a r e F i g . 3.17 a r e p r e s e n t e d in T a b l e 3.9. F i g u r e 3.17 is n o t r e q u i r e d for t h e d e s i g n c a l c u l a t i o n s ; it is s h o w n b e c a u s e a t y p i c a l g r a p h o f t h i s t y p e w a s utilized in d e v e l o p i n g E q . (3.26). T h e p l o t is p r e s e n t e d in F i g . 3.17. TABLE 3.9 Percentage of Particles with Less Than Stated Velocity vs. Settling Velocity (3)
t (min)
ω
(2) Vs (ft/hr) (Table 3.6)
3.7 6.5 14.5 25.0 39.0 56.5 77.5
97.2 55.2 24.8 14.4 9.2 6.35 4.64
%ss removal (Table 3.7)
(4) Percentage not r e m o v e d : 1 0 0 - ( % SS removal)
13.4 20.1 33.9 45.2 55.0 64.3 71.1
86.6 79.9 66.1 54.8 45.0 35.7 28.9
3.5.1. D e s i g n Calculations f r o m Laboratory Data F o r p u r p o s e s o f s c a l e - u p , t h e fact t h a t t h e efficiency o f t h e p r o c e s s in a n a c t u a l s e t t l i n g t a n k is r e d u c e d o w i n g t o t h e effect o f p a r a m e t e r s s u c h a s t u r b u l e n c e , s h o r t c i r c u i t i n g , a n d i n t e r f e r e n c e o f t h e inlet a n d o u t l e t m u s t b e t a k e n i n t o a c c o u n t . T h e n e t effect o f t h e s e f a c t o r s r e s u l t s in a d e c r e a s e o f t h e overflow r a t e a n d a n i n c r e a s e i n t h e d e t e n t i o n t i m e o v e r v a l u e s d e r i v e d f r o m t h e l a b o r a t o r y a n a l y s i s . F o r d e s i g n p u r p o s e s , it is c u s t o m a r y t o d i v i d e t h e overflow r a t e o b t a i n e d f r o m t h e l a b o r a t o r y a n a l y s i s b y a f a c t o r r a n g i n g b e t w e e n 1.25 a n d 1.75, a n d t o m u l t i p l y t h e d e t e n t i o n t i m e b y a f a c t o r in t h e s a m e r a n g e [ 3 ] . T a b l e 3.10 p r e s e n t s s o m e c o m m o n l y u s e d d e s i g n v a l u e s . T A B L E 3.10 Design Values (Primary Clarifiers) D e p t h : 7 - 1 2 ft Detention time: 1-2 hr Flow-through velocity, V= 1-5 ft/min Overflow rate: 9 0 0 - 1 2 0 0 gal/(day)(ft ) Efficiencies SS removal: 4 0 - 6 0 % B O D removal: 3 0 - 5 0 % 2
96
3.
Pretreatment and Primary Treatment
D e s i g n p r o c e d u r e o f a p r i m a r y clarifier is i l l u s t r a t e d b y E x a m p l e 3.4. E x a m p l e 3.4 I t is d e t e r m i n e d b y field o b s e r v a t i o n t h a t a r a w w a s t e w a t e r h a s a n a v e r a g e o f 4 3 0 m g / l i t e r s u s p e n d e d s o l i d s a t a flow o f 1.0 M g a l / d a y . D a t a s h o w n in T a b l e 3.3 a r e o b t a i n e d f r o m l a b o r a t o r y s e t t l i n g t e s t s . 1. D e s i g n a s e t t l i n g t a n k o f c i r c u l a r c r o s s s e c t i o n , i.e., c a l c u l a t e its d i a m e t e r a n d effective d e p t h , t o r e m o v e 5 0 % o f t h e s u s p e n d e d s o l i d s a t t h e
flow
r a t e o f 1.0 M g a l / d a y . 2. F o r t h e t a n k d e s i g n e d in (1), w h a t is t h e r e m o v a l if flow is d o u b l e d t o 2.0 M g a l / d a y ? 3. F o r t h e flow o f 1.0 M g a l / d a y , c a l c u l a t e t h e d a i l y a c c u m u l a t i o n o f s l u d g e in l b / d a y a n d t h e a v e r a g e p u m p i n g r a t e in g a l / m i n . S l u d g e c o n c e n t r a t i o n is e s t i m a t e d a s 1.5% solids f r o m tests m a d e w i t h s a m p l e s w i t h d r a w n f r o m T a p 4 of t h e l a b o r a t o r y settling c o l u m n ( F i g . 3.9). A p l o t o f % s o l i d s for t h e c o m p a c t e d s l u d g e v s . settling t i m e is c o n s t r u c t e d f r o m d a t a o b t a i n e d f r o m s a m p l e s w i t h d r a w n f r o m T a p 4 ( F i g . 3.9). A t y p i c a l p l o t o f t h i s t y p e is s h o w n in F i g . 3.18.
Settling time (min) Fig. 3.18.
Typical
plot
of % so/ids
in the sludge
vs. detention
time.
S O L U T I O N : Part 1 Step 1. D e t e r m i n e t h e m a t e r i a l b a l a n c e for SS (see F i g . 3.19). Influent: 430 mg/liter R e m o v a l : (0.50)(430) = 215 mg/liter Effluent: 430 - 215 = 215 mg/liter Step 2. D e t e r m i n e t h e overflow r a t e . F r o m F i g . 3.14 r e a d o v e r f l o w r a t e c o r r e s p o n d i n g t o a 5 0 % r e m o v a l , 2 0 0 0 g a l / ( d a y ) ( f t ) . U s i n g a 1.75 s c a l e - u p 2
f a c t o r , t a k e a d e s i g n overflow r a t e o f 2 0 0 0 / 1 . 7 5 = 1143 g a l / ( d a y ) ( f t ) . 2
3.
Sedimentation
97
Influent:
Effluent: S e t t l i n g tank
S S = 4 3 0 mg/liter
SS=2I5
mg/liter
j R e m o v a l of S S = 2 I 5 m g / l i t e r Fig.
Step
3.19.
Material
balance
for primary
clarifier
(Example
3.4).
3. D e t e r m i n e t h e d e t e n t i o n t i m e . F r o m F i g . 3.13 r e a d d e t e n t i o n
t i m e c o r r e s p o n d i n g t o a 5 0 % r e m o v a l v a l u e , t = 3 1 . 5 m i n . * U s i n g a 1.75 s c a l e - u p f a c t o r , t a k e t = (31.5)(1.75) = 55.1 m i n o r t = 5 5 . 1 / 6 0 = 0 . 9 2 h r . Step
4. C a l c u l a t e r e q u i r e d h o r i z o n t a l c r o s s s e c t i o n o f clarifier a n d its
d i a m e t e r . H o r i z o n t a l c r o s s s e c t i o n o f clarifier is 1 x 1 0 * gal/day
_
1143gal/(day)(ft ) 2
a n d d i a m e t e r is D = (4Λ/π)
1 / 2
= (874.9/0.785)
1/2
= 33.4 ft
Step 5. C a l c u l a t e effective d e p t h o f t h e clarifier. Η = volume/^ =
Qt/A
1 χ 1 0 gal/day χ ft /7.48 gal χ 0.92/24 day 6
H
874^fV
=
S O L U T I O N : Part 2
3
=
5
'
9 f t
R e m o v a l f o r a flow r a t e o f 2 M G D w i t h t h e s a m e
clarifier T h i s a m o u n t s t o d o u b l i n g t h e d e s i g n o v e r f l o w r a t e , i.e., N e w design overflow rate
(2)(1143) = 2286 gal/(day)(ft ) 2
F r o m F i g . 3.14 t h i s c o r r e s p o n d s t o a r e m o v a l o f 4 7 . 5 % o f t h e s u s p e n d e d solids. S O L U T I O N : P a r t 3 Daily accumulation of sludge a n d average p u m p i n g r a t e f o r flow o f 1.0 M G D Step 1. D e t e r m i n e t h e d a i l y a c c u m u l a t i o n o f s l u d g e . R e m o v a l of SS
215 mg/liter -> 215 χ 1 0 " lb SS/lb liquor 6
T h e r e f o r e , t h e d a i l y a c c u m u l a t i o n o f s l u d g e i n l b / d a y is 1 χ 1 0 gal liquor/day x 8.34 lb liquor/gal liquor χ 215 χ 1 0 ~ lb SS/lb liquor 6
6
= 1793 lb SS/day * From this value of the residence time (/ = 31.5 min), the % solids in the sludge is estimated as 1.5% from a curve of the type in Fig. 3.19.
98
3. Step
Pretreatment and Primary Treatment
2. C a l c u l a t e t h e a v e r a g e p u m p i n g r a t e . N o t i c e t h a t 1.5% s o l i d s
c o r r e s p o n d s t o 1.5 g S S / 1 0 0 g o f l i q u o r = 15 g S S / 1 0 0 0 g o f l i q u o r « 15 g S S / liter = 15,000 m g / l i t e r = p p m = 15,000 χ 1 0 " l b S S / l b l i q u o r = 15,000 χ 8.34 6
χ 10"
6
l b S S / g a l l i q u o r . S i n c e a c c u m u l a t i o n is 1793 l b S S / d a y , p u m p i n g r a t e
in g a l / d a y is 1793 lb SS/day 6
= 0.0143 χ 1 0 gal/day 6
(15,000 χ 8.34χ 1 0 ~ ) lb SS/gal liquor or
(0.0143 χ 10 )/(24 χ 60) = 9.93 gal/min 6
Since p u m p i n g r a t e is l o w , i n t e r m i t t e n t p u m p i n g is u s e d .
3.6. Z O N E S E T T L I N G Z o n e settling o c c u r s i n clarifiers o f a c t i v a t e d o r c h e m i c a l l y c o a g u l a t e d s l u d g e w h e n t h e c o n c e n t r a t i o n e x c e e d s 500 m g / l i t e r . T h e s l u d g e
blanket
e x h i b i t s several d i s t i n c t z o n e s . E a c h z o n e is c h a r a c t e r i z e d b y a specific s l u d g e c o n c e n t r a t i o n a n d s e t t l i n g velocity. C o n s i d e r w h a t h a p p e n s w h e n a s u s p e n s i o n w h i c h initially h a s a u n i f o r m s l u d g e c o n c e n t r a t i o n C
0
( m g / l i t e r ) is p l a c e d i n a
settling c y l i n d e r ( F i g . 3.20). S l u d g e b e g i n s t o settle o u t a n d a n i n t e r f a c e ( i n t e r f a c e 1) is e s t a b l i s h e d b e t w e e n t h e s u r f a c e o f t h e b l a n k e t o f s e t t l i n g s l u d g e a n d t h e clarified l i q u i d a b o v e . T h e z o n e b e l o w t h e clarified l i q u i d is c a l l e d t h e i n t e r f a c i a l z o n e . C o n c e n t r a t i o n o f t h e s l u d g e in t h i s z o n e is u n i f o r m , a n d it settles a s a b l a n k e t w i t h a c o n s t a n t v e l o c i t y (V ).
Simultaneously with formation of interface 1
s
(c)
(b)
(d)
Clarified Iwater zone Interface I4ftfer-focial -
Clarified water zone
zaoe_.
Clarified water zone Coalescence |of interfaces! I and 2
TfonsEQon -—zone
[Interface 2-fy~~
—,
l^mpactEn t
t >t>0
=0
Uniform sludge concentration C
2
t =t
2
Beginning of compaction
n
Clarification process Fig.
End of compaction
Thickening process 3.20. Zone
settling.
3.
99
Sedimentation
a n d interfacial zone, c o m p a c t i o n of s u s p e n d e d solids starts a t the b o t t o m of t h e c y l i n d e r ( c o m p a c t i o n z o n e ) . I n t h i s z o n e c o n c e n t r a t i o n o f S S is a l s o u n i f o r m , a n d t h e i n t e r f a c e b o r d e r i n g t h i s z o n e ( i n t e r f a c e 2) rises i n t h e c y l i n d e r with a c o n s t a n t velocity
(V).
B e t w e e n t h e i n t e r f a c i a l a n d c o m p a c t i o n z o n e s t h e r e is a t r a n s i t i o n z o n e . T h e r e , t h e velocity of settling solids decreases o w i n g t o increase of viscosity a n d density of the suspension. In this s a m e zone, sludge changes gradually in c o n c e n t r a t i o n from t h a t of the interfacial z o n e t o t h a t of t h e c o m p a c t i o n zone. C o n s i d e r interfaces 1 a n d 2 in Fig. 3.20(b). Interface 1 m o v e s d o w n w a r d with a c o n s t a n t velocity V , whereas interface 2 m o v e s u p w a r d with a c o n s t a n t s
v e l o c i t y V. E v e n t u a l l y , i n t e r f a c i a l a n d c o m p a c t i o n z o n e s m e e t , a t w h i c h t i m e ( r ) the transition z o n e fades a w a y [ F i g . 3.20(c)]. A t this time, the settled 2
sludge exhibits a uniform concentration C , 2
w h i c h is t e r m e d t h e c r i t i c a l
concentration. C o m p a c t i o n starts a n d the sludge begins to thicken, eventually reaching an ultimate concentration C
u
[Fig. 3.20(d)]. Sedimentation velocity
a t t i m e t c o r r e s p o n d s t o a v a l u e V , w h i c h is g i v e n b y t h e s l o p e o f t h e t a n g e n t 2
2
t o t h e s e t t l i n g c u r v e a t C , a s i n d i c a t e d in F i g . 3.21 w h e r e V 2
2
<
V. s
P r o c e d u r e f o r d e s i g n i n g clarifiers o p e r a t i n g u n d e r c o n d i t i o n s o f
zone
settling: 1. C a l c u l a t e t h e m i n i m u m s u r f a c e a r e a r e q u i r e d t o a l l o w for c l a r i f i c a t i o n of sludge. 2. C a l c u l a t e t h e m i n i m u m s u r f a c e a r e a r e q u i r e d t o p r o v i d e f o r t h i c k e n i n g of s l u d g e t o t h e d e s i r e d u n d e r f l o w c o n c e n t r a t i o n . 3. T a k e t h e l a r g e r o f t h e s e t w o a r e a s a s t h e d e s i g n a r e a f o r t h e clarifier.
t,
Β
t
2
t,settling time (min) Fig.
3.21.
Sludge
settling
curve.
3.
100
Pretreatment and Primary Treatment
3.6.1. Laboratory M e a s u r e m e n t s T o o b t a i n t h e p a r a m e t e r s n e c e s s a r y for d e s i g n o f t h e clarifier, a s e t t l i n g t e s t f o r t h e s l u d g e is p e r f o r m e d in t h e l a b o r a t o r y u s i n g a 1000-ml g r a d u a t e d c y l i n d e r (a s t a n d a r d g r a d u a t e c y l i n d e r h a s a h e i g h t o f 1.12 ft). T h e c y l i n d e r is filled w i t h t h e s l u r r y t o b e s t u d i e d . A t t h e b e g i n n i n g o f t h e e x p e r i m e n t (/ = 0 ) , s l u r r y c o n c e n t r a t i o n is u n i f o r m t h r o u g h o u t t h e c y l i n d e r . H e i g h t o f i n t e r f a c e 1 is r e c o r d e d a t selected t i m e i n t e r v a l s . T h i s yields t h e t y p e o f s e t t l i n g c u r v e s h o w n in F i g . 3 . 2 1 . I t is i m p o r t a n t t o stir t h e s u s p e n s i o n a t a r a t e of a b o u t 5 r p h . T h i s s t i r r i n g s i m u l a t e s t h e a c t i o n of t h e m e c h a n i c a l r a k e s utilized i n s l u d g e r e m o v a l a n d p r e v e n t s s t r a t i f i c a t i o n of t h e s l u d g e . F i g u r e 3.21 s h o w s t h a t f r o m t h e s t a r t o f t h e e x p e r i m e n t u p t o a t i m e
t
l 9
i n t e r f a c e 1 falls w i t h a c o n s t a n t velocity V given b y t h e s l o p e of t h e t a n g e n t , s
w h i c h essentially c o i n c i d e s w i t h t h e s e t t l i n g c u r v e f r o m t = 0 t o t = t t = t
x
After
v
t h i s velocity d e c r e a s e s a p p r e c i a b l y . A t t i m e / = r , t h e velocity is V 2
given by the slope of the tangent at C . A t t 2
2
2
compaction starts a n d the
v e l o c i t y is f u r t h e r r e d u c e d u n t i l it b e c o m e s e s s e n t i a l l y z e r o , t h e t a n g e n t b e i n g parallel t o the abscissa. Z o n e s e t t l i n g v e l o c i t y ( Z S V ) c o r r e s p o n d s t o t h e velocity a t w h i c h
the
s u s p e n s i o n settles p r i o r t o r e a c h i n g t h e critical c o n c e n t r a t i o n C , a n d is g i v e n 2
b y t h e s l o p e o f t h e t a n g e n t A B in F i g . 3.21 [ E q . ( 3 . 2 8 ) ] . V
s
= O A / O B = H jt
= 1.12ft/min
Q
(3.28)
3.6.2. Determination of M i n i m u m S u r f a c e Area Required to A l l o w Clarification of the Sludge M i n i m u m surface area A
c
V
s
r e q u i r e d for clarification d e p e n d s o n v e l o c i t y
a t w h i c h t h e s u s p e n s i o n settles b e f o r e r e a c h i n g t h e interfacial c r i t i c a l c o n
centration C . 2
U n d e r c o n t i n u o u s flow c o n d i t i o n s , velocity of t h e l i q u o r o v e r
t h e overflow w e i r c a n n o t e x c e e d V if c l a r i f i c a t i o n is t o t a k e p l a c e . T h i s f o l l o w s s
d i r e c t l y f r o m t h e b a s i c c o n c e p t o f t h e i d e a l s e d i m e n t a t i o n t a n k [ S e c t i o n 3.4, E q s . (3.17) a n d ( 3 . 1 9 ) ] . Therefore, A
= QIV
c
(3.29)
S
w h e r e Q is t h e flow r a t e ( f t / m i n ) ; V t h e settling velocity ( f t / m i n ) ; a n d A t h e m i n i m u m surface a r e a r e q u i r e d for clarification ( f t ) . T h e v a l u e o f t h e z o n e s e t t l i n g v e l o c i t y V is d e t e r m i n e d f r o m F i g . 3.21 a n d E q . (3.28). V a l u e o f t is r e a d d i r e c t l y f r o m t h e a b s c i s s a o f F i g . 3.21 ( p o i n t B). V is t h e n c a l c u l a t e d f r o m E q . (3.28) a n d A o b t a i n e d f r o m E q . (3.29). 3
s
c
2
s
s
c
3.
101
Sedimentation
3.6.3. Determination of M i n i m u m S u r f a c e Area Required for Thickening of the S l u d g e C o n s i d e r settling of a sludge u n d e r z o n e settling c o n d i t i o n s in a cylinder ( F i g . 3.20). A t s t a r t of t h e e x p e r i m e n t , let C
0
be the uniform sludge concentra
t i o n t h r o u g h o u t t h e c y l i n d e r . T o t a l w e i g h t of s o l i d s in t h e c y l i n d e r is w h e r e A is t h e c r o s s - s e c t i o n a l a r e a o f t h e c y l i n d e r . L e t t
2
C AH 0
09
be the time counted
from the beginning of the experiment, w h e n interfacial a n d c o m p a c t i o n zones merge together [Fig. 3.20(c)]. Let C
b e t h e critical c o n c e n t r a t i o n w h i c h is
2
u n i f o r m t h r o u g h o u t this sludge z o n e f o r m e d by the merging of interfacial and compaction zones. A graphical procedure has been p r o p o s e d [ 7 ] for d e t e r m i n i n g t . 2
Con
sider t h e s e t t l i n g c u r v e in F i g . 3.22. D r a w t w o t a n g e n t s ( A B a n d C D ) t o t h e t w o b r a n c h e s of the curve. T a n g e n t A B c o r r e s p o n d s to the c o n s t a n t velocity A|
1
Settling time (t) Fig. 3.22.
Determination
of
t. 2
o f s e t t l i n g V for t h e i n t e r f a c i a l z o n e ( z o n e s e t t l i n g v e l o c i t y , Z S V ) , a n d t a n g e n t s
C D c o r r e s p o n d s t o the settling velocity for the c o m p a c t e d sludge. P o i n t
C
2
( c o r r e s p o n d i n g t o t i m e t ) is o b t a i n e d b y b i s e c t i n g t h e a n g l e f o r m e d b y t a n g e n t s 2
A B a n d C D . T h e a b s c i s s a o f t h e p o i n t w h e r e t h e b i s e c t i n g line c u t s t h e s e t t l i n g curve c o r r e s p o n d s t o the desired value of t . 2
Consider n o w the thickening process. 1. S t a r t o f t h i c k e n i n g [ F i g . 3 . 2 0 ( c ) ] . T i m e , t \ c o n c e n t r a t i o n o f S S i n 2
s l u d g e z o n e , C ; h e i g h t of s l u d g e z o n e , 2
H. 2
2. E n d of t h i c k e n i n g [ F i g . 3 . 2 0 ( d ) ] . T h e c o m p a c t e d s l u d g e desired underflow concentration C . u
n a t e d a s t . H e i g h t of t h e s l u d g e z o n e is u
reaches
T h e t i m e a t w h i c h t h i s o c c u r s is d e s i g H. u
Consider separately the sludge zone at the start a n d e n d of thickening ( F i g . 3.23). S i n c e t h e t o t a l m a s s of s l u d g e in t h e c y l i n d e r is c o n s t a n t , t h e
102
3.
Pretreatment and Primary Treatment
{—Volume i of water squeezed out H J
at t = t
u
at, .t = ,t„
2
M
Fig.
3.23.
u
Thickening
process.
f o l l o w i n g m a t e r i a l b a l a n c e e q u a t i o n [ E q . ( 3 . 3 0 ) ] is w r i t t e n , n e g l e c t i n g t h e a m o u n t o f s u s p e n d e d solids in t h e clarified w a t e r z o n e . C
A H
0
=
0
C
A H
2
=
2
C
A H
U
(3.30)
U
or CQHQ
=
C
2
H
—
2
C
U
H
(3.31)
U
C o n s i d e r F i g . 3.23. T h e v o l u m e o f w a t e r w h i c h is s q u e e z e d o u t a n d d i s c h a r g e d o v e r t h e o v e r f l o w w e i r is c a l c u l a t e d f r o m E q . (3.32). V = A ( H
2
- H
(3.32)
)
U
T h e t i m e i n t e r v a l r e q u i r e d t o d i s c h a r g e t h i s v o l u m e o f w a t e r is t — t . u
2
A v e r a g e r a t e o f flow Q' ( f t / m i n ) o v e r t h e w e i r is [ E q . 3.33)] 3
Q' = Vj(t -t ) u
=
2
A(H -H )l(t -t ) 2
u
u
(3.33)
2
Solving for t — t , u
2
/.-fa
=
A(H -H )IQ 2
(3.34)
f
u
C o n s i d e r n o w t h e settling c u r v e a n d d e t e r m i n e g r a p h i c a l l y t h e s e t t l i n g velocity V at time t 2
2
( t a n g e n t a t p o i n t C ) . T h i s is s h o w n in F i g . 3.24. 2
L e t H± b e t h e i n t e r c e p t o f t h i s t a n g e n t . S e t t l i n g v e l o c i t y a t t
2
is s h o w n in
E q . (3.35). V
2
ι S
s
= t a n a = {H -H )lt Y
2
(3.35)
2
r
H,-H
2
J..
r
'2 Settling time (min) Fig. 3.24. Determination
of velocity
V. 2
3.
Sedimentation
103
U n d e r c o n t i n u o u s flow c o n d i t i o n s t h e velocity o f t h e l i q u o r o v e r t h e w e i r c a n n o t b e g r e a t e r t h a n V if t h i c k e n i n g is t o t a k e p l a c e . T h e r e f o r e , t h e 2
flow
r a t e Q' a t t i m e t w h e n t h i c k e n i n g s t a r t s is 2
Q = AV
= A\_(H -H )lt \
2
1
2
= ft /min 3
2
(3.36)
S u b s t i t u t i o n o f Q' given b y E q . (3.36) in E q . (3.34) yields after s i m p l i f i c a t i o n a n d r e a r r a n g e m e n t E q . (3.37). (H - H )l(t 2
u
-1 )
u
= (H, - H )/t
2
2
(3.37)
2
T h i s e q u a t i o n is t h e b a s i s for t h e g r a p h i c a l p r o c e d u r e for d e t e r m i n a t i o n o f t i l l u s t r a t e d b y F i g . 3.25.
u
Settling time (min) Fig. 3.25.
Determination
of
t. u
T o s u m m a r i z e t h e s t e p s in t h e g r a p h i c a l p r o c e d u r e for d e t e r m i n a t i o n o f t : 1. D r a w t h e t a n g e n t t o t h e settling c u r v e a t C . 2. F r o m m a t e r i a l b a l a n c e [ E q . ( 3 . 3 1 ) ] u
2
H
u
= HoColC
(3.38)
u
C a l c u l a t e H f r o m E q . (3.38). u
3. M a r k d i s t a n c e H o n t h e o r d i n a t e axis o f F i g . 3.25. D r a w t h e h o r i z o n t a l d o t t e d line f r o m H u n t i l its i n t e r s e c t i o n w i t h t h e t a n g e n t t o C . T h e a b s c i s s a o f t h i s i n t e r s e c t i o n is t h e v a l u e o f t . T h i s c a n b e s e e n b y i n s p e c t i o n of E q . (3.37) a n d c o n s i d e r a t i o n o f t h e t w o c r o s s - h a t c h e d s i m i l a r t r i a n g l e s in F i g . 3.25. u
u
2
u
M i n i m u m s u r f a c e a r e a r e q u i r e d for t h i c k e n i n g (A ) is o b t a i n e d f r o m t h e following considerations. Average rate at which the layer of c o n c e n t r a t i o n C f o r m s (in l b / m i n ) is C H A /t (3.39) t
u
u
u
t
u
104
3.
Since f r o m E q . (3.31) C H U
U
= CH 0
Pretreatment and Primary Treatment
t h e n E q . (3.39) is r e w r i t t e n a s
09
Co Ho A /t t
(3.40)
u
U n d e r c o n d i t i o n s o f c o n t i n u o u s flow a n d s t e a d y s t a t e , t h e r a t e a t w h i c h t h e l a y e r o f c o n c e n t r a t i o n C is f o r m e d m u s t e q u a l t h a t a t w h i c h s u s p e n d e d solids u
e n t e r in t h e influent (QC ). 0
Therefore QCo = C H A /t 0
0
t
(3.41)
u
Solving for A
t
A
t
= QtJHo
(3.42)
w h e r e i / = 1.12 ft. T h e d e s i g n p r o c e d u r e f o r clarifiers u n d e r z o n e s e t t l i n g c o n d i t i o n s is illus 0
t r a t e d b y E x a m p l e 3.5.
Example 3.5 D e s i g n a s e c o n d a r y settling t a n k t o p r o d u c e a n u n d e r f l o w c o n c e n t r a t i o n o f 10,900 m g / l i t e r f r o m a n influent c o n t a i n i n g 2 5 1 0 m g / l i t e r o f s u s p e n d e d solids. W a s t e w a t e r flow is 1.2 M G D . C a l c u l a t e t h e clarifier a r e a r e q u i r e d . T h e d a t a t a b u l a t e d b e l o w a r e o b t a i n e d in a l a b o r a t o r y test o f t h e s l u r r y .
/ (min)
Interface height Η (ml)
0 1 2 3 5 8 12 16 20 25
1000 850 725 600 450 350 280 240 220 210
SOLUTION Step 7. S e t t l i n g c u r v e is p l o t t e d f r o m a v a i l a b l e d a t a ( F i g . 3.26). Step 2. M i n i m u m surface a r e a r e q u i r e d f o r clarification (A ) is d e t e r m i n e d . c
1. D r a w t a n g e n t A B . R e a d t = 7.5 m i n . T h e n V = H /t s
0.149 f t / m i n . 2. A r e a r e q u i r e d f o r c l a r i f i c a t i o n :
0
= 1.12/7.5 =
3.
105
Sedimentation
ιοοο
\
\\
900 800 _ Ε CP
ω
700
\\
\
600
/
500
S>
400
ω
300
•o
\
\
/
v\
)n \\
200
/ /
'2
Η
100 t =f. b nir 5
" V
Τ
!
\\
i h
ι
t Β
2
ΙΟ
nr im
tu 15
25
20
Settling time (min) Fig.
3.26.
Graph
for Example
3.5.
Step 3. M i n i m u m surface a r e a r e q u i r e d f o r t h i c k e n i n g (A ) is c a l c u l a t e d . t
1. read t = 2. calculate 2
D e t e r m i n e t b y t h e g r a p h i c a l p r o c e d u r e s t u d i e d . F r o m F i g . 3.26 8.0 m i n . D e t e r m i n e t i m e t . D r a w t h e t a n g e n t t o t h e settling c u r v e a t C a n d H f r o m E q . (3.28). 2
u
2
u
H = (1000x2510)/10,900 = 230 ml u
3. D e t e r m i n e t b y t h e g r a p h i c a l p r o c e d u r e d e s c r i b e d . F r o m (3.26) r e a d t = 13 m i n . 4. C a l c u l a t e A f r o m E q . (3.42).
Fig.
u
u
t
1,200,000 gal/day χ ft /7.48 gal χ day/1440 min χ 13 min 3
A
t
=
1.12 ft
= 1293 ft
2
Step 4. T a k e A = A = 1293 f t . R e q u i r e d d i a m e t e r for a s e d i m e n t a t i o n t a n k of c i r c u l a r c r o s s s e c t i o n is 2
t
d = (4Α/π)
ί/2
= (1293/0.785)
1/2
= 40.6 ft
3.7. T Y P E S O F C L A R I F I E R S Clarifiers a r e classified a c c o r d i n g t o g e o m e t r y o f t h e i r h o r i z o n t a l c r o s s section a s (1) r e c t a n g u l a r a n d (2) c i r c u l a r : (a) c e n t e r a n d (b) p e r i p h e r a l feed. S k e t c h e s of t y p i c a l clarifiers a r e s h o w n in F i g s . 3 . 2 7 ( a ) , ( b ) , a n d (c).
106
3.
Pretreatment and Primary Treatment
Rotary-hoe type scraping mechanism |^J>*-Influent —
T
Clear zone
Clear solution overflow
Baffle
Discharge of thickened —sludge
(a) Rectangular clarifier
^Rotating mechanism Feed piping
h-Baffle
/
Overflow weir
Blade Plow-type scraping mechanism
Discharge of thickened sludge
(b) Circular clarifier (center feed)
m Clear solution outlet
• R o t a t i n g mechanism
" " " " L l ^ A S i l L ^ ^ ^ C o l l e c t i n g channel Influent
Plow-type scraping mechanism
•Discharge of thickened sludge (c) Circular clarifier (peripheral feed) Fig. 3.27
Types of clarifiers
(a),
(b).
and
(c).
Flotation
4.
107
1. R e c t a n g u l a r clarifier [ F i g . 3 . 2 7 ( a ) ] I n t h e t y p e s h o w n in F i g . 3 . 2 7 ( a ) , s c r a p e d s l u d g e is m o v e d t o w a r d t h e inlet e n d o f t h e t a n k . S o m e o t h e r d e s i g n s m o v e s l u d g e t o w a r d t h e effluent end of the tank. S c r a p i n g m e c h a n i s m s h o w n is o f r o t a r y - h o e t y p e , c o n s i s t i n g o f a series o f short scrapers m o u n t e d on a n endless chain, which m a k e contact with the b o t t o m of t h e t a n k . I t m o v e s slowly a t s p e e d s o f a p p r o x i m a t e l y 1 f t / m i n . 2 a . C i r c u l a r clarifier w i t h c e n t e r feed [ F i g . 3 . 2 7 ( b ) ] F e e d is a t t h e c e n t e r a n d c l e a r s o l u t i o n o v e r f l o w s t o a c o l l e c t i n g c h a n n e l a t t h e p e r i p h e r y . T h e b o t t o m o f t h e clarifier h a s a m i n i m u m s l o p e o f 1 in./ft. S c r a p i n g m e c h a n i s m is o f p l o w t y p e t o o v e r c o m e i n e r t i a a n d p r e v e n t a d h e r e n c e o f s l u d g e t o t h e b o t t o m of t h e t a n k . 2 b . C i r c u l a r clarifier w i t h p e r i p h e r a l feed [ F i g . 3 . 2 7 ( c ) ] T h e feed is a t t h e p e r i p h e r y a n d t h e c l e a r s o l u t i o n overflows t o a c o l l e c t i n g c h a n n e l a t t h e c e n t e r . T h e o t h e r d e t a i l s a r e s i m i l a r t o t h o s e for t h e t y p e s h o w n in F i g . 3 . 2 7 ( b ) . T h e inlet s e c t i o n s h o u l d b e carefully d e s i g n e d for a u n i f o r m flow d i s t r i b u t i o n across the width a n d d e p t h of the t a n k . Similarly, the outlet section should be d e s i g n e d t o collect t h e effluent u n i f o r m l y . A g o o d d e s i g n of inlet a n d o u t l e t s e c t i o n s r e d u c e s p o s s i b i l i t i e s o f flow s h o r t c i r c u i t i n g , w h i c h l e a d t o p o o r p e r f o r m a n c e of t h e clarifier. P r o p e r p o s i t i o n i n g of weirs a n d baffles, a s i n d i c a t e d in F i g . 3.27, p r e v e n t s s h o r t c i r c u i t i n g .
4. Flotation 4.1.
INTRODUCTION
F l o t a t i o n is a p r o c e s s for s e p a r a t i n g l o w d e n s i t y solids o r l i q u i d p a r t i c l e s f r o m a l i q u i d p h a s e . S e p a r a t i o n is b r o u g h t a b o u t b y i n t r o d u c t i o n o f g a s ( u s u a l l y a i r ) b u b b l e s i n t o t h e l i q u i d p h a s e . T h e l i q u i d p h a s e is p r e s s u r i z e d t o a n o p e r a t i n g p r e s s u r e r a n g i n g f r o m 30 t o 6 0 p s i a ( 2 - 4 a t m ) in p r e s e n c e o f sufficient a i r t o p r o m o t e s a t u r a t i o n o f a i r in t h e w a t e r . T h e n , t h i s a i r - s a t u r a t e d liquid is d e p r e s s u r i z e d t o a t m o s p h e r i c p r e s s u r e b y p a s s a g e t h r o u g h a p r e s s u r e reducing valve. M i n u t e air bubbles are released from the solution because of d e p r e s s u r i z a t i o n . S u s p e n d e d s o l i d s o r l i q u i d p a r t i c l e s , e.g., oil, a r e floated b y t h e s e m i n u t e a i r b u b b l e s , c a u s i n g t h e m t o rise t o t h e s u r f a c e o f t h e t a n k . C o n c e n t r a t e d s u s p e n d e d solids a r e s k i m m e d off b y m e c h a n i c a l m e a n s f r o m t h e t a n k surface. Clarified l i q u o r is w i t h d r a w n n e a r t h e b o t t o m , a n d p a r t o f it m a y b e recycled [ F i g . 3 . 2 8 ( b ) ] . A flotation s y s t e m w i t h o u t recycle is s h o w n d i a g r a m m a t i c a l l y in F i g . 3 . 2 8 ( a ) . In t h e field o f w a s t e w a t e r t r e a t m e n t , flotation is u s e d for t h e f o l l o w i n g p u r p o s e s : (1) s e p a r a t i o n o f g r e a s e s , oils, fibers, a n d o t h e r l o w d e n s i t y s o l i d s f r o m w a s t e w a t e r s ; (2) t h i c k e n i n g o f t h e s l u d g e f r o m t h e a c t i v a t e d s l u d g e
108
3.
Air injection
Pretreatment and Primary Treatment
[Retention] tank
Or
Pressurizing pump Q.MGD Influent (wastewater)
Thickened sludge (negligible volume)
Pressure reducing valve (C|,mg/liter of dissolved air)
Q,MGD Effluent ( C , m g / l i t e r of dissolved air) 2
(a) Flotation system without recycle
Air injection Thickened sludge (negligible volume) Gross effluent (Q+R), MGD Net effluent Q, MGD
R; MGD ( C , mg/liter of dissolved air) 2
(b) Flotation system with recycle Fig. 3.28.
Flotation
systems
(a) and
(b).
p r o c e s s ; a n d (3) t h i c k e n i n g of flocculated c h e m i c a l s l u d g e s r e s u l t i n g f r o m c h e m i c a l c o a g u l a t i o n t r e a t m e n t . S u p e r i o r effluent q u a l i t y , i.e., effluent c o n t a i n i n g l o w e r p e r c e n t a g e o f s u s p e n d e d s o l i d s , a n d e c o n o m y in p o w e r a r e a c h i e v e d b y flotation s y s t e m s w i t h recycle. Basic c o m p o n e n t s of a
flotation
s y s t e m a r e (1) p r e s s u r i z i n g p u m p ; (2) a i r
i n j e c t i o n facilities; (3) r e t e n t i o n t a n k ( t o p r o v i d e a i r - l i q u i d c o n t a c t ) ; (4) p r e s s u r e - r e d u c i n g v a l v e ; a n d (5)
flotation
tank.
4.2. E V A L U A T I O N O F F L O T A T I O N V A R I A B L E S FOR P R O C E S S D E S I G N 4.2.1. Parameter A / S F o r d e s i g n of
flotation
systems, a fundamental
utilized is a d i m e n s i o n l e s s a i r t o s o l i d s r a t i o (A/S) ^ ^
parameter
commonly
defined b y E q . (3.43).
lb/day of air released by depressurization lb/day of solids in the influent
^
109
Flotation
4.
T h i s p a r a m e t e r is e s t i m a t e d f r o m s t u d i e s w i t h a l a b o r a t o r y - s c a l e
flotation
cell o f p i l o t - p l a n t d a t a . T h e v a l u e o f A is o b t a i n e d f r o m d e t e r m i n a t i o n s o f d i s s o l v e d a i r ( m g / l i t e r ) a t s a m p l i n g l o c a t i o n s i n d i c a t e d a s (1) a n d (2) i n F i g . 3 . 2 8 ( a ) a n d (b). T h u s [ E q . (3.44)] A = A, - A
(3.44)
2
w h e r e A is t h e l b / d a y o f a i r r e l e a s e d b y d e p r e s s u r i z a t i o n ; A
x
d i s s o l v e d a i r a t (1) [ F i g . 3 . 2 8 ( a ) a n d ( b ) ] ; a n d A
2
t h e l b / d a y of
the lb/day of dissolved air
a t (2) [ F i g . 3 . 2 8 ( a ) a n d ( b ) ] . For A
flotation
Mgal liquor _ lb air lb liquor ^ ^„ = Q ~ ^ — x Ci — — x 8.34 — ? = 8.34QC! day M l b liquor gal liquor L
x
s y s t e m s w i t h o u t recycle [ E q . ( 3 . 4 5 ) ] , * η
L
,„ . (lb air/day)
Λ
x
(3.45) S i m i l a r l y [ E q . (3.46)] A
2
= $34QC
(lb air/day)
2
(3.46)
Therefore A = A
1
- A
2
= 8.34G(Cj - C )
(lb/day of air released)
2
(3.47)
F o r flotation s y s t e m s w i t h recycle t h e c o r r e s p o n d i n g e q u a t i o n is A = A, - A
2
= S34R(C
- C)
1
(lb/day of air released)
2
(3.48)
If S is t h e c o n c e n t r a t i o n o f s u s p e n d e d s o l i d s ( m g / l i t e r ) in t h e influent, t h e v a l u e o f S [ d e n o m i n a t o r o f E q . ( 3 . 4 3 ) ] is t
Λ
S = Q
Mgal liquor 8.34 lb liquor lb SS * — x 7~r. x St —— = 8.34QS, day gal liquor M l b liquor o
β
A
, (lb SS/day)
/ l f
0
0
/
J
(3.49) S u b s t i t u t i o n o f E q s . (3.47) [ o r E q . ( 3 . 4 8 ) ] a n d (3.49) in E q . (3.43) l e a d s t o Flotation systems without recycle A/S=(C -C )/S l
2
(3.50)
l
Flotation systems with recycle AIS = {RIQKC -C )IS x
2
t
(3.51)
4.2.2. Correlation of Flotation Variables to Parameter A / S B y u s e o f a l a b o r a t o r y flotation cell o r p i l o t - p l a n t d a t a , it is p o s s i b l e t o c o r r e l a t e m g / l i t e r o f s u s p e n d e d s o l i d s c o n t a i n e d in l i q u i d effluent t o p a r a m e t e r A/S. A t y p i c a l c o r r e l a t i o n c u r v e f o r a w a s t e w a t e r h a s t h e s h a p e i n d i c a t e d in F i g . 3.29. * N o t i c e that C
x
m g o f air/liter of liquor = C
x
lb o f air/Mlb o f liquor.
3.
110
Pretreatment and Primary Treatment
0.06
I
!
0 Fig.
3.29.
Typical
ι
I
50 m g / l i t e r of S S in e f f l u e n t
correlation
of parameter
A/S
I00
vs. concentration
of SS
in
effluent.
For a given influent, the lb/day of suspended solids (term S in ratio A/S) is fixed. Ratio A/S increases by operating at higher air rates, which results in increase of air released (A). From Fig. 3.29 it follows that a higher quality effluent is obtained. Graphs like Fig. 3.29 permit selection of the A/S ratio for a required degree of effluent clarification. These curves indicate that increasing the A/S ratio beyond an optimum value does not result in substantial reduction in effluent suspended solids. Judicious selection of the A/S ratio involves an economical balance between equipment and maintenance costs and desired effluent quality. Typical range of A/S ratios for thickening of sludges in wastewater treatment is 0.0050.060. 4.2.3. Alternative Expressions for Parameter A / S [2] Consider Eq. (3.46) for A , where C is the solubility of air in water in mg/ liter. Frequently, the solubility of air is expressed in terms of c m of air/liter of water. It is assumed that conditions at (2) [Figs. 28(a) and (b)] are atmos pheric pressure and ambient temperature. Solubility of air in water in c m of air/liter of water [hence denoted as S J is presented in Table 3.11 for atmos pheric pressure at several temperatures. This concentration of dissolved air in c m of air/liter of water is converted to the value C (mg air/liter of water) [Eq. (3.52)]. 2
2
3
3
3
2
S cm air/liter water χ ρ mg air/cm air = C 3
0
3
α
2
(mg air/liter water) (3.52)
Flotation
4.
111 T A B L E 3.11 Solubility of Air in Water at Atmospheric Pressure at Several Temperatures [6] Temperature (°C)
S (cm /liter)
0 10 20 30
29.2 22.8 18.7 15.7
3
a
U t i l i z e for t h e d e n s i t y o f a i r p 2l m e a n v a l u e o f 1.2 m g / c m . ( T h i s c o r r e s p o n d s 3
a
t o the value at 1 a t m a n d 20°C.) T h e n [ E q . (3.53)] C
= \2S
2
C o n s i d e r n o w E q . (3.45), w h e r e C
is t h e s o l u b i l i t y o f a i r in m g / l i t e r . T h e
l
cm
3
(3.53)
a
o f a i r / l i t e r o f w a t e r a t t h e r e t e n t i o n t a n k [ p o i n t ( 1 ) ] is s h o w n i n E q . (3.54).
( c m air/liter water)
(3.54)
3
a
w h e r e φ is t h e r a t i o o f s o l u b i l i t y o f a i r in w a t e r a t t h e p r e s s u r e i n t h e r e t e n t i o n t a n k [ a t ( 1 ) ] t o t h e s o l u b i l i t y a t a t m o s p h e r i c p r e s s u r e [ a t (2) in F i g s . 2 8 ( a ) a n d (b)] [Eq. (3.55)].
(p>1.0)
2
(3.55)
I t is f o u n d e x p e r i m e n t a l l y t h a t for a specific d e s i g n of t h e r e t e n t i o n t a n k , a n d w i t h i n o r d i n a r y p r e s s u r e r a n g e s u t i l i z e d in
flotation
o p e r a t i o n s , r a t i o φ is
proportional to the pressure
(3.56)
w h e r e / is t h e p r o p o r t i o n a l i t y f a c t o r a n d Ρ t h e p r e s s u r e in r e t e n t i o n t a n k i n atmospheres. A t 2 0 ° C f o r p r e s s u r e r a n g e 3 0 - 6 0 p s i a u t i l i z e d in m o s t flotation s y s t e m s a n d f o r baffled r e t e n t i o n t a n k s , t h e v a l u e o f / i n E q . (3.56) is a p p r o x i m a t e l y 0 . 5 . Consequently, values of φ vary from 30 psia ( = 30/14.7 = 2.04 a t m ) = 0.5 χ 2.04 = 1.02 60 psia ( = 60/14.7 = 4.08 a t m ) = 0.5 χ 4.08 = 2.04 Therefore for the pressure r a n g e from 30 t o 60 psia a t 20°C, solubility of air in baffled r e t e n t i o n t a n k s v a r i e s f r o m 1.02 t o 2.04 t i m e s its s a t u r a t i o n v a l u e a t 2 0 ° C a n d 1 a t m . S i n c e f r o m E q . (3.55)
Ci =
φ€
2
(3.57)
112
3.
substitution of φ a n d C
2
Pretreatment and Primary Treatment
b y t h e i r v a l u e s g i v e n b y E q s . (3.56) a n d (3.53) yields d
= fP(\2S )
(3.58)
a
S u b s t i t u t i o n of v a l u e s o f C a n d C given, respectively, b y E q s . (3.58) a n d (3.53) in E q s . (3.50) a n d (3.51) l e a d s t o x
2
Flotation systems without recycle A/S
= \2S (fP-\)/S a
(3.59)
l
Flotation systems with recycle A/S
= (R/Q)\2S (fP-
(3.60)
a
4.3. D E S I G N P R O C E D U R E F O R F L O T A T I O N UNITS WITHOUT A N D WITH RECYCLE 4.3.1. Flotation S y s t e m s w i t h o u t Recycle F r o m E q . (3.59) it follows t h a t if a r a t i o A/S
is s e l e c t e d , / b e i n g fixed for a
selected t y p e o f r e t e n t i o n t a n k a n d S f r o m c h a r a c t e r i s t i c s of t h e influent, t h i s t
a m o u n t s t o specification of o p e r a t i n g p r e s s u r e P, w h i c h is c a l c u l a t e d f r o m E q . (3.59) a s P = (llf)[(A/S)S /\2S +\l l
(3.61)
a
D e s i g n o f flotation s y s t e m s w i t h o u t recycle i n v o l v e s c a l c u l a t i n g t h e r e q u i r e d o p e r a t i n g pressure [ E q . (3.61)] a n d d e t e r m i n i n g the cross-sectional area of the flotation u n i t . T h i s a r e a is c a l c u l a t e d f r o m a selected v a l u e of t h e o v e r f l o w r a t e , u s u a l l y a v a l u e b e t w e e n 2 a n d 4 g a l / ( m i n ) ( f t ) ( E x a m p l e 3.6). 2
Example 3.6 L a b o r a t o r y flotation tests for a given w a s t e w a t e r i n d i c a t e o p t i m u m air/ s o l i d s r a t i o (A/S) a s 0.04 l b a i r / l b solids. F l o w of w a s t e w a t e r is 1.0 M G D a n d it c o n t a i n s 2 5 0 m g / l i t e r o f s u s p e n d e d solids. L a b o r a t o r y flotation tests ( w i t h o u t recycle) i n d i c a t e for a r a t i o A/S = 0.04 a n o p t i m u m effluent c o n t a i n i n g 2 5 mg/liter of suspended solids. T a k e / = 0.50 for retention t a n k a n d a n o p e r a t i n g t e m p e r a t u r e o f 2 0 ° C . D e s i g n a flotation s y s t e m w i t h o u t recycle f o r t h i s service. SOLUTION Step 1. Select A/S = 0.04, a s s t a t e d . Step 2. C a l c u l a t e Ρ f r o m E q . (3.61). Ρ = (l/0.5)(0.04 χ 250/1.2 χ 1 8 . 7 + 1 ) = 2.9 a t m Step 3. Select a n overflow r a t e , OR = 3 gal/(min)(ft ) 2
4.
Flotation
113
Step 4. C a l c u l a t e r e q u i r e d s u r f a c e a r e a , Q = 1.0 M G D or Q = 1,000,000 gal/day χ day/24 hr χ hr/60 min = 695 gal/min Surface area (ft ) = Q/OR 2
= . ™\^ ™ = 3.0 gal/(min)(ft ) ,m
2
3
2
*
ft
2
4.3.2.
Flotation Systems w i t h Recycle
I t f o l l o w s f r o m E q . (3.60) t h a t for a specific a p p l i c a t i o n (i.e., for fixed v a l u e s o f / a n d S ) o n e m u s t specify n o t o n l y t h e A/S r a t i o b u t a l s o t h e r e c y c l e b e f o r e (
t h e o p e r a t i n g p r e s s u r e Ρ b e c o m e s fixed. T h e u s u a l p r o c e d u r e is t o specify a n o p e r a t i n g p r e s s u r e Ρ a n d a n A/S
r a t i o a n d c a l c u l a t e t h e r e q u i r e d recycle
f r o m E q . (3.60), w h i c h s o l v e d for R y i e l d s E q . (3.62). R = (A/S) QSi/l.lSaUPT h e d e s i g n p r o c e d u r e for
flotation
1)
(3.62)
s y s t e m s w i t h recycle is i l l u s t r a t e d b y
E x a m p l e 3.7.
Example 3.7 F o r t h e a p p l i c a t i o n in E x a m p l e 3.6 d e s i g n a
flotation
system with recycle,
t a k i n g a n o p e r a t i n g p r e s s u r e o f 2.9 a t m . SOLUTION Step J. Select A/S
= 0.04, as stated.
Step 2 . C a l c u l a t e R f r o m E q . (3.62). R = (0.04)(1.0)(250)/1.2 χ 1 8 . 7 ( 0 . 5 x 2 . 9 - 1 ) = 0.99 M G D * 1.0 M G D T h i s m e a n s t h a t recycle r a t i o R/Q is a p p r o x i m a t e l y u n i t y . Step 3. Select a n overflow r a t e OR = 3 gal/(min)(ft ) 2
Step 4. R e q u i r e d s u r f a c e a r e a is defined a s Q + R « 2.0 M G D or 2,000,000 gal/day χ day/24 hr χ hr/60 min = Q + R = 1390 gal/min Surface area (ft ) = (Q + R)/OR 2
= 1390/3.0 = 464 ft
2
S u r f a c e a r e a is t w i c e a s l a r g e a s for t h e u n i t w i t h o u t r e c y c l e . H o w e v e r , a n effluent o f s u p e r i o r q u a l i t y (i.e., SS < 2 5 m g / l i t e r ) is o b t a i n e d .
114
3.
Pretreatment and Primary Treatment
5. Neutralization (and Equalization) 5.1. N E U T R A L I Z A T I O N I N T H E FIELD O F WASTEWATER TREATMENT N e u t r a l i z a t i o n t r e a t m e n t is often utilized in t h e f o l l o w i n g c a s e s a r i s i n g i n wastewater treatment: 1. P r i o r t o d i s c h a r g e o f t h e w a s t e w a t e r i n t o a r e c e i v i n g w a t e r . T h e j u s t i f i c a t i o n f o r n e u t r a l i z a t i o n is t h a t a q u a t i c life is sensitive t o p H v a r i a t i o n s b e y o n d a n a r r o w r a n g e a r o u n d p H 7. 2. P r i o r t o d i s c h a r g e of i n d u s t r i a l w a s t e w a t e r s t o t h e m u n i c i p a l s e w e r s y s t e m . Specification o f t h e p H of i n d u s t r i a l d i s c h a r g e s i n t o m u n i c i p a l s e w e r s y s t e m s is f r e q u e n t l y m a d e . I t is m o r e e c o n o m i c a l t o n e u t r a l i z e i n d u s t r i a l w a s t e w a t e r s t r e a m s p r i o r t o t h e d i s c h a r g e i n t o t h e m u n i c i p a l sewer, r a t h e r t h a n attempting to perform neutralization of the larger volume of c o m b i n e d domestic a n d industrial sewage. 3. P r i o r t o c h e m i c a l o r b i o l o g i c a l t r e a t m e n t . F o r b i o l o g i c a l t r e a t m e n t , p H o f t h e s y s t e m is m a i n t a i n e d w i t h i n t h e r a n g e 6 . 5 - 8 . 5 t o e n s u r e o p t i m u m b i o l o g i c a l activity. T h e b i o l o g i c a l p r o c e s s itself p r o v i d e s a n e u t r a l i z a t i o n a n d buffer c a p a c i t y a s a r e s u l t o f p r o d u c t i o n o f C 0 , w h i c h f o r m s c a r b o n a t e s a n d b i c a r b o n a t e s in s o l u t i o n . T h e d e g r e e o f p r e n e u t r a l i z a t i o n r e q u i r e d f o r b i o l o g i c a l t r e a t m e n t d e p e n d s o n t w o f a c t o r s : (1) t h e a l k a l i n i t y o r a c i d i t y p r e s e n t i n t h e w a s t e w a t e r a n d (2) t h e m g / l i t e r B O D t o b e r e m o v e d in t h e b i o l o g i c a l t r e a t m e n t . T h e l a t t e r is r e l a t e d t o t h e p r o d u c t i o n o f C 0 , w h i c h m a y p r o v i d e for partial neutralization of alkaline wastes. 2
2
5.2. M E T H O D S F O R N E U T R A L I Z A T I O N OF W A S T E W A T E R S M e t h o d s f o r n e u t r a l i z a t i o n o f w a s t e w a t e r s i n c l u d e (1) e q u a l i z a t i o n , w h i c h c o n s i s t s o f m i x i n g a c i d i c a n d a l k a l i n e w a s t e s t r e a m s a v a i l a b l e in t h e p l a n t a n d (2) d i r e c t p H c o n t r o l m e t h o d s , w h i c h c o n s i s t o f a d d i t i o n o f a c i d s ( o r b a s e s ) for neutralization of alkaline (or acidic) wastewater streams.
5.3.
EQUALIZATION
W h e n utilized for p u r p o s e o f n e u t r a l i z a t i o n , e q u a l i z a t i o n i n v o l v e s m i x i n g w a s t e w a t e r s t r e a m s o f a c i d i c a n d a l k a l i n e n a t u r e in a n e q u a l i z a t i o n b a s i n . E q u a l i z a t i o n is often u s e d f o r p u r p o s e s o t h e r t h a n n e u t r a l i z a t i o n s u c h a s (1) t o s m o o t h o u t i n d i v i d u a l w a s t e w a t e r s t r e a m flow v a r i a t i o n s , s o t h a t a c o m p o s i t e s t r e a m o f relatively c o n s t a n t flow r a t e is fed t o t h e t r e a t m e n t p l a n t ; a n d (2) t o s m o o t h o u t v a r i a t i o n s in influent B O D t o t h e t r e a t m e n t facilities. C o n s t a n t a n d v a r i a b l e level e q u a l i z a t i o n b a s i n s a r e utilized. 1. Constant level equalization basins. T h i s a r r a n g e m e n t is i l l u s t r a t e d in F i g . 3.30. T h e level in t h e e q u a l i z a t i o n b a s i n is h e l d c o n s t a n t . T h e r e f o r e a s
5.
Neutralization (and Equalization)
115
Influent
Effluent
Qi = f(t)
Acid and alkaline streams
Fig. 3.30.
Constant
level
equalization
basin.
t h e r a t e of flow o f influent v a r i e s , t h a t o f effluent is e q u a l l y affected. C o n s e q u e n t l y , t h i s is n o t a t e c h n i q u e o f flow e q u a l i z a t i o n , s i m p l y a m e t h o d o f n e u t r a l i z a t i o n . If fluctuations in t h e flow r a t e a r e t o o g r e a t , t h e effluent f r o m t h e c o n s t a n t level e q u a l i z a t i o n b a s i n is fed t o a n o t h e r e q u a l i z a t i o n b a s i n h a v i n g a s o b j e c t i v e flow e q u a l i z a t i o n . 2. Variable level equalization basins. I n t h i s m e t h o d o f e q u a l i z a t i o n , t h e effluent is t a k e n o u t a t a c o n s t a n t r a t e , a n d since t h e flow r a t e o f influent v a r i e s w i t h t i m e , t h e level o f t h e e q u a l i z a t i o n b a s i n is v a r i a b l e . T h i s m e t h o d is a l s o utilized f o r t h e p u r p o s e o f flow e q u a l i z a t i o n , a s well a s p r o v i d i n g f o r n e u t r a l i z a t i o n . A d i a g r a m o f a v a r i a b l e level e q u a l i z a t i o n b a s i n is s h o w n in Fig. 3.31. Influent Q, - f(t)
Effluent
Γ Τ Τ Τ Acid and alkaline streams
Fig. 3.31.
Variable
level equalization
basin.
Influent
Excess
Holding pond
Bleed stream
Equalization tank
I Fig. 3.32.
"Holding
pond"
method
of
equalization.
116
3.
Pretreatment and Primary Treatment
A n o t h e r m e t h o d of e q u a l i z a t i o n c o n s i s t s o f d i v e r t i n g t h e " e x c e s s " o f t h e i n c o m i n g s t r e a m t o a h o l d i n g p o n d , f r o m w h i c h a b l e e d s t r e a m is fed t o t h e e q u a l i z a t i o n t a n k . T h i s m e t h o d is n o t u s e d for n e u t r a l i z a t i o n p u r p o s e s , b u t o n l y for e q u a l i z a t i o n o f B O D c o n t e n t o r flow r a t e . T h i s is i l l u s t r a t e d b y F i g . 3.32.
5.4. D I R E C T p H C O N T R O L M E T H O D S : NEUTRALIZATION OF A C I D I C W A S T E S BY D I R E C T pH C O N T R O L M E T H O D S T h e f o l l o w i n g m e t h o d s o f d i r e c t n e u t r a l i z a t i o n of a c i d i c w a s t e s a r e t h e m o s t c o m m o n l y e m p l o y e d : (1) l i m e s t o n e b e d s , (2) s l u r r i e d l i m e n e u t r a l i z a t i o n , (3) c a u s t i c s o d a ( N a O H ) n e u t r a l i z a t i o n , (4) s o d i u m c a r b o n a t e n e u t r a l i z a t i o n , a n d (5) a m m o n i a n e u t r a l i z a t i o n . A few specific c o m m e n t s a b o u t t h e s e m e t h o d s a r e a s f o l l o w s : S l u r r i e d l i m e n e u t r a l i z a t i o n is t h e m o s t c o m m o n m e t h o d a n d is d i s c u s s e d in S e c t i o n 5.6. D e s i g n of l i m e s t o n e b e d s is d e s c r i b e d in S e c t i o n 5.5. C a u s t i c s o d a ( N a O H ) is m o r e e x p e n s i v e t h a n l i m e . It offers a n a d v a n t a g e with respect t o uniformity of the reagent, ease of storage a n d feeding, r a p i d r e a c t i o n r a t e , a n d t h e fact t h a t t h e e n d p r o d u c t s o f n e u t r a l i z a t i o n ( s o d i u m salts) a r e s o l u b l e . S o d i u m c a r b o n a t e ( N a C 0 ) is n o t a s r e a c t i v e a s c a u s t i c s o d a presents frothing problems owing to release of c a r b o n dioxide. 2
3
and
A m m o n i a ( N H O H ) p r e s e n t s t h e d i s a d v a n t a g e of b e i n g a c o n t a m i n a n t ; c o n s e q u e n t l y its u s e m a y b e r u l e d o u t b y p o l l u t i o n c o n t r o l s t a n d a r d s . 4
F a c t o r s g u i d i n g selection of a n e u t r a l i z a t i o n r e a g e n t a r e (1) p u r c h a s e c o s t , (2) n e u t r a l i z a t i o n c a p a c i t y , (3) r e a c t i o n r a t e , a n d (4) s t o r a g e a n d d i s p o s a l o f neutralization products.
5.5. L I M E S T O N E B E D S 5.5.1. Types of Equipment B o t h upflow a n d downflow types of limestone beds are employed. F o r w a s t e w a t e r s c o n t a i n i n g H S 0 , l i m e s t o n e b e d s s h o u l d n o t b e u s e d if c o n c e n t r a t i o n o f H S 0 e x c e e d s 0 . 6 % . T h e r e a s o n f o r t h i s l i m i t a t i o n is t h a t t h e l i m e s t o n e b e c o m e s c o v e r e d w i t h a n i n s o l u b l e c o a t o f C a S 0 , r e n d e r i n g it ineffective. I n a d d i t i o n , e v o l u t i o n o f C 0 c a u s e s f r o t h i n g p r o b l e m s . 2
2
4
4
4
2
U p f l o w t y p e a r r a n g e m e n t is p r e f e r a b l e t o d o w n f l o w t y p e since in u p f l o w u n i t s , C a S 0 t e n d s t o b e flushed o u t b e f o r e p r e c i p i t a t i o n o n t h e l i m e s t o n e . A l s o , e s c a p e o f C 0 g e n e r a t e d b y t h e n e u t r a l i z a t i o n r e a c t i o n is e a s i e r in u p f l o w t y p e u n i t s . F o r t h e s e r e a s o n s , m a x i m u m h y d r a u l i c r a t e for d o w n f l o w s y s t e m s is l i m i t e d t o a p p r o x i m a t e l y 5 0 g a l / ( h r ) ( f t ) . 4
2
2
P r e s e n c e o f m e t a l l i c i o n s (e.g., A l
3 +
, Fe
3 +
) in t h e w a s t e w a t e r
reduces
5.
Neutralization (and Equalization)
117
effectiveness o f t h e l i m e s t o n e b e d o w i n g t o c o a t i n g o f l i m e s t o n e w i t h p r e c i p i t a t e d h y d r o x i d e s . F i n a l l y , if d i l u t i o n of t h e a c i d i n t h e w a s t e w a t e r is i n c r e a s e d , higher residence times are required for neutralization. 5.5.2.
Design Procedure for Limestone
Beds
In this section, the laboratory procedure r e c o m m e n d e d by Eckenfelder a n d F o r d [ 3 ] for o b t a i n i n g t h e b a s i c d e s i g n d a t a is d e s c r i b e d . A n u m e r i c a l e x a m p l e is p r e s e n t e d t o i l l u s t r a t e d e s i g n o f a n a c t u a l l i m e s t o n e c o l u m n . A m o d e l o f a l a b o r a t o r y l i m e s t o n e n e u t r a l i z a t i o n c o l u m n is s h o w n i n F i g . 3.33. B e n c h scale c o l u m n s o p e r a t e w i t h h e i g h t s o f l i m e s t o n e o f 1.0-5 ft, w h i c h is t h e a c t u a l r a n g e o f h e i g h t s for p l a n t - s c a l e u n i t s . C o l u m n d i a m e t e r is a p p r o x i m a t e l y 6 in., a n d r a t e s o f flow [ g a l / ( h r ) ( f t ) ] a r e c o m p a r a b l e t o t h o s e f o r p l a n t 2
operation.
Fig. 3.33.
Laboratory
model
of limestone
neutralization
column.
Step 1. Fill n e u t r a l i z a t i o n c o l u m n s w i t h l i m e s t o n e (after w a s h i n g a n d s c r e e n i n g ) t o d e p t h s o f 1, 2, 3 , 4 , a n d 5 ft (5 c o l u m n s ) . Step 2. A d j u s t u p w a r d flow r a t e o f a c i d w a s t e w a t e r for e a c h c o l u m n . F l o w r a t e s v a r y i n g f r o m 50 t o 1000 g a l / ( h r ) ( f t ) a r e u s e d . 2
Step 3. C h e c k effluent p H f r o m e a c h c o l u m n a t e a c h flow r a t e u t i l i z e d u n t i l it is s t a b i l i z e d . Step 4. A f t e r e a c h r u n r e p l a c e l i m e s t o n e u s e d in t h e c o l u m n s . Step 5. P l o t t e r m i n a l p H a s a f u n c t i o n o f r a t e o f flow [ g a l / ( h r ) ( f t ) ] f o r e a c h d e p t h o f l i m e s t o n e . A t y p i c a l p l o t o f t h i s t y p e is s h o w n i n F i g . 3.34. 2
P u r p o s e s o f t h e d e s i g n p r o c e d u r e a r e (1) t o select t h e m o s t e c o n o m i c a l h e i g h t o f c o l u m n for a specified p H o f t h e effluent. T h i s is t h e c o l u m n h e i g h t c o r r e s p o n d i n g t o a m a x i m u m a l l o w a b l e flow r a t e , e x p r e s s e d in t e r m s o f v o l u m e of l i m e s t o n e utilized, i.e., g a l o f l i q u o r / ( h r ) ( f t o f l i m e s t o n e ) ; a n d (2) t o c a l c u l a t e a n n u a l r e q u i r e m e n t o f l i m e s t o n e u n d e r t h e s e c o n d i t i o n s , which corresponds to a m i n i m u m requirement of limestone. 3
118
3.
Pretreatment and Primary Treatment
II
IO
PH 9 8 7 6 5
4 Ο
500
I000
I500
Flow rate [gal/(hr)(f t )) 2
Fig. 3.34. Limestone
neutralization
data.
Example 3.8 I t is d e s i r e d t o n e u t r a l i z e a w a s t e w a t e r a c i d s t r e a m c o n t a i n i n g 0.1 Ν H C I t o a p H o f 7.0 t h r o u g h a l i m e s t o n e b e d . L a b o r a t o r y tests w i t h t h e w a s t e w a t e r yield d a t a p l o t t e d i n F i g . 3.34. D e s i g n a n e u t r a l i z a t i o n s y s t e m f o r 100 g a l / m i n (6000 gal/hr) of wastewater a n d estimate a n n u a l limestone r e q u i r e m e n t for the most economical operation. SOLUTION Step 1. F o r p H 7 r e a d f r o m F i g . 3.34 r a t e s o f flow [ g a l / ( h r ) ( f t ) ] c o r r e 2
s p o n d i n g t o each c o l u m n depth. T h e n calculate cross-sectional area, v o l u m e o f l i m e s t o n e b e d r e q u i r e d , a n d flow r a t e i n g a l / ( h r ) ( f t
3
of limestone bed)
( T a b l e 3.12). T A B L E 3.12 Calculations for Example 3.8
(5)
(2)
(3) Cross section 6000 gal/hr
(4) Volume o f limestone ( f t )
F l o w rate [gal/(hr)(ft )] 6000 gal/hr ~ (4)
Depth (ft)
F l o w rate [gal/(hr)(ft )] [ F r o m Fig. 3.34 for p H 7 ]
1.0 2.0 3.0 4.0
118 492 845 1047
51 12.2 7.1 5.73
51 24.4 21.3 22.9
118 246 282 262
5.0
1200
5.0
25.0
240
ω
2
{
)
~
(2)
3
3
(4) = ( / ) x ( 5 )
(
)
5.
Neutralization (and Equalization)
119
300 c
ο
/
tn
I
250
f/
to
Β 200 ο . o>.
φ o 5
150
ο
I
ι
L
LL
©
ι I f
t
s
8; I)
ft
/ft w<
/t
•<
62 ) >
>)
Λ Tt 7
i/E
1
Hi )
I00 2
3 © D e p t h (ft)
Fig. 3.35.
Determination
of optimum
bed
depth.
Step 2. P l o t flow r a t e s [ g a l / ( h r ) ( f t ) ] f r o m c o l u m n (5) o f T a b l e 3.12 vs. d e p t h s [ c o l u m n ( 7 ) ] . T h i s p l o t is s h o w n in F i g . 3.35 a n d i n d i c a t e s t h a t a 3-ft l i m e s t o n e b e d is t h e m o s t e c o n o m i c a l , c o r r e s p o n d i n g t o a m a x i m u m o n t h e c u r v e . T h e c r o s s - s e c t i o n a l a r e a in t h i s c a s e (see T a b l e 3.12 for a d e p t h o f 3.0 ft) is 7.1 f t , c o r r e s p o n d i n g t o a d i a m e t e r o f 3.0 ft. 3
2
Step 3. C a l c u l a t e t h e lb o f a c i d t o b e n e u t r a l i z e d p e r d a y ( a c i d c o n t a i n e d in t h e 6 0 0 0 g a l / h r o f t h e 0.1 Ν s o l u t i o n o f H C I ) . S i n c e t h i s is a d i l u t e a c i d s o l u t i o n , c a l c u l a t i o n is b a s e d o n t h e d e n s i t y o f w a t e r , t a k e n a s 8.34 l b / g a l . A 0.1 Ν s o l u t i o n o f H C I c o n t a i n s 3.65 g/liter o f H C I , o r a p p r o x i m a t e l y 3.65 lb o f a c i d p e r 1000 l b o f s o l u t i o n . T h e r e f o r e Mass flow r a t e : 6000 gal/hr χ 24 hr/day χ 8.34 lb/gal = 1.2 M l b / d a y Acid c o n t e n t : 3.65 χ (1,200,000/1000) = 4380 lb/day Step 4. e q u a t i o n is
Calculate
limestone
(CaC0 ) 3
required.
2HC1 + C a C 0 -» C a C l + C 0 Molecular weight: (2 χ 36.5 = 73) (100) 3
2
2
The
neutralization
+ H 0 2
T h e r e f o r e l i m e s t o n e r e q u i r e d is 4380 χ 100/73 = 6000 lb/day of limestone T h i s is t h e t h e o r e t i c a l a m o u n t o f l i m e s t o n e a s s u m i n g 1 0 0 % r e a c t i v i t y . F o r d e s i g n p u r p o s e s , a s s u m e a n 8 0 % r e a c t i v i t y . L i m e s t o n e r e q u i r e d is t h e n 6000/0.8 = 7500 lb/day or 2.738 χ 1 0 lb/year 6
120
3.
Pretreatment and Primary Treatment
5.6. S L U R R I E D L I M E T R E A T M E N T 5.6.1. Equipment for Slurried Lime S y s t e m s S l u r r i e d l i m e is t h e m o s t c o m m o n l y u s e d r e a g e n t for n e u t r a l i z a t i o n o f a c i d w a s t e w a t e r s , t h e l o w c o s t o f l i m e b e i n g t h e m a i n r e a s o n f o r its w i d e s p r e a d utilization. A
flowsheet
o f a t w o - s t a g e s l u r r i e d l i m e n e u t r a l i z a t i o n s y s t e m is
s h o w n i n F i g . 3.36. Quicklime
Recirculation line for slurried lime Slurried lime
Λ
Recirculation pump'
Water
Slurry storage tank (agitated vessel)
pH controller^ pH controllerQ-ra rJ
By-pass line
Alkaline wastewater
Fig. 3.36.
Flow
diagram
of a two-stage
slurried
lime neutralization
system.
S t e p w i s e a d d i t i o n of l i m e is r e c o m m e n d e d . F o r h i g h l y a c i d i c w a s t e s a m i n i m u m o f t w o stages is d e s i r a b l e , t h e first ( b u l k n e u t r a l i z a t i o n ) t o r a i s e p H t o a v a l u e o f 3 . 0 - 3 . 5 , a n d t h e s e c o n d (fine t u n i n g ) t o a d j u s t p H t o d e s i r e d effluent v a l u e . S o m e t i m e s a t h i r d s t a g e is d e s i r a b l e . A u t o m a t i c c o n t r o l o f t h i s p r o c e s s is n o t s i m p l e b e c a u s e t h e r e l a t i o n s h i p b e t w e e n p H a n d a m o u n t o f l i m e a d d e d is h i g h l y n o n l i n e a r , p a r t i c u l a r l y in t h e vicinity o f t h e n e u t r a l i z a t i o n p o i n t ( p H 7). T h i s is a p p r e c i a t e d b y i n s p e c t i o n o f a t y p i c a l n e u t r a l i z a t i o n c u r v e o f a n i n d u s t r i a l w a s t e w a t e r , s h o w n in F i g . 3.37. I n t h e vicinity of t h e n e u t r a l i z a t i o n p o i n t , t h e p H b e c o m e s e x c e e d i n g l y sensitive t o s m a l l a d d i t i o n s o f l i m e , v a r y i n g in a n o r d i n a r y o p e r a t i o n o f s l u r r i e d l i m e s y s t e m s a t a r a t e a s fast a s o n e p H u n i t p e r m i n u t e . A l s o , fluctuation in flow r a t e o f influent c o m p l i c a t e s o p e r a t i o n o f t h e p r o c e s s . U s e o f a n e q u a l i z a t i o n t a n k is i n d i c a t e d t o d a m p e n fluctuations, a s s h o w n in F i g . 3.36. A relatively s m a l l a m o u n t o f r e a g e n t is t h o r o u g h l y m i x e d w i t h a l a r g e l i q u i d v o l u m e in a s h o r t t i m e i n t e r v a l . M e c h a n i c a l m i x e r s a r e p r o v i d e d f o r t h i s purpose.
5.
121
Neutralization (and Equalization) 14
PH 12 10 8 7 6
I 1
4
1
2 Ο
1
2,000
--3,600
V
4,000
6,000
mg of lime/liter of wastewater Fig.
3.37.
Typical
neutralization
curve
for an industrial
wastewater.
5.6.2. D e s i g n Procedure for Slurried Lime Neutralization S y s t e m s T h e p r o c e d u r e r e c o m m e n d e d b y E c k e n f e l d e r a n d F o r d [ 3 ] is s u m m a r i z e d in t h i s s e c t i o n a n d i l l u s t r a t e d b y a n u m e r i c a l e x a m p l e . B a s i c i n f o r m a t i o n r e q u i r e d is (1) n e u t r a l i z a t i o n c u r v e f o r t h e w a s t e w a t e r (see F i g . 3.37), a n d (2) p o w e r c o n s u m p t i o n d a t a , i.e., a c u r v e o f level of a g i t a t i o n v s . d e t e n t i o n t i m e for a d e s i r e d t e r m i n a l p H (see F i g . 3.38). T h i s b a s i c i n f o r m a t i o n is o b t a i n e d by simple laboratory procedures [ 3 ] . P u r p o s e s o f t h e d e s i g n p r o c e d u r e i l l u s t r a t e d b y E x a m p l e 3.9 a r e (1) t o select t h e n u m b e r o f stages o f n e u t r a l i z a t i o n a n d t o size n e u t r a l i z a t i o n r e a c t o r s , a n d (2) t o select a p p r o p r i a t e m i x i n g e q u i p m e n t .
Example 3 . 9 2 0 0 g a l / m i n of a n a c i d i c i n d u s t r i a l w a s t e w a t e r a r e n e u t r a l i z e d t o p H 7.0. F r o m l a b o r a t o r y t e s t s , t h e n e u t r a l i z a t i o n c u r v e is p l o t t e d ( F i g . 3.37). A l s o , a c u r v e of level o f a g i t a t i o n vs. d e t e n t i o n t i m e is o b t a i n e d f o r t h i s n e u t r a l i z a t i o n ( F i g . 3.38). D e s i g n a l i m e s l u r r y n e u t r a l i z a t i o n s y s t e m . SOLUTION Step 1. F r o m t h e n e u t r a l i z a t i o n c u r v e in F i g . 3.37, l i m e s l u r r y r e q u i r e m e n t for n e u t r a l i z i n g t h e w a s t e w a t e r t o a p H o f 7 is r e a d a s 3 6 0 0 m g / l i t e r . Step 2. C a l c u l a t e l i m e s l u r r y r e q u i r e m e n t f o r 2 0 0 g a l / m i n w a s t e w a t e r flow. S i n c e 3600 m g lime/liter waste = 3600 χ 1 0 " lb lime/lb waste 6
3.
122
Pretreatment and Primary Treatment
I.Or
5 IO Residence time (min)
~"Ί.
Fig. 3.38.
Level
of agitation
vs. detention
50
I00
time.
then 200 gal waste/min χ 60 m i n / h r χ 24 h r / d a y χ 8.34 lb waste/gal waste χ 3600 χ 1 0 " lb lime/lb waste = 8647 lb lime/day 6
Step 3. N e u t r a l i z e a c i d in t w o s t e p s a s i n d i c a t e d b y flow d i a g r a m in F i g . 3.36, t h e first s t a g e f o r b u l k n e u t r a l i z a t i o n , a n d t h e s e c o n d for fine t u n i n g . Step 4. Select a d e t e n t i o n t i m e a n d size t h e r e a c t o r s . Volume of reactor (gal) = Q (gal/min) χ / (min) S e l e c t i o n o f t h e o p t i m u m d e t e n t i o n t i m e is a r r i v e d a t b y a n
(3.63) economical
balance. 1. A s s u m e a r e s i d e n c e t i m e ( u s u a l l y a v a l u e b e t w e e n 5 a n d 10 m i n ) . 2. Size t h e r e a c t o r [ E q . ( 3 . 6 3 ) ] . 3. F r o m F i g . 3.38 f o r t h e a s s u m e d r e s i d e n c e t i m e , d e t e r m i n e
power
level r e q u i r e d . 4 . Select m i x e r s (as s h o w n i n S t e p 5) f r o m k n o w l e d g e o f p o w e r level [item (3)]. 5. E s t i m a t e t o t a l c o s t s ( c a p i t a l a n d o p e r a t i n g ) c o r r e s p o n d i n g t o t h i s a s s u m e d r e s i d e n c e t i m e . M a i n i t e m s in t h e c o l u m n of c a p i t a l c o s t s a r e t h e reactors themselves (and auxiliary equipment) a n d the mixers. Energy require m e n t is t h e v a r i a b l e i t e m a m o n g o p e r a t i n g c o s t s .
Problems
123
6. R e p e a t i n g s t e p s ( l ) - ( 5 ) , a c u r v e o f t o t a l c o s t p e r y e a r v s . a series o f selected r e s i d e n c e t i m e s is p l o t t e d . T h i s c u r v e p a s s e s t h r o u g h a m i n i m u m which corresponds to the o p t i m u m detention time. Capital costs are expressed o n a y e a r l y b a s i s b y e s t i m a t i n g e q u i p m e n t life a n d u t i l i z i n g t h e c u r r e n t v a l u e for interest rate. A s s u m e for E x a m p l e 3.9 t h a t o p t i m u m r e s i d e n c e t i m e is e s t i m a t e d b y t h i s procedure as 5 min. Then Volume of each reactor (gal) = 200 gal/min χ 5 m i n = 1000 gal or 1000 gal χ ft /7.48 gal = 134 f t 3
3
S e l e c t i n g a r e a c t o r d e p t h o f 5 ft, r e q u i r e d c r o s s - s e c t i o n a l a r e a is A r e a = 1 3 4 f t / 5 ft = 26.8 ft 3
2
c o r r e s p o n d i n g t o a d i a m e t e r o f 5.84 ft. Step
5. Select m i x e r s . F r o m F i g . 3.38, p o w e r level r e q u i r e d for 5 - m i n
d e t e n t i o n t i m e is 0.15 HP/1000 gal Since e a c h t a n k h a s a v o l u m e o f 1000 g a l , specify o n e 0 . 1 5 - H P m i x e r for e a c h tank.
5.7. NEUTRALIZATION OF ALKALINE W A S T E S In principle, a n y strong acid c a n be used t o neutralize alkaline wastewaters. Cost considerations limit choice t o H S 0 (the m o s t c o m m o n ) a n d H C I . R e a c t i o n rates are essentially i n s t a n t a n e o u s . T h e basic design p r o c e d u r e for a l k a l i n e w a s t e s is s i m i l a r t o t h a t for a c i d i c w a s t e s d e s c r i b e d in S e c t i o n 5.6. 2
4
F l u e g a s e s c o n t a i n i n g 1 4 % o r m o r e o f C 0 a r e u s e d for n e u t r a l i z a t i o n o f alkaline wastewaters. W h e n bubbled t h r o u g h the wastewater the C 0 forms c a r b o n i c a c i d , w h i c h r e a c t s w i t h t h e b a s e . R e a c t i o n r a t e is s l o w b u t sufficient if p H n e e d n o t b e a d j u s t e d b e l o w 7 o r 8. E i t h e r b u b b l i n g t h r o u g h a p e r f o r a t e d p i p e o r u s i n g s p r a y t o w e r s is s a t i s f a c t o r y . 2
2
Problems I. Sedimentation {discrete settling). A particle size distribution is obtained from a sieve analysis of sand particles. For each weight fraction an average settling velocity is calculated. D a t a [6] are presented in the following tabulation. Settling velocity (ft/min) 10.0 5.0 2.0 1.0 0.75 0.50
Weight fraction remaining 0.55 0.46 0.35 0.21 0.11 0.03
124
3.
Pretreatment and Primary Treatment
1. Prepare a plot o f fraction of particles with less than stated velocity vs. settling velocity (ft/min). 2. F o r an overflow rate of 100,000 gal/(day)(ft ), calculate overall removal utilizing Eq. (3.26). 2
II. Sedimentation ulated below.
(flocculent
settling).
Time
A laboratory settling analysis gave the results tab
% suspended solids removed at indicated depth
T i m e (min)
2 ft
4 ft
6 ft
10 20 30 45 60
40 54 62 71 76
25 37 47 56 65
16 28 37 46 53
1. Perform analysis of the data and arrive at curves for % SS removal vs. detention time (min), and % SS removal vs. overflow rate [ g a l / ( d a y ) ( f t ) ] . 2. If the initial concentration of the slurry is 430 p p m , design a settling tank (i.e., calculate diameter and effective depth o f the tank) to remove 70% o f the suspended solids for a 1 Mgal/day flow. 3. What removal is attained if flow if increased to 2 M g a l / d a y ? 4. F o r the flow o f 1 Mgal/day calculate daily accumulation o f sludge in lb/day and average pumping rate in gal/min. A s s u m e sludge concentration to be 1.5% solids ( « 1 5 , 0 0 0 mg/liter). 2
III. Sedimentation (zone settling). It is desired to design a secondary settling tank to produce an underflow concentration o f 15,000 mg/liter from a mixed liquor solids content of 3750 mg/liter in the influent. Wastewater flow is 2.0 Mgal/day. Calculate clarifier area re quired. D a t a below are obtained in a laboratory test o f the slurry.
IV. Flotation.
/ (min)
Interface height, Η (ml)
0 2 4 6 8 10 15 20 25 30
1000 920 840 760 690 600 400 300 280 270
A pilot-plant flotation operation indicated o p t i m u m air/solid ratio to be
0.04 lb air/lb o f solids.
References
125
1. If a wastewater to be treated has 250 p p m suspended solids, c o m p u t e the % recycle to be pressurized to 60 psia at 20°C. Take / = 0.68. 2. F o r a wastewater flow o f 1.0 M g a l / d a y and an overflow rate o f 4.0 g a l / ( m i n ) ( f t ) c o m p u t e surface area required. 2
V. Neutralization. F o r the 3-ft limestone bed designed in Example 3.8 prepare a plot o f volume of limestone required vs. a range o f selected values o f p H for the effluent (select p H = 5, 6, 7, 8, 9, and 10).
References 1. C a m p , T. R., Trans. Am. Soc. Civ. Eng. I l l , 909 (1946). 2. Eckenfelder, W. W . , Jr., "Industrial Water Pollution Control." McGraw-Hill, N e w Y o r k , 1966. 3. Eckenfelder, W. W . , Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w Y o r k , 1970. 4. Eckenfelder, W . W . , Jr., and O'Connor, D . J., "Biological Waste Treatment." P e r g a m o n , Oxford, 1961. 5. H a z e n , Α . , Trans. Am. Soc. Civ. Eng. 5 3 , 45 (1904). 6. Metcalf & Eddy, Inc., "Wastewater Engineering." McGraw-Hill, N e w Y o r k , 1972. 7. Talmadge, W . P., and Fitch, Ε. B., Ind. Eng. Chem. 47, 38 (1955). 8. Waddel, H., J. Franklin Inst. 2 7 , 4 5 9 ^ 1 9 0 (1934).
4 Theory and Practice of Aeration in Wastewater Treatment 1. Introduction
127
2. Steps Involved in the Oxygen-Transfer Process
128
3. Oxygen-Transfer Rate Equation
128
4. Determination of the Overall Mass-Transfer Coefficient K a Unsteady State Aeration of Tap Water L
5. Integration of the between Limits
by 129
Differential Equation for Oxygen Transfer 133
6. Unsteady State Aeration of Activated Sludge Liquor
133
7. Steady State Determination of K a for the Activated Sludge Liquor
134
8. Oxygenation Capacity (OC)
135
9. Corrections for K a and Oxygenation Capacity ( O C ) with Temper ature and Pressure 9.1. Temperature Correction 9.2. Pressure Correction
135 135 135
L
L
10. Transfer Efficiency of Aeration Units
137
11. Effect of Wastewater Characteristics on Oxygen Transfer
138
12. Laboratory Determination of Oxygen-Transfer Coefficient α
140
13. Classification of Aeration Equipment—Oxygen-Transfer Efficiency
140
14. Air Diffusion Units 14.1. Type 1 . Fine Bubble Diffusers 14.2. Type 2. Large Bubble Diff users 14.3. Performance of Air Diffusion Units 14.4. Design Procedure for Aeration Systems Utilizing Air Diffusion Units 15. Turbine Aeration Units 15.1. Description of Unit 15.2. Performance of Turbine Aeration Units 15.3. Power Requirements for Turbine Aerators 15.4. Design Procedure for Aeration Systems Utilizing Turbine Aeration Units 16. Surface Aeration Units 16.1. Description of Unit 16.2. Correlation between Transfer Efficiency and Level of Agitation 16.3. Design Procedure for Aeration Systems Utilizing Surface Aeration Units Problems References
140 140 141 142
126
143 144 144 145 145 147 149 149 149 151 154 6 1 5
1.
127
Introduction
1. I n t r o d u c t i o n T h i s c h a p t e r is c o n c e r n e d w i t h t h e t r a n s f e r o f o x y g e n f r o m a i r t o a w a s t e water subjected to biological aerobic t r e a t m e n t . K n o w l e d g e of the rate of o x y g e n t r a n s f e r is essential for specification of a e r a t o r s t o b e utilized in t h e p r o c e s s . T h e o r y for o x y g e n t r a n s f e r is d i s c u s s e d , a n d t h e d e t e r m i n a t i o n o f t r a n s f e r coefficients f r o m l a b o r a t o r y e x p e r i m e n t s d e s c r i b e d . C o m m o n t y p e s o f a e r a t o r s a n d t h e p r o c e d u r e f o r specifying a e r a t o r s y s t e m s for
aerobic
wastewater treatment processes are also described. T h e b e s t k n o w n e x p l a n a t i o n for t h e m e c h a n i s m of g a s t r a n s f e r t o a l i q u i d is g i v e n b y t h e two-film t h e o r y . A c c o r d i n g t o t h i s t h e o r y , it is t h e p r e s e n c e o f t w o films, o n e liquid a n d o n e g a s , a t t h e g a s - l i q u i d i n t e r f a c e w h i c h p r o v i d e s the resistance to the passage of gas molecules from the bulk of the gas p h a s e t o t h a t of t h e l i q u i d p h a s e . F o r gases of h i g h s o l u b i l i t y in t h e l i q u i d p h a s e , e.g., a b s o r p t i o n of S 0
2
by
w a t e r , t h e m a j o r r e s i s t a n c e t o a b s o r p t i o n is t h a t offered b y t h e g a s film. F o r g a s e s o f l o w solubility in t h e l i q u i d p h a s e , e.g., a b s o r p t i o n o f o x y g e n b y a n a q u e o u s l i q u o r , t h e c o n t r o l l i n g r e s i s t a n c e resides in t h e l i q u i d film. F o r i n t e r m e d i a t e solubilities, b o t h films offer significant r e s i s t a n c e . O x y g e n s a t u r a t i o n v a l u e s ( C ) for distilled w a t e r a t s t a n d a r d c o n d i t i o n s s
(1 a t m ) a r e p r e s e n t e d in T a b l e 4 . 1 . F o r a m o r e c o m p l e t e t a b l e w i t h i n c r e m e n t s , c o n s u l t Ref. [ 7 ] . T A B L E 4.1 Oxygen Saturation Values ( C ) for Distilled Water at Standard Conditions (1 atm) [7] $
o
Temperature (°Q
Temperature (°F)
(mg/liter)
0 5 10 15 20 25 30 35 40 45 50
32.0 41.0 50.0 59.0 68.0 77.0 86.0 95.0 104.0 113.0 122.0
14.6 12.8 11.3 10.2 9.2 8.4 7.6 7.1 6.6 6.1 5.6
2
1°C
128
4.
Theory and Practice of Aeration
2. S t e p s Involved in t h e Oxygen-Transfer Process T h e p r o c e s s of o x y g e n t r a n s f e r f r o m a g a s e o u s t o a n a q u e o u s p h a s e o c c u r s in t h r e e s t e p s . Step 1. S a t u r a t i o n of t h e l i q u i d s u r f a c e b e t w e e n t h e t w o p h a s e s (let C b e s
t h i s s a t u r a t i o n c o n c e n t r a t i o n of o x y g e n ) . T h i s r a t e o f o x y g e n t r a n s f e r is v e r y r a p i d since t h e r e s i s t a n c e of t h e g a s film is negligible, a n d t h u s S t e p 1 is n e v e r the controlling one. Step
2. P a s s a g e o f t h e o x y g e n m o l e c u l e s t h r o u g h t h e l i q u i d i n t e r f a c e
film b y m o l e c u l a r diffusion. A t v e r y l o w m i x i n g levels t h e r a t e o f o x y g e n a b s o r p t i o n is c o n t r o l l e d b y S t e p 2. A t h i g h e r t u r b u l e n c e levels, t h e i n t e r f a c e film is b r o k e n u p a n d t h e r a t e o f r e n e w a l o f t h e film c o n t r o l s t h e a b s o r p t i o n o f o x y g e n . S u r f a c e r e n e w a l r a t e is t h e f r e q u e n c y a t w h i c h l i q u i d w i t h a n o x y g e n concentration C
L
( o x y g e n c o n c e n t r a t i o n in t h e b u l k o f t h e l i q u i d p h a s e )
replaces t h a t from the interface with a n oxygen c o n c e n t r a t i o n equal t o
C. s
Step 3. O x y g e n is t r a n s f e r r e d t o t h e b u l k of t h e l i q u i d b y diffusion a n d convection.
3. O x y g e n - T r a n s f e r R a t e Equation T h e b a s i c e q u a t i o n for o x y g e n - t r a n s f e r r a t e is N=
K A(C -C ) L
S
(4.1)
L
w h e r e Ν is t h e m a s s o f o x y g e n t r a n s f e r r e d p e r u n i t t i m e ( l b 0 / d a y ) ; K 2
L
the
l i q u i d film coefficient [ l b 0 / ( d a y ) ( f t ) ( u n i t A C ) ] ; A t h e interfacial a r e a for 2
2
transfer ( f t ) ; C the saturation c o n c e n t r a t i o n of oxygen (mg/liter); a n d C 2
s
L
the
c o n c e n t r a t i o n of o x y g e n i n t h e b o d y of t h e l i q u i d ( m g / l i t e r ) . E q u a t i o n (4.1) is u s u a l l y r e w r i t t e n in c o n c e n t r a t i o n u n i t s b y d i v i d i n g b y v o l u m e V of t h e s y s t e m . T h e n [ E q . (4.2)] N/V = dCJdt
= K (A/V)(C -C ) L
S
= K a(C -C )
L
L
5
(4.2)
L
w h e r e a = A/V = interfacial a r e a p e r u n i t v o l u m e ( f t / f t ) ; a n d K a is t h e o v e r a l l coefficient o f o x y g e n t r a n s f e r [ l b 0 / ( d a y ) ( f t ) ( u n i t A C ) ] . I n t h e d e t e r m i n a t i o n of t h e m a s s - t r a n s f e r coefficient, t h e o v e r a l l coefficient K a is o b t a i n e d w i t h o u t a t t e m p t i n g t o s e p a r a t e t h e f a c t o r s K a n d a. I t is a d m i t t e d l y i m p o s s i b l e t o m e a s u r e t h e interfacial a r e a A. T h e difference (C — C ) b e t w e e n s a t u r a t i o n v a l u e a n d a c t u a l c o n c e n t r a t i o n of o x y g e n ( C ) in t h e b o d y o f t h e l i q u i d p h a s e is called o x y g e n deficit ( h e n c e d e n o t e d a s O D ) . F o r a e r o b i c t r e a t m e n t p r o c e s s e s d e s i g n e d for r e m o v a l o f o r g a n i c B O D , t h e r a n g e for o p e r a t i n g d i s s o l v e d o x y g e n level C is b e t w e e n 0.5 a n d 1.5 m g / l i t e r . W h e n nitrification is t o b e a c h i e v e d , d i s s o l v e d o x y g e n level is in excess o f 2.0 m g / l i t e r . 2
3
L
3
2
L
L
S
L
L
L
4.
129
Determination of Mass-Transfer Coefficient K a L
4. D e t e r m i n a t i o n of t h e Overall M a s s - T r a n s f e r Coefficient K a by U n s t e a d y S t a t e A e r a t i o n of Tap W a t e r L
S t u d i e s o f t h e t r a n s f e r coefficient a r e u s u a l l y m a d e o n t a p w a t e r a n d t h e n c o r r e c t e d for t h e w a s t e w a t e r , a s d e s c r i b e d in S e c t i o n 11 of t h i s c h a p t e r . T h e p r o c e d u r e m o r e c o m m o n l y u s e d f o r d e t e r m i n a t i o n o f K a is t h e u n s t e a d y s t a t e a e r a t i o n o f t a p w a t e r . T h e f o u r s t e p s i n v o l v e d in t h i s d e t e r m i n a t i o n a r e given below. L
Step 1. D e o x y g e n a t e t h e w a t e r t o a n e s s e n t i a l l y z e r o c o n c e n t r a t i o n o f d i s s o l v e d o x y g e n . T h i s is d o n e b y a d d i t i o n o f d e o x y g e n a t i o n c h e m i c a l s , t h e m o s t c o m m o n l y u s e d b e i n g s o d i u m sulfite ( N a S 0 ) . C o b a l t c h l o r i d e ( C o C l ) is a d d e d as a c a t a l y s t for t h e d e o x y g e n a t i o n r e a c t i o n . 2
coci
Na S0 2
+ i0
3
2
3
2
2
> Na S0 2
4
T h e s t o i c h i o m e t r i c r a t i o is Na S0 /±0 2
3
2
= 126/16 = 7.9
T h i s m e a n s t h a t t h e o r e t i c a l l y 7.9 p p m o f N a S 0 a r e r e q u i r e d t o r e m o v e 1 p p m o f D O . B a s e d o n t h e D O o f t h e test t a p w a t e r , t h e a p p r o x i m a t e N a S 0 r e q u i r e m e n t s a r e e s t i m a t e d (a 1 0 - 2 0 % excess is u s e d ) . Sufficient c o b a l t c h l o r i d e is a d d e d t o p r o v i d e a m i n i m u m C o c o n c e n t r a t i o n o f 1.5 p p m . A n alternative deoxygenation p r o c e d u r e consists of r e m o v a l of dissolved oxygen by purging with nitrogen gas. Step 2. A f t e r D O c o n c e n t r a t i o n b e c o m e s e s s e n t i a l l y z e r o , s t a r t a e r a t i o n , m e a s u r i n g t h e i n c r e a s i n g c o n c e n t r a t i o n s o f D O a t selected t i m e i n t e r v a l s . S i n c e D O c o n c e n t r a t i o n i n c r e a s e s w i t h t i m e , t h i s m e t h o d is t e r m e d u n s t e a d y s t a t e a e r a t i o n . S t e a d y s t a t e m e t h o d s , in w h i c h D O c o n c e n t r a t i o n is k e p t c o n s t a n t , a r e d i s c u s s e d in S e c t i o n 7. D i s s o l v e d o x y g e n m e a s u r e m e n t s a r e p r e f e r a b l y p e r f o r m e d b y i n s t r u m e n t a l m e t h o d s . A p r o p e r l y c a l i b r a t e d g a l v a n i c cell o x y g e n a n a l y z e r a n d p r o b e is t h e m o s t r e l i a b l e m e t h o d . E x p e r i m e n t a l d e t e r m i n a t i o n of D O b y t h i s t e c h n i q u e is d e s c r i b e d in S e c t i o n 2.3.1 of C h a p t e r 2. C h e m i c a l a n a l y s i s of d i s s o l v e d o x y g e n ( W i n k l e r m e t h o d ) is a l s o e m p l o y e d [ 7 ] . T h e a e r a t i o n device is l o c a t e d a t t h e c e n t e r of t h e t e s t b a s i n . W h e n c i r c u l a r b a s i n s a r e e m p l o y e d , baffles a r e p l a c e d a t t h e q u a r t e r p o i n t s o f t h e b a s i n , a s i n d i c a t e d in F i g . 4 . 1 , in o r d e r t o p r e v e n t v o r t e x i n g . W h e n t e s t i n g is p e r f o r m e d in a c i r c u l a r t a n k for p i l o t o r full s c a l e t e s t s , s a m p l i n g d e p t h s for t h e D O d e t e r m i n a t i o n s a r e 1 ft f r o m t h e s u r f a c e a n d 1 ft from the b o t t o m , at the mid- a n d end-points of the radii trisecting the basin. 2
3
2
2 +
3
130
4.
Theory and Practice of Aeration
T h i s yields a t o t a l o f 12 s a m p l i n g p o i n t s , a s i l l u s t r a t e d b y F i g . 4 . 2 . S a m p l e s f r o m v a r i o u s test l o c a t i o n s a r e a n a l y z e d for D O , a n d t h e r e s u l t s a r e a v e r a g e d a n d recorded for the particular sampling time.
Aeration device
Baffles
Fig.
4.1.
Baffle
arrangement.
(For each of the six locations indicated by dots on cross section at right)
Fig.
4.2.
Location
Cross section of tank:
of sampling
points.
Step 3. R e s u l t s a r e t a b u l a t e d a s s h o w n b y s a m p l e d a t a in T a b l e 4 . 2 . T A B L E 4.2 Data for Example 4.1
w
Test time (min) 0 10 20 30 40 50 60
α
C = 10.2 mg/liter. s
C
L
(2) (mg/liter) 0.2 2.6 4.8 6.0 7.1 7.8 8.5
(3) C -C (mg/liter) s
L
10.0 7.6 5.4 4.2 3.1 2.4 1.7
e
4.
131
Determination of Mass-Transfer Coefficient K a L
Step 4. F r o m E q . (4.2) it follows t h a t a p l o t o f (C - C ) v s . t i m e i n s e m i l o g s
L
scale yields a s t r a i g h t line, t h e s l o p e o f w h i c h e q u a l s ( — K a). L
Take
Eq.
(4.2), dCJdt
=
K a(C -C ) L
s
L
Separating variables, integrating, a n d assuming K a L
t o be i n d e p e n d e n t of
the time of sampling [ E q . (4.3)]: In ( C - C ) = - K at + const. s
L
(4.3)
L
T h e p l o t o f l n ( C - C ) vs. t i m e is s h o w n i n F i g . 4 . 3 f o r d a t a i n T a b l e 4 . 2 . F o r a c c u r a c y t h e s t r a i g h t line is p l o t t e d b y t h e l e a s t - s q u a r e s m e t h o d . s
L
D e t e r m i n a t i o n o f t h e o v e r a l l m a s s - t r a n s f e r coefficient K a b y t h e m e t h o d o f u n s t e a d y s t a t e a e r a t i o n o f t a p w a t e r is i l l u s t r a t e d b y E x a m p l e 4 . 1 . L
0
Fig. 4.3.
20
Determination
40 Time (min) of K a L
60
(Example
4.1).
132
4.
Theory and Practice of Aeration
Example 4.1 D a t a p r e s e n t e d in T a b l e 4.2 a r e o b t a i n e d b y utilizing a n 8 - H P
surface
a e r a t o r in a 150,000 gal c i r c u l a r t e s t t a n k u n d e r t h e f o l l o w i n g c o n d i t i o n s : W a t e r t e m p e r a t u r e : 15°C Atmospheric pressure: 28 in. H g C : 10.2 mg/liter (at 15°C, Ρ = 28 in. Hg) s
T e s t w a t e r is d e o x y g e n a t e d u s i n g s o d i u m sulfite a n d a c o b a l t c a t a l y s t . Calculate 1. C h e m i c a l r e q u i r e m e n t s (lb N a S 0 / l b l i q u o r ) t o d e o x y g e n a t e w a t e r w i t h 9 p p m of D O a n d t o t a l l b N a S 0 2
3
2
test
3
2 . C o C l r e q u i r e m e n t s (lb) 3. V a l u e of K a l b 0 / ( h r ) ( f t ) Δ ( m g / l i t e r ) 2
3
L
2
SOLUTION Step Na S0 2
3
1. E s t a b l i s h t h e c h e m i c a l r e q u i r e m e n t s . T h e o r e t i c a l l y , 7.9 p p m o f are required to remove 1 p p m D O . Thus 7.9 χ 9.0 = 71.1 p p m of N a S 0 2
3
U t i l i z i n g a 2 0 % excess 20/100 χ 71.1 = 14.22 p p m (take 15 p p m excess) Requirements Theoretical Excess
71.1 p p m 15.0 p p m 86.1 p p m o f N a S 0 2
3
Requirements = 86.1 p p m = 86.1 χ 1 0 " lb N a S 0 / l b liquor 6
2
3
Total required: 86.1 χ 1 0 " lb N a S 0 / l b liquor χ 8.34 lb liquor/gal liquor 6
2
3
χ 150,000 gal liquor = 108 lb of N a S 0 2
3
Step 2. D e t e r m i n e t h e C o C l r e q u i r e m e n t s ( l b ) . 2
Basis: 1.5 p p m of C o
2
+
Molecular weight of C o C l : 130 2
A t o m i c weight of C o : 59 Thus 1.5 p p m C o
2 +
or 1.5 χ 130/59 = 3.3 p p m C o C l
= 3.3 χ 1 0 " lb C o C l / l b liquor 6
2
2
6.
133
Unsteady State Aeration
T h e r e f o r e , t h e l b of C o C l
required are
2
3.3 χ 1 0 " lb C o C l / l b liquor χ 8.34 lb liquor/gal liquor χ 150,000 gal liquor 6
2
= 4.13 lb C o C l
2
(minimum)
T a k e 5 lb C o C l . 2
Step
3. A s c e r t a i n t h e v a l u e of K a(hr~ ).
By p l o t t i n g i n s e m i l o g scale
l
L
g r a p h c o l u m n (5) vs. c o l u m n (1) of T a b l e 4 . 2 o n e o b t a i n s F i g . 4 . 3 . T h e n a t 15°C Ka L
= - ( s l o p e ) = - 2 . 3 0 3 [ ( l o g 1 0 - l o g 3 . 1 ) / ( 0 - 4 0 ) ] χ 60 = 1.761b0 /(hr)(ft )A(mg/liter) 3
2
5. I n t e g r a t i o n of t h e Differential E q u a t i o n for Oxygen T r a n s f e r b e t w e e n Limits I n t e g r a t i o n o f E q . (4.2) b e t w e e n t i m e s t concentrations C
L > 1
and C
L 2
ln[(C,-C
L e
and t , corresponding to
l
2
DO
, yields a)/(C -C e
L i l
-K a(t -h)
)] =
L
2
or Ka L
= 2.303 l a g [ ( C . - C
F r o m E q . (4.4) t h e v a l u e o f K a
L i l
)/(C -C s
L i 2
)]/(/ -/ ) 2
(4.4)
1
is c a l c u l a t e d f r o m o n l y t w o e x p e r i m e n t a l
L
d e t e r m i n a t i o n s o f D O . H o w e v e r , it is p r e f e r a b l e t o utilize t h e s e m i l o g l i n e a r p l o t m e t h o d with several experimental p o i n t s , since this p e r m i t s statistical averaging of errors.
6. U n s t e a d y S t a t e A e r a t i o n of A c t i v a t e d S l u d g e Liquor In aeration of activated sludge liquor, oxygen utilization (respiration rate) b y t h e m i c r o o r g a n i s m s is t a k e n i n t o a c c o u n t . E q u a t i o n (4.2) is m o d i f i e d a s follows: dCJdt
= K a(C L
sw
-C )-r
(4.5)
L
w h e r e r is t h e r a t e o f o x y g e n u t i l i z a t i o n b y t h e m i c r o o r g a n i s m s ; C s a t u r a t i o n c o n c e n t r a t i o n o f o x y g e n for t h e w a s t e w a t e r ; a n d C
L
s w
c o n c e n t r a t i o n o f d i s s o l v e d o x y g e n i n t h e a e r a t o r l i q u o r . V a l u e s o f dCJdt obtained by plotting C
L
the
the operating are
( m e a s u r e d by D O tests) vs. t i m e a n d d e t e r m i n i n g
slopes at selected t i m e intervals (Fig. 4.4). E q u a t i o n (4.5) is r e a r r a n g e d t o yield dCJdt
= (K aC -r) L
sw
-
K aC L
L
(4.6)
134
4.
E q u a t i o n (4.6) i n d i c a t e s t h a t a p l o t o f dCJdt
Theory and Practice of Aeration
(values of slopes o b t a i n e d from
F i g . 4.4) v s . C
yields a s t r a i g h t line, a s i n d i c a t e d in F i g . 4 . 5 . T h e s l o p e o f t h i s
line yields K a,
a n d r e s p i r a t i o n r a t e r is d e t e r m i n e d f r o m t h e i n t e r c e p t .
L
L
Time (t) Fig. 4.4. Determination
Fig. 4.5. Determination
of K a L
of
(unsteady
dCJdt.
state aeration
of activated
sludge
liquor).
7. S t e a d y S t a t e D e t e r m i n a t i o n of K a for t h e A c t i v a t e d S l u d g e Liquor L
A c t i v a t e d s l u d g e l i q u o r is a e r a t e d a t a r a t e j u s t sufficient t o s u p p l y t h e oxygen dCJdt
required
for
respiration
of
the
microorganisms.
In
Eq.
(4.5)
= 0, a n d t h e r e f o r e Ka L
= rl(Csw-C ) L
(4.7)
9.
135
Corrections for K a and O C L
R e s p i r a t i o n r a t e is m e a s u r e d (e.g., b y t h e W a r b u r g r e s p i r o m e t e r ) w h e n t h e d i s s o l v e d o x y g e n c o n c e n t r a t i o n b e c o m e s s t a b i l i z e d . E q u a t i o n (4.7) yields
K a. L
8. O x y g e n a t i o n C a p a c i t y (OC) I n e v a l u a t i o n o f a n a e r a t o r t h e o x y g e n t r a n s f e r r e d is e s t i m a t e d
under
s t a n d a r d conditions (SC), c o r r e s p o n d i n g t o a t e m p e r a t u r e of 20°C a n d stan d a r d a t m o s p h e r i c p r e s s u r e . R a t e o f o x y g e n t r a n s f e r r e d b y t h e a e r a t o r is r e p o r t e d a s its o x y g e n a t i o n c a p a c i t y ( O C ) , w h i c h is defined a s t h e r a t e o f o x y g e n t r a n s f e r dCjdt
a t a n initial o x y g e n c o n c e n t r a t i o n C
L
= 0 and standard
c o n d i t i o n s . F r o m E q . (4.2) O C [lb 0 / ( h r ) ( u n i t volume)] = dCJdt
= (K a) o°c(C -0)
2
L
2
=
5
(K a) o C L
20
C
s
(4.8) o r if V is t h e v o l u m e o f a e r a t i o n b a s i n , O C (lb 0 / h r ) = (K a) °c 2
L
20
C V
(4.9)
5
S i n c e t h e t e s t is p e r f o r m e d u n d e r c o n d i t i o n s o t h e r t h a n s t a n d a r d , t h e v a l u e o f Ka L
o b t a i n e d is c o r r e c t e d for t e m p e r a t u r e a n d p r e s s u r e b e f o r e a p p l i c a t i o n o f
E q . (4.9). T h e s e c o r r e c t i o n s a r e d i s c u s s e d i n S e c t i o n 9.
9. C o r r e c t i o n s for K a a n d O x y g e n a t i o n C a p a c i t y (OC) w i t h Temperature and Pressure L
9.1. T E M P E R A T U R E C O R R E C T I O N T h e o x y g e n - t r a n s f e r coefficient K a L
i n c r e a s e s w i t h t e m p e r a t u r e . T h e fol
l o w i n g t e m p e r a t u r e c o r r e c t i o n is u s e d t o d e t e r m i n e K a L
Ka L
iT)
= Ka ° L
i20
x 1.024 - > (T
C)
at 20°C: (4.10)
2O
w h e r e Τ is t h e t e m p e r a t u r e in ° C .
9.2. P R E S S U R E C O R R E C T I O N A b a r o m e t r i c c o r r e c t i o n for o x y g e n s a t u r a t i o n v a l u e C
s
in E q . (4.8) o r
(4.9) is a p p l i e d a s i n d i c a t e d b y E q . (4.11), w h i c h a s s u m e s t h a t C is d i r e c t l y s
proportional to the barometric pressure. C (corrected) = C (test) χ 29.92 in. Hg/(in. H g at test conditions) s
s
(4.11)
S i n c e o x y g e n s a t u r a t i o n is r e l a t e d t o p a r t i a l p r e s s u r e o f o x y g e n i n t h e g a s p h a s e ( H e n r y ' s l a w ) , a c o r r e c t i o n is m a d e for s a t u r a t i o n in s u b m e r g e d a e r a t i o n
136
Theory and Practice of Aeration
4.
devices (bubble aeration), where partial pressure a t t h e p o i n t of discharge exceeds atmospheric pressure d u e t o hydrostatic pressure. Oldshue [ 5 ] p r o p o s e d t h e following correction: C , s
where C C
s
m
= C , . , [ ( f t / 2 9 . 4 ) + (O /42)]
w
(4.12)
f
is t h e s a t u r a t i o n o f o x y g e n a t a e r a t i o n t a n k m i d - d e p t h ( m g / l i t e r ) ;
the saturation of oxygen at standard conditions (mg/liter); P the pressure
S5
b
( p s i a ) a t t h e d e p t h o f a i r r e l e a s e ; a n d O t h e o x y g e n i n exit g a s ( % ) . t
F o r a e r a t i o n , O = 2 1 % o f 0 , a n d E q . (4.12) yields t
2
C
= C
Stm
χ [ ( Λ / 2 9 . 4 ) + 0.5]
StS
(4.13)
T h u s , t h e value for t h e oxygenation capacity for surface a e r a t o r s ( n o h y d r o s t a t i c c o r r e c t i o n r e q u i r e d ) is g i v e n b y E q . (4.9), w h i c h is m o d i f i e d a s follows [ E q . ( 4 . 1 4 ) ] : O C = (K a) °c L
w h e r e (K a) °c L
x C (corrected) χ V
20
(4.14)
s
is c a l c u l a t e d f r o m E q . (4.10), a n d C ( c o r r e c t e d ) is g i v e n b y
20
s
E q . (4.11). T h e r e f o r e , f o r s u r f a c e a e r a t o r s OC = Ka L
χ 1.024< - > χ C (test) χ 20
iT)
τ
s
?* ' * χ V (in. H g a t test conditions) 2
9
i n
H
(4.15) F o r b u b b l e a e r a t o r s , o n e utilizes E q . (4.16). O C = (K a) ' L
w h e r e (K a) <>c L
x C
20 C
χ V
StS
(4.16)
is c a l c u l a t e d f r o m E q . (4.10) a n d C
20
f r o m E q . (4.12) [ o r
5S
Eq. (4.13)]. Therefore, for bubble aerators O C = J W > x 1.024<—>
x
[
W
2
^ ;
+
Q
5
]
χ V
(4.17)
Example 4.2 F o r t h e surface a e r a t o r in E x a m p l e 4 . 1 , calculate 1. V a l u e o f K a c o r r e c t e d t o 2 0 ° C L
2. V a l u e o f C c o r r e c t e d t o n o r m a l a t m o s p h e r i c p r e s s u r e s
3. Oxygenation capacity (lb 0 / h r ) 2
SOLUTION Step 1.
is c a l c u l a t e d f r o m E q . (4.10).
(K a) o L
20
C
(K a) « L
20
c
= 1.76 x ( 1 . 0 2 4 )
( 2 0
-
1 5 )
= 1.98 h r "
1
Step 2. C ( c o r r e c t e d ) is c a l c u l a t e d f r o m E q . (4.11). s
C (corrected) = 10.2(29.92/28.0) = 10.9 mg/liter = 10.9 χ 1 0 " lb 0 / l b liquor 6
s
2
10.
137
Transfer Efficiency of Aeration Units
Step 3. O x y g e n a t i o n c a p a c i t y is c a l c u l a t e d f r o m E q . (4.14). O C = 1.98 1/hr χ 10.9 χ 1 0 " lb 0 / l b liquor χ 150,000 gal χ 8.34 lb liquor/gal 6
2
= 27.0 lb 0 / h r 2
10. T r a n s f e r Efficiency of Aeration Units T r a n s f e r efficiency ( T E ) of a e r a t i o n u n i t s is c o m m o n l y e x p r e s s e d in t e r m s o f m a s s of o x y g e n a c t u a l l y t r a n s f e r r e d p e r ( H P χ h r ) of w o r k e x p e n d i t u r e , i.e. [ E q . (4.18)], T E = lb 0
2
transferred/(HP χ hr)
(4.18)
S o m e t i m e s , n o m i n a l H P ( n a m e p l a t e H P ) of t h e a e r a t o r is utilized s i m p l i c i t y in e v a l u a t i n g T E . I t is m o r e a c c u r a t e t o b a s e c a l c u l a t i o n
for
upon
actual H P (blade H P ) m e a s u r e d d u r i n g the test b y a w a t t meter o r a n energy c o u n t e r . W h e n t h e p o w e r f a c t o r (cos P F ) is k n o w n , b l a d e H P is c a l c u l a t e d f r o m E q . (4.19) [ 2 ] . Blade H P = (line voltage) χ (line amperage) χ [cos P F ( 3 ) J 1/2
χ (1/746) χ ( m o t o r efficiency) χ (gear efficiency)
(4.19)
w h e r e 1/746 is t h e c o n v e r s i o n f a c t o r H P / W . V a l u e s o f T E u p t o 7 l b 0 / 2
( H P χ h r ) a r e r e p o r t e d for surface a e r a t o r s , a l t h o u g h f o r m o s t u n i t s t h e v a l u e s o f T E r a n g e f r o m 2 t o 4 lb 0 / ( Η Ρ χ h r ) . F o r t u r b i n e a e r a t o r s t h e u s u a l r a n g e 2
is 2 - 3 l b 0 / ( H P x h r ) . 2
E x a m p l e 4.3 F o r t h e a e r a t o r in E x a m p l e s 4.1 a n d 4 . 2 , r e p o r t a e r a t o r efficiency in t e r m s of n a m e p l a t e H P a n d b l a d e H P . T h e f o l l o w i n g d a t a a r e a v a i l a b l e in a d d i t i o n t o t h o s e f r o m E x a m p l e s 4.1 a n d 4 . 2 : D r a w n voltage: 225 V (average) A m p e r a g e : 20 A (average) cos P F (measured): 0.85 M o t o r efficiency (estimated): 90% G e a r efficiency (estimated): 90% SOLUTION
O x y g e n t r a n s f e r r e d h a d b e e n d e t e r m i n e d in E x a m p l e 4 . 2 a s
27.0 lb 0 / h r . Therefore 2
T E = 27.0/8.0 = 3.38 lb 0 / Η Ρ χ hr 2
(nameplate)
F r o m E q . (4.19) Blade H P = (225) (20) [0.85 χ (3)
1 / 2
] ( 1/746) (0.9) (0.9) = 7.19
Then T E = (27.0/7.19) = 3.76 lb 0 / Η Ρ χ h r 2
(blade)
138
4.
Theory and Practice of Aeration
1 1 . Effect of W a s t e w a t e r C h a r a c t e r i s t i c s on Oxygen T r a n s f e r W h e n o x y g e n is s u p p l i e d for a e r o b i c b i o l o g i c a l t r e a t m e n t o f w a s t e w a t e r , it is n e c e s s a r y t o define a c o r r e c t i o n f a c t o r w h i c h r e l a t e s o x y g e n t r a n s f e r t o t h e n a t u r e of t h e w a s t e . T h i s c o r r e c t i o n f a c t o r α r e l a t e s t h e o v e r a l l m a s s t r a n s f e r coefficient (K a) o f t h e w a s t e w a t e r t o t h a t o f t h e t a p w a t e r [ E q . ( 4 . 2 0 ) ] . L
α = K a ( w a s t e w a t e r ) / ^ a (tap water)
(4.20)
L
T h e r e a r e m a n y v a r i a b l e s w h i c h affect t h e m a g n i t u d e of oc. T h e s e i n c l u d e (1) t e m p e r a t u r e o f t h e m i x e d l i q u o r ; (2) n a t u r e o f t h e d i s s o l v e d o r g a n i c a n d m i n e r a l c o n s t i t u e n t s ; (3) level o f a g i t a t i o n o f a e r a t i o n b a s i n , u s u a l l y e x p r e s s e d in t e r m s of H P p e r 1000 gal of b a s i n v o l u m e ; (4) c h a r a c t e r i s t i c s o f t h e a e r a t i o n e q u i p m e n t ; a n d (5) l i q u i d d e p t h a n d g e o m e t r y o f a e r a t i o n b a s i n . T h e t e m p e r a t u r e effect is a t t r i b u t a b l e t o t e m p e r a t u r e d e p e n d e n c e o f t h e l i q u i d film coefficient K . F i g u r e 4.8 i l l u s t r a t e s t y p i c a l t e m p e r a t u r e effect o n v a l u e s o f a. L
Since t h e n a t u r e o f d i s s o l v e d o r g a n i c a n d m i n e r a l c o n s t i t u e n t s affects a, its v a l u e is e x p e c t e d t o i n c r e a s e d u r i n g t h e c o u r s e of b i o l o g i c a l o x i d a t i o n , b e c a u s e d i s s o l v e d o r g a n i c m a t e r i a l s affecting t h e t r a n s f e r r a t e a r e r e m o v e d i n t h e b i o l o g i c a l p r o c e s s . A t y p i c a l s i t u a t i o n is s h o w n in F i g . 4.6. A s t h e final effluent a p p r o a c h e s p u r i t y o f t a p w a t e r , t h e v a l u e o f α a p p r o a c h e s u n i t y asymptotically. Effect o f m i x i n g i n t e n s i t y in a e r a t i o n b a s i n ( u s u a l l y e x p r e s s e d in t e r m s o f H P / 1 0 0 0 g a l ) is i l l u s t r a t e d b y F i g . 4 . 7 , w h i c h is a t y p i c a l c u r v e for a w a s t e w a t e r c o n t a i n i n g s u r f a c e - a c t i v e a g e n t s . A s e x p l a i n e d in S e c t i o n 2, a t l o w m i x i n g i n t e n s i t i e s t h e r a t e o f o x y g e n t r a n s f e r is c o n t r o l l e d b y t h e p a s s a g e o f t h e o x y g e n m o l e c u l e s t h r o u g h t h e l i q u i d i n t e r f a c e film b y m o l e c u l a r diffusion. T h e p r e s e n c e of s u r f a c e - a c t i v e a g e n t s i n h i b i t s m o l e c u l a r diffusion t h r o u g h
1.0
α 0.5
% B O D removed
0
100%
Raw waste Fig. 4.6.
Plot
of a vs. % BOD
removed.
11.
Wastewater Effect on 0
139
Transfer
2
Low turbulence
Moderate turbulence
High turbulence
S
I.O
0.5
H P / I O O O gal Fig. 4.7.
Plot
of a vs. mixing
intensity.
the interface, a n d t h u s α decreases. A t high mixing intensities, however, the o x y g e n t r a n s f e r is c o n t r o l l e d b y t h e r a t e of s u r f a c e r e n e w a l , a n d t h u s a t c o n ditions of high turbulence, α increases with the degree of mixing intensity. F i g u r e 4.8 s h o w s s i m u l t a n e o u s l y t h e effect o f t e m p e r a t u r e a n d m i x i n g i n t e n sity o n t h e v a l u e o f α for a t y p i c a l w a s t e w a t e r . I.O 0.9 α
0.8 0.7 0.6 0
0.05
0.I5
0.25
0.35
Power level; HP/IOOO gal Fig. 4.8.
Plot
of a vs. power
level at two
different
temperatures
[3].
A n i n t e r e s t i n g i l l u s t r a t i o n o f t h e effect o f c h a r a c t e r i s t i c s o f a e r a t i o n e q u i p m e n t o n t h e v a l u e of α is b u b b l e a e r a t i o n (air diffusion o r t u r b i n e u n i t s ) in t h e presence of surface-active agents. Presence of these agents decreases b u b b l e size, a n d t h u s i n c r e a s e s interfacial a r e a p e r u n i t v o l u m e . * U n d e r t h e s e c o n d i t i o n s t h e K a v a l u e o f t h e w a s t e w a t e r u s u a l l y i n c r e a s e s , b e c a u s e i n c r e a s e in a e x c e e d s d e c r e a s e of K c a u s e d b y t h e s u r f a c e b a r r i e r . T h i s i n c r e a s e in K a of t h e w a s t e w a t e r r e s u l t s in a c o r r e s p o n d i n g i n c r e a s e o f α [ E q . ( 4 . 2 0 ) ] . L
L
* Since a — A/V= kir /k r therefore K a) increases. 2
3
2
L
L
= K(\/r),
thus as radius (r) o f the bubble decreases, a (and
140
4.
Theory and Practice of Aeration
12. L a b o r a t o r y D e t e r m i n a t i o n of O x y g e n - T r a n s f e r Coefficient α T h i s d e t e r m i n a t i o n is b a s e d d i r e c t l y o n t h e definition o f α g i v e n b y E q . (4.20). V a l u e s o f K a o f w a s t e w a t e r a n d t a p w a t e r a r e d e t e r m i n e d a s d e s c r i b e d in S e c t i o n 4. I t is i n t e r e s t i n g t o m a k e p a r a l l e l d e t e r m i n a t i o n s for w a s t e w a t e r a n d t a p w a t e r a t different m i x i n g i n t e n s i t i e s a n d a t different t e m p e r a t u r e s , in o r d e r t o o b t a i n c u r v e s s u c h a s t h e o n e s s h o w n in F i g . 4 . 8 . L
13. Classification of A e r a t i o n E q u i p m e n t — O x y g e n - T r a n s f e r Efficiency A e r a t i o n e q u i p m e n t c o m m o n l y e m p l o y e d in w a s t e w a t e r t r e a t m e n t is classified i n t o t h r e e c a t e g o r i e s : (1) a i r diffusion u n i t s , (2) t u r b i n e a e r a t i o n u n i t s , a n d (3) surface a e r a t i o n u n i t s . W h e n c o m p a r i n g a e r a t i o n devices o r e v a l u a t i n g a b s o r p t i o n of o x y g e n in v a r i o u s w a s t e w a t e r s , it is useful t o c o n sider t h e o x y g e n - t r a n s f e r efficiency, w h i c h is defined a s [ E q . (4.21)] weight of 0 absorbed/unit time e = — , . .— (4.21) weight of
x
1 0 0
2
F o r the activated sludge process, m o s t of the p o w e r expenditure by the a e r a t o r s is for t h e p u r p o s e o f p r o v i d i n g o x y g e n t r a n s f e r . F o r l a r g e v o l u m e b i o l o g i c a l u n i t s ( n a m e l y , a e r a t e d l a g o o n s ) , t h e l a r g e r s h a r e of t h e p o w e r e x p e n d i t u r e is f o r m a i n t e n a n c e o f a n a d e q u a t e level o f a g i t a t i o n . C h a r a c t e r istics a n d specifications f o r t h e t h r e e c a t e g o r i e s o f a e r a t i o n e q u i p m e n t a r e d i s c u s s e d i n d i v i d u a l l y in t h e n e x t t h r e e s e c t i o n s .
14. Air Diffusion U n i t s 14.1. T Y P E 1 . F I N E B U B B L E D I F F U S E R S S m a l l orifice diffusion u n i t s s u c h a s p o r o u s m e d i a , p l a t e s , o r t u b e s a r e c o n s t r u c t e d o f silicon d i o x i d e ( S i 0 ) o r a l u m i n u m o x i d e ( A 1 0 ) g r a i n s , h e l d 2
2
3
in a p o r o u s m a s s w i t h a c e r a m i c b i n d e r . O t h e r u n i t s e m p l o y e d c o n s i s t o f Saran, Dacron, or nylon-wrapped tubes.
14.
141
Air Diffusion Units
Small bubbles, having a high surface a r e a per unit v o l u m e , p r o v i d e g o o d o x y g e n - l i q u i d c o n t a c t , l e a d i n g t o relatively h i g h v a l u e s o f t h e o x y g e n - t r a n s f e r efficiency. D i a m e t e r o f t h e b u b b l e s r e l e a s e d f r o m t h e s e diffusers a r e 2 . 0 - 2 . 5 m m , t h e o x y g e n - t r a n s f e r efficiency d e p e n d i n g o n b u b b l e size, (e's f r o m 5 t o 1 5 % a r e c o m m o n . ) S t a n d a r d p o r o u s diffuser u n i t s a r e d e s i g n e d t o deliver 4 - 1 5 S C F M of air per unit. A d i s a d v a n t a g e of s m a l l orifice diffusion u n i t s is t h e h i g h m a i n t e n a n c e c o s t s in s o m e a p p l i c a t i o n s o w i n g t o orifice c l o g g i n g . A i r filters a r e c o m m o n l y u s e d t o c l e a n a n d e l i m i n a t e d u s t p a r t i c l e s t h a t m i g h t c l o g t h e diffusers. A s k e t c h o f a fine b u b b l e a i r diffusion s y s t e m is s h o w n in F i g . 4 . 9 . Porous ceramic diffuser units (tubes)
Fig. 4.9. Sketch of a fine bubble of porous ceramic diffusers.
air diffusion
system
consisting
of a
series
14.2. T Y P E 2. L A R G E B U B B L E D I F F U S E R S T h e s e u n i t s e m p l o y l a r g e orifice o r h y d r a u l i c s h e a r devices. L a r g e b u b b l e u n i t s h a v e l o w e r o x y g e n - t r a n s f e r efficiency t h a n fine b u b b l e u n i t s , since t h e interfacial a r e a for o x y g e n t r a n s f e r is c o n s i d e r a b l y less. T h e y h a v e t h e a d v a n tage, however, of n o t requiring air
filters
a n d o f g e n e r a l l y r e q u i r i n g less
m a i n t e n a n c e . S k e t c h e s o f t w o t y p i c a l l a r g e b u b b l e a i r diffuser u n i t s a r e s h o w n in F i g . 4.10.
Air Fig.
4.10.
Sketches
of typical
large
bubble
diffuser
units.
142
4.
Theory and Practice of Aeration
14.3. P E R F O R M A N C E O F A I R D I F F U S I O N U N I T S P e r f o r m a n c e d a t a for a i r diffusion u n i t s a r e a v a i l a b l e a s g r a p h s f o r t h e l b of 0
2
t r a n s f e r r e d / h r p e r a e r a t i o n u n i t v s . t h e a i r flow p e r u n i t . A s a m p l e o f
t y p i c a l d a t a f o r a S a r a n - w r a p p e d t u b e ( s m a l l b u b b l e ) is s h o w n i n F i g . 4 . 1 1 .
2
5
10
20
Air f l o w / u n i t ( S C F M ) Fig. 4.11. Oxygen-transfer data from Saran tubes with permission, copyright by the University of Texas Press.)
in water
[2].
{Reprinted
Eckenfelder [ 1 ] r e c o m m e n d s the following empirical equation for corre l a t i n g p e r f o r m a n c e o f a i r diffusion u n i t s , a n d t a b u l a t e s v a l u e s o f c h a r a c t e r istic c o n s t a n t s f o r s e v e r a l t y p e s o f diffusers. Ν = CG^- (H IW ){C -C ) n)
m
sw
w h e r e Ν is t h e l b o f 0
2
χ 1.024 - > χ a
p
(Γ
L
transferred/(hr)(aeration
(4.22)
20
unit); G
s
the air
flow
( S C F M / a e r a t i o n u n i t ; S C F M are m e a s u r e d at 1 a t m a n d 6 0 ° F ) ; Η the liquid d e p t h (ft); Wthe
a e r a t i o n t a n k w i d t h (ft); C
s w
the saturation concentration
o f D O in w a s t e w a t e r ( m g / l i t e r , a t t a n k m i d - d e p t h ) ; C
L
the operating concen
tration of D O (mg/liter, usually 0.5-1.5 mg/liter); Τ the t e m p e r a t u r e (°C); α t h e o x y g e n - t r a n s f e r coefficient of t h e w a s t e w a t e r [ d e f i n e d b y E q . ( 4 . 2 0 ) ] ; a n d C,n,m,p
t h e constants characteristic of the aeration device.
F r o m Eq. (4.2), N=
K aV(C w-C ) L
S
(4.23)
L
B y c o m p a r i n g E q s . (4.22) a n d (4.23) t h e r e s u l t is K aV
= CG< - \H IW ) 1 n
L
Term Ka
has units of h r
L
If (C
sw
—C) L
m
- 1
p
χ 1.024 ( Γ
2 0 )
χ α
(ft /hr) 3
(4.24)
a n d Κ is t h e v o l u m e o f t h e a e r a t i o n t a n k i n f t . 3
is g i v e n in m g / l i t e r it is m u l t i p l i e d b y a c o n v e r s i o n f a c t o r t o
e x p r e s s it i n l b / f t , s o t h a t Ν is o b t a i n e d i n l b / h r . 3
14.
143
Air Diffusion Units
(Csw-C )
= mg/liter χ g/1000 m g χ lb/454 g χ 28.3 liter/ft
L
(C w-C ) S
F a c t o r 6.23 χ 1 0 " t h a t (C
SW
5
χ 6.23 χ I O "
L
= lb/ft
5
3
3
is c o n v e n i e n t l y i n c l u d e d in c o n s t a n t C in E q . (4.22), s o
— C ) e n t e r s t h e e q u a t i o n d i r e c t l y in m g / l i t e r , a n d Ν is o b t a i n e d in L
lb/hr.
14.4. D E S I G N P R O C E D U R E F O R A E R A T I O N S Y S T E M S UTILIZING AIR D I F F U S I O N U N I T S F u n d a m e n t a l i n f o r m a t i o n r e q u i r e d is a s f o l l o w s : 1. V o l u m e o f a e r a t i o n t a n k (V), c a l c u l a t e d f r o m b i o l o g i c a l r e a c t o r r e q u i r e m e n t s ( C h a p t e r 5, S e c t i o n 7) 2. O x y g e n r e q u i r e m e n t s (lb 0 / h r ) , a l s o c a l c u l a t e d f r o m b i o l o g i c a l r e a c t o r r e q u i r e m e n t s ( C h a p t e r 5, S e c t i o n 7) 2
3. O p e r a t i n g t e m p e r a t u r e 4. Operating D O ( C , mg/liter), usually 0.5-1.5 mg/liter except nitrification u n i t s , w h e n v a l u e s a b o v e 2 m g / l i t e r a r e e m p l o y e d 5. O x y g e n - t r a n s f e r coefficient α L
for
6. P e r f o r m a n c e d a t a for t h e a i r diffuser u n i t s [ a v a i l a b l e a s g r a p h s , see F i g . 4 . 1 1 , o r e x p r e s s e d in t e r m s of v a l u e s f o r c o n s t a n t s C, n, m, a n d ρ in E q . (4.22)] Step I. Select a t a n k d e p t h Η u s u a l l y b e t w e e n 10 a n d 15 ft. Step 2. C r o s s - s e c t i o n a l a r e a is t h e n A = VjH. Step 3. F o r a e r a t i o n t a n k s w i t h r e c t a n g u l a r c r o s s s e c t i o n , select a w i d t h W o f a p p r o x i m a t e l y twice t h e t a n k d e p t h . T h i s is n e c e s s a r y in o r d e r t o m a i n t a i n a d e q u a t e m i x i n g . T h e n t a n k l e n g t h L = A\W. F o r a e r a t i o n t a n k s w i t h circular cross section, calculate diameter from D = (4Α/π) . ί/2
Step 4. Select a i r flow r a t e G p e r a i r diffusion u n i t . U s u a l r a n g e for v a l u e s o f G is 4 - 8 S C F M / u n i t a n d 4 - 1 6 S C F M / u n i t for fine a n d l a r g e b u b b l e diffusers, respectively. s
s
Step 5. V a l u e C [ u s e d in E q . ( 4 . 2 2 ) ] is c o m p u t e d a t t a n k m i d - d e p t h f r o m E q . (4.12) [ o r E q . ( 4 . 1 3 ) ] , i.e., C = C . s w
s w
s > m
Step 6. O x y g e n a t i o n c a p a c i t y p e r a e r a t i o n u n i t \_N = lb 0 transferred/ ( h r ) ( u n i t ) ] is e s t i m a t e d f r o m m a n u f a c t u r e r ' s d a t a (e.g., F i g . 4.11) o r c o m p u t e d f r o m E q . (4.22). Step 7. F r o m o x y g e n r e q u i r e m e n t s (lb 0 / h r ) c a l c u l a t e d in C h a p t e r 5, S e c t i o n 7 a n d v a l u e o f Ν c a l c u l a t e d i n S t e p 6, c a l c u l a t e t h e n u m b e r o f a e r a t i o n u n i t s r e q u i r e d t o t r a n s f e r r e q u i r e d a m o u n t of o x y g e n . 2
2
N o . of units =
lb 0 / h r (required) 2
Ν
Step 8. P r e p a r e a l a y o u t of t h e a e r a t i o n t a n k a n d d e t e r m i n e t h e s p a c i n g b e t w e e n t h e a e r a t i o n u n i t s . M i n i m u m s p a c i n g is a b o u t 6 in. a n d m a x i m u m
144
4.
Theory and Practice of Aeration
b e t w e e n 2 4 a n d 3 0 in. T h i s is n e c e s s a r y in o r d e r t o m a i n t a i n s o l i d s i n s u s p e n s i o n a n d t o m i n i m i z e c o a l e s c e n c e of a i r b u b b l e s . If s p a c i n g s c a l c u l a t e d fall o u t s i d e t h i s r a n g e , d o u b l e r o w s o r a d j u s t m e n t in t h e n u m b e r o f u n i t s (selection o f different a i r flow r a t e G ) a r e m a d e . s
Step 9. C o m p u t e t o t a l a i r flow. Total air flow = G x (no. units) s
(SCFM)
Step 10. C o m p u t e r e q u i r e d h o r s e p o w e r o f t h e b l o w e r . H P = [(pressure d r o p , psi) χ ( S C F M ) χ 144]/(33,000)(ε ) Μ
w h e r e 33,000 a n d 144 a r e t h e c o n v e r s i o n f a c t o r s for ( f t l b f / m i n ) / H P i n . / f t , respectively, a n d e 2
2
m
and
is t h e m e c h a n i c a l efficiency ( e s t i m a t e d ) . U s u a l l y
a 6 - 1 0 psi p r e s s u r e d r o p is a d o p t e d f o r t h e b l o w e r . Step
11. C o m p u t e o x y g e n a t i o n efficiency f r o m E q . (4.21), w h e r e t h e
n u m e r a t o r w a s c a l c u l a t e d in S t e p 6 a n d t h e d e n o m i n a t o r o b t a i n e d
from
v a l u e s of G selected in S t e p 4 (it e q u a l s a p p r o x i m a t e l y 2 3 . 2 % o f t h e w e i g h t s
o f a i r c o r r e s p o n d i n g t o G ). T h e w e i g h t o f a i r c o r r e s p o n d i n g t o G is c a l c u l a t e d s
s
from the ideal gas equation. F r o m PV = NRT
= (weight/molecular weight) χ
RT
then Weight of air = (molecular weight) (PV)jR
Τ
w h e r e m o l e c u l a r w e i g h t = 29 l b / l b m o l e ( a v e r a g e m o l e c u l a r w e i g h t o f a i r ) Ρ = 1 a t m , V=
G
R = 0 . 7 3 ( a t m ) ( f t ) / ( l b m o l e ) ( ° R ) , a n d T= 3
S9
520°R (60°F).
Therefore Weight of air = (29 χ 1 χ G )/(0.73 χ 520) = 0.076G, lb/min s
a n d [ E q . (4.25)] Weight of 0 / m i n = 0.232 χ 0.076G, = 0.0176G, 2
(4.25)
15. T u r b i n e A e r a t i o n Units 15.1. D E S C R I P T I O N O F U N I T A s k e t c h of a t y p i c a l t u r b i n e a e r a t i o n u n i t is s h o w n in F i g . 4 . 1 2 . T h e s e u n i t s entrain a t m o s p h e r i c oxygen by surface aeration a n d disperse compressed air by a shearing action employing a rotating turbine or agitator. Air bubbles discharged from a pipe or sparger below the agitator are broken d o w n by the shearing action of the high speed rotating blades of the agitator. F o r systems of l o w o x y g e n u t i l i z a t i o n r a t e , o x y g e n is s u p p l i e d b y t h e flow of a i r selfi n d u c e d f r o m a n e g a t i v e h e a d p r o d u c e d b y t h e r o t o r ( s u c t i o n effect). F o r s y s t e m s o f h i g h e r o x y g e n u t i l i z a t i o n r a t e a b l o w e r o r c o m p r e s s o r is n e e d e d .
15.
145
Turbine Aeration Units
Rotor-^i, ι ^-Liquid level
Bottom Fig. 4.12. Turbine
aeration
unit.
15.2. P E R F O R M A N C E O F T U R B I N E A E R A T I O N UNITS M a i n v a r i a b l e s t o b e c o n s i d e r e d a r e a i r flow, d i a m e t e r (D), a n d s p e e d o f i m p e l l e r . T h e s e v a r i a b l e s d e t e r m i n e b u b b l e size a n d d e g r e e o f a g i t a t i o n i n t h e t a n k , t h u s affecting t h e o v e r a l l o x y g e n - t r a n s f e r coefficient K a. L
ance data for turbine aeration units are available from information
taking
these
variables
into
consideration.
Perform
manufacturer's Eckenfelder
[1]
r e c o m m e n d s t h e following empirical equation for correlating p e r f o r m a n c e of turbine aeration u n i t s : Ν = CG S D (C n
x
w h e r e Ν is t h e l b 0
- C ) 1.024 "
y
s
(r
sw
a
2 0 )
L
(4.26)
t r a n s f e r r e d / ( h r ) ( a e r a t i o n u n i t ) ; G t h e a i r flow ( S C F M /
2
s
a e r a t i o n u n i t ) ; S t h e peripheral speed of t h e impeller (ft/sec); D t h e impeller d i a m e t e r (ft, see F i g . 4 . 1 2 ) ; C water (mg/liter); C
L
s w
the saturation D O concentration in waste
t h e o p e r a t i n g D O c o n c e n t r a t i o n ( m g / l i t e r ) ; a n d C , n, x, y
the constants characteristic of t h e aeration device.* C o m p a r i n g E q s . (4.23) a n d (4.26) t h e r e s u l t is E q . (4.27). K aV L
where term K a 3
sw
n
s
x
y
χ 1.024 ( Γ
2 0 )
α
(ft /hr) 3
(4.27)
h a s u n i t s o f h r " *, a n d V is t h e v o l u m e o f t h e a e r a t o r t a n k i n
L
f t . If (C —
= CG S D
C ) in E q . (4.26) is g i v e n i n m g / l i t e r , its v a l u e is m u l t i p l i e d b y L
f a c t o r 6.23 χ 1 0 " t o o b t a i n Ν i n l b / h r , a s s h o w n i n S e c t i o n 1 4 . 3 . 5
15.3.
POWER R E Q U I R E M E N T S FOR T U R B I N E AERATORS
P o w e r is r e q u i r e d f o r t w o p u r p o s e s : ( 1 ) o p e r a t i o n o f r o t o r ( c o r r e s p o n d i n g h o r s e p o w e r is d e s i g n a t e d h e n c e a s HP )
r 9
a n d (2) o p e r a t i o n o f c o m p r e s s o r o r
* Typical values for n, JC, and y (depending o n impeller geometry) [ 4 ] : n, 0 . 4 - 0 . 9 ; x, 1.2-2.4; and y, 0 . 6 - 1 . 8 .
146 blower
4.
(corresponding
horsepower
Theory and Practice of Aeration
is d e s i g n a t e d
h e n c e a s HP ).
Power
C
d r a w n b y t h e r o t o r is c o m p u t e d f r o m t h e r e l a t i o n s h i p [ 4 ] HP
= C'D S m
r
(4.28)
p
C , m, a n d ρ a r e c o n s t a n t s c h a r a c t e r i s t i c o f t h e a e r a t i o n d ev i ce. A c t u a l d r a w n h o r s e p o w e r d e c r e a s e s a s a i r flow is i n c r e a s e d u n d e r t h e i m p e l l e r d u e
to
d e c r e a s e d d e n s i t y of t h e a e r a t e d m i x t u r e . F o r t h i s r e a s o n , h o r s e p o w e r c a l c u l a t e d f r o m E q . (4.28) is r e f e r r e d t o a s u n g a s s e d h o r s e p o w e r . E q u a t i o n (4.28) is r e w r i t t e n a s HP
= C"D n*
(4.29)
m
r
w h e r e η is e x p r e s s e d i n r e v o l u t i o n s / s e c . S i n c e S is t h e p e r i p h e r a l s p e e d i n ft/sec, η a n d S a r e r e l a t e d a s [ E q . ( 4 . 3 0 ) ] η = S/πϋ
= (ft/sec)/(ft/rev) = rev/sec
(4.30)
w h e r e %D is t h e p e r i m e t e r of t h e c i r c u m f e r e n c e d e s c r i b e d b y t h e r o t a t i o n of the impeller. Typical values of exponents m a n d ρ are [4] 4.8 ^ m ^ 5.3 2.0 < ρ ^ 2.5 U n g a s s e d h o r s e p o w e r is c o r r e l a t e d t o a c t u a l h o r s e p o w e r . A c o r r e l a t i o n is p r e s e n t e d in F i g . 4 . 1 3 . P o w e r d r a w n b y t h e c o m p r e s s o r is c a l c u l a t e d f r o m HP
C
= (pressure d r o p , psi) χ ( S C F M ) χ 144/(33,000)(e )
(4.31)
m
w h e r e s is t h e e s t i m a t e d t u r b i n e efficiency. m
Legend HP
a
:
actual HP
HP : ungassed HP r
[equations
G
s
A
R
(4.28),(4.29)]
: air flow, S C F M : area of circle described by the rotation of the impeller = ( I / 4 ) T T D 2
0
20
40
60
G /A S
Fig.
4.13. Effect
80
100
R
of air rate on turbine
horsepower
[4],
N e x t d e t e r m i n e t h e o p t i m u m p o w e r split b e t w e e n r o t o r a n d c o m p r e s s o r . A c o r r e l a t i o n b e t w e e n o x y g e n - t r a n s f e r efficiency [ e x p r e s s e d a s ( l b 0
2
trans
f e r r e d ) / ^ χ h r ) ] a n d a f a c t o r P defined a s [ E q . ( 4 . 3 2 ) ] d
P
d
= HP IHP r
c
(4.32)
15.
147
Turbine Aeration Units
h a s b e e n d e v e l o p e d b y Q u i r k [ 6 ] , a n d its u t i l i z a t i o n is s u m m a r i z e d
by
E c k e n f e l d e r [ 1 ] . P r e p r e s e n t s t h e p o w e r split b e t w e e n r o t o r a n d c o m p r e s s o r . d
A t y p i c a l c o r r e l a t i o n c u r v e is s h o w n in F i g . 4.14.
Maximum (Optimum split)
/ /
High air rates
Low air rates
Γ
Ρ3
Fig.
4.14.
Correlation
r
for power
split
\
= ..Ο
P =HP /HP d
\
c
for turbine
aeration
units.
T h e v a l u e i n d i c a t e d in F i g . 4.14 as P * is t h e o p t i m u m p o w e r s p l i t c o r r e s p o n d i n g t o t h e m a x i m u m o x y g e n - t r a n s f e r efficiency. I n m o s t c a s e s P* is approximately unity, this implying a n equal p o w e r expenditure by the turbine a n d t h e c o m p r e s s o r . A t e x t r e m e l y h i g h a i r r a t e s ( h i g h v a l u e s o f HP ) values of P a r e less t h a n 1.0, i.e., HP > HP a n d P < 1.0. U n d e r t h e s e c o n d i t i o n s , large a i r b u b b l e s a n d flooding o f t h e i m p e l l e r yield p o o r o x y g e n a t i o n efficien cies. O n t h e o t h e r h a n d , a t v e r y l o w a i r r a t e s P > 1.0 a n d t o o m u c h t u r b i n e h o r s e p o w e r is s p e n t in m i x i n g t h e l i q u o r . d
C 9
d
C
r
d
d
15.4. D E S I G N P R O C E D U R E F O R A E R A T I O N S Y S T E M S UTILIZING T U R B I N E A E R A T I O N UNITS F o r f u n d a m e n t a l i n f o r m a t i o n r e q u i r e d see i t e m s 1-5 for a i r diffusion u n i t s ( S e c t i o n 14.4), t h e n o b t a i n p e r f o r m a n c e d a t a for t h e t u r b i n e a e r a t i o n u n i t s a v a i l a b l e f r o m m a n u f a c t u r e r ' s i n f o r m a t i o n , o r e x p r e s s e d in t e r m s o f v a l u e s for t h e c o n s t a n t s in E q . (4.26). Step I. Select a t a n k d e p t h / / , u s u a l l y b e t w e e n 15 a n d 2 0 ft. I n s o m e c a s e s d e e p e r liquid d e p t h s a r e e m p l o y e d . Step 2. C r o s s - s e c t i o n a l a r e a is t h e n A = V/H. Step 3. Select a r a t i o r = D/T, w h e r e D is d i a m e t e r of t h e t u r b i n e a n d Τ t h e " d i a m e t e r " o f t h e t a n k . F o r t a n k s o f c i r c u l a r c r o s s s e c t i o n , t h e m e a n i n g of r a t i o D/T is s t r a i g h t f o r w a r d . F o r t a n k s w i t h r e c t a n g u l a r o r s q u a r e c r o s s s e c t i o n s , select a v a l u e for Τ b a s e d o n g e o m e t r y o f t h e s y s t e m . S e l e c t i o n o f Τ
148
Theory and Practice of Aeration
4.
f o r a r e c t a n g u l a r t a n k w i t h t w o t u r b i n e a e r a t o r s is i l l u s t r a t e d b y F i g . 4 . 1 5 (T e q u a l s t h e d i a m e t e r o f influence o f t h e a e r a t i o n u n i t ) . T y p i c a l D/T r a t i o s are 0.1-0.2.
Fig. 4.15.
Ratio
r = D/T
ι ι
I
ι
for a rectangular
tank
with
two
turbine
aerators.
Step 4. Select a t a n k w i d t h Τ a p p r o x i m a t e l y twice t h e t a n k d e p t h . T h e r e f o r e t u r b i n e d i a m e t e r is D = Τ χ r. split
Step 5. F r o m F i g . 4 . 1 4 ( m a n u f a c t u r e r ' s d a t a ) d e t e r m i n e o p t i m u m p o w e r P *. d
Step 6. Select a i r flow r a t e p e r u n i t G ( S C F M / a e r a t i o n u n i t ) . T y p i c a l v a l u e s a r e b e t w e e n 2 0 0 a n d 1500 S C F M . s
Step 7. V a l u e C [ t o b e u s e d i n E q . (4.26)] is c o m p u t e d a t t a n k m i d d e p t h f r o m E q . (4.12) [ o r E q . ( 4 . 1 3 ) ] , i.e., C = C . s w
s w
s > m
Step 8. O x y g e n a t i o n c a p a c i t y p e r a e r a t i o n u n i t [N = lb 0 transferred/ ( h r ) ( u n i t ) ] is e s t i m a t e d f r o m m a n u f a c t u r e r ' s d a t a o r c o m p u t e d f r o m E q . (4.26). 2
Step 9. F r o m o x y g e n r e q u i r e m e n t s ( l b 0 / h r ) c a l c u l a t e d in C h a p t e r 5, S e c t i o n 7 a n d v a l u e o f Ν c a l c u l a t e d in S t e p 8, c a l c u l a t e t h e n u m b e r o f a e r a t i o n units needed t o transfer required a m o u n t of oxygen. 2
N o . of units = lb 0 / h r ( r e q u i r e d ) / ^ 2
T h e r e s h o u l d b e o n e t u r b i n e u n i t for e v e r y 9 0 0 - 2 5 0 0 f t . By v a r y i n g a i r r a t e p e r u n i t G , o n e a d j u s t s c a l c u l a t i o n s s o t h a t s p a c i n g falls w i t h i n t h i s r a n g e . Step 10. C o m p u t e t o t a l a i r flow. 2
s
Total air flow = G χ (no. of units) s
(SCFM)
Step 11. C o m p u t e o p e r a t i n g c o m p r e s s o r h o r s e p o w e r f r o m E q . (4.31). Step 12. D e t e r m i n e
turbine
horsepower
from
optimum
power
split
e s t a b l i s h e d in S t e p 5. HP
r
=
(P *)(HP ) d
c
Step 13. C o m p u t e o x y g e n a t i o n efficiency f r o m E q . (4.21), w h e r e n u m e r a t o r w a s o b t a i n e d in S t e p 8. C a l c u l a t e d e n o m i n a t o r f r o m E q . (4.25).
16.
149
Surface Aeration Units
16. S u r f a c e A e r a t i o n U n i t s 16.1. D E S C R I P T I O N O F U N I T S u r f a c e a e r a t i o n u n i t s a r e b a s e d solely o n e n t r a i n m e n t o f o x y g e n
from
a t m o s p h e r i c air. U n l i k e a i r diffusion a n d t u r b i n e a e r a t o r s t h e r e is no s t r e a m o f a i r i n v o l v e d in t h i s s y s t e m . I m p r o v e d d e s i g n o f s u r f a c e a e r a t o r s h a s r e s u l t e d in i m p r o v e m e n t o f o x y g e n t r a n s f e r c a p a c i t y , a n d t h e i r u s e h a s i n c r e a s e d r a p i d l y in t h e p a s t few y e a r s . T h e y a r e w i d e l y u s e d in a c t i v a t e d s l u d g e p l a n t s a n d a e r a t e d l a g o o n s . T h e p r i n c i p l e o f o p e r a t i o n of s u r f a c e a e r a t o r s is i l l u s t r a t e d b y t h e s k e t c h i n F i g . 4.16. L i q u i d is d r a w n f r o m u n d e r n e a t h t h e u n i t a n d s p r a y e d u p w a r d a n d o u t w a r d by a propeller inside a vertical t u b e .
Fig.
4.16.
Cross-sectional
diagram
of a surface
aerator.
M o s t c o n v e n t i o n a l s u r f a c e a e r a t o r s a r e fixed t o p i e r s m o u n t e d a c r o s s t h e aerating tanks. Floating units are also available, the whole unit being sup p o r t e d o n a r e i n f o r c e d fiberglass float filled w i t h p l a s t i c f o a m t o m a k e it unsinkable. O x y g e n t r a n s f e r in s u r f a c e a e r a t o r s o c c u r s a c c o r d i n g t o t w o m e c h a n i s m s : (1) t r a n s f e r a t t h e t u r b u l e n t l i q u i d s u r f a c e , a n d (2) t r a n s f e r t o d r o p l e t s s p r a y e d b y t h e b l a d e s of t h e u n i t .
16.2. C O R R E L A T I O N B E T W E E N T R A N S F E R EFFICIENCY A N D LEVEL OF A G I T A T I O N A c o r r e l a t i o n h a s b e e n d e v e l o p e d [ 1 ] b e t w e e n t r a n s f e r efficiency [ e x p r e s s e d a s lb o f Ο t r a n s f e r r e d / ( H P χ h r ) ] a n d level o f a g i t a t i o n o f t h e b a s i n (in H P / 1 0 0 0 g a l b a s i n ) . T h e r e is a n a p p r o x i m a t e l i n e a r r e l a t i o n s h i p b e t w e e n t h e s e t w o p a r a m e t e r s , a s i n d i c a t e d b y t h e s t r a i g h t line in F i g . 4 . 1 7 , w h i c h is a 2
150
4.
Theory and Practice of Aeration
t y p i c a l e x a m p l e o f t h i s c o r r e l a t i o n for a specific s u r f a c e a e r a t o r u n i t . C o r r e l a t i o n b e t w e e n d i a m e t e r o f influence a n d u n i t h o r s e p o w e r , w h i c h is a l s o p l o t t e d in F i g . 4 . 1 7 , is d i s c u s s e d i n t h i s s e c t i o n . O r d i n a t e N
0
equals the lb of
0
2
transferred t o t a p water at s t a n d a r d conditions (20°C a n d 1 a t m , with initial zero dissolved oxygen) per ( H P χ hr). A correction t o o b t a i n oxygen transfer (TV) f o r a w a s t e w a t e r u n d e r o p e r a t i n g c o n d i t i o n s is p r e s e n t e d in t h i s s e c t i o n [Eq. (4.34)]. T h e l i n e a r c o r r e l a t i o n i n d i c a t e d i n F i g . 4 . 1 7 is e x p r e s s e d b y t h e r e l a t i o n s h i p [1] No = KP
V
+ N
(4.33)
s
w h e r e N * is t h e t o t a l o x y g e n t r a n s f e r r e d t o t a p w a t e r u n d e r s t a n d a r d c o n d i t i o n s p e r u n i t [ l b 0 / ( H P x h r ) ] ; P t h e H P p e r 1000 g a l o f b a s i n l i q u i d ; Κ t h e c o n s t a n t c h a r a c t e r i s t i c o f t h e a e r a t i o n device (in F i g . 4.17 t h i s c o r r e sponds to slope of the straight line); a n d N the oxygen transferred to t a p w a t e r a t s t a n d a r d c o n d i t i o n s p e r u n i t h o r s e p o w e r χ h r a t z e r o t u r b u l e n c e (in F i g . 4.17 t h i s c o r r e s p o n d s t o o r d i n a t e o f t h e s t r a i g h t line a t t h e o r i g i n ) . 0
2
v
s
I n E q . (4.33), N c o r r e s p o n d s t o o x y g e n t r a n s f e r r e d a t s t a n d a r d c o n d i t i o n s (N ) for c o n d i t i o n s o f z e r o t u r b u l e n c e (i.e., P = 0 ) . F o r a g i v e n a e r a t o r t h i s s
0
v
• F o r the specific surface aerator unit corresponding to Fig. 4.17, this relationship is #
0
=
3AP
V
+
2.65
16.
151
Surface Aeration Units
c o r r e s p o n d s t o its o p e r a t i o n in a b a s i n o f infinite v o l u m e . I n s u c h c a s e s , all o x y g e n t r a n s f e r is a c c o m p l i s h e d b y t h e s p r a y m e c h a n i s m a l o n e , since t u r b u l e n c e is negligible. P e r f o r m a n c e of surface a e r a t o r s is r e l a t e d t o t h e f o l l o w i n g f a c t o r s : (1) s u b m e r g e n c e o f i m p e l l e r , a n d (2) d i a m e t e r a n d s p e e d o f r o t o r . V a l u e s f o r t r a n s f e r efficiency [ l b 0
transferred/(HP χ h r ) ] are 2 - 4 for m o s t surface aerators,
2
a l t h o u g h v a l u e s a s h i g h a s 7 a r e r e p o r t e d . T r a n s f e r efficiency r e m a i n s e s s e n tially c o n s t a n t a t a n o p t i m u m s u b m e r g e n c e r e g a r d l e s s o f size o f t h e u n i t . F o r d e s i g n of s u r f a c e a e r a t o r s y s t e m s , s t a n d a r d t r a n s f e r efficiency
N
0
o b t a i n e d f r o m F i g . 4 . 1 7 , for e x a m p l e , is c o r r e c t e d f o r a c t u a l w a s t e w a t e r c o n d i t i o n s a n d t e m p e r a t u r e . T h i s is d o n e b y a p p l i c a t i o n o f t h e f o l l o w i n g relationship [ 1 ] : Ν =
N
C s w 0
9.2
C
l
χ 1.024 - al (T
20)
(4.34)
w h e r e Ν is t h e o x y g e n - t r a n s f e r efficiency u n d e r field c o n d i t i o n s [ l b 0 / (HPxhr)]; t h e o x y g e n - t r a n s f e r efficiency a t s t a n d a r d c o n d i t i o n s [ t a p w a t e r a t 2 0 ° C w i t h initial z e r o d i s s o l v e d o x y g e n a t a t m o s p h e r i c p r e s s u r e ; lb 0 / ( Η Ρ χ h r ) ] ; C t h e s a t u r a t i o n c o n c e n t r a t i o n o f d i s s o l v e d o x y g e n in t h e w a s t e w a t e r ; C t h e o p e r a t i n g D O level in a e r a t i o n b a s i n ; Γ t h e t e m p e r a t u r e of t h e b a s i n ( ° C ) ; a n d α = K a (wastewater)/AT a ( t a p w a t e r ) . 2
2
s w
L
L
L
I n E q . (4.34), C — C is t h e a c t u a l d r i v i n g force for o x y g e n t r a n s f e r u n d e r field c o n d i t i o n s . D r i v i n g force a t s t a n d a r d c o n d i t i o n s w i t h i n i t i a l z e r o d i s s o l v e d o x y g e n is 9 . 2 — 0 . 0 = 9.2, w h e r e 9.2 is t h e o x y g e n s a t u r a t i o n v a l u e a t 2 0 ° C in m g / l i t e r ( T a b l e 4.1). T h u s , in E q . (4.34) a p r o p o r t i o n a l i t y b e t w e e n Ν a n d N a n d t h e c o r r e s p o n d i n g d r i v i n g forces is a s s u m e d . SW
L
0
16.3. D E S I G N P R O C E D U R E F O R A E R A T I O N S Y S T E M S UTILIZING S U R F A C E AERATION UNITS F o r f u n d a m e n t a l i n f o r m a t i o n r e q u i r e d see i t e m s 1-5 for a i r diffusion u n i t s ( S e c t i o n 14.4). O b t a i n c h a r a c t e r i s t i c s for t h e a e r a t o r . T h i s i n c l u d e s (1) c o r r e l a t i o n o f N o v s . H P / 1 0 0 0 g a l , a n d (2) c o r r e l a t i o n b e t w e e n u n i t h o r s e p o w e r a n d d i a m e t e r o f influence for s o l i d s in s u s p e n s i o n (ft). F o r t h e specific d e s i g n i l l u s t r a t e d b y E x a m p l e 4 . 5 , t h e s e t w o c o r r e l a t i o n s a r e p r e s e n t e d in F i g . 4 . 1 7 . D e p t h s o f a e r a t o r b a s i n s f o r s u r f a c e a e r a t o r s a r e u s u a l l y l o w e r t h a n for diffusion o r t u r b i n e a e r a t i o n , r a n g i n g f r o m 8 t o 12 ft. Step 1. T a k e E q . (4.34) a n d c a l c u l a t e t h e t e r m m e r a n d w i n t e r c o n d i t i o n s t o d e t e r m i n e w h i c h is that [ C ] < [C ] [ t h u s (C -C ) conditions] whereas T > T [thus 1 . 0 2 4 S f r
s u m m e r
S f r
w i n t e r
s u m m e r
SW
w i n t e r
L
b e t w e e n b r a c k e t s for s u m the controlling one. N o t e is l a r g e r for t h e w i n t e r ~ is l a r g e r f o r s u m m e r
( r
2 0 )
152
4.
Theory and Practice of Aeration
c o n d i t i o n s ] . Let the results of this calculation be Ν = K
&ummcr
N=
tf
winter
χ N
(4.35)
0
χ N
(4.36)
0
w h e r e K's a r e v a l u e s o f t h e t e r m b e t w e e n b r a c k e t s in E q . (4.34). T h e lower
Κ
c o r r e s p o n d s t o controlling condition (lower transfer of oxygen). Step 2. S i n c e p o w e r level ( a b s c i s s a of F i g . 4.17) is n o t k n o w n , a t r i a l a n d e r r o r s o l u t i o n is n e c e s s a r y f o r d e t e r m i n a t i o n of N
( a n d N) b a s e d o n c o r r e
0
lation of N
0
v s . p o w e r level.
(1) A s s u m e a p o w e r level H P / 1 0 0 0 g a l . (2) F r o m F i g . 4.17 r e a d
N. 0
(3) C a l c u l a t e Ν f r o m E q . (4.35) [ o r E q . ( 4 . 3 6 ) ] , w h i c h e v e r is t h e c o n trolling one. (4) P o w e r r e q u i r e m e n t s a r e c a l c u l a t e d for a s s u m e d p o w e r level f r o m 0 required (lb 0 / h r ) Power requirements = — ^ ——— = HP JV(lb0 /HPxhr) 2
2
2
where the oxygen requirement has been previously calculated from biological reactor requirements
(item 2 o n
"Fundamental
information
required,"
S e c t i o n 14.4) (5) Select H P p e r u n i t a n d c a l c u l a t e n u m b e r o f u n i t s . (6) R e c a l c u l a t e p o w e r level. Power level = H P [Step 2(4)]/volume of aeration basin w h e r e v o l u m e o f t h e a e r a t i o n b a s i n is c a l c u l a t e d f r o m b i o l o g i c a l r e a c t o r r e q u i r e m e n t s ( i t e m 1 o n " F u n d a m e n t a l i n f o r m a t i o n r e q u i r e d , " S e c t i o n 14.4). E x p r e s s r e c a l c u l a t e d p o w e r level in t e r m s of H P / 1 0 0 0 g a l a n d c o m p a r e it w i t h t h e v a l u e a s s u m e d in S t e p 2 ( 1 ) . If a g r e e m e n t is w i t h i n 5 % , c a l c u l a t i o n s a r e s t o p p e d . O t h e r w i s e , i t e r a t e S t e p s 2(1)—(6) u n t i l a g r e e m e n t is o b t a i n e d . Step 3. S p a c i n g b e t w e e n a g i t a t o r s is d e t e r m i n e d f r o m t h e c o r r e l a t i o n i n d i c a t e d in F i g . 4.17. T h e p r o c e d u r e for t h e a e r a t o r b a s i n l a y o u t is i l l u s t r a t e d in E x a m p l e 4 . 5 .
Example 4.5 S u r f a c e a e r a t o r s a r e specified for a n a c t i v a t e d s l u d g e p l a n t t r e a t i n g a n i n d u s t r i a l w a s t e w a t e r . O x y g e n r e q u i r e m e n t s a n d v o l u m e of t h e b i o l o g i c a l r e a c t o r a r e c a l c u l a t e d b y t h e p r o c e d u r e d e s c r i b e d in C h a p t e r 5, S e c t i o n 7 ( E x a m p l e 5.7), y i e l d i n g t h e f o l l o w i n g r e s u l t s . Oxygen requirements: 665 lb 0 / h r Volume of reactor: 1,200,000 gal 2
16.
153
Surface Aeration Units
T h e f o l l o w i n g a d d i t i o n a l i n f o r m a t i o n is a v a i l a b l e . Wastewater temperature ( s u m m e r ) : 30°C, C = 7.4 mg/liter Wastewater temperature (winter): 18°C, C w = 10.3 mg/liter sw
S
T a k e t h e o p e r a t i n g D O level a t t h e b a s i n a s C
L
= 1.0 m g / l i t e r , a n d α = 0.72.
C h a r a c t e r i s t i c s o f t h e surface a e r a t o r selected a r e given b y F i g . 4 . 1 7 . D e s i g n t h e a e r a t i o n s y s t e m for t h i s a p p l i c a t i o n . SOLUTION Step 1. U t i l i z e E q . (4.34). S u m m e r : t = 30°C, C W i n t e r : / = 18°C, C
sw
sw
= 7.4 mg/liter = 10.3 mg/liter
T h u s for s u m m e r c o n d i t i o n s
a n d for w i n t e r c o n d i t i o n s
T h e r e f o r e , s u m m e r c o n d i t i o n s c o n t r o l d e s i g n (lower o x y g e n - t r a n s f e r efficiency). Step 2. (1) A s s u m e a p o w e r level, e.g., 0.25 H P / 1 0 0 0 g a l . (2) F r o m F i g . 4.17 r e a d N = 3.5 l b 0 / ( Η Ρ χ h r ) . 0
2
(3) T h e n Ν = 0 . 6 3 5 ; N = 0.635 χ 3.5 = 2.22 l b 0 / ( Η Ρ χ h r ) . (4) P o w e r r e q u i r e m e n t s a r e t h e n c a l c u l a t e d . 0
2
Oxygen requirement: 665 lb 0 / h r 2
Power requirements: 665 lb 0 / h r χ ( H P χ hr)/ 2.22 lb 0 2
2
= 299.5 H P
(5) Select six u n i t s of 50 H P e a c h ( t o t a l H P = 6 χ 50 = 3 0 0 H P ) . (6) P o w e r level is t h e n 300 HP/1200 t h o u s a n d s of gal = 0.25 HP/1000 gal w h i c h a g r e e s w i t h a s s u m e d v a l u e . T h u s d e s i g n is s a t i s f a c t o r y . Step 3. D i a m e t e r o f influence f o r 50 H P u n i t s (see F i g . 4.17) is 6 0 ft ( o r r a d i u s of influence o f 3 0 ft). S p a c i n g d i s t a n c e o f 56 ft is selected t o p r o v i d e a m i n i m u m o v e r l a p . A r r a n g e a e r a t o r s a c c o r d i n g t o l a y o u t in F i g . 4 . 1 8 . C r o s s - s e c t i o n a l a r e a of t h e b a s i n is 168 χ 112 = 18,816 f t , a n d its v o l u m e in f t is 1,200,000 gal χ ft /7.48 gal = 161,000 ft 2
3
3
T h e r e f o r e d e p t h is 161,000/18,816 = 8.6 ft
3
154
Theory and Practice of Aeration
4.
U 1|2'-0" K28'-o~—56'-0"—~28'-ο'^ " T " 28'-0"
- Φ
<£"
56'-0" lee'-o"
-Φ 56'-0"
-4>1 Ο F/flr. 4.18. Layout
Aerators
of aerator
tank (Example
4.5).
Several selections of a e r a t o r units are possible for a given
application,
leading to various layouts. Engineering j u d g m e n t and economic considera t i o n s d e t e r m i n e final s e l e c t i o n o f a e r a t o r u n i t s a n d t h e i r l a y o u t .
Problems I. T h e following results are obtained in an unsteady state aeration test utilizing a 5 - H P surface aerator. C = 9.2 p p m (measured at 2 0 ° C ; Ρ = 27 in H g ) . Aerator is a 100,000-gal circular test tank. 5
T i m e (min) 0 12 24 36 48 60
C
L
(ppm) 0 2.6 4.5 5.8 6.7 7.4
Calculate 1. Chemical requirements (lb N a S 0 / l b liquor) for deoxygenation o f test water with 8 p p m of D O 2. Value o f ^ ( h r " ) 3. If aeration tank has a capacity o f 100,000 gal, calculate the lb/hr o f oxygen transferred at standard conditions 2
1
3
155
Problems
4. If a e r a t o r h a s a n o m i n a l H P of 5, r e p o r t a e r a t o r efficiency [lb 0 / ( h r ) ( H P ) ] in t e r m s of n a m e p l a t e H P a n d b l a d e H P . T h e following i n f o r m a t i o n is a v a i l a b l e for t h e a e r a t o r : D r a w n v o l t a g e : 220 V (average) A m p e r a g e : 13.5 A (average) c o s P F ( m e a s u r e d ) : 0.8 M o t o r efficiency ( e s t i m a t e d ) : 8 5 % G e a r efficiency ( e s t i m a t e d ) : 8 5 % 2
II. U n s t e a d y s t a t e a e r a t i o n d a t a is o b t a i n e d in a diffused a e r a t o r system for w a t e r at 6.5°C a n d a w a s t e w a t e r at 0 ° C , a n d is t a b u l a t e d b e l o w . C a l c u l a t e coefficient α (at 2 0 ° C ) .
T A B L E 1a Water at 6.5°C, C = 12.3 mg/liter s
Time (min)
C
(mg/liter)
L
3 6 9 12 15 18 21
0.6 1.6 3.1 4.3 5.4 6.0 7.0
T A B L E 1b Wastewater at 0°C, C = 14.3 mg/liter s
Time (min)
C
(mg/liter)
L
0.9 1.7 2.5 3.2 3.9 4.6 5.2
3 6 9 12 15 18 21
III. A t u b i n e a e r a t o r in a n a e r a t i o n t a n k 30 χ 50 χ 15 ft transfers oxygen a c c o r d i n g t o t h e relationship K aV = 2 5 G ° - S - / ) (ft /hr) 4 5
L
1
5
1
8
3
S
w h e r e K a is in l i t e r / h r ; V is t h e t a n k v o l u m e ( f t ) ; G the air flow ( S C F M ) ; S t h e p e r i p h e r a l speed of impeller (ft/sec); a n d D t h e impeller d i a m e t e r (ft). P o w e r d r a w n by t h e t u r b i n e is defined by t h e r e l a t i o n s h i p 3
L
s
HP
r
= 0.02D
5
2 5
/?
2
7 5
w h e r e D is t h e impeller d i a m e t e r (ft) a n d η t h e r e v o l u t i o n s / s e c of r o t o r . F o r c a l c u l a t i o n of c o m p r e s s o r h o r s e p o w e r , t a k e a p r e s s u r e d r o p of 5.55 psi a n d a n efficiency ε„, = 0.65 ( 6 5 % ) .
156
4.
Theory and Practice of Aeration
1. C o m p u t e K a(hr~ ). Turbine is 4 0 in. in diameter, rotating at 15 ft/sec peripheral speed, with an air flow o f 300 S C F M . 2. Calculate 0 transfer (lb/hr) under standard conditions. Saturation solubility of oxygen in the sewage liquid at 20°C is 8.45 p p m . 3. Calculate turbine horsepower corrected from Fig. 4.13. 4. Calculate blower horsepower. 5. Calculate transfer efficiency in terms of lb of 0 transferred per H P χ hr. l
L
2
2
References 1. Eckenfelder, W. W., Jr., "Water Quality Engineering for Practicing Engineers." Barnes & N o b l e , N e w York, 1970. 2. Eckenfelder, W. W., Jr., and Ford, D . L., in "Advances in Water Quality I m p r o v e m e n t " (E. F. G l o y n a and W. W. Eckenfelder, Jr., eds.), p. 226. Univ. of Texas Press, Austin, 1968. 3. Eckenfelder, W . W., Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w York, 1970. 4. Eckenfelder, W. W., Jr., and O'Connor, D . J., "Biological Waste Treatment." Pergamon, Oxford, 1961. 5. Oldshue, J., "Biological Treatment o f Sewage and Industrial Wastes." Van N o s t r a n d Reinhold, Princeton, N e w Jersey, 1956. 6. Quirk, T. P., unpublished report, 1962 (mentioned in Eckenfelder [1]). 7. "Standard M e t h o d s for the Examination o f Water and Wastewater," 13th ed. A m . Public Health A s s o c . , Yearbook Publ., Chicago, Illinois, 1971.
5 Secondary Treatment: The Activated Sludge Process 1. Introduction
158
2. Mathematical Modeling of Activated Sludge Process
163
3. Kinetics Relationships 3.1. Introduction 3.2. Formulation of the Continuous Reactor 4. Material Balance Relationships 4.1. Design Parameters Corresponding to Net Yield of M L V S S and Oxygen Requirements for Aerobic Biological Degradation of Wastes 4.2. Material Balance for Determination of Oxygen Utilization . . . 4.3. Material Balance for Determination of Net Yield of Biological Sludge ( M L V S S ) 4.4. Total Sludge Yield 4.5. Material Balances for X and X 4.6. Typical Values of Aerobic Biological Wastewater Treatment Parameters for Different Types of Wastewaters 5. Relationship for Optimum Settling Conditions of Sludge
164 164 166 169
NVt0
Vt0
6. Experimental Determination of Parameters Needed for Design of Aerobic Biological Reactors 6.1. Bench Scale Continuous Reactors 6.2. Experimental Procedure 6.3. Calculation of Design Parameters 6.4. Numerical Examples: Determination of Design Parameters for an Activated Sludge System 7. Design Procedure for an Activated Sludge Plant 7.1. Introduction 7.2. Material Balance for Determination of Recycle Ratio of MLVSS 7.3. Material Balance for Calculation of S 7.4. Alternative Expressions for Net Yield of Biological Sludge and Oxygen Utilization in the Aerator 7.5. Calculation of Residence Time in Reactor 7.6. Equations for Sludge Recycle Ratio r in Cases When Effluent Quality and Organic Loading Control Residence Time 7.7. Neutralization Requirements 7.8. Nutrient Requirements 7.9. Design Procedure for Activated Sludge Plants 7.10. Numerical Example: Design of an Activated Sludge Plant. . . 0
169 179 181 183 184 185 185 189 189 191 193 198 205 205 206 207 208 208 209 210 211 212 213 157
158 8. The 8.1. 8.2. 8.3. 8.4. 8.5.
5.
Secondary Treatment: The Activated Sludge Process
Michaelis-Menten Relationship Derivation of M i c h a e l i s - M e n t e n Relationship Corollaries of Michaelis-Menten Relationship Significance of M i c h a e l i s - M e n t e n Constant K Determination of V : The Lineweaver-Burk Plot M i c h a e l i s - M e n t e n Relationship W h e n Nonbiodegradable Matter Is Present in the System 8.6. Michaelis-Menten Relationship in Terms of Specific G r o w t h Rate of Sludge 9. The Concept of Sludge A g e
225 226
10. Kinetics of Continuous Treatment S y s t e m s : Plug Flow, Complete M i x , and Arbitrary Flow Reactors Problems References
230 234 235
s
MAX
219 219 221 223 223 224
1. I n t r o d u c t i o n T h e h e a d i n g s e c o n d a r y t r e a t m e n t e n c o m p a s s e s all b i o l o g i c a l t r e a t m e n t processes of wastewaters, b o t h aerobic a n d anaerobic. In this c h a p t e r the a c t i v a t e d s l u d g e p r o c e s s is s t u d i e d in d e t a i l , a n d t h e m a t h e m a t i c a l m o d e l s d e v e l o p e d a r e a p p l i c a b l e , w i t h m i n o r c h a n g e s , t o all a e r o b i c p r o c e s s e s d e s c r i b e d i n C h a p t e r 6. T h e a c t i v a t e d s l u d g e p r o c e s s h a s b e e n utilized for t r e a t m e n t o f b o t h d o m e s t i c a n d i n d u s t r i a l w a s t e w a t e r s for a p p r o x i m a t e l y h a l f a c e n t u r y . D e s i g n of activated sludge plants was carried o u t to a large extent in a n empirical m a n n e r . I t w a s o n l y after t h e e a r l y 1960's t h a t a m o r e r a t i o n a l a p p r o a c h t o the design of activated sludge systems was developed. This process originated from the observation m a d e a long time ago that whenever wastewater, either d o m e s t i c o r i n d u s t r i a l , is a e r a t e d for a p e r i o d o f t i m e , t h e c o n t e n t o f o r g a n i c m a t t e r is r e d u c e d , a n d a t t h e s a m e t i m e a flocculent s l u d g e is f o r m e d . M i c r o s c o p i c e x a m i n a t i o n o f t h i s s l u d g e r e v e a l s t h a t it is f o r m e d b y a h e t e r o g e n e o u s p o p u l a t i o n o f m i c r o o r g a n i s m s , w h i c h c h a n g e s c o n t i n u a l l y in n a t u r e in r e s p o n s e t o v a r i a t i o n in t h e c o m p o s i t i o n of t h e w a s t e w a t e r a n d environmental conditions. Microorganisms present are unicellular bacteria, f u n g i , a l g a e , p r o t o z o a , a n d rotifers. O f t h e s e , b a c t e r i a a r e p o s s i b l y t h e m o s t i m p o r t a n t , b e i n g f o u n d in all t y p e s o f b i o l o g i c a l t r e a t m e n t p r o c e s s e s . T h e p u r p o s e o f t h i s c h a p t e r is t o d i s c u s s t h e d e s i g n p r i n c i p l e s f o r t h e activated sludge process a n d to apply t h e m to design of t r e a t m e n t plants. This involves development of f u n d a m e n t a l design information from l a b o r a t o r y scale r e a c t o r s . T h e a p p r o a c h u t i l i z e d is b a s e d m a i n l y o n t h e w o r k of E c k e n felder a n d a s s o c i a t e s . The activated sludge process has been developed as a c o n t i n u o u s operation b y r e c y c l i n g t h e b i o l o g i c a l s l u d g e . A flow d i a g r a m o f t h i s c o n t i n u o u s p r o c e s s
1.
159
Introduction Air
,01
! Fresh | feed
Combined ! feed
S X
S
X
X
NV,F|
X
v,o
X
NV,o
Net ! effluent
Secondary clarifier
Q'
v,a
X
V,F
Reactor effluent
e
NV,a V
F
©
®
Reactor (Aerator)
s
X
S
NV,o NV,a
Θ
ία^β^Ν Clarifi underflow
j©
Recycled sludge
γ
/
/ / / /
^
Qu
Xw., Ί
NV,u
j
Recycle and wastage pump
Or
S.
e
© Wastage Q" ΔΧ ΔΧ
ν
Ν
ν
ΔΧ,
S
e
X .. u
Fig. 5.1. Conventional definition of symbols.)
activated
sludge
process, (See
Table
5.1
for
a
is s h o w n in F i g . 5 . 1 . A l l i m p o r t a n t p r o c e s s v a r i a b l e s a r e i n d i c a t e d in F i g . 5.1 a n d defined in T a b l e 5 . 1 . T h e s e s h o u l d b e carefully e x a m i n e d b y t h e r e a d e r . I n F i g . 5 . 1 , c o m p o s i t i o n s o f t h e different s t r e a m s ( n u m b e r e d characterized by three types of concentrations:
1-7)
are
1. Concentration of soluble BOD. D e n o t e d b y t h e s y m b o l S where s u b s c r i p t i refers t o t h e specific s t r e a m u n d e r c o n s i d e r a t i o n , a s i n d i c a t e d in T a b l e 5 . 1 . S o l u b l e B O D c o m p r i s e s m a i n l y c a r b o n a c e o u s m a t e r i a l s in s o l u t i o n . i9
2. Concentrations of volatile suspended solids (VSS). T h e s e a r e d e n o t e d by s y m b o l X w h e r e s u b s c r i p t ν s t a n d s for v o l a t i l e , a n d s u b s c r i p t / refers t o t h e specific s t r e a m in q u e s t i o n ( T a b l e 5.1). V S S c o r r e s p o n d t o t h e b i o l o g i c a l sludge, constituted by the heterogeneous p o p u l a t i o n of m i c r o o r g a n i s m s . E x p e r i m e n t a l d e t e r m i n a t i o n o f V S S is p e r f o r m e d b y m e a s u r i n g t h e loss o f w e i g h t o f t o t a l s u s p e n d e d s o l i d s ( T S S ) , after i n c i n e r a t i o n in a l a b o r a t o r y o v e n a t 6 0 0 ° C . T h i s loss of w e i g h t c o r r e s p o n d s m a i n l y t o v o l a t i l i z a t i o n o f b i o logical s l u d g e . R e m a i n i n g solids after i n c i n e r a t i o n a t 6 0 0 ° C a r e r e f e r r e d t o a s n o n v o l a t i l e s u s p e n d e d s o l i d s . T h e i r n a t u r e is d i s t i n c t f r o m t h o s e in t h e biological sludge, being constituted of nonliving m a t t e r of b o t h inorganic a n d organic nature. Vii9
3. Concentrations of nonvolatile denoted by symbol X , w h e r e NV t h e specific s t r e a m in q u e s t i o n . Therefore NVfi
suspended solids (NVSS). These are s t a n d s for n o n v o l a t i l e , a n d / refers t o
TSS = VSS + N V S S Total suspended solids = Volatile suspended solids + Nonvolatile suspended solids
160
5.
Secondary Treatment: The Activated Sludge Process
T A B L E 5.1 Definition of Symbols Used in Fig. 5.1 Key For suspended solids double subscripts are utilized, e.g., X , X v,i> The first subscript (v or NV) designates volatile and nonvolatile suspended solids, respectively. The second subscript (/) refers to the specific stream in question: F, fresh feed [stream 1] 0, combined feed [stream 2 ] a, reactor effluent [stream 3] e, net effluent [stream 4 ] u, underflow from secondary clarifier [stream 5] Vt
t
N
Symbols 1. F l o w rates Q , fresh feed; M G D (million gallons per day) [stream 1] Q , recycle; M G D [stream 7] r, recycle ratio; dimensionless (r = Q IQ ) Q, combined feed; M G D ; Q = Q + Q = Q (l +/-) [stream 2 ] ( M G D of combined feed = M G D o f reactor effluent, i.e., β [stream 2] = [stream 3]) β ' , net effluent; M G D [stream 4 ] Q\ wastage; M G D [stream 6 ] ( N o t i c e that Q = β ' + β " ) β „ , clarifier underflow; M G D ; β „ = β " + β * = Q" + rQ [stream 5] F
R
R
F
F
R
F
β
F
F
2. Concentrations (mg/liter) o f soluble B O D S soluble B O D o f fresh feed S , soluble B O D o f combined feed S soluble B O D o f effluent Fi
0
ei
3. Concentrations (mg/liter) o f volatile suspended solids (VSS) A V . F , VSS in fresh feed X VSS in combined feed Xv.a, V S S in reactor. This also is equal to concentration o f VSS in reactor effluent X , , VSS in secondary clarifier underflow X V S S in net effluent (take X , » 0) V
T
0
9
v u
Vttt
v e
4. Concentrations (mg/liter) o f nonvolatile suspended solids ( N V S S ) XHV.F, N V S S in fresh feed XNV,o, N V S S in combined feed Xsv.a, N V S S in reactor (X . = XNV.o)This also equals concentration o f N V S S in reactor effluent XNV.u, N V S S in secondary clarifier underflow XNV.O N V S S in net effluent NV a
5. Wastage (lb/day) AX , V
net yield o f M L V S S in reactor (wastage o f M L V S S )
AX , wastage of N V S S AX , total sludge yield: AX = AX +AX NV
t
6. Reactor v o l u m e K, reactor volume, M G
t
V
NV
(million gallons)
+
QX, F
V F
1.
Introduction
161
A d e s c r i p t i o n o f t h e flowsheet i n F i g . 5.1 f o l l o w s , w i t h e m p h a s i s o n c o n c e n t r a t i o n s o f (1) s o l u b l e B O D , (2) v o l a t i l e s u s p e n d e d s o l i d s , a n d (3) n o n v o l a t i l e s u s p e n d e d s o l i d s for t h e different s t r e a m s . 1. Soluble BOD. F r e s h feed, i.e., t h e w a s t e w a t e r t o b e t r e a t e d [ s t r e a m 1 ] , e n t e r s t h e p r o c e s s w i t h a v a l u e of s o l u b l e B O D d e n o t e d a s S . P u r p o s e o f t h e t r e a t m e n t is t o r e d u c e t h i s v a l u e t o S (effluent B O D i n s t r e a m 4) b y o x i d a t i o n t h r o u g h aerobic biological d e g r a d a t i o n of organic m a t t e r in the wastewater. F
e
In the conventional activated sludge process, a reduction of soluble B O D t o 5 - 1 0 % o f its v a l u e in t h e fresh feed is u s u a l l y a c c o m p l i s h e d , i.e., S = 5 - 1 0 % o f S . T h i s m e a n s a s o l u b l e B O D r e m o v a l efficiency o f 9 0 - 9 5 % . e
F
F r e s h feed is c o m b i n e d w i t h r e c y c l e d s l u d g e [ s t r e a m 7 ] a n d e n t e r s t h e a e r a t o r ( c o m b i n e d feed, s t r e a m 2). B i o l o g i c a l s l u d g e is c o n t i n u o u s l y f o r m e d in t h e a e r a t o r . I t is u s u a l l y d e s i r a b l e t o o p e r a t e t h e r e a c t o r a t s t e a d y s t a t e a n d u n d e r complete mixing conditions. These t w o a s s u m p t i o n s are m a d e in m o s t mathematical models hence. C o n c e n t r a t i o n of soluble B O D in the reactor l i q u o r is d e n o t e d a s S . U n d e r s t e a d y s t a t e a n d c o m p l e t e m i x i n g c o n d i t i o n s t h e c o n c e n t r a t i o n o f s o l u b l e B O D in r e a c t o r effluent [ s t r e a m 3 ] a l s o e q u a l s S . e
e
R e a c t o r effluent e n t e r s t h e s e c o n d a r y clarifier a s i n d i c a t e d i n F i g . 5 . 1 . C o n c e n t r a t i o n o f s o l u b l e B O D is t h e s a m e i n clarifier u n d e r f l o w [ s t r e a m 5 ] a n d n e t effluent [ s t r e a m 4 ] , i.e., S . Clarifier u n d e r f l o w is split i n t o t w o s t r e a m s : wastage [stream 6] a n d recycled sludge [ s t r e a m 7 ] . F o r b o t h these streams, the concentration of soluble B O D has the same value, S . The recycled s l u d g e s t r e a m is t h e n c o m b i n e d w i t h fresh feed t o f o r m t h e c o m b i n e d feed. C o n c e n t r a t i o n o f s o l u b l e B O D in c o m b i n e d feed, d e s i g n a t e d a s S is c a l c u l a t e d b y a m a t e r i a l b a l a n c e a t t h e j u n c t i o n p o i n t o f s t r e a m s 1, 2, a n d 7. T h i s b a l a n c e is w r i t t e n in S e c t i o n 7.3. e
e
09
2. Volatile suspended solids (VSS). A t steady state, concentration of b i o l o g i c a l s l u d g e in t h e r e a c t o r is k e p t c o n s t a n t a t all t i m e s . I n t h e c o n v e n tional activated sludge process this concentration, designated as X where t h e s e c o n d s u b s c r i p t a refers t o t h e a e r a t o r , is u s u a l l y selected b e t w e e n 2 0 0 0 a n d 3 0 0 0 m g / l i t e r . S i n c e c o m p l e t e m i x i n g c o n d i t i o n s a r e p o s t u l a t e d t o exist i n t h e r e a c t o r , v o l a t i l e s u s p e n d e d solids in it a r e r e f e r r e d t o a s M L V S S ( m i x e d l i q u o r v o l a t i l e s u s p e n d e d solids). S i m i l a r l y , n o n v o l a t i l e s u s p e n d e d s o l i d s in the reactor, being also completely mixed, are referred t o as M L N V S S (mixed l i q u o r n o n v o l a t i l e s u s p e n d e d solids). T o t a l s u s p e n d e d s o l i d s i n t h e r e a c t o r are designated as M L T S S (mixed liquor total s u s p e n d e d solids). Therefore va9
MLTSS = MLVSS + MLNVSS Mixed liquor total suspended solids = mixed liquor volatile suspended solids + mixed liquor nonvolatile suspended solids
162
5.
S e c o n d a r y Treatment: The Activated S l u d g e P r o c e s s
C o n c e n t r a t i o n o f V S S i n fresh feed ( X , F ) * negligible in m a n y c a s e s , since n o a p p r e c i a b l e a m o u n t of a e r a t i o n h a s t a k e n p l a c e a t t h i s s t a g e . V S S is p r o d u c e d c o n t i n u o u s l y in t h e a e r a t o r , o w i n g t o s y n t h e s i s o f b i o l o g i c a l m a t t e r , a n d w i t h d r a w n c o n t i n u o u s l y w i t h r e a c t o r effluent. s
V
I n o r d e r t o m a i n t a i n a c o n s t a n t c o n c e n t r a t i o n o f M L V S S in t h e r e a c t o r , m o s t o f t h e clarifier u n d e r f l o w is r e c y c l e d b a c k . R e c y c l e r a t i o r is c a l c u l a t e d b y m a t e r i a l b a l a n c e ( S e c t i o n 7.2) in o r d e r t o m a i n t a i n a c o n s t a n t selected concentration X o f M L V S S w i t h i n t h e r e a c t o r a t all t i m e s . O w i n g t o s y n t h e s i s o f b i o l o g i c a l m a t t e r t h e r e is a net yield o f M L V S S in t h e r e a c t o r (AX , lb/day). Therefore t o m a i n t a i n constant concentration of M L V S S in t h e r e a c t o r a t all t i m e s , it is n e c e s s a r y t o r e m o v e f r o m t h e s y s t e m a m a s s o f M L V S S ( l b / d a y ) e q u a l t o t h i s n e t yield AX . T h i s is d o n e b y w a s t a g e o f s l u d g e [ s t r e a m 6 ] . A l t h o u g h c o n t i n u o u s w a s t a g e is i n d i c a t e d in F i g . 5 . 1 , i n p r a c t i c e it is u s u a l l y a n i n t e r m i t t e n t o p e r a t i o n . I t is s i m p l e r t o w r i t e m a t e r i a l b a l a n c e s for a s t e a d y s t a t e o p e r a t i o n ; t h u s c o n t i n u o u s w a s t a g e is a s s u m e d i n t h e r e m a i n d e r of this chapter. Intermittent wastage implies the a s s u m p t i o n of u n s t e a d y s t a t e o p e r a t i o n . Since t h e w a s t a g e s t r e a m is u s u a l l y s m a l l b y c o m p a r i s o n w i t h t h e recycle, a s s u m p t i o n o f c o n t i n u o u s w a s t a g e d o e s n o t i n t r o d u c e , in g e n e r a l , a n a p p r e c i a b l e e r r o r in t h e o v e r a l l m a t e r i a l b a l a n c e . C o n c e n t r a t i o n o f V S S i n t h e r e a c t o r effluent [ s t r e a m 3 ] is a l s o X , since complete mixing a n d steady state conditions are assumed. Vta
V
V
v$a
R e a c t o r effluent flows i n t o t h e s e c o n d a r y clarifier. U n d e r f l o w f r o m t h e l a t t e r [ s t r e a m 5 ] is a s l u r r y c o n t a i n i n g a c o n c e n t r a t i o n o f V S S d e s i g n a t e d a s X ,u (Xv,u > X ,a)> T h e v a l u e o f X is selected b y t h e d e s i g n e r , w i t h clarifier b e i n g sized t o yield t h i s specified v a l u e . U s u a l l y X is selected b e t w e e n 10,000 a n d 15,000 m g / l i t e r o f M L V S S . C o n c e n t r a t i o n s o f V S S in w a s t a g e a n d r e cycled s l u d g e a r e a l s o e q u a l t o X . I n t h e n e t effluent f r o m t h e s e c o n d a r y clarifier, c o n c e n t r a t i o n o f V S S (X ) is n e g l e c t e d in d e v e l o p m e n t o f m a t e r i a l b a l a n c e s in t h i s c h a p t e r . T h i s i m p l i e s t h a t c o m p l e t e s e p a r a t i o n o f V S S is a s s u m e d t o t a k e p l a c e in t h e s e c o n d a r y clarifier (i.e., X « 0). T h i s is u s u a l l y a g o o d a s s u m p t i o n . C o n c e n t r a t i o n o f V S S in c o m b i n e d feed, X , is c a l c u l a t e d b y a m a t e r i a l b a l a n c e a t t h e j u n c t i o n p o i n t o f s t r e a m s 1, 2, a n d 7. T h i s b a l a n c e is w r i t t e n in S e c t i o n 4 . 5 . v
v
VtU
VtU
VtU
Vt6
VfC
Vt0
3. Nonvolatile suspended solids (NVSS). C o n c e n t r a t i o n o f M L N V S S in t h e a e r a t o r is d e n o t e d a s X a n d is e q u a l t o t h o s e in b o t h c o m b i n e d feed a n d r e a c t o r effluent. T h i s is s o b e c a u s e c o m p l e t e m i x i n g is a s s u m e d a n d t h e r e is n o p r o d u c t i o n o f N V S S i n t h e a e r a t o r ( u n l i k e t h e n e t yield o f V S S ) . Thus NVta
XNV,c
=
XNV,o
C o n c e n t r a t i o n o f N V S S in fresh feed is d e s i g n a t e d a s X
NVtF
recycled sludge as X ,
NV U
a n d t h a t in t h e
( s a m e a s in u n d e r f l o w f r o m s e c o n d a r y clarifier). I n
Mathematical Modeling of Activated Sludge Process
2.
t h e c o m b i n e d feed t h i s c o n c e n t r a t i o n is d e n o t e d a s
163
X
N
V
T
0
a n d is c a l c u l a t e d b y
a m a t e r i a l b a l a n c e w h i c h is w r i t t e n i n S e c t i o n 4 . 5 . S o m e o f t h e N V S S i n t h e r e a c t o r effluent is a l s o s e p a r a t e d b y s e d i m e n t a t i o n in t h e s e c o n d a r y clarifier. C o n c e n t r a t i o n o f N V S S i n clarifier u n d e r f l o w is denoted as
X
N
V
T
a n d t h a t i n n e t effluent a s
U
In wasted sludge, besides the A X volatile sludge
( A X
N
X
N
V
>
E
.
l b / d a y o f V S S t h e r e is a l s o s o m e n o n
V
lb/day) resulting from partial sedimentation of N V S S
V
in t h e s e c o n d a r y clarifier. I n a d d i t i o n , t h e r e is t h e b i o l o g i c a l s l u d g e i n t r o d u c e d c o n t i n u o u s l y w i t h t h e fresh feed negligible since X
V F
( Q
F
X
V
Frequently, term
, F ) -
Q
is u s u a l l y v e r y s m a l l . T o t a l s l u d g e w a s t e d , A X
F
T
X
V
, F
*
S
lb/day,
is[Eq.(5.1)] AX
=
T
AX
+
V
A X
N
+
V
QF
XV,
F
(5.1)
Respective concentrations of VSS, N V S S , a n d soluble B O D are the s a m e for clarifier
underflow,
respectively, as
X
V
T
U
wastage stream, ,
X
N
V
T
U
,
and
S
E
and
recycle s l u d g e , b e i n g
denoted,
.
In s u m m a r y , c o n c e n t r a t i o n s of VSS, N V S S , a n d soluble B O D in c o m b i n e d feed
( X
V
>
0
,
X
N
V
>
0
,
and
S
0
,
respectively) are o b t a i n e d by material balances
a r o u n d t h e j u n c t i o n p o i n t o f fresh feed a n d r e c y c l e d s l u d g e s t r e a m s . T h e s e m a t e r i a l b a l a n c e s a r e w r i t t e n in S e c t i o n s 4 . 5 a n d 7 . 3 . F r o m a n o v e r a l l b a l a n c e for t h e w a s t e w a t e r [ E q . ( 5 . 2 ) ] , QF
=
ΰ
+ β"
(5.2)
W a s t e w a t e r flows a r e u s u a l l y e x p r e s s e d in m i l l i o n s o f g a l l o n s p e r d a y ( M G D ) . R e c y c l e r a t i o r is defined a s r =
QR/QF
=
recycle wastewater, M G D / f r e s h wastewater, M G D Λ
QR
= rQ
(5.3) (5.4)
F
S i n c e c o m b i n e d feed Q is e q u a l t o fresh feed plus recycle, Q = QF + Q R = Q (\+r) F
(5.5)
H e n c e , t h e d e n s i t y o f all l i q u o r s t r e a m s in F i g . 5.1 is a s s u m e d e q u a l t o t h a t o f w a t e r a t a m b i e n t t e m p e r a t u r e (8.34 l b / g a l ) . * T h i s is a g o o d a p p r o x i m a t i o n since r e l a t i v e l y d i l u t e a q u e o u s s o l u t i o n s a r e i n v o l v e d .
2. M a t h e m a t i c a l M o d e l i n g of Activated Sludge Process I t is d e s i r a b l e t o p o r t r a y t h i s p r o c e s s b y a m a t h e m a t i c a l m o d e l a n d t h e n t o d e t e r m i n e p a r a m e t e r s utilized in m a t h e m a t i c a l e q u a t i o n s f r o m e x p e r i m e n t a l d a t a o b t a i n e d u t i l i z i n g a series o f b e n c h s c a l e l a b o r a t o r y r e a c t o r s . R e l a t i o n s h i p s w h i c h a r e p e r t i n e n t t o t h e d e v e l o p m e n t o f t h i s m a t h e m a t i c a l m o d e l fall * T h i s value is a p p r o x i m a t e l y 10.0 lb/gal w h e n imperial gallons a r e utilized.
164
5.
Secondary Treatment: The Activated Sludge Process
i n t o t h r e e g r o u p s : (1) k i n e t i c s r e l a t i o n s h i p s ; (2) m a t e r i a l b a l a n c e r e l a t i o n s h i p s — m a t e r i a l b a l a n c e for d e t e r m i n a t i o n o f o x y g e n u t i l i z a t i o n a n d o f n e t yield o f M L V S S ; a n d (3) r e l a t i o n s h i p for o p t i m u m s e t t l i n g c o n d i t i o n s o f sludge.
3. Kinetics R e l a t i o n s h i p s 3.1. I N T R O D U C T I O N S t u d y o f k i n e t i c s o f a e r o b i c b i o l o g i c a l t r e a t m e n t yields t h e r a t e a t w h i c h m i c r o o r g a n i s m s d e g r a d e a specific w a s t e , a n d t h e r e f o r e p r o v i d e s t h e b a s i c i n f o r m a t i o n r e q u i r e d for sizing b i o l o g i c a l a e r o b i c r e a c t o r s . T h i s s t u d y is c o n v e n i e n t l y p e r f o r m e d in a l a b o r a t o r y scale b a t c h r e a c t o r . F i g u r e 5.2 s h o w s
Fig. 5.2.
Batch
reactor.
a d i a g r a m of four units operating in parallel, each with a capacity of approxi m a t e l y 2.0 liters [ 3 ] . R e a c t o r s a r e b u i l t o f p l e x i g l a s s . W a s t e w a t e r c o n t a i n i n g a seed o f m i c r o o r g a n i s m s * is i n t r o d u c e d i n t o t h e r e a c t o r s , a n d c o m p r e s s e d a i r is b l o w n i n t o t h e s y s t e m . T h e b i o l o g i c a l s l u d g e ( M L V S S ) is k e p t in a s t a t e of complete mixing d u e to agitation provided by air blown into the system. * Seed is either a m a s s o f biological sludge taken from an operating activated sludge plant, or settled sewage.
3.
165
Kinetics Relationships
B O D o f w a s t e w a t e r ( o r C O D , T O D , T O C ) is d e t e r m i n e d a t selected t i m e i n t e r v a l s b y w i t h d r a w i n g s a m p l e s for t h e a n a l y s i s . T h e m a s s o f a c c u m u l a t e d b i o l o g i c a l s l u d g e ( M L V S S ) is a l s o d e t e r m i n e d a t t h e s e s a m e t i m e i n t e r v a l s b y m e a s u r i n g t h e c o n c e n t r a t i o n o f M L V S S in w i t h d r a w n s a m p l e s a n d r e a d i n g t h e v o l u m e o f l i q u o r in t h e r e a c t o r a s i n d i c a t e d b y t h e v o l u m e scale. T y p i c a l c u r v e s for d e c r e a s e o f B O D a n d v a r i a t i o n o f t h e a m o u n t o f M L V S S w i t h t i m e a r e p r e s e n t e d i n F i g . 5.3.
Fig. 5.3.
Typical
BOD
and ML VSS curves
for a batch
reactor.
B O D o f t h e w a s t e w a t e r , w h i c h is a m e a s u r e o f o r g a n i c b i o d e g r a d a b l e m a t t e r c o n c e n t r a t i o n , d e c r e a s e s w i t h t i m e a s t h e o r g a n i c m a t t e r is o x i d i z e d . A p l a t e a u is e v e n t u a l l y r e a c h e d c o r r e s p o n d i n g t o t h e a m o u n t o f n o n b i o d e g r a d a b l e m a t t e r (S ). n
C o n c e n t r a t i o n o f M L V S S i n c r e a s e s a t first ( f r o m t i m e 0 t o t i m e i ) d u r i n g t h e p e r i o d w h e n a s u b s t a n t i a l c o n c e n t r a t i o n o f s u b s t r a t e (relativley h i g h B O D ) is p r e s e n t t o p r o v i d e a b u n d a n t f o o d t o s u s t a i n g r o w t h o f m i c r o o r g a n i s m s . T h i s g r o w t h c o r r e s p o n d s t o t h e s y n t h e s i s o f n e w m i c r o o r g a n i s m cells, i n d i c a t e d i n F i g . 5.3 a s " s y n t h e s i s p h a s e . " A f t e r t i m e t w h e n s u b s t r a t e c o n c e n t r a t i o n is c o n s i d e r a b l y d e p l e t e d , t h e r e is n o t e n o u g h f o o d left t o s u s t a i n g r o w t h of m i c r o o r g a n i s m s . A t this time, m i c r o o r g a n i s m s start c o n s u m i n g their "fellow m i c r o o r g a n i s m s " as food. A s this "cannibalistic feast" proceeds, concentration of M L V S S d r o p s w h e n the rate of destruction of micro o r g a n i s m cells e x c e e d s t h a t o f s y n t h e s i s o f n e w cells. T h i s c o r r e s p o n d s t o t h e "endogenous respiration phase." T h e m a x i m u m on the M L V S S curve corre sponds to time t w h e n t h e s e t w o r a t e s a r e e x a c t l y e q u a l . D i s t a n c e AX c o r r e s p o n d s t o t h e net r e d u c t i o n o f M L V S S c o n c e n t r a t i o n f r o m t t o r x
x
l9
x
2
T h e r e a r e t w o f u n d a m e n t a l differences b e t w e e n o p e r a t i o n o f c o n t i n u o u s ( F i g . 5.1) a n d b a t c h r e a c t o r s ( F i g . 5.2): (1) C o n t r a r y t o w h a t h a p p e n s in t h e
166
5.
S e c o n d a r y Treatment: The Activated S l u d g e P r o c e s s
b a t c h reactor, B O D of the wastewater in the c o n t i n u o u s reactor o p e r a t i n g at s t e a d y s t a t e c o n d i t i o n s r e m a i n s c o n s t a n t (S ). T h i s c o r r e s p o n d s g e n e r a l l y t o a l o w s u b s t r a t e c o n c e n t r a t i o n , since t h e b i o l o g i c a l r e a c t o r is u s u a l l y d e s i g n e d for r e m o v i n g m o s t o f t h e i n f l u e n t B O D . (2) C o n t r a r y t o w h a t h a p p e n s i n t h e b a t c h r e a c t o r , c o n c e n t r a t i o n o f M L V S S in t h e c o n t i n u o u s r e a c t o r o p e r a t i n g a t s t e a d y s t a t e is k e p t c o n s t a n t (X ) a t a selected v a l u e . M a i n t e n a n c e o f t h i s constant X is o b t a i n e d b y p r o v i d i n g t h e c a l c u l a t e d a m o u n t o f c o n c e n t r a t e d r e t u r n sludge. T h e m a t e r i a l balance for M L V S S , necessary t o arrive a t r e q u i r e d r e c y c l e r a t i o for t h i s p u r p o s e , is p r e s e n t e d i n S e c t i o n 7.2. e
Vta
v>a
K i n e t i c d a t a o b t a i n e d f r o m t h e b a t c h r e a c t o r is p o r t r a y e d b y t h e M i c h a e l i s M e n t e n r e l a t i o n s h i p , w h i c h is s t u d i e d in S e c t i o n 8. T w o i m p o r t a n t c o r o l l a r i e s of t h i s r e l a t i o n s h i p a r e p o s t u l a t e d n e x t , t h e s e c o n d o n e b e i n g utilized for design of the c o n t i n u o u s biological reactor. 1. A t h i g h s u b s t r a t e c o n c e n t r a t i o n s , B O D r e m o v a l follows z e r o - o r d e r k i n e t i c s . T h i s m e a n s t h a t t h e r a t e o f r e m o v a l is essentially c o n s t a n t , i n d e p e n d e n t o f s u b s t r a t e c o n c e n t r a t i o n . T h i s s i t u a t i o n is f o u n d i n e a r l y s t a g e s o f t h e b a t c h r e a c t o r o p e r a t i o n , w h e n s u b s t r a t e c o n c e n t r a t i o n is still v e r y h i g h ( h i g h B O D ) . T h i s c o r r e s p o n d s t o t h e s e c t i o n o f t h e B O D c u r v e ( F i g . 5.3) f r o m time zero to approximately time Λ In this region, the tangent to the B O D curve, which equals the rate of substrate removal, coincides essentially with t h e c u r v e itself ( c o n s t a n t s l o p e ) . 2. B O D r e m o v a l a t l o w s u b s t r a t e c o n c e n t r a t i o n s ( c o r r e s p o n d i n g t o B O D v a l u e s b e l o w 500 m g / l i t e r ) follows first-order k i n e t i c s . T h i s m e a n s t h a t r a t e o f r e m o v a l is p r o p o r t i o n a l t o r e m a i n i n g s u b s t r a t e c o n c e n t r a t i o n . T h i s c o r r e s p o n d s t o t h e s e c t i o n o f t h e B O D c u r v e b e y o n d t i m e t'. S l o p e o f t h e B O D curve (which equals rate of substrate removal) decreases with time as the B O D v a l u e is l o w e r e d . A p l o t o f t h e s e s l o p e s v s . c o r r e s p o n d i n g B O D v a l u e s yields a s t r a i g h t line r e l a t i o n s h i p , w h i c h is d i s c u s s e d in S e c t i o n 3.2. T h u s in t h i s r e g i o n , r a t e o f s u b s t r a t e r e m o v a l is d i r e c t l y p r o p o r t i o n a l t o its c o n c e n t r a t i o n (first-order k i n e t i c s ) .
3.2. F O R M U L A T I O N O F T H E C O N T I N U O U S REACTOR Since for t h e c o n t i n u o u s r e a c t o r o p e r a t i n g s u b s t r a t e c o n c e n t r a t i o n s (S ) a r e c o n s i d e r a b l y b e l o w 500 m g / l i t e r ( B O D ) , first-order k i n e t i c s is a s s u m e d in t h e f o r m u l a t i o n . C o n s i d e r t h e c o n t i n u o u s r e a c t o r o p e r a t i n g u n d e r s t e a d y s t a t e a n d c o m p l e t e m i x i n g c o n d i t i o n s . T h i s s i t u a t i o n is i l l u s t r a t e d b y F i g . 5.4. A s s u m i n g t h a t r a t e o f s u b s t r a t e r e m o v a l dS/dt follows first-order k i n e t i c s , * e
5
dSjdt = -KS
(5.6)
I t is c u s t o m a r y t o e x p r e s s s u b s t r a t e r e m o v a l r a t e p e r m g / l i t e r o f M L V S S * Minus sign in Eq. (5.6) is required since dS/dt < 0, whereas 5 > 0.
3.
167
Kinetics Relationships
Q.S
V
Q,s
0
*
~ -
Fig. 5.4. Simplified
X
v,a
diagram
p r e s e n t in t h e r e a c t o r . L e t Χ
e
——
for continuous
reactor.
b e t h i s M L V S S c o n c e n t r a t i o n . E q u a t i o n (5.6)
υα
is t h e n r e w r i t t e n (\IX , )(dSldt)
=
v a
-kS
(5.7)
T h e r e l a t i o n s h i p b e t w e e n Κ a n d k is Κ =
kX
(5.8)
v
F r o m E q . (5.7) dS/dt
=
-kX , S
(5.9)
v a
k is t h e s u b s t r a t e r e m o v a l r a t e c o n s t a n t . F o r t i m e t e q u a l t o r e s i d e n c e t i m e in t h e c o n t i n u o u s r e a c t o r , c o n c e n t r a t i o n S c o r r e s p o n d s t o 5 , a n d E q . (5.9) becomes (dS/dt) . factor = ~ kX S (5.10) e
cont
Vt
a
e
T h e f o l l o w i n g m a t e r i a l b a l a n c e for s u b s t r a t e is w r i t t e n for t h e r e a c t o r in F i g . 5.4. Change of substrate in reactor = increase d u e to influent flow - decrease d u e t o effluent flow - decrease d u e t o reaction
(5.11)
U n d e r steady state conditions, C h a n g e of substrate in reactor = 0 Increase due to influent
flow
(5.12)
= QS
(5.13)
Decrease due t o effluent flow = QS
(5.14)
Q
and e
A c c o r d i n g t o E q . (5.10), t h e d e c r e a s e in t h e a m o u n t o f s u b s t r a t e d u e t o t h e r e a c t i o n is kX S [ m i n u s sign a l r e a d y i n c l u d e d in E q . ( 5 . 1 1 ) ] . B e f o r e s u b s t i t u t i n g in E q . (5.11) t h i s v a l u e is m u l t i p l i e d b y r e a c t o r v o l u m e V since kX S r e p r e s e n t s d e c r e a s e p e r u n i t v o l u m e . va
e
y
v
a
e
Decrease due to reaction =
kX S V Vta
e
(5.15)
168
5.
Secondary Treatment: The Activated S l u d g e Process
S u b s t i t u t i o n o f v a l u e s g i v e n b y E q s . ( 5 . 1 2 ) - ( 5 . 1 5 ) in E q . (5.11) yields after manipulation (QIV)l(S.-S.)IX .J
=
9
(5.16)
kS.
However, t = VIQ =
Mgal
= day = residence time (/) in the reactor
(Mgal/day)
(5.17)
C o n s e q u e n t l y , E q . (5.16) is (S -S )IX t 0
T e r m (S -~S )/X t 0
e
e
=
0ta
kS
(5.18)
e
w h i c h a l s o a p p e a r s in o t h e r f o r m u l a t i o n s is t h e s u b
Vta
s t r a t e r e m o v a l r a t e . It c o r r e s p o n d s t o r a t e o f r e m o v a l o f s u b s t r a t e in t h e continuous reactor per mg/liter of M L V S S present. Units are (S -S )IX , t 0
e
v a
=
mg/liter of B O D removed (mg/liter of MLVSS) (day)
= m g B O D removed/(day)(mg MLVSS) = lb B O D removed/(day)(lb MLVSS)
ω co
5
Equation: (VS )/X ,a e
v
t
=
k S
e
(5.18)
Reactor No. 2
•Reactor No. 3
-Reactor No. 4
S ;mg/liter (effluent) e
Fig. 5.5.
Graphical
determination
of k (four
continuous
lab scale
reactors).
4.
169
Material Balance Relationships
E q u a t i o n (5.18) i n d i c a t e s t h a t t h e s u b s t r a t e r e m o v a l r a t e is p r o p o r t i o n a l t o substrate concentration S
(first-order k i n e t i c s ) . S u b s t r a t e r e m o v a l r a t e c o n
e
stant k
(day ) - 1
(S -S )/X t 0
e
is d e t e r m i n e d
vs. S .
Vta
e
continuous
a c c o r d i n g t o E q . (5.18) f r o m
a plot
of
F i g u r e 5.5 s h o w s a g r a p h o f d a t a o b t a i n e d f r o m f o u r
laboratory
reactors
operating at steady
state conditions.
A
n u m e r i c a l a p p l i c a t i o n is p r e s e n t e d in S e c t i o n 6.4 ( E x a m p l e 5.5). D a t a p l o t t e d i n F i g . 5.5 yield a s t r a i g h t l i n e p a s s i n g t h r o u g h t h e o r i g i n , a s s u m i n g a p p l i c a b i l i t y o f t h e m a t h e m a t i c a l m o d e l in E q . (5.18). T h e l e f t - h a n d m e m b e r , (S — S )/X t, 0
e
Vfa
v a n i s h e s a s t a p p r o a c h e s infinity (infinite r e s i d e n c e
time). Consequently, term S
e
in t h e r i g h t - h a n d m e m b e r a p p r o a c h e s
since k Φ 0. T h i s c o r r e s p o n d s t o complete
zero
r e m o v a l o f s u b s t r a t e , w h i c h is n o t
a l w a y s t h e c a s e since s o m e s u b s t r a t e s c a n n o t b e c o m p l e t e l y d e g r a d e d b y t h e a e r o b i c b i o l o g i c a l p r o c e s s , e v e n a t infinite r e s i d e n c e t i m e . I n t h e s e c a s e s , t h e s t r a i g h t line c u t s t h e a b s c i s s a a t a v a l u e o f S
e
> 0 corresponding to the con
c e n t r a t i o n o f n o n b i o d e g r a d a b l e m a t t e r . A n e x a m p l e o f t h i s s i t u a t i o n is s h o w n i n F i g . 5.14 ( S e c t i o n 6.4, E x a m p l e 5 . 5 ) . When
nonbiodegradable
m a t t e r is p r e s e n t , E q . (5.18) is m o d i f i e d
to
E q . (5.19). (S.-SJIX . t 9 m
= k(S -S ) e
n
(5.19)
w h e r e S is t h e c o n c e n t r a t i o n o f n o n b i o d e g r a d a b l e m a t t e r . n
4. Material B a l a n c e R e l a t i o n s h i p s 4.1. D E S I G N P A R A M E T E R S C O R R E S P O N D I N G TO NET YIELD OF M L V S S A N D O X Y G E N R E Q U I R E M E N T S FOR A E R O B I C BIOLOGICAL D E G R A D A T I O N OF W A S T E S 4.1.1. I n t r o d u c t i o n : M e c h a n i s m of A e r o b i c Biological D e g r a d a t i o n A c c u m u l a t i o n of M L V S S a n d utilization of oxygen are t w o i m p o r t a n t e l e m e n t s n e e d e d for d e s i g n o f a e r o b i c b i o l o g i c a l r e a c t o r s . T o o b t a i n m a t h e m a t i c a l m o d e l s w h i c h yield t h e s e t w o v a l u e s , s e v e r a l d e s i g n p a r a m e t e r s d e s i g n a t e d b y s y m b o l s a', a,a,b, a n d V a r e defined in t h i s s e c t i o n . T h e a p p r o a c h f o l l o w e d is t h a t p r o p o s e d b y E c k e n f e l d e r a n d a s s o c i a t e s [ 1 - 3 ] . E v a l u a t i o n o f t h e s e p a r a m e t e r s is a c c o m p l i s h e d b y u s i n g b e n c h scale c o n t i n u o u s b i o l o g i c a l r e a c t o r s ( S e c t i o n 6). I n t h e d i s c u s s i o n w h i c h f o l l o w s , n u m e r i c a l v a l u e s for t h e s e p a r a m e t e r s a r e u t i l i z e d for c l a r i f i c a t i o n o f s o m e c o n c e p t s . T h e s e v a l u e s a r e o b t a i n e d b y t e c h n i q u e s d i s c u s s e d i n S e c t i o n 6. T o a r r i v e a t t h e definition o f t h e s e p a r a m e t e r s , t h e b a s i c m e c h a n i s m o f aerobic d e g r a d a t i o n of a substrate m u s t b e u n d e r s t o o d . C o n s i d e r t h a t a s u b s t r a t e is c h a r g e d t o a b a t c h r e a c t o r ( F i g . 5.2), a n d t h a t c u r v e s for B O D
170
5.
Secondary Treatment: The Activated Sludge Process
r e m o v a l a n d M L V S S c o n c e n t r a t i o n a r e o b t a i n e d ( F i g . 5.3). F o r c l a r i f i c a t i o n , t a k e the hypothetical case of p u r e lactose as substrate. A s s u m e t h a t a lactose s o l u t i o n is c h a r g e d t o t h e b a t c h r e a c t o r w i t h a s e e d o f m i c r o o r g a n i s m , a n d t h a t c o m p r e s s e d a i r is b u b b l e d t h r o u g h t h e s o l u t i o n . L e t i n i t i a l c o n c e n t r a t i o n o f l a c t o s e b e e q u a l t o 1050 m g / l i t e r . S u p p o s e t h a t after a t i m e t t h i s c o n c e n t r a t i o n is r e d u c e d t o 50 m g / l i t e r . T h u s s u b s t r a t e r e m o v e d is 1050— 50 = 1000 m g / l i t e r . A s s u m e t h a t T h O D is u t i l i z e d a s a m e a s u r e o f l a c t o s e c o n c e n t r a t i o n . * T h e c h e m i c a l e q u a t i o n c o r r e s p o n d i n g t o T h O D for l a c t o s e is [ E q . ( 5 . 2 0 ) ] (CH 0) + 0 2
Molecular weight:
30
2
-» C 0
2
+ H 0
f
(5.20)
2
32
T h u s , t h e i n i t i a l T h O D o f t h e s o l u t i o n is ( 3 2 / 3 0 ) χ 1050 = 1120 m g / l i t e r . A f t e r t i m e t, r e m a i n i n g T h O D is ( 3 2 / 3 0 ) χ 50 = 53.3 m g / l i t e r . T h e r e f o r e , T h O D r e m o v e d is 1 1 2 0 - 53.3 = 1066.7 mg/liter or (32/30) ( 1 0 5 0 - 5 0 ) = 1066.7 mg/liter
(5.21)
Thus, T h O D and substrate removed are proportional, the proportionality c o n s t a n t b e i n g 3 2 / 3 0 = 1.07. S i n c e T h O D is c o r r e l a t e d t o C O D , B O D , e t c . , o n e m a y also express substrate r e m o v a l in t e r m s of these p a r a m e t e r s . Substrate oxidation
End products: C0 ,H 0,N ,P...
for energy production , Design parameter : a*
2
2
2
Substrate (e.g. lactose)
Synthesis phase Design parameters:
Endogenous respiration
New cells
Fig. 5.6. Mechanism
Design parameters:
of aerobic
biological
End products: C0 ,H 0,NH3,P nonbiodegradable products 2
2
degradation.
* A s discussed in Chapter 2, T h O D is only utilized in rare cases when complete analysis of the wastewater is known. t F o r simplicity in Eq. (5.20), lactose is represented by one sugar unit ( C H 0 ) . Multiplying this unit by a factor o f 12 o n e obtains 2
12(CH 0) = C 2
which is the molecular formula for lactose.
1 2
H
2 2
0„ ·H 0 2
4.
Material B a l a n c e
171
Relationships
M e c h a n i s m o f a e r o b i c b i o l o g i c a l d e g r a d a t i o n o f a s u b s t r a t e is r e p r e s e n t e d d i a g r a m m a t i c a l l y b y F i g . 5.6. D e s i g n p a r a m e t e r s (α', a, a, b, a n d b') i n d i c a t e d i n F i g . 5.6 a r e defined i n S e c t i o n s 4 . 1 . 2 - 4 . 1 . 9 . T h e s e v a l u e s a r e c a l c u l a t e d f r o m l a b o r a t o r y d a t a ( S e c t i o n 6). F i g u r e 5.6 i n d i c a t e s t h a t s u b s t r a t e is r e m o v e d d u r i n g t h e b i o l o g i c a l p r o c e s s in t w o w a y s . 1. P a r t o f t h e s u b s t r a t e , after b e i n g c o n s u m e d a s f o o d b y m i c r o o r g a n i s m s , is utilized t o s y n t h e s i z e n e w m i c r o o r g a n i s m cells. T h i s c o r r e s p o n d s t o t h e synthesis p h a s e . F o r the lactose example, this synthesis c o r r e s p o n d s to* synthesis
5(CH 0)
> C H N0
2
5
Molecular weight: 5 χ 30 = 150
/
7
W
T
113 ,
(5.22)
2
7 0 0
.
(MLVSS)
I n t e r m e d i a t e s t e p s in E q . (5.22) a r e c o m p l i c a t e d a n d i r r e l e v a n t . T h e e m p i r i c a l f o r m u l a C H N 0 c o r r e s p o n d s t o t h e a v e r a g e c o m p o s i t i o n o f M L V S S cells. N i t r o g e n is n e e d e d for s y n t h e s i s a n d m u s t b e p r o v i d e d . F r o m t h e a p p r o x i m a t e e m p i r i c a l f o r m u l a C H N 0 it follows t h a t % o f n i t r o g e n in t h e M L V S S cells is ( 1 4 / 1 1 3 ) x 100 = 12.4%. 5
7
2
5
7
2
2. T h e r e m a i n d e r o f t h e s u b s t r a t e is o x i d i z e d , t e r m i n a l p r o d u c t s b e i n g CO a n d H 0 . I n t h e l a c t o s e e x a m p l e , t h i s s u b s t r a t e o x i d a t i o n c o r r e s p o n d s t o E q . (5.20). T h i s t e r m i n a l o x i d a t i o n p r o c e s s is e x t r e m e l y i m p o r t a n t i n t h e p r o d u c t i o n o f c e l l u l a r e n e r g y utilized b y t h e cells t o m a i n t a i n t h e i r n o r m a l functions, such as synthesis, reproduction, a n d mobility. A s s u m e t h a t 6 5 % of t h e l a c t o s e r e m o v e d (i.e., 6 5 % o f 1000 m g / l i t e r = 6 5 0 m g / l i t e r ) is d i o x i z e d t o p r o v i d e e n e r g y r e q u i r e m e n t s , a n d t h a t 3 5 % (i.e., 3 5 0 m g / l i t e r ) is u t i l i z e d i n t h e s y n t h e s i s o f n e w cell m a t t e r . S i n c e t h e r e is a p r o p o r t i o n a l i t y c o n s t a n t r e l a t i n g s u b s t r a t e a n d T h O D r e m o v a l s [ f a c t o r (32/30) in E q . (5.20) f o r l a c t o s e ] , it f o l l o w s t h a t 6 5 % o f t h e T h O D r e m o v e d is u t i l i z e d for e n e r g y g e n e r a t i o n a n d 3 5 % for s y n t h e s i s o f n e w cells. S i m i l a r s t a t e m e n t s a r e v a l i d in t e r m s o f C O D a n d o t h e r p a r a m e t e r s defined in C h a p t e r 2 ( S e c t i o n s 2 a n d 3). 2
2
t Phosphorus is also utilized in the synthesis and b e c o m e s a constituent o f cell matter. The % o f phosphorus in the M L V S S cells is approximately 2%, s o a more accurate empirical formula for the M L V S S cells is C H N 0 / n where η is given by (atomic weight o f phosphorus = 31) >
5
7
2
31/i/(113 + 31/i) =
2/100
.·. η = 0.074 or C H N 0 P o . o 7 4 - Nitrogen and phosphorus needed are provided by addition o f a m m o n i u m phosphate to the wastewater, if it d o e s not already contain the nitrogen and phosphorus required. 5
7
2
172
5.
Secondary Treatment: The Activated Sludge Process
4.1.2. Definition of Parameter a ( S y n t h e s i s Phase) L e t α b e t h e f r a c t i o n o f s u b s t r a t e r e m o v e d t h a t is u t i l i z e d for s y n t h e s i s ( n a m e l y , α = 0.35 in l a c t o s e e x a m p l e ) . D u e t o t h e p r o p o r t i o n a l i t y b e t w e e n removal of substrate a n d those of T h O D , C O D , or B O D , α also represents f r a c t i o n s o f T h O D ( o r C O D , B O D ) u t i l i z e d f o r s y n t h e s i s o f n e w cells. Therefore, α = lb of substrate removed utilized for synthesis/lb of total substrate removed = lb T h O D removed for synthesis/lb of total T h O D removed = lb C O D removed for synthesis/lb total C O D removed = lb B O D removed for synthesis/lb total B O D removed
(5.23)
T h e n u m e r i c a l v a l u e o f α is i n d e p e n d e n t o f p a r a m e t e r s u t i l i z e d f o r e x p r e s s i n g s u b s t r a t e r e m o v a l , since α r e p r e s e n t s t h e f r a c t i o n o f s u b s t r a t e r e m o v e d utilized for s y n t h e s i s , a n d is t h e r e f o r e a d i m e n s i o n l e s s q u a n t i t y . T h e s a m e c o n v e r s i o n f a c t o r f o r c h a n g i n g p a r a m e t e r s i n w h i c h s u b s t r a t e r e m o v a l is t o be expressed appears simultaneously in the n u m e r a t o r a n d d e n o m i n a t o r of E q . (5.23), a n d t h e r e f o r e c a n c e l s o u t . P a r a m e t e r α does not a p p e a r in t h e final f o r m u l a t i o n o f a e r o b i c p r o c e s s e s d e v e l o p e d in S e c t i o n 6. I n s t e a d p a r a m e t e r a, w h i c h is r e l a t e d t o a , is u t i l i z e d .
4.1.3. Definition of Parameter a ' (Oxidation) L e t a' b e t h e f r a c t i o n o f s u b s t r a t e r e m o v e d u t i l i z e d f o r e n e r g y p r o d u c t i o n ( n a m e l y , a' = 0.65 in l a c t o s e e x a m p l e ) . Therefore, α + α'=1.0
(5.24)
a' = lb of substrate removed utilized for energy/lb of total substrate removed = lb T h O D removed for energy/lb total T h O D removed = lb C O D removed for energy/lb total C O D removed = lb B O D removed for energy/lb total B O D removed
(5.25)
where
T h e n u m e r i c a l v a l u e o f a' defined b y E q . (5.25) is i n d e p e n d e n t o f t h e p a r a m e t e r s utilized for e x p r e s s i n g s u b s t r a t e r e m o v a l . T h e s a m e
observations
m a d e for a a r e applicable here. SUMMARY
F o r the lactose example
T o t a l s u b s t r a t e r e m o v e d : 1000 m g / l i t e r T o t a l T h O D r e m o v e d : 3 2 / 3 0 χ 1000 = 1066.7 m g / l i t e r . T h e s e r e m o v a l s t a k e p l a c e in t w o w a y s :
173
Material Balance Relationships
4.
(1) S y n t h e s i s : 5(CH 0) -
C H N0
2
5
7
2
Substrate r e m o v e d utilized for synthesis: (0.35)(1000) = 350 mg/liter T h O D r e m o v e d for s y n t h e s i s : (0.35)(1066.7) = 373.3 mg/liter [ R a t i o s , 3 5 0 / 1 0 0 0 = 3 7 3 . 3 / 1 0 6 6 . 7 = 0.35 = a] (2) E n e r g y p r o d u c t i o n : (CH 0) + 0 2
2
-> C 0
2
+ H 0 2
Substrate r e m o v e d utilized for energy p r o d u c t i o n : (0.65)(1000) = 650 mg/liter T h O D r e m o v e d for e n e r g y p r o d u c t i o n : (0.65)(1066.7) = 693.4 mg/liter [ R a t i o s , 6 5 0 / 1 0 0 0 = 6 9 3 . 4 / 1 0 6 6 . 7 = 0.65 = α ' ] F r o m E q . (5.20) T h O D r e m o v e d for e n e r g y p r o d u c t i o n e q u a l s t h e l b o f o x y g e n u t i l i z e d for o x i d a t i o n o f s u b s t r a t e . T h e r e f o r e t h e d e f i n i t i o n o f a' (in t e r m s o f T h O D ) given b y E q . (5.25) is r e f o r m u l a t e d a s a'
=
0THOD
= lb of 0
2
utilized in oxidation of substrate/lb of total T h O D removed (5.26)
i.e., a' is e q u a l t o t h e l b o f o x y g e n u t i l i z e d in e n e r g y p r o d u c t i o n p e r l b o f t o t a l T h O D removed. T h e r e f o r e f r o m E q . (5.26), lb 0
2
(for energy) = a'(lb total T h O D removed) =
flTho (lb D
total T h O D removed)
(5.27)
W r i t i n g t h e r i g h t - h a n d m e m b e r o f E q . (5.27) in t e r m s o f C O D , B O D , a n d T O C b y u t i l i z i n g r a t i o s T h O D / C O D , T h O D / B O D , e t c . , yields lb 0
2
(for energy) = a\\b total C O D removed) ( T h O D / C O D ) = a\\b total B O D r e m o v e d ) ( T h O D / B O D )
(5.28)
D e f i n e s u b s c r i p t v a l u e s o f a' a s acoo = a ( T h O D / C O D )
(5.29)
= a'(ThOD/BOD)
(5.30)
ΛΒΟΟ
( w h e r e a' w i t h o u t t h e s u b s c r i p t s t a n d s for v a l u e a' =
a' ^). Th0l
174
5.
Secondary Treatment: The Activated Sludge Process
C o m b i n i n g E q s . (5.27) a n d (5.28) w i t h E q s . (5.29) a n d (5.30), lb 0
2
(for energy) = flThooOb total T h O D removed) = tfcoDOb total C O D removed) = 0BOD(lb total B O D removed)
(5.31)
H e n c e , w h e n e v e r p a r a m e t e r a' is utilized for c a l c u l a t i o n o f o x y g e n r e q u i r e m e n t s , n o s u b s c r i p t s a r e i n d i c a t e d . A n a p p r o p r i a t e v a l u e o f a' is c h o s e n t o b e c o m p a t i b l e w i t h p a r a m e t e r s for e x p r e s s i n g s u b s t r a t e r e m o v a l . F r o m
Eq.
(5.31) it follows t h a t a' e q u a l s t h e l b o f o x y g e n u t i l i z e d f o r e n e r g y p r o d u c t i o n p e r lb o f s u b s t r a t e r e m o v e d ( r e m o v a l i n t e r m s o f T h O D , C O D , a n d T O D ) . U t i l i z a t i o n o f s u b s c r i p t s C O D a n d B O D for a' m a y s e e m i n c o n s i s t e n t s i n c e a! is t h o u g h t o f a s a r a t i o , a n d t h e r e f o r e its n u m e r i c a l v a l u e s h o u l d b e pendent
inde
o f p a r a m e t e r s utilized f o r e x p r e s s i n g r e m o v a l . H o w e v e r , t h i s i n d e
p e n d e n c e a p p l i e s o n l y t o v a l u e s o f a' a s defined b y E q . (5.25). I n E q . (5.25) t h e s a m e c o n v e r s i o n f a c t o r for p a r a m e t e r s e x p r e s s i n g r e m o v a l a p p e a r s s i m u l t a n e o u s l y in t h e n u m e r a t o r a n d d e n o m i n a t o r , a n d t h e r e f o r e c a n c e l s o u t . F r o m E q . (5.31), h o w e v e r , it f o l l o w s t h a t a m o d i f i e d definition o f a' is b e i n g u t i l i z e d , i.e., flThOD
=
a' = lb 0
2
(for energy)/lb total T h O D removed
(5.32)
a'coD = lb 0
2
(for energy)/lb total C O D removed
(5.33)
a oD = lb 0
2
(for energy)/Ib total B O D removed
(5.34)
B
T h e n u m e r i c a l v a l u e o f t h e n u m e r a t o r s in E q s . (5.32), (5.33), a n d (5.34) is t h e s a m e ( l b o f o x y g e n utilized for e n e r g y r e q u i r e m e n t s ) . V a l u e s o f d e n o m i n a t o r s , h o w e v e r , v a r y d e p e n d i n g o n c h o i c e o f p a r a m e t e r s for
expressing
s u b s t r a t e r e m o v a l . C o n s e q u e n t l y , n u m e r i c a l v a l u e s o f a' f r o m E q s . (5.32), (5.33), a n d (5.34) a r e different f r o m e a c h o t h e r . T h e r e f o r e , u t i l i z a t i o n o f s u b s c r i p t s is justified. F u r t h e r m o r e , only t h e v a l u e o f a' given b y E q . (5.32) is n u m e r i c a l l y e q u a l t o t h e r a t i o s defined b y E q . (5.25), i.e., a j h O D = ' · V a l u e s o f a' g i v e n b y E q s . (5.33) a n d (5.34) a r e n o t o n l y different f r o m e a c h o t h e r , b u t a l s o n e i t h e r e q u a l s t h e f r a c t i o n o f s u b s t r a t e r e m o v e d u t i l i z e d in e n e r g y p r o d u c t i o n . a
4.1.4. Definition of Parameter a ( S y n t h e s i s Phase) P a r a m e t e r a, r e l a t e d t o a , is defined a s a = lb of M L V S S produced/lb of total substrate removed
(5.35)
C o n s e q u e n t l y , a r e p r e s e n t s yield o f b i o l o g i c a l s l u d g e p e r l b o f t o t a l s u b s t r a t e removed.
4.
Material Balance Relationships
175
T h e r e l a t i o n s h i p b e t w e e n p a r a m e t e r s a a n d a is a r r i v e d a t b y c o n s i d e r a t i o n o f t h e l a c t o s e e x a m p l e [ E q . ( 5 . 2 2 ) ] . I t is a s s u m e d t h a t 3 5 0 m g / l i t e r ( 3 5 % o f t h e t o t a l 1000 m g / l i t e r o f l a c t o s e r e m o v e d ) a r e u t i l i z e d for t h e s y n t h e s i s i n d i c a t e d b y E q . (5.22). Y i e l d o f M L V S S is c a l c u l a t e d a s M L V S S produced per 1000 m g of total substrate removed = [(0.35)(1000)] (113/150) = 263.7 mg/liter
(5.36)
T h e r e f o r e , f r o m E q . (5.36) o n e o b t a i n s a = lb M L V S S produced/lb of total substrate removed = [(0.35)(1000)] (113/150)/1000 = 263.7/1000 = 0.2637
(5.37)
i.e., 2 6 3 . 7 m g / l i t e r o f M L V S S a r e p r o d u c e d p e r 1000 m g / l i t e r o f l a c t o s e r e m o v e d ; t h u s a = 263.7/1000 = 0.2637. T h e r e l a t i o n s h i p b e t w e e n a a n d ά f r o m E q . (5.37) for t h e l a c t o s e e x a m p l e is a = a(l 13/150) .'. a =
fl(150/113)
w h e r e 113/150 is t h e s t o i c h i o m e t r i c r a t i o for E q . (5.22). S u b s t i t u t i o n o f t h i s v a l u e o f a i n E q . (5.24) yields (150/113)fl + a ' = 1.0
(5.38)
P a r a m e t e r a m a y b e w r i t t e n in t e r m s o f t o t a l T h O D r e m o v e d . L e t a
T H 0
D
O E
t h e n u m e r i c a l v a l u e o f a e x p r e s s e d in t h i s m a n n e r . tfThOD
R a t i o a/a
= lb M L V S S produced/lb of total T h O D removed
(5.39)
f r o m E q s . (5.35) a n d ( 5 . 3 9 ) , t a k i n g i n t o a c c o u n t t h e s t o i c h i o
ThOO
m e t r i c r a t i o 3 2 / 3 0 in E q . (5.20), is a/a D ThQ
= E q . (5.35)/Eq. (5.39) = lb total T h O D removed/lb total substrate removed = 32/30
(5.40)
or a =
flTho (32/30)
(5.41)
D
E q u a t i o n (5.38) w r i t t e n in t e r m s o f %
H 0
D
D V
u t i l i z i n g E q . (5.41) is
(150/113)(32/30)a
ThOD
+ a' = 1.0
a
or 1 . 4 2 f l o D + a' = Th
a
1.0
(5.42)
176
Secondary Treatment: The Activated Sludge Process
5.
M L V S S yield ( s y n t h e s i s ) is o b t a i n e d f r o m E q . (5.39). lb M L V S S produced = a h O D ( l b total T h O D removed)
(5.43)
T
E q u a t i o n (5.43) m a y b e r e w r i t t e n e x p r e s s i n g s u b s t r a t e r e m o v a l i n t e r m s o f C O D , B O D , etc., by utilizing ratios T h O D / C O D , T h O D / B O D , etc.: lb M L V S S produced = a h o ( l b total C O D removed) ( T h O D / C O D ) T
D
= 0 T h o ( l b total B O D r e m o v e d ) ( T h O D / B O D ) D
(5.44)
Define flcoo =
flxnoD
(ThOD/COD)
(5.45)
a oD =
tfihOD
(ThOD/BOD)
(5.46)
B
Therefore lb M L V S S p r o d u c e d = a D(\b CO
=
flBo (lb D
total C O D removed) total B O D removed)
(5.47)
N o s u b s c r i p t s a r e u t i l i z e d for t h e p a r a m e t e r a h e n c e . I t is u n d e r s t o o d t h a t t h e a p p r o p r i a t e v a l u e o f p a r a m e t e r a is c h o s e n t o b e c o m p a t i b l e w i t h t h e p a r a m e t e r s for e x p r e s s i n g s u b s t r a t e r e m o v a l .
4.1.5. A n Observation Concerning Factor 1.42 A l t h o u g h f a c t o r 1.42 i n E q . (5.42) is o b t a i n e d i n S e c t i o n 4 . 1 . 4 f r o m c o n s i d e r a t i o n o f t h e specific l a c t o s e e x a m p l e , it is s h o w n n e x t t h a t it a p p l i e s t o all s u b s t r a t e s , p r o v i d e d t h e a v e r a g e e m p i r i c a l f o r m u l a for t h e M L V S S is t a k e n a s C H N 0 . C o n s i d e r t h e specific l a c t o s e e x a m p l e . W r i t e E q s . (5.20) a n d 5
7
2
(5.22), m u l t i p l y i n g t h e first o n e b y a f a c t o r o f 5, i.e., 5(CH 0) + 2
Molecular weight:
5 χ 30
50
2
-+ 5 C 0
2
+ 5H 0
(5.48)
2
5x32
R e c a l l t h a t f a c t o r 1.42 o r i g i n a t e d f r o m [ E q . ( 5 . 4 2 ) ] . (150/113) (32/30) = 1.42 or
(5.49) [5(30)]/113 χ 32/30 = 1.42 T h e " m o l e c u l e " o f s u b s t r a t e is defined h e r e a s a s u g a r g r o u p ( C H 0 ) c o n 2
t a i n i n g one c a r b o n a t o m , w h i c h c o r r e s p o n d s t o a " m o l e c u l a r w e i g h t " o f 3 0 . N o t i c e t h a t in E q . (5.49), t h e m o l e c u l a r w e i g h t o f s u b s t r a t e (30 i n t h i s c a s e ) is c a n c e l e d o u t . F o r a n y s u b s t r a t e o f m o l e c u l a r w e i g h t Af, E q . (5.49) is 5A//113 χ 3 2 / M = ( 5 x 3 2 ) / 1 1 3 = 1.42
(5.50)
4.
177
Material Balance Relationships
T h u s , E q . ( 5 . 4 2 ) is a n approximate
equation for m o s t substrates, t h e only
restriction being t h e assumption t h a t t h e average empirical formula for M L V S S is C H N 0 . I n S e c t i o n 4 . 1 . 9 , i t is s h o w n t h a t v a l u e 1.42 c o r r e s p o n d s 5
7
2
to lb o f oxygen required t o oxidize 1 l b o f M L V S S d u r i n g t h e process o f endogenous respiration. 4.1.6.
Note:
Summary
A p p r o x i m a t e values of the ratio between parameters for expressing
oxygen d e m a n d are taken from Table 2.1. a. Parameter (See
a' in Different
Tabulation
ThOO
a
Units
Below) lb 0 = axhOD ( l b total T h O D removed) (energy) where a' = a' = fraction o f substrate removed utilized for energy production 2
ThOD
tfcOD
(standard C O D test)
BOD
a
(5-day B O D )
lb 0 = flcoDGb total C O D removed) (energy) where αόοο = a ' ( T h O D / C O D ) = a'(10O/83) = 1.20a' 2
lb 0 = a i o D f l b total B O D removed) (energy) where a' = a ' ( T h O D / B O D ) = α'(1Ο0/58) = 1.72a' 2
BOD
Relationships for other oxygen a n d c a r b o n p a r a m e t e r s studied in C h a p t e r 2 are readily written. b. Parameter a in Different (See Tabulation Below)
#ThOD
Units
lb M L V S S produced = a D ( l b total T h O D removed) where a D = 5/1.42; a — fraction o f substrate removed utilized for synthesis T h 0
T
h
0
(standard C O D test)
lb M L V S S produced = a D ( l b total C O D removed) where a D = A t h o d ( T h O D / C O D ) = a D ( 1 0 0 / 8 3 ) = (α/1.42)(100/83) = 0.85a
(5-day B O D )
lb M L V S S produced = a o ( l b total B O D removed) where α = A t h o d ( T h O D / B O D ) = α ποο(100/58) = (α/1.42)(100/58) = 1.21α
#COD
C O
C
T h O
O
B
ΒΟΌ
D
Τ
R e l a t i o n s h i p s f o r o t h e r o x y g e n a n d c a r b o n p a r a m e t e r s defined i n C h a p t e r 2 are readily written.
178
5.
c. Equation (5.24) the Parameters
Written
Secondary Treatment: The Activated Sludge Process
with Different
(See Tabulation
Units
for
Below)
a' = fraction of the total substrate removed utilized for energy = total substrate removed utilized for synthesis. Then a+a' = 1.0. ThOD
αχποο;
o. = fraction o f
1.42a + < i T h O D = 1.0 or 1.42fl hOD + « = 1.0 T h O D
/
T
COD (standard C O D test)
flThoo = u t c o o ( C O D / T h O D ) = α«>ο(83/100) « h o D = flcoo(COD/ThOD) = αόοο(83/100) T
.·. 1 . 4 2 ( 8 3 / 1 0 0 ) a o D + (83/100)acoD = 1.0 1 . 1 8 a o D + 0.83acoD= 1 0 C
C
BOD (5-day B O D )
tfrhOD = a o D ( B O D / T h O D ) = 0 O D ( 5 8 / 1 O O ) B
B
a' = flxhoo = ^BOD ( B O D / T h O D ) = O OD(58/100) .'. 1.42(58/100)flBOD + ( 5 8 / 1 0 0 ) f l o D = 1.0 0.82ΛΒΟΟ + 0 . 5 8 Λ Ο Ο = 1 0 B
B
Β
E q u a t i o n (5.24) is r e a d i l y w r i t t e n i n t e r m s o f o t h e r o x y g e n a n d c a r b o n p a r a m e t e r s defined i n C h a p t e r 2 .
4.1.7. D e s i g n Parameters C o r r e s p o n d i n g t o E n d o g e n o u s R e s p i r a t i o n : Introduction T w o d e s i g n p a r a m e t e r s , b a n d b', a r e defined c o r r e s p o n d i n g t o t h e e n d o genous respiration phase. E n d o g e n o u s respiration involves oxidation
of
cellular m a t t e r in o r d e r t o p r o v i d e food for t h e m i c r o o r g a n i s m s w h e n t h e concentration of substrate has decreased considerably. It corresponds t o the "cannibalistic feast" described in Section 3.1. A s s u m i n g t h a t t h e c h e m i c a l f o r m u l a f o r t h e M L V S S is C H N 0 , o x i d a t i o n 5
7
2
o f cells c o r r e s p o n d i n g t o e n d o g e n o u s r e s p i r a t i o n is g i v e n b y E q . (5.51). C H N0 113 5
Molecular weight:
7
2
+
50 -• 5 C 0 5 χ 32 = 160 2
2
+ NH + 2H 0 3
2
(5.51)
4.1.8. Definition of Parameter b ( E n d o g e n o u s Respiration) P a r a m e t e r b is defined a s f r a c t i o n o f M L V S S p e r u n i t t i m e ( d a y ~ \ h o u r " S etc.) o x i d i z e d d u r i n g p r o c e s s o f e n d o g e n o u s r e s p i r a t i o n . F o r e x a m p l e , a v a l u e o f b = 0.1 d a y "
1
m e a n s t h a t 10% of t h e total lb of M L V S S present in t h e
r e a c t o r a t a n y t i m e is o x i d i z e d p e r d a y . T h e r e f o r e , e n d o g e n o u s r e s p i r a t i o n b = lb M L V S S oxidized/(day)(lb M L V S S in reactor)
(5.52)
Consequently, the lb of M L V S S oxidized per d a y are lb M L V S S oxidized/day = b(\b M L V S S in reactor) (endogenous respiration)
(5.53)
4.
Material Balance Relationships
179
M L V S S p r e s e n t i n r e a c t o r a t a n y t i m e a s s u m i n g s t e a d y s t a t e o p e r a t i o n is constant, being given b y lb M L V S S in reactor = X , V
(5.54)
v a
where X is t h e c o n c e n t r a t i o n o f M L V S S , i.e., l b M L V S S p e r u n i t v o l u m e o f reactor; a n d Κ the reactor volume. T h u s E q s . (5.53) a n d (5.54) yield Vta
lb M L V S S oxidized/day = bX (endogenous respiration)
Vf
a
V
(5.55)
4.1.9. Definition of Parameter b' P a r a m e t e r V is defined a s t h e l b o f o x y g e n u t i l i z e d p e r d a y p e r l b o f M L V S S i n t h e r e a c t o r f o r t h e p r o c e s s o f e n d o g e n o u s r e s p i r a t i o n , i.e., [ E q . ( 5 . 5 6 ) ] b> = lb 0 / ( d a y ) ( l b M L V S S inreactor) 2
(5.56)
T h u s , o x y g e n u t i l i z a t i o n f o r e n d o g e n o u s r e s p i r a t i o n is lb 0 / d a y = b'(\b M L V S S in reactor) (endogenous respiration) 2
(5.57)
o r f r o m E q . (5.54) lb0 /day = b'X^ V (endogenous respiration) 2
a
(5.58)
T h e a p p r o x i m a t e r e l a t i o n s h i p b e t w e e n b a n d b' is w r i t t e n a s s u m i n g t h a t a v e r a g e e m p i r i c a l f o r m u l a for M L V S S is C H N 0 , a n d t h a t e n d o g e n o u s r e s p i r a t i o n c o r r e s p o n d s t o c h e m i c a l e q u a t i o n (5.51). F r o m E q s . (5.52) a n d (5.56) r a t i o b'/b is [ E q . (5.59)] 5
7
2
b'\b = lb 0 / l b M L V S S oxidized 2
(5.59)
F r o m E q . (5.51) t h i s r a t i o is b'\b = 1.42
(5.60)
C o n s e q u e n t l y , it t a k e s a p p r o x i m a t e l y 1.42 l b o f o x y g e n t o o x i d i z e 1 l b o f M L V S S . T h i s v a l u e is u s e d a s a n a p p r o x i m a t i o n for a e r o b i c d e g r a d a t i o n o f most substrates. W h e r e a s p a r a m e t e r s a a n d a' a r e r a t i o s [ E q s . (5.25), (5.32), (5.33), a n d (5.34) for a'; a n d E q s . (5.35) a n d (5.39) for a ] , b a n d b' a r e r a t e s . T i m e is n o t i n v o l v e d i n t h e definitions o f a a n d abut it is i n t h o s e o f b a n d b'.
4.2. M A T E R I A L B A L A N C E F O R D E T E R M I N A T I O N OF O X Y G E N UTILIZATION K n o w l e d g e o f o x y g e n r e q u i r e m e n t s t o effect a specified B O D r e m o v a l is n e c e s s a r y for specification o f a e r a t i o n e q u i p m e n t . F r o m d i s c u s s i o n s i n S e c t i o n s 4 . 1 . 3 a n d 4 . 1 . 9 it f o l l o w s t h a t o x y g e n is r e q u i r e d for t w o p u r p o s e s :
180
5.
Secondary Treatment: The Activated Sludge Process
(1) t o o x i d i z e s u b s t r a t e i n o r d e r t o p r o v i d e e n e r g y r e q u i r e m e n t s for cells [ E q . ( 5 . 2 0 ) ] a n d (2) for t h e e n d o g e n o u s r e s p i r a t i o n p r o c e s s [ E q . ( 5 . 5 1 ) ] . 1. Oxygen
required
for
energy.
T h e lb o f o x y g e n r e q u i r e d p e r d a y a r e
c a l c u l a t e d f r o m E q . (5.31). R e f e r r i n g t o F i g . 5.1 a n d s y m b o l s defined in T a b l e 5.1, l b 0 / d a y = a\S -S )Q (for energy) 2
0
(5.61)
e
A p p r o p r i a t e v a l u e s o f a' c o m p a t i b l e w i t h p a r a m e t e r s in w h i c h t o t a l s u b s t r a t e r e m o v a l (S — S ) is e x p r e s s e d a r e utilized in E q . (5.61). 0
e
Example 5.1 C a l c u l a t e t h e o x y g e n r e q u i r e d for e n e r g y . a
= 0.79 lb 0
BOD
(for energy)/lb total B O D
2
S
0
= 893 mg/liter
S
e
= 40 mg/liter
Q = 2.04 M G D
removed*
5
(2.04 χ 1 0 gal/day) 6
Then S - S 0
= 893 - 40 = 853 mg/liter = S
e
(total substrate removed)
r
Therefore 5
o
_
5
e
=
5 p
=
8
i ^ = liter liquor
5
853
3
' 10 g liquor m
g
B
O
D
3
g BOD. g BOD * . = 853 p p m = 853 χ 1 0 ~ — 1 0 g liquor g liquor Λ
=
853
r
r
6
6
lbBOD _ lbBOD = 853 χ Ι Ο " — = 853lb liquor M l b liquor r
o
r
6
F r o m E q . (5.61), lb 0 lb B O D l b 0 / d a y = 0.79 χ 853 χ 1 0 ~ — , \ lbBOD lb liquor (for energy) 2
r
6
2
c
r
n
, liquor lb liquor χ 2.04 χ 1 0 g a l - ~ — * 8 . 3 4 - — ^ day gal liquor = 11,500 lb 0 / d a y 6
2
* Experimental determination of parameter a' is described in Section 6.3.2. Example 5.1 is simply an illustration of unit conversion. Value a' = 0.79 is determined experimentally (Example 5.5, Section 6.4).
4.
181
Material Balance Relationships
If S is in m g / l i t e r a n d Q in M G D o w i n g t o c a n c e l l a t i o n o f f a c t o r s 10
6
r
a n d 1 0 , E q . (5.61) b e c o m e s E q . (5.62). 6
Ib0 /day
= a'S Q
2
r
χ 8.34
(5.62)
(for energy) 2. Oxygen
required for endogenous
respiration.
E q u a t i o n (5.58) is utilized
for t h i s c a l c u l a t i o n , i l l u s t r a t e d b y E x a m p l e 5.2.
Example 5.2 C a l c u l a t e t h e o x y g e n r e q u i r e d for e n d o g e n o u s r e s p i r a t i o n . L e t b = 0.15 lb 0 / ( d a y ) ( l b M L V S S in reactor)* f
2
X
Vt
a
= 300 mg/liter
V = 1.2 M G
(of MLVSS)
(1.2 χ 1 0 gal)
(reactor volume)
6
By a s i m i l a r p r o c e d u r e t o t h a t in E x a m p l e 5.1 it follows t h a t [ E q . ( 5 . 6 3 ) ] lb 0 / d a y = b'X (endogenous respiration) 2
Vt
a
V χ 8.34
w h e r e V is t h e l b 0 / ( d a y ) ( l b M L V S S in r e a c t o r ) , X 2
VyQ
(5.63)
the mg/liter of M L V S S ,
and Κ the reactor volume ( M G ) . Consequently, lb 0 / d a y = 0.15 χ 3000 χ 1.2 χ 8.34 = 4500 lb 0 / d a y (endogenous respiration) 2
SUMMARY (5.58) a s
2
T o t a l o x y g e n u t i l i z a t i o n is g i v e n b y t h e s u m o f E q s . (5.61) a n d
lb 0 / d a y = a\S 2
a
-S )Q e
+ b'X^ V = a'S Q a
r
+ b'x , v
a
V
(5.64)
F o r E x a m p l e s 5.1 a n d 5.2, lb 0 / d a y = 11,500 + 4500 = 16,000 lb 0 / d a y 2
2
4.3. MATERIAL BALANCE FOR DETERMINATION OF NET YIELD OF BIOLOGICAL S L U D G E (MLVSS) F r o m S e c t i o n s 4.1.4 a n d 4.1.8 it follows t h a t (1) a f r a c t i o n o f t h e s u b s t r a t e r e m o v e d is utilized in p r o d u c t i o n o f M L V S S , t h e lb of M L V S S p r o d u c e d b e i n g given b y E q . (5.47), a n d t h a t (2) p a r t o f t h e s l u d g e p r o d u c e d is d e s t r o y e d b y o x i d a t i o n ( e n d o g e n o u s r e s p i r a t i o n ) , t h e lb o f s l u d g e o x i d i z e d b e i n g g i v e n b y E q . (5.55). * Experimental determination of parameter b' is described in Section 6.3.2. Example 5.2 is simply an illustration o f unit conversion. Value b' = 0.15 is determined experimentally (Example 5.5, Section 6.4).
182
5.
1. Sludge
produced
Secondary Treatment: The Activated Sludge Process
from
substrate
removal.
S l u d g e p r o d u c e d in l b / d a y
is c a l c u l a t e d f r o m E q . (5.47), w h e r e t o t a l s u b s t r a t e r e m o v a l refers t o o n e - d a y p r o d u c t i o n . R e f e r r i n g t o F i g . 5.1 a n d s y m b o l s defined in T a b l e 5 . 1 , lb/day of M L V S S produced = a(S -S )Q 0
=
e
aS Q
(5.65)
r
A p p r o p r i a t e v a l u e s o f a c o m p a t i b l e w i t h p a r a m e t e r s in w h i c h t o t a l s u b s t r a t e r e m o v a l (S — S ) is e x p r e s s e d a r e utilized in E q . (5.65). 0
e
Example 5.3 Calculate M L V S S produced by substrate removal. Let a = 0.575 lb M L V S S produced/lb total B O D S
0
= 893 mg/liter
S
e
= 40 mg/liter
Q = 2.04 M G D
5
removed*
(2.04 χ 1 0 gal/day) 6
C o n v e r s i o n o f u n i t s for E q . (5.65) is s i m i l a r t o t h a t for E q . (5.61) ( E x a m p l e 5 . 1 , S e c t i o n 4.2). T h e final r e s u l t is E q . (5.66). lb/day M L V S S produced = aS Q r
(5.66)
x 8.34
w h e r e .S is in m g / l i t e r a n d Q in M G D . r
Therefore, lb/day M L V S S produced = 0 . 5 7 5 ( 8 9 3 - 4 0 ) χ 2.04 χ 8.34 = 8342 lb/day of M L V S S 2. Sludge
destroyed
by
endogenous
respiration.
e n d o g e n o u s r e s p i r a t i o n is o b t a i n e d f r o m
Sludge destroyed
E q . (5.55). T h i s c a l c u l a t i o n
by is
i l l u s t r a t e d b y E x a m p l e 5.4.
Example 5.4 Calculate M L V S S destroyed by endogenous respiration. Let b = 0.075 lb M L V S S oxidized/(day)(lb M L V S S in reactor) = d a y * 1
X
Vt0
= 3000 mg/liter V = 1.2 M G
(1.2 χ 1 0 gal; reactor volume) 6
* Experimental determination o f parameter a is described in Section 6.3.4. Example 5.3 is simply an illustration o f unit conversion. Value a = 0.575 is determined experimentally (Example 5.5, Section 6.4). Experimental determination o f the parameter b is described in Section 6.3.4. Example 5.4 is simply an illustration o f unit conversion. Value b = 0.075 is determined experimentally (Example 5.5, Section 6.4). f
4.
183
Material Balance Relationships
C o n v e r s i o n o f u n i t s for E q . (5.55) is s i m i l a r t o t h a t f o r E q . (5.58) ( E x a m p l e 5.2, S e c t i o n 4.2). T h e final r e s u l t is E q . (5.67). lb M L V S S oxidized/day = bX , V v a
where X
χ 8.34
(5.67)
is i n m g / l i t e r a n d K i n M G .
VtQ
Therefore lb M L V S S oxidized/day = 0.075 χ 3000 χ 1.2 χ 8.34 = 2252 lb/day of M L V S S SUMMARY
N e t yield o f M L V S S is o b t a i n e d b y t h e difference b e t w e e n
MLVSS
produced
[ E q . (5.65)]
and MLVSS
oxidized
(endogenous
r e s p i r a t i o n ) , g i v e n b y E q . (5.55). T h i s n e t yield i n l b / d a y is d e n o t e d a s ΑΧ
υ
[ E q . (5.68)].
lb M L V S S / d a y = AX (net yield)
V
= a(S -S )Q 0
- bX
e
Vta
V = aS Q
- bX ,
r
v a
V
(5.68)
F o r e x a m p l e s 5.3 a n d 5.4 ΑΧ
= 8342 - 2252 = 6090 lb/day
υ
4.4. T O T A L S L U D G E Y I E L D S o far, o n l y t h e yield o f b i o l o g i c a l s l u d g e ( M L V S S ) h a s b e e n c o n s i d e r e d . N o w , e x a m i n e t h e d i a g r a m f o r t h e r e a c t o r s y s t e m i n F i g . 5 . 1 . T h e fresh feed m a y contain nonvolatile suspended solids (NVSS). L e t X , F b e t h e c o n centration (mg/liter) of these N V S S . N
V
Reactor contents are under conditions of complete mixing, therefore n o settling o f M L N V S S ( o r M L V S S ) t a k e s p l a c e . C o n s e q u e n t l y , c o n c e n t r a t i o n o f N V S S i n r e a c t o r effluent is t h e s a m e a s t h a t i n c o m b i n e d feed ( X ) . In t h e s e c o n d a r y clarifier, h o w e v e r , p a r t o f t h e N V S S a s well a s m o s t o f V S S settles. L e t X b e t h e c o n c e n t r a t i o n o f N V S S i n u n d e r f l o w f r o m t h e clarifier ( s a m e a s i n w a s t a g e Q a n d recycle Q ) . C o n c e n t r a t i o n o f N V S S i n n e t effluent f r o m clarifier (Q') is X . N
N
V
T
V
T
0
U
R
N
V
T
B
Wastage of sludge corresponds t o 1. N e t yield o f b i o l o g i c a l s l u d g e ( M L V S S ) f r o m t h e r e a c t o r . T h i s is A X [ E q . ( 5 . 6 8 ) ] . S i n c e t h e r e a c t o r o p e r a t e s a t s t e a d y s t a t e , t h i s w a s t a g e is e q u a l t o n e t yield o f M L V S S , s o t h a t t o t a l l b o f M L V S S i n t h e r e a c t o r r e m a i n t h e s a m e a t all t i m e s . I n a d d i t i o n , w a s t a g e i n c l u d e s v o l a t i l e s o l i d s e n t e r i n g w i t h fresh V
feed ( Q X , F ) 9 from a n o v e r a l l b a l a n c e o f v o l a t i l e s o l i d s ( l o o p in F i g . 5.1). T h e r e f o r e , t o t a l w a s t a g e o f M L V S S is s h o w n i n E q . (5.69) [ u t i l i z i n g E q . (5.68) f o r ΑΧ ~\. a
F
s
s
e
e
n
V
υ
AX +Q X v
F
VtF
= a(S -S )Q-bX , V+
2. S e t t l e d N V S S d e n o t e d a s
0
A X
e
v a
N
V
QX, F
V F
(5.69)
( l b / d a y ) . T h i s v a l u e is d e t e r m i n e d b y
184
5.
Secondary Treatment: The Activated Sludge Process
a n o v e r a l l m a t e r i a l b a l a n c e for N V S S o v e r l o o p NVSS, I N :
in Fig. 5.1.
QFXNV,F
N V S S , O U T : Q'X , NV
e
+
Q ' X
N
V
„
T
=
e + AX
Q'XNV,
(since AX
NV
NV
= Q"X , „) (5.70) NV
T h u s t h e o v e r a l l b a l a n c e is [ E q . ( 5 . 7 1 ) ] QFXNV,F
AX
NV
=
Q'XNV,
=
Q
X
F
e+ N
V
, F
AX
NV
-
e
Q'XNV,
(5.71)
E l i m i n a t i n g Q' a n d utilizing E q . (5.2), AX v
—
N
=
Q"XNV,u
QFXNV,F
—
=
Q")XNV,e
(QF~
QF(XNV,
F ~
e) +
XNV,
Q"XNV,
e
(5.72) S u b s t i t u t i o n of AX a n d Δ Α ^ in E q . (5.1) b y t h e i r v a l u e s given b y E q s . (5.68) a n d (5.72) yields t o t a l s l u d g e yield AX [ E q . ( 5 . 7 3 ) ] . V
Κ
t
AX
t
= a(So-S )Q
- bX V+Q X ,
e
v>a
F
+
v F
+
Q (XNV,F~~XNV,e) F
Q'X v,e (5.73) N
where a(S
Q
-S )Q-
bX
e
Vt
Q
+
QF(XNV,F-Xsv,e)
F
X
a
V
V = AX
, F
Q'XNV,
= net yield of M L V S S [Eq. (5.68)]
V
= M L V S S in fresh feed
e = AX
= net yield of sludge d u e t o settling N V S S from influent [Eq. (5.72)]
NV
4.5. M A T E R I A L B A L A N C E S F O R
X
N
V
t
AND
0
X
Vr
0
T h e v a l u e of X , i.e., c o n c e n t r a t i o n of N V S S in c o m b i n e d feed, is e s t a b lished b y a m a t e r i a l b a l a n c e a r o u n d t h e j u n c t i o n of t h e fresh feed w i t h t h e recycle t o f o r m c o m b i n e d feed ( F i g . 5 . 1 , l o o p ). NVt0
NVSS, IN = Q X F
NVSS, O U T =
+
NVtF
Q X v,u R
N
QX ,o NV
Then QFXNV,F
+
QRXNV,u
U t i l i z i n g E q s . (5.4) a n d (5.5) a n d s o l v i n g for
X
N
V
<
„ ,
Xsv, ο = (XNV. r + rX , „)/(l + r) NV
A s i m i l a r m a t e r i a l b a l a n c e is w r i t t e n for X , vo
(5.74)
t h e c o n c e n t r a t i o n of V S S in
c o m b i n e d feed. F i n a l r e s u l t is Xv, ο = (Xv. F + rX , „)/(! + r) v
(5.75)
5.
185
Optimum Settling Conditions of Sludge
4.6. TYPICAL V A L U E S OF A E R O B I C BIOLOGICAL WASTEWATER TREATMENT PARAMETERS FOR DIFFERENT T Y P E S OF W A S T E W A T E R S T y p i c a l v a l u e s o f t h e s e p a r a m e t e r s a r e p r e s e n t e d in T a b l e 5.2. T A B L E 5.2 Aerobic Biological Waste—Treatment Parameters*' * Wastewater Domestic Refinery Chemical and petrochemical Brewery Pharmaceutical Kraft pulping and bleaching
a
a'
b
b'
0.73 0.49-0.62
0.52 0.40-0.77
0.075 0.10-0.16
0.106 0.142-0.227
0.017-0.03 0.074
0.31-0.72 0.56 0.72-0.77
0.31-0.76 0.48 0.46
0.05-0.18 0.10 —
0.071-0.255 0.142 —
0.0029-0.018 —
0.114
—
0.5
0.65-0.8
k
c
0.08
0.018
"Adapted from Ref. [ 2 ] . " U n i t s : a, lb M L V S S produced/lb total B O D removed; b lb M L V S S oxidized/(day) (lb M L V S S in reactor) = d a y ; a\ lb 0 (for energy)/lb total B O D r e m o v e d ; b\ lb 0 / (day)(lb M L V S S in reactor) = d a y " ; k, d a y " . Values o f b' estimated from b' = 1.426. 5
y
- 1
2
1
5
2
l
c
5. R e l a t i o n s h i p for O p t i m u m S e t t l i n g C o n d i t i o n s of S l u d g e F o r a d e q u a t e o p e r a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s , M L V S S in t h e r e a c t o r effluent s h o u l d b e r e a d i l y s e p a r a t e d in t h e s e c o n d a r y clarifier. T h e c o n d i t i o n o c c u r r i n g w h e n s l u d g e is light a n d fluffy a n d t h u s difficult t o settle is t e r m e d b u l k i n g . B u l k y s l u d g e flakes o v e r s e p a r a t i n g w e i r s a n d c o m e s o u t w i t h t h e s e c o n d a r y clarifier effluent. S i n c e c o n c e n t r a t i o n o f s u b s t r a t e in t h e effluent is s m a l l , t h e r e is n o t e n o u g h f o o d m a t e r i a l t o s u s t a i n t h e g r o w t h o f t h e microorganisms which constitute the sludge. Therefore the microorganisms are driven to endogenous respiration. O w i n g to the c o n s u m p t i o n of oxygen for e n d o g e n o u s r e s p i r a t i o n , t h e effluent h a s a relatively h i g h B O D , w h i c h is undesirable. Settling characteristics of sludge are evaluated from sedimentation tests p e r f o r m e d in t h e l a b o r a t o r y . F o r t h i s e v a l u a t i o n t w o p a r a m e t e r s a r e utilized. 1. Zone settling velocity (ZSV). T h i s p a r a m e t e r a n d its e x p e r i m e n t a l d e t e r m i n a t i o n a r e d i s c u s s e d in C h a p t e r 3 , S e c t i o n 3.6. A n easily s e t t l i n g s l u d g e h a s a h i g h Z S V o f a b o u t 2 0 ft/hr.
186
5.
2. Sludge
volume
Secondary Treatment: The Activated Sludge Process
index (SVI).
S l u d g e v o l u m e i n d e x is defined a s v o l u m e
(in c m ) o c c u p i e d b y 1 g o f d r y s l u d g e s o l i d s after s e t t l i n g for 3 0 m i n . T h e 3
s m a l l e r t h e S V I , t h e e a s i e r is t h e s e t t l i n g o f t h e s l u d g e . S e v e r a l a u t h o r s h a v e c o r r e l a t e d s e t t l i n g c h a r a c t e r i s t i c s o f s l u d g e (in t e r m s of Z S V or SVI) with a parameter designated as food to microorganism ratio ( h e n c e d e n o t e d a s F/M). FjM
T h i s p a r a m e t e r is defined a s [ E q . ( 5 . 7 6 ) ]
= lb of substrate in influent/(day)(lb M L V S S in reactor)
(5.76)
V a l u e s o f F a n d Μ a r e given b y F = (QSo) χ 8.34 Μ = (X V)
(5.77)
(lb)
(5.78)
χ 8.34
Vta
w h e r e Q is i n M G D a n d (S , X ) 0
(lb/day)
in mg/liter. Therefore
va
F/M
= QS /X , V 0
(5.79)
v a
S i n c e V/Q = t = r e s i d e n c e t i m e , F/M
= S /X , 0
v
a
t
( d a y " *)
(5.80)
I n o r d e r t o a r r i v e a t c o r r e l a t i o n s for s e t t l i n g c h a r a c t e r i s t i c s o f a s l u d g e , a series o f b e n c h scale c o n t i n u o u s r e a c t o r s a r e o p e r a t e d , e a c h a t a selected
FjM
r a t i o . S l u d g e o b t a i n e d i n e a c h r e a c t o r is s u b j e c t e d t o s e t t l i n g t e s t s ( Z S V a n d SVI). If t h e s e t w o p a r a m e t e r s , w h i c h a r e a m e a s u r e o f t h e ability o f t h e s l u d g e t o settle, a r e p l o t t e d v s . t h e c o r r e s p o n d i n g FjM
r a t i o s , c u r v e s like t h e o n e s
s h o w n in F i g . 5.7 a r e o b t a i n e d . S i n c e for o p t i m u m s e t t l i n g t h e s l u d g e s h o u l d h a v e a h i g h Z S V a n d a l o w S V I , t h e o p t i m u m FjM
r a t i o a s i n d i c a t e d in F i g . 5.7 c o r r e s p o n d s t o t h e
m a x i m u m for t h e Z S V c u r v e a n d t h e m i n i m u m for t h e S V I c u r v e . F o r m o s t w a s t e w a t e r s t h i s o p t i m u m v a l u e o f t h e FjM
r a t i o falls b e t w e e n t h e f o l l o w i n g
limits [ E q . ( 5 . 8 1 ) ] : 0.6 > FjM w h e r e FjM
is e x p r e s s e d in lb B O D
5
t i o n for t h e c o r r e l a t i o n b e t w e e n F/M
> 0.3
(5.81)
influent/(day)(lb MLVSS). A n explana ratio and sedimentation characteristics
o f t h e s l u d g e is given b e l o w . 1. A t l o w F/M r a t i o s (e.g., b e l o w F/M = 0.3) t h e a m o u n t o f f o o d ( s u b s t r a t e ) p r e s e n t in t h e s y s t e m is insufficient t o m a i n t a i n t h e g r o w t h o f t h e microorganisms. Therefore, they are driven to endogenous respiration. A t y p i c a l b a c t e r i a l cell is s h o w n in F i g . 5.8. C y t o p l a s m i c m a t e r i a l is r i c h in p r o t e i n s a n d r i b o n u c l e i c a c i d ( R N A ) , a n d it is t h e m a i n p o r t i o n o f t h e cell w h i c h is m e t a b o l i z e d d u r i n g t h e p r o c e s s o f e n d o g e n o u s r e s p i r a t i o n . T h e r e s i d u e left f r o m e n d o g e n o u s m e t a b o l i s m is c o n s t i t u t e d m a i n l y b y cell c a p s u l e s , w h i c h a r e very light a n d resist s e d i m e n t a t i o n . T h i s is w h y a t l o w F/M r a t i o s ,
Optimum Settling Conditions of S l u d g e
Bulking sludge (filamentous organisms)
Dispersed floe Flocculating sludge (cell capsules)
-poor settling-
-optimum settling range
- poor — settling
1
zsv
SVI
i /-optilmum F / M ratio. ( F / M ) p 0
Ο
0.3
F/M = S / X 0
Fig. 5.7.
V | Q
t = lb B O D
Typical
I
JL
5
I
0.6
0.9
L
1.2
influent/(day)(lb M L V S S in reactor)
correlation
of SVI and ZSV
with F/M
Nucleus
Cytoplasm
Cell c a p s u l e
Fig. 5.8.
T
Typical
bacterial
cell.
ratio.
188
5.
Secondary Treatment: The Activated Sludge Process
the sludge has p o o r settling characteristics. Sludge o b t a i n e d u n d e r these c o n d i t i o n s is r e f e r r e d t o a s d i s p e r s e d floe, a n d a m i c r o s c o p i c view o f it is s h o w n in F i g . 5.7 for t h e r e g i o n o f l o w F/M r a t i o s . 2. A t h i g h F/M r a t i o s (e.g., F/M > 0.6) t h e r e is p r e d o m i n a n c e o f a t y p e o f m i c r o o r g a n i s m w h i c h is f i l a m e n t o u s in n a t u r e (Sphaerotilus). This type of g r o w t h d o e s n o t settle well, r e m a i n i n g in s u s p e n s i o n a l m o s t indefinitely. S l u d g e u n d e r t h e s e c o n d i t i o n s is r e f e r r e d t o a s a b u l k i n g s l u d g e . 3. A t v a l u e s o f t h e F/M r a t i o b e t w e e n t h e s e t w o e x t r e m e s , s l u d g e w i t h g o o d s e t t l i n g c h a r a c t e r i s t i c s is o b t a i n e d . S l u d g e u n d e r t h e s e c o n d i t i o n s is r e f e r r e d t o a s flocculating s l u d g e . F r o m E q . (5.80) t h e r e s i d e n c e t i m e t t o yield a n o p t i m u m flocculating s l u d g e is o b t a i n e d . W r i t t e n for t h e o p t i m u m F/M r a t i o a s d e t e r m i n e d f r o m F i g . 5.7, E q . (5.80) is ( F / M W =
(5.82)
SJX t 9tU
S o l v i n g for /, t =
(5.83)
S /[X , (F/M) ] 0
v a
OPT
T h e g e o m e t r y o f t h e s y s t e m a n d t h e m a n n e r in w h i c h w a s t e w a t e r is fed t o t h e a e r a t o r h a v e a n effect o n flocculating c h a r a c t e r i s t i c s o f t h e s l u d g e . F o r e x a m p l e , if t h e a e r a t o r is a l o n g r e c t a n g u l a r t a n k w i t h relatively p o o r m i x i n g , M L V S S is initially c o n t a c t e d a t t h e feed e n d w i t h e n t e r i n g s e w a g e , a n d t h e r e fore a h i g h F/M r a t i o p r e v a i l s a t t h e e n t r a n c e . F i l a m e n t o u s g r o w t h d e v e l o p e d u n d e r t h e s e c o n d i t i o n s persists t h r o u g h o u t t h e a e r a t i o n p e r i o d , a n d s l u d g e w i t h p o o r settling c h a r a c t e r i s t i c s is o b t a i n e d ( F i g . 5.9). T h e s a m e s i t u a t i o n High F / M
c
Feed
""Effluent
Filamentous growth Fig. 5.9. flow
Effect
of
geometry
in settling
characteristics
of
MLVSS
(plug
model).
o c c u r s in a b a t c h r e a c t o r , since a h i g h F/M r a t i o p r e v a i l s a t t h e s t a r t o f t h e o p e r a t i o n . T h e r e a c t o r d e p i c t e d in F i g . 5.9 is t h e p l u g flow c o n t i n u o u s r e a c t o r . A general discussion of the kinetics of a c o n t i n u o u s t r e a t m e n t system (plug flow, c o m p l e t e m i x , a n d a r b i t r a r y flow r e a c t o r s ) is p r e s e n t e d in S e c t i o n 10. If t h e r e is c o m p l e t e m i x i n g in t h e s y s t e m , t h e F/M r a t i o is u n i f o r m t h r o u g h o u t , p o s s i b l y falling w i t h i n t h e o p t i m u m r a n g e . U n d e r s t e a d y s t a t e a n d c o m p l e t e m i x c o n d i t i o n s , s l u d g e is a l w a y s in c o n t a c t w i t h a B O D c o n c e n t r a t i o n e q u a l t o t h a t in t h e effluent. T h e r e f o r e a d e n s e s l u d g e is likely t o b e o b t a i n e d .
6.
189
Parameters for Design of Reactors
I t is i m p o r t a n t t o o b t a i n e x p e r i m e n t a l l y t h e g r a p h in F i g . 5.7 for t h e specific s u b s t r a t e u n d e r s t u d y , since c o n s i d e r a b l e v a r i a t i o n o c c u r s d e p e n d i n g
on
s u b s t r a t e c h a r a c t e r i s t i c s . S u b s t r a t e s w h i c h a r e easily d e g r a d a b l e (e.g., s o l u b l e sugars) b e c o m e immediately available as food to the m i c r o o r g a n i s m s , a n d t h e r e f o r e t h e r e s u l t is a fast g r o w t h r e s p o n s e . O n t h e o t h e r h a n d , c o m p l e x o r g a n i c s u b s t r a t e s (e.g., w a s t e w a t e r s f r o m
petroleum
and
petrochemical
plants) must u n d e r g o chemical b r e a k d o w n before being available as food to the microorganisms, g r o w t h response being therefore slower.
6. Experimental D e t e r m i n a t i o n of P a r a m e t e r s N e e d e d for Design of A e r o b i c Biological R e a c t o r s 6.1. B E N C H S C A L E C O N T I N U O U S
REACTORS
A b e n c h scale c o n t i n u o u s r e a c t o r utilized for t h e s e d e t e r m i n a t i o n s is d e s c r i b e d in t h i s s e c t i o n . P a r a m e t e r s t o b e d e t e r m i n e d a r e defined in S e c t i o n s 4.1.2 t o 4 . 1 . 9 , i.e., for k i n e t i c r e l a t i o n s h i p : k; for m a t e r i a l b a l a n c e r e l a t i o n s h i p s : a, a\ b a n d b'. A d i a g r a m o f t h e c o n t i n u o u s flow r e a c t o r is s h o w n in F i g . 5.10. T h i s u n i t is d e s i g n e d a n d b u i l t b y B i o - D e v e l o p m e n t A s s o c i a t e s , A u s t i n , T e x a s . T h e r e a c t o r is m a d e of plexiglass a n d d i v i d e d i n t o t w o s e c t i o n s : the aeration a n d settling c h a m b e r s . These simulate the reactor a n d the s e c o n d a r y clarifier for a n a c t u a l p l a n t . 9
C a p a c i t y o f t h e a e r a t i o n c h a m b e r is a p p r o x i m a t e l y 7 liters. A i r is s u p p l i e d a s i n d i c a t e d in t h e d i a g r a m . B u b b l i n g a i r k e e p s t h e c o n t e n t s o f t h e a e r a t i o n c h a m b e r in a c o m p l e t e l y m i x e d c o n d i t i o n . W a s t e w a t e r is fed c o n t i n u o u s l y f r o m a c o n s t a n t h e a d feed r e s e r v o i r b y m e a n s o f a S i g m a m o t o r p u m p , a n d overflows c o n t i n u o u s l y i n t o t h e effluent b o t t l e . T h e a e r a t i o n a n d s e d i m e n t a t i o n c h a m b e r s a r e s e p a r a t e d b y a sliding baffle w h i c h c a n b e c o m p l e t e l y r e m o v e d if desired. S t a r t - u p is p e r f o r m e d b y p l a c i n g in t h e a e r a t i o n c h a m b e r a seed o f d o m e s t i c activated sludge collected from a n o p e r a t i n g plant, a n d gradually acclimating it t o t h e w a s t e w a t e r u n d e r s t u d y . F o r w a s t e w a t e r s o f i n d u s t r i a l o r i g i n c o n taining c o m p o u n d s which are toxic to the microorganisms, mixtures of i n d u s t r i a l w a s t e w a t e r a n d d o m e s t i c s e w a g e a r e fed t o t h e r e a c t o r w i t h a g r a d u a l l y i n c r e a s e d p r o p o r t i o n o f i n d u s t r i a l w a s t e w a t e r . E v e n t u a l l y , feed is 100% i n d u s t r i a l w a s t e w a t e r w i t h o u t d e l e t e r i o u s effects o n t h e m i c r o o r g a n i s m s . F l o w r a t e is v a r i e d b y p r o p e r s e t t i n g o f t h e S i g m a m o t o r p u m p , a n d b y utilizing different i n t e r n a l d i a m e t e r s for t h e T y g o n t u b i n g . A S i g m a m o t o r p u m p operates by "squeezing" the wastewater through the Tygon tubing by m e a n s o f m e c h a n i c a l " f i n g e r s , " t h e s p e e d o f w h i c h is set. O n e p u m p p r o m o t e s w a s t e w a t e r flow t h r o u g h several r e a c t o r u n i t s in p a r a l l e l , e a c h o n e p r o v i d e d
190
5.
Secondary Treatment: The Activated Sludge Process
Fig. 5.10. Continuous flow reactor (bench scale model). Insert: Sigma pump setup for operation of five reactors in parallel.
detail
of
w i t h its o w n T y g o n feed line [ F i g . 5.10 ( i n s e r t ) ] . T h e " f i n g e r s " s i m u l t a n e o u s l y s q u e e z e t h e s e several T y g o n t u b i n g s , p r o m o t i n g different flow r a t e s for e a c h line d e p e n d i n g o n t h e i n t e r n a l d i a m e t e r o f e a c h t u b i n g . * F l o w rates are determined by calibration, either weighing or measuring the v o l u m e o f effluent o b t a i n e d d u r i n g a t i m e d p e r i o d c o r r e s p o n d i n g t o a selected settling of t h e p u m p a n d a chosen internal d i a m e t e r of tubing. F l o w rates are r e p r o d u c i b l e w i t h i n less t h a n 1% fluctuation. F l o w r a t e s v a r y c o n s i d e r a b l y , e.g., f r o m 3 5 0 d o w n t o a b o u t 1.0 l i t e r / d a y . F o r a n a e r a t o r c h a m b e r v o l u m e o f 7.0 liters, t h e s e r a t e s c o r r e s p o n d t o r e s i d e n c e times of Q = 350 liters/day, t = V/Q = 7/(350/24) = 0.48 h r * 30 min Q = 1 liter/day, t = V/Q = 7/1 = 7 days A s the section of T y g o n t u b i n g subjected t o this c o n t i n u o u s
squeezing
* Sigmamotor p u m p model T-8 (manufactured by Sigmamotor Inc., H o u s t o n , Texas) an be used to operate five units in parallel.
6.
191
Parameters for Design of Reactors
a c t i o n w e a r s o u t , it softens a n d flow r a t e s c h a n g e . I t is a d v i s a b l e t o slide t h e t u b i n g a l o n g a t p e r i o d i c i n t e r v a l s , s o t h a t a n e w s e c t i o n o f it b e c o m e s e x p o s e d t o t h e s q u e e z i n g a c t i o n . F r e q u e n t c a l i b r a t i o n is p e r f o r m e d t o e n s u r e c o n f i d e n c e in t h e r e s u l t s . T u b i n g is r e p l a c e d after it is w o r n o u t . T h e m a i n difference i n o p e r a t i n g p r i n c i p l e b e t w e e n t h i s b e n c h scale r e a c t o r a n d t h e o n e in p l a n t scale ( F i g . 5.1) is t h a t n o c o n t r o l l e d recycle o f s l u d g e is p r o v i d e d in t h e b e n c h scale u n i t . S l u d g e is r e t u r n e d t o a e r a t i o n c h a m b e r f r o m t h e s e t t l i n g c h a m b e r t h r o u g h t h e o p e n i n g b e t w e e n t h e baffle a n d t h e b o t t o m o f t h e u n i t . T h i s r a t e o f r e t u r n c a n n o t b e c o n t r o l l e d . I t is d e s i r a b l e t o m a i n t a i n t h e c o n c e n t r a t i o n o f M L V S S in t h e a e r a t i o n c h a m b e r a p p r o x i m a t e l y c o n s t a n t ( a t a selected v a l u e u s u a l l y b e t w e e n 2 0 0 0 a n d 3 0 0 0 m g / l i t e r ) . I n o r d e r t o a c h i e v e t h i s c o n s t a n t M L V S S c o n c e n t r a t i o n , t h e p r o c e d u r e is 1. D e t e r m i n e p e r i o d i c a l l y t h e M L V S S c o n c e n t r a t i o n
in t h e
aerator
l i q u o r f r o m s a m p l e s w i t h d r a w n t h r o u g h t h e d r a i n line. 2. W i t h d r a w c a l c u l a t e d w e i g h t s o f M L V S S in o r d e r t o k e e p t h i s c o n c e n t r a t i o n a t t h e selected v a l u e for a given e x p e r i m e n t . F o r a r e a c t o r o p e r a t i n g w i t h M L V S S u n d e r e n d o g e n o u s r e s p i r a t i o n c o n d i t i o n s , it is n e c e s s a r y t o a d d s l u d g e i n s t e a d o f w i t h d r a w i n g it, in o r d e r t o k e e p a c o n s t a n t
MLVSS
concentration. W h e n t h e s l i d i n g baffle is i n s e r t e d , t h e b e n c h scale r e a c t o r is u t i l i z e d t o s i m u l a t e t h e a c t i v a t e d s l u d g e u n i t a s d e s c r i b e d . By r e m o v i n g t h e s l i d i n g baffle, s i m u l a t i o n o f a n a e r a t e d l a g o o n is o b t a i n e d ( C h a p t e r 6, S e c t i o n 5).
6.2. E X P E R I M E N T A L P R O C E D U R E E a c h experiment requires 2 - 4 weeks before steady state conditions are a c h i e v e d . F o r t h i s r e a s o n it is c o n v e n i e n t t o o p e r a t e s i m u l t a n e o u s l y f o u r o r five r e a c t o r s in p a r a l l e l . S t e p s in t h e e x p e r i m e n t a l p r o c e d u r e a r e [ 3 ] 1. E a c h u n i t is filled w i t h seed s l u d g e u p t o a p r e d e t e r m i n e d v o l u m e . D i l u t i o n is m a d e w i t h w a s t e w a t e r in o r d e r t o o b t a i n a M L V S S c o n c e n t r a t i o n of 2000-3000 mg/liter. 2. A i r is t u r n e d o n a n d c o n t e n t s o f t h e a e r a t i o n c h a m b e r a r e c o m p l e t e l y m i x e d b y t h e t u r b u l e n c e t h u s p r o d u c e d . T h e sliding baffle is a d j u s t e d t o l e a v e a n o p e n i n g o f J t o i in. a t t h e b o t t o m . D u r i n g o p e r a t i o n o f t h e r e a c t o r , f u r t h e r baffle a d j u s t m e n t s a r e m a d e in o r d e r t o p r o v i d e a d e s i r e d b l a n k e t h e i g h t o f s l u d g e in t h e s e t t l i n g c h a m b e r a n d a n i n t e r c h a n g e o f s l u d g e b e t w e e n t h e t w o c h a m b e r s ( F i g . 5.11). 3. S t a r t t h e S i g m a m o t o r p u m p a t a flow r a t e n e c e s s a r y t o o b t a i n t h e d e s i r e d r e s i d e n c e t i m e in t h e a e r a t i o n c h a m b e r . A c c l i m a t i o n o f s l u d g e , if r e q u i r e d , is p e r f o r m e d a s p r e v i o u s l y d e s c r i b e d . 4. O p e r a t e t h e r e a c t o r u n t i l s t e a d y s t a t e c o n d i t i o n s a r e a c h i e v e d . A t t a i n m e n t o f s t e a d y s t a t e is a s s u m e d w h e n t w o c r i t e r i a a r e satisfied: ( a ) o x y g e n
192
5.
Secondary Treatment: The Activated Sludge Process
'—^Effluent Fig. 5.11.
Side
view
(section)
of aeration
and settling
chambers.
uptake rate of reactor contents remains unchanged (determination of oxygen u p t a k e r a t e is d e s c r i b e d i n S e c t i o n 6.3.3) a n d ( b ) B O D o f effluent b e c o m e s stable. 5. C o n c e n t r a t i o n o f M L V S S is m e a s u r e d d a i l y a n d a d j u s t e d t o a n e a r l y c o n s t a n t v a l u e for t h e d u r a t i o n o f a n e x p e r i m e n t . T o c h e c k n e t i n c r e a s e o f M L V S S , p l u g overflow weir, r a i s e baffle, a n d w i t h d r a w a s a m p l e f r o m m i x e d t a n k c o n t e n t s . If V is t h e t o t a l v o l u m e ( a e r a t i o n c h a m b e r p l u s s e t t l i n g c h a m b e r ) t
a n d t w o d e t e r m i n a t i o n s o f M L V S S a r e m a d e , e.g., 2 4 h r a p a r t y i e l d i n g v a l u e s X
l
and X, 2
r e s p e c t i v e l y , t h e n e t i n c r e a s e o f M L V S S is AX
V
V a l u e s (X
l9
X) 2
=V X ~ t
VX
2
t
1
= V (X -X ) t
2
x
(24-hr growth)
(5.84)
r e p r e s e n t " a v e r a g e d " c o n c e n t r a t i o n s o f M L V S S for t h e t o t a l
v o l u m e o f t h e t a n k , since t h e baffle h a s b e e n r a i s e d a n d t h e c o n t e n t s o f a e r a t i o n a n d s e d i m e n t a t i o n c h a m b e r s m i x e d . E s s e n t i a l l y , n o s l u d g e g r o w t h o c c u r s in t h e s e d i m e n t a t i o n c h a m b e r b e c a u s e t h e r e is n o d i r e c t a e r a t i o n t h e r e . T h e r e f o r e , t h e v a l u e o f AX
V
c a l c u l a t e d f r o m E q . (5.84) r e p r e s e n t s t h e n e t g r o w t h
occurring in the aerator. F o r a p p l i c a t i o n o f E q . (5.68), it is r e c o m m e n d e d t o t a k e X as the M L V S S c o n c e n t r a t i o n d e t e r m i n e d after t h e baffle is r a i s e d . T h i s m a y s e e m c o n t r o v e r sial since i n E q . (5.68) Χ s t a n d s for M L V S S c o n c e n t r a t i o n in a e r a t i o n c h a m b e r d u r i n g t h e o p e r a t i o n (V is t h e v o l u m e o f a e r a t i o n c h a m b e r ) . T h e c o n c e n t r a t i o n o f M L V S S in t h e s e d i m e n t a t i o n c h a m b e r is p r o b a b l y different f r o m t h a t in t h e a e r a t o r . A t t h e b o t t o m o f t h e s e d i m e n t a t i o n c h a m b e r t h e r e is a s l u d g e b l a n k e t o f v e r y h i g h M L V S S c o n c e n t r a t i o n , a n d a t t h e t o p a s u p e r n a t a n t l i q u i d w i t h negligible M L V S S c o n c e n t r a t i o n . A f t e r t h e baffle is r a i s e d , VtU
υα
193
Parameters for Design of Reactors
6.
t h i s h e t e r o g e n e o u s m a s s in t h e s e d i m e n t a t i o n c h a m b e r is m i x e d w i t h t h e c o n t e n t s o f t h e a e r a t o r . T h e w h o l e v o l u m e is t h o r o u g h l y m i x e d b y t h e b u b b l i n g a i r b e f o r e t h e s a m p l e is t a k e n . T h e d e s i g n e r s o f t h i s l a b o r a t o r y r e a c t o r c l a i m t h a t t h e r e is n o significant difference b e t w e e n M L V S S c o n c e n t r a t i o n in t h e aeration c h a m b e r during operation a n d t h a t in the whole mixed content of t h e t w o c h a m b e r s . I n a n y e v e n t , it is p r a c t i c a l l y i m p o s s i b l e t o w i t h d r a w r e p r e s e n t a t i v e s a m p l e s f r o m t h e a e r a t i o n c h a m b e r d u r i n g t h e o p e r a t i o n for a n a l y s i s of X . va
R e c a l l a l s o t h a t t h e v o l u m e o f t h e s e d i m e n t a t i o n c h a m b e r is m u c h
s m a l l e r t h a n t h a t o f t h e a e r a t i o n c h a m b e r ( r a t i o o f a b o u t 3/7). T h e r e f o r e , M L V S S c o n c e n t r a t i o n in t h e m i x e d c o n t e n t s o f t h e t w o c h a m b e r s is n o t t o o different f r o m t h a t o f t h e a e r a t i o n c h a m b e r d u r i n g o p e r a t i o n . 6. O n c e s t e a d y s t a t e o p e r a t i o n is a t t a i n e d , t h e s a m p l i n g s c h e d u l e p r e s e n t e d i n T a b l e 5.3 is f o l l o w e d . T A B L E 5.3 Sampling Schedule [3] Analysis 1. C O D , B O D , o r T O C (mg/liter) (filtered and unfiltered c o m p o s i t e samples) 2. p H 3. SS, M L V S S (mg/liter) (also determine sludge settling curves and sludge volume index o f mixed liquor at the end of test run) 4. D i s s o l v e d o x y g e n ( D O ) (mg/liter) 5. Oxygen uptake rate 6. Microscopic analysis (gram stain) 7. Color, turbidity 8. Significant ions, compounds a
b
c
Frequency
3/week
R a w waste"
Mixed liquor*
X
—
(So) daily
X
—
daily 3/week
— —
1/week 3/week
— —
3/week
X
X
OS.)
X
3/week
Effluent'
X
X
X
(Xv.a)
(keep low)
X X
X
— —
— — — X
X
Sample withdrawn from influent feed line or raw waste containers. Sample withdrawn from the unbaffled tank. Sample withdrawn from effluent bottle.
6.3. C A L C U L A T I O N O F D E S I G N P A R A M E T E R S Calculation of parameters k
9
a, a', b, a n d V is m a d e f r o m o b t a i n e d d a t a .
P r o c e d u r e is d e s c r i b e d in S e c t i o n s 6 . 3 . 1 - 6 . 3 . 4 .
194
5.
Secondary Treatment: The Activated Sludge Process
6.3.1. Determination of S u b s t r a t e Removal Rate (k) T h i s d e t e r m i n a t i o n , b a s e d o n E q . (5.18) o r E q . (5.19), is d e s c r i b e d in S e c t i o n 3.2.
6.3.2. Determination of O x y g e n Utilization Parameters a ' a n d b' T h i s d e t e r m i n a t i o n is b a s e d o n E q . (5.64) in w h i c h t h e l e f t - h a n d m e m b e r is w r i t t e n a s R V,
i.e.,
r
Rr V = a\S
-S )Q
Q
where R
+ b'X
e
Vt
a
V
(5.85)
is t h e o x y g e n u p t a k e r a t e , i.e., o x y g e n utilized p e r d a y p e r u n i t
r
volume of reactor; a n d Κ the reactor volume. E x p e r i m e n t a l d e t e r m i n a t i o n o f R is d i s c u s s e d in S e c t i o n 6 . 3 . 3 . D i v i d i n g E q . r
(5.85) b y X
v
a
K a n d l e t t i n g V/Q = t ( r e s i d e n c e t i m e ) yields R /X , r
v
a
= a'[(S
0
- S )/X , e
v
a
t] + V
(5.86)
E q u a t i o n (5.86) is t h e b a s i c r e l a t i o n s h i p for d e t e r m i n a t i o n o f o x y g e n u t i l i z a t i o n p a r a m e t e r s a' a n d V. N o t i c e t h e p r e s e n c e o f t e r m (S — S )/X t 0
e
va
(sub
s t r a t e r e m o v a l r a t e ) , w h i c h a l s o o c c u r s i n E q s . (5.18) a n d (5.19) f o r d e t e r m i n a t i o n o f k. U n i t s for R
o b t a i n e d f r o m l a b o r a t o r y scale d e t e r m i n a t i o n s a r e m e t r i c ,
r
i.e., m g 0 / ( d a y ) ( l i t e r ) . S i n c e 2
m g 0 / l i t e r liquor = lb 0 / M l b liquor 2
(Section 4.2, Example 5.1)
2
then R
= lb 0 / ( d a y ) ( M l b liquor)
r
2
X
S i m i l a r l y , for
Vta
X,
= m g MLVSS/liter liquor = lb M L V S S / M l b liquor
v a
T h e r e f o r e in E q . (5.86) RrlX ,a
=
v
T h u s R /X r
Via
lb 0 / ( d a y ) ( M l b liquor) 2
ΐ
Κ
'
T
VQQ/MIK
R
lb M L V S S / M l b liquor
=
»>
0 / ( d a y ) ( l b MLVSS) 2
is a m e a s u r e o f u t i l i z a t i o n o f o x y g e n p e r d a y a n d p e r l b o f
b i o l o g i c a l s l u d g e p r e s e n t in t h e r e a c t o r . A s s h o w n in S e c t i o n 3.2, (S -S )IX t 0
e
Ota
= l b B O D r e m o v e d / ( d a y ) ( l b MLVSS)
A c c o r d i n g t o E q . (5.86) a p l o t o f R /X v s . (S -S )/X t yields a s t r a i g h t line f r o m t h e s l o p e a n d i n t e r c e p t o f w h i c h o x y g e n u t i l i z a t i o n p a r a m e t e r s a' r
Vta
0
e
Vta
6.
195
Parameters for Design of Reactors
a n d b' a r e o b t a i n e d . A t y p i c a l p l o t is s h o w n i n F i g . 5.16, a n d a n u m e r i c a l i l l u s t r a t i o n o f its c o n s t r u c t i o n f r o m l a b o r a t o r y d a t a is p r e s e n t e d i n E x a m p l e 5.5 ( S e c t i o n 6.4).
6.3.3. Experimental Determination of the O x y g e n Uptake Rate (R ) r
P o s s i b l y t h e s i m p l e s t w a y t o d e t e r m i n e t h e o x y g e n u p t a k e r a t e is b y g a l v a n i c cell o x y g e n m e a s u r e m e n t s . T h i s is t h e o n l y m e t h o d d e s c r i b e d i n t h i s s e c t i o n . O t h e r m e t h o d s a r e p o l a r o g r a p h i c a n d W a r b u r g t e c h n i q u e s a n d off-gas a n a l y s i s . O f all t h e s e m e t h o d s , g a l v a n i c cell m e a s u r e m e n t is t h e s i m p l e s t , a n d its a c c u r a c y is u s u a l l y a d e q u a t e . T h e a p p a r a t u s for t h i s m e a s u r e m e n t is t h e dissolved oxygen analyzer ( D O analyzer) described in C h a p t e r 2 (Section 2.3.1) a n d s h o w n i n F i g . 2 . 4 . E x p e r i m e n t a l t e c h n i q u e for m e a s u r i n g o x y g e n u p t a k e r a t e (R ) is [ 6 ] r
1. Fill B O D b o t t l e w i t h a e r a t e d m i x e d l i q u o r f r o m test s o l u t i o n . 2. I n s e r t p r o b e i n t o b o t t l e , a l l o w i n g d i s p l a c e d l i q u i d t o o v e r f l o w . C a r e is t a k e n t o p r e v e n t a c c u m u l a t i o n o f a i r b u b b l e s i n s i d e b o t t l e . 3. M i x t h e c o n t e n t s u s i n g a m a g n e t i c s t i r r i n g a p p a r a t u s . 4. R e c o r d g a l v o n o m e t e r r e a d i n g s a t v a r i o u s t i m e i n t e r v a l s , u s u a l l y e v e r y 30 sec. 5. C o r r e c t r e a d i n g s b a s e d o n a p r e d e t e r m i n e d sensitivity f a c t o r (for d e t a i l s refer t o [ 6 ] ) , a n d p l o t d i s s o l v e d o x y g e n level ( o r d i n a t e ) v s . t i m e ( a b s c i s s a ) ( F i g . 5.12). 12 II 10
— I = - ( S lope ) =-(2 5-8. )/(IC -2)= 0.7 rng/(l iter)( min) = 0.7' x 6 C ) x 2 4 = I C >08r ng 0 / W ayXlit er) . = I0(D8 lb 0 / (day (Μ I b liqijor) V = 0 0 8 χ 10" l b 0 / ( day)( b liquor) 2
2
9
3
2
8
I
7
b
5 4 3 2 i 0
1 2
3 ~ 4 5 6 Time (min)
Fig. 5.12. Determination
_
7
of oxygen
8
9
uptake
Ϊ0
rate.
196
5.
Secondary Treatment: The Activated Sludge Process
6. I n F i g . 5.12, t h e s l o p e o f t h e line is o x y g e n u p t a k e r a t e in m g / ( l i t e r ) ( m i n ) . A specific u p t a k e r a t e (R /X ) r
is t h e n d e t e r m i n e d b y d i v i d i n g t h i s
va
v a l u e b y M L V S S c o n c e n t r a t i o n i n t h e test s a m p l e . I n F i g . 5.12, t h e first d a t a p o i n t s i m m e d i a t e l y after / = 0 a r e not t o b e t a k e n i n t o a c c o u n t in e v a l u a t i n g t h e s l o p e . T h e h i g h e r s l o p e o f t h i s s e c t i o n o f t h e line is d u e t o loss o f e n t r a i n e d a i r f r o m t h e l i q u o r . A f t e r a few m i n u t e s t h e s l o p e b e c o m e s s t a b i l i z e d , a n d it is t a k e n a s t h e u p t a k e r a t e . A temperature correction available from n o m o g r a p h s furnished by the m a n u f a c t u r e r is a p p l i e d t o t h e r e a d i n g s . P r o b e r e a d i n g s a r e i n a c c u r a t e a t D O c o n c e n t r a t i o n s b e l o w 0.5 m g / l i t e r . T r a n s f e r o f t h e m i x e d l i q u o r f r o m t h e reactor to the D O analyzer bottle should be rapid, a n d the test started as s o o n a s p o s s i b l e f o l l o w i n g s a m p l e w i t h d r a w a l . If o x y g e n d e p l e t i o n is t o o r a p i d , t h e s a m p l e is d i l u t e d in o r d e r t o r e d u c e M L V S S c o n c e n t r a t i o n . I t is a d v i s a b l e t o c a l i b r a t e t h e p r o b e in a s a m p l e o f w a t e r s i m i l a r t o t h a t in w h i c h t h e D O a n a l y z e r is u s e d , in o r d e r t o e l i m i n a t e e r r o r s d u e t o t h e salt effect.
6.3.4. Determination of Parameters for S l u d g e Yield (a and b) D e t e r m i n a t i o n o f p a r a m e t e r s a a n d b is b a s e d o n E q . (5.73). F o r t h e b e n c h scale r e a c t o r t h e r e is n o recycle o f s l u d g e , c o n t r a r y t o w h a t h a p p e n s for t h e r e a c t o r in F i g . 5 . 1 , for w h i c h E q . (5.73) is w r i t t e n . A simplified d i a g r a m o f t h e b e n c h scale r e a c t o r is s h o w n in F i g . 5.13. By c o m p a r i n g F i g . 5.13 w i t h F i g . 5 . 1 , t e r m s in E q . (5.73) a r e m o d i f i e d for a p p l i c a tion to the laboratory unit.
Q X
X
Q
v,o 1
1
NV,o
So Fig. 5.13.
V
Simplified
of the bench
Fig. 5.1
Q" XNV,
Fig. 5.13
zero
Χν,ο
Xy,F F
NV,e
XNV,
e
scale continuous
Q
QF
v,e
X
S
(sludge deposited) diagram
X
ο
reactor.
6.
Parameters for Design of Reactors
197
T h e r e f o r e , E q . (5.73) f o r t h e l a b o r a t o r y r e a c t o r b e c o m e s AX
t
= a(S -S )Q-bX , V+ 0
e
QX ,
v a
+
V 0
Q(X -X , ) NVt0
AX
NV e
(5.87)
AX
V
N
E q u a t i o n (5.87) is r e a r r a n g e d a s AX - Q(X v,o-X v,e t
N
V
να
(X , -X v,e NV 0
0
e
(5.88)
v a
V a n d n o t i c i n g t h a t V/Q = t ( r e s i d e n c e t i m e ) ,
Dividing through by Χ AXt/V-
bX , V
+ Xv,o) = AX = a(S -S )Q-
N
+ Xv,o)lt
N
(AXJV)
= a[(S -S )/X , t]-b 0
Xv,
e
(5.89)
v a
I n t h e n u m e r a t o r o f t h e l e f t - h a n d m e m b e r o f E q . ( 5 . 8 9 ) , t e r m AXJV
equals
t h e n e t yield o f t o t a l s l u d g e p e r u n i t v o l u m e [ i . e . , m g t o t a l s l u d g e / ( d a y ) ( l i t e r ) ] . T e r m AXJV
c o r r e s p o n d s t o t h e n e t yield o f M L V S S p e r u n i t v o l u m e . I f c o n
c e n t r a t i o n s o f N V S S a n d M L V S S i n t h e i n f l u e n t a r e n e g l i g i b l e (i.e., X
«
NVt0
X
N
V
t
e
« X
Vf0
~ 0), this e q u a t i o n reduces t o AXJV
AXJV
= a[(S -S )/X , t]-b 0
Xv,
e
(5.90)
v a
E q u a t i o n (5.89) [ o r E q . ( 5 . 9 0 ) ] is t h e b a s i c r e l a t i o n s h i p for d e t e r m i n a t i o n o f s l u d g e yield p a r a m e t e r s a a n d b. N o t i c e a g a i n t h e p r e s e n c e o f t e r m (S — S )/X t 0
e
( s u b s t r a t e r e m o v a l r a t e ) , w h i c h a l s o o c c u r r e d i n E q s . (5.18),
Vta
(5.19), a n d (5.86) f o r d e t e r m i n a t i o n o f p a r a m e t e r s k, a\ a n d b'. Note
on units for Eq. (5.89)
t i o n s , t h e v a l u e o f AXJV
lor Eq. ( 5 . 9 0 ) ] : F r o m l a b o r a t o r y d e t e r m i n a
is o b t a i n e d i n m e t r i c u n i t s , i.e., AXJV
= m g total
s l u d g e y i e l d / ( d a y ) (liter o f l i q u o r ) . F r o m s i m i l a r c o n s i d e r a t i o n s a s t h o s e f o r R
r
( S e c t i o n 6.3.2), it f o l l o w s t h a t t h i s v a l u e is n u m e r i c a l l y e q u a l t o t h a t
e x p r e s s e d i n E n g l i s h u n i t s , i.e., AXJV T h e r e f o r e t e r m (AXJV)/X
= lb sludge yield/(day)(Mlb liquor).
i n E n g l i s h u n i t s is
Vta
AXJV _ lb total sludge yield/(day)(Mlb liquor) X
~
0ta
lb M L V S S / M l b liquor
= lb total sludge yield/(day)(lb M L V S S ) Similarly, AXt/V - (X y, ο - X , N
NV
e
+ X , )/t v
0
= lb M L V S S yield/(day)(lb M L V S S )
Xv, a
A c c o r d i n g t o E q . (5.89) [ o r E q . ( 5 . 9 0 ) ] a p l o t o f ( a c c o u n t i n g f o r p r e s e n c e o f NVSS) \ AX /V-(X v, -X , t
|_
N
0
NV e
γ -^ν,α
+ X ,o)/t v
AXJV] = — — vs. -^υ,α
J
(S -S )IX at 0
e
Vt
198
5.
Secondary Treatment: The Activated Sludge Process
o r s i m p l y (if N V S S is negligible) AXJV — — vs.
(S -S )IX t 0
e
Via
yields a s t r a i g h t line f r o m t h e s l o p e a n d i n t e r c e p t o f w h i c h d e s i g n p a r a m e t e r s a a n d b a r e o b t a i n e d . A t y p i c a l p l o t is s h o w n in F i g . 5.17, a n d its c o n s t r u c t i o n f r o m l a b o r a t o r y d a t a is i l l u s t r a t e d in S e c t i o n 6.4, E x a m p l e 5.5. T h e a b s c i s s a i n t e r c e p t in F i g . 5.17 c o r r e s p o n d s t o a z e r o v a l u e for t h e o r d i n a t e . T h i s o c c u r s for a c o n d i t i o n o f n e t z e r o yield o f M L V S S , i.e., AX
V
= 0. R e f e r r i n g t o E q . (5.68), for AX
= 0 it f o l l o w s t h a t p r o d u c t i o n o f
V
M L V S S b y s y n t h e s i s , i.e., a(S
— S )Q
0
e
9
is e x a c t l y b a l a n c e d b y loss o f M L V S S
o x i d i z e d b y e n d o g e n o u s r e s p i r a t i o n , i.e., bX V.
Therefore
va
a{S -S )Q 0
e
=
bX , V v a
T h u s , t h e l e n g t h o f a b s c i s s a i n t e r c e p t is (S -S )/X t 0
e
va
= b/a, a s i n d i c a t e d i n
F i g . 5.14. I n s u m m a r y , t h e m o s t i m p o r t a n t i n f o r m a t i o n d e r i v e d f r o m b e n c h scale s t u d i e s u s i n g t h i s l a b o r a t o r y r e a c t o r is t h e o r g a n i c r e m o v a l c a p a c i t y o f a n a c c l i m a t e d b i o l o g i c a l s l u d g e r e c e i v i n g a p r e d e f i n e d w a s t e w a t e r . F u l l scale plants operating on design criteria developed using this reactor p r o d u c e a n effluent w h i c h a p p r o x i m a t e s t h e p r e d i c t e d q u a l i t y . M o r e o v e r , o x y g e n u t i l i z a t i o n r a t e s a r e s c a l e d u p w i t h r e l a t i v e a c c u r a c y f r o m b e n c h scale r e a c t o r s t o full scale u n i t s . T h e r e is s o m e difficulty, h o w e v e r , in s c a l i n g u p a n d a p p l y i n g coefficients a a n d b d e v e l o p e d f r o m b e n c h scale r e a c t o r s t o a full scale u n i t b e c a u s e o f l i m i t a t i o n s d u e t o l o w a c c u r a c y o f t h e V S S test, a n d t h e difficulty o f e s t a b l i s h i n g a solids b a l a n c e in s m a l l scale s i m u l a t i o n s t u d i e s . U s i n g l a r g e r r e a c t o r s o f p i l o t - p l a n t scale e n h a n c e s t h e a c c u r a c y o f t h e s e
coefficients.
F o r t u n a t e l y , t h e a c c u r a c y o f coefficients a a n d b is less i m p o r t a n t for t h e d e s i g n e r t h a n t h o s e for r e m o v a l r a t e c o n s t a n t (k) coefficients
and oxygen
demand
(a\b').
6.4. N U M E R I C A L E X A M P L E S : D E T E R M I N A T I O N OF D E S I G N P A R A M E T E R S FOR A N ACTIVATED SLUDGE SYSTEM Example 5.5 A n i n d u s t r i a l p l a n t is c o n s i d e r i n g a n a c t i v a t e d s l u d g e s y s t e m for t r e a t m e n t o f t h e i r w a s t e w a t e r s . P r e l i m i n a r y t e s t s a r e p e r f o r m e d i n l a b o r a t o r y scale c o n t i n u o u s r e a c t o r s ( F i g . 5.10). T h e v o l u m e o f t h e a e r a t i o n c h a m b e r in l a b o r a t o r y r e a c t o r s is 7 liters. F o u r r e a c t o r s a r e o p e r a t e d in p a r a l l e l u n t i l s t e a d y s t a t e c o n d i t i o n s a r e o b t a i n e d . D a t a t a k e n a r e p r e s e n t e d i n T a b l e 5.4. T h e influent c o n t a i n s a n a v e r a g e o f 3 0 m g / l i t e r o f N V S S . I n effluent, c o n -
Parameters for Design of Reactors
Ο
CO
Ο
Ο
Ο
>
60^ Ό < So 3 -a
h> νο ^ « Λ ο οο ο\ m οο m <Ν 00
3
53
T3 (Ν
§ ^ 3 60
^
Ο
_ σ\ οο οο
(30
5
1 S ^
«η Tt οο rn
ο 60$
Λ
8
8 > δ
3s§
(Η
3
Ε
©β C3
·-<
S "« 55 60
m fvj co
8
ε s 8 S COε ο 60 Λ J " *jj
11SS υ ο
00
00
00
η
Ν η
00
rt
03
2
OS Co
03
Ί3
ΟΙ
ο
(0
Ο
ΙΑ
*
UJ
>
» 75 —' Εο
5
Η α:
ο
?a
'4
2
6.
201
Parameters for Design of Reactors
c e n t r a t i o n o f N V S S is a p p r o x i m a t e l y 2 0 m g / l i t e r . T h e difference, 3 0 — 2 0 = 10 m g / l i t e r , c o r r e s p o n d s t o N V S S s e t t l e d i n t h e s e c o n d a r y clarifier. S l u d g e u n d e r f l o w f r o m t h e s e c o n d a r y clarifier c o n s i s t s o f t h i s N V S S s e t t l e d plus
net
yield o f V S S f r o m r e a c t o r o p e r a t i o n . F r o m d a t a in T a b l e 5.4 d e t e r m i n e d e s i g n p a r a m e t e r s k, a, a\ b, a n d V. A l s o estimate nonbiodegradable matter concentration S
n
5.5 d e t e r m i n e k a n d S .
(mg/liter). F r o m Table
A l s o p l o t c o l u m n (9) o f t h e t a b l e v s . c o l u m n (5). A
n
g r a p h o f t h i s p l o t is s h o w n in F i g . 5.14.
1
1
I
I
I
1
I
1
1
1
Slope;k = ( l . 6 l - 0 . 0 ) / ( I I O - I O ) = 0.0161 d a y = 0 . 0 0 0 6 7 hr'
1
1
-1
1
Equation:
1.6
(S -S )/X 0
e
V f Q
IQ\ ID.iy;
t=k(S -S ) e
n
O^Reactor ,
* / 3
1.2
N
= l n
q
° mg/liter
0.8 O ^ R e a c t o r No. 2 0.4
X ^ - R e a c t o r No. 3 !
>d*-Reactor No. 4
W
0
1
1
1
10
1
S Fig. 5.14.
1
1
1
1
Graphical
1
1
1
100
50 (mg/liter)
e
determination
of k and S„ (Example
5.5).
Est imc ted mir imu m (at F/l 1=0 .6)
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
F/M Fig. 5.15.
Plot
of SVI
vs. F/M
(Example
5.5).
1.8
202
5.
Secondary Treatment: The Activated Sludge Process
Step 1. D e t e r m i n e t h e r e m o v a l r a t e c o n s t a n t k ( k i n e t i c s o f B O D r e m o v a l ) [ E q . (5.19)]. k = 0.0161 d a y " S
n
A p l o t o f S V I vs. F/M
(0.00067 h r " )
1
1
= 10 mg/liter r a t i o is s h o w n in F i g . 5.15. E s t i m a t e d m i n i m u m S V I
o c c u r s a t a v a l u e o f F/M
ratio « 0.6.
Step 2. D e t e r m i n e o x y g e n u t i l i z a t i o n p a r a m e t e r s a' a n d b' [ E q . ( 5 . 8 6 ) ] . F r o m T a b l e 5.6 d e t e r m i n e a' a n d b'. P l o t c o l u m n (4) vs. c o l u m n (5) ( T a b l e 5.6). T h e g r a p h is s h o w n in F i g . 5.16. Then a' = 0.79 m g 0 / m g B O D = 0.79 lb 0 / l b B O D 2
b' = 0.15 d a y "
r
2
r
1
T A B L E 5.6 Oxygen Utilization Parameters Calculated data
Laboratory data (2)
(3)
W
Reactor
Xv, a (mg/liter)
no.
(Table 5.4)
Rr [mg 0 /(liter)(day)] (Table 5.4)
RrlXv,a (day" )
(day- ) (Table 5.5)
1 2 3 4
3100 2800 3000 2900
4025 1800 1292 780
1.298 0.643 0.431 0.269
1.440 0.620 0.350 0.145
U)
(5) (S -S )/X t 0
2
e
Vta
1
1
-τ—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—ι—r~
Slope;a' = (i.O-0.5)/(l.07-0.44) = 0.79 lb 0 ,energy/lb total B 0 D removed Equation 2
1.5
5
(5.86) ^Reactor No. I
Intercept; b'=O.I5 lb 0 /(day)(lb ML\ 2
1.0 χ
est
J^-Reactor No. 2 0.5
^ - R e a c t o r No. 3 ^-Reactor No. 4 ^"Intercept; b'=O.I5 Ju
Fig. 5.16.
ι ι Τ ι—ι
ι ι
0.2 0.4 0.6 0.8
Graphical
determination
'
1.0
1.2
1.4
I
1.6
I
I
I—I—L_
1.8 2.0
of a' and b' (Example
2.2
5.5).
«τι
• I
ι Ό
aJ
CO
w
T3 CO*
3 oo δ
•s
ν©
ί
OOJO
2
ι
φ¥
* l §: I .*i^CO
< 00
.6
υ
Ο
w
CO 1
Ι
-O
o O ^
'«»is 8
s?"^
B . l b
co
<
£
« ο
204
5.
Secondary Treatment: The Activated Sludge Process
Step 3. D e t e r m i n e s l u d g e yield p a r a m e t e r s a a n d b [ E q . ( 5 . 8 9 ) ] . (X , NV
ο- X
NVt
e
= 3 0 - 2 0 = 10 m g SS/liter)
X
Vt
0
= 0
F r o m T a b l e 5.7 d e t e r m i n e a a n d b. P l o t c o l u m n (7) vs. c o l u m n (8) ( T a b l e 5.7). T h e g r a p h is s h o w n in F i g . 5.17. a = 0.575 lb MLVSS/lb total B O D
removed
5
b = 0.075 lb MLVSS/(day)(lb MLVSS) SUMMARY
D e s i g n p a r a m e t e r s ( E x a m p l e 5.5)
k = 0.0161 d a y " (0.00067 h r " ) 1
1
S„ = 10 mg/liter
-ι
1
1
1
1
Equation; ΔΧ,/V- (X
ω ω >
N V
, -X 0
Λ
N V
,
e
* X ) / 1 _AX /V w
V
ν,α
ν,α
= a[(S -S )/X 0
e
V | Q
t]-b
(5.89)
^-Reactor No. I
J
Slope;a = (0.73-O.I55)/(l.4-0.4) =0.575 lb MLVSS/lb total BODsrernoved 0.4
_J
0.8
1
L
1
JL
2.0
( S o - S e i V X ^ t (day' ) b/a=0.075/0.575=O.I3 1
Fig. 5.17.
Graphical
determination
of a and b (Example
5.5).
205
Design Procedure for an Activated Sludge Plant
7.
a = 0.575 lb M L V S S / l b total B O D a' = 0.79 lb 0 / l b total B O D 2
removed
5
removed
5
b = 0.075 lb M L V S S / ( d a y ) ( l b M L V S S ) b' = 0.15 lb 0 / ( d a y ) ( l b MLVSS) 2
Example 5.6 If for a w a s t e w a t e r t h e l b 0 / d a y r e q u i r e d for a e r o b i c b i o l o g i c a l t r e a t m e n t is 2
lb 0 / d a y = 0.4(lb B O D removed/day) + 0.1 (lb M L V S S ) 2
5
w r i t e a n a p p r o x i m a t e e q u a t i o n for b i o l o g i c a l s l u d g e yield in l b / d a y . SOLUTION
Here a' = 0.4
(basis B O D , i.e., α' οΌ ) 5
Β
5
b' = 0.1 T h e d e s i r e d e q u a t i o n f r o m E q . (5.68) is AX„(\b M L V S S / d a y ) = a ( l b B O D removed/day) - b(\b M L V S S ) 5
A p p r o x i m a t e r e l a t i o n s h i p s for a [ S e c t i o n 4 . 1 . 6 ( c ) ] a n d b [ f r o m E q . ( 5 . 6 0 ) ] a s f u n c t i o n s o f a' a n d b' 0 . 8 2 a o D + 0.58flioD = 1.0 B
5
b = Therefore, the a p p r o x i m a t e value of
tf D BO
5
*
S
= ( l - 0 . 5 8 ^ D ) / 0 . 8 2 = [ l - ( 0 . 5 8 ) ( 0 . 4 ) ] / 0 . 8 2 = 0.94
fl D BO
5
b'\\A2
O
5
5
T h e a p p r o x i m a t e v a l u e o f b is b = 671.42 = 0.1/1.42 = 0.07 T h e a p p r o x i m a t e e q u a t i o n for M L V S S yield is AX
V
= 0.94(lb B O D removed/day) - 0.07(lb M L V S S ) 5
7. Design P r o c e d u r e for an A c t i v a t e d S l u d g e Plant 7.1.
INTRODUCTION
F r o m k n o w l e d g e o f d e s i g n p a r a m e t e r s k, a, b, a\
a n d b\
design of the
a c t i v a t e d s l u d g e p l a n t is u n d e r t a k e n . F o r t h e l a b o r a t o r y r e a c t o r i n F i g . 5.10 t h e r e is n o r e c y c l e o f s l u d g e . N e t s l u d g e yield is w i t h d r a w n i n t e r m i t t e n t l y t o m a i n t a i n a n a v e r a g e c o n s t a n t c o n c e n t r a t i o n (X ) Vf0
of M L V S S in the a e r a t i o n
c h a m b e r . F o r t h e a c t u a l p l a n t , s l u d g e is r e c y c l e d a s s h o w n i n F i g . 5 . 1 .
206
5.
Secondary Treatment: The Activated Sludge Process
A p r i m a r y v a r i a b l e selected b y t h e d e s i g n e r is c o n c e n t r a t i o n X
va
i n t h e a e r a t o r . R a t e o f recycle s l u d g e Q
R
tration. Usually X
v%a
of M L V S S
is c a l c u l a t e d t o p r o v i d e t h i s c o n c e n
is selected b e t w e e n 2 0 0 0 a n d 4 0 0 0 m g / l i t e r o f M L V S S .
A n o t h e r p r i m a r y v a r i a b l e w h i c h is selected b y t h e d e s i g n e r is t h e c o n c e n t r a t i o n X
o f M L V S S i n recycle s l u d g e ( s t r e a m 7 i n F i g . 5.1), w h i c h is a l s o e q u a l t o
VfU
M L V S S c o n c e n t r a t i o n i n u n d e r f l o w f r o m t h e s e c o n d a r y clarifier [ s t r e a m 5 ] . Concentration X
VfU
is a l s o t h e s a m e a s t h a t i n s t r e a m 6 ( w a s t a g e ) . G o o d
s e t t l i n g s l u d g e is e x p e c t e d t o a t t a i n a c o n c e n t r a t i o n X
b e t w e e n 10,000 a n d
vu
15,000 m g / l i t e r o f M L V S S . A t s t e a d y s t a t e c o n d i t i o n s t h e r e is n o a c c u m u l a t i o n o f s l u d g e . T h u s , n e t yield o f s l u d g e i n t h e a e r a t o r m u s t b e r e m o v e d i n w a s t a g e s t r e a m 6. F o r p u r p o s e s o f m a t e r i a l b a l a n c e c a l c u l a t i o n s w a s t a g e is a s s u m e d t o b e c o n t i n u o u s . I n p r a c t i c e , it is u s u a l l y p e r f o r m e d i n t e r m i t t e n t l y b y t h e a r r a n g e m e n t s h o w n i n F i g . 5.18, since it is o r d i n a r i l y t o o s m a l l t o justify c o n t i n u o u s w i t h drawal. R e t u r n a n d w a s t a g e lines a r e v a l v e d a s i n d i c a t e d . V a l v e s a r e a c t u a t e d b y a t i m e c l o c k f o r i n t e r m i t t e n t s l u d g e w a s t a g e (e.g., 5 m i n e v e r y h o u r ) .
7.2. M A T E R I A L B A L A N C E F O R D E T E R M I N A T I O N OF R E C Y C L E RATIO O F M L V S S W r i t e a m a t e r i a l b a l a n c e f o r M L V S S a r o u n d t h e s e c o n d a r y clarifier i n F i g . 5.1 ( l o o p
··). MLVSS, O U T
MLVSS, I N
1. M L V S S in net effluent [stream 4 ] zero (assuming complete sedimentation o f M L V S S in secondary clarifier)
1. M L V S S in reactor effluent [stream 3 ] QX , (S34)
(lb/day)
v a
or [from Eq. (5.5)] Q (l+r)X , (S.34) F
2. M L V S S in wastage [stream 6 ]
(lb/day)
V A
AXv+QFXV.F
(8.34)
(lb/day)
3. M L V S S in recycled sludge [stream 7 ] Q X ( Z M ) = rQ X (S34) (lb/day) R
V
T
F
U
VtU
Then Q (\+r)X , (S34) F
= 0+AX
v a
v
+ Q X , (S.34) F
V F
+ rQ ^ , « ( 8 . 3 4 ) F
S o l v i n g f o r t h e recycle r a t i o , r = ^MQFX^a-AX^^MQrXv^mMQAX^u-X^a)]
(5.91)
If n e t s l u d g e yield (AX ) a n d M L V S S c o n c e n t r a t i o n i n fresh feed (X ) V
VtF
are
negligible b y c o m p a r i s o n w i t h t e r m 8 . 3 4 2 ^ ^ , E q . (5.91) simplifies t o yield E q . (5.92). r = Xv,aKXv,u-X ,a) V
(5.92)
7.
207
Design Procedure for an Activated Sludge Plant
ω
®
Fresh feed
©
Combined feed
©
Net Reactor Secondary effluent effluent clarifier
Aerator
Xlarifier underflow
©
-^-Recycle and wastage pump
Timing device-η
\-
©
©
Recycle sludge
^
Fig. 5.18. Arrangement
for sludge
Wastage ^
wastage.
W a s t a g e flow Q" is c a l c u l a t e d b y n o t i n g t h a t it m u s t c o n t a i n t h e AX
V
o f n e t yield o f M L V S S plus t h e M L V S S f r o m fresh feed (Q X , )F
since c o n c e n t r a t i o n o f M L V S S i n s t r e a m Q" is AX
V
+ Q X , (8.34) F
V F
X , VtU
= β'%,„(8.34)
V F
(5.93)
Q" = (AX + S.34Q V
lb/day
Therefore,
X , )/S.34X
F
v
F
„
Vt
(5.94)
Q is o b t a i n e d b y c o m b i n i n g E q s . (5.2) a n d ( 5 . 9 4 ) : Q ' = Q F ~ Q" = Q F ~ (AX + 8.34QX , )/8.34X v
v F
(5.95)
utU
7.3. M A T E R I A L B A L A N C E F O R C A L C U L A T I O N OFS 0
B O D o f c o m b i n e d feed (S ) 0
is c a l c u l a t e d b y a B O D b a l a n c e a r o u n d t h e
j u n c t i o n o f fresh feed a n d recycle s l u d g e t o f o r m c o m b i n e d feed, i.e., l o o p in F i g . 5 . 1 . T h i s m a t e r i a l b a l a n c e is a s f o l l o w s : BOD IN: Q S F
+
F
BOD OUT:
QS R
e
QS
0
or
or
QS F
+
F
rQ S F
Qr{\+r)S
e
0
Then QS F
+ rQ S
F
F
=
e
Q (l+r)S F
0
Therefore S
0
From
= (S + rS.)/(l+r) F
(S — S )
E q . (5.96) t h e difference
0
e
b e t w e e n influent a n d
(5.96) effluent
s o l u b l e B O D for t h e a e r a t o r is So-S
e
= [(S + r S . ) / ( l + r ) ] - S .
So-S
e
= (S -S.)/(l+r)
F
or F
(5.97)
208
5.
S e c o n d a r y Treatment: The Activated S l u d g e P r o c e s s
7.4. A L T E R N A T I V E E X P R E S S I O N S F O R N E T YIELD OF BIOLOGICAL S L U D G E A N D O X Y G E N UTILIZATION IN T H E A E R A T O R 1. Net yield of MLVSS.
S u b s t i t u t i o n o f Q a n d (S -S ) 0
in E q . (5.68) b y
e
t h e i r v a l u e s given b y E q s . (5.5) a n d (5.97) yields after s i m p l i f i c a t i o n ΔΧ
= a(S -S )Q
υ
F
e
-
F
bX V
(5.98)
Vta
E q u a t i o n (5.98) is a n a l t e r n a t i v e e x p r e s s i o n for AX .
I t is m o r e c o n v e n i e n t
V
t h a n E q . (5.68), since it c o n t a i n s p r i m a r y v a r i a b l e s S
and Q
F
F
a n d Q. [ 5 a n d Q a r e c a l c u l a t e d f r o m k n o w l e d g e o f S , 0
Q,
F
rather than S
Q
S,
F
and r from
e
E q s . (5.96) a n d (5.5).] T h e p h y s i c a l significance o f t h e s y n t h e s i s t e r m a(S
— S )Q
F
e
is c l e a r . C o m
F
b i n e d feed Q ( F i g . 5.13) is t h o u g h t o f a s t w o h y p o t h e t i c a l s e p a r a t e s t r e a m s ( F i g . 5.19). F o r s t r e a m Q
s o l u b l e B O D is r e d u c e d f r o m S
F
to S,
F
— S)
F
T h e o t h e r s t r e a m (Q )
and
e
b i o l o g i c a l s l u d g e s y n t h e s i z e d a s a r e s u l t o f t h i s B O D r e d u c t i o n is a(S
e
Q. F
e n t e r s a n d leaves t h e r e a c t o r w i t h t h e s a m e u n c h a n g e d
R
c o n c e n t r a t i o n o f s o l u b l e B O D , i.e., S .
T h e r e f o r e it does not c o n t r i b u t e t o
e
synthesis of biological sludge.
Q ,S F
Q =Q
F
+ Q
F
Q ,S
e
QR.S
e
F
R
Q =rQ ,S R
F
e
= Q ( I +r) F
Fig.
5.19.
Diagram
corresponding
to Eq.
(5.98).
2. Oxygen utilization in the aerator. O n s u b s t i t u t i o n o f (S — S ) a n d Q b y t h e i r v a l u e s given b y E q s . (5.97) a n d (5.5), respectively, E q . (5.64) yields 0
l b 0 / d a y = a\S 2
- S) Q
F
e
+ b'X ,
F
v
e
V
a
(5.99)
Significance o f e n e r g y t e r m a\S — S )Q is p a r a l l e l t o t h a t o f t h e s y n t h e s i s t e r m in E q . (5.98). O n l y s t r e a m Q c o n s u m e s o x y g e n since s t r e a m Q e n t e r s a n d leaves t h e r e a c t o r u n c h a n g e d . E q u a t i o n (5.99) is m o r e c o n v e n i e n t t h a n E q . (5.64), since it c o n t a i n s p r i m a r y v a r i a b l e s S a n d Q r a t h e r t h a n S a n d Q. F
e
F
F
R
F
F
0
7.5. C A L C U L A T I O N O F R E S I D E N C E T I M E IN REACTOR R e s i d e n c e t i m e in t h e r e a c t o r is c a l c u l a t e d f r o m t w o c r i t e r i a i n o r d e r t o determine which o n e controls the design. These t w o criteria are 1. Effluent q u a l i t y , w h i c h m e e t s r e g u l a t o r y a u t h o r i t y s p e c i f i c a t i o n s .
7.
209
Design Procedure for an Activated Sludge Plant
Effluent q u a l i t y d e p e n d s o n s u b s t r a t e r e m o v a l r a t e g i v e n b y E q . (5.19), w h i c h s o l v e d for t yields t = (S -S )/[kX (Se-S )] 0
e
uta
(5.100)
n
2. O r g a n i c l o a d i n g , e v a l u a t e d f r o m F/M
r a t i o for o p t i m u m f l o c c u l a t i o n
a n d s e t t l i n g o f s l u d g e . T h i s is given b y E q . (5.80), w h i c h s o l v e d for t yields * = S l[X , (FIM)] 0
(5.101)
v a
R e q u i r e d r e s i d e n c e t i m e is c a l c u l a t e d f r o m E q s . (5.100) a n d (5.101), t h e l a r g e r o f t h e t w o v a l u e s o f t t h u s o b t a i n e d b e i n g a d o p t e d for d e s i g n . F o r w a s t e s w h i c h a r e easily d e g r a d a b l e (e.g., s u g a r refinery, d a i r y , b r e w e r y ) , t h e flocculation
optimum
c o n d i t i o n is c o n t r o l l i n g for r e s i d e n c e t i m e c a l c u l a t i o n s . F o r o t h e r
w a s t e s , e.g., i n p e t r o l e u m refineries a n d p e t r o c h e m i c a l p l a n t s , t h e effluent quality
criterion
controls
residence
time
requirements
since
biological
d e g r a d a t i o n is v e r y s l o w .
7.6. E Q U A T I O N S F O R S L U D G E R E C Y C L E RATIO r IN C A S E S W H E N EFFLUENT QUALITY A N D O R G A N I C L O A D I N G CONTROL RESIDENCE TIME C o n s i d e r E q . (5.91) for t h e s l u d g e recycle r a t i o . AX
is g i v e n b y E q . (5.68),
V
w h i c h is r e w r i t t e n i n c l u d i n g t h e f a c t o r 8.34 for u s e w i t h Q in M G D ; V i n M G ; and S,
S
0
and X
e9
v%a
AX
V
in m g / l i t e r a s = S.34a(S -S )Q 0
- S.34bX V
e
(5.102)
Vt0
U t i l i z i n g E q . (5.5), AX
V
= S34a(S -S )Q (\ 0
e
+ r ) - 8.34bX , V
F
(5.103)
v a
S i n c e r e a c t o r v o l u m e Κ is a v a l u e c a l c u l a t e d b y t h e d e s i g n e r , it is d e s i r a b l e t o r e w r i t e E q . (5.103) a s a f u n c t i o n o f r e s i d e n c e t i m e f, w h i c h is g i v e n b y e i t h e r E q . (5.100) o r (5.101). T e r m V in E q . (5.103) is o b t a i n e d b y c o m b i n i n g E q s . (5.17) a n d ( 5 . 5 ) : V=
Qt = Q V+r)t
(5.104)
F
S u b s t i t u t i n g in E q . (5.103) V b y its v a l u e given b y E q . (5.104), AX
= S.34a(S -S )Q (l+r)
V
Substitution
0
e
o f ΔΑ^ given
- S.34bX , Q (1
F
by
v a
Eq.
(5.105) in
+r)t
F
Eq.
(5.91) yields
(5.105) after
simplification r = [X -a(S -S )(\+r) Vta
0
e
+ bX , (\+r)tv a
X ,,]/(*,,„-X ,.) v
v
(5.106)
5.
210
Secondary Treatment: The Activated Sludge Process
R e s i d e n c e t i m e t o n t h e n u m e r a t o r o f E q . (5.106) is given b y e i t h e r E q . (5.100) o r (5.101), d e p e n d i n g o n w h e t h e r r e s i d e n c e t i m e is g o v e r n e d b y substrate removal rate or optimum
flocculation
conditions. E q u a t i o n s (5.100)
a n d (5.101) a r e w r i t t e n i n a g e n e r a l i z e d f o r m a s t = (S -a)/X , fi
(5.107)
v a
0
W h e r e effluent q u a l i t y c o n t r o l s d e s i g n ( C a s e 1), α = S
(5.108)
e
fi = k(S -S ) e
Where optimum
flocculation
(5.109)
n
conditions control design (Case 2),
(5.110)
β = ΠΜ
(5.111)
S u b s t i t u t i n g r e s i d e n c e t i m e t i n t h e n u m e r a t o r o f E q . (5.106) b y its v a l u e f r o m E q . (5.107) yields X , -a(S -Se)(l v a
Since S
0
+ r) + 6 ^ A l Ρ
0
+
r)-X ,
(X ,u-Xv,a)
v F
v
(5.112)
is not a. p r i m a r y v a r i a b l e it is d e s i r a b l e t o e l i m i n a t e it f r o m t h e
n u m e r a t o r o f E q . (5.112). S u b s t i t u t i n g (S — S ) b y i t s v a l u e g i v e n i n E q . 0
e
(5.97), a n d t h e v a l u e o f S i n t e r m ( S - a ) b y i t s v a l u e g i v e n i n E q . ( 5 . 9 6 ) , 0
0
X , -a(S -S ) v a
F
+b
e
S
F+
r
S
e p
*
(
l
+
r
)
-X
V
f
l(X -X , )
F
VtU
v a
(5.11 3)
N o w w r i t e E q . (5.113) specifically f o r C a s e s (1) a n d (2). For Case ( / ) . S u b s t i t u t i n g i n E q . (5.113) α a n d β given b y E q s . (5.108) a n d (5.109) a n d simplifying, Χν,ο - a(S -S ) F
Recycle
ratio,
e
+b
S
f
~ °
(X , -Xv,a)
- X
S
v u
VtF
(5.114)
Case (1): Effluent q u a l i t y c o n t r o l s d e s i g n
For Case (2). S u b s t i t u t i n g i n E q . (5.113) α a n d β g i v e n b y E q s . (5.110) a n d (5.111), s o l v i n g t h e r e s u l t i n g e x p r e s s i o n f o r r , a n d s i m p l i f y i n g [ E q . ( 5 . 1 1 5 ) ] , _ " Recycle
ratio,
{X , -a(S -S )-](FIM)^bS -Xv, (FIM) F
v a
e
F
F
(X , -X , )(FIM)-bSe v u
Case (2): O p t i m u m
v a
flocculation
conditions control design
7.7. N E U T R A L I Z A T I O N R E Q U I R E M E N T S O p t i m u m activity for bacteria occurs a t p H values o f 6 - 8 . I t s h o u l d b e c h e c k e d if n e u t r a l i z a t i o n is n e e d e d p r e c e d i n g b i o l o g i c a l t r e a t m e n t . F o r a l k a l i n e w a s t e s , i t is t a k e n a s a r u l e o f t h u m b t h a t u p t o 0 . 5 l b o f a l k a l i n i t y ( a s
7.
211
Design Procedure for an Activated Sludge Plant
C a C 0 ) is r e m o v e d p e r l b o f B O D r e m o v e d . T h i s h a p p e n s b e c a u s e t h e C 0 3
2
evolved from bacterial waste degradation reacts with alkalinity ( O H ~ ) present in t h e w a s t e t o f o r m b i c a r b o n a t e ( H C 0 ~ ) , w h i c h buffers t h e s y s t e m a t a p H 3
o f a b o u t 8. T h u s , n e u t r a l i z a t i o n p r e c e d i n g b i o l o g i c a l t r e a t m e n t m a y n o t b e r e q u i r e d for s o m e a l k a l i n e w a s t e w a t e r s .
7.8. NUTRIENT R E Q U I R E M E N T S T h e a p p r o p r i a t e a m o u n t o f c e r t a i n n u t r i e n t s is r e q u i r e d for b o t h s y n t h e s i s a n d respiration phases of aerobic biological degradation of wastes. R e q u i r e d nutrients include nitrogen, phosphorus, calcium, magnesium, and vitamins. M o s t of these nutrients, which are r e q u i r e d only in trace quantities, are usually present in wastewaters. However, m a n y industrial wastewaters are deficient i n n i t r o g e n a n d p h o s p h o r u s . R e q u i r e d a m o u n t s o f n i t r o g e n a n d p h o s p h o r u s are estimated by the p r o c e d u r e discussed in this section. If deficiency exists, it is c o r r e c t e d b y a d d i n g t o t h e w a s t e w a t e r c a l c u l a t e d w e i g h t s of c o m p o u n d s containing nitrogen a n d p h o s p h o r u s . A n e s t i m a t e o f r e q u i r e m e n t s for n i t r o g e n a n d p h o s p h o r u s is b a s e d o n t h e fact t h a t w a s t e d M L V S S (ΔΑ^ l b / d a y ) c o n t a i n s a p p r o x i m a t e l y 2 % o f its d r y weight as p h o s p h o r u s a n d 12% as nitrogen. A n estimate of weights of nitrogen a n d p h o s p h o r u s to be added comprises 1. W e i g h t s o f t h e s e n u t r i e n t s w h i c h a r e l o s t b y w a s t a g e o f M L V S S , i.e., N i t r o g e n : 0.12 AX
V
lb/day
P h o s p h o r u s : 0.02 AX
V
lb/day
2. W e i g h t s o f t h e s e n u t r i e n t s w h i c h a r e l o s t i n t h e effluent. ( T o t a l effluent = Q' + Q" = Q .) C o n c e n t r a t i o n s o f s o l u b l e n i t r o g e n a n d p h o s p h o r u s p r e s e n t in effluent a r e u s u a l l y e s t i m a t e d t o b e 1.0 a n d 0.5 m g / l i t e r , r e s p e c t i v e l y . T h u s , t h e a m o u n t s o f n i t r o g e n a n d p h o s p h o r u s l o s t in t h e effluent a r e F
N i t r o g e n : fi χ 1.0 χ 8.34 lb/day F
Phosphorus: Q
F
χ 0.5 χ 8.34 lb/day
w h e r e Q is t h e effluent in M G D . T h e r e f o r e , t h e t o t a l r e q u i r e m e n t s o f n i t r o g e n a n d p h o s p h o r u s a r e given b y t h e s u m o f t h e e s t i m a t e s m a d e u n d e r (1) a n d (2) [ E q s . (5.116) a n d ( 5 . 1 1 7 ) ] : F
N i t r o g e n : 0.12 AX
V
+ Q
P h o s p h o r u s : 0.02 AX
F
V
χ 1.0 χ 8.34 lb/day
+ Q
F
(5.116)
χ 0.5 χ 8.34 lb/day
(5.117)
In activated sludge plants, nitrogen a n d p h o s p h o r u s requirements are provided by the addition of a n h y d r o u s or a q u e o u s N H , H P 0 , or (NH ) P0 . 3
4
3
4
3
4
212
5.
Secondary Treatment: The Activated Sludge Process
7.9. D E S I G N P R O C E D U R E F O R A C T I V A T E D SLUDGE PLANTS Step L C a l c u l a t e t h e r e c y c l e r a t i o o f M L V S S . Select v a l u e s f o r X
and
va
X , VtU
usually within the ranges of 2000-4000 a n d 10,000-15,000 mg/liter,
respectively. F r o m s e d i m e n t a t i o n a n d S V I d a t a ( F i g . 5.7) select a n a p p r o p r i a t e v a l u e f o r t h e F/M r a t i o . O p t i m u m F/M is u s u a l l y i n t h e r a n g e 0 . 3 - 0 . 7 . T h e recycle r a t i o is c a l c u l a t e d (1) f r o m E q . (5.114), w h i c h a s s u m e s t h a t effluent q u a l i t y c o n t r o l s t h e d e s i g n , a n d (2) f r o m E q . (5.115), w h i c h a s s u m e s t h a t o p t i m u m flocculation conditions control t h e design. T h e decision o n w h i c h c o n d i t i o n c o n t r o l s d e s i g n is a r r i v e d a t i n S t e p 3 . Step 2. C a l c u l a t e B O D o f t h e c o m b i n e d feed (S ). S is c a l c u l a t e d f r o m 0
0
E q . (5.96) u t i l i z i n g both v a l u e s o f r c a l c u l a t e d i n S t e p 1 ( 1 ) a n d S t e p 1 ( 2 ) . These t w o parallel calculations a r e referred t o hence a s Steps 2 ( 1 ) a n d 2 ( 2 ) , respectively. Step 3. C a l c u l a t e r e s i d e n c e t i m e i n t h e r e a c t o r . Case
(1)
Assuming
substrate removal rate controls design
[Eq.
( 5 . 1 0 0 ) ] , w h e r e S is t h e v a l u e c a l c u l a t e d i n S t e p 2 ( 1 ) . 0
Case (2)
Assuming optimum
flocculation
conditions control design
[ E q . ( 5 . 1 0 1 ) ] , w h e r e S is t h e v a l u e c a l c u l a t e d i n S t e p 2 ( 2 ) . 0
P o s s i b l y * t h e l a r g e r o f t h e s e t w o c a l c u l a t e d r e s i d e n c e t i m e s is t h e o n e selected f o r d e s i g n . R e c y c l e r a t i o a n d B O D o f c o m b i n e d feed f o r t h e specific case which controls design a r e then a d o p t e d . Calculated values for t h e other case a r e discarded. Step 4. C a l c u l a t e t h e r e a c t o r v o l u m e . R e a c t o r v o l u m e is t h e n c a l c u l a t e d f r o m E q . (5.104) u t i l i z i n g t h e v a l u e o f t h e r e s i d e n c e t i m e selected i n S t e p 3 . A t t h i s s t a g e , d e p t h o f t a n k is selected. S e l e c t i o n d e p e n d s o n t y p e o f a e r a t o r utilized ( C h a p t e r 4 , S e c t i o n s 1 4 . 4 , 1 5 . 4 , a n d 16.3). C r o s s - s e c t i o n a l a r e a is t h e n calculated. Step 5 . C a l c u l a t e t h e n e t yield o f M L V S S . N e t yield o f M L V S S AX
V
is
c a l c u l a t e d f r o m E q . (5.105) o r E q . (5.98). Note:
A c h e c k o n t h e m a t e r i a l b a l a n c e f o r M L V S S is m a d e a t t h i s p o i n t .
(See E x a m p l e 5.7 f o r d e t a i l s . ) Step 6. C a l c u l a t e Q" a n d g ' . Q" a n d Q' a r e t h e n c a l c u l a t e d f r o m E q s . (5.94) a n d (5.95). Step 7. C a l c u l a t e ΔΧ
Νν
a n d AX . t
ΔΧ
Νν
is c a l c u l a t e d f r o m E q . (5.72) a n d
t o t a l s l u d g e yield is o b t a i n e d f r o m E q . (5.1), w h e r e ΑΧ
υ
a n d AX
NV
are the
v a l u e s c a l c u l a t e d i n S t e p s 5 a n d 7, r e s p e c t i v e l y . * T h e reason for the word possibly is that frequently a compromise is made in selection o f residence time, s o that n o t only reasonable B O D reduction is achieved (i.e., a l o w value o f S ), but also g o o d flocculation conditions for the sludge (although n o t necessarily the o p t i m u m ) are obtained. e
7.
213
Design Procedure for an Activated Sludge Plant
Note:
A c h e c k o n t h e o v e r a l l m a t e r i a l b a l a n c e for N V S S is m a d e a t t h i s
p o i n t . (See E x a m p l e 5.7 for d e t a i l s . ) Step
8. C a l c u l a t e o x y g e n r e q u i r e m e n t s ( l b / d a y ) f r o m E q . (5.64) o r E q .
(5.99). Step 9. Specify a e r a t i o n e q u i p m e n t . A e r a t i o n e q u i p m e n t is selected f r o m o x y g e n r e q u i r e m e n t s d e t e r m i n e d in S t e p 8 a n d m a n u f a c t u r e r ' s s p e c i f i c a t i o n s for a e r a t o r s . T h i s p r o c e d u r e is d e s c r i b e d in C h a p t e r 4 , S e c t i o n s 14, 15, a n d 16. C a l c u l a t e d v a l u e s n e e d e d a r e (1) t o t a l H P a n d n u m b e r o f a e r a t i o n u n i t s ; (2) p o w e r level, H P / 1 0 0 0 g a l ; a n d (3) s p a c i n g b e t w e e n a e r a t o r s . A l a y o u t for a e r a t o r s in t h e t a n k is selected. ( D e t a i l s a r e given in E x a m p l e 4.5.) Step
10. C h e c k n e u t r a l i z a t i o n r e q u i r e m e n t s . Verify if n e u t r a l i z a t i o n is
required prior t o biological treatment. F o r alkaline wastes, the rule of t h u m b s t a t i n g t h a t u p t o 0.5 l b o f a l k a l i n i t y (as C a C 0 ) is r e m o v e d p e r lb o f B O D 3
r e m o v e d is f r e q u e n t l y e m p l o y e d ( S e c t i o n 7.7). Step 11. E v a l u a t e n u t r i e n t r e q u i r e m e n t s . R e q u i r e m e n t s for n i t r o g e n a n d p h o s p h o r u s ( l b / d a y ) a r e e v a l u a t e d a s d e s c r i b e d in S e c t i o n 7.8 f r o m
Eqs.
(5.116) a n d (5.117).
7.10. N U M E R I C A L E X A M P L E : D E S I G N O F A N ACTIVATED S L U D G E PLANT Example 5.7 A n i n d u s t r i a l p l a n t ( E x a m p l e 5.5) c o n s i d e r s a n a c t i v a t e d s l u d g e s y s t e m for t r e a t m e n t o f t h e i r w a s t e w a t e r s . B a s e d e s i g n o n t h e f o l l o w i n g d a t a in a d d i t i o n t o i n f o r m a t i o n given in E x a m p l e 5 . 5 : F l o w : 1.5 M G D (1,500,000 gal/day) Influent B O D : 1200 mg/liter Effluent B O D : 40 mg/liter O H " alkalinity of raw wastewater: 90 mg/liter (as C a C 0 ) Total Kjeldahl nitrogen a n d p h o s p h o r u s in fresh feed: 85 a n d 3 mg/liter, respectively Csw' Saturation D O of wastewater at the temperature a n d barometric pressure of the test C (summer conditions at 30°C): 7.4 mg/liter C (winter conditions at 18°C): 10.3 mg/liter α = A^aiwastewaterJ/t^Laiwater)] = 0.72 Assume operating D O (level in aeration basin « 1.0 mg/liter) Characteristics of surface a e r a t o r s : given by Fig. 4.17 Select for design p u r p o s e s : X = 3000 mg/liter of M L V S S X , = 10,000 mg/liter of M L V S S Neglect VSS concentration in effluent from secondary clarifier a n d in fresh feed 5
5
3
sw
sw
Vt
a
0
u
SOLUTION A flowsheet for t h e p r o p o s e d a c t i v a t e d s l u d g e p l a n t is s h o w n in F i g . 5.20. F o l l o w d e s i g n p r o c e d u r e s u m m a r i z e d in S e c t i o n 7.9.
214
5.
S e c o n d a r y Treatment: The Activated S l u d g e P r o c e s s
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7.
215
Design Procedure for an Activated Sludge Plant
Step 1. C a l c u l a t e t h e r e c y c l e r a t i o o f M L V S S . (1) A s s u m e s u b s t r a t e r e m o v a l r a t e c o n t r o l s d e s i g n [ E q . ( 5 . 1 1 4 ) ] . H e r e X
= 3000 mg/liter
VtQ
= 0 . 5 7 5 ( 1 2 0 0 - 4 0 ) = 0.575 χ 1160 = 665 mg/liter
a(S -S ) F
e
Xv.u ~ X ,a = 10,000 - 3000 = 7000 mg/liter v
= 3000 - 665 = 2335 mg/liter
X ,a - a(S -S ) v
Xv.F
F
e
= 0
T h u s E q . (5.114) yields r = [2335 + C a l c u l a t e s e p a r a t e l y t e r m b(S
F
b(S -S )/k(S -S WOOO F
— S )/k(S e
e
e
— S ),
e
n
n
w h i c h is t h e o n l y
different
t e r m i n e q u a t i o n s for r i n S t e p 1(1) a n d S t e p 1 (2). = 0 . 0 7 5 ( 1 2 0 0 - 4 0 ) / 0 . 0 1 6 1 ( 4 0 - 1 0 ) = 180
b(S -S )lk(S -S ) F
e
e
n
r = ( 2 3 3 5 + 1 8 0 ) / 7 0 0 0 = 0.36
(2) A s s u m e
optimum
flocculation
conditions
control
design
[Eq.
(5.115)]. Here [X , -a(S v a
S )] = 2335
F
[calculated Step 1 (1)]
(X
e
Vt
u
- X , a) = 7000
[also calculated Step 1(1)]
X (F/M)
since X
v
VtF
= 0
VtF
= 0
and (F/M)
= 0.6 b = 0.075
S
e
= 40
T h u s E q . (5.115) yields r = (2335 x 0.6 + 0 . 0 7 5 x 1200)/(7000x 0 . 6 - 0 . 0 7 5 x 4 0 ) = 0.353
Step 2. C a l c u l a t e S f r o m E q . (5.96). Q
(1) A s s u m e s u b s t r a t e r e m o v a l r a t e c o n t r o l s d e s i g n (i.e., r = 0.36). = (1200 + 0.36 x 4 0 ) / ( l + 0 . 3 6 ) = 893 mg/liter
S
0
(2) A s s u m e
optimum
flocculation
conditions control
r = 0.353). S
0
= (1200 + 0.353 x 4 0 ) / ( l + 0 . 3 5 3 ) = 900 mg/liter
design
(i.e.,
216
5.
Secondary Treatment: The Activated Sludge Process
Step 3. C a l c u l a t e r e s i d e n c e t i m e . (1) A s s u m e s u b s t r a t e r e m o v a l r a t e c o n t r o l s d e s i g n (S
Q
= 893 m g / l i t e r ) .
F r o m E q . (5.100) / = ( 8 9 3 - 4 0 ) / [ 0 . 0 1 6 1 χ 3 0 0 0 ( 4 0 - 1 0 ) ] = 0.59 day or 24 χ 0.59 = 14.2 h r (2) A s s u m e o p t i m u m
flocculation
c o n d i t i o n s c o n t r o l d e s i g n (S
0
=
900 mg/liter). F r o m E q . (5.101) t = 900/3000 χ 0.6 = 0.5 day or 24 χ 0.5 = 12 h r T h u s s u b s t r a t e r e m o v a l r a t e c o n t r o l s d e s i g n , a n d c a l c u l a t i o n s i n p a r t (2) f o r S t e p s 1-3 a r e d i s c a r d e d . F/M
r a t i o for t h e r e a c t o r is [ f r o m E q . ( 5 . 8 0 ) ]
F/M = 893/3000 χ 0.59 = 0.504 F r o m F i g . 5.15, t h i s v a l u e o f F/M
is c o m p a t i b l e w i t h g o o d
flocculation
con
d i t i o n s for t h e s l u d g e . T h e r e f o r e , n o a d j u s t m e n t o f selected r e s i d e n c e t i m e ( 1 4 . 2 h r ) is r e q u i r e d t o a c h i e v e c o m p a t i b i l i t y b e t w e e n B O D r e d u c t i o n a n d g o o d flocculation
conditions.
Step 4. C a l c u l a t e r e a c t o r v o l u m e . T h r o u g h p u t r a t e [ E q . ( 5 . 5 ) ] : Q = 1.5(1+0.36) = 2.04 M G D Reactor volume [Eq. (5.104)]: V = 2.04 χ 0.59 = 1.2 M G or 1,200,000 gal χ ft /7.48 gal = 161,000 ft 3
3
F o r d e p t h s o f 10 a n d 15 ft, for e x a m p l e , c o r r e s p o n d i n g c r o s s - s e c t i o n a l a r e a s are F o r d = 10 ft
A = 161,000/10 = 16,100 ft
2
or 16,100 ft χ acre/43,500 ft 2
F o r d = 15 ft
2
= 0.37 acre
A = 161,000/15 = 10,700 ft
or 10,700/43,500 = 0.246 acre Parallel basins might be recommended.
2
217
Design Procedure for an Activated Sludge Plant
7.
Step
5. C a l c u l a t e n e t yield o f M L V S S . F r o m E q . (5.105) (a, b f r o m
E x a m p l e 5.5), AX
= 0 . 5 7 5 ( 8 9 3 - 4 0 ) χ 2.04 χ 8.34 - 0.075 χ 3000 χ 1.2 χ 8.34
ΑΧ
= 8342 - 2252 = 6090 lb/day
V
ν
O r f r o m E q . (5.98), ΑΧ
υ
ΑΧ
ν
Note:
= 0 . 5 7 5 ( 1 2 0 0 - 4 0 ) χ 1.5 χ 8.34 - 0.075 χ 3000 χ 1.2 χ 8.34 = 8342 - 2252 = 6090 lb/day A t t h i s p o i n t a c h e c k o n m a t e r i a l b a l a n c e c a l c u l a t i o n s is m a d e .
C a l c u l a t e c o n c e n t r a t i o n o f V S S in c o m b i n e d feed t o t h e r e a c t o r (X ) Vt0
E q . (5.75), w h e r e X
from
= 0.
VtF
Χν,ο = (0 + 0.36 χ 10,000)/(1 + 0.36) = 2647 mg/liter T h e difference b e t w e e n c o n c e n t r a t i o n s o f M L V S S in r e a c t o r effluent ( 3 0 0 0 m g / l i t e r ) a n d t h e v a l u e 2647 m g / l i t e r in r e a c t o r influent m u s t c o r r e s p o n d t o t h e n e t yield o f M L V S S (i.e., AX
= 6090 lb/day). Therefore, 3 0 0 0 - 2 6 4 7 =
V
353 m g / l i t e r , i.e., 353 m g o f M L V S S a r e p r o d u c e d p e r liter o f l i q u o r
flowing
t h r o u g h t h e r e a c t o r . T h e n b a s e d o n flow Q = 2.04 M G D , n e t p r o d u c t i o n o f M L V S S is 353 χ 2.04 χ 8.34 « 6 0 0 6 l b / d a y , w h i c h a g r e e s a p p r o x i m a t e l y w i t h the value 6090 lb/day of
c a l c u l a t e d in S t e p 5 ( w i t h i n 1.4%).
Step 6. C a l c u l a t e Q" a n d Q' [ E q s . (5.94) a n d (5.95), r e s p e c t i v e l y ] . Q" = 6090/8.34 χ 10,000 = 0.073 M G D
for X ,
V F
= 0
or β " = 73,000 gal/day
(9.9 gal/min)
and Q = 1,500,000 - 73,000 = 1,427,000 gal/day Step 7. C a l c u l a t e AX
NV
AX
NV
a n d AX . t
F r o m E q . (5.72),
= 1 . 5 ( 3 0 - 2 0 ) χ 8.34 + 0.073 χ 20 χ 8.34 = 125 + 12.2 « 137 lb/day
T h e t o t a l s l u d g e yield AX
is [ f r o m E q . (5.1), w h e r e X
t
VF
AX
t
Note:
= 0]
= 6090 + 137 = 6227 lb/day
C h e c k o n m a t e r i a l b a l a n c e for N V S S in t h e influent. IN = Q X F
= 1.5 χ 30 χ 8.34 = 376 lb/day
NVfF
O U T = Q"X
NVtU
Q'X ,
NV e
X
= AX
= ΔΑ^ /8.34β
NVtU
NV
= 1.427 χ 20 χ 8.34 κ
,/
= 137 lb/day = 239 lb/day (checks)
= 137/8.34 χ 0.073 = 225 mg/liter
F r o m E q . (5.74), X y, ο = (30 + 0.36 x 2 2 5 ) / ( l + 0 . 3 6 ) = 82 mg/liter N
218
5.
Step
Secondary Treatment: The Activated Sludge Process
8. C a l c u l a t e o x y g e n r e q u i r e m e n t s f r o m e i t h e r E q . (5.64) o r E q .
(5.99). F r o m E q . (5.64) (a\ V f r o m E x a m p l e 5.5) lb 0 / d a y = 0 . 7 9 ( 8 9 3 - 4 0 ) χ 2.04 χ 8.34 + 0.15 χ 3000 χ 1.2 χ 8.34 2
= 11,500 + 4500 = 16,000 F r o m E q . (5.99) lb 0 / d a y = 0 . 7 9 ( 1 2 0 0 - 4 0 ) χ 1.5 χ 8.34 + 0.15 χ 3000 χ 1.2 χ 8.34 2
= 11,500 + 4500 = 16,000 or 16,000/24 = 665 lb 0 / h r 2
Step 9. Specify a e r a t i o n e q u i p m e n t . Specification o f a e r a t i o n e q u i p m e n t a n d l a y o u t for a e r a t o r s in t h i s a p p l i c a t i o n a r e p r e s e n t e d in C h a p t e r 4 ( E x a m p l e 4.5). Step
10.
Check neutralization requirements. Utilize rule of
thumb:
0.5 l b o f a l k a l i n i t y (as C a C 0 ) a r e r e m o v e d p e r lb o f B O D r e m o v e d . C a l c u l a t e 3
lb of B O D removed per day. (1200 - 40) χ 1.5 χ 8.34 = 14,512 lb/day T h u s 14,512/2 = 7256 l b / d a y o f a l k a l i n i t y a r e r e m o v e d . C a l c u l a t e t h e l b / d a y o f a l k a l i n i t y in fresh feed. 90 χ 1.5 χ 8.34 = 1126 lb/day Since 1126 < 7 2 5 6 , n o n e u t r a l i z a t i o n is r e q u i r e d p r i o r t o t h e
biological
process. Step 11. E v a l u a t e n u t r i e n t r e q u i r e m e n t s . Nitrogen 1. N i t r o g e n lost f r o m s y s t e m t h r o u g h w a s t a g e o f s l u d g e : 0.12 AX
V
= 0.12 χ 6090
= 730 lb/day
2. N i t r o g e n l o s t in effluent (1.0 m g / l i t e r ) : 1 χ 1.5 χ 8.34
13 lb/day
(total nitrogen required)
743 lb/day
N i t r o g e n a v a i l a b l e is (85 m g / l i t e r ) 85 χ 1.5 χ 8.34 = 1070 lb/day T h u s a d d i t i o n o f n i t r o g e n is not r e q u i r e d . Phosphorus 1. P h o s p h o r u s lost f r o m s y s t e m t h r o u g h w a s t a g e o f s l u d g e : 0.02 AX
0
= 0.02 χ 6090
= 121.8 lb/day
8.
219
The M i c h a e l i s - M e n t e n Relationship
2. P h o s p h o r u s l o s t in t h e effluent
0.5 m g / l i t e r ) :
0.5 χ 1.5 χ 8.34
6
lb/day
128
lb/day
P h o s p h o r u s a v a i l a b l e is (3 m g / l i t e r ) 3 χ 1.5 χ 8.34 = 37.6 lb/day T h u s 1 2 8 - 3 7 . 6 « 91.0 lb/day of p h o s p h o r u s should be a d d e d .
8. The M i c h a e l i s - M e n t e n Relationship 8.1. D E R I V A T I O N O F M I C H A E L I S - M E N T E N RELATIONSHIP F o r m u l a t i o n o f t h e M i c h a e l i s - M e n t e n r e l a t i o n s h i p is b a s e d o n s t u d i e s o f p u r e c u l t u r e s . H o w e v e r , it is u s e d in d e t e r m i n i n g k i n e t i c s o f s u b s t r a t e d e g r a d a t i o n b y a h e t e r o g e n e o u s p o p u l a t i o n o f m i c r o o r g a n i s m s , w h i c h is t h e c a s e for the activated sludge process. Degradation
o f w a s t e s b y m i c r o o r g a n i s m s is a c c o m p l i s h e d t h r o u g h
a
c o m p l e x series o f c h e m i c a l r e a c t i o n s . T h e s e r e a c t i o n s a r e c a t a l y z e d b y o r g a n i c c a t a l y s t s ( e n z y m e s ) p r e s e n t in t h e m i c r o o r g a n i s m s . E n z y m e s a r e l a r g e m o l e c u l a r w e i g h t c o m p o u n d s . U s u a l l y e n z y m e s a r e q u i t e specific in t h e i r f u n c t i o n s a s c a t a l y s t s , i.e., a given e n z y m e o r d i n a r i l y c a t a l y z e s a specific c h e m i c a l reaction. Bacteria contains a great variety of enzymes, each one being respon sible for a m i n o r s t e p in t h e c o m p l e x p r o c e s s o f b i o l o g i c a l m e t a b o l i s m . T h e a c t i o n o f e n z y m e s is r e p r e s e n t e d b y t h e f o l l o w i n g c h e m i c a l e q u a t i o n : Substrate + Enzyme ^ E n z y m e - S u b s t r a t e complex ^
E n z y m e + Products
or symbolically [S] + [E]
^ [ E - S ] ^=f
[Ε] + [P]
(5.118)
w h e r e &'s s t a n d for t h e r e a c t i o n r a t e c o n s t a n t s . A s i n d i c a t e d b y E q . (5.118), s u b s t r a t e a n d e n z y m e u n i t e t o f o r m a n e n z y m e - s u b s t r a t e c o m p l e x . T h i s is followed by the breaking d o w n of this complex, resulting in the end p r o d u c t s . T h e e n z y m e r e m a i n s u n c h a n g e d a n d is r e a d y t o r e e n t e r t h e r e a c t i o n , a c t i n g therefore as a catalyst. T h e r a t e o f s u b s t r a t e r e m o v a l is o b t a i n e d f r o m E q . (5.118) b y m a k i n g t h e a s s u m p t i o n t h a t t h e b r e a k i n g d o w n o f t h e e n z y m e - s u b s t r a t e c o m p l e x is irreversible. T h e n E q . (5.118) is r e w r i t t e n a s [S] + [Ε] τ=±
[E-S]
[Ε] + [P]
(5.119)
220
5.
S e c o n d a r y Treatment: The Activated S l u d g e P r o c e s s
T h i s a s s u m p t i o n is essentially c o r r e c t if m e a s u r e m e n t s a r e t a k e n s h o r t l y after i n t r o d u c t i o n o f s u b s t r a t e , w h i c h m e a n s t h a t v e r y little p r o d u c t h a s b e e n a l l o w e d t o f o r m . U n d e r t h e s e c i r c u m s t a n c e s , since t h e r a t e o f t h e i n v e r s e reaction [Ε] + [P]
[E-S]
is given b y * *-2[E][P]
a n d since [ P ] « 0, it m a y b e a s s u m e d t h a t t h e b r e a k i n g d o w n o f t h e e n z y m e s u b s t r a t e c o m p l e x is irreversible. T h e r e f o r e E q . (5.118) is r e w r i t t e n a s s h o w n [ E q . ( 5 . 1 1 9 ) ] . T h e r a t e o f r e a c t i o n m e a s u r e d u n d e r t h e s e c o n d i t i o n s is t h a t o c c u r r i n g i m m e d i a t e l y o n c o n t a c t o f s u b s t r a t e a n d m i c r o o r g a n i s m , a n d is r e f e r r e d t o a s t h e initial r a t e o f r e a c t i o n . T o d e v e l o p k i n e t i c d a t a it is n e c e s s a r y t o m e a s u r e a series o f s u c h initial r a t e s , c o r r e s p o n d i n g t o different c o n c e n t r a t i o n s of s u b s t r a t e s h o r t l y after t h e s u b s t r a t e s a m p l e s a r e b r o u g h t i n t o c o n t a c t with the microorganism. S u b s t r a t e r e m o v a l r a t e is d e n o t e d h e r e b y V. F o r a b a t c h r e a c t o r , it c o r r e s p o n d s t o t h e s l o p e of t h e B O D c u r v e in F i g . 5.3 ( S e c t i o n 3.1) a t a n y specified t i m e /, c o r r e s p o n d i n g t o a c o n c e n t r a t i o n S o f s u b s t r a t e . A specific s u b s t r a t e r e m o v a l r a t e p e r m g / l i t e r o f M L V S S is utilized, i.e. [ E q . ( 5 . 1 2 0 ) ] , K = -(l/X XdSldt)
(5.120)
Vta
( M i n u s sign is n e c e s s a r y since dS/dt < 0 a n d V > 0.) F o r t h e c o n t i n u o u s r e a c t o r a s s h o w n in S e c t i o n 3.2, t h i s c o r r e s p o n d s t o [ i n finite r a t h e r t h a n differential f o r m ] (S -S )/X , t 0
e
v a
w h e r e t is t h e r e s i d e n c e t i m e in t h e c o n t i n u o u s r e a c t o r . T h e s u b s t r a t e r e m o v a l r a t e is e q u a l t o t h e r a t e o f f o r m a t i o n o f p r o d u c t P , a n d is given b y E q . (5.121). V=k [E-S]
(5.121)
2
S i m i l a r l y , t h e r a t e of f o r m a t i o n o f t h e e n z y m e - s u b s t r a t e c o m p l e x ( E - S ) is R a t e of formation of (E-S) = &i [S] [E]
(5.122)
T h e r a t e o f c o n v e r s i o n o f e n z y m e - s u b s t r a t e c o m p l e x t o Ε a n d S is [ E q . ( 5 . 1 2 3 ) ] R a t e of conversion of ( E - S ) =
fc_i[E-S]
(5.123)
T h e r e f o r e , t h e n e t c h a n g e o f c o n c e n t r a t i o n o f e n z y m e - s u b s t r a t e c o m p l e x is d[E-SVdt=
k [S] [E] x
formation
^[E-S] destruction
-
A: [E-S] 2
(5.124)
destruction
* In the formulation to follow, symbols [ S ] , [ E ] , [ E - S ] , and [ P ] are used to denote c o n centrations o f substrate, enzyme, enzyme-substrate complex, and products, respectively.
8.
221
The M i c h a e l i s - M e n t e n Relationship
L e t t h e t o t a l c o n c e n t r a t i o n o f e n z y m e in t h e r e a c t i n g s y s t e m b e d e n o t e d a s E , . T h i s i n c l u d e s n o t o n l y free e n z y m e ( E ) b u t a l s o e n z y m e in c o m b i n e d f o r m a s e n z y m e - s u b s t r a t e c o m p l e x ( E - S ) , i.e. [ E q s . (5.125) a n d ( 5 . 1 2 6 ) ] , [E ] = [E] + [E-S]
(5.125)
.·. [E] = [ E J - [ E - S ]
(5.126)
f
S u b s t i t u t i n g [ E ] in E q . (5.124) b y its v a l u e g i v e n in E q . (5.126) yields d[E-S]/dt
= A ^ t f E J - [ E - S ] ) [ S ] - k-,
[ E - S ] - k [E-S] 2
(5.127)
A t s t e a d y s t a t e c o n d i t i o n s it is u s u a l l y a s s u m e d t h a t c o n c e n t r a t i o n i n t e r m e d i a t e c o m p l e x e s ( e n z y m e - s u b s t r a t e c o m p l e x in t h i s c a s e )
of
remains
u n c h a n g e d . T h i s a s s u m p t i o n is called t h e s t e a d y s t a t e a p p r o x i m a t i o n . T h e r e fore d[E-S]/dt
= 0
(5.128)
a n d E q . (5.127) b e c o m e s E q . (5.129). &i([EJ-[E-S])[S] -
[ E - S ] - A; [E-S] = 0 2
(5.129)
S o l v i n g for [ E - S ] , [Et]
[E-S] = T e r m (k_
t
as
+ k )/k 2
l
[S]
J
(5.130)
is called t h e M i c h a e l i s - M e n t e n c o n s t a n t a n d is d e s i g n a t e d
K. s
K = (k.^^/kt
(5.131)
s
T h e n , E q . (5.130) is r e w r i t t e n a s E q . (5.132). [ E J [S] [E-S] =
(5.132)
S u b s t i t u t i n g t h i s v a l u e in E q . (5.121), t h e f o l l o w i n g e x p r e s s i o n for t h e s u b s t r a t e r e m o v a l r a t e Κ is o b t a i n e d : [ E J [S] V = & 2 — [S] + K
(Michaelis-Menten relationship)
(5.133)
s
8.2. C O R O L L A R I E S O F M I C H A E L I S - M E N T E N RELATIONSHIP T h e t w o c o r o l l a r i e s s t a t e d in S e c t i o n 3.1 a r e o b t a i n e d f r o m E q . (5.133). Corollary 1 : H i g h s u b s t r a t e c o n c e n t r a t i o n s A t high substrate concentrations, [S] > K
s
(5.134)
5.
222
Secondary Treatment: The Activated Sludge Process
N e g l e c t i n g K in t h e d e n o m i n a t o r o f E q . ( 5 . 1 3 3 ) a s c o m p a r e d t o [ S ] a n d s
simplifying, V=
* [EJ 2
= KMAX
(5.135)
E q u a t i o n ( 5 . 1 3 5 ) indicates that a t high substrate concentrations, removal of s u b s t r a t e t a k e s p l a c e a t a m a x i m u m r a t e (V ) independent of concentration. I t is a s s u m e d t h a t a t t h e s e h i g h s u b s t r a t e c o n c e n t r a t i o n s all a c t i v e sites o f t h e e n z y m e s a r e s a t u r a t e d w i t h s u b s t r a t e , a n d s o r e a c t i o n p r o c e e d s a s fast a s possible independent of substrate concentration (zero-order reaction). This c o r r e s p o n d s t o t h e section of the B O D curve in Fig. 5 . 3 (Section 3 . 1 ) from time z e r o u p t o t i m e t \ w h e r e t h e t a n g e n t t o t h e B O D c u r v e essentially c o i n c i d e s w i t h t h e c u r v e itself ( c o n s t a n t s l o p e ) . MAX
[s];from ordinate of BOD curve, figure 5.3, section 3.1 Fig.
5.21.
Plot
of V vs. [S].
F r o m F i g . 5.3, a p l o t o f s l o p e s o f t h e B O D c u r v e ( K ' s o r s u b s t r a t e r e m o v a l r a t e s ) v s . t h e c o r r e s p o n d i n g B O D v a l u e s ( [ S ] ) c a n b e c o n s t r u c t e d ( F i g . 5.21). T h e region of high substrate concentration encompasses values from t h e r i g h t - h a n d e x t r e m i t y o f t h e g r a p h d o w n t o a c o n c e n t r a t i o n S' ( c o r r e s p o n d i n g t o t i m e t' in F i g . 5.3). T h i s is t h e r e g i o n w h e r e V = V = constant irre spective o f s u b s t r a t e c o n c e n t r a t i o n . F r o m E q . (5.135) E q . (5.133) is r e w r i t t e n a s MAX
V
=
K MMAAXX
[
S
(5.136)
]
[S] + #
S
Corollary 2 : L o w s u b s t r a t e c o n c e n t r a t i o n s A t low substrate concentrations, [S] <^ K
5
(5.137)
8.
223
The M i c h a e l i s - M e n t e n Relationship
Neglecting [ S ] in the d e n o m i n a t o r of E q . ( 5 . 1 3 6 ) as c o m p a r e d t o K [SG/A.
Since V
M
A
X
and K
S
, (5.138)
a r e b o t h c o n s t a n t for a specific w a s t e , E q . ( 5 . 1 3 8 ) is r e
S
written as V=K[S]
(5.139)
where K =
(5.140)
VMAX/K,
E q u a t i o n ( 5 . 1 3 9 ) indicates that at low substrate concentrations, substrate r e m o v a l follows
first-order
kinetics. In Fig. 5 . 2 1 this corresponds t o the section
o f t h e c u r v e f r o m a v a l u e o f t h e a b s c i s s a S = 0 u p t o a v a l u e S". I n t h i s s e c t i o n , t h e c u r v e is e s s e n t i a l l y r e p l a c e d b y a s t r a i g h t line p a s s i n g t h r o u g h t h e o r i g i n (with slope = K ) . This situation corresponds to t h a t encountered in con t i n u o u s b i o l o g i c a l r e a c t o r s o p e r a t i n g a t s t e a d y s t a t e c o n d i t i o n s . I n fact, F i g . 5 . 2 1 u p t o [ S ] = S" is i d e n t i c a l t o F i g . 5 . 5 ( S e c t i o n 3 . 2 ) , w h i c h w a s u t i l i z e d f o r d e t e r m i n a t i o n o f t h e r e m o v a l r a t e c o n s t a n t f r o m a series o f
continuous
l a b o r a t o r y r e a c t o r s o p e r a t i n g in p a r a l l e l . H a d t h e s e e x p e r i m e n t s b e e n c o n t i n u e d o n h i g h e r c o n c e n t r a t i o n s o f s u b s t r a t e , t h e s t r a i g h t line w o u l d h a v e b e c o m e a c u r v e like t h e o n e in F i g . 5 . 2 1 . H o w e v e r , o p e r a t i o n o f c o n t i n u o u s r e a c t o r s is a l w a y s c o n d u c t e d a t s u b s t r a t e c o n c e n t r a t i o n s m u c h b e l o w 5 0 0 mg/liter
(expressed
as B O D ) . 5
Under
these conditions the
straight
line
relationship applies.
8.3. S I G N I F I C A N C E O F M I C H A E L I S - M E N T E N CONSTANT K s
F r o m E q . ( 5 . 1 3 6 ) it is s h o w n t h a t K is e q u a l t o t h e s u b s t r a t e c o n c e n t r a t i o n S
w h e n s u b s t r a t e r e m o v a l r a t e V e q u a l s half V /2. MAX
T h i s is s h o w n b y l e t t i n g V = V /2 MAX
t h e m a x i m u m , i.e., w h e n V = in E q . ( 5 . 1 3 6 ) a n d s o l v i n g for
[ S ] . T h e final r e s u l t is [S]
=
K
(for
S
V=V /2) MAX
T h i s is i n d i c a t e d i n F i g . 5 . 2 1 .
8.4. D E T E R M I N A T I O N O F \ Z : THE L I N E W E A V E R - B U R K PLOT M
T h e value of V
M
A
X
A
X
e s t i m a t e d f r o m F i g . 5 . 2 1 is i n a c c u r a t e since it is a n
asymptotic value. A better way of determining K inverse of Eq.
M A X
is a s f o l l o w s . T a k e t h e
(5.136), l/V
=
(A«/KMAX)(1/[S]) + (1/KMAX)
(5.141)
B a s e d o n E q . ( 5 . 1 4 1 ) a l i n e a r p l o t o f l / V v s . 1 / [ S ] is c o n s t r u c t e d , f r o m w h i c h
224
5.
Secondary Treatment: The Activated Sludge Process
the characteristic constants V
and K
MAX
are determined from the slope a n d
s
i n t e r c e p t o f t h e s t r a i g h t line. T h i s g r a p h , w h i c h is s h o w n i n F i g . 5.22, is k n o w n as the Lineweaver-Burk plot [ 5 ] . As indicated the intercept at the abscissa c o r r e s p o n d s t o (— l/K ),
since for l/V=
s
0 o n e o b t a i n s 1/[S] = — l/K
from
s
E q . (5.141).
x>
> Slope: K / V
y^—
S
M A X
J—l/K
8
intercept = l / V
M A X
i/[s] Fig. 5.22.
Lineweaver-Burk
plot.
8.5. M I C H A E L I S - M E N T E N RELATIONSHIP W H E N N O N B I O D E G R A D A B L E MATTER IS PRESENT IN THE S Y S T E M I f t h e c o n c e n t r a t i o n o f n o n b i o d e g r a d a b l e m a t t e r is i n d i c a t e d a s [ 5 J , it is a c c o u n t e d for by substituting t h e value of [ S ] by ( [ S ] — [ S J ) in E q . (5.136). A s i m i l a r s u b s t i t u t i o n in S e c t i o n 3.2 led t o E q . (5.19) f r o m E q . ( 5 . 1 8 ) . T h e r e f o r e , m o d i f i e d E q . (5.136) is y = V ^
K
s
+
[
s
]
_
[
S
n
]
(5-142)
T h e t w o c o r o l l a r i e s s t u d i e d i n S e c t i o n 8.2 d e r i v e d f r o m E q . (5.133) a r e a l s o o b t a i n e d f r o m E q . ( 5 . 1 4 2 ) . S i m i l a r l y , F i g . 5.21 is r e p l o t t e d w h e n n o n b i o d e g r a d a b l e m a t t e r is p r e s e n t ( F i g . 5.23). F r o m E q . (5.142), it is s h o w n t h a t K = [ S ] - [5„] s
w h e n V=
V /2 MAX
T h e L i n e w e a v e r - B u r k e q u a t i o n w h e n n o n b i o d e g r a d a b l e m a t t e r is p r e s e n t is w r i t t e n b y r e p l a c i n g [ S ] i n E q . (5.141) b y ( [ S ] - [ £ „ ] ) . T h e c o r r e s p o n d i n g L i n e w e a v e r - B u r k p l o t follows directly from the e q u a t i o n t h u s o b t a i n e d .
8.
225
The Michaelis Menten-Relationship
V (day"
[S]
r-[Sn]-T-K —I 8
Fig.
5.23.
Plot
of V vs. [S] when
nonbiodegradable
matter
is
present.
8.6. M I C H A E L I S - M E N T E N R E L A T I O N S H I P I N T E R M S OF SPECIFIC G R O W T H RATE OF S L U D G E T h e M i c h a e l i s - M e n t e n r e l a t i o n s h i p [ E q . ( 5 . 1 3 6 ) ] is w r i t t e n a s a f u n c t i o n o f t h e specific s u b s t r a t e r e m o v a l r a t e [ E q . ( 5 . 1 2 0 ) ] . A n e q u i v a l e n t f o r m is w r i t t e n a s a f u n c t i o n o f t h e specific g r o w t h r a t e o f s l u d g e , defined a s μ = W h e r e a s v a l u e s o f dSjdt
(l/X , )(dXldt)
(5.143)
v a
in E q . (5.120) c o r r e s p o n d t o s l o p e s o f t h e
c u r v e in F i g . 5.3, v a l u e s o f dX/dt
BOD
in E q . (5.143) c o r r e s p o n d t o s l o p e s o f t h e
M L V S S curve. A s s u m i n g t h a t t h e specific g r o w t h r a t e o f s l u d g e is p r o p o r t i o n a l t o t h e specific s u b s t r a t e r e m o v a l r a t e , i.e., t h a t a c o n s t a n t f r a c t i o n o f t h e s u b s t r a t e r e m o v e d is c o n v e r t e d t o cells (μ = aV), E q . (5.136) is r e w r i t t e n a s
F r o m E q . (5.144) it is s h o w n , f o l l o w i n g t h e s a m e p r o c e d u r e utilized i n S e c t i o n 8.3 for E q . (5.136), t h a t K is e q u a l t o t h e s u b s t r a t e c o n c e n t r a t i o n w h e n t h e s
specific g r o w t h r a t e o f t h e s l u d g e is e q u a l t o h a l f t h e m a x i m u m specific g r o w t h r a t e , i.e., K = [ S ] , w h e n s
μ — μ Α χ / 2 · All Μ
corollaries,
derivations,
and
g r a p h i c a l c o n s t r u c t i o n s s t u d i e d in S e c t i o n s 8.2 t o 8.5 b a s e d o n t h e specific s u b s t r a t e r e m o v a l r a t e a r e a l s o a p p l i c a b l e in t e r m s o f t h e specific g r o w t h r a t e of the sludge.
5.
226
Secondary Treatment: The Activated Sludge Process
9. T h e C o n c e p t of S l u d g e A g e S l u d g e a g e is defined a s t h e m e a n r e s i d e n c e t i m e o f M L V S S i n t h e r e a c t o r . F o r t h e a c t i v a t e d s l u d g e p l a n t s h o w n in F i g . 5.1 t h i s c o r r e s p o n d s t o lb M L V S S in reactor/net o u t p u t of VSS from t h e system (lb/day) (5.145) or t =
(5.146)
s
total lb/day of VSS wasted, i.e., _ input of VSS in fresh o u t p u t of VSS (lb/day) feed (lb/day) or
t s
=
(AX* Qi°J-Q Xr, +
F
( d a y S )
(
5
U
7
)
F
I n E q . (5.147) n u m e r a t o r (X V)
equals total lb of M L V S S in the reactor
Vf0
at a n y time (a constant value a t steady state conditions). T e r m s between p a r e n t h e s e s in t h e d e n o m i n a t o r r e p r e s e n t t o t a l V S S w a s t e d , i n c l u d i n g s l u d g e w a s t e d p u r p o s e l y (ΔΧ )
a n d t h a t l o s t i n effluent f r o m t h e s e c o n d a r y clarifier
(Q'X ).
c o r r e s p o n d s t o i n p u t o f s l u d g e i n fresh feed. T h e
ν
Vte
Term Q X F
VF
difference b e t w e e n t h e t e r m s w i t h i n p a r e n t h e s e s
a n d this value
QX, F
V F
represents net o u t p u t of VSS from this system. Since in t h e formulation of t h e activated sludge process c o n c e n t r a t i o n of V S S i n effluent f r o m t h e s e c o n d a r y clarifier is n e g l e c t e d (i.e., X « VtB
0), E q .
(5.147) r e d u c e s t o E q . (5.148). ts = X , V/(AX -Q Xv, ) v a
v
F
(X ,
F
v e
= 0)
(5.148)
F i n a l l y , w h e n c o n c e n t r a t i o n o f V S S i n fresh feed is a l s o n e g l i g i b l e (i.e.,
ts = Xv, a V/AXv
(Xv, e*0 X , K 9
v F
0)
(5.149)
S l u d g e a g e is a l s o r e f e r r e d t o a s m e a n cell r e s i d e n c e t i m e o r s o l i d s r e t e n t i o n time. T h e relationship between sludge age a n d hydraulic or liquid retention t i m e (t = V/Q) is p r e s e n t e d f o r t w o t y p e s o f c o m p l e t e m i x r e a c t o r s : (1) c o m p l e t e m i x — n o recycle r e a c t o r ; a n d (2) c o m p l e t e m i x r e a c t o r w i t h r e c y c l e (a) w i t h w a s t a g e d i r e c t l y f r o m r e a c t o r ( o r r e a c t o r effluent), a n d ( b ) w i t h w a s t a g e f r o m t h e s l u d g e recycle line. 1. Complete
mix—no
recycle
reactor.
I n this m o d e l , liquor in t h e reactor
u n i t is c o m p l e t e l y m i x e d a n d t h e r e is n o recycle. T h i s does not c o r r e s p o n d t o the conventional activated sludge process, b u t rather t o
flow-through
devices
s u c h a s a e r a t e d l a g o o n s ( C h a p t e r 6, S e c t i o n 5), a s s u m i n g c o m p l e t e m i x i n g t o o c c u r i n t h e l a g o o n . T h e s i t u a t i o n is d e p i c t e d b y F i g . 5.24.
9.
227
The Concept of Sludge A g e
Q Reactor
^ν,ο Fig.
5.24.
^ν,α Complete
mix reactor
without
recycle.
H y d r a u l i c o r l i q u i d r e t e n t i o n t i m e is t = V/Q, a n d t h e s l u d g e a g e is [ f r o m Eq. (5.145)] ts = Xv.aVlQ(Xv,a-Xv,o)
= ( X , J&v. a ~ Xv, .)] /
(5.150) X &0).
F r e q u e n t l y , c o n c e n t r a t i o n o f V S S in influent is negligible (i.e., T h e n , E q . (5.150) r e d u c e s t o ts = Xv,aV\QX ,
= V/Q
v a
= t
(X
vo
(5.151)
« 0)
Vt0
T h u s for t h e c o m p l e t e m i x r e a c t o r w i t h o u t recycle w h e n c o n c e n t r a t i o n o f V S S in influent is negligible, s l u d g e a g e e q u a l s h y d r a u l i c ( o r l i q u i d r e t e n t i o n ) t i m e . C o n c e n t r a t i o n o f s l u d g e in t h e r e a c t o r is k e p t a t a c o n s t a n t v a l u e X . Since c o n c e n t r a t i o n o f s l u d g e in effluent a l s o e q u a l s X , it f o l l o w s t h a t r e s i d e n c e t i m e is s u c h t h a t s l u d g e is n o t w a s h e d o u t f r o m t h e s y s t e m f a s t e r t h a n it c a n r e p r o d u c e . I n fact, since s t e a d y s t a t e is a s s u m e d ( c o n s t a n t X in r e a c t o r a n d effluent), r e s i d e n c e t i m e is s u c h t h a t s l u d g e w a s h e d o u t i n effluent is exactly replaced b y a n e q u a l m a s s o f n e t s l u d g e yield for t h e s a m e t i m e interval. Vta
va
va
2. Complete mix reactor with recycle. T h i s m o d e l c o r r e s p o n d s t o t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s ( F i g . 5.1). W a s t a g e o f s l u d g e is u s u a l l y a c c o m p l i s h e d ( F i g . 5.1) b y d r a w i n g off f r o m t h e s l u d g e recycle line. H o w e v e r , t h e possibility o f w a s t i n g s l u d g e d i r e c t l y f r o m t h e r e a c t o r ( o r r e a c t o r effluent) is a l s o c o n s i d e r e d . Q
F
Q
Χν,α 6
V
Reoctor
Q
Secondary
S
clarifier
Q'=Qp -Q" S
e
^ν,ο
e
*v,e
s X
e , Q
V,0
U
=
QR
Λ ν,υ
Λ
Fig. 5.25. Diagram of complete directly from the reactor.
mix
reactor
with
recycle,
and
wastage
228
5.
Secondary Treatment: The Activated Sludge Process
Ό" S X
Q
s X
χ
^
F
Q
V
ν,α
s
Reactor
e
E
v,a
Secondary clarifier
e
Q'= Q S
F
- Q "
E
V,F Q = C |R U
-s QR S
X
e
v,u
E
Fig. 5.26. Diagram of complete directly from the reactor effluent.
mix reactor
with
recycle,
and
wastage
a. C o m p l e t e m i x r e a c t o r w i t h recycle, a n d w a s t a g e d i r e c t l y f r o m r e a c t o r ( o r r e a c t o r effluent). T h i s c o r r e s p o n d s t o d i a g r a m s s h o w n i n F i g s . 5.25 a n d 5.26, t h e f o r m e r w i t h w a s t a g e t a k e n d i r e c t l y f r o m r e a c t o r a n d t h e l a t t e r w i t h w a s t a g e t a k e n f r o m r e a c t o r effluent. T h e h y d r a u l i c o r l i q u i d r e t e n t i o n t i m e for t h e m o d e l s i n t h e s e figures is / = V/Q w h e r e a s s l u d g e is [ f r o m E q . ( 5 . 1 4 5 ) ] 9
t s
=
lb/day VSS wasted in Q"+lb/day VSS _ input of VSS in fresh lost in effluent from secondary clarifier feed (lb/day)
(
5
1
5
2
)
or t S
=
[β"*,,« +
(QF-Q")Xv,e]
-
QfXv,F
I f t h e c o n c e n t r a t i o n o f V S S i n t h e effluent f r o m t h e s e c o n d a r y clarifier is n e g l i g i b l e (i.e., X
VfC
« 0 ) , E q . (5.153) yields ts =
X.,*VHQrx ..-QwXv.w) 9
(Xv, * 0)
(5.154)
e
F i n a l l y , w h e n t h e c o n c e n t r a t i o n o f V S S i n fresh feed is a l s o n e g l i g i b l e (i.e., X *0), VtF
ts = X...VIQTX,..
= Κ/β"
* 0; X ,
V F
* 0)
(5.155)
C o m p a r i n g E q s . (5.17) a n d (5.155), it f o l l o w s t h a t since Q" <^ β , t h e n ts > t
(5.156)
b . C o m p l e t e m i x r e a c t o r w i t h recycle, a n d w a s t a g e f r o m r e c y c l e l i n e . T h i s c o r r e s p o n d s t o t h e flow d i a g r a m i n F i g . 5 . 1 . S i n c e c o n c e n t r a t i o n o f s l u d g e i n t h e w a s t a g e s t r e a m is e q u a l t o X , VtU
w h e r e a s it is X„ a(X a f
Vt
< ^ , « ) when
9.
229
The Concept of Sludge A g e
w a s t a g e is t a k e n d i r e c t l y f r o m t h e r e a c t o r o r r e a c t o r effluent ( F i g s . 5.25 a n d 5.26), it f o l l o w s t h a t t h e v o l u m e t r i c w a s t a g e flow Q" ( w h i c h c o n t a i n s a t o t a l o f AX
Vt0
l b / h r o f s l u d g e ) is less f o r t h e c a s e o f F i g . 5 . 1 . T h i s is o n e a d v a n t a g e
o f t a k i n g w a s t a g e d i r e c t l y f r o m t h e r e c y c l e line. H y d r a u l i c o r l i q u i d r e t e n t i o n t i m e is / = V/Q w h e r e a s s l u d g e a g e is g i v e n f r o m E q . (5.152) a s 9
Xv,aV t S
" [<2'X.« +
,- *
(QF-Q")Xv,el
-
QfXv.F
I f t h e c o n c e n t r a t i o n o f V S S i n t h e effluent f r o m t h e s e c o n d a r y clarifier is n e g l i g i b l e (i.e., X
« 0 ) , E q . (5.157) y i e l d s
ve
ts = Xv,aVl(Q"X»,u-QFXv.F)
(Xv.e * 0)
(5.158)
F i n a l l y , w h e n c o n c e n t r a t i o n o f V S S i n fresh feed is a l s o n e g l i g i b l e (i.e., XV,F*0),
ts = Xv, V/Q"Xv,u
(Xv, * 0 ; Xv,F * 0)
a
(5.159)
e
C o n s e q u e n t l y , w h e n w a s t a g e is t a k e n f r o m t h e recycle l i n e , k n o w l e d g e o f b o t h mixed liquor a n d recycled sludge m i c r o o r g a n i s m c o n c e n t r a t i o n s a r e required for calculation of sludge age. F o r t h e c o m p l e t e m i x r e a c t o r w i t h r e c y c l e ( F i g s . 5 . 2 5 , 5.26, a n d 5.1), r e s i d e n c e t i m e is s u c h t h a t s l u d g e is n o t w a s t e d f r o m t h e s y s t e m f a s t e r t h a n it r e p r o d u c e s . I n fact, s i n c e a s t e a d y s t a t e c o n d i t i o n is a s s u m e d , w a s t a g e
(ΑΧ ) υ
e q u a l s e x a c t l y t h e n e t s l u d g e yield f o r t h e s a m e t i m e i n t e r v a l if l o s s o f V S S i n t h e effluent f r o m t h e s e c o n d a r y clarifier is n e g l i g i b l e .
Example 5 . 8 F o r t h e a c t i v a t e d s l u d g e p l a n t d e s i g n e d i n E x a m p l e 5.7 c a l c u l a t e t h e s l u d g e age. SOLUTION
T h i s is a c a s e o f a c o m p l e t e m i x r e a c t o r w i t h r e c y c l e , w a s t a g e
b e i n g t a k e n f r o m t h e recycle line. C o n c e n t r a t i o n o f V S S i n t h e s e c o n d a r y clarifier effluent is n e g l i g i b l e (i.e., X &0), ve
a n d also X
VF
= 0.
(5.159) is t h e n u t i l i z e d t o c a l c u l a t e t h e s l u d g e a g e . H e r e X
Vta
= 3000 mg/liter
X
VyU
= 10,000 mg/liter
V = 1.2 M G
Q" = 0.073 M G D T h e n f r o m E q . (5.159) t
s
= 3000 χ 1.2/0.073 χ 10,000 = 4.43 days
H y d r a u l i c r e s i d e n c e t i m e t is 14.2 h r ( E x a m p l e 5.7, S e c t i o n 7.10).
Equation
230
5.
Secondary Treatment: The Activated Sludge Process
A relationship between sludge age, substrate removal rate
\_(S -S^IX t\ 0
va
a n d p a r a m e t e r s a a n d b for s l u d g e yield is w r i t t e n f r o m E q . (5.149) f o r t h e c o m p l e t e m i x r e a c t o r w i t h recycle. [ I n E q . (5.149) it is a s s u m e d t h a t c o n c e n t r a t i o n s o f V S S in t h e effluent f r o m t h e s e c o n d a r y clarifier a n d i n fresh feed are negligible.] If in E q . (5.149) n e t s l u d g e yield AX
is r e p l a c e d for t h e v a l u e g i v e n b y
V
E q . (5.68), Xv,aVI[a(S -S )Q-bX , V}
h =
0
v a
e
and
ia(s -s )Q-bx , vyx . v
=
0
e
v a
v a
Since V/Q = t, t h e n l/'s = a[(S -S )/X , t] 0
e
- b
v a
(5.160)
10. Kinetics of C o n t i n u o u s T r e a t m e n t S y s t e m s : Plug Flow, C o m p l e t e Mix, a n d A r b i t r a r y Flow R e a c t o r s I n t h e f o r m a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s , t h e m o d e l utilized for t h e c o n t i n u o u s r e a c t o r w a s t h a t o f a c o m p l e t e m i x vessel. T h e p l u g flow c o n t i n u o u s r e a c t o r m o d e l w a s o n l y briefly m e n t i o n e d i n S e c t i o n 5 ( F i g . 5.9). I n t h i s s e c t i o n t h r e e m o d e l s f o r t h e c o n t i n u o u s r e a c t o r ( F i g . 5.27) a r e d e s c r i b e d : (1) p l u g flow r e a c t o r , (2) c o m p l e t e m i x r e a c t o r , a n d (3) a r b i t r a r y flow r e a c t o r . 1. Plug flow reactor.
I n t h e p l u g flow r e a c t o r fluid p a r t i c l e s t r a v e l t h r o u g h
t h e vessel w i t h o u t m i x i n g a n d t h e r e f o r e a r e d i s c h a r g e d in t h e s a m e s e q u e n c e in w h i c h t h e y e n t e r . I f a c o n t i n u o u s t r a c e r is i n t r o d u c e d s t a r t i n g a t t i m e t = 0 ( c o n c e n t r a t i o n o f t r a c e r in t h e influent b e i n g C ), 0
n o t r a c e r a p p e a r s in
effluent u n t i l a t i m e / , e q u a l t o t h e o r e t i c a l r e s i d e n c e t i m e o f t h e fluid i n t h e r
vessel, h a s e l a p s e d . T h e n , t h e c o n c e n t r a t i o n o f t r a c e r in t h e effluent j u m p s from a zero value to the value C
a n d remains at that value as long as contin
0
u o u s i n j e c t i o n o f t r a c e r is m a i n t a i n e d . If a first d o s e of s l u g t r a c e r is i n t r o d u c e d a t t i m e t = 0, n o n e o f it a p p e a r s in t h e effluent u n t i l a t i m e t h a s e l a p s e d . A t / = t r
ri
concentration of tracer in the
effluent j u m p s f r o m z e r o t o C . A t t i m e (t + dt) it is b a c k a g a i n t o z e r o . I t 0
j u m p s again to C
0
a t t i m e t + At
r
r
9
w h e r e Δ / is t h e t i m e i n t e r v a l b e t w e e n t h e
first t w o d i s c o n t i n u o u s injections o f t r a c e r . 2.
Complete
mix reactor.
In this reactor immediate dispersion of particles
t a k e s p l a c e a s t h e y e n t e r t h e vessel. F o r a c o n t i n u o u s t r a c e r , its c o n c e n t r a t i o n
10.
231
Kinetics of Continuous Treatment Systems
(a) Plug flow
(b) Complete mix
(c) Arbitrary flow
Pattern of continuous tracer input C
C
C
Pattern of slug tracer input Fig. 5.27.
Continuous
reactor
models
{adapted
from
Ref.
[7]).
in t h e effluent a s a f u n c t i o n of t i m e is d e t e r m i n e d b y t h e f o l l o w i n g m a t e r i a l b a l a n c e for t r a c e r a r o u n d t h e r e a c t o r : R a t e of change in a m o u n t of tracer in reactor = rate of input of tracer to reactor - rate of o u t p u t of tracer from reactor or (dC/dt) V = QC
— QC
0
(5.161)
w h e r e C i s t h e effluent c o n c e n t r a t i o n o f t r a c e r a t a n y t i m e t\ Κ t h e v o l u m e o f t h e r e a c t o r ; Q t h e flow r a t e ; a n d C t h e c o n c e n t r a t i o n o f t r a c e r in t h e influent. F r o m E q . (5.161), 0
dC/dt = (Q/V) ( C - C )
(5.162)
0
Since V/Q = t ( h y d r a u l i c r e s i d e n c e t i m e , w h i c h is d e n o t e d h e r e a s t s o a s t o d i s t i n g u i s h it for t i m e v a r i a b l e t), E q . (5.162) yields r
r
dC/dt =
(\lt )(C -C) r
0
or dC/{Co-C)
= (\/t )dt r
(5.163)
I n t e g r a t i n g E q . (5.163) a n d s o l v i n g for C, C = c [l-*-<'"'>] 0
(5.164)
232
5.
Secondary Treatment: The Activated Sludge Process
T h i s c o r r e s p o n d s t o t h e c u r v e for t h e c o n c e n t r a t i o n o f c o n t i n u o u s
tracer
s h o w n in F i g . 5 . 2 7 ( b ) . A s s t e a d y s t a t e c o n d i t i o n s a r e a p p r o a c h e d
(theo
retically a t / = o o ) , E q . (5.164) yields C=
Co
T h u s , the curve a p p r o a c h e s asymptotically the o r d i n a t e C = C . If a d d i t i o n 0
o f t r a c e r s t o p s w h e n a s t e a d y s t a t e c o n d i t i o n is r e a c h e d , t h e c o r r e s p o n d i n g v a l u e for t r a c e r c o n c e n t r a t i o n in t h e effluent d r o p s g r a d u a l l y f o l l o w i n g c u r v e C = C e~ \ (
also s h o w n in Fig. 5.27(b). This corresponds t o a r e a c t o r
t/tr
0
oo ( s t e a d y s t a t e ) , t h e c o n c e n t r a t i o n o f t r a c e r i n
being p u r g e d of tracer. A s t effluent a p p r o a c h e s z e r o . 3. Arbitrary
flow
reactor.
These reactors correspond to a partial mix
c o n d i t i o n b e t w e e n p l u g flow a n d c o m p l e t e m i x t y p e s . T y p i c a l p a t t e r n s for c o n t i n u o u s a n d s l u g t r a c e r i n p u t for a r b i t r a r y flow r e a c t o r s a r e s h o w n i n F i g . 5.27(c). M a t h e m a t i c a l a n a l y s i s o f t h i s t y p e o f r e a c t o r is c o n s i d e r a b l y m o r e c o m p l i c a t e d t h a n p l u g o r c o m p l e t e m i x t y p e s , a n d for t h i s r e a s o n t h e s e t w o models are usually chosen to describe reactor performance. I t is i n t e r e s t i n g t o c o m p a r e efficiency o f B O D r e m o v a l for
continuous
r e a c t o r s w i t h recycle ( t y p i c a l a c t i v a t e d s l u d g e p l a n t ) , a d o p t i n g c o m p l e t e m i x a n d p l u g flow m o d e l s t o d e s c r i b e t h e r e a c t o r in q u e s t i o n . C o m p a r i s o n is m a d e b y c o m p u t i n g for a given w a s t e w a t e r (i.e., k a n d S
fixed) t h e effluent
n
(S )
for fixed v a l u e s o f flow r a t e Q, influent B O D (S \
e
F
M L V S S c o n c e n t r a t i o n (X )
BOD
recycle r a t i o ( r ) , a n d
for v a r i o u s a s s u m e d r e s i d e n c e t i m e s t. F o r t h e
Vt0
c o m p l e t e m i x r e a c t o r , t h e k i n e t i c m o d e l is g i v e n b y E q . (5.19). If in E q . (5.19) S
0
is e l i m i n a t e d u t i l i z i n g E q . ( 5 . 9 6 ) , t h e r e s u l t is (S -S )/[(l+r)X , t] F
Solving for
e
v a
= k(S -S ) e
(5.165)
n
S, e
S
= [S
e
F
A typical plot of S
e
+ kS (\+r)X , t~\IV n
v a
+ kX ,a(l+r)t~\
(5.166)
v
v s . t o b t a i n e d f r o m E q . (5.166) is s h o w n b y t h e c u r v e
l a b e l e d " c o m p l e t e m i x m o d e l " i n d i c a t e d i n F i g . 5.28. ( F o r t = 0, S
e
for t
=
oo,
S
e
=
= S
F
and
S .) n
A k i n e t i c m o d e l for t h e c o n t i n u o u s r e a c t o r w i t h recycle u n d e r p l u g c o n d i t i o n s is m a t h e m a t i c a l l y q u i t e difficult t o d e r i v e . A m o d e l h a s
flow been
o b t a i n e d , however, by Lawrence a n d M c C a r t h y [ 4 ] . This m o d e l predicts for a given r e s i d e n c e t i m e t a l o w e r v a l u e o f effluent B O D t h a n t h a t f o r t h e c o r r e s p o n d i n g c o m p l e t e m i x m o d e l . T h i s is i n d i c a t e d b y t h e d o t t e d c u r v e i n F i g . 5.28. T h u s , t h e p l u g flow recycle s y s t e m is t h e o r e t i c a l l y m o r e efficient t h a n t h e c o m p l e t e m i x recycle s y s t e m for s t a b i l i z a t i o n o f s o l u b l e w a s t e s . I n p r a c t i c e , h o w e v e r , t h e p l u g flow m o d e l is difficult t o o b t a i n b e c a u s e o f l o n g i t u d i n a l dispersion. A l s o , the complete mix systems h a n d l e s u d d e n changes in influent
10.
233
Kinetics of Continuous Treatment Systems
t Fig. 5.28. Plot ofS mix and plug flow
vs. t for continuous models).
9
flow
reactors
with recycle
(complete
B O D ( s h o c k l o a d s ) m u c h m o r e s a t i s f a c t o r i l y t h a n p l u g flow s y s t e m s . I n a d d i t i o n , t h e r e is t h e u n f a v o r a b l e s i t u a t i o n o f v a r i a b l e F/M r a t i o s a l o n g p l u g flow r e a c t o r s , a n d its p o s s i b l e u n d e s i r a b l e effect o n t h e s e t t l i n g c h a r a c t e r i s t i c s o f t h e s l u d g e d i s c u s s e d in S e c t i o n 5. A l l t h e s e f a c t o r s t e n d t o r e d u c e differences in a c t u a l efficiency o f B O D r e m o v a l for t h e t w o m o d e l s . F i g u r e 5.29 s h o w s t h e p r o g r e s s i v e B O D r e d u c t i o n o c c u r r i n g i n a p l u g flow r e a c t o r f r o m v a l u e S a t t h e inlet t o t h e final v a l u e S . By d i v i d i n g t h e a e r a t i o n t a n k i n t o a series o f c o m p l e t e m i x r e a c t o r s ( a s s u m e a u n i f o r m s o l u b l e B O D v a l u e for t h e l i q u o r b e t w e e n a n y t w o d o t t e d p a r t i t i o n s in F i g . 5.29), a n i m p r o v e m e n t in t r e a t m e n t p e r f o r m a n c e is o b t a i n e d w i t h o u t a m a j o r loss i n a b i l i t y o f t h e s y s t e m t o h a n d l e s h o c k l o a d s . T h i s is t h e i d e a b e h i n d t h e s t e p a e r a t i o n s c h e m e ( C h a p t e r 6, S e c t i o n 4 . 1 , F i g . 6.6). 0
Fig. 5.29.
e
BOD
reduction
in a plug
flow
reactor.
234
5.
Secondary Treatment: The Activated Sludge Process
Problems I. Determination o f design parameters for an activated sludge project. A n industrial plant considers an activated sludge system for disposal o f wastewaters. Preliminary tests are per formed in laboratory scale continuous reactors. Four reactors are operated in parallel until steady state conditions are obtained. D a t a taken are presented in the following tabulations.
TABLE 1 For Removal Kinetics
Reactor no.
Average B O D of influent (mg/liter)
Average BOD5 o f effluent (mg/liter)
Average MLVSS concentration (mg/liter)
Residence time (hr)
1 2 3 4
850 800 750 700
100 50 25 15
2000 2500 3100 3100
4.81 7.32 12.7 18.4
5
TABLE 2 Oxygen Utilization and Sludge Production Reactor no.
Oxygen uptake rate R [mg 0 / ( l i t e r ) (day)]
Sludge yield AX /V [mg sludge/(liter)(day)]
1 2 3 4
3200 2187 1425 1008
2500 1450 780 403
r
V
2
F r o m these data determine design parameters k ( h r
- 1
and d a y ) * S„, a, a', b, and b'. - 1
II. A n organic chemical wastewater is t o be treated by a proposed activated sludge plant to produce an effluent B O D o f 50 mg/liter during summer conditions (20°C). Wastewater characteristics are F l o w = 2.0 M G D Influent B O D = 1000 mg/liter Treatment parameters are k a a' b b' F/M
s„
= = = = = = =
0.0005 h r " at 20°C 0.50 lb M L V S S / l b B O D 0.55 lb 0 / l b B O D 0.1 lb M L V S S / ( d a y ) ( l b M L V S S ) 0.14 lb 0 / ( d a y ) ( l b M L V S S ) 0.6 0 . 0 mg/liter 1
P
2
2
r
235
References Take X = 3000 mg/liter X = 12,000 mg/liter a
u
Neglect influent suspended solids. Calculate 1. 2. 3. 4. Base
Reactor v o l u m e (Mgal) and sludge return rate (Mgal/day) Oxygen required Ob 0 / h r ) N e t sludge yield (lb M L V S S / d a y ) H P required for surface aeration. Characteristics o f the aerator are given by Fig. 4.17. calculation o n 20°C operation and take 2
Csw = 8.0 mg/liter C = 1.0 mg/liter α = 0.8 L
Calculate required power level in H P / 1 0 0 0 gal 5. Nutrient requirements (lb/day) for nitrogen and phosphorus
References 1. Eckenfelder, W. W., Jr., "Industrial Pollution Control." McGraw-Hill, N e w Y o r k , 1966. 2. Eckenfelder, W. W . , Jr., "Water Quality Engineering for Practicing Engineers." Barnes & N o b l e , N e w Y o r k , 1970. 3. Eckenfelder, W. W., Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w York, 1970. 4. Lawrence, A . W., and McCarthy, P. L., / . Sanit. Eng. Div. Am. Soc. Civ. Eng. 4 9 , S A 3 (1970). 5. Lineweaver, H . , and Burk, D . , / . Am. Chem. Soc. 5 6 , 6 5 8 - 6 6 6 (1934). 6. "Manual for Biooxidation U n i t . " Biodevelopment Associates, P.O. B o x 1752, Austin, Texas. 7. Metcalf & Eddy, Inc., "Wastewater Engineering: Collection, Treatment, D i s p o s a l . " McGraw-Hill, N e w York, 1972. y
6 Secondary Treatment: Other Aerobic and Anaerobic Wastewater Treatment Processes 1. Introduction
237
2. Extended Aeration (or Total Oxidation Process) 2.1. Introduction 2.2. Comparison of Extended Aeration and Activated Processes 2.3. Application of Extended Aeration 2.4. Extended Aeration Units 2.5. Settling of Sludge from Extended Aeration 2.6. Nitrification in Extended Aeration 2.7. Design Criteria for Extended Aeration
238 238 Sludge 238 239 239 241 241 241
3. Contact Stabilization 3.1. Introduction 3.2. Advantage of Contact Stabilization v s . Conventional Activated Sludge Process 3.3. Solubility Index (SI) and Overall Efficiency 3.4. Design of Contact Stabilization Systems
244 245 246
4. Other Modifications of Conventional Activated Sludge Process: Step Aeration, Complete M i x Activated Sludge Process, and Tapered Aeration 4.1. Step Aeration 4.2. Complete M i x Activated Sludge Process 4.3. Tapered Aeration
247 247 248 248
5. Aerated Lagoons 5.1. Introduction 5.2. Mixing Regimes for Aerated Lagoons 5.3. Kinetics of B O D Removal 5.4. Estimate of Lagoon Temperature (T ) 5.5. Oxygen Requirements for Aerated Lagoons 5.6. Soluble Effluent B O D for an Aerated Lagoon 5.7. M L V S S Concentration in Aerated Lagoons 5.8. Retention Period Required for a Specified Effluent Soluble BOD 5.9. Total Effluent B O D for an Aerated Lagoon 5.10. Design Procedure for Aerated Lagoons w
6. Wastewater Stabilization Ponds 6.1. Introduction 6.2. Kinetics of B O D Removal for Stabilization Ponds 236
244 244
249 249 249 250 251 253 253 253 2
5
4
255 256 2
5
9
2
5
9
260
1.
Introduction
237
6.3. 6.4.
Laboratory Simulation of Stabilization Ponds Mathematical Formulation for Several Stabilization Ponds in Series 6.5. Effect of Temperature on Reaction Rate Constant Κ 6.6. Oxygen Production in Aerobic Ponds 6.7. Depth of Oxygen Penetration in Stabilization Ponds 6.8. Facultative Ponds: Hermann and Gloyna's Equation 6.9. Anaerobic Ponds 6.10. Summary of Design Criteria for Wastewater Stabilization Ponds 6.11. Design Calculations for Stabilization Ponds
7. Trickling Filters 7.1. Introduction 7.2. Thickness of Slime Layer 7.3. Comparison between Trickling Filters and Activated Sludge Process 7.4. Physical Arrangement of Trickling Filters 7.5. Trickling Filter Systems 7.6. Pretreatment for Trickling Filtration 7.7. Design Formulation for Trickling Filters 7.8. Application of Basic Mathematical Model to Trickling Filters without and with Recycle 7.9. Procedure for Design of Trickling Filters W h e n Bench Scale or Pilot-Plant Data Are Available 7.10. Design Procedure W h e n Experimental Data Are Not Available
261 261 262 262 263 263 264 264 264 268 268 269 269 270 270 271 271 272 273 282
8. Anaerobic Treatment 8.1. Introduction 8.2. A Quantitative Study of Anaerobic Degradation of an Organic Waste 8.3. Mathematical Formulation for Anaerobic Digestion Process . . 8.4. Laboratory Anaerobic Reactors for Obtaining Basic Design Information 8.5. Design Procedure for Anaerobic Digesters
282 282 283 284
Problems
293
References
294
288 289
1. I n t r o d u c t i o n T h e b a s i c m e c h a n i s m f o r a e r o b i c t r e a t m e n t o f w a s t e w a t e r s is d e s c r i b e d in C h a p t e r 5, S e c t i o n 4 . 1 . 1 . T h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s is a l s o s t u d i e d in d e t a i l i n C h a p t e r 5. I n Sections 2 - 4 of this chapter, several modifications of t h e activated sludge process are described. O t h e r types of wastewater t r e a t m e n t (aerated lagoons, s t a b i l i z a t i o n p o n d s , a n d t r i c k l i n g filters) a r e d i s c u s s e d i n S e c t i o n s 5 - 7 . T h e s e a r e m o s t l y a e r o b i c p r o c e s s e s . A n a e r o b i c t r e a t m e n t o f w a s t e w a t e r s is s t u d i e d in S e c t i o n 8.
238
6.
Secondary Treatment: Aerobic and Anaerobic Processes
F o r b o t h a e r o b i c a n d a n a e r o b i c p r o c e s s e s t h e a p p r o a c h utilized for t h e m a t h e m a t i c a l f o r m u l a t i o n , a s well a s t h e p r o c e d u r e f o l l o w e d t o o b t a i n d e s i g n d a t a f r o m b e n c h scale u n i t s , a r e t h o s e d e v e l o p e d b y E c k e n f e l d e r a n d a s s o c i a t e s .
2. Extended A e r a t i o n (or Total Oxidation P r o c e s s ) 2.1.
INTRODUCTION
T h i s p r o c e s s , a l s o r e f e r r e d t o a s t o t a l o x i d a t i o n , is a m o d i f i c a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s . T h e f u n d a m e n t a l i d e a in e x t e n d e d a e r a t i o n a s c o m p a r e d t o t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s is t o m i n i m i z e t h e a m o u n t o f excess s l u d g e . T h i s is a c h i e v e d b y i n c r e a s i n g r e s i d e n c e t i m e ; t h u s t h e r e a c t o r v o l u m e is c o m p a r a t i v e l y l a r g e r t h a n t h a t r e q u i r e d in t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s . A s a r e s u l t , essentially all d e g r a d a b l e s l u d g e f o r m e d is c o n s u m e d b y e n d o g e n o u s r e s p i r a t i o n . R e f e r r i n g t o E q . (5.68), t h e c o n d i t i o n for z e r o n e t yield o f s l u d g e is AX
« 0
(6.1)
= bX , V
(6.2)
V
or aS Q r
v a
T h e o r e t i c a l r e s i d e n c e t i m e t o a c h i e v e z e r o n e t yield o f M L V S S is o b t a i n e d f r o m E q . (6.2). / = ν/Q = aS lbX , r
v a
(6.3)
T h e m a i n a d v a n t a g e o f t h e e x t e n d e d a e r a t i o n p r o c e s s is t h a t s l u d g e h a n d l i n g facilities a r e m i n i m a l c o m p a r e d t o t h o s e r e q u i r e d for t h e a c t i v a t e d s l u d g e process.
2.2.
C O M P A R I S O N OF EXTENDED AERATION A N D ACTIVATED SLUDGE PROCESSES
There are four basic features which distinguish extended aeration from the conventional activated sludge process: 1. L o n g e r d e t e n t i o n t i m e in a e r a t o r 2. L o w e r o r g a n i c l o a d i n g s . F o r t h e e x t e n d e d a e r a t i o n p r o c e s s o r g a n i c l o a d i n g , e x p r e s s e d i n t e r m s o f f o o d t o m i c r o o r g a n i s m r a t i o (F/M) (Chapter 5, S e c t i o n 5), is u s u a l l y b e t w e e n 0.10 a n d 0 . 2 5 , a s c o m p a r e d t o v a l u e s o f 0 . 3 - 0 . 7 for t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s . 3. H i g h e r c o n c e n t r a t i o n o f b i o l o g i c a l s o l i d s in t h e a e r a t o r . T h e s e v a l u e s r a n g e f r o m 3500 t o 5000 m g / l i t e r for e x t e n d e d a e r a t i o n , a s c o m p a r e d t o 2 0 0 0 - 4 0 0 0 m g / l i t e r for t h e a c t i v a t e d s l u d g e p r o c e s s . C o m b i n a t i o n o f f e a t u r e s
2.
239
Extended Aeration
c o n s i d e r e d u n d e r (2) a n d (3) (i.e., less f o o d for g r e a t e r m i c r o o r g a n i s m p o p u l a t i o n ) r e s u l t s in s t a r v a t i o n c o n d i t i o n s for t h e m i c r o o r g a n i s m s .
Resulting
"cannibalism" (endogenous respiration conditions) reduces concentration of M L V S S , a n d t h u s a m i n i m i z a t i o n o f s l u d g e a c c u m u l a t i o n is a c h i e v e d . 4. H i g h e r c o n s u m p t i o n o f o x y g e n in e x t e n d e d a e r a t i o n p r o c e s s . F o r d o m e s t i c w a s t e w a t e r t r e a t m e n t , P a s v e e r [ 1 1 ] r e p o r t s for t h e e x t e n d e d a e r a t i o n p r o c e s s a n o x y g e n c o n s u m p t i o n a p p r o x i m a t e l y twice t h a t for t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s , n a m e l y 18 v s . 9 k w h / ( c a p i t a ) ( y e a r ) .
With
t h i s c o m p a r a t i v e l y s m a l l e x t r a c o s t for e n e r g y , s u b s t a n t i a l s a v i n g s in c a p i t a l T A B L E 6.1 Comparison of Conventional Activated Sludge and Extended Aeration Processes Characteristics F o o d to microorganism ratio [lb B O D / ( d a y ) ( l b M L V S S ) ] M L V S S concentration in reactor (mg/liter) Overall B O D removal efficiency (% includes both soluble and suspended B O D ) Effluent characteristics Soluble B O D (mg/liter) Total B O D (suspended + colloidal + soluble) (mg/liter) Suspended solids (mg/liter) Sludge yield (lb/lb B O D removed) 0 requirement (as % o f B O D removed) 5
Activated sludge
Extended aeration
0.3-0.7 2000-4000
0.10-0.25 3500-5000
90-95
85-98
10-20
10-20
15-25 <20 «0.03 90-95
20-40 <70 «0.01 120
5
5
5
5
5
2
5
e x p e n d i t u r e m a y b e a c h i e v e d . T a b l e 6.1 p r e s e n t s a c o m p a r i s o n o f t h e m a i n characteristics of conventional activated sludge a n d extended aeration processes.
2.3. A P P L I C A T I O N O F E X T E N D E D A E R A T I O N T h e extended aeration process has been applied mostly in t r e a t m e n t of w a s t e w a t e r s w h e n d a i l y v o l u m e is less t h a n 2 0 0 0 g a l / d a y . T h i s i n c l u d e s t r e a t m e n t o f d o m e s t i c s e w a g e for s m a l l c o m m u n i t i e s , h o u s i n g d e v e l o p m e n t s , recreational areas, a n d some industrial wastes. Extended aeration package units are commercially available. If well designed a n d operated, they should not present odor problems and thus can be located within populated areas.
2.4. E X T E N D E D A E R A T I O N U N I T S F i g u r e s 6.1 a n d 6.2 i l l u s t r a t e a c o n v e n t i o n a l e x t e n d e d a e r a t i o n u n i t a n d a variation k n o w n as the oxidation ditch. In the conventional aeration unit ( F i g . 6.1), t h e influent p a s s e s first t h r o u g h a s c r e e n t o r e m o v e l a r g e s u s p e n d e d
240
6.
Secondary Treatment: Aerobic and Anaerobic Processes
Q=QpQp +
Screen or influent shredder Waste
QF
\
OF
S l u d g y recycle; Q
Fig. 6.1.
Conventional
extended
^ P Y p S l u d g e wastage
R
aeration
process.
s o l i d s , in o r d e r t o p r o t e c t t h e a e r a t o r u n i t f r o m d a m a g e r e s u l t i n g f r o m c l o g g i n g . I n s o m e u n i t s a s h r e d d e r is p r o v i d e d i n s t e a d o f a s c r e e n . T h e flow d i a g r a m o f t h e c o n v e n t i o n a l u n i t is essentially i d e n t i c a l t o t h a t o f t h e a c t i v a t e d s l u d g e p r o c e s s . Effluent f r o m t h e clarifier m a y b e c h l o r i n a t e d p r i o r t o d i s c h a r g e i n the receiving water. Waste
^Aeration rotor
wastage Fig. 6.2.
Oxidation
ditch.
F i g u r e 6.2 s h o w s a d i a g r a m o f t h e o x i d a t i o n d i t c h . A n e s s e n t i a l p a r t o f t h i s s y s t e m is a n a e r a t i o n d i t c h p r o v i d e d w i t h a n a e r a t i o n r o t o r . T h i s r o t o r h a s t w o f u n c t i o n s : a e r a t i o n a n d p r o v i s i o n o f a flow velocity t o t h e m i x e d l i q u o r i n t h e d i t c h . L i q u i d flow velocity is o f t h e o r d e r o f 1 ft/sec. T h e m i x t u r e o f s e w a g e a n d activated sludge repeatedly passes over the aeration r o t o r at short inter vals. A typical r o t o r has a diameter of approximately 30 in., revolves a t a b o u t 75 r p m , h a s a d e p t h o f i m m e r s i o n o f a b o u t 6 in., a n d a n o x y g e n a t i o n c a p a c i t y (OC) of the order of 6 lb/hr.
2.
241
Extended Aeration
2.5. S E T T L I N G O F S L U D G E F R O M EXTENDED AERATION A l t h o u g h t h e o r e t i c a l l y s l u d g e yield is nil f o r t h e e x t e n d e d a e r a t i o n p r o c e s s , i n p r a c t i c e t h i s is n o t t h e c a s e since p a r t o f t h e s l u d g e is n o t b i o d e g r a d a b l e a n d t h e r e f o r e a c c u m u l a t e s . T h e n e t s l u d g e yield m u s t b e w a s t e d . F i g u r e 5.7 ( C h a p t e r 5) r e v e a l s t h a t for v a l u e s o f F/M i n t h e r a n g e u t i l i z e d for t h e e x t e n d e d a e r a t i o n p r o c e s s ( 0 . 1 0 - 0 . 2 5 ) , t h e m i c r o o r g a n i s m s a r e d r i v e n to endogenous respiration and metabolize the cytoplasmic material of their "fellow" microorganisms. T h e remains of this "cannibalist feast" are n o n d e g r a d a b l e cellular shells w h i c h a r e r e l a t i v e l y l i g h t c o m p a r e d t o c y t o p l a s m i c m a t e r i a l a n d settle w i t h difficulty. S e t t l i n g t a n k s for e x t e n d e d a e r a t i o n s y s t e m s should therefore provide longer retention t i m e t h a n for the c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s . R e t e n t i o n t i m e is a p p r o x i m a t e l y 4 v s . 2 h r f o r t h e conventional activated sludge process.
2.6. N I T R I F I C A T I O N I N E X T E N D E D A E R A T I O N A s o r g a n i c l o a d i n g (F/M r a t i o ) is l o w i n e x t e n d e d a e r a t i o n a n d s i n c e a l a r g e excess o f a i r is s u p p l i e d , nitrification m a y o c c u r t o a n a p p r e c i a b l e e x t e n t involving the conversion of a m m o n i a nitrogen t o nitrite a n d nitrate. A p r o b l e m r e l a t e d t o n i t r i f i c a t i o n is a d r o p i n p H f o r t h e s y s t e m d u e t o f o r m a t i o n o f n i t r i c a c i d . T h e p H m a y d r o p a s l o w a s 4 . 5 , in w h i c h c a s e t h e b i o l o g i c a l p r o c e s s m a y b e a d v e r s e l y affected.
2.7. D E S I G N C R I T E R I A F O R E X T E N D E D A E R A T I O N 2.7.1. Calculation of Residence T i m e for Extended Aeration U n i t s Since t h e d e t e n t i o n p e r i o d r e q u i r e d for B O D r e m o v a l is m u c h s h o r t e r t h a n t h a t for a u t o x i d a t i o n o f s l u d g e , a e r a t o r v o l u m e is c o n t r o l l e d b y t h e r a t e o f sludge oxidation. F o r m u l a t i o n o f d e s i g n p r o c e d u r e f o r e x t e n d e d a e r a t i o n is b a s e d o n E q . (6.2), w h i c h m u s t b e slightly m o d i f i e d p r i o r t o its a p p l i c a t i o n . T h e m o d i f i c a t i o n s a r e a s f o l l o w s : in E q . (6.2), t h e l e f t - h a n d m e m b e r p r e s u p p o s e s t h a t all s l u d g e f o r m e d (aS Q) is b i o d e g r a d a b l e . E x p e r i m e n t a l d a t a i n d i c a t e t h a t a p p r o x i m a t e l y 7 7 % o f t h e s l u d g e p r o d u c e d is b i o d e g r a d a b l e , t h e r e m a i n i n g 2 3 % c o m p r i s i n g n o n b i o d e g r a d a b l e c e l l u l a r shells. T h e r e f o r e , t h e l e f t - h a n d m e m b e r o f E q . (6.2) is r e w r i t t e n a s E q . (6.4). r
aSQ a
r
=faS Q
(6.4)
r
w h e r e a is t h e l b o f b i o d e g r a d a b l e M L V S S p r o d u c e d / l b t o t a l B O D r e m o v e d o r fa; f t h e l b b i o d e g r a d a b l e M L V S S p r o d u c e d / l b t o t a l M L V S S p r o d u c e d « 0.77; a n d a the lb total M L V S S p r o d u c e d / l b total B O D removed. Q
5
5
242
6.
Secondary Treatment: Aerobic and Anaerobic Processes
I n t h e r i g h t - h a n d m e m b e r o f E q . (6.2), M L V S S c o n c e n t r a t i o n c o r r e s p o n d s o n l y t o b i o d e g r a d a b l e s l u d g e , i.e., X
t o b e s u b s t i t u t e d b y fX
va
Vta
(mg/liter of
b i o d e g r a d a b l e s l u d g e ) . A s w r i t t e n in E q . (5.68) p a r a m e t e r b r e p r e s e n t s t h e fraction of total M L V S S oxidized per day. b = lb M L V S S oxidized/(day)(lb total M L V S S in reactor) Define p a r a m e t e r b referred t o lb of b i o d e g r a d a b l e sludge. 0
b = lb M L V S S oxidized/(day)(lb biodegradable M L V S S in reactor) Q
T h e relationship between b a n d b is Q
b = lb M L V S S o x i d i z e d / ( d a y ) [ / ( l b total M L V S S in reactor)] = b/f 0
If in the r i g h t - h a n d m e m b e r of E q . (6.2) b a n d X
are substituted by b
va
a n d fX , Vta
0
respectively,
ΦΙ/)(/Χ , )ν=
b (fX ,a)V= 0
bX , V
υ α
v
(6.5)
v a
T h e r e f o r e , t h e r i g h t - h a n d m e m b e r o f E q . (6.2) is left u n c h a n g e d . C o n s e q u e n t l y , t h e m o d i f i e d E q . (6.2) is o b t a i n e d b y e q u a t i n g E q . (6.4) t o E q . (6.5). faS Q
= a SQ
r
0
= bX , V
r
(6.6)
v a
R e s i d e n c e t i m e is t h e n s h o w n i n E q . (6.7). t = V/Q = a SrlbX , 0
= faS /bX ,
v a
r
= fa(S -S )/bX ,
v a
0
e
v a
(6.7)
I t is c o n v e n i e n t t o w r i t e t h e e x p r e s s i o n for r e s i d e n c e t i m e in t e r m s o f B O D for t h e fresh feed, 5 . If i n E q . (6.7) (S -S ) F
0
is s u b s t i t u t e d b y t h e v a l u e g i v e n
e
i n E q . (5.97), o n e o b t a i n s t = (fa/bX , mS -S )K\ v a
F
+r)]
e
( / * 0.77)
(6.8)
2.7.2. Expression for Recycle Ratio r C o n s i d e r E q . (5.91) for recycle r a t i o r ( l e t t i n g X
VF
« 0):
r = (SMQ X , -AX )ll834Q (Xv,u-X ,a)] F
v a
v
F
F o r e x t e n d e d a e r a t i o n , w a s t a g e AX
v
(6.9)
c o r r e s p o n d s t o n o n b i o d e g r a d a b l e cells
V
which are approximately 2 3 % of the sludge formed. ΑΧ
υ
= 8.34(1 -f)a(S -S ) 0
e
Q - (effluent loss)
where 1 - / «
1 - 0 . 7 7 « 0.23
(6.10)
2.
243
Extended Aeration N e g l e c t i n g effluent l o s s i n E q . ( 6 . 1 0 ) , * a n d s u b s t i t u t i n g AX
V
i n E q . (6.9) b y
its v a l u e f r o m E q . (6.10) y i e l d s after s i m p l i f i c a t i o n r = \.QFXv,a-i\-f)a(S -S )Q\IQ (X , -X ) 0
e
F
v u
(6.11)
Vta
S u b s t i t u t i o n o f Q a n d (S — S ) in E q . (6.11) b y t h e i r v a l u e s f r o m E q s . (5.5) 0
e
a n d (5.97) yields after s i m p l i f i c a t i o n r = [X , -
(1 -f)a'S -SMKXv.u-Xp.J
v a
(6.12)
r
2.7.3. Expression for Reactor V o l u m e S u b s t i t u t i o n o f t h e r e s i d e n c e t i m e t in E q . (5.104) b y t h e v a l u e g i v e n i n E q . (6.8) l e a d s t o E q . (6.13). V=
Q fa(S -S )/bX , F
F
e
(6.13)
v a
T h e d e s i g n p r o c e d u r e for a n e x t e n d e d
a e r a t i o n u n i t is i l l u s t r a t e d
by
Example 6.1.
Example 6 . 1 26,000 gal/day of a n industrial wastewater are t o be treated by e x t e n d e d aeration. Influent B O D
5
is S
F
= 1200 m g / l i t e r , a n d it is d e s i r e d t o r e d u c e it
t o a v a l u e n o t o v e r 50 m g / l i t e r in t h e effluent (S ). e
Take X
Vta
and X
VtU
as 4000
a n d 12,730 m g / l i t e r , r e s p e c t i v e l y . V a l u e s o f d e s i g n p a r a m e t e r s a, b, a', a n d V h a v e been estimated as 0.7, 0 . 1 , 0.5, a n d 0.142, respectively ( u n i t s : B O D , 5
d a y ) . A s s u m e t h a t 7 7 % o f t h e M L V S S f o r m e d is b i o d e g r a d a b l e a n d n e g l e c t X . VyF
C a l c u l a t e (1) r e c y c l e r a t i o , (2) r e s i d e n c e t i m e i n h r , (3) B O D
5
of c o m
b i n e d feed, (4) c o m b i n e d feed i n g a l / d a y , (5) F/M r a t i o , (6) r e a c t o r v o l u m e in gal, a n d (7) o x y g e n r e q u i r e m e n t s in l b / d a y . SOLUTION 1. R e c y c l e r a t i o f r o m E q . ( 6 . 1 2 ) : r = [ 4 0 0 0 - ( l - 0 . 7 7 ) ( 0 . 7 ) ( 1 2 0 0 - 4 0 ) ] / ( 1 2 , 7 3 0 - 4 0 0 0 ) = 0.437 2. R e s i d e n c e t i m e f r o m E q . ( 6 . 8 ) : t = [(0.77 χ 0.7)/(0.1 χ 4000)] [ ( 1 2 0 0 - 50)/(l + 0 . 4 3 7 ) ] = 1.078 days (26 hr) 3. B O D
5
o f c o m b i n e d feed f r o m E q . ( 5 . 9 6 ) : S = [1200 + (0.437) ( 5 0 ) ] / ( l + 0.437) = 850.3 mg/liter 0
4 . C o m b i n e d feed f r o m E q . ( 5 . 5 ) : Q = 2 6 , 0 0 0 ( 1 + 0 . 4 3 7 ) = 37,360 gal/day * Effluent loss in extended aeration is more significant than in the conventional activated sludge process because as explained in Section 2.5, nonbiodegradable material is difficult t o settle.
244
6.
5. F/M
Secondary Treatment: Aerobic and Anaerobic Processes
ratio from Eq. (5.80): F/M
= 850.3/(4000 χ 1.078) = 0.197
6. R e a c t o r v o l u m e f r o m E q . ( 5 . 1 0 4 ) : V = 37,360 χ 1.078 = 40,290 gal 7. O x y g e n r e q u i r e m e n t s f r o m E q . ( 5 . 8 5 ) : RV
= 0.5 ( 8 5 0 . 3 - 5 0 ) (0.03736) (8.24) + (0.142) (4000) (0.04029) 8.34
RV
= 124.7 + 190.9 = 315.6 lb/day.
r
r
3. C o n t a c t Stabilization 3.1.
INTRODUCTION
C o n t a c t s t a b i l i z a t i o n is a n o t h e r m o d i f i c a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s . A flow d i a g r a m for t h e s y s t e m is s h o w n in F i g . 6 . 3 . I n f l u e n t w a s t e w a t e r is m i x e d w i t h s t a b i l i z e d s l u d g e , a n d t h i s m i x t u r e is a e r a t e d in t h e i n i t i a l c o n t a c t t a n k f o r w h i c h d e t e n t i o n t i m e is o n l y 2 0 - 4 0 m i n . D u r i n g initial c o n t a c t a n a p p r e c i a b l e f r a c t i o n o f s u s p e n d e d a n d d i s s o l v e d B O D is r e m o v e d b y b i o s o r p t i o n after c o n t a c t w i t h t h e w e l l - a e r a t e d a c t i v a t e d s l u d g e . T h e m i x e d effluent f r o m t h e initial c o n t a c t t a n k flows i n t o a clarifier. Clarified effluent is r e m o v e d a n d u n d e r f l o w f r o m t h e clarifier is t a k e n t o a s t a b i l i z a t i o n t a n k , w h e r e it is a e r a t e d for a p e r i o d o f 1.5-5 h r . D u r i n g this stabilization period, biosorbed organics are b r o k e n d o w n by a e r o b i c d e g r a d a t i o n . S t a b i l i z e d s l u d g e l e a v i n g t h e s t a b i l i z a t i o n t a n k is i n a " s t a r v e d " condition and ready to adsorb organic waste.
3.2. A D V A N T A G E O F C O N T A C T STABILIZATION V S . CONVENTIONAL ACTIVATED SLUDGE PROCESS S i n c e o n l y r e c y c l e d s l u d g e is s u b j e c t t o l e n g t h y a e r a t i o n , t h i s s y s t e m p e r m i t s a p p r e c i a b l e r e d u c t i o n in a e r a t i o n b a s i n v o l u m e . T h i s is t h e m a i n a d v a n t a g e o f c o n t a c t stabilization vs. the conventional activated sludge process. F o r a w a s t e w a t e r flow Q ( f t / h r ) a n d a s l u d g e recycle o f 0.3Q , a p p r o x i m a t e t a n k v o l u m e s for t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s a n d c o n t a c t s t a b i l i z a t i o n a r e l\Q a n d 4Q , r e s p e c t i v e l y . T h i s c o r r e s p o n d s t o a n e a r l y t h r e e f o l d t a n k r e d u c t i o n . O v e r a l l r e m o v a l efficiencies a r e u s u a l l y l o w e r t h a n in t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s , b u t c o u l d easily r e a c h 8 5 - 9 0 % B O D removal. T h e c o n t a c t s t a b i l i z a t i o n p r o c e s s is s u i t a b l e w h e n t h e w a s t e w a t e r c o n t a i n s a high p r o p o r t i o n of B O D in suspended a n d colloidal forms. C o n t a c t stabiliza t i o n p l a n t s m a y o p e r a t e w i t h o u t n e e d o f p r i m a r y clarification. 3
F
F
F
F
5
3.
Contact Stabilization
245
Q =Q + Q =Qp (l r) +
F
R
Influent
,
A j r
Clarifier
Initial contact tank Typical f X = 4 0 0 0 mg/liter values \\ =20 to 4 0 min (Initial contact)
Effluent
v a 1
I Clarifier underflow Typical: X = 8 0 0 0 mg/liter
I Air
v u
Stabilization tank Stabilized t w 1.5 to 5 hr sludge
Sludge recycle
m
Wastage
QR
Fig.
6.3. Flow
diagram
of contact
stabilization
system.
3.3. S O L U B I L I T Y I N D E X ( S I ) A N D OVERALL EFFICIENCY T h e solubility i n d e x (SI) o f a w a s t e w a t e r is defined a s SI = soluble B O D / t o t a l B O D
(6.14)
w h e r e 0 < SI ^ 1.0. A s SI a p p r o a c h e s z e r o t o t a l B O D t e n d s t o b e o f s u s p e n d e d o r c o l l o i d a l f o r m , a n d t h e w a s t e w a t e r b e c o m e s s u i t a b l e for t r e a t m e n t b y c o n t a c t s t a b i l i z a t i o n since m o s t B O D c a n b e r e m o v e d w i t h i n a s h o r t i n i t i a l c o n t a c t p e r i o d . A s S I approaches one, total B O D tends t o be of soluble form, a n d the conventional a c t i v a t e d s l u d g e p r o c e s s is m o r e efficient.
jr-Typical residence time 1/ for initial contact tank Detention time for initial contact tank (hours) Fig.
6.4. Relationship
of SI, BOD
removal,
and initial
contact
time.
6.
246
Secondary Treatment: Aerobic and Anaerobic Processes
T h e r e l a t i o n s h i p o f S I , B O D r e m o v a l , a n d initial c o n t a c t t i m e is s h o w n i n F i g . 6.4. F o r t y p i c a l r e s i d e n c e t i m e s i n t h e initial c o n t a c t t a n k , c o n s i d e r a b l y g r e a t e r B O D r e d u c t i o n o f effluent is a c h i e v e d w h e n t h e v a l u e o f S I a p p r o a c h e s u n i t y . T h e rise in t h e c u r v e for SI = 0 f o l l o w i n g t h e initial d r o p is d u e t o o v e r o x i d a t i o n , a c o n c e p t w h i c h is d i s c u s s e d in S e c t i o n 3.4.
3.4. D E S I G N O F C O N T A C T S T A B I L I Z A T I O N SYSTEMS 3.4.1. Selection of Residence Times for C o n t a c t and Stabilization T h e m a i n o b j e c t i v e in d e s i g n o f c o n t a c t s t a b i l i z a t i o n s y s t e m s is t h e s e l e c t i o n o f r e s i d e n c e t i m e s for initial c o n t a c t a n d s t a b i l i z a t i o n t a n k s . F o r a specific w a s t e w a t e r , l a b o r a t o r y tests a r e p e r f o r m e d t o d e t e r m i n e t h e effect o f a c o m b i n a t i o n o f v a r i o u s r e s i d e n c e t i m e s for initial c o n t a c t a n d s t a b i l i z a t i o n t a n k s o n t h e % B O D r e m o v a l . A t y p i c a l set o f c u r v e s o b t a i n e d for a specific w a s t e w a t e r is s h o w n in F i g . 6.5. E a c h c u r v e c o r r e s p o n d s t o a fixed initial c o n t a c t d e t e n t i o n t i m e . T h e a b s c i s s a is s t a b i l i z a t i o n t i m e a n d t h e o r d i n a t e is t h e % B O D removal. I f t h e d e s i r e d B O D r e m o v a l c a n n o t b e o b t a i n e d in t h e initial c o n t a c t t a n k (i.e., a t s t a b i l i z a t i o n t i m e = 0 ) , s t a b i l i z a t i o n t i m e m u s t b e e x t e n d e d t o r e a c h t h e d e s i r e d r e m o v a l . F o r e a c h c o n t a c t t i m e ( t h a t is, for e a c h c u r v e in F i g . 6.4) t h e r e is a n o p t i m u m s t a b i l i z a t i o n t i m e a s i n d i c a t e d , c o r r e s p o n d i n g t o a m a x i m u m % B O D removal. F o r stabilization times longer t h a n those corre-
Fig. removal
6.5. Relationship [3]-
of
contact
time,
stabilization
time,
and
%
BOD
4.
247
Modifications of Activated Sludge Process
s p o n d i n g t o these m a x i m a , sludge d e g r a d a t i o n ( e n d o g e n o u s phase) results in s u c h a b r e a k i n g d o w n t h a t s l u d g e effluent f r o m t h e s t a b i l i z a t i o n t a n k is n o t a d e q u a t e t o p e r f o r m b i o s o r p t i o n i n t h e initial c o n t a c t . T h e r e f o r e , % B O D r e m o v a l d r o p s . T h i s is k n o w n a s o v e r o x i d a t i o n .
3.4.2. Determination of Recycle Ratio T h e recycle r a t i o is d e t e r m i n e d f r o m E q . (5.92), n e g l e c t i n g w a s t a g e . T y p i c a l values of X and X a r e 4 0 0 0 a n d 8 0 0 0 m g / l i t e r , respectively, i n w h i c h c a s e r = 0 . 5 . O n c e t h e recycle r a t i o a n d d e t e n t i o n t i m e s h a v e b e e n d e t e r m i n e d , sizing o f c o n t a c t a n d s t a b i l i z a t i o n t a n k s is s t r a i g h t f o r w a r d . VyQ
VtU
3.4.3. Determination of O x y g e n Requirements O x y g e n r e q u i r e m e n t s a r e c a l c u l a t e d f r o m E q . (5.64). C o n t a c t a n d s t a b i l i z a t i o n t a n k s a r e c o n s i d e r e d s e p a r a t e l y a n d t h e c o r r e s p o n d i n g v a l u e s o f a' a n d b' a r e u s e d in t h e c a l c u l a t i o n s .
4. O t h e r M o d i f i c a t i o n s of Conventional Activated Sludge Process: Step Aeration, Complete Mix A c t i v a t e d S l u d g e P r o c e s s , a n d Tapered Aeration 4.1. S T E P A E R A T I O N S t e p a e r a t i o n is a m o d i f i c a t i o n o f t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s in w h i c h fresh feed is i n t r o d u c e d a t s e v e r a l p o i n t s a l o n g t h e a e r a t i o n t a n k . T h i s a r r a n g e m e n t p r o v i d e s f o r a n e q u a l i z a t i o n o f t h e F/M r a t i o s a l o n g t h e t a n k . Primary clarifier
Tank length Fig. 6.6. Step
aeration
process
[8].
248
6.
Secondary Treatment: Aerobic and Anaerobic Processes
T h e a e r a t i o n t a n k is d i v i d e d b y baffles i n t o several p a r a l l e l c h a n n e l s . E a c h c h a n n e l constitutes o n e step of the process, a n d the steps are linked together i n series. T h i s p r o c e s s , a s well a s o x y g e n s u p p l y a n d d e m a n d a l o n g t h e t a n k l e n g t h , a r e i l l u s t r a t e d b y F i g . 6.6.
4.2. C O M P L E T E M I X A C T I V A T E D S L U D G E PROCESS I n t h i s m o d i f i c a t i o n o f t h e a c t i v a t e d s l u d g e p r o c e s s , fresh feed a n d r e c y c l e d s l u d g e a r e c o m b i n e d a n d t h e n i n t r o d u c e d a t several p o i n t s in t h e a e r a t i o n t a n k f r o m a c e n t r a l c h a n n e l . A e r a t e d l i q u o r leaves t h e r e a c t o r f r o m effluent c h a n n e l s o n b o t h sides o f t h e a e r a t i o n t a n k ( F i g . 6.7). Primary clarifier
Reactor
Secondary clarifier \Effluent
Wastage Recycled sludge Pump Supply Q)
Ο
Ο
Demand
ο c ο Tank length (or width)
Fig. 6.7.
Complete
mix activated
sludge
process
[8].
Oxygen supply and d e m a n d are uniform along the tank, as indicated by the g r a p h a c c o m p a n y i n g F i g . 6.7. T h e m a t h e m a t i c a l m o d e l for t h e c o n v e n t i o n a l a c t i v a t e d s l u d g e p r o c e s s d e v e l o p e d in C h a p t e r 5, S e c t i o n 3.2 a s s u m e s c o m p l e t e m i x i n g . If p l u g flow c o n d i t i o n s a r e a s s u m e d , o x y g e n d e m a n d d e c r e a s e s a l o n g the length of the aeration t a n k , whereas the oxygen supply r e m a i n s c o n s t a n t ( F i g . 6.8).
4.3. T A P E R E D A E R A T I O N T h e p u r p o s e o f t a p e r e d a e r a t i o n is t o m a t c h t h e a m o u n t o f a i r s u p p l i e d w i t h t h e o x y g e n d e m a n d a l o n g t h e a e r a t i o n t a n k . S i n c e a t t h e inlet o x y g e n d e m a n d is t h e h i g h e s t , a e r a t o r s a r e s p a c e d m o r e closely t o p r o v i d e a h i g h e r o x y g e n a t i o n r a t e . S p a c i n g b e t w e e n a e r a t o r s is i n c r e a s e d t o w a r d t h e o u t l e t a s oxygen d e m a n d decreases.
5.
249
Aerated Lagoons
Secondary clarifier
Primary clarifier
Effluent
Plug flow reactor
Wastage Recycled sludge
Γ Pump
Tank length Fig. 6.8.
Conventional
activated
sludge
process
with plug
flow
reactor
[8].
5. A e r a t e d L a g o o n s 5.1. I N T R O D U C T I O N A e r a t e d l a g o o n s a r e b a s i n s h a v i n g d e p t h s v a r y i n g f r o m 4 t o 12 ft i n w h i c h o x y g e n a t i o n o f w a s t e w a t e r s is a c c o m p l i s h e d b y a e r a t i o n u n i t s . T h e f u n d a m e n t a l difference b e t w e e n a e r a t e d l a g o o n s a n d t h e a c t i v a t e d s l u d g e s y s t e m is t h a t r e c y c l i n g o f t h e s l u d g e is p r o v i d e d i n t h e l a t t e r a s a m e a n s o f c o n t r o l l i n g t h e a m o u n t o f b i o l o g i c a l s l u d g e i n t h e a e r a t o r . A e r a t e d l a g o o n s a r e flowt h r o u g h devices, i.e., n o recycle o f s l u d g e is p r o v i d e d . S o l i d s c o n c e n t r a t i o n i n t h e l a g o o n is a f u n c t i o n o f w a s t e w a t e r c h a r a c t e r i s t i c s a n d d e t e n t i o n t i m e . I t is u s u a l l y b e t w e e n 8 0 a n d 2 0 0 m g / l i t e r , i.e., m u c h l o w e r t h a n t h a t f o r a c t i v a t e d sludge units (2000-4000 mg/liter).
5.2. M I X I N G R E G I M E S F O R A E R A T E D L A G O O N S T u r b u l e n c e level i n l a g o o n s is t h e b a s i s f o r t h e i r classification i n t o t w o t y p e s .
5.2.1. Completely M i x e d L a g o o n s T h e t u r b u l e n c e level is sufficient t o m a i n t a i n s o l i d s i n s u s p e n s i o n . D e t e n t i o n t i m e s a r e u s u a l l y less t h a n 3 d a y s , a n d p o w e r levels a r e h i g h e r t h a n 2 5 H P p e r m i l l i o n g a l l o n s o f b a s i n v o l u m e . P o w e r levels f o r a c t i v a t e d s l u d g e u n i t s a r e in t h e vicinity o f 0.25 H P / 1 0 0 0 gal ( o r 2 5 0 H P p e r m i l l i o n g a l l o n s ) , i.e., a b o u t ten times higher t h a n for a e r a t e d lagoons. A s s u m p t i o n of c o m p l e t e mixing, w h i c h e v e n f o r a c t i v a t e d s l u d g e u n i t s is a n i d e a l i z e d a p p r o x i m a t i o n , is
250
6.
Secondary Treatment: Aerobic and Anaerobic Processes
q u e s t i o n a b l e for a e r a t e d l a g o o n s . N e v e r t h e l e s s , t h i s a s s u m p t i o n is often m a d e , l e a d i n g t o a s i m p l e m a t h e m a t i c a l m o d e l for t h e l a g o o n . U t i l i z i n g a p p r o p r i a t e safety f a c t o r s , t h i s i d e a l i z e d a p p r o a c h is useful.
5.2.2. Facultative L a g o o n s T h e t u r b u l e n c e level is insufficient t o m a i n t a i n all s o l i d s in s u s p e n s i o n . P a r t o f t h e s o l i d s settle t o t h e b o t t o m o f t h e l a g o o n , w h e r e t h e y u n d e r g o anaerobic decomposition. Detention times are usually over 6 days a n d power levels a r e 4 - 2 5 H P p e r m i l l i o n g a l l o n s o f b a s i n v o l u m e . T h e r e is a g r a d u a l b u i l d u p o f r e s i d u e w h i c h h a s t o b e d e s l u d g e d a t p e r i o d s o f 1-10 y e a r s . S e l e c t i o n o f m i x i n g r e g i m e is t h e r e s u l t o f a n e c o n o m i c b a l a n c e b e t w e e n p o w e r r e q u i r e m e n t s ( g r e a t e r in c o m p l e t e l y m i x e d r e g i m e ) a n d a c r e a g e c o s t ( m o r e a c r e a g e r e q u i r e d for f a c u l t a t i v e l a g o o n s ) . T h e s e t w o m i x i n g r e g i m e s a r e i l l u s t r a t e d in F i g . 6.9. »
%
Influent \
Γ"β~7
r
/ — /
,
%
*±
* χ (Lj
• /Effluent
/
IV
Completely mixed lagoon (a)
Jk
lnfluent\
—23—
^ J~/\ ^ - ^ ^ J- S o/Effluent
v
^ '—-— Facultative lagoon
f^ffffiffigy deposition
(b) Fig. 6.9. Mixing
regimes
for aerated
lagoons.
5.3. K I N E T I C S O F B O D R E M O V A L Assuming appropriate environmental conditions ( p H , presence of nutrients, etc.), t h e r a t e o f B O D r e m o v a l is a f u n c t i o n o f d e t e n t i o n t i m e , t e m p e r a t u r e , n a t u r e of wastewater, a n d concentration of suspended volatile solids. U s u a l l y B O D r e m o v a l r a t e is a s s u m e d t o f o l l o w first-order k i n e t i c s , a n d t h e f o r m u l a t i o n for t h e c o n t i n u o u s r e a c t o r a s s u m i n g c o m p l e t e m i x i n g ( C h a p t e r 5 , S e c t i o n 3.2) is utilized. T h e r e f o r e , E q . (5.18) is t a k e n a s t h e k i n e t i c m o d e l f o r t h e l a g o o n . I t is c o n v e n i e n t t o r e w r i t e E q . (5.18) in t e r m s o f t h e r a t i o SJS i.e., t h e p e r c e n t a g e o f B O D r e m a i n i n g i n t h e effluent. L e t t i n g kX = K, r e a r r a n g e m e n t o f E q . (5.18) yields 09
Vta
SJS
0
= 1/(1 +Kt)
(6.15)
5.
251
Aerated Lagoons
w h e r e S is t h e s o l u b l e B O D o f t h e influent, S t h e s o l u b l e B O D o f t h e effluent, 0
e
Κ the removal rate constant, and (6.16)
/ (detention time) = V/Q = AD/Q
I n E q . (6.16), Κ is t h e l a g o o n v o l u m e , Q t h e flow r a t e , A t h e h o r i z o n t a l c r o s s sectional area of the lagoon, a n d D the d e p t h of t h e lagoon. I t s h o u l d b e e m p h a s i z e d t h a t t h i s m a t h e m a t i c a l m o d e l is b a s e d o n t w o f u n d a m e n t a l a s s u m p t i o n s , t h o s e o f first-order k i n e t i c s a n d t o t a l m i x i n g c o n d i t i o n s . N e i t h e r a s s u m p t i o n is v a l i d i n a l l c a s e s , n e v e r t h e l e s s , t h i s f o r m u l a t i o n is useful f o r d e s i g n p u r p o s e s . V a l u e s o f A^can b e d e t e r m i n e d f r o m b e n c h scale d a t a . A r e a c t o r like t h e o n e s h o w n i n F i g . 5.10 ( C h a p t e r 5, S e c t i o n 6.1) is u s e d t o s i m u l a t e a n a e r a t e d l a g o o n . T h e sliding baffle is r e m o v e d f o r t h i s s i m u l a t i o n . T h e v a l u e o f Κ is d e t e r m i n e d f r o m l i n e a r p l o t s s i m i l a r t o t h o s e i n F i g . 5.5 ( C h a p t e r 5, S e c t i o n 3.2) b a s e d o n E q . (5.18). Κ v a l u e s s h o u l d b e c o r r e c t e d f o r t h e l a g o o n t e m perature (summer a n d winter conditions) by the procedure described in S e c t i o n 5.4. V a l u e s o f Κ c a n a l s o b e d e t e r m i n e d f r o m p i l o t - p l a n t d a t a o r estimated from data o n operating lagoons.
5.4. E S T I M A T E O F L A G O O N T E M P E R A T U R E
(T ) w
T h e rate of B O D removal nearly doubles for every 10°C of t e m p e r a t u r e rise ( C h a p t e r 2 , S e c t i o n 7.1). T h u s i t is n e c e s s a r y t o e s t i m a t e l a g o o n t e m p e r a t u r e u n d e r a v e r a g e s u m m e r a n d w i n t e r c o n d i t i o n s . T h i s is d o n e b y p e r f o r m i n g a heat balance. Consider t h e l a g o o n represented b y Fig. 6.10. T e m p e r a t u r e v a l u e s s h o w n a r e t h o s e f o r t h e s u m m e r c o n d i t i o n s i n E x a m p l e 6.2. Let T = temperature of t h e influent t
T
w
= lagoon ( a n d effluent) temperature (°F)
Τ = atmospheric temperature (°F) a
T (air) a
(80°F) A
T:
(IOO°F)
T T
w
w
(87.I°F)
(87.I°F)
Q
Fig.
6.10. Heat
balance
for aerated
lagoon.
252
Secondary Treatment: Aerobic and Anaerobic Processes
6.
h = heat transfer coefficient [ B T U / ( d a y ) ( f t ) ( ° F ) ] between lagoon a n d a t m o s p h e r e (should take into account such factors as wind a n d humidity effects). In the absence of m o r e a c c u r a t e information a value of 100 B T U / ( d a y ) ( f t ) ( ° F ) m a y be utilized for estimate purposes 2
2
Q = flow r a t e (Mgal/day) A = lagoon surface (ft ) 2
C = specific heat of wastewater [take as 1.0 B T U / ( l b ) ( ° F ) ] T h e e n t h a l p y c h a n g e o f t h e i n f l u e n t is Λ
Mgal
Q—r— *
„,
Λ
l b liquor
4 Λ Λ
BTU
x 8.34 χ 10*
χ C
day
Mgal liquor
= β ( Γ , - 7^)8.34 x 1 0
χ (lb liquor) (°F)
(7|-r )°F w
v
1
w }
(BTU/day)
6
(6.17)
This should e q u a l h e a t loss t o s u r r o u n d i n g air given by h BTU/(day)(ft )(°F) χ A ft 2
χ (T -T )°F
2
w
= hA(T -T )
0
w
(BTU/day) (6.18)
a
E q u a t i n g E q s . (6.17) a n d ( 6 . 1 8 ) , 2 ( 7 , - 7 ^ ) 8 . 3 4 χ 1 0 = hA(T -T ) Let h χ 10- /8.34 = /
(6.19)
6
w
a
6
Note:
If h » 100 B T U / ( d a y ) ( f t ) ( ° F ) , t h e n / « 12 χ 1 0 " . 2
6
O n e then writes Q(T^T )=/A(T W
T)
W
(6.20)
a
S o l v i n g for T , w
T
w
= (AfT +QTi)/(Af+
Q)
a
(6.21)
E q u a t i o n (6.21) p e r m i t s a n e s t i m a t e o f l a g o o n t e m p e r a t u r e . T h e effect o f l a g o o n t e m p e r a t u r e (T )
o n B O D r e m o v a l r a t e Κ is g i v e n b y
w
the empirical equation K where K
Tyv
= Κ Θ »Τ
Tw
(6.22)
20
20
is t h e B O D r e m o v a l r a t e a t t e m p e r a t u r e T , K w
the B O D removal
20
r a t e a t 2 0 ° C , a n d θ t h e t e m p e r a t u r e coefficient 1.056 ( 2 0 - 3 0 ° C ) a n d
1.135
(4-20°C). C o n s i d e r E q s . (6.15), (6.16), (6.21), a n d (6.22). Substituting i n E q . (6.15) v a l u e s o f /, K , Tw
and T
w
SJS
0
g i v e n b y E q s . ( 6 . 1 6 ) , (6.22), a n d (6.21), o n e o b t a i n s
= l/{l + ( / i / > / ( 2 ) ^ 2 o ^
/ r
-
+ < 2 r
'
) / u / + < 2 ) ]
-
2 0
}
(6.23)
E q u a t i o n (6.23) p e r m i t s e v a l u a t i o n o f t h e effect o f t e m p e r a t u r e o n p e r c e n t a g e o f B O D r e m a i n i n g i n t h e effluent.
5.
253
Aerated Lagoons
5.5. O X Y G E N R E Q U I R E M E N T S F O R AERATED LAGOONS Oxygen r e q u i r e m e n t s for aerobic o x i d a t i o n processes a r e given by E q . (5.64). S i n c e t h e c o n c e n t r a t i o n o f M L V S S (X ) Vta
t h e t e r m b'X Vis
is l o w f o r a e r a t e d l a g o o n s ,
usually neglected. Therefore
va
lb 0 / d a y « a'S Q « a\\b B O D removed/day) 2
r
(6.24)
V a l u e s o f a' f o r a e r a t e d l a g o o n s v a r y f r o m 0.9 t o 1.4 d e p e n d i n g o n t h e n a t u r e of waste, mixing regime, a n d t e m p e r a t u r e .
5.6. S O L U B L E E F F L U E N T B O D F O R A N AERATED LAGOON T h e s o l u b l e effluent B O D f o r a n a e r a t e d l a g o o n is c a l c u l a t e d f r o m E q . (6.15). S o l v i n g f o r S , S = S /(l+Kt) (6.25) e
e
0
This equation does n o t take into account B O D feedback to the lagoon d u e t o a n a e r o b i c d e g r a d a t i o n of deposited solids. Usually a correction factor t o a c c o u n t f o r t h i s is i n t r o d u c e d i n E q . (6.25). S i n c e a n a e r o b i c B O D f e e d b a c k is g r e a t e r d u r i n g t h e s u m m e r , t w o m o d i f i e d f o r m s o f E q . (6.25) a r e r e c o m mended by Eckenfelder a n d F o r d [ 5 ] . S u m m e r c o n d i t i o n s : S = \2S /(\ e
0
+ Kt)
Winter c o n d i t i o n s : S = \.05S I(1+Kt) e
(6.26) (6.27)
0
F o r a c t i v a t e d s l u d g e p l a n t s , d e s i g n p r o c e d u r e c o n s i s t s o f specifying a d e s i r e d effluent q u a l i t y (S ) a n d t h e n c a l c u l a t i n g r e s i d e n c e t i m e ( / ) n e c e s s a r y t o a c h i e v e t h i s specified q u a l i t y . F o r a e r a t e d l a g o o n s t h e d e s i g n a p p r o a c h is normally t h e reverse of t h a t for activated sludge plants. U u s u a l l y w h e n a n a e r a t e d l a g o o n is b e i n g c o n s i d e r e d , o n e h a s a v a i l a b l e a g i v e n a c r e a g e o f l a n d presumably at a reasonably low cost. T h e design procedure starts from t h e k n o w n value of t h e surface area A a n d a n a s s u m e d reasonable d e p t h . T h e r e f o r e , t h e v o l u m e o f t h e l a g o o n a n d c o n s e q u e n t l y r e s i d e n c e t i m e a r e fixed. Effluent q u a l i t y S a c h i e v e d f o r t h i s r e s i d e n c e t i m e is c a l c u l a t e d f r o m E q . (6.25) [ o r E q s . (6.26) a n d ( 6 . 2 7 ) ] . F o r t h e a c t i v a t e d s l u d g e p l a n t i t is u n i m p o r t a n t t o w r i t e E q . (6.25), since S is a primary specification, rather than a calculated v a l u e . e
e
e
5.7. M L V S S C O N C E N T R A T I O N I N AERATED LAGOONS Consider a n aerated lagoon for complete m i x conditions indicated b y Fig. 6.11. Let X b e t h e c o n c e n t r a t i o n of V S S in t h e influent (mg/liter) a n d X t h e c o n c e n t r a t i o n o f M L V S S f o r t h e l a g o o n ( s a m e a s t h a t i n effluent, mg/liter). Vt0
va
254
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
V Q
Q
X
Fig.
6.11.
ν,α
Material
X
balance
ν,α
for ML VSS.
A m a t e r i a l b a l a n c e for V S S yields (Solids in) + (net synthesis in lagoon) = (solids out) or QX
Vt0
+ (aS Q-bX V) r
=
Vt0
QX
V>
D i v i d i n g t h r o u g h b y Q, l e t t i n g V/Q = / = d e t e n t i o n t i m e , a n d s o l v i n g f o r Χν,α = (X„,o + F o r facultative lagoons, X
va
aS )/(\+bt) r
(6.28)
is l o w e r t h a n t h e v a l u e e s t i m a t e d f r o m E q . (6.28)
d e p e n d i n g o n l a g o o n g e o m e t r y , a e r a t o r s p a c i n g , p o w e r level, a n d t h e n a t u r e of influent solids. R e m a r k s s i m i l a r t o t h o s e m a d e i n S e c t i o n 5.6 c o n c e r n i n g t h e difference i n design a p p r o a c h between activated sludge plants a n d aerated lagoons are a p p l i c a b l e t o E q . (6.28). F o r a c t i v a t e d s l u d g e p l a n t s , M L V S S c o n c e n t r a t i o n in t h e a e r a t o r (i.e., X ) va
X
is specified b y t h e d e s i g n e r . F o r a e r a t e d l a g o o n s ,
is a c a l c u l a t e d q u a n t i t y ( f r o m t h e specified r e s i d e n c e t i m e / f r o m w h i c h
Vta
S is c a l c u l a t e d , a n d t h e v a l u e f o r S = S — S is t h u s e s t a b l i s h e d ) . E q u a t i o n e
r
0
e
(6.28) is t h e n u t i l i z e d t o c a l c u l a t e X .
F o r a c t i v a t e d s l u d g e p l a n t s it is u n
i m p o r t a n t t o w r i t e E q . (6.28), since X
is a p r i m a r y specification r a t h e r t h a n
Vta
v>a
a calculated quantity.
5.8. R E T E N T I O N P E R I O D R E Q U I R E D F O R A SPECIFIED EFFLUENT S O L U B L E B O D F r o m t h e d e s i g n a p p r o a c h i n S e c t i o n 5.6, r e t e n t i o n p e r i o d is u s u a l l y a p r i m a r y v a r i a b l e w h i c h is i n d i r e c t l y specified b y t h e d e s i g n e r . C o n s e q u e n t l y , o b t a i n i n g a n e q u a t i o n for / f o r t h e a e r a t e d l a g o o n is less i m p o r t a n t t h a n it w a s for t h e a c t i v a t e d s l u d g e p l a n t . N e v e r t h e l e s s , a n e q u a t i o n f o r r e s i d e n c e t i m e is d e r i v e d w h i c h m a y b e u t i l i z e d t o e v a l u a t e r e s i d e n c e t i m e s f o r a specified v a l u e o f S f o r several p o s s i b l e selected v a l u e s o f l a g o o n d e p t h . T a k e E q . (5.18) a n d substitute X b y its v a l u e given i n E q . (6.28). S i m p l i f y i n g a n d s o l v i n g f o r r e s i d e n c e t i m e t, o n e o b t a i n s e
v>a
t = SMX .o v
+
aSr)kS -bSr] e
(6.29)
5.
255
Aerated Lagoons
w h e r e S = S — S . I f t h e c o n c e n t r a t i o n o f V S S i n t h e i n f l u e n t (X ) r
0
e
Vt0
is n e g l i
gible, E q . ( 6 . 2 9 ) simplifies t o yield t = XI(akS -b)
(X ,ο * 0)
e
(6.30)
v
If i n t h i s e q u a t i o n t h e u n i t s t o b e utilized a r e t ( d a y s ) , a ( l b M L V S S / l b B O D ) , r
k (hr" ), S 1
e
(mg/liter), a n d b [lb M L V S S / ( d a y ) ( l b M L V S S ) ] , a conversion
f a c t o r o f 2 4 h r / d a y is u s e d . T h e r e f o r e , E q . (6.29) is r e w r i t t e n a s E q . (6.31). t = \/(24akS -b)
(X *
e
0)
Vt0
(6.31)
5.9. T O T A L E F F L U E N T B O D F O R A N AERATED LAGOON S o far, o n l y s o l u b l e B O D o f t h e effluent (S ) h a s b e e n c o n s i d e r e d . T o t h i s , e
one m u s t a d d the B O D contribution corresponding t o volatile
suspended
solids p r e s e n t i n t h e effluent.* A s s u m i n g c o m p l e t e m i x i n g , t h e c o n c e n t r a t i o n o f V S S i n t h e effluent is e q u a l t o t h a t in t h e l a g o o n . B O D c o n t r i b u t i o n d u e t o V S S d e p e n d s o n s l u d g e a g e , w h i c h for a e r a t e d l a g o o n s is c a l c u l a t e d f r o m E q . (5.150) [ ( o r E q . (5.151) if X
vo
= 0 ) ] . A s s l u d g e s t a b i l i z e s w i t h a g e , its c o n t r i
b u t i o n t o effluent B O D l o w e r s . C o r r e l a t i o n b e t w e e n s l u d g e a g e a n d B O D c o n t r i b u t i o n b y V S S is s h o w n i n F i g . 6 . 1 2 . T h e t o t a l B O D o f t h e effluent is [ E q . (6.32)] Total B O D of effluent = S + ψΧ , e
0
0.1
0.2
(mg/liter)
υ α
0.3
0.4
(6.32)
0.5
^ = B 0 D / V S S = (lb B 0 D / l b VSS) 5
Fig. 6.12. Correlation
5
for insoluble
BOD [5].
* F o r the activated sludge process, complete settling o f M L V S S in the secondary clarifier is assumed. Therefore only soluble B O D is accounted for in the net effluent (refer t o Fig. 5.1}, where X « 0 ) . v%e
256
Secondary Treatment: Aerobic and Anaerobic Processes
6.
w h e r e S is c a l c u l a t e d f r o m E q . (6.26) [ o r E q . ( 6 . 2 7 ) ] , φ is d e t e r m i n e d f r o m F i g . 6.12, a n d X is c a l c u l a t e d f r o m E q . (6.28). e
Vtu
5.10. D E S I G N P R O C E D U R E F O R A E R A T E D LAGOONS T h e p r o c e d u r e p r o p o s e d b y E c k e n f e l d e r a n d a s s o c i a t e s is i l l u s t r a t e d b y E x a m p l e 6.2.
Example 6.2 A n a e r a t e d l a g o o n is c o n t e m p l a t e d for t r e a t i n g a n i n d u s t r i a l w a s t e w a t e r . A n a r e a o f 5 a c r e s is a v a i l a b l e . T h e f o l l o w i n g i n f o r m a t i o n is t a k e n a s a b a s i s for d e s i g n : Q = 1.5 M G D
(average flow rate)
So = 600 mg/liter Χ
υ>
ο = 20 mg/liter
( B O D of influent) 5
(VSS in influent)
Τ = average air temperature, 80°F ( s u m m e r ) ; 35°F (winter) a
T = 100°F
(influent temperature)
t
D a t a o b t a i n e d f r o m b e n c h scale e q u i p m e n t : Κ = 0.06 h r "
1
= 1.44 d a y "
a = 0.5 lb VSS/lb B O D b = 0.06 d a y -
r
1
a' = 1.1 l b 0 / l b B O D 2
h =
(at 20°C)
1
r
100BTU/(day)(ft )(°F) 2
Calculate 1. Effluent s o l u b l e B O D for s u m m e r a n d c a l c u l a t i o n s o n a l a g o o n d e p t h o f 8 ft. 2. M L V S S c o n c e n t r a t i o n a t e q u i l i b r i u m for a X for s u m m e r a n d w i n t e r c o n d i t i o n s . 3. T o t a l B O D in t h e effluent for s u m m e r a n d 4. S u r f a c e a e r a t i o n r e q u i r e m e n t s : lb 0 / d a y , level in H P / M g a l o f b a s i n v o l u m e . 5
winter conditions.
Base
completely mixed lagoon
va
5
2
S O L U T I O N : Part 1 Step 1. E s t i m a t e T
BOD w
5
winter conditions. required H P , and power
o f effluent
for s u m m e r a n d w i n t e r c o n d i t i o n s [ E q . ( 6 . 2 1 ) ] . H e r e
A = 5 acre χ 43,560 ft /acre = 217,800 ft 2
and / =
12 χ I O "
6
2
5.
257
Aerated Lagoons
corresponding to h = Summer: T
2
= (217,800 χ 12 χ 1 0 ~ χ 8 0 + 1.5 χ 100)/(217,800χ 12 χ Ι Ο " + 1 . 5 ) 6
w
= 87.1°F Winter:
100BTU/(day)(ft )(°F)
T
6
(30.6°C)
= (217,800 χ 12 χ 1 0 ~ χ 3 5 + 1 . 5 χ 100)/(217,800χ 12 χ 1 0 " + 1.5) 6
w
= 58.7°F
6
(14.8°C)
Step 2. E s t i m a t e B O D r e m o v a l r a t e Κ f o r s u m m e r a n d w i n t e r c o n d i t i o n s [Eq. (6.22)]. Summer: K . * Winter:
#
= 1.44 χ 1 . 0 5 6
30 6
c
1 4
. . 8
( 3 0
· 6
= 1.44 χ 1 . 1 3 5 < · 1 4
c
8
2 0 )
= 2.57 d a y
2 0 )
= 0.745 d a y
1
1
Step 3. C a l c u l a t e d e t e n t i o n t i m e [ E q . ( 6 . 1 6 ) ] . 217,800 ft χ 8 ft χ 7.48 gal/ft 2
t =
3 Λ
m
Λ
= 8.7 days 1,500,000 gal/day ( s o l u b l e B O D o f effluent) f o r s u m m e r a n d w i n t e r y
Step 4. C a l c u l a t e S conditions.
e
5
S u m m e r : F r o m Eq. (6.26) S
e
= 1.2 χ 600/(1 + 2.57 χ 8.7) = 30.8 mg/liter
% soluble B O D r e m o v a l : [(600 - 30.8)/600] χ 100 = 9 5 % W i n t e r : F r o m Eq. (6.27) S
e
= 1.05 χ 6 0 0 / ( 1 + 0 . 7 4 5 χ 8.7) = 84.2 mg/liter
% soluble B O D r e m o v a l : [ ( 6 0 0 - 8 4 . 2 ) / 6 0 0 ] χ 100 = 86% S O L U T I O N : Part 2
M L V S S c o n c e n t r a t i o n [ E q . (6.28)]
Summer: X ,
= [20 + 0 . 5 ( 6 0 0 - 3 0 . 8 ) ] / [ l + (0.06)(8.7)] = 200 mg/liter
Winter:
= [20 + 0 . 5 ( 6 0 0 - 8 4 . 2 ) ] / [ l + (0.06)(8.7)] = 182.6mg/liter
0 a
X,
v a
S O L U T I O N : Part 3 Soluble B O D
5
Total B O D
for effluent
o f t h e effluent h a s b e e n c a l c u l a t e d i n S o l u t i o n , P a r t 1, S t e p 4 .
Summer: S
e
= 30.8 mg/liter
Winter:
e
= 84.2 mg/liter
S
5
Step 1. E s t i m a t e ψ f r o m F i g . 6.12. F i r s t c a l c u l a t e s l u d g e a g e [ E q . ( 5 . 1 5 0 ) ] . S u m m e r : t = [200/(200 - 20)]8.7 = 9.67 days s
Winter:
/ = [ 1 8 2 . 6 / ( 1 8 2 . 6 - 2 0 ) ] 8.7 = 9.77 days s
258
Secondary Treatment: Aerobic and Anaerobic Processes
6.
T h e n f r o m F i g . 6.12, S u m m e r : F o r t = 9.67 days, read ψ = 0.332 s
Winter:
F o r t = 9.77 days, read φ = 0.330 s
Step 2 . E s t i m a t e V S S c o n t r i b u t i o n t o effluent B O D . 5
S u m m e r : ψΧ ,
ν α
= 0.332 χ 200 = 66.4 mg/liter
Winter:
υ>α
= 0.330 χ 182.6 = 60.3 mg/liter
ψΧ
Step 3. C a l c u l a t e t o t a l B O D S u m m e r : Soluble B O D
5
i n effluent.
30.8 mg/liter
5
B O D (VSS)
66.4 mg/liter
5
97.2 mg/liter Winter:
Soluble B O D
84.2 mg/liter
5
BOD5 (VSS)
60.3 mg/liter 144.5 mg/liter
S O L U T I O N : Part 4
Surface aeration requirements
Step L E s t i m a t e o x y g e n r e q u i r e m e n t s [ E q . ( 6 . 2 4 ) ] . S u m m e r : B O D = 600 - 30.8 = 569.2 mg/liter = 569.2 χ 1 0 " lb B O D / l b liquor 6
r
r
B O D / d a y = 569.2 χ 1 0 " lb B O D / l b liquor χ 1.5 χ 1 0 gal liquor/day 6
6
r
r
χ 8.34 lb liquor/gal liquor = 7121 lb B O D / d a y P
lb 0 / d a y = 1.1 lb 0 / l b B O D χ 7121 lb B O D / d a y = 7833 lb 0 / d a y 2
2
r
r
2
= 326 lb 0 / h r 2
Winter:
B O D = 600 - 84.2 = 515.8 mg/liter = 515.8 χ I O " lb B O D / l b liquor 6
r
r
B O D / d a y = 515.8 χ 1 0 " l b B O D / l b liquor χ 1.5 χ 1 0 gal liquor/day 6
6
r
r
χ 8.34 l b liquor/gal liquor = 6453 l b B O D / d a y r
= 269 lb B O D / h r r
lb 0 / d a y = 1.1 lb 0 / l b B O D χ 6453 lb B O D / d a y = 7098 lb 0 / d a y 2
2
r
r
2
= 296 lb 0 / h r 2
Step
2. E s t i m a t e l b 0 / ( H P x h r ) 2
[ E q . (4.34)]. Base estimate o n t h e
following values: N = 2.5 lb 0 / Η Ρ χ h r 0
2
α = 0.8 C
sw
C
L
= 7.0 mg/liter ( s u m m e r ) ; 9.5 mg/liter (winter) = 1.0 mg/liter
6.
Wastewater Stabilization Ponds
259
Then Summer: For T
w
= 30.6°C (Part 1, Step 1)
Ν — 2.5[(7.0-1.0)/9.2]0.8 χ 1 . 0 2 4 Winter:
For T
w
( 3 0
· -
2 0 )
· -
2 0 )
6
= 1.68 lb 0 / ( H P x hr) 2
= 14.8°C (Part 1, Step 1)
Ν = 2.5[(9.5-1.0)/9.2]0.8 χ 1 . 0 2 4
( 1 4
8
= 1.63 lb 0 / ( H P x hr) 2
Step 3. C a l c u l a t e t h e r e q u i r e d H P . lbP /hr 2
lb 0 / H P x h r 2
where the n u m e r a t o r a n d d e n o m i n a t o r have been calculated in P a r t 4 (Steps 1 a n d 2, respectively). S u m m e r : H P = 326/1.68 = 194 H P Winter:
H P = 296/1.63 = 182 H P
S u m m e r operation controls design. Step 4. E s t i m a t e t h e p o w e r level b a s e d o n 194 H P . T h e l a g o o n h a s a v o l u m e o f 2 1 7 , 8 0 0 f t χ 8 ft = 1,742,400 f t , o r 2
3
1,742,400 ft χ 7.48 gal/ft 3
3
= 13,033,152 gal
Λ H P / M g a l = 194/13.03 = 14.9 H P / M g a l
(faculative lagoon level)
6. W a s t e w a t e r S t a b i l i z a t i o n P o n d s 6.1.
INTRODUCTION
T h e b a s i c difference b e t w e e n t h e w a s t e w a t e r t r e a t m e n t p r o c e s s d e s c r i b e d i n t h i s s e c t i o n a n d t h o s e p r e v i o u s l y s t u d i e d is t h a t n o a e r a t i o n e q u i p m e n t is e m p l o y e d in s t a b i l i z a t i o n p o n d s . O x y g e n n e e d s for p o n d s a r e p r o v i d e d b y n a t u r a l surface aeration a n d by algae, which p r o d u c e oxygen by p h o t o s y n t h e s i s . O x y g e n r e l e a s e d b y a l g a e a s a r e s u l t o f p h o t o s y n t h e s i s is u t i l i z e d b y b a c t e r i a for a e r o b i c d e g r a d a t i o n o f o r g a n i c m a t t e r . P r o d u c t s o f t h i s d e g r a d a t i o n ( c a r b o n d i o x i d e , a m m o n i a , p h o s p h a t e s ) a r e in t u r n u t i l i z e d b y a l g a e . T h i s cycle s y m b i o t i c r e l a t i o n s h i p b e t w e e n a l g a e a n d b a c t e r i a is s h o w n d i a grammatically in Fig. 6.13. W a s t e w a t e r s t a b i l i z a t i o n p o n d s a r e feasible w h e n l a r g e l a n d a r e a s a v a i l a b l e a t l o w c o s t a n d h i g h q u a l i t y effluent is n o t r e q u i r e d . I f B O D o f influent is h i g h , o x y g e n d e m a n d is a b o v e t h a t p r o v i d e d b y p h o t o s y n t h e s i s n a t u r a l surface aeration. U n d e r these circumstances D O concentration in
are the and the
260
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
Newbacteria
r
Bacteria
C0 ,NH 2
Organic matter
3
Algae Solar New algae
energy Fig.
6.13.
Cyclic
symbiotic
relationship
between
algae
and
bacteria.
w a s t e w a t e r d r o p s t o a very l o w level a n d a n a e r o b i c d e c o m p o s i t i o n p r e v a i l s . T e r m i n a l p r o d u c t s for a n a e r o b i c d e c o m p o s i t i o n a r e C H + H 0 , in c o n t r a s t t o C 0 + H 0 for a e r o b i c d e c o m p o s i t i o n . C h e m i c a l e q u a t i o n s p e r t i n e n t t o a n a e r o b i c d e c o m p o s i t i o n a r e s t u d i e d in S e c t i o n 8 . 1 . 4
2
2
2
P o n d s in w h i c h t h e u p p e r l a y e r s a r e a e r o b i c a n d t h e l o w e r a r e a n a e r o b i c a r e referred t o a s f a c u l t a t i v e p o n d s . M o s t s t a b i l i z a t i o n p o n d s fall in t h i s category. W h e n e v e r o r g a n i c l o a d i n g is v e r y h i g h , o x y g e n d e m a n d m a y b e s u c h t h a t p o n d o p e r a t i o n is a n a e r o b i c . W h e n several p o n d s a r e o p e r a t i n g in series, t h e first o n e r e c e i v i n g r a w w a s t e w a t e r d i s c h a r g e is a n a e r o b i c a n d t h e s e c o n d , w h i c h receives p a r t i a l l y stabilized w a s t e w a t e r f r o m t h e first, m a y b e a f a c u l t a tive p o n d . T h e last o n e r e c e i v i n g relatively l o w B O D w a s t e w a t e r d i s c h a r g e from the preceding one might function as a n aerobic p o n d . Because of high detention time, frequently a b o u t 2 m o n t h s , r e m o v a l of refractory organic materials which c a n n o t be accomplished by activated s l u d g e o r a e r a t e d l a g o o n p r o c e s s e s m a y b e c o m e p o s s i b l e in s t a b i l i z a t i o n p o n d s . T h u s , a convenient arrangement m a y be to provide stabilization p o n d s following a n activated sludge (or aerated lagoon) unit to complete stabilization.
6.2. K I N E T I C S O F B O D R E M O V A L F O R STABILIZATION P O N D S A n i d e a l i z e d a p p r o a c h s i m i l a r t o t h a t utilized for a e r a t e d l a g o o n s is o f t e n e m p l o y e d for s t a b i l i z a t i o n p o n d s . C o n c e n t r a t i o n o f M L V S S is n o t a r e l e v a n t p a r a m e t e r in s t a b i l i z a t i o n p o n d s , a n d t h u s t h e t e r m X does not appear in t h e m a t h e m a t i c a l m o d e l . E q u a t i o n 5.18 is r e w r i t t e n a s Vf0
(S -S )lt 0
e
=
(6.33)
KS
e
E q u a t i o n (6.33) i n d i c a t e s t h a t a p l o t o f (S -S )/t 0
e
vs. S
e
yields a s t r a i g h t
line, a n d t h e v a l u e o f Κ is d e t e r m i n e d f r o m t h e s l o p e . T y p i c a l g r a p h s o f t h i s t y p e a r e s h o w n i n F i g s . 6.18 a n d 6.19.
6.
Wastewater Stabilization Ponds
261
6.3. L A B O R A T O R Y S I M U L A T I O N O F STABILIZATION P O N D S Batch or continuous models of stabilization p o n d s have been used
on
b e n c h o r p i l o t scale. T w o o f t h e s e m o d e l s , r e c o m m e n d e d b y E c k e n f e l d e r a n d F o r d [ 5 ] , a r e s h o w n in F i g . 6.14. Light system Light (JJjsys tern
"δ"
Effluent
Influentr r
Batch model
Continuous model
(a)
(b)
Fig.
6.14.
Laboratory
models
for wastewater
stabilization
ponds.
F r o m d a t a o b t a i n e d f r o m t h e s e m o d e l s , g r a p h s s u c h a s F i g s . 6.18 a n d 6.19 a r e c o n s t r u c t e d , a n d a n e s t i m a t e o f Κ v a l u e s is m a d e .
6.4. M A T H E M A T I C A L F O R M U L A T I O N F O R S E V E R A L STABILIZATION P O N D S IN S E R I E S S t a r t f r o m t h e m o d i f i e d f o r m o f t h e first-order for o n e p o n d . S o l v i n g E q . (6.15) f o r / y i e l d s :
kinetics removal equation
t = (l-SJS.)/K(SJS.)
(6.34)
R a t i o SJS r e p r e s e n t s t h e % B O D r e m a i n i n g in t h e effluent. F o r t w o s t a b i l i z a t i o n p o n d s in series ( F i g . 6.15), o n e c a n w r i t e [ E q . ( 6 . 1 5 ) ] 0
S
0
s;
Pond No. 1 •l Fig.
6.15.
for two
F o r p o n d N o . 1: S/IS
= 1/(1 +K
F o r p o n d N o . 2 : S /S '
= 1/(1+
e
e
e
*2
Diagram
0
s
Pond No. 2
X
stabilization
ponds
in
series.
ii)
(6.35)
Kt)
(6.36)
2 2
C o m b i n i n g E q s . (6.35) a n d (6.36) b y m u l t i p l i c a t i o n , SJS
0
= 1/(1 + K t )(\
+K t )
l 1
2 2
(6.37)
W h e n r e t e n t i o n p e r i o d t a n d r e m o v a l r a t e c o n s t a n t Κ a r e t h e s a m e for b o t h p o n d s , E q . (6.37) yields SJSo = 1/(1 +Kt) (6.38) 2
262
where t
Secondary Treatment: Aerobic and Anaerobic Processes
6.
= t
i
2
= t and K
= K
t
= K. S o l v i n g E q . (6.38) for /,
2
/ = [1 - (S /S ) ]/K(S /S y>
(6.39)
2
1/2
e
e
0
o
F o r η p o n d s i n series, E q s . (6.38) a n d (6.39) yield SJS
= 1/(1+Kt)
(6.40)
n
0
t = [1 - (S IS ) ]/K(S ISo) lln
e
(6.41)
1,n
0
e
F u r t h e r s i m p l i f i c a t i o n is p o s s i b l e w h e n e v e r t h e p r o d u c t Kt is m u c h less t h a n unity. Exponential e
Kt
e
Kt
is g i v e n b y t h e p o w e r series in E q . (6.42). = 1 + Kt + (Kt) /2l
+ (Kt) /3l
2
3
+ ···
(6.42)
If ATr <^ 1, o n e c a n w r i t e a s a n a p p r o x i m a t i o n e
= 1 + Kt
Kt
(6.43)
S u b s t i t u t i n g i n t h e d e n o m i n a t o r o f E q . (6.40) (1 +Kt)
by the exponential
e, Kt
gives E q . (6.44), SjSo
= lle
= e~
nKt
(6.44)
nKt
from which SolS
=
e
(6.45)
or (S /S )
=
1,n
0
e
(6.46)
6.5. E F F E C T O F T E M P E R A T U R E O N R E A C T I O N RATE C O N S T A N T Κ T h i s effect is c a l c u l a t e d f r o m a n e m p i r i c a l r e l a t i o n s h i p r e c o m m e n d e d b y Eckenfelder and F o r d [ 5 ] . K = Κ Θ25
(6.47)
25
the reaction rate at 25°C; t the tem
χ 25
t
w h e r e K is t h e r e a c t i o n r a t e a t t°C; t
K
p e r a t u r e ( ° C ) ; a n d θ t h e t e m p e r a t u r e c o n s t a n t ( 1 . 0 6 — 1 . 0 9 ; t a k e θ = 1.07).
6.6. O X Y G E N P R O D U C T I O N I N A E R O B I C P O N D S Aerobic stabilization p o n d s depend on algae to provide the oxygen necessary t o satisfy B O D r e q u i r e m e n t s . S i n c e t h i s o x y g e n is p r o d u c e d b y p h o t o s y n t h e s i s , s u n l i g h t is r e q u i r e d . T h i s r e s t r i c t s t h e d e p t h o f a e r o b i c p o n d s t o a r a n g e o f 6-18 in. T h e a m o u n t o f o x y g e n p r o d u c e d b y a l g a e is e s t i m a t e d f r o m
Oswald's
e q u a t i o n [ E q . (6.48)] [ 9 ] . O P = 0.25FI
(6.48)
L
w h e r e O P is t h e o x y g e n p r o d u c t i o n [ l b 0 / ( a c r e ) ( d a y ) ] ; F t h e light c o n v e r 2
s i o n efficiency ( % ) ; a n d I t h e light i n t e n s i t y [ c a l / ( c m ) ( d a y ) ] . 2
L
F is u s u a l l y a s s u m e d t o b e 4 % . T h u s F = 4 a n d OP « l
L
(6.49)
6.
Wastewater Stabilization Ponds
263
I v a r i e s f r o m a b o u t 100 t o 3 0 0 c a l / ( c m ) ( d a y ) d u r i n g w i n t e r a n d s u m m e r , respectively, for a l a t i t u d e o f 30°. T h i s m e a n s t h a t t h e m a x i m u m B O D l o a d i n g for a e r o b i c o p e r a t i o n o f s t a b i l i z a t i o n p o n d s t o t a k e p l a c e v a r i e s f r o m 100 t o 3001b BOD /(acre)(day). 2
L
5
6.7. D E P T H O F O X Y G E N P E N E T R A T I O N I N STABILIZATION P O N D S D e p t h of oxygen penetration has been correlated by O s w a l d [ 1 0 ] t o surface l o a d i n g , e x p r e s s e d a s l b B O D / ( a c r e ) ( d a y ) ( F i g . 6.16). T h e g r e a t e r t h e l o a d i n g , 5
t h e s h a l l o w e r t h e d e p t h o f o x y g e n p e n e t r a t i o n since o x y g e n d e m a n d is h i g h e r .
12
_
10
c .2 σ
8
Fall c onditio ns
ο α> α
ν
ί
κ
ϊ
4
ο 2
<
r
Sumn er
inter^
Ο
100
200
Loading ;lb B 0 D / ( a c r e ) ( d a y ) 5
Fig. 6.16. Correlation for depth of oxygen penetration in stabilization ponds (adapted from Oswald [10]). (Reprinted with permission, copyright by the University of Texas Press).
6.8. F A C U L T A T I V E P O N D S : H E R M A N N A N D GLOYNA'S EQUATION T h e d e s i g n f o r m u l a t i o n for s t a b i l i z a t i o n p o n d s d e s c r i b e d i n S e c t i o n s 6.2 a n d 6.4 a p p l i e s t o f a c u l t a t i v e p o n d s . F a c u l t a t i v e p o n d d e p t h s v a r y f r o m 3 t o 8 ft. A n e m p i r i c a l e q u a t i o n for f a c u l t a t i v e p o n d s h a s b e e n d e v e l o p e d b y H e r m a n n a n d G l o y n a [ 6 ] . T h i s f o r m u l a is b a s e d o n s e v e r a l a s s u m p t i o n s a n d d e v e l o p e d b y a n a l y s i s o f r e s u l t s f r o m b e n c h scale, p i l o t - p l a n t , a n d field p o n d s . I t is a p p l i c a b l e t o d o m e s t i c s e w a g e for a 8 5 - 9 5 % B O D r e d u c t i o n [ E q . ( 6 . 5 0 ) ] . V = 10.7 x 1 0 - Q S ( 1 . 0 8 5 - * ) 8
3 5
o
(6.50)
w h e r e V is t h e p o n d v o l u m e ( a c r e χ f t ) ; Q t h e w a s t e w a t e r flow ( g a l / d a y ) ; S t h e B O D o f influent ( m g / l i t e r ) ; a n d t t h e p o n d t e m p e r a t u r e ( ° C ) .
Q
M
264
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
6.9. A N A E R O B I C P O N D S L o a d i n g o f a n a e r o b i c p o n d s is s u c h t h a t a n a e r o b i c c o n d i t i o n s p r e v a i l t h r o u g h o u t the liquid. Organic loadings range between 250 a n d 4000 lb B O D / ( a c r e ) ( d a y ) . R e m o v a l efficiencies v a r y b e t w e e n 50 a n d 8 0 % . S i n c e t h i s d e g r e e o f B O D r e m o v a l is u s u a l l y n o t a d e q u a t e for d i s c h a r g i n g t h e effluent, a n a e r o b i c p o n d s are usually followed by faculative a n d aerobic ones. D e p t h s f r o m 8 t o 15 ft a r e c o m m o n , b u t g r e a t e r d e p t h s a r e r e c o m m e n d e d t o p r o v i d e m a x i m u m h e a t r e t e n t i o n , b e s i d e s t h e r e s u l t i n g e c o n o m y in t e r m s o f l a n d c o s t . 5
5
6.10. S U M M A R Y O F D E S I G N C R I T E R I A F O R WASTEWATER STABILIZATION P O N D S D e s i g n c r i t e r i a a r e s u m m a r i z e d in T a b l e 6.2. TABLE 6.2 Summary of Design Criteria for Wastewater Stabilization P o n d s ' Ponds
Criteria
D e p t h (ft) Detention time (days) Loading lb B O D / ( a c r e ) ( d a y ) % B O D removal A l g a e concentration (mg/liter) 5
a
Aerobic
Facultative
Anaerobic
0.5-1.5 2-6
3-8 7-50
8-15 5-50
100-200 80-95 100
200-500 70-95 10-50
250-4000 50-80
Adapted from Eckenfelder [4J.
6.11.
D E S I G N C A L C U L A T I O N S FOR STABILIZATION P O N D S
D e s i g n for s t a b i l i z a t i o n p o n d s is i l l u s t r a t e d b y E x a m p l e 6 . 3 . T h e d e s i g n p r o c e d u r e is t h a t r e c o m m e n d e d b y E c k e n f e l d e r a n d a s s o c i a t e s .
Example 6.3 W a s t e w a t e r s t a b i l i z a t i o n p o n d s a r e c o n s i d e r e d for t r e a t m e n t o f o r g a n i c c h e m i c a l s w a s t e . T o t a l d e s i g n flow is 1.0 M G D , a n d e s t i m a t e d p o n d t e m p e r a t u r e s a r e 15° a n d 3 0 ° C for w i n t e r a n d s u m m e r o p e r a t i o n s , r e s p e c t i v e l y . It is d e s i r e d t o r e d u c e w a s t e w a t e r C O D f r o m 2 0 0 0 t o 4 0 0 m g / l i t e r u s i n g t w o a n a e r o b i c p o n d s in series o f e q u a l d e t e n t i o n t i m e , a n d t h e n t o l o w e r t h e C O D o f t h e effluent f r o m t h e s e c o n d a n a e r o b i c p o n d t o 50 m g / l i t e r b y m e a n s o f a n
6.
265
W a s t e w a t e r Stabilization P o n d s
a e r o b i c p o n d . B e n c h scale tests a r e p e r f o r m e d w i t h w a s t e w a t e r for
both
anaerobic and aerobic p o n d conditions, and the laboratory data obtained at 25°C are tabulated below.
S (mg/liter C O D )
S (mg/liter C O D )
D e t e n t i o n time (days)
3000 2000 1200
1000 667 400
40 40 40
700 400 300
49 28 21
40 40 40
0
Anaerobic p o n d Run 1 Run 2 Run 3 Aerobic p o n d Run 4 Run 5 Run 6
e
Design the treatment system. SOLUTION
See F i g s . 6 . 1 7 - 1 9 .
Step 1. O b t a i n c o n s t a n t Κ a t 2 5 ° C for t h e a n a e r o b i c a n d a e r o b i c p o n d s , f r o m p l o t s o f l a b o r a t o r y d a t a [ ( 5 - S )/t 0
v s . SJ
e
s h o w n in t a b u l a t i o n b e l o w .
S
S
(mg/liter C O D )
(mg/liter C O D )
D e t e n t i o n time (days)
3000 2000 1200
1000 667 400
40 40 40
50 33.3 20
700 400 300
49 28 21
40 40 40
16.3 9.3 7.0
0
Anaerobic Run 1 Run 2 Run 3 Aerobic Run 4 Run 5 Run 6
e
(S -S )/t 0
e
In summary Κ (anaerobic ponds) = 0 . 0 5 d a y "
1
# (aerobic p o n d ) = 0.335 d a y "
1
Step 2. O b t a i n v a l u e s o f Κ a t 15°C since w i n t e r c o n d i t i o n s c o n t r o l t h e design [ E q . (6.47)]. For the anaerobic ponds K
ls
( w a s 0.05 d a y "
1
= 0.05 χ 1 . 0 7
( 1 5
-
2 5 )
= 0.0254 d a y "
at 25°C, laboratory conditions).
1
266
6.
Q=I.O MGD C0D=2000 mg/liter
Secondary Treatment: Aerobic and Anaerobic Processes
Aerobic pond 3
Anaerobic pond 2
Anaerobic pond 1 Si
COD
COD
4 0 0 mg/liter Fig. 6.17. Diagram 601
1
1
0
1
for Example
1
1
300
liter]Kday)
20
1———
e
6.3.
1
1
1
900
Γ
1200
(mg/liter)
of Κ (anaerobic 1
ponds)
1
for Example
1
K= 16.75/50=0.335 day"
15
\
1
600 S
Fig. 6.18. Determination
1
50 mg/liter
6.3.
J
1
S
>*#4 (49;I63) "
IO
CP
ε
(28;9.3) (2I;7)
5
k
^
Ι
0
10
Ι
Ι
-I
20
30
40
S Fig. 6.19. Determination
-
e
50
60
(mg/liter)
of Κ (aerobic
pond)
for Example
6.3.
F o r the aerobic p o n d K
l5
(was 0.335 d a y " Step
1
= 0.335 χ 1 . 0 7
( 1 5
-
2 5 )
= 0.17 d a y "
1
at 25°C, laboratory conditions).
3. C a l c u l a t e d e t e n t i o n t i m e s .
F o r t h e t w o a n a e r o b i c p o n d s i n series [ E q . ( 6 . 3 9 ) ] / = [1 - ( 4 0 0 / 2 0 ω )
1 / 2
]/0.0254(400/2000)
1 / 2
= 48.7 days
(each p o n d )
6.
Wastewater Stabilization Ponds
267
Since for t h e t w o a n a e r o b i c p o n d s in E x a m p l e 6.3 ^ = # 2 day"
1
and t = t 1
= ^ 1 5 = 0.0254
= t = 4 8 . 7 d a y s , it f o l l o w s t h a t f r o m E q s . (6.35) a n d (6.36),
2
S fS e
Λ S
f e
= (S Se)
112
0
Q
—
S /S e
e
= (2000x400)
= 894 mg/liter
1 / 2
( C O D of effluent from a n a e r o b i c p o n d 1) F o r the aerobic p o n d [ E q . (6.34)] / = ( l - 5 0 / 4 0 0 ) / 0 . 1 7 (50/400) = 41.2 days Step 4. C a l c u l a t e p o n d a r e a ( a c r e s ) . F o r t h e t w o a n a e r o b i c p o n d s a s s u m e a d e p t h o f 12 ft. S i n c e / = V/Q = Ah/Q, w h e r e / is t h e r e s i d e n c e t i m e ( d a y ) ; Κ t h e v o l u m e o f p o n d ( f t ) ; Q t h e flow r a t e ( f t / d a y ) ; A t h e a r e a o f p o n d ( f t ) ; a n d h t h e d e p t h o f p o n d (ft), t h e n A = tQ/h o r 3
3
2
A = 48.7 days χ (1 χ 1 0 gal/day χ ft /7.48 gal χ 1/12 ft x acre/43,560 f t ) 6
3
2
= 12.5 acres per p o n d T h e r e f o r e , d e s i g n t w o 12.5 a c r e p o n d s , e a c h 12 ft d e e p . S u r f a c e l o a d i n g [ l b B O D / ( a c r e ) ( d a y ) ] for e a c h o f t h e a n a e r o b i c p o n d s is s h o w n b e l o w . 5
Anaerobic
pond
1
Since 2 0 0 0 m g / l i t e r = 2 0 0 0 χ 1 0 "
6
lb C O D / l b liquor
lb C O D / d a y = 1 χ 1 0 gal liquor/day χ 8.34 lb liquor/gal liquor 6
χ 2000 χ 1 0 - lb C O D / l b liquor 6
= 1 χ 8.34 χ 2000 = 16,680 lb C O D / d a y A s s u m e B O D / C O D « 0.7. T h e n l b B O D / d a y = ( 0 . 7 ) ( 1 6 , 6 8 0 ) = 11,676 a n d lb B O D / ( a c r e ) ( d a y ) = 11,676/12.5 = 9 3 4 (surface loading for a n a e r o b i c p o n d 1). 5
5
5
Anaerobic
pond
2
lb C O D / d a y = 1 χ 8.34 χ 894 = 7456 l b B O D s / d a y = (0.7)(7456) = 5219 lb B O D / ( a c r e ) ( d a y ) = 5219/12.5 = 418 p o n d 2) 5
(surface loading for anaerobic
F o r t h e a e r o b i c p o n d , t h e d e s i g n p r o c e d u r e is a s f o l l o w s : 1. A s s u m e a d e p t h o f o x y g e n p e n e t r a t i o n , e.g., h = 3 ft. 2. F r o m k n o w l e d g e o f r e s i d e n c e t i m e a n d flow r a t e , c a l c u l a t e a first a p p r o x i m a t i o n o f t h e p o n d a r e a ( a c r e s ) : A = tQ/h. 3. C a l c u l a t e s u r f a c e a s s u m e d v a l u e o f h.
l o a d i n g in lb B O D / ( a c r e ) ( d a y ) 5
based
on
the
268
6.
Secondary Treatment: Aerobic and Anaerobic Processes
4. F r o m F i g . 6.16 d e t e r m i n e t h e d e p t h o f o x y g e n p e n e t r a t i o n a n d c o m p a r e it w i t h t h e a s s u m e d v a l u e u n d e r (1). F o r a e r o b i c o p e r a t i o n , t h e d e p t h o f o x y g e n p e n e t r a t i o n s h o u l d b e at least e q u a l t o t h e a s s u m e d d e p t h . If n e c e s s a r y , assume a n o t h e r value of h a n d iterate steps ( l ) - ( 4 ) . 5. O x y g e n p r o d u c t i o n b y a l g a e is c h e c k e d t o a s s u r e t h a t it is sufficient t o satisfy t h e s u r f a c e l o a d i n g [ E q . ( 6 . 4 9 ) ] . Calculations are as follows: 1. A s s u m e h = 3 ft. 2. C a l c u l a t e A i n a c r e s . A = 41.2 days χ (1 χ 1 0 gal/day χ ft /7.48 gal χ 1/3 ft χ acre/43,560 f t ) 6
3
2
= 42.1 acres 3. S u r f a c e l o a d i n g . F i r s t c a l c u l a t e l o a d i n g in t e r m s o f l b C O D / ( a c r e ) ( d a y ) . Since 4 0 0 m g / l i t e r = 4 0 0 χ 1 0 " l b C O D / l b l i q u o r , 6
lb C O D / d a y = 1 χ 1 0 gal liquor/day χ 8.34 lb liquor/gal liquor 6
χ 400 χ I O " lb C O D / l b liquor 6
= 3336 lb C O D / d a y Therefore, l b C O D / ( a c r e ) ( d a y ) = 3336/42.1 = 79.2 lb COD/(acre)(day) A s s u m e B O D / C O D « 0.7. T h e n t h e s u r f a c e l o a d i n g in t e r m s o f B O D 5
5
is
l b B O D / ( a c r e ) ( d a y ) = 0.7 χ 79.2 « 55 lb B O D / ( a c r e ) ( d a y ) 5
5
F r o m F i g . 6.16 for t h i s l o a d i n g , r e a d h « 3 ft. T h e r e f o r e , t h e a s s u m e d d e p t h is a p p r o p r i a t e a n d n o f u r t h e r t r i a l is n e c e s s a r y . 4. C h e c k o x y g e n p r o d u c t i o n b y a l g a e . F r o m E q . (6.49) it follows t h a t o x y g e n p r o d u c t i o n r e s t r i c t s m a x i m u m l o a d i n g s f r o m 100 t o 3 0 0 l b B O D / (acre) ( d a y ) . S i n c e a c t u a l l o a d i n g is o n l y 55 l b B O D / ( a c r e ) ( d a y ) , a n excess o f o x y g e n o v e r B O D r e q u i r e m e n t s is a v a i l a b l e . 5
5
5
7. Trickling Filters 7.1. I N T R O D U C T I O N T h e t r i c k l i n g filter is a p a c k e d m e d i a c o v e r e d w i t h b i o l o g i c a l slime t h r o u g h w h i c h w a s t e w a t e r is p e r c o l a t e d . T h e slime layer, w h i c h u s u a l l y h a s a t o t a l t h i c k n e s s b e t w e e n 0.1 a n d 2.0 m m , c o n s i s t s o f o n e a e r o b i c a n d o n e a n a e r o b i c s u b l a y e r , a s s h o w n d i a g r a m m a t i c a l l y in F i g . 6.20. T h e biological aerobic process which takes place in the aerobic sublayer is t y p i c a l ( C h a p t e r 5, S e c t i o n 4 . 1 . 1 , F i g . 5.6). T h e s u b s t r a t e is p a r t i a l l y o x i d i z e d
7.
269
Trickling Filters BOD
Oo (air)
Ν /
r
" ^ j - Aerobic \ layer -*\—J-Anaerobic / / layer / ι ,
!
—f-Slime layer / / (0.1-2.0 mm) -Organic acids
Fig.
6.20.
Diagram
of aerobic
and anaerobic
sublayers
for a trickling
filter.
t o p r o v i d e e n e r g y for t h e b i o l o g i c a l p r o c e s s . A n o t h e r p a r t o f t h e s u b s t r a t e is utilized t o s y n t h e s i z e n e w s l i m e m a t e r i a l . In the anaerobic sublayer decomposition takes place with formation of o r g a n i c a c i d s , C H , a n d H S ( S e c t i o n 8.1). I n t h e t r i c k l i n g filter, o r g a n i c a n d colloidal m a t t e r are removed by aerobic oxidation, biosorption, coagulation, a n d a n a e r o b i c d e c o m p o s i t i o n . E s s e n t i a l l y t h e r e is n o r e m o v a l b y m e c h a n i c a l filtration. T h e t e r m " t r i c k l i n g filter" is m i s l e a d i n g in t h i s r e s p e c t . 4
2
7.2. T H I C K N E S S O F S L I M E L A Y E R U s u a l l y , t h e t h i c k n e s s o f t h e s l i m e l a y e r is b e t w e e n 0.1 a n d 2.0 m m . I t h a s a n a d v e r s e effect o n t h e o p e r a t i o n o f t h e t r i c k l i n g filter if it is t h i c k e r t h a n 2.0 m m . C l o g g i n g o f m e d i a m a y o c c u r , t h u s i m p a i r i n g t h e w a s t e w a t e r flow a n d the transfer of oxygen t o aerobic m i c r o o r g a n i s m s . Operational hydraulic loadings are low [0.4-4.0 gal/(min)(ft )] a n d are n o t sufficient t o k e e p t h e slime l a y e r s c o u r e d off. T h u s , h y d r a u l i c l o a d i n g c a n n o t b e u s e d in c o n t r o l l i n g t h e t h i c k n e s s o f t h e s l i m e layer. T h i s c o n t r o l is e x e r t e d m o s t l y b y l a r v a e a n d w o r m s , w h i c h t h r i v e o n t h e a c c u m u l a t e d s l i m e . 2
A s t h e s l i m e layer i n c r e a s e s i n t h i c k n e s s , o r g a n i c m a t t e r i n t h e w a s t e w a t e r is m e t a b o l i z e d b e f o r e it c a n r e a c h t h e l a y e r o f m i c r o o r g a n i s m s c l i n g i n g t o t h e s u r f a c e o f t h e m e d i a . T h e s e m i c r o o r g a n i s m s a r e left w i t h o u t sufficient f o o d a n d tend to enter the e n d o g e n o u s respiration phase. T h u s , the slime layer loses its a b i l i t y t o c l i n g t o t h e m e d i a s u r f a c e a n d is w a s h e d a w a y . T h i s p h e n o m e n a , called s l o u g h i n g , is a f u n c t i o n o f o r g a n i c a n d h y d r a u l i c l o a d i n g o f t h e filter.
7.3. C O M P A R I S O N B E T W E E N T R I C K L I N G FILTERS A N D A C T I V A T E D S L U D G E P R O C E S S F o r B O D r e m o v a l efficiencies o f a b o u t 6 0 % , it is u s u a l l y f o u n d t h a t t r i c k l i n g filters a r e m o r e e c o n o m i c a l t h a n t h e a c t i v a t e d s l u d g e p r o c e s s , i n p a r t i c u l a r f o r s m a l l flow r a t e s o f w a s t e w a t e r . F o r h i g h e r B O D r e m o v a l efficiencies ( 9 0 % o r
270
Secondary Treatment: Aerobic and Anaerobic Processes
6.
a b o v e ) , t h e a c t i v a t e d s l u d g e p r o c e s s is m o r e e c o n o m i c a l b e c a u s e p a c k i n g material costs w o u l d be t o o high. These considerations suggest a possible t w o s t e p o p e r a t i o n : t r i c k l i n g filters f o l l o w e d b y a n a c t i v a t e d s l u d g e p l a n t , a c o m b i n a t i o n w h i c h in s o m e c a s e s m a y p r o v e a d v a n t a g e o u s . S o m e a d v a n t a g e s o f t r i c k l i n g filters o v e r t h e a c t i v a t e d s l u d g e p r o c e s s a r e (1) n o p o w e r r e q u i r e m e n t s for a e r a t i o n , (2) s i m p l e o p e r a t i o n , a n d (3) s l o w e r r e s p o n s e a n d q u i c k e r r e c o v e r y t o s u d d e n c h a n g e s o f influent B O D .
7.4. P H Y S I C A L A R R A N G E M E N T O F TRICKLING FILTERS T r i c k l i n g filters a r e b e d s f r o m 3 t o 4 0 ft d e e p filled w i t h p a c k i n g s u c h a s b r o k e n rock, clinkers, or synthetic media (trade n a m e s Surfpac, Flocor, Actifil). T h e s e p l a s t i c m a t e r i a l p a c k i n g s a r e a v a i l a b l e c o m m e r c i a l l y ( D o w Chemical C o . , Ethyl C o r p o r a t i o n , B . F . G o o d r i c h , N o r t o n Co.) in h o n e y c o m b a n d o t h e r s h a p e s . I n f l u e n t w a s t e w a t e r is u s u a l l y d i s t r i b u t e d o v e r t h e filter by a mechanical rotating a r m mechanism and percolates t h r o u g h the packing, c o m i n g in c o n t a c t w i t h t h e b i o l o g i c a l slime layer. W h e r e a s b e d s filled w i t h r o c k s , c l i n k e r s , o r o t h e r m a t e r i a l s a r e l i m i t e d in d e p t h f r o m 3 t o 8 ft, b e d s o f s y n t h e t i c m a t e r i a l s a r e c o m m o n l y 2 0 - 4 0 ft d e e p . T h e higher percentage of void space for synthetic packing allows a n easier flow a n d r e d u c e s t h e r i s k o f flooding. F o r o r d i n a r y p a c k i n g ( r o c k s , c l i n k e r s , etc.) t h e f o l l o w i n g c h a r a c t e r i s t i c s a r e t y p i c a l : d i a m e t e r s : 1^—2 i n . ; s u r f a c e a r e a : 2 4 - 3 4 f t / f t o f b u l k v o l u m e ; v o i d % : 4 5 - 5 5 % ; a n d m a x i m u m h y d r a u l i c l o a d i n g s : 0.5 g a l / ( m i n ) ( f t ) . 2
3
2
A d v a n t a g e s o f s y n t h e t i c p a c k i n g s a r e t h a t t h e y (1) a l l o w p a c k i n g d e p t h u p t o 4 0 ft; (2) a l l o w h i g h e r h y d r a u l i c l o a d i n g s , u p t o 4 g a l / ( m i n ) ( f t ) ; (3) h a v e s u r f a c e a r e a s u p t o 7 0 f t / f t o f b u l k v o l u m e ; a n d (4) a r e less likely t o b e clogged by wastewaters carrying large a m o u n t s of suspended solids. 2
2
3
D i s a d v a n t a g e s o f s y n t h e t i c p a c k i n g s a r e t h a t t h e y a r e (1) relatively e x p e n s i v e ; a n d (2) i n a p p r o p r i a t e for w a s t e w a t e r t r e a t m e n t t o a r r i v e a t a r e l a t i v e l y h i g h effluent q u a l i t y a s c o m p a r e d t o o r d i n a r y p a c k i n g .
7.5. T R I C K L I N G FILTER S Y S T E M S M o s t c o m m o n a r r a n g e m e n t s f o r t r i c k l i n g filters a r e s h o w n i n F i g . 6 . 2 1 . (a) Single filter system—May b e o p e r a t e d w i t h o r w i t h o u t recycle. R e c y c l i n g is i n d i c a t e d for h i g h e r effluent q u a l i t y . If influent B O D is g r e a t e r t h a n 500 m g / l i t e r , r e c y c l i n g is u s u a l l y r e c o m m e n d e d , (b) Alternating double filtration— T h e first filter is r e s p o n s i b l e f o r m o s t B O D r e m o v a l ; t h e s e c o n d o n e is a n effluent p o l i s h e r . C o n s e q u e n t l y , m o s t slime g r o w t h o c c u r s in t h e first filter. T h e cycle is r e v e r s e d p e r i o d i c a l l y (daily o r w e e k l y ) a s i n d i c a t e d b y d o t t e d lines in F i g . 6.21 ( b ) . I n t h i s m a n n e r c o n t r o l o f t h e slime l a y e r t h i c k n e s s is a c h i e v e d , m a i n t a i n i n g a u n i f o r m slime t h i c k n e s s in b o t h u n i t s . H i g h e r B O D effluent
7.
271
Trickling Filters Wastewater
Wastewater
Wastewater
Sedimentation! tank I
Sedimentation! tank I I
Sedimentation tank
Single filter system (a) Fig.
Alternating double filtration (b) 6.21.
Trickling
filter
Two stage filtration (c)
systems.
q u a l i t y is o b t a i n e d b y t h i s s y s t e m a s c o m p a r e d t o t h e single filters, (c) Two stage filtration—The first filter is a c o a r s e o n e , u s u a l l y filled w i t h s y n t h e t i c p a c k i n g w h i c h r e m o v e s 6 0 - 7 0 % o f t h e B O D . T h e s e c o n d filter, w h e r e s l i m e g r o w t h is c o n s i d e r a b l y less, a c t s a s t h e effluent p o l i s h e r .
7.6. P R E T R E A T M E N T FOR TRICKLING FILTRATION A p r e t r e a t m e n t s i m i l a r t o t h a t for t h e a c t i v a t e d s l u d g e p r o c e s s m a y b e r e q u i r e d for t r i c k l i n g filtration. I t m a y b e n e c e s s a r y t o a d j u s t t h e p H b y n e u t r a l i z a t i o n t o a n o p t i m u m r a n g e f r o m 7 t o 9, b e c a u s e excess a l k a l i n i t y o r acidity disturbs the biological process.
7.7. D E S I G N F O R M U L A T I O N FOR TRICKLING FILTERS T h e p u r p o s e of d e s i g n f o r m u l a t i o n is t o o b t a i n a r e l a t i o n s h i p a m o n g B O D r e m o v a l , d e p t h o f t h e filter, a n d h y d r a u l i c l o a d i n g . T h e f o l l o w i n g f o r m u l a t i o n is t h e o n e d e v e l o p e d b y E c k e n f e l d e r a n d a s s o c i a t e s . B O D r e m o v a l is u s u a l l y e x p r e s s e d a s % B O D r e m a i n i n g in t h e effluent. S /S e
= effluent B O D
0
(mg/liter)/influent B O D
(mg/liter)
T h e t r i c k l i n g filter d e p t h is d e n o t e d b y D (ft) a n d h y d r a u l i c l o a d i n g b y L [ g a l / ( m i n ) ( f t ) ] . A s s u m i n g B O D r e m o v a l t o f o l l o w first-order k i n e t i c s [ E q . (6.51)], dS/dt = -k'X S = -K'S (6.51) 2
v
w h e r e K' =
k'X . 0
272
Secondary Treatment: Aerobic and Anaerobic Processes
6.
Rearranging and integrating, \n(SelS )
= -k'X t
0
=
v
-K't
or Se/S
= e- »*
=
kx
0
(6.52)
w h e r e S is t h e effluent B O D ( m g / l i t e r ) ; S t h e influent B O D ( m g / l i t e r ) ; k' t h e e
0
r e m o v a l rate c o n s t a n t (no volatile solids i n c l u d e d ) ; X
v
K'
the volatile solids;
t h e r e m o v a l r a t e c o n s t a n t ( v o l a t i l e s o l i d s i n c l u d e d , K' = k'X );
and t the
v
residence time. R e s i d e n c e t i m e / is w r i t t e n a s t = CD/L"
(6.53)
w h e r e D is t h e filter d e p t h (ft); L t h e h y d r a u l i c l o a d i n g [ g a l / ( m i n ) ( f t ) ] ; a n d 2
C , η t h e c o n s t a n t s w h i c h a r e f u n c t i o n s o f t h e filter m e d i a a n d specific s u r f a c e . Specific s u r f a c e is defined a s f t
of p a c k i n g surface per ft
2
3
of bulk v o l u m e .
S u b s t i t u t i o n o f t [ E q . ( 6 . 5 3 ) ] in E q . (6.52) yields S IS e
0
= -
K C D
e
i
= e- "
L n
(6.54)
KD/L
w h e r e A: = K'C. E q u a t i o n ( 6 . 5 4 ) is t h e b a s i c m a t h e m a t i c a l m o d e l [ 4 ] for t r i c k l i n g filters. I t r e l a t e s % B O D r e m a i n i n g (SJS )
t o d e p t h o f t h e filter (D)
0
and
hydraulic
l o a d i n g ( L ) . P a r a m e t e r Κ ( r a t e c o n s t a n t ) is a f u n c t i o n o f t h e e a s e o f d e g r a d ability of the substrate. P a r a m e t e r η d e p e n d s o n the characteristics of the packing media.
7.8. A P P L I C A T I O N O F B A S I C M A T H E M A T I C A L M O D E L TO TRICKLING FILTERS WITHOUT A N D WITH RECYCLE S t r e a m s i n v o l v e d in o p e r a t i o n o f t r i c k l i n g filters w i t h o u t a n d w i t h r e c y c l e a r e s h o w n in t h e F i g . 6.22. R e c y c l e i m p r o v e s efficiency o f B O D r e m o v a l a n d d i l u t e s i n f l u e n t B O D t o a level c o m p a t i b l e w i t h m a i n t e n a n c e o f a e r o b i c c o n d i t i o n s . W h e n t h e r e is n o recycle, E q . (6.54) a p p l i e s d i r e c t l y . F o r t r i c k l i n g filters w i t h recycle, influent B O D (S ) is d i l u t e d t o a v a l u e S p r i o r t o e n t e r i n g F
Q
t h e filter. T h e r e l a t i o n s h i p a m o n g S
5 , S
09
a n d t h e recycle r a t i o is o b t a i n e d
e9
F
b y a m a t e r i a l b a l a n c e for t h e B O D [ l o o p ( SJS QS F
S
F
0
= e- '
0
+ QS R
(6.55)
= (QF + Q R ) S
e
= (Q S F
) of F i g . 6 . 2 2 ( b ) ] .
KD LH
+ Q S )KQ
F
R
E
F
+
Dividing b o t h n u m e r a t o r a n d d e n o m i n a t o r by Q
F
0
Q) R
a n d l e t t i n g Q /Q R
F
= r =
recycle r a t i o [ E q . (6.56)], So = ( S + r S ) / ( l + r ) F
e
(6.56)
7.
273
Trickling Filters
Q F . S
F
iQ=Q .Q .S F
R
0
Recycle
Q =rQ R
No
recycle:(6.54)
S /S =e-KD/L" e
F
S /S =e-KO/L"
[S = S ]
0
0
e
F
0
(a) Fig.
6.22.
Trickling
filter
without
recycle
(a) and with
recycle
(b).
S u b s t i t u t i n g t h i s v a l u e i n E q . (6.55), S /[(S e
+ rS W
F
+ r)] = e- '
(6.57)
KD La
e
F o r s i m p l i c i t y , let KB/ Π = X. D i v i d i n g b o t h n u m e r a t o r a n d d e n o m i n a t o r by S
F
a n d s o l v i n g for
SJS , F
SJS
= e- l(\ x
F
+r-re- )
(6.58)
x
F o r domestic sewage, a n empirical relationship has been developed
by
Balakrishnan [2] based on Eq. (6.58): S IS e
0
= exp(-0.003^°-
6 4 4
5* - D/L ) 0
5 4
n
e
(6.59)
7.9. P R O C E D U R E F O R D E S I G N O F T R I C K L I N G FILTERS W H E N B E N C H S C A L E O R PILOT-PLANT DATA A R E AVAILABLE T h r e e i t e m s will b e d i s c u s s e d : (1) A m o d e l o f b e n c h s c a l e t r i c k l i n g filter a n d p r o c e d u r e for o b t a i n i n g d e s i g n d a t a f r o m t h i s m o d e l , (2) t r e a t m e n t o f d a t a o b t a i n e d in o r d e r t o d e t e r m i n e c o n s t a n t s η a n d Kin
E q . ( 6 . 5 4 ) ; a n d (3) a p p l i
c a t i o n o f t h e s e r e s u l t s t o d e s i g n o f a p l a n t scale t r i c k l i n g filter. B o t h c a s e s , w i t h o u t a n d w i t h r e c y c l e , a r e s t u d i e d . T h e a p p r o a c h f o l l o w e d is t h a t p r o p o s e d b y E c k e n f e l d e r a n d F o r d [ 5 ] . E x a m p l e 6.4 p r e s e n t s a n u m e r i c a l i l l u s t r a t i o n .
7.9.1. M o d e l of Bench S c a l e Trickling Filter A s k e t c h o f a b e n c h scale m o d e l o f a t r i c k l i n g filter d e v e l o p e d b y E c k e n f e l d e r a n d a s s o c i a t e s is s h o w n in F i g . 6 . 2 3 . W a s t e w a t e r c o n t a i n e d i n t h e feed r e s e r v o i r is p u m p e d b y a S i g m a m o t o r p u m p t o a p e r f o r a t e d p l a t e f o r flow d i s t r i b u t i o n . T h e r e it is m i x e d w i t h t h e recycle s t r e a m f r o m t h e s e t t l i n g t a n k .
274
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
Fresh feed line ~*;(|||
»
Perforated plate for flow distribution
Simplified diagram:
7-1/2" diameter /plexiglass column 7'high iquid sampling ports (1/4" tubing)
Sigmamotor pump Recycle line-* -•-Feed (gravity Weir flow) Media sampling ports Γ^β
Overflow Constant head feed reservoir (substrate)
Media
Settling tank
Fig. 6.23.
Bench
scale
trickling
filter.
A s a p r e l i m i n a r y s t e p , it is n e c e s s a r y t o g e n e r a t e a n a c c l i m a t e d slime o n t h e filter m e d i a . T h i s m a y t a k e f r o m a few d a y s t o several w e e k s , d e p e n d i n g o n t h e n a t u r e of the wastewater. Samples are t a k e n periodically at the m e d i a a n d liquid sampling ports, a n d B O D determinations are performed. Steady state B O D v a l u e s a r e r e c o r d e d . P i l o t - p l a n t u n i t s m a y a l s o b e utilized. T h e p r o c e d u r e t o o b t a i n b a s i c d a t a is (1) select t h r e e o r f o u r h y d r a u l i c l o a d i n g s [ g a l / ( m i n ) ( f t ) ] . F o r d e e p filters (D > 10 ft) w i t h p l a s t i c p a c k i n g , flow r a t e s o f 0 . 5 - 4 g a l / ( m i n ) ( f t ) a r e selected. F o r t h e p i l o t - p l a n t u n i t ( E x a m p l e 6.4), h y d r a u l i c l o a d i n g s o f 1, 2 , 3 , a n d 4 g a l / ( m i n ) ( f t ) a r e utilized ( T a b l e 6.3). (2) F o r e a c h flow r a t e , s a m p l e a t least t h r e e d e p t h s . F o r E x a m p l e 6.4, f o u r 2
2
2
T A B L E 6.3 Data for Example 6.4* L, hydraulic loading [gal/(min)(ft )] 2
D, depth (ft)
5 10 15 20
L = 1
L = 2
L = 3
L = 4
57.5 33.5 19.5 11.3
67.5 46.0 31.0 21.0
73.0 53.0 38.7 28.2
76.0 57.0 43.0 32.8
" Values in 4 χ 4 matrix are those for (S /S ) e
0
χ 100.
7.
275
Trickling Filters
d e p t h s ( 5 , 10, 15, a n d 2 0 ft) a r e s a m p l e d . A n a l y s e s o f s a m p l e s a r e u s u a l l y e x p r e s s e d a s % B O D r e m a i n i n g , i.e., SJS
0
χ 100. S i n c e i n E x a m p l e 6.4 f o u r
h y d r a u l i c l o a d i n g s ( L ) a n d f o u r s a m p l i n g d e p t h s a r e selected, t h e SJS
0
values
f o r m a 4 χ 4 m a t r i x ( T a b l e 6.3). A n a l y s e s c a n a l s o b e m a d e i n t e r m s o f C O D o r T O C . Determinations of p H , Kjeldahl nitrogen, a n d water temperature are also performed.
7.9.2. Treatment of D a t a Obtained in Order t o Determine C o n s t a n t s η and Κ Step
1. O n s e m i l o g p a p e r , p l o t t h e % B O D r e m a i n i n g (SJS ) 0
χ 100) v s .
d e p t h (D) f o r e a c h v a l u e o f h y d r a u l i c l o a d i n g s (L). F r o m E q . (6.54), it f o l l o w s t h a t [ E q . (6.60)] ln(5 /S ) = -KD/L" e
0
= —(KIL )D
(6.60)
n
T h e r e f o r e a family o f s t r a i g h t lines is o b t a i n e d . T h e a b s o l u t e v a l u e o f t h e i r s l o p e s c o r r e s p o n d s t o K/I?. F o r E x a m p l e 6.4, t h i s p l o t is s h o w n i n F i g . 6 . 2 4 , w h e r e f o u r lines c o r r e s p o n d t o f o u r r u n s [ L = 1, 2 , 3 , a n d 4 g a l / ( m i n ) ( f t ) ] . 2
90r
15
20
D, depth (ft) Fig. 6.24. Plot
of % BOD remaining
vs.
depth.
276
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
Step 2 . O n l o g - l o g p a p e r , p l o t t h e a b s o l u t e v a l u e s o f t h e s l o p e s f o r e a c h o f t h e lines f r o m S t e p 1 v s . L ( T a b l e 6.4 a n d F i g . 6.25). Since |Slope| =
K/L
(6.61)
n
then l o g | s l o p e | = logK0.2
ι—ι
(6.62)
nlogL
1—ι—I
I I I I
|Slope| = η = (log 0.1085 -log 0.056)/(log l-log 4) K/L
n
0.1 0.08
η =0.478
j
0.06
0.04 0.03
0.02
I
1.5
Fig.
2
6.25. Plot
3 L ofK/L
4
n
5 6 7 8 9 10
vs. L.
T A B L E 6.4 vs. L
K/L"
L [gal/(min)(ft )]
"Absolute value o f slopes,A7L
1 2 3 4
0.1085 0.0779 0.0634 0.0560
2
a
rt
F r o m Fig. 6.24.
O n e s t r a i g h t line is o b t a i n e d , e a c h p o i n t o f w h i c h c o r r e s p o n d s t o o n e e x p e r i m e n t . I t s s l o p e yields t h e v a l u e o f η i n a c c o r d a n c e w i t h E q . (6.62). F o r E x a m p l e 6.4 t h i s p l o t is s h o w n i n F i g . 6 . 2 5 , w h e r e f o u r p o i n t s a r e u t i l i z e d f o r t h e construction.* * T h e value o f i f f m a y b e estimated from the ordinate intercept (for abscissa L = 1) o f Fig. 6.25 as 0.11. It is preferable, however, t o determine Κ from the slope o f the straight line in Fig. 6.26, as described in Step 4 .
7.
Trickling Filters
277
Step 3. P e r f o r m t h e f o l l o w i n g c a l c u l a t i o n s : 1. D e t e r m i n e t h e v a l u e s o f L f o r e a c h e x p e r i m e n t a l r u n u t i l i z i n g t h e n
v a l u e o f η d e t e r m i n e d in S t e p 2 ( T a b l e 6.5). T A B L E 6.5 Values of L = Ι · n
β
4 7 β
vs. L
Run no.
L(gal/(min)(ft )]
1 2 3 4
1 2 3 4
2
n
L
_
£0.478
1.0 1.393 1.690 1.941
2. C a l c u l a t e t h e v a l u e s o f DjU
for each experimental r u n at e a c h
d e p t h . F o r E x a m p l e 6.4 t h i s r e s u l t s in t h e 4 χ 4 m a t r i x ( T a b l e 6.6). T A B L E 6.6 Matrix for Values of D/L
n
=
D/L
9
Values o f L
D, depth (ft)
5 10 15 20
L = l
L = 2
L = 3
L = 4
5.0 10.0 15.0 20.0
3.59 7.17 10.75 14.35
2.96 5.91 8.87 11.83
2.57 5.15 7.71 10.30
3. C o n s t r u c t a t a b l e o f D/U v s . (SJS ) χ 100 f r o m i n s p e c t i o n o f T a b l e s 6.3 a n d 6.6. F o r E x a m p l e 6.4, t h i s is s h o w n i n T a b l e 6.7. V a l u e s o f D/Π a r e r e a d f r o m T a b l e 6.6 f r o m left t o r i g h t a n d f r o m t o p t o b o t t o m , a n d c o r r e s p o n d i n g v a l u e s o f (SJS ) χ 100 a r e r e a d f r o m T a b l e 6.3 i n t h e s a m e w a y . F o r E x a m p l e 6.4, a s u m m a r y o f c a l c u l a t e d v a l u e s is s h o w n a s a c o n v e n i e n t a r r a y i n T a b l e 6.8. Step 4. O n s e m i l o g p a p e r p l o t t h e v a l u e s o f (SJS ) χ 100 v s . D/L ( T a b l e 6.7). F r o m E q . (6.60) o b t a i n 0
0
n
0
ln (SJSo)
= - K(DIL")
T h e s l o p e o f t h e s t r a i g h t line t h u s o b t a i n e d yields t h e v a l u e o f K. F o r 6.4, t h i s p l o t is s h o w n in F i g . 6.26. S i n c e in t h i s e x a m p l e t h e r e is a 4 χ 16 p o i n t s a r e utilized in t h e c o n s t r u c t i o n o f t h i s line. T h e r e f o r e , t h e c o n s t a n t s η a n d Κ for t h e b a s i c m a t h e m a t i c a l m o d e l h a v e n o w b e e n
(6.63) Example 4 matrix, values of obtained.
6.
278
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
TABLE D/L
6.7
= D/L
n
0
D\L
n
=
v s . (SJS )
4 7 9
0
D/L
X100
(S l S ) x 100
0418
e
5 3.59 2.96 2.57 10.0 7.17 5.91 5.15 15.0 10.75 8.87 7.71 20.0 14.35 11.83 10.30
TABLE
0
57.5 67.5 73.0 76.0 33.5 46.0 53.0 57.0 19.5 31.0 38.7 43.0 11.3 21.0 28.2 32.8
6.8
S u m m a r y of Calculated Values for Example 6.4
D, depth (ft)
L [gal/(min) ( f t ) ] 2
L
0
4
7
8
D/L -* 0
78
(S /S ) e
0
χ 100
5 10 15 20
1.0 1.0 1.0 1.0
1.0 1.0 1.0 1.0
5.0 10.0 15.0 20.0
57.5 33.5 19.5 11.3
5 10 15 20
2.0 2.0 2.0 2.0
1.393 1.393 1.393 1.393
3.59 7.17 10.75 14.35
67.5 46.0 31.0 21.0
5 10 15 20
3.0 3.0 3.0 3.0
1.690 1.690 1.690 1.690
2.96 5.91 8.87 11.83
73.0 53.0 38.7 28.2
5 10 15 20
4.0 4.0 4.0 4.0
1.941 1.941 1.941 1.941
2.57 5.15 7.71 10.30
76.0 57.0 43.0 32.8
7.
279
Trickling Filters
70 60 50 Ο 2 40
^ 3 0
Ο
Slope; Κ =0.109
ε ν.
ο 2
2
0
GO
b 10
10
15
20
25
30
35
0.478
Fig.
6.26.
Plot
of (SJS )
X 1 0 0 vs.
Q
D/L
9
7.9.3. Application of Results to the D e s i g n of a Plant S c a l e Trickling Filter Case 1. T r i c k l i n g filters w i t h o u t recycle [ F i g . 6 . 2 2 ( a ) ] D e s i g n is b a s e d o n E q . (6.54), for w h i c h v a l u e s o f Κ a n d η a r e d e t e r m i n e d a s d e s c r i b e d in S e c t i o n 7.9.2. Step 1. C a l c u l a t e r e q u i r e d h y d r a u l i c l o a d i n g L for specified B O D r e m o v a l c o r r e s p o n d i n g t o r e q u i r e d SJS . A s s u m e a value for d e p t h D a n d solve for h y d r a u l i c l o a d i n g L [ g a l / ( m i n ) ( f t ) ] . S o l v i n g E q . (6.54) for L , 0
2
L = [-KDI\n(S /S )y
(6.64)
ln
e
0
E q u a t i o n (6.64) yields t h e r e q u i r e d h y d r a u l i c l o a d i n g L i n g a l / ( m i n ) ( f t ) . Step 2. T h e r e q u i r e d filter a r e a A is 2
(6.65)
280
6.
Secondary Treatment: Aerobic and Anaerobic Processes
Step 5. F i l t e r d i a m e t e r is given in E q . (6.66). d = (4Α/π)
= (Λ/0.785)
1/2
1/2
ft
(6.66)
Case 2. T r i c k l i n g filters w i t h recycle [ F i g . 6 . 2 2 ( b ) ] Step
1. C a l c u l a t e S
0
f r o m E q . (6.56) for a specified v a l u e o f t h e recycle
r a t i o r. Step 2. S o l v i n g E q . (6.55) for L , o n e o b t a i n s a n e q u a t i o n s i m i l a r t o E q . (6.64), f r o m w h i c h t h e h y d r a u l i c l o a d i n g is c a l c u l a t e d . S t e p s 1 a n d 2 m a y b e c o m b i n e d b y s o l v i n g E q . (6.58) d i r e c t l y for L. F i n a l r e s u l t is -KD
L =
(SJS )(\+r)
t
(6.67)
F
In
\+r(S /S ) J Step 3. T h e r e q u i r e d filter a r e a A is s h o w n in E q . (6.68). e
A = Q/L = Q (r+ F
F
1)/L
(ft ) 2
(6.68)
Step 4. F i l t e r d i a m e t e r is c a l c u l a t e d f r o m E q . (6.66). E x a m p l e 6.4 D a t a p r e s e n t e d in T a b l e 6.3 f o r % B O D r e m a i n i n g vs. d e p t h a r e o b t a i n e d f r o m a p i l o t - p l a n t t r i c k l i n g filter t r e a t i n g a n i n d u s t r i a l w a s t e w a t e r . 1. D e t e r m i n e v a l u e s o f p a r a m e t e r s η a n d Κ a n d w r i t e t h e c o r r e l a t i o n o f B O D remaining to depth and hydraulic loading. 2. C a l c u l a t e t r i c k l i n g filter d i a m e t e r n e c e s s a r y t o o b t a i n B O D r e d u c t i o n o f 8 0 % if w a s t e w a t e r flow is 2.0 M G D . A filter d e p t h o f 2 0 ft is p r o v i d e d w i t h recycle r a t i o o f 0 . 3 . B a s e c a l c u l a t i o n s o n a n influent B O D o f 300 m g / l i t e r . 3. If n o recycle is u s e d for t h e filter d e s i g n e d in P a r t 2, c a l c u l a t e t h e m a x i m u m w a s t e w a t e r flow p e r m i s s i b l e in M g a l / d a y t o r e a c h t h e d e s i r e d B O D reduction of 80%. S O L U T I O N : Part 1
F o l l o w p r o c e d u r e d e s c r i b e d in S e c t i o n 7.9.2.
Step 1. P l o t o f (SJS ) is s h o w n in F i g . 6.24. 0
χ 100 v s . D for a set o f f o u r v a l u e s o f L. T h i s g r a p h
Step 2. T a b l e 6.4 p r e s e n t s t h e a b s o l u t e v a l u e s of t h e s l o p e s r e a d f r o m F i g . 6.24 vs. L. F i g . 6.25 is a p l o t o f t h e s e v a l u e s . F r o m F i g . 6.25 o b t a i n η = 0 . 4 7 8 . Step 3. 1. V a l u e s o f U = L
0 , 4 7 8
for e a c h e x p e r i m e n t a l r u n a r e p r e s e n t e d i n
T a b l e 6.5. 2. M a t r i x for v a l u e s o f D / L ? = D / L e a c h d e p t h is s h o w n in T a b l e 6.6. 3. V a l u e s o f / ) / ! " = D / L
0 , 4 7 8
vs. (SJS ) 0
0 , 4 7 8
for e a c h e x p e r i m e n t a l r u n a t
χ 100 a r e p r e s e n t e d in T a b l e 6.7.
7.
281
Trickling Filters
A s u m m a r y o f c a l c u l a t e d v a l u e s is g i v e n i n T a b l e 6.8. Step 4. P l o t l o g [ ( 5 / S ) χ 100] v s . D / L ' 0
e
4 7 8
0
( F i g . 6.26). F r o m t h e s l o p e o f
t h e s t r a i g h t line in F i g . 6.26 o n e o b t a i n s # = 0 . 1 0 9 . F r o m E q . (6.54) for η = 0.478 a n d # = 0 . 1 0 9 , S /S e
=
0
^- / °* 0109D L
7e
w h i c h is t h e c o r r e l a t i o n o f B O D r e m a i n i n g t o d e p t h a n d h y d r a u l i c l o a d i n g . S O L U T I O N : Part 2
T r i c k l i n g filter d i a m e t e r
F o l l o w p r o c e d u r e d e s c r i b e d in S e c t i o n 7.9.3 ( C a s e 2). Step 1. C a l c u l a t e S
[Eq. (6.56)].
0
Here, S
e
= (0.2) (300) = 60 mg/liter
Λ S = [300 + ( 0 . 3 ) ( 6 0 ) ] / ( l + 0 . 3 ) = 244.6 mg/liter 0
Step 2. C a l c u l a t e L [ E q . ( 6 . 6 4 ) ] . Here, SelS
0
= 60/244.6 = 0.245
ln 0.245 = - 1 . 4 0 5 and η = 0.478 Therefore, L = (-0.109 χ 20/-1.405)
1 / 0
·
= 2.506 gal/(min) (ft )
4 7 8
2
or 2.506 gal/(min)(ft ) χ 6 0 m i n / h r χ 2 4 h r / d a y = 3609 gal/(day)(ft ) 2
Alternative
calculation
H e r e , SJS
F
Therefore, Λ L =
2
procedure
[ E q . (6.67)]
= 6 0 / 3 0 0 = 0.2, Κ = 0.109, η = 0 . 4 7 8 , D = 2 0 ft, a n d r = 0 . 3 . 0 . 1 0 9 x 2 I0Q
Ί1/0.478
l n [ ( 0 . 2 ) ( l + 0 . 3 ) ] / [ l -'T^m\
= 2
Step 3. D e t e r m i n e t h e filter a r e a [ E q . ( 6 . 6 8 ) ] . A = 2,000,000(0.3 + l ) / 3 6 0 9 = 720 ft Step 4. D e t e r m i n e t h e filter d i a m e t e r [ E q . ( 6 . 6 6 ) ] . d = (720/0.785) S O L U T I O N : Part 3
1/2
= 30.3 ft
N o recycle A = 720 f t SJSo
=
2
0.2
2
5 0 6
^(min)(ft ) 2
282
Secondary Treatment: Aerobic and Anaerobic Processes
6.
Step 1. C a l c u l a t e t h e a l l o w a b l e h y d r a u l i c l o a d i n g [ E q . ( 6 . 6 4 ) ] . Here, S
= S
0
F
= 300 mg/liter
L = (-0.109 x 2 0 / l n 0 . 2 )
1 / 0 4 7 8
= 1.89 gal/(min)(ft ) 2
or 1.89 gal/(min)(ft ) χ 60 m i n / h r χ 24 hr/day = 2722 gal/(day)(ft ) 2
2
Step 2. D e t e r m i n e t h e m a x i m u m w a s t e w a t e r flow [ E q . ( 6 . 6 5 ) ] . Q
= AL = 720 ft x 2 7 2 2 g a l / ( m i n ) ( f t ) = 1,960,000 gal/day 2
F
2
or Q
= 1.960 M G D
F
( w a s 2.0 M G D w i t h r = 0.3).
7.10. D E S I G N P R O C E D U R E W H E N EXPERIMENTAL DATA ARE NOT AVAILABLE Experimental d a t a necessary to determine parameters η a n d Κ by the p r o c e d u r e d e s c r i b e d in S e c t i o n 7.9.2, i.e., a n e t w o r k o f d a t a a s s h o w n i n T a b l e 6 . 3 , a r e often u n a v a i l a b l e . I t m a y n o t b e feasible t o c o n d u c t t h e t e d i o u s e x p e r i m e n t a l w o r k r e q u i r e d t o o b t a i n t h i s n e t w o r k o f d a t a . If t h i s is t h e c a s e p a r a m e t e r η is e s t i m a t e d f r o m a v a i l a b l e d a t a for different t y p e s o f filter m e d i a (n d e p e n d s only o n t h e c h a r a c t e r i s t i c s o f p a c k i n g ) . P a r a m e t e r K, w h i c h is o n l y a f u n c t i o n o f t h e n a t u r e o f t h e w a s t e w a t e r , is a l s o e s t i m a t e d f r o m a v a i l a b l e d a t a for w a s t e w a t e r s f r o m several s o u r c e s . E c k e n f e l d e r [ 4 ] p r e s e n t s a t a b u l a t i o n o f Κ v a l u e s for S u r f p a c filter m e d i a (n = 0.5) for several i n d u s t r i a l wastewaters.
8. A n a e r o b i c T r e a t m e n t 8.1. I N T R O D U C T I O N A n a e r o b i c t r e a t m e n t is utilized for t r e a t m e n t o f w a s t e w a t e r s a s well a s for d i g e s t i o n o f s l u d g e s . A n a e r o b i c t r e a t m e n t o f w a s t e w a t e r s is d e s c r i b e d in t h i s s e c t i o n . A n a e r o b i c d i g e s t i o n o f s l u d g e s is s t u d i e d in C h a p t e r 7 ( S e c t i o n s 2.6 a n d 2.7). T h e e n d p r o d u c t s o f a n a e r o b i c d e g r a d a t i o n a r e g a s e s , m o s t l y m e t h a n e ( C H ) , c a r b o n d i o x i d e ( C 0 ) , a n d s m a l l q u a n t i t i e s o f h y d r o g e n sulfide ( H S ) a n d h y d r o g e n ( H ) . T h e p r o c e s s c o m p r i s e s t w o s t a g e s : (1) a c i d fer m e n t a t i o n a n d (2) m e t h a n e f e r m e n t a t i o n . 4
2
2
2
In the acid fermentation stage, organic materials are b r o k e n d o w n t o organic acids, mainly acetic ( C H C O O H ) , p r o p i o n i c ( C H C H C O O H ) , a n d butyric ( C H C H C H C O O H ) . In the m e t h a n e fermentation stage, " m e t h a n e 3
3
2
2
3
2
8.
283
Anaerobic Treatment
m i c r o o r g a n i s m s " convert the longer chain acids to methane, c a r b o n dioxide, a n d a n a c i d h a v i n g a s h o r t e r c a r b o n c h a i n . T h e a c i d m o l e c u l e is r e p e a t e d l y b r o k e n d o w n in t h e s a m e m a n n e r . T h e r e s u l t i n g a c e t i c a c i d is d i r e c t l y c o n verted to C 0
2
and C H . 4
C H
3
C O O H ? ^ C 0 organisms
2
+ CH
4
T h e m e t h a n e fermentation stage controls the rate of a n a e r o b i c degradation. I n S e c t i o n 8.3.1 it is s h o w n t h a t a p l o t o f (S S )/X t v s . S ( F i g . 6.28) yields r a t e c o n s t a n t k. T h i s is s i m i l a r t o t h e s i t u a t i o n f o u n d in a e r o b i c t r e a t m e n t [ E q . (5.18) a n d F i g . 5 . 5 ] . H o w e v e r , o n e s t r a i g h t line is o b t a i n e d i n a e r o b i c treatment, whereas two result from anaerobic data. T h e rate constants can b e e v a l u a t e d f r o m t h e s l o p e s o f t h e s e s t r a i g h t lines. F r o m F i g . 6.28, t h e s l o p e c o r r e s p o n d i n g t o t h e a c i d f e r m e n t a t i o n s t a g e is m u c h g r e a t e r t h a n t h a t f o r t h e m e t h a n e f e r m e n t a t i o n s t a g e . If t h e t w o lines a r e p l o t t e d o n t h e s a m e scale a s in F i g . 6.28, t h e o n e c o r r e s p o n d i n g t o t h e a c i d f e r m e n t a t i o n is n e a r l y v e r t i c a l b y c o m p a r i s o n w i t h t h e o t h e r . T h i s i n d i c a t e s t h a t t h e m e t h a n e fer m e n t a t i o n s t a g e c o n t r o l s t h e p r o c e s s r a t e . T h e r e f o r e , for d e s i g n p u r p o s e s o n e should a d o p t the k value a n d other parameters evaluated from the m e t h a n e f e r m e n t a t i o n s t a g e . S i n c e m e t h a n e f e r m e n t a t i o n c o n t r o l s t h e p r o c e s s r a t e , it is i m p o r t a n t t o m a i n t a i n c o n d i t i o n s o f effective m e t h a n e f e r m e n t a t i o n . D e t e n t i o n t i m e for m e t h a n e m i c r o o r g a n i s m s m u s t b e a d e q u a t e , o r t h e y a r e washed away from the system. Experimental d a t a show that the required d e t e n t i o n t i m e v a r i e s f r o m 2 t o 2 0 d a y s . O p t i m u m p H r a n g e is 6 . 8 - 7 . 4 . 0
e
v
e
A n a e r o b i c t r e a t m e n t is relatively i n e x p e n s i v e b e c a u s e a e r a t i o n e q u i p m e n t is n o t utilized. O n t h e o t h e r h a n d , r e s i d e n c e t i m e s r e q u i r e d a r e m u c h l o n g e r t h a n for a e r o b i c p r o c e s s e s . B a d o d o r s a s s o c i a t e d w i t h a n a e r o b i c p r o c e s s e s , due mainly to production of H S a n d m e r c a p t a n s , m a y constitute a serious l i m i t a t i o n , p a r t i c u l a r l y in u r b a n a r e a s . 2
8.2. A Q U A N T I T A T I V E S T U D Y O F A N A E R O B I C D E G R A D A T I O N OF A N O R G A N I C W A S T E A quantitative study of anaerobic degradation of organic wastewaters h a s b e e n m a d e b y A n d r e w s [ 1 ] , a n d r e s u l t s a r e s u m m a r i z e d i n F i g . 6.27. T h e following observations m a y be m a d e : 1. pH. A t t h e b e g i n n i n g o f a n a e r o b i c d e g r a d a t i o n ( a c i d f e r m e n t a t i o n ) , the p H d r o p s d u e t o f o r m a t i o n of organic acids. Since at a later stage ( m e t h a n e f e r m e n t a t i o n ) t h e s e a c i d s a r e b r o k e n d o w n , p H i n c r e a s e s . T h e rise s t a r t s after a b o u t 2 d a y s , a s d e p i c t e d in F i g . 6.27. 2. COD remaining. T h i s c u r v e is relatively flat d u r i n g t h e first 2 d a y s o f t h e a c i d f e r m e n t a t i o n s t a g e , since a t t h i s t i m e o r g a n i c c o m p o u n d s a r e m e r e l y c o n v e r t e d t o s o l u b l e f o r m ; s o t h e r e is n o C O D r e d u c t i o n . C O D d r o p s m a r k e d l y after t h i s initial s t a g e .
284
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
Detention time (days) Fig. 6.27. Anaerobic degradation with a batch reactor.)
3. Methane
percentage
of an organic
and volatile
acids.
waste
[1 ] . (Data
obtained
T h e r e is a r a p i d i n c r e a s e i n
m e t h a n e p r o d u c t i o n w i t h a c o r r e s p o n d i n g d e c r e a s e in v o l a t i l e a c i d s a n d a n i n c r e a s e in p H after t h e first 2 d a y s . F o r l o n g d e t e n t i o n t i m e s , n e a r l y all v o l a t i l e a c i d s a r e c o n v e r t e d t o C H a n d C 0 . S i n c e n o t all v o l a t i l e s o l i d s a r e b i o d e g r a d a b l e , t h e c u r v e for C O D r e m a i n i n g a p p r o a c h e s a l i m i t i n g o r d i n a t e for l a r g e v a l u e s o f r e s i d e n c e t i m e .
4
2
8.3. M A T H E M A T I C A L F O R M U L A T I O N F O R ANAEROBIC DIGESTION PROCESS T h e m a t h e m a t i c a l f o r m u l a t i o n d e s c r i b e d is t h e o n e p r o p o s e d b y E c k e n f e l d e r a n d associates [ 5 ] .
8.3.1. B O D Removal Rate for A n a e r o b i c Process If B O D r e m o v a l is a s s u m e d t o f o l l o w first-order k i n e t i c s , o n e m a y utilize t h e f o r m u l a t i o n p r e s e n t e d in C h a p t e r 5, S e c t i o n 3.2 for t h e a e r o b i c p r o c e s s . E q u a t i o n (6.69), w h i c h is s i m i l a r t o E q . (5.18), is o b t a i n e d . * (S -S )IX t 0
e
v
= kS
e
(6.69)
O w i n g t o the presence of n o n r e m o v a b l e B O D a n d following considerations s i m i l a r t o t h o s e in C h a p t e r 5, S e c t i o n 3.2, o n e m a y w r i t e a m o d i f i e d f o r m o f E q . (6.69). (S -S )/X t 0
e
v
= k(S -S ) e
n
(6.70)
* N o t a t i o n X is utilized in Eq. (6.69) instead o f X , used in Eq. (5.18). Since subscript a refers to the aerator, it is omitted in the formulation of the anaerobic process. v
v a
8.
Anaerobic Treatment
285 IQ
Acid fermentation stage 2o|
Methane fermentation stage-^
^-Slope: k 7>CT
0
Fig.
6.28. Determination
of BOD removal
rate for anaerobic
degradation.
E q u a t i o n (6.70) is s i m i l a r t o E q . (5.19). F r o m E q . (6.70) a p l o t o f (S -
S )/X t
0
e
v
v s . S yields t w o s t r a i g h t lines, a s s h o w n i n F i g . 6 . 2 8 . F o r d e s i g n p u r p o s e s , t h e e
v a l u e o f k o b t a i n e d f r o m t h e s l o p e o f t h e line c o r r e s p o n d i n g t o t h e m e t h a n e f e r m e n t a t i o n s t a g e is t h e o n e a d o p t e d . I n F i g . 6.28 a s i n F i g . 5.14, t h e a b s c i s s a a t t h e o r i g i n o f t h e o r d i n a t e axis c o r r e s p o n d s t o t h e n o n b i o d e g r a d a b l e s u b strate
(S ). n
F i g u r e 6.28 c a n b e r e p l o t t e d b y c h o o s i n g a s a b s c i s s a o n l y t h e b i o d e g r a d a b l e p o r t i o n o f t h e effluent,
S '. e
S.' = S e - S
(6.71)
n
T h i s c o r r e s p o n d s t o a translation of t h e axes indicated in Fig. 6.29, so t h a t t h e s t r a i g h t line c o r r e s p o n d i n g t o m e t h a n e f e r m e n t a t i o n
passes t h r o u g h the
origin. T h e m a t h e m a t i c a l m o d e l c o r r e s p o n d i n g t o F i g . 6.29 is (S - S )/X 0
e
v
t = S /X r
v
If t h e r e is n o n o n b i o d e g r a d a b l e m a t t e r , S r e d u c e s t o E q . (6.69).
n
t = kS '
(6.72)
e
= 0, a n d S ' = S e
e9
t h e n E q . (6.72)
8.3.2. Volatile S o l i d s in the A n a e r o b i c Reactor C o n s i d e r t h e a n a e r o b i c r e a c t o r d e p i c t e d b y F i g . 6.30. L e t X
VtQ
the V S S c o n c e n t r a t i o n s in t h e influent a n d in t h e reactor,
and X
0
be
respectively.
A s s u m i n g steady state o p e r a t i o n a n d perfect mixing conditions, V S S con c e n t r a t i o n i n t h e effluent is a l s o e q u a l t o X . v
A material balance for V S S leads
286
6.
Fig. 6.29. anaerobic
Secondary Treatment: Aerobic and Anaerobic Processes
Modified plot degradation.
for
determination
of
BOD
removal
rate
for
S l u d g e out
Waste Anaerobic reactor Λ
Fig. 6.30.
ν,ο
"ν
Material
balance
for volatile
solids
in anaerobic
reactor.
t o a n e q u a t i o n s i m i l a r t o E q . (6.28) in S e c t i o n 5.7, e x c e p t for o m i s s i o n o f s u b s c r i p t α in t e r m
X. v
X = (Xv, ο + aSrW v
+ bt)
(6.73)
E q u a t i o n (6.73) is t h e e x p r e s s i o n for t h e c o n c e n t r a t i o n o f V S S in t h e a n a e r o b i c r e a c t o r . If t h e c o n c e n t r a t i o n o f V S S in t h e influent is negligible (i.e., X
Vy0
« 0),
t h i s e q u a t i o n is simplified t o yield X = aSrl(\+bt)
(X ,o*0)
v
v
(6.74)
P a r a m e t e r s a a n d b i n E q . (6.73) a r e d e t e r m i n e d b y w r i t i n g it in l i n e a r f o r m . B y c r o s s m u l t i p l y i n g , d i v i d i n g t h r o u g h b y aX
V9
a n d r e a r r a n g i n g , E q . (6.74)
yields ( l / a ) ( l -X .JX ) 9
9
+ (b/a)t
= S /X r
v
(6.75)
E q u a t i o n (6.75) is t h e b a s i s f o r d e t e r m i n a t i o n o f p a r a m e t e r s a a n d b. A p l o t of S /X = (S — S )/X v s . t yields a set o f t w o s t r a i g h t l i n e s : t h e first o n e , for l o w e r f's, c o r r e s p o n d i n g t o t h e a c i d f e r m e n t a t i o n s t a g e , a n d t h e s e c o n d , f o r r
v
0
e
v
8.
287
Anaerobic Treatment
higher / ' s , corresponding t o the m e t h a n e fermentation stage. A typical plot is s h o w n in F i g . 6 . 3 1 . T h e d e t e r m i n a t i o n o f p a r a m e t e r s a a n d b is i n d i c a t e d in this g r a p h .
(s -s )/x 0
e
v
Methane fermentation: lntercept = (l/a)(l-X
Vf0
/X ) v
Note: If X « 0 , t h e n : Intercept* l/a vo
λ .
Residence time,t (days) Fig.
6.31. Determination
of parameters
a and b.
W h e n t h e c o n c e n t r a t i o n o f V S S in t h e i n f l u e n t is negligible (i.e., X
v
o
«
0)
a s s h o w n in F i g . 6.35, S e c t i o n 8.5, E q . (6.75) yields E q . (6.76). 1/a + (b/o) t = Sr/Xv
(X
Vt
ο * 0)
(6.76)
F o r c a l c u l a t i o n of d e t e n t i o n t i m e , t h e v a l u e s o f p a r a m e t e r s α a n d b c o r r e s p o n d i n g t o the m e t h a n e fermentation are t h e ones t o be a d o p t e d since this is t h e c o n t r o l l i n g s t a g e .
8.3.3. Calculation of Detention Time for Anaerobic Treatment C o n s i d e r E q . (6.72) for B O D r e m o v a l a n d E q . (6.73) for V S S c o n c e n t r a t i o n . F r o m E q . (6.72), / = SrlkX S ' v
(6.77)
e
F r o m E q . (6.73), S
r
= | X ( 1 + bt) - X
Vt
]/a
(6.78)
0
S u b s t i t u t i n g S i n E q . (6.77) b y its v a l u e f r o m E q . (6.78) a n d s o l v i n g for t, r
t = (X v
Χ , )l(akX υ
0
v
S ' - bX ) e
0
(6.79)
I n E q . (6.79), S J is g i v e n b y E q . (6.71). W h e n t h e c o n c e n t r a t i o n o f V S S i n t h e i n f l u e n t is negligible (i.e., X
w 0 ) , E q . (6.79) is simplified t o yield
vo
/ = 1 l(akS ' - b) e
(X , ο * 0) v
(6.80)
288
6.
Secondary Treatment: Aerobic and Anaerobic Processes
8.3.4. G a s Production in A n a e r o b i c Treatment G a s p r o d u c e d b y a n a e r o b i c d e g r a d a t i o n is c o m p o s e d o f C H , C 0 , a n d small quantities of H S a n d H . Lawrence a n d M c C a r t h y [ 7 ] have s h o w n t h a t m e t h a n e g a s p r o d u c t i o n c a n b e e s t i m a t e d a s 5.62 f t ( a t S T P ) * p e r l b o f C O D r e m o v e d , excluding t h e C O D r e m o v e d w h i c h is c o n v e r t e d t o cells. T h e p r o p o s e d r e l a t i o n s h i p t o e s t i m a t e g a s p r o d u c t i o n is t h e n 4
2
2
2
3
G = 5.62(β5 -\A2QX ) Γ
= 5 . 6 2 Q ( . S - 1.42Α;)
V
(6.81)
r
w h e r e G is t h e f t o f g a s ( a t S T P ) p r o d u c e d p e r d a y ; QS t h e l b C O D r e m o v e d / d a y ; a n d \A2QX t h e l b V S S in r e a c t o r effluent/day. [ R a t i o 1.42 c o r r e s p o n d s t o l b C O D / l b V S S ; see C h a p t e r 5, E q . ( 5 . 6 0 ) . ] A t t e n t i o n s h o u l d b e g i v e n t o t h e u n i t s i n utilizing E q . (6.81). If t h e f o l l o w i n g u n i t s a r e e m p l o y e d , w h e r e G is t h e f t g a s / d a y ( a t S T P ) ; Q t h e M G D ; S t h e m g C O D / l i t e r ( « l b C O D / M l b liquor); a n d X the m g VSS/liter ( * lb V S S / M l b liquor), then 3
r
V
3
r
v
/^ S l ο COD ^ lb liquor I Q—— χ S χ 8.24 lb C O D \ day M l b liquor gal liquor lb C O D Mgal liquor lb VSS _ lb liquor \ - 1.42 χ Q—— χ X χ 8.24 lb VSS day M l b liquor gal l i q u o r /
„ *„ G = 5.62
ft3
M
a
l
l i (
u o r
l b
n
r
Λ
v
o
v
o r finally [ E q . ( 6 . 8 2 ) ] , G = 46.87e(S -1.42A ,)ft /day r
r
3
l
(STP)
(6.82)
8.4. L A B O R A T O R Y A N A E R O B I C R E A C T O R S FOR O B T A I N I N G B A S I C D E S I G N I N F O R M A T I O N T w o m o d e l s o f a l a b o r a t o r y a n a e r o b i c r e a c t o r a r e s h o w n in F i g s . 6.32 a n d 6.33. T h e r e a c t o r in F i g . 6.32 is a b a t c h t y p e a n d t h e o n e in F i g . 6.33 is continuous. T h e o p e r a t i o n a l p r o c e d u r e f o l l o w e d t o o b t a i n b a s i c d e s i g n d a t a is 1. T o p r o v i d e a seed o f m i c r o o r g a n i s m s , o b t a i n a s a m p l e o f s e t t l e d s l u d g e f r o m a m u n i c i p a l w a s t e w a t e r t r e a t m e n t p l a n t a n d p l a c e it in t h e l a b o r a t o r y reactor. A d d the wastewater to be treated a n d maintain a temperature of 35°C t h r o u g h o u t t h e test p e r i o d . 2. I f significant a m o u n t s o f a i r a r e t r a p p e d in t h e r e a c t o r , p u r g e t h e system with an inert gas. 3. T h e m i x t u r e is m i x e d c o n t i n u o u s l y , e i t h e r m e c h a n i c a l l y ( F i g . 6.32) o r b y r e c i r c u l a t i o n o f t h e g a s p r o d u c e d , b y m e a n s o f a g a s p u m p ( F i g . 6.33). D o n o t s t a r t feeding t h e w a s t e w a t e r o r w i t h d r a w i n g t h e m i x e d l i q u o r u n t i l g a s p r o d u c t i o n is a s c e r t a i n e d . * S T P stands for standard temperature and pressure, taken as Ρ = 1 atm and t = 60°F.
8.
289
Anaerobic Treatment Feed tube and g a s sampling cock Withdrawal tube
Mixer Digester
Rubber tubing—J
Fig. 6.32.
Bench
scale
Feed-pump
batch
feed
digester.
Gas^pumpp
Gas to waste
Feed-*=fl (gravity flow)
Feed-*reservoir^ (constant head)
Condensate trap
Overflow*-^ Withdrawal tube Fig. 6.33.
Digester
Continuous
feed
digester.
4. O n c e g a s p r o d u c t i o n is n o t i c e d , feed p o r t i o n s o f w a s t e w a t e r , w i t h drawing equal portions of mixed liquor. 5. D u r i n g t h e e n t i r e s t a r t - u p p e r i o d p H is o b s e r v e d closely, a n d if it falls b e l o w 6.6, a l k a l i n i t y is a d d e d t o m a i n t a i n t h e r e c o m m e n d e d r a n g e , i.e., p H 6.6-7.6. 6. C o n t i n u e t o feed a n d w i t h d r a w d a i l y u n t i l effluent B O D ( o r C O D ) stabilizes. A n a l y z e a n d r e c o r d t h e d a t a : r a w w a s t e w a t e r : p H a n d C O D ( o r B O D ) ; effluent: p H , C O D ( o r B O D ) , a n d V S S . 5
5
8.5. D E S I G N P R O C E D U R E F O R ANAEROBIC DIGESTERS T r e a t m e n t of the d a t a o b t a i n e d in the l a b o r a t o r y t o arrive at design p a r a meters a n d utilization of this i n f o r m a t i o n in the design of a n a n a e r o b i c d i g e s t e r a r e i l l u s t r a t e d b y E x a m p l e 6.5.
290
6.
Secondary Treatment: Aerobic and Anaerobic Processes
Example 6.5 200,000 gal/day of a wastewater are t o be treated by a n a e r o b i c d e g r a d a t i o n . T h e m e a n v a l u e for influent C O D is 10,000 m g / l i t e r . Effluent C O D s h o u l d b e r e d u c e d t o a t t h e m o s t 2 0 0 0 m g / l i t e r . C o n c e n t r a t i o n o f V S S in t h e influent is negligible. A b e n c h scale r e a c t o r is utilized t o s i m u l a t e t h e p r o c e s s , a n d M L V S S c o n c e n t r a t i o n is m a i n t a i n e d a t a p p r o x i m a t e l y 500 m g / l i t e r in all e x p e r i m e n t a l r u n s . D a t a o b t a i n e d a r e p r e s e n t e d in T a b l e 6.9. T A B L E 6.9 Laboratory Data for Example 6.5
Run no.
Residence time (days)
Influent COD (mg/liter)
Effluent COD (mg/liter)
1 2 3 4 5
5 10 15 20 30
8,550 8,400 9,190 10,200 12,470
3,800 2,400 1,940 1,700 1,470
1. O b t a i n d e s i g n p a r a m e t e r s k, a, a n d b. 2. C a l c u l a t e d i g e s t e r v o l u m e r e q u i r e d ( g a l ) . 3. C a l c u l a t e M L V S S c o n c e n t r a t i o n for t h e a n a e r o b i c d i g e s t e r ( m g / l i t e r ) . 4. E s t i m a t e g a s p r o d u c t i o n ( f t / d a y a t S T P ) . 3
S O L U T I O N : Part 1 Step 1. O b t a i n v a l u e s o f (S -S )/X a n d (S -S )/X t F i g s . 6.34 a n d 6 . 3 5 . T h i s is s h o w n in T a b l e 6.10. 0
e
0
0
e
necessary to plot
v
Step 2. P r e p a r e a p l o t o f (S — S )/X t vs. S a n d d e t e r m i n e k from the s l o p e o f t h e line f o r t h e m e t h a n e f e r m e n t a t i o n s t a g e . T h i s is s h o w n i n F i g . 6.34. S i n c e all r e s i d e n c e t i m e s c o n s i d e r e d a r e l o n g e r t h a n 5 d a y s , o n l y t h e s t r a i g h t line c o r r e s p o n d i n g t o t h e m e t h a n e f e r m e n t a t i o n s t a g e is s h o w n in F i g . 6.34. F r o m t h e s l o p e o f t h i s line o n e o b t a i n s k = 0 . 0 0 0 5 d a y and S « 0 ( s t r a i g h t line p a s s e s t h r o u g h o r i g i n ) . Step 5. D e t e r m i n e p a r a m e t e r s a a n d b. P l o t (S — S )/X v s . t: T h i s p l o t is s h o w n i n F i g . 6.35. A s i n d i c a t e d i n S t e p 2 , o n l y t h e s t r a i g h t line c o r r e s p o n d i n g t o t h e m e t h a n e f e r m e n t a t i o n s t a g e is o b t a i n e d . 0
e
v
e
- 1
n
0
Intercept = 1/a = 7.0 .·.
=l/7
fl
= 0.143
Slope = b/a = 0.5 Λ b = a0.5 = 0.143 χ 0.5 = 0.0715
e
v
Anaerobic Treatment
co CO
5 si
0\
Ν
O vo «o ON OO
m m h
^
^
ο
ο
ο
«η Ο «ο ρ σ< (Ν T t
Ο cn
υ CO CO
Co
Xi
ski"
ι—I
ι—« ι—I
υ
s I S O
^
^
8 oo
^
I
S8§
8
Hu
oo τ ? rn r i ^ λ
ι-^ *-T
CO
~ Q GO Π
ο
ο
οο οο"
ΙΛ
8
8 S
«ο Ο _ . _ « η Tf rt^ (Ν Tt
oC θ"
ri
9 O.
ε X UJ
ο
(ft
2 ϊ •8S 4>
c ο
(D
III
_
-I
3
<
CD
§8
•Η
Ν
(^1
Tt
ΙΛ
292
6.
Secondary Treatment: Aerobic and Anaerobic Processes
2.0
/\
Ι.Θ 16 1.4 ).000i > day" 1.2
1
>
1.0 0.8 0
χ 0.6 0.4 0.2
2000
3000
4000
S ; mg/liter [COD] e
Fig. 6.34.
Plot
S O L U T I O N : Part 2 Step S' e
e
Q
— S )/X 9
v
t vs. S
0
for Example
6.5.
D i g e s t e r v o l u m e (gal)
1. C a l c u l a t e t h e r e s i d e n c e t i m e f r o m E q . (6.80). Since
S —S
=
of (S
α
n
« 0, t h e n
S .) e
/ = 1 / ( 0 . 1 4 3 x 0 . 0 0 0 5 x 2 0 0 0 - 0 . 0 7 1 5 ) = 14 days Step 2. D i g e s t e r v o l u m e is t h e n c a l c u l a t e d . V = Qt = 200,000 gal/day χ 14 days = 2,800,000 gal SOLUTION:
Part 3
M L V S S c o n c e n t r a t i o n in a n a e r o b i c d i g e s t e r
(6 7 4 ) ] χ
ν
= 0.143(10,000 - 2 0 0 0 ) / ( l + 0 . 0 7 1 5 χ 14) = 572 mg/liter
S O L U T I O N : Part 4
G a s p r o d u c t i o n f r o m E q . (6.82)
Since Q = 200,000 gal/day = 0.2 Mgal/day then G = 46.87 x 0.2 [(10,000 - 2000) - 1.42 χ 572] = 67,380 f t / d a y at S T P 3
[Eq.
4 2
Ο
Fig.
5
6.35.
10
Plot
15
of (S
Q
20 25 t (days)
— S,)/X
v
30
35
vs. t for Example
40
45
6.5.
Problems I. A n aerated lagoon is being considered for treatment o f an industrial wastewater flow o f 1.0 M G D . T h e following data are available: Influent soluble B O D = 200 mg/liter Influent suspended solids concentration is negligible Effluent soluble B O D ^ 2 0 mg/liter Wastewater temperature = 6 0 ° F 5
5
Since in most situations summer conditions control required aerator horsepower, design u p o n an air temperature o f 85°F T a k e h = 100 B T U / ( d a y ) ( f t ) ( ° F ) D a t a obtained from laboratory simulation k = 0.025 d a y (at 2 0 ° Q , α = 0.65 lb lb B O D day, b = 0.07 d a y " a n d α' = 1.2 lb 0 / l b B O D L a g o o n depth = 10 ft Oxygen concentration to be maintained in liquid is 1.5 mg/liter Determine 1. L a g o o n surface in acres 2. Total effluent B O D (soluble + insoluble) 3. Oxygen requirements [lb 0 / ( H P ) ( h r ) ] and total required H P and p o w e r (HP/Mgal). Take C = 8 mg/liter and α = 0.85. A s s u m e aerators are conservatively at N = 3.0 lb 0 / ( H P ) ( h r ) .
base
2
- 1
r
2
2
sw
0
2
VSS/
5
level rated
294
6.
S e c o n d a r y Treatment: Aerobic and Anaerobic P r o c e s s e s
II. It is proposed to treat an industrial wastewater (flow, 1.0 M G D ) utilizing stabilization ponds. Plan the operation in three stages. Influent B O D (2000 mg/liter) should be reduced t o a value not higher than 4 0 mg/liter (effluent from third stage). Stage 1—Anaerobic pond. It should be designed for the purpose o f reducing influent B O D ( 2 0 0 0 mg/liter) to a value of 500 mg/liter in the effluent. Take K= 0.1 d a y . Plan o n a 10-ft depth. Calculate the area of the p o n d in acres. Calculate surface loading in lb B O D / (acre)(day). Stages 2 and 3—Facultative ponds. D e s i g n stages 2 and 3 for same residence time in each. T a k e Κ = 0.2 d a y " for both stages. For stage 2, the design is to be based o n a surface loading of 500 lb B O D / ( a c r e ) ( d a y ) . Calculate the area in acres and the depth in ft. Estimate depth o f o x y g e n penetration in ft. F o r stage 3, the design is t o be based o n a surface loading o f 2 5 0 lb B O D / ( a c r e ) ( d a y ) . Calculate the area in acres and the depth in ft. Estimate depth o f o x y g e n penetration in ft. 5
- 1
5
5
1
5
5
III. T h e data tabulated below for % B O D remaining (i.e., S /S pilot-plant trickling filter treating an industrial wastewater. 5
e
0
x 100) were obtained in a
Hydraulic loading [ g a l / ( m i n ) ( f t ) ] 2
Z>, depth (ft)
6 12 18 24
1
2
3
4
62 36 24 14
70 46 32 22
75 57 43 32
80 63 48.5 39
1. Correlate the data and develop a relationship between B O D removal, depth, and hydraulic loading. 2. D e s i g n a filter to obtain 80% B O D reduction from 5 M G D of settled wastewater with an initial B O D o f 2 5 0 mg/liter, using a depth o f 30 ft (a) without recycle; (b) with recycle ratio o f 1.5. 5
IV. A wastewater for which the m e a n influent C O D may be taken as 10,000 mg/liter is t o be treated by anaerobic degradation. Laboratory tests indicated that the nondegradable C O D is approximately 2000 mg/liter. Average M L V S S influent concentration is 15 mg/liter. Bench scale determinations yield the following values for design parameters: k = 0.0004 d a y " , α = 0.14 lb M L V S S yield/lb C O D removed, and 6 = 0.02 lb M L V S S oxidized/ (day)(lb M L V S S in reactor). Calculate % C O D removal for residence times o f 80, 60, 40, and 2 0 days, and plot % removal vs. residence time. O n the same graph plot the corre sponding values o f % C O D removal, neglecting the M L V S S in the influent. 1
References 1. Andrews, J. F., Cole, R. D . , and Pearson, Ε . Α . , "Kinetics and Characteristics o f Multistage Methane Fermentation," S E R L R e p . 6 4 - 1 1 . University of California, Berkeley, 1962. 2. Balakrishnan, S., Eckenfelder, W . W., and Brown, C , Water Wastes Eng. 6, N o . 1, A-22(1969). 3. Eckenfelder, W. W . , Jr., "Industrial Pollution Control." McGraw-Hill, N e w Y o r k , 1966.
295
References
4. Eckenfelder, W . W., Jr., "Water Quality Engineering for Practicing Engineers." Barnes & N o b l e , N e w Y o r k , 1970. 5. Eckenfelder, W . W., Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w York, 1970. 6. Hermann, E. J., and G l o y n a , E. F., Sewage Ind. Wastes 3 0 , N o . 8, 963 (1958). 7. Lawrence, A . W . , and McCarthy, P. L., "Kinetics o f Methane Fermentation in Anaerobic Waste Treatment," R e p . N o . 75. Department o f Civil Engineering, Stanford University, Stanford, California, 1967. 8. Metcalf & Eddy, Inc. "Wastewater Engineering: Collection, Treatment, D i s p o s a l . " McGraw-Hill, N e w York, 1972. 9. Oswald, W. J., in "Advances in Biological Waste Treatment" (W. W . Eckenfelder, Jr. and O. M c C a b e , eds.), p p . 3 5 7 - 3 9 3 . Pergamon, Oxford, 1963. 10. Oswald, W. J., in "Advances in Water Quality Improvement" (W. W. Eckenfelder, Jr. and E. F. G l o y n a , eds.), pp. 4 0 9 - 4 2 6 . Univ. o f Texas Press, Austin, 1968. 11. Pasveer, Α . , "The Oxidation D i t c h : Principles, Results and Applications," Proc. Symp. L o w Cost Waste Treatment, pp. 165-171. C P H E R I , Nagpur, India, 1969. :
7 Sludge Treatment and Disposal 1. Introduction
297
2. Aerobic and Anaerobic Digestion of Sludges 2.1. Introduction to Aerobic Digestion of Sludges 2.2. Schematic Representation of Aerobic Biological Treatment of Sludges 2.3. Concept of Sludge A g e for the Case of Sludge Digesters 2.4. Laboratory Scale Batch Reactor to Obtain Basic Design Data for Aerobic Digesters 2.5. Design Procedure for Aerobic Digesters of Sludge 2.6. Introduction to Anaerobic Sludge Digestion 2.7. Sizing of Anaerobic Sludge Digesters
297 297
301 302 307 309
3. Thickening of Sludges 3.1. Introduction 3.2. Advantages of Thickening 3.3. Gravity Thickener 3.4. Design Principles for Gravity Thickeners 3.5. Edde and Eckenfelder's Equation 3.6. Flotation Thickening
309 309 309 310 310 310 311
4. Dewatering of Sludges by Vacuum Filtration 4.1. Introduction 4.2. Variables in Vacuum Filtration 4.3. Definition of Parameter c 4.4. Filtration Equations 4.5. Laboratory Determination of Specific Resistance r and Optimum Coagulant Dosage 4.6. Units for Specific Resistance of Cake (r) 4.7. Numerical Example: Determination of Specific Cake Resist ance Using the Buchner Funnel 4.8. Specific Resistance for Compressible Cakes 4.9. Filtration Design Equation 4.10. Determination of Parameters n. s , m. and r in Eq. (7.29) . . . . 4.11. Leaf Test Laboratory Procedure for Determination of the Parameters in the Loading Equation 4.12. Illustration of Calculation Procedure for Parameters n, s, m, and r 4.13. Procedure for Rotary Filter Design 5. Pressure Filtration 6. Centrifugation 7. Bed Drying of Sludges 7.1. Introduction 7.2. Mechanisms of Dewatering Sludges on Sand Beds 7.3. Construction of Sand Drying Beds 7.4. Drying Bed Design
311 311 312 313 314
Q
0
296
299 300
315 316 317 318 319 320 321 324 327 328 328 329 329 330 331 331
2.
297
Digestion of Sludges
8. Pre-dewatering Treatment of Sludges 8.1. Chemical Coagulation 8.2. Heat Treatment of Sludges 9. Sludge Disposal 9.1. Land Disposal of Sludges 9.2. Sludge Incineration Problems References
334 334 336 340 340 340 341 341
1. I n t r o d u c t i o n M o s t p r i m a r y t r e a t m e n t p r o c e s s e s ( C h a p t e r 3) a s well a s s e c o n d a r y t r e a t m e n t s e q u e n c e s ( C h a p t e r s 5 a n d 6) yield s l u d g e s w h i c h m u s t b e d i s p o s e d o f i n s o m e a d e q u a t e w a y . S l u d g e s r e s u l t i n g solely f r o m s o l i d - l i q u i d s e p a r a t i o n p r o c e s s e s ( s e d i m e n t a t i o n , flotation) a r e r e f e r r e d t o h e n c e a s p r i m a r y s l u d g e s , a n d those resulting from biological processes are designated as secondary sludges. O n e p o s s i b i l i t y in t h e t r e a t m e n t s e q u e n c e is r e d u c t i o n o f t h e t o t a l a m o u n t of organic a n d volatile c o n t e n t by submitting the sludge t o digestion. A e r o b i c a n d a n a e r o b i c digestion of sludges are discussed in Section 2. A n o t h e r a p p r o a c h is t o i n c r e a s e t h e p e r c e n t a g e o f solid c o n t e n t s i n t h e s l u d g e b e f o r e final d i s p o s a l b y a s e q u e n c e o f p r o c e s s e s w h i c h fall u n d e r t h e h e a d i n g s o f t h i c k e n i n g a n d d e w a t e r i n g , s t u d i e d in S e c t i o n s 3 - 7 . F i g u r e 7.1 i l l u s t r a t e s t h e i n c r e a s e in s o l i d c o n t e n t w h i c h m i g h t b e e x p e c t e d i n s u c h treatment sequences. Special t r e a t m e n t p r e c e d i n g d e w a t e r i n g b e c o m e s n e c e s s a r y f o r c e r t a i n s l u d g e s w h i c h a r e difficult t o d e w a t e r . T h e s e i n c l u d e c h e m i c a l c o a g u l a t i o n a n d h e a t - t r e a t m e n t p r o c e s s e s , d e s c r i b e d in S e c t i o n 8. T h e end of the treatment sequence involves disposal of the remaining sludge, w h i c h is d i s c u s s e d in S e c t i o n 9. S l u d g e d i s p o s a l m e t h o d s fall i n t o t w o s c h e m e s involving either land disposal or incineration. All these alternatives are i n d i c a t e d in F i g . 7 . 1 , w h i c h is t h e o v e r a l l p l a n o f s t u d y f o r t h i s c h a p t e r .
2. A e r o b i c a n d A n a e r o b i c Digestion of S l u d g e s 2.1. I N T R O D U C T I O N T O A E R O B I C D I G E S T I O N OF S L U D G E S A e r o b i c d i g e s t i o n is a p r o c e s s in w h i c h a m i x t u r e o f p r i m a r y d i g e s t i b l e s l u d g e f r o m p r i m a r y clarification a n d a c t i v a t e d s l u d g e f r o m a e r o b i c b i o logical t r e a t m e n t is a e r a t e d for a n e x t e n d e d p e r i o d o f t i m e . T h i s r e s u l t s i n cellular destruction with a decrease of volatile s u s p e n d e d solids (VSS).
298
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Sludge Treatment and Disposal
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T h e p u r p o s e o f a e r o b i c d i g e s t i o n is t o r e d u c e t h e a m o u n t o f s l u d g e w h i c h is t o b e d i s p o s e d o f s u b s e q u e n t l y . T h i s r e d u c t i o n r e s u l t s f r o m c o n v e r s i o n b y oxidation of a substantial p a r t of the sludge into volatile p r o d u c t s ( C 0 , 2
N H , H ) . If b a c t e r i a l cells a r e r e p r e s e n t e d b y t h e f o r m u l a C H N 0 , o x i d a 3
2
5
7
2
t i o n t a k i n g p l a c e in a e r o b i c d i g e s t i o n is g i v e n b y E q . (5.51). T h i s o x i d a t i o n o c c u r s w h e n t h e s u b s t r a t e in a n a e r o b i c s y s t e m is insufficient f o r e n e r g y a n d s y n t h e s i s . I t c o r r e s p o n d s t o t h e e n d o g e n o u s r e s p i r a t i o n p h a s e ( F i g . 5.3).
Wastewater Primary clarifier
Aerator (Activated sludge process)
Secondary Effluent clarifier Volatile matter*
Recycled sludge Primary sludge
-a
Wastage
Aerobic
Secondary digester sludge) _ of sludge Stabilized sludge—
Fig. 7.2. Flow
diagram
showing
aerobic
digester
of
sludge.
F i g u r e 7.2 s h o w s a flow d i a g r a m o f a c o n t i n u o u s a e r o b i c d i g e s t e r for t h e treatment sequence involving primary sedimentation, the activated sludge p r o c e s s , a n d a e r o b i c d i g e s t i o n o f s l u d g e . W h e n t h e r e is a s m a l l a m o u n t o f s l u d g e t o b e d i g e s t e d , b a t c h o p e r a t i o n is u t i l i z e d w i t h i n t e r m i t t e n t d i s c h a r g e o f d i g e s t e d s l u d g e . T h e a e r o b i c d i g e s t e r s h o w n in F i g . 7.2 h a n d l e s a m i x t u r e of p r i m a r y a n d secondary sludge. D e s t r u c t i o n r a t e o f cells u s u a l l y d e c r e a s e s w h e n t h e f o o d t o m i c r o o r g a n i s m r a t i o (F/M) i n c r e a s e s . T h e r e f o r e , t h e g r e a t e r t h e p r o p o r t i o n o f p r i m a r y s l u d g e utilized in t h e p r o c e s s , t h e s l o w e r is t h e d i g e s t i o n b e c a u s e p r i m a r y s l u d g e h a s relatively h i g h B O D ( h i g h F) a n d l o w M L V S S c o n t e n t ( l o w Μ), t h u s l e a d i n g t o h i g h F/M r a t i o s .
2.2. S C H E M A T I C R E P R E S E N T A T I O N O F A E R O B I C BIOLOGICAL T R E A T M E N T OF S L U D G E S F i g u r e 5.3 s h o w s t w o v a r i a b l e s ( m a s s o f a c t i v a t e d s l u d g e a n d r e m a i n i n g s o l u b l e B O D ) p l o t t e d v s . a e r a t i o n t i m e . T h e c u r v e for r e m a i n i n g s o l u b l e B O D b e c o m e s n e a r l y flat a s t h e m a s s o f M L V S S r e a c h e s its m a x i m u m . S i n c e a e r o b i c d i g e s t i o n o f s l u d g e o c c u r s in t h e e n d o g e n o u s r e s p i r a t i o n p h a s e r e g i o n , t h e r e is essentially n o s o l u b l e B O D r e m o v a l . T h e f u n d a m e n t a l o b j e c t i v e o f a e r o b i c d i g e s t i o n is r e d u c t i o n o f t h e m a s s o f s l u d g e for d i s p o s a l , a n d n o t r e m o v a l o f soluble B O D .
300
7.
Sludge Treatment and Disposal
2.3. C O N C E P T O F S L U D G E A G E F O R T H E C A S E OF S L U D G E DIGESTERS T h e c o n c e p t o f s l u d g e a g e is d i s c u s s e d in C h a p t e r 5, S e c t i o n 9 a n d is g i v e n b y E q . (5.145), in w h i c h t h e d e n o m i n a t o r is t h e n e t o u t p u t o f V S S f r o m t h e s y s t e m . F o r all c a s e s d i s c u s s e d s o far, c o n c e n t r a t i o n o f s l u d g e i n t h e r e a c t o r effluent w a s g r e a t e r t h a n t h a t in t h e influent. A s a r e s u l t , t h e r e w a s a p o s i t i v e n e t o u t p u t o f V S S for t h e s y s t e m . F o r s l u d g e d i g e s t e r s , h o w e v e r , t h e r e is less s l u d g e l e a v i n g t h a n e n t e r i n g , t h e difference c o r r e s p o n d i n g t o s l u d g e w h i c h is b i o d e g r a d e d . A s a r e s u l t , t h e d e n o m i n a t o r in E q . (5.145) is n e g a t i v e , c o r r e s p o n d i n g t o a n e t i n p u t o f V S S t o t h e s y s t e m . T h e r e f o r e , for s l u d g e d i g e s t e r s t h i s e q u a t i o n is r e w r i t t e n a s t
s
=
lb M L V S S in the digester net input of VSS t o the system
> 0
(lb/day)
(7.1)
F r o m E q . (7.1), s l u d g e a g e c o r r e s p o n d s t o t h e a v e r a g e l e n g t h o f t i m e t h e n e t i n p u t o f s l u d g e is s u b j e c t e d t o d i g e s t i o n . A n u m e r i c a l e x a m p l e will clarify t h i s c o n c e p t ( v a l u e s f r o m E x a m p l e 7 . 1 ) : 5 0 0 0 l b / d a y o f s l u d g e a r e fed t o a n a e r o b i c digester. L a b o r a t o r y t e s t s i n d i c a t e t h a t 5 5 % o f t h i s s l u d g e is n o n degradable. This a m o u n t s t o (0.55)(5000) = 2750 lb. T h u s , the d e g r a d a b l e p o r t i o n is 5 0 0 0 - 2 7 5 0 = 2 2 5 0 l b . T h e a e r o b i c d i g e s t e r is d e s i g n e d for t h e p u r p o s e o f o x i d i z i n g 9 0 % o f t h i s d e g r a d a b l e V S S . T h e V S S w h i c h is n o t o x i d i z e d is 1 0 % o f 2 2 5 0 l b o r (0.10) (2250) = 2 2 5 l b . A s s u m e t h a t t h e d i g e s t e r is c o n t i n u o u s , o p e r a t i n g a t s t e a d y s t a t e c o n d i t i o n s , a n d t h a t t h e c o n c e n t r a t i o n o f V S S is m a i n t a i n e d a t 4 0 0 0 m g / l i t e r . D i g e s t e r v o l u m e is 1,185,000 g a l . T h e s e v a l u e s a r e s h o w n in Fig. 7.3(a). T h u s , t h e t o t a l o u t p u t o f s l u d g e is 2750 + 225 = 2975 lb/day Since t h e s e 2 9 7 5 l b / d a y e n t e r a n d leave t h e d i g e s t e r w i t h o u t a n y c h a n g e , t h e n e t s l u d g e i n p u t ( s l u d g e w h i c h is a c t u a l l y o x i d i z e d i n t h e d i g e s t e r ) is 5000 - 2975 = 2025 lb/day T h e r e f o r e , F i g . 7 . 3 ( a ) is r e d r a w n [ F i g . 7 . 3 ( b ) ] s h o w i n g t h e n e t i n p u t . S l u d g e a g e t f r o m E q . (7.1) is d e r i v e d f r o m E q . (7.2). s
t
s
Since X
v%a
t
s
(days) = X , v
a
F/(sludge input - sludge o u t p u t ) = X
Vt
= 4000 mg/liter = 4000 χ 1 0 "
6
a
V/(AX)
Mt
(7.2)
lb s l u d g e / l b l i q u o r , t h e n
(days) _ 4000 χ 1 0 " lb sludge/lb liquor χ 8.34 lb liquor/gal liquor χ 1,185,000 gal liquor 6
2025 lb sludge/day = 19.5 days
2.
301
Digestion of Sludges
V= 1,185,000 gal Aerobic digester Input:
Part of degradable sludge which is not oxidized-.225 lb/day
X = 4 0 0 0 mg/liter v a
Sludge oxidized:
5 0 0 0 lb/day sludge
(0.9)(2250)=2025 lb/day
Nondegradable sludge: 2 7 5 0 lb/day
(a)
V= 1,185,000 gal Aerobic digester Net input: 2025 lb/day
X
v a
= 4 0 0 0 mg/liter
Sludge oxidized: 2 0 2 5 lb/day (b)
Fig. 7.3. Diagrams
to illustrate
concept
of sludge
age.
F o r higher values of sludge age c o r r e s p o n d i n g t o a larger v o l u m e for t h e a e r o b i c d i g e s t e r , it is p o s s i b l e t o o x i d i z e o v e r 9 0 % o f t h e d e g r a d a b l e V S S . T h i s m e a n s less s l u d g e for d i s p o s a l , t h o u g h a t t h e e x p e n s e o f a h i g h e r i n v e s t m e n t for t h e d i g e s t e r . A n e c o n o m i c s t u d y c a n b e m a d e t o d e t e r m i n e t h e o p t i m u m f r a c t i o n o f d e g r a d a b l e s o l i d s t o b e o x i d i z e d in t h e d i g e s t e r .
2.4. L A B O R A T O R Y S C A L E B A T C H R E A C T O R TO OBTAIN B A S I C D E S I G N DATA FOR AEROBIC DIGESTERS A t y p i c a l b a t c h l a b o r a t o r y scale r e a c t o r utilized f o r o b t a i n i n g d e s i g n d a t a for a e r o b i c d i g e s t e r s is s h o w n in F i g . 5.2. E v e n w h e n d e s i g n i n g a c o n t i n u o u s a e r o b i c d i g e s t e r a s s h o w n in F i g . 7.2, t h e l a b o r a t o r y scale b a t c h r e a c t o r is needed t o o b t a i n design i n f o r m a t i o n , because residence times required for a e r o b i c d i g e s t i o n o f s l u d g e s a r e h i g h . R e q u i r e d flow r a t e s for c o n t i n u o u s o p e r a t i o n are impractically small for the low v o l u m e l a b o r a t o r y r e a c t o r a n d cannot be measured with reasonable accuracy. Therefore, continuous labora t o r y scale r e a c t o r s a s in F i g . 5.10 a r e n o t r e c o m m e n d e d . A c o n t i n u o u s digester operates, a s s u m i n g steady state conditions, with a c o n s t a n t c o n c e n t r a t i o n o f s u s p e n d e d s o l i d s . F o r t h e b a t c h l a b o r a t o r y scale u n i t utilized t o s i m u l a t e t h e p l a n t scale c o n t i n u o u s r e a c t o r , c o n c e n t r a t i o n o f suspended solids decreases with t i m e because of the g r a d u a l oxidation of V S S .
302
7.
Sludge Treatment and Disposal
F o r e x t r a p o l a t i o n o f b a t c h l a b o r a t o r y scale d a t a t o a p l a n t scale c o n t i n u o u s reactor, o n e should w o r k with a n average value of the VSS c o n c e n t r a t i o n t o s i m u l a t e t h e p r e v a i l i n g c o n s t a n t V S S c o n c e n t r a t i o n in t h e c o n t i n u o u s d i g e s t e r operating a t steady state conditions. Similarly, the oxygen u p t a k e rate for the b a t c h r e a c t o r d e c r e a s e s w i t h t i m e . A n a v e r a g e v a l u e is t a k e n t o s i m u l a t e t h e p r e v a i l i n g c o n s t a n t o x y g e n u p t a k e r a t e in t h e c o n t i n u o u s d i g e s t e r o p e r a t i n g a t steady state conditions. T h e calculation technique for utilizing b a t c h reactor d a t a in d e s i g n i n g a c o n t i n u o u s d i g e s t e r is i l l u s t r a t e d in E x a m p l e 7 . 1 . F u n d a m e n t a l d e s i g n i n f o r m a t i o n o b t a i n e d f r o m t h e b a t c h l a b o r a t o r y scale r e a c t o r is (1) s u s p e n d e d solids v s . s l u d g e a g e ( d a y s ) a n d (2) o x y g e n u p t a k e r a t e [mg/(liter)(hr)] vs. sludge age (days). Several units each with a capacity of a b o u t 2 liters a r e u s e d s i m u l t a n e o u s l y , a n d r e s u l t s o b t a i n e d a r e a v e r a g e d . T h e u n i t s a r e filled w i t h s l u d g e w i t h a n initial c o n c e n t r a t i o n w h i c h e n c o m p a s s e s the range which might be expected in the p r o p o s e d digester unit. Samples are w i t h d r a w n a t selected t i m e i n t e r v a l s , a n d V S S c o n c e n t r a t i o n s a n d o x y g e n uptake rates are determined. C a l c u l a t e d d e s i g n v a l u e s a r e (1) r e q u i r e d r e s i d e n c e t i m e ( a n d v o l u m e o f t h e d i g e s t e r ) a n d (2) o x y g e n r e q u i r e m e n t s ( l b / h r ) f r o m w h i c h n e e d e d H P is c a l c u l a t e d a n d a e r a t o r s specified.
2.5. D E S I G N P R O C E D U R E F O R A E R O B I C D I G E S T E R S O F S L U D G E [1] T h e p r o c e d u r e for u t i l i z i n g l a b o r a t o r y d a t a for t h i s d e s i g n is p r e s e n t e d i n E x a m p l e 7 . 1 . D e s i g n for a c o n t i n u o u s d i g e s t e r is i l l u s t r a t e d f r o m d a t a o b t a i n e d in a l a b o r a t o r y b a t c h r e a c t o r . F o r t h e d e s i g n o f a b a t c h r e a c t o r t h e p r o c e d u r e is s i m p l e r , b e c a u s e t h e l a b o r a t o r y scale b a t c h u n i t is a n a c t u a l m i n i a t u r e o f t h e p l a n t scale digester. T h e r a t e o f d e g r a d a t i o n o f s l u d g e is t e m p e r a t u r e d e p e n d e n t . F o r a c o n servative design, l a b o r a t o r y d a t a should be o b t a i n e d at the lowest t e m p e r a t u r e a n t i c i p a t e d i n t h e field.
Example 7.1 D a t a i n T a b l e 7.1 w e r e o b t a i n e d f r o m a l a b o r a t o r y b a t c h r e a c t o r . 5 0 0 0 l b / d a y o f s l u d g e a r e t o b e d i g e s t e d a n d it is d e s i r e d t o d e s t r o y 9 0 % o f d e g r a d a b l e VSS. A s s u m e a n operating steady state concentration of 4000 mg/liter of VSS for t h e c o n t i n u o u s d i g e s t e r t o b e d e s i g n e d . C a l c u l a t e (1) d i g e s t e r v o l u m e (gal) a n d (2) o x y g e n r e q u i r e m e n t ( l b / d a y ) . S O L U T I O N : Part 1 Step
D i g e s t e r v o l u m e (gal)
1. P l o t V S S ( m g / l i t e r ) [ c o l u m n (2), T a b l e 7.1] v s . t i m e o f a e r a t i o n
[ c o l u m n (7), T a b l e 7 . 1 ] , a s s h o w n in F i g . 7.4. T h e c u r v e is a s y m p t o t i c t o a V S S v a l u e e s t i m a t e d a t 3050 m g / l i t e r , w h i c h c o r r e s p o n d s t o n o n d e g r a d a b l e V S S in t h e s l u d g e .
2.
303
Digestion of S l u d g e s
T A B L E 7.1 Data for Example 7.1
ω Time o f aeration (days)
(2) VSS (mg/liter)
(3) R , oxygen uptake rate [mg/(liter)(hr)]
0 1 2 5 7 10 15 20 25
5550 5200 4950 4420 4170 3870 3500 3260 3200
35.0 28.0 19.0 16.0 12.5 8.8 6.1 4.2
r
8000i
7000 [Labon itory da a] 1
Κ 6000
@5000 c ε
ο 4000 ,ο,
I 3000 ο» Ε
co
£2000 1000
ί i
If 1 mρ
§
m mΜ 5
10
15
20
25
30
35
Aeration time (days) [Column φ , table 7l] Fig. 7.4.
VSS concentration
vs. aeration
time (Example
7.1).
Step 2. O b t a i n t h e o x i d i z a b l e V S S r e m a i n i n g a t a n y t i m e b y s u b t r a c t i n g 3 0 5 0 m g / l i t e r f r o m t h e v a l u e s in c o l u m n (2) o f T a b l e 7 . 1 . T h e r e s u l t is s h o w n i n T a b l e 7.2 [ c o l u m n ( 2 ) ] .
304
7.
S l u d g e Treatment and Disposal
T A B L E 7.2 Degradable V S S Remaining vs. Time of Aeration (Example 7.1)
ω Time o f aeration (days)
(2) Degradable VSS remaining (mg/liter)
0 1 2 5 7 10 15 20 25
2500 2150 1900 1370 1120 820 450 210 150
F r a c t i o n o f n o n d e g r a d a b l e s l u d g e is 3 0 5 0 / 5 5 5 0 = 0.55 ( o r 5 5 . 0 % ) , a n d t h a t o f d e g r a d a b l e s l u d g e is 1—0.55 = 0.45 ( o r 4 5 . 0 % ) . A t t i m e z e r o , c o n c e n t r a t i o n o f d e g r a d a b l e s l u d g e is 2 5 0 0 m g / l i t e r [first e n t r y in c o l u m n (2), T a b l e 7 . 2 ] . I t is d e s i r e d t o oxidize 9 0 % o f t h i s s l u d g e , w h i c h m e a n s t h a t t h e d e g r a d a b l e V S S r e m a i n i n g is 1 0 % o f 2 5 0 0 m g / l i t e r , o r ( 0 . 1 ) ( 2 5 0 0 ) = 2 5 0 m g / l i t e r . Step 3. D e t e r m i n e s l u d g e a g e ( d a y s ) for o x i d i z a b l e V S S r e m a i n i n g t o b e 2 5 0 m g / l i t e r . F o r c o n v e n i e n t i n t e r p o l a t i o n p l o t c o l u m n (2) vs. c o l u m n ( / ) (of T a b l e 7.2) o n s e m i l o g p a p e r , a s s h o w n in F i g . 7.5. F o r a n o r d i n a t e o f 2 5 0 m g / l i t e r , a s l u d g e a g e o f 19.5 d a y s is r e a d . Step 4. F r o m i n f o r m a t i o n o b t a i n e d in t h e p r e v i o u s s t e p s ( u t i l i z i n g l a b o r a t o r y d a t a o n l y ) , t h e m a t e r i a l b a l a n c e for t h e p l a n t scale a e r o b i c d i g e s t e r is w r i t t e n a s f o l l o w s : Sludge I N : 5000lb/day Sludge O U T : Nondegradable V S S : (0.55)(5000) =
2750
F r a c t i o n o f d e g r a d a b l e s l u d g e w h i c h is not o x i d i z e d : d e g r a d a b l e sludge = 5 0 0 0 - 2 7 5 0 = 2250 lb/day Fraction not oxidized: (0.1) (2250) = Total sludge output:
225 2975 lb/day
T h i s m a t e r i a l b a l a n c e is s h o w n in F i g . 7 . 3 , w h i c h w a s u s e d t o i l l u s t r a t e t h e concept of sludge age. Step 5. C a l c u l a t e t h e d i g e s t e r v o l u m e f r o m E q . (7.2), w h e r e AX
net
= (sludge i n p u t - s l u d g e output) = 5 0 0 0 - 2975 = 2025 lb/day
2.
305
Digestion of Sludges
w h e r e -X
w%a
= 4 0 0 0 m g / l i t e r a n d t = 19.5 d a y s .
Therefore, K -
19.5 days χ 2025 lb sludge/day
=
m
4000 χ 1 0 " lb sludge/lb liquor χ 8.34 lb liquor/gal liquor 6
S O L U T I O N : Part 2
Oxygen requirement (lb/day)
B e f o r e m a k i n g a n e s t i m a t e o f o x y g e n r e q u i r e m e n t s for t h e c o n t i n u o u s d i g e s t e r , c a l c u l a t e a v e r a g e v a l u e s for V S S c o n c e n t r a t i o n a n d o x y g e n u p t a k e r a t e for t h e l a b o r a t o r y u n i t o v e r a p e r i o d o f 19.5 d a y s . Step 1. C a l c u l a t e t h e a v e r a g e V S S c o n c e n t r a t i o n f o r a l a b o r a t o r y u n i t . A v e r a g e V S S c o n c e n t r a t i o n is c a l c u l a t e d f r o m F i g . 7.4. F i r s t d e t e r m i n e b y graphical integration the area b o u n d by the curve from the abscissa zero to 19.5 d a y s .
J
*f = 1 9 . 5 d a y s
*=o
(VSS concentration) dt = 78,400 (mg/liter) χ day
7.
306
Sludge Treatment and Disposal
A v e r a g e V S S c o n c e n t r a t i o n is t h e n /V=19.5days
Average VSS concentration = [ l / ( / - 0 ) ]
(VSS concentration)*// Jt=o
J
V = 19.5days
(VSS concentration)*//
f=0
= (1/19.5)78,400 = 4020 mg/liter Step
2. C a l c u l a t e t h e a v e r a g e o x y g e n u p t a k e r a t e for a l a b o r a t o r y u n i t .
O x y g e n u p t a k e r a t e [ c o l u m n (3) o f T a b l e 7 . 1 ] is p l o t t e d v s . t i m e o f a e r a t i o n [ c o l u m n (7) o f T a b l e 7 . 1 ] . T h i s is s h o w n in F i g . 7.6. D e t e r m i n e b y g r a p h i c a l i n t e g r a t i o n t h e a r e a b o u n d b y t h e c u r v e f r o m t h e a b s c i s s a z e r o t o 19.5 d a y s . /•/ = 1 9 . 5 d a y s
Area =
R dt r
Jt=o
45i
1
Ο
5
Fig. 7.6. estimate
1
= 303 m g 0 / ( l i t e r ) (hr) χ days 2
1
1
1
1
10 15 20 25 30 Time of aeration [Column®, table 7.0 of average
oxygen
uptake
rate (Example
1
35
7.1).
2.
Digestion of Sludges
307
or χ days χ 24 h r / d a y = 7272 m g 0 / l i t e r 2
A v e r a g e d a i l y o x y g e n u p t a k e r a t e is t h e n »i = 1 9 . 5 d a y s
»r = 1 9 . 5 d a y s
R dt
Averages = [l/(i-0)]
=
r
Jt=o
t=0
]
R dt r
= 1/19.5 days χ 7272 m g 0 / l i t e r 2
= 373 m g 0 / ( l i t e r ) ( d a y ) 2
Step 3. E s t i m a t e o x y g e n r e q u i r e m e n t s for a c o n t i n u o u s a e r o b i c d i g e s t e r . A v e r a g e o x y g e n u t i l i z a t i o n r a t e for t h e l a b o r a t o r y b a t c h u n i t is 373 m g 0 / ( l i t e r ) ( d a y ) 2
= 373 χ
10- lbO /(lbliquor)(day) 6
2
= 373 χ 1 0 ~ lb 0 / ( l b liquor)(day) χ 8.34 lb liquor/gal liquor 6
2
= 3.11 χ 1 0 " lb 0 / ( g a l liquor)(day) 3
2
C a l c u l a t e d a v e r a g e V S S c o n c e n t r a t i o n for t h e l a b o r a t o r y b a t c h u n i t is 4 0 2 0 m g / l i t e r ( S o l u t i o n , P a r t 2 , S t e p 1), w h e r e a s V S S c o n c e n t r a t i o n a t s t e a d y s t a t e c o n d i t i o n s for t h e c o n t i n u o u s a e r o b i c d i g e s t e r is 4 0 0 0 m g / l i t e r . O x y g e n r e q u i r e m e n t s for the c o n t i n u o u s a e r o b i c reactor a r e estimated a s s u m i n g proportionality between oxygen utilization a n d VSS concentration. 3.11 χ Ι Ο " lb 0 / ( g a l liquor)(day) χ 1.185 χ 1 0 gal liquor χ 4000/4020 3
6
2
= 3667 lb 0 / d a y 2
w h e r e 4 0 0 0 / 4 0 2 0 is t h e c o r r e c t i o n f a c t o r r e q u i r e d for t r a n s l a t i n g d a t a o b t a i n e d with the laboratory batch reactor to actual operational conditions with the c o n t i n u o u s reactor. H o r s e p o w e r requirements a n d layout for a e r a t o r s are o b t a i n e d b y t h e p r o c e d u r e d i s c u s s e d in C h a p t e r 4 , S e c t i o n s 1 4 - 1 6 .
2.6. I N T R O D U C T I O N T O A N A E R O B I C SLUDGE DIGESTION I t h a s b e e n k n o w n f o r a t l e a s t a c e n t u r y t h a t if s e t t l e d s e w a g e s o l i d s a r e k e p t in a c l o s e d t a n k for a p e r i o d o f t i m e , t h e y a r e c o n v e r t e d t o a l i q u i d s t a t e a n d a c o m b u s t i b l e g a s c o n t a i n i n g m e t h a n e is g e n e r a t e d . A p a t e n t w a s i s s u e d t o I m h o f T i n 1904 f o r t h e d e s i g n o f a n a e r o b i c d i g e s t i o n vessels, w h i c h a r e k n o w n as Imhoff tanks. M o s t sludge digestion processes in use t o d a y are anaerobic, a l t h o u g h a e r o b i c d i g e s t i o n is i n c r e a s i n g l y m o r e p o p u l a r , especially for s m a l l units.
308
7.
Sludge Treatment and Disposal
A n a e r o b i c s l u d g e d i g e s t e r s a r e u s u a l l y o f t w o t y p e s : (1) single-stage d i g e s t e r s a n d (2) t w o - s t a g e d i g e s t e r s . A t y p i c a l single-stage s l u d g e d i g e s t e r is s h o w n in F i g . 7.7. Gas removal
Gas Supernatant layer
Supernatant layer outlet
Raw sludge feed
.Actively digesting —-—~ sludge
^
Sludge heater
Digested sludge;^
Fig. 7.7. Single-stage
anaerobic
sludge
digester.
R a w s l u d g e is fed t o t h e z o n e w h e r e t h e s l u d g e is actively d i g e s t i n g a n d g a s is p r o d u c e d . A s t h e g a s rises, it lifts s l u d g e p a r t i c l e s a n d o t h e r m a t e r i a l s ( g r e a s e , oil, fats), f o r m i n g a s u p e r n a t a n t l a y e r w h i c h is d r a w n off f r o m t h e d i g e s t e r . D i g e s t e d s l u d g e is w i t h d r a w n f r o m t h e b o t t o m o f t h e t a n k . T h e d i g e s t i o n p r o c e s s is f a v o r e d b y h i g h t e m p e r a t u r e ( u s u a l l y f r o m 8 5 ° 1 0 5 ° F ) , so d i g e s t i n g s l u d g e is h e a t e d e i t h e r b y s t e a m coils w i t h i n t h e vessel o r b y m e a n s o f a n e x t e r n a l s l u d g e h e a t e r ( F i g . 7.7). G a s is r e m o v e d f r o m t h e t o p o f t h e d i g e s t e r a n d o f t e n utilized a s fuel, o w i n g t o its h i g h c o n t e n t o f m e t h a n e . T h e p u r p o s e o f t h e t w o - s t a g e u n i t is f u n d a m e n t a l l y t o p r o v i d e a b e t t e r v o l u m e Gas removal Gas
Gas
Row sludge_ feed
XIiV.f
-ι .1
Ί-
Supernatant — layer Sludge heater
Supernatant layer outlet
Digested _ sludge
outlet Stage I Fig. 7.8.
Two-stage
Stage 2 anaerobic
sludge
digester.
3.
309
Thickening of Sludges
u t i l i z a t i o n . V o l u m e u t i l i z a t i o n f o r t h e s i n g l e - s t a g e u n i t is p o o r , o w i n g t o s t r a t i f i c a t i o n a n d p o o r m i x i n g . S t a g e 1 is u s e d for d i g e s t i o n . I t is h e a t e d a n d m i x i n g is p r o v i d e d e i t h e r m e c h a n i c a l l y o r b y m e a n s o f g a s r e c i r c u l a t i o n . S t a g e 2 is u s e d for s t o r a g e a n d s e p a r a t i o n o f d i g e s t e d s l u d g e a n d t h e s u p e r n a t a n t layer.
2.7. S I Z I N G O F A N A E R O B I C S L U D G E D I G E S T E R S L a b o r a t o r y s i m u l a t i o n for a n a e r o b i c d i g e s t i o n o f s l u d g e s c a n b e m a d e b y t h e s a m e p r o c e d u r e d e s c r i b e d for a e r o b i c d i g e s t e r s , e x c e p t t h a t a e r a t i o n is n o t p r o v i d e d . C u r v e s s i m i l a r t o t h e o n e s h o w n i n F i g . 7.4 a r e o b t a i n e d f r o m l a b o r a t o r y s i m u l a t i o n , a n d t h e d i g e s t e r v o l u m e is c a l c u l a t e d b y a p r o c e d u r e s i m i l a r t o t h a t for a e r o b i c d i g e s t e r s ( E x a m p l e 7.1). E m p i r i c a l m e t h o d s a r e u s u a l l y e m p l o y e d for sizing a n a e r o b i c s l u d g e digesters. S o m e of these m e t h o d s are based o n the p o p u l a t i o n served by the s e w a g e s y s t e m (for d o m e s t i c s l u d g e ) o r o n r e c o m m e n d e d v a l u e s o f s l u d g e a g e . T h e latter values are temperature dependent, decreasing with increase of o p e r a t i n g t e m p e r a t u r e . T a b l e 7.3 p r e s e n t s s u g g e s t e d s l u d g e a g e v a l u e s a s a f u n c t i o n o f t e m p e r a t u r e . V o l u m e ( S C F M ) o f g a s g e n e r a t e d is e s t i m a t e d f r o m E q . (6.82). T A B L E 7.3 Recommended Values of Sludge A g e for Anaerobic Sludge Digester [3] Temperature (°F)
Suggested sludge age t (days)
65 75 85 95 105
28 20 14 10 10
s
3. Thickening of S l u d g e s 3.1. I N T R O D U C T I O N T h i c k e n i n g is t h e u s u a l first s t e p i n s l u d g e d i s p o s a l p r o c e s s i n g . I t c a n b e d o n e (1) b y g r a v i t y a n d (2) b y d i s s o l v e d a i r flotation.
3.2. A D V A N T A G E S O F T H I C K E N I N G 1. I t i m p r o v e s d i g e s t e r o p e r a t i o n a n d r e d u c e s c a p i t a l c o s t w h e n e v e r s l u d g e d i g e s t i o n is utilized.
7.
310
Sludge Treatment and Disposal
2. It r e d u c e s s l u d g e v o l u m e p r i o r t o l a n d o r sea d i s p o s a l . 3. I t i n c r e a s e s e c o n o m y
of sludge dewatering systems
(centrifuges,
v a c u u m filters, p r e s s u r e filters, etc.).
3.3. G R A V I T Y
THICKENER
Thickeners are t a n k s of circular cross section provided with a r o t a t i n g rake m e c h a n i s m s i m i l a r t o t h e clarifiers d i s c u s s e d in C h a p t e r 3, S e c t i o n 3.7.
3.4. D E S I G N P R I N C I P L E S F O R G R A V I T Y THICKENERS T h e p u r p o s e o f t h i c k e n e r s is t o p r o v i d e a c o n c e n t r a t e d s l u d g e u n d e r f l o w . A f u n d a m e n t a l p a r a m e t e r is t h i c k e n e r a r e a r e q u i r e d for a specific u n d e r f l o w c o n c e n t r a t i o n . T h i s is e x p r e s s e d in t e r m s o f t h e u n i t a r e a UA, defined a s [ E q . (7.3)] υ A = ft /(lb/day) = (ft )(day)/(lb) 2
(7.3)
2
[i.e., t h e a r e a ( f t ) r e q u i r e d p e r l b / d a y o f s l u d g e in t h e i n f l u e n t ] . T h e r e c i p r o c a l 2
o f t h e u n i t a r e a , w h i c h is t e r m e d m a s s l o a d i n g (ML) ML = l/UA
= (lb/day)/ft
2
is
= lb/(day)(ft )
(7.4)
2
T h e f u n d a m e n t a l design p r o b l e m consists of calculating the thickener area f r o m t h e k n o w l e d g e o f (1) flow r a t e ( M g a l / d a y ) o f s l u d g e of initial c o n c e n t r a tion C
( m g / l i t e r ) a n d (2) d e s i r e d final u n d e r f l o w c o n c e n t r a t i o n C
0
u
(mg/liter).
O n c e t h e v a l u e o f UA ( o r ML) is d e t e r m i n e d , t h e a r e a ( f t ) o f t h e t h i c k e n e r is 2
given in E q . (7.5). Area
(ft ) = (lb/day of influent) χ UA
[from Eq. (7.3)]
2
(7.5)
or Area
(ft ) = (lb/day of influent/ML) 2
[from Eq. (7.4)]
(7.6)
T h e p r o c e d u r e d i s c u s s e d in C h a p t e r 3, S e c t i o n 3.6 m a y b e utilized for d e s i g n o f t h i c k e n e r s . A specific m e t h o d for t h i c k e n e r d e s i g n b a s e d o n a n empirical equation
proposed
by Edde
and
Eckenfelder
is d e s c r i b e d
in
S e c t i o n 3.5.
3.5. E D D E A N D E C K E N F E L D E R ' S E Q U A T I O N
[2]
T h e degree t o which sludges can be thickened depends o n m a n y variables. A c o r r e l a t i o n d e v e l o p e d b y E c k e n f e l d e r a n d E d d e is [ ( C . / C o ) - 1 ] = KI(ML)» where C
u
= K/d/UAT
is t h e u n d e r f l o w c o n c e n t r a t i o n ( m g / l i t e r ) ; C
(7.7) t h e initial feed c o n
0
c e n t r a t i o n ( m g / l i t e r ) ; ML t h e m a s s l o a d i n g [ ( l b / d a y ) / f t ] ; UA t h e u n i t a r e a 2
Dewatering of Sludges
4.
311
[ f t / ( l b / d a y ) ] ; a n d Κ a n d η c o n s t a n t s . C o n s t a n t η is o n l y a f u n c t i o n o f t h e 2
r h e o l o g i c a l p r o p e r t i e s o f t h e s l u d g e . C o n s t a n t Κ is r e l a t e d n o t o n l y t o t h e initial c o n c e n t r a t i o n C
0
b u t also t o the height of settling c o l u m n .
E q u a t i o n (7.7) is w r i t t e n in l i n e a r f o r m b y t a k i n g l o g a r i t h m s o f b o t h members. l o g [ ( C / C ) - l ] = log A: - A*log(ML) = XogK - n\og(\IUA)
(7.8)
F r o m E q . (7.8) a p l o t o n l o g - l o g p a p e r o f [ ( C / C ) - 1 ] v s . (l/UA)
[or
u
0
M
0
( M L ) ] yields a s t r a i g h t line o f s l o p e = — η a n d i n t e r c e p t = K. A p r o c e d u r e f o r d e s i g n o f g r a v i t y t h i c k e n e r s s t a r t i n g f r o m d a t a o b t a i n e d f r o m b e n c h scale u n i t s a n d b a s e d o n E d d e a n d E c k e n f e l d e r ' s e q u a t i o n is d e s c r i b e d b y E c k e n felder a n d F o r d [ 1 ] .
3.6. FLOTATION T H I C K E N I N G F l o t a t i o n t h i c k e n i n g c a n b e utilized f o r s l u d g e s a n d is specifically r e c o m m e n d e d for gelatinous ones such as t h o s e from t h e activated sludge process. D e s i g n o f flotation e q u i p m e n t is d i s c u s s e d i n C h a p t e r 3 , S e c t i o n 4 .
4. D e w a t e r i n g of S l u d g e s by V a c u u m Filtration 4.1. I N T R O D U C T I O N V a c u u m filtration is t h e m o s t w i d e l y u s e d p r o c e d u r e f o r d e w a t e r i n g w a s t e w a t e r s l u d g e s . I n v a c u u m f i l t r a t i o n , w a t e r is r e m o v e d u n d e r a p p l i e d v a c u u m t h r o u g h a p o r o u s m e d i a which retains solids b u t allows liquids t o p a s s t h r o u g h . S e v e r a l t y p e s o f m e d i a a r e u s e d , s u c h a s n y l o n a n d d a c r o n c l o t h , steel m e s h , a n d t i g h t l y w o u n d stainless steel coil s p r i n g s . T h e c e n t r a l u n i t is a r o t a r y d r u m w h i c h r e v o l v e s i n a s l u r r y t a n k ( F i g . 7.9). V a c u u m is a p p l i e d t o t h e s u b m e r g e d p a r t o f t h e d r u m a n d s o l i d s a r e r e t a i n e d
Fig. 7.9. Diagram
of rotary
filter.
7.
312
Sludge Treatment and Disposal
o n t h e d r u m surface. T h e c a k e s t a r t s b u i l d i n g u p a t p o i n t A a s t h e d r u m d i v e s i n t o t h e s l u r r y t a n k , a n d r e a c h e s full t h i c k n e s s a t p o i n t Β a s t h e d r u m e m e r g e s . T i m e e l a p s e d f r o m A t o Β (i.e., s u b m e r g e n c e t i m e ) is d e s i g n a t e d a s f o r m t i m e (t ). f
F r o m Β t o A t h e c a k e is d e w a t e r e d , a n d t i m e e l a p s e d is d e s i g n a t e d a s d r y
t i m e (t ). T h e t o t a l cycle t i m e (t ) is d
c
t = t + t c
f
d
(7.9)
Since the d r u m revolves at a c o n s t a n t speed, the following relationship applies [Eq. (7.10)]: tfltc = t l(t f
f
+ t ) = (% submergence)/100 d
(7.10)
U s u a l l y d r u m s u b m e r g e n c e b e t w e e n 10 a n d 6 0 % is e m p l o y e d , i.e., 0.6/ > t > c
f
0.1/c
(7.11)
T h e r e f o r e , f o r m t i m e is u s u a l l y b e t w e e n 10 a n d 6 0 % o f t o t a l cycle t i m e . After being dried by liquid transfer t o air d r a w n t h r o u g h the cake by the a p p l i e d v a c u u m , t h e c a k e is r e m o v e d b y a knife e d g e o n t o a c o n v e y o r b e l t t o d i s p o s a l facilities. T h e filter m e d i a is t h e n w a s h e d b y a w a t e r s p r a y p r i o r t o b e i n g i m m e r s e d a g a i n in t h e s l u r r y t a n k .
4.2. V A R I A B L E S I N V A C U U M F I L T R A T I O N V a r i a b l e s t o b e c o n s i d e r e d fall i n t o t w o g r o u p s : t h o s e r e l a t e d t o s l u d g e c h a r a c t e r i s t i c s a n d t h o s e p e r t a i n i n g t o filter o p e r a t i o n . V a r i a b l e s r e l a t e d t o sludge characteristics are as follows: 1. S o l i d s c o n c e n t r a t i o n in t h e s l u d g e . T h i s is d e t e r m i n e d b y t h e n a t u r e o f t h e s l u d g e (i.e., p r i m a r y o r s e c o n d a r y , d o m e s t i c o r i n d u s t r i a l ) a n d b y t h i c k e n i n g p r o c e s s e s p r e c e d i n g t h e filtration s t e p . 2. Viscosity o f s l u d g e a n d filtrate, t h e l a t t e r b e i n g a p p r o x i m a t e l y t h e s a m e a s t h a t for w a t e r a t s i m i l a r t e m p e r a t u r e s . 3. S l u d g e c o m p r e s s i b i l i t y , w h i c h is r e l a t e d t o t h e n a t u r e o f t h e s l u d g e particles. 4. C h e m i c a l a n d p h y s i c a l c o m p o s i t i o n , i n c l u d i n g p a r t i c l e size a n d s h a p e , water content. Filter operation variables are 1. Operating vacuum. U s u a l l y f r o m 10 t o 2 0 in. o f m e r c u r y . H i g h e r v a c u u m s a r e m o r e effective w i t h i n c o m p r e s s i b l e c a k e s . F o r v e r y c o m p r e s s i b l e c a k e s , a p p l y i n g h i g h e r v a c u u m s m a y r e s u l t in c l o g g i n g t h e filter m e d i a . 2. Drum submergence. T h i s v a r i e s f r o m 10 t o 6 0 % , h i g h p o r o s i t y s l u d g e s p e r m i t t i n g h i g h e r s u b m e r g e n c e s . S l u d g e s o f l o w p o r o s i t y m u s t b e filtered w i t h low submergence, otherwise the resulting c o m p a c t a n d thick cake does n o t a l l o w a n a d e q u a t e flow o f filtrate.
4.
313
Dewatering of Sludges
3. Sludge conditioning by chemical addition. M a n y sludges require c o a g u l a n t s (e.g., F e C l , l i m e , p o l y e l e c t r o l y t e s ) t o c o a g u l a t e s m a l l e r p a r t i c l e s w h i c h m i g h t o t h e r w i s e c l o g t h e filter m e d i a , r e s u l t i n g in r e d u c t i o n o f filtration rate. 4. Type and porosity of the filter media. H i g h p o r o s i t y m e d i a r e s u l t s i n h i g h e r filtration r a t e s . 3
4.3. D E F I N I T I O N O F P A R A M E T E R c F i l t r a t i o n e q u a t i o n s in S e c t i o n 4 . 4 e m p l o y a p a r a m e t e r c, w h i c h is defined a s t h e m a s s o f solids d e p o s i t e d o n t h e filter p e r u n i t v o l u m e o f filtrate. A n e x p r e s s i o n f o r c is d e r i v e d a s f o l l o w s : L e t c b e t h e c o n c e n t r a t i o n o f s o l i d s i n t h e i n c o m i n g s l u r r y , e x p r e s s e d a s g r a m s o f s o l i d s p e r m l o f s l u r r y . S l u r r y is filtered in a l a b o r a t o r y B u c h n e r filter ( F i g . 7.10). F u r t h e r m o r e , a s s u m e t h a t for t h e filtrate t h e c o n c e n t r a t i o n o f s o l i d s is r e d u c e d t o a v a l u e i n d i c a t e d a s c ( a l s o e x p r e s s e d a s g r a m s o f solids p e r milliliter). P r e s u m a b l y c <ζ c . f
f
f
C =l/[(I-C )/C f
Fig. 7.10. Diagram
to illustrate
f
{
-(l-c,)/c,]
derivation
of Eq.
(7.12).
Since t h e s l u r r y is relatively d i l u t e d , a s s u m e t h a t t h e d e n s i t y is a p p r o x i m a t e l y t h a t o f w a t e r , i.e., 1 g p e r m l . T h u s , 1 g o f feed t o t h e filter c o n t a i n s c g r a m o f solid a n d (1 — c ) g r a m o f w a t e r . T h e m l o f w a t e r p e r g r a m o f s o l i d s i n t h e i n c o m i n g s l u r r y a r e t h e n (1 - q ) / ^ ( m l w a t e r / g s o l i d ) . S i m i l a r l y f o r t h e filtrate o n e h a s (l—c )/c ( m l w a t e r / g s o l i d ) . T h e difference b e t w e e n t h e s e t w o v a l u e s is f
t
f
f
(\-c,)lc -{\-c,)lc f
t
(ml water/g solid)
T h e i n v e r s e o f t h i s q u a n t i t y c o r r e s p o n d s t o p a r a m e t e r c, i.e., m a s s o f s o l i d s d e p o s i t e d p e r u n i t v o l u m e o f filtrate [ E q . ( 7 . 1 2 ) ] . c =
l/[(l-c,)/cr-(l-c )/o] (
(7.12)
314
7.
Sludge Treatment and Disposal
4.4. F I L T R A T I O N E Q U A T I O N S T h e b a s i c filtration e q u a t i o n d e r i v e d f r o m t h e Poiseuille a n d d ' A r c y l a w is p r e s e n t e d in E q . (7.13). dVldt
= AP/(R
+
cake
i?
medla
w h e r e Κ is t h e v o l u m e o f f i l t r a t e ; t t h e t i m e ; dV/dt
)
(7.13)
the rate of
filtration;
d r i v i n g f o r c e = p r e s s u r e difference. I f e x p r e s s e d i n psi AP e q u a l t o a p p l i e d v a c u u m in g a u g e u n i t s , b e c a u s e AP
=
H e n c e , it is i n d i c a t e d a s Ρ = a p p l i e d v a c u u m ( p s i ) ; R t h e flow o f
m e d i a
Vacuum-^atmospheric ·
t h e r e s i s t a n c e offered
cake
b y c a k e t o t h e flow o f filtrate; a n d ^
AP t h e
is n u m e r i c a l l y
t h e r e s i s t a n c e offered b y m e d i a t o
filtrate.
T h i s e q u a t i o n is r e w r i t t e n a s [ 5 ] dV/dt
=
(7.14)
PA /fr(rcV+R A)] 2
m
w h e r e V is t h e v o l u m e o f filtrate; t t h e t i m e ; Ρ t h e a p p l i e d v a c u u m ; A t h e filter a r e a ; c t h e p a r a m e t e r defined in S e c t i o n 4 . 3 , i.e., m a s s o f solids d e p o s i t e d p e r u n i t v o l u m e o f filtrate. C o n s e q u e n t l y p r o d u c t cV is t h e m a s s o f c a k e (lb) c o r r e s p o n d i n g t o v o l u m e V o f filtrate; r t h e specific r e s i s t a n c e o f c a k e t o t h e flow of filtrate (i.e., r e s i s t a n c e p e r l b o f c a k e ) . P h y s i c a l significance o f r a n d its u n i t s is d i s c u s s e d in t h i s s e c t i o n ; R t h e initial r e s i s t a n c e o f t h e filter m e d i a . T h i s r e s i s t a n c e is u s u a l l y negligible a s c o m p a r e d t o t h a t d e v e l o p e d b y t h e filter c a k e ; a n d μ t h e viscosity o f t h e filtrate. m
T h e p h y s i c a l significance o f p a r a m e t e r r c a n b e a p p r e c i a t e d if in E q . (7.14) m e d i a r e s i s t a n c e R is n e g l e c t e d . S o l v i n g f o r r, m
r =
(7.15)
PA /UicV(dV/dt)] 2
F r o m E q . (7.15) it follows t h a t r is n u m e r i c a l l y e q u a l t o t h e p r e s s u r e difference ( a p p l i e d v a c u u m P) dVjdt
required to p r o d u c e a unit rate of
filtrate
flow
(i.e.,
= 1.0) t h r o u g h a u n i t m a s s o f c a k e (i.e., cV = 1.0) a n d a u n i t y filter a r e a
(A = 1), if filtrate viscosity is u n i t y (μ = 1, e.g., 1 c P ) o r r = Ρ if dV/dt
= 1.0,
cV = 1.0, μ = 1.0, a n d A = 1.0. T h u s , t h e specific r e s i s t a n c e r m e a s u r e s t h e a b i l i t y o f t h e s l u d g e t o b e filtered; t h e h i g h e r t h e v a l u e , t h e m o r e difficult is t h e filtration. I n t e g r a t i o n o f E q . (7.14) is u s u a l l y p e r f o r m e d
assuming that
specific
r e s i s t a n c e is c o n s t a n t t h r o u g h o u t f o r m t i m e . F r o m E q . (7.14) if a t / = 0, V = 0 a n d a t t = t, V = K, i n t e g r a t i o n o f E q . 7.14 yields
A s s u m i n g t h e specific r e s i s t a n c e o f c a k e t o b e c o n s t a n t ,
4.
315
Dewatering of Sludges
or (p/A P)[rc(V /2) 2
+ R AV]
2
m
= t
Dividing both members by V a n d rearranging, jV = (prc/2PA )V+
pRJAP
2
t
(7.16)
F r o m E q . (7.16) it follows t h a t a p l o t o f t/V v s . V yields a s t r a i g h t l i n e . V a l u e s o f specific c a k e r e s i s t a n c e r a n d m e d i a r e s i s t a n c e R
m
are evaluated from the
s l o p e a n d i n t e r c e p t o f t h i s line, r e s p e c t i v e l y . r = R
m
=
(2PA /pc)s 2
(7.17)
iAP/μ
(7.18)
w h e r e s a n d / d e n o t e t h e s l o p e a n d t h e i n t e r c e p t o f t h e s t r a i g h t line. A t y p i c a l p l o t o f (t/V)
v s . Vis s h o w n in F i g . 7 . 1 3 . Specific r e s i s t a n c e is p r i m a r i l y useful
for c o m p a r i n g
filtration
c h a r a c t e r i s t i c s o f different s l u d g e s a n d d e t e r m i n i n g
o p t i m u m c o a g u l a n t r e q u i r e m e n t s t o p r o d u c e a c a k e offering a
minimum
r e s i s t a n c e ( S e c t i o n 4.5).
4.5. L A B O R A T O R Y D E T E R M I N A T I O N O F SPECIFIC RESISTANCE r A N D O P T I M U M COAGULANT DOSAGE L a b o r a t o r y d e t e r m i n a t i o n o f specific r e s i s t a n c e r is b a s e d o n c o n s t r u c t i o n o f a p l o t o f t/V v s . V a n d c a l c u l a t i o n o f r f r o m E q . (7.17). T h e l a b o r a t o r y e q u i p m e n t n e e d e d is a n o r d i n a r y B u c h n e r f u n n e l a p p a r a t u s ( F i g . 7.11). T h e p r o c e d u r e [ 1 ] is a s f o l l o w s : 1. P r e p a r e t h e B u c h n e r f u n n e l a n d filter p a p e r . 2. W e t filter p a p e r a n d a d j u s t v a c u u m t o 15 o r 2 0 in. o f H g . Buchner funnel-f Filter paper Λ Vacuum g a u g e y
Pig. 7.11.
Buchner
funnel
apparatus.
7.
316
Sludge Treatment and Disposal
3. R e c o r d filtrate v o l u m e s a t selected t i m e i n t e r v a l s u n t i l t h e v a c u u m b r e a k s . V a c u u m exists a s l o n g a s t h e r e is a p o o l o f l i q u i d o v e r t h e c a k e . A s s o o n a s t h e l i q u i d is d r a i n e d off, a i r is s u c k e d in a n d t h e v a c u u m b r e a k s . 4. M e a s u r e s o l i d s c o n t e n t in i n c o m i n g s l u r r y a n d filtrate b y e v a p o r a t i o n a n d w e i g h i n g . L e t t h e s e b e v a l u e s c (initial c o n c e n t r a t i o n , g / m l ) a n d c ( c o n c e n t r a t i o n in filtrate, g / m l ) . P a r a m e t e r c is t h e n c a l c u l a t e d f r o m E q . (7.12). {
f
5. C a l c u l a t e r f r o m a p l o t o f tjV vs. Κ u t i l i z i n g E q . (7.17). 6. R e p e a t S t e p s 1-5 u s i n g v a r i o u s c o n c e n t r a t i o n s o f c o a g u l a n t . D e p e n d i n g on the n a t u r e of the sludge, F e C l a n d / o r lime dosages are 2 - 1 0 % by weight a n d polyelectrolyte dosages 0.1-1.5% by weight. 3
7. C o m p u t e specific r e s i s t a n c e o f all s a m p l e s a s i n d i c a t e d in S t e p 5. D e t e r m i n e o p t i m u m c o a g u l a n t d o s a g e f r o m a p l o t o f specific r e s i s t a n c e v s . coagulant dosage. O p t i m u m dosage corresponds to the minimum on the specific r e s i s t a n c e c u r v e ( F i g . 7.12).
Coagulant dosage (lb/ton) Fig. specific
7.12.
Typical
curve
for
the
effect
of
coagulant
dosage
on
sludge
resistance.
4.6. UNITS FOR SPECIFIC RESISTANCE OF C A K E (r) T h e following observations should be m a d e : 1. A p p l i e d v a c u u m (in. H g ) is u s u a l l y c o n v e r t e d t o g / c m b e f o r e u t i l i z a 2
t i o n in E q . (7.17). T h e c o n v e r s i o n f a c t o r is (in. Hg)(34.5) = g / c m
2
(7.19)
If a p p l i e d v a c u u m is e x p r e s s e d in p s i , t h e c o n v e r s i o n f a c t o r is (psi)(70.1) = g / c m
2
(7.20)
4.
317
Dewatering of S l u d g e s
2. Specific r e s i s t a n c e r is u s u a l l y e x p r e s s e d in c m / g . F r o m F i g . 7 . 1 3 , 2
u n i t s for s l o p e s a r e (for t = sec, V = m l ) s = (t/V)IV
= (sec/ml)/ml « ( s e c / c m ) / c m 3
3
= sec/cm
6
U n i t s for c a r e c = g/ml « g / c m
3
a n d t h o s e for μ a r e μ = g/(cm)(sec) = Poise I f Ρ is e x p r e s s e d in g / c m
2
utilizing the conversion factors indicated in E q s .
(7.19) a n d (7.20), a n d t h e a r e a is i n c m , t h e n f r o m E q . (7.17) u n i t s f o r r a r e 2
g/cm χ c m 2
r
=
"77—77—χ
4
* sec/cm = sec /g 6
;—5
g/(cm)(sec)xg/cm
2
3
4.7. N U M E R I C A L E X A M P L E : D E T E R M I N A T I O N OF S P E C I F I C C A K E R E S I S T A N C E U S I N G THE B U C H N E R FUNNEL Example 7.2 T h e d a t a t a b u l a t e d b e l o w a r e o b t a i n e d f r o m a filtration a n activated sludge utilizing a B u c h n e r funnel a p p a r a t u s . V o l u m e o f filtrate (ml)
T i m e (sec)
25 50 75 100 125 150
48 150 308 520 788 1118
l a b o r a t o r y test for
O p e r a t i n g v a c u u m is 2 0 i n . o f H g a n d t e m p e r a t u r e is 2 5 ° C . A r e a o f t h e filter is 5 0 0 c m . A s s u m e t h a t t h e filtrate h a s t h e p r o p e r t i e s o f w a t e r a t 2 5 ° C (μ = 0.8953 c P ) . T a k e c = 0.2 g / c m . D e t e r m i n e t h e specific c a k e r e s i s t a n c e in sec /g. 2
3
2
Step 1. Set u p t h e f o l l o w i n g t a b u l a t i o n f r o m t h e d a t a g i v e n a b o v e . V (ml)
/ (sec)
tVj (sec/ml)
25 50 75 100 125 150
48 150 308 520 788 1118
1.92 3.00 4.11 5.20 6.30 7.45
318
7.
25
50
75
Sludge Treatment and Disposal
I00
I25
I50
V (ml) Fig.
7.13.
Plot
of t/V
vs. V (Example
7.2).
Step 2. P r e p a r e a p l o t o f t/V v s . V. T h i s is s h o w n in F i g . 7 . 1 3 . Step 3. F r o m F i g . 7 . 1 3 , Slope = s = (5.23 - 0 . 8 ) / ( 1 0 0 - 0.0) = 0.0443 s e c / c m
6
Since Ρ = ( 2 0 ) ( 3 4 . 5 ) = 6 9 0 g / c m , t h e n r is c a l c u l a t e d f r o m E q . (7.17). 2
2 χ 690 g / c m χ 5 0 0 c m χ 0.0443 s e c / c m 2
2
4
0.008953 g/(cm)(sec) χ 0.2 g / c m
6
3
8.53 χ 1 0 s e c / g 9
2
4.8. S P E C I F I C R E S I S T A N C E F O R COMPRESSIBLE CAKES M o s t i n d u s t r i a l w a s t e w a t e r s l u d g e s f o r m c o m p r e s s i b l e c a k e s for filtration
which
r a t e a n d specific r e s i s t a n c e a r e f u n c t i o n s o f t h e p r e s s u r e difference
a c r o s s t h e c a k e . T h i s effect is r e p r e s e n t e d b y E q . (7.21). r = rP
(7.21)
s
Q
w h e r e s is t h e coefficient o f c o m p r e s s i b i l i t y . T h e l a r g e r is s t h e m o r e c o m p r e s sible is t h e s l u d g e . W h e n s = 0, t h e specific r e s i s t a n c e is i n d e p e n d e n t o f p r e s s u r e a n d t h e s l u d g e is i n c o m p r e s s i b l e . E q . (7.21) yields 9
r = r = constant Q
Some generalizations on 1. E a s e o f
filterability
filtration
(7.22)
characteristics of sludges are as follows:
of wastewater sludges decreases with degree of
treatment. r < r < r raw primary secondary sewage sludge sludge sludge
4.
319
Dewatering of S l u d g e s
R a w s e w a g e s l u d g e is t h e e a s i e s t t o filter ( l o w e r specific r e s i s t a n c e ) , w h e r e a s s e c o n d a r y s l u d g e is t h e m o s t difficult. 2. F i l t e r a b i l i t y is i n f l u e n c e d b y p a r t i c l e size, s h a p e , a n d d e n s i t y , a n d b y electrical c h a r g e o n t h e p a r t i c l e . T h e l a r g e r t h e p a r t i c l e size, t h e h i g h e r t h e filtration
r a t e ( l o w e r specific r e s i s t a n c e ) , a n d t h u s t h e final c a k e m o i s t u r e is
lower. A d d i t i o n of coagulants p r o m o t e s agglomeration of particles, t h u s increasing
filtration
rate.
4.9. F I L T R A T I O N D E S I G N E Q U A T I O N F o r p u r p o s e o f filter d e s i g n it is c o n v e n i e n t t o m o d i f y E q . ( 7 . 1 6 ) . N e g l e c t i n g r e s i s t a n c e o f m e d i a (R
m
« 0) a n d r e c a l l i n g t h a t t is t h e f o r m t i m e (t = jV=
t \
tf
f
(7.23)
2
I t is c o n v e n i e n t t o w r i t e t h e e q u a t i o n in t e r m s o f filter l o a d i n g (L )
i.e., l b o f
f 9
d e p o s i t e d c a k e / ( f t ) ( h r ) . S i n c e p r o d u c t cV r e p r e s e n t s t h e w e i g h t o f c a k e 2
( S e c t i o n 4 . 4 ) , filter l o a d i n g (L )
b a s e d o n f o r m t i m e ( f o r m l o a d i n g ) is
f
L
f
= cV/Atf
= lb of deposited c a k e / ( f t ) ( h r )
(7.24)
2
S u b s t i t u t i n g in E q . (7.23) r b y its v a l u e g i v e n i n E q . (7.21) a n d r e a r r a n g i n g , V /A 2
= (ZPi-'tJIQir.c)
2
(7.25)
S u b s t i t u t i n g in t h e r i g h t - h a n d m e m b e r o f E q . (7.25) t h e i d e n t i t i e s t
f
a n d c = c /c
=
t /tf 2
f
and rearranging,
2
(cV/Atf)
= W-*c)l
2
(7.26)
0 r
T h e l e f t - h a n d m e m b e r o f E q . (7.26) is t h e s q u a r e o f f o r m l o a d i n g [ E q . ( 7 . 2 4 ) ] . Therefore, L
= [(^-•cVOir.'/)] ' 1
f
(7.27)
2
T A B L E 7.4 Units for Form Loading Equation (2)
(3)
Practical units
Metric units
Conversion factors
= lb/(ft )(hr)
g/(cm )(sec)
ω
L
2
f
2
L
l b / ( f t ) ( h r ) x 4 5 4 g/lb χ f t / 3 0 . 5 c m χ hr/3600 sec = 1.356 x l O " ! , / Ρ (psi) χ 70.1 g / c m / p s i = Ρ χ 70.1 [Eq. (7.20)] 2
f
2
2
2
4
P = psi
g/cm
c = g/ml = g / c m
3
r = (sec /g)xl0= practical unit / / (min)
2
g/cm g/(cm)(sec) = Poise
μ (cP) x P o i s e / 1 0 c P = μ x 1 0 "
sec /g sec
r xl0 / / (min) χ 6 0 sec/min = / / χ 6 0
—
3
μ = cP 2
2
2
7
o
2
7
o
2
320
7.
Sludge Treatment and Disposal
w h i c h is t h e e q u a t i o n for f o r m l o a d i n g . If m e t r i c u n i t s a r e u s e d ( E x a m p l e 7.2), t h e set o f u n i t s for p a r a m e t e r s in E q . (7.27) is p r e s e n t e d in c o l u m n (2), T a b l e 7.4. F o r d e s i g n p u r p o s e s it is c o n v e n i e n t t o e x p r e s s f o r m l o a d i n g in t e r m s o f l b o f c a k e / ( f t ) ( h r ) a n d o t h e r p a r a m e t e r s in t h e p r a c t i c a l u n i t s i n d i c a t e d in c o l u m n ( / ) , T a b l e 7.4. C o n v e r s i o n f a c t o r s f r o m c o l u m n ( / ) t o c o l u m n (2) a r e i n d i c a t e d in c o l u m n (5). S u b s t i t u t i o n o f t h e s e c o n v e r s i o n f a c t o r s in E q . (7.27) l e a d s t o E q . ( 7 . 2 8 ) , w h e r e all p a r a m e t e r s a r e in t h e p r a c t i c a l u n i t s listed in c o l u m n ( / ) o f T a b l e 7.4. 2
L
= 35.7 [ ( c P - )l(Mr 1
f
tf)] '
s
(7.28)
1 2
0
Since m o s t s l u d g e s h a v e specific c h a r a c t e r i s t i c s , E q . (7.28) is m o d i f i e d prediction of
filtration
for
performance. L
= 35J(P - lpr ) (c lt ) 1 S
f
ll2
m
(7.29)
n
0
f
[ F o r u n i t s see T a b l e 7.4, c o l u m n ( / ) . ] E q u a t i o n 7.29 is referred t o a s t h e f o r m l o a d i n g e q u a t i o n . T h e u s u a l r a n g e s o f v a l u e s for p a r a m e t e r s η a n d m a r e 1.0 > η > 0.4, o w i n g t o v a r i a t i o n in c a k e p e r m e a b i l i t y w h i l e a d d i t i o n a l c a k e is b e i n g f o r m e d [ E q . (7.28) c o r r e s p o n d s t o a v a l u e o f η = 0 . 5 ] ; a n d 1.0 > m > 0 . 2 5 , o w i n g t o effect o f v a r i a t i o n in s o l i d s c o n t e n t fed t o filter. [ E q . (7.28) c o r r e s p o n d s t o a v a l u e o f m = 0 . 5 . ] D e t e r m i n a t i o n o f p a r a m e t e r s n, s, m, a n d r is d i s c u s s e d in S e c t i o n 4 . 1 0 . 0
E x p e r i m e n t a l t e c h n i q u e utilized is d e s c r i b e d in S e c t i o n 4 . 1 1 . E x a m p l e 7.3 ( S e c t i o n 4.12) i l l u s t r a t e s t h e c a l c u l a t i o n p r o c e d u r e .
4.10. D E T E R M I N A T I O N O F P A R A M E T E R S n. s, m. A N D r I N E q . (7.29) Q
1. Determination
of n. If Ρ a n d c a r e h e l d c o n s t a n t (μ a n d r a r e c o n s t a n t 0
f o r a specific e x p e r i m e n t a l r u n ) , E q . (7.29) b e c o m e s L
= K (Vt )
(7.30)
n
f
1
f
where K
= 35J(P - lMr ) c 1 s
l
lf2
= constant
m
0
(7.31)
W r i t i n g E q . (7.30) in l o g a r i t h m i c f o r m , logL
= -n l o g t + l o g K ,
f
(7.32)
f
F r o m E q . (7.32), a l o g a r i t h m i c p l o t o f L v s . t yields a s t r a i g h t line o f s l o p e — Αϊ. A t y p i c a l p l o t is p r e s e n t e d in F i g . 7.16. 2. Determination of s. If t a n d c a r e h e l d c o n s t a n t (μ a n d r a r e c o n s t a n t for a specific e x p e r i m e n t a l r u n ) , E q . (7.29) b e c o m e s f
f
f
Q
L
= K P^-
S)/2
f
2
(7.33)
where K
2
= 35.7(l///r ) 0
1 / 2
( c / / / ) = constant M
(7.34)
4.
321
Dewatering of Sludges
W r i t i n g E q . (7.34) in l o g a r i t h m i c f o r m , log L
f
= [(1 - j)/2] log Ρ + log K
F r o m E q . (7.35), a l o g a r i t h m i c p l o t o f L
v s . Ρ yields a s t r a i g h t line o f s l o p e
f
(1 —s)/2.
(7.35)
2
A t y p i c a l p l o t is p r e s e n t e d i n F i g . 7.17.
3. Determination
of m. If t a n d Ρ a r e h e l d c o n s t a n t (μ a n d r a r e c o n s t a n t f
0
for a specific e x p e r i m e n t a l r u n ) , E q . (7.29) b e c o m e s L
= Kc
(7.36)
m
f
3
where #
= 35.7(/> -7/"V) 1
3
1 / 2
(l/i/)
(7.37)
W r i t i n g E q . (7.37) i n l o g a r i t h m i c f o r m , log L
f
= m l o g c + log ^ 3
F r o m E q . (7.38), a l o g a r i t h m i c p l o t o f L
f
(7.38)
v s . c y i e l d s a s t r a i g h t l i n e o f s l o p e m.
A t y p i c a l p l o t is p r e s e n t e d i n F i g . 7.18. 4. Determination (c /t/) m
ofr . 0
F r o m E q . (7.29) a p l o t o f L
yields a s t r a i g h t line o f s l o p e ( l / r ) 0
1 / 2
r
vs. 3 5 . 7 ( Ρ
1 _
7μ)
1 / 2
. A t y p i c a l p l o t is p r e s e n t e d i n
F i g . 7.19. S i n c e r is o n l y a f u n c t i o n o f t h e n a t u r e o f t h e s l u d g e , i t is a c o n s Q
t a n t for all e x p e r i m e n t s p e r f o r m e d w i t h t h e s a m e s l u d g e .
4.11. L E A F T E S T L A B O R A T O R Y P R O C E D U R E FOR D E T E R M I N A T I O N OF T H E P A R A M E T E R S IN THE L O A D I N G EQUATION E c k e n f e l d e r a n d F o r d [ 1 ] r e c o m m e n d a leaf t e s t a p p a r a t u s a s s h o w n i n F i g . 7.14 for d e t e r m i n a t i o n o f t h e p a r a m e t e r s i n t h e l o a d i n g e q u a t i o n . I t is o p e r a t e d b y a l t e r n a t i v e l y s u b m e r g i n g t h e t e s t leaf i n t h e s l u d g e t o s i m u l a t e
Fig.
7.14.
Leaf test
apparatus.
322
7.
Sludge Treatment and Disposal
f o r m t i m e t , a n d t a k i n g it o u t t o s i m u l a t e d r y t i m e t . F r o m E q . (7.10) it f o l l o w s t h a t it is p o s s i b l e t o s i m u l a t e o p e r a t i o n o f a r o t a r y filter o f k n o w n s u b m e r g e n c e b y p r o p e r c h o i c e o f t a n d t . U s u a l l y , t o t a l cycle t i m e (t ) r a n g e s from 1 to 6 min. f
d
f
d
c
I n p r a c t i c e , o n e selects a n o p e r a t i n g v a c u u m ( 1 0 - 2 0 i n . H g ) a n d a s u b m e r g e n c e ( 1 0 - 6 0 % ) . T h e n a d r y t i m e is selected t o yield t h e d e s i r e d % m o i s t u r e for t h e c a k e . T h i s is d o n e b y u s i n g t h e leaf test a p p a r a t u s a n d p r e p a r i n g a n experimental g r a p h of % m o i s t u r e for the c a k e vs. d r y time. A s a m p l e g r a p h is s h o w n in F i g . 7.15. A v a l u e o f t is t h e n selected ( F i g . 7.15) c o r r e s p o n d i n g t o the desired % m o i s t u r e for the cake. d
φ
σ ( J
Dry t i m e , t Fig.
7.15.
d
Selection
(min) of
t. d
T h e p r o c e d u r e [ 1 ] f o r d e t e r m i n a t i o n o f p a r a m e t e r s i n t h e l o a d e q u a t i o n is 1. Select t f r o m F i g . 7.15. 2. U s i n g t h e o p t i m u m c o a g u l a n t d o s a g e a s d e t e r m i n e d i n S e c t i o n 4 . 5 , r u n a series o f leaf t e s t s r e l a t i n g filter l o a d i n g t o f o r m t i m e (t ), initial s o l i d s c o n c e n t r a t i o n (c), a n d v a c u u m (P). A series o f 7 - 8 t e s t r u n s s h o u l d yield sufficient d e s i g n d a t a . T o t a l cycle t i m e is 1-6 m i n , w i t h a r a n g e o f d r u m s u b m e r g e n c e o f 1 0 - 6 0 % . T h i s c o r r e s p o n d s t o v a l u e s o f t f r o m 0.1 t o 3.6 m i n ( T a b l e 7.5). A p o s s i b l e series o f r u n s is s h o w n in T a b l e 7.6, w h i c h c o r r e s p o n d s t o d a t a in E x a m p l e 7.3. T h e f o l l o w i n g r e m a r k s a r e p e r t i n e n t t o T a b l e 7 . 6 : (a) for d e t e r m i n a t i o n o f η (Ρ a n d c c o n s t a n t ) , r u n s 1 - 3 ; ( b ) for d e t e r m i n a t i o n o f s (t a n d c c o n s t a n t ) , r u n s 6 - 8 ; (c) for d e t e r m i n a t i o n o f m (t a n d Ρ c o n s t a n t ) , r u n s 2 , 4 , a n d 5 ; a n d ( d ) f o r d e t e r m i n a t i o n o f r , r u n s 1-8. d
f
f
f
f
0
4.
323
Dewatering of Sludges
T A B L E 7.5 Usual Range of Values for t and t f
d
tc (min)
% submergence
tf = t (% submergence/100) (min)
U = t -t (min)
6 6 1 1
60% 10% 60% 10%
3.6 0.6 0.6 0.1
2.4 5.4 0.4 0.9
c
c
f
T A B L E 7.6 Leaf Test Data (5) Applied vacuum
Run no.
(2) Form time, t (min)
(3) Dry time, t (min)
(4) c
Ρ (in. H g )
P(t>si)
1 2 3 4 5 6 7 8
0.25 0.50 1.00 0.50 0.50 1.50 1.50 1.50
1.5 1.0 0.5 1.5 1.5 1.0 1.0 0.5
0.03 0.03 0.03 0.04 0.05 0.03 0.03 0.03
20 20 20 20 20 10 15 20
9.80 9.80 9.80 9.80 9.80 4.90 7.35 9.80
ω
f
d
L
(6) Loading, [lb/(ft )(hr)] 2
f
73.0 47.0 29.2 70.0 90.0 21.0 22.5 23.5
3. P r o c e d u r e f o r e a c h r u n is a s f o l l o w s . (a) A d d t h e o p t i m u m c o a g u l a n t d o s a g e a s d e t e r m i n e d i n S e c t i o n 4 . 5 . ( b ) F l o c c u l a t e t h e m i x t u r e f o r 3 0 s e c . I n s o m e c a s e s a series o f t e s t s a r e m a d e t o d e t e r m i n e o p t i m u m flocculation t i m e . (c) S u b m e r g e t h e leaf i n flocculated s l u d g e m i x t u r e f o r t h e specified f o r m t i m e (t ) ( T a b l e 7.5). M a i n t a i n g e n t l e m i x i n g t o a v o i d d e p o s i t i o n o f sludge. (d) R e m o v e leaf f r o m s l u d g e a n d h o l d it v e r t i c a l l y f o r t h e specified d r y t i m e , k e e p i n g it u n d e r full v a c u u m . (e) T r a n s f e r e n t i r e c a k e f r o m t h e filter leaf t o a t a r e d d i s h . C o m p r e s s e d a i r m a y b e g e n t l y a p p l i e d t o l o o s e n t h e c a k e f r o m t h e leaf. (f) W e i g h w e t c a k e , d r y a t 1 0 3 ° C , a n d r e w e i g h ; m e a s u r e a n d r e c o r d cake thickness. (g) T h e l o a d i n g i n l b / ( f t ) ( h r ) is f
2
L
= dry weight of sludge in g χ (cycles/hr)/[454 χ test leaf area (ft )] 2
f
324
7.
Sludge Treatment and Disposal
4. P r e p a r e t h e f o l l o w i n g g r a p h s : (a) \ogL
vs. l o g t
f
(P a n d c a r e c o n s t a n t s ) ( F i g . 7.16). S l o p e o f t h e
f
s t r a i g h t line yields p a r a m e t e r η in E q . (7.29). ( b ) l o g L y v s . l o g Ρ (t
a n d c a r e c o n s t a n t s ) ( F i g . 7.17). S l o p e o f t h e
f
s t r a i g h t line is (11 — s)/2, f r o m w h i c h p a r a m e t e r s is c a l c u l a t e d . (c) \ogL
v s . l o g c (t
f
a n d Ρ a r e c o n s t a n t s ) ( F i g . 7.18). S l o p e o f t h e
f
s t r a i g h t line is p a r a m e t e r m in E q . (7.29). (d) L
vs. 35.7(Ρ - /μ) (ο /ί ) 1 3
f
is
(l/0 > 1/2
ι/2
η
( F i g . 7.19). S l o p e o f t h e s t r a i g h t line
η
/
fr°
m
w h i c h p a r a m e t e r r is c a l c u l a t e d . 0
4.12. I L L U S T R A T I O N O F C A L C U L A T I O N P R O C E D U R E F O R P A R A M E T E R S n, s, m, AND r Q
Example 7 . 3 L a b o r a t o r y tests o n a leaf filter c o n d u c t e d for a s l u d g e yield r e s u l t s p r e s e n t e d in T a b l e 7.6. O b t a i n t h e v a l u e s for p a r a m e t e r s n, m, s, a n d r i n t h e f o r m loading equation. 0
SOLUTION (a) Determination
of n. T h e l o g a r i t h m i c p l o t o f L
f
v s . t is s h o w n in F i g . f
7.16, f r o m w h i c h s l o p e = —n = — 0 . 6 5 3 , η = 0 . 6 5 3 . 3001
1
I0L 0.1
0.2
1
1 π—Γ
0.4
ι ι ιι
0.6 0.8 1.0
1
2.0
t (min) f
Fig.
7.16.
Determination
of η (Example
7.3).
1
3.0
Dewatering of Sludges 60, 50 cqnstant R u n s : 6 . 7 and
40
R u n Mo. 8 ^ ,7-x
\ -Q
-T
Run Ν 20 -Slope = ( l - s ) / i
10
4
5
6
7
8
9
10
Ρ (psi) Fig. 7.17.
Determination
of s (Example
7.3).
100 -Rtifl -N U".
90
U
m
\
80 P u n n u n
70
Mn U.
W
Α
*τ
—*ύ v
60 Dpe=m
ccr so *
2y
1?un No.
40
30 [t , P = c o n s t a n t ] f
R u n s . 2 , 4 and 5 20 0.01
0.02
0.03
0.04
0.06
c Fig. 7.18.
Determination
of m (Example
7.3).
7.
326 ( b ) Determination 7.17.
From
Fig.
Sludge Treatment and Disposal
of s. T h e l o g a r i t h m i c p l o t o f L
v s . Ρ is s h o w n i n F i g .
f
7.17
read
s l o p e = (1 -s)/2
= 0.1635.
1 - 2 ( 0 . 1 6 3 5 ) = 0.673. (c) Determination of m. T h e l o g a r i t h m i c p l o t o f L
Therefore,
vs. c is s h o w n i n F i g .
f
7.18. F r o m F i g . 7.18 r e a d s l o p e = m = 1.265. 100 Runs: 1 throigh 8 90
Run NO.
80 Run N( ι. 1
70 -
Run Ν
ι
A
60 -Slope = ( l / r )
1/2
0
Ξ
50
-Run 1 Jo. 2
η
jf 4 0 £=Run Nu. 3 tun No 8 η No. 7 ^-Run No. 6
30 20 10 0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
35.7(p'" //x) (c /ty) s
F/flr.
7.19. Determination
,/2
m
of r
0
(Example
7.3).
T A B L E 7.7 Calculations for Determination of r (Example 7.3) c
j p l - 0 . 6 7 3
R u n no. 1 2 3 4 5 6 7 8
73.0 47.0 29.2 70.0 90.0 21.0 22.5 23.5
_
2.109 2.109 2.109 2.109 2.109 1.683 1.921 2.109
p 0 . 3 2 7
c
1.265
0.0119 0.0119 0.0119 0.0172 0.0227 0.0119 0.0119 0.0119
,0,653
f
l
0.404 0.635 1.000 0.635 0.635 1.303 1.303 1.303
s =
35J(P - M (c /t ) 1 s
lJ2
m
n
f
1.525 0.970 0.615 1.395 1.850 0.425 0.451 0.472
4.
327
Dewatering of S l u d g e s
(d) Determination
of r . 0
T h e plot of L
vs. 35J(P ' /p) (c /t/) i s
f
i/2
is
m
s h o w n in F i g . 7 . 1 9 ; t h e n e c e s s a r y c a l c u l a t i o n s a r e p r e s e n t e d in T a b l e 7.7. Take μ = 1 cP. F r o m F i g . 7.19 s l o p e = ( l / r )
= 48.5. Therefore, r = (1/48.5) = 0.000425.
1 / 2
2
0
0
I n s u m m a r y , filtration p a r a m e t e r s for t h e f o r m l o a d i n g e q u a t i o n for E x a m p l e 7.3 a r e η = 0 . 6 5 3 , s = 0 . 6 7 3 , m = 1.265, a n d r = 0 . 0 0 0 4 2 5 . 0
4.13. P R O C E D U R E F O R R O T A R Y FILTER D E S I G N [1] Step 1. F r o m F i g . 7.15 select a n a p p r o p r i a t e v a l u e o f t h e d r y t i m e t for a d
desired cake moisture. Step
2. F o r a selected % s u b m e r g e n c e a n d t h e c h o s e n v a l u e o f t > c a l d
culate form time t
f r o m E q . (7.39), o b t a i n e d b y c o m b i n i n g E q s . (7.10) a n d
f
(7.9). S o l v i n g for
t, f
t
f
= WKlOO/% submergence) - 1 ]
(7.39)
If desired, adjust the values of t [calculated from E q . (7.39)], t , a n d % s u b m e r g e n c e utilizing E q s . (7.9) a n d (7.10). T h i s is i l l u s t r a t e d i n E x a m p l e 7.4. f
c
Step 3. C a l c u l a t e f o r m l o a d i n g L f r o m E q . (7.29). F o r c a l c u l a t i o n o f L u s e (a) v a l u e s o f p a r a m e t e r s n, m, s, a n d r d e t e r m i n e d f r o m l a b o r a t o r y d a t a ( E x a m p l e 7 . 3 ) ; (b) selected v a l u e o f t h e o p e r a t i n g v a c u u m ; (c) f o r m t i m e t f r o m S t e p 2 ; a n d (d) μ a n d c f r o m d a t a o n c h a r a c t e r i s t i c s o f s l u d g e . O r d i n a r i l y take μ = 1 cP. f
f
0
f
Step 4. C a l c u l a t e cycle l o a d i n g L
c
L
c
= L (% f
from
submergence/100) χ 0.8
[lb/(ft )(hr)]
(7.40)
2
I n E q . (7.40), f a c t o r 0.8 c o m p e n s a t e s f o r t h e s e c t o r o f t h e filter d r u m w h e r e c a k e is r e m o v e d a n d m e d i a w a s h e d ( F i g . 7.9). Step 5 . C a l c u l a t e r e q u i r e d filter a r e a f r o m Filter area =
lb/hr of solids t o be removed , . L [lb/(ft )(hr)] n
/
m
2
u
(ft )
(7.41)
2
2
c
E x a m p l e 7.4 i l l u s t r a t e s t h i s d e s i g n p r o c e d u r e . E x a m p l e 7.4 L a b o r a t o r y tests for t h e s l u d g e i n E x a m p l e 7.3 i n d i c a t e d a d r y t i m e t = 1.25 m i n t o b e a n a p p r o p r i a t e c h o i c e . * I t is d e s i r e d t o d e s i g n a v a c u u m filter t o d e w a t e r 3 0 , 0 0 0 l b / d a y o f s l u d g e ( d r y w e i g h t ) . P a r a m e t e r c is 0 . 0 3 . F i l t e r o p e r a t e s 100 h r / w e e k . Select a s u b m e r g e n c e o f 2 5 % a n d a n o p e r a t i n g v a c u u m o f 15 i n . o f H g . D e t e r m i n e t h e r e q u i r e d filter a r e a i n f t . d
2
* Details o f these tests, including the graph (similar to Fig. 7.15) utilized for selection o f t , are not s h o w n in the text. d
328
7.
Sludge Treatment and Disposal
SOLUTION Step 1. t = 1.25 m i n . Step 2. C a l c u l a t e t f r o m E q . (7.39). d
f
t
f
Take t
f
= 1 . 2 5 / [ ( 1 0 0 / 2 5 ) - l ] = 0.417 min
= 0.5 m i n a n d adjust accordingly values of t a n d % submergence, c
i.e. [ f r o m E q . ( 7 . 9 ) ] , t = h + t = 0.5 + 1.25 = 1.75 min c
d
F r o m E q . (7.10), t h e a d j u s t e d % s u b m e r g e n c e is % submergence = 1 0 0 ( / / / i ) = 100(0.5/1.75) = 28.6% c
Step 3. C a l c u l a t e f o r m l o a d i n g L f r o m E q . (7.29), w h e r e p a r a m e t e r s η = 0 . 6 5 3 , s = 0 . 6 7 3 , m = 1.265, a n d r = 0 . 0 0 0 4 2 5 ( E x a m p l e 7.3). O t h e r d a t a : Ρ = 15 i n . H g ( = 7.35 p s i ) , μ = 1 c P , c = 0 . 0 3 , a n d t = 0.5 m i n . f
0
f
L
= 35.7[7.35 -°· ^(1.0)(0.0(^25)] 1
f
6 7
Step 4. C a l c u l a t e cycle l o a d i n g L
c
L
1 / 2
(0.03 · 1
2 6 5
/0.5 · 0
6 5 3
) = 44.7 lb/(ft )(hr) 2
[Eq. (7.40)].
= 44.7(28.6/100) χ 0.8 = 10.23 lb/(ft )(hr) 2
c
Step 5. D e t e r m i n e t h e r e q u i r e d filter a r e a [ E q . ( 7 . 4 1 ) ] . 30,000 lb/day χ 7 days/week χ week/100 h r F , , t e r
^
=
10.23 lb/(ft*)(hr)
=
2
0
5
3
ft2
5. P r e s s u r e Filtration F i l t r a t i o n o f s l u d g e s in filter p r e s s e s is e c o n o m i c a l l y feasible w h e n e v e r l a b o r c o s t s a r e relatively l o w , o w i n g t o t h e difficulty o f full a u t o m a t i o n o f t h e o p e r a t i o n . R e c e n t l y , s y s t e m s w i t h fully a u t o m a t i c o p e r a t i o n h a v e b e e n available commercially with a u t o m a t i c opening of the press, cake discharge, a n d w a s h i n g o f t h e filter m e d i a b e t w e e n cycles. T h e s e n e w d e v e l o p m e n t s t e n d t o m a k e filter p r e s s e s d e s i r a b l e for u s e m o r e f r e q u e n t l y in t h e f u t u r e . T h e m a i n a d v a n t a g e o f filter p r e s s e s o v e r r o t a r y v a c u u m filters is t h a t a d r i e r c a k e c a n b e o b t a i n e d . T h i s is especially a d v a n t a g e o u s if filtration is followed by incineration.
6.
Centrifugation
Dewatering of sludges by centrifugation has been applied with increasing f r e q u e n c y i n t h e last few y e a r s . A s k e t c h o f a t y p i c a l c e n t r i f u g e for t h i s service (a c o n t i n u o u s solid b o w l t y p e ) is s h o w n in F i g . 7.20. T h e c o m p o n e n t s o f t h e c e n t r i f u g e a r e (1) fixed c a s i n g , (2) r o t a t i n g b o w l , (3) r o t a t i n g i n n e r c o n v e y o r , (4) d r i v i n g c o m p o n e n t s ( m o t o r a n d g e a r s y s t e m ) , (5) s l u r r y inlet p o r t , (6) solids d i s c h a r g e p o r t , a n d (7) l i q u i d d i s c h a r g e p o r t .
7.
329
Bed Drying of Sludges
Fig. 7.20. Bird Company, Inc.)
continuous
solid
bowl
centrifuge.
(Courtesy
of Bird
Machine
S l u d g e solids a r e c o m p a c t e d b y c e n t r i f u g a l f o r c e a g a i n s t t h e i n n e r w a l l s o f the rotating bowl, then picked u p by the conveyor a n d t a k e n t o the solids d i s c h a r g e p o r t . L i q u i d is d i s c h a r g e d a t t h e o p p o s i t e e n d o f t h e b o w l . N o e s t a b l i s h e d d e s i g n p r o c e d u r e is a v a i l a b l e f r o m l a b o r a t o r y d a t a , b u t feasibility o f c e n t r i f u g a t i o n for a specific s l u d g e m a y b e e v a l u a t e d f r o m t e s t s e m p l o y i n g l a b o r a t o r y centrifuges.
7. Bed Drying of S l u d g e s 7.1. I N T R O D U C T I O N A i r d r y i n g o f s l u d g e s o n s a n d b e d s is o n e o f t h e m o s t e c o n o m i c a l m e t h o d s for d e w a t e r i n g . I t is t h e m o s t c o m m o n m e t h o d for s m a l l t r e a t m e n t p l a n t s , f o r b o t h domestic a n d industrial wastewaters.
7.
330
Sludge Treatment and Disposal
E c o n o m i c feasibility o f t h e p r o c e s s d e p e n d s g r e a t l y o n (1) a v a i l a b i l i t y o f l a n d a t a r e a s o n a b l e c o s t , a n d (2) f a v o r a b l e c l i m a t i c c o n d i t i o n s ( d r y a n d w a r m c l i m a t e ) c o n d u c i v e t o o p t i m u m e v a p o r a t i o n c o n d i t i o n s . T h e a r e a n e e d e d is a f u n c t i o n o f (1) rainfall a n d e v a p o r a t i o n r a t e s , a n d (2) s l u d g e c h a r a c t e r i s t i c s (for e x a m p l e , g e l a t i n o u s s l u d g e s r e q u i r e a l a r g e r a r e a ) .
7.2. M E C H A N I S M S O F D E W A T E R I N G ON SAND BEDS
SLUDGES
Dewatering of sludges occurs by t w o m e c h a n i s m s : 1. P e r c o l a t i o n o f w a t e r t h r o u g h t h e s a n d b e d . T h e p r o p o r t i o n o f w a t e r r e m o v e d by percolation varies from 20 t o 5 5 % d e p e n d i n g o n initial solids c o n t e n t o f s l u d g e a n d c h a r a c t e r i s t i c s o f s o l i d s . P e r c o l a t i o n is g e n e r a l l y c o m p l e t e in 1-3 d a y s , r e s u l t i n g in s o l i d s c o n c e n t r a t i o n a s h i g h a s 1 5 - 2 5 % . 2. E v a p o r a t i o n o f w a t e r . E v a p o r a t i o n o c c u r s b y m e c h a n i s m s o f r a d i a t i o n a n d c o n v e c t i o n . T h e r a t e o f e v a p o r a t i o n is s l o w e r t h a n t h a t o f d e w a t e r i n g b y p e r c o l a t i o n , a n d it is r e l a t e d t o t e m p e r a t u r e , r e l a t i v e h u m i d i t y , a n d a i r v e l o c i t y . A t y p i c a l e v a p o r a t i o n r a t e c u r v e is s h o w n i n F i g . 7.21 a n d e x h i b i t s t w o d i s t i n c t s e c t i o n s c o r r e s p o n d i n g t o c o n s t a n t a n d falling r a t e p e r i o d s .
Moisture
Fig.
7.21.
Evaporation
(%)
rate
curve.
D u r i n g t h e c o n s t a n t r a t e p e r i o d , t h e s l u d g e s u r f a c e is w e t , a n d r a t e o f e v a p o r a t i o n is relatively i n d e p e n d e n t o f t h e n a t u r e o f t h e s l u d g e . T h i s r a t e is less t h a n t h a t w h i c h is o b s e r v e d f r o m a free w a t e r s u r f a c e (free w a t e r e v a p o r a t i o n ) , o w i n g t o t h e fact t h a t t h e p l a n e o f v a p o r i z a t i o n is b e l o w t h e s u r f a c e o f t h e solid. E v a p o r a t i o n p r o c e e d s a t a c o n s t a n t r a t e u n t i l a critical m o i s t u r e c o n t e n t is r e a c h e d ( F i g . 7.21). W h e n t h e critical m o i s t u r e c o n t e n t is r e a c h e d , w a t e r n o l o n g e r m i g r a t e s t o t h e surface o f t h e s l u d g e a s r a p i d l y a s it e v a p o r a t e s , a n d t h e falling r a t e p e r i o d
7.
Bed Drying of Sludges
331
o c c u r s . R a t e o f d r y i n g d u r i n g t h i s p e r i o d is r e l a t e d t o t h i c k n e s s o f t h e s l u d g e , its p h y s i c a l a n d c h e m i c a l p r o p e r t i e s , a n d
atmospheric conditions.
Sub
s u r f a c e d r y i n g c o n t i n u e s u n t i l a n e q u i l i b r i u m m o i s t u r e c o n t e n t is r e a c h e d ( F i g . 7.21).
7.3. C O N S T R U C T I O N OF S A N D D R Y I N G B E D S T y p i c a l s a n d b e d c o n s t r u c t i o n is i n d i c a t e d i n F i g . 7.22, w h i c h s h o w s a v e r t i c a l s e c t i o n o f a d r y i n g b e d . U n d e r d r a i n p i p i n g m a y b e o f vitrified clay, w i t h a m i n i m u m d i a m e t e r o f 4 in. a n d a m i n i m u m s l o p e o f 1%. T h e filtrate is returned to the treatment plant.
.Size: 0.3-1.2 mm
-··.·•!· S a n d
: ' : ' : a to 9"
'· · diameter
Gravel
-Gravel
8 to 18"
Size: 1/8 to I inch diameter Underdrain
»
» Filtrate
piping-' Fig.
7.22.
Sand
drying
bed.
S l u d g e is u s u a l l y a p p l i e d t o d r y i n g b e d s a t d e p t h s o f 8 - 1 2 i n . I t is left t o d r y u n t i l it r e a c h e s a s o l i d s c o n t e n t b e t w e e n 3 0 a n d 5 0 % . I t is r e m o v e d w h e n s o l i d s r e a c h a liftable s t a t e , w h i c h v a r i e s w i t h i n d i v i d u a l j u d g e m e n t a s well a s t h e final d i s p o s a l m e a n s . T h e p e r i o d o f t i m e b e t w e e n a p p l i c a t i o n o f s l u d g e t o s a n d b e d a n d its r e m o v a l in a liftable s t a t e is c a l l e d b e d t u r n o v e r t i m e . I t v a r i e s b e t w e e n 2 0 a n d 7 5 d a y s , d e p e n d i n g o n t h e n a t u r e o f t h e s l u d g e . I t is p o s s i b l e t o r e d u c e s u b s t a n t i a l l y the bed t u r n o v e r time by prior t r e a t m e n t with chemical c o a g u l a n t s , for e x a m p l e , a l u m a n d p o l y e l e c t r o l y t e s . By c h e m i c a l p r e t r e a t m e n t it is p o s s i b l e t o r e d u c e d r y i n g t i m e b y a s m u c h a s 5 0 % , a n d it is feasible t o a p p l y t h e s l u d g e in a t h i c k e r l a y e r . B e d yield is r e p o r t e d t o v a r y l i n e a r l y w i t h c o a g u l a n t dosage.
7.4. D R Y I N G B E D D E S I G N In the past, drying beds have been designed o n a n empirical basis of ft of bed area/capita or lb of dry solids/(ft )(year). Values of these p a r a m e t e r s e m p l o y e d in t h e U n i t e d S t a t e s a r e given in Ref. [ 7 ] . 2
2
7.
332
Sludge Treatment and Disposal
A rational m e t h o d of design has been developed recently by Swanwick [ 6 ] a n d is r e c o m m e n d e d b y E c k f e l d e r a n d F o r d [ 1 ] . T h e p r o c e d u r e is a s f o l l o w s , a n d E x a m p l e 7.5 i l l u s t r a t e s its a p p l i c a t i n g . Step 1. Fill a glass c y l i n d e r ( 1 - 2 in. d i a m e t e r ) c o n t a i n i n g a s a n d b a s e w i t h t e s t s l u d g e t o a d e p t h o f 8 - 1 2 in. ( d e p t h e n v i s i o n e d for t h e a c t u a l u n i t t o b e designed). Step 2. A l l o w c o m p l e t e d r a i n a g e o f w a t e r f r o m s l u d g e . T h i s r e q u i r e s a d r a i n a g e p e r i o d o f 1-3 d a y s , d e p e n d i n g o n s l u d g e c h a r a c t e r i s t i c s a n d initial moisture content. This c o r r e s p o n d s t o the percolation p h a s e of the drying mechanism. Step 3. O n c e d r a i n a g e is c o m p l e t e d , r e m o v e t h e s l u d g e c o r e f r o m c y l i n d e r . U t i l i z e a s m a l l f r a c t i o n o f it t o d e t e r m i n e m o i s t u r e c o n t e n t ( b y o v e n d r y i n g a n d w e i g h i n g b e f o r e a n d after). Step 4. P l a c e t h e s l u d g e c o r e in a n o p e n d i s h t o a l l o w e v a p o r a t i o n t o o c c u r . C h e c k t h e s a m p l e p e r i o d i c a l l y u n t i l d e s i r e d m o i s t u r e c o n t e n t is r e a c h e d ( m o i s t u r e c o n t e n t w h e n s l u d g e c a k e is liftable). Step 5. T h e difference b e t w e e n w e i g h t s o f w a t e r ( m o i s t u r e c o n t e n t ) a t t h e e n d of Steps 3 a n d 4 c o r r e s p o n d s t o the w a t e r t o be e v a p o r a t e d ( e v a p o r a t i o n p h a s e in t h e d r y i n g m e c h a n i s m ) . W a t e r t o b e e v a p o r a t e d is e x p r e s s e d in i n . e v a p o r a t e d / f t o f b e d a r e a ( d e t a i l s o f c a l c u l a t i o n in S t e p 5, E x a m p l e 7.5). 2
Step 6. O b t a i n l o c a l m e t e o r o l o g i c a l r e c o r d s for rainfall (in.) a n d e v a p o r a t i o n (in.) t a b u l a t e d o n a m o n t h l y b a s i s . Step 7. P r e p a r e i n t a b u l a r f o r m a r e c o r d o f i n c h e s o f rainfall m u l t i p l i e d b y a f a c t o r 0.57 vs. m o n t h . T h i s is b a s e d o n e x p e r i m e n t a l e v i d e n c e t h a t 4 3 % o f t h e rainfall d r a i n s t h r o u g h t h e c a k e , l e a v i n g 5 7 % t o b e e v a p o r a t e d . R a i n f a l l f r a c t i o n t o b e e v a p o r a t e d v a r i e s a c c o r d i n g t o rainfall p a t t e r n s a n d i n t e n s i t y . A v a l u e o f less t h a n 5 7 % , for e x a m p l e , m i g h t b e e x p e c t e d in r e g i o n s w h e r e r a i n f a l l is i n t e n s e a n d o f s h o r t d u r a t i o n . P r e p a r e a l s o i n t a b u l a r f o r m a r e c o r d o f i n c h e s o f e v a p o r a t i o n m u l t i p l i e d b y a f a c t o r 0.75 v s . m o n t h . T h i s is b a s e d o n e x p e r i m e n t a l e v i d e n c e t h a t a v e r a g e e v a p o r a t i o n o f w e t s l u d g e is 7 5 % o f t h a t f o r free w a t e r . Step 8. C a l c u l a t e a v e r a g e e v a p o r a t i o n r a t e ( d a y s / i n . ) for e a c h m o n t h . Step 9. B a s e d o n v a l u e s c a l c u l a t e d in S t e p 8, d e t e r m i n e t i m e r e q u i r e d t o e v a p o r a t e i n c h e s o f w a t e r c a l c u l a t e d in S t e p 5 for e a c h m o n t h . Step 10. B a s e d o n v a l u e s c a l c u l a t e d in S t e p 8 a n d i n c h e s o f r a i n f a l l m u l t i p l i e d b y f a c t o r 0 . 5 7 , c a l c u l a t e t i m e r e q u i r e d t o e v a p o r a t e rainfall ( d a y s ) for e a c h m o n t h . Step 11. T h e t o t a l t i m e for e v a p o r a t i o n o f w a t e r c a l c u l a t e d in S t e p 9 p l u s t h a t t o e v a p o r a t e rainfall ( S t e p 10) is t h e n o b t a i n e d for e a c h m o n t h . Step 12. T a k e t h e l a r g e s t o f t h e t o t a l t i m e s o b t a i n e d in S t e p 11 a s t h e d e s i g n requirement. Calculate the ft 7.5 ( S t e p 12).
2
of bed area required as indicated in E x a m p l e
7.
333
Bed Drying of Sludges
E x a m p l e 7.5 Sludge drying beds are considered to dewater 4000 lb/day (dry weight) of a s l u d g e p r o d u c e d in a n i n d u s t r i a l w a s t e w a t e r t r e a t m e n t p l a n t . T h e s l u d g e is a p p l i e d t o b e d s in 10-in. lifts. A laboratory study shows t h a t percolation increased solids concentration f r o m its initial v a l u e o f 5 % t o 2 0 % i n 2 5 h r . S l u d g e is c o n s i d e r e d liftable f r o m drying beds at 25%solids. Meteorological records for the region a r e indicated in T a b l e 7.8. D e t e r m i n e t h e r e q u i r e d a r e a o f b e d s . TABLE 7 . 8 Rainfall and Evaporation Record (Example 7 . 5 ) Jan.
F e b . Mar. Apr. M a y June July A u g . Sept. Oct. N o v . D e c .
Rainfall (in.)
4.0
3.0
3.1
4.1
4.0
3.5
2.1
3.0
3.2
3.0
2.7
2.8
Evaporation (in.)
5.8
6.5
7.5
8.7
11.2
11.0
13.2
11.3
9.1
6.5
4.6
3.1
10 in. l i f t — (I f t ) 2
Fig.
7.23.
10/12 = 0 . 8 3
ft
χ ( 0 . 8 3 f t ) ( 6 2 . 4 I b / f t ) = 5 l . 8 lb 3
Basis
of calculations
for Example
7.5.
SOLUTION T a k e s l u d g e d e n s i t y a s 62.4 l b / f t . B a s e c a l c u l a t i o n s o n t o t a l l b o f w e t s l u d g e a p p l i e d i n a lift p e r f t o f b e d , a s i n d i c a t e d i n F i g . 7 . 2 3 . Steps 1 and 2. P e r f o r m l a b o r a t o r y t e s t s a s d e s c r i b e d . Step 3. S o l i d s c o n t e n t a t b e g i n n i n g o f p e r c o l a t i o n e q u a l s 5 % a s s t a t e d . B a s e d o n a 10-in. lift p e r f t o f b e d , t h i s c o r r e s p o n d s t o (51.8 l b ) ( 0 . 0 5 ) = 2.6 lb/ft (dry solids). After d r a i n a g e t o 2 0 % solids t h e total weight of wet c a k e p e r f t o f a r e a (10-in. lift) is 2.6 l b / f t / 0 . 2 = 13 l b / f t . 3
2
2
2
2
2
2
Step 4. E v a p o r a t i o n p r o c e e d s t o a final c a k e c o n t a i n i n g 2 5 % s o l i d s . T h i s c o r r e s p o n d s t o a t o t a l w e i g h t o f w e t c a k e p e r f t o f a r e a (10-in. lift) = 2.6 l b / 0 . 2 5 = 10.4 l b / f t . 2
2
334
7.
Sludge Treatment and Disposal
Step 5. W a t e r e v a p o r a t e d p e r f t o f a r e a (10-in. lift) is 2
13.0 10.4 = 2.6 lb of water evaporated/ft of bed area (Step 3) (Step 4) 2
W a t e r e v a p o r a t e d is e x p r e s s e d i n t e r m s o f i n c h e s e v a p o r a t e d p e r f t area. V o l u m e of water e v a p o r a t e d (ft /ft 3
2
of bed
o f b e d a r e a ) is
2.6 lb/ft χ ft /62.41b = 0.0417 f t / f t 2
2
3
3
2
or 0.0417 f t / f t 3
2
χ 12 in./ft = 0.5 in. of water to be evaporated/ft of bed area 2
Step 6. M a i n t a i n m e t e o r o l o g i c a l r e c o r d s ( T a b l e 7.8). Step 7. R e c o r d i n c h e s o f rainfall χ 0 . 5 7 ; see c o l u m n s (2) a n d (3) o f T a b l e 7.9. R e c o r d i n c h e s o f e v a p o r a t i o n χ 0 . 7 5 ; see c o l u m n s (4) a n d (5) o f T a b l e 7.9. Step 8. T a k e t h e a v e r a g e e v a p o r a t i o n r a t e ( d a y s / i n . ) f o r e a c h m o n t h . T h i s c a l c u l a t i o n is i n d i c a t e d in c o l u m n s (6) a n d (7) o f T a b l e 7.9. Step 9. D e t e r m i n e t h e t i m e r e q u i r e d t o e v a p o r a t e 0.5 in. o f w a t e r (see S t e p 5). T h i s c a l c u l a t i o n is i n d i c a t e d i n c o l u m n (8) o f T a b l e 7.9. Step 10. C a l c u l a t e t h e t i m e r e q u i r e d t o e v a p o r a t e rainfall ( d a y s ) . T h i s c a l c u l a t i o n is i n d i c a t e d in c o l u m n (9) o f T a b l e 7.9. Step 11. E s t i m a t e t h e t o t a l t i m e r e q u i r e d t o e v a p o r a t e 0.5 i n . o f w a t e r plus r a i n f a l l . See c o l u m n (10), T a b l e 7.9. Step 12. T a k e D e c e m b e r a s t h e c o n t r o l m o n t h [ l a r g e s t v a l u e o f t o t a l t i m e , i.e., 2 7 . 9 5 d a y s , c o l u m n (70) o f T a b l e 7 . 9 ] . R e q u i r e d a r e a o f s a n d b e d is then 4000 lb dry sludge/day χ ft /2.6 lb dry sludge χ 27.95 days 2
= 43,000 ft of bed area required 2
8. P r e - d e w a t e r i n g T r e a t m e n t of S l u d g e s F r e q u e n t l y , d e w a t e r i n g of s l u d g e s is difficult, especially w h e n o f g e l a t i n o u s c o n s i s t e n c y . D e w a t e r i n g t h e s e t y p e s o f s l u d g e s b y v a c u u m filtration, f o r e x a m p l e , is e x c e e d i n g l y difficult, a n d p r e - d e w a t e r i n g t r e a t m e n t m a y b e r e c o m m e n d e d . T w o t y p e s o f p r e d e w a t e r i n g t r e a t m e n t a r e d i s c u s s e d in t h i s s e c t i o n : (1) c h e m i c a l c o a g u l a t i o n a n d (2) h e a t t r e a t m e n t .
8.1. C H E M I C A L C O A G U L A T I O N A d d i t i o n of c h e m i c a l c o a g u l a n t s p r o m o t e s c o a l e s c e n c e o f s l u d g e p a r t i c l e s a n d t h u s i m p r o v e s t h e i r ability t o b e filtered. I t m a y b e p r e c e d e d b y w a s h i n g o f t h e s l u d g e , a n o p e r a t i o n w h i c h is k n o w n a s e l u t r i a t i o n . E l u t r i a t i o n r e d u c e s alkalinity, a n d therefore minimizes coagulant requirements. F e C l , lime, a n d polyelectrolytes are the m o s t c o m m o n coagulants. 3
8.
Pre-dewatering Treatment of S l u d g e s
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336
S l u d g e Treatment and Disposal
8.2. H E A T T R E A T M E N T O F S L U D G E S T h r e e p r o c e s s e s o f h e a t t r e a t m e n t a r e d e s c r i b e d in t h i s s e c t i o n : (1) P o r t e u s , (2) Z i m p r o , a n d (3) N i c h o l s p r o c e s s e s . T h e s e p r o c e s s e s i n v o l v e w e t o x i d a t i o n o f s l u d g e . T h i s c o n s i s t s o f c h e m i c a l o x i d a t i o n o f o r g a n i c solids in a n a q u e o u s p h a s e b y d i s s o l v e d o x y g e n in r e a c t o r s o p e r a t i n g a t h i g h t e m p e r a t u r e s a n d p r e s s u r e s . A d v a n t a g e s o f h e a t t r e a t m e n t a r e t h a t s l u d g e is sterilized, d e o d o r i z e d , a n d c a n b e easily filtered e i t h e r in v a c u u m o r p r e s s u r e
filters.
A
c o m b i n e d flow d i a g r a m for t h e P o r t e u s a n d Z i m p r o p r o c e s s e s is s h o w n in F i g . 7.24. Coagulated sludge
Sludge inlet
X
Sludge outlet (To thickening and/or filtration/or drying beds) Fig.
7.24.
Combined
flow
diagrams
for
the Porteus
and Zimpro
processes.
T h e Porteus process involves continuous operation u n d e r pressures of 180-210 psig a n d t e m p e r a t u r e s of a p p r o x i m a t e l y 400°F. After passing t h r o u g h a g r i n d e r , t h e s l u d g e is p u m p e d f r o m t h e a c c u m u l a t o r t h r o u g h a h e a t ex c h a n g e r , w h e r e it is p r e h e a t e d b y h o t s l u d g e effluent f r o m t h e r e a c t o r . H i g h p r e s s u r e s t e a m is injected i n t o t h e r e a c t o r . R e t e n t i o n t i m e in t h e r e a c t o r is approximately 30 min. A n 8 0 - 9 0 % reduction of organic m a t t e r can be accomplished; thus some organic matter and a m m o n i a are found a m o n g the end products. T h e Z i m p r o p r o c e s s differs f r o m t h e P o r t e u s p r i m a r i l y in t h e fact t h a t a i r ( i n s t e a d o f s t e a m ) is injected b y a n a i r c o m p r e s s o r ( F i g . 7.24). M a x i m u m operating temperatures are 300°-600°F, a n d design operating pressures are 1 5 0 - 3 0 0 0 p s i g . C o m b u s t i o n is 8 0 - 9 0 % c o m p l e t e . H e a t r e l e a s e p e r l b o f a i r is 1200-1400 B T U .
8.
Pre-dewatering Treatment of Sludges
337
T h e N i c h o l s h e a t - t r e a t m e n t p r o c e s s [ 4 ] is a t h e r m o m e c h a n i c a l s y s t e m t h a t c o n d i t i o n s all t y p e s o f s e w a g e s l u d g e for effective d e w a t e r i n g . S l u d g e is s u b jected t o a t e m p e r a t u r e of 395°F for a p e r i o d of 30 m i n , b r e a k i n g d o w n g e l a t i n o u s cell s t r u c t u r e s t o r e l e a s e e n t r a i n e d w a t e r . A f t e r t h i s t h e r m a l a g i n g s t e p , specific r e s i s t a n c e a n d c a p i l l a r y s u c t i o n t i m e o f t h e s l u d g e a r e l o w e r e d t o a p o i n t w h e r e it c a n easily b e d e w a t e r e d m e c h a n i c a l l y t o a h i g h s o l i d s c o n centration without addition of chemicals. H e a t t r e a t m e n t c a n b e u s e d t o c o n d i t i o n all t y p e s o f c o m b i n a t i o n s o f o r g a n i c w a s t e w a t e r s l u d g e , i n c l u d i n g t h e difficult t o h a n d l e w a s t e a c t i v a t e d s l u d g e . T h e p r e s e n c e o f i n d u s t r i a l w a s t e s h a s n o t b e e n f o u n d t o affect s l u d g e c o n ditioning. A s s h o w n i n F i g . 7.25, s l u d g e first p a s s e s t h r o u g h g r i n d e r s , t h e n i n t o h i g h p r e s s u r e feed p u m p s . I n t e r l o c k s p r e v e n t i m p r o p e r flow levels a n d r a t e s a n d a s s u r e c o n t i n u o u s feed. S l u d g e t r a v e l s d i r e c t l y t o t h e h e a t e x c h a n g e r s e c t i o n , w h e r e it is b r o u g h t t o p r o c e s s t e m p e r a t u r e i n t w o s t a g e s ; i t t h e n flows c o n tinuously through the reactor along serpentine tubes, where thermal aging takes place. T h e treated sludge then travels back t h r o u g h the heat exchanger, t h i s t i m e g i v i n g u p its h e a t , a n d i n t o a vessel t h a t c o n t r o l s p r e s s u r e a n d b a l a n c e s t h e s y s t e m o u t p u t a g a i n s t i n p u t . H e a t - t r e a t e d s l u d g e is t h e n d i s c h a r g e d i n t o a d e c a n t i n g t a n k . T h i c k e n e d s l u d g e is d r a w n c o n t i n u o u s l y o u t o f the decanting t a n k into a conditioned sludge tank equipped with mixers. F i n a l l y , c o n d i t i o n e d , t h i c k e n e d , a n d h o m o g e n i z e d s l u d g e is p u m p e d t o t h e dewatering equipment. T h e p r i m a r y sludge heater employs water t o transfer heat from o u t g o i n g t o i n c o m i n g s l u d g e . T h i s m e t h o d p e r m i t s all s l u d g e t o flow u n h i n d e r e d t h r o u g h the heat exchanger along piping designed to eliminate the high maintenance costs associated with plugging. T h e secondary heater also m a k e s use of water t o t r a n s f e r h e a t t o s l u d g e . S i n c e it is a n i n d i r e c t " c l o s e d l o o p " s y s t e m , w a t e r is r e t a i n e d , a n d all h e a t t h a t r e m a i n s a f t e r t h e s l u d g e h a s b e e n h e a t e d t o p r o c e s s t e m p e r a t u r e is r e c y c l e d t o t h e h o t w a t e r g e n e r a t o r f o r r e u s e . I n a d d i t i o n , w i t h i n d i r e c t h e a t i n g t h e r e is n o i n c r e a s e i n t h e v o l u m e o f s l u d g e t o b e d e w a t e r e d . W h e n required, the N i c h o l s system can c o m b i n e direct heating for q u i c k s t a r t - u p a n d i n d i r e c t h e a t i n g t o p r o v i d e m a x i m u m d a y - i n , d a y - o u t efficiency. B e c a u s e o f a u t o m a t i c c o n t r o l s , safety i n t e r l o c k s , levels, t e m p e r a t u r e p r o b e s , a n d centralized valving, the entire Nichols heat-treatment system c a n be c o n t r o l l e d b y o n e o p e r a t o r . N e a r l y all s y s t e m c o m p o n e n t s c a n b e l o c a t e d o u t s i d e , effectively c u t t i n g d o w n o n i n d o o r s p a c e r e q u i r e m e n t s . T h e a u t o m a t i c pressure control system prevents boiling of sludge a n d r e s u l t a n t c o r r o s i o n o f p i p i n g , i n s p i t e o f flow c o n t r o l v a l v e o p e n i n g s o r e q u i p m e n t failures. T h i s a r r a n g e m e n t a l s o a l l o w s t h e s y s t e m t o r e a c h o p e r a t i n g p r e s s u r e a u t o m a t i c a l l y , a s s o o n a s t h e feed p u m p s a r e t u r n e d o n . W h e r e a N i c h o l s - H e r r e s h o f f m u l t i p l e h e a r t h f u r n a c e ( F i g . 7.26) is u s e d , t h e
Sludge Treatment and Disposal
I a
ι
s
ο
•ε v.
I I JO IS
8.
Pre-dewatering Treatment of Sludges
339
h e a t - t r e a t m e n t s y s t e m is o p e r a t e d , e x c e p t for e l e c t r i c a l p o w e r , f r o m t h e w a s t e heat recovered from the furnace. In addition, the conditioned sludge can be d e w a t e r e d t o s u c h a n e x t e n t t h a t fuel f o r t h e f u r n a c e is r e q u i r e d o n l y a t f u r n a c e warm-up.
Fig. 7.26. Burning furnace for sludge Corporation.)
flow diagram incineration.
of Nichols-Herreshoff multiple (Courtesy of Nichols Engineering &
hearth Research
7.
340
Sludge Treatment and Disposal
9. S l u d g e Disposal T w o f u n d a m e n t a l a p p r o a c h e s a r e utilized in s l u d g e d i s p o s a l : l a n d d i s p o s a l or incineration.
9.1. L A N D D I S P O S A L O F S L U D G E S T w o t y p e s o f l a n d d i s p o s a l o f s l u d g e s a r e m e n t i o n e d h e r e : (1) l a g o o n i n g a n d (2) o x i d a t i o n p o n d s . L a g o o n i n g m a y b e a n e c o n o m i c a l d i s p o s a l m e t h o d w h e n l a r g e l a n d a r e a s a r e a v a i l a b l e a t l o w c o s t . S u p e r n a t a n t l i q u o r is r e m o v e d c o n t i n u o u s l y , a n d e v e n t u a l l y t h e l a g o o n b e c o m e s filled w i t h s o l i d s . I n a 2 - t o 3-year p e r i o d a 5 0 - 6 0 % m o i s t u r e c o n t e n t c a n b e a t t a i n e d . T h e n t h e l a g o o n is a b a n d o n e d a n d a n e w l o c a t i o n is selected. O x i d a t i o n p o n d s s i m i l a r t o t h o s e d i s c u s s e d for w a s t e w a t e r s in C h a p t e r 6, S e c t i o n 6 c a n b e utilized. A n a e r o b i c c o n d i t i o n is m a i n t a i n e d o n t h e s u r f a c e , d u e m a i n l y t o t h e p r e s e n c e o f a l g a e . A n a e r o b i c c o n d i t i o n s p r e v a i l in d e e p e r layers.
9.2. S L U D G E I N C I N E R A T I O N S l u d g e i n c i n e r a t i o n is a f r e q u e n t l y e m p l o y e d d i s p o s a l m e t h o d . Selfs u s t a i n i n g c o m b u s t i o n is s o m e t i m e s p o s s i b l e after b u r n i n g a n a u x i l i a r y fuel w h i c h raises t h e t e m p e r a t u r e o f t h e i n c i n e r a t o r a b o v e i g n i t i o n p o i n t . T h e c o m b u s t i o n p r o d u c t s are mainly c a r b o n dioxide, sulfur dioxide, a n d ash. T w o t y p e s o f i n c i n e r a t i o n o p e r a t i o n s a r e (1) m u l t i p l e h e a r t h f u r n a c e s a n d (2) fluidized b e d d r y i n g a n d b u r n i n g . A d i a g r a m o f a m u l t i p l e h e a r t h f u r n a c e for i n c i n e r a t i o n o f s l u d g e s ( N i c h o l s - H e r r e s h o f f f u r n a c e ) is s h o w n i n F i g . 7.26.
Cyclone-W
Combustion gases + ash . Fluidized : bed ..
ScrubberH
Concentrated sludge Auxiliary air blower Fig. 7.27.
Sketch
of a fluidized
bed disposal
system.
341
References
S l u d g e p a s s e s d o w n w a r d t h r o u g h a series o f h e a r t h s . V a p o r i z a t i o n o f m o i s t u r e o c c u r s in t h e u p p e r h e a r t h s , f o l l o w e d b y i n c i n e r a t i o n in t h e l o w e r o n e s . Ashes from the b o t t o m of the furnace are collected by a dry ash h a n d l i n g system. A n alternative water scrubbing system can also be provided. E x h a u s t g a s e s c o m i n g f r o m t h e u p p e r s e c t i o n o f t h e f u r n a c e flow i n t o a V e n t u r i h i g h e n e r g y t y p e s c r u b b e r f o r r e m o v a l o f fly a s h . F u r n a c e t e m p e r a t u r e is 1 0 0 0 ° - 1 8 0 0 ° F . T h e f u r n a c e is a i r c o o l e d , a i r b e i n g b l o w n b y a f a n . C o o l i n g a i r is r e c i r c u l a t e d t h r o u g h t h e a i r r e c i r c u l a t i o n d u c t . A f r a c t i o n o f t h e c o o l i n g a i r is w a s t e d t o t h e a t m o s p h e r e . A s k e t c h o f a fluidized b e d d i s p o s a l s y s t e m is s h o w n i n F i g . 7.27. S l u d g e is fed i n t o a b e d o f s a n d fluidized b y a i r . T h e t e m p e r a t u r e o f t h e fluidized b e d is 1 4 0 0 ° - 1 5 0 0 ° F . T h e r e is r a p i d d r y i n g a n d c o m b u s t i o n o f t h e s l u d g e . A s h is carried u p from the bed by the c o m b u s t i o n gases a n d separated by water s c r u b b i n g a n d c y c l o n i n g . A u x i l i a r y fuel is u t i l i z e d a t least f o r s t a r t i n g t h e combustion process.
Problems I. T h e following data are obtained from bench scale aeration o f a sludge for which a c o n tinuous digester is to be designed. T i m e o f aeration (days)
Suspended solids (mg/liter)
Oxygen uptake rate [mg/(liter)(hr)]
0 5 10 15 20 25 30
6750 5650 4750 4200 3750 3600 3550
50 21 11 7 6 5.5 5.5
6000 lb/day o f this sludge are to be treated in an aerobic digester which is designed to achieve 80% reduction o f degradable volatile suspended solids. A s s u m e a steady state concentration of 3500 mg/liter o f suspended solids in the digester. Estimate basin v o l u m e (gal) and o x y g e n requirements (lb 0 / h r ) for the aerobic digester. 2
II. Air drying is being considered to dewater 1500 lb/day (dry weight) o f a sludge utilizing 8-in. lifts. A laboratory study has s h o w n that percolation increased solids concentration f r o m its initial value o f 2% to 20% solids in 2 0 hr. Calculate the areas o f drying beds ( f t ) required for liftable sludges with 30 and 35% solids, respectively. Base calculations o n 3 in. o f rainfall and 4.5 in. evaporation for a 30-day period. 2
References 1. Eckenfelder, W. W . , Jr., and Ford, D . L., "Water Pollution Control." Pemberton Press, Austin and N e w York, 1970. 2. Edde, H . J., and Eckenfelder, W . W., Jr., J. Water Pollut. Control Fed. 4 0 , N o . 8, 1486 (1968).
342
7.
Sludge Treatment and Disposal
3. M c C a r t h y , P. L., Public Works95,9-12 (1964). 4. Nichols Engineering & Research Corporation, "Nichols Heat Treatment Process for Sludge Conditioning," F o r m . H T 123. Nichols E n g . & Res. C o r p . , Belle M e a d , New Jersey, 1975. 5. R u t h , B . F., bid. Eng. Chem. 25,157 (1933). 6. Swanwick, J. D . , Lussignea, F. W . , and Baskerville, R . C , Adv. Water Pollut. Res. 2, 387-402 (1963). 7. Water Pollution Control Federation, "Sewage Treatment Plant Design,'* M a n u a l of Practice 8. Water Pollut. Contr. Fed., Washington, D . C . , 1959.
8 Tertiary Treatment of Wastewaters 1. Introduction
344
2. Suspended Solids Removal
344
3. Carbon Adsorption 3.1. Introduction 3.2. Activated Carbons as Adsorbents 3.3. Adsorption Isotherms 3.4. Adsorption Operation 3.5. Design Procedure for Activated Carbon Adsorption Columns .
345 345 345 346 347 347
4. Ion Exchange 4.1. Introduction 4.2. Basic Mechanism of Ion Exchange: Cation and Anion Exchangers 4.3. Design of Ion Exchange Columns 4.4. Design of an Ion Exchange S y s t e m 5. Reverse Osmosis 5.1. Osmosis and Osmotic Pressure 5.2. van't Hoff Equation for Osmotic Pressure 5.3. Principle of Reverse Osmosis 5.4. Preparation of R O Membranes 5.5. Preferential Sorption-Capillary Flow Mechanism for Reverse Osmosis 5.6. Characterization of Membrane Performance 5.7. Water Flux 5.8. Rejection Factor 5.9. Effect of Shrinkage Temperature on Performance of C A Membranes 5.10. Effect of Feed Temperature on Flux 5.11. Flux Decline 5.12. Fouling: Causes and Cures 5.13. Prediction of Flux 5.14. Membrane Leakage 5.15. Solute Permeability and Concentration Polarization 5.16. Experimental Technique for Laboratory Prediction of M e m brane Performance 5.17. Final Remarks on Reverse Osmosis 6. Electrodialysis 6.1. Introduction 6.2. Voltage Required for Electrodialysis 6.3. Current Required for Electrodialysis 6.4. Pretreatment of Wastewaters in Electrodialysis
359 359 359 361 363 367 367 368 369 371 372 373 374 374 376 377 378 378 380 381 382 383 384 385 385 385 385 386 343
344
8.
Tertiary Treatment of Wastewaters
7. Chemical Oxidation Processes (Chlorination and Ozonation) 7.1. Chlorination of Wastewaters 7.2. Ozonation of Wastewaters
9. Sonozone Wastewater Purification Process Problems
387 387 390 391 391 391 394 397 397 399
References
400
8. Nutrient Removal 8.1. Introduction 8.2. Phosphorus Removal 8.3. Nitrogen Removal 8.4. Added Benefits in Nutrient Removal
1. I n t r o d u c t i o n T e r t i a r y t r e a t m e n t (also referred a s " a d v a n c e d w a s t e w a t e r t r e a t m e n t " ) c o n s i s t s o f p r o c e s s e s w h i c h a r e d e s i g n e d t o a c h i e v e h i g h e r effluent q u a l i t y t h a n c o n v e n t i o n a l s e c o n d a r y t r e a t m e n t s d e s c r i b e d in C h a p t e r s 5 a n d 6. T h e f o l l o w i n g t y p e s o f t e r t i a r y t r e a t m e n t a r e d e s c r i b e d i n t h i s c h a p t e r : (1) s u s p e n d e d s o l i d s r e m o v a l , (2) c a r b o n a d s o r p t i o n ( o r g a n i c r e m o v a l ) , (3) i o n e x c h a n g e , (4) r e v e r s e o s m o s i s , (5) e l e c t r o d i a l y s i s , (6) c h e m i c a l o x i d a t i o n ( c h l o r i n a t i o n a n d o z o n a t i o n ) , (7) n u t r i e n t r e m o v a l m e t h o d s ( n i t r o g e n a n d p h o s p h o r u s r e m o v a l ) , a n d (8) s o n o z o n e w a s t e w a t e r p u r i f i c a t i o n p r o c e s s . T h e s e p r o c e s s e s a r e n o t utilized extensively in w a s t e w a t e r t r e a t m e n t t o d a y , b u t t h e i r u s e o n a n i n c r e a s i n g l y l a r g e r scale is a n t i c i p a t e d a s effluent q u a l i t y r e q u i r e m e n t s b e c o m e m o r e s t r i n g e n t in t h e f u t u r e .
2. S u s p e n d e d Solids Removal S u s p e n d e d solids which have n o t been r e m o v e d by conventional p r i m a r y a n d s e c o n d a r y o p e r a t i o n s m a y c o n s i t u t e a m a j o r p a r t o f t h e B O D o f effluents f r o m w a s t e w a t e r t r e a t m e n t p l a n t s . T h e f o l l o w i n g r e m o v a l p r o c e s s e s for t h e s e s u s p e n d e d s o l i d s a r e a v a i l a b l e : (1) m i c r o s c r e e n i n g , (2) filtration, a n d (3) coagulation. M i c r o s c r e e n s a r e w o u n d a r o u n d r o t a t i n g d r u m s . W a s t e w a t e r is fed c o n t i n u o u s l y t o t h e i n s i d e o f t h e d r u m , flowing t o a c l e a r w a t e r s t o r a g e c h a m b e r o n t h e o u t s i d e . C l e a n i n g o f t h e i n n e r surface o f t h e d r u m is p e r f o r m e d b y s p r a y s o f c l e a r w a t e r , w a s h i n g r e q u i r e m e n t s u s u a l l y b e i n g a b o u t 5 % o f feed v o l u m e . M i c r o s c r e e n i n g r e s u l t s in 7 0 - 9 0 % r e m o v a l o f s u s p e n d e d s o l i d s . F i l t r a t i o n is c o m m o n l y u s e d for s u s p e n d e d solids r e m o v a l y i e l d i n g r e m o v a l efficiencies u p t o 9 9 % . S a n d , a n t h r a c i t e , a n d d i a t o m a c e o u s e a r t h a r e t h e m o s t c o m m o n l y e m p l o y e d filter m e d i a . C o a g u l a t i o n is p e r f o r m e d u t i l i z i n g a l u m , polyelectrolytes, lime, a n d other chemical agents.
3.
345
Carbon Adsorption
3. C a r b o n A d s o r p t i o n 3.1.
INTRODUCTION
A d s o r p t i o n is t h e c o n c e n t r a t i o n o f a s o l u t e a t t h e s u r f a c e o f a s o l i d . T h i s p h e n o m e n o n t a k e s p l a c e w h e n s u c h a s u r f a c e is p l a c e d in c o n t a c t w i t h a solution. A layer of molecules of solute a c c u m u l a t e s a t the surface of the solid d u e t o i m b a l a n c e o f s u r f a c e forces ( F i g . 8.1). N o n b a l a n c e d forces
Fig.
8.1. Representation
of forces
in a
solid.
I n t h e i n t e r i o r o f t h e solid, m o l e c u l e s a r e c o m p l e t e l y s u r r o u n d e d b y s i m i l a r m o l e c u l e s a n d t h e r e f o r e s u b j e c t e d t o b a l a n c e d forces, a s i n d i c a t e d b y t h e a r r o w s in F i g . 8 . 1 . M o l e c u l e s a t t h e s u r f a c e a r e s u b j e c t e d t o n o n b a l a n c e d forces. B e c a u s e t h e s e r e s i d u a l forces a r e sufficiently s t r o n g , t h e y m a y i m p r i s o n m o l e c u l e s o f a s o l u t e w i t h w h i c h t h e s o l i d is in c o n t a c t . T h i s p h e n o m e n o n is c a l l e d p h y s i c a l ( o r v a n d e r W a a l s ) a d s o r p t i o n . T h e solid (e.g., a c t i v a t e d c a r b o n ) is t e r m e d t h e a d s o r b e n t a n d t h e s o l u t e b e i n g a d s o r b e d is t h e a d s o r b a t e . A d s o r p t i o n c a p a c i t y is directly r e l a t e d t o t h e t o t a l s u r f a c e o f a d s o r b e n t since t h e l a r g e r t h i s s u r f a c e is, t h e m o r e r e s i d u a l ( u n b a l a n c e d ) forces a r e a v a i l a b l e for a d s o r p t i o n .
3.2. A C T I V A T E D C A R B O N S A S A D S O R B E N T S A c t i v a t e d c a r b o n s h a v e b e e n w i d e l y u s e d a s a d s o r b e n t s in w a t e r t r e a t m e n t p l a n t s t o r e m o v e t a s t e a n d o d o r c a u s i n g o r g a n i c s . I t is e x p e c t e d t h a t w i t h t h e e m p h a s i s b e i n g p l a c e d o n h i g h e r q u a l i t y effluents, use o f a c t i v a t e d c a r b o n s in t e r t i a r y t r e a t m e n t o f w a s t e w a t e r s will i n c r e a s e c o n s i d e r a b l y in t h e f u t u r e . Preparation of activated carbons. A c t i v a t e d c a r b o n s a r e p r e p a r e d f r o m c a r b o n a c e o u s r a w m a t e r i a l s s u c h a s w o o d , lignite, c o a l , a n d n u t shells b y a p r o c e s s o f t h e r m a l a c t i v a t i o n w h i c h yields a v e r y p o r o u s s t r u c t u r e w i t h l a r g e s u r f a c e a r e a s (as h i g h a s 1000 m / g ) . A d s o r p t i o n e q u i l i b r i u m is e s t a b l i s h e d w h e n t h e c o n c e n t r a t i o n o f c o n t a m i n a n t r e m a i n i n g in s o l u t i o n is in d y n a m i c b a l a n c e with that at the surface of the solid. 2
8.
346
Tertiary Treatment of Wastewaters
Reactivation of activated carbons. T h e g r e a t a d v a n t a g e o f a c t i v a t e d c a r b o n a s a n a d s o r b e n t lies i n t h e p o s s i b i l i t y o f r e a c t i v a t i o n ( u p t o 3 0 o r m o r e t i m e s ) w i t h o u t a p p r e c i a b l e loss o f a d s o r p t i v e p o w e r . U s u a l l y , r e a c t i v a t i o n is d o n e by heating spent carbon t o a b o u t 1700°F in a steam-air a t m o s p h e r e (thermal reactivation). This operation c a n be performed in multiple hearth furnaces o r r o t a r y k i l n s . A d s o r b e d o r g a n i c s a r e b u r n e d off, a n d a c t i v a t e d c a r b o n is r e stored basically t o its initial a d s o r p t i o n capacity.
3.3. A D S O R P T I O N I S O T H E R M S Equilibrium relationships between adsorbent a n d adsorbate are described by adsorption isotherms. I n this section only t h e L a n g m u i r a n d Freundlich isotherms are mentioned.
3.3.1. Langmuir Isotherm I n t h e d e v e l o p m e n t o f t h e L a n g m u i r i s o t h e r m it is a s s u m e d t h a t t h e s o l u t e is a d s o r b e d a s a m o n o m o l e c u l a r l a y e r a t t h e s u r f a c e o f t h e a d s o r b e n t . T h i s is the m o s t often used a d s o r p t i o n i s o t h e r m , being given b y t h e relationship
X/M =
KbCI(\+KC)
(8.1)
w h e r e Xis t h e w e i g h t o f s o l u t e a d s o r b e d ( a d s o r b a t e ) ( m g ) ; Μ t h e w e i g h t o f adsorbent (g); Κ the equilibrium constant ( c m of adsorbent/mg of adsor 3
bate); C the equilibrium concentration of solute (mg/liter); a n d b a constant which represents the monolayer coverage per unit weight of adsorbent ( m g o f a d s o r b a t e / g o f a d s o r b e n t ) . A t y p i c a l p l o t o f X/M
vs. C based o n
E q . ( 8 . 1 ) is s h o w n i n F i g . 8.2. E q u a t i o n ( 8 . 1 ) is r e w r i t t e n i n l i n e a r f o r m b y t a k i n g t h e r e c i p r o c a l o f b o t h members.
\KXlM) = (llKb)(llC)
+ (\lb)
X / M
Fig. 8.2. Langmuir
isotherm.
(8.2)
3.
Carbon Adsorption
347
F r o m E q . (8.2) a p l o t o f I/(X/M)
v s . 1/C yields a s t r a i g h t line, w h i c h p e r m i t s
d e t e r m i n a t i o n o f p a r a m e t e r s tfand b f r o m i t s s l o p e a n d i n t e r c e p t , r e s p e c t i v e l y .
3.3.2. Freundlich Isotherm T h e F r e u n d l i c h i s o t h e r m is e x p r e s s e d b y t h e e q u a t i o n X/M
=
kC
1,n
(8.3)
X/M a n d C h a v e t h e s a m e m e a n i n g a s i n L a n g m u i r ' s i s o t h e r m , a n d k a n d η a r e c o n s t a n t s d e p e n d e n t o n several e n v i r o n m e n t a l f a c t o r s . E q u a t i o n (8.3) is r e w r i t t e n in l i n e a r f o r m b y t a k i n g l o g a r i t h m s o f b o t h m e m b e r s .
log(AVM) = (I/n) logC + log A:
(8.4)
E q u a t i o n (8.4) r e v e a l s t h a t a l o g a r i t h m i c p l o t o f X/M
v s . C yields a s t r a i g h t
line w h i c h p e r m i t s d e t e r m i n a t i o n o f p a r a m e t e r s η a n d k f r o m its s l o p e a n d intercept.
3.4. A D S O R P T I O N
OPERATION
I n p r a c t i c e , a d s o r p t i o n o f o r g a n i c s in a c t i v a t e d c a r b o n is c o n d u c t e d e i t h e r as a batch or continuous operation. In batch o p e r a t i o n , powdered activated c a r b o n is m i x e d w i t h t h e w a s t e w a t e r a n d a l l o w e d t o settle. C o n t i n u o u s o p e r a t i o n is p e r f o r m e d i n c o l u m n s c o n t a i n i n g g r a n u l a r c a r b o n ( 4 0 - 8 0 m e s h ) . I t is more economical than batch operation a n d has found the widest application. R e m o v a l o f o r g a n i c s in a c t i v a t e d c a r b o n c o l u m n s o c c u r s b y t h r e e m e c h a n i s m s : (1) a d s o r p t i o n o f o r g a n i c m o l e c u l e s , (2) filtration o f l a r g e p a r t i c l e s , a n d (3) p a r t i a l d e p o s i t i o n o f c o l l o i d a l m a t e r i a l . P e r c e n t r e m o v a l d e p e n d s primarily on contact time between wastewater and activated carbon. A s w a s t e w a t e r flows t h r o u g h t h e b e d , c a r b o n n e a r e s t t o t h e feed p o i n t b e c o m e s s a t u r a t e d a n d m u s t b e r e p l a c e d w i t h fresh c a r b o n . T h i s is d o n e b y o p e r a t i n g several s u i t a b l y v a l v e d c o l u m n s in series. T h e first c o l u m n is r e p l a c e d w h e n e x h a u s t e d , a n d t h e flow o f w a s t e w a t e r is s w i t c h e d t o m a k e t h a t c o l u m n t h e last o n e in t h e series. I n l a r g e i n s t a l l a t i o n s r e g e n e r a t i o n o f s p e n t c a r b o n is e s s e n t i a l for e c o n o m i c feasibility.
3.5. D E S I G N P R O C E D U R E F O R A C T I V A T E D CARBON ADSORPTION COLUMNS T h e d e s i g n p r o c e d u r e d e s c r i b e d is r e c o m m e n d e d b y E c k e n f e l d e r a n d F o r d [ 5 ] a n d is b a s e d o n a n e q u a t i o n d e r i v e d b y B o h a r t a n d A d a m s [ 1 ] . T h e fol l o w i n g t o p i c s a r e d i s c u s s e d : (1) B o h a r t a n d A d a m s ' e q u a t i o n for p e r f o r m a n c e o f a c t i v a t e d c a r b o n a d s o r p t i o n c o l u m n s ( S e c t i o n 3 . 5 . 1 ) ; (2) l a b o r a t o r y t e s t s w i t h b e n c h scale c o l u m n s t o o b t a i n t h e n e c e s s a r y d e s i g n p a r a m e t e r s ( S e c t i o n 3 . 5 . 2 ) ; (3) " s c a l e - u p " o f l a b o r a t o r y d a t a a n d d e s i g n o f a p l a n t scale u n i t ( S e c t i o n 3 . 5 . 3 ) ; a n d (4) d e r i v a t i o n o f B o h a r t a n d A d a m s ' e q u a t i o n ( S e c t i o n 3.5.4).
348
8.
Tertiary T r e a t m e n t of W a s t e w a t e r s
3.5.1. Bohart and A d a m s ' Equation In the o p e r a t i o n of a n activated c a r b o n a d s o r p t i o n c o l u m n , wastewater w i t h a n influent s o l u t e c o n c e n t r a t i o n C ( m g / l i t e r ) is fed i n t o t h e c o l u m n . I t is d e s i r e d t o r e d u c e s o l u t e c o n c e n t r a t i o n in t h e effluent t o a v a l u e n o t ex ceeding C (mg/liter), as determined by water quality requirements. 0
E
A t t h e b e g i n n i n g o f t h e o p e r a t i o n , w h e n a c t i v a t e d c a r b o n is fresh, effluent c o n c e n t r a t i o n is a c t u a l l y lower t h a n a l l o w a b l e c o n c e n t r a t i o n C . A s t h e o p e r a t i o n p r o c e e d s a n d a c t i v a t e d c a r b o n a p p r o a c h e s s a t u r a t i o n , effluent c o n c e n t r a t i o n r e a c h e s v a l u e C . T h i s c o n d i t i o n is called t h e b r e a k p o i n t . L e t / b e t h e t i m e e l a p s e d t o r e a c h t h e b r e a k p o i n t (service t i m e , h o u r s ) . A t t i m e /, o p e r a t i o n is d i s c o n t i n u e d a n d a c t i v a t e d c a r b o n is r e g e n e r a t e d . E
E
A t t i m e z e r o (t = 0 ) , t h e t h e o r e t i c a l d e p t h o f c a r b o n w h i c h is sufficient t o p r e v e n t effluent s o l u t e c o n c e n t r a t i o n f r o m e x c e e d i n g v a l u e C is c a l l e d t h e critical b e d d e p t h D (ft). E v i d e n t l y , D < D, w h e r e D is t h e a c t u a l b e d d e p t h (ft) ( F i g . 8 . 3 ) . E
0
0
S e r v i c e time; t [Break
ft/hr
point]
C' >C E
E
7"" Do
"0 Fig.
8.3.
Concept
A d s o r p t i v e c a p a c i t y (N ) 0
'
of critical
bed
depth
0
(D ). 9
is a n o t h e r i m p o r t a n t d e s i g n v a r i a b l e . I t is t h e
m a x i m u m a m o u n t of c o n t a m i n a n t solute t h a t c a n be a d s o r b e d by the c a r b o n (lb solute/ft
3
t a k e n before
o f c a r b o n ) w h e n s a t u r a t i o n o c c u r s . T h e b r e a k p o i n t is u s u a l l y s a t u r a t i o n o c c u r s . A n o t h e r p a r a m e t e r w h i c h e n t e r s in B o h a r t
a n d A d a m s ' e q u a t i o n is t h e r a t e c o n s t a n t K. T h i s a s s u m e s a f i r s t - o r d e r a d sorption rate r = Kc
(8.5)
3.
349
Carbon Adsorption
m g of solute/(g of c a r b o n ) (hr) = Kmg
of solute/liter of solution
.'. Κ = liter of solution/(g of c a r b o n ) ( h r ) o r in E n g l i s h u n i t s Κ = f t of solution/(lb of c a r b o n ) (hr) 3
Performance of c o n t i n u o u s activated c a r b o n c o l u m n s m a y be evaluated by Eq. (8.6), which was developed by B o h a r t a n d A d a m s [ 1 ] . l n [ ( C / C ) - 1] = \n(e ° KN
0
1) - KC t
DiV
£
(8.6)
0
w h e r e C is t h e influent s o l u t e c o n c e n t r a t i o n ( m g / l i t e r ) ; C t h e a l l o w a b l e effluent s o l u t e c o n c e n t r a t i o n ( m g / l i t e r ) ; Κ the r a t e c o n s t a n t [ f t / ( l b o f c a r b o n ) ( h r ) ] ; N the adsorptive capacity (lb of solute/ft of c a r b o n ) ; D the d e p t h of c a r b o n b e d (ft); V t h e l i n e a r flow r a t e ( f t / h r ) ; a n d / t h e service t i m e ( h r ) . 0
E
3
3
0
A n e q u a t i o n for D is w r i t t e n f r o m E q . (8.6). D (critical d e p t h o f c a r b o n b e d , ft) is t h e t h e o r e t i c a l d e p t h o f c a r b o n sufficient t o p r e v e n t effluent s o l u t e c o n c e n t r a t i o n f r o m e x c e e d i n g v a l u e C a t / = 0. D is o b t a i n e d f r o m E q . (8.6) b y l e t t i n g t = 0 a n d s o l v i n g f o r D ( w h i c h e q u a l s D in t h i s c a s e ) . S i n c e the exponential term e is u s u a l l y m u c h l a r g e r t h a n u n i t y , t h e u n i t y t e r m w i t h i n b r a c k e t s in t h e r i g h t - h a n d m e m b e r o f E q . (8.6) is n e g l e c t e d . T h e final r e s u l t is D = (V/KNo) ln [ ( C / C ) - 1 ] (8.7) 0
0
E
0
0
K N o D , v
0
Expression
for
service
0
£
time t. S o l v i n g E q . (8.6) for t a n d n e g l e c t i n g t h e
u n i t y t e r m w i t h i n b r a c k e t s in t h e r i g h t - h a n d m e m b e r a s c o m p a r e d t o t h e exponential term. t = (No/Co
V)D - ln [ ( C / C ) - \]/KC 0
£
(8.8)
0
E q u a t i o n (8.8) is t h e b a s i s for e x p e r i m e n t a l d e t e r m i n a t i o n o f p a r a m e t e r s
N
0
a n d Κ f r o m b e n c h s c a l e c o l u m n s . T h e p r o c e d u r e f o l l o w e d is d e s c r i b e d in S e c t i o n 3.5.2.
3.5.2. Determination of Parameters N , and D f r o m Laboratory Data 0
K,
0
L a b o r a t o r y e q u i p m e n t r e c o m m e n d e d b y E c k e n f e l d e r a n d F o r d [ 5 ] is s h o w n in F i g . 8.4. R e q u i r e d d a t a for r e m o v a l o f o r g a n i c s a r e o b t a i n e d b y passing wastewater containing a k n o w n concentration of organic material ( C ) t h r o u g h a series o f c o l u m n s (e.g., t h r e e c o l u m n s in F i g . 8.4) a n d r e c o r d i n g t h e t i m e s t {t t , a n d t ) a t w h i c h c o n c e n t r a t i o n s o f effluents f r o m c o l u m n s # 1 , # 2 , a n d # 3 r e a c h t h e a l l o w a b l e effluent s o l u t e c o n c e n t r a t i o n ( C ) . E a c h set o f e x p e r i m e n t s is p e r f o r m e d a t c o n s t a n t flow r a t e [ g a l / ( m i n ) ( f t ) ] , t h e r e fore l i n e a r velocity V (ft/sec) is h e l d c o n s t a n t . Effluent f r o m c o l u m n # 1 is t h e first t o r e a c h v a l u e C ( r e c o r d t h i s t i m e ^ ) ; s o m e t i m e after, effluent f r o m c o l u m n #2 r e a c h e s v a l u e C , t h i s t i m e a l s o b e i n g r e c o r d e d ( f ) . F i n a l l y , 0
u
2
3
£
2
E
£
2
350
8. Influent
Column internal
diameter:!"
#3
Φ2 Sampling
Fig. 8.4. Activated
carbon
Tertiary Treatment of W a s t e w a t e r s
ports
columns
(bench
effluent f r o m c o l u m n # 3 r e a c h e s c o n c e n t r a t i o n C .
scale).
T h i s t i m e is a l s o r e c o r d e d
E
( i ) a n d t h e e x p e r i m e n t d i s c o n t i n u e d . C o n s e q u e n t l y , t h e e x p e r i m e n t is c a r r i e d 3
o u t u n t i l t h e b r e a k p o i n t o f t h e l a s t c o l u m n is r e a c h e d . T a b l e 8.1 s h o w s a t y p i c a l t a b u l a t i o n o f t v s . D v a l u e s a t f o u r different
flow
r a t e s . F o r E x p e r i m e n t n o . 1, t h r e e c o l u m n s o f 2.5 ft o f c a r b o n d e p t h e a c h a r e u t i l i z e d , c o r r e s p o n d i n g t o t o t a l d e p t h s o f 2 . 5 , 5.0, a n d 7.5 ft a t s a m p l i n g p o r t s . F l o w r a t e is 2 . 0 g a l / ( m i n ) ( f t ) . 2
F o r E x p e r i m e n t n o . 2, t h e first t w o c o l u m n s c o n t a i n a d e p t h o f 2 . 5 ft o f c a r b o n e a c h , a n d t h e t h i r d 5 ft. T o t a l d e p t h s c o r r e s p o n d i n g t o s a m p l i n g p o r t s a r e 2 . 5 , 5.0, a n d 10 ft. F l o w r a t e is 4 . 0 g a l / ( m i n ) ( f t ) . 2
F o r e x p e r i m e n t s n o . 3 a n d 4 , c a r b o n d e p t h s a r e , r e s p e c t i v e l y , 5-5-5 ft a n d 5-10-10 ft for t h e t h r e e c o l u m n s , c o r r e s p o n d i n g t o t o t a l d e p t h s a t s a m p l i n g p o r t s o f 5-10-15 ft a n d 5-15-25 ft. F l o w r a t e s a r e 8 a n d 16 g a l / ( m i n ) ( f t ) , 2
r e s p e c t i v e l y . E q u a t i o n (8.8) r e v e a l s t h a t a p l o t o f t v s . D yields a s t r a i g h t line for w h i c h s l o p e (s) a n d i n t e r c e p t ( i ) a r e s = .'.
N
=
0
i
=
No/CoV CQVS
(8.9)
In [(Co/CJJ) — 1]/ K C Q
Λ Κ = l n [ ( C / C ) - 1]//C 0
Values of parameters N
0
£
0
(8.10)
a n d A ' a r e d e t e r m i n e d f r o m E q s . (8.9) a n d ( 8 . 1 0 ) ,
r e s p e c t i v e l y . F i g u r e 8.5 s h o w s f o u r s t r a i g h t lines c o r r e s p o n d i n g t o t h e f o u r experiments for which d a t a are presented in T a b l e 8.1. Critical d e p t h D
0
c a l c u l a t e d f r o m E q . (8.7).
is
3.
351
Carbon Adsorption
T A B L E 8.1 Data for Example 8.1
(2) Bed depth (ft)
w
Experiment no.
F l o w rate [gal/(min)(ft )] 2
Z>! D D D D
= = = = = 2>3 = D, = Z) = D = D = £> = D =
2.0
1
3
4.0
2
t
2
8.0
3
2
3
16.0
4
t
2
3
740 1780 2780 180 560 1330 170 500 830 60 390 730
484 1164 1818 235 732 1740 445 1308 2171 314 2040 3819
(i)
2.5 5.0 7.5 2.5 5.0 10.0 5.0 10.0 15.0 5.0 15.0 25.0
2
Time, / (hr)
(4) Throughput volume (gal) e
"Calculation procedure for c o l u m n (4) o f Table 8.1. Cross-sectional area is A = (i)nUr)
= 0.00545 f t
2
2
Throughput volume = gal /(min) ( f t ) χ 0.00545 f t χ ( i 6 0 ) m i n 2
2
or (4) = ( / ) χ (5) χ 0.327
Example 8.1 D a t a in T a b l e 8.1 a r e o b t a i n e d b y u s i n g c o n t i n u o u s b e n c h scale a c t i v a t e d c a r b o n a d s o r p t i o n c o l u m n s w i t h 1-in. i n s i d e d i a m e t e r . F o u r sets o f e x p e r i ments are performed. T h e wastewater contains 20 mg/liter of a n organic solute removable by c a r b o n adsorption. Experiments are carried o u t record i n g t h e t i m e t a k e n for effluents f r o m t h e first, s e c o n d , a n d t h i r d a d s o r p t i o n c o l u m n s t o r e a c h a c o n c e n t r a t i o n C = 1.0 m g / l i t e r o f s o l u t e [ c o l u m n (3) o f T a b l e 8 . 1 ] . P r e p a r e a p l o t o f p a r a m e t e r s N , K, a n d D v s . flow r a t e [ g a l / (min(ft )]. E
0
0
2
SOLUTION A p l o t o f / v s . D [ c o l u m n (3) v s . c o l u m n (2) o f T a b l e 8.1] is p r e s e n t e d in F i g . 8.5. A d s o r p t i v e c a p a c i t y N is c a l c u l a t e d f r o m E q . (8.9), w h e r e (20 m g / l i t e r = 2 0 χ 1 0 " l b s o l u t e / l b l i q u o r ) 0
6
C
= 20 χ 1 0 " lb solute/lb liquor χ 62.4 lb liquor/ft liquor 6
0
3
= 1.248 χ 1 0 " lb solute/ft liquor 3
3
V a l u e s o f V (ft/hr) a r e Exp. n o . 1
2.0 gal/(min)(ft ) χ ft /7.48 gal χ 60 m i n / h r = 16 ft/hr 2
3
352
8.
Tertiary Treatment of Wastewaters
Exp. n o . 2
4.0 g a l / ( m i n ) ( f t ) χ ft /7.48 gal χ 60 m i n / h r = 32 ft/hr
Exp. n o . 3
8.0 g a l / ( m i n ) ( f t ) χ f t / 7 . 4 8 gal χ 60 m i n / h r = 64 ft/hr
Exp. n o . 4
16.0 g a l / ( m i n ) ( f t ) χ f t / 7 . 4 8 gal χ 60 m i n / h r = 128 ft/hr
2
3
2
3
2
T h e calculation of N
0
3
is p r e s e n t e d i n T a b l e 8.2.
3000, ;perimen t no. 1 (7.5;27 B0)f
[2.0 gpn l/ft ] 2
2500
= 2000 oo
Γ
!(5il780 © 1500 c ε ο ο, ~ 1000
iperimen \ no. 2 [4.0 gprr ,/ft ] 2
rfl0il33C srExper iment nc .3 \ [ 8 . C » gpm/ft*]
-7(2.5; 740)
(25^ '30)
ι rtl5i830 1
Αδ60) 500
«
— E x p e riment η ).4 [l6.< ) gpm/f 1
Ϊ5-60) P t e 5 ; l SO) -500L
5
10
15
20
25
Depth, D (ft) [column®, table 8.l] Fig. 8.5. Plots
of t vs. D for Example
8.1.
T A B L E 8.2 Calculation of /V (Example 8.1) 0
N
0
Exp. no.
F l o w rate [gal/(min)(ft )]
V (ft/hr)
1 2 3 4
2.0 4.0 8.0 16.0
16 32 64 128
2
Slope (Fig. 8.5) (hr/ft) 408 153 66 33
= C Vs 0
Eq.(8.9)
(*-$•£·*-«*·) (Co = 1.248x 1 0 - l b / f t ) 3
8.15 6.11 5.27 5.27
3
3.
353
Carbon Adsorption
T A B L E 8.3 Calculations of Κ and D (Example 8.1 )
a
0
Exp. no.
Flow rate [gal/(min)(ft )]
(ft/hr) (Table 8.2)
No (lb/ft ) (Table 8.2)
ι (absolute value of intercept) (Fig. 8.5)
Eq. (8.10) [ft /(lb)(hr)]
Eq. (8.7) (ft)
1 2 3 4
2.0 4.0 8.0 16.0
16 32 64 128
8.15 6.11 5.27 5.27
283 203 151 112
8.3 11.6 15.6 21.1
0.695 1.33 2.29 3.31
V
2
3
K,
D, 0
3
C = 1.248x 10" lb/ft . 3
3
0
— •
1
ο • • Δ
1
no. 1 no. 2 no. 3 no. 4
<
0(2,8-15)
20
Κ
\
1
6
^
8
(16i33l)
( 3;I5.6^T
(4;II.6W Ρ
1
Experiment Experiment Experiment Experiment
"fi;2.29l/
7 /
\
6§ '
ΙΟ
ο
(2;8.3)
/
yfoij
Ζ
>3) (8-5 27)
Λ
(1' >;5.27) 5
>4(2;0.6< >5)
5
10
15
Flow rate [ g a l / ( m i n ) ( f t ) ] 2
Fig. 8.6. Plots
of 7V K. and D 0f
9
vs. flow
rate (Example
8.1).
V a l u e s o f Κ a r e c a l c u l a t e d f r o m E q . (8.10), w h e r e ln [ ( C / C — 1] = l n [ ( 2 0 / l ) - l ] = 2.94. V a l u e s o f D a r e c a l c u l a t e d f r o m E q . (8.7). C a l c u l a t i o n s o f Κ a n d D a r e p r e s e n t e d i n T a b l e 8 . 3 . V a l u e s o f JV , K, a n d D a r e p l o t t e d v s . flow r a t e i n F i g . 8.6. 0
£
0
0
0
0
3.5.3. S c a l e - U p of Laboratory Data and D e s i g n of a Plant S c a l e Unit P l a n t scale d e s i g n is i l l u s t r a t e d b y E x a m p l e 8.2. E x a m p l e 8.2 W a s t e w a t e r utilized i n l a b o r a t o r y e x p e r i m e n t s ( E x a m p l e 8.1) is t r e a t e d f o r removal of solute (concentration = 20 mg/liter) t o a residual value of 1 mg/liter in a n a c t i v a t e d c a r b o n b e d 2.5 ft i n d i a m e t e r a n d 6 ft d e e p . F l o w is 2 5 , 0 0 0 gal/day.
354
Tertiary Treatment of Wastewaters
8.
1. C a l c u l a t e service t i m e ( h o u r s p e r cycle). 2. C a l c u l a t e n u m b e r o f c a r b o n c h a n g e s r e q u i r e d p e r y e a r a n d a n n u a l carbon volume (ft ). 3
3. E s t i m a t e s o l u t e r e m o v a l ( l b / y e a r ) . 4 . C a l c u l a t e a d s o r p t i o n efficiency b a s e d o n N a n d D . 5. E s t i m a t e % e r r o r in n e g l e c t i n g u n i t y f a c t o r in t e r m (e T h i s a p p r o x i m a t i o n is u t i l i z e d in a r r i v i n g a t E q s . (8.7)—(8.10). 0
0
—
1).
KNoD/v
SOLUTION Step
P a r t 1. C a l c u l a t i o n o f service t i m e
1. F l o w r a t e in g a l / m i n is (25,000)/(24) (60) = 17.4 g a l / m i n . C r o s s -
s e c t i o n a l a r e a o f c o l u m n is A = (£) π ( 2 . 5 ) = 4.9 f t . 2
F l o w r a t e in
2
gal/
( m i n ) ( f t ) is 17.4/4.9 = 3.6 g a l / ( m i n ) ( f t ) , o r 2
2
V = 3.6 gal/(min)(ft ) χ 60 m i n / h r χ ft /7.48 gal = 28.9 ft/hr 2
3
Step 2. F r o m F i g . 8.6, for flow r a t e 3.6 g a l / ( m i n ) ( f t ) , o n e r e a d s 2
K=
10.7ft /(lb)(hr) 3
and N
0
= 6.35 lb/ft
3
Step 5 . Service t i m e is c a l c u l a t e d f r o m E q . (8.8). / = [6.35/(1.248 χ 1 0 " χ 28.9)]6 - 2.94/(10.7 χ 1.248 χ Ι Ο " ) = 836 hr/cycle 3
SOLUTION
3
P a r t 2. N u m b e r o f c a r b o n c h a n g e s p e r y e a r a n d a n n u a l c a r b o n
volume N o . c a r b o n changes/year = (365 χ 24)/836 = 10.5 A n n u a l c a r b o n v o l u m e = (6 χ 4.9) 10.5 = 309 f t SOLUTION
3
Part 3. Estimation of solute removal (lb/year)
S o l u t e r e m o v e d p e r cycle is c a l c u l a t e d f r o m lb solute removed per cycle = lb solute in influent per cycle - lb solute in effluent per cycle Since Volume of wastewater per cycle = 25,000 gal/day χ 836 hr/cycle χ day/24 h r χ ft /7.48 gal 3
= 1.16 x 1 0 f t / c y c l e 5
3
and Co = 20 mg/liter = 1.248 χ 1 0 "
3
lb/ft
3
then lb solute in influent per cycle = 1.248 χ 1 0 ~ lb/ft χ 1.16 χ 1 0 ft /cycle 3
= 144.8 lb/cycle
3
5
3
3.
355
Carbon Adsorption
R e s i d u a l s o l u t e l e a v i n g w i t h effluent p e r cycle is c a l c u l a t e d f r o m (lb solute in effluent per cycle) = (1.16 x 1 0 ) f t / c y c l e χ C 5
= 1.16 χ 1 0 C a v e
a v e
3
(lb/cycle)
5
C
lb/ft
3
a v e
is g i v e n b y / V = 8 3 6 hr
C
= (1/836)
a v c
Cdt
Jt = 0 C s t a n d s for a series o f effluent c o n c e n t r a t i o n s ( i n c r e a s i n g v a l u e s ) , t h e h i g h e s t one being that corresponding to C
E
= 1 mg/liter (or 6.24 χ 1 0 "
lb/ft ),
5
3
w h i c h o c c u r s a f ter 836 h r o f o p e r a t i o n . If i n t e r m e d i a t e v a l u e s o f effluent c o n c e n t r a t i o n s f r o m t i m e z e r o ( s t a r t o f cycle) t o t = 836 h r ( e n d o f cycle) a r e recorded, one can evaluate the integral by graphical or numerical methods. In the absence of these values one can m a k e a conservative (low) estimate of solute removed based on the C
E
value of 1 mg/liter (6.24 χ 1 0 "
lb/ft ). The
5
3
e r r o r i n s u c h a n e s t i m a t e is less t h a n 5 % (since i n t h i s c a s e C /C 0
= 20).
E
Thus, High estimate of lb solute in effluent per cycle = 1.16 χ 10 ft /cycle χ 6.24 χ 1 0 " lb/ft 5
3
5
3
= 7.2 lb/cycle
and L o w estimate of lb solute removed per cycle = 144.8 - 7.2 = 137.6 lb/cycle or 137.6 lb/cycle x 10.5 cycle/year = 1445 lb/year P a r t 4. A d s o r p t i o n efficiency
SOLUTION Based on
N: 0
Total solute a d s o r b e d : 1445 lb/year (Solution, P a r t 3) T o t a l adsorptive capacity: (N )(annual 1962 lb Efficiency: (1445/1962) χ 100 = 73.6% 0
2
0
0
3
1.23 ft.
Efficiency: [ ( / > - D ) I D ] 100 = [ ( 6 - 1.23)/6] 100 = 79.5% 0
SOLUTION
P a r t 5. E r r o r in n e g l e c t i n g f a c t o r o f u n i t y (10.7 χ 6.35 x6.0)/28.9 = 14.11
KN D/V= 0
^i4.n e
3
=
f r o m F i g . 8.6 for flow r a t e 3.6 g a l / ( m i n ) ( f t ) , o n e r e a d s
Based on D : D=
c a r b o n volume) = 6.35 lb/ft χ 309 f t
i 4 . ii _ j
=
ι
=
ΐ 42,440
> 3 4
2,44ΐ
)3
E r r o r % = (1/1,342,440)100 = 7.45 χ 1 0 " % 5
8.
356
Tertiary Treatment of Wastewaters
3.5.4. Derivation of the Bohart and A d a m s Equation [1] C o n s i d e r a m a s s o f a d s o r b e n t . I t s r e s i d u a l c a p a c i t y (N)
diminishes at a
r a t e g i v e n b y E q . (8.11). dN/dt
= -KNC
(8.11)
w h e r e Ν is t h e r e s i d u a l a d s o r b i n g c a p a c i t y [ a t t = 0, Ν = N
0
= adsorptive
capacity ( l b / f t ) ] ; C the solute concentration (lb/ft ); t the time (hr); a n d Κ 3
3
the rate constant [ft /(lb)(hr)]. 3
C o n s i d e r n o w t h e s o l u t i o n f r o m w h i c h s o l u t e is r e m o v e d b y a d s o r p t i o n . Solute concentration diminishes at a rate given by BC/dD = -KNC/V
(8.12)
w h e r e D is t h e d e p t h o f a d s o r b e n t ( t o t a l d e p t h , D = D ) 0
(ft); a n d V t h e
flow v e l o c i t y o f s o l u t i o n p a s t t h e a d s o r b e n t (ft/hr). T h e f o l l o w i n g b o u n d a r y c o n d i t i o n s ( B C ) a p p l y : F o r B C - 1 a t t = 0, Ν = (initial s o l u t e c a p a c i t y o f a d s o r b e n t ) . F o r B C - 2 a t D = 0, C = C
0
N
0
(inlet c o n
centration). Perform the following changes of variable. Let N' = . N=
N/No
(8.13)
N N'
(8.14)
0
= C/Co
(8.15)
. c = CC
(8.16)
D' =
KN DjV 0
(8.17)
D =
D'V/KNo
(8.18)
t' =
KC t
(8.19)
t'lKCo
(8.20)
C
0
0
: t =
W i t h this change of variables, o n e writes t w o modified b o u n d a r y c o n d i t i o n s : F o r B C - 1 ' a t f' = 0 (i.e., t = 0 ) , since Ν = Λ^ , t h e n N' = N /N = 1. F o r B C - 2 ' a t D' = 0 (i.e., D = 0 ) , since C = C , t h e n C = C / C = 1. E q u a t i o n s (8.11) a n d (8.12) a r e w r i t t e n in t e r m s o f t h e n e w v a r i a b l e s N', C , D ' , a n d t \ s u b s t i t u t i n g N, C , D a n d / b y t h e i r v a l u e s given b y E q s . (8.14), (8.16), (8.18), a n d (8.20). T h e final r e s u l t after simplification is 0
0
0
0
0
0
9
F r o m Eq. (8.11) BN'Idt'
= -N'C
(8.21)
or θ In N'/dt'
= -C"
(8.22)
3.
Carbon Adsorption
357
F r o m Eq. (8.12) dC'IdD'
= -N'C
(8.23)
or dlnC'/dD'
= -N'
(8.24)
D i f f e r e n t i a t i n g - E q . (8.22) w i t h r e s p e c t t o D' a n d E q . (8.24) w i t h r e s p e c t t o t' l e a d s t o d InN'/dD'
dt' = -dC'IdD'
= N'C
(8.25)
3
dD' = -dN'/dt'
= N'C
(8.26)
2
2
In C'ldt'
S u b t r a c t i n g E q . (8.25) f r o m E q . ( 8 . 2 6 ) , 3
2
In C'ldt' dD' - B InN'/dD'
dt' = 0
2
or d \n(C'/N')ldt' 2
dD' = 0
(8.27)
+f(t')
(8.28)
I n t e g r a t i o n o f E q . (8.27) y i e l d s \n(C'/N')
= f(D')
w h e r e f { D ' ) a n d f ( t ' ) a r e , r e s p e c t i v e l y , f u n c t i o n s o f D' a n d t' alone? b o u n d a r y c o n d i t i o n s B C - Γ a n d B C - 2 ' , it f o l l o w s t h a t
Imposing
f
\n(C'/N')
= t'-D'
(8.29)
C'/N'
= e ' '
(8.30)
or r
D
E q u a t i o n (8.23) is r e w r i t t e n a s [ d C ' / i - C O l / a D ' = N'.
Dividing both mem
b e r s b y C" a n d e m p l o y i n g E q . ( 8 . 3 0 ) , -(dC'IC' )ldD 2
= N'/C
f
= e 'D
v
(8.31)
* P r o o f that Eq. (8.28) is a solution for differential equation (8.27): differentiating Eq. (8.28) with respect to D' and then with respect to yields Eq. (8.27), since dfitydD'
= 0
and a df
df(D') dD'
[since f(D') a n d / ( / ' ) are, respectively, functions of D' and / ' alone], t Application of B C - Γ and B C - 2 ' to E q . (8.29) leads to an identity In 1/1 = 0 - 0 = 0. T h e r e f o r e / ( D O and / ( / ' ) are satisfied by / ( D O = —D'
and fit')
= t'
358
8.
Tertiary Treatment of W a s t e w a t e r s
E q u a t i o n (8.31) i n t e g r a t e s t o yield* l/C" = * w h e r e ψ(ί')
D
' - ' - ^ ( 0
(8.32)
is a f u n c t i o n o f / ' a l o n e .
F u n c t i o n \l/(t')
is e v a l u a t e d b y i m p o s i n g b o u n d a r y c o n d i t i o n B C - 2 ' . ψ(ί')
= e~
-
v
1
f
(8.33)
T h u s E q . (8.32) becomes 1 / C = e '-*'
-e-*'
+ \
(8.34)
l/i^'-'-e-'+l)
(8.35)
0
F r o m E q . (8.34), C
=
M u l t i p l y i n g b o t h n u m e r a t o r a n d d e n o m i n a t o r o f E q . (8.35) b y e*\ a
= J'Ke^-l+e*')
(8.36)
S u b s t i t u t i n g i n E q . (8.36) C , D', a n d t' b y t h e i r v a l u e s g i v e n b y E q s . (8.15), (8.17), a n d (8.19), t h e r e s u l t is
C/Co
= e
K C o
7(^
o D / K
-1
e
+
K C o t
)
(8.37)
S o l v i n g E q . (8.37) for t, t h e final r e s u l t is KN D/V
E
J
0
E q u a t i o n (8.38) is r e a r r a n g e d t o yield E q . (8.6). (t is t a k e n a s t h e service t i m e a n d therefore concentration C equals
C .) E
* Proof that Eq. (8.32) is a solution for Eq. (8.31) is obtained by differentiation of Eq. (8.32) with respect to D ' . Notice that di/r(t')/dD' = 0, and therefore
d(\IC)ldD' = d ^ - ^ I d D ' or
-(dC'IdDyC
2
= e '- ' 0
1
and finally
-(dC'IC' )ldD' = 2
which is Eq. (8.31). f
Application of BC-2' to Eq. (8.34) leads to an identity 1/1 =
e
- f
Consequently, ^ ( O is satisfied by Eq. (8.33).
- -t' e
+ i
=
χ
4.
359
Ion Exchange
4. Ion E x c h a n g e 4.1. INTRODUCTION I o n e x c h a n g e is a p r o c e s s w h e r e i o n s w h i c h a r e h e l d t o f u n c t i o n a l g r o u p s o n t h e s u r f a c e o f a solid b y e l e c t r o s t a t i c forces a r e e x c h a n g e d for i o n s o f a different species in s o l u t i o n . T h i s t r e a t m e n t p r o c e d u r e h a s b e c o m e i n c r e a s i n g l y i m p o r t a n t in t h e field o f w a s t e w a t e r t r e a t m e n t . S i n c e c o m p l e t e d e m i n e r a l i z a t i o n is a c h i e v e d b y i o n e x c h a n g e , it is p o s s i b l e t o u s e split s t r e a m t r e a t m e n t p r o c e s s e s w h e r e p a r t o f t h e influent w a s t e w a t e r is d e m i n e r a l i z e d a n d t h e n c o m b i n e d w i t h b y p a s s e d influent t o p r o d u c e a n effluent o f specified q u a l i t y (e.g., a specified h a r d n e s s ) . Ion exchange resins. U n t i l t h e 1940's n a t u r a l zeolites w e r e t h e o n l y i o n e x c h a n g e r e s i n s a v a i l a b l e . E x c h a n g e c a p a c i t y w a s relatively l o w , w h i c h l i m i t e d t h e i r e c o n o m i c feasibility in t h e field o f w a s t e w a t e r t r e a t m e n t . S i n c e t h e n , n a t u r a l zeolites h a v e b e e n r e p l a c e d b y s y n t h e t i c r e s i n s . T h e s e a r e i n soluble polymers t o which acidic or basic groups are a d d e d by chemical reaction procedures. These g r o u p s are c a p a b l e of reversible exchange with i o n s p r e s e n t in a s o l u t i o n . T h e t o t a l n u m b e r o f f u n c t i o n a l g r o u p s p e r u n i t w e i g h t ( o r u n i t v o l u m e ) o f resin d e t e r m i n e s e x c h a n g e c a p a c i t y , w h e r e a s t h e t y p e o f f u n c t i o n a l g r o u p d e t e r m i n e s i o n selectivity a n d p o s i t i o n o f e x c h a n g e e q u i l i b r i u m . R e s i n p a r t i c l e s h a v e d i a m e t e r s o f a p p r o x i m a t e l y 0.5 m m a n d a r e e m p l o y e d in p a c k e d c o l u m n s w i t h w a s t e w a t e r flows o f 5 - 1 2 g a l / ( m i n ) ( f t ) . 2
4.2. BASIC M E C H A N I S M OF ION E X C H A N G E : CATION A N D A N I O N E X C H A N G E R S T h e r e a r e t w o b a s i c t y p e s o f i o n e x c h a n g e r s : (1) c a t i o n a n d (2) a n i o n exchangers. Cation exchangers. Cation exchange resins r e m o v e cations from a solu t i o n , e x c h a n g i n g t h e m for s o d i u m i o n s ( s o d i u m cycle) o r h y d r o g e n i o n s ( h y d r o g e n cycle). R e m o v a l is r e p r e s e n t e d b y E q . (8.39). [ R d e n o t e s t h e r e s i n and M t h e c a t i o n (e.g., C u , Z n , N i , C a , M g ) . ] 2
+
2 +
Sodium cycle:
Na R + M
2
+
2
2 +
2 +
^ M R + 2Na
+
2 +
2 +
(8.39a)
or Hydrogen cycle:
H R + Μ 2
2 +
^
M R + 2H
+
(8.39b)
C u , Z n , N i , C a , and M g a r e r e t a i n e d o n t h e r e s i n a n d a soft effluent is p r o d u c e d . T h i s soft effluent c o n t a i n s m a i n l y s o d i u m salts (if s o d i u m cycle is e m p l o y e d ) o r a c i d s (if h y d r o g e n cycle is e m p l o y e d ) . 2 +
2 +
2 +
2 +
2 +
W h e n t h e e x c h a n g e c a p a c i t y o f t h e resin is e x h a u s t e d , i o n i c c o n c e n t r a t i o n in effluent f r o m t h e e x c h a n g e c o l u m n e x c e e d s t h e specified v a l u e . T h i s c o n d i t i o n is called b r e a k t h r o u g h . T h e resin m u s t t h e n b e r e g e n e r a t e d . P r i o r t o
360
8.
Tertiary Treatment of Wastewaters
r e g e n e r a t i o n , t h e c o l u m n s h o u l d b e b a c k w a s h e d t o r e m o v e solid d e p o s i t s . Regeneration consists of passing t h r o u g h the c o l u m n either a brine solution ( N a C l f o r s o d i u m cycle) o r a n a c i d s o l u t i o n , u s u a l l y H S 0 2
4
o r H C I (for
h y d r o g e n cycle). R e g e n e r a t i o n r e a c t i o n s for t h e s o d i u m a n d h y d r o g e n cycles a r e
shown
below. Regenerated Regenerant resin waste MR + 2 N a C l ^ Na R 4MC1 2
M R + 2HC1 (or H S 0 ) ^ 2
2
H R
4
+
2
MC1 (or M S 0 ) 2
(8.40a) (8.40b)
4
T y p i c a l r e g e n e r a n t c o n c e n t r a t i o n s a r e 2 - 5 % b y w e i g h t w i t h flow r a t e s o f 1-2 ( g a l ) / ( m i n ) ( f t ) . A s i n d i c a t e d b y E q . ( 8 . 4 0 ) , r e g e n e r a n t w a s t e c o n s i s t s o f 2
c a t i o n salts. T h i s w a s t e s t r e a m a m o u n t s t o 1 0 - 1 5 % o f i n f l u e n t v o l u m e t r e a t e d b e f o r e b r e a k t h r o u g h . F o l l o w i n g r e g e n e r a t i o n t h e e x c h a n g e r b e d is r i n s e d w i t h water to remove residual regenerant. Regenerant "BackwasK^aste TRinsing waste)
NaCl (sodium cycle) or HCI; H S 0 ( a c i d cycle); 2
4
Reactions Removal:
2 Na* (Sodium cycle)... N a R • Μ * * M R * or or 2H (Hydrogen cycle)... H R 2
2
(8.39)
+
2
Cation exchanger
Regeneration-. MR •
2 NaCl (sodium cycle) or 2 HCI;H S0 . (Hydrogen cycle) 2
4
Regenerated resin: Na R - or H R 2
2
Legend: - Wastewater streams -Regenerant streams Rinsing streams Regenerant waste stream (cation salts)
-Backwash_ (and rinsing) Soft effluent Fig. 8.7. Cation
Sodium salts (sodium cycle) or Acids (hydrogen cycle) exchanger.
Regenerant waste-. MCI or MSO4 (8.40) 2
4.
361
Ion Exchange
C a t i o n e x c h a n g e r r e a c t i o n s a n d d i r e c t i o n s o f flow for different
streams
i n v o l v e d in t h e o p e r a t i o n o f a c a t i o n e x c h a n g e r a r e i n d i c a t e d in F i g . 8.7. H y d r o g e n cycle c a t i o n e x c h a n g e r e s i n s a r e w e a k o r s t r o n g a c i d s . M o s t a c i d i c r e s i n s u s e d in w a t e r p o l l u t i o n a b a t e m e n t a r e s t r o n g a c i d s . Anion exchangers.
A n i o n exchange resins r e m o v e anions from a solution,
e x c h a n g i n g t h e m for h y d r o x y l i o n s . R e m o v a l is r e p r e s e n t e d b y E q . (8.41) (where A ~ represents an anion). 2
R(OH) + A " ^ 2
2
RA + 2 0 H -
(8.41)
A n i o n s (e.g., S O ^ " , C r O . " ) a r e t h u s r e m o v e d f r o m s o l u t i o n . 2
R e g e n e r a t i o n is m a d e after b r e a k t h r o u g h , u s u a l l y p r e c e d e d b y b a c k w a s h i n g t o r e m o v e solid d e p o s i t s . R e g e n e r a n t s c o m m o n l y u s e d a r e s o d i u m a n d a m m o n i u m h y d r o x i d e s . T h e r e g e n e r a t i o n r e a c t i o n is Regenerated Regenerant resin waste RA + 2 N a O H ^ R(OH) + A " ' Na A ' or 2NH OH or
(8.42)
2
2
2
4
(NH ) A 4
2
Typical regenerant concentrations are 5-10% by weight. U s u a l l y , c a t i o n a n d a n i o n e x c h a n g e r s a r e u s e d in series. B y a d e q u a t e selection of ion exchangers, almost any wastewater p r o b l e m of a n inorganic n a t u r e can be h a n d l e d . Basic exchange resins are either strong or w e a k bases. A n i o n e x c h a n g e r r e a c t i o n s a n d d i r e c t i o n o f flow for t h e different s t r e a m s i n v o l v e d i n t h e o p e r a t i o n o f a n a n i o n e x c h a n g e r a r e i n d i c a t e d in F i g . 8.8.
4.3. D E S I G N O F I O N E X C H A N G E
COLUMNS
T h e first s t e p in d e s i g n i n g a n i o n e x c h a n g e s y s t e m for a specific w a s t e w a t e r is t o r u n a c o m p l e t e c a t i o n - a n i o n a n a l y s i s o f t h e i n f l u e n t t o b e t r e a t e d . I n addition, d a t a o n total dissolved solids ( T D S ) , dissolved C 0 , S i 0 , a n d p H are obtained. 2
2
C o n c e n t r a t i o n s o f i n d i v i d u a l i o n s p r e s e n t a r e e x p r e s s e d in e i t h e r o f t w o ways: 1. I n m e q / l i t e r ( m i l l i e q u i v a l e n t s p e r liter), e.g., for a s o l u t i o n c o n t a i n i n g 2 0 m g / l i t e r o f C u , t h e c o n c e n t r a t i o n in t e r m s o f m e q / l i t e r is 2 +
20/(63.5/2) = 0.63 meq/liter w h e r e 6 3 . 5 is t h e a t o m i c w e i g h t o f C u a n d 2 is t h e v a l e n c e . 2. I n t e r m s o f c a l c i u m c a r b o n a t e e q u i v a l e n t s , e.g., for a s o l u t i o n c o n taining 20 mg/liter of C u a n d the stoichiometric a m o u n t of C l ~ , the con centration of C u in t e r m s o f C a C 0 is c a l c u l a t e d a s 2 +
2 +
3
20(134.5/100) = 27 mg/liter of C a C Q
3
362
8.
Anionic waste
Tertiary Treatment of Wastewaters
NaOH or _ _Bac^was_h^waste NH4OH Regenerant
(Rinsing waste)
Reactions Removal R(OH) • A " 5 = 2 : R A + 20H~
(8.41)
2
2
Regeneration: RA • 2 NaOH 5 or 2 NH4OH
Anion exchanger
Regenerated Regenerant resin: waste. R(OH) • A " (8.42) " Na A or 2
2
2
[(NH ) Aj 4
2
Legend. Wastewater streams Regenerant streams Rinsing streams
^-Backwash (and rinsing)
Regenerant" waste]
Soft effluent Fig. 8.8. Anion
exchanger.
w h e r e 100 a n d 134.5 a r e t h e m o l e c u l a r w e i g h t s o f C a C 0 spectively.
a n d C u C l , re
3
2
D e s i g n p a r a m e t e r s d e t e r m i n e d b y l a b o r a t o r y tests p r i o r t o d e s i g n o f a n ion exchange column are 1. Exchange capacity of resin. C a t i o n - a n i o n resin c a p a c i t i e s a r e u s u a l l y e x p r e s s e d a s e q u i v a l e n t s o f i o n r e m o v e d p e r u n i t b e d v o l u m e (e.g., e q u i v a l e n t s / liter o f r e s i n o r e q u i v a l e n t s / f t o f r e s i n ) . T h e y m a y b e a l s o e x p r e s s e d p e r u n i t w e i g h t o f b e d (e.g., e q u i v a l e n t s / l b o f r e s i n ) . E x c h a n g e c a p a c i t i e s a r e a l s o e x p r e s s e d in t e r m s o f w e i g h t o f C a C 0 e q u i v a l e n t , e i t h e r p e r u n i t v o l u m e o r p e r u n i t w e i g h t o f b e d (e.g., l b C a C 0 / f t o f r e s i n , l b C a C 0 / l b o f r e s i n ) . 3
3
3
3
3
2. Regenerant requirements. R e g e n e r a n t r e q u i r e m e n t s a r e e x p r e s s e d in t e r m s o f w e i g h t p e r u n i t v o l u m e o f b e d (e.g., l b H S 0 / f t ) . T h e d e g r e e o f t h e o r e t i c a l c a p a c i t y a t t a i n e d ( w i t h r e s p e c t t o fresh resin) d e p e n d s o n t h e weight of regenerant employed. A n economic balance between degree of theoretical capacity attained a n d weight of regenerant (lb r e g e n e r a n t / f t of b e d v o l u m e ) is t a k e n i n t o c o n s i d e r a t i o n . P e r f o r m a n c e c u r v e s for r e g e n e r a n t s 3
2
4
3
4.
363
Ion Exchange
(exchange capacity of regenerated resin vs. weight of regenerant), d e t e r m i n e d from l a b o r a t o r y studies, are sometimes available from resin manufacturers. T h e exchange capacity of the c o l u m n increases with weight of regenerant utilized. 3. Rinsing water requirements. Following regeneration, the exchanger bed is r i n s e d w i t h w a t e r t o r e m o v e r e s i d u a l r e g e n e r a n t . R i n s i n g r e q u i r e m e n t s , also d e t e r m i n e d from l a b o r a t o r y studies, are s o m e t i m e s available from resin m a n u f a c t u r e r s . T h e y a r e e x p r e s s e d in t e r m s o f g a l l o n s o f w a t e r p e r f t o f resin (range, 100-200 gal/ft ). Characteristics of exchange resins are evaluated f r o m b e n c h s c a l e u n i t s [ 5 ] . Plexiglass c o l u m n s o f 1 in. d i a m e t e r a r e u s e d a t u n i t flow r a t e s c o m p a r a b l e t o p l a n t scale u n i t s . T a b l e 8.4 s h o w s t y p i c a l d a t a o b t a i n e d f r o m b e n c h scale u n i t s . 3
3
T A B L E 8.4 Characteristics of Cation and Anion Exchange Resins Utilized in Treatment of Plating Industry Wastewater
Exchange capacity Regenerant Requirement (lb/ft o f resin) Concentration (%) F l o w rate [gal/(min)(ft )] Rinsing water requirements (gal/ft o f resin)
Cation
Anion
7 0 e q / f t o f resin H S0 11.0 (inlbH S0 ) 5.0 1.0
3.5 l b C r 0 / f t o f resin NaOH 4.7 (in lb N a O H ) 10.0 1.0
130.0
100.0
3
2
3
4
2
2
3
3
3
4
Additional considerations concerning design of ion exchange systems are as follows: 1. R e c o v e r y o f v a l u a b l e w a s t e w a t e r c o n s t i t u e n t s , a n i m p o r t a n t f a c t o r i n d e t e r m i n i n g e c o n o m i c feasibility o f i o n e x c h a n g e , is i l l u s t r a t e d b y E x a m p l e 8.3. C h r o m a t e s ( C r 0 ~ ) f r o m a p l a t i n g p l a n t w a s t e w a t e r a r e h e l d b y a n a n i o n e x c h a n g e r a n d s u b s e q u e n t l y r e c o v e r e d a s c h r o m i c a c i d ( H C r 0 ) in a h y d r o gen cation exchanger. Nickel ions are salvaged from plating plant wastes. 4
2
4
2. C a l c u l a t i o n o f b e d d e p t h is i l l u s t r a t e d in E x a m p l e 8.3. A d d i t i o n a l free h e i g h t is p r o v i d e d t o a l l o w for e x p a n s i o n o f t h e b e d for b a c k w a s h i n g a n d c l e a n i n g . A s a r u l e o f t h u m b a 5 0 % a l l o w a n c e is t a k e n .
4.4. D E S I G N O F A N I O N E X C H A N G E S Y S T E M [ 5 ] E x a m p l e 8.3 D e s i g n a n i o n e x c h a n g e s y s t e m t o t r e a t 120,000 gal o f w a s t e w a t e r p e r d a y f r o m a m e t a l - p l a t i n g i n d u s t r y . T h e m a i n m e t a l i o n s p r e s e n t in t h e w a s t e w a t e r a r e c h r o m i u m , e q u i v a l e n t t o 120 m g / l i t e r o f C r 0 ( p r e s e n t a s c h r o m a t e , C r O " ) ; C u , 30 m g / l i t e r ; Z n , 15 m g / l i t e r ; a n d N i , 2 0 m g / l i t e r . 3
2
2 +
2 +
2 +
8.
364
Tertiary Treatment of Wastewaters
I t is d e s i r e d t o r e m o v e C u , Z n , a n d N i in a h y d r o g e n cycle c a t i o n e x c h a n g e r ( c a t i o n e x c h a n g e r n o . 1) a n d C r 0 ~ in a n a n i o n e x c h a n g e r d o w n s t r e a m f r o m t h e c a t i o n e x c h a n g e r . T h e h y d r o g e n cycle c a t i o n e x c h a n g e r is regenerated by a 5% solution of H S 0 . T h e anion exchanger employed to r e m o v e C K ) ~ is r e g e n e r a t e d b y a 1 0 % s o l u t i o n o f N a O H . Effluent f r o m t h i s r e g e n e r a t i o n c o n t a i n s s o d i u m c h r o m a t e ( N a C r 0 ) . V a l u a b l e C r 0 ~ is r e c o v e r e d in a n o t h e r h y d r o g e n c a t i o n e x c h a n g e r ( c a t i o n e x c h a n g e r n o . 2) a s c h r o m i c a c i d ( H C r 0 ) . T h i s c a t i o n e x c h a n g e r is a l s o r e g e n e r a t e d b y a 5 % s o l u t i o n o f H S 0 . A c i d effluents f r o m r e g e n e r a t i o n o f c a t i o n e x c h a n g e r s n o . 1 a n d 2 are c o m b i n e d , neutralized, a n d then discarded t o a sewer. Figure 8.9 s h o w s a flowsheet o f t h e p r o p o s e d p r o c e s s . 2 +
2 +
2 +
4
2
4
4
2
2
4
4
4
2
4
Characteristics of the cation a n d a n i o n exchangers t o be employed are p r e s e n t e d in T a b l e 8.4. B o t h c a t i o n a n d a n i o n u n i t s o p e r a t e 6 d a y s b e t w e e n r e g e n e r a t i o n s . D e s i g n t h e t h r e e i o n e x c h a n g e r s in t h e s y s t e m a n d e s t i m a t e regeneration a n d rinse water requirements. SOLUTION
T h e flow d i a g r a m is s h o w n in F i g . 8.9, a n d c h e m i c a l r e a c t i o n s
involved are Hydrogen cycle cation—exchanger n o . 1 Removal H R + M 2
+
2
MR + 2H
+
Start
Metal-plating wastewater
To neutralization unit CuS0 ZnS0 |NiS0
[Cu ',Zn , Ni 'CrO|-] z
2 >
2
4
4
4
CrO£ 2
Hydrogen cation exchanger #1 [to remove Cu \Zn * S Ni ] 2
_Na_2Cr_04
^ j [Na S0 ] 2
4
[to remove
Hydrogen cation exchanger #2 [to recover
CrO "]
OO /]
Anion exchanger
2
2+
4
(H Cr0 )
2
2
Η2Ο
NaOH regenerant
Legend — Wastewater streams — Regenerant streams
H Cr0 (recovered) 2
4
H S0 regenerant 2
Fig. 8.9. Ion exchanger
flow
diagram
(Example
8.3).
4
4.
365
Ion E x c h a n g e
where M = Cu Regeneration 2 +
2 +
, Zn
, and N i
2 +
2 +
.
MR + H S 0 2
-> H R + M S 0
4
2
4
A n i o n exchanger Removal R(OH) + H C r 0 2
2
4
^
RCr0
4
+ 2H 0 2
Regeneration RCr0
+ 2NaOH ^
4
R ( O H ) -f N a C r 0 2
2
4
H y d r o g e n cycle cation—exchanger n o . 2 Removal H R + Na Cr0 2
2
4
^
Na R + H Cr0 2
2
(recovered)
4
Regeneration Na R + H S 0 2
Design
of hydrogen
2
4
^
H R + Na S0 2
cycle cation—exchanger
2
4
no. 1
Step 1. C a l c u l a t e e q u i v a l e n t s o f m e t a l i o n s t o b e r e m o v e d (see t a b u l a t i o n below). Ion concentration
Equivalent wt
30 mg/liter C u 15 mg/liter Z n 2 0 mg/liter N i
63.5/2 = 31.7 65.4/2 = 32.7 58.7/2 = 29.4
2+
2+
2 +
meq/liter 30/31.7 = 15/32.7 = 20/29.4 = Total =
0.95 0.46 0.68 2.09
Step 2. D e t e r m i n e t h e t o t a l e q u i v a l e n t s / d a y t o b e r e m o v e d . 2.09 meq/liter χ 3.78 liter/gal χ 120,000 gal/day χ eq/1000 m e q = 948 e q / d a y Step 3. C a l c u l a t e t o t a l r e s i n r e q u i r e m e n t s o n t h e b a s i s o f 7 0 e q u i v a l e n t s / f t
3
o f r e s i n ( T a b l e 8.4) a n d 6 - d a y o p e r a t i o n b e t w e e n r e g e n e r a t i o n s . Resin requirement =
948 e q / d a y χ 6 days/cycle 70 e q / f t 3
=
resm/cycle
Step 4. Select a c o l u m n d i a m e t e r o f 3 ft a n d c a l c u l a t e r e q u i r e d d e p t h o f resin bed. Cross section = (i)n3
2
= 7.07 ft
2
D e p t h = 81/7.07 = 11.5 ft A l l o w i n g 5 0 % free s p a c e for b e d e x p a n s i o n for b a c k w a s h i n g a n d c l e a n i n g , t h e h e i g h t o f t h e r e q u i r e d c o l u m n is (1.50) (11.5) = 17.3 ft. U t i l i z e t w o c o l u m n s in series, e a c h 8.5 ft in h e i g h t , e a c h c o n t a i n i n g a 11.5/2 = 5.75-ft r e s i n d e p t h (free s p a c e , 2.75 ft for e a c h c o l u m n ; r a t i o , 8.5/5.75 = 1.48).
366
Tertiary Treatment of Wastewaters
8.
Step 5. C a l c u l a t e r e g e n e r a n t r e q u i r e m e n t s . R e g e n e r a n t is a 5 % s o l u t i o n of H S 0 2
4
a s i n d i c a t e d in T a b l e 8.4, a n d 11.0 l b H S 0 / f t 2
q u i r e d . T h e lb o f H S 0 2
3
4
o f resin a r e r e
required are
4
11 lb/ft χ 81 ft 3
= 891 lb of H S 0
3
2
4
per cycle
or 891 (100/5) = 17,820 lb of 5% solution Step 6. C a l c u l a t e r i n s e w a t e r r e q u i r e m e n t s . F r o m T a b l e 8.4, 130 g a l o f w a t e r a r e r e q u i r e d for r i n s i n g e a c h c u b i c f o o t o f r e s i n . R e q u i r e d r i n s e w a t e r is 130 gal/ft χ 81 ft /cycle = 10,530 gal per cycle 3
3
Design of anion exchanger C h r o m i c a c i d ( H C r 0 ) p a s s i n g t h r o u g h t h e c a t i o n u n i t is r e m o v e d in t h e a n i o n e x c h a n g e u n i t , w h i c h is d e s i g n e d a s f o l l o w s : Step 1. T o t a l c h r o m e r e m o v e d p e r d a y ( a s C r 0 ) is 2
4
3
120 mg/liter χ 3.78 liter/gal χ g / 1 0 m g χ lb/454 g χ 120,000 gal/day = 120 lb/day 3
Step
2. C a l c u l a t e resin r e q u i r e m e n t s . B a s i s : 3.5 l b C r 0 / f t
3
3
of resin
( T a b l e 8.4) a n d 6-day o p e r a t i o n b e t w e e n r e g e n e r a t i o n s . . 120 lb/day χ 6 days/cycle Resin requirement = 3 5 lb/ft 3
^
p
,
r
=
. , resm/cycle t
Step 3. Select a c o l u m n d i a m e t e r o f 3 ft a n d c a l c u l a t e r e q u i r e d d e p t h o f resin bed. Cross section = ( ± ) π 3 = 7.07 f t , d e p t h = 206/7.07 = 29 ft 2
2
A l l o w i n g a 5 0 % free s p a c e f o r b e d e x p a n s i o n , t h e h e i g h t o f t h e r e q u i r e d c o l u m n is ( 1 . 5 0 ) ( 2 9 . 0 ) = 4 3 . 5 ft. U t i l i z e f o u r c o l u m n s in series, e a c h 11 ft in h e i g h t ( 1 1 x 4 = 4 4 ) , a n d e a c h c o n t a i n i n g 2 9 / 4 = 7.25-ft r e s i n d e p t h (free s p a c e , 3.75 ft for e a c h c o l u m n ; r a t i o , 11/7.25 = 1.5). Step 4. C a l c u l a t e r e g e n e r a n t r e q u i r e m e n t s . R e g e n e r a n t is a 1 0 % s o l u t i o n o f N a O H a s i n d i c a t e d i n T a b l e 8.4, a n d 4.7 l b o f N a O H s o l u t i o n a r e r e q u i r e d per ft of resin. T h e lb of N a O H required are 3
4.7 lb/ft χ 206 ft 3
3
= 968 lb of N a O H per cycle
or 968(100/10) = 9680 lb of 10% solution Step 5 . C a l c u l a t e r i n s e w a t e r r e q u i r e m e n t s . F r o m T a b l e 8.4, 100 g a l o f w a t e r a r e r e q u i r e d for r i n s i n g e a c h c u b i c f o o t o f r e s i n . R e q u i r e d r i n s e w a t e r is 100 gal/ft χ 206 f t 3
3
= 20,600 gal
5.
367
Reverse Osmosis
Design
of hydrogen
cycle cation—exchanger
no. 2
C h r o m i c a c i d ( H C r 0 ) is r e c o v e r e d f r o m s p e n t r e g e n e r a n t l e a v i n g t h e 2
4
a n i o n e x c h a n g e r ( w h i c h c o n t a i n s N a C r 0 ) b y p a s s i n g it t h r o u g h a c a t i o n 2
4
e x c h a n g e r ( F i g . 8.9). A s c a l c u l a t e d i n S t e p 4 o f t h e d e s i g n for t h e a n i o n e x changer, 968 lb of N a O H are required. Step
1. C a l c u l a t e s o d i u m h y d r o x i d e e q u i v a l e n t s t o b e r e m o v e d b y t h e
cation exchanger. 968 lb χ 454 g/lb χ eq/40 g = 10,987 eq since t h e e q u i v a l e n t w e i g h t o f N a O H is 4 0 g. Step
2. C a l c u l a t e resin r e q u i r e m e n t s o n t h e b a s i s o f 7 0 e q u i v a l e n t s / f t
3
o f r e s i n ( T a b l e 8.4). « . 10,987eq „ , . Resin requirement = — — — r = 157 f t of resin 70 e q / f t i
r
r
3
r
3
Step 3. Select a c o l u m n d i a m e t e r o f 3 ft, a n d c a l c u l a t e r e q u i r e d d e p t h o f the resin bed. Cross section = (ί)π3
= 7.07 ft
2
2
D e p t h = 157/7.07 = 22.2 ft A l l o w i n g 5 0 % free s p a c e for b e d e x p a n s i o n , t h e h e i g h t o f t h e r e q u i r e d c o l u m n is ( 1 . 5 0 ) ( 2 2 . 2 ) = 33.3 ft. U t i l i z e t h r e e c o l u m n s i n series, e a c h 3 3 . 3 / 3 « 11 ft d e e p , e a c h c o n t a i n i n g a 2 2 . 2 / 3 = 7.4-ft r e s i n d e p t h (free s p a c e , 3.6 ft for e a c h c o l u m n ; r a t i o , 1 1 / 7 . 4 = 1.48). Step of H S 0 2
4. C a l c u l a t e r e g e n e r a n t r e q u i r e m e n t s . R e g e n e r a n t is a 5 % s o l u t i o n 4
a s i n d i c a t e d in T a b l e 8.4, a n d 11.0 l b o f H S 0 / f t 2
required. T h e lb of H S 0 2
4
4
3
of resin are
required are
11 lb/ft
3
χ 157 f t
3
=
17271bH S0 2
4
or 1727(100/5) = 34,540 lb of 5 % solution Step
5. C a l c u l a t e r i n s e w a t e r r e q u i r e m e n t s . F r o m T a b l e 8.4, 130 g a l o f
w a t e r a r e r e q u i r e d for r i n s i n g e a c h c u b i c f o o t o f r e s i n . R e q u i r e d r i n s e w a t e r is t h e r e f o r e 130 gal/ft χ 157 f t 3
3
= 20,410 gal
5. R e v e r s e O s m o s i s 5.1. O S M O S I S A N D O S M O T I C P R E S S U R E A l t h o u g h o s m o t i c p h e n o m e n a h a v e b e e n k n o w n for o v e r 2 0 0 y e a r s , t h e first p r e c i s e e x p e r i m e n t s l i n k i n g o s m o t i c p r e s s u r e t o t e m p e r a t u r e a n d s o l u t e c o n c e n t r a t i o n w e r e p e r f o r m e d in t h e l a t e 1800's b y Pfeffer. A t y p i c a l e x p e r i m e n t w i t h a s u c r o s e s o l u t i o n is i l l u s t r a t e d in F i g . 8.10.
368
Tertiary Treatment of Wastewaters
8.
Glass tube—Η
Η Dilute solution of sucrose-
-Pressure at •Pressure at A* - Ρ • 7Γ
Bag made of semipermeable membrane —
•Passage of solvent (water)
Water
Fig. 8.10.
Osmosis
experiment.
T h e b a g s h o w n in F i g . 8.10 is m a d e o f a m e m b r a n e p e r m e a b l e t o t h e s o l v e n t ( w a t e r in F i g . 8.10) b u t i m p e r m e a b l e t o t h e s o l u t e ( s u c r o s e ) . T h e s e a r e k n o w n a s s e m i p e r m e a b l e m e m b r a n e s . E a r l y s e m i p e r m e a b l e m e m b r a n e s u t i l i z e d in o s m o s i s w e r e a n i m a l m e m b r a n e s (e.g., p i g b l a d d e r s ) . S y n t h e t i c m e m b r a n e s w e r e d e v e l o p e d later, cellulose a c e t a t e m e m b r a n e s n o w b e i n g t h e m o s t w i d e l y u s e d . A d i l u t e s o l u t i o n o f s u c r o s e (e.g., a 0.001 Μ s o l u t i o n ) is p l a c e d i n s i d e t h e s e m i p e r m e a b l e b a g , w h i c h is t h e n d i p p e d i n t o a v a t c o n t a i n i n g p u r e w a t e r . T h e w a t e r diffuses s p o n t a n e o u s l y f r o m t h e v a t t o t h e i n t e r i o r o f t h e s e m i p e r m e a b l e bag, as indicated by the a r r o w . A s a result, a c o l u m n of liquid rises t h r o u g h t h e glass t u b e c o n n e c t e d t o t h e d i l u t e s u c r o s e s o l u t i o n , r e a c h i n g a t e q u i l i b r i u m a h e i g h t π a b o v e t h e level o f w a t e r in t h e v a t . A t t h i s m o m e n t , passage of solvent stops. Pressure exerted on points A ' a n d A situated at the s a m e e l e v a t i o n differs b y t h e i n c r e m e n t c o r r e s p o n d i n g t o h e i g h t π o f l i q u i d . T h i s v a l u e is c a l l e d t h e o s m o t i c p r e s s u r e o f t h e s u c r o s e s o l u t i o n . O s m o s i s is defined a s t h e s p o n t a n e o u s p a s s a g e o f a s o l v e n t f r o m a d i l u t e s o l u t i o n ( p u r e w a t e r in t h e c a s e o f F i g . 8.10) t o a m o r e c o n c e n t r a t e d o n e t h r o u g h a s e m i permeable membrane. Let Ρ be the pressure at point A (atmospheric pressure plus pressure cor r e s p o n d i n g t o a c o l u m n o f w a t e r o f h e i g h t H). T h e p r e s s u r e a t A ' is ( Ρ + π ) . O s m o t i c p r e s s u r e π is a f u n c t i o n o f t h e c o n c e n t r a t i o n o f t h e s u c r o s e s o l u t i o n a n d t e m p e r a t u r e . T h e m a t h e m a t i c a l r e l a t i o n s h i p for π a s a f u n c t i o n o f c o n c e n t r a t i o n o f s o l u t e (c) a n d a b s o l u t e t e m p e r a t u r e (T) is given b y t h e v a n ' t Hoff equation.
5.2. V A N ' T H O F F E Q U A T I O N F O R O S M O T I C PRESSURE D e r i v a t i o n o f t h e v a n ' t H o f f e q u a t i o n is f o u n d in s t a n d a r d p h y s i c a l c h e m i s t r y texts [ 3 ] . T h e e q u a t i o n is π = nRT/V
=
cRT
(8.43)
5.
369
Reverse Osmosis
w h e r e π is t h e o s m o t i c p r e s s u r e ( a t m ) ; η t h e g m o l e o f s o l u t e (e.g., s u c r o s e ) ; Vthe
v o l u m e o f t h e s u c r o s e s o l u t i o n ; n\V— c t h e c o n c e n t r a t i o n o f s u c r o s e s o l u
t i o n (g m o l e / l i t e r ) ; R t h e i d e a l g a s c o n s t a n t [ 0 . 0 8 2 ( a t m ) ( l i t e r ) / ( g m o l e ) ( ° K ) ] ; and Τ the absolute temperature (°K). T h e v a n ' t Hoff e q u a t i o n shows a startling similarity t o the ideal gas law, the solvent corresponding to the e m p t y space between gas molecules a n d these latter c o r r e s p o n d i n g t o the molecules of solute, in the case of osmosis. T h u s , one could consider osmotic pressure t o be the result of b o m b a r d m e n t exerted b y m o l e c u l e s o f s o l u t e o n t h e m e m b r a n e . D e s p i t e t h i s a n a l o g y , it is d e c e p t i v e t o c o n s i d e r o s m o t i c p r e s s u r e a s a s o r t o f p r e s s u r e w h i c h is e x e r t e d b y t h e s o l u t e . R a t h e r o s m o s i s is t h e p a s s a g e o f s o l v e n t t h r o u g h t h e m e m b r a n e d u e t o m o m e n t a r y i n e q u a l i t y o f t h e c h e m i c a l p o t e n t i a l o n t h e t w o sides o f t h e m e m b r a n e [3.] O s m o t i c pressure results from this passage of solvent. A p p l i c a t i o n o f E q . (8.43) t o a 0.001 Μ s o l u t i o n o f s u c r o s e (i.e., n/V = c = 0.001 g m o l e / l i t e r ) a t 2 0 ° C l e a d s t o a v a l u e o f t h e o s m o t i c p r e s s u r e c a l c u l a t e d as n
= RT= C
0.001 g mole/liter χ 0.082 (atm)(liter)/(g m o l e ) ( ° K ) χ 293.2°K = 0.024 a t m
A s s u m i n g t h e specific g r a v i t y o f t h e d i l u t e s o l u t i o n t o b e t h a t o f p u r e w a t e r , t h i s c o r r e s p o n d s t o a h e i g h t π ( F i g . 8.10) e q u a l t o π = 0.024 a t m χ 34 ft w a t e r / a t m = 0.82 ft
( « 1 0 in.)
If t h e v a n ' t H o f f e q u a t i o n c o u l d b e a p p l i e d t o r e l a t i v e l y
concentrated
s u c r o s e s o l u t i o n s (e.g., a 1.0 Μ s o l u t i o n ) , h e i g h t π w o u l d b e 1000 t i m e s t h a t j u s t c a l c u l a t e d , i.e., 820 ft. I n a s m u c h a s t h e i d e a l g a s l a w d o e s n o t d e s c r i b e a c c u r a t e l y g a s b e h a v i o r a t h i g h e r p r e s s u r e s , t h e v a n ' t H o f f e q u a t i o n is n o t a n a d e q u a t e m o d e l for o s m o t i c p r e s s u r e a t h i g h e r s o l u t e c o n c e n t r a t i o n s .
5.3.
PRINCIPLE OF REVERSE O S M O S I S
T h e p r i n c i p l e o f r e v e r s e o s m o s i s is i l l u s t r a t e d b y F i g . 8 . 1 1 . F i g u r e 8 . 1 1 ( a ) depicts direct osmosis [e.g., condition existing at beginning of experiment w i t h s u c r o s e s o l u t i o n ( F i g . 8 . 1 0 ) ] . S o l v e n t flows s p o n t a n e o u s l y t h r o t y g h t h e semipermeable m e m b r a n e . Figure 8.11(b) illustrates the equilibrium
con
dition. H e r e the liquid head which has developed as a result of solvent
flow
t h r o u g h t h e m e m b r a n e is e q u a l t o t h e o s m o t i c p r e s s u r e . S o l v e n t flow s t o p s . F i g u r e 8.11 (c) i l l u s t r a t e s w h a t h a p p e n s w h e n a f o r c e F i n excess o f t h e v a l u e o f o s m o t i c p r e s s u r e is a p p l i e d t o t h e s u c r o s e s o l u t i o n . S o l v e n t flow is r e v e r s e d , i.e., f r o m t h e c o m p a r t m e n t c o n t a i n i n g t h e s u c r o s e s o l u t i o n t o t h e
water
c o m p a r t m e n t . T h i s p h e n o m e n o n is c a l l e d r e v e r s e o s m o s i s ( h e n c e a b b r e v i a t e d as R O ) .
370
8.
Semipermeable U—membrane
Tertiary Treatment of Wastewaters
| Semipermeable r*—membrane '·..·•·'·.'•'·'·· . ί ' • · · . I • • ' ' · ' . ' • ' - . 1 Liquid head •' · . · ' · . ' · ' . . 1 equal to ' ' ; . ·' ' ' 1 osmotic "•'.•••'".·· I pressure .
•
Sucrose solution
.
1
1
' ' * " ' ' · -i ι
Water
' . ' ·..'·· ' ι Sucrose solution { _ . ·· · . : · · Ί ' ·' · ' . · · . ! . " · · I(a)
Water
(b)
Direct osmosis:
Equilibrium condition:
solvent flows spontaneously through
solvent flow s t o p s
the
semipermeable
membrane
ι, ι Semipermeable Force F equal to | . _ (osmotic pressure r _ — plus liquid head m
F
F
F
F
'
e
m
b
r
a
n
e
Water
M i l ! Sucrose solution '
(c) Reverse osmosis: requires applied force in excess of the osmotic pressure
Fig.
8.11.
Illustration
of the principle
of reverse
osmosis.
I n the t r e a t m e n t of wastewaters by reverse osmosis, c o n t a m i n a t e d influent is p l a c e d i n c o n t a c t w i t h a s u i t a b l e m e m b r a n e a t a p r e s s u r e i n excess o f t h e o s m o t i c p r e s s u r e o f t h e s o l u t i o n [ s a m e s i t u a t i o n a s i n F i g . 8.11(c), e x c e p t t h a t t h e left c o m p a r t m e n t c o n t a i n s w a s t e w a t e r i n s t e a d o f s u c r o s e s o l u t i o n ] . U n d e r these conditions, water with a very small a m o u n t o f c o n t a m i n a n t s permeates the m e m b r a n e . Dissolved contaminants are concentrated in t h e w a s t e w a t e r c o m p a r t m e n t . T h i s c o n c e n t r a t e , w h i c h h o p e f u l l y i s a s m a l l frac t i o n o f t h e t o t a l v o l u m e o f w a s t e w a t e r t o b e t r e a t e d , i s d i s c a r d e d . Purified w a t e r is o b t a i n e d f r o m t h e o t h e r c o m p a r t m e n t . T h e c o m p a r t m e n t s i n d i c a t e d i n F i g . 8.11 a r e a s c h e m a t i c r e p r e s e n t a t i o n o f R O p r o c e s s . I n p r a c t i c e , t h e R O p r o c e s s is c o n d u c t e d i n a t u b u l a r c o n f i g u r a t i o n s y s t e m ( F i g . 8.12). R a w w a s t e w a t e r flows u n d e r h i g h p r e s s u r e (in excess o f t h e v a l u e o f its o s m o t i c p r e s s u r e ) t h r o u g h a n i n n e r t u b e m a d e o f a s e m i -
5.
371
Reverse Osmosis
p e r m e a b l e m e m b r a n e m a t e r i a l a n d d e s i g n e d for h i g h p r e s s u r e
operation.
Purified w a t e r is r e m o v e d f r o m t h e o u t e r t u b e , w h i c h is a t a t m o s p h e r i c p r e s s u r e a n d is m a d e o f o r d i n a r y t u b u l a r m a t e r i a l . T y p i c a l v a l u e s o f o p e r a t i n g p r e s s u r e s , w a t e r fluxes (yield o f p u r i f i e d w a t e r p e r u n i t a r e a o f m e m b r a n e ) , a n d p r o d u c t quality are discussed in the following sections.
5.4. P R E P A R A T I O N O F R O M E M B R A N E S R e v e r s e o s m o s i s f o u n d its e a r l i e r a p p l i c a t i o n s in d e s a l i n a t i o n o f o c e a n water. C o n s i d e r a b l e research a n d pilot-plant w o r k are being d o n e for utiliza t i o n o f R O in r e m o v a l o f c o n t a m i n a n t s f r o m w a s t e w a t e r s . S o m e o f t h e s e (e.g., n i t r o g e n a n d p h o s p h o r u s c o m p o u n d s , c h r o m a t e s , a n d s o m e o r g a n i c c o m p o u n d s ) are not adequately removed by other processes. Consequently, usual processes m a y be complemented by R O , provided economic con siderations are favorable. Research indicates t h a t in principle, R O can be u s e d t o o b t a i n a n effluent o f v i r t u a l l y a n y d e s i r e d d e g r e e o f p u r i t y w h i l e still m a i n t a i n i n g r e a s o n a b l e flow r a t e s . M a n y natural materials have semipermeable characteristics. A n i m a l a n d plant membranes are well-known examples. Collodion, cellophanes, porous g l a s s frits, finely c r a c k e d g l a s s , a n d i n o r g a n i c p r e c i p i t a t e s s u c h a s c o p p e r ferrocyanide, a n d zinc and uranyl p h o s p h a t e s have been used. All these, however, have the shortcomings of developing leaks a n d exhibiting short lived selectivity a n d p o o r r e p r o d u c i b i l i t y . Cellulose acetate m e m b r a n e s (hence denoted as C A m e m b r a n e s ) are the m o s t successful s e m i p e r m e a b l e m e m b r a n e s d e v e l o p e d . S o u r i r a j a n a n d L o e b [ 9 ] d e v e l o p e d a t e c h n i q u e for p r e p a r a t i o n o f C A m e m b r a n e s y i e l d i n g b o t h high permeabilities a n d high degrees of solute separation from a q u e o u s s o l u t i o n s o f s o d i u m c h l o r i d e . T h e i r t e c h n i q u e is a s f o l l o w s :
^Raw wastewater in
Concentrate removal
Λ
Purified water removal Fig. 8.12. Diagram treatment by reverse
of a tubular osmosis.
configuration
system
for
wastewater
372
8.
Tertiary Treatment of Wastewaters
1. Casting step. T h e film-casting s o l u t i o n c o n t a i n s cellulose a c e t a t e d i s s o l v e d in a c e t o n e , t o w h i c h is a d d e d a n a d d i t i v e s o l u b l e in w a t e r a n d n o t affecting t h e s o l u b i l i t y o f cellulose a c e t a t e i n a c e t o n e (e.g., m a g n e s i u m p e r c h l o r a t e ) . W i t h t h i s s o l u t i o n , m e m b r a n e s a r e c a s t o n flat o r t u b u l a r s u r f a c e s (e.g., g l a s s p l a t e s o r t u b u l a r s u r f a c e s ) , e i t h e r a t l a b o r a t o r y o r l o w e r t e m p e r a t u r e s ( « — 10°C). O n e o f t h e m o s t significant d e v e l o p m e n t s in t h e field o f C A m e m b r a n e t e c h n o l o g y is c a s t i n g in t u b u l a r f o r m . T u b u l a r - s h a p e d m e m b r a n e s a r e e n t i r e l y lined w i t h i n a p o r o u s fiberglass r e i n f o r c e d t u b e . 2. Evaporation step. A f t e r c a s t i n g , p a r t o f t h e s o l v e n t ( a c e t o n e ) is a l l o w e d t o e v a p o r a t e from the surface of the m e m b r a n e at casting t e m p e r a t u r e . 3. Gelation step. T h e m e m b r a n e is i m m e r s e d in ice-cold w a t e r f o r a t least 1 h r . T h e film sets t o a gel, f r o m w h i c h t h e a d d i t i v e (e.g., m a g n e s i u m perchlorate) a n d the solvent (acetone) are leached out, leaving a t o u g h solid p o r o u s film o n t h e flat o r t u b u l a r s u r f a c e . 4. Shrinkage step. M e m b r a n e s f r o m t h e g e l a t i o n s t e p e x h i b i t p o r e s w h i c h a r e t o o large t o p e r m i t efficient o p e r a t i o n ( d i a m e t e r s « 4 0 0 0 A). T h e s e l a r g e pores are a result of the leaching process. Consequently, the m e m b r a n e receives a t h e r m a l t r e a t m e n t b y s h r i n k i n g it in h o t w a t e r f o r a b o u t 10 m i n . A d j u s t i n g t h e h o t w a t e r t e m p e r a t u r e , it is p o s s i b l e t o o b t a i n v a r i a b l e p o r o s ities, w h i c h r e s u l t s in different d e g r e e s o f w a s t e w a t e r s e p a r a t i o n . H i g h e r h o t w a t e r t e m p e r a t u r e s ( u s u a l r a n g e is 7 0 ° - 9 8 ° C ) yield s m a l l e r p o r e s .
5.5. P R E F E R E N T I A L S O R P T I O N - C A P I L L A R Y F L O W M E C H A N I S M FOR R E V E R S E O S M O S I S S e v e r a l m e c h a n i s m s h a v e b e e n p r o p o s e d b y different i n v e s t i g a t o r s t o e x p l a i n r e v e r s e o s m o s i s . O f t h e s e , o n l y t h e p r e f e r e n t i a l s o r p t i o n - c a p i l l a r y flow m e c h a n i s m is d e s c r i b e d h e r e . T h i s m e c h a n i s m , p r o p o s e d b y S o u r i r a j a n [ 9 ] , is s u m m a r i z e d a s f o l l o w s : R O s e p a r a t i o n is t h e c o m b i n e d r e s u l t o f a n i n t e r facial p h e n o m e n o n a n d fluid t r a n s p o r t u n d e r p r e s s u r e t h r o u g h c a p i l l a r y p o r e s . F i g u r e 8.13 is a c o n c e p t u a l m o d e l o f t h i s m e c h a n i s m for r e c o v e r y o f fresh w a t e r f r o m a q u e o u s salt s o l u t i o n s ( o c e a n w a t e r ) . T h e s o l u t i o n is in c o n t a c t w i t h a p o r o u s m e m b r a n e , t h e s u r f a c e o f w h i c h h a s a p r e f e r e n t i a l s o r p t i o n for w a t e r a n d / o r p r e f e r e n t i a l r e p u l s i o n for t h e s o l u t e . A c o n t i n u o u s r e m o v a l o f p r e f e r e n t i a l l y s o r b e d i n t e r f a c i a l w a t e r is effected b y flow u n d e r p r e s s u r e t h r o u g h t h e m e m b r a n e c a p i l l a r i e s . T h e p r e f e r e n t i a l l y s o r b e d w a t e r l a y e r a t t h e i n t e r f a c e is o f a m o n o m o l e c u l a r n a t u r e ( i n d i c a t e d in F i g . 8.13 b e l o w t h e single d o t t e d line), a n d r e s u l t s f r o m interaction between interfacial surface tension a n d a d s o r p t i o n of solute. F o r a m a x i m u m s e p a r a t i o n a n d p e r m e a b i l i t y , t h i s m o d e l gives rise t o t h e c o n c e p t o f critical p o r e d i a m e t e r , w h i c h is e q u a l t o t w i c e t h e t h i c k n e s s o f t h e p r e f e r e n t i a l l y s o r b e d i n t e r f a c i a l w a t e r l a y e r ( F i g . 8.14). F r o m a n i n d u s t r i a l s t a n d p o i n t , a p p l i c a t i o n o f t h e r e v e r s e o s m o s i s t e c h n i q u e for a g i v e n s e p a r a t i o n
5.
373
Reverse Osmosis
Pressure H0 2
NaXf Na Cf +
H0 H0 2
Na C f
2
Na Cf
H0 H0 H0
Na Cf
HoO
+
Bulk of the solution
Pure water interface
H0 H0 H0 2
Na CI"
2
Na Cf
2
Na Cf
H0 2
+
+
+
+
2
Na Cf +
2
Na Cf +
2
HoO H 0 ΗίΟ Porous
H0
2
2
1
\Porous film surface of appropriate chemical nature
H0 H0 H0 2
2
2
H0 2
film surfaci of appropriate chemical nature
HoO
'—Pore of c r i t i c a l size
- P o r o u s film
Fig. 8.13. Schematic representation of preferential sorption-capillary mechanism [ 9 ] . (Reprinted with permission from Ind. Eng. Chem. Copyright American Chemical Society.)
by
flow the
Demineralized water at the interface t
T"
1
P o r o u s film
11!!! · V ι ϋ !ί
Porous film
ιι
2t Critical pore diameter on the a r e a o f t h e f i l m a t t h e i n t e r f a c e
Fig. 8.14. Critical pore diameter for maximum separation and permeability [ 9 ] . (Reprinted with permission from Ind. Eng. Chem. Copyright by the American Chemical Society.)
p r o b l e m i n v o l v e s t h e c h o i c e o f t h e a p p r o p r i a t e c h e m i c a l n a t u r e o f t h e film surface a n d d e v e l o p m e n t o f m e t h o d s for p r e p a r i n g films c o n t a i n i n g t h e l a r g e s t n u m b e r of p o r e s o f t h e r e q u i r e d size. T h i s a p p r o a c h is t h e b a s i s o f t h e successful d e v e l o p m e n t o f t h e S o u r i r a j a n - L o e b t y p e of p o r o u s C A m e m b r a n e s for saline w a t e r c o n v e r s i o n a n d o t h e r a p p l i c a t i o n s .
5.6. C H A R A C T E R I Z A T I O N O F M E M B R A N E PERFORMANCE T h e t w o b a s i c p a r a m e t e r s for c h a r a c t e r i z i n g R O s y s t e m s a r e (1) p r o d u c t i o n o f purified w a t e r p e r u n i t a r e a o f m e m b r a n e ( w a t e r flux) a n d (2) p r o d u c t q u a l i t y , i.e., p u r i t y o f purified w a t e r ( r e j e c t i o n f a c t o r ) . T h e s e p a r a m e t e r s a r e d i s c u s s e d in S e c t i o n s 5.7 a n d 5.8.
374
8.
Tertiary Treatment of Wastewaters
5.7. W A T E R F L U X P r o d u c t i o n o f purified w a t e r is m e a s u r e d b y t h e w a t e r flux, defined a s quantity of p r o d u c t recovered per day per unit area of m e m b r a n e . English u n i t s a r e u s e d for w a t e r flux i n field w o r k [ g a l / ( d a y ) ( f t ) ] , w h e r e a s m e t r i c units [g/(sec) ( c m ) ] a r e used in l a b o r a t o r y tests. 2
2
F l u x t h r o u g h a specific m e m b r a n e is d e t e r m i n e d b y ( 1 ) p h y s i c a l c h a r a c teristics o f t h e m e m b r a n e , e.g., t h i c k n e s s , c h e m i c a l c o m p o s i t i o n , a n d p o r o s i t y , a n d ( 2 ) s y s t e m c o n d i t i o n s , e.g., t e m p e r a t u r e , differential p r e s s u r e a c r o s s t h e m e m b r a n e , salt c o n c e n t r a t i o n o f s o l u t i o n s t o u c h i n g t h e m e m b r a n e , a n d velocity o f feed m o v i n g a c r o s s t h e m e m b r a n e . I n p r a c t i c e , p h y s i c a l c h a r a c t e r i s t i c s o f t h e m e m b r a n e a s well a s t e m p e r a t u r e a n d c o n c e n t r a t i o n s o f s o l u t e i n feed a n d p r o d u c t s t r e a m s a r e fixed f o r a g i v e n p r o c e s s . T h e r e f o r e , w a t e r flux is a f u n c t i o n o f t h e differential p r e s s u r e a c r o s s the m e m b r a n e , being given a p p r o x i m a t e l y by FH O 2
« Λ(ΔΡ-Δπ)
(8.44)
where ΔΡ = P-
ΡΡ
(8.45)
Απ
ηρ
(8.46)
F
and F
7Tf —
=
is t h e w a t e r flux [ g a l / ( d a y ) ( f t ) ] ; A t h e p e r m e a t i o n coefficient f o r a 2
HlQ
unit area of m e m b r a n e [ g a l / ( d a y ) ( f t ) ( a t m ) ] . This term includes physical 2
v a r i a b l e s o f t h e m e m b r a n e a n d is relatively c o n s t a n t ; AP = (P — P ) F
P
p r e s s u r e e x e r t e d o n feed s o l u t i o n (P ) m i n u s p r e s s u r e o n p r o d u c t (P ) F
a n d An = (n —n ) F
P
P
t h e o s m o t i c p r e s s u r e o f feed s o l u t i o n (n ) F
p r e s s u r e o f p r o d u c t (n ) P
the
(atm);
minus osmotic
(atm).
F o r a l a b o r a t o r y e x p e r i m e n t w i t h a feed o f p u r e w a t e r An = 0 , E q . ( 8 . 4 4 ) r e d u c e s t o a classical flux e q u a t i o n : F H O = A AP 2
(8.47)
i.e., flux = r e s i s t a n c e χ d r i v i n g force. W h e n w a s t e w a t e r feed is relatively c o n c e n t r a t e d i n s o l u t e a n d p r o d u c t is a v e r y d i l u t e s o l u t i o n ( n e a r l y p u r e w a t e r ) , c o r r e c t i o n f a c t o r Δ π is s u b t r a c t e d f r o m differential p r e s s u r e AP. A c t u a l l y , An s h o u l d b e e q u a l t o t h e difference i n o s m o t i c p r e s s u r e b e t w e e n s o l u t i o n s t o u c h i n g t h e m e m b r a n e o n e a c h side (i.e., feed a n d p r o d u c t sides). T h i s is not e x a c t l y e q u a l t o n — n b e c a u s e o f c o n c e n t r a t i o n p o l a r i z a t i o n , a c o n d i t i o n d i s c u s s e d i n S e c t i o n 5 . 1 5 . T h i s is w h y E q . ( 8 . 4 4 ) is a p p r o x i m a t e . A n a c c u r a t e v e r s i o n is w r i t t e n i n S e c t i o n 5 . 1 5 . F
P
5.8. R E J E C T I O N F A C T O R I m p r o v e m e n t o f q u a l i t y b e t w e e n feed a n d p r o d u c t s t r e a m s is e x p r e s s e d q u a n t i t a t i v e l y b y t h e r e j e c t i o n f a c t o r , defined a s / =
(C -C )/C F
P
F
(8.48)
5.
Reverse Osmosis
375
w h e r e / i s t h e rejection f a c t o r ( d i m e n s i o n l e s s ) ; C t h e s o l u t e c o n c e n t r a t i o n in feed s o l u t i o n ; a n d C t h e s o l u t e c o n c e n t r a t i o n i n p r o d u c t . T h u s , / = 0.9 m e a n s t h a t C = 0 . 1 C , i.e., t h e p r o d u c t c o n t a i n s o n e - t e n t h t h e c o n c e n t r a t i o n o f s o l u t e in t h e f e e d ; t h e r e f o r e , 9 0 % o f t h e s o l u t e is rejected b y t h e m e m b r a n e . R e j e c t i o n f a c t o r is t h e r e f o r e a m e a s u r e o f m e m b r a n e selectivity. S o l u t e c o n c e n t r a t i o n s C a n d C a r e o b t a i n e d b y d e t e r m i n a t i o n o f t o t a l d i s s o l v e d solids ( e v a p o r a t i o n t o d r y n e s s ) . W h e n t h e s o l u t e is a n e l e c t r o l y t e (e.g., N a C l ) , a n a l y s i s is c o n v e n i e n t l y p e r f o r m e d b y c o n d u c t i v i t y measurements. F
P
P
f
F
P
T h e ability o f a m e m b r a n e t o reject s o l u t e s is a c o m p l i c a t e d p r o b l e m depending o n a combination of physicochemical characteristics of solute, m e m b r a n e , a n d w a t e r . P r o p e r t i e s o f t h e s o l u t e w h i c h h a v e t h e m o s t influence o n rejection o f i n d i v i d u a l species a r e (1) v a l e n c e c h a r g e — r e j e c t i o n i n c r e a s e s w i t h v a l u e o f c h a r g e o f i o n ; (2) m o l e c u l a r s i z e — r e j e c t i o n i n c r e a s e s w i t h m o l e c u l a r size o f s o l u t e ; a n d (3) h y d r o g e n b o n d i n g t e n d e n c y — p e r m e a t i o n i n c r e a s e s w i t h s t r e n g t h o f h y d r o g e n b o n d i n g . A b i l i t y o f a m e m b r a n e t o reject salts d e c r e a s e s w i t h o p e r a t i n g t i m e . V a r i a t i o n o f r e j e c t i o n w i t h t i m e is illus t r a t e d b y F i g . 8.15. 100
Monovalent rejection ions 85 1000
3000
5000
7000
Operating time (hr) Fig. 8.15. Variation of salt rejection special permission from Chemical Engineering, New York N.Y. 10020) t
with operating time [6]. (Reprinted by April 1973. Copyright by McGraw-Hill, Inc.,
t
A t first, d e c r e a s e is m o r e p r o n o u n c e d f o r s m a l l , u n i v a l e n t i o n s s u c h a s sodium ( N a ) a n d chloride (Cl~). These are normally a m o n g the most p e r m e a b l e , s h o w i n g t h e l o w e s t initial r e j e c t i o n a n d h a v e t h e h i g h e s t r a t e o f decline. D i v a l e n t i o n s s u c h a s c a l c i u m ( C a ) a n d m a g n e s i u m ( M g ) a n d a n i o n s s u c h a s sulfate ( S 0 ~ ) a r e initially rejected v e r y well a n d s h o w a v e r y +
2 +
2
4
2 +
376
8.
Tertiary Treatment of Wastewaters
s l o w r a t e o f d e c l i n e . R e j e c t i o n o f s u c h i o n s m a y a c t u a l l y i n c r e a s e for a w h i l e ( F i g . 8.15), b e c o m i n g n e a r l y c o n s t a n t after t h a t . O v e r a l l salt r e j e c t i o n t e n d s t o follow the m o n o v a l e n t curve. P r o g r e s s i v e d e c r e a s e in salt r e j e c t i o n m a y b e c a u s e d b y h y d r o l y s i s o f t h e m e m b r a n e , w i t h s u b s e q u e n t loss o f b o n d i n g sites. A n o t h e r c a u s e m a y b e i n c r e a s e in p o r e size d u e t o m e m b r a n e swelling. M o s t p r o b a b l y , b o t h effects p l a y a p a r t in t h e r e s u l t .
5.9. E F F E C T O F S H R I N K A G E T E M P E R A T U R E O N P E R F O R M A N C E OF C A M E M B R A N E S A d j u s t i n g h o t w a t e r t e m p e r a t u r e in t h e s h r i n k a g e s t e p for p r e p a r a t i o n o f C A m e m b r a n e s ( S e c t i o n 5.4) p e r m i t s c o n t r o l o f m e m b r a n e p o r o s i t y , t h u s leading to various degrees of wastewater separation. A t higher shrinkage t e m p e r a t u r e s , p o r e sizes o b t a i n e d a r e s m a l l e r , l e a d i n g t o g r e a t e r r e j e c t i o n . W a t e r flux, h o w e v e r , d e c r e a s e s a t h i g h e r s h r i n k a g e t e m p e r a t u r e s , a s e x p e c t e d . F o r specific R O a p p l i c a t i o n s o n e seeks a n e c o n o m i c b a l a n c e b e t w e e n w a t e r flux a n d rejection. F i g u r e 8.16 s h o w s r e j e c t i o n a n d flux d a t a o b t a i n e d b y K o p e c e k a n d S o u r i r a j a n for t h r e e t y p e s o f C A m e m b r a n e s . LCA-NRC-25
74
78
82
86
90
94
98
Shrinkage temperature (°C) Fig.
8.16.
Effect
CA membranes American
Chemical
of shrinkage
[ 9 ] . (Reprinted Society.)
temperature with permission
on the performance from Ind. Eng. Chem.
of Copyright
different by the
5.
377
Reverse Osmosis
5.10. E F F E C T O F F E E D T E M P E R A T U R E O N F L U X [6] F l u x is a l s o affected b y feed t e m p e r a t u r e . W a t e r p e r m e a b i l i t y for t h e m e m b r a n e i n c r e a s e s a b o u t 1.5% p e r ° F . F l u x for a m e m b r a n e is u s u a l l y specified a t 7 5 ° - 7 7 ° F , a n d a c o r r e c t i o n f a c t o r is a p p l i e d a t o t h e r t e m p e r a t u r e s . T h i s c o r r e c t i o n f a c t o r c a n b e d e r i v e d b y t h e o r e t i c a l c o n s i d e r a t i o n s f r o m diffusivity a n d viscosity values, b u t experimentally d e t e r m i n e d corrections are m o r e reliable. A c o r r e c t i o n f a c t o r w h i c h is a m u l t i p l i e r o f t h e r e q u i r e d m e m b r a n e a r e a is p r e s e n t e d i n F i g . 8.17. T h i s c u r v e w a s d e v e l o p e d b y G u l f E n v i r o n m e n t a l S y s t e m s C o . for m o d i f i e d cellulose a c e t a t e m e m b r a n e s . A p p l i c a t i o n o f t h e c o r r e c t i o n f a c t o r is i l l u s t r a t e d in E x a m p l e 8.4.
Feedwater temperature ( ° C ) Fig. 8.17. Flux/temperature correction curve [6]. (Reprinted by special permission from Chemical Engineering, April 2, 1973. Copyright by McGraw-Hill, Inc., New York, Ν. Y., 10020.)
Example 8.4 [6] I t is d e s i r e d t o specify t h e m e m b r a n e a r e a for a 100,000 g a l / d a y R O s y s t e m to treat brackish water. R e c o r d s indicate that the lowest water t e m p e r a t u r e e x p e c t e d for a n y p r o l o n g e d p e r i o d is 6 8 ° F ( 2 0 ° C ) . T h e cellulose a c e t a t e m e m b r a n e c h o s e n is e x p e c t e d t o h a v e a n a v e r a g e flux o f 15 g a l / ( d a y ) ( f t ) d u r i n g its 2 - y e a r life, o p e r a t i n g a t 6 0 0 p s i g . A v e r a g e flux w a s d e t e r m i n e d a t a b a s e t e m p e r a t u r e o f 7 7 ° F . D e t e r m i n e r e q u i r e d m e m b r a n e a r e a for o p e r a t i o n at 68°F. 2
SOLUTION
M e m b r a n e a r e a is s h o w n in E q . (8.49).
378
8.
Tertiary Treatment of Wastewaters
A t 7 7 ° F t h e r e q u i r e d m e m b r a n e a r e a is
Area (77°F) =
100,000 gal/day ' ' J = 6667 ft 15gal/(day)(ft ) 8
2
2
F o r operation at 68°F (20°C), the correction factor C C = 1.15. T h e r e f o r e ,
F
f r o m F i g . 8.16 is
F
A r e a ( 6 8 ° F ) = 1.15 χ 6667 = 7667 ft
2
If a d d i t i o n a l p r o d u c t is n o t r e q u i r e d w h e n t e m p e r a t u r e is a b o v e 6 8 ° F , o p e r a t i n g p r e s s u r e is r e d u c e d .
5.11. F L U X D E C L I N E [6] A p p l i c a t i o n o f p r e s s u r e t o t h e m e m b r a n e r e s u l t s in c o m p a c t i o n a n d c o n s e q u e n t l y i n a d e c l i n e o f flux. C o m p a c t i o n r e s u l t s f r o m d e n s i f i c a t i o n o f t h e thin m e m b r a n e layer a n d corresponds t o n a r r o w i n g of the pores t h r o u g h w h i c h w a t e r m u s t p a s s . A s t h e c h a n n e l s n a r r o w , flow d e c r e a s e s . A n o t h e r c a u s e o f flux d e c l i n e is h y d r o l y s i s o f a c e t y l g r o u p s w h i c h t a k e s p l a c e d u r i n g t h e life o f t h e m e m b r a n e . T h e r e f o r e , R O m e m b r a n e s a r e l i m i t e d to a p H operating range of 3-7, outside of which rapid hydrolysis a n d m e m b r a n e d e g r a d a t i o n o c c u r . T h e o p t i m u m r a n g e is b e l i e v e d t o b e p H 5 - 6 . H y d r o l y s i s o f a c e t y l g r o u p s r e s u l t s in loss o f h y d r o g e n b o n d i n g sites, w h i c h r e d u c e s t h e w a t e r t r a n s p o r t . T h i s h y d r o l y s i s is a l s o a s o u r c e o f salt l e a k a g e b e c a u s e t h e r e a r e fewer w a t e r b r i d g e s b l o c k i n g p a s s a g e o f f o r e i g n m a t e r i a l s through the pores. L o s s in p r o d u c t i v i t y h a p p e n s slowly t o e v e r y m e m b r a n e a n d is p e r m a n e n t . C h e m i c a l r e j u v e n a t i o n a n d l o w p r e s s u r e o p e r a t i o n t o r e l a x a n d swell t h e m a t r i x h a v e b e e n t r i e d w i t h o u t s u c c e s s ; t h e m e m b r a n e s i m p l y a g e s a n d flux decreases until economics dictate replacement.
5.12. F O U L I N G : C A U S E S A N D C U R E S [6] F o u l i n g , r e s u l t i n g in t e m p o r a r y flux r e d u c t i o n , is c a u s e d b y f o r e i g n m a t e r i a l s coating the m e m b r a n e surface, as only hydrogen b o n d i n g substances (water, a m m o n i a ) pass t h r o u g h the discriminating pores of the m e m b r a n e . N o n b o n d i n g m a t e r i a l s a r e left in t h e q u i e s c e n t film k n o w n a s t h e l i q u i d b o u n d a r y layer. T h e c o m p o s i t i o n o f d e p o s i t s in b o u n d a r y l a y e r s reflects t h e c o m p o s i t i o n o f feedwater. A s expected, the m o s t c o m m o n constituents are calcium car b o n a t e , sulfate scales, h y d r a t e s o f i r o n a n d a l u m i n u m o x i d e s , silicates, miscellaneous particulates, a n d biological growths.
5.
379
Reverse Osmosis
F o u l i n g is m i n i m i z e d b y t a k i n g t h e f o l l o w i n g p r e c a u t i o n s : (1) p r e t r e a t i n g feed t o r e m o v e i r o n a n d c o n t r o l p H , (2) l i m i t i n g t h e p r o c e s s t o n o n s c a l i n g c o n c e n t r a t i o n s o f w a s t e w a t e r , (3) filtration o f w a s t e w a t e r feed, a n d (4) injec t i o n o f s m a l l a m o u n t s o f b i o c i d e s (e.g., c h l o r i n e ) . U n f o r t u n a t e l y , n o m a t t e r h o w t h o r o u g h the protection, fouling always occurs a n d the m e m b r a n e s s h o u l d b e p e r i o d i c a l l y c l e a n e d . T h e u s u a l c l e a n i n g p r o c e d u r e is a s f o l l o w s : 1. F l u s h t h e m e m b r a n e w i t h feed w a t e r a t r e d u c e d p r e s s u r e o f t w o o r t h r e e t i m e s n o r m a l v e l o c i t y ; t h e t u r b u l e n t a c t i o n o f t h e fluid l o o s e n s f o u l i n g d e p o s i t s a n d c a r r i e s t h e m a w a y . W a t e r flushing is t h e p r e l i m i n a r y s t e p i n every cleaning operation. 2. H a r d n e s s scales ( c a r b o n a t e s a n d s u l f a t e s s a l t s o f c a l c i u m a n d m a g n e s i u m ) a r e s o m e t i m e s r e m o v e d b y s o a k i n g t h e m e m b r a n e i n distilled w a t e r for l o n g p e r i o d s o f t i m e . S i n c e t h i s is t i m e c o n s u m i n g , m o r e o f t e n a w a r m s o l u t i o n o f 1-2% citric a c i d is v i g o r o u s l y c i r c u l a t e d t h r o u g h t h e u n i t , d i s solving large a m o u n t s of metallic ions a n d keeping t h e m in solution by chelation. 3. M i c r o b i o l o g i c a l g r o w t h s o c c u r in m o s t n a t u r a l w a t e r s , a n d a r e a p a r t i c u l a r p r o b l e m w h e n t r e a t i n g effluents f r o m b i o l o g i c a l p r o c e s s e s . T h e s e a r e often r e m o v e d b y r e c i r c u l a t i n g w a s h e s o f e n z y m e d e t e r g e n t s .
Theoretical flux Averaged flux C
Actual flux
Periodic cleaning of membrane
-
Υ
Ί
Fig. 8.18. Effect of membrane cleaning on flux [6]. (Reprinted by special permiss ion from Chemical Engineering•, April 2 , 1973. Copyright by McGraw-Hill, Inc., New York, Ν. Y, 10020.)
F l u x d e g r a d a t i o n b y f o u l i n g is a n a d d i t i o n a l loss s u p e r i m p o s e d o n t h e p e r m a n e n t losses d i s c u s s e d in S e c t i o n 5 . 1 1 . T h i s is s h o w n in F i g . 8.18, w h i c h i l l u s t r a t e s t h e effect o f m e m b r a n e c l e a n i n g o n flux. A s s h o w n in F i g . 8.18, t h e a c t u a l flux c u r v e follows a d e c l i n i n g , s a w - t o o t h e d p a t t e r n w h e n t h e m e m b r a n e is c l e a n e d p e r i o d i c a l l y . W i t h o u t c l e a n i n g , flux w o u l d follow t h e l o w e s t c u r v e , a p r o j e c t i o n o f t h e initial s m o o t h d e c l i n e . F l u x f r o m a h y p o t h e t i c a l
380
8.
Tertiary Treatment of Wastewaters
m e m b r a n e t h a t is n e v e r f o u l e d ( t h e o r e t i c a l flux) is s h o w n b y t h e u p p e r line, t h e t h e o r e t i c a l flux c u r v e w h i c h t o u c h e s o n l y t h e p e a k s o f t h e s a w - t o o t h e d curve.
5.13. P R E D I C T I O N O F F L U X [6] W a t e r o u t p u t f r o m a m e m b r a n e b e g i n s t o d e c l i n e a s s o o n a s p r e s s u r e is a p p l i e d , a n d c o n t i n u e s t o d e g r a d e slowly t h e r e a f t e r . T h e loss is i r r e v e r s i b l e , a n d if m o r e flux is r e q u i r e d feed p r e s s u r e m u s t b e i n c r e a s e d . T h i s a l t e r n a t i v e is self-defeating s i n c e a d d i t i o n a l p r e s s u r e , w h i l e p r o d u c i n g m o r e w a t e r , a l s o c o m p r e s s e s t h e m e m b r a n e f u r t h e r a n d h a s t e n s flux d e c l i n e . N o r m a l p r a c t i c e is t o overspecify t h e m e m b r a n e a r e a slightly a n d t o k e e p t h e o p e r a t i n g p r e s s u r e c o n s t a n t a s l o n g a s p o s s i b l e , r e s o r t i n g t o a d d i t i o n a l p r e s s u r e l a t e in t h e life of the m e m b r a n e . T h e o u t p u t o f a m e m b r a n e is p r e d i c t a b l e b e c a u s e t h e d e c l i n e p e r u n i t a r e a o f t h e m e m b r a n e is q u i t e u n i f o r m a n d c a n b e p r o j e c t e d . A p l o t o f flux v s . o p e r a t i n g t i m e a t a specified p r e s s u r e (like t h e l o w e r c u r v e in F i g . 8.18) yields a c u r v e w i t h a n initial s t e e p d e s c e n t f o l l o w e d b y a p r o l o n g e d a n d m o d e r a t e d e c l i n e . A l o g a r i t h m i c p l o t y i e l d s a s t r a i g h t line w h i c h is a d e q u a t e f o r p r e d i c t i o n p u r p o s e s for 1 a n d p r o b a b l y 2 y e a r s . T h u s , flux c a n b e p r e d i c t e d f r o m s u c h p l o t s o n c e t h e initial flux a n d s l o p e a r e k n o w n . M a n u f a c t u r e r s p r o v i d e initial flux v a l u e s a n d e s t i m a t e s o f s l o p e s a t v a r i o u s o p e r a t i n g p r e s s u r e s . I n i t i a l flux is t h e p r o d u c t i o n for t h e first 2 4 h r d i v i d e d b y t h e m e m b r a n e a r e a in t h e t e s t u n i t . D e c l i n e s l o p e is c o m p u t e d o r d e t e r m i n e d g r a p h i c a l l y f r o m flux v a l u e s t a k e n a t t i m e i n t e r v a l s s u c h a s 10, 100, a n d 1000 h r . T i m e i n t e r v a l s in m u l t i p l e s o f 10 a r e c o n v e n i e n t l y selected b e c a u s e c o m p u t a t i o n is simplified. D e c l i n e r a t e is g i v e n b y E q . (8.50). m = ( l o g F i - l o g F J / G o g / i - l o g ^ ) = log(F,/F,)/log(/,// ) x
(8.50)
w h e r e F is t h e i n i t i a l flux [ g a l / ( h r ) ( f t ) ] ; F t h e flux a t t i m e χ [ g a l / ( h r ) ( f t ) ] ; t t h e o p e r a t i n g t i m e ( h r ) for initial flux ( o b t a i n e d b y c o m p u t i n g initial flux a n d determining from a logarithmic plot of F vs. t the time t c o r r e s p o n d i n g t o this calculated value); a n d t the operating time (hr). 2
f
2
x
t
x
t
x
D e t e r m i n a t i o n o f flux o v e r t h e life o f t h e m e m b r a n e is b a s i c t o t h e d e s i g n of R O systems, because these values are used to estimate m e m b r a n e area r e q u i r e d for a d e s i r e d p l a n t c a p a c i t y b y u t i l i z i n g E q . (8.49). T h r e e p o s s i b l e approaches are 1. T a k e a n a v e r a g e o f t h e initial a n d final flux a s a c o m p r o m i s e t o o b t a i n a n a v e r a g e a r e a . W h e n t h i s c h o i c e is m a d e , it is e x p e c t e d t h a t d u r i n g t h e l a t e r life o f t h e m e m b r a n e , flux will b e b r o u g h t u p t o its initial v a l u e b y i n c r e a s i n g the operating pressure. 2. Specify m e m b r a n e a r e a f r o m final o r s m a l l e s t flux v a l u e . A l t h o u g h fixed c o s t s a r e h i g h e r , l o w e r s y s t e m p r e s s u r e s d e c r e a s e o p e r a t i n g c o s t s . 3. Select initial flux a s t h e d e s i g n b a s i s . T h i s m i n i m i z e s m e m b r a n e a r e a
5.
381
Reverse Osmosis
a n d c a p i t a l c o s t s , b u t raises o p e r a t i n g c o s t s a s p r e s s u r e s a r e i n c r e a s e d t o m a i n t a i n p r o d u c t i o n . T h i s is a logical c h o i c e for i n t e r m i t t e n t a n d
short-term
p r o j e c t s i n w h i c h initial c o s t s m u s t b e m i n i m i z e d .
5.14. M E M B R A N E L E A K A G E [6] C a l c u l a t e d s o l u t e r e j e c t i o n is a l w a y s h i g h e r t h a n e x p e r i m e n t a l v a l u e s , e v e n those t h a t are d e t e r m i n e d u n d e r the m o s t careful conditions. F o r example, t h e o r e t i c a l r e j e c t i o n o f s o d i u m c h l o r i d e f r o m a m o d i f i e d C A m e m b r a n e is calculated as 99.7%, b u t experimental results show n o better t h a n 9 7 - 9 9 % r e j e c t i o n . T h e difference is c a u s e d b y m i n u t e i m p e r f e c t i o n s in t h e m e m b r a n e t h r o u g h w h i c h p r e s s u r i z e d b r i n e c a n flow a n d c o n t a m i n a t e t h e p r o d u c t w a t e r . All m e m b r a n e s have imperfections; these are p r o b a b l y n o t m a n u f a c t u r i n g faults, b u t a p r o p e r t y of the m e m b r a n e t h a t m u s t be adjusted o r optimized t o s u i t a specific service. H e n c e , v e r y p o r o u s C A m e m b r a n e s a r e u s e d t o screen o u t large molecules (20-500 A diameter) a n d very small particles for u l t r a f i l t r a t i o n a p p l i c a t i o n s . L e s s p o r o u s m e m b r a n e s a r e selected f o r h i g h w a t e r flow a n d m o d e r a t e salt r e j e c t i o n service in saltless s o f t e n e r s , a n d m e m b r a n e s h e a t t r e a t e d t o l o w p o r o s i t y a r e u s e d for a p p l i c a t i o n s r e q u i r i n g h i g h rejection. Fortunately, most imperfections are small a n d r u n s f r o m n u m e r o u s h o l e s w i t h d i a m e t e r s 100 l - μ ι η in d i a m e t e r . T h e m a j o r s o u r c e o f p r o d u c t s o l u t e p a s s a g e t h r o u g h l a r g e r h o l e s , since f r o m flow, salt l e a k a g e i n c r e a s e s p r o p o r t i o n a t e l y t o diameter.
easily p l u g g e d . D i s t r i b u t i o n A a n d s m a l l e r t o a few o f c o n t a m i n a t i o n results from P o i s e u i l l e ' s l a w for v i s c o u s the fourth power of pore
A n u m b e r of techniques have been tried t o reduce m e m b r a n e leakage: 1. Heat treating and modifying dope formula. T h i s is t h e o n l y p e r m a n e n t w a y t o r e d u c e l e a k a g e . M e m b r a n e s f o r sea w a t e r m u s t b e h e a t t r e a t e d a t h i g h t e m p e r a t u r e s t o yield a film t h a t c a n reject 9 9 . 5 % o f t h e salt in t h e feed. 2. Addition of certain chemicals to the feed. C h e m i c a l s o f l a r g e m o l e c u l a r size a d d e d t o t h e feed a r e utilized t o p l u g l e a k i n g p o r e s . L o e b [ 7 ] d i s c o v e r e d t h a t t r a c e a m o u n t s o f a l u m i n u m salts o c c u r r i n g in L o s A n g e l e s t a p w a t e r p l u g g e d l e a k i n g p o r e s o f test m e m b r a n e s a n d i m p r o v e d salt r e j e c t i o n . Z e p h i r a n (tetraalkylaluminum chloride) was used at University of California at Los A n g e l e s ( U C L A ) t o g a i n a s i m i l a r a n d m o r e r e p r o d u c i b l e effect. O t h e r m a t e r i a l s t h a t i m p r o v e rejection include polyvinyl methyl ether a n d D o w f a x . Unfortunately, leak-stopping additives have serious d r a w b a c k s t h a t limit t h e i r u s e : (1) t h e y a r e m o r e effective o n l o w flux t h a n o n h i g h flux s t a n d a r d C A m e m b r a n e s , n o w a l m o s t exclusively u s e d ; (2) t h e y d i s s i p a t e q u i c k l y a n d m u s t b e r e g u l a r l y r e p l e n i s h e d ; (3) m o s t o f t h e m r e d u c e w a t e r flux a s t h e y r e d u c e s o l u t e l e a k a g e ; a n d (4) t h e y a r e e x p e n s i v e a n d t h e r e f o r e useful o n l y i n special s i t u a t i o n s w h e r e c o s t is n o t a f a c t o r .
382
8.
Tertiary Treatment of Wastewaters
5.15. S O L U T E P E R M E A B I L I T Y A N D CONCENTRATION POLARIZATION T h e o r e t i c a l l y , s o l u t e flux is a f u n c t i o n o f m e m b r a n e p e r m e a b i l i t y a n d t h e difference b e t w e e n s o l u t e c o n c e n t r a t i o n s in t h e h i g h a n d l o w p r e s s u r e sides o f t h e m e m b r a n e , i.e., Fsoiuu = where F
fi(C -C ) H
= β AC «
L
fiC
H
(8.51)
is t h e s o l u t e flux [ g / ( c m ) ( s e c ) ] ; β t h e s o l u t e p e r m e a b i l i t y c o 2
solute
efficient ( c m / s e c ) ; C
H
the c o n c e n t r a t i o n of solute o n high pressure side of
m e m b r a n e , i.e., c o n c e n t r a t e side ( g / c m ) ; a n d C 3
L
the concentration of solute
o n l o w p r e s s u r e side o f m e m b r a n e , i.e., p r o d u c t side ( g / c m ) . 3
U n l i k e w a t e r flux [ E q . ( 8 . 4 4 ) ] , n o r m a l s o l u t e flux is i n d e p e n d e n t o f p r e s s u r e [ E q . ( 8 . 5 1 ) ] . T h e o r e t i c a l l y , if p r e s s u r e in t h e R O s y s t e m is i n c r e a s e d , s o l u t e diffuses a t a c o n s t a n t r a t e w h i l e w a t e r flow i n c r e a s e s . T h e r e s u l t is g r e a t e r production of pure water. Since C
L
is u s u a l l y s m a l l a s c o m p a r e d t o C , H
E q . (8.51) is w r i t t e n a p p r o x i
m a t e l y a s i n d i c a t e d , i.e., salt flux is e s s e n t i a l l y g o v e r n e d b y c o n c e n t r a t i o n o f s o l u t e in t h e b o u n d a r y l a y e r n e x t t o t h e m e m b r a n e o n t h e c o n c e n t r a t e side. Solute concentration C
H
i n t h e feed C
F
can be substantially higher than the concentration
o w i n g t o a n effect c a l l e d c o n c e n t r a t i o n p o l a r i z a t i o n . I n m e m
b r a n e p r o c e s s e s s o l u t e a c c u m u l a t e s in a relatively s t a b l e l a y e r ( b o u n d a r y layer) n e x t t o t h e m e m b r a n e . C o n c e n t r a t i o n p o l a r i z a t i o n is t h e r a t i o o f s o l u t e c o n c e n t r a t i o n a t t h i s b o u n d a r y l a y e r t o t h a t in t h e b u l k o f t h e s o l u t i o n . Initially, s o l u t e c o n c e n t r a t i o n a t t h e b o u n d a r y l a y e r is t h e s a m e a s in t h e b u l k o f t h e s o l u t i o n . H o w e v e r , since t h e m e m b r a n e is p e r m e a b l e t o s o l v e n t a n d i m p e r m e a b l e t o solute, the b o u n d a r y layer becomes heavily p o p u l a t e d with solute as solvent leaves t h r o u g h the channels o f t h e m e m b r a n e . T h e b o u n d a r y layer grows thicker a n d m o r e concentrated, because the rate of s o l u t e diffusion a w a y f r o m t h e m e m b r a n e c a n n o t k e e p p a c e w i t h s o l v e n t flow t h r o u g h t h e m e m b r a n e . T h e r e s u l t o f c o n c e n t r a t i o n p o l a r i z a t i o n follows f r o m E q . (8.51). W r i t e C
H
= C
B L
, where C
B L
s t a n d s for t h e p r o g r e s s i v e l y i n c r e a s i n g s o l u t e c o n
c e n t r a t i o n a t t h e b o u n d a r y layer. T h e r e f o r e Fsolute *
fiC
Bh
(8.52)
C o n s e q u e n t l y , c o n c e n t r a t i o n p o l a r i z a t i o n r e s u l t s in a n i n c r e a s e d s o l u t e flux o r a l o w e r p r o d u c t q u a l i t y . T h i s is u n d e s i r a b l e , s o o n e strives t o r e d u c e t h e c o n c e n t r a t i o n p o l a r i z a t i o n effect. T h i s is a c c o m p l i s h e d i n t w o w a y s : 1. Higher feed velocity. T h i s r e d u c e s t h i c k n e s s a n d c o n c e n t r a t i o n o f t h e b o u n d a r y layer a s it is s c o u r e d a w a y b y t h e feed s t r e a m a t h i g h v e l o c i t y . T h e e x t r a flow t h r o u g h t h e u n i t r e s u l t s in a n o v e r a l l l o w e r p r o d u c t r e c o v e r y , i.e., s m a l l e r r a t i o o f p r o d u c t t o feed. T h i s i n c r e a s e s p o w e r c o n s u m p t i o n a n d a m o u n t of concentrate (waste) p r o d u c e d .
5.
383
Reverse Osmosis
2. Addition
of turbulence
promoters.
T h i s is a m o r e efficient w a y t o r e d u c e
the b o u n d a r y layer. T u b u l a r m e m b r a n e units a r e p r o v i d e d with plastic balls o n t h e h i g h p r e s s u r e side t o b r e a k u p t h e s m o o t h flow o f feed s o l u t i o n . T u r b u lence p r o m o t e r s m a y r e q u i r e p r e f i l t r a t i o n o f t h e feed s o t h a t p a r t i c u l a t e s o r p r e c i p i t a t e s d o n o t g e t l o d g e d i n n a r r o w flow s p a c e s a n d p l u g t h e m e m b r a n e . T h e u s u a l c o n c e n t r a t i o n p o l a r i z a t i o n r a t i o is 1.2-2.0, w h i c h m e a n s
that
s o l u t e c o n c e n t r a t i o n a t t h e b o u n d a r y l a y e r is 1.2-2.0 t i m e s t h a t i n t h e b u l k o f t h e feed. T h e a p p r o x i m a t e e q u a t i o n f o r w a t e r flux, E q . (8.44), is r e w r i t t e n t a k i n g i n t o a c c o u n t t h e c o n c e n t r a t i o n p o l a r i z a t i o n effect, i n w h i c h c a s e Δ π is
Δπ =
(8.53)
7TBL — TCP
w h e r e n is t h e o s m o t i c p r e s s u r e o f t h e c o n c e n t r a t e d s o l u t i o n a t t h e b o u n d a r y l a y e r (n > n ). S i n c e Δ π , g i v e n b y E q . (8.53), is h i g h e r t h a n t h e v a l u e c a l c u l a t e d f r o m E q . (8.46), t h i s i m p l i e s f r o m E q . (8.44) a d e c r e a s e d w a t e r flux o w i n g t o t h e c o n c e n t r a t i o n p o l a r i z a t i o n effect. S u b s t i t u t i o n o f E q s . (8.53) a n d (8.45) i n E q . (8.44) yields BL
BL
F
F o = A[(P -P )-(n -n )] H2
F
P
Bh
(8.54)
P
T h e r e is n o s i m p l e e x p e r i m e n t a l t e c h n i q u e a v a i l a b l e t o d e t e r m i n e s o l u t e c o n c e n t r a t i o n a t t h e b o u n d a r y l a y e r ( a n d t h e r e f o r e n ). A n i n d i r e c t a p p r o a c h c o n s i s t s o f d e t e r m i n i n g e x p e r i m e n t a l l y w a t e r flux, F o, and permeation coefficient A ( S e c t i o n 5.16). T h e n n is t h e o n l y u n k n o w n i n E q . (8.54), a n d therefore BL
H2
BL
* B L = n + (P - P ) - (F /A) P
F
P
H20
(8.55)
T h e c o n c e n t r a t i o n o f s o l u t e a t t h e b o u n d a r y l a y e r is t h e n o b t a i n e d f r o m a p r e viously p r e p a r e d plot of osmotic pressure vs. solute c o n c e n t r a t i o n [ π = f(c)l
5.16. E X P E R I M E N T A L T E C H N I Q U E F O R LABORATORY PREDICTION OF MEMBRANE PERFORMANCE A laboratory a p p a r a t u s for t h e prediction of m e m b r a n e performance in r e v e r s e o s m o s i s is d e s c r i b e d b y S o u r i r a j a n a n d A g r a w a l [ 9 ] , a n d a s c h e m a t i c flow d i a g r a m is s h o w n i n F i g . 8.19. F e e d s o l u t i o n is p u m p e d t h r o u g h a s u r g e t a n k i n t o t h e cell c o n t a i n i n g t h e m e m b r a n e . P r e s s u r e is c o n t r o l l e d b y a p r e s s u r e regulator operating o n a nitrogen back pressure system. E x p e r i m e n t a l l y d e t e r m i n e d v a r i a b l e s a t a specific p r e s s u r e a r e (1) r e j e c t i o n f a c t o r [ E q . (8.48)]; (2) p r o d u c t r e c o v e r y , i.e., r a t i o o f p r o d u c t t o f e e d ; a n d (3) p e r m e a t i o n coefficient A. P e r m e a t i o n coefficient A, w h i c h i n c l u d e s p h y s i c a l v a r i a b l e s o f t h e m e m b r a n e , is relatively c o n s t a n t ( S e c t i o n 5.7). T h e r e f o r e , it
384
8.
N
Tertiary Treatment of Wastewaters
2
g a s under pressure-H
Surge tank
Feed solution inlet
Feed solution outlet
7)
Feed solution out Fig. 8.19. Schematic flow diagram permission from Ind. Eng. Chem. Copyright
of RO laboratory unit by the American Chemical
[ 9 ] . (Reprinted Society.)
with
c a n b e d e t e r m i n e d o n c e a n d for all b y o p e r a t i n g t h e s y s t e m a t a specific p r e s s u r e w i t h p u r e w a t e r a n d m e a s u r i n g t h e r a t e a t w h i c h it p e r m e a t e s t h r o u g h t h e m e m b r a n e . E q u a t i o n (8.53) f o r p u r e w a t e r o p e r a t i o n yields Δπ = n L
— 7TP
B
= 0
F r o m E q . (8.54), A = F /AP
(8.56)
H20
where AP = P and F
F
-
P
P
o is t h e p u r e w a t e r flux [ g / ( h r ) ( c m ) ] . P r o d u c t flow ( g / h r ) in t h i s 2
H 2
c a s e is called p u r e w a t e r p e r m e a b i l i t y (PWP). A = Ρ WP/(Mx
T h e r e f o r e f r o m E q . (8.56),
S χ 3600ΔΡ)
(8.57)
w h e r e A is t h e p e r m e a t i o n coefficient [ g m o l e o f w a t e r / ( s e c ) ( c m ) ( a t m ) ] ; PWP t h e p u r e w a t e r p e r m e a b i l i t y ( g / h r ) ; Μ t h e m o l e c u l a r w e i g h t o f w a t e r ( 1 8 ) ; S t h e effective m e m b r a n e a r e a ( c m ) (7.6 c m for S o u r i r a j a n ' s a p p a r a t u s in F i g . 8.19); a n d AP t h e differential p r e s s u r e ( a t m ) . 2
2
2
5.17. F I N A L R E M A R K S O N R E V E R S E O S M O S I S R e v e r s e o s m o s i s is still t o o e x p e n s i v e for w i d e s p r e a d u t i l i z a t i o n in w a s t e w a t e r t r e a t m e n t . I t is a l s o l i m i t e d t o t r e a t i n g s o l u b l e w a s t e s since s u s p e n d e d solids c l o g t h e m e m b r a n e s . C o n s e q u e n t l y , p r e t r e a t m e n t o f t h e feed is r e quired whenever there are suspended solids, t h u s raising costs.
6.
385
Electrodialysis
O p e r a t i n g p r e s s u r e s e m p l o y e d v a r y f r o m 6 0 0 u p t o 1500 p s i g . O n e o f t h e m a i n g o a l s in c u r r e n t r e s e a r c h is t o d e v e l o p b e t t e r m e m b r a n e s t h a t c a n o p e r a t e a t l o w e r p r e s s u r e s ( a r o u n d 2 5 0 p s i g ) a n d still yield relatively h i g h p r o d u c t r a t e s , o f t h e o r d e r o f 4 0 g a l / ( d a y ) ( f t ) . O n c e t h e s e difficulties a r e 2
o v e r c o m e , R O c o u l d b e c o m e a p r o c e s s o f w i d e s p r e a d a p p l i c a t i o n in t h e field of wastewater treatment.
6. Electrodialysis 6.1.
INTRODUCTION
Electrodialysis was originally developed for desalination of o c e a n water. I t is a p r o m i s i n g m e t h o d for r e m o v a l o f i n o r g a n i c n u t r i e n t s ( p h o s p h o r u s a n d n i t r o g e n ) f r o m w a s t e w a t e r s , a n d t h u s it is a p o s s i b l e final s t a g e in w a s t e w a t e r treatment processes. A d i a g r a m o f a n e l e c t r o d i a l y s i s cell is s h o w n in F i g . 8.20. T h e b a s i c c o m p o n e n t s o f t h e cell a r e a series o f m e m b r a n e s m a d e o f i o n e x c h a n g e r e s i n s . T h e s e m e m b r a n e s a r e p e r m e a b l e o n l y t o i o n i c species a n d a r e selective t o a specific t y p e o f i o n . T h e r e a r e t w o t y p e s o f m e m b r a n e s u t i l i z e d in a n e l e c t r o dialysis c e l l : (1) c a t i o n m e m b r a n e s , w h i c h p o s s e s s a fixed n e g a t i v e c h a r g e , allowing cations (positive ions) t o pass t h r o u g h t h e m b u t repelling a n i o n s ( n e g a t i v e i o n s ) ; a n d (2) a n i o n m e m b r a n e s , w h i c h p o s s e s s a fixed p o s i t i v e charge, allowing anions (negative ions) to pass t h r o u g h t h e m b u t repelling cations (positive ions). P a s s a g e o f i o n s t h r o u g h t h e m e m b r a n e s is a c c e l e r a t e d b y a p p l i c a t i o n o f a c o n s t a n t v o l t a g e a c r o s s a series o f c a t i o n - a n d a n i o n - p e r m e a b l e m e m b r a n e s , a s i n d i c a t e d in F i g . 8.20. T h e c a t h o d e a n d a n o d e a r e l o c a t e d a t t w o e x t r e m e s o f t h e cell, s o t h a t t h e m e m b r a n e c l o s e s t t o t h e c a t h o d e is c a t i o n p e r m e a b l e a n d t h a t c l o s e s t t o t h e a n o d e is a n i o n p e r m e a b l e . R a w w a s t e w a t e r is fed continuously into the concentrating compartments, and treated wastewater is w i t h d r a w n c o n t i n u o u s l y f r o m t h e d i l u t i n g c o m p a r t m e n t s .
6.2. V O L T A G E R E Q U I R E D F O R E L E C T R O D I A L Y S I S V o l t a g e r e q u i r e d is c a l c u l a t e d f r o m O h m ' s l a w [ E q . ( 8 . 5 8 ) ] . Ε = IR
(8.58)
w h e r e Ε is t h e a p p l i e d v o l t a g e ( V ) , / t h e c u r r e n t ( A ) , a n d R t h e t o t a l e l e c t r i c a l r e s i s t a n c e o f m e m b r a n e s a n d s o l u t i o n s in cells ( o h m s ) .
6.3. C U R R E N T R E Q U I R E D F Q R E L E C T R O D I A L Y S I S T h e c u r r e n t r e q u i r e d is p r o p o r t i o n a l t o t h e i o n i c s t r e n g t h o f s o l u t i o n ( e x p r e s s e d in t e r m s o f n o r m a l i t y ) , a n d t h e n u m b e r o f cells. I t is c a l c u l a t e d
386
8.
Tertiary Treatment of Wastewaters
-Diluting compartmentCathode
\
Concentrating compartment
j
A
Legend:Δ © A C Fig.
-
Anode +
C
Cation Anion Anion-permeable membrane Cation-permeable membrane
8.20. Diagram
of an electrodialysis
cell.
f r o m F a r a d a y ' s l a w : 9 6 , 5 0 0 A χ sec o f electricity t r a n s f e r o n e g r a m - e q u i v a l e n t weight of an electrolyte from o n e electrode t o another. T h u s , current required f o r e l e c t r o d i a l y s i s f o r a fixed a p p l i e d v o l t a g e Ε is g i v e n i n E q . (8.59). / = FqNe/ηε
(8.59)
w h e r e / is t h e c u r r e n t ( A ) , F t h e F a r a d a y c o n s t a n t ( 9 6 , 5 0 0 A χ s e c / g e q u i v a l e n t ) , q t h e flow r a t e (liters/sec), TV t h e n o r m a l i t y o f s o l u t i o n (g e q u i v a l e n t s / l i t e r ) , e t h e r e m o v a l efficiency (0 < e < 1.0), η t h e n u m b e r o f cells b e t w e e n e l e c t r o d e s , a n d ε t h e c u r r e n t efficiency ( 0 < ε < 1.0). I n c r e a s i n g t h e n u m b e r o f cells for a fixed a p p l i e d v o l t a g e Ε r e s u l t s i n a n i n c r e a s e o f t h e t o t a l e l e c t r i c a l r e s i s t a n c e R. C o n s e q u e n t l y , f r o m O h m ' s l a w the current / decreases.
6.4. P R E T R E A T M E N T O F W A S T E W A T E R S IN E L E C T R O D I A L Y S I S F o r p r o p e r o p e r a t i o n o f t h e e l e c t r o d i a l y s i s cell, p a r t i c u l a t e m a t t e r , l a r g e o r g a n i c i o n s , a n d c o l l o i d a l m a t t e r m u s t b e r e m o v e d p r i o r t o t h e p r o c e s s . If t h i s is n o t d o n e , t h e s e m a t e r i a l s c a u s e f o u l i n g o f m e m b r a n e s , w h i c h r e s u l t s i n a n increase of t o t a l electrical resistance. F o r a c o n s t a n t applied voltage, t h e c u r r e n t p a s s i n g t h r o u g h t h e cell is t h u s l o w e r e d . T h e r e f o r e , d e m i n e r a l i z i n g c a p a c i t y o f t h e e q u i p m e n t is l o w e r e d .
7.
387
Chemical Oxidation Processes
F o u l i n g o f m e m b r a n e s is t h e g r e a t e s t p r o b l e m t o b e o v e r c o m e i n o r d e r t o achieve e c o n o m i c operation of electrodialysis in t r e a t m e n t of wastewaters. F o u l i n g is m i n i m i z e d b y 1. P r e t r e a t m e n t o f r a w w a s t e w a t e r in o r d e r t o r e m o v e p a r t i c u l a t e a n d c o l l o i d a l m a t t e r a n d l a r g e o r g a n i c i o n s . T h i s is d o n e b y a d d i t i o n o f c o a g u l a n t s , filtration
through
microscreens,
and/or
adsorption
in
activated
carbon
c o l u m n s . C o s t of p r e t r e a t m e n t m a y r e n d e r the process u n e c o n o m i c a l . 2. F o u l i n g is m i n i m i z e d b y p e r i o d i c p l a n t s h u t d o w n f o r c l e a n i n g . 3. F r e q u e n t c u r r e n t r e v e r s a l s t e n d t o m i n i m i z e t h e effects o f f o u l i n g .
7. Chemical Oxidation P r o c e s s e s (Chlorination and Ozonation) 7.1. C H L O R I N A T I O N O F W A S T E W A T E R S 7.1.1. Utilization and Purposes of Chlorination C h l o r i n a t i o n is a w i d e l y u s e d p r o c e s s in t h e t r e a t m e n t o f d o m e s t i c a n d i n d u s t r i a l w a s t e w a t e r s . S o m e i n d u s t r i a l effluents w h i c h a r e c o m m o n l y c h l o rinated prior t o discharge into receiving waters are those from beet sugar, c a n n e r y , d a i r y , p u l p a n d p a p e r , textile, t a n n i n g , p e t r o c h e m i c a l , p h a r m a ceutical, a n d metal processing ( c h r o m i u m , electroplating) plants. Purposes of chlorination are summarized as follows: 1. Disinfection. P r i m a r i l y a d i s i n f e c t a n t o w i n g t o its s t r o n g o x i d i z i n g capacity, chlorine destroys or inhibits g r o w t h of bacteria a n d algae. 2. BOD reduction. Chlorine accomplishes B O D reduction by oxidation o f o r g a n i c c o m p o u n d s p r e s e n t in w a s t e w a t e r s . 3. Elimination or reduction of colors and odors. C o l o r a n d o d o r - p r o d u c i n g s u b s t a n c e s p r e s e n t in w a s t e w a t e r s a r e o x i d i z e d b y c h l o r i n e . T h e o x i d i z i n g ability o f c h l o r i n e is e m p l o y e d for o d o r c o n t r o l a n d c o l o r r e m o v a l i n t r e a t m e n t o f m a n y i n d u s t r i a l effluents ( b e e t s u g a r , c a n n e r y , d a i r y , p u l p a n d p a p e r , textiles). 4. Oxidation of metal ions. M e t a l i o n s w h i c h a r e in a r e d u c e d s t a t e a r e o x i d i z e d b y c h l o r i n e (e.g., f e r r o u s t o ferric i o n a n d m a n g a n o u s t o m a n g a n i c ions). 5. Oxidation of cyanides to innocuous products. T h i s a p p l i c a t i o n is d e s c r i b e d i n S e c t i o n 7.1.4.
7.1.2. Reactions of Chlorine in W a t e r A d d e d to water as either a gas or solution, chlorine reacts to form h y p o c h l o r o u s a c i d ( H O C l ) , w h i c h s u b s e q u e n t l y d i s s o c i a t e s a c c o r d i n g t o t h e fol lowing chemical equations: Cl + H 0 ^ 2
2
HOCl ^
HOCl + H H H
+
+ OCl"
Cl~
388
8.
Tertiary Treatment of Wastewaters
In the presence of a m m o n i a , h y p o c h l o r o u s acid reacts t o form m o n o c h l o r amine, dichloramine, a n d nitrogen trichloride. Relative p r o p o r t i o n s of these p r o d u c t s depend on p H a n d concentration of a m m o n i a present. C o r r e s p o n d ing chemical equations are NH
3
+ HOCl ^
NH C1 + H 0 (monochloramine) 2
2
NH C1 + HOCl ^
NHC1 + H 0 (dichloramine)
NHC1 + HOCl ^
NC1 + H 0 (nitrogen trichloride)
2
2
2
2
3
2
R e a c t i o n s o f c h l o r i n e in w a t e r a r e i l l u s t r a t e d b y F i g . 8 . 2 1 , w h i c h s h o w s t h e relationship between chlorine added and chlorine residual.
-Zone I: Destruction of 1 chlorine by \reducing compounds 0.6 0.5 0.4 0.3
-Zone 3: 1 Destruction of chloramines and \ chloro organic compounds . Zone 4 : Formation of free chlorine and presence of chloro organic compounds not destroyed
Zone 2: Formation of chloro organic compounds and chloramines
Free residual
0.2
Combined residual
0.1 0.2
0.4
0.6
0.8
Chlorine added (mg/liter) Fig. 8.21.
Reactions
of chlorine
in water
[8].
T h e i n i t i a l a m o u n t o f c h l o r i n e a d d e d is r e d u c e d b y c o m p o u n d s w h i c h r e a c t r a p i d l y w i t h c h l o r i n e (e.g., F e and M n ) . This corresponds to Z o n e 1 for w h i c h r e s i d u a l c h l o r i n e is n e a r l y z e r o . C o n t i n u e d a d d i t i o n o f c h l o r i n e r e s u l t s in r e s i d u a l c h l o r i n e i n t h e f o r m o f c h l o r o o r g a n i c c o m p o u n d s o r c h l o r a m i n e s ( c o m b i n e d r e s i d u a l ) . C h l o r i n e r e s i d u a l is a l w a y s less t h a n t h e c h l o r i n e a d d e d . T h i s c o r r e s p o n d s t o Z o n e 2 o f t h e c u r v e in F i g . 8 . 2 1 . B y adding m o r e chlorine, chloro organic c o m p o u n d s are frequently oxidized: t h e m o l e c u l e is b r o k e n d o w n a n d c h l o r i n e is l i b e r a t e d . T h i s r e s u l t s in a decrease o f r e s i d u a l c h l o r i n e ( Z o n e 3 i n F i g . 8.21). F i n a l l y , w h e n all r e d u c i n g c o m p o u n d s have been oxidized, the additional quantity of chlorine a d d e d t o the 2 +
2 +
7.
389
Chemical Oxidation Processes
w a t e r r e s u l t s in a n e q u i v a l e n t r e s i d u a l c h l o r i n e . T h i s c o r r e s p o n d s t o Z o n e 4 in F i g . 8 . 2 1 , w h i c h d i s p l a y s a 4 5 ° s t r a i g h t line for t h e free r e s i d u a l , a n d a s t r a i g h t line p a r a l l e l t o t h e a b s c i s s a for t h e c o n s t a n t c o m b i n e d r e s i d u a l .
7.1.3. Chlorine a s a Disinfectant T y p i c a l c h l o r i n e d o s a g e s r e q u i r e d for d i s i n f e c t i o n a r e s h o w n in T a b l e 8.5. T h e effectiveness o f c h l o r i n e for killing b a c t e r i a is given b y C h i c k ' s l a w [ 4 ] , w h i c h is w r i t t e n in differential f o r m a s E q . (8.60). dN/dt
= -kN
(8.60)
T A B L E 8.5 Typical Chlorine Dosages for Disinfection [8] Effluent from
D o s a g e range (mg/liter)
Untreated wastewater (prechlorination) Primary sedimentation Chemical precipitation plant Trickling filter plant Activated sludge plant Multimedia filter following activated sludge plant
6-25 5-20 2-6 3-15 2-8 1-5
w h e r e Ν is t h e b a c t e r i a c o u n t , t t h e t i m e , dN/dt t h e r a t e o f b a c t e r i a kill, a n d k t h e r a t e o f kill c o n s t a n t . S e p a r a t i n g t h e v a r i a b l e s a n d i n t e g r a t i n g f r o m t i m e t = 0 t o a n y t i m e i, N/No
= e~
(8.61)
kt
w h e r e N is t h e b a c t e r i a c o u n t a t ί = 0 a n d Ν t h e b a c t e r i a c o u n t a t t i m e t. T h e r a t e o f kill c o n s t a n t k is a f u n c t i o n o f p H , t e m p e r a t u r e , a n d a p p l i e d c o n c e n t r a t i o n o f c h l o r i n e . I t is e s t i m a t e d f r o m t h e s l o p e o f a s t r a i g h t line p l o t o f \nN/N v s . t b a s e d o n E q . (8.61), w h i c h w r i t t e n in l o g a r i t h m i c f o r m is 0
0
\n(N/N )
= -kt
0
(8.62)
T h e effectiveness o f c h l o r i n a t i o n for d e s t r u c t i o n o f v a r i o u s o r g a n i s m s c o r r e s p o n d s t o v a l u e s o f k f r o m 0.24 t o 6.3 for 9 9 % kill (i.e., N/N = 1/100 = 0.01) a t 0 ° - 6 ° C . 0
C h i c k ' s l a w is a n i d e a l i z e d p o r t r a y a l o f t h e s i t u a t i o n . U s u a l l y , i d e a l c o n d i t i o n s d o n o t exist o w i n g t o v a r i a t i o n s in cell r e s i s t a n c e , d e c r e a s e in c h l o r i n e c o n c e n t r a t i o n , e t c . R a t e s o f kill s o m e t i m e s i n c r e a s e o r d e c r e a s e w i t h t i m e . C o n s e q u e n t l y , a m o d i f i e d f o r m o f E q . (8.61) c o n t a i n i n g a n e x t r a c o n s t a n t m t o b e d e t e r m i n e d f r o m e x p e r i m e n t a l d a t a is w r i t t e n a s Ν IN
0
=
e~
ktm
(8.63)
390
8.
Tertiary Treatment of Wastewaters
If m is less t h a n 1, t h e r a t e o f kill d e c r e a s e s w i t h t i m e , a n d if m is g r e a t e r t h a n 1, it i n c r e a s e s . C o n s t a n t s i n E q . ( 8 . 6 3 ) a r e d e t e r m i n e d b y p l o t t i n g
-ln(N/N ) 0
vs. c o n t a c t t i m e t o n l o g a r i t h m i c p a p e r . T h e l i n e a r r e l a t i o n s h i p is l o g [ - ln(N/N )]
= l o g £ + m log /
0
(8.64)
T h e effect o f c h l o r i n e c o n c e n t r a t i o n is defined b y t h e r e l a t i o n s h i p C % = constant = Κ
(8.65)
w h e r e C is t h e c h l o r i n e c o n c e n t r a t i o n ( m g / l i t e r ) ; t
p
the t i m e required for a
given percentage kill; a n d η the c o n s t a n t t o be evaluated from the experi mental data. C o n s t a n t s in E q . (8.65) a r e d e t e r m i n e d b y p l o t t i n g o n l o g a r i t h m i c p a p e r t h e c o n c e n t r a t i o n o f c h l o r i n e v s . t i m e for a g i v e n p e r c e n t a g e kill. T h e l i n e a r f o r m o f E q . (8.65) is log C = - (\/n) log t + (IIn) log Κ
(8.66)
p
T h e s l o p e o f t h i s line c o r r e s p o n d s t o v a l u e o f — ( 1 / n ) .
7.1.4. Utilization of Chlorine for Destruction of C y a n i d e s C h l o r i n e is utilized t o o x i d i z e c y a n i d e t o i n n o c u o u s p r o d u c t s . T h i s is d o n e i n a n a l k a l i n e m e d i a a t v a l u e s o f p H g r e a t e r t h a n 8.5. O x i d a t i o n t a k e s p l a c e i n t w o s t a g e s a c c o r d i n g t o E q s . (8.67) a n d (8.68). First stage:
C N ~ + 2 0 H ~ + C\
-> C N O " + 2 C 1 " + H 0
2
Second stage:
2 C N O " + 4 0 H ~ + 3Cl
(8.67)
2
2
2C0
2
+ N
2
+ 6C1~ + 2 H 0 2
(8.68)
F r o m E q s . (8.67) a n d (8.68) t h e t h e o r e t i c a l s t o i c h i o m e t r i c r a t i o is 5 C 1 / 2
2 C N " o r 2 . 5 C 1 / 1 C N ~ . I n p r a c t i c e , a l a r g e excess o f c h l o r i n e is u t i l i z e d , o f 2
t h e o r d e r o f 7.5 p a r t s c h l o r i n e p e r o n e p a r t C N ~ .
7.1.5. E c o n o m i c s of Chlorination of W a s t e w a t e r s A l t h o u g h t h e u s e o f c h l o r i n a t i o n is w i d e s p r e a d , it s h o u l d b e p o i n t e d o u t t h a t c h l o r i n e is a relatively e x p e n s i v e c h e m i c a l . If e c o n o m i c s is a c o n s i d e r a t i o n for a given a p p l i c a t i o n , o t h e r m e t h o d s s h o u l d b e e v a l u a t e d . C h l o r i n e o x i d e s ( C 1 0 , C 1 0 , C 1 0 ) have been used in the disinfection of waters. A mixture 2
2
5
2
of C 1 0 / C 1 0 2
5
2
7
7
is a v a i l a b l e c o m m e r c i a l l y .
7.2. O Z O N A T I O N O F W A S T E W A T E R S C h e m i c a l o x i d a t i o n w i t h o z o n e is a n effective m e t h o d for t r e a t i n g w a s t e waters, based o n the following factors [ 5 ] : 1. O z o n e r e a c t s r e a d i l y w i t h u n s a t u r a t e d o r g a n i c s i n w a s t e w a t e r s .
8.
391
Nutrient Removal
2. F o a m i n g c h a r a c t e r i s t i c s o f w a s t e w a t e r s a r e r e d u c e d f o l l o w i n g o z o n e treatment. 3. R i n g o p e n i n g a n d p a r t i a l o x i d a t i o n o f a r o m a t i c s r e n d e r s t h e w a s t e water m o r e susceptible t o conventional biological treatment. 4 . O z o n e in t h e effluent q u i c k l y r e v e r t s t o o x y g e n o n c e it h a s s e r v e d its p u r p o s e . T h i s d i s s o l v e d o x y g e n is beneficial t o t h e r e c e i v i n g s t r e a m a n d h e l p s s u p p o r t a q u a t i c life. I n c o n t r a s t , c h l o r i n e ( w h i c h is t h e m o s t w i d e l y u s e d b a c t e r i a kill a g e n t ) lingers in t h e effluent a n d b e c o m e s a p o l l u t a n t itself). L a b o r a t o r y scale o z o n a t i o n e q u i p m e n t for e v a l u a t i n g a m e n a b i l i t y o f w a s t e w a t e r t o o z o n e o x i d a t i o n is d e s c r i b e d b y E c k e n f e l d e r a n d F o r d [ 5 ] . O z o n e m a y r e p l a c e c h l o r i n e in t r e a t m e n t o f c y a n i d e w a s t e w a t e r s . O x i d a t i o n t a k e s p l a c e in t w o s t a g e s a c c o r d i n g t o E q s . (8.69) a n d (8.70). First stage: Second stage:
CN" + 0
-+ C N O " + 0
3
2CNO~ + 3 0
3
+ H 0 2
(8.69)
2
2HC0 " + N 3
2
+ 30
2
(8.70)
8. N u t r i e n t Removal 8.1. I N T R O D U C T I O N R e m o v a l of nutrients ( p h o s p h o r u s a n d nitrogen c o m p o u n d s ) from waste w a t e r s is a n i m p o r t a n t o p e r a t i o n , b e c a u s e t h e s e c o m p o u n d s p l a y a critical r o l e i n l a k e e u t r o p h i c a t i o n ( C h a p t e r 1, S e c t i o n 7). E m p h a s i s h a s b e e n g i v e n t o p h o s p h o r u s r e m o v a l for t w o r e a s o n s : (1) p h o s p h o r u s is t h e m o s t c r i t i c a l n u t r i e n t , a n d (2) n i t r o g e n r e m o v a l p r o c e s s e s a r e less efficient a n d m o r e e x p e n s i v e . M o s t n u t r i e n t r e m o v a l t r e a t m e n t p r o c e s s e s in o p e r a t i o n t o d a y a r e d e s i g n e d for p h o s p h o r u s r e m o v a l a l o n e .
8.2. P H O S P H O R U S R E M O V A L 8.2.1. Processes for P h o s p h o r u s Removal P r o c e s s e s for p h o s p h o r u s r e m o v a l i n c l u d e (1) c h e m i c a l p r e c i p i t a t i o n , (2) a c t i v a t e d s l u d g e p r o c e s s ( C h a p t e r 5), (3) s t a b i l i z a t i o n p o n d s ( C h a p t e r 6, S e c t i o n 6), (4) r e v e r s e o s m o s i s ( C h a p t e r 8, S e c t i o n 5), a n d (5) e l e c t r o d i a l y s i s ( C h a p t e r 8, S e c t i o n 6). T h e a c t i v a t e d s l u d g e p r o c e s s , a l t h o u g h p r i m a r i l y i n t e n d e d for r e m o v a l o f o r g a n i c c o n t a m i n a n t s , r e m o v e s b o t h p h o s p h o r u s a n d n i t r o g e n , since b i o l o g ical cells c o n t a i n a p p r o x i m a t e l y 2 . 0 % p h o s p h o r u s a n d 1 2 % n i t r o g e n b y weight. F o r domestic sewage this a m o u n t s t o a p h o s p h o r u s r e m o v a l rate of 2 0 - 4 0 % ( o r 1-2 m g / l i t e r ) . S t a b i l i z a t i o n p o n d s yield relatively h i g h p h o s p h o r u s a n d n i t r o g e n r e m o v a l , p r o v i d e d light a n d t e m p e r a t u r e c o n d i t i o n s a r e f a v o r a b l e t o t h e g r o w t h o f algae. D u r i n g the summer, removal of a b o u t 80% of the nutrients m a y be
392
8.
Tertiary Treatment of Wastewaters
o b t a i n e d , w h e r e a s d u r i n g t h e w i n t e r it m a y d r o p t o 2 0 % o r less. R e v e r s e o s m o s i s a n d e l e c t r o d i a l y s i s a r e still t o o c o s t l y f o r g e n e r a l i z e d u s e . T h e d i s c u s s i o n o f p h o s p h o r u s r e m o v a l i n t h i s s e c t i o n is exclusively c o n c e r n e d w i t h chemical precipitation processes.
8.2.2. Chemical Precipitation Processes for P h o s p h o r u s Removal P r e c i p i t a n t s w h i c h h a v e b e e n m a i n l y e m p l o y e d in p h o s p h o r u s r e m o v a l a r e Fe (as F e C l ) , C a (as l i m e ) , A l [as alum, A 1 ( S 0 ) . 1 6 H 0 ] , and c o m b i n a t i o n s of F e a n d l i m e . T h e m e c h a n i s m o f p h o s p h o r u s r e m o v a l is mostly precipitation in the form of phosphates of C a , F e , a n d A l . 3 +
2 +
3 +
3
2
4
3
2
3 +
2 +
3Ca
2 +
Fe Al
3 +
3 +
+ 2P
3 _ 4
3 4
3 +
3 +
Ca (P0 ) i 3
4
2
-> F e P 0 j
(8.71)
4
-
AlP0 j 4
A d s o r p t i o n a l s o p l a y s a r o l e in t h e r e m o v a l o f s o m e p h o s p h a t e s w h i c h a r e a d s o r b e d o n t h e p r e c i p i t a t i n g floe. C o n s i d e r a t i o n s o n u t i l i z a t i o n o f different precipitants are as follows: 1. F e (as F e C l ) . F e C l (in d o s a g e s o f 10 m g / l i t e r ) is t h e m o s t c o m m o n l y e m p l o y e d p r e c i p i t a t i n g a g e n t for p h o s p h o r u s , y i e l d i n g a r o u n d 9 0 % removal. 3 +
3
3
2. C a (as l i m e ) . L i m e is less efficient t h a n F e C l . U t i l i z e d in d o s a g e s o f 5 0 0 - 7 0 0 m g / l i t e r yields r e m o v a l o f a t m o s t 8 0 % a t p H v a l u e s o f 1 0 . 5 - 1 1 . A n o t h e r d r a w b a c k o f l i m e u t i l i z a t i o n r e s i d e s in t h e l a r g e v o l u m e s o f s l u d g e p r o d u c e d , which causes a disposal p r o b l e m . Recovery of lime by calcination of this sludge m a y be performed. Because of the presence of organic materials, t h e s l u d g e m a y b e c a p a b l e o f s u p p o r t i n g its o w n c o m b u s t i o n . 2 +
3
3. A l [as alum, A 1 ( S 0 ) . 1 6 H 0 ] . Although alum precipitation yields a p p r o x i m a t e l y 9 5 % p h o s p h o r u s r e m o v a l a t d o s a g e s o f 2 0 0 - 2 5 0 m g / liter, it is less f r e q u e n t l y a p p l i e d o w i n g t o t h e h i g h c o s t o f a l u m . R e m o v a l o f 5 0 - 6 0 % o f o r g a n i c m a t e r i a l s ( c a r b o n a c e o u s a n d n i t r o g e n o u s ) is o b t a i n e d simultaneously with phosphorus removal. 4. C o m b i n a t i o n o f F e a n d lime. C o m b i n a t i o n s of F e C l solutions a n d l i m e in r e s p e c t i v e p r o p o r t i o n s o f 1 0 0 - 1 5 0 m g / l i t e r a n d 2 - 5 m g / l i t e r h a v e been used, yielding p h o s p h o r u s removal of approximately 9 5 % . T h e p r e c i p i t a t i o n o p e r a t i o n is u s u a l l y c a r r i e d o u t w i t h i n a t r e a t m e n t p r o c e s s . F i g u r e 8.22 s u m m a r i z e s t h r e e a l t e r n a t i v e p r e c i p i t a t i o n o p e r a t i o n s for p h o s p h o r u s removal within an activated sludge plant. Alternative No. 1—precipitation in primary clarifier. The precipitant, usually a c o m b i n a t i o n of F e C l a n d lime with typical dosages indicated pre v i o u s l y , is a d d e d t o r a w s e w a g e , p h o s p h a t e s b e i n g p r e c i p i t a t e d a n d r e m o v e d in 3 +
2
4
2
3 +
3
3
8.
393
Nutrient Removal -Alternative no. I Precipitation in primary clarifier -Alternative no. 2 Simultaneous precipitation
-Alternative no. 3 Subsequent precipitation
Additional clarifier for Alternative \ n o . 3 /
Wastage Fig. 8.22.
Alternative
precipitation
operations
Y •
for phosphorus
removal.
t h e p r i m a r y clarifier. P h o s p h o r u s r e m o v a l is 9 0 - 9 5 % , a n d a c o n s i d e r a b l e a m o u n t o f s l u d g e is o b t a i n e d . T h e a d d i t i o n o f l i m e in t h i s p r o c e s s r a i s e s t h e p H o f t h e effluent f r o m t h e p r i m a r y clarifier t o n e a r l y 10.0. T h i s is n o t a n obstacle to the p r o p e r operation of the biological process in the aerator, w h i c h r e q u i r e s a p H n e a r n e u t r a l i t y . T h e n a t u r a l buffering c a p a c i t y o f t h e a c t i v a t e d s l u d g e p r o c e s s is sufficient t o p r o v i d e t h e r e q u i r e d n e u t r a l i z a t i o n , owing to p r o d u c t i o n of c a r b o n dioxide. Alternative No. 2—simultaneous precipitation. P r e c i p i t a n t is a d d e d d i r e c t ly t o t h e a e r a t i o n t a n k . T h e p r e c i p i t a t e settles in t h e s e c o n d a r y clarifier t o g e t h e r with the activated sludge. It seems t h a t the chemicals d o long-term d a m a g e t o t h e b i o l o g i c a l cells, a n d t h a t t h e i r o r g a n i c r e m o v a l efficiency is i m p a i r e d t o s o m e extent. O n the other h a n d , addition of chemicals aids settling a n d c o m p a c t i o n o f t h e a c t i v a t e d s l u d g e in t h e s e c o n d a r y clarifier. Alternative No. 3—subsequent precipitation. Precipitating chemicals are a d d e d t o t h e effluent f r o m t h e s e c o n d a r y clarifier. A n a d d i t i o n a l clarifier is required to remove precipitated phosphates, thereby increasing capital costs. O n t h e o t h e r h a n d , t h e p h o s p h o r u s r e m o v a l efficiencies o b t a i n e d a r e s o m e w h a t h i g h e r t h a n t h o s e for t h e t w o p r e v i o u s a l t e r n a t i v e s . E s t i m a t i n g c h e m i c a l r e q u i r e m e n t s for p h o s p h o r u s r e m o v a l c a n n o t b e d o n e from simple stoichiometric relationships, because the actual mechanism of p h o s p h o r u s r e m o v a l is n o t k n o w n . E m p i r i c a l r e l a t i o n s h i p s h a v e b e e n d e v e l o p e d for e s t i m a t i n g c h e m i c a l r e q u i r e m e n t s , a n d t w o o f t h e s e a r e m e n t i o n e d next. 1. F o r e s t i m a t i n g F e or A l requirements, 3 +
3 +
ε
= d
+ 0.5 l o g ( m / P i )
w h e r e ε is t h e efficiency o f p h o s p h o r u s r e m o v a l [ε = (Ρι — Ρ/)/Ρ
(8.72) ί9
where P
(
394
8.
Tertiary Treatment of Wastewaters
is t h e initial p h o s p h o r u s c o n c e n t r a t i o n in w a s t e w a t e r ( m o l e s o f P / l i t e r ) ; P
f
t h e final p h o s p h o r u s c o n c e n t r a t i o n after p r e c i p i t a t i o n o p e r a t i o n ( m o l e s
of P/liter)]; C
x
a c o n s t a n t w i t h a v a l u e o f 0 . 6 1 4 for F e C l a n d 0 . 6 6 2 f o r a l u m ; 3
a n d m the required molality of precipitant (moles F e
or Al /liter).
3 +
3 +
Solving for m,
m = p lo ( e
i
c l ) / 0
-
(8.73)
5
E q u a t i o n s (8.72) a n d (8.73) a p p l y f o r a r a n g e 0.45 < ε < 0 . 9 5 . 2. F o r e s t i m a t i n g l i m e r e q u i r e m e n t s , a r u l e o f t h u m b c o n s i s t s in u t i l i z i n g a quantity of lime (moles/liter of C a
2 +
) e q u a l t o 1.5 t i m e s t h e c a r b o n a t e
hardness of the wastewater. Laboratory evaluation of p h o s p h o r u s removal can be performed.
These
tests n o t o n l y p e r m i t p l o t t i n g c u r v e s o f p h o s p h o r u s r e m o v a l efficiency v s . d o s a g e o f p r e c i p i t a n t , b u t a l s o e v a l u a t i o n o f effects o f p H a n d t e m p e r a t u r e . C u r v e s o f p h o s p h o r u s r e m o v a l efficiency v s . d o s a g e o f p r e c i p i t a n t a r e e x ponential. They reach a plateau b e y o n d a certain concentration of chemical a d d e d . I t is u s u a l l y u n e c o n o m i c a l , t h e r e f o r e , t o a t t e m p t r e d u c i n g p h o s p h o r u s c o n c e n t r a t i o n b e l o w 0.10 m g / l i t e r o w i n g t o e x t r e m e l y h i g h c h e m i c a l r e quirements.
8.3. N I T R O G E N R E M O V A L 8.3.1. Introduction N i t r o g e n together with p h o s p h o r u s contributes t o the process of lake e u t r o p h i c a t i o n . A l s o , n i t r o g e n in t h e f o r m o f N H
+ 4
or nitrites
(N0 ~) 2
e x e r t s a n o x y g e n d e m a n d b e c a u s e o f its o x i d a t i o n t o n i t r a t e s ( N 0 ~ ) . 3
NH
+ 4
-» N 0 ~ - • N 0 2
3
(8.74)
T h e s e facts j u s t i f y t h e d e s i r a b i l i t y o f n i t r o g e n r e m o v a l f r o m w a s t e w a t e r s p r i o r t o discharge into receiving waters. F o r d o m e s t i c w a s t e w a t e r s t h e split a m o n g t h e v a r i o u s f o r m s o f n i t r o g e n is relatively c o n s t a n t : N H nitrogen, 50-60%; organic nitrogen, 4 0 - 5 9 % ; and n i t r i t e s a n d n i t r a t e s , 0 - 5 % . I n t h e effluent f r o m a c t i v a t e d s l u d g e t r e a t m e n t , m o s t n i t r o g e n is p r e s e n t e i t h e r a s n i t r i t e s o r n i t r a t e s , o w i n g t o t h e o c c u r r e n c e o f nitrification. N i t r i f i c a t i o n i n v a r i a b l y o c c u r s d u r i n g a c t i v a t e d s l u d g e t r e a t m e n t a t v a l u e s o f F/M r a t i o less t h a n 1.0. ( O p t i m u m r a n g e o f F/M r a t i o s f o r t h e a c t i v a t e d s l u d g e p r o c e s s is f r o m 0.3 t o 0 . 7 ; see C h a p t e r 5, S e c t i o n 5.) +
4
8.3.2. Processes for N i t r o g e n Removal P r o c e s s e s for n i t r o g e n r e m o v a l i n c l u d e (1) n i t r i f i c a t i o n - d e n i t r i f i c a t i o n p r o c e s s ; (2) a m m o n i a s t r i p p i n g ; (3) i o n e x c h a n g e ; (4) b i o l o g i c a l p r o c e s s e s s u c h a s a c t i v a t e d s l u d g e a n d s t a b i l i z a t i o n p o n d s ; a n d (5) p r e c i p i t a t i o n p r o c e s s e s .
8.
395
Nutrient Removal
1. Nitrification-denitrification.
T h e m a i n p r o c e s s g e a r e d specifically for
n i t r o g e n r e m o v a l is n i t r i f i c a t i o n - d e n i t r i f i c a t i o n , w h i c h is a m o d i f i c a t i o n o f the activated sludge process a n d takes place in t w o steps: (a) N i t r i f i c a t i o n c o n s i s t s in a e r a t i o n e m p l o y i n g F/M
r a t i o s less t h a n
1.0, u s u a l l y F/M
« 0 . 3 , a n d a longer a e r a t i o n p e r i o d t h a n for t h e conven
tional activated
sludge process. N i t r o g e n
compounds
are converted
n i t r i t e s ( N 0 ~ ) a n d n i t r a t e s ( N 0 ~ ) in t h e p r e s e n c e o f Nitrosomonas 2
Nitrobacter
to and
3
microorganisms. NH
+ 4
" T
NCV (8.75)
^ _ T
N0 -
Nitrobacter
> N0 -
2
(b) Denitrification
_
Tj
3
is a n a n a e r o b i c s t e p w h i c h t a k e s p l a c e i n
the
s e c o n d a r y clarifier b y e x t e n d i n g r e s i d e n c e t i m e a n d a d d i n g a n o r g a n i c c a r b o n source, usually methanol. Nitrites a n d nitrates are converted t o nitrogen gas and nitrogen oxide, which are vented out. N0 ~ 2
orNCV
denitrification ^
_
T
> N
2
(8.76)
+ N 0 2
S l u d g e a g e in s e c o n d a r y clarifier is a t least 2 - 3 d a y s , h i g h e r v a l u e s b e i n g r e q u i r e d a t l o w e r o p e r a t i n g t e m p e r a t u r e s . D e n i t r i f i c a t i o n is p e r f o r m e d e v e n without addition of the organic c a r b o n source t o help meet denitrifying bacteria requirements. E n d o g e n o u s respiration provides these requirements, b u t a d d i t i o n o f t h e c a r b o n s o u r c e is h e l p f u l . A t w o - s t e p n i t r i f i c a t i o n - d e n i t r i f i c a t i o n p r o c e s s is s h o w n in F i g . 8.23. A m o r e s o p h i s t i c a t e d three-step nitrification-denitrification process is s h o w n in F i g . 8.24. T h e first s t e p is a h i g h r a t e a c t i v a t e d s l u d g e p r o c e s s w h e r e 7 5 - 8 5 % o f t h e c a r b o n a c e o u s m a t e r i a l is r e m o v e d . R e s i d e n c e t i m e in t h e a e r a t o r is a p p r o x i m a t e l y 2 h r . T h e s e c o n d s t e p is n i t r i f i c a t i o n in t h e p r e s e n c e o f a n -STEP - S T E P 2: Nitrification step: Denitrification step: N H 4 - N O g - N C £ NOil N 0 NO: 2
Raw sewage
Primary clarifier
1
j !
ί Aerator
ί
Secondary clarifier
Recycled ^sludge Fig.
8.23.
Two-step
nitrification-denitrification
Effluent
*l Wastage process.
396
8. •
S T E P I: High rate activated sludge process: Organics - C 0 • H 0 2
Primary clarifier
Raw sewage
Aerator I t=2 hr
2
Secondary clarifier No. I
[Recycled sludge (50%)
Tertiary Treatment of Wastewaters - S T E P 2: Nitrification step: N H ; * N 0 g *N03 Aerator 2 t=3 hr
Recycled sludge (50%) Wastage
I
•Np + NoO
Recycled sludge (50%) 8.24.
Three-step
Wastage
- S T E P 3.Denitrificotion step: uermr Methanol addition
Anaerobic tank
Fig.
Secondary clarifier No. 2
Secondary clarifier No. 3
Final effluent
f~X ~ Wastage
nitrification-denitrif/cation
process.
e n r i c h e d c u l t u r e o f n i t r i f y i n g b a c t e r i a . R e s i d e n c e t i m e in t h e a e r a t o r is a p p r o x i m a t e l y 3 h r . T h e t h i r d s t e p is d e n i t r i f i c a t i o n , w h i c h is a n a n a e r o b i c o p e r a t i o n , m e t h a n o l b e i n g a d d e d t o p r o v i d e a n a d e q u a t e C/N r a t i o . 2. Ammonia stripping. This process consists of adjusting p H of the w a s t e w a t e r t o a v a l u e a b o v e 10 ( l i m e is utilized for t h i s p u r p o s e ) , a n d t h e n a i r - s t r i p p i n g t h e a m m o n i a ( a t p H > 10 n i t r o g e n is p r e s e n t a s N H ) in a s t r i p p i n g t o w e r . N i t r o g e n r e m o v a l efficiencies o f a p p r o x i m a t e l y 9 0 % a r e o b t a i n e d a t a i r / l i q u i d r a t i o s o f 3 5 0 f t / g a l a n d l i q u i d r a t e s o f a b o u t 3.0 g a l / ( m i n ) ( f t ) . R e m o v a l efficiency is i n c r e a s e d close t o 9 8 % w i t h a n a i r / l i q u i d r a t i o o f 8 0 0 f t / g a l . H o w e v e r , r e m o v a l efficiency d r o p s c o n s i d e r a b l y in c o l d weather. 3
3
2
3
A m m o n i a s t r i p p i n g is e m p l o y e d e i t h e r b e f o r e o r after s e c o n d a r y t r e a t m e n t . If t h e s t r i p p i n g o p e r a t i o n is f o l l o w e d b y b i o l o g i c a l t r e a t m e n t , it is n e c e s s a r y t o l o w e r t h e p H t o a v a l u e n e a r t h e n e u t r a l p o i n t . T h i s is u s u a l l y d o n e b y r e c a r b o n a t i o n , i.e., b u b b l i n g t h r o u g h flue g a s c o n t a i n i n g c a r b o n d i o x i d e . I t is i m p o r t a n t t o leave e n o u g h n i t r o g e n u n r e m o v e d t o satisfy n u t r i t i o n a l r e q u i r e m e n t s for t h e b i o l o g i c a l p r o c e s s .
9.
397
Sonozone Wastewater Purification Process
Simultaneously with nitrogen removal, air-stripping accomplishes phos p h o r u s removal, B O D reduction, a n d r e m o v a l of suspended solids. T h e p r i m a r y v a r i a b l e s i n v o l v e d in t h e a m m o n i a - s t r i p p i n g p r o c e s s a r e p H , a i r / l i q u i d ratio, hydraulic loading [gal/(min)(ft )], packed height, a n d characteristics 2
of the p a c k i n g elements. 3. Ion exchange. for N H
+ 4
A c a t i o n e x c h a n g e r e s i n ( c l i n o p t i l o l i l e ) w h i c h is selective
is utilized i n t r e a t m e n t o f a c t i v a t e d s l u d g e effluents f r o m
the
s e c o n d a r y clarifier. T h i s r e s i n is r e g e n e r a t e d w i t h l i m e , a n d t h e r e g e n e r a n t is r e u s e d after b e i n g a i r - s t r i p p e d o f a m m o n i a i n a s t r i p p i n g t o w e r . T h e c o s t o f o p e r a t i o n is h i g h , a n d u s u a l l y , t h i s p r o c e s s is n o t e c o n o m i c a l l y
feasible.
A n i o n e x c h a n g e r e s i n s a r e u s e d for r e m o v a l o f n i t r a t e s ( N 0 ~ ) . P h o s p h a t e s 3
a n d o t h e r a n i o n s a r e s i m u l t a n e o u s l y r e m o v e d . T h e r e s i n is r e g e n e r a t e d w i t h brine a n d restored by treatment with acid a n d methanol. Pretreatment by filtration
m a y be required in ion exchange processes, thereby increasing costs.
4 . Biological
processes,
i.e., a c t i v a t e d s l u d g e a n d s t a b i l i z a t i o n
ponds,
h a v e a l r e a d y b e e n m e n t i o n e d in S e c t i o n 8.2.1 in c o n n e c t i o n w i t h p h o s p h o r u s removal. 5. Precipitation
processes.
S e c t i o n 8.2.2 d e s c r i b e s t h e s e p r o c e s s e s i n c o n
n e c t i o n w i t h p h o s p h o r u s r e m o v a l . A s f a r a s n i t r o g e n r e m o v a l is c o n c e r n e d , p r e c i p i t a t i o n m e t h o d s a r e r a t h e r inefficient, a c c o u n t i n g for less t h a n 3 0 % removal of total nitrogen.
8.4. A D D E D B E N E F I T S I N N U T R I E N T R E M O V A L A l t h o u g h initial c o s t s i n t h e p r o c e s s e s s t u d i e d a r e h i g h , t h e r e a r e a d d e d benefits i n n u t r i e n t r e m o v a l p r o c e s s e s w h i c h in s o m e c a s e s j u s t i f y t h e c o s t s . A m o n g t h e s e a r e (1) B O D r e d u c t i o n , (2) s u s p e n d e d s o l i d s r e m o v a l , (3) b a c terial a n d viral r e m o v a l , a n d (4) r e m o v a l o f v o l a t i l e o r g a n i c s .
9. S o n o z o n e W a s t e w a t e r Purification P r o c e s s This tertiary treatment process has been developed at the University of Notre D a m e (Lobund Laboratory), and sonozone plants are commercially available from the Ecology Division of T e l e c o m m u n i c a t i o n s Industries, Inc. (Lindenhurst, N e w Y o r k ) . A pilot plant at the University of I n d i a n a with a c a p a c i t y o f 2 0 , 0 0 0 g a l / d a y is b e i n g u s e d a s a r e s e a r c h m o d e l , t r e a t i n g o n c a m p u s s e w a g e . A full-size p l a n t ( c a p a c i t y , 5 7 0 , 0 0 0 g a l / d a y ) is in o p e r a t i o n at Indiantown, Florida. Telecommunications Industries, Inc. claims that the sonozone process provides tertiary treatment at costs c o m p a r a b l e to secondary treatment. T h e sonozone process combines ultrahigh frequency s o u n d a n d ozone treat-
398
8.
Tertiary Treatment of Wastewaters A C O
(2) Filtration section
(1) Physical-chemical section
2
+ O
2
(3) Sonozone section Effluent
sewage
Sludge Fig. 8.25. Flow diagram cations Industries, Inc.)
of sonozone
0 process
0
3
[10].
(Courtesy
of
3
Telecommuni
m e n t . A simplified flow d i a g r a m is s h o w n in F i g . 8.25. T r e a t m e n t i n v o l v e s t h r e e s t a g e s , o f w h i c h t h e t w o initial o n e s a r e p r e t r e a t m e n t u n i t s . 1. Physical-chemical section. S l u d g e is r e m o v e d b y a series o f p r i m a r y t r e a t m e n t contacts utilizing c o a g u l a t i o n followed by clarification. 2 . Filtration section. T h e filtration s y s t e m is d e s i g n e d t o r e m o v e m i c r o sized solids a n d o r g a n i c s f r o m clarified w a s t e w a t e r . 3. Sonozone section. T h e c e n t r a l u n i t is t h e o z o n e a n d s o n i c s u n i t . I t consists of a small vibrating metal disk a t the b o t t o m of a t a n k t h r o u g h which w a s t e w a t e r flows. A s t e a d y s t r e a m o f u l t r a s o n i c w a v e s is s e n t o u t b y t h e v i b r a t i n g d i s k , a n d s i m u l t a n e o u s l y o z o n e is b u b b l e d i n t o t h e t a n k f r o m a n e a r b y g e n e r a t o r , w h i c h p r o d u c e s o z o n e b y s h o o t i n g electric a r c s t h r o u g h t h e air. T h e e x a c t m e c h a n i s m for t h e p h e n o m e n a t a k i n g p l a c e w h e n w a s t e w a t e r is s u b j e c t e d t o a c o m b i n a t i o n o f o z o n e a n d u l t r a s o n i c w a v e s is still u n d e r i n vestigation. H i g h frequency s o u n d waves rattle bacteria a n d dissolved par ticles, b r e a k i n g t h e m i n t o s u b m i c r o n size. I n t h i s f o r m , t h e y b e c o m e h i g h l y s u s c e p t i b l e t o t h e s t r o n g o x i d i z i n g effect o f o z o n e , s o less o f it is r e q u i r e d . C a r b o n a c e o u s materials are oxidized, yielding C 0 a n d 0 . This corresponds t o t h e simplified e q u a t i o n [ E q . ( 8 . 7 7 ) ] 2
20
3
+ C -
C0
2
+ 20
2
2
(8.77)
I n tests c o n d u c t e d a t t h e 2 0 , 0 0 0 - g a l / d a y p i l o t u n i t a t t h e U n i v e r s i t y o f N o t r e D a m e , less t h a n 6 0 sec o f s o n o z o n e t r e a t m e n t d e s t r o y e d 1 0 0 % o f fecal bacteria a n d viruses, 9 3 % of p h o s p h a t e s , a n d 7 2 % of nitrogen c o m p o u n d s . Effluent p u r i t y w a s j u s t s h o r t o f t h a t o b t a i n e d b y d i s t i l l a t i o n . I n t h e effluent, o z o n e q u i c k l y r e v e r t s t o o x y g e n . T h i s d i s s o l v e d o x y g e n is beneficial t o t h e r e c e i v i n g s t r e a m a n d h e l p s s u p p o r t a q u a t i c life. I n c o n t r a s t , c h l o r i n e , t h e m o s t l a r g e l y u s e d b a c t e r i a kill, lingers i n t h e effluent a n d b e c o m e s a p o l l u t a n t itself.
Sonozone Wastewater Purification Process
9.
Sludge from
the physical-chemical
399
section
of the sonozone
system
is
processed by any of the s t a n d a r d m e t h o d s such as sand bed drying, v a c u u m filtration,
centrifugation, or incineration. T h e I n d i a n t o w n plant uses sand bed
drying, with liquid waste from the sludge reprocessed t h r o u g h the s o n o z o n e s y s t e m . S l u d g e is c o l l e c t e d a n d p e r i o d i c a l l y t r a n s p o r t e d t o a s l u d g e d i s p o s a l area. O n e i m p o r t a n t c h a r a c t e r i s t i c o f t h e p r o c e s s is its c o m p a c t n e s s . I t is e s t i m a t e d that the sonozone system requires approximately 2 0 % of the land area of conventional
systems; t h u s real estate acquisition
costs are
considerably
r e d u c e d . I n a d d i t i o n , t h e s m a l l e r h o u s i n g o f t h e p l a n t is m o r e a t t r a c t i v e t h a n the large horizontal aeration a n d sedimentation t a n k s of conventional systems.
Problems I. F o r the activated carbon column in E x a m p l e 8.2, calculate the residual concentration (C ) if depth is increased to 10 ft, keeping the same service time. Prepare a plot o f C vs. d e p t h utilizing depths o f 5 , 1 0 , 1 5 , and 2 0 ft. E
E
II. Verify if the carbon adsorption data in the tabulation below are fit by the Langmuir isotherm. If s o , determine constants Κ and b.
C (mg/liter)
A7M(g/g)
10 20 30
0.20 0.28 0.33
III. T h e wastewater stream from a plating industry has the following characteristics: flow rate, 50 gal/min for 12 hr a day; chemical c o m p o s i t i o n : copper, 30 mg/liter as C u ; zinc, 12 mg/liter as Z n ; nickel, 2 0 mg/liter as N i ; and chromate, 125 mg/liter as C r 0 . Characteristics of cation and anion exchange resins used are as given in the tabulation below. 2 +
2 +
2 +
3
Regenerant D o s a g e (lb/ft ) Concentration (%) F l o w rate [gal/(min)(ft )] Exchange capacity Rinsing water (gal/ft o f resin) 3
2
3
Cation
Anion
H S0 11.0 5 0.5 1.5 eq/liter 130
NaOH 4.7 10.0 0.5 3.8 lb C r 0 / f t 100
2
4
3
3
1. D e s i g n a n anion exchange system t o remove chromate. Calculate volume o f resin required, N a O H for daily regeneration, and rinsing water requirements. 2. D e s i g n a cation exchanger t o remove C u , Z n , and N i . Calculate v o l u m e o f resin required, H S 0 for daily regeneration, and rinsing water requirements. 2 +
2
4
2 +
2+
400
8.
Tertiary Treatment of Wastewaters
References 1. Bohart, G . S . , and A d a m s , E . Q., / . Am. Chem. Soc. 42, 523 (1920). 2. Business Week, " I s Ozone the W a y to Treat Sewage." M c G r a w - H i l l , N e w Y o r k , 1973. 3. Castellan, G . W . , "Physical Chemistry," p. 263. Addison-Wesley, Reading, M a s s a chusetts, 1964. 4. Chick, H . , / . Hyg. 8, 92 (1908). 5. Eckenfelder, W . W . , Jr., and F o r d , D . L., "Water Pollution Control." Pemberton Press, Austin and N e w Y o r k , 1970. 6. K a u p , E . C , Chem. Eng. (N.Y.) 80, N o . 8, 46 (1973). 7. Loeb, S . , "Sea Water Demineralization by Means of a Semi-permeable Membrane," Rep. 63-32. Department of Engineering, University of California, L o s Angeles, 1963. 8. Metcalf & E d d y , Inc., "Wastewater Engineering: Collection, Treatment, Disposal." M c G r a w - H i l l , N e w Y o r k , 1972. 9. Sourirajan, S . , and Agrawal, J. P., Ind. Eng. Chem. 6 1 , N o . 11, 62 (1969). 10. Telecommunications Industries, Inc., " A n Introduction to Sonozone Waste Water Treatment Systems," Tech. Bull. Telecommun. I n d . , Inc., Lindenhurst, N e w Y o r k , 1974.
Appendix C o n v e r s i o n F a c t o r s from English t o M e t r i c U n i t s LENGTH in. χ 2.54 = c m ft χ 30.48 = c m ft χ 0.3048 = c m
AREA i n . χ 6.4516 = c m f t χ 929.03 = c m f t χ 0.092903 = m acre χ 4046.8 = m 2
2
2
2
2
2
(1 acre = 43,560 f t )
2
2
V O L U M E ( U . S . G A L UTILIZED) in. in. ft ft ft gal gal
3
3
3
3
3
χ 0.01638 = liter χ 16.386 = c m χ 28.316 = liter χ 0.02831 = m χ 2.8316 χ 1 0 = c m χ 3.7854 = liter χ 3785.4 = c m 3
3
4
3
3
FLOW RATE ( U . S . G A L UTILIZED) gal/min χ 0.0631 = liter/sec gal/hr χ 1.052 χ 1 0 " = liter/sec gal/day χ 4.381 χ 1 0 " = liter/sec M G D χ 43.81 = liter/sec (Note: 3
5
M G D = million gallons/day)
FLOW RATE PER UNIT A R E A ( U . S . G A L UTILIZED) gal/(min)(ft gal/(min)(ft gal/(min)(ft gal/(min)(ft gal/(min)(ft gal/(hr)(ft ) gal/(hr)(ft ) gal/(hr)(ft ) gal/(hr)(ft ) gal/(hr)(ft ) 2
2
2
2
2
2
2
2
2
2
) ) ) ) )
X 40.75 = liter/(min) ( m ) X 0.679 = liter/(sec) ( m ) X 0.004075 = liter/(min) ( c m ) X 0.2445 = liter/(hr) ( c m ) X 2445 = liter/(hr) ( m ) X 0.679 = liter/(min) ( m ) X 0.0113 = l i t e r / ( s e c ) ( m ) X 6.79 X I O ' = l i t e r / ( m i n ) ( c m ) X 0.004075= liter/(hr) (cm ) X 40.75 = l i t e r / ( h r ) ( m ) 2
2
2
2
2
2
2
6
2
2
2
FLOW RATE PER UNIT V O L U M E ( U . S . G A L UTILIZED) g a l / ( h r ) ( f t ) X 2.2277 = liter/(min) ( m ) g a l / ( h r ) ( f t ) X 0.0371 = liter/(sec) ( m ) 3
3
3
3
401
402
Appendix
MASS lb χ 453.59 = g lb χ 0.45359 = kg
QUANTITY OF HEAT B T U χ 0.25198 = kg calorie B T U χ 251.98 = g calorie
TEMPERATURE °C = ( l / 1 . 8 ) ( ° F - 3 2 )
Index A Acclimation, 35, 189 Activated sludge p r o c e s s , 157-235 batch reactor, laboratory, 164 calculation of S , material balance, 207 comparison with e x t e n d e d aeration, 238, 239 completely-mixed activated sludge p r o c e s s , 248 contact stabilization p r o c e s s , 2 4 4 - 2 4 7 continuous reactor formulation, 166 laboratory, 189-193 sampling schedule, 193 design procedure for activated sludge plant, 2 1 2 - 2 1 9 determination of k (rate constant), 169, 194, 200 extended aeration, 2 3 8 - 2 4 4 factor 1.42, 176 kinetics relationships, 164 material balance relationships, 169 mathematical modeling, 163 neutralization requirements, 210 nonbiodegradable matter, 165, 169 nutrient requirements, 211 o x y g e n utilization, material balance for 0
determination of, 179 parametera definition (synthesis phase), 174 in different units, 177 parameter a, definition (synthesis phase), 172 parameter a ' definition (oxidation), 172 in different units, 177 parameters for design of aerobic biologi cal reactors, determination by laboratory-scale continuous reactors, 189 parameters for e n d o g e n o u s respiration, 178
parameter b, definition, 178 parameter b\ definition, 179 ratio 6 7 6 , 179 typical values of parameters for aerobic biological oxidation, for dif ferent wastewaters, 185 recycle ratio, material balance for determination of r e c y c l e ratio of M L V S S , 206, 209 relationship b e t w e e n parameters a and a , 175, 177 residence time, calculation for reactor, 208 sludge settling, optimum conditions, 185 sludge volume index, 186 z o n e settling velocity, 185 sludge yield, total, 183 step aeration, 233, 247 substrate removal rate constant, 167, 220 tapered aeration, 248 Adsorption, see Carbon adsorption A d v a n c e d treatment, see Tertiary treat ment of wastewaters Aerated lagoons, see L a g o o n s , aerated Aeration e x t e n d e d (total oxidation), 2 3 8 - 2 4 4 conventional units, 239 design criteria for, 241 nitrification in, 241 oxidation ditch, 239 settling of sludge from, 241 step, 223, 247 tapered, 248 units, see Aerators Aerators, 140-154 basic function of, in -aerobic biological p r o c e s s e s , 140 diffusion units, 140 design procedure for aerator s y s t e m s , 143 fine-bubble, 140 large-bubble, 141 performance, 142
403
Index
404 surface units, 149-154 correlation b e t w e e n transfer efficiency and level o f agitation, 149 design procedure for aerator s y s t e m s , 151 turbine units, 144-148 design procedure for aerator s y s t e m s , 147 optimum power split b e t w e e n rotor and compressor, 146 performance, 145 power requirements, 145 Aerobic degradation endogenous respiration phase, 165, 178 mechanism, 169-171 synthesis phase, 165, 172 A m m o n i a stripping, 394 Anaerobic treatment of wastewaters, 282-293 acid and methane fermentation stages, 282 bench-scale reactors, 288 B O D removal rate, 284 design procedure for anaerobic digesters, 289 detention time, calculation, 287 gas production, 288 mathematical formulation, 284 quantitative study o f anaerobic degrada tion of organic w a s t e , 283 volatile solids in anaerobic reactor, 285 Animal s p e c i e s , higher forms in receiving waters, 20 AquaRator, 31 Assimilative capacity, of river, 4 Β Bacteria (and ciliates), 19 Bacterial cell, 186 B O D , see Biochemical o x y g e n demand B o h a r t - A d a m s equation, 348, 356 Biochemical o x y g e n demand ( B O D ) , 3 3 - 3 9 , 58 acclimation and seeding, effect on B O D test, 35 algae effect on B O D test, 36 B O D test, environmental effects o n , 58 of p H , 59 of temperature, 58
dilution test, 3 3 - 3 6 g l u c o s e - g l u t a m i c acid check for B O D test, 36 manometric m e t h o d s , 38 mathematical model for B O D c u r v e , 47-61 log difference method for determina tion o f parameters k and L , 48 method of m o m e n t s for determination 0
o f parameters k and L , 51 Thomas* graphical method for deter mination of parameters k and L > 56 ratio of C O D and B O D , 35 relationship b e t w e e n k and ratio c
0
M
BODj/BODu, 58 ultimate o x y g e n demand ( B O D J , 34 Biological treatment, evaluation of feasibil ity for industrial wastewater, 61 batch reactor evaluation, 65 Warburg respirometer, 61
C Carbon adsorption, 3 4 5 - 3 5 8 activated carbons, 345 preparation, 345 reactivation, 346 adsorptive capacity, 348 B o h a r t - A d a m s equation, 348, 356 break point, 348 c o l u m n s , bench-scale, 349 critical bed depth, 349 design procedure for activated carbon c o l u m n s , 347, 353 Freundlich isotherm, 347 isotherms (adsorption), 346, 347 laboratory simulation: determination of design parameters, 349 Langmuir isotherm, 346 operation of carbon adsorption s y s t e m s , 347 Carbon parameter methods for measure ment o f organic content, 27, 4 4 - 4 7 Cell bacterial, 186 residence time, mean, 226, 300 Centrifugation of sludges, 328, 329 Clarifiers, see Sedimentation Chemical engineering, curriculum, 4 Chemical o x y g e n demand ( C O D ) , 2 8 - 3 3
Index
405
instrumental C O D m e t h o d s , 3 1 - 3 3 permanganate oxidation test, 30 rapid C O D tests, 31 standard dichromate oxidation method, 29
E n d o g e n o u s respiration, 165, 171, 177 Engineer role in water pollution abatement, 2 survey o f contribution to water pollution
Chick's l a w , 389 Chlorination of wastewaters, 3 8 7 - 3 9 0 Chick's l a w , 389 chlorine as disinfectant, 389 c y a n i d e s , destruction by, 390 e c o n o m i c s of, 390 reactions of chlorine in water, 387 utilization and purposes of, 387 Ciliates (and bacteria), 19 Clarifiers, see Sedimentation C O D , see Chemical o x y g e n demand Contact stabilization p r o c e s s , 2 4 4 - 2 4 7 advantages v s . conventional activated sludge p r o c e s s , 244 design of contact stabilization s y s t e m s , 246 solubility index (SI) and overall effi c i e n c y , 245 Cost-benefit ratio, 10 Cyanides destruction by chlorine, 390 by o z o n e , 391
Equalization, 1 1 4 - 1 1 6 constant level equalization basins, 114 variable level equalization basins, 115 Eutrophication, 22 eutrophic, mesotrophic, and oligotrophic lakes, 22
D D e c o m p o s i t i o n , of carbonaceous and ni trogenous organic matter, 17 Dichromate, standard method for C O D de mand, 29 Digesters o f sludge, see Sludge Digestion o f sludges, see Sludge D i s s o l v e d o x y g e n (DO), 3 6 - 3 8 instrumental m e t h o d s , 37 Winkler's method, 37 Drag coefficient, 7 2 - 7 4
Ε E d d e - E c k e n f e l d e r equation, 310 Effluent standards, 9 Electrodialysis, 3 8 5 - 3 8 7 current required for, 385 pretreatment of wastewaters in, 386 voltage required for, 385 End-of-pipe control, see Wastewater treat ment
abatement, 2
F Filters, trickling, see Trickling filters Filtration, sludge in v a c u u m filters, 311-328 Buchner funnel test, 315 compressible c a k e s , 318 c y c l e time, 312 design equations, 3 1 9 - 3 2 8 dry time, 312 equations, 314 form time, 312 form-loading equation, 320 leaf t e s t s , 3 2 1 - 3 2 4 optimum coagulant d o s a g e , 315 pressure filtration, 328 specific resistance o f c a k e , 314, 316, 317 variables in vacuum filtration, 312 Flotation, 107, 113 air-to-solids ratio, 108, 110 s y s t e m s with and without r e c y c l e , 108, 112, 113 Fluidized-bed disposal of sludges, 340 Food-to-microorganism ratio, 186 optimum, 186, 238 Freundlich isotherm, 347 G G l o y n a - H e r m a n n equation, 263 Growth rate, specific (of sludge), 225 Η H e n r y ' s line, statistical correlation of waste survey data, 67 H e r m a n n - G l o y n a equation, 263
406
Index I
Industrial wastewater treatment, c a s e his tory, 3 Inplant control, see Wastewater treatment, inplant Ion e x c h a n g e (exchangers), 3 5 9 - 3 6 7 anion exchangers, 361 cation exchangers, 359 design of, 3 6 1 , 3 6 3 - 3 6 7 mechanism of, 359 regeneration, 362 rinsing water requirements, 363
L Lagooning, of sludges, 340 L a g o o n s , aerated, 2 4 9 - 2 5 9 completely mixed, 249 design procedure, 256 effluent B O D soluble, 253 total, 255 facultative, 250 M L V S S concentration, 253 o x y g e n requirements, 253 retention period, 254 temperature, estimate, 251 Langmuir isotherm, 346 Light, effect o n dissolved o x y g e n , 16 L i n e w e a v e r - B u r k plot, 223 Μ Mercury, contamination of waters, 24 M i c h a e l i s - M e n t e n relationship, 2 1 9 - 2 2 5 corollaries, 166, 221 Microscreens, 344 M o o r e s method of m o m e n t s , 51 Multiple-heart furnaces, 340, 341 1
Ν Neutralization 114-123 acidic w a s t e s , by direct p H control m e t h o d s , 116-123 alkaline w a s t e s , by direct p H control methods, 123 c u r v e , 120 lime, slurried, 116, 120-123 limestone b e d s , 1 1 6 - 1 1 9
requirements, in activated sludge p r o c e s s , 210 N e w t o n ' s law (sedimentation), 73 N i c h o l s heat treatment p r o c e s s for sludges, 3 3 6 - 3 3 9 N i c h o l s - H e r r e s h o f f furnace, 340, 341 Nitrates, contamination, 24 Nitrification, 5 9 - 6 1 , 241 Nitrification-denitrification, 3 9 4 - 3 9 6 Nitrobacter microorganisms, 59 Nitrogen, 18, 3 9 4 - 3 9 7 Nitrogen removal, 3 9 4 - 3 9 7 Nitrosomonas microorganisms, 59 Nutrient removal, 3 9 1 - 3 9 7 Nutrient requirements, see Activated sludge p r o c e s s Ο
O s m o s i s , reverse, 3 6 7 - 3 8 5 concentration polarization, 382 critical pore diameter for membranes, 372 flux decline, 378 effect o f feed temperature, 377 prediction, 380 water, 374 fouling, c a u s e s and cure, 378 laboratory-scale studies, 383 mechanism of reverse o s m o s i s , 369 membranes effect o f shrinkage temperature o n performance, 376 fouling, 378 leakage, 381 performance characterization, 373 preparation, 371, 372 osmotic pressure, 367 preferential sorption, capillary flow mechanism, 372 rejection factor, 374 solute permeability, 382 turbulence promoters, 383 van't Hoff equation, 368 Oxidation ditch, 239 total oxidation p r o c e s s , 2 3 8 - 2 4 4 Oxygen demand curve, 34 carbonaceous, 6 0
Index
407
c o m b i n e d , 60 nitrification, 60 parameter m e t h o d s , for measurement of organic content, 27 sag curve, 14 saturation v a l u e s , 127 transfer coefficient, determination, 129-135 by steady state method for activated sludge liquor, 134 B y unsteady state aeration of acti vated sludge liquor, 133 o f tap water, 129 transfer effect of mixing intensity, 138 of wastewater characteristics, 138 steps i n v o l v e d , 128 two-film theory, 127 transfer efficiency, aeration units, 140 transfer p r o c e s s , 128 transfer rate equation, 128 transfer coefficient pressure correction, 135 temperature correction, 135 uptake rate, 195, 1% utilization in activated sludge p r o c e s s , see Activated sludge p r o c e s s Oxygenation capacity, 135 corrections with temperature and pres sure (for bubble and surface aerators), 135 Ozonation, 390, 397
Ρ Permanganate oxidation test, 30 Phosphorus, 18, 171, 391 Phosphorus removal, 3 9 1 - 3 9 4 Photosynthesis, 17 Ponds anaerobic, 264 facultative, 263 stabilization, 2 5 9 - 2 6 8 anaerobic, 264 depth of o x y g e n penetration, 263 design calculations, 264 design criteria, 264 facultative: Hermann and Gloyna's equation, 263 kinetics of B O D removal, 260 laboratory simulation, 261
mathematical formulation, 261 o x y g e n production in aerobic, 262 symbiotic relationship, algae and bac teria, 259 temperature, effect o n reaction rate constant K, 262 Porteus p r o c e s s , 336 Pretreatment, 70 Primary treatment, 7 0 - 1 2 3 R Reactors arbitrary-flow, 230 batch (bench-scale), 164 complete-mix, 230, 248 continuous (bench-scale), 1 8 9 - 1 9 3 , 288 plug flow, 188, 230, 233 R e c y c l e ratio of sludge activated sludge p r o c e s s , 206, 209 e x t e n d e d aeration p r o c e s s , 242 Respiration, e n d o g e n o u s , 165, 171, 177 Respirometer, see Warburg respirometer R e v e r s e o s m o s i s , see O s m o s i s , reverse S Scour velocity, 81 Screening, 71 Secondary treatment activated sludge p r o c e s s , 157-235 other aerobic and anaerobic wastewater treatment p r o c e s s e s , 2 3 6 - 2 9 5 Sedimentation, 7 1 - 1 0 7 clarifiers, t y p e s of, 105-107 c o l u m n , laboratory, 84 discrete settling, 7 2 - 8 4 flocculent settling, 8 4 - 9 8 ideal sedimentation tank c o n c e p t , 76 z o n e settling, 9 8 - 1 0 5 Segregation, 3 S e w a g e , characteristics of municipal, 65 Shredders, 71 Sludge, see also Activated sludge p r o c e s s aerobic digester, bench-scale, 301 aerobic digestion, 2 9 7 - 3 0 7 a g e , c o n c e p t of, for sludge digesters, 300 for complete-mix, no r e c y c l e reactors, 226 for complete-mix reactor with r e c y c l e , 227
Index
408 Anaerobic digestion, 3 0 7 - 3 0 9 single state digesters, 308 sizing o f anaerobic sludge digesters, 309 t w o stage digesters, 308 centrifugation of, 328, 329 chemical coagulation, 334 deposits and aquatic plants, 18 disposal, 340 fluidized bed disposal, 340 Heat treatment, 3 3 6 - 3 3 9 N i c h o l s p r o c e s s , 337 Porteus and Zimpro p r o c e s s e s , 336 Incineration, 340 fluidized bed disposal, 340 multiple-hearth furnace ( N i c h o l s - H e r r e shoff), 340, 341 Lagooning, 340 Oxidation p o n d s , 340 Pre-dewatering treatment, 334 Sand-bed drying, 3 3 1 - 3 3 4 Constant and falling rate periods, 330 Construction of b e d s , 331 design o f b e d s , 3 3 1 - 3 3 4 mechanisms o f dewatering in sandbeds, 330 settling, in extended aeration, 241 specific growth rate, 225 thickening of, 3 0 9 - 3 1 1 Edde and Eckenfelder equation, 310 flotation thickeners, 311 gravity thickeners, 310 treatment and disposal, 2 9 6 - 3 4 2 Solubility index, 245 Sonozone process, 397-399 Species diversity index, 22 Stabilization ponds, see P o n d s Standards for water quality, see Water, quality standards Statistical correlation of industrial survey data, 66 Stream standards, 9 Substrate removal rate, 166, 220 Suspended solids nonvolatile, N V S S , 159 removal (tertiary treatment), 344 total suspended solids, T S S , 161 volatile, V S S , 159, 161 Τ Tertiary treatment of wastewaters, 8, 343-400
waste
Theoretical organic carbon (ThOC), 44 Theoretical o x y g e n demand (ThOD), 27 Thickening of sludges, see Sludge T h O C , see Theoretical organic carbon T h O D , see Theoretical o x y g e n demand T h o m a s ' graphical m e t h o d , 56 T O C , see Total organic carbon T O D , see Total o x y g e n demand Total organic carbon (TOC), 27, 4 4 carbon analyzer, 44 correlation with o x y g e n d e m a n d , 46 w e t oxidation method, 44 Total o x y g e n demand ( T O D ) , 27, 39 T O D analyzer, 40 Trickling filters, 2 6 8 - 2 8 2 bench-scale, 273 comparison with activated sludge p r o c e s s , 269 data treatment to determine constants η and K, 275 design formulation, 271 design of plant-scale, 279 design procedure w h e n experimental data are not available, 282 mathematical m o d e l s , 272 packings, ordinary and synthetic, 270 physical arrangement of, 270 pretreatment, 271 slime layer, thickness of, 269 s y s t e m s , 270 V Van't Hoff equation for osmotic pressure, 368 W Warburg respirometer, 6 1 - 6 5 W a s t e w a t e r s ) , see also Water, Waste water treatment contaminants, measurement of c o n c e n tration, 26 flow measurements of streams, 66 industrial s u r v e y s , 66 sources, 9 Wastewater treatment chemical Engineering curriculum a s preparation for field, 4 degrees of, 8 e c o n o m i c s of, 10
Index industrial, case history, 3 inplant, 4-7 case histories, 6 "end-of-pipe" and, 4 what is involved in, 5 processes, selection, 70 types of (classification), 8, 9 Water, see also Water pollution, Waste water contaminants biological, 25 chemical (organic and inorganic), 24 classification, 24 physical, 25 contamination by mercury, 24 by nitrates, 24 fluorides in water, 24 quality standards, stream and effluent standards, 9 receiving, 10 reuse, economic balance, 10
409 supply ground waters, meteorological waters, and surface waters, 22 types, 22 Water pollution abatement multidisciplinary approach in, 2 role of engineer, 2 survey of contribution of engineers, 2 systems approach, 2 biological, 25 chemical, 24 effect on environment and biota, 14-22 physical, 25 thermal, 25 Winkler's method, see Dissolved oxygen (DO)
Ζ Zimpro process, 336
