Chapter 22

Chapter 22

Dairy effluents effluents Water used in domestic and industrial applications become polluted to a greater or lesser extent. Water is also used as a transport medium mediu m to carry away waste products. As awareness of the importance of improved standards of water treatment grows, process requirements re quirements become be come increasingly exacting. The food industry contributes to a great extent to pollution, particularly as the pollutants poll utants are of organic origin. origin . Organic pollutants normally consist cons ist of 1/3 dissolved, d issolved, 1/3 colloidal and an d 1/3 sus pended substances, subs tances, while whil e inorganic materials are a re usually present mainly main ly in solution. soluti on.

Dairy Processing Handbook/chapter 22

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Organic pollutants The normal way to express the concentration of a pollutant is to specify t he total quantity per unit volume of sewage. Another and more modern way of analysing the presence and quantities of organic substances in sewage effluent is the use of chromatography, such as High-Performance Liquid Chromatography (HPLC). However, the quantity of organic substances is normally determined in the form of; • biological oxygen demand (BOD) • chemical oxygen demand (COD) • calcining loss • total organic carbon (TOC)

Biological oxygen demand (BOD)

BOD is a measure of the content of biologically degradable substances in sewage.

BOD is a measure of the content of biologically degradable substances in sewage. The substances are broken down by micro-organisms in the presence of (and therefore with consumption of) oxygen. Ox ygen demand is measured in terms of the quantity of oxygen consumed by micro-organisms over a period of five days (BOD5 ) or seven days (BOD7 ), in decomposing the organic pollutants in waste water at a temperature of 20 C. BOD is measured in mg oxygen/l or g oxygen/m3. The following relationship is assumed for municipal sewage: BOD7 = 1.15 x BOD5 °

Chemical oxygen demand (COD)

COD indicates the quantity of the pollutants in waste water that can be oxidised by a chemical oxidant.

COD indicates the quantity of the pollutants in waste water that can be oxidised by a chemical oxidant. The normal reagents used for this purpose are strongly acid solutions (to ensure complete oxidation) of potassium dichromate or potassium permanganate at high temperature. Consumption of oxidant provides a measure of the content of organic substance and is converted to a corresponding quantity of oxygen, expressing the result as mg oxygen/l or g oxygen/m3 . The COD/BOD ratio indicates how biologically degradable the effluent is. Low values, i.e. < 2, indicate relatively easily degradable substances, while high values indicate the contrary. However, this relationship cannot be used generally, but a typical value of COD/BOD for municipal se wage effluent is often < 2. In the FIL-IDF Bulletin about Dairy Effluents, Document 138, 1981, it was reported (by Doedens) that the COD/BOD5 ratio for effluents generated in different groups of dairies producing liquid milk, butter or cheese ranged from 1.16 to 1.57, average 1.45, while in other groups of dairy plants producing milk powder, whey powder, lactose and casein the ratio varied from 1.67 to 2.34, average 2.14. However, the general conclusion of the FIL-IDF Bulletin referred to above is that a COD:BOD ratio established in one dairy plant cannot be transferred with sufficient reliability to anot her plant.

Calcining loss Calcining loss is obtained by first determining the dry solids content in a sample, and then calcining it so that the organic substance is burnt. The difference in weight before and after calcining represents the quantity of organic substance. The value is expressed in %.

Total organic carbon (TOC) TOC is another measure of the quantity of organic materials, determined by measuring the quantity of carbon dioxide produced from combustion of a sample. The unit is mg/l.

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Inorganic pollutants The inorganic components of sewage cons ist almost entirely of salts, and are determined largely by the ionic composition and salt concentration in the mains water. The presence of these salts in sewage is normally unimportant. Present-day effluent treatment processes concentrate on the reduction of nitrogen, phosphorus salts and heavy metals. Nitrogen and phosphorus compounds are important, as they are nutrients for organisms, e.g. algae, in recipients. As a result of the growth of algae, secondary processes can proceed in the recipient, forming further organic substances which, when they decompose, can result in considerably higher oxygen demand than is caused by primary organic pollutants in the sewage effluent.

Dairy waste water Dairy waste water can be divided into three categories: 1 Cooling water 2 Sanitary waste water 3 Industrial waste water

Cooling water As cooling water is normally free from pollutants, it is discharged into the storm water piping system i.e. the system for run-off water from rain and melting snow, etc.

Sanitary waste water The sanitary waste water is normally piped direct to the sewage treatment plant with or without first having being mixed with industrial waste water.

Industrial waste water Industrial waste water emanates from spillage of milk and products thereof, and from cleaning of equipment that has been in contact with milk products. The concentration and composition of the waste depends on the production programme, operating methods and the design of the processing plant.

Table 22.1

BOD of some milk products Product

Cream, 40% fat Whole milk, 4% fat Skimmilk, 0.05% fat Whey, 0.05% fat Whey conc., 60% DM

BOD5 mg/ l

BOD 7 mg/ l

400 000 120 000 70 000 40 000 400 000

450 000 135 000 80 000 45 000 450 000

Sewage treatment plants are dimensioned to treat a certain quantity of organic substances and also to be able to deal with certain peak loads. However, one organic substance – fat – presents specially difficult problems. Besides having a high BOD (cream with 40% fat has a BOD5 of about 400 000 mg oxygen/l while skimmilk has 70 000 mg/l), fat sticks to the walls of the mains system as well as causing sedimentation problems in the sedimentation tank as it rises to the surface. Dairy waste water should therefore pass a flotation plant where it is aerated with “dispersion water” (the method of supplying finely-dispersed air bubbles to the water at a pressure of 400 – 600 kPa is called dissolved-air


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flotation). The air bubbles attach themselves to the fat, carrying it rapidly to the surface where it is strained off, manually or mechanically depending on the size of the plant. The flotation plant is often located close to the dairy building and the waste passes through it in a continuous flow. The defatted effluent can then be mixed with the sanitary was te water going to the sewage treatment plant. Table 22.1 list the BOD of some milk products.

pH of dairy effluent

Waste water with a pH of over 10 or below 6.5 must not be discharged to the sewage system.

The pH of dairy effluent varies between 2 and 12 as a result of the use of acid and alkaline detergents for plant cleaning. Both low and high pH values interfere with the activity of the microorganisms that break down organic pollutants in the biological treatment stage of the sewage treatment plant, transforming them into biological sludge (cell detritus). As a rule, waste water with a pH of over 10 or below 6. 5 must not be discharged to the sewage system, as it is liable to corrode the pipes. Used detergents are therefore normally collected in a mixing tank, often located close to the cleaning plant, and the pH is measured and regulated to, say, pH 7.0 before it is discharged to drain.

Reducing the quantity of pollutants in waste water It is constantly necessary to control and prevent wastage of water and product in the processing plant. Hidden losses of water in subfloor and underground piping can be detected by reading the water meter and recording the quantity used at the end of the day. Daily records of water consumption should then be compared with the daily quantity of milk that has been processed. The water consumption, expressed as m3 per tonne of treated milk, should be plotted on a graph kept in an easily accessible place. A typical water/milk ratio is 2.5/1, but with intense saving of water it is possible to come down to a ratio of 1/1. The following general recommendations can serve as a guide to reducing wastage of water and product:

General milk treatment • In reception of milk, particularly when tankers are emptied, it is important that the outlet from the tankers is at least 0.5 m above the receiving container or tank, and that the connecting hose is well streched, to ensure that the tankers are completely drained. • All pipelines must be identified and marked to avoid wrong connections that would result in unwanted mixing of products as well as leakage of milk. • When pipes are installed they should be laid with a slight and correctly calculated gradient to make them self-draining. In addition, the pipes must be well suppported to prevent vibration, which could cause the couplings to work loose and thus cause leakage. • All tanks should be equipped with level controls to prevent overflow. When the highest permitted level is reached, either the feeding pump is automatically stopped and the plant operator alarmed, or an automatic valve system is activated to route the product to another preselected tank. • It is better to prevent wastage of product in the first place than to flush it away with a hose afterwards. Try to keep the floors dry; this also makes leaks easier to detect. • Make sure that the piping system and tanks are properly emptied before they are rinsed out with water.

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• Check that couplings are airtight; if air leaks into the piping system it will cause increased burning-on in heaters, erosion problems in homogenisers and foaming in milk and cream tanks (which will then be harder to empty completely).

Cheese production area • Make sure that open cheese vats are not filled to the top; stop filling when the milk level is at least 10 cm below the rim. • Collect whey carefully, and try to find commercial uses for it instead of discharging it as waste. • Curd on the floor should be swept up and treated as solid waste – not flushed into the gutter with water.

6 5

Butter production area • Cream and butter stick more readily than milk to surfaces they come in contact with, and will aggravate contamination of waste water unless they are removed before cleaning starts. • After the end of a butter production run, all accessible surfaces should be manually scraped clean. • Cream and remaining butter can then be removed with steam and hot water and collected in a container for separate treatment.

3

Milk powder production area • The evaporators should be run at the lowest possible level to prevent overcooking. • Re-use the condensate as cooling water after circulation through a cooling tower, or as feed water to t he boiler. • Spilled dry products should be swept up and treated as solid waste.

Milk packaging area • The filling machines can be provided with drains discharging into one or more containers. • Returned packages can be emptied into containers and the mixture of sweet and sour liquids used as animal feed.

4 2 1

Fig. 22.1 System for measuring the flow in an open canal with a venturi flume. 1 Waste water canal 2 Venturi flume 3 Measuring pit 4 Connection between the canal and measuring pit 5 Float 6 Measuring and recording device

6

5

Outlet control Disposal of waste water is subject to regulation in many countries. 3 Outlet control, for example, must be arranged so that the volume of waste water is continuously measured and recorded and an aliquot part, in proportion to the volume of the flow, is sampled. Figure 22.1 illustrates a system for measuring the flow in an open canal with a venturi flume. For information about the venturi 2 flume and other measuring systems, please contact the municipal authorities dealing with sewage water treatment. As to sampling, one example of the procedure is shown in figure 22.2. Signals indicating the volume of water measured in 1 the flume are transmitted via a control unit to the sampling device. An aliquot volume of the flow is sampled whenever a predetermined volume of water (say 100 l) has passed the flow transmitter The daily samples are mixed, and after an optional period a smaller volume of the mixed samples is analysed.


7 4 8

9

2

Fig. 22.2 Automatic sampling system. 1 Measuring flume 2 Measuring probe 3 Flow transmitter 4 Recorder 5 Summation device 6 Control unit 7 Subunit 8 Air 9 Sampling device

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Sewage treatment, a general survey Various arrangements are possible; t he choice of treatment is determined by the required degree of pollutant reduction. Figure 22.3 shows four possible systems.

A

B

C

1. Post-precipitation Conventional three-stage process with mechanical, A, biological, B, and chemical, C, treatment. Effective and reliable, but fairly expensive.

Grid

Sand trap

Sedimentation

A

B

C

2. Pre-precipitation A two-stage process developed during the eighties. Chemical treatment, C, is com bined with mechanical sedimentation, A, in the first stage, which results in high grade phosphorus reduction as well as about 70% BOD reduction. This relieves the load on the biological stage, B, which thus requires much less basin volume and energy input than with conventional post-sedimentation.

Chemical treatment A

Aeration Fig. 22.3 The various stages of sewage treatment can be combined in several ways.

C

3. Direct precipitation A single-stage process, with combined mechanical, A, and chemical, C, treatment as in pre-precipitation, but with no succeeding biological treatment stage.

A

B/C

4. Simultaneous precipitation A two-stage process with mechanical treatment, A, followed by a combined biological-chemical stage, B/C. A fairly cheap method of satisfying the demand for phos phorous reduction without expensive additional basin capacity, but less efficient than if the biological and chemical treatments are performed separately.

Sewage treatment in its original form consisted simply in removing the bulk of solid impurities by mechanical sedimentation (A). When this treatment was judged to be insufficient, it was supplemented with biological treatment (B) to decompose the organic compounds. Many sewage treatment plants were later extended with a third stage for chemical treatment (C) when emission of phosphorus became a serious problem. The process in plants of this type is called post-precipitation because the chemical precipitation step comes last. Later experience has however proved that it is possible to obtain the same result if chemical precipitation is combined with mechanical treatment in the first step. This system is called pre-precipitation (see figure 22.3.2). This arrangement also represents a major rationalisation of the process, as most of the sewage treatment is done in one step. The phosphorus content is already reduced by 90% and the BOD by 75% in the pre-sedi-

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5 11 8

6

10 7 1

2

9 4

3

mentation basins. As a result, the biological stage has a much lighter load to deal with and requires less bas in volume and energy input. Figure 22.4 shows a typical sewage plant layout with pre-precipitation.

Mechanical treatment The primary (mechanical) stage of sewage treatment comprises strainer grid, sand trap and primary sedimentation basins. The grid traps coarse solid matter: plastic, rags, food residues, etc. This matter is continuously scraped off the grid and disposed of separately, usually as landfill. The sand trap is a basin in which coarse separation takes place. It is dimensioned and operated in such a way that sand and other heavy particles have time to settle to the bottom, while fat and other impurities that are lighter than water float to the surface. The sediment is pumped away, and the floating scum is removed by scrapers. These waste products are likewise disposed of separately. Air is blown into the sand trap, partly to keep finer parfticles in suspension and partly to prevent putrefaction processes that cause bad smells.

Fig. 22.4 The various stages of a sew age treatment plant.

1 2 3 4 5 6 7 8 9 10 11

Inlet channel Grid Sand trap Aeration Silo for flocculant Pre-precipitation Pre-sedimentation Biological treatment Aeration Post-sedimentation Clarified effluent to recipient

Chemical treatment The principal purpose of chemical sewage treatment, also known as precipitation, is to rid the water of phosphorus. Municipal sewage systems normally collect 2.5 – 4 grams of phosphorus per person per day, mainly in the form of phosphates. Detergents account for about 30% of the phosphate content; the remaining 70% comes mainly from human excreta and food residues. Chemical precipitation with iron and aluminium based flocculants can remove almost 100% of the phosphorus present in waste water, while conventional biological trreatment only reduces the phosphorus content by 20 – 30%. The precipitation stage starts with flocculation tanks, where the flocculants are added and vigorously mixed into the water by agitators. This results in precipitation of insoluble phosphates, initially in the form of very fine particles which, however, gradually aggregate into larger flocs. The flocs settle out in pre-sedimentation basins, from which a clear effluent overflows into the basin for biological treatment. Pre-sedimentation is the final step in the combined mechanical and chemical treatment. The water is allowed to flow slowly through one or more basins where the finer particles gradually settle to the bottom as pri mary sludge. The sedimentation basins are equipped with devices that continuously


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scrape the sediment into a sump, and transverse gutters that carry off water from the clarified surface layer.

Biological treatment The remaining organic impurities in the "overflow" from the chemical treatment are broken down with the help of micro-organisms, e.g. bacteria, which feed on the organic substances present in the water. The micro-organisms must have acces s to oxygen to perform their function. This is supplied in the form of air blown into the aeration basin. The micro-organisms reproduce continuously, forming an active sludge. This sludge is removed from the water by settling in post-sedimentation basins. Most of it is recirculated to the aeration basins to keep the biological breakdown process going; the excess sl udge is removed from the process for further treatment and the clarified effluent is discharged to the recipient. An alternative to the aeration basin is the biological filter , which is a container filled with pieces of stone or plastic. The water is sprinkled over the filter by a rotating distributor, trickles down through the filte r bed, and is oxygenated by air circulation. A “skin” of micro-organisms builds up on the surfaces of the stones, etc., breaking down the organic impurities in the water.

Sludge treatment The sludge from the various stages of treatment is collected in thickening tanks to which chemicals are added to facilitate further aggregation of the solid particles. Primary sedimentation basins 100 m3 of sludge fr om primary sedimentation basins. DS2% Water content 98%

Sludge thickener 66 m3 of water removed in the sludge thickener. 34 m3 of sludge with 6% DS continues to centrifuge plant.

Decanter 26 m3 of water removed in decanter centrifuge. 8 m3 of dewatered sludge with 25% DS is discharged. Reduction in volume in centrifuge stage is 76%.

Fig. 22.5 Reduction in volume of wet sludge from the primary settling stage after treatment in a sludge thickener and decanter centrifuge. The amount of dewatered sludge discharged from the decanter centrifuge is only 8% of the volume of the wet sludge from the sedimentation basins.

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To further break down organic matter and to reduce evil-smelling substances, the sludge is eventually pumped into a digester where the organic subtances are broken down under anaerobic conditions into carbon dioxide and methane and very small amounts of hydrogen gas, ammonia and hydrogen sulphide. Carbon dioxide and methane are the main components of digester gas, which can be utilised as fuel for heating. Digester sludge is a homogeneous, practically odourless, dark-coloured substance which still has a high moisture content, 94 - 97%. It is therefore dewatered, most effectively in a decanter centrifuge, which discharges a solid phase of about one-eighth of the original volume, as shown in figure 22.5. The dewatered sludge can then be utilised as fertiliser or landfill, or simply deposited as waste.


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Chapter 22

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