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Contents 3.1 Fundamentals ....................................................................................................................... 6 3.1.1 Description .................................................................................................................. 6 3.1.2 Coagulation ................................................................................................................. 8 3.1.3 Flocculation ............................................................................................................... 12 3.1.4 Sedimentation Sedimentation .............................................................. ............................................. 13 3.1.5 Jar Testing .................................................................................................................. 13 3.1.6 The Rule and Exceptions Exceptions to the Rule ................................................................... 14 3.1.7 Equipment ................................................................................................................. 15 3.2 Coagulator Clarifier ............................................................... ............................................ 16 3.2.1 Process and Operations Overview........................................................................ 16 3.2.2 Equipment Design and Options ........................................................................... 18 3.2.2.1 Tank .................................................. ........................................................ ......... 19 3.2.2.2 Internals ..................................................... ...................................................... 19 3.2.2.3 Lining ................................................. ....................................................... ........ 21 3.2.2.4 Paint ....................................................... ........................................................... 21 3.2.2.5 Piping ................................................. ....................................................... ........ 22 3.2.2.6 Valves .................................................... ............................................................ 22 3.2.2.7 Instrumentation Instrumentation .................................................... .......................................... 22 3.2.3 Application and Design ................................................... ....................................... 22 3.2.3.1 Application ................................................... ................................................... 22 3.2.3.2 Design ....................................................... ........................................................ 24
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Clarification
3.3 Cold Col d Lim e S of ten in g Clarifier Clarifi er ............. .................... ............. ............. ............. ............. ............. ............. ............. ............. ............. ............. ........... .... 27 3.3.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 27 3.3.2 Equipment and Design Options ........................................................................... 30 3.3.2.1 Tank an k .................................................. ........................................................ ................................................................. ......... 30 3.3.2.2 Internals Internal s ............. .................... .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ...... 30 3.3.2.3 Lin in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 31 3.3.2.4 Pain t ....................................................... .................................................................................................................. ........................................................... 31 3.3.2.5 Pip in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 31 3.3.2.6 Valves Valv es .................................................... ................................................................................................................ ............................................................ 32 3.3.2.7 Instr Ins trum um en tation ............. .................... .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ....... 32 3.3.3 Ap pl icatio n and D es ig n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ......... ... 33 3.3.3.1 Ap pl icatio n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ......... 33 3.3.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 34 3.4 Lamella Clarifier ................................................................................................................ 37 3.4.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 37 3.4.2 Equipment and Design Options ........................................................................... 39 3.4.2.1 Tanks an ks ............................................ ................................................... .................. 40 3.4.2.2 Internals Internal s ............. .................... .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ...... 40 3.4.2.3 Lin in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 40 3.4.2.4 Pain t ....................................................... .................................................................................................................. ........................................................... 40 3.4.2.5 Pip in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 40 3.4.2.6 Valves Valv es .................................................... ................................................................................................................ ............................................................ 41 3.4.2.7 Instr Ins trum um en tation ............. .................... .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ....... 41 3.4.3 Ap pl icatio ns and D es ig n ................ ...................... ............. .............. .............. .............. ............. ............. .............. .............. .............. ........... .... 42 3.4.3.1 Ap pl icatio ns .............. ..................... .............. ............. ............. .............. .............. .............. ............. ............. .............. .............. .............. ........... .... 42 3.4.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 43 3.5 Che mi cal Fee d Equ ip me nt ............. .................... ............. ............. .............. ............. ............. .............. ............. ............. .............. ............. ............. ......... .. 48 3.5.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 48 3.5.2 Equipment and Design Options ........................................................................... 49 3.5.3 Ap pl icatio n and D es ig n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ......... ... 50 3.5.3.1 Ap pl icatio n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ......... 50 3.5.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 50 3.5.3.2.1 3.5.3.2.1 Bulk Bul k Che mi cal Sto rage rage Tank s ............... ...................... .............. ............. ............. .............. .............. ............. ......... ... 50 3.5.3.2.2 Measuring (Batch) Tanks ......................................................................... 50 3.5.3.2.3 Metering Pumps ........................................................................................ 51 3.5.3.2.4 3.5.3.2.4 Causti c Me tering Pum p ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. ......... 52 3.5.3.2.5 Lime Feede Fee de r ...................................................... ................................................................................................ .......................................... 53 3.6 Reso urces
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3-2
Clarification
3.3 Cold Col d Lim e S of ten in g Clarifier Clarifi er ............. .................... ............. ............. ............. ............. ............. ............. ............. ............. ............. ............. ........... .... 27 3.3.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 27 3.3.2 Equipment and Design Options ........................................................................... 30 3.3.2.1 Tank an k .................................................. ........................................................ ................................................................. ......... 30 3.3.2.2 Internals Internal s ............. .................... .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ...... 30 3.3.2.3 Lin in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 31 3.3.2.4 Pain t ....................................................... .................................................................................................................. ........................................................... 31 3.3.2.5 Pip in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 31 3.3.2.6 Valves Valv es .................................................... ................................................................................................................ ............................................................ 32 3.3.2.7 Instr Ins trum um en tation ............. .................... .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ....... 32 3.3.3 Ap pl icatio n and D es ig n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ......... ... 33 3.3.3.1 Ap pl icatio n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ......... 33 3.3.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 34 3.4 Lamella Clarifier ................................................................................................................ 37 3.4.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 37 3.4.2 Equipment and Design Options ........................................................................... 39 3.4.2.1 Tanks an ks ............................................ ................................................... .................. 40 3.4.2.2 Internals Internal s ............. .................... .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ...... 40 3.4.2.3 Lin in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 40 3.4.2.4 Pain t ....................................................... .................................................................................................................. ........................................................... 40 3.4.2.5 Pip in g .............. .................... ............. .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ........... .... 40 3.4.2.6 Valves Valv es .................................................... ................................................................................................................ ............................................................ 41 3.4.2.7 Instr Ins trum um en tation ............. .................... .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ....... 41 3.4.3 Ap pl icatio ns and D es ig n ................ ...................... ............. .............. .............. .............. ............. ............. .............. .............. .............. ........... .... 42 3.4.3.1 Ap pl icatio ns .............. ..................... .............. ............. ............. .............. .............. .............. ............. ............. .............. .............. .............. ........... .... 42 3.4.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 43 3.5 Che mi cal Fee d Equ ip me nt ............. .................... ............. ............. .............. ............. ............. .............. ............. ............. .............. ............. ............. ......... .. 48 3.5.1 Proce Proce ss and Op eration s O ve rvi ew ............. .................... .............. .............. ............. ............. .............. .............. ............. ........... ..... 48 3.5.2 Equipment and Design Options ........................................................................... 49 3.5.3 Ap pl icatio n and D es ig n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ......... ... 50 3.5.3.1 Ap pl icatio n ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. ......... 50 3.5.3.2 D es ig n ............. .................... ............. ............. .............. .............. ............. ............. .............. ............. ............. .............. .............. ............. ............. ........... .... 50 3.5.3.2.1 3.5.3.2.1 Bulk Bul k Che mi cal Sto rage rage Tank s ............... ...................... .............. ............. ............. .............. .............. ............. ......... ... 50 3.5.3.2.2 Measuring (Batch) Tanks ......................................................................... 50 3.5.3.2.3 Metering Pumps ........................................................................................ 51 3.5.3.2.4 3.5.3.2.4 Causti c Me tering Pum p ............. .................... .............. .............. ............. ............. .............. .............. ............. ............. ......... 52 3.5.3.2.5 Lime Feede Fee de r ...................................................... ................................................................................................ .......................................... 53 3.6 Reso urces
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3-2
Clarification
Module 3 CLARIFICATION In Module 1 , the reasons for water purification purification are discussed. In Module 2 , the fundamentals of water chemistry are are reviewed. In this module these ideas and concepts are applied to one of the oldest forms of water purification and treatment: clarification. Clarification is the method used to remove suspended matter from surface water and industrial wastewater. wastewater. In essence, it makes “turbid”water “clear,”as “clear,”as shown below in Figure 3.1-1 .
Figure 3.1-1: Turbid and Clarified Water
Clarification Clarifi cation also reduces iron, manganese, organic material, oil and color. It can be configured to remove hardness. hardness. It is the first step in surface water treatment, because the processes that follow normally require clear and colorless influent water. water. For the same reasons, clarification is often the last step before discharge of wastewater to a receiving stream.
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Clarification
Turbidity is the measurement of light transmission and reflection through a sample of water as shown below in Figure 3.1-2. It is an indirect measurement measurement of the amount of suspended matter in water. Turbidity is measured with a turbidimeter as shown below in Fig ure 3.1-3.
C o u r t e s y o f H a c h C h e m i c a l C o m p a n y
Figu re 3.1-2: 3.1-2: Turbidi ty M easu rem en t
C o u r t e s y o f H a c h C h e m i c a l C o m p a n y
Figure 3.1-3: Turbidimeter 3-4
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Clarification
Clarification uses chemical addition and the sedimentation process to remove suspended matter from water, as discussed below. Figure 3.1-4 below shows a collage of different clarifier designs that can be selected for use in a water purification system.
Coagulator Cl Clarifier
Cold Lime Softening Clarifier C o u r t e s y o f E c o d y n e
Collage of Clarifier Designs
Lamella Clarifier Figure 3.1-4: Collage of Clarifier Designs
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Clarification
3.1
Fundamentals
This section reviews the fundamentals of clarification technology. Coagulation, flocculation and sedimentation phenomena are discussed. The use of polymers and jar testing to optimize clarification is presented. Rules of clarification and exceptions to the rules are noted.
3.1.1
Description
Clarification removes suspended matter from water. Surface waters require clarification because they have moderate to high levels of suspended matter. Well waters do not require clarification because they have low levels of suspended matter. As shown below in Fig ure 3.1-5 , the suspended matter in water includes two kinds of particles: • Settleable Particles (macroparticles, typically visible to the eye) • Non-Settleable Particles (microparticles, normally visible through a microscope)
Settleable Macroparticles Visible to the Eye
Non-Settleable Particles Visible Though a Microscope Figu re 3.1-5: Su spe nd ed Matter in Water
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Settleable particles are particles in water that settle out over time. Figure 3.1-6 on the right shows a sample of water in which the settleable particles (suspended matter) have settled out. The water itself is clear, indicating an absence of suspended matter (turbidity). If non-settleable particles had been in the water, the water would not be clear. This “turbidity”would have indicated the presence of non-settleable particles. Turbidity is an indirect measurement of the amount of suspended matter (settleable particles and non-settleable particles) in water.
C o u r t e s y o f B e t z D e a r b o r n
Clarification uses chemicals and sedimentation to remove suspended matter (settleable particles and non-settleable particles). Several steps are involved. First, coagulation destabilizes the particle surface charge that keeps the particles in solution. Once destabilized, the particles no longer repel one another and come together as floc. Second, floc agglomerate Fig ure 3.1-6: into larger particles. Polymers are used to enhance the Suspen ded M atter flocculation process. Third, sedimentation causes agglomerated floc to settle out. The settled floc is collected and concentrated for discharge to waste, called clarifier blowdown, or recycled to the coagulation step, called sludge recycle. Clarified water is collected and flows out of the clarifier.
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3.1.2
Coagulation
The first step of the clarification process is coagulation, as shown below in Figure 3.1-7. Particles in water have a naturally occurring negative charge. This causes them to repel each other and stay in suspension. When this charge is destabilized, the particles no longer repel one another, and can come together in closer proximity. A chemical salt, called a coagulant, is mixed with the inlet water to destabilize the charge. Common coagulants are aluminum sulfate (alum), ferric sulfate and ferric chloride.
Floc
Floc
Figure 3.1-7: Coagulation
The coagulants provide a positive charge, in the form of metallic cations, that destabilize the natural negative charge of the particles. The metallic cations combine with hydroxide in the water to form a metallic hydroxide that is an insoluble compound. The destabilized particles and metal hydroxide precipitates agglomerate into small, visible particles called floc. Color, organic matter and colloids, including colloidal silica, are removed by becoming bound up in the floc. The precise mechanism for removal- absorption, adsorption, co-precipitation, or a combinationis not fully understood.
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The addition of too much coagulant can cause the suspended matter to be redispersed with the opposite charge. As shown below in Figure 3.1-8 , the amount of removal is dependent upon the coagulant dosage and the pH.
40 ppm Residual
60 ppm pH Figure 3.1-8: Particle Resi du al as a Fun ction of Coagulant Dosage and p H
Alum (aluminum sulfate), ferric sulfate and ferric chloride coagulants are acidic salts and decrease the pH of the influent water. Because of this, the pH of the water must be adjusted with caustic (sodium hydroxide) or another alkaline (high pH chemical). The adjustment is to a pH of 5.5 to 6.5 and is done to achieve the lowest residual of suspended matter. Lime is used as the coagulant when the treatment objective is hardness reduction. The dosage depends on the desired operating pH of the clarifier. For the greatest removal of hardness, the pH range is 9.5 – 10.5. Feed of coagulant alone does not produce satisfactory floc in waters having a low suspended matter concentration. In this instance, bentonite clay is added. Bentonite clay creates an artificial base of settleable macroparticles that seed the growth of floc.
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Clarification
Polymers are added to reduce the amount of coagulant required, broaden the working pH range and create denser, heavier floc that settles out more easily, as shown below in Figure 3.1-9 Polymers are long-chain organic compounds of high molecular weight that bridge floc particles together or modify their surface charge. .
C o u r t e s y o f B e t z D e a r b o r n
Figure 3.1-9: Floc Formed With Coagul ant Al on e vs. Floc Formed w ith Coagulant and Polyme r
In almost all cases, the water to be treated is disinfected with either gaseous chlorine or sodium hypochlorite. This oxidizes organic matter in the water that has taste and odor and certain metals, such as manganese and iron. When oxidized, these constituents are transformed into a form that can be removed during clarification. Their removal is important because they can cause fouling of process compo nents.
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Coagulation is carried out in a fast mix chamber, as shown below in Fig ure 3.1-10. A fast mix is required because the coagulant and water must be thoroughly mixed to allow the suspended matter and coagulant to come into contact with each other. If it is not fast mixed, some suspended matter may not come into contact with coagulant, the surface charge will not be destabilized and flocculation will not occur. As flocculated water flows into the slow mix chamber, polymer is added.
Fast Mix Chamber
Inlet Water
A d a p t e d f r o m E c o d y n e
Slow Mix Chamber Alum Feed
Polymer Feed
Figure 3.1-10: Coagu lation Step
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3.1.3
Flocculation
In the next step of clarification, the small floc (microfloc) is allowed to grow into larger floc, called macrofloc or agglomerated floc. This process, called flocculation, is shown below in Fig ure 3.1-11. Flocculation is accomplished by gently stirring the coagulated water to assure contact between microfloc particles and polymer. The polymer enhances agglomerated floc formation. As the agglomerated floc continues to grow, it becomes denser and heavier, allowing it to settle.
Floc
Agglomerated Floc
Floc
Floc
Polymer
Inlet Water
A d a p t e d f r o m E c o d y n e
Fast Mix Chamber Polymer Feed Slow Mix Chamber
Alum Feed
Figure 3.1-11: Flocculation Step
Mixing too rapidly can create what is called floc shear. Shear is the breaking apart of existing floc particles. The agglomerated floc, or macrofloc, is sheared back into microfloc.
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A d a p t e d f r o m E c o d y n e
Figu re 3.1-12: Se dim en tation Step
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3.1.4
Sedimen tation
The final step of the clarification process is sedimentation, as shown on the opposite page in Fig ure 3.1-12. In this step, agglomerated floc settles out to form sludge and the sludge is transported to the sludge concentration chamber by the scraper. The sludge thickening pickets concentrate the sludge. The sludge is discharged to waste in a process called clarifier blowdown. Above the sludge, clarified water is collected in the outlet launder and flows forward for use or further treatment.
3.1.5
Jar Testin g
Jar tests are used to establish chemical dosage requirements and predict clarifier effluent turbidity levels. They simulate the chemistry and physical operation of a clarifier. The test equipment is shown below in Fig ure 3.1-13. Varying dosages of coagulant, polymer and pH adjustment chemical (if required) are added to the beakers on the gang stirrer. The water in the beaker is stirred for the amount of time equivalent to the retention time of the fast mix chamber and slow mix chamber. The floc is then allowed to settle for the amount of time equivalent to the retention time of the sedimentation chamber. Then the turbidity of the water is measured and recorded along with the pH, stirring speed and relative volume of floc produced. Jar tests are repeated with varying dosages of chemicals and alternate chemicals to determine the appropriate combination of chemicals, pH and floc production necessary to obtain optimal performance. Jar tests should be performed prior to the final sizing of the clarifier and the chemical feeders. Once the clarifier is in operation, jar tests should be repeated if the quality of the inlet water changes significantly.
Gang Stirrer C o u r t e s y o f B e t z D e a r b o r n
Fig ure 3.1-13: Jar Tes t Equi pmen t
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Clarification
3.1.6
The Rule and Exceptions to the Rule The Rule
The general rule for clarification is feed of a disinfectant, followed by feed of the coagulant, then pH adjustment and finally addition of a polymer. The disinfectant, coagulant and pH adjustment chemical are added to initiate the coagulation process. The polymer is added after coagulation to facilitate flocculation. The coagulant is selected based on the characteristics of the water to be treated and the treatment objectives. Jar tests are used to determine the proper disinfectant, coagulant, polymer dosages and the operating pH for the clarifier.
Exceptions to the Rule
Some waters and some applications invoke exceptions to the rule. The addition of bentonite clay is required to achieve macrofloc formation in waters that have a low suspended matter concentration. The clay acts as a macroparticle base for macrofloc formation. When treating waters with a high suspended matter concentration, replacement of the coagulant with polymer is possible. The replacement is called coagulant, or alum, substitution. In this situation, there is sufficient floc formation without the feed of a coagulant. The advisability of an alum substitution treatment program must be confirmed by jar tests. If the process downstream from the clarifier is reverse osmosis, particular attention must be paid to polymer selection and dosage. Polymer carries over from the clarifier, even if the water looks perfectly clear, and can be a membrane foulant. The potential for this is determined by running a Silt Density Index (SDI) on the clarifier effluent, as described in Module 6 on reverse osmosis. Polymer carryover increases SDI. To combat this, a filter aid can be used upstream from the filtration equipment. The polymer and filter aid must be compatible with the antiscalant fed to the RO system. Many thin film membranes carry a negative surface charge (anionic). Cationic (positively charged) polymers can bind to the anionic membrane sites and cause irreversible fouling.
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3.1.7
Equipment
Clarification can be accomplished using several approaches. The equipment designs fall into two general categories. The first is the common clarifier, also called a reactivator clarifier, coagulator clarifier, cold lime softener, or solids contact clarifier, depending upon the manufacturer and performance required. The second is the inclined plate, or lamella, design that uses inclined plates to accomplish the sedimentation function. All have a fast mix chamber to ensure good contact between the treatment chemicals and suspended matter for coagulation, a slow mix chamber allowing polymer addition to facilitate flocculation, a sedimentation chamber for settling and a concentrated sludge chamber for clarifier blowdown. Required disinfectant, coagulant and pH adjustment chemicals are fed before the fast mix chamber. Clay is fed to the fast mix chamber. Polymer is fed between the fast mix and slow mix chambers. Note that the solids contact clarifiers have internal sludge recycle, and the inclined plate clarifiers can be furnished with optional external sludge recycle. The following sections present a process and operations overview, discuss equipment design and options and consider the application and design of each of these clarifiers.
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Clarification
Fig ure 3.2-1: Coag ulator Clarif ie r P&ID
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Clarification
3.2
Coagulator Clarifier
3.2.1
Process and Operations Overview
The P&ID for a coagulator clarifier is shown on the opposite page in Figure 3.2-1 and the equipment cutaway is shown below in Figure 3.2-2 Major components have been identified.
,
.
Slow Mix Tank
Sludge Scraper Drive
Variable Speed Recirculator Drive
Fast Mix Tank
A d a p t e d f r o m E c o d y n e
Sludge Layer
Concentrated Sludge Chamber
Recirculated Sludge
Figure 3.2-2: Coagulator Clarifier
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Clarification
Treatment chemicals are added to the inlet flow before it enters the fast mix chamber. The chemicals and water are thoroughly mixed in the fast mix chamber to begin the coagulation process. A variable speed recirculator drive turns the recirculator impeller to mix the inlet water and chemicals. Coagulated particles flow upward into the slow mix chamber where polymer is added to facilitate flocculation. A portion of the agglomerated floc is internally recirculated back into the fast mix chamber from the bottom of the slow mix chamber to seed the coagulation process. Agglomerated floc flows downward from the slow mix chamber into the sedimentation chamber. In the sedimentation chamber, the flow reverses to upward and agglomerated floc settles. The treated water flows upward to the outlet collection launders, then out to use or to the next process. It is important that the settling rate of the floc particles is faster than the upflow rate of the water. This gives the floc particles a relative velocity that is downward. This downward relative velocity causes the particles to move to the bottom of the sedimentation chamber, not upward into the outlet collector. Floc leaving the clarifier with the treated water is called carryover floc. In some applications, a floc barrier is provided to reduce effluent turbidity caused by floc carryover. Agglomerated floc that settles to the bottom of the sedimentation chamber is called sludge. The sludge is moved to the concentrated sludge chamber by a scraper powered by the sludge scraper drive. The collected sludge is concentrated with thickening pickets in the concentrated sludge chamber and is periodically backflushed with clarifier inlet water to keep the sludge fluidized. Concentrated sludge is blown down (discharged) to waste through the clarifier blowdown piping. The blowdown is 2-5% of the influent flow and has a 5-8% suspended solids concentration. The following is a discussion of the chemical reactions taking place as the water flows through the clarifier. Alum (Aluminum Sulfate, Al2(SO4 )3 ), is added to the water to be treated in the fast mix chamber. This consumes hydroxide in the inlet water, contributes sulfate to the treated water and forms an aluminum hydroxide precipitate (floc). Al2(SO4 )3 + 6H2O → 6H+ + 3SO42- + 2Al(OH)3 ↓ As an alternate to alum, ferric sulfate, Fe 2(SO4 )3 , can be used. This consumes hydroxide in the inlet water, contributes sulfate to the treated water and forms a ferric hydroxide precipitate (floc). Fe2(SO4 )3 + 6H2O → 6H+ + 3SO42- + 2Fe(OH)3 ↓
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Ferric sulfate is used to remove trace metals, in addition to suspended matter, in the influent water. Ferric chloride (FeCl 3 ) is an alternative to ferric sulfate and its reaction with water is similar. The primary difference is that it contributes chlorides to the treated water rather than sulfates. FeCl3 + 3H2O → 3H+ + 3Cl- + Fe(OH)3 ↓ Independent of the coagulant selected (alum, ferric sulfate or ferric chloride), the hydroxide consumed in the inlet water must be replaced by addition of an alkali, usually caustic (sodium hydroxide). The alkali adjusts the clarifier operating pH to the proper range (5.5 to 6.5). Note that two coagulants, alum and ferric sulfate, add sulfate to the treated water and one, ferric chloride, adds chloride. Chemical feeders, with associated day tanks and/or bulk storage tanks, are needed for the feed of disinfectant, coagulant, clay, pH adjustment chemical and polymer. Normally a single loop controller is used for control of the pH adjustment chemical feed. Details of chemical feed and control are discussed in Section 3.5 beginning on page 3-48. The most difficult part of operating a coagulator clarifier is the formation and maintenance of the sludge layer. Formation occurs during the startup of the coagulator clarifier as agglomerated floc is accumulated in the sedimentation chamber. Once formed, the sludge layer is subject to upset. The upset could be due to changes in influent water quality, temperature changes (particularly if the coagulator clarifier is located outdoors), or disruptions in influent flow. Once upset, it can take several hours to re-establish the sludge layer.
3.2.2
Equipment Design and Options
The clarifier vessel is designed in various diameters and straight shell heights. The operating water depth in the clarifier is one foot lower than the shell height. Final vessel height depends on site-specific requirements, such as land topography, treated water storage tank height and filter selection. It is furnished with chambers for fast mixing (coagulation), slow mixing (flocculation), settling (sedimentation) and sludge concentration. The coagulator clarifier is designed to do the following. • accomplish efficient coagulation and flocculation of influent water • collect and dispose of sludge
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This is accomplished by conservatively selecting the rise rate and retention time for the clarifier and providing robust recirculation and scraper features. The rise rate is the upward velocity of the water in the sedimentation chamber. The retention time is the amount of time the water spends in the slow mix chamber. The slow mix chamber and the sedimentation chamber are designed to achieve uniform flow through the clarifier.
3.2.2.1 Tank
Clarifiers come in 20-200 ft diameters and 13-21 ft shell heights. They are designed for operating temperatures from 35-150 ºF. The shell is field fabricated of either steel or concrete. The floor is sloped concrete and access is by ladder. Clarifier tank options are stairway access, ice design and API 650 design.
3.2.2.2 Internals
The internals on clarifiers having a diameter of 75 feet and smaller are supported from a bridge; larger units are supplied with a center post design. The clarifier uses a recirculator with a variable speed drive to fast mix the influent water and chemicals. The variable speed drive allows adjustment of the impeller speed to optimize the fast mix process. The scraper is designed to move precipitated suspended matter (sludge) to a collection chamber for disposal. The scraper drive is furnished with sufficient torque to handle dense solids. Optional scraper drive torque overload can be provided. Since the environment is not corrosive, the scraper material is carbon steel.
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An optional floc barrier can be provided to allow higher flows through the clarifier and reduce floc carryover. As shown below in Figure 3.2-3 the floc barrier is constructed of plastic and requires adequate supports to prevent it from moving. The honeycomb structure of the floc barrier creates a larger effective settling area, reducing the external dimensions of the clarifier. It also gives the clarifier a barrier to floc carryover, resulting in lower effluent turbidity. ,
A d a p t e d f r o m E c o d y n e
Figure 3.2-3: Floc Barrier
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The purpose of the outlet launder (outlet collection header) is to collect treated water. On large diameter clarifiers, they are fed by a network of radial collector pipes, as shown below in Figure 3.2-4 , that allows the treated water to be collected uniformly across the cross-sectional area of the clarifier. On smaller units the water overflows directly into the launder.
Outlet Headers
Figure 3.2-4: Ou tlet Launde r
3.2.2.3 Lining
The clarifier shell, the bridge and the internal components can be protected from corrosion with an epoxy polyamide lining or by being galvanized. The shell is rarely galvanized due to its size.
3.2.2.4 Paint
The clarifier can be painted with an epoxy polyamide primer and finish coat, an epoxy polyamide primer and siloxane finish, a siloxane epoxy and finish coat, or a custom primer and finish coat. The choice of materials depends on the environment in which the clarifier will be located.
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3.2.2.5 Pipin g
Since the water is not corrosive, carbon steel is the standard piping material. Galvanized carbon steel is an option for sites desiring better corrosion resistance. The choice of materials for chemical feed piping depends on the chemical being used; stainless steel is quite common.
3.2.2.6 Valves
Butterfly valves are the standard valves furnished for clarifier operation. The inlet isolation valve is manual; all others are air operated. Air operated valve closing action is slow to prevent water hammer. The backflush inlet and blowdown outlet valves have travel stops.
3.2.2.7 Ins trum en tation
An optional turbidity analyzer is provided to measure clarifier effluent turbidity.
3.2.3
Application and Design
3.2.3.1 Ap pli cation
Coagulator clarifiers are used to remove suspended matter. They do not remove hardness. Ferric sulfate or ferric chloride may be substituted for alum if the treatment objective includes removal of trace metals. Aluminum flocs and iron flocs are gelatinous in nature, fragile and not abrasive. This is important because the nature of the floc factors into the power requirement for the recirculator and scraper drives the amount of blowdown and the abrasive resistance of materials of construction. Compared to cold lime softening clarifiers, which have a heavy, crystalline, abrasive floc and a blowdown 2-4 times higher than a coagulator (alum) clarifier, the coagulator clarifier recirculator and scraper drives have a lower power requirement and the scraper is not subject to abrasion.
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Table 3.2-1 below gives a summary of performance for coagulator clarifiers.
Basic Equipment Select ion Coagulator Clar if ier Perf or mance Summar y Const it uent
Units
Inf luent : Alum
Ef f luent: Alum
Tur bidit y
NTU
3000
1-20
Ir on
ppm Fe
20
0.2
Mn
ppm Mn
20
0.5
Organics (dissolved) ppm O2 consumed
NO LIMIT
1-2
Oil (f loat ing)
ppm
10
0.5
TDS
ppm CaCO3
NO LIMIT
NO CHANGE
Chlor ine
Cl2
20
*
Har dness
ppm CaCO3
NO LIMIT
NO CHANGE
Alkalinit y
ppm CaCO3
90
NO CHANGE
pH
STD units
NO LIMIT
5.5 - 6.5
* Chlorine is added t o a clar if ier t o kill bact er ia and o x idize or ganic contaminant s in wat er. As the organics ar e o x idized, the chlorine is reduced to the chloride ion. The e x tent to which the chlorine content is reduced is dependent upon t he organic cont aminat ion of the water.
Table 3.2-1: Coagul ator Clarifi er Performance Su mm ary
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3.2.3.2 D esig n
Sedimentation chamber rise rate and slow mix chamber retention time are the two main factors used in sizing coagulators. Rise rate, which is the upward velocity of the treated water measured in gpm/ft 2 , determines the area of the clarifier treated water surface. Retention time determines the volume of the slow mix chamber. The area taken up by the slow mix chamber is not included in the treated water surface area. Therefore, the diameter of the clarifier is computed based on the sum of the areas of the treated water surface and the slow mix chamber. First, the slow mix chamber volume is calculated. It is a function of the clarifier flow rate and the slow mix chamber retention time. The retention time is the time it takes for the coagulation and flocculation chemical reactions to occur. The typical retention time is 20-30 minutes, based on experience. The low end of the range is used when coagulation and flocculation chemical reactions are fast; the high end when the reactions are slow. For coagulator clarifiers, the reaction is slow, so 30 minutes is selected. The equation for calculation of the slow mix chamber volume is as follows: Slow Mix Chamber Volume = (Flow rate)(Retention Time) Second, the slow mix chamber area is calculated. It is a function of the slow mix chamber volume and height. Typical heights are 12-20 ft. The height selected depends on site-specific requirements, such as land topography, treated water storage tank height, and filter selection. The equation for calculation of the slow mix chamber area is as follows: Slow Mix Chamber Area = Slow Mix Chamber Volume Slow Mix Chamber Height Third, based on the slow mix chamber volume and area calculated above, the diameter of the slow mix chamber is calculated as follows: (Slow Mix Chamber Diameter)2 = (4)(Slow Mix Chamber Area) 3.14 Fourth, the treated water surface area is calculated. It is a function of the clarifier flow rate and the treated water surface rise rate. Typical rise rates are 0.75-1.25 gpm/ ft2. Rise rates on the low end of the range are used when the settling speed of agglomerated floc is slow and/or the water temperature is low, rise rates on the high end when the agglomerated floc settles quickly and/or the water temperature is high. Coagulator clarifiers are commonly sized with a 1 gpm/ft 2 rise rate. The calculation for treated water surface area is as follows: 3-24
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Treated Water Surface Area = Clarifier Flow rate Rise Rate Fifth, the total clarifier area is calculated. It is the sum of the treated water surface area and the slow mix chamber area. Total Clarifier Area = Treated Water Surface Area + Slow Mix Chamber Area Sixth, the clarifier diameter is calculated based on its total area. (Clarifier Diameter) 2 = (4)(Total Clarifier Area) 3.14 The following example sizes a 500 gpm coagulator clarifier.
Slow Mix Chamber Volume = (500 gpm)(30 min) (7.48 gal/ft3 ) = 2000 ft3
Slow Mix Chamber Area = 2000 ft 3 12 ft = 165 ft2
(Slow Mix Chamber Diameter) 2 = d2 = (4)(165 ft2 ) 3.14 or d @ 15 ft Treated Water Surface Area = 500 gpm_ (1 gpm/ft2 ) = 500 ft2 Total Clarifier Area = 165 ft2 + 500 ft2 = 665 ft2 (Clarifier Diameter) 2 = d2 = (4)(665 ft2 ) 3.14 or d @ 29 ft.
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Figure 3.2-5 below is a sketch of the example coagulator clarifier with the
dimensions noted.
Figure 3.2-5: Example Coagulator Clarifier Dimensions
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Slow Mix Tank
Fast Mix Tank
A d a p t e d f r o m E c o d y n e
Concentrated Sludge Chamber
Sludge Layer
Figu re 3.3-2: Cold Lime So ften ing Clarifi er
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3.3
Cold Lime Softening Clarifier
The coagulator clarifier and cold lime softening clarifier have several common features. They have the same straight shell height, treated water surface rise rate and fast mix chamber retention time, but the coagulator clarifier has a longer slow mix chamber retention time than the lime softening clarifier. For this reason, the coagulator clarifier has a larger diameter than the cold lime softening clarifier, given the same influent flow rate. The two clarifiers also operate with different coagulants. This gives the coagulator clarifier a 2-5% gelatinous blowdown, whereas the cold lime softening clarifier has a 10-20% crystalline blowdown. Finally, the coagulator clarifier operates at a pH of 5.5 – 6.5 and the cold lime softening clarifier operates at a pH of 9.5 –10.5.
3.3.1
Process and Operations Overview
The P&ID for a cold lime softening clarifier is shown below in Figure 3.3-1 , and the equipment cutaway is shown on the opposite page in Figure 3.3-2. Major components have been identified.
Figure 3.3-1: Cold Lime So ften ing Clarifi er P&ID
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Treatment chemicals are added to the inlet flow before it enters the fast mix chamber. The chemicals and water are thoroughly mixed in the fast mix chamber to begin the coagulation process. A variable speed recirculator drive turns the recirculator impeller to mix the inlet water and chemicals. Coagulated particles flow upward into the slow mix chamber where polymer is added to facilitate flocculation. A portion of the agglomerated floc is internally recirculated back into the fast mix chamber from the bottom of the slow mix chamber to seed the coagulation process. Agglomerated floc flows downward from the slow mix chamber into the sedimentation chamber. In the sedimentation chamber, the flow reverses to upward and agglomerated floc settles. The treated water flows upward to the outlet collection launder, then out to use or to the next process. It is important that the settling rate of the floc particles is faster than the upflow rate of the water. This gives the floc particles a relative velocity that is downward. This downward relative velocity causes the particles to move to the bottom of the sedimentation chamber, not upward into the outlet collector. Floc leaving the clarifier with the treated water is called carryover floc. In some applications, a floc barrier is provided to reduce effluent turbidity caused by floc carryover. Agglomerated floc that settles to the bottom of the sedimentation chamber is called sludge. The sludge is moved to the concentrated sludge chamber by a scraper powered by the sludge scraper drive. The collected sludge is concentrated with thickening pickets in the concentrated sludge chamber and is periodically backflushed with clarifier inlet water to keep the sludge fluidized. The concentrated sludge is blown down (discharged) to waste through the clarifier blowdown piping. The blowdown is typically 0.2 to 1% of the influent flow rate for every 100 ppm of solids removed and has a 5-20% suspended solids concentration. The following is a discussion of the chemical reactions taking place as the water flows through the cold lime softening clarifier. Hydrated lime (Ca(OH)2 ) is added to the water to be treated prior to the fast mix chamber. This adds hydroxide and calcium to the inlet water. Ca(OH)2 + HCO3- → CaCO3 ↓ + H2O + OH-. Calcium carbonate, CaCO 3 , is formed with an attendant reduction in calcium hardness if sufficient bicarbonate alkalinity, HCO3- , is present in the inlet water. Magnesium hydroxide, Mg(OH)2 , is formed as the magnesium in the inlet water reacts with the hydroxide alkalinity, OH - , of the lime. This results in a reduction of the magnesium hardness of the inlet water. Mg + + 2OH- → Mg(OH)2↓
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However, the hydroxide alkalinity used to remove the magnesium hardness takes away hydroxide alkalinity from the conversion of bicarbonate alkalinity to carbonate alkalinity, CO32-. Therefore, more lime must be added to compensate. If sufficient bicarbonate alkalinity, greater than 35 ppm as CaCO 3 , is not present in the water, soda ash (Na 2CO3 ) must be added. Soda ash adds sodium and carbonate alkalinity to the water. Na2CO3 → Na2+ + CO32Therefore, soda ash makes carbonate alkalinity available for calcium hardness to react with to form calcium carbonate. The amount of lime and soda ash needed to reduce the inlet water hardness depends on the calcium hardness, magnesium hardness and alkalinity of the water being treated. Because the reactions are quite complicated, jar tests provide a more accurate prediction of removal performance and sludge production than calculations alone. Actual performance is an effluent with approximately 35 ppm as CaCO 3 of calcium hardness and 80% magnesium removal. Chemical feeders, with associated day tanks and/or bulk storage tanks, are needed for the feed of lime, polymer and soda ash (if required). Normally a single loop controller is used for pH control. Details of chemical feed and control are discussed in Section 3.5 beginning on page 3-48. The most difficult part of operating a cold lime softening clarifier is the formation and maintenance of the sludge. Formation occurs during the startup of the lime softening clarifier as agglomerated floc is accumulated in the sedimentation chamber. Once formed, the sludge layer is subject to upset. The upset could be due to changes in influent water quality, temperature changes (particularly if the lime softening clarifier is located outdoors) or disruptions in influent flow. Once upset, it can take several hours to re-establish the sludge layer.
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3.3.2
Equipment and Design Options
The clarifier vessel is designed in various diameters and straight shell heights. The operating water depth in the clarifier is one foot lower than the shell height. Final vessel height depends on site-specific requirements, such as land topography, treated water storage tank height and filter selection. It is furnished with chambers for fast mixing (coagulation), slow mixing (flocculation), settling (sedimentation) and sludge concentration. The cold lime softening clarifier is designed to do the following: • reduce hardness • accomplish efficient coagulation and flocculation of influent water • collect and dispose of sludge This is accomplished by conservatively selecting the sedimentation chamber (treated water surface) rise rate and slow mix chamber retention time for the clarifier, and providing robust recirculation and scraper features. The rise rate is the upward velocity of the water in the sedimentation chamber. The retention time is the amount of time the water spends in the slow mix chamber. The slow mix chamber and the sedimentation chamber are designed to achieve uniform flow through the clarifier.
3.3.2.1 Tank
Clarifiers come in 20-200 ft diameters and 13-21 ft shell heights. They are designed for operating temperatures from 35-150 oF. The shell is field fabricated of either steel or concrete. The floor is sloped concrete and access is by ladder. Clarifier tank options are stairway access, ice design, and API 650 design.
3.3.2.2 Internals
The internals on clarifiers having a diameter of 75 feet and smaller are supported from a bridge; larger units are supplied with a center post design. The clarifier uses a recirculator with a variable speed drive to fast mix the influent water and chemicals. The variable speed drive allows adjustment of the impeller speed to optimize the fast mix process.
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The scraper is designed to move precipitated suspended matter (sludge) to a collection chamber for disposal. The scraper drive is furnished with sufficient torque to handle dense solids. Optional scraper drive torque overload can be provided. Since the environment is not corrosive, the scraper material is carbon steel. An optional floc barrier can be provided to allow higher flows through the clarifier and reduce floc carryover. As shown previously in Figure 3.2-3 the floc barrier is constructed of plastic and requires adequate supports to prevent it from moving. The honeycomb structure of the floc barrier creates a larger effective settling area, reducing the external dimensions of the clarifier. It also gives the clarifier a barrier to floc carryover, resulting in lower effluent turbidity. ,
The purpose of the outlet launder is to collect treated water. On large diameter clarifiers, it is fed by a network of radial collector pipes, as shown previously in Figure 3.2-4 , that allow the treated water to be collected uniformly across the crosssectional area of the clarifier. On smaller units the water overflows directly into the launder.
3.3.2.3 Lining
The clarifier shell, the bridge and the internal components can be protected from corrosion with an epoxy polyamide lining or by being galvanized. The shell is rarely galvanized due to its size.
3.3.2.4 Paint
The clarifier can be painted with an epoxy polyamide primer and finish coat, an epoxy polyamide primer and siloxane finish, a siloxane epoxy and finish coat, or a custom primer and finish coat. The choice of materials depends on the environment in which the clarifier will be located.
3.3.2.5 Pipin g
Since the water is not corrosive, carbon steel is the standard piping material. Galvanized carbon steel is an option for sites desiring better corrosion resistance. The choice of materials for chemical feed piping depends on the chemical being used; stainless steel is quite common.
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3.3.2.6 Valves
Butterfly valves are the standard valves furnished for clarifier operation. The inlet isolation valve is manual; all others are air-operated. Air-operated valve closing action is slow to prevent water hammer. The backflush inlet and blowdown outlet valves have travel stops.
3.3.2.7 Ins trum en tation
An optional turbidity analyzer is provided to measure clarifier effluent turbidity.
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3.3.3
App lication and D esig n
3.3.3.1 Ap pli cation
Cold lime softening clarifiers remove suspended matter and reduce hardness. Lime floc is heavy, granular in nature and abrasive. This is important because the nature of the floc factors into the cold lime softening clarifier recirculator and scraper drive power requirement, blowdown amount and scraper material abrasive resistance. Table 3.3-1 below gives the performance summary for cold lime softening clarifiers.
Basic Equipment Select ion Cold Lime Sof t ening Clar if ier Perf or mance Summar y Const it uent
Units
Inf luent : Lime
Ef f luent: Lime
Tur bidit y
NTU
3000
1-20
Ir on
ppm Fe
20
0.1
Mn
ppm Mn
20
0.1
Organics (dissolved) ppm O2 consumed
NO LIMIT
1-2
Oil (f loat ing)
ppm
10
0.5
TDS
ppm CaCO3
NO LIMIT
*
Chlor ine
Cl2
20
**
Har dness
ppm CaCO3
NO LIMIT
Ca 35ppm as CaCO3 Mg 80% r educt ion
Alkalinit y
ppm CaCO3
NO LIMIT
appro x . 10ppm OHas CaCO3
pH
STD units
NO LIMIT
9.5 - 10.5
* Reduced by the amount to calcium and magnesium bicar bonate which are precipitat ed. ** Chlorine is added to a clar if ier to kill bact er ia and o x idize organic contaminants. The e x tent to which t he chlorine cont ent is reduced is dependent upon the organic contamination of t he ra w water.
Table 3.3-1: Cold Lime So ften in g Clarifi er Performance Su mmary
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3.3.3.2 D esig n
Sedimentation chamber (treated water surface) rise rate and slow mix chamber retention time are the two main factors used in sizing cold lime softening clarifiers. Rise rate, which is the upward velocity of the treated water measured in gpm/ft 2 , determines the area of the clarifier treated water surface. Retention time determines the volume of the slow mix chamber. The area taken up by the slow mix chamber is not included in the treated water surface area. Therefore, the diameter of the clarifier is computed based on the sum of the areas of the treated water surface and the slow mix chamber. First, the slow mix chamber volume is calculated. It is a function of the clarifier flow rate and the slow mix chamber retention time. The retention time is the time it takes for the coagulation and flocculation chemical reactions to occur. The typical retention time is 20-30 minutes based on experience. The low end of the range is used when coagulation and flocculation chemical reactions are fast, the high end when the reactions are slow. For cold lime softening clarifiers, the reaction is fast, so 20 minutes is selected. The equation for calculation of the slow mix chamber volume is as follows: Slow Mix Chamber Volume = (Clarifier Flow rate)*(Retention Time) Second, the slow mix chamber area is calculated. It is a function of the slow mix chamber volume and height. Typical heights are 12-20 ft. The height selected depends on site-specific requirements, such as land topography, treated water storage tank height and filter selection. The equation for calculation of the slow mix chamber area is as follows: Slow Mix Chamber Area = Slow Mix Chamber Volume Slow Mix Chamber Height Third, based on the slow mix chamber volume and area calculated above, the diameter of the slow mix chamber is calculated as follows: (Slow Mix Chamber Diameter)2 = (4)(Slow Mix Chamber Area) 3.14 Fourth, the treated water surface area is calculated. It is a function of the clarifier flow rate and the treated water surface (sedimentation chamber) rise rate. Typical rise rates are 0.75-1.25 gpm/ft 2. Rise rates on the low end of the range are used when the settling speed of agglomerated floc is slow and/or the water temperature is low, rise rates on the high end when the agglomerated floc settles quickly and/or the
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water temperature is high. Cold lime softening clarifiers are commonly sized with a 1 gpm/ft2 rise rate. The calculation for treated water surface area is as follows: Treated Water Surface Area = Clarifier Flow rate Rise Rate Fifth, the total clarifier area is calculated. It is the sum of the treated water surface area and the slow mix chamber area. Total Clarifier Area = Treated Water Surface Area + Slow Mix Chamber Area Sixth, the clarifier diameter is calculated based on its total area. (Clarifier Diameter) 2 = (4)(Total Clarifier Area) 3.14
The following example sizes a 500 gpm cold lime softening clarifier. Slow Mix Chamber Volume = (500 gpm)*(20 min) 7.48 gal/ft3 = 1333 ft3 Slow Mix Chamber Area = 1333 ft 3 12 ft = 110 ft2 (Slow Mix Chamber Diameter)2 = (4)(110 ft2 ) 3.14 or d @ 12 ft Treated Water Surface Area = 500 gpm 1 gpm/ft2 = 500 ft2 Total Clarifier Area = 500 ft2 + 110 ft2 = 610 ft2 (Clarifier Diameter) 2 = d2 = (4)(610 ft2 ) 3.14 or d @ 28 ft
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Figure 3.3-3 below is a sketch of the cold lime softening clarifier with the
dimensions noted.
Figu re 3.3-3: Exampl e Cold Lime Sof tening Clarifie r Dime nsio ns
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3.4
Lamella Clarifier
3.4.1 Process and Operations Overview
The equipment cutaway for a lamella clarifier with major equipment identified is shown below in Figure 3.4-1 .
Slow Mix Chamber
Inclined Plates
A d a p t e d f r o m E c o d y n e
Fast Mix Chamber
Sedimentation Chamber
Figure 3.4-1 Lamella Clarifier
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Treatment chemicals are added to the inlet flow before it enters the fast mix chamber. The chemicals and water are thoroughly mixed in the fast mix chamber to begin the coagulation process. Clay, if required, is fed to the fast mix tank. A variable speed mixer is used to perform the mixing. Coagulated particles overflow into the slow mix chamber where polymer is added to facilitate flocculation. A second variable speed mixer is used to perform the mixing. Agglomerated floc flows downward from the slow mix chamber, through the channel between the inclined plate chamber and the exterior wall, to the bottom of the inclined plates. Here the flow reverses to upward and agglomerated floc settle into the concentrated sludge chamber as shown below in Figure 3.4-2. Treated water flows upward to the collection flume, then out to use or to the next process.
Flow from the Channel Between the Inclined Plate Chamber and Lamella Exterior Wall into the Inclined Plate Chamber
Flow Downward Through the Channel Between the Inclined Plate Chamber and Lamella Exterior Wall
Reverse Flow Up Inclined Plates
Flow from Channel to Inclined Plate Chamber
Figure 3.4-2 Flow into the Inclined Plates
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Agglomerated floc slide down the inclined plates to the concentrated sludge chamber. Concentrated sludge is blown down to waste through the clarifier blowdown piping. The blowdown is about 2 to 5% of the influent flow and has a 5 to 8% suspended solids concentration, the same as for the coagulator clarifier. The chemistry of the lamella clarifier is the same as that of the coagulator clarifier. Refer to Section 3.2.1 on page 3-16. Chemical feeders, with associated day tanks and/or bulk storage tanks, are needed for the feed of disinfectant, coagulant, clay, pH adjustment chemical and polymer. A single loop controller is normally used for control of the pH adjustment chemical feed. Details of chemical feed and control are discussed in Section 3.5, beginning on page 3-48. The most difficult part of operating a lamella clarifier is solids carryover. This occurs when agglomerated floc is swept up through the inclined plates and carried out with the effluent. Solids carryover can be due to changes in influent water quality, temperature changes (particularly if the lamella clarifier is located outdoors), or disruptions in influent flow. This is usually eliminated by adjusting the chemical feed rates, or by reducing the flow rate of the lamella clarifier.
3.4.2
Equipment and Design Options
The lamella clarifier design includes a fast mix tank, slow mix tank, inclined plate tank and sludge concentration tank (chamber). The lamella inclined plate tank is provided with 55 º inclined plates and the sludge concentration tank with a sludge thickening picket. Inclined plates are used to separate agglomerated floc from the water and are designed to achieve uniform flow. The lamella clarifier is designed to do the following: • accomplish efficient coagulation and flocculation of influent water • collection and disposal of sludge This is accomplished by conservatively selecting the inclined plate rise rate and retention time in the slow mix tank, and providing robust mixer and sludge concentration features. The rise rate is the upward velocity of the water in the inclined plates. The retention time is the amount of time the water spends in the slow mix tank (chamber).
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3.4.2.1 Tanks
The tanks are designed for operating temperatures from 35-150 ºF. The standard material of construction is carbon steel. Optional materials are FRP, concrete, and stainless steel. Tank access is by ladder (optional). The fast mix tank provides one minute of retention time, the slow mix tank 30 minutes (the same as the coagulator clarifier).
3.4.2.2 Internals
The clarifier uses mixers with variable speed drives to fast mix and slo w mix the influent water and chemicals. The variable speed drive allows adjustment of the impeller speed to optimize mixing. Inclined plates are furnished in FRP material and are removable.
3.4.2.3 Lining
The fast mix tank, slow mix tank and inclined plate tank can be protected from corrosion with an epoxy polyamide lining or by being galvanized. The shells are rarely galvanized due to their size.
3.4.2.4 Paint
The clarifier can be painted with an epoxy polyamide primer and finish coat, an epoxy polyamide primer and siloxane finish, a siloxane epoxy and finish coat, or a custom primer and finish coat. The choice of materials depends on the environment in which the clarifier will be located.
3.4.2.5 Pipin g
Since the water is not corrosive, carbon steel is the standard piping material. Galvanized carbon steel is an option for sites desiring better corrosion resistance. The choice of materials for chemical feed piping depends on the chemical being used; stainless steel is quite common.
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3.4.2.6 Valves
Butterfly valves are the standard valves furnished for clarifier operation. The inlet isolation valve is manual, all others are air operated. Air operated valve closing action is slow to prevent water hammer. The backflush inlet and blowdown outlet valves have travel stops.
3.4.2.7 Ins trum en tation
An optional turbidity analyzer is provided to measure clarifier effluent turbidity.
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3.4.3
App lications and D esig n
3.4.3.1 Applications
Lamella clarifiers are used to remove suspended matter. They do not remove hardness. Ferric sulfate or ferric chloride may be substituted for alum if the treatment objective includes removal of trace metals. Table 3.4-1 below gives the lamella clarifier performance summary. It is identical to
the coagulator clarifier.
Basic Equipment Selection Lamella Clarif ier Perf ormance Summary Constituent
Units
Inf luent: Lime
Ef f luent: Lime
Turbidity
NTU
3000
1-20
Ir o n
ppm Fe
20
0.2
Mn
ppm Mn
20
0.5
Organics (dissolved) ppm O2 consumed
NO LIMIT
1-2
Oil (f loating)
ppm
10
0.5
TDS
ppm CaCO3
NO LIMIT
NO CHANGE
Chlorine
Cl2
20
*
Hardness
ppm CaCO3
NO LIMIT
NO CHANGE
Alkalinity
ppm CaCO3
90 NO
NO CHANGE
pH
STD units
NO LIMIT
5.5 - 6.5
* Chlorine is added t o a clar if ier t o kill bact er ia and o x idize or ganic contaminant s. The e x tent to which t he chlorine cont ent is reduced is dependent upon the or ganic contamination of t he ra w water.
Table 3.4-1 Lamel la Clarifi er Performance Su mm ary
The lamella clarifier is selected over the coagulator clarifier when either space or cost considerations are paramount. The lamella clarifier occupies a much smaller footprint than the coagulator clarifier and is significantly easier to install in the field.
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3.4.3.2 D esig n
Retention time in the fast mix tank, retention time in the slow mix tank, the rise rate in the inclined plates, and the retention time in the sludge concentration tank are the main factors used to size the lamella clarifier. Retention time determines the volume of the fast mix tank and slow mix tank. The area taken up by the fast mix tank in the slow mix tank is neglected when determining slow mix chamber area because it is so small when compared to the slow mix tank diameter. Rise rate, which is the upward velocity of the treated water measured in gpm/ft 2 , determines the area of the inclined plates. First, the fast mix tank volume is calculated. It is a function of the clarifier flow rate and the fast mix tank retention time. The retention time is the time it takes for the coagulation chemical reaction to occur. The typical retention time is 1 minute, based on experience. The equation for calculation of the fast mix tank volume is as follows: Fast Mix Tank Volume = (Clarifier Flow rate)*(Fast Mix Retention Time) Second, the slow mix tank volume is calculated. It is a function of the clarifier flow rate and the slow mix tank retention time. The retention time is the time it takes for the flocculation chemical reaction to occur. The typical retention time is 20 minutes. The equation for calculation of the fast mix tank volume is as follows: Slow Mix Tank Volume = (Clarifier Flow rate)*(Slow Mix Retention Time) Third, the slow mix tank area is calculated. It is a function of the slow mix tank volume and height. Typical heights are 12-20 ft. The height selected depends on site-specific requirements, such as land topography, treated water storage tank height, and filter selection. The equation for calculation of the slow mix tank area is as follows: Slow Mix Tank Area = Slow Mix Tank Volume Slow Mix Tank Height Fourth, the slow mix tank diameter is calculated based on its area. (Slow Mix Tank Diameter) 2 = (4)(Slow Mix Tank Area) 3.14 Fifth, the fast mix tank area is calculated based on the selected slow mix tank height. Fast Mix Tank Area = Fast Mix Tank Volume Slow Mix Tank Height 3-43
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Sixth, the fast mix tank diameter is calculated based on its area. (Fast Mix Tank Diameter) 2 = (4)(Fast Mix Tank Area) 3.14 Seventh, the inclined plate surface area is calculated. It is a function of the clarifier flow rate and the inclined plate rise rate. Typical rise rates are 0.25-0.50 gpm/ft 2. Rise rates on the low end of the range are used when the settling speed of agglomerated floc is slow and/or the water temperature is low, rise rates on the high end when the agglomerated floc settles quickly and/or the water temperature is high. Lamella clarifiers are commonly sized with a 0.25 gpm/ft 2 rise rate. The calculation for inclined plate area is as follows: Inclined Plate Surface Area =
Clarifier Flow rate ___ Inclined Plate Rise Rate
Eighth, the number of inclined plates is calculated based on the inclined plate surface area and the surface area per plate. Individual plates are 4 ft by 8 ft. (Number of Inclined Plates) = Inclined Plate Surface Area Area per Plate Ninth, the number of stacked plates is calculated. The height of the stacked plates should be slightly higher than the slow mix tank height for good hydraulics. Number of Stacked Plates = Slow Mix Tank Height 8sin(55o ), rounded up to the next integer Tenth, the height of the inclined plate tank is calculated based on the number of stacked plates. Inclined Plate Tank Height = 8(Number of Stacked Plates)sin(55 o ) Eleventh, the horizontal projection of the inclined plate tank is calculated based o n the number of stacked plates. Inclined Plate Horizontal Projection = 8(Number of Stacked Plates)cos(55 o ) Twelfth, the number of inclined plate columns is calculated based on the slow mix tank diameter.
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Number of Plate Columns = Slow Mix Tank diameter 4 rounded up to the next integer Thirteenth, the inclined plate tank width is calculated based on the number of columns. Inclined Plate Tank Width = 4(Number of Plate Columns) Fourteenth, the number of plate rows is calculated based on the number of plates, number of stacked plates, and number of plate columns. Number of Plate Rows = (Number of Plates)/(Number ofStacked Plates) Number of Plate Columns rounded up to the next integer Fifteenth, the sludge concentration tank volume is calculated based on its retention time. The retention time is the amount of time sludge is concentrated before being blown down to waste. The time selected should be approximately equal to the retention time of the slow mix tank. Sludge Concentration Tank Volume = (Clarifier Flow rate)(Sludge Concentration Retention Time) Last, the sludge concentration tank height is calculated based o n a diameter equal to that of the slow mix tank. Sludge Concentration Tank Height = 4(Sludge Concentration Tank Volume) 3.14(Slow Mix Tank Diameter) 2 Parameters in this calculation can be iterated to design a well proportioned, compact lamella clarifier. The following example sizes a 500 gpm lamella clarifier:
Fast Mix Tank Volume = (500 gpm)*(1 min) 7.48 gal/ft3 = 65 ft3 Slow Mix Tank Volume = (500 gpm)*(30 min) 7.48 gal/ft3 = 2000 ft3 3-45
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Slow Mix Tank Area = 2000 ft3 12 ft = 165 ft2 (Slow Mix Tank Diameter) 2 = (4)(165 ft2 ) 3.14 or d ≅ 15 ft Fast Mix Tank Area = 65 ft3 12 ft ≅ 5 ft2 (Fast Mix Tank Diameter) 2 = (4)(5 ft3 ) 3.14 or d ≅ 2 ft Inclined Plate Surface Area =
500 gpm___ ( 0.25 gpm/ft2 ) = 2000 ft2
(Number of Inclined Plates) = 2000 ft2 32 ft2 = 62 Number of Stacked Plates =
12 ft__ 8sin(55o ) ft = 2
Inclined Plate Tank Height = 8(2 ft)sin(55 o ) ≅ 13 ft Inclined Plate Horizontal Projection = 8(2 ft)cos(55 o ) ≅ 9 ft Number of Plate Columns = 15 ft 4 = 4 ft Inclined Plate Tank Width = 4(4 ft) = 16 ft Number of Plate Rows = (62)/(2) 4 =8
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Sludge Concentration Tank Volume = (500 gpm)*(30 min) 7.48 gal/ft3 = 2000 ft3
Sludge Concentration Tank Height = 4(2000 ft3 ) 3.14(15 ft)2 = 12 ft Figure 3.4-3 below is a sketch of the lamella clarifier with these dimensions noted.
15’ 16’
13’
12’
9’ 2’ Figure 3.4-3: Example Lamella Clarifier Dimensions
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3.5
Chemical Feed Equip men t
Clarifiers require chemical feed systems for proper operation. As these feeders often differ from the feeders used elsewhere in a water treatment system, they are described here. Refer to Ancillary Equi pme nt Section 9.3 f or more details on chemical feeders.
3.5.1
Process and Ope ration s Ove rvie w
Chemicals are required to disinfect and coagulate clarifier influent water, adjust the pH of the coagulated water for optimum flocculation and facilitate floc settling. Sodium hypochlorite disinfectant and alum coagulant are usually delivered in liquid form by tanker trucks. Hydrated lime is delivered in powder form by tanker trucks. Bentonite clay is delivered in bags. Polymer is delivered dry, in pails or cardboard shipping containers. For the coagulator clarifier and lamella clarifier, sodium hypochlorite, alum and caustic liquids are fed directly to the inlet piping with metering pumps for disinfection, coagulation and pH control. Dry clay is mixed with water and fed as a slurry to the fast mix tank, when required, for floc formation. Polymer is mixed in a measuring tank and fed with a metering pump to the slow mix tank to facilitate formation of agglomerated floc. For the cold lime softening clarifier, powdered lime is mixed with water to form a slurry and fed to the inlet piping with a centrifugal pump. Caustic for pH control and clay for floc formation are not required. Polymer is mixed in a measuring tank and fed with a metering pump to the slow mix tank to facilitate formation of agglomerated floc. Each chemical must be stored or prepared and injected into the clarifer at the appropriate rate.
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3.5.2
Equipment and Design Options
Clarifier chemical feed systems include bulk chemical storage tanks, measuring tanks, metering pumps and dry feeders. Sodium hypochlorite and alum are stored in vertical storage tanks and fed using metering pumps. Hydrated lime is stored in a silo, slurried with water and fed with a centrifugal pump. Caustic for pH adjustment is taken from a bulk liquid caustic storage tank. Polymer is provided as a liquid or prepared by blending dry powder with water in a measuring tank. Fig ure 3.5-1 below is a schematic for a typical chemical feed system arrangement for alum.
C o u r t e s y o f M i l t o n R o y
Figure 3.5-1 Typical Chemical Feed System Arrangement
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3.5.3
Application and Design
3.5.3.1 Ap pli cation
Chemical feed systems are required for coagulators, cold lime softening clarifiers and lamella clarifiers.
3.5.3.2 D esig n 3.5.3.2.1
Bulk Chemical Storage Tanks
Vertical storage tanks are provided for sodium hypochlorite, alum and Bentonite clay solutions. Silos are provided for hydrated lime The size of these tanks depends on the capacity of the clarifier and project/site specific requirements. .
3.5.3.2.2
Measuring (Batch) Tanks
Polymer is prepared by blending dry polymer and water to achieve a 1% polymer concentration, by weight.
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3.5.3.2.3
Metering Pumps
The pumps are commonly designed for the following feed rates: • • •
Sodium Hypochlorite Alum Polymer
5-10 ppm 40-60 ppm 1-2 ppm
Figure 3.5-2 below shows a metering pump with stroke control.
C o u r t e s y o f M i l t o n R o y
Figure 3.5-2 Metering Pump with Stroke Control
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Metering pumps are sized based on the clarifier flow rate and the chemical dosage. Feed rate, lb/hr = (Clarifier Flow rate, gpm)(Chemical Dosage, ppm)(8.33 lb/gal)(60 min/hr) 1,000,000 lb/million lbs Flow rate, gph = Feed rate, lbs/hr Density, lb/gal For a 500 gpm clarifier with a 40 ppm alum dosage, the feed rate is: Feed rate = (500 gpm)(40 ppm)(8.33 lb/gal)(60min/hr) 1,000,000 lb/million lbs = 10 lbs/hr Commercial alum is provided as a 26% solution with a specific gravity of 1.3. The density is 2.8 lbs/gal. The flow rate of alum is: Flow rate = 10 lbs/hr/2.8 lbs/gal = 3.5 gph A metering pump system with the ability to inject at least 3.5 gph of 26% alum is required.
3.5.3.2.4
Caustic Metering Pump
Positive displacement metering pumps are also used to feed caustic into the clarifier for pH control.
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3.5.3.2.5
Lime Feeder
A dry feeder, as shown below in Figure 3.5-3 is used to meter hydrated lime to a batch tank. There it is slurried with water for feed to a cold lime softening clarifier using a centrifugal pump. ,
C o u r t e s y o f M e t a l f a b
Figu re 3.5-3 Lime Feed er
Lime feeders are sized based on the clarifier flow rate and the chemical dosage. Feed rate, lb/hr = (Clarifier Flow rate, gpm)(Chemical Dosage, ppm)(8.33 lb/gal)(60 min/hr) 1,000,000 lb/million lbs For a 500 gpm clarifier with a 40 ppm lime dosage, the feed rate is, Feed rate = (500 gpm)(40 ppm)(8.33 lb/hr)(60min/hr) 1,000,000 lb/million lbs = 10 lbs/hr
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