clarifier manual

Clarification 34a_m3_r0 3-1

CLARIFICATION

Contents

3.1 Fundamentals ....................................................................................................................... 63.1.1 Description .................................................................................................................. 63.1.2 Coagulation ................................................................................................................. 83.1.3 Flocculation ............................................................................................................... 123.1.4 Sedimentation ........................................................................................................... 133.1.5 Jar Testing .................................................................................................................. 133.1.6 The Rule and Exceptions to the Rule ................................................................... 143.1.7 Equipment ................................................................................................................. 15

3.2 Coagulator Clarifier ........................................................................................................... 163.2.1 Process and Operations Overview........................................................................ 163.2.2 Equipment Design and Options ........................................................................... 18

3.2.2.1 Tank ................................................................................................................... 193.2.2.2 Internals ........................................................................................................... 193.2.2.3 Lining ................................................................................................................ 213.2.2.4 Paint .................................................................................................................. 213.2.2.5 Piping ................................................................................................................ 223.2.2.6 Valves ................................................................................................................ 223.2.2.7 Instrumentation .............................................................................................. 22

3.2.3 Application and Design .......................................................................................... 223.2.3.1 Application ...................................................................................................... 223.2.3.2 Design ............................................................................................................... 24

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Clarification 34a_m3_r0 3-2

3.3 Cold Lime Softening Clarifier ......................................................................................... 273.3.1 Process and Operations Overview........................................................................ 273.3.2 Equipment and Design Options ........................................................................... 30

3.3.2.1 Tank ................................................................................................................... 303.3.2.2 Internals ........................................................................................................... 303.3.2.3 Lining ................................................................................................................ 313.3.2.4 Paint .................................................................................................................. 313.3.2.5 Piping ................................................................................................................ 313.3.2.6 Valves ................................................................................................................ 323.3.2.7 Instrumentation .............................................................................................. 32

3.3.3 Application and Design .......................................................................................... 333.3.3.1 Application ...................................................................................................... 333.3.3.2 Design ............................................................................................................... 34

3.4 Lamella Clarifier ................................................................................................................ 373.4.1 Process and Operations Overview........................................................................ 373.4.2 Equipment and Design Options ........................................................................... 39

3.4.2.1 Tanks ................................................................................................................. 403.4.2.2 Internals ........................................................................................................... 403.4.2.3 Lining ................................................................................................................ 403.4.2.4 Paint .................................................................................................................. 403.4.2.5 Piping ................................................................................................................ 403.4.2.6 Valves ................................................................................................................ 413.4.2.7 Instrumentation .............................................................................................. 41

3.4.3 Applications and Design ........................................................................................ 423.4.3.1 Applications .................................................................................................... 423.4.3.2 Design ............................................................................................................... 43

3.5 Chemical Feed Equipment ............................................................................................... 483.5.1 Process and Operations Overview........................................................................ 483.5.2 Equipment and Design Options ........................................................................... 493.5.3 Application and Design .......................................................................................... 50

3.5.3.1 Application ...................................................................................................... 503.5.3.2 Design ............................................................................................................... 50

3.5.3.2.1 Bulk Chemical Storage Tanks ................................................................. 503.5.3.2.2 Measuring (Batch) Tanks ......................................................................... 503.5.3.2.3 Metering Pumps ........................................................................................ 513.5.3.2.4 Caustic Metering Pump ........................................................................... 523.5.3.2.5 Lime Feeder ................................................................................................ 53

3.6 Resources ....................................................................................................................... 54

Clarification 34a_m3_r0table of contents

In Module 1, the reasons for water purification are discussed. In Module 2, thefundamentals of water chemistry are reviewed. In this module these ideas andconcepts 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 andindustrial wastewater. In essence, it makes “turbid” water “clear,” as shown below inFigure 3.1-1.

Clarification also reduces iron, manganese, organic material, oil and color. It can beconfigured to remove hardness. It is the first step in surface water treatment,because the processes that follow normally require clear and colorless influent water.For the same reasons, clarification is often the last step before discharge ofwastewater to a receiving stream.

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Module 3CLARIFICATION

Figure 3.1-1: Turbid and Clarified Water

Clarificationtable of contents

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Turbidity is the measurement of light transmission and reflection through a sampleof water as shown below in Figure 3.1-2. It is an indirect measurement of theamount of suspended matter in water. Turbidity is measured with a turbidimeter asshown below in Figure 3.1-3.

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Figure 3.1-3: Turbidimeter

Figure 3.1-2: Turbidity Measurement

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Clarification uses chemical addition and the sedimentation process to removesuspended matter from water, as discussed below. Figure 3.1-4 below shows acollage of different clarifier designs that can be selected for use in a water purificationsystem.

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Figure 3.1-4: Collage of Clarifier Designs

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Coagulator Clarifier Cold Lime Softening Clarifier

Collage ofClarifier Designs

Lamella Clarifier


3.1 Fundamentals

This section reviews the fundamentals of clarification technology. Coagulation,flocculation and sedimentation phenomena are discussed. The use of polymers andjar testing to optimize clarification is presented. Rules of clarification and exceptionsto the rules are noted.

3.1.1 Description

Clarification removes suspended matter from water. Surface waters requireclarification because they have moderate to high levels of suspended matter. Wellwaters do not require clarification because they have low levels of suspended matter.

As shown below in Figure 3.1-5, the suspended matter in water includes two kindsof particles:

• Settleable Particles (macroparticles, typically visible to the eye)

• Non-Settleable Particles (microparticles, normally visible through a microscope)

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Figure 3.1-5: Suspended Matter in Water

Non-Settleable Particles Visible Though a Microscope

Settleable Macroparticles Visible to the Eye


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 thesettleable particles (suspended matter) have settled out. Thewater itself is clear, indicating an absence of suspended matter(turbidity). If non-settleable particles had been in the water, thewater would not be clear. This “turbidity” would have indicatedthe presence of non-settleable particles. Turbidity is an indirectmeasurement of the amount of suspended matter (settleableparticles and non-settleable particles) in water.

Clarification uses chemicals and sedimentation to removesuspended matter (settleable particles and non-settleableparticles). Several steps are involved. First, coagulationdestabilizes the particle surface charge that keeps the particles insolution. Once destabilized, the particles no longer repel oneanother and come together as floc. Second, floc agglomerateinto larger particles. Polymers are used to enhance theflocculation process. Third, sedimentation causes agglomeratedfloc to settle out. The settled floc is collected and concentrated for discharge towaste, called clarifier blowdown, or recycled to the coagulation step, called sludgerecycle. Clarified water is collected and flows out of the clarifier.

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Figure 3.1-6:Suspended Matter

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3.1.2 Coagulation

The first step of the clarification process is coagulation, as shown below in Figure3.1-7. Particles in water have a naturally occurring negative charge. This causes themto repel each other and stay in suspension. When this charge is destabilized, theparticles no longer repel one another, and can come together in closer proximity. Achemical salt, called a coagulant, is mixed with the inlet water to destabilize thecharge. Common coagulants are aluminum sulfate (alum), ferric sulfate and ferricchloride.

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The coagulants provide a positive charge, in the form of metallic cations, thatdestabilize the natural negative charge of the particles. The metallic cations combinewith hydroxide in the water to form a metallic hydroxide that is an insolublecompound. The destabilized particles and metal hydroxide precipitates agglomerateinto small, visible particles called floc. Color, organic matter and colloids, includingcolloidal silica, are removed by becoming bound up in the floc. The precisemechanism for removal- absorption, adsorption, co-precipitation, or a combination-is not fully understood.

Figure 3.1-7: Coagulation

Floc

Floc


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The addition of too much coagulant can cause the suspended matter to be re-dispersed with the opposite charge. As shown below in Figure 3.1-8, the amount ofremoval is dependent upon the coagulant dosage and the pH.

Alum (aluminum sulfate), ferric sulfate and ferric chloride coagulants are acidic saltsand decrease the pH of the influent water. Because of this, the pH of the water mustbe 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 ofsuspended 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 greatestremoval of hardness, the pH range is 9.5 – 10.5.

Feed of coagulant alone does not produce satisfactory floc in waters having a lowsuspended matter concentration. In this instance, bentonite clay is added. Bentoniteclay creates an artificial base of settleable macroparticles that seed the growth of floc.

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Figure 3.1-8: Particle Residual as a Function ofCoagulant Dosage and pH

Residual

pH

40 ppm

60 ppm


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Polymers are added to reduce the amount of coagulant required, broaden theworking pH range and create denser, heavier floc that settles out more easily, asshown below in Figure 3.1-9. Polymers are long-chain organic compounds of highmolecular weight that bridge floc particles together or modify their surface charge.

In almost all cases, the water to be treated is disinfected with either gaseous chlorineor sodium hypochlorite. This oxidizes organic matter in the water that has taste andodor and certain metals, such as manganese and iron. When oxidized, theseconstituents are transformed into a form that can be removed during clarification.Their removal is important because they can cause fouling of process components.

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Figure 3.1-9: Floc Formed With Coagulant Alonevs. Floc Formed with Coagulant and Polymer

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Coagulation is carried out in a fast mix chamber, as shown below in Figure 3.1-10. Afast mix is required because the coagulant and water must be thoroughly mixed toallow the suspended matter and coagulant to come into contact with each other. If itis 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. Asflocculated water flows into the slow mix chamber, polymer is added.

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Figure 3.1-10: Coagulation Step

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Inlet Water

PolymerFeed

Fast MixChamber

Slow MixChamber

AlumFeed


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3.1.3 Flocculation

In the next step of clarification, the small floc (microfloc) is allowed to grow intolarger floc, called macrofloc or agglomerated floc. This process, called flocculation, isshown below in Figure 3.1-11. Flocculation is accomplished by gently stirring thecoagulated water to assure contact between microfloc particles and polymer. Thepolymer enhances agglomerated floc formation. As the agglomerated floc continuesto grow, it becomes denser and heavier, allowing it to settle.

Mixing too rapidly can create what is called floc shear. Shear is the breaking apart ofexisting floc particles. The agglomerated floc, or macrofloc, is sheared back intomicrofloc.

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Figure 3.1-11: Flocculation Step

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Agglomerated Floc

Floc

Floc

Floc

Polymer

Inlet Water

PolymerFeed

Fast MixChamber

AlumFeed

Slow MixChamber

34a_m3_r0 Clarificationtable of contents

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Figure 3.1-12: Sedimentation Step

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3.1.4 Sedimentation

The final step of the clarification process is sedimentation, as shown on the oppositepage in Figure 3.1-12. In this step, agglomerated floc settles out to form sludge andthe sludge is transported to the sludge concentration chamber by the scraper. Thesludge thickening pickets concentrate the sludge. The sludge is discharged to wastein a process called clarifier blowdown. Above the sludge, clarified water is collectedin the outlet launder and flows forward for use or further treatment.

3.1.5 Jar Testing

Jar tests are used to establish chemical dosage requirements and predict clarifiereffluent turbidity levels. They simulate the chemistry and physical operation of aclarifier. The test equipment is shown below in Figure 3.1-13. Varying dosages ofcoagulant, polymer and pH adjustment chemical (if required) are added to thebeakers on the gang stirrer. The water in the beaker is stirred for the amount of timeequivalent to the retention time of the fast mix chamber and slow mix chamber. Thefloc is then allowed to settle for the amount of time equivalent to the retention timeof the sedimentation chamber. Then the turbidity of the water is measured andrecorded along with the pH, stirring speed and relative volume of floc produced. Jartests are repeated with varying dosages of chemicals and alternate chemicals todetermine the appropriate combination of chemicals, pH and floc productionnecessary to obtain optimal performance. Jar tests should be performed prior to thefinal 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.

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Figure 3.1-13: Jar Test Equipment

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Gang Stirrer


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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 thecoagulant, 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 andthe 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 watersthat have a low suspended matter concentration. The clay acts as a macroparticlebase for macrofloc formation.

When treating waters with a high suspended matter concentration, replacement ofthe 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 acoagulant. The advisability of an alum substitution treatment program must beconfirmed by jar tests.

If the process downstream from the clarifier is reverse osmosis, particular attentionmust be paid to polymer selection and dosage. Polymer carries over from theclarifier, even if the water looks perfectly clear, and can be a membrane foulant. Thepotential for this is determined by running a Silt Density Index (SDI) on the clarifiereffluent, as described in Module 6 on reverse osmosis. Polymer carryover increasesSDI. 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 ROsystem. Many thin film membranes carry a negative surface charge (anionic).Cationic (positively charged) polymers can bind to the anionic membrane sites andcause irreversible fouling.

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3.1.7 Equipment

Clarification can be accomplished using several approaches. The equipment designsfall into two general categories. The first is the common clarifier, also called areactivator clarifier, coagulator clarifier, cold lime softener, or solids contact clarifier,depending upon the manufacturer and performance required. The second is theinclined plate, or lamella, design that uses inclined plates to accomplish thesedimentation function. All have a fast mix chamber to ensure good contactbetween the treatment chemicals and suspended matter for coagulation, a slow mixchamber allowing polymer addition to facilitate flocculation, a sedimentationchamber for settling and a concentrated sludge chamber for clarifier blowdown.Required disinfectant, coagulant and pH adjustment chemicals are fed before the fastmix chamber. Clay is fed to the fast mix chamber. Polymer is fed between the fastmix and slow mix chambers. Note that the solids contact clarifiers have internalsludge recycle, and the inclined plate clarifiers can be furnished with optionalexternal sludge recycle. The following sections present a process and operationsoverview, discuss equipment design and options and consider the application anddesign of each of these clarifiers.

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Figure 3.2-1: Coagulator Clarifier P&ID


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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 componentshave been identified.

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Figure 3.2-2: Coagulator Clarifier

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SlowMixTank

SludgeScraperDrive

Variable SpeedRecirculator Drive

FastMixTank

SludgeLayer Concentrated

Sludge ChamberRecirculated

Sludge


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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 thecoagulation process. A variable speed recirculator drive turns the recirculatorimpeller to mix the inlet water and chemicals. Coagulated particles flow upward intothe slow mix chamber where polymer is added to facilitate flocculation. A portion ofthe agglomerated floc is internally recirculated back into the fast mix chamber fromthe bottom of the slow mix chamber to seed the coagulation process. Agglomeratedfloc flows downward from the slow mix chamber into the sedimentation chamber.In the sedimentation chamber, the flow reverses to upward and agglomerated flocsettles. The treated water flows upward to the outlet collection launders, then out touse or to the next process.

It is important that the settling rate of the floc particles is faster than the upflow rateof the water. This gives the floc particles a relative velocity that is downward. Thisdownward relative velocity causes the particles to move to the bottom of thesedimentation chamber, not upward into the outlet collector. Floc leaving theclarifier with the treated water is called carryover floc. In some applications, a flocbarrier is provided to reduce effluent turbidity caused by floc carryover.

Agglomerated floc that settles to the bottom of the sedimentation chamber is calledsludge. The sludge is moved to the concentrated sludge chamber by a scraperpowered by the sludge scraper drive. The collected sludge is concentrated withthickening pickets in the concentrated sludge chamber and is periodicallybackflushed with clarifier inlet water to keep the sludge fluidized. Concentratedsludge 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 solidsconcentration.

The following is a discussion of the chemical reactions taking place as the waterflows through the clarifier. Alum (Aluminum Sulfate, Al2(SO4)3), is added to thewater to be treated in the fast mix chamber. This consumes hydroxide in the inletwater, contributes sulfate to the treated water and forms an aluminum hydroxideprecipitate (floc).

Al2(SO4)3 + 6H2O → 6H+ + 3SO42- + 2Al(OH)3 ↓

As an alternate to alum, ferric sulfate, Fe2(SO4)3, can be used. This consumeshydroxide in the inlet water, contributes sulfate to the treated water and forms aferric 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 theinfluent water. Ferric chloride (FeCl3) is an alternative to ferric sulfate and itsreaction with water is similar. The primary difference is that it contributes chloridesto the treated water rather than sulfates.

FeCl3 + 3H2O → 3H+ + 3Cl- + Fe(OH)3 ↓

Independent of the coagulant selected (alum, ferric sulfate or ferric chloride), thehydroxide consumed in the inlet water must be replaced by addition of an alkali,usually caustic (sodium hydroxide). The alkali adjusts the clarifier operating pH tothe proper range (5.5 to 6.5). Note that two coagulants, alum and ferric sulfate, addsulfate to the treated water and one, ferric chloride, adds chloride.

Chemical feeders, with associated day tanks and/or bulk storage tanks, are neededfor the feed of disinfectant, coagulant, clay, pH adjustment chemical and polymer.Normally a single loop controller is used for control of the pH adjustment chemicalfeed. Details of chemical feed and control are discussed in Section 3.5 beginning onpage 3-48.

The most difficult part of operating a coagulator clarifier is the formation andmaintenance of the sludge layer. Formation occurs during the startup of thecoagulator clarifier as agglomerated floc is accumulated in the sedimentationchamber. Once formed, the sludge layer is subject to upset. The upset could be dueto changes in influent water quality, temperature changes (particularly if thecoagulator 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. Theoperating water depth in the clarifier is one foot lower than the shell height. Finalvessel height depends on site-specific requirements, such as land topography, treatedwater storage tank height and filter selection. It is furnished with chambers for fastmixing (coagulation), slow mixing (flocculation), settling (sedimentation) and sludgeconcentration. 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 forthe clarifier and providing robust recirculation and scraper features. The rise rate isthe upward velocity of the water in the sedimentation chamber. The retention timeis the amount of time the water spends in the slow mix chamber. The slow mixchamber and the sedimentation chamber are designed to achieve uniform flowthrough the clarifier.

3.2.2.1 Tank

Clarifiers come in 20-200 ft diameters and 13-21 ft shell heights. They are designedfor operating temperatures from 35-150 ºF. The shell is field fabricated of either steelor concrete. The floor is sloped concrete and access is by ladder. Clarifier tankoptions 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 supportedfrom 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 influentwater and chemicals. The variable speed drive allows adjustment of the impellerspeed to optimize the fast mix process.

The scraper is designed to move precipitated suspended matter (sludge) to acollection chamber for disposal. The scraper drive is furnished with sufficient torqueto 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 clarifierand reduce floc carryover. As shown below in Figure 3.2-3, the floc barrier isconstructed of plastic and requires adequate supports to prevent it from moving. Thehoneycomb 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 tofloc carryover, resulting in lower effluent turbidity.

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Figure 3.2-3: Floc Barrier

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The purpose of the outlet launder (outlet collection header) is to collect treatedwater. On large diameter clarifiers, they are fed by a network of radial collectorpipes, as shown below in Figure 3.2-4, that allows the treated water to be collecteduniformly across the cross-sectional area of the clarifier. On smaller units the wateroverflows directly into the launder.

3.2.2.3 Lining

The clarifier shell, the bridge and the internal components can be protected fromcorrosion with an epoxy polyamide lining or by being galvanized. The shell is rarelygalvanized due to its size.

3.2.2.4 Paint

The clarifier can be painted with an epoxy polyamide primer and finish coat, anepoxy polyamide primer and siloxane finish, a siloxane epoxy and finish coat, or acustom primer and finish coat. The choice of materials depends on the environmentin which the clarifier will be located.

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Figure 3.2-4: Outlet Launder

Outlet Headers


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3.2.2.5 Piping

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 beingused; stainless steel is quite common.

3.2.2.6 Valves

Butterfly valves are the standard valves furnished for clarifier operation. The inletisolation valve is manual; all others are air operated. Air operated valve closingaction is slow to prevent water hammer.

The backflush inlet and blowdown outlet valves have travel stops.

3.2.2.7 Instrumentation

An optional turbidity analyzer is provided to measure clarifier effluent turbidity.

3.2.3 Application and Design

3.2.3.1 Application

Coagulator clarifiers are used to remove suspended matter. They do not removehardness. Ferric sulfate or ferric chloride may be substituted for alum if thetreatment objective includes removal of trace metals.

Aluminum flocs and iron flocs are gelatinous in nature, fragile and not abrasive. Thisis important because the nature of the floc factors into the power requirement for therecirculator and scraper drives the amount of blowdown and the abrasive resistanceof materials of construction. Compared to cold lime softening clarifiers, which havea 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 lowerpower 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.

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Table 3.2-1: Coagulator Clarifier Performance Summary


3.2.3.2 Design

Sedimentation chamber rise rate and slow mix chamber retention time are the twomain factors used in sizing coagulators. Rise rate, which is the upward velocity ofthe treated water measured in gpm/ft2, determines the area of the clarifier treatedwater surface. Retention time determines the volume of the slow mix chamber. Thearea taken up by the slow mix chamber is not included in the treated water surfacearea. Therefore, the diameter of the clarifier is computed based on the sum of theareas 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 flowrate and the slow mix chamber retention time. The retention time is the time it takesfor the coagulation and flocculation chemical reactions to occur. The typicalretention time is 20-30 minutes, based on experience. The low end of the range isused when coagulation and flocculation chemical reactions are fast; the high endwhen the reactions are slow. For coagulator clarifiers, the reaction is slow, so 30minutes is selected. The equation for calculation of the slow mix chamber volume isas follows:

Slow Mix Chamber Volume = (Flow rate)(Retention Time)

Second, the slow mix chamber area is calculated. It is a function of the slow mixchamber volume and height. Typical heights are 12-20 ft. The height selecteddepends on site-specific requirements, such as land topography, treated waterstorage tank height, and filter selection. The equation for calculation of the slow mixchamber 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, thediameter 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 clarifierflow 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 ofagglomerated floc is slow and/or the water temperature is low, rise rates on the highend when the agglomerated floc settles quickly and/or the water temperature is high.Coagulator clarifiers are commonly sized with a 1 gpm/ft2 rise rate. The calculationfor treated water surface area is as follows:

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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 surfacearea 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 ft3

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 thedimensions noted.

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Figure 3.2-5: Example Coagulator Clarifier Dimensions


3-27a

Figure 3.3-2: Cold Lime Softening Clarifier

Ad

apted

from

Eco

dyne

Slow Mix Tank Fast Mix Tank

ConcentratedSludge Chamber

Sludge Layer


3.3 Cold Lime Softening Clarifier

The coagulator clarifier and cold lime softening clarifier have several commonfeatures. They have the same straight shell height, treated water surface rise rate andfast mix chamber retention time, but the coagulator clarifier has a longer slow mixchamber retention time than the lime softening clarifier. For this reason, thecoagulator clarifier has a larger diameter than the cold lime softening clarifier, giventhe same influent flow rate. The two clarifiers also operate with different coagulants.This gives the coagulator clarifier a 2-5% gelatinous blowdown, whereas the coldlime softening clarifier has a 10-20% crystalline blowdown. Finally, the coagulatorclarifier operates at a pH of 5.5 – 6.5 and the cold lime softening clarifier operates ata pH of 9.5 –10.5.


The P&ID for a cold lime softening clarifier is shown below in Figure 3.3-1, and theequipment cutaway is shown on the opposite page in Figure 3.3-2. Majorcomponents have been identified.

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Figure 3.3-1: Cold Lime Softening Clarifier P&ID


34a_m3_r0

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 thecoagulation process. A variable speed recirculator drive turns the recirculatorimpeller to mix the inlet water and chemicals. Coagulated particles flow upward intothe slow mix chamber where polymer is added to facilitate flocculation. A portion ofthe agglomerated floc is internally recirculated back into the fast mix chamber fromthe bottom of the slow mix chamber to seed the coagulation process. Agglomeratedfloc flows downward from the slow mix chamber into the sedimentation chamber.In the sedimentation chamber, the flow reverses to upward and agglomerated flocsettles. The treated water flows upward to the outlet collection launder, then out touse or to the next process.

It is important that the settling rate of the floc particles is faster than the upflow rateof the water. This gives the floc particles a relative velocity that is downward. Thisdownward relative velocity causes the particles to move to the bottom of thesedimentation chamber, not upward into the outlet collector. Floc leaving theclarifier with the treated water is called carryover floc. In some applications, a flocbarrier is provided to reduce effluent turbidity caused by floc carryover.

Agglomerated floc that settles to the bottom of the sedimentation chamber is calledsludge. The sludge is moved to the concentrated sludge chamber by a scraperpowered by the sludge scraper drive. The collected sludge is concentrated withthickening pickets in the concentrated sludge chamber and is periodicallybackflushed with clarifier inlet water to keep the sludge fluidized. The concentratedsludge 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 ofsolids removed and has a 5-20% suspended solids concentration.

The following is a discussion of the chemical reactions taking place as the waterflows through the cold lime softening clarifier. Hydrated lime (Ca(OH)2) is added tothe water to be treated prior to the fast mix chamber. This adds hydroxide andcalcium to the inlet water.

Ca(OH)2 + HCO3- → CaCO3 ↓ + H2O + OH-.

Calcium carbonate, CaCO3, is formed with an attendant reduction in calciumhardness if sufficient bicarbonate alkalinity, HCO3

-, is present in the inlet water.Magnesium hydroxide, Mg(OH)2, is formed as the magnesium in the inlet waterreacts with the hydroxide alkalinity, OH-, of the lime. This results in a reduction ofthe magnesium hardness of the inlet water.

Mg+ + 2OH- → Mg(OH)2↓

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34a_m3_r0

However, the hydroxide alkalinity used to remove the magnesium hardness takesaway hydroxide alkalinity from the conversion of bicarbonate alkalinity to carbonatealkalinity, CO3

2-. Therefore, more lime must be added to compensate.

If sufficient bicarbonate alkalinity, greater than 35 ppm as CaCO3, is not present inthe water, soda ash (Na2CO3) must be added. Soda ash adds sodium and carbonatealkalinity to the water.

Na2CO3 → Na2+ + CO3

2-

Therefore, soda ash makes carbonate alkalinity available for calcium hardness toreact with to form calcium carbonate. The amount of lime and soda ash needed toreduce the inlet water hardness depends on the calcium hardness, magnesiumhardness and alkalinity of the water being treated. Because the reactions are quitecomplicated, jar tests provide a more accurate prediction of removal performance andsludge production than calculations alone. Actual performance is an effluent withapproximately 35 ppm as CaCO3 of calcium hardness and 80% magnesium removal.

Chemical feeders, with associated day tanks and/or bulk storage tanks, are neededfor the feed of lime, polymer and soda ash (if required). Normally a single loopcontroller is used for pH control. Details of chemical feed and control are discussedin Section 3.5 beginning on page 3-48.

The most difficult part of operating a cold lime softening clarifier is the formationand maintenance of the sludge. Formation occurs during the startup of the limesoftening clarifier as agglomerated floc is accumulated in the sedimentationchamber. Once formed, the sludge layer is subject to upset. The upset could be dueto changes in influent water quality, temperature changes (particularly if the limesoftening clarifier is located outdoors) or disruptions in influent flow. Once upset, itcan take several hours to re-establish the sludge layer.

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34a_m3_r0

3.3.2 Equipment and Design Options

The clarifier vessel is designed in various diameters and straight shell heights. Theoperating water depth in the clarifier is one foot lower than the shell height. Finalvessel height depends on site-specific requirements, such as land topography, treatedwater storage tank height and filter selection. It is furnished with chambers for fastmixing (coagulation), slow mixing (flocculation), settling (sedimentation) and sludgeconcentration. The cold lime softening clarifier is designed to do the following:

• reduce hardness


• collect and dispose of sludge

This is accomplished by conservatively selecting the sedimentation chamber (treatedwater surface) rise rate and slow mix chamber retention time for the clarifier, andproviding robust recirculation and scraper features. The rise rate is the upwardvelocity of the water in the sedimentation chamber. The retention time is theamount of time the water spends in the slow mix chamber. The slow mix chamberand the sedimentation chamber are designed to achieve uniform flow through theclarifier.

3.3.2.1 Tank

Clarifiers come in 20-200 ft diameters and 13-21 ft shell heights. They are designedfor operating temperatures from 35-150 oF. The shell is field fabricated of either steelor concrete. The floor is sloped concrete and access is by ladder. Clarifier tankoptions 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 supportedfrom 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 influentwater and chemicals. The variable speed drive allows adjustment of the impellerspeed to optimize the fast mix process.

3-30


34a_m3_r0

The scraper is designed to move precipitated suspended matter (sludge) to acollection chamber for disposal. The scraper drive is furnished with sufficient torqueto 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 clarifierand reduce floc carryover. As shown previously in Figure 3.2-3, the floc barrier isconstructed of plastic and requires adequate supports to prevent it from moving. Thehoneycomb 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 tofloc carryover, resulting in lower effluent turbidity.

The purpose of the outlet launder is to collect treated water. On large diameterclarifiers, it is fed by a network of radial collector pipes, as shown previously inFigure 3.2-4, that allow the treated water to be collected uniformly across the cross-sectional area of the clarifier. On smaller units the water overflows directly into thelaunder.

3.3.2.3 Lining

The clarifier shell, the bridge and the internal components can be protected fromcorrosion with an epoxy polyamide lining or by being galvanized. The shell is rarelygalvanized due to its size.

3.3.2.4 Paint


3.3.2.5 Piping


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34a_m3_r0

3.3.2.6 Valves

Butterfly valves are the standard valves furnished for clarifier operation. The inletisolation valve is manual; all others are air-operated. Air-operated valve closingaction is slow to prevent water hammer.




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34a_m3_r0


3.3.3.1 Application

Cold lime softening clarifiers remove suspended matter and reduce hardness. Limefloc is heavy, granular in nature and abrasive. This is important because the nature ofthe floc factors into the cold lime softening clarifier recirculator and scraper drivepower requirement, blowdown amount and scraper material abrasive resistance.

Table 3.3-1 below gives the performance summary for cold lime softening clarifiers.

3-33

Table 3.3-1: Cold Lime Softening Clarifier Performance Summary


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3.3.3.2 Design

Sedimentation chamber (treated water surface) rise rate and slow mix chamberretention 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/ft2,determines the area of the clarifier treated water surface. Retention time determinesthe volume of the slow mix chamber. The area taken up by the slow mix chamber isnot included in the treated water surface area. Therefore, the diameter of the clarifieris computed based on the sum of the areas of the treated water surface and the slowmix chamber.

First, the slow mix chamber volume is calculated. It is a function of the clarifier flowrate and the slow mix chamber retention time. The retention time is the time it takesfor the coagulation and flocculation chemical reactions to occur. The typicalretention time is 20-30 minutes based on experience. The low end of the range isused when coagulation and flocculation chemical reactions are fast, the high endwhen the reactions are slow. For cold lime softening clarifiers, the reaction is fast, so20 minutes is selected. The equation for calculation of the slow mix chamber volumeis 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 mixchamber volume and height. Typical heights are 12-20 ft. The height selecteddepends on site-specific requirements, such as land topography, treated waterstorage tank height and filter selection. The equation for calculation of the slow mixchamber 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, thediameter 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 clarifierflow rate and the treated water surface (sedimentation chamber) rise rate. Typicalrise rates are 0.75-1.25 gpm/ft2. Rise rates on the low end of the range are usedwhen the settling speed of agglomerated floc is slow and/or the water temperature islow, rise rates on the high end when the agglomerated floc settles quickly and/or the

3-34


water temperature is high. Cold lime softening clarifiers are commonly sized with a1 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 surfacearea 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 ft3

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

3-35


34a_m3_r0

Figure 3.3-3 below is a sketch of the cold lime softening clarifier with thedimensions noted.

3-36

Figure 3.3-3: Example Cold Lime Softening Clarifier Dimensions


34a_m3_r0

3.4 Lamella Clarifier


The equipment cutaway for a lamella clarifier with major equipment identified isshown below in Figure 3.4-1.

3-37

Figure 3.4-1 Lamella Clarifier

Ad

apted

from

Eco

dyneInclined

Plates

SedimentationChamber

Slow MixChamber

Fast MixChamber


34a_m3_r0

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 thecoagulation process. Clay, if required, is fed to the fast mix tank. A variable speedmixer is used to perform the mixing. Coagulated particles overflow into the slowmix chamber where polymer is added to facilitate flocculation. A second variablespeed mixer is used to perform the mixing. Agglomerated floc flows downward fromthe slow mix chamber, through the channel between the inclined plate chamber andthe exterior wall, to the bottom of the inclined plates. Here the flow reverses toupward and agglomerated floc settle into the concentrated sludge chamber as shownbelow in Figure 3.4-2. Treated water flows upward to the collection flume, then outto use or to the next process.

3-38

Figure 3.4-2 Flow into the Inclined Plates

Flow from the Channel Betweenthe Inclined Plate Chamberand Lamella Exterior Wall

into the Inclined Plate Chamber

Flow Downward Through theChannel Between the InclinedPlate Chamber and Lamella

Exterior Wall

Reverse Flow UpInclined PlatesFlow from Channel to

Inclined Plate Chamber


Agglomerated floc slide down the inclined plates to the concentrated sludgechamber. Concentrated sludge is blown down to waste through the clarifierblowdown piping. The blowdown is about 2 to 5% of the influent flow and has a 5to 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 neededfor the feed of disinfectant, coagulant, clay, pH adjustment chemical and polymer. Asingle loop controller is normally used for control of the pH adjustment chemicalfeed. Details of chemical feed and control are discussed in Section 3.5, beginning onpage 3-48.

The most difficult part of operating a lamella clarifier is solids carryover. This occurswhen agglomerated floc is swept up through the inclined plates and carried out withthe effluent. Solids carryover can be due to changes in influent water quality,temperature changes (particularly if the lamella clarifier is located outdoors), ordisruptions in influent flow. This is usually eliminated by adjusting the chemical feedrates, or by reducing the flow rate of the lamella clarifier.


The lamella clarifier design includes a fast mix tank, slow mix tank, inclined platetank and sludge concentration tank (chamber). The lamella inclined plate tank isprovided with 55º inclined plates and the sludge concentration tank with a sludgethickening picket. Inclined plates are used to separate agglomerated floc from thewater and are designed to achieve uniform flow. The lamella clarifier is designed todo the following:


• collection and disposal of sludge

This is accomplished by conservatively selecting the inclined plate rise rate andretention time in the slow mix tank, and providing robust mixer and sludgeconcentration features. The rise rate is the upward velocity of the water in theinclined plates. The retention time is the amount of time the water spends in theslow mix tank (chamber).

3-39


34a_m3_r0

3.4.2.1 Tanks

The tanks are designed for operating temperatures from 35-150 ºF. The standardmaterial of construction is carbon steel. Optional materials are FRP, concrete, andstainless steel. Tank access is by ladder (optional).

The fast mix tank provides one minute of retention time, the slow mix tank 30minutes (the same as the coagulator clarifier).

3.4.2.2 Internals

The clarifier uses mixers with variable speed drives to fast mix and slow mix theinfluent water and chemicals. The variable speed drive allows adjustment of theimpeller 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 fromcorrosion with an epoxy polyamide lining or by being galvanized. The shells arerarely galvanized due to their size.

3.4.2.4 Paint


3.4.2.5 Piping


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34a_m3_r0

3.4.2.6 Valves

Butterfly valves are the standard valves furnished for clarifier operation. The inletisolation valve is manual, all others are air operated. Air operated valve closingaction is slow to prevent water hammer.




3-41


34a_m3_r0

3.4.3 Applications and Design

3.4.3.1 Applications

Lamella clarifiers are used to remove suspended matter. They do not removehardness. Ferric sulfate or ferric chloride may be substituted for alum if thetreatment objective includes removal of trace metals.

Table 3.4-1 below gives the lamella clarifier performance summary. It is identical tothe coagulator clarifier.

The lamella clarifier is selected over the coagulator clarifier when either space or costconsiderations are paramount. The lamella clarifier occupies a much smallerfootprint than the coagulator clarifier and is significantly easier to install in the field.

3-42


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Table 3.4-1 Lamella Clarifier Performance Summary


3.4.3.2 Design

Retention time in the fast mix tank, retention time in the slow mix tank, the rise ratein the inclined plates, and the retention time in the sludge concentration tank are themain factors used to size the lamella clarifier. Retention time determines the volumeof the fast mix tank and slow mix tank. The area taken up by the fast mix tank in theslow mix tank is neglected when determining slow mix chamber area because it is sosmall when compared to the slow mix tank diameter. Rise rate, which is the upwardvelocity of the treated water measured in gpm/ft2, determines the area of the inclinedplates.

First, the fast mix tank volume is calculated. It is a function of the clarifier flow rateand the fast mix tank retention time. The retention time is the time it takes for thecoagulation chemical reaction to occur. The typical retention time is 1 minute, basedon 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 flowrate and the slow mix tank retention time. The retention time is the time it takes forthe 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 tankvolume and height. Typical heights are 12-20 ft. The height selected depends onsite-specific requirements, such as land topography, treated water storage tankheight, and filter selection. The equation for calculation of the slow mix tank area isas 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 VolumeSlow Mix Tank Height

3-43


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 clarifierflow rate and the inclined plate rise rate. Typical rise rates are 0.25-0.50 gpm/ft2. Riserates on the low end of the range are used when the settling speed of agglomeratedfloc is slow and/or the water temperature is low, rise rates on the high end when theagglomerated floc settles quickly and/or the water temperature is high. Lamellaclarifiers are commonly sized with a 0.25 gpm/ft2 rise rate. The calculation forinclined 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 surfacearea and the surface area per plate. Individual plates are 4 ft by 8 ft.

(Number of Inclined Plates) = Inclined Plate Surface AreaArea per Plate

Ninth, the number of stacked plates is calculated. The height of the stacked platesshould be slightly higher than the slow mix tank height for good hydraulics.

Number of Stacked Plates = Slow Mix Tank Height8sin(55o),

rounded up to the next integer

Tenth, the height of the inclined plate tank is calculated based on the number ofstacked plates.

Inclined Plate Tank Height = 8(Number of Stacked Plates)sin(55o)

Eleventh, the horizontal projection of the inclined plate tank is calculated based onthe number of stacked plates.

Inclined Plate Horizontal Projection = 8(Number of Stacked Plates)cos(55o)

Twelfth, the number of inclined plate columns is calculated based on the slow mixtank diameter.

3-44


Number of Plate Columns = Slow Mix Tank diameter 4


Thirteenth, the inclined plate tank width is calculated based on the number ofcolumns.

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


Fifteenth, the sludge concentration tank volume is calculated based on its retentiontime. The retention time is the amount of time sludge is concentrated before beingblown down to waste. The time selected should be approximately equal to theretention 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 on a diameter equal tothat 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, compactlamella 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


34a_m3_r0

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(55o) ≅ 13 ft

Inclined Plate Horizontal Projection = 8(2 ft)cos(55o) ≅ 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

3-46


34a_m3_r0

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.

3-47

Figure 3.4-3: Example Lamella Clarifier Dimensions

16’

15’

12’

2’

9’

13’


34a_m3_r0

3.5 Chemical Feed Equipment

Clarifiers require chemical feed systems for proper operation. As these feeders oftendiffer from the feeders used elsewhere in a water treatment system, they aredescribed here. Refer to Ancillary Equipment Section 9.3 for more details onchemical feeders.


Chemicals are required to disinfect and coagulate clarifier influent water, adjust thepH of the coagulated water for optimum flocculation and facilitate floc settling.Sodium hypochlorite disinfectant and alum coagulant are usually delivered in liquidform 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 cardboardshipping containers.

For the coagulator clarifier and lamella clarifier, sodium hypochlorite, alum andcaustic liquids are fed directly to the inlet piping with metering pumps fordisinfection, coagulation and pH control. Dry clay is mixed with water and fed as aslurry to the fast mix tank, when required, for floc formation. Polymer is mixed in ameasuring tank and fed with a metering pump to the slow mix tank to facilitateformation of agglomerated floc.

For the cold lime softening clarifier, powdered lime is mixed with water to form aslurry and fed to the inlet piping with a centrifugal pump. Caustic for pH control andclay for floc formation are not required. Polymer is mixed in a measuring tank andfed with a metering pump to the slow mix tank to facilitate formation ofagglomerated floc.

Each chemical must be stored or prepared and injected into the clarifer at theappropriate rate.

3-48


34a_m3_r0


Clarifier chemical feed systems include bulk chemical storage tanks, measuringtanks, metering pumps and dry feeders. Sodium hypochlorite and alum are stored invertical storage tanks and fed using metering pumps. Hydrated lime is stored in asilo, slurried with water and fed with a centrifugal pump. Caustic for pH adjustmentis taken from a bulk liquid caustic storage tank. Polymer is provided as a liquid orprepared by blending dry powder with water in a measuring tank. Figure 3.5-1below is a schematic for a typical chemical feed system arrangement for alum.

3-49

Figure 3.5-1 Typical Chemical Feed System Arrangement

Co

urtesy of M

ilton R

oy


34a_m3_r0


3.5.3.1 Application

Chemical feed systems are required for coagulators, cold lime softening clarifiers andlamella clarifiers.

3.5.3.2 Design

3.5.3.2.1 Bulk Chemical Storage Tanks

Vertical storage tanks are provided for sodium hypochlorite, alum and Bentonite claysolutions. Silos are provided for hydrated lime. The size of these tanks depends onthe 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% polymerconcentration, by weight.

3-50


3.5.3.2.3 Metering Pumps

The pumps are commonly designed for the following feed rates:

• Sodium Hypochlorite 5-10 ppm• Alum 40-60 ppm• Polymer 1-2 ppm

Figure 3.5-2 below shows a metering pump with stroke control.

3-51

Figure 3.5-2 Metering Pump with Stroke Control

Co

urtesy of M

ilton R

oy


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. Thedensity 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 isrequired.

3.5.3.2.4 Caustic Metering Pump

Positive displacement metering pumps are also used to feed caustic into the clarifierfor pH control.

3-52


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 abatch tank. There it is slurried with water for feed to a cold lime softening clarifierusing a centrifugal pump.

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

3-53

Figure 3.5-3 Lime Feeder

Co

urtesy of M

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3.6 Resources

Demineralization by Ion Exchange, Applebaum, Academic Press, 1968.Coagulants and Flocculants – Theory and Practice, Kim, Tall Oaks Publishing, 1995.Betz Handbook of Industrial Water Conditioning.Argo Scientific Engineers Manual.

clarifier manual

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