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8-1 Chapter 8 WASTEWATER TREATMENT TECHNOLOGIES T his section discusses a number of wastewater treatment technologies considered by EPA for the development of these guidelines and standards for the CWT Industry. Many of these technologies are being used currently at CWT facilities. This section also reviews other technologies with potential application in treating certain CWT pollutants of concern. Facilities in the CWT industry use a wide variety of technologies for treating wastes received for treatment or recovery operations and wastewater generated on site. The technologies are grouped into the following five categories for this discussion: C Best Management Practices, section 8.2.1; C Physical/Chemical/Thermal Treatment, section 8.2.2; C Biological Treatment, section 8.2.3; C Sludge Treatment and Disposal, section 8.2.4; and C Zero Discharge Options, section 8.2.5. The processes reviewed here include both those that remove pollutant contaminants in wastewater and those that destroy them. Using a wastewater treatment technology that removes, rather than destroys, a pollutant will produce a treatment residual. In many instances, this residual is in the form of a sludge, that, typically, a CWT further treats on site in preparation for disposal. Section 8.2.4 discusses technologies for dewatering sludges to concentrate them prior to disposal. In the case of other types of treatment residuals, such as spent activated carbon and filter media, CWT facilities generally send those off site to a vendor facility for management. TECHNOLOGIES CURRENTLY IN USE 8.1 EPA obtained information on the treatment technologies in use in the CWT industry from responses to the Waste Treatment Industry (WTI) Questionnaire, site visits, public comments to the original proposal and the 1996 Notice of Data Availability. As described in Section 4, of the estimated 205 CWT facilities, EPA has obtained detailed facility–specific technology information for 116 of the direct and indirect discharging CWT facilities. Although EPA has facility-specific information for 145 facilities, only 116 of these facilities provided technology information. The detail provided regarding the technology information differs depending on the source. Information for the 65 facilities that completed the WTI Questionnaire was the most explicit because the questionnaire contained a detailed checklist of wastewater treatment technologies, many of which are discussed in this section. Technology information from other sources, however, is much less descriptive. Table 8-1 presents treatment technology information by subcategory for the 116 indirect and direct discharging CWT facilities for which EPA has facility-specific treatment technology information. The information in Table 8-1 has not been scaled to represent the entire population of CWT facilities. Responses to the WTI Questionnaire provide the primary basis for the technology information for the metals and the organics subcategories. Comments to the 1996 Notice of Data Availability provide the primary source of the technology information for the oils subcategory. It should be noted that a number of facilities commingle different subcategory wastes for treatment. EPA has attributed these

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Page 1: Wastewater treatment technologies EPA.pdf

8-1

Chapter

8WASTEWATER TREATMENT TECHNOLOGIES

This section discusses a number ofwastewater treatment technologies

considered by EPA for the development of theseguidelines and standards for the CWT Industry.Many of these technologies are being used

currently at CWT facilities. This section alsoreviews other technologies with potentialapplication in treating certain CWT pollutants ofconcern.

Facilities in the CWT industry use a widevariety of technologies for treating wastesreceived for treatment or recovery operationsand wastewater generated on site. Thetechnologies are grouped into the following fivecategories for this discussion:

C Best Management Practices, section 8.2.1;C Physical/Chemical/Thermal Treatment,

section 8.2.2;C Biological Treatment, section 8.2.3;C Sludge Treatment and Disposal, section

8.2.4; andC Zero Discharge Options, section 8.2.5.

The processes reviewed here include boththose that remove pollutant contaminants inwastewater and those that destroy them. Using

a wastewater treatment technology that removes,rather than destroys, a pollutant will produce atreatment residual. In many instances, thisresidual is in the form of a sludge, that, typically,a CWT further treats on site in preparation fordisposal. Section 8.2.4 discusses technologiesfor dewatering sludges to concentrate them priorto disposal. In the case of other types oftreatment residuals, such as spent activatedcarbon and filter media, CWT facilities generally

send those off site to a vendor facility formanagement.

TECHNOLOGIES CURRENTLY IN USE 8.1

EPA obtained information on the treatmenttechnologies in use in the CWT industry fromresponses to the Waste Treatment Industry(WTI) Questionnaire, site visits, publiccomments to the original proposal and the 1996

Notice of Data Availability. As described inSection 4, of the estimated 205 CWT facilities,EPA has obtained detailed facility–specifictechnology information for 116 of the direct andindirect discharging CWT facilities. AlthoughEPA has facility-specific information for 145facilities, only 116 of these facilities providedtechnology information. The detail providedregarding the technology information differsdepending on the source. Information for the 65

facilities that completed the WTI Questionnairewas the most explicit because the questionnairecontained a detailed checklist of wastewatertreatment technologies, many of which arediscussed in this section. Technologyinformation from other sources, however, ismuch less descriptive.

Table 8-1 presents treatment technologyinformation by subcategory for the 116 indirectand direct discharging CWT facilities for which

EPA has facility-specific treatment technologyinformation. The information in Table 8-1 hasnot been scaled to represent the entire populationof CWT facilities. Responses to the WTIQuestionnaire provide the primary basis for thetechnology information for the metals and theorganics subcategories. Comments to the 1996Notice of Data Availability provide the primarysource of the technology information for the oilssubcategory. It should be noted that a number

of facilities commingle different subcategorywastes for treatment. EPA has attributed these

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treatment technologies to all appropriatesubcategories.

Table 8-1. Percent Treatment In-place by Subcategory and by Method of Wastewater Disposal

Metals Subcategory Oils Subcategory Organics Subcategory

Disposal Type Direct Indirect Direct Indirect Direct Indirect

Number of Facilities withTreatment Technology Data 91 411 31,2 801,3 41 141

Equalization4 78 68 100 65 75 71Neutralization4 89 73 100 61 100 57Flocculation4 44 51 100 48 75 57Emulsion Breaking 11 29 33 56 25 50Gravity-Assisted Separation 89 61 100 85 100 64Skimming4 22 27 100 58 25 57Plate/Tube Separation4 0 10 0 19 0 21Dissolved Air Flotation 22 5 33 23 50 0Chromium Reduction4 33 76 0 48 0 57Cyanide Destruction4 33 46 100 23 25 29Chemical Precipitation 78 88 0 34 25 64Filtration 44 32 33 19 25 21Sand Filtration4 11 15 0 16 0 21Mutimedia Filtration4 11 5 0 0 0 7Ultrafiltration 0 0 0 8 0 0Reverse Osmosis4 11 0 0 3 0 0Carbon Adsorption 22 12 67 18 0 21Ion Exchange4 0 2 0 0 0 0Air Stripping 0 7 0 11 0 0Biological Treatment 56 2 100 11 100 7Activated Sludge 33 0 100 0 100 0Sequencing BatchReactors4

0 2 0 0 0 7

Vacuum Filtration4 11 17 100 6 25 7

Pressure Filtration4 67 61 100 39 75 361Sum does not add to 116 facilities. Some facilities treat wastes in multiple subcategories.2Of the 3 direct discharging oils facilities for which EPA has facility-specific information, only onecompleted the WTI Questionnaire.3Of the 80 indirect discharging oils facilities for which EPA has facility-specific information, only 31completed the WTI Questionnaire.4Information for these technologies for the oils subcategory is based on responses to the WTI Questionnaireonly.

TECHNOLOGY DESCRIPTIONS 8.2Best Management Practices 8.2.1

In addition to physical/chemical treatmenttechnologies, CWT facilities employ a number of

ancillary means to prevent or reduce thedischarge of pollutants. These efforts are termed"best management practices. EPA believes thatCWT facilities should design best management

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practices in the CWT industry with the followingobjectives in mind:

C Maximize the amount of waste materials andresiduals that are recycled rather thandisposed as residuals, as wastewater, or aswaste material.

C Maximize recycling and reuse ofwastewaters generated on site.

C Minimize the introduction of uncontaminatedwastewaters into the treatment waste stream.

C Encourage waste generators to minimize themixing of different wastes.

C Segregate wastes for treatment particularlywhere waste segregation would improvetreatment performance and maximizeopportunities for recycling.

Waste segregation is one of the mostimportant tools available for maximizing wasterecycling and improving treatment performance.For example, separate treatment of wastes

containing different types of metals allows therecovery of the individual metals from theresultant sludges. Similarly, separate treatmentcollection and treatment of waste oils will allowrecycling. Many oils subcategory facilitiescurrently practice waste oil recycling.

Physical/Chemical/Thermal Treatment 8.2.2Equalization 8.2.2.1

GENERAL DESCRIPTION

The wastes received at many facilities in theCWT industry vary considerably in both strengthand volume. Waste treatment facilities oftenneed to equalize wastes by holding wastestreamsin a tank for a certain period of time prior totreatment in order to obtain a stable waste streamwhich is easier to treat. CWT facilitiesfrequently use holding tanks to consolidate smallwaste volumes and to minimize the variability ofincoming wastes prior to certain treatmentoperations. The receiving or initial treatmenttanks of a facility often serve as equalizationtanks.

The equalization tank serves manyfunctions. Facilities use equalization tanks toconsolidate smaller volumes of wastes so that,for batch treatment systems, full batch volumesare available. For continuous treatment systems,facilities equalize the waste volumes so that theymay introduce effluent to downstream processesat a uniform rate and strength. This dampensthe effect of peak and minimum flows.Introducing a waste stream with a more uniformpollutant profile to the treatment systemfacilitates control of the operation of downstreamtreatment units, resulting in more predictable anduniform treatment results. Equalization tanks areusually equipped with agitators or aerators wheremixing of the wastewater is desired and toprevent suspended solids from settling to thebottom of the unit. An example of effectiveequalization is the mixing of acid and alkalinewastes. Figure 8-1 illustrates an equalizationsystem.

EPA does not consider the use ofequalization tanks for dilution as a legitemateuse. In this context, EPA defines dilution as themixing of more concentrated wastes with greatervolumes of less concentrated wastes in a mannerthat reduces the concentration of pollutant in theconcentrated wastes to a level that enables thefacility to avoid treatment of the pollutant.

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WastewaterInfluent

EqualizedWastewaterEffluent

Equalization Tank

Figure 8-1. Equalization System Diagram

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INDUSTRY PRACTICE

EPA found equalization being used atfacilities in all of the CWT subcategories. Ofthe 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of equalization,44 operate equalization systems. Of these,approximately 44 percent emply unstirred tanksand 56 percent use stirred or aerated tanks.

The combining of separate waste receipts

in large receiving tanks provides for effectiveequalization even though it is not necessarilyrecognized as such. Nearly every facilityvisited by EPA performed equalization, eitherin tanks specifically designed for that purposeor in waste receiving tanks. Consequently,EPA has concluded that equalization isunderreported in the data base.

Neutralization 8.2.2.2

GENERAL DESCRIPTION

Wastewaters treated at CWT facilities have

a wide range of pH values depending on thetypes of wastes accepted. Untreatedwastewater may require neutralization toeliminate either high or low pH values prior tocertain treatment systems, such as biologicaltreatment. Facilities often use neutralizationsystems also in conjunction with certainchemical treatment processes, such as chemicalprecipitation, to adjust the pH of thewastewater to optimize treatment efficiencies.

These facilities may add acids, such as sulfuricacid or hydrochloric acid, to reduce pH, andalkalies, such as sodium hydroxides, to raisepH values. Many metals subcategory facilitiesuse waste acids and waste alkalies for pHadjustment. Neutralization may be performedin a holding tank, rapid mix tank, or anequalization tank. Typically, facilities useneutralization systems at the end of a treatmentsystem to control the pH of the discharge to

between 6 and 9 in order to meet NPDES andPOTW pretreatment limitations.

Figure 8-2 presents a flow diagram for atypical neutralization system.

INDUSTRY PRACTICE

EPA found neutralization systems in-placeat facilities identified in all of the CWTsubcategories. Of the 65 CWT facilities inEPA’s WTI Questionnaire data base thatprovided information concerning the use ofneutralization, 45 operate neutralizationsystems.

Flocculation/Coagulation 8.2.2.3

GENERAL DESCRIPTION

Flocculation is the stirring or agitation ofchemically-treated water to induce coagulation. The terms coagulation and flocculation areoften used interchangeably. More specifically,“coagulation” is the reduction of the netelectrical repulsive forces at particle surfacesby addition of coagulating chemicals, whereas“flocculation” is the agglomeration of thedestabilized particles by chemical joining andbridging. Flocculation enhances sedimentationor filtration treatment system performance byincreasing particle size resulting in increasedsettling rates and filter capture rates.

Flocculation generally precedessedimentation and filtration processes andusually consists of a rapid mix tank or in-linemixer, and a flocculation tank. The wastestream is initially mixed while a coagulantand/or a coagulant aid is added. A rapid mixtank is usually designed for a detention time of15 seconds to several minutes. After mixing,the coagulated wastewater flows to aflocculation basin where slow mixing of thewaste occurs. The slow mixing allows theparticles to agglomerate into heavier, moresettleable/filterable solids. Either mechanicalpaddle mixers or diffused air provides mixing. Flocculation basins are typically designed for adetention time of 15 to 60 minutes. Figure 8-3presents a diagram of a clarification systemincorporating coagulation and flocculation.

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WastewaterInfluent

NeutralizedWastewaterEffluent

Neutralization Tank

pH monitor/control

acid

caustic

Figure 8-2. Neutralization System Diagram

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Rapid MixTank

Coagulant

Influent

FlocculatingTank

Clarifier

Sludge

Effluent

Figure 8-3. Clarification System Incorporating Coagulation and Flocculation

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There are three different types of treatmentchemicals commonly used incoagulation/flocculation processes: inorganicelectrolytes, natural organic polymers, andsynthetic polyelectrolytes. The selection of thespecific treatment chemical is highly dependentupon the characteristics and chemicalproperties of the contaminants. Many CWTfacilities use bench-scale jar tests to determinethe appropriate type and optimal dosage ofcoagulant/flocculent for a given waste stream.

INDUSTRY PRACTICE

Chemical treatment methods to enhancethe separation of pollutants from water as asolid residual may include both chemicalprecipitation and coagulation/flocculation. Chemical precipitation is the conversion ofsoluble pollutants such as metals into aninsoluble precipitate and is describedseparately. Flocculation is often an integralstep in chemical precipitation, gravityseparation, and filtration. Of the 65 CWTfacilities in EPA’s WTI Questionnaire database that provided information concerning theuse of coagulation/flocculation, 31 operatecoagulation/flocculation systems. However,due to the integral nature of flocculation inchemical precipitation and coagulation, and theinterchangeable use of the terminology, the useof coagulation/flocculation at CWT facilitiesmay have been underreported.

Emulsion Breaking 8.2.2.4

GENERAL DESCRIPTION

One process used to treat emulsifiedoil/water mixtures is emulsion breaking. Anemulsion, by definition, is either stable orunstable. A stable emulsion is one where smalldroplets of oil are dispersed within the waterand are prevented from coalescing by repulsiveelectrical surface charges that are often a resultof the presence of emulsifying agents and/orsurfactants. In stable emulsions, coalescingand settling of the dispersed oil droplets would

occur very slowly or not at all. Stableemulsions are often intentionally formed bychemical addition to stabilize the oil mixture fora specific application. Some examples of stableemulsified oils are metal-working coolants,lubricants, and antioxidants. An unstableemulsion, or dispersion, settles very rapidly anddoes not require treatment to break theemulsion.

Emulsion breaking is achieved through theaddition of chemicals and/or heat to theemulsified oil/water mixture. The mostcommonly-used method of emulsion breakingis acid-cracking where sulfuric or hydrochloricacid is added to the oil/water mixture until thepH reaches 1 or 2. An alternative to acid-cracking is chemical treatment usingemulsion–breaking chemicals such assurfactants and coagulants. After addition ofthe treatment chemical, the tank contents aremixed. After the emulsion bond is broken, theoil residue is allowed to float to the top of thetank. At this point, heat (100 to 150o F) maybe applied to speed the separation process. The oil is then skimmed by mechanical means,or the water is decanted from the bottom of thetank. The oil residue is then further processedor disposed. A diagram of an emulsionbreaking system is presented in Figure 8-4.

INDUSTRY PRACTICE

Emulsion breaking is a common process inthe CWT industry. Of the 116 CWT facilitiesin EPA’s WTI Questionnaire and NOAcomment data base that provided informationconcerning the use of emulsion breaking, 49operate emulsion breaking systems. Forty-six of the 83 oils subcategory facilities in EPA’sdata base use emulsion-breaking. As such,EPA has concluded that emulsion breaking isthe baseline, current performance technologyfor oils subcategory facilities that treatemulsified oily wastes.

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Figure 8-4. Emulsion Breaking System Diagram

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Gravity Assisted Separation 8.2.2.5

1. GRAVITY OIL/WATER SEPARATION

GENERAL DESCRIPTION

Like emulsion breaking, another in-placetreatment process used to remove oil and greaseand related pollutants from oil/water mixtures isgravity separation. Unlike emulsion breaking,gravity separation is only effective for the bulkremoval of free oil and grease. It is not effectivein the removal of emulsified or soluble oils.Gravity separation is often used in conjunctionwith emulsion breaking at CWT facilities.

Gravity separation may be performed usingspecially designed tanks or it may occur withinstorage tanks. During gravity oil/waterseparation, the wastewater is held underquiescent conditions long enough to allow the oildroplets, which have a lower specific gravitythan water, to rise and form a layer on thesurface. Large droplets rise more readily thansmaller droplets. Once the oil has risen to thesurface of the wastewater, it must be removed.

This is done mechanically via skimmers, baffles,plates, slotted pipes, or dip tubes. Whentreatment or storage tanks serve as gravityseparators, the oil may be decanted off thesurface or, alternately, the separated water maybe drawn off the bottom until the oil layerappears. The resulting oily residue from agravity separator must then be further processedor disposed.

Because gravity separation is such a widely-

used technology, there is an abundance ofequipment configurations available. A verycommon unit is the API (American PetroleumInstitute) separator, shown in Figure 8-5. Thisunit uses an overflow and an underflow baffle toskim the floating oil layer from the surface.Another oil/water gravity separation processutilizes parallel plates which shorten thenecessary retention time by shortening thedistance the oil droplets must travel before

separation occurs.

INDUSTRY PRACTICE

Of the 116 CWT facilities in EPA’s WTIQuestionnaire and NOA comment data base thatprovided information concerning the use ofoil/water gravity separation, 16 operate skimmingsystems, seven operate coalescing plate or tubeseparation systems, and 42 operate oil/watergravity separation systems. Oil/water separationis such an integral step at oils subcategoryfacilities that every oils subcategory facility

visited by EPA performed gravity oil/waterseparation, either in tanks specifically designedfor that purpose or in waste receiving or storagetanks.

2. CLARIFICATION

GENERAL DESCRIPTION

Like oil/water separators, clarificationsystems utilize gravity to provide continuous,low-cost separation and removal of particulates,flocculated impurities, and precipitates fromwater. These systems typically follow

wastewater treatment processes which generatesuspended solids, such as chemical precipitationand biological treatment.

In a clarifier, wastewater is allowed to flowslowly and uniformly, permitting the solids moredense than water to settle to the bottom. Theclarified wastewater is discharged by flowingfrom the top of the clarifier over a weir. Solidsaccumulate at the bottom of a clarifier and asludge must be periodically removed, dewatered

and disposed. Conventional clarifiers aretypically circular or rectangular tanks. Somespecialized types of clarifiers additionallyincorporate tubes, plates, or lamellar networks toincrease the settling area. A circular clarificationsystem is illustrated in Figure 8-6.

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Figure 8-5. Gravity Separation System Diagram

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Figure 8-6. Clarification System Diagram

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INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of clarificationsystems, 39 operate settling systems and sevenoperate coalescing plate or tube separationsystems. EPA did not obtain detailed enoughtreatment technology information from theNotice of Data Availability comments for the oilssubcategory facilities to determine the presence

or absence of clarification systems. In general,oils subcategory facilities are more likely toutilize gravity oil/water separation. However,oils facilities that also utilize solids generationprocesses such as chemical precipitation orbiological treatment as part of their wastetreatment train will likely utilize clarificationsystems.

3. DISSOLVED AIR FLOTATION

GENERAL DESCRIPTION

Flotation is the process of using fine bubblesto induce suspended particles to rise to thesurface of a tank where they can be collectedand removed. Gas bubbles are introduced intothe wastewater and attach themselves to theparticles, thereby reducing their specific gravity

and causing them to float. Fine bubbles may begenerated by dispersing air mechanically, bydrawing them from the water using a vacuum, orby forcing air into solution under elevatedpressure followed by pressure release. Thelatter, called dissolved air flotation (DAF), is theflotation process used most frequently by CWTfacilities and is the focus of the remainingdiscussion.

DAF is commonly used to remove

suspended solids and dispersed oil and greasefrom oily wastewater. It may effectively reducethe sedimentation times of suspended particlesthat have a specific gravity close to that of water.Such particles may include both solids withspecific gravity slightly greater than water andoil/grease particles with specific gravity slightly

less than water. Flotation processes areparticularly useful for inducing the removal ofoil-wet solids that may exhibit a combinedspecific gravity nearly the same as water. Oil-wet solids are difficult to remove fromwastewater using gravity sedimentation alone,even when extended sedimentation times areutilized. Figure 8-7 is a flow diagram of a DAFsystem.

The major components of a conventional

DAF unit include a centrifugal pump, a retentiontank, an air compressor, and a flotation tank.For small volume systems, the entire influentwastewater stream is pressurized and contactedwith air in a retention tank for several minutes toallow time for the air to dissolve. Thepressurized water that is nearly saturated with airis then passed through a pressure reducing valveand introduced into the flotation tank near thebottom. In larger units, rather than pressurizing

the entire wastewater stream, a portion of theflotation cell effluent is recycled through thepressurizing pump and the retention tank. Therecycled flow is then mixed with theunpressurized main stream just prior to enteringthe flotation tank.

As soon as the pressure is released, thesupersaturated air begins to come out of solutionin the form of fine bubbles. The bubbles attachto suspended particles and become enmeshed in

sludge flocs, floating them to the surface. Thefloat is continuously swept from the tank surfaceand is discharged over the end wall of the tank.Sludge, if generated, may be collected from thebottom of the tank. The mechanics of thebubble-particle interaction include: (1)attachment of the bubbles on the particle surface,(2) collision between a bubble and a particle, (3)agglomeration of individual particles or a flocstructure as the bubbles rise, and (4) absorption

of the bubbles into a floc structure as it forms.As such, surface chemistry plays a critical role inthe effective performance of air flotation.

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Figure 8-7. Dissolved Air Flotation System Diagram

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Other operating variables which affect theperformance of DAF include the operatingpressure, recycle ratio, detention time, theair/solids ratio, solids and hydraulic loading rates,and the application of chemical aids.

The operating pressure of the retention tankinfluences the size of the bubbles released. If thebubbles are too large, they do not attach readilyto the suspended particles. If the bubbles are toofine, they will disperse and break up fragile floc.

Wastewater treatment textbooks generallyrecommend a bubble size of 100 micrometers.The most practical way to establish the properrise rate is to conduct experiments at various airpressures.

The air-to-solids ratio in the DAF unitdetermines the effluent quality and solidsconcentration in the float. This is becauseadequate air bubbles are needed to floatsuspended solids to the surface of the tank.

Partial flotation of solids will occur if inadequateor excessive amounts of air bubbles are present.

Researchers have demonstrated that theaddition of chemicals to the water stream is aneffective means of increasing the efficiencies ofDAF treatment systems. The use of coagulantscan drastically increase the oil removal efficiencyof DAF units. Three types of chemicals aregenerally utilized to improve the efficiency of airflotation units used for treatment of produced

water; these chemicals are surface active agents,coagulating agents, and polyelectrolytes. Theuse of treatment chemicals may also enhance theremoval of metals in air flotation units. EPA’scollection of data from the CWT industry hasshown that many facilities use DAF systems toremove metals from their waste streams.

INDUSTRY PRACTICE

Of the 116 CWT facilities in EPA’s WTIQuestionnaire and NOA comment data base that

provided information concerning use of DAF, 21operate DAF systems.

Chromium Reduction 8.2.2.6

GENERAL DESCRIPTION

Reduction is a chemical reaction in whichelectrons are transferred from one chemical toanother. The main reduction application atCWT facilities is the reduction of hexavalentchromium to trivalent chromium, which issubsequently precipitated from the wastewater inconjunction with other metallic salts. A low pHof 2 to 3 will promote chromium reductionreactions. At pH levels above 5, the reductionrate is slow. Oxidizing agents such as dissolvedoxygen and ferric iron interfere with thereduction process by consuming the reducingagent.

The use of strong reducing agents such assulfur dioxide, sodium bisulfite, sodiummetabisulfite, and ferrous sulfate alsopromotesshexavalent chromium reduction. Thetwo most commonly used reducing agents in theCWT industry are sodium metabisulfite orsodium bisulfite and gaseous sulfur dioxide. Theremaining discussion will focus on chromiumreduction using these agents only. Figure 8-8 isa diagram of a chromium reduction system.

Chromium reduction using sodiummetabisulfite (Na2S2O5) and sodium bisulfite(NaHSO3) are essentially similar. Themechanism for the reaction using sodiumbisulfite as the reducing agent is:

3NaHSO3 + 3H2SO4 + 2H2CrO4 6 Cr2(SO4)3 + 3NaHSO4 + 5H2O

The hexavalent chromium is reduced totrivalent chromium using sodium metabisulfite,

with sulfuric acid used to lower the pH of thesolution. The amount of sodium metabisulfiteneeded to reduce the hexavalent chromium isreported as 3 parts of sodium bisulfite per part ofchromium, while the amount of sulfuric acid is 1part per part of chromium. The theoreticalretention time is about 30 to 60 minutes.

A second process uses sulfur dioxide (SO2)as the reducing agent. The reaction mechanismis as follows:

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3SO2 + 3H2O 6 3H2SO3

3H2SO3 + 2H2CrO4 6 Cr2(SO4)3 + 5H2O

The hexavalent chromium is reduced totrivalent chromium using sulfur dioxide, withsulfuric acid used to lower the pH of thesolution. The amount of sulfur dioxide neededto reduce the hexavalent chromium is reported as1.9 parts of sulfur dioxide per part of chromium,while the amount of sulfuric acid is 1 part perpart of chromium. At a pH of 3, the theoreticalretention time is approximately 30 to 45 minutes.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of chromiumreduction, 35 operate chromium reductionsystems. All of the 35 facilities are in the metalssubcategory. At these 35 facilities, there are foursulfur dioxide processes, 21 sodium bisulfiteprocesses, and two sodium metabisulfiteprocesses. The remaining systems use variousother reducing agents.

Cyanide Destruction 8.2.2.7

GENERAL DESCRIPTION

Electroplating and metal finishing operationsproduce the major portion of cyanide-bearingwastes accepted at CWT facilities. EPAobserved three separate cyanide destructiontechniques during site visits at CWT facilities.The first two methods are alkaline chlorinationwith gaseous chlorine and alkaline chlorinationwith sodium hypochlorite. The third method isa cyanide destruction process, details of whichthe generator has claimed are confidentialbusiness information (CBI). The two alkalinechlorination procedures are discussed here. Alkaline chlorination can destroy freedissolved hydrogen cyanide and can oxidize allsimple and some complex inorganic cyanides. It,however, cannot effectively oxidize stable iron,copper, and nickel cyanide complexes. Theaddition of heat to the alkaline chlorinationprocess can facilitate the more completedestruction of total cyanides. The use of an

extended retention time can also improve overallcyanide destruction. Figure 8-9 is a diagram ofan alkaline chlorination system.

In alkaline chlorination using gaseouschlorine, the oxidation process is accomplishedby direct addition of chlorine (Cl2) as the oxidizerand sodium hydroxide (NaOH) to maintain pHlevels. The reaction mechanism is as follows:

NaCN + Cl2 + 2NaOH 6 NaCNO + 2NaCl + H2O

2NaCNO + 3Cl2 + 6NaOH 6 2NaHCO3 + N2 + 6NaCl + 2H2O

The destruction of the cyanide takes place intwo stages. The primary reaction is the partialoxidation of the cyanide to cyanate at a pHabove 9. In the second stage, the pH is loweredto a range of 8 to 8.5 for the oxidation of thecyanate to nitrogen and carbon dioxide (assodium bicarbonate). Each part of cyaniderequires 2.73 parts of chlorine to convert it tocyanate and an additional 4.1 parts of chlorine tooxidize the cyanate to nitrogen and carbondioxide. At least 1.125 parts of sodiumhydroxide are required to control the pH witheach stage.

Alkaline chlorination can also be conductedwith sodium hypochlorite (NaOCl) as theoxidizer. The oxidation of cyanide waste usingsodium hypochlorite is similar to the gaseouschlorine process. The reaction mechanism is:

NaCN + NaOCl 6 NaCNO + NaCl

2NaCNO + 3NaOCl + H2O 6 2NaHCO3 + N2 + 3NaCl

In the first step, cyanide is oxidized tocyanate with the pH maintained in the range of 9to 11. The second step oxidizes cyanate tocarbon dioxide (as sodium bicarbonate) andnitrogen at a controlled pH of 8.5. The amountof sodium hypochlorite and sodium hydroxideneeded to perform the oxidation is 7.5 parts and8 parts per part of cyanide, respectively.

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Figure 8-8. Chromium Reduction System Diagram

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Caustic Feed Hypochlorite or Chlorine Feed

WastewaterInfluent

TreatedEffluent

First Stage

Second Stage

Acid Feed

Figure 8.9 Cyanide Destruction by Alkaline Chlorination

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INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of cyanidedestruction, 22 operate cyanide destructionsystems. All of the 22 facilities are in the metalssubcategory. Of these 22 facilities, one is athermal unit, one is the CBI unit, and the rest arechemical reagent systems.

Chemical Precipitation 8.2.2.8

GENERAL DESCRIPTION

Many CWT facilities use chemicalprecipitation to remove metal compounds fromwastewater. Chemical precipitation convertssoluble metallic ions and certain anions toinsoluble forms, which precipitate from solution.Chemical precipitation is usually performed inconjunction with coagulation/flocculationprocesses which facilitate the agglomeration ofsuspended and colloidal material. Most metalsare relatively insoluble as hydroxides, sulfides, or

carbonates. Coagulation/flocculation processesare used in conjunction with precipitation tofacilitate removal by agglomeration of suspendedand colloidal materials. The precipitated metalsare subsequently removed from the wastewaterstream by liquid filtration or clarification (orsome other form of gravity-assisted separation).Other treatment processes such as equalization,or chemical oxidation or reduction (e.g.,hexavalent chromium reduction) usually precede

the chemical precipitation process. Chemicalinteractions, temperature, pH, solubility of wastecontaminants, and mixing effects all affect theperformance of the chemical precipitationprocess.

Chemical precipitation is a two-step process.At CWT facilities, it is typically performed inbatch operations. In the first step, precipitantsare mixed with the wastewater, typically bymechanical means, such as mixers, allowing the

formation of the insoluble metal precipitants.The detention time in this step of the process is

specific to the wastewater being treated, thetreatment chemicals used, and the desiredeffluent quality. In the second step, theprecipitated metals are removed from thewastewater, typically through filtration orclarification. If clarification is used, a flocculentis sometimes added to aid the settling process.The resulting sludge from the clarifier or filtermust be further treated, disposed, or recycled. Atypical chemical precipitation system is shown in

Figure 8-10.Various chemicals may be used as

precipitants. These include lime, sodiumhydroxide (caustic), soda ash, sodium sulfide,and ferrous sulfate. Other chemicals used in theprecipitation process for pH adjustment and/orcoagulation include sulfuric and phosphoric acid,ferric chloride, and polyelectrolytes. Often,facilities use a combination of these chemicals.CWT facilities generally use hydroxide

precipitation and/or sulfide precipitation.Hydroxide precipitation is effective in removingmetals such as antimony, arsenic, chromium,copper, lead, mercury, nickel, and zinc. Sulfideprecipitation is used instead of, or in addition to,hydroxide precipitation to remove specific metalions including lead, copper, silver, cadmium,zinc, mercury, nickel, thallium, arsenic,antimony, and vanadium. Both hydroxide andsulfide precipitation are discussed in greater detail

below.

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Figure 8-10. Chemical Precipitation System Diagram

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Hydroxide precipitation using lime or causticis the most commonly-used means of chemicalprecipitation at CWT facilities. Of these, lime isused more often than caustic. The reactionmechanism for each of these is as follows:

M++ + Ca(OH)2 6 M(OH)29 + Ca++

M++ + 2NaOH 6 M(OH)29 + 2Na++

The chief advantage of lime over caustic isits lower cost. However, lime is more difficult tohandle and feed, as it must be slaked, slurried,and mixed, and can plug the feed system lines.Lime also produces a larger volume of sludge

than caustic, and the sludge is generally notsuitable for reclamation due to its homogeneousnature.

Sulfide precipitation is the next mostcommonly-used means of chemical precipitationat CWT facilities. It is used to remove lead,copper, silver, cadmium, zinc, mercury, nickel,thallium, arsenic, antimony, and vanadium fromwastewaters. An advantage of the sulfideprocess over the hydroxide process is that it can

reduce hexavalent chromium to the trivalent stateunder the same process conditions required formetals precipitation. The use of sulfides alsoallows for the precipitation of metals whenchelating agents are present. The two mostcommon sulfide precipitation processes are thesoluble sulfide process and the insoluble sulfide(Sulfex) process.

In the soluble sulfide process, either sodiumsulfide or sodium hydrosulfide, both highly

soluble, is added in high concentration either asa liquid reagent or from rapid mix tanks usingsolid reagents. This high concentration ofsoluble sulfides results in rapid precipitation ofmetals which then results in the generation offine precipitate particles and hydrated colloidalparticles. These fine particles do not settle orfilter well without the addition of coagulating andflocculating agents to aid in the formation oflarger, fast-settling floc. The high concentration

of soluble sulfides may also lead to thegeneration of highly toxic and odorous hydrogensulfide gas. To control this problem, thetreatment facility must carefully control thedosage and/or the process vessels must beenclosed and vacuum evacuated. The reactionmechanism for soluble sulfide precipitation is asfollows:

M++ + S- - 6 MS9

The basic principle governing the insoluble

sulfide process is that ferrous sulfide (FeS) willdisassociate into ferrous and sulfide ions, aspredicted by its solubility, producing a sulfideconcentration of approximately 2 mg/l undernormal conditions. In the insoluble sulfideprocess, a slurry of freshly prepared FeS(prepared by reactive FeSO4 and NaHS) is addedto the wastewater. As the sulfide ions areconsumed in precipitating the metal pollutants,additional FeS will disassociate. This will

continue as long as other heavy metals withlower equilibrium constants are present insolution. Because most heavy metals havesulfides that are less soluble than ferrous sulfate,they will precipitate as metal sulfides. Inaddition, if given enough time, any metalhydroxides present will dissolve and precipitateout as sulfides. If the operation is performedunder alkaline conditions, the released ferrousion will precipitate out as a hydroxide. The

following reactions occur when FeS is added toa solution that contains dissolved metal and metalhydroxide:

FeS 6 Fe+ + + S- -

M+ + + S- - 6 MS9

M(OH)2 6 M+ + + 2(OH)-

Fe+ + + 2(OH)- 6 Fe(OH)29

One advantage of the insoluble sulfideprocess over the soluble sulfide process is that

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the insoluble sulfide process generates nodetectable H2S gas odor. This is because thedissolved sulfide concentration is maintained at arelatively low concentration. Disadvantages ofthe insoluble sulfide process include considerablyhigher than stoichiometric reagent consumptionand significantly higher sludge generation thaneither the hydroxide or soluble sulfide process.

Wastewater treatment facilities often chooseto combine hydroxide precipitation and sulfide

precipitation for optimal metals removal. Acommon configuration is a two-stage process inwhich hydroxide precipitation is followed bysulfide precipitation with each stage followed bya separate solids removal step. This will producethe high quality effluent of the sulfideprecipitation process while significantly reducingthe volume of sludge generated and theconsumption of sulfide reagent.

In addition to the type of treatment chemical

chosen, another important operational variable inchemical precipitation is pH. Metal hydroxidesare amphoteric, meaning they can reactchemically as acids or bases. As such, theirsolubilities increase toward both lower and higherpH levels. Therefore, there is an optimum pHfor hydroxide precipitation for each metal, whichcorresponds to its point of minimum solubility.Figure 8-11 presents calculated solubilities ofmetal hydroxides. For example, as demonstrated

in this figure, the optimum pH range where zincis the least soluble is between 8 and 10. Thesolubility of metal sulfides is not as sensitive tochanges in pH as hydroxides and generallydecreases as pH increases. The typical operatingpH range for sulfide precipitation is between 7and 9. Arsenic and antimony are exceptions tothis rule and require a pH below 7 for optimumremoval. As such, another advantage of sulfideprecipitation over hydroxide precipitation is that

most metals can be removed to extremely lowconcentrations at a single pH.

For wastewater contaminated with a singlemetal, selecting the optimum treatment chemical

and treatment pH for precipitation simplyrequires the identification of the treatmentchemical/pH combination that produces thelowest solubility of that metal. This is typicallydone using a series of bench-scale treatabilitytests. However, when wastewater iscontaminated with more than one metal, as isoften the case for wastewaters at CWT facilities,selecting the optimum treatment chemical andpH for a single-stage precipitation process

becomes more difficult and often involves atradeoff between optimal removal of two ormore metals. In general, for wastewatercontaminated with multiple metals, EPA hasconcluded that a single-stage precipitationprocess does not provide for adequate treatment.In such cases, a series of chemical treatmentsteps using different pH values and/or differenttreatment chemicals may be more appropriate.Each of these treatment steps needs to be

followed by a solids separation step in order toprevent the resolubilization of metal precipitatesduring the subsequent treatment step.

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0.0001

0.001

0.01

0.1

1

10

100

0 2 4 6 8 10 12 14

pH

Sol

ub

le M

etal

Con

cen

trat

ion

(m

g/l)

Cr(OH)3

Ni(OH)2

Cu(OH)2

Cd(OH)2

Zn(OH)2

Pb(OH)2

Figure 8-11. Calculated Solubilities of Metal Hydroxides

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In order to take advantage of the effects ofpH and treatment chemical selection on metalsprecipitation, a facility may hold its wastes andsegregate them by pollutant content fortreatment. This type of waste treatmentmanagement, called selective metalsprecipitation, may be adopted in order tooptimize the recovery of specific metalpollutants. In instances where the segregatedwastes contain several metals, the pH of the

precipitation process may be adjusted so that thedesired metal for recovery is precipitated ingreater proportion than the other metals.Multiple precipitation steps are then performed inseries on a single waste stream using different pHvalues, resulting in different metals beingselectively precipitated into separate sludges.The production of specific sludges containingonly the target metals makes the sludges moresuitable for reclamation. If the sludge is to be

sold to a smelter for re-use, then hydroxideprecipitation using only caustic should beperformed. The calcium compounds from limewould interfere with the smelting process.

Selective precipitation is advantageousbecause the metals may be reclaimed and re-used rather than disposed as a sludge in a landfilland because it allows for optimal removal of themetals of concern. However, selective metalsprecipitation does have additional costs such as

those associated with the extra tanks andoperating personnel required for wastesegregation.

INDUSTRY PRACTICE

Of the 116 CWT facilities in EPA’s WTIQuestionnaire and NOA comment data base thatprovided information concerning the use ofchemical precipitation, 57 operate chemicalprecipitation systems. Fifty-one of these

facilities treat metals subcategory wastewaters.As discussed previously, a single facility may useseveral chemical precipitation steps, dependingupon the type of waste being treated. Of the 51

chemical precipitation systems at metalssubcategory facilities, 13 operate secondaryprecipitation processes, four operate tertiaryprecipitation processes, and one employsselective chemical precipitation processes.

Filtration 8.2.2.9

Filtration is a method for separating solidparticles from a fluid through the use of a porousmedium. The driving force in filtration is apressure gradient caused by gravity, centrifugalforce, pressure, or a vacuum. CWT facilities use

filtration treatment processes to remove solidsfrom wastewaters after physical/chemical orbiological treatment, or as the primary source ofwaste treatment. Filtration processes utilized inthe CWT industry include a broad range ofmedia and membrane separation technologies.

To aid in removal, the filter medium may beprecoated with a filtration aid such as groundcellulose or diatomaceous earth. Polymers aresometimes injected into the filter feed piping

downstream of feed pumps to enhanceflocculation of smaller flocs to improve solidscapture. The following sections discuss thevarious types of filtration in use at CWTfacilities.

1. SAND FILTRATION

GENERAL DESCRIPTION

Sand filtration processes consist of either afixed or moving bed of media that traps andremoves suspended solids from water passingthrough the media. There are two types of fixed

sand bed filters: pressure and gravity. Pressurefilters contain media in an enclosed, watertightpressure vessel and require a feed pump to forcethe water through the media. A gravity filteroperates on the basis of differential pressure of astatic head of water above the media, whichcauses flow through the filter. Filter loading ratesfor sand filters are typically between 2 to 6gpm/sq-ft.

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Fixed media filters have influent and effluentdistribution systems consisting of pipes andfittings. A stainless steel screen covered withgravel generally serves as the tank bottom andsupport for the sand. Dirty water enters the topof the filter and travels downward.

Moving bed filters use an air lift pump anddraft tube to recirculate sand from the bottom tothe top of the filter vessel, which is usually openat the top. Dirty water entering the filter at the

bottom must travel upward, countercurrently,through the downward moving fluidized sandbed. Particles are strained from the rising waterand carried downward with the sand. Due to thedifference in specific gravity, the lighter particlesare removed from the filter when the sand isrecycled through a separation box often locatedat the top of the filter. The heavier sand fallsback into the filter, while the lighter particles arewashed over a weir to waste.

Both fixed media and moving bed filtersbuild up head loss over time. Head loss is ameasure of solids trapped in the filter. As thefilter becomes filled with trapped solids, theefficiency of the filtration process falls off, andthe filter must be backwashed. Reversing theflow will backwash filters so that the solids in themedia are dislodged and may exit the filter.Sometimes air is dispersed into the sand bed toscour the media.

Fixed bed filters may be automaticallybackwashed when the differential pressureexceeds a preset limit or when a timer starts thebackwash cycle. A supply of clean backwashwater is required. Backwash water and trappedparticles are commonly discharged to anequalization tank upstream of the wastewatertreatment system’s gravity separation system orscreen for removal. Moving bed filters arecontinuously backwashed and have a constant

rate of effluent flow.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning use of sand filtration,eight operate sand filtration systems.

2. MULTIMEDIA FILTRATION

GENERAL DESCRIPTION

CWT facilities may use multimedia, orgranular bed, filtration to achieve supplementalremoval of residual suspended solids from theeffluent of chemical and biological treatmentprocesses. In granular bed filtration, thewastewater stream is sent through a bedcontaining two or more layers of differentgranular materials. The solids are retained in thevoids between the media particles while the

wastewater passes through the bed. Typicalmedia used in granular bed filters includeanthracite coal, sand, and garnet.

A multimedia filter is designed so that thefiner, denser media is at the bottom and thecoarser, less dense media at the top. A commonarrangement is garnet at the bottom of the bed,sand in the middle, and anthracite coal at the top.Some mixing of these layers occurs and isanticipated. During filtration, the removal of the

suspended solids is accomplished by a complexprocess involving one or more mechanisms suchas straining, sedimentation, interception,impaction, and adsorption. The medium size isthe principal characteristic that affects thefiltration operation. If the medium is too small,much of the driving force will be wasted inovercoming the frictional resistance of the filterbed. If the medium is too large, small particleswill travel through the bed, preventing optimum

filtration.By designing the filter bed so that pore size

decreases from the influent to the effluent side ofthe bed, different size particles are filtered out atdifferent depths (larger particles first) of the filterbed. This helps prevent the build up of a single

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layer of solids at the bed surface which canquickly increase the pressure drop over the bedresulting in shorter filter runs and more frequentbackwash cycles. Thus, the advantage ofmultimedia filtration over sand filtration is longerfilter runs and less frequent backwash cycles.

The flow pattern of multimedia filters isusually top-to-bottom. Upflow filters, horizontalfilters, and biflow filters are also used. Figure 8-12 is a top-to-bottom multimedia filter. The

classic multimedia filter operates by gravity.However, pressure filters are occasionally used.

The complete filtration process involves twophases: filtration and backwashing. As the filterbecomes filled with trapped solids, the efficiencyof the filtration process falls off. Head loss is ameasure of solids trapped in the filter. As thehead loss across the filter bed increases to alimiting value, the end of the filter run is reachedand the filter must be backwashed to remove the

suspended solids in the bed. Duringbackwashing, the flow through the filter isreversed so that the solids trapped in the mediaare dislodged and can exit the filter. The bedmay also be agitated with air to aid in solidsremoval. Backwash water and trapped particlesare commonly discharged to an equalization tankupstream of the wastewater treatment system’sgravity separation system or screen for removal.

An important feature in filtration and

backwashing is the underdrain. The underdrainis the support structure for the filtration bed.The underdrain provides an area for theaccumulation of the filtered water without itbeing clogged from the filtered solids or themedia particles. During backwash, theunderdrain provides even flow distribution overthe bed. This is important because the backwashflowrate is set so that the filter bed expands butthe media is not carried out with the backwashed

solids. The media with different densities thensettle back down in somewhat discrete layers atthe end of the backwash step.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning use of multimediafiltration, four operate multimedia filtrationsystems.

3. PLATE AND FRAME PRESSURE FILTRATION

GENERAL DESCRIPTION

Another filtration system for the removal ofsolids from waste streams is a plate and framepressure filtration systems. Although plate andframe filter presses are more commonly used fordewatering sludges, they are also used to removesolids directly from wastewater streams. Theliquid stream plate and frame pressure filtrationsystem is identical to the system used for the

sludge stream (section 8.4.1) with the exceptionof a lower solids level in the influent stream.The same equipment is used for bothapplications, with the difference being the sizingof the sludge and liquid units. See section 8.4.1for a detailed description of plate and framepressure filtration. No CWT facilities in EPA’sdatabase use plate and frame filtration.

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Figure 8-12. Multi-Media Filtration System Diagram

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4. MEMBRANE FILTRATION

GENERAL DESCRIPTION

Membrane filtration systems are processeswhich employ semi-permeable membranes anda pressure differential to remove solids inwastestreams. Reverse osmosis andultrafiltration are two commonly-used membranefiltration processes.

A. ULTRAFILTRATION

GENERAL DESCRIPTION

CWT facilities commonly use ultrafiltration(UF) for the treatment of metal-finishing

wastewater and oily wastes. It can removesubstances with molecular weights greater than500, including suspended solids, oil and grease,large organic molecules, and complexed heavymetals. UF can be used when the solutemolecules are greater than ten times the size ofthe solvent molecules, and are less than one-halfmicron. In the CWT industry, UF is applied inthe treatment of oil/water emulsions. Oil/wateremulsions contain both soluble and insoluble oil.

Typically the insoluble oil is removed from theemulsion by gravity separation assisted byemulsion breaking. The soluble oil is thenremoved by UF. Oily wastewater containing 0.1to 10 percent oil can be effectively treated byUF. Figure 8-13 shows a UF system.

In UF, a semi-permeable microporousmembrane performs the separation. Wastewateris sent through membrane modules underpressure. Water and low–molecular –weight

solutes (for example, salts and some surfactants)pass through the membrane and are removed aspermeate. Emulsified oil and suspended solidsare rejected by the membrane and are removedas concentrate. The concentrate is recirculatedthrough the membrane unit until the flow ofpermeate drops. The permeate may either bedischarged or passed along to another treatmentunit. The concentrate is contained and held for

further treatment or disposal. An importantadvantage of UF over reverse osmosis is that theconcentrate may be treated to remove theconcentrated solids and the separated water maythen be retreated through the UF system.

The primary design consideration in UF isthe membrane selection. A membrane pore sizeis chosen based on the size of the contaminantparticles targeted for removal. Other designparameters to be considered are the solids

concentration, viscosity, and temperature of thefeed stream, pressure differential, and themembrane permeability and thickness. The rateat which a membrane fouls is also an importantdesign consideration.

INDUSTRY PRACTICE

Of the 116 CWT facilities in EPA’s WTIQuestionnaire and NOA comment data base thatprovided information concerning use ofultrafiltration, six operate ultrafiltration systems.

B. REVERSE OSMOSIS

GENERAL DESCRIPTION

Reverse osmosis (RO) is a process forseparating dissolved solids from water. CWTfacilities commonly use RO in treating oily ormetal-bearing wastewater. RO is applicablewhen the solute molecules are approximately thesame size as the solvent molecules. Asemi–permeable, microporous membrane andpressure are used to perform the separation. RO

systems are typically used as polishing processes,prior to final discharge of the treated wastewater.Reverse osmosis systems have beendemonstrated to be effective in removingdissolved metals.

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Figure 8-13. Ultrafiltration System Diagram

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Osmosis is the diffusion of a solvent (such aswater) across a semi-permeable membrane froma less concentrated solution into a moreconcentrated solution. In the reverse osmosisprocess, pressure greater than the normalosmotic pressure is applied to the moreconcentrated solution (the waste stream beingtreated), forcing the purified water through themembrane and into the less concentrated streamwhich is called the permeate. The low-

molecular-weight solutes (for example, salts andsome surfactants) do not pass through themembrane. They are referred to as concentrate.The concentrate is recirculated through themembrane unit until the flow of permeate drops.The permeate can either be discharged or passedalong to another treatment unit. The concentrateis contained and held for further treatment ordisposal. Figure 8–14 shows an RO system.

The performance of an RO system is

dependent upon the dissolved solidsconcentration and temperature of the feedstream, the applied pressure, and the type ofmembrane selected. The key RO membraneproperties to be considered are: selectivity forwater over ions, permeation rate, and durability.RO modules are available in various membraneconfigurations, such as spiral-wound, tubular,hollow-fiber, and plate and frame. In addition tothe membrane modules, other capital items

needed for an RO installation include pumps,piping, instrumentation, and storage tanks. Themajor operating cost is attributed to membranereplacement. A major consideration for ROsystems is the disposal of the concentrate due toits elevated concentrations of salts, metals, andother dissolved solids.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that provided

information concerning use of reverse osmosis,two operate reverse osmosis systems.

5. LANCY FILTRATION

GENERAL DESCRIPTION

The Lancy Sorption Filter System is apatented method for the continuous recovery ofheavy metals. The Lancy sorption filtrationprocess may reduce metals not removed byconventional waste treatment technologies to lowconcentrations.

In the first stage of the Lancy filtrationprocess, a soluble sulfide is added to thewastewater in a reaction tank, converting most of

the heavy metals to sulfides. From the sulfidereaction tank, the solution is passed through thesorption filter media. Precipitated metal sulfidesand other suspended solids are filtered out. Anyremaining soluble metals are absorbed by themedia. Excess soluble sulfides are also removedfrom the waste stream.

The Lancy filtration process reportedlyreduces zinc, silver, copper, lead, and cadmiumto less than 0.05 mg/l and mercury to less than 2

Fg/l. In addition to the effective removal ofheavy metals, the system has a high solidsfiltration capacity and a fully automatic,continuous operation. The system continuouslyrecycles and reuses the same filter media therebysaving on operating costs. The system may beinstalled with a choice of media discharge - slurryor solid cake. Figure 8-15 illustrates the LancySorption Filtration System.

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Figure 8-14. Reverse Osmosis System Diagram

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Figure 8-15. Lancy Filtration System Diagram

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INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning use of filtration systems,only one operates the Lancy Sorption FiltrationSystem. This unit is used for polishing effluentfrom a treatment sequence including chemicalprecipitation, clarification, and sand filtration.EPA obtained performance data for this systemduring a sampling episode at one of the metals

subcategory facilities. The performance datashowed that some metals were reduced to thetarget levels while the concentration of somepollutants increased. This may not representoptimal performance of the system, however,because the facility reported that they wereexperiencing operational problems throughout thesampling episode.

Carbon Adsorption 8.2.2.10

GENERAL DESCRIPTION

Activated carbon adsorption is a

demonstrated wastewater treatment technologythat uses activated carbon to remove dissolvedorganic pollutants from wastewater. Theactivated carbon is made from manycarbonaceous sources including coal, coke, peat,wood, and coconut shells. The carbon sourcematerial is “activated” by treating it with anoxidizing gas to form a highly porous structurewith a large internal surface area. CWT facilitiesgenerally use granular forms of activated carbon

(GAC) in fixed bed columns to treat wastewater.However, some use powdered activated carbon(PAC) alone or in conjunction with biologicaltreatment. Figure 8-16 presents a diagram of afixed-bed GAC collumn.

In a fixed bed system, the wastewater entersthe top of the unit and is allowed to flowdownward through a bed of granular activatedcarbon. As the wastewater comes into contactwith the activated carbon, the dissolved organic

compounds adsorb onto the surface of the

activated carbon. In the upper area of the bed,the pollutants are rapidly adsorbed. As morewastewater passes through the bed, this rapidadsorption zone moves downward until itreaches the bottom of the bed. At this point, allof the available adsorption sites are filled and thecarbon is said to be exhausted. This conditioncan be detected by an increase in the effluentpollutant concentration, and is calledbreakthrough.

GAC systems are usually comprised ofseveral beds operated in series. This designallows the first bed to go to exhaustion, while theother beds still have the capacity to treat to anacceptable effluent quality. The carbon in thefirst bed is replaced, and the second bed thenbecomes the lead bed. The GAC system pipingis designed to allow switching of bed order.

After the carbon is exhausted, it can beremoved and regenerated. Usually heat or steam

is used to reverse the adsorption process. Thelight organic compounds are volatilized and theheavy organic compounds are pyrolyzed. Spentcarbon may also be regenerated by contacting itwith a solvent which dissolves the adsorbedpollutants. Depending on system size andeconomics, some facilities may choose to disposeof the spent carbon instead of regenerating it.For very large applications, an on-siteregeneration facility is more economical. For

smaller applications, such as in the CWTindustry, it is generally cost-effective to use avendor service to deliver regenerated carbon andremove the spent carbon. These vendorstransport the spent carbon to their centralizedfacilities for regeneration.

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Figure 8-16. Carbon Adsorption System Diagram

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The carbon adsorption mechanism iscomplicated and, although the attraction isprimarily physical, is a combination of physical,chemical, and electrostatic interactions betweenthe activated carbon and the organic compound.The key design parameter for activated carbon isthe adsorption capacity of the carbon. Theadsorption capacity is a measure of the mass ofcontaminant adsorbed per unit mass of activatedcarbon and is a function of the compound being

adsorbed, the type of carbon used, and theprocess design and operating conditions. Ingeneral, the adsorption capacity is inverselyproportional to the adsorbate solubility.Nonpolar, high molecular weight organics withlow solubility are readily adsorbed. Polar, lowmolecular weight organics with high solubilitiesare more poorly adsorbed.

Competitive adsorption between compoundshas an effect on adsorption. The carbon may

preferentially adsorb one compound overanother. This competition could result in anadsorbed compound being desorbed from thecarbon. This is most pronounced when carbonadsorption is used to treat wastewater with highlyvariable pollutant character and concentration.

INDUSTRY PRACTICE

Of the 116 CWT facilities in EPA’s WTIQuestionnaire and NOA comment data base thatprovided information concerning use of carbon

adsorption, 17 operate carbon adsorptionsystems.

Ion Exchange 8.2.2.11

GENERAL DESCRIPTION

A common process employed to removeheavy metals from relatively low–concentrationwaste streams, such as electroplating wastewater,is ion exchange. A key advantage of the ionexchange process is that the metal contaminantscan be recovered and reused. Anotheradvantage is that ion exchange may be designed

to remove certain metals only, providingeffective removal of these metals from highly-contaminated wastewater. A disadvantage is thatthe resins may be fouled by some organicsubstances.

In an ion exchange system, the wastewaterstream is passed through a bed of resin. Theresin contains bound groups of ionic charge onits surface, which are exchanged for ions of thesame charge in the wastewater. Resins are

classified by type, either cationic or anionic. Theselection is dependent upon the wastewatercontaminant to be removed. A commonly-usedresin is polystyrene copolymerized withdivinylbenzene.

The ion exchange process involves foursteps: treatment, backwash, regeneration, andrinse. During the treatment step, wastewater ispassed through the resin bed and ions areexchanged until pollutant breakthrough occurs.

The resin is then backwashed to reclassify thebed and to remove suspended solids. During theregeneration step, the resin is contacted witheither an acidic or alkaline solution containinghigh concentrations of the ion originally presentin the resin. This "reverses" the ion exchangeprocess and removes the metal ions from theresin. The bed is then rinsed to remove residualregenerating solution. The resultingcontaminated regenerating solution must be

further processed for reuse or disposal.Depending upon system size and economics,some facilities choose to remove the spent resinand replace it with resin regenerated off-siteinstead of regenerating the resin in-place.

Ion exchange equipment ranges from simple,inexpensive systems such as domestic watersofteners, to large, continuous industrialapplications. The most commonly-encounteredindustrial setup is a fixed-bed resin in a vertical

column, where the resin is regenerated in-place.Figure 8-17 is a diagram of this type of system.These systems may be designed so that theregenerant flow is concurrent or countercurrent

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to the treatment flow. A countercurrent design,although more complex to operate, provides ahigher treatment efficiency. The beds maycontain a single type of resin for selectivetreatment, or the beds may be mixed to providefor more complete deionization of the wastestream. Often, individual beds containingdifferent resins are arranged in series, whichmakes regeneration easier than in the mixed bedsystem.

INDUSTRY PRACTICE

EPA is aware of only one CWT facilityusing ion exchange.

Electrolytic Recovery 8.2.2.12

GENERAL DESCRIPTION

Another process for reclaiming metals fromwastewater is electrolytic recovery. It is acommon technology in the electroplating, mining,and electronic industries. It is used for therecovery of copper, zinc, silver, cadmium, gold,and other heavy metals. Nickel is poorly

recovered due to its low standard potential.The electrolytic recovery process uses an

oxidation and reduction reaction. Conductiveelectrodes (anodes and cathodes) are immersedin the metal-bearing wastewater, with an electricpotential applied to them. At the cathode, ametal ion is reduced to its elemental form(electron-consuming reaction). At the sametime, gases such as oxygen, hydrogen, ornitrogen form at the anode (electron-producing

reaction). After the metal coating on the cathodereaches a desired thickness, it may be removedand recovered. The metal-stripped cathode canthen be used as the anode.

The equipment consists of anelectrochemical reactor with electrodes, a gas-venting system, recirculation pumps, and apower supply. Figure 8-18 ia a diagram of anelectrolytic recovery system. Electrochemicalreactors are typically designed to produce high

flow rates to increase the process efficiency.A conventional electrolytic recovery system

is effective for the recovery of metals fromrelatively high-concentration wastewater. Aspecialized adaptation of electrolytic recovery,called extended surface electrolysis, or ESE,operates effectively at lower concentration levels.The ESE system uses a spiral cell containing aflow-through cathode which has a very openstructure and therefore a lower resistance to fluid

flow. This also provides a larger electrodesurface. ESE systems are often used for therecovery of copper, lead, mercury, silver, andgold.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning use of electrolyticrecovery, three operate electrolytic recoverysystems.

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Figure 8-17. Ion Exchange System Diagram

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Figure 8-18. Electrolytic Recovery System Diagram

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Stripping 8.2.2.13

Stripping is a method for removing dissolvedvolatile organic compounds from wastewater.The removal is accomplished by passing air orsteam through the agitated waste stream. Theprimary difference between air stripping andsteam stripping is that steam stripping is operatedat higher temperatures and the resultant off-gasstream is usually condensed and recovered orincinerated. The off-gas from air strippingcontains non-condenseable air which must be

either passed through an adsorption unit orincinerated in order to prevent transfer of thevolatile pollutants to the environment. EPA isnot aware of any applications of steam strippingtechnologies in the CWT industry.

1. AIR STRIPPING

GENERAL DESCRIPTION

Air stripping is effective in removingdissolved volatile organic compounds fromwastewater. The removal is accomplished bypassing high volumes of air through the agitated

wastewater stream. The process results in acontaminated off-gas stream which, dependingupon air emissions standards, usually requires airpollution control equipment. Stripping canbe performed in tanks or in spray or packedtowers. Treatment in packed towers is the mostefficient application. The packing typicallyconsists of plastic rings or saddles. The twotypes of towers that are commonly used, cross-flow and countercurrent, differ in design only in

the location of the air inlets. In the cross-flowtower, the air is drawn through the sides for thetotal height of the packing. The countercurrenttower draws the entire air flow from the bottom.Cross-flow towers have been found to be moresusceptible to scaling problems and are lessefficient than countercurrent towers. Figure 8-19is a countercurrent air stripper.

The driving force of the air stripping mass-

transfer operation is the difference inconcentrations between the air and waterstreams. Pollutants are transferred from themore concentrated wastewater stream to the lessconcentrated air stream until equilibrium isreached. This equilibrium relationship is knownas Henry's Law. The strippability of a pollutantis expressed as its Henry's Law Constant, whichis a function of both its volatility or vaporpressure and solubility.

Air strippers are designed according to thestrippability of the pollutants to be removed. Forevaluation purposes, organic pollutants can bedivided into three general strippability ranges(low, medium, and high) according to theirHenry's Law Constants. The low strippabilitygroup (Henry's Law Constants of 10-4 [mg/m3

air]/[mg/m3 water] and lower) are not effectivelyremoved. Pollutants in the medium (10-1 to 10-4)and high (10-1 and greater) groups are effectively

stripped. Pollutants with lower Henry's lawconstants require greater column height, moretrays or packing material, greater temperature,and more frequent cleaning than pollutants witha higher strippability.

The air stripping process is adverselyaffected by low temperatures. Air strippersexperience lower efficiencies at lowertemperatures, with the possibility of freezingwithin the tower. For this reason, depending on

the location of the tower, it may be necessary topreheat the wastewater and the air feed streams.The column and packing materials must becleaned regularly to ensure that low effluentlevels are attained.

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Figure 8-19. Air Stripping System Diagram

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Air stripping has proved to be an effectiveprocess in the removal of volatile pollutants fromwastewater. It is generally limited to influentconcentrations of less than 100 mg/l organics.Well-designed and operated systems can achieveover 99 percent removals.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning use of air stripping, 11

operate air stripping systems.

Liquid Carbon Dioxide Extraction 8.2.2.14

GENERAL DESCRIPTION

Liquid carbon dioxide (CO2) extraction is aprocess used to extract and recover organiccontaminants from aqueous waste streams. Alicensed, commercial application of thistechnology is utilized in the CWT industry underthe name “Clean Extraction System” (CES).The process may be effective in the removal oforganic substances such as hydrocarbons,

aldehydes and ketones, nitriles, halogenatedcompounds, phenols, esters, and heterocyclics.It is not effective in the removal of somecompounds which are very water-soluble, suchas ethylene glycol, and low molecular weightalcohols. It may provide an alternative in thetreatment of waste streams which historicallyhave been incinerated.

In liquid carbon dioxide extraction, the wastestream is fed into the top of a pressurized

extraction tower containing perforated plates,where it is contacted with a countercurrentstream of liquefied CO2. The organiccontaminants in the waste stream are dissolved inthe CO2; this extract is then sent to a separator,where the CO2 is redistilled. The distilled CO2

vapor is compressed and reused. Theconcentrated organics bottoms from theseparator can then be disposed or recovered.The treated wastewater stream which exits the

extractor (raffinate) is pressure-reduced and maybe further treated for residual organics removalif necessary to meet discharge standards. Figure8-20 is a diagram of the CES is presented in.

INDUSTRY PRACTICE

EPA is aware of only one facility using thistechnology in the CWT industry. Pilot–scaleinformation submitted to EPA by the CWTfacility showed effective removal for a variety oforganic compounds. EPA sampled this

commercial CWT CES unit during thisrulemaking effort. Performance was notoptimal, however, as the facility reportedoperational problems with the unit throughout thesampling episode.

Biological Treatment 8.2.3

A portion of the CWT industry acceptswaste receipts that contain organic pollutants,which are often amenable to biologicaldegradation. This subset of CWT facilities isreferred to as the organics subcategory. In

addition, a portion of the facilities in the oilssubcategory also use biological treatment to treatwastewater separated from oily wastes.

Biological treatment systems use microbeswhich consume, and thereby destroy, organiccompounds as a food source. The microbes usethe organic compounds as both a source ofcarbon and as a source of energy. Thesemicrobes may also need supplemental nutrientsfor growth, such as nitrogen and phosphorus, if

the waste stream is deficient in these nutrients.Aerobic microbes require oxygen to grow,whereas anaerobic microbes will grow only in theabsence of oxygen. Facultative microbes are anadaptive type of microbe that can grow with orwithout oxygen.

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Figure 8-20. Liquid CO2 Extraction System Diagram

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The success of biological treatment isdependent on many factors, such as the pH andtemperature of the wastewater, the nature of thepollutants, the nutrient requirements of themicrobes, the presence of inhibiting pollutants,and variations in the feed stream loading.Certain compounds, such as heavy metals, maybe toxic to the microorganisms and must beremoved from the waste stream prior tobiological treatment. Load variations are a major

concern, especially in the CWT industry, wherewaste receipts vary over time in bothconcentration and volume.

There are several adaptations of biologicaltreatment. These adaptations differ in threebasic ways. First, a system may be aerobic,anaerobic, or facultative. Second, themicroorganisms may either be attached to asurface (as in a trickling filter), or be unattachedin a liquid suspension (as in an activated sludge

system). Third, the operation may be eitherbatch or continuous.

Of the 116 facilities in the WTIQuestionnaire and NOA comment data base thatresponded to EPA’s inquiry concerning the useof biological treatment, 17 operate biologicaltreatment systems. There were no anaerobicsystems reported. Theses systems includesequencing batch reactors, attached growthsystems (biotowers and trickling filters) and

activated sludge systems. With the exception oftrickling filters, EPA sampled at least oneapplication of each of the following biologicaltreatment technologies during the development ofthese effluent guidelines.

Sequencing Batch Reactors 8.2.3.1

GENERAL DESCRIPTION

A sequencing batch reactor (SBR) is asuspended growth system in which wastewater ismixed with existing biological floc in an aerationbasin. SBRs are unique in that a single tank acts

as an equalization tank, an aeration tank, and a

clarifier. An SBR is operated on a batch basiswhere the wastewater is mixed and aerated withthe biological floc for a specific period of time.The contents of the basin are allowed to settleand the supernatant is decanted. The batchoperation of an SBR makes it a useful biologicaltreatment option for the CWT industry, wherethe wastewater volumes and characteristics areoften highly variable. Each batch can be treateddifferently depending on waste characteristics.

Figure 8-21 shows an SBR.The SBR has a four cycle process: fill,

react, settle, and decant. The fill cycle has twophases. The first phase, called static fill,introduces the wastewater to the system understatic conditions. This is an anaerobic period andmay enhance biological phosphorus uptake.During the second phase of the fill cyclewastewater is mechanically mixed to eliminatethe scum layer and prepare the microorganisms

to receive oxygen. In the second cycle, the reactcycle, aeration is performed. The react cycle isa time–dependent process where wastewater iscontinually mixed and aerated, allowing thebiological degradation process to occur. Thethird cycle, called the settling cycle, providesquiescent conditions throughout the tank andmay accommodate low settling rates byincreasing the settling time. During the last ordecant cycle, the treated wastewater is decanted

by subsurface withdrawal from below the scumlayer. This treated, clarified effluent may thenbe further treated or discharged.

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Figure 8-21. Sequencing Batch Reactor System Diagram

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When the quantity of biomass in the SBRexceeds that needed for operation, the excessbiomass is removed. The sludge that is removedfrom the SBR may be reduced in volume bythickening and dewatering using any of thesludge treatment processes discussed in section8.2.4. The dewatered sludge may be disposed ina landfill or used as an agricultural fertilizer.

An SBR carries out all of the functions of aconventional continuous flow activated sludge

process, such as equalization, biologicaltreatment, and sedimentation, in a time sequencerather than a space sequence. Detention timesand loadings vary with each batch and are highlydependent on the specific raw wastewaterloadings. Typically, an SBR operates with ahydraulic detention time of 1 to 10 days and asludge retention time of 10 to 30 days. Themixed liquor suspended solids (MLSS)concentration is maintained at 3,500 to 10,000

mg/l. The overall control of the system may beaccomplished automatically by using levelsensors or timing devices. By using a single tankto perform all of the required functionsassociated with biological treatment, an SBRreduces land requirements. It also provides forgreater operation flexibility for treating wasteswith viable characteristics by allowing thecapability to vary detention time and mode ofaeration in each stage. SBRs also may be used

to achieve complete nitrification/denitrificationand phosphorus removal.

INDUSTRY PRACTICE

EPA is aware of only one CWT facility thatuses an SBR. This facility is in the organicssubcategory, and its SBR unit was sampledduring the development of these effluentguidelines.

Attached Growth Biological Treatment Systems 8.2.3.2

Another system used to biodegrade theorganic components of a wastewater is theattached growth biological treatment system. Inthese systems, the biomass adheres to thesurfaces of rigid supporting media. Aswastewater contacts the supporting medium, athin-film biological slime develops and coats thesurfaces. As this film (consisting primarily ofbacteria, protozoa, and fungi) grows, the slimeperiodically breaks off the medium and isreplaced by new growth. This phenomenon oflosing the slime layer is called sloughing and isprimarily a function of organic and hydraulicloadings on the system. The effluent from thesystem is usually discharged to a clarifier to settleand remove the agglomerated solids.

Attached growth biological systems areappropriate for treating industrial wastewatersamenable to aerobic biological treatment. Whenused in conjunction with suitable pre- and post-treatment processes, attached growth biologicalsystems remove suspended and colloidalmaterials effectively. The two major types ofattached growth systems used at CWT facilitiesare trickling filters and biotowers. This sectiondescribes these processes.

1. TRICKLING FILTERS

GENERAL DESCRIPTION

Trickling filtration is an aerobic fixed-filmbiological treatment process that consists of astructure, packed with inert medium such asrock, wood, or plastic. The wastewater isdistributed over the upper surface of the mediumby either a fixed spray nozzle system or arotating distribution system. The inert mediumdevelops a biological slime that absorbs andbiodegrades organic pollutants. Air flowsthrough the filter by convection, therebyproviding the oxygen needed to maintain aerobicconditions. Figure 8-22 is a flow diagram of atrickling filter.

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TricklingWastewater

Filter Material

Underdrain

Filter Material

Distributer

Figure 8-22. Trickling Filter System Diagram

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Trickling filters are classified as low-rate orhigh-rate, depending on the organic loading.Typical design organic loading values range from5 to 25 pounds and 25 to 45 pounds BOD5 per1,000 cubic feet per day for low-rate and high-rate, respectively. A low-rate filter generally hasa media bed depth of 1.5 to 3 meters and doesnot use recirculation. A high-rate filter may havea bed depth from 1 to 9 meters and recirculatesa portion of the effluent for further treatment.

INDUSTRY PRACTICE

EPA is aware of only one CWT facility thatuses a trickling filter. This facility is in the oilssubcategory.

2. BIOTOWERS

GENERAL DESCRIPTION

A variation of a trickling filtration process isthe aerobic biotower. Biotowers may beoperated in a continuous or semi-continuousmanner and may be operated in an upflow ordownflow manner. In the downflow mode,

influent is pumped to the top of a tower, whereit flows by gravity through the tower. The toweris packed with plastic or redwood mediacontaining the attached microbial growth.Biological degradation occurs as the wastewaterpasses over the media. Treated wastewatercollects in the bottom of the tower. If needed,additional oxygen is provided via air blowerscountercurrent to the wastewater flow. In theupflow mode, the wastewater stream is fed into

the bottom of the biotower and is passed upthrough the packing along with diffused airsupplied by air blowers. The treated effluentexits from the top of the biotower.

Variations of this treatment process involvethe inoculation of the raw influent with bacteriaand the addition of nutrients. Wastewatercollected in the biotowers is delivered to aclarifier to separate the biological solids from thetreated effluent. A diagram of a biotower is

presented in Figure 8-23.

INDUSTRY PRACTICE

EPA is aware of two biotowers in operationin the CWT Industry. One system treats a wastestream which is primarily composed of leachatefrom an on-site landfill operation. The othersystem treats high-TOC wastewater from ametals recovery operation. EPA conductedsampling at this facility during the developmentof these effluent guidelines.

Activated Sludge 8.2.3.3

GENERAL DESCRIPTION

The activated sludge process is acontinuous-flow, aerobic biological treatmentprocess that employs suspended-growth aerobicmicroorganisms to biodegrade organiccontaminants. In this process, a suspension ofaerobic microorganisms is maintained bymechanical mixing or turbulence induced bydiffused aerators in an aeration basin. Thissuspension of microorganisms is called the mixed

liquor. Figure 8-24 is a diagram of aconventional activated sludge system.

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Figure 8-23. Biotower System Diagram

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Figure 8-24. Activated Sludge System Diagram

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Influent is introduced into the aeration basinand is allowed to mix with the contents. A seriesof biochemical reactions is performed in theaeration basin, degrading organics and generatingnew biomass. Microorganisms oxidize thesoluble and suspended organic pollutants tocarbon dioxide and water using the availablesupplied oxygen. These organisms alsoagglomerate colloidal and particulate solids.After a specific contact period in the aeration

basin, the mixture is passed to a settling tank, orclarifier, where the microorganisms are separatedfrom the treated water. A major portion of thesettled solids in the clarifier is recycled back tothe aeration system to maintain the desiredconcentration of microorganisms in the reactor.The remainder of the settled solids is wasted andsent to sludge handling facilities.

To ensure biological stabilization of organiccompounds in activated sludge systems,

adequate nutrient levels must be available to thebiomass. The primary nutrients are nitrogen andphosphorus. Lack of these nutrients can impairbiological activity and result in reduced removalefficiencies. Certain wastes may have lowconcentrations of nitrogen and phosphorusrelative to the oxygen demand. As a result,nutrient supplements (e.g., phosphoric acidaddition for additional phosphorus) have beenused in activated sludge systems at CWT

facilities. The effectiveness of the activated sludge

process is governed by several design andoperation variables. The key variables areorganic loading, sludge retention time, hydraulicor aeration detention time, and oxygenrequirements. The organic loading is describedas the food-to-microorganism (F/M) ratio, orkilograms of BOD5 applied daily to the systemper kilogram of mixed liquor suspended solids

(MLSS). The MLSS in the aeration tank isdetermined by the rate and concentration ofactivated sludge returned to the tank. Theorganic loading (F/M ratio) affects the BOD5

removal, oxygen requirements, biomassproduction, and the settleability of the biomass.The sludge retention time (SRT) or sludge age isa measure of the average retention time of solidsin the activated sludge system. The SRT affectsthe degree of treatment and production of wastesludge. A high SRT results in a high quantity ofsolids in the system and therefore a higher degreeof treatment while also resulting in theproduction of less waste sludge. The hydraulic

detention time determines the size of the aerationtank and is calculated using the F/M ratio, SRT,and MLSS. Oxygen requirements are based onthe amount required for biodegradation oforganic matter and the amount required forendogenous respiration of the microorganisms.The design parameters will vary with the type ofwastewater to be treated and are usuallydetermined in a treatability study.

Modifications of the activated sludge process

are common, as the process is extremelyversatile and can be adapted for a wide variety oforganically contaminated wastewaters. Thetypical modification may include a variation ofone or more of the key design parameters,including the F/M loading, aeration location andtype, sludge return, and contact basinconfiguration. The modifications in practicehave been identified by the major characteristicsthat distinguish the particular configuration. The

characteristic types and modifications are brieflydescribed as follows:

C Conventional The aeration tanks are longand narrow, with plug flow (i.e., littleforward or backwards mixing).

C Complete Mix The aeration tanks areshorter and wider, and the aerators,diffusers, and entry points of the influent

and return sludge are arranged so that thewastewater mixes completely.

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C Tapered Aeration A modification of theconventional process in which the diffusersare arranged to supply more air to theinfluent end of the tank, where the oxygendemand is highest.

C Step Aeration A modification of theconventional process in which thewastewater is introduced to the aeration tankat several points, lowering the peak oxygen

demand.

C High Rate Activated Sludge A modificationof conventional or tapered aeration in whichthe aeration times are shorter, the pollutantsloadings are higher per unit mass ofmicroorganisms in the tank. The rate ofBOD5 removal for this process is higher thanthat of conventional activated sludgeprocesses, but the total removals are lower.

C Pure Oxygen An activated sludge variationin which pure oxygen instead of air is addedto the aeration tanks, the tanks are covered,and the oxygen-containing off-gas isrecycled. Compared to normal air aeration,pure oxygen aeration requires a smalleraeration tank volume and treats high-strengthwastewaters and widely fluctuating organicloadings more efficiently.

C Extended Aeration A variation of completemix in which low organic loadings and longaeration times permit more completewastewater degradation and partial aerobicdigestion of the microorganisms.

C Contact Stabilization An activated sludgemodification using two aeration stages. Inthe first, wastewater is aerated with the

return sludge in the contact tank for 30 to 90minutes, allowing finely suspended colloidaland dissolved organics to absorb to theactivated sludge. The solids are settled out

in a clarifier and then aerated in the sludgeaeration (stabilization) tank for 3 to 6 hoursbefore flowing into the first aeration tank.

C Oxidation Ditch Activated Sludge Anextended aeration process in which aerationand mixing are provided by brush rotorsplaced across a race-track-shaped basin.Waste enters the ditch at one end, is aeratedby the rotors, and circulates.

INDUSTRY PRACTICE

Because activated sludge systems aresensitive to the loading and flow variationstypically found at CWT facilities, equalization isoften required prior to activated sludgetreatment. Of the 65 CWT facilities in EPA’sWTI Questionnaire data base that providedinformation concerning use of activated sludge,four operate activated sludge systems.

Sludge Treatment and Disposal 8.2.4

Several of the waste treatment processesused in the CWT industry generate a sludge.These processes include chemical precipitation ofmetals, clarification, filtration, and biologicaltreatment. Some oily waste treatment processes,such as dissolved air flotation and centrifugation,also produce sludges. These sludges typicallycontain between one and five percent solids.They require dewatering to concentrate them andprepare them for transport and/or disposal.

Sludges are dewatered using pressure,gravity, vacuum, or centrifugal force. There areseveral widely-used, commercially-availablemethods for sludge dewatering. Plate and framepressure filtration, belt pressure filtration, andvacuum filtration are the primary methods usedfor sludge dewatering at CWT facilities. A plateand frame filter press can produce the driest filtercake of these three systems, followed by the beltpress, and lastly, the vacuum filter. Each of

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these sludge dewatering methods are discussedbelow.

In some instances, depending upon thenature of the sludge and the dewatering processused, the sludge may first be stabilized,conditioned, and/or thickened prior todewatering. Certain sludges require stabilization(via chemical addition or biological digestion)because they have an objectionable odor or area health threat. Sludges produced by the CWT

industry usually do not fall into this category.Sludge conditioning is used to improvedewaterability; it can be accomplished via theaddition of heat or chemicals. Sludge thickening,or concentration, reduces the volume of sludgeto be dewatered and is accomplished by gravitysettling, flotation, or centrifugation.

Plate and Frame Pressure Filtration 8.2.4.1

GENERAL DESCRIPTION

Plate and frame pressure filtration systems isa widely used method for the removal of solids

from waste streams. In the CWT industry, plateand frame pressure filtration system are used forfiltering solids out of treated wastewater streamsand sludges. The same equipment is used forboth applications, with the difference being thesolids level in the influent stream and the sizingof the sludge and liquid units. Figure 8-25 is aplate and frame filter press.

A plate and frame filter press consists of anumber of recessed filter plates or trays

connected to a frame and pressed togetherbetween a fixed end and a moving end. Eachplate is constructed with a drainage surface onthe depressed portion of the face. Filter cloth ismounted on the face of each plate and then theplates are pressed together. The sludge ispumped under pressure into the chambersbetween the plates of the assembly while waterpasses through the media and drains to thefiltrate outlets. The solids are retained in the

cavities of the filter press between the cloth

surfaces and form a cake that ultimately fills thechamber. At the end of the cycle when thefiltrate flow stops, the pressure is released andthe plates are separated. The filter cake dropsinto a hopper below the press. The filter cakemay then be disposed in a landfill. The filtercloth is washed before the next cycle begins.

The key advantage of plate and framepressure filtration is that it can produce a drierfilter cake than is possible with the other

methods of sludge dewatering. In a typical plateand frame pressure filtration unit, the filter cakemay exhibit a dry solids content between 30 and50 percent. It is well-suited for use in the CWTindustry as it is a batch process. However, itsbatch operation results in greater operating laborrequirements.

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INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of pressurefiltration, 34 operate pressure filtration systems.Of these 34 facilities, 25 operate plate and framepressure filtration systems, three operate beltpressure filtration systems, and six did notspecify the type of presure filtration systemsutilized.

Belt Pressure Filtration 8.2.4.2

GENERAL DESCRIPTION

A belt pressure filtration system uses gravityfollowed by mechanical compression and shearforce to produce a sludge filter cake. Belt filterpresses are continuous systems which arecommonly used to dewater biological treatmentsludge. Most belt filter installations are precededby a flocculation step, where polymer is added tocreate a sludge which has the strength towithstand being compressed between the belts

without being squeezed out. Figure 8-26 showsa typical belt filter press.

During the press operation, the sludge streamis fed onto the first of two moving cloth filterbelts. The sludge is gravity-thickened as thewater drains through the belt. As the belt holdingthe sludge advances, it approaches a secondmoving belt. As the first and second belts movecloser together, the sludge is compressedbetween them. The pressure is increased as the

two belts travel together over and under a seriesof rollers. The turning of the belts around therollers shear the cake which furthers thedewatering process. At the end of the rollerpass, the belts move apart and the cake dropsoff. The feed belt is washed before the sludgefeed point. The dropped filter cake may then bedisposed.

The advantages of a belt filtration system areits lower labor requirements and lower power

consumption. The disadvantages are that the

belt filter presses produce a poorer qualityfiltrate, and require a relatively large volume ofbelt wash water.

Typical belt filtration applications maydewater an undigested activated sludge to a cakecontaining 15 to 25 percent solids. Heat-treated,digested sludges may be reduced to a cake of upto 50 percent solids.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTI

Questionnaire data base that providedinformation concerning the use of pressurefiltration, 36 operate pressure filtration systems.Of these 34 facilities, 25 operate plate and framepressure filtration systems, three operate beltpressure filtration systems, and six did notspecify the type of presure filtration systemsutilized.

Vacuum Filtration 8.2.4.3

GENERAL DESCRIPTION

A commonly-used process for dewatering

sludge is rotary vacuum filtration. These filterscome in drum, coil, and belt configurations. Thefilter medium may be made of cloth, coil springs,or wire-mesh fabric. A typical application is arotary vacuum belt filter; a diagram of thisequipment is shown in Figure 8-27.

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Figure 8-26. Belt Pressure Filtration System Diagram

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Figure 8-27. Vacuum Filtration System Diagram

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In a rotary vacuum belt filter, a continuousbelt of filter fabric is wound around a horizontalrotating drum and rollers. The drum isperforated and is connected to a vacuum. Thedrum is partially immersed in a shallow tankcontaining the sludge. As the drum rotates, thevacuum which is applied to the inside of thedrum draws the sludge onto the filter fabric. Thewater from the sludge passes through the filterand into the drum, where it exits via a discharge

port. As the fabric leaves the drum and passesover the roller, the vacuum is released. Thefilter cake drops off of the belt as it turns aroundthe roller. The filter cake may then be disposed.

Vacuum filtration may reduce activatedsludge to a cake containing 12 to 20 percentsolids. Lime sludge may be reduced to a cake of25 to 40 percent solids.

Because vacuum filtration systems arerelatively expensive to operate, they are usually

preceded by a thickening step which reduces thevolume of sludge to be dewatered. Anadvantage of vacuum filtration is that it is acontinuous process and therefore requires lessoperator attention.

INDUSTRY PRACTICE

Of the 65 CWT facilities in EPA’s WTIQuestionnaire data base that providedinformation concerning the use of vacuumfiltration, eight operate vacuum filtration

systems.

Filter Cake Disposal 8.2.4.4

After a sludge is dewatered, the resultantfilter cake must be disposed. The most commonmethod of filter cake management used in theCWT industry is transport to an off–site landfillfor disposal. Other disposal options areincineration or land application. Land applicationis usually restricted to biological treatmentresiduals.

Zero or Alternate DischargeTreatment Options 8.2.5

This section discusses zero dischargewastewater treatment and disposal methods. Inthis context, zero discharge refers to anywastewater disposal method other than indirectdischarge to a POTW or direct discharge to asurface water. A common zero dischargemethod employed by CWT facilities thatgenerate small volumes of wastewater istransportation of the wastewater to an off-sitedisposal facility such as another CWT facility.

Other methods discussed below include deepwell disposal, evaporation, and solidification.

Deep well disposal consists of pumping thewastewater into a disposal well, that dischargesthe liquid into a deep aquifer. Normally, theseaquifers are thoroughly characterized to insurethat they are not hydrogeologically-connected toa drinking water supply. The characterizationrequires the confirmation of the existence ofimpervious layers of rock above and below the

aquifer. Pretreatment of the wastewater usingfiltration is often practiced to prevent theplugging of the face of the receiving aquifer.

Traditionally used as a method of sludgedewatering, evaporation (or solar evaporation)also can involve the discharge and ultimatestorage of wastewater into a shallow, lined, on-site basin or ditch. Because the system is opento the atmosphere, the degree of evaporation isgreatly dependent upon climatic conditions. This

option is generally available only to thosefacilities located in arid regions.

Solidification is a process in which materials,such as fly ash, cement, and lime, are added tothe waste to produce a solid. Depending on boththe contaminant and binding material, thesolidified waste may be disposed of in a landfillor incinerated.

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INDUSTRY PRACTICE

EPA has information for 24 CWT facilities notdischarging directly to surface waters or POTWsthat employ zero and alternate dischargemethods. Of those 24 facilities, seven dispose ofwastewater by deep well injection, 13 transportwastewater to an off-site commercial or intra-company wastewater treatment facility, twodispose of wastewater by evaporation, onesolidifies wastewater and landfills it on-site, and

one discharges wastewater to a privately-ownedtreatment works.

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REFERENCES 8.3

Standard Methods for Examination of Water and Wastewater, 15th Edition, Washington DC.

Henricks, David, Inspectors Guide for Evaluation of Municipal Wastewater Treatment Plants,Culp/Wesner/Culp, El Dorado Hills, CA, 1979.

Technical Practice Committee, Operation of Wastewater Treatment Plants, MOP/11, Washington, DC,1976.

Clark, Viesman, and Hasner, Water Supply and Pollution Control, Harper and Row Publishers, NewYork, NY, 1977.

Environmental Engineering Division, Computer Assisted Procedure For the Design and Evaluation ofWastewater Treatment Systems (CAPDET), U. S. Army Engineer Waterways Experiment Station,Vicksburg, MS, 1981.

1991 Waste Treatment Industry Questionnaire, U.S. Environmental Protection Agency, Washington,DC.

Osmonics, Historical Perspective of Ultrafiltration and Reverse Osmosis Membrane Development,Minnetonka, MN, 1984.

Organic Chemicals and Plastics and Synthetic Fibers (OCPSF) Cost Document, SAIC, 1987.

Effluent Guidelines Division, Development Document for Effluent Limitations Guidelines & Standardsfor the Metal Finishing , Point Source Category, Office of Water Regulation & Standards, U.S. EPA,Washington, DC, June 1983.

Effluent Guidelines Division, Development Document For Effluent Limitations Guidelines andStandards for the Organic Chemicals, Plastics and Synthetic Fibers (OCPSF), Volume II, Point SourceCategory, EPA 440/1-87/009, Washington, DC, October 1987.

Engineering News Record (ENR), McGraw-Hill Co., New York, NY, March 30, 1992.

Comparative Statistics of Industrial and Office Real Estate Markets, Society of Industrial and OfficeRealtors of the National Association of Realtors, Washington, DC, 1990.

Effluent Guidelines Division, Development Document for Effluent Limitations Guidelines & Standardsfor the Pesticides Industry, Point Source Category, EPA 440/1-85/079, Washington, DC, October,1985.

Peters, M., and Timmerhaus, K., Plant Design and Economics for Chemical Engineers, McGraw-Hill,New York, NY, 1991.

Chemical Marketing Reporter, Schnell Publishing Company, Inc., New York, NY, May 10, 1993.

Palmer, S.K., Breton, M.A., Nunno, T.J., Sullivan, D.M., and Supprenaut, N.F., Metal/CyanideContaining Wastes Treatment Technologies, Alliance Technical Corp., Bedford, MA, 1988.

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Freeman, H.M., Standard Handbook of Hazardous Waste Treatment and Disposal, U.S. EPA,McGraw-Hill, New York, NY, 1989.

Corbitt, Robert, Standard Handbook of Environmental Engineering, McGraw-Hill Publishing Co., NewYork, NY, 1990.

Perry, H., Chemical Engineers Handbook, 5th Edition, McGraw-Hill, New York, NY, 1973.

Development Document for BAT, Pretreatment Technology and New Source PerformanceTechnology for the Pesticide Chemical Industry, USEPA, April 1992.

Vestergaard, Clean Harbors Technology Corporation to SAIC - letter dated 10/13/93.

Brown and Root, Inc., "Determination of Best Practicable Control Technology Currently Available toRemove Oil and Gas," prepared for Sheen Technical Subcommittee, Offshore Operators Committee,New Orleans, (March 1974).

Churchill, R.L., "A Critical Analysis of Flotation Performance," American Institute of ChemicalEngineers, 290-299, (1978).

Leech, C.A., "Oil Flotation Processes for Cleaning Oil Field Produced Water," Shell Offshore, Inc.,Bakersfield, CA, (1987).

Luthy, R.C., "Removal of Emulsified Oil with Organic Coagulants and Dissolved Air Flotation," JournalWater Pollution Control Federation, (1978), 331-346.

Lysyj, I., et al., "Effectiveness of Offshore Produced Water Treatment," API et al., Oil Spillprevention, Behavior Control and Clean-up Conference (Atlanta, GA) Proceedings, (March 1981).

Pearson, S.C., "Factors Influencing Oil Removal Efficiency in Dissolved Air Flotation Units," 4thAnnual Industrial Pollution Conference, Houston, TX, (1976).

Viessman, W., And Hammer, M.J., Water Supply and Pollution Control, Harper Collins Publishers,New York, NY, 1993.

Wyer, R.H., et al., "Evaluation of Wastewater Treatment Technology for Offshore Oil ProductionFacilities," Offshore Technology Conference, Dallas, TX, (1975).

Eckenfelder, Welsey, Industrial Pollution Control, New York: McGraw-Hill, 1989.

Joint Task Force, Design of Municipal Wastewater Treatment Plants, MOP 8, Alexandria: WaterEnvironment Federation, 1991.

Tchobanoglous, George, Wastewater Engineering, 2nd Ed., New York: McGraw-Hill, 1979.

Development Document for the Proposed Effluent Limitations Guidelines and Standards for theLandfills Point Source Category, USEPA, January, 1998.

Development Document for the Proposed Effluent Limitations Guidelines and Standards for IndustrialWaste Combustors, USEPA, December 1997.