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SECTION 6

PLANNING AND DESIGN OFHAZARDOUS WASTE TREATMENT CENTERS

Planning and designing a hazardous waste treatment center (HWTC) is a necessarycomplement to construction of common effluent treatment plants (CETPs) (see Section 5). Thepurpose of an HWTC is to provide a safe alternative to uncontrolled disposal of hazardousindustrial liquid and solid wastes. A related purpose is to direct toxic discharges from industryaway from CETPs to prevent impairment of their plants' treatment processes. An HWTC is asingle centralized facility that serves a relatively large geographic area including manyindividual SMSEs to take advantage of economies of scale that allow the use of hazardouswaste treatment options, such as incineration, that are beyond the technical and financialcapabilities of individual SMSEs. Although this chapter focuses on centralized treatment, thepossibility of installing treatment processes for specific types of wastes at the industrial facilityshould not be overlooked. For example, relatively small distillation units for solvent recoveryare available, and some SMSEs might be able to take advantage of this waste minimizationoption.

This section focuses on the planning and engineering design of HWTCs and includesinformation on the major types of hazardous waste treatment alternatives available. Volume IIincludes an exercise to estimate waste generation from an industry producing lacqueredwooden utensils and to analyze the appropriate treatment technologies.

6.1 DESIGN BASIS

A centralized HWTC is actually only part of a larger system that can include: (1) acollection system (road, rail, or a combination of the two) for obtaining wastes from individualSMSEs (and larger enterprises), (2) transfer centers where wastes with similar characteristicscan be identified and combined, (3) a system for transport of bulk wastes from transfer centersto the centralized treatment plant, (4) an HWTC designed to handle a wide range of hazardouswastes, and (5) a secure landfill for disposal of solidified wastes and other treatment residuals(e.g., incinerator ash, residual sludges).

The major differences involved in designing an HWTC compared with a CETP are (1)HWTCs only use mobile collection systems are never sewer systems, and (2) treatmentsystems at HWTCs need to be flexible enough to handle wastes with varying characteristicsbecause of the diversity of waste sources.

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6.1.1 Solid and Liquid Waste Characteristics and Volume

Section 3 provides basic information on the types of wastes that an HWTC is likely tohandle. Because a large number of SMSEs might be involved, obtaining detailed informationon the waste characteristics and volumes for each enterprise may not be feasible. Detailedaudits of randomly selected enterprises in a particular industrial category can provide a basisfor estimating the overall waste characteristics and volume for that category, provided aninventory of all existing enterprises is available. Efforts should concentrate on identifying thevolume of wastes in the major categories identified in Table A-2 of Worksheet A in Volume II(e.g., cyanide wastes, heavy metal sludges and solutions, halogenated solvents, nonhalogenatedsolvents). For the purpose of treatment method selection and the design of operatingparameters, it is important to have some idea of the range of concentrations of the major wastestreams (i.e., percent organics and the maximum concentrations of metals in wastewater, etc.).

6.1.2 Collection and Transportation

For the following reasons, mobile rather than pipe systems are most cost-effective intransporting wastes from SMSEs to an HWTC:

n Both solid and liquid wastes must be collected, so mobile pickup is still requiredeven if a pipe system is in place.

n When multiple liquid waste streams (spent solvents, contaminated wastewaters)have to be handled separately, multiple pipe collection systems are prohibitivelyexpensive.

n Outlying SMSEs can be more easily served by mobile systems than a pipe network.n More expensive construction materials are generally required for pipe systems

handling hazardous wastes.

Just as pretreatment before discharge to a sewer is essential for effective functioning ofa CETP, segregation of hazardous waste streams at the SMSE is essential for effectiveoperation of an HWTC. Segregation maximizes opportunities for recovery and recycling ofwastes such as solvents and metals, and minimizes potential hazards from mixing ofincompatible wastes. Liquid wastes at the SMSE are generally stored in holding tanks that canbe pumped into portable tanks for transport to an HWTC, while solid wastes can be stored indrums or other containers that can be transported or possibly reused.

Large quantities of liquid wastewaters and dilute sludge slurries are most economicallytransported using vacuum inductor tank trucks. Tank trucks are manufactured in a variety ofcapacities ranging from 1,000 to 6,000 gallons. Small volumes of waste (less than 500 gallons)are most economically stored in drums and transported on flatbed trucks. A standard industrialdrum holds 55 gallons. Drums are especially applicable in situations where several smallvolumes of different, incompatible wastes are generated at a single facility.

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During the design of an HWTC, transportation maps are developed showing locationsof individual SMSEs in relation to roads, rail lines, and navigable waterways. Specific designelements of a collection and transportation system include: 1) the selection of containermaterials suitable for the types of wastes to be transported; 2) choosing types and sizes ofvehicles compatible with available transport routes; 3) choosing the number of vehicles toensure a waste pickup that is commensurate with the volume of waste generated with a safetymargin for delays and maintenance; and 4) the development of safe operating procedures fortracking, handling, and transporting the hazardous materials.

6.1.3 Transfer Centers or Stations

An HWTC capable of handling the full range of hazardous wastes generated in a regionmight be too far from many SMSEs to allow efficient transportation directly to the center.Transfer centers allow preliminary classification and mixing of compatible wastes, thusfacilitating processing and treatment upon arrival at the HWTC. Waste storage at transfercenters also facilitates equalization of the volumes of different waste types to further enhancethe efficient operation of the HWTC.

Location is a key consideration in the design of transfer centers. Centers should be nearmajor transportation corridors (rail lines, highways) to facilitate transporting large volumes ofwaste from the transfer center to the HWTC. A transfer center should have enough land areaavailable to provide temporary storage of containerized waste and possibly larger volumestorage of several types of liquid wastes. Other important design elements for these centersinclude: (1) designing facilities for chemical analysis of wastes for classification andcompatibility testing (see Section 6.1.4), and (2) designing containment systems to protectonsite workers and nearby populations from exposure to hazardous materials.

6.1.4 Characterization and Classification of Received Wastes

Wastes received at an HWTC must be accurately characterized and classified to avoidhazards from mixing of incompatible wastes and to ensure that waste volumes do not exceedthe design limits of treatment processes. Potential hazards from mixing incompatible hazardouswastes include:

n Explosionsn Firen Generation of flammable gasn Generation of toxic gasn Generation of heatn Solubilization of toxins

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Figure 6-1 identifies potentially incompatible combinations of 12 types of hazardouswaste. Compatibility is primarily a concern when different wastes are mixed to create largerbatches for treatment by a particular process. Compatibility might also be of concern indesigning waste storage facilities. These should be designed to minimize the possibility ofaccidental releases being incompatible.

Screening tests that do not require sophisticated and expensive laboratory equipment areusually used for compatibility testing. These tests are normally used during collection if liquidwastes are mixed in the transport container, or at transfer centers. An HWTC should have alaboratory capable of performing accurate analyses of a wide range of organic and inorganicsubstances. Laboratory analyses allow identification of wastes that exceed specifications forcontinuous treatment processes. Off-specification wastes must either be treated using analternative method or modified by mixing them with other wastes until they fall withinspecifications. Laboratory analyses are also used to characterize wastes that are treated usingbatch chemical processes for the purpose of determining required chemical inputs to completereactions.

The U.S. Environmental Protection Agency (EPA) (1984a) provides detailed guidanceof development of waste analysis plans. Appendix A contains several checklists that may beuseful in the development of waste analysis plans and a list of simple American Society forTesting and Material (ASTM) test methods for screening waste characteristics. Appendix Ealso includes a bibliography of major references on methods for chemical analysis of wastesand wastewaters.

Figure 6-1. Compatibility of selected hazardous wastes (Batstone et al., 1989)

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6.2 HAZARDOUS WASTEWATER TREATMENT PROCESS ALTERNATIVES

As with CETPs, methods for treating hazardous industrial wastewater can be broadlyclassified as physical, chemical, and biological. Physical methods for component separation(e.g., gravity, filtration) discussed in Section 5.2 for CETPs are equally applicable tohazardous wastewaters and are generally used for the same purposes, including: (1)preliminary (see Section 5.2.1) and primary treatment (see Section 5.2.2) to remove settleablesolids, and (2) clarification systems (see Section 5.2.2) to remove flocculated impurities andprecipitates following chemical treatment processes that generate suspended solids.

Figure 6-2 identifies the operational size ranges of different methods for physicaltreatment of industrial wastewater. Physical treatment methods can be classified as componentseparation methods that use size or density as the primary separation factor and phaseseparation methods that generally operate on the ionic and molecular level to separatecontaminants from the liquid matrix. As shown in Figure 6-2, however, some phase separationmethods also operate within the molecular size range.

This section focuses on physical phase separation and chemical treatment methods thatare commonly used for treating hazardous industrial wastewaters. Most of the chemicalprocesses discussed in this section also are suitable for pretreatment of wastewater on site at anSMSE prior to discharging to a CETP. When used on site, it is mainly the residuals(concentrated liquid wastes and sludges) generated by these processes that would be collectedfor treatment at a HWTC. In the absence of pretreatment and discharge to a CETP, toxicwastewater should be collected and transported to an HWTC where the same or similartreatment processes would be used. The advantage of pretreatment of industrial wastewater onsite is that a much smaller volume of more concentrated waste can be sent to the HWTC.

Physical phase separation methods covered in this section include:

n Air stripping (Section 6.2.1)n Carbon adsorption (Section 6.2.2)n Reverse osmosis (Section 6.2.3)n Ultrafiltration (Section 6.2.3)n Liquid carbon dioxide extraction (Section 6.2.3)

Chemical treatment methods covered in this section include:

n Neutralization (Section 6.2.4)n Chemical precipitation (Section 6.2.5)n Cyanide destruction (Section 6.2.6)n Chromium reduction (Section 6.2.7)n Electrolytic recovery (Section 6.2.8)n Ion exchange (Section 6.2.9)

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Figure 6-2. Operational size ranges of methods for treating industrial wastewaters(Fresenius et al., 1989)

The descriptions of these materials are drawn largely from EPA's DevelopmentDocument for Proposed Effluent Limitations Guidelines and Standards for the CentralizedWaste Treatment Industry (EPA). This report contains much useful performance data ontreatment methods used in the centralized waste treatment industry.

Biological treatment methods applicable to toxic wastewaters are discussed briefly inSection 6.2.10. Appendix B identifies references that include more detailed information onengineering design for specific processes.

6.2.1 Air Stripping

Air stripping is an effective treatment method for removing dissolved volatile organiccompounds from wastewater. The removal is accomplished by passing high volumes of airthrough the agitated wastewater stream. The process results in a contaminated off-gas streamthat, depending upon air emissions standards, usually requires treatment in air pollution controlequipment.

Stripping can be performed in tanks or in spray or packed towers. Treatment in packedtowers is the most efficient application. The packing typically consists of plastic rings orsaddles. Two commonly used types of towers, cross-flow and countercurrent, differ in designonly in the location of air inlets. Cross-flow towers draw air through the sides for the total

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height of the packing. The countercurrent tower draws the entire air flow from the bottom. Thecross-flow towers are more susceptible to scaling problems and are less efficient than thecountercurrent towers. A countercurrent air stripper is shown in Figure 6-3.

Figure 6-3. Air stripping system diagram (U.S. EPA, 1995)

Figure 6-3a shows a variance of the countercurrent air stripper system.

Figure 6-3a. Compact bed scrubberSource: Productos químicos. Planes de acción para mejoramiento ambiental. Manual paraempresarios de la PYME. Santafé de Bogotá: Sir Speedy. Impresiones Daza Aragón Ltda.

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The driving force of the air stripping mass-transfer operation is the difference inconcentrations between the air and liquid streams. Pollutants are transferred from the moreconcentrated wastewater stream to the less concentrated air stream until equilibrium is reached;this equilibrium relationship is known as Henry's Law. The strippability of a pollutant isexpressed as its Henry's Law Constant, which is a function of both its volatility and solubility.

Air strippers are designed according to the characteristics of the pollutants to beremoved. Organic pollutants can be divided into three general strippability ranges (low,medium, and high) according to their Henry's Law Constants. The low strippability group,with Henry's Law Constants of 10-3 (mg/m3 air)/(mg/m3 water) and lower, are not effectivelyremoved by air stripping. Pollutants in the medium (10-1 to 10-3) and high (greater than 10-1)groups are effectively stripped. Pollutants with lower Henry's law constants require greatercolumn height, more trays or packing material, greater pressure and temperature, and morefrequent cleaning than pollutants with a higher strippability.

Low temperatures adversely affect the air stripping process. Air strippers experiencelower efficiencies at lower temperatures, with the possibility of freezing occurring within thetower. For this reason, depending on the location of the tower, it may be necessary to preheatthe wastewater and the air feed streams. The column and packing materials must be cleanedregularly to ensure that low effluent levels are attained.

Air stripping has proved to be an effective process in the removal of volatile pollutantsfrom wastewater. It is generally limited to influent concentrations of less than 100 mg/Lorganics. Well-designed and operated systems can achieve over 99 percent removals.

6.2.2 Carbon Adsorption

Activated carbon adsorption is a demonstrated treatment technology for the removal oforganic pollutants from wastewater. Most applications use granular activated carbon (GAC) incolumn reactors. Sometimes powdered activated carbon (PAC) is used alone or in conjunctionwith another process, such as biological treatment. GAC is the more commonly used method;however, a diagram of a downflow fixed-bed GAC system is presented in Figure 6-4.

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Figure 6-4. Carbon adsorption system diagram (U.S. EPA, 1995)

The mechanism of adsorption is a combination of physical, chemical, and electrostaticinteractions between the activated carbon and the adsorbate, although the attraction is primarilyphysical. Activated carbon can be made from many carbonaceous sources including coal, coke,peat, wood, and coconut shells.

The key design parameter is adsorption capacity, a measurement of the mass ofcontaminant adsorbed per unit mass of carbon, which is a function of the compound beingadsorbed, the type of carbon used, and the process design and operating conditions. In general,the adsorption capacity is inversely proportional to the adsorbate solubility. Nonpolar, high-molecular-weight organics, with low solubility, are readily adsorbed. Polar, low-molecular-weight organics, with high solubilities, are more poorly adsorbed. Competitive adsorption ofother compounds affects adsorption. The carbon may preferentially adsorb one compound overanother with the competition resulting in an adsorbed compound being desorbed from thecarbon.

In a fixed-bed system, pollutants are removed in increasing amounts as wastewaterflows through the bed. In the upper area of the bed, pollutants are rapidly adsorbed. As morewastewater passes through the bed, this rapid adsorption zone increases until it reaches thebottom of the bed. At this point, all available adsorption sites are filled and the carbon is said

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to be exhausted. This condition can be detected by an increase in the pollutant concentration ofthe effluent from the bed and is called breakthrough.

GAC systems usually comprise several beds operated in series. This design allows thefirst bed to go to exhaustion, while the other beds still have the capacity to treat to anacceptable effluent quality. The carbon in the first bed is replaced, and the second bed thenbecomes the lead bed. The GAC system piping is designed to allow switching of bed order.

After the carbon is exhausted, it can be removed and regenerated. Usually, heat orsteam is used to reverse the adsorption process. The light organic compounds are volatilized,and the heavy organic compounds are pyrolyzed. Spent carbon can also be regenerated bycontacting it with a solvent that dissolves the adsorbed pollutants. Depending on system sizeand economics, some facilities may choose to dispose of the spent carbon instead ofregenerating it. For very large applications, as may occur at an HWTC, construction of anon-site regeneration facility may be justified. For smaller applications, it is generallycost-effective to use a vendor service to deliver regenerated carbon and remove the spentcarbon. These vendors transport the spent carbon to their centralized facilities for regeneration.

GAC adsorption is a widely used wastewater treatment technology. Generally, thechemical oxygen demand (COD) of the waste stream can be reduced to less than 10 mg/L andthe biological oxygen demand (BOD) to less than 2 mg/L. Removal efficiencies typically are inthe range of 30 to 90 percent.

Poor GAC system performance sometimes results from competitive adsorption betweencompounds in the waste stream. The pollutant methylene chloride is often used as a measure ofadsorption competition in a GAC system because it is readily adsorbed and also desorbed bycompetitive compounds. Thus, low methylene chloride removals indicate competitiveadsorption effects. Oil and grease can adversely affect GAC performance by coating the carbonparticles, thereby inhibiting the adsorption process. A commonly applied limit on oil andgrease loading to a GAC system is 10 mg/L. Suspended solids also adversely affect GACperformance by plugging the bed, resulting in excessive head loss. A commonly used totalsuspended solids (TSS) loading limit to a GAC system is 50 mg/L. Poor performance of GACunits used at centralized waste treatment plants in the United States of America has beenobserved and attributed to the inherent difficulty of operating carbon adsorption units forvariable waste streams (EPA, 1995).

6.2.3 Other Physical Treatment Technologies

Other less commonly used physical treatment technologies used in the U.S. centralizedwaste treatment industry include:

n Reverse osmosisn Ultrafiltrationn Carbon dioxide liquid extraction

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Reverse osmosis (RO) is a process for separating dissolved solids from water. It iscommonly used to treat oily or metal-bearing wastewater. RO is applicable when the solutemolecules are approximately the same size as the solvent molecules. A semipermeable,microporous membrane and pressure are used to perform the separation. RO systems aretypically used as end-of-pipe polishing processes, prior to final discharge of the treatedwastewater.

Osmosis is the diffusion of a solvent (such as water) across a semipermeable membranefrom a less concentrated solution into a more concentrated solution. In the reverse osmosisprocess, pressure greater than the normal osmotic pressure is applied to the more concentratedsolution (the waste stream being treated), forcing the purified water through the membrane andinto the less concentrated stream, which is called the permeate. Low-molecular-weight solutes(for example, salts and some surfactants) do not pass through the membrane. They are referredto as concentrate. The concentrate is recirculated through the membrane unit until the permeateflow drops. The permeate can either be discharged or passed along to another treatment unit.The concentrate is contained and held for further treatment or disposal. An RO system isshown in Figure 6-5a.

Figure 6-5. Other physical treatment technologies: (a) reverse osmosis, (b) liquid carbondioxide extraction (EPA, 1995)

Performance of an RO system is dependent upon the dissolved solids concentration andtemperature of the feed stream, the applied pressure, and the type of membrane selected. Thekey RO membrane properties to be considered are selectivity for water over ions, permeationrate, and durability. RO modules are available in various membrane configurations, such asspiral-wound, tubular, hollow-fiber, and plate and frame. In addition to the membrane

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modules, other capital items needed for an RO installation include pumps, piping,instrumentation, and storage tanks. The major operating cost is attributed to membranereplacement.

EPA (1995) presents performance data for a single unit with an average reduction in theconcentration of oil and grease by 87.4 percent. Aluminum, barium, calcium, chromium,cobalt, iron, magnesium, manganese, nickel, and titanium were all reduced in this unit bymore than 98 percent.

Ultrafiltration (UF) is used for the treatment of metal-finishing wastewater and oilywastes. It can remove substances with molecular weights greater than 500, including suspendedsolids, oil and grease, large organic molecules, and complex heavy metals. UF is used whenthe solute molecules are greater than 10 times the size of the solvent molecules and are lessthan one-half micron. The centralized waste treatment industry applies UF to treat oil/wateremulsions. Oil/water emulsions contain both soluble and insoluble oil. Typically, the insolubleoil is removed from the emulsion by gravity separation assisted by chemical addition. Thesoluble oil is then removed through UF. Oily wastewater containing 0.1 to 10 percent oil canbe effectively treated using UF. A UF system is typically used as an in-plant treatmenttechnology, treating the oil/water emulsion prior to mixing with other wastewater. A schematicUF system is similar to reverse osmosis (see Figure 6-5a), with the difference being in thecharacteristics of the membrane.

In UF, a semipermeable, microporous membrane performs the separation. Wastewateris sent through membrane modules under pressure. Water and low-molecular-weight solutes(e.g., salts, some surfactants) pass through the membrane and are removed as permeate. Themembrane rejects emulsified oil and suspended solids, which are removed as concentrate. Theconcentrate is recirculated through the membrane unit until the permeate flow drops. Thepermeate can either be discharged or passed along to another treatment unit. The concentrate iscontained and held for further treatment or disposal.

The primary design consideration in UF is membrane selection. Membrane pore size ischosen based on the size of the contaminant particles targeted for removal. Other designparameters to be considered are solids concentration, viscosity, and temperature of the feedstream, and membrane permeability and thickness.

U.S. EPA (1995) presents performance data for a UF system that treats oilywastewater. The system removed 87.5 percent of the influent oil and grease and 99.9 percentof the TSS. Removal of several organic and metal pollutants exceeded 90 percent.

Liquid carbon dioxide (CO2) extraction is used to extract and recover organiccontaminants from aqueous waste streams. A licensed, commercial application of thistechnology, the "Clean Extraction System (CES)," is used in the centralized waste treatmentindustry. The process can be effective in removing organic substances such as hydrocarbons,aldehydes and ketones, nitriles, halogenated compounds, phenols, esters, and heterocyclics. It

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is not effective in removing some compounds that are very water-soluble, such as ethyleneglycol, and low-molecular-weight alcohols. It can provide an alternative in the treatment ofwaste streams that historically have been incinerated.

The waste stream is fed into the top of a pressurized extraction tower containingperforated plates, where it is contacted with a countercurrent stream of liquefied CO2. Theorganic contaminants in the waste stream are dissolved in the CO2; this extract is then sent to aseparator, which redistills the CO2. The distilled CO2 vapor is compressed and reused. Theconcentrated organics bottoms from the separator can then be disposed or recovered. Thetreated wastewater stream that exits the extractor (raffinate) is pressure-reduced and may befurther treated for residual organics removal if necessary to meet discharge standards. Adiagram of the CES is presented in Figure 6-5b.

Pilot-scale operational data for a commercial CES unit show high removals for theorganic compounds chloroform, 1,2-dichloroethane, ethylbenzene, methylene chloride, andtoluene, with rates generally exceeding 99 percent (phenol removal was poorest with 83percent). EPA sampled a CES operating unit and found significantly lower removal rates,ranging from 48 to 88 percent.

6.2.4 Neutralization

Acidic corrosive wastes (pH less than 2) and alkaline corrosive wastes (pH greater than12.5) typically require neutralization prior to use of subsequent treatment processes to limitcorrosion of processing equipment and to improve treatment efficiency. Neutralization or pHadjustment is often required for wastes that do not classify as corrosive in order to optimizechemical treatment such as precipitation (see Section 6.2.5) and biological treatment.

Major neutralization processes include (1) mixing of acid and alkali waste streams, (2)use of alkaline materials to neutralize acids (limestone, lime, and caustic soda), and (3) used ofacidic reagents to neutralize alkaline wastes (sulfuric acid, hydrochloric acid, carbonic acidsand liquid carbon dioxide). Table 6-1 provides summary information on these processes,including:

n Applicable waste streamsn Stage of developmentn Performancen Residuals generatedn Cost

Mixing acid and alkali waste streams is the simplest and least expensive method,provided the wastes are compatible. Cyanide-containing wastes generally require treatment todestroy cyanides (see Section 6.2.6) before neutralization. Typically, a tradeoff exists betweenthe cost of neutralization reagents and length of time required for neutralization and the volume

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of sludge the process creates. Cheaper methods generally take a longer time to completeneutralization due to more dilute concentrations of reagents. Cheaper reagents, such aslimestone and lime, and sulfuric acid also tend to produce larger volumes of sludges.

Selection of a neutralization method requires evaluation of the compatibility of wastestreams with each other and available reagents. Selection of a method also required weighingthe tradeoffs of the reagent cost versus the speed of neutralization and the sludge disposal cost.Section 6.4.2 discusses further options for treatment and use of corrosive wastes.

6.2.5 Chemical Precipitation

Chemical precipitation is used to remove metal compounds from wastewater. In thechemical precipitation process, soluble metallic ions and certain anions are converted toinsoluble forms, which precipitate from the solution. The precipitated metals are subsequentlyremoved from the wastewater stream by liquid filtration or clarification. The performance ofthe process is affected by chemical interactions, temperature, pH, solubility, and mixingeffects.

Various chemicals can be used as precipitants, including sodium hydroxide (NaOH),lime (Ca(OH)2), soda ash, sulfide, ferrous sulfate, and acid. Hydroxide precipitation iseffective in removing such metals as antimony, arsenic, chromium, copper, lead, mercury,nickel, and zinc. Sulfide precipitation primarily removes mercury, lead, and silver.

Hydroxide precipitation using lime or caustic is the most commonly used means ofchemical precipitation, and of these, lime is used more often than caustic. The chief advantageof lime over caustic is its lower cost. Lime is more difficult to handle and feed, however, as itmust be slaked, slurried, and mixed, and can plug the feed system lines. Lime also produces alarger volume of sludge, and the sludge is generally not suitable for reclamation due to itshomogeneous nature. Also, dewatered metal sludge is typically sold to smelters for reuse, andthe calcium compounds in lime precipitation sludge interfere with smelting. The metals fromcaustic precipitation sludge can often be recovered. The reaction mechanism for precipitationof a divalent metal using lime is shown below:

M++ + Ca(OH)2 _ M(OH)2 + Ca++

And, the reaction mechanism for precipitation of a divalent metal using caustic is:

M++ + 2NaOH _ M(OH)2 + 2Na++

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Table 6-1. Summary of neutralization technologies (Wilk et al., 1988)

Process Applicable WasteStreams

Stage ofDevelopment

Performance Residuals Generated Cost

Acid/alkalimutualneutralization

All acid/alkalicompatible wastestreams exceptcyanide

Well developed Generally slower thancomparable technologiesdue to dilute concentrationof reagents. May evolvehazardous constituents ifincompatible wastes aremixed

Variable, dependent onquantity of insolubles andproducts contained in eachwaste stream

Least expensive ofall neutralizationtechnologies

Limestone Dilute acid wastestreams of less than5,000 mg/l mineralacid strength andcontaining lowconcentration of acidsalts

Well developed Requires stone sizes of0.074 mm or less. Requires45 minutes or more ofretention time. Can onlyneutralize acidic wastes topH 6.0. Must be aerated toremove evolved CO2

Will generate voluminoussludge product whenreacted with sulfate-containing wastes. Stonesover 200 mesh willsulfonate, be renderedinactive, and add to sludgeproduct

Most cost-effectivein treatingconcentrated wastes.May be cost-effectivein treating diluteacidic wastes

Lime All acid wastes Well developed Requires 15 to 30 minutesof retention time. Must beslurried to a concentrationof 10 to 35% solids prior touse. Can under-(below pH7) or over- (above pH 7)neutralize

Will generate voluminoussludge similar to limestone

More expensive thancrushed limestone(200 mesh)

Caustic soda All acid wastes Well developed Requires 3 to 15 minutes ofretention time. In liquidform, easy to handle andapply. Can under- or over-neutralize including pH 13or higher

Reaction products aregenerally soluble, however,sludges do not dewater asreadily or as easily as limeor limestone

Most expensive of allwidely used alkalinereagents (five timesthe cost of lime)

Sulfuric acid All alkaline wastesexcept cyanide

Well developed Requires 15 to 30 minutesof retention time. In liquidform, but presents burnhazard. Highly reactive andwidely available

Will generate largequantities of gypsum sludgewhen reacted with calcium-based alkaline wastes

Least expensive ofall widely used acidicreagents

Hydrochloricacid

All alkaline wastes Well developed,but rarelyapplied due tohigh reagent cost

Requires 5 to 20 minutes ofretention time. Liquid formpresent burn and fumehazard. More reactive thansulfuric

Reaction products aregenerally soluble

Approximately twiceas expensive assulfuric on aneutralizationequivalent basis

Carbonic acids,liquid carbondioxide

All alkaline wastesexcept cyanide

Emergingtechnology

Retention time 1 to 11/2minutes. In liquid form,must be vaporized prior touse. Can only neutralizealkaline wastes to pH 8.3and point

Will form calciumcarbonate precipitate whenreacted with calcium-basedalkaline wastes

Approximately 3 to 4times as expensive assulfuric. Therefore,limited toapplications usingmore than 200 tonsof reagents per yearor with flow rategreater than 100,000gpd

In addition to the type of treatment chemical chosen, another important design factor inthe chemical precipitation operation is pH. Metal hydroxides are amphoteric, meaning that they

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can react chemically as acids or bases. As such, their solubilities increase toward both lowerand higher pH levels. Therefore, each metal has an optimum pH for precipitation thatcorresponds to its point of minimum solubility. Another key consideration in a chemicalprecipitation application is the detention time, which is specific to the wastewater being treatedand the desired effluent quality. It may take from less than an hour to several days to achieveadequate precipitation of the dissolved metal compounds.

Chemical precipitation is a two-step process. It is typically performed in batchoperations where the wastewater is first mixed with the treatment chemical in a tank. Themixing is typically achieved by mechanical means such as mixers or recirculation pumping.Then, the wastewater undergoes a separation/dewatering process such as clarification orfiltration, where the precipitated metals are removed from solution. In a clarification system, aflocculent is sometimes added to aid in the settling process. The resulting sludge from theclarifier or filter must be further treated, disposed, or recycled. A typical chemicalprecipitation system is shown in Figure 6-6.

Figure 6-6. Chemical precipitation system diagram (U.S. EPA, 1995)

The batch operation aspect of chemical precipitation makes it an easily adapted processfor treatment at HWTCs, where the waste receipts can be highly variable. A facility can holdits wastes and segregate them by pollutant content for treatment. This type of waste treatment

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management, called selective metals precipitation, can be implemented to concentrate on oneor two major pollutants of concern. This application of chemical precipitation uses severaltanks to allow the facility to segregate its wastes into smaller batches, thereby avoidinginterference with other incoming waste receipts and increasing treatment efficiency. Thesespecific operations also produce specific sludges containing only the target metals, makingthem suitable for reclamation.

The effluent quality achievable with chemical precipitation depends upon the metalspresent in the wastewater and the process operating conditions. This technology is widely usedwith possible removal efficiencies greater than 99 percent, and it often removes metalpollutants down to levels of 1 µg/L or less.

6.2.6 Cyanide Destruction

Cyanide is a very toxic pollutant and, therefore, wastes containing cyanide are animportant environmental concern. Electroplating and metal finishing operations produce mostcyanide-bearing wastes. At least a dozen cyanide destruction technologies are available, butonly six are used commonly: alkaline chlorination, ozonation, ozonation with irradiation,electrolytic hydrolysis, hydrogen peroxide oxidation, and precipitation processes (Weathington,1988). The most commonly used method is alkaline chlorination with either gaseous chlorineor sodium hypochlorite.

A diagram of an alkaline chlorination system is presented in Figure 2-9. Alkalinechlorination can destroy free dissolved hydrogen cyanide and can oxidize all simple and somecomplex inorganic cyanides; however, it cannot effectively oxidize stable iron, copper, andnickel cyanide complexes. The addition of heat to the alkaline chlorination process canfacilitate the more complete destruction of total cyanides.

In alkaline chlorination using gaseous chlorine, the oxidation process is accomplishedby direct addition of chlorine (Cl2) as the oxidizer and sodium hydroxide (NaOH) to maintainpH levels (see Figure 2-9). The reaction mechanism is:

NaCN + Cl2 + 2NaOH NaCNO + 2NaCl + H202NaCNO + 3Cl2 + 6NaOH 2NaHCO3 + N2 + 6NaCl + 2H20

Destruction of the cyanide takes place in two stages. The primary reaction is partialoxidation of the cyanide to cyanate at a pH above 9. In the second stage, the pH is lowered tothe 8 to 8.5 range for the oxidation of the cyanate to nitrogen and carbon dioxide (as sodiumbicarbonate). Each part of cyanide requires 2.73 parts of chlorine to convert it to cyanate andan additional 4.1 parts of chlorine to oxidize the cyanate to nitrogen and carbon dioxide. Atleast 1.125 parts of sodium hydroxide are required to control the pH with each stage.

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

NaCN + NaOCl NaCNO + NaCl2NaCNO + 3NaOCl + H20 2NaHCO3 + N2 + 3NaCl

Cyanide destruction efficiencies of greater than 99 percent are possible with thistechnology but can vary greatly depending on the forms of cyanide present.

6.2.7 Chromium Reduction

Reduction is a chemical reaction in which electrons are transferred from one chemicalto another. The main application of chemical reduction in wastewater treatment is the reductionof hexavalent chromium to trivalent chromium. This is a commonly used pretreatment processin the leather tanning industry (see Section 2.3.3) and the electroplating industry (see Section2.3.5). The reduction enables the trivalent chromium to be precipitated from solution inconjunction with other metallic salts. Sulfur dioxide, sodium bisulfite, sodium metabisulfite,and ferrous sulfate are strong reducing agents commonly used in industrial wastewatertreatment applications. Two types of chromium reduction are discussed here:

n Reduction through the use of sodium metabisulfite or sodium bisulfiten Reduction through the use of gaseous sulfur dioxide

A diagram of a chromium reduction system using gaseous sulfur dioxide is presented inFigure 2-9.

These chromium reduction reactions are favored by a low pH of 2 to 3. At pH levelsabove 5, the reduction rate is slow. Oxidizing agents such as dissolved oxygen and ferric ironinterfere with the reduction process by consuming the reducing agent. After the reductionprocess, the trivalent chromium is removed by chemical precipitation.

Chromium reduction using sodium metabisulfite (Na2S205) and sodium bisulfite(NaHSO3) are essentially similar. The mechanism for the reaction using sodium bisulfite as thereducing agent is:

3NaHSO3 + 3H2S04 + 2H2CrO4 Cr2(S04)3 + 3NaHSO4 + 5H20

The hexavalent chromium is reduced to trivalent chromium using sodium metabisulfite,with sulfuric acid used to lower the pH of the solution. The amount of sodium metabisulfiteneeded to reduce the hexavalent chromium is reported as 3 parts of sodium bisulfite per part of

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chromium, while the amount of sulfuric acid is 1 part 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 reactionmechanism is:

3SO2 + 3H20 3H2SO3

3H2SO3 + 2H2CrO4 Cr2(S04)3 + 5H20

The hexavalent chromium is reduced to trivalent chromium using sulfur dioxide, withsulfuric acid used to lower the pH of the solution. The amount of sulfur dioxide needed toreduce the hexavalent chromium is reported as 1.9 parts of sulfur dioxide per part ofchromium, while the amount of sulfuric acid is 1 part per part of chromium. At a pH of 3, thetheoretical retention time is approximately 30 to 45 minutes.

U.S. EPA (1995) reported hexavalent chromium reduction efficiency of 99.99 percentfor the sulfur dioxide process based on one centralized waste treatment plant. Another plantusing the chromium reduction process with sodium metabisulfite actually showed an increase inhexavalent concentration, indicating the importance of careful process control to achievetreatment objectives.

6.2.8 Electrolytic Recovery

Electrolytic recovery is used for the reclamation of metals from wastewater. It is acommon technology in the electroplating, mining, and electronic industries and is used for therecovery of copper, zinc, silver, cadmium, gold, and other heavy metals. Nickel is poorlyrecovered due to its low standard potential.

The electrolytic recovery process uses an oxidation and reduction reaction. Conductiveelectrodes (anodes and cathodes) are immersed in the metal-bearing wastewater, with electricalenergy applied to them. At the cathode, a metal ion is reduced to its elemental form(electron-consuming reaction). At the same time, gases such as oxygen, hydrogen, or nitrogenform at the anode (electron-producing reaction). After the metal coating on the cathode reachesa desired thickness, it may be removed and recovered. The metal-plated cathode can then beused as the anode.

The equipment consists of an electrochemical reactor with electrodes, a gas ventingsystem, recirculation pumps, and a power supply. A diagram of an electrolytic recoverysystem is presented in Figure 6-7. Electrochemical reactors are typically designed to producehigh flow rates to increase the process efficiency.

A conventional electrolytic recovery system is effective for the recovery of metals fromrelatively high-concentration wastewater. A specialized adaptation of electrolytic recovery,

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called extended surface electrolysis (ESE), operates effectively at lower concentration levels.The ESE system uses a spiral cell containing a flow-through cathode that has a very openstructure and therefore a lower resistance to fluid flow. This also provides a larger electrodesurface. ESE systems are often used for the recovery of copper, lead, mercury, silver, andgold.

6.2.9 Ion Exchange

Ion exchange is commonly used for the removal of heavy metals from relativelylow-concentration waste streams, such as electroplating wastewater. A key advantage of theion exchange process is that it allows for the recovery and reuse of the metal contaminants. Ionexchange can also be designed to be selective to certain metals and can provide effectiveremoval from wastewater that has high background contaminant levels. A disadvantage is thatsome organic substances can foul the resins.

In an ion exchange system, the wastewater stream is passed through a bed of resin. Theresin contains bound groups of ionic charge on its 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 wastewater contaminant to be removed. A commonly usedresin is polystyrene copolymerized with divinylbenzene.

The ion exchange process involves four steps: treatment, backwash, regeneration, andrinse. During the treatment step, wastewater is passed through the resin bed. The ion exchangeprocess continues until pollutant breakthrough occurs. The resin is then backwashed toreclassify the bed and to remove suspended solids. During the regeneration step, the resin iscontacted with either an acidic or alkaline solution containing the ion originally present in theresin. This "reverses" the ion exchange process and removes the metal ions from the resin.The bed is then rinsed to remove residual regenerating solution. The resulting contaminatedregenerating solution must be further processed for reuse or disposal. Depending upon systemsize and economics, some facilities choose to remove the spent resin and replace it with resinregenerated off site instead of regenerating the resin in-place.

Ion exchange equipment ranges from simple, inexpensive systems such as domesticwater softeners, to large, continuous industrial applications. The most commonly encounteredindustrial setup is a fixed-bed resin in a vertical column, where the resin is regeneratedin-place. A diagram of this type of system is presented in Figure 6-8. These systems can bedesigned so that the regenerant flow is concurrent or countercurrent to the treatment flow. Acountercurrent design, although more complex to operate, provides a higher treatmentefficiency. The beds can contain a single type of resin for selective treatment, or the beds canbe mixed to provide for more complete deionization of the waste stream. Often, individualbeds containing different resins are arranged in series, which makes regeneration easier than inthe mixed bed system.

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Figure 6-7. Electrolytic recovery system diagram (U.S. EPA, 1995)

Ion exchange is very effective in the treatment of low-concentration, metal-bearingwastewater. A common application, chromic acid recovery, has a demonstrated performanceof 99.5 percent. Copper removal from metal finishing rinsewaters can also exceed 99 percent,and nickel removals range from 82 to 96 percent.

6.2.10 Ozonization

Ozone (O3) is a blue gas generated by the passage of air through a high potentialelectric field (10,000 to 20,000 v). Ozone is used to disinfect wastewater because of itsoxidizing properties.

In industrial wastewater treatment, several contact units or chambers with ozone mustbe available to guarantee oxidation of pollutants, virus and bacteria. If wastewater containsflocculated material and an ozone disinfection is desired, it is appropriate to use a turbinecontact system. Indeed, bubbles produced by a porous diffuser system cannot create sufficientturbulence to disintegrate the agglomerated matter or completely oxidize bacteria and virus.The estimation of ozone dosage often requires a previous laboratory test.

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Figure 6-8. Ion exchange system diagram (U.S. EPA, 1995)

6.2.10 Biological Treatment

Conventional biological treatment processes for wastewater are discussed in Section5.3. These processes generally require pretreatment of industrial wastewater to reduceconcentrations of heavy metals and toxic organics to levels that will not impair the performanceof the biological treatment system.

If CETPs or conventional sewage treatment plants are able to treat the bulk ofnonhazardous organics in industrial wastewaters, biological treatment methods are not likely tobe a major component in an HWTC. If the HWTC does receive a significant volume ofnonhazardous industrial wastewater with organics, the treatment options would be similar tothose discussed for CETPs in Section 5.3. Biological treatment process choices might well bedifferent for an HWTC compared with a CETP because the need for maintenance is less of aconstraint.

Rotating biological contactor systems (RBCs) (see Section 5.3.4) are the conventionalbiological treatment process that is most suitable for specific treatment of industrialwastewaters containing up to 1 percent soluble organics, including solvents, halogenatedorganics, acetone, alcohols, phenols, phthalates, ammonia, and petroleum products. RBCs alsocan treat inorganic cyanides (EPA, 1992).

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Slurry biodegradation is a treatment process where an aqueous slurry is created bycombining sludge with water and biodegraded aerobically using a self-contained reactor or alined aerated lagoon. The process is similar to the conventional activated sludge process or anaerated lagoon, except that the system can handle highly contaminated soils or sludges thathave contaminant concentrations ranging from 2,500 mg/kg to 250,000 mg/kg. The mainapplications of this technology are treating coal tars, refinery wastes, hydrocarbons, wood-preserving wastes, and organic and chlorinated organic sludges. The required operationalparameters of the process are similar to the activated sludge process. As with activated sludge,the presence of heavy metals may inhibit microbial metabolism of the slurry.

A promising innovative technology, the anaerobic, expanded-bed, GAC bioreactor,currently being developed by EPA's Risk Reduction Engineering Laboratory in Cincinnati,Ohio, uses both GAC biological treatment to overcome the problems created by wastewatersthat contain both biodegradable organics and toxic organics. Figure 6-9 shows a schematic ofthe system. The GAC sorbs the toxic organics, and the expanded bed configuration enhancesbiomass attachment to the GAC, allowing decomposition of the wastewater's readilybiodegradable constituents and providing regeneration of more slowly degraded substances thatare sorbed on to the GAC. This design, combined with heating to optimize microbial activityrates, allows hydraulic retention times of 3 to 12 hours, representing a significant reduction inbioreactor volume compared with conventional technologies. Table C-3 in Worksheet C inVolume II provides additional information about suitable wastes for this technology.

Figure 6-9. Schematic of anaerobic, expanded-bed GAC bioreactor

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6.3 OTHER HAZARDOUS WASTE TREATMENT PROCESS ALTERNATIVES

Alternative treatment methods for hazardous wastes can be broadly classified as:

n Solidification/stabilization technologies (see Section 6.3.1)n Thermal treatment technologies

Incineration (see Section 6.3.2) is the most commonly used thermal treatment method,but other thermal treatments are available (see Section 6.3.3). Use of bothsolidification/stabilization and thermal treatment (most likely incineration) is an integral part ofan HWTC.

6.3.1 Solidification/Stabilization (S/S)

Solidification or fixation refers to techniques that incorporate hazardous waste into asolid material of high structural integrity. Encapsulation involves either fine waste particles(microencapsulation) or a large block or container of wastes (macroencapsulation).Stabilization refers to techniques that treat hazardous waste by converting it into a less soluble,mobile, or toxic form. Solidification/stabilization (S/S) processes use one or both of thesetechniques.

In situ S/S processes are applied to in place wastes or contaminated soils, and ex situS/S processes involve in-tank treatment. Ex situ S/S processes are most likely to be used at anHWTC. Figure 6-10 shows generic elements of an in-tank treatment system. The key elementsof this system include separating and crushing large particles, and the mixing stage wherebinding agents and water are added. At an HWTC, S/S technologies are most likely to be usedfor two types of waste streams: (1) as-received solid wastes (e.g., plastics, resins, tars andsludges that are not suitable for treatment using other processes, and (2) as a final treatmentstep for residual solids and sludges generated from other treatment processes on site. The finalstep in handling S/S treated wastes usually involve disposal in a secure landfill.

Table C-4 in Worksheet C (Volume II) provides summary information on major S/Sprocesses. Solidification through the addition of cement, lime, or other pozzolanic materialssuch as flyash are the most commonly used and are suitable for the large majority of inorganicwastes. Other S/S processes, such as embedding waste in thermoplastic materials such asbitumen, paraffin or polyethylene, and microencapsulation are more expensive and are usuallyused only for problem-causing wastes such as those with a high organic content. Physicalstabilization involves blending sludge or semiliquid wastes with a bulking agent such aspulverized fly ash to produce a soil-like consistency that can be readily transported by truck,conveyor, or rail car to a disposal site.

A key consideration in evaluating the suitability of S/S technologies is whether thewaste to be treated has physical or chemical properties that would interfere with the

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stabilization or solidification process. Table 6-2 identifies factors that might interfere withthese processes.

Figure 6-10. Elements of a typical ex situ solidification/stabilization process (U.S. EPA,1993)

S/S binding agents

Excavation

(1)

Classification

(2)

Mixing

(3)

Off-gastreatment(optional)

(4)

Residuals

WaterStabilized and solidified

mediaOversize rejects

Crusher

VOCcapture andtreatment

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Table 6-2. Summary of factors that may interfere with stabilization and solidificationprocesses (U.S. EPA)

Characteristics Affecting Processing Feasibility Potential Interference

VOCs Volatiles not effectively immobilized; driven off by heatof reaction. Sludges and soils containing volatileorganics can be treated using a heated extruderevaporator or other means to evaporate free water andVOCs prior to mixing with stabilizing agents.

Use of acidic sorbent with metal hydroxide wastes Solubilizes metal.

Use of acidic sorbent with cyanide wastes Releases hydrogen cyanide

Use of acidic sorbent with waste containingammonium compounds

Releases ammonia gas

Use of acidic sorbent with sulfide wastes Releases hydrogen sulfide

Use of alkaline sorbent (containing carbonates suchas calcite or dolomite) with acid waste

May create pyrophoric waste.

Use of siliceous sorbent (soil, fly ash) withhydrofluoric acid waste

May produce soluble fluorosilicates

Presence of anions in acidic solutions that formsoluble calcium salts (e.g., calcium chloride acetate,and bicarbonate)

Cation exchange reactions – leach calcium from S/Sproduct increases permeability of concrete, increases rateof exchange reactions.

Presence of halides Easily leached from cement and lime.

Organic compounds Organics may interfere with bonding of waste materialswith organic binders

Semivolatile organics or polyaromatic hydrocarbons(PAHs)

Organics may interfere with bonding of waste materials.

Oil and grease Weaken bonds between waste particles and cement bycoating the particles. Decrease in unconfinedcompressive strength with increased concentrations of oiland grease.

Fine particle size Insoluble material passing through a No. 200 mesh sievecan delay setting and curing. Small particles can alsocoat larger particles, weakening bonds between particlesand cement or other reagents. Particle size > ¼ inch indiameter not suitable.

Halides Reduced physical setting, easily leached for cement andpozzolan S/S. May dehydrate thermoplasticsolidification.

Soluble salts of manganese, tin, zinc, copper and lead Reduced physical strength of final product caused bylarge variations in setting time and reduced dimensionalstability of the cured matrix, thereby increasingleachability potential.

Cyanides Cyanides interfere with bonding of waste materials.

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Characteristics Affecting Processing Feasibility Potential Interference

Sodium arsenate, borates, phosphates Retard setting and curing and weaken strength of finalproduct.

Sulfate Retard setting and cause swelling and spalling in cementS/S. With thermoplastic solidification may dehydrate andrehydrate, causing splitting.

Phenols Marked decreases in compressive strength for highphenol levels.

Presence of coal or lignite Coals and lignites can cause problems with setting,curing, and strength of the end product.

Sodium borate, calcium sulfate, potassiumdichromate, and carbohydrates.

Interferes with pozzolanic reactions that depend onformation of calcium silicate and aluminate hydrates.

Nonpolar organics (oil, grease, aromatichydrocarbons, PCBs)

May impede setting of cement, pozzolan, or organic-polymer S/S. May decrease long-term durability andallow escape of volatiles during mixing. Withthermoplastic S/S, organics may vaporize from heat.

Polar organics (alcohols, phenols, organic acids,glycols)

With cement or pozzolan S/S, high concentrations ofphenol may retard setting and may decrease short-termdurability; all may decrease long-term durability; all maydecrease long-term durability. With thermoplastic S/S,organics may vaporize. Alcohols may retard setting ofpozzolans.

Solid organics (plastics, tars, resin) Ineffective with urea formaldehyde polymers; may retardsetting of other polymer S/S.

Oxidizers (sodium hypochlorite, potassiumpermanganate, nitric acid, or potassium dichromate)

May cause matrix breakdown or fire with thermoplasticor organic polymer S/S.

Metals (lead, chromium, cadmium, arsenic, mercury) May increase setting time of cements if concentration ishigh.

Nitrates, cyanides Increase setting time, decrease durability for cement-based S/S.

Soluble salts of magnesium, tin, zinc, copper andlead

May cause swelling and cracking within inorganic matrixexposing more surface area to leaching.

Environmental/waste conditions that lower the pH ofmatrix

Eventual matrix deterioration.

Flocculants (e.g. ferric chloride) Interference with setting of cements and pozzolans.

Soluble sulfates > 0.01% in soil or 150 mg/L inwater

Endangerment of cement products due to sulfur attack.

Soluble sulfates > 0.5% in soil or 2000 mg/L inwater

Serious effects on cement products from sulfur attacks.

Oil, grease, lead, copper, zinc and phenol Deleterious to strength and durability of cement, lime/flyash, fly ash/cement binders.

Aliphatic and aromatic hydrocarbons Increase set time for cement.

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Characteristics Affecting Processing Feasibility Potential Interference

Chlorinated organics May increase set time and decrease durability of cementif concentration is high.

Metal salts and complexes Increase set time and decrease durability for cement orclay/cement.

Inorganic acids Decrease durability for cement (Portland Type I) orclay/cement.

Inorganic bases Decrease durability for clay/cement; KOH and NaOHdecrease durability for Portland cement Type III and IV.

6.3.2 Incineration

Incineration is the most commonly used method for thermal treatment of organicliquids, and solids and sludges contaminated with toxic organics. Figure 6-11 shows a flowdiagram with the following key elements of an incinerator system: (1) waste processing, whichincludes screening, size reduction, and waste mixing, (2) a waste feed system, (3) acombustion unit, (4) air pollution control equipment to collect/treat products of incompletecombustion, particulate emissions, and acid gases, and (5) facilities for handling and disposingresidual ash from the combustion unit, and particulates and residual wastewater from the airpollution control system.

Incinerators are usually classified by the type of combustion unit, with rotary kiln,liquid injection, fluidized bed, and infrared units being those most commonly used forhazardous wastes. Existing industrial boilers and kilns, especially cement kilns, are alsosometimes used for thermal treatment of hazardous wastes. Table 6-3 identifies major wasteproperties that affect the performance of an incinerator. Table C-5 in Worksheet C (Volume II)provides additional information about qualifying factors relevant to different types ofincinerator units. Appendix B identifies major references that provide more detailedinformation about the selection and design of incinerators.

Incineration is a relatively expensive treatment option, but the economies of scalecreated by a large HWTC mean an incinerator is likely to be a key element in the design of afacility capable of treating a wide range of hazardous wastes. Incineration is also sometimesused to treat sludge from conventional wastewater treatment plants serving large cities. Asdiscussed in Section 6.5, sludge from CETPs normally is put to beneficial use; however, itmight be necessary to transport highly contaminated sludges to an HWTC for furthertreatment, and possibly incineration.

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Figure 6-11. Incineration system concept flow diagram (U.S. EPA, 1991)

Table 6-3. Waste properties affecting incineration system performance (U.S. EPA, 1991)

Property Hardware Affected Operating ParameterAffected

Effect of Performance Example Feeds ofConcern

Heating value Rotary kiln Rotary kiln temperature,flue gas residence time.

Feed capacity, fuel usage. Plastics, trash.

Density Rotary kiln Weight of material held bykiln.

Feed capacity. Brominated sludge (highdensity sludge).

Halogen andsulfur content

Quench system, airpollution controlequipment design andoperation.

Pump cavitation, pHcontrol, blowdown rate,particulate emissions.

Feed capacity, causticusage.

Trial burn mixture,brominated sludge.

Moisture Feed system Increased fuel usage tomaintain temperature.

Particle sizedistribution

Cyclone, SCC, ducts,wet electrostaticprecipitation (WEP),instrumentation.

Kiln draft, particulateemissions excess oxygencontrol, temperaturecontrol.

Fouling of duct, cyclone,SCC, process water system,and instruments.

Soils, brominatedsludge, vermiculite.

H:Cl ratio ofPOHCs.

_____ Incinerator’s ability tothermally destroyPOHCs/PICs.

As H:Cl ratio decreases,thermal stability of POHCsincreases and oxidation ofPICs is reduced. Underoxygen starved conditions,the tendency to form PICsincreases as the N:Cl ratiodecreases.

C2Cl6C6l6, C2HCl andsimilar compounds.

Any fusioncharacteristics(determined bychemicalcharacteristics,e.g., alkalis).

Rotary kiln, cyclone,ducts, quench elbow,instrumentation

Kiln draft, temperature,excess O2 control.

Slagging of kiln, plugging ofinstruments and downstreamequipment.

Plastic, trash,brominated sludge.

Exhaust to Atmosphere

Acid Gas

Control

Particulate

Removal

GasConditioning

ResidualTreatment

Wastewater To Disposal

Ash

Removal

Combustion

Unit

WasteFeeding

WasteProcessing Auxiliary

FuelCombustion

Air

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6.3.3 Other Thermal Treatment Technologies

Other thermal treatment technologies include a variety of methods that use heat (but notprimarily oxidation by direct air combustion as with incineration) to remove or destroy organiccontaminants. Available technologies include:

n Pyrolysisn Wet air oxidationn Thermal desorption

Supercritical water oxidation is an emerging thermal treatment technology that hasreceived considerable bench-scale and pilot-scale testing.

Pyrolysis is a thermal process that transforms hazardous organic materials in an oxygen-poor atmosphere into gaseous components and a solid residue (coke) containing fixed carbonand ash. Upon cooling, the gaseous components condense, leaving an oil/tar residue. Pyrolysisis applicable to a wide range of organic wastes in soil and sludge, including polychlorinatedbiphenyls (PCBs) and dioxins/furans.

Wet air oxidation uses elevated temperature and pressure to oxidize dissolved or finelydivided organics. Its main application is to treat waste streams that are too dilute (less than 5percent organics) to treat economically by incineration and that have contaminant levels abovethose considered ideal for biological treatment. This technology can also be used to treatwastewaters with pesticides, phenolics, organic sulfur, and cyanide. It is not suitable forhalogenated aromatic organics or for treating large volumes of waste.

Thermal desorption is used to physically separate volatile and some semivolatilecontaminants from soil, sediments, sludges, and filter cakes by heating them at temperatureshigh enough to volatilize the organic contaminants. Desorbed organics in the gas stream arethen treated by being burned in an afterburner, condensed in a single- or multistage condenser,or captured by carbon adsorption beds. Thermal desorption systems are usually classified aslow temperature (200 to 600oF/93 to 215oC) or high temperature (600 to 1,000oF/315 to538oC) systems.

The main difference between the two is that low-temperature systems target volatileorganic compounds, whereas high-temperature systems target semivolatile organics.

Supercritical water oxidation (SCWO) uses oxidants (air, oxygen, or hydrogenperoxide) to decompose organics in an aqueous waste stream that is above the critical point ofwater (364oC/221 atmospheres). Supplemental fuel may be required at startup and for dilutewastes, but waste streams with a COD greater than 15,000 mg/L generally are self-sustaining.SCWO can be used for liquid wastes, sludges, and slurried solid wastes.

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Table C-5 in Worksheet C (Volume II) provides additional information about the abovethermal treatment methods and a number of innovative thermal treatment technologies (electricreactor, molten glass, molten salt, and radiofrequency thermal heating), and also identifiesreferences where more detailed information can be obtained.

6.4 SELECTION OF TREATMENT PROCESSES

Major considerations in the selection of hazardous waste treatment processes at HWTCsinclude:

n Waste medium. Wastewater, organic liquids, sludge, and solids containing the sametype of contaminants may often require different treatment processes (see Section6.4.1).

n Contaminant type. The physical and chemical properties of contaminants in a wasteaffect the suitability of available treatment processes. For example, precipitation is achemical treatment method that applies mainly to inorganics such as metals andcyanides. Air stripping and thermal treatment methods, on the other hand, are moresuited for treatment of wastewaters and solids contaminated with volatile andsemivolatile organics. Whether organic contaminated wastes are halogenated ornonhalogenated also may influence the suitability of a particular treatment option.Mixed organic and inorganic wastes are often the most difficult to treat, frequentlyrequiring a series of different treatment steps. Contaminant-specific waste treatmentoptions are covered further for corrosive wastes in Section 6.4.2, solvent wastes inSection 6.4.3, and other contaminants in Section 6.4.4.

n Contaminant concentration. The successful operation of some treatment processesdepend on the concentration of contaminants in the waste stream. Section 6.4.1provides information on the range of applicability of treatment techniques as afunction of the organic concentration in liquid waste streams.

n Waste volume. Some treatment methods, such as incineration, require large volumesof waste to be cost-effective. Other methods, such as wet air oxidation (see Section6.3.3), are better suited for small volumes of waste. Process efficiency requires areasonably good match between the volume of waste at which a process worksefficiently and the volume of waste to be treated.

n Waste variability. Continuous treatment processes generally operate more efficientlyif the waste stream does not vary greatly in flow rate and chemical composition.Equalization tanks (see Section 5.2.1) are often used to control variability forcontinuous wastewater treatment processes. Batch treatment processes are wellsuited for wastes that vary in chemical composition.

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n Availability. The importance of reliable performance of treatment technologiesgenerally restricts technology choices to those that are commercially developed.Testing and development of innovative and emerging technologies may be possiblebut probably not as a central feature of any treatment train that handles largevolumes of waste.

n Cost. Cost will probably be the major factor for choosing between two or moretreatment options that satisfy all the other criteria. As discussed in Section 5.4.2,evaluating treatment costs requires considering the overall cost and the relativesignificance of capital, and operation and maintenance costs.

n Residuals. Most treatment processes produce residuals that may require furthertreatment (e.g., off gases) or disposal (e.g., ash, sludges). The type and volume ofresiduals generated should be considered when selecting a treatment technology.

The types of wastes received at an HWTC depend on the specific industrial processesused by the industries sending wastes to the facility. Numerous treatment options could besuitable for a particular batch or stream of waste. Worksheet A in Volume II describes aprocedure for identifying waste characteristics and treatment options for specific industrialcategories.

The screening procedure described in Worksheet A is applicable for selecting potentialtreatment options for CETPs and HWTCs. Criteria for selecting onsite pretreatment options forSMSEs prior to discharging wastes is more similar to the criteria for CETPs discussed inSection 5.4.1. An HWTC generally mixes similar wastes from many individual sources. Animportant step in the process of selecting treatment options for an HWTC is to determine themajor types of waste streams that will be handled by the facility through combining thesewastes.

6.4.1 Media-Specific Options

Table 6-4 identifies potential treatment alternatives for various types of liquid and solidhazardous wastes for (1) waste minimization, (2) pretreatment, and (3) treatment and disposal.Table 6-4 indicates that treatment options are often dependent on the concentration of the wastestream. Thus, options identified for recycling of concentrated inorganic liquids differ fromthose for treating dilute inorganic liquids, except for electrodialysis. Figure 6-12 identifies theapproximate ranges of applicability of commercially available treatment techniques (solid bar)as a function of organic concentration in liquid waste streams. The dashed lines indicatepotential extensions of available technologies and the emerging technology of supercriticalwater oxidation.

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The pretreatment options for various waste media identified in Table 6-4 are mostlyphysical methods, many of which are discussed in Sections 5.2.1, 5.2.2, and 5.5.2. Majorchemical pretreatment methods include neutralization, cyanide destruction, and chromiumreduction. Treatment and disposal methods identified in Table 6-4 are discussed in Sections 6.2and 6.3.

Table 6-5 is a matrix showing the potential applicability of 17 treatment technologiesfor general types of contaminants in three media: (1) aqueous wastes, (2) organic liquids, and(3) sludges and soils. Although this table was developed for screening technologies for onsiteremediation of contaminated sites, all of the treatment categories could be used in HWTCs.

6.4.2 Corrosive Wastes

Neutralization of corrosive wastes was discussed in Section 6.2.4. Table 6-6 providesthe following information on eight treatment technologies for recovery and reuse of corrosivewastes:

n Applicable waste streamsn Stage of developmentn Performancen Residuals generatedn Cost

Because of the problems involved in transporting corrosive wastes to an HWTC, thesetechnologies would generally be best applied at the industrial facility where they are generated.Unfortunately, financial constraints would probably limit the use of such technologies bySMSEs.

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Table 6-4. Hazardous waste management alternatives (Wilk et al., 1988)

Waste ManagementObjective

Applicable WasteType(s)

Potential Waste Management/Treatment Alternative

Waste minimization

Source reduction All Raw material substitutionProduct reformulation

Process redesignWaste segregation

Recycling Concentrated inorganicliquids (e.g. plating,etching solutions)

CrystallizationIon exchange

Evaporation/distillationElectrodialysis

Solvent extractionThermal decomposition

Dilute inorganic liquids(e.g. plating rinses)

Ion exchangeElectrodialysis

Reverse osmosis Donna dialysis/coupledtransport

Concentrated organicliquids (e.g. solvents withacid/alkali)

Neutralization followed by recovery such as distillation, evaporation, streamstripping, or use as a fuel.

Waste exchange Concentrated liquids Recycling Reuse in process withlower raw materialspecifications

Mutual neutralization

Dilute organic liquids Mutual neutralization

Pretreatment Liquid with solids ScreeningDistillationCentrifugation

SedimentationFlotationEqualization

FiltrationSetting

Liquid-two-phase DecantingExtraction

FlotationDistillation

CentrifugationEqualization

Liquid or sludge withcyanide

Cyanide destructionthrough chlorination

Liquid or sludge withhexavalent chromium

Chromium reduction

Sludge Vacuum filtrationOther dewatering

Filter press Centrifugation

Bulky solids Shredders Hammermills Crushers

Neutralization Acidic waste Limestone Lime Caustic sodaAlkaline waste Sulfuric acid Hydrochloric acid Carbonic acid (CO2)All Mutual neutralization

Treatment and disposal Metal-containing liquid Precipitation andclarification

Trace organic-containingliquids

Adsorption

Dilute organic-containingliquid

Biological treatmentAir stripping

Chemical oxidationIncineration

Ozonation

Concentrated organicliquid

DistillationExtractionIncineration

Steam strippingSupercritical fluidsUse as a fuel

EvaporationWet air oxidation

Inorganic sludges andsolids

Solidification Encapsulation Landfill

Organic sludges and solids Incineration Wet air oxidation

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Figure 6-12. Approximate ranges of applicability of treatment techniques as a function of organic concentration in liquid waste streams (Breton et al., 1987)

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Table 6-5. Onsite waste treatment technology matrix (U.S. EPA, 1991)

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Table 6-6. Summary of recovery/reuse technologies for corrosive wastes (Wilk et al., 1988)

ProcessApplicable Waste

StreamsStage of

DevelopmentPerformance

ResidualsGenerated

Cost

Evaporation/Distillation

Metal platingrinses; acidpickling liquors

Well-establishedfor treating platingrinses

Plating solutionrecovered forreuse in platingbath. Rinse watercan be reused

Impurities will beconcentrated,therefore,crystallization/filtration systemmay be required

Can be cost-effective forrecoveringcorrosive platingsolutions fromrinse waters

Crystallization H2SO4 picklingliquors;HNO3/HF;pickling liquors;caustic aluminumetch solutions

20 to 25 systemscurrently inoperation (fewerapplications forcaustic recovery)

97-98% recoveryfor H2SO4 (80-95% metalremoval)

Ferrous sulfateheptahydeatecrystals (can betraded or sold)

Cost-effective iftreating largequantities of waste

99% HNO3 and50% HFrecovered

Metal fluoridecrystals (can recoveradditional HF bythermaldecomposition)

80% recoveryNaOH

Aluminumhydroxide crystals(can be traded orsold)

Ion exchange Plating rinses:acid picklingbaths; aluminumetching solutions;H2SO4; anodizingsolutions; rack-stripping solutions(HF/HNO3)

Several RFIE unitsin operation fortreatment ofcorrosives

Cocurrent systemsnot technicallyfeasible for directtreatment ofcorrosives; can beused inconjunction withneutralizationtechnologies tolower overall costs

Cocurrent processgenerates spentregenerant, which isalso corrosive

RFIE and APUare cost-effective

Units for directtreatment of acidbath only availablefrom ECO-TECLtd.

RFIE units showgood results.ConventionalRFIE performsbest with dilutesolutions. APUperforms best withhigh metalconcentration (30to 100 g/L)

Recovered metalswhich can bereused, treated,disposed, ormarketed

Electrodialysis Recovery ofchromic/sulfuricacid etchingsolutions

Units currentlybeing sold, butlimited area ofapplication. 5 inoperation

85% recovery ofetching solution.45% copperremoval; 30%zinc removal

Metals which can betreated, disposed, orregenerated forreuse

Cost-effective forspecificapplications(chromic/sulfateacid etchants)

Recovery ofplating rinses(particularlychromic acid rinse

Several inoperation

Works best whencopperconcentrations arein the 2 to 4 oz/gal

Chromic acid can bereturned to platingbath; rinse watercan be reused

Low capitalinvestment; costeffective forspecific

6-38

ProcessApplicable Waste

StreamsStage of

DevelopmentPerformance

ResidualsGenerated

Cost

waster) usage application(chromic acidrinses)

Recovery ofHNO3/HF picklingliquors

Marketed, none inoperation to date

3 M HF/HNO3

recorded2M KOH Solnwhich can berecycled back to thepretreatment stepfor this EDapplication

Cost-effective forlarge quantitygenerator

Reverse osmosis Plating rinses Corrosive wastemembranesmarketed by fourcompanies. RDmodule systemsapplicable tocorrosivesavailable from twocompanies

90% conversionachieved withcyanide platingrinses

Recovered platingsolution returned toplating bath (afterbeing concentratedby an evaporator).Rinsewater reused

Cost-effective forlimitedapplications.Development of amore chemicallyresistantmembrane wouldmake it very cost-effective for awider area ofapplication

Donnadialysis/coupledtransport

Plating rinses;potentiallyapplicable to acidbaths

Donnan analysisonly lab-scaletested

Data not availablefor Donna analysis(further testingrequired)

Data not availablefor Donnan analysis

No cost dataavailable forDonna analysis

Coupled transportlab and fieldtested. Coupledtransport system iscurrently beingmarketed

Coupled transporthas demonstrated99% recovery forchromate fromplating rinses.Other platingrinses should beapplicable, but notfully tested

For chromateplating rinseapplications, sodiumchromate isgenerated: can beused elsewhere inplant or subjected toion exchange torecover chromicacid for recycle toplating solution

Average capitalcost for platingshop is $20,000.Can be cost-effective forspecificapplications

Solventextraction

HNO3/HF picklingliquors

Commercial-scalesystems installedfor developmentpurposes inEurope and Japan.No commercial-scale installationsin U.S.

95% recovery ofHNO3; 70%recovery of HF

Metal sludge (95%iron can berecovered bythermaldecomposition)

Not available

Thermaldecomposition

Acid wastes Well-establishedfor recoveringspent pickleliquors generatedby steel industry.Pilot-scale stagefor organic wastes

99% regenerationefficiency forpickling liquors

98-99% purity ironoxide which can bereused, traded, ormarketed

Expensive capitalinvestment. Onlycost-effective forlarge quantitywaste acidgenerators

(1) RFIEL Reverse flow ion exchange

6.4.3 Solvent Wastes

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Many industries generate solvent wastes through cleaning equipment. Solvents are usedextensively in the metals treatment/finishing industries and the electronics industries. Freshsolvents are organic liquids and can be either halogenated (such as tetrachloroethylene) ornonhalogenated (such as methanol and toluene). Solvent wastes can be either, single- or multicomponent waste streams (i.e., mixed with water, solids, or both).

Table 6-7 provides an overview of solvent waste minimization and treatment options,and Table 6-8 provides the following information on more than two dozen specificrecovery/treatment options: (1) applicable waste streams, (2) stage of development of theprocess, (3) performance, and (4) residuals generated.

Solvent reclamation technologies, such as distillation, evaporation, and steam stripping,are applied to spent solvents to remove water and other liquid contaminants before they arereused. If a solvent is only contaminated with solids, reclamation can be accomplished byfiltration or other physical component separation methods. Sometimes, contaminated solventscan be reused without treatment by shifting use to applications with lower purity requirements.Organic liquid solvents also have potential for use as a supplemental fuel in industrial kilns andhigh-temperature industrial boilers. Special care is required when using halogenated solvents asa fuel to ensure that concentrations of chlorine in the fuel blend do not exceed levels that willcorrode the system.

The main treatment option for concentrated waste solvents for which reclamation is notfeasible is thermal treatment. Table 6-8 identifies four incineration options and seven otherthermal technologies; of these seven technologies, only pyrolysis (see Section 6.3.3) iscommercially available. Most of the physical and chemical treatment options identified inTable 6-9 are for aqueous waste streams contaminated by relatively low concentrations ofsolvents. Figure 6-12 can be used as a guide for identification of potential technologies basedon the concentration of solvents and other organics in wastewater.

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Table 6-7. Solvent waste management alternatives to land disposal (Breton et al., 1987)

Wastemanagement

objective

Applicable wastetype(s)

Potential waste management alternative

WasteMinimization

All Raw materialsubstitutionProductreformulation

Process redesignWaste segregation

Recycling All Reclamation Reuse (e.g., as a fuel or process solvent)

PretreatmentLiquid with solids Screening

FloatationSedimentationSettling

FiltrationCentrifugation

Distillation

Liquid – Two Phase DecantingDistillation

Floatation Centrifugation Extraction

Sludge Vacuum filtration Filter press Centrifugation Otherdewatering

Bulky solids Shredders Hammermills CrushersLow Btu/HighViscosity

Blending

Treatment Physical Liquid Distillation

Steam strippingEvaporationAir stripping

FractionationCarbonadsorption

ExtractionResinadsorption

Chemical Liquid Wet air oxidationOther chemicaloxidations

Supercritical water oxidation Chlorinolysis

Ozonation

Biological Liquid Activated sludge Aerated lagoon Tricklingfilter

Incineration All Liquid inyection Rotary kiln Fluidized-bed

Starved air

Otherthermal

All PyrolysisprocessesPlasma systems

Molten glassElectric reactor

Circulatingfluid bedMolten salt

Post-treatmentOrganic liquid Decanting Dehydrating Fractionatio

nThermaldestruction

Solid/sludge Solidification Encapsulation Thermaldestruction

Aqueous Liquid Carbon adsorptionOzonation

Resin adsorptionOther oxidations

Air stripping BiologicalTreatment

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Table 6-8. Summary of solvent treatment processes (Breton et al., 1987)

ProcessApplicable waste

streamsStage of development Performance Residuals generated

Incineration

Liquid injectionincineration

All pumpable liquidsprovided wastes can beblended to Btu level of6500 Btu/lb. Some solidsremoval may benecessary to avoidplugging nozzles

Estimated that over 219units are in use. Mostwidely usedincineration technology

Excellent destructionefficiency (>99.99%).Blending can avoidproblems associated withresiduals, e.g., HCl

TSP, possibly some PICs andHCl if halogenated organicsare fired. Only minor ash ifsolids removed inpretreatment processes

Rotary kiln incineration All wastes provided Btulevel is maintained

Over 40 units inservice; most versatilefor waste destruction

Excellent destructionefficiency (>99.99%)

Requires APCDs. Residualsshould be acceptable ifcharged properly

Fluidized bed incineration Liquids or nonbulky solids Nine units reportedly inoperation-recirculatingbed units underdevelopment

Excellent destructionefficiency (>99.99%)

As above

Fixed/multiple hearths Can handle a wide varietyof wastes

Approximately 70 unitsin use. Old technologyfor municipal wastecombustion

Performance may bemarginal for hazardouswastes, particularlyhalogenated wastes

As above

Use as a Fuel

Industrial kilns Generally all wastes, butBtu level, chlorinecontent, and otherimpurity content mayrequire blending tocontrol chargecharacteristics andproduct quality

Only a few units nowburning hazardouswaste

Usually excellentdestruction efficiency(>99.99%) because of longresidence times and hightemperatures

Requires APCDs. Residualsshould be acceptable

High temperatureindustrial boilers

All pumpable fluids, butshould blend halogenatedorganics. Solids removalparticularly important toensure stable burneroperation

Several units in use Most units tested havedemonstrated high DRE(>99.99%)

Waste must be blended tomeet emission standards forTSP and HCl unless boilersequipped with APCDs

Òther Thermal Technologies

Circulating bed combustor Liquids or nonbulky solids Only one U.S.manufacturer. No unitstreating hazardouswaste

Manufacturer reports highefficiencies (>99.99%)

Bed material additives canreduce HCl emissions.Residuals should beacceptable

Molten glass incineration Almost all wastes,provided moisture andmetal impurity levels arewithin limitations

Technology developedfor glassmanufacturing. Notavailable yet as ahazardous waste unit

No performance dataavailable, but DREs shouldbe high (>99.99%)

Will need APC device forHCl and possibly PICs; solidretained (encapsulated) inmolten glass

Molten salt destruction Not suitable for high(>20%) ash contentwastes

Technology underdevelopment since1969, but furtherdevelopment on hold

Very high destructionefficiencies for organics (sixnines for PCBs)

Needs some APC devices tocollect material not retained insalt. Ash disposal may be aproblem

Furnace pyrolysis units Most designs suitable forall wastes

One pyrolysis unitRCRA permitted.Certain designsavailable commercially

Very high destructionefficiencies possible(>99.99%). Possibility ofPIC formation

TSP emissions lowers thanthose from conventional willneed APC devices for HCl

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ProcessApplicable waste

streamsStage of development Performance Residuals generated

Plasma arc pyrolysis Present design suitableonly for liquids

Commercial designappears imminent, withfuture modificationsplanned for treatment ofsludges or solids

Efficiencies exceeded sixnines in tests with solvents

Requires APC devices forHCP and TSP, needs flare forH2 and CO destruction

Fluid wall advancedelectric reactor

Suitable for all wastes ifsolids pretreated to ensurefree flow

Ready for commercialdevelopment. Test unitpermitted under RCRA

Efficiencies have exceededsix nines in tests withsolvents

Requires APC devices forHCP and TSP, needs flare forH2 and CO destruction

In situ vitrification Technique for treatingcontaminated soils, couldpossibly be extended toslurries. Also use assolidification process

Not commercial,further work planned

No date available, butDREs of over six ninesreported

Off gas system needed tocontrol emissions to air. Ashcontained in vitrified soil

Physical Treatment MethodsDistillation This is a process used to

recover and separatesolvents. Fractionaldistillation will requiredsolids removal to avoidplugging columns

Technology welldeveloped andequipment availablefrom many suppliers;widely practicedtechnology

Separation depends uponreflux (99 + percentachievable). This is arecovery process

Bottoms will usually containlevels of solvent in excess of1,000 ppm; condensate mayrequire further treatment

Evaporation Agitated thin film unitscan tolerate higher levelsof solids and higherviscosities than othertypes of stills

Technology is welldeveloped andequipment is availablefrom several suppliers;widely practicedtechnology

This is a solvent recoveryprocess. Typical recoveryof 60 to 70 percent

Bottoms will containappreciable solvent. Generallysuitable for incineration

Stream Stripping A simple distillationprocess to remove volatileorganics from aqueoussolutions. Preferred forlow concentrations andsolvents with lowsolubilities

Technology welldeveloped andavailable.

Not generally considered afinal treatment, but canachieve low residual solventlevels

Aqueous treated stream willprobably require polishing.Further concentration ofoverhead steam generallyrequired

Air Stripping Generally used to treatlow concentration aqueousstreams

Technology welldeveloped and available

Not generally considered afinal treatment, but may beeffective for highly volatilewastes

Air emissions may requiretreatment

Liquid-liquid extraction Generally suitable onlyfor liquids of low solidcontent

Technology welldeveloped for industrialprocessing

Can achieve high efficiencyseparations for certainsolvents/waste combinations

Solvent solubility in aqueousphase should be monitored

Carbon adsorption Suitable for low solid,low concentration aqueouswaste streams

Technology welldeveloped; used aspolishing treatment

Can achieve low levels ofresidual solvent in effluent.

Adsorbate must be processedduring regeneration. Spentcarbon and wastewater mayalso need treatment.

Resin Adsorption Suitable for low solidwaste streams. Considerfor recovery of valuablesolvent

Technology welldeveloped in industryfor special resin/solventcombinations.Applicability to wastestreams notdemonstrated

Can achieve low levels ofresidual solvent in effluent.

Adsorbate must be processedduring regeneration.

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ProcessApplicable waste

streamsStage of development Performance Residuals generated

Chemical Treatment Processes

Wet air oxidation Suitable for aqueousliquids, also possible forslurries. Solventconcentrations up to 15%

Hightemperature/pressuretechnology, widely usedas pretreatment formunicipal sludges, onlyone manufacturer

Pretreatment for biologicaltreatment. Some compoundsresist oxidation

Some residues likely whichneed further treatment

Supercritical wateroxidation

For liquids and slurriescontaining optimalconcentrations of about10% solvent

Supercritical conditionsmay impose demandson system reliability.Commercially availablein 1987

Supercritical conditionsachieve high destructionefficiencies (>99.99%) forall constituents

Residuals not likely to be aproblem. Halogens can beneutralized in process

Ozonation Oxidation with ozone(possibly assisted by [UV]suitable for low solid,dilute aqueous solutions

Now used as apolishing step forwastewaters

Not likely to achieveresidual solvent levels in thelow ppm range for mostwastes

Residual contamination likely;will require additionalprocessing of off gases

Other chemical oxidationprocesses

Oxidizing agents may behighly reactive forspecific constituents inaqueous solution

Oxidation technologywell developed forcyanides and otherspecies (phenols), notyet established forgeneral utility

Not likely to achieveresidual solvent levels in thelow ppm range for mostwastes

Residual contamination likely;will require additionalprocessing

Chlorinolysis Suitable for any liquidchlorinated wastes

Process produces aproduct (e.g., carbontetrachloride). Notlikely to be available

Not available Air and wastewater emissionswere estimated as notsignificant

Dechlorination Dry soils and solids Not fully developed Destruction efficiency ofover 99% reported fordioxin

Residual contamination seemslikely

Biological TreatmentMethods

Aerobic technologysuitable for dilute wastesalthough someconstituents will beresistant

Conventional treatmentshave been used foryears

May be used as finaltreatment for specificwastes, may bepretreatment for resistantspecies

Residual contamination likely;will usually require additionalprocessing

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Figure 6-9. Industrial wastewater process applicability matrix (McArdle et al., 1987)

6-45

6.4.4 Other Contaminant-Specific Options

Table 6-9 is a matrix identifying the applicability of 19 physical, chemical, andbiological technologies for treating the following wastewater characteristics: (1) suspendedsolids, (2) oil, grease, and immiscible liquids, (3) pH, (4) total dissolved solids, (5) metals,(6) cyanides, (7) volatile organics, (8) semivolatile organics, (9) pesticides and PCBs, and (10)pathogens.

Table 6-10 is a similar matrix rating the potential effectiveness of 16 thermal,chemical, physical, and biological treatment technologies for eight types of organiccontaminants and eight types of inorganic contaminants in soils and sludge. This matrix wasdeveloped for screening technologies at contaminated sites, but all the ex-situ technologies (11of the 16) are equally applicable to treatment of hazardous waste at HWTCs.

Table 6-11 is a matrix rating the effectiveness of six technologies for treatingcontaminated solids for 11 major types of hazardous contaminants. These 11 groups weredeveloped for the EPA Superfund program to facilitate tests for the treatability of materials atuncontrolled hazardous waste sites. The nine categories for organic contaminants represent amore detailed breakdown than shown in other tables in this guide. A more complete list ofapplicable treatment technologies for these treatability groups as they relate to pesticidechemical waste groups is contained in Table B-5 in Worksheet B (Volume II).

Finally, Table C-8 in Worksheet C (in Volume III) provides a screening matrix formore than 50 treatment technologies in relation to the following five major groups ofcontaminants: (1) volatile organic compounds (VOCs), (2) semivolatile organic compounds(SVOCs), (3) fuels (petroleum hydrocarbons), (4) inorganics, and (5) explosives. Informationon how to use this matrix is presented in Worksheet C.

Although the matrices in Tables 6-5, 6-9, 6-10, 6-11, and C-9 overlap somewhat, eachcontains significant information that is not covered elsewhere.

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Table 6-10. Treatment technology screening guide contaminants in soil and sludges(U.S. EPA, 1988)

6-47

Table 6-11. Predicted treatment effectiveness for contaminated solids(Offut and Knapp, 1990)

6-48

6.4.5 Industry-Specific Options

Table 6-12 is a matrix that identifies the applicability of 27 candidate treatment andcontrol technologies for 34 industries. This table provides a somewhat quicker way to preparea preliminary list of candidate treatment technologies for a particular industry than theprocedure described in Worksheet A (Volume II), but should not be used as a substitute for amore detailed screening process.

6.4.6 Most Commonly Used Treatment Processes

Typically, HWTCs need to have the ability to treat the full range of hazardous wastesthat are produced by industrial processes, unless industrial production in a region is sospecialized that certain categories of hazardous waste are not generated. Compared withCETPs, treatment processes at HWTCs need to handle concentrated liquid, sludge, and solidwastes, which mainly require physical, chemical, and thermal treatment, and also use of S/Stechnologies. If an HWTC also receives large amounts of industrial wastewater, thenbiological treatment processes are also likely to be significant, and the HWTC would alsofunction as a CETP.

Table 6-13 summarizes the results of a survey that EPA conducted in 1994 of 85centralized waste treatment facilities in the United States to identify the types of treatmenttechnologies in actual use. All of these facilities mainly treat liquid wastes received fromindustries. The 22 technologies in Table 6-13 are classified as (1) physical pretreatment, (2)physical phase separation, (3) chemical, (4) biological, and (5) sludge dewatering.

Equalization is the most commonly used pretreatment method (81 facilities) withclarification/flocculation (35 facilities) and gravity separation (18 facilities) also commonlyused. Granular media filtration (equally divided between sand filters and multimedia filters)and carbon adsorption are the most commonly used physical treatment methods.

Precipitation is by far the most commonly used chemical treatment method, andmultiple applications at a single facility are typical. Cyanide destruction and chromiumreduction were performed at a little less than half the facilities.

Activated sludge was the most commonly used biological treatment process and wasusually used only at facilities with onsite manufacturing operations that produced a relativelyconstant waste stream that could support a continuous biological treatment system.

Plate and frame press filtration was the most commonly used sludge dewatering method(34 facilities), followed by vacuum filtration (10 facilities) and belt pressure filtration (6facilities).

Chapter 8 includes a case study of an existing HWTC that outlines the mix oftechnologies likely to be required at HWTCs.

6-49

Table 6-12. Candidate treatment and control technology for 34 industries (Saltzbergand Cushnie, 1985)

6-50

Table 6-13. Frequency of use of treatment technologies at industrial centralized wastetreatment facilities

Technology Number (Out of 85)

Physical Pretreatment

Gravity separation 18

Clarification/flocculation 35

Dissolved air flotation 5

Emulsion breaking Most oils subcategory facilities

Equalization 81 (36 unstirred, 45 stirred or aerated)

Physical (Phase Separation)

Air stripping 1

Granular media filtration 10 sand filters, 9 multimedia filters

Carbon adsorption 11

Reverse osmosis 3

Ultrafiltration 3

Liquid carbon dioxide extraction 1

Chemical

Precipitation 184 individual applications (more than one perfacility)

Cyanide destruction 30

Chromium reduction 38 (4 sulfur dioxide, 21 sodium bisulfite, 2 sodiummetabisulfite, 11 other reagents)

Electrolytic recovery 3

Ion exchange 1

Biological

Sequencing batch reactors 1

Biotowers 2

Activated sludge 12

Sludge Dewatering

Plate and frame pressure filtration 34

Belt pressure filtration 6

Vacuum filtration 10

Source: EPA (1995) WTI Survey

6-51

6.5 RESIDUALS MANAGEMENT AND DISPOSAL

Several types of residuals (i.e., waste products) are generated from the treatmentprocesses employed at CETPs and HWTCs including: sludges, solids, incinerator ash, airemissions, and concentrated liquid waste streams.

Table 6-14 provides an overview of the major types of residuals associated withwastewater treatment processes. Table 6-1 identifies residuals associated with specificneutralization processes, and Table 6-8 identifies residuals associated with solvent treatmentprocesses. Treatment and disposal options for the major types of residuals produced by CETPsand HWTCs are discussed below.

Table 6-14. Residual generated by various wastewater treatment processes (McArdle etal., 1987)

Residuals

Treatment process SludgesAir

emissionsConcentrated

liquid waste streamSpentcarbon

Pretreatment operations Sedimentation X Granular media filtration X Oil/water separation XPhysical/chemical treatment operations

Neutralization XPrecipitation flocculation/sedimentation XOxidation/reduction XCarbon adsorption X XAir stripping XSteam stripping XReverse osmosis XUltrafiltration XIon exchange XWet-air oxidation X

Biological treatment operationsActivated sludge X XSequencing batch reactor X XPowdered activated carbon treatment (PACT) X XRotating biological contactor X XTrickling filter X

6.5.1 Overview of Sludge Treatment Options

The treatment processes described in Sections 5 and 6 concentrate solids from liquidwastes into sludges that must be treated (e.g., stabilized and dewatered) before being finallydisposed. The following discussion addresses sludge treatment processes relevant to bothCETPs and HWTCs.

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Figure 6-13 shows a general schematic for handling sludge at municipal wastewatertreatment plants. Most processes shown in this figure are intended to remove water, reducevolume, or reduce the mass of solids in the initial sludge, which is typically only a few percentsolids and the rest water. Many of these processes, such as aerobic and anaerobic digestion,lime stabilization, and composting, also reduce pathogens in municipal wastewater sludges.Table 6-15 describes the effect of major sludge treatment processes on sludge and theirsignificance for sludge use and disposal options. If CETPs are likely to receive human as wellas industrial wastewaters, reduction of pathogens should be a significant design consideration,especially if land application for agricultural purposes is desired. In HWTCs, pathogenreduction generally is not a concern unless the facility processes medical wastes.

Sludge thickening methods typically increase sludge solids from a few percent solids toas much as 10 percent solids. Raw primary sludges that have not received biological treatmentalso require stabilization to control odors and pathogens. WPCF/WEF (1980) provides moredetailed information on sludge thickening.

Sludge stabilization involves digestion or oxidation of sludge to reduce the mass ofsolids and pathogens. Lime stabilization, during which lime (hydrated lime, Ca(OH)2;quicklime, CaO; or lime-containing kiln dust or fly ash) is added in sufficient amounts to raisethe pH above 12, is a method for pathogen reduction. Major references for additionalinformation on sludge stabilization include EPA (1977) and WPCF/WEF (1985, 1987b).

Figure 6-13. General schematic for solids handling showing most commonly usedmethods of treatment and disposal (U.S. EPA, 1987)

THICKENINGPRIMARYSLUDGE

THICKENINGSECONDARY

SLUDGE

STABILIZATION CONDITIONING DEWATERING

TREATMENT

ULTIMATEDISPOSAL

* Compost* Incineration* Drying

* Source thickening in the primary clarifier* Gravity

* Anaerobic digestion* Aerobic digestion* Wet air oxidation* Anaerobic -aerobic digestion* Chlorine oxidation* Lime stabilization

* Ferric chloride* Lime* Lime and ferric chloride* Polymer* Heat treatment* Elutriation* Freeze-thaw

* Solid-bowl centrifuge* Belt filter-press* Vacuum filter* Filter press* Drying beds* Sludge lagoons* Gravity/low pressure devices

* Land spread* Landfill* Land injection

* Dissolved air flotation* Solid-bowl centrifuge* Belt or drum thickener

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Table 6.15. Effects of pretreatment and sludge treatment processes on sludge andsludge use/disposal options

Treatment process and definition Effect on sludge Effect on use/disposal options

Pretreatment: Reduction incontaminant levels in industrialwastewater discharge

Reduces levels of heavy metals andorganics in industrial wastewaterdischarge, thereby lowering theconcentration of these constituents inthe sludge

Increases the viability of landapplication, distribution and marketing,and ocean disposal. Reduces need forpollution control devices duringincineration, and prevents problemswith incinerator ash disposal

Thickening: Low-force separation ofwater and solids by gravity or flotation

Increases solids concentration of sludgeby removing water, thereby loweringsludge volume

Lowers sludge transportation costs forall options

Digestion (Aerobic and Anaerobic):Biological stabilization of sludgethrough conversion of some of theorganic matter to water, carbondioxide, and methane.

Reduces the volatile and biodegradableorganic content of sludge by convertingit to soluble material and gas. Reducespathogen levels and controlsputrescibility

Reduces sludge quantity. Preferredstabilization method prior to landfillingand land application. Reduces heatvalue for incineration, but anaerobicdigestion produces recoverablemethane

Lime Stabilization: Stabilization ofsludge through the addition of lime

Raises sludge pH. Temporarilydecreases biological activity. Reducespathogen levels and controlsputrescibility. Increases the dry solidsmass of the sludge

Increases the amount of auxiliary fuelrequired in incineration if the amountof inert material in the sludge isincreased

Conditioning: Alteration of sludgeproperties to facilitate the separation ofwater from sludge. Conditioning canbe performed in many ways, e.g.,adding inorganic chemicals such aslime and ferric chloride; addingorganic chemicals such as polymers; orbriefly raising sludge temperature andpressure. Thermal conditioning alsocauses disinfection

Improves sludge dewateringcharacteristics. Conditioning mayincrease the mass of dry solids to behandled and disposed of withoutincreasing the organic content of thesludge

Increases the amount of auxiliary fuelrequired in incineration if the amountof inert material in the sludge isincreased

Dewatering: High-force separation ofwater and solids

Increases solids concentration of sludgeby removing much of the entrainedwater, thereby lowering sludge volume.Some nitrogen and other solublematerials are removed with the water

Reduces fuel costs for incineration.Reduces land requirements and bulkingsoil requirements for landfilling.Lowers sludge transportation costs forall options. Dewatering may beundesirable during land application inregions where the water itself is avaluable agricultural resource.Reduction of nitrogen levels may ormay not be an advantage

Composting: Aerobic processinvolving the biological stabilization ofsludge in a windrow, in an aeratedstatic pile, or in a vessel

Lowers biological activity. Can destroyall pathogens. Degrades sludge to ahumus-like material. Increases sludgemass due to addition of bulking agent

Useful prior to land application anddistribution and marketing. Often notappropriate for other use or disposaloptions due to cost

Heat drying: Application of heat tokill pathogens and eliminate most ofthe water content

Disinfects sludge. Slightly lowerspotential for odors and biologicalactivity

Generally used only prior todistribution and marketing

6-54

Sludge conditioning involves physical (heat, freeze-thaw) or chemical treatment toreduce moisture content and modify sludge characteristics to increase the rate of subsequentdewatering processes. WPCF/WEF (1988) provides more detailed information on sludgeconditioning.

Sludge dewatering increases solids content to the point where sludge can be more orless handled as a solid for use or disposal (see Section 6.5.1.1). The solids content ofdewatered sludges varies greatly depending on the characteristics of the sludge and thedewatering method used but typically ranges from 20 to 50 percent. Table 6-16 outlinesoperational selection criteria for sludge dewatering processes based on plant size. Majorreferences for additional information on municipal sludge dewatering include EPA (1987) andWPCF/WEF (1983, 1987a). The next section provides additional information on commonlyused dewatering methods at hazardous waste treatment facilities.

6.5.1.1 Sludge Dewatering at HWTCs

Hazardous waste treatment processes such as sedimentation, neutralization,precipitation, and oxidation/reduction produce sludges that often must be dewatered beforefurther treatment or disposal. This section describes three dewatering systems that arecommonly used at industrial waste treatment facilities in the United States (see Table 6-13):

n Plate and frame pressure filtrationn Vacuum filtrationn Belt pressure filtration

Plate and Frame Pressure Filtration

A plate and frame filter press consists of a number of filter plates or trays connected toa frame and pressed together between a fixed end and a moving end (Figure 6-14a). Filtercloth is mounted on the face of each plate. The sludge is pumped into the unit under pressurewhile the plates are pressed together. The solids are retained in the cavities of the filter pressand begin to attach to the filter cloth until a cake is formed. The water or filtrate passesthrough the filter cloth and is discharged from a drainage port in the bottom of the press. Thesludge influent is pumped into the system until the cavities are filled. Pressure is applied to theplates until the flow of filtrate stops. At the end of the cycle, the pressure is released and theplates are separated. The filter cake drops into a hopper below the press. The filter cake canthen be disposed of in a landfill. The filter cloth is washed before the next cycle begins. Thekey advantage of plate and frame pressure filtration is that it can produce a drier filter cakethan other methods of sludge dewatering can, and the fact that it is a batch process is anadvantage when sludges from different waste streams need to be handled separately. Becauseof the batch operation, however, a plate and frame filter press is more labor intensive.

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Table 6-16. Operational Selection Criteria for Sludge Dewatering Processes (U.S. EPA,1987)

Plant Size Key Criteria

Small Minimum Mechanical Complexity< 0.08 m3/s Local Repairs and Parts(< 2 mgd) Minimum operator attendance

Reliable without skilled serviceUnaffected by climatic factorsLarge excess capacityHandleable cake

Medium Low operator attendance0.08 – 0.44 m3/s Local repair and parts(2 – 10 mgd) Transportable cake without nuisance

Mechanical reliabilityCompetitive O&M costsDrier cake

Large Lowest O&M costs/ton dry solids> 0.44 m3 /s Lowest capital costs/ton dry solids

Drier cakeHigh output/unitMechanical reliabilityTransportable cake without nuisance

General Considerations

• Compatibility with existing equipment with long-term sludge disposal.• Long-term serviceability/utility• Acceptable environmental factors• Good experience at other operating installations.• Competence and quality of local operator and service personnel• Compatibility with plant size• Acceptance by user and regulatory agency.• Availability and need of manufacturer’s services.

Rotary Vacuum Filtration

Rotary vacuum filters come in drum, coil, and belt configurations. The filter mediumcan be made of cloth, coil springs, or wire-mesh fabric. A typical application is a rotaryvacuum belt filter (see Figure 6-14b). A continuous belt of filter fabric is wound around ahorizontal rotating drum and rollers. The drum is perforated and is connected to a vacuum.The drum is partially immersed in a shallow tank containing the sludge. As the drum rotates,the vacuum that is applied to the inside of the drum draws the sludge onto the filter fabric. Thewater from the sludge passes through the filter and into the drum, where it exits via adischarge port. As the fabric leaves the drum and passes over the roller, the vacuum isreleased. The filter cake drops off the belt as it turns around the roller. The filter cake can thenbe disposed of. Because vacuum filtration systems are relatively expensive to operate, they are

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usually preceded by a thickening step that reduces the volume of sludge to be dewatered. It is acontinuous process and therefore requires less operator attention.

Belt Pressure Filtration

Belt pressure filtration uses gravity followed by mechanical compression and shearforce to produce a sludge filter cake. Belt filter presses are continuous systems that arecommonly used to dewater biological treatment sludge. Most belt filter installations arepreceded by a flocculation step, where polymer is added to create a sludge that has the strengthto withstand being compressed between the belts without being squeezed out. A typical beltfilter press is illustrated in Figure 6-14c. During the press operation, the sludge stream is fedonto the first of two moving cloth filter belts. The sludge is gravity-thickened as the waterdrains through the belt. As the belt holding the sludge advances, it approaches a secondmoving belt. As the first and second belts move closer together, the sludge is compressedbetween them. The pressure is increased as the two belts travel together over and under aseries of rollers. The turning of the belts around the rollers shears the cake, which furthers thedewatering process. At the end of the roller pass, the belts move apart and the cake drops off.The feed belt is washed before the sludge feed point. The dropped filter cake can then bedisposed of. The advantages of a belt filtration system are its lower labor requirements andlower power consumption. One disadvantage is that belt filter presses produce a poorer qualityfiltrate and require a relatively large volume of belt wash water.

6.5.2 Final Use and Disposal Options for Sludge

Residual sludges that remain after any recyclable constituents have been removed aredewatered as much as possible before being ultimately disposed. Further stabilization, such asadding a bulking agent (see Section 6.3.1), may be required to create a sludge consistency thatis easier to handle for transport to the location of ultimate disposal. Final disposal options forsludge include beneficial uses such as (1) land application and (2) distribution and marketing ofcomposted sludge, or non-beneficial uses such as (1) landfilling, (2) incineration (see Section6.3.2), and (3) ocean disposal.

The main difference between CETPs and HWTCs is that if SMSE pretreatmentprograms are successful for a CETP, it should be possible to put sludge to a beneficial use byland application for agricultural production, forestry, or land reclamation, whereas sludgesgenerated at HWTCs generally are unsuitable for beneficial uses. The general approach toresiduals management at an HWTC it is to treat them on site to minimize volume and toxicity,and dispose of the remainder in a secure landfill. Figure 6-15 rates the relative importance ofsludge constituents, major sludge characteristics, and costs for the five use/disposal options.

Solids created using solidification processes, and incinerator ash, also requireplacement in a final disposal area. The main disposal options for such residuals are landfillingand ocean disposal.

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Major general references for additional information on sludge treatment, use, anddisposal include HMCRI (1974-1989), EPA (1979, 1984b, 1985b), and WPCF/WEF (1989).

Figure 6-14. Selected sludge dewatering systems: (a) plate and press pressure filtration,(b) vacuum filtration, and (c) belt pressure filtration (U.S. EPA, 1995)

6.5.2.1 Land Application

Where land is available, and sludges are uncontaminated or contaminants exist inconcentrations that are within acceptable limits, land application is the method of choice.Timing and rates of application may differ depending on whether the land is used foragricultural crops, pasture, or forestry. Sludge is especially valuable for reclamation ofseverely disturbed or degraded lands. Some capital costs for sludge treatment are showed inSection 7.3.2 Table 6-16 presents pollutant concentration limits for land application of sludges.

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Table 6-17. Pollution limit for land application of sludge

Description (*)

PollutantMaximum concentration

limit (**)(mg/kg)

Pollutant accumulationrate

(kg/ha)

Permissibleconcentration limit (***)

(mg/kg)

Annual pollutantaccumulation rate

(kg/ha/year)Arsenic 75 41 41 2.0Cadmium 85 39 39 1.9Chrome 3000 3000 1200 150Copper 4300 1500 1500 75Lead 840 300 300 15Mercury 57 17 17 0.85Molibdene 75 18 18 0.90Nickel 420 420 420 21Selenium 100 100 36 5.0Zinc 7500 2800 2800 140

(*) All limits in a dry basis; (**) absolute values; (***) monthly averages.USEPA. 40 CFR Part 503, Standards for the use or disposal of sewage sludge. 1993.

Major references for additional information on land application of sewage sludge includeHMCRI (1974-1989), EPA (1983, 1994), and WCPF/WEF (1989).

6.5.2.2 Distribution and Marketing

Composting of sludge produces a stable product that can be bagged and marketed.Composting requires mixing of dewatered sludge with a bulking agent such as wood chips,bark, rice hulls, or straw, and further aerobic decomposition. Major references for additionalinformation on sludge composting include Benedict et al. (1987), HMCRI (1974-1989), andEPA (1985a).

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Figure 6-15. Importance factors affecting sludge use/disposal options: (a) sludgeconstituents, (b) sludge characteristics, and (c) cost factors

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6.5.2.3 Landfilling

Modern landfills are coming under increased regulatory scrutiny, and as a result, willbe more protective of the environment in the future. A variety of specific technologies areassociated with a state-of-the-art landfill including: liner systems, leachate collection systems,leachate treatment, landfill gas control and recovery, improved closure techniques, provisionsfor post-closure care, and monitoring systems, all of which are discussed below. If landfillsare properly planned and operated, a completed landfill site can ultimately be used by theowner for recreational or other purposes, such as open space. Batstone et al. (1989) addresstechnical requirements for safe disposal of hazardous waste in landfills. Table B-1, inAppendix B, identifies major references on the design of secure landfills.

Siting a Landfill

Siting a hazardous waste landfill involves analyzing a number of factors associated withlocation alternatives. Because of environmental concerns, careful scientific and engineeringanalysis must take place during potential site evaluations. Surface and subsurface geology,hydrogeology, and the environmental nature of surrounding areas must be evaluated forpotential impacts. Ground water resources must be protected, and the integrity of soils must bepreserved. A substantial hydrogeological investigation and prediction of leachate quantities areusually performed early in the planning stages. When siting a new landfill, decision makersalso will have to consider logistical factors such as access roads, travel distance, and traveltime.

Hazardous Waste Landfill Components

Cells are the basic building blocks of landfills. During daily operations, waste isconfined to defined areas where it is spread and compacted throughout the day. At the end ofthe day (or several times a day), the waste is covered by a thin layer of soil, which also iscompacted. This unit of compacted and covered waste is called the cell. Several adjacent cells(all the same height) are referred to as a lift. A landfill consists of a series of lifts.

The components of a hazardous waste landfill include:

n Foundationsn Dikesn Liner Systems

- Low-permeability soil liners- Flexible membrane liners (FMLs)- Leachate collection systems

n Final cover systems

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Foundations

The foundations for hazardous waste landfills should provide structurally stablesubgrades for the overlying landfill components. The foundations also should providesatisfactory contact with the overlying liner or other system components. In addition, thefoundations should resist settlement, compression, and uplift resulting from internal or externalpressures, thereby preventing distortion or rupture of the landfill components.

Dikes

The purpose of a dike in a hazardous waste landfill is to function as a retaining wall,resisting the lateral forces of the stored wastes. A dike is the aboveground extension of thefoundation and provide support to the overlying landfill components. Dikes therefore must bedesigned, constructed, and maintained with sufficient structural stability to prevent theirfailure. Dikes also may be used to separate cells for different wastes within a large landfill.

Dikes may be constructed of soil material that is compacted as necessary to a specifiedstrength. Materials other than soil may be used to construct dikes, as long as the design of thedike accommodates the particular properties of the selected materials and proper installationprocedures are followed. Drainage layers and structures may be included in the dike design ifconditions warrant control of seepage. Although seepage through a dike should be preventedby a liner system, a dike must be designed to maintain its integrity if the liner fails andseepage occurs.

Liner Systems

The primary function of a liner system is to minimize and control the flow of leachatefrom the site to the environment, particularly towards ground water. Liners are made of low-permeability soils (typically clays) or synthetic materials (e.g., plastic). Landfills can bedesigned with more than one liner, and a mix of liner types may be used (these are referred toas composite liners).

There are two types of liner systems currently used in land disposal facilities in theU.S. A single liner system consists of one liner and one leachate collection system as shown inFigure 6-16. A double liner system includes two liners (primary and secondary), with aprimary leachate collection system above the primary (top) liner and a secondary leakdetection/leachate collection system between the two liners, as shown in Figure 6-17.

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Figure 6-16. Schematic of a single clay liner system for a landfill (U.S. EPA, 1988)

Figure 6-17. Schematic of a double liner and leak detection system for a landfill(U.S. EPA, 1988)

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The term "liner system" includes the liner(s), leachate collection system(s), and anyspecial additional structural components such as filter layers or reinforcement. The majorcomponents of both single and double liner systems are the following:

n Low-permeability soil linersn Flexible membrane liners (FML)n Leachate collection and removal systems (LCRS)

Low-Permeability Soil Liners. The purpose of a low-permeability soil liner dependson the overall liner system design. In the cases of single liners constructed of soil or doubleliner systems with soil secondary liners, the purpose of the soil liner is to prevent constituentmigration through the soil liner. In the case of soil liners used as the lower component of acomposite liner, the soil component serves as a protective bedding material for the FML uppercomponent and minimizes the rate of leakage through any breaches in the FML uppercomponent. An objective shared by all low-permeability soil liners is to serve as long-term,structurally stable bases for all overlying materials.

Low-permeability soil liner design is site- and material-specific. Prior to design, manyfundamental yet important criteria should be considered such as: in-place permeability of theliner; liner stability against slope failure, settlement, and bottom heave; and the long-termintegrity of the liner. Natural and manmade soil amendments (e.g., soil-cement, bentonite,lime) may be specified in a soil liner design to enhance the performance of natural soil.

Flexible Membrane Liners. The purpose of a FML in a hazardous waste landfill is toprevent the migration of any hazardous constituents into the liner during the period that thefacility is in operation and typically during a 30-year postclosure monitoring period. Inaddition, FMLs should be compatible with the waste liquid constituents that may contact themand be of sufficient strength and thickness to withstand the forces expected to be encounteredduring construction and operation.

The design of a lined sewage sludge surface disposal site requires consideration of morethan the performance requirements of the FML; it also requires careful design of thefoundation supporting the FML. The foundation provides support for the liner system,including the FMLs and the leachate collection and removal systems. If the foundation is notstructurally stable, the liner system may deform, thus restricting or preventing its properperformance.

The performance requirements of an FML include:

n Low permeability to waste constituentsn Strength or mechanical compatibility of the sheetingn Durability for the lifetime of the facility

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The designer must specify the necessary criteria for each of these properties based onengineering requirements, performance requirements, and the specific site conditions.

Leachate Collection Systems. Leachate refers to liquid that has passed through oremerged from landfilled waste and contains dissolved, suspended, or immiscible materialsremoved from the waste. The purpose of a primary leachate collection system in a landfill is tominimize the leachate head on the top liner during operation and to remove liquids from thelandfill through the postclosure monitoring period. The leachate collection system should becapable of maintaining a leachate head of less than 30 cm (1 foot). The purpose of a secondaryleachate collection system (sometimes referred to as a leak detection system) between the twoliners of a landfill is to rapidly detect, collect, and remove liquids entering the system throughthe postclosure monitoring period. If uncontrolled, landfill leachate can be responsible forcontaminating ground water and surface water.

Leachate is generally collected from the landfill through sand drainage layers, syntheticdrainage nets, or granular drainage layers with perforated plastic collection pipes, and is thenremoved through sumps or gravity drain carrier pipes.

Once leachate has been collected and removed from the landfill, it must undergo sometype of treatment and disposal. The most common methods of management are:

n Discharge to a wastewater treatment plantn On-site treatment followed by dischargen Recirculation back into the landfill

An extensive body of literature has been developed on the design of liners and leachatecollection systems. For additional information on these systems, including information onmaterials specifications, construction procedures, and quality control issues, see the referencesU.S. EPA, 1988, and U.S. EPA, 1993.

Landfill Closure and Final Cover Systems

Closure is the procedure, once waste placement in a landfill ends, that renders the sitesafe and acceptable to the public. Closure is intended to minimize the environmental andpublic health and safety hazards, and prepares the site for the post-closure period. During thepost-closure period, the site may be secured to allow degradation of the waste to proceed.Once the site has stabilized, it is converted to its planned final end use.

Final cover systems for hazardous waste landfills are designed to provide long-termminimization of liquid migration and leachate formation in the closed landfill by preventing theinfiltration of surface water into the landfill for many years. Final cover systems also controlthe venting of gas generated in the facility and isolate the wastes from the surfaceenvironment. These cover systems are constructed in layers, the most important of which arethe barrier layers. Other layers are included to protect or to enhance the performance of thebarrier layers. A final cover system must be constructed so that it functions with minimum

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maintenance, promotes drainage and minimizes erosion or abrasion of the cover,accommodates settlement and subsidence so that the cover's integrity is maintained, and has apermeability less than or equal to the permeability of the bottom liner system component withthe lowest permeability.

Environmental Safeguards at Landfill Sites

Ground-water protection is the most difficult and costly environmental control measurerequired at many hazardous waste landfills. Additionally, contamination of surface water andmethane gas buildup must be avoided.

Monitoring

To ensure the components of a landfill are performing their designed function, surfacewater and ground water monitoring should be included at all landfills. By sampling fromground water wells located near the solid waste disposal facility, the presence, degree, andmigration of any leachate can be detected. The main concern with environmental monitoring isensuring that the number and location of sampling points are adequate to characterizebackground levels (for ground water) and that sampling is frequent enough to determinewhether any performance or other environmental quality standards are being met.

Run-on/Runoff Controls for Surface Waters

The purpose of a run-on control system is to collect and redirect surface waters tominimize the amount of surface water entering the landfill. Run-on control can beaccomplished by constructing berms and swales above the filling area that will collect andredirect the water to the stormwater control structures.

Surface water management also is necessary at landfill sites to minimize erosiondamage to earthen containment structures. Design of a surface water management systemrequires a knowledge of local precipitation patterns, surrounding topographic features,geologic conditions, and facility design. Surface water management systems do not have to beexpensive or complex to be effective. The equipment and materials used for construction of thesurface water management system are the same as those used for general earthwork andfoundation construction.

Explosive Gases Control

Methane gas is a product of the anaerobic decomposition of organic waste. At andaround landfills, methane can migrate through soil and accumulate in closed areas (e.g.,building basements). The accumulation of methane gas in landfills can potentially result in fireand explosions that can endanger employees, users of the site, and occupants of nearbystructures, or cause damage to containment structures (methane is explosive in confined spaceswhen found in concentrations between 5 and 15 percent). These hazards are preventablethrough monitoring and through corrective action should methane gas levels exceed specified

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limits in the facility structures. Once methane is collected, it is usually vented into theatmosphere, flared (burned), or recovered as an energy source.

6.5.2.4 Ocean Disposal

In ocean disposal, municipal wastewater sludge is released into a designated area of theocean either from outfall pipes or vessels at the ocean surface. This option has the potential forseverely degrading the local marine environment. Batstone et al. (1989) address the technicalrequirements for safe disposal of hazardous waste in the ocean.

6.5.3 Air Emissions

Conventional aerobic biological treatment processes generally do not produce noxiousoff gases. Anaerobic biological treatment, however, can produce hydrogen sulfide, a toxic gasthat requires offgas treatment, and methane, a combustible gas that needs to be either collectedfor its energy value or diluted below its explosive limit. Biological treatment of wastewatercontaining toxic organics typically requires collection and treatment of the offgases to removeorganic contaminants (see Figure 5-6). Similar sorts of treatment are required for air andthermal stripping technologies that separate contaminants into their gaseous phase. Incinerationtechnologies require the collection of particulates in stack gases, and treatment of products ofincomplete combustion and acid gases. These air pollution control technologies result in solidparticulate residues that require disposal and liquid wastes that usually require furthertreatment.

6.5.4 Concentrated Liquid Waste Streams

Processes such as reverse osmosis and ultrafiltration create concentrated liquid wastestreams. Concentrated wastewaters containing inorganic constituents can be further treated byprecipitation and then dewatered. Evaporation is an alternative method for obtaining residualsolids from concentrated liquid waste streams. Concentrated organic liquid waste streams areusually incinerated. Deep well injection is another alternative for concentrated liquid wastes.

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6.6 REFERENCES

Batstone, R., J.E. Smith, Jr., and D. Wilson, eds. 1989. The Safe Disposal of HazardousWaste: the special needs and problems of developing countries. World Bank Technical PaperNumber 93.

Benedict, A.H., E. Epstein, and J. Alpert. 1987. Composting Municipal Sludge: A TechnologyEvaluation. EPA/600/2-87/021.

Breton, M., et al. 1987. Technical Resource Document: Treatment Technologies for SolventContaining Wastes. EPA/600/2-86/095 (NTIS PB87129821). Washington, DC.

Fresenius, W., W. Schneider, B. Böhnke, and K. Pöppinghaus (eds.). 1989. Waste WaterTechnology: Origin, Collection, Treatment and Analysis of Waste Water. Springer-Verlag,New York, NY.

Hazardous Material Control Research Institute. 1974. Municipal sludge management. HMCR.Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1975. Municipal Sludge Management anddisposal. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1977. Disposal of residues on land. HMCRI.Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1977. Composting of municipal residues andsludges. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1978. Acceptable sludge disposal techniques.HCMRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1978. Treatment and disposal of industrialwastewater HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1978. Design of municipal sludge compostfacilities. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1979. Municipal management impact ofindustrial toxic material on POTW sludge. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1980. Composting of municipal and industrialsludges: design, operations, marketing, health, rules & regs. HMCRI. Sludge/WastewaterManagement Series.

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Hazardous Material Control Research Institute. 1980. Municipal and industrial sludgecomposting-materials handling. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1982. Composting of Municipal and IndustrialSludges. HMCRI. Sludge/Wastewater Management Series.

Hazardous Material Control Research Institute. 1980; 1983. Municipal and Industrial SludgeUtilization and Disposal Proceedings Series. HMCRI. Municipal and Industrial SludgeUtilization and Disposal Proceeding Series.

Hazardous Material Control Research Institute. 1986; 1987; 1988; 1989). Municipal SewageTreatment Plant Sludge Management Proceedings Series.

McArdle, J.L., M.M. Arozarena, and W.E. Gallagher. 1987. A Handbook on Treatment ofHazardous Waste Leachate. EPA/600/S8-87/006 (NTIS PB87152328). Washington, DC.

Offut, C.K., and J.O. Knapp. 1990. The Challenge of Treating Contaminated Superfund Soil.In: Superfund '90, Hazardous Material Control Research Institute, Silver Spring, MD, pp.700-711.

Saltzberg, E.R., and J.C. Cushnie, Jr. 1985. Centralized Waste Treatment of IndustrialWastewater. Noyes Data Corporation, Park Ridge, NJ.

U.S. EPA. 1977. Operator Manual: Stabilization Ponds. EPA/430/9-77-005. Washington,DC.

U.S. EPA. 1979. Sludge Treatment and Disposal (Process Design Manual). EPA/625/1-79-011 (NTIS PB80200546). Washington, DC. [See also U.S. EPA (1974, 1978a).]

U.S. EPA. 1983. Land Application of Municipal Sludge (Process Design Manual).EPA/625/1-83-016. Washington, DC.

U.S. EPA. 1984a. Waste Analysis Plans: A Guidance Manual. EPA/530-SW-84-012.Washington, DC.

U.S. EPA. 1984b. Use and Disposal of Municipal Wastewater Sludge (EnvironmentalRegulations and Technology). EPA/625/10-84-003. Washington, DC.

U.S. EPA. 1985a. Composting of Municipal Wastewater Sludges. Seminar PublicationEPA/625/4-85-014 (NTIS PB88186119). Washington, DC.

U.S. EPA. 1985b. Estimating Sludge Management Costs at Municipal Wastewater TreatmentFacilities (Handbook). EPA/625/6-85/010 (NTIS PB86124542). Washington, DC.

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U.S. EPA. 1987. Dewatering Municipal Wastewater Sludges (Design Manual). EPA/625/1-87/014. Washington, DC.

U.S. EPA. 1988. Guide to Technical Resources for the Design of Land Disposal Facilities.EPA/625/6-88/018. Cincinnati, OH.

U.S. EPA. 1991. Issues Affecting the Applicability and Success of Remedial/RemovalIncineration Projects. Superfund Engineering Issue. EPA/540/2-91/004. Washington, DC.

U.S. EPA. 1992. Rotating Biological Contactors. Engineering Bulletin EPA/540/S-92/007.Washington, DC

U.S. EPA. 1993. Solid Waste Disposal Facility Criteria, Technical Manual. EPA/530/R-93/017 (NTIS PB94-100-450). Washington, DC.

U.S. EPA. 1993. 40 CFR Part 503, Standards for the use or disposal of sewage sludge.

U.S. EPA. 1994. Process Design Manual for Land Application of Sewage Sludge andDomestic Septage. Draft Report Submitted by Eastern Research Group to U.S. EPA Center forEnvironmental Research Information, September 16, 1994.

U.S. EPA. 1995. Development Document for Proposed Effluent Limitations Guidelines andStandards for the Centralized Waste Treatment Industry. EPA/821-R-95-006. Washington, DC.

Weathington, B.C. 1988. Destruction of Cyanide in Wastewaters: A Review and Evaluation.EPA/600/2-88/031 (NTIS PB88213046). Washington, DC.

Wilk, L., S. Palmer, and M. Breton. 1988. Technical Resource Document: TreatmentTechnologies for Corrosive-Containing Wastes, Volume II. EPA/600/2-87/099 (NTISPB88131289). Washington, DC.

Water Pollution Control Federation; Water Environment Federation. 1980. Sludge thickening.MOP FD-1. WPCF/WEF. Alexandria, VA.

Water Pollution Control Federation; Water Environment Federation. 1983. Sludge dewatering.MOP 20. WPCF/WEF. Alexandria, VA.

Water Pollution Control Federation; Water Environment Federation. 1985. Sludgestabilization. MOP FD-9. WPCF/WEF. Alexandria, VA.

Water Pollution Control Federation; Water Environment Federation. 1987a. Operation andmaintenance of sludge dewatering systems. MOP OM-8. WPCF/WEF. Alexandria, VA.

Water Pollution Control Federation; Water Environment Federation. 1987b. Activated sludge.MOP OM-9. WPCF/WEF. Alexandria, VA.

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Water Pollution Control Federation; Water Environment Federation. 1988. Sludgeconditioning. MOP FD-14. WPCF/WEF. Alexandria, VA.

Water Pollution Control Federation; Water Environment Federation.. 1989. Beneficial use ofwaste solids. MOP FD-15. WPCF/WEF. Alexandria, VA.