SECTION 6 PLANNING AND DESIGN OF HAZARDOUS WASTE TREATMENT CENTERS

Transcription

1 SECTION 6 PLANNING AND DESIGN OF HAZARDOUS WASTE TREATMENT CENTERS Planning and designing a hazardous waste treatment center (HWTC) is a necessary complement to construction of common effluent treatment plants (CETPs) (see Section 5). The purpose of an HWTC is to provide a safe alternative to uncontrolled disposal of hazardous industrial liquid and solid wastes. A related purpose is to direct toxic discharges from industry away from CETPs to prevent impairment of their plants' treatment processes. An HWTC is a single centralized facility that serves a relatively large geographic area including many individual SMSEs to take advantage of economies of scale that allow the use of hazardous waste treatment options, such as incineration, that are beyond the technical and financial capabilities of individual SMSEs. Although this chapter focuses on centralized treatment, the possibility of installing treatment processes for specific types of wastes at the industrial facility should not be overlooked. For example, relatively small distillation units for solvent recovery are available, and some SMSEs might be able to take advantage of this waste minimization option. This section focuses on the planning and engineering design of HWTCs and includes information on the major types of hazardous waste treatment alternatives available. Volume II includes an exercise to estimate waste generation from an industry producing lacquered wooden 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) a collection system (road, rail, or a combination of the two) for obtaining wastes from individual SMSEs (and larger enterprises), (2) transfer centers where wastes with similar characteristics can be identified and combined, (3) a system for transport of bulk wastes from transfer centers to the centralized treatment plant, (4) an HWTC designed to handle a wide range of hazardous wastes, 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) treatment systems at HWTCs need to be flexible enough to handle wastes with varying characteristics because of the diversity of waste sources. 6-1

2 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 to handle. Because a large number of SMSEs might be involved, obtaining detailed information on the waste characteristics and volumes for each enterprise may not be feasible. Detailed audits of randomly selected enterprises in a particular industrial category can provide a basis for estimating the overall waste characteristics and volume for that category, provided an inventory of all existing enterprises is available. Efforts should concentrate on identifying the volume 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, nonhalogenated solvents). For the purpose of treatment method selection and the design of operating parameters, it is important to have some idea of the range of concentrations of the major waste streams (i.e., percent organics and the maximum concentrations of metals in wastewater, etc.) Collection and Transportation For the following reasons, mobile rather than pipe systems are most cost-effective in transporting wastes from SMSEs to an HWTC: Both solid and liquid wastes must be collected, so mobile pickup is still required even if a pipe system is in place. When multiple liquid waste streams (spent solvents, contaminated wastewaters) have to be handled separately, multiple pipe collection systems are prohibitively expensive. Outlying SMSEs can be more easily served by mobile systems than a pipe network. 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 of a CETP, segregation of hazardous waste streams at the SMSE is essential for effective operation of an HWTC. Segregation maximizes opportunities for recovery and recycling of wastes such as solvents and metals, and minimizes potential hazards from mixing of incompatible wastes. Liquid wastes at the SMSE are generally stored in holding tanks that can be pumped into portable tanks for transport to an HWTC, while solid wastes can be stored in drums or other containers that can be transported or possibly reused. Large quantities of liquid wastewaters and dilute sludge slurries are most economically transported using vacuum inductor tank trucks. Tank trucks are manufactured in a variety of capacities 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 industrial drum holds 55 gallons. Drums are especially applicable in situations where several small volumes of different, incompatible wastes are generated at a single facility. 6-2

3 During the design of an HWTC, transportation maps are developed showing locations of individual SMSEs in relation to roads, rail lines, and navigable waterways. Specific design elements of a collection and transportation system include: 1) the selection of container materials suitable for the types of wastes to be transported; 2) choosing types and sizes of vehicles compatible with available transport routes; 3) choosing the number of vehicles to ensure a waste pickup that is commensurate with the volume of waste generated with a safety margin for delays and maintenance; and 4) the development of safe operating procedures for tracking, handling, and transporting the hazardous materials Transfer Centers or Stations An HWTC capable of handling the full range of hazardous wastes generated in a region might be too far from many SMSEs to allow efficient transportation directly to the center. Transfer centers allow preliminary classification and mixing of compatible wastes, thus facilitating processing and treatment upon arrival at the HWTC. Waste storage at transfer centers also facilitates equalization of the volumes of different waste types to further enhance the efficient operation of the HWTC. Location is a key consideration in the design of transfer centers. Centers should be near major transportation corridors (rail lines, highways) to facilitate transporting large volumes of waste from the transfer center to the HWTC. A transfer center should have enough land area available to provide temporary storage of containerized waste and possibly larger volume storage of several types of liquid wastes. Other important design elements for these centers include: (1) designing facilities for chemical analysis of wastes for classification and compatibility testing (see Section 6.1.4), and (2) designing containment systems to protect onsite workers and nearby populations from exposure to hazardous materials Characterization and Classification of Received Wastes Wastes received at an HWTC must be accurately characterized and classified to avoid hazards from mixing of incompatible wastes and to ensure that waste volumes do not exceed the design limits of treatment processes. Potential hazards from mixing incompatible hazardous wastes include: Explosions Fire Generation of flammable gas Generation of toxic gas Generation of heat Solubilization of toxins 6-3

4 Figure 6-1 identifies potentially incompatible combinations of 12 types of hazardous waste. Compatibility is primarily a concern when different wastes are mixed to create larger batches for treatment by a particular process. Compatibility might also be of concern in designing waste storage facilities. These should be designed to minimize the possibility of accidental releases being incompatible. Screening tests that do not require sophisticated and expensive laboratory equipment are usually used for compatibility testing. These tests are normally used during collection if liquid wastes are mixed in the transport container, or at transfer centers. An HWTC should have a laboratory capable of performing accurate analyses of a wide range of organic and inorganic substances. Laboratory analyses allow identification of wastes that exceed specifications for continuous treatment processes. Off-specification wastes must either be treated using an alternative method or modified by mixing them with other wastes until they fall within specifications. Laboratory analyses are also used to characterize wastes that are treated using batch chemical processes for the purpose of determining required chemical inputs to complete reactions. The U.S. Environmental Protection Agency (EPA) (1984a) provides detailed guidance of development of waste analysis plans. Appendix A contains several checklists that may be useful in the development of waste analysis plans and a list of simple American Society for Testing and Material (ASTM) test methods for screening waste characteristics. Appendix E also includes a bibliography of major references on methods for chemical analysis of wastes and wastewaters. Figure 6-1. Compatibility of selected hazardous wastes (Batstone et al., 1989) 6-4

5 6.2 HAZARDOUS WASTEWATER TREATMENT PROCESS ALTERNATIVES As with CETPs, methods for treating hazardous industrial wastewater can be broadly classified as physical, chemical, and biological. Physical methods for component separation (e.g., gravity, filtration) discussed in Section 5.2 for CETPs are equally applicable to hazardous 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 settleable solids, and (2) clarification systems (see Section 5.2.2) to remove flocculated impurities and precipitates following chemical treatment processes that generate suspended solids. Figure 6-2 identifies the operational size ranges of different methods for physical treatment of industrial wastewater. Physical treatment methods can be classified as component separation methods that use size or density as the primary separation factor and phase separation methods that generally operate on the ionic and molecular level to separate contaminants from the liquid matrix. As shown in Figure 6-2, however, some phase separation methods also operate within the molecular size range. This section focuses on physical phase separation and chemical treatment methods that are commonly used for treating hazardous industrial wastewaters. Most of the chemical processes discussed in this section also are suitable for pretreatment of wastewater on site at an SMSE 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 collected for treatment at a HWTC. In the absence of pretreatment and discharge to a CETP, toxic wastewater should be collected and transported to an HWTC where the same or similar treatment processes would be used. The advantage of pretreatment of industrial wastewater on site 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: Air stripping (Section 6.2.1) Carbon adsorption (Section 6.2.2) Reverse osmosis (Section 6.2.3) Ultrafiltration (Section 6.2.3) Liquid carbon dioxide extraction (Section 6.2.3) Chemical treatment methods covered in this section include: Neutralization (Section 6.2.4) Chemical precipitation (Section 6.2.5) Cyanide destruction (Section 6.2.6) Chromium reduction (Section 6.2.7) Electrolytic recovery (Section 6.2.8) Ion exchange (Section 6.2.9) 6-5

6 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 Development Document for Proposed Effluent Limitations Guidelines and Standards for the Centralized Waste Treatment Industry (EPA). This report contains much useful performance data on treatment methods used in the centralized waste treatment industry. Biological treatment methods applicable to toxic wastewaters are discussed briefly in Section Appendix B identifies references that include more detailed information on engineering design for specific processes Air Stripping Air stripping is an effective treatment method for removing dissolved volatile organic compounds from wastewater. The removal is accomplished by passing high volumes of air through the agitated wastewater stream. The process results in a contaminated off-gas stream that, depending upon air emissions standards, usually requires treatment in air pollution control equipment. Stripping can be performed in tanks or in spray or packed towers. Treatment in packed towers is the most efficient application. The packing typically consists of plastic rings or saddles. Two commonly used types of towers, cross-flow and countercurrent, differ in design only in the location of air inlets. Cross-flow towers draw air through the sides for the total 6-6

7 height of the packing. The countercurrent tower draws the entire air flow from the bottom. The cross-flow towers are more susceptible to scaling problems and are less efficient than the countercurrent 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 scrubber Source: Productos químicos. Planes de acción para mejoramiento ambiental. Manual para empresarios de la PYME. Santafé de Bogotá: Sir Speedy. Impresiones Daza Aragón Ltda. 6-7

8 The driving force of the air stripping mass-transfer operation is the difference in concentrations between the air and liquid streams. Pollutants are transferred from the more concentrated 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 is expressed 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 be removed. 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/m 3 air)/(mg/m 3 water) and lower, are not effectively removed 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 greater column height, more trays or packing material, greater pressure and temperature, and more frequent cleaning than pollutants with a higher strippability. Low temperatures adversely affect the air stripping process. Air strippers experience lower efficiencies at lower temperatures, with the possibility of freezing occurring within the tower. For this reason, depending on the location of the tower, it may be necessary to preheat the wastewater and the air feed streams. The column and packing materials must be cleaned regularly to ensure that low effluent levels are attained. Air stripping has proved to be an effective process in the removal of volatile pollutants from wastewater. It is generally limited to influent concentrations of less than 100 mg/l organics. Well-designed and operated systems can achieve over 99 percent removals Carbon Adsorption Activated carbon adsorption is a demonstrated treatment technology for the removal of organic pollutants from wastewater. Most applications use granular activated carbon (GAC) in column reactors. Sometimes powdered activated carbon (PAC) is used alone or in conjunction with 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

9 Figure 6-4. Carbon adsorption system diagram (U.S. EPA, 1995) The mechanism of adsorption is a combination of physical, chemical, and electrostatic interactions between the activated carbon and the adsorbate, although the attraction is primarily physical. 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 of contaminant adsorbed per unit mass of carbon, which is a function of the compound being adsorbed, 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, highmolecular-weight organics, with low solubility, are readily adsorbed. Polar, low-molecularweight organics, with high solubilities, are more poorly adsorbed. Competitive adsorption of other compounds affects adsorption. The carbon may preferentially adsorb one compound over another with the competition resulting in an adsorbed compound being desorbed from the carbon. In a fixed-bed system, pollutants are removed in increasing amounts as wastewater flows through the bed. In the upper area of the bed, pollutants are rapidly adsorbed. As more wastewater passes through the bed, this rapid adsorption zone increases until it reaches the bottom of the bed. At this point, all available adsorption sites are filled and the carbon is said 6-9

10 to be exhausted. This condition can be detected by an increase in the pollutant concentration of the effluent from the bed and is called breakthrough. GAC systems usually comprise several beds operated in series. This design allows the first bed to go to exhaustion, while the other beds still have the capacity to treat to an acceptable effluent quality. The carbon in the first bed is replaced, and the second bed then becomes 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 or steam 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 by contacting it with a solvent that dissolves the adsorbed pollutants. Depending on system size and economics, some facilities may choose to dispose of the spent carbon instead of regenerating it. For very large applications, as may occur at an HWTC, construction of an on-site regeneration facility may be justified. For smaller applications, it is generally cost-effective to use a vendor service to deliver regenerated carbon and remove the spent carbon. These vendors transport the spent carbon to their centralized facilities for regeneration. GAC adsorption is a widely used wastewater treatment technology. Generally, the chemical oxygen demand (COD) of the waste stream can be reduced to less than 10 mg/l and the biological oxygen demand (BOD) to less than 2 mg/l. Removal efficiencies typically are in the range of 30 to 90 percent. Poor GAC system performance sometimes results from competitive adsorption between compounds in the waste stream. The pollutant methylene chloride is often used as a measure of adsorption competition in a GAC system because it is readily adsorbed and also desorbed by competitive compounds. Thus, low methylene chloride removals indicate competitive adsorption effects. Oil and grease can adversely affect GAC performance by coating the carbon particles, thereby inhibiting the adsorption process. A commonly applied limit on oil and grease loading to a GAC system is 10 mg/l. Suspended solids also adversely affect GAC performance by plugging the bed, resulting in excessive head loss. A commonly used total suspended solids (TSS) loading limit to a GAC system is 50 mg/l. Poor performance of GAC units used at centralized waste treatment plants in the United States of America has been observed and attributed to the inherent difficulty of operating carbon adsorption units for variable waste streams (EPA, 1995) Other Physical Treatment Technologies Other less commonly used physical treatment technologies used in the U.S. centralized waste treatment industry include: Reverse osmosis Ultrafiltration Carbon dioxide liquid extraction 6-10

11 Reverse osmosis (RO) is a process for separating dissolved solids from water. It is commonly used to treat oily or metal-bearing wastewater. RO is applicable when the solute molecules are approximately the same size as the solvent molecules. A semipermeable, microporous membrane and pressure are used to perform the separation. RO systems are typically used as end-of-pipe polishing processes, prior to final discharge of the treated wastewater. Osmosis is the diffusion of a solvent (such as water) across a semipermeable membrane from a less concentrated solution into a more concentrated solution. In the reverse osmosis process, pressure greater than the normal osmotic pressure is applied to the more concentrated solution (the waste stream being treated), forcing the purified water through the membrane and into 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 referred to as concentrate. The concentrate is recirculated through the membrane unit until the permeate flow 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 is shown in Figure 6-5a. Figure 6-5. Other physical treatment technologies: (a) reverse osmosis, (b) liquid carbon dioxide extraction (EPA, 1995) Performance of an RO system is dependent upon the dissolved solids concentration and temperature of the feed stream, the applied pressure, and the type of membrane selected. The key RO membrane properties to be considered are selectivity for water over ions, permeation rate, and durability. RO modules are available in various membrane configurations, such as spiral-wound, tubular, hollow-fiber, and plate and frame. In addition to the membrane 6-11

12 modules, other capital items needed for an RO installation include pumps, piping, instrumentation, and storage tanks. The major operating cost is attributed to membrane replacement. EPA (1995) presents performance data for a single unit with an average reduction in the concentration 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 by more than 98 percent. Ultrafiltration (UF) is used for the treatment of metal-finishing wastewater and oily wastes. It can remove substances with molecular weights greater than 500, including suspended solids, oil and grease, large organic molecules, and complex heavy metals. UF is used when the solute molecules are greater than 10 times the size of the solvent molecules and are less than one-half micron. The centralized waste treatment industry applies UF to treat oil/water emulsions. Oil/water emulsions contain both soluble and insoluble oil. Typically, the insoluble oil is removed from the emulsion by gravity separation assisted by chemical addition. The soluble oil is then removed through UF. Oily wastewater containing 0.1 to 10 percent oil can be effectively treated using UF. A UF system is typically used as an in-plant treatment technology, treating the oil/water emulsion prior to mixing with other wastewater. A schematic UF system is similar to reverse osmosis (see Figure 6-5a), with the difference being in the characteristics of the membrane. In UF, a semipermeable, microporous membrane performs the separation. Wastewater is 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. The membrane rejects emulsified oil and suspended solids, which are removed as concentrate. The concentrate is recirculated through the membrane unit until the permeate flow 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. The primary design consideration in UF is membrane selection. Membrane pore size is chosen based on the size of the contaminant particles targeted for removal. Other design parameters to be considered are solids concentration, viscosity, and temperature of the feed stream, and membrane permeability and thickness. U.S. EPA (1995) presents performance data for a UF system that treats oily wastewater. The system removed 87.5 percent of the influent oil and grease and 99.9 percent of the TSS. Removal of several organic and metal pollutants exceeded 90 percent. Liquid carbon dioxide (CO2) extraction is used to extract and recover organic contaminants from aqueous waste streams. A licensed, commercial application of this technology, the "Clean Extraction System (CES)," is used in the centralized waste treatment industry. The process can be effective in removing organic substances such as hydrocarbons, aldehydes and ketones, nitriles, halogenated compounds, phenols, esters, and heterocyclics. It 6-12

13 is not effective in removing some compounds that are very water-soluble, such as ethylene glycol, and low-molecular-weight alcohols. It can provide an alternative in the treatment of waste streams that historically have been incinerated. The waste stream is fed into the top of a pressurized extraction tower containing perforated plates, where it is contacted with a countercurrent stream of liquefied CO2. The organic contaminants in the waste stream are dissolved in the CO2; this extract is then sent to a separator, which redistills the CO2. The distilled CO2 vapor is compressed and reused. The concentrated organics bottoms from the separator can then be disposed or recovered. The treated wastewater stream that exits the extractor (raffinate) is pressure-reduced and may be further treated for residual organics removal if necessary to meet discharge standards. A diagram of the CES is presented in Figure 6-5b. Pilot-scale operational data for a commercial CES unit show high removals for the organic compounds chloroform, 1,2-dichloroethane, ethylbenzene, methylene chloride, and toluene, with rates generally exceeding 99 percent (phenol removal was poorest with 83 percent). EPA sampled a CES operating unit and found significantly lower removal rates, ranging from 48 to 88 percent Neutralization Acidic corrosive wastes (ph less than 2) and alkaline corrosive wastes (ph greater than 12.5) typically require neutralization prior to use of subsequent treatment processes to limit corrosion of processing equipment and to improve treatment efficiency. Neutralization or ph adjustment is often required for wastes that do not classify as corrosive in order to optimize chemical 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 of acidic reagents to neutralize alkaline wastes (sulfuric acid, hydrochloric acid, carbonic acids and liquid carbon dioxide). Table 6-1 provides summary information on these processes, including: Applicable waste streams Stage of development Performance Residuals generated 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 to destroy cyanides (see Section 6.2.6) before neutralization. Typically, a tradeoff exists between the cost of neutralization reagents and length of time required for neutralization and the volume 6-13

14 of sludge the process creates. Cheaper methods generally take a longer time to complete neutralization due to more dilute concentrations of reagents. Cheaper reagents, such as limestone and lime, and sulfuric acid also tend to produce larger volumes of sludges. Selection of a neutralization method requires evaluation of the compatibility of waste streams with each other and available reagents. Selection of a method also required weighing the tradeoffs of the reagent cost versus the speed of neutralization and the sludge disposal cost. Section discusses further options for treatment and use of corrosive wastes Chemical Precipitation Chemical precipitation is used to remove metal compounds from wastewater. In the chemical precipitation process, soluble metallic ions and certain anions are converted to insoluble forms, which precipitate from the solution. The precipitated metals are subsequently removed from the wastewater stream by liquid filtration or clarification. The performance of the process is affected by chemical interactions, temperature, ph, solubility, and mixing effects. Various chemicals can be used as precipitants, including sodium hydroxide (NaOH), lime (Ca(OH)2), soda ash, sulfide, ferrous sulfate, and acid. Hydroxide precipitation is effective 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 of chemical precipitation, and of these, lime is used more often than caustic. The chief advantage of lime over caustic is its lower cost. Lime is more difficult to handle and feed, however, as it must be slaked, slurried, and mixed, and can plug the feed system lines. Lime also produces a larger volume of sludge, and the sludge is generally not suitable for reclamation due to its homogeneous nature. Also, dewatered metal sludge is typically sold to smelters for reuse, and the calcium compounds in lime precipitation sludge interfere with smelting. The metals from caustic precipitation sludge can often be recovered. The reaction mechanism for precipitation of 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 NaOH _ M(OH)2 + 2Na

15 Table 6-1. Summary of neutralization technologies (Wilk et al., 1988) Process Acid/alkali mutual neutralization Applicable Waste Streams All acid/alkali compatible waste streams except cyanide Stage of Development Well developed Performance Residuals Generated Cost Generally slower than comparable technologies due to dilute concentration of reagents. May evolve hazardous constituents if incompatible wastes are mixed Variable, dependent on quantity of insolubles and products contained in each waste stream Least expensive of all neutralization technologies Limestone Dilute acid waste streams of less than 5,000 mg/l mineral acid strength and containing low concentration of acid salts Well developed Requires stone sizes of mm or less. Requires 45 minutes or more of retention time. Can only neutralize acidic wastes to ph 6.0. Must be aerated to remove evolved CO2 Will generate voluminous sludge product when reacted with sulfatecontaining wastes. Stones over 200 mesh will sulfonate, be rendered inactive, and add to sludge product Most cost-effective in treating concentrated wastes. May be cost-effective in treating dilute acidic wastes Lime All acid wastes Well developed Requires 15 to 30 minutes of retention time. Must be slurried to a concentration of 10 to 35% solids prior to use. Can under-(below ph 7) or over- (above ph 7) neutralize Caustic soda All acid wastes Well developed Requires 3 to 15 minutes of retention time. In liquid form, easy to handle and apply. Can under- or overneutralize including ph 13 or higher Will generate voluminous sludge similar to limestone Reaction products are generally soluble, however, sludges do not dewater as readily or as easily as lime or limestone More expensive than crushed limestone (200 mesh) Most expensive of all widely used alkaline reagents (five times the cost of lime) Sulfuric acid All alkaline wastes except cyanide Well developed Requires 15 to 30 minutes of retention time. In liquid form, but presents burn hazard. Highly reactive and widely available Will generate large quantities of gypsum sludge when reacted with calciumbased alkaline wastes Least expensive of all widely used acidic reagents Hydrochloric acid All alkaline wastes Well developed, but rarely applied due to high reagent cost Requires 5 to 20 minutes of retention time. Liquid form present burn and fume hazard. More reactive than sulfuric Reaction products are generally soluble Approximately twice as expensive as sulfuric on a neutralization equivalent basis Carbonic acids, liquid carbon dioxide All alkaline wastes except cyanide Emerging technology Retention time 1 to 11/2 minutes. In liquid form, must be vaporized prior to use. Can only neutralize alkaline wastes to ph 8.3 and point Will form calcium carbonate precipitate when reacted with calcium-based alkaline wastes Approximately 3 to 4 times as expensive as sulfuric. Therefore, limited to applications using more than 200 tons of reagents per year or with flow rate greater than 100,000 gpd In addition to the type of treatment chemical chosen, another important design factor in the chemical precipitation operation is ph. Metal hydroxides are amphoteric, meaning that they 6-15

16 can react chemically as acids or bases. As such, their solubilities increase toward both lower and higher ph levels. Therefore, each metal has an optimum ph for precipitation that corresponds to its point of minimum solubility. Another key consideration in a chemical precipitation application is the detention time, which is specific to the wastewater being treated and the desired effluent quality. It may take from less than an hour to several days to achieve adequate precipitation of the dissolved metal compounds. Chemical precipitation is a two-step process. It is typically performed in batch operations where the wastewater is first mixed with the treatment chemical in a tank. The mixing is typically achieved by mechanical means such as mixers or recirculation pumping. Then, the wastewater undergoes a separation/dewatering process such as clarification or filtration, where the precipitated metals are removed from solution. In a clarification system, a flocculent is sometimes added to aid in the settling process. The resulting sludge from the clarifier or filter must be further treated, disposed, or recycled. A typical chemical precipitation 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 process for treatment at HWTCs, where the waste receipts can be highly variable. A facility can hold its wastes and segregate them by pollutant content for treatment. This type of waste treatment 6-16

17 management, called selective metals precipitation, can be implemented to concentrate on one or two major pollutants of concern. This application of chemical precipitation uses several tanks to allow the facility to segregate its wastes into smaller batches, thereby avoiding interference with other incoming waste receipts and increasing treatment efficiency. These specific operations also produce specific sludges containing only the target metals, making them suitable for reclamation. The effluent quality achievable with chemical precipitation depends upon the metals present in the wastewater and the process operating conditions. This technology is widely used with possible removal efficiencies greater than 99 percent, and it often removes metal pollutants down to levels of 1 µg/l or less Cyanide Destruction Cyanide is a very toxic pollutant and, therefore, wastes containing cyanide are an important environmental concern. Electroplating and metal finishing operations produce most cyanide-bearing wastes. At least a dozen cyanide destruction technologies are available, but only 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 chlorine or sodium hypochlorite. A diagram of an alkaline chlorination system is presented in Figure 2-9. Alkaline chlorination can destroy free dissolved hydrogen cyanide and can oxidize all simple and some complex inorganic cyanides; however, it cannot effectively oxidize stable iron, copper, and nickel cyanide complexes. The addition of heat to the alkaline chlorination process can facilitate the more complete destruction of total cyanides. In alkaline chlorination using gaseous chlorine, the oxidation process is accomplished by direct addition of chlorine (Cl2) as the oxidizer and sodium hydroxide (NaOH) to maintain ph levels (see Figure 2-9). The reaction mechanism is: NaCN + Cl2 + 2NaOH 2NaCNO + 3Cl2 + 6NaOH NaCNO + 2NaCl + H20 2NaHCO3 + N2 + 6NaCl + 2H20 Destruction of the cyanide takes place in two stages. The primary reaction is partial oxidation of the cyanide to cyanate at a ph above 9. In the second stage, the ph is lowered to the 8 to 8.5 range for the oxidation of the cyanate to nitrogen and carbon dioxide (as sodium bicarbonate). Each part of cyanide requires 2.73 parts of chlorine to convert it to cyanate and an additional 4.1 parts of chlorine to oxidize the cyanate to nitrogen and carbon dioxide. At least parts of sodium hydroxide are required to control the ph with each stage. 6-17

18 Alkaline chlorination can also be conducted with sodium hypochlorite (NaOCl) as the oxidizer. The oxidation of cyanide waste using sodium hypochlorite is similar to the gaseous chlorine process. The reaction mechanism is: NaCN + NaOCl 2NaCNO + 3NaOCl + H20 NaCNO + NaCl 2NaHCO3 + N2 + 3NaCl Cyanide destruction efficiencies of greater than 99 percent are possible with this technology but can vary greatly depending on the forms of cyanide present Chromium Reduction Reduction is a chemical reaction in which electrons are transferred from one chemical to another. The main application of chemical reduction in wastewater treatment is the reduction of hexavalent chromium to trivalent chromium. This is a commonly used pretreatment process in the leather tanning industry (see Section 2.3.3) and the electroplating industry (see Section 2.3.5). The reduction enables the trivalent chromium to be precipitated from solution in conjunction with other metallic salts. Sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sulfate are strong reducing agents commonly used in industrial wastewater treatment applications. Two types of chromium reduction are discussed here: Reduction through the use of sodium metabisulfite or sodium bisulfite Reduction through the use of gaseous sulfur dioxide A diagram of a chromium reduction system using gaseous sulfur dioxide is presented in Figure 2-9. These chromium reduction reactions are favored by a low ph of 2 to 3. At ph levels above 5, the reduction rate is slow. Oxidizing agents such as dissolved oxygen and ferric iron interfere with the reduction process by consuming the reducing agent. After the reduction process, 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 the reducing 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 metabisulfite needed to reduce the hexavalent chromium is reported as 3 parts of sodium bisulfite per part of 6-18

19 chromium, while the amount of sulfuric acid is 1 part per part of chromium. The theoretical retention time is about 30 to 60 minutes. A second process uses sulfur dioxide (SO2) as the reducing agent. The reaction mechanism is: 3SO2 + 3H20 3H2SO3 + 2H2CrO4 3H2SO3 Cr2(S04)3 + 5H20 The hexavalent chromium is reduced to trivalent chromium using sulfur dioxide, with sulfuric acid used to lower the ph of the solution. The amount of sulfur dioxide needed to reduce the hexavalent chromium is reported as 1.9 parts of sulfur dioxide per part of chromium, while the amount of sulfuric acid is 1 part per part of chromium. At a ph of 3, the theoretical retention time is approximately 30 to 45 minutes. U.S. EPA (1995) reported hexavalent chromium reduction efficiency of percent for the sulfur dioxide process based on one centralized waste treatment plant. Another plant using the chromium reduction process with sodium metabisulfite actually showed an increase in hexavalent concentration, indicating the importance of careful process control to achieve treatment objectives Electrolytic Recovery Electrolytic recovery is used for the reclamation of metals from wastewater. It is a common technology in the electroplating, mining, and electronic industries and is used for the recovery of copper, zinc, silver, cadmium, gold, and other heavy metals. Nickel is poorly recovered due to its low standard potential. The electrolytic recovery process uses an oxidation and reduction reaction. Conductive electrodes (anodes and cathodes) are immersed in the metal-bearing wastewater, with electrical energy 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 nitrogen form at the anode (electron-producing reaction). After the metal coating on the cathode reaches a desired thickness, it may be removed and recovered. The metal-plated cathode can then be used as the anode. The equipment consists of an electrochemical reactor with electrodes, a gas venting system, recirculation pumps, and a power supply. A diagram of an electrolytic recovery system is presented in Figure 6-7. Electrochemical reactors are typically designed to produce high flow rates to increase the process efficiency. A conventional electrolytic recovery system is effective for the recovery of metals from relatively high-concentration wastewater. A specialized adaptation of electrolytic recovery, 6-19

20 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 open structure and therefore a lower resistance to fluid flow. This also provides a larger electrode surface. ESE systems are often used for the recovery of copper, lead, mercury, silver, and gold Ion Exchange Ion exchange is commonly used for the removal of heavy metals from relatively low-concentration waste streams, such as electroplating wastewater. A key advantage of the ion exchange process is that it allows for the recovery and reuse of the metal contaminants. Ion exchange can also be designed to be selective to certain metals and can provide effective removal from wastewater that has high background contaminant levels. A disadvantage is that some organic substances can foul the resins. In an ion exchange system, the wastewater stream is passed through a bed of resin. The resin contains bound groups of ionic charge on its surface, which are exchanged for ions of the same charge in the wastewater. Resins are classified by type, either cationic or anionic; the selection is dependent upon the wastewater contaminant to be removed. A commonly used resin is polystyrene copolymerized with divinylbenzene. The ion exchange process involves four steps: treatment, backwash, regeneration, and rinse. During the treatment step, wastewater is passed through the resin bed. The ion exchange process continues until pollutant breakthrough occurs. The resin is then backwashed to reclassify the bed and to remove suspended solids. During the regeneration step, the resin is contacted with either an acidic or alkaline solution containing the ion originally present in the resin. 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 contaminated regenerating solution must be further processed for reuse or disposal. Depending upon system size and economics, some facilities choose to remove the spent resin and replace it with resin regenerated off site instead of regenerating the resin in-place. Ion exchange equipment ranges from simple, inexpensive systems such as domestic water softeners, to large, continuous industrial applications. The most commonly encountered industrial setup is a fixed-bed resin in a vertical column, where the resin is regenerated in-place. A diagram of this type of system is presented in Figure 6-8. These systems can be designed so that the regenerant flow is concurrent or countercurrent to the treatment flow. A countercurrent design, although more complex to operate, provides a higher treatment efficiency. The beds can contain a single type of resin for selective treatment, or the beds can be mixed to provide for more complete deionization of the waste stream. Often, individual beds containing different resins are arranged in series, which makes regeneration easier than in the mixed bed system. 6-20

21 Figure 6-7. Electrolytic recovery system diagram (U.S. EPA, 1995) Ion exchange is very effective in the treatment of low-concentration, metal-bearing wastewater. A common application, chromic acid recovery, has a demonstrated performance of 99.5 percent. Copper removal from metal finishing rinsewaters can also exceed 99 percent, and nickel removals range from 82 to 96 percent Ozonization Ozone (O3) is a blue gas generated by the passage of air through a high potential electric field (10,000 to 20,000 v). Ozone is used to disinfect wastewater because of its oxidizing properties. In industrial wastewater treatment, several contact units or chambers with ozone must be available to guarantee oxidation of pollutants, virus and bacteria. If wastewater contains flocculated material and an ozone disinfection is desired, it is appropriate to use a turbine contact system. Indeed, bubbles produced by a porous diffuser system cannot create sufficient turbulence to disintegrate the agglomerated matter or completely oxidize bacteria and virus. The estimation of ozone dosage often requires a previous laboratory test. 6-21

22 Figure 6-8. Ion exchange system diagram (U.S. EPA, 1995) Biological Treatment Conventional biological treatment processes for wastewater are discussed in Section 5.3. These processes generally require pretreatment of industrial wastewater to reduce concentrations of heavy metals and toxic organics to levels that will not impair the performance of the biological treatment system. If CETPs or conventional sewage treatment plants are able to treat the bulk of nonhazardous organics in industrial wastewaters, biological treatment methods are not likely to be a major component in an HWTC. If the HWTC does receive a significant volume of nonhazardous industrial wastewater with organics, the treatment options would be similar to those discussed for CETPs in Section 5.3. Biological treatment process choices might well be different for an HWTC compared with a CETP because the need for maintenance is less of a constraint. Rotating biological contactor systems (RBCs) (see Section 5.3.4) are the conventional biological treatment process that is most suitable for specific treatment of industrial wastewaters containing up to 1 percent soluble organics, including solvents, halogenated organics, acetone, alcohols, phenols, phthalates, ammonia, and petroleum products. RBCs also can treat inorganic cyanides (EPA, 1992). 6-22

23 Slurry biodegradation is a treatment process where an aqueous slurry is created by combining sludge with water and biodegraded aerobically using a self-contained reactor or a lined aerated lagoon. The process is similar to the conventional activated sludge process or an aerated lagoon, except that the system can handle highly contaminated soils or sludges that have contaminant concentrations ranging from 2,500 mg/kg to 250,000 mg/kg. The main applications of this technology are treating coal tars, refinery wastes, hydrocarbons, woodpreserving wastes, and organic and chlorinated organic sludges. The required operational parameters 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 wastewaters that contain both biodegradable organics and toxic organics. Figure 6-9 shows a schematic of the system. The GAC sorbs the toxic organics, and the expanded bed configuration enhances biomass attachment to the GAC, allowing decomposition of the wastewater's readily biodegradable constituents and providing regeneration of more slowly degraded substances that are sorbed on to the GAC. This design, combined with heating to optimize microbial activity rates, allows hydraulic retention times of 3 to 12 hours, representing a significant reduction in bioreactor volume compared with conventional technologies. Table C-3 in Worksheet C in Volume II provides additional information about suitable wastes for this technology. Figure 6-9. Schematic of anaerobic, expanded-bed GAC bioreactor 6-23

24 6.3 OTHER HAZARDOUS WASTE TREATMENT PROCESS ALTERNATIVES Alternative treatment methods for hazardous wastes can be broadly classified as: Solidification/stabilization technologies (see Section 6.3.1) 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 both solidification/stabilization and thermal treatment (most likely incineration) is an integral part of an HWTC Solidification/Stabilization (S/S) Solidification or fixation refers to techniques that incorporate hazardous waste into a solid 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 these techniques. In situ S/S processes are applied to in place wastes or contaminated soils, and ex situ S/S processes involve in-tank treatment. Ex situ S/S processes are most likely to be used at an HWTC. Figure 6-10 shows generic elements of an in-tank treatment system. The key elements of this system include separating and crushing large particles, and the mixing stage where binding agents and water are added. At an HWTC, S/S technologies are most likely to be used for two types of waste streams: (1) as-received solid wastes (e.g., plastics, resins, tars and sludges that are not suitable for treatment using other processes, and (2) as a final treatment step for residual solids and sludges generated from other treatment processes on site. The final step 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/S processes. Solidification through the addition of cement, lime, or other pozzolanic materials such as flyash are the most commonly used and are suitable for the large majority of inorganic wastes. Other S/S processes, such as embedding waste in thermoplastic materials such as bitumen, paraffin or polyethylene, and microencapsulation are more expensive and are usually used only for problem-causing wastes such as those with a high organic content. Physical stabilization involves blending sludge or semiliquid wastes with a bulking agent such as pulverized 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 the waste to be treated has physical or chemical properties that would interfere with the 6-24

25 stabilization or solidification process. Table 6-2 identifies factors that might interfere with these processes. S/S binding agents Excavation (1) Classification (2) Mixing (3) Off-gas treatment (optional) Residuals (4) VOC capture and treatment Oversize rejects Water Stabilized and solidified media Crusher Figure Elements of a typical ex situ solidification/stabilization process (U.S. EPA, 1993) 6-25

26 Table 6-2. Summary of factors that may interfere with stabilization and solidification processes (U.S. EPA) Characteristics Affecting Processing Feasibility VOCs Use of acidic sorbent with metal hydroxide wastes Use of acidic sorbent with cyanide wastes Use of acidic sorbent with waste containing ammonium compounds Use of acidic sorbent with sulfide wastes Use of alkaline sorbent (containing carbonates such as calcite or dolomite) with acid waste Use of siliceous sorbent (soil, fly ash) with hydrofluoric acid waste Presence of anions in acidic solutions that form soluble calcium salts (e.g., calcium chloride acetate, and bicarbonate) Presence of halides Organic compounds Semivolatile organics or polyaromatic hydrocarbons (PAHs) Oil and grease Potential Interference Volatiles not effectively immobilized; driven off by heat of reaction. Sludges and soils containing volatile organics can be treated using a heated extruder evaporator or other means to evaporate free water and VOCs prior to mixing with stabilizing agents. Solubilizes metal. Releases hydrogen cyanide Releases ammonia gas Releases hydrogen sulfide May create pyrophoric waste. May produce soluble fluorosilicates Cation exchange reactions leach calcium from S/S product increases permeability of concrete, increases rate of exchange reactions. Easily leached from cement and lime. Organics may interfere with bonding of waste materials with organic binders Organics may interfere with bonding of waste materials. Weaken bonds between waste particles and cement by coating the particles. Decrease in unconfined compressive strength with increased concentrations of oil and grease. Fine particle size Halides Insoluble material passing through a No. 200 mesh sieve can delay setting and curing. Small particles can also coat larger particles, weakening bonds between particles and cement or other reagents. Particle size > ¼ inch in diameter not suitable. Reduced physical setting, easily leached for cement and pozzolan S/S. May dehydrate thermoplastic solidification. Soluble salts of manganese, tin, zinc, copper and lead Reduced physical strength of final product caused by large variations in setting time and reduced dimensional stability of the cured matrix, thereby increasing leachability potential. Cyanides Cyanides interfere with bonding of waste materials. 6-26

27 Characteristics Affecting Processing Feasibility Sodium arsenate, borates, phosphates Sulfate Phenols Potential Interference Retard setting and curing and weaken strength of final product. Retard setting and cause swelling and spalling in cement S/S. With thermoplastic solidification may dehydrate and rehydrate, causing splitting. Marked decreases in compressive strength for high phenol levels. Presence of coal or lignite Sodium borate, calcium sulfate, potassium dichromate, and carbohydrates. Nonpolar organics (oil, grease, aromatic hydrocarbons, PCBs) Polar organics (alcohols, phenols, organic acids, glycols) Solid organics (plastics, tars, resin) Oxidizers (sodium hypochlorite, potassium permanganate, nitric acid, or potassium dichromate) Coals and lignites can cause problems with setting, curing, and strength of the end product. Interferes with pozzolanic reactions that depend on formation of calcium silicate and aluminate hydrates. May impede setting of cement, pozzolan, or organicpolymer S/S. May decrease long-term durability and allow escape of volatiles during mixing. With thermoplastic S/S, organics may vaporize from heat. With cement or pozzolan S/S, high concentrations of phenol may retard setting and may decrease short-term durability; all may decrease long-term durability; all may decrease long-term durability. With thermoplastic S/S, organics may vaporize. Alcohols may retard setting of pozzolans. Ineffective with urea formaldehyde polymers; may retard setting of other polymer S/S. May cause matrix breakdown or fire with thermoplastic or organic polymer S/S. Metals (lead, chromium, cadmium, arsenic, mercury) May increase setting time of cements if concentration is high. Nitrates, cyanides Soluble salts of magnesium, tin, zinc, copper and lead Environmental/waste conditions that lower the ph of matrix Flocculants (e.g. ferric chloride) Increase setting time, decrease durability for cementbased S/S. May cause swelling and cracking within inorganic matrix exposing more surface area to leaching. Eventual matrix deterioration. Interference with setting of cements and pozzolans. Soluble sulfates > 0.01% in soil or 150 mg/l in water Soluble sulfates > 0.5% in soil or 2000 mg/l in water Oil, grease, lead, copper, zinc and phenol Aliphatic and aromatic hydrocarbons Endangerment of cement products due to sulfur attack. Serious effects on cement products from sulfur attacks. Deleterious to strength and durability of cement, lime/fly ash, fly ash/cement binders. Increase set time for cement. 6-27

28 Characteristics Affecting Processing Feasibility Chlorinated organics Metal salts and complexes Inorganic acids Inorganic bases Potential Interference May increase set time and decrease durability of cement if concentration is high. Increase set time and decrease durability for cement or clay/cement. Decrease durability for cement (Portland Type I) or clay/cement. Decrease durability for clay/cement; KOH and NaOH decrease durability for Portland cement Type III and IV Incineration Incineration is the most commonly used method for thermal treatment of organic liquids, and solids and sludges contaminated with toxic organics. Figure 6-11 shows a flow diagram with the following key elements of an incinerator system: (1) waste processing, which includes screening, size reduction, and waste mixing, (2) a waste feed system, (3) a combustion unit, (4) air pollution control equipment to collect/treat products of incomplete combustion, particulate emissions, and acid gases, and (5) facilities for handling and disposing residual ash from the combustion unit, and particulates and residual wastewater from the air pollution 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 for hazardous wastes. Existing industrial boilers and kilns, especially cement kilns, are also sometimes used for thermal treatment of hazardous wastes. Table 6-3 identifies major waste properties 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 of incinerator units. Appendix B identifies major references that provide more detailed information about the selection and design of incinerators. Incineration is a relatively expensive treatment option, but the economies of scale created by a large HWTC mean an incinerator is likely to be a key element in the design of a facility capable of treating a wide range of hazardous wastes. Incineration is also sometimes used to treat sludge from conventional wastewater treatment plants serving large cities. As discussed in Section 6.5, sludge from CETPs normally is put to beneficial use; however, it might be necessary to transport highly contaminated sludges to an HWTC for further treatment, and possibly incineration. 6-28

29 Exhaust to Atmosphere Acid Gas Control Waste Processing Auxiliary Fuel Combustion Air Particulate Removal Waste Feeding Combustion Unit Gas Conditioning Ash Removal Residual Treatment Wastewater To Disposal Figure 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 Parameter Affected Heating value Rotary kiln Rotary kiln temperature, flue gas residence time. Density Rotary kiln Weight of material held by kiln. Halogen and sulfur content Quench system, air pollution control equipment design and operation. Pump cavitation, ph control, blowdown rate, particulate emissions. Effect of Performance Feed capacity, fuel usage. Feed capacity. Feed capacity, caustic usage. Moisture Feed system Increased fuel usage to maintain temperature. Particle size distribution H:Cl ratio of POHCs. Any fusion characteristics (determined by chemical characteristics, e.g., alkalis). Cyclone, SCC, ducts, wet electrostatic precipitation (WEP), instrumentation. Rotary kiln, cyclone, ducts, quench elbow, instrumentation Kiln draft, particulate emissions excess oxygen control, temperature control. Incinerator s ability to thermally destroy POHCs/PICs. Kiln draft, temperature, excess O2 control. Fouling of duct, cyclone, SCC, process water system, and instruments. As H:Cl ratio decreases, thermal stability of POHCs increases and oxidation of PICs is reduced. Under oxygen starved conditions, the tendency to form PICs increases as the N:Cl ratio decreases. Slagging of kiln, plugging of instruments and downstream equipment. Example Feeds of Concern Plastics, trash. Brominated sludge (high density sludge). Trial burn mixture, brominated sludge. Soils, brominated sludge, vermiculite. C2Cl6C6l6, C2HCl and similar compounds. Plastic, trash, brominated sludge. 6-29

30 6.3.3 Other Thermal Treatment Technologies Other thermal treatment technologies include a variety of methods that use heat (but not primarily oxidation by direct air combustion as with incineration) to remove or destroy organic contaminants. Available technologies include: Pyrolysis Wet air oxidation Thermal desorption Supercritical water oxidation is an emerging thermal treatment technology that has received considerable bench-scale and pilot-scale testing. Pyrolysis is a thermal process that transforms hazardous organic materials in an oxygenpoor atmosphere into gaseous components and a solid residue (coke) containing fixed carbon and ash. Upon cooling, the gaseous components condense, leaving an oil/tar residue. Pyrolysis is applicable to a wide range of organic wastes in soil and sludge, including polychlorinated biphenyls (PCBs) and dioxins/furans. Wet air oxidation uses elevated temperature and pressure to oxidize dissolved or finely divided organics. Its main application is to treat waste streams that are too dilute (less than 5 percent organics) to treat economically by incineration and that have contaminant levels above those considered ideal for biological treatment. This technology can also be used to treat wastewaters with pesticides, phenolics, organic sulfur, and cyanide. It is not suitable for halogenated aromatic organics or for treating large volumes of waste. Thermal desorption is used to physically separate volatile and some semivolatile contaminants from soil, sediments, sludges, and filter cakes by heating them at temperatures high enough to volatilize the organic contaminants. Desorbed organics in the gas stream are then 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 as low temperature (200 to 600 o F/93 to 215 o C) or high temperature (600 to 1,000 o F/315 to 538 o C) systems. The main difference between the two is that low-temperature systems target volatile organic compounds, whereas high-temperature systems target semivolatile organics. Supercritical water oxidation (SCWO) uses oxidants (air, oxygen, or hydrogen peroxide) to decompose organics in an aqueous waste stream that is above the critical point of water (364 o C/221 atmospheres). Supplemental fuel may be required at startup and for dilute wastes, 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. 6-30

31 Table C-5 in Worksheet C (Volume II) provides additional information about the above thermal treatment methods and a number of innovative thermal treatment technologies (electric reactor, molten glass, molten salt, and radiofrequency thermal heating), and also identifies references where more detailed information can be obtained. 6.4 SELECTION OF TREATMENT PROCESSES Major considerations in the selection of hazardous waste treatment processes at HWTCs include: Waste medium. Wastewater, organic liquids, sludge, and solids containing the same type of contaminants may often require different treatment processes (see Section 6.4.1). Contaminant type. The physical and chemical properties of contaminants in a waste affect the suitability of available treatment processes. For example, precipitation is a chemical treatment method that applies mainly to inorganics such as metals and cyanides. Air stripping and thermal treatment methods, on the other hand, are more suited for treatment of wastewaters and solids contaminated with volatile and semivolatile organics. Whether organic contaminated wastes are halogenated or nonhalogenated also may influence the suitability of a particular treatment option. Mixed organic and inorganic wastes are often the most difficult to treat, frequently requiring a series of different treatment steps. Contaminant-specific waste treatment options are covered further for corrosive wastes in Section 6.4.2, solvent wastes in Section 6.4.3, and other contaminants in Section Contaminant concentration. The successful operation of some treatment processes depend on the concentration of contaminants in the waste stream. Section provides information on the range of applicability of treatment techniques as a function of the organic concentration in liquid waste streams. Waste volume. Some treatment methods, such as incineration, require large volumes of waste to be cost-effective. Other methods, such as wet air oxidation (see Section 6.3.3), are better suited for small volumes of waste. Process efficiency requires a reasonably good match between the volume of waste at which a process works efficiently and the volume of waste to be treated. Waste variability. Continuous treatment processes generally operate more efficiently if 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 for continuous wastewater treatment processes. Batch treatment processes are well suited for wastes that vary in chemical composition. 6-31

32 Availability. The importance of reliable performance of treatment technologies generally restricts technology choices to those that are commercially developed. Testing and development of innovative and emerging technologies may be possible but probably not as a central feature of any treatment train that handles large volumes of waste. Cost. Cost will probably be the major factor for choosing between two or more treatment options that satisfy all the other criteria. As discussed in Section 5.4.2, evaluating treatment costs requires considering the overall cost and the relative significance of capital, and operation and maintenance costs. Residuals. Most treatment processes produce residuals that may require further treatment (e.g., off gases) or disposal (e.g., ash, sludges). The type and volume of residuals generated should be considered when selecting a treatment technology. The types of wastes received at an HWTC depend on the specific industrial processes used by the industries sending wastes to the facility. Numerous treatment options could be suitable for a particular batch or stream of waste. Worksheet A in Volume II describes a procedure for identifying waste characteristics and treatment options for specific industrial categories. The screening procedure described in Worksheet A is applicable for selecting potential treatment options for CETPs and HWTCs. Criteria for selecting onsite pretreatment options for SMSEs prior to discharging wastes is more similar to the criteria for CETPs discussed in Section An HWTC generally mixes similar wastes from many individual sources. An important step in the process of selecting treatment options for an HWTC is to determine the major types of waste streams that will be handled by the facility through combining these wastes Media-Specific Options Table 6-4 identifies potential treatment alternatives for various types of liquid and solid hazardous 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 waste stream. Thus, options identified for recycling of concentrated inorganic liquids differ from those for treating dilute inorganic liquids, except for electrodialysis. Figure 6-12 identifies the approximate ranges of applicability of commercially available treatment techniques (solid bar) as a function of organic concentration in liquid waste streams. The dashed lines indicate potential extensions of available technologies and the emerging technology of supercritical water oxidation. 6-32

33 The pretreatment options for various waste media identified in Table 6-4 are mostly physical methods, many of which are discussed in Sections 5.2.1, 5.2.2, and Major chemical pretreatment methods include neutralization, cyanide destruction, and chromium reduction. Treatment and disposal methods identified in Table 6-4 are discussed in Sections 6.2 and 6.3. Table 6-5 is a matrix showing the potential applicability of 17 treatment technologies for 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 onsite remediation of contaminated sites, all of the treatment categories could be used in HWTCs Corrosive Wastes Neutralization of corrosive wastes was discussed in Section Table 6-6 provides the following information on eight treatment technologies for recovery and reuse of corrosive wastes: Applicable waste streams Stage of development Performance Residuals generated Cost Because of the problems involved in transporting corrosive wastes to an HWTC, these technologies would generally be best applied at the industrial facility where they are generated. Unfortunately, financial constraints would probably limit the use of such technologies by SMSEs. 6-33

34 Table 6-4. Hazardous waste management alternatives (Wilk et al., 1988) Waste Management Objective Waste minimization Applicable Waste Type(s) Potential Waste Management/Treatment Alternative Source reduction All Raw material substitution Product reformulation Process redesign Waste segregation Recycling Concentrated inorganic liquids (e.g. plating, etching solutions) Dilute inorganic liquids (e.g. plating rinses) Concentrated organic liquids (e.g. solvents with acid/alkali) Crystallization Ion exchange Evaporation/distillation Electrodialysis Solvent extraction Thermal decomposition Ion exchange Electrodialysis Reverse osmosis Donna dialysis/coupled transport Neutralization followed by recovery such as distillation, evaporation, stream stripping, or use as a fuel. Waste exchange Concentrated liquids Recycling Reuse in process with lower raw material specifications Dilute organic liquids Mutual neutralization Mutual neutralization Pretreatment Liquid with solids Screening Distillation Centrifugation Liquid-two-phase Decanting Extraction Liquid or sludge with Cyanide destruction cyanide through chlorination Liquid or sludge with Chromium reduction hexavalent chromium Sludge Vacuum filtration Other dewatering Sedimentation Flotation Equalization Flotation Distillation Filter press Filtration Setting Centrifugation Equalization Centrifugation Bulky solids Shredders Hammermills Crushers Neutralization Acidic waste Limestone Lime Caustic soda Alkaline waste Sulfuric acid Hydrochloric acid Carbonic acid (CO2) All Mutual neutralization Treatment and disposal Metal-containing liquid Precipitation and clarification Trace organic-containing Adsorption liquids Dilute organic-containing Biological treatment Chemical oxidation Ozonation liquid Air stripping Incineration Concentrated organic liquid Distillation Extraction Steam stripping Supercritical fluids Evaporation Wet air oxidation Incineration Use as a fuel Inorganic sludges and Solidification Encapsulation Landfill solids Organic sludges and solids Incineration Wet air oxidation 6-34

35 Figure Approximate ranges of applicability of treatment techniques as a function of organic concentration in liquid waste streams (Breton et al., 1987) 6-35

36 Table 6-5. Onsite waste treatment technology matrix (U.S. EPA, 1991) 6-36

37 Table 6-6. Summary of recovery/reuse technologies for corrosive wastes (Wilk et al., 1988) Process Evaporation/ Distillation Crystallization Ion exchange Electrodialysis Applicable Waste Streams Metal plating rinses; acid pickling liquors H2SO4 pickling liquors; HNO3/HF; pickling liquors; caustic aluminum etch solutions Plating rinses: acid pickling baths; aluminum etching solutions; H2SO4; anodizing solutions; rackstripping solutions (HF/HNO3) Recovery of chromic/sulfuric acid etching solutions Stage of Development Well-established for treating plating rinses 20 to 25 systems currently in operation (fewer applications for caustic recovery) Several RFIE units in operation for treatment of corrosives Units for direct treatment of acid bath only available from ECO-TEC Ltd. Units currently being sold, but limited area of application. 5 in operation Performance Plating solution recovered for reuse in plating bath. Rinse water can be reused 97-98% recovery for H2SO4 (80-95% metal removal) 99% HNO3 and 50% HF recovered 80% recovery NaOH Cocurrent systems not technically feasible for direct treatment of corrosives; can be used in conjunction with neutralization technologies to lower overall costs RFIE units show good results. Conventional RFIE performs best with dilute solutions. APU performs best with high metal concentration (30 to 100 g/l) 85% recovery of etching solution. 45% copper removal; 30% zinc removal Residuals Generated Impurities will be concentrated, therefore, crystallization/ filtration system may be required Ferrous sulfate heptahydeate crystals (can be traded or sold) Metal fluoride crystals (can recover additional HF by thermal decomposition) Aluminum hydroxide crystals (can be traded or sold) Cocurrent process generates spent regenerant, which is also corrosive Recovered metals which can be reused, treated, disposed, or marketed Metals which can be treated, disposed, or regenerated for reuse Cost Can be costeffective for recovering corrosive plating solutions from rinse waters Cost-effective if treating large quantities of waste RFIE and APU are cost-effective Cost-effective for specific applications (chromic/sulfate acid etchants) Recovery of plating rinses (particularly chromic acid rinse Several in operation Works best when copper concentrations are in the 2 to 4 oz/gal Chromic acid can be returned to plating bath; rinse water can be reused Low capital investment; cost effective for specific 6-37

38 Process Applicable Waste Stage of Residuals Performance Streams Development Generated Cost waster) usage application (chromic acid rinses) Recovery of HNO3/HF pickling liquors Marketed, none in operation to date 3 M HF/HNO3 recorded 2M KOH Soln which can be recycled back to the pretreatment step for this ED application Cost-effective for large quantity generator Reverse osmosis Plating rinses Corrosive waste membranes marketed by four companies. RD module systems applicable to corrosives available from two companies Donna dialysis/coupled transport Solvent extraction Thermal decomposition Plating rinses; potentially applicable to acid baths HNO3/HF pickling liquors Acid wastes (1) RFIEL Reverse flow ion exchange Solvent Wastes Donnan analysis only lab-scale tested Coupled transport lab and field tested. Coupled transport system is currently being marketed Commercial-scale systems installed for development purposes in Europe and Japan. No commercialscale installations in U.S. Well-established for recovering spent pickle liquors generated by steel industry. Pilot-scale stage for organic wastes 90% conversion achieved with cyanide plating rinses Data not available for Donna analysis (further testing required) Coupled transport has demonstrated 99% recovery for chromate from plating rinses. Other plating rinses should be applicable, but not fully tested 95% recovery of HNO3; 70% recovery of HF 99% regeneration efficiency for pickling liquors Recovered plating solution returned to plating bath (after being concentrated by an evaporator). Rinsewater reused Data not available for Donnan analysis For chromate plating rinse applications, sodium chromate is generated: can be used elsewhere in plant or subjected to ion exchange to recover chromic acid for recycle to plating solution Metal sludge (95% iron can be recovered by thermal decomposition) 98-99% purity iron oxide which can be reused, traded, or marketed Cost-effective for limited applications. Development of a more chemically resistant membrane would make it very costeffective for a wider area of application No cost data available for Donna analysis Average capital cost for plating shop is $20,000. Can be costeffective for specific applications Not available Expensive capital investment. Only cost-effective for large quantity waste acid generators 6-38

39 Many industries generate solvent wastes through cleaning equipment. Solvents are used extensively in the metals treatment/finishing industries and the electronics industries. Fresh solvents are organic liquids and can be either halogenated (such as tetrachloroethylene) or nonhalogenated (such as methanol and toluene). Solvent wastes can be either, single- or multi component 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 specific recovery/treatment options: (1) applicable waste streams, (2) stage of development of the process, (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 are reused. If a solvent is only contaminated with solids, reclamation can be accomplished by filtration or other physical component separation methods. Sometimes, contaminated solvents can 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 and high-temperature industrial boilers. Special care is required when using halogenated solvents as a fuel to ensure that concentrations of chlorine in the fuel blend do not exceed levels that will corrode the system. The main treatment option for concentrated waste solvents for which reclamation is not feasible is thermal treatment. Table 6-8 identifies four incineration options and seven other thermal technologies; of these seven technologies, only pyrolysis (see Section 6.3.3) is commercially available. Most of the physical and chemical treatment options identified in Table 6-9 are for aqueous waste streams contaminated by relatively low concentrations of solvents. Figure 6-12 can be used as a guide for identification of potential technologies based on the concentration of solvents and other organics in wastewater. 6-39

40 Table 6-7. Solvent waste management alternatives to land disposal (Breton et al., 1987) Waste management objective Waste Minimization All Applicable waste type(s) Raw material substitution Product reformulation Potential waste management alternative Process redesign Waste segregation Recycling All Reclamation Reuse (e.g., as a fuel or process solvent) Pretreatment Liquid with solids Screening Sedimentation Filtration Distillation Floatation Settling Centrifugation Liquid Two Phase Decanting Floatation Centrifugation Extraction Distillation Sludge Vacuum filtration Filter press Centrifugation Other dewatering Bulky solids Shredders Hammermills Crushers Low Btu/High Viscosity Blending Treatment Physical Liquid Distillation Steam stripping Chemical Liquid Wet air oxidation Other chemical oxidations Evaporation Air stripping Fractionation Carbon adsorption Supercritical water oxidation Chlorinolysis Biological Liquid Activated sludge Aerated lagoon Trickling filter Incineration All Liquid inyection Rotary kiln Fluidizedbed Other thermal Post-treatment All Pyrolysis processes Plasma systems Molten glass Electric reactor Circulating fluid bed Molten salt Organic liquid Decanting Dehydrating Fractionatio n Solid/sludge Solidification Encapsulation Thermal destruction Aqueous Liquid Carbon adsorption Resin adsorption Air stripping Ozonation Other oxidations Extraction Resin adsorption Ozonation Starved air Thermal destruction Biological Treatment 6-40

41 Table 6-8. Summary of solvent treatment processes (Breton et al., 1987) Incineration Process Liquid injection incineration Rotary kiln incineration Fluidized bed incineration Fixed/multiple hearths Use as a Fuel Industrial kilns High temperature industrial boilers Òther Thermal Technologies Applicable waste streams All pumpable liquids provided wastes can be blended to Btu level of 6500 Btu/lb. Some solids removal may be necessary to avoid plugging nozzles All wastes provided Btu level is maintained Stage of development Performance Residuals generated Estimated that over 219 units are in use. Most widely used incineration technology Over 40 units in service; most versatile for waste destruction Liquids or nonbulky solids Nine units reportedly in operation-recirculating bed units under development Can handle a wide variety of wastes Generally all wastes, but Btu level, chlorine content, and other impurity content may require blending to control charge characteristics and product quality All pumpable fluids, but should blend halogenated organics. Solids removal particularly important to ensure stable burner operation Approximately 70 units in use. Old technology for municipal waste combustion Only a few units now burning hazardous waste Several units in use Circulating bed combustor Liquids or nonbulky solids Only one U.S. manufacturer. No units treating hazardous waste Molten glass incineration Molten salt destruction Furnace pyrolysis units Almost all wastes, provided moisture and metal impurity levels are within limitations Not suitable for high (>20%) ash content wastes Most designs suitable for all wastes Technology developed for glass manufacturing. Not available yet as a hazardous waste unit Technology under development since 1969, but further development on hold One pyrolysis unit RCRA permitted. Certain designs available commercially Excellent destruction efficiency (>99.99%). Blending can avoid problems associated with residuals, e.g., HCl Excellent destruction efficiency (>99.99%) Excellent destruction efficiency (>99.99%) Performance may be marginal for hazardous wastes, particularly halogenated wastes Usually excellent destruction efficiency (>99.99%) because of long residence times and high temperatures Most units tested have demonstrated high DRE (>99.99%) Manufacturer reports high efficiencies (>99.99%) No performance data available, but DREs should be high (>99.99%) Very high destruction efficiencies for organics (six nines for PCBs) Very high destruction efficiencies possible (>99.99%). Possibility of PIC formation TSP, possibly some PICs and HCl if halogenated organics are fired. Only minor ash if solids removed in pretreatment processes Requires APCDs. Residuals should be acceptable if charged properly As above As above Requires APCDs. Residuals should be acceptable Waste must be blended to meet emission standards for TSP and HCl unless boilers equipped with APCDs Bed material additives can reduce HCl emissions. Residuals should be acceptable Will need APC device for HCl and possibly PICs; solid retained (encapsulated) in molten glass Needs some APC devices to collect material not retained in salt. Ash disposal may be a problem TSP emissions lowers than those from conventional will need APC devices for HCl 6-41

42 Process Plasma arc pyrolysis Fluid wall advanced electric reactor Applicable waste streams Present design suitable only for liquids Suitable for all wastes if solids pretreated to ensure free flow In situ vitrification Technique for treating contaminated soils, could possibly be extended to slurries. Also use as solidification process Physical Treatment Methods Distillation This is a process used to recover and separate solvents. Fractional distillation will required solids removal to avoid plugging columns Evaporation Agitated thin film units can tolerate higher levels of solids and higher viscosities than other types of stills Stream Stripping Air Stripping Liquid-liquid extraction Carbon adsorption Resin Adsorption A simple distillation process to remove volatile organics from aqueous solutions. Preferred for low concentrations and solvents with low solubilities Generally used to treat low concentration aqueous streams Generally suitable only for liquids of low solid content Suitable for low solid, low concentration aqueous waste streams Suitable for low solid waste streams. Consider for recovery of valuable solvent Stage of development Performance Residuals generated Commercial design appears imminent, with future modifications planned for treatment of sludges or solids Ready for commercial development. Test unit permitted under RCRA Not commercial, further work planned Technology well developed and equipment available from many suppliers; widely practiced technology Technology is well developed and equipment is available from several suppliers; widely practiced technology Technology well developed and available. Technology well developed and available Technology well developed for industrial processing Technology well developed; used as polishing treatment Technology well developed in industry for special resin/solvent combinations. Applicability to waste streams not demonstrated Efficiencies exceeded six nines in tests with solvents Efficiencies have exceeded six nines in tests with solvents No date available, but DREs of over six nines reported Separation depends upon reflux (99 + percent achievable). This is a recovery process This is a solvent recovery process. Typical recovery of 60 to 70 percent Not generally considered a final treatment, but can achieve low residual solvent levels Not generally considered a final treatment, but may be effective for highly volatile wastes Can achieve high efficiency separations for certain solvents/waste combinations Can achieve low levels of residual solvent in effluent. Can achieve low levels of residual solvent in effluent. Requires APC devices for HCP and TSP, needs flare for H2 and CO destruction Requires APC devices for HCP and TSP, needs flare for H2 and CO destruction Off gas system needed to control emissions to air. Ash contained in vitrified soil Bottoms will usually contain levels of solvent in excess of 1,000 ppm; condensate may require further treatment Bottoms will contain appreciable solvent. Generally suitable for incineration Aqueous treated stream will probably require polishing. Further concentration of overhead steam generally required Air emissions may require treatment Solvent solubility in aqueous phase should be monitored Adsorbate must be processed during regeneration. Spent carbon and wastewater may also need treatment. Adsorbate must be processed during regeneration. 6-42

43 Process Chemical Treatment Processes Wet air oxidation Supercritical water oxidation Ozonation Other chemical oxidation processes Chlorinolysis Applicable waste streams Suitable for aqueous liquids, also possible for slurries. Solvent concentrations up to 15% For liquids and slurries containing optimal concentrations of about 10% solvent Oxidation with ozone (possibly assisted by [UV] suitable for low solid, dilute aqueous solutions Oxidizing agents may be highly reactive for specific constituents in aqueous solution Suitable for any liquid chlorinated wastes Stage of development Performance Residuals generated High temperature/pressure technology, widely used as pretreatment for municipal sludges, only one manufacturer Supercritical conditions may impose demands on system reliability. Commercially available in 1987 Now used as a polishing step for wastewaters Oxidation technology well developed for cyanides and other species (phenols), not yet established for general utility Process produces a product (e.g., carbon tetrachloride). Not likely to be available Pretreatment for biological treatment. Some compounds resist oxidation Supercritical conditions achieve high destruction efficiencies (>99.99%) for all constituents Not likely to achieve residual solvent levels in the low ppm range for most wastes Not likely to achieve residual solvent levels in the low ppm range for most wastes Not available Dechlorination Dry soils and solids Not fully developed Destruction efficiency of over 99% reported for dioxin Biological Treatment Methods Aerobic technology suitable for dilute wastes although some constituents will be resistant Conventional treatments have been used for years May be used as final treatment for specific wastes, may be pretreatment for resistant species Some residues likely which need further treatment Residuals not likely to be a problem. Halogens can be neutralized in process Residual contamination likely; will require additional processing of off gases Residual contamination likely; will require additional processing Air and wastewater emissions were estimated as not significant Residual contamination seems likely Residual contamination likely; will usually require additional processing 6-43

44 Figure 6-9. Industrial wastewater process applicability matrix (McArdle et al., 1987) 6-44

45 6.4.4 Other Contaminant-Specific Options Table 6-9 is a matrix identifying the applicability of 19 physical, chemical, and biological technologies for treating the following wastewater characteristics: (1) suspended solids, (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 organic contaminants and eight types of inorganic contaminants in soils and sludge. This matrix was developed for screening technologies at contaminated sites, but all the ex-situ technologies (11 of 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 treating contaminated solids for 11 major types of hazardous contaminants. These 11 groups were developed for the EPA Superfund program to facilitate tests for the treatability of materials at uncontrolled hazardous waste sites. The nine categories for organic contaminants represent a more detailed breakdown than shown in other tables in this guide. A more complete list of applicable treatment technologies for these treatability groups as they relate to pesticide chemical 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 for more than 50 treatment technologies in relation to the following five major groups of contaminants: (1) volatile organic compounds (VOCs), (2) semivolatile organic compounds (SVOCs), (3) fuels (petroleum hydrocarbons), (4) inorganics, and (5) explosives. Information on 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, each contains significant information that is not covered elsewhere. 6-45

46 Table Treatment technology screening guide contaminants in soil and sludges (U.S. EPA, 1988) 6-46

47 Table Predicted treatment effectiveness for contaminated solids (Offut and Knapp, 1990) 6-47

48 6.4.5 Industry-Specific Options Table 6-12 is a matrix that identifies the applicability of 27 candidate treatment and control technologies for 34 industries. This table provides a somewhat quicker way to prepare a preliminary list of candidate treatment technologies for a particular industry than the procedure described in Worksheet A (Volume II), but should not be used as a substitute for a more detailed screening process Most Commonly Used Treatment Processes Typically, HWTCs need to have the ability to treat the full range of hazardous wastes that are produced by industrial processes, unless industrial production in a region is so specialized that certain categories of hazardous waste are not generated. Compared with CETPs, treatment processes at HWTCs need to handle concentrated liquid, sludge, and solid wastes, which mainly require physical, chemical, and thermal treatment, and also use of S/S technologies. If an HWTC also receives large amounts of industrial wastewater, then biological treatment processes are also likely to be significant, and the HWTC would also function as a CETP. Table 6-13 summarizes the results of a survey that EPA conducted in 1994 of 85 centralized waste treatment facilities in the United States to identify the types of treatment technologies in actual use. All of these facilities mainly treat liquid wastes received from industries. 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) with clarification/flocculation (35 facilities) and gravity separation (18 facilities) also commonly used. 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, and multiple applications at a single facility are typical. Cyanide destruction and chromium reduction were performed at a little less than half the facilities. Activated sludge was the most commonly used biological treatment process and was usually used only at facilities with onsite manufacturing operations that produced a relatively constant 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 (6 facilities). Chapter 8 includes a case study of an existing HWTC that outlines the mix of technologies likely to be required at HWTCs. 6-48

49 Table Candidate treatment and control technology for 34 industries (Saltzberg and Cushnie, 1985) 6-49

50 Table Frequency of use of treatment technologies at industrial centralized waste treatment facilities Technology Number (Out of 85) Physical Pretreatment Gravity separation 18 Clarification/flocculation 35 Dissolved air flotation 5 Emulsion breaking Equalization Air stripping 1 Granular media filtration Most oils subcategory facilities 81 (36 unstirred, 45 stirred or aerated) Physical (Phase Separation) Carbon adsorption 11 Reverse osmosis 3 Ultrafiltration 3 Liquid carbon dioxide extraction 1 Precipitation 10 sand filters, 9 multimedia filters Chemical Cyanide destruction 30 Chromium reduction Electrolytic recovery 3 Ion exchange 1 Sequencing batch reactors 1 Biotowers individual applications (more than one per facility) 38 (4 sulfur dioxide, 21 sodium bisulfite, 2 sodium metabisulfite, 11 other reagents) Biological Activated sludge 12 Sludge Dewatering Plate and frame pressure filtration 34 Belt pressure filtration 6 Vacuum filtration 10 Source: EPA (1995) WTI Survey 6-50

51 6.5 RESIDUALS MANAGEMENT AND DISPOSAL Several types of residuals (i.e., waste products) are generated from the treatment processes employed at CETPs and HWTCs including: sludges, solids, incinerator ash, air emissions, and concentrated liquid waste streams. Table 6-14 provides an overview of the major types of residuals associated with wastewater treatment processes. Table 6-1 identifies residuals associated with specific neutralization processes, and Table 6-8 identifies residuals associated with solvent treatment processes. Treatment and disposal options for the major types of residuals produced by CETPs and HWTCs are discussed below. Table Residual generated by various wastewater treatment processes (McArdle et al., 1987) Residuals Treatment process Sludges Air Concentrated Spent emissions liquid waste stream carbon Pretreatment operations Sedimentation X Granular media filtration X Oil/water separation X Physical/chemical treatment operations Neutralization X Precipitation flocculation/sedimentation X Oxidation/reduction X Carbon adsorption X X Air stripping X Steam stripping X Reverse osmosis X Ultrafiltration X Ion exchange X Wet-air oxidation X Biological treatment operations Activated sludge X X Sequencing batch reactor X X Powdered activated carbon treatment (PACT) X X Rotating biological contactor X X Trickling filter X Overview of Sludge Treatment Options The treatment processes described in Sections 5 and 6 concentrate solids from liquid wastes into sludges that must be treated (e.g., stabilized and dewatered) before being finally disposed. The following discussion addresses sludge treatment processes relevant to both CETPs and HWTCs. 6-51

52 Figure 6-13 shows a general schematic for handling sludge at municipal wastewater treatment plants. Most processes shown in this figure are intended to remove water, reduce volume, or reduce the mass of solids in the initial sludge, which is typically only a few percent solids 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 their significance for sludge use and disposal options. If CETPs are likely to receive human as well as industrial wastewaters, reduction of pathogens should be a significant design consideration, especially if land application for agricultural purposes is desired. In HWTCs, pathogen reduction generally is not a concern unless the facility processes medical wastes. Sludge thickening methods typically increase sludge solids from a few percent solids to as much as 10 percent solids. Raw primary sludges that have not received biological treatment also require stabilization to control odors and pathogens. WPCF/WEF (1980) provides more detailed information on sludge thickening. Sludge stabilization involves digestion or oxidation of sludge to reduce the mass of solids 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 raise the ph above 12, is a method for pathogen reduction. Major references for additional information on sludge stabilization include EPA (1977) and WPCF/WEF (1985, 1987b). THICKENING PRIMARY SLUDGE * Source thickening in the primary clarifier * Gravity TREATMENT * Compost * Incineration * Drying THICKENING SECONDARY SLUDGE * Dissolved air flotation * Solid-bowl centrifuge * Belt or drum thickener STABILIZATION CONDITIONING DEWATERING * 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 ULTIMATE DISPOSAL * Land spread * Landfill * Land injection Figure General schematic for solids handling showing most commonly used methods of treatment and disposal (U.S. EPA, 1987) 6-52

53 Table Effects of pretreatment and sludge treatment processes on sludge and sludge use/disposal options Treatment process and definition Effect on sludge Effect on use/disposal options Pretreatment: Reduction in contaminant levels in industrial wastewater discharge Thickening: Low-force separation of water and solids by gravity or flotation Digestion (Aerobic and Anaerobic): Biological stabilization of sludge through conversion of some of the organic matter to water, carbon dioxide, and methane. Lime Stabilization: Stabilization of sludge through the addition of lime Conditioning: Alteration of sludge properties to facilitate the separation of water from sludge. Conditioning can be performed in many ways, e.g., adding inorganic chemicals such as lime and ferric chloride; adding organic chemicals such as polymers; or briefly raising sludge temperature and pressure. Thermal conditioning also causes disinfection Dewatering: High-force separation of water and solids Composting: Aerobic process involving the biological stabilization of sludge in a windrow, in an aerated static pile, or in a vessel Heat drying: Application of heat to kill pathogens and eliminate most of the water content Reduces levels of heavy metals and organics in industrial wastewater discharge, thereby lowering the concentration of these constituents in the sludge Increases solids concentration of sludge by removing water, thereby lowering sludge volume Reduces the volatile and biodegradable organic content of sludge by converting it to soluble material and gas. Reduces pathogen levels and controls putrescibility Raises sludge ph. Temporarily decreases biological activity. Reduces pathogen levels and controls putrescibility. Increases the dry solids mass of the sludge Improves sludge dewatering characteristics. Conditioning may increase the mass of dry solids to be handled and disposed of without increasing the organic content of the sludge Increases solids concentration of sludge by removing much of the entrained water, thereby lowering sludge volume. Some nitrogen and other soluble materials are removed with the water Lowers biological activity. Can destroy all pathogens. Degrades sludge to a humus-like material. Increases sludge mass due to addition of bulking agent Disinfects sludge. Slightly lowers potential for odors and biological activity Increases the viability of land application, distribution and marketing, and ocean disposal. Reduces need for pollution control devices during incineration, and prevents problems with incinerator ash disposal Lowers sludge transportation costs for all options Reduces sludge quantity. Preferred stabilization method prior to landfilling and land application. Reduces heat value for incineration, but anaerobic digestion produces recoverable methane Increases the amount of auxiliary fuel required in incineration if the amount of inert material in the sludge is increased Increases the amount of auxiliary fuel required in incineration if the amount of inert material in the sludge is increased Reduces fuel costs for incineration. Reduces land requirements and bulking soil requirements for landfilling. Lowers sludge transportation costs for all options. Dewatering may be undesirable during land application in regions where the water itself is a valuable agricultural resource. Reduction of nitrogen levels may or may not be an advantage Useful prior to land application and distribution and marketing. Often not appropriate for other use or disposal options due to cost Generally used only prior to distribution and marketing 6-53

54 Sludge conditioning involves physical (heat, freeze-thaw) or chemical treatment to reduce moisture content and modify sludge characteristics to increase the rate of subsequent dewatering processes. WPCF/WEF (1988) provides more detailed information on sludge conditioning. Sludge dewatering increases solids content to the point where sludge can be more or less handled as a solid for use or disposal (see Section ). The solids content of dewatered sludges varies greatly depending on the characteristics of the sludge and the dewatering method used but typically ranges from 20 to 50 percent. Table 6-16 outlines operational selection criteria for sludge dewatering processes based on plant size. Major references for additional information on municipal sludge dewatering include EPA (1987) and WPCF/WEF (1983, 1987a). The next section provides additional information on commonly used dewatering methods at hazardous waste treatment facilities Sludge Dewatering at HWTCs Hazardous waste treatment processes such as sedimentation, neutralization, precipitation, and oxidation/reduction produce sludges that often must be dewatered before further treatment or disposal. This section describes three dewatering systems that are commonly used at industrial waste treatment facilities in the United States (see Table 6-13): Plate and frame pressure filtration Vacuum filtration Belt pressure filtration Plate and Frame Pressure Filtration A plate and frame filter press consists of a number of filter plates or trays connected to a frame and pressed together between a fixed end and a moving end (Figure 6-14a). Filter cloth is mounted on the face of each plate. The sludge is pumped into the unit under pressure while the plates are pressed together. The solids are retained in the cavities of the filter press and begin to attach to the filter cloth until a cake is formed. The water or filtrate passes through the filter cloth and is discharged from a drainage port in the bottom of the press. The sludge influent is pumped into the system until the cavities are filled. Pressure is applied to the plates until the flow of filtrate stops. At the end of the cycle, the pressure is released and the plates are separated. The filter cake drops into a hopper below the press. The filter cake can then be disposed of in a landfill. The filter cloth is washed before the next cycle begins. The key advantage of plate and frame pressure filtration is that it can produce a drier filter cake than other methods of sludge dewatering can, and the fact that it is a batch process is an advantage when sludges from different waste streams need to be handled separately. Because of the batch operation, however, a plate and frame filter press is more labor intensive. 6-54

55 Table Operational Selection Criteria for Sludge Dewatering Processes (U.S. EPA, 1987) Plant Size Key Criteria Small Minimum Mechanical Complexity < 0.08 m 3 /s Local Repairs and Parts (< 2 mgd) Minimum operator attendance Reliable without skilled service Unaffected by climatic factors Large excess capacity Handleable cake Medium Low operator attendance m 3 /s Local repair and parts (2 10 mgd) Transportable cake without nuisance Mechanical reliability Competitive O&M costs Drier cake Large Lowest O&M costs/ton dry solids > 0.44 m 3 /s Lowest capital costs/ton dry solids Drier cake High output/unit Mechanical reliability Transportable 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 medium can be made of cloth, coil springs, or wire-mesh fabric. A typical application is a rotary vacuum belt filter (see Figure 6-14b). A continuous belt of filter fabric is wound around a horizontal 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. The water from the sludge passes through the filter and into the drum, where it exits via a discharge port. As the fabric leaves the drum and passes over the roller, the vacuum is released. The filter cake drops off the belt as it turns around the roller. The filter cake can then be disposed of. Because vacuum filtration systems are relatively expensive to operate, they are 6-55

56 usually preceded by a thickening step that reduces the volume of sludge to be dewatered. It is a continuous process and therefore requires less operator attention. Belt Pressure Filtration Belt pressure filtration uses gravity followed by mechanical compression and shear force to produce a sludge filter cake. Belt filter presses are continuous systems that are commonly used to dewater biological treatment sludge. Most belt filter installations are preceded by a flocculation step, where polymer is added to create a sludge that has the strength to withstand being compressed between the belts without being squeezed out. A typical belt filter press is illustrated in Figure 6-14c. During the press operation, the sludge stream is fed onto the first of two moving cloth filter belts. The sludge is gravity-thickened as the water drains through the belt. As the belt holding the sludge advances, it approaches a second moving belt. As the first and second belts move closer together, the sludge is compressed between them. The pressure is increased as the two belts travel together over and under a series of rollers. The turning of the belts around the rollers shears the cake, which furthers the dewatering 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 be disposed of. The advantages of a belt filtration system are its lower labor requirements and lower power consumption. One disadvantage is that belt filter presses produce a poorer quality filtrate and require a relatively large volume of belt wash water Final Use and Disposal Options for Sludge Residual sludges that remain after any recyclable constituents have been removed are dewatered as much as possible before being ultimately disposed. Further stabilization, such as adding a bulking agent (see Section 6.3.1), may be required to create a sludge consistency that is easier to handle for transport to the location of ultimate disposal. Final disposal options for sludge include beneficial uses such as (1) land application and (2) distribution and marketing of composted sludge, or non-beneficial uses such as (1) landfilling, (2) incineration (see Section 6.3.2), and (3) ocean disposal. The main difference between CETPs and HWTCs is that if SMSE pretreatment programs are successful for a CETP, it should be possible to put sludge to a beneficial use by land application for agricultural production, forestry, or land reclamation, whereas sludges generated at HWTCs generally are unsuitable for beneficial uses. The general approach to residuals 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 of sludge constituents, major sludge characteristics, and costs for the five use/disposal options. Solids created using solidification processes, and incinerator ash, also require placement in a final disposal area. The main disposal options for such residuals are landfilling and ocean disposal. 6-56

57 Major general references for additional information on sludge treatment, use, and disposal include HMCRI ( ), EPA (1979, 1984b, 1985b), and WPCF/WEF (1989). Figure Selected sludge dewatering systems: (a) plate and press pressure filtration, (b) vacuum filtration, and (c) belt pressure filtration (U.S. EPA, 1995) Land Application Where land is available, and sludges are uncontaminated or contaminants exist in concentrations 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 for agricultural crops, pasture, or forestry. Sludge is especially valuable for reclamation of severely disturbed or degraded lands. Some capital costs for sludge treatment are showed in Section Table 6-16 presents pollutant concentration limits for land application of sludges. 6-57

58 Table Pollution limit for land application of sludge Pollutant Maximum concentration limit (**) (mg/kg) Description (*) Pollutant accumulation rate (kg/ha) Permissible concentration limit (***) (mg/kg) Annual pollutant accumulation rate (kg/ha/year) Arsenic Cadmium Chrome Copper Lead Mercury Molibdene Nickel Selenium Zinc (*) All limits in a dry basis; (**) absolute values; (***) monthly averages. USEPA. 40 CFR Part 503, Standards for the use or disposal of sewage sludge Major references for additional information on land application of sewage sludge include HMCRI ( ), EPA (1983, 1994), and WCPF/WEF (1989) 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 additional information on sludge composting include Benedict et al. (1987), HMCRI ( ), and EPA (1985a). 6-58

59 Figure Importance factors affecting sludge use/disposal options: (a) sludge constituents, (b) sludge characteristics, and (c) cost factors 6-59

60 Landfilling Modern landfills are coming under increased regulatory scrutiny, and as a result, will be more protective of the environment in the future. A variety of specific technologies are associated with a state-of-the-art landfill including: liner systems, leachate collection systems, leachate treatment, landfill gas control and recovery, improved closure techniques, provisions for post-closure care, and monitoring systems, all of which are discussed below. If landfills are properly planned and operated, a completed landfill site can ultimately be used by the owner for recreational or other purposes, such as open space. Batstone et al. (1989) address technical requirements for safe disposal of hazardous waste in landfills. Table B-1, in Appendix 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 with location alternatives. Because of environmental concerns, careful scientific and engineering analysis must take place during potential site evaluations. Surface and subsurface geology, hydrogeology, and the environmental nature of surrounding areas must be evaluated for potential impacts. Ground water resources must be protected, and the integrity of soils must be preserved. A substantial hydrogeological investigation and prediction of leachate quantities are usually performed early in the planning stages. When siting a new landfill, decision makers also will have to consider logistical factors such as access roads, travel distance, and travel time. Hazardous Waste Landfill Components Cells are the basic building blocks of landfills. During daily operations, waste is confined to defined areas where it is spread and compacted throughout the day. At the end of the day (or several times a day), the waste is covered by a thin layer of soil, which also is compacted. 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: Foundations Dikes Liner Systems - Low-permeability soil liners - Flexible membrane liners (FMLs) - Leachate collection systems Final cover systems 6-60

61 Foundations The foundations for hazardous waste landfills should provide structurally stable subgrades for the overlying landfill components. The foundations also should provide satisfactory contact with the overlying liner or other system components. In addition, the foundations should resist settlement, compression, and uplift resulting from internal or external pressures, 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 the foundation and provide support to the overlying landfill components. Dikes therefore must be designed, constructed, and maintained with sufficient structural stability to prevent their failure. 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 specified strength. Materials other than soil may be used to construct dikes, as long as the design of the dike accommodates the particular properties of the selected materials and proper installation procedures are followed. Drainage layers and structures may be included in the dike design if conditions warrant control of seepage. Although seepage through a dike should be prevented by a liner system, a dike must be designed to maintain its integrity if the liner fails and seepage occurs. Liner Systems The primary function of a liner system is to minimize and control the flow of leachate from the site to the environment, particularly towards ground water. Liners are made of lowpermeability soils (typically clays) or synthetic materials (e.g., plastic). Landfills can be designed with more than one liner, and a mix of liner types may be used (these are referred to as composite liners). There are two types of liner systems currently used in land disposal facilities in the U.S. A single liner system consists of one liner and one leachate collection system as shown in Figure A double liner system includes two liners (primary and secondary), with a primary leachate collection system above the primary (top) liner and a secondary leak detection/leachate collection system between the two liners, as shown in Figure

62 Figure Schematic of a single clay liner system for a landfill (U.S. EPA, 1988) Figure Schematic of a double liner and leak detection system for a landfill (U.S. EPA, 1988) 6-62

63 The term "liner system" includes the liner(s), leachate collection system(s), and any special additional structural components such as filter layers or reinforcement. The major components of both single and double liner systems are the following: Low-permeability soil liners Flexible membrane liners (FML) Leachate collection and removal systems (LCRS) Low-Permeability Soil Liners. The purpose of a low-permeability soil liner depends on the overall liner system design. In the cases of single liners constructed of soil or double liner systems with soil secondary liners, the purpose of the soil liner is to prevent constituent migration through the soil liner. In the case of soil liners used as the lower component of a composite liner, the soil component serves as a protective bedding material for the FML upper component and minimizes the rate of leakage through any breaches in the FML upper component. 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, many fundamental yet important criteria should be considered such as: in-place permeability of the liner; liner stability against slope failure, settlement, and bottom heave; and the long-term integrity 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 to prevent the migration of any hazardous constituents into the liner during the period that the facility is in operation and typically during a 30-year postclosure monitoring period. In addition, FMLs should be compatible with the waste liquid constituents that may contact them and be of sufficient strength and thickness to withstand the forces expected to be encountered during construction and operation. The design of a lined sewage sludge surface disposal site requires consideration of more than the performance requirements of the FML; it also requires careful design of the foundation 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 not structurally stable, the liner system may deform, thus restricting or preventing its proper performance. The performance requirements of an FML include: Low permeability to waste constituents Strength or mechanical compatibility of the sheeting Durability for the lifetime of the facility 6-63

64 The designer must specify the necessary criteria for each of these properties based on engineering requirements, performance requirements, and the specific site conditions. Leachate Collection Systems. Leachate refers to liquid that has passed through or emerged from landfilled waste and contains dissolved, suspended, or immiscible materials removed from the waste. The purpose of a primary leachate collection system in a landfill is to minimize the leachate head on the top liner during operation and to remove liquids from the landfill through the postclosure monitoring period. The leachate collection system should be capable of maintaining a leachate head of less than 30 cm (1 foot). The purpose of a secondary leachate collection system (sometimes referred to as a leak detection system) between the two liners of a landfill is to rapidly detect, collect, and remove liquids entering the system through the postclosure monitoring period. If uncontrolled, landfill leachate can be responsible for contaminating ground water and surface water. Leachate is generally collected from the landfill through sand drainage layers, synthetic drainage nets, or granular drainage layers with perforated plastic collection pipes, and is then removed through sumps or gravity drain carrier pipes. Once leachate has been collected and removed from the landfill, it must undergo some type of treatment and disposal. The most common methods of management are: Discharge to a wastewater treatment plant On-site treatment followed by discharge Recirculation back into the landfill An extensive body of literature has been developed on the design of liners and leachate collection systems. For additional information on these systems, including information on materials specifications, construction procedures, and quality control issues, see the references U.S. EPA, 1988, and U.S. EPA, Landfill Closure and Final Cover Systems Closure is the procedure, once waste placement in a landfill ends, that renders the site safe and acceptable to the public. Closure is intended to minimize the environmental and public health and safety hazards, and prepares the site for the post-closure period. During the post-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-term minimization of liquid migration and leachate formation in the closed landfill by preventing the infiltration of surface water into the landfill for many years. Final cover systems also control the venting of gas generated in the facility and isolate the wastes from the surface environment. These cover systems are constructed in layers, the most important of which are the barrier layers. Other layers are included to protect or to enhance the performance of the barrier layers. A final cover system must be constructed so that it functions with minimum 6-64

65 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 a permeability less than or equal to the permeability of the bottom liner system component with the lowest permeability. Environmental Safeguards at Landfill Sites Ground-water protection is the most difficult and costly environmental control measure required at many hazardous waste landfills. Additionally, contamination of surface water and methane gas buildup must be avoided. Monitoring To ensure the components of a landfill are performing their designed function, surface water and ground water monitoring should be included at all landfills. By sampling from ground water wells located near the solid waste disposal facility, the presence, degree, and migration of any leachate can be detected. The main concern with environmental monitoring is ensuring that the number and location of sampling points are adequate to characterize background levels (for ground water) and that sampling is frequent enough to determine whether 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 to minimize the amount of surface water entering the landfill. Run-on control can be accomplished by constructing berms and swales above the filling area that will collect and redirect the water to the stormwater control structures. Surface water management also is necessary at landfill sites to minimize erosion damage to earthen containment structures. Design of a surface water management system requires a knowledge of local precipitation patterns, surrounding topographic features, geologic conditions, and facility design. Surface water management systems do not have to be expensive or complex to be effective. The equipment and materials used for construction of the surface water management system are the same as those used for general earthwork and foundation construction. Explosive Gases Control Methane gas is a product of the anaerobic decomposition of organic waste. At and around 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 fire and explosions that can endanger employees, users of the site, and occupants of nearby structures, or cause damage to containment structures (methane is explosive in confined spaces when found in concentrations between 5 and 15 percent). These hazards are preventable through monitoring and through corrective action should methane gas levels exceed specified 6-65

66 limits in the facility structures. Once methane is collected, it is usually vented into the atmosphere, flared (burned), or recovered as an energy source Ocean Disposal In ocean disposal, municipal wastewater sludge is released into a designated area of the ocean either from outfall pipes or vessels at the ocean surface. This option has the potential for severely degrading the local marine environment. Batstone et al. (1989) address the technical requirements for safe disposal of hazardous waste in the ocean Air Emissions Conventional aerobic biological treatment processes generally do not produce noxious off gases. Anaerobic biological treatment, however, can produce hydrogen sulfide, a toxic gas that requires offgas treatment, and methane, a combustible gas that needs to be either collected for its energy value or diluted below its explosive limit. Biological treatment of wastewater containing toxic organics typically requires collection and treatment of the offgases to remove organic contaminants (see Figure 5-6). Similar sorts of treatment are required for air and thermal stripping technologies that separate contaminants into their gaseous phase. Incineration technologies require the collection of particulates in stack gases, and treatment of products of incomplete combustion and acid gases. These air pollution control technologies result in solid particulate residues that require disposal and liquid wastes that usually require further treatment Concentrated Liquid Waste Streams Processes such as reverse osmosis and ultrafiltration create concentrated liquid waste streams. Concentrated wastewaters containing inorganic constituents can be further treated by precipitation and then dewatered. Evaporation is an alternative method for obtaining residual solids from concentrated liquid waste streams. Concentrated organic liquid waste streams are usually incinerated. Deep well injection is another alternative for concentrated liquid wastes. 6-66

67 6.6 REFERENCES Batstone, R., J.E. Smith, Jr., and D. Wilson, eds The Safe Disposal of Hazardous Waste: the special needs and problems of developing countries. World Bank Technical Paper Number 93. Benedict, A.H., E. Epstein, and J. Alpert Composting Municipal Sludge: A Technology Evaluation. EPA/600/2-87/021. Breton, M., et al Technical Resource Document: Treatment Technologies for Solvent Containing Wastes. EPA/600/2-86/095 (NTIS PB ). Washington, DC. Fresenius, W., W. Schneider, B. Böhnke, and K. Pöppinghaus (eds.) Waste Water Technology: Origin, Collection, Treatment and Analysis of Waste Water. Springer-Verlag, New York, NY. Hazardous Material Control Research Institute Municipal sludge management. HMCR. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Municipal Sludge Management and disposal. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Disposal of residues on land. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Composting of municipal residues and sludges. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Acceptable sludge disposal techniques. HCMRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Treatment and disposal of industrial wastewater HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Design of municipal sludge compost facilities. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Municipal management impact of industrial toxic material on POTW sludge. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Composting of municipal and industrial sludges: design, operations, marketing, health, rules & regs. HMCRI. Sludge/Wastewater Management Series. 6-67

68 Hazardous Material Control Research Institute Municipal and industrial sludge composting-materials handling. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute Composting of Municipal and Industrial Sludges. HMCRI. Sludge/Wastewater Management Series. Hazardous Material Control Research Institute. 1980; Municipal and Industrial Sludge Utilization and Disposal Proceedings Series. HMCRI. Municipal and Industrial Sludge Utilization and Disposal Proceeding Series. Hazardous Material Control Research Institute. 1986; 1987; 1988; 1989). Municipal Sewage Treatment Plant Sludge Management Proceedings Series. McArdle, J.L., M.M. Arozarena, and W.E. Gallagher A Handbook on Treatment of Hazardous Waste Leachate. EPA/600/S8-87/006 (NTIS PB ). Washington, DC. Offut, C.K., and J.O. Knapp The Challenge of Treating Contaminated Superfund Soil. In: Superfund '90, Hazardous Material Control Research Institute, Silver Spring, MD, pp Saltzberg, E.R., and J.C. Cushnie, Jr Centralized Waste Treatment of Industrial Wastewater. Noyes Data Corporation, Park Ridge, NJ. U.S. EPA Operator Manual: Stabilization Ponds. EPA/430/ Washington, DC. U.S. EPA Sludge Treatment and Disposal (Process Design Manual). EPA/625/ (NTIS PB ). Washington, DC. [See also U.S. EPA (1974, 1978a).] U.S. EPA Land Application of Municipal Sludge (Process Design Manual). EPA/625/ Washington, DC. U.S. EPA. 1984a. Waste Analysis Plans: A Guidance Manual. EPA/530-SW Washington, DC. U.S. EPA. 1984b. Use and Disposal of Municipal Wastewater Sludge (Environmental Regulations and Technology). EPA/625/ Washington, DC. U.S. EPA. 1985a. Composting of Municipal Wastewater Sludges. Seminar Publication EPA/625/ (NTIS PB ). Washington, DC. U.S. EPA. 1985b. Estimating Sludge Management Costs at Municipal Wastewater Treatment Facilities (Handbook). EPA/625/6-85/010 (NTIS PB ). Washington, DC. 6-68

69 U.S. EPA Dewatering Municipal Wastewater Sludges (Design Manual). EPA/625/1-87/014. Washington, DC. U.S. EPA Guide to Technical Resources for the Design of Land Disposal Facilities. EPA/625/6-88/018. Cincinnati, OH. U.S. EPA Issues Affecting the Applicability and Success of Remedial/Removal Incineration Projects. Superfund Engineering Issue. EPA/540/2-91/004. Washington, DC. U.S. EPA Rotating Biological Contactors. Engineering Bulletin EPA/540/S-92/007. Washington, DC U.S. EPA Solid Waste Disposal Facility Criteria, Technical Manual. EPA/530/R- 93/017 (NTIS PB ). Washington, DC. U.S. EPA CFR Part 503, Standards for the use or disposal of sewage sludge. U.S. EPA Process Design Manual for Land Application of Sewage Sludge and Domestic Septage. Draft Report Submitted by Eastern Research Group to U.S. EPA Center for Environmental Research Information, September 16, U.S. EPA Development Document for Proposed Effluent Limitations Guidelines and Standards for the Centralized Waste Treatment Industry. EPA/821-R Washington, DC. Weathington, B.C Destruction of Cyanide in Wastewaters: A Review and Evaluation. EPA/600/2-88/031 (NTIS PB ). Washington, DC. Wilk, L., S. Palmer, and M. Breton Technical Resource Document: Treatment Technologies for Corrosive-Containing Wastes, Volume II. EPA/600/2-87/099 (NTIS PB ). Washington, DC. Water Pollution Control Federation; Water Environment Federation Sludge thickening. MOP FD-1. WPCF/WEF. Alexandria, VA. Water Pollution Control Federation; Water Environment Federation Sludge dewatering. MOP 20. WPCF/WEF. Alexandria, VA. Water Pollution Control Federation; Water Environment Federation Sludge stabilization. MOP FD-9. WPCF/WEF. Alexandria, VA. Water Pollution Control Federation; Water Environment Federation. 1987a. Operation and maintenance 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. 6-69

70 Water Pollution Control Federation; Water Environment Federation Sludge conditioning. MOP FD-14. WPCF/WEF. Alexandria, VA. Water Pollution Control Federation; Water Environment Federation Beneficial use of waste solids. MOP FD-15. WPCF/WEF. Alexandria, VA. 6-70