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