United States Office of Research and EPA/625/R-97/002 Environmental Protection Development December 1997 Agency Washington DC 20460 Technology Transfer x>EPA Capsule Report Sources and Air Emission Control Technologies at Waste Management Facilities ------- Technology Transfer EPA/625/R-97/002 Capsule Report Sources and Air Emission Control Technologies at Waste Management Facilities December 1997 U.S. Environmental Protection Agency Off ice of Research and Development Cincinnati, Ohio 45268 Printed on Recycled Paper ------- Disclaimer The information in this capsule report has been funded by the U.S. Environmental Protection Agency and the U.S. Department of Energy. It has been subjected to the EPA's peer and administrative review, and it has been approved for publication as an EPA document. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by EPA. ------- Foreword The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to formulate and implement actions leading toacompatible balance between human activities and the ability of natural systems to support and nurture life. To meet this mandate, EPA's research program is providing data andtechnical support for solving environmental problems today and building a science knowledge base necessary to manage our ecological resources wisely, understand how pollutants affect our health, and prevent or reduce environmental risks in the future. The National Risk Management Research Laboratory is the Agency's center for investigation of technological and management approaches for reducing risks from threats to human health and the environment. The focus of the Laboratory's research program is on methods for the prevention and control of pollution to air, land, water and subsurface resources; protection of water quality in public water systems; remediation of contaminated sites and ground water; and prevention and control of indoor air pollution. Thegoal of this research effort is to catalyze development and implementation of innovative, cost- effective environmental technologies; develop scientific and engineering information needed by EPA to support regulatory and policy decisions; and provide technical support and information transfer to ensure effective implementation of environmental regulations and strategies. This publication has been produced as part of the Laboratory's strategic long-term research plan. It is published and made available by EPA's Office of Research and Development to assist the user community and to link researchers with their clients. E. Timothy Oppelt, Director National Risk Management Research Laboratory ------- Contents Figures vi Acknowledgments v'i' introduction 1 Sources and Control Technologies 1 Sources of Air Emissions 1 Surface Impoundments 1 Tanks -2 Fixed Roof Tanks 3 Floating Roof Tanks 4 Pressure Tanks -^ Containers 7 ' Treatment Devices 10 Distillation 1° Solvent Extraction 11 Air Stripping 11 Steam Stripping 11 Thin-Film Evaporation 12 Waste Incineration 13 Control Devices 13 Adsorption 13 Condensation 15 Absorption 15 Combustion Equipment 15 Thermal Incineration 16 Catalytic Incineration 17 Existing Boilers and Process Heaters 17 Unconventional Technologies 17 References 20 ------- Figures 1. Emissions from impoundments and other open liquid surfaces 1 2. Major factors affecting emissions .2 3. Fate of organics in a surface impoundment: emissions, effluent, biodegradation, sludge .2 4. Typical fixed roof tank .3 5. Tankworking losses .3 6. Covered tanks (working losses) .3 7. Tank breathing losses 4 a. Covered tanks (breathing losses) .4 9. Closed-vent system and control device .4 10. Floating roof -4 11. Internal floating roof .5 12. External floating roof .5 13. External floating roof tank -6 14. IFR mechanical shoe seal .6 15. IFR liquid-mounted seal. .7 16. IFR vapor-mounted seal with secondary seal .7 17. IFR fittings that require controls -8 18. EFR mechanical shoe seal with secondary seal 8 19. EFR liquid-mounted seal with secondary seal .9 20. EFR fittings that require controls -9 21. Splash loading method 9 22. Submerged fill pipe 9 23. Bottom loading reduces liquid turbulence and vapor and/or liquid contact 10 24. Typical bottom loading system with vapor collection 10 VI ------- Figures (continued) 25. Emission sources for distillation 11 26. Emissions from solvent extraction 11 27. Schematic diagram of an air stripping system 12 26. Steam stripper for ethylene dichloride (EDC)/vinyl chloride 12 29. Preliminary treatment prior to stripping 12 30. Flow path of thin-film evaporator 13 31. Carbon canisters 13 32. General process flow diagram of an adsorption process for organic recovery 14 33. Fixed-bed regenerative carbon adsorption system process flow diagram and potential emission sources 14 34. Schematic diagram of a contact condenser 15 35. Schematic diagram of a shell-and-tube surface condenser 15 / 36. Packed tower for gas absorption 16 37. Steam-assisted elevated flare system 16 36. Thermal vapor incinerator 17 39. Schematic diagram of a catalytic incinerator system 18 40. Fluid-bed catalytic incinerator 18 41. Typical rotary carousel system 18 42. Carousel system with incinerator 19 43. Schematic of an open single-bed biofilter system 19 VII ------- Acknowledgments Thiscapsule reportwasdeveloped from technical material prepared by Research Triangle Institute (RTI) for a seminar publication on air emissions control at waste management facilities. The original seminar publication was prepared under contract to the U.S. EPA Office of Research and Development (ORD), through Eastern Research Group, Inc., and the Office of Air Quality Planning and Standards (OAQPS). Justice A. Manning, Center for Environmental Research Information, ORD, coordinated the original seminars with assistance from ERG, Inc. Peer reviewers who provided the final review of this capsule report are Michele Aston, OAQPS, and John 0. Burckle and Scott R. Hedges, ORD, National Risk Management Research Laboratory. The tireless efforts of Robert. A. Zerbonia of RTI were greatly appreciated. VIII ------- Introduction The chemicals processed during waste management operations can volatilize into the atmosphere and cause carcinogenic or other toxic effects or contribute to ozone formation. Regulations have been developed to control air emissions from these operations. The EPA has promul- gated standards under the authority of Section 3004(n) of the Hazardous and Solid Waste Amendments to the Re- source Conservation and Recovery Act (RCRA) that limit organic air emissions from waste management units at hazardous waste treatment, storage, and disposal facilities (TSDF); the rules also apply to 90-day accumulation units at hazardous waste generator facilities. In June 1990, EPA promulgated standards for process vents and equipment leaks; additional RCRA standards were promulgated on December 6, 1994, (effective December 6, 1996) and amended on November 25, 1996, to limit air emission from tanks, surface impoundments, and containers used in man- aging hazardous wastes. Implementation of air emission controls on many types of waste management operations are required by these RCRA air rules. This capsule report focuses on the major sources and controls of air emissions at waste management facilities, how these emissions occur, and how they can be con- trolled. The major sources that are discussed in detail include surface impoundments, the very broad and diverse category of tanks and ancillary equipment, containers, and treatment devices. As each source is described, controls that are inherent to that source or commonly found on that particular source are presented. In addition, details are provided on the basic mechanisms by which emissions occur and the major factors that affect the emissions. After the discussion of sources and their inherent con- trols, air pollution control devices that may be generally applicable to any enclosed or vented source (i.e., add-on controls) are described. The discussion of control devices focuses on their applicability, control performance, and the major factors affecting performance. Organic removal (i.e., pretreatment) and destruction processes are also discussed as a means of controlling air emissions and reducing or eliminating the emission potential. This discussion de- scribes processes that remove or destroy the organics in the waste, which may eliminate the need to control subse- quent waste processing steps. Sources and Control Technologies The types of sources found at waste management facili- ties, inherent controls that are typically part of the con- struction and operation of the sources, and emission mechanisms are discussed in this chapter. As each source is discussed, covers and enclosures that are specifically applicable to the source are described, as well as simple work practices that reduce emissions. Other emission con- trols that are broadly applicable to many of the individual sources are discussed collectively in the last part of the chapter. These controls include traditional air pollution control devices, processes that remove the organics be- fore the waste is placed in units with a high emission potential, and waste incineration. Sources of Air Emissions The discussion of emission sources is divided into four categories: surface impoundments, the very broad and diverse group of tanks and ancillary equipment, contain- ers, and treatment devices. The major focus is on the first three categories because they are the most directly im- pacted by the RCRA air emission regulations. Surface Impoundments A surface impoundment is defined under RCRA as "a natural topographical depression, man-made excava- tion, or diked area formed primarily of earthen materials (although it may be lined with man-made materials) which is designed to hold an accumulation of liquid wastes or wastes containing free liquids and which is not an injection well. Examples of surface impoundments are holding, stor- age, settling, and aeration pits, ponds, and lagoons." Impoundments are simply ponds and lagoons that are used primarily for managing aqueous wastes and sludges. Specific uses include storage, equalization, neutralization, evaporation, solids settling, and biodegradation. A surface impoundment is below grade, usually has berms with sloping sides to contain wastes, and has a liquid surface that is exposed to the atmosphere. It may be operated as a flowthrough system with liquid flowing in at one point and out at the same time at another point, or the liquid may be pumped out or evaporated, leaving behind a sludge. Figure 1 illustrates emissions from impoundments and other open liquid surfaces as caused by diffusion and wind flow across the surface. Figure 2 lists several of the factors that affect emissions from impoundments. These same factors are applicable to emissions from open tanks. The constituent's volatility has a direct effect on emissions from impoundments and other sources with exposed liquid surfaces. Highly volatile com- pounds such as benzene are readily emitted from open sources, whereas relatively nonvolatile compounds such as phenol tend to stay in the water. The residence time in the impoundment has an obvious effect on emissions: longer residence times result in higher emissions. If the waste is in the impoundment long enough, even relatively nonvolatile compounds are evaporated. For impoundments with relatively short residence times, a higher percentage of the organics may be removed with the effluent and emitted later in other units in the treatment sequence. From surface to air Wind Flow in Diffusion through liquid Flow Out - Figure 1. Emissions from impoundments and other open liquid surfaces. ------- Constituent volatility Residence time Surface area Turbulence (aeration, agitation) Windspeed and temperature Extent of competing mechanisms (such as biodegradation) Figure 2. Major factors affecting emissions. Many impoundments and tanks are agitated for mixing, air stripping, or biodegradation. Agitation and aeration in- crease emissions by creating turbulent zones and increase contact between the waste and air. There are a highly turbulent area and water spray around the agitators used in mechanically aerated units. Essentially all of the highly volatile compounds can be emitted when the impound- ment is mechanically agitated. Approximately half of the impoundments used to treat hazardous waste are aerated or agitated. As Figure 3 illustrates, organic removal from impound- ments also occur from mechanisms other than air emis- sions resulting from wind blowing across the exposed surface of the waste. Organics can also be removed by biodegradation, adsorption onto sludge, or removed with the effluent. Emission models have been developed to estimate the extent of each of these different removal mechanisms. The models developed for open liquid surfaces are applicable to impoundments and open tanks. These mod- els can account for the emissions from relatively calm surfaces or from the turbulence created by aeration or agitation. The emissions are modeled as two mass trans- fer steps in series: (1) diffusion through the liquid, and (2) mass transfer from the surface of the liquid to the air. The approach can account for removal in flowthrough systems and removal in units designed for disposal or evaporation. The extent of biodegradation, if any, can also be esti- mated. One of the controls demonstrated for impoundments is an air-supported structure, which uses fans to maintain a positive pressure to inflate the structure. For effective control, the air vented from the structure must be sent to a control device, such as a carbon adsorber. Air-supported Emissions Wind Flow- in Biodegradation Sludge Flow in Sludge out Figure 3. Fate of organics in a surface impoundment: emissions, effluent, biodegradation, sludge. structures have been used as enclosures for conveyors, open top tanks, and storage piles, as well as impound- ments. An air-supported structure and control device has been installed on a l-acre aerated lagoon that is used for biodegradation at a pharmaceutical manufacturing facility. The cover material is PVC-coated polyester with a Tedlar backing. An agitator system in the structure provides oxy gen and keeps carbon and biomass suspended. In this application, the exhaust from the structure is vented to a carbon adsorber. Very few leaks were found around the structure; consequently, the control efficiency is deter- mined primarily by how well the control device works; it may exceed 95 percent. This plant's experience with the air-supported structure has found that corrosion can be accelerated inside the structure, condensation may occur, temperatures may be high, and special worker safety pre- cautions are needed. Floating membrane covers are another control option and have been demonstrated on various types of im- poundments, including water reservoirs in the western parts of the United States. For proper operation as a control technique for organic compounds, the membrane must provide a seal at the edge of the impoundment and provisions made to remove rainwater. If gas is generated under the cover, vents and a control device may be needed. In addition, if sludge accumulates, some means for peri- odic sludge removal may be required, such as a sludge pump. Emission control depends primarily on the type of mem- brane, its thickness, and the individual organic compounds in the waste. Theoretical estimates based on diffusion through the membrane indicate worst-case control effi- ciencies of 50 to over 95 percent. Laboratory studies indi- cate that the cover is an efficient control for some organic compounds, but, for specific compounds that permeate the membrane, the control efficiency is lower. The floating membrane cover has been demonstrated on an impoundment that is used as an anaerobic digester. The impoundment is about 7 acres in size with a depth of approximately 14 feet. The membrane material is 100-mil high-density polyethylene. The cover is anchored over a concrete ring wall that extends above grade level around the perimeter of the impoundment. The membrane ex- tends over the concrete wall and is covered with backfill to anchor and seal it. Punctures or tears in the membrane can be patched. This installation has been in operation for over 4 years, and the company supplying the membrane offered a 20-year warranty on the life of the material. Tanks The most diverse group of waste management sources falls into the category of tanks, which is broadly defined under RCRA. If the unit is not a land disposal source, it is probably a tank. A tank is defined as "a stationary device, designed to contain an accumulation of hazardous waste, which is constructed primarily of nonearthen materials (e.g., wood, concrete, steel, plastic) that provide structural support." A tank system is defined as a tank and its ancillary equipment, where ancillary equipment includes such devices as piping, fittings, flanges, pumps, and valves. ------- The category of tanks and tank systems includes two broad classes, open top tanks and covered tanks. The latter class is divided into fixed roof tanks, floating roof tanks, and pressure tanks. Fixed roof tanks are the most common type of storage tank found at hazardous waste facilities. Emissions occur through the tank's vent, which may be open to the atmosphere, equipped with a pres- sure-vacuum relief valve, or vented to a pollution control device. Fixed Roof Tanks The fixed roof may have several openings in addition to the vent, such as a manhole for tank entry, a hatch used for measuring the liquid level, or an overflow pipe (Fig- ure 4). The pressure-vacuum relief valve is also called a conservation vent, which permits small changes in the liquid level without expelling the tank's vapors. If the tank has a conservation vent or is vented to a control device, it is important that the other openings on the tank be kept closed and sealed for the emission controls to be effective. As an emission control option, fixed roofs can be retrofit- ted to open tanks, or a fixed roof tank can be used to replace an open tank or impoundment. Compared to an open tank, a fixed roof tank can provide additional control of 66 to 99 percent, depending on the waste volatility and the operating characteristics of the tank openings. If the fixed roof tank is constructed to withstand pressures of 2.5 psig, an additional control of 20 to 45 percent can be obtained. (Most tanks are not designed and constructed to withstand this^pressure.) Emissions from fixed roof tanks occur primarily from working! (loading) losses and, to a lesser extent, from breathing losses. The quantity emitted is most directly affected by the rate at which vapors are expelled from the tank and the volatility of the tank's contents. These emis- sions are increased by heating or aeration. Working losses occur when waste is pumped into the tank and vapors are expelled by the rising level of liquid (Figures 5 and 6). Breathing losses occur when the volume of vapor in the tank is increased because of changes in temperature or pressure (Figures 7 and 8). Equations developed by the American Petroleum Insti- tute (API) are used to estimate emissions for organic liquids. The basic form of the equation, which can be used for other types of wastes, estimates the volume of vapor Pressure/vacuumvalve (for venting) Manhole H Nozzle (for submerged H fill or drainage) Figure 5. Tank working losses. Working losses , due to loading J Volume of displaced - vapors Vapor space : Liquid waste : New liquid . level Original liquid level >* £3- Liquid in Figure 4. Typical fixed roof tank. Figure 6. Covered tanks (working losses). expelled from the tank based on the amount of liquid pumped in and on the vapor concentration. The concen- tration of constituents in the vapor can be measured or estimated from volatility data. One error in using the API tank equations for aqueous wastes is to estimate the concentration in the vapor from the mole fraction of the compound in the liquid, which significantly underestimates concentration. Henry's law constant should be used for dilute aqueous wastes. Breathing losses are usually very low compared to working losses. Note that if the tank is operated at a constant liquid level, as some separators and collection tanks are, very little vapor is displaced and working losses are small. Two primary emission control approaches exist for fixed roof tanks. The vapor space above the liquid can be eliminated by keeping the liquid level at the roof line (or vice versa), or the vapor space can be vented to a control device. An alternative to keeping the tank at a constant liquid level is to install a floating roof, as discussed as follows. Figures 9 and 10 illustrate the .application of a closed vent system with control device and two arrange- ments of floating roof tanks. ------- Figure 7. Tank breathing losses. Breathing losses due to ambient pressure and temperature fluctuations Vapor phase concentration in equilibrium with waste liquid Liquid waste Head space - volume decreases as temperature increases Figure 9. Closed-vent system and control device. Figure 10. Floating roof. Figure 8. Covered tanks (breathing losses). Floating Roof Tanks Floating roof tanks are common at petroleum refineries and gasoline marketing facilities for the storage of volatile liquids. The floating roof can be installed internally in a fixed roof tank or externally without a fixed roof. Figures 11 and 12 show internal floating roof and external floating roof tanks in cross-section. The roof floats on the liquid and moves with changes in the liquid level, thus reducing vapor displacement and controlling working losses. The major requirement for a floating roof is an effective seal between the roof and the tank walls. Emissions from a properly maintained floating roof are very low and occur from standing losses and withdrawal losses. Figure 13 shows the equipment associated with an ex- ternal floating roof. Standing losses occur at the deck seals and at openings for fittings in the floating roof. If retrofitted to a hazardous waste tank, the floating roof materials must be compatible with the waste; also floating roofs cannot be used in hazardous waste treatment tanks with surface mixers or aeration equipment. The emission reductions achieved by a floating roof relative to a fixed roof have been evaluated for volatile organic liquids by using empirical models. Depending on the type of deck and seal system selected, emission reductions of 93 to 97 percent can be obtained. For the smaller size tanks and varieties of wastes found at hazardous waste facilities, reductions of 74 to 82 percent can be obtained relative to a fixed roof. Convening an open top tank to a floating roof tank is estimated to reduce emissions by 96 to 99 percent. Internal and external floating roofs have different design and operating requirements. Typical EPA requirements relating to roof seals and fittings for floating roof tanks are discussed as follows. An internal floating roof must be equipped with either a mechanical shoe seal, or a continuous liquid-mounted seal, or double-seal system. Figures 14, 15, and 16 illus- trate the different kinds of seals. Mechanical shoe seals (Figure 14} operate through a scissors design with a weighted leg to maintain pressure between the seal and the tank wall. A flexible envelope covers the area between ------- Gauge float system F i g . 1 1 . Internal floating roof. Gauge float system Mechanics* shoe seal Automatic Aleeder vent Emergency roof drain Figure 12. External floating roof. the roof deck and the seal. Liquid-mounted seals (Figure 15) maintain pressure between the roof and the tank wall by means of a liquid-filled tube that rides between the two. The term "liquid-mounted" refers to the fact that the seal in this case contacts the liquid directly. A scuff band, sealed to the roof, protects the tube from abrasion against the wall. A variation of this seal uses a resilient foam ring instead of a liquid-filled tube. Double-seal systems are illustrated by a vapor-mounted seal with secondary seal (Figure 16). This seal is similar to the liquid-mounted seal with two exceptions. The liquid is replaced by foam and a flexible rim-mounted seal is added to the top of the floating deck. The term "vapor-mounted" refers to the fact that the seal does not come into direct contact with the liquid. Fittings that typically require controls to achieve emis- sion reductions are shown in Figure 17. A seal is required around each of these vents, wells, or hatches. Further, each opening must project below the liquid surface. Ac- cess openings and gauge wells must be equipped with gasketed covers equipped with bolts, and the cover must be closed and bolted except when in use. Ladder wells must have a gasketed sliding cover. Other requirements include a gasketed sliding cover or flexible fabric seal for column wells and slitted fabric covers that cover at least 90 percent of the opening for sample wells. Automatic bleeder vents must be gasketed and must be closed except when the internal floating roof is floated off or landed. Rim space vents must be gasketed, and open only ------- Main drain Pontoon manhole Seal envelope Figure 13. External floating roof tank ' Tank wall Envelope Figure 14. IFR mechanical shoe seal. when the roof is floated off or as directed by the manufac- turers for proper and safe operation. External floating roof (EFR) requirements are similar in that the EFRs must use a primary seal with continuous, roof rim-mounted secondary seal. This specification means that the EFR must have a primary and a secondary seal. The primary seal can be either liquid-mounted or equipped with a mechanical shoe seal. Figure 18 is an illustration of the EFR mechanical shoe seal with secondary seal. This seal is identical to the internal floating roof seal except for the addition of a flexible secondary seal mounted to the top of the deck rim. Figure 19 shows EFR liquid-mounted seals with added secondary seals at the deck rim. These seals also are identical to their internal floating roof counterparts except for the added secondary seals. The EPA rules that apply to storage tanks have very specific requirements for the EFR fittings. The EFR fittings requiring controls are shown in Figure 20. Access open- ings, sampling wells, and gauge wells must project below the liquid surface, their covers must be gasketed, and the cover must be closed and sealed except when in use. Emergency roof drains, must be equipped with a slotted membrane fabric cover that covers at least 90 percent of the opening. Automatic bleeder vents must be gasketed and closed except when the EFR is floated off or landed. The rim space vents must be gasketed and may be open only when the roof is floated off. ------- Tank wall /IT Tank wall Floating roof Seal fabric Figure 15. IFR liquid-mounted seal. -Tank wall L Rim-mounted secondary seal Floating roof Scuff band Foam filled tube Vapor space -A Figure 16. IFR vapor-mounted seal with secondary seal. Pressure Tanks Pressure tanks are designed to operate safely at inter- nal pressures above atmospheric pressure. Consequently, these tanks can often be operated as closed systems and do not have emissions at normal storage conditions or during routine loading and withdrawal. Pressure-relief valves on the tanks open only in the event of improper operation or an emergency to relieve excess pressure. These tanks are most common for the storage of gases; however, they can also be used to store liquids. Low pressure tanks can be defined as those operating at up to 2 atm, and high-pressure tanks are those operat- ing at more than 2 atm. However, these values are not specified in the RCRA air rules; pressure tanks are de- fined in the rules as designed not to vent to the atmo- sphere as a result of compression of the vapor headspace in the tank during filling of the tank to its design capacity. In summary, tank emissions primarily occur due to am- bient temperature changes and tank filling operations. Con- trol of tank emissions is accomplished by reducing vapor displacement or by venting vapors to closed systems. Tank controls include use of floating roofs with specific seal types and fittings or installation of leak-tight closed- vent systems and control devices that achieve 95 percent or more control. Containers Containers are defined under RCRA as, "any portable device in which a material is stored, transported, treated, disposed of, or otherwise handled." Examples of typical containers are drums, dumpsters or roll-off bins, and tank trucks. Emissions occur from loading these containers, from uncovered containers during storage or transport, from treatment operation losses, and from spills. Sources of emissions associated with drums include air emissions from the evaporation of leaks and spills; and poor housekeeping practices that can make spill detection and cleanup difficult. If the drums are well maintained on a diked pad, emissions from spills or ruptures can be identi- fied by routine inspection procedures and promptly cleaned up. Dumpsters or roll-off bins can be a source of emis- ------- Gauge float system Ladder well ff Access hatch Bleeder vent s^_MM Column /well Sample well Figure 17. IFR fittings that require controls. Tank wall Rim-mounted secondary seal Envelope Floating roof Shoe Figure 18. EFR mechanical shoe seal with secondary seal. sions if they are left uncovered with the surface of the waste exposed to the atmosphere. Emissions from containers also occur when they are loaded, and emissions are greatest when splash filling (shown in Figure 21) is used. When splash filling, the vapors displaced from the container by loading can quickly become saturated with volatiles caused by the splashing. Submerged filling (shown in Figure 22) uses an influent pipe that is below the liquid surface, which reduces splash- ing and the degree of saturation of the displaced vapors. A study of submerged filling of tank trucks indicates that emissions are reduced by about 65 percent relative to splash filling. A third method of filling containers, bottom loading, is shown in Figure 23. In this case, a tight connection is made between the fill pipe or hose and the container. Also, the container hatches remain closed and all vapors vented from the container can be routed to a control device. A typical bottom loading system with vapor collec- tion is shown in Figure 24. Advantages for bottom loading include improved safety, faster loading, reduced labor costs, and lower emissions. Other basic controls for containers include using simple covers during storage, placing containers used for treat- ment operations in an enclosure vented to a control device during treatment, or ensuring that appropriate transport and routine housekeeping practices are followed, with daily inspections and prompt cleanup of spills. For enclosures vented to a control device, EPA has provided criteria for design and operation of the enclosure (40 CFR 52.571, Appendix B). Efficiency of the system depends on the type and operation of both the control device and the enclo- sure. ------- - Tank wall Rim-mounted secondary seal -Tank Wall Rim-mounted secondary seal Figure 19. EFR liquid-mounted seal with secondary seal. Automatic bleeder vent Emergency roof drain Guide-pole well Figure 20. EFR fittings that require controls. Gauge float svstem 1 Fill pipe Organic vapor emissions Fill pipe Hatch cover Cargo tank Organic vapor emissions Hatch cover Cargo tank Figure 21. Splash loading method. Figure 22. Submerged fill pipe. ------- Vapor vent to recovery or atmosphere Hatch closed Waste Cargo tank Figure 23. Fill pipe Bottom loading reduces liquid turbulence and vapor and/or liquid contact. Treatment Devices Several treatment technologies may be used to remove organic compounds from waste. The most common of these treatments include steam stripping, air stripping, thin-film evaporation, solvent extraction, distillation, and waste incineration. Features common to all the technologies include avoid- ing the need for controls on subsequent processes (hence pretreatment), removal efficiency dependant on waste con- stituents and process design, removal of essentially 100 percent of highly volatile compounds, and applicability to many wastes and compounds. The control efficiency achieved by waste treatment de- pends on several factors including the following: percent removed from waste and emissions from removal process, e.g., emissions from a steam stripping operation. Overall, a 98 to 99 percent removal efficiency can be obtained. For example, benzene being steam stripped from wastewater would typically achieve a 98 to 99 percent removal. Distillation One of the most commonly used treatment process for organic liquids is distillation. This separation process is based on the differences in volatility of components, and can be performed as continuous or batch operations at atmospheric pressure, under vacuum, or at greater than atmospheric pressure. Batch distillation is the most com- mon of the three for TSDF sites. For each type of distilla- Level sensor Island wiring \ Figure 24. Typical bottom loading system with vapor collection. Product supply To vapor processor 10 ------- Vapors Waste in sun Vent t Condenser Residual out Vent t Receiver Figure 25. Emission sources for distillation. tion operation, the organics are transferred from the liquid waste to the vapor phase, then condensed and collected as a separate liquid. Emissions from distillation typically occur from the over- head venting system, the collection tank vents, or the vacuum system. The primary condenser is an inherent control for these emissions, but pollution control devices can be added to the still vents to provide additional con- trol. Examples include a second condenser or an activated carbon canister. Figure 25 is a schematic showing emis- sion sources for the distillation process. Solvent Extraction Solvent extraction is another type of separation process used for organic liquids. In this process, the constituent to be removed preferentially dissolves in a solvent chosen for its high capacity for dissolving the constituent combined with easy physical separation from the remainder of the treated waste. The solvent in the extract is typically recov- ered by distillation, while the still bottoms containing the constituent are decanted for reclamation or disposal. How- ever, this solvent recovery stage is associated with emis- sions as shown in Figure 26 at the still vent. Any collection tanks associated with the unit may also have emission points. Although 80 to 100 percent .of the target organics can be removed from the waste by the solvent, the overall control efficiency is probably less. Also, waste residuals usually require further treatment. In common industrial applications, solvent extraction is used to remove phenols, acetic acid, hydroxy aromatic acids, and petroleum oils. Extraction is also applicable for organic sludges such as benzene from petroleum refinery sludge. Air Stripping Air stripping is a common treatment process used for aqueous wastes. The stripper may be a spray tower, a packed column, or simply an aerated tank used to provide contact between the waste and air. Strippers are most commonly used to remove parts per million or lower levels of volatiles from dilute aqueous wastes. Many strippers are simply vented directly to the atmosphere, while others are controlled by carbon adsorption or incineration. Condens- ers on air strippers are uncommon and are generally inef- fective because of low vapor phase concentrations and high volumetric flow rates. Figure 27 is a schematic dia- gram of an air stripping system and shows the system elements and vents that are emission points. Steam Stripping Steam stripping is another method used to treat aque- ous wastes with concentrations on the order of hundreds of parts per million or higher. Steam is injected directly into the wastewater, the overhead vapors are condensed, or- ganics are separated from the condensed water, and the decanted water is returned to the feed stream. Emissions occur from the vent on the condenser/decanter and from collection tank vents. Continuous steam strippers operate with either trays or packing and require a low solids con- tent in the feed to prevent plugging and fouling. Figure 28 is a schematic of an actual steam stripping system and illustrates the use of a heat exchanger to preheat the feed (to recover energy) and to cool the bot- toms stream from the stripper before additional wastewater treatment. This particular system has a high level of or- ganic recovery because both a primary and much colder secondary condenser are used. Overall emission control should be excellent because noncondensibles are vented to an incinerator. For continuous steam strippers, pretreatment as shown in Figure 29 may sometimes be required to adjust pH or to remove solids, which can foul the column packing or trays and cause plugging problems. Any separate organic phase that can be decanted from the wastewater is removed prior to stripping. Solvent Waste in 1 idr^ ^b Solvent extractor Emissions Extracted Extracted organics f and solvent J \ _. .... .. \ Solvent recycle Figure 26. Emissions from solvent extraction. 11 ------- Overhead vapors Liquid distributor Vent Control device > Vent air I Control device residue (e.g., spent carbon) Waste in Figure 27. Schematic diagram of an air stripping system. Feed To incinerator Water to feed tank EDO Bottoms Figure 28. Steam stripper for ethylene dichloride (EDC)/vinyl chloride. Additives Vent 1 t Wastewater - Decanter Organics Decanted water Sludge Figure 29. Preliminary treatment prior to stripping. Emissions from steam stripping come from the con- denser/decanter vent and collection tanks. The primary condenser provides an inherent control for this process, but additional control may be obtained by installing control devices on the equipment vents. Thin-Film Evaporation Thin-film evaporation typically is used for viscous liq- uids, sludges, and slurries that often cannot be treated with other technologies. A thin layer of waste is spread over a moving or wiped surface that is heated to volatilize organics. This configuration makes the technology adapt- able to many physical forms and waste compositions. The vertical thin-film evaporator (TFE) looks like a distil- lation column. In the TFE, the vapors with volatile organics are removed overhead to a condenser, and the treated waste is discharged from the bottom as shown in Figure 30. As with distillation units, emissions occur from vents on condensers, decanters, collection tanks, and vacuum systems if used. The primary condenser is an inherent control, but additional control may be obtained from con- trol devices on the system vents. 12 ------- Drive systemi vapor \ To condenser/ outlet / decanter Modular heating bodies Adsorption In the adsorption process, organics are selectively col- lected on the surface of a porous solid adsorbent (acti- vated carbon, silica gels, molecular sieves). Activated carbon is the most commonly used adsorbent because it has high internal surface area, low cost, and insensitivity to water. One gram of activated carbon can have a sur- face area equal to that of a football field and typically can adsorb up to half its weight in organics. The adsorber will remove essentially all of the target volatiles from the vented vapors until breakthrough, which is when the volatiles are first detected in the cleaned vapor leaving the adsorption bed. Carbon adsorbers can achieve control efficiencies of at least 95 percent, and control levels of 97 to 99 percent have been demonstrated in many applications. The two common types of adsorbers are carbon canisters and regenerable fixed beds. Carbon canisters, shown in Figure 31, are used for low vent flows, usually less than 100 ft3/min, and are not regenerated onsite. They are usually discarded or re- turned to the supplier. The canisters are fairly compact units and can be removed easily and fresh canisters in- stalled. However, their compact size implies limited capac- ity and a possible requirement for frequent changes. Fixed-bed carbon adsorbers that can be regenerated (shown as a general process flow diagram in Figure 32 and in more detail in Figure 33) are used for controlling continuous vent streams, with flows exceeding 100,000 ft3/ min, and can handle a wide range of organic concentra- Product outlet Figure 30. Flow path of thin-film evaporator. Waste Incineration Waste incineration is also an emission control option that can be used instead of processing the waste in units with a high emission potential. Waste incineration is com- monly used for wastes that were previously land-disposed. The technology provides destruction of 99.9 percent or higher as demonstrated in many units and is applicable to organic wastes and sludges. This technology should not be confused with vapor incineration, which is discussed in a later section. Control Devices Conventional, or commonly used, air pollution control technologies applicable to waste management operations are discussed in this section. The conventional air pollu- tion control devices are divided into two types: recovery devices, e.g., adsorption, condensation, and absorption; and combustion devices, e.g., flares, thermal incineration, catalytic incineration, and boilers or process heaters. The recovery devices recover organics for reuse when they are of value, while the combustion devices oxidize the organ- ics to destroy them. . For vent flows less than 100 CFM . Cannot be regenerated in canister Activated carbon Figure 31. Carbon canisters. 13 ------- Organic- containing gas stream On-line. Sorbent Clean gas Organfci ,sensor Regenerating gas - .Qff-jjne. Sorbent bed Organic + regenerating gas Organic (liquid) Byproducts Figure 32. General process flow diagram of an adsorption process for organic recovery. Exhaust vent Vent Filter Blower tream /^ \ \ =>y>=i_r^ =n f- j leal exchanger rvmiinn °P*>nal> dSng9 air Ambient air intake for cooling/drying I" L Filter Blower .^tnnm 1 E t*fl- -j- T H> I Z] ; L i * ^ 1 absorber Steam Carbon absorber / ^St= «-cSj- N l| /\ <2> Vent Vlnt Condenser ~ \3/ Organ; phase- o recovery 0 Aqueousphase to disposal or treatment Figure 33. Fixed-bed regenerative carbon adsorption system process flow diagram and potential emission sources. tions. A common procedure is to have dual beds with one desorbing while the other is adsorbing. The carbon capac- ity for organics is affected by the concentration of organics in the vapor. Carbon manufacturers generally have equi- librium data for specific compounds and specific carbons. The bed design is important and must be deep enough to prevent rapid breakthrough, yet not so deep as to cause excessive pressure drop. Flow rate is important in the bed design and in determining carbon capacity requirements. Humidity has an adverse effect when water occupies some of the adsorption sites. For a relative humidity of 50 per- cent or more, dehumidification or dilution may be neces- sary to lower the relative humidity of the inlet stream. The bed's operating temperature affects capacity, and some compounds such as aldehydes and ketones may generate heat in adsorbers. For these special cases, some means of removing the excess heat may be necessary. For effective emission control by adsorption, it is neces- sary either to monitor for breakthrough or to replace the carbon periodically before breakthrough occurs (based on design and operating experience). In addition, any emis- sions from the disposal or regeneration of the carbon should be controlled. It is of little value to control emis- sions from a vent stream only to emit the collected organ- 14 ------- ics later in the wastewater treatment associated with re- generation. Condensation One of the most common devices for organics recovery is the condenser. Condensers work by cooling the vented vapors to the dew point and removing the organics as a liquid. Typical coolants include cooling tower water, refrig- erated water, brines, and glycols. The efficiency of a con- denser is determined by the vapor phase concentration of the specific organics and the condenser temperature. Two common types of condensers are contact condensers and surface condensers (also called non-contact or shell and tube condensers). The contact condenser (shown in Figure 34 ) is cheap and efficient. However, the cooling liquid that directly con- -vtet Vapojt inlet » "^ ^ Distribution tray Liquid level Liquid outlet (water and VOCs) Figure 34. Schematic diagram of a contact condenser. tacts the vented vapors can present a disposal problem. For example, if the coolant is water that is sent to waste- water treatment, the volatiles may be emitted in open tanks. The shell and tube condenser (Figure 35) does not allow contact between the vented vapors and the cooling medium. In this type of condenser, a concentrated organic liquid can be recovered for recycle or other use. Absorption Absorption also is used as an air pollution control tech- nology. In absorption, the organics in. the vent gas are dissolved in a liquid by direct contact as shown for the packed tower absorber in Figure 36. The contact between the absorbing liquid and the vent gas is accomplished in spray towers or packed or plate columns. Some common solvents that may be useful for volatile organics include water, mineral oils, or other nonvolatile petroleum oils. Absorption efficiency is affected by temperature of the gas and sorbent (higher temperatures give poorer absorption), pressure (higher pressures increase solubility), solubility of the constituent in the sorbent (higher solubilities require less sorbent and allow higher efficiencies at equilibrium), and reaction kinetics (for reactions that can effectively remove the constituent from the surface of the absorbing liquid). Absorption efficiencies of 60 to 96 percent have been reported for organics. For example, methylene chlo- ride removal from vented vapors has been measured at 87 percent using water as the absorbing liquid. Commonly, absorbers also are used to remove inorganic gases such as SO,, H2S, HCI, and NH,. When choosing a solvent, it must be compatible with the target constituent at the operating conditions of the absorber. The material removed from the absorber may present a disposal or separation problem. For example, organics must be removed from the water or nonvolatile oil without losing them as emissions during the solvent recovery or treatment process. Combustion Equipment Vapor combustion is another control technique for vented vapors. The destruction of organics can be accomplished Coolant inlet Cooling tower or refrigeration unit Coolant outlet Vapor outlet Vapor inlet Condensed VOC (to decanter or receiving tank) Figure 35. Schematic diagram of a shell-and-tube surface condenser. 15 ------- 3 c> Cleaned gas to final control device Absorbing liquid in Organic laden gas in Absorbing liquid with organics to disposal or organic solvent recovery in flares; thermal oxidizers, such as incinerators, boilers, or process heaters; and in catalytic oxidizers. Flares are an open combustion process in which oxygen is supplied by the air surrounding the flame. Surrounding the flame with steam improves combustion. Flares are operated either at ground level (usually with enclosed multiple burner heads) or are elevated. Elevated flares often use steam injection to improve combustion by in- creasing mixing or turbulence and by pulling in additional combustion air. Properly operated flares can achieve de- struction efficiencies of at least 98 percent. Figure 37 is a schematic of the basic components of a flare system. The EPA has developed regulations for the design and opera- tion of flares that include tip exit velocities for different types of flares and different gas stream heating values (above 200 Btu/scf). These design and operating criteria were established because of the difficulty and cost associ- ated with measuring flare emissions. Figure 36. Packed tower for gas absorption. Thermal Incineration Thermal incineration is a common air pollution control technology used to destroy organic vapors. Thermal incin- Steam nozzles Gas barriers Helps prevent flashback Pilot burners Rare stack' Gas collection header and ollection header n^ I transfer line A £3Il_r-"-i I Knock-out_w. drum ^^ seal Steam line f - Ignition device i f - - Air line Gas line Drain Figure 37. Steam-assisted elevated flare system. 16 ------- eration requires high temperatures, good mixing, sufficient oxygen, and adequate resident time for proper operation and complete combustion of the organics. Auxiliary fuel is needed if the heating value of the waste gas is less than 50 Btu/scf. Thermal vapor incinerators are available in a wide range of sizes; capacity from 200 to 50,000 scfm are available. Destruction efficiency of at least 98 percent is achievable for an inlet concentration greater than 2,000 ppm. Below 2,000 ppm, EPA studies have found that combustion or destruction efficiency decreases as concentration decreases below 2,000 ppmv. A heat recov- ery unit, such as a steam generator, may be used to recover some of the energy from a thermal incinerator. Figure 38 is a depiction of the major components of a thermal vapor incinerator. The two types of thermal incinerators are recuperative and regenerative. Recuperative incinerators have an exte- rior shell and tube type heat recovery system and process gases with heats of combustion sufficiently high to sustain high temperatures without auxiliary fuel. Regenerative in- cinerators have internal heat recovery by means of two or more, usually ceramic, gas paths through the incinerator. While gases are burned and heating one side, the previ- ously heated side is used to heat the incoming gas. The two sides are switched to maintain an operating equilib- rium. Regenerative incinerators are effective for waste gases with low concentrations of the constituent to be destroyed. Catalytic Incineration Catalytic incinerators provide oxidation at temperatures lowe*1han those required by thermal incinerators (typically 350°C to 500°C versus 750°C to 1 ,000°C). Design consid- erations are important because the catalyst may be ad- versely affected by high temperatures, high concentrations of organics, fouling from particulate matter or polymers, and deactivation by halogens or certain metals. Destruc- tion efficiencies in catalytic incinerators are on the order of 98 percent. The basic components of a catalytic oxidizer are shown in Figure 39 and are similar to those of a thermal unit except that a catalyst bed is used. Sizes of catalytic incinerators range from large, high-volume units to small, low-volume packaged units. A fluid-bed catalytic incinerator is shown in Figure 40. The energy require- ments (and therefore, operating costs) of a catalytic oxi- dizer are lower than those of a thermal unit because of the lower operating temperatures. However, capital costs are higher. Existing Boilers and Process Heaters The organics in vented vapors can also be destroyed with a high level of efficiency in existing boilers or process heaters. In this case, no dedicated or new control equip ment is required. In these devices, vapors with halogens or sulfur are sometimes avoided because of potential cor- rosion problems when acid gases are formed. These de- vices recover the heating value of the vent stream and also offer the advantage of using existing equipment to control emissions. Vent streams are added as fuel, as secondary combustion air, or as diluent. Other advantages include recovery of the heating value of vent stream and destruction efficiencies of 98 percent or higher. Unconventional Technologies New, unconventional technologies are being used more frequently in the United States. One of these technologies, a modified two-stage system, combines adsorption with incineration or further adsorption. The first stage collects the target constituent and concentrates it in a regenerated gas from less than 100 ppm up to more than 2,000 ppm. The second stage involves more conventional treatment and depends on the concentration and chemical charac- teristics of the constituent. For example, if the constituent Waste gas Auxiliary fuel burner (discrete) Stack Mixing section Combustion section optional Heat Recovery Figure 38. Thermal vapor incinerator. 17 ------- can be reused, fixed bed adsorption would be used to collect it. Otherwise, the constituent would be thermally or catalytically incinerated. The most common of the two- stage systems with rotary carousels is shown in Figure 41. In this system either activated carbon or zeolite is used as the first-stage sorbent. While one segment of the wheel is adsorbing, another segment is being desorbed. While the second stage shows incineration or recovery in the figure, the specific process that is chosen depends on site condi- tions. A carousel system in line with an incinerator is shown in Figure 42. In this configuration, combustion gas is used to produce hot desorption gas by means of a heat exchanger. Many of these units are in operation in Japan and Europe, and several have recently begun operation in the United States. Their use is driven by air standards and air toxics rules. These systems do not tolerate particulate loadings well and may require filtration of the incoming gas stream. Another nonconventional device that is finding increased use for air pollution control is biofiltration. Vapors are vented through biologically active material where organics are digested to carbon dioxide and water by microorgan- isms. This technology, similar to compost in a garden, has been successfully applied in Europe to control odors, vola- tile organic compounds, and air toxic emissions. However, the process has not been used extensively in the United States. One of the properties of biofiltration is a general limitation to organic concentrations of 1,000 ppm or less, although new systems are reportedly treating higher con- centrations. Control efficiencies greater than 90 percent are achieved in various applications, and the literature indicates low operating costs that provide an economic advantage over other technologies. Figure 43 is a sche- matic of an open single-bed biofilter system. In practice, industrial applications typically involve multiple closed or covered beds. Emission source Supplementary Tuei Catalytic incinerator Preheater Stack - Catalyst bed1 Heat exchanger (optiona Figure 39. Schematic diagram of a catalytic incinerator system. t Outlet Stack Transition H eatexchanger r Process gas inlet Catalyst bed ,. Temperature T indicating controller Process air in Concentrate Blower Desorptionflow 1(| Incinerator Recovery Preheat burner » Return to process Solvent Figure 40. Fluid-bed catalytic incinerator. Figure 41. Typical rotary carousel system. 18 ------- Mist removal filters Adsorption cylinder Fan Preheater Catalytic combustion Figure 42. Carousel system with incinerator. Hot desorbing air Ducting Raw gas Blower Clean gas tJ Or o Humidifier Drainage Figure 43. Schematic of an open single-bed biofilter system. Filter material VVVVVSA* xVVVVVVV Air distribution system Biofilter 19 ------- References U.S. Environmental Protection Agency, CERI. "Handbook: Control Technologies for Hazardous Air Pollutants." EPA/ 625/6-86/014. NTIS PB91-228809/AS. Cincinnati, OH. September 1986. U.S. Environmental Protection Agency, Control Technol- ogy Center. "Industrial Wastewater Volatile Organic Com- pound Emissions-Background Information for BACT/ LAER Determinations." EPA-450/3-90-004. January 1990. U.S. Environmental Protection Agency, OAQPS, "Alterna- tive Control Technology Document-Organic Waste Pro- cess Vents." EPA-450/3-91-007. December 1990. U.S. Environmental Protection Agency, OAQPS. "Hazard- ous Waste, Treatment, Storage, and Disposal Facilities (TSDF)-Air Emission Models." EPA-45013-87-026. No- vember 1989. U.S. Environmental Protection Agency, OAQPS, Hazard- ous Waste Treatment, Storage, and Disposal Facilities (TSDF)-Background Information for Promulgated Organic Emission Standards for Process Vents and Equipment Leaks. EPA-45013-89-009, July 1990. U.S. Environmental Protection Agency, OAQPS. "Hazard- ous Waste TSDF-Background Information Document for Proposed RCRA Air Emission Standards." EPA-450/3- 89-023. (Will be available to the public upon proposal of the standard.) U.S. Environmental Protection Agency, OAQPS. "Hazard- ous Waste TSDF-Technical Guidance Document for RCRA Air Emission Standards for Process Vents and Equipment Leaks." EPA-450/3-89-021. July 1990. U.S. Environmental Protection Agency, ORD/HWERL "Pre- liminary Assessment of Hazardous Waste Pretreatment as an Air Pollution Control Technique." EPA-600/2-86- 028. NTIS PB86-172095/AS. March 1986. U.S. Environmental Protection Agency, OAQPS. "VOC Emissions from Petroleum Refinery Wastewater Sys- tems-Background Information for Proposed Standards." EPA-450/3-85-001a. February 1985. "Hazardous Waste Treatment, Storage, and Disposal Fa- cilities; Air Emission Standards for Volatile Organics Control." Federal Register, Vol. 52, pp. 3748-3770. Feb- ruary 5, 1987. "Hazardous Waste Treatment, Storage, and Disposal Fa- cilities-Organic Air Emission Standards for Process Vents and Equipment Leaks." Federal Register, Vol. 55, pp. 25454-25519. June 21, 1990. U.S. Environmental Protection Agency. "Air Stripping of Contaminated Water Sources-Air Emissions and Con- trols." Control Technology Center. Research Triangle Park, NC. Publication No. EPA-450/3-87-017. August 1987. U.S. Environmental Protection Agency. "Distillation Opera- tions in Synthetic Organic Chemical Manufacturing-Back- ground Information for Proposed Standards." EPA Publication No. EPA-450/3-83-005a. December 1983. U.S. Environmental Protection Agency, Air Pollution Train- ing Institute, RTP, NC 27711. "APTI Course 415 Control of Gaseous Emissions." EPA-450/2-81-005. December 1981. U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. "Hazardous Waste Treatment, Storage, and Disposal Facilities (TSDF)- Background Information for Promulgated Organic Emission Standards for Process Vents and Equip- ment Leaks." EPA-450/3-89-009, July 1990. U.S. Environmental Protection Agency, Office of Air Qual- ity Planning and Standards. "Hazardous Waste TSDF- Technical Guidance Document for RCRA Air Emission Standards for Process Vents and Equipment Leaks." EPA-450/3-89-021. July 1990. U.S. Environmental Protection Agency, Office of Air Qual- ity Planning and Standards. "OAQPS Control Cost Manual, 4th Edition." EPA-450/3-90-006. Research Tri- angle Park, NC 27711. January 1990. U.S. Environmental Protection Agency, Office of Air Qual- ity Planning and Standards. "RCRA TSDF Air Emis- sions-Background Technical Memoranda for Proposed Standards." EPA-450/3-86-009. October 1990. U.S. Environmental Protection Agency, Office of Research and Development, Hazardous Waste Engineering Re- search Laboratory. "Air Strippers and Their Emissions Control at Superfund Sites." Publication No. EPA-600/D- 88-153, NTIS PB88-239082. Cincinnati, OH. August 1988. U.S. Environmental Protection Agency, Office of Research and Development, Hazardous Waste Engineering Re- search Laboratory. "Preliminary Assessment of Hazard- ous Waste Pretreatment as an Air Pollution Control Technique." Publication No. EPA-600/2-86-028, NTIS PB86-172095/AS. March 1986. U.S. Environmental Protection Agency, Office of Research and Development, Industrial Effects Research Labora- tory. "Process Design Manual for Stripping of Organics." Cincinnati, OH. Publication No. EPA-600/2-84-I 39. Au- gust 1984. 20 ^GOVERNMENT PRINT^r III V fetM»i/30174 ------- |