4.1.2.3 Description of Primary Treatment Technology for Alternative 3, Desorption and Vaporization Extraction System Alternative 3 consists of excavation and transportation of wastes to the CTF as described in Section 4.1.1. Wastes would then be treated by the DAVES process developed by RSI. Treated wastes would be disposed of in the landfill at the disposal facility, and treatment residuals such as condensed oils would be treated off site in a TSCA incinerator. The DAVES process and general support requirements for the DAVES are discussed below. Figures 4-12 and 4-13 present material flow diagrams for Scenario 1 and Scenario 2 respectively. Desorption and Vaporization Extraction System The DAVES process is designed to separate PCBs, VOCs, SVOCs, and volatile inorganic compounds (such as tetraethyl and tetraethyl lead) from soil, sludge, and sediment so that the treated material can be disposed of as a nonhazardous material. The PCBs are removed in a fluidized-bed reactor (vapor extractor). Hot turbulent air from a fuel-fired furnace contacts contaminated feed to vaporize the PCBs. The vaporized PCBs are removed in gas and liquid treatment systems and are subsequently concentrated into a pasty centrifuge sludge. DAVES is a modular, transportable system. One DAVES has been constructed and is currently inactive. This system is capable of nominally treating 8.5 tons per hour. Assuming a 70 percent mechanical online availability, a treatment rates of 80 percent of the nominal design treatment rate, and 24-hour-per-day treatment, the DAVES process would be able to treat the PCB- contaminated soil, sediment, and sludge from the six sites in about 7.0 years under Scenario 1 and 9.7 years under Scenario 2. RSI claims to have designed large-capacity DAVES that can treat The system can be moved from site to site by tractors that pull five flat-bed (8- by 25-foot) trailers. The process equipment is assembled at the site by crane. Most of the process equipment is permanently mounted on the trailers. Figure 4-14 presents the process flow diagram for the DAVES process. The DAVES process mainly consists of the following four smaller systems: Waste feed system Vapor extraction system Gas treatment system Liquid treatment system Each of these systems is discussed below. Waste Feed System The waste feed system measures and transfers contaminated materials from a stockpile to the vapor extractor. The contaminated material is transported to the waste storage hopper in small transportable bins. The bins are lifted with a small crane and dumped into the waste storage hopper. If convenient, the contaminated material can also be transported to the waste storage hopper using a front-end loader and dumped directly into the hopper. An enclosed belt conveyor transports contaminated material from the waste storage hopper to a weigh hopper. The weigh hopper uses weigh cells to weigh the materials. From the weigh hopper, the contaminated material is transferred to the waste feed hopper by an enclosed belt conveyor. The waste feed hopper transfers the material to the vapor extraction system. The feed rate into the DAVES process is metered and controlled by adjusting the speed of the belt that conveys the waste sediments from the waste feed hopper to the vapor extractor. The entire DAVES waste feed system, including hoppers and conveyors, is enclosed and a slight vacuum is maintained on the feed system to reducd the potential for organic vapor or dust emissions. Vapor Extraction System The vapor extractor vaporizes contaminants from the feed waste. Contaminated materials are fed to a fluidized-bed roller mill where the materials come in contact with a hot air stream. The hot air is generated by a fuel-fired burner located next to the extractor. The temperature range of the hot air flowing to the extractor is between 1,000 澹 and 1,500 澹. The hot air from the burner is blown into the vapor extraction unit through a manifold inlet at the bottom of the extractor at a rate sufficient to fluidize the feed solids. The temperature in the extractor is maintained at 320 澹 by adjusting the hot air flow rate. A grinding area in the vapor extractor uses plow blades and a grinding roller device to grind and turn the material while it is simultaneously fluidized by the hot air, thus providing effective hot air-to-waste contact. Moisture and organic contaminants are vaporized from the materials. The residence time in the vapor extraction unit is approximately 3 minutes at a maximum feed rate of 8.5 tons per hour. The volume (level) of the materials in the extractor is kept constant. The differential pressure measured across the volume of the bed indicates the level of the material. As feed materials accumulate in the extractor, the pressure drop across the bed increases, transmitting a signal to adjust the removal rate of clean, treated solids by the treated solids conveyor. The treated solids are withdrawn from the bottom of the vapor extractor at a rate controlled by the speed of the belt conveyor that transfers the treated solids to the storage bins. The treated solids are sprayed with water to prevent dust emissions before being discharged into storage bins. The treated solids may be disposed of on site or in a landfill in accordance with ARARs. The pressure in the extractor is maintained at a slightly negative pressure to control escape of fugitive emissions. An induced draft blower downstream of the extractor moves the gas out of the extractor to the gas treatment system and maintains the pressure balance in the extractor. Gas Treatment System The gas from the extractor contains hot air, vaporized water and organics, and entrained solid particles. The gas passes through a cyclone to remove large entrained particles and then through a baghouse to remove fine particles. The particles are collected in lock hoppers at the bottom of the cyclone and baghouse. The pressure in the gas treatment system is maintained at a slightly negative pressure by a series of small blowers to prevent fugitive emissions. The gas exiting the baghouse is fed by a blower to a venturi scrubber. The scrubber water is recycled and cooled by a finned tube cooler. The pressure drop forces most of the remaining entrained particulates, liquids, and condensable vapors to fall out of the gas stream. The temperature of the gas exiting the venturi scrubber is about 150 澹. Approximately half of the steam in the condensable vapor entering the venturi scrubber is condensed in this unit. The condensate is transferred to the contaminated water storage tank. The gas stream exiting the venturi scrubber is then sent to a direct-contact, counter-current washer with three stages of dual- flow decks. Almost all of the remaining condensable vapors are condensed by the washer's water stream, which is recycled and cooled in a second finned cooler. This condensate is also sent to the contaminated water storage tank where it is stored until it is treated in the liquid treatment system. The exit gas from the counter-current washer is fed by a blower to a chiller unit consisting of coils carrying coolant circulated through a refrigeration unit. The remaining condensable material is removed in this unit. This condensate is sent to the contaminated water storage tank. The chilled gases are passed over a reheat coil carrying recirculating water from the venturi scrubber to reduce its relative humidity before the gas enters the carbon adsorption unit, which consists of two activated carbon adsorbers in series. Each bed contains 3,600 pounds of fresh activated carbon. A third adsorber is available as a standby. During carbon replacement, the first unit in the series is taken off line and the carbon is replaced. The second unit in series then becomes the first unit in series, and the unused carbon adsorber becomes the second unit in series. Thus, two adsorbers are always in series. After the gas passes through the adsorbers, it is exhausted to the atmosphere through a stack. If an emergency shutdown of the carbon adsorbers occurred, the stack gas would be recycled to the vapor extractor and recirculated through the system. Liquid Treatment System The condensate from the venturi scrubber; direct-contact, counter-current washer; and chiller unit in the contaminated water storage tank is fed to a centrifuge, where the solids and the discrete organic liquid phase are separated from the water and form a concentrated sludge. The sludge is collected in drums and stored for off-site disposal. The water from the centrifuge is treated with polymer and alum filter aids and sent to a pressure filter system consisting of a prefilter and a multimedia filter. These units remove most of the colloidally suspended contaminants and any remaining solids. The filter effluent passes through two activated carbon adsorption beds arranged in series to remove dissolved organic contaminants and remaining colloidal contaminants. This treated effluent is stored in a tank. Backwash from the unit is returned to the contaminated water storage tank for treatment. Desorption and Vaporization Extraction System Support Requirements DAVES support requirements include site mobilization and access requirements, utilities, contaminated material pretreatment, and residuals post-treatment. The DAVES process requires a relatively level area of about 5,500 square feet. Additional area is also required for office space, on-site laboratory work, and pretreatment and post-treatment of soil. The DAVES process requires a gravel or concrete pad area and steel plate supports to support trailers and to prevent equipment from leaning or sliding in soft soil. Site access requirements for the DAVES process are minimal. The DAVES is transported to a site on five tractor trailer trucks. The roadbed must be able to support vehicles that may deliver the vapor extractor, tanks, APC equipment, and other equipment required for system operation. Utility requirements for the DAVES include electricity, fuel, and water for cooling, quenching treated soil, and fire protection. The system requires 800 kilowatts (kW) with 460-volt, three-phase electrical service. The system also requires about 250 gallons of makeup water per day. Approximately 100,000 cubic feet of natural gas per day is required to fuel the air heater. Pretreatment requirements include size separation and moisture content reduction. Size separation typically includes removing large debris from excavated wastes and screening to remove oversized (greater than 1-inch diameter) material. Oversized material may be crushed and treated in the DAVES or disposed of off site. Extreme care must be taken when screening contaminated materials to ensure that VOCs, asbestos, and metals are not emitted during screening operations. Several methods can be used to reduce soil moisture content. Blending is one option; however, soil blending may be difficult if large amounts of clay or very wet materials are blended. Air drying, preferably in an enclosed tent or building, can be used to dry soil at the CTF. Adequate space must be available on site to allow air drying. Pretreatment for soil moisture content reduction should only be considered if air emissions can be adequately controlled; otherwise, moisture would be removed in the DAVES at the expense of additional fuel. Post-treatment requirements include treated soil quenching, treatment of inorganics, disposal of treated soil, and treatment residuals disposal. The treated soil is very dry; therefore, soil quenching is needed to reduce emissions of dust contaminated with inorganics. Moisture removed from feed materials can be treated and used as quench water, or else an off-site source of quench water may be provided. Soil quenching should be conducted in an enclosed conveyor at the CTF to reduce dust emissions. Treated soil from the sites may contain high levels of metals and asbestos. Wastes containing inorganics must be treated to immobilize the inorganics before final disposal. Treatment typically involves solidification/stabilization of the treated solids because contaminants in these solids will not be affected by the DAVES. The treated soil may be backfilled on site or disposed of off site in a solid or hazardous waste landfill in accordance with ARARs. The DAVES process generates treatment residuals that require off- site treatment. The majority of these residuals are oils or condensed organics generated by the cooling and condensing units of the DAVES. The condensed organics may be stored in an on-site tank located in the materials handling building in accordance with applicable TSCA and RCRA regulations. The oils and condensed organics can be transported by tanker truck for treatment off site, usually in an approved incinerator. Other solid waste streams such as cyclone and baghouse fines are typically blended with the feed material to reduce the moisture content of the feed material and to retreat the fines. If the fines meet cleanup goals, they are typically disposed of along with the treated soil.