SVE is the preferred presumptive remedy. SVE has been selected most frequently to address VOC contamination at Superfund sites, and performance data indicate that it effectively treats waste in place at a relatively low cost. In cases where SVE will not work or where uncertainty exists regarding the ability to obtain required cleanup levels, thermal desorption may be the most appropriate response technology. In a limited number of situations, incineration may be most appropriate.
Another commonly used technology, bioventing, uses a similar approach to vapor extraction in terms of equipment type and layout but uses air injection rather than extraction and has a different objective: The intent of bioventing is to use air movement to provide oxygen for aerobic degradation using either indigenous or introduced microorganisms. While some organic materials are usually brought to the surface for treatment with the exhaust air, additional degradation is encouraged in situ. This difference in approach renders less volatile materials (particularly fuel products such as diesel fuel) amenable to the process because volatilization into the soil air is not the primary removal process.
The AFCEE Bioventing Initiative currently encompasses more than 145 fuel sites at 56 military installations, including one Marine, three Army, and one Coast Guard facility. Approximately 50% of the current systems are full scale. As of September 1995, approximately 125 are installed and operating. The remainder are to be installed.
It is necessary to know other subsurface information to provide remediation of VOCs in the ground water. Treatability studies to characterize the biodegradability may be needed for any biodegradation technology. Treatability studies are usually necessary to ensure that the contaminated ground water can be treated effectively at the design flow. A subsurface geologic characterization would be needed for any isolation or stabilization technologies. Ground water models are also often needed to predict flow characteristics, changes in contaminant mixes and concentrations, and times to reach cleanup levels.
The most commonly used technologies to treat nonhalogenated VOCs in ground water, surface water, and leachate are air stripping and carbon adsorption. These are both ex situ technologies requiring ground water extraction.
Air stripping involves the mass transfer of volatile contaminants from water to air. This process is typically conducted in a packed tower or an aeration tank. The generic packed tower air stripper includes a spray nozzle at the top of the tower to distribute contaminated water over the packing in the column, a fan to force air countercurrent to the water flow, and a sump at the bottom of the tower to collect decontaminated water. Auxiliary equipment that can be added to the basic air stripper includes a feed water heater (normally not incorporated within an operational facility because of the high cost) and an air heater to improve removal efficiencies, automated control systems with sump level switches and safety features such as differential pressure monitors, high sump level switches and explosion proof components, and discharge air treatment systems such as activated carbon units, catalytic oxidizers, or thermal oxidizers. Packed tower air strippers are installed either as permanent installations on concrete pads, or as temporary installations on skids, or on trailers.
Liquid phase carbon adsorption is a full-scale technology in which ground water is pumped through a series of vessels containing activated carbon to which dissolved contaminants adsorb. When the concentration of contaminants in the effluent from the bed exceeds a certain level, the carbon can be regenerated in place; removed and regenerated at an off-site facility; or removed and taken off-site for disposal. Carbon used for explosives- or metals-contaminated ground water must be removed and properly disposed of. Adsorption by activated carbon has a long history of use in treating municipal, industrial, and hazardous wastes.
Other fairly common technologies used in separating contaminants from ground water are distillation and membrane pervaporation.
Membrane pervaporation is a process that uses permeable membranes that preferentially adsorb volatile organic compounds (VOCs) from contaminated water. Contaminated water first passes through a heat exchanger, raising the water temperature. The heated water then enters the pervaporation module, containing membranes composed of a nonporous organophilic polymer, similar to silicone rubber, formed into capillary fibers. VOCs diffuse by vacuum from the membrane-water interface through the membrane wall. Treated water exits the pervaporation module, while the organic vapors travel from the module to a condenser where they return to the liquid phase. The condensed organic materials represent only a fraction of the initial wastewater volume and may be subsequently disposed of at a cost savings.
Three technologies that are most commonly used to treat nonhalogenated VOCs in air emissions/off-gases are carbon adsorption, catalytic oxidation, and thermal oxidation.
Carbon adsorption is a remediation technology in which pollutants are removed from air by physical adsorption onto the carbon grain. Carbon is 'activated' for this purpose by processing the carbon to create porous particles with a large internal surface area (300 to 2,500 square meters per gram of carbon) that attracts and adsorbs organic molecules as well as certain metal and other inorganic molecules.
Commercial grades of activated carbon are available for specific use in vapor-phase applications. The granular form of activated carbon is typically used in packed beds through which the contaminated air flows until the concentration of contaminants in the effluent from the carbon bed exceeds an acceptable level. Granular activated carbon systems typically consist of one or more vessels filled with carbon connected in series and/or parallel operating under atmospheric, negative, or positive pressure. The carbon can then be regenerated in place, regenerated at an off-site regeneration facility, or disposed of, depending upon economic considerations.
Catalytic oxidation is a relatively new alternative for the treatment of VOCs in air streams resulting from remedial operations. VOCs are thermally destroyed at temperatures typically ranging from 600 to 1,000 ° by using a solid catalyst. First, the contaminated air is directly preheated (electrically or, more frequently, using natural gas or propane) to reach a temperature necessary to initiate the catalytic oxidation of the VOCs. Then the preheated VOC-laden air is passed through a bed of solid catalysts where the VOCs are rapidly oxidized.
In most cases, the process can be enhanced to reduce auxiliary fuel costs by using an air-to-air heat exchanger to transfer heat from the exhaust gases to the incoming contaminated air. Typically, about 50% of the heat of the exhaust gases is recovered. Depending on VOC concentrations, the recovered heat may be sufficient to sustain oxidation without additional fuel. Catalyst systems used to oxidize VOCs typically use metal oxides such as nickel oxide, copper oxide, manganese dioxide, or chromium oxide. Noble metals such as platinum and palladium may also be used. However, in a majority of remedial applications, nonprecious metals (e.g., nickel, copper, or chromium) are used. Most commercially available catalysts are proprietary.
Thermal oxidation equipment is used for destroying contaminants in the exhaust gas from air strippers and SVE systems. Probably fewer than 100 oxidizers have been sold to treat air stripper effluents; most of these units are rated less than 600 scfm. Typically, the blower for the air stripper or the vacuum extraction system provides sufficient positive pressure and flow for thermal oxidizer operation.
Thermal oxidation units are typically single chamber, refractory-lined oxidizers equipped with a propane or natural gas burner and a stack. Lightweight ceramic blanket refractory is used because many of these units are mounted on skids or trailers. Thermal oxidizers are often equipped with heat exchangers where combustion gas is used to preheat the incoming contaminated gas. If gasoline is the contaminant, heat exchanger efficiencies are limited to 25 to 35% and preheat temperatures are maintained below 530 ° to minimize the possibility of ignition occurring in the heat exchanger. Flame arrestors are always installed between the vapor source and the thermal oxidizer. Burner capacities in the combustion chamber range from 0.5 to 2 million Btus per hour. Operating temperatures range from 1,400 to 1,600 °, and gas residence times are typically 1 second or less.
A treatment train is the combination of different treatment technologies. A system diagram of a common treatment train for nonhalogenated VOCs is illustrated below. A soil vapor extraction system is used to pull VOC vapor from the contaminated soil. An air sparging system delivers air below the water table to enhance bioremediation. After passing through a liquid/vapor separator, the VOC contaminated water and gas are pumped to a liquid phase granulated activated carbon (GAC) adsorption system. The GAC system removes VOCs in the water. The effluent is either discharged or pumped back as fluid recycling. Meanwhile, VOCs in the gas stream are completely destroyed through catalytic oxidation. Carbon dioxide and water vapor are discharged to the atmosphere as the final products of oxidation.