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7 20 March 1996 AGE-NC Project No 95-0130 Page 10 of 23 611 PRINCIPLES ' For most SVE systems, a vacuum blower is used to volatilize hydrocarbons and draw the vapors into extraction wells installed and screened within the contaminant plume The vapors are drawn from the wells and treated in one of several possible treatment units (carbon canisters, internal combustion engines, thermal oxidizers, and catalytic oxidizers) Air infection wells may be installed outside the contaminant plume to increase air flow to the extraction unit ` An internal-combustion(IC) system draw vapors into a carburetor system of an engine and mixes the vapors with air prior to combustion Exhaust gasses (oxides of carbon and hydrogen) are broken down further in a catalytic converter before being emitted to the atmosphere IC systems require supplemental fuel, such as propane or natural gas, which can become increasingly expensive as vapor concentrations decrease below levels required for combustion Typically, internal combustion engines work effectively where in-situ hydrocarbon vapor concentrations approach the 40,000 ppmv required for peak performance of the engine As vapor concentrations decrease below 20,000 ppmv, supplemental fuel requirements become burdensome Another disadvantage of these systems is the noise generated by the engine, however, the noise can be mitigated by placing the unit inside an enclosure, such as a building Hydrocarbon vapors are routed through activated carbon filters in a carbon-canister system, where adsorption of the hydrocarbons onto activated carbon removes the hydrocarbons from the vapor stream These systems work best for soil impacted by low concentrations of hydrocarbons, but become expensive when high concentrations of hydrocarbon vapors necessitate frequent canister replacement and disposal of spent carbon Thermal oxidation systems destroy hydrocarbons by open-flame combustion Natural gas is commonly used as supplemental fuel, heating the extracted vapors to a combustion temperature of approximately 1400°F Relative to internal combustion systems, these systems may be slightly more expensive to purchase and install However, they can operate at higher air flow rates, and therefore remediate sites at a faster rate For this system to be effective, hydrocarbon concentrations should range from 5,000 ppmv to 30,000 ppmv They also operate at much lower noise levels than internal combustion systems Catalytic oxidation units provide another option for treating vadose zone contamination, particularly after other extraction systems have reached their effective limits due to lowered hydrocarbon concentrations These systems operate at temperatures of approximately 700°F, requiring less supplemental fuel than either thermal oxidizers or internal combustion engines Other requirements and limitations are similar to those for internal combustion and thermal oxidizer systems I