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19 June 2000 AGE-NC Project No 96-0232 Page 19 of 35 Thermal oxidation systems destroy hydrocarbons by open-flame combustion Natural gas is commonly used as supplemental fuel, heating the extracted vapor stream to a combustion temperature of approximately 1,400°F Relative to internal combustion systems,these systems may be sllghtlymore 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 8 12 Required Soil Conditions The blower in a soil vapor extraction system creates an induced vacuum in the subsurface,generating vapor flow towards the area of lowered air pressure (the extraction point) through the soil pore spaces The efficiency of this process is proportional to soil porosity and intrinsic permeability Intrinsic permeability Is the measure of the ability of soils to transmit fluids, porosity is the percentage of soil that Is void of material A high moisture content in soils can reduce the effectiveness of SVE by restricting the flow of air through soil pores Generally,coarser grained soils (I e , sands) are the desired material for a SVE technology because coarser grained soils have a greater intrinsic permeability than finer grained soils (I e , silts and clays) 8 13 Monitoring Activities Monitoring of vapor extraction systems involves weekly measurement of vapor concentrations at the inlet to the destruction unit and periodic service and repair Systems can be equipped with an automatic telecommunication system to alert the operator of system malfunction or failure Vapor samples should be collected at least monthly for laboratory analysis to monitor the efficiency of the remediation program When the concentrations of extracted vapor have decreased and stabilized after a certain period of operation,confirmation borings are drilled to collect soil samples for quantitative analysis In the laboratory to document the degree of cleanup obtained Advanced GcoEmironmental,Inc