Vapor Extraction/Bioventing Sequential Treatment of Soil Contaminated with Volatile and SemiVolatile Hydrocarbon Mixtures

A cost-effective removal strategy was studied in bench-scale columns that involved vapor extraction (SVE) and bioventing (SBV) sequential treatment of toluene- and decane-contaminated soil. By using GC analysis to measure hydrocarbon concentrations, CO₂, and O₂ content values in the outlet gas, the removal kinetics were determined as was the contribution of evaporation and biodegradation to the removal of contaminants from soil. The effect of operating mode on treatment performance was studied at a continuous air flow and consecutively at two different flow rates, and compared with an intermittent (pulse) flow rate. The two-rate flow was required due to the inhibitory effect of toluene on indigenous microorganisms at above 75% of the toluene saturation concentrations in the gas phase. The intermittent flow was controlled by the O₂ content values in the soil gas, which at above 4% did not limit biodegradation. To reach comparable removal efficiency at the constant flow, about three times less air was required for toluene than for decane. This air volume could be reduced, in the case of decane, by a factor of 1.6 and 2.9, at the two-rate and intermittent flow, respectively. A higher contribution of biodegradation to the overall removal of hydrocarbon will lower hydrocarbon concentrations in the off-gases to be treated. Together with the decreased amount of air used, this can reduce the overall remediation costs. The overall process can be better understood by determining the degree of contaminants removal by evaporation and biodegradation in the experimental set up.