Effect of gas velocity and influent concentration on biofiltration of gasoline off-gas from soil vapor extraction

This study was conducted to evaluate the effects of gas inlet concentration and velocity on the biofiltration of gasoline vapor. Gasoline vapor was treated using a compost biofilter operated in an upflow mode for about 3 months. The inlet concentration of gasoline total petroleum hydrocarbon (TPH) ranged from about 300 to 7000 mg m−3 and gas was injected at velocities of 6 and 15 m h−1 (empty bed residence time (EBRT) = 10 and 4 min, respectively). The maximum elimination capacities of TPH at 6 and 15 m h−1 found in this research were over 24 and 19 g m−3 of filling material h−1, respectively. TPH removal data was fit using a first-order kinetic relationship. In the low concentration range of 300–3000 mg m−3, the first-order kinetic constants varied between about 0.10 and 0.29 min−1 regardless of gas velocities. At TPH concentrations greater than 3000 mg m−3, the first-order kinetic constants were about 0.09 and 0.07 min−1 at gas velocities of 6 m h−1 and 15 m h−1, respectively. To evaluate microbial dynamics, dehydrogenase activity, CO2 generation and microbial species diversity were analyzed. Dehydrogenase activity could be used as an indicator of microbial activity. TPH removal corresponded well with CO2 evolution. The average CO2 recovery efficiency for the entire biofilter ranged between 60% and 70%. When the gas velocity was 6 m h−1, most of the microbial activity and TPH removal occurred in the first quarter of the biofilter. However, when the gas velocity was 15 m h−1, the entire column contributed to removal. Spatial and temporal variations in the biofilter microbial population were also observed. Nearly 60% of the colonies isolated from the compost media prior to biofiltration were Bacillus. After 90 days of biofiltration, the predominant species in the lower portion (0–50 cm) of the filter were Rhodococcus, while Pseudomonas and Acinetobacter dominated the upper portion (75–100 cm).