Performance of a Pilot-Scale Compost Biofilter Treating Gasoline Vapor

Abstract

A pilot-scale compost biofiltration system was operated at a gasoline soil vapor extraction site in Hayward, Calif. for one year. The media was composed of equal volumes of compost and perlite, a bulking agent. Supplements added included nitrogen (as KNO₃), a gasoline degrading microbial inoculum, buffer (crushed oyster shell), and water. The biofiltration system was composed of four identical units with outside dimensions of 1.2 × 1.2 × 1.2 m (4 × 4 × 4 ft) operated in an up-flow mode. The units were configured in parallel during the first eight months and then reconfigured to two parallel systems of two units in series. Air flux values ranged from 0.29 to 1.0 m³/m² per min. Inlet total petroleum hydrocarbon (TPH_gas) concentrations ranged from 310 to 2,700 mg/m³. The average empty bed contact time was 2.2 min. Following start-up, performance of the individual biofilters varied considerably for a seven-month period. The principal factor affecting performance appeared to be bed moisture content. Overall TPH_gas removals reached 90% for short periods in one unit, and BTEX removals were typically above 90%. Drying resulted in channeling and loss of bed activity. Management of bed moisture content improved over the study period, and recovery of system performance was achieved without replacement of bed media. Overall TPH_gas removals exceeded 90% during the final 50 days of the study.