Biodegradation of trichloroethylene in continuous-recycle expanded-bed bioreactors.

Experimental bioreactors operated as recirculated closed systems were inoculated with bacterial cultures that utilized methane, propane, and tryptone-yeast extract as aerobic carbon and energy sources and degraded trichloroethylene (TCE). Up to 95% removal of TCE was observed after 5 days of incubation. Uninoculated bioreactors inhibited with 0.5% Formalin and 0.2% sodium azide retained greater than 95% of their TCE after 20 days. Each bioreactor consisted of an expanded-bed column through which the liquid phase was recirculated and a gas recharge column which allowed direct headspace sampling. Pulses of TCE (20 mg/liter) were added to bioreactors, and gas chromatography was used to monitor TCE, propane, methane, and carbon dioxide. Pulsed feeding of methane and propane with air resulted in 1 mol of TCE degraded per 55 mol of substrate utilized. Perturbation studies revealed that pH shifts from 7.2 to 7.5 decreased TCE degradation by 85%. The bioreactors recovered to baseline activities within 1 day after the pH returned to neutrality.