Removal of tetrachloroethylene in an anaerobic column bioreactor

Removal of tetrachloroethylene (perchloroethylene; C₂Cl₄) by microbial consortia from two sites with different C₂Cl₄ exposure histories was examined in a bench-scale anaerobic column bioreactor. It was hypothesized that optimal removal would be observed in the reactor packed with sediments having an extensive exposure history. Microbial consortia were enriched from hyporheic-zone (HZ) sediments from the Portneuf aquifer near Pocatello, Idaho, and from industrial-zone (IZ) sediments from a highly contaminated aquifer in Portland, Oregon. Lactate and acetate were the electron donors during experiments conducted over 9 and 7 months for HZ and IZ sediments, respectively. In the HZ bioreactor, the retention time ranged from 31 h to 81 h, and inlet C₂Cl₄ concentrations ranged from 0.1 ppm to 1.0 ppm. Dechlorination of C₂Cl₄ averaged 60% and reached a maximum of 78%. An increase in C:N from 27:1 to 500:1 corresponded to an 18% increase in removal efficiency. Trichloroethylene production corresponded to decreased effluent C₂Cl₄; further intermediates were not detected. In the IZ bioreactor, the retention time varied from 34 h to 115 h; the inlet C₂Cl₄ concentration was 1.0 ppm. C₂Cl₄ removal averaged 70% with a maximum of 98%. Trichloroethylene and cis-dichloroethylene were detected in the effluent. Increases in C:N from 50:1 to 250:1 enhanced dechlorination activity.