Dechlorination of high concentrations of tetrachloroethene in microcosms and a laboratory sand column

High concentrations (up to 88 mg/L) of tetrachloroethene (PCE) were shown to be dechlorinated in anaerobic microcosms and in a laboratory sand column operated under anaerobic, methanogenic conditions. Initial experiments were conducted in microcosms prepared using sediment from two sites known to have been contaminated with chlorinated hydrocarbons. Anaerobic sand column experiments were performed using an inoculum from the secondary anaerobic digester of a local municipal sewage treatment plant. Experiments that attempted to couple the anaerobic and aerobic processes were carried out in a single sand column in which anaerobic and aerobic zones were established. All experiments monitored the disappearance of PCE and the formation and degradation of PCE dechlorination products. Acetate was provided as the primary substrate. Results of this study add to the body of knowledge necessary to evaluate in situ bioremediation as an alternative for clean up of sites where high concentrations of PCE are present in groundwater.

The extent of PCE dechlorination in anaerobic sediment microcosms varied depending upon the source of the sediments. PCE removal rates in active sediment microcosms averaged 1.58 µmole/L/day and proceeded stepwise to trichloroethene (TCE) and cis-1,2-dichloroethene (CIS). No vinyl chloride (VC) was detected in any sediment microcosms.

PCE dechlorination in the anaerobic sand column occurred, after a lag time, at influent PCE concentrations of 32 and 88 mg/L. Dechlorination proceeded at a minimum volumetric removal rate of 160 µmole/L/day. At the higher PCE concentration, dechlorination proceeded through TCE to both CIS and trans-1,2-dichloroethene (TRANS). At the lower influent PCE concentration, VC and traces of ethene (ETH) were formed.

Methane content of the column offgas varied from 40 to 60%. Acetate utilization in the column was rapid and complete. Electron balance calculations indicated that ~1% of the reducing power of acetate was used in the reductive dechlorination of PCE.

During operation of the sand column in a combined anaerobic/aerobic mode, methanogenesis was sustained in the lower portion of the column while a mixed culture of methanotrophs was inoculated into the upper portion. Methanotrophs proved difficult to establish in the sand column environment. However, detection of both the CIS and TRANS epoxides showed that small amounts of the dichloroethene (DCE) isomers were degraded by the methanotrophs.