Naturally occurring populations of Dehalococcoides ethenogenes (DE), as well as bioaugmentation cultures, have been used in situ to biodegrade dichloroethene (DCE) and vinyl chloride (VC), the recalcitrant anaerobic daughter products of solvents such as tetrachloroethene (PCE) and trichloroethene (TCE). Bioremediation Consulting Inc. (BCI), of Watertown, MA, has identified DE at about 50% of 200 U.S. sites contaminated with these solvents. For sites that did not contain these bacteria, BCI has developed culturing procedures to adapt its DE culture to a variety of site-specific conditions.
At complex industrial sites, the ability of microbial systems to degrade multiple chlorinated contaminants is the key to successful application of low-cost bioremediation. At these sites, chlorinated ethenes, such as TCE, may occur in the presence of additional chemicals, particularly 1,1,1-trichloroethane (TCA), which has been shown to inhibit many of the naturally-occurring strains of DE. Duhamel et al. (2002, Water Research vol 36) reported inhibition of DE by as little as 700 parts per billion of TCA. For mixtures of chlorinated ethenes and ethanes, additional special strain of bacteria, Dehalobacter (Db), is required for dechlorination of 1,1-DCA, the recalcitrant daughter product of TCA. TCA has been found in at least 809 of the more than 1,600 sites placed on the National Priorities List, and is the seventh most prevalent volatile organic compound in groundwater at concentrations over 0.2 parts per billion.
BCI has identified at least four naturally-occurring microbial communities at industrial sites in which TCA-tolerant DE and Dehalobacter species are simultaneously dechlorinating TCE to non-toxic ethene, and TCA to chloroethane (CA). (CA can subsequently be degraded to CO2 under aerobic conditions, as well as in soil under sulfate-reducing conditions.) Using a “co-culturing” process, BCI has developed several microbial cultures that simultaneously dehalogenate chlorinated ethenes and chlorinated ethanes. Continuing the co-culturing process in site groundwater, robust site-specific cultures have been created.
A site-specific co-culture of DE and Db is currently being grown for the cleanup for a Pennsylvania Superfund site containing not only 100 ppm DCE and 17 ppm TCA to, but also up to 15,000 ppm of methanol in some well areas. Working with O’Brien and Gere Engineers, BCI conducted a microcosm study in site groundwater in which the BCI co-culture was initially inhibited by methanol, but was eventually successful in degrading DCE and TCA, as well as methanol, to non-detectable concentrations. BCI is growing this site-specific co-culture for a pilot remediation at this site. We describe our approach to developing this co-culture, as well as strategies for sites having a variety of conditions.