Feasibility study relative to the aerobic cometabolic bioremediation of a chlorinated solvent-contaminated aquifer

The focus of this slurry microcosm study was to investigate the feasibility of the aerobic cometabolic bioremediation of an aquifer contaminated by a mixture of chlorinated aliphatic hydrocarbons (CAHs) containing vinyl chloride (VC: 1.5 mg/L in the aq. phase), cis- and trans-1,2-dichloroethylene (c-DCE and t-DCE: 0.5 and 0.4 mg/L), trichloroethylene (TCE: 0.2 mg/L), 1,1,2-trichloroethane (1,1,2-TCA: 0.12 mg/L) and 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA: 0.05 mg/L). Methane and propane were tested as growth substrates. Some microcosms were not bioaugmented, whereas others were additioned with either of two types of inocula derived from the aquifer’s indigenous biomass. In the non-bioaugmented tests the CAH biodegradation process began after remarkably long and non-reproducible lag-phases (36-264 days), while in the inoculated microcosms the lag-phases proved significantly shorter and more reproducible (0-10 days with the methane-utilizing inoculum, 10-15 days with the propane-utilizing inoculum). After the onset of the biodegradation processes, the entire 6-CAH mixture was completely degraded in the methane-utilizing microcosms, whereas a t-DCE residual-concentration effect was observed in several propane-utilizing microcosms. In all the tests the degradation rate of each CAH was characterized by sharp pulse-to-pulse increases during the initial 2 or 3 CAH spikes and by the subsequent achievement of a pseudo steady-state value, which proved to be influenced mainly by the number of chlorine atoms in the molecule (VC: 22-46 mg/(L d); cis-DCE: 1.6-4.1 mg/(L d); trans-DCE: 0.08-5.3 mg/(L d); TCE: 0.07-0.46 mg/(L d); 1,1,2-TCA: 0.05-0.65 mg/(L d); 1,1,2,2-TeCA: 0.01-0.07 mg/(L d)). The steady-state rates achieved in the bioaugmented microcosms were about equal to those attained in the non-inoculated microcosms.