Aerobic cometabolic biodegradation of chlorinated aliphatic hydrocarbons by mixed cultures grown on methane or propane: slurry microcosm tests with and without bioaugmentation.

The long-term aerobic cometabolic biodegradation of a mixture of chlorinated aliphatic hydrocarbons (CAHs) including vinyl chloride (VC), cis- and trans-1,2-dichloroethylene (cis-DCE and trans-DCE), trichloroethylene (TCE), 1,1,2-trichloroethane (1,1,2-TCA) and 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) was monitored in slurry microcosms constructed with soil and groundwater sampled from a CAH-contaminated aquifer and fed with methane or propane as growth substrates. The efficacy of 2 types of inocula obtained from the indigenous biomass of the same aquifer was tested. The lag-phases for the onset of the CAH mixture degradation were significantly shorter and more reproducible in the inoculated tests (0-15 days) than in the non-inoculated ones (36-264 days). The cometabolic transformation of the 6 CAHs, successfully continued for up to 410 days, generally followed a first-order kinetic. The slopes of the linear interpolations of the plots of degradation rate versus concentration had the following average values respectively for the methane- and propane-utilizing consortia (1/day): VC, 29-24; trans-DCE, 10-0.59; cis-DCE, 3.1-8.3; TCE, 0.4-1.6; 1,1,2-TCA, 0.44-5.7; 1,1,2,2-TeCA, 0.29-1.1. The long-term rates obtained in the inoculated microcosms were analogous to those obtained in the non-inoculated ones. The second part of the study was aimed at studying the effect of the presence or absence of the CAH mixture on the process of production of significant quantities of CAH-degrading methane- and propane-utilizing biomass starting from small amounts of inoculum sampled from the non-inoculated slurry microcosms. To this goal, a methane-utilizing and a propane-utilizing consortium were grown in batch liquid/gas reactors both in the presence and in the absence of the CAH mixture. At different times during the growth process, inocula were sampled from the reactors and introduced into slurry microcosms set up with 5 types of aquifer materials. In all these tests the inocula led to the immediate onset of the CAH-mixture biodegradation. In the microcosms constructed with the aquifer material utilized in the first part of the study, the inoculation led to slopes of the rate-concentration plots generally higher than those obtained in the microcosms were the inocula were initially developed. The consortia grown in the liquid/gas reactors in the presence and in the absence of the CAH mixture were characterized by the same efficacy in terms of lag-times for the onset of the CAH degradation process and of long-term CAH degradation rates in the inoculated slurry microcosms. The results of this study confirm the effectiveness of methane and propane as growth substrates for the aerobic cometabolic biodegradation of CAHs. We consider of particular interest the long-term aerobic transformation of 1,1,2,2-TeCA, a compound that is generally considered non biodegradable in aerobic conditions.