Degradation of tetrachloroethene by an anaerobic/aerobic bioprocess

In cooperation with the University of Stuttgart, Germany, we have developed an economic two-stage process for the biodegradation of chlorinated ethenes. The process consists of an anaerobic step for the partial reductive dehalogenation of tetrachloroethene (PCE) and trichloroethene (TCE). In a subsequent aerobic step the products of anaerobic dehalogenation cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) are completely dechlorinated by an ethene-utilizing culture. A 30-liter loop reactor (anaerobic stage) was combined with a 30-liter trickling filter bioreactor (aerobic stage). The anaerobic stage was inoculated with the strictly anaerobic bacterium Dehalospirillum multivorans. For the aerobic stage we used an ethene-utilizing mixed culture (K20) which exhibits high cometabolic activity against cDCE and VC. Based on the structural analogy between ethene and the chlorinated ethenes the compounds are oxidized by the same initial enzymes. Ethene serves as source of carbon and energy, electron donor and thus maintains the viability of the cells. It also serves as an inducer of an initial monooxygenase which is responsible for the cooxidative activity of the cells. The mixed culture K20 consists of at least five bacterial strains. All of them can cooxidize chloroethenes and show high tolerance against cDCE (up to 6 mM in the liquid phase). The dechlorination of cDCE (200 æM) was not perturbed by the presence of TCE (100 mu M) or PCE (100 mu M). The transformation yields [mol of chlorinated ethene/mol of ethene] exceeds comparative data in the literature (0.51 and 0.61 for cDCE and VC, respectively). The lab scale bioreactors have now been operated for more than one year under non-sterile conditions. During this continuous process the rates of reductive dechlorination were constant, being 2.7 mu mol l(exp -1) h(exp -1) for PCE and 3.6 mu mol l(exp -1) h(exp -1) for TCE. The cDCE degradation rate was 3.9 æmol l(exp -1) h(exp -1) in the aerobic reactor. Basing on these results a pilot scale experiment with a reactor volume of 2 x 10 m3 was started.