Development of two-reactor system for cometabolic degradation of chlorinated ethylenes in groundwater

Abstract

Groundwater contaminated, predominantly with aromatics (m/p-xylene: 1750 μg/l, toluene: 1200 μg/l, ethyl benzene: 650 μg/l) and chlorinated ethylenes (cis-DCE: 1800 μg/l, TCE: 500 μg/l) were biologically treated in a fluidized-bed reactor with immobilized cells. The aromatics were completely decomposed, while the decomposition of cis-DCE and TCE were only approx. 20 and 15 %, respectively. Since the adsorption capacity for chlorinated ethylenes is quite low it limits the lifetime of adsorbents such as activated carbon, which is used for post treatment. Inefficiency increases costs and the costs for post treatment become a critical concern for the cost of the entire treatment process. This experiment achieved a significant improvement in decomposition efficiency for chlorinated ethylenes using cometabolism. The effectiveness of the various substrates used differed highly. Methanol and toluene were used as the substrate for a one-stage reactor. Methanol increased the decomposition efficiency up to 40 and 60 % for cis-DCE and TCE, respectively, at a retention time of 10 h. The substrate concentration in the feed was 200 mg/L. Toluene gave better decomposition efficiency, 92 % for cis-DCE and 76 % for TCE, with 20 mg/L of toluene in the feed. Methanol and methane were used as the substrate for a two reactor system. In this system, cells grown on methanol or methane in the first reactor were continuously fed into the second reactor and groundwater was fed only into the second reactor. When methanol $(384 mg/L^*d)$ was used, the decomposition efficiency for cis-DCE and TCE were 60 and 70 %, respectively. Similar decomposition efficiency was observed with only small amount of methane $(20.1 mg/L^*d)$. Further investigations on TCE degradation with a two reactor system applying model groundwater showed better decomposition efficiency while using toluene as substrate. 500 ㎛g/L$ TCE in the influent could be decomposed completely, for 1000 ㎛g/L$ TCE the decomposition ef...