Biotransformation of tetrachloroethylene by anaerobic attached-films at low temperatures

Abstract

This study was conducted to examine the feasibility of using an anaerobic attached-film expanded-bed (AAFEB) process for the treatment of tetrachloroethylene (PCE) at 15°C. A laboratory-scale continuous-flow reactor, with an expanded-bed volume of 900 ml, was operated at hydraulic retention times of 1.8-4 h and influent PCE concentrations of 8–12 mg/l. Small samples (50 ml) of attached film media were used for batch testing 3–7 mg/l of PCE in a separate 300 ml AAFEB reactor. The attached films were a mixed anaerobic consortium grown on diatomaceous earth support particles under methanogenic conditions. Sucrose was used as an external electron donor and growth substrate. Reductive dechlorination of PCE to trichloroethylene (TCE), cis-1,2-dichloroethylene (DCE), vinyl chloride (VC) and ethylene (ETH) was observed. The conversion efficiency of PCE and TCE to lesser chlorinated compounds and ETH was above 98% during continuous-flow testing. VC accumulated as the major dechlorination product and ETH was produced at very low rates. The maximum PCE dechlorination rate, q max, was 5.33 mg PCE/g volatile solids-day (32.1 μmol/g VS-d) and the one-half velocity coefficient, K s, was 0.009 mg PCE/l (0.054 μM) under continuous-flow conditions. Since the AAFEB carried more than 20 g volatile solids per liter of bed, low temperature conversion rates would be expected to exceed 60 mg PCE/l bed-day. This indicates removal efficiencies greater than 99% could be obtained at hydraulic retention times of less than 1 h at ambient groundwater temperatures with this process.