Effect of Hydrogen on Reductive Dechlorination of Chlorinated Ethenes

Abstract

A methanogenic fluidized bed reactor (FBR) fed with lactate and tetrachloroethene (PCE) was operated for 14 months to study the effect of electron donor and PCE loading on chloroethene dechlorination rates. Lactate was fed continu ously at 200 mg/L (2.2 mmol/L), and the influent PCE feed concentration was increased stepwise from 3.5 to 160 μmol/L. Vinyl chloride (VC) and ethene accounted for 80% and 20%, respectively, of the PCE dechlorination. Batch tests with various electron donors showed that H2, propionate, and lactate supported dechlorination of PCE, trichloroethene (TCE), cis-dichloroethene (c-DCE), and VC, whereas no dechlorination was observed with acetate or in the absence of an electron donor. Different short-term steady H2 concentrations were obtained by adjusting the FBR influent lactate feed concentration, and the effect of H2 concentra tion on the rate of chloroethene dechlorination was determined. Dechlorination rates for PCE, TCE, c-DCE, and VC showed a Michaelis−Menten relationship with H2 partial pressure. The half-velocity coefficients for H2 utilization by dechlorinators ranged from 12 to 28 ppm for the chloroethenes and are at least an order of magnitude lower than values reported for methanogens. This implies that dechlorinating bacteria can out-compete methanogens for H2 utilization at low H2 concentration.