COMPARATIVE KINETICS OF HYDROGEN UTILIZATION FOR REDUCTIVE DECHLORINATION OF TETRACHLOROETHENE AND METHANOGENESIS IN AN ANAEROBIC ENRICHMENT CULTURE

Anaerobic microorganisms that reductively dechlorinate tetrachloroethe ne (PCE) must often compete with methanogens for H-2. This study compa red the kinetics of H-2 utilization between the two types of organisms at 35 degrees C under conditions of continuous agitation. Limiting le vels of H-2 were administered to 160-mL serum bottles seeded with a PC E/butyric acid enrichment culture; H-2, methane, and vinyl chloride we re tracked over time using headspace samples. Measured half-velocity c onstants with respect to H-2-K-s(H-2) values +/- 95% Cl-for methanogen esis and dechlorination were 960 +/- 180 and 100 +/- 50 nM, respective ly. Mass-transfer equations were used to calculate aqueous H-2 concent rations at the half-velocity point from headspace measurements. The po ssible effect on K-s(H-2) values arising from interconversion between H-2 and formate through an active formate/H-2 lyase system was examine d by comparing results from formate-fed and H-2-fed bottles. Only meth anogens in the culture were apparently capable of using formate; hence , the measured methanogenic K-s(H-2) was dependent on which electron d onor was administered. The nearly 10-fold difference in K-s(H-2) betwe en methanogens and dechlorinators suggests that the deliberate choice of an electron donor whose fermentation results in a slow, steady, and low-level release of H-2 over time could maximize dechlorination pote ntial while minimizing methanogenic competition for H-2.