MODELING THE PRODUCTION OF AND COMPETITION FOR HYDROGEN IN A DECHLORINATING CULTURE

A comprehensive biokinetic model employing Michaelis-Menten-type kinet ics, H-2 thresholds, and thermodynamic limitations oh donor fermentati on was used to describe both fermentation of electron donors and compe tition for the evolved H-2 between hydrogenotrophic tetrachloroethane dechlorinators and methanogens. Model simulations compared favorably t o experimental data where delivery of H-2 to a tetrachloroethene dechl orinator was accomplished using the donors butyric acid, ethanol, lact ic acid, and propionic acid. Fermentations of the different donors wer e characterized by different dynamic patterns, of H-2 generation that were captured successfully by the model. Experimental data and model s imulations show that the ability to use H-2 at appreciable rates at lo w levels provides a competitive advantage to dechlorinators over metha nogens. Slowly fermented substrates producing lower H-2 levels-kinetic ally accessible to dechlorinators, but too low for significant use by methanogenic competitors-were more effective and persistent ''selectiv e'' stimulators of dechlorination than rapidly fermented substrates pr oducing higher Ha levels-accessible to both dechlorinators and methano gens. Model simulations suggest that adding excessive levels of rapidl y fermented, high H-2-level-generating donors in an attempt to overcom e competition, instead results in a dominant methanogen population and an eventual failure of dechlorination. When stimulating dechlorinatio n, the quality of the donor as well as; the quantity added must be con sidered.