Modeling the production of and competition for hydrogen in a dechlorinating culture

A comprehensive biokinetic model employing Michaelis-Menten-type kinetics, H(2) thresholds, and thermodynamic limitations oh donor fermentation was used to describe both fermentation of electron donors and competition for the evolved H(2) between hydrogenotrophic tetrachloroethane dechlorinators and methanogens. Model simulations compared favorably to experimental data where delivery of H(2) to a tetrachloroethene dechlorinator was accomplished using the donors butyric acid, ethanol, lactic acid, and propionic acid. Fermentations of the different donors were characterized by different dynamic patterns, of H(2) generation that were captured successfully by the model. Experimental data and model simulations show that the ability to use H(2) at appreciable rates at low levels provides a competitive advantage to dechlorinators over methanogens. Slowly fermented substrates producing lower H(2) levels-kinetically accessible to dechlorinators, but too low for significant use by methanogenic competitors-were more effective and persistent "selective" stimulators of dechlorination than rapidly fermented substrates producing higher Ha levels-accessible to both dechlorinators and methanogens. Model simulations suggest that adding excessive levels of rapidly fermented, high H(2)-level-generating donors in an attempt to overcome competition, instead results in a dominant methanogen population and an eventual failure of dechlorination. When stimulating dechlorination, the quality of the donor as well as; the quantity added must be considered.