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

A comprehensive biokinetic model employing Michaelis-Menten-type kinetics, H₂ thresholds, and thermodynamic limitations on donor fermentation was used to describe both fermentation of electron donors and competition for the evolved H₂ between hydrogenotrophic tetrachloroethene dechlorinators and methanogens. Model simulations compared favorably to experimental data where delivery of H₂ 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 H₂ generation that were captured successfully by the model. Experimental data and model simulations show that the ability to use H₂ at appreciable rates at low levels provides a competitive advantage to dechlorinators over methanogens. Slowly fermented substrates producing lower H₂ 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 H₂ levels-accessible to both dechlorinators and methanogens. Model simulations suggest that adding excessive levels of rapidly fermented, high H₂-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.