Modeling the Production of and Competition for Hydrogen in a Dechlorinating Culture

Abstract

A comprehensive biokinetic model employing Michaelis−Menten-type kinetics, H 2 thresholds, and thermodynamic limitations on donor fermentation was used to describe both fermentation of electron donors and competition for the evolved H 2 between hydrogenotrophic tetrachloroethene 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 H 2 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.