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

TLDR

Dechlorination stimulation depends on both donor quality and quantity. A biokinetic model incorporating Michaelis–Menten kinetics, H₂ thresholds, and thermodynamic limits was developed to capture donor fermentation and H₂ competition between dechlorinators and methanogens. Simulations matched experiments and showed that low‑level, slowly fermented donors give dechlorinators a competitive edge, whereas high‑H₂ donors favor methanogens and hinder dechlorination.

Abstract

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.