INTERACTIONS AMONG SULFATE REDUCERS, ACETOGENS, AND METHANOGENS IN ANAEROBIC PROPIONATE SYSTEMS

The complex process of propionate oxidation in anaerobic systems in the presence of excess sulfate is based on combinations of six biological reactions. Laboratory batch and continuous experiments were conducted to study which of these reactions were predominant in propionate oxidation by sulfate-reducing bacteria in the presence of acetogens and methanogens. The engineering significance of the feed propionate to sulfate ratio was studied. In batch serum bottle experiments, sulfide was more toxic to sulfate-reducing bacteria (SRB) than to acetogens and methanogens. Acetate was the least favored substrate for sulfate reduction. Two mechanisms appear possible under sulfate-limited conditions: (1) propionate use by both SRB and non-SRB acetogens and acetate and hydrogen use by methanogens and (2) propionate use by non-SRB acetogens followed by acetate use by methanogens and hydrogen use by both SRB and methanogens. In the chemostats, at a feed propionate:sulfate ratio of 2.2 (sulfate-limited condition), the acetate formed was primarily used by methanogens. With a gradual decrease in feed propionate:sulfate ratio from 2.2 to 0.44 (sulfate-rich condition), SRB outcompeted methanogens for acetate. In the chemostat study un-ionized HIS concentrations of up to 178 mg S/L (total sulfide 464 mg S/L) did not inhibit sulfate reduction.