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

The complex process of propionate oxidation in anaerobic systems in th e presence of excess sulfate is based on combinations of six biologica l reactions. Laboratory batch and continuous experiments were conducte d to study which of these reactions were predominant in propionate oxi dation by sulfate-reducing bacteria in the presence of acetogens and m ethanogens. The engineering significance of the feed propionate to sul fate ratio was studied. In batch serum bottle experiments, sulfide was more toxic to sulfate-reducing bacteria (SRB) than to acetogens and m ethanogens. Acetate was the least favored substrate for sulfate reduct ion. Two mechanisms appear possible under sulfate-limited conditions: (1) propionate use by both SRB and non-SRB acetogens and acetate and h ydrogen use by methanogens and (2) propionate use by non-SRB acetogens followed by acetate use by methanogens and hydrogen use by both SRB a nd methanogens. In the chemostats, at a feed propionate:sulfate ratio of 2.2 (sulfate-limited condition), the acetate formed was primarily u sed by methanogens. With a gradual decrease in feed propionate:sulfate ratio from 2.2 to 0.44 (sulfate-rich condition), SRB outcompeted meth anogens for acetate. In the chemostat study un-ionized HIS concentrati ons of up to 178 mg S/L (total sulfide 464 mg S/L) did not inhibit sul fate reduction.