Sulfate Reduction Relative to Methane Production in High-Rate Anaerobic Digestion: Technical Aspects

The effect of different substrates and different levels of sulfate and sulfide on methane production relative to sulfate reduction in high-rate anaerobic digestion was evaluated. Reactors could be acclimated so that sulfate up to a concentration of 5 g of sulfate S per liter did not significantly affect methanogenesis. Higher levels gave inhibition because of salt toxicity. Sulfate reduction was optimal at a relatively low level of sulfate, i.e., 0.5 g of sulfate S per liter, but was also not significantly affected by higher levels. Both acetoclastic and hydrogenotrophic methane-producing bacteria adapted to much higher levels of free H 2 S than the values reported in the literature (50% inhibition occurred only at free H 2 S levels of more than 1,000 mg/liter). High levels of free H 2 S affected the sulfate-reducing bacteria only slightly. Formate and acetate supported the sulfate-reducing bacteria very poorly. In the high-rate reactors studied, intensive H 2 S formation occurred only when H 2 gas or an H 2 precursor such as ethanol was supplied.