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.
acetate (medium A) or acetate together with ethanol (medium AE) digested in high-rate anaerobic reactors. The purpose of this paper is to examine in more detail why the SRB were apparently not able to scavenge the major part of the electron flow under those conditions. Several factors which could affect the activities of the SRB were investigated. These include the possible feedback inhibition of H2S on SRB, iron availability for the SRB, the effect of the SRB inocula, substrate concentration, and the capacity of the SRB to colonize and adhere in the carrier matrix of the reactor. The effect of iron availability was investigated because it has been reported that the SRB have an absolute requirement for iron (2, 13, 17). A deficiency of this element could severely affect the growth of the SRB.
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