Sulphate stimulated the rate of conversion of acetic acid to methane in enriched cultures as well as in liquid from anaerobic fermenters fed with wastes, but the effect depended on the soluble iron content of the culture. At low soluble iron contents, i.e. 0–02–0–05 mM, sulphate had little or no immediate (after 7–10 d) effect, but at 0–5 mM or higher soluble iron content, sulphate (0–6 mM or more) significantly increased the rate of acetic acid conversion over that obtained with 0–5 mM soluble iron without sulphate. Long‐term continuous culture tests (100–160 d) showed that 0–6 mM or more sulphate increased the maximum growth rate of the culture from 0–07–0–08 d without sulphate to 0–14–0–16/d, even at iron levels of 0–02–0–05 mM. Hydrogen was inhibitory to the conversion of acetic acid to methane under all conditions tested, but stimulated t
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