Effect of COD/SO42− ratio and sulfide on thermophilic (55 °C) sulfate reduction during the acidification of sucrose at pH 6

This study investigated the effect of the COD/SO42− ratio (4 and 1) and the sulfide concentration on the performance of thermophilic (55 °C) acidifying (pH 6) upflow anaerobic sludge bed reactors fed with sucrose at an organic loading rate of 4.5 g COD lreactor−1 day−1. Sulfate reduction efficiencies amounted to 65% and 25-35% for the COD/SO42− ratios of 4 and 1, respectively. Acidification was complete at all the tested conditions and the electron flow was similar at the two COD/SO42− ratios applied. The stepwise decrease of the sulfide concentrations in the reactors with a COD/SO42− ratio of 1 by N2 stripping caused an immediate stepwise increase in the sulfate reduction efficiencies, indicating a reversible inhibition by sulfide. The degree of reversibility was, however, affected by the growth conditions of the sludge. Acidifying sludge pre-grown at pH 6, at a COD/SO42− ratio of 9 and exposed for 150 days to 115 mg l−1 sulfide, showed a slower recovery from the sulfide inhibition than a freshly harvested sludge from a full scale treatment plant (pH 7 and COD/SO42−=9.5) exposed for a 70 days to 200 mg l−1 sulfide. In the latter case, the decrease of the sulfide concentration from 200 to 45 mg l−1 (35 mg l−1 undissociated sulfide) by N2 stripping caused an immediate increase of the sulfate reduction efficiency from 35% to 96%.