Biological sulfate (SO
42−) reduction was examined in anaerobic sequential batch reactors (ASBRs) operated under different hydraulic retention times (HRTs) ranging from 12 to 36 h and COD (Chemical Oxygen Demand)/SO
42−) ratios of 2.4, 1.6 and 0.8. Competition
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Biological sulfate (SO
42−) reduction was examined in anaerobic sequential batch reactors (ASBRs) operated under different hydraulic retention times (HRTs) ranging from 12 to 36 h and COD (Chemical Oxygen Demand)/SO
42−) ratios of 2.4, 1.6 and 0.8. Competition between SO
42− reducing bacteria (SRBs), methane producing archaea (MPAs) and homoacetogens (HACs) was examined in controls and cultures treated with linoleic acid (LA). The ASBR performance was influenced by the COD/SO
42− ratio in control cultures with a SO
42− reduction of 87% at a COD/SO
42− ratio of 0.8. At a 12 h HRT, in both control and LA treated cultures, greater than 75% SO
42− removal was observed under all the conditions examined. In control reactors operating at a 36 h HRT, high levels of MPAs belonging to Methanobacteriales and Methanosarcinales were detected; however, in comparison, under low COD/SO
42− ratio and with decreasing HRT conditions, a relative increase in SRBs belonging to Desulfovibrio and Desulfatibacillum was observed. Adding 0.5 g·L
−1 LA suppressed Methanobacteriales, while increasing the LA concentration to 1 g·L
−1 completely suppressed MPAs with a relative increase in SRBs. HACs belonging to Bacteroidetes were observed in the control and in cultures operated at 12 h HRT with a COD/SO
42− ratio of 1.6 and fed 0.5 g·L
−1 LA; however, with all other LA levels (0.5 and 1.0 g·L
−1) and HRTs (12, 24 and 36 h), HACs were not detected.
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