Effect of pH and low temperature shocks on the competition between sulphate reducing bacteria and methane producing bacteria in uasb reactors

During the anaerobic treatment of sulphate-containing wastewaters, sulphate-reducing bacteria (SRB) compete with methane-producing bacteria (MPB) for the available electron donors. In this work the treatment of a synthetic wastewater, namely, a mixture of acetate, propionate and butyrate with high concentrations of sulphate (COD:sulphate ratio of 0.5-2.5), was studied in upflow anaerobic sludge bed reactors (UASB) operating at 30 degrees C. The influences on the competition between SRB and MPB of two levels of pH (6.75 and 7.75) and low temperature shocks (12-15 degrees C for 3 days) were investigated. The results showed that SRB were able to outcompete MPB at both levels of pH with high (up to 350 mg l(-1)) and low (below 100 mg l(-1)) concentrations of free hydrogen sulphide in the digesters. Short-term low temperature shocks had no significant effect on the proportions of feed COD used by SRB and MPB. The COD:sulphate ratio was a key factor affecting the competition between SRB and MPB. At values higher than 0.67 the fraction of degraded COD consumed by SRB decreased, due to sulphate limitation.