Effect of influent COD/SO42− ratios on UASB treatment of a synthetic sulfate-containing wastewater

• UASB performance was investigated at COD/SO42− ratio ranging from 0.5 to 20.
• MPA out-competed SRB in COD and electrons utilization at COD/SO42− ratios applied.
• All the SRB species were found to be incomplete oxidizers.
• Degradation pathways at different COD/SO42− ratios were identified.

The effect of the chemical oxygen demand/sulfate (COD/SO42−) ratio on the anaerobic treatment of synthetic chemical wastewater containing acetate, ethanol, and sulfate, was investigated using a UASB reactor. The experimental results show that at a COD/SO42− ratio of 20 and a COD loading rate of 25.2 gCOD L−1 d−1, a COD removal of as high as 87.8% was maintained. At a COD/SO42− ratio of 0.5 (sulfate concentration 6000 mg L−1), however, the COD removal was 79.2% and the methane yield was 0.20 LCH4 gCOD−1. The conversion of influent COD to methane dropped from 80.5% to 54.4% as the COD/SO42− ratio decreased from 20 to 0.5. At all the COD/SO42− ratios applied, over 79.4% of the total electron flow was utilized by methane-producing archaea (MPA), indicating that methane fermentation was the predominant reaction. The majority of the methane was produced by acetoclastic MPA at high COD/SO42− ratios and both acetoclastic and hydrogenthrophic MPA at low COD/SO42− ratios. Only at low COD/SO42− ratios were SRB species such as Desulfovibrio found to play a key role in ethanol degradation, whereas all the SRB species were found to be incomplete oxidizers at both high and low COD/SO42− ratios.