Effect of influent COD/SO₄^2- ratios on UASB treatment of a synthetic sulfate-containing wastewater

The effect of the chemical oxygen demand/sulfate (COD/SO₄^2-) 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/SO₄^2- ratio of 20 and a COD loading rate of 25.2gCODL^-1d^-1, a COD removal of as high as 87.8% was maintained. At a COD/SO₄^2- ratio of 0.5 (sulfate concentration 6000mgL^-1), however, the COD removal was 79.2% and the methane yield was 0.20LCH₄gCOD^-1. The conversion of influent COD to methane dropped from 80.5% to 54.4% as the COD/SO₄^2- ratio decreased from 20 to 0.5. At all the COD/SO₄^2- 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/SO₄^2- ratios and both acetoclastic and hydrogenthrophic MPA at low COD/SO₄^2- ratios. Only at low COD/SO₄^2- 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/SO₄^2- ratios.