Effects of operational variations of micro-oxygenation and pH shock on the competition between methane production and sulfate reduction in a UASB reactor

To investigate the effect of oxygen, pH shock, and H2S inhibition on the competition between sulfate-reducing bacteria (SRB) and methane-producing archaea (MPA), a UASB reactor treating wastewater containing sulfate, ethanol and acetate was operated for 360 days. With micro-oxygenation performed at a flow rate of 100 mL/min in the upper part of the up-flow anaerobic sludge blanket (UASB) reactor, the sulfate removal and total sulfide concentration increased to 68.6% and 500 mg S/L, while the methane production rate slightly reduced from 2.5 to 2.1 L/L/d. However, methane production was still the main reaction in the reactor. The pH shock and high H2S concentration finally changed the competition results between SRB and MPA. Sulfate reduction was not affected while methane production was completely inhibited at H2S concentration more than 200 mg/L. Complete sulfate reduction (99.0%) was maintained over the long term with a stable organic COD removal (70.4%). The experimental results proved that in the complete sulfate-reducing stage, ethanol was mainly utilized by incomplete-oxidizing SRB for sulfate reduction.