Combined inhibition of anaerobic digestion by sulfate, salinity, and ammonium: potential inhibitory factors in forward osmosis-concentrated municipal wastewater

This study investigated the combined and interactive effects of sulfate, salinity (NaCl), and ammonium on mesophilic anaerobic digestion using synthetic wastewater simulating concentrated municipal wastewater from the forward osmosis (FO) process. Batch anaerobic digestion experiments were conducted with varying concentrations of sulfate, NaCl, and ammonium. Complete sulfate reduction was observed in all test systems, regardless of the NaCl and ammonium concentration, indicating no significant inhibitory effect on sulfate-reducing bacteria (SRB). However, the increased toxicity of hydrogen sulfide produced by SRB under high concentrations of sulfate, NaCl, and ammonium inhibited methanogenic activity, resulting in reduced methane production. Despite this, methanogens, primarily Methanosarcina, tolerated low and moderate levels of sulfate, NaCl, and ammonium; thus, their coexistence with SRB (Desulfotomaculales) enabled efficient acetate utilization and methane production. The enhanced Methanosarcina activity was further confirmed through the antagonistic effects between NaCl and ammonium. No significant decrease in methane production was observed in the co-presence of 0.5 g/L sulfate, 10 g/L NaCl, and 1 g/L ammonium-nitrogen compared to the reference condition without the addition of these components. This study identified the inhibitory mechanisms resulting from sulfate, NaCl, and ammonium interactions, which may occur in FO-concentrated municipal wastewater. These findings offer insights for optimizing the FO process to maintain sulfate, NaCl, and ammonium concentrations below inhibitory levels, thereby ensuring efficient methane production.