Effect of molybdate on methanogenic and sulfidogenic activity of biomass

The effect of molybdate, a sulfate analog, on the total methanogenic activity (TMA) and total sulfidogenic activity (TSA) of biomass metabolizing synthetic sucrose based substrate containing sulfate was investigated in batch assays. In Phase I of the study, TMA and TSA were assessed twice for four feed changes at a chemical oxygen demand to sulfate ratio of 3.5. In Phase II, long-term experiments were conducted for 10–13 feed changes with varying chemical oxygen demand (COD) concentration, sulfate concentration, ratio, molybdate dose and biomass with different growth histories. Assays with 3 mM molybdate showed TSA inhibition over 85%. Dose dependency was observed for sulfate concentration, ratio, and biomass history. The minimum concentration that gave over 93% TSA inhibition was 0.25 mM. However, intermediate concentrations of molybdate inhibited methane producing bacteria (MPB) activity. TMA stimulation was observed at 0.75–2.0 mM molybdate.