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 (COD/SO(4)(2-)) 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, COD/SO(4)(2-) ratio, molybdate dose and biomass with different growth histories. Assays with 3mM molybdate showed TSA inhibition over 85%. Dose dependency was observed for sulfate concentration, COD/SO(4)(2-) 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.