ORP-based oxygenation for sulfide control in anaerobic treatment of high-sulfate wastewater

A series of chemostat studies were conducted at a constant influent total organic carbon of 3750 mg/L (equivalent chemical oxygen demand (COD) of 10,000 mg/L) but at different influent sulfates of 1000, 3000 and 5000 mg/L in order to investigate the feasibility of online sulfide toxicity control through periodic oxygenation to the recycled biogas stream. The oxygen dosing for sulfide oxidation was regulated by using oxidation-reduction potential (ORP) as a controlling parameter. During oxygenation at elevated ORPs of -230 and -180 mV (50 and 100 mV above natural ORP of -280 mV, respectively), the dissolved and gaseous sulfides were completely eliminated which resulted in a concomitant improvement in methane yield by 56.3% at 5000 mg/L influent sulfate. However, at influent sulfates of 1000 and 3000 mg/L, both methane generation rate and sulfate removal efficiency were dropped appreciably at elevated ORPs. Facultative heterotrophs were found to consume as high as 66.3% of the influent COD during oxygenation. For effective sulfide oxidation at lower sulfate levels, it was no longer required to raise the ORP by as much as 50 or 100 mV. The actual needed ORP increase depended on the influent sulfate. This study had proven that the ORP-controlled oxygenation was reliable for achieving consistent online sulfide control during anaerobic treatment of high-sulfate wastewater.