In this study, a preset oxidation-reduction potential (ORP) was employed to regulate oxygen dosing for online sulfide toxicity control during anaerobic treatment of high sulfate wastewater. The experiment was conducted in an upflow anaerobic reactor (UAF), which was operated at a constant influent total organic carbon (TOC) of 6740 mg/L (equivalent to a chemical oxygen demand (COD) of 18000 mg/L) but with different influent sulfates of 1000, 3000 and 6000 mg/L. The reactor was initially run without oxygen injection at a natural ORP of about −290 to −300 mV and then was followed by oxygenation to raise ORP by +25 mV above the natural level for each influent sulfate level. With 1000 and 3000 mg/L influent sulfates, the produced sulfide levels did not impose any appreciable toxicity to methanogens even without oxygen injection. However, with 6000 mg/L influent sulfate without oxygen injections, the dissolved sulfide level quickly went up to over 800 mg/L which imposed a significant suppression on methanogenesis. Upon oxygen injection to raise the ORP by +25 mV, the dissolved sulfide was quickly reduced to 160 mg/L. If more oxygen was injected to raise the ORP by +50 mV, then dissolved sulfide further decreased to 12.2 mg/L with a concomitant improvement in methane yield by 45.9%.
With a +25 mV preset ORP increase, it was found that at the 1000 mg/L influent sulfate the supplied oxygen was more than that needed for sulfide oxidation. The excess oxygen could be utilized readily by facultative heterotrophs for organic oxidation, which contributed 13.5% of the total COD removal in the UAF. At the higher sulfate levels of 3000 and 6000 mg/L, the facultative contributions to the total COD removal were only 4.7 and 4.1%, respectively. This study has clearly demonstrated that ORP is a workable and reliable means for regulating oxygen injection to achieve online sulfide toxicity control in a single-stage anaerobic system treating high sulfate wastewater.
