Enhancing methanogenesis in fed-batch anaerobic digestion of high-strength sulfate-rich wastewater using zero valent scrap iron

The aim of this study was to evaluate the addition of zero valent iron (ZVI) in scrap form to an anaerobic digestion reactor in order to enhance methane production from high-strength, sulfate-rich wastewater. The wastewater contained 6,000 mg/L COD and 3,000 mg/L sulfate. From an initial set of batch tests, the addition of either ZVI powder or ZVI scrap to the medium resulted in comparable methane yields of around 0.13 L CH4/g COD added. In a fed-batch operation treating the sulfate-rich wastewater for 60 days, the reactor with added ZVI scrap was found to have higher media pH (6.3-8.1), increased sulfate reduction efficiency (65-85%) and higher concentrations of dissociated sulfide (106-1,020 mg S/L) than a fed-batch operation without added ZVI scrap. The reactor with added ZVI had a maximum methane yield of 0.25 L CH4/g COD added.day and a maximum methane composition in the biogas of 53%. In contrast, the control reactor without added ZVI gave a methane yield of 0.07 L CH4/g COD added.day and a methane composition in the biogas of 27%. The control reactor also had higher concentrations of undissociated sulfide (153-612 mg S/L) and relatively high volatile fatty acids (VFA) to alkalinity ratios (0.4-0.7 g CH3COOH/g CaCO3). The experimental results suggested that ZVI provided favorable pH values for methanogenesis, reduced the toxicity of sulfides and thus enhanced the competitiveness of methanogens with sulfate-reducing bacteria (SRB). Moreover, the larger negative oxygen reduction potential (ORP) values in the reactor with added ZVI scrap indicated that the ZVI might enhance electron transfer activities of microbes in the reactor.