Insights into mechanisms of red mud promoting biogas production from waste activated sludge anaerobic digestion

Anaerobic digestion of waste activated sludge is usually limited by the low overall methane yield. The addition of exogenous materials is considered to be a potential method to improve anaerobic methane production. In this study, red mud was added into waste activated sludge anaerobic digester and its effects on anaerobic digestion were investigated. Results showed that methane production was improved with the addition of red mud, and almost reached maximum in 10 g/L red mud added reactor (124.4 mL/g VSS), which was about 24.0 % higher than control. Further investigation indicated that the key genes related to methane metabolism, the activity of the corresponding coenzymes and the rate of extracellular electron transfer were also increased in red mud added reactor. Batch experiments on the effects of red mud in each stage of waste activated sludge anaerobic digestion showed that red mud could improve solubilization, hydrolysis, and acidification processes, but inhibit the methanogenesis process. The potential mechanism behind the enhancement of methane production during whole waste activated sludge anaerobic digestion is that the addition of red mud can intensify the hydrolysis and acidification processes, thereby providing more substrates for methanogens and subsequently enhancing the activity of coenzymes related to methane production.