Red mud enhances methanogenesis with the simultaneous improvement of hydrolysis-acidification and electrical conductivity

The role of red mud in the improvement of methanogenesis during sludge anaerobic digestion was innovatively investigated in this study. The results demonstrated that the addition of 20 g/L red mud resulted in a 35.5% increase in methane accumulation. Red mud effectively promoted the hydrolysis-acidification of organic compounds in the sludge, which resulted in the increase of protein, polysaccharide, and VFAs by 5.1-94.5%. The activities of key enzymes were improved by 41.4-257.3%. Electrochemical measurements presented direct evidence that the electrical conductivity was significantly improved with red mud. More conductive magnetite was formed during the secondary mineralization after Fe(III) reduction by Fe(III)-reducing genes such as Clostridiaceae and Ruminococcaceae. The higher conductivity enhanced the electron transfer between the syntrophic bacteria (Geobacteraceae) and methanogens (Methanosaeta and Methanosarcina), and then improved the methanogenesis. This research provides a novel perspective on the synergism between sludge and red mud for methane production.