Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe3O4 was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe3O4. The maximal SCFA production of the Fe3O4-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe3O4 was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe3O4 stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.