Enhanced two-phase anaerobic digestion of waste-activated sludge by combining magnetite and zero-valent iron

Previous studies have found that magnetite can promote the hydrolysis-acidification but inhibit the methanogenesis, while zero-valent iron (ZVI) only promoted the methanogenesis. Therefore, a new two-phase anaerobic digestion model, in which magnetite was added to the first phase, and ZVI was added to the second phase, was proposed to promote both hydrolysis-acidification and methanogenesis and avoid magnetite inhibition. The results showed that in the new model, methane production was improved by 10.2% and 18.1% and chemical oxygen demand (COD) removal was improved by 7.9% and 10.9% compared with reactors that included only magnetite and only ZVI, respectively. In the new model reactors, inhibition of methanogenesis by magnetite was avoided compared with that of the magnetite-only reactors, and hydrolysis efficiency was improved via dissimilatory iron reduction (DIR) compared with that of ZVI-only reactors. The data on volatile fatty acids (VFAs), coenzyme F-420 and electron transfer system (ETS) further confirmed these conclusions.