Dual roles of zero-valent iron in dry anaerobic digestion: Enhancing interspecies hydrogen transfer and direct interspecies electron transfer.

Although commonly viewed as a promising method, dry anaerobic digestion is not been widely applied to dispose of food wastes, especially in developing countries because of its insufficiency in handling with lower mass transfer and high acidic accumulation of the system. Zero valent iron (ZVI) has been found to demonstrate superior performance such as enhancing methane production. However, up to date, the mechanism of ZVI remains unclear. In this study, adding 5 g/L ZVI could improve interspecies hydrogen transfer (IHT) to enhance the dry anaerobic digestion of food wastes, but was unable to resist the shocks of high organic loading. With increasing ZVI dosage to 10 g/L, the performances of digestion systems were improved to maintain the systems stable. With 10 g/L of ZVI addition, electron transfer capacity of the sludge increased by 5.4 folds, and electroactive proteins of sludge increased by 2.3 folds. Microbial community analysis also indicated that the relative abundances of Methanothrix and Methanosarcina performing direct interspecies electron transfer were enriched to 67.5% and 27.2% with 10 g/L ZVI addition, respectively. These results suggested that direct interspecies electron transfer could be established with a proper dosage of ZVI that served as a conductive material to connect electron exchange among microorganisms. Thus, ZVI played a dual role including improving interspecies hydrogen transfer and promoting direct interspecies electron transfer to keep the systems efficient to treat high-solid food wastes.