Exploring the roles of zero-valent iron in two-stage food waste anaerobic digestion

[Display omitted] •ZVI enhanced the performance of two-stage food waste anaerobic digestion.•ZVI accelerated FW hydrolysis-acidification, H2 and CH4 generations.•ZVI strengthened significantly bioenergy recovery efficiency.•Syntrophic bacteria, Pseudomonas and methanogens were enhanced by ZVI addition. This research investigated the roles of zero-valent iron (ZVI) in a two-stage food waste digestion process. ZVI was added separately to hydrolytic-acidogenic (HA) and methanogenic (MG) stages to understand its impacts on FW hydrolysis-acidification, methanogenesis and bioenergy recovery efficiency. Results showed that ZVI effectively enhanced the overall performance of digestion as compared with the controls without ZVI. Supplementing with ZVI could facilitate the HA process along with faster hydrogen generation. In addition, ZVI shortened the lag phase of MG stage by 42.43–57.23% and raised the maximum methane production rate and yield by 33.99–38.20% and 11–13%, respectively, compared with the controls. Supplementing ZVI to the HA stage could simultaneously raise the bioenergy recovery efficiency of the HA and MG stages by 71.92% and 96.96%, respectively. Further studies demonstrated that iron corrosion contributed little to hydrogen and methane production. The enrichment of syntrophic bacteria, Pseudomonas, and methanogens, and the enhancement of electron transfer among those microbes was supposed to be the main possible mechanism for the enhancement of methanogenesis with ZVI assisted.