Enhancement effects and mechanisms of iron filings on the performance of anaerobic co-digestion of hulless barley straw and pig manure

In order to study the enhancing effects and mechanisms of iron filings in anaerobic co-digestion (co-AD), we investigated the effects of different concentrations of iron filings (0 g L−1, 6 g L −1, 10 g L −1, 14 g L −1, 18 g L −1) on the co-AD of hulless barley straw and pig manure, based on medium-temperature batch anaerobic digestion. The results revealed that the addition of iron filings improved the methane production performance of co-AD, and that cumulative methane production was the highest (375.26 mL g−1 VS) when the addition concentration was 14 g L −1, which was 21.33 % higher than that of the treatment group without added iron filings. The addition of an appropriate amount iron filings can also provide certain economic benefits. The addition of iron filings increased the relative abundances of bacterial groups such as Fermentimonas, Proteiniphilum, Syner-01, Ercella, Thermovirga, unclassified Synergistaceae and unclassified Anaerolineaceae as well as acetate trophic methanogens such as Methanothrix and Methanosarcina, but it did not promote the growth of hydrogenotrophic methanogens. The addition of iron filings increased the electron transport system (ETS) activity of the co-AD and enhanced the enrichment of microorganisms involved in direct interspecies electron transfer (DIET), such as Methanothrix, Methanosarcina and unclassified Anaerolineaceae. This may represent one of the key reasons behind the observed increase in methane production. A metabolomic analysis found that the presence of 47 metabolites may have been related to the addition of iron filings, suggesting that their addition significantly affected the related metabolic pathways of the co-AD. The differential metabolites were mainly enriched in metabolic pathways such as ATP-binding cassette (ABC) transport, purine metabolism, linoleic acid metabolism, and alpha-linolenic acid metabolism.