Enhancement of methane production and antibiotic resistance genes reduction by ferrous chloride during anaerobic digestion of swine manure

In this study, the effects of ferrous chloride (FeCl2) addition on methane production and antibiotic resistance genes were investigated during anaerobic digestion (AD) of swine manure. FeCl2 could improve the accumulative methane production by maximum 21.51%. High-throughput quantitative PCR (HT-qPCR) results indicated that FeCl2 increased the abundance reduction of antibiotic resistance genes (ARGs) by the maximum of 33.3%, and strengthened the reduction of pathogenic microorganisms and heavy metal resistance genes (MRGs). FeCl2 could intensify the utilization of acetate and propionate by enhancing H2 utilization and direct interspecies electron transfer (DIET), where DIET was further enhanced by the magnetite that was formed through hydroxy iron reacted with acetic acid. The acetoclastic methanogenesis at early stage (D9) was strengthened, but the hydrogenotrophic methanogens was enhanced after D16. 68.26% of the variables of the evolution of ARGs could be explained by the changes of bacterial community and others was affected by MRGs, MGEs, environmental variables, etc. Accordingly, FeCl2 addition is an attractive and feasible way to increase methane production.