Effects of ferric oxide on the microbial community and functioning during anaerobic digestion of swine manure

Iron-based materials have been suggested as environmentally-friendly additives that can enhance methane production during anaerobic digestion (AD). In this study, the effects of ferric oxide (Fe2O3) addition on methane production were investigated during swine manure AD. In addition, the effects of Fe2O3 addition on the AD ternary pH buffer system and microbial community were evaluated. Fe2O3 could improve the accumulative methane production by maximum 11.06% when adding 75 mmol of Fe2O3. Higher methane production could be attributed to the enhancement of direct interspecies electron transfer (DIET) and the formation of Fe-S precipitates, but not the addition of Fe2O3 as a nutrient. Furthermore, Fe2O3 addition enhanced methanogenesis rather than acetogenesis, as evinced by analysis of functional genes. Nevertheless, high-throughput sequence analysis of microbial community composition revealed the lack of a significant influence by Fe2O3 addition, and Fe2O3 addition did not significantly affect the ternary pH buffer system.