Effects of waste rusted iron shavings on enhancing anaerobic digestion of food wastes and municipal sludge

Abstract In this study, rusted iron shavings with Fe0 and iron oxides were added to the mixture of food wastes and municipal sludge to improve methane (CH4) production in the fermentation system. The effectiveness of such treatments was evaluated based on methane yield, biogas production, and production of volatile fatty acids (VFAs) and Fe2+ in the sludge supernatant. The results showed that the maximum daily methane yield (DCH4) and specific methane production potential (SCH4) for the group treated with the rusted iron shavings reached 13.28 mL g−1VSS0·d−1 and 408.1 mL·g−1VSSremoved (on day 36), respectively. These values were 12.2% and 41.2% higher than those obtained from the control group (nothing added). During digestion, the highest VFAs level of the rusted iron shavings group was 13,694 mg L−1 on day 3, which was 48% higher than that of the control group. The volatile suspended solids (VSS) removal rates of the group with rusted iron shavings (45.8%) were 18.0% higher than that of the control group (27.8%). The Fe2+ content of the rusted iron shavings group was 132% and 282%, respectively, higher than those of the clean iron shavings and non-rusted iron shavings groups on day 5. Analysis data showed that the rusted iron mainly consisted of FeOOH with 53.5% hydroxyl groups and 41.4% Fe2O3 due to the porous and loose structures in the sludge system. These results indicated that rusted iron shavings, as a reductive material, could produce a more reductive environment that promoted methanogenic activity in the system, leading to improved methane yield.