The Research on Different Strategies of Fe₃O₄ Additive Utilization during Semi-Continuous Anaerobic Co-Digestion: A Comparison of Magnetic Separation Recycling and Replenishment without Recycling

Fe₄O₃ has shown great potential as an enhancer of batch anaerobic digestion performance. In this study, the effect of two types of Fe₃O₄ additive utilization strategies (magnetic separation recycling strategy and replenishment without recycling strategy) on semi-continuous anaerobic co-digestion of sludge and corn straw were investigated. A maximum methane yield of 163.88 ± 12.18 mL/g·VS·d was obtained at the dosage of 10 g/kg Fe₃O₄ with magnetic separation recycling strategy compared to the control without the addition of Fe₃O₄. As compared to the replenishment strategy, the recycling strategy improved specific daily methane yields by 38.45% and 45.75% for Fe₃O₄ addition amounts of 5 g/kg and 10 g/kg, respectively. The results show that the recycling of Fe₃O₄ additives can further optimize the composition of VFAs. Although the total VFAs were not elevated, the proportion of acetic acid increased and the proportion of propionic acid as well as butyric acid decreased. The present research supports the magnetic recycling of Fe₃O₄ as a new strategy to solve the problem of additive loss in semi-continuous anaerobic digestion process.