Enhanced anaerobic co-digestion of food waste and sewage sludge by co-application of biochar and nano-Fe₃O₄

Conductive materials have been utilized to facilitate direct interspecies electron transfer (DIET) in anaerobic digestion (AD) to enhance methane production. However, the impact and efficacy of the co-application of biochar and nano-Fe₃O₄ have not been adequately elucidated, particularly their interaction on electron transfer efficiency. In this investigation, we examined the influence of simultaneously or independently adding biochar and nano-Fe₃O₄ to food waste (FW) and sewage sludge (SS) anaerobic co-digestion. A synergistic effect was observed under the co-application condition. Methane production reached 300.3 ± 19.8 mL/gCOD with the co-application of biochar and nano-Fe₃O₄, representing a 43.3%, 35.4%, and 5.4% increase compared to the sole Fe₃O₄, biochar, and nano-Fe₃O₄, respectively. Mechanistic analysis revealed that, in comparison to sole biochar and nano-Fe₃O₄, their co-occurrence significantly accelerated hydrolysis and acidogenesis, thereby enhancing the release of soluble organic components. Furthermore, the application of nano-Fe₃O₄ improved system stability and significantly promoted propionate degradation, maintaining a favorable condition for methane production. Additionally, the noteworthy increase in INT-ETS activity and cytochrome c concentration indicated that the co-application of biochar and nano-Fe₃O₄ stimulated electron transfer. Correspondingly, the activity of coenzyme F420, which indicates the performance of methanogenesis, exhibited a 2.44-fold increase compared to the control. This indicated that nano-Fe₃O₄ and biochar co-amendment can serve as a robust platform to strengthen DIET. This study provided a new insight regarding the application of biochar and nano-Fe₃O₄ in the AD system for strengthening electron transfer to promote methane production.