Static magnetic field coupled with magnetic biochar boosts methane production in sludge anaerobic digestion by activating dual electron transfer mechanisms

Anaerobic digestion (AD) of sewage sludge provides a sustainable route for sludge reduction and energy recovery, yet its efficiency is often hindered by limited electron transfer and poor syntrophic metabolism. Here, the synergistic effect of static magnetic field (SMF) coupled with magnetic biochar (MBC) on methane production was investigated. The combined treatment markedly increased the conductivity and capacitance of AD systems, creating an electrochemically favorable microenvironment and boosting methane yield to 175.39 mL CH₄ gVS⁻¹_added, compared to 80.70 mL CH₄ gVS⁻¹_added in the control. Mechanistically, SMF-MBC optimized microbial respiratory chain electron transfer by promoting the conversion of NADH to NAD⁺, activating coenzyme F420 and the electron transport system, thereby accelerating acidification and methanogenesis. Moreover, structural variations in sludge proteins, including loosened secondary structures and disrupted C-N/N-H bonds in amide III regions both confirmed that coupled technology facilitated proton-coupled electron transfer (PECT) and strengthening the electron exchange. The enrichment of Methanobacterium, Methanosaeta , and Chloroflexi, along with the upregulation of functional genes encoding key enzymes, provided evidence that direct interspecies electron transfer (DIET) was promoted by SMF-MBC. These findings indicated that SMF-MBC coupling offers a robust approach for optimizing sludge-to-energy conversion by manipulating electron transfer such as DIET and PECT mechanisms.