New insights into the mechanisms underlying biochar-assisted sustained high-efficient co-digestion: Reducing thermodynamic constraints and enhancing extracellular electron transfer flux

To clarify the roles of biochar in the anaerobic co-digestion of waste activated sludge (WAS) and food waste (FW), batch tests were conducted coupled with thermodynamics, extracellular electron transfer flux and microbial community analysis. Compared with the control group, biochar significantly facilitated the co-digestion at three periods, but its sustainable facilitation was mainly in the syntrophic methanogenesis of volatile fatty acids (VFAs). The thermodynamic analysis confirmed that biochar could alleviate limitations imposed by high hydrogen partial pressure during interspecies hydrogen transfer (IHT), the thermodynamic windows was expanded 137% and 92% in the syntrophic methanogenesis of acetate and propionate, respectively. Meanwhile, due to the redox capacity of biochar (4.85 and 0.35 μmol e⁻/g biochar), the equivalent current of direct interspecies electron transfer (DIET) flux for syntrophic methanogenesis of acetate and propionate obtained were 1.0 × 10⁻⁴ A and 0.9 × 10⁻⁴ A, which were 10⁸ times than that of IHT. It should be noticed that the functional microorganisms like Methanosarcina which could participate DIET were only enriched on the surface of biochar, the dominant Methanothermobacter in suspended sludge probably indicate IHT was still the main pathway for syntrophic methanogenesis. Nevertheless, the DIET triggered by the redox-active moieties on the surface of biochar and the enhanced IHT by alleviating thermodynamic restrictions, promoted the syntrophic methanogenesis synergistically.