Abstract:Direct interspecies electron transfer (DIET) offers a faster electron transfer rate and reduced dependence on diffusive mass transfer, potentially becoming the primary mechanism in the anaerobic digestion of urban organic solid waste, such as food waste and excess sludge. However, digestate, the byproduct of anaerobic digestion, poses environmental hazards and hinders the achievement of a waste-free city. This study investigates the morphological and structural characteristics of iron-containing digestate-based biochar (Fe-BC) produced at various carbonization temperatures, and its impact on the anaerobic digestion process of urban organic solid wastes, toward achieving "waste-to-value-added product". The results showed that increasing the carbonization temperature led to a more developed pore structure in the Fe-BC but reduced the complexity of its functional group composition. Fe-BC prepared at 800 °C exhibited the most developed pore structure and the highest iron content, with more uniform iron distribution throughout the biochar. In anaerobic digestion experiments, the group with biochar prepared at 800 °C demonstrated a methane cumulative yield of 184.24 mL/g volatile solids (VS), a 19.78% increase compared to the control group (without biochar). The VS removal rate reached 45.75%, a 6.73% increase over the control group. The group with biochar prepared at 500 °C showed a 15.66% increase in methane cumulative yield and a 5.79% increase in VS removal rate, while the other experimental groups showed no significant improvement. The effect of Fe-BC on promoting methane production was more effective in the middle of the experiment, peaking on the ninth day. Furthermore, Fe-BC prepared at 800 ℃ improved both the methane content and quality of the biogas produced. Electrochemical analysis indicated that the sludge in the Fe-BC group prepared at 800 °C exhibited greater capacitance and lower electrical resistance, suggesting enhanced interspecies electron transfer, microbial electron exchange, and electrical conductivity. Additionally, Fe-BC increased the relative abundance of Methanosaeta and Fastidiosipila. In conclusion, Fe-BC derived from digestate possesses a pore structure, with Fe-BC prepared at 800 ℃ exhibiting the most developed porosity. The addition of Fe-BC prepared at 800 °C most effectively promoted the anaerobic digestion of urban organic solid waste. Moreover, Fe-BC enriched specific microorganisms, thereby promoting DIET and enhancing the anaerobic digestion of urban organic solid waste. Close-