Preparation of Iron-Containing Digestate-Based Biochar and Its Impacts on Anaerobic Digestion Performance of Urban Organic Solid Wastes

Direct interspecies electron transfer (DIET) offers a faster electron transfer rate and a reduced dependence on diffusive mass transfer, potentially becoming a primary mechanism for the anaerobic digestion of urban organic solid wastes, 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 of urban organic solid wastes, with the goal of achieving "waste-to-value-added products". 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 ℃ 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 ℃ demonstrated a cumulative methane 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 ℃ showed a 15.66% increase in cumulative methane yield and a 5.79% increase in the VS removal rate, while the other experimental groups showed no significant improvement. The effect of Fe-BC on promoting methane production was most pronounced in the middle of the experiment, peaking on the ninth day. Furthermore, Fe-BC prepared at 800 ℃ improved both the methane content and the quality of the biogas produced. Electrochemical analysis indicated that the sludge in the Fe-BC group prepared at 800 ℃ 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 well-developed pore structure, with Fe-BC prepared at 800 ℃ exhibiting the most developed porosity. The addition of Fe-BC prepared at 800 ℃ was the most effective in promoting the anaerobic digestion of urban organic solid wastes. Moreover, Fe-BC enriched specific microorganisms, thereby promoting DIET and enhancing the anaerobic digestion of urban organic solid wastes.