Biochar as an electron bridge: mechanistic insights into enhanced anammox performance in wastewater treatment

Abstract Anaerobic ammonium oxidation (anammox) process constitutes an energy-efficient nitrogen removal pathway critical for sustainable wastewater treatment. Anammox bacteria possess extracellular electron transfer capabilities relevant to their metabolism and potential interactions within microbial consortia. Enhanced electron transfer has been associated with anammox bacteria enrichment and improved nitrogen removal efficiency. Biochar addition has been reported to enhance electron transfer in anammox systems. This review summarizes the current understanding of biochar-induced electron transfer mechanisms during anammox processes. The proposed mechanisms include: (1) extracellular-polymeric-substance-mediated enrichment of redox-active compounds that facilitate biofilm-mediated electron transfer, (2) direct interspecies electron transfer through conductive biochar networks, and (3) mediated interspecies electron transfer via redox-active functional groups on biochar surfaces. Future research should integrate machine learning with mechanistic investigations to improve the prediction of biochar performance and guide the rational design of biochar materials. Such research efforts may provide a framework for optimizing electron transfer pathways and for future validation in scalable anammox systems. Highlights Biochar supports extracellular electron transfer in anammox systems. Biochar stimulates EPS secretion, enriching redox-active compounds in the biofilm. Biochar provides conductive surfaces and redox-active groups for DIET and MIET.