Biochar-enhanced direct interspecies electron transfer in anaerobic digestion: A review

Anaerobic digestion (AD) is a sustainable technology for treating organic waste and generating renewable energy. However, its efficiency remains constrained as reported by limited methane production rate that could be increased in the presence of conductive materials, potentially participating to electron transfer processes. The concept of direct interspecies electron transfer (DIET) has emerged recently and is considered as more efficient electron-transfer mechanism than hydrogen-based pathways. Among DIET-enhancing materials, biochars show particular promise owing to their physicochemical properties, cost-effectiveness, and environmental compatibility. This review examines experimental evidence and hypotheses explaining how biochars could facilitate DIET in AD and illustrates more specifically the role of biochar acting potentially as electron conduits, microbial habitats, and buffering agents. Key factors influencing biochars' chemical make-up and performance as an additive in AD, such as pyrolysis temperature, feedstock type, and dosage, are discussed in depth. Experimental evidences supporting biochar-mediated DIET are usually microbial interactions, electrochemical behavior, and improved conductivity in both sludge and biochars. While conductivity could appear as a critical parameter to explain DIET in studies with other materials, biochar conductivity values are relatively low compared to engineered carbon materials, suggesting a gap in our understanding of the reactivity of biochars. This review identifies evidences and key knowledge gaps and proposes future research directions to better understand how biochars can facilitate DIET in AD.