Interspecies electron transfer stimulating approaches in anaerobic digestion process: recent advancements, challenges, and future perspectives

Anaerobic digestion (AD) is a biological process of converting complex organic matter into biogas (methane) and digestate in the absence of oxygen by microorganisms. In this regard, interspecies electron transfer (IET) accelerates metabolic activities between fermentative bacteria and methanogenic archaea by promoting direct electron transfer. However, its stability and efficiency of methane production are affected due to the imbalances between acidification and methanization. Previously, various review works have been done regarding the approaches of stimulating IET, especially direct interspecies electron transfer (DIET) between syntrophic partners in AD to enhance methane production and system stability. However, existing studies mainly addressed conductive materials (CMs) and simpler substrates (acetate, butyrate, ethanol, butanol), focusing on efficiency while ignoring industrial applicability and environmental impact. Advanced approaches for stimulating IET in AD of complex organic substrates remain poorly reviewed. Therefore, this review aimed to analyze the status of various advanced IET stimulating approaches for enhanced methane production from various complex organic substrates. It is also to discuss their advantages, challenges, industrial applicability, environmental impacts, and finally, to recommend possible future perspectives. As a result, it enables us to understand the effects of various IET stimulating approaches in AD and also becomes an invaluable resource for scholars, professionals, and decision-makers. The future studies should focus on cost-effective, environmentally sustainable, and scalable IET-stimulating methods like enriched electroactive microbial consortia, nanotechnology, and hybrid systems for complex organic substrates, while assessing industrial feasibility, long-term stability, and environmental impacts.