Current Trends and Future Prospects of Biochar Use to Improve Anaerobic Digestion: An Up-to-Date Critical Review

Biochar supplementation has gained a lot of interest in recent years as a strategy to improve anaerobic digestion. As a result, research on the field has expanded in diverse directions, yet a clear pathway is not being followed, which can lead to unexpected or contradictory results. This review analyzed the most recent literature (2020-2024) on this topic and identified three major research trends: (i) investigating the mechanisms behind biochar enhancement of anaerobic digestion (analysis of microbial communities, interspecies electron transfer, metabolic pathways, enzymatic activity, gene expression, extracellular polymeric substances, quorum sensing, and antibiotic resistance genes); (ii) maximizing biochar applications in anaerobic digestion through the use of novel tools (biochar engineering, modeling and optimization, and integration of anaerobic digestion and other technologies); (iii) advancing towards the large-scale implementation of biochar addition to anaerobic digestion (continuous operation, biochar effects on digestate, techno-economic analysis, and life cycle assessment). By investigating these topics, key knowledge gaps and challenges to be addressed in future research were defined and discussed. This review aims to provide a clear and insightful picture of the current state and future prospects of scientific research in this field, which may be of great relevance given the current rise in this technology.