Conductive Materials as Promoters of Direct Interspecies Electron Transfer in Biocatalytic Anaerobic Digestion: A Review

This review presents a mechanistically focused overview of the role of conductive materials in promoting direct interspecies electron transfer (DIET) during anaerobic digestion. A central interpretative issue addressed in this review is whether the improved process performance observed after material addition reflects enhanced DIET or overlapping physicochemical and biological effects. The review systematises the current state of knowledge on the relationships between material properties, the structure of anaerobic consortia, process response, and the strength of evidence supporting DIET involvement. It discusses indirect and direct electron transfer and the criteria used to interpret the process, microbiological, electrochemical, structural, and molecular data. It also addresses functional interactions at the material–microorganism interface and non-DIET pathways of process improvement, including biomass immobilisation, inhibitor adsorption, buffering, micronutrient effects, and biofilm reorganisation. Conductive materials are also systematised into carbon-based, iron-based, composite and engineered, and organic conductive groups, with their roles related to process limitations, practical constraints, and their applicability in reactor-oriented systems. The distinctive contribution of this review lies in moving beyond simple cataloguing of materials and technological effects towards a framework for mechanistic evaluation, evidence grading, and process translation in conductive-material-assisted anaerobic digestion.