Challenges in engineering direct interspecies electron transfer for enhanced methanogenesis

Interspecies electron transfer (IET) between fatty acid-oxidizing bacteria and hydrogenotrophic methanogens is key to stable and efficient methanogenesis in the anaerobic digestion (AD) process. Recently discovered direct IET (DIET) enables energetically and kinetically advantageous methanogenesis and facilitating cell-to-cell electrical connections using conductive additives (mostly metal- or carbon-based) has been recognized a promising approach to enhancing methanogenesis. Over the last decade, extensive efforts have been made to understand the mechanism and role of DIET in AD and to use it to improve AD performance and stability. However, challenges and limitations remain and must be addressed for the practical application of DIET-AD technology, for example, (i) how to prevent the washout loss of particulate conductive materials in continuous operation, (ii) how to further promote DIET so as to achieve better and more stable methanogenic performance, and (iii) how to evaluate the effects of DIET promotion separately from enhanced biomass retention by adding conductive materials with high specific surface areas. This paper discusses these challenges in both research and practical application of engineering DIET in AD using conductive additives, as well as the efforts to address them. Reviewing current technologies and recent advances to tackle the challenges and improve the practicability of the DIET-AD process, it has been suggested that significant further research is needed, especially on the retention of conductive additives and their biocompatibility and interactions with microorganisms. DIET-AD technology is still in its early stages, and ongoing efforts are required to improve its scalability and economic feasibility at the industrial scale.