Carbon, iron, and polymer-based conductive materials for improving methane production in anaerobic wastewater treatment systems: A review on their direct interspecific electron transfer mechanism

Organic compounds in wastewater have a high potential to be converted to (bio)methane (CH4) via anaerobic processes for energy recovery. Facilitating direct interspecies electron transfer (DIET) between microorganisms with the addition of conductive materials, as opposed to the conventional anaerobic biological process, has been viewed as an efficient strategy for boosting the efficiency of CH4 production. In recent years, extensive research has been conducted on the performance, mechanisms, and technical uses of conductive materials, such as carbon, iron and polymer-based materials in anaerobic wastewater treatment systems. In this review, the properties of conductive materials have been comprehensively reviewed and summarized, along with their application strategies to enhance CH4 production. In addition, the paper focuses on the methane production mechanism in anaerobic bioreactors, in the presence of carbon, iron, and polymer-based materials. The DIET theory employed for methane synthesis and the involvement of various organic wastes including ethanol, butyrate, propionate, and complex organic compounds are also elaborated upon. The prospective future research directions were also identified and stated in this review.