Carbon brush-activated electron shuttling accelerates acetoclastic methanogenesis in palm oil mill effluent

Palm oil, an essential dietary component consumed daily by over three billion people, originates from the most productive oil crop requiring less land and water than other crops. However, its production causes environmental issues such as water pollution from palm oil mill effluent (POME) and greenhouse gas emissions during treatment. Efficient recovery of biogas produced from POME is important for maintaining the sustainability of oil palm industry. Direct interspecies electron transfer (DIET) with conductive materials represents a promising technology for enhancing biogas, but this approach has never been applied to POME treatment. This study aims to evaluate the effect of conductive material on acetoclastic methanogenesis by comparing 220-day triplicate semi-batch anaerobic POME with (CB culture) and without (P culture) carbon brush. Biogas production and sCOD degradation were rapid in CB culture, although the final CH₄ volumes were similar, indicating accelerated but not increased methane generation. Cyclic voltammetry revealed two primary redox mediators in CB culture with half redox potentials of approximately -36 mV and 364 mV, higher than CH₄/CO₂ redox potential, indicating DIET to methanogens was unlikely and suggesting VFA degradation via electron shuttling to abundant humic substances (HS). Thauera, which reduces HS, and Methanosaeta, an acetoclastic methanogen, dominated in only the CB cultures. These results suggest CB may facilitate the coupling of HS reduction with sCOD degradation to acetate, supporting acetoclastic methanogenesis in POME. This study presents first report of electron shuttles activated with conductive material that possibly enhances acetoclastic methanogenesis attributed to the unique HS abundance in POME.