Mitigating over-acidification and enhancing methane production in hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) treating fruit and vegetable wastewater: performance and mechanisms

Fruit and vegetable wastewater (FVWW) generated during the pretreatment of fruit and vegetable waste (FVW), exhibits high biomethane potential (BMP). However, it encounters challenges including over-acidification and low methane productivity during anaerobic digestion (AD). Although microbial electrolysis cell assisted anaerobic digestion (MEC-AD) is an effective enhancement method, its application in FVWW processing remains underexplored. This study investigated the treatment of FVWW at three concentrations using MEC-AD bioreactor (R_MEC-AD) at a voltage of 0.6 V. The results showed that under high organic load (COD 15000 mg/L), the methane productivity and yield of R_MEC-AD increased by 65.4 % and 40.0 % respectively compared to traditional AD (R_AD), while the accumulation of volatile fatty acids (VFAs) decreased by 59.1 %. Thermodynamic analysis confirmed that efficient propionate acetogenesis and acetoclastic methanogenesis emerged as key factors in enhancing methane productivity in R_MEC-AD. Microbial community analysis identified hydrogenotrophic methanogenesis as the dominant pathway at the cathode, the enrichment of exoelectrogens, propionate-degrading bacteria, and acetoclastic methanogens on the anode was central to facilitating rapid syntrophic propionate acetogenesis and efficient acetoclastic methanogenesis. Electron promoted electromethanogenesis and synergistic methanogenesis with the microbial community. This study confirms the anti-overacidification advantage of R_MEC-AD and provides feasible technical strategies for FVWW treatment and resource recovery. © 2025 Published by Elsevier B.V.