Strategies and mechanisms for electron transfer enhancement in anaerobic membrane bioreactors: A critical review.

Abstract

Anaerobic membrane bioreactor (AnMBR) is a promising technology for simultaneous wastewater treatment and energy recovery, yet its performance is critically constrained by inefficient extracellular electron transfer (EET) between microorganisms and electron acceptors, which limits pollutant degradation and methanogenic conversion. To overcome this bottleneck, this review systematically summarizes two major enhancement strategies: (i) supplementation of exogenous redox mediators (RMs), including soluble electron shuttles and insoluble conductive materials (e.g., carbon-based and iron-based additives); and (ii) electrochemical coupling, where membranes are configured either as separate electrodes or as integrated conductive filtration units. Based on these strategies, we further elucidate the underlying mechanisms in three key aspects: (a) enhanced removal of various pollutants via reductive cleavage, adsorption, and anodic oxidation; (b) promoted methane production through direct interspecies electron transfer (DIET), iron-mediated chemical electron donation, and bioelectrochemical methanogenesis; and (c) mitigated membrane fouling by electrostatic repulsion, in-situ gas scouring, and degradation of extracellular polymeric substances. Moreover, this review identifies remaining knowledge gaps, including mediator loss, cost-effectiveness, long-term operational stability, and microbial adaptation, and outlines future research directions to facilitate practical translation. The comprehensive analysis presented herein aims to provide a valuable reference for advancing AnMBR towards energy-efficient, low-cost, and sustainable wastewater treatment, thereby bridging the gap between laboratory innovation and engineering implementation.