Review on intensified treatment of refractory wastewater in anaerobic digestion based on extracellular electron transfer: Mechanisms, strategies, and applications

In anaerobic digestion (AD) system for the treatment of refractory wastewater, extracellular electron transfer (EET) by microorganisms is critical in influencing pollutant degradation and wastewater resourcing. However, when removing pollutants from refractory wastewater using AD technology, hydrolysis by extracellular enzymes is limited by EET efficiency, which often difficult to achieve their efficient degradation. Meanwhile, the anaerobic bacterial community was impacted by the toxicity of pollutants, which also affected the interspecies electron transfer, which seriously affected the effluent water quality and methane production. Therefore, intensifying the EET efficiency of the anaerobic system has become the key to break through the bottleneck of refractory wastewater treatment and resource utilization technology. In this paper, the EET mechanisms in the AD system for treating refractory wastewaters are summarized and a systematic review of EET intensification strategies for AD systems for treating refractory wastewaters, including EET from microorganisms to pollutants and electroactive bacteria to methanogens, is presented. In addition, this paper discusses in detail the application of EET based on enhanced treatment of refractory wastewaters containing pollutants with electron-absorbing groups (azo dyes, halogenated compounds, nitroaromatic compounds) and heavy metal oxides, and it also looks forward to the research direction and application prospect of regulating EET. This paper will provide a reference for treating and resourcing refractory wastewaters by AD technology.