Application of exogenous redox mediators in anaerobic biological wastewater treatment: A critical review

In anaerobic biological wastewater treatment , extracellular electron transfer (EET) by microorganisms is one of the essential factors affecting pollutant degradation and wastewater resource utilization. However, this process can be inhibited by disadvantages such as pollutants' toxicity or external environmental fluctuations, which reduces anaerobic biological treatment efficiency and resource recycling. By dosing exogenous redox mediators (RMs) into anaerobic biological treatment, electron transfer channels can be established between microorganisms and electron acceptors (e.g., pollutants, ions or other microorganisms), which will accelerate the anaerobic reaction process. So it is considered to have great potential in anaerobic biological wastewater treatment. The present paper reviews the types, mechanisms and applications of exogenous RMs systematically. The exogenous RMs were classified into exogenous electron shuttles (ESs) and conductive materials according to their structural characteristics. And the properties, mechanisms, methods of modification and immobilization for different exogenous RMs were also expounded. In addition, in this paper, the applications of exogenous RMs in the treatment of azo dyes , nitroaromatic, halogenated organic pollutants, nitrogenous wastewater, as well as stimulating direct interspecies electron transfer (DIET) to enhance the methanogenic performance of anaerobic digestion were discussed in detail, and the remaining shortcomings in these studies were pointed out. Finally, the possible future applications of exogenous RMs were prospected. This paper aims to further enhance the applicability of exogenous RMs in anaerobic biological wastewater treatment. • Modification and immobilization methods of exogenous redox mediators are elucidated. • Application of electron shuttle enhanced anaerobic biological treatment is reviewed. • Conductive materials as DIET medium to enhance anaerobic digestion is discussed. • Research directions and future application prospects of exogenous RMs are summarized.