Electrochemical membrane bioreactors: State-of-the-art and future prospects

Integration of an electrochemical process with membrane bioreactor (MBR) has attracted considerable attention in the last decade for simultaneous improvement in pollutant removal and hydraulic performance of MBR. Electrochemical MBR (eMBR) with sacrificial anodes has been observed to achieve enhanced phosphorus (up to 40%) and micropollutant removal (5-60%). This is because direct anodic oxidation, indirect oxidation by reactive oxygen species and electrocoagulation can supplement the biological process. The application of an electric field can substantially reduce membrane fouling by 10% to 95% in the eMBR as compared to the conventional MBR. Sacrificial electrodes (e.g., iron or aluminium) have been reported to be more suitable for fouling mitigation than non-sacrificial electrodes (e.g., titanium). However, during prolonged operation, metal ions released from sacrificial electrodes can adversely affect microbial activity and could accumulate in activated sludge. Depending on the current density and electrode material (sacrificial or non- sacrificial), anodic oxidation, electrocoagulation, electrophoresis and/or electroosmosis mechanisms are responsible for suppressing membrane fouling propensity. This paper critically reviews the current status of the electrochemical MBR technology and presents a concise summary of eMBR configurations and electrode materials. Comparative removal of bulk organics, nutrients and micropollutants in the eMBR and conventional MBR is discussed, and performance governing factors are elucidated. Impacts of operating conditions such as current density on mixed liquor properties (e.g., floc size and zeta potential) and microbial activity are elucidated. The extent of membrane fouling mitigation along with associated mechanisms as well as energy consumption is explained and critically analysed. Future research directions are suggested to fast track the scalability of eMBR, which include but are not limited to electrode lifetime, development of self-cleaning conductive membranes, optimisation of operating parameters, removal of emerging micropollutants, accumulation of toxic metals in activated sludge, and degradation by-products and ecotoxicity.