Balancing sludge reduction and membrane fouling mitigation by tuning electrical voltages of a side-flow electrochemical oxidation system during MBR processing

In this study, a side-flow electrochemical oxidation (EO) process was innovatively integrated with the A2O-MBR, and it was found that sludge reduction could be significantly enhanced, however, membrane fouling mitigation depended on the applied voltages. For the EMBR-20 V system, the strong EO ability realizes an excellent sludge reduction performance (60.3%). However, the secondary organic substances released by lysed sludge cells in the EO process exacerbate membrane fouling, with a 14-day shortened operation time before reaching the 30 kPa TMP limit. Moderate sludge reduction (17.8%) and membrane fouling mitigation (6.3% with a 3-day extended operational time) were simultaneously achieved in the EMBR-3 V system. Notably, returning the lysed sludge did not influence the organics and nitrogen removal in either system, but caused phosphorus concentration in the effluent to increase, mostly because of the substantial reduction of sludge discharge in the EMBR-20 V system. Meanwhile, key enzyme activity levels (ROS and LDH) under 20 V were significantly higher than those under 3 V and the blank group, whereas ATP showed a downward trend, indicating that sludge microorganisms were effectively destroyed under high voltage. In addition, the electrochemical effect can also affect SMP organic component and microbial community structures, which play an important role in membrane fouling and effluent quality. Interestingly, the analysis showed that SMP, not EPS, was the main cause of membrane fouling in the integrated system. After economic evaluation, the operational cost of the EMBR-3 V system was slightly lower (0.50–0.57 ¥/t of water) than that of the single A2O-MBR system (0.56–0.65 ¥), while the EMBR-20 V system cost was significantly higher (1.61–2.03 ¥).