Low voltage electric field mediating gravity-driven membrane bioreactor in treating saline wastewater: removal improvement and membrane fouling control

Gravity-driven membrane bioreactor (GDMBR) endowed with low maintenance and energy consumption owing to its combinations between biological degradation and membrane rejection, however, has not been reported in treating the saline wastewater due to the potential biological inhibition. This study investigated the effects of salt on GDMBR performance, and introduced a novel low voltage electric field (eGDMBR) with hopes to mediate the biological activity and improve the filtration performance. The influence of different low voltage electric fields (0.25 V, 0.50 V, and 0.75 V) on the flux variations and contaminants removal of eGDMBR were investigated. The results demonstrated that integrating low voltage electric field into GDMBR would not impact the appearance of flux stabilization, but delivered a significant influence on its stabilized flux level, and 0.50 V was the optimized voltage, contributing to a substantial flux improvement of 47.10 % in eGDMBR-0.50. Moreover, with the assistance of low voltage electric fields, eGDMBR process obtained an effective removal of chemical oxygen demand (COD), ammonia nitrogen (NH₄⁺-N) and total phosphorus (TP), accounting for average removal efficiency of 54.01 %, 73.08 % and 25.74 %, respectively. Compared to the control, coupling 0.50 V voltage electric field to GDMBR was beneficial to engineer a loose, porous and heterogeneous structure of membrane biofilm and also reduce the accumulation of extracellular polymeric substance (EPS), significantly contributing to alleviating the membrane fouling. Furthermore, with the assistance of low voltage electric fields, the Proteobacteria was enriched by a relative abundance of 58.58 % in the membrane biofilm of eGDMBR-0.50, which was in favor of the degradation of COD and NH₄⁺-N. Therefore, these findings indicated that eGDMBR conferred significant superiorities in treating the saline wastewater and mitigating membrane fouling.