Enhanced nitrogen and phosphorus removal in gravity driven membrane bioreactor: Performance and mechanism

The effective removal of nitrogen and phosphorus is a technical bottleneck in rural wastewater treatment. In this work, Gravity-Driven Membrane Bioreactor (GDMBR) was upgraded by integrating electrocoagulation (EC), aerobic-anaerobic (OA) tanks and ceramic membrane to fabricate EC-OA-GDMBR. This enhancement aimed to improve the long-term and stable removal efficiency of nitrogen and phosphorus in rural wastewater treatment. First, Fe³⁺ generated from EC facilitated Anammox (Candidatus Brocadia and Candidatus Kuenenia) and Feammox (Pseudomonas, Exiguobacterium, Geobacteria, and Geobacteria) in the anaerobic tank. Denitrification occurred either in the anaerobic tank or in the deep layer of biofilm in the aerobic tank, resulting in a synergy on the nitrogen removal even at low carbon source. Second, EC enhanced the phosphorus removal by combining with PO₄^3- to form insoluble phosphate precipitates. Additionally, the carriers created more attachment areas for microbial growth in the aerobic tank, promoting organic degradation. Accordingly, the EC, biodegradation, Anammox/Feammox and denitrification in EC-OA-GDMBR synergistically improved water purification efficiency, achieving COD_Cr < 38.8 ± 1.2 mg/L, NH₃-N < 0.05 mg/L, TN < 8.8 mg/L and TP < 0.02 mg/L in enfluent. Third, EC-OA-GDMBR exhibited desirable filtration efficiency (5.5–8.3 LMH), as the biofilm with large porosity (191.1∼273.6 μm) on the ceramic membrane surface delayed the formation of cake layer. Furthermore, after 120 day s of system operation, this, this setup demonstrated strong strong stability and adaptability to the characteristics of dispersed and discontinuous inflow for rural wastewater. This work elucidates the efficiency improvement of nitrogen and phosphorus removal for EC-OA-GDMBR, and provides a new vision for rural wastewater treatment.