Effects of Moringa oleifera-based pre-coagulation and hydraulic flushing on the reduction of submerged nanofiltration membrane fouling

Membrane fouling is challenging for the direct nanofiltration (NF) treatment of surface water when pre-filtration is not performed. This study aimed to determine the efficacy of pre-coagulation using a natural coagulant, Moringa oleifera (MO), and hydraulic flushing to reduce membrane fouling during submerged NF treatment of dam water. MO-based coagulation considerably reduced turbidity from 8–12 to 1–2 NTU. However, the transmembrane pressure (TMP) of submerged NF treatments with and without pre-coagulation typically increases over time, indicating that turbid substances were not the major contributors to hydraulic resistance during submerged NF treatment. Hydraulic flushing, simulated by pouring water onto the surface of the membrane in the laboratory, resulted in near-full recovery of TMPs. Regardless of pre-coagulation, dissolved organic carbon concentrations in the permeate were comparable, dropping from 1.2 mg/L in the dam water to 0.4 mg/L in the permeates. Bacteria were deposited on the surface of the NF membrane when pre-coagulation was applied. This suggests the potential for biofouling during long-term operation. This study highlights the potential of MO-based pre-coagulation for fouling mitigation of submerged NF membranes when used with hydraulic flushing, but warns that residual biopolymers or high microorganism concentrations may offset the reduction in membrane fouling during long-term operations. • Pre-coagulation using a plant-based coagulant (MO) substantially decreased turbidity. • MO pre-coagulation did not markedly reduce the increase in TMP of the NF membrane. • Expected TMP benefit of turbidity removal was masked by MO organics and biofouling. • Hydraulic flushing was more effective when MO pre-coagulation was applied.