Overlooked influence of phosphate on the performance of a dual membrane process with coagulation pretreatment

The coagulation-ultrafiltration-nanofiltration (CUF-NF) treatment system is a promising approach for water treatment. However, the effects of phosphate on this system remain inadequately understood. In this study bench scale tests were conducted with both aluminium and iron salts using jar testing and membrane filtration, and a range of phosphate doses. Results in this study reveal that, in the CUF treatment stages, phosphate inhibits floc formation and reduces organic matter removal efficiency by competing for "active sites" and increasing the negative surface charge of flocs. Specifically, the flocculation index values decreased by 56 % for Fe³⁺ and 37 % for Al³⁺ as phosphate concentrations increased from 0 to 0.2 mM, leading to a reduction in dissolved organic carbon (DOC) removal from 34.4 % to 14 % due to phosphate interference. In contrast, the quality of NF effluent improved substantially as phosphate concentrations increased, with the NF reaching a maximum of 20.2 % DOC removal efficiency. At low phosphate concentrations (<0.05 mM), NF flux showed minimal change; however, at higher concentrations, a notable decline in NF flux was observed. Additionally, phosphate presence significantly reduced the formation potentials of trihalomethanes (THMFP) and haloacetic acids (HAAFP) in the NF effluent by over 80 % at 0.2 mM phosphate. This reduction may be attributed to the enhanced ability of the NF membrane to intercept low molecular weight organics, mainly due to the presence of macromolecules that CUF failed to remove. Furthermore, the presence of phosphate is thought to increase the electronegativity and hydrophilicity of the fouling layer on the NF surface, which in turn further enhances the rejection of negatively charged dissolved organic matter (DOM).This study provides a mechanistic understanding of how phosphate influences coagulation, floc formation, DOM removal, and membrane fouling within the CUF-NF treatment system. The findings offer valuable insights for optimizing the treatment of phosphate-containing raw waters using dual membrane processes with coagulation pretreatment in practical applications.