Effects of microplastic on submerged nanofiltration for advanced drinking water treatment

The removal of organic matter from drinking water is critical for protecting public health. However, their removal by an emerging process, submerged nanofiltration (NF), can be affected by competing substances, such as microplastics (MPs). This study aimed to investigate the impact of MPs in reservoir water on the removal of dissolved organic matter (DOM) using a submerged NF membrane. In the experiments, a loose NF membrane (NF270) was operated at a pressure below 0.1 MPa. Polyethylene terephthalate microplastics (PET-MPs) were introduced into the feed water at concentrations of 0, 10, 100, and 1,000 mg/L for filtration cycles 1, 2, 3, and 4, respectively. The results showed that the permeate flux did not significantly differ across all cycles, ranging between 12.7 and 14.7 L/m²·h. Increasing PET-MP concentrations adversely affected DOM removal efficiency, which was analyzed in terms of dissolved organic carbon (DOC) and ultraviolet absorbance at 254 nm (surrogate indicator of humic acid concentrations). Specifically, as the PET-MP concentration increased from 0 to 1,000 mg/L, the DOC removal decreased from 87.9% to 86.6%, and the UV-254 absorbance removal decreased from 97.5% to 90.0%. This study demonstrated that the submerged NF270 membrane effectively removed DOM via a size exclusion phenomenon. Humic acid promoted electrostatic repulsion and encouraged ion retention by increasing the negative electrical charge on the membrane. In contrast, water molecules, which are small and nearly uncharged, can permeate fouled membranes more readily than organic molecules. However, the removal efficiency decreased with increasing PET-MP concentration. Sponge cleaning reduced membrane surface fouling, as indicated by the stable permeate flux. Overall, this study demonstrated that submerged NF treatment can be stably operated without the effects of MPs on membrane fouling.