Loose nanofiltration membranes for selective rejection of natural organic matter and mineral salts in drinking water treatment

Loose nanofiltration (LNF) is a promising technology for advanced treatment of drinking water, primarily owing to the potentially high rejection of natural organic matter (NOM) and high passage of mineral salts, particularly divalent cations, along with the low operating pressure. This study was devoted to relating the performance with the properties of LNF membranes, and obtaining the membrane properties most suitable for drinking water treatment. Cluster analysis indicated that the commercially available LNF membranes would behave quite differently, mainly depending on the molecular weight cut-off (MWCO). By using a treated natural water as the feed, a best-performing membrane had a mineral salts rejection lower than 30% and an NOM rejection higher than 70%, with the membrane water permeance at ∼17 L/m2/h/bar. The LNF membranes with high mineral salts/NOM selectivity should have an MWCO of around 1000 Da, sufficiently uniform pore sizes, and high surface negative charge density. The rejection characteristics for mineral ions by LNF membranes were similar with the relatively dense conventional NF membranes, but with a lower rejection difference among the ions, manifesting less strong co-ion and counter-ion competition effects. The LNF membranes with an MWCO of ∼1000 Da could satisfactorily remove (>80%) disinfection byproduct precursors, with small-sized protein-like and other substances remaining in the permeate water. This study helps to understand the rejection characteristics and mechanisms of LNF membranes, which will benefit the synthesis of high-performance LNF membranes for drinking water treatment and other applications.