Performance evaluation of polyamide nanofiltration membranes for phosphorus removal process and their stability against strong acid/alkali solution

In this study, a quantitative performance of three commercial polyamide nanofiltration (NF) membranes (i.e., NF, NF90, and NF270) for phosphorus removal under different feed conditions was investigated. The experiments were conducted at different feed phosphorus concentrations (2.5, 5, 10, and 15 mg·L^-1) and elevated pHs (pH 1.5, 5, 10, and 13.5) at a constant feed pressure of 1 MPa using a dead-end filtration cell. Membrane rejection against total phosphorus generally increased with increasing phosphorus concentration regardless of membrane type. In contrast, the permeate flux for all the membranes only decreased slightly with increasing phosphorus concentration. The results also showed that the phosphorus rejections improved while water flux remained almost unchanged with increasing feed solution pH. When the three membranes were exposed to strong pHs (pH 1.5 and 13.5) for a longer duration (up to 6 weeks), it was found that the rejection capability and water flux of the membranes remained very similar throughout the duration, except for NF membrane with marginal decrement in phosphorus rejection. Adsorption study also revealed that more phosphorus was adsorbed onto the membrane structure at alkaline conditions (pH 10 and 13.5) compared to the same membranes tested at lower pHs (pH 1.5 and 5). In conclusion, NF270 membrane outperformed NF and NF90 membranes owing to its desirable performance of water flux and phosphorus rejection particularly under strong alkali solution. The NF270 membrane achieved 14.0 L·m^-2·h^-1 and 96.5% rejection against 10 mg·L^-1 phosphorus solution with a pH value of 13.5 at the applied pressure of 1 MPa.