Breaking the Symmetry: Mitigating Scaling in Tertiary Treatment of Waste Effluents Using a Positively Charged Nanofiltration Membrane

When salinity of municipal wastewater increases and approaches the limits of toxicity for plants, moderate desalting of wastewater becomes vital for keeping it suitable for irrigation. Nanofiltration (NF) is an attractive solution, as it partially removes NaCl. Unfortunately, commercial NF membranes (e.g., NF270) strongly reject multivalent ions present in wastewater, especially, scale-forming calcium and phosphate. This results in undesired demineralization, severe membrane scaling, and unacceptably low water recovery. To address this problem, we report here that a positively charged NF (p-NF) performs significantly better than NF270, owing to overall lower rejection of scale-forming ions. Therefore, for a commensurate flux and NaCl rejection, p-NF shows much less scaling than NF270, even at recoveries as large as 80%–85%. This suggests that p-NF may have an advantage over standard NF for moderate desalting of wastewater and other water sources with high scaling potential.