Effect of influent matrices on the performance of polyelectrolyte hollow fiber nanofiltration membranes for drinking water treatment

The widespread presence of hardness ions in surface water poses potential challenges for drinking water treatment, which can be efficiently removed by nanofiltration (NF) membranes. Compared with mainstream spiralwound polyamide ones, hollow fiber (HF) NF membranes present a promising alternative due to the potential advantages in operation. This study investigated the ion rejection properties of a commercial polyelectrolyte multilayer (PEM)-based HF NF membrane by treating surface water with varying hardness levels, respectively resembling the raw water hardness of northern and southern areas of China. Results show that compared to NF270 membrane, the PEM HF NF membrane had a thicker separation layer with higher surface hydrophobicity, leading to a lower water permeance. Despite the predominantly negatively charged surface, the PEM HF NF membrane achieved high CaCl2 and MgCl2 rejections of 85.4 % and 90.2 %, respectively. During surface water treatment, the PEM HF NF membrane maintained stable water purification performance after several cycles of backwashing and outperformed NF270 membrane in removing organic matters and hardness ions. The distinct ion rejection characteristics and underlying mechanisms for the PEM HF NF membrane treating surface water with different hardness levels were comprehensively elucidated. Moreover, the membrane negative rejection of Na+ in low hardness water affected by the Na+/Cl- molar ratio in the feed was particularly interpreted. This study provides performance demonstration and mechanistic insights for promoting the potential application of PEM HF NF membranes for drinking water treatment.