Effect of urea-based chemical cleaning on TrOCs rejection by nanofiltration membranes

Nanofiltration (NF) is an effective method for removing trace organic compounds (TrOCs) from drinking and reused water. However, chemical cleaning of NF membranes may impact their properties, leading to a decrease in TrOCs rejection. To better understand the mechanisms behind changes in membrane rejection after chemical cleaning, we investigated the changes in surface properties and TrOCs rejection of two commercial membranes (NF270 and NF90) after urea/ hydrochloric acid (HCl) cleaning, and compared them to conventional sodium hydroxide (NaOH)/HCl cleaning. The findings indicated that urea cleaning increased the membrane pore size and hydrophilicity while marginally affecting membrane crosslinking and zeta potential. Possibly due to the thicker and denser active layer of NF90, neither urea-based nor NaOH-based cleaning had a significant impact on its TrOCs rejection and permeability. In contrast, NF270 was more susceptible to chemical cleaning, and urea-based cleaning had less impact on its TrOCs rejection than NaOH-based cleaning. Although urea cleaning increased the pore size of the NF270 membrane, its pore size recovered well after HCl cleaning, resulting in good recovery of the rejection of hydrophilic neutral TrOCs. Moreover, urea cleaning had limited effects on the electrical properties of the NF270 and might enhance the hydrophilicity of the membrane, resulting in full recovery of the rejection of hydrophobic and negatively charged TrOCs after urea/HCl cleaning.