Role of organic fouling layer on the rejection of trace organic solutes by nanofiltration: mechanisms and implications.

To investigate how the organic fouling layers on nanofiltration (NF) membrane surface and the strong matrix effect (particularly by Ca2+) influence the rejection of trace organic compounds (TOrCs), filtration experiments with two TOrCs, bisphenol A (BPA) and sulfamethazine (SMT), were carried out with virgin and organic-fouled NF membrane. Organic fouling layer on the membrane was induced by sodium alginate (SA) at different concentrations of Ca2+. The results indicated that NF membrane maintained consistently rejection of TOrCs with little influence by membrane fouling at lower Ca2+ concentration. In contrast, organic fouling caused at higher concentration of Ca2+ observably restrained the rejections of both BPA and SMT. Furthermore, based on the cake-enhanced concentration polarization (CECP) model, the rejection of TOrCs was divided to the real rejection and the mass transfer coefficient. Moreover, it was found that the decrease in rejection resulted by organic fouling was due to the real rejection that was restrained by fouling layer with irregular impact on the mass transfer coefficient. Although the mechanism of trace compounds rejection was complex, the controlling factors varied among foulants. Nevertheless, the steric effect of the cake layer played an important role in determining solute rejection by organic-fouled NF membrane.