Effects of organic, biological and colloidal fouling on the removal of pharmaceuticals and personal care products by nanofiltration and reverse osmosis membranes

The impact of different foulant composition and pHs on the removal of pharmaceutical and personal care products (PPCPs) by nanofiltration (NF270 and NF90) and reverse osmosis (XLE) membranes was investigated in this study. Humic acid (HA), alginate (SA) and silica (Si) were used for the formation of organic, biological and colloidal fouling, respectively, and were further mixed for the formation of composite fouling on membrane surfaces. The results show that significant flux decline was observed with decreasing pH and all kinds of foulant composition for the three membranes, which was contributed mostly by irreversible fouling, especially for the tight XLE. Moreover, the trends of flux decline and particle size of composite foulants at different pHs were dominant by HA for all membranes, implying that the control of organic matrix in water before reaching NF/RO process is crucial to the performance of membrane filtration. On the other hand, all types of membrane fouling caused the decrease of PPCP rejection by NF270, especially for the hydrophilic-ionized and hydrophobic-ionized compounds at low pHs because of the shield of dominant electrostatic repulsion mechanism between PPCPs and membrane surface, causing the cake-enhanced concentration polarization phenomenon. The results were verified by dense and thick cake layers observed using scanning electron microscopy, the corresponding change of contact angles, and high adsorption amount of very hydrophobic triclosan on the active layer of NF270. However, steric hindrance and electrostatic repulsion worked synergistically for the increasing PPCP rejection by NF90 with increasing pH and membrane fouling, while steric hindrance dominates PPCP rejection by XLE. Therefore, one order less triclosan adsorption happened on membrane surface and the fouling layer of NF90 and XLE. The fouling mechanisms were illustrated using the modified Hermia model as gel layer formation for the loose NF270, intermediate blocking for NF90 and complete blocking for XLE.