Seasonal variations of nanofiltration (NF) foulants: identification and control

Studies were conducted to develop correlation between nanofiltration (NF) membrane fouling, feed water quality, and membrane characteristics. The water quality of the three seasonal NF feed samples was characterized by UV, DOG, HPLC-OUVA-DOC, XAD, IC and ICP. Surface charge, specific flux variation at different pH and ionic strengths, molecule weight cut-off (MWCO), and functional groups of the NF 200 membrane surface were determined. The extent of fouling was significantly dependent on water quality, membrane properties, and operational conditions. Higher flux decline was observed at high DOG, high divalent cations, high alkalinity and low temperature. Temperature strongly affected the specific flux and natural organic matter (NOM) rejection. Inorganic salt precipitation was proven by FTIR, X-ray diffraction, X-ray fluorescence and SEM. NOM fouling was observed during long run pilot tests at relatively high temperature while scale fouling was observed at both high and low temperatures. Due to the decrease in solubility of CaCO3(s) and CaSO4(s) at high temperature, homogeneous crystallization occurred that led to less nux decline. At low temperature, in spite of the increased solubility, heterogeneous crystallization with NOM absorption was observed. This is due to an increase in pH and formation of nucleus components induced by a temperature decrease. Heterogeneous crystallization caused more significant flux decline than homogeneous crystallization.