Ultrafiltration as pretreatment of seawater desalination: Critical flux, rejection and resistance analysis

In this study, the concept “critical flux” was used in the seawater desalination system, which could used to enhance the performance of ultrafiltration (UF) process regarding membrane fouling. The impacts of various factors, such as seawater properties (natural organic matter 0.012–0.082 cm−1 of UV254, ionic strength 409–470 mmol L−1, calcium concentration 19–29 mmol L−1), membrane property (molecular weight cut-off, 20–30 and 80–100 KDa) and hydrodynamic conditions (cross-flow velocity 0.02–0.17 ms−1), on the UF performance were systematically investigated in terms of the critical flux, rejection and filtration resistance under Jcrit conditions. The results showed that the critical flux declined with an increase in NOM, calcium concentration and membrane MWCO, but the critical flux was improved by elevated feed turbidity and cross-flow velocity. Ionic strength was found to have less impact on the critical flux than divalent ion concentration, and both impacts of ionic strength and divalent ion concentration depended on the NOM concentration in the feed. The turbidity removal efficiency maintained at approximately 100% under all the testing conditions, while the NOM removal efficiency increased with the NOM concentration and feed turbidity, and decreased with the membrane MWCO, regardless of ionic strength and calcium concentration in the feed. As for the filtration resistance under Jcrit conditions, it showed that the resistance caused by fouling (adsorption) was minimal and could be ignored under all the testing conditions except for ultra low cross-flow velocity (0.02 ms−1).

► Concept “Jcrit” was used for optimization of UF system in treating seawater.
► Impacts of factors on the UF performance under Jcrit conditions were investigated.
► Dosing of HA decreases Jcrit and enhances UV254 removal under Jcrit conditions.
► Salt concentration has little impact on removal efficiency under Jcrit conditions.
► Lower Jcrit and lower resistance are obtained by UF membrane with larger MWCO.
► Enhanced Jcrit and lower resistance is observed at higher cross-flow velocities.