Performance of UF–NF integrated membrane process for seawater softening

Nanofiltration (NF) membranes are playing a more and more important role in brackish and seawater softening process, wastewater reclamation and other industrial separation. In this study, ultrafiltation (UF) and NF (ESNA3, Hydranautics) were assembled to an integrated membrane system (IMS) as pretreatment prior to seawater reverse osmosis (SWRO) to evaluate the NF efficiency of seawater softening. The influences of NF feed property (the feed from UF1 with 100 kDa MWCO and UF2 with 20 kDa MWCO, respectively), operating pressure (0.5–3.0 MPa), cross-flow velocity (0.028–0.048 m/s) and feed temperature (5–17 °C) on the softening efficiency in terms of the NF flux and ion rejection were investigated. The results showed that the high softening efficiency of NF membrane could be achieved using UF membrane with smaller MWCO (20 kDa). NF system using UF2 as pretreatment process yielded a higher flux of 61.4 L/(m2 h) with Ca2+, Mg2+, SO42− rejection of 44.37%, 73.26% and 99.82%, respectively, under the optimum operating conditions of operating pressure at 1.5 MPa, cross-flow velocity at 0.035 m/s and feed temperature at 12.5 °C. And, the relatively excellent selectivity of divalent ions and univalent ions can be obtained by ESNA3 membrane compared to that of other NF membranes reported in the literature, indicating that the ESNA3 membrane seems to be appropriate for seawater softening process.

Research Highlights
► An integrated UF-NF process to soft seawater.
► The effects of different factors on the softening efficiency of the UF-NF process.
► ESNA3 NF membrane seemed to be appropriate for seawater softening.