Comprehensive pilot-scale investigation of seawater nanofiltration softening by increasing permeate recovery with recirculation

Abstract

Two commercial nanofiltration (NF) modules and one tight ultrafiltration module (with molecular weight cut-off of 20 kDa) were assembled to two integrated membrane systems to investigate comprehensively the NF efficiency of seawater softening. The effect of increasing NF permeate recovery (RNF) by recirculation and dosage of chemicals on the observed and the real NF separation performance, concentration polarization extent was evaluated. The results showed that under both HCl acid and antiscalant dosage condition (termed as Scheme II), the superior softening performances with 87.7–93.5% of the observed and 88.5–93.9% of the real total hardness removals were achieved at constant inlet cross-flow velocity of 0.05 m s−1, pressure of 2.02–2.67 MPa, temperature of 21–22°C, and NF retentate recycle ratio (Rr) of 1.69–37.16. The Ca2+, Mg2+, and SO42- ionic concentrations and the total hardness value in NF permeate with RNF at 65%, and NF permeate flux at 6.24 L m–2 h–1 in Scheme II were only around 176, 84, 45, and 798 mg L−1, which was much lower than that in typical SWRO feed streams, and indicated that NF90-4040 (termed as NF1) membrane under chemicals dosage yielded a relatively high efficiency of seawater softening.