Performance Of Uf-nf Integrated Membrane Process For Seawater Softening

As the pretreatment technology in seawater desalination, Ultrafiltration (UF) can provide highly permeate which can totally satisfy the feed quality requirements, such as SDI15, turbidity. But Ultrafiltration (UF) cannot remove the low molecular weight organic matters (such as the molecular weight between hundereds and thousands of organic matters in seawater). It was reported that these low molecular weight organic matters can produce new fouling for RO membrane surface. Because of NF membrane with molecular weight cut-off (MWCO) of 200～2000 Da, it has the separation characteristics to remove these matters and the divalent ions (the scale proning ions) effectively. So it is significant to adopt UF-NF systems as integrative pretreatment techniques to evaluate for seawater softening.In this study, two UF-NF systems were adopted as integrative pretreatment techniques to evaluate for seawater softening. They are UF1 (PES 100 kDa MWCO)-NF and UF2 (PS 20 kDa MWCO)-NF. Their nanofiltration performance was evaluated by operating pressure (0.5 MPa-3.0 MPa), cross-flow velocity (0.03 m·s-1～0.05 m·s-1) and feed temperature (5℃～17℃). Several parameters are examined including the DOC of UF2 (PS 20 kDa MWCO)-NF process and UV254 under long-term operation with the variance of seawater. The main content and conclusions of present study are as follows:1) In the UF1-NF coupling process, the experimental data showed that the flux, the recovery and the rejection of nanofiltration continued to rise for both processes when the operating pressure increased from 0.5 MPa to 3.0 MPa gradually. The rejection of ESNA3 was 10.07% when the operating pressure was 0.5 MPa for UF1-NF process; the rejection of Mg2+, Ca2+ reached to 19.58%,45.39% at the 0.5 MPa operating pressure. Then the similarly phenomena were occurred in the cross-flow velocity (0.03 m·s-1～0.05 m·s-1) and feed temperature (5℃～17℃) processes.2) In the UF2-NF coupling process, the experimental data showed that the flux, the recovery and the rejection of nanofiltration continued to rise for both processes when the operating pressure increased from 0.5 MPa to 3.0 MPa gradually. The rejection of ESNA3 was 10.22% when the operating pressure was 0.5 MPa for UF2-NF process; the rejection of Mg2+, Ca2+ reached to 28.17%,54.27% at the 0.5 MPa operating pressure. Then the similarly phenomena were occurred in the cross-flow velocity (0.03 m·s-1～0.05 m·s-1) and feed temperature (5℃～17℃) processes.3) Comparing with the UF1-NF coupling process, the experimental data showed that the flux, the recovery and the rejection of nanofiltration continued to rise for both processes. But the performance of the UF2-NF process was higher than the UF1-NF process all the time. No matter what in the operating pressure (0.5 MPa-3.0 MPa) or cross-flow velocity (0.03 m·s-1～0.05 m·s-1) or feed temperature (5℃～17℃) processes. UF2-NF coupling process showed better performance in seawater softening and salt rejection.4) In the long-term operation with the variance of seawater for the UF2-NF process, each parameters including operating pressure, flux and recovery were detected continuously.