Study On Seawater Nanofiltration

Nanofiltration which is one kind of membrane separation technology is usually used in softening water. The unique property is the ability of NF membranes to selectively reject divalent ions, while passing monovalent ions. Treating seawater using NF membrane not only removed the scaling ions in seawater, decreasing the of scaling tendency and increasing the recovery of the system,but also greatly removed the salinity.This paper treated the seawater of different salinity used the flat membrane and spiral wound membrane NF experimental device which were designed and machined, by the NF90-2540 flat NF membrane, NF90-4040 and NF270-4040 spiral wound NF membrane module. The results show that the ion retention increased according to the following sequence Na⁺,Cl^-,Ca²⁺, Mg²⁺,SO₄^2-. The reduction of hardness, ion retention, rejection of the system, recovery rate of the system, the permeating flux increased when boosting the operating pressure. While raising the inlet temperature,the reduction of hardness, ion retention, rejection of the system decreased, the permeating flux and the recovery rate of the system increased. When raising the influent flux,the reduction of hardness, ion retention, rejection of the system increased, the permeating flux increased, the recovery rate of the system decreased. The rejection of NF membrane increased with the inlet salinity decreasing. The effect of the operating pressure was highly significant. The paper softened the seawater of different salinity with NF90-4040 and NF270-4040 spiral wound NF membrane module. The reduction of hardness was greater than 75% and the removal of SO₄^2- was more than 80%, when treated the feed with NF90-4040 NF membrane. When softened the feed with NF270-4040 NF membrane, the reduction of hardness was larger than 78% and the reduction of SO₄^2- was more than 83%. NF270-4040 NF membrane had a large flux comparing with the NF90-4040 NF membrane.The paper treated the simulated seawater of 35‰(wt‰)by NF90-4040 NF membrane module. During the first-stage nanofiltration, the rejection of Ca²⁺ and Mg²⁺ was higher than 85%, the removal of SO₄^2- was more than 93.40%, the reduction of Cl^- and Na+ was higher than 67%, the rejection of the system was greater than 71%. After the second-stage nanofiltration, the rejection of the whole system was 94%. The conductivity of the second-stage nanofiltration permeating water was 371μs/cm, realizing the seawater desalination.