Discussion on calculation of maximum water recovery in nanofiltration system

• A method has been proposed to calculate the maximum recovery for NF2 system.
• Water recovery for a NF element should be lower than 25% to maintain it stable.
• The CP degree was recommended to be 1.15 at the recovery of 25% in NF system.

In desalination process, the effect of concentration factor (CF) and concentration polarization degree (CP degree) on water recovery in nanofiltration (NF) system displays a significant difference from that in reverse osmosis (RO) system due to their different salt rejections. In this work, the relationship between CF, salt rejection and water recovery was firstly discussed in a single NF element. In order to keep a stable performance, a spiral NF2 membrane was taken as an example, and water recovery for the single NF2 element should be about 25% instead of 15%, which is generally considered suitable for the single RO element. Then, the CP degree was calculated according to the theoretical model and experimental test, and the ultimate value was pointed out to be below 1.2. Thus, when calculating the scaling tendency of the sparingly soluble salts, the salt concentration near the membrane surface was recommended to be 1.15 times as high as that in bulk solution. Finally, a calculation method for maximum recovery was proposed for NF system. Because of the varied rejection with different NF membranes for the same feed water, which is significantly different from that in RO system, the pre-test is very necessary for NF system design.