Effect of surface roughness on retention of reverse osmosis membranes

The performances of two commercially available Thin Film Composite reverse osmosis membranes i.e. TFC-ULP and TFC-SR2 from Fluid Systems were studied. For the TFC-ULP membrane low water flux ( around 7 lit/m(2).h) and high ion retention (around 70%) were obtained. However the TFC-SR2 membrane showed much higher flux (90 lit/m(2).h) and much lower retention (around 15%). Unlike the common membranes, TFC-ULP and TFC-SR2 membranes are composed of three layers. The skin layers of both membranes are made from polyvinyl alcohol and the two support layers are made from poly aryl sulfur ether and polyester. However contradictory behaviors of the membranes cannot be explained by the characteristics of materials used for membrane preparation and/or membrane morphology. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that the TFC-ULP membrane possesses a rough surface while the TFC-SR2 membrane predominantly has a smooth surface. Contradictory membrane performances may be due to higher trapping of the ions on the rough surface of TFC-ULP membrane resulting in higher retention. The deposition causes higher resistance against the flow leading to lower flux. The smooth surface of the TFC-SR2 membrane has less ability to trap the ions resulting in lower retention, lower resistance against the flow and therefore higher flux.