An investigation into the relationship between flux and roughness on RO membranes using scanning probe microscopy

The performance of four commercial reverse osmosis (RO) membranes was studied to ascertain if there is a universal relationship between membrane surface roughness and flux. The atomic force microscope (AFM) was used to image the membrane surfaces. It was found that for these membranes (Osmonics SG, AD, AG and TriSep X20) a general correlation between surface roughness and membrane flux does not exist. Osmonics AG and AD have similar surface roughness values but the flux using Osmonics AG is 10 times higher than with Osmonics AD. The flux with TriSep X20 membrane is similar to Osmonics AD yet the surface roughness of TriSep X20 is 75% higher than Osmonics AD. Although a relation between surface roughness and flux was not found in this study it cannot be generalized that this is the case for all membranes. More intensive research needs to be carried out in this area to understand fully the effect of surface roughness on flux with special attention being paid to AFM roughness results and the methods used to characterize surface roughness. That is because 1) RO membranes surfaces are not even and in this study imaging different areas of one membrane resulted in tip to 33% variation in surface roughness. Gaining representative values of surface roughness is therefore an important aspect in surface characterization; 2) AFM imaging in water showed that TriSep X20 surface roughness increased by 35% after 2 h of soaking in water. This indicates that membrane surface morphology changes upon exposing the membrane to different environments and so the conditions under which the roughness is measured are very important.