Comparison of nonhomogeneous and homogeneous mass transfer in reverse osmosis membrane processes

Reverse osmosis (RO) membranes are pressure driven, diffusion controlled processes. Current diffusion controlled solute mass transfer models assume a homogeneous membrane surface. This study evaluated mass transfer processes assuming mass transport is not a homogeneous process, which is dependent on the thickness variation of the membrane's active layer. Three-dimensional ridge and valley active layer morphologies were created numerically using Gaussian random vectors. A nonhomogeneous solution diffusion model (NHDM) was then developed to account for surface variation through the active layer. NHDM was further modified by incorporating concentration polarization (CP) effects. A comparison of the NHDM and the NHDMCP with the commonly accepted homogeneous solution diffusion model (HSDM) using pilot-scale brackish water RO operating data indicated that the NHDM is more accurate, when the solute concentration in the feed stream is low, while NHDMCP appears to predict better with a high solute feed concentration.