Transport phenomena in membrane processes have been theoretically studied for a lab-scale flat geometry membrane module, which was continuously fed with fruit juice solutions. The proposed model is based on the solution of continuity, momentum and mass transfer equations, modified according to the boundary layer theory coupled with the osmotic pressure model. The aim of the present study is to determine the range of operating conditions in the laminar flow regime, where the concentration polarization phenomena, typical of membrane operation, are maintained within acceptable limits. Membrane hydraulic permeability, generalized Reynolds number and fluid rheological properties were shown to play a crucial role in the system performances. Both the diffusion coefficient and the free stream velocity are critical in evaluating membrane performances. As far as the permeate flux decay is concerned, an opposite trend between the diffusion coefficient and axial velocity, is observed at a fixed Reynolds number. The presented model is general: it could be applied to characterize the concentration polarization phenomena whenever a solution, modeled by non-Newtonian constitutive equations, is fed to a membrane module. (C) 2001 Elsevier Science Ltd. All rights reserved.