AbstractThe performance of reverse osmosis and tight nanofiltration with flat‐sheet membranes can be predicted accurately. The proposed numerical model solves the local momentum and mass conservation equations in the module's feed channel with solution‐diffusion boundary conditions. Both qualitative and quantitative predictions of the permeate flux and of the rejection rate are obtained with an accuracy depending on the limitations of the solution‐diffusion model for describing membrane mass transport and on the value of solute permeability. As an extension of the applications to plate‐and‐frame modules, the ability to describe the performance of processes carried out with spiral‐wound modules is also tested with own desalination experiments and with data from the literature.