In order to optimize the performance of NF and RO membrane filtration plants, the influence of the plant design and operating conditions has been studied. For this purpose a simplified mathematical model has been developed, which takes into account the difference in rejection of monovalent and bivalent ions, and the hydraulics of SPW membrane modules. This study clearly shows that an increase in permeate productivity of 20% can be achieved, by lowering the number of membrane modules (N<2-3) per pressure vessel.