Reverse osmosis desalination membranes can be utilized to purify seawater creating clean water. To meet purity requirements multiple membrane modules are typically required and the configuration should be chosen to minimize energy consumption and costs. Here a numerical model is proposed based on a tanks-in-series formulation of model equations. This model was validated against reverse osmosis system analysis (ROSA®) simulation software and used to investigate the performance of a number of different configurations. Systematic evaluation was made on how the performance of membrane systems is influenced by the arrangement of multiple vessels for the multi-module design of membranes systems.