An RO water desalination system consists of the pre treatment section, the desalination section and the post treatment section. In the new design plants different energy recovery systems are adopted to reduce the energy consumption of desalination section, Among the various energy recovery systems the pressure exchanger (PX) system produces notable benefits not only to the energy consumption but also regarding the high pressure pump size and the consequent scale-up of the system. A secondary effect of the use of PX is the increasing of feed salinity to the desalination section due to the mixing of the concentrate coming from RO section with the portion of feed water passing trough the PX device, on the contact layer between the two streams. The different software supplied by the membrane manufactures does not take into account this phenomenon, making chemical dosing calculation on RO train inlet feed analysis that is different from the raw water analysis. An original model calculation to take into account these different seawater conditions has been developed based on the classical carbonate system equilibrium allowing production of a complete chemical and mass balance of the entire system including the chemicals dosing rate based on raw water characteristics. The model is not affected by the membrane characteristics and can be utilized for an easy and quick basic design of the plant.