This paper discusses the performance and characteristics of desalination membranes used in treatment of water for potable use. In this study, The Desalination Economic Evaluation Program (DEEP) is a spreadsheet tool originally developed for the IAEA by General Atomics and later expanded in scope by the IAEA, The DEEP-5.1 version program allows designers and decision makers to compare performance and cost estimates of various desalination and power configurations. Desalination options modeled include MSF, MED, VC,RO and hybrid systems. This paper presents the results of the operational performance of RO units in experimental scale. The rate of decline of productivity for a period 38 Days is examined and described by simple power law function. The evaluation of the solvent and salt permeability coefficients with the time of operation is quantified. The results show the recovery, pressure and conductivity across the membrane for 38 days test run. Also, the results show the effect of chemical additives and operational parameters on the performance of the membrane during the cleaning. Based on such individual RO module data (one year) the product flow rate and Total Dissolved Solids (TDS) performance is calculated and RO module replacement ratio can be estimated. These developments can be utilized to improve the quality of surface seawater feed to the level comparable to, or better than the water quality from the well water sources. These new developments enable a more advanced RO system design which should result in increased reliability and lower water cost. And compare the result of reverse osmosis desalination conventional system with a mathematical model is built and solved with ROSA computer programs to get results for the design calculations of the reverse osmosis mode used to remove salt from the feed water. The principles of the reverse osmosis mode, the conditions and factors affecting their operation, are discussed. The number of stages with their arrangements in the reverse osmosis mode is calculated using the ROSA Program. A mathematical model and computer programs including all the mentioned cases are built and applied on the reverse osmosis mode of water stations. The results of these calculations are compared with the practical results and showed a fair good agreement.