Nanofiltration membranes (NF) and Reverse osmosis have applications in several areas. One of the main applications has been in water treatment for drinking water production as well as wastewater treatment. The Reverse osmosis (RO) and nanofiltration (NF) plants use a semi permeable membrane that allows water to pass but not salts. The thermodynamic analysis of desalination processes along with actual plant operational data can be useful to put various desalination techniques in perspective, and the best analytical tool to do this is the second law analysis. Thus the analysis in this article starts from a basic separation process which is the main objective of all desalination process, and conducts a comprehensive analysis using actual operation data from major desalination processes.