It is a process by which water is demineralized or deionized through the use of pressure via a semi-permeable membrane. Semi-permeable membranes are membranes that allow some molecules and atoms to go through but not others. In reverse osmosis, water passes through a semi-permeable membrane but most of the dissolved salts, bacteria and other contaminants.Pressure is required for the reverse osmosis process to be completed in order to allow pure water to pass whilekeeping contaminants at bay.