In order to understand reverse osmosis, we must first define osmosis. Osmosis is the passage of a liquid through a semi-permeable membrane. A semi-permeable membrane is a membrane which allows one component of a solution to pass through it and not the others. In osmosis, there is a tendency for a liquid to go from an area of less concentration to an area of more concentration through a semi-permeable membrane.
If sufficient pressure is applied to the concentrated solution, reverse osmosis will take place. The pressure causes a flow through the semi-permeable membrane into the dilute solution. The semi-permeable membrane acts as a barrier to most ions and many other contaminants and does not allow them to pass through into the dilute solution. When applied to water, this means that the product water has a reduced total dissolved solids content as a result of the passage of water molecules through the membrane while the mineral ions are rejected.
The Deionization Process Deionization or demineralization is the process of removing up to 95 percent of the dissolved minerals such as calcium, magnesium, sodium, manganese, sulfates, chlorides, nitrates and silica from the water supply by way of ion exchange. Dissolved minerals in the water consist of two parts-a metallic part which is a positively charged ion and a non-metallic part which is a negatively charged ion. Two separate resins are utilized for the Deionization process; the cation resin exchanges the positive ions for hydrogen ions (H+) while the anion resin exchanges the negative ions for hydroxide ions (OH-). The cation resin is regenerated with hydrochloric acid (HCl), and the anion resin is regenerated with sodium hydroxide (NaOH), also known as caustic soda.