It is the removal of mineral ions by using the ion exchange process. Using a mixed bed of anion and cation resins; ions, salts and minerals can be entirely removed from water. The RO membrane has a direct impact on the life-span of the DI. The RO membrane can remove about 95-99% of TDS (minerals) in your water. When a DI is used after the RO membrane, you can expect almost 100% removal of the TDS.