Deionized water (DI) is deeply demineralized, ultra-pure water with the resistivity close to 18 megohm-cm. It is used in microelectronics, printed circuit boards, instrument manufacture, pharmacy, washing liquids, etc. In order to obtain the high quality pure deionized water, a multi-stage water purification process can be used. After pre-cleaning, the water is supplied to the reverse osmosis membrane, and then the water is filtered through a special deionization medium, which removes the rest of the ions in the water. The purity of deionized water can exceed the purity of distilled water.