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Deionisation is a method of water purification, entailing the use of canisters or columns of resin beads which are capable of removing dissolved ionic species from the feed water. Two resins are used - one (cationic resin) to attract and remove positive ions (cations) and another (anionic resin) to remove negative ions (anions). The two resins may be contained seperatly or may be blended into a mixed bed resin. The benefit of seperating the resins is that they may be regenerated once they are exhausted to enable the resins to be reused. DI filters do not produce any waste water while operating, unlike reverse osmosis (RO) and purify 100% of the water that passes through them.

DI can be used on its own (eg Aquarium Pharmaceuticals Water Purifier) or may be used as a final stage to further treat the purified water exiting a RO filter. (Referred to as an RO/DI system). RO/DI systems enable much longer life for the DI resins and are capable of reducing contaminants to extremely low levels.