In mixed-bed deionizers the cation and anion-exchange resins are intimately mixed and contained in a single pressure tank. The thorough mixture of cation-exchangers and anion- exchangers in a single column makes a mixed-bed deionizers equivalent to a lengthy series of twin bed deionizers. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a twin bed deionizer.

Although more efficient in purifying the incoming feedwater, mixed-bed deionizer are more sensitive to impurities in the water supply and involve more complicated regeneration process. Mixed-bed deionizers are normally used to ‘polish’ the water to higher levels of purity after it has been initially treated by either a twin bed deionizer or a reverse osmosis unit