In mixed-bed deionizers the cation-exchange and anion-exchange resins are commonly mixed in a single pressure vessel. The thorough mixture of cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. The modular construction permits flexibility in the selection of cation and anion units to cater to different types of raw water. The applications have it uses in food and beverages, pharmaceuticals and paper and pulp industry. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant. Mixed-bed deionizers are normally used to ‘polish' the water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis RO unit.