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Deionizersare most commonly used when ionic contamination is such that reverse osmosisalone cannot be relied upon to produce water of acceptable quality. In suchmost instances, mixed bed deionizers may be placed downstream of the reverseosmosis unit, completing the purification process. A wide variety of public water vending machines as well as many industrial applications in the electronics industry operate in this manner. The common application of deionizers are us under
The two-bed deionizer consists of two vessels - one containing acation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form. Water flows through the cation column, whereupon all the cations are exchanged for hydrogenions. To keep the water electrically balanced, for every monovalent cation,e.g. Na+, one hydrogen ion is exchanged and for every divalentcation, e.g. Ca2+, or Mg2+, two hydrogen ions are exchanged. The same principle applies when considering anion-exchange. The decationised water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).
In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of cation-exchangers and anion-exchangers in a single column makes amixed-bed deionizer equivalent to a lengthy series of two-bed plants. As aresult, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.
Although more efficient in purifying the incoming feed water, mixed-bed plants are more sensitive to impurities in the water supply and involve a 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 two-bed deionizer or a reverse osmosis unit.