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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 publicwater vending machines as well as many industrial applications in theelectronics industry operate in this manner.Â 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 othercontaining an anion resin in the hydroxyl (OH-) form. Water flowsthrough 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 areexchanged. The same principle applies when considering anion-exchange. Thedecationised water then flows through the anion column. This time, all thenegatively charged ions are exchanged for hydroxide ions which then combinewith the hydrogen ions to form water (H2O).
In mixed-bed deionizers the cation-exchange and anion-exchange resins areintimately mixed and contained in a single pressure vessel. The thoroughmixture 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 appreciablyhigher than that produced by a two-bed plant.
Although more efficient in purifying the incoming feed water, mixed-bed plantsare more sensitive to impurities in the water supply and involve a morecomplicated regeneration process. Mixed-bed deionizers are normally used toÃ¢ÂÂpolishÃ¢ÂÂ the water to higher levels of purity after it has been initiallytreated by either a two-bed deionizer or a reverse osmosis unit.