Water demineralizing units, such as two-bed demineralizers, are used to remove dissolved minerals such as calcium, magnesium, sodium, sulfate, chloride, and alkalinity from water. Typically, ion exchange resins such as the strong base anion, weak base anion, and strong acid cation types are used in the demineralization of water.
As a disadvantage, strong base demineralizers normally leak a few parts per million of alkalinity, caused mostly by the presence of sodium ions which are not neutralized by the presence of strong acid anions, so that at least some of the residual sodium ions present may be considered to be in the form of sodium hydroxide.
In some industrial uses, it is desired to remove this residual alkalinity from the effluent water to obtain a product having a pH that is very close to neutral (between 6.8 and 7.2) and having an electrical resistivity on the order of one megaohm. Such a neutral pH and high resistivity cannot be achieved in the presence of traces of sodium hydroxide.
Such small amounts of alkalinity can be effectively removed by either a strong acid or weak acid ion exchange resin, both having been employed in the past for such a purpose. However, the use of such "polishing" units (for removing last traces of alkalinity in effluent water) requires added, expensive ion exchange cells having high ion exchange capacity, so that there is no need for frequent regeneration of the cells. For example, in the prior art, one might "polish" the water effluent stream from a 42 inch two-bed demineralizer flowing at 100 gallons per minute with a 30-inch to 40-inch cation polishing unit.
In accordance with this invention, an improvement is proposed in the method of "polishing" effluent water from a strong base demineralizer which carries a residual amount of alkalinity. By the improvement of this invention, the size and expense of the "polishing" unit can be significantly reduced without a significant loss of water processing capacity. Thus, large quantities of water may be demineralized by this invention to desired pH and high resistivity at reduced cost.