Dealkalization is the addition of acid in some form, to drive the alkalinity equilibrium to the left (see the Chemical Equilibrium discussion in Section I), so that some of the alkalinity is lost as CO2, To do that with ion exchange requires loading the resin’s (or zeolite’s) active sites with H+, which is the embodiment of acidity. That is accomplished by regenerating the bed with a 5% acid solution (50,000 ppm H+) instead of a salt brine. The resin’s active sites have a “preference” for Ca++ and Mg++ and other divalent cations, but the high concentration of H+ ions overwhelms the preference and eventually removes all of the calcium and magnesium if the regeneration lasts long enough. During the service cycle, the resin exercises its preference and readily exchanges each Ca++ and Mg++ ion in the water for two H+ ions. These affect the alkalinity equilibrium instantly and cause some to be lost as CO2 gas. Thus, the overall result is removal of both hardness and alkalinity, and a reduction in TDS as well.
Dealkalization can be done with the same type of ion exchange resin as that used for softening, called strong-acid or strongly acidic cation exchange resin, but that is risky: when there is more hardness in the water than alkalinity (a common condition), more H+ will be produced than there is’ alkalinity to be neutralized, resulting in an acidic, corrosive, over-treated water. Therefore, it is more prudent to use a different type of resin, called weak-acid or weakly acidic cation exchange resin, sometimes referred to as “WAC resin,” for Weak Acid Cation. The difference between them is the strength to overpower the effects of weak acids, or buffers, in the water. (See the discussion of buffers in Section I.) When a WAC resin is regenerated with acid and used to exchange H+ for hardness ions, it works only as long as alkalinity remains available to neutralize the H+. If the water has more hardness than alkalinity, WAC resins in H+ form work only up to the point of destroying all of the alkalinity, and then they stop. For example, in treating a water supply with 300 ppm of hardness as CaCO3 but only 250 ppm of alkalinity as CaCO3, WAC resin would exchange only 250 of the 300 ppm, leaving 50 ppm of hardness and zero alkalinity, with no risk of over-treating and creating a corrosive condition. If the water has more alkalinity than hardness, WAC resin will exchange all of the hardness and neutralize the same amount of alkalinity.
There is also a third ion exchange process for dealkalization, and that is using a strong-base or strongly-basic anion exchange resin in chloride form. It is regenerated with NaCI salt brine just like softening; the difference is that the resin exchanges two CI– ions for CO3-2 ions instead of two Na+ ions for Ca++ or Mg++ ions. The system is set up and run exactly like a softener. This is hardly ever done, because softener resin is among the cheapest and strong-base anion resin the most costly media available.