Cation and Anion Leakage and the Factors Affecting Them

As feed water flows down the cation bed, cations are exchanged for H+, thus reducing the pH. The higher the total dissolved solids (TDS), the lower the pH and the higher the leakage of sodium. Higher percentages of influent sodium also will increase the sodium leakage because more sodium will be left on the cation exchanger after regeneration. This is why it is not recommended to soften feed water going to a DI system. Since the acid produced by the cation exchanger affects leakage, water that is high in chlorides and sulfates will cause a higher Na+ leakage. Conversely, waters that are high in bicarbonate will give more favorable cation leakage characteristics. One way to reduce overall cation leakage is to increase the acid dosage during regeneration. In fact, the regenerant level is the only controllable variable. However, there is a rapid drop off in the economics for increasing the acid dose compared to the increase in capacity and reduction of leakage. Typical acid levels are approximately 8 pounds per cubic feet of resin.

On the other hand, anion resins love strong acidic anions such as chlorides and sulfates but have a tougher time removing the weak ionized acidic anions such as bicarbonates and silica. This especially is true if there is a considerable Na+ leakage from the cation, which converts to NaOH in the anion exchanger. Efficient operation of the anion exchanger is not concerned with the overall TDS level because the removal of the acidic anions results in a neutral pH, as shown in the reaction:

HCl + R*OH → RCl– + HOH(H2O)

However, the anion is affected adversely by high levels of weak acids (which we already have deemed beneficial to the operation of the cation.) Again, increasing the regenerant level will improve the operation of an anion but the gains are limited by economics. Typical caustic levels are approximately 8 to 9 pounds per cubic feet of anion.

Typical two-bed effluent with 400 to 500 ppm feed water would be a conductivity of about < 5 microsiemens, a pH of 9.5 and silica leakage of 20 ppb.