Weak Acid Softening

The Process

The weak acid cation resin is a polyacrylic cross-linked cation with carboxylic acid functionality and has a Pka between 5 and 5.5. The weak acid cation resin being most different from its strong-acid cation counterpart, offers a distinct advantage for the application considered. With this resin, it is possible to remove the temporary hardness as well as reduce the total dissolved solids without the addition of any sodium.

Exhaustion Reaction

The hardness reduction reaction which is most applicable with the resin, designated by RCOOH +, is as follows:

2RCOOˉ H++ Ca++(HC03)2 = (RCOO)2Ca++ + 2H2CO3

The resin in the hydrogen form reacts with the calcium bicarbonate in the influent water to produce calcium-form resin and carbonic acid, which is carbon dioxide and water.

Regeneration Reaction

A major advantage of the weak acid resin is the efficiency with which it can be regenerated. Near stoichiometric or theoretical quantities of acid will regenerate the resin. The reaction of the resin with hydrochloric acid is as follows:

(RCOO)2Ca++ + 2H+Cl– = 2(RCOO)H+ + Ca++Cl2

The hydrogen in the hydrochloric acid replaces the calcium on the weak acid resin. The chloride of the hydrochloric acid is very soluble and avoids the problems of calcium sulfate precipitation which occurs when sulfuric acid is used.

Limiting the quantity of regenerant to the capacity of the resin virtually eliminates the presence of acid in either the weak acid resin effluent or in the regeneration waste, which makes this process both very efficient and very attractive from a waste disposal standpoint.


In order to remove the carbon dioxide which has been generated when passing the water through the cation resin, a forced draft decarbonator is used. The softener effluent water is sprayed over plastic packing, passes downflow, and is distributed through the packing. At the same time air is blown upwards, completely contacting the trickling water, srubbing out the carbon dioxide. The decarbonator effluent usually contains less than 15 mg/L (ppm) carbon dioxide.

pH Adjustment

The effluent from the decarbonator will contain less than 15 ppm CO2, but because the alkalinity is low and the pH is borderline (6.5 – 6.8), a small quantity of alkali can be fed to produce a more stabilized water. Also, in some cases, a portion of the high pH, high alkalinity raw water can by-pass the decarbonator and blend with the decarbonated water.

For additional information on this process please contact your local Hungerford & Terry represenative; or you may email or call Hungerford & Terry headquarters today.