Three-cell gravity filter system for a power plant in Florida.

Sodium and Hydrogen Cycle Softeners
Operating in the sodium cycle, the sodium in the treated water is increased proportionately to the amount of calcium and magnesium in the untreated water. For applications where sodium increase is a concern, Hungerford & Terry produces a line of weak-acid softeners that operate in the hydrogen cycle instead of the sodium cycle. Where total hardness is numerically equal to the alkalinity in the raw water, the hardness can be economically removed using an acid-regenerated weak-acid (carboxylic) cation exchange resin. The quantity of hardness which exceeds the alkalinity will not be removed. The ion exchange mechanism in the weak-acid softening process is an exchange of calcium and magnesium ions for hydrogen ions. Therefore, no sodium is added to the treated water. Either hydrochloric acid or sulfuric acid can be used for regeneration.

During the ion exchange process with weak acid resin, carbonic acid is formed and can be stripped off using a forced draft degasifier following the softener. Also, a small quantity of sodium hydroxide can be added following the decarbonator to neutralize any remaining CO₂ and to provide non-corrosive water with a neutral pH. The system can be designed for stoichiometric quantities of acid so that the waste produced does not require neutralization prior to disposal.