Water softening involves the use sodium form cation exchange resins in an ion exchange column the to exchange hardness causing ions such as calcium, magnesium and barium, etc. Hard water - water containing calcium and magnesium ions is admitted to the ion - exchange column.

Before being contacted by the water, the resin particles have associated with them only mobile, replaceable sodium cations and fixed, irreplaceable anions. There are, of cause, thousands of particles in a typical exchanger unit, each particle containing not just a few ions, but literally billions.

Exchange is initiated. Some of the calcium and magnesium ions are adsorbed by the resin which in turn, releases sodium ions. Two sodium ions are released for each calcium or magnesium ion.

The resins particle is exhausted of its sodium ions; now contains calcium and magnesium. In actual practice, a few sodium ions generally remain on the exchanger.

The effluent water is now soft - does not contain hardness producing cations. The resin bed may now be regenerated by adding to it a solution containing sodium ions - most frequently, sodium chloride.

Softening resin is not capable of removing 100% of the calcium and magnesium hardness. Some leakage will occur. The degree of leakage is affected by the total dissolved solids (TDS) of the raw water and the regeneration level of the resin. Typical regeneration level are from 6 to 15 pounds of salt per cu. ft. of resin. As the TDS content increases, the regeneration level must also increase to maintain the hardness leakage at an acceptable low level. From a practical view, water softeners are designed and operated to produce effluent water quality of less than 1 ppm of total hardness.

Water softening can be used not only to remove scale-forming compounds (such as calcium, barium, strontium, and iron) but also to enhance the stability of colloidal silt particles.