A water softener is an ion exchange device that removes calcium and magnesium from fresh water and replaces those ions with sodium ions. The catalyst within a water softener is the resin. Strong acid resin in the sodium form is referred to as zeolite. The process of softening takes place until the resin has exchanged all of the sodium ions for all of the calcium and magnesium ions. At this point, the resin needs to be regenerated.
Regeneration is achieved by introducing a concentrated sodium solution into the resin bed.
The sodium replaces the calcium and magnesium on the resin beds, and the softening process beings anew. Although sodium cycle softeners are capable of producing zero hardness leakage, leakage into the effluent water can occur. Some of the factors affecting leakage are the total dissolved solids content of the raw water, the sodium concentration during regeneration, the temperature of the raw water, and the depth of the resin bed. In general the carbonate cycle is recommended for systems with operating pressures below 125 psig. The phosphate cycle is recommended for systems with operating pressures above 125 psig.