Water softeners are primarily used to remove calcium and magnesium ions from boiler makeup water. If not removed, these ions can be the basis for forming insoluble compounds that will precipitate in boiler systems. A water softener eliminates this problem by replacing precipitate-forming ions with sodium ions.
Since sodium based compounds are very soluble, precipitates formed by hard water can be eliminated. In a water softener, ion exchange occurs as the water runs through a bed of small plastic resin beads which are saturated with sodium ions. As the water, containing calcium and magnesium ions, flows past the resin beads, calcium and magnesium ions “stick” to the ion exchange resin, liberating the sodium ions, which then go into solution. Eventually, the resin beads contain nothing but calcium and magnesium ions and no sodium. At this point, softening no longer occurs and hardness ions (Ca and Mg) begin to leak through the bed. When leakage occurs, or prior to the resin being exhausted, it is time to regenerate the media.
The most common means for removing water hardness rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents.
Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which "hardness ions" - mainly Ca2+ and Mg2+ - are exchanged for sodium ions. As described by NSF/ANSI Standard 44, ion exchange devices reduce the hardness by replacing magnesium and calcium (Mg2+ and Ca2+) with sodium or potassium ions (Na+ and K+)."
Ion exchange resins are organic polymers containing anionic functional groups to which the dications (Ca++) bind more strongly than monocations (Na+). Inorganic materials called zeolites also exhibit ion-exchange properties. These minerals are widely used in laundry detergents. Resins are also available to remove carbonate, bi-carbonate and sulphate ions which are absorbed and hydroxide ions released from the resin.
When all the available Na+ ions have been replaced with calcium or magnesium ions, the resin must be re-charged by eluting the Ca2+ and Mg2+ ions using a solution of sodium chloride or sodium hydroxide depending on the type of resin used. For anionic resins, regeneration typically uses a solution of sodium hydroxide (lye) or potassium hydroxide. The waste waters eluted from the ion exchange column containing the unwanted calcium and magnesium salts are typically discharged to the sewage system.
Reverse osmosis (RO) takes advantage of hydrostatic pressure gradients across a special membrane. The membrane has pores large enough to admit water molecules for passage; hardness ions such as Ca2+ and Mg2+ remain behind and are flushed away by excess water into a drain. The resulting soft water supply is free of hardness ions without any other ions being added. Membranes have a limited capacity, requiring regular replacement.
Chelators are used in chemical analysis, as water softeners, and are ingredients in many commercial products such as shampoos and food preservatives. Citric acid is used to soften water in soaps and laundry detergents. A commonly used synthetic chelator is ethylenediaminetetraacetic acid (EDTA).
Since Ca2+ and Mg2+ exist as nonvolatile salts, they can be removed by distilling the water. Distillation is too expensive in most cases. Rainwater is soft because it is naturally distilled during the water cycle of evaporation, condensation and precipitation.
Lime softening is when lime is added to hard water to make it softer.