Hardness in water is caused by certain salts. The main hardness causing ions are Calcium (Ca2+), Magnesium (Mg2+) and Bicarbonate (HCO3-). These ions or minerals are normally addressed as scale in the water causing scaling of pipes and equipment in drinking water and process water systems.

A water softener is a unit that is used to soften water, by removing the minerals that cause the water to be hard. They are specific ion exchangers that are designed to remove ions, which are positively charged.

Softeners mainly remove calcium (Ca2+) and magnesium (Mg2+) ions. Calcium and magnesium are often referred to as 'hardness minerals'. Softener are Sometimes even applied to remove iron. The softening devices are able to remove up to five milligrams per litre (5 mg/L) of dissolved iron.

Softeners can operate automatic, semi-automatic, or manual. Each type is rated on the amount of hardness it can remove before regeneration is necessary.A water softener collects hardness minerals within its conditioning tank and from time to time flushes them away to drain.

Ion exchangers are often used for water softening. When an ion exchanger is applied for water softening, it will replace the calcium and magnesium ions in the water with other ions, for instance sodium or potassium. The exchanger ions are added to the ion exchanger reservoir as sodium and potassium salts (NaCl and KCl).

In many applications, e.g. the preparation of drinking water, water in breweries and sodas, but also cooling water and boiler feed water, the hardness of the water is of importance.

The hardness is normally measured in German degree, French Hardness or (equivalent) concentration Ca2+. softner are specific ion exchangers which are designed to remove ions with a multiple positive charge.Brackish water softeners are also available using specific resins and regenerated with HCl and NaOH.

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