Water softening methods or treatment of hard water : 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.

 Ion-exchange resin device

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.^[5] As described by NSF/ANSI Standard 44,^[6] ion exchange devices reduce the hardness by replacing magnesium and calcium (Mg²⁺ and Ca²⁺) with sodium or potassium ions (Na⁺ and K⁺).

So how does Ion Exchange work?

Ion exchange resins are used to produce deionized (demineralized or “DI”) water. These resins are small plastic beads that are composed of organic polymer chains that have charged functional groups built into the resin bead. Each functional group has either a fixed positive or negative charge.

Cation resin has a negative functional group and therefore attracts positively charged ions. There are two types of cation resins, weak acid cation (WAC) and strong acid cation (SAC). Weak acid cation resin is used mainly in dealkalization and other unique applications. For this reason we will focus on explaining the role of strong acid cation resins used for producing deionized water.

Anion resin has a positive functional group and therefore attracts negatively charged ions. There are two types of anion resins; weak base anion (WBA) and strong base anion (SBA). Both types of anion resins are used to produce deionized water, however, they have different characteristics listed below:

• WBA resins do not remove silica, C02 or have the ability to neutralize weak acids and has a lower than neutral pH when used in a dual separate bed system.

• SBA resins remove all anions in the above table, including C02, and has a higher than neutral pH when used in a dual separate bed system due to sodium leakage.

• Mixed Bed resins use SAC and SBA resins combined together.

Reverse osmosis

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.