Water Softening

Softening of water by ion exchange process involves the exchange or substitution of sodium minerals/ions for hardness minerals, mainly calcium and magnesium. The exchange is made possible because the minerals are ionic in nature (often called ionized impurities), which means they have an electrical charge. The ion exchange process is based on the fact that like charges repel one another and unlike charges attract one another. Calcium and magnesium are positively charged ions known as “Cations”.

Calcium and magnesium ions in water are dissolved in solution. They have been dissolved into the water, as a result of the water trickling down through strata of rock and soil. The water actually dissolves the calcium and magnesium deposits as it goes. These dissolved minerals, in the water, eventually find their way into underground aquifers. As water from the aquifers is brought to the earth’s surface either naturally from springs or pumped it contains the dissolved calcium and magnesium. Calcium and magnesium are considered to be “hard” minerals and are detrimental to water using equipment and appliances such as boilers, steam generators, hot water heaters, commercial dishwashers, etc.

How water softening works

An ion exchange water softener uses a man-made media called sodium zeolite resin to remove hard minerals (calcium and magnesium) from water. The resin, made of polystyrene divinyl benzene, consists of millions of tiny plastic beads all of which contain many negatively charged exchange sites designed to attract positively charged “Cations”. When the resin is in the regenerated state, the negatively charged exchange sites hold positively charged “sodium” cations. The sodium cations are weakly bonded onto the resin exchange sites.

As water, containing hard minerals such as calcium and magnesium, is passed through the resin the resin beads displace their sodium ions from the exchange sites and attract the calcium and magnesium ions. During the ion exchange process, relatively small amounts of other strongly charged cations, such as iron and manganese, are also removed along with the calcium and magnesium. The water existing the resin and water softener now contains sodium ions which are less harmful to equipment and appliances.

This process of ion exchange is possible for two reasons:

  1. All Cations do not have the same strength of positive charge
  2. The resin prefers the more strongly charged Cations calcium and magnesium than it does the weaker charged sodium Cations

Although resin has an extremely vast number of exchange sites, eventually all of the resin exchange sites become occupied by calcium and magnesium, and no future exchange can take place. The resin is then said to be exhausted and must be regenerated. The resin of the softener is regenerated with a dilute solution of sodium chloride (common salt) and water — brine. During regeneration, the flow of service water from the softener is first stopped. Brine is drawn from the brine tank mixing with a separate stream of water. The brine solution flows downward through the resin, contacting the resin beads which are loaded with calcium and magnesium ions. Even though the calcium and magnesium are more strongly charged than the sodium, the concentrated brine solution contains literally billions of the more weakly charged sodium ions which have the power to displace the smaller number of calcium and magnesium ions. When the calcium and magnesium ions are displaced (exchanged), the positive sodium ions are then attracted to the negative exchange sites. Eventually all sites are taken up by sodium ions and the resin is said to be regenerated and ready for the next softening cycle.

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