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How does an ion exchange water softener work?

HomeHow does an ion exchange water softener work?
HomeHow does an ion exchange water softener work?

How does an ion exchange water softener work?

It is known that only the ion exchange process truly softens water, by extracting the minerals that cause lime-scale from hard water. But how does that work exactly? Find out below!

The ion exchange process involves the exchange of the hardness minerals in water, chiefly calcium and magnesium, for sodium minerals. The exchange is made possible because the minerals are ionic in nature, which means they have an electrical charge. The ion exchange process is based on the fact that like charges repel one another and opposite charges attract. 

An ion exchange water softener exchanges the hardness minerals (calcium and magnesium) dissolved in water for sodium. This soft mineral is contained on the softener resin beads and does not build up on surfaces as scale deposits.

Ion exchange resin in water softeners

How ion exchange works

The exchange takes place by passing hard water over man-made ion exchange resin beads, in a suitable pressure vessel tank. The resin in most modern softeners (polystyrene divinyl benzene) consists of millions of tiny plastic beads, all of which are negatively charged exchange sites. The ions considered in this process (calcium, magnesium and sodium) are all positivly charged ions. 

When the resin is in the base state, the negatively charged resin beads hold positively charged sodium ions.  As the calcium and magnesium contact the resin beads in their travel through the resin tank, they displace the sodium ions from the exchange sites. 

 

What happens next

Ion exchange is possible for two reasons:

  1. All positively charged ions do not have the same strength of positive charge, and
  2. The resin prefers the more strongly charged calcium and magnesium ions, rather than the weaker sodium ions.

The displaced sodium ions then pass through the resin ‘bed’ and out the softener outlet, thus the softener delivers ‘soft’ water. 

How does a water softener work?

Recharge

Eventually all of the resin exchange sites are occupied by calcium and magnesium and no further softening exchange can take place. The resin is said to be exhausted and must be regenerated. 

The resin of the softener is regenerated with a brine solution. During regeneration the flow of service water from the softener is stopped.

Brine is drawn from the brine tank, mixing water with the salt in the reservoir. The brine solution flows through the resin, contacting the resin beads 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.

This way, the sodium ions have the power to displace the smaller number of calcium and magnesium ions. When the calcium and magnesium ions are displaced, the positive sodium ions are then attracted to the negatively charged exchange sites on the resin.

Eventually, all exchange sites are taken up by sodium ions. The resin is said to be regenerated and ready for the next softening cycle.

 

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