How does a water softener soften the water and remove calcium that is responsible for lime scale build up in plumbing?

A water softener softens the water by removing the hardness elements in the water supply. Calcium (Ca2+) and Magnesium (Mg2+) are the main two culprits causing hard water. In South Africa much of our water supply is hard water and the hardness just differs from area to area.

The water softener works on the basis of ion exchange, which means it removes an element from the water, in this case Ca2+ and Mg2+ which have a positive charge and replaces it with a different element, Sodium (Na2+) with the same charge. The replacement of a ion with the same charge is needed to keep the balance in the water itself.


The water that needs to be treated passes through a bed of resin. The resin, which is an ion exchange material, contains the sodium ions (Na2+) which are electrostatically bound to it. These ions are readily replaced by the two hardness ions.

The exchange is known as ion-exchange and it is this process that removes the hardness elements from the water and creates what we know as ‘soft water’.

With the calcium and magnesium removed from the water lime scale build up if effectively stopped and over time the ‘soft water’ will also remove the lime build up in the plumbing of your home.

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Over time, the system needs to regenerate. This is different to the backwash of a swimming pool where the water flow is simply reversed. In the case of the water softener the backwash is actually regeneration. As Mg2+ and Ca2+ are removed from the water it sticks to the resin. The resin’s capacity is gradually exhausted as the concentration of the Calcium and Magnesium increases.

The regeneration is done by passing a concentrated brine of sodium chloride back through the resin. When the regeneration takes place the Magnesium and Calcium ions are removed from the resin and replaced by Sodium ions, ready for the next round.

This in short is how a water softener works.