A water softener uses a process called ion exchange to remove the calcium and magnesium ions that cause the water to be hard.  In a water softener, ion exchange takes place within resin-filled tanks which remove the calcium and magnesium ions, thereby making it soft.  Once the resin tank is full of hardness ions, the water softener will flush them down the drain using a salt solution or ‘brine’ which is passed through the resin tank when the softener cleans itself or ‘regenerates’.

The regeneration process begins after a given volume of water has passed through the water softener, or alternatively it may be governed by an electrical timer depending on the type of softener.  The majority of electrical softeners have a single resin tank whereas most non-electric units tend to have two resin tanks working together.

Twin tank water softeners use their own soft water for regeneration so they protect themselves against hard water problems.  This is why they have a greater life expectancy and often carry far longer manufacturer guarantees.