Zeolites are naturally occurring sodium aluminium silicates having different amounts of water of crystallization. They are represented as $Na_2O.Al_2O_3.xSiO_2.yH_2O$ where x and y varies from 2 to 10 and 2 t 6 respectively. They are produced synthetically as well. They have the property of exchanging their Na ions for hardness causing ions like $Ca^{++} \ \ and \ \ Mg^{++}$.
The reactions taking place the softening process are as follows-
$Ca(HCO_3)_2 + Na_2Ze → CaZe + 2NaHCO_3 \\
MgSO_4+ Na_2Ze → MgZe +Na_2SO_4 \\
CaCl_2+ Na2Ze → CaZe + 2NaCl$
where Ze represents zeolite.
The Ze mineral gets exhausted when all the Na+ are replaced by $Ca^+$ and $Mg^{++}$ ions. Now Ze can be regenerated by passing Nacl solution,
$CaZe +2 NaCl → CaCl_2 + Na_2 Ze \\
MgZe+ 2 NaCl → MgCl_2+Na_2 Ze$
The regenerated Ze can now be used for replacing $Ca^{++}$ and $Mg^{++}$ from hard water. Zeolite softening is carried out in large cylindrical tank as shown, holding ze material on a perforated platform. The tank has two inlets for feeding raw water and passing saturated NaCl solution. There are two outlets for softened water and removing $CaCl_2, mgcl_2,$ the wash water formed during the regeneration.
Limitations-
1) Raw material must be free from turbidity and suspended impurities.
2) Highly acidic water is not suitable as it affects mineral.
3) Zeolites of iron and manganese cannot be easily regenerated by passing NaCl solution. Hence iron and manganese impurities in the water to be treated must be minimum.
Advantages-
1) Water of about less than 15 ppm is obtained.
2) The process automatically adjusts for different hardness of incoming water.
3) Water obtained is quiet clear.
4) The equipment is compact and requires less space.
5) It requires less skill in maintenance as well as operation.
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