There are a number of methods to remove the hardness present in water. One those methods are being followed, the hard water gets converted to soft water. Some of the methods to remove hardness from water are,
Given above are some of the hardness reduction methods.
We can boil water to remove temporary hardness. Temporary hardness in water can be easily removed by boiling. On boiling, calcium/magnesium bicarbonate decomposes to give
calcium/magnesium carbonate, which is insoluble in water. Therefore, it precipitates out.
In Clark's process, slaked lime, Ca(OH)2 is added to temporary hard water. Insoluble calcium carbonate precipitate out and no longer produce hardness.
Calcium and magnesium ions present in hard water react with sodium carbonate to produce insoluble carbonates. The water now contains soluble and harmless sodium salts.
Calgon is a trade name of a complex salt, sodium hexametaphosphate (NaPO3)6. It is used for softening hard water. Calgon ionizes to give a complex anion:
The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon.
This results in the removal of calcium and magnesium ions from hard water in the form of a complex with Calgon. The water is softened and sodium ions are released into water.
Permutit or sodium aluminium silicate is a complex chemical compound, which occurs as a natural mineral called Zeolite. Permutit or zeolites are insoluble in water and have the property of exchanging ions present in them with the ions present in the solution.
Permutit or zeolites are packed in a suitable container and a slow stream of hard water is passed through this material. As a result, calcium and magnesium ions present in hard water are exchanged with sodium ions in the permutit (Na+Al-Silicate). The outgoing water contains sodium salts, which do not cause hardness.
This is the basic concept of ion exchange and hardness removal.
Giant organic molecules having acidic or basic groups are known as Ion-exchange resins. Acid resins contain the acid group (- COOH).
Acid resins exchange their H+ ions with other cations such as Ca2+, Mg2+, etc., present in hard water. Acid resins are, therefore known as base-exchange resins.
Basic resins exchange their OH-ions with the other anions such as HCO3-, Cl-, SO42-, present in hard water. Basic resins, therefore, are also known as acid exchange resins.
Fig: 11.5 - Ion-exchange process for water softening
In the ion exchange process, hard water is passed through two tanks
'A' and 'B'. Tank- A contains acid resin and tank- B is filled with basic resin. All the cations present in hard water (except H+) are removed by the acid resin present in Tank- A, and the basic resin present in Tank- B removes all the anions (except OH-) present in hard water. Water obtained after passage through both the tanks is free from all the cations and anions that make it hard. The water obtained after passing through the ion-exchangers is called deionised water or demineralised water. This is as good as distilled water. The water becomes soft after this process.