Why Is Zeolite Used In The Purification Of Water?

Purification: This is done for drinking water (potable/domestic) — freed from suspended impurities (sand filtration), bacteria (disinfection by chlorine and ozone.) and odour (treatment with charcoal and silver compounds). Scientific — distilled water and conductivity water, is free from suspended impurities, and dissolved salts. It involves boiling the water and condensing the steam.

Softening: Water which will only with difficulty give a lather with soap (not detergent) is called `hard water. The `hardness' is invariably due to the dissolved salts of calcium, magnesium, aluminium and iron. Hardness of water is no longer classified as `temporary' and permanent. Now it is `Carbonate' and `Non-carbonate' hardness.

Presence of the above mentioned salts in water is useful to animal and plants.

When it exceeds, five degree hardness, it is not suitable for cooking or washing and also in steam boilers. Prolonged intake by humans may lead to kidney stones also. Paradoxically, very soft waters cause mineral deficiency leading to certain debilities.

The process of removing the above mentioned salts from hard water is called `softening'. Normally, it is enough if the salts are converted into the corresponding sodium salts. The softening is carried out by treating with lime — soda, hexa-metaphosphates, or zeolite. The zeolite processing involves natural zeolites, glauconites (`green sand') or artificial zeolites (permutit) . However the exhausted permutit is revived by treating with common salt. This softening is adequate for raising steam in low pressure boilers.

But, certain processes, eg., high pressure steam for steam turbines, need water, free of all dissolved salts: `De-mineralised' water. This can be had by combining basic and acidic synthetic ion exchange resins, based on the zeolite process.

Manufacturing drinking water from sea by demineralisation is not economically viable. Better methods such as (the fuel intensive). De-salination (in the petroleum-rich middle east.) and (the power intensive) reverse-osmosis process are available. However, they have their own limitations. Zeolite is used to soften/demineralise, and not to purify, water.

Published in The Hindu on Mar 21, 2002.