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Chapter: Environmental Engineering - Water Treatment

Water Tretment: Water Softenning

In almost every raw water supply, hardness is present as calcium and magnesium bicarbonate, often referred to as carbonate hardness or temporary hardness.


 

WATER TRETMENT: WATER SOFTENNING

 

In almost every raw water supply, hardness is present as calcium and magnesium bicarbonate, often referred to as carbonate hardness or temporary hardness. These compounds result from the action of acidic, carbon dioxide laden rain water on naturally occurring minerals in the earth, such as limestone. For example:

CO2 + H2O = H2CO3

Carbon dioxide + water = carbonic acid

H2CO3    + CaCO3 = Ca(HCO3)2

Carbonic acid  + calcium carbonate = calcium bicarbonate

Hardness may also be present as a sulfate or chloride salt, referred to as noncarbonate or permanent hardness. These salts are caused by mineral acids present in rain water or the solution of naturally occurring acidic minerals.

The significance of "carbonate" or "temporary" hardness as contrasted to "noncarbonate" or "permanent" hardness is that the former may be reduced in concentration simply by heating. In effect, heating reverses the solution reaction:

Ca(HCO3)2  + Heat = CaCO3 + H2O  + CO2

Calcium bicarbonate + calcium carbonate = water + carbon dioxide

 

Reduction of noncarbonate hardness, by contrast, requires chemical addition. A combination of lime and soda ash, along with coagulant and flocculant chemicals, is added to raw water to promote a precipitation reaction. This allows softening to take place.

 

 

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