Short Note on Chlorine demand

Chlorine demand

When chlorine and chlorine compounds are added to a water source, it purifies the water by damaging the cell structure of bacterial pollutants, thereby destroying them. The amount of chlorine needed to do this is called the chlorine demand of the water. The chlorine demand varies with the amount of impurities in the water and therefore the chlorine demand of a water source will vary as the quality of the water varies. Chlorine and chlorine compounds are consumed by inorganic and organic matters present in water before any disinfection is achieved.

For achieving proper disinfection of water by killing the pathogenic organisms, it is essential to provide sufficient time and dose of chlorine to satisfy various chemical reactions and leave adequate amount of chlorine as residual in the form of free or combined chlorine. The difference between the amount of chlorine added to water and amount of residual chlorine after a specified contact time is the chlorine demand. In other words it is the amount Cl2 that is needed to destroy bacteria and to oxidise all organic matter and ammoniacal substances present in the water.