VPE 511: Environment and Environmental Hygiene (2+1)

CHLORINATION

• Chlorination is essentially the addition of “free” chlorine to water; it may be introduced as chlorine gas, as sodium hypochlorite, or as bleaching powder.
• Chlorine gas is used in large scale practice while sodium hypochlorite and bleaching powder find use in small scale sterilizers. The two recognized procedures for the treatment of water by free chlorine are normal chlorination and super chlorination.
• In normal chlorination the concentration of chlorine is brought to such a level that a residuum is left after a contact time between chlorine and water of at least half an hour; this contact time is extended as along as practicable.
• It is arranged for the residuum of chlorine to be so small that it imparts no detectable smell or taste to the water , so that the water is available for use without further treatment.
• Difficulties occasionally arise where the water before treatment contains substances that react with chlorine to yield products possessing a powerful and objectionable taste.
• Traces of phenol compounds for example , even in concentrations as low as 1 part in 2000 millions, react with chlorine to produce chlorphenols which impart a very objectionable and persistent iodoform like taste and smell to the water.
• These difficulties may sometimes be obviated by super chlorination which results in the complete destruction by oxidation of the substances concerned, but the safest procedure appears to be removal of such compounds before chlorination by filtering the water through charcoal.
• Super chlorination is adopted where the contact time between water and sterilizing agent is necessarily reduced (where storage tanks are not available or their use is inconvenient. In this process chlorine is introduced into the water to a concentration about ten times that used in normal chlorination.
• Rapid and complete sterilization is effected in a few minutes but the treated water contains a very high residuum of the free chlorine and must be dechlorinated.
• Sulphur dioxide (in large scale practice) and sodium thiosulphate ( in small scale practice) are the dechlorinating agents in general use.
• The advantages of super chlorination lie in the very short contact time required for sterilization and also in the fact that as the high concentration of chlorine tends to oxidize organic matter completely there is less tendency for taints of the chlorphenol type to develop.