5 Principles of Chlorination

For adequate chlorination, following rules should be followed:
1. The water to be chlorinated should be clear and free from turbidity.
– Turbidity impedes efficient chlorination
2. The chlorine demand of the water should be estimated first
– The chlorine demand of water is the difference between
• the amount of chlorine added to the water and the amount of residual chlorine remaining in a sample of water
• at the end of a specific period of contact (usually 60 min),
• at a given temperature and pH of the water
– In other words, it is the amount of chorine that is needed to destroy bacteria and to oxidize all the organic matter and ammonical substances present in the water
– The point at which the chlorine demand of the water is met is called the ‘break point’
– If further chlorine is added beyond the break point, ‘free chlorine begins to appear in the water
3. The contact period;
– presence of free residual chlorine
– for a contact period of at least one hour
– is essential to kill bacteria and viruses
• Chlorine has no effect on spores, protozoal cysts and helminthic ova, except in higher doses
4. The minimum recommended concentration of free chlorine is 0.5 mg/L for one hour
– The free residual chlorine provides a margin of safety against subsequent microbial contamination such as may occur during storage and distribution
5. The sum of the ‘chlorine demand’ of the specific water plus the ‘free residual chlorine’ of 0.5 mg/L constitutes the correct dose of chlorine to be applied