Amount of residual chlorine to be present after contact time should: (Repeat 2013)

Ans: A (0.5 mg/L) Ref: Park V Textbook of Preventive and Social Medicine, 20th edition, page 625 Explanation: Chlorination Chlorine kills pathogenic bacteria but has no effect on certain spores, protozoal cyst, helminthic ova and virus (polio & viral hepatitis) Oxidises iron, manganese and hydrogen sulfide. Destroys odor and taste producing constituents. Control algae & slime organism and aid coagulation. The action of chlorine is due to hypochlorous acid. And acts best at pH 7. Action: H2O + Cl2 - HCl + HOCl -Disinfectant activity HOCl - + + - Principles of chlorination Water should be clear as turbidity impedes effective chlorination. Chlorine demand of water: Amount of chlorine added- residual chlorine (after a contact period of 60 minutes) Q. Breakpoint chlorination point at which chlorine demand of the water is met and if further chlorine is added free chlorine appears in water. Minimum recommended concentration of free chlorine is 0.5mg/L. Chlorine dose= chlorine demand of water+ free residual chlorine.