Minimum level of free chlorine in potable water is: (Repeat)

Ans: B (0.5mg) Ref: Park 'Textbook of Preventive and Social Medicine', 22nd edition, Pg: 663 & Community medicine with recent advances,, Suryakantha. 3rd edition, Pg: 32.Explanation:The residual chlorine that remains at the end of one hour contact period is called the 'free residual chlorine'.The purpose of residual chlorine is to provide margin of safety against contamination during storage and distribution.The minimum concentration of free residual chlorine for drinking purpose should be 0.5 mg/ litre or 0.5 parts per million.Free residual chlorine is detected by ortho toludine test using chloroscope / chlorotex apparatus.During a threat of outbreak of water borne epidemic we do super chlorination when the free residual chlorine is more than 2mg/litre. Such high levels may irritate throat, nose and eye. Hence this is followed with dechlorination by controlled addition of sodium thiosulphate or activated carbon.