Recommended residual chlorine level after 1 hr contact period is:

Ref: Park 'Textbook of Preventive and Social Medicine',22nd edition, Pg: 663 3c Community medicine with recent advances,Survakantha, 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 mgl litre or (L5 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 superchlorination when the free residual chlorine is more than 2mg/litre. Such high levels may irritate throat, nose and eye. Hence this is follow ed with dechlorination by controlled addition of sodium thiosulphate or activated carbon.