Resdiual chlorine in chlorination of water should be -

Chlorination is process of purification of water kills pathogenic bacteria. The presence of free residual chloribe for contact period of one hour is recommended.The minimum recommended level of free residual chlorine is 0.5 mg/L for one hour. If residual chlorine after 1 hour contact period is less than 0.5 mg/L ,chlorintaion has to be repeated. Parks textbook of preventive and social medicine.K Park. Edition 23.Pg no: 715