<prompt> Residual chlorine in chlorination of water should be:1 mgL after 1 hr0.5 mgL after 1 hr1 mgL after 30 mins0.5 mgL after 30 mins  <response>: The minimum recommended concentration of free chlorine is 0.5 mgL for one hour. The sum of the chlorine demand of the specific water plus the free residual chlorine of 0.5 mgL constitutes the correct dose of chlorine to be applied. The chlorine has no effect on spores, protozoal cysts and helminthic ova, except in higher doses. Ref: Parks Textbook of Social and Preventive Medicine, 19th edition, Page 575. 