Chlorine is the poisonous and bleaching chemical,why is it used for treatment in drinking water supply ?

Chlorination is nessecary for descinifiction from odor and bacteria.. but we must added the optimum dose of it to privent enversly effect.

as a final stage of water treatment, chlorine is added as disinfectant (germicide), as an oxidization agent, it kills many types of microbes that cause water-born diseases, chlorine by itself in water is not harmful to human health unless it react with THM components which will produce cancerogenic products, for this reason, in some countries they use ozone as alternative for chlorine, but chlorine has advantage of chlorine-residual which prevents any water pollution that may occur in the network pipelines due to leakage. 

Actually after the biological treatment Chlorination is the process to be adopted before the water is to be supplied for drinking purpose. This helps in killing the remaining bacteria from the process. Chlorine is highly toxic and hence prevent in spreading of any water borne diseases. 

clorine is highly disinfectant of water and also bleach is oxidised matter.