During the water treatment process, what is the purpose of adding chlorine to water?
It kills bacteria/microbes.
Chlorine, along with ozone and iodine, are some of the most effective antimicrobial chemicals. In pools, we use chlorine (and sometimes ozone as well) to keep it clear of bacteria and algae growth. In water treatment, its the same idea, but the chorine is removed later because it's toxic to drink. If you look at those emergency water purification tablets, they also use iodine to kill any potential pathogens in the water.