Chlorination is a process used for water-purification. The disinfecting action of chlorine is mainly due to

When chlorine gas dissolves in water it hydrolyzes to give hypochlorous acid (HOCl) and hydrochloric acid (HCl): Cl2 + H2O → HOCl + HCl. The primary disinfecting species is hypochlorous acid (and its dissociated forms), which is the active oxidant that inactivates bacteria, viruses and other pathogens by oxidative damage to cellular components. The relative amounts of HOCl and hypochlorite ion (OCl–) depend on pH, and HOCl is substantially more biocidal than OCl–; thus the disinfecting action of chlorine in water is mainly due to formation of HOCl rather than nascent oxygen or hydrogen formation [1].

Sources

1. [1] https://www.fao.org/fileadmin/templates/agns/pdf/jecfa/cta/61/NaDCC.pdf