In the Water supply to houses factories etc.,
Chlorine is dissolved in the water as a bacteriocide.
Chlorine has the ability to disproportionate in water.
H2O + Cl2 = HCl + HClO
HCl is hydrochloric acid and HClO is Bleach. Such is the amount put in the water supply you don't smell or feel it. However, if has the effect of killing bacteria.