Which part of chlorine disinfection is most effective?
Chlorination is one of the greatest advances in water purification.

Chlorine kills pathogenic bacteria, but has no effect on spores, certain viruses (e.g., polio, viral hepatitis) and cyst of E. histolytica, in usual doses.
 
Mechanism of action

When chlorine is added to water, there is formation of hypochloric and hypochlorous acid

The hypochloric acid is neutralized by the alkanity of the water.

The hypochlorous acid ionizes to form hydrogen ions and hypochlorite ions.

The disinfecting action of chlorine is mainly due to the hypochlorous acid and to a small extent due to the hypochlorite ions.
The hypochlorous acid is the most effective form of chlorine for water disinfection, it is 70-80 times more effective than hypochlorite ion.
 
Method of Chlorination

For disinfecting large bodies of water, following methods of chlorination are available:

1. Chlorine gas → First choice
 
Chloramine
Perchloron or high test hypochlorite (HTH)
Hypochlorous acid
Hypochlorousion
Hypochlorite ion
None