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Since 1910 chlorine has been used in the disinfection process of wastewater because of its success in eliminating high rates of water-borne illnesses.
Chlorine (Cl2) reacts with water (H2O):
Cl2 + H2O → HOCl + HCl
The resulting hypochlorous acid (HOCl) disassociates into hydrogen (H+) and hypochlorite (OCl-) ions:
HOCl ---> H+ + OCl¯
Hypochlorous acid (HOCl) is a weak acid that penetrates the cell membrane of wastewater pathogens, destroying the enzymes within the cytoplasm. Wastewater pathogens killed through chlorination include bacteria, viruses, protozoa, and amoebic cysts. 
Chlorine’s effectiveness as a disinfectant depends upon:
- chlorine detention time and concentration
- wastewater pH and temperature
- total suspended solids (turbidity)
- other reactive species in the water (hydrogen sulfide, organics)
- pathogen (cryptosporidium, for example, is highly resistant to chlorine)
- Water Quality and Health Council. Wastewater Chlorination: An Enduring Public Health Practice. Accessed online 5/02/08. http://www.waterandhealth.org/wastewater/chlorination.php3
- Chlorination. Accessed online March 27th, 2008. http://en.wikipedia.org/wiki/Chlorination
- Davis, Mackenzie, and Susan Masten. Principles of Environmental Engineering and Science. McGraw Hill: New York, NY 2004.