Since 1910 chlorine has been used in the disinfection process of wastewater because of its success in eliminating high rates of water-borne illnesses.[1]

Chlorine (Cl2) reacts with water (H2O):[2]

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.[3]

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Chlorine’s effectiveness as a disinfectant depends upon:[3]

  • 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)
  1. Water Quality and Health Council. Wastewater Chlorination: An Enduring Public Health Practice. Accessed online 5/02/08.
  2. Chlorination. Accessed online March 27th, 2008.
  3. 3.0 3.1 Davis, Mackenzie, and Susan Masten. Principles of Environmental Engineering and Science. McGraw Hill: New York, NY 2004.

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