Shock chlorination

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From swimming pools to wells, chlorine is a common chemical used to disinfect water sources.

Due to safety concerns, hypochlorite (bleach) is the most commonly used compound to conduct shock chlorination1. Hypochlorite is used in one of three forms: commercial bleach (approx. 3.5-5% concentration), calcium hypochlorite (Ca(OCl)2; 65-70% concentrated), or sodium hypochlorite (NaOCl; about 12% concentration)2. These forms of hypochlorite are not as pure as chlorine gas, and will degrade in strength when in storage. However, like chlorine gas, they also react with water to form a disinfectant, hypochlorous acid (HOCl).

Microbial agents

Frequently, microbial factors infiltrate water sources through fecal matter. Many types of bacterial pathogens can initiate waterborne illnesses, including enteric bacteria, protozoa, or viruses3.
Due to the variety of species that can inhabit water sources, it comes as no surprise that chlorine has varying affects on the types of microbes. Though time and concentration can eliminate virtually every species, some species remain resistant to the process. Species such as Vibrio cholera have been tested to last approximately 30 days in drinking water sources, while toxigenic E.coli can last 90 days4. Survival techniques include cysts, spores, absorption, and intracellular survival and growth. Other species will remain viable, but no longer culturable.

Helicobacter pylori

Electron micrograph of Helicobacter pylori, a microbe commonly found in public water sources. Courtesy: Timothy Hoover (Franklin College)

Helicobacter pylori is known to cause gastritis and peptic ulcers.
Studies done in Peru5 and Japan6 have shown the presence of the bacteria in public water sources, proving its possibility as a waterborne microbe.


Immunofluorescence of Cryptosporidium, the microbe that caused an epidemic in Milwaukee in 1993. Over 104 deaths were credited to the waterborne microbe . Courtesy: H.D.A Lindquist (EPA)

Cryptosporidium parvum is a type of parasite capable of causing gastrointestinal illness. Unlike Helicobacter pylori, however, Cryptosporidium has been proven to be unresponsive to chlorination7.




Success rates

Alternative methods

Scientists are not content with shock chlorination. As technology advances, methods to improve both testing and disinfection are created.


1 Rutala W., Weber D. "Uses of Inorganic Hypochlorite (Bleach) in Health-Care Facilities". 1997. Clinical Microbiology Reviews 10(4). p. 597-610.

2 "Chlorination Chemistry"

3 Leclerc H., Schwartzbrod L., Dei-Cas E. "Microbial agents associated with waterborne diseases". 2002. Crit Rev Microbiol 28(4). p. 371-409

4 Ford T. "Microbiological Safety of Drinking Water: United States and Global Perspectives". February 1999. Environmental Health Perspectives 107(1). p. 191-206

5 Hulten K., Han S.W., Enroth H., Klein P.D., Opekun A.R., Gilman R.H., Evans D.G., Graham D.Y., El-Zaatari F.A. "Helicobacter pylori in the drinking water in Peru". Gastroenterology. April 1996. Volume 110(4). p. 1031-5.

6 Horiuchi T., Ohkusa T., Watanabe M., Kobayashi D., Miwa H., Eishi Y. "Helicobacter pylori DNA in dirnking water in Japan". Microbol Immunol. 2001. Volume 45(7). p. 515-9.

7 Bukhari Z., Marshall M.M., Korich D.G., Fricker C.R., Smith H.V., Rosen J., Clancy J.L. "Effects of ozone, chlorine dioxide, chlorine, and monochloramine on Cryptosporidium parvum oocyst viability". 1990. Appl Environ Microbiol 56(5). p. 1423-8.

Edited by Erika Jensen, student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2013, Kenyon College.