Shock chlorination

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From swimming pools to wells, chlorine is a common chemical used to disinfect water sources. About a century ago, the process of chlorination was introduced in order to prevent the spread of diseases such as typhoid or cholera. Over time, the process has modified, with the EPA (Environmental Protection Agency) beginning to implement the use of chloramine, a closely related compound, instead1.

Due to safety concerns, hypochlorite (bleach) is the most commonly used compound to conduct shock chlorination2. Hypochlorite is used in one of three forms: commercial bleach (approx. 3.5-5% concentration), calcium hypochlorite (Ca(OCl)3; 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 viruses4.
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 days5. 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 Peru6 and Japan7 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 50 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 most chlorination8.
In 1993, Milwaukee (Wisconsin) experienced one of the worst waterborne outbreaks after one of the city's water plants failed to remove Cryptosporidium oocysts from the Lake Michigan water collected for public use. Over 403,000 residents and visitors were affected, experiencing the watery diarrhea, nausea, fever, vomiting, and abdominal cramps associated with infection. Approximately 54 deaths occurred, although there is some debate regarding other preexisting conditions that affected patient survival9.
In October 2013, a resurgence of the Cryptosporidium protozoan appeared in Milwaukee, this time in local swimming pools10.




Suggested chlorine levels to sanitize domestic wells. Courtesy: Keith Smith. (OSU Extension)

Shock chlorination of drinking water may be recommended for domestic use due to several different problems. In some instances, groundwater will be infiltrated by bacteria, requiring consistent disinfecting. Occasionally, shock chlorination may be an isolated necessity due to some foreign contamination during installation or repairs to a pump. Unlike the process for drinking water, which requires an initial shock and subsequent allowance of the chlorine concentration to drop, pool treatment usually requires a constant level to be maintained (barring some extreme contamination, such as waste or vomit). For example, the recommended amount of chlorine for disinfecting drinking water is 200 ppm, while swimming pools are usually maintained from 1.0-3.0 ppm (

Safety during domestic shock chlorination requires protective gear such as gloves, goggles, and other clothing items, such as an apron. Concentrated chlorine levels are extreme irritants, so contact with skin should be avoided. Mixing chlorine bleach with other chemicals may produce toxic fumes.

Success rates

In 1998, using Raman and ultraviolet spectroscopic radiation, scientists proved that the bactericidal activity of chlorine treatment was quantitatively linked to the concentration of the hypochlorous acid in the chemical solution. Further, the pH dependence discovered indicates an equilibrium amongst the chlorine, hypochlorous, and hypochlorite11. Though typically chlorine activity decreases at higher pH, compromise with other factors (such as prevention of skin and eye irritation) places the ideal pH at around 7.2-7.812.

The success of shock chlorination, however, is not that simple. Due to the process of horizontal gene transfer (in which bacteria can pick up DNA from other species), some species of bacteria previous susceptible to chlorine are evolving to be resistant.

Related and Alternative methods

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

Quality Monitoring

DNA microarray can be used to monitor water quality more precisely13.

Chlorine Replacements

Various groups complain about the toxic residues created by chlorination. However, various alternatives that have been created appear to have faults of their own, producing the carcinogenic compounds similar to the ones scientists sought to remove.

Bromine is used as a disinfectant in swimming pools. However, unlike chlorine, it cannot be used to disinfect drinking water.


1 "Chlorine Substitutes In Water May Have Risks". 7 January 2011. NPR.

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

3 "Chlorination Chemistry"

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

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

6 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.

7 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.

8 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.

9 Hoxie N.J., Davis J.P., Vergeront J.M., Blair K.A. "Cryptosporidiosis-associated mortality following a massive waterborne outbreak in Milwaukee, Wisconsin". December 1997. Am J Public Health 87(12). p. 2032-5.

10 Karen Herzog. "4 new cases of Cryptosporidium confirmed on North Shore". 1 October 2013. Milwaukee Journal Sentinel.

11 Nakagawara S., Goto T., Nara M., Ozawa Y., Hotta K., Arata Y. "Spectroscopic Characterization and the pH Dependence of Bactericidal Activity of the Aqueous Chlorine Solution". 1998. Analytical Sciences 14(4). p. 691-8.

12 "Your Disinfection Team: Chlorine & pH". Centers for Disease Control and Prevention.

13 Lemarchand K., Masson L., Brousseau R. "Molecular biology and DNA microarray technology for microbial quality monitoring of water.". 2004. Crit Rev Microbiol 30(3). p. 145-72.

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