Shock chlorination

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Shock chlorination is the process of reducing bacterial and algal contaminants in varying water sources, such as swimming pools and wells, through chemical means (some form of chlorine). 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), the active chemical in the process of shock chlorination.

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 microbes. Though time and concentration can eliminate virtually every species, some species remain resistant to the process. Species such as Helicobacter pylori have been tested to last approximately 25 days in drinking water sources, while toxigenic E.coli can last 90 days. Others, meanwhile, are considered resistant to chlorination, such as Cryptosporidium parvum, and will persist indefinitely. Survival techniques include cysts, spores, absorption, and intracellular survival and growth. Lastly, some species will remain viable, but no longer culturable.

Listed below are a couple of microbes that are found in water sources and are capable are causing disease in humans. They represent a variety of forms as well as response to the process of shock chlorination. Other species not detailed here but worth mentioning include Vibrio cholerae, Salmonella, and Giardia lambria5.

Helicobacter pylori

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

Helicobacter pylori is a bacterium 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. H.pylori is commonly used in studies regarding microbial survival in chlorine and other disinfectants, along with E.coli.


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 ever reported 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.


Approximate disinfectant times for microbes with a variety of resistance. Note the parameters, including temperature, pH, and concentration. Courtesy: Centers for Disease Control and Prevention
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 solid waste or vomit, in which extreme levels must temporarily be reached as well). 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 ppm11.

If a household is not on city water, and instead relies on a well, samples should be tested every year. After shocking, the water source should not be used for a minimum of eight hours, though some chlorine may persist for seven to ten days. Because of this, faucets should be run until the chlorine smell disappears (the water should be safe for uses other than consumption if the smell lingers). Safety during domestic shock chlorination requires protective gear such as gloves, goggles, and other clothing items, such as a chemical-resistant apron. Concentrated chlorine levels are extreme irritants, so contact with skin should be avoided. Mixing chlorine bleach with other chemicals may produce toxic fumes. Thus, ensure that the area is well ventilated before beginning12.

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 hypochlorite13. Though typically chlorine activity decreases at higher pH, compromise with other factors (such as prevention of skin and eye irritation) places the ideal pH for swimming pools at approximately 7.2-7.814.

DNA microarray can be used to monitor water quality more precisely15. A study in 2011 established 26 different microbial probes, and claimed to drastically reduce the time and labor required with current methods of identifying the presence of microbes in drinking water sources16.

The success of shock chlorination, however, is not straightforward or universal. Due to the process of horizontal gene transfer (in which bacteria can pick up DNA from other species), some species of bacteria previously susceptible to chlorine are evolving to be resistant. Natural selection may also cause higher fitness in organisms that have mutations that allow higher survival in chlorine. For instance, scientists have discovered two forms of Vibrio cholerae (a gram-negative bacterium causing the disease Cholera), rugose (referencing its wrinkly texture) and smooth. Rugose colonies, unlike smooth, are capable of producing a slime covering. This covering consists of long polysaccharides. Experimentation demonstrated that these colonies can withstand chlorine concentration ten to twenty times higher than their counterparts17. In 1993, WHO reported approximately 6,800 deaths worldwide from cholera. By 1997, that number was up to 10,000, demonstrating the decrease in success by current chlorination methods as populations of Vibrio cholerae adapt18.

Related and Alternative methods

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. This is where other methods are used, such as carbon filtration and enhanced coagulation, physical-chemical processes, though this is expensive19.

Bromine is used as a disinfectant in swimming pools. However, unlike chlorine, it cannot be used to disinfect drinking water20. In contrast, the EPA references three different oxidizing disinfectants for drinking water: chlorine, chloramines, and ozone. Monochloramine, NH2Cl, is formed during a reaction between ammonia and chlorine (a process called chloramination). Ozone is a newer method, but offers greater levels of oxidation with quicker decomposition. In a study done in 1997, it was found that H.pylori and E.coli respond similarly to hypochlorous acid21. However, a different study in 2002 demonstrates the imperfection of any of the types suggested by the EPA. H.pylori is more resistant to chlorine and ozone than E.coli, but less resistant to monochloramine22.

One compound that seems to have the greatest success rate, chlorine dioxide, is capable of killing even Cryptosporidium. However, the compound is extremely expensive to make, and must be produced on site, making it inefficient. It is also highly flammable, posing major health risks in comparison to the chemicals seen above23.


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 drinking 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 Dozier M., McFarland M. "Shock Chlorination of Stored Water Supplies". AgriLife Extension.

12 Wright M. "Disinfecting a Domestic Well with Shock Chlorination". October 1997.

13 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.

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

15 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.

16 Zhou G, Wen S, Liu Y, Li R, Zhong X, Feng L, Wang L, Cao B. "Development of a DNA microarray for detection and identification of Legionella pneumophila and ten other pathogens in drinking water". 2011. Int J Food Microbiol 145(1):293-300.

17 Leslie M. "Stanford researchers discover where cholera bacteria hides between outbreaks". April 1999. Stanford Report.


19 Sedlak D., von Gunten U. "The Chlorine Dilemma". January 2011. Science. 331(6013):42-43

20 "Disinfectants-Bromine"

21 Johnson, C.H., Rice E.W., Reasoner D.J. "Inactivation of Helicobacter pylori by chlorination". Appl. Environ. Microbiol. 1997. 63:4969–4970

22 Baker K., Hegarty J., Redmond B., Reed N., Herson D. "Effect of Oxidizing Disinfectants (Chlorine, Monochloramine, and Ozone) on Helicobacter pylori". February 2002. Appl. Environ. Microbiol. 68. p.981-984

23 "Chlorination Chemistry"

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