Chlorination (Encyclopedia of Environmental Issues, Revised Edition)
Drinking water, wastewater, and water in swimming pools are the most common water sources where chlorination is used to kill bacteria and prevent the spread of diseases. Viruses are generally more resistant to chlorination than are bacteria, but they can be eliminated with an increase in the chlorine levels needed to kill bacteria. Common chlorinating agents include elemental chlorine gas and sodium or calcium hypochlorite. In water these substances generate hypochlorous acid, which is the chemical agent responsible for killing microorganisms by inactivating bacteria proteins or viral nucleoproteins.
Public drinking water was chlorinated in most large U.S. cities by 1914. The effectiveness of chlorination in reducing outbreaks of waterborne diseases in the early twentieth century was clearly illustrated by the drop in typhoid deaths: 36 per 100,000 in 1920 to 5 per 100,000 by 1928. Chlorination has remained the most economical method of purifying public water, although it is not without potential risks. Chlorination has also been widely used to prevent the spread of bacteria in the food industry.
In its elemental form, high concentrations of chlorine are very toxic, and solutions containing more than 1,000 milligrams per liter (mg/l) are lethal to humans. Chlorine has a characteristic odor that is detectable at levels of 2-3 mg/l of water. Most public water supplies contain chlorine levels of 1-2 mg/l, although the actual...
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Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Bull, Richard J. “Drinking Water Disinfection.” In Environmental Toxicants: Human Exposures and Their Health Effects, edited by Morton Lippmann. New York: John Wiley & Sons, 2000.
Gray, N. F. Drinking Water Quality: Problems and Solutions. 2d ed. New York: Cambridge University Press, 2008.
(The entire section is 41 words.)
Chlorination (Encyclopedia of Public Health)
Disinfection is the most important step in the water treatment process to destroy pathogenic bacteria and other harmful agents. Chlorination is a very common and effective method for the disinfection of drinking water, and it has been the single most important process for assuring the bacteriological safety of potable water supplies. The practice was introduced in Belgium in 1903, and was first used in the United States in 1908 in Chicago. A sharp decline in typhoid deaths was noted following the onset of chlorination, as was also the case with cholera, dysentery, and hepatitis A. Chlorination has also contributed to a major decline in infant mortality rates due to waterborne illness. Waterborne epidemics have virtually disappeared in the industrialized world. When waterborne disease outbreaks have occurred, they have generally been traced back to a failure of the chlorination system.
Chlorine is a very cost-effective disinfection process. Chlorine concentration is generally 1 miligram per liter, which is about 1 part per million. Chlorine can be added directly as chlorine gas, or indirectly as sodium-hypochlorite solution. Chlorine is applied both to drinking water and to wastewater. Chlorine demand is a measure of the chlorine added to the water system that combines with the impurities and is not available for disinfection. The lower the pH of the water, the more effective chlorine is for disinfection. After the chlorine is added to the water, there must be sufficient contact time for the chlorine to effectively destroy the bacteria.
The chlorination system generally includes chlorine storage and feed equipment. In most cases, a metering device (a chlorinator) allows the chlorine to mix via a small side stream of water. In contrast to a short-acting disinfectant (such as ozone), chlorine also has the benefit of being residual in the system. Should a pipe break or another type of accident occur, there is usually enough residual chlorine in the system to provide for protection of the water supply. Chlorine is used extensively when interruptions of water piping or cross-connections occur. Chlorine used at higher amounts will kill all potential organisms present, but at usual treatment levels certain resistant organisms, such as cryptosporidia, may survive. Chlorine is also used to clean reservoirs, basins, wells, and pipes. Algae can also be controlled with the use of chlorine.
Chlorine does have some undesirable characteristics, including imparting undesirable taste and odors to the water, especially when phenol is preset. Also, the reaction of chlorine with the organic material that can be present in the water results in a group of disinfectant by-products, known as the trihalomethanes (THMs). The most common THM is chloroform, which has been shown to cause cancer in laboratory animals. Chlorine itself is highly toxic and must be handled with extreme care at the water treatment facility.
MARK G. ROBSON
(SEE ALSO: Disinfection By-Products in Drinking Water; Drinking Water; Water Quality; Water Treatment)
Craun, G. (1993). Safety of Water Disinfection-Balancing Chemical and Microbial Risks. Washington, DC: ILSI Press.
Koren, H., and Bisesi, M. (1995). Handbook of Environmental Health and Safety, 3rd edition, Vol. 2. Boca Raton, FL: Lewis Publishers.
Wallace, M. (1998). Maxcy-Rosenau-Last Public Health and Preventive Medicine. Stamford, CT: Appleton & Lange.
Chlorination (World of Microbiology and Immunology)
Chlorination refers to a chemical process that is used primarily to disinfect drinking water and spills of microorganisms. The active agent in chlorination is the element chlorine, or a derivative of chlorine (e.g., chlorine dioxide). Chlorination is a swift and economical means of destroying many, but not all, microorganisms that are a health-threat in fluid such as drinking water.
Chlorine is widely popular for this application because of its ability to kill bacteria and other disease-causing organisms at relatively low concentrations and with little risk to humans. The killing effect occurs in seconds. Much of the killing effect in bacteria is due to the binding of chlorine to reactive groups within the membrane(s) of the bacteria. This binding destabilizes the membrane, leading to the explosive death of the bacterium. As well, chlorine inhibits various biochemical reactions in the bacterium. In contrast to the rapid action of chlorine, other water disinfection methods, such as the use of ozone or ultraviolet light, require minutes of exposure to a microorganism to kill the organism.
In many water treatment facilities, chlorine gas is pumped directly into water until it reaches a concentration that is determined to kill microorganisms, while at the same time not imparting a foul taste or odor to the water. The exact concentration depends on the original purity of the water supply. For example, surface waters contain more organic material that acts to absorb the added chlorine. Thus, more chlorine needs to be added to this water than to water emerging from deep underground. For a particular treatment facility, the amount of chlorine that is effective is determined by monitoring the water for the amount of chlorine remaining in solution and for so-called indictor microorganisms (e.g., Escherichia coli).
Alternatively, chlorine can be added to water in the form of a solid compound (e.g., calcium or sodium hypochlorite). Both of these compounds react with water, releasing free chlorine. Both methods of chlorination are so inexpensive that nearly every public water purification system in the world has adopted one or the other as its primary means of destroying disease-causing organisms.
Despite this popularity, chlorination is not without drawbacks. Microorganisms such as Cryptosporidium and Giardia form dormant structures called cysts that are resistant to chlorination. The prevalence of these protozoans in worldwide drinking water supplies is increasing. Thus, the effectiveness of chlorination may be compromised in some water systems. As well, adherent bacterial populations of bacteria such as Escherichia coli that form in distribution pipelines are extremely resistant to chlorine, and so can contaminate the disinfected water that flows from the treatment plant to the tap. A third concern with chlorination is the reaction between chlorine and methane gas, which produces one or more chlorinated derivatives. The best known are trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride). These chlorinated hydrocarbons have been shown to have adverse health effects in humans when ingested in sufficient quantity for a long time.
Furthermore, from an engineering point of view, excess chlorine can be corrosive to pipelines. In older water treatment systems in the United States, for example, the deterioration of the water distribution pipelines is a significant problem to water delivery and water quality.
See also Infection control; Water quality