Ask a question about 'Chlorination'
Start a new discussion about 'Chlorination'
Answer questions from other users
Full Discussion Forum
Chlorination is the process of adding the element chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 to water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 as a method of water purification
Water purification
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...

 to make it fit for human consumption as drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

. Water which has been treated with chlorine is effective in preventing the spread of waterborne disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...


The chlorination of public drinking supplies was originally met with resistance, as people were concerned about the health effects of the practice. The use of chlorine has greatly reduced the prevalence of waterborne disease as it is effective against almost all bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, as well as amoeba
Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...


Chlorination is also used to sanitize the water in swimming pool
Swimming pool
A swimming pool, swimming bath, wading pool, or simply a pool, is a container filled with water intended for swimming or water-based recreation. There are many standard sizes; the largest is the Olympic-size swimming pool...

s and as a disinfection stage in sewage treatment
Sewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...


Shock chlorination is a process used in many swimming pools, water well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...

s, springs
Spring (hydrosphere)
A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...

, and other water sources to reduce the bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l and algal
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 residue in the water. Shock chlorination is performed by mixing a large amount of sodium hypochlorite, which can be in the form of a powder or a liquid such as chlorine bleach, into the water. Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three ppm or less.


The first scientists to suggest disinfecting water with chlorine were Louis-Bernard Guyton de Morveau
Louis-Bernard Guyton de Morveau
Louis-Bernard Guyton de Morveau was a French chemist and politician...

 (in France) and William Cumberland Cruikshank
William Cumberland Cruikshank
William Cumberland Cruikshank was a British chemist and anatomist. He was the author of The Anatomy of the Absorbing Vessels of the Human Body, which was first published in 1786....

 (in England), both around the year 1800.

The technique of purification of drinking water by use of compressed liquefied chlorine gas was developed in 1910 by U.S. Army Major
Major is a rank of commissioned officer, with corresponding ranks existing in almost every military in the world.When used unhyphenated, in conjunction with no other indicator of rank, the term refers to the rank just senior to that of an Army captain and just below the rank of lieutenant colonel. ...

 (later Brig. Gen.
Brigadier General
Brigadier general is a senior rank in the armed forces. It is the lowest ranking general officer in some countries, usually sitting between the ranks of colonel and major general. When appointed to a field command, a brigadier general is typically in command of a brigade consisting of around 4,000...

) Carl Rogers Darnall
Carl Rogers Darnall
Brigadier General Carl Rogers Darnall was a United States Army chemist and surgeon credited with originating the technique of liquid chlorination of drinking water...

 (1867–1941), Professor of Chemistry at the Army Medical School
Army Medical School
Founded by U.S. Army Brigadier General George Miller Sternberg, MD in 1893, the Army Medical School was by some reckonings the world's first school of public health and preventive medicine...

. Shortly thereafter, Major (later Col.
Colonel , abbreviated Col or COL, is a military rank of a senior commissioned officer. It or a corresponding rank exists in most armies and in many air forces; the naval equivalent rank is generally "Captain". It is also used in some police forces and other paramilitary rank structures...

) William J. L. Lyster (1869–1947) of the Army Medical Department used a solution of calcium hypochlorite
Calcium hypochlorite
Calcium hypochlorite is a chemical compound with formula 2. It is widely used for water treatment and as a bleaching agent...

 in a linen bag to treat water.

For many decades, Lyster's method remained the standard for U.S. ground forces in the field and in camps, implemented in the form of the familiar Lyster Bag (also spelled Lister Bag). Darnall's work became the basis for present day systems of chlorination of municipal water supplies, which were perfected in the 1930s and widely established in the United States by World War II.

Chemistry in water

When chlorine is added to water, it reacts to form a pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 dependent equilibrium mixture of chlorine, hypochlorous acid
Hypochlorous acid
Hypochlorous acid is a weak acid with the chemical formula HClO. It forms when chlorine dissolves in water. It cannot be isolated in pure form due to rapid equilibration with its precursor...

 and hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

Cl2 + H2O → HOCl + HCl

Depending on the pH, hypochlorous acid partly dissociates
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...

 to hydrogen and hypochlorite
The hypochlorite ion, also known as chlorate anion is ClO−. A hypochlorite compound is a chemical compound containing this group, with chlorine in oxidation state +1.Hypochlorites are the salts of hypochlorous acid...

HOCl → H+ + ClO-

In acidic solution, the major species are Cl2 and HOCl while in alkaline solution effectively only ClO- is present. Very small concentrations of ClO2-, ClO3-, ClO4- are also found.


Disinfection by chlorination can be problematic, in some circumstances. Chlorine can react with naturally occurring organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s found in the water supply to produce compounds known as disinfection byproducts (DBPs). The most common DBPs are trihalomethane
Trihalomethanes are chemical compounds in which three of the four hydrogen atoms of methane are replaced by halogen atoms. Many trihalomethanes find uses in industry as solvents or refrigerants. THMs are also environmental pollutants, and many are considered carcinogenic...

s (THMs) and haloacetic acids (HAAs). Due to the potential carcinogenicity of these compounds, drinking water regulations across the developed world require regular monitoring of the concentration of these compounds in the distribution systems of municipal water systems. The World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 has stated that the "Risks to health from DBPs are extremely small in comparison with inadequate disinfection."

There are also other concerns regarding chlorine, including its volatile nature which causes it to disappear too quickly from the water system, and aesthetic
Aesthetics is a branch of philosophy dealing with the nature of beauty, art, and taste, and with the creation and appreciation of beauty. It is more scientifically defined as the study of sensory or sensori-emotional values, sometimes called judgments of sentiment and taste...

 concerns such as taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

 and odour.


Chlorine in water is more than three times more effective as a disinfectant against Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

than an equivalent concentration of bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

, and is more than six times more effective than an equivalent concentration of iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....


Several alternatives to traditional chlorination exist, and have been put into practice to varying extents. Ozonation is used by many European countries and also in a few municipalities in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. Due to current regulations, systems employing ozonation in the United States still must maintain chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 residuals comparable to systems without ozonation.

Disinfection with chloramine
Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen atoms with chlorine atoms. Monochloramine is an inorganic compound with the formula NH2Cl. It is an unstable colourless liquid at its melting point of -66° temperature, but it is usually handled as a dilute...

 is also becoming increasingly common. Unlike chlorine, chloramine has a longer half life in the distribution system and still maintains effective protection against pathogens. The reason chloramines persist in the distribution is due to the relatively lower redox potential in comparison to free chlorine. Chloramine is formed by the addition of ammonia into drinking water to form mono-, di-, and trichloramines. Whereas Helicobacter pylori
Helicobacter pylori
Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...

can be many times more resistant to chlorine than Escherichia coli, both organisms are about equally susceptible to the disinfecting effect of chloramine.

Water treated by filtration may not need further disinfection
Disinfectants are substances that are applied to non-living objects to destroy microorganisms that are living on the objects. Disinfection does not necessarily kill all microorganisms, especially nonresistant bacterial spores; it is less effective than sterilisation, which is an extreme physical...

; a very high proportion of pathogens are removed by materials in the filter bed. Filtered water must be used soon after it is filtered, as the low amount of remaining microbes may proliferate over time.

The advantage of chlorine in comparison to ozone is that the residual persists in the water for an extended period of time. This feature allows the chlorine to travel through the water supply system, effectively controlling pathogenic backflow contamination. In a large system this may not be adequate, and so chlorine levels may be boosted at points in the distribution system, or chloramine
Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen atoms with chlorine atoms. Monochloramine is an inorganic compound with the formula NH2Cl. It is an unstable colourless liquid at its melting point of -66° temperature, but it is usually handled as a dilute...

 may be used, which remains in the water for longer before reacting or dissipating.

Another method which is gaining popularity is UV disinfection. UV treatment leaves no residue in the water due to use of light instead of chemical disinfectants. However, this method alone (as well as chlorination alone) will not remove bacterially produced toxins, pesticides, heavy metals, etc. from water. Often, multiple steps are taken in commercially sold water.

See also

  • chloramination
    Chloramination is the treatment of drinking water with a chloramine disinfectant. Both chlorine and small amounts of ammonia are added to the water one at a time which react together to form chloramine , a long lasting disinfectant...

  • water fluoridation
    Water fluoridation
    Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay. Fluoridated water has fluoride at a level that is effective for preventing cavities; this can occur naturally or by adding fluoride...

  • water pollution
    Water pollution
    Water pollution is the contamination of water bodies . Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds....

  • saltwater pool
    Saltwater pool
    Salt water chlorination is a process that uses dissolved salt as a store for the chlorination system. The chlorinator uses electrolysis to break down the salt . The resulting chemical reaction eventually produces hypochlorous acid , and sodium hypochlorite , which are the sanitizing agents already...

  • sodium hypochlorite
  • Symclosene/trichloroisocyanuric acid
    Trichloroisocyanuric acid is the organic compound with the formula . It is used as an industrial disinfectant, bleaching agent and a reagent in organic synthesis...

     is the chemical in chlorination tablets
  • water filtration

External links