Chlorine is one of the most commonly used disinfectants for water disinfection. Chlorine can be applied for the deactivation of most microorganisms and it is relatively cheap.

When was chlorine discovered?

Chlorine gas was presumably discovered in the thirteenth century. Chlorine (Cl2) was first prepared in pure form by the Swedish chemist Carl Wilhelm Scheele in 1774. Scheele heated brown stone (manganese dioxide; MnO2) with hydrochloric acid (HCl). When these substances are heated the bonds are broken, causing manganese chloride (MnCl2), water (H2O) and chlorine gas (Cl2) to form.

Reaction mechanism:

MnO2 + 4HCl -> MnCl2 + Cl2 + 2H2O

Figure: Carl Wilhelm Scheele discovered chlorine in 1774
Scheele discovered that chlorine gas was water-soluble and that it could be used to bleach paper, vegetables and flowers. It also reacted with metals and metal oxides. In 1810 sir Humphry Davy, an English chemist who tested fundamental reations of chlorine gas, discovered that the gas Scheele found must be an element, given that the gas was inseperable. He named the gas ‘chlorine’ (Cl), after the Greek word ‘chloros’, which means yellow-greenish and refers to the color of chlorine gas .

Where can chlorine be found?

Chlorine can be found on many different locations all over the world. Chlorine is always found in compounds, because it is a very reactive element. Chlorine can usually be found bond to sodium (Na), or in kitchen salt (sodium chloride; NaCl). Most chlorine can be found dissolved in seas and salty lakes. Large quantities of chlorine can be found in the ground as rock salts or halite.

The properties of chlorine

Chlorine (Cl2) is one of the most reactive elements; it easily binds to other elements. In the periodic chart chlorine can be found among the halogens. Other halogens are fluorine (F), bromine (Br), iodene (I) and astatine (At). All halogens react with other elements in the same way and can form a large quantity of substances. Halogens often react with metals to form soluble salts.
Chlorine atoms contain 17 negative electrons (negatively charged particles). These move around the heavy core of the atom in three shells. Within the inner shell there are two electrons, within the middle shell there are eight and within the outer shell there are seven. In the outer shell there is space left for another electron. This causes free, charged atoms, called ions, to form. It can also cause an extra eletron to form (a covalent bond; a chlorine bond), causing the outer shell to complete.

Figure: chlorine atoms contain 17 electrons
Chlorine can form very stable substances, such as kitchen salt (NaCl). Chlorine can also form very reactive products, such as hydrogen chloride (HCl). When hydrogen chloride dissolves in water it becomes hydrochloric acid. The hydrogen atom gives off one electron to the chlorine atom, causing hydrogen and chlorine ions to form. These ions react with any kind of substance they come in contact with, even metals that are corrosion resistant under normal circumstances. Concentrated hydrochloric acid can even corrode stainless steel. This is why it is stored either in glass or in plastic.

Disinfection requirements

Disinfection reduces pathogenic microorganisms in the water to levels designated safe by public health standards. This prevents the transmission of disease.
An effective disinfection system kills or neutralizes all pathogens in the water. It is automatic, simply maintained, safe, and inexpensive. An ideal system treats all the water and provides residual (long term) disinfection. Chemicals should be easily stored and not make the water unpalatable. State and federal governments require public water supplies to be biologically safe.
The U.S. Environmental Protection Agency (EPA) recently proposed expanded regulations to increase the protection provided by public water systems. Water supply operators will be directed to disinfect and, if necessary, filter the water to prevent contamination from Giardia lamblia, coliform bacteria, viruses, heterotrophic bacteria, turbidity, and Legionella.
Private systems, while not federally regulated, also are vulnerable to biological contamination from sewage, improper well construction, and poor-quality water sources. Since more than 30 million people in the United States rely on private wells for drinking water, maintaining biologically safe water is a major concern.


Testing water for biological quality

The biological quality of drinking water is determined by tests for coliform group bacteria. These organisms are found in the intestinal tract of warm-blooded animals and in the soil. Their presence in water indicates pathogenic contamination, but they are not considered to be pathogens. The standard for coliform bacteria in drinking water is “less than 1 coliform colony per 100 milliliters of sample” (< 1/ 100ml).
Public water systems are required to test regularly for coliform bacteria. Private system testing is done at the owner’s discretion. Drinking water from a private system should be tested for biological quality at least once each year, usually in the spring. Testing is also recommended following repair or improvements in the well.
Coliform presence in a water sample does not necessarily mean that the water is hazardous to drink. The test is a screening technique, and a positive result (more than 1 colony per 100 ml water sample) means the water should be retested. The retested sample should be analyzed for fecal coliform organisms. A high positive test result, however, indicates substantial contamination requiring prompt action. Such water should not be consumed until the source of contamination is determined and the water purified.
A testing laboratory provides specific sampling instructions and containers. The sampling protocol includes the following:
run cold water for a few minutes (15 minutes) to clear the lines;
use sterile sample container and handle only the outside of container and cap; andupon collecting the sample, immediately cap bottle and place in a chilled container if delivery to lab exceeds 1 hour (never exceed 30 hours). Many laboratories do not accept samples on Friday due to time limits.

Chlorine treatment

Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors. These components “use up” chlorine and comprise the chlorine demand of the treatment system. It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection.
The chlorine that does not combine with other components in the water is free (residual) chlorine, and the breakpoint is the point at which free chlorine is available for continuous disinfection. An ideal system supplies free chlorine at a concentration of 0.3-0.5 mg/l. Simple test kits, most commonly the DPD colorimetric test kit (so called because diethyl phenylene diamine produces the color reaction), are available for testing breakpoint and chlorine residual in private systems. The kit must test free chlorine, not total chlorine. We also recommend monitoring the ORP (Oxidation Reduction Potential) of the water. Paper – Use of ORP Monitoring for Disinfection University of California and YSI.


(1) Free Chlorine
Chlorine Gas i.e. Cl2 + Pure water
(a) Hydrolysis Cl2 + H2O HOCl + HCl
(b) Ionisation HOCl H+ + OCl- (Free Chlorine Residuals)
Hypochlorous Hypochlorite Acid Ion
Form of Free Chlorine
depends on pH



Chlorine added to water is not necessarily available for disinfection.
Lowland surface waters
– chlorine demand of 6 – 8 mg/l
• Chlorine Reacts with:
– Ammonia
• breakpoint chlorination
– Organic Matter
• Dissolved, colour
• particulate
– Metal ions
• pipe materials
• from source water

(2) Combined Chlorine

Cl2 + NH3 (1 – 50 PPM)
Sequential substitution of H in NH3
NH2 Cl (Monochloramine)
NHCl2 (Dichloramine)
NCl3 (Nitrogen trichloride)
Low pH NHCl2 and NCl3 become more
High Cl:NH3 ratio abundant
NHCl2 Good disinfectant but nasty to taste in water.
NCl3 is particularly offensive
High Cl:NH3 ratios also give increased rate of breakdown reactions
Wt. ratio Cl:NH3
< 5:1 HOCl + NH3 NH2Cl + H2O
< 10:1 HOCl + NH2Cl NHCl2 + H2O
> 10:1 HOCl + NHCl2 NCl3 + H2O
2 NH3 + 3 Cl2 N2 + 6 HCl
Mole ratio 2 : 3 gives complete oxidation = Breakpoint
ie. Wt. ratio 1 : 7.6 gives complete oxidation = Breakpoint
Other products of oxidation include:
– NO3- (Nitrate ion)
– Organo- chloramines (protein amino groups)
If NH3 concentration in water (including organic nitrogen) is known
can calculate amount HOCL required for “breakpoint”
Theoretically Chlorine requirement = Wt. NH3-N x 7.6
in practice (Margin of safety) = Wt. NH3-N x 10



Combined Residual

(a) Simple, Marginal chlorination
Suitable for Upland waters
(b) Ammonia-chlorine treatment. (Add NH3, then HOCl)
Suitable for groundwaters
Ensures combined residuals in distribution.

Free Residual

(a) Breakpoint chlorination
Suitable for Lowland surface waters.
(b) Superchlorination + Dechlorination (SO2, S2O32- or Act. Carbon. )
• For industrially polluted surface waters
destroys tastes + odours + colour
• Short contact time or pollution load variable (wells).
Desirable to have chlorine Residual in the Distribution System (in U.K.)

Combined chlorine preferable. Most persistent.

Chlorine also reacts with H2S, Fe(II), Mn(II) (groundwaters or hypolimnetic water
H2S + 4 Cl2 + 4 H2O H2SO4 + 8 HCl
H2S + Cl2 S + 2HCl
2Fe(HCO3)2 + Cl2 + Ca(HCO3)2
2Fe(OH)3 (s) + CaCl2 + 6 CO2
(associated pH rise. Useful for: iron removal; coagulant production.)
MnSO4 + Cl2 + 4 NaOH MnO2 (s) + 2 NaCl + Na2SO4 + 2 H2O
(precipitate takes 2-4 hours to form, longer for complex Mn ions)
Where H2S, Mn or Fe present:
previous practice used PRECHLORINATION + FILTRATION
But T.H.M. problems, therefore now discouraged.


(1) pH influences effectiveness
(2) THM formation (CARCINOGEN)
1 ug/l MAC (EC) and 100 ug/l MCL (USEPA) ug/l = ppb
Therefore Chlorination practice now modified
– Aim to remove THM PRECURSORS
using O3 + GAC/PAC
before final chlorination
Alternative Strategy: replace Cl2 by other oxidants
or remove micro-organisms by more efficient clarification.




Operational Factors Affecting Chlorination Practice

• Form of Chlorine
– Storage and decomposition
• Mixing Efficiency
– baffled mixing chambers
• Temperature
– slower at low temps
– seasonal variation significant
– pH
• Concentration
• Time

Kinetics of Disinfection

Ideally: All cells equally mixed with disinfectant
All cells equally susceptible to disinfectant.
Disinfectant concentration unchanged in contact tank.
No interfering substances present
Then: Disinfection is a function of:
(1) Time of Contact
(2) Concentration of Disinfectant
(3) Temperature of Water

Minimum Bactericidal Chlorine Residuals Based on Coliform Removal at 20-25oC

Practical Disinfection

How does Chlorine Inactivate Microorganisms?

Chlorine inactivates a microorganism by damaging its cell membrane. Once the cell membrane is weakened, the chlorine can enter the cell and disrupt cell respiration and DNA activity (two processes that are necessary for cell survival).

When/How do We Chlorinate Our Waters?

Chlorination can be done at any time/point throughout the water treatment process – there is not one specific time when chlorine must be added. Each point of chlorine application will subsequently control a different water contaminant concern, thus offering a complete spectrum of treatment from the time the water enters the treatment facility to the time it leaves.
Pre-chlorination is when chlorine is applied to the water almost immediately after it enters the treatment facility. In the pre-chlorination step, the chlorine is usually added directly to the raw water (the untreated water entering the treatment facility), or added in the flash mixer (a mixing machine that ensures quick, uniform dispersion of the chlorine). Chlorine is added to raw water to eliminate algae and other forms of aquatic life from the water so they won’t cause problems in the later stages of water treatment. Pre-chlorination in the flash mixer is found to remove tastes and odours, and control biological growth throughout the water treatment system, thus preventing growth in the sedimentation tanks (where solids are removed from the water by gravity settling) and the filtration media (the filters through which the water passes after sitting in the sedimentation tanks). The addition of chlorine will also oxidize any iron, manganese and/or hydrogen sulphide that are present, so that they too can be removed in the sedimentation and filtration steps.
Disinfection can also be done just prior to filtration and after sedimentation. This would control the biological growth, remove iron and manganese, remove taste and odours, control algae growth, and remove the colour from the water. This will not decrease the amount of biological growth in the sedimentation cells.
Chlorination may also be done as the final step in the treatment process, which is when it is usually done in most treatment plants. The main objective of this chlorine addition is to disinfect the water and maintain chlorine residuals that will remain in the water as it travels through the distribution system. Chlorinating filtered water is more economical because a lower CT value is required. This is a combination of the concentration (C) and contact time (T). The CT concept is discussed later on in this fact sheet. By the time the water has been through sedimentation and filtration, a lot of the unwanted organisms have been removed, and as a result, less chlorine and a shorter contact time is required to achieve the same effectiveness. To support and maintain the chlorine residual, a process called re-chlorination is sometimes done within the distribution system. This is done to ensure proper chlorine residual levels are maintained throughout the distribution system.

Residual Chlorine, Breakpoint

Any type of chlorine that is added to water during the treatment process will result in the formation of hypochlorous acid (HOCl) and hypochlorite ions (OCl-), which are the main disinfecting compounds in chlorinated water. More detail is provided later on in this fact sheet.
A Form of Chlorine + H2O -> HOCl + OCl-
Of the two, hypochlorous acid is the most effective. The amount of each compound present in the water is dependent on the pH level of the water prior to addition of chlorine. At lower pH levels, the hypochlorous acid will dominate. The combination of hypochlorous acid and hypochlorite ions makes up what is called ‘free chorine.’ Free chlorine has a high oxidation potential and is a more effective disinfectant than other forms of chlorine, such as chloramines. Oxidation potential is a measure of how readily a compound will react with another. A high oxidation potential means many different compounds are able to react with the compound. It also means that the compound will be readily available to react with others.

Combined chlorine is the combination of organic nitrogen compounds and chloramines, which are produced as a result of the reaction between chlorine and ammonia. Chloramines are not as effective at disinfecting water as free chlorine due to a lower oxidation potential. Due to the creation of chloramines instead of free chlorine, ammonia is not desired product in the water treatment process in the beginning, but may be added at the end of treatment to create chloramines as a secondary disinfectant, which remains in the system longer than chlorine, ensuring clean drinking water throughout the distribution system.
The amount of chlorine that is required to disinfect water is dependent on the impurities in the water that needs to be treated. Many impurities in the water require a large amount of chlorine to react with all the impurities present. The chlorine added must first react with all the impurities in the water before a chlorine residual is present. The amount of chlorine that is required to satisfy all the impurities is termed the ‘chlorine demand.’ This can also be thought of as the amount of chlorine needed before free chlorine can be produced. Once the chlorine demand has been met, breakpoint chlorination (the addition of chlorine to water until the chlorine demand has been satisfied) has occurred. After the breakpoint, any additional chlorine added will result in a free chlorine residual proportional to the amount of chlorine added. Residual chlorine is the difference between the amount of chlorine added and the chlorine demand. Most water treatment plants will add chlorine beyond the breakpoint.
If ammonium is present in the water at the time of chlorine addition breakpoint chlorination will not occur until all the ammonium has reacted with the chlorine. Between 10 and 15 times more chlorine than ammonia is required before free chlorine and breakpoint chlorination can be achieved. Small water treatment plants frequently only add a fraction of the required chlorine (in relation to ammonium ions) and end up not properly disinfecting their water supplies.
The type of chloramines that are formed is dependent on the pH of the water prior to the addition of chlorine. Between the pH levels 4.5 and 8.5, both monochloramine and dichloramine are created in the water. At a pH of 4.5, dichloramine is the dominant form, and below that trichloramine dominates. At a pH above 8.5 monochloramine is the dominant form. Hypochlorous acid reacts with ammonia at its most rapid rate at a pH level around 8.3.
The chlorine to ammonia nitrogen ratio characterizes what kind of residual is produced.

Is Chlorine All the Same?

The chlorination process involves adding chlorine to water, but the chlorinating product does not necessarily have to be pure chlorine. Chlorination can also be carried out using chlorine-containing substances. Depending on the pH conditions required and the available storage options, different chlorine-containing substances can be used. The three most common types of chlorine used in water treatment are: chlorine gas, sodium hypochlorite, and calcium hypochlorite.


Chlorine Gas

Chlorine gas is greenish yellow in colour and very toxic. It is heavier than air and will therefore sink to the ground if released from its container. It is the toxic effect of chlorine gas that makes it a good disinfectant, but it is toxic to more than just waterborne pathogens; it is also toxic to humans. It is a respiratory irritant and it can also irritate skin and mucus membranes. Exposure to high volumes of chlorine gas fumes can cause serious health problems, including death. However, it is important to realize that chlorine gas, once entering the water, changes into hypochlorous acid and hypochlorite ions, and therefore its human toxic properties are not found in the drinking water we consume.
Chlorine gas is sold as a compressed liquid, which is amber in color. Chlorine, as a liquid, is heavier (more dense) than water. If the chlorine liquid is released from its container it will quickly return back to its gas state. Chlorine gas is the least expensive form of chlorine to use. The typical amount of chlorine gas required for water treatment is 1-16 mg/L of water. Different amounts of chlorine gas are used depending on the quality of water that needs to be treated. If the water quality is poor, a higher concentration of chlorine gas will be required to disinfect the water if the contact time cannot be increased.
When chlorine gas (Cl2) is added to the water (H2O), it hydrolyzes rapidly to produce hypochlorous acid (HOCl) and the hypochlorous acid will then dissociate into hypochlorite ions (OCl-) and hydrogen ions (H+).
Cl2 + H2O -> HOCl + H+ + OCl-
Because hydrogen ions are produced, the water will become more acidic (the pH of the water will decrease). The amount of dissociation depends on the original pH of the water. If the pH of the water is below a 6.5, nearly no dissociation will occur and the hypochlorous acid will dominate. A pH above 8.5 will see a complete dissociation of chlorine, and hypochlorite ions will dominate. A pH between 6.5 and 8.5 will see both hypochlorous acid and hypochlorite ions present in the water. Together, the hypochlorous acid and the hypochlorite ions are referred to as free chlorine. Hypchlorous acid is the more effective disinfectant, and therefore, a lower pH is preferred for disinfection. 

Calcium Hypochlorite 



Calcium hypochlorite (CaOCl) is made up of the calcium salts of hypochlorous acid. It is produced by dissolving chlorine gas (Cl2) into a solution of calcium oxide (CaO) and sodium hydroxide (NaOH). Calcium hypochlorite is a white, corrosive solid that comes either in tablet form or as a granular powder. Calcium hypochlorite is very stable, and when packaged properly, large amounts can be purchased and stored until needed. The chemical is very corrosive however, and thus requires proper handling when being used to treat water. Calcium hypochlorite needs to be stored in a dry area and kept away from organic materials. It cannot be stored near wood, cloth or petrol because the combination of calcium hypochlorite and organic material can create enough heat for an explosion. It must also be kept away from moisture because the tablets/granular powder readily adsorb moisture and will form (toxic) chlorine gas as a result. Calcium hypochlorite has a very strong chlorine odour – something that should be kept in mind when placing them in storage.
When treating water, a lesser amount of calcium hypochlorite is needed than if using chlorine gas. Compared to the 1-16 mg/L required with chlorine gas, only 0.5-5 mg/L of calcium hypochlorite is required. When calcium hypochlorite is added to water, hypochlorite and calcium ions are produced.
Ca(OCl)2 -> Ca+2 + 2OCl-
Instead of decreasing the pH like chlorine gas does, calcium hypochlorite increases the pH of the water (making the water less acidic). However, hypochlorous acid and hypochlorite concentrations are still dependent on the pH of the water; therefore by decreasing the pH of the water, hypochlorous acid will still be present in the water. As a result, calcium hypochlorite and chlorine gas both produce the same type of residuals.

Sodium Hypochlorite

Sodium hypochlorite (NaOCl) is made up of the sodium salts of hypochlorous acid and is a chlorine-containing compound that can be used as a disinfectant. It is produced when chlorine gas is dissolved into a sodium hydroxide solution. It is in liquid form, clear with a light yellow color, and has a strong chlorine smell. Sodium hypochlorite is extremely corrosive and must be stored in a cool, dark, and dry place. Sodium hypochlorite will naturally decompose; therefore it cannot be stored for more than one month at a time. Of all the different types of chlorine available for use, this is the easiest to handle.
The amount of sodium hypochlorite required for water treatment is much less than the other two forms of chlorine, with 0.2-2 mg of NaOCl/L of water being recommended. Like calcium hypochlorite, sodium hypochlorite will also produce a hypochlorite ion, but instead of calcium ions, sodium ions are produced. NaOCl will also increase the pH of the water through the formation of hypochlorite ions. To obtain hypochlorous acid, which is a more effective disinfectant, the pH of the water should be decreased.
NaOCl -> Na+ + OCl-

Is Chlorine a Sure Way of Eliminating Pathogens?

Chlorination has been proven to be very effective against bacteria and viruses. However, it cannot disinfect all waterborne pathogens. Certain pathogens, namely protozoan cysts, are resistant to the effects of chlorine. Cryptosporidium and Giardia, two examples of protozoan cysts, have caused great concern due to the serious illnesses they can cause. Cryptosporidium was the cause of the outbreak in North Battleford in 2001, and Milwaukee in April 1993. In raw water with high Giardia and Cryptosporidium levels, another method of disinfection should be considered. For more information on these protozoa, please read their self-titled fact sheets in the public information section.

Is Chlorinating Water ‘Fool-proof’?

There are a number of factors that affect the disinfection process. Of these, the concentration or dosage of chlorine and the chlorine contact time (the time that chlorine is allowed to react with any impurities in the water) are the most important factors.
Chlorine needs time to inactivate any microorganisms that may be present in the water being treated for human consumption. The more time chlorine is in contact with the microorganisms, the more effective the process will be. The contact time is the time from when the chlorine is first added until the time that the water is used or consumed.
The same positive relationship is seen when considering the chlorine concentration. The higher the concentration of chlorine, the more effective the water disinfection process will be. This relationship holds true because as the concentration increases, the amount of chlorine for disinfection is increased. Unlike the relationship between chlorine concentration and disinfection effectiveness, the chlorine concentration and the contact time of chlorine with water show an inverse relationship. As the chlorine concentration increases, the required water-chlorine contact time ultimately decreases. To determine the level of disinfection (D), a CT value can be calculated. This value is the product of the chlorine concentration (C) and contact time (T). The formula is as follows: C*T=D. This concept shows that an increase in chlorine concentration (C) would require less contact time to achieve the same desired level of disinfection. Another possibility would be an increase in contact time that would in turn require a lower chlorine concentration in order for the level of disinfection to stay the same.
The required CT value depends on several factors, including: the type of pathogens in the water, the turbidity of the water, the pH of the water and the temperature of the water. Turbidity is the suspended matter in the water and the types of pathogens can range from bacteria like E.coli and Campylobacter to viruses including Hepatitis A. At lower temperatures, higher turbidity, or higher pH levels, the CT value (i.e. the disinfection level) will have to be increased, but at lower turbidity, there is less suspended material in the water that will prevent contact of the disinfectant with the microorganisms, thus requiring a lower CT value. A higher water temperature and a lower pH level will also allow for a lower CT value.

Impurity Reactions

Chlorine can react with a number of different substances. In raw water, there may be a number of different impurities to react with the added chlorine, resulting in an increase of the chlorine demand. As a result, more chlorine will need to be added for the same level of inactivation. Some major impurities that may exist in water include: dissolved iron, hydrogen sulphide, bromine, ammonia, nitrogen dioxide, and organic material. In some cases, the result of chlorine reacting with impurities will increase the quality of the water (by eliminating the undesired elements), while in other cases, the chlorine-impurity reactions will create undesired side products that are harmful to human health. Chlorine will first react with inorganic impurities (dissolved iron, bromine, ammonia, etc.) before reacting with the organic compounds (dissolved organic material, bacteria, viruses, etc.).
Iron, which will give water an undesirable metallic taste if present, is one of the inorganic compounds that will react with hypochlorous acid (the stronger form of free chlorine that is produced after pure chlorine is added to water). By reacting with hypochlorous acid, the dissolved iron will go from a soluble state to an insoluble state, as a precipitate is formed as a result of the reaction. The iron precipitate, in its insoluble state, can be removed by filtration process within the water treatment centre.
2 Fe2+ (liquid) + HOCl + 5H2O -> 2 Fe(OH)3 (solid) + 5H+ + Cl-
Hypochlorous acid can also react with hydrogen sulphide (H2S), if it is present in the water being treated. Hydrogen sulfide is an undesirable impurity in water because it gives water an undesired smell. At levels below 1 mg/L hydrogen sulphide generates a musty smell to the water, while at levels above 1 mg/L a rotten egg smell will prevail. Hydrogen sulphide is also toxic. The hypochlorous acid and H2S reaction gives hydrochloric acid and sulphur ions as its products.
H2S + HOCl -> H+ + Cl- + S + H2O
Bromine in the water can result in the production of undesired compounds. Bromine ions can react with hypochlorous acid to create hypobromous acid. Hypobromous acid also has disinfectant properties and is more reactive than hypochlorous acid. Hypochlorous acid or hypobromous acid will react with organic material in the water and create halogenated by-products, such as trihalomethanes.
Br- + HOCl -> HOBr + Cl-
Ammonia is a compound that may exist in the water. It is a nutrient to aquatic life, but one that will become toxic in high concentrations. Ammonia is produced as a result of decaying matter and therefore naturally exists in the water; however, human activity also releases a large amount of ammonia into the water, which contributes to an increasing level of ammonia that may cause concern. Some ‘human activity sources’ include: municipal wastewater treatment plants, agricultural releases, and industrial releases, such as pulp and paper mills, mines, food processing, and fertilizer production. Reactions between ammonia and chlorine will produce monochloramines, dichloramines, and trichloramines, which are collectively known as chloramines. These compounds are beneficial to the water treatment process as they have disinfection capacity, but they are not as effective as chlorine although chloramines will last longer in the water.
Chlorine also reacts with phenols to produce monochlorophenols, dichlorophenols, or trichlorophenols, which cause taste and odour problem at low levels. At higher levels, chlorophenols are toxic and affect the respiration and energy storage process. Chlorophenols are mainly man-made compounds, but can be found naturally in animal wastes and decomposing organic material.

Are there Health Concerns with Chlorinating Water?

Chlorine can be toxic not only for microorganisms, but for humans as well. To humans,
chlorine is an irritant to the eyes, nasal passages and respiratory system. Chlorine gas must be carefully handled because it may cause acute health effects and can be fatal at concentrations as low as 1000 ppm. However, chlorine gas is also the least expensive form of chlorine for water treatment, which makes it an attractive choice regardless of the health threat.
In drinking water, the concentration of chlorine is usually very low and is thus not a concern in acute exposure. More of a concern is the long term risk of cancer due to chronic exposure to chlorinated water. This is mainly due to the trihalomethanes and other disinfection by-products, which are by-products of chlorination. Trihalomethanes are carcinogens, and have been the topic of concern in chlorinated drinking water. Chlorinated water has been associated with increased risk of bladder, colon and rectal cancer. In the case of bladder cancer, the risk may be doubled. Although there are concerns about carcinogens in drinking water, Health Canada’s Laboratory Centre for Disease Control says that the benefits of chlorinated water in controlling infectious diseases outweigh the risks associated with chlorination and would not be enough to justify its discontinuation. In Europe, however, chorination has been discontinued in many communities.

Chlorination By-products

A number of different by-products can be produced from the reactions in the disinfection process. By-products created from the reactions between inorganic compounds and chlorine are harmless and can be easily removed from the water by filtration. Other by-products, such as chloramines, are beneficial to the disinfection process because they also have disinfecting properties. However, there are undesired compounds that may be produced from chlorine reacting with organic matter. The compounds of most concern right now are trihalomethanes (THMs) and haloacetic acids (HAAs). THMs and HAAs are formed by reactions between chlorine and organic material such as humic acids and fulvic acids (both generated from the decay of organic matter) to create halogenated organics. A greater level of THM formation has been found in surface water or groundwater influenced by surface water.
Trihalomethanes are associated with several types of cancer and are considered carcinogenic. The trihalomethane of most concern is chloroform, also called trichloromethane. It was once used as an anaesthetic during surgery, but is now used in the process of making other chemicals. About 900 ppm of chloroform can cause dizziness, fatigue, and headaches. Chronic exposure may cause damage to the liver and kidneys. Other harmful disinfection by-products are: trichloracetic acid, dichloroacetic acid, some haloacetonitriles, and chlorophenols.
Trichloracetic acid is produced commercially for use as a herbicide and is also produced in drinking water. This chemical is not classified as a carcinogen for humans, and there is limited information for animals. Dichloroacetic acid is an irritant, corrosive, and destructive against mucous membranes. This is also not currently classified as a human carcinogen. Haloacetonitriles were used as pesticides in the past, but are no longer manufactured. They are produced as a result of a reaction between chlorine, natural organic matter, and bromide. Chlorophenols cause taste and odour problems. They are toxic, and when present in higher concentrations, affect the respiration and energy storage process in the body.

Contact time with microorganisms

The contact (retention) time (Table 1) in chlorination is that period between the introduction of the disinfectant and when the water is used. A long interaction between chlorine and the microorganisms results in an effective disinfection process. The contact time varies with chlorine concentration, the type of pathogens present, pH, and temperature of the water. The calculation procedure is given below.
Contact time must increase under conditions of low water temperature or high pH (alkalinity). Complete mixing of chlorine and water is necessary, and often a holding tank is needed to achieve appropriate contact time. In a private well system, the minimum-size holding tank is determined by multiplying the capacity of the pump by 10. For example, a 5-gallons-per-minute (GPM) pump requires a 50-gallon holding tank. Pressure tanks are not recommended for this purpose since they usually have a combined inlet/outlet and all the water does not pass through the tank.An alternative to the holding tank is a long length of coiled pipe to increase contact between water and chlorine. Scaling and sediment build-up inside the pipe make this method inferior to the holding tank.

To calculate contact time, one should use the highest pH and lowest water temperature expected. For example, if the highest pH anticipated is 7.5 and the lowest water temperature is 42 °F, the “K” value (from the table below) to use in the formula is 15. Therefore, a chlorine residual of 0.5 mg/l necessitates 30 minutes contact time. A residual of 0.3 mg/l requires 50 minutes contact time for adequate disinfection.

Chlorination levels

If a system does not allow adequate contact time with normal dosages of chlorine, superchlorination followed by dechlorination (chlorine removal) may be necessary.
Superchlorination provides a chlorine residual of 3.0-5.0 mg/l, 10 times the recommended minimum breakpoint chlorine concentration. Retention time for superchlorination is approximately 5 minutes. Activated carbon filtration removes the high chlorine residual.
Shock chlorination is recommended whenever a well is new, repaired, or found to be contaminated. This treatment introduces high levels of chlorine to the water. Unlike superchlorination, shock chlorination is a “one time only” occurrence, and chlorine is depleted as water flows through the system; activated carbon treatment is not required. If bacteriological problems persist following shock chlorination, the system should be evaluated.


Chlorine solutions lose strength while standing or when exposed to air or sunlight. Make fresh solutions frequently to maintain the necessary residual.
Maintain a free chlorine residual of 0.3-0.5 mg/l after a 10-minute contact time. Measure the residual frequently.
Once the chlorine dosage is increased to meet greater demand, do not decrease it.
Locate and eliminate the source of contamination to avoid continuous chlorination. If a water source is available that does not require disinfection, use it.
Keep records of pertinent information concerning the chlorination system and we recommend that you monitor the ORP of the water.

Types of chlorine used in disinfection

Public water systems use chlorine in the gaseous form, which is considered too dangerous and expensive for home use. Private systems use liquid chlorine (sodium hypochlorite) or dry chlorine (calcium hypochlorite). To avoid hardness deposits on equipment, manufacturers recommend using soft, distilled, or demineralized water when making up chlorine solutions.

Equipment for continuous chlorination

Continuous chlorination of a private water supply can be done by various methods. The injection device should operate only when water is being pumped, and the water pump should shut off if the chlorinator fails or if the chlorine supply is depleted. A brief description of common chlorination devices follows.

chlorine pump (see Fig.):

• commonly used, positive displacement or chemical-feed device,
• adds a small amount, of chlorine to the water,
• dose either fixed or varies with water flow rates
• recommended for low and fluctuating water pressure,
chlorine drawn into the device then pumped to water delivery line

Figure 1. Pump type (positive displacement) chlorinator
Figure 2. Injector (aspirator) chlorinator

suction device:
• The line from chlorine supply to the suction side of water pump,
• chlorine drawn into the water held in the well pump,
• dosage uniformity not assured with this system,
• some suction devices inject chlorine directly into well water, increasing the contact time between microorganisms and disinfectant; water/chlorine mixture is then drawn into the well pump

aspirator (see Fig. 2):
• simple, inexpensive mechanism,
• requires no electricity,
• the vacuum created by water flowing through a tube draws chlorine into a tank where it mixes with untreated water,
• treated solution fed into the water system,
• chlorine doses not consistently accurate

solid feed unit:
• waste treatment and swimming pool disinfection,
• requires no electricity,
• controlled by a flow meter,
• the device slowly dissolves chlorine tablets to provide continuous supply of chlorine solution

batch disinfection:
• used for fluctuating chlorine demand,
• three tanks, each holding 2 to 3 days’ water supply, alternately filled, treated and used

Disinfection by-products

Trihalomethanes (THMS) are chemicals that are formed, primarily in surface water, when naturally occurring organic materials (humic and fulvic acids from degradation of plant material) combine with free chlorine. Some of the THMs present in drinking water are chloroform, bromoform, and bromodichloromethane. Since groundwater rarely has high levels of humic and fulvic acids, chlorinated private wells contain much lower levels of these chemicals.
THMs are linked to increases in some cancers, but the potential for human exposure to THMs from drinking water varies with season, contact time, water temperature, pH, water chemistry and disinfection method. Although there is a risk from consuming THMs in chlorinated drinking water, the health hazards of undisinfected water are much greater. The primary standard (maximum contaminant level) for total THMs in drinking water is 0.10 mg/l, and activated carbon filtration removes THMs from water. Our comprehensive water quality
test kit for City Water or Well Water.
Important tools, Chlorine Monitoring and ORP Monitoring.

How is chlorine transported?

Chlorine is a very reactive and corrosive gas. When it is transported, stored or used, safety precautions must be taken. In Holland for example, chlorine is transported in separate chlorine trains.

How can chlorine be stored?

Watery chlorine should be protected from sunlight. Chlorine is broken down under the influence of sunlight. UV radiation in sunlight provides energy which aids the break-down of underchloric acid (HOCl) molecules. First, the water molecule (H2O) is broken down, causing electrons to be released which reduce the chlorine atom of underchloric acid to chloride (Cl-). During this reaction an oxygen atom is released, which will be converted into an oxygen molecule:

2HOCl -> 2H+ + 2Cl- + O2

How is chlorine produced?

Chlorine is produced from chlorine bonds by means of electrolytic or chemical oxidation. This is often attained by electrolysis of seawater or rock salt. The salts are dissolved in water, forming brine. Brine can conduct a powerful direct current in an electolytic cell. Because of this current chlorine ions (which originate from salt dissolving in water) are transformed to chlorine atoms. Salt and water are divided up in sodium hydroxide (NaOH) and hydrogen gas (H2) on the cathode and chlorine gas on the anode. These cathode and anode products should be separated, because hydrogen gas reacts with chlorine gas very agressively.

Which methods can be used to produce chlorine?

To produce chlorine, three different electrolysis methods are used.

1. The diaphragm cell-method, which prevents products to mix or react by means of a diaphragm. The electrolysis barrel contains a positive pole, made of titanium and a negative pole, made of steel. The electrodes are separated by a so-called diaphragm, which is a wall that only lets fluids flow through, causing gasses that form during a reaction to be separated. The application of the countercurrent principle prevents hydroxide ions from reaching the positive pole. However, chlorine ions can pass through the diaphragm, causing the sodium hydroxide to become slightly polluted with chlorine. This causes the following reactions to take place:
+ pole : 2Cl- -> Cl2 + 2e-
– pole : 2 H2O + 2 e- -> 2OH- + H2

2. The mercury cell-methode uses one mercury electrode, causing the reaction products to be purer than those of the diaphragm cell-methode. With this method an electrolysis barrel is used which contains a positive titanium pole and a negative flowing mercury pole. On the negative pole a reaction with sodium (Na+) takes place, causing sodium amalgams to be formed. When the amalgams flow through a second reaction barrel, sodium reacts with water to sodium hydroxide and hydrogen. This causes the hydrogen gas to remain separated from the chlorine gas, which is formed on the positive pole.
Within the electrolysis barrel the following reactions take place:

+ pole : 2 Cl- -> Cl2 + 2e-
– pole : Na+ + e- -> Na
second reaction barrel: 2Na + 2H2O -> 2 Na+ + 2OH- + H2

3. The membrane-method resembles the diaphragm method. The only difference is that the membrane only allows positive ions to pass, causing a relatively pure form of sodium hydroxide to form.
During the mercury electrolysis process a solution containing 50 mass-% of sodium hydroxide is formed. However, during the membrane and diaphragm
processes the solution must be evaporated using steam.
Sixty percent of the European chlorine production takes place by means of mercury electrolysis, whereass 20% takes place in the diaphragm process and 20% takes place in the membrane process.
Chlorine can also be produced by means of hydrogen chloride oxidation with oxygen from air. Copper(II)chloride (CuCl2) is used as a cathalyser during this so
called ‘Deaconprocess’:
4HCl + O2 -> 2H2O + 2Cl2
Finally, chlorine can be produced by means of molten salts electrolysis and, mainly in laboratories, by means of hydrochloric acid and manganese dioxide oxidation:

MnO2 + 4HCl -> MnCl2 + 2H2O + Cl2
When gaseous chlorine is added to water the following hydrolysis reaction takes place:
Cl2 + H2O = H+ + Cl- + HOCl

Chlorine applications

Chlorine is applied on a massive scale. Chlorine is a very reactive element, causing it to quickly form compounds with other substances. Chlorine also has the ability to develop a bond between two substances that do not normally react with one another. When chlorine bonds to a substance that contains carbon atoms, organic substances are formed. Examples are plastic, solvents and oils, but also several human body fluids. When chlorine chemically binds to other elements, it often replaces a hydrogen atom during a so-called substitution reaction. Multiple hydrogen atoms in the same molecule can be replaced by chlorine atoms, causing new substances to form one after another.
Chlorine plays an important role in medical science. It is not only used as a disinfectant, but it is also a constituent of various medicines. The majority of our medicines contain chlorine or are developed using chlorine-containing byproducts. Medical herbs also contain chlorine. The first anaesthetic used during surgery was chloroform (CHCl3).
The chemical industry creates ten thousands of chlorine products using a small number of chlorine containing chemicals. Emaples of products which contain chlorine are glue, paints, solvents, foam rubbers, car bumpers, food additives, pesticides and antifreeze. One of the most commonly used chlorine-containing substances is PVC (poly vinyl chloride). PVC is widely used, for example in drainpipes, insulation wires, floors, windows, bottles and waterproof clothes.

Figure: products containing chlorine

Chlorine-based bleach is applied as a disinfectant on a large scale. The substances are also used to bleach paper. Bleaching occurs as a result of chlorine or hypochlorite oxidation.
About 65% of industrialized chlorine is used to produce organic chemicals, such as plastics. About 20% is used to produce bleach and disinfectants. The remaining chlorine is used to produce inorganic compounds from chlorine and several different elements, such as zinc (Zn), iron (Fe) and titanium (Ti).


Chlorine as a disinfectant

Chlorine is one of the most widely used disinfectants. It is very applicable and very effective for the deactivation of pathogenic microorganisms. Chlorine can be easily applied, measures and controlled. Is is fairly persistent and relatively cheap.
Chlorine has been used for applications, such as the deactivation of pathogens in drinking water, swimming pool water and wastewater, for the disinfection of household areas and for textile bleaching, for more than two hundred years. When chlorine was discovered we did not now that disease was caused by microorganisms. In the nineteenth century doctors and scientists discovered that many diseases are contagious and that the spread of disease can be prevented by the disinfection of hospital areas. Very soon afterward, we started experimenting with chlorine as a disinfectant. In 1835 doctor and writer Oliver Wendel Holmes advised midwifes to wash their hands in calcium hypochlorite (Ca(ClO)2-4H2O) to prevent a spread of midwifes fever. However, we only started using disinfectants on a wider scale in the nineteenth century, after Louis Pasteur discovered that microorganisms spread certain diseases.hlorine has played an important role in lenghthening the life-expectancy of humans.
For more information about pathogens in aquatic systems, please take a look at pathogens in freshwater ecosystems.

Chlorine as a bleach

Surfaces can be disinfected by bleaching. Bleach consists of chlorine gas dissolved in an alkali-solution, such as sodium hydroxide (NaOH). When chlorine is dissolved in an alkalic solution, hypochlorite ions (OCl-) are formed during an autoredox reaction. Chlorine reacts with sodium hydroxide to sodium hypochlorite (NaOCl). This is a very good disinfectant with a stable effect.
Bleach cannot be combined with acids. When bleach comes in contact with acids the hypochlorite becomes instable, causing poisonous chlorine gas to escape. The accompanying underchloric acid is not very stable.

Figure \: chlorine is often used as a bleach

Bleaching powder (CaOCl2) can also be used. This is produced by directing chlorine through calcium hydroxide (CaOH). The benefit of bleaching powder is that it is a solid. This makes it easier to apply as a disinfectant in medical areas, next to its use as a bleach. When bleaching powder dissolves, it reacts with water to underchloric acid (HOCl) and hypochlorite ions (OCl-).

How does chlorine disinfection work?

Chlorine kills pathogens such as bacteria and viruses by breaking the chemical bonds in their molecules. Disinfectants that are used for this purpose consist of chlorine compounds which can exchange atoms with other compounds, such as enzymes in bacteria and other cells. When enzymes come in contact with chlorine, one or more of the hydrogen atoms in the molecule are replaced by chlorine. This causes the entire molecule to change shape or fall apart. When enzymes do not function properly, a cell or bacterium will die.
When chlorine is added to water, underchloric acids form:
Cl2 + H2O -> HOCl + H+ + Cl-
Depending on the pH value, underchloric acid partly expires to hypochlorite ions
Cl2 + 2H2O -> HOCl + H3O + Cl-
HOCl + H2O -> H3O+ + OCl-
This falls apart to chlorine and oxygen atoms:
OCl- -> Cl- + O
Underchloric acid (HOCl, which is electrically neutral) and hypochlorite ions (OCl-, electrically negative) will form free chlorine when bound together. This results in disinfection. Both substances have very distinctive behaviour. Underchloric acid is more reactive and is a stronger disinfectant than hypochlorite. Underchloric acid is split into hydrochloric acid (HCl) and atomair oxygen (O). The oxygen atom is a powerful disinfectant.
The disinfecting properties of chlorine in water are based on the oxidising power of the free oxygen atoms and on chlorine substitution reactions.

Figure: the neutral underchloric acid can better penetrate cell walls of pathogenic microorganisms that the negatively charged hypochlorite ion
The cell wall of pathogenic microorganisms is negatively charged by nature. As such, it can be penetrated by the neutral underchloric acid, rather than by the negatively charged hypochlorite ion. Underchloric acid can penetrate slime layers, cell walls and protective layers of microorganisms and effectively kills pathogens as a result. The microorganisms will either die or suffer from reproductive failure.

The effectivity of disinfection is determined by the pH of the water. disinfection with chlorine will take place optimally when the pH is between 5,5 and 7,5. underchloric acid (HOCl) reacts faster than hypochlorite ions (OCl-); it is 80-100% more effective. The level of underchloric acid will decrease when the pH value is higher. With a pH value of 6 the level of underchloric acid is 80%, whereass the concentration of hypochlorite ions is 20%. When the pH value is 8, this is the other way around.
When the pH value is 7,5, concentrations of underchloric acid and hypochlorite ions are equally high.

Underchloric acid (left) : hypochlorite ions (right)

What is free and bound active chlorine?

When chlorine is added to water for disinfection purposes, it usually starts reacting with dissolved organic and inorganic compounds in the water. Chlorine can no longer be used for disinfection after that, because is has formed other products. The amount of chlorine that is used during this process is referred to as the ‘chlorine enquiry’ of the water.
Chlorine can react with ammonia (NH3) to chloramines, chemical compounds which contain chlorine, nitrogen (N) and hydrogen (H). These compounds are referred to as ‘active chlorine compounds’ (contrary to underchloric acid and hypochlorite, which are referred to as ‘free active chlorine’) and are responsible for water disinfection. However, these compounds react much more slowly than free active chlorine.

What doses of chlorine does one apply?

When dosing chlorine one has to take into acount that chlorine reacts with compounds in the water. The dose has to be high enough for a significant amount of chlorine to remain in the water for disinfection. Chlorine enquiry is determined by the amount of organic matter in the water, the pH of the water, contact time and temperature. Chlorine reacts with organic matter to disinfection byporducts, such as trihalomethanes (THM) and halogenated acetic acids (HAA).
Chlorine can be added for disinfection in several different ways. When ordinary chlorination is apllied, the chlorine is simply added to the water and no prior treatment is necessary. Pre- and postchlorination means adding chlorine to water prior to and after other treatment steps. Rechlorination means the addition of chlorine to treated water in one or more points of the distribution system in order to preserve disinfection.

What is breakpoint chlorination?

Breakpoint chlorination consists of a continual addition of chlorine to the water upto the point where the chlorine enquiry is met and all present ammonia is oxidized, so that only free chlorine remains. This is usually applied for disinfection, but it also has other benefits, such as smell and taste control. In order to reach the breakpoint, a superchlorination is applied. To achieve this, one uses chlorine concentrations which largely exceed the 1 mg/L concentration required for disinfection.

Figure: breakpoint chlorination

Which chlorine concentration is applied?

Chlorine gas can be obtained as fluid gas in 10 bar pressure vessels. It is highly water soluble (3 L chlorine/ 1 L water). To kill bacteria little chlorine is required; about 0,2-0,4 mg/L. the concentrations of chlorine added to the water are usually higher, because of the chlorine enquiry of the water.
Nowadays chlorine gas is only used for large municipal and industrial water purification installations. For smaller applications one usually ads calcium or sodium hypochlorite.

Which factors determine the effectivity of chlorine disinfection?

Factors which determine chlorine disinfection effectivity:
Chlorine concentrations, contact time, temperature, pH, number and types of microorganisms, concentrations of organic matter in the water.
Table: disinfection time for several different types of pathogenic microorganisms with chlorinated water, containing a chlorine concentration of 1 mg/L (1 ppm) when pH = 7,5 and T = 25 °C
Disinfection time of fecal pollutants with chlorinated water

What are the health effects of chlorine?

The reaction of the human body to chlorine depends on the concentration of chlorine present in air, and on the duration and frequency of exposure. Effects also depend on the health of an individual and the environmental conditions during exposure.
When small amounts of chlorine are breathed in during short time periods, this can affect the respirational system. Effects vary from coughing and chest pains, to fluid accumulation in the lungs. Chlorine can also cause skin and eye irritations. These effects do not take place under natural conditions. When chlorine enters the body it is not very persistent, because of its reactivity.
Pure chlorine is very toxic, even small amounts can be deadly. During World War I chlorine gas was used on a large scale to hurt or kill enemy soldiers. The Germans were the first to use chlorine gas against their enemies.
Chlorine is much denser than air, causing it to form a toxic fume above the soil. Chlorine gas affects the mucous membrane (nose, throat, eyes). Chlorine is toxic to mucous membranes because it dissolves them, causing the chlorine gas to end up in the blood vessels. When chlorine gas is breathed in the lungs fill up with fluid, causing a person to sort of drown.