Chlorination (centralised)

The control of infectious diseases through clean water and improved sanitation is one of the most important public health achievements of the 20th century. The use of chlorine in the treatment of drinking water played a major role in reducing or even virtually eliminating waterborne diseases in developed countries, such as typhoid fever, cholera, dysentery, and other gastroenteritic diseases.

Centralised chlorination involves the use of chlorine for disinfection within a centralised drinking water supply system, either as primary or secondary disinfection. Chlorine is the most commonly used disinfectant in the world for treating drinking water worldwide, due to its advantages of being an inexpensive yet effective water purification method.

How does it work?

Chlorine can be added to water in different forms, depending on the pH conditions required and the available storage options. The three most common types of chlorine used in water treatment are: chlorine gas, sodium hypochlorite, and calcium hypochlorite.

Once added to the water, chlorine kills microorganisms such as bacteria, algae, and fungi (read more about pathogens and contaminants). It kills cells by first damaging the cell membrane, entering the cell, and disrupting cell respiration and DNA activity, two processes that are necessary for cell survival. In addition to killing microorganisms, chlorine also oxidises iron, manganese, taste and odour compounds, removes colour in the water, and destroys hydrogen sulphide.

What is it used for?

The ability of chlorine to kill many types of microorganisms growing in water makes chlorination suitable for the following uses:

• To prevent algal, fungal, and bacterial growth (see pathogens and contaminants).
• To control slime growth in distribution systems (to learn about residual chlorine, see also preventing recontamination).
• To maintain clean filter media at the treatment plant (see also slow and rapid sand filtration).
• To restore and preserve pipeline capacity (to learn about residual chlorine, see also preventing recontamination).
• To restore well capacity, to disinfect water mains (see also well development and rehabilitation).
• To control taste and odours

At household level

Chlorination is one of many household water treatment systems that can be done to ensure safe drinking water at the point of use. To learn more about how chlorination can be done at a household level, see point of use chlorination.

At municipal level

Chlorination is the most common disinfection method in drinking water treatment plants, and can be done at any stage throughout the water treatment process. Each point of chlorine application will 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.

The chlorination process is integrated into water treatment plants as a primary (within a water purification plant) or secondary (within distribution system) disinfection method.

Primary disinfection

Primary disinfection is the application of a disinfectant in the drinking water treatment plant. The amount of chlorine needed and time needed to react and disinfect is called the Contact Time (CT), and is a product of the concentration of residual chlorine (mg/l) and the disinfectant contact time. CT values required to achieve the necessary disinfection depends on the microorganism targeted, pH, and temperature. Other design factors influencing the amount of chlorine required are contact chamber design, adequate mixing, and the presence of sunlight.

The following are possibilities for chlorination as a primary disinfection method:

Pre-chlorination

In pre-chlorination, chlorine is applied to the water almost immediately after it enters the treatment facility to eliminate algae and other organisms from water so they won’t cause a problem in later treatment stages. Pre-chlorination is found to remove tastes and odours and control biological growth throughout the water treatment system, thus preventing growth in the sedimentation tanks and the filtration media. The addition of chlorine also oxidises any iron, manganese and/or hydrogen sulphide that are present, so that they too can be removed in the sedimentation and filtration steps.

After sedimentation and before filtration

This controls the biological growth, removes iron and manganese, removes taste and odours, controls algae growth, and removes the colour from the water.

Final treatment step

The most common stage for chlorination is as a final treatment step to disinfect the water and maintain chlorine residuals that will remain in the water as it travels through the distribution system. Chlorinating as a final step is more economical because a lower CT value is required, as by the time the water has been through sedimentation and filtration, a lot of the unwanted organisms have been removed, so less chlorine and a shorter contact time is required to achieve the same effectiveness.

Secondary disinfection

Secondary disinfection may be applied to the treated water as it leaves the treatment plant or at rechlorination points throughout the distribution system, to introduce and maintain a chlorine residual in the drinking water distribution system. Overall, a chlorine residual provides two main benefits:

• It can limit the growth of biofilm within the distribution system and its associated taste and odour problems.
• A rapid drop in disinfectant residual may provide an immediate indication of treatment process malfunction or a break in the integrity of the distribution system

A chlorine residual may also reduce the risk of recontamination in the event of an intrusion into the distribution system.

Chlorination disinfection by-products (CDBPs)

A number of different by-products can be produced from reactions in the chlorination process. Some by-products, such as chloramines, are beneficial to the disinfection process because they also have disinfecting properties. However, there are several undesired compounds that may be produced from chlorine reacting with natural organic matter such as humic and fulvic acids, which are generated from the decay of organic matter:

• Trihalomethanes (THMs): considered carcinogenic. The trihalomethane of most concern is chloroform (also called trichloromethane). Chronic exposure may cause damage to the liver and kidneys.
• Haloacetic acids (HAAs; includes trichloroacetic acid, dichloroactic acid):Trichloracetic acid is produced commercially for use as a herbicide and is also produced in drinking water, but is not classified as a carcinogen for humans. Dichloroacetic acid is an irritant, corrosive, and destructive against mucous membranes.
• Haloacetonitriles: were used as pesticides in the past, but are no longer manufactured, and form from 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.

In order to avoid the formation of CDBPs, it is recommended to remove organic precursors or optimise the treatment system to so that chlorine is added after organic precursors have been removed.

Taste and odour problems

While chlorination can help improve taste and odour through the reaction with organic materials and iron, it can also generate chlorinous flavours caused by the presence of the disinfectant itself or by the occurrence of other CDBPs formed by the reaction with other compounds in the water. For example, the reaction of chlorine with certain nitrogen compounds (e.g., amino acids, ammonium, urea) present in source water may lead to the formation of strong-smelling compounds such as aldehydes, nitriles, and some chloramines, which can cause pronounced chlorinous tastes and odours, sometimes even at very low levels. Chlorophenols can also be formed at the plant or in the distribution system and can impart taste and odours to the water.

The consequence of these compounds in drinking water is consumer dissatisfaction, turning to other water sources (such as bottled water), purchasing home filtration devices (MACKEY 2009), and the rejection of a water source that is actually safe to drink. This is particularly a problem when no other safe drinking water sources are available.

While chlorination can result in CDBPs that pose health risks and cause taste and odour problems, risks to health from these by-products are extremely small in comparison with the risks associated with inadequate disinfection.

Chlorine itself is non-toxic, and while drinking water typically contains around 1 mg/L, even 50 mg/L has been shown to have no health impact. However, some CDBPs are carcinogenic and should be avoided by removing organic precursors before chlorination. Despite these risks, the effectiveness in killing pathogens far outweighs the risk of CDBPs.

While most microorgansisms are rapidly deactivated by chlorine (e.g. E. coli), others are chlorine-resistant (e.g. Giardia and Cryptosporidium), and thus are not killed when chlorine is added to water. If these pathogens are present in large concentrations, additional treatment is needed such as membrane filtration or boiling (for more information pathogens in drinking water, see pathogens and contaminants).

Chlorine is a low-cost way to effectively disinfect drinking water. Centralised chlorination as primary disinfection is most economical when added as a final treatment step, as less chlorine is needed to achieve the same level of disinfection.

Chlorination is an effective water purification method, but the presence of inorganic compounds (e.g. iron, manganese, etc.) or natural organic matter may result in decreased efficiency in pathogen deactivation and in the formation of harmful CDBPs. Therefore, removal of these compounds before the addition of chlorine is recommended, or other water purification methods preferentially employed, such as solar disinfection (see solar pasteurisation, UV tubes, and SODIS), membrane filtration, or boiling to kill pathogens.

To ensure effectiveness and prevent recontamination, a chlorine residual should be maintained throughout the distribution system. Water leaving a treatment plant should be tested daily for chlorine residual, turbidity, total coliforms, and E. coli to confirm the microbiological safety of the supply. In the distribution system, the presence of adequate chlorine residuals should be confirmed when sampling for total coliforms and E. coli.