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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967

Disinfection of Water Supplies


This information sheet sets out the general principles of disinfection of water supplies.  Disinfection is a regulatory requirement and is essential for the protection of public health from waterborne disease.



Disinfection is always carried out on public water supplies in the UK to protect public health, by killing or inactivating harmful microorganisms, including bacteria and viruses, which may be present in drinking water. Worldwide, many thousands of people continue to die each year from waterborne diseases.  Sickness and diarrhoea and diseases including cholera and typhoid are spread by drinking water which is contaminated with human excrement.

Disinfection of surface water is carried out by a two stage process of physical removal of pathogens followed by inactivation with an oxidising agent. For water stored in underground aquifers the first stage is carried out by filtering through the ground. There are several ways of disinfecting water supplies, and the main methods are listed below.

  1. Physical removal of bacteria and viruses in conjunction with:
  2. Chlorine
  3. Ultraviolet light
  4. Ozone

Chlorination is commonly used in the UK because it is very effective at disinfecting the water, is harmless to humans at the concentrations used and is relatively cheap.  It also provides protection for the water after it leaves the treatment works and as it travels through the network of pipes and reservoir into people's homes. Chlorine gas or liquid sodium hypochlorite is injected into the water during treatment and allowed contact time for the disinfection process to work.  In some areas chloramine is the preferred agent to provide a residual disinfection in the network. Dissolved organic matter can react with chlorine to form harmful by products, and therefore organic matter may need removing before disinfection with chlorine.

Other disinfection methods have different advantages, and can be used in conjunction with chlorine or as a substitute, depending on the specific circumstances.   In rural locations for private water supplies the most effective option may be disinfection with a small ultraviolet light unit.


Key considerations

1) Water sources vary greatly in the amount of microorganisms present, and the risks of pollution. For example groundwater can be completely free of microorganisms, whereas a river may contain many thousands of bacteria and viruses in every 100 millilitres. A risk assessment of the water quality is carried out to determine the type of treatment and disinfection required, both under normal conditions and in the event of deterioration in quality due to events such as extreme weather or pollution.

2) The risk assessment should consider the potential for harmful bacteria and viruses in the untreated water. In practice this will usually involve identifying sources of pathogens within the catchment and measuring the raw water loading of coliform and Escherichia coli bacteria. Coliform and Escherichia coli bacteria are indicators of environmental and faecal contamination. The risk assessment must also consider the presence of Cryptosporidium which can cause the disease cryptosporidiosis. Cryptosporidium is widely present in the environment particularly in agricultural catchments.

3) The risk assessment will inform the level and type of treatment required, and the best method of disinfection.

4) Surface water (rivers and reservoirs) must be prepared for disinfection. This can be a number of stages including clarification and filtration, plus sometimes secondary filtration. These processes must be closely controlled to ensure that they are removing the required amount of microorganisms, and other impurities.

5) Turbidity is measured through the treatment process. Turbidity is the cloudiness of the water caused by the amount of small particles present. Clear water is very important for the pleasing appearance of drinking water, but turbidity monitoring also fulfils another important function. Turbidity is used in water treatment as a 'surrogate' for measuring microbiological loading, because it can give an instant measurement of the amount of particles being removed by the water treatment.

6) Disinfection needs to be monitored to ensure it is effective. Samples of water are taken from the outlet of treatment works, the service reservoirs and customer taps and analysed for coliforms and Escherichia coli. Cryptosporidium should be monitored at sites where there is a risk.

7) There is no regulatory standard for the amount of chlorine in the water supply. A residual concentration of around 0.2 milligrammes per litre of free chlorine provides an effective disinfectant at the tap. In practice there is a balance between maintaining a chlorine residual effective enough to protect the water and providing water which is aesthetically acceptable in terms of taste and odour.  People may notice a slight taste or smell of chlorine. Water can be stood in a jug in the fridge for a few hours to remove the chlorine taste. The chlorine is not harmful at the concentrations used and it is not associated with cancer.

8) There are other forms of disinfectant which can be used instead or alongside chlorine; the choice of disinfectant will depend on the local circumstances and the risk assessment.

9) Cryptosporidium is not killed by chlorine at the concentrations at which it is used. Physical removal by treatment processes aimed at particle removal, plus an alternative disinfection agent such as ultraviolet light where required provide effective cryptosporidium control.

10) Disinfectants including chlorine and ozone can react with dissolved organic matter to form disinfection by-products. Organic matter can affect the UV transmission and hence the efficacy of UV disinfection. Many private supplies have coloured water, and effective removal of organic matter is important. By-product formation is a consideration in the choice of disinfection method.

11) It may be necessary to issue public health advice if there is a problem with the disinfection process, or any other indication that the water supply may be contaminated or unfit to drink. Such decisions are usually taken after discussion with local health authorities. This may result in the customers being given advice to boil the water before drinking or brushing teeth. Boiling is an effective way of killing harmful microorganisms including bacteria, viruses and Cryptosporidium.


Further Information

More specific guidance is given in the CIWEM Policy Position Papers: "Chlorine Disinfection of Water Supplies in the UK" and "Ultraviolet Disinfection of Drinking Water Supplies".

A good reference for all aspects of drinking water quality is the World Health Organisation "Guidelines for Drinking Water Quality" 4th Edition.


August 2012


The Chartered Institution of Water and Environmental Management (CIWEM) is the leading professional body for the people who plan, protect and care for the environment and its resources, providing educational opportunities, independent information to the public and advice to government. Members in 98 countries include scientists, engineers, ecologists and students.


Water Supply (Water Quality) (England and Wales) Regulations 2000. SI 3184.

UKWIR, 2005.  UK Water Industry Research Report 05-DW02-37 'A review of Ct in water disinfection.'

UKWIR, 2010.  UK Water Industry Research Report 10-DW-02-59 'Removal of micro-organisms during water treatment'.



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Chartered Institution of Water and
Environmental Management (CIWEM)

106-109 Saffron Hill, London, EC1N 8QS  
Tel: 020 7831 3110   Fax: 020 7405 4967