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.
- Physical removal of bacteria and viruses in conjunction
- Ultraviolet light
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.
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
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
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
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.
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.
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'.