This Policy Position Statement (PPS) outlines the rationale of
using chlorine as a disinfectant for water supplies within the
United Kingdom, taking into account the history of waterborne
disease, subsequent legislation requiring utilities to disinfect
drinking waters, and maintaining good operational practices by
water supply utilities with regard to the requirements of
regulators, and the risk to public health.
CIWEM's Position on Chlorine Disinfection:
CIWEM believes that drinking untreated water from sources that
may be, or are contaminated by harmful micro-organisms, presents an
unacceptable risk of disease, and is a danger to health. It is
safer to drink water from supplies treated with chlorine according
to current guidelines.
The expansion of population - especially in city or urban
environments, has had a history of cross-contamination of drinking
water sources and the various delivery systems, principally from
domestic sewage and animal wastes. An additional introduced risk to
sources and transmission systems of treated water is from
'carriers' of infectious diseases who may not necessarily exhibit
symptoms of the disease themselves, but nevertheless can cause
outbreaks of waterborne infections by coming into contact with - or
their bodily wastes contaminating - water intended for drinking or
domestic use. An example of this is the typhoid outbreak in Croydon
in 1938. Since then, it has become a necessary precaution to
disinfect public water supplies within the UK as part of a diligent
- In the UK there is a legal requirement (monitored by government
drinking water regulators) to disinfect public water supplies.
- In the concentrations used within water utilities in the UK,
the addition of chlorine does not present a recognisable health
hazard, although it is acknowledged that some customers may dislike
a chlorinous taste or odour. The primary reason for using chlorine
is to protect public health, and the positive health benefits far
outweigh the risks of long-term ingestion of disinfection
- Note, that within the UK, chlorine is used to Disinfect the
water; it does not Sterilise water; i.e. not all organisms or their
dormant stages are inactivated by chlorine dosed according to
current practice; however most infective organisms are effectively
inactivated or destroyed.
- The operational use of chlorine in gas or liquid form is
widespread, having a good safety record. There are many delivery,
dosing, control and monitoring technologies and systems
- Chlorine is a relatively low cost, and powerful
- Generally, viruses are more resistant to chlorine than
- Harmful organisms can be shielded by particulates, therefore
chlorination should be linked to a low water turbidity standard.
This may require pre-treatment processes if source waters are
- Chlorine species dissociate in solution, with the more active
Hypochlorous Acid constituent being more concentrated at a lower,
or more acidic, pH i.e. disinfection is more efficient at a lower
- The action of chlorine is rapid, but not always instantaneous;
for example, some chemical constituents or bacteriological and
viral contaminants will require time for the chemical reactions to
complete, or for the micro-organisms to be inactivated.
- This "reaction time" part of the disinfection process is
usually termed the "Contact Time", and varies with the chlorine
concentration. (The term Ct may be encountered; this is
concentration x contact time usually specified within a range of pH
values, and is expressed in units of mg.min/l.)
- Some processes also use the more persistent, but less active
species of Chloramine (note, after the primary disinfection
process) to provide a residual disinfectant e.g. within the
transmission or distribution system.
- In the concentrations of chlorine used today, coupled with
adequate contact times, harmful viruses and bacteria will be
inactivated. Some encysted or spore forms of micro-organisms (e.g.
the parasite Cryptosporidium species) would resist inactivation.
This also supports having a linked turbidity standard to minimise
Chlorinated hydrocarbons and chlorine disinfection
It is accepted that there can be adverse health effects if
sufficiently high concentrations of chlorinated organics (e.g. such
as chloroform) are ingested over a long period of time.
Toxicological data suggests that under current UK practice (e.g.
with treatment processes minimising dissolved organics), this risk
is minimal compared to the high probability of illness - or even
death - caused by drinking untreated contaminated water.
In support of the above, previous instances of other countries
reducing or omitting chlorine disinfection have been attributed to
the desire to reduce chlorinated hydrocarbons. This has
sometimes had a disastrous public health result, in that
waterborne disease such as typhoid or cholera have spread though
the communities supplied with inadequately disinfected water.
This PPS is aimed at current practices within England, Scotland,
Wales and Northern Ireland using chlorine gas or solutions as a
primary disinfection process in an approved treatment process train
or system. This PPS, therefore omits booster or secondary or
point-of-use chlorination. It does not include Chlorine Dioxide
processes. It is realised that within Europe, some areas do not
normally chlorinate public supplies, and have alternative process
facilities e.g. long term dune filtration, or have seasonal
cessation of chlorine disinfection, and that monitoring and
maintenance programmes aim to ensure a robust transmission
integrity in the absence of water mains bursts and repairs etc.
Conclusion - the future of Chlorine Disinfection within the
Water companies, public health professionals, researchers and
regulators continually evaluate current toxicological and process
technology information regarding the use of chlorine as a potable
water disinfectant, and health risk.
In the medium to long term, the use of chlorine based compounds
to disinfect drinking water, and provide a residual disinfectant
protection against harmful micro-organisms within transmission
systems, will continue to provide safe drinking water to the public
in compliance with requirements of the health-based standards
overseen by the relevant drinking water regulators of the country
Note: CIWEM Policy Position Statements (PPS) represent the
Institution's views on issues at a particular point in time.
It is accepted that situations change as research provides new
evidence. It should be understood, therefore, that CIWEM
PPS's are under constant review, that previously held views may
alter and lead to revised PPS's.