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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

Chlorine Disinfection of Water Supplies in the UK


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 treatment programme.

Key Issues

  1. In the UK there is a legal requirement (monitored by government drinking water regulators) to disinfect public water supplies.
  2. 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 by-products.
  3. 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.
  4. 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 available.
  5. Chlorine is a relatively low cost, and powerful disinfectant.
  6. Generally, viruses are more resistant to chlorine than bacteria.
  7. 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 turbid.
  8. 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 pH Value.
  9. 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.
  10. 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.)
  11. 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.
  12. 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 particulates.

Chlorinated hydrocarbons and chlorine disinfection by-products

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 UK

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 or region.

October 2006

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.


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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