Chlorination of Drinking Water
DOH 331-253 (05/04) Click for
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Note: This page deals with the
practice of continuously adding chlorine to water, not the occasional
use of chlorine to disinfect wells, pipes, and other water system
equipment.
Why water
systems continuously add chlorine to their water
Many public water
systems add chlorine (a process known as "chlorination") to their water
supply for the purpose of disinfection. Disinfection kills or inactivates
harmful microorganisms which can cause illnesses such as typhoid, cholera,
hepatitis and giardiasis. Sometimes, water systems use chlorination for
taste and odor control, iron and manganese removal, and to stop nuisance
growths in wells, water pipes, storage facilities and conduits.
Chlorine is also
added for its "residual" properties. Chlorine remaining in the water
supply, or added after disinfection is first accomplished, is available to
fight against potential contamination in water distribution and storage
systems that might enter through leaks and pipe breakages. This is called
"secondary disinfection."
Chlorine can be added
to water as a gas or in the form of hypochlorite either as liquid or
solid. Gas chlorination requires more sophisticated equipment and more
training to apply safely. Adding chlorine as a hypochlorite is much
simpler, requires less training and is safer. If large amounts of
chlorine are required, the overall cost of gas chlorination may be lower.
 History of chlorination
Techniques for
chlorination of water supplies were developed in the late 19th and early
20th centuries. In 1908, Jersey City Water Works became the first system
in the United States to practice large-scale chlorination on a permanent
basis.
As more water systems
adopted the practice of chlorination, there was a corresponding decrease
in the number of waterborne disease outbreaks. Health professionals
regard the chlorination of water as one of the most important advances in
the field of public health. As the graph illustrates, waterborne
diseases, such as typhoid, caused thousands of deaths annually in the
United States in the early 1900s and are now considered rare.
Does
chlorination result in harmful compounds?
When chlorine is
added to water, it reacts with organic substances that occur naturally in
the water. The compounds formed are called "disinfection byproducts (DBPs)." The amount formed depends on the amount of chlorine used and contact time
between the organic substances and the chlorine.
Some studies of
human health effects from exposure to chlorinated water show increased
risk to cancer and reproductive and developmental effects. Other studies
show no additional risk. Since there may be potential health effects, the
U.S. Environmental Protection Agency (EPA) adopted the Stage 1 Disinfectants and Disinfection Byproducts Rule (DBPR), which specifies
maximum allowable levels and monitoring requirements for disinfectants and
DBPs. (See additional resources below.)
Other
disinfectants or methods of disinfection
There are other
disinfectants used for treatment of water. These include:
- Chloramines: These are formed by a combination of chlorine (from gas or hypochlorite)
and ammonia.
- Chlorine dioxide (ClO2): This compound is always produced on-site using sodium chlorite and
either chlorine or hydrochloric acid.
- Ozone (O3): This compound is produced by an electrical discharge through air or
oxygen.
- Ultraviolet
Radiation (UV): This is a
non-chemical method of disinfection by using ultraviolet radiation at
certain wavelengths.
All disinfectants
have advantages and disadvantages in their effectiveness, residual
qualities and byproduct formation, depending on the application
circumstances and water quality. For a more thorough discussion of other
disinfectants, request or download DOH publication "Alternate
Disinfectants."
Is chlorination
right for your water system?
The reason your
water system chlorinates depends upon the needs of the water system. Most
public water systems in the state of Washington use groundwater
exclusively. Disinfection alone is sufficient treatment against bacteria
and viruses. Since most groundwater has low levels of dissolved organic
substances, the formation of DBPs is not a serious health concern. All
factors considered - cost , simplicity of operation, and residual
qualities - the use of chlorine (as gas or hypochlorite) is a good choice
of disinfectant for most water systems.
Future
challenges
Disinfection of
drinking water, especially when the supply originates from a surface
source, is needed to maintain water quality and protect public health. The challenge is to maintain the level of microbial protection while
minimizing the exposure of the consumers to DBPs. Research still
continues, especially in the field of health effects of disinfectants
other than chlorine.
For more
information
Stage 1 Rule: Disinfectants and
Disinfection Byproducts, DOH 331-254
Disinfection Byproducts, DOH 331-251
Alternate Disinfectants, DOH 331-252
Transition from Stage 1 to Stage 2 Disinfection Byproducts Rule
Monitoring DOH 331-377
Eastern Region: Spokane Valley - (509) 329-2100
Northwest Region: Kent - (253) 395-6750
Southwest Region: Tumwater - (360) 236-3030 |
