I LLI NOIS -I NOlANA
Water Issues
And Concerns
Fact Sheet Series
Drinking Water:
Disinfection with Chlorine
by Leslie E. Dorworth
A Brief History
Water has been treated for many centuries. First, it
was boiled and filtered to improve the taste and
appearance. We know today that these treatments
make water safer for human use, but no one knew
it those many centuries ago. Research scientists
discovered that water could contain harmful
bacteria that resulted in diseases.
Due to the microscopic size of pathogens, filters
are generally ineffective in stopping them from
entering water supplies. Therefore, the most
appropriate step is a disinfectant to kill the
pathogenic micro-organisms. Transmission of
certain pathogenic diseases through drinking water
has been a recognized public health problem since
before the turn of the century. The presence of
these organisms in water supplies has frequently
led to the transmission of deadly diseases such as
cholera, typhoid, dysentery and hepatitis.
Chlorine, one of 90 naturally occurring elements is
a basic building block of our planet. It was first
used as a disinfectant in Europe and North
America in the early part of this century. Since
then, widespread epidemics of the most severe
forms of diseases usually do not occur in the
United States. Unfortunately, there are problem
areas in the world where microbiological problems
still exist in the water supplies. This problem
usually leads to contaminated drinking
water supplies.
In the United States, Congress enacted the Safe
Drinking Water Act (SDWA) in 1974. The law
was amended in 1986 to expand the U. S.
Environmental Protection Agency's (U.S. EPA)
role in protecting public health from contaminated
drinking water. The amendments require the
agency to enforce control of specific disease­
causing organisms and indicators that may be
present in drinking water and require public water
suppliers to disinfect water. Amendments enacted
in 1996 make it clear that any federal agency is
subject to penalties for past violations of
the SDWA.
Chlorine for Drinking Water Disinfection
The spread of waterborne diseases such as cholera
and typhoid fever is prevented or controlled by
adding chlorine to water. Although chlorine is not
the only disinfecting agent available to the water
supply industry, it is the most widely used
disinfectant in North America. The wide use of
chlorine is due primarily to its effectiveness, the
scientific understanding of its properties, and the
technical capabilities (most plants are modeled for
the chlorine treatment process) of most treatment
plants in North America.
Disinfection, in this case chlorination of drinking
water, is traditional in public health protection.
Water suppliers usually use chlorination in
combination with source protection and water
filtration against the microbiological contamination
of drinking water. Another form of disinfection is
ozonation. Both chlorination and ozonation kill
organisms by oxidation. Ultra violet radiation,
another method, is used to kill the microorganisms.
In the United States, chlorination is the most
common disinfectant in use.
Chlorine gas is used in 90 percent of all water
disinfection applications. In order for chlorine to
be effective against microorganisms, chlorine must
be present in sufficient quantity, and it must have a
sufficient amount of time to react. This reaction
time period is called the contact time. For most
water systems, the best contact time is usually 30
minutes. To ensure continued protection against
harmful organisms, a certain amount of chlorine
must remain in the water after the treatment
process. The remaining chlorine is known as a
residual chlorine.
Suggestions have been made to use the alternative
treatment processes listed above instead of chlorine
as the primary means of disinfection. These
techniques obviously have many advantages, but,
in some situations, water suppliers will still need to
use chlorine in some form to provide the necessary
disinfectant residual.
Different organisms are resistant to chlorine at
different levels, therefore, varying contact times are
required to kill them. Bacteria tend to be the least
resistant and die quickly upon exposure. Viruses
require a longer contact time for chlorine treatment
to be effective. Treatment of the protozoan
Giardia LambLia requires filtration plus
chlorination to ensure complete removal of the
cells and cysts from the water supply. The
chlorination treatment has been proven relatively
ineffective against Cryptosporidium, another
protozoan. The best treatment in this case
is filtration.
Ways that Chlorine Disinfects the Water
Chlorine kills the organisms following a two step
manner. First, the chlorine molecule penetrates the
cell wall of the organism. Second, chlorine kills
bacteria by forming hypochlorus acid which attacks
the respiratory, transport and nucleic acid activity
of the bacteria. In contrast, the protein coat of
viruses is affected by chlorine.
Concerns Using Chlorine in Drinking Water
Chlorine is element number 17 on the periodic
table. It exists naturally as part of a wide range of
substances, from simple table salt to hydrochloric
acid that the stomach secretes to aid in the
digestion of food. Chlorine used for disinfection in
water and wastewater treatment is minor in terms
of volume, even though most public water supplies
in North America use chlorine based disinfectants.
The chlorine used in drinking water disinfection
accounts for less than 1.5 percent of the total
chlorine used in today's world.
Chlorine can combine with natural organic
compounds in raw water to create some
undesirable by-products, however, on its own, it
usually is not a problem to public health. The
SDWA regulates the by-products. One concern
with chlorinated water is the tendency to form
trihalomethanes, or THMs, a carcinogenic by­
product of the disinfection process. In 1979, the
U.S. EPA adopted the THMs regulation, limiting
the allowable level of this by-product in drinking
water supplies. In 1992, the U.S. EPA established
federally enforceable standards for 89
contaminants, including THMs, that may be found
in drinking water.
In order to address the U.S. EPA regulations, in
this case specifically THMs, the water treatment
plants changed operations to minimize THM
production without compromising public health.
Some of the methods used include reducing the
amounl of chlorine, changing the timing during
disinfection so that chlorine is added in either
sooner or later during process, changing the
chlorine type used and removing the organic
material that reacts with the chlorine to
produce THMs.
Safe Drinking Water Cannot be Taken
for Granted
Most of us never think about getting sick or even
dying from drinking water. In many developing
countries where people do not have access to safe
drinking water, diseases associated with dirty
water kill more than 5 million people per year
around the world, according to the World Health
Organization. Without proper disinfection
procedures, waterborne disease outbreaks in the
U.S. would significantly increase.
Drinking water treatment plants must comply with
U.S. EPA regulations set forth in the SDWA and
provide adequate microbial protection to ensure
the public health, reduce the levels of disinfection
by-products and limit corrosion control.
Chlorination, the treatment of choice in the U.S.
when used in combination with other treatment
methods in a well operated treatment facility can
consistently meet public health goals.
Through a learning process about water treatment
process, chlorine use in treatment plants has been
reduced. The reduction of chlorine as an additive
has been balanced by providing microbial
protection and reducing the by-products produced
through the treatment process.
Recommended Resources
Chlorine: Viewing A Phaseout From Different
Vantage Points. N. Riggs. The HELM, Vol. II,
No.3, 1995. Illinois-Indiana Sea Grant Program.
Water Quality Issues and Concerns is an ongoing
series addressing relevant water quality issues. For
water quality information, contact Leslie Dorworth, Sea
Grant aquatic ecology specialist, at 219 989-2726;
[email protected]
Standard Methods For The Examination Of
Water and Wastewater, 18th Edition 1992. A.
E Greenberg, L. S. Cleseri, A. DEaton (eds.).
APHA, AWWA, WEF.
State Of Knowledge Report On
Environmental Contaminants And Human
Health In The Great Lakes. D. Riedel, N.
Tremblay and E. Tompkins (eds.). Great Lakes
Health Effects Program, Environmental Health
Effects Division, Health Canada, 1997.
Water Treatment Plant Operation: A Field
Study Training Guide. Prepared by California
State University, Sacramento School of
Engineering; K. D. Kerri, Project Director.
Hornet Foundation Inc. California State
University, Sacramento. 1992
Originally published in Scientific American, Ask
the Experts www.sciam.com May 4, 1998.
Produced by the Illinois-Indiana Sea Grant College
Program, one of 29 National Sea Grant College
Programs. Created by Congress in
..""0....
1966, Sea Grant combines university,
/~\ government, business and industry
l~~J expertise to address coastal and
Great Lakes needs. Funding is
o""'"'~d~
provided by the National Oceanic
Atmospheric Administration, U. S.
Department of Commerce, Purdue University, West
Lafayette, Indiana, and the University of Illinois at
Urbana-Champaign.
'0-'
For additional copies or reprint information, contact
Nancy Riggs, Sea Grant public information manager,
217333-8055; [email protected]
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