How Does Chlorine Added to Drinking Water Kill Bacteria and Other Harmful Organisms_ Why Doesn't It Harm Us_ - Scientific American

8/10/2019 How Does Chlorine Added to Drinking Water Kill Bacteria and Other Harmful Organisms_ Why Doesn't It Harm Us_ - Scientific American

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8/3/2014 How does chlorine added to drinking water kill bacteria and other harmful organisms? Why doesn't it harm us? - Scientific American

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Image: Chlorine Chemistry Council

TYPHOID FEVERhas been virtually eliminated through the

chlorination of water.

Energy & Sustainability Ask the Experts

How does chlorine added to drinkingwater kill bacteria and other harmfulorganisms? Why doesn't it harm us?May 4, 1998

Jon J. Calomiris, Water Research Program Manager at the UnitedStates Air Force

Research Laboratory, and Keith A. Christman, Director, Disinfectionand Government

Relations at the Chlorine Chemistry Council, collaborated on this answer.

While quenching your thirst with a glass of tap water, enjoying your morning shower

or swimming in a pool, you most likely are, at one time or another, aware of the

chlorine used to disinfect your municipal water. Although its distinctive aroma may be

unpleasant to some, it is an indication that your water supply is being adequately

treated to stave off harmful or deadly microorganisms.

Chlorine effectively kills a large variety of microbial waterborne pathogens, including

those that can cause typhoid fever, dysentery, cholera and Legionnaires' disease.

Chlorine is widely credited with virtually eliminating outbreaks of waterborne disease

in the United States and other developed countries. AndLifemagazine recently cited

the filtration of drinking water and use of chlorine as "probably the most significant

public health advance of the millennium."

Health officials began treating

drinking water with chlorine

in 1908. Prev iously, typhoid

fever had killed about 25 out

of 100,000 people in the U.S.

annually, a death rate close to

that now associated with

automobile accidents. Today,

typhoid fever has been

virtually eliminated.

Chlorine is currently

employed by over 98 percent of all U.S. water utilities that disinfect drinking water. It

has proved to be a powerful barrier in restricting pathogens from reaching your faucet

and making you ill. Chlorine and chlorine-based compounds are the only disinfectants

that can efficiently kill microorganisms during water treatment, and maintain the

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have no outer membrane. Although these bacterial types are, in general, more chlorine

tolerant than gram-negative bacteria, most waterborne species do not normally pose a

health threat.

Certain waterborne viruses, such as enteric viruses and hepatitis A, may be ev en more

tolerant to chlorine disinfection than some bacterial species. But the means by which

chlorine inactivates viruses is not well understood.

In recent years, the parasitic protozoans Cryptosporidium parvum and Giardialambliahave emerged as formidable waterborne pathogens. These protozoa are

remarkably resistant to chlorine disinfection and consequently, present a great

challenge to the water industry and health officials, who are responsible for providing

safe drinking water to the public. Currently, filtration is the most effective process for

removing these protozoa from drinking water. To fully protect the public, however,

effective disinfection methods must be developed.

If chlorine kills so many species of microorganisms, why doesn't it harm humans?

Fortunately, when we ingest chlorinated drinking water, food in our stomachs and the

materials normally present in the intestinal tract quickly neutralize the chlorine. So

chlorine concentrations along cell membranes in the gastrointestinal tract are probablytoo low to cause injury.

This example may simply be another case of "dose makes the poison." Like medicine, a

little bit of chlorine, such as the levels used in drinking water or swimming pools, kills

relatively simple, but potentially deadly, microorganisms. At much higher

concentrations, chlorine could damage the cells in our body.

Water utilities carefully regulate chlorine levels so that they effectively kill disease-

causing microorganisms but do not harm people. The Environmental Protection

Agency (EPA), with the help of water utilities, environmentalists and chlorine

manufacturers, recently proposed a regulation that would reduce the chlorineconcentrations in drinking water to assure that the disinfectant does not approach

unsafe levels.

Some additional details are provided by Leslie E. Dorworth of the Illinois-

Indiana Sea Grant College Program.

Water has been treated for many centuries. First, it was boiled and filtered to improve

the taste and appearance. Chlorine, one of 90 naturally occurring elements, 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 have become

exceedingly rare in the U.S.

In the U.S., Congress enacted the Safe Drinking Water Act in 1974. The law was

amended in 1986 to expand the EPA's role in protecting public health from

contaminated drinking water. The amendments require the agency to control specific

disease-causing organisms and indicators that may be present in drinking water and to

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 Act.

Chlorine can combine with natural organic compounds in raw water to create some

undesirable by-products; on its own, however, it does not usually pose a problem to

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public health. The legislation regulates the by-products. One concern with chlorinated

water is its tendency to form trihalomethanes (THMs), carcinogenic by-products of

the disinfection process. In 1979, the EPA adopted the THM regulation, limiting their

allowable level in drinking water supplies. In 1992, the EPA established federally

enforceable standards that now cover 83 contaminants, including THMs, that may be

found in drinking water.

In order to address the EPA regulations--in this case THMs specifically- -water

treatment plants changed operations to minimize THM production without

compromising public health. Some of the methods used include reducing the amount of

chlorine; changing the timing during disinfection so that chlorine is added in either

sooner or later during process; changing the type of chlorine used; and removing the

organic material that reacts with the chlorine to produce THMs.

Although chlorine is not the only disinfecting agent available to the water supply

industry, it is the most widely used disinfectant in North America. Another form of

disinfection is ozonation. Both chlorination and ozonation kill organisms by oxidation.

Ultraviolet treatment, another method, uses UV radiation to kill microorganisms.

For chlorine to be effective against microorganisms, it must be present in a sufficientquantity, and it must have a sufficient amount of time to react. This reaction time 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 treatment. The remaining chlorine is known

as a residual chlorine. It is this tiny amount that you sometimes smell in your tap

water.

Most of us never think about getting sick or even dying from drinking water. But in

many developing countries around the world, diseases associated with dirty water kill

more than 5 million people each year, according to the World Health Organization.

Without proper disinfection procedures, outbreaks in the U.S. would significantlyincrease.

As researchers and officials have learned more about water disinfection, the use of

chlorine in treatment plants has been reduced. This reduction has been balanced by

providing microbial protection and reducing the by-products created through the

treatment process.

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How Does Chlorine Added to Drinking Water Kill Bacteria and Other Harmful Organisms_ Why Doesn't It Harm Us_ - Scientific American