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How does chlorine added to drinking water kill bacteria and other harmful organisms? Why doesn't it harm us?

Jon J. Calomiris, Water Research Program Manager at the United States Air Force Research Laboratory, and Keith A. Christman, Director, Disinfection and 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. And Life magazine recently cited the filtration of drinking water and use of chlorine as "probably the most significant public health advance of the millennium."

Typhoid graph
Image: CHLORINE CHEMISTRY COUNCIL
TYPHOID FEVER has been virtually eliminated through the chlorination of water.

Health officials began treating drinking water with chlorine in 1908. Previously, 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 quality of the water as it flows from the treatment plant to the consumer's tap.

Although chlorine's value has been known for nearly a century, the mechanism by which the compound kills or inactivates microorganisms is not clearly understood. The bulk of chlorine disinfection research, conducted from the 1940s to the 1970s, focused on bacteria. Though limited, this work gave rise to some speculation. Researchers postulated that chlorine, which exists in water as hypochlorite and hypochlorous acid, reacts with biomolecules in the bacterial cell to destroy the organism.

Further work led to the so-called "multiple hit" theory of chlorine inactivation. It asserted that bacterial death probably results from chlorine attacking a variety of bacterial molecules or targets, including enzymes, nucleic acids and membrane lipids.

Early research efforts focused on how chlorine attacks enzymes. The disinfectant was able to inactivate extracts of various enzymes because it is highly reactive with sulfur-containing and aromatic amino acids. But it had no effect on cytoplasmic enzymes, suggesting that it might not reach biomolecules within the bacterium. Thus, researchers redirected their attention to the molecules on the surface of the bacterial cell.

A new hypothesis proclaimed that perhaps chlorine acted by attacking the bacterial cell wall. Proponents of this idea suggested that chlorine exposure might destroy the cell wall--by altering it physically, chemically and biochemically--and so terminate the cell's vital functions, killing the microorganism.

A possible sequence of events during chlorination would be:

  • disruption of the cell wall barrier by reactions of chlorine with target sites on the cell surface
  • release of vital cellular constituents from the cell
  • termination of membrane-associated functions
  • termination of cellular functions within the cell.

    During the course of these events, the microorganism dies, meaning it is no longer capable of growing or causing disease.

    Bacteria
    Image: CHLORINE CHEMISTRY COUNCIL
    E. COLI. are commonly found in contaminated water. They are destroyed by chlorination.

    Although chlorine's disruption of the cell wall appears to be the fundamental event leading to the demise of the bacterium, the mechanism by which chlorine disrupts the cell wall had not been determined. Recently, though, scientists have studied how chlorine affects the cell walls of "gram-negative" bacteria, organisms including those causing typhoid fever, dysentery, cholera and Legionnaires' disease

    By definition, gram-negative bacteria possess cell walls that consist of an outer membrane and a cytoplasmic membrane. The outer membrane, being the outermost region that has direct contact with the organism's environment, functions as a protective barrier. The investigation revealed that, for each bacterial species, chlorination significantly increased the permeability of the outer membrane, leaving the bacterium vulnerable to destruction.

    How chlorine inactivates other types of bacteria has not been determined. Scientists do not understand much about spore-forming bacteria or gram-positive bacteria, which 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 even 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 Giardia lamblia have 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 probably too 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 chlorine concentrations 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 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 sufficient quantity, 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 significantly increase.

    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.

    Answer posted May 04, 1998

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