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Water for Our Future

IISG-98-14 Drinking Water: Disinfection with Chlorine
L. E. Dorworth, Department of Biological Sciences, Purdue University Calumet, Hammond, Indiana

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 byproduct 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 amount 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. 11, No.3, 1995. Illinois-Indiana Sea Grant Program.

Standard Methods For The Examination Of Water and Wastewater, 18th Edition 1992. A. E Greenberg, L. S. Cleseri, A. D Eaton (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