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Drinking-Water Disinfection: Closing the Curtain on Chlorine?

by Kristine Bradof

This article originally appeared in the July 1996 issue of the Wellspring newsletter, published by the MTU Regional Groundwater Education in Michigan (GEM) Center, now the Center for Science and Environmental Outreach at Michigan Technological University.

Chlorine has been hailed for its critical role in one of the greatest public health success stories of this century, safe drinking water. In 1908, Jersey City, New Jersey, became the first municipality in the U.S. to disinfect its drinking water with chlorine (University of California (UC) at Berkeley 1995). Until that time, epidemics of waterborne diseases, such as cholera, typhoid, dysentery, and hepatitis, were responsible for many deaths. Today, we associate those diseases with Third World countries that have inadequate or nonexistent water and wastewater treatment. For almost 90 years, chlorine has been the disinfectant of choice for public water supplies. It kills the microorganisms responsible for many waterborne illnesses at relatively low cost. A residual level of chlorine also remains after treatment to disinfect the water as it passes through the distribution pipes.

Yet, drinking-water disinfection accounts for only a small part of chlorine use. Almost half of all U.S. industries use chlorine or related compounds. Together, those industries generate 45 million jobs and $1.6 trillion in economic activity. "Nearly 40 percent of the U.S. gross national product is in some way reliant on chlorine chemistry," according to a spokesperson for the Dow Chemical Company (Bond 1994). Chlorine is used in processes like bleaching of paper pulp and in a wide range of products, including crop-protection chemicals, plastics, adhesives, deodorants, paints, films, perfumes, cosmetics, tires, household cleaners, brake fluid, rocket fuel, and drugs for treating diseases such as cancer, pneumonia, malaria, and high blood pressure (Bond 1994).

But, what’s this? The International Joint Commission on Great Lakes water quality and the American Public Health Association both recommend that use of chlorine in industrial processes be phased out or banned (UC Berkeley 1995). The Clinton Administration says that chlorine should ultimately be eliminated from use as a disinfectant, as well as in the chemical and pharmaceutical industries (Haskett 1995). What has shifted opinion so sharply against chlorine?

Chlorine’s tarnished halo

Unfortunately, the property of reactivity that makes chlorine so useful also has a darker side. The 1970s brought the discovery that chlorine reacts with organic material in water to form byproducts, especially trihalomethanes like chloroform, that are toxic in large amounts (UC Berkeley 1995). The effects of long-term exposure to smaller doses are less certain. After reviewing the evidence, the National Academy of Sciences recommended in 1987 that the U.S. Environmental Protection Agency reduce the maximum allowable level of chlorination byproducts in the drinking water consumed by more than 100 million Americans (Olson 1993).

By the early 1990s, epidemiological studies at the Harvard University School of Public Health, the Medical College of Wisconsin, and the National Cancer Institute had linked chlorinated drinking water to bladder and rectal cancers in humans (Chlorine and cancer 1993). The researchers estimated that disinfection byproducts cause at least 10,700 cases of those cancers each year. Follow-up animal studies by the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, showed kidney and liver toxicity and high rates of kidney and colorectal cancer in rats who were exposed to trihalomethanes in their food (Raloff 1993).

Another study, published in the American Journal of Epidemiology, associated chlorination byproducts in drinking water with pancreatic cancer (Olson 1993). The U.S. Public Health Service and the New Jersey Department of Health in 1992 found a possible connection between chlorination byproducts and some birth defects of the spine and nervous system (Olson 1993).

As the human health effects of chlorine compounds were becoming known, so too were their contributions to global environmental problems. These problems include reproductive failures in birds and thinning of the protective layer of ozone above the Earth. An alphabet soup of chlorine-based chemicals—DDT, PCBs, CFCs—have since been banned or are being phased out in the U.S. and other developed countries.

Championing chlorination

In spite of the evidence linking chlorination byproducts to cancer and other health concerns, most experts are reluctant to condemn chlorine. "The potential health threat of microbial contamination of drinking water, which chlorination prevents, is much greater" (Chlorine and cancer 1993). The author of one of the largest studies on the chlorine-cancer connection, Kenneth Cantor of the National Cancer Institute’s Environmental Epidemiology Branch, notes that the cancer risk associated with chlorination byproducts is "highly uncertain" (UC Berkeley 1995). The September 1995 University of California at Berkeley Wellness Letter concludes that "for some jobs, such as the disinfection of drinking water, chlorine has no good substitute just now. Typhoid and cholera are real and very deadly risks. The cancer risk remains unclear. On the basis of the evidence, there’s no reason to conclude that chlorination of the water supply is unsafe."

Disinfection with ozone or UV light

Fortunately, chlorination is not the only method available for disinfecting water. In fact, as the 1993 outbreak of the waterborne disease cryptosporidiosis in Milwaukee proved, chlorine is not effective against all microbes. The other major disinfection options, treatment with ultraviolet (UV) light or ozone, are becoming more common, but they, too, have their limitations.

Disinfection systems that use ultraviolet light kill "almost 100 percent of all microorganisms," but only if the unit includes a prefilter to remove suspended matter or turbidity (Stewart 1990). Disease-causing organisms are killed only in direct contact with UV light. Particles in unfiltered water can shield pathogens from the light, allowing them to pass through unharmed. The filter must be fine enough to remove Giardia cysts, which may not be killed by UV light (Stewart 1990). A light sensor should also be part of the disinfection unit to ensure that UV light levels are sufficient to kill the microbes.

Since the first commercial use of ozone for disinfection of drinking water in Nice, France, in 1908, more than 1,300 ozone water treatment plants have been built worldwide. The oldest operating ozone system in the U.S. was built in Whiting, Indiana, in 1940 (Dimitriou 1995). By 1990, 40 ozone plants were operating in the U.S. as an alternative to chlorination. An additional 100 plants were expected to be in the design or construction phase by 1995.

In Hackensack, New Jersey, ozone disinfects 200 million gallons of drinking water per day and also removes iron and other objectionable tastes and odors. Some municipalities rely on ozonation to disinfect wastewater before discharging it to surface waters. Ozone is also replacing chlorine as a bleaching agent in many new or retooled paper mills (Dimitriou 1995).

With sufficient ozone concentration and contact time, most bacteria and Giardia cysts in water are killed, but viruses may be more resistant (Stewart 1990). Like UV light, ozone leaves no chemical residual to continue disinfection in the water distribution system. Ozone may also react with organic material in the water to form toxic compounds, including formaldehyde and ketones (Behm 1993).

Safe drinking water’s bottom line

The key to safe drinking water is not to rely entirely on any method of disinfection. As Northwestern University Professor of Environmental Engineering Bruce Rittmann notes, "one of the foundations of drinking water treatment is that we should have multiple barriers to prevent pathogens from reaching consumers" (Russell 1995). The other barriers are starting with a water supply likely to contain the lowest number of disease-causing organisms and  physically removing, by filtration, any particles in the water, including the microorganisms themselves. Although public and private wells generally are less likely than surface water to contain problem microorganisms, filtration and disinfection are becoming more common even for those supplies. Groundwater is less protected from contamination than once thought. The companion article in this issue of Wellspring on fighting microbes with microbes describes how certain filters produce "biologically stable" water that may not require disinfection or at least doesn’t produce harmful byproducts when it is disinfected.


Behm, D. Choose your poison. Chlorination brings risks of its own. The Milwaukee Journal, September 23, 1993, pp. A1, A8.

Bond, G. C. A world without chlorine. The Copper Nugget, July 20, 1994, p. 18.

Chlorine and cancer. In Huntoon, E. (ed.) "Ground Water Protection"—A Learning Experience. 1993 Continuing Education Program for Well Drillers & Pump Installers. Technical Guide. Lodi, WI: Wisconsin Water Well Association. 1993, p. 48. (Reprinted from Water Technology magazine.)

Dimitriou, M. A. Ozone treatment comes of age in water, wastewater applications. In Huntoon, E. (ed.) Improve Ground Water Protection by Sharing Information. 1995 Continuing Education Program for Well Drillers & Pump Installers. Technical Guide. Lodi, WI:Wisconsin Water Well Association. 1995, pp. 65-67. (From Environmental Solutions magazine.)

Haskett, D. The chlorine issue. Shall we throw the baby out with the bathwater...or not? In Huntoon, E. (ed.) Improve Ground Water Protection by Sharing Information. 1995 Continuing Education Program for Well Drillers & Pump Installers. Technical Guide. Lodi, WI:Wisconsin Water Well Association. 1995, p. 26. (From Pollution Equipment News 1994.)

Olson, E. D. Think Before You Drink. The Failure of the Nation’s Drinking Water System to Protect Public Health. New York: Natural Resources Defense Council, September 1993.

Raloff, J. Chlorination products linked to cancer. Science News 143, May 29, 1993, p.343.

Russell, S. On tap. Environmental engineer Bruce Rittmann explains how biological processes can reduce chlorine in drinking water. Northwestern Perspective [Evanston, IL:Northwestern University], Fall 1995, pp. 24-27.

Stewart, J. C. Drinking Water Hazards: How to Know if There are Toxic Chemicals in Your Water and What to Do if There Are. Hiram, OH:Envirographics. 1990.

University of California at Berkeley School of Public Health. Sunset for chlorine? University of California at Berkeley Wellness Letter 11(12), September 1995, p. 5.