Disinfection: Chlorine's Role in Public Health
Chlorine-based disinfectants have played a critical role in protecting America's drinking water supply from waterborne infectious diseases for nearly a century. Chlorination began in the early years of the 20th century in Great Britain, where its application sharply reduced typhoid deaths. Shortly after this dramatic success, chlorination and filtration were introduced into the United States, which resulted in the virtual elimination of waterborne diseases such as cholera, typhoid, dysentery and hepatitis A.(1) According to the World Health Organization, the adoption of drinking water chlorination has been one of the most significant advances in public health protection.(2) While chlorine's most important attributes are its broad-spectrum germicidal potency and persistence in water distribution systems, its ability to efficiently and economically address many other water treatment concerns has also supported its widespread use. Chlorine-based compounds are the only major disinfectants exhibiting lasting residual properties to provide continual protection against microbial regrowth.(1)
Chlorine-based chemicals are the disinfectants of choice for treating drinking water. In fact, some 98 percent of all systems that disinfect their water employ chlorine-based disinfectants. Chlorine-based disinfectants are available as a gas, sodium hypochlorite solution and solid calcium hypochlorite. Chlorine dioxide is another very strong chlorine-based disinfectant that has been shown to be effective against cryptosporidium.
Facilities use chlorine because it does its job extremely well, is safe to use when handled properly and is very cost effective. After its initial introduction in 1908 in New Jersey, chlorine was adopted as a disinfectant by most water treatment plants in the United States and Canada. More than 200 million Americans and Canadians receive chlorine-disinfected drinking water every day.
Groundwater, although filtered by natural processes, is often susceptible to microbial contamination and may need disinfection. A major groundwater pathogen occurrence study, supported by the American Water Works Association (AWWA) Research Foundation and the U.S. Environmental Protection Agency (EPA), indicates that about 60 percent of vulnerable wells and about half of wells initially considered not vulnerable have been found to be positive for one or more indicators of fecal contamination in tests for total coliform bacteria, E. coli, coliphage and human viruses. Viruses were found about ten times more often than fecal bacteria, calling into question the adequacy of current coliform monitoring (Macler, Bruce A., Fredrick W. Pontius, Journal AWWA,January 1997).
Groundwaters are also the source of nearly half of all waterborne disease outbreaks in the United States. In each decade since 1920, 43 to 56 percent of the outbreaks reported in all types of water systems were caused by contaminated, inadequately treated groundwater. In community water systems, inadequate disinfection of groundwater and untreated groundwater were the identified causes of 25 percent of the U.S. waterborne disease outbreaks reported between 1971 and 1992.(3)
The EPA is developing a Groundwater Disinfection Rule to address the public health risks from microbial contamination of groundwater systems. Chlorination is being considered by EPA as one of the likely Best Available Technologies for disinfection along with W, ozone and ultrafiltration. Another approach under consideration would require a measurable chlorine residual in the distribution system (Macler, Bruce A., Fredrick W. Pontius, Journal AWWA, January 1997).
CHLORINE FOR GROUNDWATER DISINFECTION
Chlorine is often the best choice for groundwater disinfection because it is cost-effective, reliable, relatively simple, measurable and provides a residual. This residual helps protect water from microbial contamination all the way to the tap, providing an indicator of contamination in the distribution system. Chlorine-based disinfectants are the only disinfectants that provide this residual protection.
A 1985 study of the role of chlorine in water treatment conducted by J. Carrell Morris of the Harvard University School of Medicine identified many of chlorine's benefits:4
Potent germicide. The demonstrated use of chlorine reduces the level of disease-causing microorganisms in drinking water to almost immeasurable levels.
Residual qualities. Chlorine produces a sustained residual disinfection action "unique among available large-scale water disinfectants. Chlorine's superiority as a residual disinfectant remains true today. The presence of a sustained residual maintains the hygienicity of the finished drinking water from the treatment plant to the consumer's tap.
Biological growth control. Chlorine's powerful germicidal action eliminates slime, bacteria, molds and algae. Chlorine controls these nuisance organisms, which typically can grow in reservoirs, on the walls of transmission water mains and in storage tanks.
Chemical control. Chlorine in water treatment destroys hydrogen sulfide and removes ammonia and other nitrogenous compounds that have unpleasant tastes and hinder disinfection.
According to the AWWA white paper, Chlorine for Drinking Water Disinfection, "Chlorine disinfection technology is far simpler than other disinfection technology. Experience shows that reliable operation using chlorine disinfection can be achieved in treatment plants of all sizes."
DISINFECTION BYPRODUCTS AND GROUNDWATER
There has been concern in recent years over disinfection byproducts (DBPs) which are produced by the reaction of any chemical disinfectant and organic matter in the water. In fact, EPA has proposed new regulations to reduce DBP levels. The stage I DBP rule would set new maximum contaminant levels (MCLs) of 80 parts per billion (ppb) for total trihalomethanes, 60 ppb for haloacetic acids and 10 ppb for bromate. Because groundwaters are generally very low in organic DBP precursors, low levels of DBPs are generally produced. A survey by the American Water System reported an average groundwater trihalomethane level of 19 ppb compared to 60 ppb for surface waters and a haloacetic acid level of 8 ppb for groundwater versus 48 ppb for surface waters. (Journal AWWA, June 1997). The EPA also predicts that 88 percent of groundwater systems can meet these MCLs without changes to their treatment processes.
CHLORINE DISINFECTION WORLDWIDE
Chlorinated drinking water's chief benefit is the protection of public health through the control of waterborne diseases. It plays a paramount role in controlling pathogens in water that cause human illness, as evidenced by the virtual absence of waterborne diseases such as typhoid and cholera in developed countries. Untreated or inadequately treated drinking water supplies remain the greatest threat to public health, especially in developing countries, where nearly half the population drinks contaminated water. In these countries, diseases such as cholera, typhoid and chronic dysentery are endemic and kill young and old alike. In 1990, over three million children under the age of five died of diarrhea! diseases in developing countries. Unfortunately, the availability of safe drinking water supplies in many areas is practically nonexistent due to poverty, poor understanding of water contamination and lack of a treatment and delivery infrastructure.
A recent example of the continuing public health threat from waterborne disease outbreaks occurred in Peru in 1991, where a major causative factor was the absence or inadequacy of drinking water disinfection. This failure to disinfect was partly based on concern about U.S. reports regarding DBPs. The result: a five-year epidemic of cholera, its first appearance in the Americas in this century. The epidemic spread to 19 Latin American countries and has been only partially abated through public health interventions supported by the Pan American Health Organization's advice and technical assistance. Nearly one million cases and 10,000 deaths have been reported.(3) These statistics strongly reinforce the concept that water disinfection must be a primary tool in protecting public health worldwide. As noted by the American Academy of Microbiology, "The single, most important requirement that must be emphasized is that disinfection of a public water supply should not be compromised."(5)
According to the World Health Organization, disinfection by chlorine is still the best guarantee of microbiologically safe water (Wl-lO Regional Office for Europe, Drinking Water Disinfection).
Chemistry Division of the American Chemistry Council
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