Disinfection is the most important step in the water treatment process to destroy pathogenic bacteria and other harmful agents. Chlorination is a very common and effective method for the disinfection of drinking water, and it has been the single most important process for assuring the bacteriological safety of potable water supplies. The practice was introduced in Belgium in 1903, and was first used in the United States in 1908 in Chicago. A sharp decline in typhoid deaths was noted following the onset of chlorination, as was also the case with cholera, dysentery, and hepatitis A. Chlorination has also contributed to a major decline in infant mortality rates due to waterborne illness. Waterborne epidemics have virtually disappeared in the industrialized world. When waterborne disease outbreaks have occurred, they have generally been traced back to a failure of the chlorination system.
Chlorine is a very cost-effective disinfection process. Chlorine concentration is generally 1 miligram per liter, which is about 1 part per million. Chlorine can be added directly as chlorine gas, or indirectly as sodium-hypochlorite solution.
The chlorination system generally includes chlorine storage and feed equipment. In most cases, a metering device (a chlorinator) allows the chlorine to mix via a small side stream of water. In contrast to a short-acting disinfectant (such as ozone), chlorine also has the benefit of being residual in the system. Should a pipe break or another type of accident occur, there is usually enough residual chlorine in the system to provide for protection of the water supply. Chlorine is used extensively when interruptions of water piping or cross-connections occur. Chlorine used at higher amounts will kill all potential organisms present, but at usual treatment levels certain resistant organisms, such as cryptosporidia, may survive. Chlorine is also used to clean reservoirs, basins, wells, and pipes. Algae can also be controlled with the use of chlorine.
Chlorine does have some undesirable characteristics, including imparting undesirable taste and odors to the water, especially when phenol is preset. Also, the reaction of chlorine with the organic material that can be present in the water results in a group of disinfectant by-products, known as the trihalomethanes (THMs). The most common THM is chloroform, which has been shown to cause cancer in laboratory animals. Chlorine itself is highly toxic and must be handled with extreme care at the water treatment facility.
MARK G. ROBSON
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