Drinking water Chlorination
Chlorine is a disinfectant added to drinking water to reduce or eliminate microorganisms, such as bacteria and viruses, which can be present in water supplies. The addition of chlorine to our drinking water has greatly reduced the risk of waterborne diseases.
For more than a century, the safety of Canadian drinking water supplies has been greatly improved by the addition of chlorine. Disinfecting our drinking water ensures it is free of the microorganisms that can cause serious and life-threatening diseases, such as cholera and typhoid fever. To this day, chlorine remains the most commonly used drinking water disinfectant, and the disinfectant for which we have the most scientific information.
Chlorine is added as part of the drinking water treatment process. However, chlorine also reacts with the organic matter, naturally present in water, such as decaying leaves. This chemical reaction forms a group of chemicals known as disinfection by-products. The most common of these by-products are trihalomethanes (THMs), which include chloroform. The amount of THMs found in drinking water depends on a number of things, including the season and the source of the water. For example, THM levels are generally lower in winter than in summer, because the amount of natural organic matter is lower and less chlorine is needed to disinfect at colder temperatures. THM levels are also low when wells or large lakes are the drinking water source, and higher when rivers or other surface waters are the source, because they generally contain more organic matter
The Benefits of Chlorine
Current scientific data shows that the benefits of chlorinating our drinking water (less disease) are much greater than any health risks from THMs and other by-products. Although other disinfectants are available, chlorine remains the choice of water treatment experts. When used with modern water filtration methods, chlorine is effective against virtually all microorganisms. Chlorine is easy to apply and small amounts of the chemical remain in the water as it travels in the distribution system from the treatment plant to the consumer’s tap,This level of effectiveness ensures that microorganisms cannot recontaminate the water after it leaves the treatment plant.
Alternatives to Chlorination
A number of cities use ozone to disinfect their water, because ozonation does not produce THMs. Although ozone is a very effective disinfectant, it breaks down quickly and cannot be used to maintain disinfection in the distribution system. Small amounts of chlorine or other disinfectants still must be added. Renovating water treatment plants so they can use ozone can be expensive.
Examples of other disinfectants include chloramines and chlorine dioxide. Chloramines are weaker disinfectants than chlorine, but are very effective in the distribution system. Chlorine dioxide can be used in the treatment plant, but it is not very effective in the distribution system .
All chemical disinfectants used in drinking water can be expected to form by-products that could affect human health. In general, we know less about the by-products of other disinfectants than about chlorination by-products.