Chlorine has saved countless lives
By Caroline Alepin, P.Eng., M.Eng., and Michael McNulty, CET, ProMinent Fluid Controls Ltd.
The fact that water is a mode of disease transmission, has been known for only a little more than a hundred years. Water-borne diseases such as typhoid fever and cholera were responsible for taking the lives of hundreds of thousands of people around the World, including North America, at the beginning of the 20th century, so the need to disinfect potable water to preserve human life emerged. By definition, the purpose of disinfection of potable water is to destroy pathogenic organisms and thereby eliminate and prevent water-borne diseases.1
Chlorine, in the form of hypochlorites, was first used in 1896, as a temporary relief to typhoid epidemics at the Austria-Hungary naval base of Pola in the Adriatic Sea. As a continuous disinfection treatment, chlorination was first introduced in 1905, in Lincoln, England, to eliminate a typhoid fever epidemic, which was traced to the water supply.
Shortly after this success, chlorination and filtration were introduced in the United States and resulted in the virtual elimination of water-borne diseases such as typhoid, cholera, dysentery and hepatitis A. It is considered by many that the adoption of chlorination has been the most significant advance in public health protection.2
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| Disinfection time versus free available chlorine residual (for 99.6 to 100 percent kill). Temperature 20 - 29°C, pH as indicated1. |
Forms of Chlorine
There are three forms of chlorine used to disinfect water. Chlorine gas (Cl2) contains over 99% pure chlorine and is delivered as a liquid in pressurized containers. Calcium hypochlorite (Ca(OCl)2) contains 65% available chlorine by weight, is available in powdered form or in compressed tablets, and requires special storage to avoid contact with organic material as they can react to generate enough heat and oxygen to start and support a fire.
The third form of chlorine, which will be the focus of this article, is sodium hypochlorite (NaOCl). Sodium hypochlorite is supplied as a transparent, light-yellow solution of sodium hypochlorite and water, containing from 9 to 15 percent (more commonly 12.5%) by weight available chlorine. Sodium hypochlorite used for drinking water disinfection is similar to household bleach, except it is about double the concentration.
As a reference, adding 0.5 ppm of chlorine to a 10 m by 5 m, by 2 m deep swimming pool requires only 2 cups of household bleach or 1 cup of 12.5% sodium hypochlorite. As a second reference, adding 1 ppm of chlorine to 3.8 million litres (1 million US gallons) requires the addition of only 25 litres (6.7 US gallons) of 12.5% sodium hypochlorite.
Sodium hypochlorite is an alkaline chemical, which possesses a pungent odour and a low surface tension. Sodium hypochlorite storage does not present any fire hazard. However, it is quite corrosive and should be kept away from equipment that can be corroded. Overall, sodium hypochlorite possesses less restrictive storage and handling requirements than chlorine gas or calcium hypochlorite; the risk is minimized due to the degree of dilution.
Sodium hypochlorite is an effective disinfectant and a strong oxidant, which is also used for iron and manganese removal, taste and odour control, and biological growth control, while maintaining a safe residual throughout the distribution system.
1White, G.C. The Handbook of Chlorination, 2nd Ed. Von Nostrand Reinhold. New York, 1986.
2Leidholdt, R. Chlorine - "Special Agent" for Disinfecting Water. American Water Works Association. Vol. 26. No. 6. Pp 40-43. June 2000.
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