Although three-quarters of the Earth's surface is water, only 1% is available for drinking, and this often requires treatment before it can be used safely. Water contains many kinds of microbes and organisms that can cause disease. It is estimated that 80% of all sickness and disease in developing countries is caused by unsafe water and inadequate sanitation. According to the World Health Organization (WHO), diseases associated with dirty water cause some 25,000 deaths per day. Chlorine and its derivative water treatment products, sodium hypochlorite and chlorine dioxide, are very powerful disinfection agents and, when added to water in minute quantities, rapidly destroy bacteria and other microorganisms.
Chlorine compounds have been used in many countries for almost a century to treat drinking water. Where they are used, they have virtually eradicated serious waterborne diseases. Different water disinfection techniques, which include disinfection using ozone, U.V. and ultrafiltration, are used around the world. These methods are often used in tandem with chlorine. A major advantage of chlorine is that it has a residual disinfection effect, and it is the only technique that can ensure disinfection right up to the tap. In fact, the residual ability to destroy and inhibit the activity of pathogenic agents is a specific characteristic of chlorine.
The action of other disinfectants, such as ozone, U.V. is only temporary. This residual property of chlorine means that it is chosen as the main technique for protecting drinking water circulating in supply networks. This is particularly important in cities, where leaks can allow germs into the system beyond the treatment station. Emergency situations involving accidental contamination of drinking water supply systems continue to demonstrate the need for chlorine as an ultimate defense against waterborne microbiological infection. In addition to its disinfecting properties, chlorine has other benefits for water treatment - these include preventing the growth of algae and slime in pipes and storage tanks. Chlorination systems are also a comparatively economical technique and safe to use, requiring minimum maintenance. Properly used in pre-treated water (eg. pH adjustment, flocculation, sedimentation and filtration), only very small quantities of chlorine are needed for effective purification.
Generally, it is possible to recognize the smell and taste of chlorine only in water which has not been processed properly. This is rare in modern water systems. Over decades of chlorine use, epidemiological studies carried out on populations drinking chlorinated water have failed to establish a causal relationship with any disease (including cancer), a conclusion that has been recognized by the WHO. However, it is expected that chlorine will remain a perfectly acceptable disinfectant, provided the appropriate techniques are applied. Public health is the main benefit provided by the use of chlorine to disinfect drinking water. Applied to water treatment, chlorine has four essential functions :
Elimination of undesirable matter from the water by oxidation
Permanent protection of the hygienic and sanitary quality of the water throughout the distribution phase
Active, immediate disinfection in cases of accidental pollution
Continuous monitoring (of the chlorine demand) to warn of pollution.
For over 100 years, chlorination of drinking water has provided a world-wide demonstration of its effectiveness in protecting public health.
See also: Calcium carbonate, Carbon dioxide, Chlorination, Dissolved oxygen, Langelier calculation, Langelier index, Larson-Skold index, Oddo-Tomson index, pH, Puckorius index, Ryznar index, Scaling Indices, Stiff-Davis index, Total dissolved solids, Water corrosivity