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For example, if a water has 2.0 ppm chlorine demand, and a chlorine dosage of 5.0 ppm is fed into the water, the chlorine residual would be 3.0 ppm.

The rate of feed is normally adjusted with a chemical feed pump to provide a chlorine residual of 0.5-1.0 ppm after 20 minutes of contact time. This is enough to kill coliform bacteria, but may or may not kill any viruses or cysts which may be present. Such a chlorine residual not only serves to overcome intermittent trace contamination from coliform bacteria, but also provides for minor variations in the chlorine demand of the water. The pathogens causing such diseases as typhoid fever, cholera and dysentery succumb most easily to chlorine treatment. Cyst-forming protozoa which cause amoebic dysentery and giardiasis are most resistant to chlorine.

As yet little is known about viruses, but some authorities place them at neither extreme in resistance to chlorination.


For emergency purposes iodine may be used for treatment of drinking water. Much work at present is being done to test the effect of iodine in destroying viruses, which are now considered among the pathogens most resistant to treatment. Tests show that 20 minutes exposure to 8.0 ppm of iodine is adequate to render a potable water. As usual, the residual required varies inversely with contact time. Lower residuals require longer contact time, while higher residuals require shorter contact time. While such test results are encouraging, not enough is yet known about the physiological effects of iodine­treated water on the human system. For this reason its use must be considered only on an emergency basis.


Silver in various forms has been used to inhibit the growth of microorganisms. It is most frequently found combined with activated carbon in filters. When some bacteria species come into contact with this silver, they are rendered inactive. There is disagreement among the experts as to the effectiveness of this process because silver ions in water kill E.coli very well and probably also salmonella, shigella, and vibro bacteria, but it has found lesser effect on viruses, cysts, and other bacteria species. Silver does not produce offensive tastes or odors when used in water treatment. Further, organic matter does not interfere with its effectiveness as is the case with free chlorine. Its high cost, interferences by chlorides and sulfides, need for long periods of exposure, and incomplete bactericidal action have hindered its widespread acceptance.


Copper ions are used quite frequently to destroy algae in surface waters. But these ions are relatively ineffective in killing bacteria. Copper sulfate, for example, is also used to kill algae in reservoirs.


Disease-bearing organisms are strongly affected by the pH of a water. They will not survive when water is either highly acid or highly alkaline. Thus treatment which sharply reduces or increases pH in relation to the normal range of 6.5 to 7.5 can be an effective means of destroying organisms.

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Related Articles:

Water Disinfection Methods. Part I
Water Disinfection Methods. Part III
What is Solar Water Disinfection?


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