Science of Water : Concepts and Applications

316 The Science of Water: Concepts and Applications
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It must not add unpleasant taste or odor to the water.
It must be readily available at a reasonable cost and be safe and easy to handle, transport,
store, and apply.
It must be quick and easy to determine the concentration of the disinfectant in the treated
water.
It should persist within the disinfected water at a high enough concentration to provide
residual protection through the distribution.
√ Note: Disinfection is effective in reducing waterborne diseases because most pathogenic organisms
are more sensitive to disinfection than are nonpathogens. However, disinfection is only as
effective as the care used in controlling the process and assuring that all of the water supply is
continually treated with the amount of disinfectant required, producing safe water.
The methods of disinfection are as follows:
1. Heat—Possibly the fi rst method of disinfection. Disinfection is accomplished by boiling
water for 5–10 min. Good, obviously, only for household quantities of water when bacteriological
quality is questionable.
2. UV light—While a practical method of treating large quantities, adsorption of UV light
is very rapid, so the use of this method is limited to nonturbid waters close to the light
source.
3. Metal ions—Silver, copper, and mercury.
4. Alkalis and acids.
5. pH adjustment—To under 3.0 or over 11.0.
6. Oxidizing agents—Bromine, ozone, potassium permanganate, and chlorine.
The vast majority of water systems in the United States use chlorine for disinfection. Along with
meeting the desired characteristics listed above, chlorine has the added advantage of a long history
of use—it is fairly well understood. Although some small water systems may use other disinfectants,
we concentrate on chlorine.
CHLORINATION
Chlorine is the most commonly used substance for disinfection of water in the United States. The
addition of chlorine or chlorine compounds to water is called chlorination. Chlorination is considered
to be the single most important process for preventing the spread of waterborne disease.
Chlorine deactivates microorganisms through several mechanisms, assuring that it can destroy
most biological contaminants:
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It causes damage to the cell wall.
It alters the permeability (the ability to pass water in and out through the cell wall) of the
cell.
It alters the cell protoplasm.
It inhibits the enzyme activity of the cell so it is unable to use its food to produce energy.
It inhibits cell reproduction.
Chlorine is available in a number of different forms: (1) as pure elemental gaseous chlorine
(a greenish-yellow gas possessing a pungent and irritating odor that is heavier than air, nonfl ammable,
and nonexplosive); when released to the atmosphere, this form is toxic and corrosive; (2) as
solid calcium hypochlorite (in tablets or granules); or (3) as a liquid sodium hypochlorite solution
(in various strengths).