The most common use of chlorine in water treatment is to disinfect water. As a disinfectant, it has drawbacks, but it also has advantages. Other methods of disinfection such as ultraviolet and ozonation are effective disinfectants but they do not provide a residual to prevent pathogen regrowth as chlorination does. When treatment plants are distant from the point of use, chlorination is the best way to provide safe water to the end user. Municipal water providers usually rely on measurements of “chlorine residual”—the amount of chlorine remaining in the water after it reaches its destination—as proof of safety. Residual requirements vary, but typical residual goal would be for 0.2 to 1 mg/L.
In addition to disinfection, chlorine can be effectively used to oxidize iron, manganese and hydrogen sulfide to facilitate their removal, to reduce color in water, and to aid in such treatment processes as sedimentation and filtration.
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Chlorine and pH
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In general terms, the lower the pH of the water, the more effective chlorine is as a disinfectant. Again, speaking generally, a reason for dosing effectively is that chlorination raises the pH of water, so overdosing often raises the pH to levels where chlorine does not work effectively as a disinfectant. More is not always more powerful. Chemically, this has to do with the relationship between the two constituents of chlorine that together are often referred to as “free chlorine”—hypochlorus acid and hypochlorite ions. Hypochlorus acid is the more effective disinfectant and it dominates at lower pH levels, so a lower pH is preferred for disinfection. Conversely, a higher pH is needed for water treatment strategies that depend on chlorination to oxidize iron and manganese.
Types of Chlorine Used in Water Treatment
“Pure chlorine” is seldom used for water treatment. The three most common chlorine-containing substances used in water treatment are chlorine gas, sodium hypochlorite, and calcium hypochlorite. The choice of the chlorine type to be used often depends on cost, on the available storage options and on the pH conditions required. Chlorination affects pH and pH affects results—a fact that is commonly overlooked in residential water treatment.
Chlorine Gas
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Chlorine gas is greenish yellow in color and heavier than air. Its high toxicity makes it an excellent disinfectant for water but also a hazard to humans who handle it. Chlorine gas, of course, is a deadly weapon when used in chemical warfare. It is a respiratory irritant and can irritate skin and mucous membranes and can cause death with sufficient exposure. Because of chemical changes that occur when it is introduced into water, chlorine gas is no more toxic to humans when used to treat drinking water than other forms of chlorine. Chlorine gas, which is actually sold as an amber-colored compressed liquid, is the least expensive form of chlorine and is, consequently, the preferred type for municipal water systems.
Calcium Hypochlorite
Calcium hypochlorite is manufactured from chlorine gas. It is best known as chlorine pellets and granules in residential water treatment. It is a white solid with a very pungent odor and it can create enough heat to explode, so it must not be stored near wood, cloth or petroleum products. Calcium hypochlorite increases the pH of the water being treated.
Sodium Hypochlorite
Sodium hypochlorite is a chlorine-containing compound most easily recognized as household bleach. It is a light yellow liquid that has a relatively short shelf life. It is the easiest to handle of all the types of chlorine. Sodium hypochlorite also increases the pH of the water being treated. A lower concentration of chlorine in this form is needed to treat water than with calcium hypochlorite or chlorine gas.
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