Chlorination involves adding a measured amount of chlorine to water to produce a residual sufficient to kill bacteria, viruses, and cysts. The killing effect of chlorine depends on the pH of the water, temperature, chlorine level and contact time (i.e., the time the chlorine is in the water before consumption). Show
 Typically, chlorine is added to public drinking water as the final stage of treatment, often following an upstream filtration step which removes sediment that can tie up chlorine and shield organisms from its effect. Chlorine has been used for over a century as a primary water disinfectant and is largely responsible for elimination of water-borne diseases such as typhoid and dysentery in developed countries. Unfortunately, chlorine reacts with many organic compounds to form chlorine disinfection by-products that are recognized as potent carcinogens at low levels of concentrations. Such organic compounds include humic and fulvic acids, which derive from rotting vegetation common in surface waters. Reactions between free chlorine and these acids may produce a class of compounds called trihalomethanes. Strategies to reduce these in public water supplies include enhanced filtration for better organic removal and use of ammonia together with chlorine to produce chloramines for use in lieu of chlorine. Chloramines have longer half-life in the water and are less likely to produce trihalomethanes. Chlorine is typically added to water using chemical feed systems to inject liquid sodium hypochlorite (bleach) solution or added as gaseous chlorine (typical of larger public water treatment plants). Chlorine cannot be used in most high purity water loops without contaminating the process or interfering with the end use of the water. These systems often rely on ultraviolet disinfection as an alternative to water chlorination. The Water Professionals® can help you specify chlorination systems based on water chemistry, downstream use and biological conditions. 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. Chlorine and pHIn 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 GasChlorine 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 HypochloriteCalcium 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 HypochloriteSodium 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. What is the purpose of adding chlorine to water during the water treatment process quizlet?What is the purpose of adding chlorine to water during the water treatment process? Chlorine is added to prevent bacteria from growing in the water.
Which of the following does not contribute to water conservation?Cutting trees to check water loss due to transpiration is not help in conservation of water.
What material would be the best to use for the layer that will hold water?What material would be the best to use for the layer that will hold water? a permeable material, such as gravel.
Why is groundwater pollution a greater problem than surface water pollution?If we compare an organic pollutant in groundwater to one in surface water water, groundwater has fewer microbes to digest organic pollutants, less oxygen no sunlight and surface from which organic pollutant can evaporate. Especially in slow moving groundwater, pollutants can persist indefinitely.
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Chlorine is often added during water treatment to
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