Water Treatment

Treatment of Water Supply

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What, then, must be removed from public water supplies, and what other chemicals are added to the water? How do public water facilities treat our water to make it safe for us to drink and appropriate for other human uses? There are six major steps in the treatment of our water: screening, sedimentation, precipitation, filtration, adsorption, and disinfection. Some of these steps, such as precipitation, involve chemical reactions among the aqueous species dissolved in the water; others, such as screening, involve only separation of particles on the basis of physical characteristics like size. Many of these steps depend on one another. For instance, precipitation generates solids in the water from particles that had been dissolved; these solids must then be removed through sedimentation or filtration. We shall discuss each of the six steps in water treatment below, and then present a schematic showing how the steps work together to produce clean, usable freshwater.

Screening

Surface water (water from lakes and rivers) often has large debris, such as sticks, leaves, fish, and trash, floating in it. These objects can clog the water-treatment system and must be removed before the water enters the treatment plant. Treatment facilities that use surface water have large screens (Figure 4) covering the site of water intake. The debris is too large to pass through the holes in the screens. Thus, as the water enters the plant, the large debris is removed. The screens must be cleaned periodically to remove any objects that have become stuck, so that they do not clog the screen and impede water flow into the plant. (Another problem for water-treatment plants in the Great Lakes and Mississippi Valley is zebra mussels, which can "congregate" at the water intake and clog the screen, eventually sealing it off. Potassium permanganate, KMnO₄, can be used to kill these mollusks.)

Figure 4 This drawing shows some of the large objects in surface water that are removed as the water passes through a screen into the water-treatment facility. The large black arrows show the direction of water flow through the screen.

Sedimentation

Other suspended (insoluble) particles, such as sand and dirt, are small enough to pass easily through the screens. These particles must be removed from the water by another process known as sedimentation(Figure 5). When water is allowed to sit, heavy suspended particles (e.g., sand) will settle to the bottom over time because they are denser than water. The water, now free of the suspended impurities, can be collected from the top without disturbing the layer of sediment at the bottom (which is eventually discarded).

Figure 5 Particles that are insoluble in water may be suspended in the water, particularly if the water is turbulent (stirred up). If the particles are heavy enough, they will settle to the bottom when the water is allowed to sit still over time.

Sometimes the insoluble particles are too small to settle out quickly enough to use sedimentation alone. Two processes, known as flocculation and coagulation, are used to create larger particles that will settle quickly to the bottom. In flocculation, small particles with non-rigid surfaces are made to agglomerate by mixing the water (and thus bringing the particles into contact with one another so that the surfaces can become stuck together). When the agglomeration of the particles gets large enough, the aggregate can settle in still water by sedimentation. Other suspended particles do not agglomerate well by flocculation. To remove these particles from the water, coagulation must be used. Coagulation is the process of gathering particles into a cluster or clot, often achieved by the addition of special chemicals known as coagulants. The most common coagulant used in water-treatment facilities is aluminum sulfate (alum, Al₂(SO₄)₃). Other Al and Fe salts, including poly-aluminum chloride, ferric chloride, and ferric sulfate, may be used as well. These salts react with ions naturally found in the water to produce a solid precipitate (Equation 2). As this precipitate forms, other particles are caught in the solid, forming a mass that will settle to the bottom via sedimentation (Figure 6).

(2)

Figure 6 When coagulants such as Al2(SO4)3 are added to the water supply, they form solid precipitates (green), as shown in Equation 2, above. These precipitates catch other impurities (red) in the water, forming a solid mass containing the precipitate formed by coagulation and the trapped impurities. This mass will settle to the bottom by sedimentation, and the water (with the trapped impurities now removed) can be drained off of the top.

Precipitation

The steps in the water-treatment process described above are used to remove insoluble particles from the water supply. But recall from above ("Species (Other Than H₂O) Contained in Water") that water also contains many molecules and ions in solution. Many of the ions in solution can be removed by precipitation: reacting the ions (to be removed) with other ions to produce insoluble solids that can be removed by sedimentation (see above) or filtration (see below).

How does the precipitation step of the water-treatment process differ from the coagulation reaction described above? ANSWER: In the precipitation step, the particles to be removed are part of the chemical reaction forming the precipitate. In the coagulation step, the particles to be removed are not part of the reaction forming the precipitate; they are simply trapped in the precipitate that is formed from added chemicals.

A typical precipitation reaction used to remove ions in water treatment follows the reaction shown in Equation 3, below. This is the same reaction type that you performed in the Experiment when the reaction between ions from two aqueous solutions produced a solid precipitate.

(3)

Two major classes of ions are typically removed via precipitation:

• Calcium (Ca²⁺) ions and magnesium (Mg²⁺) ions that have been leached from minerals in the ground cause the condition known as "water hardness". These ions do not pose any health threat, but they can engage in reactions that leave insoluble mineral deposits, such as scum rings on bathtubs and cooking vessels, or scale on industrial boilers, which decreases the boilers' efficiency. These deposits can make hard water unsuitable for many uses.
• Iron (Fe²⁺) ions and manganese (Mn²⁺) ions can stain plumbing fixtures and laundered clothes. These ions may also promote the growth of certain bacteria, which give foul tastes and odors to the water.