Domestic Water Treatment

The typical steps in treating water for household use in the United States and many other developed countries consist of (1) settling of suspended material, (2) aeration to aid in the oxidation of easily oxidized organic matter, (3) a preliminary chlorine treatment to remove bacteria and other organic material through oxidation, (4) use of a flocculating agent such as alum (aluminum sulfate) to carry down particulate matter, (5) addition of lime [Ca(OH)2] to

Settling

1

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Oxidation

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Flocculation/ Filtration/ pH Adjustment

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Disin!

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FIGURE 11-4 The common steps in the treatment of domestic water.

control pH and assist in the flocculation process, (6) filtration (through sand beds), and (7) a final chlorine treatment. Activated carbon may be used in some circumstances to absorb traces of material that give odor or taste, and a fluoride source (e.g., Na2SiF6) may be added as a dental health measure. Figure 11-4 shows a schematic block diagram of the process.

Suspensions of small particles in a liquid (colloids, Section 9.6) are often stabilized by adsorption of ions on the surface of the particles; if ions of a particular charge are adsorbed preferentially, the colloidal particles will repel each other and coagulation will be impeded. Various chemicals can be used that form insoluble precipitates that carry down the suspended materials. Examples are compounds of Fe(III) and Al(III), which on hydrolysis form the insoluble hydrated oxides discussed in Chapter 10. As mentioned earlier, aluminum sulfate, Al2(SO4)3, is a widely used chemical for water treatment. Ferrous sulfate is another; it is oxidized by oxygen in the water to ferric ions. Hydrolysis of both Al(III) and Fe(III) produces polynuclear hydroxo species that are readily adsorbed on the surface of the suspended material, and as these hydroxo compounds further condense to insoluble forms, bind the particles with the floc as it settles. If the pH is near neutrality, both Al(III) and Fe(III) are completely hydrolyzed and the added materials are completely removed from the water (see Section 10.6.3). The pH may be adjusted to ensure this by the addition of a base such as Ca(OH)2 (lime). The reaction can be summarized:

The small amounts of calcium and sulfate ions left in solution are not harmful.

The best-known step in the treatment process for drinking water is chlorin-ation. Chlorine is a strong oxidizing agent and in water produces hypochlorite, which is widely used in bleaches (Section 10.6.13):

This reaction is essentially complete at normal pH (the room temperature equilibrium constant is 4.5 x 10-4mol2/liter2). HOCl is a weak acid (Ka 3 x 10~8) and will be present as both HOCl molecules and OCP ions, with relative amounts depending on the pH. (Concentrations will be equal at pH = pKa; Chapter 9.) These species are referred to as free available chlorine. The HOCl molecule is most effective as a disinfectant, presumably because it can enter bacterial cells more easily than the ion.

The primary purpose of the chlorine treatment is to oxidize organic materials in the water, especially bacteria. The redox half-reaction of HOCl is

Oxidizing conditions must be maintained throughout the entire distribution system to prevent redevelopment of septic conditions while ensuring that the chlorine content is not excessive near the addition point. The stability of the hypochlorite with respect to disproportionation to CP and ClO3 is dependent on the pH, which may require adjustment for this reason, as well as to reduce the corrosive properties of the water.

Although chlorination oxidizes many organic materials, it does not oxidize all. It can result in chlorination of some molecules, with the possibility of producing chlorinated hydrocarbons with significant health hazards (Section 8.3.1). Chloroform is one of the potential by-products of water chlorination that is of concern. It may arise from the well-known haloform reaction, in which compounds that have structures CH3C(O)R or CH3CH(OH)R, or can be oxidized to form these structures (e.g., olefins) undergo a reaction in which the hydrogens on the carbon adjacent to the one carrying the oxygen undergo a dissociation to form the carbanion intermediate [CH2C(O)R]~ followed by displacement of the Cl of HOCl to give the chlorinated product. This continues until all the hydrogens have been replaced, whereupon hydrolytic cleavage of the C—C bond releases chloroform, CHCl3, as illustrated in the following reaction scheme.

ii .11.: x ii RePeat rcch3-—► h + rcch2 + ci:oh —- :oh + rcccih2-—-

twice

O O more

II II

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