FIGURE 17.3 Chlorination for the breakdown of phenol; numbers in brackets are odor threshold concentrations in Jg/L.
then is simply to chlorinate at low pH, and this in fact, would be fortunate, since HOCl predominates at low pH values instead of at high pH values. Based on this fact, if superchlorination is to be conducted, it should be done at low pH values. Further research, however, should be performed to establish the accuracy of this assumption.
Chlorinated waters and wastewaters can contain not only chloroform and bro. mochloromethane but also other brominated compounds. In addition, iodinated com. pounds may also be produced; that is, this is the case if iodine (or bromine in the case of brominated compounds) is present, in the first place. In general, the products formed from the halogen family to produce the derivative products of methane are called trihalomethanes. The formula is normally represented by CHX3, where X can be Cl, Br, or I. Examples of other brominated species are bromodichloromethane, chlorodibromomethane, and bromoform. The most commonly observed iodinated trihalomethane is iododichloromethane. The reason why brominated and iodinated trihalomethanes can be formed is that bromine and iodine are below chlorine in the halogen family of the periodic table. It is an observed fact in chemistry that stronger acids drive the weaker acids. The acid precursor of stronger acids (Cl in HOCl) are higher in the series than those of the precursor of the weaker acids (Br and I in HOBr and HOI, respectively). For this reason, HOCl drives the weaker acids HOBr and HOI. These two acids then react in the same way as HOCl when it produces the brominated and iodinated trihalomethanes.
Example 17.10 In Figure 17.4, when hydrogen is abstracted from the methyl group, what happens to the double between carbon and oxygen?
Solution: As shown, the double bond is ruptured making the oxygen end negative and single bonded. The double bond switches to become a carbon. to. carbon double bond. As indicated in subsequent reactions, this flip. flopping of the double bond continues until the formation of chloroform. Ans
Acid generation. Whether or not acid will be produced depends upon the form of chlorine disinfectant used. Using chlorine gas will definitely produce hydrochloric acid. Sodium hypochlorite and calcium hypochlorite will not produce any acid; on the contrary, it can result in the production of alkalinity. Superchlorination using HOCl will definitely produce acids.
As shown in Equation (17.9), a mole of hydrochloric acid is produced per mole of chlorine gas that reacts. Chlorination uses up the disinfectant, so this reaction would be driven to the right and any mole of chlorine gas added will be consumed. Thus, if a mmol/L of the gas is dosed, this will produce a mmol/L of HCl. This is equivalent to one mgeq of the acid, which must also be equivalent to a mgeq of alkalinity. The analytical equivalent mass of alkalinity in terms of CaCO3 is 50 mg CaCO3 per mgeq. Thus, the mmol/L of hydrochloric acid produced will need 50 mg/L of alkalinity expressed as CaCO3 for its neutralization. Or, simply, one mmol of hydrochloric acid requires 50 mg of alkalinity expressed as CaCO3 for its neutralization.
In superchlorination, breakpoint reactions like Eqs. (17.37) to (17.42) will transpire. A host of other reactions may also occur such that all these, as shown by the preceding reactions, produce acids. The number of reactions are many, therefore, it is not possible to predict the amount of acids produced by using simple stoichiometry. The only way to determine this amount is to run a jar test as is done in the determination of the optimum alum dose. Metcalf & Eddy, Inc. wrote that in practice it is found that 15.0 mg/L of alkalinity is needed per mg/L of ammonia nitrogen (Metcalf & Eddy, Inc., 1972). But, again, the best method would be to run the jar test.
Example 17.11 A flow of 25,000 m /d of treated water is to be disinfected using chlorine in pressurized steel cylinders. The raw water comes from a reservoir where the water from the watershed has a very low alkalinity. With this low raw-water alkalinity, coupled with the use of alum in the coagulation process, the alkalinity of the treated water when it finally arrives at the chlorination tank is practically zero. Calculate the amount of alkalinity required to neutralize the acid produced during the addition of the chlorine gas.
Solution: Because the dose of the chlorine is not given, assume it to be 1.0 mg/L, which is equal to:
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