Page 322 - Water and wastewater engineering

LIME–SODA SOFTENING 7-39

CO 2 that is not dissolved. Exposure to a 5 percent CO 2 concentration over a prolonged period
may cause unconsciousness.

Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of photos.

7-9 CHAPTER REVIEW
When you have completed studying this chapter, you should be able to do the following without
the aid of your textbooks or notes:
1. Define hardness in terms of the chemical constituents that cause it and in terms of the
results as seen by the users of hard water.
2. Using diagrams and chemical reactions, explain how water becomes hard.
3. Given the total hardness and alkalinity, calculate the carbonate hardness and noncar-
bonate hardness.
4. Explain the significance of alkalinity in lime-soda softening.
2 2
5. State the proper pH for removal of Ca and Mg and explain how the reactions
“ensure” the proper pH.
6. Explain why the solubility relationships do not fully explain the pH required to achieve
2
2
satisfactory precipitation of Ca and Mg .
7. Explain to a client why lime-soda softening cannot produce a water completely free of
hardness.
8. Explain to a client why a magnesium concentration of 40 mg/L as CaCO 3 is a design
objective for lime-soda softening.
9. Describe to a client under what circumstances CO 2 in raw water is to be removed by
precipitation or by stripping.
10. Given a water analysis, select an appropriate lime-soda softening process, that is,
selective calcium removal, excess lime softening, or split treatment.
11. Explain the purpose of recarbonation.

12. Explain why the softening process may be of benefit in removing constituents of
concern other than calcium and magnesium.

With the use of this text, you should be able to do the following:
13. Estimate the CO 2 concentration of a water given the pH, alkalinity, and water temperature.

14. Estimate the amount of lime and soda ash required to soften water of a stated composition.
15. Calculate the fraction of the “split” for a lime-soda softening system.
16. Draw bar graphs to describe a water during different stages of softening.