Page 379 - Water and wastewater engineering

9-18 WATER AND WASTEWATER ENGINEERING

9-5 ELECTRODIALYSIS

Unlike NF/RO that are pressure driven, electrodialysis (ED) and electrodialysis reversal (EDR) pro-
cesses are electrical voltage-driven. Alternating anion and cation transfer ion exchange membranes in
flat-sheet form are placed between positive and negative electrodes. With the application of a direct
current voltage, positively charged ions move toward the negative electrode (cathode), and negatively
charged anions move toward the positive electrode (anode). This causes alternating compartments to
become demineralized and the intervening compartments to become concentrated with ions.
ED and EDR do not remove electrically neutral substances such as silica, particulate matter,
or pathogens. They are capable of removing the smallest charged contaminant ions. Although ED
and EDR processes will soften water, they more often find special application in treating specific
contaminants such as arsenic and sulfate and more general application in the treatment of brack-
ish water with total dissolved solids less than 3,000 mg/L (Bergman, 2005).

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

9-6 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 the following abbreviations: MF, UF. NF, RO.
2. Explain why NF/RO membranes are suitable for softening and MF/UF are not.

3. Using a diagram, explain to a lay audience what the terms osmosis and reverse osmosis
mean.
4. Describe the two common configurations of membrane material and identify the one
most frequently used for water softening.
5. List the four generally recognized mechanisms of membrane fouling.
6. Using a sketch you have drawn, identify the following terms that describe an NF/RO
water treatment system: membrane element, stage, and array.
7. Sketch a NF/RO system including pretreament and post-treatment processes.
8. Explain the the concept of “limiting salt” and the means to reduce its effect.

9. Explain how electrodialysis differs from NF/RO treatment.
With the use of this text, you should be able to do the following:
10. Calculate the osmotic pressure given the appropriate parameters.

11. Calculate the fraction of the “split” for an NF/RO softening system.
12. Calculate the rejection and concentration factors from a mass balance.
13. Estimate the recovery rate for a given set of water constituents.

14. Estimate the dose of sulfuric acid to achieve a given product recovery rate.