1. d. In cocurrent brine regeneration, the bottom portion of the resin bed is not regenerated as completely as the top portion of the resin bed, and especially at lower saltings, there remains a band of only slightly regenerated resin on the bottom of the resin bed. When softened water, during the subsequent service run, reaches this bottom band, it will tend to remove a small amount of the hardness, which will show up as a hardness leakage. In countercurrent regeneration, the least regenerated section of the bed is near the top, while the bottom of the resin bed is very thoroughly regenerated. This eliminates hardness leakage at the beginning of the service run. Even high TDS water can be treated with lower hardness leakage by countercurrent regeneration systems.

2. b. The displacement rinse in countercurrent regeneration systems must be done using softened water to prevent hardness deposits on the bottom of the resin bed; otherwise, these deposits show up as hardness leakage in the downflow service cycle.

3. a. When final (fast) rinse to a salt-free end point is done in a downflow mode, it can be done with raw or hard water.

4. a. Since the very well regenerated resin at the bottom portion of the resin bed in countercurrent regeneration softeners removes most all traces of remaining hardness that may not have been removed in the upper portion of the resin bed, the hardness leakage at the end of the service cycle occurs very rapidly when it is reached. If all conditions are met for the countercurrent regeneration cycle to keep the bed compacted, there is generally a greater utilization of capacity for the amount of salt used in regeneration than there is in cocurrent regeneration.

5. c. Upflow regeneration tends to cause resin bed expansion with space or voids being formed through which the regenerant flow will not make as complete of contact with the resin beads. Unless some means is found, during countercurrent regeneration, to hold the resin bed immobile, an inefficient use of the regenerant can occur.

6. b. The development of the mixed bed ion exchange resin system has enabled industries, such as the semiconductor and condensate polishing, to have available large volumes of 18 megohm-cm resistance deionized water that is necessary in their operations.

7. False. Anion resin, in most cases, is of lower density (lighter weight) than cation resin.

8. True. The key to operation of a mixed bed ion exchange system is the ability to separate the anion from the cation resins after each service cycle so that they can be regenerated separately. During the backwash step it is most important to get a clean separation of the lighter anion resin from the heavier cation resin, with a minimum of mixing of the two, since the cation acid regenerant (HCl or H2SO4) would convert the anion resin to the completely exhausted form. The same problem would also be found with the cation resin, where the sodium hydroxide regenerant for the anion resin would convert the cation resin completely to the sodium form.

9. True. For cation exchange resins, a common replacement point is when their cross-linkage has been attacked and their moisture content has gone up above 50 percent.

10. False. Cation exchange resins are generally based on the same resin structure as many of the anion exchange resins, but since they have more chemically durable exchange sites than anion exchange resins, they also exhibit longer usable life. Under reasonable conditions, cation exchange resins may have a life expectancy of five to 10 years as compared to that of anion resins from one year to five years.