1. e. Effective backwash is often a very critical step to maintaining longevity and continued working order to a filter or ion exchange media bed. The backwash cycle serves to:
a. Expand the media bed from its settled and sometimes packed condition that can result from swelling or shrinkage and also from dirt cementing.
b. Clean the media by flushing out particulate matter that have been filtered from the feed water. The scrubbing action of the media particles or beads against each other during backwash helps to remove materials that may have coated the media. The faster the upward flow rate that can be maintained while still keeping the top of the media bed slightly below the top exit port, the more desirable it will be.
c. Remove media fines and broken media particles. Water treatment media and resins do tend to break down physically over time and use from turbulent forces of flowing water; it is necessary to remove these media fines to maintain good hydraulic conditions of the media bed and to prevent channeling and pressure drop.
d. Classify the media bed with larger media particles separated from the finer media particles. This classification according to media particle size provides the best conditions for uniform service flow (as well as flow of regenerants and fines flows) across the entire area of the resin bed.
2. False. It is best to maintain some back pressure by control of the flow on the outlet or discharge line during the backwash cycle. This helps to prevent the release of any air or gas from the backwash water and thus prevent flotation of the media. Any gas released can attach in bubbles to the media particles and cause clumps of the media to float out of the tank.
3. b. Normally, 10 to 15 minutes is sufficient, but there may be cases where 20 to 30 minutes are required for particularly tenacious dirt buildups. If longer backwash times than these appear necessary, special cleanup procedures might be considered.
4. True. Especially with strong base anion resins, the use of hard water for backwash can cause precipitation of calcium and magnesium foulants in the anion resin bed because of high pH zones in the exhausted anion resin bed.
5. True. Regeneration is the attempt to reverse the chemical reaction that occurred in the service cycle. The main factors that must be considered are concentration and quantity of regenerant chemicals, contact time and temperature.
6. True. Some rock salt and even some sea salt have calcium and magnesium contents, which when dissolved can produce saturated brine solutions with up to 1,900 milligrams per liter (mg/L) of calcium and up to 140 mg/L of magnesium. Use of high hardness brine reduces the efficiency for removing the calcium and magnesium from the resin during the regeneration cycle, and results in higher leakage of hardness during the service cycle.
7. c. 260,000 milligrams of sodium chloride can be dissolved in one liter of water. However, most often, if there is a rapid use of brine and therefore a short time for the salt to dissolve in the water, the brine concentration that is reached in a water softener brine tank may be no more than 230,000 to 240,000 mg/L.
8. True. Exhausted resin beads have a lower salt concentration and a higher water concentration than does the strong saturated brine solution. Through the process of osmosis, any substance with a higher water concentration (lower salt concentration) will try to pass water to the solution with the lower water content (higher salt concentration) so as to equalize the salt concentration in each. As the brine dehydrates (removes water from) the resin bead, the bead will shrink. The outer surface of the resin bead will try to shrink first, and if the osmotic pressure, caused by highly concentrated brine, is great enough the surface of the bead can crack (like the drying of a mud flat during a long drought), and the beads can split or crack. This phenomenon is known as osmotic shock.
10. a. About 30 minutes of brine solution contact with the resin is desirable in softener resin regeneration. The contact time is calculated from the time the brine is introduced into the resin bed until it is displaced from the resin bed by the slow displacement rinse.