1. e. Many standard water softeners now in use are probably co-current systems (downflow service and downflow regeneration). They are typically operated with five cycles: 1) service run; 2) backwash; 3) salt regeneration; 4) displacement rinse (or slow rinse); and 5) fast rinse (or final rinse to quality).
2. False. Calcium having the same +2 valence but higher atomic weight (40.1 a.m.u. for Ca2+ vs. 24.3 a.m.u. for Mg2+), has a stronger affinity for cation resin exchange sites than does magnesium. Generally, the ions with greater valence and/or greater atomic/molecular weight exhibit higher selectivity preferences for resin exchange sites and are less likely to dissolve or hydrolyze off the resin.
3. True. As hard water is passed through a water softener resin bed, the top resin layer converts from the regenerated sodium form to progressively growing layers of the exhausted calcium and magnesium forms. However, since magnesium is not held as tightly to the resin exchange sites as is calcium, the calcium displaces the magnesium causing it to move farther down the resin bed. The very top layer of resin becomes predominantly calcium form, with the magnesium in a layer below that, and the bottom remainder of the resin bed in a progressively diminishing layer of the sodium form.
4. False. The various bands of exhausted resin are not completely distinct and of well-defined shape, since that also depends largely on flow rate and the uniformity of flow distribution. The higher the flow rate, the less defined or distinct the bands will be, and the more they will be spread out down the resin bed. The flow of water down the column of resin takes the path of least resistance; there may be portions of the resin bed that will have become compacted, caked or channeled (possibly due to improper backwash or tank wall effects). Thus, the bands or layers of exhausted resin may tilt or bulge. This could cause an earlier breakthrough or leakage than if the exhausted bands are narrowly condensed, horizontal and flat as they tend to become with a slower flow rate and a more uniform flow distribution.
5. d. The higher the ratio of sodium to total cations, the larger will be the spread of the sodium band below the calcium and magnesium bands, which results in an earlier breakthrough of sodium and potentially a greater leakage of sodium from hydrogen regenerated cation exchange. The TDS is a measure of the total cations in the feedwater. The greater amount of total cations causes greater amounts of acids in the sodium exchange zone. The sodium’s relatively weak bond to resin is further weakened by the presence of these acids (especially strong acids), which causes a spreading out of the sodium band, and higher sodium leakage. The higher the alkalinity (HCO3–) and the lower the mineral acidity (Cl– and SO42-), the more weak carbonic acid (H2CO3) and the lesser amounts of strong acids (HCl and H2SO4) that will be formed ahead of the sodium exchange zone. Since carbonic acid is a weak acid that will not cause sodium to be hydrolyzed or exchanged off the resin as readily, the sodium leakage decreases and the achievable capacity increases for waters with higher percentages of alkalinity anions.
6. True. The affinity of the weak acid cation (WAC) carboxylic exchange groups for divalent cations is considerably greater than the affinity of the strong acid cation (SAC) resin sulfonic exchange groups for divalent cations because of the very large difference in affinity of the WAC resins for divalent ions over monovalent ions, WAC resins in the sodium form can soften even concentrated — up to 10,000 ppm — (but not saturated) brine solutions.
7. True. Strong acid cation (SAC) resins change in volume about five to eight percent in going from one ionic form to another (regeneration causes shrinkage). Weak acid cation (WAC) resins on the other hand change from 20 to 100 percent in volume. This difference can require changes in equipment design to accommodate the larger changes in volume of the WAC resins, both in service cycle (swelling) and in regeneration (shrinking). Such swelling in the service cycle can also cause a potential problem in pressure drop across the resin bed. For this reason, more shallow bed depths are designed for WAC resins.
8. a. Silica is one of the major constituents (about 28 percent) of the earth's crust. It is found in all waters, but because of its very low solubility it is generally present under 30 ppm. Municipal water supplies in the U.S. average about seven ppm of silica.
9. False. Silica is considered to be in solution in water as the extremely weak silicic acid (H2SiO3 and H3SiO4), which is barely ionized. It is sometimes represented as a nonionized species: SiO2 • (H2O)n (where n represents an unknown number of water molecules). Further, as the amount of silica increases in the soluble SiO2 • (H2O) form, there is a tendency for the silica molecules to bond together or polymerize into two or more units, and it can combine into a large number of units to become an insoluble polymer, which is classified as colloidal silica. The polymerization of silica is enhanced at low pH.
10. True. Fortunately, the majority of silica in water supplies is of the soluble H2SiO3, H3SiO4, and HSiO3– forms, and can be removed by the strongly basic anion (SBA) exchange resins that are in the hydroxide form. If, however, the insoluble polymerized or colloidal silica is present in the water supply, it will not be removed to any great extent by the usual SBA resins. Chemical precipitation, ultrafiltration, reverse osmosis, and/or very large pore (macroporous) SBA resin may be required for colloidal silica.
