Answers for silica, sulfides and membranes

Nov. 7, 2012

1. d. At low pH, silica exists in the silicic acid (H4SiO4 or Si(OH)4) form and can polymerize to form a colloid (colloid silica). At …

1. d. At low pH, silica exists in the silicic acid (H4SiO4 or Si(OH)4) form and can polymerize to form a colloid (colloid silica). At water pH above 8, silicic acid dissociates into the silicate anion (SiO32-), and can then precipitate as a salt with calcium, magnesium, iron or aluminum. Because of their stability once they fall out of solution, silica and silicates are difficult to redissolve.

2. c. The potential for silica precipitation should be evaluated if the concentration exceeds 20 mg/L in feedwater to an RO system.

3. b. Shallow wells in many parts of Florida are frequently found to contain the rotten-egg smelling hydrogen sulfide.

4. a. If oxidized by chlorine injection, or even by oxygen from the atmosphere, the sulfides will convert to the elemental sulfur, which can then be removed by media filtration.

5. False. Phosphates, having a strong negative charge, do react with multivalent cations, such as iron, calcium, etc.; however, unless RO feedwater is acidified, phosphates will tend to fall out of solution in reverse osmosis concentrate water because of the increasing concentration of phosphate as well as the usual increase in concentrate water pH.

6. a. Unlike the polyamide and charged polysulfone RO membranes, the functional groups on the ends of the cellulose acetate (CA) membrane polymer chains are not extremely polar. This gives the CA membranes an advantage in that they are less likely to attract charged foulants to the membrane surface.

7. f. Membrane hydrolysis is the natural break-off of the acetyl functional groups from a cellulose acetate membrane structure. It is caused by natural reactions with water but occurs most rapidly at higher pH conditions. The optimum pH for the slowest rate of CA membrane hydrolysis is 4.8. As hydrolysis occurs, the CA membrane will lose its salt rejection capabilities.

8. b. The cellulose acetate (CA) membrane is not appropriate for applications requiring very high pressures. When operated above about 500 psig, the CA membrane tends to compact, which reduces water permeation.

9. a. Special polyamide (PA) membranes that use a thicker PA membrane layer are manufactured for extremely high pressure applications, such as sea water desalination. However, PA thin film membranes have negative charge characteristics that tend to attract cationic type foulants that can increase the membrane fouling rate. Only anionic surfactants should be used to clean a PA membrane system. Also, PA membranes are sensitive to oxidizing agents such as chlorine and iodine. Such oxidants need to be removed by activated carbon prior to continuous exposure to PA membranes.

10. e. Charged polysulfone membranes are chemically sulfonated to affix negatively charged sulfonate groups on the membrane surface. Even though pore sizes in the membrane are large enough to pass salts, the charged anion sites reject anions in the feed stream, which indirectly repels monovalent cations as well due to their attraction to the anions in solution. These membranes have pH tolerance similar to polyamide/thin film membranes, and also a high tolerance to chlorine and other oxidants like cellulose acetate membranes. If exposed to divalent or trivalent cations, such as calcium, magnesium or iron in the water, however, the charged polysulfone membrane will hold onto these ions like an ion exchange softening resin will.

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