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January 2011, Vol. 23, No.1
Certification Quiz
Test your knowledge of membranes
True or False Questions:
- Transmembrane pressure (TMP) is defined as the difference between the feed pressure and the permeate pressure.
- A gradual decrease in TMP usually indicates membrane fouling.
- Oxidants, such as sodium hypochlorite, are typically used to remove scaling from the membranes.
- Acids are typically used to remove inorganic foulants from the membranes.
- In a membrane bioreactor (MBR) system, the activated sludge process does not affect the operational strategy of the membrane system.
Multiple Choice Questions:
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Membrane cleaning solutions are typically prepared by mixing a cleaning chemical — sodium hypochlorite, citric acid, etc. — with what?
A. Potable water.
B. Nonpotable water.
C. Groundwater.
D. Membrane permeate.
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In an immersed MBR, what is the main purpose of the membrane air scour?
A. To maintain a consistent solids concentration in the membrane tank.
B. To continuously remove solids from the membrane surface.
C. To provide oxygen for the biological process.
D. To create foam.
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Cartridge filters, commonly used as pretreatment for nanofiltration (NF) or reverse-osmosis (RO) membranes, should be replaced when the pressure loss across the cartridge filters exceeds what value? (Note: System feed pressure is greater than 150 lb/in.2.)
A. 1 lb/in.2.
B. 3 lb/in.2.
C. 15 lb/in.2.
D. 50 lb/in.2.
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Symptoms of O-ring or seal failure in pressure membrane systems can be what?
A. A sharp increase in TMP and a sharp decrease in rejection.
B. A gradual increase in TMP and no change in rejection.
C. A sharp decrease in TMP and a sharp decrease in rejection.
D. A gradual decrease in TMP and no change in rejection.
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Cleaning the membranes should cause the permeability/specific flux to do what?
A. Increase.
B. Decrease.
C. Not change.
D.
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Which of the following methods cannot be used to continuously monitor membrane integrity for MBR, microfiltration, ultrafiltration, NF, and RO systems?
A. Permeate turbidity.
B. Permeate temperatures.
C. Permeate conductivity.
D. Permeate particle count.
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What is the maximum silt density index (SDI) in the feed water recommended by manufacturers of spiral-wound membranes?
A. 0.5.
B. 1.
C. 5.
D. 10.
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Calculate the SDI (ASTM D4185), given the following information:
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Ti, time to collect initial 500 mL
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85 seconds
|
|
Tf, time to collect second 500 mL
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100 seconds
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Tt, total test time
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15 minutes
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A. 0.1.
B. 1.0.
C. 1.2.
D. 5.0.
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Calculate the membrane permeability (gal/ft2•d per lb/in.2) for a pressurized MF membrane system, at reference conditions/temperature, given the following information:
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Feed flow = 1000 gal/min
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Average feed pressure = 60 lb/in.2
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Permeate flow = 850 gal/min
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Permeate pressure = 15 lb/in.2
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Membrane area = 50,000 ft2
|
|
A. 0.40 gal/ft2•d per lb/in.2.
B. 0.48 gal/ft2•d per lb/in.2.
C. 0.54 gal/ft2•d per lb/in.2.
D. 0.64 gal/ft2•d per lb/in.2.
Tony Greiner is a senior associate in the Charlotte, N.C., office of Hazen and Sawyer (New York).
Answer Key:
Click here to show or hide the answer key
1. True.
2. False. A gradual increase in TMP is one of the indicators that membrane fouling is occurring; other process parameters (such as flux, specific flux/permeability) also should be reviewed.
3. False. Oxidants are used to remove biological growth/fouling. Acids remove scaling.
4. True.
5. False. The characteristics of the mixed liquor can directly affect the membrane operation, affecting the flux rate, fouling potential, and cleaning strategy/frequency.
6. d. Using water other than the membrane permeate is generally not recommended, and operations staff should consult with the membrane manufacturer prior to doing so.
7. b. The air scour does cause the effects listed in answers a, c, and d; however, these are secondary effects and in some instances detrimental.
8. c.
9. c.
10. a. Cleaning should restore most of the membrane permeability; membrane permeability gradually decreases over time due to irreversible/irrecoverable fouling.
11. b.
12. c. The maximum SDI recommended is 5; however, in general practice, an SDI of less than 3 is preferred.
13. b. SDI = (100 × [1 – Ti ÷ Tf]) ÷ Tt. So, SDI = (100 × [1 – 85 seconds ÷ 100 seconds]) ÷ 15 minutes = 1.0.
14. c. Membrane permeability = flux ÷ driving pressure. First, calculate flux. Flux = (permeate flow × 1440 min/d) ÷ membrane area. So, flux = 850 gal/min × 1440 min/d) ÷ 50,000 ft2 = 24.48 gal/ft2•d. Next, calculate permeability. Permeability = flux ÷ driving pressure. Driving pressure = feed pressure – permeate pressure. So, permeability = 24.48 gal/ft2•d ÷ (60 lb/in.2 – 15 lb/in.2) = 0.54 gal/ft2•d per lb/in.2. (Note that typically the flux would have to be corrected to the reference temperature prior to proceeding with Step 2.)
References:
American Water Works Association (2005). Microfiltration and Ultrafiltration Membranes for Drinking Water, Manual of Water Supply Practices M53, First Edition. Denver: American Water Works Association.
American Water Works Association (2007). Reverse Osmosis and Nanofiltration, Manual of Water Supply Practices M46, Second Edition. Denver: American Water Works Association.
Water Environment Federation (2006). Membrane Systems for Wastewater Treatment. Alexandria, Va.: Water Environment Federation.
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