Waste Water Engineering Multiple Choice Questions on “Reverse Osmosis Design”.
1. The ________ the salt rejection, the better the system is performing.
a) Higher
b) Lower
c) Moderate
d) Poor
Answer: a
Clarification: The higher the salt rejection, the better the system is performing. A low salt rejection can mean that the membranes require cleaning or replacement.
2. What is the osmotic pressure for sea water?
a) 1.5 Mpa
b) 1.8 Mpa
c) 2.3 Mpa
d) 2.45 Mpa
Answer: d
Clarification: The osmotic pressure of sea water is 2.45 Mpa. The concentration of salt in sea water is 32000-35000 mg/l. The applied pressure on the RO equipment is 5.6-6.
3. Membrane permeation rate increases proportionally to effective pressure.
a) True
b) False
Answer: a
Clarification: Although membrane permeation rate increases proportionally to effective pressure, the highest recommended applied pressure of commercial membranes presently available is 7.0 Mpa.
4. RO membranes are made of _______
a) Plastic
b) Cotton
c) Silk
d) Polymer
Answer: d
Clarification: The highest recommended applied pressure of commercial membranes presently available is 7.0 Mpa; beyond which compaction will start to occur due to the fact that RO membranes are made of polymers.
5. Concentrated seawater which is called _________
a) Permeate
b) Brine
c) RO water
d) Salt water
Answer: b
Clarification: When fresh water is separated from seawater, concentrated seawater which is called brine and is retained on the surface of the RO membrane.
6. The osmotic pressure of the brine increases proportionally to the salt concentration.
a) True
b) False
Answer: a
Clarification: The salt concentration and the osmotic pressure of the brine are related proportionally, i.e., the osmotic pressure of the brine increases as the salt concentration increases.
7. The upper limit of brine concentration is ______ times the normal sea water.
a) 1.5
b) 2
c) 2.5
d) 3
Answer: c
Clarification: The upper limit of the brine concentration is usually fixed. The upper limit is normally 1.6-2.5 times that of the normal seawater.
8. The water flux of RO membranes ______ as water viscosity lowers.
a) Increases
b) Decreases
c) Alters
d) Fluctuates
Answer: a
Clarification: When the water viscosity is lowered, the water flux of RO membranes increases and the water viscosity is lowered by an increase in temperature. Thus, the higher the temperature better is the flux.
9. What is the maximum acceptable limit of temperature for RO?
a) 20˚C
b) 30˚C
c) 40˚C
d) 50˚C
Answer: c
Clarification: In order to reduce the effects of temperature to a minimum, the acceptable upper limit is 40°C. RO systems operate at 25°C usually.
10. What is the design temperature of RO systems?
a) 20˚C
b) 25˚C
c) 30˚C
d) 35˚C
Answer: b
Clarification: RO systems are usually designed to operate at 25°C and water flux falls by 20–25 percent when water temperature is lowered by 10°C.
11. What is the operating pH for RO systems?
a) 2-3
b) 4-6
c) 5-7
d) 8-9
Answer: c
Clarification: The pH of seawater is 8–8.5 in general. The pH is reduced to 5-7 in order to reduce scaling on the surface and chemical deterioration of the RO membrane.
12. Calculate the number of membranes required for the following data:
Flow: 535 m3/h.
Area: 400 m2
Flux considered: 12 lmh
a) 112
b) 115
c) 117
d) 120
Answer: a
Clarification: Number of membranes is calculated based on the flow and area. Flux is generally fixed based on the TDS present in the raw water. Number of membranes = Flow x 1000/flux/area. A number of membranes = 535/400/12=112.
13. Calculate the recovery for the following data:
Product Flow: 535 m3/h.
Feed flow : 635 m3/h.
Flux considered: 12 lmh
a) 80%
b) 85%
c) 90%
d) 75%
Answer: b
Clarification: Recovery= Feed flow/Permeate flow. In this case recovery is 85%. This is arrived by 535/635*100=85%.
14. What is the thumb rule to consider for osmotic pressure?
a) 1 psi for every 86 ppm of Na
b) 2 psi for every 86ppm of Na
c) 1 psi for every 43 ppm of Na
d) 2 psi for every 43 ppm of Na
Answer: a
Clarification: The tendency of a pure solution constituting low solute concentration to flow into solution constituting higher solute concentration is known as osmosis. When pressure greater than osmotic pressure is applied then reverse osmosis takes place. The thumb rule for the consideration of osmotic pressure is 1 psi for every 86 ppm of Na.
15. What is the flux considered while designing a reverse osmosis system for treating RO permeate?
a) 30-40 LMH
b) 12-18 LMH
c) 20-25 LMH
d) 25-30 LMH
Answer: a
Clarification: Flux is the amount of water flowing through a particular area. This is dependent both on the nature of water and the membrane surface area. The flux considered for RO permeate is 30-40 LMH.
16. What is the flux considered while designing a reverse osmosis system for treating bore well water?
a) 30-40 LMH
b) 40-50 LMH
c) 20-30 LMH
d) 12-18 LMH
Answer: c
Clarification: The flux increases with increase in pressure. It also increases with increase in temperature. In case of bore well water, the flux considered is 20-30 LMH.
17. What is the flux considered while designing a reverse osmosis system for treating surface water?
a) 15-22 LMH
b) 22-30 LMH
c) 12-15 LMH
d) 30-35 LMH
Answer: a
Clarification: The flux to be considered depends on the nature of the water to be treated. The flux is used to determine the number of membranes required for the treatment system. In the case of treating surface water by a conventional method the flux to be considered is 15-22 LMH.
18. What is the flux considered while designing a reverse osmosis system for treating surface water with UF system?
a) 15-20 LMH
b) 20-30 LMH
c) 12-15 LMH
d) 30-35 LMH
Answer: b
Clarification: The flux to treat surface water by a conventional method and to treat by UF differs. Conventional method includes Dual media filters. In case the surface water is treated by conventional, the flux considered should be around 20-30 LMH.
19. How is Net driving pressure mathematically expressed?
a) Net driving pressure = Pf – 0.5 * DP – Pp -Posmavg
b) Net driving pressure = 0.5 * DP –Pp – Posmavg
c) Net driving pressure = Pf – 0.5 * DP – Pp
d) Net driving pressure = Pf * DP – Pp -Posmavg
Answer: a
Clarification: Net driving pressure = Pf – 0.5 * DP – Pp -Posmavg. The net driving pressure is the pressure required to adequately sustain osmotic pressure. Here Pf is the feed pressure. DP is the pressure drop and Pp is the Permeate pressure.