# Waste Water Engineering Questions and Answers – Fouling and Scaling of Membranes

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Fouling and Scaling of Membranes”.

1. What is the percentage of earth’s salt water?
a) 97%
b) 97.1%
c) 97.2%
d) 97.3%

Explanation: 97.2% of earth’s water is salt water which cannot be used for drinking, agricultural and many industrial purposes. Only 0.6% of earth’s water resources are fresh water and due to the unequal spatial distribution of water reserves and increasing populations, the per capita availability of fresh water is decreasing.

2. The formation of the alkaline scale is a function of the operational temperature.
a) True
b) False

Explanation: In addition to pH, the formation of alkaline scale is a function of the operational temperature. Calcium carbonate may form up to a temperature of 95 °C while Mg(OH)2 mainly forms above temperatures of 95-100°C due to the increase in hydroxyl formation with increasing temperature.

3. What is the average permeating of SiO2 in reverse osmosis?
a) 0.2-0.3
b) 0.4-0.6
c) 0.6-0.8
d) 0.8-1

Explanation: SiO2 scaling has been observed in reverse osmosis permeators despite a silica scaling index of only 0.6-0.8 i.e. the concentrate is under saturated with Si02.

4. ________ is a process whereby a solution or a particle is deposited on a membrane surface or in membrane pores.
a) Deposition
b) Absorption
c) Membrane fouling
d) Membrane accumulation

Explanation: When a solution or a particle is deposited on a membrane surface or in membrane pores it is called membrane fouling. It usually occurs in processes such as membrane bioreactor, ultrafiltration, microfiltration or nanofiltration.

5. What is the average pore diameter of NF270?
a) 0.52 nm
b) 0.64 nm
c) 0.76 nm
d) 0.84 nm

Explanation: The average pore diameter of NF270 is 0.84 nm, the percentage of sodium ion rejection is 35.8% and that of calcium ion rejection is 57.5%.

6. What is the surface roughness of the BW30 membrane?
a) 60 nm
b) 62.6 nm
c) 64.5 nm
d) 66.2 nm

Explanation: The surface roughness of BW30 is 62.6 nm and that of NF270 is 4.1 nm. The sodium ion rejection is 35.8% and that of calcium ions is 57.5%.

7. What is the surface roughness of NF270 membrane?
a) 3.6 nm
b) 4.1 nm
c) 5.3 nm
d) 6.8 nm

Explanation: The surface roughness of NF270 membrane is 4.1 nm and that of BW30 is 62.6 nm. The percentage of sodium ion rejection is 35.8% and that of calcium ion rejection is 57.5%.

8. Operating conditions also affect fouling of the membrane.
a) True
b) False

Explanation: membrane fouling is influenced by numerous factors such as system hydrodynamics, operating conditions, membrane properties, and material property (solute).

9. Which of the following factors does membrane fouling does not depend on?
a) Solute
b) Operating conditions
c) Climate
d) Hydrodynamics

Explanation: At low pressure, low feed concentration, and high feed velocity, concentration polarisation effects are minimal and flux is almost proportional to trans-membrane pressure difference.

10. Which of these pollutants cause biofilm formation?
a) Calcium sulphate
b) Silica
c) Calcium carbonate
d) Oxides

Explanation: Fouling is caused by choking of the membranes. This reduces the efficiency of the membranes. Oxides forms biofilm which fouls the membranes.

11. Which of these do not cause scaling of the membranes?
a) Organic and inorganic colloids
b) Calcium sulphate
c) Silica
d) Calcium carbonate

Explanation: The organic and inorganic colloids do not cause scaling of the membranes. Scaling is caused by silica and calcium salts. This is also caused by metal oxide formation.

12. How can biofilm formation be prevented?
a) By installing a microfilter prior to Reverse Osmosis
b) By installing a nanofilter prior to Reverse Osmosis
c) By installing a ultrafilter prior to Reverse Osmosis
d) By installing a pressure sand filter prior to Reverse Osmosis

Explanation: Biofilm formation causes fouling in membranes. Fouling reduces the efficiency of membranes. This can be reduced by installing a microfilter prior to Reverse Osmosis.

13. How can scaling be prevented?
a) By adding sodium hydroxide
b) By adding lime
c) By adding HCl
d) By adding alum

Explanation: Scaling is another factor which results in fouling. This can be prevented by adding HCl. Adding acids prevents fouling.

14. What is the SDI assumed for nanofiltration?
a) 4-6
b) 0-2
c) 2-4
d) 6-8

Explanation: The SDI assumed for nanofiltration is 0-2. The SDI is the silt density index. SDI is a measure of the fouling index.

15. What is the SDI assumed for reverse osmosis spiral wound?
a) 0-2
b) 5-9
c) 9-13
d) 13-15

Explanation: The SDI assumed for reverse osmosis is 0-2. The SDI is the silt density index. SDI is a measure of the fouling index.

16. What is the SDI assumed for reverse osmosis hollow fibre?
a) 4-6
b) 0-3
c) 6-8
d) 8-10

Explanation: The SDI assumed for reverse osmosis hollow fibre is 0-2. The SDI is the silt density index. SDI is a measure of the fouling index.

17. What is the MFI in s/L2 assumed for nanofiltration?
a) 0-10
b) 20-25
c) 12-15
d) 15-20

Explanation: MFI is known as the modified fouling index. This is an index to measure membrane fouling. It is measured in s/L2. The MFI in s/L2 assumed for nanofiltration is 0-10.

18. What is the MFI in s/L2 assumed for reverse osmosis hollow fibre?
a) 0-2
b) 6-8
c) 8-12
d) 12-16

Explanation: MFI is known as a modified fouling index. This is an index to measure membrane fouling. It is measured in s/L2. The MFI in s/L2 assumed for reverse osmosis for the spiral wound is 0-2.

19. What is the MFI in s/L2 assumed for reverse osmosis spiral wound?
a) 2-6
b) 6-10
c) 0-2
d) 14-18

Explanation:MFI is known as a modified fouling index. This is an index to measure membrane fouling. It is measured in s/L2. The MFI in s/L2 assumed for reverse osmosis for the spiral wound is 0-2.

Sanfoundry Global Education & Learning Series – Waste Water Engineering.

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