Waste Water Engineering Multiple Choice Questions on “Sand Filters – 1”.
1. What is the size of the small rapid sand filter bed?
a) 50 sqm
b) 80 sqm
c) 100 sqm
d) 120 sqm
Answer: c
Clarification: The size of the small rapid sand filter is 100 sqm. The minimum size required is 100 sqm for the efficient working of the sand filter with a gravel depth of 500 mm and 0.7-1 m of sand.
2. Pre-treatment of beds is not necessary.
a) True
b) False
Answer: b
Clarification: The penetration of suspended solids is deep through the filter bed. The length of run is two days. Before starting the operation again, pre-treatment of the rapid sand filter beds is done by coagulation.
3. What is the percentage of bacterial removal in filter beds?
a) 60-70 %
b) 75-80 %
c) 80-90 %
d) 90-99 %
Answer: d
Clarification: Practically no biological action occurs but some nitrification occurs where the filtration velocity is relatively slow, where the oxygen content is adequate and where the nitrifying bacteria find favourable nutrients in the water. Bacterial removal efficiency is 90-99 %.
4. What is the effective size of sand used in rapid sand filters?
a) 0.2-0.3 mm
b) 0.3-0.6 mm
c) 0.6-1.2 mm
d) 12-2.4 mm
Answer: c
Clarification: The size of the small rapid sand filter bed is 100 sqm. Gravel and sand are used in layers in the bed. The effective size of the sand particles used in the filter bed lies between 0.6-1.2 mm.
5. What is the head loss found in a rapid sand filter?
a) <3
b) >3
c) 0
d) <4
Answer: b
Clarification: When the head loss becomes excessive and beyond 1.5-2.5 m, the filter should be cleaned. The maximum head loss experienced in rapid sand filters is usually 3 metres.
6. What is the surface wash rate?
a) 5-6 cum/sqm h
b) 6-7 cum/sqm h
c) 8-10 cum/sqm h
d) 10-12 cum/sqm h
Answer: d
Clarification: The specified minimum backwash time for a rapid filter is 5 minutes. The amount of water required to wash a rapid filter may vary from 3-6% of the total amount of water filtered. The surface wash rate is usually 10-12 m3/m2 h.
7. What is the porosity range during expansion of the filter bed?
a) 0.4-0.5
b) 0.5-0.6
c) 0.1-0.2
d) 0.68-0.7
Answer: d
Clarification: There are different types of backwashing recommended for the removing of clogged particles from the filter bed. During backwashing, the filter bed expands and the porosity is the range of 0.68-0.7.
8. Even by high-rate of backwash, efficiency of cleaning is poor.
a) True
b) False
Answer: a
Clarification: There are different types of backwashing. High and low rates and water backwashing is done and air scour is carried out prior to water backwashing in few cases and efficiency of cleaning is poor.
9. What is the air scour rate which is provided before a water backwash?
a) 12 cum/ sqm h
b) 15 cum/ sqm h
c) 20 cum/ sqm h
d) 27 cum/ sqm h
Answer: d
Clarification: Different types of backwashing. High and low rates and water backwashing is done and air scour is carried out prior to water backwashing in few cases and the air scours rate is 27 m3/m2 h.
10. What is the depth of washing during high-rate water backwash?
a) 3-4 mm
b) 3-6 mm
c) 2-3 mm
d) 1-2 mm
Answer: b
Clarification: During high rates of backwashing by water the depth of bed that usually is cleaned is 3-6 mm. 2-3 mm of depth is cleaned by air scour which is followed by low and high rates of water backwashing.
11. What does sand filter remove?
a) TDS
b) TSS
c) BOD
d) COD
Answer: b
Clarification: Sand filters remove TSS. This usually constitutes gravel, pebbles and sand as the medium. This is usually prior to an Activated Carbon Filter (ACF).
12. What is the TSS at the outlet of the Pressure Sand Filter (PSF)?
a) <10 ppm
b) 25 ppm
c) 100 ppm
d) >150 ppm
Answer: a
Clarification: At the outlet of the filter the TSS is <10 ppm. In case of Side stream filter, we can get an outlet of <5ppm. In case of Dual Media Filter we can get an outlet of <1ppm.
13. While designing a sand filter what is the velocity that is assumed?
a) 10-12 m/h
b) 15 m/h
c) 20 m/h
d) 30 m/h
Answer: a
Clarification: The rise velocity assumed is 10-12 m/hr. Based on this the sizing of the filter is carried out. Based on this the diameter of the vessel is derived.
14. While designing a dual media filter what is the velocity that is assumed?
a) 10-12 m/h
b) 15-18 m/h
c) 20 m/h
d) 30 m/h
Answer: b
Clarification: The rise velocity assumed is 15-18 m/hr. Based on this the sizing of the filter is carried out. Based on this the diameter of the vessel is derived.
15. While designing a multimedia filter what is the velocity that is assumed?
a) 10-12 m/h
b) 15-18 m/h
c) 18-20 m/h
d) 30 m/h
Answer: c
Clarification: The rise velocity assumed is 18-20 m/hr. Based on this the sizing of the filter is carried out. Based on this the diameter of the vessel is derived.
16. Calculate the diameter of a sand filter with the following data.
Flow: 2200 m3/hr
Velocity: 12 m/hr
a) 15.3 m
b) 15.5 m
c) 16.3 m
d) 16.5 m
Answer: a
Clarification: The calculated diameter is 15.3 m. Area is first calculated by dividing the flowrate with velocity. That is 2200/12 =184 m2. Then square root (184 x (4/3.14)) = 15.3 m.