Waste Water Engineering Multiple Choice Questions on “Aerated Grit Chambers”.
1. In aerated grit chambers, there are no moving parts below the water surface.
a) True
b) False
Answer: a
Clarification: Aerated grit chambers consist of extremely simple design. It consists of no moving parts and air lifting pumps can be used as well as air blowers to pump air. Maintenance is significantly reduced.
2. A grit chamber is usually installed ______ primary sedimentation tanks.
a) Before
b) After
c) In between
d) In
Answer: a
Clarification: A grit chamber is usually installed before primary sedimentation tanks and it is sensible to say that it should be placed well before wastewater pumps. There are basically three different types of grit chambers.
3. At what velocity will the particles settle down?
a) 0.5 m/s
b) 0.3 m/s
c) 0.1 m/s
d) 0.25 m/s
Answer: b
Clarification: The design of a horizontal flow type is such that as wastewater flows through in horizontal direction at a certain velocity (0.3m/s) particles will start to settle at the channel before reaching the outlet point.
4. Grit chamber is constructed to protect the further mechanical equipment of the water treatment plant.
a) True
b) False
Answer: a
Clarification: Grit chamber is needed to protect major mechanical equipment from wear and tear including damage. Other than that, it also ensures that formation of deposit especially in pipelines can be avoided and overall this can save the time that otherwise would be spent to clean the accumulation of waste.
5. Grit chambers are constructed to prevent their accumulation in _______
a) Sedimentation tank
b) Sludge digesters
c) Coagulation tank
d) Sand filter
Answer: b
Clarification: Grit chamber is needed to protect major mechanical equipment from wear and tear including and remove the inorganic particles to prevent damage to the pumps and to prevent their accumulation in sludge digesters.
6. What is the cleaning period for manual grit chambers?
a) 3 days
b) 5 days
c) 48 hours
d) 1 week
Answer: d
Clarification: Manually cleaned grit chambers should be cleaned at least once a week. The simplest method of cleaning is by means of shovel. The grit washing mechanisms are also of several designs most of which are agitation devices using either water or air to produce washing action.
7. Grit chamber is similar to which of the following?
a) Screening unit
b) Sedimentation tank
c) Coagulation tank
d) Sand bed
Answer: b
Clarification: Sedimentation tanks and grit chambers are similar, designed to separate the intended heavier inorganic materials (specific gravity about 2.65) and to pass forward the lighter organic materials.
8. What is the specific gravity of settleable particles in the grit chamber?
a) 2.98
b) 2.56
c) 2.65
d) 2.89
Answer: c
Clarification: The specific gravity of heavier particles is 2.65. Grit chambers are nothing but like sedimentation tanks, designed to separate the intended heavier inorganic materials (specific gravity about 2.65) and to pass forward the lighter organic materials.
9. Stoke’s law holds good for Reynolds’s number ______
a) Less than 1
b) Equal to 1
c) Greater than 1
d) Nearer to 1
Answer: a
Clarification: The settling velocity of discrete particles can be determined using an appropriate equation depending upon Reynolds number. Stoke’s law holds good for Reynolds number, Re below 1.
10. What is the horizontal velocity considered for grit chamber?
a) 0.25-0.4m/sec
b) 0.6-0.9 m/sec
c) 1-2 m/sec
d) 3-4m/sec
Answer: a
Clarification: The horizontal velocity considered while designing a grit chamber is 0.25-0.4m/sec. In most cases, it is considered as 0.3 m/sec. The setting velocity for the removal of 21mm material is 1-1.3m/ sec.
11. What is the detention time considered for aerated grit chamber?
a) 5-6 mins
b) 2-5 mins
c) 1-2 mins
d) 3-4 mins
Answer: b
Clarification: The detention time for aerated grit chambers is 2-5 mins. Usually, in most cases, a detention time of 3 mins is considered. The release of VOC is one application of these aerated grit chambers.
12. Which of these is not a grit removal equipment installed in the aerated grit chamber?
a) Screw conveyers
b) Tubular conveyers
c) Jet pumps
d) Communitors
Answer: d
Clarification: Communitors are used to grind the materials. These are prior to a screen. Screw conveyers, tubular conveyers and jet pumps are used to remove the grit.
13. What is the amount of air supplied for these grit chambers?
a) 1-2 m3/m.min
b) 0.2-0.5 m3/m.min
c) 0.5-0.8 m3/m.min
d) 0.8-1 m3/m.min
Answer: b
Clarification: The air supplied for an aerated grit chamber ranges from0.2-0.5 m3/m.min. There are air diffusers installed in these chambers. These aid in diffusing the air supplied to the chamber.
14. At what depth are the air diffusers placed in the grit chamber?
a) 0.6-0.9 m
b) 1-2 m
c) 0.1-0.4m
d) 0.45-0.6 m
Answer: d
Clarification: The air diffusers are placed at 0.45-0.6m at the chamber. Influent and effluent baffles are also placed at the chamber. These improve the grit removal efficiency.
15. What is the amount of grit quantities captured by the aerated grit chamber?
a) 0.004-0.20 m3/103.m3
b) 0.25 m3/103.m3
c) <0.004 m3/103.m3
d) 0.5 m3/103.m3
Answer: a
Clarification: The amount of grit quantities captured by the aerated grit chamber ranges from 0.004-0.20 m3/103.m3.Usually it is in the range 0.015 m3/103.m3. In terms of US unit the typical value is 2 ft3/Mgal.
16. Usually while designing what is the range of the width of the aerated grit chamber assumed?
a) 0.5-2 m
b) 8-10 m
c) 2.5-7.0m
d) 12m
Answer: c
Clarification: While designing an aerated grit chamber the width is assumed in the range 2.5-7.0m. The ratio of the width: depth is 1:1 to 5:1. Typical ratio is generally assumed as 1.5:1.
17. Usually while designing what is the range of length of the aerated grit chamber assumed?
a) 7.5-20
b) 4-6 m
c) 25 m
d) 50 m
Answer: a
Clarification: While designing an aerated grit chamber the width is assumed in the range 7.5-20 m. The ratio of the length: width is 3:1 to 5:1. Typical ratio is generally assumed as 4:1.
18. Usually while designing what is the range of depth of the aerated grit chamber assumed?
a) 0.5-2 m
b) 8-10 m
c) 2-5 m
d) 12 m
Answer: c
Clarification: While designing an aerated grit chamber the width is assumed in the range 2-5 m. When expressing the same in US unit it is 7-16 ft. The release of VOC by theses aerated grit chambers is one thing which we have to consider and take proper measures to reduce the same.
19. Aerated grit chambers are usually designed to remove particles in which range of size?
a) 0.1mm diameter
b) 0.35 mm diameter
c) 0.21 mm diameter
d) 0.41 mm diameter
Answer: c
Clarification: The aerated grit chambers are designed to remove particles in the range 0.21mm diameter. The cross section of these tanks is similar to that of an activated sludge process tank. The only difference is that these chambers have hoppers of about 0.9m in depth.