Environmental Engineering Problems on “Design Elements of Sedimentation”.
1. The detention period of a rectangular tank is given by _______
a) t0 = LBH/Q
b) t0 = LB/HQ
c) t0 = Q/LBH
d) t0 = HQ/LB
Answer: a
Clarification: The detention period of a rectangular tank is given by t0 = LBH/Q where L is the length, B, is the breadth and H is the height of rectangular tank and Q = discharge rate.
2. The average time required by water to pass through the settling tank is called _______
a) Detention time
b) Flowing through period
c) Time of flow
d) Mean time
Answer: b
Clarification: The average time required by water to pass through the settling tank is called ‘flowing through period’ which is always less than the detention period.
3. Range of Displacement efficiency in the plain sedimentation tank is ___________
a) 0.25-0.5
b) 0.1-0.2
c) 0.5-0.8
d) 0.3-0.6
Answer: a
Clarification: Displacement efficiency is defined as the ratio of flowing through period to the detention period and varies from 0.25 to 0.5 in the plain sedimentation tank.
4. The quantity of water flowing per hour per unit horizontal area is called ________
a) Detention time
b) Flowing through period
c) Displacement time
d) Overflow rate
Answer: d
Clarification: Overflow rate = Q/A where Q is discharge rate in litre per day, A is the horizontal area in m2.
5. Settling tank efficiency is reduced by _______
i. Eddy current
ii. Surface current
iii. Density current
a) i, ii
b) i, ii, iii
c) ii, iii
d) i, iii
Answer: b
Clarification: Eddy current is set up by the inertia of the incoming fluid, surface current is induced due to wind in open tank and density current cause cold water to under run the warm water to flow across its surface.
6. 1cm per second settling velocity corresponds to a surface loading of ______ litre per day per m2.
a) 36000
b) 864000
c) 24000
d) 6000
Answer: b
Clarification: Surface loading = 864000*settling velocity = 864000*1=864000litre per day per m2 where settling velocity is in cm/Sec.
7. What will be the surface area of the basin for a flow of 2*106 litre per day having a surface loading rate 10,000 litre/day/m2?
a) 100
b) 200
c) 300
d) 400
Answer: b
Clarification: Surface area of basin = flow of water/surface loading rate = 2000000/10000=200m2.
8. The detention period of the vertical flow clarifier is _______
a) 1-1.5
b) 2
c) 3.5
d) 4
Answer: a
Clarification: Typical design value of the detention period for the vertical flow clarifier is between 1 to 1.5 hours. They normally remove the flocculent.
9. The design value of surface loading for horizontal flow circular sedimentation is _______
a) 30-40m3/m2/d
b) 20-30m3/m2/d
c) 10-20m3/m2/d
d) 50-100m3/m2/d
Answer: a
Clarification: For the design of horizontal flow circular sedimentation, the surface loading rate is 30-40m3/m2/d and its design detention period period is 2-2.5 hours.
10. Detention time for a circular tank is given by _______
a) t = D (D + 0.785H) /Q
b) t = D (0.1D + 0.785H) /Q
c) t = D2 (0.01D + 0.785H)
d) t = D2 (0.01D + 0.785H) /Q
Answer: d
Clarification: Detention time for a circular tank is given by t = D2 (0.01D + 0.785H) /Q where D is the diameter of the tank, H is the vertical depth of wall and Q is the discharge rate.
11. The sedimentation basin should be made as deep as possible.
a) True
b) False
Answer: b
Clarification: The particle settling efficiency is independent of the depth of the basin and should be made as shallow as possible.
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