# Waste Water Engineering Questions and Answers – Sedimentation Tanks – 2

This set of Waste Water Engineering Interview Questions and Answers for Experienced people focuses on “Sedimentation Tanks – 2”.

1. In a primary sedimentation tank in percentage what is the amount of TSS removed?
a) 40-50%
b) 50-70%
c) 80%
d) 85%

Explanation: A primary sedimentation tank removes around 50-70% of TSS. This is a type of preliminary treatment. This is usually followed after the screens.

2. In a primary sedimentation tank in percentage what is the amount of BOD removed?
a) 40-50%
b) 20-40%
c) 60-70%
d) 20-30%

Explanation: In a primary sedimentation tank, around 20-40% of BOD is removed. This primary sedimentation is prior to the clarifiers and flocculators. Also, it is classified as preliminary treatment.

3. What is the detention time considered while designing a primary sedimentation tank which is followed by a secondary treatment tank?
a) 1hr
b) 1.5-2.5hrs
c) 3hrs
d) 3-4hrs

Explanation: The detention time considered while designing a primary sedimentation tank is 1.5-2.5 hrs. This majorly depends on the amount of TSS and BOD present in the raw waste water. In most cases, it is considered as 2 hrs.

4. What is the detention time considered while designing a primary sedimentation tank which is followed by an activated sludge treatment tank?
a) 1hr
b) 1.5-2.5hrs
c) 3hrs
d) 3-4hrs

Explanation: The detention time considered while designing a primary sedimentation tank, which is followed by activated sludge treatment tank is 1.5-2.5 hrs. This majorly depends on the amount of TSS and BOD present in the raw waste water. In most cases, it is considered as 2.5 hrs.

5. What is the weir loading rate assumed while designing a primary sedimentation tank which comes prior to a secondary settling tank?
a) 125-500 m3/m.d
b) 50-100 m3/m.d
c) 25-50 m3/m.d
d) >500 m3/m.d

Explanation: The weir loading rate for a primary sedimentation tank is assumed as 125-500 m3/m.d. Rectangular tanks have bridge mechanisms which is used for cleaning. These also have a chain and solid collector to clean the tank.
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6. What is the range of width assumed while designing a rectangular tank?
a) 1-3 m
b) 3-24m
c) 24-48m
d) 48-60m

Explanation: The width of the rectangular tank is assumed as 3-24m while designing it. In the case of width is greater than 6m than more people have to be employed to clean the tank. The typical value assumed is 5m.

7. While designing a rectangular primary sedimentation tank in what range the length should be assumed?
a) 10-12m
b) 15-90m
c) 100-150m
d) 150-200m

Explanation: While designing a rectangular primary sedimentation tank, 15-90m is the range in which the length of the tank should be assumed. The typical depth value for a rectangular tank is around 4.3 m.

8. What is the range of diameter assumed while designing a circular tank?
a) 60-80m
b) 3-60m
c) 80-150m
d) 150-300m

Explanation: The diameter of the circular tank is assumed as 3-60 m while designing it. In the US units it is around 40-150 ft. The typical value assumed is in between12-45 m.

9. What is the flight speed assumed while designing a circular tank?
a) 0.01r/min
b) 0.02-0.05 r/min
c) 0.06 r/min
d) 0.06-0.09 r/min

Explanation: The range of diameter assumed while designing a circular tank is 0.02-0.05 r/min. The typical value for the circular tank would be 0.03 r/min. The typical value for bottom slope would be 1/12mm/mm.

10. What is the flight speed assumed while designing a rectangular tank?
a) 0.1r/min
b) 0.2-0.5 r/min
c) 0.6-1.2 r/min
d) 1.2-1.5 r/min

Explanation: The range of diameter assumed while designing a circular tank is 0.6-1.2 r/min. The typical value for the circular tank would be 0.9 r/min. The velocity in these tanks should be maintained between 3-9 m/min.

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