Waste Water Engineering Multiple Choice Questions & Answers on “Secondary Clarifiers – 2”.
1. Which of these require a flash mixer and flocculation prior to it?
a) High rate solid contact clarifier
b) Clariflocculator
c) Secondary clarifier
d) Dissolved aeration flotation
Answer: c
Clarification: The secondary clarifier requires a flashmixer prior to it. It also requires flocculation prior to it. In this type of clarifier only clarification takes place.
2. Which of these require a flash mixer prior to it?
a) High rate solid contact clarifier
b) Clariflocculator
c) Secondary clarifier
d) Dissolved aeration flotation
Answer: b
Clarification: A clariflocculator requires a flash mixer prior to it. A clarifier requires both a flash mixer and flocculator prior to it. Clarifiers are used to remove suspended solids.
3. Which of these don’t require a flash mixer and a flocculator prior to it?
a) High rate solid contact clarifier
b) Clariflocculator
c) Secondary clarifier
d) Dissolved aeration flotation
Answer: a
Clarification: High rate solid contact clarifiers don’t require a flash mixer or a flocculator prior to it. This has both the flocculation zone and flash mixing zone inside. It also has a draft tube separating the flocculation zone and the settling zone.
4. Which of these is the most efficient?
a) High rate solid contact clarifier
b) Primary clarifier
c) Clariflocculator
d) Tube settler
Answer: a
Clarification: High rate solid contact clarifier is the most efficient. It removes around 97% of the suspended solids present in the waste water. The capital cost for the same is very high.
5. While designing a clarifier what is the maximum limit velocity that is assumed?
a) 1.5 m/h
b) 2.5 m/h
c) 3 m/h
d) 3.5 m/h
Answer: a
Clarification: While designing a clarifier, the velocity assumed is 1.5 m/h. Based on this the diameter has arrived. Also the diameter for a clarifier would be greater compared to that of a clariflocculator.
6. While designing a clariflocculator what is the maximum limit velocity that is assumed?
a) 1.5 m/h
b) 2.0 m/h
c) 3 m/h
d) 3.5 m/h
Answer: b
Clarification: While designing a clariflocculator, the velocity assumed is 2.0 m/h. Based on this the diameter arrives. Also, the diameter for a clariflocculator would be greater compared to that of a HRSCC.
7. While designing an HRSCC what is the velocity assumed?
a) 1.5 m/h
b) 2.5 m/h
c) 3 m/h
d) 3.5 m/h
Answer: c
Clarification: While designing an HRSCC, the velocity assumed is 3.0 m/h. Based on this the diameter has arrived. Also the diameter for an HRSCC would be smaller than the other clarifier or clariflocculator.
8. While designing a clariflocculator what is the maximum detention time assumed for the flocculation zone?
a) 40 mins
b) 60 mins
c) 90 mins
d) 120 mins
Answer: a
Clarification: While designing a clariflocculator, the time assumed for the flocculation zone is 40 mins. This usually ranges from 10-40 mins. For a better and efficient process generally, the time is chosen as 40 mins.
9. While designing an HRSCC what is the detention time assumed for the draft tube?
a) 60 secs
b) 90 secs
c) 120 secs
d) 150 secs
Answer: a
Clarification: While designing an HRSCC, the detention time assumed for the draft tube is around 60 secs. Based on the detention time and the flow rate of the water, the volume of the draft tube is calculated. From this and the height, the area is calculated.
10. While designing an HRSCC what is the detention time assumed for the detention hood?
a) 60 secs
b) 90 secs
c) 120 secs
d) 150 secs
Answer: c
Clarification: While designing an HRSCC, the detention time assumed for the detention hood is around 120 secs. Based on the detention time and the flow rate of the water, the volume of the detention hood is calculated. From this and the height, the area is calculated.
11. While designing an HRSCC what is the Solid loading rate assumed for the Clarification zone?
a) 1 m/h
b) 2 m/h
c) 0.75 m/h
d) 0.9 m/h
Answer: d
Clarification: While designing an HRSCC, the solid loading rate for the clarification zone is assumed as 0.9 m/h. Area of the clarification zone = Flow/Solid loading rate. From the area, the diameter for the clarification zone is calculated.
12. Calculate the diameter of the draft tube for the following data.
Flow: 1100 m3/h.
Height of the draft tube: 1.5 m
a) 4 m
b) 5 m
c) 6 m
d) 7 m
Answer: a
Clarification: The volume is calculated by multiplying flow x time. (1100 x 60/3600=18.33). The volume is divided by the height to obtain the area. Square root (area x 1.27) =3.98 (rounded off to 4m).
13. Calculate the clarifier volume for a HRSCC for the following data.
Flow: 500 m3/h.
a) 40 m2
b) 90 m2
c) 75 m2
d) 100 m2
Answer: c
Clarification: The retention time is considered as 90 mins for the flocculation zone. The volume is calculated by multiplying retention time and flow. The area is then calculated by dividing the volume by the height. From the area the diameter can be calculated.
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