# Waste Water Engineering Questions and Answers – Screens and Comminutors – 2

This set of Waste Water Engineering Questions and Answers for Experienced people focuses on “Screens and Comminutors – 2”.

1. With respect to the operation which of these screens is the most expensive?
a) Chain driven
b) Continuous belt
c) Catenary
d) Reciprocating

Explanation: For a continuous belt replacement of screen is a time consuming process. Also this is a very expensive process. Hence the Continuous belt screen is considered the most expensive type of screen.

2. Which of these type of screen can be used both as a fine and coarse screen?
a) Continuous belt
b) Catenary
c) Reciprocating
d) Chain driven

Explanation: A continuous belt type screen can be used both as a fine and coarse screen. This is because the screen openings range from 0.5-30 mm. This is a continuous self- cleaning screen.

3. What is the maximum allowable approach velocity for a bar screen at peak flow rates?
a) >0.9 m/s
b) <0.9m/s
c) 1-2 m/s
d) 2.5m/s

Explanation: The approach velocity at peak flow rates for a bar screen should not exceed 0.9m/s. This is to prevent the pass-through of debris. These screens should be designed in such a way to prevent accumulation of debris or grit.

4. What should be installed in order to control the velocity of the bar screens?
a) Sluice gate
b) Grit chamber
c) Parshall flume
d) Hoppers

Explanation: Parshall flumes are installed at the downstream of a bar screen. These are installed in order to control the approach velocity of a bar screen. The approach velocity is a very important factor while designing the screens to prevent the accumulation of grit and debris.

5. What is the mathematical representation of hydraulic loses through a screen?
a) hL= 1/C(V2/2g)
b) hL= 1/C(V2-v2/2g)
c) hL= 1/C(V2-v2/g)
d) hL= 1/C(V2/g)

Explanation: The hydraulic loss/ head loss is represented as hL= 1/C(V2-v2/2g) .Where hL is the head loss of the screen and C is the empirical discharge coefficient. V is the velocity of the bar screen through the openings, v is the approach velocity in the upstream and g is the acceleration due to gravity (9.81m/s).
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6. Which of these screens is used to remove secondary suspended solids?
a) Medium drum rotary screen
b) Fine drum rotary screen
c) Tangential fine screen
d) Horizontal reciprocating screen

Explanation: Fine drum rotary screens are used to remove secondary suspended solids. The mesh is generally made up of stainless steel. The screen cloth would be polyester material.

7. What is the headloss for fine screens?
a) 0.8-1.4m
b) 2-2.5m
c) 3-3.5m
d) <0.8 m

Explanation: The head loss for fine screens would be 0.8-1.4m. In many applications fine screens is limited to plants where head loss through screens is not a problem. Water sprays are supplied in order to keep the screens clean continuously.

8. What is the amount of BOD removed by a fixed parabolic fine screen?
a) 25-40 %
b) 40-50%
c) 5-20%
d) 30-40%

Explanation: The amount of BOD removed by a fixed parabolic fine screen is 5-20%. The amount of TSS is removed 5-30%. Compared to a rotary drum fine screen, the efficiency of these screens is low.

9. How is the headloss for a fine screen calculated?
a) hL= 1/2g(Q/CA)2
b) hL= 1/g(Q/CA)2
c) hL= 1/2g(Q/C)2
d) hL= 1/2g(Q/CA)

Explanation: The headloss for a fine screen is calculated as hL= 1/2g (Q/CA)2. Where hL is the headloss through the fine screen. Q is the flow rate, A is the effective open are of the submerged screen.

10. What is the size of the openings for a microscreen?
a) 35-50µm
b) 10-35µm
c) 50-60µm
d) 60-6µm

Explanation: The size of the openings for a microscreen range from 10-35µm. Microscreens utilize the use of variable low speed. This speed is up to 4r/min.

11. What is the removal efficiency of TSS in case of microscreens?
a) 80-85%
b) 85-90%
c) <10%
d) 10-80%

Explanation: The removal efficiency of the TSS by microscreens is around 10-80%. Usually it is around 55 % in most of the cases. Microscreens can’t handle solid fluctuations.

12. What is the typical headloss through the microscreens?
a) 150-200mm
b) 75-150mm
c) <75mm
d) >200mm

Explanation: The typical headloss through the microscreens is 75-150mm. In case the headloss exceeds 200 mm then bypasss should be provided. The mesh is generally made up of stainless steel.

a) 1-3 m3/m2.min
b) 7-9 m3/m2.min
c) 3-6 m3/m2.min
d) 10-12 m3/m2.min

Explanation: The hydraulic loading rate for a microscreen is 3-6 m3/m2.min. The hydraulic loading rate is based on the submerged surface area of the drum. The drum diameter is 2-2.5 m.

14. Comminutors are usually installed in which size of water treatment plants?
a) 0.2 m3/sec
b) <0.2 m3/sec
c) 0.5 m3/sec
d) 1 m3/sec

Explanation: Comminutors are usually installed in plants which are <0.2 m3/sec. These are installed in order to shred material. These require high operating and maintenance cost.

15. What is the headloss through a communitor?
a) 0.3-0.4 m
b) 0.1-0.3 m
c) 0.5-0.6 m
d) 0.6-0.7 m

Explanation: The headloss through the communitor is 0.1-0.3m. In large units it can approach 0.9m. These may be preceded by grit chambers.

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