# Environmental Engineering Questions and Answers – Types of Screens and Aerators

This set of Environmental Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Types of Screens and Aerators”.

1. Which process of water treatment is done to avoid floating debris, branches, trees or other large particles suspended in water?
a) Screening
b) Aeration
c) Primary sedimentation
d) Secondary sedimentation

Explanation: Screens are used as a pretreatment process for purification of water to avoid large size suspended materials from water.

2. What is the size of the coarse screen used in pretreatment of water?
a) 25mm
b) 50mm
c) 75mm
d) 100mm

Explanation: Coarse screens are used in the form of bars of 25mm diameter size and are spaced at 75 to 100mm centers.

3. The coarse screen is inclined at _____ to facilitate the cleaning or raking.
a) 300
b) 350
c) 450
d) 900

Explanation: The coarse screen is inclined at 450-600 so that they can be easily cleaned with a rake. Moreover, it reduces the flow velocity which also helps in cleaning the screen.

4. The head loss through screen depends on _______________
a) Shape of screen elements
b) Open area, block area
c) Approach velocity
d) Nature of their construction, approach velocity

Explanation: The head loss through the screen is given by, h=B (w/b) 4/3h1*sinx Where h=head loss, h1=approach velocity head, b=minimum width, x=angle of rack, B=shape factor. It also depends on the nature of their construction.

5. The shape factor for circular rods in finding the head loss through the screen is _____
a) 1.79
b) 2.42
c) 1.83
d) 1.45

Explanation: The shape factor rectangular bar with sharp edges is 2.42, for semicircular face is 1.83 and for circular rod is 1.79.

6. Micro strainers are used to remove color in water.
a) True
b) False

Explanation: Micro strainers are used for screening stored water which contains plankton, algae and other microscopic particles.

7. Identify the incorrect statement regarding aeration process.
a) It removes taste and odor
b) It increases the dissolved oxygen content of water
c) It increases the carbon dioxide content of water
d) It is used for mixing chemicals with water

Explanation: Aeration reduces the carbon dioxide content and corrosiveness of water and hence, raises the pH value of water.

8. The reduction of carbon dioxide by cascade aerators is in the range of __________
a) 10-20%
b) 20-30%
c) 40-50%
d) 50-60%

Explanation: Cascade aerators are the gravity aerators in which water falls from 1-3m height such that it comes in contact with air and reduces carbon dioxide content to 50-60%.

9. In which type of aerator, the flow of water is divided into fine streams and small droplets?
b) Inclined apron aerator
c) Spray aerator
d) Gravel bed aerator

Explanation: In Cascade aerator, the flow of water is divided into fine streams and small droplets such that they come in contact with air in their trajectories and reduces CO2 by 70-90%.

10. The elapsed time in gravity filter is 1sec. What is the height of fall in a single descent?
a) 4.4m
b) 4.9m
c) 8.8m
d) 9.8m

Explanation: t = 1sec, g = 9.8
Elapsed time, t = (2h/g) 1/2 in a single descent.
Height of fall, H = t2g/2 = 1*1*9.8/2 = 4.9m.

11. ______ are used in fine screen so that they do not get clogged up.
a) Automatic strainers
b) Spray aerators
d) Gravel bed aerators

Explanation: Automatic strainers are self cleaning devices that remove solids from the flowing liquid continuously and help in a fine screen so that it does not get clogged up.

Sanfoundry Global Education & Learning Series – Environmental Engineering.

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