This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Screens and Comminutors – 1”.
1. What is the opening of fine screens?
a) 0.5 inch
b) 1 inch
c) 0.25 inch
d) 0.15 inch
View Answer
Explanation: Fine screens have openings of less than 0.25 in. and are used to remove solids smaller than those retained on bar racks. They are used primarily in water or wastewater containing little or no coarse solids.
2. In drum type screen, which axis does the drum rotate?
a) Horizontal
b) Vertical
c) Irregular
d) Horizo-vertical
View Answer
Explanation: The drum-type screen consists of a cylinder covered by a screen cloth with the drum rotating on a horizontal axis, slightly less than half submerged. Wastewater enters the inside of the drum at one end and flows outward through the screen cloth.
3. Installing screens at an angle allows easier cleaning.
a) True
b) False
View Answer
Explanation: Screens are placed either vertically or at an angle in the flow channel. Installing screens at an angle allows easier cleaning (particularly if by hand) and more screen area per channel depth, but obviously requires more space.
4. Which of the following is true for screens and comminutors?
a) Unit operation
b) Unit process
c) Chemical treatment
d) pH balancer
View Answer
Explanation: The primary treatment incorporates unit operations for removal of floating and suspended solids from the wastewater. They are also referred to as the physical unit operations. The unit operations used are screening for removing floating papers, rages, cloths, plastics, cans stoppers, labels, etc.
5. Screen is the first unit operation in wastewater treatment plant.
a) True
b) False
View Answer
Explanation: Screen is the first unit operation in wastewater treatment plant. This is used to remove larger particles of floating and suspended matter by coarse screening. A set of inclined parallel bars, fixed at certain distance apart in a channel helps to accomplish this.
6. __________ wastewater treatment plant may or may not need the screens.
a) Domestic
b) Industrial
c) Sewage
d) Chemical
View Answer
Explanation: Industrial wastewater treatment plant may or may not need the screens. Screens must be provided even for industrial wastewater treatment plant to separate labels, stopper, cardboard, and other packing materials.
7. How many types of screens are present?
a) 3
b) 4
c) 2
d) 5
View Answer
Explanation: Screens can be broadly classified into two types depending upon the opening size provided as coarse screen (bar screens) and fine screens. There are manually cleaned screens and mechanically cleaned screens based on the cleaning operation.
8. Lower the velocity through the screen, the ________ is the amount of screening that would be removed.
a) Lower
b) Greater
c) Lesser
d) Stronger
View Answer
Explanation: The velocity of flow ahead of and through a screen varies materially and affects its operation. Lower the velocity through the screen, the greater is the amount of screening that would be removed.
9. What is the approach velocity for grit bearing wastewaters?
a) 0.2-0.3 m/s
b) 0.6-0.75 m/s
c) 0.5-0.75 m/s
d) 0.45-0.60 m/s
View Answer
Explanation: The approach velocity of 0.6 to 0.75 m/sec for the grit bearing wastewaters is suggested. Accordingly, the bed slope of the channel should be adjusted to develop this velocity.
10. Which of the following does the quantity of screening does not depend on?
a) Type of rack
b) Type of screen
c) Type of sewer system
d) Temperature
View Answer
Explanation: The quantity of screening depends on the type of rack or screen used as well as sewer system and geographic location. The quantity of screening removed by bar screen is 0.0035 to 0.0375 m3 / 1000 m3 of wastewater treated (Typical value = 0.015 m3 /1000 m3 of wastewater).
11. The head lost by water flowing in a channel as a result of turbulence caused by the velocity of the flowing water is called ________
a) Putrefaction
b) Buffer
c) Head
d) Head loss
View Answer
Explanation: The head, pressure or energy lost by water flowing in a pipe or channel as a result of turbulence caused by the velocity of the flowing water, channel walls or restrictions caused by fittings.
12. What is the maximum allowable head loss for a manual coarse screen?
a) 150mm
b) 300 mm
c) 200 mm
d) 100mm
View Answer
Explanation: The maximum allowable head loss for a manual coarse screen is 150 mm. For a mechanical screen, it ranges from 150-600mm. The maximum approach velocity for a manual screen is 0.3-0.6 m/s.
13. At what angle generally a coarse manual screen is placed?
a) 60 degree
b) 50 degree
c) 65-70 degree
d) 30-45 degree
View Answer
Explanation: The coarse screens are generally placed at 30-45 degree. This holds good only for a manual screen. For a mechanical screen the angle is between 0-30 degrees.
14. Which of these screens can handle very large particles?
a) Reciprocating rake
b) Continuous belt
c) Front clean/Front return
d) Front clean/back return
View Answer
Explanation: Reciprocating rake is a type of mechanically driven screen. This can handle very large particles. This has relatively low operating and maintenance cost.
15. From which of these type of screens there is a possibility of odour emission?
a) Catenary
b) Reciprocating rake
c) Continuous belt
d) Chain driven screen
View Answer
Explanation: Catenary screens have an open design. Due to this construction, there is a possibility that they might emit odour. These screens also remove large objects.
16. Which of these screens have a very large footprint?
a) Reciprocating
b) Continuous belt
c) Catenary
d) Chain driven
View Answer
Explanation: Catenary screens are placed at an angle 45-75 degree. Due to this reason, it has a very large footprint.
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