Waste Water Engineering MCQ - Unit Operations and Unit Processes Part-2

1. Which type of mixer is used when the flow needs to be changed often?
a) Paddle mixer
b) Static mixer
c) Mechanical mixer
d) Mechanical aerator

Answer: b
Explanation: When the flow needs to be changed /reversed often static mixers are used. These can be comprised of over and under narrow channels. The flow reversals in these mixers produce the energy required for flocculation

2. In order to prevent clogging what should be provided prior to tube settlers?
a) Clarifiers
b) Coarse screen
c) Fine screen
d) Grit chamber

Answer: c
Explanation: Fine screens are placed before a tube settler. This is done in order to prevent clogging. Tube settlers are placed at an angle of 45-60 degree.

3. Which of these is not a type of grit chamber
a) Horizontal flow
b) Vortex
c) Vertical flow
d) Aerated

Answer: c
Explanation: Grit chambers are of the type horizontal flow, aerated, vortex. There’s no type such as vertical flow. These protect mechanical equipment from abrasion and reduces the formation of heavy particles in pipes.

4. What is the detention time considered while designing a Grit chamber?
a) 20-40 sec
b) 10-20 sec
c) 90-120 sec
d) 45-90 sec

Answer: d
Explanation: The detention time considered while designing a Grit chamber is 45-90 sec. Usually, 60 secs is considered as the detention time in most of the cases. The typical head loss considered is 30-40%.

5. In which of the following systems no flow is added or allowed to leave during the design detention time?
a) Arbitrary flow reactor
b) Fluidized bed reactor
c) Mixed batch reactor
d) Packed bed reactor

Answer: c
Explanation: Arbitrary reactor is any degree of partial mixing between plug-flow and complete-mix. Where no flow is added or allowed to leave during the design detention time is called mixed batch reactor.

6.In __________ reactor the bed is filled with a packing material that expands when wastewater moves upwards.
a) Fluidized bed reactor
b) Packed bed reactor
c) Plug flow reactor
d) Mix batch reactor

Answer: a
Explanation: Fluid particles pass through the tank and are discharged in the same sequence in which they enter in plug flow reactor. The packing material that expands and gets fluidized when wastewater moves upward in the reactor is provided in fluidized bed reactor.

7. What is the aspect ratio of a stirred tank bioreactor?
a) 1-2
b) 2-3
c) 2-4
d) 3-5

Answer: d
Explanation: The aspect ratio of a stirred tank bioreactor is usually between 3-5. However, for animal cell culture applications, the aspect ratio is less than 2. The number of impellers depends on the size of the bioreactor and is variable.

8. The diameter of the impeller is usually 1/3 rd of the vessel diameter.
a) True
b) False

Answer: a
Explanation: The diameter of the impeller is usually 1/3 rd of the vessel diameter. The distance between two impellers is approximately 1.2 impeller diameter. Different types of impellers (Rustom disc, concave bladed, marine propeller etc.) are in use.

9. In bubble column bioreactor, the flow rate of air/ gas influences the performance of oxygen transfer.
a) True
b) False

Answer: a
Explanation: The air or gas is introduced at the base of the column through perforated pipes or plates, or metal micro porous sparger is in the bubble column bioreactor. The flow rate of the air/gas influences the performance factors —O2 transfer, mixing.

10. The vessel used for bubble column bioreactors is _______
a) Square
b) Rectangular
c) Cylindrical
d) Circular

Answer: c
Explanation: The bubble column bioreactors may be fitted with perforated plates to improve performance. The vessel used is usually cylindrical with an aspect ratio of 4-6 (i.e., height to diameter ratio).