250+ TOP MCQs on Unit Operations and Unit Processes and Answers

Waste Water Engineering Multiple Choice Questions on “Unit Operations and Unit Processes”.

1. Separation is a unit operation.
a) True
b) False
Answer: a
Clarification: In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerization, and other reactions.

2. Refrigeration is a mass transfer process.
a) True
b) False
Answer: b
Clarification: Processes like gas liquefaction and refrigeration are thermodynamic processes. Mass transfer processes include gas absorption, distillation, extraction, adsorption and drying.

3. Solids concentration will be ________ in water than wastewater.
a) More
b) Less
c) Equal to
d) Similar
Answer: b
Clarification: Water and waste water differ only in the way that solids concentration will be more in waste water. Straining the finer particles in water treatment is called filtration and is towards the biological action of bacteria on the trickling filter to remove organic solids.

4. Unit operations are the ______ operations to remove the impurities.
a) Physical
b) Chemical
c) Biological
d) Biochemical
Answer: a
Clarification: The physical operations to remove the impurities present in the water and waste water are called unit operations where as the unit processes are the chemical and biological conversion on the status of the impurities that they will be converted to a form that can be easily separated.

5. What did the structures use to prevent floating matter that enters into pumps and pumping systems?
a) Aeration units
b) Screens
c) Grit chamber
d) Bioreactor
Answer: b
Clarification: Screens are in use from the intake structure where they prevent the floating matter enters into the pumping units, and fine and coarse screens are in use to treat waste water to prevent the entry of floating wastes and coarse solids into the treatment.

6. _________ is simply detaining water for a sufficient time.
a) Coagulation
b) Flocculation
c) Sedimentation
d) Filtration
Answer: c
Clarification: Sedimentation is simply detaining water for a sufficient time mostly in stagnant or relatively stagnant position to make the flow velocity of water less than the settling velocity of the solid particles that they without being driven by horizontal force settles down by gravity.

7. What is the removal of fine particle sand dissolved solids called?
a) Filtration
b) Sedimentation
c) Deoxidation
d) Disinfection
Answer: a
Clarification: Filtration is to the removal of fine particle sand dissolved solids where the fine sand layer and coarse sand layer below serves as the media to remove colloidal solids and the water remains completely free of solids.

8. How are colour and odour removed?
a) Adsorption
b) Sedimentation
c) Filtration
d) Coagulation
Answer: a
Clarification: Odour and colour present in water and waste water are removed by aeration and adsorption process. The odour and colour causing elements are adsorbed and aerated that the water is free from impurities for use and wastewater for reuse and recycling.

9. The law of ________ states that mass can neither be created nor destroyed.
a) Radiation
b) Wastewater
c) Conversion of mass
d) Water
Answer: c
Clarification: The law of conservation of mass states that mass can neither be created nor destroyed. Thus in a processing plant, the total mass of material entering the plant must equal the total mass of material leaving the plant, less any accumulation left in the plant.

10. What is the size of openings of a coarse screen?
a) 6mm-150mm
b) 150-200mm
c) >200mm
d) < 6mm
Answer: a
Clarification: The size of the coarse screens ranges from 6-150mm. Coarse screens are further classified as hand cleaned and mechanically cleaned. Coarse screens remove large particles and prevent clogging in pipes and pumps.

11. While designing a mechanical screen, the clear space between the bars would be in what range?
a) 15-75mm
b) 25-50mm
c) 20-40mm
d) >75mm
Answer: a
Clarification: The clear space between the bars for a mechanical screen would be in the range 15-75mm. The clear space between the bars for a hand screen would be 25-50mm. The slope from the vertical for these screens would range from 0-30 degree.

12. What is the maximum approach velocity to be considered for a mechanical screen while designing it?
a) 0.3-0.5m/s
b) 0.6-1m/s
c) 1-1.5m/s
d) 2 m/s
Answer: b
Clarification: The maximum approach velocity for a mechanical screen would be in the range 0.6-1m/s. The minimum approach velocity would be in the range 0.3-0.5m/s. These mechanical screens reduce manual labour.

13. What is the size of the openings for a fine screen?
a) 0.1 mm
b) 0.8-1mm
c) 0.2-0.6mm
d) 1-3mm
Answer: c
Clarification: The size of openings for a fine screen is 0.2-0.6mm. These are used as a substitute for a primary clarifier. These are used to remove the suspended particles.

14. What is the percentage of TSS removed by a rotary drum fine screen?
a) 50%
b) 60%
c) 65-75%
d) 25-45%
Answer: d
Clarification: 25-45 percentage of TSS is removed by rotary drum fine screens. This however actually depends on the nature of the waste water. Also it depends on the waste water travel time.

15. Which type of mixers are used to mix coagulants like alum, ferric ammonium sulphate and coagulant aids such as polyelectrolyte and lime?
a) Static mixers
b) Mechanical mixers
c) Mechanical aerators
d) Paddle mixers
Answer: d
Clarification: Paddle mixers are used to mix coagulants and coagulant aids. Paddle flocculators contain appropriately spaced paddles mounted on a shaft. Flocculation is brought about by the movement of the slow moving paddles.

16. 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
Clarification: 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.

17. 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
Clarification: 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.

18. Which of these is not a type of grit chamber
a) Horizontal flow
b) Vortex
c) Vertical flow
d) Aerated
Answer: c
Clarification: 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.

19. 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
Clarification: 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%.