Engineering Questions with Answers - Multiple Choice Questions

Environmental Engineering MCQ – Digestion of Sludge, Sludge Digestion Tank

1 - Question

How many stages are there in the sludge digestion process?
a) 2
b) 3
c) 4
d) 5

View Answer

Answer: b
Explanation: The sludge digestion process consists of acid fermentation, acid regression and alkaline fermentation.




2 - Question

The form of sludge which is free from pathogenic bacteria is ___________
a) Undigested sludge
b) Digested sludge
c) Primary sludge
d) Supernatant liquor

View Answer

Answer: b
Explanation: The digested sludge is free from pathogenic bacteria. It has the behavior of rich humus soil.




3 - Question

The BOD of supernatant liquor in a sludge digestion process is ____ ppm.
a) 3000
b) 0
c) 1000
d) 250

View Answer

Answer: a
Explanation: The range of BOD of supernatant liquor in a sludge digestion process is between 2000 and 3000 ppm. It is a form of sludge that is obtained in the sludge digestion process.




4 - Question

Which of the following is correct regarding pH in different stages of the sludge digestion process?
a) Acid fermentation > Acid regression > Alkaline fermentation
b) Acid regression > Acid fermentation > Alkaline fermentation
c) Acid fermentation < Acid regression < Alkaline fermentation
d) Acid fermentation < Acid regression > Alkaline fermentation

View Answer

Answer: c
Explanation: The pH value in Alkaline fermentation stage is 7.5 whereas in Acid regression and Acid fermentation stage, the pH>7, having the least in acid fermentation stage.




5 - Question

Which gas evolves the most in the sludge digestion process?
a) SO2
b) CO2
c) N2
d) CH4

View Answer

Answer: d
Explanation: Methane gas evolves the most during the sludge digestion process by approximately 70% of the total gas evolved.




6 - Question

Which of the following is incorrect about aerobic digestion in the sludge digestion process?
a) It has a low capital cost
b) It is easier to manage.
c) It has high residual energy
d) It decreases the BOD

View Answer

Answer: c
Explanation: The residual energy in aerobic digestion is very less in comparison to anaerobic digestion.




7 - Question

The color of digestive sludge is ___________
a) Blue
b) Black
c) Brown
d) Gray

View Answer

Answer: b
Explanation: The color of the digestive sludge is black. It has an inoffensive odor.




8 - Question

Which of the following is correct regarding anaerobic digestion in the sludge digestion process?
a) It contains high amount of biomass
b) It is a low energy process
c) It causes infertility in soil
d) Aerobic bacteria oxidize the organic matter

View Answer

Answer: b
Explanation: Anaerobic digestion is a low energy process that contains low amount of biomass and where both facultative and anaerobic bacteria solubilize the organic matter.




9 - Question

The maximum temperature at which thermophilic microorganisms remain active in sludge digester is ___________
a) 60oC
b) 40oC
c) 30oC
d) 20oC

View Answer

Answer: a
Explanation: The maximum and minimum temperature at which thermophilic microorganisms remain active in sludge digester are 60oC and 40oC respectively.




10 - Question

Which of the following is obtained in the secondary tank in two stage sludge digester?
a) Partially digested sludge
b) Partially digested sludge and supernatant liquor
c) Digested sludge and supernatant liquor
d) supernatant liquor

View Answer

Answer: c
Explanation: In two stage sludge digester, the partially digested sludge is obtained in primary tank, whereas digested sludge is obtained in the secondary tank along with supernatant liquor.




11 - Question

The high rate sludge digestion tanks are used when the capacity of the tank is ____ million liters per day.
a) 1
b) 3
c) 4
d) 10

View Answer

Answer: c
Explanation: High rate digesters are used when the capacity of tank is 4 million litres per day. Complete mixing and uniform feeding of raw sludge take place in high rate digesters.




12 - Question

In which of the following, both primary and secondary tanks are used?
a) Two stage sludge digesters
b) Single stage sludge digesters
c) Low rate digesters
d) High rate digesters

View Answer

Answer: a
Explanation: In two stage sludge digesters, the sludge is passed in the primary tank for 7 days and then it is passed in the secondary tank followed by the removal of residual matter.




13 - Question

The minimum size of sludge digestion tank is ___________
a) 0.5m
b) 1m
c) 2m
d) 3m

View Answer

Answer: d
Explanation: The minimum size of sludge digestion tank is 3m. They are cylindrical in shape with a funnel shaped at the bottom.




14 - Question

The range of temperature at which mesophilic microorganism becomes active in sludge digester is ___________
a) 30-40oC
b) 10-40oC
c) 20-50oC
d) 20-40oC

View Answer

Answer: d
Explanation: The mesophilic microorganism becomes active at 20-40oC in the sludge digester. The rate of digestion, increases with increase in temperature




15 - Question

The sludge digestion process is independent of which of the following?
a) Temperature of sludge
b) pH of sludge
c) Shape of tank
d) Mixing of sludge

View Answer

Answer: c
Explanation: The sludge digestion process depends on the temperature of sludge, pH conditions and mixing of sludge and is independent of shape of tank.

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