# Environmental Engineering Questions and Answers – Design of Activated Sludge Plant

This set of Environmental Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Design of Activated Sludge Plant”.

1. Which of the following is the correct expression of the aeration period in hours?
a) V/Q
b) 24V/Q
c) V/24Q
d) Q/V

Explanation: Aeration period = 24V/Q where V is the volume of the tank and Q is the quantity of sewage. It is measured in hours.

2. Which of the following activated sludge process has minimum food to microorganism ratio?
a) Extended aeration
b) Step aeration
c) Modified aeration
d) Conventional

Explanation: Extended aeration process has minimum food to microorganism ratio of 0.5 which means it has the maximum BOD removal efficiency.

3. The maximum aeration period for a conventional activated sludge system is ___________
a) 2 hours
b) 3 hours
c) 4 hours
d) 6 hours

Explanation: The detention period for a conventional activated sludge system is between 4 to 6 hours.

4. The organic loading in an activated sludge plant is defined as ___________
a) Mass of BOD per day/Volume of the aeration tank
b) Volume of aeration tank/Mass of BOD per day
c) The mass of BOD per day/Volume of secondary clarifier
d) Aeration period * Volume of aeration tank

Explanation: The organic loading in an activated sludge plant is defined as the mass of BOD to the aeration tank in grams per unit volume of aeration tank in m3.

5. _________ is defined as a volume in ml occupied by 1 gram of solid in the mixed liquor after 30 minutes of settling.
b) Suspended solid volume
c) Sludge volume index

Explanation: Sludge volume index is used to indicate the settling behavior of sludge. It is measured in ml/gm.
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6. The design value of food to microorganism ratio in the tapered aeration sludge process is ___________
a) 0.1
b) 0.2
c) 0.4
d) 0.8

Explanation: The design value of food to microorganism ratio is 0.3-0.4 for a tapered aeration sludge process.

7. Which of the following sludge process has maximum aeration period?
a) Conventional process
b) Extended process
c) Step aeration process
d) Tapered aeration

Explanation: Extended process has a maximum aeration period of 12 to 24 hours.

8. The maximum BOD removal efficiency of the Step aeration process of the activated sludge system is ___________
a) 92%
b) 80%
c) 99%
d) 78%

Explanation: The BOD removal efficiency of the step aeration process of the activated sludge system is between 85% to 92%.

9. BOD of sewage is dependent on food to microorganism ratio.
a) True
b) False

Explanation: If the food to microorganism ratio increases, BOD removal from the sewage will decrease.

10. _________ is the time at which sewage is detained in the aeration tank.
a) Average time
b) Detention time
c) Aeration period
d) Reaeration period

Explanation: Aeration period decides the loading rate at which sewage is introduced in the aeration tank.

11. Which of the following has minimum hydraulic retention time?
a) Conventional process
b) Extended process
c) Modified aeration process
d) Complete mix process

Explanation: Modified aeration process has a minimum hydraulic retention time with an aeration period of 1.5 hours.

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