This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Activated Sludge Treatment Systems – 1”.
1. Why is the sludge aerated?
a) To avoid bacterial growth
b) To increase bacterial growth
c) To maintain pH
d) To maintain temperature
View Answer
Explanation: An activated-sludge reactor is a system in which pre-treated sewage (i.e. having passed through primary treatment) is aerated to promote the growth of bacteria (cells) that gradually consume the organics in the sewage.
2. ________ is treated in activated sludge reactor.
a) Pre-treated sludge
b) Treated sludge
c) Macronutrients
d) Micro-organisms
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Explanation: Pre-treated sludge is treated in an activated sludge reactor. This helps to reduce a load of treatment on the activated sludge process.
3. After the treatment, the BOD demand ______
a) Remains constant
b) Decreases
c) Increases
d) Alters
View Answer
Explanation: After the treatment, the BOD demand decreases. BOD stands for biological oxygen demand and COD stands for chemical oxygen demand.
4. Which is the next reactor after activated sludge reactor in the treatment process?
a) Flocculation unit
b) Aeration unit
c) Clarifier
d) Disinfection unit
View Answer
Explanation: Clarifier (settling tank) is the next reactor where the solids (mostly cells, called sludge at this stage) are separated from the water. The system is commonly operated in continuous mode (as opposed to batch mode).
5. Where is the sludge at the bottom of the clarifier processed to?
a) Settling unit
b) Aerator
c) Flocculation unit
d) Disinfection unit
View Answer
Explanation: The sludge coming from the bottom of the clarifier is processes to aerator unit and this clearly indicates activated sludge process.
6. Which of these is not an alternative to activated sludge treatment systems?
a) Stabilization ponds
b) Rotating biological reactors
c) Trickling filter
d) Screening units
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Explanation: In the trickling filter, pre-treated sewage is sprayed on the substrate. Cells can grow by the available oxygen. Stabilization ponds are similar to aerated lagoons and the process is natural.
7. An activated sludge system consists of two components, an aerator and ________
a) Screening units
b) Disinfection unit
c) Flocculation unit
d) Clarifier
View Answer
Explanation: Aerator and clarifier are the two components present in activated sludge systems. An activated sludge system consists of two components such as aerator and clarifier.
8. How is air pumped in the aerator unit?
a) Bubbled from bottom
b) Sides
c) Bubbled from top
d) Sprayed
View Answer
Explanation: As cells need oxygen for their metabolism, air is injected from the bottom of the aerator. Rising bubbles agitate the water well and create good contact between the three ingredients: cells, sewage and oxygen.
9. Which are the three ingredients in activated sludge systems?
a) Cells, sewage and oxygen
b) Cells, sewage and nitrogen
c) Solids, sewage and oxygen
d) Solids, water and oxygen
View Answer
Explanation: The cells need oxygen for their metabolism, air is injected from the bottom of the aerator. The water is well agitated by the rising bubbles and creates good contact between the three ingredients: cells, sewage and oxygen.
10. Mechanical stirring can be done instead of injection of air from the bottom.
a) True
b) False
View Answer
Explanation: Activated-sludge aerators are well agitated by mechanical stirring from the top or injection of air from the bottom.
11. The sludge particles concentration is increased by the growth of the organism in aeration tanks.
a) True
b) False
View Answer
Explanation: The wastewater is first screened and then it is mixed with different amounts of the recycled liquid containing high proportion of organisms and sludge concentration decreases as organism growth increases.
12. The organisms feed on _____ in aeration tanks.
a) Water
b) Air
c) Sludge particles
d) Bacteria
View Answer
Explanation: The organisms in the reactor multiply by feeding of the organic solids present in the reactor. The reactor consists of wastewater containing organic solids.
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