Question

Assertion (A): Activated sludge contains large population of bacteria.
Reason (R): These bacteria help in anaerobic digestion of human waste in biogas plant.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

"Activated Sludge" is generated inside highly agitated aeration tanks to reduce BOD, meaning its microbial community is strictly aerobic. Because these microbes cannot function or drive processes in an anaerobic biogas digester, any statement claiming they actively perform anaerobic digestion is false.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question evaluates two statements about secondary sewage treatment and biogas production. We need to determine if they are individually true and if the stated reason correctly explains the assertion.

Step 2: Key Formula or Approach:
* Activated Sludge: The sedimented mass of bacterial and fungal flocs formed during the aerobic biological treatment phase of sewage.
* Anaeriobic Digesters: Anaerobic bioreactors where anaerobic methanogenic microbes digest the sludge to produce biogas.

Step 3: Detailed Explanation:
Evaluating Assertion (A): During the secondary aeration phase of sewage treatment, useful aerobic heterotrophic microbes multiply into visible structural aggregates called "flocs" (masses of bacteria associated with fungal filaments). After the biochemical oxygen demand (BOD) is significantly reduced, the effluent is passed into a settling tank where these bacterial flocs are allowed to sediment. This sedimented biological mass is formally termed activated sludge. It contains a dense, highly active population of aerobic bacteria. Thus, Assertion (A) is true.
Evaluating Reason (R): The reason states that *these* bacteria help in the anaerobic digestion of human waste in a biogas plant. This is factually false. The bacteria dominant inside the activated sludge are strictly aerobic organisms. When transferred to an anaerobic sludge digester or biogas plant, these aerobic bacteria are broken down and digested by a completely different group of microbes—strictly anaerobic bacteria (such as *Methanobacterium*). Therefore, the bacteria making up the activated sludge do not carry out anaerobic digestion; they are the substrate being digested. Thus, Reason (R) is false.
Since the assertion is a true statement but the reason is completely false, it matches the configuration of option (C).

Step 4: Final Answer:
Assertion (A) is true, but Reason (R) is false, which corresponds to option (C).