Wastewater treatment terminology: in the activated sludge process, which option correctly defines “activated sludge” at the secondary stage?

Introduction / Context:
The activated sludge process is a cornerstone of biological wastewater treatment. Understanding what “activated sludge” actually refers to is fundamental to process control, return rates, and sludge age management.

Given Data / Assumptions:

• Mixed liquor from the aeration tank flows to a secondary clarifier (final settling tank).
• Microbial flocs (biomass) settle and are separated from treated effluent.
• A portion of settled sludge is returned (RAS) to maintain mixed liquor suspended solids; the rest is wasted (WAS).

Concept / Approach:
“Activated sludge” specifically denotes the biologically active, flocculated microbial mass produced in the system—most clearly identified as the settled biomass in the secondary clarifier after aeration. It is “activated” because it has a high concentration of microorganisms ready to oxidize organics upon return to the aeration tank.

Step-by-Step Solution:
Aeration supplies oxygen; microorganisms form flocs while consuming organics.Mixed liquor flows to the secondary clarifier where biomass settles.The settled biomass (activated sludge) is partly returned as RAS; excess is WAS.Hence, the most precise definition here is the post-aeration settled microbial mass in the secondary tank.

Verification / Alternative check:
Standard texts: activated sludge = biologically active solids separated in secondary settling, recycled to aeration.

Why Other Options Are Wrong:

• “Removable from aeration unit”: sloppy; separation occurs in the clarifier.
• “Humus tank” pertains to trickling filters, not the activated sludge process.
• “Rich in nutrients” mischaracterizes; it is rich in microorganisms, not nutrients.

Common Pitfalls:

• Confusing humus tank (TF) with secondary clarifier (ASP).
• Assuming activated sludge refers to any sludge in the plant.

Final Answer:
sludge in the secondary tank post-aeration rich in microbial mass