Activated sludge recirculation — The activated sludge process consists of returning a portion of the clarifier underflow (settled sludge) to the aeration basin influent. Which option most closely captures this concept?

Introduction:
The activated sludge process (ASP) is a suspended-growth biological treatment in which mixed liquor from the aeration basin flows to a secondary clarifier. Settled biomass (sludge) is then returned to the aeration basin as return activated sludge (RAS) to maintain sufficient microbial concentration. This question focuses on that hallmark recirculation step.

Given Data / Assumptions:

• Aspiration: maintain target mixed liquor suspended solids (MLSS) in aeration.
• Process flow: aeration basin → secondary clarifier → sludge return to aeration influent.
• Wasting (WAS) is a separate, controlled withdrawal.

Concept / Approach:
RAS is drawn from the clarifier underflow and is returned to the aeration basin inlet (influent stream). Although the provided options use the word “effluent,” the intended idea is the clarifier return to the reactor feed line. Among the choices, the most accurate representation is returning a portion from the clarifier back to the stream entering the reactor (i.e., to the influent of the aeration tank).

Step-by-Step Solution:
Identify the ASP loop: aeration → clarification → RAS to aeration influent.Select the option that describes sending a portion of the clarifier outflow/return to the reactor inlet.Note: “Effluent” in the option should be understood as the clarifier return line rather than final plant effluent.

Verification / Alternative check:
Process and instrumentation diagrams (P&IDs) show RAS pumps routing settled sludge from the clarifier back to the front of the aeration basin, confirming the recirculation concept.

Why Other Options Are Wrong:

• (b) and (d) refer to streams exiting units, not the required recycle.
• (c) merely names the influent stream without indicating recycle from the clarifier.
• (e) No recycle would collapse MLSS and treatment performance.

Common Pitfalls:
Confusing final plant effluent with clarifier underflow; overlooking that RAS is biomass-rich, not just water.

Final Answer:
Effluent (clarifier return) sent to the stream entering the reactor