In a conventional activated sludge process, the secondary clarifier is primarily used for:

Question

In a conventional activated sludge process, the secondary clarifier is primarily used for:

1. A. Aeration of mixed liquor
2. B. Settling and separation of biomass (sludge) from treated effluent (Correct answer)
3. C. Equalization of flow and BOD
4. D. Disinfection of effluent

Correct Answer

Option B — Settling and separation of biomass (sludge) from treated effluent

Detailed Solution & Explanation

The correct answer is Settling and separation of biomass (sludge) from treated effluent.

Key Points

• In the conventional activated sludge process, the aeration tank produces mixed liquor — treated wastewater with a dense suspension of microbial floc. That floc must be removed before discharge, and the secondary clarifier does exactly that by gravity settling.
• The clarifier performs two duties at once: it clarifies the effluent for discharge, and it thickens the settled sludge so that a concentrated stream can be returned to the aeration tank as Return Activated Sludge (RAS), with the surplus withdrawn as Waste Activated Sludge (WAS).
• The other options belong to different units — aeration happens upstream in the aeration tank, equalisation in a preliminary basin, and disinfection after clarification.

Additional Information

• Without RAS the process would fail: the returned sludge is what maintains the MLSS concentration and hence the F/M ratio and sludge age (SRT) on which treatment efficiency depends.
• Settling quality is measured by the Sludge Volume Index (SVI), the volume in mL occupied by one gram of sludge after 30 minutes' settling. Values of roughly 80-120 mL/g indicate good settling; above about 150 mL/g indicates bulking, usually caused by filamentous organisms.
• Bulking sludge is the classic operational failure of the process, and it shows up first as solids carrying over the clarifier weir.
• A conventional plant of this type typically achieves 85-95% BOD removal.