Activated sludge process: why is biomass recycled from the secondary clarifier back to the aeration basin?

Introduction / Context:
The activated sludge process for wastewater treatment relies on a dense, active microbial community to remove organics and nutrients. Recycling settled biomass (return activated sludge, RAS) from the clarifier to the aeration basin is fundamental to maintaining treatment performance.

Given Data / Assumptions:

• Aeration basin followed by secondary clarification.
• Settled sludge is partially returned (RAS) and partially wasted (WAS).
• Objective: sustain a target mixed liquor suspended solids (MLSS) and sludge age.

Concept / Approach:
Returning biomass keeps the aeration basin rich in active organisms, which boosts reaction rates and removal efficiency. By controlling RAS and WAS, operators manage sludge retention time (SRT), which influences nitrification, settling properties, and overall stability. Efficient recycling also reduces the volume of sludge requiring disposal per unit of treatment by optimizing biomass growth and decay dynamics.

Step-by-Step Solution:
Recycle increases MLSS → higher volumetric removal rates.SRT control via RAS/WAS balances growth vs. decay, improving process stability.Optimized solids handling reduces net sludge production for disposal.

Verification / Alternative check:
Plant data show deterioration in effluent quality when RAS is interrupted, and restoration upon resuming recycle; modeling confirms sensitivity of effluent BOD/TSS to MLSS and SRT settings.

Why Other Options Are Wrong:
Each single-option rationale is true but incomplete; collectively they capture the full purpose, hence “All of the above.”

Common Pitfalls:
Confusing RAS (process control) with WAS (inventory control/disposal); neglecting impact of RAS flow on clarifier hydraulic loading and sludge blanket stability.

Final Answer:
All of the above