Activated sludge process (ASP): Which statement best describes the essence of the activated sludge process in biological wastewater treatment?

Introduction / Context:
The activated sludge process is the workhorse of secondary (biological) wastewater treatment. Understanding its core feature—mixing influent with return activated sludge under aeration—helps differentiate ASP from other processes like trickling filters or primary sedimentation.

Given Data / Assumptions:

• Conventional ASP with aeration tank and secondary clarifier.
• Use of return activated sludge (RAS) to maintain biomass concentration.
• Air (or oxygen) supply supports aerobic oxidation of organics.

Concept / Approach:
In ASP, mixed liquor (influent + RAS) is aerated to oxidize organics and synthesize biomass. After clarification, part of the settled sludge is returned (RAS) to the aeration tank; the remainder is wasted (WAS). This loop is central to maintaining the right mixed liquor suspended solids (MLSS) and sludge age.

Step-by-Step Solution:
Identify the hallmark: RAS + aeration.Select the option explicitly stating aeration with previously aerated sludge.

Verification / Alternative check:
Standard ASP schematics show influent + RAS entering aeration, followed by secondary clarification and RAS return—confirming the correct statement.

Why Other Options Are Wrong:
Pure stirring without aeration is insufficient; oxygen is needed for aerobic metabolism.Centrifugation is a solids-liquid separation, not the ASP core mechanism.“Aeration until full chemical stability” is vague and omits the critical RAS loop.

Common Pitfalls:

• Confusing primary treatment (settling) with secondary biological oxidation.
• Overlooking the importance of sludge age and MLSS control via RAS/WAS.

Final Answer:
Aeration is carried out with an admixture of previously aerated sludge (return activated sludge)