Activated sludge process — treatment stage classification The activated sludge system (aeration basin plus secondary clarification) is categorized as which stage of wastewater treatment?

Introduction / Context:
The activated sludge process is the workhorse of municipal wastewater treatment worldwide. Properly identifying it as secondary treatment is essential for regulatory reporting and process optimization.

Given Data / Assumptions:

• Activated sludge uses suspended-growth microbial communities in aeration tanks.
• Oxygen is supplied to support oxidation of dissolved and colloidal organics.
• Secondary clarifiers separate biomass from clarified effluent, with return activated sludge (RAS) recycled.

Concept / Approach:
Secondary treatment is defined by biological oxidation of BOD. Activated sludge fits this definition precisely. It is distinct from primary settling and tertiary nutrient removal (for example, chemical phosphorus precipitation, denitrification filters) and from anaerobic sludge digestion (solids handling).

Step-by-Step Solution:
Identify activated sludge as a biological oxidation process.Associate biological BOD removal with secondary treatment.Exclude primary and tertiary stages based on function.

Verification / Alternative check:
Process schematics and textbooks classify activated sludge under secondary treatment alongside trickling filters and RBCs.

Why Other Options Are Wrong:
Primary is physical settling; tertiary is advanced polishing; “any of these” is inaccurate; sludge digestion is a solids treatment step, not the activated sludge process itself.

Common Pitfalls:
Confusing the term “sludge” with solids handling; here it refers to the mixed liquor suspended solids used for biological oxidation.

Final Answer:
Secondary treatment.