Wastewater treatment stages — what does solids removal represent? In the standard classification of sewage treatment, removal of settleable and floatable solids is generally considered:

Introduction / Context:
Primary treatment is the first major stage in a wastewater plant after preliminary screening and grit removal. It aims to reduce suspended solids and some portion of BOD by physical processes, setting the stage for effective biological treatment.

Given Data / Assumptions:

• Primary clarifiers rely on gravity settling and skimming of scum.
• Typical performance: 50–70% removal of suspended solids and 25–40% removal of BOD.
• Downstream processes further oxidize dissolved organics and remove nutrients.

Concept / Approach:
Solids removal by primary sedimentation basins is quintessential primary treatment. Biological oxidation (secondary) and nutrient polishing (tertiary) occur later. Correct classification avoids conflating physical and biological unit processes.

Step-by-Step Solution:
Identify the process described: removal of solids via settling and skimming.Map to treatment stage: that is primary treatment.Exclude secondary (biological oxidation) and tertiary (nutrient polishing, filtration, disinfection).

Verification / Alternative check:
Plant process flow diagrams consistently list primary clarifiers between preliminary treatment and secondary aeration basins.

Why Other Options Are Wrong:
Secondary focuses on BOD removal by microbes; tertiary targets nutrients/pathogens; “none” and “disinfection” are not solids removal steps.

Common Pitfalls:
Assuming any clarifier is “primary.” Secondary clarifiers also exist but are paired with biological reactors and remove biomass, not raw settleable solids.

Final Answer:
Primary treatment.