Within the conventional “tertiary” (advanced) treatment stage of a wastewater plant, which of the following unit operations is most representative of tertiary polishing of secondary effluent?

Introduction / Context:
Tertiary or advanced treatment refers to processes applied after secondary biological treatment to polish effluent and meet stringent limits for solids, turbidity, nutrients, or emerging contaminants. Clarifying which unit operations typify this stage helps in selecting appropriate technologies for reuse and discharge permits.

Given Data / Assumptions:

• Secondary clarifier effluent still contains fine suspended solids and colloids.
• Tertiary processes aim to reduce TSS/turbidity and sometimes nutrients.
• Conventional tertiary trains often include media filtration or membranes.

Concept / Approach:
Filtration (e.g., rapid sand, dual-media, cloth disk, or membrane filtration) is a hallmark tertiary operation that removes fine particles not captured by sedimentation. While chemical coagulation and flocculation can precede filtration as aids, filtration itself provides the polishing step that characterizes tertiary treatment in many plants.

Step-by-Step Solution:

Verification / Alternative check:
Plant P&IDs and process schematics commonly show rapid sand filters or membranes after secondary clarifiers, confirming filtration’s role in tertiary polishing.

Why Other Options Are Wrong:

• Chemical coagulation/flocculation: Often used as a pretreatment for filtration but not, by itself, the polishing step.
• Sedimentation: Typically primary or secondary clarification, not tertiary.
• None of these: Incorrect because filtration is a standard tertiary unit.

Common Pitfalls:
Assuming disinfection alone is tertiary; while post-treatment, it does not remove particulates and may be impaired by high turbidity.

Final Answer:
filtration