For secondary sedimentation tanks (secondary clarifiers) following biological treatment, what is the typical range of surface overflow rate used for design (expressed as litres per square metre per day)?

Introduction / Context:
Surface overflow rate (SOR) is a key criterion for sizing secondary clarifiers. It relates the allowable solids loading and clarifier settling performance to plan area.

Given Data / Assumptions:

• Municipal secondary clarifiers after activated sludge or trickling filter.
• Normal temperature and solids characteristics.

Concept / Approach:
Design practice uses SOR values derived from experience and pilot data. Typical municipal guidance for secondary clarifiers is in the range of roughly 25–35 m³/m²·day (equivalently 25,000–35,000 L/m²·day) for average flow.

Step-by-Step Solution:
Step 1: Identify the standard SOR range for secondary clarifiers.Step 2: Convert units: 1 m³/m²·day = 1,000 L/m²·day.Step 3: Choose the option that matches typical practice.

Verification / Alternative check:
Cross-check with typical state and national manuals which list similar SOR bands for average loading; peak conditions are handled via increased allowable SOR with safety factors.

Why Other Options Are Wrong:

• 40,000–60,000+ L/m²·day: Too high for average flow; risk of solids carryover.
• 80,000–100,000 L/m²·day: Characteristic of peak/overload, not design average.
• 10,000–20,000 L/m²·day: Too conservative for typical municipal design.

Common Pitfalls:
Using primary clarifier SOR values or peak hour SOR directly for average design without considering return sludge and MLSS properties.

Final Answer:
25,000 to 35,000 L/m²/day