Typical detention time in a water sedimentation (clarifier) tank In conventional water treatment, what is the usual detention period range selected for primary sedimentation of raw water?

Introduction / Context:
Detention time in sedimentation tanks (clarifiers) is a key design parameter that determines the opportunity for suspended solids to settle under gravity before filtration. Selecting an appropriate range balances performance, footprint, and cost in municipal water treatment plants.

Given Data / Assumptions:

• Conventional rapid sand filtration train with coagulation/flocculation followed by sedimentation.
• Steady flow, typical raw water quality.
• Standard overflow rates and tank geometries.

Concept / Approach:
For plain sedimentation with prior coagulation and flocculation, detention times of approximately 2–4 hours are commonly adopted. Longer times increase capital cost and can promote sludge putrefaction; much shorter times may reduce removal efficiency. The choice is interrelated with surface overflow rate and particle/floc settling velocities.

Step-by-Step Solution:

Verification / Alternative check:
Design manuals and codes specify overflow rates that, when combined with tank dimensions, yield detention times in this range for municipal systems.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing wastewater primary clarifiers (which can have different criteria) with potable water clarifiers; ensure the right context.

Final Answer:
2 to 4