Detention time guidelines for plain sedimentation What is the usual range of detention time adopted for a plain sedimentation tank (without coagulation)?

Introduction / Context:
Detention time is the nominal period a water parcel spends in a sedimentation basin. It affects removal efficiency of settleable solids and must be coordinated with surface overflow rate and tank hydraulics.

Given Data / Assumptions:

• Plain sedimentation (no coagulants).
• Continuous flow rectangular or circular basins with inlet/outlet baffles.

Concept / Approach:
Empirical practice and design standards recommend a detention time range that balances particle settling with reasonable tank volumes. For plain sedimentation, a 4–8 hour range is widely used to achieve acceptable removal for typical raw water conditions.

Step-by-Step Solution:

Verification / Alternative check:
Design examples consistently adopt 6 hours within the 4–8 hour band as an initial estimate, fine-tuned via pilot data.

Why Other Options Are Wrong:

• 2–4 hours: commonly too short for plain sedimentation.
• 8–12 or higher: possible in special cases but not the usual guideline.

Common Pitfalls:
Ignoring short-circuiting; effective detention can be lower than nominal, so inlet/outlet baffles are critical.

Final Answer:
4 to 8 hours