Detention time guidelines for plain sedimentation What is the usual range of detention time adopted for a plain sedimentation tank (without coagulation)?
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A2 to 4 hours
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B4 to 8 hours
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C6 to 10 hours
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D8 to 12 hours
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E12 to 16 hours
Answer
Correct Answer: 4 to 8 hours
Explanation
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:
Identify customary ranges: 4–8 hours for plain sedimentation.Shorter times (2–4 hours) risk poor removal; much longer times increase size and cost without proportional benefit.Select 4–8 hours as the standard answer.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