Sedimentation design: which of the following statements are correct for conventional settling tanks?
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ADetention period for plain sedimentation tanks ranges between 4–8 hours
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BDetention period for sedimentation with coagulants typically ranges between 2–4 hours
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CHorizontal flow velocity in sedimentation tanks is generally limited to about 0.3 m/min
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DAll the above
Answer
Correct Answer: All the above
Explanation
Introduction: Sedimentation removes suspended solids by gravity prior to filtration. Design ranges for detention time and permissible horizontal velocities promote quiescent conditions without short-circuiting or re-suspension.
Given Data / Assumptions:
- Plain sedimentation (no coagulant) vs. coagulation–sedimentation.
- Typical design ranges are being tested.
Concept / Approach: Plain sedimentation requires longer detention for natural settleability (4–8 h). With coagulation, floc forms and settles faster (2–4 h). Horizontal velocities are limited (~0.3 m/min) to maintain laminar/near-laminar flow and avoid scour.
Step-by-Step Solution: Validate detention times for both cases against standard design practice. Check horizontal velocity limit typical of rectangular basins. All statements match design norms; choose “All the above”.
Verification / Alternative check: Design manuals corroborate similar ranges; exact values vary with temperature, particle size, and floc properties.
Why Other Options Are Wrong:
- Single statements omit the rest of the correct design guidelines; the comprehensive option is right.
Common Pitfalls:
- Applying plain-sedimentation detention times to coagulated water (overdesign).
- Allowing excessive horizontal velocities that resuspend floc.
Final Answer: All the above.