Sedimentation design: which of the following statements are correct for conventional settling tanks?

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.