Design velocity in plain sedimentation tanks What is the typical horizontal flow velocity maintained in a plain sedimentation tank to promote particle settling without resuspension?

Introduction / Context:
In plain sedimentation (without chemical coagulation), the tank hydraulics must be gentle enough to allow suspended particles to settle while avoiding scouring of settled solids. Horizontal velocity is a key parameter along with surface overflow rate and detention time.

Given Data / Assumptions:

• Rectangular continuous-flow sedimentation basin.
• Uniform, laminar-like bulk flow regime desired through baffle design.

Concept / Approach:
Practical design guidelines keep horizontal velocities modest—commonly around 20 cm/min—to limit turbulence. This complements design of detention time and overflow rate (e.g., 4–8 hours detention and typical surface loading limits) to achieve target removal efficiencies of suspended matter.

Step-by-Step Solution:

Verification / Alternative check:
Design texts list horizontal velocities around 15–30 cm/min, with 20 cm/min commonly adopted for preliminary sizing.

Why Other Options Are Wrong:

• 3 cm/min: unnecessarily low, causing large tank sizes.
• 30–40 cm/min: may disturb settling and increase carryover.
• 10 cm/min: feasible but not the most typical nominal figure.

Common Pitfalls:
Ignoring inlet/outlet baffles; even with correct velocity, poor inlet/outlet design can short-circuit flow and reduce performance.

Final Answer:
20 cm/min