For continuous-flow primary sedimentation tanks used in municipal wastewater treatment, which of the following design statements is generally adopted in practice?

Introduction / Context:
Primary sedimentation tanks remove settleable solids and reduce the organic load before biological treatment. Their geometric proportions and hydraulic limits are chosen to prevent sludge resuspension, short-circuiting, and scum carryover.

Given Data / Assumptions:

• Continuous-flow rectangular tanks are under discussion.
• Common design heuristics for municipal plants are acceptable.
• Velocity limits are intended to avoid scouring.

Concept / Approach:
Standard practice uses rules-of-thumb to set width, length-to-width ratio, and surface overflow rates. Keeping velocities low (around 0.3 m/min) maintains quiescent conditions to promote settling. Sizing by volume against maximum daily flow ensures adequate detention during peak conditions.

Step-by-Step Solution:
Width ≈ 6 m: provides practical construction modules and manageable sludge collection spans.Length ≈ 4–5 × width: improves plug-flow behavior and reduces short-circuiting.Max horizontal velocity ≈ 0.3 m/min: limits turbulence and resuspension.Daily capacity matched to maximum day flow: ensures minimum detention under peak loading.

Verification / Alternative check:
These figures align with many municipal design handbooks that target stable hydraulic conditions and effective solids capture while keeping construction economical.

Why Other Options Are Wrong:
Each individual statement is valid; hence only the combined choice captures the complete set of accepted practices.

Common Pitfalls:
Overemphasizing a single parameter (e.g., only velocity) while neglecting geometric ratios that control short-circuiting and sludge transport.

Final Answer:
All of the above.