Plain sedimentation design — typical surface overflow rates: Normal values of overflow rate (surface loading rate) for plain sedimentation tanks are generally in which range, expressed in L/h per m^2 of surface area?

Introduction / Context:
In water treatment plant design, the key sizing parameter for primary (plain) sedimentation tanks is the surface overflow rate (SOR), also called surface loading rate. It represents the volumetric flow per unit plan area and governs particle removal by gravity settling under quiescent conditions.

Given Data / Assumptions:

• Plain sedimentation (without tube settlers or high-rate coagulation enhancements).
• Cold-to-moderate water temperatures and typical municipal water qualities.
• Conventional rectangular or circular clarifiers.

Concept / Approach:
Sedimentation theory (ideal settling) relates the critical settling velocity to the SOR. For plain sedimentation, many design manuals present typical SOR ranges equivalent to roughly 15–24 m/day. Converting to L/h·m^2: 1 m/day = 1000 L/m^2/day = 41.67 L/h·m^2. Therefore 18–24 m/day corresponds to approximately 750–1000 L/h·m^2, a commonly cited range for design and performance checks in all-weather operation.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with practical loading: many plants run 0.8–1.0 m^3/m^2·h under normal conditions; up-rating beyond this may reduce capture of slow-settling flocs.

Why Other Options Are Wrong:

• 100–250 and 250–500 L/h·m^2 are too low for typical municipal clarifiers (would oversize tanks).
• 500–750 L/h·m^2 is low side of enhanced-performance or conservative design, but industry norms for plain sedimentation usually extend to about 1000 L/h·m^2.
• 1200–1500 L/h·m^2 is more characteristic of high-rate clarification with tube settlers or lamella plates, not plain basins.

Common Pitfalls:
Confusing SOR (flow/area) with weir loading (flow/length); ignoring temperature effects on viscosity; applying high-rate clarifier values to plain tanks without polymer or plate packs.

Final Answer:
750 to 1000 L/h·m^2