Design of plain sedimentation tanks (water treatment): What is the typical surface overflow rate (also called surface loading rate) used for plain sedimentation units?

Introduction / Context:
In water treatment plant design, the surface overflow rate (SOR) or surface loading rate is a primary sizing parameter for plain sedimentation tanks. It represents the volumetric flow per unit plan area and controls the settling environment for suspended particles prior to filtration.

Given Data / Assumptions:

• Plain sedimentation (no tube settlers, no high-rate coagulation clarifiers).
• Conventional design practice in SI units.
• Steady average design flow conditions.

Concept / Approach:

SOR is chosen so that the effective settling velocity of target particles is less than or equal to the overflow rate. For conventional plain sedimentation, many handbooks recommend daily surface loadings of roughly 24 to 30 m/day. Converting to hourly units gives about 1,000 to 1,250 litres/hr/m^2 for typical designs, providing a practical balance between tank size and clarified water quality.

Step-by-Step Solution:

Verification / Alternative check:

Cross-check with design texts: plain sedimentation (without enhanced settlers) typically lies near 20–30 m/day; plants operating at the higher end require good coagulation–flocculation control to maintain effluent quality.

Why Other Options Are Wrong:

• 250–1,000 L/hr/m^2: These are low for plain sedimentation and would oversize tanks.
• >1,250 L/hr/m^2: Generally too high for plain settling without high-rate applications.

Common Pitfalls:

• Confusing high-rate clarifier loadings with plain sedimentation values.
• Ignoring seasonal temperature (viscosity) impacts; colder water can warrant conservative SOR.

Final Answer:

1,000 to 1,250 litres/hr/m^2.