Surface overflow rate (SOR) for sedimentation with coagulation What is the normal range of surface overflow rate for a plain sedimentation tank when coagulants are used?

Introduction / Context:In clarifier design, the surface overflow rate (SOR) is a key sizing parameter. With coagulation, fine particles form settleable floc, allowing higher SOR than plain sedimentation without coagulant.

Given Data / Assumptions:

• Conventional coagulation–flocculation followed by sedimentation.
• Municipal water quality of moderate turbidity.
• Design for average conditions (not peak shock loads).

Concept / Approach:Typical exam and handbook values place SOR for coagulated water around 1000–1250 L/hr/m^2 (which equals about 24–30 m^3/m^2/day). This range balances tank area versus settling performance and floc quality.

Step-by-Step Solution:Recall standard SOR ranges for coagulated waters.Select the range 1000–1250 L/hr/m^2 as the normal design band.

Verification / Alternative check:Converting to m/day: 1000–1250 L/hr/m^2 ≈ 24–30 m/day, matching many design manuals for conventional clarifiers.

Why Other Options Are Wrong:

• 750–1000 L/hr/m^2 is on the conservative/low side for coagulated water.
• 1250–1500 or a single figure of 1500 L/hr/m^2 are relatively high and not “normal” baseline values.

Common Pitfalls:Confusing SOR for primary sedimentation in wastewater with potable-water clarifiers; also mixing up hr-based and day-based units.

Final Answer:1000 to 1250 litres/hr/m^2