Settling tank design terminology: The ratio of the overflow (effluent) discharge to the plan surface area of a continuous-flow settling tank is commonly termed what?

Introduction / Context:
Clarifier sizing uses the concept of surface overflow rate (SOR), which is the volumetric flow divided by the plan area. This parameter relates hydraulic loading to settling performance of particles and flocs in primary/secondary clarifiers and water treatment basins.

Given Data / Assumptions:

• SOR is expressed typically as m³/m²·day or litres/hr/m².
• We refer to conventional continuous-flow settling basins.

Concept / Approach:

Different texts use “surface loading rate,” “overflow rate,” or simply “overflow” (as shorthand) to represent the same design ratio, Q/A. Because these are synonymous in the clarifier design context, an option including all terms is correct.

Step-by-Step Solution:

Verification / Alternative check:

Design standards, manuals, and textbooks interchange these terms, though some prefer “surface overflow rate” for clarity.

Why Other Options Are Wrong:

• Each single term is correct, but the most complete answer is the inclusive choice.
• “Downflow index” is not a standard term for Q/A in settling tanks.

Common Pitfalls:

• Confusing SOR with weir loading rate, which uses effluent weir length.
• Using inappropriate units—always check for consistent units.

Final Answer:

all the above.