Sedimentation tank design parameter: The discharge per unit plan area of a sedimentation (settling) tank is generally called what?

Introduction / Context:
Settling basin performance is governed by the surface overflow parameter that compares inflow to plan area. Different terms are used interchangeably in practice. Recognizing these synonyms avoids confusion when consulting varied standards and texts.

Given Data / Assumptions:

• Parameter is discharge divided by plan area (Q/A).
• Used as a sizing criterion for sedimentation tanks and clarifiers.

Concept / Approach:
Surface overflow rate, surface loading rate, and overflow velocity all denote the same conceptual parameter: the upward superficial velocity against which particle settling must compete. If the particle settling velocity exceeds this value (with safety factors), it will be removed in an ideal clarifier.

Step-by-Step Solution:
Define the parameter: v_s = Q / A.Recognize synonymous terminology in literature: overflow rate = surface loading = overflow velocity (superficial).Choose the option that includes all synonymous terms.

Verification / Alternative check:
Design manuals routinely use these terms interchangeably and provide typical design ranges by process type (primary, secondary, tertiary clarification).

Why Other Options Are Wrong:
Individual terms (a, b, or c) are correct but incomplete; the question asks what it is “generally called,” acknowledging synonyms.Basin turnover number is unrelated.

Common Pitfalls:

• Confusing overflow rate with weir loading rate (Q per unit weir length) or hydraulic retention time.
• Ignoring that real clarifier performance also depends on flocculation, inlet/outlet design, and sludge removal.

Final Answer:
All of the above