Hydraulic structures — retardation of flow: In which of the following treatment units is the flow of water intentionally retarded to promote settling and clarification?

Introduction / Context:
Clarification processes in water treatment rely on reducing water velocity so that suspended particles can settle under gravity, or can be captured by flocculation and subsequent settling. Various terms are used for these units, but the hydraulic principle is the same: retard the flow to achieve settling.

Given Data / Assumptions:

• Conventional treatment train including coagulation–flocculation and sedimentation.
• Goal is to remove settleable solids and reduce turbidity prior to filtration.

Concept / Approach:
Settling tanks, sedimentation tanks/basins, and clarifiers are functionally similar. They provide quiescent conditions by enlarging plan area (lower surface overflow rate) and increasing detention time, allowing particles with settling velocity greater than the critical velocity to be removed.

Step-by-Step Solution:

Verification / Alternative check:
Design parameters such as surface overflow rate and detention time are specified for each of these unit types and share overlapping ranges depending on process configuration.

Why Other Options Are Wrong:
Each individual unit listed indeed retards flow; selecting only one would be incomplete.

Common Pitfalls:
Confusing mixing/flocculation basins (which intentionally maintain gentle velocity gradients) with sedimentation, which favors very low velocities and uniform flow distribution.

Final Answer:
All of the above