In municipal and industrial wastewater treatment, sedimentation (gravity settling) is a physical unit operation used primarily to remove which type of suspended particles from water?

Introduction / Context:
Sedimentation, also called gravity settling, is one of the most common primary treatment steps in water and wastewater engineering. It exploits density differences between suspended solids and the surrounding fluid to separate solids without chemical reaction or biological conversion. Understanding what types of particles sedimentation can remove clarifies where it fits in a treatment train and why flotation and filtration are used for other particle classes.

Given Data / Assumptions:

• Sedimentation tanks (clarifiers) operate under quiescent to mildly turbulent flow.
• Particles are discrete or flocculent, with densities relative to water.
• No chemical coagulation or air addition is assumed for the basic mechanism.

Concept / Approach:
Gravity acts on particles more dense than water, giving them a positive settling velocity according to Stokes’ law for small Reynolds numbers: v_s ∝ (ρ_p − ρ_w) * d_p^2. Such particles can be removed by settling in clarifiers if the hydraulic surface overflow rate is low enough. Particles less dense than water have negative apparent settling velocity (they rise) and are typically removed by flotation rather than sedimentation.

Step-by-Step Solution:

Verification / Alternative check:
Plant experience shows grit, sand, and heavy organic flocs are readily removed in primary clarifiers, whereas fats, oils, and greases float and are skimmed, not settled.

Why Other Options Are Wrong:

• Particles less dense than water: they rise, calling for flotation/skimming.
• “Pertinacious material”: vague and not a standard engineering category.
• None of the above: incorrect because gravity settling targets denser particles.

Common Pitfalls:
Expecting sedimentation to remove true colloids; these usually require coagulation–flocculation prior to settling. Also, excessive hydraulic loading reduces capture even for dense particles.

Final Answer:
remove particles that are more dense than water