Primary sedimentation settling of sand-size inorganics: In primary settling tanks, 0.2 mm inorganic particles (typical sand) will separate out if their specific gravity (relative density) is approximately what value?

Introduction / Context:

Primary sedimentation targets removal of settleable solids from sewage before biological treatment. Grit chambers and primary clarifiers remove heavier inorganic particles such as sand. The settling behavior of these particles depends on size, shape, and specific gravity.

Given Data / Assumptions:

• Particle size ≈ 0.2 mm (sand-size range).
• Quiescent conditions representative of primary clarifiers/grit chambers.
• Particle is largely quartzose (common in municipal grit).

Concept / Approach:

Quartz sand typically has a specific gravity near 2.65. At 0.2 mm, with adequate detention and low turbulence, such particles settle effectively. Design of grit chambers often assumes quartz specific gravity to size channels and detention times.

Step-by-Step Solution:

Verification / Alternative check:

Engineering manuals list grit specific gravity in the 2.60–2.70 range, with 2.65 as the standard design value.

Why Other Options Are Wrong:

• Values like 2.25–2.55 underestimate typical quartz sand and suggest lighter organics/minerals.
• 2.75 is somewhat higher than common municipal grit assumptions.

Common Pitfalls:

• Confusing specific gravity of organics (≈ 1.0–1.3) with inorganic grit (≈ 2.65).

Final Answer:

2.65