Hazen’s settling relation in water and wastewater treatment is typically applicable to particles of which size range?

Introduction / Context:
Different empirical/analytical laws govern settling velocity depending on particle size and flow regime. Recognizing which law applies is essential for sedimentation basin design.

Given Data / Assumptions:

• Hazen’s relation is empirical for larger, non-colloidal particles.
• Stokes’ law applies to very small, laminar-settling particles.

Concept / Approach:
Stokes’ law is valid for small particles (Reynolds number for particle settling in laminar regime), commonly for diameters below about 0.1 mm in water. For larger grains (sand-sized), inertial effects increase and Stokes’ assumptions break; empirical relations like Hazen's are used.

Step-by-Step Solution:
Classify size → boundary near 0.1 mm for water treatment practice.Hazen’s relation → applicable for particles larger than approximately 0.1 mm.Choose the option reflecting this threshold.

Verification / Alternative check:
Design manuals differentiate Stokes (fine) vs. Hazen or Newton regimes (coarser).

Why Other Options Are Wrong:

• Options (a)–(c) refer to smaller sizes where Stokes is more appropriate.
• (e) Colloidal particles do not follow simple gravity settling; they require coagulation-flocculation.

Common Pitfalls:
Using Stokes for coarse sand leads to significant errors because laminar assumptions fail.

Final Answer:
Diameters greater than 0.1 mm.