In environmental engineering of air and water systems, particulate matter is often classified by size. What is the standard name used for particles with aerodynamic diameter greater than 75 µm (micrometre), i.e., coarse solids typically removed by grit chambers and primary settling devices?

Introduction / Context:
Engineers classify airborne and waterborne particulates by characteristic size because removal technologies depend strongly on particle diameter and density. Very coarse solids behave ballistically and settle quickly, while fine aerosols require filtration or electrostatic methods. This question asks for the standard term used for very coarse particles larger than 75 micrometres, a range especially relevant to preliminary treatment and grit removal equipment.

Given Data / Assumptions:

• Particle diameter considered: > 75 µm.
• Context includes environmental engineering devices such as grit chambers, bar racks, cyclones, and settling basins.
• Terminology is conventional: grit vs. dust vs. smoke vs. powder.

Concept / Approach:
Coarse particles with diameters in the order of tens to hundreds of micrometres typically separate readily by gravity or inertia. In water and wastewater treatment, “grit” refers to heavy inorganic solids like sand, silt, and small stones. In air pollution control, “dust” usually denotes smaller solids generated by handling and crushing; “smoke” describes fine combustion-generated particles; “powder” is a general materials term, not specific to environmental classification. Thus, particles larger than 75 µm are most appropriately called “grit.”

Step-by-Step Solution:
Identify the size regime: > 75 µm indicates coarse, fast-settling particles.Map common environmental terms to size ranges: grit (coarse), dust (finer), smoke (very fine), powder (generic).Select the term used in preliminary treatment and coarse particle removal: grit.

Verification / Alternative check:
Design manuals for municipal wastewater plants specify grit chambers targeting sand-sized particles, often 2100–75 µm. Air handling systems classify particulate matter; PM100/PM10/PM2.5 terminology also implies that “grit” is coarser than typical regulated dust fractions.

Why Other Options Are Wrong:
Dust: Usually smaller, often < 75 µm, generated by mechanical action.Powder: Generic materials term without environmental sizing specificity.Smoke: Very fine combustion aerosols, typically submicron to a few micrometres.

Common Pitfalls:

• Assuming all particulates are “dust” regardless of size.
• Confusing water treatment terminology (grit) with general materials handling (powder).

Final Answer:
grit