Detritus (grit) tanks in preliminary treatment: Which statements correctly describe their operating targets and function?

Introduction / Context:
Detritus or grit tanks are preliminary treatment units designed to remove fine inorganic particles (sand, silt, grit) that would otherwise abrade pumps and reduce downstream capacity. They operate at controlled velocities and short detention times to favor settling of heavier inorganics over organics.

Given Data / Assumptions:

• Very short detention (order of minutes).
• Carefully controlled low horizontal velocity.
• Target removal: inorganics, not organics.

Concept / Approach:

By maintaining a low approach velocity (often cited near 0.09 m/s for detritus tanks and about 0.3 m/s for conventional grit channels) and a detention of a few minutes, heavier grit settles while lighter organics are carried forward, limiting septic conditions and odors.

Step-by-Step Solution:

Verification / Alternative check:

Field measurements of velocity and grit capture efficiency validate design; adjust weirs/baffles to tune velocities.

Why Other Options Are Wrong:

In this set each individual statement describes a recognized design/operation feature; hence “All the above” is correct.

Common Pitfalls:

Velocities too high causing carryover of grit; too low causing organic settling and odor; long detention times promoting septic conditions.

Final Answer:

All the above