In the design of a grit chamber for sewage treatment, which set of typical design parameters is appropriate to ensure grit removal without carrying organics?

Introduction / Context:
Grit chambers remove heavy inorganic particles (sand, silt) to protect downstream pumps and clarifiers. The hydraulic conditions must allow grit to settle while keeping most organics in suspension.

Given Data / Assumptions:

• Municipal wastewater with typical grit characteristics.
• Preliminary treatment stage before primary sedimentation.
• Constant approach velocity through the chamber.

Concept / Approach:
Effective grit removal is achieved by controlling velocity and detention time. Velocities around 0.15–0.30 m/s let sand settle but keep organics moving. Shallow to moderate depths (≈0.9–1.2 m) and detention ≈1 minute are widely used baselines in horizontal-flow grit chambers.

Step-by-Step Solution:
Select a target horizontal velocity: 0.2–0.3 m/s is common.Set detention time near 1 minute to limit basin size while achieving settling of grit sizes 0.2–0.25 mm.Adopt a practical depth around 1 m to simplify cleaning and maintain stable flow.

Verification / Alternative check:
Design manuals present similar ranges, with aerated grit chambers using controlled spiral flows but comparable effective velocities.

Why Other Options Are Wrong:

• Very high velocities (≥0.8–1.0 m/s): Carry grit through; cause scouring.
• Very low velocities (≤0.05 m/s): Settle organics and cause odor.
• Extreme depths or detention times are unnecessary and uneconomical.

Common Pitfalls:
Ignoring seasonal flow variation; not providing velocity control (proportional weirs or parshall flumes) leading to variable performance.

Final Answer:
Maintain flow velocity about 0.15–0.30 m/s, provide 0.9–1.2 m depth, and about 1 minute detention