Detritus (grit) tanks in wastewater treatment: What is the typical design flow velocity maintained through a detritus (grit) tank to allow heavy inorganic particles to settle while keeping organics in suspension?

Introduction / Context:
A detritus (grit) tank is a preliminary wastewater treatment unit that removes heavy inorganic solids such as sand, gravel, and silt. The design objective is to maintain a controlled low velocity that lets grit settle without causing excessive deposition of organic matter. This question checks the typical target velocity used by designers for efficient grit removal and odor control.

Given Data / Assumptions:

• Unit type: detritus or grit-removal channel/tank in a sewage treatment works.
• Objective: settle heavy inorganics, carry organics forward.
• Uniform section and steady approach flow are assumed for design.

Concept / Approach:
Settling behavior of sand-sized particles is sensitive to horizontal velocity. Too high a velocity carries grit through; too low a velocity causes organic deposition and septic conditions. Practice commonly adopts a low, near-laminar regime velocity. While simple grit channels may use around 0.3 m/s, detritus tanks that focus on finer grit often target lower velocities near about 0.09 m/s, striking a balance between capture and preventing septicity.

Step-by-Step Solution:
Identify the unit: detritus/grit tank with controlled low velocity.Recall typical low-velocity design bands used in practice.Select the representative value 0.09 m/s that aligns with low-velocity detritus removal while minimizing organic settlement.

Verification / Alternative check:
Design references list low horizontal velocities for detritus tanks, often an order of magnitude below pipe velocities, ensuring sand-sized particle capture. Field experience confirms that the value chosen yields effective removal without sludge-like deposition when detention and aeration (if used) are adequate.

Why Other Options Are Wrong:
0.05 m/s: Can be too low, risking organic deposition and septic conditions.1.25 m/s: Far too high; grit will not settle at such velocities.0.30 m/s: More typical of simple grit channels; detritus tanks often use lower velocities for finer grit.None of these: Incorrect because 0.09 m/s is a valid design choice.

Common Pitfalls:

• Confusing velocities used in aerated grit chambers or channels with those in detritus tanks.
• Assuming “slower is always better”; excessively low velocity causes organic deposition and odor.

Final Answer:
0.09 m/s