Grit chamber sizing using detention time: If the recommended horizontal velocity is 0.2 m/s and the detention period is 2 minutes, what should be the chamber length?

Introduction / Context:
Grit chambers remove sand and other heavy inorganics. A standard sizing step is fixing approach velocity and detention time so that grit settles while organics remain in suspension. Length follows simply from velocity multiplied by time, assuming near plug flow behavior.

Given Data / Assumptions:

• Horizontal (non-aerated) grit chamber.
• Design velocity v = 0.2 m/s.
• Detention time t = 2 minutes = 120 seconds.

Concept / Approach:

For steady, uniform flow, the travel distance during detention is length = velocity * time. This yields the minimum chamber length that provides the specified detention period at the chosen velocity profile.

Step-by-Step Solution:

Verification / Alternative check:

Check against typical ranges: For v ≈ 0.2–0.3 m/s and t ≈ 1–2 min, lengths often fall within 12–36 m depending on loading—24 m is reasonable.

Why Other Options Are Wrong:

20 m or 16 m correspond to shorter detentions; 30 m is longer than specified; 12 m is clearly too short for the given parameters.

Common Pitfalls:

Forgetting to convert minutes to seconds; confusing overflow rate with horizontal velocity; neglecting allowance for inlet/outlet zones (which may be added in detailed design).

Final Answer:

24 m