A grit chamber of dimensions 15 m × 2.0 m × 0.6 m carries a flow of 540 m³/h. The detention time is approximately
- A2 minutesCorrect
- B3 minutes
- C5 minutes
- D10 minutes
Solution
Concept
Hydraulic detention time of grit chambers
Formula
t = V / Q = (L × B × H) / Q
Step-by-step solution
1. Basin volume V = 15 m × 2.0 m × 0.6 m = 18 m³. 2. Discharge Q = 540 m³/h = 540 / 60 m³/min = 9 m³/min. 3. Detention time t = V / Q = 18 / 9 = 2 minutes. 4. Thus, detention time is approximately 2 minutes. Answer: A
Explanation
Detention time (t) in a sedimentation basin or grit chamber is calculated by dividing tank volume by volumetric flow rate. Volume = 15 m × 2.0 m × 0.6 m = 18 m³. Flow rate Q = 540 m³/h = 540 / 60 = 9 m³/min. Detention time = 18 m³ / (9 m³/min) = 2 minutes.
Why the other options are wrong
B) 3 minutes overestimates detention time by 50%. C) 5 minutes corresponds to a smaller flow rate. D) 10 minutes is typical for primary clarifiers rather than grit chambers.
Shortcut
V = 18 m³; Q = 9 m³/min -> t = 18 / 9 = 2 min.
Common mistake
Forgetting to convert flow rate from hours to minutes, resulting in 0.033 hours and calculation confusion.
Correct answer: A) 2 minutes