Question

a sedimentation tank is 70 ft long, 40 ft wide and 12 ft deep. what is the detention time, in minutes, if the flow is 3000 gpm?

84 minutes
74 minutes
102 minutes
97 minutes

Explanation:

Calculate the volume of the sedimentation tank in cubic feet

$$ V_{\text{ft}^3} = 70\text{ ft} \times 40\text{ ft} \times 12\text{ ft} = 33,600\text{ ft}^3 $$

Convert the volume to gallons

$$ V_{\text{gal}} = 33,600\text{ ft}^3 \times 7.48\text{ gal/ft}^3 = 251,328\text{ gal} $$

Calculate the detention time in minutes

$$ t = \frac{V_{\text{gal}}}{Q} = \frac{251,328\text{ gal}}{3000\text{ gpm}} \approx 83.78\text{ minutes} \approx 84\text{ minutes} $$

Answer:

• (A) 84 minutes (Correct answer)
• (B) 74 minutes
• (C) 102 minutes
• (D) 97 minutes