A settling tank measuring 60ft by 20ft by 12ft treats a flow of 2.4 MGD. What is its detention time?

A settling tank measuring 60ft by 20ft by 12ft treats a flow of 2.4 MGD. What is its detention time?

• A. 15 min
• B. 30 min
• C. 1.1 hr
• D. 2.3 hr

Answer: (C)

To determine the detention time of the settling tank, you first need to calculate its volume and then divide that by the flow rate being treated. The volume of the tank can be calculated using the formula for the volume of a rectangular prism: \[ \text{Volume} = \text{Length} \times \text{Width} \times \text{Height} \] For this tank: - Length = 60 ft - Width = 20 ft - Height = 12 ft Calculating the volume: \[ \text{Volume} = 60 \text{ ft} \times 20 \text{ ft} \times 12 \text{ ft} = 14,400 \text{ ft}^3 \] Next, to convert the flow rate from million gallons per day (MGD) to cubic feet per minute (cfm), you use the conversion factors where 1 gallon is approximately 0.133681 cubic feet: \[ \text{Flow Rate} = 2.4 \text{ MGD} \times 1,000,000 \text{ gallons/MG} \times 0.133681 \text{ ft}^3/\text{gallon} \

To determine the detention time of the settling tank, you first need to calculate its volume and then divide that by the flow rate being treated.

The volume of the tank can be calculated using the formula for the volume of a rectangular prism:

[ \text{Volume} = \text{Length} \times \text{Width} \times \text{Height} ]

For this tank:

• Length = 60 ft

• Width = 20 ft

• Height = 12 ft

Calculating the volume:

[ \text{Volume} = 60 \text{ ft} \times 20 \text{ ft} \times 12 \text{ ft} = 14,400 \text{ ft}^3 ]

Next, to convert the flow rate from million gallons per day (MGD) to cubic feet per minute (cfm), you use the conversion factors where 1 gallon is approximately 0.133681 cubic feet:

[ \text{Flow Rate} = 2.4 \text{ MGD} \times 1,000,000 \text{ gallons/MG} \times 0.133681 \text{ ft}^3/\text{gallon} \