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Colorado Wastewater (WW) Operator D Practice Exam

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Detention time in a stabilization pond with influent flow rate 0.785 MGD, depth 4.5 ft, and area 17 acres is approximately how many days?
Correct Answer:
32 days
Explanation:
Detention time, or hydraulic retention time, is the time water spends in the pond and is found by dividing the pond’s volume by the influent flow. First find the volume from area and depth: 17 acres at 4.5 ft deep gives 76.5 acre-feet. Convert to gallons since the flow is in gallons per day: 1 acre-foot is about 325,851 gallons, so the volume is 76.5 × 325,851 ≈ 24,930,000 gallons. The inflow is 0.785 MGD, or 785,000 gallons per day. So the detention time is 24,930,000 ÷ 785,000 ≈ 31.8 days, which rounds to about 32 days.

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