Wastewater New York State Practice Exam

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What is the detention time of a rectangular primary sedimentation tank that is 50 ft. x 25 ft. x 10 ft. with an influent flow of 2.25 mgd?

0.5 hours
1.0 hours
1.5 hours
2.0 hours

The correct answer is: 1.0 hours

To determine the detention time of a rectangular primary sedimentation tank, the volume of the tank needs to be calculated, and then this volume is divided by the influent flow rate. First, let’s find the volume of the tank. The dimensions are given as 50 feet by 25 feet by 10 feet. The volume can be calculated using the formula for the volume of a rectangular prism: Volume = length × width × height Volume = 50 ft × 25 ft × 10 ft Volume = 12,500 cubic feet Next, to convert this volume into gallons, it is useful to know that 1 cubic foot is approximately equal to 7.48 gallons: Volume in gallons = 12,500 cubic feet × 7.48 gallons/cubic foot Volume in gallons ≈ 93,100 gallons Now, the influent flow rate is given as 2.25 million gallons per day (mgd). To find the detention time in hours, you can use the formula: Detention Time (hours) = Volume (gallons) / Flow Rate (gallons per hour) First, convert the flow rate from mgd to gallons per hour: Flow Rate in gallons per