1. A rectangular clarifier is 75 feet long, 22 feet wide, and 10 feet deep. If it receives a flow of 1,250,000 gallons per day, what is the detention time in hours?

2. A rectangular secondary clarifier is 60 feet long, 30 feet wide and 8 feet deep. If the flow into the clarifier is 1,400,000 gallons per day, what is the detention time in hours?

3. A clarifier is 70 feet long, 25 feet wide and 10 feet deep. If the flow is 2.78 MGD, what is the detention time in hours?

4. If a final clarifier is 95 feet in diameter, 11 feet deep and receives flow of 7,000,000 gallons per day, what is the detention time?

5. A clarifier receives a flow of 4,320,000 gallons per day. If the clarifier is 65 ft in diameter and 12 ft deep, what is the detention time in hrs?

Answers:

Detention time formula: (Volume) / (Flow Rate) – just make sure the Volume and Flow are in the same units – either MGD or GPH – DON’T MIX UNITS

1. 2.37 hrs

* Find Volume of a rectangle = L, ft X W, ft X H, ft = 75 ft X 22 ft X 10 ft =

16,500 cuft

* Convert Cuft to Gallons = 16,500 cuft X 7.48 gal/cuft = 123,420 gal

* Convert Gallons Per Day to Hours = (1,250,000 gal) / (24 hrs/day) = 52,083.33 gal/hr

(You know there are 24 hours per day)

* Complete the formula (123,420 gal) / (52,083.33 gal/hr) =

2. 1.85 hrs

* Find Volume = 60 ft X 30 ft X 8 ft = 14,400 cuft

* Convert cuft to gallons = 14,400 cuft X 7.48 gal/cuft = 107,712 gal

* Convert Gallons Per Day to Hours = (1,400,000 gpd) / (24 hrs/day) = 58,333.33 gal/hr

* Complete the formula (107,712 gal) / (58,333.33 gal/hr) =

3. 1.13 hrs

* Find Volume = 70 ft X 25 ft X 10 ft = 17,500 cuft

* Convert cuft to gallons = 17,500 cuft X 7.48 gal/cuft = 130,900 gal

* Convert MGD to GPD = 2.78 MGD X 1,000,000 = 2,780,000 gpd (or just shuffle decimal 6 places to the right)

* Convert Gallons Per Day to Gallons per Hour = (2,780,000 gpd) / (24 hrs/day) = 115,833.33 gph

* Complete the formula = (130,900 gal) / (115,833.33 gph) =

4. 2.07 hrs

* (Draw your circle and name your parts)

* Find Volume of a circular tank = 3.14 X R, ft X R, ft X H, ft

* Find Radius = (95 ft) / (2) = 47.5 ft

* Find Volume = 3.14 X 47.5 ft X 47.5 ft X 11 ft = 77,930.87 cuft

(OR 0.785 X D, ft X D, ft X H, ft = 0.785 X 95 ft X 95 ft X 11 ft)

* Convert Cuft to Gallons = 77,930.87 cuft X 7.48 gal/cuft = 582,922.94 gal

* Convert Gallons Per Day to Gallons Per Hour = (7,000,000 gpd) / (24 hrs/day) = 291,666.67 gph

* Complete the formula = (582,922.94 gal) / (281,666.67 gph) =

5. 1.65 hours

* Find Radius = (65 ft) / (2) = 32.5 ft

* Find Volume = 3.14 X 32.5 ft X 32.5 ft X 12 ft = 39.799.5 cuft

(OR 0.785 X 65 ft X 65 ft X 12 ft)

* Convert Cuft to Gallons = 39,799.5 cuft X 7.48 gal/cuft = 297,700.26 gal

* Convert Gallons Per Day to Gallons Per Hour = (4,320,000 gpd) / (24 hr/day) =

180,000 gph

* Complete the formula = ((297,700.26 gal) / (180,000 gph) =