1. What is the detention time, in hours, of a clarifier that is 40 ft in diameter, 12 feet deep and receives a flow of 1.11 MGD?

Lab Results for remaining 4 problems:
Influent BOD = 225 mg/L
Influent TSS = 238 mg/L
Primary Effluent BOD = 153 mg/L
Primary Effluent TSS = 114 mg/L
Plant Effluent BOD = 18 mg/L
Plant Effluent TSS = 23 mg/L

2. What is the primary unit’s BOD removal efficiency?

3. What is the primary unit’s TSS removal efficiency?

4. What is the total plant’s BOD removal efficiency?

5. What is the total plant’s TSS removal efficiency?

Answers:

1. 2.44 hrs
(Volume) / (Flow, rate)
* Draw your circle and name your parts and find Radius (40 ft) / (2) = 20 ft
* Formula for Volume of a circular tank =
3.14 X R, ft X R, ft X H, ft = 3.14 X 20 ft X 20 ft X 12 ft = 15,072 cuft
(OR 0.785 X D, ft X D, ft X H, ft = 0.785 X 40 ft X 40 ft X 12 ft)
* Convert Cuft to Gallons = 15,072 cuft X 7.48 gal/cuft = 112,738.56 gal
* Convert MGD to GPD = 1.11 MGD X 1,000,000 = 1,110,000 gpd
* Convert Gallons Per Day to Gallons Per Hour (there are 24 hours in a day) =
(1,110,000 gpd) / (24 hrs/day) = 46,250 gph
* Work formula = (112,738.56 gal) / (46,250 gph) =

Efficiency = (In – Out) / (In) X 100 %

2. 32 %
* (225 mg/L – 153 mg/L) = 72 mg/L
* (72 mg/L) / (225 mg/L) = 0.32
* Convert Decimal to Percent = 0.32 X 100 % =
(or just shuffle the decimal 2 places to the right and add the % sign)

3. 52 %
* (238 mg/L – 114 mg/L) = 124 mg/L
* (124 mg/L) / (238 mg/L) = 0.52 mg/L
* Convert Decimal to Percent

4. 92 %
* (225 mg/L – 18 mg/L) = 207 mg/L
* (207 mg/L) / (225 mg/L) = 0.92 mg/L
* Convert Decimal to Percent

5. 90 %
* (238 mg/L – 23 mg/L) = 215 mg/L
* (215 mg/L) / (238 mg/L) = 0.90 mg/L
* Convert Decimal to Percent