To calculate the different loadings on a pond, the information listed under known must be available. Keep in mind math is math – so work these problems even if you don’t have Ponds on your level exam. Good practice.
Known
Average Depth = 4 ft
Average Width = 400 ft
Average Length = 600 ft
Flow = 0.5 MGD
BOD = 150 mg/L
Population = 5,000 persons
1. Find Detention Time, days
2. Find Population Loading, persons/acre
3. Find Hydraulic Loading, inches/day
4. Find Organic Loading, lbs BOD/day/ac
Answers:
1. Detention Time, days = (Pond Volume, ac-ft) / (Flow Rate, ac-ft/day)
Pond Volume, ac-ft = (Avg. Length, ft X Avg. Width, ft X Depth, ft) / (43,560 sqft /acre) =
(400 ft X 600 ft X 4 ft) / (43,560 sqft/acre) =
22. ac-ft
Flow Rate, ac-ft/day = (gal/day) / (7.48 gal/cuft X 43,560 sq ft/acre)
0.5 MGD X 1,000,000 = 500,000 gpd
(500,000 gpd) / (7.48 gal/cuft X 43,560 sqft/acre) = 1.53 ac-ft/day
Detention Time, days = (22.0 ac-ft) / (1.53 ac-ft/day) = 14.4 days
2. Population Loading, persons/acre = (Population served, persons) / (Pond Area, acres)
Pond Area, acres = (Avg. Length, ft X Avg. Width, ft) / (43,560 sqft/acre)
(400 ft X 600 ft) / (43,560 sqft/acre) = 5.51 acres
(5,000 persons) / (5.51 acres) = 907 persons/acre
3. Hydraulic Loading, in/day = (Depth of Pond, inches) / (Detention Time, days)
(4 ft X 12 in/ft) / (14.4 days) = 3.33 in/day
4. Organic Loading, lbs BOD/day/ac = (Flow, MGD X BOD, mg/L X 8.34 lbs/gal) / (Area, ac)
(0.5 MGD X 150 mg/L X 8.34 lbs/gal) / (5.51 acres) = 114 lbs BOD/day/acre