CIV 220F ENGINEERING ECOLOGY
Pr actice Pr oblems #5: BOD and Sag Cur ve
1. A sample of wastewater was accidentally incubated for seven days at 20°C instead of five
days. The BOD 7 was 208 mg/L. Assume k 1 = 0.15 day–1.
a. Calculate BOD 5 .
b. Calculate BOD 10 .
c. Calculate the BOD in the sample (L o ).
2. IGNORE THIS ONE IN 2010 -- A wastewater initially had a dissolved oxygen (DO)
concentration of 0.5 mg/L. The BOD measurement was conducted with this wastewater by
placing 5 mL of sample into a standard 300 mL sample bottle. The DO of the dilution water
was 9.2 mg/L. After 5 days of incubation at 20°C, the DO of the sample bottle was 5.8
mg/L. If k 1 = 0.21 day–1 at 20°C, what was the initial oxygen consumption rate in this
wastewater at 20°C?
3. IGNORE THIS ONE IN 2010 -- A tanker car containing vegetable oil derails near a small
pond with a volume of 25,000 m3. Approximately 475 kg of vegetable oil enters the pond.
The pond temperature is 18°C. Assume that the vegetable oil biodegradation stoichiometry
is:
C 8 H 16 O + 11.5 O 2 → 8 CO 2 + 8 H 2 O
MW:
128
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and that k 1 at 18°C is 0.23 day–1.
a. Estimate the theoretical BOD in the pond immediately after the spill. Assume the
vegetable oil mixes completely in the pond.
b. A sample of the pond water taken immediately after the spill is subjected to the BOD
test. 50 mL of pond water is diluted to 300 mL. The initial dissolved oxygen
concentration in the diluted mixture is 8.3 mg/L. After 5 days of incubation at 20°C,
the dissolved oxygen concentration is 3.5 mg/L.
(1) What is the BOD 5 of the pond water sample?
(2) What L o is predicted for the pond immediately after the spill? Explain any
differences between L o and your answer in part a. Assume θ = 1.047.
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4. A wastewater treatment facility discharges a flow of 1.5 m3/s with L = 44 mg/L, dissolved
oxygen concentration equal to 1 mg/L, and a temperature of 18°C. The receiving stream
minimum flow is 12 m3/s with an upstream dissolved oxygen concentration of 7.2 mg/L, an
upstream temperature of 15°C and an upstream L of 14 mg/L. The reaeration coefficient k 2
is 0.30 day–1 at 20°C and the deoxygenation rate k 1 downstream of the discharge is 0.25 day–
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at 20°C. If the minimum allowable dissolved oxygen concentration downstream of the
discharge is 4.0 mg/L, does the plant meet this requirement? Use the attached dissolved
oxygen concentration tables as needed.
5. A stream with a minimum flow of 7.5 m3/s, an upstream dissolved oxygen concentration of
7.5 mg/L, an upstream temperature of 17°C and an upstream L of 10 mg/L receives treated
effluent from a wastewater treatment facility. The treated effluent flow is 0.25 m3/s with
L = 50 mg/L, dissolved oxygen concentration equal to 1.5 mg/L, and a temperature of 20°C.
The stream reaeration coefficient k 2 is 0.30 day–1 at 20°C and the deoxygenation rate k 1
downstream of the discharge is 0.40 day–1 at 20°C. The average cross-sectional area of the
stream is 50 m2. What is the dissolved oxygen concentration 40 km downstream from the
discharge? Is this upstream or downstream of the point of minimum dissolved oxygen
concentration?
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