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Waste water mass balance problem

BOD: Biochemical Oxygen Demand
BOD5 is the total amount of oxygen consumed by microorganisms during the first five days of biodegradation

BOD5 = (DOi - Dof)/(P) this is the five-day BOD of a diluted sample
Where DOi = the initial dissolved oxygen (DO) of the diluted wastewater
DOf = the final DO of the diluted wastewater, 5 days later
P = the dilution fraction = (volume of wastewater)/(volume of wastewater plus dilution water)
A standard BOD bottle holds 300 mL, so P is just the volume of wastewater divided by 300 mL

Consider a lake with 100 X 10^6 m^2 of surface area for which the only source of phosphorus is the effluent from a wastewater treatment plant. The effluent flow rate is 0.4 m^3/s and its phosphorus concentration is 10.0 mg/L (10.0 g/m^3). The lake is also fed by a stream having 20 m^3/s of flow with no phosphorus. If the phosphorus settling rate is estimated to be 10 m/yr, estimate the average phosphorus concentration in the lake. What level of phosphorus removal at the treatment plant would be required to keep the average lake concentration below 0.010 mg/L?


Solution Preview

Although not stated in the problem, I would assume an outflow to keep the lake level unchanged. Then this is essentially a mass balance problem. Draw a diagram showing the input from the wastewater treatment plant and the stream. Fill in each of the numbers given in the problem, and leave blanks for those you're ...

Solution Summary

Waste water mass balance problem