Consider a 10 x 10^6 m3 lake fed by a polluted stream having a flow rate of 5.0 m3
/s and pollutant
concentration equal to 10 mg/l. There is also a sewage outfall that discharges 0.5 m3
/s of
wastewater having a pollutant concentration of 100 mg/L. The stream of sewage wastes have a
decay rate coefficient of 0.20 / day.
a) Assuming the pollutant is completely mixed in the lake and assuming to evaporation or
other water losses or gains, find the steady-state pollutant concentration in the lake.
b) The sewage discharge was eliminated as a source of pollution. The incoming stream still has
flow Qs= 5 m3
/s and concentration Cs= 10 mg/L. To solve the problem, find the new final
steady-state concentration. Assuming complete mix conditions, find the concentration of
pollutant in the lake on week after the diversion.