Question

Consider a 10x108 m3 lake fed by a polluted stream having a flow rate of 2.0 m3 /s and pollutant concentration equal to 10.0 mg/L. There is also a sewage outfall that discharges 0.5 m3 /s of wastewater that has a pollutant concentration of 100 mg/L into the lake. The stream and sewage pollutants have a decay rate coefficient of 0.20 day-1 (first order). Under these flow conditions the lake remains the same volume (e.g. a stream enters and exits the lake) and that the lake shallow and in an area which significant wind (i.e. always has white caps). Assume there is no evaporation or other water losses or gains.

a) Which ideal reactor would be best use mathematically model this system? Why? (1 sentence)

b) What is the steady-state concentration of pollutant in the lake under current conditions?

c) If one day the wastewater plant treatment process were to be improved, releasing a pollutant concentration of 25 mg/L at the 0.5 m3 /s (which is assumed to be become immediately completely mixed in the lake), how long would it take for the system to most nearly reach steady-state.

Answer 1