A small lake with volume 20.0 x 103 m3) has no natural pollutant sources. However, a sewage pipe discharges sewage containing a pollutant into the lake. Conditions in the sewage pipe ate Qs = 0.5 m3/s, and Cs = 100.0 mg/L The pollutant is destroyed in the lake with a first-order rate constant of 0.2Wday. The flow rate out of the lake is 5.0 m3/s. Assume that the lake is well-mixed.
(a) Calculate the steady-state pollutant concentration in the lake.
(b) What is the retention time for this lake?