2. The concentration of UBOD in a river entering the first of two lakes that are connected in series is equal to 20 mg/L. If the first-order BOD reaction rate

constant (K) equal to 0.35/d and complete mixing in each lake, what is the concentration of UBOD at the outlet of each lake? The flow in the river is equal to 4000 m³/d and the volume of the first and second lake is 20,000 and 12,000 m², respectively. Assume steady-state conditions.