The concentration of UBOD in a river entering the first of two lakes that are connected in a series is equal to 20m/L. If the first-order BOD reaction rate constant (K) equals 0.35 d^-1 and complete mixing occurs in each lake, what is the concentration of UBOD at the outlet of each lake? The flow in the river is equal to 4000 m^3/d and the volume of the first and second lake is 20,000 and 12,000 m^3, respectively. Assume steady-state conditions.
If the length of the river connection between the two lakes is equal to 3km and the velocity in the river is equal to 0.4 m/s, determine the concentration of UBOD in the effluent from the second lake.