A well-mixed sewage lagoon (a shallow pond) is receiving 430 m^3/d of sewage out of a sewer pipe. The lagoon has a surface area of 10 ha (hectares) and a depth of 1 m. The pollutant concentration in the raw sewage discharging into the lagoon is 180 mg/L. The organic matter in the sewage degrades biologically (decays) in the lagoon according to first- order kinetics. The reaction rate constant (decay coefficient) is 0.7 d^-1. Assuming no other water losses or gains (evaporation, seepage, or rainfall) and that the lagoon is completely mixed, find the steady-state concentration of the pollutant in the lagoon effluent.