The municipal wastewater treatment process for a hypothetiical town consists of two separate lagoons being operated in series. The first lagoon has a volume of 4 million gallons, while the second lagoon has a volume of 2.5 million gallons. Average flow into the first lagoon is 650,000 gallons/day. The concentration of BOD (material to be degraded) in the infuent is 175 mg/L, and its concentration in such a system has been observed to decrease according to first-order kinetics with a rate constant of 0.25/day. Presume that both lagoons can be considered well-mixed and to be operating at steady state.
(a) Find the BOD concentrations at the outlet of both lagoons.
(b) If the NPDES permit specifies an outlet BOD of 40 mg/L, might this lagoon system be meeting requirements?