Average influent pollutant concentration

Question

Problem:

A wastewater treatment lagoon is able to reduce pollutant concentrations at a rate of 84% per day with a process that follows 2nd-order reaction kinetics. The treatment system currently operates at a uniform, steady flow of 100 m³/d.

Required:

Question: Assuming the average influent pollutant concentration equals 45 mg/L and beginning with a general mass balance expression, derive a relationship describing pollutant concentrations in this system.

Note: Explain in detail.