2. A pond is used to treat sewage wastewater before the liquid is discharged into a river. The inflow to the pond is sewage at a flow rate of 4000 m3 /day and with a BOD concentration of Cin = 25 mg/L. The volume of the pond is 20,000 m3. The purpose ofthe pond is to allow time for the decay of BOD to occur before discharge into the environment. BOD decays in the pond with a first-order rate constant equal to 0.25/day. What is the BOD concentration at the outflow of the pond, in units of mg/L?