A pond is used to treat a dilute municipal waste-water before the liquid is discharged into a river. The inflow to the pond has a flow rate of Q = 4,000 m3/day and a BOD concentration of Cin = 25 mg/L. The volume of the pond is 20,000 m3. The purpose of the pond is to allow tie 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?