4.1 A waste stabilization pond is used to treat a dilute municipal wastewater before the liquid is dis- charged 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 time for the decay of BOD to occur before discharge into the environ- ment. BOD decays in the pond with a first-order rate constant equal to 0.25/day. What is the BOD concen- tration at the outflow of the pond, in units of mg/L?