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?