The saturation of dissolved oxygen concentration of a stream is 9.1 mg,/l.. At a sewage outfall. the dissolved oxygen concentration of the stream is 8.0 mg/L. The stream has a reaeration rate constant of 4 /day and a deoxygenation rate constant of 0.1 /day. initial BOD ultimate in the mixture zone is 200 mg/L. The time after discharge at which the water will reach its minimum dissolved oxygen concentration is :