An industrial plant discharges treated effluent into the New River. The effluent becomes instantaneously and completely mixed with the New River and the BOD of the mixture is 24 mg/L, the DO is 5.5 mg/L, and the saturated DO is 9.2 mg/L. The deoxygenation rate coefficient (kd) is 0.24/day and the reaeration rate coefficient (kr) is 0.71/day (both already corrected for temperature). The river is moving with a velocity of 1 m/s and a flow of 18 m3/s. The minimum DO occurs 133 km downstream of the industrial plant effluent discharge. Find the minimum downstream DO in mg/L.