Ariver has a flow velocity of 3 cm/s, an aeration coefficient of 0.5 d–1, and a temperature of 20°c. after initial mixing with wastewater, the river has a do concentration of 8.0 mg/l, an ultimate bod of 25 mg/l, and a bod decay constant of 0.8 d–1. neglecting the effect of sedimentation on bod removal, determine the distance over which the do is less than or equal to 7 mg/l.

Air enters a horizontal, constant-diameter heating duct operating at steady state at 290 k, 1 bar, with a volumetric flow rate of 0.25 m°/s, and exits at 325 k, 0.95 bar. the flow area is 0.04 m2. assuming the ideal gas model with k = 1.4 for the air, determine (a) the velocity at the inlet and exit, each in m/s, and (c) the rate of heat transfer, in kw flow rate, in kg/s, (b) the mass kg 0.3