A wastewater treatment plant (WWTP) discharges to a river. The wastewater effluent characteristics are: flow = 1.5 m3/s, BOD = 60 mg/L, DO = 0 mg/L, and temperature = 25oC. The river characteristics just upstream of the wastewater discharge are: flow = 7.5 m3/s, BOD = 5.5 mg/L, DO = 90% of saturation, and temperature = 25oC. For this water, kd = 0.18 d-1 and kr = 0.22 d-1.
A. Find the BOD (= L0), DO, and oxygen deficit (= D0) just after the wastewater effluent and river mix.
B. Plot the DO from the point of discharge to 20 days downstream of the discharge point as a function of travel time.
C. Fish hatcheries required more than 4 mg/L of dissolved oxygen. If the river flows at 2 km/day, where in the river should you avoid siting a fish hatchery? Express your answer as a range in km downstream from the discharge point