5.18 the ultimate bod of a river just below a sewage outfall is 50.0 mg/l and do is at the saturation value of 10.0 mg/l. the deoxygenation rate coefficient kd is 0.3/day, and the reaeration rate coefficient kr is 0.90 /day. the river is flowing at the speed of 48.0 miles per day. the only source of bod on this river is this single outfall. (a) find the critical distance downstream at which do is a minimum. (b) find the minimum do. (c) if a wastewater treatment plant is to be built, what fraction of the bod would have to be removed from the sewage to assure a minimum of 5.0 mg/l everywhere downstream?