58. A municipality is considering upgrading its POTW to provide secondary treatment so that the receiving stream has better aesthetics and provides improved habitat for fish and other aquatic organisms. While fishes differ in their tolerance of low DO, 7 mg/liter or higher is necessary for many species to thrive, and even short-term survival of some species is threatened when DO levels decrease to approximately 5 mg/liter. Wastewater enters the POTW at a rate of 0.09 m sec and has a typical BOD of 200 mg/liter, which is decreased 20% by the existing primary treatment prior to disinfection and discharge to a stream. During dry periods, typical streamflow is 0.2 me, and average stream width and depth are 10 m and 0.3 m, respectively. The rearation co-efficient, based on a propane gas tracer experiment, has been measured as 2.1/ day. The decay rate of BOD in effluent from this facility can be approximated as 0.25/day; DO upstream of the POTW is 9 mg/liter. a. How far downstream from the POTW is the maximum DO sag expected? b. What is the minimum DO in this stream? Could a population of northern pike, which need a minimum of 6 mg/liter DO for short-term survival, survive a dry period in this stream? C. What minimum DO would be expected in this stream under the flow conditions specified if the municipality were to implement secondary treat-ment, which would remove approximately 80% of the BOD from the wastewater entering the POTW?