A facultative pond is designed for a community of 5000 people. Summer wastewater flow is 2000 m3/d and BOD5 is 180 mg/l. Winter flow and BOD5 values are 6000 m3/d and 90 g/m3 respectively. Average pond temperatures are 20 oC in the summer and 10 oC in the winter. Assuming that a CFSTR model is appropriate and that oxygen transfer will not be a problem, determine the pond volume required to reduce the influent BOD5 to 20 mg/l.