A 6 ml sample of wastewater is diluted to 300 ml in a standard BOD bottle. The initial DO level in the bottle is 8.5 mg/l and DO after 5 days at 20C was found to be 0.5 mg/1. The BOD of the waste is reduced to 40 g/m when treated in an aerated lagoon, find the aeration period. Given value of k is 0.23 d' at 20C.