A $5-$day BOD test is performed on a sample of chlorinated wastewater effluent. Sixty milliliters of

wastewater sample and $2$ ml of $$seed$$ are added to $300-$ml BOD bottles and mixed with enough dilution

water to fill the bottles. $$Seed$$ must be added, since wastewater effluent is chlorinated, which kills

pathogens and bacteria. For the blanks or controls, $2$ ml of $$seed$$ are added to $300-$ml BOD bottles and

mixed with enough dilution water to fill the bottles. The average dissolved$-$oxygen concentrations of the

diluted wastewater samples and blanks on day $1$ of the test were $6\mathrm{.}8$ and $7\mathrm{.}4$ mg$/$L$,$ respectively. After

incubation at $20$C for $5$ days, the average DO concentrations of the diluted wastewater samples and

blanks were $2\mathrm{.}5$ and $7\mathrm{.}1$ mg$/$L$,$ respectively. Calculate the $5-$day BOD of the chlorinated effluent.