$4\mathrm{.}31$ Fifteen $(15\mathrm{.}0)mL$ of a sample of wastewater was put into a $300mL$ BOD bottle, and then the bottle was filled to its full volume of $300mL.$ The initial dissolved oxygen $($DO$)$ was $9\mathrm{.}3m\frac{g}{L}$ and five days later the $DO$ was $5\mathrm{.}4m\frac{g}{L}.$

Water Resources

$165$

a$.$ What is the $BO{D}_5$ of this wastewater, in $m\frac{g}{L}?$

b$.$ If the BOD decay rate constant is $0\mathrm{.}25$ day ${?}^{-1},$ what is the ultimate BOD, in $m\frac{g}{L}?$