$2.$ Assuming that the mixed oxygen deficit $($Da$)$ is zero and that the ultimate BOD $($Lr$)$ of a river above the wastewater outfall is zero, calculate the amount of ultimate BOD $($in kg$/$day$)$ that can be discharged if the DO must be kept at $4\mathrm{.}00$ mg$/$L at a point $8\mathrm{.}05$ km downstream. The stream deoxygenation rate $($kd$)$ is $1\mathrm{.}80$ days$-1$ at $12$C$,$ and the reaeration rate $($kr$)$ is $2\mathrm{.}20$ days$-1$ at $12$C$.$ The river temperature is $12$C and saturation value of DO at $12$C is $10\mathrm{.}83$ mg$/$L$.$ The river flow is $5\mathrm{.}95$ m$3/$s with a speed of $0\mathrm{.}300$ m$/$s$.$ The wastewater flow is $0\mathrm{.}0130$ m$3/$s$.$