$9-25$ The initial ultimate BOD after mixing of the Bergelmir River is

I$2\mathrm{.}0mg*L^{-\text{I}}.$ The DO in the Bergelmir River after the wastewater

and river have mixed is at saturation. The river temperature is

$10C.$ At $IOC,$ the deoxygenation rate constant $(k_d)$ is $0\mathrm{.}30$ day ${?}^{-\text{I}},$

and the reaeration rate constant $(k_r)$ is $0\mathrm{.}40$ day ${?}^{-1}.$ Determine the

critical point $(t_c)$ and the critical DO$.$ Assume the river temperature is not significantly changed after the wastewater effluent mixes with it$.$