Remember, the mass balance equation for a continuous flow, completely mixed system in which a reaction is occurring $(k=$ rate constant, $n=$ reaction order$)$:

${\displaystyle \underset{?}{\overset{?}{}}}(Q_{in}{C}_{in})-{\displaystyle \underset{?}{\overset{?}{}}}(Q_{\text{o}\text{u}\text{t}}{C}_{\text{o}\text{u}\text{t}})+-k{C}^{n}V=d\frac{M}{d}t$

A single activated sludge unit is designed as a completely$-$mixed, continuous$-$flow system $)=(Q_{\text{o}\text{u}\text{t}}$ at steadystate. The first$-$order rate constant describing production of biomass in the unit is $4\mathrm{.}8$ day ${?}^{-1}.$ Virtually no biomass enters the activated sludge unit. What is the hydraulic retention time of the activated sludge unit? Remember hydraulic retention time $=$ volume $\frac{?}{?}$ flowrate.