$2\mathrm{.}2.$ An activated sludge process is to be used for secondary treatment of $15,000\frac{m^3}{d}$ of municipal wastewater. After primary clarification, the BOD is $170m\frac{g}{L},$ and it is desired to have no more than $25m\frac{g}{L}$ soluble BOD in the effluent from the activated sludge process. Preliminary analysis has established the following values:

$HRT=10d$

$Y=0\mathrm{.}5k\frac{g}{k}g$

$k_d=\frac{0\mathrm{.}05}{d}$

$MLSS=4500m\frac{g}{L}$

Concentration of solids from the secondary clarifier $=12,000m\frac{g}{L}$

Determine $($a$)$ the volume of the activated sludge reactor, $($b$)$ the mass and volume of solids that must be wasted each day, and $($c$)$ recycle ratio i$.$e$.\frac{Q_r}{Q}.$

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