$1.$ The following data are available for a WWTP $(8)$

Q$=0\mathrm{.}045$ m$3/$s$.$ The BOD$5$ from the primary treatment effluent $=225$ mg$/$L$,$ S $=5$ mg$/$L$,$

MLVSS $=2,600$ mg$/$L$,$ Y $=0.5$ mg VSS$/$mg BOD$5,$ kd $=0\mathrm{.}056/$day$,$ Ks $=80$ mg$/$L BOD$5,$

$\backslash$mu m$=3/$day$.$ Determine the following:

a$.$ Mean cell residence time

b$.$ Hydraulic retention time

c$.$ Aeration tank volume

d$.$ F$/$M ratio