Problem statement: Aeration tank volume $=0\mathrm{.}898$ m$3,$ Wastewater flow $=3\mathrm{.}67$ m$3/$day$,$ Waste sludge flow $=0\mathrm{.}0189$ m$3/$day$,$ MLSS $=$ MLVSS $=2,350$ mg$/$L$,$ SS in waste sludge $=11,000$ mg$/$L$,$ Influent BOD $=128$ mg$/$L$,$ Effluent BOD $=22$ mg$/$l$,$ Effluent SS $=26$ mg$/$L

Calculate:

$($a$)$ Return sludge flow; $($b$)$ F$/$M$,($c$)$ Sludge age; $($d$)$ Aeration period $($hydraulic retention time$)$