A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids shown in the figure.
$S_0$, $S$ = readily biodegradable soluble BOD, mg/L
$Q, \: Q_r, \: Q_w$ = flow rates $m^3/d$
$X, \: X_0, \: X_e, \: X_u$= microorganisms concentrations (mixed liquor soils volatile suspended solids or MLVSS ), mg/l
The mean cell residence time (in days, up to one decimal place) is _____