A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids is shown in the figure.
S0, S = readily biodegradable soluble BOD, mg/L
Q, Qr, Qw = flow rates, m3/d
X0, X, Xe, Xu = microorganism concentrations (mixed-liquor volatile suspended solids or MLVSS), mg/L
The mean cell residence time (in days, up to one decimal place) is ______