Question

A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids is shown in the figure.



So, $S=$ readily biodegradable soluble BOD mg/l
$Q,Q_1,Q_W=$ flow rates, $m^3/d$

$X_0,X,X_e,X_u=$
microorganism concentrations (mixed-liquor volatile suspended solids or MLVSS), mg/L
The mean cell residence time (in days, up to one decimal place is______

• A. 7.5

Answer 1

The correct option is A 7.5
Mean cell residence time is given by

${\theta }_c=\frac{VX}{(Q_O-Q_W)X_e+Q_W{X}_u}$
Detention time $=2h$

Volume of tank

$=Q_o{t}_d=15000\times \frac{2}{24}=1250m^3$
Since, $X_e=0,{\theta }_c$ will be calculated as follows:

${\theta }_c=\frac{1250\times 3000}{50\times 10000}=7.5days$