![A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids](https://www.gkseries.com/blog/wp-content/uploads/2023/09/A-schematic-flow-diagram-of-a-completely-mixed-biological-reactor-with-provision-for-recycling-of-solids.jpg)
Q. A schematic flow diagram of a completely mixed biological reactor with provision for recycling of solids is shown in the figure.
![](https://www.gkseries.com/blog/wp-content/uploads/2023/09/Screenshot-947.png)
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
Ans: 7.5
Sol:
![](https://www.gkseries.com/blog/wp-content/uploads/2023/09/image-82.png)
![](https://www.gkseries.com/blog/wp-content/uploads/2023/09/image-83.png)