Recycle, Purge, and Bypass

Question. True or false: cell recycle uncouples the hydraulic retention time from the biomass residence time, enabling operation at dilution rates higher than $\mu_{max}$ without washout.

Solution.

1. Without recycle, HRT and biomass residence time are the same thing (cells leave with the liquid at rate $D$), so washout is unavoidable once $D > \mu_{max}$.
2. With recycle, the separator returns cells to the tank, so HRT stays $1/D$ (liquid-only) while biomass residence time stretches out — the two are no longer equal.
3. Because cells are no longer actually leaving at rate $D$, the reactor can run at $D > \mu_{max}$ without the population being flushed out faster than it can regrow.

Answer: True.