For industrial processes, cells, broth, or both, may be reused. Recycle of spent broth has the added advantage of saving expensive substrate that would otherwise be needed for new cell growth. Usually the carbohydrates in a bioprocess are pretty well exhausted, but much of the expensive nitrogenous ingredients remains. Some other nutrients must be supplemented, but adding sugar to the spent broth restores most of its nutrient value. Of course, total recycle is bad because undesirable materials can build up and impair the process.
Without recycle, washout occurs when D is greater than the maximum specific growth rate while S can be low. With recycle, D can greatly exceed the maximum specific growth rate. Some of the assumptions may not be rigorous. For example, a constant yield coefficient is assumed although it it known that starving cells at very low dilution rate differ from rapidly dividing cells. Further research with continuous cultivation of microorganisms is needed to provide relationships on which to base improved computer analysis.
When concentration factor times recycle fraction approaches 1, the washout can be many times that for no recycle.
End of this topic.