Biomass feedback systems

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

This complicated commensalistic system is shown to be less stable, with limit-cycle response occurring after dilution rate changes, as would be expected from a model with more parameters. Though virtually any feedback from the dependent to the independent species will cause some overshoot after a step change in D, the most pronounced oscillatory behavior is caused by feedback inhibition and feedforward activation. Limited agreement with experimental data was obtained even though the analysis was somewhat limited due to the complexity of the system and the large number of differential equations. Similar results were obtained by Sheintuch (1980), who examined the dynamics of commensalistic systems with self- and cross-inhibition. Multiplicity of steady states was observed as well as oscillatory states with these complex kinetics and in the case of a reactor with biomass recirculation. Stability and dynamics are summarized in a qualitative phase plane by this author. 

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