Culture effluent

Culture effluent

Culture effluent to effluent collection or to subsequent stages

Fig. 2.7. A multistage chemostat.

advantageous in the utilization of multiple carbon sources and in the production of secondary metabolites. Harte and Webb (1967) demonstrated that when Klebsiella aerogenes was grown on a mixture of glucose and maltose only the glucose was utilized in the first stage and maltose in the second. Secondary metabolism may occur in the second stage of a dual system in which the second stage acts as a holding tank where the growth rate is much smaller than that in the first stage. The adoption of multistage systems in research and industry has been extremely limited, due to the complexity of the systems. One example of the industrial application of the technique is in continuous brewing which is described in a later section.

Feedback systems

A chemostat incorporating biomass feedback has been modified such that the biomass in the vessel reaches a concentration above that possible in a simple chemostat, that is, greater than Y(SR — s). Biomass concentration may be achieved by:

(i) Internal feedback. Limiting the exit of biomass from the chemostat such that the biomass in the effluent stream is less concentrated than in the vessel.

(ii) External feedback. Subjecting the effluent stream to a biomass separation process, such as sedimentation or centrifugation, and returning a portion of the concentrated biomass to the growth vessel.

Pirt (1975) gave a full kinetic description of these feedback systems and this account summarizes his analysis.

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