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Design of Continuous Bioreactors - Lesson Summary

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Key points in this module are:

Continuous culture can be operated as a chemostat or turbidostat.

Mass balance in a chemostat can be calculated as Accumulation = Rate of cell input - Rate of cell output + Rate of cell growth.

The most important features of a chemostat are that microorganisms can be grown in a physiological steady state under constant environmental conditions.

• For steady-state CSTR with sterile feed, the specific growth rate of the culture can be controlled by the dilution rate.
• The residence time can be used to compare the effectiveness of a fermenter.
• The productivity of a fermenter is expressed as the amount of a product per unit of time and volume.
• At very high dilution rates, the microorganisms are washed out faster than they multiply
• At very low dilution rates, nutrient supply becomes limited.

Comparison in productivity between batch and continuous bioreactors include the following:

• Batch Bioreactor - Easy to operate
• Continuous Bioreactor - Gives high productivity compared to batch bioreactor
• Continuous bioreactors require skill to operate
• Frequent contamination will reduce the productivity of continuous bioreactor
• If the final cell concentration to be reached is in the late log phase, the batch fermenter is a better choice than CSTR
In multiple CSTRs connected in series, if the cell concentration is less than Cxopt one fermenter is better than two fermenters connected in series. If cell concentration is more than Cxopt the best combination is a CSTR operated at Cxopt followed by PFR.