Operating a Chemostat (Continuous Stirred-Tank Bioreactor)

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Cells are grown in a chemostat (continuous stirred-tank bioreactor) according to the reaction: cells + substrate ⟶ more cells + product. Cell growth (for a given death rate and maximum growth rate) depends on the dilution rate and the nutrient/substrate concentration. The cell (, dashed) and substrate () concentrations are plotted in blue, and the production rate () is plotted in green. Washout occurs when the dilution rate is so high that the cells are flushed out of the reactor, so that the cell concentration drops to zero. Use the sliders to vary the feed substrate concentration, the Monod constant, the maximum growth rate, and the death rate.

Contributed by: Garrison J. Vigil  (April 2018)
Additional contributions by: Rachael L. Baumann and John L. Falconer
(University of Colorado Boulder, Department of Chemical and Biological Engineering)


Snapshots


Details

Rates of cell growth , cell death , and substrate consumption are:

,

,

,

,

where is the specific cell growth rate (), and are the cell and substrate concentrations (g/L), is the specific death rate constant (), is the yield coefficient, is the maximum specific growth rate (), and is the Monod constant (g/L).

The cell and substrate concentrations are found by solving the CSTR mass balances at steady state:

,

.

Solving for and :

,

,

where is the dilution rate (), is the feed substrate concentration (g/L), and .

Washout occurs when the cell concentration is zero:

.

The maximum rate of cell production is where is greatest, or:

,

,

where and are in units of .



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