Startup and Steady State in a Chemostat

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This Demonstration shows the operation of a chemostat. Starting up as a batch reactor, the concentrations change with time. After this short transient period, the bioreactor settles into a steady state.


The behavior of a chemostat, also called a continuous reactor, can be characterized by the concentration profiles of the reactants and products.

In this Demonstration, is time, is the concentration of a reactant consumed (also called the substrate), is the biomass (or cell) concentration, is the concentration of a product and is the dilution rate. The concentration in a perfectly mixed tank is uniform throughout the vessel and is therefore identical to the concentration of the effluent stream.


Contributed by: R. Ricardo Sánchez (August 2022)
Open content licensed under CC BY-NC-SA



The characteristics of continuous operation are as follows:

Snapshot 1: steady state; after an initial startup period, there is no variation of concentrations with time

Snapshot 2: constant reaction rates, where is the biomass reaction rate, is the reaction rate of the consumed reactant and is the product reaction rate

Snapshot 3: Washout of the organisms (cells) will occur when the dilution rate is greater than the specific growth rate . This corresponds to the complete removal of cells by flow out of the tank. In many references, including [1], the variables , , and productivity are plotted versus the dilution rate , which characterizes the steady state in a chemostat. See Related Links.

Some suggestions for the user:

Increase interactively to note effect on washout.

Change the feed substrate concentration to alter the steady state.

Investigate the influence of maintenance requirements on the steady-state biomass concentration.

Operate initially as a batch reactor with , and switch to chemostat operation with . is the maximum specific growth rate, and in this case was set up to .

Calculate , the biomass-substrate yield, making use of the yield graph.


[1] I. J. Dunn, E. Heinzle, J. Ingham and J. E. Prvenosil, Biological Reaction Engineering: Dynamic Modelling Fundamentals with Simulation Examples, Second Edition, Weinheim, Germany: Wiley-VCH, 2003.

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