## Tuesday, April 19, 2011

### Chemostat Culture Vs Batch Culture: Microbial Growth and Kinetics

A chemostat is a bioreactor to which fresh medium is continuously added, while culture liquid is continuously removed to keep the culture volume constant.By changing the rate at which medium is added to the bioreactor the growth rate of the microorganism can be easily controlled.

The residence time distribution of a chemical reactor is a probability distribution function that describes the amount of time a fluid element could spend inside the reactor.

Dilution Rate

The dilution rate is defined as the rate of flow of medium over the volume of culture in the bioreactor, it is denoted by D.

Dilution rate, D = f / V [Medium Flow rate / Culture Volume]

Growth and Dilution Rate

If the limiting nutrient is the source of energy, growth ceases at low dilutions and the cells are washed out.
If the limiting nutrient is an amino acid or other precursor in macromolecular synthesis, chemostat can be operated at dilution rates leading to a mean residence time of several days or weeks.

Equations in Chemostat culture

Mean resident time, MRT = V / f
Dilution rate, D = f / V [Medium Flow rate / Culture Volume]
μ= D (dx / dt =μx – xf / V =μx – Dx = 0  in the chemostat)

c = KsD / (μmax – D) (solving Monod equation for c gives the relationship between nutrient concentration in the growth vessel and the dilution rate)
Y = x / (cr – c), cr: nutrient conc. in the reservoir
dc / dt = Dcr – Dc,  dc / dt = (dx / dt) (dc / dx),
dx / dt = μx,  dc / dx = 1/Y, μx / Y = Dcr – Dc,       D = μ

Batch Culture Vs Continuous Culture

In batch culture, the culture environment continuously changes
• Growth, product formation, and substrate utilization all terminate after a certain time interval.

In continuous culture, fresh nutrient medium is continually supplied to a well-mixed culture, and products and cells are simultaneously
withdrawn
• Growth and product formation can be maintained for
prolonged periods of time
• At steady state, cell, product, and substrate
concentrations remain constant

Continuous culture provides constant environmental conditions for growth and product formation, and supplies uniform quality product

• Has higher productivity than batch culture for growth associated products.

Batch culture predominantly used in the biotech industry because

• Many secondary products are not growth associated
• Genetic instability
• Operability and reliability
• Economic considerations

Reactors for Continous Culture

Chemostat:

• Also known as a continuous stirred tank reactor (CSTR)

• Cellular growth is typically limited by one essential nutrient; other nutrients are in excess

• When operated at steady state, nutrient, product and cell concentrations are constant.

Plug Flow Reactor (PFR):

• Substrate and cell concentrations vary with axial position in the reactor

• An ideal PFR resembles a batch reactor in which distance along the fermenter replaces incubation time

The Ideal Chemostat

An ideal chemostat is a perfectly mixed, continuous-flow, stirred-tank reactor

• Most chemostats require some control elements (e.g. pH and DO controllers)

• Fresh sterile medium is fed to the completely mixed and aerated reactor, and cell suspension is removed at the same rate.

• Liquid volume in the reactor is kept constant.

References:
Principles of Fermentation Technology
chemeng.mcmaster.ca/courses/che3bk3/Lecture