Chemostat

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A chemostat. A flow of nutrient enters the chemostat. The growth of bacteria consumes some of the nutrient. In the outflow bacteria are harvested (most likely not all of the nutrient is consumed so some exits the reactor as well.)

A chemostat (from Chemical environment is static) is a device used in microbiology for growing and harvesting bacteria. It consists of two primary parts: a nutrient reservoir and a growth chamber. The volume of growth medium for the bacteria is kept constant in order to run a controlled experiment with a known and well defined population dynamic.

Some sources of concern are:

Foaming results in overflow with the volume of liquid not exactly constant
Some very fragile cells are ruptured when caught between the magnetic stirring bar and the glass of the vessel. Suspending the stirring bar usually corrects this fault.
Changing pumping rate by turning the pump on and off over short time periods may not work because cells respond to sudden changes by altering their rates. Very short intervals of on/off are OK.
Bacteria travel upstream quite easily. They will reach the reservoir of sterile medium quickly unless the liquid path is interrupted by an air break in which the medium falls in drops through air.

The mathematical model behind the chemostat is a dynamic system, often with two variables. The first describing the concentration of nutrient and the second describing the concentartion of bacteria.