If a medium is fed continuously to a culture at a suitable rate, a steady state is achieved eventually, that is, formation of new biomass by the culture is balanced by the loss of cells from the vessel.
A chemostat is a nutrient- limited self balancing culture system which may be maintained in a steady state over a wide range of sub-maximum specific growth rates. Feedback Systems: It is operated in two ways:
i) Internal feedback- Limiting the exit of biomass from the chemostat such that the biomass in the effluent stream is less concentrated than in the vessel.
ii) External feedback- Subjecting the effluent stream to a bioprocess separation process, such as sedimentation or centrifugation, and returning a portion of the concentrated biomass to the growth vessel.
Feedback contributes significantly to the process efficiency. Also feedback systems are attractive for animal cell culture where the low growth rates and low cell densities limit productivity.