Chemostats With Recycle
A recycling of cells from the chemostat effluent provides a means of continuously inoculating the vessel. Such recycling adds stability to the system and minimizes the effect of process perturbations. It allows a chemostat to be operated at higher steady state cell concentrations than in a chemostat without recycling.
The nomenclature is the same as for the chemostat except for two additional parameters, a, the recycle ratio, and C, the concentration factor:
Cells in feed +Cells in recycle – Cells out +Cells growth = Cell accumulation
F/ V(Xo) + aFC/ V(X1) -1 + a/ V(X1)+ μ X1 = dX/dt
Cell maintenance and death are assumed to be small. Solving the equation for the steady state where dX / dt = 0, and substituting D = F/V,
μ = (1 + a - aC)D
With the recycle, thus, D does not equal m anymore, since the (1+a-aC) is always lesser than 1 and tthe concentration factor 1 > 1, and μ < D. As a consequence of the cell recycle, it is possible to increase the overall productivity of the system because a dilution rate greater than the maximum specific growth rate may be employed.
Recycling systems can be very expensive in certain industries, so can be the requirement for each batch inoculation. The inoculum cascading technique has been trialed and found suitable for certain processes, whereby one batch is inoculated from the previous batch fermentation. Timing and culture conditions are, however, of greatest importance. Often, inoculum cascading has to be performed after only a short fermentation time to avoid carry-over of lysed or dead or contaminating cells.
Solid-state cultivation is generally characterized by the growth of microorganisms on water insoluble substrates in the absence of free water, whereas solid-substrate cultivation is characterized by the growth of microorganisms on water insoluble substrates in the presence of free water. With increasing amounts of free water, solid-state cultivation progresses via solid-substrate to liquid substrate cultivations in the presence of solid particles.