Multiphase Bioreactor Design

are a very interesting technique due to their low operational costs (Sousa et al., 1994a)
and simplicity—no complex mechanical devices (e.g. centrifuges) are needed. In fact,
construction costs are low and energy input is not very significant. Moreover, it allows a
natural selection of the flocculating organisms in continuous fermentations, as the non-
flocculating ones will be dragged out of the system together with the effluent. Further, the
use of flocculating cells may provide important contributions for the improvement of
separation processes in fermentation; besides being less aggressive than other separation
techniques, a reduction in production costs is obtained since the amount of cells to be
separated by centrifugation or filtration is significantly reduced (Teixeira and Mota,
1992). An additional advantage is obtained when using bioreactors operating with
genetically modified flocculent microorganisms. In fact, such reactors are able to retain
the genetically modified biomass, thus minimising the danger of spreading it into the
environment, while exerting a permanent selective stress in the organism by expelling all
the individuals that have lost flocculation ability.
Among the several possible bioreactor designs, reactors in which the power for mixing
and mass transfer is introduced by injecting gas (such as bubble columns and airlifts) are
better adapted to the culture conditions of flocculating microorganisms and present other
advantages summarised by Michalski (1992):

the construction is simpler and cheaper;

compressors can easily be maintained or removed without the need to stop the process

(as would be the case of e.g. a stirring motor);

maintenance is simpler because there are no rotating parts below the liquid level; the

maximum local shear stresses are much smaller than in other types of bioreactors;

gas dispersion efficiency is good.

If a flocculent organism is used, the absence of an immobilisation support is also an
advantage. In fact, it usually poses problems either in downstream processing (if it has to
be separated from the product) or while recovering the biomass or the support itself (as it
has to be separated from the cells), contributing to increased production costs. The
environmental pollution can also become a worry when the disposal of the unrecoverable
support is to be considered. Under such circumstances flocculation can be a solution,
having no need for biomass/support separation and no additional environmental worries.
However, in general and strongly associated with the flocculation ability of the
microorganism used, there is a drawback concerning flocculation bioreactors when these
are compared with those using an immobilisation support; while in continuous operation,
the biomass concentration in the outlet stream is higher in the former than in the latter.
Also, although present in most immobilisation systems, diffusional limitations may be a
high concern in flocculation bioreactors.

Flocculation

To clearly understand the operation of flocculation bioreactors, it is convenient to clarify
the mechanism of cell flocculation.
Cell properties are the result of the permanent interaction between environmental,
nutritional and genetic control; in the specific case of yeast, the effects on cell wall
composition are the ones mainly responsible for the capacity of forming aggregates—

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