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264 Bioreactors

clean-in-placemethods.In-placecleaningdoesnotrequire
dismantling the bioreactor and saves time. Automation as-
sures consistency of cleaning. Cleaning between batches
is essential to preventing cross-contamination of products.
Also, a clean bioreactor is easier to sterilize. Cleaning
is achieved by physical action of high velocity flow, jet
sprays, agitation, and chemical action of cleaning agents
enhanced by heat. While mechanical forces are necessary
to remove gross soil and to ensure adequate penetration
of cleaning solutions to all areas, most of the cleaning ac-
tion is provided by chemicals—surfactants, acids, alkalis,
and sanitizers. A generally applicable cleaning scheme for
bioreactors utilizes a water pre-rinse to remove gross soil;
a hot alkali recirculation step to digest and dissolve away
the remaining soil; and a water wash to remove residual
alkali. A bioreactor that processes injectable drugs, should
be rinsed with a hot water-for-injection (WFI) wash as the
final cleaning step. Optional acid wash and sanitization
FIGURE 23 Bioreactor operational modes: (a) a continuous flow
steps may be added in some applications. well-mixed bioreactor, (b) a continuous flow well-mixed bioreactor
with biomass recycle.
D. Operational Modes of Bioreactors

The way a culture is operated and fed has a profound im-
In a well-mixed steady-state continuos culture
pact on the outcome of the fermentation. Also, the reactant
(Fig. 23a), the broth composition does not vary with time
feeding and reactor operational strategies affect perfor-
and position in the bioreactor. Typically, a continuous
manceofenzymebioreactorsandthosethatusenon-viable
fermentation starts as a batch culture and switches to
biomass as a biocatalyst. Most bioreactors for microbial
continuous feeding when a sufficient concentration of
growth and culture of other cells are operated as batch
biomass has been obtained. The feed is added contin-
and fed-batch devices. A batch fermentation is initiated
uously and at a constant rate. The broth volume does
by inoculating a presterilized and cooled medium that is
not change, as the rate of harvest matches the feeding
contained typically in a well-mixed bioreactor (Fig. 22a).
rate (Fig. 23a). Sometimes, a well-mixed continuos
The medium composition in the fermenter changes con-
culture may be carried out with recycle of a part of the
tinuously as nutrients are consumed to produce biomass
harvested biomass to the bioreactor (Fig. 23b). This
and metabolites. The broth is harvested after the desig-
strategy increases the steady state biomass concentration
nated batch time. The broth volume in a batch fermenter
in the reactor, improves conversion of the substrate, and
remains essentially constant (Fig. 22a), discounting any
enhances the productivity. Some continuous flow reaction
evaporative loss. A fed-batch culture is identical to a batch
and production schemes use a number of well-mixed
operation, except that a feed is added to the broth continu-
reactors in series (Fig. 24a–c). The harvest of one reactor
ously or intermittently (Fig. 23b). The volume of the broth
becomes the feed for the next. This arrangement is es-
increases with time. Also, the feeding rate generally in-
pecially useful when different environmental conditions
censes with time, to satisfy the demand of an exponentially
are needed at different stages of a process, for example,
increasing cell population. The broth is harvested at the
when the requirements for growing the biomass differ
end of the batch period.
from the ones for synthesis of a metabolite by the cells. A
multistage continuous array of well-mixed reactors may
be fed with different feeds and precursor compounds at
different stages, as shown in Fig. 24b. Also, such a series
of reactors my employ recycle of the biomass (Fig. 24c)
to the first stage, or one or more of the other stages.
Continuous flow processes sometimes use a plug flow
bioreactor in which there is little mixing of fluid elements
in the direction of flow (Fig. 25a). This type of flow is
FIGURE 22 Bioreactor operational modes: (a) a well-mixed batch typically achieved in long tubes and channels. The com-
bioreactor, (b) a well-mixed fed-batch bioreactor. position of the broth does not change with time at a fixed