However, employing immobilized cells implies that they do not reproduce during
reactor operation. The growth means that cell layers are accumulated on the
support surface until the moment they start to deattach from the solid phase leading
to the system destabilization. To avoid this, the necessary conditions for the
cells not to grow (nonviable cells) are ensured. In addition, if aeration is needed,
a constant air supply has to be available, which can be difficult when a fixed bed
of biocatalysts is used (Figure 7.5a). For this reason, an auxiliary tank is used to
supply air to part of the effluent stream, which is then recirculated to the reactor to
ensure the aerobic conditions of the culture broth within the bed. One alternative
configuration is the fluidized-bed reactor where the liquid feed stream flows up
inside the reactor containing mobile biocatalyst particles. In this way, the bed is
expanded, as shown in Figure 7.5b. If aerobic conditions are required, the air can
be directly injected into the bioreactor. Despite these advantages, processes using
immobilized cells are not widespread in industrial microbiology today due to the
complexity of the systems involved.