Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 641

Substituting the kinetic and operating parameters into Eqn (E12-5.6) , we obtain

S 0 S
1 þ Rð1 c R Þþk d V

800 18:97
1 þ 0:4 ð1 2:5Þþ0:005 3200
400

X ¼ YF X = S ¼ 0:5 mg = L

Q

¼ 887:53 mg = L

And the total sludge production G X ,

G X ¼ð1 þ RÞQX c R RQX ¼½1 þ Rð1 c R ÞQX (E12-5.7)

G X ¼ 1 þ 0:4 ð1 2:5Þ 400 887:53 = ¼ 142005 = ¼ 142 600=

g h g h kg-CS h

12.4. IMMOBILIZED CELL SYSTEMS

Immobilization of cells as biocatalysts are almost as common as enzyme immobilization.
Immobilization is the restriction of cell mobility within a defined space. Immobilized cell
cultures have the following potential advantages over suspension cultures:

(1) Immobilization provides high cell concentrations.
(2) Immobilization provides cell reuse and eliminates the costly processes of cell recovery
and cell recycle.
(3) Immobilization eliminates cell washout problems at high dilution rates.
(4) The combination of high cell concentrations and high flow rates (no washout restrictions)
allows high volumetric productivities.
(5) Immobilization may also provide favorable microenvironmental conditions (i.e. cell e cell
contact, nutrient-product gradients, and pH gradients) for cells, resulting in better
performance of the biocatalysts (e.g. higher product yields and rates).
(6) In some cases, immobilization improves genetic stability.
(7) For some cells, protection against shear damage is important.

The major limitation on immobilization is that the product of interest should be excreted
by the cells. A further complication is that immobilization often leads to systems for which
diffusional limitations are important. In such cases, the control of microenvironmental condi-
tions is difficult, owing to the resulting heterogeneity in the system. With living cells, growth
and gas evolution present significant problems in some systems and can lead to significant
mechanical disruption of the immobilizing matrix.
In Chapter 8, we discussed enzyme immobilization. Figure 8.16 provides a useful
summary of immobilization strategies. Many of the ideas in enzyme immobilization have
a direct counterpart in whole cells. However, the maintenance of a living cell in such a system
is more complex than maintaining enzymatic activity. The primary advantage of immobi-
lized cells over immobilized enzymes is that immobilized cells can perform multistep,
cofactor-requiring, biosynthetic reactions that are not practical using purified enzyme
preparations.