Figure 3. A typical flux decay profile for microfiltration systems
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From this point on, it is typical to carry out further optimization at pilot scale so that the membrane configuration and
channel width are representative of the large-scale operation. These experiments are carried out under non-steady state operating
conditions (i.e., no recycle) so that concentration varies over the course of the experiment. Membrane loading (i.e., volume
of broth processed per unit membrane surface area) is another important parameter for optimization. An effort should be made
to conduct experiments with loadings in the correct ballpark of what is ultimately going to be operated at large-scale. A
curve that is typically measured is the flux decay profile, which plots flux versus time (Figure 3). It is often difficult
to predict how TMP will influence flux decay under these conditions from the steady state experiments conducted earlier. A
higher TMP might result in a higher initial flux but cause more rapid flux decay or might have a beneficial effect because
a majority of the total permeate might be collected in the very initial stages of the filtration. It is typical to attempt
to optimize the area under the flux-versus-time curve while maintaining an upper limit on the processing time. Cross-flow
velocities influence the filtration in-line with what was observed during the steady state screening experiments with the
caveat that excessively high cross-flow velocities can cause undesirable effects of cell breakage, which may influence product
quality (i.e., Chinese hamster ovary proteins (CHOP) and DNA levels) even before a significant impact is seen on the filtration.
DEPTH FILTRATION
Depth filtration (also called prefiltration or media filtration) refers to the use of a porous medium that is capable of retaining
particles from the mobile phase throughout its matrix rather than just on its surface.15 These filters are frequently used when the feedstream contains a high content of particles.16 In such cases, depth filters can remove larger, insoluble contaminants before final filtration through a microfiltration
membrane that would otherwise clog relatively quickly—hence the term prefiltration.6,17
Figure 4. Schematic of depth filter operation in removing particulates
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Depth filters used in bioprocessing typically are composed of a fibrous bed of cellulose or polypropylene fibers along with
a filter aid (e.g., diatomaceous earth) and a binder that is used to create flat sheets of filter medium. The filter aids
provide a high surface area to the filter and are sometimes used by themselves in clarification applications.18 An additional charge can be imparted to some depth filters, either from the binder polymer or from other charged polymers
incorporated into the filter.19 Sometimes, a microfiltration membrane with an absolute pore size rating is integrated into the depth filter sheet as the
bottommost layer. Porous depth filters can retain particles in their tortuous flow channels to a level that size-based screening
alone cannot achieve. A schematic of how a depth filter operates is shown in Figure 4.
For process-scale applications, depth filters are often fabricated into cells consisting of two layers of filters separated
from each other such that flow occurs from the outside into the space between the layers and is then collected. Multiple cells
can be stacked into a housing in which pressure is used to drive flow through the assembly. Depth filters are usually single-use
devices that enable a reduction in the extent of process validation required for their use in biopharmaceutical applications.
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