Biomedical Engineering Reference
In-Depth Information
And perhaps there is no need for it if the process can be isolated and, more
particularly, if a single product is handled in a single facility.
Depth Filtration
Cell removal by filtration leaves media and its entire component in the same
volume, and this requires depth filtration. Depth filters are filters that use a
porous filtration medium to retain particles throughout the medium, rather
that just on the surface of the medium. These filters are commonly used
when the fluid to be filtered contains a high load of particles because, com-
pared to other types of filters, they can retain a large mass of particles before
becoming clogged. These filters are discussed in Chapter 10.
The performance of depth filters is largely dependent on the colloid content
of the bioreactor offload and the cell debris removal capacity of the upstream
centrifuge. Usually, depth filters are operated with a constant flow of 100-200
L/(m
2
·h) and up to 150 Lfeed/m
2
of filter depending on the composition of the
feed stream. The Millipore Millistak+ Pod system has a maximum capacity of
33 m
2
filter area, resulting in a batch capacity of 3000-5000 L. The Millipore
Mobius FlexReady process equipment supports offers a larger 55 m
2
filter area.
Since the washing of these filters requires very large volumes of buffers, appro-
priate size holding tanks are required that can be lined with disposable PE
liners.
Ultrafiltration
Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic
pressure forces a liquid against a semipermeable membrane. Suspended
solids and solutes of high molecular weight are retained, while water and
low-molecular-weight solutes pass through the membrane. This separation
process is used in industry and research for purifying and concentrat-
ing macromolecular (103-106 Da) solutions, especially protein solutions.
Ultrafiltration is not fundamentally different from microfiltration, nanofiltra-
tion, or gas separation, except in terms of the size of the molecules it retains.
Ultrafiltration is applied in cross-flow or dead-end mode, and separation in
ultrafiltration undergoes concentration polarization.
Diafiltration is a membrane-based separation that is used to reduce,
remove, or exchange salts and other small-molecule contaminant from a pro-
cess liquid or dispersion. In batch diafiltration, the process fluid is typically
diluted by a factor of two using “clean” liquid, brought back to the original
