Introduction
Membrane filtration is currently one of the most widely used
separation techniques employed
by the biotechnology industry as a means of recovering desired
products from cellular lysate
and fermentation medium. As these biologically-derived products
become more important as
pharmaceutical products, it in turn becomes increasingly
important to understand how to
optimize membrane filtration processes. One of the most
important aspects of optimizing
membrane filtration of protein-containing solutions is
understanding the extraordinary fouling
effect that proteins have upon membranes. Optimizing the
filtration of protein solutions
requires specific techniques different from those used during the
filtration of non-protein
solutions.
Membrane Construction and Characteristics
Synthetic membranes used for the recovery of biological products
are usually composed of
polymeric materials such as: polyethersulfone, nylon, cellulose
acetate, and polyvinylidene
fluoride. The properties and structure of the various membranes
are dependent upon their
specific material of construction.
Microfiltration vs. Ultrafiltration
Two classes of membrane filtration systems are commonly used to
filter protein solutions.
These two classes are: microfiltration and ultrafiltration.
Microfiltration and ultrafiltration
systems are mostly similar, with the one major difference between
the two being the average
pore size of the membranes. Microfiltration membranes have an
average pore size of
approximately one micrometer. The proteins, water, and other
smaller solutes pass through
the membrane, while the larger cellular components are retained
by the membrane.
Ultrafiltration membranes have an average pore size ranging
approximately from 1 to 250
nanometers. The proteins and larger solutes are retained by the
membrane while water and
smaller solutes passes through the membrane. Often, these
processes are performed in series,
microfiltration followed by ultrafiltration, to recover and
concentrate the protein.
Modes of Membrane Fouling
There are three different modes by which
membranes foul. These
modes include: pore narrowing/constriction, pore plugging, and
gel/cake layer formation.
Cross-flow Filtration: An Overview
For large-scale, industrial membrane filtration, cross-flow or tangential-
flow filtration is commonly employed.
Protein Structure Within Solution
The first step in attempting to understand the mechanism by which
proteins foul membranes
which is different than that for other types of particles is to
understand the structure of the protein molecule .
Protein Aggregation
Proteins in solution have the ability to interact with one
another. When proteins interact, they
tend to mass together to form large particles called protein
aggregates. Protein aggregation is
similar to coagulation of colloidal particles in solution. It is
this aggregation of proteins, the
formation of particles much larger than individual protein
molecules which is one of the
reasons protein solutions tend to foul filtration membranes to an
extent greater than non-
protein containing solutions. There appear to be two mechanisms
by which proteins
aggregate: main hydrophobic interactions
and
thiol oxidation / thiol-disulfide interchange reactions.
Peculiarities of Protein Filtration
During the filtration on non-protein solutions (suspensions), the
values of the system
parameters can be adjusted to enhance the flux as the membrane
becomes fouled. Often,
during the filtration of suspensions, a cake layer is formed at
the surface of the membrane
which is non-compressible. Increasing the transmembrane pressure
will increase the flux
across the membrane for these systems. However, proteins tend to
form compressible cakes at
the surface of the membrane. As the transmembrane pressure is
increased, the particle cake
formed at the surface of the membrane compresses, thereby
decreasing the flux across the
membrane and increasing the sieving of the proteins.
During the filtration of suspensions, the shear across the
surface of the membrane is often
increased to remove the cake built up at the surface of the
membrane. This is done either by
increasing the velocity of the feed across the surface of the
membrane, or by using system
geometries which generate vortices which essentially "scour" the
particles from the surface of
the membrane. While this may work for suspensions, it appears to
have the opposite effect
upon protein solutions. Increasing the shear across the membrane
surface during the filtration
of protein solutions tends to increase the rate of aggregation of
the proteins in the solution.
Therefore, since conventional methods to optimize flux do not
work well during protein
filtration, other methods to optimize flux during protein
filtration must be developed.
Methods Used to Control Fouling of Membranes During Filtration
of Protein
Solutions
Backflushing/Backpulsing
Membrane Surface Modification