AALTO UNIVERSITY SCHOOL OF CHEMICAL TECHNOLOGY
Master’s Programme in Biomass Refining
CHEM-E3140 Bioprocess technology II
Filtration
Tomi Eilamo
347381
Oskari Kilpi
349208
Tommi Levo
295695
Elmeri Pienihäkkinen 354361
Report submitted: 5.12.2016
1.Abstract
Filtration is one of the most common methods for the downstream processing of fermentation products
and it is used at all production scales. Basic principle in filtration is to separate the suspended particles
or larger molecular mass components from bulk liquid with porous medium. Filtration is applicable as a
separation method when molecular sizes of substances in process stream differ significantly. Several
parameters affect effectivity of filtration processes, such as pore size, pressure and temperature.
Filtration processes and equipment are often classified by pore size, direction of flow in relation to filter,
type of filter and geometry of filters.
2.Background
Typically in the field of bioprocess technology, the highest equipment and operating cost exist in the
downstream processing of fermentation products. Manufacturing costs, including capital and operational
expenses, of downstream processing part can be up to 50 % of the overall costs. Often the situation is
that operational costs exceed clearly all other expenses and thus any improvements in the efficiency of
downstream processing can have significant effects in the profitability of the process. [Lee et. al. 2011]
3.Theory
Principles of filtration
Filtration is one of the most common methods for the downstream processing of fermentation products
and it is used at all production scales. Basic principle in filtration is to separate the suspended particles
or larger molecular mass components from bulk liquid with porous medium. [Stanbury et al. 2016]
Driving force can be pressure difference, vacuum, concentration gradient, valence or electrochemical
affinity [Ballew et. al. 2002].
In principle, filtration and membrane separation technologies can be used four different ways: clarifying
the cells from the fermentation broth and conditioning the cells for mechanical or chemical disruption
(1), clarifying the products from homogenate of cellular debris after the a disruption process (2), clarifying
extracellular product from the culture after fermentation (3) and concentration and diafiltration of a
clarified product for chromatography (4). [Lee et. al. 2011]
Filtration materials can be classified in depth filters, screen filter and membranes. Depth filters and
screen filter are commonly used in dead-end filtration, while membranes are used in tangential flow
filtration. [Lee et. al. 2011] Depth filters do not have a precise pore size or structure and thus they are
not absolute. This means that particles with wider range in size will permeate through the filter.
Components that are larger than the apertures of the filter will be trapped on the surface of the filter. In
the case of smaller particles, random entrapment and adsorption of matter occurs within the structure
of the media. Depth filter may also have an electric charge which aids in the entrapment of smaller
particles. Depth filters have thicker construction and higher porosity. This leads to some advantages,
such as higher flow rate and dirt loading capacity when compared to screens and membrane filters.
They are also cheaper than many screens and membrane filters. Depth filters are manufactured from
fibrous materials, woven or nonwoven polymeric material or inorganic materials. [Ballew et. al. 2002]
Fig 1. Depth vs. screen filter [Ballew et. al. 2002].
In the case of screen filters (or mesh filters), particles are retained directly on the surface of the screen.
Their pore size is precisely defined, so that only particles with diameter below pore size of the filter will
permeate the filter. Screens are used when there is need for low nonspecific binding or low adsorption
or absorption of the filtrate. [Ballew et. al. 2002]
Membrane filters are thin membranes often manufactured from polymeric material but also other
materials can be used. Due to manufacturing techniques, they relatively well defined flow rate, pore
size, pore structure, pore density, bubble point and tensile strength. Membrane filter mainly function by
trapping the particles on its surface, but some entrapment into the membrane also occurs, [Ballew et.
al. 2002] Typical housings for membrane filters are tubular, flat sheet, hollow fiber, spiral wound and
vibrating membrane systems [Lee et. al. 2011].
Parameters affecting filtration
Temperature is one important parameter affecting filtration. Increasing temperature increases the
disruption rate which is beneficial if the product is intracellular product. However, high temperature can
cause deactivation or degradation of proteins which can be the desired product. Temperature also
reduces the viscosity of the fermentation broth which leads to reduced filtration time. Viscosity can also
be reduced by chemical additives. [Lee et. al. 2011]
Another important parameter is the specific cake resistance of cellular material. Different types of cells
disrupt differently. Other are weaker than others and disrupted cell can block the pores. [Lee et. al. 2011]
Some filter aids can be used to reduce the cake resistance. These filter aids increase the porosity of the
cake and thus increase the flow rate through the cake. [Stanbury et al. 2016]
Transmembrane pressure is important in dead-end filtration because it is the driving force that forces
the filtrate pass the filter. During operation it is controlled by the inlet, outlet and filtrate pressure valves.
Cross-flow velocity is important in tangential flow filtration. It needs to be sufficiently high to provide
enough shear stress on the membrane surface to prevent settling. [Lee et. al. 2011]
Fig 2. Filtration time [Lee et. al. 2011].
Microfiltration
Microfiltration (MF) is a well-known separation process, which is capable of removing particle sizes of
micro-scale (from 0,02μm up to 10μm) in contaminated liquid or gas streams by a semi-permeable
membrane of certain pore size. The separable size of particle has different specifications depending on
the study, but the scale is similar. [Lee et. al. 2011]
The most prominent process designs for MF include in-line (dead-end) filtration and cross-flow
(tangential) filtration. In in-line filtration the feed stream is forced completely through the membrane
which results in particle accumulation on membrane surface and a particle free permeate. In cross-flow
filtration the feed is circulated across the membrane surface which results in particle free permeate and
in retentate concentrated in particles. [Baker et. al. 2000]
The membranes utilized in MF can be divided in two classes: In depth and screen membranes. The
difference between them is the pore size and particle capture site: In depth membranes have small
pores that capture the particles on the surface on the membrane and screen membranes have larger
pores which capture the particles in the membrane interior. The membranes are usually fitted in special
cartridges, which enable folding of the membranes and thus larger surface area. Modern cartridge filter
units have multiple cartridges installed. [Baker et. al. 2000]
Fig 3. Tangential and dead-end filtration. (Ballew et. al. 2002)
Fig 4. Cartridge filters [Direct industry, 2016]
MF processes in industry usually consist of two cartridge filters (pre filter and final filter) consisting of
two or more membranes, a feed-pump and pressure difference measuring and control equipment. The
prefilter is the main filtering unit, which captures the largest of particles and the final filter is used to
capture the residual fine particles. The main purpose of prefilter is to prevent plugging of the fine filter
and thus extend its operation time. By measuring and controlling the pressure difference across the
filters their stage of plugging can be controlled: Pressure difference rises as the filters are covered with
particles until the preset value of pressure difference is met and the filters are changed. [Baker et. al.
2000]
To extend filter life some modern cross-flow filter a back-flushing (long flow reversal) or -pulsing (short
reversal) method is used. In back-flushing the filter membrane is flushed opposite to filtrate flow direction
with water or solvent in order to remove particles from filter. [Baker et. al. 2000]
Fig 5. Two cartridge filter system. [Baker et. al. 2000]
Ultrafiltration
Ultrafiltration (UF) is a separation process that makes use of filters with pore size between 1 and 20
nanometers [van Reis & Zydney, 2007]. Membrane permeability in UF is however often specified by
molecular weight cut-off (MWCO) rather than by pore diameter. The separation is often done using
hydraulic pressure of 2 to 10 atmospheres to force low molecular weight solutes through membrane.
[Stanbury et al. 2016] UF membranes are designed to provide high retention of proteins and other
macromolecules by small pore size and sometimes by electrostatic interactions. Amino acids generally
pass through UF membranes, while proteins are too large to pass through. UF is typically used for
protein concentration and buffer exchange, processes which were earlier performed using size
exclusion chromatography. [van Reis & Zydney, 2007] However, UF is practical only when separating
molecules whose molecular weights differ by at least a factor of ten as the membranes often have
significant variance in pore size. [Stanbury et al. 2016]
At industrial scale, UF processes are generally carried out as fed batch operations followed by batch
operation. Membrane material is typically composite regenerated cellulose in industrial scale, but also
regenerated cellulose, polysulfone and polyethersulfone can be used. [van Reis & Zydney, 2007]
Membranes can be either anisotropic or isotropic. Anisotropic filters have a thin skin with small pores
on top of a thicker, highly porous layer that provides mechanical strength. [Shuler & Kargi, 2002]
Isotropic, self-supporting filters have homogenous structure that allows them to be cleaned by
backflushing. Anisotropic filters are geometrically generally flat sheets while isotropic filters are hollow
fibers. [van Reis & Zydney, 2007]
4. Industrial significance and General applications
Both ultrafiltration and microfiltration are applied in dairy industry. Microfiltration can be applied in dairy
industry for bacteria removal or protein extraction process for manufacturing. Microfiltration can be used
in cheese making process to extract proteins for the manufacture of whey protein isolates and micellar
casein products. Another MF application in dairy industry is treatment of dairy effluents and waste
streams. Those effluents represent a risk of pollution, and cross-flow microfiltration is used for bacteria
removal, which provides a lower-temperature approach for microbial growth control. That leads to
extended shelf life of the products. [Fernández et. al. 2012]
Microporous membranes can be also applied on testing for microorganisms, clinical and general
laboratory applications and in the cold sterilization of fluids.
In the cold sterilization of the fluids the membranes are used to remove microorganisms from the fluid
with the dead-end flow. The required pore size is 0.2-0.45 um. [Ballew et. al. 2002]
Ultrafiltration method has three group of applications: concentration, desalting and fractionation.
Concentration will decrease the solvent volumes during isolation and purification. The solvent is
removed through the membrane. Ultrafiltration is used for decreasing the solvent volumes during
isolation and purification, replacing older techniques such as evaporation, precipitation, gel filtration and
dialysis. [Ballew et. al. 2002]
When ultrafiltration is used for desalting, small molecules quantitatively passes through the membrane
along with the solvent and macromolecules are retained by the membrane. The simplest desalting
method is to replace the solvent that had passed through the membrane with contaminant free solvent.
[Ballew et. al. 2002]
Fractionation is the separation of molecules with different size. The dilute mixture of components with
higher and lower molecular weights is filtered through the membrane, and the component with higher
molecular mass (retentate) is rejected when the component with smaller molar mass (filtrate) passes
the membrane. [Ballew et. al. 2002]
Ultrafiltration method is applied on deproteinization of blood and urine samples. The advantage of
ultrafiltration method in deproteinization is that UF can be used for all analyses without any change of
the solute concentration. Furthermore, ultrafiltration method adds no ions to the sample, unlike the
standard technique. [Ballew et. al. 2002]
5. Conclusions
Filtration is effective and relatively cheap method for separating or concentrating products in
downstream processing. It can be applied at several different points in downstream processing and
combined with other separation techniques. Filtration is also often relatively easy process to scale up
and the equipment doesn’t take as much space as many other separation processes. However, it is not
always applicable as it requires the molecules in incoming stream to have significant difference in their
molecular weight.
References
Baker, Richard W. Membrane technology. John Wiley & Sons, Inc., 2000.Chapter 7
Ballew, H.W., Martinez F.J., Markee, C. and Eddleman R.T., eds, 2002. The ABCs of filtration and
bioprocessing for the third millenium. Spectrum Laboratories, Inc.
Shuler, M. L., and F. Kargi. Bioprocess engineering. New York: Prentice Hall. 2002.
Direct industry. http://www.directindustry.com/prod/hilliard-corporation/product-29728-137164.html,
visited 27.11.2016 17:00
Fernández García, L., Álvarez Blanco, S., & Riera Rodríguez, F. A. (2013). Microfiltration applied to
dairy streams: removal of bacteria. Journal of the Science of Food and Agriculture, 93(2), 187-196.
Lee, T. and D’Amore, T., 2011. Membrane Separation Theoretical and Applicable Considerations for
Optimum Industrial Bioprocessing. 1(2),.
Stanbury, P., A. Whitaker and S. Hall. Principles of Fermentation Technology. 2016.
van Reis, R. and A. Zydney. "Bioprocess membrane technology." Journal of Membrane Science 297.1.
2007. 16-50.