Ultrafiltration is a variety of membrane filtration in which forces like pressure
or concentration gradients lead to a separation through a semipermeable
membrane. Suspended solids and solutes of high molecular weight are retained in
the so-called retentate, while water and low molecular weight solutes pass
through the membrane in the permeate. This separation process is used in
industry and research for purifying and concentrating macromolecular solutions,
especially protein solutions. Ultrafiltration is not fundamentally
different from microfiltration. Both of these separate based on size exclusion
or particle capture. It is fundamentally different from membrane gas separation,
which separate based on different amounts of absorption and different
rates of diffusion. Ultrafiltration membranes are defined by the molecular
weight cut-off of the membrane used. Ultrafiltration is applied in cross-flow
or dead-end mode. Applications
Industries such as chemical and pharmaceutical manufacturing, food and
beverage processing, and waste water treatment, employ ultrafiltration in
order to recycle flow or add value to later products. Blood dialysis also
utilizes ultrafiltration. = Drinking water=
UF can be used for the removal of particulates and macromolecules from raw
water to produce potable water. It has been used to either replace existing
secondary and tertiary filtration systems employed in water treatment
plants or as standalone systems in isolated regions with growing
populations. When treating water with high suspended solids, UF is often
integrated into the process, utilising primary and some secondary treatments as
pre-treatment stages. UF processes are currently preferred over traditional
treatment methods for the following reasons:
No chemicals required Constant product quality regardless of
feed quality Compact plant size
Capable of exceeding regulatory standards of water quality, achieving
90-100% pathogen removal UF processes are currently limited by
the high cost incurred due to membrane fouling and replacement. Additional
pretreatment of feed water is required to prevent excessive damage to the
membrane units. In many cases UF is used for pre
filtration in reverse osmosis plants to protect the RO membranes.
= Protein concentration= UF is used extensively in the dairy
industry; particularly in the processing of cheese whey to obtain whey protein
concentrate and lactose-rich permeate. In a single stage, a UF process is able
to concentrate the whey 10-30 times the feed.
The original alternative to membrane filtration of whey was using steam
heating followed by drum drying or spray drying. The product of these methods had
limited applications due to its granulated texture and insolubility.
Existing methods also had inconsistent product composition, high capital and
operating costs and due to the excessive heat used in drying would often denature
some of the proteins. Compared to traditional methods, UF
processes used for this application: Are more energy efficient
Have consistent product quality, 35-80% protein product depending on operating
conditions Do not denature proteins as they use
moderate operating conditions The potential for fouling is widely
discussed, being identified as a significant contributor to decline in
productivity. Cheese whey contains high concentrations of calcium phosphate
which can potentially lead to scale deposits on the membrane surface. As a
result substantial pretreatment must be implemented to balance pH and
temperature of the feed to maintain solubility of calcium salts.
= Other applications= Filtration of effluent from paper pulp
mill Cheese manufacture, see ultrafiltered
milk Removal of pathogens from milk
Process and waste water treatment Enzyme recovery
Fruit juice concentration and clarification
Dialysis and other blood treatments Desalting and solvent-exchange of
proteins Laboratory grade manufacturing
Principles The basic operating principle of
ultrafiltration uses a pressure induced separation of solutes from a solvent
through a semi permeable membrane. The relationship between the applied
pressure on the solution to be separated and the flux through the membrane is
most commonly described by the Darcy equation:
where J is the flux,TMP is the transmembrane pressure, μ is solvent
viscosity, Rt is the total resistance. Membrane fouling
= Concentration polarization= When filtration occurs the local
concentration of rejected material at the membrane surface increases and can
become saturated. In UF, increased ion concentration can develop an osmotic
pressure on the feed side of the membrane. This reduces the effective TMP
of the system, therefore reducing permeation rate. The increase in
concentrated layer at the membrane wall decreases the permeate flux, due to
increase in resistance which reduces the driving force for solvent to transport
through membrane surface. CP affects almost all the available membrane
separation process. In RO, the solutes retained at the membrane layer results
in higher osmotic pressure in comparison to the bulk stream concentration. So the
higher pressures are required to overcome this osmotic pressure.
Concentration polarisation plays a dominant role in ultrafiltration as
compared to microfiltration because of the small pore size membrane. It must be
noted that concentration polarization differs from fouling as it has no
lasting effects on the membrane itself and can be reversed by relieving the
TMP. It does however have a significant effect on many types of fouling.
= Types of fouling= Particulate deposition
The following models describe the mechanisms of particulate deposition on
the membrane surface and in the pores: Standard blocking: macromolecules are
uniformly deposited on pore walls Complete blocking: membrane pore is
completely sealed by a macromolecule Cake filtration: accumulated particles
or macromolecules form a fouling layer on the membrane surface, in UF this is
also known as a gel layer Intermediate blocking: when
macromolecules deposit into pores or onto already blocked pores, contributing
to cake formation Scaling
As a result of concentration polarization at the membrane surface,
increased ion concentrations may exceed solubility thresholds and precipitate on
the membrane surface. These inorganic salt deposits can block pores causing
flux decline, membrane degradation and loss of production. The formation of
scale is highly dependent on factors affecting both solubility and
concentration polarization including pH, temperature, flow velocity and
permeation rate. Biofouling
Microorganisms will adhere to the membrane surface forming a gel layer –
known as biofilm. The film increases the resistance to flow, acting as an
additional barrier to permeation. In spiral-wound modules, blockages formed
by biofilm can lead to uneven flow distribution and thus increase the
effects of concentration polarization. Membrane arrangements
Depending on the shape and material of the membrane, different modules can be
used for ultrafiltration process. Commercially available designs in
ultrafiltration modules vary according to the required hydrodynamic and
economic constraints as well as the mechanical stability of the system under
particular operating pressures. The main modules used in industry include:
= Tubular modules= The tubular module design uses polymeric
membranes cast on the inside of plastic or porous paper components with
diameters typically in the range of 5 – 25 mm with lengths from 0.6 - 6.4 m.
Multiple tubes are housed in a PVC or steel shell. The feed of the module is
passed through the tubes, accommodating radial transfer of permeate to the shell
side. This design allows for easy cleaning however the main drawback is
its low permeability, high volume hold-up within the membrane and low
packing density. = Hollow fibre=
This design is conceptually similar to the tubular module with a shell and tube
arrangement. A single module can consist of 50 to thousands of hollow fibres and
therefore are self-supporting unlike the tubular design. The diameter of each
fibre ranges from 0.2 – 3 mm with the feed flowing in the tube and the product
permeate collected radially on the outside. The advantage of having
self-supporting membranes as is the ease at which it can be cleaned due to its
ability to be backflushed. Replacement costs however are high, as one faulty
fibre will require the whole bundle to be replaced. Considering the tubes are
of small diameter, using this design also makes the system prone to blockage.
= Spiral-wound modules= Are composed of a combination of flat
membrane sheets separated by a thin meshed spacer material which serves as a
porous plastic screen support. These sheets are rolled around a central
perforated tube and fitted into a tubular steel pressure vessel casing.
The feed solution passes over the membrane surface and the permeate
spirals into the central collection tube. Spiral-wound modules are a compact
and cheap alternative in ultrafiltration design, offer a high volumetric
throughput and can also be easily cleaned. However it is limited by the
thin channels where feed solutions with suspended solids can result in partial
blockage of the membrane pores. = Plate and frame=
This uses a membrane placed on a flat plate separated by a mesh like material.
The feed is passed through the system from which permeate is separated and
collected from the edge of the plate. Channel length can range from 10 – 60 cm
and channel heights from 0.5 – 1 mm. This module provides low volume hold-up,
relatively easy replacement of the membrane and the ability to feed viscous
solutions because of the low channel height, unique to this particular
design. Process characteristics
The process characteristics of a UF system are highly dependent on the type
of membrane used and its application. Manufacturers’ specifications of the
membrane tend to limit the process to the following typical specifications:
Process design considerations When designing a new membrane separation
facility or considering its integration into an existing plant, there are many
factors which must be considered. For most applications a heuristic approach
can be applied to determine many of these characteristics to simplify the
design process. Some design areas include:
= Pre-treatment= Treatment of feed prior to the membrane
is essential to prevent damage to the membrane and minimize the effects of
fouling which greatly reduce the efficiency of the separation. Types of
pre-treatment are often dependent on the type of feed and its quality. For
example in wastewater treatment, household waste and other particulates
are screened. Other types of pre-treatment common to many UF
processes include pH balancing and coagulation. Appropriate sequencing of
each pre-treatment phase is crucial in preventing damage to subsequent stages.
Pre-treatment can even be employed simply using dosing points.
= Membrane specifications= Material
Most UF membranes use polymer materials however ceramic membranes are used for
high temperature applications. Pore size
A general rule for choice of pore size in a UF system is to use a membrane with
a pore size one tenth that of the particle size to be separated. This
limits the amount of smaller particles entering the pores and adsorbing to the
pore surface. Instead they block the entrance to the pores allowing simple
adjustments of cross-flow velocity to dislodge them.
= Operation strategy= Flow type
UF systems can either operate with cross-flow or dead-end flow. In dead-end
filtration the flow of the feed solution is perpendicular to the membrane
surface. On the other hand in cross flow systems the flow passes parallel to the
membrane surface. Dead-end configurations are more suited to batch
processes with low suspended solids as solids accumulate at the membrane
surface therefore requiring frequent backflushes and cleaning to maintain
high flux. Cross-flow configurations are preferred in continuous operations as
solids are continuously flushed from the membrane surface resulting in a thinner
cake layer and lower resistance to permeation.
Flow velocity Flow velocity is especially critical for
hard water or liquids containing suspensions in preventing excessive
fouling. Higher cross-flow velocities can be used to enhance the sweeping
effect across the membrane surface therefore preventing deposition of
macromolecules and colloidal material and reducing the effects of
concentration polarization. Expensive pumps are however required to achieve
these conditions. Flow temperature
To avoid excessive damage to the membrane, it is recommended to operate a
plant at the temperature specified by the membrane manufacturer. In some
instances however temperatures beyond the recommended region are required to
minimise the effects of fouling. Economic analysis of the process is
required to find a compromise between the increased cost of membrane
replacement and productivity of the separation.
Pressure Pressure drops over multi-stage
separation can result in a drastic decline in flux performance in the
latter stages of the process. This can be improved using booster pumps to
increase the TMP in the final stages. This will incur a greater capital and
energy cost which will be offset by the improved productivity of the process.
With a multi-stage operation, retentate streams from each stage are recycled
through the previous stage to improve their separation efficiency.
Multi-stage, multi-module Multiple stages in series can be applied
to achieve higher purity permeate streams. Due to the modular nature of
membrane processes, multiple modules can be arranged in parallel to treat greater
volumes. = Post-treatment=
Post-treatment of the product streams is dependent on the composition of the
permeate and retentate and its end-use or government regulation. In cases such
as milk separation both streams can be collected and made into useful products.
Additional drying of the retentate will produce whey powder. In the paper mill
industry, the retentate is incinerated to recover energy and permeate is
discharged into waterways. It is essential for the permeate water to be
pH balanced and cooled to avoid thermal pollution of waterways and altering its
pH. = Cleaning=
Cleaning of the membrane is done regularly to prevent the accumulation of
foulants and reverse the degrading effects of fouling on permeability and
selectivity. Regular backwashing is often conducted
every 10 min for some processes to remove cake layers formed on the
membrane surface. By pressurising the permeate stream and forcing it back
through the membrane, accumulated particles can be dislodged, improving
the flux of the process. Backwashing is limited in its ability to remove more
complex forms of fouling such as biofouling, scaling or adsorption to
pore walls. These types of foulants require chemical
cleaning to be removed. The common types of chemicals used for cleaning are:
Acidic solutions for the control of inorganic scale deposits
Alkali solutions for removal of organic compounds
Biocides when bio-fouling is evident When designing a cleaning protocol it is
essential to consider:Cleaning time – Adequate time must be allowed for
chemicals to interact with foulants and permeate into the membrane pores.
However if the process is extended beyond its optimum duration it can lead
to denaturation of the membrane and deposition of removed foulants. The
complete cleaning cycle including rinses between stages may take as long as 2
hours to complete.Aggressiveness of chemical treatment – With a high degree
of fouling it may be necessary to employ aggressive cleaning solutions to remove
fouling material. However in some applications this may not be suitable if
the membrane material is sensitive, leading to enhanced membrane
ageing.Disposal of cleaning effluent – The release of some chemicals into
wastewater systems may be prohibited or regulated therefore this must be
considered. For example the use of phosphoric acid may result in high
levels of phosphates entering water ways and must be monitored and controlled to
prevent eutrophication.Summary of common types of fouling and their respective
chemical treatments New developments
In order to increase the life-cycle of membrane filtration systems, energy
efficient membranes are being developed in membrane bioreactor systems.
Technology has been introduced which allows the power required to aerate the
membrane for cleaning to be reduced whilst still maintaining a high flux
level. Mechanical cleaning processes have also been adopted using granulates
as an alternative to conventional forms of cleaning; this reduces energy
consumption and also reduces the area required for filtration tanks.
Membrane properties have also been enhanced to reduce fouling tendencies by
modifying surface properties. This can be noted in the biotechnology industry
where membrane surfaces have been altered in order to reduce the amount of
protein binding. Ultrafiltration modules have also been improved to allow for
more membrane for a given area without increasing its risk of fouling by
designing more efficient module internals.
The current pre-treatment of seawater desulphonation uses ultrafiltration
modules that have been designed to withstand high temperatures and
pressures whilst occupying a smaller footprint. Each module vessel is self
supported and resistant to corrosion and accommodates easy removal and
replacement of the module without the cost of replacing the vessel itself.
References