ULTRAFILTRATION
Roger Kuan
Casey Hunt
Connie Perkins
ENVE 436
December 4, 1998
1. Introduction
Filtration is the separation of two or more components from a fluid stream. A
filtration membrane acts as a selective barrier, allowing the passage of certain
components and retaining others components of a mixture. The most common membrane
processes are microfiltration, ultrafiltration, and hyperfiltration (reverse osmosis) (1). An
ultrafiltration (UF) membrane removes undissolved, suspended or emulsified solids from
water supplies without requiring coagulation. The particles that UF retains have a
molecular weight of 1000 to 1,000,000. It is most generally used in specialized
applications requiring extremely high purity water, as pretreatment prior to reverse
osmosis or for removal of colloidal silica from boiler feed water. Removal includes
colloidal silica, synthetic and natural organics, including taste and odor causing
compounds, SOCs and natural organic compounds that can serve as precursors to
trihalomethanes and other DBPs (7).
Ultrafiltration generally works by pumping the feeding solution under pressure
over the surface of a suitably supported membrane. The pressure gradient forces solvent
and smaller species through the pores of the membrane, while the larger molecules are
retained. The membranes are cleaned either by backflushing alone or with the addition of
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a chlorine solution, which restores the original porosity and allows continuous use for
indefinite periods (1).
There are advantages and disadvantages to types of ultrafiltration membranes and
the applications they are used for. This paper will discuss types of membranes, UF
applications, case studies, and a critique of an UF application.
2. Ultrafiltration Membrane Modules
There are four primary configurations of ultrafiltration membrane modules: tubular,
hollow-fiber (HF), spiral-wound and flat plate. Each configuration has their inherent
strengths and weaknesses and varies in their industrial and commercial applications (2).
Choosing the right module configuration is important to the overall effectiveness of the
ultrafiltration process.
2.1 Tubular Membrane Modules
Tubular membrane devices are similar to their HF module counterparts, but
employ much larger internal diameter tubes typically in the range of 0.3 to 2.5 cm. In
this design, the semi-permeable membrane is cast in place within a porous support tube
made of fiberglass or other appropriate materials. The individual tubes are then packed
in small bundles, which are kept in place by two end plates. This is then encapsulated
into a plastic or stainless steel sleeve to form a single tube cartridge. During the filtration
process the feed or “unfiltered” liquid flows into one end plate and the resulting permeate
or “filtered” liquid flows radially through the membrane tube and towards the sleeve
collection area and out the permeate outlet ports (7).
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The primary advantage of the tubular membrane module design is the large
internal diameter of its tubes, which prevent the problem of particulate plugging.
Generally speaking, the internal diameters of the tubes are 10 times the size of the largest
expected particulate in the feed liquid. The primary disadvantages of tubular modules are
their very low membrane packing density and high energy costs (7).
The tubular membrane is generally used in small flow, high solids loading
applications. Its design is inherently easy to clean, therefore it is the membrane of choice
when severe fouling is expected (2).
2.2 Hollow-Fiber Modules
The HF modules consist of an array of narrow-bore fibers with a dense skin layer
which is bound into bundles of fibers (e.g. 50–10,000 fibers) and potted at the ends with
an epoxy or polyurethane resin to form a tube sheet. The tube sheet is machined to
expose the open bores of the fibers and then encapsulated into a plastic cylinder housing.
These modules work in a similar fashion to the tubular modules. The feed liquid flows
into the fibers through the end plates of the cylinder and the resulting permeate flows
radially outward through the fiber walls into a collection area and then out through the
permeate outlet ports (7).
The primary advantages of the HF modules are their high mass transfer rates, low
energy costs and high packing densities. All of these are the result of the small internal
diameter of the packed fibers. The main disadvantage of the HF module design is that it
is susceptible to particulate plugging. This requires a pre-filtration of the feed liquid to
remove large particulate matter prior to loading into the HF ultrafiltration process (7).
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The HF module is generally used in high flow, low solids loading applications.
Its design is inherently easy to clean, therefore it is also the design of choice in
applications which require frequent cleaning (2).
2.3 Spiral-Wound Modules
Spiral modules are constructed using flat sheet membranes in the form of a
“pocket” consisting of two membrane sheets separated by a porous support plate and a
permeable mesh. The membranes are sealed along the edges to form a pocket using an
adhesive. Several of these pockets are spirally wound around a single collecting tube
using a feed-side mesh as a spacer to establish the desired feed channel thickness. The
entire spiral is then wrapped with fiberglass tape and fitted into a tubular steel or plastic
pressure vessel with an antitelescoping device placed at both ends of each element to
prevent distortion of the spiral during operation (7).
The pressurized feed solution is fed into one end of the vessel and flows through
the plastic screens along the surface of the membranes. The permeate flows into the
closed membrane pockets and spirals radially inward where it is collected and through
the central tube (7).
The main advantages of these modules are their high membrane packing density,
effective mass transfer characteristics and low energy costs. This is due to the spacing
between the membrane sheets and the low flow rates of the feed solutions. The main
disadvantages of the spiral modules are that they are highly susceptible to fouling and are
generally difficult to clean. This eliminates them from consideration for applications
where Total Suspended Solids (TSS) loading is high (2).
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2.4 Flat-Plate Modules
Flat-plate modules use multiple flat sheet membranes in a sandwich arrangement
consisting of a support plate, membrane and channel separator. The membranes are
sealed to the plates using gaskets and hydraulically clamped to form a tight fit. Several
of these membranes are stacked together and clamped to form a complete module. The
feed channel is typically 0.03–0.1 cm in height. The feed solution flows radially over the
membranes, alternating between an inward and outward flow as it moves through the
stack of membranes. The permeate is collected around the periphery of the unit (7).
The main advantages of the flat-plate module design are that they have high
membrane packing densities and low hold-up volumes. This is due to the small channel
height in the flat-plate modules. The main disadvantage of the flat-plate design is that
they are susceptible to particulate plugging and the modules are difficult to clean (7).
The main application for the flat-plate module is in recovering biological
products. This is because of the low hold-up volume of the design (2).
3. Applications
Over one billion dollars is spent each year on Ultrafiltration (UF) systems, with
uses ranging from filtering blood to painting cars. The uses, which vary excessively, fall
into two categories: concentration and clarification processes (5).
If the product of interest is the material being retained by the filter, then the
process is classified as a concentration process. However, clarification processes are
those that are concerned with the streams passing through the filters. Hazardous waste
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management involves both processes (7). Some ultrafiltration applications pertaining to
hazardous waste management are:

Dairy processing

Oily wastewater from washing operations

Bleach effluents in paper industry

Leather and tanning effluents

Water containing chlorinated aliphatic hydrocarbons
3.1 Dairy Processing
Dairy applications are the largest applications of ultrafiltration in the world. UF is
used to produce reduced-fat milk, lactose-free milk products, and is used for purifying
by-products of cheese. The by-product of cheese-making, whey, is a dilute solution of
lactose, salts, minerals, proteins, and some fat. Whey cannot be discharged to the
municipal wastewater treatment plant because of the high concentration of protein and
lactose, which have a high BOD and COD. In order to deal with this problem, dairy
plants use ultrafiltration to refine whey. Whey is repeatedly passed through filters in
order to separate the different constituents. UF in this case is a concentration process
involving the collection of different whey constituents from the filters. Through
ultrafiltration, constituents such as protein and lactose can be retained and reused for
other purposes such as animal feed (7).
3.2 Oily Wastewater Treatment
Many industries produce oil-containing effluents from their manufacturing
processes. Examples include: metal-working operations for cleaning metal surfaces,
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cooling purposes, removal of oils from textiles, degreasing of mechanical parts, and
naturally occurring oils in food processing operations. The majority of oil in waste
streams is free-floating on the surface of the water and is easily skimmed off. A small
percentage of the oil however, is somewhat soluble and cannot easily be removed.
Chemical treatment is one means of removing the remaining oil, but remains inferior to
UF because of the remaining COD levels (6).
During the cleaning process of wool, oil from glandular secretions is rinsed into
the waste stream. The resulting stream has a high COD and therefore cannot be
discharged to local waste treatment plants. To reduce COD levels in the waste stream,
UF is used, resulting in additional benefits. Because the UF permeate contains nonionic
detergent, it is recycled for use as cleaning water, thus decreasing total detergent usage
and effluent volume. The retentate (recovered oil) is then dried and disposed of as solid
waste (6).
3.3 Chlorinated Aliphatic Hydrocarbon (CAH) Removal
CAHs are commonly produced chemicals for use as solvents. PCE, DCA, and
TCE are the most commonly used CAHs. The widespread use of CAHs has resulted in
pollution of soils, aquifers, and surface waters. UF is an integral part of treating water
contaminated with CAH in the parts per million to parts per billion range; this is done
through biological means. Biological removal of CAH involves using ultrafiltration in
conjunction with bioreactors. The purpose of UF in biological removal of CAH is the
collection of the microbial cells on the filters. The microorganisms are removed after the
removal of the CAHs, producing a clean permeates stream. While other forms of
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chemical treatment can remove CAH, UF in association with bioreactors produces a
contaminant-free permeate stream requiring no further treatment (4).
4. Critique of an UF Application
An auto coating plant uses 1.5 million liters of water a year for the production of
water-based paints and the regular cleaning of their equipment. The plant proposed a
water treatment process using a filtering system to cut down on the cost of hazardous
waste disposal and water consumption. It would first take the wastewater and run it
through a pre-filter to remove large particles. Next, the wastewater would be placed in
ultrafiltration tubular module membrane where suspended solids and high molecular
weight particles would be removed. Last, the wastewater would be placed in a
hyperfiltration membrane (RO) to remove small impurities. After this treatment, the
plant is able to retain 90% of their wastewater for reuse (2).
This is a very good technique for reducing the amount of water used and
eliminating the amount of hazardous waste produced. On the contrary, one weakness to
this process is that ketones still remain in the water making the water not suitable for
human consumption.
One way to improve the efficiency of this system is to change the type of UF
membrane used. A more suitable UF membrane would be the hollow-fiber module
instead of the tubular module membrane. The hollow-fiber membrane is more suitable for
high flows and low solid application. The tubular membrane is more efficient generally
for small flows.
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5. Case Studies
5.1 Electronics Industry
In order to deal with hazardous chemicals such as PCB, Ericsson, a manufacturer
of power-supply equipment, built an on-site wastewater treatment plant to minimize
effluent in a local river. The treatment plant uses UF as part of the contaminant removal
process. The plant removes water contamination from degreasing, ferric phosphatizing,
and chrome passivation. The function of UF is to remove contaminants before the waste
stream passes to an ion exchanger. Because the permeate is pure water, it is recycled for
use in the pretreatment plant. The system reduces water-treatment chemical consumption
by 90% and fresh water consumption by 6.5% (3).
5.2 Paper Mill Whitewater
Metsa-Serla Kirkniemi Mills, a paper mill in Finland, recently installed an
ultrafiltration system for treatment of presswater from its magazine mill operations.
Sedimentation was used previously. Based on the UF systems initial performance, more
filters were added. The plant saves resources by reusing permeate for shower, sealing,
and makeup water. Currently, the system runs 24 hours per day producing 200 m 3 of
permeate per hour (3).
With uses ranging from filtering beer to removing CAH from contaminated
ground water, ultrafiltration is an extremely dynamic system. UF is appealing to many
industries because of its ability to produce such a pure permeate. Through use of UF,
water can be recycled conserving both water and money. In addition, UF systems can be
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run continuously and do not require constant maintenance. As environmental laws
become more stringent, UF will continue to find its way into industry.
WORKS CITED
1. Chryan, Munir, Ph.D., (1986) Ultrafiltration Handbook. Technomic Publishing Co.,
Lancaster.
2. Department of Environmental Management, Office of Environmental Coordination,
Pollution Prevention Program. (1997), “Fact Sheet: Ultrafiltration”, State of
Rhode Island and Providence Plantations, http://es.epa.gov.
3. Elias, L, (1998) "High Shear Membrane Process and Wastewater," Chemical
Engineering, 105(10), 94-6.
4. Ingura, Subbalaxmi, Boensch, Melissa, and Shreve, Gina, (1998) "Microbial
Enhancement of TCE and 1,2-DCA Solute Flux in UF Membrane Bioreactors,"
AichE Journal, 44(9), 2112-23.
5. LaGrega, Michael D., Buckingham, Philip L., and Evans, Jeffrey C., (1986)
Hazardous Waste Management. McGraw-Hill, Inc: New York, NY.
6. Lin, Shing H., (1998) "Treatment of Waste Oil/Water Emulsion by Ultrafiltration and
Ion Exchange," Water Research, 32(9), 2680-8.
7. Zeman, L.J. and Zydney, A.L., (1996) Microfiltration and Ultrafiltration: Principles
and Applications, Marcel Dekker Inc: New York, NY.
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