1
Introduction
Historical Development of Membranes
The first membrane developments were achieved using readily
available membranes in nature,
such as bladders of pigs or sausage casings made from animal gut
(Baker, 2004). But later
research led to the usage of nitrocellulose to manufacture
membranes which were preferred as
they could be manufactured in series (Baker, 2004). In the
beginning of the XX century,
Bechhold, Elford and Bachmann developed a method to manufacture
nitrocellulose membranes
of specific pore sizes, and by the 1930s microporous
nitrocellulose membranes were
commercially available (Baker, 2004).
A key discovery that converted membrane separations from a
laboratory technique to an
industrial application was the development of the
Loeb-Sourirajan process to manufacture
defect free, high flux, reverse osmosis membranes (Baker, 2004).
These membranes consisted
of a selective film over a more thick, permeable and porous
support that provided high
mechanical resistance (Cheryan, 1998). The flux through this
membrane resulted larger than
any other available in the market at that time and made possible
the application of reverse
osmosis as a practical method. The work of Loeb and Sourirajan,
and high investments of the
US government were an important factor in the further
development of ultrafiltration,
microfiltration and electrodialysis resulting in membranes with
selective layers as thin as 0.1 m
(Baker, 2004).
In the subsequent years, packing methods for membrane
applications were developed, such as
spiral wound, hollow fiber and plate and frame configurations
which enabled a broader industrial
utilization. By 1980, ultrafiltration, reverse osmosis,
microfiltration and electrodyalisis were
established processes with broad application in the industry.
The principal development during
2
that decade was gas separation membrane technologies. Companies
such as Monsanto and
Dow introduced the first membranes for hydrogen separation,
nitrogen from air separation and
carbon dioxide from natural gas (Baker, 2004). Gas separation
membrane technologies have
been in constant development and are spreading at a high
rate.
Types of Membranes
There are different types of synthetic membranes that differ in
their chemical and physical
composition and in their operation mechanisms. Basically, a
membrane is a discrete interface
that moderates the penetration of different chemical substances
in contact with it. A membrane
can be either physically or chemically heterogeneous or it can
be uniform in its composition
(Baker, 2004). The basic types of membranes are described below
and shown in figure 1.
Nonporous, Dense Membranes
Although membranes classified as nonporous or dense might have
pores in their structure in the
range of 5 to 10 angstroms, the model in which permeation occurs
differs from other types of
membranes and is better explained by solution-diffusion
phenomena (Brschke, 1995), and the
driving force for the separation using these type of membranes
can be an applied pressure, a
difference in concentration or an electrical potential gradient.
Since these types of membranes
do not rely on the size of the pores to achieve the separation
process, components of similar
molecular size can be separated if their solubility in the
membrane is different. Dense
membranes are widely used in gas separation and reverse osmosis
(Baker, 2004).
Microporous Membranes
A microporous membrane has a solid matrix and a random
distribution of connected pores.
3
Figure 1. Membrane types (Baker, 2004)
Separation of components in this case is achieved by a sieving
mechanism in which particles
larger than the pores are rejected by the membrane, while
particles smaller can be partially
rejected according to the pore size distribution in the
membrane. These types of membranes
are used mainly for microfiltration and ultrafiltration (Baker,
2004). Microporous membranes can
be either symmetric or asymmetric (anisotropic, as shown in
figure 1), where the latter are
composed of a thin layer which acts as the selective part of the
membrane and a thick support
or substructure which provides physical strength and
stability.
4
Electrically Charged Membranes
The ion-exchange membranes used in electrodialysis and diffusion
dialysis are essentially
sheets of ion-exchange resins. Cation-exchange membranes have
negatively charged groups
chemically attached to the polymer chains, ions with an opposite
charge can permeate through
these sites and since their concentration is high they are able
to carry the electric current
through the membrane. Ions of the same charge are repelled.
Attachment of positive fixed
charges to the polymer chains forms anion-exchange membranes,
which are selectively
permeable to negative ions. Electrically charged membranes may
be either nonporous or
porous and the separation is affected by the ionic strength in
the solution (Porter, 1990).
Ceramic, Metal and Liquid Membranes
The interest in membranes made from unconventional materials
which can be stronger and
withstand severe conditions such as very high or low pH values,
broader operation
temperatures or strong solvent management have been continuously
growing as technological
advances allow their fabrication, and microporous ceramic and
metallic membranes are being
used in ultrafiltration and microfiltration applications where
these kinds of conditions are present.
Dense metal membranes are also being considered in gas
separation processes (Baker, 2004).
Membrane Processes
The more developed industrial membrane separation processes are
microfiltration, ultrafiltration,
reverse osmosis, electrodialysis diffusion dialysis and gas
separation. These processes are
well established and the market is served by experienced
companies, like Millipore and General
Electric (Baker, 2004). Different application ranges for the
pressure driven separation
processes; microfiltration, ultrafiltration and reverse osmosis
are shown in figure 2.
5
Figure 2. Pressure driven membrane separation spectrum.
(Suppliers of Liquid Filtration
Products, 2011)
Ion Exchange Membrane Processes
The basic principles of electrodialysis and diffusion dialysis
processes are very similar to those
of ion exchange, in which positive and negative ions diluted in
a solution are driven through ion
exchange membranes with opposite charged constituents, while
ions with the same charge are
mostly rejected. The driving force for these separation
processes are chemical potential in the
case of diffusion dialysis, or an applied electrical potential
in the case of electrodialysis.
Membranes are usually placed in a stack and alternating between
cation or anion, selective in a
6
way that the feed solution is ion depleted throughout the
process. A schematic of diffusion
dialysis is shown in figure 3.
Figure 3. Diffusion dialysis. (Functional Membranes and Plant
Technology, 2012)
Because both positive and negative ions move in opposite
directions under the effect of an
electrical potential, in the case of electrodialysis the process
is often analyzed by the number of
electric charges transported through the membrane, and not by
the material permeated (Baker,
2004).
Microfiltation
This process is used to remove particles in the size range of
0.1 to 10 micrometers from liquids
(figure 2, Cheryan, 1998). There are two main types of
microfiltration techniques: dead-end and
cross flow microfiltration (Figure 4). Dead-end is a common type
of microfiltration encountered
in the industry, where it finds application in sterile
filtration and clarification (Cheryan, 1998). It
employs depth or surface membranes. In this type of filtration,
retained particles build up in the
7
membrane void spaces by a sieving action on the fibrous
materials from which they are
fabricated.
Figure 4. Dead end and Cross flow Microfiltration. (Ridgelea,
2012)
In surface microfiltration, the particles are retained on the
upstream surface of the filter by a
sieving mechanism (Cheryan, 1998). Build-up of particles during
dead-end filtration requires the
replacement or cleaning of the filter medium when the flow
decreases. For this reason, dead-
end filtration is a batch process. The cross flow configuration
on the other hand, has the
advantage that particles do not build up in the same intensity
on the membranes surface
because the feed flows tangentially to the surface of the
membrane and they are sloughed off
by the high shear imposed by the tangential flow of bulk
suspension. For this reason higher flux
rates can be maintained for longer periods of time.
Nevertheless, fouling of the membrane will
occur over time and the flux rate will decline (Baker,
2004).
Appropriate membrane selection is an important factor in
microfiltration, as well as all other
membranes separation processes, as adsorption can play a
fundamental part in fouling. For
example, hydrophobic membranes (e.g., PTFE) generally show a
greater tendency to be fouled,
especially by proteins (Cheryan, 1998).
DEAD END FILTRATION CROSS FLOW FILTRATION
8
Ultrafiltration
Ultrafiltration is a membrane separation process in cross-flow
operation. In a solution containing
low molecular weight and high molecular weight solutes, the
latter will be retained by the
membrane, while the smaller low molecular weight particles will
permeate through. The driving
force in order to achieve the separation is a pressure
difference applied to a solution on the feed
side of a membrane. Ultrafiltration membrane pore sizes are
usually classified according to the
molecular weight of the species that will be retained by
assigning to them a molecular weight cut
off (MWCO). A schematic of this process is shown in figure 5.
The solvent and low molecular
weight species passes through the membrane while solutes with a
larger weight than the
MWCO are retained.
Figure 5. Ultrafiltration principle of operation. (Functional
Membranes and Plant Technology, 2012)
Since micro molecular components have significantly lower
molecular weights, it is possible to
separate them from other macromolecular compounds in aqueous
solution by using
ultrafiltration. Membrane pore diameters in this case are
typically between 0.1 and 0.005
micrometers and are able to retain proteins, polymers, and
chelates of heavy metals (Figure 2)
(Cheryan, 1998). Since low-molecular-weight solutes flow through
the membrane, osmotic
pressure is not an issue. However, since retained large
molecules and colloidal particles have
low diffusivities in the liquid medium, ultrafiltration
membranes are more susceptible to fouling
Permeate Retentate
9
and concentration polarization than reverse osmosis or
microfiltration membranes (Cheryan,
1998).
Usually, not all the particles larger than the molecular weight
cut off of the membrane are
rejected, and some particles smaller than this parameter may be
partially rejected also
(Paterson, 1993). In order to estimate the separation degree
attained by the process, a
mathematical model has been developed for the rejection of the
solutes (Cheryan, 1998):
= 1
Where R is the rejection coefficient
CP is the concentration in the permeate
CR is the concentration in the retentate
During this process, the total volume of a solution will be
reduced as the solvent and low
molecular weight components are being removed resulting in the
concentration of the
macromolecular species, since their quantity remain unchanged.
The concentration and volume
relationship in ultrafiltration systems are characterized by the
following equation (Cheryan,
1998):
0
= 0 =
Where Cf is the final concentration of the feed
C0 is the initial concentration of the feed
V0 is the initial feed volume
Vf is the final feed volume
CF is the concentration factor
10
R is the rejection coefficient
These mathematical models can also be applied in the same way to
the microfiltration process
(Cheryan, 1998).
Ultrafiltration membranes can be either polymeric of ceramic.
Polymeric membranes are
asymmetric and are available in different configurations, such
as tubular, plate and frame,
hollow fiber or spiral wound (Cheryan, 1998). Some
ultrafiltration membranes are illustrated in
figure 6.
Figure 6. Ultrafiltration membranes
Reverse Osmosis
Reverse osmosis can be defined as the movement of solvent
molecules through a
semipermeable membrane into a region of higher solvent
concentration, or lower solute
concentration. The driving force for osmosis is the difference
in the chemical potential of the
solutions at both sides of the membrane, where molecules will
tend to move from a higher
chemical potential zone (pure solvent) to a lower chemical
potential one (solution). This
Polyethersulfone
(Sterlitech, 2010)
Regenerated Cellulose
(Bioxys, 2005)
Ceramic
(Unceram, 2006)
11
difference will generate an osmotic pressure that depends on the
concentration of the solute, its
molecular weight, the number of ions for ionized solutes and the
temperature of the system
(Cheryan, 1998). As other membrane separation processes, in
reverse osmosis the solvent
moves from a high solute concentration zone to a low
concentration one, overcoming the
osmotic pressure of the solution by means of an applied external
pressure (Figure 7). The basic
relationship between the applied pressure by a pump, the osmotic
pressure, and the flow of
solvent through a membrane is expressed in terms of the rate of
solvent transport per unit area
per unit time, also called flux, and also the driving force and
resistances, described by the
following equation (Baker, 2004):
= ( )
Where J is the flux through the membrane
A is the water transport coefficient
p is the pressure differential across the membrane
is the osmotic pressure differential across the membrane
Osmotic pressure increases as concentration increases and the
molecular weight of the solute
decreases. Because the typical particle sizes involved in
microfiltration and ultrafiltration
processes, the osmotic pressure due to their presence is usually
low enough to be negligible. In
reverse osmosis, on the other hand, osmotic pressure effects are
likely to be the dominant
resistance (Cheryan, 1998).
Reverse osmosis membranes are non-porous and asymmetric, as
described in the introduction
section of this paper and consist of a thin skin, which is
supported by a porous substructure.
The membranes can be made of a single polymer such as cellulose
acetate, non-cellulosic
12
polymer or of thin-film composites (Baker, 2004). Due to the
small pore size, reverse osmosis
membranes are susceptible to plugging and it is necessary to
pretreat the feed. In addition,
there are limitations on the allowable pH and temperature of
feed due to physical instability of
the membrane materials in harsh environments (Baker, 2004).
Figure 7. Reverse osmosis principle of operation and reverse
osmosis in cross flow configuration
(Aquatruewater, 2008)
Gas Membrane Separation
Membranes can be used for gas and vapor separation in a variety
of applications, including
VOC removal and/or recovery. The driving force for the
separation of a gas mixture by a
membrane process is a concentration difference between the two
sides of the membrane,
where the permeable species will move from the high pressure
side to the low pressure side.
Membranes for gas separation can be either polymeric, including
materials such as
polyethersulfone, polyamides and other cellulosic derivatives,
or ceramic and even metallic.
Membranes used for gas separation can be of two different kinds;
porous and nonporous
(Figure 8).
In the case of porous membranes, depending on the size of the
pores, the mathematical models
that govern the separation and hence the separation itself will
be affected, and a molecular
13
sieving separation can be achieved with pore diameters in the
order of 5 to 20 angstroms
(Baker, 2004).
With non-porous membranes gases are separated due to their
different diffusivity and solubility
values in the membrane (Porter, 1990). Gases dissolve into the
material, diffuse through, and
desorb on the other side. Both the molecular size and the
chemical nature of the gas will
influence the separation process. As polymer science has
developed during the past years,
many have been tested and some have very good selectivity
(Porter, 1990).
Figure 8. Gas separation membranes. (CO2CRC, 2011)
The most important elements that will determine the economic
feasibility of a gas membrane
separation process are the permeability, selectivity and
membrane life (Baker, 2004)
14
Discussion
Membrane Technology Limitations
The main limitations for membrane separation processes are the
concentration polarization and
membrane fouling. Concentration polarization controls the
performance of electrodialysis,
diffusion dialysis, microfiltration, ultrafiltration, and to a
lower extent reverse osmosis and gas
separation processes, because of the high diffusion coefficient
of gases (Baker, 2004). It is an
effect where particles rejected by the membrane tend to form a
layer near the surface causing
further resistance to the flow of the permeate. The flux
decrease is usually explained by two
mechanisms: The first one is an increase in the osmotic pressure
due to the increased solute
concentration near the surface of the membrane in comparison to
the bulk concentration in the
feed, and the second one is the hydrodynamic resistance of the
boundary layer (Cheryan,
1998). To reduce the effect of concentration polarization
several factors such as pressure, feed
concentration, temperature and turbulence in the feed channel
must be optimized.
Membrane fouling on the other hand is characterized by an
irreversible decline in the flux that
cannot be counteracted with fluid management techniques. It is
due to the accumulation of feed
components on the membrane surface or within the pores of the
membrane and is influenced by
the chemical natures of both the membrane and the solutes and
membrane-solute and solute-
solute interactions (Cheryan, 1998). Usually the only way of
restoring the flux of a fouled
membrane is through cleaning. Fouled membranes and auxiliary
equipment are generally
cleaned by clean-in-place procedures (Lindau and Jnson, 1993)
which are usually based on
various chemical or enzymatic treatments to restore the membrane
to its original state. Many
appropriate cleaning agents are available. Acids, such as nitric
acid or ethylenediaminetetra-
acetic acid (EDTA), are used to remove salt deposits (Cheryan,
1998). Caustic-based
detergents are used to remove proteinaceous deposits. Enzyme
cleaning agents containing
15
hydrolytic enzymes, such as amylases, proteases, or
glucogenases, are sometimes used for
specific applications, and are used at the optimal pH for the
respective enzyme. Rinsing with
water at high circulation rates and reduced pressure, or
back-flushing from the permeate side of
the membrane are also used to clean membranes (Baker, 2004).
Ion Exchange Membrane Applications
Electrodialysis is the most used ion-exchange membrane
separation process today and its most
common application is brackish water desalination to obtain
potable water and sea salt (Baker,
2004). Other uses for electrodialysis are found in the food
industry for whey desalination, fruit
juice demineralization, control of the cation balance in milk
and the replacement of strontium by
calcium to reduce the radioactive elements in milk or related
products (Cheryan, 1998). In the
pulp and paper industry, for the treatment of bleaching waste
water solutions, in the glass
manufacturing industry for the processing of a waste stream of
ammonium fluoride solution, and
similarly in effluents containing hydrogen fluoride solutions in
the quartz tube manufacturing
process (Leitz, 1976).
The degree of water recovery in each case is limited by
precipitation of insoluble salts in the
feed. There are additional applications for microfiltration in
wastewater treatment including
regeneration of waste acid streams used in metal pickling
processes and the removal of heavy
metals from other waste waters (Gering and Scamehorn, 1988),
where electrodialysis
membranes separate electrolytes and can also separate
multivalent ions. The arrangement of
membranes in these systems depends on the application.
Regarding electrodialysis application in the production of table
salt by concentration of
seawater, several processes have been developed along with
electrodialysis such as reverse
16
osmosis electrodialysis (Tanaka, Ehara, Itoi and Goto, 2003) and
reverse electrodialysis
(Turek, 2002). This process is mainly practiced in Japan, which
rely on the sea as the only salt
supplier (Baker, 2004).
Additional applications for electrodialysis can be found in the
preparation of ultrapure water for
the electronics industry (Yang, 2004) where salt concentrations
must be reduced to the ppb
range. A problem with electrodialysis in this case is that the
feed streams are diluted and
separation becomes inefficient, in these cases the addition of
ion exchange beads in the stacks
can further aid the separation to the objective values.
Microfiltration Applications
The use of microfiltration technology has many practical
applications. Most of them are based
on the properties of semi permeable microfiltration membranes
that allow separation and/or
concentration of ultrafine particles, large molecules (0.1 to 10
micrometers) and microorganisms
(Cheryan, 1998). The process is widely used in dairy and
beverage industry as well as
pharmaceutical industry to produce sterile water (Porter,
1990).
Considering environmental pollution prevention, microfiltration
helps to reduce the amounts of
wastewater and concentrate pollutants generated by industries
like: landfill leachate treatment,
metal finishing industry and laundry industry (Cheryan, 1998).
Wastewater treatment is one of
the major applications of microfiltration technology. Landfill
leachate - is a by-product generated
by precipitation and degradation at solid waste disposal
facilities. Managing leachate is
considered one of the most important problems with designing and
maintaining a landfill. Many
different organic and inorganic compounds dissolved or suspended
in leachate pose a potential
pollution problem for local ground and surface water. Current
leachate treatment options include
on-site treatment, recycling and re-injection, biological
treatment, discharge to a municipal water
17
treatment facility or a combination of these processes. Typical
systems used for treatment of
leachate are: activated sludge, fixed film and constructed
wetlands. Modern on-site treatment of
relatively dilute landfill leachate includes the use of
microfiltration process to concentrate
leachate after chemical precipitation of toxic metals. The use
of cross flow filtration allows high
level of solids (2-4%) to be processed (Zenon Environmental,
1994). Microfiltration is usually
followed by reverse osmosis of the permeate which concentrates
remaining inorganic and
organic contaminants. The cost of application of membrane
filtretion technology to treat landfill
leachate varies depending on the composition of the leachate. In
the end a treatment process
which incorporates precipitation, microfiltration and reverse
osmosis estimates to be more cost-
effective, compared to biological and other treatments, that
allows to meet new standards of
released wastewater (Zenon Environmental, 1994). In the metal
finishing industry microfiltration
found its application in electroplating rinse bathe maintenance.
This is a relatively new area of
application of microfiltration. The main reason the technology
was not used before is the lack of
membranes that could tolerate hostile conditions of
electroplating process (Cushnie, 2009).
Polymeric membranes deteriorate at high temperatures and
corrosive nature of washing
solutions. Ceramic membranes, on the other hand being chemically
inert, are capable of
working under these conditions (Baker, 2004).
Prior to the application of microfiltration technology, the
contents of an aqueous degreasing bath
supposed to be discarded after 80 hours of constant use
(Cushine, 2009). The process allowed
removal of fine oil emulsion and colloidal particles from
degreasing baths, thus making the
contents reusable for longer (Porter, 1990). Microfiltration
application in metal finishing industry
also has some limitations. Some of the cleaning formulations
used in the process contain
colloidal silicic acid, which has a tendency to plug the pores
of the ceramic membrane. Also
aluminum cleaning solutions cannot be used together with
microfiltration, as dissolved
aluminum concentration will build up because it is unaffected by
filtration process. Examples of
18
microfiltration process use in electroplating industry estimate
around 2.1 years of return on
investment with initial investment of around 27 000$ and
operating cost of 6250$ (Cushine,
2009). Laundry industry is a major generator of wastewater.
Wastewater from laundry sources
accounts for 10% of municipal sewer release (Porter,1990).
Laundry wastewater contains large
amount of suspended solids, a high BOD load, oil, grease, heavy
metals, and other organic
compounds which in sum largely exceed municipal discharge
standards. A common method for
such wastewater treatment consists of lime coagulation and
flocculation followed by clarification
by dissolved air (Porter,1990). Application of cross-flow
microfiltration allows the recycle of
permeate back to the plant, thus reducing the amounts of
discharged water. Furthermore, the
process allows reusing of up to 90% of the wastewater with good
washing results by use of a
modular washing system (Hoinkis, Panten, 2008).
Ultrafiltration Applications
As with microfiltration process applications of ultrafiltration
are based on ability of membranes
to separate the retained material because of small pores on
their surface.
The largest area of application of the ultrafiltration
technology is in electrocoat painting.
Ultrafiltration helps to recover more than 90% of the paint
drag-out, and substantially reduces
the load on wastewater treatment (Nath, 2008). It is widely used
in the automotive and
appliance fields (Porter, 1990). In electrocoating process the
paint is applied to metal parts in a
tank containing 15-20% of the paint emulsion (Baker, 2004).
After coating, the part is removed
and rinsed to remove the excess of paint. Ultrafiltration system
removes ion impurities from the
paint tank carried over from earlier steps of the process and
recovers clean rinse water for
countercurrent rinse operation. The retentate containing paint
emulsion is returned back to the
tank (Baker, 2004). The savings in recovered paint alone cover
the cost of process operation.
19
The estimated payback period of ultrafiltration system
installation is less than one year not to
mention the savings in sewage treatment and deionized water cost
(Cheryan, 1998).
Another application of ultrafiltration technology is the use of
membrane bio-reactors. The use
of membrane bio-reactors (MBR) in wastewater treatment becomes
more common, due to lower
space requirements, lower operation involvement, modular
expansion capabilities and
consistent quality of output water. The technology allows to
treat high strength waste with poor
biodegradability and old sludges. MBR technology combines common
activated sludge
treatment with low-pressure membrane filtration (AMTA, 2007).
The ultrafiltration process
creates a barrier to contain microorganisms and makes possible
to treat raw sewage and
wastewater. The process ensures an effluent free of solids, due
to a membrane barrier and
helps to overcome the problems associated with poor sludge
setting in common activated
sludge processes (AMTA, 2007). The high quality permeate
produced by MBRs is suitable for
variety of applications for industrial and municipal purposes.
The operation of MBR also has
some limitations. Those include the need of fine screening to
remove abrasive, stringy and
fiborous material as it can damage the membrane or can increase
fouling. Other pretreatement
of industrial wastewater may vary depending on factors like COD,
temperature, TDS or high
content of inorganic solids. Because of the variable parameters
of operation, the cost of
implementing a MBR technology also varies. For smaller
facilities lesser than 1 MGD general
guidelines estimate expected equipment cost of 2-6$ peer gallon
of plant capasity and plant
construction cost of 12-20$ per gallon of plant capasity (AMTA,
2007). Estimated operation
costs range from 350$ to 550$ per million gallons treated (AMTA,
2007). Facilities larger than 1
MGD can expect equipment cost of 0.75-1.50$ peer gallon of plant
capasity and plant
construction cost of 5-12$ per gallon of plant capasity (AMTA,
2007). Estimated operation costs
range from 300$ to 500$ per million gallons treted (AMTA,
2007).
20
Reverse Osmosis Applications
Approximately one-half of the reverse osmosis systems currently
installed are used for
desalination of brackish or seawater. The remaining half is used
in the production of ultrapure
water for the power generation, pharmaceutical, and electronics
industries and for applications
such as pollution control and food processing (Baker, 2004).
Since we aim to discuss
applications related to pollution prevention, desalination will
not be covered in this paper.
An established and growing application for reverse osmosis is
the production of ultrapure water
for the electronics and pharmaceutic industries. In this case,
the feed is usually municipal water
which contains less than 200 ppm of dissolved solids (Baker,
2004). Reverse osmosis typically
removes more than 98% of the salts and other dissolved
particles, additional processing with
carbon absorption and ion exchange will remove the remaining
impurities (Ganzi, 1989).
Apparently, pollution control should be a major application for
reverse osmosis but in practice,
membrane fouling, one of the limitations of membrane processing,
can cause low plant
reliability. This has inhibited its widespread use in this area.
On the other hand, reverse osmosis
has several advantages that make it attractive such as
simplicity in design and operation,
modern units require very low maintenance if used properly,
inorganic and organic pollutants
can be removed at the same time, the process do not affect the
nature of the material being
recovered, and depending on the application waste streams can be
considerably reduced and
can be further treated in a more efficient and cost effective
way if needed (Williams, 2003).
One of the successful uses of reverse osmosis is in the recovery
of nickel from nickel-plating
rinse tanks, where a stream used to rinse the material after
nickel-plating ends up containing
around 3000 ppm of nickel, which represent a pollution problem,
as it cannot be directly wasted,
21
and a valuable material lost for the industry, the application
of reverse osmosis allows to
produce a permeate stream with only around 50 ppm of nickel that
can be reused in the process
and a concentrate that is sent to the plating tank (Baker,
2004). The same principle can be
applied for the recovery of copper, zinc, copper cyanide,
chromium, aluminum and gold and in
general the metal finishing industry, allowing recoveries
between 75 up to 95% (Benito and
Ruis, 2001).
One of the areas of research for the reverse osmosis membranes
is its use in the recovery and
tertiary treatment of water to produce drinking water from
sewage (Abel-Jawad, 2002). Although
the process is economically feasible, particularly in water
limited regions, psychological barriers
are still the biggest obstacle for its implementation. Attempts
have been made in the US to
introduce this operation, injecting treated water into the
aquifer and mixing it with natural
groundwater which somewhat has helped to its acceptance (Baker,
2004).
Because of high rejection of inorganic compounds, reverse
osmosis membranes have also
been studied for treatment of radioactive effluents (Arnal,
Sancho, Verdu, 2003) and the
removal of other toxic componds (Ning, 2002) and have been used
for the treatment
of uranium conversion process effluents containing corrosive,
toxic and radioactive compounds.
Gas Membrane Separation Applications
The principal established and developed gas separation processes
at industrial level are used
for Hydrogen and Nitrogen separation, carbon dioxide and methane
separation, nitrogen from
air and water from air. After the first gas membrane separation
units proved to work successfully
for hydrogen separation, further development lead to a process
to separate carbon dioxide from
natural gas during extraction, after which it is reinjected into
the ground (Baker, 2004). This
application is an example in the mitigation of greenhouse gases
emissions to the atmosphere
22
and is widely spread in wells that use carbon dioxide as a
pressurization medium. The largest
application for membrane separation is the production of
Nitrogen from air, process that uses
polysulfone and ethyl cellulose membranes.
A growing application for these membrane systems is the removal
of volatile organic
compounds from air and other streams. In this case, rubbery
membranes are used, which are
more permeable to organic compounds. Most of the plants of this
type installed aim to recover
gasoline vapors from air vented during transfer operations,
although this technology is also
applied for the recovery of fluorinated hydrocarbons from
refrigeration streams (Freeman,
1995).
23
Conclusions and Recomendations
Since the appearance and industrial application of membrane
separation processes, several
decades ago, there has been a period of very rapid growth (Nath,
2008). In the areas of
microfiltration, ultrafiltration, reverse osmosis,
electrodialysis and diffusion dialysis we can say
that the technology is relatively mature in terms of their
utilization. However, significant
advances have been made as membranes continue to displace
conventional separation
techniques. The most rapidly expanding area is the use and
development of gas separation
membrane techniques; although its market share is still very
small in comparison to the other
technologies, it is projected to grow further as development of
more selective and high flux
membranes allow its economic use in the petrochemical and
natural gas processing areas. In
terms of market development and applications, gas separation
processes can be divided in two
groups; the first one includes established applications, such as
nitrogen-air separation and
hydrogen recovery, which represent up to 80% of the current
market and have undergone
significant improvements in membrane selectivity and flux,
increasing efficiency and decreasing
costs (Baker, 2004). Another group is comprised by developing
processes, which include
carbon dioxide separation from natural gas, volatile organic
component separation from air and
recovery of hydrocarbons from petrochemical plant purge gases,
all these are already used on a
commercial scale and their application is directly related for
pollution prevention in a very
important and relevant area; control of greenhouse gas
emissions. Significant expansion in
these applications and process designs is occurring. The
combination of a gas separation
process with others, such like distillation of organic vapor
mixtures, for example, is other of the
developing areas.
A 2001 market analysis for membrane separation technologies
confirms that the expanding use
of membranes mainly in water and wastewater treatment and gas
separation technologies has
made possible important advances in the area. Also, increasingly
strict environmental
24
regulations and awareness, applied during the past decades have
increased the adoption of
membrane separation processes, influenced also by the reduction
in waste disposal costs and
the increased opportunity of materials recovery and recycling
(Atkinson, 2002). Table 1 shows
the summary of membrane materials demand and their growth.
Table 1. Summary of membrane materials demand in US$ million
(Atkinson, 2002)
% Annual growth
Item 1996 2001 2006 2011 01/96 06/01 Gross domestic product (bil
US$) 7813 10208 13100 16800 5.5 5.1 Membrane demand 950 1480 2110
2940 9.3 7.4 Microfiltration 520 740 980 1290 7.3 5.8 Reverse
osmosis 180 310 490 740 11.5 9.6 Ultrafiltration 150 270 420 630
12.5 9.2 Pervaporation 15 40 65 95 21.7 10.2 Others 85 120 155 185
7.1 5.3 US$/sq ft 1.45 1.55 1.65 1.75 1.3 1.3 Membrane demand (mil
sq ft) 661 948 1270 1660 7.5 6.0
According to the data, microfiltration membranes account for the
largest share of the market, as
it is a very popular and low-cost alternative in applications
that do not require high levels of
purity. Its use is common, many times as pretreatment for other
more specific separation
processes. There is still a good opportunity for the growth of
the industry in the bacterial control
of drinking water and other beverages and treatment of sewage
(Baker, 2004). So we can
conclude that municipal water treatment is likely to develop
into a major future application of this
technology.
The reverse osmosis industry is one of the better established
when considering membrane
separation processes. It has the second largest share of the US
market. Demand for reverse
osmosis membranes have advanced rapidly because this process can
deliver a high level of
purity, demanded in wastewater treatment systems and other
applications in the industry. Two
25
of the main industries served are the electronics and
pharmaceutical, but the desalination
market to produce fresh water has been growing over the past
years. Recent developments
have also lowered water desalination costs and increased
membrane unit fluxes, as well as
improved resistance (Elimelech, 2011).
Ultrafiltration accounts for the third largest share of the
membrane market, the expansion of this
technology is limited due to the high cost per liter of permeate
produced in most wastewater and
industrial process stream applications. Since membrane fluxes
are not high, and large amounts
of energy are used for the feed recirculation in order to
control fouling and concentration
polarization, costs are usually high (Baker, 2004). Research and
development of fouling
resistant membranes is now the preferred approach, changing the
membrane surface
absorption characteristics. Although ceramic membranes do not
present these disadvantages,
costs are still very high in comparison to polymeric membranes
and should be reduced by an
order of magnitude to be competitive (Cheryan, 1998).
Electrodialysis is by far the largest used of ion exchange
membranes, although it accounts for a
very small share of the market. Both desalting brackish water
and salt production are well
established processes and major technical innovations that will
change their competitive
position of the industry do not appear likely. And the total
market is small.
In figure 9, the membrane demand by market is presented.
26
Figure 9. Membrane demand by market, 2001 (Atkinson, 2002)
As shown in the figure, water and wastewater treatment accounted
for 55% of the membrane
demand in the year 2001, this is due to the emphasis on reducing
contaminants in water feed
streams and reclaiming process components and recycling
water.
It is evident, by the data provided by this market study that
the most used membrane technology
in wastewater and water treatment is microfiltration, followed
by reverse osmosis. Both
processes have found broad and successful applications in
pollution prevention. The fact that
the membrane market forecast is to keep growing during the next
years and that applications
such as gas separation have still a long way to go in terms of
research and development, tells
us that they will play a fundamental role in pollution
prevention and even in pollution
remediation. But in addition to new improved membranes and
membrane processes, there is
also the need for application know-how, which often requires the
cooperation of various
scientific disciplines. Also it appears to be a lack of
education in membrane science technology,
Water and wastewater
treatment, 55%
Industrial gas, 3%
Chemical production, 5%
Pharmaceutical/medical, 9%
Food and beverage
processing, 22%
Others, 6%
27
while other unit operations are included in technical schools
and university programs,
membrane science and technology seldom is.
28
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