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).

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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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