During the past few decades nanotechnology has come up leaps and bounds, as this multidisciplinary scientific field is undergoing explosive development. It is widely in used in industry, referring to engineering and manufacturing (and other scientific) activities involving studies at a nanomolecular scale.
Nanotechnology is the design characterisation, production and applications of structures, devices, and systems by controlled manipulation of size and shape at the nanometre scale (atomic, molecular, and macromolecular scale) that produces structures, devices, and systems with at least one novel/superior characteristic or property.
The sudden growth in filtration (and other related filtration activity), led to the establishment of three membrane separation processes which are reverse osmosis (RO), Ultrafiltration (UF) and more currently Microfiltration, which allowed nanotechnology which introduced nanofiltration in the mix, of which nanofiltration has taken a main role in the association of nanotechnology in filtration (commonly in both liquid and gas phases).
Fig. 1. A human hair fragment sample and a network of a single walled carbon nanotubes. (Nanowerks, 2010)
In perspective (Fig. 1.), a human hair is 80000 nm wide in comparison to nanomolecules and nanotubes at a scale below 100 nm. For further comparison, a red blood cell is approximately 7000 nm wide. Atoms are smaller than 1 nm, whereas many molecules including some proteins range between 1 nm and larger.
Engineering and other disciplines have great interest in the study of nanomolecules to enhance its current applications of nanotechnology, such as …. (Anon, 2004)
This paper, examines nanofiltration …
What the importance
What is
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References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Nanowerks. 2010 (copyright). Introduction to Nanotechnology. Available: www.nanowerk.com. Last Accessed: 20/09/2010.
Anon, Elsevier Ltd., 2004, Filtration & Separation, Applying nanotechnology to filtration applications, Volume 41, Issue 6, Page 8.
Nanofiltration (139W)
Nanofiltration (NF) can be defined as the ability to purify, separate and target macromolecules in continuous systems, which can be further described as a membrane separation process that is essentially in liquid phase, as it separates a wide range of both organic and inorganic substances that are dissolved in solutions which may or may not have distinct suspended particles in the liquid. NF is used primarily to separate low molecular weight organics and multivalent salts from monovalent salts and water. (Koch Membrane Systems, 2008)
It is a relatively recent development in the range of separation processes via membrane, as membrane development grew rapid during the 1970's and 1980's (Sunderland, 2008), and is also one of the most widely used membrane separation processes and can be applied to many different chemical engineering related industries, which is further described in Current Applications in Industry.
However, determining the features and functions of nanofiltration process is important in understanding its applications in industry.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration? Volume 45, Issue 8, Pages 32-35.
Koch Membrane Systems, 2008 (copyright), Nanofiltration - filtration overview (A KMS Leadership Category). Available: www.kochmembrane.com/sep_nf.html. Last Accessed: 20/09/2010.
2.1 Nanofiltration Membrane
The separation of solutes from solutions cannot be done without membranes. They are the key to the performance and separation of nanofiltration (cross-flow) systems.
Fig. 2. An SEM view of Alstrom's Disruptor nanofiltration membrane (Sunderland, 2008)
At an engineering and chemistry perspective, the separation between solute and solvent is a pressure driven process of which molecules of the solvent are diffused through the semi-permeable membrane material, which is driven mainly by a high transmembrane pressure, typically 150-500 psig, not including through any distinct physical holes in the membrane (KOCH Membrane Systems, 2008), at nanoscales that is exemplified in Fig.2. (Sunderland, 2008)
Nanofiltration's recognition as a membrane separation process came from the development of a thin film membrane. The remarkable growth in popularity in today's industry is largely due to its unique ability to separate and fractionate relatively low molecular weight organic and ionic species. It is possible for solute molecules to diffuse through the membrane, either the solute has a finite (yet small) diffusion coefficient in the membrane or because the process has been intentionally modified to suit the designer/engineers requirements In most cases, a separation between two different non-charged solutes is determined predominantly by the difference in their size and shape.
NF membranes (often categorised as "loose" RO) transport and rejection mechanisms is quite complex and is still debated between scientists. (Nystrom, M., kallpa, L. & Luque, S., 1995,) One theory is called solution-diffusion theory. It states that the membrane is described as a porous film of which water and solute ion's are dissolved where the solutes concentration gradient forces moves into the membrane. In this case, the transport of wateris depended on the gradient of the hydraulic pressure applied in the filtration process. The transport of the solute through the membrane depends on hindered diffusion and convection. The transportation of a non-charged solute through an NF membrane is considered to be determined by a steric exclusion mechanism. (Yacubowicz & Yacubowicz, 2005)
The performance of NF membranes generally requires five operating parameters: pressure, temperature, crossflow velocity, pH levels and salinity. As mentions, pressure is the driving force responsible for the process. This driving pressure is the supplied hydraulic pressure that is less than the osmosis pressure applied by the solutes in the membrane. Ideally, good separation is supplied at net pressures of 150 psi (10 bar) or higher. Increasing the process temperature increases the NF membrane flux due to viscosity reduction. The rejection of NF membranes is not dependent significantly on the process temperature. Increasing the crossflow velocity in an NF membrane process increases the average flux due to efficient removal of fouling layer from the membrane surface. However, the mechanical strength of the membrane, and construction of the element and system hardware will determine the maximum crossflow velocity that can be applied. Running a NF membrane at too high a crossflow velocity may cause premature failure of membranes and modules. (Yacubowicz & Yacubowicz, 2005)
The effect of pH on the membrane can be responsible for changes in the feed solution, causing changes in membrane performance. Two examples are change of solubility of ions at different pH regimes, causing different rejection rate; and change in the dissociation state of ions at different pH ranges. In industry, different membrane manufacturers use different chemistries to produce their thin film composite layer, the pH dependency of a membrane should be determined for each membrane type.With salinity, the effective pore radius of a charged pore will increase as the ionic strength of the surrounding liquid increases. Therefore, the rejection of monovalent ions will decrease as their concentration in the feed solution increases.
Recent developments of NF membranes have exceptional stability in very low or high pH, very high temperature, or organic solvent media, required membrane manufacturers to seek new materials for membrane manufacturing. The materials used for these innovative membranes are highly crosslinked, to allow long term stability and practical membrane life in aggressive environments. Nanofiltation
membranes have a slightly charged surface. Most NF membranes are negatively charged at neutral pH. This surface charge plays a major role in the transportation mechanism and separation properties of NF membranes.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Yacubowicz, J. & Yacubowicz J., 2005, Filtration & Separation, Nanofiltration: properties and uses, Volume 42, Issue 7, Page 16-21.
Koch Membrane Systems, 2008 (copyright), Nanofiltration - filtration overview (A KMS Leadership Category). Available: www.kochmembrane.com/sep_nf.html. Last Accessed: 20/09/2010.
Nystrom, M., kallpa, L. & Luque, S., 1995, Journal of Membrane Science, Fouling and retention of nanofiltration membranes, Volume 98, Issue 3, Pages 249-262.
Nanomaterials & Nanofibres in Filtration
NF membranes are manufactured using two preparation techniques:
• Polymer phase inversion resulting in a homogeneous asymmetric membrane;
• Interfacial polarisation of a thin film composite layer on top of a substrate ultrafiltration membrane or other porous substrate. Cellulose acetate and sulfonated polysulfone are two common materials used for making homogeneous asymmetric NF membranes.
Thin film composite NF membranes use crosslinked polyamide polymers, reacted to carboxylic group or other charged "pendant." Substrate materials commonly used for thin film composite membranes are polysulfone (PS), polyethersulfone (PES), polyvinyledene fluoride (PVDF), polyacrylonitrile (PAN), and Polyether ether Ketone (PEEK).
"Air filter assembly for filtering an air stream to remove particulate matter entrained in the stream" and US Patent 6,673,136, "Air filtration arrangements having fluted media constructions and methods". The former patent covers the company's latest innovations in Ultra-Web nanofiber filter media technology for a wide range of self cleaning pulse jet filtration systems, while the latter covers the Ultra-Web nanofibre filter media when used in conjunction with the Donaldson PowerCoreâ„¢ filtration technology. PowerCore provides engine OE customers with added value and design flexibility by reducing the filter and air cleaner package size by up to 60%. (1)
References:
Anon, Elsevier Ltd., 2004, Filtration & Separation, Applying nanotechnology to filtration applications, Volume 41, Issue 6, Page 8.
2.3 Nanofiltration vs. Ultrafiltration & Reverse Osmosis
A brief comparison between NF, UF & RO is used to describe and differentiate each processes main characteristics and roles in filtration, Futher detailed explanations can be found from references.
In terms of separation size and pressure differences, nanofiltration takes the lower end values of ultrafiltration (MWCO values of 100-1000 Daltons) and the upper end of reverse osmosis, and pressure differences that is considerably greater in ultrafiltration and significantly more in reverse osmosis, respectively.
These processes took the separation spectrum from the traditional cut point limit of standard filtration of around 0.01 mm (10 μm) to the very finest distinct solids, a few nanometres in size, and enabled the separation of large molecules from solution. The actual size ranges vary somewhat from source to source, but there is general agreement that microfiltration covers the range 10 μm down to 0.1 μm, while ultrafiltration covered 0.1 μm down to 0.005 μm (5 nm) in terms of discrete particles or Molecular Weight Cut-Off (MWCO) figures of 300,000 down to around 300 Daltons for dissolved materials. Reverse osmosis, of course, was designed to retain the very small sodium chloride molecule, which meant passing nothing else but water. The key difference between nanofiltration and reverse osmosis is that the latter retains monovalent salts (such as sodium chloride), whereas nanofiltration allows them to pass, and then retains divalent salts such as sodium sulphate.
Unfortunately the term has entered the public consciousness with a component of "fear of the unknown" attached to it. This does not concern nanofiltration, since the media involved in it are mostly continuous and indistinguishable from RO or
UF membranes. It does concern nanofibre production and use, however, and the makers and users of nanofibres will have to takecare not to magnify the concern.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Koch Membrane Systems, 2008 (copyright), Nanofiltration - filtration overview (A KMS Leadership Category). Available: www.kochmembrane.com/sep_nf.html. Last Accessed: 20/09/2010.
Membrane Fouling
Fouling of the membrane is a limitation of the process. Over time, the flow channels of the membrane become blocked by the formation of a slowly thickening layer on the membrane surface, reducing the effective diameter of the membrane pores.
This results in a continuous decline in the rate of membrane permeation as well as an increase in the rejection of the low molecular weight solute (Coulson, Harker & Backhurst, 2002).
There are several types of membrane fouling,
References:
Coulson. J.F., Harker, J.H. & Barckhurst, J.R. 2002. Chemical Engineering Volume 2, Particle Technology and Separation Process, 5th Edition, Pages 879-880, Pergamon Press plc, Oxford.
2.5 Concentration Polarisation
The phenomena, Concentration Polarisation is another limiting
In this case both membrane fouling and concentration polarisation …
Current Applications in Industry
Industrial applications of nanofiltration are quite common in the food and dairy sector, in chemical processing, in the pulp and paper industry, and in textiles, although the chief application continues to be in the treatment of fresh, process and waste waters.
Industrial applications of NF membranes are common in food and dairy, chemical process, pulp and paper, electronic and textile industries. The primary application of NF membranes continues to be in water treatment.
NF membrane processes have gradually found their way into Industrial applications, to serve as a viable alternative to more traditional separation processes like extraction, evaporation and distillation. The first industrial systems using NF membranes were installed in 1978 using tubular membranes for desalination of dyes and brighteners.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
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Filtration in Water - Water Treament: Groundwater:
In the treatment of water, NF finds use in the polishing at the end of conventional processes. It cannot be used for water desalination, but it is an effective means of water softening, as the main hardness chemicals are divalent. At first sight, NF would not seem to have much place in MBR processes, because the higher transmembrane pressure differentials needed for NF are not available in most bioreactor systems, but there are some specialised uses for MBRs in which NF is finding a place.
Smith's review(2) covers the whole field of nanotechnology well, including reference to Argonide's NanoCeram fibres of 2 nm alumina, used for the filtration of 99.9999% of bacteria, viruses and protozoan cysts (now available as Ahlstrom's Disruptor technology). NF membranes are also used for the removal of natural organic matter from water, especially tastes, odours and colours, and in the removal of trace herbicides from large water flows. They can also be used for the removal of residual quantities of disinfectants in drinking water.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Van der Bruggen, B. & Vandecasteele, C., 2003, Environmental Pollution, Removal of pollutants from surface water and groundwater by nanofiltration; overview of possible applications in the drinking water industry, Volume 122, Issue 3, Pages 435-445.
From the very start, the drinking water industry has been the major application area for nanofiltration. The historical reason for this is that NF membranes were essentially developed for softening, and to this date NF membranes are still sometimes denoted as ''softening'' membranes (Duran and Dunkelberger, 1995; Fu et al.,1994). The first nanofiltration plants that were developed were essentially meant for softening, and NF became a concurrent to lime softening. Softening was mainly of interest for groundwater in contrast to surface waters, where the major problem is usually a high organic content. Hardness removal is still one of the major purposes of nanofiltration today. However, the removal of dissolved organics soon became an essential part of the process. The removal of natural organic matter (NOM) is necessary for most production units, especially when surface water is treated, and can efficiently be done by nanofiltration. Rejection of organics
0269-7491/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0269-7491(02)00308-1
Environmental Pollution 122 (2003) 435-445
www.elsevier.com/locate/envpol
* Corresponding author. Tel.: +32-16-32-23-40; fax: +32-16-32-
29-91.
E-mail address: [email protected] (B. Van
der Bruggen).
was lower than with reverse osmosis membranes, but
NF membranes were still capable to remove natural
organic matter (NOM) and color (Fu et al., 1994; Lo
and Sudak, 1992; Watson and Hornburg, 1989; Taylor
et al., 1987). This combination of organics and inorganics
removal changed somewhat the purpose of
nanofiltration for drinking water production: from a
pure softening process to a combinatory process for
removal of a whole range of different compounds. At
the present time, nanofiltration is rather seen as a combinatory
process capable of removing hardness and a
wide range of other components in one step.
The possibility of replacing many different treatment
processes by a single membrane treatment was the
engine for intense research and an enhanced interest
from drinking water companies. New applications were
found, such as disinfection by the removal of viruses
(Yahya et al., 1993), removal of pesticides and other
micropollutants (Montovay et al., 1996; Taylor et al.,
1995) and of arsenic (Waypa et al., 1997). Research focused on the understanding of the transport mechanisms of different compounds through the membranes, on the exploration of typical nanofiltration applications (description, modelling and economic evaluation), and
on the development of new applications. Among the
new subjects that have been studied recently is the
reduction of nitrate concentrations by NF (Van der
Bruggen et al., 2001a), although this is not a featured
application for NF (nitrate ions are monovalent). In the
same way, NF membranes are used in seawater desalination
(Al-Sofi et al., 2001; Hassan et al., 1998) for
partial removal of ions as a pretreatment to reverse
osmosis.
Pilot studies and full-scale plants show that NF is a
reliable process for the combined removal of a wide
range of components from groundwater as well as from
surface water (Mulford et al., 1999; Gaid et al., 1998;
Madireddi et al., 1997; Ventresque and Bablon, 1997;
Ventresque et al., 1997; Bertrand et al., 1997; Lozier et
al., 1997).
3.2 Food & Drink Industry
Food industry applications are quite numerous. In the dairy sector, NF is used to concentrate whey, and permeates from other whey treatments, and in the recycle of clean-in-place solutions. In the processing of sugar, dextrose syrup and thin sugar juice are concentrated by NF, while ion exchange brines are demineralised. NF is used for degumming of solutions in the edible oil processing sector, for continuous cheese production, and in the production of alternative sweeteners.
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References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
3.3 Chemical & Pharmaceutical Industry
There are probably as many different applications in the whole chemical sector (including petrochemicals and pharmaceuticals) as in the rest of industry put together. Many more are still at the conceptual stage than are in plant use, but NF is a valuable contributor to the totality of the chemicals industry. The production of salt from natural brines uses NF as a purification process, while most chemical processes produce quite vicious wastes, from which valuable chemicals can usually be recovered by processes including NF. The high value of many of the products of the pharmaceutical and biotechnical sectors allows the use of NF in their purification processes.
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
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3.4 Paper Pulp Industry
The paper pulp industry uses a very great quantity of water in its production processes, a quantity that the industry is striving to reduce, mainly by "closing the water cycle" - a system in which the purification properties of NF have a major role.
All of these specifically mentioned applications have been water-based, but nanofiltration is not restricted to the treatment of aqueous suspensions. Indeed one of the largest NF plants was installed at a petroleum refinery for the dewaxing of oils.
Boam and Nozari, in their review(3) of organic solvent nanofiltration, point out that many organic system separation processes are quite highly energy intensive, and that, by contrast,
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
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3.5 Energy
OSN can be quite an energy saving alternative (for example, by comparison with distillation). In aqueous systems, nanofiltration uses hydrophilic polymeric materials, such as polyether-sulphone, polyamides and cellulose derivatives. These materials, in contact with organic solvents, quickly lose their stability. Special membranes have therefore been developed to provide the same kind of performance as in aqueous systems, and they are now used for solvent exchange, solvent recovery and separation, for catalyst recovery and for heavy metal removal.
3.6 Textiles
Appendix A - other applications of nanofiltration
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Discussion:
Limitations and implications
Advantages & Disadvantages
Potential Future Uses of Nanotechnology in Filtration
1000-2000 words
In spite of all promising perspectives for nanofiltration, not only in drinking water production but alsoin wastewater treatment, the food industry, the chemical and pharmaceutical industry, and many otherindustries, there are still some unresolved problems that slow down large-scale applications. This paper
identifies six challenges for nanofiltration where solutions are still scarce: (1) avoiding membrane fouling,
and possibilities to remediate, (2) improving the separation between solutes that can be achieved, (3) further
treatment of concentrates, (4) chemical resistance and limited lifetime of membranes, (5) insufficient
rejection of pollutants in water treatment, and (6) the need for modelling and simulation tools.
A complete list of the potential applications of nanotechnology is too vast and diverse to discuss in detail, but without doubt, one of the greatest values of nanotechnology will be in the development of new and effective medical treatments
Human health-care nanotechnology research can definitely result in immense health benefits. The genesis of nanotechnology can be traced to the promise of revolutionary advances across medicine, communications, genomics, and robotics.
Other relevant works:
References:
Sunderland, K., 2008, Developments in filtration, What is Nanofiltration?, Volume 45, Issue 8, Pages 32-35.
Case Study A:
500
Conclusions: (400W)
NF is a suitable method for the removal of a wide range of pollutants from groundwater or surface water, in view of drinking water production. The major application is softening, but NF is usually applied for the combined removal of NOM, micropollutants, viruses and bacteria, nitrates and arsenic, or for partial desalination. Industrial full-scale installations have proven the reliability of NF in these areas.
However, it should be taken into account that a relatively large concentrated fraction is obtained (up to 20% of the feed volume), where the initial pollutants are present in elevated concentrations. Easy methods for concentrate disposal are discharge to salt water bodies, transport to wastewater treatment plants, the use of deep injection wells, and blending for use as irrigation water (possibly after purification of the concentrate with UF). More complex applications may require the implementation of a hybrid system, e.g. in combination with adsorption or biodegradation. The environmental fate of the pollutants in the concentrate is usually unclear; research and practical applications should therefore focus on the further treatment of the concentrated fraction, which is inextricably bound up with the application of NF.
NF is a suitable method for the removal of a wide range of pollutants from groundwater or surface water, in view of drinking water production. The major application is softening, but NF is usually applied for the combined removal of NOM, micropollutants, viruses and bacteria, nitrates and arsenic, or for partial desalination. Industrial full-scale installations have proven the reliability of NF in these areas.
However, it should be taken into account that a relatively large concentrated fraction is obtained (up to 20% of the feed volume), where the initial pollutants are present in elevated concentrations. Easy methods for concentrate disposal are discharge to salt water bodies, transport to wastewater treatment plants, the use of deep injection wells, and blending for use as irrigation water (possibly after purification of the concentrate with UF). More complex applications may require the implementation of a hybrid system, e.g. in combination with adsorption or biodegradation. The environmental fate of the pollutants in the concentrate is usually unclear; research and practical applications should therefore focus on the further treatment of the concentrated fraction, which is inextricably bound up with the application of NF.
NF is a suitable method for the removal of a wide range of pollutants from groundwater or surface water, in view of drinking water production. The major application is softening, but NF is usually applied for the combined removal of NOM, micropollutants, viruses and bacteria, nitrates and arsenic, or for partial desalination. Industrial full-scale installations have proven the reliability of NF in these areas.
However, it should be taken into account that a relatively large concentrated fraction is obtained (up to 20% of the feed volume), where the initial pollutants are present in elevated concentrations. Easy methods for concentrate disposal are discharge to salt water bodies, transport to wastewater treatment plants, the use of deep injection wells, and blending for use as irrigation water (possibly after purification of the concentrate with UF). More complex applications may require the implementation of a hybrid system, e.g. in combination with adsorption or biodegradation. The environmental fate of the pollutants in the concentrate is usually unclear; research and practical applications should therefore focus on the further treatment of the concentrated fraction, which is inextricably bound up with the application of NF.
