Nanofiltration (NF) is a relatively recent membrane filtration process
used most often with low total dissolved solids water such as surface
water and fresh groundwater, with the purpose of softening (polyvalent
cation removal) and removal of disinfection by-product precursors such
as natural organic matter and synthetic organic matter.
Nanofiltration is also becoming more widely used in food processing
applications such as dairy, for simultaneous concentration and partial
(monovalent ion) demineralisation.
2 Range of applications
3 Advantages and disadvantages
4 Design and operation
4.1 Concentration polarisation
4.2 Spiral wound module
4.3 Tubular module
Flux enhancing strategies
5.1 Performance parameters
5.2 Morphology parameters
5.3 Solute transport and rejection
6 Typical figures for industrial applications
7.2 Disinfection and stabilisation
8 New developments
9 See also
11 External links
Nanofiltration is a membrane filtration-based method that uses
nanometer sized through-pores that pass through the membrane.
Nanofiltration membranes have pore sizes from 1-10 nanometers, smaller
than that used in microfiltration and ultrafiltration, but just larger
than that in reverse osmosis. Membranes used are predominantly created
from polymer thin films. Materials that are commonly used include
polyethylene terephthalate or metals such as aluminum. Pore
dimensions are controlled by pH, temperature and time during
development with pore densities ranging from 1 to 106 pores per cm2.
Membranes made from polyethylene terephthalate and other similar
materials, are referred to as "track-etch" membranes, named after the
way the pores on the membranes are made. "Tracking" involves
bombarding the polymer thin film with high energy particles. This
results in making tracks that are chemically developed into the
membrane, or "etched" into the membrane, which are the pores.
Membranes created from metal such as alumina membranes, are made by
electrochemically growing a thin layer of aluminum oxide from aluminum
metal in an acidic medium.
Range of applications
Historically, nanofiltration and other membrane technology used for
molecular separation was applied entirely on aqueous systems. The
original uses for nanofiltration were water treatment and in
particular water softening. Nanofilters can "soften" water by
retaining scale-forming, hydrated divalent ions (e.g. Ca2+, Mg2+)
while passing smaller hydrated monovalent ions.
In recent years, the use of nanofiltration has been extended into
other industries such as milk and juice production. Research and
development in solvent-stable membranes has allowed the application
for nanofiltration membranes to extend into new areas such as
pharmaceuticals, fine chemicals, and flavour and fragrance
industries. Development in organic solvent nanofiltration
technology and commercialization of membranes used has extended
possibilities for applications in a variety of organic solvents
ranging from non-polar through polar to polar aprotic.
Fine chemistry and Pharmaceuticals
Non-thermal solvent recovery and management
Room temperature solvent exchange
Oil and Petroleum chemistry
Removal of tar components in feed
Purification of gas condensates
Continuous recovery of homogeneous catalysts
Natural Essential Oils and similar products
Fractionation of crude extracts
Enrichment of natural compounds Gentle Separations
Able to extract amino acids and lipids from blood and other cell
Advantages and disadvantages
One of the main advantages of nanofiltration as a method of softening
water is that during the process of retaining calcium and magnesium
ions while passing smaller hydrated monovalent ions, filtration is
performed without adding extra sodium ions, as used in ion
exchangers. Many separation processes do not operate at room
temperature (e.g. distillation), which greatly increases the cost of
the process when continuous heating or cooling is applied. Performing
gentle molecular separation is linked with nanofiltration that is
often not included with other forms of separation processes
(centrifugation). These are two of the main benefits that are
associated with nanofiltration.
Nanofiltration has a very favorable
benefit of being able to process large volumes and continuously
produce streams of products. Still,
Nanofiltration is the least used
method of membrane filtration in industry as the membrane pores sizes
are limited to only a few nanometers. Anything smaller, reverse
osmosis is used and anything larger is used for ultrafiltration.
Ultrafiltration can also be used in cases where nanofiltration can be
used, due to it being more conventional. A main disadvantage
associated with nanotechnology, as with all membrane filter
technology, is the cost and maintenance of the membranes used.
Nanofiltration membranes are an expensive part of the process. Repairs
and replacement of membranes is dependent on total dissolved solids,
flow rate and components of the feed. With nanofiltration being used
across various industries, only an estimation of replacement frequency
can be used. This causes nanofilters to be replaced a short time
before or after their prime usage is complete.
Design and operation
Industrial applications of membranes require hundreds to thousands of
square meters of membranes and therefore an efficient way to reduce
the footprint by packing them is required. Membranes first became
commercially viable when low cost methods of housing in 'modules' were
achieved. Membranes are not self-supporting. They need to be
stayed by a porous support that can withstand the pressures required
to operate the NF membrane without hindering the performance of the
membrane. To do this effectively, the module needs to provide a
channel to remove the membrane permeation and provide appropriate flow
condition that reduces the phenomena of concentration polarisation. A
good design minimises pressure losses on both the feed side and
permeate side and thus energy requirements. Leakage of the feed into
the permeate stream must also be prevented. This can be done through
either the use of permanent seals such as glue or replaceable seals
such as O-rings.
Concentration polarisation describes the accumulation of the species
being retained close to the surface of the membrane which reduces
separation capabilities. It occurs because the particles are convected
towards the membrane with the solvent and its magnitude is the balance
between this convection caused by solvent flux and the particle
transport away from the membrane due to the concentration gradient
(predominantly caused by diffusion.) Although concentration
polarisation is easily reversible, it can lead to fouling of the
Spiral wound module
Spiral wound modules are the most commonly used style of module and
are 'standardized' design, available in a range of standard diameters
(2.5", 4" and 8") to fit standard pressure vessel that can hold
several modules in series connected by O-rings. The module uses flat
sheets wrapped around a central tube. The membranes are glued along
three edges over a permeate spacer to form 'leaves'. The permeate
spacer supports the membrane and conducts the permeate to the central
permeate tube. Between each leaf, a mesh like feed spacer is
inserted. The reason for the mesh like dimension of the spacer
is to provide a hydrodynamic environment near the surface of the
membrane that discourages concentration polarisation. Once the leaves
have been wound around the central tube, the module is wrapped in a
casing layer and caps placed on the end of the cylinder to prevent
'telescoping' that can occur in high flow rate and pressure
Tubular modules look similar to shell and tube heat exchangers with
bundles of tubes with the active surface of the membrane on the
inside. Flow through the tubes is normally turbulent, ensuring low
concentration polarisation but also increasing energy costs. The tubes
can either be self-supporting or supported by insertion into
perforated metal tubes. This module design is limited for
nanofiltration by the pressure they can withstand before bursting,
limiting the maximum flux possible. Due to both the high
energy operating costs of turbulent flow and the limiting burst
pressure, tubular modules are more suited to 'dirty' applications
where feeds have particulates such as filtering raw water to gain
potable water in the Fyne process. The membranes can be easily cleaned
through a 'pigging' technique with foam balls are squeezed through the
tubes, scouring the caked deposits.
Flux enhancing strategies
These strategies work to reduce the magnitude of concentration
polarisation and fouling. There is a range of techniques available
however the most common is feed channel spacers as described in spiral
wound modules. All of the strategies work by increasing eddies and
generating a high shear in the flow near the membrane surface. Some of
these strategies include vibrating the membrane, rotating the
membrane, having a rotor disk above the membrane, pulsing the feed
flow rate and introducing gas bubbling close to the surface of the
Many different factors must be taken into account in the design of NF
membranes, since they vary so much in material, separation mechanisms,
morphology and thus application. Two important parameters should be
investigated during preliminary calculations, performance and
Retention of both charged and uncharged solutes and permeation
measurements can be categorised into performance parameters since the
performance under natural conditions of a membrane is based on the
ratio of solute retained/ permeated through the membrane.
For charged solutes, the ionic distribution of salts near the
membrane-solution interface plays an important role in determining the
retention characteristic of a membrane. If the charge of the membrane
and the composition and concentration of the solution to be filtered
is known, the distribution of various salts can be found. This in turn
can be combined with the known charge of the membrane and the
Gibbs–Donnan effect to predict the retention characteristics for
Uncharged solutes cannot be characterised simply by Molecular Weight
Cut Off (MWCO,) although in general an increase in molecular weight or
solute size leads to an increase in retention. The valence charge,
chemical structure, functional end-groups as well as pH of the solute,
all play an important role in determining the retention
characteristics and as such detailed information about the solute
molecule characteristics must be known before implementing a NF
The morphology of a membrane must also be known in order to implement
a successful design of a NF system, and this is usually done by
Atomic force microscopy
Atomic force microscopy (AFM) is one method used to
characterise the surface roughness of a membrane by passing a small
sharp tip (<100 Ă) across the surface of a membrane and measuring
Van der Waals force
Van der Waals force between the atoms in the end of the
tip and the surface. This is useful as a direct correlation
between surface roughness and colloidal fouling has been developed.
Correlations also exist between fouling and other morphology
parameters, such as hydrophobe, showing that the more hydrophobic a
membrane is, the less prone to fouling it is. See membrane fouling for
Methods to determine the porosity of porous membranes have also been
found via permporometry, making use of differing vapour pressures to
characterise the pore size and pore size distribution within the
membrane. Initially all pores in the membrane are completely filled
with a liquid and as such no permeation of a gas occurs, but after
reducing the relative vapour pressure some gaps will start to form
within the pores as dictated by the Kelvin equation. Polymeric
(non-porous) membranes cannot be subjected to this methodology as the
condensable vapour should have a negligible interaction within the
Solute transport and rejection
Mechanisms through which solutes in nanofiltration transport through
Unlike membranes with larger and smaller poor sizes, passage of
solutes through nanofiltration is significantly more complex.
Because of the pore sizes, there are three modes of transport of
solutes through the membrane. These include 1) diffusion (molecule
travel due to concentration potential gradients, as seen through
reverse osmosis membranes), 2) convection (travel with flow, like in
larger pore size filtration such as microfiltration), and 3)
electromigration (attraction or repulsion from charges within and near
Additionally, the exclusion mechanisms in nanofiltration are more
complex than in other forms of filtration. Most filtration systems
operate solely by size (steric) exclusion, but at small length scales
seen in nanofiltration, one must also consider the impacts of surface
charge on the small charged solutes, and also the impacts of
hydration, where molecules in solution have a solvation shell of
surrounding water molecules. The exclusion due to hydration is
referred to as dielectric exclusion, a reference to the different
dielectric constants (energy) associated with a particles precense in
solution versus within a membrane substrate.
Primary rejection mechanisms that prevent solutes from entering the
pores in nanofiltration.
The transport and exclusion mechanisms are heavily influenced by
membrane pore size, solvent viscosity, membrane thickness, solute
diffusivity, solution temperature, solution pH, and membrane
dielectric constant. The pore size distribution is also important.
Modeling rejection accurately for NF is very challenging. It can be
done with applications of the Nernst–Planck equation, although a
heavy reliance on fitting parameters to experimental data is usually
In general, charged solutes are much more effectively rejected in NF
than uncharged solutes, and multivalent solutes like SO2−
4 (valence of 2) experience very high rejection.
Typical figures for industrial applications
Keeping in mind that NF is usually part of a composite system for
purification, a single unit is chosen based off the design
specifications for the NF unit. For drinking water purification many
commercial membranes exist, coming from different chemical families,
having different structures, chemical tolerances and salt rejections
and so the characterisation must be chosen based on the chemical
composition and concentration of the feed stream.
NF units in drinking water purification range from extremely low salt
rejection (<5% in 1001A membranes) to almost complete rejection
(99% in 8040-TS80-TSA membranes.) Flow rates range from 25–60 m3/day
for each unit, so commercial filtration requires multiple NF units in
parallel to process large quantities of feed water. The pressures
required in these units are generally between 4.5-7.5 bar.
For seawater desalination using a NF-RO system a typical process is
Because of the fact that NF permeate is rarely clean enough to be used
as the final product for drinking water and other water purification,
is it commonly used as a pre treatment step for reverse osmosis
(RO) as is shown above.
As with other membrane based separations such as ultrafiltration,
microfiltration and reverse osmosis, post-treatment of eitherpermeate
or retentate flow streams (depending on the application) – is a
necessary stage in industrial NF separation prior to commercial
distribution of the product. The choice and order of unit operations
employed in post-treatment is dependent on water quality regulations
and the design of the NF system. Typical NF water purification
post-treatment stages include aeration and disinfection &
Polyvinyl chloride (PVC) or fibre-reinforced plastic (FRP)
degasifier is used to remove dissolved gases such as carbon dioxide
and hydrogen sulfide from the permeate stream. This is achieved by
blowing air in a countercurrent direction to the water falling through
packing material in the degasifier. The air effectively strips the
unwanted gases from the water.
Disinfection and stabilisation
The permeate water from a NF separation is demineralised and may be
disposed to large changes in pH, thus providing a substantial risk of
corrosion in piping and other equipment components. To increase the
stability of the water, chemical addition of alkaline solutions such
as lime and caustic soda is employed. Furthermore, disinfectants such
as chlorine or chloroamine are added to the permeate, as well as
phosphate or fluoride corrosion inhibitors in some cases.
Contemporary research in the area of
Nanofiltration (NF) technology is
primarily concerned with improving the performance of NF membranes,
minimising membrane fouling and reducing energy requirements of
already existing processes. One way in which researchers are
attempting to improve NF performance – more specifically increase
permeate flux and lower membrane resistance – is through
experimentation with different membrane materials and configurations.
thin film composite membranes (TFC), which consist of a number of
extremely thin selective layers interfacially polymerized over a
microporous substrate, have had the most commercial success in
industrial membrane applications due to the capability of optimizing
the selectivity and permeability of each individual layer. Recent
research has shown that the addition of nanotechnology materials such
as electrospunnanofibrous membrane layers (ENMs) to conventional TFC
membranes results in an enhanced permeate flux. This has been
attributed to inherent properties of ENMs that favour flux, namely
their interconnected pore structure, high porosity and low
transmembrane pressure. A recently developed membrane
configuration which offers a more energy efficient alternative to the
commonly used spiral wound arrangement is the hollow fibre membrane.
This format has the advantage of requiring significantly less
pre-treatment than spiral wound membranes, as solids introduced in the
feed are displaced effectively during backwash or flushing. As a
result, membrane fouling and pre-treatment energy costs are reduced.
Extensive research has also been conducted on the potential use of
Titanium Dioxide (TiO2, titania) nanoparticles for membrane fouling
reduction. This method involves applying a nonporous coating of
titania onto the membrane surface. Internal fouling/pore blockage of
the membrane is resisted due to the nonporosity of the coating, whilst
the superhydrophilic nature of titania provides resistance to surface
fouling by reducing adhesion of emulsified oil on the membrane
List of nanotechnology applications
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(2017). "Effect of temperature on ion transport in nanofiltration
membranes: Diffusion, convection and electromigration". Desalination.
Elsevier BV. 420: 241–257. doi:10.1016/j.desal.2017.07.020.
^ Raymond D. Letterman (ed.)(1999). "Water Quality and Treatment." 5th
Ed. (New York: American Water Works Association and McGraw-Hill.)
^ Dow Chemical Co.
Nanofiltration Membranes and Applications
^ Baker, L.A.; Martin (2007). "
Nanotechnology in Biology and Medicine:
Methods, Devices and Applications". Nanomedicine: Nanotechnology,
Biology and Medicine. 9: 1–24.
^ Apel, P.Yu; et al. (2006). "Structure of Polycarbonate Track-Etch:
Origin of the "Paradoxical" Pore Shape". Journal of Membrane Science.
282 (1): 393–400. doi:10.1016/j.memsci.2006.05.045.
^ a b Rahimpour, A; et al. (2010). "Preparation and Characterisation
of Asymmetric Polyethersulfone and Thin-Film Composite Polyamide
Nanofiltration Membranes for Water Softening". Applied Surface
Science. 256 (6): 1657–1663. doi:10.1016/j.apsusc.2009.09.089.
^ Labban, O.; Liu, C.; Chong, T.H.; Lienhard V, J.H. (2017).
"Fundamentals of low-pressure nanofiltration: Membrane
characterization, modeling, and understanding the multi-ionic
interactions in water softening". Journal of Membrane Science. 521:
^ Baker, L.A.; Martin, Choi (2006). "Current Nanoscience".
Nanomedicine: Nanotechnology, Biology and Medicine. 2 (3):
^ a b Mohammed, A.W.; et al. (2007). "Modelling the Effects of
Nanofiltration Membrane Properties on System Cost Assessment for
Desalination Applications". Desalination. 206 (1): 215–225.
^ a b Baker, Richard (2004). Membrane Technology and Applications.
West Sussex: John Wiley & Sons. ISBN 0470854456.
^ a b c d e f g h Schafer, A.I (2005).
Nanofiltration Principles and
Applications. Oxford: Elsevier. ISBN 1856174050.
^ a b c Wiley, D.E.; Schwinge, Fane (2004). "Novel Spacer Design
Improves Observed Flux". Journal of Membrane Science. 229 (1-2):
53–61. ISSN 0376-7388.
^ a b Schwinge, J.; Neal, P.R.; Wiley,D.E.; Fletcher, D.F.; Fane, A.G.
(2004). "Spiral Wound Modules and Spacers: Review and Analysis".
Journal of Membrane Science. 242 (1-2): 129–153.
doi:10.1016/j.memsci.2003.09.031. ISSN 0376-7388.
^ Grose, A.B.F; Smith, A.J.; Donn, A.; O'Donnell, J.; Welch, D.
(1998). "Supplying High Quality Drinking Water to Remote Communities
in Scotland". Desalination. 117 (1-3): 107–117.
doi:10.1016/s0011-9164(98)00075-7. ISSN 0011-9164.
^ a b American Water Works Association (2007). Manual of Water Supply
Reverse Osmosis and Nanofiltration. Denver: American
Water Works Association. pp. 101–102.
^ Misdan, N.; Lau, W.J.; Ismail, A.F.; Matsuura, T. (2013). "Formation
of Thin Film Composite
Nanofiltration Membrane: Effect of Polysulfone
Substrate Characteristics". Desalination. 329: 9–18.
^ Subramanian, S; Seeran (2012). "New Direction is Nanofiltration
Applications- Are Nanofibres the Right Materials as Membranes in
Desalination". Desalination. 308: 198.
^ Pearce, G (2013). Nifty Nanofiltration, New Developments Show
Promise (26 ed.). Water World Magazine.
^ Dražević, E.; Košutić, K.; Dananić, V.; Pavlović, D.M. (2013).
"Coating Layer Effect on Performance of Thin Film Nanofiltration
Membrane in Removal of Organic Solutes". Separation and Purification
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Project ETAP-ERN, that uses renewable energies for desalinization. (in
Nano based methods to improve water quality - Hawk's Perch Tech