Separation and Purification Technology B. Van der Bruggen

Separation and Purification Technology 63 (2008) 251–263
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Separation and Purification Technology
journal homepage: www.elsevier.com/locate/seppur
Review
Drawbacks of applying nanofiltration and how to avoid them: A review
¨
¨ b,1 , M. Nystrom
¨ b,1
B. Van der Bruggen a,∗ , M. Mantt
ari
a
K.U.Leuven, Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, W. de Croylaan 46, B – 3001 Leuven, Belgium
Lappeenranta University of Technology, Department of Chemical Technology, Laboratory of Membrane Technology and Technical Polymer Chemistry,
P.O. Box 20 FI-53851 Lappeenranta, Finland
b
a r t i c l e
i n f o
Article history:
Received 26 February 2008
Received in revised form 6 May 2008
Accepted 10 May 2008
Keywords:
Membrane filtration
Nanofiltration
Fouling
Concentrates
Fractionation
Water treatment
Drinking water
Wastewater
a b s t r a c t
In spite of all promising perspectives for nanofiltration, not only in drinking water production but also
in wastewater treatment, the food industry, the chemical and pharmaceutical industry, and many other
industries, 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.
The implementation of nanofiltration in the industry is a success story because these challenges can be
dealt with for many applications, whereas more research would result in many more possible applications.
It is suggested that these challenges should be among the main priorities on the research agenda for
nanofiltration. This paper offers an overview of the state-of-the-art in these areas, without going into
details about specific observations in individual studies, but rather aiming at giving the overall picture
of possible drawbacks. This leads to suggestions which direction the nanofiltration research community
should follow, and where research questions can be found. Evidently, the six identified challenges are
to some extent interrelated; mutual influences are explained as well as possible solutions, or possible
pathways to solutions.
© 2008 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Insufficient separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Treatment of concentrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Membrane lifetime and chemical resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Insufficient rejection for individual compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modelling and simulation of nanofiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
The introduction of new technologies always involves transition
phenomena, such as unexpected start-up problems, discussions
between believers and non-believers, and research efforts leading
∗ Corresponding author. Tel.: +32 16 32 23 40; fax: +32 16 32 29 91.
E-mail addresses: [email protected] (B. Van der Bruggen),
¨
¨
¨
[email protected]fi (M. Mantt
ari),
[email protected]fi (M. Nystrom).
1
Tel.: +358 5 621 2192; fax: +358 5 621 2199.
1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2008.05.010
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to fundamental understanding, suggestions for practical solutions
and technical improvements. Nanofiltration was defined as “a process intermediate between reverse osmosis and ultrafiltration that
rejects molecules which have a size in the order of one nanometer”
[1]. It was introduced in the late 1980s, mainly aiming at combined
softening and organics removal [1]. Since then, the application
range of nanofiltration has extended tremendously. New possibilities were discovered for drinking water production, providing
answers to new challenges such as arsenic removal [2–7], removal
of pesticides, endocrine disruptors and chemicals [8–11,6,12,13],
and partial desalination [14–17]. Large plants were constructed, the
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B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
´
best documented example being the Mery-sur-Oise
plant in France,
which was started in the second half of the 1990s [18,19].
During the last decade, the interest in the use of membrane technology in general and nanofiltration in particular has emerged in
wastewater treatment as well as drinking water and process water
production. This growth can be explained by a combination of (1)
growing demand for water with high quality, (2) growing pressure
to reuse wastewater, (3) better reliability and integrity of the membranes, (4) lower prices of membranes due to enhanced use, and (5)
more stringent standards, e.g., in the drinking water industry.
The number of applications for nanofiltration increases steadily;
nevertheless, several challenges remain to be solved to allow the
use of nanofiltration in more demanding applications. Drinking
water production, still the largest application of nanofiltration in
terms of volumes, currently faces new challenges. The notion of
impeccable drinking water developed in the Water Quality 21
research programme in the Netherlands [20] and the growing concerns in the USA on the presence of emerging contaminants [21]
induced a shift towards less permeable, high rejection membranes,
which can be denoted as (low pressure) reverse osmosis membranes. For ‘tight’ nanofiltration membranes and reverse osmosis
membranes, knowledge on which solutes are removed to what
extent is needed.
The potential for nanofiltration in wastewater treatment and
water reuse is noteworthy [22–25], but hindered by unstabilities in
operation caused by membrane fouling. Extensive research projects
in which nanofiltration was used for water reclamation have been
carried out; in the majority of these, membrane fouling was studied
as a potential problem. Industrial plants may be successful [26,27],
but their success depends on a thorough understanding of possible
interactions between the feed solution and the membrane, causing organic fouling, scaling, biofouling, or particulate fouling. When
wastewater is to be treated, the concentrate is usually another problem [28,29]. The discharge option is often compromised by the
increase of concentrations in the remaining fraction after membrane treatment. This can also be problematic for drinking water
production, when discharge is not possible or not allowed, or when
the yield is considered too low for a valuable, permit-protected
source such as groundwater.
The chemical processing industry and the pharmaceutical
industry are other potential beneficiaries of nanofiltration. Huge
savings could be obtained by implementing membrane technology; environmental benefits due to reduced energy consumption
make nanofiltration particularly attractive [30,31]. Drawbacks in
this area are of a different kind; for solvent filtration, one of the
emerging applications, these are mainly related to membrane stability and lifetime [32], and the lack of fundamental understanding
of the process performance that can be translated to modelling and
simulation tools [33–36].
The food industry traditionally adopts new technologies relatively fast. The dairy industry was among the first users of
nanofiltration [37]. Nevertheless, the challenges in the food industry are high. Standards for food products are very high and emerging
applications, such as low fat products, low calories products, and
products suitable for special diets require more and improved
separations. Based on its potential to separate monovalent and multivalent ions, and to separate organic solutes with different size
from one another, nanofiltration could be the promise for the future.
However, separation factors are often insufficient, which limits the
potential.
This review gives a systematic overview of reported impediments for nanofiltration, and possible solutions to solve these
problems. Solutions may be directly suggested from the literature,
or can be derived from a critical assessment of the state-of-theart in nanofiltration. Challenges that will be covered in this review
are (1) membrane fouling, its causes and possibilities to remediate, (2) separation between solutes that can be achieved, (3)
further treatment of concentrates, (4) chemical resistance of membranes, (5) insufficient rejection in water treatment, and (6) the
need for modelling and simulation tools. It must be stressed that
many applications are already running regardless of these suggested improvements. Nevertheless, one should be aware that more
can be done if the limitations can be overcome.
2. Membrane fouling
Fouling is one of the main problems in any membrane separation, but for nanofiltration it might be even somewhat more
complex because of the interactions leading to fouling take place
at nanoscale, and are therefore difficult to understand [38–44]. Its
negative consequences are obvious and include the need for pretreatment, membrane cleaning, limited recoveries and feed water
loss, and short lifetimes of membranes. In that sense, membrane
fouling is closely related to other problems such as concentrate
treatment and membrane stability and lifetime: a total control of
fouling would reduce the need for cleaning and would enhance the
permeate yield.
Foulants playing a role for nanofiltration membranes can be
organic solutes, inorganic solutes, colloids, or biological solids
[45]. An extensive description of the consequences of fouling in
nanofiltration can be found in the literature [46], including indices
describing the feed water fouling potential and the post factum
analysis by membrane autopsy. Boussu et al. [47–49] extended the
study of membrane characteristics to prediction and interpretation of fouling caused by organic solutes, colloids and surfactants.
Fouling of organic solutes is thought to be mainly caused by adsorptive interactions with the membrane material [50–52]. Fouling
and adsorption can be related to component properties, which is
reflected by the correlation between the octanol–water partition
coefficient (log P) and adsorption; adsorption is also related to the
dipole moment and the water solubility [52]. Concerning the membrane characteristics, the hydrophobicity of the top layer is believed
to cause the most flux decline [53,54]. For charged organic compounds, electrostatic attraction or repulsion forces between the
component and the membrane influence the degree of fouling. A
necessary condition for this is that the membrane surface charge is
large enough; otherwise hydrophobic forces overcome the electrostatic forces resulting in more fouling of hydrophobic membranes
[55].
Depending on the relative size of colloidal particles and membrane pores, colloidal fouling may occur either due to accumulation
of particles on the membrane surface and build-up of a cake or by
penetration within the membrane pores [56–59]. It is assumed that
colloidal fouling is related to membrane roughness [60,61]: colloids
are thought to be preferentially transported into the valleys, which
results in “valley clogging”. In addition, surface hydrophobicity and
permeability also play a role [62–64].
The size, charge and concentration of the colloids also influence
fouling in nanofiltration. An increase in colloid concentration leads
to an increase in fouling [58,63,65–67]; a larger colloid size may
have either a negative [63] or a positive effect [56,68] on fouling in
comparison with smaller colloids.
Inorganic fouling is related to scaling, i.e., precipitation of salts
on the membrane surface [46]. Nanofiltration membranes retain
ions, causing an increase of the concentration at the membrane
surface, which may exceed the solubility limit at a certain point
in the filtration module. The most common constituents of scale
are calcium carbonate, gypsum, barium/strontium sulphate and silica, although other potential scalants exist [46]. Scaling is a purely
thermodynamic process involving a phase change, which requires
B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
a degree of supersaturation. In general, the point of saturation can
be estimated from the activities of the ions involved in the precipitation reaction; nevertheless, it is difficult to determine the ‘critical’
point of supersaturation.
Biofouling is a general problem with many membrane processes
and involves all biologically active organisms, mainly bacteria
and (in some cases) fungi [46]. Biofouling is a dynamic process
and involves the formation and growth of a biofilm attached to
the membrane. The biofilm may reduce the water flux and even
totally prevent water passage. For nanofiltration of wastewater, the
biofilms were found to have a thickness of 20–30 ␮m [69]. Biofouling is not a specific problem for nanofiltration, in contrast to
scaling and adsorption of small organic solutes: these may (partly)
penetrate into the membrane, whereas bacteria are too large and
will remain in the superficial biofilm. Nevertheless, it is suggested
[69,70] that the formation and accumulation of exopolymeric substances (EPS) is the real cause of flux decline when biofouling
occurs.
Classical solutions to fouling are the optimization of pretreatment methods and cleaning of membranes. Suggested
pretreatment methods often make use of other pressure driven
membrane separations such as ultrafiltration and microfiltration
[71–73]; other options include ozonation or UV/H2 O2 oxidation,
adsorption (PAC) and flocculation [74,75]. An extensive overview
of pretreatment methods can be found in the literature [76].
Cleaning of nanofiltration membranes has become a research
area on itself [77–79]. Nevertheless, in practical applications cleaning is usually considered in a very pragmatic way. Physical cleaning
may be a significant part of the cleaning protocol and includes flushing (backflush, forward flush, reverse flush), scrubbing, air sparging,
vibrations and sonication [46,80,81]. Membrane design and process
conditions may help in this by increasing the efficiency of physical
cleaning, up to the point where direct nanofiltration without pretreatment can be applied [82]. Chemical cleaning involves chemical
reactions such as hydrolysis, saponification, solubilisation, dispersion, chelation, and peptisation [83]. Membrane manufacturers
often develop specific cleaning strategies and products suitable for
their own membranes. However, it should be taken into account
that the cleaning protocol should also depend on the characteristics
of the feed solution. This leads to a wide variety of cleaning mixtures and protocols in the literature [84–86]. In addition, enzymatic
cleaning may be considered [87].
Membrane modification is potentially the most sustainable
solution to obtain fouling-resistant membranes [77]. The idea is to
insert hydrophilic groups into a polymeric structure, so that the
overall material becomes more hydrophilic and thus less prone
to (organic) fouling. Ultrafiltration membranes are often taken as
starting point; a hydrophilic nanofiltration membrane is obtained
by grafting [88–90]. Nanofiltration membranes can be modified by
ion beam irradiation in view of obtaining fouling-resistant membranes [91]. However, it is not clear to what extent the newly
obtained membranes are stable.
Colloidal fouling may be reduced by developing membranes
with lower surface charge or surface charge similar to that of
the foulant. Increasing the hydrophilicity may also be beneficial
to reduce colloidal fouling. Surface roughness may also increase
membrane fouling by increasing the rate of attachment onto the
membrane surface [77]; it is accepted that membranes with a rough
surface are more prone to fouling than membranes with a smoother
surface [92]. Biological fouling can be reduced by the addition of,
e.g., silver nanoparticles in the membrane structure [93].
An intrinsic solution to the problem of membrane fouling could
be the concept of critical or sustainable flux [94]. The critical flux
is the maximal flux where fouling interactions remain reversible;
when operating below the critical flux, flux decline can be reversed
253
Fig. 1. Critical flux for paper mill effluent for a flat sheet membrane module (temperature: 40 ◦ C, cross-flow velocity 2.7 m/s). Open circles: pressure increase; black
circles: pressure decrease; black squares: pressure decrease after the maximum
pressure.
by non-destructive measures. The critical flux concept has a sound
theoretical basis; it represents the shift from repulsive interaction
(dispersed matter-polarised layer) to attractive interaction (condensed matter-deposit) [94]. The concept of a sustainable flux
evolved from the critical flux theory and can be considered a generalisation: the sustainable flux is defined as the flux above which the
rate of fouling is economically and environmentally unsustainable.
The sustainable flux depends on hydrodynamics, feed conditions
and process time, and is therefore difficult to determine. Nevertheless, the understanding of this principles leads to guidelines for
operational conditions where fouling is minimal [95,96]. A typical
example is shown in Fig. 1 for paper mill effluent, where the pressure was stepwise increased and decreased; it can be clearly seen
that the critical flux in this case is around 50 l/m2 h.
3. Insufficient separation
Internationally, nanofiltration has known a breakthrough since
the last decade in areas related to water treatment and drinking water production [97], where it is used for softening and
removal of pollutants (micropollutants such as pharmaceutically
active compounds, pesticides and other relatively small organic
solutes). Nanofiltration can also be applied for more challenging
applications, involving fractionation rather than purification. It is
well known that nanofiltration membranes can be used for salt fractionation [98–101] since the rejection of monovalent salts is lower
than that of multivalent salts. An extreme case of charge-induced
separation is the observation of negative rejections of monovalent
ions in the presence of multivalent ions or polyelectrolytes [102].
Typically, the rejection of a divalent ion of the same charge as the
membrane is above 95%, whereas the rejection of a monovalent ion
of the same charge can be anywhere between 20 and 80% [103].
Thus, nanofiltration membranes allow ion fractionation, which is
a significant advantage and one of the reasons of the fast commercial growth of the process. Nevertheless, the separation factors
obtained with nanofiltration are relatively modest, typically 5–10.
Many applications of ion separation can be found [104–107]. A wellknown application is the separation of peptides based on charge
differences [108]. In the latter case, the solution pH is often the key
to control the desired separation.
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B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
Fig. 2. Typical sigmoidal rejection curve obtained for rejection of uncharged solutes
with a nanofiltration membrane.
For uncharged solutes, however, (nanofiltration) membranes are
characterised by a sigmoidal rejection curve (rejection as a function
of molar mass) [109], which results in an insufficient separation
between different compounds on the basis of molecular size. A
typical sigmoidal rejection curve is given in Fig. 2. Furthermore,
the separation depends on hydrophobicity and charge interactions
[110]. Therefore, the permeate contains molecules with variable
size, both below and above the claimed pore size of the membrane.
Either the permeate or the retentate is to be considered as a waste
fraction.
Fractionation using membranes (including nanofiltration) is
considered by many authors, but usually in the sense that nanofiltration is preceded by either ultrafiltration or microfiltration, or
followed by reverse osmosis [111–114]. The fractions obtained in
this way are orders of magnitude different in molecular size, and
a finer fractionation (between solutes with size in the nanometer
range and below) is seldomly reported.
In the pharmaceutical industry, many possible applications of
fractionation in the nanorange are to be found. Pharmaceutically
active compounds and intermediates are often thermally labile
(which makes a distillative separation difficult or impossible), and
have to be separated from smaller or larger side products, remaining reagents and the solvent. A single membrane separation is often
insufficient to obtain the desired separation because it is impossible to retain one component completely and at the same time
allow a second component, slightly different in size or charge, to
pass completely. The incompleteness of the separation is a major
impediment for a wide application of membrane processes, including nanofiltration. A multiple membrane passage may improve
the overall rejection but not the separation between different
individual compounds. Diafiltration is sometimes a good solution
when product recovery is considered, such as in solvent exchange
[115], but is not applicable for separation of individual solutes. The
use of continuous counter current integrated membrane cascades
with recycle, in analogy with (conventional) separations based on
thermodynamic equilibrium (presented in Fig. 3), may allow better separations between individual compounds, or fractionation
of a mixture. This should allow realising any separation, i.e., to
obtain simultaneously a nearly complete rejection of component
A, and no removal of component B, slightly different in molecular
size (or any other parameter playing a role in transport through
the membrane). To this date the separation that can be attained
with a nanofiltration cascade has not received much attention.
For gas separations with membranes, cascades have been studied and are well known as integrated separation processes [116].
For liquid separations there is no knowledge on cascades, apart
from some exploratory studies concerning module configurations
in reverse osmosis [117], which is a more or less similar idea. For
nanofiltration, cascades have not been considered before; a multi-
Fig. 3. Schematic representation of the principle of a membrane cascade (adapted
from [116]).
step approach was recently suggested for purification of solvents
[118].
Fractionation is also of importance in the food industry. Again,
membranes are the key for these separations: the share of the
food industry is 20–30% of the entire membrane market [119].
Nanofiltration for food applications is the second largest after ultrafiltration [119]. Dairy applications were among the very first where
nanofiltration was used [120]. Using nanofiltration, desalted lactose containing whey could be produced with a single process; ca.
40% of the salts in whey can be removed. By using diafiltration, salt
removal can be even up to 90%. However, as previously stated, there
is a significant product loss to permeate (lactose, in this case) when
diafiltration is used. Another example is skim milk modification. A
precise control of the milk composition would open new possibilities in the area of tailor-made milk products; however, in spite of
the unsharp separation that can be achieved in one step, this seems
to be the first and continuing success story of nanofiltration [120].
In the sweetener industry, purification of xylose is an emerging application. This requires a challenging separation between
xylose and glucose, two compounds with only a slight difference in
molecular size and with similar properties such as, e.g., polarity. A
separation can be achieved, but it was shown to be extremely difficult [121]; the process is feasible when a single fraction (xylose) is to
be recovered with enhanced purity, but again, a sharp separation
is impossible using simple one-step solutions. Nanofiltration can
be used more easily for separation of oligosaccharides [122–124]
in combination with ultrafiltration, or even the separation of saccharides and salts (in diafiltration mode) [125].
Nanofiltration cascades can possibly also be used for purification of natural sweeteners. Stevia rebaudiana Bertoni is a plant
that contains very sweet steviol glycosides, of which stevioside
and rebaudioside A are the most abundant (Fig. 4). Stevioside and
rebaudioside A can be used as a natural sweetener in low doses
(maximum 200–300 mg/day), without a significant caloric value. It
B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
Fig. 4. Chemical structure of stevioside.
is also safe for phenylketonuria patients who might be in danger
by consumption of large amounts of aspartame. High doses stevioside (750–1500 mg/day) might be used in the treatment of the
metabolic syndrome (hypertension, diabetes type 2) [126].
It is obvious that food additives have to meet very strict requirements. During the isolation of the sweeteners from Stevia, safety
and sustainable techniques are needed. The use of solvents should
be avoided if possible. The sweeteners can be extracted with water.
After extraction, purification of the crude extract is needed to separate the products with a molecular mass between 800 and 1000
(sweetener fraction). This could in principle be done with nanofiltration, but the separation that can be realised in this way results in
either a too low purity, or a large loss of the fraction between 800
and 1000. Nanofiltration cascades are a possible solution to this. The
final solution can be further concentrated by using reverse osmosis,
upon which the sweeteners can be crystallised.
4. Treatment of concentrates
The generation of a concentrate (or retentate) stream is an
intrinsic problem for pressure driven membrane processes, including nanofiltration. For aqueous streams, the concentrate is often
an unwanted by-product of the purification process and has to be
discharged or further treated. This is per definition an unsolved
problem when the feed solution contains ‘unwanted’ compounds
or a fraction that cannot be reused, since membranes only achieve
a separation and not a destruction or transformation. The composition of the concentrate is similar to the feed composition, but with
increased concentrations for components rejected by the membrane. The concentration factor CF can be calculated from the mass
balance for component i [29]:
Cr,i =
(Qf × Cf,i ) − (Qp × Cp,i )
Qr
so that
CF =
Cr,i
Q
= f
Cf,i
Qr
1 − (REC ×
Cp,i
Cf,i
,
where REC = recovery, Q = volumetric flow (l/h), C = concentration
(mg/l); the subscripts r, f, p and i refer to the retentate, the feed, the
permeate and the component used, respectively.
255
Additives such as anti-scalants (polyacrylates, polyacrylic acids,
polyphosphates) also end up in the concentrate; the addition of
sulphuric acid or hydrochloric acid influences the pH of the concentrate. Chemical cleaning for removal of scaling, organic fouling
and biofouling from the membrane surface [79,127] results in
a (relatively small) additional waste stream, containing cleaning
chemicals such as acids (phosphoric acid or citric acid), bases
such as sodium hydroxide, complexing agents such as EDTA, polyacrylates, sodium hexametaphosphate) and disinfectants (H2 O2 ,
NaOCl).
Possibilities to treat or to discharge the concentrate [29] include
reuse, further treatment by removal of contaminants, incineration,
direct or indirect discharge in surface water, direct or indirect discharge in groundwater, and landfilling.
Reuse is the most attractive option, but only applicable in few
cases where the concentrated fraction is the desired product, such
as in the food industry. The beverage industry uses a concentration step prior distribution, allowing to reduce the volume and the
related costs. Water is added at the point of usage; although some
flavours may be lost, this method is generally used as the most efficient solution. Nanofiltration is a cheap concentration method, used
as an alternative for reverse osmosis when salt permeation is not a
problem. The permeate is relatively pure water that can be used or
treated as a waste fraction. The concentrate is further dehydrated
to obtain a viscous liquid ready for distribution. Nanofiltration is
applied to this purpose in several applications [128,129]. Other
examples in food processing where the concentrate can be reused
are to be found in the dairy industry, as already discussed. Nanofiltration is used for the recovery of organic nutrients in so-called
‘second cheese whey’ [130]. The whey is processed by nanofiltration to recover a rich lactose fraction in the concentrate and a
process water with a high salt content in the permeate. It should
be concluded that the challenge of obtaining a good separation is
intrinsically interconnected with the concentrate problem.
An example of a closed cycle in wastewater treatment can be
found in the tanning industry, where nanofiltration is used for
the recuperation of chromium from exhausted chromium baths
[131–134]. A combination of ultrafiltration and nanofiltration can
then be used to recycle the tanning baths a concentration of Cr (III)
is obtained in the concentrate fraction; the concentrate can directly
be reused for retaining baths or further concentrated by precipitation (at high pH, addition of NaOH required) and dissolution (in a
concentrated sulphuric acid solution). The nanofiltration permeate
contains a high chloride concentration, because monovalent ions
are almost not retained by the nanofiltration membrane. This is an
advantage when the permeate is reused in pickle baths (saving in
chemicals to be added). The permeate is then a side product that
can be reused as a rinsing water, or discharged.
An integrated treatment system has been considered for water
reuse in a textile company based on nanofiltration, in which the
concentrate generated from purification of exhausted dye baths
is entirely recycled by systematically separating all constituents
[135]. Nanofiltration is applied after a classical wastewater treatment, and produces high-quality process water. The idea of a
zero-discharge system in the textile industry was already suggested
by a combination of chemical, biological and membrane processes,
but appeared to be quite challenging [136]. A combination of membrane processes was suggested for design of new productive cycles
[137], a concept now adopted in the terminology of process intensification. At present, the use of membrane processes in the textile
industry is still limited to one-step designs, which solves the problem of fresh water supply but not the waste (water) problem, since
the pollutant load is unchanged after concentration.
An integrated approach should comprise two main steps [135]:
removal of the organic fraction (dyes, additives), and removal of
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B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
the inorganic fraction (salts). After a pretreatment using microfiltration, the removal of the organic fraction can be done by
nanofiltration using a membrane with low salt rejection at a high
temperature, close to the temperature of the dye bath. The permeate fraction contains a large fraction of inorganics; the organic
fraction should be low. The concentrate is mainly organic in nature.
Membrane distillation can be applied to separate the organic fraction from water, taking advantage of the elevated temperature of
the feed. The distillate is recycled to the finishing process; the
remaining organic fraction has an added value by utilizing its
energy content in an incineration process. The energy yield makes
up for the loss of energy by losses in the different treatment steps.
The nanofiltration permeate feeds a second nanofiltration unit,
where salts are retained using a relatively ‘tight’ nanofiltration
membrane with high salt rejection. The permeate from the second nanofiltration unit is pure enough for reuse as process water.
The concentrate is a salt solution, comparable to the brine from
desalination processes, and can be used for salt production in a
membrane crystallizer [138]. The combination of all these membrane processes results in a zero-discharge system with energy
recuperation. A detailed description and calculation of this system
can be found in the literature [135].
If reuse of the concentrate is not feasible, further treatment can
be necessary before discharge. Two options for further treatment
can be distinguished: (a) further concentration, and (b) removal
of specific components by a proper choice of a selective treatment method. The first option leads to a sludge or solid waste
that has to be reused (if possible), landfilled (if necessary after
solidification/stabilisation or a similar pretreatment to avoid leaching of contaminants), or incinerated. The second option leads to
a (treated) wastewater, that has to be reused (if possible) or discharged in surface water (direct or indirect via sewage systems) or
in groundwater.
Concentrates resulting from drinking water production are a
special case. A distinction should be made between groundwater
and surface water. Nanofiltration is not often used for production
of drinking water from groundwater, because (a) groundwater is
almost exclusively used when a source of good quality, not requiring extensive further (membrane) treatment, is available, and (b)
the concentrate that is generated is a large waste fraction, expensive and technically challenging to dispose of. For surface water,
nanofiltration is a valuable option when the concentrate can easily
be discharged. A study in the Netherlands [139] revealed that disposal of the concentrate is a serious problem, especially in those
cases where no large surface water is present. In general, concentrate disposal as such was feasible, as long as a limited number of
parameters such as sulphate, chloride, phosphate, iron and antiscalant were under control.
Other factors than the volume and composition that have to
be taken into account are legal requirements such as allowances
and conditions; cost of further treatment; local factors such as the
proximity and size of a wastewater treatment plant, the presence of
surface water or open land, soil characteristics and geological structure; flexibility of the disposal method in case of an expansion of the
existing plant; and public acceptance. Release of micropollutants to
the concentrate was also mentioned as a risk [140].
5. Membrane lifetime and chemical resistance
Membrane lifetime and chemical resistance of nanofiltration
membranes is related to the occurrence of fouling (and therefore,
the need for cleaning), and the application of nanofiltration in
demanding circumstances such as in solvent resistant nanofiltration. These are well-known problems for nanofiltration and other
membrane processes, and their impact is usually studied from a
pragmatic point of view, i.e., as a solution to specific filtration
problems. This includes the choice of membrane materials, operating conditions, energy consumption, cleaning chemicals, permeate
yield and overall environmental impact.
For aqueous applications, membrane lifetime depends significantly on the cleaning frequency and the overall strategy against
membrane fouling. Applications where fouling requires frequent
cleaning often face a faster membrane deterioration, because cleaning agents also damage the membrane to some extent. This has
resulted in various cleaning protocols proposed by membrane
manufacturers, as explained above. Examples of these cleaning protocols can be found in the literature [46,84–87]. Alkaline cleaning
[46] is essential for the removal of organic foulants, or inorganic
colloids coated by organics from the surface of the membrane and
from the pores of the membrane. It was found that ca. 50% of all
foulants are organic in nature [141], therefore, alkaline cleaning
is the first measure to be taken in general. In most cases a high
pH is obtained by using sodium hydroxide and sodium carbonate; this is often combined with a anionic or nonionic surfactant
allowing to emulsify fat containing particles and to prevent foulants
from adhering to the surface. Alkaline cleaning requires a pH often
above the window of chemical resistance of the membrane, which
is possible by limiting the contact time with the cleaning solution.
Nevertheless, repetition of alkaline cleaning may damage the membrane. The outcome may be positive as well: increased fluxes and
unchanged rejections were observed after alkaline cleaning [142].
Acid cleaning [46] follows the same principles, using nitric acid,
citric acid, phosphonic acid or phosphonic acid to obtain a low pH
(1–2) The purpose in this case is the removal of scale, since precipitated salts are more soluble at low pH. Again, this requires going
outside the applicable pH window for a short time, which determines the membrane’s lifetime in the long run. Enzymatic cleaning
can be applied in more mild conditions, but can only be applied
for specific foulants (often polysaccharides as excretion products
of biofoulants). Finally, biocides may be necessary to destroy biofoulants; these products are mainly based on oxidation (chlorine,
ozone) and therefore also attack the membrane to some extent.
This is usually indicated by the manufacturer as a maximal chlorine
tolerance.
A solution where membrane deterioration is completely absent
does not exist. However, a good strategy should minimise the
impact of cleaning by using well-chosen cleaning agents, tailored
for the specific application. This usually requires trial-and-error.
It must be pointed out that in this procedure, the only parameter is usually the cleaning efficiency by the water flux recovery,
the clean water flux recovery or the change in membrane resistance [46]. Considerations about membrane lifetime are not usually
taken into account, although the importance is known and recognised. Membrane autopsy could help in making the optimal choice,
especially when biofouling is the problem [143]. Inorganic foulants
(scale) can be determined by ICP-MS [144]; determination of the
specific nature of organic foulants is difficult, although in many
cases conclusions can be made, especially when some information about the possible foulants is available. This is, for example,
the case for determination of EPS deposits [145], NOM [146], trace
contaminants [147] and even organic deposits in general [148].
Compatibility of polymeric membranes with a wide range of
organic solvents for solvent resistant nanofiltration is a new and
even more challenging issue in discussions about membrane lifetime. Membranes can be made more stable by, e.g., increasing the
degree of crosslinking of the polymeric top layer [149], by using
alternative membrane materials such as poly(organophosphazene)
[150], crosslinked poly(urethanes) [151], filled PDMS [152], and
polyimide [153], or by improving more common materials such
B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
257
Table 1
Solvent resistant nanofiltration membranes and membrane characteristics as specified by the manufacturers
Membrane
N30F
NF-PES-010
MPF-44
MPF-50
Desal-5-DK
Desal-5-DL
SS-030505
SS-169
SS-01
HITK-1T
StarMem-120
StarMem-122
StarMem228
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
Manufacturer
a
Nadir
Nadira
Kochd
Kochd
Osmonicsh
Osmonicsh
SolSepj
SolSepj
SolSepj
HITKo
METp
METp
METp
Material
PES
PES
PDMS
PDMS
PA
PA
–k
–k
–k
TiO2
PI
PI
PI
MWCO (Da)
400
1000
250
700
150–300
150–300
–g
–g
–g
–g
200
220
280
Tmax (◦ C)
95
95
40
40
90
90
90
150
150
–g
60
60
60
L (l/h m2 bar)
b
1.0–1.8
5–10b
1.3b
1.0f
5.4b
9.0b
1.0l
10l
10l
5b
1.0q
1.0q
0.26q
R (%)
70–90c
30–50c
98e
–g
98i
96i
>90m
95m
97n
–g
–g
–g
–g
Nadir Filtration GmbH, Wiesbaden, Germany.
Pure water permeability.
4% lactose (MW 342).
Koch Membrane Systems, Wilmington, MA, USA.
5% sucrose (MW 342).
Methanol permeability.
Not specified.
GE Osmonics, Vista, CA, USA.
MgSO4 .
SolSep BV, Apeldoorn, The Netherlands.
Covered by secrecy and non-analysis agreement.
Ethanol permeability.
MW ∼ 500 in ethanol.
MW ∼ 1000 in acetone.
¨ Technische Keramik, Hermsdorf/Thuringen,
¨
Hermsdorfer Institut fur
Germany.
Membrane Extraction Technology, London, UK.
Toluene permeability.
as poly(acrylonitrile) [154]. An overview of solvent resistant
nanofiltration materials can be found in the literature [32,155];
commercially available membranes are summarised in Table 1
[156].
Solvent resistant nanofiltration is successful even on large scale
in many applications [31]. Nevertheless, its success depends on
a careful analysis of the separation problem, a good membrane
choice and small-scale testing. Recurrent problems are dissolution, deformation or swelling of the membrane. Swelling values
of up to 170% have been reported [157]. It has been shown that
swelling follows very complex mechanisms and may be significantly influenced by pressure, which indicates that compaction
plays a secondary role. The properties of the solvent itself determine
the degree of swelling for a given membrane. For example, it was
observed that for mixtures of xylene and heptane with methanol,
ethanol or propanol, reduced swelling occurred as the concentration of alcohol increased [157]. Other studies describe the changes
in performance as complex solvent–solute–membrane interactions
[158] involving pore solvation (and solute solvation) rather than
swelling. These approaches may explain the dynamic behaviour of
polymeric membranes when applied in organic solvents; however,
it is not clear to what extent swelling shortens the membrane’s
lifetime. Nevertheless, it can be assumed that swelling may lead to
membrane deterioration in the long run.
Changes in the membrane performance or instability of polymeric membranes in organic solvents is not always visible [159].
Even when there was no apparent interaction between membrane
and solvent (damage), membrane properties might have changed.
Pore sizes may have changed, or the hydrophobic (hydrophilic)
character of the membranes may have shifted towards a more
hydrophilic (hydrophobic) one. An early study of nanofiltration
membranes assumed to be stable in at least some organic solvents
[159], three out of four first generation membranes showed visible defects after exposure to one or more organic solvents (Fig. 5),
and the characteristics of all four membranes changed notably after
exposure to the solvents. Therefore, stability of polymeric membranes in organic solvents is a very relative concept: the membrane
might look unchanged, but membrane characteristics could have
changed to a certain extent, ranging from a slight difference to a
total loss of selectivity.
The development of ceramic nanofiltration membranes for
applications in organic solvents may solve the problems of swelling,
changes in performance, and limited lifetime, due to their superior chemical resistance. Ceramic membranes are substantially
more expensive, but this may be compensated by higher fluxes
(especially at high temperatures) and the prolonged lifetime.
However, only few ceramic nanofiltration membranes are commercially available today, in spite of the good performance of
Fig. 5. Effect of exposure to a range of organic solvents (methylene chloride,
n-hexane, ethyl acetate, ethanol, acetone) of a first generation solvent resistant
nanofiltration membrane.
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B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
these membranes. Hydrophilic ceramic nanofiltration membranes
in asymmetric multilayer configurations have been successfully
developed since the late 1990s [160–162]. These consist of an open
porous support, mesoporous interlayers, and defectless microporous top layers made of (hydrophilic) alumina, zirconia or titania.
Attempts have been made to make these ceramic membranes
hybrophobic, so that filtration of non-polar solvents would be feasible. However, this is work in progress and needs to be further
developed.
6. Insufficient rejection for individual compounds
Being regarded as ‘low pressure reverse osmosis’ membranes
in the 1980s, nanofiltration had the advantage of lower energy
consumption due to higher fluxes, resulting from a more ‘loose’
membrane structure with more free volume in the polymer.
Reduced cost for applications where complete rejection of (mainly)
ions was not necessary has paved the way to many applications
of nanofiltration. However, one of the new trends in water treatment is the demand for complete absence of all possible pollutants,
even at ultra-low concentrations. This may be a subjective customer
criterion not necessarily based on risks or toxicity, but it is a reality that has to be recognised. It is to some extent related to new
advances in analytical chemistry, allowing detection of pollutants
at concentrations in the range of ng/l, but the trend can also be
seen for ‘suspicious’ compounds such as nitrate. International standards for nitrate – ranging from 10 mg/l (USEPA) to 50 mg/l (EU)
– are based on possible health effects for infants under 6 months
(methemoglobinemia). Having a stomach pH above 4 (which causes
a partial reduction of nitrate to nitrite), they might suffer from
lack of oxygen in their blood due to reaction of hemoglobine with
nitrite. Nitrate itself does not pose any risk; no clear health effects
can be observed [163]. Nonetheless, nitrates remain on the list
of unwanted species in (drinking) water; removal techniques are
considered in many studies. Since nitrate is a monovalent ion, it
can only partially be removed by nanofiltration [164–168]. Therefore, reverse osmosis membranes are often preferred to ensure a
(nearly) full removal of nitrate. This may be unnecessary, but on
the other hand, synergetic toxicity effects may occur in combination with, e.g., pesticides [170], and some effects may be unknown
[163] so that the precaution principle can be defended. Table 2
shows nitrate rejection values from the literature obtained with
typical nanofiltration membranes. Although these rejections would
Table 2
Experimental nitrate rejections for typical nanofiltration membranes
Membrane
Nitrate rejection (%)
Reference
NF90
HG19
SX10
SV10
SX01
BQ01
MX07
NF70
NF45
UTC-20
UTC-60
MPS44
NF70
Desal
ESNA-1 LF
NF
NF90
OPMN-K
OPMN-P
94–98
9
32
28
25
12
8
76
16
32
11
90 → 50
90 → 85
60 → 33
75–80
65–80
85–95
25–50
40–70
[169]
[168]
[168]
[168]
[168]
[168]
[168]
[167]
[167]
[167]
[167]
[166]
[166]
[166]
[165]
[164]
[164]
[164]
[164]
depend on the experimental concentrations, it can be concluded
that all membranes – with the exception of NF90 – have moderate
nitrate rejections. Nanofiltration for lowering nitrate concentrations to some extent, in combination with another objective such
as softening or NOM removal, is certainly feasible, but if nearly
complete nitrate removal is wanted, nanofiltration is not the appropriate process.
An even more challenging pollutant is boron. Boron is an important micronutrient for plants, animals and humans, although the
range between deficiency and excess is narrow [171]. In aqueous
environments (i.e., neutral pH) boron is mainly present as boric
acid, which is mostly undissociated and therefore only partially
rejected even by reverse osmosis membranes. In acid conditions (at
pH values between 3 and 4.5), boron can be removed by nanofiltration based on charge interactions [172]. This was investigated
for liquid waste streams in coal-fired power plant, which contain
a wide spectrum of trace elements, most of which originate in the
coal and remain in the fly ash or bottom ash when the coal is burned.
Another study, for removal of boron from chemical landfill leachate,
used high pH values (around 11) and also obtained good results,
although the rejection for nanofiltration was relatively low compared to reverse osmosis [173]. Under neutral conditions, boron
can only be removed as a complex. Complexation of boron with
mannitol allows a good removal with nanofiltration membranes
[171]. Nevertheless, for most applications, boron is not a target compound, but it is monitored as a species of interest with a very low
concentration by preference. A comparison of different treatment
trains would yield a disadvantage for nanofiltration, if boron is a
(possible) problem.
For organic micropollutants, rejections with nanofiltration may
range from high to low. Organic micropollutants are a very
broad class of compounds, comprising natural and synthetic hormones; industrial pollutants such as phthalates, alkylphenols,
bisphenol-A, PCBs (polychlorinated biphenyls), PAHs (polyaromatic hydrocarbons), NDMA (N-nitrosodimethylamine) and MTBE
(methyl tertiarybutyl ether); pesticides; pharmaceuticals; personal
care products and disinfection by-products (DBPs) [174]. It is not
always clear which are the most important compounds to look at;
a recent study made an attempt to define ‘priority compounds’ in
view of drinking water production, based on maximum observed
concentrations in surface water, toxicity and production volumes
[175]. A large variation of physico-chemical parameters can be
found among these various types of micropollutants, and because
some of these parameters have a significant influence on rejection,
the removal of micropollutants from aqueous solution can be very
different from component to component. Modelling and prediction
of rejections is still difficult (see also next section), so that extensive experimental research has to be carried out to assess removal
of individual micropollutants. A qualitative appraisal of rejections
was obtained through a classification of compound/membrane
combinations [176]. A further semi-quantitative assessment of the
rejection of organic compounds in aqueous solution was derived in
an attempt to quantify the range of rejections that can reasonably
be expected based on a limited number of parameters [174]. This
classification can be used for practical conclusions. Both studies
[174,176] are based on up to ten classes of compounds. Parameters used for this classification are molecular weight, molecular
weight cut-off of the membrane, pKa (solute charge) and log Kow
(hydrophobicity). It was shown that the lowest rejection should be
expected for uncharged hydrophobic compounds with low molar
mass, which can be explained by the absence of steric hindrance
effects and electrostatic interactions. Examples are 2-naftol, 4phenylphenol, estradiol, ibuprofen, fluoranthene and bisphenol-A,
estradiol, estrone, atrazine, simazine, diuron, and isoproturon. For
these compounds, a rejection decrease as a function of time, due
B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
to adsorption in the membrane matrix, was observed, which may
lead to misinterpretations: observed rejections may be overestimated when the time of measurement was not sufficiently long
[177].
Micropollutants from other classes, however, have also been
identified as problematic compounds. It should be understood that
the classification is not absolute: membrane pore sizes are usually
unknown, and when they are known, they represent an effective
pore size, dependent on the determination method. Furthermore,
molar mass is not a good measure for the size of a solute; it is
a pragmatic parameter because of its availability, but may yield
wrong conclusions. The (largely unknown) interplay between steric
hindrance, charge repulsion and hydrophobic interactions are a further complication. A typical example is NDMA: this is a relatively
hydrophilic solute, but has a low molar mass, which leads to low
rejections in nanofiltration. Research into the removal mechanisms
playing a role in the nanofiltration process can help to improve
the insights into the removal of the organic pollutants, which may
contribute to the development of better barriers, even if other pollutants should arise in the drinking water sources. This requires
a better understanding of interactions between solutes and membranes involved in transport through the membrane, development
of model equations and their translation to a simulation tool that
can be used to provide realistic predictions of concentrations in
permeates.
7. Modelling and simulation of nanofiltration
Modelling the performance of a nanofiltration membrane comprises two aspects: flux prediction and rejection prediction. These
two allow full understanding of a lab-scale membrane module.
Scaling up to larger installations requires that changes along the
filtration module are taken into account, i.e., the influence of the
permeate yield. This can be done by taking concentration increases
into account [178].
Different models have already been proposed for the description of the flux through a (nanofiltration) membrane. For relatively
porous nanofiltration membranes, simple pore flow models based
on convective flow can be used. The Hagen–Poiseuille model
and the Jonsson and Boesen model, which are commonly used
for aqueous systems permeating through porous media, such as
microfiltration and ultrafiltration membranes, take no interaction
parameters into account, and viscosity is the only solvent parameter. Nevertheless, these expressions are usually sufficient for use
in nanofiltration, because they basically express the fundamental
Darcy’s law (flux proportional to pressure gradient) with an empirical proportionality constant, the permeability. The latter parameter
is not expressed as a function of membrane characteristics, as it is
done in other processes, because some parameters (porosity, pore
size) are difficult to measure or even doubtful as concept in nanofiltration.
Two aspects remain to be modelled: the influence of membrane
fouling on flux, and prediction of fluxes for organic solvents. Not
much work has been done on modelling of fouling in nanofiltration.
For surfactants, a correlation has been proposed [179], although it
was recommended not to replace experimental testing by the proposed equation. A more general model, based on characteristics
of individual models, was proposed for aqueous solutions containing well-known organic solutes [51]. In most applications, however,
the feed composition is unknown, so that models are not helpful. In
these circumstances, it may be better to use a pragmatic approach
by using estimates of flux decline to be expected. Practical measures to minimise flux decline and membrane fouling were already
discussed in a previous section.
259
In contrast with aqueous solutions, the Hagen–Poiseuille or
Jonsson and Boesen equation are insufficient to describe the
performance of solvent resistant nanofiltration membranes. A
resistance-in-series model based on convective transport of the solvent for the permeation of pure solvents and solvent mixtures can
be used [180]:
J=
P
[(c − l ) + f1 ] + f2 where f1 and f2 are solvent independent parameters characterising
the nanofiltration and ultrafiltration sublayers, a solvent parameter, c the critical surface tension of the membrane material and l
the surface tension of the solvent. This model takes solvent viscosity
and the difference in surface tension between the solid membrane
material and the liquid solvent into account. However, this model
is developed for hydrophobic membranes, but seems inadequate
for the description of fluxes through hydrophilic membranes [181].
A further disadvantage is that for each solvent–membrane combination an empirical parameter has to be determined as a
measure for the interaction between a solvent and the membrane
material.
Polymeric nanofiltration membranes can also be described by a
solution-diffusion mechanism, possibly corrected for the influence
of convective transport [34]. A description of solvent transport in
this case is necessarily based on the solution-diffusion (SD) model
[182]. With respect to flux modelling of organic solvents, a possible
equation would be [183]:
J∝
V 1 m
n m
This model is based on solvent viscosity, the molar volume
Vm (as a measure for the molecular size), the surface tension of
the solid membrane material and a sorption value (as a measure for membrane–solvent interactions). An alternative equation
[184] is:
J∝
Vm
· where is the difference in surface tension (mN/m), is the
dynamic viscosity (Pa s), and Vm is the solvent molar volume
(m3 /mol).
It is evident that these models for describing fluxes in solvent
resistant nanofiltration have not yet converged; more experience is
needed before a translation to a ‘universal’ model can take place.
Transport of solutes through nanofiltration membranes can be
described by the equations of Spiegler and Kedem, which combine
both diffusive and convective effects:
Js = L (P − )
Jc = Ps x
dc
+ (1 − )Js c
dx
leading to an expression for the rejection R:
R=
(1 − F)
1 − F
1− with F = exp −
Ps
Js
The permeability Ps is a measure of the transport of a molecule
by diffusion. The reflection coefficient of a given component is the
maximal possible rejection for that component (at infinite solvent
flux). Various models have been proposed for the reflection coefficient [185–188]. If a lognormal distribution can be assumed for
the pore size, a molecule may permeate through every pore that is
larger than the diameter of the molecule [188]. The reflection curve
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B. Van der Bruggen et al. / Separation and Purification Technology 63 (2008) 251–263
can then be expressed as:
=
0
rc
1
1
exp
√
Sp 2 r
−
2
(ln(r) − ln(¯r ))
2Sp2
dr
with rc = dc /2. This equation comprises two variables, Sp and r¯ ,
where Sp is the standard deviation of the distribution. This standard deviation is a measure for the distribution of the pore sizes. r¯
is a mean pore size, namely the size of a molecule that is retained
for 50%.
This relatively simple case for uncharged solutes in water
already reflects the difficulties in developing a reliable and generally applicable model for nanofiltration. When rejections in organic
solvents are considered, the problem is even larger, given the
complex interactions between solutes, solvents and membranes,
leading to differences in solvation and therefore also in effective
size. The rejection of organic solutes in water is influenced by
partitioning effects [189,190], and sorption in the membrane was
assumed to be one of the factors that govern the selectivity of membranes towards small organic molecules. Therefore, quantitative
sorption data are crucial for understanding this effect [191]. For
ion rejection in water, many models have been developed [192],
but these calculations tend to become so complex that a simple
translation to a simulation tool is not straightforward. Model-based
tools to design membrane processes for new industrial applications or to optimise existing membrane installations are needed,
and in spite of all efforts in unravelling flux and separation phenomena, these tools are not yet available. An ambitious attempt to
use Maxwell–Stefan equations and all current knowledge to come
to a predictive model led to the conclusion that much depended on
fitting parameters and not on physically relevant parameters [193],
so that further research is needed if a generally applicable simulation tool is envisaged. Membrane manufacturers often develop
their own simulation tool, but this is based on empirical rejection
and flux data and cannot be applied for other than standard configurations, membranes or solutions. For electrolytes in water, an
interesting approach to simulation has been the NANOFLUX tool
[194]. Nevertheless, further extension of this tool or development
of new tools will be the major key to industrial implementation of
nanofiltration.
8. Conclusions
It is clear that nanofiltration still has to grow more in terms
of understanding, materials, and process control. Regardless of
reviewed drawbacks, NF is widely used in industry and special
properties of the NF membranes make possible novel separations that are difficult or expensive to achieve with other
separation methods. Furthermore, the potential of nanofiltration in industrial application is still underdeveloped because
of these drawbacks. In anticipation of new insights and generally applicable solutions, using nanofiltration with the current
knowledge will offer a considerable lead to more conservative
players. Practical problems requiring a pragmatic solution are
membrane fouling and the need for cleaning (and, related to
this, membrane lifetime). A number of options have been proposed; using the expertise of a membranologist, feasible solutions
can be elaborated for many applications. Applications where
an enhanced separation between solutes is required, or a complete removal of contaminants, can be solved by using novel
process configurations or by selecting an adequate membrane.
On the long run, process control can be obtained by translating insights in transport to models, and then translating these
models into simulation tools. A number of attempts in this direction have proven to be successful, and further extension of these
would allow prediction of the process performance in all circumstances.
In summary, it can be stated that further implementation of
nanofiltration will go hand in hand with the elaboration of the
research objectives mentioned. In that sense, it can be expected
that large steps forward will be made during the coming decade.
Acknowledgements
The Fund for Scientific Research Flanders (FWO-Vlaanderen) is
gratefully acknowledged for a travel grant (B. Van der Bruggen).
NanoMemPro is acknowledged for financial support.
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