Separation and Puriﬁcation Technology 63 (2008) 251–263 Contents lists available at ScienceDirect 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.2008.05.010 251 252 253 255 256 258 259 260 260 260 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” . It was introduced in the late 1980s, mainly aiming at combined softening and organics removal . 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 252 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  and the growing concerns in the USA on the presence of emerging contaminants  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 , 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 . 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 . An extensive description of the consequences of fouling in nanoﬁltration can be found in the literature , 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 . 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 . 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  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 . 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 . 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 . 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 . 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 . 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 . Chemical cleaning involves chemical reactions such as hydrolysis, saponiﬁcation, solubilisation, dispersion, chelation, and peptisation . 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 . Membrane modiﬁcation is potentially the most sustainable solution to obtain fouling-resistant membranes . 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 . 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 ; it is accepted that membranes with a rough surface are more prone to fouling than membranes with a smoother surface . Biological fouling can be reduced by the addition of, e.g., silver nanoparticles in the membrane structure . An intrinsic solution to the problem of membrane fouling could be the concept of critical or sustainable ﬂux . 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) . 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 , 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 . 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% . 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 . In the latter case, the solution pH is often the key to control the desired separation. 254 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) , 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 . 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 , 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 . For liquid separations there is no knowledge on cascades, apart from some exploratory studies concerning module conﬁgurations in reverse osmosis , 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 ). step approach was recently suggested for puriﬁcation of solvents . 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 . Nanoﬁltration for food applications is the second largest after ultraﬁltration . Dairy applications were among the very ﬁrst where nanoﬁltration was used . 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 . 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 ; 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) . 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) . 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 : 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  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’ . 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 . 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 . A combination of membrane processes was suggested for design of new productive cycles , 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 : removal of the organic fraction (dyes, additives), and removal of 256 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 . 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 . 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  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 . 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  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 , 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 . Acid cleaning  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 . 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 . Inorganic foulants (scale) can be determined by ICP-MS ; 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 , NOM , trace contaminants  and even organic deposits in general . 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 , by using alternative membrane materials such as poly(organophosphazene) , crosslinked poly(urethanes) , ﬁlled PDMS , and polyimide , 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) . An overview of solvent resistant nanoﬁltration materials can be found in the literature [32,155]; commercially available membranes are summarised in Table 1 . Solvent resistant nanoﬁltration is successful even on large scale in many applications . 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 . 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 . Other studies describe the changes in performance as complex solvent–solute–membrane interactions  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 . 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 , 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. 258 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 . 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 , and some effects may be unknown  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                    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 . 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 . 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 . Under neutral conditions, boron can only be removed as a complex. Complexation of boron with mannitol allows a good removal with nanoﬁltration membranes . 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) . 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 . 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 . 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 . 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 . 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 . 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 , 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 . 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 : 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 . 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 . A description of solvent transport in this case is necessarily based on the solution-diffusion (SD) model . With respect to ﬂux modelling of organic solvents, a possible equation would be : 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  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 . The reﬂection curve 260 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 . For ion rejection in water, many models have been developed , 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 , 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 . 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. 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