Review pubs.acs.org/IECR
Integrated Membrane Separation Processes for Recycling of Valuable Wastewater Streams: Nanofiltration, Membrane Distillation, and Membrane Crystallizers Revisited Bart Van der Bruggen* Process Engineering for Sustainable Systems (ProcESS), Department of Chemical Engineering, KU Leuven W. de Croylaan 46 B-3001 Leuven, Belgium ABSTRACT: One of the suggestions made by Enrico Drioli to the membrane community for many years is to work on a radical change from conventional separation and conversion methods to membrane-based separation methods, making use of integrated systems combining a range of membrane separation processes. This paper looks back at these ideas, taking a joint paper written by myself with Efrem Curcio and Enrico Drioli and published in 2004, as a reference. Three processes were central in this paper: the use of nanofiltration for fractionation, membrane distillation, and membrane contactors combined with crystallization as membrane crystallizers. The objective of this paper is to monitor the progress in these three aspects of the proposed overall integrated process. This is done based on the literature and our own expertise. The intention is not to focus on a new and limited subset of results but to evaluate the overall idea of process integration one decade after being proposed. Based on this, it is concluded that the required fractionation in nanofiltration appears to be possible by process engineering rather than membrane engineering. Membrane distillation is today in a clear exponential growth phase and has emerged from research laboratories into larger scale applications, although most still in desalination whereas the potential is larger. Membrane crystallization remains an undiscovered process for many, in spite of its proven technical performance and interest in the scientific literature.
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INTRODUCTION Although process integration is generally accepted to be a more sustainable approach in process design than the traditional approach of unit operations, the principle has not yet often been applied to wastewater treatment. This may be related to the fact that processes not requiring large investments are preferred for wastewater treatment (i.e., biological conversion processes). Wastewater treatment plants worldwide are generally based on biodegradation by oxidation (and to a lesser extent, reduction). Retrofitting such plants with an entirely new approach, not based on oxidation, is rarely possible. Furthermore, combinations with other processes are usually limited to pre- and post-treatment, and handling of the byproducts generated during the treatment.1 Thus, an analysis of integrated technologies for wastewater treatment should also take benefits associated with the environmental effect of avoided byproducts into account.2 This should help to improve the sustainability of new treatment facilities and allow the selection of the most feasible technologies. Studies based on conventional treatment technologies rarely focus on integration, even though a combination of processes is typically necessary for minimization of the overall environmental impact. An evident hybrid concept is the membrane bioreactor (MBR), in which an improvement of the overall efficiency is obtained by integrating the bioconversion with the separation, by using a microfiltration or ultrafiltration membrane. Although this fits within the integration concept, an MBR does not change the concept of biodegradation and sludge production. Nevertheless, the improvement offered by MBR is appreciated, which has made this a mature technology today.3 Biological technologies are still sometimes thought to be a cheaper and more environmental friendly method, but it is © XXXX American Chemical Society
understood that, for treatment of textile effluents, an additional method to remove recalcitrant compounds is needed. For example, Lotito et al.4 developed a sequencing batch biofilter granular reactor with integrated ozone oxidation for a synergetic biological and chemical oxidation activity. This can be also integrated in an MBR, particularly when difficult wastewater such as pharmaceutical wastewater is to be treated. Laera et al.5 operated an MBR with oxidation by either ozonation or UV/H2O2 in the recirculation stream and found higher removals for 34 out of 55 degradation products considered in the study. Thus, even for biological systems, process integration can be of importance. More advanced physicochemical processes can be of interest for process integration. One example is adsorption, which may complement photocatalysis. The integration of adsorption and photocatalytic degradation system as a hybrid treatment process results in a synergetic enhancement for the chemical removal efficiency, as shown by Vimonses et al.6 for a fluidized bed reactor with formulated clay mixture absorbents and a slurry photoreactor using TiO2 impregnated kaolin catalysts. Membrane processes are thought to be ideal candidates for process integration; several studies have been reported on combinations of membrane processes for wastewater treatment. These are not always applied as integrated processes, but rather as a sequence of complementary processes. Vergili et al.7 Special Issue: Enrico Drioli Festschrift Received: October 21, 2012 Revised: January 4, 2013 Accepted: January 4, 2013
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Figure 1. Reference flowsheet for wastewater treatment in the textile industry. Full line, water flow; dashed line, energy flow; MD, membrane distillation; NF, nanofiltration; MCr, membrane crystallizer. Reprinted with permission from Van der Bruggen et al.13 Copyright 2004, Elsevier.
which are not optimized for the application in FO (membranes are often the same as in RO). Progress on this so far is limited to the identification of the problem; solutions essentially depend on the development of optimized membrane structures for FO. Interesting is that the principle of FO is a good basis for developing new integrated processes, and it can even be argued that this will be the only realistic application of FO. Lutchmiah et al.12 extracted water from sewage by means of forward osmosis (FO), in combination with a reconcentration system (i.e., reverse osmosis (RO)). The purpose of this integration is not only in water production but also in the generation of renewable energy from concentrated sewage. Wastewater is then not considered as waste but as a resource of water, of energy, or of chemicals (after postprocessing of the purified fractions). This paper considers a selection of novel processes for wastewater treatment, taking ideas on process integration suggested by Enrico Drioli and published jointly by myself, Curcio, and Drioli in 2004 as a reference.13 The most prominent processes proposed in this paper, nanofiltration fractionation, membrane distillation, and membrane contactors and crystallizers, are reconsidered in the underlying paper, on the basis of research progress during the past decade. It is not the intention to review these technologies in detail, nor to provide scientific evidence for specific aspects of the processes considered, but to provide some new inspiration by looking back at those suggestions made in 2004 and more relevant today than ever before. This may further trigger the overall concept of using wastewater as a resource, which will require major further efforts in research in the years to come.
studied various combinations of ultrafiltration (UF), loose nanofiltration, tight nanofiltration, and reverse osmosis, each followed by membrane distillation (MD). The eventual sludge was produced after MD was incinerated, and it was shown that the limitation to the feasibility of the concept was in fact related to the incineration cost and not to the technological performances. Other combinations, not necessarily based on membranes, are possible and have been proposed. Zhao et al.,8 for example, used a combination of powdered activated carbon adsorption and microfiltration for treatment of reverse osmosis brines; the main advantage was in the full usage of the adsorption capacity of the carbon. During the past decade, new processes emerged that do not rely on oxidation but on new separation principles. The fundamental challenge in this approach is that it intrinsically depends on postprocessing of separated streams and eventual reuse of the purified products. This approach almost naturally leads to an integrated design for the separation, yielding a range of product streams for further processing. Probably the most obvious example is forward osmosis, a new technology that suffers from inherent problems when used as a stand-alone process (i.e., the regeneration of the draw solution) but with entirely new possibilities when operated as a hybrid treatment system. Ge et al.9 studied the concept of a polyelectrolytepromoted forward osmosis−membrane distillation (FO-MD) hybrid system and applied it for treatment of wastewater containing acid dyes. They used a poly(acrylic acid) sodium salt as the draw solute of the FO to dehydrate the wastewater, while the MD was employed to reconcentrate the draw solution. This allowed a continuous operation of the wastewater treatment process, due to the integration. Similar work on forward osmosis focused on hybridization of wastewater treatment and seawater desalination. Hancock et al.10 considered a process consisting of FO operated in osmotic dilution mode and seawater reverse osmosis (SWRO) and demonstrated its feasibility, although further progress is needed in module design and cleaning. These issues are also mentioned by Zhao et al.,11 in addition to reverse solute diffusion. A specific challenge in FO is related to concentration polarization and fouling; this is intrinsic to the morphology of the membranes,
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PROCESS INTEGRATION FOR WASTEWATER TREATMENT: REFERENCE FLOWSHEET A reference flowsheet was elaborated for wastewater treatment in 2004, on the basis of a case study on textile processing effluents.13 The proposed membrane-based integrated water treatment system is reproduced in Figure 1. The case of textile wastewater is of particular interest, since it has a heavy load of organic as well as inorganic pollutants. Organic compounds are added to finishing baths for dyeing of B
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al. for the possible recycling of rinsing wastewater from indigo dyeing to the process itself, using dead-end microfiltration with a 5 μm filter to remove coarse particles and to minimize fouling of further NF and RO membranes.16 Compared to the reference flowsheet, this corresponds to the two first processes. This can be considered today’s standard in advanced textile wastewater treatment. Vergili et al.17 advanced closer to the reference flowsheet, by adopting the zero liquid discharge (ZLD) approach in treating textile dye bath wastewater via integrated membrane processes. This process integration included various combinations of ultrafiltration, loose nanofiltration, tight nanofiltration, and reverse osmosis. It was considered both technically feasible and economically viable. Many other treatment processes keep relying on biological processes, which implies that sludge generation is inherent to the approach, and a fully closed cycle cannot be obtained. Resource recovery in this context could comprise the use of wastewater as an energy source.18 However, omission of the biological treatment requires the development and use of new separation or transformation processes. The requirements for such processes are (a) to offer a separation more selective than can be achieved by conventional processes, (b) not to generate waste byproducts, and (c) to allow full integration of all process streams including recycle streams. The three key processes for integrated physicochemical wastewater treatment, nanofiltration, membrane distillation, and membrane crystallization, will be reconsidered below in terms of their use in an integrated, (nearly) zero discharge approach. Fractionation by Nanofiltration. Nanofiltration was traditionally used only as a process for softening and removal of organics and micropollutants in potable water production. The application of nanofiltration today has been extended to other areas as well, including not only fractionation as suggested in the scheme, but also in solvent filtration, water recycling, and treatment of process streams in a wide range of industries.19 Application in the textile industry as a new wastewater treatment technology was studied for the first time in 1996. In the eight years preceding 2004, 40 papers were published on nanofiltration of textile effluents, or 5 per year on average. In the eight years from 2005 to 2012, 161 papers were published on the same topic, or 20 per year on average. Initially, nanofiltration was considered as an alternative method for advanced wastewater treatment; water recycling soon became an important objective. Nevertheless, nanofiltration is today still considered a purification process rather than a fractionation process when wastewater applications are considered. The fractionation idea suggested in the reference flowsheet refers to a selective removal of a selection of compounds (organic− inorganic, but this can be extended to a narrow range of organic compounds) while keeping the remaining compounds unchanged. This can be achieved by cascading membranes and recycling the product streams. Van der Bruggen and Braeken20 demonstrated that the rejection curve, reflecting the rejection of uncharged compounds as a function of molar mass, is significantly influenced by a cascade design, as shown in Figure 2. The rejection curve using a single-step nanofiltration, and a two and three step cascade is shown in Figure 3 for the nanofiltration membrane Desal-5-DL.20 The sigmoidal curve for a single passage is reshaped to a more step-function-like curve for a two and three step cascade, corresponding to a better defined separation between smaller compounds that are not rejected (at all) and somewhat larger compounds that are
textile or for other purposes, such as improving moth resistance; furthermore, the textile itself introduces organic compounds to the baths as well. Salts, on the other hand, are essential to regulate the attachment of dyes or other compounds to the textile; concentrations of salts can be often high and easily reach 10 g/L. Finishing baths are often applied at elevated temperature (80−90°), which may also allow for (waste) energy recovery. Figure 1 is based on a systematic fractionation of all constituents of the wastewater. This was (and is) a revolutionary concept, going far beyond the current objectives of water recycling. Of course one should be aware of the current constraints, balancing economic added value to cost. These, however, are rather volatile and may change significantly over a decade; phosphates, for example, have increased their price by a factor 4 since 2004 with peaks of an increase by a factor 8. Phosphates, among others, are increasingly scarce and therefore strategic compounds, with a production centralized in a very small number of countries (Morocco and the Western Sahara contain an estimated 70% of the remaining world phosphate reserves; the remaining production is in China, the Middle East and the U.S.A.; igneous reserves are found in Brazil, Canada, Russia, and South Africa). Geopolitical motivations may then surpass economic fluctuations. In brief, the wastewater is pretreated in a dead-end microfiltration (MF) unit for removal of suspended solids; the organic fraction is removed by a loose nanofiltration membrane with low salt rejection at a high temperature (NFA). The permeate fraction contains a large fraction of salts with the remainder of organics not retained by NF-A. The retained fraction is a warm, aqueous solution containing a high concentration of organics. In the MD unit, water is further removed by using the temperature difference as driving force. The pure distillate is recycled as fresh water; the thickened organic fraction is energetically valorized. The NF-A permeate is fed to a second nanofiltration unit (NF-B), using a tight NF membrane or an RO membrane, removing the salt fraction from the water. Remaining organic compounds, not completely removed in NF-A have a second barrier to prevent them from intruding the NF-B permeate, which is recycled as process water. The NF-B retentate is a concentrated salt solution, comparable to desalination brine. The salts are recovered in a membrane crystallizer as highpurity crystals (membrane crystallization has been shown to yield crystals with superior properties14), and reused directly for new finishing baths. For a more detailed, quantitative description of the flowsheet, the reader is referred to Van der Bruggen et al.,13 where the background and context are also extensively described.
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CURRENT AND FUTURE TRENDS IN PROCESS INTEGRATION FOR WASTEWATER TREATMENT It is interesting to re-evaluate the membrane processes taking part in this scheme and how research interests during the past decade have evolved. With the exception of microfiltration, the application of all processes involved was new, unusual, and challenging for wastewater treatment. The concept of process integration is well-known but, apart from some exceptions, not yet applied in wastewater treatment. Tahri et al.15 studied different combinations of selected dyeing cycle baths by using the combination of microfiltration with nanofiltration in order to reuse the treated water in the dyeing process. Similarly, a membrane based treatment strategy was developed by Uzal et C
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continuous solvent exchange was demonstrated by Lin and Livingston.26 However, the application of cascades in wastewater treatment still remains a challenge; furthermore, inorganic−organic fractionation using membrane cascades remains undescribed (for the removal of organic compounds without changing the ion concentrations, electrodialysis is an alternative; for the application considered here, it is rather the organic fraction that should be removed). Other remaining challenges for the implementation of cascades include the development of membranes with uniform pore size, membranes with high rejection for organic solutes but no rejection for inorganic ions, and positively charged nanofiltration membranes. Membrane Distillation. As early as in 1986, a workshop on membrane distillation was organized by Enrico Drioli in Rome, Italy. Membrane distillation developed slowly in the 1990s but remained a niche application for a long time. Today, the interest in membrane distillation has grown significantly, and although some drawbacks still remain, large-scale applications for seawater desalination and brine recovery are considered.27 In the scientific literature, the average number of publications on membrane distillation in the 1990s was 14.6. This decreased slightly to 13.6 during the five years before the reference year 2004. The next period of five years saw an increase by a factor 3 to an average of 42.4, and this was again doubled by 2011 to 85. Thus, it can be stated that there has been a sharp increase in the attention to membrane distillation recently. This is partly due to the general trend of increased numbers of publications but also because of some limitations that have been overcome. Sirkar and Li28 managed to solve issues related to long-term pore wetting and reduced brine-side heat and mass transfer coefficients. Membrane synthesis has made progress, although operational problems remain. However, the largest boost for membrane distillation was the demonstration of the technology on larger scale, mainly by the Memstill process, establishing a 2 m3/d pilot plant at the Senoko Refuse Incineration Plant from February 2006 to June 2007 (by TNO, The Netherlands, and Keppel Seghers, Singapore). This was later to be scaled up to 600 m2. Other demonstration plants included a plant in Aqaba, Jordan, established by the Fraunhofer Institute for Solar Energy Systems, using 4 × 10 m2 of membrane surface area,29 and a plant for direct contact membrane distillation with membrane surface areas between 1.30 m2 and 6.6 m2.30 The reader is referred to the literature for a more extensive discussion on membrane distillation.31,32 Again, progress has been made in application fields other than wastewater treatment. This is related to the added value of the product on one side, because wastewater is still perceived as a no-value product in contrast with seawater (although people’s minds are changing); on the other hand, it has to be recognized that there are specific challenges for membrane distillation when wastewater is applied, such as the increased risk for fouling effects due to the organic fraction in the wastewater. More general aspects that would further stimulate the breakthrough of membrane distillation in a wide range of applications, including wastewater treatment, are as follows. (a) Enhanced heat integration/heat transfer: Operation of membrane distillation at low temperature gradients would allow to increase the efficiency of the process and therefore, the process economics.
Figure 2. Principle of a membrane cascade (based on the principle of a distillation column21).
Figure 3. Effect of cascading nanofiltration membranes on the rejection as a function of molar mass, using one, two, and three membranes with recycle. Reprinted with permission from ref 20. Copyright 2007, American Chemical Society.
nearly completely retained. The transition between both is very narrow. Lightfoot et al.22 further elaborated the concept of integrated cascades for liquid separations and developed an algorithm for describing ideal membrane cascades for fractionation of binary and pseudobinary mixtures. They conclude that the development of efficient diafilters is needed if membrane cascades are to achieve their full potential in competing with both chromatography and simulated moving bed operations. Vanneste et al.23 made a techno-economical comparison of such cascades with simulated moving beds and concluded that membrane cascades seem the most promising for large scale continuous processes for producing pure products. These observations pushed more studies to make membrane process designs based on an integrated countercurrent membrane cascade.24 Intrinsic disadvantages are related to the increased complexity of such designs, which have been found difficult to operate on a small scale, and the energy requirements. The mass yield should be high for an integrated cascade, but with more simple diafiltration designs, the mass yield is low. Today, membrane cascades mainly attract the interest of applications with high added value. For example, large-scale protein purifications can be carried out economically by using cascade ultrafiltration systems run at constant flux for efficient purification of lysozyme from chicken egg white.25 A second example is in solvent filtration; the feasibility of membrane cascades using organic solvent nanofiltration membranes for D
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(b) Membrane materials with increased water flux and low sensitivity to fouling: several improved membrane materials with remarkable features have been proposed in the literature, using, for example, carbon nanotubes33 or electrospun fibres.34 Both methods rely on increased hydrophobicity combined with accelerated transport; this is shown in Figure 4 for a membrane filled with carbon nanotubes. Such membranes should be made available commercially on a sufficiently large scale and at realistic cost.
only a modest 50. Membrane crystallization is even less known and can therefore be considered a process for membrane connoisseurs: less than 10 papers per year are published on this subject so far. However, they attract interest and are well cited; interesting to note is that Enrico Drioli is author of 7 of the 10 most cited papers on membrane crystallization, enough to consider him the father of this process. It can be assumed that membrane crystallization is still hindered by the low economical value of crystals when wastewater treatment is considered. However, this is an evaluation based on a single aspect of an overall new approach, not taking benefits related to costs for discharge, environmental effects, and sustainability into account. Furthermore, the economic value of products is volatile and even when low value products such as salts are considered, remarkable market mechanisms can be observed. Nonetheless, applications of membrane crystallization might be easier in economically more attractive applications for, for example, proteins purification or in the production of pharmaceutical compounds, which will allow the process to become more mature. This would open the door toward other applications as well. One of these could be in CO2 capture for the production of carbonate-containing minerals. A significant benefit of using membrane crystallization is the high purity of the crystals that can be obtained (an example is shown in Figure 5), and the ease of crystallization at lower levels of
Figure 4. Transport of water vapor by membrane distillation through a membrane filled with carbon nanotubes (Reprinted with permission from ref 33. Copyright 2012, Elsevier.
(c) Composite membrane structures designed especially for membrane distillation: this includes the dual layer approach in which hydrophobic and hydrophilic layers are combined35,36 and the use of surface modifying macromolecules.37 Such membranes are focused on a stable operation of membrane distillation, particularly in large-scale plants.
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MEMBRANE CRYSTALLIZERS In parallel with membrane distillation, Drioli’s group actively promoted the more general idea of membrane contactors since the 1990s.38 Step by step academics and industrialists are becoming aware of the potential of membrane contactors, and research and investments in this area increased considerably. The growing interest is much related to the application of membrane contactors for CO2 capture and recovery, often by using a system based on ionic liquids.39 After absorption of CO2, a solution containing salts is supersaturated by removing water; the membrane itself induces heterogeneous crystallization, yielding high-quality crystals. These can be simple salts, but also, for example, proteins, which opens a wide range of potential applications. Membrane crystallizations allows operating at lower supersaturation levels, because the crystallization is heterogeneous due to the presence of membrane pores promoting the crystallization process. Publications on membrane contactors did not appear before the mid-1990s. Before the turn of the century, only 23 papers in this field were published. Remarkably, a review paper by Gabelman and Hwang (“Hollow fiber membrane contactors”)40 was the most cited paper in Journal of Membrane Science in 1999, today approaching 500 citations. Since 2004, the number of publications has increased, but the average number of publications per year between 2004 and 2012 is still
Figure 5. Na2CO3 crystals obtained in a membrane crystallizer (magnification 5×).
supersaturation. The technological potential of this process is obvious, the waiting is for the market of suppliers and end users.
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CONCLUSIONS The analysis of ongoing research clearly demonstrates the emerging interest in new and integrated processes for advanced wastewater treatment and related applications. The potential of engineered, integrated membrane cascades for fractionation of organic solutes in aqueous and nonaqueous solution has been recognized during the past decade as an alternative to the development of membrane materials with uniform pore size or membranes specifically designed for selective transport of a given compound. This is particularly the case for nanofiltration and ultrafiltration: it has been shown that engineering rather than membrane synthesis is the most promising pathway E
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(3) Judd, S. The MBR Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment; Elsevier: Oxford, U.K., 2006. (4) Lotito, A. M.; Fratino, U.; Bergna, G.; Di Iaconi, C. Integrated biological and ozone treatment of printing textile wastewater. Chem. Eng. J. 2012, 195, 261. (5) Laera, G.; Cassano, D.; Lopez, A.; Pinto, A.; Pollice, A.; Ricco, G.; Mascolo, G. Removal of organics and degradation products from industrial wastewater by a membrane bioreactor integrated with ozone or UV/H2O2 treatment. Environ. Sci. Technol. 2012, 46 (2), 1010. (6) Vimonses, V.; Jin, B.; Chow, C. W. K.; Saint, C. An adsorption− photocatalysis hybrid process using multi-functional-nanoporous materials for wastewater reclamation. Water Res. 2010, 44 (18), 5385. (7) Vergili, I.; Kaya, Y.; Sen, U.; Gonder, Z. B.; Aydiner, C. Technoeconomic analysis of textile dye bath wastewater treatment by integrated membrane processes under the zero liquid discharge approach. Resourc. Conserv. Recycl. 2012, 58, 25. (8) Zhao, C. X.; Gu, P.; Cui, H. Y.; Zhang, G. H. Reverse osmosis concentrate treatment via a PAC-MF accumulative countercurrent adsorption process. Water Res. 2012, 46 (1), 218. (9) Ge, Q. C.; Wang, P.; Wan, C. F.; Chung, T. S. Polyelectrolytepromoted forward osmosis-membrane distillation (FO-MD) hybrid process for dye wastewater treatment. Environ. Sci. Technol. 2012, 46 (11), 6236. (10) Hancock, N. T.; Black, N. D.; Cath, T. Y. A comparative life cycle assessment of hybrid osmotic dilution desalination and established seawater desalination and wastewater reclamation processes. Water Res. 2012, 46 (4), 1145. (11) Zhao, S. F.; Zou, L.; Tang, C. Y. Y.; Mulcahy, D. Recent developments in forward osmosis: Opportunities and challenges. J. Membr. Sci. 2012, 396, 1. (12) Lutchmiah, K.; Cornelissen, E. R.; Harmsen, D. J. H.; Post, J. W.; Lampi, K.; Ramaekers, H.; Rietveld, L. C.; Roest, K. Water recovery from sewage using forward osmosis. Water Sci. Technol. 2011, 64 (7), 1443. (13) Van der Bruggen, B.; Curcio, E.; Drioli, E. Process intensification in the textile industry: The role of membrane technology. J. Environ. Manage. 2004, 73 (3), 267. (14) Ji, X.; Curcio, E.; Al Obaidani, S.; Di Profio, G.; Fontananova, E.; Drioli, E. Membrane distillation-crystallization of seawater reverse osmosis brines. Sep. Purif. Technol. 2010, 71 (1), 76. (15) Tahri, N.; Masmoudi, G.; Ellouze, E.; Jrad, A.; Drogui, P.; Ben Amar, R. Coupling microfiltration and nanofiltration processes for the treatment at source of dyeing-containing effluent. J. Cleaner Prod. 2012, 33, 226. (16) Uzal, N.; Yilmaz, L.; Yetis, U. Nanofiltration and reverse osmosis for reuse of indigo dye rinsing waters. Sep. Sci. Technol. 2010, 45 (3), 331. (17) Vergili, I.; Kaya, Y.; Sen, U.; Gonder, Z. B.; Aydiner, C. Technoeconomic analysis of textile dye bath wastewater treatment by integrated membrane processes under the zero liquid discharge approach. Resour., Conserv. Recycl. 2012, 58, 25. (18) Heubeck, S.; De Vos, R. M.; Craggs, R. Potential contribution of the wastewater sector to energy supply. Water Sci. Technol. 2011, 63 (8), 1765. (19) Van der Bruggen, B.; Mänttäri, M.; Nyström, M. Drawbacks of applying nanofiltration and how to avoid them: A review. Sep. Purif. Technol. 2008, 63, 251. (20) Van der Bruggen, B.; Braeken, L. Comparison of methods to enhance separation characteristics in nanofiltration. Ind. Eng. Chem. Res. 2007, 46, 2236. (21) Seader, J. D.; Henley, E. J. Separation Process Principles, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2006. (22) Lightfoot, E. N.; Root, T. W.; O’Dell, J. L. Emergence of ideal membrane cascades for downstream processing. Biotechnol. Prog. 2008, 24 (3), 599. (23) Vanneste, J.; De Ron, S.; Vandecruys, S.; Soare, S. A.; Darvishmanesh, S.; Van der Bruggen, B. Techno-economic evaluation of membrane cascades relative to simulated moving bed chromatog-
toward enhanced fractionation. Nevertheless, practical implementation of integrated membrane cascades has to be further elaborated. Lab-scale testing has demonstrated difficulties with, for example, flow coupling; furthermore, the separation of organic and inorganic fractions as intended in the flowsheet for textile wastewater treatment, remains a challenge due to the typical high rejections of ions for membranes with low cutoff values for organics. Membrane distillation, on the other hand, was suggested as a candidate process for the integrated approach with the benefit of exploiting the waste heat present in aqueous flows. After two decades in anonymity, the process has been taken to a higher level during the past decade by considering the use of membrane distillation for desalination purposes; this can be based on seawater but may be of even more interest when industrial brines are considered. These brines often contain waste heat, or may be combined with available waste heat, so that the production of fresh water can be achieved as a bonus, with membrane distillation as a sustainable part of an overall industrial activity. For desalination purposes, this is feasible today; for organic brines resulting from wastewater fractionation, more challenges have to be met: organic fouling of the hydrophobic membrane and the selectivity of the separation particularly for the lower molar mass fraction of the organic compounds. The technology of other types of membrane contactors was shown feasible, but these are not yet widely applied nor understood. This may be related to the limited availability of specialty membranes on the market; and the fact that potential benefits are not yet fully understood by the scientific community. Membrane crystallizers, which are in fact a combined contactor/heterogeneous crystallization process, are among the most remarkable new technologies: they allow the production of high-purity crystals, which can be proteins but also common salts present in wastewater. Such new processes as described here may still suffer from cost issues today when applied for wastewater treatment, but it has been shown that this is a matter of various factors, including process development and scarcity of products that can be obtained from wastewater (such as phosphates). This makes the development of new technologies to a strategical choice with geopolitical dimensions.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +32 16 322340. Fax: +32 16 322991. E-mail: bart.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Wenyuan Ye is acknowledged for experimental work on membrane crystallizers leading to Figure 5.
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REFERENCES
(1) Rosso, D.; Stenstrom, M. K. The carbon-sequestration potential of municipal wastewater treatment. Chemosphere 2008, 70 (8), 1468. (2) Molinos-Senante, M.; Garrido-Baserba, M.; Reif, R.; HernandezSancho, F.; Poch, M. Assessment of wastewater treatment plant design for small communities: Environmental and economic aspects. Sci. Total Environ. 2012, 427, 11. F
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