Article pubs.acs.org/IECR
Membrane Engineering for Water Engineering Enrico Drioli†,‡,§ and Francesca Macedonio*,†,‡ †
Institute on Membrane Technology, National Research Council of Italy, ITM-CNR c/o University of Calabria, via P. Bucci cubo 17/C, Rende (CS), Italy ‡ Department of Chemical Engineering and Materials, University of Calabria, via P. Bucci, Rende (CS), Italy § WCU Energy Engineering Department, Hanyang University, Seoul 133-791, S. Korea ABSTRACT: Today membrane engineering is giving interesting solutions to some of the major problems of our modern industrialized society. Membrane bioreactors (MBR) in municipal water treatments, membrane operations and integrated membrane systems in seawater and brackish water desalination are some important cases where membrane engineering is playing a dominant role. This manuscript discusses how membrane engineering contributes to the solution of this water problem. It gives also a vision on the current and future developments of membrane operations in the field of water treatment.
1. INTRODUCTION Currently modern membrane engineering represents one of the possible ways for implementing a process intensif ication strategy, i.e. for developing processes and methods aimed at decreasing raw materials utilization, energy consumption, production costs, equipment size, and waste generation.1 Membrane engineering is already recognized worldwide as a powerful tool for solving some of the major problems of our industrialized and highly populated society. Membrane bioreactors (MBR) in municipal water treatments, membrane operations and integrated membrane systems in seawater and brackish water desalination are some important cases where membrane engineering is playing a dominant role for solving the situation of freshwater demand at low costs and minimum environmental impact. In many regions of the world, conventional thermal desalination plants have been changed to use membrane processes because they are 10-fold more energetically efficient then thermal options; conventional activated sludge plants have been turned into membrane bioreactors due to their compactness (up to 5 times more compact than conventional plants), reduced sludge production, and considerable level of physical disinfection. Pervaporation is another well-developed membrane technology, having great potential for the intensification of various industrial processes, such as breaking azeotropes2 and removing volatile organic compounds (VOCs) from liquids. Moreover, pervaporation and vapor permeation coupled with a conventional distillation column offers several advantages including reduction of energy consumption, improvement of product quality, and avoidance of entrainers, making this technology especially suitable for close boiling or azeotropic mixtures.3−5 Most of its industrial applications focus on the dehydration of solvents (e.g., for acetonitrile dehydration in which the use of a hybrid membrane distillation process leads total cost reductions of up to 60%6). Natural gas dehumidification and separation of air components are some other examples in which membrane technology is already applied at the industrial scale. On the contrary, the application of membrane contactor technology for the selective recovery of evaporated wastewater © 2012 American Chemical Society
from industrial gases in order to recycle process streams thus minimizing fresh water requirements, is still at research level.7 The above-reported examples show membrane engineering has a much wider spectrum of potential applications as unit operations in process engineering than any other available technology. Membrane operations can be used to conduct molecular separations, chemical transformations, and mass and energy transfer between different phases, showing a higher efficiency than conventional separation and reaction unit operations. There are also some interesting opportunities to integrate membrane operations into existing industrial processes to achieve the benefits of process intensification. In the following sections, an overview on the current developments of membrane operations in the field of water treatment and their place in the intensification of water recovery and exploitation is presented.
2. MEMBRANE ENGINEERING FOR WATER PROBLEM Water is vital for all known forms of life and for all types of industrial development. Although it covers 70.9% of the Earth’s surface, only 3% of the Earth’s water is freshwater and around 99% of this is locked in polar ice and groundwater. Therefore, less than 1.0% of all freshwater (≈0.01% of global waters) is available for people and ecosystems.8 Despite the fact that access to safe drinking water has improved over the last decades in almost every part of the world, in 2008 over 2.6 billion people were yet living without access to improved sanitation facilities, 3.3 million people die from water-related health problems each year, and 46% of people on earth do not have water piped to their homes.9 The forecasts are that by 2025 two-third of people will live in regions with water scarcity. This means that people will not have sufficient water resources to maintain their current level of Special Issue: APCChE 2012 Received: Revised: Accepted: Published: 10051
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process is not economical for waters with high salts concentrations14,15 but is competitive for brackish waters with up to 3000 ppm salt. Integrating the properties of membranes with that of a biological catalyst is the basis of a piece of separation process equipment called a membrane bioreactor (MBR). It is being used in increasing applications for industrial and municipal water treatment. Conventional MBR uses low-pressure membrane filtration, either MF or UF, to retain the mixed liquor of the bioreactor and delivers particle-free treated effluent.16 Because the membrane is an absolute barrier for bacteria and in the case of UF also for viruses, the MBR process provides a considerable level of physical disinfection. The resulting high quality and disinfected effluent implies that MBR processes can be especially suitable for reuse and recycling of wastewater. More recently, membrane bioreactors with membrane distillation (MD) membranes in place of MF/UF have been developed for the treatment of industrial and municipal used waters.17 The development of MDBR system was due to the fact that, in a conventional MBR, the molecular weight cutoff of the utilized MF/UF membranes delivers a portion of the organic species of the feed. For overcoming this, the development of a number of innovative high retention membrane bioreactors (HRMBRs) have been developed such as membrane distillation MBR (MDBR), nanofiltration MBR (NFMBR),18,19 and the osmotic MBR (OMBR).20,21 The HRMBR systems are able, in principle, to retain effectively small size and persistent contaminants, which facilitates their biodegradation in the bioreactor, thereby producing higher quality product water.16 Water treatment processes were revolutionized by the introduction of membrane operations. In particular seawater desalination was transformed by RO technology. However, the main limitation of membrane systems has to be also considered. The latter is membrane fouling, i.e. components present in the feedwater deposited and/or absorbed on the membrane surface. While well-known technical solutions are usually available for the design and the manufacture of a membrane based unit, the pretreatment of the water to be processed represents a crucial aspect of each water treatment process, that determining the success or the failure of a plant, depending on the raw water characteristics and dosage of chemical agents for the cleaning of membranes. 2.1. Forward Osmosis, Pressure-Retarded Osmosis, Reverse Electrodialysis. Recent Developments. Forward osmosis (FO), pressure-retarded osmosis (PRO), and reverse electrodialysis (RED) are some membrane processes which are receiving increasing attention in the recent years. FO is a membrane process which uses the osmotic pressure difference across the membrane to induce a flow of water through the membrane, from a feed into a highly concentrated draw solution. The challenge of the process is to identify a concentrated draw solution that could be removed efficiently from clean water and, then, reused. FO can be employed in many fields of science and engineering including water and wastewater treatment, seawater/brackish water desalination, food processing, drug delivery, and electric power production. The main FO advantages are the low energy input and the low membrane-fouling propensities with respect to evaporation and pressure-driven membrane processes. The advancing of the
per capita food production from irrigated agricultureeven at high levels of irrigation efficiencyand also to meet reasonable water needs for domestic, industrial, and environmental purposes. Considering water shortage as a global matter, the option of water production/purification/reuse processes is the only possible solution for future generations. The purification and reuse of wastewaters is crucial for rising the exploitation of potable water and reducing its consumption. The treatment and recycling of process waters are indispensable for preventing further contamination of water resources. The production of potable water from saline and polluted waters is essential for increasing the amount of available good quality water. Today membrane engineering plays a dominant role in water desalination and in municipal water treatment. In fact, 60.0% of the total worldwide reclaimed water (around 65.2 million m3/ d) is produced through reverse osmosis (RO) technology. Success of RO technology is due to the lower energy consumption and the cheaper price (around 23%) with respect to conventional water purification technologies. Other advantages of membrane processes are their relatively small footprint and modularity which enables easy adaptation of process scale. Successful already applied membrane operations for water treatment are both pressure-driven (e.g., microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis, and membrane bioreactor) and electrically driven (e.g., electrodialysis) processes. These processes have already industrial applications becauseif correctly designed, manufactured, and operatedthey show a high operating stability, since the process is started.10 Microfiltration use include, among other things, sterilization, clarification, and the treatment of oily wastewaters (due to the high oil removal efficiency, low energy cost, and compact design of MF compared with traditional treatments such as mechanical separation, filtration, and chemical de-emulsification).11 In general, MF is utilized for the elimination of particulates with particle sizes in the submicrometer range. Membranes for ultrafiltration have been developed and proven for many years in a wide range of applications, such as highly polluted municipal and industrial wastewaters. In recent years UF has been also considered in seawater desalination installations, especially when treating surface seawater and for retrofit upgrades to existing conventional reverse osmosis pretreatment systems. Nanofiltration is a type of pressure-driven membrane operation that has properties in between those of UF and RO. NF membranes have relatively high charge and are typically characterized by lower rejection of monovalent ions than that of RO membranes, but maintaining high rejection of divalent ions. NF membranes have been employed in pretreatment unit operations in both thermal and membrane seawater desalination processes, for softening brackish and seawaters, as well as in membrane mediated wastewater reclamation and other industrial separations. Reverse osmosis is usually used to separate dissolved salts and ions. Its applications range from the production of ultrapure water for semiconductor and pharmaceutical use to the desalination of seawater for drinking water production and the purification of industrial wastewater. Electrodialysis (ED) has been in commercial use for desalination of brackish water for the past three decades, particularly for small- and medium-scale processes.12,13 The ED 10052
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is 23% lower (approximately 20 atm) and its transmembrane flux is 14−15% higher (approximately 0.08 m3/m2·h) when the applied operational pressure remains constant (68 atm). Moreover, according to refs 24 and 25, the recovery factor of a coupled NF + RO seawater desalination system is 10−12% higher than that of an SWRO plant based on conventional pretreatment in the same operative conditions (i.e., feed flow rate and RO operational pressure) whereas the desalted water cost is approximately 20% lower. Further improvement can be achieved using ultrafiltration and nanofiltration as pretreatment prior to SWRO. Since UF is effective for rejecting suspended particles, colloids, macromolecules, algae, and bacteria, the NF membrane fouling can be partly avoided by using UF pretreatment. The UF efficiency on NF performance was investigated by Song et al.26 Their results showed that NF system using UF as pretreatment process yields a higher flux, and, if UF membrane with smaller MWCO is utilized, higher softening efficiency of NF membrane could be achieved. Proof of the benefits in the adoption of integrated membrane systems and, in particular, of MF/UF membranes as SWRO pretreatment are their recent proliferation. Only in 2010, CH2M Hill’s Rob Huehmer has identified 94 such SWRO installations in operation or contracted.27 Integrated membrane systems offer also the possibility to handle the retentate stream (the brine) of the desalination plants, thus further improving current plants. Brine represents a crucial aspect of water treatment systems whose handling has to be adapted to the specific conditions at the construction site of a plant. For desalination plants located near coastal areas, brine can be discharged directly in the sea. In these cases particular attention should be given to the possibility of dangerous ecosystem modifications. For brackish water desalination plants, brine is frequently released into solar evaporation ponds. Recently, ZLD (zero liquid discharge) or near-ZLD systems are being considered for the recovery of clean water from brine streams thus minimizing their disposal. ZLD or near-ZLD systems consist mainly of thermal methods (such as brine concentrators, crystallizers, thermal evaporators, and spray driers) that are capable of recovering 95−99% high purity distillate from the waste streams.28,29 Moreover, these processes reduce concentrate to a slurry that can be disposed of in landfills or produce/generate solid mineral salts. Although attractive for volume minimization, these processes are typically not used because their capital and operating costs often exceed the cost of the desalting facility.23,30−33 Different studies have demonstrated that membrane contactor processes (such as membrane distillation (MD) and membrane crystallization (MCr)), as well as the previously mentioned forward osmosis, can potentially minimize brine volume at lower energy expenditure.23−25,34−38 Martinetti et al.23 proved that FO can achieve water recoveries up to 90% from the brackish water reverse osmosis brine and vacuum-enhanced direct contact membrane distillation (VEDCMD) up to 81%; Drioli and co-workers24,25,36−38 showed that MD and MCr can reach water recoveries up to 80 and 97%, respectively, from seawater or brackish water NF/RO brine. MD is a membrane process which combines both membrane technology and evaporation processing in one unit. It involves the transport of water vapor through the pores of hydrophobic membranes via a partial pressure difference across the membrane. For almost three decades, MD has been considered
process requires, however, the identification of draw solutions nonreactive with polymeric membrane and easily separable from the product fresh water. Appropriate membranes with substantially reduced internal concentration polarization, high water permeability, high rejection of solutes, high chemical stability, and high mechanical strength in both flat-sheet and hollow fiber configurations must be also developed in order to advance the field of FO. Pressure-retarded osmosis (PRO) is another known membrane process which, together reverse electrodialysis (RED), can be used to generate power from salinity gradients. PRO and RED have their own field of application: PRO seems to be more adapt to generate power from concentrated saline brines because of the higher power density combined with higher energy recovery. For the same reason, RED seems to be more attractive for power generation using seawater and river water.22 Until now, the main drawback of these techniques was the high price of membranes. However, a reconsideration of these membrane processes is worthwhile due to the declining membrane costs, to the increasing prices of fossil fuels and to the increasing necessity to have sustainable conversion of salinity-gradient energy available for the future. Moreover, PRO, RED, FO and, in general, the salinity gradient based technologies, might become really competitive only in integrated systems where energy production, water production, and brine reduction are the final objectives. An example of desalination system using FO with the objectives of increasing water recovery and decreasing brine volume can be found in ref 23. Martinetti et al.23 investigated forward osmosis for water recovery enhancement in desalination of brackish water. In particular, they found that high water recoveries (greater than 98%) can be achieved in processes in which reverse osmosis brine streams are further desalinated by FO. 2.2. Integrated Schemes. Membrane Distillation and Membrane Crystallizer: Potentialities As Stand-Alone Processes and in Integrated Systems. Key characteristics of membrane operations are, in fact, their great flexibility and mutual compatibility for integration, thus offering the possibility of combining membrane technologies different but complementary each other. The efficiency of the resulting integrated membrane process is enhanced due to the improvement of the single membrane units. For example, nanofiltration membranes are playing a more and more important role in brackish and seawater softening and wastewater reclamation. Various studies can be found in literature proving that the high NF rejection toward hardness, impurities, and dissolved salts can be used as pretreatment in seawater reverse osmosis (SWRO) desalination (i) to decrease the organic and inorganic fouling of RO membranes, (ii) to reduce the osmotic pressure of the seawater fed to RO, and, therefore, the operational pressure of RO, and (iii) to improve system recovery. In fact, the removal of bivalent ions by NF leads to a decrease in the osmotic pressure of the stream fed to the RO units (which is directly proportional to the solute concentration) and thus to an increase in the RO transmembrane flux if the applied hydrostatic pressure is constant. For example, if standard seawater is considered (i.e., seawater at 20 °C and TDS = 35.000 ppm), its osmotic pressure is equal to 26 atm and RO units operating at 68 atm have a transmembrane flux on the order of 0.07 m3/m2·h. If NF is used as RO pretreatment, the osmotic pressure of the RO feed 10053
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Water vapor passes through the membrane and condenses in the distillate channel. The latent and sensible heat is transferred through the condenser foil to preheat the feedwater in the condenser channel by internal heat recovery. An external heat source (e.g., solar collector) heats the feedwater until the desiderated temperature (only from 73 to 80 °C in Figure 2). The activities carried out by Winter et al.39 prove that, in this configuration, the main effect of the active membrane area increase is the significant reduction in the specific energy demand. This justifies the transfer of the PGMD channel configuration into a compact spiral package where, even without module insulation, only minimal heat losses to ambient occur. In recent years, a further MD enhancement was developed and produced by MEMSYS.40 The latter invented vacuum multieffect membrane distillation (V-MED), an MD device based on a multieffect desalination process under vacuum (Figure 3).
an alternative approach for conventional desalination technologies such as multistage-flash distillation and RO. These two techniques involve high thermal energy and high operating pressure respectively, which result in excessive operating costs if oil prices continuously move up. MD offers the attractiveness of operation at atmosphere pressure and low temperatures (30− 90 °C), with the theoretical ability to achieve 100% salt rejection. A MCr is conceived as an alternative technology for producing crystals and pure water from supersatured solutions; the use of the membrane distillation technique in the concentration of a solution by solvent removal in the vapor phase is utilized in this application. Owing to their low energy requirement, MD/MCr coupled with solar energy, geothermal energy, or waste heat can achieve cost and energy efficiency. For example, the Fraunhofer Institute for Solar Energy Systems (ISE) works on the development of energy selfsufficient desalination systems based on solar driven MD technology since many years. Recently, it is studying and producing spiral wound MD-modules in permeate gap membrane distillation (PGMD) (Figure 1).
Figure 3. Basic principles of V-ME membrane distillation. Reprinted with permission from ref 40. Copyright 2011 ITM-CNR.
V-MEMD has defined stages and heat is transported by evaporation and condensation. In stage 1, steam from evaporator condenses on a condensor foil at pressure P1 and temperature T1. This foil and a microporous hydrophobic membrane represent the channel in which the feed solution flows. The solution is heated thanks to the condensation energy from stage 1 and evaporates in stage 2 at pressure P2 and temperature T2 which are lower than T1 and P1. This process can be replicated in further stages at always reduced pressure and temperature. In the last stage the steam is finally condensed in a condenser which is the cold side of the system. The reported benefits of V-MEMD are the high water flux, the low heat loss, the modular construction, and the internal heat recovery. If as single units membrane contactor operations are more efficient than corresponding traditional unit operations (i.e., water recovery from conventional thermal desalination processes is not more than 40%41), their combination into existing water treatment processes is expected to generate important synergistic effects and to improve the overall process efficiency. The total water recovery of a brackish water RO desalination plant in which RO brine is further desalinated by VEDCMD can be greater than 96%;23 the total recovery of a seawater NF + RO desalination plant can increase up to about 93% if MCr is used for the exploitation of NF and RO brines;24,25,36,37 water recoveries as high as 76.6−88.9% can be achieved in a brackish water desalination system constituted by pretreatment/RO/ Wind evaporation/MCr, while less than 0.75−0.27% of the raw brackish water fed to the plant is discharged to the environment.38
Figure 1. Basic channel arrangement and temperature profile for PGMD. Reprinted with permission from ref 39. Copyright 2011 Elsevier.
PGMD is one of the possible MD configurations in which a third channel is introduced by an additional nonpermeable foil. The advantage of this arrangement is the separation of the distillate from the coolant. Therefore, the coolant can be any other liquid, such as cold feedwater. Winter et al.39 report that the presence of the distillate channel reduces sensible heat losses due to an additional heat transfer resistance; the disadvantage is the reduction of the effective temperature difference across the membrane, which slightly lowers the permeation rate.39 In module development, PGMD opens the opportunity to integrate an efficient internal heat recovery system as can be seen in Figure 2.
Figure 2. Module arrangement for PGMD. Reprinted with permission from ref 39. Copyright 2011 Elsevier. 10054
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For what concerns costs, previous studies24,25,38 showed that the introduction of a MCr unit on NF and RO retentate streams of an integrated membrane-based desalination system constituted by MF/NF/RO generate so many high quality mineral salts that the gain from their sale might cover more than entirely the cost of the desalination process. However, the development and commercial availability of proper membranes and modules manufactured especially for MD purposes, with improved transmembrane fluxes through the minimization of boundary layer heat/mass transfer resistances and heat losses through the membrane, is necessary for the industrial exploitation of this technology.
(2) Van Baelen, D; Van der Bruggen, B.; Van den Dungen, K.; Degreve, J.; Vandecasteele, C. Pervaporation of water-alchool mixtures and aceticacid-water mixtures. Chem. Eng. Sci. 2005, 60, 1583−1590. (3) Moganti, S.; Noble, R. D.; Koval, C. A. Analysis of a membrane/ distillation column hybrid process. J. Membr. Sci. 1994, 93, 31−44. (4) Stephan, W.; Noble, R. D.; Koval, C. A. Design methodology for a membrane/distillation column hybrid process. J. Membr. Sci. 1995, 99, 259−272. (5) Lipnizki, F.; Field, R. W.; Ten, P.-K. Pervaporation-based hybrid process: A review of process design, application and economies. J. Membr. Sci. 1999, 153, 183−210. (6) Fontalvo, J.; Cuellar, P.; Timmer, J. M. K.; Vorstman, M. A. G.; Wijers, J. G.; Keurentjes, J. T. F. Comparing pervaporation and vapor permeation hybrid distillation processes. Ind. Eng. Chem. Res. 2005, 44, 5259−5266. (7) Macedonio, F.; Brunetti, A.; Barbieri, G.; Drioli, E. Membrane Condenser as a new technology for water recovery from humidified “waste” gaseous streams. Ind. Eng. Chem. Res. 2012, submitted forpublication. (8) Lean, G. Hinrichsen, D. Atlas of the enviroment; Harper Perennml: New York, 1994; pp 57−64; available online at http://www. infoforhealth.org/pr/m14/m14.pdf (accessed Apr 2012). (9) http://www.unwater.org/downloads/UN-Water GLAAS 2010 Report.pdf. (10) Peters, T. Membrane Technology for Water Treatment. Chem. Eng. Technol. 2010, 33 (No. 8), 1233−1240. (11) Cui, J.; Zhang, X.; Liu, H.; Liu, S.; Yeung, K. L. Preparation and application of zeolite/ceramic microfiltration membranes for treatment of oil contaminated water. J. Membr. Sci. 2008, 325, 420−426. (12) AlMadani, H. M. N. Water desalination by solar powered electrodialysis process. Renewable Energy 2003, 28, 1915−1924. (13) Charcosset, C. A review of membrane processes and renewable energies for desalination. Desalination 2009, 245, 214−231. (14) Van der Bruggen, B.; Vandecasteele, C. Distillation vs. membrane filtration: overview of process evolutions in seawater desalination. Desalination 2002, 143, 207−218. (15) Fritzmann, C.; Löwenberg, J.; Wintgens; Melin, T. State-of-theart of reverse osmosis desalination. Desalination 2007, 216, 1−76. (16) Lay, W. C. L.; Liu, Y.; Fane, A. G. Impacts of salinity on the performance of high retention membrane bioreactors for water reclamation: A review. Water Res. 2010, 44, 21−40. (17) Phattaranawik, J.; Fane, A. G.; Pasquier, A. C. S.; Bing, W. A novel membrane bioreactor based on membrane distillation. Desalination 2008, 223 (1−3), 386−395. (18) Choi, J. H.; Dockko, S.; Fukushi, K.; Yamamoto, K. A novel application of a submerged nanofiltration membrane bioreactor (NF MBR) for wastewater treatment. Desalination 2002, 146 (1−3), 413− 420. (19) Choi, J. H.; Lee, S. H.; Fukushi, K.; Yamamoto, K. Comparison of sludge characteristics and PCR−DGGE based microbial diversity of nanofiltration and microfiltration membrane bioreactors. Chemosphere 2007, 67 (8), 1543−1550. (20) Cornelissen, E. R.; Harmsen, D.; de Korte, K. F.; Ruiken, C. J.; Qin, J. J.; Oo, H.; Wessels, L. P. Membrane fouling and process performance of forward osmosis membranes on activated sludge. J. Membr. Sci. 2008, 319 (1, 2), 158−168. (21) Oo, H.; Kekre, K. A.; Qin, J. J.; Tao, G.; Lay, C. L.; Lew, C. H.; Cornelissen, E. R.; Ruiken, C. J.; de Korte, K. F.; Wessels, L. P. Osmotic membrane bioreactor: preliminary pilot study on effects of osmotic pressure on membrane flux and air scouring on fouling. IWA Regional Conference “Membrane Technologies in Water and Waste Water Treatment”, Moscow, June 2−4, 2008. (22) Post, J. W.; Veerman, J.; Hamelers, H. V. M.; Euverink, G. J. W.; Metz, S. J.; Nymei-jer, K.; Buisman, C. J. N. Salinity-gradient power: evaluation of pressure-retarded osmosis and reverse electrodialysis. J. Membr. Sci. 2007, 288, 218−230. (23) Riziero Martinetti, C.; Childress, A. E.; Cath, T. Y. High recovery of concentrated RO brines using forward osmosis and membrane distillation. J. Membr. Sci. 2009, 331, 31−39.
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CONCLUSIONS AND FUTURE DEVELOPMENTS Nanostructured artificial membranes represent today one of the most interesting examples of applications of nanotechnology in fields of strategic importance for our society. Artificial membranes and membrane operations in fact are already dominant technology in brackish and seawater desalination and wastewater treatment and reuse thus contributing to the end of the thermal era. Moreover membrane technology offers also the possibility of utilizing salinity gradient for energy production (the so-called blue energy). For approaching the ambitious objective of reaching “zero liquid discharge” and “low energy consumption”, different membrane operations can be coupled in integrated systems. Integrated membrane systems allow the development of more cost-effective and environmentally acceptable processes and the use of their synergic effects in terms of better performance of the overall system. At present, future development in water treatment systems requires the following: (1) the development of membranes particularly adapt to specific applications; (2) the handling and control of concentrate discharge; (3) the development of water treatment systems coupled with renewable energy sources for a significant reduction in energy consumption and cost; (4) the reconsideration of pressure-retarded osmosis and reverse electrodialysis as membrane techniques to generate power from salinity gradients; (5) the realization of advanced water management systems, at closed-circuit, based on graduated quality requirements.10,41 Among the technological developments, carbon nanotubes, fullerene, aquaporin channels, new protein-based membranes, and graphene membranes have emerged in recent years as innovative water technologies and developed membranes with superior permeability, durability, and selectivity for water purification.41
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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