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Desalination 391 (2016) 43–60

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Desalination journal homepage: www.elsevier.com/locate/desal

Recent trends in membranes and membrane processes for desalination P.S. Goh a, T. Matsuura b,⁎, A.F. Ismail a,⁎, N. Hilal c,d a

Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia, 81310, Johor, Malaysia Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur Private, Ottawa, Ontario, K1N 6N5, Canada c Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK d Qatar Energy and Environment Research Institute (QEERI), Doha, State of Qatar b

H I G H L I G H T S • Current trends of membrane materials for desalination • Emerging membrane-based desalination processes • Challenges and future outlook of novel membranes and membrane processes

a r t i c l e

i n f o

Article history: Received 14 October 2015 Received in revised form 15 December 2015 Accepted 20 December 2015 Available online 4 January 2016 Keywords: Membranes Membrane processes Desalination

a b s t r a c t Access to clean water resource continues to be the most urgent and pressing global issue where hiking economic and ecological needs have urged for more water-efficient technologies. Membrane-based separations for desalination are playing an increasingly important role to provide adequate water resources of desirable quality for a wide spectrum of designated applications. The engagement of multidisciplinary research areas into the commercial membrane and membrane systems offers an opportunity to refine and optimise current techniques as well as provides new insight and novel methods of purifying water. The advancement of material science and engineering reveals the potentials to solve real-world practical problems and heighten the current technologies. This review highlights some of the latest notable achievements of novel advanced membrane materials and emerging membrane processes for water solution. The unique characteristics of advanced membranes and emerging membrane processes in leading the state-of-the-art desalination are presented. Lastly, the future directions for research, development and commercialization of membrane and membrane processes are critically discussed. It is expected that, the promising and well-adapted characteristics possessed by the novel membranes and advanced membrane processes can provide meaningful inspiration for breakthrough technologies and solutions where soon they will be translated into exploitable innovations in industries. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The burgeoning population growth, urbanization and industrialization have increased the demands for reliable water resources. The effects of weather change, which is also closely tied to the rapid growth, have further complicated and exacerbated the balance between water supply and demand. With this joint pressure, the issues linked to water stress and shortage are expected to be widespread and increase significantly for the years to come. The growing awareness towards the far-reaching impacts of water scarcity and declining water quality on the environment and mankind has pushed considerable efforts of searching sustainable solutions to counter the on-going global challenge. Among the many strategies adopted to provide affordable and ⁎ Corresponding authors. E-mail addresses: [email protected] (T. Matsuura), [email protected] (A.F. Ismail).

http://dx.doi.org/10.1016/j.desal.2015.12.016 0011-9164/© 2015 Elsevier B.V. All rights reserved.

safe drinking water, waste water treatment is considered a promising solution to meet the ever rising demand and alleviate the alarming problems [1,2]. Undoubtedly, desalination and wastewater treatment have provided valuable opportunity to avoid the complete depletion and diminishment of fresh water resources. Particularly, desalination has been recognised as the essential contributor to reduce water stress in coastal and inland regions through the production of fresh water from seawater, saline groundwater, drainage water and treated wastewater. Desalination and wastewater treatment based on membrane technology is one of the approaches which has been extensively explored to tackle the challenge of increasing access to clean drinking water in order to sustain the rapidly growing global population as well as to ensure economic progress. The research and industrial community has been thinking strategically and looking beyond the fence line to fully utilize membrane technology as cost-effective candidate to an expanding range of purification and separation needs such as water

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and wastewater treatment, brackish and seawater desalination. In fact, recent progresses have witnessed the potential and reliability of applying commercially available membranes and membrane systems to relieve water issues [3,4]. Pressure-driven membrane technologies such as reverse osmosis (RO) have long gained great global interest to offer affordable yet effective solutions to achieve promising product water quality while imposing a smaller footprint of water treatment plant. Generally, membranes dedicated for desalination are designed and formulated to achieve high throughput and selectivity while exhibiting high mechanical integrity and resistance towards fouling at the minimum processing costs [5]. However, the development of membrane that can one size fit all at commercially interesting level remains a great challenge. Typically, the membrane separation efficiency is primarily impeded by two physical limitations, i.e. i) concentration polarisation which is caused by the increased concentration at the membrane surface resulted from selective transfer of some species under the effect of transmembrane driving forces and ii) membrane fouling which is strongly related to factors such as solution chemistry, level of pretreatment, membrane properties and operating conditions [6]. Additionally, commercially available polymeric membranes for water separation are still suffering from several drawbacks and challenges including the trade-off relationship between productivity and selectivity [4]. Normally, membrane fouling represents a greater and complex problem which can detrimentally deteriorate membrane lifespan, hence poses a serious threat to the application and popularity of membrane separation technology [7]. The irreversible organic and biological fouling of a membrane, which inevitably results in the permanent loss of the membrane's permeate flux, has prompted numerous contemporary investigations to enhance the membrane's anti-fouling properties through membrane formulations or surface modifications [8–10].Considering the adverse impacts of membrane fouling, approaches for fouling mitigation have been arousing great research efforts. Furthermore, research has also been actively pursued to resolve the trade-off issue. As part of the solutions, new membrane materials have been developed to tailor and optimize the parameters which are known to influence the fouling mechanism and propensity [11]. Enhanced membrane properties in terms of their pore size and pore size distribution as well as the surface charge, hydrophilicity and roughness are fine-tuned through the innovative and multidisciplinary efforts adopted from the holistic interplay between membrane engineering and other emerging fields such as advanced materials and nanotechnology. Nano-engineered materials have found numerous applications that can potentially improve the existing technologies. Owing to their special properties relative to bulk material, the introduction of nanomaterials and nanotechnology in the field of membrane-based water and waste water treatment has been a hot topic of research and development in the past decade, and is likely to continue to be for a long time [12,13]. While the existing technology for water treatment is suffering from the inherent limitations, the emerging nanotechnology provides a versatile platform to furnish affordable and safe drinking water to address global water shortage issues. With the advancement that involves cross disciplinary science and engineering subjects, nanotechnology can favourably render a variety of options to tailormake solutions through different top-down or bottom-up approaches to tackle a wide range of conventional and emerging pollutants such as heavy metals, biological toxins, organic and inorganic solutes, just to name a few. Mounting evidences from the past studies have indicated that this new material revolution poses profound impact on heightening the membrane-based separation performance. Despite the outstanding achievement made by the commercially available membrane technologies, research continues to seek for more rooms of improvements in terms of water cost, particularly the energy consumption and recovery which are directly associated to the unit water cost. For instance, regardless the efforts made, the average energy consumption for seawater desalination using RO is still higher than that of theoretical energy required. Thus, based on this reason, there is a

substantial need to call for innovative membrane processes and technologies to improve the current water treatment systems. Currently, osmotically driven membrane processes (ODMPs) which are endowed with energy-saving features for water purification and clean renewable energy technologies have emerged as one of the most prominent and fast-moving researches in its field [6]. Recently, forward osmosis (FO) and pressure retarded osmosis (PRO) have been addressed as remarkable and revolutionary ODMPs for power generation, desalination and wastewater treatment. The key advantages of ODMPs over the pressure driven membrane processes are their low energy consumption, low fouling propensity hence minimum cleaning, high salt rejection and high water flux [14]. On the other hand, owing to their several unique characteristics such as total rejection for ultrapure water production, insensitivity to feed concentration and stable performance at high contaminant concentrations, thermally driven membrane distillation (MD) has gained equally important attentions as one of the most highly researched membrane-based separation processes to overcome the limitations at the conditions where those conventional processes cannot practically operate [15,16]. It is worth mentioning that, membrane capacitive deionisation (MCDI) has also been prominently explored to overcome some limitations of the conventional CDI desalination process. However, due to some inherent bottlenecks such as extensive energy consumption in thermally driven separation and the challenge to operate FO alone for a municipal scale application at the current stage, the stand alone separation processes with a single step could not match the actual challenges [17]. By considering the pluses and minuses of these systems, integrated membrane system allows more efficient processes as the strength and advantages of each system can be optimally harnessed and exploited [18,19]. This review focuses on the recent development of novel membranes and membrane processes for desalination. The modification approaches of membrane materials through the incorporation of advanced materials, particularly nanomaterials, as well as surface modification are presented. In the second part of this review, the basic features and applications of some emerging membrane-based separation technologies namely FO, PRO, MD and MCDI as well as their roles in the integrated membrane system are briefly overviewed. Subsequently, the challenges and future outlook of novel membranes and membrane systems for desalination and wastewater treatment are critically discussed. Obviously, tremendous research efforts have been dedicated towards understanding the dynamics of novel membrane materials and advanced membrane-based separation processes. With the ongoing research, it is possible that more advancements and more conclusive findings can be noted in the years ahead, especially in view of economic and environmental sustainability [20]. 2. Novel advanced membrane for desalination Thanks to the ceaseless advances made in science and technology, a wide range of polymers and production techniques have been used in a great diversity of separation membranes which were tailored for an effective desalination process [21,22].The ultimate performance of a desalination membrane is intimately dependant on the materials they are made from in which the composition of the membrane plays a key role to determine the important properties such as salt rejection, fouling propensity, mechanical strength, and reactivity. Commercially, thin film composite (TFC) polyamide (PA) and cellulose acetate (CA) membranes have been widely used for RO and FO whereas hydrophobic polytetrafluoroethylene (PTFE) and polyvinylidene-fluoride (PVDF) have been commonly used in MD [20]. Despite the efforts made for the selection of the best membrane materials and their preparation technique based on the objective and requirement of the process, they may not suffice to reach the best performance. It is due to the fact that the inherent properties of the used materials may have adverse effect on the separation [23]. Typically, selectivity-flux trade off exists as an inherent problem for all polymeric membranes, owing to the intrinsically

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contradicting effects between the polymer chain stiffness and interchain spacing [24,25]. Besides that, due to the destructive effects of membrane fouling, different strategies have been established to enhance the fouling resistance of desalination membranes, particularly that for pressure-driven processes, in order to lower the energy usage, enhance process reliability and minimize the environmental impact of sea-water desalination [26].The findings from numerous investigations evidently showed that the membrane fouling resistance can be improved by smoothing membrane surface, increasing surface hydrophilicity and introducing strong electrostatic repulsion between membrane surface and charged foulants [27,28].The development of anti-biofouling membrane with smooth, hydrophilic surfaces has shown resistance to the adhesion of protein and bacteria, hence successfully reduced the use of cleaning protocols and biocides in water treatment process. Since membranes that are intrinsically incorporated with advanced functionalities to resolve the above-mentioned issues are uncommon, modification of membrane through the incorporation of additive with desired functionalities or the surface modification of membrane is necessary to achieve the goal. In brief, the current common goal of membrane modification is to render enhanced membrane properties, particularly in terms of product water quality, permeability and antifouling properties, through facile and straightforward methods that feature the ease of synthesis, reproducibility and most importantly, high potential to be upscaled or mass-produced for practical application. This section presents the recent progresses made in the development of novel advanced membrane through the fabrication of nanocomposite membrane and modification of membrane surface to cater for the desired properties of membranes for desalination. Table 1 summarizes the current trend in polymeric membranes for desalination [29–55]. Certainly, the application and progress of these novel membranes in water treatment further affirm scientists' commitment by clearly articulating the possibility of resolving water-related issues with the heightened membrane performance. 2.1. Nanocomposite membrane A significant number of studies on nano-enhanced membranes have focused on the introduction of nanomaterials into polymeric membranes. In general, when they are incorporated into membrane,

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nanomaterials open doors to new properties that macroscale materials do not share. It is expected that more emerging new contaminants can be treated effectively even at low concentrations, due to the increased specificity of nanotechnology and the development of smart membrane tailored for specific uses to render novel reactions at the nanoscale with the increased number of surface atoms. Generally, two approaches have been utilized to introduce nanomaterials as additive in the polymer matrix i.e. i) incorporation of the nanomaterials during the formation of dope and ii) coating of nanoparticles on the surface of membrane via chemical bonding or self-assembly [23].The different characteristics with desired purposes can be invoked through the interaction between the nanomaterials surface and polymeric chains and/or solvent during the membrane fabrication process. It is also well accepted that the type and the size of nanoparticles as well as their concentration in the dope are of great importance in the membrane morphology and permeability [23,56]. Nanomaterials that hold potentials to create synergism or multifunction when integrated into the polymer system are metal/metal oxide nanoparticles, carbon based nanomaterials and biomimetic aquaporin (AQP). he incorporation of a wide range of metal/metal oxide nanoparticles into the polymeric membranes has been the focus of numerous investigations since last decade [57]. The nanoparticles which are commonly reported in desalination membrane fabrication are titanium dioxide (TiO2), zeolites, silica and silver. Among the metal/metal oxides' host of remarkable traits, some of the useful ones for desalination include antimicrobial, super-hydrophilic and photocatalytic properties. Silver nanoparticles are well known as antibacterial materials based on its interaction with sulfur and phosphorous to further damage bacterial proteins, which allows effective mitigation of biofouling when they are incorporated in the membrane. On the other hand, due to the attractive super-hydrophilicity and photocatalytic properties, the addition of TiO2 nanoparticles can not only improve the hydrophilicity but also alleviate the biofouling problem [58]. Fullerene-based nanomaterials such as carbon nanotubes (CNTs) and graphene-based nanomaterials are emerging as versatile new materials with properties that suggest a great potential for improving water treatment technologies, particularly in membrane separation. Fullerene nanomaterials are intrinsically endowed with some exciting properties in terms of their mechanical strength, ability to tailor size, flexibility in modifying functionality, and

Table 1 Current trend in polymeric membranes for desalination. Membrane configurations

Material description

Operating mode

Reference

Asymmetric flat sheet/hollow fiber membranes Flat sheet/hollow fiber mixed matrix membranes

Cellulose acetate (CA) and cellulose triacetate (CTA) membrane through phase inversion Incorporation of nanomaterials into CA and CTA, membrane formation through phase inversion Nanofillers: silica, carbon nanotubes, silver Active layer-PA from Interfacial polymerization (eg. m-phenylenediamine (MPD) and trimesoyl chloride (TMC)) Substrate- porous UF/NF (PSf, PES, PAN) flat sheet membrane Active layer-PA from Interfacial polymerization Substrate- electrospun nanofiber (polyvinylidene fluoride (PVDF),polyvinyl alcohol (PVA)) as substrate Active layer- layer-by-layer deposition of polyelectrolytes (Poly(styrene sulfonate), Poly(allylamine hydrochloride)) Active layer-PA from Interfacial polymerization incorporated with nanofillers Substrate- porous UF/NF (PSf, PES) flat sheet membrane Active layer-PA from Interfacial polymerization Substrate- porous UF/NF (PSf, PES) flat sheet membrane incorporated with nanofillers Nanofillers: silica, graphene oxide, titania nanotube

RO, ODMP RO, ODMP

[29,30] [31–34]

RO, ODMP

[35,36]

RO, ODMP

[37,38]

RO, ODMP

[39]

RO, ODMP

[40–44]

RO, ODMP

[45,46]

MD

[47–49]

MD

[50–52]

MCDI

[53–55]

Polyamide-based TFC

Polyelectrolyte-based TFC Polyamide-based TFN

Flat sheet/hollow fiber porous hydrophobic membranes Porous hydrophobic mixed matrix membranes

Ion exchange membrane

PTFE, polyethylene (PE), PES and PVDF Incorporation of nanomaterials into hydrophobic, membrane formation through phase inversion Nanofillers: carbon nanotubes, nanodiamond, clay Anion exchange membrane: aminated PSf, aminated PVDF Cation exchange membrane: sulfonated polyphenylene (PP) oxide

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electron affinity that enables the profound contributions of their potentials for new membrane-based desalination technologies [59].The hydrophobic surface in the interior of a defect-free carbon-based nanomaterials also allow for a nearly frictionless flow [60]. Simultaneously, the membrane surface hydrophilicity and roughness can also be tailored through the attachment of modified carbon-based materials such as functionalized CNTs and graphene oxide (GO) [61]. One of the important structural properties that merits CNTs as a favourable nanomaterial for desalination is their ability to reject dissolved ions while enabling water molecules to pass through [20,62].The small and controllable diameter of CNTs allows the design of desalination membrane in a fashion where fluid can selectively flow through the hollow core of the CNT. Graphene-based nanomaterials have shown potential application for making an ultra-fast, energy-efficient and environmentally friendly generation of desalination membrane [60,63]. Numerous computational studies have predicted that nanoporous graphene could outperform the commercial water desalination membrane where it can work under lower pressures and provide well-defined channels that can filter salty water at a faster rate than RO membranes [64]. Graphene sheets perforated with micropores have been proposed as a potential star to allow ultrafast separation of atomic species where water molecules may permeate between graphene sheets across the polymeric matrix [65,66]. Over the last five years, the participation of aquaporin-based membranes in desalination technology has opened up an exciting direction in desalination due to the high AQP water transport and excellent selectivity. Biomimetic AQP membranes represent a rapid moving field where the main attractions of AQP relevant for biomimetic membrane technology for water purification are i) highly permeable, ii) highly selective, iii) defect free (iv) mechanically stable to withstand the required pressure for target applications, (v) chemically and biologically stable for long term use, and (v) easy to scale up at reasonable cost [67, 68].The pioneering work conducted by Kumar et al. [69] indicated that AQP-incorporated triblock copolymer membranes could lead to more controllable, productive and sustainable water treatment membranes. Following that, systematic researches have been conducted on active AQP based composite membranes, which exhibited competitive water permeability and enhanced ion rejections for existing RO, FO or NF system [70–72]. Several approaches for fabricating AQP-based composite membranes with compatible NaCl rejection have been reported [73,74]. 2.1.1. Thin film nanocomposite Over the last 30 years, TFC, which is generally synthesized through interfacial polymerization between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) on a porous substrate layer, remains as the primary choice for the state-of-the-art RO desalination processes [27]. Although TFC membrane has demonstrated promising characteristics to produce very pure water from highly contaminated sources with good water flux and salt rejection as well as good mechanical strength and tolerance to a wide range of feed pH [75], the hydrophobic aromatic groups and high cross-linking degree at the selective layer have inherently limited water flux. This type of membrane is also found to be highly susceptible towards fouling, which ultimately increases the energy consumption and desalination cost. Recently, several routes to increase water permeability and anti-fouling properties without compromising selectivity have been proposed to architecturally modify the chemistry or morphology of the active layer of TFC membranes. One of the new approaches was implemented through the integration of functional nanoparticles with super-hydrophilicity and anti-microbial properties within the polymer films, which has led to the creation of an emerging class of thin film nanocomposite (TFN) RO membranes. The continual studies have revealed that while TFN membrane can purify water as well as TFC membranes, it is value-added with some significantly improved properties in which the inorganic additives have contributed not only to the improvement of diffusion features of the formed membrane but also to the enhancement of the degree of hydrophilicity

and hence its fouling resistance. The inorganic nanomaterials are found to improve film formation by offering the following benefits: i) increasing the diffusion rate of monomers to the interface ii) expanding the wet zone on the top of support layer and iii) capturing by products and controlling reaction pH (buffer agent) [41,76,77]. AquaporinZ-containing proteoliposomes were added to the MPD aqueous solution, followed by the reaction with TMC in the organic phase to form a salt-rejection PA layer via interfacial polymerization [73]. When benchmarked against commercial RO membranes, it is found that the high performance biomimetic membranes show a comparable NaCl rejection and an order of magnitude higher flux than the seawater RO membrane SW30HR. Due to the ease of TFC fabrication, the resulting membrane has an area greater than 200 cm2 which can be easily scaled up to produce membrane areas at an industrial scale. Furthermore, the PA layer may also serve as a protection layer to the proteoliposomes against environmental factors and provide a mechanical support to them. This leads to high mechanical stability of membranes, which is an important requirement for desalination applications. Hollow fiber TFN membrane embedded with AQP-incorporated proteoliposomes has been recently studied for their FO separation performance [78]. As depicted in Fig. 1, a new protocol has been established where the vesicles with AQPs are immobilized on the hollow fiber polyethersulfone substrate and subsequently coated by a layer of PA to cover or connect these vesicles through interfacial polymerization. The defect free PA layer immobilised with AQP could effectively function as a selective layer, which determines the water permeability and salt rejection. It was found that, The biomimetic membrane with a high AQP covering density exhibited a 40 L m−2 h−1 permeate flux at 5 bar, almost 200% as much as the flux of a typical commercial BW30 RO membrane, and rejection of 97.5% to a 500 ppm NaCl solution, which outperformed the commercial membrane tested under the same conditions. Due to the different roles played by the PA selective thin layer and porous substrate, the incorporation of nanomaterials in these layers may impart different impacts to the separation properties. The effects of the incorporation of Zeolite A in PA selective layer and polysulfone (PSf) substrate have been investigated [79]. Overall, it was found that regardless of the position of nanoparticles, the resultant TFN exhibited smoother and more hydrophilic surface, higher water permeability and salt rejection as well as greater resistance to physical compaction. However, the incorporation of zeolites in the substrate could resist physical compaction better via enhanced mechanical stability meanwhile the embedment of zeolites A in the selective layer resisted compaction by mitigating densification of polymer film. NaY zeolite nanoparticles have also been embedded into the PA layer via interfacial polymerization to form a novel zeolite–PA TFN membrane [44].Owing to the hydrophilicity of NaY zeolite, the orientated distribution of the nanoparticles at the bottom of the PA thin layer on the PSf side, instead of the PA surface side, allowed the nanoparticles to play a greater role in heightening the membrane separation performance. Under the optimum conditions of 0.15 wt% NaY zeolite, the water flux increased from 0.95 to 1.78 m3/m2/day with the incorporation of the zeolite nanoparticles, while providing a high salt rejection of 98.8%. TiO2 has also been used to structurally modify the PSf substrate of FO TFN membrane [80]. Compared to the control TFC membrane, the FO water flux of TFN was reported to increase significantly from 4.2 to 8.1 L/m2.h and from 6.9 to 13.8 L/m2 h (AL-DS orientation), respectively, when seawater was used as feed solution and 2 M NaCl was used as draw solution. The increase in water flux can be attributed to the formation of fingerlike macrovoids that connect the top and bottom layer of the substrate and reduce the tortuosity, consequently decrease the internal concentration polarisation effect. During a typical desalination treatment, the biofouling of RO membranes is resolved through the addition of free chlorine during the pretreatment of feed solution. Despite the dechlorination stage prior to the RO system, a minimum amount of free chlorine is intentionally left to prevent the growth of the microorganisms. Unfortunately, the chemical

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Fig. 1. The preparation of AQP hollow fiber membrane through the formation of polyamide layer to cover the AQP through interfacial polymerization [78].

interactions of aromatic polyamide with residual chlorine oxidant can degrade the polyamide layer and subsequently deteriorate the performance of the TFC membrane. Very recently, the hybrid of reduced graphene oxide (rGO)/TiO2 has been embedded in the PA layer aimed to enhance chlorine resistance and antifouling property, hence improve the overall RO performance of the TFN [43]. At an optimum loading of 0.02 wt%, it was found that water flux was improved by 21% compared to the TFC counterpart meanwhile the salt rejection of 99.45% was achieved. In this hybrid system, rGO sheet served as a platform to restrict the aggregation of TiO2. As such, the well dispersed TiO2 could effectively render chlorine resistance and antifouling property and also improve the separation performance of resulted membrane. TFC membranes have also been modified with antimicrobial nanomaterials to increase the antimicrobial activity and improve the biofouling resistance of the resultant TFN. GO is one of the promising nanofillers to render antimicrobial properties while potentially increasing the TFN's permeability and mechanical strength [81,82]. In a recent work, GO nanosheets

were covalently bound to the polyamide active layer to promote efficient bacterial cell inactivation in which the number of viable E. coli cells was reduced by 64.5% after contact with the TFN for 1 h, without affecting the intrinsic transport properties of the TFN membrane for desalination (Fig. 2). [83]. 2.1.2. Mixed matrix membrane Mixed matrix membranes (MMMs), which have been fabricated through the direct incorporation of inorganic fillers into polymeric host matrix, have been widely used as an alternative approach to achieve synergetic effects between the nanofillers and host polymer matrix. Compared to TFN, MMMs enjoy the ease of fabrication as the nanofillers are typically added to the polymer dope, followed by phase inversion technique to produce the flat sheet or hollow fiber MMMs. CA/polyethylene glycol (PEG) RO membrane embedded with fused silica particles (FSP) has been reported [34]. The optimum performance of desalination process was shown by 30 wt.% FSP in which the

Fig. 2. (a) GO nanosheets were covalently bound to the polyamide active, b) SEM images showed that the TFN promote efficient bacterial cell inactivation of E. coli cells after 1 h contact time [83].

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permeation flux of 0.66 L/m2h and salt rejection of 98.4% were obtained. The increase of permeation flux was mainly due to hydrophilic nature of FSP in which the doping of FSP changed the permeate mobility and also the polymer chain segmental motion and subsequently increased the free volume of the MMMs. On the other hand, the electrostatic interactions between the FSP and dense membrane prevented the formation of undesired voids hence increased the salt rejection. Denotation nanodiamond (DND) is a new generation of carbonbased nanomaterial that has potential to enhance conventional hydrophobic membranes for water vapor flux in MD process [52]. Since the outer core of the DNDs is graphitic and quite hydrophobic, they decrease pore wetting while enhancing the transport of pure water vapor. As such, the incorporation of DNDs has favorably altered the water–membrane interactions to enhance vapor permeability while preventing liquid penetration into the membrane pores. Additionally, the DNDs possess a graphitic ring structure with additional -COOH and -OH groups on its surface which leads to specific interactions with the water vapor molecules further leading to enhanced flux as shown in the illustration in Fig. 3. As a result, the desalination performance was consistently higher compared to that of PVDF in which maximum permeate flux of 13.8 L/m2h and salt reduction of 99.9% could be achieved. The structural, mechanical, interfacial, and separation characteristics of the nanocomposite membranes have been historically associated with the types of nanoparticles and their size as well as the interaction of the nanoparticles with the host polymer and solvent during the dope preparation. Particularly, the degree of dispersion of nanomaterials in polymer matrix showed great impacts on the mechanical strength of composite membrane performances [84]. Improving the compatibility of CNTs with polymer membrane matrix through CNTs functionalization is a straight forward approach to prevent the leakage of CNTs from the resultant nanocomposite membrane in order to preserve the membrane tensile strength and minimize CNTs loss into the environment from economic and environmental points of view. The incorporation of dodecylamine (DDA) functionalized multiwalled carbon nanotubes (MWCNTs) in PSf membrane has been attempted for the mitigation of membrane biofouling [85]. The

amide functionalized MWCNTs facilitated the dispersion of nanotubes in the casting solutions and improved the interfacial compatibility and stability of composite membranes. Owing to the antimicrobial properties rendered by the DDA/MWCNTs, the composite membrane embedded with 0.5 wt.% of MWCNTs exhibited the highest flux recovery (83%) and lowest total flux loss (29%) with reduced irreversible fouling resistance (17%). Due to its capability to render hydrophilicity, mechanical strength, chemical stability, ion exchange capacity and conductivity, rGO has been incorporated into polyaniline (PANI) conductive anion exchange membranes for capacitive deionisation process [55]. The experimental results indicated that the salt removing efficiency of membrane assisted CDI was much higher than CDI without ion exchange membranes. This feature was mainly attributed to the added rGO which has helped to improve the dispersion of the PANI in the solvent to produce membrane with higher electrical conductivity and also high ion exchange capacity. 2.2. Surface modified membrane Surface modification that allows the structural engineering of polymeric membranes is performed to introduce abundant hydrophilic functional groups onto the hydrophobic membranes by means of several well established approaches such as i) adsorption and surface coating; ii) chemical reactions induced by high energy substances (UV, plasma) or oxidative treatment by strong acids and iii) surface grafting of a functional monomer or polymer on a base membrane [8]. The concept of surface hydrophilicity enhancement has been extended to the surface modification of TFC. Hydrophilic additives such as poly(m-aminostyrene-co-vinyl alcohol) and o-aminobenzoic acid– triethylamine (o-ABA–TEA) salt have been introduced into the aqueous phase during the interfacial polymerization of PA selective layer [27]. During the RO desalination process, these hydrophilic additives could create an additional pathway to enhance water transport and provide charge repulsion to increase salt rejection. As a result, the resultant surface-modified membrane showed a considerable water flux increase as well as lower flux decline. The antifouling mechanism of a

Fig. 3. The mechanism that takes place at the DND immobilized membranes. The enhanced flux is mainly attributed to the enhance hydrophilicity and interaction of water molecules with functional groups on DND [52].

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hydrophilic membrane surface can be attributed to the formation of a compact hydration layer on the surface that may hinder the direct contact of foulants with the membrane surfaces [86]. The introduction of zwitterionic polymers (ZPs) such as polyphosphobetaine, polysulfonbetaine and polycarboxybetaine which possess both anionic and cationic groups in a single monomer unit onto the polyamide thin film represents an attractive alternative to simultaneously enhance permeability and antifouling properties. The intra- and inter- association between opposite charges of zwitterion monomers facilitates the formation of “free water” hydration layers which can be attributed to the ionic solvation of these “free water” molecules [87]. The strength of this hydration layer is thought to surpass the hydrogen bonded water molecules in neutral hydrophilic polymers [88].However, some studies also reasonably pointed out that, the high surface free energy of hydrophilic membrane could induce the adhesion strength of organic foulant, hence complicate the cleaning process, therefore membrane with lower surface free energy may be more beneficial in this aspects. One of the approaches to reduce the surface free energy while maintaining the surface hydrophilicity is the modification of membrane surface with surfactant that shows both hydrophilic and oleophobic surface properties [8]. 2.3. Graphene/graphene oxide membrane While the development of nanocomposite and surface modified membranes which consist of mainly polymeric materials is progressing steadily towards practical applications, graphene membranes have also attracted tremendous interest to elevate the performance of desalination membrane technology to offer significant improvement and energy saving. Currently, several computational methods have been employed to provide further insight of graphene membranes for desalination [89]. Several exciting computational work have outwardly shown that a graphene membrane with sub-nanometer pores holds great potential as desalination membrane when the channel pores are carefully tailored to selectively control the passage of ions [90,91]. Also, being the thinnest material, the two-dimensional sheets of graphene can be favourably used as ultra-thin membranes to increase water flux [92]. The graphene membranes which containing continuous channels allow the passage of greater volume of water at the given pressure far lower than that of required by conventional polymeric membranes. While maintaining the high salt rejection, single-sheet graphene membranes are 250 times thinner than the selective layer in commercial RO membranes. As such, a significant improvement in water flux of at least 250 times could be expected under the same driving force [93]. The precisely controlled pores can be custom formed on the graphene monolayers through ion etching, self-assembly or doping. In order to tailor for their desired applications, surface functionalities can also be introduced through chemical modification such as oxidation and fluoridation. For instance, the dangling bonds in the graphene pores can be saturated with F atoms under a vacuum fluorine atmosphere [94]. One of the interesting feature of graphene when it is applied as FO membrane is the exceptional strength of porous graphene which requirement of a support layer can be ruled out. As such, when the external hydraulic pressure is not necessary, the graphene FO membrane exhibits nearly zero ICP [94]. It has been reported that for the FO system using fluorinated porous graphene the water flux is about 1.8 × 104 times higher than that of a typical cellulose triacetate membrane. While great efforts have been attempted to address the challenge to achieve large-area, high quality and single-crystal graphene, free standing GO membrane has also opened a brand new era for assembling membranes with enhanced ion selectivity performance due to their facile and large-scale production in solution. Moreover, the assembling of GO thin film is much more feasible as GO can be readily dispersed to form well-dispersed aqueous colloids in the absence of any surfactants or stabilizing agents [95]. Recently, the tunability of water desalination across graphene oxide framework (GOF) membranes, which consist of

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layers of GO sheets covalently interconnected by linear boronic acid linkers, has been reported [96]. Assessment using classical MD simulations revealed that 100% salt rejection coupled to a water permeability two orders of magnitude higher than any commercial RO membranes can be achieved. 3. Emerging membrane processes The application of RO process has rapidly bloomed since the 1960's and has become the leading and preferred technology for seawater desalination by surpassing conventional thermal processes in many new plant installations [97]. Currently, RO accounts for 60% of desalination erections across the globe, thanks to its order of magnitude less energy requirement than its thermal counterparts. Despite the unprecedented success and significant growth of RO in desalination, it is also associated with several severe drawbacks including relatively low water recovery factors, scaling and biofouling, and high electrical energy cost, which typically represents half of the total water production cost. [98].The process limitations have eventually made RO less favourable for energy-effective desalination as it presents a big challenge to make RO desalination affordable for poorer countries [21,99,100]. Additionally, higher energy consumption is also directly linked to a corresponding increase in greenhouse gas emissions which further deteriorate the quality of the environment. Besides that, RO also suffers greatly from a high rate of brine production in which current methods of brine disposal are still inadequate to ensure sustainable management. Research and commercial attentions in emerging ODMP, MD and MCDI as illustrated in Fig. 4 are increasing globally due to the potential of the processes to render significantly reduced operational cost without sacrificing the desalination performance. The features and advances include simple process design, minimal and reversible fouling due to low or no hydraulic pressure application, as well as energy recovery and reduction in energy consumption [101]. Albeit pressure driven membrane processes are expected to maintain their leadership in the near future, it is anticipated that emerging technologies such as FO, PRO, MD and MCDI will soon find their values and places in the market and industries. This section provides a brief overview on the features and performance of the evolving FO, PRO, MD and MCDI processes by comparing and contrasting with their counterpart RO process. 3.1. Forward osmosis Research and development of FO have been actively carried out to provide an alternative solution to the existing water problems through desalination, power generation, food processing and wastewater treatment. The exponentially increasing number of research papers and comprehensive reviews published over the last few years has indicated the increasing level of academic interest in this ODMP [14,102–108]. Recently, the involvement of a few number of commercial companies, such as Oasys Water Inc. and Hydration Technology Innovations (HTI) Inc. through the investment of significant funding has implied the eagerness of industry to exploit the technology to a greater extend. The reason for the looming of FO in widespread application is primarily attributed to its advantages over the conventionally available processes. According to thermodynamic principles, FO process can occur spontaneously, hence it requires minimal energy input to achieve the separation. As FO does not operate with hydraulic pressure but with osmotic pressure difference, when compared to traditional pressure-driven RO membrane processes, FO has less membrane fouling, scaling, and brine discharge. Moreover, the greenhouse gas emissions are considerably less in compared to that of thermal techniques. Also, FO has the prospect to effectively reduce the amount of rejected brine when high osmotic pressure gradient is sufficiently maintained across the membrane to help attain reasonable water flux and water recovery, hence potentially reduces the environmental burden of brine disposal as what typically is faced by many desalination plants, particularly for

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offshore desalination [109]. Another astonishing element of FO is that when combined with industrial or power production processes that produce waste heat, the electricity requirement is remarkably less than for RO. Worth mentioning that, while RO membranes do not necessarily provide good rejection for boron, which is usually in the form of boric acid in the normal operating pH ranges, the re-circulating draw solution dispenses an opportunity to improve boron rejection which has long presented a challenge to the membrane industry. In brief, FO process, similar to RO, requires a selectively permeable membrane separating two fluids with different osmotic pressures to participate in two major steps, i.e. the osmotic dilution of the draw solution and the generation of fresh water from the diluted draw solution [109]. However, unlike those pressure-driven membrane processes, FO only requires minimal external energy input for liquid circulation. As such, a desalination plant operating on FO technology can be constructed at 90% of the construction cost and operated at 80% of the operation cost of an RO plant [110]. Nevertheless, when reconcentration process for water recovery and draw solution reuse is concerned, an energy input is needed for the process [111,112]. In order to ensure the

operation of FO plant on a continuous and economic basis, several substantial conditions must be fulfilled, i.e. i) appropriate selection of the draw solution; ii) control of a constant osmotic pressure for the concentrated draw solution; iii) prevention of contamination of the draw solution with salts from the feed solution and back diffusion of the draw solution to the feed solution and iv) robust FO membrane that can adequately deal with flow on both sides. While the operating conditions seem trivial to be solved, the properties hence capability of membrane and draw solution are the two key factors that play crucial role in dictating the efficiency of the FO performance. Extensive efforts have been directed to develop high performing FO membranes which can offer better flux performance and also draw solutions with desired properties in all aspects for FO applications [113]. With the availability of desired membranes and draw solutions that are economically and technically viable, it is expected that the construction and operating cost of FO plants can be greatly reduced [110]. FO membranes are normally characterized with a thin rejection layer and a support layer with high porosity and low tortuosity. Currently, the CA and TFC FO membranes produced commercially are specially

Fig. 4. Schematic illustration of a) FO, b) PRO c) MD and d) MCDI (a–c: internet sources, d: [163]).

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Fig. 4 (continued).

designed to mitigate internal concentration polarization (ICP) by using a much more open substrate than their RO counterpart [114,115]. During the fabrication of FO membranes, the polymer dope and membrane formation conditions are manipulated to obtain finger-like macrovoid structure so that the impact of ICP can be greatly reduced [116,35, 117].Similar to the RO counterpart, FO membranes can be moduled into different common geometries such as spiral wound, flat sheet, tubular or hollow fibre. Due to the absence of hydraulic pressure, FO is less susceptible to severe fouling as the deposited layer is less compacted. Unlike fouling in RO membranes, most of the organic and inorganic foulants found on the FO membranes can be easily removed by osmotic backwashing, hence the need for any chemical reagents for cleaning is completely eliminated [118]. More than 98% water flux recovery could be obtained after water rinsing which is much higher than that commonly observed in RO [6]. While research is devoted to developing membranes feasible for FO, the search for suitable draw solutions is also of great importance. In general, the draw solutes can be broadly categorized into organic-based, inorganic-based and some emerging draw solutions such as micellar solution, magnetic nanoparticles (MNPs) and RO brines [109,119–122]. Typically, the primary challenges associated with draw solutions are the availability of a suitable solution that is capable to provide strong driving force for mass transport and the minimum energy consumption involved in the

reconcentration of the draw solution for continuous FO operation [6]. Precisely, a promising draw solution should exhibit high osmotic pressure with minimum reverse diffusion and can be easily and economically reconcentrated and recovered. Most importantly the draw solution must be of zero toxicity and should not degrade the membranes or cause scaling or fouling on the membrane surface [123]. The sweet spots of FO process are unavoidably accompanied with some practical challenges. One of the disadvantages of FO is a lower quantity of freshwater per unit of water treated. The commercial polymeric membranes applied for pressure driven RO or NF system cannot be suitably used for ODMP due to the presence of severe ICP within the support layer. The new generation of FO membranes has overcome this problem by reducing the thickness of the membrane support layer which is thinner than conventional RO membrane [124–126]. As a result, FO membrane is not tailored to tolerate high feed pressures. Concentration polarization on both sides of the asymmetric FO membrane, i.e. ICP occurs within the membrane support layer and external concentration polarization (ECP) exists at the membrane active layer surface, should be responsible to the significant decrease of the effective transmembrane osmotic pressure which in turn serves as one of the major factors that contribute to deteriorating water flux and recovery across the membranes [127]. A debate has been raging recently where the low energy and low cost feature of FO are seemingly too good to be

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true as significant amount of energy may still be required to regenerate the draw solution. As a result, for seawater desalination, the energy requirement of a standalone FO process is much higher than that of RO [109]. Due to the concentration gradient nature of FO, the occurrence of reverse solute flux across the membrane from the draw to the feed solution is almost unavoidable. Recent researches have unfavourably pointed out that the reverse solution diffusion of some multivalent ions may expedite membrane fouling by interfering with the fouling agents in the feed [128]. Furthermore, the lower solution diffusion coefficients and larger ion sizes of multivalent ions also resulted in severe ICP. Despite the limiting factors mentioned, in overall, the merits of FO still outweigh its disadvantages. Therefore, if efficiently used, FO can prove to be an efficient way of desalinating seawater [110]. Besides that, FO hybrid systems can be used for the desalination of highsalinity waters, which is not possible when using standalone RO process [107]. 3.2. Pressure retarded osmosis PRO is an ODMP which possesses a great potential to provide sustainable solutions for the burning needs of both clean water and green energy. The osmotic power harvested by PRO represents a new class of emerging renewable energy that is less periodic than other renewable sources like wind and solar energy [129,130]. In principle, PRO is a method of producing renewable energy from the mixture of two streams with different salinity. Typically, a simple PRO system takes in surface water and seawater on either side of the membrane in which permeate from a dilute feed stream enters a concentrated draw stream in a pressurized state via osmosis. Through the pressure of drawpermeate mixture, osmotic energy is converted into hydraulic energy and the pressurized water which permeates through a PRO membrane is used to drive a hydro-turbine for useful power extraction [131,132]. While exhibiting similar physical principles and process components as FO, the key contribution of the unique PRO process is to facilitate conversion of osmotic pressure difference to hydraulic pressure in order to generate electricity when the hydraulic pressure is released through a turbine or other devices. As it ought to be, the main application of PRO is to generate power, rather than to perform separation [6]. The world's first prototype osmotic power plant equipped with 2000 m2 of membranes with marginal power output capacity between 2 kW and 4 kW was opened by Statkraft in Norway. This potential source of energy is capable to generate approximately 2000 TWh per year of electric power to cater for approximately 10% of the current world energy demand [133].The power company claims that the PRO energy cost can be further brought down to an attractive level as the osmotic power plant can be designed to operate at full capacity almost continuously. Hence this technology can be cost competitive with other renewable power sources such as wind power and biomass [134]. One of the profound challenges to meet the technical and economic feasibility of PRO is the development of a PRO-specific membrane which is yet to be commercially available. Structurally, PRO membrane is similar to that of FO membrane in which the porous support layer in the PRO membrane is thinner than that of the conventional RO membrane. Additionally, excellent mechanical strength is one of the essential factor to be considered in fabrication of the PRO membrane to withstand the applied hydraulic pressure on the draw solution side of the membrane [75,135]. It has been reported that the pressure applied can unfavourably initiate membrane deformation thus lead to severe salt leakage and reduce the power density [136]. As mentioned earlier, the FO membrane is less prone to fouling on the feed side and even if it occurs, it can be easily tackled with simple hydrodynamic cleaning [137–139]. Whereas in PRO, as the membrane active layer is placed against draw water, it was demonstrated that organic and inorganic species in the feed water can easily deposit within the porous support layer where hydrodynamic shear induced by cross-flow is absent. This phenomenon is detrimental to the efficiency of PRO membrane

processes [136,140,141]. In recent years, the development of osmotic membranes and membrane modules which can produce higher power densities with PRO processes has been extensively investigated [114, 142–145]. So far, TFC membranes post-treated with ethanol or TFC membrane subjected to NaOCl exposure [142] and solvent immersion [146,147] have exhibited significant improvement in the water permeability. Besides, in order to avoid heterogeneous compaction and uneven deformation under a high pressure, membranes with sponge-like structure are more favourable for PRO process for the advantages of strong mechanical strength. Due to the large unsupported distance over feed spacers, flat sheet PRO membranes tend to experience uncontrollable mechanical deformation [136]. Hence, attention has been switched to the fabrication of hollow fiber membranes using polymer with strong mechanical properties to enhance membrane mechanical stability [148]. A recent study evidenced that an impressive power density of 20.9 W/m2 can be obtained by polyetherimide (PEI) hollow fiber membranes when 1 mM NaCl and 1 M NaCl solution were used as feed and draw solution, respectively [148]. 3.3. Membrane distillation MD, a third generation desalination technique, is a hybrid thermalmembrane desalination process that utilises low-grade waste heat or renewable energy and hydrophobic membrane to produce high quality distillate [149].The main benefit of MD lies in the ability to operate at lower hydrostatic pressure than conventional pressure-driven membrane processes and a lower operating feed temperature compared to conventional distillation [150]. As MD process can take place at normal pressure and temperature lower than the boiling point of water, it could be versatilely used to solve various wastewater problems. Over the last decade, concerns on poor removal efficiency of small molecule contaminants and heavy metal ions have urged the research community and industries to seek for promising alternatives [151]. MD stands out as a feasible option because it can provide higher removal efficiency for these contaminants. For instance, MD demonstrated its excellent capability in the removal of boron contaminants with a rejection 99.8% and almost complete rejections of heavy metals such as arsenic, chromium or gold [151]. MD also emerges as a potential element to address the issue of disposal of produced brine and extensive energy consumption, which are the inherent drawbacks of conservative RO process, for the sustainable growth of desalination technology [152]. Another strength of this technology is that the high solute concentrations can be achieved and ultrapure water can be produced in a single step. The world's first seawater desalination plant based on MD was operated by Aquever in 2014 at Maldives. The plant has a capacity of 10,000 l of drinking water per day and utilizes waste heat of a local power generator. Several research projects also focused on the coupling of MD with solar energy for seawater desalination and provided new insights into the large potential for applications such as stand-alone desalination systems from remote sites [153,154]. Due to the independency of trans-membrane flux from feed concentration, MD holds the advantage of overcoming the feed concentration limit of around 70,000 mg/L in conventional RO plant, thus enables MD to serve as a compact low pressure operation for further recovery of RO brines to reduce the significant cost associated to high discharge volumes particularly in inland groundwater desalination system [155]. Recently, computational fluid dynamics simulations have been widely applied to study the transport phenomena which include the momentum, heat and mass transfer in conventional and newly designed MD modules in order to improve MD performance [156]. MD is a breakthrough technology which combines membrane separation and distillation, resulting in pure water production from seawater using low-grade source of energy such as solar and geo-thermal energy for a cost efficient, energy efficient liquid separation system. [152,157]. Principally, MD is a thermally driven membrane process which involves transport of water vapor through microporous hydrophobic membrane

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where the driving force of MD is supplied by the vapor pressure difference generated by temperature gradient imposed between the liquid/ vapor interfaces [150]. MD processes can be broadly classified into four configurations, i.e. direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), vacuum membrane distillation (VMD) and sweep gas membrane distillation (SGMD), depending on the methods to induce vapor pressure gradient across the membrane and to collect the transported vapors from the permeate side. Out of the four configurations, DCMDis the most favourable mode due to its inherent simplicity without the need of external condensers [158]. Previously, the relatively slow progress and constraint in the commercialization of MD were related to i)the lack of commercially available high performance membranes ii) high energy consumption with respect to RO and iii) limited investigations carried out on module designing [151]. However, owing to the recent and growing extensive research activities carried out in various areas of MD, particularly the growing availability of commercial hydrophobic membranes, the process has become much more attractive due to the availability of better membranes and the possibility to reap alternative energy sources [158]. Lately, the evaluation of the practicability of MD seawater desalination has been extended to pilot scale which focused on the harnessing of solar energy either by photovoltaic panels or solar collectors with heat storage mediums [149,159,160]. In recent years, the MD research attention has gone into preparing customized membranes for MD applications. Some approaches applied for the current and perspective trends in membrane fabrication for MD applications have been comprehensively reviewed [158]. The ultimate performance of MD is largely associated to the membrane properties. In general, membranes used in MD process should have low resistance against mass transfer and low thermal conductivity to avoid heat losses. Typically, porous hydrophobic membrane that is not wetted by the liquid transport but only allow the transportation of vapour is of particular interests. Membrane materials, membrane thickness and porosity are some of the parameters that need to be carefully optimized to ensure excellent features for MD applications. The phenomenon of flux deterioration in MD is common in long-term operation and transmembrane flux decline as a consequence of fouling and temperature polarization when the system is operated under high convection flux [157]. As MD involves different transport phenomena, the nature of fouling in MD is different from other pressure-driven membrane processes [158]. Furthermore, the hydrophobic nature of the membranes has also intensified the susceptibility of MD towards organic fouling. While most of the conventional techniques applied for fouling and concentration polarization reduction in pressure driven membrane processes are inappropriate for MD process due to high energy consumption, several hydraulic techniques have been established to reduce fouling and thermal polarization in MD. Some interesting and promising techniques include induction of secondary flow, pulsating and intermittent flows and air sparging [161]. However, one of the exasperating impacts is the additional operational and capital cost associated with the equipment used. 3.4. Membrane capacitive deionization CDI is a long existing desalination technology which has never been made in industrial deployment due to deficiencies in cost and salinity limits, despite its less energy intensive compared to RO and thermal desalination. In brief, the salt solution is allowed to flow through the MCD module which consists of numerous pairs of high-surface-area electrodes with high specific surface area (400–1100 m2/g) and a very low electrical resistivity (less than 40 mΩ cm) [99]. The polarization of each electrode pair is then initiating the electrosorption of anions and cations in solution and later on, the saturated electrodes are regenerated by desorption of the adsorbed ions under zero electrical potential or reverse electric field. On the other hand, as the name implies, MCDI is a modified system obtained by inserting two ion exchange membranes

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into the conventionally used CDI to tackle the shortages of CDI in several ways [99,162]. In principle, MCDI relies on the applications of an electrical potential difference between the two oppositely placed porous electrodes where in front of which ion-exchange membranes are placed. Operating MCDI at constant current mode can produce freshwater with a stable effluent concentration. While the conventional CDI suffers a major problem during the regeneration step where the residual ions left from the incomplete regeneration accumulated and blocked the way of other ions during the next purification step, the ion exchange membranes introduced in MCDI system can assist in preventing adsorption during the regeneration process [163]. Furthermore, these ion exchange membranes also capable to keep the co-ions in the interparticle pores of the electrodes which in turn enhance the salt adsorption efficiency. Most impressively, the required energy input of MCDI is lower than that of CDI as well as very energy efficient compared to its RO and MD counterparts, particularly for salt removal from streams where the ion concentration is already relatively low [164]. Like other desalination technologies, the performance of MCDI greatly depends on the optimisation of the material and process. A proper selection and optimization of ion-exchange membranes with high ion selectivity and good chemical durability is crucial to further enhance the ion exchange capacity and electrical conductivity to render low resistance [165]. Additionally, research have also been dedicated to the development of electrode materials with improved charge and discharge kinetics. The electric capacity electrodes must be increased and the proper sorption mechanism should be selected in order to increase the storage of ions in the electrodes and to improve the rate and efficiency of salt removal. Recently, MCDI device using CNT electrodes is of favours to minimize ion desorption during electrosorption. With the right combinations of enhanced materials and optimized system operation, the salt removal capacity of MCDI was found to be 50% higher than that of CDI system [166]. 3.5. Integrated membrane system There has been growing emphasis on membrane-based seawater desalination technology, particularly integrated membrane system (IMS) that combines multiple classes of desalination systems with different beneficial characteristics [167]. The IMS is a breakthrough solution that is win-win as it can achieve consistently more than the individual system could. The interconnections of the systems that complement each other can provide a route to minimize the limitations and barriers that restrict their widespread applications. In a broad context, IMS is a promising way of improving the overall membrane operation by lowering membrane fouling rates, energy consumption, and operating cost, hence resulting in a more stable and reliable desalination operation. Table 2 summarizes some of the advantages of the IMS. A hybrid system which consists of FO followed by conventional RO has recently been investigated [108,138,168]. In this system, FO can be effectively utilized as an RO pre-treatment process, where the retained brine from RO is re-circulated to the FO modules and then used as a draw solution [98,169]. When the FO unit is installed prior to the RO desalination plant, the seawater is used as the draw solution and is diluted before entering the RO system. Low hydraulic pressure can be applied to the subsequent RO as a driving force to produce fresh water from the diluted seawater, hence significantly lowering the energy required for the RO desalination. Furthermore, the fouling potential in the RO membrane can be also dramatically lowered with the presence of FO as the pretreatment unit. As no or only low hydraulic pressure is applied, FO exhibits good fouling reversibility for which a pilot study has demonstrated that FO membrane fouling in a pilot-scale FO–RO hybrid system was largely reversible after 1300 h operation [138,170].Through the implementation of FO in the commercial pressure driven RO system, the desalination plant could exhibit low fouling tendency, improved recovery factor and reduction in the use of chemicals at the pre-treatment stage [98,171]. Since the feed solutions for the FO process can be diverse

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Table 2 The features of various hybrid modes for desalination. Hybrid mode

Features/advantage

FO–RO

• • • • • • • • • •

FO–PRO FO–MD

PRO–RO

FO as the pretreatment of RO Lower applied pressure for subsequent RO Lower RO fouling Reduce the concentration of saline waste Power generation Draw solution generation Increase FO flux Allow robust feed condition PRO as the pretreatment of RO Reduce the concentration of saline waste

and high quality product water can be obtained, FO–RO hybrids system can be promisingly used along coastal locations. A dual-stage FO–PRO process for hypersaline solution treatment and power generation has also been proposed [172].The treatment process can significantly reduce the concentration of saline wastewater to allow direct disposal to sea. The hybrid system can not only reduce the TDS of the hypersaline solution but also generate a useful power from the osmotic pressure gradient across the FO membrane through the PRO unit. To expand and fully harvest the advantages of MD process, new MD applications and hybrid systems have be explored. Incorporation of MD into the desalination process can dramatically reduce brine discharge and therefore enhance water recovery. When coupled with other desalination processes such as RO, MD can increase the recovery factor and enhance the overall process efficiency by treating the RO brine [173,174]. Meanwhile, as mentioned earlier, a sustainable FO system must be equipped with second separation process to generate draw solute and produce clean water. An integrated FO–MD system as shown in Fig. 5 has been attempted to achieve this purpose [175].As a process driven by extensive thermal energy, the FO–MD process is of great interest when solar energy or waste heat is abundantly available. Since MD is usually operated at high temperatures with the aid of waste heat or solar panels, the draw solution can be recycled at higher temperature compared to stand-alone FO unit, hence increases the FO flux. Owing to the low susceptibility of FO process towards fouling, the hybrid FO–MD process can be sustainable under robust feed conditions [151]. In a typical FO–MD process, the FO process serves to draw clean water from the feed solution to the draw solution side, meanwhile the MD process is utilized to reconcentrate the diluted draw solution [176].The FO–MD desalination process investigated by Wang et al. using hydroacid as draw solution demonstrated the highest seawater desalination flux of 6 LMH and 32 LMH for FO and MD, respectively [175].

PRO can effectively function as a supplementary technology to improve the performance and sustainability of conventional desalination processes. PRO hybridization with desalination technologies, especially FO and RO, was found to be very promising to simultaneously ease water scarcity and energy stresses. Currently, there is an increasing trend of coupling PRO process with RO process to reduce the cost of desalination while enhancing the power generation in PRO [130,177–179]. This target can be achieved through either diluting the seawater with PRO permeates or utilise the concentrated brine of RO as a draw solution of PRO [75]. Furthermore, the PRO dilutes the concentration of the brine and eventually reduces environmental impact of brine discharge to sea. As such, the integrated system has the potential to reduce the cost of seawater desalination as well as the cost of post-treatment for brine disposal [180]. Although the hybrid system requires more membrane area, overall the system can still be beneficial to reduce the total desalinated water cost as the energy cost is much more than the membrane cost [181]. However, it should be noted that the advantages of PRO–RO hybrid system for practical application can only be realized if an optimized low cost fouling control strategy such as anti-scaling pretreatment is employed and high fouling resistant PRO membrane is developed [182]. RO–PRO hybrid configurations can be varied in terms of the sequence of each system and the influent concentration [179]. In brief, when the RO process is located prior to the PRO, the direct use of low-saline water as a feed solution of PRO is less advantageous for the power production of PRO since the concentrated brine of the RO significantly reduces the driving force for the power generation of PRO. However, the effective brine management can be achieved when the concentrated brine of the RO is used for a draw solution of the PRO plant where PRO process is configured to dilute the brine concentration. Furthermore, the concentrated brine of RO can improve the ability of a draw solution to draw water from the feed channel, which is beneficial for the PRO power generation. On the other hand, when the RO process goes after PRO process, the PRO process can act as a pre-treatment of the RO process. 4. Challenges and future outlook With water shortage issue that is lingering around the world, communities are turning to desalination as the ultimate strategy for reliable water supply. Membrane technologies are playing a growing role in meeting water supply and water treatment needs for municipalities and industry. The various techniques both as stand-alone units and in integration can address the different water qualities. Moreover, the possibility of operating plants with different capacity together with centralized and decentralized systems according to the specific requirements in the given area makes membrane technologies an interesting answer for water issues [98]. As the cost and energy consumption for desalination

Fig. 5. An integrated FO–MD system which allows the FO process serves to draw clean water from the feed solution to the draw solution side, meanwhile the MD process is utilized to reconcentrate the diluted draw solution [175].

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are important factors to minimise the environmental impact of desalinated fresh water supply especially in remote areas where little options are available, the extent to which the role of membrane technology further expands depends largely on their cost-effectiveness. Other than the desalination performance and efficiency, the energy use, concentrate disposal options and environmental and health concerns are among the top issues dictating the technology's adoption. The focus of the membrane research community is geared toward designing and creating a reliable membrane that is very suitable for desalination in which it can continue to function in all commercial environments to handle the hiking expectations of the desalination industries. While nanotechnology is turning into a mature technology, the emerging nanomaterials are directly participating in the water treatment field by providing rapid and effective removal of contaminants. The convergence between nanochemistry and membrane science will likely yield a new generation of active membranes endowed with different attractive functionalities. A membrane that possesses all desired properties to achieve complete rejection and improve the flux in multiple folds would represent an attractive proposition to industry. However, one of the biggest constraints to transform the advanced functionalities of these novel membranes into industrial realization is associated to their scaling-up into designed products that can be economically mass produced [183]. The investors should recognize that it is not a one-off exercise to develop an economically viable manufacturing platform that is capable of delivering a product with consistent quality to the marketplace. The findings and achievement at fundamental level provide the basic framework to discover the true innovative potential of the materials. Several challenges still need to be addressed to optimize the design of the advanced membranes for practical applications. Despite their astonishing properties in ideal conditions, most of the nano-enabled membranes suffer greatly from some inherent limitations such as poor solubility and processability, which hinders their commercialization for industrial applications [87]. Hence, extra efforts should be endeavoured to resolve the issues. Also, the fundamental understandings must first be developed to systematically evaluate the effects of nanomaterials on the membrane properties such as surface hydrophilicity, pore size, charge density and membrane porosity so that they can correspond effectively to the membrane performance changes [4]. For example, since the selectivity across the active layer of TFN membranes is directly related to the availability of free volumes [65], the interactions between the incorporated nanomaterial and the polymer matrix must be well comprehended in order to design a desalination membrane that can enhance the liquid permeation while retaining the salt diffusion across the membranes. Although the membrane surface properties can be improved by modifications, either through direct incorporation of the materials within the polymer matrix such as nanocomposite membranes or through the attachment of materials with surface functionalities on the membrane surface, several underlying issues should still be taken into consideration. The first issue is related to the direct contact of the modified membranes with contaminants during the separation process, since membrane fouling can only be minimized but cannot be completely avoided. The next issue is linked to the potential release of nanomaterials from the nanocomposite membrane, which would raise health and safety concerns [7]. In this concern, it is of high priority to ensure that nanotechnology is incorporated into desalination technology in a safe, responsible and sustainable manner. As such, an informed assessment of the potential human health and environmental impacts upon the contact of nanomaterials with water body must be carefully conducted. In spite of their capability to offer several advantages, another obvious issue is the high production cost associated to complicity of nanomaterial synthesis, which has also prompted the search for more economical yet effective option [28]. It is also worth noting that the development of novel membranes is mainly based on the studies performed from the perspective of materials science, with focuses placed on the physicochemical and morphological characteristics of the

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newly designed and synthesized materials. Prior to their commercialization, still insufficient attention has been devoted to the complexity and interactions of the membrane materials in water body. Additionally, through the progresses made in molecular simulation, the outstanding features of graphene for the development of novel desalination membranes have been well acknowledged. The unique properties such as high tensile strength as well as controlled pore and porosity open the doors to construct extremely thin membrane with size tunable pores to facilitate high water passage rate. Besides that the ability to manipulate and fully exploit the properties of some of the discussed nanomaterials, such as aquaporin and graphene nanosheets remain limited hence further innovations and revolutionary approaches are highly required to develop efficient synthesis and up-scaling method as well as to enhance reproducibility. The positive impacts of these nanomaterials in water treatment are likely to be based on the ability to conceive of and build devices that actively function at the scale of the material itself. Such devices might combine functions such as energy harvesting and treatment, detection or control [184]. Presently, the assessment on the relationships among the properties of nanomaterials, the characteristics of the resultant nanocomposite membranes and the membrane system and operations are still inadequate to provide further insights on the practicability of the materials for commercial desalination processes. Currently, only nanocomposite membranes have been commercialized while others are still under fundamental developmental stages [99].When the cost of energy and membrane is considered, the benefit of developing high performance membranes with higher permeability and fouling resistance is insufficient to be main driver for lowering seawater desalination costs. Thus, other factors such as process improvements which include lesser pretreatment and effective brine management need to be considered to lower the overall desalination costs [99]. With increasing number of plants built in water-stressed countries to complement existing water resources, RO is unquestionably the most ubiquitous technology for desalination. The emergence of these new and advanced technologies which are still in the laboratory phase implies that despite the maturity and reliability of the existing commercial processes for desalination, the advancement in material sciences and engineering still strive to diversify the options for desalination. Currently, ODMPs, MD and MCDI may not be the main stream in their realms, but they are increasingly becoming a topic of some interest and the attention devoted to the related research apparently hikes at a steady pace. Through the advancement made in all aspects, the emerging desalination technologies offer unprecedented desalination performance that outperforms the conventional technologies. Owing to their interesting features, it is clear that ODMPs can pose an impressive array of potential benefits not only in the laboratories, but also when they are up-scaled and deployed in the real field. The low hydraulic pressure needed during the process is beneficial to render low energy consumption, which eventually ensures process costs cut down. While the security of global energy supplies continues to be problematic, the striking points of FO seem in place to offer alternative solutions. Meanwhile, a properly designed MD process features unique benefit to treat highly saline brines that cannot be treated by conventional desalination processes and makes it an ideal candidate to desalinate concentrated brines from thermal desalination plants to augment fresh water production from existing facilities. Albeit the growing numbers of studies are conducted, the applications are still explored at a laboratory scale. For instance, the current research of FO is mainly directed to enhance the intrinsic transport properties of membranes on a molecular level [161].The implications of these improvements on the energy efficiency of different FO desalination processes remain unexplored. Further detailed investigations and conceptual proofs are highly needed to promote FO as a mainstream treatment process [6]. Additionally, despite the similar osmotic concept that lies behind RO, FO and PRO, development of versatile membrane materials and structure is still a great challenge. Although the osmotic power technology has been growing

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rapidly, the generation and utilization of osmotic power still remain several years away from commercial viability. For example, although the potential of PRO technology has prospered. Recent studies have also pointed out that the power density should be at least higher than 5 W/m2 to make the system economically feasible [114]. The goal to attain high performance membrane with high water permeability and salt selectivity, which are coupled with customized support layer that suppresses the detrimental effect of ICP, can be reached with the breakthrough in the material sciences and engineering to make a big leap for the production of membranes that will be useful for commercial osmotic power production. In terms of IMS, the positive and encouraging findings open up the prospective of employing hybrid systems that integrate FO, PRO, MD and conventional pressure driven processes for seawater/brackish desalination. Nevertheless, as the development of RO–PRO or FO–RO hybrid processes is still in their infancy stage, more research and real field data are important to ensure the viability of this hybrid system before it marches into the industry large scale operation. Above all, in order to foresee the economic niche of these emerging technologies, a reliable cost analysis is crucial to assess the efficiency of the various hybrid system configurations by considering all key parameters and factors such as energy production/consumption, water production, capital construction costs, labor costs, and maintenance costs. A holistic analysis that covers not only the thermodynamic comparison, but also details such as pressure fluctuation and pretreatment and solution recovery can reflect the practical but not the idealised conditions of the system, hence can provide further understanding on how they will fit within the desalination industry. Despite the impressive potential and effectives of these emerging membrane desalination processes, membrane fouling still exists as the most detrimental hiccup of membrane-based desalination technologies. In view of the adverse consequences of membrane fouling to the overall performance and practicability of the processes, the fouling mechanisms and the control strategies must be carefully studied to mitigate the persistent issue. Similar to their RO counterpart, these membrane processes are inevitably susceptible to macromolecules organic fouling, inorganic scaling and biofouling. However, due to the unique features with respect to their working principle and applied hydraulic pressure among these desalination processes, the membrane fouling behaves differently in some cases. Therefore, in depth research and evaluation must be performed in accordance to their working principle and operating conditions. From an industrial perspective, the challenges of a sustainable water future present a high potential for growth, as well as business opportunities in sectors related to water. Although it may not be immediately apparent that these new membrane and membrane systems are beneficial to invest in, the deterioration of competency and reliability of the existing desalination technologies seemingly far outweighs the cost of investment. However, the adoption of new technologies strongly depends on the cost effectiveness. Therefore, it is important to implement the technologies with minimal changes to the existing infrastructure. In this context, retrofitting and upgrading of the current desalination plants seems to be a more promising approach. Incorporating innovative and sustainable materials and processes into currently existing facilities can seemingly treat the challenging water needs at only a fraction of the cost of expanding or building more expensive water treatment plants. The feasibility and versatility of emerging technologies have brought about huge leap forward both in the industrial world and in society. Leading edge membrane technology has advanced to such an extent that the emerging separation systems can operate extremely reliably through more efficient and cost-effective techniques to produce fresh water of high quality.

technologies based on membrane and membrane system are the answers to this global issue. In fact, we have seen an increase in technology breakthroughs and investments in the past 10 years where the advancements made in membrane and membrane system are aimed at alleviating stressed water supplies, as well as minimizing overall consumption, energy use and costs associated with producing and recycling water. The interdisciplinary research offers leapfrogging opportunities to develop next-generation membranes for water supply system. As highlighted in this review, the intrinsic properties of desalination membranes can be drastically altered through the incorporation of nanomaterials or membrane surface modifications where some of the drawbacks of current desalination techniques could be avoided by inventing more efficient and customized membrane materials. The introduction of nanomaterials during the membrane formation has been the subject of interest as the presence of the carefully selected nanomaterials with desired properties could render favourable changes in the membrane morphology and performance. The growing impact of advanced membranes in desalination has arisen from the new functionalities offered by the additive materials or surface functional groups which serve as the tools for providing the desired features. Surface modifications of polymeric membranes allow the tailored alteration of surface properties in terms of hydrophilicity, roughness and porosity, hence allow emergence of a new class of novel membranes to potentially combat some underlying issues of the commercial membranes. Fabrication of the desalination membrane with specific functional properties is scientifically promising and commercially profitable. However, one should bear in mind that, nanotechnology-based solutions in the water sector will find wide applications only when they are low cost, highly efficient, and able to provide clean drinking water in very remote regions. It is expected that substantial initial investment would be needed to incorporate or switch to nano-enhanced desalination. However, once adopted, these techniques could considerably lower water treatment costs over the long term. In this article, several advanced technologies for desalination namely FO, PRO and MD are reviewed. Some of these stand-alone or hybrid technologies have been commercialized or are close to commercialization but some are still at their early exploration stage. Inevitably, all emerging membrane desalination processes might be plagued by the potential challenges associated with limited experience on scale-up, process design and mode of operations as well as some pretreatment issues on seawater/brine sources. In order to advance the technical knowledge in these technologies, field studies must be deployed to evaluate their feasibility at a pilot scale. When we put aside the overwhelming momentum and expectations, on a longer time scale, key questions still remain as to the technology's long-term durability and performance in a full-scale operation. Would these novel membranes survive from the harsh conditions in the commercial environment? Could these emerging desalination technologies provide holistic solutions to the limitations of the existing pressure driven membrane processes? It is obvious that, clear protocols and assessment indexes that serve as the guideline for the selection of the most promising materials and operating conditions, accurate modelling for an easy scale-up and full mode operation as well as significant multidisciplinary research efforts are desired to contribute to the progresses of the technologies. To conclude, not all breakthroughs made in the field of desalination science and engineering can be certainly transformed into realities to break the existing boundaries. While some advances have been made to move from laboratory to industry in order to close the gaps between fundamental research and practical applications, it is anticipated that those which currently strive at the stage for innovations will surely emerge in the not-too-distant future.

5. Concluding remarks References Recognizing the plethora of alarming crisis associated with water scarcity, industry and academia are now seeking for more options to enhance water treatment efficiency. Undoubtedly, desalination

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