The methods of fabrication include techniques such as IP, solution mixing and polymer melt blending (Kim and Deng, 2011) of which IP has been the most common since its introduction in 2007 (Jeong et al., 2007). Generally an asymmetric membrane prepared through phase inversion is the support layer (Ghosh and Hoek, 2009). Polymers generally used to form the support layers are polysulfone (PSF), polyimide (PI), polyethersulfone (PES), polyacrylonitrile (PAN), and polypropylene (PP) (Ghosh and Hoek, 2009). Various techniques can be used to deposit a thin layer on a porous sub-layer one of these is IP. The traditional synthesis technique requires the dissolution of a difunctional amine in water and a trifunctional acid halide in an organic solvent. The solvent is usually chosen such that water and the solvent are immiscible. The support membrane (usually PSF) is initially immersed in the aqueous solution and, after saturation, is then immersed into the organic phase. Upon immersing in the organic phase, there is a polymerization reaction between the monomers to form the thin layer (Ghosh and Hoek, 2009). Alternatively, there is another method of IP known as the SIM method. In this method, the organic phase is poured over the membrane as it already has amines present on it due to phase inversion. This method is more efficient as there is more complete wetting, which results in a more homogenous, defect-free layer as compared to the traditional methods (Hermans et al., 2014).

TFCs have been a boon to membrane based technologies. However, they do have some limitations. These have been discussed in sub Section 3.2.

One of the biggest advantages of a TFC is that one can control and optimize each layer individually to improve its function. A high solvent flux can be obtained without compromising on the salt rejection. In addition, they are also much stronger and more stable (Petersen, 1993). However, in spite of the above advantages, TFCs have a few limitations which cannot be neglected.

One of the main drawbacks is its low resistance to chlorine (Lau et al., 2012). Chlorination of the main group present in the PA layer increases the hydrophobic nature of the membrane, which in turn decreases flux (Hermans et al., 2015). Chlorine is abundantly present in waters to be treated as chlorination is an important disinfection and pre-treatment step. Therefore, this is a significant performance inhibitor. TFCs are also very susceptible to fouling over time by microorganisms or organic compounds which leads to decline or deterioration of membrane performance. The process of de-fouling increases costs and energy consumption. In addition, high temperatures can also cause collapsing or compactness of pores in the membrane (Hermans et al., 2015).

To overcome these limitations, various methods were developed with the intent to improve the fouling resistance and chlorine resistance of TFCs thereby increasing their performance. This can primarily be done through modifications in the substrate. The following subsection discusses this aspect in details.

It is difficult to regulate the position of the filler in TFNs. It has been reported that when NaA nanoparticles are dissolved in the aqueous phase, they are more concentrated near the porous sublayer as compared to near the surface of thin film. On the other hand, when NaA nanoparticles were dissolved in the organic phase, they were found to be homogeneously distributed with equal concentrations near the surface and the sublayer (Huang et al., 2013b). It has been reported that the position of the fillers in the membrane can be tuned by selecting an appropriate type of nanoparticle.

Additive nanomaterials have been demonstrated to improve the performance of TFNs. This has been discussed in the following section.

Membranes can be enabled to adsorb heavy metals from water by the incorporation of NPs inside the polymer matrix. Daraei developed a method to remove copper from aqueous solutions by incorporating PANI/Fe₃O₄ NPs inside the PES matrix through PI method (Daraei et al., 2012). Elsewhere, Fe-Mn binary oxide (FMBO) is used to remove As (III)] (Jamshidi Gohari et al., 2013). These highlight the possibility of nanocomposite membranes being incorporated with adsorbents for removal of contaminants.

TiO₂ has been known for its photocatalytic properties and as a result has been used for applications such as water splitting, treatment of water and self-cleaning of surfaces. Its stability, ease of preparation and commercial availability add to its functionality (Mills and Le Hunte, 1997; Paz, 2010). Rahimpour found that UV-irradiated TiO₂/PES nanocomposite membranes had higher flux and improved fouling resistance compared to ordinary nanocomposite membranes, attributing the improvement to the photocatalysis and high hydrophilicity of TiO₂ under UV irradiation (Rahimpour et al., 2008).

Membrane biofouling, which is caused by microbial growth, has been a major challenge to membrane technology (Zhu et al., 2010). It increases energy costs, decreases permeability, and reduces permeate quality. Hence, developing antimicrobial membranes can result in significant enhancement in efficiency of the separation process. Silver (Ag), due to its impressive biocidal properties, is a highly explored antimicrobial agent and has proven applications in avenues such as antimicrobial coatings, plastics, and wound dressings (Liu et al., 2008; Lind et al., 2009b). Addition of Ag₂O nanoparticles not only acts as biocidal agent but also improves the desalination performance of the membrane (Al-Hobaib et al., 2015). Chou used Ag NPs in CA matrix and Zodrow incorporated Ag NPs into PSU matrix to improve biofouling resistance (Chou et al., 2005; Zodrow et al., 2009).Table 2 provides a summary of the variety of nanoparticles that are used in TFNs (Mills and Le Hunte, 1997; Chou et al., 2005; Liu et al., 2008; Rahimpour et al., 2008; Zodrow et al., 2009; Paz, 2010; Zhu et al., 2010; Daraei et al., 2012; Kim E. S. et al., 2013; Huang S. G. et al., 2013; Jamshidi Gohari et al., 2013; Duan et al., 2015a).The various performance parameters that are utilized in order to characterize TFNs have been discussed in detail in Section 6.

The measurement of water flux in thin film nanocomposite membranes was performed using a cross-flow membrane module. The water flux was calculated using Equation 1: F = V / ( At ) , (1) Where F is the pure water flux, V is total volume of permeated pure water, A is area of the membrane, and t is the operation time (Wu et al., 2010).

Nanoparticles in the thin film membrane can lead to an increase in hydrophilic nature and decrease in cross-linking of the membrane which consequently contributes to high flux of water through the membrane. Increased water flux can also depend upon the structure of the nanoparticle used. For example, MCM-41 silica nanoparticles are porous in nature (Yin et al., 2012). These pores inside the nano-particles present in the thin film membrane can act as short pathways for preferential passage of water molecules through the nanoparticle (Yin et al., 2012). Water tends to move faster through hydrophobic porous particles than hydrophilic non-porous particles (Duan et al., 2015b).The nanomaterials possess high surface porosity resulting in improved salt rejection capacity and reduced macro void formation.

Carbon nanotubes also facilitate high flux of water, as they act like channels for transportation of water (Safarpour et al., 2015). This reduces the water transport route as the water molecules can enter into part of the multiwalled carbon nanotubes (MWCNTs) instead of passing through the entire PA film. Aggregation of these nanoparticles might also lead to the formation of a network inter-connected with other pores in the membrane leading to a higher increase in water flux (Zhang et al., 2011).

For all the data reported, salt rejection is calculated using the formula: R =   1 − C p / C f , (2) Where R is salt rejection, C_p is concentration of salt in the permeate solution, and C_f is concentration of salt in the feed solution (Wu et al., 2010).

Salt rejection is governed by factors such as defects and molecular sieving. Also, it is known that for transport of ions of different valences through carbon channels with negatively charged functional groups, ion exclusion depends more upon electrostatic interactions (Donnan exclusion) rather than steric hindrance (Shen et al., 2013). It is also known that high crosslinking in the MPD-TMC (m-phenylene diamine—trimesoyl chloride) layer causes higher salt rejection and lowerNaClpermeance (Duan et al., 2015b).

As reported by Safarpour, increase in rGo/TiO₂ loading in thin film membrane leads to decrease in roughness of membrane (Safarpour et al., 2015). This is due to the increase in hydrogen bonding between the hydrophilic nanoparticles and polyamide layer. It was also observed that increase in nanoparticle loading causes decrease in contact angle with water, which is an indication of increasing hydrophilicity of membrane. This can be understood based on the increase of surface charge density. Another trend observed is how the pure water flux increases with the increase in nanoparticle loading. This is expected as an increase in hydrophilicity leads to increased flux (Safarpour et al., 2015).

Permeability and selectivity are two major performance factors for membrane technology. In improving the performance of membranes, there is always a trade-off between these two factors. In an attempt to achieve high levels of water flux, a decrease in salt rejection is obtained. For example, work done by Jun Yin shows a large trade-off between flux and salt rejection (Yin et al., 2012). Pure water flux increases with percentage loading of MWCNTs, from 20 LMH at zero loading of MWCNTs to 71 LMH at 0.1% w/v of MWCNTs. Salt rejection is observed to decrease with increase in loading of MWCNTs, from 94% at zero loading of MWCNTs to 82% at 0.1% w/v of MWCNTs.

The incorporation of nanomaterials could alter the physicochemical properties of the membrane such as cross linkage, charge density and hydrophilicity. This provides specific water pathways that could conquer the permeability-selectivity trade-off. These facts are evident by the advantage of polydopamine coating that provides ZIF-8 nanoparticles with dispersibility in water (You et al., 2021) and other recent works on TFN membranes (Jeon and Lee, 2020; Saleem and Zaidi, 2020; Siew Khoo et al., 2021). Such TFN membranes exhibited improved permeability in desalination.

Almost all studies that used hydrophilic nanofillers showed a decreased contact angle in the TFNs, proving improved surface hydrophilicity. With increase in zeolite loading from 0 to 0.4% (w/v) the contact angle of zeolite-PA TFN membrane has been seen to decrease from around 70^o to 40^o (Jeong et al., 2007). Other examples of oxidized MWNTs-PA TFN membranes and silica-PA TFN membranes have also displayed a significant reduction in contact angle with increase in loading up to certain specific values (Zhang et al., 2011; Yin et al., 2012). This indicates enhanced water permeability with an increase in nanofiller loading.

Two possible explanations have been proposed for the decrease in the contact angle due to the nanoparticles (NPs). The first ascribes the increased surface hydrophilicity to the hydrolysis of acyl chloride resulting in the generation of carboxylic acid functional groups. This phenomenon occurs due to increasedcount of surface acyl chloride groups in TMC that remain without reacting with the dimethyl pimelimidate’s (DMP) amine group as a result of NPs hydrating and releasing heat when in contact with the MPD aqueous solution (Kim et al., 2000; Ghosh et al., 2008). The second suggests that the presence of embedded hydrophilic NPs on the membrane surface offers larger number of hydrophilic functional groups to the surface.

Along with hydrophilicity, the thickness and the cross-linkage condition of the thin-film layer are key factors in determining the water permeability and selectivity (Ghosh et al., 2008). Generally, a lesserextent of cross-linking and thinner films offer higher permeability. Incorporation of NPs in the PA matrix could reduce cross-linking in the thin-film layer by disrupting the reaction between amine groups and acyl chloride groups. Lind showed through FTIR and XPS results that the cross lining in all their TFN membranes were less compared to the corresponding TFC membranes (Lind et al., 2010). However, it was put forward that molecular sieving or defects could have played an importantpart in the performance.

Furthermore, the inclusion of NPs may also produce additional channels for the flow of water while excluding the solutes. A higher value of water flux with constant salt rejection in zeolite-A NP TFNs was reported by Jeong(Jeong et al., 2007). The hydrophilic molecular sieving NPs may provide preferential flow paths for water molecules. Yin showed that mesoporous silica NPs with highly ordered hexagonal pores exhibit higher permeability compared to nonporous silica NPs (Yin et al., 2012). The nonporous silica NPs resulted in cross linking thereby increasing permeability compared to regular TFCs. The increase in permeability was even higher for the mesoporous silica NP TFNs.

It is desired to significantly improve permeability while maintaining the same salt rejection. However, in order to exploit the favorable properties of the NPs, it is necessary to optimize the internal structure, size and surface properties while ensuring suitable interfacial interactions with the polymer.

Extensive research has been conducted on incorporating nanoparticles that reduce the fouling of membranes, hence prolonging the duration for which the membrane can be used. It is known that the antifouling ability is related to the hydrophilic nature, negative charge and smoothness of the membrane (Shen et al., 2013). The anti-fouling performance of membranes can be evaluated by conducting filtration experiments. The membrane is first compacted using distilled water. This ensures that an almost constant permeate flux is obtained. This is necessary as the initial flux affects the extent of fouling. A protein solution, for example BSA (bovine serum albumin) that possesses synergistic fouling effects, is then taken in the reservoir in order to conduct the permeating experiment for a definite time period. The fouling capacity is measured based on the decline in the flux with time (Shen et al., 2013).

The incorporation of hydrophilic NPs into the PA structure has been shown to increase hydrophilicity of the surface and usually helps diminish surface fouling. Long term fouling tests with silica particles, chloroform, and sodium humate have shown TFN membranes to have a much lower flux reduction when compared to TFC membranes (Rana D. et al., 2011). Extensive research work has been conducted to investigate the antifouling ability of TFNas shown in Figure 6 (Safarpour et al., 2015; Siew Khoo et al., 2021). In order to compare the antifouling properties, water flux through a pristine RO membrane was compared to flux through a TFN with 0.02 % wt. loading of rGo/TiO₂ under the same applied pressure. The flux through the former dropped to 49% of its value whilst the flux through later decreased to 75% of its value after 180 min of filtration (Safarpour et al., 2015). Hence the presence of fillers in thin film membrane leads to better antifouling resistance. Similarly, recent reports show how the introduction of carboxyl-functionalized MWCNTs reduces fouling (Zhao et al., 2014). This has been attributed to the negative surface charge and greater surface hydrophilicity (Lau et al., 2015).

Polyamide (PA) based membranes do not have a good resistance to continuous exposure of oxidizing agents. Chlorine is widely used as disinfectant in water treatment (Park et al., 2008). However, chlorine is also a strong oxidizing agent and feed water coming to membrane units from disinfection plants contains chlorine. PA membranes cannot tolerate water having chlorine even in the order of few parts per billion, and the chlorine treatment is required to check the development of biofilms on the membrane. Removing the chlorine from water is an undesired option as this will increase the number of treatment steps, and the overall cost of water treatment. Hence there is a need to increase the resistance of membranes to chlorine (Lau et al., 2015). In order to study the chlorine resistance of a membrane, it is subjected to a high concentration of free chlorine for a short time, which is essentially analogous to the exposure of the membrane for a long time to free chlorine of low concentration (Shen et al., 2013). In order to conduct this study, chlorinate solution is prepared and, the water flux and salt rejection of the required membrane, before and after chlorination is measured and compared (Shen et al., 2013).

As reported by Safarpour et al. (2015), salt rejections before and after exposure to sodium hypochlorite for pristine membrane and TFN were compared to study the chlorine resistance (Safarpour et al., 2015). The bare membrane (without any modifications) showed a decrease in salt rejection from 95.4 to 65.38%, while the TFN membrane with 0.02% loading of rGo/TiO₂ showed decrease from 99.45 to 96.4% (Safarpour et al., 2015).

Exposure to chlorine can cause ring-chlorination and N-chlorination reactions which disrupt the symmetry of the PA layer converting it from a crystalline state to an amorphous one (Safarpour et al., 2015). This leads to larger free volumes and flexibility of the PA layer that allow salt molecules to pass through easily. On the other hand, the intermolecular hydrogen bonds are amplified by interaction between rGo/TiO₂ particles and membrane active layer (Safarpour et al., 2015). This can provide obstruction to the substitution of hydrogen with chlorine on the amide groups of the aromatic polyamide membranes resulting in an increase in the chlorine resistance of the membrane. TFNs having carboxyl-functionalized MWCNTs as fillers have also been known to show high chlorine resistance (Park J. et al., 2010). Interactions between the carboxyl group and amide layer is understood to be the reason causing this behavior. Table 4 attempts to summarize a majority of the TFNs reported in literature (Petersen, 1993; Mills and Le Hunte, 1997; Kim et al., 2000; Williams, 2003; Kim et al., 2004; Chou et al., 2005; Wang et al., 2005; Louie et al., 2006; Jeong et al., 2007; Lee et al., 2007; Ghosh et al., 2008; Lee et al., 2008; Liu et al., 2008; Park et al., 2008; Rahimpour et al., 2008; Singh and Aswal, 2008; Ghosh and Hoek, 2009; Jadav and Singh, 2009; Lind et al., 2009b; Zhou et al., 2009; Zodrow et al., 2009; Jadav et al., 2010; Li and Wang, 2010; Lind et al., 2010; Ma et al., 2010; Park K. T. et al., 2010; Park J. et al., 2010; Paz, 2010; Wu et al., 2010; Zhu et al., 2010; Rana D. et al., 2011; Rana T. M. D. et al., 2011; Roy et al., 2011; Kim and Deng, 2011; Kong et al., 2011; Pendergast and Hoek, 2011; Zhang et al., 2011; Baroña et al., 2012; Daraei et al., 2012; Kim et al., 2012; Lau et al., 2012; Ma et al., 2012; Saleh and Gupta, 2012; Yin et al., 2012; Zhao et al., 2012; Alam et al., 2013; Amini et al., 2013; Bao et al., 2013; Baroña et al., 2013; Chan et al., 2013; Daraei et al., 2013; de Lannoy et al., 2013; Hu et al., 2013; Huang et al., 2013a; Huang et al., 2013b; Jamshidi Gohari et al., 2013; Kim S. G. et al., 2013; Kim E. S. et al., 2013; Kim S. G. et al., 2013; Li and Wang, 2013; Lu et al., 2013; Li et al., 2014; Pendergast et al., 2013; Rajaeian et al., 2013; Shen et al., 2013; Wu et al., 2013b; You et al., 2013; Yu et al., 2013; Wu et al., 2013a; Hermans et al., 2014; Subramani et al., 2014; Zhao et al., 2014; Al-Hobaib et al., 2015; Duan et al., 2015a; Duan et al., 2015b; Ghanbari et al., 2015; Hermans et al., 2015; Lau et al., 2015; Safarpour et al., 2015; Shenvi et al., 2015; NanoH2O and Inc. InterNano, 2021). The arrows under the application column denote the performance of the TFN membrane as compared to the parent TFC membrane. For example, in the first row, the fouling resistance of the membrane with Oxidized MWNT nano-fillers increases when compared with the fouling resistance of the TFC membrane without the nano-fillers.

The following section discusses various desalination processes and the use of TFNs along with specific modifications to improve the desalination performance of the same in detail.

Energy demand is one of the most important considerations that need to be taken into account for desalination.RO consumes the least amount of energy among MED, MSF and RO (Wade, 1993; Wade, 2001). For RO membranes, energy is consumed in pumping water across the membranes while in MSF and MED energy is consumed in converting water to steam and for running pumps (Sagle and Freeman, 2004). Energy consumption is also affected by the salinity of the feed water (Ettouney et al., 2002). Higher concentration of salt in feed water leads to larger osmotic pressure. Larger osmotic pressure means a larger trans-membrane pressure needs to be applied which in turn means larger pressure needs to be applied by the pumps, leading to an increase in the energy consumption (Sagle and Freeman, 2004). The energy consumption of TFNs are improved compared to TFCs for the separation process requirement (Subramani et al., 2014).

While MED and MSF require thermal and electrical energy, RO requires only electrical energy. Assuming water production of 290,000 m³/day, average total energy consumption (ATEC) in MW (Mega Watt) for the three processes is graphically shown in Figure 7A.

Figure 7B allows us to compare the three desalination technologies: MED, MSF and RO, based on the energy consumption and the water cost.The energy cost calculations are based on a plant capacity of a volume 32,000 m³/day; and a total dissolved solids (TDS) concentration of 42,000 mg/L. The energy cost used is $0.053/kWh. The specific heat consumption (abbreviated as SHC in the figure) is expressed in kW_th h/m³ and the specific electricity consumption (abbreviated as SEC in the figure) is expressed in kW_e h/m³. The water cost (abbreviated as WC in the figure) calculations are based on a plant capacity of volume 31,822 m³/day and a TDS concentration of 37,000 mg/L. Energy costs are expressed in $1.5/m³ (Sagle and Freeman, 2004).

It can be seen that RO has the lowest cost per unit volume. RO membranes have a higher initial cost, but the end result is a higher production rate as compared to MED and MSF (Ettouney et al., 2002; Sagle and Freeman, 2004; Kesieme et al., 2013). In fact, it has been concluded by Kesieme that even with the inclusion of carbon tax in Australia, RO continues to be the most cost effective of the discussed desalination processes (Kesieme et al., 2013).

RO is the predominant technology used for desalination. Figure 8 shows the (global) cumulative desalination capacity trends (Zhao et al., 2020) and forecast up to 2030 (Shahzad et al., 2019).It is a common practice to improve parameters like salt rejection and permeate flux using various kinds of NPs. The following subsection discusses such modifications that have been reported in the recent times in detail.

It is possible to improve the desalination performance of TFNs by incorporating NPs like functionalized silica nanoparticles (SNs) into the membrane. SNs can be synthesized in various sizes and with differing surface functionalities like epoxy, amine and hydroxyl. The chemistry, surface hydrophilicity and morphology of TFNs are affected by the various factors like size, concentration and surface functionality of the used SNs resulting in an increase in the permeate flux without any drastic change in the salt rejection. Similarly, it is possible to incorporate various other kinds of NPs in order to improve the desalination performance of TFNs (Zargar et al., 2017).

A polyamide TFN has also been reported that has been incorporated with multiwalled carbon nanotubes-titania nanotubes hybrids (MWCNT-TN). The use of acid treated MWCNT-TN as a filler in the PA membrane results in improvement in the surface properties of the membrane (surface charge, contact angle and roughness) resulting in an increase in the water permeability with negligible change in the salt rejection (Wan Azelee et al., 2017).

ZnO nanostructures like nanorods (R-ZnO), nanoflowers (F-ZnO) and nanospheres (S-ZnO) have been shown to improve the hydrophilicity of the TFN membrane with increase in the ZnO loading. Amongst three nanostructures, S-ZnO has the largest surface area and smaller size. TFNs with S-ZnO incorporated in them were seen to possess the highest permeate flux with good salt rejection, out of three types of ZnO nanostructures (Rajakumaran et al., 2020).

Similarly, Na + functionalized carbon quantum dot (Na-CQD) incorporated into the PA layer, resulting in a TFN hollow fiber membrane has been shown to possess a much larger effective surface area, a larger number of hydrophilic O-containing groups in the PA layer and a less thickness of the PA layer. These results in a better permeate flux of water enabling use in the desalination of brackish water (Gai et al., 2019).

Another research work done on amino-phenyl modified mesoporous silica NP based TFNs reported a 21.6% increase in water permeability with a marginal decrease in salt rejection (0.29%) compared to pure PA membrane under optimum AMSN dosage of 0.25 g/L (Wang et al., 2019).

TFC membranes modified with Cu-Al layered double hydroxide nanofillers were found to be less negative than the usual TFC membranes and a substantial improvement in the anti-fouling properties was observed indicated by an improvement in water flux by 14% (Tajuddin et al., 2019).

Biopolymer based nanocomposite films, which consist of single dimensional palygorskite (PAL) nanorods and double dimensional montmorillonite (MMT) nanoplatelets in the sodium alginate (SA) film resulted in an enhancement in the tensile stress and the capacity of water resistance of the film (Huang et al., 2018).

All of these above examples showcase some of the numerous recently developed mechanisms to enhance the applicability of thin film nanocomposites in desalination applications by improving the various properties like water permeability and salt rejection of the membrane(Figure 9).However, TFNs continue to have some limitations, especially those concerning the adverse effects of desalination processes on the environment. Some of these concerns have been discussed in Section 7.4.

Thermal emissions, brine discharge and chemicals used in the process all contribute towards environmental pollution (Van der Bruggen and Vandecasteele, 2002). Thermal emissions are directly related to the electric power consumption in the process. Therefore, RO membranes show the least amount of thermal emissions (Van der Bruggen and Vandecasteele, 2002). The impact of brine discharge on the environment can be determined by the temperature and salinity of the waste stream (Van der Bruggen and Vandecasteele, 2002). Brine released at high temperatures in fresh water bodies can decrease the oxygen content of the water hence harming the microorganisms present in the water. Since RO process does not involve heating up the water like in MED or MSF, it causes less harm to the environment in this aspect. When evaluating the environmental impact, all chemical additives that have been added to the water have to be considered (Van der Bruggen and Vandecasteele, 2002). Chemicals that might be used to reduce fouling or scaling, enter fresh water bodies, causing water pollution (Van der Bruggen and Vandecasteele, 2002). Taking all factors into account, it can be seen that RO is the least destructive to surroundings (Sagle and Freeman, 2004).

However, as seen in the previous section, RO involves water wastage. In addition, the demineralized water obtained is unhealthy and the process requires energy input. RO also tends to make the water acidic and is unable to remove chlorine, chloramines, volatile organic compounds (VOCs) and pharmaceuticals. In fact, all conventional methods of water treatment have drawbacks. Ultrafiltration is incapable of removing dissolved inorganic components and involves high energy costs. In microfiltration, fouling is a very serious issue and requires regular cleaning. Similarly, in nanofiltration, membrane fouling results in these membranes having a very limited lifetime. Pre-treatment is often a necessity in the case of nanofiltration (Das et al., 2014).

It has been suggested that CNT membranes possess various favorable properties like anti-fouling function, low energy consumption and self-cleaning functions (Das et al., 2014). This makes them a good alternative to conventional water treatment technologies and they might help in eradicating the fresh water crisis very soon (Das et al., 2014).In the following Section 7.5, the future scope of thin film nanocomposite membrane and their requisite for design and development towards commercialization for various applications is elucidated.

Hoek introduced thin film nanocomposite (TFN) RO membranes by incorporating nanoparticles in polyamide layer in 2007 (Jeong et al., 2007). Since then, the new nanoparticles and nanocomposites were researched. The TFN membranes exhibited greater potential in overcoming trade-off between permeability and selectivity. The TFN membranes provided improvement in chlorine resistance and antifouling properties (wang et al., 2011).Despite such unique properties, dispersion of hydrophilic nanoparticles and leaching of nanoparticles into retentate and permeate has raised the environmental concerns and need further research. As the world is facing shortage of freshwater, thin film nanocomposite (TFN) membranes are anticipated to accelerate desalination industry and it can be extended as selective membranes for CO₂ separation (Wong et al., 2016).The incorporation of functionalized fillers such as GO, CNT, TiO₂, Ag-TiO₂, MOFs ( Figure 10 ) or organic fillers for specific applications has the potential to enhance membrane performance (Kumar et al., 2020).Recently, an interlayer of nanomaterials (TFNi) membranes showed extraordinary improvement in water flux and selectivity that can be used for the removal of heavy metals and micropollutants at a large scale (Yang et al., 2020).In the near future, high impact TFN membranes with antifouling and biofouling properties, chemical resistance, improved mechanical strength and thermal stability need to be produced by either predisposition of new types of functionalized nanofillers or organic fillers compatible with selective layers of respective membranes. Such design and development of TFN membranes may pave way for more robust membrane systems with increased performance and long term durability providing precise solutions for various commercial water treatment challenges.