4.1. The Critical Flux Concept and Impact of Flux on Fouling in Forward Osmosis
The critical flux concept states that significant membrane fouling occurs only if the flux is above some critical value [59
] or the permeate flux, at which an irreversible deposit on the membrane surface appears [60
]. In more general definition, is the first permeate flux at which fouling becomes noticeable [61
]. Until now, very few FO publications (Figure 4
) discuss the relationship between critical flux and fouling behaviour. Wang et al. [62
] demonstrated that the critical flux concept in pressure-driven membrane processes could also be applied to osmotically-driven membrane processes, such as the FO process. Wang and his co-workers carried out a direct microscopic observation of the FO process using latex particles as a model foulant. The observations revealed that at a flux of 15 L/m2
h, surface coverage of the membrane by the foulant was negligible. At a flux of 28 L/m2
h, a small amount of coverage appeared on the FO membrane. When the flux value exceeded 41 L/m2
h, the surface coverage by the latex particles was drastically increased. This suggested that the critical flux value of the FO process was somewhere close to 28 L/m2
According to Zou et al. [59
], the critical flux value in the FO process is decreased when draw solution containing divalent ions such as MgCl2
is used as draw solution. On the other hand, when NaCl was used as a draw solution, significant flux decline was not observed for flux as high as 30 L/m2
h. Some researchers have associated critical flux value with a critical draw solution concentration (concentration of draw solution bove which significant fouling occurs) [59
]. Interestingly, while the presence of divalent calcium ions in the feed solution exacerbate fouling, keeping the initial flux value below the critical flux will have negligible effect of calcium ions on fouling behaviour [64
Feed spacer and membrane orientation also have a significant impact on critical flux behaviour in the FO process [62
]. Feed spacers are reported to enhance critical flux significantly [62
]. According to Wang and his co-workers [62
], critical flux was enhanced to about 52 L/m2
h in the presence of feed spacer, whereas in the absence of feed spacers, a critical flux of 28 L/m2
h was observed. In the absence of feed spacer, the membrane will experience a severe external concentration polarization (ECP), which can indirectly promote internal concentration polarization (ICP) and thus lead to a dramatic flux decline [62
]. The high fouling propensity of the membrane in the PRO mode, and low fouling propensity in the FO mode, can also be explained in terms of critical flux. At a similar baseline flux value, significant fouling deposition and flux decline is observed in the PRO mode, whereas for the same membrane, less fouling deposition and stable flux is achieved in the FO mode [63
]. Similar results were reported by Wang et al. [62
] and Zhao et al. [65
]. The impact of different spacer designs, feed spacer location, and impact of operating parameters on critical flux behaviour is still unknown and can be future work on the FO process.
4.2. Effects of Hydrophilicity, Charge and Morphology on FO Membrane Fouling
Generally, if water contact angle is less than 90°, the surface is considered hydrophilic and if the contact angle is greater than 90°, the surface would be hydrophobic. A contact angle of 0° would ideally result in complete hydrophilicity or wetting of the surface. Hydrophilic enhancement of TFC FO membrane is an effective approach to improve FO membrane performance [66
], and resilience against fouling. It is generally assumed that increasing hydrophilicity of a membrane will provide more opportunity for water rather than foulants to chemically associate with a membrane surface [67
]. Increasing hydrophilicity of a membrane is preferred over decreasing the thickness, because it can selectively increase the water flux without an increase in reverse salt flux [68
]. A number of studies have focused on modifying the support layer of TFC FO membrane by incorporating hydrophilic functionalized nanomaterials, such as graphene oxide, carbon nanotubes, titanium dioxide and silica nanoparticles [66
]. Some of these studies are listed in Table 2
. One study found that the higher the titanium dioxide loading on the Psf-TiO2
substrate of a TFC membrane, the lower the contact angle (high hydrophilicity) and greater the porosity [69
]. However, higher loading of the nanoparticles compromised the NaCl rejection of the membrane. A simpler way to increase hydrophilicity of a membrane is by using coatings of hydrophilic polymers like PVA (polyvinyl alcohol). However, to render PVA stable in aqueous phase it must be cross linked with another material, such as glutaraldehyde, to reduce its water solubility [66
]. Hydrophilic polymers, such as PVA, PVP (polyvinyl pyrrolidone) and PEG (polyethylene glycol), often act as pore formers and improve hydrophilicity of the membrane surface [67
The surface charge of a membrane also plays a vital role in fouling. Most natural organic matter (NOM), proteins and colloidal particles are negatively charged in aqueous solution at high pH [24
]; the presence of negatively charged groups on a membrane surface can electrostatically repel these foulants. However, to achieve high resistance to biofouling, by both positively and negatively charged foulants, a neutrally charged surface with high hydrophilicity is preferred [67
]. Some researchers have fabricated hollow fibre FO membranes with positively charged NF-like skin using polyamide-imides [70
]. Compared to a neutral membrane, the positively charged membrane provided double isoelectric points to the salt transfer through the membrane in the FO mode, leading to a reduced salt penetration, whereas in the PRO mode the positively charged surface facilitated salt transportation.
Controlling the support layer morphology during membrane fabrication can significantly enhance performance of FO membrane [85
]. Membrane surface morphology also has a great influence on foulant–membrane interaction [43
]. TFC FO membranes fabricated with a sulphonated material in the substrate can exhibit a fully sponge-like (if 50% sulphonated material) or finger-like structure (if less than 50%) [86
]. Such membranes have increased hydrophilicity and good flux and antifouling behaviour. A fully sponge-like structure with good antifouling properties is preferred for long-term stability of the membrane [86
]; while a finger-like morphology with large macrovoids has been proved to maximize porosity [87
4.3. Other Factors Limiting Membrane Performance
Besides fouling of the membrane, there are a number of other factors that limits forward osmosis membrane performance and, hence, cause reduction in permeate flux across the semi-permeable membrane. Due to these factors, the water flux is much lower than anticipated, based on the osmotic pressure difference between the draw and feed side and the water permeability coefficient of the membrane. Water flux is of critical importance in all osmotic-driven membrane processes and, according to Lay et al. [88
], flux determines the productivity and, ultimately, the viability of the process.
In osmotic-driven membrane processes, concentration polarization can take place on both sides of the membrane [89
]. On the feed side, the solute is concentrated at the membrane surface. This is referred to as concentrative external concentration polarization or CECP. CECP is similar to concentration polarization in pressure-driven membrane processes [90
]. On the draw side, the solute is diluted at the membrane surface and is referred to as dilutive external concentration polarization or DECP. In most flux models for FO, the effects of ECP are assumed to be negligible because of low fluxes, high mass transfers [1
] and no hydraulic pressures [90
]. It has been shown that ECP plays a minor role in osmotic-driven membrane processes compared to pressure driven membrane processes, and is not the main cause of flux decline in osmotic-driven processes [91
]. ECP effects were ruled out when NaCl dissolved in deionized water was used as the feed solution in the study conducted by McCutheon et al. [91
]; however, ECP severely impact feeds with high total dissolved solids [1
]. Waste from different industries, such as food processing, mining operations, oil and gas operations, power plants, landfills and pharmaceutical manufacturing are large sources of total dissolved solids (TDS). According to a research by Wang et al. [92
], the dominant factor for osmotic pressure drop in FO is internal concentration polarization (ICP); however, the effects of ECP cannot be ignored when treating high salinity solutions using FO. Therefore, ECP effects should be considered when treating complex feeds such as wastewater.
It is generally known that high cross flow velocities, turbulence or manipulating the water flux can mitigate ECP [93
]. According to Gruber et al. [94
], increasing the cross flow velocities reduces ECP at the membrane, which in turn leads to higher permeate flux, and significant ECP is observed on the draw side when cross flow velocity is less than one meter per second, whereas ECP on the feed side is insignificant using realistic cross flow velocities. Results from this study further revealed that concentrative ECP on the feed side would only become significant when cross flow velocity on the feed side is almost comparable to membrane flux. Simulations done in this study showed that ECP is more significant when low cross flow velocities are used and mass transfer promoting spacers are absent. It must be kept in mind that increasing cross flow velocities entail additional energy consumption [95
]. Another way to reduce the effects of ECP is by manipulation of flux. But since water flux in FO is already low, the ability to diminish ECP effects by reducing flux is limited [90
While ECP can be mitigated by high cross flow velocities and well-designed hydrodynamics, as discussed above, internal concentration polarization, or ICP, occurs inside the porous support layer in asymmetric membranes and is challenging to mitigate by simply changing cross flow velocities or hydrodynamics. ECP occurs in both pressure-driven and osmotic-driven membrane processes, on the other hand ICP is exclusive to FO [96
]. ICP is considered a major challenge in FO and it leads to reduced water flux and increased reverse salt diffusion [1
]. ICP can be further categorized into concentrative internal concentration polarization, or CICP, and dilutive internal concentration polarization, or DICP.
When the active layer faces the feed solution (AL-FS mode or FO mode), the water permeates through the porous support layer and dilutes the draw solution inside the support layer, this leads to dilutive internal concentration polarization, or DICP. At the same time, concentrative ECP is present on the active layer in the FO mode. On the other hand, when the active layer faces the draw solution (AL-DS mode or PRO mode), as water permeates through the membrane the solutes are concentrated inside the porous support layer, giving rise to concentrative internal concentration polarization, or CICP. At the same time, dilutive ECP takes place on the active layer.
Several researchers have investigated the use of ultrasound waves to mitigate internal concentration polarization. One such effort was done by Choi et al. [97
] using frequencies of 25, 45 and 72 KHz over an output power range of 10–70 W. Experimental results indicated that ultrasound can only mitigate the adverse effects of ICP, but cannot overcome it completely. Another effort using ultrasound was done by Heikkinen et al. [98
], in which a novel ultrasound-assisted forward osmosis system was developed. The study demonstrated that sonification was effective to mitigate ICP and enhance water flux (35 LMH with ultrasound and 20 LMH without ultrasound for TFC membrane using sodium sulphate as DS). However, using ultrasound waves had drawbacks in both the works mentioned above. In the first study by Choi et al. [97
], membrane damage was reported at frequency of 25 KHz, regardless of the intensity. In the second publication by Heikkinen et al. [98
], high water flux was accompanied by high reverse salt flux. Several studies have associated high reverse salt flux with membrane damage as well [12
Alternatively, use of spacers have been investigated to overcome ICP effects in the FO. According to Hawari et al. [102
], CICP could be mitigated by using a spacer and increasing feed solution flow rate, and DICP is aggravated by increasing draw solution flow rate. Zhang et al. [95
] investigated the effect of spacer location to mitigate dilutive ICP without energy input. Results demonstrated that placing the spacer (1 × 1 mm) in the draw channel, with one end of spacer connected to the membrane, can mitigate DICP, and placing a spacer in both feed and draw channels, with one end connected to the membrane, can be a method to reduce the effects of CECP and DICP in the FO mode. The location of placing a spacer; however, seems controversial, as another study by Wang et al. [92
] recommends placing a small spacer in contact with the active layer in the feed channel and 2.7 mm away from the support layer in the draw channel. However, spacers are reported to induce membrane deformation in FO in presence of gypsum scaling [100
] and PRO [12
] under high pressures, while increasing feed solution flow rate leads to loss in recovery rate [103
There has been tremendous research done in the field of membrane fabrication to reduce the effects of ICP. These efforts are using double-skinned membranes, nanofiber composite membranes, increasing hydrophilicity of membranes, increasing porosity, reducing thickness of support layer or reducing the tortuosity of the support layer [69
]. Several researchers have investigated the use of symmetric FO membranes in which the support layer is eliminated completely, resulting in no internal concentration polarization [110
]. Porous single layer graphene oxide membranes also exhibited zero internal concentration polarization and high water flux (three times higher than cellulose triacetate FO membrane) [111
]. However, thin membranes exhibit low mechanical strength and may require frequent replacement in the event of damage. Apart from this, most novel membrane fabrication techniques are quite expensive, time consuming, require a long time to scale up and have intricate processes [95
]; therefore a simple, effective and efficient way needs to be investigated to minimize ICP in future forward osmosis applications.
In forward osmosis, water permeates from the feed side to the draw side due to the high osmotic pressure of the draw solution. However, no membrane is perfect and a small amount of draw solute also diffuses back to the feed side [112
]. This phenomenon occurs because of the high concentration difference between the draw solution and the feed solution [113
] and is; therefore, inevitable in the FO process [114
]. As a result of this reverse salt diffusion, there is a decrease in the net driving force across the membrane and, hence, reverse salt diffusion is considered a major bottleneck in the FO operation.
Reverse salt diffusion is a unique mass transport phenomena, which has a potential to impact FO membrane fouling [115
]. Wastewater contains a variety of foulants depending on the type of wastewater used. Major foulants in impaired water are microorganisms, organic matter and in-organic matter [1
]. All these foulants have tendency to form a fouling/gel-type layer on the membrane surface. Once salt diffuses from the draw side to the feed side, it accumulates on the fouling layer formed on the membrane surface, leading to a net reduction in driving force and permeate flux decline. Lee et al. [26
] suggests that this reduction in water flux, due to reverse salt diffusion, is mainly due to a cake-enhanced osmotic pressure rather than increased resistance of the fouling layer formed on the membrane. Reverses salt diffusion in the FO exacerbates the cake-enhanced osmotic pressure within the fouling layer, leading to an elevated osmotic pressure on the feed side, as a result of which there is a net reduction in driving force and, hence, leads to substantial flux decline.
In forward osmosis, reverse salt diffusion is generally attributed to two main factors, the type of draw solution and the selectivity of semi-permeable membrane used. An ideal draw solute for forward osmosis should have osmotic pressure high enough to promote a high water flux across the membrane and to limit reverse salt diffusion [116
]. According to Achilli et al. [117
], the lowest reverse salt flux is exhibited by draw solutions containing larger-sized hydrated anions, such as MgSO4
, regardless of their paired cations, and reverse salt diffusion through the negatively charged CTA membrane is likely controlled by the anion hydrated size. Based on the solution diffusion mechanism for transport through a semi-permeable FO membrane, it is likely that cations and anions pass through the membrane as a pair to maintain electro-neutrality [118
]. However, NH4
showed the highest reverse salt flux despite larger size (450 × 10−12
m) of HCO3−
anion and KHCO3
, as well as NaHCO3
exhibited the lowest reverse salt flux, which shows that reverse salt flux is not dependent on the size of hydrated anion, or cation, rather overall molecular size of the solute may be a factor. For instance, draw solutions with high molecular size, such as TMA-CO2
], has less reverse salt diffusion compared to NH3
and; therefore, draw solutes which have high osmotic pressure and large molecular sizes needs further investigation to minimize reverse salt diffusion issues.
Reverse diffusion is also a crucial factor to consider when draw solutions containing nitrogen and phosphorous are used, as these cause eutrophication in the receiving water environment [121
]. In fertilizer-driven forward osmosis, by Phuntso et al. [121
exhibited the lowest reverse salt flux, whereas NH4
showed the highest reverse solute flux amongst the selected fertilizers. The lowest flux of NH4
was attributed to the smaller hydrated diameter of both ions. Reverse diffusion of draw solutes have also an impact on fouling, as well as fouling reversibility in forward osmosis. Reverse ionic flux by NaCl is also reported to promote humic acid fouling [40
], and divalent cations, such as Ca2+
, are shown to promote organic fouling in comparison with monovalent, such as Na+
]. Moreover, another study reports that the reverse diffusion of draw solutes (especially divalent cations) can change the feed solution chemistry and promote alginate fouling [122
]. Additionally, reverse permeation of divalent cations results in dramatically different biofouling behaviour [115
While fouling in FO is reversible using simple physical cleaning, reverse diffusion of salt can hinder the reversibility [58
]. Therefore, in selecting draw solutes for forward osmosis, the reverse diffusion of draw solutes into the feed side, and the risk of induced fouling should be evaluated [122
]. Membranes with high selectivity should be coupled with the selected draw solute to reduce reverse salt flux and fouling in forward osmosis.
In an effort to reduce reverse salt flux and internal concentration polarization, Zhang et al. [123
] investigated a phase inversion process of CA membranes by introducing different casting conditions and coagulant baths. Membranes with ultra-thin selective layer and a fully support layer were fabricated. Amongst the different membranes, the double-dense layer membrane exhibited the lowest reverse salt flux of about 1 g/m2
h, which implies its great suitability for seawater desalination and wastewater treatment. Another novel approach that has recently attracted some attention is assisted forward osmosis, also known as AFO. AFO has been recently investigated and claimed to reduce reverse salt flux [124
]. Though careful considerations should be given to keep membrane integrity, AFO seems promising in minimizing reverse salt leakage and enhancing water flux in forward osmosis.
4.4. Coupled Effects of Concentration Polarization and Fouling on Flux Behavior in Forward Osmosis
Concentration polarization and fouling are the main factors responsible for flux decline in the FO process. Tang et al. [125
] systematically investigated the coupled effects of ICP and fouling on flux in the FO process. Results revealed that the stable flux in the FO mode is at the expense of severe initial ICP, whereas the PRO mode under fouling condition is subject to pore clogging of the support layer, which enhance the effects of ICP and CEOP and reduce the membrane permeability. However, this study did not explore the combined effects of external concentration polarization (ECP) and fouling on flux decline in the FO process. The effects of ECP cannot be ignored in the FO process when treating high saline streams or feeds with high fouling propensity, such as wastewater. Particularly in the FO mode, the effect of concentrative ECP on the feed side is higher when feeds with high total dissolved solids (TDS) are used [126
]. According to Parida and Ng [35
], in the PRO mode, increasing organic foulants concentration in the feed solution increased external concentration polarization effects at the membrane surface, leading to more severe organic fouling and flux reduction. On the other hand, increasing organic loading in the feed solution had minimal impact on flux decline in the FO mode. It should be noted that in the FO mode, a high cross flow velocity of 50 cm s−1
was used in this study, and hence the effect of ECP was negligible.
Fouling in a broad scope could be caused by cake-enhanced osmotic pressure, concentrative CP on the feed, reverse salt flux, or even due to the dilution of draw solution. Information about the type of feed and draw solution should be available in order to understand the reason for water flux. Several lab size FO tests are performed on NaCl draw solution and DI feed water and, hence, decline in water flux is mainly due to CP. In general, ionic draw solution prepared in lab, such as NaCl, has very low fouling propensity, but ion diffusion across the membrane and reaction with organic and inorganic matters in the feed solution may cause fouling problems. Scaling is also possible when there is an interaction between the components of DS and FS, due to diffusion across the membrane. Membrane charge and surface morphology are responsible for membrane fouling, as demonstrated in experimental studies.
The best approach to minimize fouling; therefore, should be through conducting a pilot study to understand the best of operating parameters and membrane options. This includes type and concentration of DS, type of FO membrane, recovery rate, pre-treatment, etc. This approach is similar to pilot studies in commercial RO desalination plants that are carried out before RO plant design and commissioning. Pilot studies will help to avoid any major problems and provide skills for trouble shooting. In case of commercial FO plants, pilot studies are recommended to select the type and concentration of DS, membrane type and any other requirements, such as pre-treatment.