Forward Osmosis, Reverse Electrodialysis and Membrane Distillation

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation

CHAPTER 15 Forward Osmosis, Reverse Electrodialysis and Membrane Distillation: New Integration Options in Pretreatment and Post-treatment Membrane De...

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Forward Osmosis, Reverse Electrodialysis and Membrane Distillation: New Integration Options in Pretreatment and Post-treatment Membrane Desalination Process Ramato Ashu Tufa1, Gianluca Di Profio2, Enrica Fontananova2, Ahmet H. Avci3 and Efrem Curcio2,3 1

Department of Inorganic Technology, University of Chemistry and Technology Prague, Prague, Czech Republic 2Institute on Membrane Technology of the National Research Council (ITM-CNR), University of Calabria, Rende, Italy 3Department of Environmental and Chemical Engineering, University of Calabria (DIATIC-UNICAL), Rende, Italy

1 Introduction According to reports by United States Geological Survey, only 2.5% of all global water sources are considered as fresh water, and 69% of the fresh water is in frozen form [1]. Membrane processes for water desalination treat useless saline water sources into fresh water that can be used as drinking or irrigation purposes. However, in membrane-based technologies, as well as in conventional technologies, water and energy are strongly interconnected. Even though membrane-based processes are already more sustainable alternatives to conventional equivalent processes, there is still margin for improvement from the process intensification point of view [2]. Today, the technological spectrum of conventional pressure-driven membrane separation units [microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO)], representing the core of modern membrane desalination systems and widely used in many industrial applications, is complemented by new emerging membrane operations. The possibility to redesign current desalination plants by combining forward osmosis (FO),

Current Trends and Future Developments on (Bio-) Membranes. DOI: © 2019 Elsevier Inc. All rights reserved.


366 Chapter 15 membrane distillation (MD), and reverse electrodialysis (RED) in already existing desalination schemes, so realizing next-generation integrated membrane processes can be a promising strategy due to the synergistic potential that can be achieved in both pretreatment and post-treatment stages. Integration of membrane processes can help to overcome drawbacks of standalone processes. Moreover, considering the waterenergy nexus, more efficient systems can be implemented. For instance, RO, which leads the desalination market with more than half of the overall plants, endures from pre- and post-treatment of process effluents [3]. Membrane foulants present in the RO feed solution causes low water flux and water recovery [4] while disposal of outflowing RO brines creates environmental problems [5]. Therefore, integration of FO and RED as pretreatment and post-treatment, respectively, can enhance RO performance from the energy and environmental points of view. FO has the potential to remove foulants and dilute the feed solution by consuming less energy compared with conventional pretreatment units [6], while RED allows exploiting salinity gradient power (SGP) while diluting concentrated brines to the disposable salinity levels [7]. Moreover, mutually, RED power output is benefited from elevated salinity gradient of concentrated RO brine. MD, which is a temperature driven process, can be integrated to utilize RO retentate for higher solute rejections by using largely available waste heat. In addition to higher water recovery, enhanced RED power can be obtained from MD brines [7]. In this chapter, integration of developing membrane-based technologies [i.e., FO, MD, RED, and capacitive deionization (CD)] and developed membrane-based technologies (i.e., RO and electrodialysis) have been discussed and possible benefits were revealed by particular case studies in the literature.

2 Forward Osmosis in Pretreatment Stage FO is a relatively new membrane separation process where the selective transport of solvent (typically water) is naturally driven by the osmotic pressure gradient established across two solutions having different concentration put in contact with an ideally semipermeable membrane [8]. The solution having the lowest salt concentration is denominated the feed solution while the highly concentrated solution used to extract water from feed is denominated the draw solution. FO operation results in the progressive increase of feed concentration and in the corresponding dilution of the draw solution that, therefore, needs to be regenerated. In principle, unlike RO, FO does not need energy to be operated, with exception of the relatively low amount of energy required by recirculation pumps (Fig. 1).

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 367




Transmembrane flux







ΔP = Δπ



Fig. 1 Conceptual difference between FO and RO.

The transmembrane water flux (Jw) in FO is expressed as a function of the difference between the bulk osmotic pressure of the draw solution (πD,b) and the bulk osmotic pressure of the feed solution (πF,b):  (1) Jw 5 A πD;b 2 πF;b where A is the membrane water permeability coefficient. However, the concentration polarization phenomenon, i.e., the accumulation or depletion of solutes in the boundary layer adjacent to the membrane surface, affects the extent of driving force to mass transfer. Similar to RO membranes, FO membranes are also typically asymmetric, with a thin dense top layer supported by a porous sublayer: therefore, due to different water and solute mass transport rates in solutions, dense and porous membrane structure, two types of concentration polarization occurs; (1) external concentration polarization (ECP) at the feed solutionmembrane and draw solutionmembrane interfaces, and (2) internal concentration polarization (ICP) in the porous sublayer of the membrane. Since, in FO, the active layer faces the feed solution, a concentrative ECP takes place, so concentration at the membrane interface is higher than bulk concentration; as a consequence, the effective osmotic gradient decreases [9]. In addition, under the same membrane-feed orientation, transported water from feed to draw solution and inefficient mixing in the pores result in dilutive ICP where the draw solution is diluted in the interior of the porous sublayer (Fig. 2). Overall, transmembrane water flux is expressed as [10]: Jw 5

1 K lnðAπD

1 BÞ AπF 1 B 1 Jw


368 Chapter 15 DENSE TOP LAYER


Solute 1

Feed solution WATER FLUX

Effective Driving Concentration

Theoretical Driving Concentration

Solute 2

C=0 Draw solution

Fig. 2 Transport through asymmetric FO membrane.

where B is the permeability coefficient of the solute, and K is the solute resistivity, a measure of salt transport in the support layer of the membrane. K is defined as [11]: K5

δtl τ εD


where δtl is the thickness of the dense thin layer, τ the membrane tortuosity, ε the membrane porosity, and D the diffusion coefficient of the solute composing the draw solution. When selecting the solute for the preparation of a draw solution, the most relevant parameters are solubility, concentration, van’t Hoff coefficient, operational temperature, molecular weight, particle size, and cost. NaCl draw solution is frequently used because of its high solubility, nontoxicity, wide availability, and relatively low price; in addition, other salts have also been suggested and explored as draw solutes: MgSO4, KHCO3, NaHCO3, Na2SO4, (NH4)2SO4, K2SO4, MgCl2, Ca(NO3)2, NH4Cl, CaCl2, KCl, NH4HCO3, KBr [12]. McCutcheon et al. (2006) used a thermolytic draw solution based on ammonium bicarbonate [13] due to its regeneration potential. In recent years, the increasing interest in FO for desalination applications was driven by its low energy consumption (as it is operated under a naturally established osmotic pressure gradient) and its low propensity to fouling. Under the generally accepted assumption that membrane fouling intensity is directly proportional to the applied pressure, FO exhibits a

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Fig. 3 The different extent of cake-layer compaction in: (A) FO and (B) RO, is at the origin of the lower fouling propensity of FO.

less detrimental effect in terms of reversibility and cake layer compaction of suspended matter. The different cake layer formation is illustrated under osmotic pressure (Fig. 3A) and under applied pressure (Fig. 3B). However, a negative impact on the membrane performance is observed, mostly related to flux decrease resulting from pore blocking in the support layer [14]. The complexity of fouling phenomena promoted by natural organic matter (NOM), i.e., a complex mixture of humic and fulvic acids, polysaccharides, amino acids, proteins, fatty acids, phenols, carboxylic acids, quinines, lignins, carbohydrates, alcohols, etc., is correlated to the high number of variables characterizing organic macromolecules: molecular size, charge properties, intermolecular interactions, hydrophobic/hydrophilic domains, foulantmembrane interactions, morphological and physicochemical membrane properties (porosity, roughness, surface tensions), etc. [15]. Investigations carried out by Mi and Elimelech using alginate, bovine serum albumin, and humic acid as model organic foulants showed a strong correlation between organic fouling and foulantfoulant intermolecular adhesion [16]. The importance of molecular charge was elucidated by Gu et al. through experimental tests involving oppositely charged lysozyme and alginate on the surface of cellulose triacetate and thin-film composite membranes [17]: under severe fouling conditions (feed water containing a mixture of both proteins), fouling was dominated by foulantdeposited foulant interactions. It is well accepted that physicochemical membrane properties play a crucial role in determining the extent and nature of fouling. The evidence that hydrophobic substrates adsorb higher amount of polysaccharides and proteins with respect to hydrophilic surfaces is extensively reported in the literature for several membrane operations [1720]. From an operational point of view, membrane orientation in FO operations has been observed to exert a significant influence on the extent of fouling. Zhao et al. observed that active layer facing feed side (AL-FS) provides a more stable and higher water flux in comparison to the opposite membrane orientation [6]. Analogously, the experimental

370 Chapter 15 activity of Honda et al. proved that, when applying FO to microalgae cultivation, the rapid flux decrease observed in active layerfacing draw solution (AL-DS) mode was due to pore clogging, adsorption, and ICP in the support layer [21]. A study of Mi and Elimelech [22] proved that alginate fouling in FO is almost fully reversible: more than 98% water flux was recovered by simple water rinse. This surprising behavior, exhibited even in absence of chemical cleaning agents, was explained in terms of a less compact organic fouling layer formed as a result of the unpressurized operating conditions [22]. Moreover, Kim et al. confirmed the lower propensity of FO operation to organiccolloidal fouling when compared with RO, again attributed to absence of applied hydraulic pressure on the feed side [23].

3 Integrated Forward Osmosis Configurations The reversibility of fouling, in conjunction with a low energy input, represent the most prominent advantages of FO technology with respect to current seawater reverse osmosis (SWRO) membrane pretreatment operations such as MF and UF. MF is a membrane unit operated at 13 bar and able to remove suspended solids, to reduce the chemical oxygen demand (COD) below 25 mg/L, and to achieve a SDI (silt density index) ,5. UF employs membranes with molecular weight cut-off (MWCO) usually ranging from 5 to 100 kDa and operated at pressures between 2 and 7 bar, showing ability to retain bacteria, viruses, macromolecules, and colloids; turbidity is also decreased down to 0.4 NTU and SDI below 2 in the permeate stream. The advantages of integrated MF/UF pretreatment unit can be projected as (1) higher RO flux and water recovery factor potential, (2) lower footprint compared to conventional coagulation/flocculation/dual media filtration method, (3) extended membrane lifetime, and (4) significant reduction of chemical dosing [24,25]. The pretreatment energy requirements ranges from 0.24 to 0.40 kWh/m3 and for 8%12% of the total energy consumption of SWRO plant [26]; the most part of the energy input is required to drive pressure pumps at filtration stages. In this context, the integration of FO in a seawater membrane desalination plant has the potential to replace conventional pretreatment stages. The integration between FO, NF, and MD for high water recovery ratio in seawater desalination was proposed by Curcio et al. [27]: the retentate stream produced by MD is used as a draw solution in the FO stage; once diluted, FO permeate is fed to NF. The NF permeate, softened, is sent to the SWRO train [27].

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 371 concentrated seawater outlet

seawater inlet


diluted draw solution


permeate (drinking water)

concentrated draw solution

Fig. 4 Integrated scheme FORO (closed-loop draw solution).

A general desalination scheme integrating FO and RO is presented in Fig. 4: the draw solution, progressively diluted by water permeating FO stage, is regenerated by RO. According to this configuration, fouling ideally affects only the FO unit. Among the various alternative schemes proposed, Tan and Ng considered a hybrid FONF process for seawater desalination; the possibility to achieve water fluxes of about 10 L/m2h and to maintain solute rejection of the FO membrane over 99.4% proved the feasibility of this approach [28]. Recently, the conceptual combination of water reuse and desalination gained an interest driven by the potential opportunities offered by innovative FORO hybrid schemes that, in principle, might synergistically reduce water intake cost and energy input [29,30]. Giving up the advantage of a better control of fouling, FO might be simply operated on existing pretreatment train to reduce the salinity of feed seawater and, consequently, produce desalted water with lower energy consumption and higher recovery factor. This concept is schematically illustrated in Fig. 5: water is transferred through the FO membrane from a low salinity feed solution (e.g., secondary or tertiary treated effluent) to seawater (used as draw solution) by osmosis. Similar systems have been proposed to treat urban water run off in a coastal region [31] or, in combination with RO, electrodialysis (ED) or MD for draw solution reconcentration, for sewer mining [32]. As a further evolution of the conceptual design illustrated in Fig. 5, FO can be implemented within the secondary (biological) treatment in conjunction with biological degradation and clarification, in analogy with membrane bioreactors: this unit is known as an osmotic membrane bioreactor (OMBR) [33,34].

372 Chapter 15 concentrated wastewater outlet

wastewater inlet


diluted seawater


permeate (drinking water)

retentate pretreated seawater inlet

Fig. 5 Synergistic exploitation of pretreated wastewater and seawater sources in a hybrid FORO scheme.

4 Integrated Application of Reverse Electrodialysis in Membrane Desalination Integrated application of RED systems with other membrane technologies has recently emerged as an attractive opportunity for simultaneous generation of renewable energy and drinking water [35,36]. RED requires a concentrated solution as an input for power generation, and this makes it beneficial for integrated application with membrane-based seawater desalination processes where high salinity streams (retentate) are produced, particularly in MD and RO operations [7,3739]. The global desalination market is largely dominated by seawater RO operations having a water recovery in the range of 30%50%. Therefore, a noticeable amount of RO concentrates have to be taken care of due to potential environmental threat. Fortunately, RO brine can be a valuable source for power generation in RED. The use of RO brine for power generation also has an advantage in terms of reducing the energy consumed for desalination. For example, theoretically, production of 1 m3 drinkable water with 50% water recovery factor requires 1.1 kWh energy, and salinity gradient energy potential of concentrated retentate can be recovered to compensate a fair amount of energy demanded by the RO unit to move toward this goal. Furthermore, the net CO2 emissions associated with the generation of thermoelectric energy required to operate seawater RO plants is about 1.41.8 kg/m3 [40,41], and this is expected to double the yearly CO2 emissions in two decades. Thus, the use of clean energy generated by RED using brine discharges can be a viable alternative to polluting power sources.

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 373

Fig. 6 Schematic illustration of the integrated application of RED in desalination technologies.

Fig. 6 shows a schematic illustration of the integrated application of RED with membrane desalination systems (such as RO, MD, FO, and ED) as well as solar evaporation systems (ponds). In principle, the integrated application of RED in desalination technologies involves the use of concentrate brine as high concentrated compartment (HCC) solution, and seawater (or, if available, river water/brackish water/treated wastewater) as low concentration compartment (LCC) solution. When integrated with RO, for example, the retentate can be directly fed to the HCC of RED system, or after further concentration by MD (or other evaporative technologies like thermal vapor compression, mechanical vapor compression, etc.) [7,35,36]. The use of abundantly available solar energy to raise the temperature of feed solutions can be a strategy to increase the power density of RED. Finally, the RED outlet can be either recycled back to desalination systems or discharged after further dilution if required.

5 Hybrid Reverse ElectrodialysisReverse Osmosis Schemes Mixing RO brine (1 M in NaCl) with feed seawater (0.5 M NaCl) has a significant potential for SGP generation. Having a high salinity in the concentrated compartment can lead to higher membrane potential and lower RED stack resistance. Besides that, the consequent dilution effect (during mixing) is beneficial in terms of minimizing environmental problems associated with brine discharge [7,42]. Therefore, integrated operation RORED not only enables the energy-efficient production of pure water but also reduces environmental problems. There are studies demonstrating this approach by evaluating system performance and efficiency on the theoretical basis. Li et al. investigated various designs of REDRO

374 Chapter 15

Fig. 7 Possible configurations of an integrated REDRO process: (A) RED operated using seawater as HCC solution; (B) RED operated using RO brine as an HCC solution [43].

hybrid process that exploits the synergy of both systems [43]. Different configurations were explored, and in one of the scenarios (REDRO), seawater is first treated by RED and then fed to a RO system (Fig. 7A). During the RED process, the free energy of mixing is extracted as seawater salinity is reduced, which is subsequently fed to the RO unit. The use of low salinity seawater in RO will not only reduce the pumping energy but also provide extra energy from brine discharge. In another scenario (RORED), the RED process can be utilized after RO to treat effluents (Fig. 7B). The RO effluent with higher salinity is fed to the RED unit resulting in higher power output, and additionally, the salts in the concentrated seawater are partially diluted during the RED process, which eases the process of brine management. Other complex designs can also be implemented, as for example, a REDRORED configuration in which two RED units are used for pre- and posttreatment. Such configuration has synergetic advantage of both the RORED and REDRO configurations, although a relatively high capital cost is expected. Alternatively, a portion of the RO retentate can also be recycled back to the HCC of the initial RED unit instead of using the additional RED unit. The performance of the proposed combinations of the RED and RO system was evaluated through a modeling approach [43]. As shown in Fig. 8A, considering a RED stack of 50 cells and 600 cm2 active membrane area in 20 parallel branches (each branch consists of 5 RED stacks), model predictions show the possibility of achieving 56% reduction, that is, from 1.8 kWh/m3 without pretreatment to 0.8 kWh/m3 with pretreatment in RO energy

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 375

Fig. 8 Comparison of the different RED and RO integrated configurations in terms of specific energy consumption: (A) REDRO configuration; (B) RORED configuration. Reproduced with permission Li et al. (2013) [43]. Copyright Elsevier 2013.

consumption when using RED effluent as an RO feed. If the RO brine is recycled back to RED, the specific energy consumption (SEC) of RO can be significantly reduced by 78%. There is no net reduction in SEC of RO for RORED configuration (Fig. 8B) since seawater is the feed solution for RO. However, energy recovery of the RED unit is increased due to an increase in the driving force or the salinity gradient when using RO brine. But this recovery in the exploited energy does not suffice for the energy consumption of the RO process, which remains at a value of B0.7 kWh/m3 for the RORED configuration. Kwon et al. [44] performed experimental investigations on RED using brine solutions from RO and FO desalination plants by mixing with HCC solutions of seawater (0.6 M NaCl) and river water (0.01 M NaCl) [44]. As shown in Fig. 9A, a net power density of 1.56 and 2.07 W/m2 was recorded at a flow rate of B4 mL/min for RO brine (1 M NaCl) and FO brine (2.4 M NaCl), respectively by using an HCC solution of river water (0.01 M NaCl). Moreover, SEC analysis of the RO (at a water recovery rate of 50%) was performed based on the energy recovery of RO brine by RED, and considering the energy consumption of typical RO process to be 3.0 kWh/m3. As shown in Fig. 9B, a minimum SEC of 2.77 and 2.98 kWh/m3, corresponding to an energy recovery of 13.5% and 2.2%, were determined by using river water and seawater as an LCC feed, respectively. For the FO process (at a water recovery rate of 75% and SEC of 0.84 kWh/m3), a minimum SEC of 7.3 and 0.82 kWh/m3, corresponding to an energy recovery of 13.5% and 2.2%, was computed by using river water and seawater, respectively (Fig. 9C).

6 Hybrid Reverse ElectrodialysisMembrane Distillation Schemes Integrated application of RED with MD benefits from the use of hypersaline MD brine with a concentration of up to 5 M NaCl for power generation, which limits the ohmic losses,

376 Chapter 15

Fig. 9 (A) Variations in gross, net, and pumping power densities with flow rate; experiments with HCC solution of RO brine (1 M NaCl) and FO brine (2.4 M NaCl), and LCC solution of river water (0.01 M NaCl). Change in the specific energy consumption of (B) RO and (C) FO with varying flow rate. The black and red lines (light gray in print version) represent the process using seawater and river water as an LCC solution, respectively. Reproduced with permission Kwon et al. (2015) [44]. Copyright Elsevier 2015.

increases the driving force and enhances the power output of RED. There is quite a lot of literature demonstrating this concept through experiments and modeling [7,45]. Tufa et al. experimentally demonstrated an innovative approach integrating RED and MD to produce clean energy and potable water, simultaneously, under near-zero liquid discharge concept and energy-efficient seawater RO desalination [7]. Batch experimental tests on direct contact membrane distillation (DCMD) over 35 h of operation at feed and distillate average temperatures of 40 C and 20 C, respectively, resulted in a final NaCl concentration of 3.41 M, corresponding to 70.7% volume reduction factor starting from an initial NaCl

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 377 (1 M) volume of 20 L. A concentration of up to supersaturation (5.4 M NaCl) was reached in 27 h by increasing the temperature gradient to 25 C, corresponding to a volume reduction factor of 83.6% within 27 h. A 21% decrease in transmembrane flux with respect to the initial 1.7 kg/m2 h was measured under this scenario. The salinity gradient energy potential of the hypersaline MD brines (45.4 M) was further tested in RED. The power density of RED monotonically increased with increase in the concentration of MD brine used as HCC solution due to increase in driving force (Fig. 10A). A maximum power density of 2.4 W/m2MP (membrane pair) was achieved when mixing MD brine (5.4 M NaCl) and seawater (0.5 M NaCl) under ambient conditions [7]. However, it is worth noting that MD operated at high concentration factor has a potential membrane scaling risk due to the precipitation of slightly soluble sulfate and carbonate salts when running RED at high concentrations [46]. The temperature of MD brine depends on that of the feed solution, which varies depending on the employed temperature gradient. Therefore, investigation of the variation of the power output of RED with operative temperature, regarding its impact on the ionic mobility and membrane resistivity, is essential [47]. As shown in Fig. 10B, when increasing the temperature from 20 C to 50 C, the maximum power density almost doubles increasing from 1.44 to 2.08 W/m2MP. Similarly, Daniilidis et al. [48] reported the potential improvement in performance of RED with feed temperature, reaching a maximum power density of 3.8 W/m2 at 20 C, which increased to 6.7 W/m2 at 60 C [48].

Fig. 10 Variation gross power density vs. current density (A) at different HCC concentrations (MD brine/ seawater: 45.4 M/0.5 M NaCl, 20 C) and (B) at different temperatures (MD brine/seawater: 5 M/0.5 M NaCl). Reproduced with permission Tufa et al. (2015) [7]. Copyright Elsevier 2015.

378 Chapter 15

7 Closed Loop Membrane DistillationReverse Electrodialysis Scheme The thermal energy required to drive MD can be obtained from the low-grade waste heat that is freely available from industrial sources. In this regard, integrated MDRED can potentially be an alternative for conversion of low-grade waste heat into electricity in a closed loop. Fig. 11 illustrates this concept in which MD is operated by low-grade waste heat, and reject brine is fed to HCC of RED along with pretreated seawater to LCC of RED. The RED effluent can be concentrated by either MD or external evaporator using low-grade waste heat. Recently, REDMD was modeled in this concept for conversion of low-grade waste heat to electricity, considering both the heat and mass transfer characteristics under the thermal separation (MD) stage and electricity generation (RED) stage [45]. In the model simulations, NaCl molality was varied between 1.0 and 5.0 mol/kg and the heat sink and source temperature were assumed at 20 C and 60 C. Model calculation shows that B0.6 is a critical value for relative permeate/feed-flow rate (α) under model conditions; for α , 0.6, the mass recovery rate (ξ) is proportional to α and permeate flow rate dominates the regime (permeate limiting regime) while for α . 0.6, feed-flow rate starts to dominate the regime (feed limiting regime) therefore ξ reaches a plateau (Fig. 12A) [45]. Due to fact that ξ is proportional to max power, it is expected to follow the same trend for the same range of MD feed concentration. Even though they follow the same trend, it is worth noting that max power produced on a RED stack is more sensitive to MD feed NaCl concentration (Fig. 12B) [45]. Clearly, high MD feed concentration induces more power output due to a reduction in ohmic losses and increase in the driving force. Although the high concentration of feed limits MD mass recovery rate, the initial concentration of MD brine is enough to maintain a sufficiently large voltage for RED. The electrical efficiency of the MDRED can be depicted from heat absorbed by MD and RED power output. As shown in Fig. 12C, the variation of electrical efficiency with α displays a concave-shaped curve, implying that there exists an optimal α value for Desalted water



Pretreated seawater

Waste heat


Waste heat

Fig. 11 Conceptual illustration of an MDRED system for conversion of low-grade waste heat into electricity in a closed loop.

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 379

Fig. 12 The variation of (A) mass recovery rate and (B) power output. (C) Electrical efficiency for 1.05.0 mol/kg relative MD permeate/feed-flow rate in the integrated MDRED system. (D) The variation of heat absorbed, specific heat duty, and electrical efficiency of the hybrid MDRED system for 5.0 mol/kg relative permeate/feed-flow rate in the integrated MDRED system. Reproduced with permission Long et al. (2017) [45]. Copyright Elsevier 2017.

maximal electrical efficiency. Moreover, maximum of electrical efficiency and minimum of specific heat duty overlap at the same α value, whereas minimum of absorbed heat has a lower value (Fig. 12D). In general, a high concentration of the MD feed solution results in high specific duty, which leads to a high electrical efficiency of the hybrid process. This shows that the benefit of RED in terms of output power bypasses the energy requirement of MD module (β) in the thermal separation step [45].

8 Hybrid Reverse OsmosisMembrane DistillationReverse Electrodialysis Scheme Integrated ROMDRED process can also be applied for simultaneous production of pure water and energy. In such a scenario (Fig. 13), RO brine is first concentrated by MD before it is fed to RED. Basically, an RO brine from seawater could have a concentration

380 Chapter 15 permeate



Blowdown recycle to MD brine (HCC)





pre-treated seawater

desalted water


Fig. 13 Integrated ROMDRED for simultaneous production of water and renewable energy. Reproduced with permission Tufa et al. (2015) [7]. Copyright Elsevier 2015.

of about 1 M NaCl at 50% water recovery. Operation of MD on 1 M RO brine could result in a hypersaline solution with a concentration of up to 5 M NaCl. This implies a huge potential of enhancing the power density of RED by working at high salinity gradient starting from an RO brine. For example, a RED operating on seawater as a LCC solution and RO brine as a HCC solution assumes a salinity ratio of about 2. But this salinity ratio increases to 10 (fivefold) by using MD brine instead of RO brine. This would have a huge benefit in terms of OCV and RED stack resistance. The use of highly concentrated brine solution increases the Nernst membrane potential and reduces the Ohmic losses in the RED stack, therefore generated power can be enhanced accordingly. The drawback of using highly concentrated solutions might involve the risk of fouling particularly the scaling of slightly soluble salts when working with real solutions. Moreover, highly concentrated brine could also limit membrane permselectivity thereby reducing system efficiency.

9 Coupling Reverse Electrodialysis With Other Membrane Desalination Units There is also a huge potential in integrating RED with other membrane desalination technologies like ED [49,50] and CD [51]. The RED operating on the reject (concentrate) stream of ED is used as a power source to reduce energy consumption at ED stage. The integration of REDED can be operated by a closed-loop recirculation of the RED outlets back to ED with the possibility of regenerating the HCC solution. The concept remains similar when integrating RED with CD as well. However, such processes are not yet sufficiently covered in literature requiring further investigation on the theoretical and experimental basis.

Forward Osmosis, Reverse Electrodialysis and Membrane Distillation 381

10 Conclusions and Future Trends Lower fouling propensity and energy demand compared with pressure-driven membrane units make FO a suitable option to boost the efficiency of current seawater membrane desalination systems. Recent optimistic investigations indicate that FO can potentially decrease the energy input to RO seawater desalination down to 1.5 kWh/m3, not far from the thermodynamic threshold (B1 kWh/m3). However, preliminary studies indicate that the present commercial FO membranes do not guarantee a sustainable implementation of FO in the desalination industry due to the high capital investment cost (CAPEX). The inversion point is predicted in proximity of a targeted membrane cost of 25 h/m2 c.a. or a transmembrane flux higher than 15 L/m2 h. Integration of RED in membrane desalination systems is a promising approach for concurrent production of fresh water and electrical energy. In theory, such a concept might enable a technological solution to low energy desalination. Similarly, MD integration can increase water recovery factor above 90% [52]. Moreover, the adverse effects of brine discharge to the ecosystem and the pollution of the environment by the greenhouse gases released from power plants that supply energy to desalination plants could be minimized. Such advantage from the synergistic integration of RED and MD with other membranebased technologies is consistent with the process intensification strategy [2] and zero liquid discharge paradigm [7,52,53]. However, implementation of these integrated membrane systems at the industrial level requires a significant research effort. It is well recognized that the power output at SGP-RED stage is highly influenced by the presence of divalent ion and fouling phenomenon during operations with natural feeds; therefore, the development of new materials, particularly ion exchange membranes able to overcome the adverse effect of divalent ions as well as fouling, is necessary. Ion exchange membranes for RED should have low resistance ion-conductive membrane materials at a low cost (,2 h/m2) and with high permselectivity ( . 95%) for operations under real conditions [54]. Addressing these issues will have a significant impact on the possibility of commercial implementation of RED technology.


active layer facing draw solution capital investment cost capacitive deionization chemical oxygen demand direct contact membrane distillation external concentration polarization electrodialysis forward osmosis high concentration compartment


internal concentration polarization low concentration compartment membrane distillation molecular weight cut-off microfiltration natural organic matter osmotic membrane bioreactor reverse electrodialysis specific energy consumption seawater reverse osmosis ultrafiltration

List of Symbols A B D JW K P

water permeability coefficient (m3/m2 s Pa) solute permeability coefficient (m/s) diffusion coefficient (m2/s) transmembrane water flux (m3/m2 s) solute resistivity (s/m) hydraulic pressure (Pa)

List of Greek Symbols α δ tl ε ξ π D,b π F,b τ

relative permeate/feed-flow rate () thickness of the dense thin layer (m) membrane porosity () mass recovery rate () bulk osmotic pressure of the draw solution (Pa) bulk osmotic pressure of the feed solution (Pa) membrane tortuosity ()

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