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Energy Consumption of Brackish Water Desalination: Identifying the Sweet Spots for Electrodialysis and Reverse Osmosis
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Energy Consumption of Brackish Water Desalination: Identifying the Sweet Spots for Electrodialysis and Reverse Osmosis
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  • Sohum K. Patel
    Sohum K. Patel
    Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States
    Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520-8286, United States
    More by Sohum K. Patel
    Orcidhttp://orcid.org/0000-0001-5228-9449
  • P. Maarten Biesheuvel
    P. Maarten Biesheuvel
    European Centre of Excellence for Sustainable Water Technology, Wetsus, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands
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    Orcidhttp://orcid.org/0000-0002-5468-559X
  • Menachem Elimelech*
    Menachem Elimelech
    Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States
    Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520-8286, United States
    *Menachem Elimelech. Email: [email protected]. Phone: (203) 432-2789.
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    Orcidhttp://orcid.org/0000-0003-4186-1563
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ACS ES&T Engineering

Cite this: ACS EST Engg. 2021, 1, 5, 851–864
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https://doi.org/10.1021/acsestengg.0c00192
Published January 22, 2021

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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Though electrodialysis (ED) and reverse osmosis (RO) are both mature, proven technologies for brackish water desalination, RO is currently utilized to desalinate over an order of magnitude more brackish water than ED. This large discrepancy in the adoption of each technology has yet to be thoroughly justified in the literature, particularly from the perspective of energy consumption. Hence, in this study, we performed a direct and systematic comparison of the energy consumption of RO and ED for brackish water desalination, precisely mapping out the ideal operational space of each technology for the first time. Using rigorous system-scale models for RO and ED, we determine the specific energy consumption and energy efficiency of each process over a wide range of brackish water conditions. Specifically, we investigate the effects of varying feed salinity, extent of salt removal, water recovery, and productivity to ultimately identify the operational sweet spots of each technology. By maintaining the same separation parameters (i.e., feed salinity, salt removal, water recovery) and productivity between RO and ED throughout the study, we ensure that our comparison of the technologies is valid and fair. Our results indicate that both RO and ED are capable of operating with high energy efficiency (>30%) for brackish water desalination, though for differing conditions. Particularly, we show that whereas ED excels for low feed salinities (<3 g L–1) and extents of salt removal, RO operates optimally for high salinity feeds (>5 g L–1), which require more extensive desalination. Through our in-depth energetic analysis, we provide guidance for future applications of RO and ED, emphasizing that increased implementation of ED will require significant reduction in the cost of ion-exchange membranes.

Copyright © 2021 The Authors. Published by American Chemical Society

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INTRODUCTION

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While the planet’s limited freshwater resources incessantly shrink due to pollution and climate change, the global demand for freshwater continues to rapidly increase with population growth. In effect, global water scarcity persists as one of the greatest challenges of our time, already affecting two-thirds of the world population and expanding at an alarming rate. (1−3) Though improved water management and conservation are essential countermeasures, effectively combatting water scarcity will require further intervention, specifically through the harnessing of unconventional water sources to augment dwindling freshwater supplies. Alternative water sources, however, are generally characterized by high salinities, requiring salt to be extracted from the water prior to utilization.
The process of water–salt separation, termed desalination, inherently requires the consumption of energy, making the minimization of energy consumption a long-standing goal in the development of desalination technologies. (4−6) A simple, yet effective approach to reducing the minimum energy requirement of desalination is to target lower salinity waters; (7) hence, as reliance on desalinated water has recently surged, brackish waters (salinity range of approximately 1 g L–1 to 10 g L–1) have increasingly been utilized as feedwater, as opposed to the vastly abundant, but considerably more saline, seawater. Therefore, the identification and implementation of the most energy efficient brackish water desalination technology is critical toward ensuring sustainable water production in the coming decades.
Currently, reverse osmosis (RO) dominates global desalination capacity, accounting for over 80% of the desalinated brackish water. (8) In contrast, the second most utilized technology for brackish water desalination, electrodialysis (ED), commands only 8% of the brackish water desalination market share, (8) despite suggestions that the energy consumption of ED is comparable or even superior to RO for certain brackish water conditions. (9) The highly distinct mechanisms of water–salt separation in RO and ED, however, make the conditions for which each technology is energetically advantageous difficult to discern.
Reverse osmosis, a pressure-driven process, makes use of a semipermeable membrane which allows for the permeation of water molecules while rejecting the passage of solutes. Hence, as pressurized saline feedwater is passed along a membrane module, a freshwater permeate stream and concentrated brine stream are produced. (10,11) The energy consumption of RO is dictated by the pressurization of the feedwater, with the process inherently requiring the application of hydraulic pressures in excess of the osmotic pressure of the exiting brine. Though the first viable cellulose acetate RO membrane was demonstrated in the 1960s, (12) it was not until the development of the polyamide thin-film-composite (TFC) membrane in the late 1970s that RO proliferated in application. (13) Current TFC RO membranes, combined with state-of-the-art energy recovery devices and pumps, facilitate seawater RO to operate within a remarkable 2-fold of the thermodynamic minimum energy requirements, making RO the unrivaled technology for seawater desalination. (10,14) However, at lower feed salinities, such as the brackish water regime, the energy efficiency of RO has been demonstrated to be comparable to other technologies. (7) Furthermore, efforts to reduce the energy consumption of brackish water RO through enhancement of membrane material properties (i.e., water permeability) have been shown to be relatively insignificant, providing alternative technologies the opportunity to compete for brackish water desalination capacity. (15,16)
Electrodialysis, also a relatively mature membrane-based desalination process, is an electrically driven technology that relies on the transmembrane transport of ions, rather than water, to achieve desalination. (17−19) Specifically, in ED an electric field is applied across a stack of ion-exchange membranes to drive ions toward the oppositely charged electrode. As the feedwater is passed through the flow channels, cations and anions selectively pass through alternating cation- and anion-exchange membranes, respectively, thereby forming ion-concentrated and depleted solution streams on each side of the membrane. Since the first demonstration of multicompartment ED in 1940, (20) the technology has gained considerable momentum, being primarily adopted for brackish water desalination, but also applied in the bioprocessing, pharmaceutical, and food industries. (17,21,22) The gradual development of highly selective and conductive ion-exchange membranes (23) has made ED an energy efficient technology for brackish water desalination, as demonstrated by our previous study. (24) Nonetheless, compared to RO, ED’s penetration of the brackish water desalination market is minimal and warrants further assessment.
Previous works have investigated the optimal operation of ED, (25,26) while others have discussed the relative merits of RO and ED (17) and proposed RO-ED hybrid systems. (27,28) Nonetheless, precise and rigorous identification of the operational sweet spots of RO and ED, with respect to one another, has yet to be demonstrated. Hence, there is a critical need for such an analysis to assist in guiding the future direction of the growing brackish water desalination sector.
In this study, we systematically compare the energy consumption of ED and RO over a wide range of brackish water operating conditions to determine the optimal applications of each desalination technology with regard to energy consumption. To ensure a meaningful comparison, the feed salinity, water recovery, and salt removal are fixed across both technologies, thereby setting the theoretical minimum energetic demand of a given water–salt separation. Additionally, the kinetic efficiency of desalination is held constant across ED and RO by fixing the system productivity, defined as the rate of water production normalized by the projected membrane area. Our comparison ultimately provides fundamental insight into the current RO dominated brackish water desalination market and reveals opportunities for the growth of ED in particular applications. We conclude by highlighting practical considerations, outside of energy consumption, which largely influence the employment and outlook of each technology.

ED PROCESS MODEL AND CALCULATION OF ENERGY CONSUMPTION

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ED Operation Mode

In ED, numerous cell pairs—each consisting of an anion-exchange membrane (AEM), cation exchange membrane (CEM), concentrate channel, and diluate channel—are placed between a pair of electrodes. Here, we consider the conventional plate-and-frame stack configuration, in which the flow channels and ion-exchange membranes are aligned between the electrodes, as shown in Figure 1. To represent commercial-scale brackish water desalination, we simulate a fifty-cell pair stack operating under steady state continuous flow conditions. (19,29)

Figure 1

Figure 1. Schematic illustration of the variable water recovery electrodialysis (ED) process. A portion of the multicell pair ED stack is shown. Each cell pair contains an anion-exchange membrane (AEM), cation-exchange membrane (CEM), and spacer channels between the membranes. An external voltage is applied across the ED stack as (an equal flow rate of) saline water passes through each of the flow channels. The generated electric field causes ions in the channels to migrate toward the oppositely charged electrode. The AEM and CEM enable selective permeation of anions and cations, respectively. Due to nonideal membrane selectivity, however, the transport of counterions across the membrane (Jct) is also accompanied by some degree of counterproductive co-ion transport (Jco). The overall selective transport of ions results in alternating product and brine channels. Redox reactions at the surface of the electrodes convert the ionic current into an electrical current (i). For a system-scale water recovery greater than 50%, a fraction of the brine effluent (αR) is recycled, thus reducing the overall volume of brackish feedwater and increasing the system-scale water recovery.

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Upon polarization of the electrodes, anions and cations in the flow channels migrate toward the oppositely charged electrode, selectively traversing the anion- and cation-exchange membranes, respectively. This counterion flux (Jct) is the underlying principle of ED desalination and results in alternating channels of diluate and concentrate. Practical ion-exchange membranes, however, also permit some degree of co-ion flux (Jco) from the concentrate to diluate channel, detracting from the efficacy of salt removal in ED. Redox reactions at the electrodes enable the conversion of the ionic current in the ED stack to an electrical current, which flows from the cathode to the anode via an external circuit. Here, we assume reduction of hydrogen ions at the cathode, and oxidation of chloride ions at the anode, corresponding to a combined electrode potential (Vel) of 1.4 V throughout the analysis.
Co-current and equal flow is maintained through each of the channels to minimize transmembrane hydrostatic pressure differences, which would otherwise exacerbate water transport across the ion-exchange membranes. (29,30) Hence, the module-scale water recovery is fixed at 50% throughout the analysis. To vary the system-scale water recovery, we employ the commonly practiced “feed-and-bleed” operation mode. (17−19,31) Particularly, a fraction of the produced brine (αR) is recirculated and mixed with the brackish feedwater supplied to the concentrate channels. In effect, the volume of brackish feedwater required to produce a given volume of product water is reduced, thus allowing the system-scale water recovery to exceed the module-scale recovery of 50%. In Figure S1, we provide a schematic illustration of the feed-and-bleed operation mode on a single pair of concentrate and diluate channels to clearly demonstrate this operation mode.

ED Process Model

We utilize a two-dimensional (2-D) Nernst–Planck model to fundamentally capture the underlying ion transport phenomena in the ED stack. The modeling framework utilized was originally demonstrated by Sonin and Probstein (32) in 1968 and was later extended to include co-ion transport in the ion-exchange membranes, Donnan potentials at the membrane–solution interfaces, water transport through the membranes, and the effects of porosity and tortuosity in a spacer-filled channel. (30,33−35) These advancements of the Sonin–Probstein model have facilitated more realistic description of desalination performance, cell pair voltage, and current distribution, as has been confirmed through rigorous validation against several experimental data sets. (30,34) In this 2-D model, the applied cell pair voltage is invariant across the length of the membrane, and the total current in the stack self-consistently distributes over the membrane surface (i.e., more current may go to parts closer to the channel inlets, where the transmembrane concentration differences are lower, than to later parts of the channel). Thus, all positions in the 2-D geometry are numerically coupled together. Further model illustration is provided in Figure S2, where we demonstrate the capability of predicting key ED transport phenomena, such as concentration polarization and the limiting current density.
As demonstrated by previous studies, the model relies on the numerical solution of the extended Nernst–Planck equation, both in the spacer channels and in the ion-exchange membranes. (30,32−34) Two-dimensional Nernst–Planck modeling effectively minimizes the number of independent variables and circumvents the assumption of boundary layer thicknesses commonly used to describe concentration polarization. Specifically, in our 2-D modeling approach we generate concentration profiles for each discretized position down the length of the channel. The concentration at each point in the spacer channels—with the exception of the membrane–solution interfaces, where thin electrical double layers form—is governed by the condition of electroneutrality and the convection–diffusion equation (eq S6). Concentrations at the membrane–solution interfaces are determined according to the continuity of ion flux between the ion-exchange membranes and solution, coupled with the Donnan potentials on each side of the membrane. Thus, the effects of concentration polarization are effectively captured without applying the concept of separate boundary layers next to a thin core “bulk” region, which is not particularly applicable for the narrow spacer-filled channels of an ED membrane stack.
Whereas ion transport across the thickness of the spacer channels (i.e., perpendicular to the membranes) is governed by diffusion and migration, ion transport along the length of the channel (i.e., parallel to the membranes) is solely related to advection. Notably, we neglect water transport across the ion-exchange membranes, as the effects of both osmotic and electro-osmotic transport are insignificant for analysis of brackish water ED. (29,30) A fully developed Hagen–Poiseuille velocity profile is applied for the convective fluid flow through the entirety of the spacer channels. Though the fluid streamlines in a spacer-filled channel may be more accurately described by different flow profiles, it has been shown that the fluid profile applied in the 2-D modeling approach leads to minimal deviation in the prediction of ED process performance. (33) We also confirm the relative lack of importance in selection of the velocity profile in Figure S3, in which we show that changing the Hagen–Poiseuille profile to plug flow has nearly no effect on the predicted energy consumption of ED.
In contrast to the fluid flow profile, we emphasize that the estimation of the porosity (ε) and tortuosity (τ) in the spacer-filled channels plays a more significant role in the ED process modeling. Though a typical mesh-filled channel provides enhanced fluid dispersion, the inclusion of a spacer results in retarded ion mobility from the reduced amount of cross-sectional area (porosity) and the elongation of the transport pathways (tortuosity). Hence, to accurately predict the potential drops in the spacer channels, it is critical to use an effective diffusion coefficient for electromigration (De), determined by scaling the diffusion coefficient in free (bulk) solution (Dd) by a factor of . Here, we use an experimentally verified value of 0.314. (30) We also note that our determination of the full concentration profiles across the thickness of the channel, with the use of an experimentally verified effective diffusion coefficient for electromigration, intrinsically captures the so-called spacer shadow effect. (36,37) Further details regarding the ED process modeling—including specified values for system geometry, mass transport properties, and membrane properties—are provided in the Supporting Information.
Throughout the modeling, feedwater is considered as sodium chloride solution, and the sodium and chloride ions are assumed to possess the same diffusivity, as is common in the modeling of electro-driven desalination processes, such as electrodialysis and capacitive deionization. (27,29,30,33,35,36,38,39) Additionally, the properties of the anion- and cation-exchange membranes (other than the sign of their intrinsic charge) are assumed to be the same. As a result of the assumptions, a plane of symmetry is established in each cell pair, allowing for the computational space to be limited to only half of a cell pair. We note that a more rigorous calculation with unequal diffusion coefficients and ion-exchange membrane properties (34) shows that dropping the assumption of symmetry does not have a considerable effect on the prediction of ED process performance.

ED Performance Metrics

The ED model is used to predict the specific energy consumption (SEC) of ED across a broad range of brackish water desalination conditions. Specific energy consumption, which is the energy consumption normalized by the volume of product water, is calculated by
(1)
where N is the number of cell pairs in the stack, Vcp is the voltage per cell pair, Vel is the combined redox reaction potential of the electrodes (previously defined as a fixed value of 1.4 V), I̅ is the average current density in a cell pair, Am is the projected area of the ion-exchange membrane, and QP is the volumetric flow rate of the product water per cell pair. For stacks with a large number of cell pairs, as in any practical application of ED, (17) the term for the redox potential of the electrodes becomes negligible, simplifying the expression. Nonetheless, to ensure the rigor of our analysis, we maintain the use of eq 1 in its presented form throughout our study.
The calculated SEC is compared to the thermodynamic minimum specific energy consumption (SECmin) to calculate the thermodynamic energy efficiency (η) as
(2)
where SECmin represents the theoretical energy required by a thermodynamically reversible process and is a function of only the separation conditions, namely the system-scale water recovery (WR), salt removal (Rs), and feed salinity (cF). Particularly, for a fully dissociated 1:1 salt, SECmin can be expressed as (6)
(3)
where Rg is the universal gas constant, T is the absolute temperature, cB is the salinity of the concentrated brine solution, and cP is the salinity of the desalinated product water.
The specific energy consumption of ED is dependent on several water–salt separation parameters—namely, the salt removal, water recovery, and productivity. Throughout the study we utilize the conventional definition of salt removal, defined in terms of the diluate channel effluent (product water) salinity (cP) and the feed salinity (cF):
(4)
The water recovery of a desalination process is the volume of product water generated per volume of feedwater supplied to the system. Thus, in the feed-and-bleed operation described, the system-scale water recovery is given by
(5)
in which QB,f is the volumetric flow rate of the “makeup” brackish feedwater supplied to the concentrate (brine) channels, which is required in addition to the recirculated brine to equalize the flow rates in the diluate and concentrate channels (a schematic illustration of the feed-and-bleed operation mode is shown in Figure S1).
Lastly, the productivity is a measure of the kinetic efficiency of the separation process and relates the rate of desalinated water production to the system size. For ED, though a cell pair consists of two membranes, the system size is most accurately represented by the projected area of a single ion-exchange membrane (Am). Hence, the productivity is expressed as
(6)

RO PROCESS MODEL AND CALCULATION OF ENERGY CONSUMPTION

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RO Operation Mode

In RO, saline feedwater is pressurized and passed along a dense semipermeable membrane, which allows the permeation of water while blocking the passage of virtually all solutes. The hydraulic pressure applied to the feedwater must be greater than the osmotic pressure of the brine (retentate) at the end of the membrane module to ensure use of the entire membrane area, with higher pressures increasing the transmembrane water flux at the expense of greater energy consumption and salt passage. (40) The membrane, being the core of the RO process, largely dictates the overall performance. Current state-of-the-art thin film composite (TFC) polyamide membranes used in brackish water desalination possess operational permeabilities in the range of 3–5 L m–2 h–1 bar–1 and are capable of salt removals in excess of 99%, with some as high as 99.8%. (16)
In order to tune the salt removal of RO for fair comparison with ED, we employ a single-stage bypass system (Figure 2), as introduced in our previous study, (41) in which only a portion of the feedwater is treated by the RO module. The remainder of the feedwater circumvents the high-pressure pump and RO module by passing through either a brine or product bypass stream. The product bypass stream, which is mixed with the permeate from the RO module to form the overall product water, is used to adjust the system-scale salt removal. Alternatively, the brine bypass stream is mixed with the RO retentate and serves to minimize the load on the RO module. The flow rate fractions of each of the three streams are governed by the specified system-scale salt removal and water recovery, with further details provided in the RO Performance Metrics subsection.

Figure 2

Figure 2. Schematic illustration of the variable salt removal reverse osmosis (RO) process. The brackish feedwater is split among three streams (with the following flow rate fractions): RO module feed (αRO), brine bypass (αB), and product bypass (αP). The hydraulic pressure of the feedwater fed to the RO module is increased (by ΔP) via a high-pressure pump, requiring the input of electrical energy. Water permeates through the RO membrane with a flux of Jw, while complete salt rejection is assumed. The permeate and retentate streams from the RO module are mixed with the corresponding bypass streams, generating the overall product and brine streams, respectively. The system-scale salt removal is controlled by tuning the fraction of feedwater sent to the product bypass stream. Alternatively, the brine bypass stream serves to minimize the flow to the RO module, and thus the energy consumption. An energy recovery device (ERD) is applied to the retentate stream exiting the RO module to recover and reuse a significant portion of the mechanical energy provided by the high-pressure pump. However, an ERD is only applied to certain cases throughout the study, as its use in brackish water RO is circumstantial.

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A key advantage of RO is that an energy recovery device can be applied to the retentate stream to recover a large fraction of the stored mechanical energy. However, we note that in brackish water RO an energy recovery device (ERD) is often not economically justifiable (especially for lower feed salinities), as the energy savings are not substantial enough to outweigh the incurred capital cost. (42) Thus, in this study we consider scenarios both with and without the application of an energy recovery device, specifically indicating cases in which energy recovery is utilized.

RO Process Model

To describe the mass transfer and concentration polarization phenomena in the RO module, we utilize a classical solution–diffusion-based model coupled with film theory. (43−45) Particularly, we apply one-dimensional finite element analysis to discretize the membrane module and account for the varying flow rate and solution concentrations along the length of the membrane. With the system-scale salt removals of interest in this study (<95%) being considerably lower than the rejection of typical brackish water RO membranes (>99%), salt flux across the membrane is neglected throughout the analysis. This assumption of ideal salt rejection is often applied to simplify RO modeling (46) and is especially valid for the relatively low-salt-removal bypass system utilized, as the system-scale salt removal is primarily controlled by the product bypass stream, rather than the performance of the RO membrane.
With the assumption of complete salt rejection, mass transfer in the RO module is solely described by water transport across the membrane. According to solution–diffusion theory, the water flux in each finite element is given by
(7)
in which A is the membrane permeability coefficient, ΔP is the hydraulic pressure applied to the feedwater, and πF,m is the osmotic pressure corresponding to the salt concentration at the feed-side membrane surface. Throughout the analysis, a constant A value of 5 L m–2 h–1 bar–1 is utilized, representative of a typical brackish water RO membrane. (16)
To determine the water flux, the concentration at the membrane surface must be simultaneously solved. Here, we use film theory to describe concentration polarization effects near the membrane–solution interface. With the assumption of ideal salt rejection, the concentration at the membrane surface (cF,m) can be determined by
(8)
where cF,b is the local feed-side bulk concentration and kF is the feed-side mass transfer coefficient. The mass transfer coefficient is directly proportional to the boundary layer (film) thickness, which is a function of the hydrodynamic conditions in the RO module. However, with the mass transfer coefficient being highly system-dependent, in this study we use a fixed value of 150 L m–2 h–1, as is typical of current RO systems. (16,46)
As the feedwater flows along the length of the membrane, water is transported from the feed stream to the product stream, affecting the volumetric flow rates on each side of the membrane. Accordingly, in each finite element the change in the product-side flow rate (QP) with respect to the membrane surface area (Am) is equal to the water flux:
(9)
In contrast, the change in the flow rate on the feed-side (QF) is the opposite of the water flux:
(10)
By numerically integrating eqs 9 and 10 over the total membrane area, we can calculate the permeate and retentate flow rates at the exit of the module. Thus, the module-scale water recovery (r), the ratio of the permeate and RO feedwater flow rates, can be determined.

RO Performance Metrics

The RO model is used to determine the specific energy consumption (SEC) over various desalination conditions. In the described RO bypass system, the SEC is governed by the fraction of feedwater, which is directed to the RO module (αRO), and hence must be pressurized. Thus, the SEC is given by
(11)
where ΔP is the applied hydraulic pressure, ηERD is the efficiency of the energy recovery device (if applied), WRmod is the module-scale water recovery, WR is the system-scale water recovery, and ηP is the efficiency of the high pressure pump. The efficiency of the energy recovery device and high-pressure pump are both set to 0.8 throughout the analysis. We note that an efficiency of 0.8 may be considered low with respect to the performance of state-of-the-art pressure exchanger devices and high-pressure pumps; (11,47) however, for the comparative purposes of this study, we opt for relatively conservative values. As with ED, the SEC of RO is compared to the SECmin (eq 3) to determine the energy efficiency of desalination.
The flow rate fractions for the RO module feed (αRO), brine bypass (αB), and product bypass (αP) streams are critical parameters toward the system-scale performance as they are directly related to the energy consumption, salt removal, and water recovery. We note that the system-scale salt removal is defined in the same manner as ED (eq 4), while the system-scale water recovery (WR) is defined as the ratio of the volumetric flow rates of the product water (QP) and feedwater (QF):
(12)
The flow rate fraction of the product bypass stream controls the system-scale salt removal and also influences the system-scale water recovery. Accordingly, a system mass balance reveals that the flow rate fraction of the product water bypass stream is given by
(13)
The RO module flow rate fraction is also related to the system-scale salt removal and water recovery, while additionally affecting the module-scale water recovery:
(14)
After solving the product bypass and RO module flow rate fractions, the brine bypass flow rate fraction can be determined by realizing the sum of the flow rate fractions must equal unity.
Though the system-scale salt removal and water recovery are specified parameters for a given separation, the module-scale water recovery remains a degree of freedom. Affecting both the recoverable energy as well as the required hydraulic pressure (as shown by eq 11), the module-scale water recovery directly influences the SEC of the RO bypass system. Hence, for each investigated separation condition, the module-scale water recovery which minimizes the SEC is determined through numerical optimization. For further details regarding the bypass system modeling, we refer the reader to our previous work. (41)
In order to ensure a fair comparison, the productivity (or average water flux) of the RO system must be consistent with that of ED. The productivity, as previously defined, relates the total rate of water production to the system size. Hence, for the RO system, the productivity is determined in the same manner as ED (eq 6), with the membrane area, Am, being the projected surface area of the RO membrane.

COMPARISON OF ENERGY CONSUMPTION IN RO AND ED

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Effect of Varying Feed Salinity and Salt Removal

Increasing the feed salinity or the extent of salt removal demands greater energy consumption in both RO and ED, despite their highly distinct mechanisms of water–salt separation. In ED, treating higher salinity feeds or increasing the extent of desalination ultimately requires the transport of more ions across the ion-exchange membranes. Thus, a greater driving force, in the form of amplified electrical potential, must be applied, increasing SEC. For RO, SEC is dictated by the volume of feedwater that must be pressurized and the magnitude of pressure that must be applied by the pump. To make effective use of the membrane area, the hydraulic pressure applied must exceed the osmotic pressure of the retentate stream at the end of the RO module. Thereby, higher salinity feedwaters, which in turn generate retentates of higher concentration and osmotic pressure, require the application of larger hydraulic pressures. The extent of salt removal in RO, unlike ED, cannot readily be adjusted by manipulating the driving force. Rather, we enable variable salt removal through the bypass system described in Figure 2. In such a bypass system, increasing the degree of desalination requires a larger fraction of the feedwater to pass through the RO module, thereby increasing the energy demand of the pump.
In Figure 3, we show the energy consumption, SEC, of RO (without energy recovery) and ED as a function of salt removal for various salinity feedwaters. Feed salinities of 1 g L–1 (orange), 3 g L–1 (red), 5 g L–1 (blue), and 10 g L–1 (purple) are assessed to encompass the range of brackish water salinities, while water recovery and productivity are fixed at 80% and 20 L m–2 h–1, respectively. As expected, the SEC of both RO and ED consistently increases with the extent of salt removal and feed salinity, albeit at varying rates. Overall, Figure 3 highlights that ED and RO outperform one another for particular feed salinities and salt removals. Whereas ED performs best for relatively low feed salinities and salt removals, RO has an energetic advantage over ED for higher feed salinities and salt removals.

Figure 3

Figure 3. Specific energy consumption (SEC) for RO and ED as a function of salt removal (Rs). The water recovery and productivity are fixed at 80% and 20 L m–2 h–1, respectively. The various colored lines show different feed salinities: 1 g L–1 (orange), 3 g L–1 (red), 5 g L–1 (blue), and 10 g L–1(purple). (A) Specific energy consumption of RO with a water permeability and feed-side mass transfer coefficient of 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The module-scale water recovery is optimized for each feed salinity and salt removal to minimize SEC. Energy recovery is not applied. (B) Specific energy consumption of ED with the modeling parameters specified in Table S1. The cell pair voltage is tuned to achieve varying degrees of salt removal. Effluent from the brine channels is recycled at a recycle ratio of 3.0 to achieve a water recovery of 80%.

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Specifically, for feed salinities of 1 and 3 g L–1, ED has lower energy consumption than RO throughout the entire range of salt removals (20% to 90%), though the disparity between the SEC decreases as the salt removal and feed salinity are increased. For example, for a 1 g L–1 feed and 30% salt removal, ED utilizes 0.013 kWh m–3 while RO requires more than 3-fold the energy (0.042 kWh m–3). When salt removal is increased to 80%, however, this factor decreases to nearly 2-fold. Similarly, the energy consumption of RO and ED converges when assessing a 3 g L–1 feed salinity. For a salt removal of 80%, for instance, the SEC of ED is 0.29 kWh m–3, while that of RO is only 24% higher at 0.36 kWh m–3.
Though ED requires less energy for the treatment of relatively low feed salinities, RO becomes the energetically superior technology as feed salinity is increased, particularly at high salt removals. When the feed salinity is increased to 5 g L–1, for example, ED maintains lower SEC than RO up to 80% salt removal. However, further increasing the feed salinity to 10 g L–1 shifts this transition point to only 65% salt removal. Notably, for high feed salinities and salt removals, the magnitude of the SEC considerably grows, thereby also making the difference in the performance of RO and ED more substantial. For instance, in the case of 90% salt removal of a 10 g L–1 feed, the SEC of ED is 2.99 kWh m–3 while that of RO is only 1.51 kWh m–3, a sizable difference of 1.48 kWh m–3. In contrast, for a 3 g L–1 feed, the maximum difference in the SEC of ED and RO is only 0.093 kWh m–3.
The superior performance of RO at higher feed salinities and salt removals can readily be discerned by noting the relatively linear trend of its SEC curves with respect to those of ED. The near linear relation observed for the RO curves is a result of the bypass system utilized, by which increasing the salt removal generally requires the direction of more feedwater through the RO module. By contrast, the effects of increasing the salt removal in ED are more complex, leading to deviation from linearity. Particularly, as greater extents of desalination are achieved, the diluate solution becomes depleted of ions, thereby increasing the solution resistance and the magnitude of the associated potential drop. Greater salt removal (for a fixed productivity) also necessitates an increase of the counterion flux, leading to sharper concentration gradients at the membrane–solution interface and, thus, intensified driving force for the passage of co-ions. Additionally, higher degrees of salt removal generate more severe transmembrane concentration gradients, increasing the rate of back-diffusion of counterions from the concentrate channel to the diluate channel. (17) The compromised selectivity of the ion-exchange membranes and the larger rate of back-diffusion that result from an increase in salt removal ultimately deteriorate the current efficiency of the ED process, thus requiring more energy consumption. Notably, these detrimental effects are exacerbated at higher feed salinities, as is apparent by the increasingly exponential behavior of the SEC curves in Figure 3B. For example, for a 3 g L–1 feed, increasing the salt removal from 30% to 80% results in a decline of current efficiency from 88% to 76%. However, for a 10 g L–1 feed, the current efficiencies for 30% and 80% salt removal are reduced to 70% and 49%, respectively.

Effect of Varying Water Recovery

Maximizing water recovery, the volume of product water generated per volume of feedwater, is a primary objective of desalination processes. However, with an increase in water recovery corresponding to a greater degree of water–salt separation, additional energy input is typically required. In RO, increasing the system-scale water recovery necessitates that either more water be directed through the RO module or the module-scale recovery be increased, both of which increase SEC. We note that for each unique set of operating conditions, the module-scale recovery, and thus the flow rate fraction to the RO module, are numerically optimized to minimize SEC.
In ED, the volumetric flow rates through the diluate and concentrate channels are held equal to minimize transmembrane water flux, thus fixing the module-scale water recovery at 50%. Therefore, the system-scale water recovery is increased by recirculating a fraction of the produced brine to the feed of the concentrate channels, in effect reducing the volume of fresh feedwater required to generate a given volume of product water. However, the implementation of such brine recirculation further increases the salinity in the concentrate channels, intensifying the effects of performance deteriorating phenomena (i.e., co-ion transport and back-diffusion). The resulting decline in current efficiency thus requires a larger SEC for a given salt removal.
As determined in the previous section, the energetic performance of RO and ED begins to notably diverge for a feed salinity of 5 g L–1. Hence, we continue our analysis of a 5 g L–1 feed salinity in Figure 4, where we show the thermodynamic energy efficiency of RO and ED for varying water recovery and salt removal. The productivity, as before, is maintained at 20 L m–2 h–1 throughout. In each color map, the energy efficiency for a given water recovery and salt removal corresponds to the provided scale bars. Contour lines, representing fixed SEC values, are also shown to illustrate the energetic penalty of increasing either salt removal or water recovery.

Figure 4

Figure 4. Energy efficiency color maps of RO and ED for varying salt removal (Rs) and system-scale water recovery (WR). The energy efficiency (η) is defined as the ratio of the thermodynamic minimum specific energy consumption of separation (SECmin) and the actual specific energy consumption (SEC). The values for energy efficiency correspond to the color legends shown on the right side of each plot. The productivity is set to 20 L m–2 h–1 throughout. (A) Energy efficiency of RO for 5 g L–1 feed salinity. The water permeability coefficient and the feed-side mass transfer coefficient are set to 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The module-scale water recovery is optimized for each salt removal and system-scale water recovery to maximize the energy efficiency. Energy recovery is not applied. (B) Energy efficiency of ED for 5 g L–1 feed salinity. The salt removal and water recovery are varied by tuning the cell pair voltage and brine effluent recycle ratios, respectively. The black contour lines shown in (A) and (B) represent fixed SEC values.

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The scale bars in Figure 4 reveal that RO and ED perform over a similar range of energy efficiency values. Specifically, the energy efficiency of RO over the conditions shown ranges from 9.8% to 27.0%, while ED operates at 5.5% to 29.5% energy efficiency. Nonetheless, the salt removal conditions over which each technology operates with maximum energy efficiency are in stark contrast. Whereas RO attains optimal energy efficiency when salt removal is maximized, ED displays its best performance for moderate salt removals, in agreement with the findings of Figure 3. Notably, both RO and ED display their highest energy efficiency for relatively high-water recoveries, ranging from approximately 70% to 80%. Moving away from these optimal regions leads to gradual decline in energy efficiency, as the rate of growth of the SECmin overtakes that of the SEC. However, ED, unlike RO, maintains high energy efficiency for most conditions outside of its optimal operating zone, showing particularly low energy efficiencies only when both the water recovery and salt removal are increased beyond 90%.

Effect of Varying Productivity

In addition to energy consumption, the productivity, which relates a system’s throughput to its size, is a key performance metric for assessing desalination process performance. Whereas the energy consumption determines operational costs, the productivity is indicative of the required capital, with higher productivities corresponding to more compact and cost-efficient systems. However, in membrane separation processes, increased productivities generally come at the expense of energy efficiency. (48−52) For RO, an increase in productivity requires a faster rate of water transport across the membrane, for which larger hydraulic pressures must be applied. To increase the productivity in ED, in contrast, the volumetric flow rate through each of the spacer channels is increased, decreasing the hydraulic retention time. Thus, for a fixed salt removal, a larger current density must be realized by increasing the applied voltage.
In Figure 5, we show the specific energy consumption, SEC, and energy efficiency, η, of RO and ED for productivities of 20, 40, and 60 L m–2 h–1, covering the typical range of values reported in brackish water desalination. (36,46,50,52−56) For each productivity, we show salt removals of 50% (red), 70% (blue), and 90% (orange), while fixing the feed salinity and water recovery at 5 g L–1 and 80%, respectively. As expected, for a fixed salt removal, the SEC of both RO and ED increase with productivity, with ED showing slightly greater sensitivity to changes in productivity than RO. For example, for a salt removal of 70%, increasing the productivity from 20 to 60 L m–2 h–1 increases the SEC of ED by 33.9%, whereas RO increases by 27.3%. Nonetheless, the magnitude of the SEC of ED remains smaller than RO, except for in the case of 90% salt removal.

Figure 5

Figure 5. (A) Specific energy consumption and (B) corresponding energy efficiency of ED (open bars) and RO with no energy recovery (hatched bars) for varying productivity. The energy efficiency is defined as the ratio of the thermodynamic minimum specific energy consumption of separation and the actual specific energy consumption. Salt removals of 50% (red bars), 70% (blue bars), and 90% (orange bars) are shown. The water recovery and feed salinity are set to 80% and 5 g L–1, respectively. Productivity is defined as the rate of water production normalized by the projected membrane area (eq 6). For ED, the productivity is the flow rate through an individual spacer channel divided by the projected area of an ion-exchange membrane. In RO, the productivity is the product water flow rate divided by the membrane area of the module.

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Though increasing the productivity results in a higher SEC by RO and ED, it is important to note that it has no effect on the SECmin. Thereby, the energy efficiency of RO and ED (for a fixed salt removal) declines with increasing productivity, as is shown by Figure 5B. As previously discussed, the energy efficiency of ED is reduced with increasing salt removal, whereas the opposite effect is observed for RO. It is notable, however, that as the productivity is increased, the energetic advantage of ED for lower salt removals begins to narrow. For instance, for 70% salt removal and 20 L m–2 h–1 productivity, the energy efficiency of ED is 5.3% higher than that of RO. However, when the productivity is increased to 60 L m–2 h–1 the difference in the energy efficiency of ED and RO is reduced to 3.1%. Hence, in cases where very high throughputs are required, ED’s superior performance for low salt removal may potentially be overturned by RO.

Identifying the Operational Sweet Spots of RO and ED

Thus far, we have demonstrated the general trends in the energy consumption of RO and ED for varying separation and operating conditions. Specifically, we showed that ED outperforms RO for lower feed salinities and extents of salt removal and that both technologies are capable of performing efficiently at similarly high water recoveries. Nonetheless, the particular conditions under which each process exhibits superior performance have yet to be identified. Hence, in this section we focus on locating the operational sweet spots of RO and ED.
Notably, in the following analysis we investigate only the effects of varying separation conditions (i.e., feed salinity, salt removal, and water recovery), while fixing the productivity at 20 L m–2 h–1, a typical value for brackish water desalination. (29,46,53,55) We choose to exclude productivity as a variable on the basis of our results from the previous section, which showed both technologies demonstrate remarkable stability in their energy efficiency, even when the productivity is increased up to 3-fold. Instead, we now consider the application of energy recovery, a feature exclusive to RO. The energy recovery device (ERD) is assumed to operate at 80% efficiency, representative of the performance of a turbocharger or Pelton wheel type ERD. (47) Though such ERDs operate at considerably lower efficiencies than modern pressure exchangers, they continue to serve as economical alternatives for energy recovery in brackish water RO. (11,42,57)
To make the optimal operating conditions for each technology apparent, in Figure 6 we show color maps of the difference between the energy efficiency of ED and RO. The sign of the values signifies the energetically superior technology, with positive and negative values corresponding to higher energy efficiency in ED and RO, respectively. The magnitude of the values, in contrast, indicates the extent of the advantage the technology provides. White contour lines are provided on each of the color maps to clearly show the conditions at which RO and ED operate with equal energy efficiency. Hence, these transition lines effectively demarcate the operational sweet spots of each technology. In Figure 6A,C,E, energy recovery is not used, whereas in Figure 6B,D,F the application of energy recovery in RO is included. The difference in ED and RO energy efficiency is shown for varying water recovery and salt removal, while the feed salinity is specified on the individual plots. We continue with the evaluation of 3, 5, and 10 g L–1 feed salinities, but no longer consider a feed salinity of 1 g L–1 since ED was shown to considerably outperform RO across the entire range of salt removals. Furthermore, at 1 g L–1 feed salinities and below, the values for SEC are very small, making energy recovery in RO impractical and the comparison of energy consumption among ED and RO of minimal significance.

Figure 6

Figure 6. Energy efficiency phase diagrams of ED and RO for varying salt removal (Rs) and system-scale water recovery (WR). The difference between the ED and RO energy efficiencies is plotted, with values corresponding to the color legends shown to the right of each panel. The white contour lines show the transition between ED and RO being more energy efficient: above the line, RO is more energy efficient, while below the line, ED is more energy efficient. The energy efficiency (η) is defined as the ratio of the thermodynamic minimum specific energy consumption of separation and the actual specific energy consumption. The productivity is set to 20 L m–2 h–1 throughout. The water permeability coefficient and the feed-side mass transfer coefficient for RO are set to 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The RO module-scale water recovery is optimized for each salt removal and system-scale water recovery to maximize the energy efficiency. In ED, the salt removal and water recovery are varied by tuning the cell pair voltage and brine effluent recycle ratios, respectively. (A), (C), (E) Energy efficiency phase-diagram of ED-RO with no energy recovery in RO for 3, 5, and 10 g L–1 feed salinity, respectively. (B), (D), (F) Energy efficiency phase-diagram of ED-RO with an RO energy recovery device operating at 80% efficiency (ηERD = 0.8) for 3, 5, and 10 g L–1 feed salinity, respectively.

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In Figure 6A,B, color maps for a feed salinity of 3 g L–1 are shown. For the case of no energy recovery in RO (Figure 6A), the transition line extends across the top right corner of the plot, indicative of ED being the dominant technology up to very high salt removals and water recoveries. Specifically, until a water recovery of 73%, the energy efficiency of ED remains higher than RO across all the shown salt removals (up to 95%). As the water recovery is further increased, the transition line trends toward lower salt removal values, demonstrating that when high recoveries and salt removals are needed, RO is the more favorable technology. For example, at 90% water recovery and 95% salt removal, the energy efficiency of RO is 8.4% higher than that of ED. Nonetheless, it is important to note that desalination of a 3 g L–1 feed to drinking water standards requires only 83% salt removal, and that further extending the depth of desalination incurs greater energetic and operational costs. Hence, for the case of the more practical 83% salt removal, the energy efficiency of ED is superior to RO across all water recoveries. When energy recovery is applied to RO (Figure 6B), the transition line shifts considerably downward, providing RO with a much larger region of favorable conditions. However, even with energy recovery, at 83% salt removal ED retains an advantage over RO, albeit marginal.
Upon increasing the feed salinity to 5 g L–1 in Figure 6C,D, the transition lines shift further downward from the 3 g L–1 counterparts, demonstrating the increasing advantage of RO. Nonetheless, when energy recovery is not applied, ED remains competitive with RO. Particularly, in the case of 90% salt removal to achieve a 0.5 g L–1 product water salinity, ED remains energetically superior up to a water recovery of 67%. However, in the case of energy recovery and 90% salt removal, RO overtakes ED with a notable difference in energy efficiency of up to 11%. When the feed salinity is further increased to 10 g L–1, any practical advantage of ED is eliminated, even in the case of no energy recovery in RO. With a 10 g L–1 feed salinity, 95% salt removal is required to achieve drinking water quality. For such high salt removals, RO becomes the superior technology, showing over a 17% difference in energy efficiency in the case of no energy recovery and 80% water recovery. The application of energy recovery further widens the gap in energy efficiency to 22%, highlighting RO’s dominance for high salinity and salt removal applications.

IMPLICATIONS AND OUTLOOK

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As mature desalination technologies, ED and RO have both been proven effective and reliable for full-scale brackish water desalination. Nonetheless, RO has established dominance over the brackish water desalination market share in recent years, with little discussion in the literature regarding the limited adoption of electrodialysis. In this study, we presented the first direct and thorough comparison of the energy consumption of RO and ED for brackish water desalination, providing insight and guidance toward the application of each technology. The operational sweet spots for ED and RO were identified, revealing that ED and RO are both capable of operating with similarly high energy efficiencies, though under different conditions. Here, we summarize the determined energetic advantages of each technology and extend our discussion to include additional practical factors that may influence which process is preferred. To make the advantages of each technology apparent, we rank RO and ED on a five-point scale for each of the considered metrics, as shown in Table 1.
Table 1. Rating of ED and RO across Several Key Brackish Water Desalination Performance Metrics
As demonstrated throughout our analysis, ED and RO operate with high energy efficiency in brackish water desalination, exceeding values of 30% for operation within each technology’s respective sweet spot. When compared to efficiencies of emerging alternatives such as capacitive deionization (CDI) or thermal distillation technologies, which typically operate with single-digit thermodynamic energy efficiency, the energy efficiencies of ED and RO are remarkable. (7) However, it is important to note that whereas ED operates most efficiently for low feed salinity separations, for which the magnitude of SEC is small, RO provides energy efficient operation for energy intensive separations of higher feed salinities and salt removals, for which energy consumption is a major concern. Hence, we discern that RO is the overall more energy efficient technology compared to ED, as shown by RO and ED’s five- and four-point ratings in Table 1, respectively.
We note that in this study we considered only single-pass systems for RO and ED. Alternative process designs, particularly for RO, offer the potential to significantly improve energy efficiency, though at greater capital cost. For example, it is well established that significant energy savings can be realized in brackish water RO by implementing brine staging, in which the brine produced by one stage serves as the feed for the following stage. (10,11,58) With such a multistage configuration, the applied hydraulic pressure in each stage can be varied according to the osmotic pressure of the respective feed, bringing the process closer to the ideal case of thermodynamic reversibility. (6) Splitting the RO process into just two stages, for example, has been shown to decrease the specific energy consumption for brackish water desalination by up to 40%. (59) Similarly, closed-circuit RO, in which the hydraulic pressure is temporally varied as the brine is recirculated and mixed with the feed, has been shown to provide up to 45% reduction in energy consumption as compared to single-stage BWRO. (59,60) Though the energy efficiency of RO can effectively be enhanced by such system configurations, the energy consumption of ED is relatively insensitive to similar approaches due to the large current densities (and resulting high rates of entropy generation) characteristic of cost-effective, compact ED stacks. (61,62) For instance, implementing two-stage ED for brackish water desalination was shown to result in less than a 5% decrease in SEC. (63) Accordingly, we further emphasize RO’s superior ranking to ED in terms of energy efficiency.
A key outcome of our analysis is the identification of precise feed salinity ranges that are most suitable for efficient operation of ED and RO. As shown in Figures 1 and 6, ED outperforms RO across all practically relevant water recoveries and salt removals for feed salinities up to 3 g L–1, even when energy recovery is applied in RO. With the treatment of such low feed salinities constituting nearly 30% of brackish water desalination, (8) expanded application of ED is warranted from the perspective of energy efficiency. However, we reiterate that the specific energy consumption for the desalination of low feed salinities is relatively small, often making capital investment a greater consideration than operational costs.
In our analysis, we further showed that the energetic performances of ED and RO begin to notably diverge at a feed salinity of 5 g L–1, with RO gaining a considerable advantage for practically relevant salt removals beyond this point. It is important to note that though RO only outperforms ED for feed salinities of 5 g L–1 and greater, it retains relatively strong performance even in the treatment of lower feed salinity applications. In contrast, the energy efficiency of ED deteriorates rapidly outside its optimal salinity range (i.e., high feed salinities), restricting its practical applicability across a wide range of feed salinities. For example, whereas RO still operates with 13% energy efficiency for 50% salt removal of a 1 g L–1 feed, the energy efficiency of ED for 95% salt removal of a 10 g L–1 feed is less than 7% (with productivity and water recovery in both scenarios fixed at 20 L m–2 h–1 and 80%, respectively). Thus, RO provides greater versatility for applications in which the feed salinity considerably varies.
Though high feed salinities inherently require large degrees of salt removal to achieve drinking water quality, applying such extensive desalination to lower feed salinities unnecessarily expends additional energy. Additionally, due to its corrosivity and low alkalinity, completely deionized water is unsuitable for widespread distribution and consumption, thus requiring remineralization. (13) Desalination technologies that readily facilitate variable degrees of salt removal are therefore essential toward efficient brackish water desalination. In ED, the applied voltage to the stack can easily be adjusted, thereby allowing for precise tunability in the amount of salt removal. (18,26,64) In contrast, the degree of salt removal in RO is difficult to control. Though in this study we utilized an RO bypass system that employs both brine and product water bypass streams to control the extent of salt removal, we note that such a process has yet to be implemented in practice and would require complex control systems to optimize and manipulate the feed flow rate distribution in real-time. Hence, for relatively low feed salinity applications, ED holds a significant advantage over RO by avoiding inefficient overtreatment and remineralization.
Another important distinction between ED and RO worth noting is their ability to remove various types of solutes, specifically charged and uncharged species. Whereas RO relies on a combination of charge and size exclusion to reject both charged and uncharged solutes in the feedwater, ED’s reliance on an electric field driving force inherently limits its applicability to the removal of only charged species. Therefore, RO, unlike ED, is capable of effectively removing many uncharged micropollutants, viruses, and micro-organisms to simultaneously meet multiple water quality objectives. (65) Nonetheless, it is worth noting that RO membranes still struggle with the rejection of smaller uncharged species, such as 1,4-dioxane and N-nitrosodimethylamine (NDMA)—contaminants of growing concern. (66,67) The selection of RO or ED for brackish water desalination should therefore be source-water specific, with complex or polluted feedwaters opting for the use of RO due to its rejection of a broad spectrum of contaminants.
Through our analysis, we revealed that RO and ED are both capable of operating efficiently at high water recoveries (∼80%). We note, however, that our study considered water recoveries only up to 90%, as further increasing the recovery results in severe membrane fouling and scaling. (68) Particularly, brackish groundwaters typically contain high concentrations of scale-forming inorganic species, such as calcium, sulfate, and silica, which precipitate on the membrane surface when concentrated at high water recoveries. (69) Similarly, the propensity for colloidal and organic fouling on membrane surfaces increases as the concentration of foulants is elevated with increasing water recovery. In ED, fouling and scaling of the ion-exchange membranes results in increased electrical resistance and reduced ion selectivity, increasing SEC for a given separation. (17,70) Fouling and scaling in RO also results in increased process energy consumption, as higher hydraulic pressures are required to maintain the necessary water flux. (40)
Though fouling and scaling remain a challenge in both ED and RO, severity and effectiveness of mitigation strategies for each technology vary. In RO, typical approaches to control membrane fouling and scaling include pretreatment, mechanical or chemical cleaning, and membrane surface modification. (71) Such methods are also applicable to ED, in addition to exclusive strategies that exploit the use of the electric field driving force. Specifically, periodic reversal or pulsation of the electric field have been shown to be highly effective methods to prevent fouling and scaling of the ion-exchange membranes. (72) Though pulsed current operation is still under investigation at the lab-scale, current reversal is commonly practiced at the industrial scale and has been shown to minimize the frequency of membrane cleaning and replacement. (73) An additional aspect of ED that reduces the severity of fouling and scaling with respect to RO is the tendency of only charged foulants to be drawn to the ion-exchange membranes via electromigration. Conversely, in RO, the bulk convective permeate flow transports all foulants, including uncharged species, to the membrane surface, upon which permeation drag further exacerbates fouling rates. (74) Thus, neutral foulants that continue to plague RO operation, such as silica, (75,76) are less of a concern in ED.
The primary focus of this study was to distinguish ED and RO in terms of energy consumption. However, an equally important consideration in the determination of a preferred technology is capital cost. Here, we estimate the capital cost of each technology on the basis of the respective membranes utilized. Estimates from industry reveal that the area normalized cost of current ion-exchange membranes (>200 $ m–2) (28) is more than an order of magnitude greater than thin-film composite RO membranes (<10 $ m–2), (77) severely hampering the practical implementation of ED. By fixing the system productivity between RO and ED throughout our analysis, we ensured the technologies were fairly compared with regard to system size. However, it should be noted that the productivity in ED is normalized by the projected area of only one of the ion-exchange membranes in a cell pair, which actually consists of both an anion- and cation-exchange membrane. Thus, for a given productivity, ED requires double the membrane area compared to RO, inherently putting ED at an even larger disadvantage in terms of cost. Furthermore, capital cost becomes increasingly important as the magnitude of the energy consumption, and thus the operational costs, decrease. It is therefore sensible why ED, despite being more energy efficient for low-feed salinity desalination, has been dominated by the more cost-effective RO. Though the extremely high cost of ion-exchange membranes currently hampers the application of ED, economies of scale and renewed interest in the research and development of ion-exchange membranes—largely due to their application in energy conversion and storage technologies—could optimiztically drive down the future cost of ion-exchange membranes to be more economically competitive with RO membranes.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.0c00192.

  • Description of ED process model; parameters utilized for ED process modeling (Table S1); schematic illustration of feed-and-bleed ED operation mode (Figure S1); ED model validation (Figure S2); effect of changing velocity profile on prediction of ED process performance (Figure S3); ED model-predicted ion-exchange membrane resistances as a function of feedwater salinity (Figure S4) (PDF)

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Author Information

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  • Corresponding Author
    • Menachem Elimelech - Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States;  Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520-8286, United States;  Orcidhttp://orcid.org/0000-0003-4186-1563;  Email: [email protected]
  • Authors
    • Sohum K. Patel - Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States;  Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520-8286, United States;  Orcidhttp://orcid.org/0000-0001-5228-9449
    • P. Maarten Biesheuvel - European Centre of Excellence for Sustainable Water Technology, Wetsus, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands;  Orcidhttp://orcid.org/0000-0002-5468-559X
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the NSF Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500).

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    Wang, Li; Violet, Camille; DuChanois, Ryan M.; Elimelech, Menachem
    Journal of Chemical Education (2020), 97 (12), 4361-4369CODEN: JCEDA8; ISSN:0021-9584. (American Chemical Society and Division of Chemical Education, Inc.)
    Minimizing the energy consumption of desalination processes is an important goal for augmenting freshwater prodn. and mitigating water scarcity. Chem., civil, mech., and environmental engineering students can derive and analyze the energy consumption of desalination processes by applying engineering fundamentals such as thermodn., transport phenomena, and process design. We explore the fundamental thermodn. limits of the most prominent desalination technologies in a format designed for engineering students and instructors. Two conceptual processes for reverse osmosis (RO) and electrodialysis (ED) are developed to demonstrate that thermodynamically reversible processes consume the theor. min. energy, which is the Gibbs free energy of sepn. We then quantify the practical min. energy consumption for RO and ED, showing that the energy consumption of these processes approaches the min. thermodn. limit with increased process staging.
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  7. 7
    Lin, S. H. Energy Efficiency of Desalination: Fundamental Insights from Intuitive Interpretation. Environ. Sci. Technol. 2019, 54 (1), 76– 84,  DOI: 10.1021/acs.est.9b04788
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  8. 8
    Jones, E.; Qadir, M.; van Vliet, M. T. H.; Smakhtin, V.; Kang, S. M. The state of desalination and brine production: A global outlook. Sci. Total Environ. 2019, 657, 1343– 1356,  DOI: 10.1016/j.scitotenv.2018.12.076
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    The state of desalination and brine production: A global outlook
    Jones, Edward; Qadir, Manzoor; van Vliet, Michelle T. H.; Smakhtin, Vladimir; Kang, Seong-mu
    Science of the Total Environment (2019), 657 (), 1343-1356CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    Rising water demands and diminishing water supplies are exacerbating water scarcity in most world regions. Conventional approaches relying on rainfall and river runoff in water scarce areas are no longer sufficient to meet human demands. Unconventional water resources, such as desalinated water, are expected to play a key role in narrowing the water demand-supply gap. Our synthesis of desalination data suggests that there are 15,906 operational desalination plants producing around 95 million m3/day of desalinated water for human use, of which 48% is produced in the Middle East and North Africa region. A major challenge assocd. with desalination technologies is the prodn. of a typically hypersaline conc. (termed 'brine') discharge that requires disposal, which is both costly and assocd. with neg. environmental impacts. Our ests. reveal brine prodn. to be around 142 million m3/day, approx. 50% greater than previous quantifications. Brine prodn. in Saudi Arabia, UAE, Kuwait and Qatar accounts for 55% of the total global share. Improved brine management strategies are required to limit the neg. environmental impacts and reduce the economic cost of disposal, thereby stimulating further developments in desalination facilities to safeguard water supplies for current and future generations.
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  9. 9
    Tanaka, Y. A computer simulation of batch ion exchange membrane electrodialysis for desalination of saline water. Desalination 2009, 249 (3), 1039– 1047,  DOI: 10.1016/j.desal.2009.06.055
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    A computer simulation of batch ion exchange membrane electrodialysis for desalination of saline water
    Tanaka, Yoshinobu
    Desalination (2009), 249 (3), 1039-1047CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    A computer simulation program is developed for predicting desalinating performance of a batch electrodialysis process. The program includes the principle of (1) mass transport, (2) c.d. distribution, (3) cell voltage, (4) mass balance/energy consumption and (5) limiting c.d. In this simulation the following parameters are inputted; (1) membrane characteristics such as overall transport no., overall solute permeability, overall electro-osmotic permeability, overall hydraulic permeability, d.c. elec. resistance etc., (2) electrodialyzer specifications such as flow-pass thickness, flow-pass width and flow-pass length in a desalting cell etc. and (3) electrodialytic conditions such as voltage, electrolyte concn. in a feeding soln., linear velocity in desalting cells, std. deviation of normal distribution of soln. velocity ratio etc. The following phenomena were computed and discussed; (1) Changes of electrolyte concn. and c.d. with operation time. (2) Influence of cell voltage on operation time (batch duration), water recovery and energy consumption. (3) Influence of vol. of an electrolyte soln. prepd. at first on operation time. (4) Influence of cell voltage, electrolyte concn. and std. deviation of soln. velocity ratio in desalting cells on limiting c.d. (5) Energy consumption in a reverse osmosis process. (6) Excepting limiting c.d., the performance of an electrodialyzer is never influenced by the std. deviation of normal distribution of soln. velocity ratio in desalting cells. (7) Energy consumption in electrodialysis is less than that in reverse osmosis at feeding saline water concn. less than about 2000 mg/l.
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  10. 10
    Elimelech, M.; Phillip, W. A. The future of seawater desalination: energy, technology, and the environment. Science 2011, 333 (6043), 712– 717,  DOI: 10.1126/science.1200488
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    The Future of Seawater Desalination: Energy, Technology, and the Environment
    Elimelech, Menachem; Phillip, William A.
    Science (Washington, DC, United States) (2011), 333 (6043), 712-717CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    A review. In recent years, numerous large-scale seawater desalination plants were built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, the authors review the possible redns. in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technol. soln. to global water shortages.
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    Greenlee, L. F.; Lawler, D. F.; Freeman, B. D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water sources, technology, and today’s challenges. Water Res. 2009, 43 (9), 2317– 2348,  DOI: 10.1016/j.watres.2009.03.010
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    11
    Reverse osmosis desalination: Water sources, technology, and today's challenges
    Greenlee, Lauren F.; Lawler, Desmond F.; Freeman, Benny D.; Marrot, Benoit; Moulin, Philippe
    Water Research (2009), 43 (9), 2317-2348CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    A review. Reverse osmosis membrane technol. has developed over the past 40 years to a 44% share in world desalting prodn. capacity, and an 80% share in the total no. of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technol. for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (conc.) disposal options, and plant location, have created significant differences in process development, implementation, and key tech. problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technol. in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.
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    Loeb, S. The Loeb-Sourirajan Membrane - How It Came About. ACS Symp. Ser. 1981, 153, 1,  DOI: 10.1021/bk-1981-0153.ch001
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    The Loeb-Sourirajan membrane: how it came about
    Loeb, Sidney
    ACS Symposium Series (1981), 153 (Synth. Membr., Vol. 1), 1-9CODEN: ACSMC8; ISSN:0097-6156.
    A review with 16 refs.
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    Kucera, J. Reverse Osmosis: Industrial Processes and Applications; John Wiley & Sons, 2015.
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    Porter, C. J.; Werber, J. R.; Zhong, M.; Wilson, C. J.; Elimelech, M. Pathways and Challenges for Biomimetic Desalination Membranes with Sub-Nanometer Channels. ACS Nano 2020, 14 (9), 10894– 10916,  DOI: 10.1021/acsnano.0c05753
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    Pathways and challenges for biomimetic desalination membranes with sub-nanometer channels
    Porter, Cassandra J.; Werber, Jay R.; Zhong, Mingjiang; Wilson, Corey J.; Elimelech, Menachem
    ACS Nano (2020), 14 (9), 10894-10916CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A review. Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrixes on a robust support. While biol. components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers. However, the exptl. performance of biomimetic membranes remains far below that of biol. membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection. We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examn. of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m-2 h-1 bar-1, which is comparable to that of current state-of-the-art polymeric desalination membranes. Through a quant. assessment of biomimetic membrane formats, we anal. show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains obsd. subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.
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  15. 15
    Patel, S. K.; Ritt, C. L.; Deshmukh, A.; Wang, Z. X.; Qin, M. H.; Epsztein, R.; Elimelech, M. The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies. Energy Environ. Sci. 2020, 13 (6), 1694– 1710,  DOI: 10.1039/D0EE00341G
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    The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies
    Patel, Sohum K.; Ritt, Cody L.; Deshmukh, Akshay; Wang, Zhangxin; Qin, Mohan; Epsztein, Razi; Elimelech, Menachem
    Energy & Environmental Science (2020), 13 (6), 1694-1710CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    As the threat of global water scarcity continues to grow, a myriad of scientific effort is directed towards advancing water desalination technologies. Reverse osmosis (RO), solar thermal desalination (STD), and capacitive deionization (CDI), have dominated recent pressure-, thermal-, and electro-driven desalination research efforts, resp. Despite being based on distinctive driving forces and sepn. mechanisms, research of these three processes has primarily shared the same fundamental goal and approach: the minimization of energy consumption for desalination through the development of novel materials. A variety of materials have been studied and proposed to enhance RO membrane permeability, STD solar absorptivity, and CDI electrode capacitance. Here, we critically discuss the advanced materials investigated and assess their efficacy in augmenting the energy efficiency of desalination. Through our systematic anal., we show that materials have relatively insignificant impact on further increasing energy efficiency, regardless of the process applied. We provide insights into the inherent limitations of advanced materials for improving the energy efficiency of each of the evaluated technologies and propose more effective materials-based research directions. We conclude by highlighting the opportunity for considerable improvement in energy efficiency via system design, reinforcing the crit. need for a paradigm shift in desalination research.
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    Werber, J. R.; Deshmukh, A.; Elimelech, M. The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes. Environ. Sci. Technol. Lett. 2016, 3 (4), 112– 120,  DOI: 10.1021/acs.estlett.6b00050
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    16
    The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes
    Werber, Jay R.; Deshmukh, Akshay; Elimelech, Menachem
    Environmental Science & Technology Letters (2016), 3 (4), 112-120CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    A review is given. Desalination membranes are essential for the treatment of unconventional water sources, such as seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel membrane materials may yield significant performance gains over state-of-the-art thin-film composite (TFC) membranes, which are constrained by the permeability-selectivity trade-off. However, little guidance currently exists on the practical impact of such performance gains, namely enhanced water permeability or enhanced water-solute selectivity. We 1st discuss the performance of current TFC membranes. We then highlight and provide context for recent module-scale modeling studies that have found limited impact of increased water permeability on the efficiency of desalination processes. We cover several important examples of water treatment processes in which inadequate membrane selectivity hinders process efficacy. We conclude with a discussion of how the need for enhanced selectivity may influence the design strategies of future membranes.
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    Strathmann, H. Ion-exchange membrane separation processes; Elsevier, 2004; Vol. 9.
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    Strathmann, H. Electrodialysis, a mature technology with a multitude of new applications. Desalination 2010, 264 (3), 268– 288,  DOI: 10.1016/j.desal.2010.04.069
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    Electrodialysis, a mature technology with a multitude of new applications
    Strathmann, H.
    Desalination (2010), 264 (3), 268-288CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Electrodialysis is a mature process which is applied since more than 50 years on a large industrial scale for the prodn. of potable water from brackish water sources. But more recently electrodialysis in combination with bipolar membranes or with ion-exchange resins has found a large no. of new interesting applications in the chem. process industry, in the food and drug industry as well as in waste water treatment and the prodn. of high quality industrial water. In this paper the principle of electrodialysis is described and its advantages and limitations in various applications are pointed out. More recent developments in electrodialysis as well as in related processes such as electrodialytic water dissocn. or continuous electrodeionization are discussed and their present and potential future applications are indicated. Research needs for a sustainable growth of electrodialysis and related processes are pointed out.
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    Campione, A.; Gurreri, L.; Ciofalo, M.; Micale, G.; Tamburini, A.; Cipollina, A. Electrodialysis for water desalination: A critical assessment of recent developments on process fundamentals, models and applications. Desalination 2018, 434, 121– 160,  DOI: 10.1016/j.desal.2017.12.044
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    Electrodialysis for water desalination: A critical assessment of recent developments on process fundamentals, models and applications
    Campione, A.; Gurreri, L.; Ciofalo, M.; Micale, G.; Tamburini, A.; Cipollina, A.
    Desalination (2018), 434 (), 121-160CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    A review. The need for unconventional sources of fresh water is pushing a fast development of desalination technologies, which proved to be able to face and solve the problem of water scarcity in many dry areas of the planet. Membrane desalination technologies are nowadays leading the market and, among these, electrodialysis (ED) plays an important role, esp. for brackish water desalination, thanks to its robustness, extreme flexibility and broad range of applications. In fact, many ED-related processes have been presented, based on the use of Ion Exchange Membranes (IEMs), which are significantly boosting the development of ED-related technologies. This paper presents the fundamentals of the ED process and its main developments. An important outlook is given to operational aspects, hydrodynamics and mass transport phenomena, with an extensive review of literature studies focusing on theor. or exptl. characterization of the complex phenomena occurring in electromembrane processes and of proposed strategies for process performance enhancement. An overview of process modeling tools is provided, pointing out capabilities and limitations of the different approaches and their possible application to optimization anal. and perspective developments of ED technol. Finally, the most recent applications of ED-related processes are presented, highlighting limitations and potentialities in the water and energy industry.
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    Meyer, K. H.; Straus, W. La perméabilité des membranes VI. Sur le passage du courant electrique a travers des membranes sélectives. Helv. Chim. Acta 1940, 23 (1), 795– 800,  DOI: 10.1002/hlca.19400230199
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    Permeability of membranes. VI. The passage of the electric current through selective membranes
    Meyer, Kurt H.; Straus, W.
    Helvetica Chimica Acta (1940), 23 (), 795-800CODEN: HCACAV; ISSN:0018-019X.
    cf. C. A. 31, 7450.1. Selective membranes may be the principal cause of action currents. To test this theory regenerated cellulose (cellophane) was chosen as a membrane permeable to cations and its const. of selectivity detd. as described in earlier papers of this series and found to be 0.02. The artificial catgut, "Naturin," prepd. from waste hides was chosen as a membrane permeable to anions but its const. of selectivity was only 0.007. Methylation of this membrane raised the const. of selectivity to 0.017. A cell having 4 cellophane membranes alternating with 3 methylated "Naturin" membranes was constructed, filled with 0.01 N KCl, and connected with 2 reversible electrodes. After equil. was attained a current of 0.75 ma. at 250 v. was passed for 10-30 min., the current interrupted, and the polarization measured by a compensation method about 20 secs. later and found to be about 100 mv. but dropping quickly to about 0 in 30 min. This difference of potential after 20 sec. is certainly due to an electrolytic effect and therefore supports M.'s theory. H. Rein (Z. Biol. 85, 217(1926)) who measured similar effects 10-4 sec. after interrupting the current found much larger values, 0.390 to 0.555 v., which were undoubtedly due to electrostatic effects not shown in M. and S.'s measurements after 20 sec.
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    Gurreri, L.; Tamburini, A.; Cipollina, A.; Micale, G. Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives. Membranes 2020, 10 (7), 146,  DOI: 10.3390/membranes10070146
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    Electrodialysis applications in wastewater treatment for environmental protection and resources recovery: a systematic review on progress and perspectives
    Gurreri, Luigi; Tamburini, Alessandro; Cipollina, Andrea; Micale, Giorgio
    Membranes (Basel, Switzerland) (2020), 10 (7), 146CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
    A review. This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of sepn. under the action of an elec. field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aq. solns., including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high sepn. efficiency, and chem.-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concn., diln., desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and orgs., or elec. energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liq. discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.
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    Xu, T.; Huang, C. Electrodialysis-based separation technologies: a critical review. AIChE J. 2008, 54 (12), 3147– 3159,  DOI: 10.1002/aic.11643
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    Electrodialysis-based separation technologies: a critical review
    Xu, Tongwen; Huang, Chuanhui
    AIChE Journal (2008), 54 (12), 3147-3159CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
    A review. To support a sustainable industrial growth, chem. engineering today faces a crucial challenge of meeting the increasing demand for materials and energy. One possible soln. is to decrease the equipment size/productivity ratio, energy consumption, and waste generation via process integration and optimization. This review focuses on the integration of electrodialysis with traditional unit operations and other membrane sepns. Such integrations, due to their diversity and practicability, can be versatile tools to meet specific needs from chem., biochem., food, and pharmaceutical industries.
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    Xu, T. W. Ion exchange membranes: State of their development and perspective. J. Membr. Sci. 2005, 263 (1–2), 1– 29,  DOI: 10.1016/j.memsci.2005.05.002
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    Ion exchange membranes: State of their development and perspective
    Xu, Tongwen
    Journal of Membrane Science (2005), 263 (1-2), 1-29CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    A review. During the last 50 years, ion exchange membranes have evolved from a lab. tool to industrial products with significant tech. and com. impact. Today ion exchange membranes are receiving considerable attention and are successfully applied for desalination of sea and brackish water and for treating industrial effluents. They are efficient tools for the concn. or sepn. of food and pharmaceutical products contg. ionic species as well as the manuf. of basic chem. products. The evolvement of an ion exchange membrane not only makes the process cleaner and more energy-efficient but also recovers useful effluents that are now going to wastes, and thus makes the development of society sustainable. Therefore, the intention of this review is to give a brief summary of the different prepn. and characteristics of ion exchange membrane as well as their potential applications. The most relevant literatures in the field are surveyed and some elucidating case studies are discussed, also accounting for the results of some research programs carried out in the author's lab.
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    Patel, S. K.; Qin, M.; Walker, W. S.; Elimelech, M. Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization. Environ. Sci. Technol. 2020, 54 (6), 3663– 3677,  DOI: 10.1021/acs.est.9b07482
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    24
    Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization
    Patel, Sohum K.; Qin, Mohan; Walker, W. Shane; Elimelech, Menachem
    Environmental Science & Technology (2020), 54 (6), 3663-3677CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Electro-driven technologies are viewed as a potential alternative to the current state-of-the-art technol., reverse osmosis, for the desalination of brackish waters. Capacitive deionization (CDI), based on the principle of electrosorption, has been intensively researched under the premise of being energy efficient. However, electrodialysis (ED), despite being a more mature electro-driven technol., has yet to be extensively compared to CDI in terms of energetic performance. In this study, we utilize Nernst-Planck based models for continuous flow ED and const.-current membrane capacitive deionization (MCDI) to systematically evaluate the energy consumption of the two processes. By ensuring equivalently sized ED and MCDI systems-in addn. to using the same feed salinity, salt removal, water recovery, and productivity across the two technologies-energy consumption is appropriately compared. We find that ED consumes less energy (has higher energy efficiency) than MCDI for all investigated conditions. Notably, our results indicate that the performance gap between ED and MCDI is substantial for typical brackish water desalination conditions (e.g., 3 g L-1 feed salinity, 0.5 g L-1 product water, 80% water recovery, and 15 L m-2 h-1 productivity), with the energy efficiency of ED often exceeding 30% and being nearly an order of magnitude greater than MCDI. We provide further insights into the inherent limitations of each technol. by comparing their resp. components of energy consumption, and explain why MCDI is unable to attain the performance of ED, even with ideal and optimized operation.
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  25. 25
    McGovern, R. K.; Zubair, S. M.; Lienhard, J. H. The cost effectiveness of electrodialysis for diverse salinity applications. Desalination 2014, 348, 57– 65,  DOI: 10.1016/j.desal.2014.06.010
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    25
    The cost effectiveness of electrodialysis for diverse salinity applications
    McGovern, Ronan K.; Zubair, Syed M.; Lienhard V, John H.
    Desalination (2014), 348 (), 57-65CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    We provide a thermoeconomic assessment of electrodialysis indicating that the technol. is most productive and efficient for the partial desalination of feed streams at the higher end of the brackish range of salinities. After optimizing the c.d. to minimise the sum of energy and equipment costs, we demonstrate that at low feed salinities the productivity, and hence equipment costs, of electrodialysis are hampered by the limiting c.d. By contrast, at higher feed salinities both productivity and efficiency are hampered by the reduced chem. p.d. of salt in the diluate (low salinity) and conc. (high salinity) streams. This anal. indicates the promise of further developing electrodialysis for the treatment of waters from oil, gas and coal-bed methane as well as flue-gas de-sulphurisation, where the partial desalination of streams at the high-end of the brackish range can be beneficial.
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  26. 26
    Chehayeb, K. M.; Farhat, D. M.; Nayar, K. G.; Lienhard, J. H. Optimal design and operation of electrodialysis for brackish-water desalination and for high-salinity brine concentration. Desalination 2017, 420, 167– 182,  DOI: 10.1016/j.desal.2017.07.003
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    26
    Optimal design and operation of electrodialysis for brackish-water desalination and for high-salinity brine concentration
    Chehayeb, Karim M.; Farhat, Daniel M.; Nayar, Kishor G.; Lienhard, John H. V.
    Desalination (2017), 420 (), 167-182CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Electrodialysis (ED) is a desalination technol. that has been deployed com. for decades. However, few studies in the literature have looked at the optimal design and operation of these systems, esp. for the concn. of high-salinity brines. In this paper, a set of constraints is defined to allow a fair comparison between different system sizes, designs, and operating conditions. The design and operation of ED are studied for the applications of brackish-water desalination and of high-salinity brine concn. for a fixed system size. The set of variables that det. the power consumption of a fixed-size system is reduced to include only the channel height and the velocity, with all the other design and operation variables depending on these two variables. After studying the minimization of power consumption for a fixed system size, the min. costs assocd. with the different system sizes are studied, and the differing trends in brackish-water and high-salinity applications are compared. Finally this paper presents the effect of the cost modeling parameters on the trends of the optimal system size, c.d., length, channel height, and velocity for the two applications studied.
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  27. 27
    McGovern, R. K.; Zubair, S. M.; Lienhard, V. J. H. The benefits of hybridising electrodialysis with reverse osmosis. J. Membr. Sci. 2014, 469, 326– 335,  DOI: 10.1016/j.memsci.2014.06.040
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    The benefits of hybridizing electrodialysis with reverse osmosis
    McGovern, Ronan K.; Zubair, Syed M.; Lienhard V, John H.
    Journal of Membrane Science (2014), 469 (), 326-335CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    A cost anal. reveals that hybridization of electrodialysis with reverse osmosis is only justified if the cost of water from the reverse osmosis unit is <40% of that from a stand-alone electrodialysis system. In such cases the addnl. reverse osmosis costs justify the electrodialysis cost savings brought about by shifting salt removal to higher salinity, where current densities are higher and equipment costs lower. Also, the anal. suggests that a simple hybrid configuration is more cost effective than a recirculated hybrid, a simple hybrid being one where the reverse osmosis conc. is fed to the electrodialysis stack and the products from both units are blended, and a recirculated being one hybrid involving recirculation of the electrodialysis product back to the reverse osmosis unit. The underlying rationale is that simple hybridization shifts salt removal away from the lowest salinity zone of operation, where salt removal is most expensive. Further shifts in the salinity at which salt is removed, brought about by recirculation, do not justify the assocd. increased costs of reverse osmosis.
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  28. 28
    Nayar, K. G.; Fernandes, J.; McGovern, R. K.; Dominguez, K. P.; McCance, A.; Al-Anzi, B. S.; Lienhard, V. J. H. Cost and energy requirements of hybrid RO and ED brine concentration systems for salt production. Desalination 2019, 456, 97– 120,  DOI: 10.1016/j.desal.2018.11.018
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    28
    Cost and energy requirements of hybrid RO and ED brine concentration systems for salt production
    Nayar, Kishor G.; Fernandes, Jenifer; McGovern, Ronan K.; Dominguez, Kyle P.; McCance, Adriene; Al-Anzi, Bader S.; Lienhard, John H. V.
    Desalination (2019), 456 (), 97-120CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    A new concept to conc. seawater up to 200 g/kg for producing vacuum salt using a reverse osmosis (RO) system hybridized with an electrodialysis (ED) system is presented. The RO system operates up to pressures of 120 bar and concs. seawater up to 120 g/kg with the ED system concg. RO brine to 200 g/kg. A parametric anal. to minimize the specific cost of brine concn. was conducted. Parameters varied were: the degree of RO-ED hybridization, ED c.d., electricity prices and water prices. Optimal hybrid RO-ED designs reduced brine concn. costs by 33-70% over standalone ED systems, with revenue generated from water co-prodn. further subsidizing costs by 1-6%. Optimizing ED c.d. reduced costs the most. Including a crystallizer, the total redn. in prodn. cost over a standalone ED-crystallizer system was 19-55%, with the prodn. cost for a typical case being $111/tonne-salt. The proposed RO-ED-crystallizer (REC) systems were found to be techno-economically feasible in Cyprus, Japan, Kuwait, Saudi Arabia, and the USA. At a road transportation distance of 735 km, REC based seawater vacuum salt was competitive with conventional vacuum salt. REC systems may open up the potential of small-scale decentralized salt prodn.
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    Wright, N. C.; Shah, S. R.; Amrose, S. E.; Winter, A. G. A robust model of brackish water electrodialysis desalination with experimental comparison at different size scales. Desalination 2018, 443, 27– 43,  DOI: 10.1016/j.desal.2018.04.018
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    A robust model of brackish water electrodialysis desalination with experimental comparison at different size scales
    Wright, Natasha C.; Shah, Sahil R.; Amrose, Susan E.; Winter, Amos G. V.
    Desalination (2018), 443 (), 27-43CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    This paper presents a robust anal. model for brackish water desalination using electrodialysis (ED), with prediction of the desalination rate, limiting c.d., and total energy use including pumping energy. Several assumptions reduce computation time and accurately model ED system behavior. The predicted desalination rate, limiting c. d., and total energy usage agree with measurements across two diverse ED stack designs, differing in total membrane area (0.18 m2, 37.1 m2), membrane manufacturers (GE Water, PCA GmbH), and flow channel spacers. The com.-scale stack was addnl.tested with real groundwater, demonstrating that brackish groundwater may be modeled as an equiv. concn. NaCl soln. Sensitivity to the membrane diffusion coeff., area available for ion transport, level of discretization along the flow channel length, boundary layer and membrane resistances, and water transport are analyzed to guide empirical characterization when higher accuracy is required. No single existing model for pressure drop in the membrane spacers could accurately predict pumping power in both stacks. One model for each stack was found to reasonably approx.pressure drop, however exptl. validation of specific spacer designs is recommended. The fully quant., parametric description of electrodialysis behavior presented forms a useful tool to design, evaluate, and optimize ED systems.
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    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: II. Effect of water transport through ion-exchange membranes. J. Membr. Sci. 2017, 531, 172– 182,  DOI: 10.1016/j.memsci.2017.02.031
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    Nernst-Planck transport theory for (reverse) electrodialysis: II. Effect of water transport through ion-exchange membranes
    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
    Journal of Membrane Science (2017), 531 (), 172-182CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Transport of water through ion-exchange membranes is of importance both for electrodialysis (ED) and reverse electrodialysis (RED). In this work, we extend our previous theory [J. Membrane Sci., 510 (2016) 370-381] and include water transport in a two-dimensional model for (R)ED. Following a Maxwell-Stefan (MS) approach, ions in the membrane have friction with the water, pore walls, and one another. We show that when ion-ion friction is neglected, the MS-approach is equiv. to the hydrodynamic theory of hindered transport, for instance applied to nanofiltration. After validation against exptl. data from literature for ED and RED, the model is used to analyze single-pass seawater ED, and RED with highly concd. solns. In the model, fluxes and velocities of water and ions in the membranes are self-consistently calcd. as function of the driving forces. We investigate the influence of water and coion leakage under different operational conditions.
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    Tanaka, Y. A computer simulation of feed and bleed ion exchange membrane electrodialysis for desalination of saline water. Desalination 2010, 254 (1–3), 99– 107,  DOI: 10.1016/j.desal.2009.12.008
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    A computer simulation of feed and bleed ion exchange membrane electrodialysis for desalination of saline water
    Tanaka, Yoshinobu
    Desalination (2010), 254 (1-3), 99-107CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    A computer simulation program is developed to predict desalinating performance of a feed and bleed electrodialysis process, inputting membrane characteristics, electrodialyzer specifications and electrodialytic conditions. Computing results enable to discuss the phenomena such as influence of cell voltage on c.d., ionic fluxes, soln. fluxes, current efficiency, ohmic potential and membrane potential, and further the influence of cell voltage and electrolyte concn. on the output of desalted solns., energy consumption and water recovery. Excepting limiting c.d., the performance of an electrodialyzer is scarcely influenced by the std. deviation of the normal distribution of the soln. velocity ratio in desalting cells. Energy consumption in a feed and bleed process is larger than that in a batch process for higher feed concn., and it is less than that in a reverse osmosis process at feed concn. less than one thousand and hundred-odd mg/l.
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    Sonin, A. A.; Probstein, R. F. A Hydrodynamic Theory of Desalination by Electrodialysis. Desalination 1968, 5 (3), 293,  DOI: 10.1016/S0011-9164(00)80105-8
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    Hydrodynamic theory of desalination by electrodialysis
    Sonin, Ain A.; Probstein, Ronald F.
    Desalination (1968), 5 (3), 293-329CODEN: DSLNAH; ISSN:0011-9164.
    A hydrodynamic theory of demineralization by electrodialysis was developed for a multichannel system with steady laminar flow between plane, parallel membranes. The modeling of the system is found to be governed by 4 basic similarity parameters: (1) a dimensionless applied potential, (2) the product of the channel aspect ratio and the inverse Peclet no., (3) the ratio of brine and dialyzate inlet concns., and (4) a parameter measuring membrane resistance. For sufficiently long channels it is shown that there are 2 distinct regions: a "developing" region where the concn. diffusion layers are growing, and a "developed" region where the diffusion layers fill the channel. Parabolic and uniform velocity profiles are considered and self-consistent solns. are derived for the distributions of salt concn., elec. field, and c.d. in the system, as well as for the total current. An integral method of soln. is used. In the limits of low and high polarization, anal. solns. are obtained which when matched at their point of equality closely approx. the complete numerical solns. Under a wide range of operating conditions, the soln. for the total current is represented by the empirical formula I = [1 - exp(- 3)]1/3, where I and are, resp., a dimensionless current and potential embodying the 4 similarity parameters mentioned. Comparison is made of the calcd. limiting total current with expt.
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    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: I. Effect of co-ion transport through the membranes. J. Membr. Sci. 2016, 510, 370– 381,  DOI: 10.1016/j.memsci.2016.03.012
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    Nernst-Planck transport theory for (reverse) electrodialysis: I. Effect of co-ion transport through the membranes
    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
    Journal of Membrane Science (2016), 510 (), 370-381CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Electrodialysis (ED) and Reverse Electrodialysis (RED) are related technologies for H2O desalination and energy conversion, both based on the selective transport of ions through ion exchange membranes. Fundamental understanding of these processes requires the description of ion transfer phenomena both along and through the membranes. The authors develop a simple 2-dimensional model valid for ED and RED, extending the approach by Sonin and Probstein (Desalination 5, 1968, 293) by using the Nernst-Planck equation not only in the flow channels but also in the membranes. This model requires as only input parameters the geometrical features of the system, the membrane charge d., and the diffusion coeffs. of ions in the channel and in the membrane. The effect of nonideal behavior of the membrane due to the co-ion transport is discussed, evaluating the performance of the processes in terms of current efficiency and energy consumption (for ED), and salt flux efficiency and power d. (for RED). Membrane properties such as permselectivity are calcd. as outputs of the model, and depend on position in the channel.
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    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: III. Optimal membrane thickness for enhanced process performance. J. Membr. Sci. 2018, 565, 480– 487,  DOI: 10.1016/j.memsci.2018.07.090
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    Nernst-Planck transport theory for (reverse) electrodialysis: III. Optimal membrane thickness for enhanced process performance
    Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
    Journal of Membrane Science (2018), 565 (), 480-487CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    The effect of the thickness of ion exchange membranes has been investigated for electrodialysis (ED) and reverse electrodialysis (RED), both exptl. and through theor. modeling. By developing a two-dimensional model based on Nernst-Planck theory, we theor. find that reducing the membrane thickness benefits process performance only until a certain value, below which performance drops. For ED, an optimum thickness can be identified in the range of 10-20 μm, while for RED the max. power d. is found for membranes that are three times as thick. Model calcns. compare well with exptl. data collected with a series of homogeneous membranes with the same chem. compn. and a thickness in the range of 10-100 μm. Our results show that the classical picture that membranes should be as thin as possible (as long as they remain pinhole-free and structurally stable) is insufficient, and must be replaced by a more accurate theor. framework.
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    Biesheuvel, P. M.; Dykstra, J. E. Physics of Electrochemical Processes ; 2020; http://www.physicsofelectrochemicalprocesses.com/.
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    Lee, H. J.; Sarfert, F.; Strathmann, H.; Moon, S. H. Designing of an electrodialysis desalination plant. Desalination 2002, 142 (3), 267– 286,  DOI: 10.1016/S0011-9164(02)00208-4
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    Designing of an electrodialysis desalination plant
    Lee, Hong-Joo; Sarfert, F.; Strathmann, H.; Moon, Seung-Hyeon
    Desalination (2002), 142 (3), 267-286CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
    The design and operation of an electrodialysis desalination process are based on a set of fixed and variable parameters such as stack construction, feed and product concn., membrane properties, flow velocities, c.d., recovery rates, etc. These parameters are interrelated and may be rather different for different applications. For an efficient operation of an electrodialysis desalination plant, the process has to be optimized in terms of overall costs considering component properties and operating parameters. The design and optimization of an electrodialysis plant to be used for brackish water desalination was studied. The required equations were derived or, as in the case of the limiting c.d., were exptl. detd. As an example, an electrodialysis plant with a sheet-flow stack construction and given feed soln. compn. was designed and optimized in terms of overall costs and the sensitivities of the different parameters were analyzed.
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    Fidaleo, M.; Moresi, M. Optimal strategy to model the electrodialytic recovery of a strong electrolyte. J. Membr. Sci. 2005, 260 (1–2), 90– 111,  DOI: 10.1016/j.memsci.2005.01.048
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    Optimal strategy to model the electrodialytic recovery of a strong electrolyte
    Fidaleo, Marcello; Moresi, Mauro
    Journal of Membrane Science (2005), 260 (1-2), 90-111CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Mostly Nernst-Planck derived relations were used to simulate the electrodialytic recovery of a strong electrolyte, namely sodium chloride. It was set up a 5-step exptl. procedure consisting of 0-current leaching, osmosis, and dialysis, electro-osmosis, desalination, current-voltage and validation tests. The contribution of leaching and solute diffusion across the electro-membranes was negligible with respect to the electro-migration. On the contrary, solvent diffusion tended to be important as the solute concn. difference at the membrane sides increased or c.d. was reduced. The electro-osmosis and desalination tests yielded the water and solute transport nos. By performing several limiting current tests at different solute concns. and feed flow rates using anionic or cationic membranes, it was possible to det. simultaneously the limiting current intensity, the ratio of the differences between the counter-ion transport nos. in the anion- and cation-exchange membranes and soln., the overall resistance of the electro-membranes, the effective membrane surface area, and the solute mass transfer coeff. All these process and design parameters allowed the time course of the solute concn. in the concg. (C) and dilg. (D) compartments, as well as the voltage applied to the electrodes, to be reconstructed quite accurately without any further correction factors. The capability of the above parameters to simulate the performance of the electrodialysis (ED) unit was checked by resorting to a few validation tests, that were performed in quite different operating conditions from those used in the training tests, that is by filling tank C with a low feed vol. with a low solute concn. and applying a const. current intensity to magnify the effect of electro-osmosis or by changing the current intensity step-wisely to simulate the continuous-mode operation of a multistage ED unit. A parameter sensitivity anal. made the different contribution of the process and design parameters to be assessed, thus yielding a straightforward procedure for designing or optimizing accurately ED desalination units up to a final salt concn. of ∼1.7 Kmol/m3.
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    Wang, L.; Dykstra, J. E.; Lin, S. H. Energy Efficiency of Capacitive Deionization. Environ. Sci. Technol. 2019, 53 (7), 3366– 3378,  DOI: 10.1021/acs.est.8b04858
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    Energy Efficiency of Capacitive Deionization
    Wang, Li; Dykstra, J. E.; Lin, Shihong
    Environmental Science & Technology (2019), 53 (7), 3366-3378CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A review. Capacitive deionization (CDI) as a class of electrochem. desalination has attracted fast-growing research interest in recent years. A significant part of this growing interest is arguably attributable to the premise that CDI is energy efficient and has the potential to out-compete other conventional desalination technologies. In this review, systematic evaluation of literature data reveals that while the abs. energy consumption of CDI is in general low, most existing CDI systems achieve limited energy efficiency from a thermodn. perspective. The authors also analyze the causes for the relatively low energy efficiency and discuss factors that may lead to enhanced energy efficiency for CDI.
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    Wang, L.; Lin, S. Membrane capacitive deionization with constant current vs constant voltage charging: Which is better?. Environ. Sci. Technol. 2018, 52 (7), 4051– 4060,  DOI: 10.1021/acs.est.7b06064
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    Membrane Capacitive Deionization with Constant Current vs Constant Voltage Charging: Which Is Better?
    Wang, Li; Lin, Shihong
    Environmental Science & Technology (2018), 52 (7), 4051-4060CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Membrane capacitive deionization (MCDI) can be typically operated with const. voltage (CV) or const. current (CC) mode in the charging stage. While a series of previous studies have compared both charging modes to identify the better operating mode, neither their performance evaluation protocols were consistent, nor did their conclusions unanimously converge. This study presents a new framework to evaluate and compare MCDI performance, considering the kinetic efficiency, the energetic efficiency, and the intrinsic trade-off between the two. A key prerequisite for making rational comparison of performance between MCDI operations is that the operations being compared should all result in the same target adsorption. With this key prerequisite and the new evaluation framework based on the trade-off curve between kinetic and energetic efficiencies, our exptl. assessment and theor. anal. suggest that whether CC or CV charging is more efficient is strongly dependent on the target adsorption and, to a less extent, on the kinetic rate of charging. However, the advantage in energy or kinetic efficiency of one charging mode over that of the other is relatively small in all cases. Our study also reveals that, for a given MCDI system, there exist regimes of target adsorptions and kinetic rates that can only be achieved by either CC or CV charging, or even regimes that can be achieved by neither charging mode. In summary, this study revises our current understanding regarding the comparison of the two typical charging modes in MCDI, and introduces a new framework for comparing the performance of different MCDI and CDI operations.
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    Baker, R. W. Membrane technology and applications; John Wiley & Sons, 2012.
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    Qin, M.; Deshmukh, A.; Epsztein, R.; Patel, S. K.; Owoseni, O. M.; Walker, W. S.; Elimelech, M. Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis. Desalination 2019, 455, 100– 114,  DOI: 10.1016/j.desal.2019.01.003
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    Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis
    Qin, Mohan; Deshmukh, Akshay; Epsztein, Razi; Patel, Sohum K.; Owoseni, Oluwaseye M.; Walker, W. Shane; Elimelech, Menachem
    Desalination (2019), 455 (), 100-114CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Capacitive deionization (CDI), which is based on the electrosorption of ions by porous electrodes, is an emerging technol. for brackish water desalination. Understanding the key drivers of energy consumption in CDI and benchmarking CDI with reverse osmosis (RO), the current state-of-the-art for brackish and seawater desalination, is crucial to guide the future development of desalination technologies. In this study, we develop system-scale models to analyze the energy consumption and energy efficiency of CDI and RO over a wide range of material properties and operating conditions. Using our models, we explore how the energetic performance of CDI and RO compare as a function of feed salinity, water recovery, salt rejection, and av. water flux, which is normalized by electrode and membrane area in CDI and RO, resp. Our anal. shows that RO is significantly more energy efficient than CDI, particularly when targeting higher salinity feed streams and higher salt rejection values. For brackish water with a salt concn. of 2000 mg L-1, achieving 50% water recovery and 75% salt rejection, with an av. water flux of 10 L m-2 h-1 using CDI requires a specific energy consumption of 0.85 kWh m-3, more than eight times that of RO (0.09 kWh m-3). Importantly, our results also indicate that current efforts to improve electrode materials can only marginally reduce the energy consumption of CDI. We conclude with a discussion highlighting other important factors, such as capital cost, electrode stability, and membrane fouling, which affect the efficacy of CDI and RO for low-salinity desalination.
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    Drak, A.; Adato, M. Energy recovery consideration in brackish water desalination. Desalination 2014, 339, 34– 39,  DOI: 10.1016/j.desal.2014.02.008
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    Energy recovery consideration in brackish water desalination
    Drak, Alexander; Adato, Matan
    Desalination (2014), 339 (), 34-39CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    In brackish water RO desalination, the low feed water TDS and relatively low brine flow make the use of energy recovery devices ambiguous. The decision to implement energy recovery device must always be based on the Life Cycle Cost estn. of the plant. Design considerations concerning the energy recovery device selection and field experience in Lahat brackish water desalination plant (40,000 m3/day) are presented in this article. Two types of energy recovery device are generally considered in the brackish water RO desalination, turbocharger and isobaric energy recovery devices. Taking into consideration the simplicity of the turbocharger, it was selected for the 1st phase of the Lahat brackish water desalination plant with the design recovery range of 80%-88%. The turbocharger was designed for max recovery and external bypass line was added to operate the plant at low recoveries. For such wide recovery range the turbocharger entire efficiency range of 30%-40% was achieved. Due to the limitation of the turbocharger to operate efficiently at the broad recovery range and Life Cycle Cost benefits of isobaric energy recovery device, the isobaric energy recovery device was selected for the 2nd phase of the Lahat brackish water desalination plant.
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    Geise, G. M.; Paul, D. R.; Freeman, B. D. Fundamental water and salt transport properties of polymeric materials. Prog. Polym. Sci. 2014, 39 (1), 1– 42,  DOI: 10.1016/j.progpolymsci.2013.07.001
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    Fundamental water and salt transport properties of polymeric materials
    Geise, Geoffrey M.; Paul, Donald R.; Freeman, Benny D.
    Progress in Polymer Science (2014), 39 (1), 1-42CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
    A review. Fundamental water and salt transport properties of polymers are crit. for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissoc. in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Addnl., the influence of other polymer structure characteristics, such as crosslinking and morphol. in phase sepd. systems, on water and salt transport properties is discussed. The role of free vol. on water and salt transport properties is discussed. The soln.-diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.
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    Wijmans, J. G.; Baker, R. W. The Solution-Diffusion Model - a Review. J. Membr. Sci. 1995, 107 (1–2), 1– 21,  DOI: 10.1016/0376-7388(95)00102-I
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    The solution-diffusion model: a review
    Wijmans, J. G.; Baker, R. W.
    Journal of Membrane Science (1995), 107 (1-2), 1-21CODEN: JMESDO; ISSN:0376-7388. (Elsevier)
    A review, with refs., is given on the soln.-diffusion model for describing transport phenomena in relation to membranes. The soln.-diffusion model has emerged over the past 20 yr as the most widely accepted explanation of transport in dialysis, reverse osmosis, gas permeation, and pervaporation. In this paper we will derive the phenomenol. equations for transport in these processes using the soln.-diffusion model and starting from the fundamental statement that flux is proportional to a gradient in chem. potential. The direct and indirect evidence for the model's validity will then be presented, together with a brief discussion of the transition between a soln.-diffusion membrane and a pore-flow membrane seen in nanofiltration membranes and some gas permeation membranes.
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    Porter, M. C. Concentration Polarization with Membrane Ultrafiltration. Ind. Eng. Chem. Prod. Res. Dev. 1972, 11 (3), 234,  DOI: 10.1021/i360043a002
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    Concentration polarization with membrane ultrafiltration
    Porter, Mark C.
    Industrial & Engineering Chemistry Product Research and Development (1972), 11 (3), 234-48CODEN: IEPRA6; ISSN:0196-4321.
    Unusually high ultrafiltrate fluxes were obsd. by using thin-channel ultrafiltration in the dewatering and purification of colloidal suspensions. Polymer latexes, paints, metal oxides, starch, and even cellular suspensions all exhibited higher fluxes than those predicted from the gel-polarization model. Theoretical reasons for these anomalies are discussed in conjunction with exptl. data obtained with thin-channel devices utilizing anisotropic noncellulosic membranes.
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    Bartholomew, T. V.; Mauter, M. S. Computational framework for modeling membrane processes without process and solution property simplifications. J. Membr. Sci. 2019, 573, 682– 693,  DOI: 10.1016/j.memsci.2018.11.067
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    Computational framework for modeling membrane processes without process and solution property simplifications
    Bartholomew, Timothy V.; Mauter, Meagan S.
    Journal of Membrane Science (2019), 573 (), 682-693CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
    Accurately modeling membrane processes is crit. to evaluating novel process configurations, designing scalable membrane systems, informing process cost ests., and directing future research. Most membrane process models trade accuracy for computational efficiency by employing simplified approxns. of the process (i.e. no salt flux, no pressure drop) and soln. properties (i.e. ideal soln., and const. d., viscosity, and diffusivity). This work presents a detailed one-dimensional finite difference model for evaluating membrane processes that avoids these common simplifications. We apply this model to quantify the error introduced by these simplifications for case studies of reverse osmosis, osmotically assisted reverse osmosis, forward osmosis, and pressure retarded osmosis. While the magnitude of error introduced by these simplifications is dependent on the case study parameters and specifications, we find that existing model formulations can underestimate or overestimate av. water flux by nearly 50% for some membrane processes operating under std. conditions. Finally, we investigate the error introduced by simplified inlet-outlet models that do not solve the governing system of differential equations, and we assess the accuracy of novel inlet-outlet formulations that use a log and geometric mean, instead of the typical arithmetic mean, to represent non-linear water flux profiles.
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    Manth, T.; Gabor, M.; Oklejas, E. Minimizing RO energy consumption under variable conditions of operation. Desalination 2003, 157 (1–3), 9– 21,  DOI: 10.1016/S0011-9164(03)00377-1
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    Minimizing RO energy consumption under variable conditions of operation
    Manth, Thomas; Gabor, Michael; Oklejas, Eli, Jr.
    Desalination (2003), 157 (1-3), 9-21CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
    Specific energy consumption (SEC) for reverse osmosis (RO) desalination systems has usually been estd. using simplistic analyses that consider an av. duty point of operation for a certain plant. A more sophisticated and comprehensive approach that accounts for the effects of variable parameters of operation on SEC was recently described, introducing the concept of the hydraulic envelope. Variable parameters include flow rates at variable recoveries, feed temp. and salinity with their resulting pressure requirements, pressure losses caused by membrane fouling, and pressure losses caused by system controls such as feed throttle valves. This paper will explore in greater detail various energy recovery strategies under variable parameters of operation. Particular attention will be paid to a recently developed, innovative energy recovery configuration that uses a motor-driven booster pump coupled to a Pelton turbine, the so-called PROP, instead of a single-component high-pressure feed pump. This new energy recovery concept can not only be applied to single-stage RO plants, but also as a highly effective interstage booster for dual-stage Brine Conversion Systems (BCS). The concept was submitted for patenting. Results of the anal. suggest that the key issue for minimizing SEC is to control the plant over the entire width of the operational range without creating throttling losses. This can only be achieved by using hydraulic equipment that allows for feed pressure adjustment at min. energy dissipation and eliminates the need for throttling valves. The newly developed PROP concept provides min. SEC over the entire range of the hydraulic envelope of a plant, while at the same time allowing for max. hydraulic dynamic control efficiency of the RO unit. The PROP offers a significant savings potential in terms of capital costs compared to conventional energy recovery strategies.
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    Lin, S. H.; Elimelech, M. Kinetics and energetics trade-off in reverse osmosis desalination with different configurations. Desalination 2017, 401, 42– 52,  DOI: 10.1016/j.desal.2016.09.008
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    Kinetics and energetics trade-off in reverse osmosis desalination with different configurations
    Lin, Shihong; Elimelech, Menachem
    Desalination (2017), 401 (), 42-52CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Optimizing system design and operation of reverse osmosis (RO) systems requires an in-depth comprehension of the intrinsic tradeoff between RO mass transfer kinetics and energetics. In this study, we demonstrate that this crit. trade-off can be quantified using the relationship between the av. water flux and the specific energy consumption (SEC). We derive anal. expressions to quantify the av. water flux and SEC for single stage, two stage, and closed circuit RO processes. These anal. expressions are useful for system design and operation optimization as they facilitate direct comparison of the kinetic and energetic efficiencies between RO processes with different operation conditions and system configurations. Finally, we compare the kinetics and energetics of the three system configurations using these anal. expressions and discuss their relative advantages and disadvantages in RO desalination.
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    Wang, L.; Lin, S. Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization. Water Res. 2018, 129, 394– 401,  DOI: 10.1016/j.watres.2017.11.027
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    Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization
    Wang, Li; Lin, Shihong
    Water Research (2018), 129 (), 394-401CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Significant progress has been made over recent years in capacitive deionization (CDI) to develop novel system configurations, predictive theor. models, and high-performance electrode materials. To bring CDI to large scale practical applications, it is important to quant. understand the intrinsic tradeoff between kinetic and energetic efficiencies, or the relationship between energy consumption and the mass transfer rate. In this study, we employed both exptl. and modeling approaches to systematically investigate the tradeoff between kinetic and energetic efficiencies in membrane CDI (MCDI). Specifically, we assessed the relationship between the av. salt adsorption rate and specific energy consumptions from MCDI expts. with different applied current densities but a const. effluent salinity. We investigated the impacts of feed salinity, dild. water salinity, dild. water vol. per charging cycle, and electrode materials on the kinetics-energetics tradeoff. We also demonstrate how this tradeoff can be employed to optimize the design and operation of CDI systems and compare the performance of different electrode materials and CDI systems.
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    Hawks, S. A.; Ramachandran, A.; Porada, S.; Campbell, P. G.; Suss, M. E.; Biesheuvel, P. M.; Santiago, J. G.; Stadermann, M. Performance metrics for the objective assessment of capacitive deionization systems. Water Res. 2019, 152, 126– 137,  DOI: 10.1016/j.watres.2018.10.074
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    Performance metrics for the objective assessment of capacitive deionization systems
    Hawks, Steven A.; Ramachandran, Ashwin; Porada, Slawomir; Campbell, Patrick G.; Suss, Matthew E.; Biesheuvel, P. M.; Santiago, Juan G.; Stadermann, Michael
    Water Research (2019), 152 (), 126-137CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    In the growing field of capacitive deionization (CDI), a no. of performance metrics have emerged to describe the desalination process. Unfortunately, the sepn. conditions under which these metrics are measured are often not specified, resulting in optimal performance at minimal removal. Here we outline a system of performance metrics and reporting conditions that resolves this issue. Our proposed system is based on volumetric energy consumption (Wh/m3) and throughput productivity (L/h/m2) reported for a specific av. concn. redn., water recovery, and feed salinity. To facilitate and rationalize comparisons between devices, materials, and operation modes, we propose a nominal std. sepn. of removing 5 mM from a 20 mM NaCl feed soln. at 50% water recovery. We propose this particular sepn. as a std., but emphasize that the rationale presented here applies irresp. of sepn. details. Using our proposed sepn., we compare the desalination performance of a flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI) device, showing how significantly different systems can be compared in terms of generally desirable desalination characteristics. In general, we find that performance anal. must be considered carefully so to not allow for ambiguous sepn. conditions or the maximization of one metric at the expense of another. Addnl., for context and clarity, we discuss a no. of important underlying performance indicators and cell characteristics that are not performance measures in and of themselves but can be examd. to better understand differences in performance.
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    Chehayeb, K. M.; Lienhard, J. H. On the electrical operation of batch electrodialysis for reduced energy consumption. Environ. Sci-Wat Res. 2019, 5 (6), 1172– 1182,  DOI: 10.1039/C9EW00097F
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    Hand, S.; Shang, X.; Guest, J. S.; Smith, K. C.; Cusick, R. D. Global Sensitivity Analysis To Characterize Operational Limits and Prioritize Performance Goals of Capacitive Deionization Technologies. Environ. Sci. Technol. 2019, 53 (7), 3748– 3756,  DOI: 10.1021/acs.est.8b06709
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    Global Sensitivity Analysis To Characterize Operational Limits and Prioritize Performance Goals of Capacitive Deionization Technologies in water desalination
    Hand, Steven; Shang, Xia; Guest, Jeremy S.; Smith, Kyle C.; Cusick, Roland D.
    Environmental Science & Technology (2019), 53 (7), 3748-3756CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Capacitive deionization (CDI) technologies couple electronic and ionic charge storage, enabling improved thermodn. efficiency of brackish desalination by recovering energy released during discharge. However, insight into CDI has been limited by discrete exptl. observations at low desalination depths (Δc, typically reducing influent salinity by 10 mM or less). The performance and sensitivity of 3 common CDI configurations [std. CDI, membrane CDI (MCDI), and flowable electrode CDI (FCDI)] were evaluated across the operational and material design landscape by varying 8 common input parameters (electrode thickness, influent concn., c.d., electrode flow rate, specific capacitance, contact resistance, porosity, and fixed charge). All combinations of designs were evaluated for 2 influent concns. with a calibrated and validated 1-dimensional (1-D) porous electrode model. Sensitivity analyses were carried out via Monte Carlo and Morris methods, focusing on 6 performance metrics. Across all performance metrics, high sensitivity was obsd. to input parameters which impact cycle length (current, resistance, and capacitance). Simulations demonstrated the importance of maintaining both charge and round-trip efficiencies, which limit the performance of CDI and FCDI, resp. Accounting for energy recovery, only MCDI was capable of operating at thermodn. efficiencies similar to reverse osmosis.
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    Wang, L.; Lin, S. H. Theoretical framework for designing a desalination plant based on membrane capacitive deionization. Water Res. 2019, 158, 359– 369,  DOI: 10.1016/j.watres.2019.03.076
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    Theoretical framework for designing a desalination plant based on membrane capacitive deionization
    Wang, Li; Lin, Shihong
    Water Research (2019), 158 (), 359-369CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Despite significant progress made in multiple aspects of capacitive deionization (CDI), a rational framework is in need for optimizing the design and operation of a large desalination system based on CDI. In this work, we develop a theor. framework for guiding the design of a desalination plant based on CDI with ion exchange membranes (i.e. membrane CDI, or MCDI). This framework is established by identifying (1) the practical design constraints, (2) the inter-relationships between different design and operating parameters, (3) a set of independent variables, and (4) the key performance metrics. The proposed design framework reduces the degrees of freedom of the system and facilitates more focused and systematic anal. of the overall performance of an MCDI-based desalination plant. Careful anal. using the proposed design framework suggests the presence of an optimal tradeoff curve that comprises all the possible optima of design and operating conditions with which an MCDI-based desalination plant is the most cost-effective. We also show that the typical practice of using equal flowrates for charging and discharge yields very good performance compared to the optima, as long as water recovery is not too high. Finally, we also briefly explain the implication of this framework on cost-based optimization of the design and operation of an MCDI-based desalination plant.
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    Hand, S.; Guest, J. S.; Cusick, R. D. Technoeconomic Analysis of Brackish Water Capacitive Deionization: Navigating Tradeoffs between Performance, Lifetime, and Material Costs. Environ. Sci. Technol. 2019, 53 (22), 13353– 13363,  DOI: 10.1021/acs.est.9b04347
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    Technoeconomic Analysis of Brackish Water Capacitive Deionization: Navigating Tradeoffs between Performance, Lifetime, and Material Costs
    Hand Steven; Guest Jeremy S; Cusick Roland D
    Environmental science & technology (2019), 53 (22), 13353-13363 ISSN:.
    Capacitive deionization (CDI), a class of electrochemical separation technologies, has been proposed as an energy-efficient brackish water desalination method. Previous studies have focused on improving capacity and energy consumption through material (e.g., ion-selective membranes [IEMs], charged carbon) and operational modifications, but there has been no analysis that directly links lab-scale experimental performance to capital and operating costs of full-scale water production. In this study, we developed a parameterized process model and technoeconomic analysis framework to project capital and operating costs at the million gallon per day scale based on reported material and operational characteristics for constant current CDI with and without low ($20 m(-2))- and high-cost ($100 m(-2)) IEMs. Using this framework, we conducted global sensitivity and uncertainty analyses for water price across the reported CDI design space. Our results show that the operating constraints of brackish water desalination lead to capital costs 2-14 times greater than operating costs (particularly for MCDI). While MCDI outperforms CDI, IEM prices dictate the threshold at which MCDI is more cost-effective. The high relative capital costs highlight the importance of achieving system lifetimes at 2 years or beyond. Last, we set performance and areal cost benchmarks for material-based CDI performance and lifetime improvements.
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    Lopez, A. M.; Williams, M.; Paiva, M.; Demydov, D.; Do, T. D.; Fairey, J. L.; Lin, Y. J.; Hestekin, J. A. Potential of electrodialytic techniques in brackish desalination and recovery of industrial process water for reuse. Desalination 2017, 409, 108– 114,  DOI: 10.1016/j.desal.2017.01.010
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    Potential of electrodialytic techniques in brackish desalination and recovery of industrial process water for reuse
    Lopez, Alexander M.; Williams, Meaghan; Paiva, Maira; Demydov, Dmytro; Do, Thien Duc; Fairey, Julian L.; Lin, Yu Po J.; Hestekin, Jamie A.
    Desalination (2017), 409 (), 108-114CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Large demands for water in industry and consumer markets have led to the development of seawater desalination plants worldwide. Electrodialysis allows the removal of ions at a much lower specific energy consumption than pressure-driven systems and holds the potential to move the desalination industry to greater water yields, lowering the degree of water wasted and energy required for sepns. This study investigates the use of traditional electrodialysis as well as electrodeionization for the removal of contaminant ions from brackish water as well as samples from industrial sources. Results indicated that conventional electrodeionization can successfully remove ion contaminants from brackish water at specific energy consumptions of approx. 0.9-1.5 kWh/m3 water recovered with high water productivity at 40-90 L/m2 h. Ion-exchange resin wafer electrodeionization showed greater promise with specific energy consumption levels between 0.6-1.1 kWh/m3 water recovered and productivity levels between 10-40 L/m2 h. From these results, electrodialysis and electrodeionization have demonstrated viability as alternatives to pressure-driven membrane systems for brackish water desalination.
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    Hand, S.; Cusick, R. D. Emerging investigator series: capacitive deionization for selective removal of nitrate and perchlorate: impacts of ion selectivity and operating constraints on treatment costs. Environ. Sci-Wat Res. 2020, 6 (4), 925– 934,  DOI: 10.1039/C9EW01105F
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    Andrews, W. T.; Pergande, W. F.; McTaggart, G. S. Energy performance enhancements of a 950 m(3)/d seawater reverse osmosis unit in Grand Cayman. Desalination 2001, 135 (1–3), 195– 204,  DOI: 10.1016/S0011-9164(01)00150-3
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    Energy performance enhancements of a 950 m3/d seawater reverse osmosis unit in Grand Cayman
    Andrews, W. T.; Pergande, Wil F.; McTaggart, Gregory S.
    Desalination (2001), 135 (1-3), 195-204CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
    This paper describes the upgrade of a seawater reverse osmosis unit by replacing the hydraulic turbocharger energy-recovery system with a dual work-exchanger energy-recovery (DWEER) system. Before the upgrade, the unit operated at a capacity of 1071 m3/day, and a specific electricity of 3.00 KWh/m3. Tests conducted on the unit after the upgrade, when operated at essentially the same membrane conditions, showed a 59% capacity increase to 1699 m3/day, and a 26% redn. in specific electricity to 2.22 KWh/m3. A further test was then conducted on the unit, by temporarily modifying it such that it operated at essentially the same conditions as before the upgrade. The test resulted in a specific electricity of 2.32 KWh/m3, a redn. in specific electricity of 23% compared to the original performance. This confirmed that the redn. in specific electricity was due to the DWEER system and not economies of scale from the increased capacity. A detailed efficiency anal. showed that the upgrade unit had an overall desalination energy efficiency of 45%, which was broken down into 85% for pumping, 92% for energy recovery, and 57% for the membrane array. This indicates that further improvements in efficiency are most likely to be achieved in the membrane array. The operating experience of the 1st 6 mo is presented.
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    Zhu, A. Z.; Christofides, P. D.; Cohen, Y. Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination. Ind. Eng. Chem. Res. 2009, 48 (13), 6010– 6021,  DOI: 10.1021/ie800735q
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    Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination
    Zhu, Aihua; Christofides, Panagiotis D.; Cohen, Yoram
    Industrial & Engineering Chemistry Research (2009), 48 (13), 6010-6021CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Advances in highly permeable reverse osmosis (RO) membranes have enabled desalting operations, in which it is practically feasible for the applied pressure to approach the osmotic pressure of the exit brine stream. However, energy cost remains a major contributor to the total cost of water produced by RO membrane desalination. Redn. of the overall cost of water prodn. represents a major challenge and, in the present work, various elements of water prodn. cost are evaluated from the viewpoint of optimization, with respect to various costs (energy, membrane area and permeability, brine management, and pressure drop), as well as the important thermodn. crossflow constraint, utilization of energy recovery devices, and operational feed and permeate flow rate constraints. More specifically, in this study, an approach to the optimization of product water recovery at pressures that approach the osmotic pressure of the exit brine stream is presented via several simple RO process models that utilize highly permeable membranes. The results suggest that it is indeed feasible to refine RO processes to target for operation under the condition of min. energy consumption, while considering the constraint imposed by the osmotic pressure, as specified by the thermodn. crossflow restriction. Although it is shown that multistage RO provides energy savings, this is at the expense of greater membrane area cost. Overall, as process costs above energy costs are added, the operational point for achieving min. water prodn. cost shifts to higher recoveries. Although the newer generation of RO membranes can allow high recovery operations at lower pressures, limitations due to mineral scaling and fouling impose addnl. constraints. The incorporation of these phenomena in the optimization approach is the subject of ongoing research.
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    Lin, S. H.; Elimelech, M. Staged reverse osmosis operation: Configurations, energy efficiency, and application potential. Desalination 2015, 366, 9– 14,  DOI: 10.1016/j.desal.2015.02.043
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    Staged reverse osmosis operation: Configurations, energy efficiency, and application potential
    Lin, Shihong; Elimelech, Menachem
    Desalination (2015), 366 (), 9-14CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodn. of sepn. of saline solns. Therefore, pushing the energy consumption towards the thermodn. limit of sepn. would lead to significant long-term savings in energy cost. In this work, we quant. demonstrate the potential of energy redn. for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the min. specific energy of desalination (i.e., the min. energy required to generate a unit vol. of permeate water) to the no. of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our anal. suggests that although it is theor. impossible to reach the thermodn. min. energy of sepn. with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.
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    Werber, J. R.; Deshmukh, A.; Elimelech, M. Can batch or semi-batch processes save energy in reverse-osmosis desalination?. Desalination 2017, 402, 109– 122,  DOI: 10.1016/j.desal.2016.09.028
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    Can batch or semi-batch processes save energy in reverse-osmosis desalination?
    Werber, Jay R.; Deshmukh, Akshay; Elimelech, Menachem
    Desalination (2017), 402 (), 109-122CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the min. practical energy of desalination substantially exceeds the thermodn. min. Batch processes can theor. approach the thermodn. min., suggesting the possibility for further energy savings. In this study, we aim to quantify what energy redns. may be possible for batch-like processes when process inefficiencies such as frictional losses and concn. polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive anal. approxns. and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this anal., we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and two-stage RO would save 13% and 15% energy, resp., over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.
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    Chehayeb, K. M.; Lienhard, J. H. Entropy generation analysis of electrodialysis. Desalination 2017, 413, 184– 198,  DOI: 10.1016/j.desal.2017.03.001
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    Entropy generation analysis of electrodialysis
    Chehayeb, Karim M.; Lienhard, John H. V.
    Desalination (2017), 413 (), 184-198CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Electrodialysis (ED) is a desalination technol. with many applications. In order to better understand how the energetic performance of this technol. can be improved, the various losses in the system should be quantified and characterized. This can be done by looking at the entropy generation in ED systems. In this paper, we implement an ED model based on the Maxwell-Stefan transport model, which is the closest model to fundamental equations. We study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In addn., we study the effect of the channel height, membrane thickness, and cell-pair voltage on the specific rate of entropy generation. We express the second-law efficiency of ED as the product of current and voltage utilization rates, and study its variation with c.d. Further, we define the useful voltage that is used beneficially for sepn. We derive the rate of entropy generation that is due to the passage of ions through a voltage drop, and we investigate whether voltage drops themselves can provide a good est. of entropy generation.
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    Mistry, K. H.; McGovern, R. K.; Thiel, G. P.; Summers, E. K.; Zubair, S. M.; Lienhard, J. H. Entropy Generation Analysis of Desalination Technologies. Entropy 2011, 13 (10), 1829– 1864,  DOI: 10.3390/e13101829
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    Entropy generation analysis of desalination technologies
    Mistry, Karan H.; McGovern, Ronan K.; Thiel, Gregory P.; Summers, Edward K.; Zubair, Syed M.; Lienhard V., John H.
    Entropy (2011), 13 (), 1829-1864CODEN: ENTRFG; ISSN:1099-4300. (MDPI AG)
    Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies. Entropy generation anal., and specifically, Second Law efficiency, is an important tool for illustrating the influence of irreversibilities within a system on the required energy input. When defining Second Law efficiency, the useful exergy output of the system must be properly defined. For desalination systems, this is the min. least work of sepn. required to ext. a unit of water from a feed stream of a given salinity. In order to evaluate the Second Law efficiency, entropy generation mechanisms present in a wide range of desalination processes are analyzed. In particular, entropy generated in the run down to equil. of discharge streams must be considered. Phys. models are applied to est. the magnitude of entropy generation by component and individual processes. These formulations are applied to calc. the total entropy generation in several desalination systems including multiple effect distn., multistage flash, membrane distn., mech. vapor compression, reverse osmosis, and humidification-dehumidification. Within each technol., the relative importance of each source of entropy generation is discussed in order to det. which should be the target of entropy generation minimization. As given here, the correct application of Second Law efficiency shows which systems operate closest to the reversible limit and helps to indicate which systems have the greatest potential for improvement.
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    Chehayeb, K. M.; Nayar, K. G.; Lienhard, J. H. On the merits of using multi-stage and counterflow electrodialysis for reduced energy consumption. Desalination 2018, 439, 1– 16,  DOI: 10.1016/j.desal.2018.03.026
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    On the merits of using multi-stage and counterflow electrodialysis for reduced energy consumption
    Chehayeb, Karim M.; Nayar, Kishor G.; Lienhard, John H. V.
    Desalination (2018), 439 (), 1-16CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    The cost of electrodialysis (ED) systems can be decreased by decreasing their power consumption. Such redns. may be achieved by using degrees of freedom in the system's configuration to obtain a more uniform spatial distribution of the rate of entropy generation, as explained by the theorem of equipartition of entropy generation. In this paper, we study possible improvements to the energy efficiency of electrodialysis through the use of two elec. stages with different voltages, and through operation in a counterflow configuration. We first consider how a two-stage ED system should be operated. In particular, we look at how the voltages and current densities should be chosen. In addn., we quantify the effect of operating under two voltages in brackish-water desalination and in high-salinity brine concn. Finally, we quantify the effect of operating ED in counterflow for the same applications. We show that high ED fixed costs prevent the achievement of significant improvements in energy efficiency. If fixed costs are reduced, and larger systems become cost-effective, we show that a power redn. of up to 29% is possible by going from a single-stage to a two-stage configuration.
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    Parulekar, S. J. Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis. J. Membr. Sci. 1998, 148 (1), 91– 103,  DOI: 10.1016/S0376-7388(98)00148-3
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    Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis
    Parulekar, Satish J.
    Journal of Membrane Science (1998), 148 (1), 91-103CODEN: JMESDO; ISSN:0376-7388. (Elsevier Science B.V.)
    Cost-effective operations of a batch electrodialyzer for removal of salt from a single salt soln. are investigated. It is desired to minimize the operating cost for a particular batch. The operating cost for an electrodialysis (ED) stack is comprised of cost related to energy consumption and cost of maintenance of the ED stack. In effective operations of an ED stack, the maintenance cost is a small fraction of the total operating cost. The bulk of the operating cost is therefore proportional to total energy consumption, which is the sum of the elec. energy needed for salt removal and the energy required to pump various solns. through the ED stack. For fixed feed compn. and the desired percent salt recovery, the total energy required is influenced by trajectories of current flowing through and the voltage applied across the ED stack and the operating time. In this regard, the following operations are studied: (I) const. current operation, (II) const. voltage operation, (III) const. current operation followed by const. voltage operation, (IV) const. voltage operation followed by const. current operation, and (V) operation with time-variant current and voltage. For arbitrary relations among salt concn., current utilization, and stack resistance, optimal current and voltage trajectories that lead to min. energy requirement are identified for each of the five operations. It is established anal. that operation V is superior to operations III and IV, which in turn are superior to operations I and II. Numerical illustrations reveal that the performance differences in these operations are enhanced as the percent salt recovery is increased.
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    Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for next-generation desalination and water purification membranes. Nat. Rev. Mater. 2016, 1 (5), 16018,  DOI: 10.1038/natrevmats.2016.18
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    Materials for next-generation desalination and water purification membranes
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    Nature Reviews Materials (2016), 1 (5), 16018CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    Membrane-based sepns. for water purifn. and desalination have been increasingly applied to address the global challenges of water scarcity and the pollution of aquatic environments. However, progress in water purifn. membranes has been constrained by the inherent limitations of conventional membrane materials. Recent advances in methods for controlling the structure and chem. functionality in polymer films can potentially lead to new classes of membranes for water purifn. In this Review, we first discuss the state of the art of existing membrane technologies for water purifn. and desalination, highlight their inherent limitations and establish the urgent requirements for next-generation membranes. We then describe mol.-level design approaches towards fabricating highly selective membranes, focusing on novel materials such as aquaporin, synthetic nanochannels, graphene and self-assembled block copolymers and small mols. Finally, we highlight promising membrane surface modification approaches that minimize interfacial interactions and enhance fouling resistance.
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    Li, W.; Patton, S.; Gleason, J. M.; Mezyk, S. P.; Ishida, K. P.; Liu, H. Z. UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse. Environ. Sci. Technol. 2018, 52 (11), 6417– 6425,  DOI: 10.1021/acs.est.7b06042
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    UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse
    Li, Wei; Patton, Samuel; Gleason, Jamie M.; Mezyk, Stephen P.; Ishida, Kenneth P.; Liu, Haizhou
    Environmental Science & Technology (2018), 52 (11), 6417-6425CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    A sequential combination of membrane treatment and UV-based advanced oxidn. processes (UV/AOP) has become the industry std. for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addn., persulfate (S2O82-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH2Cl and S2O82- on 1,4-D degrdn. were examd. at various levels of oxidant dosage, chloride, and soln. pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D degrdn. rate. At the optimal ratio, the degrdn. rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to the soln. pH due to a disproportionation of NH2Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride transformed HO• and SO4•- to Cl2•- that is less-reactive with 1,4-D, while the presence of dissolved O2 promoted gaseous nitrogen prodn. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.
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    Plumlee, M. H.; Lopez-Mesas, M.; Heidlberger, A.; Ishida, K. P.; Reinhard, M. N-nitrosodimethylamine (NDMA) removal by reverse osmosis and UV treatment and analysis via LC-MS/MS. Water Res. 2008, 42 (1–2), 347– 355,  DOI: 10.1016/j.watres.2007.07.022
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    N-nitrosodimethylamine (NDMA) removal by reverse osmosis and UV treatment and analysis via LC-MS/MS
    Plumlee, Megan H.; Lopez-Mesas, Montserrat; Heidlberger, Andy; Ishida, Kenneth P.; Reinhard, Martin
    Water Research (2008), 42 (1-2), 347-355CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    N-nitrosodimethylamine (NDMA) is a probable human carcinogen found in ng/L concns. in chlorinated and chloraminated water. A method was developed for the detn. of ng/L levels of NDMA using liq. chromatog.-tandem mass spectrometry (LC-MS/MS) preceded by sample concn. via solid-phase extn. with activated charcoal. Recoveries were >90% and allowed a method reporting limit as low as 2 ng/L. Using this method, the removal of NDMA was detd. for the Interim Water Purifn. Facility (IWPF), an advanced wastewater treatment facility operated by the Orange County Water District (OCWD) in Southern California. The facility treats effluent from an activated sludge treatment plant with microfiltration (MF), reverse osmosis (RO), and an UV-H2O2 advanced oxidn. process (UV-AOP). Six nitrosamines were surveyed: NDMA, N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosopiperidine (NPip), and N-nitrosopyrrolidine (NPyr). Only NDMA was detected and at all treatment steps in the IWPF, with influent concns. ranging from 20 to 59 ng/L. Removals for RO and UV ranged from 24 to 56 and 43 to 66%, resp. Overall, 69±7% of the original NDMA concn. was removed from the product water across the advanced treatment process and, in combination with blending, the final concn. did not exceed the California drinking water notification level of 10 ng/L. NDMA removal data are consistent with findings reviewed for other advanced treatment facilities and lab. studies.
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    Thompson, D. W.; Tremblay, A. Y. Fouling in Steady and Unsteady State Electrodialysis. Desalination 1983, 47 (May), 181– 188,  DOI: 10.1016/0011-9164(83)87071-4
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    Thompson, D. W.; Tremblay, A. Y.
    Desalination (1983), 47 (), 181-8CODEN: DSLNAH; ISSN:0011-9164.
    The performance of a cyclic electrodialysis process (with sealed membrane-pairs and periodically reversing flow and elec. polarity) was compared with a steady-state process when both were operating under fouling conditions. A 2000 mg/L NaCl soln. was circulated through the bench-scale test cells for 20 h in each run. Fe(II) was introduced into the feed as FeCl2 at concns. of 0, 1, and 5 ppm. IONAC MC 3142 cationic membranes and IONAC MA 3148 anionic membranes were used in each type of process, with the flow channels defined by polyethylene-mesh separators through which the fluid velocity was 1.25 cm/s. Potentials of 20 and 30 V were applied across each stack, which consisted of 13 or 14 active membranes plus 2 membranes in each of the electrode rinse compartments. Reddish-brown deposits were visible on all of the test membranes in all steady and unsteady-state runs having Fe in their feed streams. Deposits for the steady-state runs were uneven and showed clear evidence of flow channelling. In the unsteady-state runs the deposits were obsd. as a fainter, even, coloring over the whole surface of the membrane. The deposition process is partly irreversible so that the polarity switching does not prevent build up of this type of fouling. Some fouling occurred inside the sealed membrane-pairs.
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    Jiang, S. X.; Li, Y. N.; Ladewig, B. P. A review of reverse osmosis membrane fouling and control strategies. Sci. Total Environ. 2017, 595, 567– 583,  DOI: 10.1016/j.scitotenv.2017.03.235
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    Jiang, Shanxue; Li, Yuening; Ladewig, Bradley P.
    Science of the Total Environment (2017), 595 (), 567-583CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    A review. Reverse osmosis (RO) membrane technol. is one of the most important technologies for water treatment. However, membrane fouling is an inevitable issue. Membrane fouling leads to higher operating pressure, flux decline, frequent chem. cleaning and shorter membrane life. This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments. The fundamentals of fouling are discussed in detail, including biofouling, org. fouling, inorg. scaling and colloidal fouling. Furthermore, fouling mitigation technologies are also discussed comprehensively. Pretreatment is widely used in practice to reduce the burden for the following RO operation while real time monitoring of RO has the advantage and potential of providing support for effective and efficient cleaning. Surface modification could slow down membrane fouling by changing surface properties such as surface smoothness and hydrophilicity, while novel membrane materials and synthesis processes build a promising future for the next generation of RO membranes with big advancements in fouling resistance. Esp. in this review paper, statistical anal. is conducted where appropriate to reveal the research interests in RO fouling and control.
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    Advances in Colloid and Interface Science (2016), 229 (), 34-56CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
    The environmentally friendly ion-exchange membrane (IEM) processes find more and more applications in the modern industries in order to demineralize, conc. and modify products. Moreover, these processes may be applied for the energy conversion and storage. However, the main drawback of the IEM processes is a formation of fouling, which significantly decreases the process efficiency and increases the process cost. The present review is dedicated to the problematic of IEM fouling phenomena. Firstly, the major types of IEM fouling such as colloidal fouling, org. fouling, scaling and biofouling are discussed along with consideration of the main factors affecting fouling formation and development. Secondly, the review of the possible methods of IEM fouling characterization is provided. This section includes the methods of fouling visualization and characterization as well as methods allowing investigations of characteristics of the fouled IEMs. Eventually, the reader will find the conventional and modern strategies of prevention and control of different fouling types.
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    Desalination (1979), 28 (1), 31-40CODEN: DSLNAH; ISSN:0011-9164.
    A brief description, history of development, and operating data on the 1st experience with the EDR process are given. EDR constitutes an improvement on the electrodialysis process by providing automatic self-cleaning, thereby eliminating need for acid or complexing agent feeds.
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    Journal of Colloid and Interface Science (1995), 173 (1), 165-80CODEN: JCISA5; ISSN:0021-9797. (Academic)
    A theor. study of particle deposition on a permeable surface in a parallel-plate channel is presented. The convective diffusion equation is formulated rigorously with the inclusion of lateral transport due to permeation drag and inertial lift, and transport due to gravitational, double layer, and van der Waals forces. A numerical procedure for solving the governing equation is also presented. The effects of particle size, permeation velocity, soln. ionic strength, cross-flow velocity, and particle d. on the initial rate of particle deposition are studied. The results indicate that the local and av. particle deposition rates on a permeable surface are detd. by an interplay between several transport and interaction mechanisms, among which permeation drag, elec. double layer repulsion, and inertial lift are most important.
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    Koo, T.; Lee, Y. J.; Sheikholeslami, R.
    Desalination (2001), 139 (1-3), 43-56CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
    Desalination plays an important role in producing pure water from brackish water. Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concns. in brackish water. The presence of silica and its ability to foul membranes limits the use of silica bearing waters for desalination and when used, it has many economic penalties. This study examines the effect of silica polymn. in the presence of polyvalent cations and anions in RO systems. Source of silica in the experimentation was from com. grade sodium metasilicate (Na2O3Si.9H2O). The membranes used were polyamide and thin film manufd. by Osmonics. Use of glassware is minimized to avoid the possibility of any contribution by silica leaching into soln. The feed soln. consists of silica, calcium and magnesium ions in various concns. to det. the effect of polyvalent ions on polymn. and the appropriate pre-treatment technol. The expt. was set up in a way as to simulate the conditions that would be encountered in a desalination plant. Concn. polarization (C/P) in the system was exptl. detd. with a simple technique that was developed and its effects on fouling are examd. In addn. tests were carried out to examine the actual fouling mechanism in reverse osmosis units under various exptl. conditions. Furthermore the effect of cleaning, with distd. water and with pulsations as well as with com. available cleaners were examd. Some com. available cleaners were capable at partially restoring the flux. Further investigation is underway to examine the effectiveness of new silica specific inhibitors.
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  76. 76
    Lisitsin, D.; Hasson, D.; Semiat, R. Critical flux detection in a silica scaling RO system. Desalination 2005, 186 (1–3), 311– 318,  DOI: 10.1016/j.desal.2005.06.007
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    76
    Critical flux detection in a silica scaling RO system
    Lisitsin, Dmitry; Hasson, David; Semiat, Raphael
    Desalination (2005), 186 (1-3), 311-318CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Desalination of brackish water contg. silica at high recovery levels leads to a rapid flux decline, due to the pptn. of a colloidal soln. consisting of polymd. silica nano-particles. The crit. flux (CF) phenomenon postulates that there is a certain permeate flux level below which the rate of flux decrease due to membrane fouling becomes negligible. The aim was to develop a simple technique for detecting the existence of a crit. flux threshold limit under conditions at which the membrane surface is exposed to a const. pptn. potential. A series of expts. was carried out in a silica scaling tubular RO system. Exptl. conditions were controlled such that the dissolved silica content on the membrane was held at the same supersatn. ratio level of 2.1-2.2 while varying the initial permeation flux value over the range 17-45 L/h-m2. The rate of flux decline remained const., irresp. of the permeate flux level examd.
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  77. 77
    Shi, L.; Rossi, R.; Son, M.; Hall, D. M.; Hickner, M. A.; Gorski, C. A.; Logan, B. E. Using reverse osmosis membranes to control ion transport during water electrolysis. Energy Environ. Sci. 2020, 13 (9), 3138– 3148,  DOI: 10.1039/D0EE02173C
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    77
    Using reverse osmosis membranes to control ion transport during water electrolysis
    Shi, Le; Rossi, Ruggero; Son, Moon; Hall, Derek M.; Hickner, Michael A.; Gorski, Christopher A.; Logan, Bruce E.
    Energy & Environmental Science (2020), 13 (9), 3138-3148CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The decreasing cost of electricity produced using solar and wind and the need to avoid CO2 emissions from fossil fuels has heightened interest in hydrogen gas prodn. by water electrolysis. Offshore and coastal hydrogen gas prodn. using seawater and renewable electricity is of particular interest, but it is currently economically infeasible due to the high costs of ion exchange membranes and the need to desalinate seawater in existing electrolyzer designs. A new approach is described here that uses relatively inexpensive com. available membranes developed for reverse osmosis (RO) to selectively transport favorable ions. In an applied elec. field, RO membranes have a substantial capacity for proton and hydroxide transport through the active layer while excluding salt anions and cations. A perchlorate salt was used to provide an inert and contained anolyte, with charge balanced by proton and hydroxide ion flow across the RO membrane. Synthetic seawater (NaCl) was used as the catholyte, where it provided continuous hydrogen gas evolution. The RO membrane resistance was 21.7 ± 3.5 Ω cm2 in 1 M NaCl and the voltages needed to split water in a model electrolysis cell at current densities of 10-40 mA cm-2 were comparable to those found when using two commonly used, more expensive ion exchange membranes.
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  • Abstract

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    Figure 1

    Figure 1. Schematic illustration of the variable water recovery electrodialysis (ED) process. A portion of the multicell pair ED stack is shown. Each cell pair contains an anion-exchange membrane (AEM), cation-exchange membrane (CEM), and spacer channels between the membranes. An external voltage is applied across the ED stack as (an equal flow rate of) saline water passes through each of the flow channels. The generated electric field causes ions in the channels to migrate toward the oppositely charged electrode. The AEM and CEM enable selective permeation of anions and cations, respectively. Due to nonideal membrane selectivity, however, the transport of counterions across the membrane (Jct) is also accompanied by some degree of counterproductive co-ion transport (Jco). The overall selective transport of ions results in alternating product and brine channels. Redox reactions at the surface of the electrodes convert the ionic current into an electrical current (i). For a system-scale water recovery greater than 50%, a fraction of the brine effluent (αR) is recycled, thus reducing the overall volume of brackish feedwater and increasing the system-scale water recovery.

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    Figure 2

    Figure 2. Schematic illustration of the variable salt removal reverse osmosis (RO) process. The brackish feedwater is split among three streams (with the following flow rate fractions): RO module feed (αRO), brine bypass (αB), and product bypass (αP). The hydraulic pressure of the feedwater fed to the RO module is increased (by ΔP) via a high-pressure pump, requiring the input of electrical energy. Water permeates through the RO membrane with a flux of Jw, while complete salt rejection is assumed. The permeate and retentate streams from the RO module are mixed with the corresponding bypass streams, generating the overall product and brine streams, respectively. The system-scale salt removal is controlled by tuning the fraction of feedwater sent to the product bypass stream. Alternatively, the brine bypass stream serves to minimize the flow to the RO module, and thus the energy consumption. An energy recovery device (ERD) is applied to the retentate stream exiting the RO module to recover and reuse a significant portion of the mechanical energy provided by the high-pressure pump. However, an ERD is only applied to certain cases throughout the study, as its use in brackish water RO is circumstantial.

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    Figure 3

    Figure 3. Specific energy consumption (SEC) for RO and ED as a function of salt removal (Rs). The water recovery and productivity are fixed at 80% and 20 L m–2 h–1, respectively. The various colored lines show different feed salinities: 1 g L–1 (orange), 3 g L–1 (red), 5 g L–1 (blue), and 10 g L–1(purple). (A) Specific energy consumption of RO with a water permeability and feed-side mass transfer coefficient of 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The module-scale water recovery is optimized for each feed salinity and salt removal to minimize SEC. Energy recovery is not applied. (B) Specific energy consumption of ED with the modeling parameters specified in Table S1. The cell pair voltage is tuned to achieve varying degrees of salt removal. Effluent from the brine channels is recycled at a recycle ratio of 3.0 to achieve a water recovery of 80%.

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    Figure 4

    Figure 4. Energy efficiency color maps of RO and ED for varying salt removal (Rs) and system-scale water recovery (WR). The energy efficiency (η) is defined as the ratio of the thermodynamic minimum specific energy consumption of separation (SECmin) and the actual specific energy consumption (SEC). The values for energy efficiency correspond to the color legends shown on the right side of each plot. The productivity is set to 20 L m–2 h–1 throughout. (A) Energy efficiency of RO for 5 g L–1 feed salinity. The water permeability coefficient and the feed-side mass transfer coefficient are set to 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The module-scale water recovery is optimized for each salt removal and system-scale water recovery to maximize the energy efficiency. Energy recovery is not applied. (B) Energy efficiency of ED for 5 g L–1 feed salinity. The salt removal and water recovery are varied by tuning the cell pair voltage and brine effluent recycle ratios, respectively. The black contour lines shown in (A) and (B) represent fixed SEC values.

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    Figure 5

    Figure 5. (A) Specific energy consumption and (B) corresponding energy efficiency of ED (open bars) and RO with no energy recovery (hatched bars) for varying productivity. The energy efficiency is defined as the ratio of the thermodynamic minimum specific energy consumption of separation and the actual specific energy consumption. Salt removals of 50% (red bars), 70% (blue bars), and 90% (orange bars) are shown. The water recovery and feed salinity are set to 80% and 5 g L–1, respectively. Productivity is defined as the rate of water production normalized by the projected membrane area (eq 6). For ED, the productivity is the flow rate through an individual spacer channel divided by the projected area of an ion-exchange membrane. In RO, the productivity is the product water flow rate divided by the membrane area of the module.

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    Figure 6

    Figure 6. Energy efficiency phase diagrams of ED and RO for varying salt removal (Rs) and system-scale water recovery (WR). The difference between the ED and RO energy efficiencies is plotted, with values corresponding to the color legends shown to the right of each panel. The white contour lines show the transition between ED and RO being more energy efficient: above the line, RO is more energy efficient, while below the line, ED is more energy efficient. The energy efficiency (η) is defined as the ratio of the thermodynamic minimum specific energy consumption of separation and the actual specific energy consumption. The productivity is set to 20 L m–2 h–1 throughout. The water permeability coefficient and the feed-side mass transfer coefficient for RO are set to 5 L m–2 h–1 bar–1 and 150 L m–2 h–1, respectively. The RO module-scale water recovery is optimized for each salt removal and system-scale water recovery to maximize the energy efficiency. In ED, the salt removal and water recovery are varied by tuning the cell pair voltage and brine effluent recycle ratios, respectively. (A), (C), (E) Energy efficiency phase-diagram of ED-RO with no energy recovery in RO for 3, 5, and 10 g L–1 feed salinity, respectively. (B), (D), (F) Energy efficiency phase-diagram of ED-RO with an RO energy recovery device operating at 80% efficiency (ηERD = 0.8) for 3, 5, and 10 g L–1 feed salinity, respectively.

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      The state of desalination and brine production: A global outlook
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      Science of the Total Environment (2019), 657 (), 1343-1356CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      Rising water demands and diminishing water supplies are exacerbating water scarcity in most world regions. Conventional approaches relying on rainfall and river runoff in water scarce areas are no longer sufficient to meet human demands. Unconventional water resources, such as desalinated water, are expected to play a key role in narrowing the water demand-supply gap. Our synthesis of desalination data suggests that there are 15,906 operational desalination plants producing around 95 million m3/day of desalinated water for human use, of which 48% is produced in the Middle East and North Africa region. A major challenge assocd. with desalination technologies is the prodn. of a typically hypersaline conc. (termed 'brine') discharge that requires disposal, which is both costly and assocd. with neg. environmental impacts. Our ests. reveal brine prodn. to be around 142 million m3/day, approx. 50% greater than previous quantifications. Brine prodn. in Saudi Arabia, UAE, Kuwait and Qatar accounts for 55% of the total global share. Improved brine management strategies are required to limit the neg. environmental impacts and reduce the economic cost of disposal, thereby stimulating further developments in desalination facilities to safeguard water supplies for current and future generations.
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      A computer simulation of batch ion exchange membrane electrodialysis for desalination of saline water
      Tanaka, Yoshinobu
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      A computer simulation program is developed for predicting desalinating performance of a batch electrodialysis process. The program includes the principle of (1) mass transport, (2) c.d. distribution, (3) cell voltage, (4) mass balance/energy consumption and (5) limiting c.d. In this simulation the following parameters are inputted; (1) membrane characteristics such as overall transport no., overall solute permeability, overall electro-osmotic permeability, overall hydraulic permeability, d.c. elec. resistance etc., (2) electrodialyzer specifications such as flow-pass thickness, flow-pass width and flow-pass length in a desalting cell etc. and (3) electrodialytic conditions such as voltage, electrolyte concn. in a feeding soln., linear velocity in desalting cells, std. deviation of normal distribution of soln. velocity ratio etc. The following phenomena were computed and discussed; (1) Changes of electrolyte concn. and c.d. with operation time. (2) Influence of cell voltage on operation time (batch duration), water recovery and energy consumption. (3) Influence of vol. of an electrolyte soln. prepd. at first on operation time. (4) Influence of cell voltage, electrolyte concn. and std. deviation of soln. velocity ratio in desalting cells on limiting c.d. (5) Energy consumption in a reverse osmosis process. (6) Excepting limiting c.d., the performance of an electrodialyzer is never influenced by the std. deviation of normal distribution of soln. velocity ratio in desalting cells. (7) Energy consumption in electrodialysis is less than that in reverse osmosis at feeding saline water concn. less than about 2000 mg/l.
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      Elimelech, M.; Phillip, W. A. The future of seawater desalination: energy, technology, and the environment. Science 2011, 333 (6043), 712– 717,  DOI: 10.1126/science.1200488
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      The Future of Seawater Desalination: Energy, Technology, and the Environment
      Elimelech, Menachem; Phillip, William A.
      Science (Washington, DC, United States) (2011), 333 (6043), 712-717CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A review. In recent years, numerous large-scale seawater desalination plants were built in water-stressed countries to augment available water resources, and construction of new desalination plants is expected to increase in the near future. Despite major advancements in desalination technologies, seawater desalination is still more energy intensive compared to conventional technologies for the treatment of fresh water. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants. Here, the authors review the possible redns. in energy demand by state-of-the-art seawater desalination technologies, the potential role of advanced materials and innovative technologies in improving performance, and the sustainability of desalination as a technol. soln. to global water shortages.
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      Greenlee, Lauren F.; Lawler, Desmond F.; Freeman, Benny D.; Marrot, Benoit; Moulin, Philippe
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      A review. Reverse osmosis membrane technol. has developed over the past 40 years to a 44% share in world desalting prodn. capacity, and an 80% share in the total no. of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technol. for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (conc.) disposal options, and plant location, have created significant differences in process development, implementation, and key tech. problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technol. in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.
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      Loeb, S. The Loeb-Sourirajan Membrane - How It Came About. ACS Symp. Ser. 1981, 153, 1,  DOI: 10.1021/bk-1981-0153.ch001
      12
      The Loeb-Sourirajan membrane: how it came about
      Loeb, Sidney
      ACS Symposium Series (1981), 153 (Synth. Membr., Vol. 1), 1-9CODEN: ACSMC8; ISSN:0097-6156.
      A review with 16 refs.
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      Porter, C. J.; Werber, J. R.; Zhong, M.; Wilson, C. J.; Elimelech, M. Pathways and Challenges for Biomimetic Desalination Membranes with Sub-Nanometer Channels. ACS Nano 2020, 14 (9), 10894– 10916,  DOI: 10.1021/acsnano.0c05753
      14
      Pathways and challenges for biomimetic desalination membranes with sub-nanometer channels
      Porter, Cassandra J.; Werber, Jay R.; Zhong, Mingjiang; Wilson, Corey J.; Elimelech, Menachem
      ACS Nano (2020), 14 (9), 10894-10916CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrixes on a robust support. While biol. components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers. However, the exptl. performance of biomimetic membranes remains far below that of biol. membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection. We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examn. of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m-2 h-1 bar-1, which is comparable to that of current state-of-the-art polymeric desalination membranes. Through a quant. assessment of biomimetic membrane formats, we anal. show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains obsd. subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.
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    15. 15
      Patel, S. K.; Ritt, C. L.; Deshmukh, A.; Wang, Z. X.; Qin, M. H.; Epsztein, R.; Elimelech, M. The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies. Energy Environ. Sci. 2020, 13 (6), 1694– 1710,  DOI: 10.1039/D0EE00341G
      15
      The relative insignificance of advanced materials in enhancing the energy efficiency of desalination technologies
      Patel, Sohum K.; Ritt, Cody L.; Deshmukh, Akshay; Wang, Zhangxin; Qin, Mohan; Epsztein, Razi; Elimelech, Menachem
      Energy & Environmental Science (2020), 13 (6), 1694-1710CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      As the threat of global water scarcity continues to grow, a myriad of scientific effort is directed towards advancing water desalination technologies. Reverse osmosis (RO), solar thermal desalination (STD), and capacitive deionization (CDI), have dominated recent pressure-, thermal-, and electro-driven desalination research efforts, resp. Despite being based on distinctive driving forces and sepn. mechanisms, research of these three processes has primarily shared the same fundamental goal and approach: the minimization of energy consumption for desalination through the development of novel materials. A variety of materials have been studied and proposed to enhance RO membrane permeability, STD solar absorptivity, and CDI electrode capacitance. Here, we critically discuss the advanced materials investigated and assess their efficacy in augmenting the energy efficiency of desalination. Through our systematic anal., we show that materials have relatively insignificant impact on further increasing energy efficiency, regardless of the process applied. We provide insights into the inherent limitations of advanced materials for improving the energy efficiency of each of the evaluated technologies and propose more effective materials-based research directions. We conclude by highlighting the opportunity for considerable improvement in energy efficiency via system design, reinforcing the crit. need for a paradigm shift in desalination research.
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      Werber, J. R.; Deshmukh, A.; Elimelech, M. The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes. Environ. Sci. Technol. Lett. 2016, 3 (4), 112– 120,  DOI: 10.1021/acs.estlett.6b00050
      16
      The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes
      Werber, Jay R.; Deshmukh, Akshay; Elimelech, Menachem
      Environmental Science & Technology Letters (2016), 3 (4), 112-120CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
      A review is given. Desalination membranes are essential for the treatment of unconventional water sources, such as seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel membrane materials may yield significant performance gains over state-of-the-art thin-film composite (TFC) membranes, which are constrained by the permeability-selectivity trade-off. However, little guidance currently exists on the practical impact of such performance gains, namely enhanced water permeability or enhanced water-solute selectivity. We 1st discuss the performance of current TFC membranes. We then highlight and provide context for recent module-scale modeling studies that have found limited impact of increased water permeability on the efficiency of desalination processes. We cover several important examples of water treatment processes in which inadequate membrane selectivity hinders process efficacy. We conclude with a discussion of how the need for enhanced selectivity may influence the design strategies of future membranes.
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      Strathmann, H. Ion-exchange membrane separation processes; Elsevier, 2004; Vol. 9.
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      Strathmann, H. Electrodialysis, a mature technology with a multitude of new applications. Desalination 2010, 264 (3), 268– 288,  DOI: 10.1016/j.desal.2010.04.069
      18
      Electrodialysis, a mature technology with a multitude of new applications
      Strathmann, H.
      Desalination (2010), 264 (3), 268-288CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Electrodialysis is a mature process which is applied since more than 50 years on a large industrial scale for the prodn. of potable water from brackish water sources. But more recently electrodialysis in combination with bipolar membranes or with ion-exchange resins has found a large no. of new interesting applications in the chem. process industry, in the food and drug industry as well as in waste water treatment and the prodn. of high quality industrial water. In this paper the principle of electrodialysis is described and its advantages and limitations in various applications are pointed out. More recent developments in electrodialysis as well as in related processes such as electrodialytic water dissocn. or continuous electrodeionization are discussed and their present and potential future applications are indicated. Research needs for a sustainable growth of electrodialysis and related processes are pointed out.
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    19. 19
      Campione, A.; Gurreri, L.; Ciofalo, M.; Micale, G.; Tamburini, A.; Cipollina, A. Electrodialysis for water desalination: A critical assessment of recent developments on process fundamentals, models and applications. Desalination 2018, 434, 121– 160,  DOI: 10.1016/j.desal.2017.12.044
      19
      Electrodialysis for water desalination: A critical assessment of recent developments on process fundamentals, models and applications
      Campione, A.; Gurreri, L.; Ciofalo, M.; Micale, G.; Tamburini, A.; Cipollina, A.
      Desalination (2018), 434 (), 121-160CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      A review. The need for unconventional sources of fresh water is pushing a fast development of desalination technologies, which proved to be able to face and solve the problem of water scarcity in many dry areas of the planet. Membrane desalination technologies are nowadays leading the market and, among these, electrodialysis (ED) plays an important role, esp. for brackish water desalination, thanks to its robustness, extreme flexibility and broad range of applications. In fact, many ED-related processes have been presented, based on the use of Ion Exchange Membranes (IEMs), which are significantly boosting the development of ED-related technologies. This paper presents the fundamentals of the ED process and its main developments. An important outlook is given to operational aspects, hydrodynamics and mass transport phenomena, with an extensive review of literature studies focusing on theor. or exptl. characterization of the complex phenomena occurring in electromembrane processes and of proposed strategies for process performance enhancement. An overview of process modeling tools is provided, pointing out capabilities and limitations of the different approaches and their possible application to optimization anal. and perspective developments of ED technol. Finally, the most recent applications of ED-related processes are presented, highlighting limitations and potentialities in the water and energy industry.
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    20. 20
      Meyer, K. H.; Straus, W. La perméabilité des membranes VI. Sur le passage du courant electrique a travers des membranes sélectives. Helv. Chim. Acta 1940, 23 (1), 795– 800,  DOI: 10.1002/hlca.19400230199
      20
      Permeability of membranes. VI. The passage of the electric current through selective membranes
      Meyer, Kurt H.; Straus, W.
      Helvetica Chimica Acta (1940), 23 (), 795-800CODEN: HCACAV; ISSN:0018-019X.
      cf. C. A. 31, 7450.1. Selective membranes may be the principal cause of action currents. To test this theory regenerated cellulose (cellophane) was chosen as a membrane permeable to cations and its const. of selectivity detd. as described in earlier papers of this series and found to be 0.02. The artificial catgut, "Naturin," prepd. from waste hides was chosen as a membrane permeable to anions but its const. of selectivity was only 0.007. Methylation of this membrane raised the const. of selectivity to 0.017. A cell having 4 cellophane membranes alternating with 3 methylated "Naturin" membranes was constructed, filled with 0.01 N KCl, and connected with 2 reversible electrodes. After equil. was attained a current of 0.75 ma. at 250 v. was passed for 10-30 min., the current interrupted, and the polarization measured by a compensation method about 20 secs. later and found to be about 100 mv. but dropping quickly to about 0 in 30 min. This difference of potential after 20 sec. is certainly due to an electrolytic effect and therefore supports M.'s theory. H. Rein (Z. Biol. 85, 217(1926)) who measured similar effects 10-4 sec. after interrupting the current found much larger values, 0.390 to 0.555 v., which were undoubtedly due to electrostatic effects not shown in M. and S.'s measurements after 20 sec.
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    21. 21
      Gurreri, L.; Tamburini, A.; Cipollina, A.; Micale, G. Electrodialysis Applications in Wastewater Treatment for Environmental Protection and Resources Recovery: A Systematic Review on Progress and Perspectives. Membranes 2020, 10 (7), 146,  DOI: 10.3390/membranes10070146
      21
      Electrodialysis applications in wastewater treatment for environmental protection and resources recovery: a systematic review on progress and perspectives
      Gurreri, Luigi; Tamburini, Alessandro; Cipollina, Andrea; Micale, Giorgio
      Membranes (Basel, Switzerland) (2020), 10 (7), 146CODEN: MBSEB6; ISSN:2077-0375. (MDPI AG)
      A review. This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of sepn. under the action of an elec. field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aq. solns., including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high sepn. efficiency, and chem.-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concn., diln., desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and orgs., or elec. energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liq. discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.
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    22. 22
      Xu, T.; Huang, C. Electrodialysis-based separation technologies: a critical review. AIChE J. 2008, 54 (12), 3147– 3159,  DOI: 10.1002/aic.11643
      22
      Electrodialysis-based separation technologies: a critical review
      Xu, Tongwen; Huang, Chuanhui
      AIChE Journal (2008), 54 (12), 3147-3159CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
      A review. To support a sustainable industrial growth, chem. engineering today faces a crucial challenge of meeting the increasing demand for materials and energy. One possible soln. is to decrease the equipment size/productivity ratio, energy consumption, and waste generation via process integration and optimization. This review focuses on the integration of electrodialysis with traditional unit operations and other membrane sepns. Such integrations, due to their diversity and practicability, can be versatile tools to meet specific needs from chem., biochem., food, and pharmaceutical industries.
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    23. 23
      Xu, T. W. Ion exchange membranes: State of their development and perspective. J. Membr. Sci. 2005, 263 (1–2), 1– 29,  DOI: 10.1016/j.memsci.2005.05.002
      23
      Ion exchange membranes: State of their development and perspective
      Xu, Tongwen
      Journal of Membrane Science (2005), 263 (1-2), 1-29CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      A review. During the last 50 years, ion exchange membranes have evolved from a lab. tool to industrial products with significant tech. and com. impact. Today ion exchange membranes are receiving considerable attention and are successfully applied for desalination of sea and brackish water and for treating industrial effluents. They are efficient tools for the concn. or sepn. of food and pharmaceutical products contg. ionic species as well as the manuf. of basic chem. products. The evolvement of an ion exchange membrane not only makes the process cleaner and more energy-efficient but also recovers useful effluents that are now going to wastes, and thus makes the development of society sustainable. Therefore, the intention of this review is to give a brief summary of the different prepn. and characteristics of ion exchange membrane as well as their potential applications. The most relevant literatures in the field are surveyed and some elucidating case studies are discussed, also accounting for the results of some research programs carried out in the author's lab.
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    24. 24
      Patel, S. K.; Qin, M.; Walker, W. S.; Elimelech, M. Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization. Environ. Sci. Technol. 2020, 54 (6), 3663– 3677,  DOI: 10.1021/acs.est.9b07482
      24
      Energy Efficiency of Electro-Driven Brackish Water Desalination: Electrodialysis Significantly Outperforms Membrane Capacitive Deionization
      Patel, Sohum K.; Qin, Mohan; Walker, W. Shane; Elimelech, Menachem
      Environmental Science & Technology (2020), 54 (6), 3663-3677CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Electro-driven technologies are viewed as a potential alternative to the current state-of-the-art technol., reverse osmosis, for the desalination of brackish waters. Capacitive deionization (CDI), based on the principle of electrosorption, has been intensively researched under the premise of being energy efficient. However, electrodialysis (ED), despite being a more mature electro-driven technol., has yet to be extensively compared to CDI in terms of energetic performance. In this study, we utilize Nernst-Planck based models for continuous flow ED and const.-current membrane capacitive deionization (MCDI) to systematically evaluate the energy consumption of the two processes. By ensuring equivalently sized ED and MCDI systems-in addn. to using the same feed salinity, salt removal, water recovery, and productivity across the two technologies-energy consumption is appropriately compared. We find that ED consumes less energy (has higher energy efficiency) than MCDI for all investigated conditions. Notably, our results indicate that the performance gap between ED and MCDI is substantial for typical brackish water desalination conditions (e.g., 3 g L-1 feed salinity, 0.5 g L-1 product water, 80% water recovery, and 15 L m-2 h-1 productivity), with the energy efficiency of ED often exceeding 30% and being nearly an order of magnitude greater than MCDI. We provide further insights into the inherent limitations of each technol. by comparing their resp. components of energy consumption, and explain why MCDI is unable to attain the performance of ED, even with ideal and optimized operation.
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    25. 25
      McGovern, R. K.; Zubair, S. M.; Lienhard, J. H. The cost effectiveness of electrodialysis for diverse salinity applications. Desalination 2014, 348, 57– 65,  DOI: 10.1016/j.desal.2014.06.010
      25
      The cost effectiveness of electrodialysis for diverse salinity applications
      McGovern, Ronan K.; Zubair, Syed M.; Lienhard V, John H.
      Desalination (2014), 348 (), 57-65CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      We provide a thermoeconomic assessment of electrodialysis indicating that the technol. is most productive and efficient for the partial desalination of feed streams at the higher end of the brackish range of salinities. After optimizing the c.d. to minimise the sum of energy and equipment costs, we demonstrate that at low feed salinities the productivity, and hence equipment costs, of electrodialysis are hampered by the limiting c.d. By contrast, at higher feed salinities both productivity and efficiency are hampered by the reduced chem. p.d. of salt in the diluate (low salinity) and conc. (high salinity) streams. This anal. indicates the promise of further developing electrodialysis for the treatment of waters from oil, gas and coal-bed methane as well as flue-gas de-sulphurisation, where the partial desalination of streams at the high-end of the brackish range can be beneficial.
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    26. 26
      Chehayeb, K. M.; Farhat, D. M.; Nayar, K. G.; Lienhard, J. H. Optimal design and operation of electrodialysis for brackish-water desalination and for high-salinity brine concentration. Desalination 2017, 420, 167– 182,  DOI: 10.1016/j.desal.2017.07.003
      26
      Optimal design and operation of electrodialysis for brackish-water desalination and for high-salinity brine concentration
      Chehayeb, Karim M.; Farhat, Daniel M.; Nayar, Kishor G.; Lienhard, John H. V.
      Desalination (2017), 420 (), 167-182CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Electrodialysis (ED) is a desalination technol. that has been deployed com. for decades. However, few studies in the literature have looked at the optimal design and operation of these systems, esp. for the concn. of high-salinity brines. In this paper, a set of constraints is defined to allow a fair comparison between different system sizes, designs, and operating conditions. The design and operation of ED are studied for the applications of brackish-water desalination and of high-salinity brine concn. for a fixed system size. The set of variables that det. the power consumption of a fixed-size system is reduced to include only the channel height and the velocity, with all the other design and operation variables depending on these two variables. After studying the minimization of power consumption for a fixed system size, the min. costs assocd. with the different system sizes are studied, and the differing trends in brackish-water and high-salinity applications are compared. Finally this paper presents the effect of the cost modeling parameters on the trends of the optimal system size, c.d., length, channel height, and velocity for the two applications studied.
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    27. 27
      McGovern, R. K.; Zubair, S. M.; Lienhard, V. J. H. The benefits of hybridising electrodialysis with reverse osmosis. J. Membr. Sci. 2014, 469, 326– 335,  DOI: 10.1016/j.memsci.2014.06.040
      27
      The benefits of hybridizing electrodialysis with reverse osmosis
      McGovern, Ronan K.; Zubair, Syed M.; Lienhard V, John H.
      Journal of Membrane Science (2014), 469 (), 326-335CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      A cost anal. reveals that hybridization of electrodialysis with reverse osmosis is only justified if the cost of water from the reverse osmosis unit is <40% of that from a stand-alone electrodialysis system. In such cases the addnl. reverse osmosis costs justify the electrodialysis cost savings brought about by shifting salt removal to higher salinity, where current densities are higher and equipment costs lower. Also, the anal. suggests that a simple hybrid configuration is more cost effective than a recirculated hybrid, a simple hybrid being one where the reverse osmosis conc. is fed to the electrodialysis stack and the products from both units are blended, and a recirculated being one hybrid involving recirculation of the electrodialysis product back to the reverse osmosis unit. The underlying rationale is that simple hybridization shifts salt removal away from the lowest salinity zone of operation, where salt removal is most expensive. Further shifts in the salinity at which salt is removed, brought about by recirculation, do not justify the assocd. increased costs of reverse osmosis.
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    28. 28
      Nayar, K. G.; Fernandes, J.; McGovern, R. K.; Dominguez, K. P.; McCance, A.; Al-Anzi, B. S.; Lienhard, V. J. H. Cost and energy requirements of hybrid RO and ED brine concentration systems for salt production. Desalination 2019, 456, 97– 120,  DOI: 10.1016/j.desal.2018.11.018
      28
      Cost and energy requirements of hybrid RO and ED brine concentration systems for salt production
      Nayar, Kishor G.; Fernandes, Jenifer; McGovern, Ronan K.; Dominguez, Kyle P.; McCance, Adriene; Al-Anzi, Bader S.; Lienhard, John H. V.
      Desalination (2019), 456 (), 97-120CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      A new concept to conc. seawater up to 200 g/kg for producing vacuum salt using a reverse osmosis (RO) system hybridized with an electrodialysis (ED) system is presented. The RO system operates up to pressures of 120 bar and concs. seawater up to 120 g/kg with the ED system concg. RO brine to 200 g/kg. A parametric anal. to minimize the specific cost of brine concn. was conducted. Parameters varied were: the degree of RO-ED hybridization, ED c.d., electricity prices and water prices. Optimal hybrid RO-ED designs reduced brine concn. costs by 33-70% over standalone ED systems, with revenue generated from water co-prodn. further subsidizing costs by 1-6%. Optimizing ED c.d. reduced costs the most. Including a crystallizer, the total redn. in prodn. cost over a standalone ED-crystallizer system was 19-55%, with the prodn. cost for a typical case being $111/tonne-salt. The proposed RO-ED-crystallizer (REC) systems were found to be techno-economically feasible in Cyprus, Japan, Kuwait, Saudi Arabia, and the USA. At a road transportation distance of 735 km, REC based seawater vacuum salt was competitive with conventional vacuum salt. REC systems may open up the potential of small-scale decentralized salt prodn.
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    29. 29
      Wright, N. C.; Shah, S. R.; Amrose, S. E.; Winter, A. G. A robust model of brackish water electrodialysis desalination with experimental comparison at different size scales. Desalination 2018, 443, 27– 43,  DOI: 10.1016/j.desal.2018.04.018
      29
      A robust model of brackish water electrodialysis desalination with experimental comparison at different size scales
      Wright, Natasha C.; Shah, Sahil R.; Amrose, Susan E.; Winter, Amos G. V.
      Desalination (2018), 443 (), 27-43CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      This paper presents a robust anal. model for brackish water desalination using electrodialysis (ED), with prediction of the desalination rate, limiting c.d., and total energy use including pumping energy. Several assumptions reduce computation time and accurately model ED system behavior. The predicted desalination rate, limiting c. d., and total energy usage agree with measurements across two diverse ED stack designs, differing in total membrane area (0.18 m2, 37.1 m2), membrane manufacturers (GE Water, PCA GmbH), and flow channel spacers. The com.-scale stack was addnl.tested with real groundwater, demonstrating that brackish groundwater may be modeled as an equiv. concn. NaCl soln. Sensitivity to the membrane diffusion coeff., area available for ion transport, level of discretization along the flow channel length, boundary layer and membrane resistances, and water transport are analyzed to guide empirical characterization when higher accuracy is required. No single existing model for pressure drop in the membrane spacers could accurately predict pumping power in both stacks. One model for each stack was found to reasonably approx.pressure drop, however exptl. validation of specific spacer designs is recommended. The fully quant., parametric description of electrodialysis behavior presented forms a useful tool to design, evaluate, and optimize ED systems.
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    30. 30
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: II. Effect of water transport through ion-exchange membranes. J. Membr. Sci. 2017, 531, 172– 182,  DOI: 10.1016/j.memsci.2017.02.031
      30
      Nernst-Planck transport theory for (reverse) electrodialysis: II. Effect of water transport through ion-exchange membranes
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
      Journal of Membrane Science (2017), 531 (), 172-182CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Transport of water through ion-exchange membranes is of importance both for electrodialysis (ED) and reverse electrodialysis (RED). In this work, we extend our previous theory [J. Membrane Sci., 510 (2016) 370-381] and include water transport in a two-dimensional model for (R)ED. Following a Maxwell-Stefan (MS) approach, ions in the membrane have friction with the water, pore walls, and one another. We show that when ion-ion friction is neglected, the MS-approach is equiv. to the hydrodynamic theory of hindered transport, for instance applied to nanofiltration. After validation against exptl. data from literature for ED and RED, the model is used to analyze single-pass seawater ED, and RED with highly concd. solns. In the model, fluxes and velocities of water and ions in the membranes are self-consistently calcd. as function of the driving forces. We investigate the influence of water and coion leakage under different operational conditions.
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    31. 31
      Tanaka, Y. A computer simulation of feed and bleed ion exchange membrane electrodialysis for desalination of saline water. Desalination 2010, 254 (1–3), 99– 107,  DOI: 10.1016/j.desal.2009.12.008
      31
      A computer simulation of feed and bleed ion exchange membrane electrodialysis for desalination of saline water
      Tanaka, Yoshinobu
      Desalination (2010), 254 (1-3), 99-107CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      A computer simulation program is developed to predict desalinating performance of a feed and bleed electrodialysis process, inputting membrane characteristics, electrodialyzer specifications and electrodialytic conditions. Computing results enable to discuss the phenomena such as influence of cell voltage on c.d., ionic fluxes, soln. fluxes, current efficiency, ohmic potential and membrane potential, and further the influence of cell voltage and electrolyte concn. on the output of desalted solns., energy consumption and water recovery. Excepting limiting c.d., the performance of an electrodialyzer is scarcely influenced by the std. deviation of the normal distribution of the soln. velocity ratio in desalting cells. Energy consumption in a feed and bleed process is larger than that in a batch process for higher feed concn., and it is less than that in a reverse osmosis process at feed concn. less than one thousand and hundred-odd mg/l.
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    32. 32
      Sonin, A. A.; Probstein, R. F. A Hydrodynamic Theory of Desalination by Electrodialysis. Desalination 1968, 5 (3), 293,  DOI: 10.1016/S0011-9164(00)80105-8
      32
      Hydrodynamic theory of desalination by electrodialysis
      Sonin, Ain A.; Probstein, Ronald F.
      Desalination (1968), 5 (3), 293-329CODEN: DSLNAH; ISSN:0011-9164.
      A hydrodynamic theory of demineralization by electrodialysis was developed for a multichannel system with steady laminar flow between plane, parallel membranes. The modeling of the system is found to be governed by 4 basic similarity parameters: (1) a dimensionless applied potential, (2) the product of the channel aspect ratio and the inverse Peclet no., (3) the ratio of brine and dialyzate inlet concns., and (4) a parameter measuring membrane resistance. For sufficiently long channels it is shown that there are 2 distinct regions: a "developing" region where the concn. diffusion layers are growing, and a "developed" region where the diffusion layers fill the channel. Parabolic and uniform velocity profiles are considered and self-consistent solns. are derived for the distributions of salt concn., elec. field, and c.d. in the system, as well as for the total current. An integral method of soln. is used. In the limits of low and high polarization, anal. solns. are obtained which when matched at their point of equality closely approx. the complete numerical solns. Under a wide range of operating conditions, the soln. for the total current is represented by the empirical formula I = [1 - exp(- 3)]1/3, where I and are, resp., a dimensionless current and potential embodying the 4 similarity parameters mentioned. Comparison is made of the calcd. limiting total current with expt.
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    33. 33
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: I. Effect of co-ion transport through the membranes. J. Membr. Sci. 2016, 510, 370– 381,  DOI: 10.1016/j.memsci.2016.03.012
      33
      Nernst-Planck transport theory for (reverse) electrodialysis: I. Effect of co-ion transport through the membranes
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
      Journal of Membrane Science (2016), 510 (), 370-381CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Electrodialysis (ED) and Reverse Electrodialysis (RED) are related technologies for H2O desalination and energy conversion, both based on the selective transport of ions through ion exchange membranes. Fundamental understanding of these processes requires the description of ion transfer phenomena both along and through the membranes. The authors develop a simple 2-dimensional model valid for ED and RED, extending the approach by Sonin and Probstein (Desalination 5, 1968, 293) by using the Nernst-Planck equation not only in the flow channels but also in the membranes. This model requires as only input parameters the geometrical features of the system, the membrane charge d., and the diffusion coeffs. of ions in the channel and in the membrane. The effect of nonideal behavior of the membrane due to the co-ion transport is discussed, evaluating the performance of the processes in terms of current efficiency and energy consumption (for ED), and salt flux efficiency and power d. (for RED). Membrane properties such as permselectivity are calcd. as outputs of the model, and depend on position in the channel.
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    34. 34
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M. Nernst-Planck transport theory for (reverse) electrodialysis: III. Optimal membrane thickness for enhanced process performance. J. Membr. Sci. 2018, 565, 480– 487,  DOI: 10.1016/j.memsci.2018.07.090
      34
      Nernst-Planck transport theory for (reverse) electrodialysis: III. Optimal membrane thickness for enhanced process performance
      Tedesco, M.; Hamelers, H. V. M.; Biesheuvel, P. M.
      Journal of Membrane Science (2018), 565 (), 480-487CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      The effect of the thickness of ion exchange membranes has been investigated for electrodialysis (ED) and reverse electrodialysis (RED), both exptl. and through theor. modeling. By developing a two-dimensional model based on Nernst-Planck theory, we theor. find that reducing the membrane thickness benefits process performance only until a certain value, below which performance drops. For ED, an optimum thickness can be identified in the range of 10-20 μm, while for RED the max. power d. is found for membranes that are three times as thick. Model calcns. compare well with exptl. data collected with a series of homogeneous membranes with the same chem. compn. and a thickness in the range of 10-100 μm. Our results show that the classical picture that membranes should be as thin as possible (as long as they remain pinhole-free and structurally stable) is insufficient, and must be replaced by a more accurate theor. framework.
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    35. 35
      Biesheuvel, P. M.; Dykstra, J. E. Physics of Electrochemical Processes ; 2020; http://www.physicsofelectrochemicalprocesses.com/.
      There is no corresponding record for this reference.
    36. 36
      Lee, H. J.; Sarfert, F.; Strathmann, H.; Moon, S. H. Designing of an electrodialysis desalination plant. Desalination 2002, 142 (3), 267– 286,  DOI: 10.1016/S0011-9164(02)00208-4
      36
      Designing of an electrodialysis desalination plant
      Lee, Hong-Joo; Sarfert, F.; Strathmann, H.; Moon, Seung-Hyeon
      Desalination (2002), 142 (3), 267-286CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
      The design and operation of an electrodialysis desalination process are based on a set of fixed and variable parameters such as stack construction, feed and product concn., membrane properties, flow velocities, c.d., recovery rates, etc. These parameters are interrelated and may be rather different for different applications. For an efficient operation of an electrodialysis desalination plant, the process has to be optimized in terms of overall costs considering component properties and operating parameters. The design and optimization of an electrodialysis plant to be used for brackish water desalination was studied. The required equations were derived or, as in the case of the limiting c.d., were exptl. detd. As an example, an electrodialysis plant with a sheet-flow stack construction and given feed soln. compn. was designed and optimized in terms of overall costs and the sensitivities of the different parameters were analyzed.
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    37. 37
      Fidaleo, M.; Moresi, M. Optimal strategy to model the electrodialytic recovery of a strong electrolyte. J. Membr. Sci. 2005, 260 (1–2), 90– 111,  DOI: 10.1016/j.memsci.2005.01.048
      37
      Optimal strategy to model the electrodialytic recovery of a strong electrolyte
      Fidaleo, Marcello; Moresi, Mauro
      Journal of Membrane Science (2005), 260 (1-2), 90-111CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Mostly Nernst-Planck derived relations were used to simulate the electrodialytic recovery of a strong electrolyte, namely sodium chloride. It was set up a 5-step exptl. procedure consisting of 0-current leaching, osmosis, and dialysis, electro-osmosis, desalination, current-voltage and validation tests. The contribution of leaching and solute diffusion across the electro-membranes was negligible with respect to the electro-migration. On the contrary, solvent diffusion tended to be important as the solute concn. difference at the membrane sides increased or c.d. was reduced. The electro-osmosis and desalination tests yielded the water and solute transport nos. By performing several limiting current tests at different solute concns. and feed flow rates using anionic or cationic membranes, it was possible to det. simultaneously the limiting current intensity, the ratio of the differences between the counter-ion transport nos. in the anion- and cation-exchange membranes and soln., the overall resistance of the electro-membranes, the effective membrane surface area, and the solute mass transfer coeff. All these process and design parameters allowed the time course of the solute concn. in the concg. (C) and dilg. (D) compartments, as well as the voltage applied to the electrodes, to be reconstructed quite accurately without any further correction factors. The capability of the above parameters to simulate the performance of the electrodialysis (ED) unit was checked by resorting to a few validation tests, that were performed in quite different operating conditions from those used in the training tests, that is by filling tank C with a low feed vol. with a low solute concn. and applying a const. current intensity to magnify the effect of electro-osmosis or by changing the current intensity step-wisely to simulate the continuous-mode operation of a multistage ED unit. A parameter sensitivity anal. made the different contribution of the process and design parameters to be assessed, thus yielding a straightforward procedure for designing or optimizing accurately ED desalination units up to a final salt concn. of ∼1.7 Kmol/m3.
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    38. 38
      Wang, L.; Dykstra, J. E.; Lin, S. H. Energy Efficiency of Capacitive Deionization. Environ. Sci. Technol. 2019, 53 (7), 3366– 3378,  DOI: 10.1021/acs.est.8b04858
      38
      Energy Efficiency of Capacitive Deionization
      Wang, Li; Dykstra, J. E.; Lin, Shihong
      Environmental Science & Technology (2019), 53 (7), 3366-3378CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A review. Capacitive deionization (CDI) as a class of electrochem. desalination has attracted fast-growing research interest in recent years. A significant part of this growing interest is arguably attributable to the premise that CDI is energy efficient and has the potential to out-compete other conventional desalination technologies. In this review, systematic evaluation of literature data reveals that while the abs. energy consumption of CDI is in general low, most existing CDI systems achieve limited energy efficiency from a thermodn. perspective. The authors also analyze the causes for the relatively low energy efficiency and discuss factors that may lead to enhanced energy efficiency for CDI.
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    39. 39
      Wang, L.; Lin, S. Membrane capacitive deionization with constant current vs constant voltage charging: Which is better?. Environ. Sci. Technol. 2018, 52 (7), 4051– 4060,  DOI: 10.1021/acs.est.7b06064
      39
      Membrane Capacitive Deionization with Constant Current vs Constant Voltage Charging: Which Is Better?
      Wang, Li; Lin, Shihong
      Environmental Science & Technology (2018), 52 (7), 4051-4060CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Membrane capacitive deionization (MCDI) can be typically operated with const. voltage (CV) or const. current (CC) mode in the charging stage. While a series of previous studies have compared both charging modes to identify the better operating mode, neither their performance evaluation protocols were consistent, nor did their conclusions unanimously converge. This study presents a new framework to evaluate and compare MCDI performance, considering the kinetic efficiency, the energetic efficiency, and the intrinsic trade-off between the two. A key prerequisite for making rational comparison of performance between MCDI operations is that the operations being compared should all result in the same target adsorption. With this key prerequisite and the new evaluation framework based on the trade-off curve between kinetic and energetic efficiencies, our exptl. assessment and theor. anal. suggest that whether CC or CV charging is more efficient is strongly dependent on the target adsorption and, to a less extent, on the kinetic rate of charging. However, the advantage in energy or kinetic efficiency of one charging mode over that of the other is relatively small in all cases. Our study also reveals that, for a given MCDI system, there exist regimes of target adsorptions and kinetic rates that can only be achieved by either CC or CV charging, or even regimes that can be achieved by neither charging mode. In summary, this study revises our current understanding regarding the comparison of the two typical charging modes in MCDI, and introduces a new framework for comparing the performance of different MCDI and CDI operations.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXktlOhurY%253D&md5=b6e49bb9bc6ce1c64db8bae7212d66ae
    40. 40
      Baker, R. W. Membrane technology and applications; John Wiley & Sons, 2012.
      There is no corresponding record for this reference.
    41. 41
      Qin, M.; Deshmukh, A.; Epsztein, R.; Patel, S. K.; Owoseni, O. M.; Walker, W. S.; Elimelech, M. Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis. Desalination 2019, 455, 100– 114,  DOI: 10.1016/j.desal.2019.01.003
      41
      Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis
      Qin, Mohan; Deshmukh, Akshay; Epsztein, Razi; Patel, Sohum K.; Owoseni, Oluwaseye M.; Walker, W. Shane; Elimelech, Menachem
      Desalination (2019), 455 (), 100-114CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Capacitive deionization (CDI), which is based on the electrosorption of ions by porous electrodes, is an emerging technol. for brackish water desalination. Understanding the key drivers of energy consumption in CDI and benchmarking CDI with reverse osmosis (RO), the current state-of-the-art for brackish and seawater desalination, is crucial to guide the future development of desalination technologies. In this study, we develop system-scale models to analyze the energy consumption and energy efficiency of CDI and RO over a wide range of material properties and operating conditions. Using our models, we explore how the energetic performance of CDI and RO compare as a function of feed salinity, water recovery, salt rejection, and av. water flux, which is normalized by electrode and membrane area in CDI and RO, resp. Our anal. shows that RO is significantly more energy efficient than CDI, particularly when targeting higher salinity feed streams and higher salt rejection values. For brackish water with a salt concn. of 2000 mg L-1, achieving 50% water recovery and 75% salt rejection, with an av. water flux of 10 L m-2 h-1 using CDI requires a specific energy consumption of 0.85 kWh m-3, more than eight times that of RO (0.09 kWh m-3). Importantly, our results also indicate that current efforts to improve electrode materials can only marginally reduce the energy consumption of CDI. We conclude with a discussion highlighting other important factors, such as capital cost, electrode stability, and membrane fouling, which affect the efficacy of CDI and RO for low-salinity desalination.
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    42. 42
      Drak, A.; Adato, M. Energy recovery consideration in brackish water desalination. Desalination 2014, 339, 34– 39,  DOI: 10.1016/j.desal.2014.02.008
      42
      Energy recovery consideration in brackish water desalination
      Drak, Alexander; Adato, Matan
      Desalination (2014), 339 (), 34-39CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      In brackish water RO desalination, the low feed water TDS and relatively low brine flow make the use of energy recovery devices ambiguous. The decision to implement energy recovery device must always be based on the Life Cycle Cost estn. of the plant. Design considerations concerning the energy recovery device selection and field experience in Lahat brackish water desalination plant (40,000 m3/day) are presented in this article. Two types of energy recovery device are generally considered in the brackish water RO desalination, turbocharger and isobaric energy recovery devices. Taking into consideration the simplicity of the turbocharger, it was selected for the 1st phase of the Lahat brackish water desalination plant with the design recovery range of 80%-88%. The turbocharger was designed for max recovery and external bypass line was added to operate the plant at low recoveries. For such wide recovery range the turbocharger entire efficiency range of 30%-40% was achieved. Due to the limitation of the turbocharger to operate efficiently at the broad recovery range and Life Cycle Cost benefits of isobaric energy recovery device, the isobaric energy recovery device was selected for the 2nd phase of the Lahat brackish water desalination plant.
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    43. 43
      Geise, G. M.; Paul, D. R.; Freeman, B. D. Fundamental water and salt transport properties of polymeric materials. Prog. Polym. Sci. 2014, 39 (1), 1– 42,  DOI: 10.1016/j.progpolymsci.2013.07.001
      43
      Fundamental water and salt transport properties of polymeric materials
      Geise, Geoffrey M.; Paul, Donald R.; Freeman, Benny D.
      Progress in Polymer Science (2014), 39 (1), 1-42CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
      A review. Fundamental water and salt transport properties of polymers are crit. for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissoc. in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Addnl., the influence of other polymer structure characteristics, such as crosslinking and morphol. in phase sepd. systems, on water and salt transport properties is discussed. The role of free vol. on water and salt transport properties is discussed. The soln.-diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.
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    44. 44
      Wijmans, J. G.; Baker, R. W. The Solution-Diffusion Model - a Review. J. Membr. Sci. 1995, 107 (1–2), 1– 21,  DOI: 10.1016/0376-7388(95)00102-I
      44
      The solution-diffusion model: a review
      Wijmans, J. G.; Baker, R. W.
      Journal of Membrane Science (1995), 107 (1-2), 1-21CODEN: JMESDO; ISSN:0376-7388. (Elsevier)
      A review, with refs., is given on the soln.-diffusion model for describing transport phenomena in relation to membranes. The soln.-diffusion model has emerged over the past 20 yr as the most widely accepted explanation of transport in dialysis, reverse osmosis, gas permeation, and pervaporation. In this paper we will derive the phenomenol. equations for transport in these processes using the soln.-diffusion model and starting from the fundamental statement that flux is proportional to a gradient in chem. potential. The direct and indirect evidence for the model's validity will then be presented, together with a brief discussion of the transition between a soln.-diffusion membrane and a pore-flow membrane seen in nanofiltration membranes and some gas permeation membranes.
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    45. 45
      Porter, M. C. Concentration Polarization with Membrane Ultrafiltration. Ind. Eng. Chem. Prod. Res. Dev. 1972, 11 (3), 234,  DOI: 10.1021/i360043a002
      45
      Concentration polarization with membrane ultrafiltration
      Porter, Mark C.
      Industrial & Engineering Chemistry Product Research and Development (1972), 11 (3), 234-48CODEN: IEPRA6; ISSN:0196-4321.
      Unusually high ultrafiltrate fluxes were obsd. by using thin-channel ultrafiltration in the dewatering and purification of colloidal suspensions. Polymer latexes, paints, metal oxides, starch, and even cellular suspensions all exhibited higher fluxes than those predicted from the gel-polarization model. Theoretical reasons for these anomalies are discussed in conjunction with exptl. data obtained with thin-channel devices utilizing anisotropic noncellulosic membranes.
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    46. 46
      Bartholomew, T. V.; Mauter, M. S. Computational framework for modeling membrane processes without process and solution property simplifications. J. Membr. Sci. 2019, 573, 682– 693,  DOI: 10.1016/j.memsci.2018.11.067
      46
      Computational framework for modeling membrane processes without process and solution property simplifications
      Bartholomew, Timothy V.; Mauter, Meagan S.
      Journal of Membrane Science (2019), 573 (), 682-693CODEN: JMESDO; ISSN:0376-7388. (Elsevier B.V.)
      Accurately modeling membrane processes is crit. to evaluating novel process configurations, designing scalable membrane systems, informing process cost ests., and directing future research. Most membrane process models trade accuracy for computational efficiency by employing simplified approxns. of the process (i.e. no salt flux, no pressure drop) and soln. properties (i.e. ideal soln., and const. d., viscosity, and diffusivity). This work presents a detailed one-dimensional finite difference model for evaluating membrane processes that avoids these common simplifications. We apply this model to quantify the error introduced by these simplifications for case studies of reverse osmosis, osmotically assisted reverse osmosis, forward osmosis, and pressure retarded osmosis. While the magnitude of error introduced by these simplifications is dependent on the case study parameters and specifications, we find that existing model formulations can underestimate or overestimate av. water flux by nearly 50% for some membrane processes operating under std. conditions. Finally, we investigate the error introduced by simplified inlet-outlet models that do not solve the governing system of differential equations, and we assess the accuracy of novel inlet-outlet formulations that use a log and geometric mean, instead of the typical arithmetic mean, to represent non-linear water flux profiles.
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    47. 47
      Manth, T.; Gabor, M.; Oklejas, E. Minimizing RO energy consumption under variable conditions of operation. Desalination 2003, 157 (1–3), 9– 21,  DOI: 10.1016/S0011-9164(03)00377-1
      47
      Minimizing RO energy consumption under variable conditions of operation
      Manth, Thomas; Gabor, Michael; Oklejas, Eli, Jr.
      Desalination (2003), 157 (1-3), 9-21CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
      Specific energy consumption (SEC) for reverse osmosis (RO) desalination systems has usually been estd. using simplistic analyses that consider an av. duty point of operation for a certain plant. A more sophisticated and comprehensive approach that accounts for the effects of variable parameters of operation on SEC was recently described, introducing the concept of the hydraulic envelope. Variable parameters include flow rates at variable recoveries, feed temp. and salinity with their resulting pressure requirements, pressure losses caused by membrane fouling, and pressure losses caused by system controls such as feed throttle valves. This paper will explore in greater detail various energy recovery strategies under variable parameters of operation. Particular attention will be paid to a recently developed, innovative energy recovery configuration that uses a motor-driven booster pump coupled to a Pelton turbine, the so-called PROP, instead of a single-component high-pressure feed pump. This new energy recovery concept can not only be applied to single-stage RO plants, but also as a highly effective interstage booster for dual-stage Brine Conversion Systems (BCS). The concept was submitted for patenting. Results of the anal. suggest that the key issue for minimizing SEC is to control the plant over the entire width of the operational range without creating throttling losses. This can only be achieved by using hydraulic equipment that allows for feed pressure adjustment at min. energy dissipation and eliminates the need for throttling valves. The newly developed PROP concept provides min. SEC over the entire range of the hydraulic envelope of a plant, while at the same time allowing for max. hydraulic dynamic control efficiency of the RO unit. The PROP offers a significant savings potential in terms of capital costs compared to conventional energy recovery strategies.
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    48. 48
      Lin, S. H.; Elimelech, M. Kinetics and energetics trade-off in reverse osmosis desalination with different configurations. Desalination 2017, 401, 42– 52,  DOI: 10.1016/j.desal.2016.09.008
      48
      Kinetics and energetics trade-off in reverse osmosis desalination with different configurations
      Lin, Shihong; Elimelech, Menachem
      Desalination (2017), 401 (), 42-52CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Optimizing system design and operation of reverse osmosis (RO) systems requires an in-depth comprehension of the intrinsic tradeoff between RO mass transfer kinetics and energetics. In this study, we demonstrate that this crit. trade-off can be quantified using the relationship between the av. water flux and the specific energy consumption (SEC). We derive anal. expressions to quantify the av. water flux and SEC for single stage, two stage, and closed circuit RO processes. These anal. expressions are useful for system design and operation optimization as they facilitate direct comparison of the kinetic and energetic efficiencies between RO processes with different operation conditions and system configurations. Finally, we compare the kinetics and energetics of the three system configurations using these anal. expressions and discuss their relative advantages and disadvantages in RO desalination.
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    49. 49
      Wang, L.; Lin, S. Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization. Water Res. 2018, 129, 394– 401,  DOI: 10.1016/j.watres.2017.11.027
      49
      Intrinsic tradeoff between kinetic and energetic efficiencies in membrane capacitive deionization
      Wang, Li; Lin, Shihong
      Water Research (2018), 129 (), 394-401CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Significant progress has been made over recent years in capacitive deionization (CDI) to develop novel system configurations, predictive theor. models, and high-performance electrode materials. To bring CDI to large scale practical applications, it is important to quant. understand the intrinsic tradeoff between kinetic and energetic efficiencies, or the relationship between energy consumption and the mass transfer rate. In this study, we employed both exptl. and modeling approaches to systematically investigate the tradeoff between kinetic and energetic efficiencies in membrane CDI (MCDI). Specifically, we assessed the relationship between the av. salt adsorption rate and specific energy consumptions from MCDI expts. with different applied current densities but a const. effluent salinity. We investigated the impacts of feed salinity, dild. water salinity, dild. water vol. per charging cycle, and electrode materials on the kinetics-energetics tradeoff. We also demonstrate how this tradeoff can be employed to optimize the design and operation of CDI systems and compare the performance of different electrode materials and CDI systems.
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    50. 50
      Hawks, S. A.; Ramachandran, A.; Porada, S.; Campbell, P. G.; Suss, M. E.; Biesheuvel, P. M.; Santiago, J. G.; Stadermann, M. Performance metrics for the objective assessment of capacitive deionization systems. Water Res. 2019, 152, 126– 137,  DOI: 10.1016/j.watres.2018.10.074
      50
      Performance metrics for the objective assessment of capacitive deionization systems
      Hawks, Steven A.; Ramachandran, Ashwin; Porada, Slawomir; Campbell, Patrick G.; Suss, Matthew E.; Biesheuvel, P. M.; Santiago, Juan G.; Stadermann, Michael
      Water Research (2019), 152 (), 126-137CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      In the growing field of capacitive deionization (CDI), a no. of performance metrics have emerged to describe the desalination process. Unfortunately, the sepn. conditions under which these metrics are measured are often not specified, resulting in optimal performance at minimal removal. Here we outline a system of performance metrics and reporting conditions that resolves this issue. Our proposed system is based on volumetric energy consumption (Wh/m3) and throughput productivity (L/h/m2) reported for a specific av. concn. redn., water recovery, and feed salinity. To facilitate and rationalize comparisons between devices, materials, and operation modes, we propose a nominal std. sepn. of removing 5 mM from a 20 mM NaCl feed soln. at 50% water recovery. We propose this particular sepn. as a std., but emphasize that the rationale presented here applies irresp. of sepn. details. Using our proposed sepn., we compare the desalination performance of a flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI) device, showing how significantly different systems can be compared in terms of generally desirable desalination characteristics. In general, we find that performance anal. must be considered carefully so to not allow for ambiguous sepn. conditions or the maximization of one metric at the expense of another. Addnl., for context and clarity, we discuss a no. of important underlying performance indicators and cell characteristics that are not performance measures in and of themselves but can be examd. to better understand differences in performance.
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    51. 51
      Chehayeb, K. M.; Lienhard, J. H. On the electrical operation of batch electrodialysis for reduced energy consumption. Environ. Sci-Wat Res. 2019, 5 (6), 1172– 1182,  DOI: 10.1039/C9EW00097F
      There is no corresponding record for this reference.
    52. 52
      Hand, S.; Shang, X.; Guest, J. S.; Smith, K. C.; Cusick, R. D. Global Sensitivity Analysis To Characterize Operational Limits and Prioritize Performance Goals of Capacitive Deionization Technologies. Environ. Sci. Technol. 2019, 53 (7), 3748– 3756,  DOI: 10.1021/acs.est.8b06709
      52
      Global Sensitivity Analysis To Characterize Operational Limits and Prioritize Performance Goals of Capacitive Deionization Technologies in water desalination
      Hand, Steven; Shang, Xia; Guest, Jeremy S.; Smith, Kyle C.; Cusick, Roland D.
      Environmental Science & Technology (2019), 53 (7), 3748-3756CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Capacitive deionization (CDI) technologies couple electronic and ionic charge storage, enabling improved thermodn. efficiency of brackish desalination by recovering energy released during discharge. However, insight into CDI has been limited by discrete exptl. observations at low desalination depths (Δc, typically reducing influent salinity by 10 mM or less). The performance and sensitivity of 3 common CDI configurations [std. CDI, membrane CDI (MCDI), and flowable electrode CDI (FCDI)] were evaluated across the operational and material design landscape by varying 8 common input parameters (electrode thickness, influent concn., c.d., electrode flow rate, specific capacitance, contact resistance, porosity, and fixed charge). All combinations of designs were evaluated for 2 influent concns. with a calibrated and validated 1-dimensional (1-D) porous electrode model. Sensitivity analyses were carried out via Monte Carlo and Morris methods, focusing on 6 performance metrics. Across all performance metrics, high sensitivity was obsd. to input parameters which impact cycle length (current, resistance, and capacitance). Simulations demonstrated the importance of maintaining both charge and round-trip efficiencies, which limit the performance of CDI and FCDI, resp. Accounting for energy recovery, only MCDI was capable of operating at thermodn. efficiencies similar to reverse osmosis.
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    53. 53
      Wang, L.; Lin, S. H. Theoretical framework for designing a desalination plant based on membrane capacitive deionization. Water Res. 2019, 158, 359– 369,  DOI: 10.1016/j.watres.2019.03.076
      53
      Theoretical framework for designing a desalination plant based on membrane capacitive deionization
      Wang, Li; Lin, Shihong
      Water Research (2019), 158 (), 359-369CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Despite significant progress made in multiple aspects of capacitive deionization (CDI), a rational framework is in need for optimizing the design and operation of a large desalination system based on CDI. In this work, we develop a theor. framework for guiding the design of a desalination plant based on CDI with ion exchange membranes (i.e. membrane CDI, or MCDI). This framework is established by identifying (1) the practical design constraints, (2) the inter-relationships between different design and operating parameters, (3) a set of independent variables, and (4) the key performance metrics. The proposed design framework reduces the degrees of freedom of the system and facilitates more focused and systematic anal. of the overall performance of an MCDI-based desalination plant. Careful anal. using the proposed design framework suggests the presence of an optimal tradeoff curve that comprises all the possible optima of design and operating conditions with which an MCDI-based desalination plant is the most cost-effective. We also show that the typical practice of using equal flowrates for charging and discharge yields very good performance compared to the optima, as long as water recovery is not too high. Finally, we also briefly explain the implication of this framework on cost-based optimization of the design and operation of an MCDI-based desalination plant.
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    54. 54
      Hand, S.; Guest, J. S.; Cusick, R. D. Technoeconomic Analysis of Brackish Water Capacitive Deionization: Navigating Tradeoffs between Performance, Lifetime, and Material Costs. Environ. Sci. Technol. 2019, 53 (22), 13353– 13363,  DOI: 10.1021/acs.est.9b04347
      54
      Technoeconomic Analysis of Brackish Water Capacitive Deionization: Navigating Tradeoffs between Performance, Lifetime, and Material Costs
      Hand Steven; Guest Jeremy S; Cusick Roland D
      Environmental science & technology (2019), 53 (22), 13353-13363 ISSN:.
      Capacitive deionization (CDI), a class of electrochemical separation technologies, has been proposed as an energy-efficient brackish water desalination method. Previous studies have focused on improving capacity and energy consumption through material (e.g., ion-selective membranes [IEMs], charged carbon) and operational modifications, but there has been no analysis that directly links lab-scale experimental performance to capital and operating costs of full-scale water production. In this study, we developed a parameterized process model and technoeconomic analysis framework to project capital and operating costs at the million gallon per day scale based on reported material and operational characteristics for constant current CDI with and without low ($20 m(-2))- and high-cost ($100 m(-2)) IEMs. Using this framework, we conducted global sensitivity and uncertainty analyses for water price across the reported CDI design space. Our results show that the operating constraints of brackish water desalination lead to capital costs 2-14 times greater than operating costs (particularly for MCDI). While MCDI outperforms CDI, IEM prices dictate the threshold at which MCDI is more cost-effective. The high relative capital costs highlight the importance of achieving system lifetimes at 2 years or beyond. Last, we set performance and areal cost benchmarks for material-based CDI performance and lifetime improvements.
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    55. 55
      Lopez, A. M.; Williams, M.; Paiva, M.; Demydov, D.; Do, T. D.; Fairey, J. L.; Lin, Y. J.; Hestekin, J. A. Potential of electrodialytic techniques in brackish desalination and recovery of industrial process water for reuse. Desalination 2017, 409, 108– 114,  DOI: 10.1016/j.desal.2017.01.010
      55
      Potential of electrodialytic techniques in brackish desalination and recovery of industrial process water for reuse
      Lopez, Alexander M.; Williams, Meaghan; Paiva, Maira; Demydov, Dmytro; Do, Thien Duc; Fairey, Julian L.; Lin, Yu Po J.; Hestekin, Jamie A.
      Desalination (2017), 409 (), 108-114CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Large demands for water in industry and consumer markets have led to the development of seawater desalination plants worldwide. Electrodialysis allows the removal of ions at a much lower specific energy consumption than pressure-driven systems and holds the potential to move the desalination industry to greater water yields, lowering the degree of water wasted and energy required for sepns. This study investigates the use of traditional electrodialysis as well as electrodeionization for the removal of contaminant ions from brackish water as well as samples from industrial sources. Results indicated that conventional electrodeionization can successfully remove ion contaminants from brackish water at specific energy consumptions of approx. 0.9-1.5 kWh/m3 water recovered with high water productivity at 40-90 L/m2 h. Ion-exchange resin wafer electrodeionization showed greater promise with specific energy consumption levels between 0.6-1.1 kWh/m3 water recovered and productivity levels between 10-40 L/m2 h. From these results, electrodialysis and electrodeionization have demonstrated viability as alternatives to pressure-driven membrane systems for brackish water desalination.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1Wrtrc%253D&md5=690c6dec3463d1dc17a8d40fe9953051
    56. 56
      Hand, S.; Cusick, R. D. Emerging investigator series: capacitive deionization for selective removal of nitrate and perchlorate: impacts of ion selectivity and operating constraints on treatment costs. Environ. Sci-Wat Res. 2020, 6 (4), 925– 934,  DOI: 10.1039/C9EW01105F
      There is no corresponding record for this reference.
    57. 57
      Andrews, W. T.; Pergande, W. F.; McTaggart, G. S. Energy performance enhancements of a 950 m(3)/d seawater reverse osmosis unit in Grand Cayman. Desalination 2001, 135 (1–3), 195– 204,  DOI: 10.1016/S0011-9164(01)00150-3
      57
      Energy performance enhancements of a 950 m3/d seawater reverse osmosis unit in Grand Cayman
      Andrews, W. T.; Pergande, Wil F.; McTaggart, Gregory S.
      Desalination (2001), 135 (1-3), 195-204CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
      This paper describes the upgrade of a seawater reverse osmosis unit by replacing the hydraulic turbocharger energy-recovery system with a dual work-exchanger energy-recovery (DWEER) system. Before the upgrade, the unit operated at a capacity of 1071 m3/day, and a specific electricity of 3.00 KWh/m3. Tests conducted on the unit after the upgrade, when operated at essentially the same membrane conditions, showed a 59% capacity increase to 1699 m3/day, and a 26% redn. in specific electricity to 2.22 KWh/m3. A further test was then conducted on the unit, by temporarily modifying it such that it operated at essentially the same conditions as before the upgrade. The test resulted in a specific electricity of 2.32 KWh/m3, a redn. in specific electricity of 23% compared to the original performance. This confirmed that the redn. in specific electricity was due to the DWEER system and not economies of scale from the increased capacity. A detailed efficiency anal. showed that the upgrade unit had an overall desalination energy efficiency of 45%, which was broken down into 85% for pumping, 92% for energy recovery, and 57% for the membrane array. This indicates that further improvements in efficiency are most likely to be achieved in the membrane array. The operating experience of the 1st 6 mo is presented.
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    58. 58
      Zhu, A. Z.; Christofides, P. D.; Cohen, Y. Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination. Ind. Eng. Chem. Res. 2009, 48 (13), 6010– 6021,  DOI: 10.1021/ie800735q
      58
      Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination
      Zhu, Aihua; Christofides, Panagiotis D.; Cohen, Yoram
      Industrial & Engineering Chemistry Research (2009), 48 (13), 6010-6021CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Advances in highly permeable reverse osmosis (RO) membranes have enabled desalting operations, in which it is practically feasible for the applied pressure to approach the osmotic pressure of the exit brine stream. However, energy cost remains a major contributor to the total cost of water produced by RO membrane desalination. Redn. of the overall cost of water prodn. represents a major challenge and, in the present work, various elements of water prodn. cost are evaluated from the viewpoint of optimization, with respect to various costs (energy, membrane area and permeability, brine management, and pressure drop), as well as the important thermodn. crossflow constraint, utilization of energy recovery devices, and operational feed and permeate flow rate constraints. More specifically, in this study, an approach to the optimization of product water recovery at pressures that approach the osmotic pressure of the exit brine stream is presented via several simple RO process models that utilize highly permeable membranes. The results suggest that it is indeed feasible to refine RO processes to target for operation under the condition of min. energy consumption, while considering the constraint imposed by the osmotic pressure, as specified by the thermodn. crossflow restriction. Although it is shown that multistage RO provides energy savings, this is at the expense of greater membrane area cost. Overall, as process costs above energy costs are added, the operational point for achieving min. water prodn. cost shifts to higher recoveries. Although the newer generation of RO membranes can allow high recovery operations at lower pressures, limitations due to mineral scaling and fouling impose addnl. constraints. The incorporation of these phenomena in the optimization approach is the subject of ongoing research.
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    59. 59
      Lin, S. H.; Elimelech, M. Staged reverse osmosis operation: Configurations, energy efficiency, and application potential. Desalination 2015, 366, 9– 14,  DOI: 10.1016/j.desal.2015.02.043
      59
      Staged reverse osmosis operation: Configurations, energy efficiency, and application potential
      Lin, Shihong; Elimelech, Menachem
      Desalination (2015), 366 (), 9-14CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodn. of sepn. of saline solns. Therefore, pushing the energy consumption towards the thermodn. limit of sepn. would lead to significant long-term savings in energy cost. In this work, we quant. demonstrate the potential of energy redn. for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the min. specific energy of desalination (i.e., the min. energy required to generate a unit vol. of permeate water) to the no. of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our anal. suggests that although it is theor. impossible to reach the thermodn. min. energy of sepn. with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.
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    60. 60
      Werber, J. R.; Deshmukh, A.; Elimelech, M. Can batch or semi-batch processes save energy in reverse-osmosis desalination?. Desalination 2017, 402, 109– 122,  DOI: 10.1016/j.desal.2016.09.028
      60
      Can batch or semi-batch processes save energy in reverse-osmosis desalination?
      Werber, Jay R.; Deshmukh, Akshay; Elimelech, Menachem
      Desalination (2017), 402 (), 109-122CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the min. practical energy of desalination substantially exceeds the thermodn. min. Batch processes can theor. approach the thermodn. min., suggesting the possibility for further energy savings. In this study, we aim to quantify what energy redns. may be possible for batch-like processes when process inefficiencies such as frictional losses and concn. polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive anal. approxns. and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this anal., we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and two-stage RO would save 13% and 15% energy, resp., over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.
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    61. 61
      Chehayeb, K. M.; Lienhard, J. H. Entropy generation analysis of electrodialysis. Desalination 2017, 413, 184– 198,  DOI: 10.1016/j.desal.2017.03.001
      61
      Entropy generation analysis of electrodialysis
      Chehayeb, Karim M.; Lienhard, John H. V.
      Desalination (2017), 413 (), 184-198CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Electrodialysis (ED) is a desalination technol. with many applications. In order to better understand how the energetic performance of this technol. can be improved, the various losses in the system should be quantified and characterized. This can be done by looking at the entropy generation in ED systems. In this paper, we implement an ED model based on the Maxwell-Stefan transport model, which is the closest model to fundamental equations. We study the sources of entropy generation at different salinities, and locate areas where possible improvements need to be made under different operating conditions. In addn., we study the effect of the channel height, membrane thickness, and cell-pair voltage on the specific rate of entropy generation. We express the second-law efficiency of ED as the product of current and voltage utilization rates, and study its variation with c.d. Further, we define the useful voltage that is used beneficially for sepn. We derive the rate of entropy generation that is due to the passage of ions through a voltage drop, and we investigate whether voltage drops themselves can provide a good est. of entropy generation.
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    62. 62
      Mistry, K. H.; McGovern, R. K.; Thiel, G. P.; Summers, E. K.; Zubair, S. M.; Lienhard, J. H. Entropy Generation Analysis of Desalination Technologies. Entropy 2011, 13 (10), 1829– 1864,  DOI: 10.3390/e13101829
      62
      Entropy generation analysis of desalination technologies
      Mistry, Karan H.; McGovern, Ronan K.; Thiel, Gregory P.; Summers, Edward K.; Zubair, Syed M.; Lienhard V., John H.
      Entropy (2011), 13 (), 1829-1864CODEN: ENTRFG; ISSN:1099-4300. (MDPI AG)
      Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies. Entropy generation anal., and specifically, Second Law efficiency, is an important tool for illustrating the influence of irreversibilities within a system on the required energy input. When defining Second Law efficiency, the useful exergy output of the system must be properly defined. For desalination systems, this is the min. least work of sepn. required to ext. a unit of water from a feed stream of a given salinity. In order to evaluate the Second Law efficiency, entropy generation mechanisms present in a wide range of desalination processes are analyzed. In particular, entropy generated in the run down to equil. of discharge streams must be considered. Phys. models are applied to est. the magnitude of entropy generation by component and individual processes. These formulations are applied to calc. the total entropy generation in several desalination systems including multiple effect distn., multistage flash, membrane distn., mech. vapor compression, reverse osmosis, and humidification-dehumidification. Within each technol., the relative importance of each source of entropy generation is discussed in order to det. which should be the target of entropy generation minimization. As given here, the correct application of Second Law efficiency shows which systems operate closest to the reversible limit and helps to indicate which systems have the greatest potential for improvement.
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    63. 63
      Chehayeb, K. M.; Nayar, K. G.; Lienhard, J. H. On the merits of using multi-stage and counterflow electrodialysis for reduced energy consumption. Desalination 2018, 439, 1– 16,  DOI: 10.1016/j.desal.2018.03.026
      63
      On the merits of using multi-stage and counterflow electrodialysis for reduced energy consumption
      Chehayeb, Karim M.; Nayar, Kishor G.; Lienhard, John H. V.
      Desalination (2018), 439 (), 1-16CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      The cost of electrodialysis (ED) systems can be decreased by decreasing their power consumption. Such redns. may be achieved by using degrees of freedom in the system's configuration to obtain a more uniform spatial distribution of the rate of entropy generation, as explained by the theorem of equipartition of entropy generation. In this paper, we study possible improvements to the energy efficiency of electrodialysis through the use of two elec. stages with different voltages, and through operation in a counterflow configuration. We first consider how a two-stage ED system should be operated. In particular, we look at how the voltages and current densities should be chosen. In addn., we quantify the effect of operating under two voltages in brackish-water desalination and in high-salinity brine concn. Finally, we quantify the effect of operating ED in counterflow for the same applications. We show that high ED fixed costs prevent the achievement of significant improvements in energy efficiency. If fixed costs are reduced, and larger systems become cost-effective, we show that a power redn. of up to 29% is possible by going from a single-stage to a two-stage configuration.
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    64. 64
      Parulekar, S. J. Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis. J. Membr. Sci. 1998, 148 (1), 91– 103,  DOI: 10.1016/S0376-7388(98)00148-3
      64
      Optimal current and voltage trajectories for minimum energy consumption in batch electrodialysis
      Parulekar, Satish J.
      Journal of Membrane Science (1998), 148 (1), 91-103CODEN: JMESDO; ISSN:0376-7388. (Elsevier Science B.V.)
      Cost-effective operations of a batch electrodialyzer for removal of salt from a single salt soln. are investigated. It is desired to minimize the operating cost for a particular batch. The operating cost for an electrodialysis (ED) stack is comprised of cost related to energy consumption and cost of maintenance of the ED stack. In effective operations of an ED stack, the maintenance cost is a small fraction of the total operating cost. The bulk of the operating cost is therefore proportional to total energy consumption, which is the sum of the elec. energy needed for salt removal and the energy required to pump various solns. through the ED stack. For fixed feed compn. and the desired percent salt recovery, the total energy required is influenced by trajectories of current flowing through and the voltage applied across the ED stack and the operating time. In this regard, the following operations are studied: (I) const. current operation, (II) const. voltage operation, (III) const. current operation followed by const. voltage operation, (IV) const. voltage operation followed by const. current operation, and (V) operation with time-variant current and voltage. For arbitrary relations among salt concn., current utilization, and stack resistance, optimal current and voltage trajectories that lead to min. energy requirement are identified for each of the five operations. It is established anal. that operation V is superior to operations III and IV, which in turn are superior to operations I and II. Numerical illustrations reveal that the performance differences in these operations are enhanced as the percent salt recovery is increased.
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    65. 65
      Werber, J. R.; Osuji, C. O.; Elimelech, M. Materials for next-generation desalination and water purification membranes. Nat. Rev. Mater. 2016, 1 (5), 16018,  DOI: 10.1038/natrevmats.2016.18
      65
      Materials for next-generation desalination and water purification membranes
      Werber, Jay R.; Osuji, Chinedum O.; Elimelech, Menachem
      Nature Reviews Materials (2016), 1 (5), 16018CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      Membrane-based sepns. for water purifn. and desalination have been increasingly applied to address the global challenges of water scarcity and the pollution of aquatic environments. However, progress in water purifn. membranes has been constrained by the inherent limitations of conventional membrane materials. Recent advances in methods for controlling the structure and chem. functionality in polymer films can potentially lead to new classes of membranes for water purifn. In this Review, we first discuss the state of the art of existing membrane technologies for water purifn. and desalination, highlight their inherent limitations and establish the urgent requirements for next-generation membranes. We then describe mol.-level design approaches towards fabricating highly selective membranes, focusing on novel materials such as aquaporin, synthetic nanochannels, graphene and self-assembled block copolymers and small mols. Finally, we highlight promising membrane surface modification approaches that minimize interfacial interactions and enhance fouling resistance.
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    66. 66
      Li, W.; Patton, S.; Gleason, J. M.; Mezyk, S. P.; Ishida, K. P.; Liu, H. Z. UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse. Environ. Sci. Technol. 2018, 52 (11), 6417– 6425,  DOI: 10.1021/acs.est.7b06042
      66
      UV Photolysis of Chloramine and Persulfate for 1,4-Dioxane Removal in Reverse-Osmosis Permeate for Potable Water Reuse
      Li, Wei; Patton, Samuel; Gleason, Jamie M.; Mezyk, Stephen P.; Ishida, Kenneth P.; Liu, Haizhou
      Environmental Science & Technology (2018), 52 (11), 6417-6425CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      A sequential combination of membrane treatment and UV-based advanced oxidn. processes (UV/AOP) has become the industry std. for potable water reuse. Chloramines are used as membrane antifouling agents and therefore carried over into the UV/AOP. In addn., persulfate (S2O82-) is an emerging oxidant that can be added into a UV/AOP, thus creating radicals generated from both chloramines and persulfate for water treatment. This study investigated the simultaneous photolysis of S2O82- and monochloramine (NH2Cl) on the removal of 1,4-dioxane (1,4-D) for potable-water reuse. The dual oxidant effects of NH2Cl and S2O82- on 1,4-D degrdn. were examd. at various levels of oxidant dosage, chloride, and soln. pH. Results showed that a NH2Cl-to-S2O82- molar ratio of 0.1 was optimal, beyond which the scavenging by NH2Cl of HO•, SO4•-, and Cl2•- radicals decreased the 1,4-D degrdn. rate. At the optimal ratio, the degrdn. rate of 1,4-D increased linearly with the total oxidant dose up to 6 mM. The combined photolysis of NH2Cl and S2O82- was sensitive to the soln. pH due to a disproportionation of NH2Cl at pH lower than 6 into less-photoreactive dichloramine (NHCl2) and radical scavenging by NH4+. The presence of chloride transformed HO• and SO4•- to Cl2•- that is less-reactive with 1,4-D, while the presence of dissolved O2 promoted gaseous nitrogen prodn. Results from this study suggest that the presence of chloramines can be beneficial to persulfate photolysis in the removal of 1,4-D; however, the treatment efficiency depends on a careful control of an optimal NH2Cl dosage and a minimal chloride residue.
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    67. 67
      Plumlee, M. H.; Lopez-Mesas, M.; Heidlberger, A.; Ishida, K. P.; Reinhard, M. N-nitrosodimethylamine (NDMA) removal by reverse osmosis and UV treatment and analysis via LC-MS/MS. Water Res. 2008, 42 (1–2), 347– 355,  DOI: 10.1016/j.watres.2007.07.022
      67
      N-nitrosodimethylamine (NDMA) removal by reverse osmosis and UV treatment and analysis via LC-MS/MS
      Plumlee, Megan H.; Lopez-Mesas, Montserrat; Heidlberger, Andy; Ishida, Kenneth P.; Reinhard, Martin
      Water Research (2008), 42 (1-2), 347-355CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      N-nitrosodimethylamine (NDMA) is a probable human carcinogen found in ng/L concns. in chlorinated and chloraminated water. A method was developed for the detn. of ng/L levels of NDMA using liq. chromatog.-tandem mass spectrometry (LC-MS/MS) preceded by sample concn. via solid-phase extn. with activated charcoal. Recoveries were >90% and allowed a method reporting limit as low as 2 ng/L. Using this method, the removal of NDMA was detd. for the Interim Water Purifn. Facility (IWPF), an advanced wastewater treatment facility operated by the Orange County Water District (OCWD) in Southern California. The facility treats effluent from an activated sludge treatment plant with microfiltration (MF), reverse osmosis (RO), and an UV-H2O2 advanced oxidn. process (UV-AOP). Six nitrosamines were surveyed: NDMA, N-nitrosomethylethylamine (NMEA), N-nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA), N-nitrosopiperidine (NPip), and N-nitrosopyrrolidine (NPyr). Only NDMA was detected and at all treatment steps in the IWPF, with influent concns. ranging from 20 to 59 ng/L. Removals for RO and UV ranged from 24 to 56 and 43 to 66%, resp. Overall, 69±7% of the original NDMA concn. was removed from the product water across the advanced treatment process and, in combination with blending, the final concn. did not exceed the California drinking water notification level of 10 ng/L. NDMA removal data are consistent with findings reviewed for other advanced treatment facilities and lab. studies.
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    68. 68
      Stanford, B. D.; Leising, J. F.; Bond, R. G.; Snyder, S. A. Inland desalination: Current practices, environmental implications, and case studies in Las Vegas, NV. Sustainability Science and Engineering 2010, 2, 327– 350,  DOI: 10.1016/S1871-2711(09)00211-6
      There is no corresponding record for this reference.
    69. 69
      Brady, P. V.; J, K. R.; M, M. T.; Hightower, M. M. Inland desalination: Challenges and research needs. Journal of Contemporary Water Research & Education 2005, 132, 46– 51,  DOI: 10.1111/j.1936-704X.2005.mp132001007.x
      There is no corresponding record for this reference.
    70. 70
      Thompson, D. W.; Tremblay, A. Y. Fouling in Steady and Unsteady State Electrodialysis. Desalination 1983, 47 (May), 181– 188,  DOI: 10.1016/0011-9164(83)87071-4
      70
      Fouling in steady- and unsteady-state electrodialysis
      Thompson, D. W.; Tremblay, A. Y.
      Desalination (1983), 47 (), 181-8CODEN: DSLNAH; ISSN:0011-9164.
      The performance of a cyclic electrodialysis process (with sealed membrane-pairs and periodically reversing flow and elec. polarity) was compared with a steady-state process when both were operating under fouling conditions. A 2000 mg/L NaCl soln. was circulated through the bench-scale test cells for 20 h in each run. Fe(II) was introduced into the feed as FeCl2 at concns. of 0, 1, and 5 ppm. IONAC MC 3142 cationic membranes and IONAC MA 3148 anionic membranes were used in each type of process, with the flow channels defined by polyethylene-mesh separators through which the fluid velocity was 1.25 cm/s. Potentials of 20 and 30 V were applied across each stack, which consisted of 13 or 14 active membranes plus 2 membranes in each of the electrode rinse compartments. Reddish-brown deposits were visible on all of the test membranes in all steady and unsteady-state runs having Fe in their feed streams. Deposits for the steady-state runs were uneven and showed clear evidence of flow channelling. In the unsteady-state runs the deposits were obsd. as a fainter, even, coloring over the whole surface of the membrane. The deposition process is partly irreversible so that the polarity switching does not prevent build up of this type of fouling. Some fouling occurred inside the sealed membrane-pairs.
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    71. 71
      Jiang, S. X.; Li, Y. N.; Ladewig, B. P. A review of reverse osmosis membrane fouling and control strategies. Sci. Total Environ. 2017, 595, 567– 583,  DOI: 10.1016/j.scitotenv.2017.03.235
      71
      A review of reverse osmosis membrane fouling and control strategies
      Jiang, Shanxue; Li, Yuening; Ladewig, Bradley P.
      Science of the Total Environment (2017), 595 (), 567-583CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      A review. Reverse osmosis (RO) membrane technol. is one of the most important technologies for water treatment. However, membrane fouling is an inevitable issue. Membrane fouling leads to higher operating pressure, flux decline, frequent chem. cleaning and shorter membrane life. This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments. The fundamentals of fouling are discussed in detail, including biofouling, org. fouling, inorg. scaling and colloidal fouling. Furthermore, fouling mitigation technologies are also discussed comprehensively. Pretreatment is widely used in practice to reduce the burden for the following RO operation while real time monitoring of RO has the advantage and potential of providing support for effective and efficient cleaning. Surface modification could slow down membrane fouling by changing surface properties such as surface smoothness and hydrophilicity, while novel membrane materials and synthesis processes build a promising future for the next generation of RO membranes with big advancements in fouling resistance. Esp. in this review paper, statistical anal. is conducted where appropriate to reveal the research interests in RO fouling and control.
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    72. 72
      Mikhaylin, S.; Bazinet, L. Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control. Adv. Colloid Interface Sci. 2016, 229, 34– 56,  DOI: 10.1016/j.cis.2015.12.006
      72
      Fouling on ion-exchange membranes: Classification, characterization and strategies of prevention and control
      Mikhaylin, Sergey; Bazinet, Laurent
      Advances in Colloid and Interface Science (2016), 229 (), 34-56CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
      The environmentally friendly ion-exchange membrane (IEM) processes find more and more applications in the modern industries in order to demineralize, conc. and modify products. Moreover, these processes may be applied for the energy conversion and storage. However, the main drawback of the IEM processes is a formation of fouling, which significantly decreases the process efficiency and increases the process cost. The present review is dedicated to the problematic of IEM fouling phenomena. Firstly, the major types of IEM fouling such as colloidal fouling, org. fouling, scaling and biofouling are discussed along with consideration of the main factors affecting fouling formation and development. Secondly, the review of the possible methods of IEM fouling characterization is provided. This section includes the methods of fouling visualization and characterization as well as methods allowing investigations of characteristics of the fouled IEMs. Eventually, the reader will find the conventional and modern strategies of prevention and control of different fouling types.
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    73. 73
      Katz, W. E. The electrodialysis reversal (EDR) process. Desalination 1979, 28 (1), 31– 40,  DOI: 10.1016/S0011-9164(00)88124-2
      73
      The electrodialysis reversal (EDR) process
      Katz, William E.
      Desalination (1979), 28 (1), 31-40CODEN: DSLNAH; ISSN:0011-9164.
      A brief description, history of development, and operating data on the 1st experience with the EDR process are given. EDR constitutes an improvement on the electrodialysis process by providing automatic self-cleaning, thereby eliminating need for acid or complexing agent feeds.
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    74. 74
      Song, L. F.; Elimelech, M. Particle Deposition onto a Permeable Surface in Laminar-Flow. J. Colloid Interface Sci. 1995, 173 (1), 165– 180,  DOI: 10.1006/jcis.1995.1310
      74
      Particle deposition onto a permeable surface in laminar flow
      Song, Lianfa; Elimelech, Menachem
      Journal of Colloid and Interface Science (1995), 173 (1), 165-80CODEN: JCISA5; ISSN:0021-9797. (Academic)
      A theor. study of particle deposition on a permeable surface in a parallel-plate channel is presented. The convective diffusion equation is formulated rigorously with the inclusion of lateral transport due to permeation drag and inertial lift, and transport due to gravitational, double layer, and van der Waals forces. A numerical procedure for solving the governing equation is also presented. The effects of particle size, permeation velocity, soln. ionic strength, cross-flow velocity, and particle d. on the initial rate of particle deposition are studied. The results indicate that the local and av. particle deposition rates on a permeable surface are detd. by an interplay between several transport and interaction mechanisms, among which permeation drag, elec. double layer repulsion, and inertial lift are most important.
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    75. 75
      Koo, T.; Lee, Y. J.; Sheikholeslami, R. Silica fouling and cleaning of reverse osmosis membranes. Desalination 2001, 139 (1–3), 43– 56,  DOI: 10.1016/S0011-9164(01)00293-4
      75
      Silica fouling and cleaning of reverse osmosis membranes
      Koo, T.; Lee, Y. J.; Sheikholeslami, R.
      Desalination (2001), 139 (1-3), 43-56CODEN: DSLNAH; ISSN:0011-9164. (Elsevier Science B.V.)
      Desalination plays an important role in producing pure water from brackish water. Reverse osmosis (RO) is by far the most efficient way to remove colloidal and dissolved silica, which can be found in high concns. in brackish water. The presence of silica and its ability to foul membranes limits the use of silica bearing waters for desalination and when used, it has many economic penalties. This study examines the effect of silica polymn. in the presence of polyvalent cations and anions in RO systems. Source of silica in the experimentation was from com. grade sodium metasilicate (Na2O3Si.9H2O). The membranes used were polyamide and thin film manufd. by Osmonics. Use of glassware is minimized to avoid the possibility of any contribution by silica leaching into soln. The feed soln. consists of silica, calcium and magnesium ions in various concns. to det. the effect of polyvalent ions on polymn. and the appropriate pre-treatment technol. The expt. was set up in a way as to simulate the conditions that would be encountered in a desalination plant. Concn. polarization (C/P) in the system was exptl. detd. with a simple technique that was developed and its effects on fouling are examd. In addn. tests were carried out to examine the actual fouling mechanism in reverse osmosis units under various exptl. conditions. Furthermore the effect of cleaning, with distd. water and with pulsations as well as with com. available cleaners were examd. Some com. available cleaners were capable at partially restoring the flux. Further investigation is underway to examine the effectiveness of new silica specific inhibitors.
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    76. 76
      Lisitsin, D.; Hasson, D.; Semiat, R. Critical flux detection in a silica scaling RO system. Desalination 2005, 186 (1–3), 311– 318,  DOI: 10.1016/j.desal.2005.06.007
      76
      Critical flux detection in a silica scaling RO system
      Lisitsin, Dmitry; Hasson, David; Semiat, Raphael
      Desalination (2005), 186 (1-3), 311-318CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Desalination of brackish water contg. silica at high recovery levels leads to a rapid flux decline, due to the pptn. of a colloidal soln. consisting of polymd. silica nano-particles. The crit. flux (CF) phenomenon postulates that there is a certain permeate flux level below which the rate of flux decrease due to membrane fouling becomes negligible. The aim was to develop a simple technique for detecting the existence of a crit. flux threshold limit under conditions at which the membrane surface is exposed to a const. pptn. potential. A series of expts. was carried out in a silica scaling tubular RO system. Exptl. conditions were controlled such that the dissolved silica content on the membrane was held at the same supersatn. ratio level of 2.1-2.2 while varying the initial permeation flux value over the range 17-45 L/h-m2. The rate of flux decline remained const., irresp. of the permeate flux level examd.
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    77. 77
      Shi, L.; Rossi, R.; Son, M.; Hall, D. M.; Hickner, M. A.; Gorski, C. A.; Logan, B. E. Using reverse osmosis membranes to control ion transport during water electrolysis. Energy Environ. Sci. 2020, 13 (9), 3138– 3148,  DOI: 10.1039/D0EE02173C
      77
      Using reverse osmosis membranes to control ion transport during water electrolysis
      Shi, Le; Rossi, Ruggero; Son, Moon; Hall, Derek M.; Hickner, Michael A.; Gorski, Christopher A.; Logan, Bruce E.
      Energy & Environmental Science (2020), 13 (9), 3138-3148CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The decreasing cost of electricity produced using solar and wind and the need to avoid CO2 emissions from fossil fuels has heightened interest in hydrogen gas prodn. by water electrolysis. Offshore and coastal hydrogen gas prodn. using seawater and renewable electricity is of particular interest, but it is currently economically infeasible due to the high costs of ion exchange membranes and the need to desalinate seawater in existing electrolyzer designs. A new approach is described here that uses relatively inexpensive com. available membranes developed for reverse osmosis (RO) to selectively transport favorable ions. In an applied elec. field, RO membranes have a substantial capacity for proton and hydroxide transport through the active layer while excluding salt anions and cations. A perchlorate salt was used to provide an inert and contained anolyte, with charge balanced by proton and hydroxide ion flow across the RO membrane. Synthetic seawater (NaCl) was used as the catholyte, where it provided continuous hydrogen gas evolution. The RO membrane resistance was 21.7 ± 3.5 Ω cm2 in 1 M NaCl and the voltages needed to split water in a model electrolysis cell at current densities of 10-40 mA cm-2 were comparable to those found when using two commonly used, more expensive ion exchange membranes.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsestengg.0c00192.

    • Description of ED process model; parameters utilized for ED process modeling (Table S1); schematic illustration of feed-and-bleed ED operation mode (Figure S1); ED model validation (Figure S2); effect of changing velocity profile on prediction of ED process performance (Figure S3); ED model-predicted ion-exchange membrane resistances as a function of feedwater salinity (Figure S4) (PDF)


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