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Visual analysis of membranes (B to E) in contact with fresh water after 54 days experiment. For comparison, a clean membrane is shown (A), with no visual distinction between a clean AEM and CEM.
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Reverse electrodialysis (RED) is a process to harvest renewable energy from the salinity gradient obtained by the controlled mixing of river and seawater. When using natural waters, (bio)fouling is an inevitable process which has a negative impact on the obtained power density. Specific characteristics of RED do not allow the direct transfer of kno...
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... the end of the experiment (day 54) membrane samples were taken from the stacks and were prepared for visual analyses, and representative pictures are shown in Fig. 5. AEMs were covered by a brown coloured layer (Fig. 5B and D), which was not observed for the CEMs (Fig. 5C and E) and can be attributed to the presence of humic acids. The negative charge of humic acids favours its attachment to AEMs which are positively charged, as reported before ...
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... the end of the experiment (day 54) membrane samples were taken from the stacks and were prepared for visual analyses, and representative pictures are shown in Fig. 5. AEMs were covered by a brown coloured layer (Fig. 5B and D), which was not observed for the CEMs (Fig. 5C and E) and can be attributed to the presence of humic acids. The negative charge of humic acids favours its attachment to AEMs which are positively charged, as reported before ...
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... the end of the experiment (day 54) membrane samples were taken from the stacks and were prepared for visual analyses, and representative pictures are shown in Fig. 5. AEMs were covered by a brown coloured layer (Fig. 5B and D), which was not observed for the CEMs (Fig. 5C and E) and can be attributed to the presence of humic acids. The negative charge of humic acids favours its attachment to AEMs which are positively charged, as reported before ...
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... fouling was observed on both membrane types on stacks DM I and II (Fig. 5B and D) but not after the microfiltration step ( Fig. 5C and E). The same can be seen for the spacers, where more particulate fouling accumulated after dual media filtration than after microfiltration ( Fig. S3.3A and ...
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... fouling was observed on both membrane types on stacks DM I and II (Fig. 5B and D) but not after the microfiltration step ( Fig. 5C and E). The same can be seen for the spacers, where more particulate fouling accumulated after dual media filtration than after microfiltration ( Fig. S3.3A and ...
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... It is possible that the microorganisms that are able to pass the microfiltration step find less competition and more space to grow on the available membrane surface [29]. Another reason why we do not observe the same pattern observed in Micro stacks as in DM stacks could be related to the presence of particulate fouling (as clearly observed in Fig. 5) on the membrane surface. The adhesion of particles to the membrane surface form a sort of protective layer to bacterial adhesion, since it creates a less ideal surface for microbial growth [30]. Most probably the difference in EPS and biofouling is a combination of these two factors. The water analysis presented in Section 3.2 showed ...
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Reverse Electrodialysis (RED) has shown to be a promising technology for electricity production but membrane fouling can influence the energy yield, thus, it is crucial to gain knowledge on the impact and prevention of fouling. We used a sequence of pre-treatments (dual-media filter, microfilter and activated carbon filter) to fractionate foulants...
Citations
... In the extremities of the stack, electrode compartments are placed and with the use of electrolyte solutions, redox reactions occur which allow the potential of moving ions to be harvested into electrical energy if an external load is connected (Fig. 10). The byproduct of the process is somewhat a mix of the two feed solutions, with the high salinity solution becoming less concentrated than the original feed, while the low salinity solution becomes more concentrated [157]. However, the difference in concentration of the solutions after the process are not major, so it is unlikely that their classification into new categories of salinity change. ...
... Another known factor that can reduce the power output of RED systems is membrane fouling. A dual-media filter of anthracite and sand was proven to be a suitable and cost-effective solution for fresh and seawater pre-treatment in RED applications [157,164]. When using brines, the possibility of scaling also has to be considered. ...
... The desired pre-treatment should be robust, durable, with simple operation and maintenance and, most importantly, with a low energy demand [11]. In previous studies we showed that a dual media filter, using anthracite and sand as media, could fulfill many of these requirements, being effective for both seawater and fresh water pre-treatment [12]. In our work, it was also shown that the formation of a fouling layer composed by particulate matter was the main contributor to pumping power losses in the process, but at the same time, its formation could also limit attachment and growth of microorganisms and thus biofouling [12,13]. ...
... In previous studies we showed that a dual media filter, using anthracite and sand as media, could fulfill many of these requirements, being effective for both seawater and fresh water pre-treatment [12]. In our work, it was also shown that the formation of a fouling layer composed by particulate matter was the main contributor to pumping power losses in the process, but at the same time, its formation could also limit attachment and growth of microorganisms and thus biofouling [12,13]. Thus, the presence of small amounts of particulate matter can be beneficial, and fouling management through cleaning should consider this aspect. ...
... Vermaas [6] observed an accumulation of diatoms and mineral silica at the AEMs, but in that study both AEM and CEM were profiled and that could interfere with the fouling attachment and formation. As previous studies showed [12,13], when there is less incidence of fouling by particulate matter, there is more available ideal surface area for attachment and growth of bacteria and EPS layer, as well as diatoms, resulting in pronounced biofouling. Biofouling is more difficult to remove during the cleaning procedure, even when using air sparging [15]. ...
Reverse electrodialysis (RED) is a promising technology to harvest salinity gradient power (SGP) that is available where fresh and sea water mix, using anion (AEM) and cation exchange membranes (CEM) in a stack. Fouling of the membranes is one of the main challenges for RED, since it leads to a reduction in attainable net power output. In this study, we combined the use of profiled ion-exchange membranes (200 μm thick compartments) with a pre-treatment by a dual media filter for both natural water streams (Lake Ijssel and Wadden Sea), with four different cleaning procedures: (i) increased flow, (ii) reverse and increased flow, (iii) reverse flow and feed switch, and (iv) air sparging. Cleaning with air sparging was the most effective technique, limiting the pumping losses and not influencing the power generation capacity. The cleaning with reverse flow and feed switch also showed to be suitable, keeping the pressure drop losses lower than 100 mbar for both water streams. Post experiment membrane autopsy showed that CEMs were more subjected to particulate fouling than AEMs, and that a lower accumulation of fouling by particulate resulted in a higher concentration of humic acids and biofouling on the membrane surface.
... Biological fouling occurs when microorganisms attach to the membrane or spacer surface and form biofilms that obstruct the flow of ions or solutions through the membrane channels. This results in a significant decrease in the net power density generated by RED, ranging from 28% to 60%, depending on the water quality and operating conditions (Santoro et al. 2021;Vital et al. 2021). Moreover, biological fouling causes a substantial increase in the pressure drop along the membrane channels, which in turn increases the pumping power required to maintain a constant flow rate (Cosenza et al. 2022). ...
Reverse electrodialysis (RED) generates energy from salinity gradients, such as the one between seawater and river water, by selectively transporting ions through ion-exchange membranes. This paper discusses the challenges facing RED in four areas: ion exchange membranes, stacks, fouling, and processes, although it may not cover all issues as new advancements arise. Although bench-scale RED has impressive power generation, pilot-scale RED operations present challenges for widespread adoption. However, the main advantage of RED lies in its potential for synergetic applications with other processes like energy conversion/storage, wastewater treatment, and desalination. The development of low-cost, innovative membranes and integration processes with high energy efficiency and power generation capabilities for both scaled-up and-down approaches is essential for RED's continued advancement. Dedicated research will contribute to its potential for integration with other processes and viability as a renewable energy source. RED has the potential to be a significant player in the renewable energy market, but new advancements also present new challenges.
... During the 25 day continuous test, they observed a significant resistance rise, along with a slight decrease in membrane permselectivity, which results in a power loss of 60% of the RED system. This result was confirmed by the work of Vital et al., 205 where after 54 days, the power density decrease by 25%. This fouling comes in two major parts of the RED stack, the IEMS and the spacers. ...
... The first barrier can be a simple pretreatment a 20 μm pore size mesh filter, proposed by Vermaas et al., 204 to filtrate the biggest particles. Vital et al. 205 fouling by Vermass et al. 191 where they replaced the traditional spacers and IEMs by profiled membranes, and they found that the pressure drop increased four times slower and the power density remained relatively higher. He et al. 206 confirmed this importance of the spacers and proposed a new type with different inlet and outlet to enhance the hydraulic performance and the power density. ...
A large amount of untapped energy sources surrounds us. In this review, we summarize recent works of water-based energy harvesting systems with operation scales ranging from miniature systems to large scale attempts. We focus particularly on the triboelectric energy, which is produced when a liquid and a solid come into contact, and on the osmotic energy, which is released when salt water and fresh water are mixed. For both techniques we display the state of the art understanding (including electrical charge separation, electro-osmotic currents and induced currents) and the developed devices. A critical discussion of present works confirms the significant progress of these water-based energy harvesting systems in all scales. However, further efforts in efficiency and performance amelioration are expected for these technologies to accelerate the industrialization and commercialization procedure.
... In laboratory experiments using pure NaCl solutions-with salinities comparable to river and sea water-power densities are achieved of more than 2 W/m 2 [3]. However, with natural feed waters, there is the problem of biological matter, solid particles, and dissolved matter as sources of membrane fouling and spacer blockage, resulting in a reduction in power density [4][5][6][7][8]. Possible threats to the anion exchange membranes (AEM) and the 2 of 23 cation exchange membranes (CEM) from divalent ions were mentioned as early as 1970 by Lacey in an article citing the Great Salt Lake in Utah as a possible source of RED, along with freshwater from one of the incoming rivers [9]. ...
Divalent ions have a negative effect on the obtained power and efficiency of the reverse electrodialysis (RED) process when using natural waters. These effects can largely be attributed to the interaction between the various ions and the membranes, resulting in a decreased membrane voltage, an increased membrane resistance, and uphill transport of divalent ions. The aim of this study was to investigate the causes of these differences and, if possible, to find underlying causes. The approach mainly followed that in literature articles that specifically focused on the effect of divalent ions on RED. It transpired that seven publications were useful because the methodology was well described and sufficient data was published. I found two widely shared misconceptions. The first concerns the role of the stack voltage in uphill transport of divalent ions; itis often thought that the open circuit voltage (OCV) must be taken into account, but it is plausible that the voltage under working conditions is the critical factor. The second debatable point concerns the methodology used to make a series of solutions to study the effect of divalent ions. Typically, solutions with a constant number of moles of salt are used; however, it is better to make a series with a constant ratio of equivalents of those salts. Moreover, it is plausible that the decreased voltage can be explained by the inherently lower Donnan potential of multi-charged ions and that increased resistance is caused by the fact that divalent ions—with a lower mobility there than the monovalent ions—occupy relatively much of the available space in the gel phase of the membrane. While both resistance and voltage play a decisive role in RED and probably also in other membrane processes like electrodialysis (ED), it is remarkable that there are so few publications that focus on measurements on individual membranes. The implications of these results is that research on the effect of divalent ions in RED, ED and similar processes needs to be more structured in the future. Relatively simple procedures can be developed for the determination of membrane resistance in solutions of mixtures of mono- and divalent salts. The same applies to determining the membrane potential. The challenge is to arrive at a standard method for equipment, methodology, and the composition of the test solutions.
... Research in RED with natural waters is still new and the effect of foulants from each water stream is not fully understood [5]. Vital and colleagues [8] showed how RED has four possible different interactions of foulants from water streams and types of membrane, highlighting the importance to study them separately and understand the effect of each of these combinations in the overall performance. A foulant fractionation study in RED for fresh water was performed, where results showed that particulate fouling was the main cause for a decrease in performance over long-term operation. ...
... Feed waters for each stack and inlet untreated waters were characterized at 7 different sampling points twice a week by various methods already described in [8]. In summary, the characterization was carried out for concentration of ions, in/organic carbon, suspended solids concentration, particle size distribution (PSD) and turbidity, pH and conductivity. ...
... Camera pictures were taken from cell pairs located in the centre of the membrane pile to visualize fouling appearance by naked eye. Membrane pieces were cut from an inlet central of the membrane in duplicate for observation under optical microscope and scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (SEM-EDX) as described in [8]. For optical microscopy, the membrane pieces were stained with crystal violet 0.1 % solution (Crystal Violet/ammonium oxalate solution, Boom B.V., Netherlands) and Alcian Blue 8 GX 0.1 % solution (Sigma Aldrich, the Netherlands) for visualization of organic fraction and the acidic polysaccharides within the EPS biofilm matrix, respectively [21,22]. ...
Reverse Electrodialysis (RED) has shown to be a promising technology for electricity production but membrane fouling can influence the energy yield, thus, it is crucial to gain knowledge on the impact and prevention of fouling. We used a sequence of pre-treatments (dual-media filter, microfilter and activated carbon filter) to fractionate foulants in seawater based on size and investigated its effect on RED. Fouling reversibility was studied by applying a cleaning procedure with reversal and increased flow and air sparging at the end of the experiment. After 63 days of operation, when using only the dual-media filter as pre-treatment, the pressure drop increased by 520 mbar in the seawater compartments without affecting the electrochemical measurements. The application of the cleaning procedure allowed to recover 90 % of the pumping power losses. A membrane autopsy showed the development of a biofilm layer on the membrane of the stacks receiving the most treated water while it was not detected when only the dual-media filter was used, which showed that the presence of particulate matter could help limit biofouling development. Together with the performance recovery, we concluded that the application of the dual media filtration system is a suitable and cost effective pre-treatment for RED.
... Rijnaarts et al. (2018) [154] reported that Donnan dialysis can reduce the concentration of divalent cations, but the cost of IEMs should be decreased to 10.7 USD/m 2 (or 10 €/m 2 ). Vital et al. (2021) adopted a DMF for the pretreatment of fresh water at the Afsluidijk RED pilot system [155]. They reported that the stand-alone DMF is a simple pre-treatment method for RED application. ...
Reverse electrodialysis (RED) is an emerging renewable energy technology that generates electricity by combining concentrated and diluted streams with varying salinities. Ion-exchange membranes (IEMs) have undergone significant advancements in RED, with an enhanced understanding of system configuration and operation conditions for increased power generation. This comprehensive review focuses on recent advances in IEMs, process design, and optimization of RED systems over the last five years. Challenges in the pilot-scale and field-scale systems are discussed, as well as practical limitations such as IEM fouling and electrochemical reactions on electrodes. Future research directions for enhancing overall performance, power generation, and economic feasibility of RED for salinity gradient power (SGP) generation are also proposed. Future advances in the following directions will increase the economic feasibility of RED application in SGP: 1) development of scalable IEMs with high anti-fouling efficiency, mechanical strength, and ion selectivity/conductivity, 2) process optimization (including pre-treatment) for IEM and electrode fouling mitigation, and 3) control of undesirable irreversible faradaic reactions.
... Other studies, focused on the fouling in natural waters, indicate that, when no anti-fouling strategies are in place, organic matter can decrease the gross power density up to 40 %, whereas inorganic fouling can decrease up to 8 % [18,19]. anti-fouling strategies, such as water pre-treatment [20], cleaning strategies [21,22] or RED monitors [23,24] show a prospect of solving fouling issues and maintaining low stack pressure drops with the challenge of using less energy. The use of profiled membranes [25,26] was also suggested to decrease stack pressure drops. ...
... Pictures of the AEM, CEM, spacers and electrodes were taken and membrane and spacer pieces were cut for optical and electron microscopic investigation, always on an inlet, central area of the membrane. The procedure was the same as described in Vital et al. [20]. In short, membrane and spacer pieces were fixed and dried for microscopy analyses under scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), while other pieces were stained with Alcian Blue 8 GX 0.1 % solution (Sigma Aldrich, the Netherlands) and observed with phase-contrast microscopy (Olympus BX40, Japan) at 10x and 20x magnification, for visualization of organic fraction of extracellular polymeric substances (EPS) and biofilm [47]. ...
... More details can be seen during the cleaning and stack autopsy in section 3.4. Interestingly, the gross electrical performance was not affected by the pressure increase and fouling [20]. We conclude that the electrical performance was dependent on the water's composition and concentration and mostly independent of the fouling for the period of testing. ...
A multistage reverse electrodialysis system was studied at the REDstack research facility (the Afsluitdijk, the Netherlands) for over 30 days to describe the performance of such configuration under natural water conditions. The experiments were done with two 0.22 x 0.22 m² stacks in series comprising 32 cell pairs (3.1 m² of membrane area) for stage 1 and 64 cell pairs (6.2 m² membrane area) for stage 2. The total gross power density at the available salinity gradient was stable at around 0.35 W∙m⁻². The total net power density, corrected for the initial pressure drop of the stacks, was 0.25 W∙m⁻² at an energy efficiency of 37%. Throughout the operation, due to increased stack pressure drop, the actual total net power density lowered to 0.1 W∙m⁻². A distinct behaviour was found for multivalent ions in each stage. For stage 1, Ca²⁺ and SO4²⁻ were transported from the river water to the seawater side, so-called uphill transport. For stage 2, uphill transport was not found, in line with Donnan potential calculations. Stack autopsy revealed microorganisms with sizes ten times larger than the cartridge filter nominal pore size (5 µm) and biofilm covering part of the spacer open area, both contributing to the increasing pressure drop in the stacks. This study showed that stable gross power densities and high energy efficiencies were obtained from feeding natural waters to a multistage reverse electrodialysis system, independent of fouling. In addition, it emphasized the importance of maintaining pumping power losses low for a viable deployment of the technology.
... Besides fouling by organic compounds, it is worth mentioning that biofilms can also grow over the surface of AEMs during their exposure to natural feedwaters, thus hampering the overall performance of RED for blue energy harvesting. It was recently demonstrated in a 54-day-long study with natural feedwaters that the presence of biopolymers caused a reduction in gross power density [19]. Therefore, deep knowledge on the causes of biofouling and the development of adequate strategies for its mitigation are highly required [20]. ...
This study covers the modification, (bio)fouling characterization, use, and cleaning of commercial heterogeneous anion exchange membranes (AEMs) to evaluate their feasibility for reverse electrodialysis (RED) applications. A surface modification with poly (acrylic) acid resulted in an improved monovalent perm-selectivity (decreased sulfate membrane transport rate). Moreover, we evaluated the (bio)fouling potential of the membrane using sodium dodecyl sulfate (SDS), sodium dodecyl benzenesulfonate (SDBS), and Aeromonas hydrophila as model organic foulants and a biofoulant, respectively. A detailed characterization of the AEMs (water contact angle, ion exchange capacity (IEC), scanning electron microscopy (SEM), cyclic voltammetry (CV), and Fourier Transform Infrared (FTIR) spectra) was carried out, verifying that the presence of such foulants reduces IEC and the maximum current obtained by CV. However, only SDS and SDBS affected the contact angle values. Cleaning of the biofouled membranes using a sodium hypochlorite aqueous solution allows for (partially) recovering their initial properties. Furthermore, this work includes a fouling characterization using real surface and sea water matrixes, confirming the presence of several types of fouling microorganisms in natural streams. A lower adhesion of microorganisms (measured in terms of total bacteria counts) was observed for the modified membranes compared to the unmodified ones. Finally, we propose a cleaning strategy to mitigate biofouling in AEMs that could be easily applied in RED systems for an enhanced long-term process performance.
... A power production decrease of~40% with respect to the system fed with artificial solutions was found. Recently, Vital et al. [35] carried out a long-run test (54 days) with natural seawater and fresh water and claimed that suspended particles with an average diameter of 10 µm reduced the gross power density of the system by about 25%. ...
Wastewaters generated by crude oil extraction processes, called “produced waters” (PWs), are complex solutions that contain organic compounds, mainly hydrocarbons, and often exhibit high salinity. The large amounts of PWs represent a global issue because of their environmental impact. An approach widely used in the oil industry is the reinjection of this wastewater into the extraction wells after a suitable treatment. The high salt concentration of such solutions may be used in salinity gradient technologies to produce green electricity. Among these technologies, reverse electrodialysis (RED) is one of the most promising. In this work, the application of RED for energy generation from two different real oil industry brines was investigated. An experimental campaign was performed by testing 10 × 10 cm2 units in long-run continuous operations, monitoring the performance for more than 25 days. Fouling phenomena, occurring during the continuous operation, decrease the unit performance and several anti-fouling strategies were adopted to tackle this issue. As a result, a positive net power density for up to 18 days of continuous operation was obtained. A maximum power density of about 2.5 W/m2 was observed, demonstrating how the RED technology could be an important strategy to harvest energy from an industrial waste.
