Article pubs.acs.org/est
Periodic Feedwater Reversal and Air Sparging As Antifouling Strategies in Reverse Electrodialysis David A. Vermaas,†,‡ Damnearn Kunteng,§ Joost Veerman,§ Michel Saakes,† and Kitty Nijmeijer*,‡ †
Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC, Leeuwarden, The Netherlands Membrane Science & Technology, University of Twente, MESA+ Institute for Nanotechnology, P.O. Box 217, 7500 AE, Enschede, The Netherlands § REDstack B.V., Pieter Zeemanstraat 6, 8606 JR, Sneek, The Netherlands ‡
S Supporting Information *
ABSTRACT: Renewable energy can be generated using natural streams of seawater and river water in reverse electrodialysis (RED). The potential for electricity production of this technology is huge, but fouling of the membranes and the membrane stack reduces the potential for large scale applications. This research shows that, without any specific antifouling strategies, the power density decreases in the first 4 h of operation to 40% of the originally obtained power density. It slowly decreases further in the remaining 67 days of operation. Using antifouling strategies, a significantly higher power density can be maintained. Periodically switching the feedwaters (i.e., changing seawater for river water and vice versa) generates the highest power density in the first hours of operation, probably due to a removal of multivalent ions and organic foulants from the membrane when the electrical current reverses. In the long term, colloidal fouling is observed in the stack without treatment and the stack with periodic feedwater switching, and preferential channeling is observed in the latter. This decreases the power density further. This decrease in power density is partly reversible. Only a stack with periodic air sparging has a minimum of colloidal fouling, resulting in a higher power density in the long term. A combination of the discussed antifouling strategies, together with the use of monovalent selective membranes, is recommended to maintain a high power density in RED in short-term and long-term operations.
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INTRODUCTION Renewable energy can be generated from mixing seawater and river water. The potential in this application is huge, as the theoretically obtained power from mixing the global river water runoff with seawater is well over 2 TW,1−3 i.e., close to the present worldwide electricity demand.1 The energy that is required to separate fresh water and ions when seawater evaporates can be captured when fresh and salt water mix again. Although practical application is still under development, the advantages of such potential power source are evident, as sustainable and stable power generation is possible using the continuous river water discharge to the sea. This electrical power can be captured in reverse electrodialysis using ion exchange membranes. A RED module (i.e., RED stack) is composed of a series of alternating cation exchange membranes (CEMs) and anion exchange membranes (AEMs), with flows of seawater and river water supplied in the compartments between these membranes.2 Due to the salinity gradient over the membranes, the ions are transported from the seawater compartment toward the river water compartment. As the membranes are selective for either cations (CEM) or anions (AEM), each membrane creates an electrical potential. This potential can be used to drive an electrical current, when electrodes with a (reversible) redox reaction3 are connected to an electrical consumer. Alternatively, capacitive electrodes can © 2014 American Chemical Society
be used to transfer the ionic current into an electrical current without the necessity of a redox reaction.4 This requires periodic switching of the feedwaters to reverse the direction of the electrical current The performance of a RED stack is well investigated under laboratory conditions2,8−14 or in idealized models.5−9 Hence, the obtained gross power density, expressed in W per m2 of membrane area, has reached recently a record of 2.2 W/m2 for mixing artificial seawater and river water10 and reached up to 7.5 W/m2 for increased salinity gradients.11 The latter results suggest that practical applications are close to a positive net present value (NPV) during the lifetime of a RED power plant, especially if membrane prices decrease.12 However, research on RED under practical conditions, using natural streams of seawater and river water, is sparse. In general, practical applications with ion exchange membrane technologies suffer from several fouling types, such as scaling, biofouling, adhesion of organic substances, and deposition of colloids.13−15 A recent fouling study16 without any specific antifouling strategies showed that fouling in RED is dominated by colloids Received: Revised: Accepted: Published: 3065
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Figure 1. Schematic illustration of RED stack setup in a plane perpendicular to the membranes (A) and in a plane parallel to the membranes (B), which shows the cross-wise flow orientation as used for the experiments. The profiled ridges on the profiled membrane are only illustrated in panel B, using darker shading, and are not on scale. The lighter gray area indicates the area available for water flow, in this case river water. The seawater compartment faces the other side of this membrane.
up to approximately 5 μm, such as clay minerals and remnants of diatom shells. Scaling of calcium phosphate and biofouling (intact micro-organisms) are detected in lesser degree at the fouled membranes.16 As a consequence, the power density decreases by approximately 50% during the first day, followed by a slow further decrease in power density in the next 3 weeks. Moreover, the pressure loss between the feedwater inlet and outlet increased rapidly, to a maximum of approximately 2 bar.16 For a stack with spacers in between the membranes, this maximum pressure drop was reached within 5 days. For a stack with profiled membranes, which integrate the membrane and spacer, the maximum pressure drop was reached after 20 days. Although these profiled membranes reduced the effects of fouling, a thick layer of fouling material was attached to the membranes in both designs.16 Antifouling strategies are required to reduce fouling and to maintain a sustainable high power density and a low pressure drop. However, regular antifouling strategies are too expensive (e.g., pretreatment with ultrafiltration) or environmentally undesired (e.g., disinfectants or coagulatants) considering the large quantities of feedwater involved in a future commercial RED power plant. Therefore, only cheap and environmentally friendly antifouling treatments can be considered. Previous research showed that periodic switching of feedwaters (i.e., swapping seawater for river water and vice versa) tempers the increase in feedwater pressure due to biological material under artificial accelerated biofouling conditions.17 Such an osmotic shock not only prevents this biofouling but also induces a reversal of the electrical field, which can reduce fouling of organic acids and charged colloids.13 Moreover, a periodic feedwater switch enables the use of capacitive electrodes.4 Alternatively, fouling may be reduced using disturbances in the flow conditions. Using natural feedwaters, the pressure drop temporarily decreased for approximately one day when the feedwater flow rate was disturbed for a short time (e.g., 5 min).16 This observation gives reason to assume that artificial disturbances, for example introduced by air bubbles in the feedwater compartments, offer a possibility to reduce fouling. This research investigates experimentally the performance of RED stacks using natural feedwaters using two antifouling strategies; namely 1) periodic switching of the feedwaters and
2) periodic injection of pressurized air (i.e., air sparging). The performance of these systems is compared to that of a stack without any antifouling strategy. The results indicate that both antifouling strategies significantly influence the degree and type of the fouling in a RED stack and as such generally increase the corresponding performance. Furthermore, the results give directions for additional new antifouling strategies in order to maintain a high power density in RED.
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EXPERIMENTAL SETUP RED Stacks. Three identical RED stacks were built according to Figure 1. The housings of these stacks were supplied by REDstack B.V. (The Netherlands). Profiled membranes were used, which make the use of spacers unnecessary. Each stack comprised 10 cells composed of a CEM, an AEM (Ralex CMH/AMH, MEGA AS, Czech Republic), a compartment for seawater, and a compartment for river water. The membranes were standard grade (i.e., not monovalent selective). All membranes had an area of 10 cm × 10 cm available for ion exchange. The profiled membranes were hot pressed into an aluminum mold, such that the membrane surface contained profiles (i.e., ridges) of 230 μm in height and 300 μm in width. The created feedwater channels were 230 μm in height and 1.9 mm in width. The hot pressing was performed at a temperature of 140 °C and a pressure of 200 bar, for 10 min, according to the previously explained procedure.18 The stack contains 10 CEMs and 11 AEMs, as the last compartment was closed with an AEM to shield the electrode rinse solution. The membrane pile was fixed between two end plates with an inserted working electrode made of Ti−Ru/Ir mesh and a dimension of 10 cm × 10 cm (MAGNETO Special Anodes B.V., The Netherlands). The electrode compartments were rinsed with 0.25 M NaCl solution. The potential over the RED cells was measured using an Ag/AgCl reference electrode (QIS, The Netherlands) connected to the middle of each electrode compartment. More details on the electrode system are described in the Supporting Information. Feedwater was supplied to the stack via distribution chambers at all sides of the membrane pile and hence creating manifolds to distribute the feedwater uniformly over all feedwater channels (Figure 1B). The river water feed was
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Figure S1. For most of the time during the 30-min chronopotentiometric cycles, a current density of 5 A/m2 was generated by the stacks. The open circuit voltage (OCV, in V) was derived from a 90 s stage without electrical current. The stack area resistance was derived from the steady voltages during open circuit and during the stages with electrical current (2.5 A/m2, 5 A/m2, and 10 A/m2), in combination with Ohm’s law. The power density was derived from the OCV, the stack area resistance, and the total membrane area, according to
oriented 90° with respect to the seawater feed, i.e., cross-flow is applied. After assembling of the stacks, the membranes were brought in equilibrium with artificial seawater (0.5 M NaCl) and river water (0.017 M NaCl) for at least 1 week, after which natural feedwaters were supplied. Feedwater. Natural feedwater was supplied to all stacks; fresh water was obtained from a nearby canal (Van Harinxmakanaal, The Netherlands), and salt water was obtained from the nearby harbor in the Wadden Sea (Harlingen harbor, The Netherlands). These sources are referred to as river water and seawater in this paper, respectively. The river water was stored in a buffer tank (33 h capacity during the first month of the experiment, 400 h capacity for the later part of the experiment) to prevent a stop in the less reliable river water supply line. Both feedwaters were filtered through microfilters with a median diameter of 10 μm. The water quality of the feedwater and pressure drop over the stack was monitored, as described in the Supporting Information. Both feedwater streams had a constant flow rate of 200 mL/min for each stack, which is equivalent to an average velocity in the feedwater compartments of 1.7 cm/s and a Reynolds number of 8. Antifouling Strategies. Two antifouling strategies were evaluated, i.e., periodic feedwater switching and periodic air sparging. Therefore, one stack was supplied with 3-way valves in the feedwater line, such that every 30 min the supply of seawater to that stack was switched for river water and vice versa. A second stack was equipped with a T-joint and a valve, to which an air compressor was connected. The air was injected every 30 min for 30 s, at a pressure difference over the stack of approximately 1 bar. Although the water flow was not stopped when air was injected, the volumetric air flow dominated the water flow rate during this 30 s period. A third stack was used as a reference, where no antifouling strategy was applied. Both antifouling strategies consume some power to apply them or reduce the power output compared to clean stacks. In case of feedwater switching, the concentration difference is smaller during a short period, which results in a temporarily lower power density. Switching the feeds each 30 min reduced the average power density by approximately 2%. Based on the pressure drop during air sparging and assuming a volumetric flow rate of 2 L/min, approximately 50% of the obtained energy from the RED stack without fouling was consumed for air sparging. Although this is a significant consumption of power, the conditions for air sparging in this research were chosen to demonstrate its effect. The duration and frequency of air sparging can be optimized for the most efficient antifouling treatment at large scale applications. Therefore, the power losses due to feedwater switching or air sparging are not included in the (gross) power density. The fouling experiments were conducted at the Wetsalt demo site in Harlingen, The Netherlands, during autumn 2012 (from September 17 to November 23). After dismantling, two membrane pairs from the middle of each stack were used for analyzing the foulants. The remaining eight membrane pairs from each stack were cleaned manually with a brush and stored in demineralized water or 5 M NaCl (brine). After that treatment, these membranes were reused in RED stacks to operate again from December 13 to December 17, 2012. Electrical Measurements. To measure the open circuit voltage, stack resistance, and power density, chronopotentiometry was applied, using individual potentiostats (Ivium technologies, The Netherlands) for all stacks, as described in
Pdens =
OCV2 4R stack ·Nm
(1)
in which Pdens is the power density (W/m2), Rstack is the stack area resistance (Ω·m2), and Nm is the number of membranes in the stack (-). The final membrane, between the last feedwater compartment and the electrode compartment, is not considered here because this last membrane would be insignificant in large scale applications with many membrane pairs.19 Power Density Normalization. As the open circuit voltage and stack resistance are dependent on the ionic concentrations and temperature of the feedwater11,20 and these parameters fluctuate in time, the power density is normalized using a theoretical power density based on the actual concentrations and temperature. This theoretical power has been calculated using the Nernst equation for determining the OCV, corrected for the apparent membrane permselectivity α (-)20 and the activity coefficient γ (-) OCV = α ·
Nm·R ·T ⎛ γs·cs ⎞ ⎟⎟ ·ln⎜⎜ F ·z ⎝ γr·cr ⎠
(2)
in which R is the universal gas constant (8.314 J/(mol·K)), T is the absolute temperature (K), F is the Faraday constant (96485 C/mol), z is the valence of the transported ions, and c is the ionic concentration in the feedwater (M). The subscripts s and r indicate seawater and river water, respectively. The apparent membrane permselectivity represents the actual voltage compared to the theoretical obtained voltage over a RED stack.21 The valence of the ions (e.g., z = ± 1 for Na+ and Cl− and z = ± 2 for Mg2+ and SO42‑) influences the OCV. Multivalent ions, which are present as minor species in the natural feedwaters, actually reduce the OCV22−24 and increase the electrical resistance of the system.23,24 Because a mixture of these ions is present in natural feedwater, while the individual ionic concentrations are sampled intermittent, the theoretical voltage is calculated based on the total salinity (measured by the actual conductivity) and using the valence of the dominant species, i.e., z = ± 1. To account for the presence of multivalent ions, a relatively low apparent permselectivity of 0.85 is used for the calculation of the theoretical power density.16 The stack resistance is estimated based on the ohmic resistance, which is dependent on the membrane resistance and feedwater conductivity, and the nonohmic resistance, which is due to the decrease in potential as ions are transported from seawater to river water in the stack.8 Although the nonohmic resistance is also influenced by preferential channeling,25 i.e., the nonuniform feedwater distribution in the feedwater compartments which can occur due to fouling, the nonohmic resistance is initially estimated assuming uniform flow. Hence, the theoretical power density 3067
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to each electrical measurement are used to calculate the theoretical power density. The concentrations of organic matter and humic substances are relatively high compared to typical concentrations of natural waters,26 which motivates to consider humics acids as a possible foulant for AEMs.27,28 Prevention of Colloidal Fouling. Previous research showed that visible deposition develops in RED stacks at a time scale of several days or weeks.16 The time series for the average pressure drop over the feedwater channels, indicating clogging of feedwater channels due to fouling, is shown in Figure 2A for the full duration of the experiment. Figure 2B−G show photos of the fouled profiled membranes after the experiment. All stacks experience a pressure drop that is an order of magnitude lower than stacks with spacer filled channels under practical conditions,16 because of the use of profiled membranes and the design with wide inflow manifolds. Nevertheless, the pressure drop increases significantly from day 10 for the stack without antifouling strategy (Figure 2A) due to fouling. Analysis of this fouling with scanning electron microscopy (SEM) revealed that the fouled surfaces mainly contain clay particles and diatom remnants (Supporting Information, Figure S2), which is similar as found in previous research.16 These clay particles and diatom remnants are grouped as colloidal fouling. The pressure drop in the stack with feedwater switching started to rise later, most pronounced from day 22 on. The feedwater switching creates disturbances in the feedwater flow, which reduce colloidal fouling. However, this antifouling strategy is apparently not sufficient to avoid fouling in the longer term. In contrast, the stack with air sparging keeps a remarkably low pressure drop over the full duration of the experiment (Figure 2A), indicating that periodic air sparging prevents or removes colloidal fouling. The pressure drop increases very slowly, to a maximum of only 62 mbar after more than 2 months. A SEM analysis of the fouling in the stack with air sparging (Supporting Information, Figure S2) indicated microorganisms at a minor part of the membrane area, although the
should be considered as the power density that could be obtained when no fouling would occur. The response time to establish a steady voltage is used to indicate preferential channeling.25
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RESULTS Feedwater Conditions. The salinity, temperature, and feedwater composition of the natural feedwater sources as used in the present research are presented in Table 1. Table 1. Ion Concentrations, Temperature, Dissolved Organic Carbon (DOC), and Humic Substances (HS), which Is the Sum of Humic Acids and Fulvic Acidsa
river water
seawater
temp (°C)
cations (mEq/L)
anions (mEq/L)
Na+: 37 ± 25 Mg2+: 9 ± 5 Ca2+: 7 ± 1 total: 55 ± 30 Na+: 199 ± 32 Mg2+: 43 ± 7 Ca2+: 13 ± 2 total: 264 ± 41
Cl−: 43 ± 28 SO42‑: 5 ± 3 total: 46 ± 29
16 ± 4
Cl−: 254 ± 40 SO42‑: 27 ± 4 total: 268 ± 42
12 ± 3
organic compds (mg-C/L) DOC: 28 ± 8 HS: 19 ± 9 DOC: 12 ± 1 HS: 10 ± 3
a
The presented values are average values and standard deviations considering all samples. The total cation and anion concentrations include minor ionic species that are not shown individually (e.g. K+, PO43‑). Details on measurement techniques for cations, anions, and organic compounds are given in the Supporting Information.
The large standard deviations in the measured concentrations (Table 1) demonstrate that the salinity strongly fluctuates, which is due to the fact that the inlet of the seawater and that of the river water are located relatively close together. Therefore, the sources for seawater and river water partly mix near the river mouth, dependent on the tide and the river water discharge. The fluctuations in salinity and temperature result in a fluctuating theoretical and practical power production, as a higher salinity ratio and a higher temperature increase the power density in RED.11,20 The actual salinities corresponding
Figure 2. Average pressure drop as function of the time (A) and photos of profiled cation exchange membranes from the fouled stacks at the end of the experiment (B−G). The left series of photos (B, D, and F) show the full area of 10 × 10 cm2, while the right photos (C, E, and G) show images of approximately 1 × 1 cm2, zoomed close to the inflow of the feedwater compartments. The original membrane color is beige. 3068
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channeling as observed in the stack after dismantling (Figure 2D,E). The increase in response time is remarkably rapid, which suggests that a few large colloids clogged the feedwater channels in this stack suddenly, rather than that a slow accumulation of a colloid layer occurs. As the response time is very sensitive when a small membrane area is inaccessible for feedwater,25 the response time increases rapidly when such nonuniform colloidal fouling occurs. The response time does not increase above 20 s for the stacks without treatment or air sparging. This is partly due to the high pressure drop in the stack without treatment and in particular due to the air sparging in the other stack, as both phenomena create larger hydraulic driving forces than in the stack with feedwater switching. This large driving force reduces the chance that a feedwater channel is blocked. However, the prompt increase at particularly day 46 for the stack with feedwater switch, and the absence of preferential channeling for the stack without treatment, cannot be predicted for a future experiment. This indicates that preferential channeling is unpredictable and partly coincidental. The occurrence of preferential channeling as indicated by the large response time is in agreement with the nonohmic resistance, which is due to the concentration changes in the bulk and diffusive boundary layer, as shown in Figure S3. The nonohmic resistance increases in particular for the stack with feedwater switching, to approximately 500% of the theoretical value. This effect is included in the power density, which we will show later. Obtained Power Density. For the obtained voltage and power, the short-term effects (when no significant colloidal fouling occurs) are separated from the long-term effects. The short-term effects on the obtained voltage and power are discussed first. The normalized open circuit voltage, i.e., the apparent permselectivity when considering monovalent ions only, and the normalized power density are shown in Figure 4 as a function of time for the first 4.5 h of operation. For all cases, the apparent permselectivity decreases from approximately 95% at the start of the experiment to 74−81% after 4 h. The fluctuations visible at this small time scale for the stack with feedwater switch stem from the voltage reversal after every feedwater switch and a slight bias in the reference electrodes that measure the voltage. The rapid decrease in apparent permselectivity in the first hours (Figure 4A) was also observed in previous research, although in less detail, and attributed to the presence of multivalent ions and organic substances such as humic acids.16 At the start of the experiment, when the membranes are equilibrated with NaCl solutions, the apparent permselectivity is relatively high; even higher than the expected 85%. This indicates that the membranes are not yet in equilibrium with the actual feedwater, which is composed of a mixture of monovalent and multivalent ions. The presence of multivalent ions in the natural feedwater reduces the open circuit voltage, as can be deduced from the role of the valence z in eq 2.22,23 In addition, organic substances such as humic acids may adhere electrostatically to the ion exchange membranes, specifically AEMs due to the negative charge and large molecular size, and hence shield charged moieties in the membrane. The lower available fixed membrane charge reduces the apparent permselectivity.27 The stack with feedwater switch has the highest apparent permselectivity after 4 h, followed by the stack without any
deposition is still dominated by colloids that are similar to those in the other RED stacks. Photos of the membranes after dismantling the stacks support the observed pressure drops. The membranes in the stack without any treatment are almost completely covered with a thick layer of colloids (Figure 2B,C), whereas in the stack with feedwater switch a few channels are still relatively clean (Figure 2D,E). These open channels explain the slightly lower pressure drop for the stack with feedwater switching. This nonuniform distribution of fouling forces a nonuniform distribution of feedwater, which is known as preferential channeling.29 Such preferential channeling is known to induce a large nonohmic resistance and will decrease the power density in RED.25 Preferential channeling starts at spots that are vulnerable to fouling, such as spacer knits in spacer filled channels29 or arbitrary rough spots in profiled membranes. As a consequence, the local velocity around this initial adhered deposit is lower, due to the local obstruction of the flow. This lower local velocity intensifies the fouling at these spots, leading to preferential channeling. The membranes from the stack with air sparging also contain fouled colloids (Figure 2F) but in a much thinner layer and smaller fractions compared to the other stacks (Figure 2G). Therefore, fouling is not completely prevented using air sparging but distributed such that the feedwater can still flow through the stack at a relatively low pressure drop. The periodically applied high pressure during air sparging and the water−air interfaces scour the feedwater channels, such that all channels remain open for feedwater flow. Preferential channeling can be electrochemically deduced in situ from the response time of the system to establish a steady voltage of the stack when the electrical circuit is suddenly opened or closed.25 This response time is presented in Figure 3 as a function of time.
Figure 3. Response time, which is the typical time scale to establish a steady voltage after a sudden change in electrical current, as a function of the time of operation. A moving average with a span of 4 h was applied.
The short response time at the start of the experiment (response time
