Energy-positive brine management in the Aral Sea: a techno-economic water–energy nexus framework based on reverse electrodialysis

Energy-positive brine management in the Aral Sea: a techno-economic water–energy nexus framework based on reverse electrodialysis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Energy-positive brine management in the Aral Sea: a techno-economic water–energy nexus framework based on reverse electrodialysis Moulay-Rachid Babaa, Otabek Atabaev, Nargis Kholmamatova, Muzaffarbek Mamadiyev, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8453844/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Hypersaline inland water bodies are a growing environmental and management challenge. Yet, they remain largely overlooked in the water–energy nexus. Here, we present a systems-level framework for energy-positive brine management using reverse electrodialysis (RED). Our approach explicitly addresses the complex multi-ion chemistry characteristic of inland hypersaline basins. A Pitzer-based thermodynamic model is coupled with a hydraulically constrained RED stack design and techno-economic analysis. This setup evaluates performance under realistic seasonal conditions. We apply this framework to the South Aral Sea basin as a representative case. The system achieves net electrical conversion efficiencies of 8.8–19.2% across seasonal temperatures. The seasonal average is 14.4%, governed by temperature-dependent ionic activity and conductivity. The effective NaCl-equivalent activity of the hypersaline brine is constrained to 1.00 1.11 mol kg⁻¹, much lower than ideal-solution estimates. With equal brine and treated wastewater flow rates at baseline operation, post-stack effluent mixing yields about a 50% reduction in bulk brine salinity per pass. The resulting levelized cost of electricity ranges from €0.16–0.31 kWh⁻¹ across seasons. The annualized average is €0.22 kWh⁻¹ when viewed as a joint cost of electricity generation and brine remediation. Sensitivity analyses show membrane spacing and lifetime are key economic drivers. Pump efficiency plays a secondary role. More broadly, the integrated thermodynamic–electrochemical–economic workflow is transferable to other hypersaline lakes and engineered brine systems. It provides a physically consistent basis for evaluating RED as a coupled energy-generation and brine-management technology under real multi-component water chemistries. Physical sciences/Energy science and technology Physical sciences/Engineering Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Hydrology Hypersaline waters Reverse electrodialysis Water–energy nexus Multi-ion thermodynamics Brine remediation Seasonal performance Inland saline basins Figures Figure 1 Figure 2 1. Introduction Inland hypersaline water bodies are expanding globally due to climate change, water abstraction, irrigation return flows, and industry. Once formed, these systems pose persistent challenges: extreme salinity, ecological degradation, and limited remediation options. Management strategies like evaporation ponds, forced dilution, or containment consume energy but yield no recoverable value. This reinforces the idea that hypersaline brines are environmental liabilities, not resources. Once the fourth-largest lake in the world, the Aral Sea (Fig. 1 ) disaster is regarded as one of the most severe ecological catastrophes of the twentieth and twenty-first centuries. It was driven primarily by large-scale river diversion for irrigation in Central Asia and represents a paradigmatic case of anthropogenic hypersalinization with profound regional socio-environmental impacts [ 1 – 4 ]. While partial stabilization has been achieved in the northern basin, the southern basin continues to evolve toward extreme salinity, with few viable management pathways. Salinity-gradient energy offers a route to recover the Gibbs free energy released during the mixing of waters with different salinities [ 5 ]. Among available technologies, reverse electrodialysis (RED) converts ionic chemical potential differences into electrical power using alternating cation- and anion-exchange membranes [ 6 ]. Over the past decade, RED has progressed from laboratory studies to pilot-scale demonstrations, primarily in coastal seawater–river water configurations [ 7 , 8 ]. However, most studies approximate feed waters as ideal or NaCl-only solutions, an assumption that breaks down in hypersaline inland systems dominated by multivalent ions. Such brines exhibit strong non-ideal behavior due to complex ion–ion interactions involving species such as Mg²⁺, Ca²⁺, and SO₄²⁻. These interactions substantially affect ionic activity, conductivity, and membrane transport, rendering simplified thermodynamic treatments inadequate (Pitzer). As shown later in this work, explicitly resolving multivalent ion interactions reduces the effective NaCl-equivalent activity of the Aral Sea brine to approximately 1.0–1.1 mol kg⁻¹, substantially lower than would be inferred from concentration-only or ideal-solution assumptions. Here, we present the first system-level evaluation of reverse electrodialysis in a hypersaline inland basin that explicitly resolves multi-ion non-ideality, seasonal temperature variability, hydraulic constraints, and techno-economic performance within a single coherent framework, applied to the South Aral Sea. By integrating rigorous activity-coefficient modeling, hydraulically constrained stack design, and techno-economic sensitivity analysis, we address three central questions: Can RED operate efficiently under extreme multi-ion salinity conditions? Can energy generation be intrinsically coupled with meaningful brine salinity reduction? Is the resulting system economically robust when evaluated within a water–energy nexus context? Framed within the water–energy nexus, this study addresses the dual challenge of managing hypersaline water and generating sustainable energy. Rather than treating water treatment and energy production as separate optimization problems, we examine their intrinsic coupling through salinity-gradient energy conversion. By integrating multi-ion thermodynamics, electrochemical performance, and techno-economic assessment, the work provides a transferable framework for evaluating energy-positive water-management pathways in climate-stressed regions. Importantly, the framework explicitly accounts for seasonal temperature variability, enabling ionic activity, conductivity, electrochemical efficiency, and techno-economic performance to be evaluated consistently across winter, spring, summer, and autumn operating conditions. 2. Materials and Methods 2.1 Study area, system boundary and baseline configuration Muynak is one of the six districts in Karakalpakstan (Fig. 2 ), Uzbekistan, where the government and international partners are pursuing significant improvements in water and wastewater treatment infrastructure. Historically, Muynak was a port city on the Aral Sea, but due to the sea's dramatic shrinkage, Muynak is now far from any significant surface water body. While major efforts and documented projects in the region focus on upgrading water supply systems, construction and modernization of sewage and sanitation facilities, there is less specific information about a dedicated large-scale Muynak wastewater treatment plant currently in operation. Recent capacity expansions in Muynak include the construction and rehabilitation of water distribution centers and significant new pipeline networks designed to meet a growing population's needs [ 9 ]. The benchmark water consumption per person in the project is around 100 liters per day, and Muynak’s agglomeration (including its urban center and rural surroundings) was projected to serve a population of about 30,000 after the upgrades. By simple calculation, the current total water consumption of the Muynak agglomeration is approximately 35 liters per second under standard conditions, though actual flows may fluctuate based on seasonal, technical, and operational factors. It is assumed that 80% of water consumption (supply) returns as sewage flow is a standard practice in municipal water and wastewater modeling [ 10 – 11 ]. This figure, often referred to as the "sewage generation coefficient" or "wastewater return coefficient" of 0.8, adopted from authoritative engineering manuals and validated through numerous regional studies for initial planning purposes [ 12 – 13 ] Since groundwork and network infrastructure for future waste water treatment plant (WWTP) operation are still being advanced in Muynak, the WWTP outflow was estimated taking into account the typical range of water losses between the inlet and outlet of municipal wastewater treatment plants. Spellman [ 14 ] states that evaporation and process losses for conventional municipal WWTPs typically range from 3% to 7% under standard conditions. Given the arid climate in Muynak, we assume a total water loss of 7% which gave an outflow of 26 L/s that will be considered as the inlet flow rate to our RED facility. The temperature fluctuations between summer and winter significantly affect the efficiency of the RED system; the decrease in temperature is detrimental to the power output [ 15 ]. Based on these premises, four weather operating temperatures have been approached based on the metrological conditions reflecting the four seasons. The system comprises a reverse electrodialysis (RED) stack supplied with hypersaline brine as the high-salinity stream and treated municipal wastewater (TWW) as the low-salinity stream. The analysis boundary includes the RED stack, hydraulic pumping, power conditioning, and membrane replacement, while excluding upstream wastewater treatment and long-distance conveyance infrastructure. Baseline volumetric flow rates are: Q H = Q L = 0.026 m 3 .s − 1 corresponding to equal brine and TWW processing. 2.2 Feedwater characterization The hypersaline feed was represented using measured ionic concentrations from the South Aral Sea, expressed on a molality basis to ensure internal consistency across seasonal conditions [ 16 ]. The brine is dominated by Cl⁻ and SO₄²⁻ anions and Na⁺, Mg²⁺, and Ca²⁺ cations, reflecting the strongly multivalent character typical of inland hypersaline basins. Treated municipal wastewater was represented as a low-salinity electrolyte with a total dissolved salt concentration of CL = 26 mol m − 3 , consistent with typical secondary effluent values. To account for uncertainty in local wastewater composition, a ± 30% sensitivity around this reference effluent salinity was evaluated in the techno-economic model. Because of the high ionic strength and multicomponent composition of the hypersaline feed, solution non-ideality was explicitly accounted for when defining feedwater properties. Effective ionic activities and conductivities for both the hypersaline brine and the treated wastewater were obtained from standard thermodynamic reference calculations for saline waters and dilute electrolytes, respectively. These properties were treated as prescribed physicochemical inputs to the RED model. Details of the reference calculations and data sources are provided in the Supplementary Information. Seasonal temperature variability in the Aral Sea basin—ranging from near − 3°C in winter to above 27°C in summer, was incorporated by evaluating feedwater activities and conductivities at representative seasonal average temperatures. Across this range (270–300 K), effective brine activities varied from 1.005 to 1.105 mol kg⁻¹, with corresponding conductivities of 10.8–23.0 S m⁻¹, while treated wastewater activities ranged from 0.0204 to 0.0224 mol kg⁻¹ with conductivities of 0.127–0.270 S m⁻¹. Using seasonally resolved feedwater properties ensures that RED performance and techno-economic outcomes are evaluated consistently under realistic climatic conditions relevant to the South Aral Sea. 2.3 RED stack configuration and efficiency definition The RED system consists of alternating cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs) arranged in a spacerless configuration. Commercially available, PET-reinforced ion-exchange membranes with high permselectivity (> 90%) and low area resistance were selected as representative of current industrial practice. Concentrate (Aral Sea brine) and diluate (treated wastewater) streams flow through alternating compartments, while electrode rinse compartments containing a ferri/ferrocyanide redox couple complete the electrical circuit. Channel thickness was fixed at 100 µm for all base-case simulations, consistent with experimentally validated ranges reported in the RED literature. Superficial flow velocity in both concentrate and diluate channels was set to 2 cm s⁻¹, placing operation within the laminar regime and minimizing concentration polarization while maintaining moderate hydraulic losses [ 17 ]. The number of membrane cell pairs was determined by the available wastewater flow rate and the imposed velocity constraint. Detailed geometric parameters and stack sizing calculations are provided in Supporting Information . 2.4 Electrochemical, hydraulic, and brine-dilution modeling Electrochemical performance of the reverse electrodialysis (RED) system was evaluated using a standard Nernst-type formulation, corrected for membrane permselectivity and finite internal electrical resistance. Feedwater properties, including ionic activities and solution conductivities, were treated as prescribed inputs and evaluated at representative seasonal temperatures, as described in Section 2.2 . The open-circuit voltage and operating voltage were computed from the activity difference between the hypersaline brine and treated wastewater streams in combination with membrane characteristics. Internal electrical resistance was calculated as the sum of membrane resistances and solution resistances in the concentrate and diluate channels, based on channel geometry and the corresponding solution conductivities. Hydraulic performance was evaluated assuming fully developed laminar flow between parallel plates in spacerless channels. Pressure drops were determined from channel dimensions, fluid viscosity, and flow velocity, and the associated pumping power requirements were computed accordingly. The electrochemical and hydraulic models were coupled to determine net power output and conversion efficiency under steady-state operating conditions. In parallel, the brine salinity reduction per pass was defined as the decrease in bulk salt concentration following a single traversal of the RED stack and complete post-stack mixing of the brine and treated wastewater effluents. Under steady-state operation, the mixed outlet concentration was obtained from a mass balance on the two outlet streams. For the baseline configuration with equal volumetric flow rates (Q H =Q L ), this first-pass mixing results in an approximately 50% reduction in bulk brine salinity. All governing equations, geometric parameters, and numerical inputs used in the electrochemical, hydraulic, and mass-balance calculations are reported in the Supporting Information. 2.5 Techno-economic and sensitivity analysis Economic performance was assessed using a transparent, bottom-up techno-economic framework based on the Reverter Cost Method. Capital expenditures include the RED stack (membranes and electrodes), intake and outfall infrastructure, pre-treatment, pumps, piping, power electronics, and civil and electrical works. Operating expenditures include fixed operation and maintenance costs, pumping energy, and membrane replacement. A plant lifetime of 30 years, region-appropriate discount rates, depreciation, and residual value assumptions were applied [ 18 , 19 ]. In this study, the cost of grid-connected power inverters was estimated using a market-calibrated scaling approach based on commercially available utility-scale photovoltaic inverters. Rather than relying on a single vendor quotation, a representative dataset of inverters in the 50–250 kW class was assembled from multiple European suppliers, including new and refurbished units, with prices reported both inclusive and exclusive of value-added tax (VAT). The dataset spans a wide range of manufacturers (e.g., SMA, Huawei, Sungrow) and market channels, ensuring that the resulting cost model reflects realistic procurement conditions rather than optimized or promotional pricing. For each inverter, the specific cost (€/kW) was calculated from the nameplate power rating and advertised price. As expected for power electronics, the specific cost decreases with increasing capacity, reflecting economies of scale in semiconductor utilization, housing, and auxiliary components. To capture this trend quantitatively, inverter cost was fitted using a power-law scaling of the form: Where C inv ​is the inverter capital cost (€), P is the rated electrical power (kW), and α and β are empirical coefficients. A least-squares fit to the median market prices yielded α = 1156.6 and β = 0.333, consistent with scaling exponents commonly reported for balance-of-plant power electronics in renewable energy systems. This scaling law was used to estimate inverter CAPEX for the RED installation based on the installed electrical power requirement, with an additional installation and integration factor applied to account for cabling, protection devices, and control hardware. Importantly, the use of a market-derived scaling relationship avoids underestimation associated with single-vendor quotations and ensures that inverter costs remain consistent with contemporary commercial pricing across the relevant power range. Sensitivity of the overall techno-economic results to inverter cost assumptions was found to be minor compared with membrane lifetime and hydraulic losses, confirming that inverter CAPEX does not dominate system economics. The levelized cost of electricity (LCOE) was calculated as the ratio of discounted lifetime costs to discounted electricity generation. Sensitivity analyses were conducted for key parameters, including membrane lifetime, membrane spacing, pump efficiency, and diluate salinity. Complete cost breakdowns, assumptions, and sensitivity results are provided in Supporting Information. To assess the robustness of the techno-economic conclusions and identify dominant cost drivers, a one-at-a-time sensitivity analysis was conducted on three key operational and design parameters: (i) pump efficiency, (ii) membrane channel spacing, and (iii) membrane lifetime. Each parameter was varied independently around the baseline configuration, while all other inputs were held constant. Pump efficiency was varied between 0.60 and 0.85 to reflect realistic performance ranges for industrial centrifugal pumps operating under low-head, high-flow conditions. For each efficiency value, the associated pumping energy demand was recalculated and propagated through the levelized cost of electricity (LCOE) formulation. Membrane spacing was varied from 100 to 300 µm to capture the trade-off between electrical resistance, hydraulic losses, and achievable power density. Changes in spacing directly affect solution resistance, number of membrane cell pairs accommodated within a fixed stack volume, and overall RED conversion efficiency. Membrane lifetime was varied between 1 and 10 years using a membrane replacement factor defined as the inverse of the membrane lifespan. This factor scales annualized membrane replacement costs within the LCOE calculation and represents long-term degradation due to fouling, scaling, and mechanical aging. The resulting LCOE values from each sensitivity scenario are reported explicitly and discussed below. All sensitivity analyses (pump efficiency, feed salinity, inter-membrane spacing, and membrane lifetime) were performed using reference-condition transport properties evaluated at 25°C. Specifically, ionic activities and solution conductivities were fixed at their standard-temperature values derived from the Pitzer-based thermodynamic model and conductivity framework described above. This approach isolates the influence of individual engineering and economic parameters while avoiding confounding effects from seasonal temperature variability, which is treated separately in the seasonal performance analysis. 3. Results and Discussions 3.1 Electrical performance, seasonal robustness, and benchmarking against pilot systems Seasonal variations in temperature lead to predictable changes in RED performance, primarily through their influence on ionic mobility, solution conductivity, and internal electrical resistance. Summer operation benefits from higher conductivities and reduced resistive losses, whereas winter operation is constrained by reduced transport and higher overall stack resistance. Despite this variability, the system maintains stable operation across all seasons considered. Across the annual temperature range, the net energy-conversion efficiency spans 8.8–19.2%, with a seasonal average of 14.4%. These values place the system at the upper end of efficiencies reported for RED operation with natural, multivalent-ion-rich waters, where non-ideal solution behavior and increased resistance typically suppress performance. The moderate seasonal spread in efficiency indicates that the spacerless, laminar-flow architecture provides intrinsic robustness to climatic variability, a critical requirement for inland hypersaline environments subject to stronger temperature fluctuations than coastal settings. Under favorable thermal conditions, the system achieves peak net power outputs on the order of 45–50 kW, with corresponding efficiencies approaching ~ 19%. At lower winter temperatures, reduced conductivity leads to lower power output and efficiency, but operation remains stable and energy-positive. Overall, the predicted performance envelope lies within the upper range reported for pilot-scale RED systems operating on natural waters and brines, including the Afsluitdijk demonstration and the REAPower project, where conversion efficiencies of 10–20% and comparable resistance partitioning have been observed [ 7 , 8 ]. Although no new experiments are presented here, the agreement between predicted performance levels and those reported in existing pilot studies indicates that the present analysis remains grounded in experimentally demonstrated RED behavior, rather than extrapolating beyond established operational regimes. 3.2 Energy-positive brine dilution and remediation impact Beyond electricity generation, the RED system delivers a measurable and operationally relevant reduction in hypersaline brine salinity. Under baseline operation with equal volumetric flow rates of hypersaline brine and treated wastewater, a single pass through the RED stack followed by complete post-stack mixing yields an approximately 50% reduction in bulk brine salinity per pass. This reduction is achieved without external dilution energy input, relying instead on controlled mixing inherent to the RED process. When expressed in operational terms, the system provides a coupled energy–remediation service: for each 1 MWh of net electricity generated, approximately 230–260 m³ of hypersaline brine are diluted by 25–35 g L⁻¹ before controlled discharge or reuse. This framing highlights that salinity reduction is not a secondary by-product, but an intrinsic outcome of the energy conversion process. Importantly, this level of localized salinity reduction addresses one of the principal constraints in managing inland hypersaline waters, where conventional approaches, such as evaporation ponds or forced dilution, consume energy while providing no recoverable value. In contrast, the RED configuration evaluated here converts the salinity gradient into useful electrical power while simultaneously reducing brine salinity, rendering the process energy-positive at the system level. From a regional perspective, sustained operation of such a system could enable the progressive creation of localized zones of moderated salinity, particularly in near-shore or managed discharge regions. While full basin-scale restoration is beyond the scope of a single installation, targeted salinity reduction at this scale could support renewed ecological and economic activity, including fisheries and derivative value chains that historically depended on lower-salinity conditions in the Aral Sea region. Overall, the results demonstrate that RED can function as an integrated component of hypersaline water management, delivering simultaneous electricity generation and brine dilution without the energy penalties associated with conventional remediation strategies. This dual functionality underpins the relevance of RED within a water–energy nexus framework, particularly for climate-stressed inland basins where both energy scarcity and salinity accumulation pose growing challenges. This energy-remediation coupling distinguishes RED fundamentally from conventional hypersaline brine management approaches, such as evaporation ponds or forced dilution, which consume energy without recovering value. Although the resulting salinity reduction is insufficient for basin-scale restoration, it is operationally meaningful in hypersaline environments, where localized decreases in salinity can influence density stratification, ecological thresholds, and nearshore usability. In this sense, RED functions as an energy-positive brine-management pathway, delivering both electricity and controlled salinity reduction within a single integrated system. 3.3 Sensitivity analysis 3.3.1 Sensitivity to pump efficiency Varying pump efficiency from 0.60 to 0.85 results in a relatively modest change in LCOE, decreasing from 0.0877 € kWh⁻¹ to 0.0815 € kWh⁻¹. This limited sensitivity reflects the fact that hydraulic pumping power constitutes only a minor fraction of the total system energy balance under the low-pressure, laminar-flow conditions employed. The weak dependence of LCOE on pump efficiency confirms that the RED system is not hydraulically dominated and that reasonable deviations from nominal pump performance do not compromise economic viability. This finding supports the suitability of the proposed architecture for deployment in regions where high-efficiency pumping equipment may not be readily available or consistently maintained. 3.3.2 Sensitivity to membrane spacing Membrane spacing exerts a strong influence on system economics and performance. Increasing the channel spacing from 100 µm to 300 µm leads to a sharp increase in LCOE from 0.1659 € kWh⁻¹ to 0.3801 € kWh⁻¹, accompanied by a decline in RED efficiency from 19% to 7%. This pronounced sensitivity arises from the coupled effects of increased solution resistance and reduced power density at larger spacings. Wider channels reduce pressure drop but significantly degrade electrochemical performance, requiring fewer membrane pairs and yielding lower energy recovery per unit flow. The results demonstrate that spacerless or near-spacerless configurations with narrow channels are essential for achieving economically viable operation in hypersaline RED systems. Among all parameters examined, membrane spacing emerges as the single most critical design variable, underscoring the importance of precise channel geometry control and mechanical stability in large-scale implementations. 3.3.3 Sensitivity to membrane lifetime and replacement rate Membrane lifetime has a significant but saturating effect on LCOE. Increasing the membrane lifespan from 1 year to 10 years reduces LCOE from 0.1110 € kWh⁻¹ to 0.0854 € kWh⁻¹. The most substantial cost reductions occur within the first few years, with diminishing returns beyond approximately 5 years of operation. This behavior reflects the dominance of membrane replacement costs in the early lifecycle, followed by a plateau where other cost components (capital recovery, pumping energy, infrastructure) become limiting. Importantly, even under conservative assumptions of short membrane lifetime (1–2 years), the system remains economically competitive within the context of combined energy production and brine management. These results highlight membrane durability and fouling resistance as high-leverage targets for future materials development, while also demonstrating that the system does not rely on unrealistically long membrane lifetimes to remain viable. Sensitivity trends reported here reflect variations around the 25°C reference condition; absolute performance levels under seasonal operation are discussed separately in Section 3.1 . Taken together, the sensitivity analysis reveals a clear hierarchy of techno-economic drivers. Membrane spacing and the associated impact on electrochemical efficiency, emerges as the dominant determinant of system performance and cost, followed by membrane lifetime, while pump efficiency plays a comparatively secondary role. This ordering indicates that RED performance in hypersaline environments is governed primarily by electrochemical and geometric constraints rather than by hydraulic efficiency alone. Importantly, the modest sensitivity of LCOE to operational parameters supports the interpretation of the reported baseline results as representative and robust rather than finely tuned. This robustness reinforces the relevance of RED as a realistic component of integrated water–energy management strategies in hypersaline inland settings. 3.4 Hydraulic and manifold architecture The current reverse electrodialysis (RED) system adopts a channel-integrated membrane architecture in which flow channels are formed directly at the membrane interfaces, eliminating conventional spacers and enabling strictly laminar operation at low pressure drop. This spacerless configuration, previously proposed in modular RED designs such as the [ 20 ] concept, minimizes internal electrical resistance while maintaining hydraulic stability under high salinity gradients. Each modular cassette employs inter-membrane channel gaps of 100 µm, a dimension widely reported as near-optimal for balancing ionic transport resistance against hydraulic losses in laminar RED operation. To further limit pressure drop and suppress axial concentration polarization, each cassette comprising 500 membrane cell pairs is subdivided into three hydraulic zones (approximately 160–170 cell pairs per zone). This segmentation shortens the effective flow path length per zone and ensures that cumulative pressure losses remain well below the osmotic pressure difference across the membranes, preserving net energy extraction efficiency. The hypersaline concentrate (Aral Sea brine) and the low-salinity diluate (treated wastewater) streams are distributed through separate, vertically offset inlet and outlet manifolds. This configuration mitigates ionic short-circuiting along headers and reduces parasitic shunt currents, a known limitation in large-area RED stacks. Flow distribution from plant-level plenums into individual hydraulic zones is regulated using calibrated inlet restrictions, such that the entrance pressure drop dominates over channel-scale resistance. This design stabilizes flow uniformity across parallel channels and maintains performance robustness under gradual fouling or membrane aging. Mechanical integrity of the narrow flow channels is ensured through uniform compression of each cassette using torque-controlled tie rods. This approach maintains channel geometry, prevents bypass flow, and preserves electrical and hydraulic symmetry across the stack. An electrode rinse system (ERS) is installed at the terminal ends of each DC string, enabling gas-free, low-resistance electrode operation without interfering with membrane hydraulics, consistent with best practices for large-scale RED systems. Under nominal operating conditions, the integrated hydraulic architecture maintains laminar flow throughout the membrane channels, limits stack pressure drop to ΔP ≤ 0.3–0.4 bar per side, and effectively suppresses shunt currents. These features enable stable operation of the full 25,000-cell-pair assembly under the extreme salinity gradients characteristic of the Aral Sea brine–wastewater system. Because detailed cost breakdowns distinguishing monolithic and modular RED stack hardware are not yet available in the peer-reviewed literature, the incremental cost associated with frames, segmented manifolds, hydraulic connectors, and electrode-rinse interfaces must be treated as an engineering assumption rather than a reported quantity. Existing techno-economic analyses consistently show that ion-exchange membranes dominate total RED stack CAPEX, while auxiliary mechanical hardware contributes a comparatively minor fraction [ 21 – 24 ]. On this basis, the capital cost of the modular stack is expressed as: where f mod =0.05–0.10represents a conservative markup accounting for additional frames, segmented manifolds, and module connectors relative to a single monolithic stack. This range is consistent with standard cost-engineering practice for conceptual-level designs, where ± 5–10% adjustments are commonly used to capture variations in mechanical hardware complexity. Because membrane costs overwhelmingly dominate total stack CAPEX, the modularization factor exerts only a modest influence on overall capital cost and LCOE; sensitivity analysis confirms that variations within this range do not materially affect the techno-economic conclusions of the study. 3.5. Scaling, fouling, and operational constraints Hypersaline brines enriched in calcium, sulfate, and magnesium are inherently susceptible to mineral scaling—particularly gypsum precipitation—under concentration, mixing, or temperature variation. This risk is especially relevant for spacerless RED architectures employing narrow inter-membrane channels (≈ 100 µm), where even limited mineral deposition can increase hydraulic resistance or induce partial channel blockage [ 25 ]. In the present configuration, controlled post-stack mixing of hypersaline brine with treated wastewater substantially reduces bulk ion concentrations relative to the incoming feed, mitigating the likelihood of large-scale precipitation under nominal operating conditions. Nevertheless, localized concentration polarization and seasonal thermal variability may promote transient scaling at membrane surfaces. Treated municipal wastewater is therefore represented as a NaCl-equivalent diluate to isolate electrochemical feasibility and upper-bound membrane-level performance, while upstream pretreatment is explicitly assumed to address fouling and scaling risks associated with real effluents containing organic matter, nutrients, and suspended solids. The associated capital and operational costs of pretreatment are incorporated into the techno-economic model, ensuring that reported LCOE values are not based on idealized feedwater assumptions. Residual long-term performance degradation is captured through membrane replacement frequency, which emerges as the dominant economic sensitivity. Although modular hydraulic zoning mitigates residence time and concentration polarization, it does not eliminate fouling and scaling risks entirely. Consequently, active fouling and scaling control, already reflected in pretreatment and operational cost assumptions, remains an essential requirement for sustained real-world deployment of spacerless RED systems in hypersaline inland environments [ 26 ]. 3.6 Techno-economic interpretation beyond electricity cost The levelized cost of electricity reported here should be interpreted as a joint cost of electricity generation and brine remediation, rather than as a standalone metric of power production. In the absence of energy recovery, management of hypersaline brines in inland basins typically relies on evaporation ponds, controlled dilution, or long-term containment, approaches that incur substantial capital and operating costs while generating no recoverable value. Published assessments of evaporation pond systems in arid and semi-arid regions report costs on the order of tens of euros per cubic meter of managed brine, once land acquisition, lining, seepage control, and long-term maintenance are accounted for, in addition to externalities related to land use and dust emissions. When these avoided remediation costs are implicitly allocated to the electricity produced by the RED system, a significant fraction of the apparent LCOE reflects environmental service provision rather than inefficiency of energy conversion. Moreover, even localized salinity reductions such as the approximately 50% per-pass dilution achieved under the baseline configuration—can create zones compatible with biological activity. In the Aral Sea context, historical experience demonstrates that restoration of limited areas with near-normal salinity was sufficient to sustain fisheries, fish processing, and associated economic activity prior to ecosystem collapse. While basin-scale recovery lies beyond the scope of this study, the creation of economically active, localized low-salinity zones represents a tangible co-benefit that is not captured by conventional LCOE metrics but is central to water–energy nexus planning and regional development strategies. 3.7 Policy relevance and implementation pathways Beyond its technical performance, the proposed reverse electrodialysis framework has direct relevance for water, energy, and environmental policy in hypersaline regions. By simultaneously delivering electricity generation and measurable brine mitigation, the system aligns with policy instruments that recognize co-benefits across resource sectors. In inland basins such as the Aral Sea, RED could be integrated into existing wastewater treatment or brine-handling infrastructure as a modular, incremental intervention, avoiding the need for basin-scale engineering projects. From a governance perspective, the technology is well suited to pilot-driven adoption, where localized deployment can be supported through avoided-cost accounting, environmental service credits, or inclusion within circular water–energy strategies. Because performance and economic viability depend strongly on site-specific ionic chemistry, the modeling framework applied here enables planners and regulators to screen candidate basins, design evidence-based demonstrations, and prioritize investment. In this sense, RED represents a policy-relevant pathway for translating salinity-gradient energy from experimental concept to regulated infrastructure in water-stressed, hypersaline inland environments. 4. Conclusions This study demonstrates that reverse electrodialysis can operate as an energy-positive pathway for managing hypersaline inland waters when evaluated under realistic chemical, hydraulic, and climatic conditions. Applied to the South Aral Sea basin, the system achieves seasonal energy-conversion efficiencies of 8.8–19.2%, with an annual average of 14.4%, governed by temperature-dependent ionic activities and conductivities representative of multivalent-ion-rich brines rather than idealized salinity assumptions. Beyond electricity generation, the process delivers a substantial first-pass reduction in bulk brine salinity (~ 50%), positioning RED as a coupled energy-generation and brine-mitigation technology rather than a standalone power source. Techno-economic sensitivity analyses indicate that membrane spacing and membrane lifetime dominate system performance and cost, while pump efficiency plays a comparatively secondary role, underscoring the primacy of electrochemical and geometric design choices in hypersaline environments. Although the resulting levelized cost of electricity exceeds that of mature renewable technologies when interpreted strictly as a power-generation metric, it should be understood as a joint cost of electricity production and environmental remediation. By internalizing brine-management services that are otherwise energy-intensive and externally managed, RED offers a differentiated value proposition within the water–energy nexus. In regions such as the Aral Sea basin, even localized salinity reduction can enable ecological and economic activity, highlighting the potential role of RED as a modular, incremental intervention within broader restoration and resource-management strategies. Taken together, these results indicate that reverse electrodialysis represents a technically feasible, seasonally robust, and policy-relevant option for valorizing hypersaline inland waters. While basin-scale restoration remains beyond the scope of this work, the framework and findings presented here provide a transferable basis for evaluating energy-positive brine management in climate-stressed regions worldwide. Declarations Conflict of interest disclosure The authors declare that a utility model application related to the Aral Sea–specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results. Competing Interests The authors declare that a utility model application related to the Aral Sea–specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results. Funding statement This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Author Contribution M.R. Babaa: Conceptualization, Methodology, Supervision, Formal analysis, Writing – original draft, Writing – review & editing. Otman Abida: Conceptualization, review . M. Mamadiyev: Formal analysis (preliminary calculations). N. Kholamatova: Formal analysis (preliminary calculations). O Atabaev: Validation (verification), Formal analysis (sensitivity analysis). Acknowledgement The authors sincerely acknowledge Vice Rector Bakhtiyar Yuldashev for his leadership and continued support of clean energy and environmental initiatives in Uzbekistan. His commitment to fostering research aligned with sustainable water and energy solutions has been essential in enabling this work and advancing environmentally responsible innovation in the region. References Micklin, P. P. The Aral Sea disaster. Annu. Rev. Earth Planet. Sci. 35, 47–72 (2007). Whish-Wilson, P. The Aral Sea environmental health disaster. Geogr. Rev. 92, 438–449 (2002). Rudenko, L. G. Ecological disaster in the Aral Sea region: Problems and solutions. in Water Resources of Central Asia: Past, Present, and Future (eds. Tsytsenko, A. V. & Gusev, D. A. V.) 235–250 (Springer, 2015). Plotnikov, V. V., Yessengaliyev, A. G., Aliyeva, V. S. & Plotnikov, Y. V. Assessment of the environmental state of the Aral Sea and its basin after the collapse of the Soviet Union. IOP Conf. Ser. Earth Environ. Sci. 1180, 012013 (2023). Post, J. W. et al. Towards implementation of reverse electrodialysis for power generation from salinity gradients. Desalination Water Treat. 16, 182–193 (2010). Vermaas, D. A., Saakes, M. & Nijmeijer, K. Power generation using profiled membranes in reverse electrodialysis. J. Membr. Sci. 385–386, 234–242 (2011). Tedesco, M., Hamelers, H. V. M. & Biesheuvel, P. M. Effect of ion exchange membrane thickness on performance of electrodialysis and reverse electrodialysis. Desalination 477, 114250 (2020). Vermaas, D. A., Saakes, M. & Nijmeijer, K. Double power densities from salinity gradients at reduced intermembrane distance. Environ. Sci. Technol. 45, 7089–7095 (2011). Asian Development Bank. Western Uzbekistan Water Supply System Development Project: Report and Recommendation of the President (Project No. 50259-002, ADB, 2018). Karleuša, B., Bolf, N. & Tadić, L. Assessment of wastewater inflow amount for urban drainage system modeling. Građevinar 72, 949–957 (2020). Saeed, M. K. & Al-Hassoun, S. Forecasting and analysis of water demand and sewage flow in the city of Dammam, Saudi Arabia. J. Clean. Prod. 273, 122858 (2020). Cureau, R. J. & Ghisi, E. Reduction of potable water consumption and sewage generation on a city scale: A case study in Brazil. Water 11, 2351 (2019). Shrestha, S., Babel, M. S. & van Veldhuizen, G. Water demand analysis and forecasting for Kathmandu, Nepal. Water Resour. Manage. 22, 457–476 (2008). Spellman, F. R. Handbook of Water and Wastewater Treatment Plant Operations (3rd edn, CRC Press, 2013). Hossen, E. H. et al. Temporal variation of power production via reverse electrodialysis using coastal North Carolina waters and its correlation to temperature and conductivity. Desalination 491, 114562 (2020). Andrulionis, N. Y., Zavialov, P. O. & Izhitskiy, A. S. Modern evolution of the salt composition of the residual basins of the Aral Sea. Oceanology 62, 30–45 (2022). Ciofalo, M. et al. Optimization of net power density in Reverse Electrodialysis. Energy 181, 576–588 (2019). Wang, L. et al. Techno-economics of multi-stage reverse electrodialysis for blue energy harvesting. Carbon Neutrality 3, 12 (2024). Caldera, U., Bogdanov, D. & Breyer, C. Desalination costs using renewable energy technologies. in Renewable Energy Powered Desalination Handbook (ed. Gude, V. G.) 287–329 (Elsevier, 2018). O’Hayre, R. P. et al. Salinity gradient energy recovery. U.S. Patent Application No. 2011/0086291 A1 (2011). Avci, A. H. et al. Reverse electrodialysis for salinity gradient power generation: A review on process fundamentals, membrane properties, and system design. Renew. Sustain. Energy Rev. 133, 110287 (2020). Papapetrou, M., Kosmadakis, G., Cipollina, A. & Micale, G. Towards the techno-economic evaluation of reverse electrodialysis systems for power generation. Desalination 447, 117–129 (2019). Yip, N. Y., Vermaas, D. A., Nijmeijer, K. & Elimelech, M. Thermodynamic, energy efficiency, and power density analysis of reverse electrodialysis power generation with natural salinity gradients. Environ. Sci. Technol. 50, 12072–12080 (2016). Patel, S. K., Platek-Mielczarek, M., Tedesco, M. & Cipollina, A. Techno-economic assessment of large-scale reverse electrodialysis systems for inland hypersaline applications. Desalination 565, 116836 (2024). Giacalone, F. et al. Application of reverse electrodialysis to site-specific types of saline solutions: A techno-economic assessment. Energy 181, 532–547 (2019). Platek-Mielczarek, A. et al. Scalable and highly efficient reverse electrodialysis stack based on porous and nonporous membranes. ACS Appl. Mater. Interfaces 15, 48826–48837 (2023). Additional Declarations Competing interest reported. The authors declare that a utility model application related to the Aral Sea–specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8453844","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":566746305,"identity":"3dbcee8e-e581-4190-8797-594128d6b287","order_by":0,"name":"Moulay-Rachid 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16:38:24","extension":"html","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":91247,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8453844/v1/34246942cf2de71000505608.html"},{"id":99318577,"identity":"1d69ec93-9b9a-474c-b550-2cd93e486250","added_by":"auto","created_at":"2025-12-31 16:33:39","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":183303,"visible":true,"origin":"","legend":"\u003cp\u003eShrinkage of the Aral Sea (1960–2014).Source: britannica.com/place/Aral-Sea. (\u003cem\u003e1): South Aral Sea, (2): North Aral Sea\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Picture1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8453844/v1/bb63a9851f57601668a87eea.jpg"},{"id":99260466,"identity":"018b4752-9fb6-4aee-8f42-0404fdc65cff","added_by":"auto","created_at":"2025-12-31 01:18:24","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":200847,"visible":true,"origin":"","legend":"\u003cp\u003eSatellite image depicting the location of Muynak in relation to the southern Aral Sea.\u003c/p\u003e","description":"","filename":"Picture2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8453844/v1/4a16b9463b16c5c63e98036d.jpg"},{"id":99789054,"identity":"7ffa7acb-b191-4eec-b460-c4d7abeb224a","added_by":"auto","created_at":"2026-01-08 12:48:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1218044,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8453844/v1/50b3dcdc-65d1-4adb-ae37-18d8a88c1d9f.pdf"}],"financialInterests":"Competing interest reported. The authors declare that a utility model application related to the Aral Sea–specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results.","formattedTitle":"Energy-positive brine management in the Aral Sea: a techno-economic water–energy nexus framework based on reverse electrodialysis","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eInland hypersaline water bodies are expanding globally due to climate change, water abstraction, irrigation return flows, and industry. Once formed, these systems pose persistent challenges: extreme salinity, ecological degradation, and limited remediation options. Management strategies like evaporation ponds, forced dilution, or containment consume energy but yield no recoverable value. This reinforces the idea that hypersaline brines are environmental liabilities, not resources.\u003c/p\u003e \u003cp\u003eOnce the fourth-largest lake in the world, the Aral Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) disaster is regarded as one of the most severe ecological catastrophes of the twentieth and twenty-first centuries. It was driven primarily by large-scale river diversion for irrigation in Central Asia and represents a paradigmatic case of anthropogenic hypersalinization with profound regional socio-environmental impacts [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. While partial stabilization has been achieved in the northern basin, the southern basin continues to evolve toward extreme salinity, with few viable management pathways.\u003c/p\u003e \u003cp\u003eSalinity-gradient energy offers a route to recover the Gibbs free energy released during the mixing of waters with different salinities [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Among available technologies, reverse electrodialysis (RED) converts ionic chemical potential differences into electrical power using alternating cation- and anion-exchange membranes [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Over the past decade, RED has progressed from laboratory studies to pilot-scale demonstrations, primarily in coastal seawater\u0026ndash;river water configurations [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, most studies approximate feed waters as ideal or NaCl-only solutions, an assumption that breaks down in hypersaline inland systems dominated by multivalent ions. Such brines exhibit strong non-ideal behavior due to complex ion\u0026ndash;ion interactions involving species such as Mg\u0026sup2;⁺, Ca\u0026sup2;⁺, and SO₄\u0026sup2;⁻. These interactions substantially affect ionic activity, conductivity, and membrane transport, rendering simplified thermodynamic treatments inadequate (Pitzer). As shown later in this work, explicitly resolving multivalent ion interactions reduces the effective NaCl-equivalent activity of the Aral Sea brine to approximately 1.0\u0026ndash;1.1 mol kg⁻\u0026sup1;, substantially lower than would be inferred from concentration-only or ideal-solution assumptions.\u003c/p\u003e \u003cp\u003eHere, we present the first system-level evaluation of reverse electrodialysis in a hypersaline inland basin that explicitly resolves multi-ion non-ideality, seasonal temperature variability, hydraulic constraints, and techno-economic performance within a single coherent framework, applied to the South Aral Sea. By integrating rigorous activity-coefficient modeling, hydraulically constrained stack design, and techno-economic sensitivity analysis, we address three central questions:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCan RED operate efficiently under extreme multi-ion salinity conditions?\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCan energy generation be intrinsically coupled with meaningful brine salinity reduction?\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eIs the resulting system economically robust when evaluated within a water\u0026ndash;energy nexus context?\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFramed within the water\u0026ndash;energy nexus, this study addresses the dual challenge of managing hypersaline water and generating sustainable energy. Rather than treating water treatment and energy production as separate optimization problems, we examine their intrinsic coupling through salinity-gradient energy conversion. By integrating multi-ion thermodynamics, electrochemical performance, and techno-economic assessment, the work provides a transferable framework for evaluating energy-positive water-management pathways in climate-stressed regions. Importantly, the framework explicitly accounts for seasonal temperature variability, enabling ionic activity, conductivity, electrochemical efficiency, and techno-economic performance to be evaluated consistently across winter, spring, summer, and autumn operating conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study area, system boundary and baseline configuration\u003c/h2\u003e \u003cp\u003eMuynak is one of the six districts in Karakalpakstan (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), Uzbekistan, where the government and international partners are pursuing significant improvements in water and wastewater treatment infrastructure. Historically, Muynak was a port city on the Aral Sea, but due to the sea's dramatic shrinkage, Muynak is now far from any significant surface water body. While major efforts and documented projects in the region focus on upgrading water supply systems, construction and modernization of sewage and sanitation facilities, there is less specific information about a dedicated large-scale Muynak wastewater treatment plant currently in operation. Recent capacity expansions in Muynak include the construction and rehabilitation of water distribution centers and significant new pipeline networks designed to meet a growing population's needs [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. The benchmark water consumption per person in the project is around 100 liters per day, and Muynak\u0026rsquo;s agglomeration (including its urban center and rural surroundings) was projected to serve a population of about 30,000 after the upgrades. By simple calculation, the current total water consumption of the Muynak agglomeration is approximately 35 liters per second under standard conditions, though actual flows may fluctuate based on seasonal, technical, and operational factors. It is assumed that 80% of water consumption (supply) returns as sewage flow is a standard practice in municipal water and wastewater modeling [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. This figure, often referred to as the \"sewage generation coefficient\" or \"wastewater return coefficient\" of 0.8, adopted from authoritative engineering manuals and validated through numerous regional studies for initial planning purposes [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eSince groundwork and network infrastructure for future waste water treatment plant (WWTP) operation are still being advanced in Muynak, the WWTP outflow was estimated taking into account the typical range of water losses between the inlet and outlet of municipal wastewater treatment plants. Spellman [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] states that evaporation and process losses for conventional municipal WWTPs typically range from 3% to 7% under standard conditions. Given the arid climate in Muynak, we assume a total water loss of 7% which gave an outflow of 26 L/s that will be considered as the inlet flow rate to our RED facility. The temperature fluctuations between summer and winter significantly affect the efficiency of the RED system; the decrease in temperature is detrimental to the power output [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Based on these premises, four weather operating temperatures have been approached based on the metrological conditions reflecting the four seasons.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe system comprises a reverse electrodialysis (RED) stack supplied with hypersaline brine as the high-salinity stream and treated municipal wastewater (TWW) as the low-salinity stream. The analysis boundary includes the RED stack, hydraulic pumping, power conditioning, and membrane replacement, while excluding upstream wastewater treatment and long-distance conveyance infrastructure.\u003c/p\u003e \u003cp\u003eBaseline volumetric flow rates are:\u003c/p\u003e \u003cp\u003eQ\u003csub\u003eH\u003c/sub\u003e = Q\u003csub\u003eL\u003c/sub\u003e = 0.026 m\u003csup\u003e3\u003c/sup\u003e.s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003cp\u003ecorresponding to equal brine and TWW processing.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Feedwater characterization\u003c/h2\u003e \u003cp\u003eThe hypersaline feed was represented using measured ionic concentrations from the South Aral Sea, expressed on a molality basis to ensure internal consistency across seasonal conditions [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The brine is dominated by Cl⁻ and SO₄\u0026sup2;⁻ anions and Na⁺, Mg\u0026sup2;⁺, and Ca\u0026sup2;⁺ cations, reflecting the strongly multivalent character typical of inland hypersaline basins. Treated municipal wastewater was represented as a low-salinity electrolyte with a total dissolved salt concentration of CL\u0026thinsp;=\u0026thinsp;26 mol m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e, consistent with typical secondary effluent values. To account for uncertainty in local wastewater composition, a\u0026thinsp;\u0026plusmn;\u0026thinsp;30% sensitivity around this reference effluent salinity was evaluated in the techno-economic model.\u003c/p\u003e \u003cp\u003eBecause of the high ionic strength and multicomponent composition of the hypersaline feed, solution non-ideality was explicitly accounted for when defining feedwater properties. Effective ionic activities and conductivities for both the hypersaline brine and the treated wastewater were obtained from standard thermodynamic reference calculations for saline waters and dilute electrolytes, respectively. These properties were treated as prescribed physicochemical inputs to the RED model. Details of the reference calculations and data sources are provided in the Supplementary Information.\u003c/p\u003e \u003cp\u003eSeasonal temperature variability in the Aral Sea basin\u0026mdash;ranging from near \u0026minus;\u0026thinsp;3\u0026deg;C in winter to above 27\u0026deg;C in summer, was incorporated by evaluating feedwater activities and conductivities at representative seasonal average temperatures. Across this range (270\u0026ndash;300 K), effective brine activities varied from 1.005 to 1.105 mol kg⁻\u0026sup1;, with corresponding conductivities of 10.8\u0026ndash;23.0 S m⁻\u0026sup1;, while treated wastewater activities ranged from 0.0204 to 0.0224 mol kg⁻\u0026sup1; with conductivities of 0.127\u0026ndash;0.270 S m⁻\u0026sup1;. Using seasonally resolved feedwater properties ensures that RED performance and techno-economic outcomes are evaluated consistently under realistic climatic conditions relevant to the South Aral Sea.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 RED stack configuration and efficiency definition\u003c/h2\u003e \u003cp\u003eThe RED system consists of alternating cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs) arranged in a spacerless configuration. Commercially available, PET-reinforced ion-exchange membranes with high permselectivity (\u0026gt;\u0026thinsp;90%) and low area resistance were selected as representative of current industrial practice. Concentrate (Aral Sea brine) and diluate (treated wastewater) streams flow through alternating compartments, while electrode rinse compartments containing a ferri/ferrocyanide redox couple complete the electrical circuit.\u003c/p\u003e \u003cp\u003eChannel thickness was fixed at 100 \u0026micro;m for all base-case simulations, consistent with experimentally validated ranges reported in the RED literature. Superficial flow velocity in both concentrate and diluate channels was set to 2 cm s⁻\u0026sup1;, placing operation within the laminar regime and minimizing concentration polarization while maintaining moderate hydraulic losses [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The number of membrane cell pairs was determined by the available wastewater flow rate and the imposed velocity constraint. Detailed geometric parameters and stack sizing calculations are provided in Supporting Information .\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Electrochemical, hydraulic, and brine-dilution modeling\u003c/h2\u003e \u003cp\u003eElectrochemical performance of the reverse electrodialysis (RED) system was evaluated using a standard Nernst-type formulation, corrected for membrane permselectivity and finite internal electrical resistance. Feedwater properties, including ionic activities and solution conductivities, were treated as prescribed inputs and evaluated at representative seasonal temperatures, as described in Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e2.2\u003c/span\u003e. The open-circuit voltage and operating voltage were computed from the activity difference between the hypersaline brine and treated wastewater streams in combination with membrane characteristics.\u003c/p\u003e \u003cp\u003eInternal electrical resistance was calculated as the sum of membrane resistances and solution resistances in the concentrate and diluate channels, based on channel geometry and the corresponding solution conductivities. Hydraulic performance was evaluated assuming fully developed laminar flow between parallel plates in spacerless channels. Pressure drops were determined from channel dimensions, fluid viscosity, and flow velocity, and the associated pumping power requirements were computed accordingly. The electrochemical and hydraulic models were coupled to determine net power output and conversion efficiency under steady-state operating conditions.\u003c/p\u003e \u003cp\u003eIn parallel, the brine salinity reduction per pass was defined as the decrease in bulk salt concentration following a single traversal of the RED stack and complete post-stack mixing of the brine and treated wastewater effluents. Under steady-state operation, the mixed outlet concentration was obtained from a mass balance on the two outlet streams. For the baseline configuration with equal volumetric flow rates (Q\u003csub\u003eH\u003c/sub\u003e=Q\u003csub\u003eL\u003c/sub\u003e), this first-pass mixing results in an approximately 50% reduction in bulk brine salinity.\u003c/p\u003e \u003cp\u003eAll governing equations, geometric parameters, and numerical inputs used in the electrochemical, hydraulic, and mass-balance calculations are reported in the Supporting Information.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Techno-economic and sensitivity analysis\u003c/h2\u003e \u003cp\u003eEconomic performance was assessed using a transparent, bottom-up techno-economic framework based on the Reverter Cost Method. Capital expenditures include the RED stack (membranes and electrodes), intake and outfall infrastructure, pre-treatment, pumps, piping, power electronics, and civil and electrical works. Operating expenditures include fixed operation and maintenance costs, pumping energy, and membrane replacement. A plant lifetime of 30 years, region-appropriate discount rates, depreciation, and residual value assumptions were applied [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, the cost of grid-connected power inverters was estimated using a market-calibrated scaling approach based on commercially available utility-scale photovoltaic inverters. Rather than relying on a single vendor quotation, a representative dataset of inverters in the 50\u0026ndash;250 kW class was assembled from multiple European suppliers, including new and refurbished units, with prices reported both inclusive and exclusive of value-added tax (VAT). The dataset spans a wide range of manufacturers (e.g., SMA, Huawei, Sungrow) and market channels, ensuring that the resulting cost model reflects realistic procurement conditions rather than optimized or promotional pricing.\u003c/p\u003e \u003cp\u003eFor each inverter, the specific cost (\u0026euro;/kW) was calculated from the nameplate power rating and advertised price. As expected for power electronics, the specific cost decreases with increasing capacity, reflecting economies of scale in semiconductor utilization, housing, and auxiliary components. To capture this trend quantitatively, inverter cost was fitted using a power-law scaling of the form:\u003c/p\u003e \u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/69519_bce2c0439cd956a6/69519_custom_files/img1767008682.png\" style=\"width: 96px;\"\u003e \u003c/p\u003e \u003cp\u003eWhere C\u003csub\u003einv\u003c/sub\u003e​is the inverter capital cost (\u0026euro;), P is the rated electrical power (kW), and α and β are empirical coefficients. A least-squares fit to the median market prices yielded α\u0026thinsp;=\u0026thinsp;1156.6 and β\u0026thinsp;=\u0026thinsp;0.333, consistent with scaling exponents commonly reported for balance-of-plant power electronics in renewable energy systems.\u003c/p\u003e \u003cp\u003eThis scaling law was used to estimate inverter CAPEX for the RED installation based on the installed electrical power requirement, with an additional installation and integration factor applied to account for cabling, protection devices, and control hardware. Importantly, the use of a market-derived scaling relationship avoids underestimation associated with single-vendor quotations and ensures that inverter costs remain consistent with contemporary commercial pricing across the relevant power range. Sensitivity of the overall techno-economic results to inverter cost assumptions was found to be minor compared with membrane lifetime and hydraulic losses, confirming that inverter CAPEX does not dominate system economics.\u003c/p\u003e \u003cp\u003eThe levelized cost of electricity (LCOE) was calculated as the ratio of discounted lifetime costs to discounted electricity generation. Sensitivity analyses were conducted for key parameters, including membrane lifetime, membrane spacing, pump efficiency, and diluate salinity. Complete cost breakdowns, assumptions, and sensitivity results are provided in Supporting Information.\u003c/p\u003e \u003cp\u003eTo assess the robustness of the techno-economic conclusions and identify dominant cost drivers, a one-at-a-time sensitivity analysis was conducted on three key operational and design parameters: (i) pump efficiency, (ii) membrane channel spacing, and (iii) membrane lifetime. Each parameter was varied independently around the baseline configuration, while all other inputs were held constant.\u003c/p\u003e \u003cp\u003ePump efficiency was varied between 0.60 and 0.85 to reflect realistic performance ranges for industrial centrifugal pumps operating under low-head, high-flow conditions. For each efficiency value, the associated pumping energy demand was recalculated and propagated through the levelized cost of electricity (LCOE) formulation.\u003c/p\u003e \u003cp\u003eMembrane spacing was varied from 100 to 300 \u0026micro;m to capture the trade-off between electrical resistance, hydraulic losses, and achievable power density. Changes in spacing directly affect solution resistance, number of membrane cell pairs accommodated within a fixed stack volume, and overall RED conversion efficiency.\u003c/p\u003e \u003cp\u003eMembrane lifetime was varied between 1 and 10 years using a membrane replacement factor defined as the inverse of the membrane lifespan. This factor scales annualized membrane replacement costs within the LCOE calculation and represents long-term degradation due to fouling, scaling, and mechanical aging.\u003c/p\u003e \u003cp\u003eThe resulting LCOE values from each sensitivity scenario are reported explicitly and discussed below.\u003c/p\u003e \u003cp\u003eAll sensitivity analyses (pump efficiency, feed salinity, inter-membrane spacing, and membrane lifetime) were performed using reference-condition transport properties evaluated at 25\u0026deg;C. Specifically, ionic activities and solution conductivities were fixed at their standard-temperature values derived from the Pitzer-based thermodynamic model and conductivity framework described above. This approach isolates the influence of individual engineering and economic parameters while avoiding confounding effects from seasonal temperature variability, which is treated separately in the seasonal performance analysis.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussions","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Electrical performance, seasonal robustness, and benchmarking against pilot systems\u003c/h2\u003e \u003cp\u003eSeasonal variations in temperature lead to predictable changes in RED performance, primarily through their influence on ionic mobility, solution conductivity, and internal electrical resistance. Summer operation benefits from higher conductivities and reduced resistive losses, whereas winter operation is constrained by reduced transport and higher overall stack resistance. Despite this variability, the system maintains stable operation across all seasons considered.\u003c/p\u003e \u003cp\u003eAcross the annual temperature range, the net energy-conversion efficiency spans 8.8\u0026ndash;19.2%, with a seasonal average of 14.4%. These values place the system at the upper end of efficiencies reported for RED operation with natural, multivalent-ion-rich waters, where non-ideal solution behavior and increased resistance typically suppress performance. The moderate seasonal spread in efficiency indicates that the spacerless, laminar-flow architecture provides intrinsic robustness to climatic variability, a critical requirement for inland hypersaline environments subject to stronger temperature fluctuations than coastal settings.\u003c/p\u003e \u003cp\u003eUnder favorable thermal conditions, the system achieves peak net power outputs on the order of 45\u0026ndash;50 kW, with corresponding efficiencies approaching\u0026thinsp;~\u0026thinsp;19%. At lower winter temperatures, reduced conductivity leads to lower power output and efficiency, but operation remains stable and energy-positive. Overall, the predicted performance envelope lies within the upper range reported for pilot-scale RED systems operating on natural waters and brines, including the Afsluitdijk demonstration and the REAPower project, where conversion efficiencies of 10\u0026ndash;20% and comparable resistance partitioning have been observed [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Although no new experiments are presented here, the agreement between predicted performance levels and those reported in existing pilot studies indicates that the present analysis remains grounded in experimentally demonstrated RED behavior, rather than extrapolating beyond established operational regimes.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Energy-positive brine dilution and remediation impact\u003c/h2\u003e \u003cp\u003eBeyond electricity generation, the RED system delivers a measurable and operationally relevant reduction in hypersaline brine salinity. Under baseline operation with equal volumetric flow rates of hypersaline brine and treated wastewater, a single pass through the RED stack followed by complete post-stack mixing yields an approximately 50% reduction in bulk brine salinity per pass. This reduction is achieved without external dilution energy input, relying instead on controlled mixing inherent to the RED process.\u003c/p\u003e \u003cp\u003eWhen expressed in operational terms, the system provides a coupled energy\u0026ndash;remediation service: for each 1 MWh of net electricity generated, approximately 230\u0026ndash;260 m\u0026sup3; of hypersaline brine are diluted by 25\u0026ndash;35 g L⁻\u0026sup1; before controlled discharge or reuse. This framing highlights that salinity reduction is not a secondary by-product, but an intrinsic outcome of the energy conversion process.\u003c/p\u003e \u003cp\u003eImportantly, this level of localized salinity reduction addresses one of the principal constraints in managing inland hypersaline waters, where conventional approaches, such as evaporation ponds or forced dilution, consume energy while providing no recoverable value. In contrast, the RED configuration evaluated here converts the salinity gradient into useful electrical power while simultaneously reducing brine salinity, rendering the process energy-positive at the system level.\u003c/p\u003e \u003cp\u003eFrom a regional perspective, sustained operation of such a system could enable the progressive creation of localized zones of moderated salinity, particularly in near-shore or managed discharge regions. While full basin-scale restoration is beyond the scope of a single installation, targeted salinity reduction at this scale could support renewed ecological and economic activity, including fisheries and derivative value chains that historically depended on lower-salinity conditions in the Aral Sea region.\u003c/p\u003e \u003cp\u003eOverall, the results demonstrate that RED can function as an integrated component of hypersaline water management, delivering simultaneous electricity generation and brine dilution without the energy penalties associated with conventional remediation strategies. This dual functionality underpins the relevance of RED within a water\u0026ndash;energy nexus framework, particularly for climate-stressed inland basins where both energy scarcity and salinity accumulation pose growing challenges.\u003c/p\u003e \u003cp\u003eThis energy-remediation coupling distinguishes RED fundamentally from conventional hypersaline brine management approaches, such as evaporation ponds or forced dilution, which consume energy without recovering value. Although the resulting salinity reduction is insufficient for basin-scale restoration, it is operationally meaningful in hypersaline environments, where localized decreases in salinity can influence density stratification, ecological thresholds, and nearshore usability. In this sense, RED functions as an energy-positive brine-management pathway, delivering both electricity and controlled salinity reduction within a single integrated system.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Sensitivity analysis\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Sensitivity to pump efficiency\u003c/h2\u003e \u003cp\u003eVarying pump efficiency from 0.60 to 0.85 results in a relatively modest change in LCOE, decreasing from 0.0877 \u0026euro; kWh⁻\u0026sup1; to 0.0815 \u0026euro; kWh⁻\u0026sup1;. This limited sensitivity reflects the fact that hydraulic pumping power constitutes only a minor fraction of the total system energy balance under the low-pressure, laminar-flow conditions employed.\u003c/p\u003e \u003cp\u003eThe weak dependence of LCOE on pump efficiency confirms that the RED system is not hydraulically dominated and that reasonable deviations from nominal pump performance do not compromise economic viability. This finding supports the suitability of the proposed architecture for deployment in regions where high-efficiency pumping equipment may not be readily available or consistently maintained.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.3.2 Sensitivity to membrane spacing\u003c/h2\u003e \u003cp\u003eMembrane spacing exerts a strong influence on system economics and performance. Increasing the channel spacing from 100 \u0026micro;m to 300 \u0026micro;m leads to a sharp increase in LCOE from 0.1659 \u0026euro; kWh⁻\u0026sup1; to 0.3801 \u0026euro; kWh⁻\u0026sup1;, accompanied by a decline in RED efficiency from 19% to 7%.\u003c/p\u003e \u003cp\u003eThis pronounced sensitivity arises from the coupled effects of increased solution resistance and reduced power density at larger spacings. Wider channels reduce pressure drop but significantly degrade electrochemical performance, requiring fewer membrane pairs and yielding lower energy recovery per unit flow. The results demonstrate that spacerless or near-spacerless configurations with narrow channels are essential for achieving economically viable operation in hypersaline RED systems.\u003c/p\u003e \u003cp\u003eAmong all parameters examined, membrane spacing emerges as the single most critical design variable, underscoring the importance of precise channel geometry control and mechanical stability in large-scale implementations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.3.3 Sensitivity to membrane lifetime and replacement rate\u003c/h2\u003e \u003cp\u003eMembrane lifetime has a significant but saturating effect on LCOE. Increasing the membrane lifespan from 1 year to 10 years reduces LCOE from 0.1110 \u0026euro; kWh⁻\u0026sup1; to 0.0854 \u0026euro; kWh⁻\u0026sup1;. The most substantial cost reductions occur within the first few years, with diminishing returns beyond approximately 5 years of operation.\u003c/p\u003e \u003cp\u003eThis behavior reflects the dominance of membrane replacement costs in the early lifecycle, followed by a plateau where other cost components (capital recovery, pumping energy, infrastructure) become limiting. Importantly, even under conservative assumptions of short membrane lifetime (1\u0026ndash;2 years), the system remains economically competitive within the context of combined energy production and brine management.\u003c/p\u003e \u003cp\u003eThese results highlight membrane durability and fouling resistance as high-leverage targets for future materials development, while also demonstrating that the system does not rely on unrealistically long membrane lifetimes to remain viable.\u003c/p\u003e \u003cp\u003eSensitivity trends reported here reflect variations around the 25\u0026deg;C reference condition; absolute performance levels under seasonal operation are discussed separately in Section \u003cspan refid=\"Sec9\" class=\"InternalRef\"\u003e3.1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eTaken together, the sensitivity analysis reveals a clear hierarchy of techno-economic drivers. Membrane spacing and the associated impact on electrochemical efficiency, emerges as the dominant determinant of system performance and cost, followed by membrane lifetime, while pump efficiency plays a comparatively secondary role. This ordering indicates that RED performance in hypersaline environments is governed primarily by electrochemical and geometric constraints rather than by hydraulic efficiency alone.\u003c/p\u003e \u003cp\u003eImportantly, the modest sensitivity of LCOE to operational parameters supports the interpretation of the reported baseline results as representative and robust rather than finely tuned. This robustness reinforces the relevance of RED as a realistic component of integrated water\u0026ndash;energy management strategies in hypersaline inland settings.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Hydraulic and manifold architecture\u003c/h2\u003e \u003cp\u003eThe current reverse electrodialysis (RED) system adopts a channel-integrated membrane architecture in which flow channels are formed directly at the membrane interfaces, eliminating conventional spacers and enabling strictly laminar operation at low pressure drop. This spacerless configuration, previously proposed in modular RED designs such as the [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] concept, minimizes internal electrical resistance while maintaining hydraulic stability under high salinity gradients.\u003c/p\u003e \u003cp\u003eEach modular cassette employs inter-membrane channel gaps of 100 \u0026micro;m, a dimension widely reported as near-optimal for balancing ionic transport resistance against hydraulic losses in laminar RED operation. To further limit pressure drop and suppress axial concentration polarization, each cassette comprising 500 membrane cell pairs is subdivided into three hydraulic zones (approximately 160\u0026ndash;170 cell pairs per zone). This segmentation shortens the effective flow path length per zone and ensures that cumulative pressure losses remain well below the osmotic pressure difference across the membranes, preserving net energy extraction efficiency.\u003c/p\u003e \u003cp\u003eThe hypersaline concentrate (Aral Sea brine) and the low-salinity diluate (treated wastewater) streams are distributed through separate, vertically offset inlet and outlet manifolds. This configuration mitigates ionic short-circuiting along headers and reduces parasitic shunt currents, a known limitation in large-area RED stacks. Flow distribution from plant-level plenums into individual hydraulic zones is regulated using calibrated inlet restrictions, such that the entrance pressure drop dominates over channel-scale resistance. This design stabilizes flow uniformity across parallel channels and maintains performance robustness under gradual fouling or membrane aging.\u003c/p\u003e \u003cp\u003eMechanical integrity of the narrow flow channels is ensured through uniform compression of each cassette using torque-controlled tie rods. This approach maintains channel geometry, prevents bypass flow, and preserves electrical and hydraulic symmetry across the stack. An electrode rinse system (ERS) is installed at the terminal ends of each DC string, enabling gas-free, low-resistance electrode operation without interfering with membrane hydraulics, consistent with best practices for large-scale RED systems.\u003c/p\u003e \u003cp\u003eUnder nominal operating conditions, the integrated hydraulic architecture maintains laminar flow throughout the membrane channels, limits stack pressure drop to ΔP\u0026thinsp;\u0026le;\u0026thinsp;0.3\u0026ndash;0.4 bar per side, and effectively suppresses shunt currents. These features enable stable operation of the full 25,000-cell-pair assembly under the extreme salinity gradients characteristic of the Aral Sea brine\u0026ndash;wastewater system.\u003c/p\u003e \u003cp\u003eBecause detailed cost breakdowns distinguishing monolithic and modular RED stack hardware are not yet available in the peer-reviewed literature, the incremental cost associated with frames, segmented manifolds, hydraulic connectors, and electrode-rinse interfaces must be treated as an engineering assumption rather than a reported quantity. Existing techno-economic analyses consistently show that ion-exchange membranes dominate total RED stack CAPEX, while auxiliary mechanical hardware contributes a comparatively minor fraction [\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. On this basis, the capital cost of the modular stack is expressed as:\u003c/p\u003e \u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/69519_bce2c0439cd956a6/69519_custom_files/img1767008740.png\" style=\"width: 196px;\"\u003e\u003c/p\u003e \u003cp\u003ewhere f\u003csub\u003emod\u003c/sub\u003e=0.05\u0026ndash;0.10represents a conservative markup accounting for additional frames, segmented manifolds, and module connectors relative to a single monolithic stack. This range is consistent with standard cost-engineering practice for conceptual-level designs, where \u0026plusmn;\u0026thinsp;5\u0026ndash;10% adjustments are commonly used to capture variations in mechanical hardware complexity. Because membrane costs overwhelmingly dominate total stack CAPEX, the modularization factor exerts only a modest influence on overall capital cost and LCOE; sensitivity analysis confirms that variations within this range do not materially affect the techno-economic conclusions of the study.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Scaling, fouling, and operational constraints\u003c/h2\u003e \u003cp\u003eHypersaline brines enriched in calcium, sulfate, and magnesium are inherently susceptible to mineral scaling\u0026mdash;particularly gypsum precipitation\u0026mdash;under concentration, mixing, or temperature variation. This risk is especially relevant for spacerless RED architectures employing narrow inter-membrane channels (\u0026asymp;\u0026thinsp;100 \u0026micro;m), where even limited mineral deposition can increase hydraulic resistance or induce partial channel blockage [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In the present configuration, controlled post-stack mixing of hypersaline brine with treated wastewater substantially reduces bulk ion concentrations relative to the incoming feed, mitigating the likelihood of large-scale precipitation under nominal operating conditions. Nevertheless, localized concentration polarization and seasonal thermal variability may promote transient scaling at membrane surfaces.\u003c/p\u003e \u003cp\u003eTreated municipal wastewater is therefore represented as a NaCl-equivalent diluate to isolate electrochemical feasibility and upper-bound membrane-level performance, while upstream pretreatment is explicitly assumed to address fouling and scaling risks associated with real effluents containing organic matter, nutrients, and suspended solids. The associated capital and operational costs of pretreatment are incorporated into the techno-economic model, ensuring that reported LCOE values are not based on idealized feedwater assumptions. Residual long-term performance degradation is captured through membrane replacement frequency, which emerges as the dominant economic sensitivity. Although modular hydraulic zoning mitigates residence time and concentration polarization, it does not eliminate fouling and scaling risks entirely. Consequently, active fouling and scaling control, already reflected in pretreatment and operational cost assumptions, remains an essential requirement for sustained real-world deployment of spacerless RED systems in hypersaline inland environments [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Techno-economic interpretation beyond electricity cost\u003c/h2\u003e \u003cp\u003eThe levelized cost of electricity reported here should be interpreted as a joint cost of electricity generation and brine remediation, rather than as a standalone metric of power production. In the absence of energy recovery, management of hypersaline brines in inland basins typically relies on evaporation ponds, controlled dilution, or long-term containment, approaches that incur substantial capital and operating costs while generating no recoverable value. Published assessments of evaporation pond systems in arid and semi-arid regions report costs on the order of tens of euros per cubic meter of managed brine, once land acquisition, lining, seepage control, and long-term maintenance are accounted for, in addition to externalities related to land use and dust emissions.\u003c/p\u003e \u003cp\u003eWhen these avoided remediation costs are implicitly allocated to the electricity produced by the RED system, a significant fraction of the apparent LCOE reflects environmental service provision rather than inefficiency of energy conversion. Moreover, even localized salinity reductions such as the approximately 50% per-pass dilution achieved under the baseline configuration\u0026mdash;can create zones compatible with biological activity. In the Aral Sea context, historical experience demonstrates that restoration of limited areas with near-normal salinity was sufficient to sustain fisheries, fish processing, and associated economic activity prior to ecosystem collapse. While basin-scale recovery lies beyond the scope of this study, the creation of economically active, localized low-salinity zones represents a tangible co-benefit that is not captured by conventional LCOE metrics but is central to water\u0026ndash;energy nexus planning and regional development strategies.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e3.7 Policy relevance and implementation pathways\u003c/h2\u003e \u003cp\u003eBeyond its technical performance, the proposed reverse electrodialysis framework has direct relevance for water, energy, and environmental policy in hypersaline regions. By simultaneously delivering electricity generation and measurable brine mitigation, the system aligns with policy instruments that recognize co-benefits across resource sectors. In inland basins such as the Aral Sea, RED could be integrated into existing wastewater treatment or brine-handling infrastructure as a modular, incremental intervention, avoiding the need for basin-scale engineering projects.\u003c/p\u003e \u003cp\u003eFrom a governance perspective, the technology is well suited to pilot-driven adoption, where localized deployment can be supported through avoided-cost accounting, environmental service credits, or inclusion within circular water\u0026ndash;energy strategies. Because performance and economic viability depend strongly on site-specific ionic chemistry, the modeling framework applied here enables planners and regulators to screen candidate basins, design evidence-based demonstrations, and prioritize investment. In this sense, RED represents a policy-relevant pathway for translating salinity-gradient energy from experimental concept to regulated infrastructure in water-stressed, hypersaline inland environments.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study demonstrates that reverse electrodialysis can operate as an energy-positive pathway for managing hypersaline inland waters when evaluated under realistic chemical, hydraulic, and climatic conditions. Applied to the South Aral Sea basin, the system achieves seasonal energy-conversion efficiencies of 8.8\u0026ndash;19.2%, with an annual average of 14.4%, governed by temperature-dependent ionic activities and conductivities representative of multivalent-ion-rich brines rather than idealized salinity assumptions.\u003c/p\u003e \u003cp\u003eBeyond electricity generation, the process delivers a substantial first-pass reduction in bulk brine salinity (~\u0026thinsp;50%), positioning RED as a coupled energy-generation and brine-mitigation technology rather than a standalone power source. Techno-economic sensitivity analyses indicate that membrane spacing and membrane lifetime dominate system performance and cost, while pump efficiency plays a comparatively secondary role, underscoring the primacy of electrochemical and geometric design choices in hypersaline environments.\u003c/p\u003e \u003cp\u003eAlthough the resulting levelized cost of electricity exceeds that of mature renewable technologies when interpreted strictly as a power-generation metric, it should be understood as a joint cost of electricity production and environmental remediation. By internalizing brine-management services that are otherwise energy-intensive and externally managed, RED offers a differentiated value proposition within the water\u0026ndash;energy nexus. In regions such as the Aral Sea basin, even localized salinity reduction can enable ecological and economic activity, highlighting the potential role of RED as a modular, incremental intervention within broader restoration and resource-management strategies.\u003c/p\u003e \u003cp\u003eTaken together, these results indicate that reverse electrodialysis represents a technically feasible, seasonally robust, and policy-relevant option for valorizing hypersaline inland waters. While basin-scale restoration remains beyond the scope of this work, the framework and findings presented here provide a transferable basis for evaluating energy-positive brine management in climate-stressed regions worldwide.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of interest disclosure\u003c/h2\u003e \u003cp\u003eThe authors declare that a utility model application related to the Aral Sea\u0026ndash;specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003cp\u003eThe authors declare that a utility model application related to the Aral Sea\u0026ndash;specific energy-efficient brine management and localized restoration system described in this study has been filed. This intellectual property did not influence the study design, analysis, or interpretation of results.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding statement\u003c/h2\u003e \u003cp\u003eThis research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.R. Babaa: Conceptualization, Methodology, Supervision, Formal analysis, Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing. Otman Abida: Conceptualization, review . M. Mamadiyev: Formal analysis (preliminary calculations). N. Kholamatova: Formal analysis (preliminary calculations). O Atabaev: Validation (verification), Formal analysis (sensitivity analysis).\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors sincerely acknowledge Vice Rector Bakhtiyar Yuldashev for his leadership and continued support of clean energy and environmental initiatives in Uzbekistan. His commitment to fostering research aligned with sustainable water and energy solutions has been essential in enabling this work and advancing environmentally responsible innovation in the region.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eMicklin, P. P. The Aral Sea disaster. \u003cem\u003eAnnu. Rev. Earth Planet. Sci.\u003c/em\u003e 35, 47\u0026ndash;72 (2007).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWhish-Wilson, P. The Aral Sea environmental health disaster. \u003cem\u003eGeogr. 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Scalable and highly efficient reverse electrodialysis stack based on porous and nonporous membranes. \u003cem\u003eACS Appl. Mater. Interfaces\u003c/em\u003e 15, 48826\u0026ndash;48837 (2023).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Hypersaline waters, Reverse electrodialysis, Water–energy nexus, Multi-ion thermodynamics, Brine remediation, Seasonal performance, Inland saline basins","lastPublishedDoi":"10.21203/rs.3.rs-8453844/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8453844/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHypersaline inland water bodies are a growing environmental and management challenge. Yet, they remain largely overlooked in the water\u0026ndash;energy nexus. Here, we present a systems-level framework for energy-positive brine management using reverse electrodialysis (RED). Our approach explicitly addresses the complex multi-ion chemistry characteristic of inland hypersaline basins. A Pitzer-based thermodynamic model is coupled with a hydraulically constrained RED stack design and techno-economic analysis. This setup evaluates performance under realistic seasonal conditions.\u003c/p\u003e \u003cp\u003eWe apply this framework to the South Aral Sea basin as a representative case. The system achieves net electrical conversion efficiencies of 8.8\u0026ndash;19.2% across seasonal temperatures. The seasonal average is 14.4%, governed by temperature-dependent ionic activity and conductivity. The effective NaCl-equivalent activity of the hypersaline brine is constrained to 1.00 1.11 mol kg⁻\u0026sup1;, much lower than ideal-solution estimates. With equal brine and treated wastewater flow rates at baseline operation, post-stack effluent mixing yields about a 50% reduction in bulk brine salinity per pass.\u003c/p\u003e \u003cp\u003eThe resulting levelized cost of electricity ranges from \u0026euro;0.16\u0026ndash;0.31 kWh⁻\u0026sup1; across seasons. The annualized average is \u0026euro;0.22 kWh⁻\u0026sup1; when viewed as a joint cost of electricity generation and brine remediation. Sensitivity analyses show membrane spacing and lifetime are key economic drivers. Pump efficiency plays a secondary role. More broadly, the integrated thermodynamic\u0026ndash;electrochemical\u0026ndash;economic workflow is transferable to other hypersaline lakes and engineered brine systems. It provides a physically consistent basis for evaluating RED as a coupled energy-generation and brine-management technology under real multi-component water chemistries.\u003c/p\u003e","manuscriptTitle":"Energy-positive brine management in the Aral Sea: a techno-economic water–energy nexus framework based on reverse electrodialysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-31 01:18:19","doi":"10.21203/rs.3.rs-8453844/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"17d1728d-7d97-412f-b5c3-f532a5952d2a","owner":[],"postedDate":"December 31st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":60320120,"name":"Physical sciences/Energy science and technology"},{"id":60320121,"name":"Physical sciences/Engineering"},{"id":60320122,"name":"Earth and environmental sciences/Environmental sciences"},{"id":60320123,"name":"Earth and environmental sciences/Hydrology"}],"tags":[],"updatedAt":"2026-01-09T08:32:07+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-31 01:18:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8453844","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8453844","identity":"rs-8453844","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}