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The valorization of ultra-concentrated seawater brines, named bitterns, requires preliminary purification processes, such as membrane filtration, before they can be fully exploited. This study investigates the performance of an ultrafiltration pilot plant aimed at separating organic matter and large particles from real bitterns. An empirical model for the bittern viscosity was developed to better characterize the membrane. Distinct variations in permeability, fouling resistance and rejection coefficient were observed under operational pressures ranging from 2 to 4 bar. Working at low pressure (2 bar), the pilot plant achieves permeability and rejection coefficient values of 17 L/m2hbar and 95%, respectively. Foulant behavior was characterized by determining a “fouling resistance”, obtaining an average value of 1013 m−1. Tests with three distinct bittern samples were conducted to assess the influence of chemical composition and organic matter content on membrane permeability and fouling characteristics. The collected data enabled a comprehensive characterization of the ultrafiltration pilot unit working with this particular saline feed solution, which has very high technical–economic potential.

This article presents an experimental validation of modeling approaches for the AB-FB battery, an innovative technology with significant potential for large-scale energy storage applications. The results demonstrate, through experimental analyses that a simplified zero-order equivalent circuit model (ECM) provides adequate accuracy in predicting the electrical behavior of AB-FB cells, particularly in scenarios requiring constant current. In comparison, the more complex electrochemical model, while offering detailed insights into material and electrolyte interactions, is excessively intricate for the sole purpose of describing electrical behavior. Additionally, the study confirms that the discharge characteristics of the AB-FB align with those of established battery technologies, indicating familiarity with its operational principles, mitigating technical risks associated with large-scale deployment. The main contribution is therefore the assessment of the potential use of this technology and the possibility of modeling it with already established approaches.

Electrodialysis with Bipolar Membranes (EDBM) is a promising process to produce acidic and basic solutions that are important in circular treatment chains. However, there is a lack of knowledge regarding the long-run performance of EDBM units fed multi-ionic solutions. This study aims to fill this gap by performing a long-run test, via an innovative feed and bleed configuration, with real brines coming from Trapani (Italy) saltworks and pre-treated to remove Mg and Ca. A laboratory EDBM unit with 5 triplets with an active membrane area of 100 cm2 was used. The results show that a steady-state concentration of 0.44 mol/L on average of OH- and H+ (in the form of NaOH and HCl) was reached in both acid and base outlet solutions, respectively, with a maximum deviation of 2 %. Stable performances were observed with a current efficiency of 69 % and 56 % and specific energy consumption of 1.78 kWh/kg and 2.19 kWh/kg on average for base and acid, respectively. These results demonstrate the potential of EDBM processes to maintain competitive and efficient performance.

During the crude oil extraction processes, for each barrel of oil turns out an equivalent of 3 barrels of wastewaters on average. These wastes are known as Produced Waters (PWs) and their dramatic impact on the environment has attracted the attention of researchers in order to find an economic and efficient method for their treatment. Dealing with PWs is not easy: the long exposure with oil increases their hydrocarbon fraction, while the contact with the underground wells increases their concentration in salts and minerals. The direct discharge of PWs into the sea is obviously not allowed by law and PWs are usually re-injected into the well. The present work deals with a novel and innovative treatment chain (including assisted reverse electrodialysis (ARED) as dilution step) able to reduce both the salinity and organic content of PWs. The innovative scheme includes an ultrafiltration unit as pre-treatment, upstream an ARED unit for the PW dilution. Once the salinity level has been reduced down to a value affordable for a bioremediation step, PWs are sent to a bio-reactor, where the organic compounds are digested. Finally, a reverse osmosis unit is used to recover water from the treated PWs and to recycle it as diluted stream in the ARED unit. A techno-economic model was purposely developed in the present work to assess the economic feasibility of the proposed scheme. Preliminary results suggest that the treatment costs are lower than 5 € m-3PW and fully competitive with current PWs treatment technologies.

The evaporative crystallization of minerals in saltworks is one of the oldest industrial processes still in operation and has the potential to achieve high solar energy conversion efficiencies. While salt production from seawater has been extensively studied, the circular economy of brines exploitation is opening new fields of investigation. This article focuses on the design and simulation of a process for selective precipitation and mineral recovery from reverse osmosis brines. The process involves a two-step evaporative system followed by reactive crystallization of magnesium hydroxide. Laboratory tests and simulations using PHREEQC (pH-REdox-Equilibrium C-program) were conducted to validate the thermodynamics of the process and forecast the optimal operative conditions.

Nowadays scientists and communities are particularly worried about climate change and the transition towards renewable energies is a matter of crucial importance. In this context, European Community is strongly promoting the production of green hydrogen, i.e. the electrolysis of water coupled with renewable electrical sources. In addition to the energy issue, United Nations have identified the lack of water as another big issue for modern society. Electrodialysis (ED) is an electro-membrane process able to desalinate a salty feed by the application of an external power supply. It takes advantage of the use of ionic exchange membranes which allow a controlled separation of ions from the salty feed to obtain freshwater and brine as outputs. The present work tries to address the above issues by proposing the simultaneous production of fresh water and hydrogen with an integrated hydrogen-electrodialysis (HED) unit. The aim of the present work is that of assessing the economic feasibility of the process. To this purpose, a techno-economic model has been developed to predict the behaviour of the HED unit. The model is able to predict experimental data and should be regarded as a simple yet reliable tool to assess the process economic feasibility. Preliminary analyses suggest that the simultaneous production of hydrogen and fresh water may be profitable.

Electrodialysis with bipolar membranes (EDBM) has been recently proposed as a promising strategy for the valorization of desalination waste brine. This process enables the production of chemicals (i.e., acids and bases) starting from a salty stream via the application of an electric field. Several process configurations have been reported in the literature, operating both in continuous and discontinuous mode. The continuous feed & bleed configuration has resulted the best option for the process scale-up. In this work, a performance evaluation of a large scale EDBM unit (19 m2 of total membrane area) operated in feed & bleed mode is presented. Time profile of acid and base concentrations as well as of voltage applied to the stack are reported, thus proving the stability of the process in galvanostatic condition. Four tests have been conducted at four different current density (200- 500 A m-2) and the results have been analysed utilizing appropriate performance parameters (i.e., Current Efficiency, Specific Energy Consumption and Specific production). Results suggest that increasing the current density, the acid concentration rises at fixed base concentration of 1 mol l-1. Furthermore, increasing the current density, the current efficiency of base remains fairly constant (60-63 %), while for acid a growth is observed. Specific energy consumption and specific production show an increasing trend with the current density: at 500 A m-2, values of 3.6 and 2.7 kWh kg-1 and 0.94 and 1.3 ton y-1 m-2 were obtained, respectively.

Since the last century, humanity has been facing challenging scenarios, like global warming, environmental pollution and the dramatic increase in energy demand. In this framework, green hydrogen has been identified as the most promising energy vector to achieve carbon neutrality. With this respect, the idea of the present work is to combine the Reverse Electrodialysis (RED) membrane process with hydrogen production. Experimental RED tests were carried out by feeding the unit with different concentrated solutions to study the process performance. Collected results suggest that this approach is a viable way to produce hydrogen with high faradic efficiencies, up to a maximum of 99 %, highlighting also the technology advantage of producing hydrogen by exploiting the salinity gradient energy, thus leading to a production with Specific Energy Consumption close to zero.

Water, energy, and minerals are fundamental pillars for the future of humankind. Sustainable and renewable productive processes and resources are the only possibility to face the continuously growing global population and high living standards requirements. The present work introduces a proof of concept for the pilot scale production of magnesium hydroxide, Mg(OH)2, from waste saltworks bitterns located in the district of Trapani, Italy. Mg(OH)2 was produced by adopting a proprietary “Magnesium Crystals Granulometry Controlled Reactor”, Mg-CGCR. The influence of several parameters was investigated on Mg(OH)2 suspensions and powders characteristics: (i) the bittern flow rate, (ii) alkaline sodium hydroxide, NaOH, concentrations solutions (adopted as the precipitant agent) and (iii) the final suspension pH value (stoichiometric or OH- excess amounts). A Mg2+ recovery >99 % can be achieved thanks to the adoption of a product recycling strategy in the reactor. Furthermore, highly pure Mg(OH)2 powders, addressed by cationic purity, were synthesized. Results demonstrate the possibility of producing highly pure Mg(OH)2 products from waste-concentrated saline solutions, thus turning waste into valuable compounds.

Recently, the utilisation of renewable energy sources is a matter of increasing importance in Europe for Energy Transition and to achieve energy independence. To this aim, tailored Electric Energy Storage (EES) devices must be employed to tackle the issue of fluctuating production from renewables. The Acid/Base Flow Battery (AB-FB) is a cutting-edge technology that allows energy to be stored in the form of acidic and alkaline solutions (van Egmond et al., 2018). This method employs two membrane processes, one for the charge phase and one for the discharge phase, namely Electrodialysis with Bipolar Membrane (EDBM) and Reverse Electrodialysis with Bipolar Membrane (REDBM), respectively. The polymeric membranes and the two electrodes are the main components of this battery.

Produced waters (PWs) are waste streams generated during the crude oil extraction processes. The management of these wastewaters is complicated by the large volumes extracted during the oil recovery operations: these depends on the life of the oil-well: typically, 3 barrels of PWs on average are produced for each barrel of oil extracted. After oil separation, PWs are usually re-injected into the well, but this approach is not always possible without a preliminary and suitable treatment. Bioremediation techniques might be a good option, but they fail due to the PWs high salinity, which inhibit bacteria growth and metabolism. Thus, reducing their salinity upstream a bioremediation unit is a matter of crucial importance. To this aim, Assisted Reverse electrodialysis (ARED) along with the use of a dilute stream typically available on site is here proposed as a novel solution. In ARED an additional voltage is applied in the same direction of the salinity gradient through the membranes in order to enhance the passage of ions from the PW to the diluted solution, thus significantly reducing the required membrane area. An experimental campaign was carried out in order to assess the process feasibility. A fixed volume of real PWs was fed to a laboratory scale ARED unit. Each experimental test lasted for three days to reduce the salinity down to about 20 g l-1, a value compatible with the biomass metabolism for a downstream bioremediation step. Two different types of commercial membranes were tested and relevant energy consumptions were calculated. The long-runs performed did not show a significant loss of efficiency due to fouling, thus suggesting that ARED might a suitable technology for a pre-dilution of produced waters.

Electrical energy storage is critical for a broader penetration of renewable energies with intermittent nature, such as solar and wind energy. The Acid/Base Flow Battery (AB-FB) is a unique, sustainable, and environmental-friendly storage technology with high electrolyte solution energy density. The method relies on reversible electrodialytic technologies using bipolar membranes to transform electrical energy into chemical energy related to pH gradients and vice versa. The charge phase is accomplished by using bipolar membrane electrodialysis, whereas the discharge phase is performed via bipolar membrane reverse electrodialysis. In a previous work, we developed an advanced multi-scale process model (Culcasi et al., 2021b), revealing the importance of operating conditions and design features for the AB-FB battery performance. For the first time, the current work attempts to optimize the AB-FB. The net Round Trip Efficiency and average net discharge power density were maximized in a two-objective optimization. The e-constraint method was used to construct curves of Pareto optimal solutions under various scenarios, thereby systematically assessing the effect of decision variables consisting of operating and design parameters. The gPROMS Model Builder® software package's optimization tool was used. This optimization study demonstrated that in a closed-loop configuration, optimized operating conditions and design features can be chosen to maximize net Round Trip Efficiency up to 64% and average net discharge power density up to 19.5 W m-2 using current commercial membranes.

Magnesium hydroxide nanoparticles are widely employed in numerous industrial applications. Several preparation methods have been proposed using mainly synthetic Mg2+ containing solutions. In the present work, the possibility of producing Mg(OH)2 nanoparticles from real bitterns, the by-product of sea salt production, is investigated. Bitterns are highly concentrated Mg2+ containing solutions whose exploitation can turn a waste into valuable products embracing the circular economy idea. Two bitterns collected from Galia and Margi saltworks of the district of Trapani (Italy) were studied. Galia and Margi bitterns had a 1 M and 2.5 M Mg2+ concentration, respectively. A 2 mm diameter circular-cross sectional T-mixer was adopted to ensure fast reactant mixing. NaOH solutions were employed as precipitant agents. Mg(OH)2 nanoparticles characterized by cationic and mass purity higher than 99 % and 90 %, respectively, were successfully produced when treating Galia bitterns, while the excessive Margi Mg2+ concentration yielded stronger micrometer Mg(OH)2 agglomerates.

In agreement with the Water Framework Directive, Circular Economy and European Union (EU) Green Deal packages, the EU-funded WATER-MINING project aims to validate next-generation water resource solutions at the pre-commercial demonstration scale in order to provide water management and recovery of valuable materials from alternative sources. In the framework of the WATER-MINING project, desalination brines from the Lampedusa (Italy) seawater reverse osmosis (SWRO) plant will be used to produce freshwater and recover valuable salts by integrating different technologies. In particular, electrodialysis with bipolar membranes (EDBM) will be used to produce chemicals (NaOH and HCl). A novel EDBM pilot plant (6.4 m2, FuMa-Tech) has been installed and operated. The performance of EDBM for single pass under different flowrates (2–8 L·min−1) for acid, base and saline channels, and two current densities (200 and 400 A·m−2), has been analyzed in terms of specific energy consumption (SEC) and current efficiency (CE). Results showed that by increasing the flowrates, generation of HCl and NaOH slightly increased. For example, ΔOH− shifted from 0.76 to 0.79 mol·min−1 when the flowrate increased from 2 to 7.5 L·min−1 at 200 A·m−2. Moreover, SEC decreased (1.18–1.05 kWh·kg−1) while CE increased (87.0–93.4%), achieving minimum (1.02 kWh·kg−1) and maximum (99.4%) values, respectively, at 6 L·min−1.

Concentrated bitterns discharged from saltworks have extremely high salinity, often up to 300 g/L, thus their direct disposal not only has a harmful effect on the environment, but also generates a depletion of a potential resource of renewable energy. Here, reverse electrodialysis (RED), an emerging electrochemical membrane process, is proposed to capture and convert the salinity gradient power (SGP) intrinsically conveyed by these bitterns also aiming at the reduction of concentrated salty water disposal. A laboratory-scale RED unit has been adopted to study the SGP potential of such brines, testing ion exchange membranes from different suppliers and under different operating conditions. Membranes supplied by Fujifilm, Fumatech, and Suez were tested, and the results were compared. The unit was fed with synthetic hypersaline solution mimicking the concentration of natural bitterns (5 mol/L of NaCl) on one side, and with variable concentration of NaCl dilute solutions (0.01–0.1 mol/L) on the other. The influence of several operating parameters has also been assessed, including solutions flowrate and temperature. Increasing feed solutions’ temperature and velocity has been found to lower the stack resistance, which enhances the output performance of the RED stack. The maximum obtained power density (corrected to account for the effect of electrodic compartments, which can be very relevant in five cell pairs laboratory stacks) reached around 10.5 W/m2cellpair, with FUJIFILM Type 10 membranes, temperature of 40 °C, and a fluid velocity of 3 cm s−1 (as empty channel, considering 270 μm thickness). Notably, the present study results confirm the large potential for SGP generation from hypersaline brines, thus providing useful guidance for the harvesting of SGP in seawater saltworks all around the world.

The SEArcularMINE project aims to recover critical raw materials (CRMs) from brines from saltworks, thus facing a CRM shortage within Europe. To promote a fully circular scheme, the project valorises concentrated brines using electrodialysis with bipolar membranes (EDBM) to generate the required amounts of reactants (i.e., acids and bases). Regarding the performances of new non-woven cloth ion-exchange membranes (Suez): (i) an ultra-thin non-woven polyester cloth and (ii) a thin polypropylene cloth acting as the support structures were assessed. Additionally, the anion layer includes a catalyst to promote the water dissociation reaction. The effect of current density (100, 200, and 300 A m−2) on the performance of two combinations of membranes in an inter-laboratory exercise using 2 M NaCl was evaluated. According to statistical analysis ANOVA, there was an agreement on the results obtained in both laboratories. NaOH/HCl solutions up to 0.8 M were generated working at 300 A m−2 using both combinations of membranes. Regarding the performance parameters, stack set-ups incorporating thin polypropylene membranes showed lower specific energy consumption (SEC) and higher specific productivity (SP) than ultra-thin polypropylene ones. Hence, for ultra-thin polypropylene membranes, SEC was reported to be between 2.18 and 1.69 kWh kg−1NaOH and SP between 974 and 314 kg m−2 y−1.

Precipitation processes have been successfully proved to be a feasible route to extract high-value products from wastes. However, reactive crystallization is characterized by fast kinetics, especially when highly concentrated solutions are used or sparingly soluble compounds precipitate, thus requiring a fast mixing of the reactants. In this context, T-shaped mixers have been extensively investigated both experimentally and numerically as a mean to achieve a rapid reactants homogenization. Many research efforts have been focused on the analysis of rectangular cross-sectional T-shaped mixers, while less attention has been devoted to the study of circular cross-sectional ones. In the present work, mixing times in a 3 mm circular cross-sectional T-shaped mixer have been optically estimated by exploiting the extremely fast neutralization reaction between sodium hydroxide and hydrochloric acid solutions. Reynolds numbers ranging between 1,000 and 6,000 were investigated. Mixing phenomena were (i) captured by detecting the colour change of a pH indicator by means of a high frame rate camera and (ii) quantified by a suitable image analysis technique. The assessed mixing times were found in accordance with available data in the literature.

In the context of renewable energy sources, storage systems have been proposed as a solution to the issues related to fluctuations in the production and consumption of electric power. The EU funded BAoBaB project is aimed at developing the Acid/Base Flow battery (AB-FB), an environment-friendly, cost-competitive, grid-scale battery storage system based on the cyclic coupling of Bipolar Membrane ElectroDialysis (BMED) and its reverse, the Bipolar Membrane Reverse ElectroDialysis (BMRED) (Pärnamäe et al., 2020). Bipolar membranes promote catalytically water dissociation, thus allowing the storage of electric power in the form of acidic and alkaline solutions (pH gradient), obtained from their corresponding salt (charging mode – BMED), which are then recombined to provide electrical power (discharging mode – BMRED). The membranes are key elements for the process performance; however, the energy conversion efficiency is also affected by the operating parameters of the process and the design features of the stack. In this work, we performed a sensitivity analysis by a mathematical multi-scale model previously developed (Culcasi et al., 2020a). The performance of AB-FB systems was predicted, focusing on the Round Trip Efficiency. Results showed that proper design features made the effect of parasitic currents negligible. Moreover, proper operating conditions maximized the RTE up to 66%.

Pickling is one of the most important steps in steel manufacturing industry. During the process, an acid reacts with the surface oxides causing metal ions accumulation in the pickling solution. Disposal of the waste acid represents a critical issue for the hot-dip galvanizing industry in terms of environmental damage and high costs. Recovering of the main products by using an integrated process, with the perspective of a circular approach, could minimize the wastewater production, leading to reinvigorate this industrial sector economy. In this context, recovery of the metal ions mainly present in the pickling solution, such as Fe and Zn, becomes a critical issue. In this work, the reactive precipitation process reliability was proved through lab-scale experiments, in order to collect information for the design of a pilot-plant to be installed in the Tecnozinco SrL (Carini, Italy) hot-dip galvanizing plant. Experiments were carried out in a continuous stirred tank reactor by feeding the acidic metals-rich stream, an alkaline reactant and an oxidant. The quantity of Fe in the outlet solution and its speciation in the precipitate were detected to evaluate the recovery efficiency and the process quality, whereas Zn concentration in the precipitated was detected for determining the product purity. Some key parameters, such as temperature and pH, were studied by varying inlet streams flow rates. A very effective metals separation was observed by obtaining Fe(III) hydroxide at a high purity of 99 %. Zn ions were successfully separated by keeping them in solution with the aim of generating a zinc/ammonium chloride stream, to be reused in the fluxing baths of the hot-dip galvanizing plant.

Direct disposal of concentrate brines, generated by many industrial sectors, causes serious environmental concerns. An industrial sector that produces high concentrate brines is softening water industry, in particular during the regeneration of ion exchange resins. These brines are mainly rich in sodium, magnesium and calcium chlorides, thus, their regeneration, by removal/recovery of magnesium and calcium, can be a valuable option to turn the brine into a source of minerals and reduce disposal costs and environmental concerns. In this regards, one of the goals of EU-funded ZERO BRINE project is to develop a treatment chain for the valorization and regeneration of spent brines from water softening plants. The treatment chain consists of Nanofiltration, crystalization and evaporation to regenerate spent brines and recover high purity magnesium and calcium hydroxides. In this work, a wide experimental campaign was carried out to recover high purity magnesium and calcium via selective chemical precipitation of relevant hydroxides adding by injection of an alkaline solution in two consecutive steps at controlled pH. Precipitations were performed using synthetic brine solution, mimicking the retentate produced by nanofiltration, some of them were carried out considering a pre-treatment step to remove bicarbonate ions.

Wastewaters generated by crude oil extraction processes, called “produced waters” (PWs), are complex solutions that contain organic compounds, mainly hydrocarbons, and often exhibit high salinity. The large amounts of PWs represent a global issue because of their environmental impact. An approach widely used in the oil industry is the reinjection of this wastewater into the extraction wells after a suitable treatment. The high salt concentration of such solutions may be used in salinity gradient technologies to produce green electricity. Among these technologies, reverse electrodialysis (RED) is one of the most promising. In this work, the application of RED for energy generation from two different real oil industry brines was investigated. An experimental campaign was performed by testing 10 × 10 cm2 units in long-run continuous operations, monitoring the performance for more than 25 days. Fouling phenomena, occurring during the continuous operation, decrease the unit performance and several anti-fouling strategies were adopted to tackle this issue. As a result, a positive net power density for up to 18 days of continuous operation was obtained. A maximum power density of about 2.5 W/m2 was observed, demonstrating how the RED technology could be an important strategy to harvest energy from an industrial waste.

An integrated hybrid membrane process, composed of a diffusion dialysis (DD), a membrane distillation (MD) and a reactive precipitation unit (CSTR), is proposed as a promising solution for the valorization and onsite recycling of pickling waste streams. An economic analysis was performed aiming to demonstrate the feasibility of the developed process with a NPV of about EUR 40,000 and a DPBP of 4 years. The investment and operating costs, as well as the avoided costs and the benefits for the company operating the plant, were analyzed with an extensive cost tracking exercise and through face-to-face contact with manufacturers and sector leaders. A mathematical model was implemented using the gPROMS modelling platform. It is able to simulate steady state operations and run optimization analysis of the process performance. The impact of key operating and design parameters, such as the set-point bath concentration and the DD and MD membrane areas, respectively, was investigated and the optimal arrangement was identified. Finally, operating variables and design parameters were optimized simultaneously in a nonlinear framework as a tradeoff between profitability and environmental impact. We show how the integration of new technologies into the traditional pickling industry could provide a significant benefit for the issues of process sustainability, which are currently pressing.

Commercial hemodialyzers are hollow-fiber cylindrical modules with dimensions and inlet–outlet configurations dictated mostly by practice. However, alternative configurations are possible, and one may ask how they would behave in terms of performance. In principle, it would be possible to depart from the standard counter-flow design, while still keeping high clearance values, thanks to the increase in the shell-side Sherwood number (Sh) due to the cross-flow. To elucidate these aspects, a previously developed computational model was used in which blood and dialysate are treated as flowing through two interpenetrating porous media. Measured Darcy permeabilities and mass transfer coefficients derived from theoretical arguments and CFD simulations conducted at unit-cell scale were used. Blood and dialysate were alternately simulated via an iterative strategy, while appropriate source terms accounted for water and solute exchanges. Several module configurations sharing the same membrane area, but differing in overall geometry and inlet–outlet arrangement, were simulated, including a commercial unit. Although the shell-side Sherwood number increased in almost all the alternative configurations (from 14 to 25 in the best case), none of them outperformed in terms of clearance the commercial one, approaching the latter (257 vs. 255 mL/min) only in the best case. These findings confirmed the effectiveness of the established commercial module design for the currently available membrane properties.

Stirred tanks typically employed in process industries are provided with baffles. Although the presence of baffles is known to guarantee good mixing rates, unbaffled vessels may be compulsory in some applications as crystallization, bioremediation, biotechnology and ore industry. A better understanding of unbaffled stirred vessels flow dynamics may allow (i) a proper design to be performed and (i) conditions/processes where baffle presence can be avoided to be recognized. In the present study, the k-? SST was used to simulate an unbaffled tank from early to fully turbulent regime (Re(600-33,000). The unbaffled tank simulated has a diameter T=0.19m and is stirred by a standard six-bladed Rushton turbine with diameter D=T/2 and clearance C=T/3. A corresponding baffled tank was also simulated in order to compare the the two systems. A time dependent Sliding Grid approach was employed for the baffled tank to account for the impeller-to-baffle relative rotation. Conversely, for the case of the unbaffled vessel, a reference frame rotating with the impeller was adopted. Experimental literature data concerning the power and pumping numbers were employed for the simulation validation. RANS results were in good agreement with the experimental data for the baffled case at the largest Re, whereas predictions for the unbaffled vessel exhibited a less satisfactory agreement with experimental data. The latter finding may be due to the poor capability of the two-equations model to manage the anisotropic turbulence typical of high swirling flows.

Salinity Gradient Heat Engines (SG-HEs) have been proposed as a promising technology for converting low-temperature heat into electricity. The SG-HE includes two different processes: (i) a salinity gradient process where the salinity gradient between two solutions is converted into electricity and (ii) a thermal regeneration process where low-grade heat (T

This Special Issue of Membranes journal focuses on electromembrane processes and is motivated by the increasing interest of the scientific community towards their characterization by experiments and modelling for several applications [...]

Bipolar Membrane Reverse Electrodialysis (BMRED) can be used to produce electricity exploiting acid-base neutralization, thus representing a valuable route in reusing waste streams. The present work investigates the performance of a lab-scale BMRED module under several operating conditions. By feeding the stack with 1 M HCl and NaOH streams, a maximum power density of ~17 W m−2 was obtained at 100 A m−2 with a 10-triplet stack with a flow velocity of 1 cm s−1, while an energy density of ~10 kWh m−3 acid could be extracted by a complete neutralization. Parasitic currents along feed and drain manifolds significantly affected the performance of the stack when equipped with a higher number of triplets. The apparent permselectivity at 1 M acid and base decreased from 93% with the five-triplet stack to 54% with the 38-triplet stack, which exhibited lower values (~35% less) of power density. An important role may be played also by the presence of NaCl in the acidic and alkaline solutions. With a low number of triplets, the added salt had almost negligible effects. However, with a higher number of triplets it led to a reduction of 23.4–45.7% in power density. The risk of membrane delamination is another aspect that can limit the process performance. However, overall, the present results highlight the high potential of BMRED systems as a productive way of neutralizing waste solutions for energy harvesting.

A novel technology, the ion exchange membrane crystallizer (CrIEM), that combines reactive and membrane crystallization, was investigated in order to recover high purity magnesium hydroxide from multi-component artificial and natural solutions. In particular, in a CrIEM reactor, the presence of an anion exchange membrane (AEM), which separates two-compartment containing a saline solution and an alkaline solution, allows the passage of hydroxyl ions from the alkaline to the saline solution compartment, where crystallization of magnesium hydroxide occurs, yet avoiding a direct mixing between the solutions feeding the reactor. This enables the use of low-cost reactants (e.g., Ca(OH)2) without the risk of co-precipitation of by-products and contamination of the final crystals. An experimental campaign was carried out treating two types of feed solution, namely: (1) a waste industrial brine from the Bolesław Śmiały coal mine in Łaziska Górne (Poland) and (2) Mediterranean seawater, collected from the North Sicilian coast (Italy). The CrIEM was tested in a feed and bleed modality in order to operate in a continuous mode. The Mg2+ concentration in the feed solutions ranges from 0.7 to 3.2 g/L. Magnesium recovery efficiencies from 89 up to 100% were reached, while magnesium hydroxide purity between 94% and 98.8% was obtained.

This paper presents a comprehensive review of studies on electrodialysis (ED) applications in wastewater treatment, outlining the current status and the future prospect. ED is a membrane process of separation under the action of an electric field, where ions are selectively transported across ion-exchange membranes. ED of both conventional or unconventional fashion has been tested to treat several waste or spent aqueous solutions, including effluents from various industrial processes, municipal wastewater or salt water treatment plants, and animal farms. Properties such as selectivity, high separation efficiency, and chemical-free treatment make ED methods adequate for desalination and other treatments with significant environmental benefits. ED technologies can be used in operations of concentration, dilution, desalination, regeneration, and valorisation to reclaim wastewater and recover water and/or other products, e.g., heavy metal ions, salts, acids/bases, nutrients, and organics, or electrical energy. Intense research activity has been directed towards developing enhanced or novel systems, showing that zero or minimal liquid discharge approaches can be techno-economically affordable and competitive. Despite few real plants having been installed, recent developments are opening new routes for the large-scale use of ED techniques in a plethora of treatment processes for wastewater.

Acid recovery from pickling waste solutions is an important step to enhance hot-dip-galvanizing industry process sustainability. Diffusion dialysis (DD) can be used to separate acids and heavy metals (e.g., iron and zinc) from pickling waters, promoting the circular use of such raw materials. In the present study, a laboratory scale unit operating in batch and a continuous large scale unit, both equipped with Fumasep anionic exchange membranes, were tested. Results obtained show that zinc and iron concentration affect the HCl recovery in opposite ways. Iron chlorides enhance acid recovery, while zinc chlorides considerably tend to diffuse through the membrane because of negatively charged chloro-complexes formation and slightly reduce the acid diffusion. A multi-components mathematical model, with a time-dependent and distributed-parameters architecture, was adopted enabling the prediction of operations with hydrochloric acid, zinc, and iron metals both in batch and in continuous dialyzers. As a result, a good comparison between model simulations and experiments was achieved in both configurations.

Magnetic-field-assisted fluidization is starting to be considered as a viable alternative to standard fluidized beds for those operations (such as particle separations, filtration, adsorption) in which the solid phase can be made of magnetic particles or, alternatively, the fluidizing agent is a ferro-fluid; thus the fluid bed responds to the action of magnetic fields, and stabilized fluidization regimes can be generated.One of the major difficulties to be tackled is the development of a predictive model capable of estimating the stabilized-to-bubbling transition velocity for a given magnetic field or, on the other hand, the magnetic field intensity required to stabilize the bed to a quiescent condition. The fluid dynamics prediction of a stabilized bed is also a challenging task at the moment.On this basis, a very simple model for the description of MSFB was derived in this contribution starting from basic fluid dynamics and magnetodynamics equations. The model was implemented in a commercial CFD code in order to simulate the effect of the magnetic field onset on a freely bubbling fluidized bed.

The hydrodynamics of electrodialysis and reverse electrodialysis is commonly studied by neglecting membrane deformation caused by transmembrane pressure (TMP). However, large frictional pressure drops and differences in fluid velocity or physical properties in adjacent channels may lead to significant TMP values. In previous works, we conducted one-way coupled structural-CFD simulations at the scale of one periodic unit of a profiled membrane/channel assembly and computed its deformation and frictional characteristics as functions of TMP. In this work, a novel fluid−structure interaction model is presented, which predicts, at the channel pair scale, the changes in flow distribution associated with membrane deformations. The continuity and Darcy equations are solved in two adjacent channels by treating them as porous media and using the previous CFD results to express their hydraulic permeability as a function of the local TMP. Results are presented for square stacks of 0.6-m sides in cross and counter flow at superficial velocities of 1 to 10 cm/s. At low velocities, the corresponding low TMP does not significantly affect the flow distribution. As the velocity increases, the larger membrane deformation causes significant fluid redistribution. In the cross flow, the departure of the local superficial velocity from a mean value of 10 cm/s ranges between −27% and +39%.

In the examined heat engine, reverse electrodialysis (RED) is used to generate electricity from the salinity difference between two artificial solutions. The salinity gradient is restored through a multi-effect distillation system (MED) powered by low-temperature waste heat at 100 °C. The current work presents the first comprehensive economic and environmental analysis of this advanced concept, when varying the number of MED effects, the system sizing, the salt of the solutions, and other key parameters. The levelized cost of electricity (LCOE) has been calculated, showing that competitive solutions can be reached only when the system is at least medium to large scale. The lowest LCOE, at about 0.03 €/kWh, is achieved using potassium acetate salt and six MED effects while reheating the solutions. A similar analysis has been conducted when using the system in energy storage mode, where the two regenerated solutions are stored in reservoir tanks and the RED is operating for a few hours per day, supplying valuable peak power, resulting in a LCOE just below 0.10 €/kWh. A life-cycle assessment has been also carried out, showing that the case with the lowest environmental impact is the same as the one with the most attractive economic performance. Results indicate that the material manufacturing has the main impact; primarily the metallic parts of the MED. Overall, this study highlights the development efforts required in terms of both membrane performance and cost reduction, in order to make this technology cost effective in the future.

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models.

In electro-membrane processes, a pressure difference may arise between solutions flowing in alternate channels. This transmembrane pressure (TMP) causes a deformation of the membranes and of the fluid compartments. This, in turn, affects pressure losses and mass transfer rates with respect to undeformed conditions and may result in uneven flow rate and mass flux distributions. These phenomena were analyzed here for round pillar-type profiled membranes by integrated mechanical and fluid dynamics simulations. The analysis involved three steps: (1) A conservatively large value of TMP was imposed, and mechanical simulations were performed to identify the geometry with the minimum pillar density still able to withstand this TMP without collapsing (i.e., without exhibiting contacts between opposite membranes); (2) the geometry thus identified was subject to expansion and compression conditions in a TMP interval including the values expected in practical applications, and for each TMP, the corresponding deformed configuration was predicted; and (3) for each computed deformed configuration, flow and mass transfer were predicted by computational fluid dynamics. Membrane deformation was found to have important effects; friction and mass transfer coefficients generally increased in compressed channels and decreased in expanded channels, while a more complex behavior was obtained for mass transfer coefficients.

A performance analysis of a salinity gradient heat engine (SGP-HE) is presented for the conversion of low temperature heat into power via a closed-loop Reverse Electrodialysis (RED) coupled with Multi-Effect Distillation (MED). Mathematical models for the RED and MED systems have been purposely developed in order to investigate the performance of both processes and have been then coupled to analyze the efficiency of the overall integrated system. The influence of the main operating conditions (i.e., solutions concentration and velocity) has been quantified, looking at the power density and conversion efficiency of the RED unit, MED Specific Thermal Consumption (STC) and at the overall system exergy efficiency. Results show how the membrane properties (i.e., electrical resistance, permselectivity, water and salt permeability) dramatically affect the performance of the RED process. In particular, the power density achievable using membranes with optimized features (ideal membranes) can be more than three times higher than that obtained with current reference ion exchange membranes. On the other hand, MED STC is strongly influenced by the available waste heat temperature, feed salinity and recovery ratio to be achieved. Lowest values of STC below 25 kWh/m3 can be reached at 100 °C and 27 effects. Increasing the feed salinity also increases the STC, while an increase in the recovery ratio is beneficial for the thermal efficiency of the system. For the integrated system, a more complex influence of operating parameters has been found, leading to the identification of some favorable operating conditions in which exergy efficiency close to 7% (1.4% thermal) can be achieved for the case of current membranes, and up to almost 31% (6.6% thermal) assuming ideal membrane properties.