Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals

i i Equation Chapter 1 Section 1 Trabajo Fin de Máster Máster universitario en Sistemas de Producción de Potencia Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals Autor: Eva Sánchez Carceller Tutor: Lourdes García Rodríguez Dpto. Máquinas Térmicas Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2020 ii iii Trabajo Fin de Máster Máster universitario en sistemas de producción de potencia iv Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals Autor: Eva Sánchez Carceller Tutor: Lourdes García Rodríguez Profesor titular Dpto. de Máquinas Térmicas Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2020 v vi vii Proyecto Fin de Carrera: Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals Autor: Eva Sánchez Carceller Tutor: Lourdes García Rodríguez El tribunal nombrado para juzgar el Proyecto arriba indicado, compuesto por los siguientes miembros: Presidente: Vocales: Secretario: Acuerdan otorgarle la calificación de: Sevilla, 2020 El Secretario del Tribunal viii ix A ellos, xvi xvii xviii Índice de Figuras Fig. 1. Changes of bottled water consumption in Spain (thousands of m3) [1] 1 Fig. 2. Simple Seawater desalination process by Reverse Osmosis (SWRO) [108] 3 Fig. 3. Mass flow in a common desalination plant [3] 3 Fig. 4. General Electric’s Aquasel NTBC 9 Fig. 5. Brine concentrator functional layout 12 Fig. 6. System diagram 23 Fig. 7. m3/m1 tendency with T1, T2 variations 28 Fig. 8. S3/S1 tendency with T1, T2 variations 29 Fig. 9 m3/m1 evolution with T1, T2 and m2/m1=0.1 30 Fig. 10 m3/m1 evolution with T1, T2 and m2/m1=0.2 30 Figura 11 S3/S1 evolution with T1, T2 and m2/m1=0.1 31 Figura 12 S3/S1 evolution with T1, T2 and m2/m1=0.2 31 Figura 13 S3/S1 evolution with T1, T2 and S1=0.07 32 Figura 14 S3/S1 evolution with T1, T2 and S1=0.085 32 1 1. INTRODUCTION e water demand on all civilized countries has risen due to the population growth with the pass of the years (Fig. 1). This fact is undoubtedly related with the increase in brine flows discharged by the desalination plants, which causes a greater environmental impact. Fig. 1. Changes of bottled water consumption in Spain (thousands of m3) [1] Therefore, and more than ever, is necessary to correctly manage the brine discharge of the desalination processes, in order to achieve a correct disposal of the output, brine minimization or an efficient reutilization of this. Many investigations came out because of this, with the objective of reducing the brine disposal of the desalination plants. The management processes consist, commonly, in minimization techniques that use various technologies, such as membranes to do physical-chemical separations, heat treatment methods (distillation, vaporization, etc.) and some innovative technologies that are appearing nowadays. This project also makes a point about the process sustainability showing up different ways of brines disposal and even other applications that the output may have, promoting its reuse. 1.1. Background of desalination water Reverse osmosis on desalination process of seawater is one of the biggest sources of drinking water of the world. Sea Water Reverse Osmosis (SWRO) or Brackish Water Reverse Osmosis (BWRO), the reverse osmosis is a fairly normalized process at the current industry, and it is present in most of countries without natural sources of fresh water. The ancient background of the process makes that the optimization no longer focus on the process T 2 Introduction 2 itself, instead of that, it is searching for a better energy efficiency, a better separation processes or low fouling and scaling. The continuous upgrades and optimizations of this method have made of this technology something solid and economical, largely replacing other processes for obtaining drinking water like industrial distillation, which needs more expensive equipment, bigger and with low operating flexibility. During the past decades, the SWRO technology has greatly risen its production capacity, in 2011 the global processing capacity up to 3.5e7 of water each day. In addition to optimizing energy consumption, reducing it by a large percentage, from 20 to 2.5 kWh (approximately) per m3 of treated water [2]. The RO is so extended due to its capacity to be useful in a lot of different situations, this technology is not only used for desalination processes. The wide use and versatility of the RO has made this process studied for use in very promising fields for the industry. The membrane technology can be used, for example, to obtain molecular hydrogen, separating it from the other components of the flow [3]. Another possibility is the use of palladium based or ceramic membranes, useful for ionic transport or CO2 catching on natural gas treatment. There are, also, more applications of this technology at the pharmaceutical or biotechnological fields isolating amino acids, proteins, or enzymatic substances. All this history and the actual possibilities in this technology makes it remain on the spotlight of current industry, being a constantly improving and optimizing process. 1.2. Current trends in brine disposal In nature, water is found in three different states: solid, in the form of ice; gaseous, like steam in the atmosphere; and liquid, as the water of rivers, lakes and seas. However, of these three states, only the ice can be considered like pure water. The liquid state of water is impossible to find in the nature, and the drinking water is, in fact, a solution of salts using water as solvent element. The taste of drinking water depends on the composition of salts that it has diluted. In this way, the water prevenient from coastal zones will had a different, and generally worst, flavor than the water at the mountains, with less concentration of salts. Therefore, water solutions found in nature can range from 0.2 grams per liter of salts in mountain water, to about 35 grams per liter on average in seawater. In other way, the salts concentration in output brines can achieve the 45 grams per liter, which are dumped directly to the ocean. In order to obtaining drinking water from seawater it is necessary to reduce the salts concentration in seawater under 0.5 grams per liter. This objective is pursued by the desalination plants of reverse osmosis (RO), getting a waste current from the membranes reject flow. This reject flow is a dissolution of seawater highly concentrate in salts (Fig. 2). Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 3 3 Fig. 2. Simple Seawater desalination process by Reverse Osmosis (SWRO) [108] In the previous image can be seen the feed flow (the seawater), the permeate flow (the product of the process) and the concentrate flow (the brine). As usual, it is obtained a 50 % of drinking water of all the seawater fed to the plant. Because of that, something more than a half is treated as waste, and is bumped to the sea as brine flow. For example, if some plant were fed with 1 L of seawater, it would produce 0.45 L of drinking water, approximately, and 0.55 L of brine. Focusing in a desalination plant placed at Alicante, Spain, it is obtained the next flow scheme (Fig. 3): Fig. 3. Mass flow in a common desalination plant [3] 4 Introduction 4 Some studies from 2017 [4] confirm that almost 76 million m3 of brine water per day are produced by desalination plants, although nowadays this figure can be larger. It is obvious to come to the conclusion that it is necessary to study a method that considerably reduces the brine flows thrown into the sea by desalination plants. A reducing method based on the elimination or, better still, the reuse of the waste stream. 1.3. Objectives and range This project aims to carry out a study of the management of brines from the rejection of desalination processes by reverse osmosis. A collection of different bibliographic sources and an investigation of the state of art in this field is carried out, delving into the most common aspects, such as the direct elimination of reject brines, the minimization of them and, finally, their reuse. The latter alternative is the most interesting and the one with the most prominence in this work. The range of the project encompasses the bibliographic search, the drafting of the most interesting and important points in it and, finally, a simulation in MATLAB of a system that reuse the exhaust gases of a microturbine to increase the salinity of a brine. 5 2 BRINE MANAGEMENT METHODS esalination technology is widely used in places where drinking water resources are limited, an example of this is Middle East and, specially, the United Arab Emirates (UAE). The environmental issue generated by the desalination processes around the world is one of the biggest cons in the production of fresh water from desalination plants based on either, distillation and SWRO processes [5]. This is mainly due to three causes: chemical products added to the mainstream of the process that keep existing in the waste flow; high temperature (only in distillation plants) and salinity in the blowdown (brine), much higher than the environmental ones. On the other hand, the outlet properties can be highly modified by the quality of the water fed to the plant. Depending on the quality of the water resources different previous treatments or other steps in the process would be necessary to achieve the adequate product properties. The main damages of reject brine to the environment include eutrophication, pH fluctuations, incrementing concentration of heavy metals and sterilizing characteristics due to the disinfectants which can cause a series of problems to marine and underground habitat [6]. In order to achieve the best way to manage reject brine, it has been considered different factors like total reject flow, the composition of the reject flow, the ubication of the desalination plant, etc. [5]. All of this makes the management of the project very difficult and expensive. There are different reject brine management strategies: brine minimization, direct disposal, or direct reuse. This project aims to investigate these three strategies. 2.1 Brine minimization The desalination plant rejects large volumes of brine because the present technology can’t achieve the complete recuperation of the permeate [6]. It is a priority to focus on this field through the treatment of the brine itself, attempting to reduce its volume. 2.1.1 Chemical pretreatment Chemical pretreatment is commonly used for the removal of scaling or inorganic components such as Ca2+, Mg2+, Ba2+ and Sr2+, before the secondary treatment of the flow. The advantage of this method is found in the high recovery of the permeate (> 95%). Some studies [8] have achieved a very high recovery of the permeate with this method, obtaining up to 83% recovery in the first step and more than 95% in the second. In other way, it is necessary a chemical pretreatment to the brine after the first step of the process, to obtain a better recovery and a bigger reduction of the volume in the end of the process. This second treatment is also due to the high concentration of chemical compounds that the flow is bringing with itself from the first step, these chemicals can damage the membrane of the second step or reduce the efficiency of the process. This “post-treatment” can enhance the recovery from an initial 85 % up to a 95 % with a second step. Moreover, with this method the scaling precursors such as Ba2+, Ca2+ and Sr2+ were eliminated in a solid contact pilot-scale reactor in a 67, 97, 94 and 88 % respectively. This RO two-step process with an Intermediate Chemical Demineralization (ICD) was used to minimize the reject and enhance the permeate recovery of the brackish water in Colorado River, in the Unites States [8]. D 6 Brine management methods 6 During the ICD step, the scaling precursors can reach a high concentration, seriously exceeding their solubility and making very difficult the pH control. Rahardianto et al. [7] offers a solution to this problem through the incorporation of a Accelerated Precipitation Softening (APS) system between the first and the second step. The “APS” system involves a pH adjustment to the alkaline range (10.5) through the addition of Sodium Hydroxide (NaOH) followen by the seeding of calcite crystals. This process ensures the solution desaturation of the scaling precursors. The seeding of the crystals is also followed by microfiltration and normalization of the pH by HCl addition before the secondary step to prevent calcite scales formation. The demineralization of the solution enhances the efficiency of the process from a 90 % up to a 98 % of global recuperation. The “APS” also reaches the reduction of compounds like Ba, Ca, Sr, Mg/Si in a >95, >90, 78 and 10-20 % respectively [9]. 2.1.2 Electro kinetic treatment and ionic exchange Electro kinetic separation is a new method used to achieve softening (mineral concentration reduction) of primary brines before the secondary step of the RO [10, 11]. Electrocoagulation is one of the purification methods in electro kinetic applications [12, 13]. Electrocoagulation is a process that changes surface charge of particles by continuously providing metallic ions as coagulant sources with the aid of a sacrificial anodic electrode [14-16]. Such anodic electrode expels a continuous source of metallic ions, forming metal hydroxides that destabilizes the floating particles. At the same time, an electrophoretic process leads a negatively charged particles current to the anodic electrode surface. By this way, attraction forces were created between the anode and the charged flocs in the diffuse layer, and, subsequently, charged particles become deposited on the electrode. During this process, some flocs reach the necessary weight to gravitational precipitation. This system has generated great advances in RO processes, going to be called "high efficiency RO systems”, for example, by electro kinetic pretreatment 80% of silica of brackish water has been eliminated using a current of 0.5A and an hydraulic retention time of 30 minutes [17]. Another example is in the water treatment of Big Bear Valle, California, where permeate recovery has been significantly improved thanks to a high-efficiency RO system [18]. A proposed improvement to join electro kinetic systems consists of the filtration and recycling of concentrate. Sometimes the concentrate treated after the last step is filtered and recirculated to the intermediate step between the stages of the process, where it can be reused as second-step feed water (brackish water). Various simulations have shown that with this system a very high recovery can be achieved (from 99.5 to 99.9%). However, this method does not eliminate the hardness of the current, since the recirculated current implies a high salinity, which makes the process possibly unprofitable [19]. On the other hand, ion exchange has also been used to remove silica and silicate from nanofiltration concentrate (NF) [20, 21]. Bivalent ions, such as calcium, barium or magnesium are removed with this method to prevent the formation of scales during filtration process. With this system, a 98% permeate recovery is achieved. However, by using this methodology forms a layer of fouling composed, among other compounds, by silica, aluminum and iron, which reduces the percentage of effective recovery of the membranes after washing and makes it necessary to carry out more studies and tests to verify their long-term economic validity. Finally, it is worth mentioning that the Electrodialysis Reversal system (EDR) is suitable for high efficiency when is applied as pretreating [22]. EDR can achieve maximum recoveries of up to 92- 94% for one-step systems and up to 97% in two-step systems [22], resulting in a huge decrease in concentrate flow, which can be poured, treated, or reused. Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 7 7 2.1.3 Biological techniques Biological pretreatment techniques have proven to be viable ways of removing some foulants or scalants from low-salinity brine. A biological process using sulfate reducing bacteria has been tested to reduce the sulfate content in the RO concentrate [23]. To effectively remove sulfates through these bacteria, it is necessary both an electron donor and a carbon source, in a study taken as an example [24], acetate is used as an electron donor. One option for the reduction of sulfates by biological means is the use of a Fluidized Bed Bioadsorver Reactor system (FBBR) [25]. The source of fed water is water from the Colorado River in Southern California, with a TDS over 700 mg/L. With this kind of water the reduction of sulfate is a priority to improve the overall recovery of permeate, as well as for the reduction of the precipitation potential of barium sulfate, calcium sulfate and/or strontium sulfate in the membrane, during the second step of the process [25]. It is demonstrated that FBBR is an effective and cost-effective technology that can be used to remove sulfate from the concentrated stream of an RO, with a recovery greater than 95% [25]. However, if heavy metals (which inhibit microbial activity by poisoning them) are majority components in the composition of brine concentrate, it would be convenient to look for an alternative to the use of FBBR. A resolution to this problem could be, for example, granulated active carbon that can adsorb such heavy metals, resolving the problem [25]. The FBBR has also been used to remove ammonia content from the concentrate of a RO process with the aim of reducing the components dissolved in it by denitrification [26]. As an experimental method, brackish water product from a RO process was treated with a biological method. Using a biological willow bed and, as a feeding stream to the system, a previously treated effluent of wastewater [27]. In this experiment, total COD, nitrogen, and phosphorus were eliminated by 20, 32 and 32%, respectively. After this biological treatment, an electrodialysis system is added, with the aim of treating the rejection concentrate, which is redirected back to the wastewater treatment plant. This technique ensured a high total recovery of permeate water, a minimization of waste brine and the transfer of xenobiotics in the final concentrate discharged to the wastewater treatment plant, rather than sending all of that, directly, to the environment. 2.1.4 Vibratory Shear Enhanced Processing (VSEP) Vibration Shear Enhanced Processing (VSEP) is a technology consisting of surface oscillation of the membrane and is used to improve membrane-based minimization strategies [28, 29]. A system to treat the concentrated flow from a RO process in a VSEP unit is designed, using the classic plate and frame configuration [28, 29]. The mechanism by which the system operates consist in the application of a vibrating shear performed by torsional oscillation at 50 Hz frequency in the membrane surface, so that the shear size is ten times larger compared to the typical cross-flow membrane systems [29]. Through this method a significant reduction of brine final volume is achieved and prevents polarization and colloidal fouling by creating a considerably shorter route for feed water across the membrane surface thanks to the vibration produced. Unlike conventional RO processes, VSEP is not limited by salts composition and concentration in the treated flow, the presence of suspended solids or colloids in the brine. Therefore, the great advantages of this method are the high resistance to membrane flake, the high filtration rates, and the low decrease of effective area in the membrane [30]. On the other hand, the energy to maintain active vibration is 14 Brine management methods 14 2.1.11 Multi-effect Distillation (MED) and Mechanical Vapor Compression (MVC) MED consists of a set of tanks in series that perform a distillation process at increasingly less pressures, so that the steam produced in one step is used to heat and evaporate the solution found in the next step and is at lower pressure. This process is used for the treatment of brines from RO. The performance of this process depends to a large extent on the number of steps (or effects) available, in the greater number of steps, the better, although it is estimated that the combination of RO-MED processes can achieve a yield of 93% of the recovered permeate flow. On the other hand, to carry out this process it is necessary the pretreatment of the inlet flow, with the aim of reducing the concentration of silicates and preventing the formation of scaling on the heat exchange surface. To improve process efficiency, it is common to install a steam compressor between the different steps, in this way, the compressor provides steam energy through a compression process during evaporation, in addition to recirculating it. This combination of techniques is very useful if you want to choose to completely eliminate the discharge of liquids (Zero Liquid Discharge, ZLD). A mechanical compression of steam can be used to treat brines when it is interesting to obtain distilled water and more concentrated salts [60], the concentrate obtained from this process can be stored and subsequently treated by thickeners and press filters. As a general rule, this equipment usually operate at a temperature of 80 ºC, however, recent studies have shown that operating at 172 ºC, lower pressure and less heat transfer area are needed, resulting in smaller equipment size, lower hazards, lower costs [61]. At 172 ºC, the salinity of the concentrate is increased by 15%, with a feeding concentration of 35000 mg/L. 2.1.12 Membrane Distillation (MD) Membrane distillation processes combine two of the most promising fields in brine reduction or removal, membrane separation and heat treatments, an alternative process designed to dehydrate solutions with high salt concentrations. Compared to simple membrane separation, where the driving force was provided by a pressure, concentration, or electrical gradient, the driving force in membrane distillation (MD) is about steam pressure, which is directly related to the temperature differential across the entire membrane surface [42, 62]. Permeate can be produced at temperatures as low as 10 or 20 ºC, provided that the differential step between hot and cold currents is maintained [63]. The mechanism of MD operation is based on a liquid-vapor balance where volatile components are diffused through a micro porous and hydrophobic membrane. This system has a high-water recovery capacity from concentrates and can be installed based on various configurations. The different configurations of an MD system can be direct contact, air bursts, low pressure, and vacuum, although the most widely used system uses direct contact configuration (DCMD). The operation of this configuration is as follows, on one side of the membrane is the flow of concentrate to the highest temperature (of the respective gradient), in this way, the volatile components evaporate and pass through the micropores of the hydrophobic membrane, separating from the liquid phase that does not pass through it and is retained along with the dissolved salts. On the other side, condensation permeate is collected by cooling the steam. Among the advantages of this system are: the vapor pressure gradient, one of the driving forces, is not affected by the concentration of salts carried by the concentrate; low presence of fouling and bio-fouling; the process has the ability to receive heat from the source that best suits the system, solar, geothermal, electrical, etc.; low operating and maintenance costs. On the other hand, the main drawback of this is the low permeate flow obtained in most cases [64]. Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 15 15 The performance and sustainability of a membrane distillation system in the recovery of permeate from concentrates from a thermal desalination plant [63] has been investigated in a pilot plant. Three different samples are taken for the pilot plant, Persian Gulf salt water, synthetic saline solutions and, finally, brines from desalination plants, all of which are operating under different operating conditions. In addition, five different types of membranes are used, already marketed and compatible with membrane distillation systems, these membranes are composed of an active layer of polytetrafluorethylene (PTFE) on a polypropylene (PP), supported with different pore sizes and thickness. The temperature range is maintained at 70 and 30 ºC for the feed current and distillate, respectively, in addition, it is operated with a flow rate of 1.8 L/min for feeding and distilling. Under these conditions, a constant permeate flow of 25 L/m2h is achieved using synthetic salt water with a salt concentration of 70000 mg/L. However, if the salt concentration is increased beyond this level, the permeate flow obtained decreases considerably to less than 20 L/m2h. None of the experiments performed detect fouling, in addition, it is shown that both the permeation flow and the efficiency of the system increase directly proportional to the temperature of the feed current, leaving constant (to maintain the gradient) the temperature of the distillate. All membranes achieve 99.9% of rejected salts in the concentrate flow. The researchers involved in the experiment recommend the use of the MD system to treat the concentrated rejection of thermal desalination plants, because it is a very efficient system for the production of distillate with very low salt concentration (10 µS/cm conductivity), in addition, because the concentrate of the thermal desalination plants retains some of the heat of the process, this combination is ideal for the recovery of that heat. On the other hand, a reduction in the rejection flow has also been tested by a distillation system with membranes by direct contact but improved with vacuum (Vacuum-Enhanced Direct Contact Membrane Distillation, VEDCMD) [44]. This system is compared to the direct osmosis process by carrying out a process under the same operating conditions and with the same feed. The membrane used in VEDCMD is one manufactured from PTFE on a PP support, as explained above, with a pore size of 0.22 µm, a feeding temperature between 40 and 60 ºC and a distillate of 20 ºC. High permeate recoveries are recorded, reaching up to 81% efficiency from brine concentrates, compared to 96% achieved when dealing with a combination of RO with VEDCMD. It also compares the lowest and highest temperatures recorded for both VEDCMD and FO, and it is shown that VEDCMD has better performance operating at lower temperature for feeds with high salinity and low scaling presence. However, direct osmosis is above VEDCMD when it comes to feedings with high tendency to form scaling and less saline concentration. It is concluded, therefore, that a scaling inhibitory treatment is convenient in processes to improve the performance of VEDCMD. On the other hand, experiments have also been conducted with a Vacuum Membrane Distillation system (VMD), yielding interesting results [65]. In this test, brines of synthetic production with high saline concentrations (up to 300,000 mg/L) are tested under different operating conditions. The results shown by this process indicate that high percentages of permeate recovery can be achieved, even if there are high saline concentrations in the feed by using a highly permeable membrane, a low permeate collection pressure and, finally, raising the temperature of the feed current as much as possible. The permeate flow obtained varies between 7 and 19 L/m2h when the feed salt concentration is between 64,000 and 300,000 mg/L at 50ºC and 6 kPa of pressure in the permeate. In addition, it is demonstrated that scaling is only presented when working with high salinity feeds, which may be due to the presence of low solubility compounds that precipitate in the solution, such as sulfate or calcium carbonate, yet this event is reversible and the formed crystals can simply be collected and removed when the membrane is being cleaned with clean 16 Brine management methods 16 water. It is concluded that, for the treatment of brines from RO with a VMD system, a high recovery of permeate, up to 89%, can be achieved, minimizing the volume of brine considerably. Among other processes that use membrane distillation technology, the system of air bursts in a multistage membrane distillation (AGMD) and using hollow fiber membranes, is widely used to achieve high permeate ratios from the brine rejection of RO processes [66]. In this way, it is observed that the performance of the process increases as the number of stages rises, being 5 to 8% recovery for systems with one stage, and up to 22.2 or 24.7% recovery when 4 is available. The maximum recovery percentage is found when using a system with 14 stages, where you get an 88.2% recovery of permeate with a concentration in the feed of 352,000 mg/L. Thanks to all this data, it is concluded that the AGMD system can be used to treat currents with high saline concentrations. 2.2 Direct removal of brines A conventional approach in the brine treatment is the direct disposal to large water bodies like seas, oceans, etc. or injection into inland wells [67]. However, this kind of solution has the worst consequences to marine environment and underground water due to, as indicated by “P. Oceánica” [68], the elimination of reject brine with high salinity could affect to different kind of marine species, the high temperature of the reject current also negatively affects to the surrounding areas decreasing the concentration of dissolved oxygen in water and, therefore, helping to create a hypoxic environment, which harms all living beings who inhabit it. In addition, there is a risk of leaks with high salinity concentrations when the disposal of the brine is being produced to the underground wells, which can gravely affect the terrestrial environment [69]. Therefore, the direct disposal method is not recommended unless it is the only feasible alternative. The direct disposal of the rejected brine into large bodies of water have been practiced since past years by several countries. In order to carry out this procedure, the discharge must be very diluted to the legal level determined by the properties of the discharge current itself and the characteristics of the receiving aquatic environment. The concentrate properties are defined by the TDS, the density and the discharge flow, on the other hand, the environment properties where this current is rejected are determined by physical characteristics such as the volume of the water body and the biological content of the surrounding area. Different factors such as the location of the underwater emissary, extent of mixing and the water currents capacity to transport the mixed brine must be studied in order to realize the direct discharge of the reject brine into any sea. Due to the high salinity content in the reject current in comparison with the sea water, there is a possibility that the current sinks and be diluted into the bottom of the ocean, as it is much denser. Although the most common result in most cases is horizontal spreading [70]. To solve this situation, an alternative method is proposed to avoid the direct discharge of rejected brine into the seas [71]. The conventional method requires a discharge angle of 60º to the horizontal but, however, extent of dilution when the sea is at maximum sea rise is not provided with this angle. In addition, it is shown that an angle of 30 or 45º to the horizontal can seriously improve the operation design. Some of these benefits are a better degree of dilution, better brine dilution when sea currents are weak and the possibility of placing the underwater emissary in shallow places or near the coast to cost reduction [71]. Normally, a plume develops on the sea surface due to the large difference in concentrations between the rejected brine and the pouring medium. This generates negative environmental effects, so have been studied the best way to achieve the optimized disperse of the plume [72]. First, this plume is considered to come from the rejected brine of a desalination plant in the Persian Gulf, dilution through multiple outlets is shown as the most feasible method to improve mixing, eliminate the plume and reduce environmental impacts. Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 17 17 Another studied option consists in the mixing of brine concentrate with sewage before it is discharged into the sea, which decreases the salinity of the concentrate. Sometimes, there are systems to inject brine concentrate directly into the sewer, which causes it to mix naturally with wastewater, producing a single effluent of lower salinity and temperature, however, there is a possibility that treatment of this mixture is necessary before it is discharged into the environment, which only depends on the characteristics of the wastewater flow, as the retention capacity of organic compounds in the brine concentrate flow may cause the flow mixture to not be safely discharged at sea [73]. In addition, the high salinity of the concentrate can cause wastewater to be treated under certain conditions [74]. Due to all these factors, brine rejection has been attempted to mix with cooling water from power plants rather than wastewater, which is possible as long as the plant in question is located near the desalination facilities. This technique seems quite feasible because both currents are compatible, so that a cold and dense stream of concentrate is being mixed with another hot and light cooling water. As a result of this mixture, the time required for dilution at sea is reduced, increasing the degree of dilution of the discharge. Another way of removing the brine concentrate is the grounding technique, which is an economically viable option for desalination plants that are placed far from the coast. Injection into deep groundwater reserves is a method of elimination by which rejected brine is injected into an underground aquifer containing non-drinking water, however, this system could create uncertainty about the environmental impact by being very complex measuring it [76, 75]. Deep well injection is an advanced disposal technique used worldwide. Some of the most important factors to consider for a good discharge are the geological strata, the desired underground aquifer depth, the type of well used, the availability of the well, the compatibility between the well water and the brine and the possible reception that this may have for the population [77]. A porous injection site able to withstand the injection flow rates during the installation life is required. In addition, a geological layer of hydrophobic materials, such as clay or mother rock, is required to prevent leakage into freshwater streams or terrestrial environment of the injected brine [76, 75]. Another alternative under study is the injection of brine concentrate into a depleted oil well, which is feasible to improve the reduced pressure normally resulting from oil wells after constant production [78]. Six wells were studied for brine injection recovery research, with different rates of oil production recorded as a result of injection. It is therefore concluded that brine injection into low-level oil wells is a viable form of pressure recovery approach for all wells [78]. Studies such as Badescu et al [79] have developed different ways of enhancing the sustainability the brine removal through submerged pipes. Technical, economical, and environmental restrictions are defined in order to discover the best way of brine removal, with the aim of reducing the energy spent with this method. Among other aspects, the optimal pipe length and depth of the discharge point are determined. In the study of Schuiling et al [80] the brine removal from Strait of Ormuz to the Arabic Sea without any energy expenditure is examined. 2.3. Reuse application of reject brine Despite the fact that some processes may be effective in recovering and minimizing the volume of brines, it should be considered that this current can be a valuable resource for the production of mineral salts, metals and/or chemicals. Currently, different technologies are focused on the production of salt and chemicals from brine to ensure the reduction of the total cost of the desalination process. In addition to the economic benefits, rejected brine recovery can reduce the environmental impacts associated with this process [81, 82]. However, the capital and operating costs of these systems are obviously higher than those associated with a common desalination process with no brine recovery at all. There is a wide range of research to recover salts from the residual brine to use reuse them for other applications [81 - 88], this idea is explained more extensively below. 18 Brine management methods 18 2.3.1. Chemical precipitation Chemical precipitation is a feasible method for recovering salt from rejected brine. This method has been carried out by using various precipitating agents [85]. As an experiment for salt recovery, different samples such as untreated seawater, synthetic seawater and two concentrates of reverse osmosis processes obtained from desalination plants located around the Mediterranean and Red Sea coasts were used, sodium phosphate and sodium carbonate were used as chemical precipitants. Magnesium and calcium salts were recovered thanks to the precipitating agents used. Using sodium phosphate, calcium recovery was 98% and 75% from seawater and RO concentrate, respectively, 47% and 24% of magnesium recovered from seawater and RO concentrate, respectively as well. Using sodium carbonate to achieve precipitation, calcium reduction was achieved from 690 mg/L to 36 mg/L and 400 mg/L to 20 mg/L for RO concentrate and seawater, respectively. In addition, magnesium concentration decreased from 2600 mg/L to 2375 mg/L and 1460 mg/L to 1250 mg/L for RO concentrate and seawater, respectively. Per kg of sodium carbonate used, 2 kg/m3 of tracing and magnesium deposits were obtained from the RO concentrate. In addition, 1.43 kg/m3 of seawater tanks were obtained. Prior to chemical precipitation, pretreated with membrane/thermal and ion exchange technology were carried out to improve the obtained results. This technology is suitable for new plants, or to modernize current desalination plants with nanofiltration membranes in such a way that brine obtained with this method can be treated with a two-stage chemical treatment and an ion exchange purification. Rejected brine can also be decalcified in such a way that ion exchange can be used to remove magnesium from the decalcified current. In order to implement these changes at the industrial level, the optimization and economic analysis of these treatments are a priority. The valorization of calcium and magnesium subproducts of concentrated RO brine through chemical precipitation using sodium carbonate precipitants and sodium hydroxide at 25ºC and 65ºC is another salt recovery technique [89]. Casas et al. [89] observed that calcium removal was affected by temperatures above 65ºC, but magnesium removal was not affected. In addition, calcium and magnesium recovery was masked by antifouling agents and metal ions present in the rejection current; therefore, it was necessary to concentrate even more the rejected brine by using electrodialysis systems, which helps reducing the amount of antifouling agents. 0.35-14 g/L sodium carbonate and 0.85 g/L sodium hydroxide were used to achieve the greatest calcium and magnesium removal. 10 mg/L of calcium and magnesium remained in the residue of the ED-RO process and elimination efficiencies of >95% were achieved in all experiments. The order of treatment processes did not influence overall disposal efficiency. Although low concentrations of calcium and magnesium were observed in residual subproducts, they may not be suitable for some applications because they still contain NaCl. Meanwhile, these subproducts can still be used as agents to control pH and recover fertilizers containing wastewater treatment phosphates. 2.3.2. Adsorption The rejection of brines from desalination processes is a potential source of metal collection. A simple and effective way to get those metals can be through an adsorption process. Although the following properties of the metals must be taken into account in determining the metal to be recovered from rejected brines: physical properties, chemical characteristics, technical feasibility of the use of adsorbents to remove metals from the brine resource and economic and environmental benefits derived from the recovery of such metals [90, 91]. The feasibility of using various adsorbents to recover metals such as rubidium, cesium, uranium and lithium from seawater RO brine [90] has been investigated. It was concluded that the hexacyanoferrate-based extracting medium (CsTreat) was the best adsorbent for cesium and rubidium extraction. Cesium Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 19 19 extraction using CsTreat was not significantly influenced by salinity. In addition, CsTreat was able to select only cesium and rubidium for adsorption. However, different results were obtained for adsorption in metallic and bimetallic systems, performing worse in bimetallic systems. Lithium could not be eliminated by any adsorbent and remained in the brine. In addition to the associated environmental benefits, high returns on investment in metal recovery from brine rejection can be obtained, especially for uranium, a very valuable material as an energy source through nuclear energy. However, metal recovery technologies require more research in order to improve the extraction process and the performance in near future [92]. A similar investigation has evaluated the extraction of valuable metal ions from the RO concentrate using selective sorbents [93]. Cesium, lithium, rubidium and uranium have been included in the group of metal ions. These metal ions (Cs(I), Li(I), Rb(I) and U(I)) were extracted using commercial sorbents, namely CsTreat, ZrP and S910. It was reported that CsTreat was able to achieve the highest extraction capacity for Rb(I) and a reasonably high adsorption of Cs(I). S910 had a good adsorption capacity for U(VI) but that of Li(I) by ZrP was unsuccessful. It was determined that there is a need for further research and studies aimed at recovering valuable metals from residual brine using different adsorbent agents and adsorbent-based materials in order to achieve an economically feasible and a technically simple method [93]. An adsorbent resin (chelating resin crosslinked polymer resin) has been developed and experimentally characterized in discontinuous fixed-bed column operation to select and extract uranium from brine [94]. The polymer chain has stereoselectivity towards the widely populated tricarbonyl complexes in uranilo compounds that may be present in some concentrated solutions. This is especially important when the source of concentrate is residual brine of RO, which normally has a mild acid pH and contains a lean amount of target uranium. In addition, there is competition between the uranium complex and other ions present in the brine to occupy the resin. This resin was characterized by a remnant of hydroxyamic acid because it was discovered that the acid achieved an adequate selectivity, good adsorption speed, adequate mechanical resistance, good load resistance, simple elution and detection by chromatographic means in such a way that it can be altered by polymers of different sizes and shapes. This process was shown to exhibit better economic viability, but it was concluded that more research is needed to make the extraction process more efficient by reducing the steps involved, which will eventually reduce the cost of treatment. Another study used BAMBP [4-tercbutil-2-(a-methylbencil) phenol] or 4-sec-butyl-2-(amethylbencil) phenol] to extract the rejected brine rubidium obtained from a RO desalination plant with a capacity of 100,000 m3/d [91]. More than 80% rubidium was recovered. NaCl was also extracted from the rejected brine. In the same way, an attempt was made to extract phosphorus with the extractor agent, but this step was not economically feasible. This can be achieved on an industrial scale if restrictions on phosphorus discharge are increased. The extraction of phosphorus from the rejected brine would result in significant environmental advantages because it would reduce eutrophication in the natural environment. 2.3.3. Membrane and electrokinetic processes The behavior of nanofiltration for the removal of sulfates and chlorides from brine [95] has been tested in pilot-scale experiments. Experimental results were theoretically compared to the Donnan Steric Pore Model (DSPM). The membrane used for the process (NF270) provided good results with a sulfate rejection of 70-96% and a chloride rejection of 2-11% under an operating pressure of 500-2000 kPa, an ion force of 0.1-1.3 M and an initial chloride concentration of 0.2-1.2 M. Through the mathematical model an accurate description of the experimental ion rejection of the membrane was obtained using the pore radius, the thickness of the membrane layer and the load density of the membrane. This study concluded that the nanofiltration separation process to 20 Brine management methods 20 remove sulfate and chloride was effective for highly concentrated saline solutions and efficiency of the process could be predicted through the use of the mathematical model used. Bipolar membrane electrodialysis (BMED) has also been applied to reuse RO rejection brine generated from a seawater desalination plant, turning it into products with both possibilities, acidic and basic pHs [96]. It was shown that it is possible to produce acids or bases with concentrations of 1 M or higher with an efficiency of between 60 and 90 % thanks to this process. On the other hand, efficiency is affected by the transfer of ions to the highest concentration of acid through the anionic exchange membrane as opposed to the slow movement of the co-ions and to the most basic parts through the membrane. The inlays and scales of the membrane were caused by multivalent anions and cations; therefore, it was recommended a pretreatment prior to the principal treatment with BMED. The acid and base produced by this method include hydrochloric acid and sodium hydroxide [96]. Sodium hypochlorite can be generated through on-site electrochlorination from brine residues [97]. An electrolysis cell can be used for the process by feeding brine and exposing it to a minimum voltage of direct current (DC). In the work of Abdul-Wahab and Al-Weshahi [97], sodium hypochlorite appears instantly as the electrolyte process took place. This method prevents hazardous chlorine gases and the risks of transporting it since the production occurs in the form of sodium hypochlorite. These benefits improve the process environmentally. In addition, hydrogen gas could also be produced and transported through a pneumatic transport system [97]. Because hypochlorrite is preferred over chlorine as an anti-incrustatne and disinfectant, the cost of purchasing hypochlorite for the main desalination process can be saved by recycling hypochlorite from treating the same brine. This method can be a solution for brine waste management and a business opportunity to environmentally improve the desalination process, making it more sustainable. The separation of brine salts by electrodialysis is improved by the use of cationic and anionic exchange membranes located between the electrodes and connected to the electrical current. This process can be used as a pretreatment step before other methods, such as evaporative cooling or crystallization, so that brine can be dehydrated first. This step is also necessary to remove calcium or sulfate ions that may cause gypsum crystallization [82, 83]. Inlays due to organic and colloidal particles should be considered when designing electrodialysis cell to remove salt from rejected brine [82]. This mechanism has also been used to study the possibility of water recovery, in addition to the salts recovery [84]. This process consists of four anionic exchange membranes and five cationic membranes. The performance impact of variation on operating conditions, such as membrane characteristics, current density, and initial brine concentration, was investigated. It was observed that the electro-osmotic potential influenced the displacement of water from the saline solution, leading to a recovery of 67.78% fresh water and a desalination rate of 72.74%. In addition, a highly concentrated output brine was obtained, reaching 27.13% m/V, and can be used for the production of edible sea salt by crystallization. 2.3.4. Crystallization and evaporation One approach to salt recovery from brines is Selective Salt Recovery (SSR) by crystallization. SSR could also be called fractional crystallization, as it involves precipitation of dissolved salts through alterations in the concentrations of these, through the influence of temperature changes. The SSR involves a sequence of processes designed to recover specific salts from concentrates based on differences in their solubility and their ease of being precipitated or adsorbed [98, 99, 88]. The marketing of salts produced from SSR makes this approach cost-effective and beneficial to the environment, since instead of discharging these salts to the environment they are recovered for commercial applications. The performance of the SSR process is influenced by operating conditions, such as pressure, pH and other components, in addition to the characteristics of the salts to be obtained. The wide variety of forms of saline particles and their ability to remain in an Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 21 21 unstable balance (metastability) are key factors in the kinetics of the process and must be studied before choosing any operating conditions for SSR. Selective salt precipitation is particularly influenced by process pH, temperature, residence time, the presence of antifouling agents, mixing efficiency and the physicochemical nature of salts in terms of crystal shape, size, and purity [98, 99]. Currently, the use of the SAL-PROC ™ system and brine concentrators are the most common steps in a SSR process [88]. The most important salts are those that are cost-effective, such as gypsum, calcium carbonate, magnesium hydroxide, sodium sulfate, sodium, and calcium chlorides. These can be recovered from salt water by selective extraction using the SAL-PROC process ™ [88]. This approach requires some sequential steps of chemical processing and evaporation cooling. Theoretically, the salt recovery obtained from the process was the 100% and the ZLD of the rejected brine can be achieved with this method [92, 99, 81, 88]. One point of improvement to consider with respect to the SSR process is the optimization of salt production for human consumption, which guarantees greater respect for the environment due to the reduction of brine discharges and a higher profitability of the desalination process [99]. Mineral salts can also be segregated from brine using evaporation or distillation technologies [92, 87]. In addition, the thermal energy required for these technologies can be supplied by renewable energy production sources, such as solar energy. The RO concentrate of a brackish water desalination plant has been used to produce salts with the help of a solar pond [87]. The plant produced about 12,024 m3 of rejection. Before salt recovery, salt forms were produced in liquid, crystalline and solid suspension states thanks to the solar pond. These salts contained K+, Na+, Ca2+, Mg2+, SO4 and Cl2. Research was conducted using potentiometry, gravimetry, SEM and XRD analysis to analyze these components, demonstrating that the evaporation of the concentrate at a certain temperature and pressure can be used to recover certain salts, including potassium chloride, sodium chloride, magnesium sulfate heptahydrate, and calcium sulfate dehydrated. Salts are important for agricultural and industrial applications [81]. Solar evaporation ponds can also be combined with other brine management practices to produce energy or salts. Salts produced from this type of processes include potassium chloride, potassium sulfate, sodium sulfate, lithium sulfate and boric acid [100]. Another approach to thermally segregating water and salts from the saline solution is the use of a heat pump [82]. Using isothermal evaporation, one study investigated the recovery of mineral salts present in the rejection brine of an RO process that operated with seawater and reported that calcium, magnesium and silicon were first eliminated by a pretreatment with soda lime [101]. The volume of the concentrate was subsequently reduced by evaporation and, at the same time, profitable salts such as sodium sulfate were produced. It is established by the results obtained that 81-88% of Na2SO4 was contained in the by-products of sodium sulfate. Only 1% of the original brine volume remained after evaporation. In addition, the complete extraction of Na2SO4 was achieved. Although the total cost of desalination can be reduced by recovering salts through this method, additional economic analysis is required to study the economic viability of this approach on a long-term and industrial scale. 2.3.5. Regeneration of wetlands and agriculture applications Rejection brine can be used to regenerate wetlands and salty ecosystems that suffer from progressive degradation due to drought and human action. These wetlands can be useful for agricultural, livestock, industrial, and local fauna and flora development. Two possible variants of regeneration processes are noteworthy: the regeneration of both coastal and continental wetlands. In the regeneration of coastal wetlands, brine from a seawater-based desalination process should be used, while, in the regeneration of continental wetlands, brine must come from RO processes working with brackish water. The economic and environmental effects of this approach depend on the used brine and the characteristics of the wetland being restored. Working 22 Brine management methods 22 with this method would with no doubt improve the quality and availability of water resources [102, 82, 103, 104]. Rejection brine can be used for the cultivation or irrigation of a diverse number of species such as plants, animals or microorganisms that are tolerant to slightly higher saline concentrations than freshwater or than seawater. Two of the most studied options are the cultivation of Artemias (brine shrimp) and the dunaliella saline microalgae [102, 105]. Artemia is a small crustacean that is no more than two centimeters long and only appears in high salinity waters (70 g/L to 340 g/L), although it can also develop in seawater. It is used as a natural food for larvae or young and adult species of fish and shellfish [102]. On the other hand, Dunaliella salina is a microalgae chlorophyte that stores large amounts of carotenoids (up to 14% of its dry mass) and contains high amounts of glycerol, which makes it a natural source of carotenoids. In addition, it generates products that are useful for pharmaceutical or food applications [106]. Research by Raja et al. [106] has shown that higher rates of carotenoid production can be achieved using this method compared to commonly used crop techniques. Although the mechanism in which brine stimulates the production of carotenoids in Dunaliellas has not yet been described, it opens up the possibility of using this type of crop as an alternative, especially in areas where there is a high presence of brine from desalination processes due to lack of fresh water [102, 106]. In addition to the previous mentioned, algae farming in ponds in natural brine conditions has also been investigated in a pilot-scale unit [107]. The unit consisted of three parallel rafts for open spaces in natural conditions. Scendesmus algae were grown in laboratory in controlled media to optimize their growth in saline water, after this process, they were introduced into rafts, which contained water with a salinity range of 40,000-80,000 mg/L under constants measurements in order to obtain the efficiency in the removal of Dissolved Total Solids (DTS). The results showed that the DTS elimination efficiency of algae species varied in a range of 13% and 63% [107]. The variation in disposal efficiency was due to the concentration of DTS, the retention time on the raft, and the conditions. This method was considered to have achieved high DTS elimination and the highest final concentration was at permissible limits for safe injection into the sea. An additional option is to use the brine to grow fish species that are able to live in saline environments. Finally plants species that are tolerant to high salinity levels or brackish water (up to 8000 mg/L) such as beetroot, sugar cane, cotton, dates, barley, spinach, asparagus, or wheat can be grown using reject brine [102]. Some grasses nowadays such as Paspalum can also be irrigated with high salinity water. However, additional research works are still needed to evaluate the ability of biological cells to grow in brine with or without pretreatment and the environmental cum economic implications of this type of application. 23 3. BRINE CONCENTRATION This part of the Project has been studied due to the “Turbomachinery conference and exhibition” which took place in Sevilla, in June of this same year (2020). Many interesting projects were presented in this exhibition, but it is one of them that is especially strinking and causes the inclusion of these calculations in the work. This simulation is reflected in an article called "Micro Gas Turbines Supporting Renewable Energy Systems in Off-Grid Water Treatment Facilities in the Mining Industry", which introduces the outlet gases from a microturbine system to help heat salt water, causing its evaporation and thus obtaining water with higher concentration in salts, and evaporated water of higher purity level. In this way, the residual heat of these gases can be used at a relatively low temperature to obtain a considerable source of drinking water. This section details the calculations and equations for a saltwater concentration system heated with flue gases, adapting the article system to project-specific operating conditions. The system is as follows: Fig. 6. System diagram Siendo: • Current 1: input of pre-heated brine. • Current 2: gases input. • Current 3: brine outlet flow already concentrated. • Current 3’: water los due to evaporation due to the heat incorporated by gases. • Current 3’’: water losses from drag due to gas speed. In distillation plants, the gases formed pass through a dehumidifying mesh, referred to as demister, that retains moisture droplets. For this particular case and due to the large flow rates that are handled, calculations for drag losses are considered negligible. • Current 4: outlet of cold gases. It is detailed a matter and energy balance to define the system. 3’’ 30 Conclusions 30 4.3. Effect of m2/m1 ratio in the mass flow m3 Figure 9 and figure 10 shows the behavior of the oversized flow of the brine current to the output versus the evolution of the temperatures of the combustion gases and brine at the inlet of the control volume for two different values of the ratio m2/m1. Fig. 9 m3/m1 evolution with T1, T2 and m2/m1=0.1 Fig. 10 m3/m1 evolution with T1, T2 and m2/m1=0.2 The effect of increase the ratio m2/m1 shows that it is no related to T1 temperature due to the different between the graph has the same behavior. Nevertheless, as the amount of m2 increase the heat transferred to the brine increase so the effect of T2 is more important and increase the amount of water evaporated. Concentrate treatments in reverse osmosis desalination plants: Status and innovative proposals 31 31 4.4. Effect of m2/m1 ratio in the salinity S3 Figure 11 and figure 12 shows the salinity behavior of the brine current (S3/S1), versus the evolution of the temperatures of the combustion gases and brine at the inlet of the control volume for two different values of the ratio m2/m1. Figura 11 S3/S1 evolution with T1, T2 and m2/m1=0.1 Figura 12 S3/S1 evolution with T1, T2 and m2/m1=0.2 The effect of increase the ratio m2/m1 shows that it is no related to T1 temperature due to the different between the graph has the same behavior. Nevertheless, as the amount of m2 increase the heat transferred to the brine increase so the effect of T2 is more important and increase the amount of water evaporated so the salinity S3 increase. 32 Conclusions 32 4.5. Effect of S1 in the salinity S3 Figure 13 and figure 14 shows the salinity behavior of the brine current (S3/S1), versus the evolution of the temperatures of the combustion gases and brine at the inlet of the control volume for two different values of the salinity S1. Figura 13 S3/S1 evolution with T1, T2 and S1=0.07 Figura 14 S3/S1 evolution with T1, T2 and S1=0.085 The salinity S1 affect BPE value. For t=100ºC and S=0.07 BPE= 2.16ºC, if S=0.085 then BPE= 2.94ºC. 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