Method for recovering water and eliminating nitrate ions in aqueous saline flows and device for carrying out the method

Abstract

The present invention relates to a method and a corresponding device for recovering water and eliminating nitrate ions in aqueous currents, based on denitrification by means of electrochemical reduction. The method allows nitrate ions to be converted into N2 gas and the majority of the treated water to be recovered as low-mineralised water, which helps minimise the environmental impact associated with the discharge of generated wastewater. Advantageously, the invention allows some of the ions in the treated water to be converted into recoverable insoluble salts, while other ions are managed as concentrated solutions useful in steps of the method. The invention also allows the gas mixture obtained in one of the steps of the method to be used to supply a fuel cell, thereby reducing energy costs and helping minimise fossil fuel use in the process.

Description

[0001] DESCRIPTION PROCEDURE FOR RECOVERING WATER AND REMOVING NITRATE IONS FROM SALINE AQUEOUS STREAMS AND EQUIPMENT FOR CARRYING OUT SAID PROCEDURE FIELD OF THE INVENTION The present invention falls within the field of environmental remediation, more specifically, in the treatment of water with a high saline concentration and contaminated with nitrate ions (NO3-), additionally allowing the valorization of the by-products generated in the different stages of the process of the invention. The invention that is proposed relates, in a specific formulation, to a multi-stage process and the equipment for carrying out said process, preferably for application in the treatment of reject water from Reversible Electrodialysis (EDR) desalination plants, which have a high content of nitrate anions (NO3-).Advantageously, the proposed invention achieves the complete elimination of nitrate anions, primarily in the form of gaseous nitrogen (N2), as well as the utilization of water and the valorization of the remaining saline components to achieve an overall process close to zero discharge. The central stage of the procedure is the direct electrochemical reduction of nitrate ions (REN), also contemplating a series of pre- and post-conditioning stages of the reject water to be treated, with which the following are achieved: minimizing the electrical consumption of the electro-reduction (REN) stage, increasing the amount of recovered water, achieving an almost complete transformation of NO3- into N2 (with minimal quantities of other products such as NH4. +or NO2-) and valorize the H2 generated as a byproduct during the electrochemical reduction of NO3-. Additionally, the recovery of insoluble calcium and magnesium salts and saline solutions for the regeneration of the different resins involved in the process is enabled. BACKGROUND Groundwater contamination is becoming a major global problem. One of the main causes of groundwater contamination is the presence of high nitrate contents in aquifers [R.F. Spalding, M.E. Exner, “Occurrence of Nitrate in Groundwater - A Review, Journal of Environment Quality”, 22 (1993) 392–402.; I. Karlović, K. Posavec, O. Larva, T. Marković, “Numerical groundwater flow and nitrate transport assessment in alluvial aquifer of Varaždin region”, NW Croatia, J. Hydrol. Reg. Stud.41 (2022) 101084].The origin of this pollution is mostly due to anthropogenic activities, mainly the use of fertilizers [P.N. Vinod, P.N. Chandramouli, M. Koch, “Estimation of Nitrate Leaching in Groundwater in an Agriculturally Used Area in the State Karnataka, India, Using Existing Model and GIS”, Aquatic Procedia, 4 (2015) 1047–1053; F. Beeckman, H. Motte, T. Beeckman, “Nitrification in agricultural soils: impact, actors and mitigation”, Curr. Opin. Bio-technol.50 (2018) 166–173] and waste from intensive livestock farming [P.K. Sahoo, K. Kim, M.A. Powell, “Managing Groundwater Nitrate Contamination from Livestock Farms: Implication for Nitrate Management Guidelines”, Current Pollution Reports, 2 (2016) 178–187]. The increase in nitrate concentrations in surface and groundwater leads to negative impacts on the environment and human health [NS Bryan, J.Loscalzo, “Nitrite and Nitrate in Human Health and Disease”, Humana Press, Cham, 2017; Internation Agency for Research on Cancer (IARC), World health organization international agency for research on cancer, “Iarc Monogr. Eval. Carcinog. Risks To Humans”. 94 (2010) 1–464; R. Picetti, M. Deeney, S. Pastorino, MR Miller, A. Shah, DA Leon, AD Dangour, R. Green, “Nitrate and nitrite contamination in drinking water and cancer risk: A systematic review with meta-analysis", Environ. Res. 210 (2022) 112988]. Nitrate can pose a risk to human health as it can trigger cases of methemoglobinemia (hemoglobin incapable of transporting oxygen), cancer, as well as thyroid problems. For these reasons, different legislations limit the concentration of nitrate in drinking water. For example, The Nitrate Directive (EC, 1991) is the European legislation that establishes a maximum concentration of 50 mg L. -1for nitrate [European Union (EU) Nitrates Directive (1991), European Commission]. Other directives are: 2015 / 1787 / EU, 2006 / 118 / EC and 2013 / 39 / EC. The World Health Organization (WHO) also recommends a maximum contaminant level (MCL) of 50 mg L -1of nitrate in drinking-water [World Health Organization (WHO), Nitrate and Nitrite in Drinking-water. Background document for development of WHO Guidelines for Drinking-water Quality, 2003]. As a consequence of the problem generated by the presence of nitrates in water, there has been a growing interest in the development of efficient denitrification technologies. There are two approaches available in the field of wastewater denitrification: separation techniques and transformation techniques. Currently, processes based on separation techniques are the most widespread methods for removing nitrates from groundwater to obtain water suitable for human consumption. These are based on the displacement of nitrates from the contaminated stream to be concentrated in an auxiliary waste stream. The most widely used techniques are membrane desalination technologies (reverse osmosis and electrodialysis) and anion exchange resins.As mentioned, the main drawback shared by all these known technologies is the creation of secondary wastewater streams containing high concentrations of nitrates, which must be subsequently managed. Transformation techniques are based on the destruction of nitrate by chemically converting it into other compounds. Within this group, there are two different approaches: biological removal and (electro)catalytic denitrification. Biological removal uses bacteria to carry out the denitrification process. On the other hand, (electro)catalytic processes such as heterogeneous catalysis and electrocatalytic reduction use a reducing agent (chemical reagent or electrons) in combination with an (electro)catalyst to reduce nitrate ions.The main limitations of biological denitrification technology are its inability to treat concentrated nitrate waste streams, the formation of sludge, and the risk of pathogenic bacterial contamination in drinking water. Ideally, electrochemical denitrification can circumvent the limitations of the other techniques discussed (separation and biological denitrification technologies). It involves carrying out the electrochemical reduction of the ion directly on the cathode of an electrolytic cell. However, due to the complex chemistry of nitrate, the process can produce a wide range of products: NO2-, NH3, NO, NO2, N2O, N2H4, NH2OH, and N2, with nitrogen gas (N2) being the only one of interest as it is harmless. The accumulation of any of the other products must be avoided, as their toxicity is even greater than that of nitrate, which would exacerbate the initial problem.The half-reaction of nitrate reduction (NO3-) to nitrogen gas (N2) is as follows: NO3- + 5 e- + 6 H. +→ ½ N2+ 3 H2O The selectivity of the process towards nitrogen gas depends on many factors such as the design of the electrolytic cell, the working mode or the specific composition of the water to be treated, but the most important variable is the cathode material that will act as electrocatalyst (Duca and Koper, 2012). The international patent application WO2020128121A1 entitled 'Process for the electrochemical reduction of nitrates in water by means of combinations of Bi and Sn', which claims the priority of the patent application ES2713374A1, describes a process for the electrochemical removal of nitrates in contaminated water, using a cathode composed of an alloy of bismuth and tin on which the nitrate ion is mostly reduced to nitrogen gas.From the point of view of the energy efficiency of the electrochemical nitrate reduction (ERN) process, as the nitrate reduction takes place in an aqueous medium, it will always compete (to a greater or lesser extent depending on the experimental conditions) with the reduction of the solvent to generate H2 H2O + 1 e- → ½ H2 + OH- (in a basic medium) H. ++ 1 e- → ½ H2 (in an acidic medium), consuming part of the electric current applied to the electrochemical reactor and constituting the main energy inefficiency of the NO3- reduction process. Thus, the aforementioned patent document WO2020128121A1 discloses a process for the electrochemical reduction of nitrates with high selectivity in the reduction of NO3- to N2. However, this process presents a series of common limitations of NO3- electro-reduction processes for their industrial implementation, which include: - inability to treat water with high hardness, such as that presented by the type of water object of this application (220-320 ºF) - moderate energy efficiencies and treatment capacity at low NO3- concentrations (less than 500 ppm) - the management of small quantities of NH3 generated as a by-product of the reduction of NO3-.Thus, the present invention offers a comprehensive treatment system for wastewater from Reverse Electrodialysis (RED) plants, which have high NO3- ion contents, which resolves or minimizes the drawbacks described above, allowing for a viable and effective process. To this end, the method of the invention is based on a REN process and includes a series of pre- and post-treatment stages aimed at addressing the practical limitations of REN processes mentioned in the previous paragraph and recovering the remaining saline content present in the treated process water.The provision of an ammonia sensor, preferably a solid colorimetric ammonia sensor such as that disclosed in patent ES2619356B1 'Passive device for the in situ detection and / or determination of ammonia in gases' and in European patent EP3001184 'Passive colorimetric sensor for the detection and / or determination of ammonium, ammonia or aliphatic amines in gases, manufacturing method and use', whose priority corresponds to patent application ES2519891, guarantees the control of ammonia release. Finally, in order to minimize the negative effect of the solvent (water) reduction reaction on the energy efficiency of the process, the electrochemical stage of NO3- ion reduction has been designed to allow the energy recovery of the generated H2.DESCRIPTION OF THE INVENTION The studies carried out to date on electrochemical reduction as a technique for eliminating NO3- in contaminated waters of different nature address water treatment from an individualized perspective of this technology; being incapable of solving a series of practical problems inherent to its application to real waters on an industrial scale. At this time, there are no precedents for treatment systems based on electrochemical reduction integrated into a global system with a series of pre- and post-conditioning stages of the water stream to be treated applied to real waters.The method and equipment proposed in the present invention are particularly advantageous for the treatment of nitrate-contaminated wastewater from reverse electrodialysis (RDA) desalination plants, although they can also be applied to any stream with a high nitrate concentration. The composition of wastewater from an EDR plant can vary depending on the characteristics of the inlet water and the plant's operating conditions. However, they are generally characterized by an approximately neutral pH (6.8-7.3) and a conductivity in the range of 3.2 to 4.7 mScm. -1 . They also have high hardness values, between 220 and 320 ºF (Ca concentrations 2+ and Mg 2+greater than 750 ppm and 250 ppm), nitrate concentrations in the range of 320 to 460 ppm and sulfates, chlorides and bicarbonates, mainly sodium, up to 2-3 g / l. The inclusion, order and operating particularities of the pre- and post-treatment stages of the reject and concentrate waters that constitute the present invention respond to the characteristics of the REN process as follows. In accordance with the essence of the invention, the claimed process comprises the following stages: - Reduction of the hardness of the saline aqueous stream to be treated, such as the reject from an EDR process, by means of ion exchange in a Na-cation exchanger +, obtaining a softening of at least 98% of the initial concentration of calcium and magnesium ions present in the saline aqueous stream to be treated. - Next, the outlet stream from the softening stage is subjected to an osmosis stage to concentrate the nitrate ion content of the outlet stream from the cation exchanger by means of double-stage reverse osmosis with a recirculation of the concentrated outlet flow of at least 75%, generating a permeate stream with a total dissolved solids (TDS) concentration of less than 500 mg / L and a concentrate stream with a nitrate ion concentration of at least 1000 ppm. Preferably, the osmosis stage has a 5 µm microfiltration pretreatment, with antiscalant and acid dosing and high-pressure pumping.Advantageously, this osmosis concentration stage allows the subsequent REN stage to be fed with a stream with a high NO3- ion content. Equally advantageously, the permeate stream generated in osmosis can be used to feed the EDR process or any other process through which the saline aqueous stream to be treated was generated. - pH adjustment, between values ​​of 8 and 13, of the concentrate stream generated in the previous osmosis stage and feeding it as a catholyte stream into an electrochemical reactor provided with a cathode consisting of a combination of bismuth and tin.- Electrochemical reduction at the cathode of the nitrate ions contained in the catholyte of the electrochemical reactor, which are converted into N2 gas in a percentage of at least 60%, and reduction of the aqueous solvent generating H2 gas, obtaining, on the one hand, an output gaseous stream with N2, at least 97% of H2 and other gases; and, on the other hand, an output aqueous stream with a maximum percentage of ammonium ions of 40% with respect to the initial nitrate ion content and a nitrate and nitrite ion content below 50 and 5 ppm, respectively. - Checking the ammonia / ammonium levels in the output gaseous and aqueous streams of the electrochemical reduction. - Addition of sodium hypochlorite to the output aqueous stream of the electrochemical reduction for the oxidation of ammonium NH4. + and nitrite to N2 and NO3-, respectively, so that the nitrate content of the outlet water from this stage is less than 50 ppm and the ammonium content NH4 +is less than 5 ppm, - Checking the ammonia / ammonium levels in the output stream of the oxidation stage. - Demineralization of the output stream of the oxidation stage using two ion exchange stages, a cation exchanger-H + with Na resins + and an ion exchanger-OH- with Cl- resins. - Activation of the cation exchange resins- Na + from the previous stage with HCl and obtaining solutions concentrated in Ca and Mg and activation of the anion exchange resins-Cl- with NaOH obtaining saline solutions enriched in Na + and Cl-. - Regeneration of the resin used in the stage of hardness reduction of the saline aqueous stream by using solutions enriched in Na +and Cl- and recovery of insoluble Ca and Mg salts. - Purification of the gaseous by-product obtained from the electrochemical reduction by removing traces of ammonia using a trap with an ammonia-capturing solution. - Checking the ammonia levels in the output stream from the purification stage. - Drying and deoxygenation of the stream obtained in the previous stage to obtain a hydrogen stream. - Conditioning and storage of the stream obtained in the previous stage. - Energy recovery from the hydrogen stream from the previous stage by means of a fuel cell for the generation of electrical energy. With regard to the essential stage of the procedure described, the electrochemical reduction of NO3- occurs at the cathode of the electrochemical reactor, where electrons are supplied from the electrode to the species to be reduced.Depending on the number of electrons exchanged, the reaction will give rise to a variety of reduction products: NO2-, NH3, NO, NO2, N2O, N2H4, NH2OH and N2. For all the possible cathodes to be used in this process and independently of the reaction product formed, the reduction of NO3- always entails an increase in the pH of the solution in the vicinity of the cathode (the half-reaction consumes one H. + for each electron transferred from the cathode). On the other hand, it should be noted that, in the electrochemical reduction stage, it must be considered that in addition to the REN reaction of interest on the cathode, the reduction of the solvent takes place simultaneously to form H2. This reaction also consumes an H +for each electron transferred. Another aspect to consider in the REN process is that an electrochemical reactor with the anode and cathode compartments separated (either by an ion-selective membrane or a non-selective porous separator) is preferably used. The reason is that, regardless of the cathode material used, the reduction of the NO3- ion always occurs in several stages that involve the exchange of a single electron in each step. This leads to the appearance of reaction intermediates within the solution that will continue to be reduced. The most important of these intermediates is the nitrite ion NO2-. If an electrochemical reactor without compartment separation is used, the generated NO2- ion can reach the anode and be oxidized back to NO3-, resulting in a decrease in process efficiency.As indicated above, the saline aqueous reject streams to be treated have a high concentration of Ca ions. 2+ and Mg 2+Under the operating conditions of the REN process, where a strongly basic environment is generated in the vicinity of the cathode, these ions will precipitate primarily in the form of highly insoluble hydroxides and carbonates. The precipitates formed pose an extremely serious problem that must be solved if the process is to be implemented viably. The precipitates are deposited on all elements of the system. However, the problem is critical in the electrochemical reactor due to the blockage of the cathode surface and the separator pores, since both elements are quickly and irreversibly rendered useless (as mentioned, the greatest increase in pH occurs at the cathode surface). To address this problem, the present invention incorporates the initial step of hardness reduction or softening of the described aqueous saline stream by means of ion exchange.The water softening achieved is around 98% of the initial concentration of calcium and magnesium ions in the saline aqueous stream to be treated, through a complete bed prototype of a cation-Na exchanger. + which works continuously and allows to remediate the volume indicated in equation 1, expressing the retention capacity of the resin with respect to Ca, in one cycle: Likewise, the resin is regenerated for successive cycles with 10% NaCl (3 L per kg of resin and 1.5 L of reverse osmosis permeate water for washing). The concentrate obtained in each regeneration cycle, selecting the volume corresponding to the removal of Ca and Mg, is used to recover calcium and magnesium salts or hydroxides, and the resulting solution, with a high sodium content, will be reused along with the surplus from the regeneration process for further regenerations of this same resin. The concentrated mixture resulting from the activation / regeneration of the demineralization resins, which is carried out at the exit of the REN process, is also useful. Calcium / magnesium carbonates, sulfates, or hydroxides in particular have commercial uses in agriculture, livestock, and construction. After softening, the next step in the process is to increase the concentration of NO3- ions.The concentration of nitrate ions in the saline aqueous stream to be treated, preferably from an EDR process, is relatively low for the typical requirements of an electrochemical process. This results in limited transport of NO3- ions to the cathode surface and an increase in the extent to which the parallel H2 formation reaction takes place. This phenomenon results in decreased process efficiency, along with higher energy consumption and lower treatment capacity, making the electrochemical process unviable from an energy and economic standpoint. To avoid these drawbacks, the present invention utilizes the hardness-free saline stream from the hardness reduction stage to insert a reverse osmosis system prior to the REN stage.The reverse osmosis system consists of a double stage with high recirculation of the concentrate (around 75%), generating an osmotized flow with a total dissolved solids (TDS) concentration of less than 500 mg / L and a concentrated flow with high levels of nitrate ions above 1000 ppm. The osmosis system optionally features 5 µm microfiltration, high-pressure pumping to allow the flow to pass through the membranes, antiscalant and acid dosing to work with high ion concentrations, as well as a system for displacing the concentrate from the membranes with permeate water to prevent scaling. .The reverse osmosis process allows for the recovery of a high percentage of the demineralized water contained in the saline stream (estimated at around 75%). The main objective is to achieve a nitrate ion content exceeding 1,000 ppm, preferably 1,500 ppm, in the concentrated water at this stage. This way, most of the water is recovered, and a smaller volume of saline stream is obtained, which can be treated under better conditions in the subsequent electrochemical stage.In the pre-treatment stages prior to the REN stage, a low-hardness saline solution with a high concentration of NO3- ions is generated, which has important advantages for its electrochemical treatment: - The low hardness allows for the effective electrochemical treatment of the concentrate water since, as mentioned, in practice it is not feasible to carry out the REN process in an electrochemical system due to the precipitation of calcium and magnesium compounds on the cathode, separator and other elements of the electrochemical system. - The increase in the NO3- concentration allows the use of higher working current densities and obtain greater efficiency of the electro-reduction process. This translates into a significant reduction in energy consumption and treatment time, as well as in the reduction of investment costs by reducing the electrode area required to carry out the treatment.- The higher conductivity of the concentrate solution from reverse osmosis decreases the operating voltage of the electrochemical reactor and therefore the electricity consumption. The lower volume of concentrate water generated also results in lower investment costs by requiring a smaller electrochemical system. The electrochemical denitrification treatment (REN) used has been previously described in patents ES2713374 and WO2020128121, based on the use of Bi / Sn cathodes in a reactor with compartment separation. The present invention proposes a modification to this process based on the collection and storage of the H2 stream generated on the cathode during the parallel H2O reduction reaction. Since H2 is generated simultaneously with the REN process, its collection must be carried out simultaneously with the destruction of nitrate ions.This requires a modification of the experimental equipment for the electrochemical stage so that the generated H2 can be separated, purified, and stored under appropriate conditions for subsequent use in generating electrical energy. It should be noted that the process of the invention is viable when the electrochemical system operates either in continuous, single-step operation or in discontinuous, batch operation. Furthermore, the electrochemical reactor must have separate compartments to avoid explosive mixtures between the cathodic H2 and the O2 generated on the anode. The reuse of the H2 byproduct produced in the REN electrochemical process is proposed as a step subsequent to the REN process, since the gas stream obtained in the cathodic process is primarily H2 gas.In fact, it has been observed that the gas content of this stream in a typical formulation of the REN process reaches values ​​above 97% while the rest of the gases present are N2 (which comes from the transformation of NO3- ions), O2 (comes from the passage of O2 generated in the anodic process and which passes through the ion exchange membrane), H2O vapor (in equilibrium with the solution treated in the REN process) and traces of NH3 (come from the over-reduction of NO3- ions). This mixture of gases N2, O2, H2O vapor and traces of NH3 do not exceed 3% of the total content of the gaseous mixture generated in the cathodic branch of the REN process.The reuse of this gaseous stream poses the sequence of the following stages for its energy exploitation: 1) Purification of the gaseous by-product obtained in the electrochemical reduction by eliminating traces of ammonia with a trap with an ammonia capturing solution as the main by-product of the nitrate elimination of the REN process. To check the elimination of the NH3 content in the gaseous stream, the colorimetric solid passive ammonia sensor described in patents ES2519891B1, ES259891B2, EP3001184B1 is preferably used. 2) Drying and deoxygenation by means of a process of elimination of H2O vapor and O2 present in the hydrogen stream produced.Water vapor is present in the hydrogen stream due to the vapor phase equilibrium, as hydrogen is generated in the aqueous phase, and oxygen, present in very low percentages, is due to the transfer of oxygen from the anodic process through the membrane that separates the cathodic and anodic compartments. 3) Conditioning of hydrogen for storage according to the technologies currently on the market. 4) Recovery of energy from the hydrogen stream using a fuel cell or any other technology on the market that uses hydrogen as an energy vector. The immediate advantage of storing the hydrogen gas stream generated in the electrochemical stage for feeding into a fuel cell (air / H2) is to obtain electrical energy that minimizes the electrical expenditure of the electrochemical stage.On the other hand, electrochemical denitrification treatment based on the use of Bi / Sn cathodes achieves almost complete nitrate removal and high selectivity toward nitrogen gas generation (greater than 50% and potentially reaching 90% depending on experimental conditions). The main reaction byproducts are ammonium and nitrite (the latter to a lesser extent), both considerably more toxic and with more stringent maximum discharge concentrations than nitrate.This is a problem common to all REN processes, regardless of the cathode, electrochemical reactor or experimental working conditions, a problem that the process of the present invention solves by including a stage of treatment of the water coming from the electrochemical denitrification stage by adding sodium hypochlorite for the elimination of the ammonium and nitrite content by oxidizing them to N2 and NO3- respectively; also considering the process of nitrification of ammonium to nitrate having as an intermediate the formation of nitrite, according to the following reactions: 2NH4. + + 3NaClO → N2 + 3H2O + 2H + + 3Na + + 3Cl- NO2- + 2NaClO → NO3- + 2Na ++ 2Cl- In this process, the determined dosage of the oxidant established for the elimination of ammonium / ammonia is added. To do this, it will be necessary to determine the concentration of ammonium ions in the stream coming from the electrochemical reduction stage REN, which will preferably be carried out by using a passive solid colorimetric ammonia sensor such as the one disclosed in patents ES2519891B1, ES259891B2, EP3001184B1. At the end of this oxidation stage, the nitrate content of the treated water will not exceed 50 ppm and the NH4 content + will be less than 5 ppm; for monitoring the elimination of NH4 content + In the aqueous stream, the colorimetric sensor indicated above will preferably be used. Treated water that has undergone the REN denitrification process and the subsequent elimination of the NH4 ion content +, will contain anions and cations that were present in the concentrate water and that have not been treated by the REN process. In order to achieve a process with 'zero' discharge to the environment, the process of the invention includes a stage of valorization of this ionic content (demineralization post-treatment). This demineralization post-treatment process of the water from the electrochemical denitrification cell (REN) and the subsequent oxidation with NaClO, includes two ion exchange stages with two complete bed prototypes, one containing a cation exchanger-H + and the other an OH- ion exchanger. The treated volume is estimated from equation 2 for the first stage and equation 3 for the second: Demineralization uses approximately 150 g of each of the resins per liter of water to be treated, and achieves a reduction of approximately 97% of sodium ion and 99% of chloride ion, the Ca content is less than 10 ppm, the pH and conductivity are approximately 11.8 and 2.5 mS / cm in the demineralized water. This water can be mixed with the reverse osmosis permeate to be sent as header water, in accordance with a preferred embodiment of the invention. The activation and subsequent regeneration of the activated resins with Na is proposed. +and Cl- for successive cycles with 12% HCl and 12% NaOH. The waste obtained with both resins will be preferably used for the regeneration of the resin used in the softening. The proposed procedure implies that the saline aqueous stream to be treated, which has a saline content (nitrate ions and others) that prevents its discharge into public channels and reuse, will be subjected to the following stages: 1) Softening of the saline aqueous stream (such as the reject from an EDR plant) by means of ion exchange techniques and obtaining Ca and Mg salts in the activation / regeneration residues. 2) Obtaining high quality water (permeate) and water with a higher nitrate ion content (concentrate) by means of reverse osmosis.3) Elimination of nitrate ions present in the concentrate from the previous stage, transforming them primarily into nitrogen gas (an inert and non-toxic species) by means of electro-denitrification (ERN). 4) Transformation of ammonium ions present in the solution treated in the electro-denitrification process into nitrogen gas by oxidation with sodium hypochlorite. 5) Obtaining quality water (treated water) in the post-treatment process that uses ionic resins (demineralization). This stage includes the regeneration of the ionic resins, which will provide a concentrated NaCl solution for use in the regeneration of the softening resin. 6) Treatment and recovery of the gaseous stream obtained in the electrochemical reduction stage as an H2 stream suitable for feeding a fuel cell and obtaining electrical energy.Furthermore, the permeate from the osmosis stage and the treated water obtained in the demineralization stage can be mixed, providing water of sufficient quality for use in various operations at the plant that uses the EDR technique to obtain drinking water, which has a problem arising from the management of the EDR's reject water. Furthermore, the claimed procedure makes it possible to use the gas stream obtained in the catholyte during the REN process of nitrate destruction to power a fuel cell, thereby reducing the electrical consumption of the stages involved in the overall process.This proposed overall process concept makes it possible to make use of most of the treated water, generating a series of precipitated calcium and magnesium salts that can be valued on the market, in addition to reducing energy consumption by being able to use the gaseous stream, which essentially contains H2 gas, as feed for an air / H2 fuel cell. Finally, it only remains to indicate that the present invention also claims equipment for the recovery of water and elimination of nitrate ions in saline aqueous streams, in accordance with the described procedure, said equipment preferably being integrated by the following components: - a cation-Na exchanger. +, - a double-stage osmosis equipment, - an electrochemical reactor provided with a cathode comprising a combination of bismuth and tin - a sensor for the detection of ammonia / ammonium at the outlet of the electrochemical reactor, - a hypochlorite dosing device for oxidation in the outlet stream of the electrochemical reactor - a sensor for the detection of ammonia / ammonium in the outlet stream of the electrochemical reactor after the addition of hypochlorite, - a cation exchanger-H +, - an anion exchanger -OH-, - a trap with ammonia capturing solution, - a sensor for detecting ammonia at the outlet of the ammonia trap, - a system for condensing water and eliminating all oxygen from the gas stream, - a hydrogen storage system of hydrides, by means of a hydrogen compressor, - a fuel cell for generating electrical energy. BRIEF DESCRIPTION OF THE FIGURES In order to complement the description that follows and in order to help better understand the characteristics of the invention, in accordance with a preferred example of practical embodiment thereof, some figures are attached as an integral part of said description, in which the following has been represented, for illustrative and non-limiting purposes: Figure 1.- Shows a diagram of the process according to a preferred embodiment of the invention, in which the saline aqueous stream to be treated is the reject from an EDR plant. Figure 2.- Shows a diagram of the steps followed to utilize the hydrogen produced as a by-product in the electrochemical reduction stage of denitrification REN. Figure 3.- Shows a graph of the behavior of different ion exchange materials, such as zeolites and Amberlite® type ion exchange resins, where the volume of treated water in ml is represented on the abscissa axis and the % softening on the ordinate axis, data series A corresponding to the Ameberlite® type resin and data series B to the zeolite, and where 0.3 g of Amberlite® or zeolite and 30 ml of water to be treated were used, collecting 5 ml aliquots.DETAILED EXPOSURE OF EMBODIMENTS Figure 1 shows a diagram of the process according to a preferred embodiment of the invention, which includes all the pre- and post-treatment stages of the aqueous saline stream to be treated and the concentrate with respect to the REN process. Thus, the process illustrated in Figures 1 and 2 comprises the steps of: - Reducing the hardness (1) of the aqueous saline stream (7) in a cation-Na exchanger. +, obtaining a softening of at least 98% of the initial concentration of calcium and magnesium ions. - Concentration of the nitrate ion content of the cation exchanger outlet stream by means of double-stage reverse osmosis (2) with a recirculation of the concentrated outlet flow of at least 75%, generating a permeate stream (8) with a total dissolved solids (TDS) concentration of less than 500 mg / L and a concentrate stream (9) with a nitrate ion concentration of at least 1000 ppm. - Adjustment of the pH, between values ​​of 8 and 13, of the concentrate stream (9) generated in the previous osmosis stage (2) and feeding it as a catholyte stream in an electrochemical reactor with a bismuth and tin cathode. - Electrochemical reduction (3) at the cathode of the nitrate ions contained in the catholyte of the electrochemical reactor to N2 gas in a percentage of at least 60%.In parallel, water reduction occurs at the cathode, generating H2 gas, so that in the electrochemical reduction (3) a gaseous output stream (10) is obtained that contains N2, O2, water vapor, traces of NH3 and at least 97% of H2; and an aqueous output stream (11) with a maximum percentage of ammonium ions of 40% with respect to the initial nitrate ion content, and a nitrate and nitrite ion content below 50 and 5 ppm, respectively. - Checking the ammonia / ammonium levels in the gaseous and aqueous output streams of the electrochemical reduction (3). - Addition of sodium hypochlorite to the aqueous output stream (11) of the electrochemical reduction (3) for the oxidation (4) of ammonium (NH4. + ) and nitrite (NO 2- ) to nitrogen (N2) and nitrate ion (NO3-), respectively, so that the nitrate content of the output water (12) of this stage (4) is less than 50 ppm and the ammonium content (NH4 +) is less than 5ppm. - Checking the ammonia / ammonium levels in the output stream (12) of the oxidation stage (4). - Demineralization (5) of the output stream (12) of the oxidation stage (4) by two ion exchange stages, a cation exchanger-H + with Na resins + and an ion exchanger-OH- with Cl- resins. The demineralized water (13) can be conducted to the head of the EDR process where the saline aqueous stream (7) to be treated originated. - Activation of the cation exchange resins- Na + from the previous step (5) with HCl and obtaining solutions concentrated in Ca and Mg and activation of the anion exchange resins-Cl- with NaOH obtaining saline solutions enriched in Na + and Cl- (14). - Regeneration of the resin used in the hardness reduction stage (1) of the saline aqueous stream (7) by using solutions enriched in Na +and Cl- (14) and recovery of insoluble salts of Ca and Mg (15). - Purification (6) of the gaseous by-product obtained in the electrochemical reduction (3) by eliminating traces of ammonia with a trap with an ammonia capturing solution. - Checking the ammonia levels in the output stream of the purification stage (6). - Drying and deoxygenation (16) of the stream obtained in the previous stage to obtain a hydrogen stream. - Conditioning (17) and storage (18) of the hydrogen stream obtained in the previous drying and deoxygenation stage (16). - Energy recovery (19) from the hydrogen stream of the previous stage by means of a fuel cell for the generation of electrical energy.With regard to the ion exchange resins used in the hardness reduction (1) and demineralization (5) stages, 'Amberlite ®' type resins are preferably used, since they offer superior performance in the process of the invention with respect to other types of ion exchange material such as zeolites. The softenings achieved using Amberlite® and Zeolite were estimated. To this end, different experiments were carried out with continuous monitoring of the softening by means of a GILSON Minipuls 3 peristaltic pump, TYGON LMT-55 tubing models SC0021T (Purple-Purple, ID=2.06 mm) and SC0222T (Black-white, ID=3.17 mm) and the sigma column of 10 cm length and 0.7 cm in diameter that contained an amount of exchangers of 0.3 g. 30 mL of the reject water was processed at a flow rate of 2 mL / min, 5 mL aliquots were collected and the total hardness was determined by titrating these aliquots at pH 10 (adjusted with NH3 / NH4 buffer).+ ) using 0.01M EDTA and Eriochrome Black T (NET) as indicator. The total hardness of the untreated reject water expressed in mg CaCO3 / L was 2550. The result of this experiment is shown in Figure 3, where it is observed that the Amberlite® type resin provides better results than zeolite. Additionally, Amberlite® resin showed better flow behavior. Another experiment was carried out with quantities around 0.5 g of Amberlite® and zeolite for which 20 mL of treated water were collected and the total hardness was analyzed (Table 1), also obtaining better softening results for Amberlite® 82.4 % versus 61.8 %. Table 1. Result of an experiment carried out with varying amounts of ion exchange material for softening 20 ml of water in a 10 x 0.7 cm column. In accordance with the preferred embodiment of the detailed invention, 2000 L of a saline aqueous stream (7) were treated, specifically water from the reject of an EDR plant that had a conductivity of 4.00 mS / cm and a pH of 7.0 and whose main analytical characteristics are shown in Table 2, were subjected to a softening stage (1) in a filtration system by passing a flow rate of 320 liters / hour for a time of 6.25 hours through a 50-liter filter with Amberlite® IRC120 Na Ion Exchange Resin Gaussian, Gel, Strong Acid Cation Exchange Resin, which is regenerated in the flow direction with 10% NaCl.The treated stream presented a conductivity of 4.27 mS / cm, a pH of 7.5 and a composition as shown in table 3 where it can be observed that the level of elimination of calcium and magnesium ions is greater than 98% in both cases. Table 2. Ionic composition of the saline aqueous stream (7) rejected by an EDR. Table 3. Ionic composition of the outlet stream from the hardness reduction or softening stage (1). This stream, which has a very low content of calcium and magnesium ions, is subjected to the reverse osmosis stage (2). To this end, the 2000 liters coming from stage (1) were subjected to the reverse osmosis process in a two-stage equipment, with the addition of acid and antiscalant. In the first stage, with a pressure of 10 bar, an LG BW 4040 ES membrane with recirculation close to 89% was installed, and in the second stage, with a pressure of 18 bar, an LG BW 4040 R membrane with recirculation around 80%, with the aim of increasing the nitrate concentration from around 420 ppm to 1500 ppm. This stage (2) allows a recovery of 75% of the water as permeate (8) of low conductivity, for a total time of 8 hours to treat the 2,000 liters.Stage (2) provided two streams, a 1500 litre permeate stream (8) (pH=5.4 and 0.2 mS / cm specific conductivity) whose analytical characteristics are shown in Table 4 and which can be redirected to the header of the reverse electrodialysis (RDA) process that gave rise to the saline aqueous stream (7) to be treated or, where appropriate, used as treated water for consumption. Another 500 litre stream (pH=6.5 and 18.28 mS / cm specific conductivity), labelled as concentrate (9), whose analytical characteristics are shown in Table 5. As can be seen, most of the nitrate ion content (>93%) has remained in the concentrate stream (9). (LOD = Limit of Detection) Table 4. Ionic composition of the PERMEATE stream (8) from the reverse osmosis stage (2)

[0002] Table 5. Ionic composition of the CONCENTRATE stream (9) from the reverse osmosis stage (2). It is this concentrate stream (9) from stage (2) that is subjected to stage (3) for the destruction of nitrate ions by the electro-reduction process. For this purpose, two possible alternatives for treatment in stage (3) REN were proposed, which are based on treating the concentrate stream (9) coming from stage (2): either in a batch operation mode (English term for batch operation mode) (EXAMPLE 1) or in a single-step operation mode (EXAMPLE 2). EXAMPLE 1 Treatment in Batch Operation Mode. The batch size to be treated was set at 50 liters of CONCENTRATE water (9) coming from stage (2). To these 50 liters, 0.32 liters of 50% NaOH were added to adjust the pH to a value of 12.8 to carry out stage (3) efficiently.This solution was fed as a catholyte stream into a filter press type electrochemical reactor with the following characteristics: Sn-Bi cathode, DSA O2 anode, Nafion® 450 membrane, active geometric area of ​​3300 cm. 2 The anolyte stream was a 0.06M sulfuric acid solution. The recirculation flow rate of both streams through the electrochemical reactor was 300 L h -1. A current of 165 A was circulated for 7 hours and 45 minutes. The applied potential difference between the two electrodes varied between 6.15 volts and 5.24 volts at the end of the process. The temperature of the anolyte changed from the initial 20.2 ° C to 27.6 ° C while that of the catholyte reached 31.6 ° C. During the process, the catholyte is recirculated to the electrochemical reactor, passing through a tank equipped with a degassing system that allows the separation of the gaseous mixture (10) that will be subjected to the process described in step (6) and treated water (11) that will be subjected to step (4). 50.35 L of treated water were obtained, the most important analytical characteristics of which are shown in table 6. Table 6. Ionic composition of the catholyte stream treated in the REN stage (E3). Batch mode (LOD = Limit of Detection). Using the data in tables 5 and 6 and the volumes of the solutions, it can be seen that the nitrate content has been reduced by more than 99%. In addition, the treatment stream in stage (3) contains nitrite (10 ppm) and ammonium (128 ppm) as a result of the electrochemical process itself. This means that 69% of the nitrogen initially present as nitrate has disappeared as N2 and most of the remaining N in the sample will be transformed into N2 gas in stage (4) of oxidation of the remaining ammonium present in the water treated in this stage (3). In this sense, 486 mL of 37% HCl are added to 50.32 L liters (catholyte), which come from stage (3), to raise the pH to a value of 8.5 units. Next, 689 mL of 10% NaClO are added, the water stream is stirred and the mixture is allowed to cool for 1 hour.After that time, there are 51.50 liters whose composition is shown in table 7 and corresponds to the result of stage (4). Table 7. Ionic composition of the treated stream (12) in the ammonium ion oxidation stage (4) (LOD = Limit of Detection). The concentrate (12) is processed through two beds of cation / Na resins. + and anionic / Cl- activated in the form of H + and OH- (5). For the first activation, 6 L of 12% HCl per kg of resin are used, followed by washing with 2.5 L of reverse osmosis permeate water. For the second resin activation, 7 L of 12% NaOH per kg of resin are used, followed by washing with 2.5 L of reverse osmosis permeate water. The concentrates are collected and mixed for later use in the regeneration of the cation / Na resin. +used in the softening stage, with a pH of around 3 and sodium and chloride concentrations of around 32,800 ppm and 51,200 ppm. During regeneration, a high-concentration calcium solution is obtained from which Ca and Mg carbonate or hydroxide can be obtained. The composite resulting from the precipitation is used for further resin regenerations. The concentrate (12) from stage (4) is first passed through the cationic resin and then through the anionic resin, the resulting composition being that shown in Table 8. Table 8. Ionic composition of the treated stream (13) in the demineralization stage (5). (LOD = Limit of Detection) EXAMPLE 2 Treatment in Single-Step Operation Mode. The batch size to be treated was set at 50 liters of concentrate water (9) from stage (2). To these 50 liters, 0.32 liters of 50% NaOH were added to adjust the pH to a value of 12.8 to carry out stage (3) efficiently. This solution was fed as a catholyte stream into a filter press type electrochemical reactor with the following characteristics: Sn-Bi cathode, DSA O2 anode, Nafion® 450 membrane, active geometric area of ​​3300 cm 2. The anolyte stream was a 0.06M sulfuric acid solution. The anolyte recirculation flow rate through the electrochemical reactor was 50 L / h while the catholyte stream was circulated only once through the reactor at a circulation flow rate of 10 L / h. Once the catholyte stream leaves the electrochemical reactor, it passes to an auxiliary tank equipped with a degassing system that allows the separation of the gaseous mixture (10) that will be subjected to the process described in step (6) and treated water (11) that will be subjected to step (4). During step (3) in this continuous regime mode in a single step, a current of 165 A was circulated for 3 hours. The potential difference applied between the two electrodes varied between 5.47 volts and 5.16 volts at the end of the process. The temperature of the anolyte changed from the initial 23.5 ºC to 32.4 ºC while that of the catholyte reached 28.4 ºC.Once the process is complete, the catholyte collected in the auxiliary tank of the pilot plant containing the electrochemical reactor is measured and analyzed. This resulted in 50.32 liters of treated water, the most important analytical characteristics of which are shown in Table 9. Table 9. Ionic composition of the CATHOLITE stream treated in the REN stage (3). Single-step mode (LOD = Limit of Detection) Using the data in tables 5 and 9 and the volumes of the solutions, it can be seen that the nitrate content has been reduced by more than 90%. In addition, it contains nitrite (28 ppm) and ammonium (128 ppm) as a result of the electrochemical process itself. This means that 65% of the nitrogen initially present as nitrate has disappeared as N2 and most of the remaining N remaining in the sample will be transformed into N2 gas in stage (4) of oxidation of the remaining ammonium present in the water treated in this stage (3). In this sense, 486 mL of 37% HCl are added to 50.32 liters (catholyte), which come from stage (3), to bring the pH to a value of 8.5 units. Next, 689 mL of 10% NaClO are added, the water stream is stirred and the mixture is allowed to cool for 1 hour.After that time, there are 51.50 liters whose composition is shown in table 10 and corresponds to the result of stage (4). Table 10. Ionic composition of the treated stream (12) in the ammonium ion oxidation stage (4). (LOD = Limit of Detection) Demineralization (5) is carried out in the same manner as indicated in example 1 and the water obtained (13) has a composition similar to that shown in Table 8. In both examples described above, a gaseous stream (1.2 L / min) is obtained from the catholyte line of the REN process (3). The gases produced in said denitrification process are given by the production of, mainly, H2 (550 NL, 94.8%), N2 (2.5 NL, 0.4%), and a residual amount of NH3 in gaseous form (0.050 NL, <0.01%), in addition to the volume of water vapor resulting from obtaining these gases in an aqueous medium (16.5 NL, 2.8%). The presence of O2 has also been detected in this current due to its passage through the membrane of the anodic compartment (11 NL, 1.9%).All gas volume data are referenced to standard conditions (NS). This gaseous stream was bubbled through a 2.6 g / L H2SO4 trap to retain all NH3 from the stream as ammonium sulfate, allowing the remaining gases to pass through. The resulting stream was passed through a system to condense the water and remove all oxygen from the stream (Orion Z01 model), leaving a stream with a flow rate of 1.15 L / min composed of 99.5% H2 and 0.5% N2. This stream is conditioned to the pressure stipulated for storage (5-12 bar pressure) in a hydride hydrogen storage system (MyH2 model, 600 liter hydrogen capacity, 5-12 bar recharge pressure) using a hydrogen compressor (KNF mod.630.15SP.13E, 8.5 bar operating temperature) and thus a fuel cell was used. This flow rate determines the power of the fuel cell to be used.In this exemplification, the Horizon Fuel Cell Technology FCS-C100 H-100 model fuel cell was used as a model. This is an open cathode fuel cell, capable of delivering a maximum power of 100 W (12 V at 8.3 A) at a hydrogen flow rate of 1.3 L / min, with a stack temperature of 65 °C and a hydrogen pressure of 0.45-0.55 bar obtained from the hydride storage system. The gas stream fed the fuel cell at a current density of 136 mA / cm. 2 obtaining a load of 69 A h, demonstrating the concept that the gaseous stream obtained in the denitrification process can be valorized.

Claims

1. - Procedure for the recovery of water and elimination of nitrate ions in saline aqueous streams comprising the following steps: - Reduction of the hardness (1) of the saline aqueous stream (7) by means of ion exchange in a cation exchanger-Na +, obtaining a softening of at least 98% of the initial concentration of calcium and magnesium ions. - Concentration of the nitrate ion content of the outlet stream from the cation exchanger by means of double-stage reverse osmosis (2) with a recirculation of the concentrated outlet flow of at least 75%, generating a permeate stream (8) with a total dissolved solids (TDS) concentration of less than 500 mg / L and a concentrate stream (9) with a nitrate ion concentration of at least 1000 ppm. - Adjustment of the pH, between values of 8 and 13, of the concentrate stream (9) generated in the previous osmosis stage (2) and feeding it as a catholyte stream into an electrochemical reactor provided with a cathode formed by a combination of bismuth and tin.- Electrochemical reduction (3) at the cathode of the nitrate ions contained in the catholyte of the electrochemical reactor, which are converted into N2 gas in a percentage of at least 60%, and reduction of the aqueous solvent, generating H2 gas, obtaining an output gaseous stream (10) with at least N2, traces of ammonia, and at least 97% H2; and an output aqueous stream (11) with a maximum percentage of ammonium ions of 40% with respect to the initial nitrate ion content and a nitrate and nitrite ion content below 50 and 5 ppm, respectively. - Checking the ammonia / ammonium levels in the output gaseous and aqueous streams of the electrochemical reduction (3). - Addition of sodium hypochlorite to the output aqueous stream (11) of the electrochemical reduction (3) for the oxidation (4) of ammonium (NH4. + ) and nitrite (NO 2-) to nitrogen (N2) and nitrate ion (NO3-), respectively, so that the nitrate content of the output water (12) of this stage (4) is less than 50 ppm and the ammonium content (NH4 + ) is less than 5ppm. - Checking the ammonia / ammonium levels in the output stream (12) of the oxidation stage (4). - Demineralization (5) of the output stream of the oxidation stage (4) by two ion exchange stages, one cation-H exchanger + with Na resins + and an ion exchanger-OH- with Cl- resins. - Activation of the cation exchange resins- Na +of the previous stage (5) with HCl and obtaining solutions concentrated in Ca and Mg and activation of the anion exchange resins-Cl- with NaOH obtaining saline solutions enriched in Na + and Cl- (14). - Regeneration of the resin used in the hardness reduction stage (1) of the aqueous saline stream (7) by using the solutions enriched in Na +and Cl- (14) and recovery of insoluble salts of Ca and Mg (15). - Purification (6) of the gaseous by-product obtained in the electrochemical reduction (3) by removing traces of ammonia using a trap with an ammonia-capturing solution. - Checking the ammonia levels in the output stream from the purification stage (6). - Drying and deoxygenation (16) of the stream obtained in the previous stage to obtain a hydrogen stream. - Conditioning (17) and storage (18) of the stream obtained in the previous stage. - Energy recovery (19) from the hydrogen stream from the previous stage by means of a fuel cell for the generation of electrical energy. 2.- A method for recovering water and removing nitrate ions from saline aqueous streams, according to claim 1, characterized in that the electrochemical reactor in which the electrochemical reduction takes place has separate anodic and cathodic compartments.

3. A method for recovering water and removing nitrate ions from saline aqueous streams, according to claim 2, characterized in that the separation between compartments in the electrochemical reactor is achieved by an ion-selective membrane or a non-selective porous separator.

4. A method for recovering water and removing nitrate ions from saline aqueous streams, according to claim 1, characterized in that the double-stage osmosis stage comprises a 5µ microfiltration pretreatment, antiscalant and acid dosing, and high-pressure pumping. 5.- Method for the recovery of water and elimination of nitrate ions in saline aqueous streams, according to claim 1, characterized in that the permeate stream (8) generated in the osmosis stage (2) is conducted to the head of an installation from which the saline aqueous stream (7) originates in order to reduce discharges. 6.- Method for the recovery of water and elimination of nitrate ions in saline aqueous streams, according to claim 1, characterized in that the checking of the levels of ammonia and / or ammonium in the output streams of the electrochemical reduction stage (3), of the oxidation stage (4) and of the purification stage (6) is carried out by means of a passive colorimetric sensor for the detection of ammonia and aliphatic amines in gases. 7.- Equipment for the recovery of water and elimination of nitrate ions in saline aqueous streams, according to the method according to any of the preceding claims, characterized in that it comprises: - a cation-Na exchanger. + , - a double-stage osmosis equipment, - an electrochemical reactor provided with a cathode comprising a combination of bismuth and tin - a sensor for the detection of ammonia / ammonium at the outlet of the electrochemical reactor, - a hypochlorite dosing device for oxidation in the outlet stream of the electrochemical reactor - a sensor for the detection of ammonia / ammonium in the outlet stream of the electrochemical reactor after the addition of hypochlorite, - a cation exchanger-H +, - an anion exchanger -OH-, - a trap with ammonia capturing solution, - a sensor for the detection of ammonia at the outlet of the ammonia trap, - a system for condensing water and eliminating all oxygen from the gas stream, - a hydrogen storage system of hydrides, by means of a hydrogen compressor, - a fuel cell for the generation of electrical energy. 8.- Equipment for the recovery of water and elimination of nitrate ions in aqueous streams salines, according to claim 7, characterized in that the double-stage osmosis equipment is provided with a pretreatment that has a 5µm microfiltration equipment, antiscalant and acid dosing and with a high-pressure pump. 9.- Equipment for the recovery of water and elimination of nitrate ions in saline aqueous streams, according to claim 7, characterized in that the sensor for the detection of ammonia / ammonium is a passive colorimetric sensor.

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