EP4097275A1

EP4097275A1 - System and method for electrochemical stabilization of urine with concomitant production of an oxidized compound

Info

Publication number: EP4097275A1
Application number: EP21700816.8A
Authority: EP
Inventors: Korneel Rabaey; Pieter NAERT; Jolien DE PAEPE; Peter CLAUWAERT
Original assignee: Universiteit Gent
Current assignee: Universiteit Gent
Priority date: 2020-01-29
Filing date: 2021-01-29
Publication date: 2022-12-07
Also published as: WO2021152090A1

Abstract

The invention relates to a system for stabilizing a solution comprising urine and for producing at least one oxidized compound. The system comprises at least one vessel for receiving and storing a solution comprising urine and at least one electrochemical cell. A solution comprising urine is mixed with the outlet flow of the cathode compartment and/or with the outlet flow of the anode compartment of the electrochemical cell. At the anode compartment an outlet flow comprising at least one oxidized compound is provided. The oxidized compound is produced at the anode and is preferably derived from urine or a constituent of urine. The invention further relates to a method to stabilize urine and to produce at least one oxidized compound.

Description

System and method for electrochemical stabilization of urine with concomitant production of an oxidized compound

Field of the invention [0001] The present invention relates to a system for electrochemical stabilization of urine or a solution comprising urine and for producing an oxidized compound at the anode of the electrochemical cell. The stabilization of urine and the production of an oxidized compound occur preferably simultaneously. The oxidized compound preferably comprises an oxidized compound derived from urine or a constituent of urine. The invention also relates to a method using an electrochemical cell to stabilize urine and to produce an oxidized compound at the anode of the electrochemical cell.

Background art

[0002] Urine is widely available and is generally considered as an interesting source for recovery of nutrients such as nitrogen and phosphorus.

[0003] The key issue to recover nutrients from urine is the high instability of urine. Immediately after release, urea decomposes due to microbial action and ammonia and/or (bi)carbonate are released with a concomitant pH increase which triggers precipitation of (divalent) cations as for example calcium and/or magnesium. This leads to odor issues, scaling problems and a loss of nitrogen (by volatilization) and phosphorus (by uncontrolled precipitation). These problems can be avoided by adding chemicals such as Ca(OH)2. However, as the addition of chemicals requires additional costs of the chemicals, the supply and storage of the chemicals, methods using additional chemicals are not preferred to stabilize urine. Moreover, typical additives such as calcium oxide leave considerable residues to be disposed of as well. Addition of for example NaOH may increase the salinity which may complicate downstream biological treatment (for example nitrification, plant/microalgae production). The increase in divalent cation concentration for example by the addition of Ca(OH)2 and the precipitation with for example phosphate, carbonate or sulphate may cause scaling problems. [0004] A second issue that arises in the context of urine is the risk of pathogen spreading.

Therefore, methods to stabilize urine require additional precautions to avoid the risk of pathogen spreading. Summary of the invention

[0005] It is an object of the present invention to provide a system and a method to stabilize urine or a solution comprising urine thereby avoiding the problems of the prior art.

[0006] It is an object of the present invention to provide a system and a method to stabilize urine or a solution comprising urine avoiding or limiting scaling problems.

[0007] It is another object of the present invention to provide a system and a method to stabilize urine or a solution comprising urine avoiding odor nuisance and/or avoiding the spread of pathogens related to urine.

[0008] It is another object of the present invention to provide a system and a method to stabilize urine or a solution comprising urine and to (simultaneously) produce at least one oxidized compound. The oxidized product is for example derived from the (stabilized) urine or a constituent of (stabilized) urine.

[0009] It is a further object of the present invention to provide a method allowing to produce a disinfectant from urine or from a solution comprising urine.

[0010] It is a further object of the present invention to provide a system comprising an electrochemical cell, whereby an alkaline solution produced at the cathode is used to stabilize urine or a solution comprising urine and whereby an oxidized product (for example a disinfectant) is produced at the anode. The oxidized product is for example suitable for toilet flushing or cleaning.

[0011] It is still a further object of the present invention to provide a system and method to stabilize urine or a solution comprising urine allowing controlled precipitation of divalent ions so that scaling problems are avoided, and, in particular, that phosphorus can be recovered, as the precipitate comprising divalent ions typically also comprises phosphate. The recovered precipitate may be used as fertilizer or as a source of phosphorus.

[0012] It is also an object of the present invention to provide a chemical-free method to stabilize urine or a solution comprising urine.

[0013] According to a first aspect of the present invention a system for stabilizing urine and for producing at least one oxidized compound is provided. The system comprises at least one vessel for receiving and storing urine or a solution comprising urine and at least one electrochemical cell. The vessel comprises at least one inlet X for providing an inlet flow X’ into the vessel and at least one outlet Y for providing an outlet flow Y’ from the vessel. Preferably, the vessel comprises at least one additional inlet Z for introducing urine or a solution comprising urine either continuously or discontinuously. The urine or solution comprising urine is preferably stabilized in the vessel and results in stabilized urine or a solution comprising stabilized urine. The electrochemical cell comprises a cathode compartment comprising a cathode, a middle compartment and an anode compartment comprising an anode.

The cathode compartment and the middle compartment are separated by a first separator.

The middle compartment and the anode compartment are separated by a second separator.

The middle compartment of the at least one electrochemical cell is provided with at least one inlet B for introducing an inlet flow B’ into the middle compartment; at least one outlet E for providing an outlet flow E’ from the middle compartment.

The anode compartment of the at least one electrochemical cell is provided with at least one inlet C for introducing an inlet flow C’ into the anode compartment;- at least one outlet F for providing an outlet flow F’ from the anode compartment, the outlet flow F’ comprising an oxidized compound formed at the anode. The oxidized compound is preferably derived from urine or a constituent of urine and/or from a compound of inlet flow C’. More preferably, the oxidized compound is derived from urine or a constituent of urine present in the electrochemical cell, in particular in the middle compartment of the electrochemical cell.

The cathode compartment of the at least one electrochemical cell is provided with at least one inlet A for introducing an inlet flow A’ in the cathode compartment; at least one outlet D for providing an outlet flow D’ from the cathode compartment.

The system according to the present invention is characterized in that the at least one outlet Y of the vessel is connected to the at least one inlet B of the middle compartment of the electrochemical cell for introducing the inlet flow B’ comprising urine, preferably stabilized urine, from the vessel into the middle compartment of the electrochemical cell; and the at least one of the outlet D or the at least one outlet E is connected with the at least one inlet X of the vessel for introducing at least part of the outlet flow D’ or at least part of the outlet flow E’ into the vessel, wherein the outlet flow D’ and/or the outlet flow E’ that is at least partially introduced in the vessel is more alkaline than the inlet flow B’. In particular, an outlet D is connected with an inlet X of the vessel for introducing at least part of at least one of the outlet flow D’ into the vessel, optionally via a second electrochemical cell, wherein the outlet flow D’ that is at least partially introduced in the vessel is more alkaline than the inlet flow B’

Preferably, the outlet flow D’ and/or the outlet flow E’ that is at least partially introduced in the vessel has a pH of at least 11 , more preferably a pH of at least 12. Most preferably, the outlet flow D’ and/or the outlet flow E’ that is at least partially introduced in the vessel has a pH of at least 12, for example a pH of 13. [0014] The outlet flow D’ can either be continuously or discontinuously, for example at predetermined time intervals, introduced into the vessel. Similarly, the outlet flow E’ can either be continuously or discontinuously, for example at predetermined time intervals, introduced into the vessel.

[0015] In particular embodiments, the system further comprises a second electrochemical cell, comprising a cathode compartment and an anode compartment, wherein an outlet D (of the cathode compartment of a first electrochemical cell) is connected with an inlet G of the cathode compartment of the second electrochemical cell, for providing a flow D’ to the cathode compartment of the second electrochemical cell and wherein an outlet I of the cathode compartment of the second electrochemical cell is connected with the at least one inlet X of the vessel, for providing an outlet flow G to the vessel. In particular, the outlet flow G that is at least partially introduced in the vessel has a pH of at least 11, more preferably a pH of at least 12, for example a pH of 13. In certain embodiments, an outlet W of the vessel is connected to the inlet H of the anode compartment of the second electrochemical cell, for providing an outlet flow W of a solution comprising stabilized urine to the anode compartment of the second electrochemical cell. During operation, the pH of the stabilized urine in the anode compartment of the second electrochemical cell is lowered. In particularly preferred embodiments, outlet E of the middle compartment of the first electrochemical cell is further connected with inlet A of the cathode compartment of the first electrochemical cell, for providing the outlet flow E’ to the cathode compartment of the first electrochemical cell, and outlet D of the cathode compartment of the first electrochemical cell is connected with inlet G of the cathode compartment of the second electrochemical cell.

[0016] It is clear that the vessel can be provided with more than one inlet X and/or with more than one outlet Y. Furthermore, it is clear that the anode compartment, the middle compartment and/or the cathode compartment can also be provided with more than one inlet and/or with more than one outlet.

[0017] The electrochemical cell preferably comprises a power supply to apply a voltage between the anode and the cathode and to drive the overall process.

[0018] The solution comprising urine present in the vessel is mixed with at least part of the outlet flow D’ and/or with at least part of the outlet flow E’. In particular embodiments, where the system comprises a second electrochemical cell, the solution comprising urine present in the vessel is mixed with at least part of the outlet flow G of the cathode compartment of the second electrochemical cell. The solution comprising urine is preferably stored in the vessel at a pH of at least 11 , more preferably at a pH of at least 12. Preferably, the solution comprising urine is preferably stored in the vessel at a pH below 13, most preferably at a pH between 12 and 13. [0019] The system according to the present invention preferably allows to stabilize a solution comprising urine and to simultaneously produce at least one oxidized compound. More preferably, the system according to the present invention allows to stabilize a solution comprising urine while simultaneously producing an oxidized compound derived from urine or a constituent of urine, preferably from the urine or a constituent of urine that is introduced from the vessel in the middle compartment of the electrochemical cell.

[0020] In preferred embodiments the cathode compartment is provided with at least one inlet A for introducing an inlet flow A’ in the cathode compartment of the electrochemical cell; at least one outlet D for providing an outlet flow D’ from the cathode compartment. In such embodiments the at least one outlet D is preferably connected with the at least one inlet X of the vessel for introducing at least part of outlet flow D’ into the vessel. It is understood that the outlet D may optionally be connected with an inlet X of the vessel via a second electrochemical cell, particularly wherein the outlet D of the cathode compartment of a first electrochemical cell is connected to the inlet G of the cathode compartment of a second electrochemical cell and wherein the outlet I of the cathode compartment of a second electrochemical cell is connected to an inlet X of the vessel. The outlet flow D’ can either be continuously or discontinuously, for example at predetermined time intervals, optionally via a second electrochemical cell, introduced into the vessel. The outlet flow D’ is preferably more alkaline than the inlet flow B’. It is clear that in such embodiment also the at least one outlet E might be connected with an inlet of the vessel, for example with the at least one inlet X of the vessel, to allow to introduce at least part of the outlet flow E’ into the vessel. The outlet flow E’ can either be continuously or discontinuously, for example at predetermined time intervals introduced into the vessel.

[0021] In certain embodiments of the system envisaged herein, the at least a first electrochemical cell comprises a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet X of the vessel is connected to at least inlet A of the cathode compartment and inlet B1 of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E1 of the first middle compartment, and/or an outlet W of the vessel is connected to an inlet C2 of the second anode compartment. During operation, the pH of the stabilized urine in the second anode compartment is lowered. [0022] Preferably, inlet flow B’ comprises a solution comprising urine, for example stabilized urine, having a pH of at least 11 , more preferably a pH of at least 12 and/or a concentration of divalent metal ions preferably below 20 % of the concentration of divalent metal ions of fresh urine, more preferably below 10 % or below 5 % of the concentration of divalent metal ions of fresh urine. The inlet flow B’ comprises preferably a solution comprising urine, for example stabilized urine, from the vessel.

[0023] The inlet flow A’ comprises preferably an aqueous solution and/or comprises a solution comprising urine, for example stabilized urine. More preferably, the inlet flow A’ comprises a solution comprising urine, for example stabilized urine, having a pH of at least 11 , preferably a pH of at least 12 and/or a concentration of divalent metal ions preferably below 20 % of the concentration of divalent ions of fresh urine, more preferably below 10 % or below 5 % of the concentration of divalent metal ions of fresh urine. In particular preferred embodiments the inlet flow A’ comprises a solution comprising urine, for example stabilized urine from the vessel.

[0024] In particular embodiments the inlet flow A’ and the inlet flow B’ both comprise a solution comprising urine, for example stabilized urine, preferably a solution comprising stabilized urine from the vessel. In certain embodiments, inlet flow G’ into the cathode compartment of a second electrochemical cell may also comprise a solution comprising urine, for example stabilized urine, preferably a solution comprising stabilized urine from the vessel or, more particularly, from the cathode compartment of a first electrochemical cell. [0025] It is further understood that, in particular embodiments, the outlet E of the middle compartment is connected with the at least inlet A of the cathode compartment for introducing an outlet flow E’ into the cathode compartment. Accordingly, inlet flow A’ may comprise a solution comprising urine, for example stabilized urine, from the middle compartment. Advantageously, this enables the initial removal of chloride from the stabilized urine in the middle compartment towards the anode, after which, in the cathode compartment, the urine can be made more alkaline to create a strong alkaline fluid.

[0026] The outlet flow D’ comprises an alkaline flow having a pH higher than inlet flow A’, preferably an alkaline flow having a pH of at least 11. More preferably the flow D’ comprises an alkaline flow having a pH of at least 12. Most preferably, the outlet flow D’ comprises an alkaline flow having a pH of at least 12, for example a pH of 13. In certain embodiments, the outlet flow D’ is provided to the cathode compartment of a second electrochemical cell, via inlet G. In certain embodiments, the outlet flow G of the cathode compartment of a second electrochemical cell comprises an alkaline flow having a pH higher than inlet flow A’, preferably an alkaline flow having a pH of at least 11. More preferably the flow G comprises an alkaline flow having a pH of at least 12. Most preferably, the outlet flow G comprises an alkaline flow having a pH of at least 12, for example a pH of 13.

[0027] The inlet flow C’ comprises preferably water or an aqueous solution, for example rainwater, potable water, ground water, (treated) grey water or any type of aqueous solution. In certain embodiments, a buffer tank provides the aqueous solution for inlet flow C’. Alternatively, the inlet flow C’ may comprise a solution comprising urine or stabilized urine for example an aqueous solution comprising urine or stabilized urine. In particular embodiments the inlet flow C’ comprises a solution comprising urine for example stabilized urine from the vessel. Optionally, the inlet flow C’ may further comprise one or more additives, for example compounds that are able to be oxidized at the anode. The inlet flow C’ comprises for example sodium chloride and/or sodium sulfate. In certain embodiments, inlet flow C’ comprises water used in an air conditioning or other water consuming system associated with heat management.

[0028] The outlet flow F’ preferably comprises the at least one oxidized compound produced at the anode of the electrochemical cell. Preferably, the at least one oxidized compound is produced from a compound derived from the (stabilized) urine or from a constituent of (stabilized) urine. The at least one oxidized compound is preferably produced from urine or from a constituent of urine that is introduced from the vessel into the middle compartment of the electrochemical cell. The at least one oxidized compound is for example produced from chloride present in the (stabilized) urine, for example the urine introduced from the vessel into the middle compartment of the electrochemical cell. Preferred oxidized compounds comprise chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or other reactive species derived from oxygen. Outlet flow F’ may be provided to a buffer tank. [0029] The at least one oxidized compound can also be produced from a compound present in inlet flow C’. It is clear that the oxidized compound or oxidized compounds may also comprise a combination of a compound or compounds produced from urine or from a constituent of urine and a compound or compounds produced from a compound of the inlet flow C’.

[0030] As first and second separator, present in the at least first electrochemical cell, any means to separate two compartments of a vessel while allowing ion transport can be considered. Preferred examples of first and second separators comprise ion selective membranes. Ion selective membranes such as cation exchange membranes or anion exchange membranes are preferred although also bipolar membranes can be considered as first and/or as second separator. In particular embodiments, the second electrochemical cell comprises a third separator, which may be an ion selective membrane.

[0031] The first separator, i.e. the separator between the cathode compartment and the middle compartment(s) preferably comprises a separator allowing cation transport. More preferably, the first separator comprises a cation exchange membrane. Also, a bipolar membrane can be considered as first separator. [0032] The second separator, i.e. the separator between the middle co part ent(s) and the anode co part ent(s) preferably comprises a separator allowing anion transport. More preferably, the second separator comprises an anion exchange membrane.

[0033] As cathode any type of cathode known in the art can be considered.

[0034] Preferably, the cathode comprises stainless steel, nickel or any other material enabling the reduction of water can be considered.

[0035] As anode any type of anode known in the art can be considered.

[0036] Preferably, the anode comprises a dimensionally stable anode, an anode comprising a catalyst such as iridium oxide or ruthenium oxide or another electrocatalyst enabling the production of oxidants in an aqueous medium.

[0037] The system according to the present invention may comprise one or more controllers and/or one or more adjusting means. The system comprises for example a pH controller and/or means to adjust the pH. Alternatively or additionally, the system may comprise a flow rate controller and/or means to adjust the flow rate and/or means to adjust the concentration of the oxidized compound. Any of the flows A’, B’, C’, D’, E’, F’ can be provided with one or more controllers and/or with one or more adjusting means of the flow rate, the pH, and/or the concentration of the oxidized compound. In certain embodiments, wherein the system comprises a second electrochemical cells, any of the flows G’, H’, G and/or J’ can be provided with one or more controllers and/or with one or more adjusting means of the flow rate, the pH, and/or the concentration of the oxidized compound.

[0038] In a particular embodiment the pH of the outlet flow D’ of the cathode compartment and/or the pH of the outlet flow E’ of the middle compartment is controlled by a pH controller. In case the pH of the outlet flow D’ and/or the pH of the outlet flow E’ does not fall within a predetermined range, the process can be adjusted for example by modifying the current level of the power supply. In a further particular embodiment, the outlet flow G of the cathode compartment of a second electrochemical cell is controlled by a pH controller. In case the pH of the outlet flow G does not fall within a predetermined range, the process can be adjusted for example by modifying the current level of the power supply for either the first and/or the second electrochemical cell.

[0039] In a particularly preferred embodiment, the pH of inlet flow B’ of the middle compartment is controlled by a pH controller. In case the pH of the inlet flow B’ does not fall within a predetermined range, the process can be adjusted for example by modifying the current level of the power supply.

[0040] In another embodiment the flow rate of outlet flow D’ and/or the flow rate of outlet flow E’ is controlled by one or more flow rate controllers. In case the flow rate of outlet flow D’ and/or the flow rate of outlet flow E’ does not fall within a predetermined range, the process can be adjusted for example by adjusting the flow rate(s) of one or both the outlet flow D’ and/or the outlet flow E’. In a further particular embodiment, the flow rate of outlet flow G of the cathode compartment of a second electrochemical cell is controlled by a flow rate controller. In case the flow rate of outlet flow G does not fall within a predetermined range, the process can be adjusted for by adjusting flow rate G.

[0041] It is clear that embodiments provided with one or more pH controllers and one or more flow rate controllers can be considered as well.

[0042] As vessel any vessel known in the art suitable for receiving and storing urine or a solution comprising urine can be considered.

[0043] The vessel can be adapted to allow the precipitation of compounds comprising divalent metals, for example comprising calcium and/or magnesium from the urine or from the solution comprising urine. The vessel is for example provided with a conically shaped bottom part.

[0044] The vessel can be provided with one or more additional inlets Z, for example to introduce urine or a solution comprising urine either continuously or discontinuously into the vessel. The vessel may also be provided with one or more additional outlets V for example for harvesting compounds from the vessel, for example compounds comprising divalent metal ions, such as compounds comprising calcium and/or magnesium and/or with one or more outlets W for providing stabilized urine or a solution comprising stabilized urine for example periodically.

[0045] In particular embodiments the vessel comprises more than one compartment, for example a first compartment provided with the inlet X or Z and a second compartment provided with the outlet Y or W. The vessel may have an overflow to bring (stabilized) urine from the first compartment to the second compartment. Such embodiment has the advantage to allow limited mixing in the first compartment and enables better precipitation. Such embodiment furthermore has the advantage that the first compartment can be emptied without removal of all fluid from the system. It is clear that a vessel with more than two compartments, for example three or four compartments can also be considered.

[0046] The system according to the present invention is suitable to stabilize urine or to stabilize a solution comprising urine.

[0047] The system according to the present invention can be used to produce at least on oxidized compound. Outlet flow F’ from the anode compartment preferably comprises one or more oxidized compounds. Outlet flow F’ comprises for example an aqueous flow comprising one or more oxidized compounds. Preferred oxidized compound comprise chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or other reactive species derived from oxygen. The outlet flow F’ is for example suitable for toilet flushing, rainwater, surface water, (treated) grey water or other streams in which the presence of one or more oxidized products is attractive for example for disinfection purpose, for removal of unwanted compounds or for both disinfection purposes and removal of unwanted compounds. Outlet flow F’ may be collected in a suitable buffer tank. Any vessel known in the art suitable for receiving and storing an aqueous solution comprising the oxidized compounds can be considered as a suitable buffer tank. Advantageously, when used for toilet flushing, the toilet is disinfected after each use. In certain embodiments, the concentration of residual chlorine, hypochlorous acid or hypochlorite ions in outlet flow F’ is at least 3 mg/L, preferably 5 mg/L as also recommended by the World Health Organization for different disinfection purposes. In other embodiments, the main target of the oxidant production is not disinfection but removal of colour from the fluid.

[0048] In certain embodiments, the system may further comprise a nitrogen removal unit, i.e. a unit capable of removing nitrogen, such as urea such as from a stabilized urine stream. A nitrogen removal unit typically comprises a sorption unit, including a sorbent such as activated carbon, ion exchange resins or other materials as known to a person skilled in the art. Typically, the nitrogen removal unit may be connected to an outlet W of the vessel, or to an outlet J of the anode compartment of the second electrochemical cell, or to an outlet F2 of the second anode compartment, in case the first electrochemical cell comprises a second anode and second middle compartment.

[0049] In certain embodiments, the system may further comprise a vessel or bioreactor containing bacteria for converting nitrogen compounds present in the treated or stabilized urine. The vessel or bioreactor may be positioned before or after the second anode or the second anode compartment. The bacteria can in the presence of oxygen convert nitrogenous species present in the stabilized or treated urine towards nitrate via nitrification or to nitrogen gas via partial nitritation/anammox, or via similar processes as known to a person skilled in the art.

[0050] An advantage of the system is that it allows to combine the stabilization of urine or a solution comprising urine and the production of a flow comprising at least one oxidized product. The system according to the present invention allows to produce an oxidized compound while stabilizing urine or a solution comprising urine.

[0051] The system according to the present invention is particularly interesting for use in buildings with separate urine collection, (temporary) toilets for example on festivals, events or construction sites, community toilets, refugee camps, field hospitals, ... These applications particularly benefit of using the flow comprising oxidized products for toilet flushing purposes, to disinfect the toilet after use, thereby improving the hygiene conditions at these locations.

[0052] In terms of overall system architecture, the proposed system can be integrated at public toilets or buildings or similar places where toilets are in use. At least part of the toilets collects urine separately which is brought to the aforementioned vessel. The collected urine is stabilized and brought through the one or more cathode compartments and middle compartment of the one or more electrochemical cells as set out in the different embodiments of the present invention, and, optionally also through a second anode compartment (if present) before leaving the system. The water stream in which oxidants are generated is stored in a separate buffer tank which, in the case of rainwater, can be the rainwater tank, in the case of greywater, potable water or other water streams, a separate buffer tank. From the tank, the water is recirculated over the electrochemical cell, e.g. via flows C’ and F\ From the buffer tank, the water is provided to other water consuming systems, such as toilets (for flushing, for air-conditioning systems or other systems using water at different temperatures and for different uses.

[0053] According to a second aspect of the present invention, a method to stabilize urine and to produce at least one oxidized compound at the anode of an electrochemical cell is provided. The at least one oxidized compound comprises preferably a disinfectant. Preferably, the stabilization of the solution comprising urine and the production of the oxidized compound occurs simultaneously.

[0054] The method according to the present invention uses a system comprising a vessel for receiving and storing a solution comprising urine and comprises at least one electrochemical cell. The electrochemical cell comprises a cathode compartment comprising a cathode, a middle compartment and an anode compartment comprising an anode. The cathode compartment and the middle compartment are separated by a first separator. The middle compartment and the anode compartment are separated by a second separator. The method comprises the steps of introducing and/or storing a solution comprising urine in the vessel; introducing an inlet flow B’ into the middle compartment of the electrochemical cell, the inlet flow B’ comprising a solution comprising urine from the vessel. Preferably, inlet flow B’ comprises a solution comprising stabilized urine from the vessel; introducing an inlet flow C’ into the anode compartment of the electrochemical cell; introducing an inlet flow A’ into the cathode compartment of the electrochemical cell; activating the electrochemical cell by applying a voltage between the cathode and the anode of the electrochemical cell; reducing water at the cathode of the electrochemical cell; oxidizing at least one compound at the anode of the electrochemical cell to produce at least one oxidized compound; providing an outlet flow E’ from the middle compartment of the electrochemical cell; providing an outlet flow F’ from the anode compartment of the electrochemical cell, the outlet flow F’ comprising the at least one oxidized compound produced at the anode; providing an outlet flow D’ from the cathode compartment of the electrochemical cell; introducing at least part of the outlet flow D’ from the cathode compartment and/or at least part of the outlet flow E’ from the middle compartment into the vessel, whereby the outlet flow D’ or the outlet flow E’ that is/are introduced into the vessel is more alkaline than the inlet flow B’. In particular, outlet flow D’ from the cathode compartment is introduced into the vessel, optionally via a second electrochemical cell, thereby generating an outlet flow G, whereby the outlet flow D’ and/or outlet flow G that is introduced in the vessel is more alkaline than the inlet flow B’.

[0055] In particular embodiments of the methods according to the present application, wherein the system further comprises a second electrochemical cell, comprising a cathode compartment and an anode compartment, wherein an outlet D of the cathode compartment of the first electrochemical cell is connected with an inlet G of the cathode compartment of the second electrochemical cell, and wherein an outlet I of the cathode compartment of the second electrochemical cell is connected with the at least one inlet X of the vessel, the step of introducing at least part of the outlet flow D’ from the cathode compartment of the first electrochemical cell into the vessel comprises the steps of (1) introducing at least part of the outlet flow D’ of the cathode compartment of a first electrochemical cell into the cathode compartment of a second electrochemical cell; (2) reducing water at the cathode of a second electrochemical cell by applying a voltage between the cathode and the anode of the second electrochemical cell; and (3) introducing at least part of the outlet flow G via outlet I from the cathode compartment of the second electrochemical cell into the vessel, wherein the outlet flow G is more alkaline than the inlet flow B’. In certain embodiments, the method further comprises the steps of introducing a flow H’ comprising a solution comprising stabilized urine into the anode compartment of the second electrochemical cell; generating oxidants, such as oxygen, and protons at the anode of the second electrochemical cell, and removing an outlet flow J’ from the anode compartment of the second electrochemical cell. It is understood that in the anode compartment of the second electrochemical cell, the pH of the stabilized urine is lowered. Outflow J’ of the anode compartment of a second electrochemical cell may be discharged e.g. to a sewer system, or may be directed to other uses. In particular, the urine flow through the anode compartment of the second electrochemical cell may be controlled based on incoming urine to enable control of fluid volume in the buffer tank.

[0056] In particular embodiments, an outlet flow E’ of the middle compartment is provided to the inlet A of the cathode compartment of a first electrochemical cell. Advantageously, this enables the initial removal of chloride from the stabilized urine in the middle compartment towards the anode, after which, in the cathode compartment, the urine can be made more alkaline to create a strong alkaline fluid. The outlet flow D’ of the cathode compartment may be provided to inlet X of the vessel or to inlet G of the cathode compartment of the second electrochemical, thereby generating an outflow G, which in its turn is provided to inlet X of the vessel.

[0057] In particular embodiments, wherein the system comprises a first electrochemical cell further comprising a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet X of the vessel is connected to at least inlet A of the cathode compartment and inlet B1 of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E1 of the first middle compartment, and/or an outlet W of the vessel is connected to an inlet C2 of the second anode compartment, the method further comprises the steps of providing a solution comprising stabilized urine from the vessel or from the outlets from the middle compartment and/or the cathode compartment, to the second anode compartment, and generating protons at the second anode. This enables a lowering of the pH of the stabilized urine. The outflow of the second anode compartment via outlet F2 may be discharged e.g. to a sewer system, or may be directed to other uses.

[0058] The urine or the solution comprising urine can be introduced either continuously or discontinuously into the vessel. The urine or the solution comprising urine in the vessel is preferably mixed with the outlet flow E’ and/or with the outlet flow D’ of the electrochemical cell. In particular embodiments, the urine or the solution comprising urine in the vessel is mixed with the outlet flow I’ of the cathode compartment of the second electrochemical cell. Preferably, the urine or the solution comprising urine is stored in the vessel at a pH of at least 11 , more preferably at a pH of at least 12. The urine or the solution comprising urine is preferably stored in the vessel at a pH lower than 13. Most preferably, the urine or the solution comprising urine is stored at a pH of at least 12 but lower than 13.

[0059] The one or more electrochemical cells can be activated continuously or discontinuously, for example at predetermined time intervals. [0060] The one or more electrochemical cells are preferably activated by a power supply applying a voltage between the anode and the cathode.

[0061] The inlet flow B’ is preferably introduced into the middle compartment of the electrochemical cell by at least one inlet B. The inlet flow B’ can be introduced into the middle compartment either continuously or discontinuously, for example at predetermined time intervals. In particular embodiments, during operation, once inlet flow B’ is introduced in the middle compartment of the first electrochemical cell, cations are exchanged by the cation exchange membrane, positioned between the middle compartment and the cathode compartment, and move to the cathode compartment, where a more alkaline solution is created due to water reduction. Consequently, outlet flow D’ of the cathode compartment comprises a more alkaline solution than the inlet flow B’. In additions, anions are exchanged by the anion exchange membrane, positioned between the middle compartment and the anode compartment, and move to the anode compartment. In particular embodiments, anodic oxidation of the anions, for example the chloride ions, to chlorine, hypochlorite or hypochlorous acid occurs at the anode. The oxidized compound or compounds produced at the anode leave the electrochemical cell with outlet flow F’.

[0062] The inlet flow C’ is preferably introduced into the anode compartment of the electrochemical cell by at least one inlet C. The inlet flow C’ can be introduced into the anode compartment either continuously or discontinuously, for example at predetermined time intervals.

[0063] Inlet flow A’ is introduced into the cathode compartment of the electrochemical cell by at least one inlet A, either continuously or discontinuously, for example at predetermined time intervals.

[0064] The outlet flow E’ from the middle compartment of the electrochemical cell is preferably provided through at least one outlet E. The outlet flow E’ from the middle compartment can be provided continuously or discontinuously, for example at predetermined time intervals. In particular embodiments, outlet flow E’ from the middle compartment is provided to inlet A of the cathode compartment of the electrochemical cell. In certain embodiments, at least part of outlet flow E’ may be introduced into the vessel via inlet X of the vessel.

[0065] The outlet flow F’ from the anode compartment of the electrochemical cell is preferably provided through at least one outlet F. The outlet flow F’ from the anode compartment can be provided continuously or discontinuously, for example at predetermined time intervals.

[0066] The outlet flow D’ is preferably provided through at least one outlet D. The outlet flow D’ can be provided continuously or discontinuously, for example at predetermined time intervals. In certain embodiments, outlet flow D’ from the cathode compartment is at least partially provided to inlet X of the vessel. In certain other embodiments, outlet flow D’ from the cathode compartment of a first electrochemical cell is at least partially introduced in the cathode compartment of the second electrochemical cell via inlet G.

[0067] In certain embodiments, at least part of the outlet flow D’ from the cathode compartment and/or the at least part of the outlet flow E’ from the middle compartment may be introduced into the vessel through inlet X of the vessel.

[0068] In certain embodiments, wherein the system further comprises a second electrochemical cell, outlet flow G from the cathode compartment of the second electrochemical cell is provided through outlet I and outlet flow J’ from the anode compartment of the second electrochemical cell is provided through outlet J. The outlet flows G and J’ can be provided continuously or discontinuously, for example at predetermined time intervals. Outlet flow J’ may be directed to a sewer system. Inlet flow G’ that is introduced in the cathode compartment of the second electrochemical cell is preferably connected with the outlet of the cathode compartment of the first electrochemical cell to introduce a solution comprising urine, preferably a solution comprising stabilized urine in the cathode compartment of the second electrochemical cell. Inlet flow H’ that is introduced in the anode compartment of the second electrochemical cell is preferably connected with an outlet of the vessel to introduce a solution comprising urine and preferably comprising stabilized urine in the anode compartment of the second electrochemical cell.

[0069] The inlet flow B’ that is introduced into the middle compartment of the electrochemical cell is preferably connected with an outlet of the vessel to introduce a solution comprising urine and preferably comprising stabilized urine in the middle compartment of the electrochemical cell.

[0070] The method according to the present invention preferably allows to stabilize a solution comprising urine and to simultaneously produce at least one oxidized compound. More preferably, the method according to the present invention allows to stabilize a solution comprising urine while simultaneously producing an oxidized compound derived from urine or a constituent of urine.

[0071] In a preferred method according to the present invention, an inlet flow A’ is introduced into the cathode compartment of the electrochemical cell, preferably through at least one inlet A, and an outlet flow D’ is provided from the cathode compartment of the electrochemical cell, preferably through an outlet D. The inlet flow A’ may comprise an aqueous solution and/or a solution comprising urine. More preferably, the inlet flow A’ comprises a solution comprising (stabilized) urine from the vessel. Most preferably, the inlet flow A’ comprises outlet flow E’ from the middle compartment of the (first) electrochemical cell. Preferably, the outlet flow D’ is at least partially introduced into the vessel. In particularly preferred embodiments, outlet flow D’ from the cathode compartment of a first electrochemical cell is at least partially introduced in the cathode compartment of a second electrochemical cell via inlet G. It is clear that in such methods also at least part of the outlet flow E’ can be introduced in the vessel.

[0072] Preferably, inlet flow B’ comprises a solution comprising (stabilized) urine having a pH of at least 11, more preferably a pH of at least 12 and/or a concentration of divalent metal ions preferably below 20 % of the concentration of divalent metal ions of fresh urine, more preferably below 10 % or below 5 % of the concentration of divalent metal ions of fresh urine. The inlet flow B’ comprises preferably a solution comprising urine from the vessel.

[0073] The inlet flow A’ comprises preferably an aqueous solution and/or comprises a solution comprising urine. More preferably, the inlet flow A’ comprises a solution comprising (stabilized) urine having a pH of at least 11, preferably a pH of at least 12 and/or a concentration of divalent metal ions below 20 % of the concentration of divalent metal ions of fresh urine, more preferably below 10 % or below 5 % of the concentration of divalent metal ions of fresh urine. In particularly preferred embodiments the inlet flow A’ comprises (stabilized) urine from the vessel or from the middle compartment (as outlet flow E’).

[0074] In particular embodiments the inlet flow A’ and the inlet flow B’ both comprise a solution comprising (stabilized) urine, preferably a solution comprising (stabilized) urine from the vessel. More in particular, also inlet flow G’ into the cathode compartment of a second electrochemical cell comprises a solution comprising (stabilized) urine, preferably a solution comprising (stabilized) urine from the vessel or from the cathode compartment of the first electrochemical cell.

[0075] The outlet flow D’ comprises preferably an alkaline flow having a pH of at least 11. More preferably the flow D’ comprises an alkaline flow having a pH of at least 12. Most preferably, the outlet flow D’ comprises an alkaline flow having a pH of at least 13. In particular embodiments, the outlet flow G of the cathode compartment of a second electrochemical cell comprises preferably an alkaline flow having a pH of at least 11. More preferably the flow G comprises an alkaline flow having a pH of at least 12. Most preferably, the outlet flow G comprises an alkaline flow having a pH of at least 13.

[0076] The inlet flow C’ comprises preferably water or an aqueous solution, for example rainwater, potable water, ground water, (treated) grey water or any type of aqueous solution. Alternatively, the inlet flow C’ may comprise a solution comprising urine or stabilized urine for example an aqueous solution comprising urine, for example a solution comprising (stabilized) urine from the vessel. Optionally, the inlet flow C’ may further comprise one or more additives, for example compounds that are able to be oxidized at the anode. The inlet flow C’ comprises for example sodium chloride and/or sodium sulfate.

[0077] The outlet flow F’ preferably comprises the at least one oxidized compound produced at the anode of the electrochemical cell. Preferably, the at least one oxidized compound is produced from a compound derived from the (stabilized) urine, for example from chloride present in the (stabilized) urine. Preferred oxidized compounds comprise chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or other reactive species derived from oxygen. In certain embodiments, outlet flow F’ is connected to a flushing means.

[0078] The at least one oxidized compound can also be produced from a compound present in inlet flow C’. It is clear that the oxidized compound or oxidized compounds may also comprise a combination of a compound or compounds produced from a compound derived from urine and a compound or compounds produced from a compound of the inlet flow C’. [0079] The method according to the present invention preferably uses a first and/or second separator as described above. In certain embodiments, in case a second electrochemical cell, a third separator as described above is used.

[0080] The method according to the present invention preferably uses one or more anodes and/or one or more cathodes as described above.

[0081] The method according to the present invention preferably uses a vessel as described above. The vessel can be adapted to allow the precipitation of compounds comprising divalent metals, for example comprising calcium and/or magnesium from the urine or from the solution comprising urine. The vessel is for example provided with a conically shaped bottom part.

[0082] The vessel can be provided with one or more additional inlets Z, for example to introduce urine or a solution comprising urine either continuously or discontinuously into the vessel. The vessel may also be provided with one or more additional outlets V for example for harvesting compounds from the vessel, for example compounds comprising divalent metal ions, such as compounds comprising calcium and/or magnesium and/or with one or more outlets W for providing stabilized urine or a solution comprising stabilized urine, for example periodically. Advantageously, the compounds comprising divalent metal ions may comprise phosphate, and can be used as a source of phosphorus for various uses.

[0083] In particular embodiments the vessel comprises more than one compartment, for example a first compartment provided with the inlet X or Z and a second compartment provided with the outlet Y or W. The vessel may have an overflow to bring (stabilized) urine from the first compartment to the second compartment. Such embodiment has the advantage to allow limited mixing in the first compartment and enables better precipitation. Such embodiment furthermore has the advantage that the first compartment can be emptied without removal of all fluid from the system.

[0084] The method according to the present invention may further comprise one or more steps to control or adjust the method, for example by means of one or more controllers and/or one or more adjusting means as for example a pH controller and/or means to adjust the pH and/or a flow rate controller and/or means to adjust the flow rate, and/or means to adjust the concentration of the oxidized compound or the flow rate, the pH and/or the concentration of the oxidized compound. Any of the flows A’, B’, C’, D’, E’, F’ can be provided with one or more controllers and/or with one or more adjusting means. In certain embodiments comprising a second electrochemical cells, any of the flows G’, H’, G and/or J’ can be provided with one or more controllers and/or with one or more adjusting means of the flow rate, the pH, and/or the concentration of the oxidized compound.

[0085] The method according to the present invention can be used to produce at least on oxidized compound. Outlet flow F’ from the anode compartment preferably comprises one or more oxidized compounds, for example an aqueous flow comprising one or more oxidized compounds. Preferred oxidized compound comprise chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or other reactive species derived from oxygen. The outlet flow F’ is for example suitable for toilet flushing, rainwater, surface water, (treated) grey water or other streams in which the presence of one or more oxidized products is attractive for example for disinfection purpose, for removal of unwanted compounds or for both disinfection purposes and removal of unwanted compounds.

[0086] In certain embodiments, the method may further comprise the step of removing nitrogen from a stabilized urine stream or flow. Such stabilized urine flow may be an outlet flow W from the vessel, or an outlet flow J’ from the anode compartment of a second electrochemical cell. In certain embodiments, the method may further comprise the step of removing the nitrogen compounds in the stabilized urine, particularly by bacteria.

[0087] An advantage of the method is that it allows to combine the stabilization of urine or a solution comprising urine and the production of a flow comprising at least one oxidized product. The system according to the present invention allows to produce an oxidized compound while stabilizing urine or a solution comprising urine.

Brief description of the drawings

[0088] The present invention will be discussed in more detail below, with reference to the attached drawings. Different connections between elements in a system may be represented by lines with different appearance (full line or differently dashed lines). It is further understood that the different connections between the elements in the system may further be outfitted with valves (not shown). In particular:

Figures 1 to 5 show different embodiments of systems according to the present invention;

Figure 6 shows an embodiment of a vessel for receiving and storing urine;

Figure 7 shows the cumulative electric charge needed to increase the pH of fresh (non-hydrolyzed) male urine to a pH above 12;

Figure 8 shows the stability of the electrochemically stabilized urine over 18 months. Less than 8 % of the total nitrogen (TN, dashed lines) is hydrolyzed into total ammoniacal nitrogen (TAN, solid lines).

Figure 9 and 10 show different embodiments of systems according to the present invention.

Description of embodiments

[0089] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. The size of some of the elements in the drawing may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0090] When referring to the endpoints of a range, the endpoints values of the range are included.

[0091] When describing the invention, the terms used are construed in accordance with the following definitions, unless indicated otherwise.

[0092] The term ‘and/or’ when listing two or more items, means that any one of the listed items can by employed by itself or that any combination of two or more of the listed items can be employed.

[0093] The terms ‘first’, ‘second’ and the like used in the description as well as in the claims, are used to distinguish between similar elements and not necessarily describe a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention.

[0094] For the purpose of this invention ‘stabilized urine’ is defined as urine having at least less that than 50% hydrolysis in 48 hours, or less than 50% hydrolysis in 24 hours, for example less than 20% hydrolysis in 48 hours or less than 20% hydrolysis in 24 hours, for example less than 10% hydrolysis in 48 hours or less than 10% hydrolysis in 24 hours. More preferably, stabilized urine has less than 50 % of hydrolysis of urea in one week, for example less than 20 % or less than 10% of hydrolysis of urea in one week. Urine or the solution comprising urine is preferably kept at room temperature or a temperature lower than room temperature, such as between 0°C and 30°C or between 5°C or 10°C and 20°C or 25°C.

EXAMPLE 1

[0095] Figure 1 shows a first embodiment of a system 100 according to the present invention for stabilizing urine or a solution comprising urine and for producing at least one oxidized compound. The system 100 comprises a vessel 101 and an electrochemical cell 102. The vessel 101 has at least one inlet X (for introducing an inlet flow X’ into the vessel 101) and at least one outlet Y (for providing an outlet flow Y’ from the vessel 101). Preferably, the vessel 101 has at least one additional inlet Z for introducing urine (inlet flow Z’) or a solution comprising urine either continuously or discontinuously into the vessel 101. The vessel 101 may also be provided with one or more additional outlets V for example for harvesting compounds comprising divalent metal ions, for example compounds comprising calcium and/or magnesium and/or with one or more outlets W for providing stabilized urine or a solution comprising stabilized urine for example periodically.

[0096] The electrochemical cell 102 comprises a cathode compartment 104 comprising a cathode 105, a middle compartment 106 and an anode compartment 108 comprising an anode 109. The cathode compartment 104 and the middle compartment 106 are separated by a first separator 110, for example a cation exchange membrane. The middle compartment 106 and the anode compartment 108 are separated by a second separator 112, for example an anion exchange membrane.

[0097] A power supply 114 applies a voltage between the cathode 105 and the anode 109. [0098] The cathode compartment 104 is provided with at least one inlet A for introducing an inlet flow A’ in the cathode compartment 104 and with at least one outlet D for providing an outlet flow D’ from the cathode compartment 104.

[0099] The middle compartment 106 is provided with at least one inlet B for introducing an inlet flow B’ in the middle compartment 106 and with at least on outlet E for providing an outlet flow E’ from the middle compartment 106.

[00100] The anode compartment 108 is provided with at least one inlet C for introducing an inlet flow C’ in the anode compartment 108 and with at least one outlet F for providing an outlet flow F’ from the anode compartment 108.

[00101] In the system of Figure 1 a solution comprising urine in vessel 101 is mixed with the outlet flow D’ from the cathode compartment 104 of the electrochemical cell 102. The pH of the resulting solution in the vessel 101 is preferably more than 12 and more preferably between 12 and 13. From the vessel 101 a fluid is pumped through outlet Y of the vessel 101 to provide a flow B’ that is introduced through inlet B into the middle compartment 106 of the electrochemical cell 102.

[00102] The inlet flow A’ introduced through inlet A of the cathode compartment 104 comprises for example an aqueous solution.

[00103] The inlet flow B’ comprises (stabilized) urine or a solution comprising (stabilized) urine from the vessel 101.

[00104] The inlet flow C’ introduced through inlet C of the anode compartment 106 comprises for example an aqueous solution, for example rainwater or potable water or (treated) grey water.

[00105] Once inlet flow B’ is introduced in the middle compartment 106 of the electrochemical cell 102 and a voltage is applied, cations are exchanged by the cation exchange membrane 110 and move to the cathode compartment 104, where a more alkaline solution is created due to water reduction. Consequently, outlet flow D’ comprises a more alkaline solution than the inlet flow B’. Anions are exchanged by the anion exchange membrane 112 and move to the anode compartment 108. Anodic oxidation causes the conversion of the anions, for example the chloride ions, to chlorine, hypochlorite or hypochlorous acid at the anode 109. The oxidized compound or compounds produced at the anode 109 leave(s) the electrochemical cell 102 with outlet flow F\

EXAMPLE 2

[00106] Figure 2 shows an alternative embodiment of a system 100 according to the present invention similar to the embodiment shown in Figure 1. The system 100 shown in Figure 2 differs from the one in Figure 1 in that both the outlet flow D’ and the outlet flow E’ are at least partially introduced in the vessel 101.

EXAMPLE 3

[00107] Figure 3 shows a further embodiment of a system 100 according to the present invention similar to the embodiments shown in Figure 1 and Figure 2. The system 100 shown in Figure 3 differs from the one in Figure 1 in that outlet flow E’ is at least partially introduced in the cathode compartment 104. This enables initial removal of chloride from the stabilized urine towards the anode, after which in the cathode compartment 104 the urine can be made more alkaline to create a strong alkaline fluid.

EXAMPLE 4 [00108] Figure 4 shows a further embodiment of a system 100 according to the present invention. The solution comprising (stabilized) urine of the vessel 101 is introduced through the inlet A to provide inlet flow A’ in the cathode compartment and through inlet B to provide inlet flow B’ in the middle compartment. Outlet flow D’ is pumped from the cathode compartment 104 to the vessel. Optionally, also outlet flow E’ is pumped from the middle compartment 106 to the vessel 101.

EXAMPLE 5

[00109] Figure 5 shows still a further embodiment of a system 100 according to the present invention. The first separator 110 comprises a bipolar membrane or an anion exchange membrane and the second separator comprises an anion exchange membrane. The solution comprising (stabilized) urine of the vessel 101 is introduced through the inlet B to provide inlet flow B’ in the middle compartment 106 of the electrochemical cell. Outlet flow E’ from the middle compartment 106 of the electrochemical cell 101 is introduced in the vessel 101. Hydroxyde ions, produced in the cathode compartment 104 by water reduction at the cathode 105, are exchanged by the anion exchange membrane or bipolar membrane 110 and move to the middle compartment 106, where a more alkaline solution is created.

EXAMPLE 6

[00110] Figure 6 shows a vessel 600 for receiving and storing urine or a solution comprising urine. The vessel comprises two chambers 601 , 602 partially or fully separated by a wall 603. In a first chamber 601 (at the left side of the vessel 600) urine or a solution comprising urine is introduced through inlet Z. A flow X’ from the first and/or second electrochemical cell is introduced in the first chamber 601 via inlet X, and a flow Y’ is pumped from the second chamber 602 of the vessel 600 to the electrochemical cell. The vessel 600 may have an overflow between the two chambers 601, 602. Such overflow allows to bring stabilized urine from the first chamber 601 into the second chamber 602 (at the right side of the vessel) from which stabilized urine is brought to the electrochemical cell.

[00111] A vessel 600 comprising multiple chambers as shown in Figure 6 allows limited mixing in the first chamber 601 to enable better precipitation, and enables emptying of the first chamber 601 without removing all fluid from the vessel 600.

[00112] Flow X’ corresponds for example with outlet flow D’ of the cathode compartment of an electrochemical cell of the above described examples, with outlet flow E’ from the middle compartment of an electrochemical cell of the above described example or with a combination of outlet flow D’ from the cathode compartment and outlet flow E’ from the middle compartment of an electrochemical cell of the above described examples. Flow X’ may also correspond to flow G of the cathode compartment of a second electrochemical cell as described in example 9 below.

[00113] Flow Y’ corresponds for example with inlet flow B’ to the middle compartment of an electrochemical cell of the herein described examples or with a combination of inlet flow A’ to the cathode compartment and inlet flow B’ to the middle compartment of an electrochemical cell of the herein described examples.

EXAMPLE 7

[00114] Figure 7 shows the cumulative electric charge needed to increase the pH of fresh (non-hydrolyzed) male urine to a pH above 12. About 4 kC (kilo coulomb) or 6-7 kC per liter of urine is required to reach a pH of 11 or 12, respectively. The hydroxyde requirement can fluctuate depending on the urine composition and dilution.

EXAMPLE 8

[00115] Figure 8 shows the stability of the electrochemically stabilized urine over time. After increasing the pH to 12 with an electrochemical cell, the urine was stored in closed falcon tubes at room temperature. Samples were taken regularly to measure the TAN (total ammoniacal nitrogen, i.e. NH3 + NH4⁺) concentration. The experiment was performed with three different (1:1 diluted) urine batches shown by different line types in Figure 8. After 18 months, less than 8% of the total nitrogen (TN) was hydrolyzed into TAN, demonstrating the effectiveness of the electrochemical urine stabilization. The total nitrogen content is for the three batches indicated in Figure 8 by the lines indicated with TN.

EXAMPLE 9

[00116] Figure 9 shows an embodiment, wherein the system comprises a first electrochemical cell, comprising three compartments, and a second electrochemical cell, comprising two compartments. Stabilized urine from the buffer tank or vessel is brought to the middle compartment and cathode compartment of the first electrochemical cell and to the cathode compartment of the second electrochemical cell. In the anode compartment of the first electrochemical cell, the aforementioned reaction occurs leading to the formation of oxidized species in a water stream. In the anode compartment of the second electrochemical cell, urine from the buffer tank is brought and at the anode, oxidants preferably oxygen are generated with concomitant proton production. This enables a lowering of the pH of the stabilized urine. The effluent of the anode compartment of the second anode is brought towards discharge or other use. In certain embodiments of the system represented in Figure 9, the system may be controlled with a power supply diverting part of the current away from the anode of the first electrochemical cell towards the anode of the second electrochemical cell. The distribution of the current towards the anode of the first electrochemical cell can be controlled based on the pH or the concentration of the oxidized compound or another parameter as relevant for the specific setting of the fluid going through the anode compartment of the first electrochemical cell, whereby if the target pH or the concentration of the oxidized compound is reached the additional current is diverted towards the anode of the second electrochemical cell. In some further embodiments, the urine flow through the second anode compartment is controlled based on the incoming urine to enable control of the fluid volume in the buffer tank or vessel.

[00117] The particular embodiment of the system according to the present application for stabilizing urine or a solution comprising urine and for producing at least one oxidized compound and presented in Figure 9 can be described as follows. The system 100 comprises a vessel 101 , an electrochemical cell 102 and an electrochemical cell 103. The vessel 101 has at least one inlet X, for introducing an inlet flow X into the vessel 101 , and at least one outlet Y, for providing an outlet flow Y’ from the vessel 101. Preferably, the vessel 101 has at least one additional inlet Z for introducing urine (inlet flow Z’) or a solution comprising urine either continuously or discontinuously into the vessel 101. The vessel 101 may also be provided with one or more additional outlets V for example for harvesting compounds comprising divalent metal ions, for example compounds comprising calcium and/or magnesium and/or with one or more outlets W for providing stabilized urine or a solution comprising stabilized urine for example periodically. The electrochemical cell 102 comprises a cathode compartment 104 comprising a cathode 105, a middle compartment 106 and an anode compartment 108 comprising an anode 109. The cathode compartment 104 and the middle compartment 106 are separated by a first separator 110. The first separator preferably comprises a separator allowing cation transport. More preferably, the first separator comprises a cation exchange membrane. Also, a bipolar membrane can be considered as first separator. The middle compartment 106 and the anode compartment 108 are separated by a second separator 112. The second separator preferably comprises a separator allowing anion transport. More preferably, the second separator comprises an anion exchange membrane. A power supply (not shown) is used to apply a voltage between the cathode 105 and the anode 109.

[00118]The electrochemical cell 103 comprises a cathode compartment 116 comprising a cathode 117, and an anode compartment 118 comprising an anode 119. The cathode compartment 116 and the anode compartment 118 are separated by a separator 120. The separator, i.e. the separator between the cathode compartment 116 and the anode compartment 118 preferably comprises a separator allowing ion transport. More preferably, the separator comprises a cation exchange membrane. A power supply (not shown) is used to apply a voltage between the cathode 117 and the anode 119.

[00119] The cathode compartment 104 is provided with at least one inlet A for introducing an inlet flow A’ in the cathode compartment 104 and with at least one outlet D for providing an outlet flow D’ from the cathode compartment 104. The middle compartment 106 is provided with at least one inlet B for introducing an inlet flow B’ in the middle compartment 106 and with at least on outlet E for providing an outlet flow E’ from the middle compartment 106. The anode compartment 108 is provided with at least one inlet C for introducing an inlet flow C’ in the anode compartment 108 and with at least one outlet F for providing an outlet flow F’ from the anode compartment 108, the outlet flow F’ comprising an oxidized compound formed at the anode. The oxidized compound is preferably derived from urine or a constituent of urine and/or from a compound of inlet flow C’. More preferably, the oxidized compound is derived from urine or a constituent of urine present in the electrochemical cell, in particular in the middle compartment of the electrochemical cell.

[00120] The cathode compartment 116 is provided with at least one inlet G for introducing an inlet flow G’ in the cathode compartment 116 and with at least one outlet I for providing an outlet flow G from the cathode compartment 116. The anode compartment 118 is provided with at least one inlet H for introducing an inlet flow H’ in the anode compartment 118 and with at least one outlet J for providing an outlet flow J’ from the anode compartment 118.

[00121] In the system of Figure 9, a solution comprising urine in vessel 101 is mixed with the outlet flow G from the cathode compartment 116 of the electrochemical cell 103. The pH of the resulting solution in the vessel 101 is preferably more than 12 and more preferably between 12 and 13. From the vessel 101 a fluid is pumped through outlet Y of the vessel 101 to provide a flow B’ that is introduced through inlet B into the middle compartment 106 of the electrochemical cell 102. The inlet flow B’ comprises (stabilized) urine or a solution comprising (stabilized) urine from the vessel 101. The inlet flow A’ comprises (stabilized) urine or a solution comprising (stabilized) urine from the outlet E of middle compartment 106. The inlet flow G’ comprises (stabilized) urine or a solution comprising (stabilized) urine from the outlet D of cathode compartment 104. The inlet flow C’ introduced through inlet C of the anode compartment 106 comprises for example an aqueous solution, for example rainwater or potable water or (treated) grey water. The inlet flow H’ comprises (stabilized) urine or a solution comprising (stabilized) urine from the vessel 101.

[00122] Stated differently, in the system of Figure 9:

- the at least one outlet Y of the vessel 101 is connected with the at least one inlet B of the middle compartment 106 for introducing the inlet flow B’ comprising urine, preferably stabilized urine, from the vessel 101 into the middle compartment 106. Once inlet flow B’ is introduced in the middle compartment 106 of the electrochemical cell 102 and a voltage is applied, cations are exchanged by the cation exchange membrane 110 and move to the cathode compartment 104, where a more alkaline solution is created due to water reduction. Consequently, outlet flow D’ comprises a more alkaline solution than the inlet flow B’. Anions are exchanged by the anion exchange membrane 112 and move to the anode compartment 108. Anodic oxidation causes the conversion of the anions, for example the chloride ions, to chlorine, hypochlorite or hypochlorous acid at the anode 109. The oxidized compound or compounds produced at the anode 109 leave(s) the electrochemical cell 102 with outlet flow P;

- the at least one of the outlet E is connected with the at least one inlet A of the cathode compartment 104 for introducing the inlet flow A’ comprising urine, preferably stabilized urine, from the middle compartment 106 into the cathode compartment 104 of electrochemical cell 102. This enables initial removal of chloride from the stabilized urine towards the anode, after which in the cathode compartment 104 the urine can be made more alkaline to create a strong alkaline fluid;

- the at least one of the outlet D is connected with the at least one inlet G of the cathode compartment of the electrochemical cell 103 for introducing the inlet flow G’ comprising urine, preferably stabilized urine, from the cathode compartment of electrochemical cell 102 into the cathode compartment of electrochemical cell 103;

- the at least one of the outlet I is connected with the at least one inlet X of the vessel for introducing the outlet flow G into the vessel, wherein the outlet flow G is more alkaline than the inlet flow B’. Preferably, the outlet flow G that is introduced in the vessel has a pH of at least 11, more preferably a pH of at least 12. Most preferably, the outlet flow G that is introduced in the vessel has a pH of at least 12, for example a pH of 13;

- the at least one outlet W of the vessel 101 is connected with the at least one inlet H of the anode compartment 118 for introducing the inlet flow H’ comprising urine, preferably stabilized urine, from the vessel 101 into the anode compartment 118. The urine flow through anode compartment 118 is controlled based on incoming urine to enable control of fluid volume in the buffer tank.

At anode 119, urine from the vessel 101 is brought and at the electrode, oxidants preferably oxygen are generated with concomitant proton production. This enables a lowering of the pH of the stabilized urine. The effluent of anode 119 is brought towards discharge or other use.

EXAMPLE 10 [00123] Figure 10 shows an embodiment of a system (200) according to the present application wherein the electrochemical cell (202) is supplemented with a second anode and second middle compartment. The electrochemical cell 202 comprises a cathode compartment 208 comprising a cathode, a first middle compartment 206 and a second middle compartment (210), a first anode compartment 212 comprising a first anode and a second anode compartment 204 comprising a second anode. The cathode compartment and the adjacent middle compartments are separated by a respective separator. Each anode compartment is separated from the adjacent middle compartment by a separator. At the first anode, in the first anode compartment 212, the aforementioned reaction occurs leading to the formation of oxidized species in a water stream. Water is provided to the first anode compartment from a buffer tank 220 via inlet C1 , and the water comprising the oxidized species is returned to buffer tank 220 via outlet F1. At the second anode, in the second anode compartment 204, urine from vessel 201 or at least partially from the outflow of middle compartment 206, cathode compartment 208, and middle compartment 210 is introduced via inlet C2, and at the electrode, oxidants preferably oxygen are generated with concomitant proton production. This enables a lowering of the pH of the stabilized urine. The effluent of the second anode compartment 204 is brought via outlet F2 towards discharge (e.g. a sewer) or other use. Further, a flow is provided from vessel 201 to the cathode compartment 208 via inlet A and/or to the first middle compartment 206 and/or to the second middle compartment 210 via inlets B1 and B2, respectively. The outflow from the cathode compartment 208, via outlet D, and/or to the first middle compartment 206 and/or to the second middle compartment 210, via outlets E1 and E2, respectively, may at least partially returned to the vessel 201 and/or at least partially provided to the second anode compartment 204. However, depending on the conditions, the fluid flows can be altered. The embodiment presented in Figure 10 is merely an example of how the flows can be directed.

[00124] In a further embodiment relating to Figure 10, the system is controlled with a power supply diverting part of the current away from the first anode towards the second anode (not shown in Figure 10). The distribution of current towards the first anode can be controlled based on the pH or the concentration of the oxidized compound of the fluid going through the first anode compartment, whereby if the target pH or the concentration of the oxidized compound is reached, the additional current is diverted towards the second anode. In particular, the urine flow through the second anode compartment is controlled based on the incoming urine to enable the control of the fluid volume in the vessel or buffer tank. [00125] The present application also provides aspects and embodiments as set forth in the following Statements:

[00126] Statement 1. A system for stabilizing a solution comprising urine and for producing at least one oxidized compound, the system comprising:

- At least one vessel for receiving and storing a solution comprising urine and at least one electrochemical cell, the vessel being provided with at least one inlet X and at least one outlet Y; at least a first electrochemical cell comprising a cathode compartment, a middle compartment and an anode compartment, the cathode compartment and the middle compartment being separated by a first separator, the middle compartment and the anode compartment being separated by a second separator, wherein: o the middle compartment is provided with at least one inlet B for introducing an inlet flow B’ into the middle compartment and is provided with at least one outlet E for providing an outlet flow E’ from the middle compartment, o the anode compartment is provided with at least one inlet C for introducing an inlet flow C’ into the anode compartment and is provided with at least one outlet F for providing an outlet flow F’ from the anode compartment; o the cathode compartment is provided with at least one inlet A for introducing an inlet flow A’ into the cathode compartment and is provided with at least one outlet D for providing an outlet flow D’ from the cathode compartment;

- wherein the at least one outlet Y of the vessel is connected with the at least one inlet B of the middle compartment of the electrochemical cell for introducing the inlet flow B’ comprising urine, preferably stabilized urine, from the vessel into the middle compartment of the electrochemical cell, and

- wherein an outlet D is connected with the at least one inlet X of the vessel for introducing at least part of at least one of the outlet flow D’ into the vessel, optionally via a second electrochemical cell, wherein the outlet flow D’ that is at least partially introduced in the vessel is more alkaline than the inlet flow B’.

[00127] Statement 2. The system according to statement 1, further comprising a second electrochemical cell, comprising a cathode compartment and an anode compartment, wherein an outlet D is connected with an inlet G of the cathode compartment, for providing a flow D’ to the cathode compartment and wherein an outlet I of the cathode compartment is connected with the at least one inlet X of the vessel, for providing an outlet flow I’ to the vessel. [00128] Statement 3. The system according to statement 2, wherein an outlet W of the vessel is connected to the inlet H of the anode compartment of the second electrochemical cell, for providing an outlet flow W of a solution comprising stabilized urine to the anode compartment.

[00129] Statement 4. The system according to any one of statements 1 to 3, wherein the at least one outlet E is connected with the at least one inlet X of the vessel for introducing at least part of outlet flow E’ into the vessel, wherein the outlet flow E’ that is at least partially introduced in the vessel is more alkaline than the inlet flow B’.

[00130] Statement 5. The system according to any one of statements 1 to 3, wherein the at least one outlet E of the middle compartment is connected with the at least one inlet A of the cathode compartment of the first electrochemical cell for introducing an outlet flow E’ into the cathode compartment.

[00131] Statement 6. The system according to statement 1, wherein the at least a first electrochemical cell comprises a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet of the vessel is connected to at least inlet A of the cathode compartment and inlet B of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E of the first middle compartment, and/or an outlet W of the vessel is connected to an inlet of the second anode compartment.

[00132] Statement 7. The system according to any one of the preceding statements, wherein:

* the inlet flow B’ comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine; and/or

* the flow A’ comprises an aqueous solution and/or comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine; and/or

* the outlet flow D’ comprises an alkaline flow, having a pH of at least higher than the inlet flow A’, preferably having a pH of at least 11 or 12; and/or the inlet flow C’ comprises an aqueous solution optionally comprising additives, (stabilized) urine optionally comprising additives or an aqueous solution and (stabilized) urine optionally comprising additives; and/or

* the outlet flow F’ comprises at least one oxidized compound selected from the group consisting of chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or a reactive species derived from oxygen; and/or * the outlet flow F’ comprises at least one oxidized compound that is derived from urine or from a constituent of urine.

[00133] Statement 8. The system according to any one of the preceding statements, wherein the first separator and/or second separator comprise an ion selective membrane.

[00134] Statement 9. The system according to any one of the preceding statements further comprising a nitrogen removal unit and/or a vessel or reactor comprising bacteria for the removal of nitrogen compounds from the stabilized urine.

[00135] Statement 10. A method to stabilize urine and to produce at least one oxidized compound using a system comprising (i) a vessel for receiving and storing a solution comprising urine and (ii) at least a first electrochemical cell, the electrochemical cell comprising a cathode compartment, a middle compartment and an anode compartment, the cathode compartment and the middle compartment being separated by a first separator, the middle compartment and the anode compartment being separated by a second separator, the method comprising the steps of

(a) introducing and/or storing a solution comprising urine in the vessel;

(b) introducing an inlet flow B’ into the middle compartment of the (first) electrochemical cell, wherein said inlet flow B’ comprises a solution comprising urine, preferably a solution comprising stabilized urine, from the vessel;

(c) introducing an inlet flow C’ into the anode compartment of the electrochemical cell;

(d) introducing an inlet flow A’ into the cathode compartment of the electrochemical cell;

(e) activating the at least first electrochemical cell by applying a voltage between the cathode and the anode of the electrochemical cell;

(f) reducing water at the cathode of the electrochemical cell;

(g) oxidizing at least one compound at the anode of the electrochemical cell to produce at least one oxidized compound;

(h) providing an outlet flow E’ from the middle compartment of the electrochemical cell;

(i) providing an outlet flow F’ from the anode compartment of the electrochemical cell, wherein the outlet flow F’ comprises the at least one oxidized compound produced at the anode;

(j) providing an outlet flow D’ from the cathode compartment of the electrochemical cell; and (k) introducing at least part of the outlet flow D’ from the cathode compartment into the vessel, optionally via a second electrochemical cell, whereby the outlet flow D’ that is introduced into the vessel is more alkaline than the inlet flow B’.

[00136] Statement 11. The method according to statement 10, wherein the system further comprises a second electrochemical cell comprising a cathode compartment and an anode compartment, wherein step (k) comprises the steps of (k1) introducing at least part of the outlet flow D’ into the cathode compartment of the second electrochemical cell; (k2) reducing water at the cathode of the second electrochemical cell by applying a voltage between the cathode and the anode of the second electrochemical cell; and (k3) introducing at least part of the outlet flow G from the cathode compartment of the second electrochemical cell into the vessel, wherein the outlet flow G is more alkaline than the inlet flow B’.

[00137] Statement 12. The method according to statement 11, further comprising the steps (11) of introducing a flow H’ comprising a solution comprising stabilized urine into the anode compartment of the second electrochemical cell; (I2) generating oxidants, particularly oxygen, and protons at the anode of the second electrochemical cell, and (I3) removing an outlet flow J’ from the anode compartment of the second electrochemical cell.

[00138] Statement 13. The method according to any one of statements 10 to 12, wherein an outlet flow E’ of the middle compartment is provided to the inlet A of the cathode compartment. [00139] Statement 14. The method according to statement 10, wherein the at least first electrochemical cell comprises a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet of vessel is connected to at least inlet A of the cathode compartment and inlet B of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E of the first middle compartment, and/or an outlet W of the vessel is connected to an inlet of the second anode compartment, the method further comprising the steps of providing a solution comprising stabilized urine from the vessel or from the outlets from the middle compartment and/or the cathode compartment, to the second anode compartment, and generating protons at the second anode. [00140] Statement 15. The method according to any one of statements 10 to 14, wherein the stabilization of a solution comprising urine and the production of at least one oxidized compound at the anode of the electrochemical cell occur simultaneously.

[00141] Statement 16. The method according to any one of statements 10 to 15, wherein the solution comprising urine is stored in the vessel (101) at a pH of at least 11. [00142] Statement 17. The method according to any one of statements 10 to 16, wherein the at least one oxidized compound produced at the anode of the first electrochemical cell is derived from urine or from a constituent of urine.

[00143] Statement 18. The method according to any one of statements 10 to 17, wherein: * the inlet flow B’ comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine; and/or

* the outlet flow D’ comprises an alkaline flow having a pH of at least 12; and/or

* the inlet flow C’ comprises an aqueous solution optionally comprising additives, (stabilized) urine optionally comprising additives or an aqueous solution and (stabilized) urine optionally comprising additives; and/or * the outlet flow F’ comprises at least one oxidized compound selected from the group consisting of chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or a reactive species derived from oxygen; and/or

* the outlet flow F’ comprises at least one oxidized compound that is derived from urine or from a constituent of urine. [00144] Statement 19. Use of the system as defined in any one of statement 1 to 9 to stabilize a solution comprising urine and to produce a flow comprising at least one oxidized compound, preferably at least one disinfectant.

Claims

1. A system (100) for stabilizing a solution comprising urine and for producing at least one oxidized compound, the system comprising: - At least one vessel (101) for receiving and storing a solution comprising urine and at least one electrochemical cell (102), the vessel being provided with at least one inlet X and at least one outlet Y; at least a first electrochemical cell (102) comprising a cathode compartment (104), a middle compartment (106) and an anode compartment (108), the cathode compartment (104) and the middle compartment (106) being separated by a first separator (110), the middle compartment (106) and the anode compartment (108) being separated by a second separator (112), wherein: o the middle compartment (106) is provided with at least one inlet B for introducing an inlet flow B’ into the middle compartment (106) and is provided with at least one outlet E for providing an outlet flow E’ from the middle compartment (106), o the anode compartment (108) is provided with at least one inlet C for introducing an inlet flow C’ into the anode compartment (108) and is provided with at least one outlet F for providing an outlet flow F’ from the anode compartment (108); o the cathode compartment (104) is provided with at least one inlet A for introducing an inlet flow A’ into the cathode compartment (104) and is provided with at least one outlet D for providing an outlet flow D’ from the cathode compartment (104); - wherein the at least one outlet Y of the vessel (101) is connected with the at least one inlet B of the middle compartment (106) of the electrochemical cell (102) for introducing the inlet flow B’ comprising urine, preferably stabilized urine, from the vessel into the middle compartment (106) of the electrochemical cell (102), and - wherein an outlet D is connected with the at least one inlet X of the vessel (101) for introducing at least part of at least one of the outlet flow D’ into the vessel

(101), optionally via a second electrochemical cell (103), wherein the outlet flow D’ that is at least partially introduced in the vessel (101) is more alkaline than the inlet flow B’.

2. The system (100) according to claim 1, further comprising a second electrochemical cell (103), comprising a cathode compartment (116) and an anode compartment (118), wherein an outlet D is connected with an inlet G of the cathode compartment (116), for providing a flow D’ to the cathode compartment (116) and wherein an outlet I of the cathode compartment (116) is connected with the at least one inlet X of the vessel (101), for providing an outlet flow G to the vessel (101).

3. The system according to claim 2, wherein an outlet W of the vessel (101) is connected to the inlet H of the anode compartment (118) of the second electrochemical cell (103), for providing an outlet flow W of a solution comprising stabilized urine to the anode compartment (118).

4. The system (100) according to any one of claims 1 to 3, wherein the at least one outlet E is connected with the at least one inlet X of the vessel (101) for introducing at least part of outlet flow E’ into the vessel (101), wherein the outlet flow E’ that is at least partially introduced in the vessel (101) is more alkaline than the inlet flow B’.

5. The system (100) according to any one of claims 1 to 3, wherein the at least one outlet E of the middle compartment (106) is connected with the at least one inlet A of the cathode compartment (104) for introducing an outlet flow E’ into the cathode compartment (104).

6. The system according to claim 1, wherein the at least a first electrochemical cell comprises a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet of vessel (101) is connected to at least inlet A of the cathode compartment and inlet B of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E of the first middle compartment, and/or an outlet W of the vessel (101 ) is connected to an inlet of the second anode compartment.

7. The system (100) according to any one of the preceding claims, wherein the inlet flow B’ comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10% of the concentration of divalent metal ions of fresh urine.

8. The system (100) according to any one of the preceding claims, wherein the flow A’ comprises an aqueous solution and/or comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine.

9. A system (100) according to any one of the preceding claims, wherein the outlet flow D’ comprises an alkaline flow, having a pH of at least higher than the inlet flow A’, preferably having a pH of at least 11 or 12.

10. A system (100) according to any one of the preceding claims, wherein the inlet flow C’ comprises an aqueous solution optionally comprising additives, (stabilized) urine optionally comprising additives or an aqueous solution and (stabilized) urine optionally comprising additives.

11. A system (100) according to any one of the preceding claims, wherein the outlet flow F’ comprises at least one oxidized compound selected from the group consisting of chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide ora reactive species derived from oxygen.

12. A system (100) according to any one of the preceding claims, wherein the outlet flow F’ comprises at least one oxidized compound that is derived from urine or from a constituent of urine.

13. A system (100) according to any one of the preceding claims, wherein the first separator (110) and/or second separator (112) comprise an ion selective membrane.

14. A system (100) according to any one of the preceding claims further comprising a nitrogen removal or recovery unit, and/or a vessel comprising bacteria for the removal of nitrogenous compounds.

15. A method to stabilize urine and to produce at least one oxidized compound using a system (100) comprising (i) a vessel (101) for receiving and storing a solution comprising urine and (ii) at least a first electrochemical cell (102), the electrochemical cell (102) comprising a cathode compartment (104), a middle compartment (106) and an anode compartment (108), the cathode compartment (104) and the middle compartment (106) being separated by a first separator (110), the middle compartment (106) and the anode compartment (108) being separated by a second separator (112), the method comprising the steps of

(a) introducing and/or storing a solution comprising urine in the vessel (101); (b) introducing an inlet flow B’ into the middle compartment (106) of the electrochemical cell (102), wherein said inlet flow B’ comprises a solution comprising urine, preferably a solution comprising stabilized urine, from the vessel (101);

(c) introducing an inlet flow C’ into the anode compartment (108) of the electrochemical cell (102);

(d) introducing an inlet flow A’ into the cathode compartment (104) of the electrochemical cell (102);

(e) activating the at least first electrochemical cell (102) by applying a voltage between the cathode and the anode of the electrochemical cell; (f) reducing water at the cathode of the electrochemical cell (102);

(g) oxidizing at least one compound at the anode of the electrochemical cell (102) to produce at least one oxidized compound;

(h) providing an outlet flow E’ from the middle compartment (106) of the electrochemical cell (102); (i) providing an outlet flow F’ from the anode compartment (108) of the electrochemical cell (102), wherein the outlet flow F’ comprises the at least one oxidized compound produced at the anode;

0 providing an outlet flow D’ from the cathode compartment (104) of the electrochemical cell (102); and (k) introducing at least part of the outlet flow D’ from the cathode compartment

(104) into the vessel (101), optionally via a second electrochemical cell, whereby the outlet flow D’ that is introduced into the vessel (101) is more alkaline than the inlet flow B’. 16. The method according to claim 15, wherein the system (100) further comprises a second electrochemical cell (103) comprising a cathode compartment (116) and an anode compartment (118), wherein step (k) comprises the steps of (k1) introducing at least part of the outlet flow D’ into the cathode compartment (116) of the second electrochemical cell (103); (k2) reducing water at the cathode of the second electrochemical cell (103) by applying a voltage between the cathode and the anode of the second electrochemical cell; and (k3) introducing at least part of the outlet flow G from the cathode compartment (116) of the second electrochemical cell into the vessel (101), wherein the outlet flow G is more alkaline than the inlet flow B’.

17. The method according to claim 16, further comprising the steps (11) of introducing a flow H’ comprising a solution comprising stabilized urine into the anode compartment (118) of the second electrochemical cell (103); (I2) generating oxidants, particulary oxygen, and protons at the anode of the second electrochemical cell (103), and (I3) removing an outlet flow J’ from the anode compartment (118) of the second electrochemical cell (103). 18. The method according to any one of claims 15 to 17, wherein an outlet flow E’ of the middle compartment (106) is provided to the inlet A of the cathode compartment (104).

19. The method according to claim 15, wherein the at least first electrochemical cell comprises a second anode compartment and a second middle compartment, positioned between the cathode compartment and the second anode compartment; wherein an outlet of vessel (101) is connected to at least inlet A of the cathode compartment and inlet B of the first middle compartment, and wherein outlet D of the cathode compartment and/or outlet E of the first middle compartment, and/or an outlet W of the vessel (101 ) is connected to an inlet of the second anode compartment, the method further comprising the steps of providing a solution comprising stabilized urine from the vessel (101) or from the outlets from the middle compartment and/or the cathode compartment, to the second anode compartment, and generating protons at the second anode.

20. The method according to any one of claims 15 to 19, wherein the stabilization of a solution comprising urine and the production of at least one oxidized compound at the anode of the electrochemical cell (103) occurs simultaneously.

21. The method according to any one of claims 15 to 20, wherein the solution comprising urine is stored in the vessel (101) at a pH of at least 11.

22. The method according to any one of claims 15 to 21, wherein the at least one oxidized compound produced at the anode of the first electrochemical cell is derived from urine or from a constituent of urine.

23. The method according to any one of claims 15 to 22, wherein the inlet flow B’ comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine. 24. The method according to any one of claims 15 to 23, wherein the flow A’ comprises an aqueous solution and/or comprises a solution comprising urine having a pH of at least 11 and/or a concentration of divalent metal ions lower than 10 % of the concentration of divalent metal ions of fresh urine. 25. The method according to any one of claims 15 to 24, wherein the outlet flow D’ comprises an alkaline flow having a pH of at least 12.

26. The method according to any one of claims 15 to 25, wherein the inlet flow C’ comprises an aqueous solution optionally comprising additives, (stabilized) urine optionally comprising additives or an aqueous solution and (stabilized) urine optionally comprising additives.

27. The method according to any one of claims 15 to 26, wherein the outlet flow F’ comprises at least one oxidized compound selected from the group consisting of chlorine, hypochlorite, hypochlorous acid, hydrogen peroxide or a reactive species derived from oxygen.

28. The method according to any one of claims 15 to 27 wherein the outlet flow F’ comprises at least one oxidized compound that is derived from urine or from a constituent of urine.

29. Use of a system (100) as defined in any one of claims 1 to 14 to stabilize a solution comprising urine and to produce a flow comprising at least one oxidized compound, preferably at least one disinfectant.