Bio-electrochemical process for the production of methane and / or acetate using a microbial electrolyzer and associated system

Abstract

Bio-electrochemical process for the production of methane and / or acetate using a microbial electrolyzer and associated system. The present invention relates to a bio-electrochemical process for the production of methane and / or acetate using a microbial electrolyzer (17), said process comprising: - the oxidation of organic matter to carbon dioxide in an anodic compartment (20) of the microbial electrolyzer (17); - the reduction of carbon dioxide to methane and / or acetate depending on the pH of a catholyte (30) received in a cathodic compartment (18) of the microbial electrolyzer (17); characterized in that the process comprises adjusting the pH of the catholyte (30) to a desired value by adding carbon dioxide to the catholyte (30). Figure for the abstract: Figure 1

Description

Title of the invention: Bioelectrochemical process for the production of methane and / or acetate using a microbial electrolyzer and associated system

[0001] The present invention relates to a bio-electrochemical production process for methane and / or acetate using a microbial electrolyzer.

[0002] Environmental protection requires the implementation of treatment for greenhouse gases, particularly for carbon dioxide (CO2).

[0003] Carbon dioxide treatment aims to valorize carbon dioxide by capturing it and, ideally, transforming it into valuable molecules of interest, such as methane, volatile fatty acids, and ethanol.

[0004] There are processes for producing methane and acetate by reducing carbon dioxide under anaerobic conditions through the metabolism of microorganisms. However, these processes are carried out at 37 °C or 55 °C, which requires significant energy consumption. Furthermore, such processes also require substantial infrastructure, which further increases the cost of the process.

[0005] Alternative methods have therefore been developed. For example, carbon dioxide reduction has been considered using a microbial electrolyzer. The electrolyzer has electrodes coated with microorganisms that catalyze the reduction of carbon dioxide. Consequently, carbon dioxide is reduced at the cathode to produce methane or acetate. The bioelectrochemical system makes it possible to limit energy costs and to have more compact reactors compared to systems operating under anaerobic conditions.

[0006] For example, the process of reducing carbon dioxide to methane by a bio-electrochemical system was reported by Zhen et al., in the article, Bioresource Technology, 2018, volume 266, pages 382-388. According to the authors, a stream of carbon dioxide is injected into the cathode compartment, in which the carbon dioxide is reduced by the microorganisms in the presence of an electric current to produce methane.

[0007] However, such a process is not entirely satisfactory. Indeed, during the reduction of carbon dioxide at the cathode, acetates are also generated as an intermediate product. Zhen's process does not allow the acetates to be extracted from the system and is therefore unable to produce acetates as the main reduction product. Thus, such a process only allows the conversion of carbon dioxide to methane and does not offer the versatility to choose between methane and acetate as the reduction product. Furthermore, such a process requires an external supply of carbon dioxide, which increases the cost as well as the size of the equipment.

[0008] One object of the invention is therefore to propose a bio-electrochemical process using a microbial electrolyzer to reduce carbon dioxide molecules, in order to produce methane and / or acetate.

[0009] The method according to the invention also aims to limit the energy costs and the land footprint of such a system.

[0010] In particular, the production process according to the invention has the advantage of limiting, or even avoiding, the external supply of carbon dioxide.

[0011] Thus, the method according to the invention comprises: - the oxidation of organic matter into carbon dioxide in an anodic compartment of the microbial electrolyzer; - the reduction of carbon dioxide into methane and / or acetate depending on the pH of a catholyte received in a cathode compartment of the microbial electrolyzer;

[0012] characterized in that the process includes adjusting the pH of the catholyte to a desired value by adding carbon dioxide to the catholyte.

[0013] Organic matter is defined as substances composed of molecules containing carbon and hydrogen. In particular, organic matter is oxidizable and may also include one or more elements such as oxygen, nitrogen, sulfur, and phosphorus. Organic matter may include natural and synthetic compounds, such as hydrocarbons, polymers, biomolecules, such as proteins, lipids, carbohydrates, nucleic acids, and their derivatives. Organic matter may be of biological origin, resulting from metabolic processes in living organisms, or of non-biological origin, resulting from chemical syntheses.

[0014] In particular, organic matter can also include organic pollution of aquatic environments, the quantity of which can be characterized by the Chemical Oxygen Demand (COD), which corresponds to the amount of oxygen required to oxidize all the oxidizable matter in an environment. Organic pollution of aquatic environments includes, for example, organic compounds resulting from the decomposition of plant waste or excrement.

[0015] A "microbial electrolyzer" is understood to be an electrolyzer in which the electrodes comprise microorganisms. Microorganisms may include, for example, archaea, bacteria, fungi, and microscopic algae. In particular, the microorganisms used according to the invention are microorganisms capable of supplying or consuming electrons; in other words, electroactive microorganisms.

[0016] According to one embodiment, the microbial electrolyzer can be an electrolyzer in which at least one of the electrodes comprises biofilms of electroactive microorganisms.

[0017] The term "anodic compartment" of a microbial electrolyzer means an assembly comprising one or more anodes and an anolyte, in which the electrochemical oxidation reaction takes place in the presence of an electric current.

[0018] The term "anolyte" means an electrolyte solution comprising water and free ions, located in the anodic compartment.

[0019] The term "cathodic compartment" of a microbial electrolyzer means an assembly comprising one or more cathodes and a catholyte, in which the electrochemical reduction reaction takes place in the presence of an electric current.

[0020] The term "catholyte" means an electrolyte solution comprising water and free ions, located in the cathode compartment.

[0021] The term “acetate” means an ion with the formula CH3COO, resulting from the deprotonation of acetate acid CH3COOH, or from the dissociation of acetate salts such as CH3COONa, CH3COOK.

[0022] More particularly, the process according to the invention makes it possible to control the amount of carbon dioxide added in the cathodic compartment, in order to adjust the pH of the catholyte according to the molecule to be produced.

[0023] Indeed, when the amount of carbon dioxide added to the cathode compartment is high, the catholyte generally has a pH below 7.5, which favors methane production. On the other hand, when the amount of carbon dioxide added to the cathode compartment is low, in particular when the rate of carbon dioxide introduction is lower than the rate of carbon dioxide consumption at the anode, the catholyte generally has a pH above 7.5, which favors acetate generation.

[0024] Thus, thanks to this controlled addition step of carbon dioxide, it is possible to produce methane or acetate by a single bio-electrochemical reactor, maintaining the pH of the catholyte at the appropriate value to obtain the desired reduction product.

[0025] In addition, the process according to the invention also makes it possible to achieve a production that can be switched between methane and acetate by adjusting the pH of the catholyte by modifying the amount of carbon dioxide added.

[0026] According to particular embodiments of the invention, the bio-electrochemical production process according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination: - The process according to the invention includes a step in which the production of methane is replaced by the production of acetate by decreasing the amount of carbon dioxide introduced into the catholyte; - The process according to the invention includes a step to produce methane, in which the pH of the catholyte is adjusted to a value less than or equal to 7.5; - The process according to the invention includes a step in which the production of acetate is replaced by the production of methane by increasing the amount of carbon dioxide introduced into the catholyte. - The process according to the invention includes a step to produce the acetate, in which the pH of the catholyte is adjusted to a value greater than 7.5. - The process according to the invention includes a step in which the carbon dioxide produced in the anodic compartment is collected and introduced into the catholyte; - The process according to the invention includes a step in which the anodic compartment is placed under an anaerobic environment. - The process according to the invention includes a step in which a difference in electrical potentials is applied between the cathode and the anode of the microbial electrolyzer to produce an electric current, and the difference in potentials between the cathode and the anode is maintained between 0.6 V and 3 V; - The process according to the invention includes a step in which the cathode comprises methanogenic archaea and homoacetogenic bacteria, and the anode comprises electrogenic bacteria, such as Geobacter suifurre duc ens; - The process according to the invention includes a step in which the anolyte has a chemical oxygen demand (COD) value between 200 and 30,000 mg O2 / L; and / or - The process according to the invention includes a step in which the anolyte is wastewater comprising organic matter, preferably acetate, and the catholyte is wastewater or a phosphate buffer solution.

[0027] The term "cathodic potential" refers to an electrochemical potential, which characterizes the potential difference at the interface between the cathode and the catholyte.

[0028] The term "anodic potential" refers to an electrochemical potential, which characterizes the potential difference at the interface between the anode and the anolyte.

[0029] The term "potential difference" means the potential difference calculated by subtracting the cathodic potential from the anodic potential.

[0030] The electrode potentials can be determined by comparison with the standard hydrogen electrode, whose potential is defined at zero volts.

[0031] Electrode potential measurements are for example carried out with the positive terminal of the electrometer connected to the electrode to be measured and the negative terminal to the reference electrode, which can be a hydrogen electrode.

[0032] Methanogenic archaea are defined as microorganisms that produce methane as a metabolic by-product of life under anoxic conditions, examples include Methanobacterium and Methanospirillum.

[0033] Homoacetogenic bacteria are defined as microorganisms that produce acetate as a metabolic by-product of life under anoxic conditions, Acetobacterium can be cited as an example.

[0034] Electrogenic bacteria are defined as microorganisms capable of interfacing their metabolism with a conductive surface, transferring electrons from the oxidation of organic matter, such as acetic acid. Geobacter sulfurreducens is an example. The term "chemical oxygen demand (COD)" refers to a value that characterizes the amount of matter, such as organic matter, that can be oxidized by oxygen in a solution. This value can be determined by spectrophotometric analysis. A phosphate buffer solution can be an aqueous solution in which the solute comprises sodium chloride, disodium phosphate, monopotassium phosphate, and potassium chloride.

[0035] Wastewater can include industrial or municipal wastewater or water from agriculture or the agri-food industry.

[0036] The invention further relates to a bio-electrochemical production system for methane and / or acetate comprising a microbial electrolyzer, the microbial electrolyzer comprising: - an anodic compartment to receive an anolyte containing organic matter, and a cathodic compartment to receive a catholyte, the anodic compartment and the cathodic compartment being separated by at least one ion exchange membrane; - one or each anode received in the anodic compartment and covered with initial microorganisms for the oxidation of organic matter contained in the anolyte into carbon dioxide, - one or each cathode received in the cathode compartment and covered with secondary microorganisms for the reduction of carbon dioxide contained in the catholyte to methane and / or acetate depending on the pH of the catholyte; and - an electrical circuit connecting the anode(s) to the cathode(s);

[0037] being characterized in that it comprises a device for injecting carbon dioxide into the catholyte, to adjust the pH to the desired value according to the desired reduction product at the cathode.

[0038] The term “first microorganisms” means microorganisms comprising electrogenic bacteria as defined according to the invention.

[0039] The term “second microorganisms” means microorganisms comprising methanogenic and homoacetogenic archaea as defined according to the invention.

[0040] The production system according to the invention may further comprise at least one of the following features or any technically feasible combination thereof: - The cathodic compartment and the anodic compartment are separated by at least two ion exchange membranes delimiting an intermediate compartment between them. - The at least two ion exchange membranes include at least one cation exchange membrane. - The at least two ion exchange membranes include at least one cation exchange membrane and at least one anion exchange membrane. - The anode and cathode are made of identical or different materials, carbon-based and independently chosen from graphite fabric, graphite fiber brushes, graphite felt, graphite granules and activated carbon granules, and combinations thereof.

[0041] The ion exchange membrane is generally a polymeric membrane, for example fluoropolymers

[0042] An electrical circuit is an electrical assembly comprising electrical elements such as a voltage source, a conductor, a resistor, and a capacitor. The electrical circuit is configured to establish a closed loop through which an electric current flows, characterized by its current intensity and voltage.

[0043] The terms "including" and "includes" as used herein are synonymous with "including", "includes" or "contains", "containing", and are inclusive or boundless and do not exclude additional features, elements or unspecified method steps.

[0044] Unless otherwise indicated, the percentages used are percentages by mass, and the pressures are absolute pressures.

[0045] The expressions % by mass and % mass have an equivalent meaning and refer to the proportion of the mass of a product relative to 100 g of a composition comprising it.

[0046] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0047] [Fig-1] [Fig.1] is a schematic representation of a bio production system electrochemical according to the invention.

[0048] According to one embodiment, a bio-electrochemical production system 10 according to the invention is schematically illustrated in [Fig. 1].

[0049] The production system 10 includes an anolyte source 12, configured to introduce a flow of anolyte 14a into a microbial electrolyzer 17.

[0050] The anolyte 14a stream is, for example, an aqueous solution comprising organic compounds, such as treated wastewater.

[0051] The treated wastewater is for example derived from industrial or municipal wastewater or from agricultural or agri-food industry, previously treated by a wastewater treatment plant.

[0052] According to one embodiment, the production system 10 also includes a carbon dioxide injection device 15, configured to introduce a flow of carbon dioxide 16 into the microbial electrolyzer 17.

[0053] The carbon dioxide introduction rate by the carbon dioxide injection device 15 can be adjusted according to the desired carbon dioxide reduction product, namely methane and / or acetate.

[0054] According to one embodiment, the production system 10 comprises a microbial electrolyzer 17 where electrolysis takes place, configured to oxidize the organic compounds in the anolyte 14b to carbon dioxide and to reduce the carbon dioxide supplied by the carbon dioxide injection device 15 to methane and / or acetate.

[0055] The microbial electrolyzer 17 may include a cathode compartment 18, an anodic compartment 20, an ion exchange membrane 22 and an electrical circuit 24.

[0056] Electrolysis takes place within the microbial electrolyzer 17 by: - establishment of an electrical circuit 24 between the cathode compartment 18 and the anodic compartment 20; - oxidation of organic compounds in the anolyte 14b to carbon dioxide in the anodic compartment 20; and - reduction of carbon dioxide supplied by the carbon dioxide injection device 15 into methane and / or acetate in the cathode compartment 18.

[0057] The electrical circuit 24 includes an electrical source configured to establish an electric current between the cathode compartment 18 and the anode compartment 20.

[0058] The electrical circuit 24 includes an electrical source such as a renewable energy source, such as a solar park, a tidal park or a wind farm, or a battery system, an electrical network or any other device supplying electricity.

[0059] The anodic compartment 20 is configured to accommodate the anolyte 14b supplied by the anolyte source 12, an anode 26, immersed in the anolyte 14b and covered by first microorganisms 28.

[0060] The first microorganisms 28 are introduced onto the surface of the anode 26, forming an electrogenic biofilm. The first microorganisms 28 are capable of releasing electrons when supplied with a source of electron-enriched compounds, such as acetic acid.

[0061] The cathodic compartment 18 is configured to accommodate a catholyte 30, the carbon dioxide flow 16 supplied by the carbon dioxide injection device 15, a cathode 32 immersed in the catholyte 30 and covered by second microorganisms 34.

[0062] The catholyte 30 is, for example, an aqueous solution. The catholyte 30 may comprise wastewater or a buffer solution having a pH of 7.

[0063] Typically, the carbon dioxide introduced into the cathode compartment 18 is at least partially dissolved in the catholyte 30, in the form of carbonic acid H2CO3 or CO32 and / or HCO3 anions.

[0064] According to one embodiment, the second microorganisms 34 comprise at least methanogenic archaea and homoacetogenic bacteria.

[0065] Methanogenic and homoacetogenic archaea develop, for example, in digested sludge obtained from wastewater treatment.

[0066] The term "digested sludge" means sludge from the treatment of waste or wastewater, obtained after a step of degradation of organic matter by microorganisms under an anaerobic environment.

[0067] The digested sludge is then introduced to the cathode 32 to form a cathode covered by microorganisms.

[0068] The second microorganisms 34 are sensitive to the concentration of carbon dioxide dissolved in the catholyte. Methanogenic and homoacetogenic archaea each have an optimal carbon dioxide concentration at which bacterial activity is highest for producing methane or acetate, respectively.

[0069] The ion exchange membrane 22 is configured to separate the anodic compartment 20 and the cathodic compartment 18 and prevent the immigration of acetate produced at the cathode 32 to the anode 26.

[0070] The cathode compartment 18 may further include a sensor, not illustrated in [Fig.1], configured to determine the pH value of the catholyte 30.

[0071] The carbon dioxide introduced into the cathode compartment 18 is at least partially dissolved in the catholyte 30, first forming carbonic acid:

[0072] [Equation 1] Cü? +

[0073] Carbonic acid dissociates by releasing its hydrogen atoms through two successive steps:

[0074] [Equation 2] HCO% and

[0075] [Equation 3] HCO^ H * + coj"

[0076] Therefore, increasing the amount of carbon dioxide in catholyte 30 can induce increased formation of carbonic acid and promote its dissociation. Consequently, the concentration of hydrogen cation in catholyte 30 increases, leading to a decrease in the pH value, as measured by a pH sensor.

[0077] On the other hand, the decrease in the amount of carbon dioxide in the catholyte 30 can induce an increase in the pH value.

[0078] Thus, the pH of the catholyte is related to the concentration of carbon dioxide dissolved in the catholyte 30.

[0079] The carbon dioxide concentration can be adjusted to a range where the activity of methanogenic or homoacetogenic microorganisms is optimal for reducing carbon dioxide to methane or acetate, respectively.

[0080] Said range of concentration of dissolved carbon dioxide in catholyte 30 can therefore be determined as a function of the pH range of the catholyte.

[0081] Typically, the optimal pH range for activating methanogenic microorganisms is less than 7.5, preferably less than 7.0, in particular between 6.0 and 7.0.

[0082] Typically, the optimal pH range for activating homoacetogenic microorganisms is greater than 7.5, preferably greater than 8.0, in particular between 8.0 and 9.0.

[0083] Optionally, the carbon dioxide generated in the anodic compartment 20 is collected and introduced into the carbon dioxide source, in order to be reinjected into the cathodic compartment 18 to produce methane or acetate.

[0084] Thus, the production system according to the invention makes it possible to produce molecules of interest such as methane and acetate from organic compounds dissolved in wastewater, while limiting the supply of external carbon dioxide.

[0085] According to one embodiment, instead of having a single ion-exchange membrane 22, the production system according to the invention may comprise a compartment intermediate, which is connected on one side to the anodic compartment 20 and on the other to the cathodic compartment 18.

[0086] The intermediate compartment generally includes an anion exchange membrane that separates the intermediate compartment and the cathodic compartment, and a cation exchange membrane that separates the intermediate compartment and the anodic compartment.

[0087] The anion exchange membrane allows acetate ions to pass from the cathode compartment to the intermediate compartment.

[0088] The cation exchange membrane prevents acetate ions from passing from the intermediate compartment to the anodic compartment.

[0089] Thus, the intermediate compartment is configured to collect the acetate ions produced during electrolysis and prevent them from oxidizing at the anode 26.

[0090] An example of a bio-electrochemical production process for methane and / or acetate using a microbial electrolyzer, implemented in a system according to the invention shown in [Fig. 1], will now be described:

[0091] As mentioned above, the microorganisms are first introduced onto the surface of the electrodes.

[0092] For the anode 26, according to one embodiment, a biofilm containing the electrogenic microorganisms is prepared beforehand and introduced into the virgin anode 26, to obtain the anode 26 covered by the first microorganisms 28.

[0093] The anode 26 is immersed in the anolyte 14b. Optionally or alternatively, the anolyte 14b includes the electrogenic microorganisms, such that the biofilm including the first microorganisms 28 forms at the anode 26 in the presence of the anolyte 14b.

[0094] For the cathode 32, according to one embodiment, a biofilm containing the second microorganisms 34 is prepared by applying the digested sludge to the surface of the cathode 32.

[0095] During the formation of cathodic and anodic biofilms, the temperature is maintained between 20°C and 35°C, preferably between 25°C and 30°C.

[0096] The electrodes coated with microorganisms are immersed in the corresponding electrolytes.

[0097] The anolyte 14b introduced into the anodic compartment 20 has a chemical oxygen demand (COD) of between 200 and 30,000 mg O2 / L, preferably between 50 and 30,000 mg O2 / L

[0098] Preferably, acetate can be introduced into the anolyte 20, for a concentration of between 0.5 g / L and 1 g / L.

[0099] The catholyte 30 introduced into the cathodic compartment 18 has an initial pH value between 6.5 and 8, preferably between 7 and 8.

[0100] Preferably, the anodic compartment 20 is placed in an anaerobic environment, such that the concentration of dioxygen in the anodic compartment is maintained below 10,000 ppm, preferably between 0 ppm and 10,000 ppm, by a flow of inert gas. In particular, the inert gas is nitrogen.

[0101] Carbon dioxide is introduced from the carbon dioxide source into the cathode compartment 18. The pH of the catholyte 30 is determined and monitored by a pH sensor (not shown). The amount of carbon dioxide is controlled to maintain the pH of the catholyte 30 between 6 and 7, in order to optimize the activity of methanogenic archaea for producing methane in the cathode compartment 18.

[0102] An electrical potential difference between the cathode 32 and the anode 26 is established by the electrical circuit 24. The electrical potential difference is maintained between 0.6 V and 3 V. The potential at the cathode 32 is maintained between -0.5 and -1 V relative to the standard hydrogen electrode (SHE). The production system 10 thus prepared and adjusted, in particular with a catholyte 30 of pH between 6 and 7 and a potential at the cathode 32 between -0.5 and -1 V, produces methane at the cathode 32.

[0103] To change the production of methane to acetate, the amount of carbon dioxide is decreased so that the pH of the catholyte 30 increases up to 7.5. Then, the pH is maintained between 7.5 and 8.5 by the introduction of carbon dioxide, optionally, the potential at the cathode 32 is also adapted and maintained between -0.5 and -1 V relative to the standard hydrogen electrode (HSE), to activate the homoacetogenic bacteria and produce the acetate.

[0104] To change the production of acetate to methane, the amount of carbon dioxide is increased so that the pH of the catholyte 30 decreases to 7. Then, the pH is maintained between 6 and 7 by the introduction of carbon dioxide. Optionally, the potential at the cathode 32 is also adjusted and maintained between -0.5 and -1 V relative to the standard hydrogen electrode (HEE) to activate homoacetogenic bacteria and produce methane. The amount of carbon dioxide in the catholyte 30 is increased by an external supply of carbon dioxide introduced into the cathode compartment 18.

[0105] During the production of acetate by the production system 10, the ion exchange membrane 22 prevents the acetate ions produced at the cathode from moving towards the anode 26.

[0106] During the production of acetate and / or methane by the production system 10, the anolyte is supplied by the anolyte source 12.

[0107] Alternatively or optionally, the amount of carbon dioxide in the catholyte 30 is increased by an input of recycled carbon dioxide produced in the anodic compartment 20, this recycled carbon dioxide being collected and reinjected into the cathode compartment 18, for example via the carbon dioxide injection device 15.

[0108] Recycled carbon dioxide can for example be transported by a conduit which connects the anodic compartment 20 and the carbon dioxide injection device 15.

[0109] The process according to the invention makes it possible to reduce carbon dioxide to methane and / or acetate by microorganisms in an electrolyzer. Such a process is simpler and easier to operate compared to carbon dioxide reduction processes by anaerobic digestion with microorganisms alone. Furthermore, the process according to the invention makes it possible to select the reduction product between methane and acetate, and to switch from one to the other. This flexibility makes it easier to adapt the carbon dioxide reduction process to the actual requirements of industrial production.

Claims

Demands

1. A bio-electrochemical production process for methane and / or acetate using a microbial electrolyzer (17), said process comprising: - the oxidation of organic matter to carbon dioxide in an anodic compartment (20) of the microbial electrolyzer (17); - the reduction of carbon dioxide to methane and / or acetate as a function of the pH of a catholyte (30) received in a cathodic compartment (18) of the microbial electrolyzer (17); characterized in that the process comprises adjusting the pH of the catholyte (30) to a desired value by adding carbon dioxide to the catholyte (30).

2. A process according to claim 1 wherein the production of methane is replaced by the production of acetate by decreasing the amount of carbon dioxide introduced into the catholyte (30).

3. A method according to claim 1 or 2, wherein the pH of the catholyte (30) is adjusted to a value greater than 7.

5.

4. A process according to claim 1, wherein the production of acetate is replaced by the production of methane by increasing the amount of carbon dioxide introduced into the catholyte (30).

5. A method according to claim 1 or 4, wherein the pH of the catholyte (30) is adjusted to a value less than or equal to 7.

5.

6. A method according to any one of the preceding claims, wherein the carbon dioxide produced in the anodic compartment (20) is collected and introduced into the catholyte (30).

7. A method according to any one of the preceding claims, wherein the anodic compartment (20) is placed under an anaerobic environment.

8. A method according to any one of the preceding claims, wherein an electrical potential difference is applied between the cathode (32) and the anode (26) of the microbial electrolyzer (17) to produce an electric current, and the potential difference between the cathode (32) and the anode (26) is maintained between 0.6 V and 3 V.

9. A method according to any one of the preceding claims, wherein the cathode (32) comprises methanogenic archaea and homoacetogenic bacteria, and the anode (26) comprises electrogenic bacteria, such as Geobacter sulfurreducens.

10. A process according to any one of the preceding claims, wherein the anolyte (14b) has a chemical oxygen demand (COD) value between 200 and 30,000 mg O2 / L.

11. A process according to claim 10, wherein the anolyte (14b) is wastewater comprising organic matter, preferably acetate, and the catholyte (30) is wastewater or a phosphate buffer solution.

12. Bio-electrochemical production system (10) of methane and / or acetate comprising a microbial electrolyzer (17), the microbial electrolyzer (17) comprising: - an anodic compartment (20) for receiving an anolyte (14b) containing organic matter, and a cathodic compartment (18) for receiving a catholyte (30), the anodic compartment (20) and the cathodic compartment (18) being separated by at least one ion exchange membrane (22); - one or each anode (26) received in the anodic compartment (20) and covered with first microorganisms (28) for the oxidation of organic matter contained in the anolyte (14b) into carbon dioxide, - one or each cathode (32) received in the cathodic compartment (18) and covered with second microorganisms (34) for the reduction of carbon dioxide contained in the catholyte (30) into methane and / or acetate depending on the pH of the catholyte;- an electrical circuit (24) connecting the anode(s) (26) to the cathode(s) (32); characterized in that the bio-electrochemical production system (10) includes a device for injecting carbon dioxide (15) into the catholyte (30), to adjust the pH to the desired value according to the desired reduction product at the cathode (32).

13. Bio-electrochemical production system (10) according to claim 12, the cathode compartment (18) and the

14.

15.

16. anodic compartment (20) are separated by at least two ion exchange membranes delimiting between them an intermediate compartment. System according to claim 13, wherein the at least two ion exchange membranes include at least one cation exchange membrane. System according to claim 13 or 14, wherein the at least two ion exchange membranes include at least one cation exchange membrane and at least one anion exchange membrane. System according to any one of claims 12 to 15, wherein the anode (26) and the cathode (32) are made of identical or different materials, based on carbon and independently selected from graphite fabric, graphite fiber brushes, graphite felt, graphite granules and activated carbon granules, and combinations thereof.

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