Use of magnesium hydroxide in an anaerobic digester

Abstract

Use of magnesium hydroxide in an anaerobic digester The present invention relates to the use of magnesium hydroxide to improve the conversion of organic carbon from incoming organic matter into biomethane in an anaerobic digester.

Description

Title of the invention: Use of magnesium hydroxide in an anaerobic digester Technical field

[0001] The present invention relates to the field of anaerobic digestion (methanization) of fermentable (digestible) materials with a view to quantitative production of biogas and / or reduction of carbon pollution within an agricultural input, a municipal or industrial effluent and a waste source. It further relates to a formulation of additive with a magnesium component making it possible to improve the efficiency of the methanization reaction for the same incoming organic load in order to be able to treat more organic load - and therefore produce more biogas - in a methanizer of a given size.

[0002] The present invention is thus aimed at players in the “Methanization” technical sector whose potential users are part of the following sectors: agriculture, industry, household waste and municipal treatment plants.

[0003] More particularly, it is intended for operators of a dry or liquid methanization unit equipped with a continuous perfectly stirred reactor (CSTR), a plug flow reactor (PFR), a fluidized sludge bed (UASB) or a granular sludge bed (EGSB) for the purpose of producing biogas, even more particularly when this biogas is subsequently directly injected into the network or recovered in the form of electricity and heat by cogeneration and contributes to creating a source of income for the unit in question. Prior art

[0004] To meet the environmental and energy challenges of the current situation, anaerobic digestion is an interesting and promising waste treatment technique since it allows the combination of “carbon pollution treatment” and “energy production”.

[0005] Nevertheless, the sector also faces technical challenges that can destabilize the economic relevance of a project: - Methanizers operate empirically, with digestion conditions that are complicated to find because there is a lot of variability: inputs (mainly), seasonality, conditions from one farm to another. - Methanizers must operate in degraded conditions to meet feeding plans on the one hand and biogas production targets on the other, inducing potential stress on microorganisms and therefore an inevitable loss of potential production. - a methanizer does not convert all of the available C into CH4. There is therefore a loss of productivity and a lower environmental and financial benefit.

[0006] To date, the major concern facing the sector is maximizing the yield of a unit in relation to the total incoming load. That is to say, the effective expression rate of the methanogenic potential of the incoming load (BMP) is not optimal or the conversion of incoming organic carbon (TOCg) into biomethane (CH4) is not optimized at the process scale.

[0007] Maximizing biogas production for a fixed quantity of input organic matter involves increasing the effective expression rate of the methanogenic potential of the inputs considered.

[0008] The state of the art reveals several strategies already in place. Among them, we find co-digestion which consists of establishing strategies for mixing inputs to take advantage of the particular characteristics of each. Livestock effluents (slurry and manure), abundant in agricultural methanization, have a lower methanogenic power but allow better maintenance of the pH in the digestion medium. Plant inputs, for their part, promote the good development of microbial biomass. Among them are intermediate crops for energy purposes (CIVE), vegetable residues, green waste, etc. Finally, other types of inputs such as residues from the agri-food industry can be used for their particularly high methanogenic power in order to rebalance loads so that biogas production objectives are achieved.

[0009] Co-digestion is a particularly important and strategic technique for units that have to process a wide variety of inputs. This is particularly the case, for example, for units installed in energy centers collecting substrates from the agri-food industry.

[0010] However, it is not always possible to have a wide variety of inputs, which can restrict the use of co-digestion, nor to have access to the precise characterization of the inputs (%MS, %MO, BMP, composition) available, which can lead to an increased risk of imbalance if there is an antagonism between constituents.

[0011] Another approach to maximize biogas production is to choose and maintain very strict operating conditions in the methanizer, such as temperature, pH, volatile organic acid to buffer capacity ratio (FOS / TAC), residence time, % incoming feed, % dry matter (DM), % organic matter (OM). However, this imposes constraints that are difficult to meet due to the empirical operation of methanizers.

[0012] It is also possible to maximize this biogas production by using physical techniques such as grinding or recirculation. However, this requires modifying the methanizers and / or using more energy, which is expensive. and the result is not always up to expectations.

[0013] Finally, it is possible to use chemical additivation techniques, mainly in remediation, that is to say as a corrective action in the event of a problem. It is thus possible to add to the methanizer, for example, iron oxides and hydroxides to correct an accumulation of sulfur in the medium, and nutrients such as selenium and other micronutrients to correct the nutrient medium of the bacteria.

[0014] Thus, it is known from application WO2009055793 to use alkaline agents in order to correct the pH of the methanizer. This application thus mentions Mg(OH)2 among the list of said alkaline agents. However, magnesium hydroxide is therefore only used as a pH corrector in methanizers encountering a pH problem.

[0015] However, the inventors surprisingly discovered that magnesium hydroxide, in particular standard magnesium hydroxide, more particularly in powder form, made it possible to improve biogas production by improving the conversion of organic carbon from an input organic matter into biomethane. In particular, standard magnesium hydroxide, i.e. not having a very high specific surface area, in particular a specific surface area of ​​between 1 and 100 m2 / g measured by the multipoint BET method by N2 adsorption, in powder form was just as effective as magnesium hydroxide in the form of an aqueous suspension. However, this is quite surprising because magnesium hydroxide in powder form mixes more difficultly with the inputs than magnesium hydroxide in the form of an aqueous suspension.

[0016] Magnesium hydroxide in powder form, however, has the advantage of being easier to handle, requiring less storage capacity, and not requiring any special or dedicated equipment, particularly if it is packaged in digestible packaging that can also be added to the methanizer. The dosage of magnesium hydroxide is also made easier since it can thus be simply indexed to the incoming organic matter, and the product simply added to it. Finally, the magnesium hydroxide according to the invention does not require a high specific surface area, which makes it possible to manufacture it with a simpler and less expensive process (without the need to calcine the magnesium carbonate into MgO, a step that emits a lot of CO2) than a magnesium hydroxide with a high specific surface area or even to use a natural product.

[0017] Furthermore, the magnesium hydroxide according to the invention makes it possible to accelerate the kinetics of biogas production in the anaerobic digester for a given incoming organic matter. Statement of the invention

[0018] The present invention therefore relates to the use of magnesium hydroxide, advantageously in powder form, to improve the conversion of organic carbon from an incoming organic matter into biomethane in an anaerobic digester.

[0019] For the purposes of the present invention, the term "organic carbon" or "TOCg" means any carbon derived from living organisms. Organic carbon is therefore linked to other carbons or to elements such as hydrogen (H), oxygen (O), nitrogen (N) or phosphorus (P) in organic molecules. It can be measured according to the NF EN ISO 10694 standard dated 1995 by dry combustion. The total carbon content can also be measured according to the NF EN ISO 10694 standard dated 1995 by dry combustion.

[0020] For the purposes of the present invention, the term "incoming organic matter" means the organic matter (OM) or carbonaceous matter constituting the inputs of the anaerobic digester which will be likely to degrade into biogas and produce biomethane. The proportion of organic matter is generally measured by loss on ignition at 550°C of the dry matter (i.e. of the input dried at 105°C) according to standard NF EN 15935 dated 2021. Indeed, the input contains water, organic matter and mineral matter.

[0021] For the purposes of the present invention, the term “inputs” means any organic biomass intended to be used in an anaerobic digester. In particular, it is defined in the decree of November 23, 2011 (amended by the decree of June 24, 2014) setting the nature of the inputs in the production of biomethane for injection into natural gas networks. Advantageously, the input is chosen from industrial waste (such as wash water from industrial processes, industrial sludge), in particular from the agri-food industry (such as lignocellulosic residues, grease, sludge, biowaste, fruits and vegetables, slaughterhouse waste, etc.), agricultural waste (such as livestock effluents and plant waste, in particular manure, slurry, animal by-products, crop residues), energy crops (such as whole plants, silage, hay, etc.)), urban waste (such as sorted household biowaste, or from collective catering, large and medium-sized stores, materials resulting from the treatment of wastewater (such as sludge, grease, organic liquids)) and mixtures thereof, in particular among livestock effluents (such as slurry, droppings, manure, etc.), plant inputs, residues from the agri-food industry, sewage treatment plant sludge and mixtures thereof, more advantageously among slurry, in particular from dairy cows, silage, in particular from corn and / or CIVE, co-products of vegetable processing, co-products of cameo processing, sewage treatment plant sludge and mixtures thereof.

[0022] Thus advantageously the incoming organic matter is chosen from the organic matter of industrial waste, agricultural waste, energy crops and their mixtures, in particular from the organic matter of livestock effluents, plant inputs, residues from the agri-food industry, sludge from sewage treatment plants and their mixtures, more advantageously from the organic matter of slurry, silage, co-products of vegetable processing, co-products of cameo processing, sludge from the sewage treatment plant and their mixtures.

[0023] In the context of the present invention the terms "between ... and ..." and "in the range from ... to ..." mean that the limits of the range are included.

[0024] For the purposes of the present invention, the expression "improving the conversion of organic carbon from an incoming organic matter into biomethane" means increasing the methanogenic potential of an incoming organic matter in an anaerobic digester, in particular obtaining a gain in methanogenic potential of at least 5% for the incoming organic matter compared to the same incoming organic matter without the addition of magnesium hydroxide according to the invention. The methanogenic potential is measured in standard cubic meters (Nm3) of methane under standard conditions of temperature (15°C) and pressure (latm), relative to the quantity of organic matter (MO in tonnes) initially present in the sample used for the test (NmVtMO) and is evaluated by a methanogenic potential test (BMP - Biochemical Methane Potential) over a period of 20 days. The methanogenic potential represents the quantity of biogas and methane (CH4) that can be produced by an input in an anaerobic digester.The methanogenic potential test is used to determine the maximum biogas production of a biomass sample. During this test, the biogas production rate (fermentation kinetics) and the methane (CH4), carbon dioxide (CO2), H2S and O2 composition of the biogas produced are measured. This test can be carried out on different biomass samples: livestock effluents (slurry, droppings, manure, etc.), agri-food waste (lignocellulosic residues, grease, sludge, etc.), municipal waste (biowaste, WWTP sludge, etc.), energy crops (whole plant, silage, hay, etc.) as well as any other organic residue or by-product. This test is specifically described in Example 1. For this test, a fixed quantity and composition of substrate (ration) is placed in a reactor in contact with active anaerobic microorganisms (inoculum).In particular the inoculum contains a consortium of bacteria making possible the digestion of organic carbon entering under anaerobic conditions such as: . - Hydrolytic and fermentative bacteria, the main species of which belong to the genera Clostridium, Bacillus, Ruminococcus, Enterobacteroides, Propio-nibacterium and Butivibrio. - Acetogenic bacteria, the main species of which are homoacetogens of genera Clostridium, Acetobacterium, Sporomusa, Acetogenium, Acetoanaerobicum, Pelobacter Butyribacterium, Eubacterium..., syntrophs of the genera Syntro-phobacter, Syntrophomonas, Syntrophus... and sulfate-reducers of the genera Desul-fovibrio, Desulfobacter, Desulfotomaculum, Desulfomonas... - Methanogenic bacteria belonging to the group of archaea whose main species are Methanobacterium, Methanobrevibacter, Methanosphaera, Me-thanolobus, Methanococcus, Methanosarcina, Methanocorpusculum, Methanoculleus, Methanogenium, Methanoplanus, Methanospirillum, Methanococcoides, Methanoha-lophilus, Methanohalobium. The medium is maintained in optimal fermentation conditions until the cumulative quantity of biogas generated no longer varies. The methanogenic potential is the quantity of methane produced.

[0025] The magnesium hydroxide according to the invention may be in the form of an aqueous suspension or in the form of a powder, advantageously, it is in the form of a powder. Advantageously, the density of the powder is in the range of 0.2 to 0.7 g / cm3, measured according to the ASTM D7481-09 standard.

[0026] In a particular embodiment, the magnesium hydroxide according to the invention is in the form of an aqueous suspension, advantageously obtained by precipitation of magnesium hydroxide in water.

[0027] Advantageously, the solid content of the aqueous suspension of magnesium hydroxide according to the invention is at least 40% by mass, advantageously at least 50% by mass, more advantageously at least 53% by mass, relative to the total mass of the suspension.

[0028] In another embodiment, the aqueous suspension of magnesium hydroxide according to the invention comprises water as the sole solvent. It advantageously further comprises at least one dispersant, advantageously selected from the group consisting of homopolymers or copolymers of acrylic acid or methacrylic acid and their salts and a mixture thereof and lignosulfonates and their mixtures, in particular chosen from polycarboxylates, polyether polycarboxylates, polyacrylates, polyacrylate copolymers, acrylic and / or methacrylic copolymers, salts (in particular polyacrylate salts) and a mixture thereof, and / or at least one anti-foaming agent, advantageously selected from the group consisting of silicone oils (such as polydimethylsiloxanes in particular marketed by the company Kurita or by the company ArrMaz) or organic oils, polyglycols,fatty acid esters or polyesters and mixtures thereof, said oils or polyglycols being able to contain organic or mineral particles in suspension, and / or a non-associative thickener or gelling agent (for example cellulose derivatives, xanthan gum, alginates, polyvinyl alcohols (PVA), polyethylene glycol (PEG) or polyoxyethylenes (POE), polyvi-, nylpyrrolidones (PVP) or certain members of the acrylic (co)polymer (ASE) family), alkali salts (e.g. magnesium acetate, potassium nitrate) and / or acid conditioners (e.g. acetic acid, citric acid) or a mixture thereof. In particular, the aqueous suspension comprises at least one dispersant.

[0029] In one embodiment, the viscosity of the aqueous suspension of magnesium hydroxide according to the invention, measured at 1.7 s 1 at 20°C with a Brookfield viscometer, is less than 1000 mPa.s, preferably less than 700 mPa.s, in particular less than 300 mPa.s.

[0030] In particular, it may be the aqueous suspension described in the applicant's application PCT / FR2022 / 050886 or marketed by the applicant under the name MH53S.

[0031] In an advantageous embodiment, the magnesium hydroxide in the form of an aqueous suspension according to the invention has a purity (in dry matter) of at least 80%, advantageously at least 90%.

[0032] In another particularly advantageous embodiment, the magnesium hydroxide according to the invention is in powder form.

[0033] In a particular embodiment of the invention, the humidity level of the magnesium hydroxide powder, measured by the gravimetric method by loss of mass at 105°C, according to the invention is less than 5%, advantageously less than 1%.

[0034] In one embodiment, the magnesium hydroxide particles according to the invention have a volume average size, specifically a volume average diameter, D90 of less than 200 pm, preferably less than 100 pm, in particular less than 45 pm, more preferably less than 25 pm, even more preferably less than 15 pm, measured by a laser granulometer, specifically the Mastersizer 2000 from Malvern Instruments, more specifically obtained after suspending the powder and diluting the suspension to 20,000th and passing it through the granulometer.

[0035] In another embodiment, the magnesium hydroxide particles according to the invention have a volume average size, specifically a volume average diameter, D50 of less than 40 pm, preferably less than 20 pm, in particular less than 7 pm, measured by a laser granulometer, specifically the Mastersizer 2000 from Malvern Instruments, more specifically obtained after suspending the powder and diluting the suspension to 20,000th and passing it through the granulometer.

[0036] Advantageously, the specific surface area of ​​the magnesium hydroxide particles according to the invention measured by the multipoint BET method by N2 adsorption, in particular with Micromeritics Gemini VII equipment, is between 1 and 100 m2 / g, more advantageously between 1 and 50 m2 / g, even more advantageously between 5 and 20 m2 / g, in particular between 15 m2 / g and 10 m2 / g, more particularly 10 m2 / g.

[0037] In an advantageous embodiment, the dry matter content of magnesium hydroxide used, in mg / g of incoming organic matter, is between 3 and 30, advantageously between 3.5 and 15, even more advantageously between 3.5 and 10.

[0038] In an advantageous embodiment, the magnesium hydroxide in powder form according to the invention has a purity of at least 80%, advantageously at least 90%.

[0039] Magnesium hydroxide in powder form may be of natural origin (such as brucite ore) or synthetic and may be commercially available from the applicant.

[0040] The magnesium hydroxide in powder form according to the invention can be packaged in digestible packaging.

[0041] In an advantageous embodiment, the magnesium hydroxide, in particular in powder form, according to the invention also makes it possible to accelerate the kinetics of biogas production in the anaerobic digester for a given incoming organic material.

[0042] For the purposes of the present invention, the expression "accelerate the kinetics of biogas production in the anaerobic digester for a given incoming organic matter" means increasing the kinetics of biomethane production for a given incoming organic matter, in particular under the conditions of Example 3, i.e. increasing the speed at which a given incoming organic matter reaches its methanogenic potential and therefore reducing the time required to reach this methanogenic potential.

[0043] In a particular embodiment, the anaerobic digester or methanizer is part of a dry or liquid methanization unit equipped with a continuous perfectly stirred reactor (CSTR), a plug flow reactor (PFR), a fluidized sludge bed (UASB) or a granular sludge bed (EGSB). In particular, the anaerobic digester is intended for the quantitative production of biogas, more particularly said biogas being intended to be injected into the network or recovered in the form of electricity and heat by cogeneration, and / or for the reduction of carbon pollution within an agricultural input, a municipal effluent, for example from a municipal or industrial wastewater treatment plant or a waste source, for example household.

[0044] The present invention will be better understood on reading the description of the figures and examples which follow, which are given for information purposes only. Brief description of the drawings

[0045] [Fig.l] [Fig.l] represents the quantity of methane produced (cumulative volumes in NmVtMO) as a function of time (in days) under the conditions of example 3 for a given ration (input) without the addition of magnesium hydroxide according to the invention (control) or with the addition of 125 mg of powdered magnesium hydroxide according to the invention (3.8 mg / g of MO) (MHP 125 mg) or 180 mg of liquid magnesium hydroxide according to the invention (5.5 mg / g of MO) EXAMPLES

[0046] Example 1: Effect of Mg(OH)2 on the methanogenic potential of a ration derived from agricultural inputs

[0047] The effect of magnesium hydroxide addition on biogas production is assessed by a methanogenic potential (BMP) test. This test determines the maximum methane production that can be expected from the methanization of a substrate. This test can be performed on any type of biomass, waste or digestate sample.

[0048] For this test, a fixed quantity and composition of substrate (ration) is placed in a reactor in contact with active anaerobic microorganisms (inoculum). The medium is maintained under optimal fermentation conditions until the cumulative quantity of biogas generated no longer varies. The methanogenic potential is the quantity of methane produced.

[0049] The equipment used is a laboratory pilot consisting of 1000cm3 tanks, equipped with a gas outlet and a stirring blade. All tanks are independent and positioned in an oven at 42°C for the entire duration of the test. A neutral atmosphere (N2) is injected beforehand before connection to the reactor to ensure the anaerobic nature of the reaction.

[0050] The ration used is representative of a methanizer on farm and is composed of the following raw materials: - Dairy cow slurry from a farm; - A 50 / 50 mass mixture of corn silage (freeze-dried and crushed) and CIVE silage (fresh)

[0051] The raw materials used are characterized according to: - The dry matter content (%MS) measured by the difference in mass between the raw material and the raw material after drying in an oven at 105°C; - The organic matter rate (%MO) measured by the difference in mass between the raw material and the raw material after drying at 550°C; - the MO ratio measured by the ratio between the %MO and the %MS - The Carbon rate (%C) measured on raw substrate according to standard NF ISO 10694 of 1995; - The nitrogen rate (%N) measured on raw substrate according to standard NF ISO 13654-1 of 2002 and - the C / N ratio measured by the ratio between %C and %N.

[0052] They have the characteristics indicated in the following table 1:

[0053] [Tableauxl] Slurry Silage %MS % 2.50 93.00 ratio MO % 99.10 96.00 %MO % 2.48 89.28 %C total % 0.84 42.69 %N Total % 0.08 0.96 %C / N % 10.5 44.5

[0054] The inoculum is obtained by direct collection of the methanization digestate from a methanizer located on the farm from which the slurry used in the ration comes. It has the characteristics indicated in the following table 2:

[0055] [Tables2] Digestate MS % 5.00 MO % 4.92

[0056] The final formulation is calculated in such a way as to provide optimal conditions for the generation of biogas.

[0057] In certain tanks, the medium is added with a magnesium hydroxide according to the invention, liquid (MHL) or solid (MHP).

[0058] The MHL additive (according to the invention) used is an aqueous suspension of magnesium hydroxide in water containing 53% by mass relative to the total mass of the suspension of magnesium hydroxide particles whose Mg(OH)2 purity is 98%, whose particle size (measured by a Mastersizer 2000 laser granulometer from Malvern Instruments) is less than 100 pm, and whose Specific Surface Area (SSA measured by multipoint BET method by adsorption of N2 on Quantachrome Flowin) is 15 m2 / g.

[0059] The MHP additive (according to the invention) used is a magnesium hydroxide powder whose Mg(OH)2 purity is 94%, whose particle size (measured by a Mastersizer 2000 laser granulometer from Malvern Instruments) is less than 100 pm, and whose Specific Surface Area (SSA measured by the multipoint BET method by adsorption of N2 on Quantachrome Flowin) is 10 m2 / g.

[0060] They have the characteristics indicated in the following table 3:

[0061] [Tables3] Solids content Water Purity Mg(OH) 2 (solid base) Particles passing 100pm SSA (m2 / g) MHL (liquid) 53% 47% 98% >99% 15 MHP (powder) 100% <1% 94% >99% 10

[0062] Several tanks are used in parallel and organized as follows: - One tank is left empty, in order to constitute a blank. - Three tanks, hereinafter referred to as control tanks, do not contain any additives and are loaded as follows: 500 g of digestate, 140 g of slurry and 5 g of silage (50 / 50 corn / CHIVE). - Three tanks are supplemented with liquid magnesium hydroxide (MHL) (according to the invention) and three tanks are supplemented with solid magnesium hydroxide (MHP) according to the invention and are loaded as follows: 500 g of digestate, 140 g of slurry, 5 g of silage (50 / 50 corn / chive) and from 110 to 540 mg (dry basis) (measured on a dry basis) of additive (MHL or MHP).

[0063] The test is conducted over a period of 20 days. This duration is sufficient for the cumulative quantity of biogas to no longer vary.

[0064] The biogas produced is collected in bags, allowing the quantity generated to be measured. At the end of the test, the bags are analyzed by gas chromatography (GC) carried out with an Agilent Micro GC system to determine the proportion of the different gases present (CO2, CH4, H2S and O2). The Methanogenic Potential can thus be determined. It is expressed in standard cubic meters (Nm3) of methane under standard conditions of temperature (15°C) and pressure (latm), relative to the quantity of organic matter initially present in the sample used for the test (NmVtMO). The values ​​are averaged over the three identical tanks.

[0065] The gain in Methanogenic Potential provided by the use of additives is then expressed as the ratio between the values ​​of the control tanks and the tanks with additives.

[0066] The results for the MHL additive (according to the invention) are gathered in the following table 4

[0067] [Tables4] Comparative examples Quantity of additive MHL mg (dry basis) content in mg / g OM Methanogenic potential (Nm3 / t OM) Control Methanogenic potential (Nm3 / t OM) Additive Gain from additivatio n Ex MHL A 540 16.5 489 537 +9.8% Ex MHL B 110 3.8 567 582 +2.6%

[0068] The results for the MHP additive (according to the invention) are gathered in the following table 5

[0069] [Tables5] Examples Quantity of MHP additive mg (dry basis) content in mg / g OM Methanogenic Potential (Nm3 / t OM) Control Methanogenic Potential (Nm3 / t OM) Additive Gain from additivatio n Ex MHP A 380 11.5 506 547 +8.1% Ex MHP B 125 3.8 854 905 +6.0%

[0070] It therefore appears that the addition of magnesium hydroxide, in particular in solid form, to an agricultural input containing 4.92% organic matter makes it possible to increase its Methanogenic Potential. Furthermore, the additivation with a magnesium hydroxide powder proves to be as effective as the additivation with a magnesium hydroxide suspension.

[0071] Furthermore, measurements of the pH of the medium are carried out at the end of the test. They are collected in the following table 6:

[0072] [Tableauxô] Example PH Control PH Additive Ex MHL A 7.90 7.90 Ex MHP A 7.96 7.96 Ex MHL B 8.10 8.10 Ex MHP B 8.12 8.08

[0073] It therefore appears that the additivation of magnesium hydroxide does not influence or does not influence significantly the pH of the medium, and that the methane gain effect is not therefore not linked to a remediation mechanism of a pH that is too low.

[0074] Example 2: Effect of Mg(OH)2 on the methanogenic potential of a mixed ration from industrial and agricultural waste.

[0075] In this example, we seek to evaluate the repeatability of the effect observed in example 1 on a different formulation.

[0076] The material used is the same as in example 1.

[0077] The ration used this time comes from an industrial methanization unit which processes 60,000 tonnes of organic matter all from the different activities of the territory: co-product of vegetable processing, co-product of cameo processing, sludge from sewage treatment plant etc. The so-called "animal" inputs underwent a thermal hygienization treatment in order to get rid of possible pathogens. The animal and plant inputs were then finely ground and mixed. They have the characteristics indicated in the following table 7:

[0078] [Tables7] Ration %MS % 13.9 ratio MO % 98 %MO % 13.7 %C total % 8.64 %N Total % 0.69 %C / N % 12.5

[0079] The inoculum is obtained by direct collection of digestate from this same methanization unit. It has the characteristics indicated in the following table 8:

[0080] [Tables8] Digestate MS % 6.0 MO % 5.7

[0081] In some tanks, the medium is added with a liquid (MHL) or solid (MHP) magnesium hydroxide. The MHP and MHL additives are identical to those described in Example 1.

[0082] Several tanks are used in parallel and organized as follows: - One tank is left empty, in order to constitute a blank. - Three tanks, hereinafter referred to as control tanks, do not contain any additives and are loaded as follows: 600.0 g of digestate and 83.4 g of ration. - Three tanks are supplemented with solid magnesium hydroxide (MHP) and are loaded as follows: 600.0 g of digestate, 83.4 g of ration and 125 mg (dry basis) of MHP additive. - Three tanks are supplemented with liquid magnesium hydroxide (LMH) and are loaded as follows: 600.0 g of digestate, 83.4 g of ration and 180 mg (dry basis) of MHL additive.

[0083] The test is conducted over a period of 19 days. This duration is not sufficient for the cumulative quantity of biogas to no longer vary, and is therefore not a measure of the complete Methanogenic Potential but of the production of CH4 at 19 days.

[0084] The results are collected in the following table 9

[0085] [Tables9] Examples Additive (mg - dry basis) content in mg / g MO CH4 production at 19 days (Nm3 / t MO) Control CH4 production at 19 days (Nm3 / t MO) Additive Gain from additivation Ex MHL C 180 mg MHL 5.5 164 459 +180% Ex MHP C 125 mg MHP 3.8 164 457 +179%

[0086] This example confirms that the addition of 3.5 to 10 mg of magnesium hydroxide / mg of incoming organic matter to an input composed of industrial and agricultural waste with 5.70% organic matter makes it possible to increase the production of biomethane. Furthermore, the additivation with a magnesium hydroxide powder proves to be as effective as the additivation with a magnesium hydroxide suspension.

[0087] Example 3: Effect of Mg(OH)2 on the kinetics of biomethane production

[0088] This test seeks to evaluate the effect of the addition of magnesium hydroxide on the kinetics of biomethane production.

[0089] The material used is the same as in examples 1 and 2.

[0090] The ration used is the same as in example 2.

[0091] The additive used (MH) is the same as that of example 1.

[0092] Several tanks are used in parallel and organized as follows: - A tank is left empty, in order to create a blank. - Three tanks, hereinafter referred to as control tanks, do not contain any additives and are loaded as follows: 600.0 g of digestate and 83.4 g of ration. - Three tanks are supplemented with magnesium hydroxide in powder form (MHP) according to the invention and are loaded as follows: 600.0 g of digestate, 83.4 g of ration and 125 mg (dry basis) of MHP additive. - Three tanks are supplemented with magnesium hydroxide in liquid form (MHL) according to the invention and are loaded as follows: 600.0 g of digestate, 83.4 g of ration and 180 mg (dry basis) of MHL additive.

[0093] The amount of methane produced as a function of time is measured. The results are shown in [Fig.l].

[0094] These results show that in addition to the effect on the methanogenic potential of the ration, the magnesium hydroxide in powder form according to the invention also has an accelerating effect on its decomposition kinetics. The gain in biogas for the additive methods compared to the control is in fact maximum after only a few days, before gradually reducing. The additive tanks reach their methanogenic potential more quickly.

Claims

Claims

1. Use of magnesium hydroxide for improving the conversion of organic carbon from an incoming organic matter into biomethane in an anaerobic digester, characterized in that the specific surface area of ​​the magnesium hydroxide particles measured by the multipoint BET method by N2 adsorption is between 1 and 100 m2 / g and that the magnesium hydroxide is in powder form.

2. Use according to claim 1, characterized in that the dry matter content of magnesium hydroxide, in mg / g of incoming organic matter, is between 3 and 30.

3. Use according to any one of claims 1 or 2, characterized in that the specific surface area of ​​the magnesium hydroxide particles measured by the multipoint BET method by N2 adsorption is between 1 and 50 m2 / g, more advantageously between 5 and 20 m2 / g.

4. Use according to any one of claims 1 to 3, to further accelerate the kinetics of biogas production in the anaerobic digester for a given incoming organic material.

5. Use according to any one of claims 1 to 4, characterized in that the incoming organic matter is chosen from organic matter from industrial waste, agricultural waste, energy crops, urban waste and mixtures thereof, in particular from organic matter from livestock effluents, plant inputs, residues from the agri-food industry, sewage treatment plant sludge and mixtures thereof, advantageously from organic matter from silage slurry, co-products from vegetable processing, co-products from cameo processing, sewage treatment plant sludge and mixtures thereof.

6. Use according to any one of claims 1 to 5, characterized in that the anaerobic digester is part of a dry or liquid methanization unit equipped with a continuous perfectly stirred reactor, a piston reactor, a fluidized sludge bed or a granular sludge bed.

7. Use according to any one of claims 1 to 6, characterized in that the anaerobic digester is intended for the quantitative production of biogas and / or the reduction of carbon pollution within an agricultural input, a municipal or industrial effluent or a source of waste.

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