New agent to accelerate the degradation of organic waste

Abstract

The present invention relates to the technical field of the fermentation of waste of organic origin, in particular agricultural and agro-industrial waste, and more particularly that comprising lignocellulosic residues. It has been discovered, surprisingly, that the regular addition of at least one superabsorbent polymer in digesters during anaerobic fermentations makes it possible to effectively and very simply increase the degradation of the lignocellulosic residues present. The invention thus allows operators not only to overcome a major technical problem but also to achieve substantial savings by reducing residence times and thanks to the gain generated in biogas production. The invention also relates to the process using this new agent.

Description

Title of the invention: New agent for accelerating the degradation of waste of organic origin

[0001] The present invention relates to the technical field of the fermentation of waste of organic origin, in particular agricultural and agro-industrial waste, and more particularly those comprising lignocellulosic residues. The invention also relates to a new agent for accelerating the degradation of this type of organic waste as well as a method implementing it. Technical field of the invention

[0002] Methane is a gas that can be produced by the fermentation of biodegradable material, such as organic waste, particularly agricultural and agro-industrial waste. This biological process is called methanization. It consists of transforming, in the absence of oxygen, organic matter into:

[0003] - a renewable energy, called biogas, which includes methane among other things (CH4), generally 50% to 70%, and carbon dioxide (CO2),

[0004] - as well as a digestate that can be used as fertilizer.

[0005] The biogas thus produced can be transformed into heat, electricity and / or fuel. Technical background

[0006] In anaerobiosis, organic matter decomposes due to the presence of numerous species of bacteria. This reaction occurs in a sealed tank, called a digester or methanizer, in which organic waste is stored to be subjected to the action of microorganisms (bacteria) in the absence of oxygen.

[0007] The main stages that occur during fermentation are:

[0008] • hydrolysis and acidogenesis: acidogenic microorganisms transform the complex organic chains into simpler compounds: peptides, amino acids, fatty acids, sugars;

[0009] • acetogenesis: the products of acidogenesis are converted into acetic acid;

[0010] • methanogenesis: methanogenic microorganisms are responsible for the gas production (gasification): the acetic acid obtained during acetogenesis is transformed into methane and carbon dioxide.

[0011] Methane fermentation therefore allows the elimination of a significant quantity of organic matter. Methanization produces on average 3 times less CO2 than conventional aerobic fermentation, it is therefore a very efficient source of renewable energy. The biogas produced by methanization can replace natural gas, for example to produce heat, electricity and / or fuel. for vehicles. The quantity of biogas generated is representative of the quality of fermentation. When this is well controlled, 1 kg of fermented sugar leads to the production of 600 liters of biogas composed mainly of methane CH4 (generally > 60 v / v) and carbon dioxide CO2. Other elements may also be present in very small proportions. The calorific value (LHV) of the biogas depends on the proportion of methane; for example, for a biogas containing 65% methane, the LHV will be 6.46 kWh / m3.

[0012] Furthermore, once methanized, the residual material (digestate) is easily recyclable, particularly in the form of fertilizer because it is mainly made up of ammonia, a product of the transformation of the nitrogen contained therein before fermentation.

[0013] According to some theories, the bacteria involved in methanization could have been the first living organisms to appear on Earth, 3 billion years ago, when there was still no oxygen in the atmosphere. As today, they degraded the organic molecules present (CO2 and hydrogen) into methane and oxygen. The bacteria producing biogas could therefore be at the origin of the appearance of oxygen on Earth and, by extension, of life.

[0014] The British H. Davy demonstrated the presence of methane in the gases produced during the decomposition of slurry as early as 1808. Nearly 100 years later, in 1897, the first digester was built in India with the aim of producing vehicle fuel.

[0015] The current sector is mainly based on the use of methanization processes which include the introduction of solid organic matter, typically liquid agricultural effluents (slurry in particular) to which other waste (called co-substrates or inputs) are added and which can be up to 40% dry matter. The former provide the water and microorganisms ensuring the methanization reactions, the latter the material with a higher biogas yield. The methanization process consists of conveying the organic matter to be treated, most often by means of pumping systems, a hopper or an endless screw, inside a digester. The organic matter is then continuously mixed by one or more agitators in order to avoid the phenomena of settling, flotation or crusting of the biomass.

[0016] The influence of temperature is decisive for the proper functioning of fermentation. In fact, digesters are generally heated. The most frequently used fermentation, called mesophilic, takes place at around 35 °C. There is also thermophilic fermentation (50-60 °C) which allows the size of methanizers to be reduced as well as better elimination of pathogenic germs. A two-stage solution is also sometimes used: a first thermophilic reactor with a short stay followed by a second mesophilic reactor.

[0017] The organic matter remains in the digester for a period of several weeks. The solid organic matter supplied is often ground before being incorporated into the digestion tank in order to facilitate its transport and mixing. These processes require a significant expenditure of energy to be continuously mixed inside the digester.

[0018] From an operational point of view, the supply of organic matter is done regularly (several times a day) by feeding stages in order to preserve the optimal physicochemical conditions for methanogenic activity (temperature, pH). This progressive addition of solid co-substrate in the digester results in a phenomenon of accumulation of organic matter which is closely linked to its degradation rate. To control this phenomenon, it is often necessary to pre-treat the solid organic matter before its introduction, for example by grinding the solid part and removing (sorting) as much of the unwanted matter as possible.

[0019] Furthermore, in order to allow mechanical agitation, in certain processes, the solid content of the reaction medium is fixed and must therefore not exceed 10 to 15%. A withdrawal of the digestate is therefore carried out regularly in order to maintain the solid content below this threshold. A reintroduction of the liquid digestate into the digester after phase separation can also be carried out. Presentation of the invention

[0020] Despite its many advantages, the methanization process still needs to improve its efficiency and robustness. Indeed, certain wastes are not destroyed or are poorly destroyed by methanization and / or can cause malfunctions in the process.

[0021] This is the case, in particular, when the effluents have a high ammonia content. Indeed, the microorganisms present in anaerobic digesters are very sensitive to the digestion of nitrogen-rich effluents, such as livestock waste, and the presence of free ammonia in high quantities can lead to inhibition or toxicity which will eventually lead to process failure.

[0022] Similarly, lignocellulosic residues (based on cellulosic and / or hemi-cellulosic fibers and lignin) are among the slowest to be degraded during anaerobic digestion. Thus, the anaerobic biodegradation of lignocellulosic residues, such as straw, generally requires residence times of 40 days or more in the digester, which has the consequence of significantly reducing its energy yield. This is the case, in particular, for cereal straws, which greatly hinders their recovery by methanization.

[0023] To date, the main solutions proposed to resolve this problem are based on

[0024] - sorting organic materials before they are introduced into the digester,

[0025] - physical and / or chemical processes leading to the destructuring of the ligno- matrix cellulosic,

[0026] - and biological pretreatments by enzymes or special microorganisms specific.

[0027] All are relatively expensive in time, price and / or energy.

[0028] There is therefore still an unmet need to help with the degradation of this lignocellulosic biomass directly in anaerobic digesters and without generating additional costs for the methanizer operator. Summary of the invention

[0029] The inventors have discovered, surprisingly, that the regular addition of at least one superabsorbent polymer, added dry or hydrated, at a very low dose, separately or in a mixture during the supply(s) of solid organic matter makes it possible to effectively and very simply increase the degradation of the lignocellulosic residues present in the digester, thus reducing their residence time. The invention thus allows operators not only to overcome a major technical problem but also to make substantial savings thanks to the gain generated in biogas production. Detailed description of the invention

[0030] A first aim of the invention is to propose a new agent, in this case a superabsorbent polymer, making it possible to accelerate the degradation of lignocellulosic residues.

[0031] Another aim of the invention is also to propose a method for treating these lignocellulosic residues using this agent to facilitate or accelerate their digestion in methanizers. The invention also relates to a composition for degrading said lignocellulosic residues as well as its use or the use of the treatment method for the degradation of lignocellulosic residues.The present invention therefore relates to the use of a superabsorbent polymer to accelerate the fermentation of waste of organic origin, in particular agricultural and agro-industrial waste, and is characterized in that this waste comprises a source of lignocellulosic residues, of the straw type, and in that the superabsorbent polymer is a water-retaining polymer, of natural or synthetic origin which has a water retention capacity greater than or equal to 10 times its weight in demineralized water, preferably greater than or equal to 20 times, advantageously greater than or equal to 30 times.

[0032] Another aim of the invention is also to propose a process for the fermentation of waste of organic origin, in particular agricultural and agro-industrial, comprising lignocellulosic residues, with a view to accelerating their degradation in digesters, such as anaerobic methanization digesters.

[0033] The present invention also relates to a composition for the fermentation of waste of organic origin, in particular agricultural and agro-industrial, characterized in that it comprises a liquid mixture of an aqueous solution of organic matter containing - a source of lignocellulosic residues, present directly in the organic waste used during fermentation, in particular slurry and manure, and / or added from an external source, - and a superabsorbent polymer in solid form, said superabsorbent polymer being chosen from the group of water-retaining polymers of natural or synthetic origin.

[0034] This type of water-retaining polymer, having a water retention capacity greater than or equal to 10 times its weight in demineralized water, preferably greater than or equal to 20 times, advantageously greater than or equal to 30 times, is generally known under the name of superabsorbent or under the abbreviation: SAP ("superabsorbent polymer"). It is generally in the form of powder, agglomerated or not. Their structure based on a three-dimensional network similar to a multitude of small cavities, each of which has the capacity to deform and absorb water, gives them the property of absorbing very large quantities of water and therefore of swelling. The superabsorbent polymers of natural origin, which can be used in the context of the present invention, are for example those described in patents US358364, US 1693890, US3846404, US3935099 or US3661815...Examples include, but are not limited to: guar gum, alginates, carboxymethyl cellulose, dextran, xanthan gum, etc. SAPs of synthetic origin that can be used in the context of the present invention are, for example, water-soluble polymers that are crosslinked or can be crosslinked. There are many types. Such polymers are, for example, described in patent FR 2559158, which describes crosslinked polymers of acrylic or methacrylic acid, crosslinked graft copolymers of the polysaccharide / acrylic or methacrylic acid type, crosslinked terpolymers of the acrylic or methacrylic acid / acrylamide / sulfonated acrylamide type, and their alkaline earth or alkali metal salts.In a preferred embodiment, the monomers used for the preparation of the superabsorbent polymers are chosen from acrylamide and / or partially or totally salified acrylic acid and / or partially or totally salified ATBS (acrylamido tertio butylsulfonate) and / or NVP (N vinylpyrrolidone) and / or acryloylmorpholine and / or partially or totally salified itaconic acid. In a preferred embodiment, the superabsorbent polymers are crosslinked homopolymers or copolymers based on partially or totally salified acrylic acid. Other hydrophilic monomers, such as for example cationic monomers, but also monomers with hydrophobic characteristics, may be used. to produce superabsorbent polymers. Examples of cationic monomers include diallyldialkyl ammonium salts and monomers of the dialkylaminoalkyl (meth)acrylate, dialkylaminoalkyl (meth)acrylamide type and their quaternary ammonium or acid salts. In particular, quaternized or salified dimethylaminoethyl acrylate (ADAME) and / or dimethylaminoethyl methacrylate (MADAME), acrylamidopropyltrimethylammonium chloride (APTAC) and / or methacrylamidopropyltrimethylammonium chloride (MAPTAC). Synthetic superabsorbent polymers are generally crosslinked with 100 to 6000 ppm (parts per million) of at least one crosslinking agent chosen from the group comprising acrylic compounds such as methylene bis acrylamide, allylic compounds such as tertra allylammonium chloride, vinyl compounds such as divinyl benzene, diepoxy, metal salts, etc.Some may also have double crosslinking, for example by an acrylic crosslinker. The superabsorbent polymers of the invention may also be post-treated by post-crosslinking the surface of the polymer particles in order to increase their absorption capacity under the effect of pressure, as described, for example, in patent applications DE 4020780 Cl, DE 19909653 Al and DE 199098838 AL.

[0035] For cost reasons, absorbent materials of synthetic origin of the crosslinked sodium or potassium acrylate (co)polymer type with or without post-crosslinking will be preferred.

[0036] SAP can be obtained by all the polymerization techniques well known to those skilled in the art: gel polymerization, precipitation polymerization, emulsion polymerization (aqueous or inverse) followed or not by a distillation step, suspension polymerization, solution polymerization, these polymerizations being followed or not by a step making it possible to isolate a dry form of the (co)polymer by all types of means well known to those skilled in the art.

[0037] The above-mentioned absorbent materials can also be combined with each other.

[0038] From an operational point of view, the quantity of superabsorbent polymer added to the digester each day will depend on the size of the digester as well as the quantity of lignocellulosic organic matter added. Ideally, it should be between 10 g and 500 g per m3 of daily organic co-substrate addition to the digester, which represents a concentration of between 0.01 g / L and 0.5 g / L, preferably between 0.05 g / L and 0.2 g / L. The use of a higher quantity of superabsorbent polymer is possible without affecting the fermentation, however the economic benefit may be limited.

[0039] To limit the number of applications, the superabsorbent polymer is advantageously added to the digester at the same time as the addition of co-substrate, preferably in a mixture, by regular feeding steps, preferably several times a week and advantageously several times a day in order to preserve the optimal physicochemical conditions for methanogenic activity (temperature, pH). It can be added either dry or hydrated.

[0040] The present invention also relates to the process for methanization of organic materials comprising lignocellulosic residues, of the straw type, characterized in that it comprises a step of bringing said lignocellulosic residue into contact, in an aerobic or anaerobic environment, with at least one superabsorbent polymer as described above, said treatment leading to an increase in the degradability of said lignocellulosic residues present in the digester, thus reducing their residence time in the latter. The invention thus makes it possible to solve a major technical problem but also to improve the performance of digesters by increasing the production of biogas.

[0041] The lignocellulosic residues can be chosen from cereal straws such as wheat, corn, rapeseed, etc. and / or from all types of woody residues (wood, miscanthus, etc.).

[0042] The invention thus makes it possible to contribute to the recovery of large quantities of lignocellulosic biomass that is currently poorly exploited (combustion sector) and thus advantageously supplement the source of material to be digested, this being an inexhaustible resource. It also makes it possible to avoid or limit the use of so-called "food" biomass, potentially edible by humans, which is currently used for its energy potential.

[0043] The mechanism of the effect of the superabsorbent polymer is not known, but it could for example either limit undesirable bacteria, or provide a substrate or support for desirable bacteria, or initiate enzymatic stimulation...

[0044] The present invention also relates to any variant or adaptation which will appear clearly to those skilled in the art, if necessary by resorting to a few routine tests.

[0045] In addition to the foregoing description, the invention will be better understood with the aid of the examples which follow and which are given for illustrative and non-limiting purposes. Examples

[0046] [fig. 1] represents a diagram of an exemplary reactor used to implement the present invention.

[0047] Operation of the reactor:

[0048] The reactor is equipped with:

[0049] - a cover 2a comprising a bubbler 2b, - and an agitator comprising an electric motor 3a and a propeller 3b,

[0050] The cylindrical reactor 1 is thermostatically controlled by a double jacket 4 at 35°C.

[0051] The bubbler is the name of the cap that closes the fermentation tank while allowing the biogas produced during fermentation to escape. The principle of a bubbler consists of a small pipe that forms a downward bend. This bend is filled with a liquid (in this case water). The ends of the pipe are connected to one inside and the other to the outside of the reactor. The bubbler acts as a valve: the liquid prevents air and external microorganisms from entering the tank but thanks to the overpressure in the tank, the biogas produced inside will manage to pass through the liquid to get out.

[0052] In addition, regular mixing is ensured inside the reactor thanks to a stirrer. Effective mixing reduces the temperature and organic matter concentration differences in the mass of the digester and increases the chances of encounters between microorganisms, superabsorbents and materials to be degraded.

[0053] To avoid any discrepancy that can be observed between the performance of laboratory digesters, we chose a single 10-liter reactor. The simulation is carried out with parameters conventionally used for mesophilic methanization (35°C) in a wet process (< 10% DM). We neglected the effect of pH on this simulation and considered the reactor as perfectly mixed at a constant stirring speed of 16 rpm and filled to its maximum capacity.

[0054] The substrate consists exclusively of cattle manure rich in lignocellulosic residues (generated by dairy cows) and raw (without phase separation step): 16% dry matter (DM). This fresh manure was then diluted with water heated to 35°C to reach the desired dry matter contents.

[0055] All comparative tests were carried out using the same reactor and under strictly identical conditions. To determine the methane production by simulation, we measured the reduction in the dryness of the substrate, which is representative of the efficiency of the bioconversion of organic matter into methane. Indeed, for the same sludge, the reduction in dryness will be a function of the degradation rate of the organic matter present as well as its biodegradability. Table 1 below shows the variations in dryness obtained after 25 days, in the absence and presence of superabsorbent polymer. Dryness is determined by an index used in the field of wastewater treatment. Dryness is the mass percentage of dry matter. Thus, a sludge with a dryness of 10% has a humidity of 90%. It is evaluated by the quantity of solid remaining after heating at 110°C for two hours.It is expressed as a weight percentage. [Tables 1] Dryness of the starting substrate (% DM) Superabsorbent (SAP) used: trade name / chemical nature (% anionicity) Quantity of SAP used (mass used / concentration in the substrate) Reduction in dryness of the substrate after 25 days Ex 1 10 Apromud P150 (100%) Sodium polyacrylate 0.5 g 0.05 g / L 52.9% Ex 2 10 Apromud G300 (100%) Sodium polyacrylate 2g / 0.2 g / L 54.3% CEx 1-2 10 non na 49.5% Ex 3 7 Aprodev 06 (30%) Potassium acrylamide-acrylate copolymer 0.5 g 0.05 g / L 52.1% Ex 4 7 Aprodev 27 Mixture of sawdust with Apromud P150 (100% cf ex 1) 0.5 g 0.05 g / L 53.2% CEx 3-4 7 no na 50.2%

[0057] When carrying out the examples, it was possible to observe that:

[0058] - given the very small quantity of SAP used, this, even once inflated by the water present in the substrate is not visually distinguishable once the mixture is made.

[0059] - at the beginning of digestion (0-3 days), the bubbler lets out very little gas which which means that methane production is slow: this is explained by the fact that very little substrate is already hydrolyzed

[0060] - from 4 days, the bubbling in the bubbler is very intense and corresponds to a maximum methane production (when the products of acetogenesis are present in large quantities).

[0061] - after 15 days, the activity of the bubbler begins to decrease because there is no progress left sively more than inert organic matter in the particulate and soluble phases.

[0062] The examples described in Table 1 (annotated Ex) show that in the presence of a superabsorbent polymer, compared to the counter-examples carried out without (Cex), and whatever the nature of the SAP used, the biomethanizations of organic materials comprising lignocellulosic residues, of the straw type, carried out according to the invention make it possible to significantly increase the reduction in the dryness of the substrate used. It is indeed noted that the pretreatment of the substrates with a very small quantity of SAP leads to an increase in the degradability of the organic matter present in the reactor and in particular of the lignocellulosic residues.

[0063] Unexpectedly, the use of SAP according to the invention consequently reduces the residence time of substrates based on lignocellulosic residues, known to be among the slowest to be degraded during anaerobic digestion, in the digester by improving their digestibility and thus allows operators not only to overcome a major technical problem but also to make substantial savings thanks to the gain generated in biogas production.

[0064] The invention thus makes it possible to contribute to the recovery of large quantities of currently poorly exploited lignocellulosic biomass, which is an inexhaustible resource. It also makes it possible to avoid or limit the use of so-called "food" biomass, potentially edible by humans, which is currently used for its energy potential.

Claims

Claims

1. Composition for the fermentation of waste of organic origin, in particular agricultural and agro-industrial waste, characterized in that it comprises a liquid mixture of an aqueous solution of organic matter containing a source of lignocellulosic residues and a superabsorbent polymer in solid form, said superabsorbent polymer being chosen from the group of water-retaining polymers of natural or synthetic origin.

2. Composition according to claim 1, characterized in that the source of lignocellulosic residues is chosen from cereal straws, in particular wheat, corn, rapeseed and / or all types of woody residues, in particular wood and miscanthus.

3. Composition according to any one of claims 1 or 2, characterized in that the lignocellulosic residues are present directly in the organic waste used during fermentation, in particular slurry and manure, and / or added from a source external to them.

4. Composition according to any one of the preceding claims, characterized in that said superabsorbent polymer is based on a crosslinked synthetic (co)polymer comprising one or more monomers, chosen from the group consisting of - partially or totally salified acrylic acid, partially or totally salified acrylamido tertio butylsulfonate, partially or totally salified itaconic acid, acrylamide, N-vinyl pyrrolidone, dialkylaminoalkyl (meth)acrylate and / or diallylaminoallyl (meth)acrylamide, their quaternary ammonium salts or their acid salts, such as for example dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, quaternized or salified, acrylamidopropyltrimethylammonium chloride and methacrylamidopropyltrimethylammonium chloride.

5. Composition according to any one of claims 3 or 4, characterized in that said superabsorbent polymer is a crosslinked synthetic (co)polymer comprising one or more monomers, chosen from the group of partially or totally salified acrylic acid monomers of the crosslinked sodium or potassium acrylate (co)polymer type with or without post-crosslinking.

6. Method for treating organic waste comprising residues lignocellulosic waste characterized in that it comprises a step of bringing said organic waste into contact with a superabsorbent polymer leading to an increase in the degradation of the lignocellulosic residues.

7. Method according to claim 6, characterized in that the lignocellulosic residues are chosen from cereal straws.

8. Method according to any one of claims 6 or 7, characterized in that the superabsorbent polymer is advantageously added to the digester at the same time as the addition of co-substrate, preferably in a mixture, by regular feeding stages, preferably several times per week and advantageously several times per day.

9. Method according to any one of claims 6 to 8, characterized in that the treatment is carried out in an anaerobic environment for the production of methane.

10. Method according to any one of claims 6 to 9, characterized in that the quantity of superabsorbent polymer added to the digester is carried out at a concentration of between 0.01 g / L and 0.5 g / L, preferably between 0.05 g / L and 0.2 g / L relative to the volumes of additions of organic co-substrate in the digester.

11. Use of the composition according to any one of claims 1 to 5 or of the process according to any one of claims 6 to 10, for the fermentation of waste of organic origin, in particular agricultural and agro-industrial waste, comprising lignocellulosic residues, with a view to accelerating their degradation in digesters, such as anaerobic methanization digesters.

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