FR3091879A1 - New agent to accelerate the degradation of organic waste - Google Patents

Abstract

The present invention relates to the technical field of the fermentation of organic waste, in particular agricultural and agro-industrial waste, and more particularly waste 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 enables operators not only to overcome a major technical problem but also to achieve substantial savings by reducing residence times and thanks to the savings generated in the production of biogas. The invention also relates to the method implementing this novel agent.

Description

New agent to accelerate the degradation of organic waste

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

Technical field of the invention

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

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

- as well as a digestate that can be used as a fertilizer.

The biogas thus produced can be transformed into heat, electricity and/or fuel.

Technical background

In anaerobiosis, organic matter is decomposed by the presence of many species of bacteria. This reaction takes place in a sealed tank, called a digester or methanizer, in which the organic waste is stored to be subjected to the action of micro-organisms (bacteria) in the absence of oxygen.

The main steps involved in fermentation are:

• hydrolysis and acidogenesis: acidogenic micro-organisms transform complex organic chains into simpler compounds: peptides, amino acids, fatty acids, sugars;

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

• methanogenesis: methanogenic micro-organisms are responsible for the production of gas (gasification): the acetic acid obtained during acetogenesis is transformed into methane and carbon dioxide.

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

In addition, 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 which was contained therein before fermentation.

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. Biogas-producing bacteria could therefore be at the origin of the appearance of oxygen on Earth and, by extension, of life.

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, a first digester was built in India with the aim of producing fuel for vehicles .

The current sector is mainly based on the use of methanation processes which include the introduction of solid organic matter, typically liquid agricultural effluents (slurry in particular) to which other wastes are added (called co-substrates or inputs) and which can go as far as at 40% dry matter. The former provide the water and microorganisms ensuring the methanisation reactions, the latter the material with the highest yield of biogas. The methanation 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 materials are then stirred continuously by one or more agitators in order to avoid the phenomena of settling, flotation or crusting of the biomass.

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 around 35°C. There is also thermophilic fermentation (50-60°C) which makes it possible to reduce the size of methanizers as well as better elimination of pathogenic germs. A two-step solution is also sometimes used: a first thermophilic reactor with a short residence time followed by a second mesophilic reactor.

The organic matter remains for a period of several weeks in the digester. The solid organic materials brought in are often crushed before being incorporated into the digestion tank in order to facilitate their conveyance and mixing. These processes require a significant expenditure of energy to be stirred continuously inside the digester.

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 physico-chemical conditions for methanogenic activity (temperature, pH). This gradual addition of solid co-substrate in the digester results in a phenomenon of accumulation of organic matter which is closely linked to its rate of degradation. 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 undesirable matter as possible.

In addition, in order to allow mechanical stirring, in certain processes, the solid content of the reaction medium is fixed and must therefore not exceed 10 to 15%. A racking of the digestate is therefore carried out regularly in order to keep the solids level below this threshold. A reintroduction of the liquid digestate into the digester after phase separation can also be carried out.

Presentation of the invention

Despite its many advantages, the anaerobic digestion process still needs to improve its efficiency and robustness. Indeed, some waste is not or poorly destroyed by anaerobic digestion and/or can cause malfunctions in the process.

This is the case, in particular, when the effluents have a high ammonia content. Indeed, the micro-organisms 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 by inducing a failure of the process.

Similarly, lignocellulosic residues (based on cellulosic and/or hemicellulose 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 and more in the digester, which has the consequence of greatly reducing its energy yield. This is the case, in particular for cereal straw, which greatly hinders their recovery through methanation.

To date, the main solutions proposed to solve this problem are based on:

- the sorting of organic matter before it is introduced into the digester,

- physical and/or chemical processes leading to the breakdown of the ligno-cellulosic matrix,

- and biological pre-treatments with specific enzymes or micro-organisms.

All of them are relatively expensive in terms of time, price and/or energy.

There is therefore still an unmet need that would help to degrade this ligno-cellulosic biomass directly in anaerobic digesters and without generating additional costs for the operator of the methanizer.

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 as a mixture during the addition(s) of solid organic matter makes it possible to increase effectively and very simply the degradation of 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

A first object 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.

Another object of the invention is also to provide a process for treating these lignocellulosic residues using this agent to facilitate or accelerate their digestion in methanizers. The invention also relates to a composition making it possible to degrade said ligno-cellulosic residues as well as its use or the use of the treatment method for the degradation of ligno-cellulosic 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, 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 or equal to 30 times.

Another object of the invention is also to provide 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 digesters methanization anaerobes.

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 ligno- cellulose, 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.

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 generally comes in the form of powder, agglomerated or not. Their structure based on a three-dimensional network comparable to a multitude of small cavities, each of which has the ability 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, US1693890, US3846404, US3935099 or US3661815, etc. Mention will be made, without limitation: guar gum, alginates , carboxymethyl cellulose, dextran, xanthan gum, etc. The 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 that can be crosslinked. There are many types. Such polymers are for example described in patent FR 2559158 in which there are described 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 type / acrylamide / sulfonated acrylamide 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 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, could be used to produce the superabsorbent polymers. Among the cationic monomers, mention will be made, by way of example, of diallyldialkyl ammonium salts and monomers of the dialkylaminoalkyl (meth)acrylate, dialkylaminoalkyl (meth)acrylamide type as well as their quaternary ammonium or acid salts. Mention will be made in particular of 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 for example methylene bis acrylamide, allylic compounds such as for example tertra allylammonium chloride, vinyls such as divinyl benzene, diepoxy, metal salts, etc. Some may also have double crosslinking, such as an acrylic crosslinker. The superabsorbent polymers of the invention may also be post-treated by post-crosslinking of the surface of the polymer particles in order to increase their absorption capacity under the effect of pressure as described for example in the applications patent DE 4020780 C1, DE 19909653 A1 and DE 199098838 A1.

Preference will be given, for cost reasons, to absorbent materials of synthetic origin of the (co)polymer type of crosslinked sodium or potassium acrylate with or without post crosslinking.

The 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, polymerization in solution, these polymerizations being optionally followed 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.

The absorbent materials mentioned above can also be combined with each other.

From an operational point of view, the quantity of superabsorbent polymer brought into the digester each day will depend on the size of the latter as well as the quantity of ligno-cellulosic organic matter added. Ideally, it should be between 10 g and 500 g per m ³ of daily organic co-substrate addition in the digester, which represents a concentration 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 greater quantity of superabsorbent polymer is possible without this affecting the fermentation, however the economic interest may be limited.

To limit the number of inputs, the superabsorbent polymer is advantageously added to the digester at the same time as the addition of cosubstrate, preferably as a mixture, by regular feed levels, preferably several times a week and advantageously several times a day in order to preserve the optimum physico-chemical conditions for methanogenic activity (temperature, pH). It can be added either dry or hydrated.

The present invention also relates to the process for the methanization of organic materials comprising ligno-cellulosic residues, of the straw type, characterized in that it comprises a step of bringing said ligno-cellulosic residue into contact, in an aerobic or anaerobic medium, 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.

The lignocellulosic residues can be chosen from cereal straw such as wheat, corn, rapeseed, etc. and/or all types of ligneous residues (wood, miscanthus, etc.).

The invention thus makes it possible to contribute to the recovery of large quantities of lignocellulosic biomass currently poorly exploited (combustion sector) and as well as to advantageously supplement the source of matter to be digested, the latter 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.

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

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

In addition to the foregoing description, the invention will be better understood with the aid of the examples which follow and which are given by way of non-limiting illustration.

Examples

shows a diagram of an example of a reactor used to implement the present invention.

Operation of the reactor:

The reactor is equipped with:

• a cover 2a comprising a bubbler 2b,
• and a stirrer comprising an electric motor 3a and a propeller 3b,

The cylindrical reactor 1 is thermostated by a double jacket 4 at 35°C.

The bubbler is the name of the cap that closes the fermentation tank while letting the biogas produced during fermentation escape. The principle of a bubbler consists of a small pipe that forms a downward bend. This elbow is filled with a liquid (here water). The ends of the pipe are connected one inside and the other outside the reactor. The bubbler acts like 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 succeed in passing through the liquid to exit.

In addition, regular mixing is ensured inside the reactor thanks to a stirrer. Efficient mixing reduces the differences in temperature and concentration of organic matter in the mass of the digester and increases the chances of encounters between micro-organisms, superabsorbents and materials to be degraded.

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

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 content.

All the comparative tests were carried out using the same reactor and under strictly identical conditions. To determine by simulation the production of methane, we measured the reduction of 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 depend on the rate of degradation of the organic matter present as well as its biodegradability. Table 1 below shows the dryness variations obtained after 25 days, in the absence and in the presence of superabsorbent polymer. The 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 percentage by weight.

Dryness of the starting substrate
(% DM) Superabsorbent (SAP) used
: trade name
/ chemical nature
(% anionicity) Amount of SAP used (mass used / concentration in substrate) Reduction in substrate dryness after 25 days Ex 1 10 Apromud P150 (100%) Sodium Polyacrylate 0.5g
0.05g/L 52.9% Ex 2 10 Apromud G300 (100%) Sodium Polyacrylate 2g/
0.2g/L 54.3% CEx 1-2 10 no n / A 49.5% Ex 3 7 Aprodev 06 (30%) Acrylamide-Potassium Acrylate Copolymer
0.5g
0.05g/L 52.1% Ex 4 7 Aprodev 27
Mixture of sawdust with Apromud P150 (100% cf ex 1) 0.5g
0.05g/L 53.2% CEx 3-4 7 no n / A 50.2%

During the realization of the examples, one could note that:

- given the very small quantity of SAP used, it even once swollen by the water present in the substrate is not visually distinguishable once the mixture has been made.

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

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

- after 15 days, the activity of the bubbler begins to decrease because there is gradually only inert organic matter left in the particulate and soluble phases.

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 matter comprising ligno-cellulosic residues, of the straw type, produced according to the invention make it possible to substantially increase the reduction in the dryness of the substrate used. It is in fact observed 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.

Unexpectedly, the use of SAP according to the invention consequently reduces the residence time of the 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.

The invention thus makes it possible to contribute to the valorization of large quantities of ligno-cellulosic biomasses currently poorly exploited, 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 (11)

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 material containing a source of lignocellulosic residues and a superabsorbent polymer in the form of in solid form, said superabsorbent polymer being chosen from the group of water-retaining polymers of natural or synthetic origin. Composition according to Claim 1, characterized in that the source of lignocellulosic residues is chosen from cereal straws, in particular of wheat, corn, rapeseed and / or by all types of wood residues, in particular wood and miscanthus . 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 these. Composition according to any one of the preceding claims, characterized in that the 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 acrylic acid salified, acrylamido tertio butylsufonate partially or totally salified, itaconic acid partially or totally salified, acrylamide, N-vinyl pyrrolidone, dialkylaminoalkyl (meth) acrylate and / or diallylaminoallyl (meth) acrylamide, their salts of quaternary ammonium or their acid salts, such as, for example, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, quaternized or salified, acrylamidopropyltrimethylammonium chloride and methacrylamidopropyltrimethylammonium chloride. Composition according to any one of Claims 3 or 4, characterized in that the 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 type (co) polymer of sodium or potassium acrylate crosslinked with or without postcrosslinking. Process for treating organic waste comprising lignocellulosic residues, 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. Process according to Claim 6, characterized in that the lignocellulosic residues are chosen from cereal straw. Process 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 as a mixture, by regular feed stages, preferably several times a week and advantageously several times a day. Process according to any one of Claims 6 to 8, characterized in that the treatment is carried out in an anaerobic medium for the production of methane. Process 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 compared to the volumes of organic co-substrate additions in the digester. 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.