Composting process aimed at reducing greenhouse gas emissions

Abstract

The invention relates to a process for composting organic waste comprising the following steps: - applying a particle size sorting (8) to raw compost from a previous composting cycle in order to separate said raw compost into at least a compost fraction (10) and rejects (9), said rejects (9) having a particle size greater than that of the compost fraction; - applying a treatment (11) to the rejects (9) so as to reduce the concentration of soluble nitrogen in the treated rejects (3); - preparing a composting mixture (1) by mixing organic waste (2) with structuring agents (5) comprising the treated rejects (3); - piling the composting mixture (1) thus prepared in a composting unit (6) and composting it. Figure to be published with the abstract: Fig. 1

Description

Title of the invention: Composting process for reducing greenhouse gas emissions. Field of the invention

[0001] The invention relates to the field of waste treatment and more particularly concerns a process for composting organic waste. Technological background

[0002] Composting is a biological process ensuring aerobic decomposition, by microorganisms, of organic waste into an organic product rich in humic compounds: compost.

[0003] In a composting process, it is known to add structuring agents to the composting mixtures in order to improve their physical structure. The purpose of structuring agents is to prevent compaction and ensure homogeneous and efficient decomposition of organic matter. By creating air spaces in the piles of material to be composted, they allow optimal oxygen circulation, which is essential for the aerobic microorganisms responsible for decomposition.

[0004] After the composting mixture has been piled up, composting conditions are regularly managed to maintain optimal decomposition conditions: the piles are regularly turned over or forced aeration systems are used to ensure adequate oxygen circulation, water is added as needed to maintain optimal moisture levels and the temperature is controlled.

[0005] At the end of the composting process, when the material is stabilized, that is, when its degradability potential is low, the raw compost undergoes screening operations to separate, on the one hand, the compost fraction and, on the other hand, the rejects, which include, in particular, an undegraded woody fraction. The rejects are then used, at least partially, as structuring agents in a subsequent composting cycle.

[0006] Biological composting processes generate gas emissions that can represent between 70 and 80% of the total greenhouse gas emissions of the entire treatment process. Furthermore, it has been observed that nitrous oxide (N2O) emissions represent the majority of the greenhouse gas emissions from the biological process, measured in CO2 equivalent.

[0007] Several strategies have been explored experimentally to reduce greenhouse gas emissions during the composting process. These different strategies can be classified into two groups: the first focus on the composition of the initial mixture to be composted, while the latter focus on managing composting conditions. However, while these strategies offer potential ways to reduce greenhouse gas emissions, they are not entirely satisfactory: they can involve additional costs, sometimes require precise distribution of additives in the compost, and can have varying effects depending on the specific composting conditions. Summary

[0008] Also, a problem which arises and which the present invention aims to solve is to provide a composting process which makes it possible to reduce greenhouse gas emissions in a simple and effective way.

[0009] Confidential experiments have shown that rejects constitute a significant source of greenhouse gas emissions when used as a structuring agent in a subsequent composting cycle. This phenomenon can be explained by two distinct factors. First, the rejects have very high concentrations of soluble nitrogen (ammonium (NH4+) and nitrate (NO3-) ions), which leads to increased nitrous oxide emissions. Second, these rejects carry a specific microbial community whose reintroduction into the mixture to be composted also promotes nitrous oxide emissions.

[0010] The graph in [Fig. 2], resulting from this work, illustrates the cumulative amount of nitrous oxide emitted during the composting of dewatered sewage sludge as a function of the amount of soluble nitrogen initially present in the structuring agents. It can be seen that the high concentration of soluble nitrogen in the residues used as structuring agents is, at least in part, responsible for the significant nitrous oxide emissions.

[0011] Also, an idea at the base of the invention consists of reducing the amount of soluble nitrogen present in the rejects, used as a structuring agent in a composting mixture, in order to limit nitrous oxide emissions.

[0012] In order to solve this problem, and according to a first objective, a process for composting organic waste is proposed comprising the following steps: - applying a particle size sorting to a raw compost from a previous composting cycle in order to separate said raw compost into at least a compost fraction and into rejects, said rejects having a particle size greater than that of the compost fraction; - apply a treatment to the rejects so as to decrease the concentration of soluble nitrogen in rejects treated by said treatment; - prepare a composting mixture by mixing organic waste with structuring agents including treated rejects; - pile up the prepared composting mixture in a composting unit and compost it.

[0013] Thus, thanks to the reduction in the concentration of soluble nitrogen in the rejects, greenhouse gas emissions are reduced during the composting of the mixture incorporating said rejects. The composting process therefore uses the rejects from the particle size sorting of the raw compost as a structuring agent, without suffering the drawbacks associated with such use, with regard to the generation of greenhouse gases. The composting process is therefore inexpensive compared to other solutions for reducing greenhouse gas emissions and has a lower impact on climate change.

[0014] According to embodiments, such a composting process may include one or more of the following characteristics.

[0015] According to one embodiment, the treatment applied to the rejects is chosen from: - a mechanical cleaning operation during which the surface of the rejects is cleaned; - a stabilization operation during which the rejects are stored at least until said rejects have a soluble nitrogen concentration less than or equal to a soluble nitrogen concentration threshold or for a period greater than or equal to a duration threshold; - a particle size reduction operation during which the particle size of the rejects is reduced; and - their combinations.

[0016] According to one embodiment, the composting process includes a second particle size sorting operation to separate fine particles from the treated residues, either simultaneously or after the treatment applied to the residues. This makes it possible to eliminate the fine particles that have the highest concentrations of soluble nitrogen and microorganisms.

[0017] According to one embodiment, the mechanical cleaning operation is carried out by brushing and / or agitating the residues. This allows the fine particles to be separated from the surface of the residues.

[0018] According to one embodiment, the mechanical cleaning operation is carried out using at least one piece of equipment selected from: - equipment with rotating brushes; - a drum screening equipment, optionally equipped with at least one rotary brush mounted to rotate freely inside said drum; - a star screening equipment; and - a spar screening equipment.

[0019] According to one embodiment, the drum of the drum screening equipment is a bar drum. The use of a bar screening drum is advantageous because it allows for better separation efficiency of fine particles. Preferably, the bars of the screening drum have a polyhedral cross-section, such a cross-section further increasing the separation efficiency compared to a circular cross-section.

[0020] According to one embodiment, the operation of stabilizing the rejections comprises: - measure or estimate the concentration of soluble nitrogen in the rejects; - compare said soluble nitrogen concentration to a concentration threshold; and - store the rejects at least until the nitrogen concentration is less than or equal to the nitrogen concentration threshold.

[0021] According to one embodiment, the rejects are stored for a period greater than or equal to a duration threshold and the duration threshold is determined according to a correlation relationship, for example established experimentally, between a variation in the concentration of soluble nitrogen in rejects and a stabilization time of said rejects and / or according to a correlation relationship, for example established experimentally, between a quantity of greenhouse gas emitted by composting a mixture to be composted including rejects and a stabilization time of said rejects.

[0022] According to one embodiment, the treatment applied to the rejects comprises the particle size reduction operation. This particle size reduction operation is also advantageous in that it promotes the transformation of soluble nitrogen into ammonia rather than into nitrous oxide, which has a high global warming potential (GWP).

[0023] According to one embodiment, the particle size reduction operation is carried out by grinding.

[0024] According to one embodiment, the treated residues from the particle size reduction operation have a particle size between 100 and 200 mm, and preferably between 100 and 150 mm. Such a particle size allows the structural character of the treated residues to be preserved while ensuring that their size is sufficiently small to increase the temperature at the beginning of fermentation, thus promoting the emission of ammonia in favor of nitrous oxide.

[0025] According to one embodiment, the rejects obtained by the particle size sorting of the raw compost have a particle size greater than a particle size threshold, said particle size threshold being between 100 and 200 mm and advantageously between 100 and 150 mm.

[0026] According to one embodiment, the composting unit is a closed, forced-aeration installation equipped with a washing tower designed to capture and treat ammonia emitted preferentially to nitrous oxide during composting.

[0027] According to other embodiments, the composting unit can also be an open installation and / or one with turning / mixing of the piles.

[0028] According to one embodiment, the structuring agents further include first-use structuring agents comprising woody materials, for example green waste containing woody residues.

[0029] According to one embodiment, the treatment applied to the rejects makes it possible to reduce the concentration of microorganisms in said rejects and more particularly in nitrifying and denitrifying microorganisms. Brief description of the figures

[0030] Other features and advantages of the invention will become apparent from the following description of particular embodiments of the invention, given by way of example but not limitation, with reference to the accompanying drawings in which:

[0031] [Fig-1] is a flowchart representing the different stages of a process of composting according to a method of implementation.

[0032] [Fig.2] is a graph representing the cumulative amount of nitrous oxide emitted as a function of the amount of soluble nitrogen initially present in the structuring agents of the mixture to be composted.

[0033] [Fig.3] is a schematic representation of a rotary brush equipment.

[0034] [Fig.4] is a schematic representation of a drum screening equipment.

[0035] [Fig.5] is a schematic representation of a screening equipment with a barred drum.

[0036] [Fig.6] is a schematic representation of a star screening equipment. Description of the implementation methods

[0037] In relation to [Fig. 1], a process for composting organic waste is described below.

[0038] The term "organic waste" means any biodegradable material, that is, material capable of being broken down by microorganisms. For example, organic waste includes sewage sludge, food waste, agricultural waste, livestock waste, by-products of the agri-food industry, lignocellulosic materials (paper, cardboard, sawdust and wood chips), digestate from methanization, and green waste. It should be noted, however, that the composting process according to the invention is more specifically designed for organic waste that requires the addition of structuring agents for composting. Furthermore, the reduction in greenhouse gas emissions attributable to the process according to Since the invention is correlated with the quantity of residues used as structuring agents, this process is particularly suitable for composting organic waste requiring a significant addition of structuring agents, that is, for almost all organic waste streams to be composted, with the exception of green waste containing woody residues. This includes, in particular, sewage sludge, food waste, kitchen and table scraps, livestock waste, by-products of the agri-food industries, digestate from methanization, and other similar materials.

[0039] A composting mixture 1 is prepared by mixing organic waste 2 to be composted with structuring agents 5 which are at least partially composed of treated rejects 3, i.e., screening rejects that have undergone one or more of the treatments described below. In the embodiment shown, the structuring agents 5 comprise both first-use structuring agents 4 and treated rejects 3. The organic waste 2 is, for example, dewatered sewage sludge. The first-use structuring agents 4 comprise woody materials and are, for example, green waste containing woody residues, sawdust, and wood chips.

[0040] The composting mixture 1 may also include additives, not illustrated in [Fig. 1], such as: - one or more physical additives, for example chosen from biochar, zeolite, bentonite, perlite, vermiculite, sand, clay, activated carbon, fly ash and mixtures thereof; - one or more chemical additives, for example selected from gypsum, superphosphate, calcium and magnesium phosphate, citric acid, ferrous sulfate and mixtures thereof; and / or - one or more microbial additives, for example chosen from mature compost, nitrite-oxidizing bacteria, nitrogen-renewing bacterial agents, ammonia-oxidizing bacteria and mixtures thereof.

[0041] The composting mixture 1 is piled in a composting unit 6 for the decomposition phase. The mixture may, in particular, be arranged in windrows, i.e., in long rows several meters long. During this decomposition phase, the temperature is controlled and aeration is ensured by turning or mixing the piles and / or by forced ventilation systems.

[0042] When the material has reached sufficient stability, the raw compost is extracted from the composting unit 6. The raw compost is then subjected to one or more particle size sorting operations 8 aimed at isolating, on the one hand, the rejects 9 containing the largest particles, and on the other hand, a fraction of compost 10 composed of the finest particles and intended to be used as a soil amendment.

[0043] According to one embodiment, the particle size sorting operation 8 of the raw compost is carried out by screening. It can, for example, be carried out with: - a drum screening equipment, i.e. comprising a perforated cylindrical drum, which rotates freely and allows the compost particles to pass through; - a vibrating screen screening equipment, i.e. one comprising a perforated surface subjected to vibrations, allowing the passage of compost particles; - a star screening equipment; or - a stringer screening equipment.

[0044] According to one embodiment, the particle size sorting operation 8 is carried out so that the rejects 9 have a minimum size between 100 and 200 mm and preferably between 100 and 150 mm.

[0045] The rejects 9 are then subjected to at least one treatment 11 during which their concentration of soluble nitrogen is reduced.

[0046] According to a first embodiment, the treatment 11 is a mechanical cleaning operation during which the surface of the rejects 9 is cleaned, which makes it possible to detach from the surface of the rejects 9 the fine particles which are the most concentrated in soluble nitrogen and microorganisms since they are the parts of the rejects which have been in direct contact with the material to be degraded during the previous fermentation phase.

[0047] This mechanical surface cleaning operation can in particular be carried out by means of brushing equipment allowing the surface of the rejects 9 to be rubbed and / or of agitation equipment allowing them to be stirred.

[0048] According to an advantageous embodiment, the rejects 9 undergo a second particle size sorting operation, for example by screening, simultaneously or after this mechanical cleaning operation. This makes it possible to remove the fine particles from the rejects 9.

[0049] Preferably, the mechanical cleaning operation does not use water to clean the rejects 9. Indeed, the use of water would not only increase the cost of treatment, but would also be likely to have adverse effects on greenhouse gas emissions by increasing the moisture of the treated rejects 3 used as structuring agents.

[0050] According to one embodiment, the brushing equipment is a rotary brush unit 12, as schematically represented in [Fig. 3]. This equipment includes, in particular, a waste loading system 13, optionally equipped with an infeed conveyor, rotary brushes 14 through which the waste 9 passes, and an unloading system 15, optionally equipped with an outfeed conveyor. The brushing action removes fine particles from the surface of the waste 9, which are advantageously collected in receptacles 16. Advantageously, The brushing equipment does not use water for the mechanical cleaning of the rejects 9. Optionally, the rotary brushes 14 can however be cleaned with water to prevent them from becoming clogged.

[0051] According to one embodiment, schematically illustrated in [Fig. 4], the agitation of the residues 9 is achieved by means of a drum screening unit 17, i.e., comprising a perforated cylindrical drum 18, which is rotatable and allows the fine particles to pass through. According to an advantageous embodiment, the drum screening unit 17 is equipped with at least one rotating brush 19, illustrated in dashed lines in [Fig. 4], which is mounted for rotatable movement inside the perforated cylindrical drum 18 and is arranged to be driven in rotation in the opposite direction to that of the perforated cylindrical drum 18.

[0052] According to another embodiment, schematically illustrated in [Fig. 5], the agitation of the rejects 9 is achieved by means of a screening unit 20 having a bar drum. The bars 21 advantageously have a polyhedral cross-section. It can also be equipped with at least one rotating brush 22, driven in rotation in the opposite direction to the bar drum.

[0053] According to yet another embodiment, schematically illustrated in [Fig. 6], the agitation of the residues 9 is achieved by means of a star screening device 23 which offers an even more advantageous fine particle separation efficiency. A star screening device comprises a plurality of parallel rotating shafts, each equipped with a plurality of star-shaped discs. The discs are regularly spaced from one another so as to form regular openings allowing the passage of the finest particles. The larger particles are transported to one end of the device where they are discharged. The rotary motion of the star-shaped discs causes agitation of the residues 9.

[0054] According to another embodiment, not shown, the agitation of the residues 9 is achieved by means of a slatted screening device. A slatted screening device comprises a screen mounted on a frame and metal slats arranged in the longitudinal or transverse directions of the device and supporting the frame. The slats thus increase the impacts with the residues 9 during their processing, which promotes the separation of the finest particles.

[0055] According to a second embodiment, the treatment 11 is a stabilization operation during which the rejects 9 are stored, which makes it possible to reduce their concentration of soluble nitrogen.

[0056] The stabilization operation is implemented: - either until said refusals 9 have a concentration of soluble nitrogen less than or equal to a threshold of soluble nitrogen concentration (first variant); - either for a period greater than or equal to a duration threshold (second variant).

[0057] In the first variant, the concentration of soluble nitrogen in the rejects 9 is estimated or measured and compared to a concentration threshold. As soon as the estimated or measured concentration of soluble nitrogen in the rejects 9 is less than or equal to said concentration threshold, the stabilization operation can be stopped. The treated rejects 3 from this stabilization operation can then be used as a structuring agent 5 in a subsequent composting cycle.

[0058] The threshold concentration of soluble nitrogen is advantageously less than 2.7 gN / Kg of dry matter, preferably between 0.1 and 2 gN / Kg of dry matter.

[0059] By way of example, the concentration of soluble nitrogen in the residues 9 is determined according to the method described below. A representative number of samples are taken from the residues 9. The soluble nitrogen from each sample is extracted by mixing it in a calcium chloride solution, which is then stirred and filtered. The solution is then analyzed by an analytical instrument, such as a spectrophotometer or a chromatograph, which determines the total nitrogen concentration (ammoniacal nitrogen (NH4+), nitrate (NO3), and nitrite (NO2)) of the sample. The concentration of soluble nitrogen in the residues 9 is then calculated by taking the average of the concentrations of all the analyzed samples.

[0060] According to the second embodiment, the operation to stabilize the rejections 9 is implemented for a duration greater than or equal to a duration threshold.

[0061] Several methods can be implemented to determine this duration threshold.

[0062] According to a first example, the duration threshold is determined based on an experimentally established correlation relationship between a variation in the concentration of soluble nitrogen in the rejects 3 and the stabilization time of said rejects 3. The duration threshold is, for example, chosen such that the concentration of soluble nitrogen in the rejects 9 for a stabilization treatment of a duration corresponding to said duration threshold is less than or equal to the soluble nitrogen concentration threshold mentioned above in relation to the first embodiment.

[0063] According to a second example, the duration threshold is established based on an experimentally established correlation relationship between the quantity of greenhouse gases emitted by the composting of a mixture to be composted including rejects and the stabilization time of said rejects.

[0064] Furthermore, according to a third embodiment, the treatment of 11 is a mechanical operation of particle size reduction of the rejects 9, for example by grinding.

[0065] Such a mechanical particle size reduction operation makes it possible, like the mechanical cleaning operation described above, to remove the particles from the residue The finest particles are the most concentrated in soluble nitrogen and microorganisms. It also has an additional effect: the use of structuring agents with a smaller average size increases the thermophilic phase of composting, which promotes the emission of ammonia, which is not a greenhouse gas, and consequently reduces the emission of nitrous oxide, which has a very high Global Warming Potential (GWP).

[0066] According to an advantageous embodiment, the rejects 9 are subjected to a second screening operation, simultaneously or after this mechanical particle size reduction operation, which makes it possible to remove fine particles, which are not suitable as a structuring agent and / or those most concentrated in soluble nitrogen and microorganisms.

[0067] Advantageously, the particle size of the treated residues 3 from the particle size reduction operation is less than 200 mm and preferably less than 150 mm. Advantageously, the particle size of the treated residues 3 is between 50 and 200 mm and preferably between 100 and 150 mm.

[0068] By way of example, the crushing equipment may include a hammer mill, a knife mill or a disc mill.

[0069] Although treatment 11 has been described below in several distinct forms, it can also be achieved by combining several of these methods.

[0070] Furthermore, the composting unit 6 can be of any type. In particular, it can be a turning composting unit 6, that is, one in which the piles are regularly turned using mechanical equipment to aerate and mix the material, or a forced-aeration composting unit, that is, one using systems, such as ventilation systems, to force air through the piles of the mixture to be composted. The composting unit 6 can be an open-air installation or an enclosed installation, that is, one with a closed enclosure in which the mixture to be composted is piled.

[0071] In an advantageous embodiment, the composting unit 6 is a closed, forced-aeration installation equipped with a washing tower 7 designed to capture and treat the ammonia (NH3) emitted during the composting operations. Such a composting unit is particularly advantageous when used in combination with a composting process in which the rejects 9 are subjected to a mechanical particle size reduction operation in order to promote ammonia emissions over nitrous oxide emissions.

[0072] Although the invention has been described in connection with several particular embodiments, it is clearly evident that it is by no means limited to them and that it includes all technical equivalents of the means described as well as their combinations if these fall within the scope of the invention, as defined by the claims.

Claims

Demands

1. A process for composting organic waste comprising the following steps: - applying a particle size sorting (8) to raw compost from a previous composting cycle in order to separate said raw compost into at least a compost fraction (10) and rejects (9), said rejects (9) having a particle size greater than that of the compost fraction; - applying a treatment (11) to the rejects (9) so as to reduce a concentration of soluble nitrogen in the rejects treated (3) by said treatment (11); - preparing a composting mixture (1) by mixing organic waste (2) with structuring agents (5) comprising the treated rejects (3); - piling the composting mixture (1) thus prepared in a composting unit (6) and composting it.

2. A composting process according to claim 1, wherein the treatment (11) applied to the rejects (9) is selected from: - a mechanical cleaning operation in which a surface of the rejects (9) is cleaned; - a stabilization operation in which the rejects (9) are stored at least until said rejects (9) have a soluble nitrogen concentration less than or equal to a soluble nitrogen concentration threshold or for a duration greater than or equal to a duration threshold; - a particle size reduction operation in which the particle size of the rejects (9) is reduced; and - combinations thereof.

3. Composting process according to claim 2, comprising a second particle size sorting operation to separate fine particles from the treated rejects (3), simultaneously or after the treatment (11) applied to the rejects (9).

4. Composting process according to claim 3 or 4, wherein the treatment (11) applied to the rejects (9) comprises the mechanical cleaning operation and wherein said mechanical cleaning operation is carried out by brushing and / or agitating the rejects (9).

5. Composting process according to claim 4, wherein the mechanical cleaning operation is carried out by means of at least one piece of equipment selected from: - a rotary brush equipment (12); - a drum screening equipment (17, 20), optionally equipped with at least one rotary brush (19, 22) mounted movably in rotation inside said drum; - a star screening equipment (23); and - a stringer screening equipment.

6. Composting process according to any one of claims 2 to 5, wherein the treatment (11) applied to the rejects (9) comprises the operation of stabilizing the rejects and wherein the operation of stabilizing the rejects comprises: - measuring or estimating the concentration of soluble nitrogen in the rejects (9); - comparing said concentration of soluble nitrogen to a concentration threshold; and - storing the rejects (9) at least until the nitrogen concentration is less than or equal to the nitrogen concentration threshold.

7. Composting process according to any one of claims 2 to 5, wherein the treatment (11) applied to the rejects (9) comprises the operation of stabilizing the rejects during which the rejects (9) are stored for a period greater than or equal to a threshold duration and wherein the threshold duration is determined according to a correlation relationship between a variation in the concentration of soluble nitrogen in rejects (9) and a stabilization time of said rejects (9) and / or according to a correlation relationship between a quantity of greenhouse gas emitted by composting a mixture to be composted including rejects (9) and a stabilization time of said rejects (9).

8. Composting process according to any one of claims 2 to 7, wherein the treatment (11) applied to the rejects (9) comprises the particle size reduction operation and wherein the particle size reduction operation is carried out by grinding.

9. A composting process according to any one of claims 2 to 7, wherein the treatment (11) applied to the rejects (9) comprises the particle size reduction operation and wherein the treated rejects (3) originate from said particle size reduction operation have a particle size between 100 and 200 mm, and preferably between 100 and 150 mm.

10. Composting process according to any one of claims 1 to 9, wherein the rejects obtained by the particle size sorting of the raw compost have a particle size greater than a particle size threshold, said particle size threshold being between 100 and 200 mm.

11. Composting process according to any one of claims 1 to 10, wherein the composting unit (6) is a closed, forced-aeration installation equipped with a washing tower (7) designed to capture and treat the ammonia emitted during composting.

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