PROCESS FOR BIOLOGICAL DRY TREATMENT OF ORGANIC WASTE

Description

[0001] The invention relates to a process for the dry biological treatment of organic waste with a view to producing, by fermentation, biogas rich in methane or other compounds with energy or biochemical value and optionally, with the fermentation residue, a marketable fertilizing material.

[0002] To optimize the methanization yield of industrial biological reactors and ensure the agronomic recovery of by-products from biological treatment of organic waste in industrial reactors, a separation between non-biodegradable materials, considered undesirable, and biodegradable materials, must be carried out.

[0003] There are two main categories of industrial biological reactors for biological treatment: fermenters / digesters operating in the dry process in which the biological reactions will be carried out at dry matter contents of between 12 and 40%; fermenters / digesters operating in the wet process in which the biological reactions will be carried out at dry matter contents of between 1 and 15%.

[0004] Undesirable waste separation techniques can be applied upstream or downstream of biological treatment in an industrial reactor itself. These separation techniques may require dilution of the waste and prior suspension of the solid fraction in a "liquid process" or may be used directly in a "dry process". Thus, two separation techniques are currently used: ▪ A so-called wet separation used upstream of biological reactors that operate in the wet process, and this with a liquor generally containing less than 15% dry matter inside the biological reactor. ▪ A so-called dry separation generally used upstream of biological reactors that operate in the dry process, and this with a liquor generally containing more than 15% dry matter inside the biological reactor.

[0005] One of the separation techniques operating in the dry process involves dry pretreatment in an aerobic rotating tube where a particle size reduction of the incoming waste is carried out by mechanical effect and pre-composting with O 2 consumption and CO 2 release, this pretreatment can have a residence time ranging from a few hours to a few days and be followed by adapted mechanical and particle size sorting steps. This pretreatment makes it possible to produce purified organic matter suitable for methanization followed by composting or simple composting with, in both cases, agronomic recovery of the final compost, but it is only suitable for organic waste with a high dry matter content and containing a significant quantity of cardboard. In addition, during the short aerobic degradation occurring in the rotating tube, part of the biodegradable organic matter is permanently lost.The purified organic fraction resulting from this separation technique can be methanized in biological reactors that operate in the dry process and can also be suspended for use in biological reactors that operate in the wet process.

[0006] This type of separation is not effective on wet and sticky organic waste collected selectively (biowaste) or in mixtures (household waste from North Africa and Asia).

[0007] Another dry separation technique is particle size sorting, generally with a mesh size of 40 to 80 mm. This sorting is carried out dry on the organic waste which may have undergone a prior particle size reduction, generally grinding, or not, with an additional step of ballistic separation of heavy inerts, or not. This purified organic fraction can be used in anaerobic biological reactors which operate in the dry process. However, the technique does not show sufficient efficiency when the waste contains a lot of undesirable materials, i.e. 15 to 20%, which generally leads to introducing some of the undesirable materials into the biological reactors. In addition, to consider an agronomic recovery of the digestate, additional treatments are necessary. This type of separation is not effective on wet and sticky organic waste.

[0008] One of the separation techniques suitable for wet and sticky organic waste consists of a wet pretreatment combined with an initial particle size reduction, aqueous suspension as well as one or more particle size separations, and various sedimentation / decantation stages. The final suspension obtained is rich in organic matter. This suspension contains very little undesirable matter and between 6 and 12% dry matter. This technique is suitable for wet and sticky organic waste such as kitchen waste or food industry waste often collected in plastic bags. However, due to the high water content of the fraction resulting from this pretreatment, it is however impossible to directly combine this technique with an anaerobic biological reactor operating in a dry process. The use of an anaerobic biological reactor operating in a wet process is necessary.

[0009] However, the use of an anaerobic biological reactor that operates in a dry or wet process conditions the use of a technique for separating unwanted substances, respectively in a dry or wet process. In the prior art, there is no process combining a separation of unwanted substances carried out in a wet process and fermentation carried out using a biological reactor operating in a dry process. For example, we can cite the process described in patent CA 2063777, which describes a waste treatment process combining the preparation of a suspension of biogenic substances that can be easily dehydrated by mechanical separation with anaerobic digestion, without however describing an intermediate dehydration step.

[0010] At the industrial infrastructure level, the volumes of wet-process anaerobic biological reactors are larger, even for the same substrate load. For the same organic load, which corresponds to the quantity of organic matter introduced into the biological reactor per cubic meter of reacting material per day, the volume of wet-process reactors is larger due to the significant dilution of waste, on average two to five times larger than dry-process fermenters / digesters. Dry-process reactors are more compact and less energy-intensive due to the smaller quantity of water to be heated.

[0011] Compared to a dry-process anaerobic biological reactor, a wet-process anaerobic biological reactor produces a larger quantity of water to be treated. Before recovery, mainly for agricultural purposes, the fermented organic residues after wet-process anaerobic biological treatment must undergo a liquid / solid separation step. This separation is generally carried out mechanically and the volumes of effluent produced require the installation of correctly sized and efficient treatment plants in order to be able to recycle all or part of the water at the start of the process and discharge the non-recycled volumes into the public network or into the natural environment.Furthermore, the partial mineralization of organic nitrogen during the fermentation / digestion phase of organic waste introduces significant quantities of ammoniacal nitrogen into these excess effluents; this requires treatment, because ammoniacal nitrogen is toxic to bacteria, particularly methanogenic bacteria, and must not exceed certain concentrations in anaerobic biological reactors. The treatment of ammoniacal nitrogen in situ by stripping or nitrification / denitrification is complex and expensive. The use of a biological reactor operating in a wet process always generates a quantity of aqueous effluent loaded with ammoniacal nitrogen requiring specific treatment before recycling it upstream of the process. On the other hand, biological treatment in an anaerobic biological reactor operating in a dry process does not always require a liquid / solid separation step and produces a smaller quantity of effluent.In some parts of the world, particularly in Asia, methanization facilities are required to treat very humid organic waste containing few undesirable substances. Current practice involves the use of an anaerobic biological reactor operating in a wet process in association with a separation step of the undesirable substances that can be carried out downstream or upstream of the biological treatment. These solutions have several disadvantages, such as a large footprint, high water and energy consumption, and the production of contaminated effluent that is very expensive and complex to treat.

[0012] There is therefore a need to develop a more energy- and water-efficient process for treating wet and sticky waste, which requires a more compact installation.

[0013] Hartman et al. (Water Science and Technology, 2000(41), 3, 145-153) present a study on the relationship between manure pretreatment methods and biogas yield from fibers separated from manure. Hartman et al.thus describe a process in which 1) the manure is first macerated, then an effluent from the macerated manure is filtered to obtain the fibers, 2) the latter are then pressed to further reduce the water content in the fibers, and 3) the fibers thus obtained are used for the production of biogas, by anaerobic biological treatment. However, manure, which is a mixture of bedding (straw, fodder, etc.) and animal excrement, cannot be considered as organic waste containing impurities composed of non-biodegradable materials. In addition, the process of Hartman et al. does not allow the recovery of the organic part purified by anaerobic biological treatment (biogas production), but the insoluble part (fibers).

[0014] EP 1261432 describes a method for treating solid waste comprising an organic fraction. The method consists of diluting the waste followed by various sieving processes and applying magnets. This is followed by dehydration, in particular by centrifugation, of the purified organic fraction resulting from said step of separating unwanted materials. From said dehydration step comes a purified and dehydrated organic fraction which will then be treated in a composting unit or in a fermentation unit if the waste does not come from a fermentation device. The dehydration step also produces an aqueous fraction composed essentially of water which will be reinjected upstream of the process to dilute the organic waste.

[0015] However, this document does not disclose the possibility of combining wet separation with dry biological treatment.

[0016] Furthermore, dehydration by centrifugation is only suitable for a suspension with a dry matter content of less than 25%.

[0017] Furthermore, due to the very high rotation speed of a centrifuge, it is very sensitive to the presence of abrasive elements in the waste to be pressed and therefore is not a sufficiently robust, high-performance system for this type of waste.

[0018] The present invention aims to propose a technical solution to overcome this lack.

[0019] The invention relates to a method for the biological treatment of organic waste containing impurities composed of non-biodegradable materials, said method comprising: a first step of wet mechanical separation of the non-biodegradable materials present in the above-mentioned organic waste to obtain a purified organic fraction, a second step of dehydration of said purified organic fraction to obtain a purified and dehydrated organic fraction and an effluent, a third step of dry anaerobic biological treatment of the dehydrated organic fraction to obtain organic residues.

[0020] For the purposes of this application, “organic waste” means all waste, whether sorted at source or not, containing a solid fraction and composed of both biodegradable organic matter and non-biodegradable matter.

[0021] “Biodegradable material” means any waste that can be degraded by biological treatment (aerobic or anaerobic).

[0022] The term "non-biodegradable materials" means any waste that cannot be rapidly degraded by biological treatment (aerobic or anaerobic), i.e. that cannot be degraded within a period of less than 5 years, and preferably within a period of less than 1 year. Examples of non-biodegradable materials according to the invention include glass, plastic, textiles, pebbles and fragments of tiles, pottery, etc. and metal elements.

[0023] For the purposes of the present invention, the terms "non-biodegradable material", "non-biodegradable material" and "undesirables" or "impurities" are interchangeable.

[0024] Preferably, said organic waste is moist organic waste and more or less sticky depending on its nature (vegetable, animal) and its state of conservation linked to the season and the collection methods and frequencies implemented with craftsmen, restaurateurs, industrialists or supermarkets or even individuals.

[0025] The term "wet mechanical separation" means a process for separating organic waste consisting of an optional first step of crushing / opening the bags and packaging containing the organic waste followed by a second step of dilution and aqueous suspension and then by a third step comprising one or more particle size separations, such as screening and / or sieving, and / or densimetric separations. The aqueous suspension obtained in this wet mechanical separation is characterized in particular by a relatively low viscosity, such that the suspension can be treated as a low-viscous liquid. In particular, equipment such as pumps capable of transferring liquids is sufficient for transporting the suspension from one treatment device to another in the installation implementing the process.

[0026] The final suspension obtained is purified of unwanted materials and rich in organic matter. Typically, the final suspension obtained has a content of 2% or less, and in particular 0.3% or less, by mass relative to the mass of dry matter for plastics with a particle size > 5 mm in diameter, and 4% or less, and in particular 0.8% or less, by mass relative to the mass of dry matter for heavy inert waste with a particle size > 2 mm in diameter.

[0027] In the following, the dry matter content is expressed in %. The dry matter content corresponds to the MS / MB ratio of the mass of dry matter (MS) obtained after 24 hours of drying at 105°C to the gross mass (GM) which corresponds to the mass of raw material before drying at 105°C, and expressed in %.

[0028] The term "dehydration" refers to a mechanical process that allows part of the liquid phase contained in the suspension to be removed and a purified organic fraction that is pasty to solid and is called dehydrated to be obtained. The dehydrated organic fraction is thus relatively viscous, that is to say that it cannot be considered a liquid. In other words, equipment such as pumps capable of transferring liquids is not sufficient for transporting the suspension from one treatment device to another: specific equipment, capable of transferring viscous products, is necessary.

[0029] Typically, the dehydration step makes it possible to significantly increase the dry matter content of the purified organic fraction: the dehydrated purified organic fraction (FOED) generally comprises at least 10%, generally at least 15% and up to at least 20 to 25% more dry matter (DM) than the purified organic fraction before dehydration (i.e. at the end of step 1) and before step 2), FOE): DM(FOED) = DM(FOE) + 10% to 25%. The increase in dry matter varies depending on the nature of the initial purified organic fraction (i.e. obtained at the end of step 1), its dry matter content and the type of device used to implement this step. A person skilled in the art will in particular know how to adapt the device used to obtain the desired increase in dry matter content.

[0030] “Dry anaerobic biological treatment” means a treatment using anaerobic fermenters / digesters operating in a dry process, in other words without significant addition of water and dilution, in which the anaerobic biological reactions are carried out at dry matter contents of between 10 and 50%, in particular between 12 and 40%.

[0031] In one embodiment, the organic waste containing non-biodegradable materials is wet waste having a dry matter content of less than 50%.

[0032] In another embodiment, the organic waste containing non-biodegradable materials is waste containing less than 20% of non-biodegradable materials (i.e., between 0 and 20% by mass of dry matter of undesirables relative to the total dry matter mass of the raw waste). Typically, the organic waste containing non-biodegradable materials is waste containing between 5% and 20% by mass of dry matter of non-biodegradable materials relative to the total dry matter mass of the raw waste.

[0033] Organic waste can in particular come from selective collections.

[0034] In a more particular embodiment, the organic waste containing non-biodegradable materials is waste from selective collection containing a maximum of 20% of non-biodegradable materials (i.e. between 0 and 20% by mass of dry matter of undesirable materials, and typically between 5% and 20% by mass of dry matter of non-biodegradable materials, these percentages being expressed relative to the total mass of dry matter of the raw waste) and with a dry matter content of less than or equal to 30%.

[0035] In accordance with the present invention, wet mechanical separation is implemented by a variable supply of dilution water for the raw waste, generally recycled and pre-treated water, and by at least one means chosen from grinding, granulometric sorting, decantation, desanding and flotation.

[0036] This step allows the extraction of unwanted materials from organic waste to obtain a purified organic fraction. Mechanical separation methods can be implemented alone or in combination.

[0037] Examples of grinding methods include a grinder or a pulper which defibrates the waste.

[0038] As a means of granulometric sorting, we can cite a cylindrical sieve which can retain, for example, plastics or a sand trap which can be used in this step to separate heavy non-biodegradable materials, such as sand, glass or metals.

[0039] The dry matter content of the waste at the end of the wet mechanical separation stage is approximately 6% to 20%, and preferably approximately 8% to 15%.

[0040] The implementation of a dehydration step on the purified organic fraction obtained at the end of the wet mechanical separation step makes it possible to reduce or eliminate the effluent from the purified organic fraction and to obtain a purified and dehydrated organic fraction which can be treated in a biological reactor operating in the dry process.

[0041] The dry matter content of the waste at the end of the dehydration step of said purified organic fraction is approximately 25% to 50%, preferably approximately 25% to 35%, more preferably approximately 25% to 30%.

[0042] In accordance with the present invention, the third stage of the process is implemented in an anaerobic biological reactor operating in a dry process, in particular an anaerobic fermenter or digester operating in a dry process.

[0043] The dry matter content of the waste at the end of the dry anaerobic biological treatment stage is approximately 12% to 40%.

[0044] Organic residues from the biological reactor can undergo a second dehydration to obtain dehydrated organic residues.

[0045] In an advantageous embodiment of the invention, the dehydration of the purified organic fraction is carried out by mechanical pressing, which is suitable for heterogeneous, fibrous fluids with high dry matter contents.

[0046] Advantageously, mechanical pressing is carried out using a worm screw opposing a force against a filter, or a piston exerting a compressive force against a filter, the purified organic fraction being pressed against the filter, or possibly between several filters. It is thus possible to use a filter press, in particular a piston filter press, or a plate filter press such as equipment from the company FAURE EQUIPEMENT. Mechanical pressing can also be carried out using a worm screw press such as, for example, the STRAINPRESS press marketed by the company HUBER.

[0047] More advantageously, mechanical pressing is carried out by a piston press, such as the Bücher piston press. This type of press makes it possible to extract a significant quantity of water from a purified organic fraction while retaining the majority of the biodegradable solid elements for processing in industrial biological reactors operating in the dry process.

[0048] Compared to dehydration by centrifugation, a piston press is particularly advantageous, because a piston press can work on a suspension with a dry matter content of up to 50% (in particular between 2% and 35%), whereas centrifugation can generally only work on a suspension with a dry matter content of less than 25% (in particular between 2% and 15%).

[0049] Furthermore, the piston press (especially a piston filter press) is more robust and less sensitive to the presence of abrasive elements in the waste to be pressed compared to a centrifuge.

[0050] According to one embodiment, the method of the present invention further comprises an additional step after the second step (dehydration of the purified organic fraction) and before the third step (biological treatment), said additional step consisting of a step of recirculating part of the effluent obtained at the end of the second step to the first step of wet mechanical separation.

[0051] The proportion of effluent to be recycled in the recirculation stage varies depending on the initial dry matter content of the waste and the overall process applied. Those skilled in the art will be able to determine these proportions in light of general knowledge.

[0052] The effluent from the second stage, particularly from pressing, can be more easily recycled at the start of the process.

[0053] Thanks to this recirculation, the process of the invention consumes very little water.

[0054] Furthermore, the ammoniacal nitrogen which is produced mainly during the biological treatment stage in fermenters / digesters is only very little present in the effluent obtained after the dehydration stage of the purified organic fraction which is not or only slightly fermented.

[0055] The effluent obtained after the dehydration stage can undergo biological treatment and / or physicochemical treatment before being recycled to the first stage of wet mechanical separation.

[0056] Said biological treatment can be nitrification / denitrification.

[0057] The said physicochemical treatment can be stripping and / or ultrafiltration as well as reverse osmosis.

[0058] These treatments allow for the removal of more suspended solids from the effluent. Physicochemical or biological treatments produce sludge that can be recycled to the dewatering stage.

[0059] Physicochemical treatment and / or biological treatment can be applied only to part of the effluent obtained in the second stage of the process and requires smaller structures.

[0060] In an advantageous embodiment, the method of the invention comprises an additional step of physicochemical treatment and / or biological treatment of at least part of the effluent before recirculation. This additional step is implemented after the step of dehydration of the purified organic fraction and aims to treat the effluent from a dehydration device. At the end of the physicochemical treatment and / or biological treatment, the effluent is recycled to the first step of the process.

[0061] In another advantageous embodiment, the effluent from the second stage (the dehydration stage) undergoes physicochemical treatment and / or biological treatment after several recycling loops.

[0062] Within the framework of the invention, the method may further comprise a step of pasteurization of the purified organic fraction obtained at the end of the first step (the mechanical separation step) and / or a step of pasteurization of the purified and dehydrated organic fraction obtained at the end of the second step (the dehydration step).

[0063] Pasteurization allows the purified organic fractions to be sanitized in order to comply with regulations on animal by-products insofar as the purified organic fractions to be treated are partly of animal origin and comparable to category 2 or 3 animal by-products as described in the European regulation.

[0064] This sanitization is carried out at 70°C for at least one hour and also allows the purified organic fraction to be preheated before methanizing it.

[0065] In another embodiment, the method of the invention further comprises, after the third step, a step of composting the organic residues.

[0066] According to a preferred embodiment, the carbon / nitrogen ratio of the dehydrated organic fraction is controlled so as to optimize the third stage of anaerobic biological treatment.

[0067] The carbon to nitrogen (C / N) ratio is a parameter commonly used by those skilled in the art, who know how to measure it by conventional methods. It is typically measured using a CHN analyzer or a continuous flow isotope mass spectrometer (CF-IRMS).

[0068] Preferably, the carbon / nitrogen (C / N) ratio is between 20 and 35%, more preferably between 24 and 28%.

[0069] Indeed, particularly in the case of recirculation of effluents such as dehydration effluent, the dehydrated organic fraction may have a nitrogen content that is too high compared to the carbon content, which may have a negative impact on the anaerobic biological treatment stage.

[0070] Thus, the method of the invention may comprise an optional step of adjusting the carbon / nitrogen ratio of the dehydrated organic fraction, typically by increasing the carbon level by adding carbon-rich organic materials, for example by adding waste with a high carbon content.

[0071] For the purposes of the present invention, the term "high-carbon waste" means waste having a C / N rate greater than 20%, preferably between 20% and 100%. Examples of high-carbon waste typically used in the invention are plant waste such as wood chips or woody green waste, but also paper and / or cardboard.

[0072] Furthermore, when the carbon / nitrogen ratio is adjusted by adding high carbon waste, the person skilled in the art knows how to determine the quantity of waste to be added to obtain the optimal carbon / nitrogen ratio range for the third stage of anaerobic biological treatment.

[0073] According to another embodiment of the invention, at least a portion of the effluent from the anaerobic biological treatment - which contains anaerobic microorganisms - is recirculated to the first wet mechanical separation stage or to the second dehydration stage, or is reused in the third anaerobic biological treatment stage.

[0074] The purpose of such recirculation is to maintain the bacterial population in the third stage of anaerobic biological treatment, so as to maintain the yield (or performance level) of said stage.

[0075] There The proportion of effluent from the anaerobic biological treatment to be recycled in the recirculation stage varies depending on the initial dry matter content of the waste and the overall process applied. Those skilled in the art will be able to determine these proportions in the light of general knowledge.

[0076] The effluent from anaerobic biological treatment can be more easily recycled to the head of the process. Thanks to this recirculation, the process of the invention consumes very little water.

[0077] Also described is an installation for implementing the method of the invention as described above.

[0078] The said installation includes: a device or a combination of devices for wet mechanical separation of non-biodegradable materials to obtain a purified organic fraction; a dehydration device for dehydrating the purified organic fraction to obtain a purified and dehydrated organic fraction; a device for dry anaerobic biological treatment of the purified and dehydrated organic fraction, said dehydration device being installed downstream of a mechanical separation device or combination of devices and upstream of a dry biological treatment device.

[0079] In one embodiment, said mechanical separation device is selected from a grinder, an agitator, a cylindrical sieve, a decanter, a flotation tank, a sand trap. These devices can be used alone or in combination.

[0080] In an advantageous embodiment, said dehydration device is a piston press, such as the Bücher piston press. Alternatively, it may be a filter press, in particular a piston filter press, or a plate filter press such as, for example, equipment from the company FAURE EQUIPEMENT. It may also be a worm screw press such as, for example, the STRAINPRESS press marketed by the company HUBER.

[0081] According to an advantageous embodiment, the installation comprises a recirculation loop making it possible to recirculate at least part of the effluent containing anaerobic microorganisms obtained at the outlet of the anaerobic biological treatment device to the inlet of the anaerobic biological treatment device, or to the inlet of the device or combination of wet mechanical separation devices.

[0082] In an advantageous embodiment, said anaerobic biological treatment device is an anaerobic fermenter or digester operating in a dry process.

[0083] Other advantages and particularities of the invention will emerge from reading the description which follows, with reference to the appended figure, which illustrates: The Figure 1illustrates a particular embodiment of the method of the invention. According to this embodiment, the organic waste 100 first undergoes mechanical separation implemented by several mechanical separation devices, namely a grinder 20, an agitator 21, a cylindrical sieve 22 and a sand and floating trap 23.

[0084] The purified organic fraction 200 at the end of this mechanical separation is introduced into a first dehydration device 30. At the end of this device 30, the purified and dehydrated organic fraction 300 is pasteurized in a device 60 and then treated in an anaerobic biological reactor operating in the dry process 40. Organic residues 400 at the end of this treatment are dehydrated in a second dehydration device 31. Dehydrated organic residues 310 at the end of this step are introduced into a composting device 70.

[0085] The effluent at the end of the first dehydration 800 and the effluent at the end of the second dehydration 810 successively undergo a physicochemical treatment 51 and a biological treatment 52. Sludge 900 obtained in these treatments is introduced into the first dehydration device 30. The effluent 820 at the end of these treatments is recycled to the mechanical separation devices, namely the grinder 20, the agitator 21, the cylindrical sieve 22.

Claims

1. A method for biological treatment of organic waste containing impurities composed of non-biodegradable materials, comprising: • a first step of wet mechanical separation of the non-biodegradable materials present in the aforesaid organic waste such as to obtain a purified organic fraction having a dry matter content of 6% to 20%, • a second step of dewatering said purified organic fraction such as to obtain a purified, dewatered organic fraction having a dry matter content of 25% to 50% and an effluent, • a third step of dry anaerobic biological treatment of the dewatered organic fraction such as to obtain organic residues.

2. The method as claimed in claim 1, wherein the organic waste containing non-biodegradable materials is wet waste having a dry matter content of less than 50%.

3. The method as claimed in claim 1 or 2, wherein the organic waste containing non-biodegradable materials is waste containing less than 20% non-biodegradable materials.

4. The method as claimed in any one of claims 1 to 3, wherein the purified organic fraction is dewatered by means of mechanical pressing.

5. The method as claimed in claim 4, wherein mechanical pressing is implemented by a piston press, a worm-gear press, a piston filter press or a plate filter press.

6. The method as claimed in any one of claims 1 to 5, further comprising a supplementary step after the second step and before the third step, said supplementary step consisting in a step of recirculating a portion of the effluent obtained on completion of the second step toward the first wet mechanical separation step.

7. The method as claimed in claim 6, further comprising a supplementary step of physico-chemical treatment and / or a biological treatment of at least a portion of the effluent prior to recirculation.

8. The method as claimed in any one of claims 1 to 7, further comprising a step of pasteurizing the purified organic fraction obtained on completion of the first step and / or a step of pasteurizing the purified, dewatered organic fraction obtained on completion of the second step.

9. The method as claimed in any one of claims 1 to 8, wherein the third step is implemented in a dry digester or fermenter.

10. The method as claimed in any one of claims 1 to 9, wherein wet mechanical separation is implemented via at least one means chosen from grinding, sorting by particle size, settling, desilting and flotation.

11. The method as claimed in any one of claims 1 to 10, characterized in that it comprises an optional step of adjustment of the carbon / nitrogen ratio of the dewatered organic fraction obtained on completion of the second step, typically via increasing the carbon content by adding carbon-rich organic materials.

12. The method as claimed in any one of claims 1 to 11, characterized in that at least a portion of the effluent originating from the anaerobic biological treatment - containing anaerobic microorganisms - is recirculated to the first, wet mechanical separation step or to the second, dewatering step, or is reused in the third step of anaerobic biological treatment.

13. The method as claimed in any one of claims 1 to 12, further comprising, after the third step, a step of composting the organic residues.