Waste treatment process for waste containing biodegradable organic matter

Abstract

The invention relates to a process for treating waste containing biodegradable organic matter. The process comprises a step (a) of separating the solid impurities from said waste within a chamber, followed by a step (b) of anaerobic digestion of the treated waste separated in step (a), during which biogas and a digestate containing dissolved CO2 are produced. The separation step (a) comprises: (a1) a flotation step in which a portion of the digestate from step (b) is brought into contact with said waste within the chamber under pH conditions sufficient to generate CO2 bubbles, at least a portion of the solid impurities being transported by the CO2 bubbles to the surface of said mixture and forming a floating phase; and (a2) a separation of the floating phase. Abstract figure: Fig. 1

Description

Title of the invention: Process for treating waste containing biodegradable organic matter. Field of the invention

[0001] The present invention relates to a method and installation for treating waste containing biodegradable organic matter comprising acidic organic compounds and solid impurities. The method and installation are particularly suitable for removing unwanted materials, including plastics, present in the treated waste. State of the art

[0002] A traditional waste treatment line includes a waste preparation stage (sorting, crushing, etc.), a methanation stage by dry or wet process, and a digestate treatment stage from the methanation of waste, by traditional composting and / or stabilization and / or spreading and / or bio-drying in tunnels, windrows or bags.

[0003] The waste processed by the treatment line consists of organic waste containing inert impurities. This organic waste includes waste of both plant and animal origin. It specifically includes biowaste as defined by Directive 2008 / 98 / EC, namely food waste, particularly from catering establishments, but also pulp from industrial waste depackaging facilities, biodegradable garden or park waste, and mixtures of urban, agricultural, and industrial organic waste. This biowaste thus includes organic compounds forming a fermentable fraction, as well as solid impurities that are not degradable by fermentation.

[0004] The preparation stage, also called "pre-treatment", usually includes one or more of the following treatments: - grinding, - dilution with water, - mixing and / or pulping, - one or more mechanical separations, - sanitization.

[0005] Grinding makes the organic matter present in packaging accessible. It can utilize separator grinders (with integrated screen), possibly with the addition of water.

[0006] Dilution with water makes the waste stream more fluid and the organic matter more accessible. The latter can then be recovered with a better yield.

[0007] However, dilution requires significant water consumption and an increase in the volume of flows to be treated in the various subsequent stages, which reduces the profitability of the treatment lines.

[0008] Mechanical separation(s) make it possible to remove some of the solid impurities present in waste, such as glass, plastics, and metals. Mechanical separation can involve screening, possibly integrated into the crusher, debarking, aeration, flotation, sedimentation, or centrifugation. However, the separation efficiency can vary depending on the quality of the incoming waste.

[0009] Sanitization is a step often mandated by legislation. It consists of killing at least some human and animal pathogens by heating. It also allows for the elimination, at least in part, of phytopathogens, quarantine organisms and plant pests, fungi and viruses, as well as seeds present in waste.

[0010] Existing waste treatment technologies require a compromise between increasing the quality of the treated waste stream and reducing the amount of untreated waste exiting the entire treatment line. Indeed, improving the quality of the treated waste stream necessitates more selective sorting, thus increasing the amount of untreated waste exiting the line and consequently reducing the volume of the treated waste stream. Conversely, reducing the amount of untreated waste exiting the line requires less selective sorting, which will decrease the quality of the treated waste stream.

[0011] The invention aims to remedy at least in part the disadvantages mentioned above, and in particular aims to optimize the waste treatment process in order to be able to treat all qualities of incoming waste. Summary of the invention

[0012] In order to solve this problem, a process for treating waste containing biodegradable organic matter is proposed, said waste comprising acidic organic compounds and solid impurities, said process comprising the following steps: - a) a step of separating solid impurities from said waste within an enclosure during which purified waste is produced; - b) an anaerobic digestion stage of the treated waste separated in stage a) during which biogas and digestate containing dissolved CO2 are produced.

[0013] According to the invention, the separation step a) comprises: a1) a flotation step during which a portion of the digestate from step b) is brought into contact with said waste within the enclosure under pH conditions sufficient to generate CO2 bubbles, at least a portion of the solid impurities being transported by the CO2 bubbles to the surface of said mixture, and, a2) a separation of a floating phase located within the enclosure to the surface of said mixture, the floating phase containing a portion of the solid impurities and / or solid impurities adhering to CO2 bubbles rising to the surface of said mixture.

[0014] According to the invention, circulating the digestate, namely returning a portion of the digestate produced in step a) of separation, increases the efficiency of solid impurity removal. Indeed, the digestate contains dissolved CO2, particularly in the form of carbonates, which will be volatilized upon contact with the waste, the latter being acidic due to the presence of acidic organic compounds (humic acids, volatile fatty acids, long-chain fatty acids, etc.). This volatilization generates CO2 bubbles in the digestate-waste mixture. The generated CO2 bubbles accelerate the flotation of solid impurities by adhering to them, thus lowering the density of the solid impurities. CO2 bubbles can also be generated by the fermentation of organic compounds present in the waste, as the digestate contains fermentative bacteria capable of carrying out biological fermentation.

[0015] Furthermore, the addition of digestate to the waste reduces the viscosity of the mixture, as digestate is less viscous than waste. This reduction in viscosity results in a more fluid flow of the resulting mixture, and therefore better separation of solid impurities. This reduction in viscosity can also result from the heat potentially provided by the digestate when it is directly conveyed from the digester to the separation chamber. In addition, the digestate also provides hydrolytic enzymes that can accelerate the size reduction of certain biodegradable organic compounds, further reducing the viscosity of the mixture and improving the separation of light impurities (which tend to float).

[0016] Circulating the digestate also reduces surface tension, which promotes the flotation of the lightest solid impurities. Low surface tension promotes the adhesion of CO2 bubbles to solid impurities, reducing the density of the bubble-solid impurity complex. This reduction in surface tension stems primarily from the humic acids provided by the digestate, known for their surfactant effect, but also from other surfactants naturally present in waste, such as volatile fatty acids. long-chain fatty acids and proteins. In addition, when the digestate provides heat, surface tension decreases as the temperature increases, and a higher temperature promotes the flotation of the lightest solid impurities.

[0017] Furthermore, circulating the digestate within the separation chamber, then recirculating it within a digester mixed with biodegradable organic matter during step b), makes it possible to reduce the CO2 content in the biogas produced in step b) and therefore to increase the percentage of methane in the biogas, without changing the residence time of the digestate in the digester.

[0018] Thus, the addition of digestate to the waste during step a) results in the flotation of the lightest solid impurities without the need to use (inject) a flotation gas, such as air, due to the synergistic effect of spontaneous CO2 bubbling, viscosity reduction, and surface tension reduction. Implementing flotation separation without air injection also avoids the degradation of organic matter by such air injection and eliminates the need for air diffusion systems, which are known to be costly in terms of both investment and maintenance.

[0019] Preferably, the amount of digestate introduced into the enclosure of step a) of separation can be adjusted so as to maintain the pH of the mixture at a value sufficient to cause the generation of CO2 bubbles.

[0020] This improves pH control and optimizes CO2 bubble generation and consequently the flotation of the lightest solid impurities.

[0021] Advantageously, the digestate produced during step b) can be sent to step a) without an intermediate step and / or without cooling.

[0022] In other words, the digestate does not undergo any chemical or physical treatment before entering the separation chamber and / or is not cooled by a device installed between the digester and the separation chamber. The temperature of the digestate exiting the digester is therefore approximately equal to the temperature of the digestate entering the separation chamber. The digestate thus imparts heat to the waste. The digestate and / or the mixture can then advantageously be at a temperature of 30°C to 60°C. This temperature increase causes a reduction in the viscosity of the mixture as well as a decrease in the surface tension of the CO2 bubbles, promoting the adhesion of solid impurities to them.

[0023] Advantageously, prior to step a) of separation, the waste may be subjected to at least one step chosen from (i) a step of grinding said waste, (ii) a hygienization step, (iii) a gravity separation step and (iv) a hydrolysis step.

[0024] Such pretreatment of waste makes it possible to reduce incoming solid impurities and / or to prepare the waste for treatment such as digestion.

[0025] In one embodiment, the separation step a) may include a3) a gravity separation at the bottom of the enclosure of a portion of the solid impurities.

[0026] Gravity separation a3) allows the removal of solid impurities that are too dense to float, which settle to the bottom of the separation chamber from where they can be discharged. It should be noted that the presence of hydrolytic enzymes in the digestate can accelerate the size reduction of certain biodegradable organic compounds, also leading to a reduction in the viscosity of the mixture and improved separation of both heavy impurities (tendency to settle) and light impurities (tendency to float).

[0027] Optionally, the process may include a step c) of phase separation, for example by filter press, of the floating phase to separate a first liquid phase containing organic compounds sent to step b) of digestion and a second phase containing solid impurities.

[0028] Phase separation makes it possible in particular to separate the solid impurities from the organic compounds present in the floating phase in order to return the latter to the step b) of digestion in which they can be transformed into methane, thus increasing the overall biogas yield.

[0029] The invention also relates to a waste treatment plant for waste containing biodegradable organic matter, said waste comprising acidic organic compounds and solid impurities, capable of implementing the process as described above, said plant comprising: - a separation chamber for at least part of the solid impurities of said waste, said chamber including an outlet for the purified waste produced; - a digester capable of implementing anaerobic digestion of the treated waste to produce biogas and a digestate containing dissolved CO2, said digester being connected to the outlet of the treated waste from the enclosure, and said digester comprising a biogas outlet and a digestate outlet.

[0030] According to the invention, said enclosure comprises: - a digestate inlet connected to the digestate outlet of the digester, - a management device configured to bring the digestate into contact with the waste located inside the enclosure under pH conditions sufficient to generate CO2 bubbles, and - at least one device for separating a floating phase located within the enclosure at the surface of said mixture, the floating phase containing some of the solid impurities and / or solid impurities adhering to CO2 bubbles rising to the surface of said mixture.

[0031] The method according to the invention can in particular be implemented by the installation according to the invention.

[0032] Advantageously, the management device can, in addition, be configured to adjust the amount of digestate sent into the enclosure and maintain sufficient pH conditions to generate CO2 bubbles.

[0033] Advantageously, the digestate inlet of the enclosure can be connected to the digestate outlet of the digester without an intermediate enclosure capable of implementing an intermediate stage and / or without an intermediate cooling device.

[0034] Advantageously, the installation may include at least one of the following features: - at least one waste treatment unit located upstream of the unit in relation to the waste flow and chosen from (I) a grinding unit, (II) a sanitizing unit, (III) a gravity separation unit, (IV) a hydrolysis unit; - the separation unit includes at least one device for mixing the digestate with the waste, - the separation chamber includes a gravity separation device at the bottom of the chamber for part of the solid impurities; - the installation includes a phase separation device, for example by filter press, of the floating phase comprising an inlet connected to an outlet of the floating phase of the separation device of the enclosure, an outlet of a first liquid phase containing organic compounds connected to an inlet of the digester and an outlet of a second phase containing solid impurities.

[0035] Advantageously, the installation according to the invention, and in particular its separation enclosure, can be devoid of air diffusion systems. Detailed description of the invention Definitions / abbreviations

[0036] Volatile matter (VM) refers to the portion of matter that can be volatilized at 550 °C. The volatile matter content of a sample is determined gravimetrically by calcination at 550 °C of the material obtained after drying at 105 °C. The volatile matter content is expressed in g / L of sample, most often in g / kg for waste containing biodegradable organic matter.

[0037] Dry matter (denoted DM) includes both suspended solids and dissolved salts. The dry matter content is expressed in g / L of sample, most often in g / kg for waste containing biodegradable organic matter, and can be determined according to standard NF EN 12880- Nov 2000.

[0038] In what follows, the dry matter content is expressed as a percentage. The dry matter content corresponds to the ratio DM / MB of the mass of dry matter (DM) 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 is expressed as a percentage. Waste treated

[0039] The waste to be treated by the present invention contains biodegradable organic matter.

[0040] The waste may include: - Dry matter (dryness) 20 to 45% by mass; - Volatile matter / dry matter: 60 to 95% by mass; - Undesirable rate: from 2 to 10% by mass; - Undesirable materials: glass, metals, plastics, textile fibers. Among the undesirable materials, glass represents approximately 5% by mass.

[0041] In addition, the waste treated by the present invention includes organic compounds, typically proteins and acidic organic compounds such as humic acids, volatile fatty acids such as acetic acid or propionic acid, and long chain fatty acids such as palmitic acid.

[0042] The waste treated by the present invention thus generally has an acidic pH, typically less than or equal to 5, for example 4. Detailed description of the process

[0043] The process according to the invention is a waste treatment process for removing solid impurities, particularly the lightest ones, that the waste may contain, and in particular lightweight plastics, especially macroplastics (>2-5 mm) and / or microplastics. This removal is achieved in particular by recirculating digestate from a digester to generate bubbles that serve as carriers for the solid impurities, especially the small ones. Preliminary step to the separation step a)

[0044] Prior to step a) of separation, the waste may be subjected to at least one step chosen from (i) a step of grinding said waste, (ii) a hygienization step, (iii) a gravity separation step and (iv) a hydrolysis step.

[0045] These preliminary steps allow the waste to be prepared before treatment in order, for example, to remove some of the solid impurities or to make the organic matter more accessible. They can also reduce the viscosity of the waste before its treatment by separation according to step a) of the invention.

[0046] The grinding step (i) makes it possible to make accessible the organic matter present for example in packaging.

[0047] Step (i) can be carried out in a grinding chamber. The grinding chamber may include one or more mills, optionally with a water supply. These mills may also be separator mills comprising integrated screens to perform separation by particle size.

[0048] Step (ii) of sanitization consists of eliminating at least some of the human and animal pathogens. It also allows for the elimination, at least in part, of phytopathogens, quarantine organisms and plant pests, fungi and viruses, as well as seeds present in the waste.

[0049] The objective of sanitization is therefore to eliminate or inactivate at least some of the microorganisms in waste that have pathogenic effects on plants, animals, and humans, so that there is a minimal risk of disease transmission. Sanitizing these substances ensures their regulatory compliance in terms of epidemic hygiene.

[0050] The sanitization step (ii) is typically carried out in a sanitization chamber comprising one or more tanks in which the waste is brought to a minimum high temperature of 50 °C, for example from 50 °C to 140 °C. Preferably, the temperature is at least 70 °C.

[0051] During the sanitization step (ii), the waste is brought to a high temperature for 1 minute to 48 hours, preferably for at least one hour.

[0052] Step (iii) of gravity separation allows for the separation of so-called "heavy" solid impurities and / or so-called "light" solid impurities from the rest of the waste. Indeed, some solid impurities such as metals, glass, and pebbles can be heavier than the rest of the waste present, while some plastics are generally lighter than the rest of the waste. They can therefore be separated from the waste using a gravity-based principle.

[0053] Step (iii) of separation can be carried out in a gravity separation chamber. The gravity separation chamber may include one or more settling tanks and / or flotation chambers and / or one or more hydrocyclones. A portion of the solid impurities is then recovered from the bottom of the separation chamber (heavier impurities such as stones, pebbles, gravel, glass, metal, etc.) or from the top of the chamber on the surface of the waste (lighter impurities, such as plastics). In general, at least some of the heavier solid impurities are separated first before at least some of the lighter solid impurities are separated.

[0054] Step (iv) of hydrolysis consists of breaking down complex molecules, mainly organic molecules (e.g. carbohydrates, proteins and lipids) into molecules of lower molecular mass, such as sugars, amino acids and fatty acids.

[0055] Hydrolysis can be carried out by acidic means, for example with sulfuric acid, by basic means, for example with hydrated lime, or by biological means using natural enzymes supplied or produced in situ by bacteria or fungi.

[0056] This step can be carried out in a hydrolysis chamber with an inlet line of an acidic compound, a basic compound, or natural enzymes, typically for a duration of 3 hours to 7 days for biological hydrolysis or 5 minutes to 1 hour for acidic or basic hydrolysis. The temperature of this step can be from 30 to 60 °C.

[0057] In the case of acid hydrolysis, the pH of the waste in the hydrolysis chamber can be from 0 to 4, while in the case of basic hydrolysis, the pH of the waste in the hydrolysis chamber can be from 10 to 14.

[0058] In the case of biological hydrolysis, the natural enzymes used may be, for example, cellulases, lipases or proteases.

[0059] The pre-step may include one or more of steps (i) to (iv), in various combinations. For example, the pre-step may include only a grinding step (i) or a grinding step (i) followed by a sanitizing step (ii). In another embodiment, the pre-step may include (i) a grinding step followed by (ii) a sanitizing step and then a hydrolysis step (iv). In these various combinations, at least one gravity separation step (iii) may be carried out before or after the grinding step (i), preferably before.

[0060] Step a) of separation

[0061] The separation step a) aims to separate the solid impurities, particularly the lightest ones, present in the waste to produce purified waste which is then sent to the digestion step b).

[0062] The waste treated in step a) typically has a dry matter content of 15 to 30%.

[0063] Step a) of separation is typically implemented in a separation chamber equipped with a waste inlet and a purified waste outlet. The separation chamber is further equipped with at least one separation device to implement a separation a2) of a floating phase, as described below.

[0064] The separation enclosure can optionally be equipped with at least one mixing device.

[0065] By floating phase, we mean a phase located on the surface of the liquid medium present in the containment, namely on the surface of the waste and digestate mixture. This floating phase contains some of the solid impurities, and in particular some of the solid impurities less dense than the waste, such as plastics, and in particular highly fragmented plastics, e.g. macroplastics (especially small ones) and / or microplastics. This floating phase may also contain solid impurities adhering to CO2 bubbles and thus forming a foam, as explained below.

[0066] Microplastics are plastic fragments whose largest external dimension is 5 millimeters. Typically, their smallest external dimension is 1 micrometer. Macroplastics are fragments whose largest external dimension is at least a millimeter, in particular at least 2 to 5 mm.

[0067] The separation device may, for example, include an arm for scraping the surface of the mixture and removing it, for example via an overflow.

[0068] An example of a chamber that can be used for implementing step a) is, for example, a chamber with an overflow. This chamber is typically cylindrical in shape. The chamber may advantageously have a conical bottom for the removal of heavier impurities. However, the invention is not limited to a particular chamber shape, and non-cylindrical chambers are conceivable. In particular, the bottom of the chamber may then have a tapering shape, similar to a funnel, but not necessarily conical.

[0069] During step a) of separation, the waste may also be subjected to a3) gravity separation at the bottom of the enclosure. Gravity separation allows for the separation of another portion of the solid impurities that are denser than the waste. These "dense" solid impurities may be, for example, glass and / or pebbles and / or metals and / or larger pieces of plastic.

[0070] The gravity separation of solid impurities is carried out at the bottom of the enclosure, for example, by a gravity separation device. The gravity separation device may include a conduit for removing the "dense" solid impurities. For this purpose, the bottom of the enclosure may advantageously be conical or funnel-shaped to facilitate the recovery and removal of these denser impurities. Any narrowed shape of the bottom is thus conceivable, as described above.

[0071] This gravity separation step a3) can however be omitted if the heaviest solid impurities have already been removed, for example by one or more prior gravity separation steps (iii) as previously described.

[0072] The purified waste produced during step a) of separation is evacuated from the enclosure via a waste outlet pipe and conveyed to a digester.

[0073] Step b) of anaerobic digestion

[0074] Once separated, the purified waste is subjected to an anaerobic digestion step to produce a digestate containing dissolved CO2 and biogas.

[0075] Anaerobic digestion, or methanation, corresponds to a cascade of well-known biochemical reactions that allow microorganisms to convert the organic matter present in a digester into biogas. The biogas can be used, possibly after cleansing and purification. The remaining matter is called digestate.

[0076] Biogas is a gaseous mixture generally saturated with water and typically composed of approximately 50% to 70% by volume of methane (CH4), 30% to 50% by volume of carbon dioxide (CO2), and some trace gases (NH3, N2, H2S, etc.). Biogas is a renewable energy source that can be used for the production of electricity and heat and / or fuel.

[0077] The anaerobic digestion step is advantageously an anaerobic digestion step carried out in liquid form.

[0078] In general, anaerobic digestion can be carried out at a temperature of 30 to 60°C, under mesophilic or thermophilic conditions, preferably from 35 to 55°C. The conditions for carrying out this step, in particular the temperature, pH and residence time, can advantageously be chosen in order to maximize biogas production.

[0079] Waste digestate containing biodegradable organic matter, rich in stabilized organic matter, is alkaline, typically with a pH of 7 to 9.

[0080] The digestate may include fermentative bacteria as well as hydrolytic enzymes capable of carrying out fermentation and accelerating the degradation of organic matter. It may also include organic compounds, for example, proteins, volatile fatty acids, humic acids, and long-chain fatty acids.

[0081] The digestate also includes dissolved carbon dioxide (CO2) in the form of carbonates or bicarbonates. The pKa of the CO2aq / HCO3 couple in the aqueous phase is approximately 6.4, while the pKa of the HCO3 / CO32 couple is 10.3. Thus, at the pH of the digestates, the dissolved CO2 is present essentially in the form of hydrogen carbonate.

[0082] The digestion step b) is typically implemented in a digester.

[0083] For example, the digester can be an enclosure closed by an upper wall, defining two volumes: a first volume containing the purified waste and the digestate generated, and a second volume between the first volume and the upper wall, also called the gaseous head, and towards which the generated biogas rises.

[0084] The digester includes a treated waste inlet connected to the treated waste outlet of the separation chamber. At the digester inlet, the treated waste, mixed with the digestate that has been sent to the separation chamber, may have a dry matter content of 10 to 25%.

[0085] The digester further comprises a biogas outlet and a digestate outlet. At the outlet, the digestate may have a dry matter content of 5 to 10%.

[0086] Recirculation of the digestate in step a) of separation

[0087] Once produced, a portion of the digestate is introduced into the separation chamber and brought into contact with the waste under pH conditions sufficient to generate CO2 bubbles. For this purpose, the separation chamber includes a digestate inlet connected to the digestate outlet of the digester.

[0088] Preferably, the digestate is sent to the separation chamber without an intermediate step and / or without cooling. This means that the digestate does not undergo any chemical or physical treatment before entering the separation chamber, such as dehydration, thickening, or other processes. It is also not cooled by a device installed between the digester and the separation chamber. The temperature of the digestate leaving the digester is then substantially equal to the temperature of the digestate entering the separation chamber, which reduces the viscosity of both the digestate and the waste.

[0089] However, it is possible to subject the digestate to separation in order to recover a liquid fraction of the digestate, which would then be recirculated within the separation unit. It is also possible to store the digestate, or a liquid fraction thereof, in a buffer tank before sending it to the separation unit.

[0090] Once in the separation chamber, the digestate and waste can optionally be mixed by a mixing device.

[0091] A control device is further configured to carry out the contact under pH conditions sufficient to generate CO2 bubbles.

[0092] Indeed, the digestate has a basic pH, while the waste has an acidic pH. Thus, when the waste and the digestate come into contact, the pH is lowered, allowing the release of carbon dioxide in gaseous form, thereby generating CO2 bubbles. Degassing can be observed, in particular, when the pH is approximately 8.5 or lower, for example, between 8.5 and 4, preferably from 8.2 to 4.5. Thus, conditions sufficient for generating CO2 bubbles typically include a mixture pH of 8.5 or lower, preferably 8.2 or lower. It should be noted that the maximum pH below which degassing can be observed may depend on the nature of the waste containing biodegradable organic matter and / or the digestate and can be determined by those skilled in the art through tests and / or models.

[0093] At least some of the solid impurities in the waste can then be transported by the CO2 bubbles to the surface of the mixture and form a floating phase. Indeed, the CO2 bubbles will adhere to solid impurities such as plastics, and in particular microplastics and / or macroplastics (especially smaller ones), thus lowering the density of the entire bubble-solid impurity mixture, causing it to float. A foam then forms on the surface of the waste and digestate mixture. Furthermore, Solid impurities less dense than waste can naturally rise to the surface, this rise being favored by the rise of CO2 bubbles associated with solid or non-solid impurities.

[0094] The introduction of digestate into the enclosure thus allows a separation by flotation a1) of the lightest solid impurities.

[0095] The transport of solid impurities by CO2 bubbles can also be facilitated by organic compounds such as volatile fatty acids, humic acids, proteins, and long-chain fatty acids present in the digestate and waste. Indeed, organic compounds such as humic acids, volatile fatty acids, long-chain fatty acids, and proteins act as surfactants, thus reducing the surface tension of the solid impurities. This reduction in surface tension promotes the adhesion of CO2 bubbles to the solid impurities and therefore their transport to the surface of the mixture.

[0096] Furthermore, the digestate exiting the digester has a relatively high temperature, for example, from 30°C to 60°C. Introducing digestate directly from the digester, without an intermediate step or device, into the waste increases the temperature of the waste. Since the temperature is higher, the surface tension is also reduced, thus promoting the adhesion of CO2 bubbles to solid impurities.

[0097] A sufficient pH for generating CO2 bubbles can be obtained by adjusting the amount of digestate introduced into the waste. This adjustment can be made by the management system based on the pH of the waste, and / or the pH of the digestate, and / or the amount of dry matter present in the treated waste and / or in the digestate, and / or the operating conditions of the digester, which can be measured and / or estimated. In particular, the quantities of digestate to be added can be determined beforehand through tests and / or models, depending on the nature of the waste, the nature of the digestate, and / or the operating conditions of the digester.

[0098] In addition to pH control, the management device can also regulate the residence time, temperature and mass ratio between waste and digestate during the implementation of step a).

[0099] For example, the residence time of the waste / digestate mixture within the separation chamber can be from a few minutes to 7 days, preferably from a few minutes to 4 days, and more preferably from 30 minutes to 2 hours. A person skilled in the art may advantageously choose a residence time short enough to prevent hydrolysis reactions from occurring during step a).

[0100] To improve separation, step a) can advantageously be carried out at a temperature of 30 °C to 60 °C, preferably 35 to 55 °C. Preferably, the digestate introduced into the separation chamber is at a temperature of 30 °C to 60 °C, preferably 35 to 55 °C. It is then unnecessary to equip the chamber with a heating device. Nor is it necessary to heat the digestate when it comes directly from the digester.

[0101] The mass ratio between waste and digestate within the separation chamber can be from 0.25 to 0.75.

[0102] Preferably, in order to promote the generation of CO2 bubbles as close as possible to the bottom of the separation chamber and thus optimize the separation of the lightest solid impurities, the digestate can advantageously be introduced into the separation chamber in the lower part thereof, in particular under the waste inlet.

[0103] Optional step c) of phase separation of the floating phase

[0104] The floating phase exiting the separation step contains some of the solid impurities and / or CO2 bubbles associated with the solid impurities that have risen to the surface of the mixture. It may also contain organic compounds such as long-chain fatty acids, proteins, and / or particulate organic matter that have been carried to the surface of the mixture.

[0105] At the exit of the separation step, the floating phase can then be subjected to phase separation, for example using a filter press or any other suitable phase separation device, in order to separate a first liquid phase containing these organic compounds and a second phase having a reduced liquid content and the solid impurities initially present in the floating phase.

[0106] Phase separation thus makes it possible to separate solid impurities from organic compounds. The first phase is then sent to step b) of digestion during which the organic compounds will be transformed into methane, thereby increasing the overall biogas yield.

[0107] Step c) of phase separation is typically implemented by a phase separation device, for example by filter press, comprising an inlet connected to an outlet of the floating phase of the separation device of the enclosure, an outlet of a first liquid phase containing organic compounds connected to the inlet of the treated waste of the digester and an outlet of a second phase containing the solid impurities.

[0108] A filter press separation device may include one or more filter presses.

[0109] Preferably, the filter press used is a plate filter press comprising a bank of vertical hollow plates pressed together by a moving head to form trays. Filter cloths cover each tray.

[0110] The floating phase is typically introduced via a pump to a pressure sufficient to ensure the operation of the filter press (generally 7 to 15 bar). Under this pressure, the liquid passes through the filter cloth, which retains the solid particles. This filtrate is collected either at each tray or at the end of the filter.

[0111] A filter press can also be equipped with membrane plates. The membrane, deformable under the action of air or pressurized water, allows the filter cakes to be compacted following the filtration phase. In this case, the filtration pressure is most often 8 bar, and it is the compaction that ensures the pressurization of the cakes to 15 bar. Description of the drawings

[0112] The invention will be better understood with reference to [Fig.1] representing a waste treatment plant according to an embodiment of the invention.

[0113] In the figure, the arrows represent the direction of waste and / or flow within the installation. The waste flows between the different enclosures of the installation via pipes or conveyor belts, depending on the viscosity of the waste entering and exiting the enclosures of the installation.

[0114] With reference to [Fig. 1], the treatment plant 1 comprises a separation chamber A connected to a waste inlet 3. A pump and / or a valve (not shown in the figure) may be provided to regulate the flow rate of the waste feeding the inlet 3.

[0115] The waste inlet 3 can be located at any level within the enclosure. In the example shown, the waste inlet is located at the top of the separation enclosure A.

[0116] Optionally, prior to their entry into enclosure A, the waste is sent to at least one enclosure D chosen from (I) a grinding enclosure, (II) a sanitizing enclosure, (III) a gravity separation enclosure, (IV) a hydrolysis enclosure.

[0117] The enclosure D includes a waste inlet 5 and a waste outlet 9 connected to the waste inlet 3 of the separation enclosure A, and optionally an inlet 7 of ancillary compounds depending on the prior step used.

[0118] The grinding chamber (I) may include one or more mills, optionally with a water supply. These mills may also be separator mills comprising integrated screens to perform separation by particle size.

[0119] The sanitization enclosure (II) may include one or more tanks in which the waste is brought to a high temperature, as previously described.

[0120] The gravity separation enclosure (III) may include one or more settling tanks and / or flotation tank and / or one or more hydrocyclones.

[0121] The hydrolysis enclosure (IV) may include one or more tanks comprising an inlet line for an acidic compound, a basic compound or natural enzymes.

[0122] The separation enclosure A is also equipped with an outlet 10 for the purified waste produced.

[0123] The separation enclosure A may comprise one or more tanks in series and / or in parallel.

[0124] The separation chamber A here includes a digestate inlet 11 connected to a digestate outlet 13 of a digester B to receive a portion of the digestate produced by the latter. This digestate inlet 11 is located below the waste inlet 3, near the bottom of chamber A. It should be noted that the digestate may not be introduced directly into chamber A but may be stored in an intermediate buffer storage chamber. Furthermore, it may also be possible to send only a liquid fraction of the digestate, recovered via a separation device, into the separation chamber A.

[0125] In this embodiment, the separation chamber A further comprises a mixing device Al for the digestate and the waste. The mixing device is selected according to the viscosity of the mixture. It can be chosen, for example, from a vertical mixer, a paddle mixer, or a ribbon mixer. Such a mixing device Al helps to prevent dead zones within the chamber but is not essential and can be omitted.

[0126] The separation chamber A also includes a separation device A2 for the floating phase formed on the surface of the mixture. The invention is not limited to a specific separation device A2, and any device suitable for separating a floating phase in a flotation chamber may be used (scraping, overflow, or spillway device, etc.). The separation device A2 includes, in particular, an outlet 14 for the floating phase.

[0127] The invention is not limited to a particular number of mixing devices A1 and / or separation devices A2. Depending on the dimensions of the separation chamber A, one or more mixing and / or separation devices may be provided. Alternatively, the separation chamber A may comprise only one or more separation devices A2.

[0128] To control the installation, the separation enclosure A includes a configured management device A3, in particular programmed, to bring the digestate into contact with the waste under pH conditions sufficient to generate CO2 bubbles.

[0129] The A3 control device is described in detail below. It typically comprises one or more processors, for example, microprocessors or microcontrollers. The processor(s) may have storage means, which may be random access memory (RAM), electrically erasable programmable read-only memory (EEPROM), flash memory, external memory, or other. These storage means may, among other things, store received data, a control model, and one or more computer programs. The A3 control device also includes communication means, optionally bidirectional, with a control system and / or with sensors and / or measuring and / or determining means.

[0130] The control system typically includes means for adjusting the operating parameters of the installation, and in particular the quantity of digestate entering the separation chamber A. These adjustment means are, for example, one or more valves, pumps, etc., in communication with the management device A3.

[0131] The control system can thus typically include one or more valves, solenoid valves and / or pumps, regulating the quantities of digestate and / or waste entering and leaving the separation chamber A, and optionally a temperature maintenance system for the separation chamber A, one or more sensors, for example chosen from a pH meter of the digestate, a pH meter of the mixture within the chamber A, a temperature sensor, a flow sensor of the flows entering and leaving the separation chamber A, and / or a sensor for measuring the organic matter of the digestate, cooperating with each other and with the management device A3.

[0132] The control system may further include a control loop allowing the operating parameter(s) to be modified according to data received from the sensors and operating parameter(s) received from the determination system.

[0133] Optionally, the control device A3 is configured, in particular programmed, to adjust the quantity of digestate sent into the chamber. For this purpose, the control system may, for example, include a pH meter 24 installed within the digestate / waste mixture, for example on the wall of the separation chamber A, and a valve 25 mounted at or upstream of the digestate inlet 11. Thus, when the measured pH of the mixture is above a threshold pH sufficient to generate CO2 bubbles, the valve 25 is closed, and vice versa. However, the invention is not limited to this arrangement, and other configurations of the control system may be provided.

[0134] Optionally, the separation chamber A includes a gravity separation device A4. This device A4 allows the removal of solid impurities deposited at the bottom of the chamber. It is therefore installed at the bottom of the chamber A, which is conical in shape. The A4 device may then include a solid impurity removal line 26 and / or a scraping system for removing them. In the latter case, it may be preferable for the bottom of the enclosure A not to be conical. Such a gravity separation device may be omitted if the densest solid impurities have been removed during one or more prior gravity separation stages.

[0135] Installation 1 also includes a digester B.

[0136] The digester includes a treated waste inlet 15 connected to the treated waste outlet 10, which is discharged from the separation chamber A. The digester further includes a biogas outlet 17 and a digestate outlet 13 connected to the digestate inlet 11 of the separation chamber A.

[0137] Preferably, as shown in [Fig. 1], the installation does not include an intermediate chamber capable of implementing an intermediate stage and / or a cooling device between the digestate outlet 13 and the digestate inlet 11 of the separation chamber A. In other words, the digestate exiting digester B is sent directly to the separation chamber A.

[0138] Optionally, the floating phase is recycled back into the process. The installation then includes a phase separation device C, for example by filter press, for the floating phase, comprising an inlet 19 connected to an outlet 14 of the floating phase from the separation device A2. The device C also includes an outlet 21 of a first liquid phase containing organic compounds connected to the inlet 15 of the treated waste from digester B (or to another inlet of the digester not shown), and an outlet 23 of a second phase containing the solid impurities. Example

[0139] Example: Treatment of waste consisting of ground-up food biowaste.

[0140] The waste treated in this example consists entirely of food biowaste. The waste to be treated has undergone a preliminary grinding and sieving stage to obtain waste particles with a diameter of no more than 2 millimeters. The pH of the waste entering the separation chamber is 3.7. Before entering the separation chamber, the waste is also preheated to a temperature of 38°C.

[0141] A first test is carried out using a digestate from an anaerobic digester in which wastes as described above have been treated. The digestate from this first test has a pH of 8.3 and is at a temperature of 38°C.

[0142] In this first test, plastics are added separately to the separation chamber in order to be able to easily analyze their distribution in the chamber.

[0143] These plastics are derived from a mixture of food-grade plastics comprising equal mass amounts of polyethylene terephthalate (PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene high-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS). The plastics were also shredded separately to obtain waste with a diameter of no more than 2 millimeters.

[0144] The separation chamber used is a cylindrical pilot chamber with a flat bottom having two guillotine valves allowing the chamber to be separated into a lower zone, a central zone and an upper zone, representing respectively 20, 60 and 20% of the total useful volume of the mixture inside the chamber.

[0145] Seven liters of waste and seven liters of digestate are introduced into the separation chamber. Sixty grams of plastics, larger than 2 mm, are homogeneously added to the mixture. The test is carried out without external heating of the mixture.

[0146] In the separation chamber, an agitator is used to homogenize the mixture, with the guillotine valves open. During mixing, significant foaming was observed due to the generation of CO2 bubbles. The foaming was observed at the surface of the mixture and corresponds to the floating phase. This foaming represents approximately 10% by volume of the useful liquid.

[0147] The separation is carried out for 1 hour, then the enclosure is divided into three zones by closing the guillotine valves. The floating material from each zone (i.e., the floating phases in each zone) is then collected and washed with bleach to remove the organic matter present and recover only the plastics. The recovered plastics are then dried before being weighed.

[0148] At the outlet of the separation stage, the plastic recovery rate is 72% in the upper zone, 22% in the middle zone, and 6% in the lower zone. In the lower, middle, and upper zones, the concentration of plastics larger than 2 mm is 2.4, 2.9, and 28.3 g DM / L mixture, respectively.*

[0149] Thus, it is noted that more than three-quarters of the plastics present have been recovered in the upper part of the mixture and can therefore be recovered in the floating phase.

[0150] Fat separation and flotation of solid organic matter were also observed. Pre-separation hydrolysis can at least partially degrade this solid organic matter and lipids, reducing their flotation and allowing their subsequent processing in the digester. Alternatively, or in combination, the fats and solid organic matter present in the floating phase can be recovered by a filter press or similar device and then sent to the digester.

[0151] A comparative test was carried out under the same conditions as the first test using a digestate containing an inhibitor and having an acidic pH (pH 4.7). During this comparative test, no foaming was observed. This is due to the characteristic The acid in the digestate prevented the formation of CO2 bubbles. In this comparative test, only 16% of plastics smaller than 2 mm could be recovered in the upper zone.

Claims

Demands

1. A process for treating waste containing biodegradable organic matter, said waste comprising acidic organic compounds and solid impurities, said process comprising the following steps: - a) a step of separating the solid impurities from said waste within an enclosure during which purified waste is produced;- b) an anaerobic digestion step of the treated waste separated in step a) during which biogas and a digestate containing dissolved CO2 are produced, characterized in that the separation step a) comprises: a1) a flotation step during which a part of the digestate from step b) is brought into contact with said waste within the enclosure under pH conditions sufficient to generate CO2 bubbles, at least a part of the solid impurities being transported by the CO2 bubbles to the surface of said mixture, and a2) a separation of a floating phase located within the enclosure at the surface of said mixture, the floating phase containing a part of the solid impurities and / or solid impurities adhering to CO2 bubbles rising to the surface of said mixture.;

2. Processing method according to claim 1, characterized in that the quantity of digestate introduced into the enclosure of the separation step a) is adjusted so as to maintain the pH of the mixture at a value sufficient to cause the generation of CO2 bubbles.

3. Processing method according to claim 1 or 2, characterized in that the digestate produced at step b) is sent to step a) without intermediate step and / or without cooling.

4. A treatment process according to any one of claims 1 to 3, characterized in that, prior to step a) of separation, the waste is subjected to at least one step selected from (i) a step of grinding said waste, (ii) a hygienization step, (iii) a gravity separation step and (iv) a hydrolysis step.

5. Processing method according to any one of claims 1 to 4, characterized in that the separation step a) comprises a3) a gravity separation at the bottom of the enclosure of a portion of the solid impurities.

6. Processing method according to any one of claims 1 to 5, characterized in that the process comprises a step c) of phase separation of the floating phase to separate a first liquid phase containing organic compounds sent to step b) of digestion and a second phase containing solid impurities.

7. Installation (1) for the treatment of waste containing biodegradable organic matter, said waste comprising acidic organic compounds and solid impurities, capable of carrying out the process according to any one of the preceding claims, said installation comprising: - a chamber (A) for separating at least a part of the solid impurities of said waste, said chamber (A) comprising an outlet (10) of the purified waste produced;- a digester (B) capable of carrying out the anaerobic digestion of treated waste to produce biogas and a digestate containing dissolved CO2, said digester (B) being connected to the treated waste outlet (10) of the enclosure (A), and said digester (B) comprising a biogas outlet (17) and a digestate outlet (13), characterized in that said enclosure (A) comprises: - a digestate inlet (11) connected to the digestate outlet (13) of the digester (B), - a management device (A3) configured to bring the digestate into contact with the waste located inside the enclosure (A) under pH conditions sufficient to generate CO2 bubbles, and - at least one separation device (A2) for a floating phase located within the enclosure (A) on the surface of said mixture, the floating phase containing some of the solid impurities and / or solid impurities adhering to CO2 bubbles rising to the surface of said mixture.

8. Treatment plant according to claim 7, characterized in that the management device (A3) is further configured to adjust the quantity of digestate sent into the enclosure and maintain sufficient pH conditions to generate CO2 bubbles.

9. Treatment plant according to claim 7 or 8, characterized in that the digestate inlet (11) of the enclosure (A) is connected to the digestate outlet (13) from digester (B) without an intermediate enclosure capable of implementing an intermediate stage and / or without an intermediate cooling device.

10. A treatment plant according to any one of claims 7 to 9, characterized in that it comprises at least one of the following features: - at least one waste treatment unit located upstream of the unit (A) with respect to the waste flow and selected from (I) a grinding unit, (II) a hygienization unit, (III) a gravity separation unit, (IV) a hydrolysis unit; - the separation unit (A) comprises at least one device for mixing the digestate with the waste, - the separation unit (A) comprises a gravity separation device (A4) at the bottom of the unit for part of the solid impurities;- the installation includes a floating phase separation device (C) comprising an inlet (19) connected to an outlet (14) of the floating phase of the separation device (A2) of the enclosure (A), an outlet (21) of a first liquid phase containing organic compounds connected to an inlet of the digester (B) and an outlet (23) of a second phase containing solid impurities.;