AU2013239188B8

AU2013239188B8 - Method for treating waste, in particular household refuse, and unit for implementing same

Info

Publication number: AU2013239188B8
Application number: AU2013239188A
Authority: AU
Inventors: Arnaud SOMMAIN
Original assignee: Suez International SAS
Current assignee: Suez International SAS
Priority date: 2012-03-27
Filing date: 2013-03-25
Publication date: 2017-11-23
Anticipated expiration: 2033-03-25
Also published as: CA2867656C; AU2013239188B2; TR201904830T4; FR2988624A1; HK1202481A1; CN104302404B; AU2013239188A8; EP2830774A1; WO2013144817A1; PL2830774T3; AU2013239188A1; CA2867656A1; CN104302404A; FR2988624B1; EP2830774B1

Abstract

The invention relates to a method for treating waste, particularly household refuse, containing organic materials mixed with undesirable products, in particular metals, mineral materials, plastics, glass. According to the method, waste is subjected to a first screening process (4). The fraction of waste passing through the screening is subjected to a pre-fermentation treatment in a rotating tube (1) and the material leaving the tube (1) is subjected to a methanisation treatment in a digester (2). The waste, as collected and without preliminary shredding, is subjected to a first screening process (4) using a screen of mesh size L1. Following said screening process, the fraction of waste is sent for treatment in the rotating tube (1). The rotating tube is supplied with waste regularly over time. The retention time (T) of the waste in the rotating tube is substantially constant and at least equal to the time required for almost all of the organic material to be of a particle size fraction smaller than that of the undesirable products. Upon leaving the rotating tube, and before entering the digester, the waste is subjected to screening (9) using a screen of mesh size L2, ensuring that the organic materials of a finer particle size are separated from the undesirable products.

Description

METHOD FOR TREATING WASTE, IN PARTICULAR HOUSEHOLD REFUSE, AND UNIT FOR IMPLEMENTING SAME

The invention relates to a process for treating waste, in particular household refuse, containing organic matter mixed with undesirable products, especially metals, mineral matter, plastics, and glass, a process of the type of those according to which: - the waste is subjected to a first sorting by screening, - the fraction of waste passing through the screening is subjected to a prefermentation treatment in a rotating tube with feed at one end and extraction at the other end, - and the material leaving the prefermentation treatment tube is subjected to a methanization treatment in a digester.

The household waste resulting from non-selective collections contains different categories of material such as putrescible waste (food waste, green waste), paper, cardboard, glass, plastics, ferrous and nonferrous metals, fabrics, sanitary textiles, toxic products (batteries, pots of paint, etc).

The methanization and composting of this household waste with a view to converting it to reusable biogas and compost generally comprises four main steps: - a mechanical preparation of the waste which aims to separate the biodegradable organic matter from the other fractions that cannot be upgraded to biogas or to compost; a prefermentation rotating tube, generally that is essentially horizontal, constitutes a means for such a preparation; - methanization, which aims to produce renewable energy and is carried out in horizontal or vertical chambers, which may or may not be mechanically stirred; - composting, or aerobic maturation, which generally involves a prior operation of pressing the digestate originating from the methanization, in order to achieve a level of solids content and a porosity that enable ) the self-composting of the digestate, or then requires the provision of a structuring agent and the mixing thereof with the digestate in order to obtain a compostable substrate; - final refining which aims to additionally remove the i contaminants remaining after the preceding two operations, and to prepare the compost to a particle size that enables the agronomical upgrading thereof.

The mechanical preparation may comprise a preliminary ) crushing followed by particle size sorting steps as taught by EP 0 131 319. Such a process has the major drawback of separating contaminants or the reusable materials (glass, plastic, fabric, etc.), which generally leads to sorting difficulties and generates a i high risk of contamination of the compost and requires equipment of very large sizes with respect to the requirements . FR 2 951 095 discloses a process for treating household ) waste that comprises several particle size sorting steps and which avoids the use of a prefermentation rotating tube which mixes waste that is reusable and waste that is not reusable in the form of compost. i However, the prefermentation rotating tube followed by particle size sorting steps has the advantage of a fragmentation of papers and cardboards and a start of biodegradation subsequently enabling a separation of the methanizable function by particle size sorting ) steps.

The quality level of the waste leaving the rotating tube needs to be improved since it is not generally constant or optimal. Indeed, the screening mesh at the tube outlet is a constant: if the organic matter is not ) sufficiently reduced, it will be removed and will result in a significant loss for the methanization. A contrario, if physical impurities pass through the mesh, these impurities will be found in the substrate introduced into the digesters. Such impurities have no i methanogenic interest and may compromise both the safety and durability of the methanization process (accumulation of floating plastics, sedimentation of glass in the bottom of the digesters, etc.). Finally, these impurities will be very difficult to extract from ) the digestate at the end of the aerobic maturation in order to allow a standardization of the compost produced.

Embodiments of the invention seek to obtain a quality i homogenous organic fraction, which is as free of undesirable products as possible, having an optimal biogas production with respect to the sizing of the digester used. The invention also aims to produce a digestate containing a high solids content in order to ) be able to render optional the pressing thereof before aerobic maturation, without adversely affecting the digestate, and to provide a compost that conforms to the standards applied in the countries concerned by the process . i

According to a first aspect of the invention, there is provided: a process for treating waste, in particular household refuse, containing organic matter mixed ) with undesirable products, especially metals, mineral matter, plastics, and glass, according to which process: - the waste is subjected to a first sorting by screening, - the fraction of waste passing through the screening is subjected to a prefermentation treat) ment in a rotating tube with feed at one end and extraction at the other end, - and the material leaving the prefermentation treatment tube is subjected to a methanization treatment in a digester, i wherein: - the waste as collected, and without prior crushing, is subjected to the first sorting by screening through screens having a mesh size LI; - the fraction of waste that has passed through ) this first sorting operation, which comprises elements of which the large size is less than the value LI, is sent to the prefermentation treatment in the rotating tube; - the rotating tube is fed with waste regularly i over time, by smoothing out the fits and starts of collection; - the residence time of the waste in the rotating tube is substantially constant and at least equal to the time needed so that almost all of the ) organic matter is in a particle size fraction smaller than that of the undesirable products; - and the waste, after leaving the rotating tube and before entering the digester, is subjected to a screening through screens having a mesh size L2 i that ensures a separation between the organic matter of finer particle size and the undesirable products .

According to a second aspect of the present invention, ) there is provided: a waste treatment plant for carrying out a process as claimed in any one of the preceding claims, comprising: - a first station for sorting by screening, ) - a rotating tube for the prefermentation treatment of the fraction of waste that has passed through this first sorting operation, - and a digester in order to subject the material leaving the prefermentation treatment tube to a i methanization treatment, wherein : - the first station for sorting by screening has a mesh size LI for the waste as collected, and without prior crushing; ) - means for feeding the rotating tube with waste comprise at least one metering hopper suitable for smoothing out the possible fits and starts of feeding originating from sudden variations in particle size and/or density of the collected i waste stream, in order to ensure regular feeding, without interruption over time; - the rotating tube is provided so that the residence time of the waste is at least equal to the time needed so that almost all of the organic ) matter is in a particle size fraction smaller than that of the undesirable products; - and, upstream of the digester, a station for screening through screens having a mesh size L2 is provided in order to ensure a separation between i the organic matter of finer particle size and the undesirable products.

The residence time of the waste in the rotating tube is at least equal to the time needed so that at least 75% ) by weight of the organic matter leaving the prefermentation tube is in a particle size fraction smaller than that of the undesirable products.

The process, via a permanent and regular feeding of the rotating tube, makes it possible to smooth out the irregular supplies of material over time caused in ) particular by the fact that the collection generally takes place only five days out of seven, and eight to twelve hours per day.

Advantageously, the feeding of the rotating tube with i waste and the extraction of the treated waste are carried out continuously, the extraction flow rate corresponding to the feed flow rate, less the loss of matter and loss by evaporation, and being substantially constant over a duration of several hours, preferably ) of at least 10 h and advantageously of 24 h.

As a variant, the rotating tube may be fed with waste in batches, at regular relatively reduced time intervals, preferably of less than 14 hours, and the i weight of waste introduced into the tube with each loading batch is substantially the same.

The size LI of the mesh of the first sorting operation may be between 200 and 600 mm, preferably between 350 ) and 500 mm and advantageously equal to 450 mm.

The residence time of the waste in the rotating tube may be between two and four days, preferably between two and a half days and three and a half days and i advantageously three days. The size L2 of the mesh of the screen before entry into the digester may be between 5 and 14 mm, preferably between 7 and 12 mm and advantageously equal to 8 mm. ) The feeding of the rotating tube may use at least one metering hopper suitable for smoothing out the possible fits and starts of feeding that originate from sudden variations in particle size and/or density of the collected waste stream.

Several sorting stages with smaller mesh sizes at each ) next stage may be provided between the outlet from the rotating tube and the last screening through screens having a mesh size L2.

The plant may comprise a flip-flow screen providing the i last screening before the digester.

Preferably, the digester is horizontal, mechanically stirred, and means are provided to ensure that a fraction of the outgoing digestate is recirculated to ) the inlet of the digester, advantageously at a recirculation ratio of at least 200%.

Downstream of the rotating tube and upstream of the last screening, the waste may be subjected to a i bounce/adherence sorting. A mixing hopper may be provided downstream of the last screening and upstream of the digester, and the recirculated digestate fraction is introduced, for a portion of greater than 50%, into the mixing hopper, ) and for the other portion at the inlet of the digester, downstream of the mixing hopper.

Embodiments of the present invention also relate to a waste treatment plant for carrying out the process i defined previously, this plant comprising: - a first station for sorting by screening, - a rotating tube for the prefermentation treatment of the fraction of waste that has passed through this first sorting operation, ) - and a digester in order to subject the material leaving the prefermentation treatment tube to a methanization treatment, and being characterized in that: - the first station for sorting by screening has a mesh size LI for the waste as collected, and without prior crushing; ) - means for feeding the rotating tube with waste are provided in order to ensure regular feeding, without interruption over time, by smoothing out the fits and starts of collection; - the rotating tube is provided so that the residence i time of the waste is at least equal to the time needed so that almost all of the organic matter is in a particle size fraction smaller than that of the undesirable products; - and, upstream of the digester, a station for ) screening through screens having a mesh size L2 is provided in order to ensure a separation between the organic matter of finer particle size and the undesirable products. i The plant advantageously comprises several rotating tubes operating in parallel, in particular three rotating tubes.

The size LI of the mesh of the screen of the first ) sorting operation may be between 200 and 600 mm, preferably between 350 and 500 mm and advantageously equal to 450 mm.

The size L2 of the mesh of the screen before entry into i the digester may be between 5 and 14 mm, preferably between 7 and 12 mm and advantageously equal to 8 mm.

The means of feeding the rotating tube may comprise at least one metering hopper suitable for smoothing out ) the possible fits and starts of feeding originating from sudden variations in particle size and/or density of the collected waste stream.

Several sorting stages, with smaller mesh sizes at each following stage, may be provided between the outlet of the rotating tube and the last screening by a screen ) having a mesh size L2.

The plant may comprise a flip-flow screen providing the last screening before the digester. i Advantageously, the digester is horizontal, mechanically stirred, and means are provided to ensure that a fraction of the outgoing digestate is recirculated to the inlet of the digester. ) A mixing hopper may be provided downstream of the last screening and upstream of the digester, and the recirculated digestate fraction is introduced, for a portion of greater than 50%, into the mixing hopper, and for the other portion at the inlet of the digester, i downstream of the mixing hopper.

Embodiments of the invention consist, apart from the provisions set out above, of a certain number of other provisions, that will be mentioned more explicitly ) below regarding an exemplary embodiment described with reference to the appended drawings, but which is in no way limiting. In these drawings: fig. 1 is a diagram of a treatment process according to the invention, and i fig. 2 is a graph that schematically illustrates the change in the particle size of the organic matter and of the inorganic or synthetic organic matter of the same sizes in a prefermentation treatment rotating tube, as a function of the residence time shown on the ) x-axis, while the mean diameter of the particle size is shown on the y-axis.

One objective of certain embodiments of the invention is to obtain: - a quality of organic fraction that is homogeneous, after passing into a prefermentation treatment rotating ) tube 1 (fig. 1) and as free as possible of undesirable products after screening, - and an optimal biogas production with respect to the sizing of the digester 2 used. i Embodiments of the invention also aim to provide a digestate with a high solids content, so as to be able to render optional the pressing thereof before aerobic maturation without adversely affecting this digestate, and a compost that conforms to the standards applied in ) the countries affected by the process.

The term "digestate" denotes the product leaving the digester, consisting of organic matter after anaerobic digestion . i

It turned out that the quality level of the material leaving the prefermentation rotating tube was not constant. It was able to be established that the irregular supply of material over time, especially only ) five days out of seven, and eight to twelve hours per day, led to a variable residence time of the material in the rotating tube and prevented the desired quality level from being achieved. i The screening mesh at the outlet of the tube 1 is a constant; if the organic matter is not sufficiently reduced, it will be removed and will result in a significant loss for the methanization. A contrario, if the physical impurities, constituting the undesirable ) products, pass through this mesh, these impurities will be found in the substrate introduced into the digester, which is to be avoided.

According to certain embodiments, the size of a screen mesh corresponds, for a square or rectangular mesh, to the length of the diagonal of the mesh, and for a ) circular mesh, to the diameter of this mesh.

It has been able to be established that the particle size of the organic matter and of the undesirable products changed differently as a function of the i residence time in the rotating tube. In fig. 2 the variation of the particle size shown on the y-axis, in the form of a mesh size through which the product leaving the tube 1 may pass, as a function of the residence time of the product in the tube 1, shown on ) the x-axis, is illustrated schematically.

Cl corresponds to the particle size curve of the organic matter, while C2 corresponds to the particle size curve of the undesirable mineral, inorganic or i synthetic organic matter. Beyond a residence time To, the curve Cl breaks away downward with respect to the curve C2. After a residence time Ti, the particle size of the organic matter (curve Cl) is substantially smaller than that of the inorganic or synthetic organic ) matter. By choosing a screen for which the mesh has a size Ml corresponding to the particle size of the curve Cl, or which is slightly greater, in particular by 1 mm, than this particle size, an effective separation between the organic matter of the undesirable products i is ensured, with a minimal loss of organic matter.

By way of non-limiting example, Tl may be between 2 and 3.5 days, and Ml may be between 7 and 9 mm, advantageously equal to 8 mm. ) Fig. 1 of the drawings illustrates a diagram of the whole of a treatment line implementing the process of the invention.

The waste collected, especially by dump trucks, is introduced into a treatment line inlet 3 and is sent to one or more primary screens 4, especially using belt ) conveyors or metal apron conveyors. Each screen 4 consists of a screen in which the meshes have a size LI provided in order to retain the coarse fractions, the large size of which is greater than LI. The size LI is advantageously between 400 and 500 mm, preferably equal i to 450 mm. The fraction 5 that passes through the screens 4 constitutes a fine and intermediate fraction which is used to feed the essentially horizontal rotating tube 1. ) The same amounts of waste are introduced, at constant flow rate, into the tube 1 and extracted continuously, quasi-permanently. The feeding of the tube 1 is optimized by use of hoppers 6, in particular metering hoppers (dosing hopper, the bottom of which is equipped i with sequentially moving slats (or equivalent) allowing the product to advance), which make it possible to smooth out the possible fits and starts of feeding that originate from sudden variations in particle size and/or density of the waste stream. According to the ) diagram, the smoothing hopper 6 is installed downstream of the outlet of the fraction 5 and upstream of the tube 1. As a variant, the smoothing hopper 6 could be located between the line inlet 3 and the primary screens 4. In the latter case, the equipment does not i fulfill the action of limiting the fits and starts as well (the break of load permitted by the hopper upstream of the tubes, according to the diagram of fig. 1, allows maintenance actions, a discontinuous treatment of the waste). )

The coarsest fraction 7 having a size greater than that of the meshes of the screens 4, for example greater than 450 mm, and which has not passed through the screens, may be subjected to a coarse shredding 8 before being reintroduced into the prefermentation tubes 1 or as a variant into the primary screens 4, or ) being treated separately.

Industrial water Ei is introduced into the prefermentation tubes 1 in order to promote biological activity. The temperature rise of the biological reactions causes i the evaporation Ev of all or some of this industrial water .

The rotating tube 1 is horizontal and has a closed cylindrical wall, the inner surface of which is ) equipped with profiles, for example U-shaped profiles that turn their concavity radially inward, which are regularly spaced apart, extending along the generatrices of the cylinder, and are suitable for rotating the waste introduced at the end la. The waste i progresses through the tube 1 under the effect of the thrust of the material loaded at the inlet la and is extracted at the outlet lb after a residence time T in the tube 1. ) By ensuring a suitable residence time T of the waste in the rotating tube 1 (T is equal to or greater than Tl), combined with regular feeding, a mixture is obtained at the outlet lb in which the organic matter has a particle size equal to or less than Ml, in particular i less than 8 mm, while the undesirable products have a larger particle size. Separation of the organic matter to be introduced at the inlet 2a of the digester 2, relative to the undesirable products, is provided by a screen 9 having meshes whose size corresponds to that ) of the particle size of the organic matter, in particular 8 mm. The screen 9 is advantageously a flip-flow screen; such a screen is provided in order to carry out screening through a successively taut and slackened elastic cloth, offering a slight deformation of the meshes which are unclogged, this system allowing screening of wet product in very small particle sizes. )

Several prefermentation rotating tubes 1 may be provided in order to operate in parallel, especially three prefermentation tubes. The regularity of the feed and the extraction from the tubes makes it possible to i establish a finished product of homogeneous composition and constant particle size profile at the fraction 27 to be introduced into the digester 2. The amount of undesirable products (plastics, glass, pebbles, metals) is practically zero in the fraction having a particle ) size of less than Ml.

In order to avoid overloading the screen 9 with undesirable products, the sizes of which are substantially greater than the mesh of the screen 9, i several sorting stages are provided upstream of the screen 9. A first sorting stage at the outlet lb of the tube 1 is carried out by a drum screen 10, or perforated drum, ) which allows a fraction 11, for example of particle size 0-80 mm, to pass through and rejects an oversize 12 corresponding for example to a particle size 80-450 mm. This oversize 12 is subjected to an iron-removal step 13 that makes it possible to separate the i steels 14 from the light oversize 15 having high net calorific value NCV.

The fraction 11 which has passed through the screen 10 is introduced into a second drum screen 16 having ) meshes with a size smaller than that of the screen 10, in particular 20 mm meshes. The oversize from the screen 16, which corresponds in the example considered to a particle size of 20-80 mm, is subjected to an iron-removal step 13a which separates the steels 14a from another fraction 17. This fraction 17 is subjected to a bounce/adherence sorting step 18. This sorting ) operation consists in projecting the material onto a rigid plate or an inclined belt conveyor, which causes heavy oversize to bounce and lighter portions, especially organic matter, to adhere. The heavy fraction 19 resulting from the sorting 18 is subjected to a i eddy current separation 20 which makes it possible to separate the non-ferrous metals 21 from the heavy oversize 22. The lightest fraction 18a resulting from the bounce/adherence sorting step 18 constitutes the raw material for the manufacture of solid recovered ) fuel.

The fraction 23 which has passed through the meshes of the screen 16, for example the fraction of particle size 0-20 mm, is subjected to a sorting 24, preferably i of bounce/adherence type, which separates heavy oversize 22 from a lighter fraction 25 containing organic matter .

This fraction 25 is then subjected to a separation by ) the screen 9 having meshes with a size equal to or less than Ml corresponding to the separation between the particle size of the organic matter and that of the undesirable products according to fig. 2, after a residence time at least equal to T1 in the rotating i drum 1. The screen 9 separates an oversize 26, the particle size of which is, for example, 8-20 mm, and a fine fraction 27, the particle size of which is, for example, 0-8 mm. The screen 9 is advantageously a flip-flow screen. )

The fine fraction 27, leaving the screen 9, is introduced into a mixing hopper 28 that gives, at the outlet 29, a homogeneous mixture which is introduced at the inlet of the mechanically stirred horizontal digester 2. The digester 2 consists of a cylindrical tube of horizontal axis in which means of stirring the material ) introduced are arranged. The stirring means may comprise a coaxial rotating shaft equipped with spaced out radial arms, provided with mixing paddles. The digester 2 produces the biogas recovered at an outlet 30 located in the upper part of the digester. The i digestate is discharged through an outlet 31 opposite the digester inlet. A fraction 32 of the digestate is recirculated according to a portion 32a into the mixing hopper 28 and, according to another portion 32b, to the inlet of the digester 2, downstream of the hopper 28. )

The digestate serves as a carrier for transporting the organic substrate constituted by the fraction 27. The solids content of the mixture is controllable as the incorporation of liquid (mains water, industrial i waters, etc.) is also possible, and the mixture produced in the hopper 28 is an intimate and homogeneous mixture.

The recirculation ratio R/Q of the digestate is ) preferably greater than 200% so as to ensure an infinitely mixed material. The recirculation ratio R/Q corresponds to the ratio of the mass flow R of the recirculated digestate fraction 32 to the flow Q of the fraction 27 of material introduced into the hopper 28. i

The fraction 32a recirculated into the hopper 28 is preferably greater than the fraction 32b recirculated to the inlet of the digester, downstream of the hopper 28. The fraction 32a is advantageously at least equal ) to 60% by weight of the recirculated fraction 32.

By way of a non-limiting numerical example, the hopper 28 may have a volume of 2 m3; the fraction 27 introduced into the hopper 28 may be 1200 t/month, the fraction 32a 1800 t/month and the fraction 32b 1740 t/month. The ) total fraction 32 recirculated is 3540 t/month. According to this example, the recirculation ratio is 3540/1200, i.e. around 300%.

According to an embodiment of the invention, the i composition, or recipe, of the mixture introduced into the digester 2 is improved by the recirculation of the digestate which replaces a mixing fluid such as industrial water. The wetting power of the digestate is greater than that of the liquid. It is thus possible to ) significantly increase the organic load in the mixture while reducing the overall volume.

The recirculation of the digestate into the mixing hopper 28 increases the treatment capacity of the i digester by around 30%. Indeed, the volume needed to introduce one ton of organic substrate, originating from the fraction 27, is considerably reduced. By way of non-limiting example, the capacity of introducing organic substrate has changed from around 25 t/day to ) 40 t/day according to the invention.

At identical viscosity, the solids content of the mixture 29 resulting from the new composition is higher. It is thus possible to significantly increase i the organic load in the mixture while reducing the reduced overall volume.

The digestate has a solids content of around 35% which may range up to 40%, and which is satisfactory for ) allowing post-treatment by composting; this dryness very greatly simplifies the exploitation and improves the yields. Good management of the viscosity of the liquor entering into the digester makes it possible to satisfy the mechanical mixing conditions in the digester, without excessive stresses. ) The fraction 33 of the digestate, which is not recirculated, is used for the composting step 34. For this, the fraction 33 is introduced into a mixer 35, the output of which is sent to the composting stage 34. The material leaving the composting stage 34 is sent to a i final refining step 36 which provides, at the outlet, the compost 37. A fraction 38 of the refining step is advantageously recirculated to the inlet of the mixer 35. ) The mixer 35 and the refining step 36 constitute a structuring treatment 39 for the compost, shown schematically by a dotted-line outline. This treatment could be replaced by a dehydration step, at the outlet of the composting step 34. The fraction 33 would then be i directly introduced at the inlet of the composting stage 34.

Certain embodiments of the invention make it possible to obtain a quality of organic fraction 29, sent to the ) digester, which is homogeneous and free as far as possible from undesirable products, which is highly favorable to the stability of the methanization.

The small particle size of the organic fraction, 0-8 mm, increases the contact area between the bacteria i and the substrate, which is also favorable to the methanization. The reduced content of undesirable products and the constant homogeneity of the product make it possible to guarantee the conformity of all the production batches of compost. This small particle size ) is optimally exploited in the horizontal digester 2, the mechanical stirring of which enables the degassing of the biogas produced in the liquor.

The production of biogas by the digester 2 is optimal with regard to the sizing of the digester used and makes it possible to obtain a digestate having a high ) solids content, of at least 30% SC up to 38% SC, which makes it possible to render optional a mechanical pressing before aerobic maturation, without adversely affecting the digestate, and to obtain a compost that conforms to the standards. i

Owing to the mechanical-biological separation process involving the rotating tube 1, the steps for treating the product at the inlet and outlet of the tube by screening make it possible to optimize the quality of ) each of the categories of materials contained in the waste with respect to its upgrading route: - the most unstable, highly reactive organic matter is composted rapidly in the tube 1; - the organic substrate (fraction 27) intended for the i methanization is optimized; - the separation of the ferrous and non-ferrous metals of great cleanliness is optimal; - the streams having a high calorific value are well separated; ) - the cleanliness of the final compost is ensured.

The improvement in the stability of the digester 2 obtained according to the invention has an impact on the electricity production output per ton of organic i substrate introduced into the digester. According to one example, the mean output has experienced an increase of around 10%.

The process of certain embodiments of the invention can ) be used industrially for sizing a waste treatment unit with methanization, by maximizing both: - the degree of extraction of the non-synthetic organic matter initially present in the waste, owing to a continuous feeding of the rotating tube 1 and an optimal residence time in this tube, while improving ) the cleanliness thereof; - the production of biogas from the methanization process using a horizontal digester and driving it in a mode similar to an infinitely mixed mode, due to the recirculation ratio, instead of the customary plug flow i mode, and maintaining the liquor present in the digester at an optimal solids content in the vicinity of 35%.

Claims

1. A process for treating waste, in particular household refuse, containing organic matter mixed with undesirable products, especially metals, mineral matter, plastics, and glass, according to which process: - the waste is subjected to a first sorting by screening, - the fraction of waste passing through the screening is subjected to a prefermentation treatment in a rotating tube with feed at one end and extraction at the other end, - and the material leaving the prefermentation treatment tube is subjected to a methanization treatment in a digester, wherein : - the waste as collected, and without prior crushing, is subjected to the first sorting by screening through screens having a mesh size LI; - the fraction of waste that has passed through this first sorting operation, which comprises elements of which the large size is less than the value LI, is sent to the prefermentation treatment in the rotating tube; - the rotating tube is fed with waste regularly over time, by smoothing out the fits and starts of collection; - the residence time of the waste in the rotating tube is substantially constant and at least equal to the time needed so that almost all of the organic matter is in a particle size fraction smaller than that of the undesirable products; - and the waste, after leaving the rotating tube and before entering the digester, is subjected to a screening through screens having a mesh size L2 that ensures a separation between the organic matter of finer particle size and the undesirable products .

2. The process as claimed in claim 1, wherein the residence time of the waste in the rotating tube is at least equal to the time needed so that at least 75% by weight of the organic matter leaving the prefermentation tube is in a particle size fraction smaller than that of the undesirable products .

3. The process as claimed in claim 1 or 2, wherein feeding of the rotating tube with waste and the extraction of the treated waste are carried out continuously, the extraction flow rate corresponding to the feed flow rate, less the loss of matter and loss by evaporation, and being substantially constant over a duration of several hours, preferably of at least 10 h and advantageously of 24 h.

4. The process as claimed in any one of the preceding claims, wherein the size LI of the mesh of the first sorting operation is between 200 and 600 mm, preferably between 350 and 500 mm and advantageously equal to 450 mm.

5. The process as claimed in any one of the preceding claims, wherein the residence time of the waste in the rotating tube is between two and four days, preferably between two and a half days and three and a half days and advantageously three days.

6. The process as claimed in any one of the preceding claims, wherein the size (L2) of the mesh of the screen before entry into the digester is between 5 and 14 mm, preferably between 7 and 12 mm and advantageously equal to 8 mm.

7. The process as claimed in any one of the preceding claims, wherein the feeding of the rotating tube uses at least one metering hopper suitable for smoothing out the possible fits and starts of feeding originating from sudden variations in particle size and/or density of the collected waste stream.

8. The process as claimed in any one of the preceding claims, wherein several sorting stages, with smaller mesh sizes at each following stage, are provided between the outlet of the rotating tube and the last screening by a screen having a mesh size L2.

9. The process as claimed in any one of the preceding claims, wherein the last screening before the digester is provided by a flip-flow screen.

10. The process as claimed in any one of the preceding claims, wherein the digester is horizontal, mechanically stirred, and a fraction of the digestate leaving the digester is recirculated to the inlet, preferably at a recirculation ratio of at least 200%.

11. The process as claimed in claim 10, wherein a mixing hopper is provided downstream of the last screening and upstream of the digester, and the recirculated digestate fraction is introduced, for a portion of greater than 50%, into the mixing hopper, and for the other portion at the inlet of the digester, downstream of the mixing hopper.

12. A waste treatment plant for carrying out a process as claimed in any one of the preceding claims, comprising: - a first station for sorting by screening, - a rotating tube for the prefermentation treatment of the fraction of waste that has passed through this first sorting operation, - and a digester in order to subject the material leaving the prefermentation treatment tube to a methanization treatment, wherein : - the first station for sorting by screening has a mesh size LI for the waste as collected, and without prior crushing; - means for feeding the rotating tube with waste comprise at least one metering hopper suitable for smoothing out the possible fits and starts of feeding originating from sudden variations in particle size and/or density of the collected waste stream, in order to ensure regular feeding, without interruption over time; - the rotating tube is provided so that the residence time of the waste is at least equal to the time needed so that almost all of the organic matter is in a particle size fraction smaller than that of the undesirable products; - and, upstream of the digester, a station for screening through screens having a mesh size L2 is provided in order to ensure a separation between the organic matter of finer particle size and the undesirable products.

13. The plant as claimed in claim 12, wherein it comprises several rotating tubes operating in parallel, in particular three rotating tubes.

14. The plant as claimed in claim 12 or 13, wherein the size LI of the mesh of the screen of the first sorting operation is between 200 and 600 mm, preferably between 350 and 500 mm and advantageously equal to 450 mm.

15. The plant as claimed in any one of claims 12 to 14, wherein the size L2 of the mesh of the screen before entry into the digester is between 5 and 14 mm, preferably between 7 and 12 mm and advantageously equal to 8 mm.

16. The plant as claimed in any one of claims 12 to 15, wherein it comprises a flip-flow screen providing the last screening before the digester.

17. The plant as claimed in any one of claims 12 to 17, wherein the digester is horizontal, mechanically stirred, and that means are provided to ensure that a fraction of the outgoing digestate is recirculated to the inlet of the digester .