CULTURE SUBSTRATE FOR METHANISATION METHOD

Abstract

A methanisation unit includes a culture substrate to be used in a method for methanising liquid effluents with structured packing, the culture substrate being made up of more than 50% of wood elements, of which at least one dimension is greater than 80 mm, the porosity of the culture substrate being greater than 50%. Embodiments relate to a method for preparing a culture substrate intended to be used in a methanisation unit according to the invention, and a methanisation method in a methanisation unit.

Claims

1. A methanisation unit comprising: a culture substrate intended for being used in a method for methanising liquid effluents with structured packing, said culture substrate being made up of more than 50% of wood elements of which at least one dimension is greater than 80 mm, the porosity of said culture substrate being greater than 50%.

2. The methanisation unit according to claim 1 wherein said wood elements comprise branched and/or skewed branches.

3. A method for preparing a culture substrate intended for being used in a methanisation unit according to claim 1, comprising in order the following steps: collecting green waste, inserting green waste into said methanisation unit so as to form a structured packing.

4. The method for preparing according to claim 3, further comprising a grinding step after the collecting step and before the insertion step, the grinding being a slow grinding, so as to obtain that a majority of the ground elements have a largest dimension that is less than a metre.

5. The method for preparing according to claim 3, comprising a screening step before the insertion step, the screening being done at a dimension comprised between 30 mm and 80 mm.

6. The method for preparing according to claim 5, comprising a composting step before the screening step.

7. A method for methanising in a methanisation unit according to claim 1.

8. The method for methanising according to claim 7, wherein said culture substrate is prepared by the following steps in order: collecting green waste; and inserting green waste into said methanisation unit so as to form a structured packing.

9. The method for methanising according to claim 7 comprising in order the following steps: inserting a culture substrate into an upstream methanisation tunnel (3a), sending liquid effluents into the upstream methanisation tunnel, inserting a culture substrate into a downstream methanisation tunnel, sending effluents treated at the output of the upstream methanisation tunnel into the downstream methanisation tunnel.

10. The method for methanising according to claim 7, wherein a portion comprised between 30% and 60% of the effluents treated at the output of the downstream methanisation tunnel is sent back as input of the upstream methanisation tunnel.

Description

[0035] The present invention will be understood better when reading the following detailed description, in reference to the accompanying figures wherein:

[0036] is a schematic view of a wood element of a culture substrate according to a preferred embodiment of the invention.

[0037] is a schematic view of a wood element of a culture substrate according to a second embodiment of the invention.

[0038] is a schematic view of a wood element of a culture substrate according to a third embodiment of the invention.

[0039] is a schematic view of a methanisation installation according to a preferred embodiment of the invention.

[0040] The culture substrate 1 according to the invention is intended for being used in a method for methanising liquid effluents with structured packing. The culture substrate 1 is made up of more than 50% of wood elements 2 of which at least one dimension is greater than 80 mm.

[0041] The dimension of the wood elements 2 allows the culture substrate 1 to have a high porosity, which favours the flow of the fluids and the obtaining of a high exchange surface for bacterial development on the one hand, and between the culture substrate and the liquid effluents to be treated on the other hand. This porosity is further improved by using wood elements of diverse and irregular shapes. If the elements are of smaller dimensions, they nest together more easily by leaving little space between them, which results in most cases in a culture substrate 1 that does allow for a sufficient flow of the fluids. This can cause a compaction of the substrate, even an occlusion, reducing the exchange surface.

[0042] In the present invention, the term “porosity”, applied to a culture substrate, means the fraction of volume occupied by air in the total volume of the culture substrate.

[0043] The measurement of the porosity can for example be carried out in the following way: [0044] the culture substrate 1 is disposed in a container, [0045] the container is filled with water, quickly enough so that the water does not have the time to infiltrate inside the elements comprising the culture substrate 1, in particular the wood elements 2, so that the porosity internal to these elements is not taken into account, [0046] the porosity is then equal to the volume of water added in the container, divided by the volume of the container.

[0047] The porosity of the culture substrate 1 is preferably greater than 50%, which allows for an exchange surface that is satisfactory for the effectiveness of the methanisation method between on the one hand the culture substrate 1 and the bacteria that it hosts, and on the other hand the liquid effluents. Such a porosity also makes it possible to reduce the risks of the culture substrate 1 being obstructed or clogged by biological and/or mineral deposits, requiring cleaning operations or replacement of the culture substrate 1. A longer service life of the culture substrate 1 is thus obtained.

[0048] A higher porosity allows, up to a certain point, the maintaining of a sufficient flow with the maintaining of the exchange surface, or surface available for the development of bacteria, which allows for a greater effectiveness of the methanisation reaction. It is indeed the maintaining of the exchange surface that allows for the greatest effectiveness, but it is very difficult to measure; that is why the porosity is used to characterise an effective culture substrate.

[0049] Examples of measured porosities give values of 63%, 65%, and 70%. These are effectively values that are on the average higher than for wood chips, where the porosity is generally comprised between 40% and 60%.

[0050] In a preferred embodiment of the invention, the wood elements 2 are branched and/or skewed branches. In the scope of the present invention, the expression “branched branch” designates a branch including at least one ramification, i.e. it includes at least two linear parts, not necessarily straight, forming an angle between them. An example is shown in FIG. 1. The expression “skewed branch” designates a branch of which the shape is such that it is not contained in a plane. An example is shown in FIG. 2.

[0051] The wood elements 2 of the branched and/or skewed branch type have the advantage when they are superposed, of leaving free spaces between them, so that a higher global porosity of the culture substrate 1 and a higher exchange surface are obtained.

[0052] They are moreover easy to find and inexpensive: this can be for example green waste formed mostly of wood, or the non-degraded portion of a composting of green waste, that can be recovered at the output of the composting. This can be for example screening residues of the compost.

[0053] In the present invention, the term “green waste” designates all of the materials coming from ligno-cellulosic plants and coming from trimming, cutting or the maintenance of these plants, for example during the maintaining of gardens, green areas, forests, hedges, or trees. The green waste can also include the by-products coming from the transformation and the reclaiming of wood, a by-product being a substance or an object coming from a production process of which the first purpose is not the production of this substance or this object.

[0054] Other types of wood elements 2 can be used, as long as their shape provides a substantial porosity to the culture substrate and a good exchange surface. This can be for example elements in the form of a tetrapod, as shown in FIG. 3, forestry chips, shreddings, or of yet another form.

[0055] The culture substrate 1 according to the invention can be prepared by implementing a method comprising in order the following steps: [0056] collecting green waste, [0057] inserting green waste into a methanisation unit so as to form a structured packing.

[0058] The culture substrate 1 is thus obtained inexpensively. In addition it is obtained locally, and allows for a reclaiming of this waste.

[0059] A grinding step can be implemented between the collection step and the insertion step. The grinding is then a slow grinding, so as to obtain that a majority of the ground elements have a largest dimension that is less than a metre. This grinding can be necessary if the green waste collected is too large, which can cause a problem for the insertion step, or which can result in an excessively porous culture substrate 1, the presence of grosses branches preventing the introduction of thinner branches. An excessively porous culture substrate 1, for example greater than 90%, results in a reduced exchange surface between the effluents to be treated and the bacteria, and therefore degrades the performance of the methanisation method.

[0060] A screening step can be implemented before the insertion step, after the grinding if there is one. The screening, which can be done for example at a dimension comprised between 30 mm and 80 mm, makes it possible to get rid of the fine particles that would participate in the occlusion of the culture substrate, which would reduce the effectiveness of the method of methanisation.

[0061] A composting step can take place before the screening step. This makes it possible to degrade the green waste so that once the compost is eliminated by the screening step, there only remains the material that is not degradable by compost, rich in lignin. As these materials rich in lignin are not, or are very little, degradable by methanisation, they make it possible to obtain a stable culture substrate 1.

[0062] The culture substrate 1 can thus be obtained by recycled materials, treated if necessary by conventional steps of treating green waste. The culture substrate 1 is thus inexpensive, obtained locally and is environmentally friendly, and makes it possible to obtain good performance when it is used in a methanisation method.

[0063] The culture substrate 1 can be used in a continuous methanisation method, shown in FIG. 4 and comprising in order the following steps: [0064] inserting a culture substrate 1 into an upstream methanisation tunnel 3a, [0065] sending liquid effluents into the upstream methanisation tunnel 3a, [0066] inserting a culture substrate 1 into a downstream methanisation tunnel 3b, [0067] sending effluents treated at the output of the upstream methanisation tunnel 3a at the input of the downstream methanisation tunnel 3b.

[0068] Since it was set into operation, the culture substrate 1 of the upstream methanisation tunnel 3a has had the time to be colonised by bacteria. The effluents sent into the downstream methanisation tunnel 3b are then charged with bacteria, and the colonisation of the downstream methanisation tunnel 3b is very fast, which improves the performance of the method of methanisation.

[0069] When the upstream methanisation tunnel 3a is at the end of its life, the downstream methanisation tunnel 3b, of which the culture substrate 1 is younger, continues to operate alone, the time to replace the culture substrate in the first methanisation tunnel 3a. The downstream methanisation tunnel 3b then becomes the upstream methanisation tunnel 3a, and vice versa. The method is thus repeated continuously, each methanisation tunnel 3a, 3b having in turn a culture substrate 1 that is older than the other methanisation tunnel 3b, 3a, with the methanisation tunnel 3a, 3b having the oldest culture substrate being placed upstream from the other methanisation tunnel 3b, 3a.

[0070] Using two methanisation tunnels 3a, 3b makes it possible to have a method that operates without interruption, not only during the end of the life of one of the methanisation tunnels 3a, 3b, but also during maintenance operations or when undesirable items are removed from one of the methanisation tunnels 3a, 3b.

[0071] In a preferred embodiment of the invention, a portion comprised between 30% and 60% of the effluents treated at the output of the downstream methanisation tunnel 3b are sent back into the upstream methanisation tunnel 3a. This allows for a dilution of the effluents as input, which are sometimes excessively thick to allow for a fast and effective methanisation.

[0072] Such a dilution can also be implemented when the upstream methanisation tunnel 3a operates alone, in particular before the implementation of the downstream methanisation tunnel 3b. There is then a portion comprised between 30% and 60% of effluents treated at the output of the upstream methanisation tunnel 3a sent back as input of the upstream methanisation tunnel 3a.

[0073] As shown in FIG. 4, before the methanisation in the methanisation tunnels 3a, 3b the method of methanisation according to the invention may comprise one of the following two steps: [0074] preparing in a deconditioner and/or grinder 4, [0075] hygienising in a hygienisation unit 5.

[0076] After methanisation, the biogas can be the object of post-storage in a gasometer 6.

[0077] The method for methanisation according to the invention allows the retention time of the solids (SRT) in the methanisation tunnels 3a, 3b to be maximised. Thus an optimum colonisation of the substrate is obtained and the conservation of the bacteria of interest by decoupling the retention time of the liquid effluents (biowaste) by that of the solids (bacteria/culture substrate).

[0078] Although the description hereinabove is based on particular embodiments, it in no way limits the scope of the invention, and modifications can be made, in particular by substitution of technical equivalents or by different combinations of all or a part of the characteristics developed hereinabove.