BE1029541B1 - FILTERING MATERIAL FOR WASTEWATER TREATMENT - Google Patents

Abstract

Vegetable filter material intended for the treatment of waste water comprising cellulose and lignin, said vegetable filter material being characterized in that it comprises pieces of wood forming a population of pieces, said population of pieces having a distribution of sizes of particles between 3 mm and 25 mm and said pieces of wood being pieces of torrefied wood.

Description

FILTERING MATERIAL INTENDED FOR THE TREATMENT OF WASTEWATER The present invention relates to a vegetable filtering material intended for the treatment of wastewater comprising cellulose and lignin.

Wastewater treatment includes three main areas, domestic, municipal and industrial wastewater. Domestic wastewater includes wastewater from private homes, but also wastewater from small communities such as, for example, a hotel complex, campsite, housing estate, offices, residential buildings, small villages.

Biodegradable domestic wastewater cannot be directly discharged into a superficial hydraulic environment but must first undergo purification treatments whose function is to reduce their polluting nature, so as to allow them to meet the standards which have been established by the legislation.

Conventionally, wastewater is subjected to Anaerobic pre-treatment, combined with a settling process, in a settling tank, commonly known as a septic tank or septic tank, which separates a large part of the suspended solids (MES). ) and possibly floats. This pre-treatment can be followed by an aerobic treatment of the water clarified by bacteria.

Various manufacturers currently offer on the market wastewater treatment devices formed by an association between a septic tank or all-water tank and a filter mounted downstream of this tank.

Such a filter comprises a sealed enclosure intended to be buried, is provided with an inlet pipe for the effluents originating from the septic tank or septic tank and with an outlet pipe for the filtered effluents.

This enclosure accommodates a mass or filtering bed formed by an aerobic biomass support filtering material.

The ideal filter material has a long service life and adequate purification performance. Purification performances can be direct or indirect. For example, activated carbon has a direct purification performance by directly neutralizing pollutants present in the effluents when these pollutants are in contact with the activated carbon. Alternatively, a filter material that allows the growth of bacteria that adhere to it to digest pollutants will have an indirect purification performance. Of course, the two types of purifying performance are sometimes combined. For example, different physical and chemical properties have proven to be important, such as: - water retention by mass or volume, which defines the capacity of the filtering material to retain a large quantity of bound water within the intra-particle porosity after drying (elimination of free water), to keep the material wet. A humidity maintained in the material over time allows the material to create an environment favorable to the bacterial activity of the bacteria which adhere to it, and this even in the absence of a supply of effluents containing matter to be digested. On the other hand, too high water retention can also create accelerated degradation of the material. In addition, when the material forms a bed or filter bed, it is also important to have a profile of water content in the filter material that is homogeneous in the volume of the bed or filter bed, so as to have a uniform biofilm and effective in the volume of the filter bed or the filter bed.

- The hydraulic compaction corresponding to the compaction of the filtering material after pouring a large quantity of water on the filtering material, the large quantity of water corresponding for example to 2 times the total volume of the filtering material in less than 5 minutes. High hydraulic settlement will make the filter material less resistant to degradation and cause a reduction in the efficiency of the filter material over time. Indeed, a very low hydraulic settlement, for example less than 3% relative to an initial volume of the filtering material, ensures good conservation of the filtering material without risk of clogging and/or without reducing the purification performance of the filtering material.

- The content of water-soluble matter present in the filtering material makes it possible to evaluate the mass percentage of matter which will be leached by the circulation of water. This reflects two phenomena, on the one hand a release of water-soluble materials from the filter material, causing its degradation, and on the other hand, the release by leaching of these water-soluble materials which are found in the filtered water. Too high a percentage of water-soluble materials, for example greater than 10%, does not give a stable filtering material over time.

- The air capacity representing the interstitial space available between particles or grains of the filter material for circulation of the oxygen required for aerobic digestion by bacteria. A good air capacity allows optimal oxygenation of the material and therefore the development of an aerobic biofilm favorable to effective purification.

Regarding the performance of aerobic digestion, a filter material is adequate if it allows a lasting maintenance of the bacterial population for optimal aerobic digestion once colonized by the bacterial population. A filtering material colonized by the bacterial population decreases in wastewater: - carbon pollution defined by the biological oxygen demand over 5 days (BODs), - the quantity of suspended solids, - the chemical oxygen demand, - Ja pollution nitrogen resulting from the treatment of the nitrogen present in wastewater by bacteria which degrade ammonium (NH4) and produce nitrates (NO3).

In addition to having good physical and chemical properties and good purification performance when it is colonized, it is important that the filter material be inexpensive so as to be economically profitable, that it be as abundant, easy to access whatever the given territory, and ecological.

A filter whose filtering material is prepared based on pine bark is known for example from document EP2976301. According to this document, a filtering material based on pine bark is light and makes it possible to obtain purification performance in accordance with domestic sanitation regulations. However, pine bark is sensitive to hydraulic settlement, thus impacting the durability of the filter material.

Another filter whose filtering material consists exclusively of bulk fibers of hemp shiv is known from document WO201713312. This document proposes a compact filtration system while avoiding the clogging of the filter, and making it possible not to replace the filtering material too frequently. However, hemp availability is non-uniform globally with China producing nearly half of the world's production. The rest being grown mainly in France, the Democratic Republic of Korea and Spain.

Furthermore, a filter whose filter material contains at least one layer of crushed hazelnut shells is known from document EP3008018. According to this document, this filter is intended for a domestic wastewater treatment system based on a filter bed containing a material available in large quantities, therefore inexpensive and at the same time presenting completely satisfactory filtration performance. Although this paper touts its technology, the worldwide availability of hazelnut shells extends along a band of latitude between 30 and 45° north. Moreover, according to this document, it is preferable for the filter to have a re-aeration layer of plastic material between two layers of hazelnut shells, in order to promote the development of aerobic bacteria. The use of a plastic layer therefore represents a detrimental environmental impact of this filter.

Finally, hazelnut crops are often attacked by insects such as the hazelnut balanin. To combat this pest in intensive crops, insecticides are used. These will inevitably end up on hazelnut shells, which can lead to pollution in wastewater treatment when hazelnut shells are used as filter material. Finally, the document FR2331515 mentions A Gacterial bed support for purifying domestic and industrial waste water using charcoal of vegetable origin which has a low apparent density and before a high hydrophilicity, The process for manufacturing this charcoal of hydrophilic plant origin includes a step of treating the wood with sulfuric acid to defibrate it. Then, this fibrous wood is pyrolyzed to form charcoal of vegetable origin. according to this document, this coal has good purification capacities. On the other hand, knowing that the wood must be treated with chemicals such as sulfuric acid and must be pyrolyzed resulting in a significant consumption of thermal energy, this process for obtaining this charcoal of vegetable origin is not ecological and has limited economic returns.

Unfortunately, as can be seen, filter materials as described above still have many drawbacks. Indeed, if some have an acceptable lifespan and/or purification performance in accordance with the legislation, the availability of certain materials remains problematic. However, it is important, all the more in fields of application with environmental impact, to enter into a process of circular economy implying that the filtering material is cheap so as to be economically profitable, that it is also abundant, easily accessible, and ecological.

The object of the present invention is to overcome these drawbacks by providing a filter material that is inexpensive, abundant, easy to access, ecological and has characteristics of durability and adequate purification characteristics, both in the colonized state and in bulk. For this, the present invention provides a vegetable filtering material characterized in that it comprises pieces of wood forming a population of pieces, said population of pieces having a distribution of particle sizes between 3 mm and 25 mm and said pieces of wood being pieces of roasted wood.

Within the meaning of the present invention, the term “pieces” is understood to mean, for example, plates, grains, fibers, briquettes, beads or granules.

According to the present invention, a filtering material comprising pieces of torrefied wood according to a particle size distribution of the pieces of torrefied wood as mentioned above has physico-chemical characteristics which make it possible to obtain a sufficiently low mass water retention to avoid rapid degradation of the material. and sufficient for the development of aerobic biofilm and also has a low percentage of water-soluble material compatible with optimal conservation of the filtering material. According to the present invention, the filter material also has, in the form of a filter bed, an air capacity favorable to the development of an aerobic biofilm and a very good resistance to hydraulic compaction.

Indeed, against all expectations, on the one hand, the pieces of torrefied wood allow development and maintenance of the bacterial population which allows aerobic digestion of the organic matter in suspension in the waste water and on the other hand, the pieces of torrefied wood are resistant to hydraulic settlement and allow the preparation of a suitable filter bed.

Torrefied wood is known as a fuel because it has a high energy value and is therefore increasingly used in the bioenergy sector. Torrefied wood is also increasingly used in furniture such as fences, garden furniture, floors, etc...

It is also known that a torrefied biomass has greater grindability characteristics than the corresponding raw biomass (Baptiste Colin. Modeling of the forrefaction of wood chips in a rotary kiln and experimental validation on the scale of a continuous laboratory pilot Process engineering Albi-Carmaux School of Mines,

2014. French. NNT: 201 4EMAC0015). Greater grindability of torrefied wood makes it more manageable.

It has been demonstrated according to the present invention that despite the easier grindability of the torrefied material, the pieces of torrefied wood selected to have a particle size distribution between 3 and 25 mm have sufficient resistance to hydraulic compaction to form a durable filter bed.

In addition, torrefied pieces of wood having a particle size distribution between 3 and 25 mm also provide adequate water retention properties while having a reduced water-soluble matter content thanks to the torrefaction which reduces the content of water-soluble matter present in the wood. wood and destroys the parietal hemicelluloses as well as the accessible sugars.

As a result, the wood becomes resistant to attack by fungi, wood-boring insects and bacteria, which increases its durability.

However, despite the fact that torrefied wood is resistant to attack by microorganisms, it proves to be a completely satisfactory support for the bacterial population and the development and maintenance of the biofilm is favorable.

The filter bed formed from pieces of torrefied wood also has an air capacity suitable for the development and maintenance of the bacterial biofilm.

In addition, torrefaction of wood makes it possible to modify the structure of wood at a lower temperature than pyrolysis, therefore using less energy and without the addition of chemicals, which makes the torrefaction process more profitable and more ecological compared to to the aforementioned wood pyrolysis process.

In addition, wood, the raw material for torrefied wood, is inexpensive and durable.

In addition, the pieces of torrefied wood are formed from sawmill, production and cutting waste and thus form a by-product for which the treatment of waste water becomes a means of recovery.

Since wood is an abundant resource in many territories, the filtering material according to the present invention is in perfect harmony with an efficient, profitable and ecological circular economy by allowing an industrial application whatever the territory.

Preferably, in the filter material intended for the treatment of waste water according to the present invention, said pieces of torrefied wood are torrefied wood chips.

Within the meaning of the present invention, the term "torrefied wood slab" is understood to mean a body of torrefied wood having a shape defined by three dimensions, for example a thickness, a length and a width, and in which one of the dimensions, for example the thickness, is smaller than the other two dimensions.

Advantageously, in the filtering material intended for the treatment of waste water according to the present invention, said population of pieces comprises a first population of pieces having a distribution of particle sizes between 3 mm and 10 mm and a second population of pieces having a distribution of particle sizes greater than 10 mm and less than or equal to 25 mm, said fitrant material has a mass of the first population greater than that of the second population. Indeed, an adequate distribution of the size distribution of the pieces of torrefied wood by having more pieces whose size distribution is between 3 mm and 10 mm makes it possible to optimize the specific surface of the filter bed, i.e. both the interstitial surface between the pieces and the surface taking into account the porosity of the pieces, for the development of a bacterial biofilm, which will increase the purification performance of the filtering material. Moreover, such a distribution of the sizes of the pieces also allows better resistance to compaction and to degradation.

Preferably, the filtering material intended for the treatment of waste water according to the present invention has a water retention volume capacity of between 5% and 35%, more preferably of between 7% and 25% by volume of water calculated with respect to the volume of a bed of dry matter formed from the filter material. A volumetric water retention capacity in this range of values makes it possible to have (i) a sufficient conservation of humidity to maintain the bacterial biofilm, and (ii) a slow degradation of the filtering material because the volumetric retention capacity of water is low enough to slow putrefaction or degradation of the filter material.

Favorably, the filtering material intended for the treatment of wastewater according to the present invention has a bulk density of between 0.1 and 0.3 g/cm°.

Preferably, the filter material intended for the treatment of waste water according to the present invention has, in the form of a filter bed, an air capacity of between 20 and 50%, preferably between 30 and 40% by volume of air calculated with respect to the volume a bed of dry matter formed from the filter material. This range of values for the air capacity allows the filter material to be sufficiently aerated, which promotes optimal growth of the bacterial biofilm.

Advantageously, in the filtering material intended for the treatment of waste water according to the present invention, said pieces of torrefied wood are chosen from the group of pieces of beech wood, pine wood, poplar wood, ash wood, acacia, eucalyptus wood, softwood, and combinations thereof. Indeed, the pieces of torrefied wood can be made from different types of wood depending on the availability of wood waste, for example sawmill waste, in the territory in which the filter material according to the present invention is used.

Preferably, the filtering material intended for the treatment of waste water according to the present invention has, in the form of a filtering bed of dry matter, an initial volume and a resistance to degradation when it forms a bed of filtering material measured by a lower average settlement percentage to 5%, preferably less than 4%, more preferably less than 3%, even more preferably less than 2%, favorably less than 1% relative to the initial volume. Indeed, the average settlement percentage is calculated by a ratio between the difference between the initial volume occupied by a bed of dry matter formed by the filtering material and the volume occupied by the bed of dry matter formed by the filtering material after at least 56 days. and the initial volume of the bed of dry matter formed from the filter material. Indeed, a very low average compaction percentage ensures good conservation of the filtering material without risk of clogging and/or without reducing the purification performance of the filtering material.

Advantageously, the filtering material intended for the treatment of waste water according to the present invention has a water-soluble material content of between 1 and 6%, preferably between 2 and 4% by weight relative to the weight of filtering material.

Indeed, such a percentage of water-soluble matter in the filtering material allows good conservation of the filtering material and ensures that compounds of the filtering material which could possibly be a source of pollution are not released with filtered water.

Advantageously, the filtering material intended for the treatment of waste water according to the present invention comprises a content of pieces of torrefied wood of between 35 and 100% by weight relative to the weight of filtering material.

Indeed, a filtering material comprising pieces of torrefied wood and one or more other types of compounds can prove to be advantageous depending on the availability of the different types of compounds, and this so as to use, depending on the given territory, compounds locally available to form the filter material, reducing transport costs.

Preferably, the filtering material intended for the treatment of wastewater according to the present invention forms a filtering bed comprising one or more layers of said filtering material superimposed over a predetermined height, in which said one or more layers comprise said first population of pieces and/or said second population of pieces.

Other embodiments of the filter material intended for the treatment of waste water according to the present invention are indicated in the appended claims.

The present invention also relates to a purification tank intended for the treatment of waste water.

Advantageously, the purification tank intended for the treatment of waste water comprises: - A waste water settling chamber arranged to allow the suspended solids to sediment in a lower zone of the settling chamber, and comprising at least one inlet waste water and at least one clarified water outlet, - vre filtration chamber comprising at least one clarified water inlet, in fluid communication with said at least one clarified water outlet from the settling chamber, and at least one at least one filired water outlet, between which extends a filter it comprising the motériou fittrant according to the invention, said H of filter material having an upstream face and a downstream face in a fluidic direction of filtration.

Within the meaning of the present invention, the term “clarified water” designates waste water at least partially depleted of suspended solids.

Within the meaning of the present invention, the term “fluidic direction of filtration” denotes an average direction of the water through the bed of filter material between said at least one clarified water inlet and said at least one filtered water outlet. This average direction represents a resultant of the water displacement vectors, these water displacement vectors through the it of filtering material can be vertical, horizontal or oblique according to different positions of entry of clarified waters and positions of exits of filtered waters .

A purification tank comprising an upstream settling chamber and a downstream filtration chamber makes it possible to reduce the load of organic matter and suspended solid matter in the waste water while avoiding clogging of the filtering material.

Advantageously, in the purification tank intended for the treatment of waste water, the settling chamber also allows a degradation of organic matter by means of the metabolism of anaerobic bacteria.

Preferably, in the purification tank intended for the treatment of waste water, said at least one clarified water inlet of the filtration enclosure is located in an upper zone of said filtration enclosure and said at least one water outlet filtered is located in a lower zone of said filtration enclosure. Indeed, it is advantageous for gravity to be a main force field to define the fluidic direction of filtration.

Preferably, in the purification tank intended for the treatment of waste water, the fluid communication between said filtration enclosure and said settling enclosure is achieved through a clarified water distributor which comprises a plurality of outlets clarified water, the clarified water outlets distributing said clarified water into a plurality of positions on the upstream face of the filter bed. Indeed, a good distribution of clarified water upstream of the filter bed will make it possible to use a maximum of the filtration surface of the filter bed, which will increase the purification performance of the filter bed.

Advantageously, the purification tank intended for the treatment of waste water comprises a means for distributing clarified water, located downstream of said distributor, and comprising a tipping bucket. Indeed, the presence of a tipping bucket will further promote a good distribution of clarified water on the filter bed.

Preferably, the purification tank intended for the treatment of waste water comprises at least one pipe for supplying a gas containing oxygen arranged to supply oxygen into the filtration enclosure to allow aerobic digestion by a bacterial population distributed over the filter material.

Advantageously, in the purification tank intended for the treatment of waste water, said settling chamber and said filtration chamber have walls made of a material which is inert with respect to corrosion.

Advantageously, in the purification tank intended for the treatment of waste water, the bed of filtering material has a filtration surface of between 1 m? and 15 m2, more preferably between 2 m? and 10 met has a filtration bed height preferably between 60 cm and 1 m, more preferably between 80 cm and 95 cm, preferably 90 cm. Indeed, such a filtration surface combined with such a filtration height allows filtration of a volume of waste water corresponding to current domestic use.

Preferably, in the purification tank intended for the treatment of waste water, the filtering bed preferably comprises a homogeneous filtering material but said filtering bed can also comprise at least one additional filtering material chosen from the group of Xylit, crushed hulls of nuts, pyrolyzed wood, raw wood, and vegetable fibers, said filtering material and said additional filtering material being arranged in a mixture or in superposed layers according to the fluidic direction of filtration.

Other embodiments of the sewage tank for waste water treatment according to the present invention are indicated in the appended claims.

The present invention also relates to a process for purifying waste water in a purification tank.

The method for purifying wastewater in a purification tank according to the present invention comprises a settling zone and a filtration zone comprising a filter bed of filter material according to the present invention, said filter bed having an upstream face filter and a downstream face of the filter bed in a fluidic filtration direction, said method comprising the steps of: - an admission of waste water into the settling zone, - a settling of suspended solids in the settling zone forming a bed of decanted solids and a clarified liquid phase, - a transfer of the clarified liquid phase to the filtration zone, - a distribution of the clarified liquid phase in a plurality of positions on the upstream face of the filter bed, - a filtration of the clarified liquid phase through the filter bed according to the fluidic direction of filtration with obtaining filtered water, and

- an evacuation of filtered water by at least one filtered water outlet. Advantageously, in the process for purifying wastewater in a purification tank, the wastewater has an initial nitrate ion content and the filtered water has a nitrate ion content at least 2 times higher, preferably at least 3 times higher. higher, favorably at least 5 times higher than the initial nitrate ion content of the wastewater. Favorably, in the process for purifying wastewater in a purification tank, the filtered water has a biochemical oxygen demand, BODs, of less than or equal to 35 mgOa!, preferably less than or equal to 30 mgOa.! , favorably less than or equal to 20 mgOa.l.

Preferably, in the process for purifying waste water in a purification tank, the waste water has an initial ammonium ion content and the filtered water has an ammonium ion content reduced by at least 50%, preferably at least 60%, favorably at least 70% relative to the initial ammonium ion content of the wastewater.

Other characteristics, details and advantages of the invention will become apparent from the description given below, on a non-limiting basis and with reference to the appended drawings.

Figure 1 shows a photograph of pieces of torrefied wood as filter material according to the present invention.

Figure 2 is a cross-sectional schematic representation of several filter beds comprising pieces of torrefied wood.

Figure 3 is a schematic sectional representation of a purification tank according to the present invention. Figure 4 shows the water retention capacities of a filter bed made up of different filter materials.

Figure 5 shows the average hydraulic settlement of a filter bed according to the present invention compared to the average hydraulic settlement of a pine bark filter bed.

Figure 6 shows the content of water-soluble materials of filter materials according to the present invention in comparison with different comparative filter materials.

FIG. 7 shows a temporal evolution of the nitrate concentration of water filtered by a filter bed of filter materials according to the present invention in comparison with filter beds of different comparative filter materials.

In the figures, identical or similar elements bear the same references.

Figure 1 shows pieces of torrefied wood obtained by a torrefaction process as described in the document US10526543, and coming from Saniflor A or coming from LMK Energy B. To form the pieces of torrefied wood, raw wood is first crushed and cut. This crushed raw wood is then previously dried to less than 15% humidity before undergoing the roasting process.

The crushed torrefied wood is then classified according to its size, for example by sieving to retain pieces of torrefied wood having a grain size between 3 mm and 25 mm.

Advantageously, this raw wood is selected from a group of pieces of beech wood, pine wood, poplar wood, ash wood, acacia wood, eucalyptus wood, softwood, and their combinations.

Figure 2 illustrates a section of a filter bed comprising a filter material. The filter bed comprises, for example, loose pieces whose particle size is variable. Advantageously, a loose, random arrangement of the pieces creates a plurality of interstitial spaces, which promotes the development of a bacterial biofilm and a flow of clarified water through the filter bed. Figure 2A illustrates a section of a filter bed composed solely of pieces of torrefied wood 1. Figure 2B illustrates a section of a filter bed comprising, starting from the top downwards: - A first layer of pieces of torrefied wood 1, - a second layer comprising pieces of a second filtering material 2, - a third layer of pieces of torrefied wood, and - a fourth layer comprising pieces of a third filtering material 3. Advantageously, according to the present invention, the pieces of torrefied wood are chips of torrefied wood.

FIG. 2C illustrates a section of a filter bed comprising pieces of torrefied wood 1, pieces of a second filtering material 2 and pieces of a third filtering material which are randomly distributed over the height and/or the width and/or length of said filter bed.

Advantageously, the filter bed comprises a homogeneous filter material but it can also comprise at least one additional filter material, as for example illustrated in FIGS. 2B and 2C by the second filter material and the third filter material, chosen from the group of Xylit , crushed shells of nuts, pyrolyzed wood, raw wood, and vegetable fibers, said filtering material and said additional filtering material being arranged in a mixture or in superposed layers according to the fluidic direction of filtration.

As shown in Figure 2, the filter material comprises a population of pieces.

Advantageously, said population of pieces comprises a first population of pieces having a distribution of particle sizes between 3 mm and 10 mm and a second population of pieces having a distribution of particle sizes greater than 10 mm and less than or equal to 25 mm , said filter material has a mass of the first population greater than that of the second population.

Arrows 4 in Figure 2 illustrate the water displacement vectors through the filter material.

As illustrated, these water displacement vectors 4 through the filtering material can be vertical,

horizontal or oblique according to different clarified water inlet positions and filtered water outlet positions. The fluidic direction of filtration is the sum of these different vectors of water displacement through the filtering material. Advantageously, this fluidic direction is substantially oriented from the top of the filter bed to the bottom of the filter bed according to the force of gravity.

The filter material according to the present invention has certain chemical and physical properties. Indeed, the filtering material advantageously has a water retention capacity of between 7% and 25% by volume of water calculated with respect to the volume of a bed of dry matter formed from the filtering material. The filter material also preferably has an apparent dry matter bed density of between 0.1 and 0.3 g/cm. The filter material in the form of a filter bed is also advantageously defined by an air capacity of between 30 and 40% by volume of air calculated with respect to the volume of a bed of dry matter formed from the filter material. The filtering material according to the present invention also preferably has a resistance to degradation when it forms a bed of filtering material measured by an average percentage of settlement of less than 1% relative to the initial volume of a bed of dry matter formed. filter material. The filtering material also advantageously has a water-soluble material content of between 2 and 4% by weight relative to the weight of filtering material. Preferably, the filtering material according to the present invention comprises a content of torrefied wood pieces of between 35 and 100% by weight relative to the weight of filtering material.

FIG. 3 illustrates a sectional view of the purification tank 5 intended for the treatment of waste water according to the present invention. This purification tank 5 comprises a settling enclosure 6 and a filtration enclosure 7 having walls 8 made of a material inert with respect to corrosion such as concrete, or a polymer such as polyethylene or polypropylene. The settling chamber 6 and the filtration chamber 7 are separated by a wall 9. The purification tank 5 is also provided with a first opening system 10 allowing access to the settling chamber 6 and a second opening system 11 allows access to the filtration enclosure 7. The settling enclosure é comprises a waste water inlet 12 and a clarified water outlet through a filter 13. A pipe 14 is connected to the fire 13 providing fluid communication between the settling enclosure 6 and the filtration enclosure 7. The filtration enclosure 7 comprises a disthbuteur 15 connected to the iuyau 14. Advantageously, the distributor 15 is attached to an upper wall of the filtration enclosure 7 using fasteners 16 and 16'. The distributor 15 also has a clarified water inlet 17. The filtration enclosure 7 also has a fifred water outlet 18, between which extends a filtering bed 19 comprising the filtering material according to the present invention. The filtration enclosure 7 also comprises a means of distributing the clarified water, located downstream of the distributor 15 and comprising a tipping bucket {not shown in FIG. 3}. Said distribution means comprises a plurality of outlet orifices 20, 20° which end in pipes 21, 21° to be discharged onto elifiitrani 19.

The waste water enters the settling chamber 6 via the waste water inlet 12. The settling chamber 6 is arranged to allow the suspended solids to settle in a lower zone 22 of the settling chamber 6 The clarified liquid phase, the ciorified water, leaves the settling chamber 6 or through the filter 13 and then arrives in the distributor 15 through the pipe 14 surmounting the distributor 15. The pipe 14 allows the inflow of ciorfied water donations the filtration tank 7. the filtration enclosure 7 comprising said it of filter material 19 has an upstream face and a downstream face in a fluid direction of filtration. The fluidic direction of filtration is the sum of the displacement vectors 4 of the water through the filtering material. These displacement vectors 4 are also represented in FIG. 2. Advantageously, the entry of clarified water through pipe 14 into the filtration enclosure is located in an upper zone of said filtration enclosure 7 and said water outlet 18 is located in a lower zone of said filtration enclosure 7. Advantageously, fluid communication between said filtration enclosure 7 and said settling enclosure 6 is achieved through a clarified water distributor 15 which comprises a plurality of clarified water outlets 20, 20' and 21, 21', the clarified water outlets distributing said clarified water into a plurality of positions on the upstream face of the filter bed. Said clarified water passes through the filter bed 19 in the fluidic direction of filtration to give filtered water. The filtered water is then evacuated through the filired water outlet 18. Preferably, the purification tank 5 also comprises at least one pipe for supplying a gas (not shown in FIG. 3) containing oxygen arranged to provide oxygen in the filtration enclosure 7 to allow aerobic digestion by a bacterial population distributed over the bed of filtering material 19. Advantageously, the bed of filtering material 19 has a filtration surface of between 2 m2 and 10 m2.

Advantageously, said suspended solids which have sedimented in the lower zone 22 of the settling chamber 6 are evacuated from the settling chamber 6 in a predetermined manner at regular intervals.

Advantageously, the waste water has an initial nitrate ion content and the filtered water has a nitrate ion content at least 5 times greater than the initial nitrate ion content of the waste water.

Preferably, the filtered water has a biochemical oxygen demand, BODs, of less than or equal to 20 mgOa.

Preferably, the waste water has an initial ammonium ion content and the filtered water has an ammonium ion content reduced by at least 70% relative to the initial ammonium ion content of the waste water.

Examples.- Example 1.- Temperature and roasting time of the wood to obtain pieces of forrefied wood according to the present invention The raw wood from sawmill waste is cut beforehand before drying into pieces having a grain size ranging from 3 mm to 10 cm . This cut wood is then dried at a temperature of 80° C. in a reducing atmosphere to obtain a dried cut wood having a humidity of less than or equal to 15%.

After drying, the wood is roasted at a gradually increasing temperature up to 240°C for 60 minutes in a reducing atmosphere.

The pieces of torrefied wood are then sieved using a sieve to collect pieces with a grain size between 3 mm and 25 mm.

These pieces of torrefied wood are separated into a first population of pieces having a particle size between 3 mm and 10 mm and a second population of pieces having a particle size greater than 10 mm and less than or equal to 25 mm.

Example 2—Preparation of a Filtering Bed Comprising Pieces of Forrefied Wood According to the Present Invention A filter bed comprising the pieces of torrefied wood obtained in Example 1 is placed in the filtration enclosure.

This filter bed has a diameter of 200 mm and a bed height of 480 mm.

This filter bed comprises 3.15 kg of dry matter of pieces of torrefied wood, of which 70% by mass corresponds to the first population of pieces and 30% by mass corresponds to the second population of pieces.

The first and the second population of pieces are mixed in bulk throughout the volume constituting the filter bed.

Example 3. Purifying copacity in a ballast pilot A test pilot according to the present invention having a volume of 15 dm and a filter bed according to example 2, has a wastewater treatment capacity defined by a nominal daily organic load of 150 Li".EH and a nominal daily hydraulic flow of 3.6 | per day (1d). The term "EH" means "equivalent inhabitant". This test pilot makes it possible to obtain filtered water at the outlet of the enclosure filter with a nominal daily organic load less than or equal to 35 mg/l and a suspended solids content less than or equal to 30 mg/l.

Example 4.- Comparison of the volumetric water retention capacities of the filter bed made up of different filter materials

The volumetric water retention capacities of the materials listed in Table 1 were compared by applying 23 | of water for 23 dm of filter material in the form of a bed of 2.875 dm? over a height of 0.8 m.

Filter materials composed of pieces of torrefied wood from Saniflor (Example 4.1}, pieces of torrefied wood from LMK Energy (Example 4.2), coconut coir (Example 4.3), pine bark (Example 4.4) and hazelnut shells (Example 4.5). (Saniflor) Example 4.2 According to the invention Pieces of | 6.5 mm 196 torrefied wood kg/m3 (LMK Energy) Example 4.3 Example Coconut coir | 4 mm 94 kg/ms CEE leon [07 (TOI Example 4.4 Example Coconut bark | 3mm 169 PTE (esse on am Example 4.5 Example Shells of | 10 mm 225 [TE men [rare | [am To determine the volumetric water-holding capacity, a filter bed was immersed in a predetermined volume of water for 24h.

The filter bed is then drained until no more water flows from the filter bed (stable mass of water in a scaled collection tank). We then recovered a determined volume of filtration material (less than 5 dm') waterlogged which was then placed in an oven at 110°C for 12 hours.

The quantity of water retained by the filtration material is obtained by subtracting the mass of material after drying in an oven (110°C, 12h) from the initial mass of material saturated with water (after draining) and carried by unit volume of filtration material. The volumetric water holding capacities of the filter bed are calculated as the difference between the wet mass of the sample and the dry mass of the sample divided by the volume of the sample. In some cases, it may be advantageous to sample at different locations in the column and average. Figure 4 illustrates the volumetric water retention capacity of a filter bed made up of pieces of torrefied wood from Saniflor (Ex 4.1) or LMK Energy (Ex 4.2), coconut coir (Ex 4.3), pine bark (Ex 4.4), hazelnut shells (Ex 4.5). Figure 4 shows the volumetric water retention capacity of the filter bed at a predetermined height of the filter bed. The filtering materials used in Examples 4.1, 4.2 and 4.5 show a lower water retention volume capacity than the filtering materials used in Examples 4.3 and 4.4. It is important to have a volumetric water retention capacity greater than 5% to allow good growth of the bacterial biofilm but less than 25% to avoid too rapid degradation of the filter material. A filter bed consisting of coconut coir (Ex 4.3) and pine bark (Ex 4.4) shows too high a volume water retention capacity compared to pieces of roasted wood [Ex 4.1 and 4.2) and hazelnut shells (Ex 4.5). Each black bar in Figure 4 represents an average over 3 experimental measurements. Example 5.- Comparison of the hydraulic settlement of the filter bed formed by different filtering materials The hydraulic settlement of the materials indicated in table 2 was compared by pouring 11.5 | of water on a filter bed of 23 | in less than 5 minutes. Filtering materials composed of pieces of torrefied wood from Saniflor (Example 4.1) and pine bark (Example

4.4). Hydraulic settlement is measured by comparing the height of the filter bed after water discharge with the height of the filter bed of dry matter before water discharge. Table 2.-

Example 4.1 According to the Invention Wood Pieces 01 Figure 5 illustrates the percentage of hydraulic settlement for the filter material of Example 4.1 and Example 4.4. As shown in Figure 5, a filter bed made of pine bark (Ex 4.4) is more sensitive to settlement than a filter bed made of pieces of torrefied wood (Ex 4.1). The torrefied pieces of wood are particularly resistant to hydraulic compaction because they have a great hardness following their torrefaction. In addition, these pieces of torrefied wood advantageously being torrefied wood chips have a rectangular shape, short whose arrangement is unlikely to vary. Figure 5 shows for examples 4.1 and 4.2 an average of the percentage of hydraulic settlement of 3 experimental measurements and a standard deviation corresponding to each average.

Example 6.- Comparison of water-soluble material content of different filter materials The water-soluble material content of different filter materials shown in Table 1 was compared. To determine the water-soluble material content of a filter material, the Van Soest's technique for the determination of the parietal constituents of plants for the analysis of compost products and various litters for agronomic use using an aqueous solvent (htn://ep.onigres.free.fr/aite/himi/coursé/ coursA- 3/resources/Methode7%320- R20Tschnique%20Van%20S0oest%20manuelle7%20et%20Fibertec.vof and Van Soest P.J. and Wine R.H., “Determination of lignin and cellulose in acid-detergent liber with permanganate”, J. Assoc. Off. Anal. Chem. 1968, 51: 780-785).

Figure 6 shows the water-soluble content of different filter media as shown in Table 1. As shown in Figure 6, the pieces of torrefied wood (Ex 4.1 and 4.2) have a water-soluble content of 3 to 3.5% , which is relatively low and allows good conservation of the filtering material. In comparison, a filter material made of coconut coir (Ex 4.3) has a percentage of water-soluble matter of 13%, which is too high and harmful for adequate conservation of the filter material. FIG. 6 shows for each example an average of the water-soluble matter content of 3 experimental measurements and a standard deviation corresponding to each average. Example 7.- Evolution over time of the production of nitrates by the aerobic biomass contained in the filter bed formed of different filtering materials The evolution over time of the nitrate content of water filtered by different filtering materials indicated in table 3 was compared. To do this, the nitrate content of the filtered water is measured by a photometric method (DMP 1.0-50.0 mg/l! NOz-N) Once a week. Table 3.- Example 4.1 According to the invention Pieces of torrefied wood CE (TT | EE Example 4.2 According to the invention Pieces of torrefied wood OT loen As shown in Figure 7, a filter bed consisting of pieces of torrefied wood (Ex 4.1 and 4.2) makes it possible to obtain filtered water with an increased concentration of nitrates after 5 weeks of treatment compared to the filter bed made up of coconut coir (Ex 4.3) or pine bark (Ex

4.4). The production of nitrate ions (NO3) or nitrification is the result of the degradation of ammonium ions. It is ensured by aerobic bacteria which intervene at the very end of the cycle. Also, the presence of nitrification is an indication of good biological health of the filter and of effective degradation of nitrogen. Figure 7 shows for each example, and each week,

an average of the nitrate ion content of 3 experimental measurements and a standard deviation corresponding to each average.

It is understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the appended claims.

Claims (21)

1. Vegetable filtering material intended for the treatment of waste water comprising cellulose and lignin, said vegetable filtering material being characterized in that it comprises pieces of wood forming a population of pieces, said population of pieces having a distribution of particle faults between 3 mm and 25 mm and said pieces of wood being pieces of torrefied wood. 2. Filter material according to claim 1, wherein said population of pieces comprises a first population of pieces having a distribution of particle faults between 10 3 mm and 10 mm and a second population of pieces having a distribution of particle sizes greater than 10 mm and less than or equal to 25 mm, said filter material has a mass of the first population greater than that of the second population. 3. Filter material according to claim 1 or claim 2, having a water retention volume capacity of between 5% and 35%, more preferably between 7% and 25% by volume of water calculated with respect to the volume of water. a bed of dry matter formed from the filter material. 4. Filter material according to any one of claims 1 to 3, having a bulk density of between 0.1 and 0.3 g/cm* 5. Filter material according to any one of claims 1 to 4, having in the form of a filter bed an air capacity of between 20 and 50%, preferably between 30 and 40% by volume of air calculated relative to the volume of a bed of dry matter formed from the filter material. 6. Filter material according to any one of the preceding claims, in which said pieces of torrefied wood are chosen from the group of pieces of beech wood, pine wood, poplar wood, ash wood, oak wood. acacia, eucalyptus wood, softwood, and combinations thereof. 7. Filter material according to any one of the preceding claims, having, in the form of a filter bed of dry matter, an initial volume and a resistance to degradation when it forms a bed of filter material measured by an average percentage of settlement of less than 5 %, preferably less than 4%, more preferably less than 3%, even more preferably less than 2%, favorably less than 1% relative to the initial volume. 8. Filter material according to any one of the preceding claims, having a water-soluble material content of between 1 and 6%, preferably between 2 and 4% by weight relative to the weight of filter material. 9. Filter material according to any one of the preceding claims, comprising a content of torrefied wood pieces of between 35 and 100% by weight relative to the weight of filter material. 10. Purification tank intended for the treatment of waste water, kidite purification tank comprising: - a waste water settling chamber arranged to allow the suspended solids to settle in a lower zone of the settling chamber, and comprising at least one waste water inlet and at least one clarified water outlet, - vre filtration enclosure comprising at least one clarified water inlet, in fluid communication with said at least one chlorified water outlet of said enclosure decantation, and at least one filtered water outlet, between which extends a filter bed comprising the filter material according to any one of the preceding claims, the said filter material bed having an upstream face and a downstream face in a direction filtration fluidics. 11. Purification tank according to claim 10, wherein said at least one clarified water inlet of the filtration enclosure is located in an upper zone of said filtration enclosure and said at least one filtered water outlet is located in a lower zone of said filtration enclosure. 12. Purification tank according to claim 10 or claim 11, wherein the fluid communication between said filtration enclosure and said settling enclosure is carried out through a clarified water distributor which comprises a plurality of outlets of clarified water, the clarified water outlets distributing said clarified water into a plurality of positions on the upstream face of the filter bed. 13. Purification tank according to any one of claims 10 to 12, comprising a means for distributing clarified water, located downstream of said distributor, and comprising a tipping bucket. 14. Purification tank according to any one of claims 10 to 13, comprising at least one supply pipe for a gas containing oxygen arranged to supply oxygen into the filtration enclosure to allow a aerobic digestion by a bacterial population distributed over the filter material. 15. Purification tank according to any one of claims 10 to 14, wherein said settling chamber and said filtration chamber have walls made of a material inert with respect to corrosion. 16. Purification tank according to any one of claims 10 to 15, wherein the bed of filter material has a filtration surface of between 1] m? and 15 m2, more preferably between 2 m? and 10 m? and has a filtration bed height preferably between 60 cm and 1 m, more preferably between 80 cm and 95 cm, preferably 90 cm. 17. Purification tank according to any one of claims 10 to 16, wherein the filter bed preferably comprises a homogeneous filter material but said filter bed may also comprise at least one additional filter material chosen from the group of Xylit, shells crushed nuts, pyrolyzed wood, raw wood, and vegetable fibers, said filtering material and said additional filtering material being arranged in a mixture or in superimposed layers according to the fluidic direction of filtration. 18. Process for purifying waste water in a purification tank comprising a settling zone and a filtration zone comprising a filter bed of filter material according to any one of claims 1 to 9, said filter bed having a side upstream of the filter bed and a face downstream of the filter bed in a fluidic filtration direction, said method comprising the steps of: - an admission of waste water into the settling zone, - a settling of suspended solids in the decantation forming a bed of decanted solids and a clarified liquid phase, - a transfer of the clarified liquid phase to the filtration zone, - a distribution of the clarified liquid phase in a plurality of positions on the upstream side of the filter bed, - filtration of the clarified liquid phase through the filtering bed according to the fluidic filtration direction with obtaining filtered water, and - evacuation of the filtered water by at least ins a filtered water outlet. 19. Process for purifying waste water in a purification tank according to claim 18, in which the waste water has an initial nitrate ion content and the filtered water has a nitrate ion content at least 2 times higher, preferably at least 3 times higher, favorably at least 5 times higher than the initial nitrate ion content of the waste water. 20. Process for purifying waste water in a purification tank according to claim 18 or claim 19, in which the filtered water has a biochemical oxygen demand, BODs, of less than or equal to 35 mgOa.!, preferably less or equal to 30 mgO:2.l, favorably less than or equal to 20 mgOa.. 21. Process for purifying waste water in a purification tank according to any one of claims 18 to 20, in which the waste water has an initial ammonium ion content and the filtered water has a reduced ammonium ion content. at least 50%, preferably at least 60%, favorably at least 70% relative to the initial ammonium ion content of the waste water.