Method for making biofilm supports intended for use in a biological water treatment device

Abstract

The invention relates to a method for making biofilm supports intended for use in a biological water treatment device, the method comprising the following steps: a) producing a mixture consisting of an alginate, a plasticiser and water, wherein the alginate and the plasticiser are present in the mixture in an alginate / plasticiser weight ratio of 2 to 3, and wherein the mixture comprises 20% to 60% by weight of alginate and 15% to 65% by weight of water; b) stirring the mixture until a powder or aggregates are obtained; b') optionally, an additional step of grinding or crushing the aggregates; c) optionally hot-extruding the compound obtained at the end of step b) or b'), if necessary after adjusting its moisture content to a value of between 5% and 40%, to obtain an extruded product, and dividing the extruded product into granules; d) co-extruding, thermocompressing or thermoforming a mixture consisting of 50% to 95% by weight of the compound obtained at the end of step b) or b') or of the granules obtained at the end of step c), and 5% to 50% of a biodegradable aliphatic polyester, resulting in an extruded, thermocompressed or thermoformed material; e) shaping the extruded, thermocompressed or thermoformed material to obtain the supports.

Description

Method for manufacturing biofilm supports intended for use in a biological water treatment system Technical Field

[0001] This presentation concerns the field of biological water treatment for purification or drinking water production using biomass attached to mobile supports. More specifically, this presentation addresses the design, construction, and use of such mobile supports. Previous Technique

[0002] Biological water treatment processes utilize one or more biomasses capable of degrading all or part of the organic pollution contained in the water. This biomass can be placed in an aerobic, anoxic, or alternating aerobic and anoxic environment, depending on the nature of the pollutants to be removed.

[0003] This biomass can be used in its free form, for example in the activated sludge technique, in granular form, or as a biofilm attached to mobile supports. In the treatment plants, these supports form beds that can be fixed or fluidized by the flow of water being treated and, where applicable, by the biomass oxygenation fluid.

[0004] Fixing biomass to supports offers many advantages. It allows for a reduction in the footprint of installations, minimizes biomass leakage, and increases biomass efficiency. Indeed, fixing biomass limits its time suspended in water, a form in which it is ineffective or only minimally efficient. It also improves oxygen availability when the biomass is operating aerobically.

[0005] However, in order to fulfill their functions, the media must meet numerous criteria.

[0006] Therefore, the material used to make them must be "compatible" with biomass, meaning it must allow biomass to grow on its surface. In practice, the biomass must be able to adhere to the material both during seeding and throughout the substrate's lifespan. Furthermore, this material must not It must absorb water, which would make the supports heavier. This material must also have good mechanical resistance.

[0007] The supports themselves must have a large protected surface area. A protected surface area is defined as the surface of the support accessible to the biomass but not likely to come into contact with external solid elements, primarily other moving supports and the walls of the tanks containing them. This protected surface area can be optimized by treating or shaping the material. These supports must also have a large specific surface area and be sufficiently mechanically strong to withstand the compressive and bending stresses imposed on them by the weight of other supports when stored or when present in the empty tanks of inactive systems. They must also be resistant to abrasion, i.e., impacts generated by interaction between them, and to erosion, i.e., wear related to their use.Their dimensions must be compatible with the installations and their maintenance, and in practice should be between 1 and 10 cm. Their density must allow for easy fluidization in water, i.e., with little energy, and in practice less than 1 kg / m³. 3 Finally, their manufacturing process must be able to guarantee large volumes and low production costs.

[0008] Some very common plastics, such as HDPE or polyurethane, make it possible to meet these demanding specifications, which is why these supports are now mostly made up of them.

[0009] Materials other than plastics have been proposed for biomass substrates. The literature describes substrates made of porous inorganic materials, sometimes ground, such as certain zeolites, volcanic rocks, or expanded clays. These materials have the advantage of being naturally porous, very hard, and therefore having a high specific surface area. However, the porous nature of these materials has the disadvantage of giving the substrates made from them poor resistance to erosion and abrasion. Furthermore, they are difficult to streamline at low cost. Finally, their supply in large quantities can be subject to numerous uncertainties (production, availability, etc.).

[0010] Other reactive natural organic materials, that is, those that provide nutrients to biomass as they decompose, were also considered. Supports in the form of wood chips were tested. However, while such materials are inexpensive and can be used to manufacture easily fluidizable supports, they also exhibit poor mechanical properties and degrade rapidly in aqueous environments. They also have the disadvantage of releasing significant amounts of soluble carbon in an uncontrolled manner when in contact with water.

[0011] Slow-release carbon biomass carriers (SLCs) have also been proposed. This category of carriers, which also serve as a source of nutrients for biomass, is primarily obtained from biodegradable insoluble polymers. These polymers can be "petroleum-based," meaning derived from fossil fuels such as polycaprolactone (PCL), or "bio-based," meaning composed of natural molecules such as polysaccharides (e.g., starch) or derived from synthetic materials such as polylactic acid (PLA) or polybutylene succinate (PBS). These carriers eventually degrade in reactors, but at a slower rate than reactive natural organic materials. However, this degradation varies from one polymer to another.Some polymers such as polylactic acid (PLA) are only biodegradable under industrial composting conditions, whereas polysaccharides and proteins are generally biodegradable at low temperatures in home compost.

[0012] Such supports, which can take many forms, are also a lever for limiting the environmental impact of biofilm supports.

[0013] The use of PCL as a biofilm support with progressive degradation of the latter by enzymatic hydrolysis is thus described by L. Chu (see reference 1 at the end of this presentation) and AL. Rodrigues (reference 2).

[0014] The use of PLA as a coating on a conventional petroleum-based polymer support, in order to promote the adhesion and development of biofilm, has been described by N. Yu et al. (reference 3).

[0015] The use of PBS as a biofilm support has been studied by Lua et al. (reference 4) and Shu et al. (reference 5).

[0016] Such biodegradable polyesters offer a number of advantages: thermoplasticity, thermal stability, good mechanical strength, good water resistance, dimensional stability.

[0017] However, their cost is high, and their manufacturing process consumes as much energy as that of traditional petroleum-based polymers.

[0018] A potentially promising method for reducing the cost and environmental impact of this type of substrate is to combine biodegradable aliphatic polyesters with less expensive natural materials. However, a challenge lies in finding polyesters and natural materials that are compatible with each other to ensure the long-term stability of their blend. Good compatibility improves the properties of the final material.

[0019] However, since polysaccharides are hydrophilic and aliphatic polyesters are rather hydrophobic, these compounds are not very compatible with each other. To improve their compatibility, it is necessary to create new chemical interactions at the interfaces or to increase the surface area of ​​these interfaces. This can be achieved by modifying the mechanical means used to mix them and / or by adding compatibilizing reagents.

[0020] Several families of such compatibilizing reagents have been studied to optimize the interface between aliphatic polyesters and natural polysaccharides. However, the phenomena involved are complex and not yet fully understood. Therefore, there is currently no proven, generic solution for compatibilizing natural polysaccharides and aliphatic polyesters.

[0021] In practice, it is primarily the combination of aliphatic polyesters and starch that has been studied. For example, Shen et al. described PCL supports with rapid degradation of starch on the surface (see reference 6).

[0022] However, starch has reduced thermal stability, low mechanical strength, and absorbs water. These drawbacks, which are also common to many other natural polysaccharides, make their use for forming biofilm supports in industrial water treatment plants difficult to envision.

[0023] An objective of the present invention is to propose a process for manufacturing biomass supports from a biodegradable aliphatic polyester and a natural polysaccharide allowing the use of these supports in industrial wastewater treatment facilities or devices in fluidized bed.

[0024] In particular, one objective is to propose such a process which allows the production of supports which have a lifespan compatible with industrial-scale use.

[0025] Another objective is to describe such a process which is inexpensive to implement and which allows for the mass production of such media.

[0026] Another objective of the invention is to propose such a process to lower the carbon footprint of water treatment facilities in which biofilm supports are used. Description of the invention All or part of these objectives are achieved through the invention, which relates to a method for manufacturing biofilm supports for use in a biological water treatment device, said method comprising the following steps: a) preparing a mixture composed of an alginate, a plasticizer, and water, said alginate and said plasticizer being present in said mixture according to a a) alginate / plasticizer weight ratio of 2 to 3, said mixture comprising 20 to 60% by weight of alginate and 15 to 65% by weight of water; b) stirring said mixture until a powder or aggregates are obtained; b 7 (c) possibly the additional step of grinding or crushing said aggregates; (b) possibly the hot extrusion of said compound obtained at the end of step (b) or (b) 7 (d) where appropriate after adjusting its moisture content to a value between 5 and 40%, to obtain an extruded product, and the fractionation of said extruded product into granules; (d) the co-extrusion, thermocompression or thermoforming of a mixture consisting of 50 to 95% by weight of said compound obtained at the end of step (b) or (b) 7) or said granules obtained at the end of step c) and 5 to 50% of a biodegradable aliphatic polyester leading to an extruded, thermocompressed or thermoformed material; e) shaping said extruded, thermocompressed or thermoformed material to obtain said supports.

[0027] Thus, the invention proposes to combine an alginate with an aliphatic polyester to form biofilm supports.

[0028] The specific conditions and treatment of this association make it possible to obtain a material with mechanical properties, including resistance to erosion and abrasion, and water resistance, better than those of prior art materials obtained by combining aliphatic polyesters with other natural polysaccharides such as starch.

[0029] In this regard, it should be noted that, to the inventors' knowledge, no study on the association of alginate with biodegradable aliphatic polyesters has yet made it possible to define the conditions for improving their compatibility in a stable way over time.

[0030] More specifically, the conditions indicated in the step of mixing the alginate with water and a plasticizer, and in the step of stirring this mixture, allow for the production of a powder or aggregates of plasticized alginate. This powder or these plasticized aggregates, reduced by crushing or grinding, can then be intimately combined with bio-based aliphatic polyester to obtain a stable material that can be transformed into biofilm supports. The low-speed, low-turbulence stirring step can be carried out in a mixer, for example a turbo mixer, or in any other device to obtain a homogeneous mixture with reduced energy consumption.

[0031] During the plasticization process, water acts as both a destructuring and plasticizing agent. Its concentration in the mixture strongly impacts the properties of the resulting product. For this reason, the compound obtained at the end of step b) or step b 7 ) above will preferably be hot extruded to obtain a better plasticized extruded product, ensuring that the moisture content of the compound is within an acceptable range.

[0032] According to one variant, the adjustment of the hygrometry of the compound obtained in step b) or b') is carried out by a drying step of the compound.

[0033] Preferably, said biodegradable aliphatic polymer is polylactic acid (PLA) or said polybutylene succinate (PBS), said coextrusion, said compression or said thermoforming being carried out on a mixture of 50 to 95% by weight of said compound obtained at the end of step b) or b 7) or said granules obtained at the end of step c) and 5 to 50% by weight of polylactic acid or polybutylene succinate.

[0034] According to one variant, said coextrusion, said thermocompression or said thermoforming is carried out with 75% by weight of said compound obtained at the end of step b) or b') or of said granules obtained at the end of step c) and 25% by weight of polylactic acid.

[0035] According to another variant, said coextrusion, said thermocompression, said thermoforming is carried out with 85% by weight of said compound obtained at the end of step b) or b') or of said granules obtained at the end of step c) and 15% by weight of polybutylene succinate.

[0036] Preferably, said alginate is sodium alginate.

[0037] Advantageously, the process includes a step of adding a calcium salt. By treating the sodium alginate with an aqueous solution containing calcium ions (Ca 2+ ) an ion exchange takes place and leads to the formation of an insoluble calcium alginate gel. This step of adding a calcium salt is preferably carried out on the said extruded, thermocompressed or thermoformed product.

[0038] The substrates produced using the proposed process preferentially have a density close to 1 to be more easily fluidized.

[0039] When the mixture of plasticized alginate powder or granules and biodegradable aliphatic polyester is made by co-extrusion, the process may include a foaming or expansion step carried out during said co-extrusion in order to bring the density of said extruded-foamed product to a value close to 1.

[0040] When the mixture of plasticized alginate powder or granules and biodegradable aliphatic polyester is produced by coextrusion, thermocompression, or thermoforming, a filler may be added during the coextrusion, thermocompression, or thermoforming step to bring the density of the co-extruded, thermocompressed, or thermoformed material close to 1. The process will then include a step during the coextrusion, thermocompression, or thermoforming of at least one filler that brings the density of the material close to 1.

[0041] Although steps a) to d) of the above process already provide good compatibility between the alginate and the polyester, the process may include an additional step of adding during said co-extrusion or said thermocompression or said thermoforming at least one compatibilizing agent to improve the compatibility of said alginate and said biodegradable aliphatic polymer.

[0042] In this case, said at least compatibilizing agent shall be chosen from maleic anhydride, succinic anhydride, malic acid, ascorbic acid, citric acid.

[0043] Finally, preferably, the plasticizer used will be chosen from glycerol, sorbitol, xylitol or a mixture of these.

[0044] The invention also relates to any biofilm support for a biological water treatment device that has been obtained by the process according to the invention described above.

[0045] Finally, the invention also relates to any biological water treatment device comprising at least one reactor accommodating such biofilm supports. Brief description of the drawings

[0046] The invention will be better understood from the following description of a non-limiting embodiment thereof, given only by way of example with reference to the drawings in which: Fig.1 is a photograph of pieces of a material obtained by means of a first embodiment of the process according to the present exposition; Fig. 2 is a scanning electron micrograph of an alginate / PLA composite. Detailed description of a method of implementation.

[0047] A mixture was made containing 48.6% sodium alginate, 19.4% glycerol and 32% water.

[0048] This mixture was then placed in a turbo-mixer, where it underwent low-speed, low-turbulence agitation for less than 30 minutes, preferably between 2 and 20 minutes. At the end of this step, an alginate powder with a moisture content of approximately 35% was obtained.

[0049] This powder was then hot-extruded in an extruder set to an extrusion temperature of 110 to 130 °C. The extruded product was in the form of a solid tube with a diameter between 0.5 and 5 cm, or a rod 2 to 6 mm in diameter, flexible enough to be coiled. The tube was then split into multiple smaller tubes of equal and controlled dimensions, or the rod was split into The granules were of a size close to the diameter of the rush and were precisely controlled. The moisture content of these granules was measured at 25%.

[0050] These granules were used to make two mixtures, one consisting of plasticized alginate granules and PLA in a 50 / 50 weight ratio and the other consisting of plasticized alginate granules and PBS in an 80 / 20 weight ratio.

[0051] Each of these mixtures was then hot co-extruded in an extruder set to an extrusion temperature of 140 to 150 °C for the mixture containing PLA and 115 °C to 130 °C for the mixture containing PBS.

[0052] This yielded an extruded material in the form of a hollow tube (or granules) with rough surfaces. Pieces of this hollow tube are shown in Figure 1.

[0053] These pieces of tubing were placed for 4 hours in a 3% aqueous calcium chloride solution and then rinsed. This treatment improves the material's water resistance.

[0054] The material obtained from the co-extrusion of the plasticized alginate / PLA mixture was observed using a scanning electron microscope. This observation confirmed the compatibility of the two materials. Referring to Figure 2, alginate grains with a size of 50 micrometers were observed in the matrix, indicating very good integration of this compound into the PLA.

[0055] The tube-shaped element obtained after the co-extrusion step and treatment with calcium chloride exhibits good mechanical properties, a high protected surface area inside the tube, and internal and external surfaces with asperities suitable for accommodating and retaining biomass. These characteristics allow it to be used in water treatment reactors as biofilm supports. Such supports can be implemented in industrial water treatment plants. 1 : Chu, L .Nitrogen removal using biodegradable polymers as carbon source and biofilm carriers in a moving bed biofilm reactor, Chemical Engineering Journal 170, Issue 1, 2011, 220-225 2: Rodrigues, A,L. A poiy-e-caproiactone based biofilm carrier for nitrate removal from water, International journal of Environmental Science and Technology, 2014 3: N. Yu, Innovative Coating-Etching Method of Biocarrier Fabrication for Treating Wastewater with a Low C / N Ratio, Polymers 2022, 14(15), 3010 4 : Luo, G., Li, L., Liu, Q., Xu, G., Tan, H., 2014a. Effect of dissolved oxygen on heterotrophic denitrification using poiy(butyiene succinate) as the carbon source and biofilm carrier. Bioresour. Technol. 171, 152-158 5 : Zhu, S.M., Deng, Y.L., Ruan, Y.J., Guo, X.S., Shi, M.M., Shen, J.Z., 2015. Biological denitrification using poiy(butyiene succinate) as carbon source and biofilm carrier for recirculating aquaculture system effluent treatment. Bioresour. Technol. 192, 603-610 6 : Shen, Z., Wang, J., 2011. Biological denitrification using cross-linked starch / PCL blends as solid carbon source and biofilm carrier. Bioresour. Technol. 102, 8835- 8838.

Claims

DEMANDS

1. A method for manufacturing biofilm supports for use in a biological water treatment device, said method comprising the following steps: a) preparing a mixture of an alginate, a plasticizer, and water, said alginate and plasticizer being present in said mixture in an alginate / plasticizer weight ratio of 2 to 3, said mixture comprising 20 to 60% by weight of alginate and 15 to 65% by weight of water; b) stirring said mixture until a powder or aggregates are obtained; b 7 (c) possibly the additional step of grinding or crushing said aggregates; (b) possibly the hot extrusion of said compound obtained at the end of step (b) or (b) 7(d) where appropriate after adjusting its moisture content to a value between 5 and 40%, to obtain an extruded product, and the fractionation of said extruded product into granules; (d) the co-extrusion, thermocompression or thermoforming of a mixture consisting of 50 to 95% by weight of said compound obtained at the end of step (b) or (b) 7 ) or said granules obtained at the end of step c) and 5 to 50% of a biodegradable aliphatic polyester leading to an extruded, thermocompressed or thermoformed material; e) shaping said extruded, thermocompressed or thermoformed material to obtain said supports.

2. A method according to claim 1 wherein said adjustment of the hygrometry of said compound obtained in step b) or b' is carried out by a drying step of said compound.

3. A method according to claim 1 or 2 wherein said biodegradable aliphatic polymer is polylactic acid or polybutylene succinate, said coextrusion, said thermocompression or said thermoforming being carried out on a mixture of 50 to 95% by weight of said compound obtained at the end of step b) or b 7 ) or said granules obtained at the end of step c) and 5 to 50% by weight of polylactic acid or polybutylene succinate.

4. A process according to claim 3 wherein said coextrusion, said thermocompression or said thermoforming is carried out with 75% by weight of said compound obtained at the end of step b) or b') or of said granules obtained at the end of step c) and 25% by weight of polylactic acid.

5. A process according to claim 3 wherein said coextrusion or said thermocompression, said thermoforming is carried out with 85% by weight of said compound obtained at the end of step b) or b') or of said granules obtained at the end of step c) and 15% by weight of polybutylene succinate.

6. A method according to any one of claims 1 to 5 wherein said alginate is a sodium alginate.

7. A method according to any one of claims 1 to 6 comprising a step of adding a calcium salt.

8. A method according to claim 7 wherein said step of adding a calcium salt is carried out on said extruded, thermocompressed or thermoformed product, before said step of shaping the product.

9. A method according to any one of claims 1 to 8 comprising a foaming step carried out during said co-extrusion step enabling the density of the extruded and foamed material to be brought to a value close to 1.

10. A method according to any one of claims 1 to 9 comprising an addition step during said coextrusion, thermocompression or thermoforming of at least one filler enabling the density of said material to be brought to a value close to 1.

11. A method according to any one of claims 1 to 10 comprising an addition step during said coextrusion or thermocompression or said thermoforming of at least one compatibilizing agent to improve the compatibility of said alginate and said biodegradable aliphatic polymer.

12. A method according to claim 11 wherein said at least compatibilizing agent is selected from maleic anhydride, succinic anhydride, malic acid, ascorbic acid, citric acid.

13. A method according to any one of claims 1 to 12 characterized in that said plasticizer is selected from glycerol, sorbitol, xylitol or a mixture thereof.

14. Biofilm support for biological water treatment device, characterized in that it was obtained by the process according to any one of claims 1 to 13.

15. Biological water treatment device characterized in that it comprises at least one reactor hosting biofilm supports according to claim 14.

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