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

Abstract

A process for manufacturing biofilm supports intended for use in a biological water treatment system, the process comprising the following steps: a) preparing a mixture composed of an alginate, a plasticizer and water, said alginate and the plasticizer being present in said mixture in an alginate / plasticizer weight ratio of 2 to 3, the mixture comprising 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 the additional step of grinding or crushing the aggregates; c) optionally hot extrusion of said compound obtained at the end of step b) or b'), where appropriate after adjusting its moisture content to a value between 5 and 40%, to obtain an extruded product, and fractionating said extruded product into granules;d) co-extrusion, thermocompression or thermoforming of a mixture consisting of 50 to 95% by weight of the compound obtained at the end of step b) or b') or the 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 of the extruded, thermocompressed or thermoformed material to obtain said supports. Fig. 1.;

Description

Title of the invention: Method for manufacturing biofilm supports for use in a biological water treatment device. Technical field

[0001] This presentation relates to the field of biological water treatment for purification or drinking water treatment using biomass fixed on mobile supports. More specifically, this presentation concerns 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 alternately aerobic and anoxic environment, depending on the nature of the pollutants to be removed.

[0003] This biomass can be used in free form, for example in the so-called activated sludge technique, in granular form, or as a biofilm attached to mobile supports. Such supports form beds in the installations that can be fixed or fluidized by the flow of water to be treated and, where applicable, by the biomass oxygenation fluid.

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

[0005] In order to fulfill their functions, the supports must, however, meet many criteria.

[0006] Thus, the material constituting them must be "compatible" with biomass, that is, allow the growth of biomass on its surface. In practice, the biomass must be able to adhere to the material at the time of seeding and throughout the use of the supports. Furthermore, this material must not absorb water, which would make the supports heavier. This material must also exhibit good mechanical strength.

[0007] The supports must, for their part, have a large protected surface area. The protected surface area is understood to be the surface of the support accessible to biomass but not likely to come into contact with external solid elements, primarily Other movable supports and the walls of the tanks containing them. This protected surface 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 the other supports when stored or when they are present in the empty tanks of the inactive equipment. 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, range from approximately 1 to 10 cm. Their density must allow for easy fluidization in water, i.e., with little energy, and in practice, less than 1 kg / m³.Finally, their manufacturing process must be able to guarantee large volumes and low production costs.

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

[0009] Materials other than these plastics have been proposed for biomass supports. The literature describes supports 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 supports made from them poor resistance to erosion and abrasion. Furthermore, they are difficult to fluidize 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, materials which provide nutrients to biomass as they decompose, have also been considered. Supports in the form of wood chips have been tested. However, while such materials are inexpensive and can be used to manufacture easily fluidizable supports, they also have poor mechanical properties and degrade rapidly in aqueous environments. They also have the disadvantage of releasing significant quantities of soluble carbon in an uncontrolled manner when in contact with water.

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

[0012] Such supports, which can have a multitude of shapes, are also a lever to limit 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 have 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 those 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 in this case is finding polyesters and natural materials that are compatible with each other to ensure the long-term stability of their mixture. 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. In order to improve their compatibility, it is necessary to create new chemical interactions at the interfaces or to increase the surface area of ​​these interfaces. To do this, it is possible to modify the mechanical means used to mix them and / or to add compatibilizing reagents.

[0020] Different 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. and understood. Therefore, there is currently no proven generic solution for making natural polysaccharides and aliphatic polyesters compatible.

[0021] In practice, it is mainly 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 method 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 use at the industrial stage.

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

[0026] Another objective of the invention is to propose such a process making it possible to lower the carbon footprint of water treatment facilities in which biofilm supports are used. Description of the invention

[0027] All or part of these objectives are achieved through the invention, which relates to a method for manufacturing biofilm supports intended for use in a biological water treatment device, said method comprising the following steps:

[0028] a) the production of a mixture composed of an alginate, a plasticizer and water, said alginate and said 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;

[0029] b) agitating said mixture until a powder or aggregates are obtained;

[0030] b') possibly the additional step of grinding or crushing said aggregates;

[0031] c) optionally hot extrusion of said compound obtained at the end of step b) or b'), where appropriate after adjusting its moisture content to a value between between 5 and 40%, to obtain an extruded product, and the fractionation of said extruded product into granules;

[0032] 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') or of 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;

[0033] e) shaping said extruded, thermocompressed or thermoformed material to obtain said supports.

[0034] Thus, the invention proposes to associate an alginate with an aliphatic polyester to form biofilm supports.

[0035] The particular conditions and treatment of this association make it possible to obtain a material having mechanical properties, in particular resistance to erosion and abrasion, and water resistance, better than those of prior art materials obtained by association of aliphatic polyesters with other natural polysaccharides such as starch.

[0036] 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.

[0037] 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, make it possible to obtain a powder or aggregates of plasticized alginate. This powder or these plasticized aggregates, reduced by crushing or grinding, can then be intimately combined with the bio-based aliphatic polyester to obtain a stable material that can be transformed into biofilm supports. The stirring step at low speed and low turbulence can be carried out in a mixer, for example a turbo-mixer, or in any other device in order to obtain a homogeneous mixture with reduced energy consumption.

[0038] During the plasticizing 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') above will preferably be hot-extruded to obtain a better-plasticized extruded product, ensuring that the moisture content of the compound remains within an acceptable range.

[0039] According to one variant, said adjustment of the hygrometry of said compound obtained in step b) or b') is carried out by a drying step of this compound.

[0040] 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') or of said granules obtained at the end of step c) and 5 to 50% by weight of polylactic acid or polybutylene succinate.

[0041] 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.

[0042] 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.

[0043] Preferably, said alginate is a sodium alginate.

[0044] Advantageously, the process includes a step of adding a calcium salt. By treating sodium alginate with an aqueous solution containing calcium ions (Ca2+), an ion exchange occurs and leads to the formation of an insoluble calcium alginate gel.

[0045] This step of adding a calcium salt is preferably carried out on said extruded, thermocompressed or thermoformed product.

[0046] The supports produced using the proposed process preferentially have a density close to 1 in order to be more easily fluidized.

[0047] When the mixture of plasticized alginate powder or granules and biodegradable aliphatic polyester is carried out 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.

[0048] 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 said coextrusion, thermocompression, or thermoforming step to bring the density of said co-extruded, thermocompressed, or thermoformed material close to 1. The process will then include a step of adding, during said coextrusion, thermocompression, or thermoforming, at least one filler to bring the density of said material close to 1.

[0049] 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 coextrusion or said thermocompression or said thermoforming of at least one compatibilizing agent to improve the compatibility of said alginate and said biodegradable aliphatic polymer.

[0050] In this case, said at least compatibilizing agent shall be chosen from maleic anhydride, succinic anhydride, malic acid, ascorbic acid, citric acid.

[0051] Finally, preferably, the plasticizer used will be chosen from glycerol, sorbitol, xylitol or a mixture of these.

[0052] The invention also relates to any biofilm support for a biological water treatment device which has been obtained by the process according to the invention described above.

[0053] Finally, the invention also relates to any biological water treatment device comprising at least one reactor housing such biofilm supports. Brief description of the drawings

[0054] 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:

[0055] [Fig. 1] is a photograph of pieces of a material obtained by means of a first embodiment of the process according to the present description;

[0056] [Fig.2] is a scanning electron micrograph of an alginate / PLA composite. Detailed description of a method of implementation.

[0057] A mixture was made containing 48.6% sodium alginate, 19.4% glycerol and 32% water.

[0058] This mixture was then placed in a turbo-mixer, in which 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.

[0059] This powder was then hot-extruded in an extruder set to an extrusion temperature of 110 to 130 °C. At the extruder's outlet, an extruded product was obtained 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, sufficiently flexible to be coiled. The tube was then divided into multiple smaller tubes of equal and controlled dimensions, or the rod was divided into granules of a size close to the diameter of the rod and with controlled dimensions. The moisture content of these granules was measured at 25%.

[0060] These granules were used to make two mixtures, one consisting of plasticized alginate granules and PLA in a weight ratio of 50 / 50 and the other consisting of plasticized alginate granules and PBS in a weight ratio of 80 / 20.

[0061] 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.

[0062] An extruded material in the form of a hollow tube (or granules) with asperities on its (their) outer surface was thus obtained. Pieces of this hollow tube are shown in [Fig. 1].

[0063] These pieces of tubing were placed for 4 hours in a 3% aqueous calcium chloride solution and then rinsed. This treatment improves the water resistance of the material.

[0064] The material obtained after 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 [Fig. 2], alginate grains with a size of 50 micrometers were observed in the matrix, indicating very good integration of this compound into the PLA.

[0065] The tube-shaped element obtained after the co-extrusion step and after treatment with calcium chloride exhibits good mechanical properties, a high protected surface area inside the tube, and an inner and outer surface with asperities suitable for receiving and retaining biomass. These characteristics allow it to be used in water treatment reactors to form biofilm supports. Such supports can be implemented in industrial water treatment plants.

[0066] 1 : Chu, L .Nitrogen removal using biodégradable polymers as carbon source and biofilm carriers in a moving bed biofilm reactor, Chemical Engineering Journal 170, Issue 1,2011,220-225

[0067] 2: Rodrigues, A,L. A poly-e-caprolactone based biofilm carrier for nitrate removal from water, International journal of Environmental Science and Technology, 2014

[0068] 3: N. Yu, Innovative Coating-Etching Method of Biocarrier Fabrication for Treating Wastewater with a Eow C / N Ratio, Polymers 2022, 14(15), 3010

[0069] 4 : Luo, G., Li, L., Liu, Q., Xu, G., Tan, H., 2014a. Effect of dissolved oxygen on heterotrophic dénitrification using poly(butylene succinate) as the carbon source and biofilm carrier. Bioresour. Technol. 171, 152-158

[0070] 5 : Zhu, S.M., Deng, Y.L., Ruan, Y.J., Guo, X.S., Shi, M.M., Shen, J.Z., 2015. Biological dénitrification using poly(butylene succinate) as carbon source and biofilm carrier for recirculating aquaculture System effluent treatment. Bioresour. Technol. 192, 603-610

[0071] 6 : Shen, Z., Wang, J., 2011. Biological dénitrification 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 composed of an alginate, a plasticizer and water, said alginate and said 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') optionally the additional step of grinding or crushing said aggregates; c) optionally hot extrusion of said compound obtained at the end of step b) or b'), where appropriate after adjusting its moisture content to a value between 5 and 40%, to obtain an extruded product, and 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') or of 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) the shaping of 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 process 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 wt% of said compound obtained at the end of step b) or b') or of said granules obtained at the end of step c) and 5 to 50 wt% 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 process according to any one of claims 1 to 10 comprising a step of adding during said coextrusion or said thermocompression or said thermoforming 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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