System and method for recovering textile microfibres from a waste liquid by means of dissolved air flotation

Abstract

The present invention relates to a system and a method for recovering microfibres contained in a liquid effluent (L) from a textile treatment device. The system comprises at least one device (2, 4, 10, 20, 30, 5) for dissolving a gas directly in at least a portion of the liquid effluent (L) by means of pressurisation, and a device (5, 6, 7, 40, 50, 60, 61, 62) for separation by means of dissolved gas flotation, which are connected to one another. The device (5, 6, 7, 40, 50, 60, 61, 62) for separation by means of dissolved gas flotation further comprises means (60, 61, 62) for collecting the microfibres (MF) on the surface of the liquid effluent (L) contained in its enclosure (50).

Description

[0001] SYSTEM AND METHOD FOR RECOVERING TEXTILE MICROFIBERS FROM A DRAIN LIQUID BY DISSOLVED AIR FLOTATION

[0002] Technical field

[0003] The present invention relates to the field of removing microfibers contained in a drain liquid from a textile treatment device, such as a washing machine, a laundry (industrial or not), a device for dyeing textiles, or a device for waterproofing textiles.

[0004] Plastic is omnipresent in our daily lives and our clothes are no exception, as around 60% of the fibers used in the textile industry worldwide are made from plastics such as polyester, polyamide, or acrylic. Due to abrasion during washing, these synthetic fibers can break into microfibers. It is generally accepted that plastic microfibers are fragments of synthetic fibers between 1 m and 5 mm in length. According to recent studies, more than 700,000 plastic microfibers can be released each time a domestic washing machine is used. Discharged into wastewater, they are only partially retained in wastewater treatment plants (WWTPs), and some end up in rivers and then the oceans.It is estimated that 500kT / year of plastic microfibers from washing machines are released into the aquatic environment worldwide, or nearly 33% of the estimated release of primary microplastics.

[0005] In addition to discharges into the aquatic environment, it is important to note that the majority of plastic microfibers retained in wastewater treatment plants end up in sludge during the initial treatment stages. However, the main outlet for this sludge in France, and in most countries, is spreading on agricultural land to improve and fertilize the soil. These microfibers therefore also end up in the natural environment. A post-treatment solution in wastewater treatment plants to reduce the discharge of plastic microfibers into the oceans would only solve part of the problem. To be effective, plastic microfiber capture solutions must be deployed as close as possible to the emission sources, i.e., at the outlet of textile washing systems.

[0006] Several studies have shown the presence of these microplastics in our food (fish, seafood, etc.) or in drinking water. The University of Newcastle in Australia, for example, estimated that a human being ingests an average of 5g of microplastic per week, the equivalent of a credit card. The consequences for human health are still unknown, but we already know that microplastics adsorb organic pollutants and metals (whose toxicity has been proven) on their surface and alter the growth and reproduction of certain living organisms. Furthermore, on February 11, 2020, France enacted a law on the fight against waste and the circular economy, in which Article 79 stipulates that new washing machines must be equipped with a plastic microfiber filter from January 1, 2025.France was thus the first country in the world to take regulatory measures on the issue of microfiber discharge from washing machines. An amendment was subsequently passed, specifying that the professional sector would also be affected, and that solutions external to washing machines could be considered.

[0007] Prior art

[0008] We are particularly aware of application WO2017 / 173215A1, which describes a spherical plastic object that is inserted into the drum of a washing machine with the clothes to be cleaned and that can capture the released fibers in situ thanks to numerous protuberances. However, the geometry of these protuberances is not currently designed to capture small objects such as microfibers, the retention efficiency of microfibers whose length is greater than 100 pm being only 26%.

[0009] Patent application WO2021 / 197937A1 is also known, which relates to a system and method for filtering microfibers contained in a liquid effluent from a textile treatment device. More specifically, this system comprises a granular medium arranged in an enclosure, means for the percolation of the liquid effluent through the granular medium, means for discharging the liquid effluent under the granular medium, and means for connection to means for regenerating the granular medium by gaseous fluidization. Thus, this system allows the granular medium to be regenerated, particularly in the event of clogging. However, the presence of long natural fibers and dirt can quickly clog the granular medium and the regeneration frequency can become too high, particularly in the professional sector where the rate of washing does not allow for too frequent filtration stops.Patent application WO20057820A1 is also known, which relates to a device and a method for recovering microplastic fibers using a hydrocyclone placed upstream of a cartridge filtration system. Such a system requires operation with a liquid pump that can generate pressure drop (involving high electricity consumption), and a filter cleaning process (requiring regular maintenance, consuming additional water). In addition, this method is limited by the nature or density of the material constituting the fiber. Indeed, this method can only recover fibers having either a density lower than that of water (in the case of polyethylene for example) or the opposite (in the case of PET) but cannot recover both simultaneously.More specifically in the field of industrial laundries in France, the filtration technologies used are technologies known to those skilled in the art, such as screening baskets, static curved screens, rotating screens or continuous chain self-cleaning screens.

[0010] Existing filtration methods used in laundries can be continuously regenerated, the solids are isolated and recovered as waste, but their filtration mesh is too large (between 500pm and 1mm) to effectively capture microfibers, which are small objects, most often less than 200pm in length. However, it is not technically possible to significantly reduce the mesh size of these sieve-based technologies, due to machining and / or clogging reasons.

[0011] The present invention overcomes these drawbacks. More specifically, the present invention relates to a method and a system for recovering microfibers contained in a liquid effluent from a textile treatment device, by means of generating gas microbubbles obtained by expanding dissolved gas directly in the liquid effluent to be treated. In particular, the generation of gas bubbles directly in the liquid effluent allows a majority of the microfibers attached to the microbubbles to rise to the surface, and what is more, quickly and without risk of fouling or clogging of the system.

[0012] Summary of the invention

[0013] The present invention relates to a system for the recovery of fine particles, preferably microfibers or microplastics, contained in a liquid effluent, in particular a liquid effluent from a textile treatment device, said system comprising at least:

[0014] A) A device for dissolving a gas in said liquid effluent by pressurization, comprising a first enclosure, means for supplying at least a portion of said liquid effluent into said first enclosure, means for supplying said gas, means for compressing said gas, and means for discharging said at least a portion of said liquid effluent comprising said dissolved gas;

[0015] B) A device for separation by dissolved gas flotation, comprising a second enclosure, means for expanding said gas, means for supplying said at least a portion of said liquid effluent comprising said dissolved gas into said second enclosure, means for collecting fine particles on the surface of the liquid effluent in said second enclosure and means for discharging the clarified liquid effluent from said second enclosure. According to one implementation of the invention, said means for collecting said fine particles may comprise means for collecting by scraping, by suction or by overflow.According to an implementation of the invention, said second enclosure of said device for separation by dissolved gas flotation may comprise at least one separator extending perpendicularly to the base of said enclosure and arranged between said means for supplying said at least a portion of said liquid effluent comprising said dissolved gas into said second enclosure and said means for discharging the clarified liquid effluent from said second enclosure.

[0016] According to an implementation of the invention, said second enclosure of said device for separation by dissolved gas flotation may comprise at least one separator in the form of a hollow cylinder, said hollow cylinder being arranged in such a way that an opening of said means for supplying said at least a portion of said liquid effluent comprising said dissolved gas into said second enclosure is inside said hollow cylinder, and that an opening of said means for discharging the clarified liquid effluent from said second enclosure is outside said hollow cylinder.

[0017] According to one implementation of the invention, said device for separation by dissolved gas flotation may further comprise means for supplying another portion of said liquid effluent into said enclosure of said device for separation by dissolved gas flotation.

[0018] According to an implementation of the invention, a geometry of said first enclosure and said gas compression means of said device for dissolving a gas in said liquid effluent by pressurization can be configured so that the volume fraction of said gas is between 0.1 to 10%, preferably between 0.5 to 5%.

[0019] According to one embodiment, said system comprises means for injecting a flocculating agent, preferably located between the gas expansion means and the means for bringing at least part of the liquid effluent comprising the dissolved gas into the enclosure.

[0020] The invention further relates to a method for recovering fine particles, preferably microfibers or microplastics, contained in a liquid effluent, in particular a liquid effluent from a textile treatment device, said method being implemented by means of the system for recovering fine particles contained in a liquid effluent as described above, said method comprising at least the following steps: a) at least a portion of said liquid effluent from said textile treatment device is introduced into said first enclosure of said device for dissolving a gas in said liquid effluent by pressurization, and, by means of said device for dissolving a gas in said liquid effluent by pressurization, said at least a portion of said liquid effluent comprising a dissolved gas is produced in said first enclosure;b) introducing said at least a portion of said liquid effluent comprising said dissolved gas into said device for separation by dissolved gas flotation, and by means of said device for separation by dissolved gas flotation, depressurizing said at least a portion of said liquid effluent comprising the dissolved gas and separating by flotation said fine particles from at least said at least a portion of said liquid effluent; c) collecting said fine particles present on the surface of said liquid effluent in said second enclosure by means of said means for collecting said fine particles of said device for separation by dissolved gas flotation; d) discharging said clarified liquid effluent from said second enclosure, by means of said means for discharging said clarified liquid effluent from said device for separation by dissolved gas flotation.;

[0021] According to one implementation of the invention, at the end of step d), said clarified liquid effluent can be introduced into at least one device for filtering and / or separating fine particles contained in a liquid effluent.

[0022] According to one implementation of the invention, said filtration and / or separation device may be chosen from a granular medium filtration device, a membrane filtration device, a hydrocyclone filtration device, a flocculation filtration device or a decantation filtration device.

[0023] According to one implementation of the invention, said granular medium filtration device may further comprise means of connection to means of regeneration by gaseous fluidization of said granular medium.

[0024] According to one embodiment, the method comprises an injection of a flocculating agent, preferably between the depressurization step and the flotation separation step.

[0025] Other characteristics and advantages of the system and method according to the invention will appear on reading the following description of non-limiting examples of embodiments, with reference to the figures appended and described below.

[0026] List of figures

[0027] Figure 1 schematically illustrates a first implementation of the system for the recovery of microfibers contained in a liquid effluent according to the invention.

[0028] Figure 2 schematically illustrates a second implementation of the system for the recovery of microfibers contained in a liquid effluent according to the invention.

[0029] Figure 3 schematically illustrates a third implementation of the system for the recovery of microfibers contained in a liquid effluent according to the invention.

[0030] Figure 4 schematically illustrates a fourth implementation of the system for the recovery of microfibers contained in a liquid effluent according to the invention. Description of the embodiments

[0031] According to a first aspect, the invention relates to a system for the recovery of fine particles (preferably microfibers or microplastics) contained in a liquid effluent, in particular a liquid effluent from a textile treatment device.

[0032] According to a second aspect, the invention relates to a method for the recovery of fine particles, preferably microfibers or microplastics, contained in a liquid effluent, in particular a liquid effluent from a textile treatment device, which can be implemented by means of the system for the recovery of fine particles contained in a liquid effluent according to the invention, or not.

[0033] By "fine particles" is meant particles with dimensions between 1 pm and 5 mm of any shape or material. ... The fine particles may preferably be microfibers or microplastics. The fine particles may also be mineral or metallic particles. In the remainder of the description, the application to microfibers is illustrated, however, the invention relates to all fine particles.

[0034] By "microfibers", or "textile microfibers", we mean particles from woven (or non-woven) or knitted materials, composed of natural fibers (cotton, wool, etc.) and / or synthetic fibers (polyester, polyamide, acrylic, etc.) such as clothing or fabrics used in clothing or for any other application (for example, sheets, curtains, etc.) in homes or industry. Microfibers, normally carried in washing machine drain water, are generally elongated, with diameters generally between 1 and 50 microns. The length of the fibers can range from a few fiber diameters to several mm depending on the nature and condition of the materials that are washed upstream.

[0035] By "liquid effluent" is meant any aqueous effluent containing fine particles in suspension and originating from a device for treating, manufacturing, transforming or extracting the material. The liquid effluent may in particular originate from a textile treatment device, or originate from an industrial process.... In the remainder of the description, the application to effluents from textile treatment devices is illustrated, however the invention relates to any liquid effluent comprising fine particles.

[0036] By "textile treatment device" is meant in particular a textile washing device, for example an individual washing machine (or a washing machine), for domestic or commercial use, a set of washing machines (for example in laundries), an industrial laundry (for example a laundry), etc. However, a textile treatment device according to the invention generally comprises any device bringing a textile into contact with a liquid, the liquid then being separated from the textile, such as a device for dyeing a textile, or a device for waterproofing a textile.

[0037] By "liquid effluent from at least one textile treatment device" is meant the liquid from the emptying of the textile treatment device, for example the liquid after washing and / or rinsing and / or spinning in the case of a washing machine. This is subsequently referred to equivalently as "drain liquid". Conventionally, the microfiber load of liquid effluents leaving textile treatment devices is generally limited, with contents between 0.1 and 1000 ppm by weight, generally between 1 and 500 ppm by weight. By "clarified liquid effluent" is meant a liquid effluent separated from at least a portion of the fine particles it contains. As a preferred variant, the clarified liquid effluent is a liquid effluent from a textile treatment device separated from at least a portion of the microfibers.

[0038] The system for recovering microfibers contained in a liquid effluent from a textile treatment device according to the invention comprises at least:

[0039] A device for dissolving a gas in said liquid effluent by pressurization, comprising a first enclosure, means for supplying at least a portion of said liquid effluent into said first enclosure, means for supplying said gas, means for compressing said gas, and means for discharging said liquid effluent comprising said dissolved gas;

[0040] A device for separation by dissolved gas flotation, comprising a second enclosure, means for expanding said gas, means for supplying said at least part of said liquid effluent comprising said dissolved gas into said second enclosure, means for collecting microfibers on the surface of the liquid effluent in said second enclosure and means for discharging the clarified liquid effluent from said second enclosure.

[0041] Thus, the system according to the invention differs from the prior art in that it implements the principle of flotation to separate the microfibers contained in a liquid effluent from a textile treatment device. Moreover, the system according to the invention is configured for dissolution of a gas directly in at least a portion of the liquid effluent to be treated, this liquid effluent comprising the dissolved gas then being injected into a device for separation by dissolved gas flotation. In processes involving flotation in fields other than that of the invention, the gas is dissolved in a liquid (generally water; this is then referred to as white water) separate from the liquid effluent to be treated.Generating these microbubbles directly within the liquid effluent to be treated allows for better attachment of the microbubbles to the microfibers (because some of the bubbles will nucleate directly on the suspended fibers at the time of expansion) and therefore better efficiency in the rise and recovery of the microfibers.

[0042] Indeed, flotation kinetics, that is to say the kinetics describing the local phenomena of bubble-microfiber interaction, is classically broken down as the product of three probabilities:

[0043] A probability of collision of bubbles with microfibers, essentially dependent on the hydrodynamics of the system considered (length to diameter aspect ratio of fibers, diameter of bubbles, gas volume fraction, property of the liquid (density, viscosity)); a probability of attachment of microfibers to bubbles, dependent on surface forces (wettability of microfibers, van der Waals force, electrostatic forces, etc.) and therefore on the physicochemical properties of the chemical system considered and in particular the presence of surfactants in the continuous liquid phase.a probability of detachment (quantifying the risk of detachment of microfibers from the bubbles), dependent on surface forces and hydrodynamics, and in particular if the regime of the continuous liquid phase is turbulent or if the gas volume fraction rate is too high (which can generate induced turbulence ("bubbles induced turbulence") if the gas volume fraction rate is too high (greater than 10%).

[0044] The Applicant has been able to observe, through laboratory tests, that generating microbubbles directly within the liquid effluent to be treated increases the probability of attachment of the microbubbles to the microfibers. Indeed, during the rapid expansion of the gas in the effluent, the nucleation phenomenon appears preferentially on the microfibers. In the case of application of the invention, the probability of collision and attachment is increased by this nucleation phenomenon directly on the textile microfibers, thus providing better capture and recovery efficiency on the surface of the microfibers.

[0045] Furthermore, the Applicant was able to observe that, by this same phenomenon, hydrophilic microfibers, either by nature or by treatment, are captured by the bubbles (by direct nucleation of the bubbles on the microfibers), whereas the probability of attachment between the bubbles and the hydrophilic particles would be much lower in a conventional flotation separation process (gas dissolved in water).

[0046] Furthermore, the system according to the invention makes it possible to dispense with the use of a circulation device (pump) and control means to generate white water. Finally, the device according to the invention also has the advantage of being compact, and in particular of having a reduced footprint compared to flotation devices known in other fields of application, thus making it perfectly suited to the treatment of effluents from textile treatment devices.

[0047] Figure 1 schematically illustrates a first implementation of the system for the recovery of microfibers contained in a liquid effluent according to the invention. For this implementation, the entire liquid effluent to be treated L is compressed by a pump 10 to enter the enclosure 30 of the device for dissolving a gas G by pressurization 2, 4, 5, 20, 30 by a liquid supply 2. The enclosure 30 is also connected to a compressed gas supply 4. The gas G is compressed by a compressor 20 supplied by a pipe 3. The pressurized enclosure 30 allows the dissolution of the gas G in the liquid effluent L to be treated. The liquid effluent comprising the dissolved gas LG is expanded by means of expansion equipment 40 upstream of the enclosure 50 of the device for separation by flotation by dissolved gas 5, 6, 7, 40, 50, 60, allowing the generation of microbubbles MB and the rise of the microfibers MF in the enclosure 50.The enclosure 50 of the device for separation by dissolved gas flotation 5, 6, 7, 40, 50, 60 is also equipped with a microfiber collection system 60 allowing them to be evacuated via the evacuation pipe 7. The clarified effluent LC is evacuated from the enclosure 50 via the evacuation pipe 6.

[0048] Description of the device for dissolving a gas in the liquid effluent by pressurization

[0049] The device for dissolving a gas in the liquid effluent by pressurization is intended to receive at least a portion of the liquid effluent from the textile treatment device. According to the invention, it comprises at least one enclosure, means for supplying at least a portion of the liquid effluent into the enclosure, means for supplying the gas, means for compressing the gas, and means for evacuating said at least a portion of the liquid effluent comprising the dissolved gas.

[0050] According to one implementation of the invention, the enclosure (also called saturator) of the device for dissolving a gas in the liquid effluent by pressurization may be cylindrical or parallelepipedal in shape, preferably cylindrical.

[0051] Preferably, the enclosure may be formed from metal so as to provide pressure resistance. Preferably, the enclosure may be formed from stainless steel.

[0052] According to an implementation in which the gas and the liquid effluent are brought into contact in the enclosure in a co-current manner (flow of the liquid effluent and gas in the same direction) or counter-current manner (flow of the liquid effluent and gas in opposite directions), the enclosure is preferably substantially elongated along the vertical axis. Subsequently, the terms "upper" or "lower" part or wall are defined relative to the median plane of the enclosure of the device for dissolving a gas in the liquid effluent by pressurization in the operating position.

[0053] According to an implementation according to which the gas and the liquid effluent are brought into contact in the co-current enclosure, the openings of the means for supplying the gas and of the means for supplying at least a portion of the liquid effluent into the enclosure may be arranged in an upper part of the enclosure, advantageously in the upper wall of the enclosure, and very preferably in a central part of this upper wall (for example located in a zone centered on the barycenter of the upper wall, and the radius of which may correspond to 30% of the smallest dimension of the upper wall). An opening arranged in a central part of the upper wall of the enclosure allows better distribution of the liquid and the gas, and thus better contact.

[0054] According to an implementation according to which the gas and the liquid effluent are brought into contact in the enclosure in counter-current, the opening of the means for bringing at least a portion of the liquid effluent (respectively means for bringing the gaseous effluent) into the enclosure may be arranged in an upper (respectively lower) part of the enclosure, advantageously in the upper (respectively lower) wall of the enclosure, and very preferably in a central part of this upper (respectively lower) wall, for example located in a zone centered on the barycenter of the upper (respectively lower) wall, and the radius of which may correspond to 30% of the smallest dimension of the upper (respectively lower) wall.An opening made in a central part of the upper (respectively lower) wall of the enclosure allows better distribution of the liquid (respectively of the gas), and thus better contact.

[0055] According to one implementation of the invention, the liquid effluent is brought into contact with the gas by a gaseous headspace. According to this implementation, the enclosure is preferably substantially elongated along the horizontal axis, and the opening of the means for bringing at least part of the liquid effluent into the enclosure is located in a lower part of the enclosure and the opening of the means for bringing the gas is located in an upper part of the enclosure. Preferably, the enclosure according to this implementation may have a length of at least 20 cm (for a liquid effluent flow rate of 120 L / h), which is a sufficient length for gas / liquid contact.

[0056] According to one implementation of the invention, the geometry of the enclosure and the gas compression means can be configured so that the volume fraction of gas is between 0.1 and 10%, preferably between 0.5 and 5%. In particular, any geometry making it possible to increase the exchange surface (addition of baffles, film flowing on walls, etc.) makes it possible to increase the quantity of dissolved gas (until saturation given by Henry's law is reached). A person skilled in the art has perfect knowledge of the means for configuring the device for dissolving a gas in the liquid effluent by pressurization so that the volume fraction of gas is between 0.1 and 10%, preferably between 0.5 and 5%.

[0057] According to a preferred implementation, the opening of the means for bringing at least part of the liquid effluent into the enclosure may be located in an upper part of the enclosure. This makes it possible to create within the saturator a laminar or turbulent liquid jet or even a liquid film falling through a pressurized gaseous sky, up to a liquid guard located at the bottom of the saturator, the latter being necessary to ensure that the supply pipes located downstream are constantly under load, that is to say filled with liquid effluent.

[0058] According to one implementation of the invention, the opening of the means for bringing at least part of the liquid effluent into the enclosure may have a diameter of at least 8 mm to avoid any risk of clogging.

[0059] According to an exemplary embodiment, for a flow rate of 4L / min, an internal saturator diameter of 150 mm and a liquid inlet orifice diameter of 10 mm, a drop height of at least 20 cm in a gaseous atmosphere at 7 bar and a temperature of around 20°C can provide a volume fraction making it possible to recover and collect by flotation microfibers in a concentration of around 200 ppm in a liquid effluent from a textile treatment device.

[0060] Preferably, the device for dissolving a gas in the liquid effluent by pressurization comprises means for controlling the liquid level in the enclosure (such as a valve controlled by a level sensor or a liquid pipe of a height equivalent to the height of the desired level in the enclosure) and configured so as to obtain a gaseous ceiling occupying at least 30%, preferably at least 50% of the total height of the enclosure. This allows a sufficient height of the liquid for gas / liquid contact.

[0061] The compression means of the device for dissolving a gas in said liquid effluent by pressurization may be of any type of compressor known to those skilled in the art. According to one implementation of the invention, the compression means are sized to allow pressurization of between 2 and 20 bar, preferably 4 to 10 bar.

[0062] According to one implementation of the invention, the gas may be air. In this way, the gas may be simply taken from outside the flotation separation device according to the invention, which avoids means for storing the gas. According to an implementation in which the liquid effluent from the textile treatment device has a low pressure (for example close to atmospheric pressure), this effluent may be pressurized beforehand using a pump, before being brought into the enclosure of the device to dissolve a gas in the liquid effluent by pressurization. This may be useful in the case where the liquid effluent is collected, at the outlet of the textile treatment device, in a recovery tank. A pump can then pump the liquid effluent to be treated into this tank and send it to the saturator.

[0063] According to one implementation, packing elements (loose or structured) may be arranged in the enclosure, to allow better gas / liquid contact. For example, Raschig rings, of the Ralu type or of the Pali type, commercially available from RASCHIG GmbH and JAEGER PRODUCTS, Inc., may be used. According to one implementation of the invention, the means for evacuating the part of the liquid effluent comprising the dissolved gas comprise at least one opening, arranged in a lower part of the enclosure, and a conduit for connecting the enclosure of the device for dissolving a gas in said liquid effluent by pressurization to the enclosure of the device for separation by dissolved gas flotation described below.

[0064] Description of the device for separation by dissolved gas flotation

[0065] According to the invention, the device for separation by dissolved gas flotation comprises an enclosure (also called a flotation column), means for expanding the gas, means for bringing said at least part of the liquid effluent comprising the dissolved gas into the enclosure, means for collecting microfibers on the surface of the liquid effluent and means for discharging the clarified liquid effluent.

[0066] Thus, by means of the device for separation by dissolved gas flotation, the liquid effluent comprising the dissolved gas is returned to a pressure lower than the compression pressure of the gas, which will generate microbubbles which will rise to the surface. The bubbles obtained by such a device can be between 30 and 70 microns.

[0067] According to one implementation of the invention, the means for supplying at least one portion of the liquid effluent comprising the dissolved gas may comprise a pipe connecting the opening of the means for discharging at least one portion of the liquid effluent comprising the dissolved gas of the means for dissolving a gas in the liquid effluent by pressurization to the opening of the means for supplying the device for separation by flotation by dissolved gas.

[0068] According to the invention, the gas expansion means are arranged along the means for supplying at least part of the liquid effluent comprising the dissolved gas, preferably close to the opening provided in the enclosure of the device for separation by dissolved gas flotation. The expansion means may correspond to a valve, preferably a needle-type regulating valve. Such valves make it possible to create a strong shear and to maintain a constant pressure difference regardless of the variation in the flow rate of the effluent.

[0069] According to one implementation of the invention, the enclosure of the device for separation by dissolved gas flotation according to the invention may be cylindrical or parallelepipedal in shape, preferably cylindrical. This geometry makes it possible to promote a homogeneous rise of the gas microbubbles. Preferably, the enclosure may be formed from glass or metal or any other material preferably limiting the accumulation of electrostatic charges which could interact with the microfibers.

[0070] Preferably, the enclosure may have a height of at least 10 cm, which is a sufficient height to allow the capture and re-emergence of the microfibers by the microbubbles as will be demonstrated in the application example below.

[0071] Advantageously, the opening of the means for supplying at least part of the liquid effluent comprising the dissolved gas is located in a lower part of the enclosure, preferably in a lower wall of the enclosure. It is clear that this allows the interaction time between the microbubbles and the microfibers to be increased and thus increases their probability of collision.

[0072] According to one implementation of the invention, the enclosure of the device for separation by dissolved gas flotation may further comprise at least one separator (or a separating blade) extending perpendicularly to the base of the enclosure and arranged between the opening for supplying at least one portion of the liquid effluent comprising the dissolved gas and the means for discharging the clarified liquid effluent. According to one implementation of the invention, the separator may also be a hollow cylinder arranged so that an opening of the means for supplying at least one portion of the liquid effluent comprising the dissolved gas is inside the hollow portion of the cylinder (preferably in its center), and an opening of the means for discharging the clarified liquid effluent from the second enclosure is outside the hollow cylinder.In other words, the liquid effluent containing the dissolved gas is injected into the center of the hollow cylinder, and the clarified liquid effluent is recovered in the annular part between the hollow cylinder and the walls of the enclosure of the separation device. This configuration makes it possible to increase the recovery efficiency of the microfibers, by limiting the possibilities of mixing between the effluent to be treated and the clarified effluent, while optimizing the compactness of the separation device as well as facilitating the recovery of the supernatant (supernatant fine particles).

[0073] According to an alternative embodiment of this implementation of the invention, the section of the annular part (formed by the hollow cylinder) may be greater than or equal to the section of the central column (hollow cylinder) where the injection takes place. Typically, the annular section may be between 1 and 10 times the size of the section of the central column (hollow cylinder). The height of the central column may be determined as the product of the velocity of the liquid effluent by a characteristic time for capturing fine particles by the bubbles. The characteristic time may typically be between 10s and 5min. The velocity of the liquid effluent must be sufficiently low, and preferably lead to a laminar type regime possibly to be evaluated with a Reynolds number.

[0074] Figure 2 schematically illustrates this implementation of the system for the recovery of microfibers contained in a liquid effluent from a textile treatment device according to the invention. More specifically, Figure 2 reproduces the elements of Figure 1 (thus the elements in common will not be described again), to which is added a cylindrical separator 51 arranged in the enclosure 50. The separator 51 makes it possible to physically separate the zone containing the ascending microbubbles to which the microfibers are attached (so-called “contact” zone) and the zone for recovering the clarified effluent (so-called “withdrawal” zone). This separation makes it possible to greatly limit the possibilities of mixing between the effluent to be treated and the clarified effluent, and thus to increase the efficiency of recovery of the microfibers.

[0075] According to one implementation of the invention, the device for separation by dissolved gas flotation may comprise means for injecting a flocculating agent located between the gas expansion means and the means for bringing at least part of the liquid effluent comprising the dissolved gas into the enclosure.

[0076] According to one implementation of the invention, the microfiber collection means may correspond to suction means, scraping means, or overflow collection means.

[0077] According to an implementation in which the microfiber collection means correspond to scraping means, these may correspond to a rotating system equipped with semi-rigid blades powered by a motor. The blades may be partially immersed in water and may be designed to recover and push the microfibers located on the surface towards the walls of the flotation column. Geometric reducers may be installed along the walls of the enclosure, so that the semi-rigid blades, encountering these geometric reducers, deform and compress, dry and evacuate the microfibers from the enclosure. Advantageously, in this design, the liquid level in the enclosure can be kept constant so that the scraping system, which is located at a fixed height, is effective.The liquid level can be kept constant by regulation means known to those skilled in the art, such as a valve controlled by a level sensor or a liquid leg of a height equivalent to the height of the desired level in the enclosure of the device for separation by dissolved gas flotation.

[0078] Figure 3 schematically illustrates a third implementation of the system for recovering microfibers contained in a liquid effluent from a textile treatment device according to the invention. More specifically, Figure 3 reproduces the elements of Figure 2 (thus the elements in common will not be described again), to which are added a rotating system provided with blades 61 and a motor 62 to power the rotating system provided with blades 61.

[0079] According to one implementation of the invention, the device for separation by dissolved gas flotation may further comprise means for supplying another portion of the liquid effluent, in which gas has not been dissolved.

[0080] Figure 4 schematically illustrates a fourth implementation of the system for the recovery of microfibers contained in a liquid effluent from a textile treatment device according to the invention. More specifically, Figure 4 reproduces the elements of Figure 1 (thus the elements in common will not be described again), to which are added a circuit comprising a pump 70 and two pipes 7, 8 to bring a portion of the liquid effluent L directly into the enclosure 50 of the device for separation by flotation by dissolved gas 5, 6, 7, 40, 50, 60, without passing through the device to dissolve a gas G by pressurization 2, 4, 5, 20, 30.

[0081] According to an implementation in which the microfiber collection means correspond to gas suction means, the suction means may correspond to a water vacuum cleaner configured to suck up the foam phase containing the microfibers and located on the surface of the liquid effluent. This has the advantage of being able to use the same means to compress the gas as to evacuate the microfibers, for example by adding a venturi-type device to create the vacuum necessary for sucking up the foam phase.

[0082] According to one implementation, the means of collecting the microfibers may correspond to means of overflowing the foam, along an inclined plane before being collected.

[0083] According to one implementation of the invention, the means for discharging the treated liquid effluent comprise at least one opening, arranged in a lower part of the enclosure. This opening allows evacuation by simple gravity of the liquid effluent comprising the clarified liquid effluent in the lower part of the enclosure. Advantageously, the opening of the means for discharging the clarified liquid effluent is arranged in the lower wall of the enclosure of the system according to the invention, to avoid an accumulation of the treated liquid effluent in the bottom of the enclosure. The method according to the invention comprises at least the following steps:

[0084] 1) Dissolution of a gas in the liquid effluent by pressurization

[0085] 2) Depressurization of the liquid effluent including the dissolved gas and separation by flotation of the microfibers from the liquid effluent

[0086] 3) Collection of microfibers

[0087] 4) Evacuation of clarified liquid effluent

[0088] The method according to the invention can be applied continuously (i.e. all the above steps are carried out simultaneously) or discontinuously (i.e. the above steps are carried out sequentially). The method according to the invention can be implemented by means of the system for the recovery of microfibers contained in a liquid effluent from a textile treatment device as described above, or any other system.

[0089] 1) Dissolution of a gas in the liquid effluent by pressurization

[0090] During this step, at least a portion of the liquid effluent from said textile treatment device is introduced into said first enclosure of said device for dissolving a gas in said liquid effluent by pressurization, and, by means of said device for dissolving a gas in said liquid effluent by pressurization, at least a portion of the liquid effluent comprising a dissolved gas is produced in said first enclosure.

[0091] Thus, during this step, it is a question of dissolving a gas in at least one part of the liquid effluent, by means of the device for dissolving a gas in the liquid effluent by pressurization according to the invention. The purpose of this step is to bring into contact, in the enclosure, at least one part of the liquid effluent comprising microfibers with a pressurized gas.

[0092] According to one implementation, the means for supplying at least part of the liquid effluent may comprise a recovery tank arranged under the textile treatment device in the case of gravity drainage (as generally encountered for professional washing machines) and a pump allowing the sending and pressurization of the liquid effluent to be treated towards the enclosure of the device to dissolve a gas in the liquid effluent by pressurization.

[0093] According to one implementation of the invention, the pressure in the enclosure of the device for dissolving a gas in the liquid effluent by pressurization can be predetermined so as to obtain a sufficient quantity of gas dissolved in the liquid effluent to be treated (i.e. a gas volume fraction of at least 0.1% and at most 10%) to carry out separation by flotation once the liquid effluent to be treated has been decompressed.

[0094] According to a discontinuous implementation of the method according to the invention, the effluent from the textile treatment device can be brought directly into the device to dissolve a gas in the liquid effluent by pressurization before being maintained at static pressure for a period of between 1 and 15 minutes.

[0095] 2) Depressurization of the liquid effluent including the dissolved gas and separation by flotation of the microfibers from the liquid effluent

[0096] During this step, at least a portion of the liquid effluent comprising the dissolved gas is introduced into the device for separation by dissolved gas flotation and, by means of the device for separation by dissolved gas flotation, the liquid effluent comprising the dissolved gas is depressurized and the microfibers are separated by flotation from the at least a portion of the liquid effluent.

[0097] In other words, the depressurization of the liquid effluent including the dissolved gas will lead to the generation of microbubbles. The microfibers will then attach themselves to the microbubbles that rise to the surface of the liquid effluent, to float on the surface of the liquid effluent. Note that synthetic fibers have a significant hydrophobic character, this will facilitate their attachment to the microbubbles.

[0098] After numerous tests, the Applicant was able to observe that the range of microbubble dimensions obtained by the system according to the invention (30-70 pm) is suitable for separation by flotation of at least 70% in number of microfibers from a liquid effluent from a textile treatment device.

[0099] According to a discontinuous implementation of the method according to the invention, at the end of step 1), the part of the liquid effluent comprising the dissolved gas is decompressed and injected into the enclosure of the device for separation by dissolved gas flotation, then the injection of the liquid effluent comprising the dissolved gas is stopped for a predetermined duration. According to an implementation of the invention, the predetermined duration may be a function of the rise time of the bubble-microfiber couplings (between 3 cm / minute and 16 cm / minute). For example, the predetermined duration may be 7 minutes for an enclosure of a dissolved gas flotation separation device with a height of 20 cm.

[0100] According to an implementation of the invention according to which the device for separation by dissolved gas flotation comprises means for injecting a flocculating agent as described above, step 2) may comprise a sub-step of injecting at least one flocculating agent between the depressurization sub-step and the flotation separation sub-step. The injection of a flocculating agent carried out at the injection of the depressurized liquid effluent makes it possible to obtain a satisfactory mixture of the flocculating agent and the liquid effluent to be treated. A flocculating agent makes it possible to promote the agglomeration of the smallest fibers, which facilitates their separation.For example, a flocculating agent can be used in the form of mineral salts with polyvalent cations such as alumina sulfate or ferric chloride, activated silica, or natural organic polyelectrolytes (starches, alginate) or synthetic polyelectrolytes (high molecular weight polymers such as polycrylamides or polyvinylamines). The injection of a flocculating agent will preferably be carried out at a low content, generally between 1 and 100 ppm relative to the liquid effluent to be treated.

[0101] 3) Collection of microfibers on the surface of the liquid effluent

[0102] During this step, the microfibers present on the surface of the liquid effluent are collected in the enclosure of the device for separation by dissolved gas flotation according to the invention, using the microfiber collection means of the device for separation by dissolved gas flotation according to the invention.

[0103] This step can be facilitated by the application field of the method according to the invention which concerns the treatment of a liquid effluent from a textile treatment device. Indeed, due to the use, in general, of detergents (surfactants) in this application field (however not necessarily in the case of the textile dyeing field), a foam phase will naturally form on the surface of the liquid effluent. This foam can then promote the aggregation of the microfibers that have reached the surface of the liquid effluent, facilitating their collection. In addition, in general, natural fibers will naturally form flocs by entangling with each other.

[0104] This step can be carried out using any collection method of the device for dissolved gas flotation separation, for example scraping, suction or overflow collection means. Figure 3 already described illustrates an implementation of the collection of microfibers present on the surface of the liquid effluent by scraping means.

[0105] According to an implementation in which the method according to the invention is implemented continuously, the liquid level in the enclosure can be regulated and kept constant to allow the recovery of the microfibers by collection means of the scraping, suction or overflow type.

[0106] According to an implementation in which the method according to the invention is implemented discontinuously, the collection means of the scraping, suction or overflow type can be implemented after an observation time corresponding to the time required for the bubble-microfiber couplings to rise. 4) Evacuation of the clarified liquid effluent

[0107] During this step, the clarified liquid effluent (at least in part) of the microfibers is evacuated from the enclosure of the device for separation by dissolved gas flotation, by means of the means for evacuating the clarified liquid effluent from the device for separation by dissolved gas flotation according to the invention.

[0108] According to an implementation of the invention in which the method according to the invention is implemented continuously, the means for discharging the clarified liquid effluent are sized according to the means for supplying the liquid effluent (and vice versa), so as to allow a constant volume of liquid effluent inside the flotation column according to the invention.

[0109] According to an implementation in which the method according to the invention is implemented discontinuously, the means for discharging the clarified liquid effluent may comprise a pump and a valve arranged downstream of the enclosure of the device for separation by dissolved gas flotation according to the invention. Alternatively, the means for discharging the treated liquid effluent may be configured to exploit a gravity flow of the clarified liquid effluent. In this case, they may for example be arranged at the base of the enclosure of the device for separation by dissolved gas flotation according to the invention. According to this design, the means for discharging the liquid effluent from the device for separation by dissolved gas flotation may comprise a valve to allow the filtered liquid effluent to be discharged from the enclosure in a controlled manner.

[0110] According to one implementation, the method according to the invention may further comprise at least one additional step in which the clarified effluent is injected at the outlet of the system for the recovery of microfibers according to the invention into an additional separation and / or filtration device, such as for example a granular media filtration device, a membrane filtration device (microfiltration, membrane ultrafiltration, reverse osmosis), a hydrocyclone separation device (or a cyclonic separation chamber), a flocculation separation device or a decantation filtration device. This additional step, downstream of steps 1) to 4) described above, may make it possible to improve the overall recovery of the microfibers contained in the liquid effluent to be treated.Conversely, prior separation of the microfibers contained in the liquid effluent to be treated via the system for the recovery of microfibers according to the invention makes it possible to limit the fouling and / or clogging of the filters arranged downstream (in particular the granular and / or membrane filters).

[0111] Preferably, the at least one additional microfiber filtration step can be implemented by means of a system described in patent application WO2021 / 197937 A1, which relates to a system and a method for filtering microfibers contained in a liquid effluent from a textile treatment device. More specifically, this system comprises a granular medium arranged in an enclosure, means for the percolation of the liquid effluent through the granular medium, means for discharging the liquid effluent under the granular medium, and means for connection to means for regenerating the granular medium by gaseous fluidization. Thus, this system allows the granular medium to be regenerated, in particular in the event of clogging, and to recover the plastic microfibers in the gas flow.Thus, the system for the recovery of microfibers according to the invention makes it possible to eliminate a majority (at least 50% and preferably at least 70% in number) of the microfibers contained in the drain liquid, and the filtration by granular medium makes it possible to further improve this separation rate as will be demonstrated in the application example below. The filtration step on granular medium is in this design implemented at the end of the separation steps by flotation in order to limit the fouling of the filter on granular medium.

[0112] According to one implementation of the invention, the method according to the invention may further comprise an additional step in which the clarified effluent is injected at the outlet of the system according to the invention, or at the outlet of an additional separation and / or filtration device where appropriate, into a bacterial treatment device, for example by UV or ozonation.

[0113] Thus, the method and system according to the invention allow efficient and rapid recovery of microfibers contained in a liquid effluent from a textile treatment device.

[0114] Furthermore, unlike flotation separation processes known in other application fields, the process according to the invention does not necessarily require a prior coagulation / flocculation step, which are carried out using polyacrylamide-type chemical additives and which have the function of creating flocs before the flotation step (collision of bubbles with flocs). Indeed, in the case of water from a textile treatment device, the coagulation / flocculation step is not obligatory because:

[0115] Natural fibers will naturally form flocks by entangling with each other

[0116] Synthetic fibers have a significant hydrophobic character which will facilitate the attachment of bubbles to the fibers.

[0117] In addition, some flotation processes known in other application fields use, in an effluent conditioning step, surfactants whose function is to create a foam phase on the surface of the separator to stabilize the pollutant on the surface. In the case of water from a textile treatment device, the presence of residual detergent (linked to washing) will allow this function of stabilizing the fibers on the surface. This foam also makes it possible to recover the pollutants in a "drier" state than a conventional overflow.

[0118] Furthermore, compared to the prior art known in the application field, the method according to the invention has the advantage of not being subject to the problems of fouling or clogging which are conventionally encountered when the technical solution involves a mechanical filter.

[0119] Finally, the method according to the invention makes it possible to capture a wide variety of fibers present in the effluent from the laundry and is not limited by the nature or density of the material constituting the fiber (PET, PE, natural cotton, etc.), unlike a centrifugation treatment system (hydrocyclone for example).

[0120] Furthermore, the invention relates to a textile treatment device comprising at least one system for recovering microfibers contained in a liquid effluent from a textile treatment device as described above, and in which said textile treatment device is in fluid connection with said system for recovering microfibers contained in a liquid effluent from a textile treatment device.

[0121] The invention also relates to a system and a method for recovering fine particles in a liquid effluent from an industrial process. In this embodiment, the invention can be located downstream of a primary pretreatment (screen, gravity settling, etc.) and upstream of a membrane separation system. In this embodiment, it may be advantageous to add flocculating and coagulating agents to improve the recovery of fine particles. Indeed, the invention makes it possible to effectively separate small particles not captured during the primary pretreatment and to limit the impact (fouling, wear, efficiency, etc.) of the membrane separation.

[0122] Examples

[0123] The characteristics and advantages of the method and system according to the invention will appear more clearly on reading the application example below.

[0124] The process according to the invention was implemented on a liquid effluent from an industrial laundry. The properties of this feed are described in Table 1. More specifically, Table 1 describes the total suspended matter and the microfiber concentration of the effluent to be treated.

[0125] The process according to the invention was implemented under the following conditions: 100L of effluent to be treated were injected by a pump (flow rate of 4L / min) into a saturator (15 liters) whose function is to dissolve air in the effluent to be treated. For this, the saturator was pressurized with air at 7 bar continuously. A gaseous ceiling in the saturator of approximately 30 cm allowed sufficient saturation of the water.

[0126] The effluent from the saturator was then conveyed to the treatment column. To do this, the effluent containing dissolved air leaves the saturator, then passes through a so-called depressurization valve (located just below the treatment column), the function and geometry of which ensure the depressurization of the effluent and the appearance of microbubbles, whose diameter is between 30 and 70 pm.

[0127] The effluent to be treated is then injected through the base of the column, in its center. The effluent arrives in the so-called treatment compartment (approximately 3 liters) where the effluent is treated by microbubbles. Thus, the microbubbles and microfibers are captured by the surface of the water, this supernatant part being recovered by suction. The clarified liquid effluent descends through the peripheral crown, formed by the space between the treatment compartment (approximately 13 liters), and the flotation column. The clarified liquid effluent is then drawn off from the bottom and recovered. Throughout the process, the liquid level was kept constant to allow the recovery of microfibers (constituting the supernatant) by suction.

[0128] The flotation kinetics was measured using a camera capable of measuring opacity (determination of a gray level) of the surface of the liquid effluent present in the column. Indeed, as the microfibers are captured on the surface of the liquid effluent, the surface of the liquid effluent becomes increasingly opaque over time, until it becomes completely obstructed by the microfibers. Adapted image processing (measurement of obstruction over time) made it possible to determine a kinetics of approximately 200 seconds.

[0129] Table 2 gives the properties of the clarified effluent from the process according to the invention. More specifically, Table 2 describes the total suspended matter and the microfiber concentration of the effluent at the outlet of the process according to the invention.

[0130] The efficiency of flotation separation was estimated by comparing the properties of the liquid effluent to be treated (Table 1) and the clarified effluent from the system according to the invention (Table 2). Thus, the application of the flotation separation process made it possible to reduce suspended matter (> 25 pm) by 78% in weight and to reduce microfibers by 82% in number, which demonstrates the relevance of the process according to the invention for this type of effluent.

Claims

Claims 1. System for the recovery of fine particles, preferably microfibers or microplastics, contained in a liquid effluent (L), in particular a liquid effluent from a textile treatment device, said system being characterized in that it comprises at least: A) A device for dissolving a gas in said liquid effluent by pressurization (2, 4, 10, 20, 30, 5), comprising a first enclosure (30), means for supplying (2, 10) at least a portion of said liquid effluent (L) into said first enclosure (30), means for supplying (4) said gas (G), means for compressing said gas (20), and means for discharging (5) said at least a portion of said liquid effluent comprising said dissolved gas (LG); B) A device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62), comprising a second enclosure (50), means for expanding (40) said gas, means for supplying (5) said at least one part of said liquid effluent comprising said dissolved gas (LG) into said second enclosure (50), means for collecting (7, 60, 61, 62) the fine particles (MF) on the surface of the liquid effluent (L) in said second enclosure (50) and means for discharging (6) the clarified liquid effluent (LC) from said second enclosure (50).

2. System according to one of the preceding claims, in which said means for collecting (7, 60, 61, 62) said fine particles (MF) comprise means for collecting by scraping (61, 62), by suction or by overflow.

3. System according to one of the preceding claims, wherein said second enclosure (50) of said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62) comprises at least one separator extending perpendicularly to the base of said enclosure (50) and arranged between said means for supplying (5) said at least a portion of said liquid effluent comprising said dissolved gas (LG) into said second enclosure (50) and said means for discharging (6) the clarified liquid effluent (LC) from said second enclosure (50).

4. System according to one of claims 1 to 2, wherein said second enclosure of said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62) comprises at least one separator (51) in the form of a hollow cylinder, said hollow cylinder (51) being arranged so that an opening of said means for supplying (5) said at least a portion of said liquid effluent comprising said dissolved gas (LG) into said second enclosure (50) is inside said hollow cylinder (51), and that an opening of said means for discharging (6) the clarified liquid effluent (LC) from said second enclosure (50) is outside said hollow cylinder (51).

5. System according to one of the preceding claims, wherein said device for separation by dissolved gas flotation further comprises means for supplying (7, 8, 70) another part of said liquid effluent into said enclosure (50) of said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62).

6. System according to one of the preceding claims, wherein a geometry of said first enclosure (30) and said gas compression means (20) of said device for dissolving a gas in said liquid effluent by pressurization are configured so that the volume fraction of said gas is between 0.1 to 10%, preferably between 0.5 to 5%.

7. System according to one of the preceding claims, in which said system comprises means for injecting a flocculating agent, preferably located between the gas expansion means and the means for supplying at least part of the liquid effluent comprising the dissolved gas into the enclosure.

8. Method for the recovery of fine particles, preferably microfibers or microplastics, contained in a liquid effluent, in particular a liquid effluent from a textile treatment device, said method being implemented by means of the system for the recovery of fine particles contained in a liquid effluent according to any one of the preceding claims, characterized in that said method comprises at least the following steps: a) at least a portion of said liquid effluent (L) is introduced into said first enclosure (30) of said device for dissolving a gas in said liquid effluent by pressurization (2, 4, 10, 20, 30, 5), and, by means of said device for dissolving a gas in said liquid effluent by pressurization (2, 4, 10, 20, 30, 5), said at least a portion of said liquid effluent comprising a dissolved gas (LG) is produced in said first enclosure (30);b) introducing said at least one portion of said liquid effluent comprising said dissolved gas (LG) into said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62), and by means of said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62), depressurizing said at least one portion of said liquid effluent comprising the dissolved gas (LG) and separating said fine particles (MF) by flotation from at least said at least one portion of said liquid effluent (L); c) collecting said fine particles (MF) present on the surface of said liquid effluent (L) in said second enclosure (50) by means of said means (60, 61, 62) for collecting said fine particles (MF) from said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62);d) said clarified liquid effluent (LC) is evacuated from said second enclosure (50), by means of said means for evacuating (6) said clarified liquid effluent (LC) from said device for separation by dissolved gas flotation (5, 6, 7, 40, 50, 60, 61, 62).; 9. Method according to claim 8, in which, at the end of step d), said clarified liquid effluent (LC) is introduced into at least one device for filtering and / or separating fine particles contained in a liquid effluent.

10. Method according to claim 9, in which said filtration and / or separation device is chosen from a granular medium filtration device, a membrane filtration device, a hydrocyclone filtration device, a flocculation filtration device or a decantation filtration device.

11. Method according to claim 10, wherein said granular medium filtration device further comprises means for connection to means for regeneration by gaseous fluidization of said granular medium.

12. Method according to one of claims 8 to 11, in which the method comprises an injection of a flocculating agent, preferably between the depressurization step and the flotation separation step.

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