Method for stirring a fluid in a tank for treating and / or storing the fluid, and associated installation

Abstract

The method comprises the following steps: - injecting gas into the fluid via a gas injection system in the tank (16) comprising a plurality of openings (54) for injecting gas into the tank (16), - controlling the injection of gas by the gas injection system in order to generate a shear field (80) of the fluid on at least the bottom (21) of the tank (16), the shear field (80) being non-uniform by moving angularly about a central axis of the tank (16). The method comprises pivoting the non-uniform shear field (80) about the central axis.

Description

[0001] TITLE: Process for mixing a fluid in a fluid treatment and / or storage tank and associated installation

[0002] The present invention relates to a method for stirring a fluid in a fluid treatment and / or storage tank, the tank comprising a bottom and at least one side wall delimiting an interior volume containing the fluid, the method comprising the following step:

[0003] - injecting gas into the fluid by a gas injection system in the tank comprising a plurality of gas injection openings in the tank.

[0004] Such a tank is, for example, integrated into a sludge digester intended to generate biogas, containing in particular biomethane, by anaerobic methane fermentation.

[0005] Methane fermentation has a cellular processing power that is capable of removing a significant amount of organic matter, producing biogas. Biogas production is favored because the biogas can then be used in a biomethane production unit by separating the carbon dioxide and methane contained in the biogas (for example by a membrane process), and / or as an energy source to produce electricity, to heat buildings, and / or to power vehicles.

[0006] In this regard, it is known to introduce sludge from water treatment into a sludge digester tank to carry out fermentation producing biogas.

[0007] Many parameters influence the efficiency of biogas production using sludge. In particular, the sludge must be efficiently mixed in the digester, particularly to reduce temperature and organic matter concentration differences within the digester, and to increase the chances of contact between microorganisms and the materials to be degraded.

[0008] The mixing operation must also be efficient, in order to guarantee a residence time in the digester providing complete mixing. It must allow the solid layer that could form on the sludge-gas interface in the upper part of the digester to be broken up. In addition, it must guarantee sufficient mixing at the bottom of the digester to minimize the accumulation of deposits, so as not to lose useful volume and sufficiently agitate the fluid to facilitate the continuous rise of the biogas.

[0009] In known mixing devices, biogas is reinjected into vertical tubes distributed around the central axis of the digester tank. The biogas is injected continuously through downward-facing injection openings distributed in a circle around the central axis.

[0010] Thanks to the buoyancy of the gas in the sludge, the gas rises to promote mixing in the digester.

[0011] Such a mixing device is very reliable and requires minimal maintenance, which is very useful for limiting interventions within the digester.

[0012] However, it is not entirely satisfactory. Indeed, even if the rise of the gas in the tank produces a movement of the mud and a resulting shear at the bottom of the tank, the shear generated by the movement of the mud at the bottom of the tank is limited and is localized in certain high shear zones, while dead zones may remain at the bottom of the tank.

[0013] To overcome this problem, it is necessary to increase the number of injection rods, which increases the manufacturing and operating cost, while limiting the volume available for storage / treatment in the tank.

[0014] An aim of the invention is therefore to provide a stirring device intended to be placed in a fluid treatment tank, which allows very efficient stirring of the fluid, in particular at the bottom of the tank, while limiting the space taken up within the interior volume of the tank.

[0015] To this end, the invention relates to a method of the aforementioned type, characterized by the following step:

[0016] - controlling the injection of gas by the gas injection system to generate a shear field of the fluid on at least the bottom of the tank, the shear field being non-uniform by moving angularly around a central axis of the tank, the method comprising pivoting the non-uniform shear field around the central axis.

[0017] The method according to the invention may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination;

[0018] - the non-uniform shear field comprises at least one high shear region, with an angular extent of less than 180° around the central axis, and at least one lower or locally zero shear region, angularly adjacent to the high shear region, the pivoting of the non-uniform shear field comprising the joint rotational driving of the high shear region and the lower or locally zero shear region around the central axis;

[0019] - the non-uniform shear field comprises two high shear regions diametrically opposed with respect to the central axis, the pivoting of the non-uniform shear field causing the high shear regions to rotate together around the central axis;

[0020] - the or each high shear region has a butterfly wing shape, the pivoting of the non-uniform shear field causing the high shear regions to rotate together in a butterfly wing shape around the central axis;

[0021] - the rotation speed of the non-uniform shear field is greater than 0.5 revolutions / day and is for example between 0.8 revolutions / day and 1.2 revolutions / day;

[0022] - the pivoting of the non-uniform shear field around the central axis is achieved by controlling the gas injection system;

[0023] - the gas injection system comprises a plurality of gas injection openings distributed around the central axis, the creation of the non-uniform shear field comprising the injection of gas through at least one injection opening at at least a first angular position around the central axis at a nominal flow rate, and the injection of a reduced flow rate of gas or the absence of gas injection through at least one other injection opening at a second angular position angularly offset from the first angular position;

[0024] - the pivoting of the non-uniform shear field comprises successive steps of reducing the injection flow rate by at least one injection opening at a first angular position from a gas injection at the nominal flow rate to a gas injection at the reduced flow rate or to an absence of gas injection, and of increasing the injection flow rate by at least one injection opening at a second angular position from a gas injection at the reduced flow rate or from an absence of gas injection to an injection at the nominal flow rate;

[0025] - successive steps of decreasing the gas injection occur on successive injection openings angularly moving in a direction of rotation, the successive steps of increasing the gas injection occurring on successive injection openings in the same direction of rotation;

[0026] - the injection openings are distributed over at least one circle around the central axis;

[0027] - at least one additional injection opening is located at the periphery of the tank, in the vicinity of the side wall;

[0028] - at least one movable deflector is arranged in the tank, the pivoting of the non-uniform shear field comprising an actuation of the movable deflector between a first position and a second position; - the treatment and / or storage tank is a tank of a digester, the fluid being a sludge, advantageously from a water treatment unit, the gas being biogas, produced by fermentation of the sludge.

[0029] The invention also relates to an installation for mixing a fluid, comprising:

[0030] - a fluid treatment and / or storage tank, the tank comprising a bottom and at least one side wall delimiting an interior volume containing the fluid;

[0031] - a system for injecting gas into the tank comprising a plurality of gas injection openings into the tank, characterized in that the gas injection system comprises a controller configured to control the injection of gas in order to generate a shear field of the fluid on at least the bottom of the tank, the shear field being non-uniform by moving angularly around a central axis of the tank, the installation being configured to control the injection of gas in order to pivot the non-uniform shear field around the central axis.

[0032] The installation according to the invention may include one or more of the following characteristics, taken in isolation or in any technically possible combination:

[0033] - the gas injection system comprises a plurality of injection rods distributed around the central axis, each injection rod opening downwards through a gas injection opening of the plurality of gas injection openings, the controller being capable of selectively controlling the injection of gas into each of the injection rods and through each gas injection opening between a configuration of gas injection at a nominal flow rate and a configuration of gas injection at a reduced flow rate or a configuration of no gas injection.

[0034] The invention will be better understood by reading the following description, given solely by way of example, and made with reference to the appended drawings, in which:

[0035] - [Fig.1] Figure 1 is a schematic view, taken in section along a vertical plane, of a fluid treatment and / or storage installation comprising a gas injection system for implementing the method according to the invention;

[0036] - [Fig.2] Figure 2 is a partial perspective view of the gas injection system of the installation of Figure 1;

[0037] - [Fig .3] Figure 3 is a view of the non-uniform shear field produced by the gas injection system of Figure 2, when implementing the method according to the invention, the non-uniform shear field comprising high shear regions in the shape of butterfly wings; - [Fig .4] Figure 4 is a view similar to Figure 2, illustrating the configuration of the gas injection system to pivot the non-uniform shear field produced in Figure 3;

[0038] - [Fig.5] Figure 5 is a view similar to Figure 3, showing the pivoting of the non-uniform shear field under the effect of the gas injection controlled by the gas injection system.

[0039] A first mixing method according to the invention is intended to be implemented in an installation 12 for treating and / or storing a fluid 14, with a view to mixing the fluid 14 within the installation 12.

[0040] The treatment and / or storage installation 12 is, for example, an installation for treating sludge from a water treatment unit, for example for treating industrial water, drinking water, or wastewater.

[0041] The treatment and / or storage facility 12 is intended in particular to carry out fermentation, preferably anaerobic, to eliminate organic matter present in the fluid, producing a biogas. The biogas comprises in particular biomethane, produced by methane fermentation. The biogas is intended to be separated to produce biomethane and / or is intended to be used as fuel or as a fuel.

[0042] More generally, the stirring method is intended for stirring a fluid 14 in the form of a liquid with a viscosity greater than that of water. The viscosity of the fluid 14 is generally greater than 20 mPa.s, in particular between 10 mPa.s and 80 mPa.s, measured for a shear rate of 100 s -1 .

[0043] The fluid 14 is preferably non-Newtonian, in particular shear-thinning, its viscosity decreasing as a function of the shear to which it is subjected.

[0044] The treatment and / or storage installation 12 comprises a tank 16 for treating and / or storing the fluid 14, an assembly 18 for recovering and recirculating the gas produced in the tank 16, and a gas injection system 20, arranged in the tank 16.

[0045] The tank 16 comprises a bottom 21, a side wall 22 and a roof 24 defining a closed interior volume 26. The interior volume 26 is intended to contain fluid 14, and a gaseous ceiling 28 above the surface 30 of the fluid.

[0046] The internal volume 26 of the tank 16 is for example greater than 400 m 3 and is notably between 400 m 3 and 20,000 m 3 .

[0047] The tank 16 is provided with an inlet 32 ​​for injecting fluid to be treated into the interior volume 26 and an outlet 34 for discharging the treated fluid, advantageously located opposite the injection inlet 32, or at the lowest point of the bottom 21. An upward drive of the fluid to be treated by a column of bubbles can be provided opposite the injection inlet 32.

[0048] The recovery and recirculation assembly 18 comprises a circuit 40 for recovering gas from the gaseous headspace 28, and a circuit 42 for recirculating gas to the gas injection system 20 to supply the gas injection system 20 with gas.

[0049] The recovery circuit 40 comprises at least one pipe suitable for taking gas from the gaseous airspace 28, for example to lead it to a gas storage facility 44, in particular a gasometer.

[0050] The recirculation circuit 42 is connected to the recovery circuit 40, upstream of the gas storage 44. It comprises at least one compressor 46, for compressing the recirculated gas and supplying the gas injection system 20 with compressed gas at a pressure greater than 1.2 bar absolute, and for example between 1.4 bar absolute and 3.5 bar absolute.

[0051] The 46 compressor typically draws between 1 Nm 3 / h and 2 Nm 3 / h of gas per m 2 surface area of ​​the tank 16, taken horizontally. The compressor 46 is preferably a volumetric machine, which pumps a constant flow rate.

[0052] Alternatively, the recirculation circuit 42 is tapped directly into the gaseous airspace 28, separately from the tapping of the recovery circuit 40.

[0053] In the example shown in FIG. 1, the gas injection system 20 into the tank 16 comprises a plurality of gas injection rods 50, arranged around a central axis AA' of the tank 16, in the vicinity of the central axis A-A'.

[0054] The rods 50 are located at a distance from the central axis A-A' less than 50%, in particular less than 30%, of the minimum distance separating the central axis A-A' at the point closest to the side wall 22.

[0055] The gas injection system 20 further comprises a gas distributor 52 connected upstream to the circulation conduit 42 and connected downstream to each of the rods 50, to control the quantity of gas injected by each rod 50.

[0056] In the example shown in Figure 1, each rod 50 defines at its lower end an opening 54 for distributing gas into the tank 16, directed downwards.

[0057] Each rod 50 comprises at least one upper section 56, connected to the gas distributor 52, which extends vertically from the roof 24 downwards, parallel to the central axis AA'.

[0058] With reference to Figure 2, a first group 55 of outer rods 50 further comprises a lower section 58 extending the upper section 56 downwards. The lower section diverges radially away from the central axis AA' by moving away from the axis A-A'. Each rod 50 of the first group 55 of rods 50 thus has a general J shape.

[0059] In this example, the rods 50 of the first group 55 are distributed over an outer circle 60. They are distributed angularly in a regular manner around the axis A-A'.

[0060] Advantageously, the rods 50 of the gas distribution system 20 further comprise a second group 62 of rods 50 arranged inside the first group 55 of rods 50.

[0061] The rods 50 of the second group 62 have only one section 56 parallel to the axis A-A', which opens vertically downwards through a gas injection opening 54.

[0062] The rods 50 of the second group 62 are distributed over an inner circle 64 concentric with the outer circle 60 and arranged inside the outer circle 60. They are distributed angularly in a regular manner around the axis A-A'.

[0063] In the example shown in Figures 1 to 5, the number of rods 50 of the first group 55 opening onto the outer circle 60 is greater than or equal to 4, and is in particular greater than 8, preferably between 10 and 20.

[0064] The number of rods 50 of the second group 62 opening onto the inner circle 64 is for example greater than or equal to 2 and is notably between 5 and 20.

[0065] The injection opening 54 of each rod 50 of the second group 62 opens onto the inner circle 64 above the gas injection opening 54 of each rod 50 of the first group 55 on the outer circle 60.

[0066] The gas distributor 52 comprises, for each gas injection opening 54 associated with a rod 50, a valve 70 for controlling the gas flow rate through the rod 50 and consequently, the gas flow rate injected into the fluid 14 through the gas injection opening 54.

[0067] The gas distributor 52 includes a controller 72 configured to drive each valve 70 to selectively control the gas flow rate through each rod 50 and each gas injection opening 54.

[0068] In this example, the valves 70 are arranged outside the interior volume 26, for example above the roof 24. Each valve 70 is connected upstream to the compressor 46.

[0069] The controller 72 is configured here to control each valve 70 between a configuration of nominal gas injection through the corresponding gas injection opening 54, and a configuration of reduced gas injection or a configuration of no gas injection through the gas injection opening 54. As will be seen below, the controller 72 is configured to selectively control at least a first group of valves 70 in the nominal injection configuration to place a first group 76A, 76B of injection openings 54 in a configuration of nominal gas injection into the fluid 14.

[0070] The controller 72 is configured to selectively control at least a second group of valves 70 in the reduced injection configuration or in the no injection configuration, so that at least a second group 78A, 78B of injection openings 54 is placed in the reduced injection configuration or in the no injection configuration (which is symbolized by a cross at the opening 54 in FIGS. 2 and 4).

[0071] Thus, as visible in figures 3 and 5, the controller 72 is configured to generate in the fluid 14, in the vicinity of the bottom 21, a shear field 80 of the fluid 16 on the bottom 21, which is non-uniform in moving angularly around the central axis A-A'.

[0072] This shear field is taken tangentially relative to the bottom 21 following the bottom 21.

[0073] The non-uniform shear field 80 comprises in particular at least one high shear region 82A, 82B, with an angular extent of less than 180° around the central axis A-A', which in this example has a butterfly wing shape. The angular extent around the axis A-A' of the or each high shear region 82A, 82B is greater than 20°, preferably greater than 90°.

[0074] The non-uniform shear field 80 comprises, on either side of each high shear region 82A, 82B, a region 84A, 84B of lower or locally zero shear.

[0075] The high shear regions 82A, 82B are generally characterized by a shear stress at the bottom 21, taken parallel to the bottom 21, greater than 1.2 Pa, in particular between 1.3 Pa and 1.7 Pa.

[0076] On the contrary, the regions of lower or zero shear locally 84A, 84B are characterized by a shear stress at the level of the bottom 21, taken parallel to the bottom 21, strictly less than 1.2 Pa and in particular between 0.8 Pa and 1.1 Pa.

[0077] The shear stresses are for example determined by simulation, for example by means of a finite volume calculation code taking into account the fluid mechanics equations, and with a user interface, using “Computational Fluid Dynamics” or “Computational Fluid Dynamics” techniques. The controller 72 is also configured to successively modify the configuration of the valves 70, to switch an injection opening 54 of each second group 78A, 78B of injection openings 54, adjacent to an injection opening 54 located at a first end of a first group 76A, 76B of injection openings 54, from the reduced injection configuration or the no injection configuration to the nominal injection configuration.

[0078] The controller 72 is also configured to successively modify the configuration of the valves 70 to simultaneously switch an injection opening 54 of the first group 76A, 76B located at a second end of the first group 76A, 76B of injection openings opposite the first end, from the nominal injection configuration to the reduced injection configuration or to the no injection configuration.

[0079] Each first group 76A, 76B of injection openings 54 in the nominal injection configuration therefore moves in rotation around the axis A-A' in a direction of rotation marked by an arrow in FIG. 5.

[0080] Correspondingly, each second group 78A, 78B of injection openings 54 in the reduced injection configuration or in the no injection configuration also moves in rotation about the axis A-A' in the same direction of rotation marked by an arrow in FIG. 5

[0081] Thus, the controller 72 is configured to rotate the non-uniform shear field 80 about the axis A-A', causing the high shear regions 82A, 82B and the lower or locally zero shear regions 84A, 84B to rotate about the axis AA', as seen in FIG. 5.

[0082] A method of stirring a fluid 14 in the tank 16 of the treatment and / or storage installation 12 will now be described.

[0083] Fluid 14 is supplied discontinuously, for example periodically, or continuously into the interior volume 26 through the injection inlet 32.

[0084] Correspondingly, treated fluid is discharged through the discharge outlet 34. The fluid flow rate 14 is adjusted so that the surface of the fluid 30 in the interior volume 26 extends below the roof 24 to delimit a gaseous sky 28.

[0085] During its passage through the interior volume 26, the fluid 14 is treated. For example, if it is sludge from water treatment, the sludge undergoes fermentation, in particular anaerobic fermentation eliminating the organic matter contained in the sludge and producing biogas which accumulates in the gaseous atmosphere 28.

[0086] The gas produced by the fluid 14 is then evacuated through the recovery circuit 40 to be brought to the gas storage 44. A portion of the gas circulating in the recovery circuit 40 is recycled through the recirculation circuit 42 to the compressor 46, where it is compressed to a pressure for example included in the ranges defined above.

[0087] The compressed gas is then supplied to the gas distributor 52. The controller 72 then controls the valves 70 to maintain at least a first group of valves 70 in their nominal injection configurations, so that a first group 76A, 76B of injection openings 54 injects gas into the tank 16 at a nominal injection flow rate, relative to the surface area of ​​the tank 16, taken horizontally, for example between 0.3 Nm 3 / h / m 2 and 0.7 Nm 3 / h / m 2 .

[0088] The controller 72 controls at least a second group of valves 70 in their reduced injection configurations or in their no injection configurations so that a second group 78A, 78B of injection openings 54 injects gas into the tank 16 at a reduced injection rate or has an absence of gas injection, for example at least 20% lower than the injection rate through the injection openings 54 of the first group 76A, 76B.

[0089] In the example shown in Figures 2 and 3, all the gas injection openings 54 of the inner circle 64 advantageously inject gas at a nominal flow rate.

[0090] In the example of Figure 2, the controller 72 also controls the injection openings 54 of the outer circle 62 so that two groups 76A, 76B of diametrically opposed injection openings 54 inject gas into the tank at the nominal flow rate, while two groups 78A, 78B of diametrically opposed injection openings 54 inject gas into the tank 16 at a reduced injection flow rate or have no gas injection.

[0091] For example, when the number of injection openings is equal to 12, as in the example of FIG. 2, two groups 76A, 76B of three consecutive injection openings 54 inject gas at the nominal flow rate, while two groups 78A, 78B of three consecutive injection openings 54, arranged between the groups 76A, 76B, inject gas at a reduced flow rate or have an absence of gas injection.

[0092] Under the effect of such a gas injection, as illustrated by FIG. 3, two high shear regions 82A, 82B, here in the shape of butterfly wings, are created on the bottom 21 of the tank 16. Each high shear region 82A, 82B has an angular extent of less than 180° around the central axis A-A' and generally greater than 45°.

[0093] Locally lower or zero shear regions 84A, 84B are interposed between the high shear regions 82A, 82B.

[0094] The shear field 80 thus obtained is non-uniform by moving angularly around the axis AA' Then, as illustrated by FIG. 4, successive steps of switching the valves 70, as described above, are carried out by the controller 72 to progressively pivot around the axis A-A' the first group 76A, 76B of injection openings 54 injecting gas into the tank 16 at the nominal flow rate, and jointly the second group 78A, 78B of injection openings 54 injecting gas at a reduced flow rate or having an absence of injection into the tank 16, in a first given direction of rotation.

[0095] As illustrated by Figure 5, this translates correspondingly into an angular displacement around the axis AA' of each high shear region 82A, 82B and a corresponding displacement of each lower or locally zero shear region 84A, 84B in the first given direction of rotation.

[0096] The angular rotation speed around the central axis A-A' of the non-uniform shear field 80, and consequently, of each high shear region 82A, 82B is generally greater than 0.5 revolutions / day and is for example between 0.8 revolutions / day and 1.2 revolutions / day.

[0097] By creating a non-uniform shear field 80 by means of the gas distribution system 20, it is possible to create high shear regions 82A, 82B and to rotate them around the central axis A-A' to sweep the entire surface of the bottom 21 of the tank 16.

[0098] Thus, a high shear is obtained at the bottom 21 of the tank 16, which minimizes deposits over the long term, covers almost the entire surface of the bottom 21, with a minimum of injected gas possible, while sequencing the gas injections over time.

[0099] This also reduces the variable costs of the installation 12, since the number of cleanings of the bottom 21 of the tank 16 by operators can be reduced and their frequency reduced. This very surprising result is obtained in a very simple way, just by controlled injection of gas, even in the absence of moving parts in the tank 16, which can remain completely closed.

[0100] In the preceding example, the flow rate injected through the injection openings of the second group 78A, 78B is preferably zero. Alternatively, the injection of gas into the second group 78A, 78B of injection openings 54 produces a flow rate lower than that of the injection openings 54 of the first group 76A, 76B, but not zero. Thus, the controller 72 varies the flow rate circulating through each valve 70, without completely canceling it.

[0101] In one variant, at least a portion of the gas injection openings 54 are located at the periphery of the vessel 16, in the vicinity of the side wall 22 or through the side wall 22. In another variant, the direction of rotation of the groups 76A, 76B; 78A, 78B of injection openings 54 is changed over time, to create an alternating rotational movement of the high shear regions 82A, 82B.

[0102] In yet another variant, a plurality of moving parts, for example deflectors movable between a first retracted position and a second deployed position, are used to generate the non-uniform shear field 82 in combination with the gas injection, and to drive it in rotation about the axis A-A'.

[0103] At least part of the deflectors can, for example, be deployed through the side wall 22 of the tank 16.

[0104] In another variant, at least one gas injection device 20 comprises, in combination with the rods 50 or as a substitute for at least one rod 50, a geyser mixer (or "geyser pump" in English) comprising an injection tube opening upwards, and a device for injecting gas bubbles into the tube from the bottom of the tube.

[0105] In another variant, only the outer circle 60 is used for gas injection, the inner circle 64 remaining without gas injection.

[0106] In another variant, the treatment and / or storage tank 16 is also a tank of a digester, but the fluid introduced into the tank 16 includes or is a substrate of organic matter formed by biowaste.

[0107] Biowaste is, for example, defined in Article 541-1-1 of the French Environmental Code.

[0108] Examples of biowaste include biodegradable garden or park waste, food or kitchen waste from households, offices, restaurants, wholesale trade, canteens, catering or retail stores, as well as comparable waste from food processing plants.

[0109] To obtain the organic matter substrate introduced into the tank 16, the biowaste generally undergoes pre-treatment. This includes, for example, mechanical treatment such as grinding and / or shredding to reduce the size of the solid particles and increase the contact surface, sorting to remove non-biodegradable materials such as plastics, homogenization, etc.

Claims

CLAIMS 1. Method for stirring a fluid (14) in a tank (16) for treating and / or storing the fluid (14), the tank (16) comprising a bottom (21) and at least one side wall (22) delimiting an interior volume (26) containing the fluid (14), the method comprising the following step: - injecting gas into the fluid (14) by a gas injection system (20) into the tank (16) comprising a plurality of openings (54) for injecting gas into the tank (16), the method being characterized by the following step: - controlling the injection of gas by the gas injection system (20) to generate a shear field (80) of the fluid (14) on at least the bottom (21) of the tank (16), the shear field (80) being non-uniform by moving angularly around a central axis (A-A') of the tank (16), the method comprising pivoting the non-uniform shear field (80) around the central axis (A-A').

2. Method according to claim 1, in which the non-uniform shear field (80) comprises at least one high shear region (82A, 82B), of angular extent less than 180° around the central axis (A-A'), and at least one lower or locally zero shear region (84A, 84B), angularly adjacent to the high shear region (82A, 82B), the pivoting of the non-uniform shear field (80) comprising the joint rotational driving of the high shear region (82A, 82B) and the lower or locally zero shear region (84A, 84B) around the central axis (A-A').

3. The method of claim 2, wherein the non-uniform shear field (80) comprises two high shear regions (82A, 82B) diametrically opposed with respect to the central axis (A-A'), the pivoting of the non-uniform shear field (82) jointly rotating the high shear regions (82A, 82B) around the central axis (A-A').

4. A method according to any one of claims 2 or 3, wherein the or each high shear region (82A, 82B) has a butterfly wing shape, the pivoting of the non-uniform shear field (80) jointly rotating the high shear regions (82A, 82B) in a butterfly wing shape around the central axis (A-A').

5. Method according to any one of the preceding claims, in which the rotation speed of the non-uniform shear field (80) is greater than 0.5 revolutions / day and is for example between 0.8 revolutions / day and 1.2 revolutions / day.

6. Method according to any one of the preceding claims, in which the pivoting of the non-uniform shear field (80) around the central axis (A- A') is carried out by controlling the gas injection system (20).

7. A method according to any one of the preceding claims, wherein the gas injection system (20) comprises a plurality of gas injection openings (54) distributed around the central axis (A-A'), the creation of the non-uniform shear field (80) comprising injecting gas through at least one injection opening (54) at at least a first angular position around the central axis (A-A') at a nominal flow rate, and injecting a reduced flow rate of gas or no gas injection through at least one other injection opening (54) at a second angular position angularly offset from the first angular position.

8. The method of claim 7, wherein pivoting the non-uniform shear field (80) comprises successive steps of reducing the injection flow rate through at least one injection opening (54) to a first angular position from gas injection at the nominal flow rate to gas injection at the reduced flow rate or to no gas injection, and increasing the injection flow rate through at least one injection opening (54) to a second angular position from gas injection at the reduced flow rate or from no gas injection to injection at the nominal flow rate.

9. Method according to claim 8, in which successive steps of decreasing the gas injection occur on successive injection openings (54) while moving in a direction of rotation, the successive steps of increasing the gas injection occurring on successive injection openings (54) in the same direction of rotation.

10. Method according to any one of claims 7 to 9, in which the injection openings (54) are distributed over at least one circle (60, 64) around the central axis (A-A').

11. Method according to claim 10, in which at least one additional injection opening (54) is located at the periphery of the tank (16), in the vicinity of the side wall (22).

12. A method according to any one of claims 10 or 11, wherein at least one movable deflector is disposed in the tank (16), pivoting the non-uniform shear field (80) comprising actuation of the movable deflector between a first position and a second position.

13. Method according to any one of the preceding claims, in which the treatment and / or storage tank (16) is a tank (16) of a digester, the fluid (14) being a sludge, advantageously originating from a water treatment unit, and / or the fluid being a substrate of organic matter formed by biowaste, the gas being biogas, produced by fermentation of the sludge and / or the substrate of organic matter formed by biowaste.

14. Installation for mixing a fluid (14), comprising: - a tank (16) for treating and / or storing the fluid (14), the tank (16) comprising a bottom (21) and at least one side wall (22) delimiting an interior volume (26) containing the fluid (14); - a gas injection system (20) into the tank (16) comprising a plurality of openings (54) for injecting gas into the tank (16), characterized in that the gas injection system (22) comprises a controller (72) configured to control the injection of gas in order to generate a shear field (80) of the fluid (14) on at least the bottom (21) of the tank (16), the shear field (80) being non-uniform by moving angularly around a central axis (A-A') of the tank (16), the installation (14) being configured to control the injection of gas in order to pivot the non-uniform shear field (80) around the central axis (A-A').

15. Installation (14) according to claim 14, in which the gas injection system (20) comprises a plurality of injection rods (50) distributed around the central axis (A-A'), each injection rod (50) opening downwards through a gas injection opening (54) of the plurality of gas injection openings (54), the controller (72) being able to selectively control the injection of gas into each of the injection rods (50) and through each gas injection opening (54) between a configuration of gas injection at a nominal flow rate and a configuration of gas injection at a reduced flow rate or a configuration of no gas injection.

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