Installation for treating wastewater using activated sludge comprising a spiral separator, and associated treatment method

Abstract

The invention relates to an installation (10) for treating a stream (12) of wastewater, comprising: - an activated sludge biological reactor (14), - a liquid / solid separation device (16), - a gravimetric separation device (18) comprising a first spiral separator (56) configured to classify part of the mixed liquor stream (24) or part of the activated sludge stream (46) into three element streams (50, 52, 54), - a first return line (20) intended to circulate one of the three streams (50, 52, 54) to upstream of the biological reactor (14) or into the biological reactor (14), - a first discharge line (22) intended to circulate one of the three streams (50, 52, 54) to a first treatment unit or a storage unit (72, 74).

Description

WASTEWATER TREATMENT INSTALLATION USING ACTIVATED SLUDGE INCLUDING A SPIRAL SEPARATOR AND ASSOCIATED TREATMENT PROCESS

[0001] The present invention relates to the field of wastewater treatment, municipal or industrial, and in particular biological treatment by activated sludge.

[0002] The general principle of this type of treatment relies on removing carbon, nitrogen, and phosphorus pollution from wastewater using a complex culture of microorganisms called "activated sludge." The wastewater, after typically undergoing mechanical and sometimes chemical pretreatment, is brought into contact with the activated sludge in a biological reactor. In the presence of oxygen, the pollution is transferred from the liquid phase (wastewater) to the solid phase (activated sludge). After a contact time necessary for the biochemical reactions to occur, the mixture of water and sludge, generally called "mixed liquor," is separated to produce a stream of treated water on one side and a stream of activated sludge on the other. For example, in conventional continuous activated sludge treatment, this operation takes place in a settling tank.Part of the thickened activated sludge stream is then recirculated to the top of the biological reactor to maintain continuous biochemical reactions with the wastewater. A portion of the excess activated sludge is extracted from the settling tank and removed for further treatment.

[0003] The proper functioning of activated sludge treatment depends on operating conditions such as the applied loading rate, sludge age, hydraulic retention time, etc. These conditions influence the selection of microbial species in the sludge and determine its quality. The sludge's quality and composition, in turn, influence its settling capacity. Thus, the limiting step in the process is often the separation by settling, resulting from the low settling capacity of certain sludges. For example, under certain conditions, high quantities of filamentous microorganisms, characterized by a low settling capacity, can develop.

[0004] One solution to this problem is to densify the activated sludge recirculated in the biological reactor by creating a pressure that selects for the densest particles. EP 2 925 676 B1, for example, proposes using a hydrocyclone to separate the activated sludge stream into two streams: a dense stream containing sludge with good settling capacity, such as granular sludge, and a less dense stream containing smaller particles, filaments, and colloids with lower settling capacity. The dense stream is recirculated into the biological reactor, and the less dense stream is removed from the system for further treatment.

[0005] Depending on the activated sludge treatment variant, the mixed liquor includes elements such as:

[0006] - elements present in wastewater such as fats, oils and greases (designated by the acronym "FOG" in English for "fats / oils / grease"), inert organic fibers, inorganic fibers, mineral particles such as grains of sand or microsand,

[0007] - elements resulting from the biological treatment of pollution by activated sludge such as dense sludge, granular sludge, flocs, filamentous bacteria, biologically stable foam and scum,

[0008] - metallic salts such as iron orthophosphate (FePO4), iron(III) hydroxide (Fe(OH)3), iron(II) hydroxide (Fe(OH)2), aluminium phosphate (AlPO4) or aluminium hydroxide (Al(OH)3), resulting from the addition of chemical substances for the conditioning of sludge or water,

[0009] - elements with non-specific adsorption properties, such as activated carbon in granular, powder or micro-grain form, particles with specific adsorption properties such as zeolite,

[0010] - elements added to the biological reactor and intended to ballast the flocs, such as clay particles, bentonite, diatomaceous earth, or any micronized mineral particle,

[0011] - mobile supports added in the biological reactor to fix the biomass.

[0012] By "inert" we mean elements or materials that do not actively participate in the degradation of organic pollution, as opposed to purifying biomass made up of microorganisms that carry out biological transformation.

[0013] Some components of the mixed liquor, such as granular sludge, are necessary to promote sludge settling. Others, however, like mineral particles, accumulate in the biological reactor and limit treatment efficiency by reducing the reactor's usable volume. The presence of inert organic or inorganic fibers promotes the formation of fibrous deposits (tow) that can accumulate on equipment or measuring sensors.

[0014] One aim of the invention is to provide a more efficient and optimized activated sludge wastewater treatment plant that overcomes the disadvantages mentioned above.

[0015] To this end, the invention relates to a wastewater treatment installation comprising:

[0016] - at least one activated sludge biological reactor configured to be fed by the wastewater stream and to produce a mixed liquor stream,

[0017] - at least one liquid / solid separation device configured to separate the mixed liquor stream into a purified water stream and an activated sludge stream,

[0018] - at least one gravimetric separation device comprising at least one first spiral separator, the gravimetric separation device being configured to classify at least a part of the mixed liquor stream or at least a part of the activated sludge stream into at least three element streams, the first spiral separator being configured to classify at least a part of the mixed liquor stream or at least a part of the activated sludge stream into at least two of the three element streams,

[0019] - at least one first return line intended to circulate at least one of the three flows upstream of the biological reactor or within the biological reactor,

[0020] - at least one first discharge pipe intended to circulate at least one of the three streams to at least one first treatment unit or storage unit.

[0021] Thus, thanks to the gravimetric separation device, and in particular the spiral separator, it is possible to classify the mixed liquor stream or the activated sludge stream into a plurality of distinct streams, each containing a majority of a specific type of element. Each of these streams can then be directed either to the biological reactor to promote activated sludge treatment or, conversely, to a treatment or storage unit to remove elements detrimental to the activated sludge process or to the equipment. In particular, the removed elements detrimental to biological treatment can be advantageous when reintroduced into a specific treatment unit. The invention also allows for the recovery of certain elements intentionally added to the biological reactor, such as activated carbon, in order to reduce operating costs.

[0022] The spiral separator allows for the efficient separation of elements present in the mixed liquor or in the activated sludge stream, and more finely than a state-of-the-art hydrocyclone can achieve.

[0023] The treatment installation according to the invention may comprise one or more of the following features, taken individually or in any technically possible combination:

[0024] - at least one first spiral separator extends along a substantially vertical principal axis and comprises:

[0025] * at least one upper inlet intended for the introduction of at least part of the mixed liquor flow or at least part of the activated sludge flow and,

[0026] * at least two lower outputs for two of the at least three element streams,

[0027] the spiral separator defining a helical surface winding around the main axis from the upper inlet to the lower outlets;

[0028] - the activated sludge biological reactor is a sequential activated sludge biological reactor and the liquid / solid separation device is formed by a tank of said activated sludge biological reactor;

[0029] - the activated sludge biological reactor is a continuous activated sludge biological reactor and the liquid / solid separation device includes a secondary settling tank or includes at least one membrane unit disposed in or outside a basin of the activated sludge biological reactor;

[0030] - at least one first spiral separator is configured to classify at least a part of the mixed liquor stream or at least a part of the activated sludge stream into at least three element streams;

[0031] - the gravimetric separation device includes a second spiral separator connected downstream of the first spiral separator, one of the flows from the first spiral separator feeding the second spiral separator, the first spiral separator and the second spiral separator having at least one distinct geometric characteristic;

[0032] - one of the three element streams is an element stream comprising the majority of the inert non-biodegradable mineral particles present in at least part of the mixed liquor stream or at least part of the activated sludge stream, the first discharge pipe being intended to circulate said stream to a treatment and / or storage unit;

[0033] - one of the three element streams is an element stream comprising the majority of dense activated sludge containing granular sludge present in at least a portion of the mixed liquor stream or at least a portion of the activated sludge stream, the first return line being intended to circulate said element stream upstream of the biological reactor or within the biological reactor; and

[0034] - The gravimetric separation device also includes:

[0035] * a hydrocyclone configured to separate at least a part of the mixed liquor stream or at least a part of the activated sludge stream into a second stream of elements comprising a majority of light activated sludge including bacterial flocs and a dense intermediate stream of elements mostly denser than the light activated sludge, the first spiral separator being configured to classify the dense intermediate stream into a first stream of elements comprising the majority of dense activated sludge from the mixed liquor stream or activated sludge stream and a third stream of elements comprising the majority of inert non-biodegradable mineral particles from the mixed liquor stream or activated sludge stream.

[0036] The invention also relates to a method for treating wastewater comprising the following steps:

[0037] - to feed a biological activated sludge reactor with a wastewater stream and produce a mixed liquor stream,

[0038] - separate the mixed liquor stream into a purified water stream and an activated sludge stream using at least one liquid / solid separation device,

[0039] - classify at least a portion of the mixed liquor stream or at least a portion of the activated sludge stream into at least three element streams with at least one gravimetric separation device comprising at least one first spiral separator, said classification step comprising the classification of at least a portion of the mixed liquor stream or at least a portion of the activated sludge stream into at least two of the three element streams with the first spiral separator,

[0040] - to circulate at least one of the three streams through the biological reactor,

[0041] - to circulate at least one of the three streams to at least one first processing unit or a storage unit.

[0042] The method according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0043] - one of the three element streams is an element stream comprising the majority of inert, non-biodegradable mineral particles present in at least part of the mixed liquor stream or at least part of the activated sludge stream, the process comprising a step of circulating said stream to the first treatment unit or a storage unit;

[0044] - one of the three element streams is an element stream comprising the majority of dense activated sludge including granular sludge present in at least part of the mixed liquor stream or at least part of the activated sludge stream, the process comprising a step of circulating said stream upstream of the biological reactor or in the biological reactor;

[0045] - the process includes a step of introducing particles upstream of or into the biological reactor, one of the three element streams being an element stream comprising the majority of said particles present in at least a part of the mixed liquor stream or at least a part of the activated sludge stream, the process comprising a step of circulating said stream comprising the majority of the particles upstream of or into the biological reactor; and

[0046] - the particles are chosen from: adsorbent particles intended to adsorb micropollutants present in the wastewater stream and / or particles intended to ballast the bacterial flocs present in the biological reactor and / or biomass supports intended for the fixation and growth of biomass in the biological reactor.

[0047] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the accompanying drawings, among which: Figure 1 is a schematic view of an installation according to a first embodiment of the invention; Figure 2 is a schematic cross-sectional representation of the spiral separator of the installation; Figure 3 is a perspective view of the lower end of the spiral separator; Figure 4 is a schematic representation of an installation according to a second embodiment of the invention; Figure 5 is a schematic representation of an installation according to a third embodiment of the invention; Figures 6 and 7 are partial schematic views of the spiral separator of the installation; Figure 6 is a schematic view of an installation according to a third embodiment of the invention; Figure 7 is a schematic view of an installation according to a fourth embodiment of the invention;Figure 1 is a schematic view of an installation according to a fifth embodiment of the invention; Figure 2 is a schematic view of an installation according to a sixth embodiment of the invention.

[0048] The illustration schematically shows an installation 10 for treating a wastewater stream 12 according to a first embodiment of the invention.

[0049] Wastewater flow 12 is, for example, a municipal wastewater flow. Alternatively, the wastewater flow is an industrial wastewater flow.

[0050] According to the invention, the treatment installation 10 comprises at least one activated sludge biological reactor 14, at least one liquid / solid separation device 16, at least one gravimetric separation device 18, at least one first return line 20, and at least one first discharge line 22.

[0051] The biological reactor 14 is configured to be fed by a wastewater stream 12 and to produce a mixed liquor stream 24. In the example of the, the activated sludge biological reactor 14 is a continuous activated sludge reactor.

[0052] Preferably, as shown in the figure, the installation 10 includes upstream of the biological reactor 14, a pretreatment unit 26, a primary treatment unit 28, an activated sludge recirculation line 30 and a first bypass line 32.

[0053] The pretreatment unit 26 is configured to pretreat the raw wastewater stream 12 received upstream of said unit 26 and provide a pretreated wastewater stream 34. Pretreatment protects the pipes of the installation 10 from blockages and the various components of the installation 10 from abrasion. Pretreatment also removes anything that could interfere with subsequent treatment processes. The pretreatment unit 26 includes, for example (not shown in the figure), a screening device and optionally a sieving device to separate and remove bulky materials from the wastewater stream 12; a grit removal device to remove sand from the wastewater stream 12; and a grease and oil removal device, frequently used in conjunction with the grit removal device, to remove substances with a density slightly lower than water from the wastewater stream 12.

[0054] The primary treatment unit 28 includes a primary clarifier 36 configured to receive the pre-treated wastewater stream 34 from the pretreatment unit 26, separate by settling a portion of the suspended solids forming primary sludge, and provide a settled wastewater stream 38. In a known manner, the installation 10 may include a primary sludge treatment unit 40 such as a thickening unit before being directed to a dewatering, anaerobic digestion and / or incineration unit.

[0055] In various embodiments, the biological reactor 14 comprises one or more anaerobic zones and / or one or more anoxic zones and / or one or more aerobic zones (not shown) to treat the carbon, nitrogen, and phosphorus present in the wastewater stream 12. The anaerobic, anoxic, and aerobic zones are fluidically connected. It is understood that these zones may be different volumes within the same biological reactor 14 or formed by different fluidly connected reactors.

[0056] In the example of the, the liquid / solid separation device 16 is a secondary decanter 42 configured to receive the mixed liquor stream 24 and separate by decantation the mixed liquor stream 24 into a purified water stream 44 and an activated sludge stream 46.

[0057] In the example of the, the activated sludge stream 46 includes at least dense activated sludge, light activated sludge and inert mineral particles.

[0058] Dense activated sludge comprises granular sludge, also known as granules. These are compact, pseudo-spherical microbial aggregates. The size of the granular sludge typically ranges from 200 µm to 2000 µm. The density of dense activated sludge is around 1.025 g / cm³. 3 and 1.08 g / cm² 3 Granular sludges have a compact structure which gives them resistance to shear stresses.

[0059] Light activated sludge contains bacterial flocs. Flocs are clumps formed by the agglomeration of microorganisms and suspended matter held together by extracellular polymeric substances produced by the microorganisms.

[0060] Light activated sludge has a density, for example, of between 1.015 g / cm³ 3 and 1.035 g / cm² 3 The flocs typically range in size from 50 µm to 300 µm at shear levels commonly found in biological basins. The flocs have a loosely compacted, flaky structure that breaks down when shear stress is applied.

[0061] Inert mineral particles include sand, gravel, and other mineral particles, commonly referred to as "grits." Mineral particles may also include fragments of glass or shells.

[0062] The size of residual inert mineral particles after a desanding step is typically between 10 µm and 200 µm. For example, it is between 10 µm and 100 µm for microsand particles and between 100 µm and 200 µm for sand particles. The density of inert mineral particles is generally greater than 2.5 g / cm³ 3 but can vary depending on the type of material.

[0063] The activated sludge recirculation line 30 is designed to recirculate a major portion of the activated sludge flow 46 from the liquid / solid separation device 16, i.e., the secondary clarifier 42 in this example, into the biological reactor 14 to maintain an adequate concentration of microorganisms in the biological reactor 14, necessary for the biochemical reactions to continue. A portion of the activated sludge flow 46, i.e., the excess activated sludge, is sent to the gravimetric separation device 18 to remove the excess sludge in order to control the sludge age.

[0064] The first bypass line 32 is intended to possibly circulate part of the excess activated sludge flow 46 to a treatment or storage unit 48 of the installation 10, in particular if the gravity separation device 18 cannot treat the entire flow of excess activated sludge.

[0065] According to the invention, the gravimetric separation device 18 is configured to classify at least a part of the activated sludge stream 46, in particular the excess activated sludge stream that has not been recirculated in the biological reactor 14, into at least three element streams 50, 52, 54. The gravimetric separation device 18 is fluidically connected to the liquid / solid separation device 16.

[0066] According to the invention, the gravimetric separation device 18 comprises at least one first spiral separator 56. The first spiral separator 56 is configured to classify the activated sludge stream 46 into at least two of the three element streams 50, 52, 54.

[0067] In the example of the, the gravimetric separation device 18 includes a single first spiral separator 56 configured to classify the activated sludge stream 46 into three distinct element streams: a first stream 50 comprising the majority of the dense activated sludge from the activated sludge stream 46, a second stream 52 comprising the majority of the light activated sludge from the activated sludge stream and a third stream 54 comprising the majority of the inert mineral particles from the activated sludge stream 46.

[0068] As schematically illustrated in the figure, the first spiral separator 56 extends along a substantially vertical principal axis P. The first spiral separator 56 comprises at least one upper inlet 58 for the introduction of the activated sludge stream 46 and at least three lower outlets 60 for each of the at least three element streams 50, 52, 54. In the first embodiment, the first spiral separator 56 comprises exactly three lower outlets 60 to recover the first stream 50, the second stream 52, and the third stream 54, respectively.

[0069] The first spiral separator 56 defines a helical surface 62 winding around the main axis P in a helix. The helical surface 62 extends from the upper inlet 58 to the lower outlets 60. The helical surface 62 delimits a trough 63 receiving the mixture to be classified, i.e. a part of the activated sludge flow 46 or a part of the mixed liquor flow 24, as the case may be.

[0070] Preferably, as shown in the figure, the first spiral separator 56 comprises a plurality of deflectors 64 mounted on the helical surface 62 to direct the different flows 50, 52, 54 towards the lower outlets 60.

[0071] For example, in the first embodiment, the first spiral separator 56 comprises two deflectors 64 to delimit three channels 66, each directed towards a lower outlet 60. Preferably, the position of the deflectors 64 relative to the helical surface 62 is adjustable. For example, each deflector 64 is rotatable relative to the helical surface 62 about an axis substantially perpendicular to the helical surface 62.

[0072] The first spiral separator 56 is configured to classify the different elements of a mixture on the basis of the density, size and hydrodynamic properties of the elements that compose it, as the mixture travels up and down the helical surface 62.

[0073] Several forces contribute to the ranking of the elements in the mixture:

[0074] - the force of gravity which tends to pull the elements back towards the main axis P, at the center of the spiral,

[0075] - the centrifugal force which tends to move the elements radially outwards at a distance from the main axis P,

[0076] - the drag force exerted by the water on the elements which displaces the elements present on the water surface outwards, away from the main axis P, and the elements which are on the helical surface 62 inwards, in the direction of the main axis P,

[0077] - Bagnold forces which lift the elements near the helical surface 62, creating an expansion and contraction of the water that facilitates their classification,

[0078] - the friction forces generated by the helical surface 62 which oppose the movement of the elements.

[0079] The vector sum of these five forces determines the direction taken by an element during its journey through the spiral separator 56.

[0080] The helical surface is characterized by a plurality of geometric features that influence the classification performance of the first spiral separator 56.

[0081] The geometric characteristics include for example the number of turns of the helical surface 62, the height H of the helical surface taken along the principal axis P, the pitch, the radius r, the slope α and the geometry of the helical surface 62 in a radial plane (figures 2 and 3).

[0082] The number of turns of the helical surface 62 is, for example, between 3 and 15, or between 3 and 10. The number of turns influences the ability of the spiral separator 56 to classify different particle sizes. With more turns, the separator 56 can be more efficient at separating fine particles, which require more time to separate from the liquid. Furthermore, a spiral separator with more turns can classify a larger volume of mixture because it offers a larger separation surface. Increasing the number of turns also allows for the processing of mixtures with a higher solids concentration, as separation can be more efficient over a longer distance.

[0083] The height H measured along the main axis P is for example between 1 m and 5 m.

[0084] The pasuest is the distance taken along the principal axis P traveled by the helical surface 62 in one turn.

[0085] The radius of the helical surface 62 is, for example, between 0.15 m and 2.5 m. A larger radius increases the separation area, which improves separation efficiency, especially for fine or light particles.

[0086] The slope α, also called the inclination, of the helical surface 62 is the angle formed by the spiral with respect to the horizontal. The slope α is expressed as a function of the pitch and the radius according to the formula: tan(α) = u / (2πr). The slope α ranges from 0.1° to 45°, for example, from 3° to 20°. A steeper slope α increases the centrifugal force applied to the solid particles and facilitates their movement towards the outer region of the separator, thus improving solids separation. It also allows for a higher flow rate. A shallower slope increases the time the liquid-solid mixture remains in the spiral. This can improve separation by providing more time for fine and light particles to separate from the liquid.

[0087] The geometry of the helical surface 62 in a radial plane substantially perpendicular to the principal axis P influences the classification of the mixture elements. In particular, the geometry of the helical surface can be characterized by the radial inclination θ, which is the angle formed between the helical surface projected onto an axial plane and a transverse plane perpendicular to the principal axis P. The radial inclination θ can vary locally radially.

[0088] In one particular embodiment, the radius remains variable along the principal axis P. For example, the radius increases from the upper inlet to the lower outlets. This allows for the classification of mixtures containing particles with a very wide size range, for example, between 40 µm and 3 mm.

[0089] Advantageously, the first spiral separator 56 comprises a plurality of riffles 68 and / or grooves on the helical surface 62.

[0090] A riffle 68 is a rib extending outward from the helical surface 62. Each riffle 68 has a height, for example, between 100 µm and 5000 µm.

[0091] Each groove has a depth, for example, between 100 µm and 5000 µm.

[0092] Each riffle 68 or each groove extends along the helical surface 62, for example in a helix, i.e. the distance between each point of the riffle 68 or the groove and the principal axis P is constant all along the helical surface 62, or in an arc, i.e. the distance between a point of the riffle 68 or the groove and the principal axis P increases or decreases along the helical surface 62. In the latter case, the angle formed between a radius of the spiral and a tangent to the riffle 68 or the groove is for example between 5° and 45°.

[0093] The riffles 68 and grooves allow the elements present in the mixture to be intercepted according to their size and / or density to guide them towards certain regions of the helical surface 62 and facilitate their separation.

[0094] Furthermore, according to certain embodiments, the first spiral separator 56 further comprises at least one protrusion extending in projection from the helical surface 62 in a helix, that is to say that the distance between each point of the protrusion and the main axis P is constant all along the helical surface 62.

[0095] A person skilled in the art can refer to the following articles for the choice of geometric characteristics of the first spiral separator 56 according to the nature of the particles they wish to classify: Sivamohan and Forssberg, 1985, “Principles of spiral concentration”, International Journal of Mineral Processing, 15, 173-181; Falconer, 2003, “Gravity separation: old techniques / new methods”, Physical Separation in Science and Engineering, 12, 31-48; Burt, 1984, “Gravity Concentration Technology”, Elsevier Science; Honaker et al., 2007, “Ultrafine coal cleaning using spiral concentrators”, Mineral Engineering, 20, 1315-1319; Atasoy and Spottiswood, 1995, “A study of particle separation in a spiral concentrator”, Minerals Engineering, 8, 1197-1208; Richards et al., 2000, “Gravity separation of ultra-fine (~0.1 mm) minerals using spiral separators”, MineralsEngineering, 13, 65-77.

[0096] As the activated sludge flow 46 moves down the helical surface 62, the heavy particles tend to migrate towards the main axis P of the first spiral separator 56 while the lighter particles move radially in the opposite direction from the main axis P of the separator 56, towards a peripheral region of the helical surface 62.

[0097] Inert mineral particles, including sand and microsand particles with the highest density and a size less than 200 µm, are mainly located near the main axis of the separator 56; dense activated sludge, including granular sludge with an intermediate density and a size greater than that of inert mineral particles, is located in a central axial region of the helical surface 62; and light activated sludge with a low density and a small size is located at the periphery of the helical surface 62, in a region furthest from the main axis P of the spiral.

[0098] In the first embodiment, the first return pipe 20 is intended to circulate the first flow 50 comprising the majority of the dense activated sludge upstream of the biological reactor 14 or in the biological reactor 14 to allow densification of the activated sludge in the reactor 14.

[0099] In particular in the first embodiment, the installation 10 includes in addition to the first discharge pipe 22 and a second discharge pipe 70.

[0100] The first discharge line 22 is intended to circulate at least part of the second stream 52 comprising the majority of the light activated sludge to a treatment unit, in particular a sludge treatment and / or storage unit 72. In particular, said unit 72 may be similar to the treatment and storage unit 48 connected to the first bypass line 32.

[0101] The second discharge pipe 70 is intended to circulate the third stream 54 comprising the majority of inert mineral particles to a treatment or storage unit for inert mineral particles 74. Thus, the inert mineral particles which unnecessarily occupy a volume in the biological reactor 14 and which are likely to degrade the pipes because of their abrasive nature are purged from the biological reactor 14.

[0102] According to an advantageous embodiment, the first spiral separator 56 includes a foam or floating matter separation element from the activated sludge stream 46 introduced into the spiral separator 56. For example, the foam separation element includes a horizontally adjustable separation plate, the positioning of the edge of which is set at water level or a few millimeters below the water level, and preferably disposed on an outer edge of the helical surface 62. This horizontal separation plate is intended to collect the foam floating on the surface of the activated sludge stream and direct it to a treatment unit.

[0103] A process for treating a wastewater stream 12 according to a first embodiment of the invention is now described.

[0104] The process includes a step of feeding the activated sludge biological reactor 14 with the wastewater stream 12 and producing a mixed liquor stream 24.

[0105] Preferably, the wastewater stream 12 comes from a pretreatment stage and a primary treatment stage carried out upstream of the feed stage.

[0106] The pretreatment includes, for example, screening and sieving sub-steps to separate and remove bulky materials contained in the wastewater stream 12, a grit removal sub-step to remove sands from the wastewater stream 12, and degreasing and oil removal sub-steps to remove products with a density slightly lower than water.

[0107] The primary treatment includes a settling stage of the pre-treated wastewater stream 34 to remove some of the suspended solids forming primary sludge, and provide a settled wastewater stream 38 which feeds the biological reactor 14.

[0108] According to the invention, the process then comprises separating the mixed liquor stream 24 into a purified water stream 44 and an activated sludge stream 46 with at least one liquid / solid separation device 16.

[0109] In the example of the, the liquid / solid separation is carried out with a secondary clarifier 42. The activated sludge stream 46 is recovered in the underflow of the secondary clarifier 42.

[0110] Then, at least a part of the activated sludge stream 46 is classified into three streams of elements 50, 52, 54 with at least one gravimetric separation device 18 comprising a single first spiral separator 56. In particular, the classification step includes the classification of the activated sludge stream 46 into three streams of elements 50, 52, 54 with the first spiral separator 56.

[0111] The first flow 50, comprising the majority of the dense activated sludge, is circulated with the first return pipe 20 upstream of the biological reactor 14 or in the biological reactor 14 to allow densification of the activated sludge in the reactor 14.

[0112] At least part of the second stream 52, comprising the majority of the light activated sludge, is circulated with the first discharge pipe 22 to the sludge treatment and / or storage unit 72.

[0113] The third stream 54, comprising the majority of inert mineral particles, is circulated with the second discharge pipe 70 towards the inert mineral particle treatment and / or storage unit 74.

[0114] A second embodiment according to the invention, represented in the figure, is now described by differences with respect to the first embodiment.

[0115] In this embodiment, the installation 10 includes at least one first addition unit 75 of elements upstream of the biological reactor 14 or in the biological reactor 14.

[0116] For example, the element addition unit 75 is an adsorbent particle addition unit capable of adsorbing micropollutants configured to inject a quantity of adsorbent particles upstream of the biological reactor 14 and downstream of the primary treatment unit 28, and / or into the biological reactor 14 and / or into the activated sludge recirculation line 30. The adsorbent particles are intended to adsorb a quantity of micropollutants present in the wastewater stream 12.

[0117] In operation, the activated sludge stream 46 includes at least light activated sludge and / or dense activated sludge, inert mineral particles and adsorbent particles.

[0118] Light activated sludge, dense activated sludge and inert mineral particles have the same properties as those mentioned in the first embodiment.

[0119] For example, the adsorbent particles include granular activated carbon.

[0120] Activated carbon grains are larger than inert mineral particles. The size of the activated carbon grains is deliberately chosen to allow for separation with the spiral separator, preferably greater than 100 µm and less than 3 mm. This definition may also include grains commonly referred to as micrograins. The density of activated carbon (the density of the material, excluding the pores within the particles) is between 2.0 g / cm³ 3 and 2.3 g / cm² 3 The apparent density of activated carbon grains or micro-grains suspended in water is lower due to porosity.

[0121] The first spiral separator 56 is configured to classify part of the activated sludge stream 46 into three distinct element streams: a first stream 50 comprising the majority of the adsorbent particles, in particular the activated carbon grains, present in the activated sludge stream 46, a second stream 52 comprising the majority of the light activated sludge and / or dense activated sludge present in the activated sludge stream 46 and a third stream 54 comprising the majority of the inert mineral particles present in the activated sludge stream 46.

[0122] As the activated sludge flow 46 descends along the helical surface 62, the inert mineral particles, including sand and microsand particles with the highest density and a size less than 200 µm, are mainly located near the main axis P of the separator 56; the activated carbon grains with an intermediate density and a size greater than the inert mineral particles are located in a central axial region of the helical surface 62; and the light activated sludge with a low density and a small size is located at the periphery of the helical surface 62, in a region furthest from the main axis P of the spiral.

[0123] In this embodiment, the first return line 20 is intended to circulate the first stream 50 comprising the majority of the adsorbent particles, in particular activated carbon grains, upstream of the biological reactor 14 or into the biological reactor 14. This makes it possible to control the retention time of the adsorbent particles in the biological reactor 14, independently of the age of the sludge, to maximize the adsorption of micropollutants present in the mixed liquor stream 24 and to minimize operating costs by recycling the adsorbent particles.

[0124] Optionally, the installation includes an adsorbent particle regeneration unit (not shown) connected to the first return line 20 between the first lower outlet 60 of the first spiral separator 56 and the biological reactor 14 to allow for the regeneration of the adsorbent particles. Of course, according to a particular embodiment, the installation 10 may include a second bypass line (not shown) to avoid the passage of the adsorbent particles through the regeneration unit and direct them directly to the biological reactor 14.

[0125] As shown in the figure, in addition, the installation 10 includes a second return line 76 intended to recirculate all or part of the second stream 52 comprising the majority of the light activated sludge and / or light activated sludge to the biological reactor 14. The installation 10 may then include a control device configured to distribute the second stream 52 between the second return line 76 and the first discharge line 22.

[0126] The treatment process includes a preliminary step of injecting a quantity of adsorbent particles upstream of the biological reactor 14 and downstream of the primary treatment unit 28, into the biological reactor 14 and / or into the activated sludge recirculation line 30.

[0127] Once the classification has been carried out with the first spiral separator 56, the first stream 50, comprising the majority of the adsorbent particles, is circulated with the first return line 20 upstream of the biological reactor 14 and downstream of the primary treatment unit 40, or into the biological reactor 14.

[0128] The second and third streams 52, 54 are treated in a similar way to what is done in the first embodiment.

[0129] Optionally, all or part of the first stream 50 is circulated and sent to an adsorbent particle regeneration unit to regenerate the adsorbent particles. Of course, according to a particular embodiment, the adsorbent particles are bypassed by the regeneration unit and directed directly to the biological reactor 14.

[0130] In addition, consideration can be given to recirculating all or part of the second stream 52 comprising the majority of the light activated sludge and / or dense activated sludge to the biological reactor 14. According to a particular embodiment, the third stream 54 is distributed between the third return line 76 and the first discharge line 22.

[0131] Alternatively, the adsorbent particles include zeolite particles.

[0132] Alternatively, the first unit of element addition is a unit of chemical agent addition upstream of biological reactor 14 or in biological reactor 14.

[0133] For example, the chemical agent addition unit is configured to inject an aluminum or iron-based chemical agent to form with the phosphorus present in the wastewater stream 12 metallic salts such as iron orthophosphate (FePO4) and / or aluminum phosphate (AlPO4).

[0134] The gravimetric separation device 18, and in particular the spiral separator 56, is configured to classify the activated sludge stream 46 or the mixed liquor 24 into at least one stream containing the majority of the metallic salts. This stream is then recirculated upstream or into the biological reactor 14 or directed to a specific treatment unit, for example, to recover and utilize the phosphorus, iron, and / or aluminum present in the metallic salts.

[0135] Alternatively, the first unit for adding elements is a unit for adding particles intended to ballast light activated sludge and in particular bacterial flocs and to form ballasted activated sludge.

[0136] The gravimetric separation device 18, and in particular the spiral separator 56, is configured to classify the activated sludge stream 46 or the mixed liquor 24 into at least one stream comprising the majority of the ballasted activated sludge. This stream is then recirculated upstream or into the biological reactor 14.

[0137] Alternatively, the first unit for adding elements is a unit for adding biomass supports intended to promote the fixation and development of biomass.

[0138] The gravimetric separation device 18, and in particular the spiral separator 56, is configured to classify the activated sludge stream 46 or the mixed liquor 24 into at least one stream comprising the majority of the biomass support material. This stream is then recirculated upstream or into the biological reactor 14.

[0139] A third embodiment according to the invention, represented in the figure, is now described by differences with respect to the second embodiment.

[0140] In operation, the activated sludge stream includes at least light activated sludge comprising flocs, dense activated sludge comprising granular sludge, inert mineral particles and adsorbent particles, in particular activated carbon grains.

[0141] Light activated sludge, dense activated sludge, inert mineral particles and activated carbon grains have the same properties as those mentioned in the first embodiment and the third embodiment.

[0142] The first spiral separator 56 is configured to classify the activated sludge stream into four distinct element streams: a first stream 50 comprising the majority of the activated carbon grains present in the activated sludge stream, a second stream 52 comprising the majority of the light activated sludge present in the activated sludge stream, a third stream 54 comprising the majority of the inert mineral particles present in the activated sludge stream and a fourth stream 80 comprising the majority of the dense activated sludge present in the activated sludge stream.

[0143] The first spiral separator 56 includes four lower outlets 60. Preferably, the first spiral separator 56 includes three deflectors 64 to delimit four channels 66 each directed towards a lower outlet 60.

[0144] As the activated sludge flow descends along the helical surface 62, the inert mineral particles, including sand and microsand particles with the highest density and a size less than 200 µm, are mainly located near the main axis P of the separator 56; the activated carbon grains with an intermediate density and a size greater than that of the inert mineral particles are located in a central axial region of the helical surface 62; the dense activated sludge, which is lighter than the granular activated carbon but denser than the light activated sludge, is located in a region further axially from the central axial region; and the light activated sludge, with a low density and a small size, is located at the periphery of the helical surface 62, in the region furthest from the main axis P of the spiral.

[0145] In this embodiment, the installation 10 includes a third return pipe 78 intended to circulate the fourth stream 80 comprising the majority of the dense activated sludge upstream of the biological reactor 14 or in the biological reactor 14.

[0146] Alternatively, the lower outlets 60, corresponding to the first stream 50 containing the majority of the activated carbon grains and the third stream 54 containing the majority of the inert particles, are arranged at an intermediate distance between the upper inlet 58 and the other lower outlets 60. Indeed, given their size and density characteristics, these elements are classified fairly quickly after a limited number of rotations. It is also conceivable that the second part of the spiral separator 56, located between these intermediate outlets 60 and the other lower outlets 60 corresponding to the second and fourth streams 52, 80, has distinct geometric properties, for example, the inclination, the position of the riffles 68 or grooves, etc., from the first part of the first spiral separator 56, located between the upper inlet 58 and the outlets 60 corresponding to the first and third streams 50, 54.

[0147] A fourth embodiment according to the invention, with reference to Figures 6, 7 and 8, is now described by differences with respect to the third embodiment.

[0148] In operation, in this embodiment, the activated sludge stream 46 comprises at least light activated sludge including flocs, dense activated sludge including granular sludge, inert mineral particles, activated carbon grains, and inert organic and inorganic fibers.

[0149] Light activated sludge, dense activated sludge, inert mineral particles and activated carbon grains have the same properties as those mentioned previously.

[0150] Inert organic fibers include, for example, textile fibers such as cotton, wool or cellulose, polyester, or even hair.

[0151] Inorganic fibers include, for example, glass, asbestos, or ceramic fibers.

[0152] The density of inert or inorganic organic fibers, for example, is between 0.2 g / cm³ 3 and 1.07 g / cm² 3 Their size varies considerably, but is generally greater than 500 µm and can reach several millimeters, for example 3 mm (larger fibrous elements, on the order of a centimeter, are mostly retained during the pretreatment step, but may also be present in the mixed liquor). Their size and geometric shape make them very sensitive to drag forces as they travel across the helical surface 62 of the spiral separator 56.

[0153] In this embodiment, the gravimetric separation device 18 is configured to classify the activated sludge stream into five element streams: a first stream 50 comprising the majority of the activated carbon grains present in the activated sludge stream 46, a second stream 52 comprising the majority of the light activated sludge present in the activated sludge stream 46, a third stream 54 comprising the majority of the inert mineral particles present in the activated sludge stream 46, a fourth stream 80 comprising the majority of the dense activated sludge present in the activated sludge stream 46, and a fifth stream 82 comprising the majority of the inert organic fibers and inorganic fibers present in the activated sludge stream 46.

[0154] In particular, the gravimetric separation device 18 includes a first spiral separator 56 and a second spiral separator 84 connected downstream of one of the lower outlets 60 of the first spiral separator 56, and in particular to the outlet 60 of the least dense element flow.

[0155] Thus, the first spiral separator 56 is configured to separate the activated sludge stream 46 into three streams: the first stream 50 comprising the majority of the activated carbon grains present in the activated sludge stream 46, the third stream 54 comprising the majority of the inert mineral particles present in the activated sludge stream, and an intermediate stream 86 comprising a mixture of the second stream 52 comprising the majority of the light activated sludge present in the activated sludge stream 46, the fourth stream 80 comprising the majority of the dense activated sludge present in the activated sludge stream 46, and the fifth stream 82 comprising the majority of the inert organic fibers and inorganic fibers present in the activated sludge stream 46.

[0156] The second spiral separator 84 is configured to separate the intermediate stream 86 into three streams: the second stream 52, the fourth stream 80 and the fifth stream 82.

[0157] As an example, to perform this classification, the helical surface 62 of the first spiral separator 56 has a fairly steep slope, for example, between 10° and 45°. As shown in Figures 6 and 7, the first spiral separator 56 includes a protrusion 88 extending along the helical surface 56, at a constant distance from the main axis P, for example, 1 / 3 of the spiral radius. The protrusion 88 delimits a channel located in a first region 90 on the side of the main axis P, which preferentially concentrates the inert mineral particles. The activated carbon grains, subject to greater drag due to their diameter, concentrate in a second central region 92, further axially from the main axis P than the first region 90.

[0158] Preferably, the first spiral separator 56 further comprises, in the illustrated example, a plurality of grooves 94 and a plurality of riffles 96.

[0159] The grooves 94 extend in circular arcs across the helical surface 62. Each groove 94 extends from a first end 98 located near the first region 90, specifically near an outer edge of the first region 90, to a second distal end 100 located near a third region 102, further axially from the second region 92. Each groove 94 has a depth, for example, between 200 µm and 1000 µm and allows the inert mineral particles present in the second region 92 or the third region 102 to be directed towards the first region 90. The depth of the grooves 94 is chosen to be greater than the size of the inert mineral particles, typically greater than 200 µm, and close to or less than the size of the activated carbon grains, so that the activated carbon grains can be carried over the grooves without being intercepted.

[0160] The riffles 96 extend in an arc across the helical surface 62. Each riffle 96 extends from a first end 104 located near the second region 92, specifically from an outer edge of the second region 92, and a second end 106 located near an outer edge of the third region 102. Each riffle 96 has a height, for example, between 500 µm and 5000 µm and directs the activated carbon particles and the inert mineral particles towards the second central region 92. The inert mineral particles are also intercepted by the grooves 94, which direct them towards the first region 90, while the activated carbon particles remain in the second central region 92.

[0161] Dense activated sludge, light activated sludge and inert and inorganic organic fibers remain preferentially located in the third region 102.

[0162] The second spiral separator 84 has different geometric characteristics from the first spiral separator 56 to facilitate the classification of less dense materials. For example, the helical surface 62 has a shallower slope and radial inclination than that of the first separator 56. The radius of the spiral of the second spiral separator 84 is advantageously larger than that of the spiral of the first spiral separator 56.

[0163] Inert organic fibers and inorganic fibers with low density but a geometry that makes them more susceptible to drag forces are collected in a peripheral region of the second spiral separator 84. Dense activated sludge is moved to a proximal region located near the main axis P, and light activated sludge to a central region located between the proximal and peripheral regions.

[0164] Alternatively, the gravimetric separation device 18 comprises a single first spiral separator with an upper portion having the geometric characteristics of the first spiral separator 56, and a lower portion connected to the upper portion, having the geometric characteristics of the second spiral separator 84 described above. The upper portion includes two intermediate outlets for extracting the first stream 50, comprising the majority of the activated carbon grains present in the activated sludge stream 46, and the third stream 54, comprising the majority of the inert mineral particles present in the activated sludge stream 46.The intermediate stream 86 is classified in the lower portion of the separator which includes three lower outlets to extract the second stream 52 comprising the majority of the light activated sludge present in the activated sludge stream 46, the fourth stream 80 comprising the majority of the dense activated sludge present in the activated sludge stream 46, and the fifth stream 82 comprising the majority of the inert organic fibers and inorganic fibers.

[0165] In the embodiment of the, the first spiral separator 56 includes a first return pipe 20, a first rejection pipe 22, a transfer pipe 108.

[0166] The first return line 20 is intended to circulate the first stream 50 comprising the majority of the activated carbon grains upstream of the biological reactor 14 or into the biological reactor 14.

[0167] Optionally, the installation includes an activated carbon regeneration unit 110 connected to the first return line 20 between the first lower outlet 60 of the first spiral separator 56 and the biological reactor 14 to regenerate the activated carbon granules. Of course, according to a particular embodiment, the installation 10 may include a second bypass line 112 to avoid the passage of the activated carbon granules through the regeneration unit 110 and direct them directly to the biological reactor 14.

[0168] The first discharge pipe 22 is intended to circulate the third stream 54 comprising the majority of inert mineral particles to the inert material treatment and / or storage unit 74 of the installation 10.

[0169] The transfer line 108 is intended to circulate the intermediate flow 86 towards the second spiral separator 84.

[0170] The second spiral separator 84 includes a second return pipe 76, a second discharge pipe 70 and a third discharge pipe 114.

[0171] The second return pipe 76 is intended to circulate the fourth stream 80 comprising the majority of the dense activated sludge upstream of the biological reactor 14 or in the biological reactor 14.

[0172] The second discharge pipe 70 and the third discharge pipe 114 are intended to circulate respectively the second stream 52 comprising the majority of the light activated sludge present in the activated sludge stream 46 and the fifth stream 82 comprising the majority of the inert organic fibers and inorganic fibers to a recovery unit 116 of the installation 10, as shown on the.

[0173] The valorization unit 116 includes a thickening unit 118, an anaerobic digestion unit 120, and a dehydration unit 122.

[0174] The thickening unit 118 is intended to receive the second stream 52 comprising the majority of the light activated sludge and to form a thickened sludge stream 124. The thickening unit 118 is intended to increase the solids concentration of the second stream 52. For example, the thickening unit 118 includes a table or a drum or a draining grid, or a static thickener, or a centrifuge.

[0175] The anaerobic digestion unit 120 is designed to receive the thickened sludge stream 124 and to produce biogas and digestate 126.

[0176] The dehydration unit 122 is intended to dehydrate the digestate 126 to form a sludge cake.

[0177] Advantageously, the third discharge pipe 114 is connected downstream of the anaerobic digestion unit 120 and upstream of the dewatering unit 122. Inert organic or inorganic fibers do not participate in biological activity during anaerobic digestion. On the contrary, they can improve the dewatering properties of the sludge. They help structure the sludge cake and thus improve its final dryness while preventing the formation of fibrous accumulation in the anaerobic digestion unit 120.

[0178] As an alternative or in addition, the installation 10 includes a diversion line 128 intended to circulate all or part of the third stream 54 comprising the majority of the inert mineral particles upstream of the dewatering unit 122. The presence of the inert mineral particles helps to improve the dryness of the sludge cake.

[0179] Alternatively, or in addition, the recovery unit 116 includes a hydrolysis treatment unit 130, preferably thermal or chemical, designed to hydrolyze all or part of the inert organic fibers of the fifth stream 82, which comprises the majority of the inert organic fibers and inorganic fibers, and to circulate the hydrolyzed stream 132 downstream of the thickening unit 118 and upstream of the anaerobic digestion unit 120. The hydrolysis process renders some of the inert organic fibers biodegradable. This allows the fifth stream 82 to contribute to biogas production during anaerobic digestion.

[0180] A fifth embodiment according to the invention, with reference to the, is now described by differences with respect to the first embodiment.

[0181] In this embodiment, the gravimetric separation device 18 is configured to classify at least a part of the mixed liquor stream 24 into at least three streams of elements 50, 52, 54. In particular, the first spiral separator 56 is configured to classify the mixed liquor stream 24 into these three streams of elements 50, 52, 54 in a manner similar to that described in the first embodiment.

[0182] A sixth embodiment according to the invention, with reference to the, is now described in relation to the first embodiment.

[0183] In this embodiment, the gravimetric separation device 18 includes a hydrocyclone 134 and a first spiral separator 56 fluidically connected downstream of the hydrocyclone 134.

[0184] The hydrocyclone 134 is configured to separate part of the activated sludge flow 46 into the second flow 52 of elements comprising the majority of light activated sludge and a dense intermediate flow 136 of elements mostly denser than light activated sludge in particular inert mineral particles and dense activated sludge, and where appropriate adsorbent particles or particles intended to ballast the flocs.

[0185] The first spiral separator 56 is configured to classify the dense intermediate stream 136 into the first stream 50 of elements comprising the majority of dense activated sludge and the third stream 54 of elements comprising the majority of inert mineral particles.

[0186] The different flows 50, 52, 54 are treated in the same way as in the first embodiment.

[0187] As an alternative, the gravimetric separation device 18 can be configured to classify at least part of the mixed liquor stream 24 and not the activated sludge stream 46.

[0188] A seventh embodiment according to the invention, with reference to the, is now described in relation to the first embodiment.

[0189] In this embodiment, the activated sludge biological reactor 14 is a sequential activated sludge biological reactor and the liquid / solid separation device 16 is formed by a basin of said activated sludge biological reactor 14.

[0190] In this type of biological reactor, the biological treatment steps take place in a single tank according to the following sequence: feeding the biological reactor with a wastewater flow 12, reaction phase (anaerobic and / or anoxic and / or aeration by introduction of air or a gas containing oxygen) in the biological reactor, decantation of the mixed liquor, and draining of the clarified water.

[0191] According to a particular embodiment, the feeding and emptying steps take place simultaneously.

[0192] According to a particular embodiment, the gravimetric separation device 18 is configured to classify at least a part of the activated sludge flow 46 taken from at least one height in the tank, during the settling and / or feeding stage.

[0193] Alternatively or in addition, the gravimetric separation device 18 is configured to classify at least a part of the mixed liquor flow 24 taken from at least one height in the tank, during the reaction step.

[0194] Alternatively (not shown), for all the above embodiments, the liquid / solid separation device 16 includes at least one membrane unit disposed in or outside a basin of the activated sludge biological reactor 14.

Claims

A wastewater (12) treatment plant (10) comprising: - at least one activated sludge biological reactor (14) configured to be fed by the wastewater (12) stream and to produce a mixed liquor (24) stream, - at least one liquid / solid separation device (16) configured to separate the mixed liquor (24) stream into a treated water (44) stream and an activated sludge (46) stream, - at least one gravimetric separation device (18) comprising at least one first spiral separator (56), the gravimetric separation device (18) being configured to classify at least a portion of the mixed liquor (24) stream or at least a portion of the activated sludge (46) stream into at least three element streams (50, 52, 54), the first spiral separator (56) being configured to classify at least a portion of the mixed liquor (24) stream or at least a portion of the activated sludge stream. activated sludge (46) in at least two of the three element streams (50, 52, 54),- at least one first return line (20) intended to circulate at least one of the three streams (50, 52, 54) upstream of the biological reactor (14) or within the biological reactor (14), - at least one first discharge line (22) intended to circulate at least one of the three streams (50, 52, 54) to at least one first treatment unit or storage unit (72, 74). Treatment installation (10) according to claim 1, wherein at least one first spiral separator (56) extends along a substantially vertical principal axis (P) and comprises: - at least one upper inlet (58) for the introduction of at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46) and, - at least two lower outlets (60) for two of the at least three element streams (50, 52, 54), the spiral separator (56) defining a helical surface (62) winding around the principal axis (P) from the upper inlet (58) to the lower outlets (60). Treatment installation (10) according to claim 1 or 2, wherein the activated sludge biological reactor (14) is a sequential activated sludge biological reactor and the liquid / solid separation device (16) is formed by a tank of said activated sludge biological reactor (14). Treatment plant according to claim 1 or 2, wherein the biological activated sludge reactor (14) is a continuous biological activated sludge reactor and the liquid / solid separation device (16) includes a secondary settling tank (42) or includes at least one membrane unit disposed in or outside a basin of the biological activated sludge reactor (14). Treatment plant (10) according to any one of claims 1 to 4, wherein at least one first spiral separator (56) is configured to classify at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46) into at least three element streams. Processing installation according to any one of claims 1 to 5, wherein the gravimetric separation device (18) comprises a second spiral separator (84) connected downstream of the first spiral separator (56), one of the streams from the first spiral separator (56) feeding the second spiral separator (84), the first spiral separator (56) and the second spiral separator (84) having at least one distinct geometric feature. Treatment plant (10) according to any one of claims 1 to 6, wherein one of the three element streams (50, 52, 54) is an element stream comprising the majority of the inert non-biodegradable mineral particles present in at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46), the first discharge line (22) being intended to circulate said stream to a treatment and / or storage unit (74). Treatment plant (10) according to any one of claims 1 to 7, wherein one of the three element streams is an element stream (50, 52, 54) comprising the majority of dense activated sludge including granular sludge present in at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46), the first return line (20) being intended to circulate said element stream upstream of the biological reactor (14) or into the biological reactor (14). Treatment installation (10) according to any one of claims 1 to 8, wherein the gravimetric separation device (18) further comprises: - a hydrocyclone (134) configured to separate at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46) into a second stream (52) of elements comprising a majority of light activated sludge containing bacterial flocs and a dense intermediate stream (136) of elements mostly denser than the light activated sludge,the first spiral separator (56) being configured to classify the dense intermediate stream (136) into a first stream of elements (50) comprising the majority of dense activated sludge from the mixed liquor stream portion (24) or the activated sludge stream portion (46) and a third stream (54) of elements comprising the majority of non-biodegradable inert mineral particles from the mixed liquor stream portion (24) or the activated sludge stream portion (46). A wastewater treatment process (12) comprising the following steps: - feeding a biological activated sludge reactor (14) with a wastewater stream (12) and producing a mixed liquor stream (24), - separating the mixed liquor stream (24) into a treated water stream (44) and an activated sludge stream (46) with at least one liquid / solid separation device (16), - classifying at least a portion of the mixed liquor stream (24) or at least a portion of the activated sludge stream (46) into at least three element streams (50, 52, 54) with at least one gravimetric separation device (18) comprising at least one first spiral separator (56), said classification step comprising classifying at least a portion of the mixed liquor stream (24) or at least a portion of the activated sludge stream (46) into at least two of the three element streams (50, 52, 54) with the first spiral separator (56),- circulate at least one of the three streams in the biological reactor (14),- to circulate at least one of the three streams to at least one first processing unit or a storage unit (72, 74). A treatment process according to claim 10, wherein one of the three element streams (50, 52, 54) is an element stream comprising the majority of inert, non-biodegradable mineral particles present in at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46), the process comprising a step of circulating said stream to the first treatment unit or a storage unit (72, 74). A treatment process according to claim 10 or 11, wherein one of the three element streams is an element stream comprising the majority of the dense activated sludge including the granular sludge present in at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46), the process comprising a step of circulating said stream upstream of the biological reactor (14) or in the biological reactor (14). A treatment process according to any one of claims 10 to 12, comprising a step of introducing particles upstream of the biological reactor (14) or into the biological reactor (14), one of the three element streams being an element stream comprising the majority of said particles present in at least a part of the mixed liquor stream (24) or at least a part of the activated sludge stream (46), the process comprising a step of circulating said stream comprising the majority of the particles upstream of the biological reactor (14) or into the biological reactor (14). Treatment process according to claim 13, wherein the particles are selected from: adsorbent particles intended to adsorb micropollutants present in the wastewater stream (12) and / or particles intended to ballast bacterial flocs present in the biological reactor (14) and / or biomass supports intended for the fixation and growth of biomass in the biological reactor (14).

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