EP4572872A1 - Method for treating dirty overflow water from a water treatment plant, and corresponding facility - Google Patents

Abstract

The invention relates to a method for treating dirty overflow water in order to separate suspended matter and insoluble granular material that is heavier than water. This method involves a degassing step (1), two steps of liquid-solid separation within a hydrocyclone (4) and a recirculation cylinder (5), and a step of recirculating the water to be treated, carried out by a hydroejector (3).

Description

DESCRIPTION

TITLE: Process for treating dirty overflow water from a water treatment system, and corresponding installation

TECHNICAL AREA

The field of the invention is that of the treatment of water with a view to its purification or its purification.

More precisely, the invention relates to the treatment of dirty water previously coagulated and potentially flocculated, coming from a water treatment sector.

PRIOR ART

A well-known process for treating water, in particular surface water to be made potable and urban or industrial wastewater to be depolluted, consists of coagulating the water to be treated with a coagulating reagent, such as for example a trivalent metal salt, to be made flocculating the organic matter contained in the coagulated water with a flocculating reagent, usually consisting of an organic polymer, and decanting or filtering the flocs and other solid materials in a decanter or through a filter. In the case of the decanter, the sludge is extracted at the bottom of the decanter, and the treated water is extracted in the overflow of the decanter; while in the case of a filter, the filtered water is extracted from another side of the filter, and the sludge/dirty water is extracted by backwashing through the overflow of the filter.

More particularly, an example of technology implementing coagulation and flocculation steps consists of adding coagulants and/or flocculants to raw water, in order to obtain flocs. The water thus coagulated and/or flocculated can be filtered through an ultrafiltration membrane, the flocs forming a cake on the surface of the membrane, and the clarified water passing through it. Subsequently, the floc cake can be evacuated by backwashing the membrane and overflowing with a tarpaulin of dirty washing water.

Thus dirty water is recovered by overflowing the filtration installation, by filter or membranes, this dirty water can then be thickened and/or dehydrated in post-treatment systems. However, such a process induces high water losses and expensive and cumbersome post-treatment.

Another example of technology implementing coagulation and flocculation stages is offered by the Actiflo® process, which also uses a ballast made of a fine, high-density granular material such as microsand for example. , injected into or upstream of the flocculation zone, in order to increase the speed of floc formation by serving as a flocculation initiator, and also to increase by increasing their density the speed of settling of the flocs formed during the phase flocculation. The weighted sludge is extracted at the bottom of the decanter.

Microsand, with an average diameter of between approximately 20 and 300 micrometers, most often 80 and 200 micrometers, is the ballast used most frequently for reasons of availability and cost.

The ballast is usually, for economic reasons of reuse, separated from the sludge extracted from the settling structure and recycled in the process. Ballast losses are usually divided between losses through treated water overflowing from the decanter and losses with sludge extracted from the structures.

An injection of fresh ballast intended to compensate for ballast losses is planned.

Ballast losses entrained with the sludge are important to control, both to minimize expenses for fresh ballast and to avoid degrading the quality of the extracted sludge.

The means used to separate the ballast from the extracted sludge and recycle this ballast in the process while minimizing ballast losses are generally chosen from static gravity separation techniques (such as decantation) or dynamic (such as centrifugation and cycloning), most often by hydrocycloning of the sludge/ballast mixture.

The ballast losses in the overflow of the hydrocyclone, the means most often used in practice to separate the ballast from the extracted sludge, are in general and for a hydrocyclone geometry and fixed operating conditions, approximately proportional to the concentration of the ballast in the mixture entering the hydrocyclone.

In order to reduce these ballast losses, it has already been proposed in the state of the art to treat this overflow in a second hydrocyclone. Such a use of two hydrocyclones mounted in cascade is notably disclosed in WO03053862A1.

However, this process leads to volumes of sludge at the outlet of the second hydrocyclone still containing a significant proportion of water. Thus, this diluted sludge is the cause of water loss.

This sludge must therefore be thickened and dehydrated during post-treatment.

Such post-processing increases the costs of implementing the process. The corresponding equipment also increases the footprint of the installations.

To limit water losses, we also know the invention disclosed in W02011103651A1 known under the name Actiflo® HCS. It concerns a ballasted flocculation - decantation installation including a simplified system for recirculating part of the overflow of the hydrocyclone for separating the ballasted sludge. Part of the overflow from the hydrocyclone is returned, after degassing, to the suction of the extraction pump of the mud/ballast mixture supplying the hydrocyclone. This regulated recirculation system makes it possible to reduce the volume of water extracted with the sludge and therefore to concentrate this sludge. It also helps reduce ballast losses. But it cannot be used on all types of water and must be selected according to the load and flow rate of the water to be treated.

OBJECTIVES OF THE INVENTION

The objective of the invention is to propose a method making it possible to overcome these drawbacks of the prior art.

More precisely, the main objective of the invention is to propose a process for treating dirty overflow water coming from a water treatment sector, making it possible to limit water losses.

Another objective of the invention is to reduce the volume of sludge produced by such a water treatment system.

Another objective of the invention is to propose a process which makes it possible to reduce the quantities of chemicals which must be used in such a sector and therefore to limit the ecological impact of the corresponding treatment.

Another objective of the invention is to propose a device for implementing such a method making it possible to limit the size of such a sector. PRESENTATION OF THE INVENTION

These objectives, as well as others which will appear later, are achieved using a process for treating dirty overflow water used to separate, on the one hand, treated water, and on the other hand, solid particles contained in the dirty overflow water, said solid particles being composed of suspended matter and/or an insoluble granular material heavier than water. This process includes the following steps:

- a step of degassing the dirty overflow water;

- a step of supplying dirty water from degassed overflow to a hydrocyclone via a hydroejector powered by an accelerator pump, the accelerator pump being able to adjust a flow rate and a pressure;

- a first step of separating suspended matter and/or granular material present in the dirty degassed overflow water, by hydrocycloning within the hydrocyclone;

- a step for recovering an overflow of the hydrocyclone, the overflow mainly comprising water and residual suspended matter;

- a second step of separating residual suspended matter from the water recovered from the overflow of the hydrocyclone, within a recirculation cylinder;

- a step of recirculating water towards the hydrocyclone comprising the residual suspended matter coming from the recirculation cylinder by suction by the hydroejector;

- a step of extracting clarified water, leaving the recirculation cylinder;

- a step of continuous measurement of the flow rate at the inlet and outlet of said hydroejector, and at the outlet of the recirculation cylinder.

According to the invention, in order to be able to eliminate the suspended matter present at the overflow outlet of an upstream installation, the invention proposes to implement a process for treating this dirty overflow water comprising two liquid separation stages -solid using a hydrocyclone and a recirculation cylinder, coupled with a recirculation of the water at the outlet of the recirculation cylinder.

According to the present description, the terms "recirculation cylinder" designate a cylinder comprising the following elements: a water supply close to one of the ends of the cylinder (connected to the overflow of the hydrocyclone), a clarified water outlet positioned at the other end of the cylinder, an evacuation of solid particles positioned at the periphery of the cylinder between the supply and the outlet of the cylinder and connected to the hydroejector, and a centrifugation system.

The function of this recirculation cylinder is similar to that of a hydrocyclone in the sense that the recirculation cylinder uses a vortex creating centrifugation of the water contained in the cylinder, allowing liquid-solid separation. The centrifugal force created by the swirling flow in which the fluid rotates around an axis causes the ejection of solid particles around the periphery of the cylinder wall. The water discharged through a tangential outlet becomes loaded with solid particles. The clarified water drains into the center of the cylinder.

According to the present invention, the dirty overflow water therefore undergoes at least a first stage of liquid-solid separation (or hydrocycloning) in the hydrocyclone; then the overflow water from the hydrocyclone also undergoes at least a second stage of liquid-solid separation in the recirculation cylinder, the latter having a lower cut-off threshold than the hydrocyclone, and making it possible to separate the majority of the suspended particles residuals which have not been separated from the water during the first stage of liquid-solid separation.

In order to limit water losses, the water containing the residual suspended particles recovered on the wall of the cylinder and separated from the clarified water during the second stage of liquid-solid separation is reintroduced during stage d. power to the hydrocyclone via the recirculation loop. Thus, the water coming from the recirculation cylinder and containing suspended matter is sucked up by a hydroejector (device using the Venturi effect to create suction), and is re-circulated towards the hydrocyclone in order to optimize its treatment and to remove residual suspended matter. Thus, this process makes it possible to optimize liquid-solid separation, in particular for extremely fine solid particles (suspended matter), while limiting water losses.

In addition, the implementation of continuous measurement of flow and pressure also makes it possible to monitor the proper functioning of the process.

Furthermore, in order to avoid any malfunction linked to the presence of air, the process describes the implementation of a degassing step upstream of the supply step.

According to a preferred characteristic of the invention, the method comprises a step of continuously measuring the flow rate of the accelerator pump and a step of adjusting this flow rate continuously as a function of the result of the measurement recorded during the measurement step. ; the step of adjusting the flow rate of said accelerator pump being such that, for a flow rate at the inlet of said hydroejector of xm ³ /h, then the flow rate at the outlet of the hydroejector is between l.lx and l.5x m ³ / h, the flow rate at the underflow outlet of the hydrocyclone is less than 0.lx m ³ /h, the recirculation flow rate is between 0.lx and 0.5x m ³ /h, and the flow rate at the outlet of overflow of the recirculation cylinder is between 0.9x and 0.95x m ³ /h.

In order to set up the recirculation loop making it possible to suck up the water containing residual suspended matter coming from the recirculation cylinder, the invention proposes to maintain a specific flow rate at each stage of the process. Thus, advantageously, the optimization of the process according to the invention is made possible by the use of a variable speed accelerator pump and the implementation of continuous measurement of the flow rate. Indeed, the process according to the invention has a very specific operating point, requiring continuous maintenance of adequate conditions. In particular, all of the parameters implemented in this process are co-dependent (flow rate, pressure, characteristics of the hydroejector and hydrocyclones, etc.). Thus, in order to limit water losses and maximize the elimination of suspended matter, the process according to the invention proposes to regulate the flow rate during each stage of the process to obtain at the outlet of the recirculation cylinder a flow rate close to the flow rate. at the hydroejector inlet, that is to say approximately equal to xm ³ /h, and more precisely, a flow rate of between 0.9x and 0.95x m ³ /h.

According to another characteristic of the invention, the method comprises a step of adding polymer to the dirty degassed overflow water upstream of said hydroejector and/or at the inlet of the recirculation cylinder.

Advantageously, the addition of polymer to the water to be treated makes it possible to improve the elimination of suspended matter. In fact, the addition of polymers to the dirty overflow water to be treated, before it arrives in the hydroejector, makes it possible to agglomerate together the particles of suspended matter, and thus increase the size of these particles. particles, which facilitates their separation from water during the different stages of liquid-solid separation.

This step of adding polymers can be carried out automatically as a function of a level of suspended matter at the inlet of the hydroejector, and/or at the inlet of the recirculation cylinder. According to an advantageous characteristic of the invention, the flow rate at the hydroejector inlet x is between 4 and 50 m ³ /h.

Thus, the process according to the invention advantageously makes it possible to control the flow rate and adapt it for optimization of the liquid-solid separation and recirculation stages of the water to be treated. In addition, respecting a predetermined flow rate at the outlet of the pump makes it possible to maintain the conditions corresponding to the specific operating point (flow rates, pressure) of the process, and thus to optimize its efficiency, by limiting the water losses and maximizing the elimination of suspended matter.

Preferably, the hydraulic residence time is between 2 s and 4 s within the hydrocyclone and within the recirculation cylinder.

Thus, respecting a specific hydraulic residence time makes it possible to optimize the separation of suspended matter and water during the liquid-solid separation stages.

According to a preferred variant, the hydrocyclone has a cut-off threshold of between 10 and 25 pm, and the recirculation cylinder has a cut-off threshold of between 5 and 15 pm.

Thus, this process makes it possible to eliminate extremely fine particles (for example flocs), whose density is relatively close to water. These cut-off thresholds make it possible to limit the use of chemical products because of their effectiveness on fine particles. In fact, these two mechanical stages of liquid-solid separation do not require the use of coagulating or flocculating products to effectively separate extremely fine contaminants from the water.

According to an advantageous characteristic, the pressure of the water leaving said accelerator pump is between 2 and 5 bars.

This pressure is adapted to water treatment, and is part of the specific parameters of the process, making it possible to respect the specific operating point of the process. Indeed, a pressure of 2 to 4 bars at the accelerator pump outlet allows, after the water has passed through the hydroejector, to obtain a pressure of between 1.5 and 2.5 bars at the hydrocyclone inlet. , due to pressure losses induced by the Venturi effect (approximately 1 to 1.5 bar). Thus, the pressure obtained at the inlet of the hydrocyclone and the recirculation cylinder allows the implementation of cut-off thresholds allowing optimal separation of suspended matter.

According to another preferred aspect of the invention, the hydroejector comprises a nozzle whose outlet diameter is between 1/5th and half of its inlet diameter.

In order to create a Venturi effect generating a sufficient suction force to allow the recirculation of part of the water coming from the recirculation cylinder towards the hydrocyclone, the process proposes adjusting the diameter of the nozzle in order to respect the specific operating point of the process. In particular, the pressure losses and the suction force depend on the inlet and outlet diameters of the hydroejector nozzle. In fact, the smaller the outlet diameter compared to the inlet diameter, the greater the pressure losses and the suction force, while for an outlet diameter relatively close to the inlet diameter, the pressure losses load and suction force are lower.

According to a preferred variant of the invention, the dirty overflow water comes from a hydrocyclone of a ballasted flocculation-settling installation.

According to other variants, the dirty overflow water may also come from a filtration system, said filtration system belonging to the group comprising:

- sand filters;

- granular active carbon filters; - ultrafiltration membranes;

- biological filters using biomass fixed on a granular support such as for example filters marketed under the registered trademark Biostyr;

- mechanical filters, such as Hydrotech or others.

The invention also relates to a device for treating dirty overflow water with a view to separating suspended matter and/or an insoluble granular material heavier than water for the implementation of a process such as described previously. The device includes:

- degassing means at the inlet of said installation;

- an accelerator pump capable of adjusting a flow rate and a pressure;

- a hydroejector;

- a hydrocyclone powered by water coming from the hydroejector;

- a recirculation cylinder powered by water coming from the overflow of the hydrocyclone;

- pressure and/or flow measurement means; the hydroejector being capable of recirculating part of the water coming from the recirculation cylinder towards the hydrocyclone.

According to one characteristic of the invention, the device comprises means for adding polymer provided upstream of the hydroejector and/or at the inlet of the recirculation cylinder.

According to a preferred characteristic of the invention, the recirculation cylinder is positioned horizontally.

Indeed, the device can be installed at the top of a ballasted flocculation-settling device of the prior art, and be connected to the overflow of the hydrocyclone thereof, such a configuration making it possible to minimize the bulk of the device. all.

According to a preferred characteristic of the invention, the hydrocyclone and the recirculation cylinder each have a length of between 0.8 and 2m.

Thus, the device is relatively compact and takes up little space, and can be placed in the upper part of a pre-existing installation of the prior art.

PRESENTATION OF FIGURES

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given by way of a simple illustrative and non-limiting example and described with reference to the drawings in which:

[Fig. 1] represents a device for treating dirty overflow water, according to an exemplary embodiment of the invention;

[Fig. 2] represents a ballasted flocculation-settling installation equipped with such a device. DETAILED DESCRIPTION OF THE INVENTION

1. Description of an embodiment of a device according to the invention

The device according to the invention is supplied with dirty water coming from a water treatment system. This water treatment sector can for example take the form of a ballasted flocculation-settling installation, a biofilter, an ultrafiltration membrane, a sand filter, or any other water treatment facility. Within the water treatment sector, the water has previously undergone physicochemical treatments. The dirty water may thus have been coagulated and potentially flocculated and/or weighted, before being filtered, and/or decanted, and/or hydrocyclonated. The dirty water supplying the device according to the invention can therefore come from the overflow 100 under pressure of a hydrocyclone of a weighted flocculation-settling installation, or even from the overflow 100 (under pressure, or gravity) of counter- washing a filter (biofilter, ultrafiltration, sand filter, activated carbon filter, etc.) or any type of separator used in a water treatment sector.

With reference to Figure 1, the DISP device according to the invention comprises degassing means 1 to eliminate any air from the dirty water entering the device. This device also includes an acceleration pump 2 playing the role of driving force, this making it possible to bring the water coming from the degassing means to a hydroejector 3 with Venturi effect. The water is then conveyed to a hydrocyclone 4. In the underflow 41 of this hydrocyclone, granular material which can be used to ballast the flocs, as well as a large part of the suspended matter are recovered in the form of sludge. The granular material used as ballast can optionally be reused. In overflow 42 of the hydrocyclone, pretreated water, containing only extremely fine residual suspended matter, is recovered. The water may also potentially contain residual granular materials. This pretreated water is routed to a recirculation cylinder 5 within which the residual particles are separated from the treated water by centrifugation thanks to the creation of a vortex. The particles are therefore found on the walls of the recirculation cylinder, then are sucked up by the Venturi effect by the hydroejector 3, connected to the wall of the recirculation cylinder. The residual suspended solids coming from the recirculation cylinder are thus mixed with the untreated dirty water coming from the tank or degassing column, within the hydroejector, then the mixture is conveyed to the hydrocyclone 4, etc. This recirculation loop therefore makes it possible to reprocess these residual suspended materials until the contaminants are optimally eliminated.

The flow rate within the device implemented by the method according to the invention changes depending on the stage of the process. Thus, at the accelerator pump outlet, the flow rate is preferably between 4 and 50 m3/h. Then taking this feed flow as a reference and assigning the value x to it, the feed flow of the hydrocyclone is advantageously between l.lx and l.5x, the underflow flow of the hydrocyclone is preferably less than 0.lx, the recirculation flow rate is advantageously between 0.lx and 0.5x, and finally, the outlet flow rate is preferably between 0.9x and 0.95x. It should be noted that the flow rate does not depend on the size of the nozzle 31, unlike the circulation speed.

It is possible to put several processing lines according to the invention in parallel to process a greater total flow rate. 2. Characteristics of the elements constituting the device according to an exemplary embodiment of the invention

2.1. Sensors

Flow and pressure sensors 6 can be placed between each processing step. These sensors make it possible to check the direction of water circulation (in the event of a malfunction), as well as the quantity of water recirculated. This makes it possible to adapt the pressure at the inlet of the device to maintain optimal treatment conditions.

2.2. Degassing means

The degassing means 1 make it possible to eliminate the air or any gas potentially contained in the dirty water to be treated. These means are for example a tank, a tarpaulin or a degasser. They thus make it possible to avoid any presence of air which could be the cause of phenomena causing malfunctions within the installation (for example, the phenomenon of pump cavitation).

Hydraulic means, such as vents or bleeders, can also be used to ensure the absence of air and/or gas in the device. These means can be placed at any processing stage of the device, in particular, at the liquid-solid separation stages. This makes it possible, among other things, to avoid cavitation phenomena (which could cause malfunctions in particular at the level of the supply pump and the Venturi effect hydroejector).

2.3. Accelerator pump

The accelerator pump 2 plays the role of driving force, and makes it possible to supply the device with dirty degassed overflow water.

Accelerator pump 2 allows very fine control of the flow and pressure parameters. This pump works preferentially in relation to the sensors 6. This pump can be frequency-varying.

In the embodiment implemented by the inventor during his tests, the pressure delivered by the accelerator pump was 3 to 4.5 bars.

2.4. Venturi effect hydroejector

The hydroejector 3 is powered by the accelerator pump 2, and also feeds into the recirculation cylinder 5. In fact, the pump provides the flow of degassed water necessary for the hydroejector to suck up, by Venturi effect, a portion water contained in the recirculation cylinder (this water containing solid particles to be extracted from the water), the hydroejector being connected to the wall of the recirculation cylinder.

The hydroejector is composed of a nozzle 31 whose conical shape (combined with the flow generated by the accelerator pump) allows the creation of the Venturi effect. At the nozzle outlet (inlet side of the hydrocyclone) the outlet diameter is between 1/5th and half of the inlet diameter of the nozzle (accelerator pump side). This ratio must be respected because, in the event of an outlet diameter that is too small, the pressure loss would be too great, and for a diameter that is too large, the Venturi effect would disappear. The nozzle outlet diameters used by the inventor during his tests were between 2 and 15mm. In addition, the angle formed by the nozzle also influences the Venturi effect.

The speed of circulation of the water within the device is advantageously 0.5 to 2 m/s, apart from within the Venturi effect hydroejector, where the water is accelerated 4 to 25 times to obtain a speed from 2 to 50 m/s. The speed of circulation of the water within the hydroejector 3 is adjustable and depends on the diameter of the nozzle 31. The speed of circulation at the outlet of the hydroejector 3 is therefore between 0.5 and 2 m/s.

The passage of the water to be treated through the hydroejector induces a reduction in pressure. The smaller the nozzle diameter, the greater the pressure loss. Likewise, the higher the flow rate, the greater the pressure loss. In the embodiment implemented by the inventor during his tests, the pressure losses due to the Venturi effect were 1 to 1.5 bars.

The materials used for the manufacture of the nozzle (and all the elements of the device) must resist abrasion and be adapted to the viscosity of the water.

The Venturi effect has a precise operating point to allow the suction of suspended solids from the recirculation cylinder. This suction depends on the pressure and flow rate provided by the accelerator pump, as well as the parameters inherent to the hydroejector (diameter, angle of the nozzle).

2.5. Hydrocyclone

The hydrocyclone 4 makes it possible to separate and extract the solid particles contained in the water to be treated coming from the hydroejector 3. The heaviest particles are thus extracted via the underflow of the hydrocyclone 4, while the overflow containing the finest particles (residual particles) is transferred to the recirculation cylinder 5.

The solid particles extracted from the hydrocyclone underflow may include granular materials serving as ballast. These granular materials can be recycled for reuse in steps upstream of the device.

The hydrocyclone used in the device has a cut-off threshold of between 5 and 25pm. This threshold depends on the flow rate, the pressure, and the hydraulic residence time.

The cut-off threshold of the hydrocyclone can be adapted to the separation of microsand particles and particles of equivalent density (10 to 25 pm), or to the separation of particles of lower densities (5 to 15 pm). Thus, this cut-off threshold, like the cut-off threshold of the recirculation cylinder, can be adapted according to the particles (density, size) contained in the water to be treated.

The hydrocyclone preferably measures between 0.8 and 2 m in length, with a hydraulic residence time of between 1 and 4 s, and a speed within it of 0.5 to 1 m/s. Optimal performance for such a hydrocyclone is obtained for 4s of hydraulic residence time. Indeed, the longer the hydraulic residence time, the more effective the liquid-solid separation. According to an exemplary embodiment, the hydrocyclone is similar to a hydrocyclone of a ballasted flocculation-settling installation, but is more spindle-shaped. The hydrocyclone may possibly be larger than 2m, depending on the desired use. Thus, the hydraulic residence time can be increased and allow an improvement in the liquid-solid separation efficiency.

In an exemplary embodiment, the hydrocyclone is positioned vertically. In the embodiment implemented by the inventor during his tests, the hydrocyclone operated at a pressure of 1 to 1.5 bars. In particular, in the case of using the device of the invention following a ballasted flocculation-settling installation comprising a first hydrocyclone, it is necessary to work at higher pressures than for the first hydrocyclone of the installation ballasted flocculation-decantation.

2.6. Recirculation cylinder

According to the present description, the terms "recirculation cylinder" designate a cylinder comprising the following elements: a water supply close to one of the ends of the cylinder (connected to the overflow of the hydrocyclone), a clarified water outlet positioned at the other end of the cylinder, an evacuation of solid particles positioned at the periphery of the cylinder between the supply and the outlet of the cylinder and connected to the hydroejector, and a centrifugation system.

The recirculation cylinder uses a vortex allowing the centrifugation of the water resulting from the overflow of the hydrocyclone, and allowing the separation of residual particles from the water. Thus, the particles group together on the walls of the recirculation cylinder and are sucked up by the hydroejector 3.

The function of this recirculation cylinder is similar to that of a hydrocyclone in that the recirculation cylinder implements a vortex creating centrifugation of the water contained in the cylinder, allowing liquid-solid separation. The centrifugal force created by the swirling flow in which the fluid rotates around an axis causes the ejection of solid particles around the periphery of the cylinder wall. The water discharged through a tangential outlet becomes loaded with solid particles. The clarified water drains into the center of the cylinder.

The recirculation cylinder 5 used in the device has a very fine cut-off threshold (of the order of 5 to 15 pm) adapted to the fineness of the residual suspended matter. This threshold depends on the flow rate, the pressure, and the hydraulic residence time. In addition, the recirculation cylinder being positioned downstream of the hydrocyclone, it has a lower threshold than this. Indeed, the hydrocyclone having carried out a first liquid-solid separation, only the finest particles remain present in the water transmitted to the recirculation cylinder. Therefore, in order to perform a second effective solid-liquid separation, the cutoff threshold must be decreased.

Furthermore, to meet both the space requirement, while optimizing the treatment of suspended matter, in one embodiment, the inventor set the length of the recirculation cylinder at lm20. The hydraulic residence time within this recirculation cylinder is between ls and 4s, with optimal performance for 4s, like the hydrocyclone. According to this embodiment, the recirculation cylinder had a size similar to the hydrocyclone, however, another configuration, in which their size differs is also possible. The recirculation cylinder was positioned horizontally.

The recirculation cylinder may be provided with a vent or a purge allowing the evacuation of air having been introduced during the liquid-solid separation stages. Indeed, this air purging, carried out by a vent for example, makes it possible to avoid the accumulation of air at the high point which could cause a malfunction.

In the embodiment implemented by the inventor during his tests, the recirculation cylinder operated at a pressure of 1 to 1.5 bars. 2.7. Other items

In a variant of the invention, a complementary step of adding polymer 7 can be implemented downstream of the acceleration pump, or at the inlet of the recirculation cylinder. To further improve the performance of the device, an automatic addition could be implemented depending on the rate of suspended matter entering the device.

A coagulant addition step 7 can also be implemented upstream of the polymer addition step.

A diaphragm valve can be positioned downstream of the accelerator pump, which can possibly allow the pressure to be adjusted.

3. Description of a filtration installation equipped with a device according to the invention

According to one embodiment of the invention, the DISP device can be implemented at the outlet of the backwashing overflow of a filter, this overflow being able to be either under pressure or gravity. Within the installation upstream of the device, the water is previously coagulated and potentially flocculated. It can also contain solid mineral particles such as sand or clay.

Thus, the dirty water can be backwash water from an ultrafiltration membrane, this ultrafiltration membrane allowing for example the filtration of raw water or pre-treated water. According to an exemplary embodiment, raw water (e.g. lake water) is coagulated before being routed to an ultrafiltration membrane. The suspended solids are thus coagulated and form a cake by sticking to the ultrafiltration membrane. This cake is then removed by backwashing and is evacuated with the backwashing water by overflowing the dirty water tank, towards the device of the invention. Subsequently, the device according to the invention makes it possible to separate the water from the solid materials contained in the dirty overflow water, in order to limit water losses from the installation.

According to another embodiment, the dirty water can be backwash water from a sand or granular activated carbon filter, or even backwash water from a biofilter (example Biostyr™).

4. Description of a ballasted flocculation-settling installation equipped with a device according to the invention

In relation to Figure 2, one embodiment of the invention comprises the implementation of the DISP device within a ballasted flocculation-settling installation. Such a ballasted flocculation-settling installation implements:

- an optional coagulation step of the suspended matter contained in the water to be treated, by the addition of coagulants 1000 in the water to be treated, forming agglomerates of particles;

- a step of flocculating the suspended matter contained in the water to be treated, in a flocculation zone 2000, thus forming flocs;

- an addition of granular material serving as ballast for the flocs, in flocculation zone 2000; - decantation of the suspended matter, flocs and granular material, in a decanter 3000, thus forming a sludge;

- a supply of mud to a first hydrocyclone 4000, the first hydrocyclone separating part of the solid particles (suspended matter, flocs and granular material) from the mud;

- recovery of a sludge composed of the heaviest particles (largely granular material) in the underflow 4001 of the first hydrocyclone, the granular material being recycled for reuse;

- recovery of a mud composed of the lightest overflow particles (100) from the first hydrocyclone, this mud (or dirty overflow water) always containing suspended matter, flocs and potentially granular material.

The installation also includes a DISP device according to the invention as described above in connection with Figure 1 (to simplify, certain elements of the device as described in Figure 1 have been omitted in Figure 2), and implementing performs the following steps:

- degassing 1 of the dirty overflow water 100 from the first hydrocyclone 4000;

- supply of a hydroejector 3 with dirty water degassed by an accelerator pump 2;

- hydrocycloning of water at the outlet of hydroejector 3, within a second hydrocyclone 4;

- recovery of granular material, flocs and suspended matter, in underflow 41 of the second hydrocyclone;

- centrifugation of the overflow water 42 from the second hydrocylone 4, within a recirculation cylinder 5;

- recirculation 200 of solid particles (granular material, flocs, suspended matter) by suction via the Venturi effect produced by the hydroejector 3, the water containing these solid particles being sucked by the hydroejector 3 into the recirculation cylinder 5 , to be mixed, within the hydro-ejector 3, with the dirty degassed overflow water supplying the hydro-ejector. The mixture then continues its circulation within the DISP device to supply the second hydrocyclone 4, etc.

- recovery of clarified water 300 at outlet 51 of the recirculation cylinder, this water can be recycled at the head of the flocculation installation - ballasted decantation;

- continuous measurement and adjustment of the flow rate, to comply with the following conditions: for a flow rate at the accelerator pump outlet of xm ³ /h, then the flow rate at the hydroejector outlet is between l.lx and l.5x m ³ /h, the flow rate at the underflow outlet of the second hydrocyclone is less than 0.lx m ³ /h, the recirculation flow rate is between 0.lx and 0.5x m ³ /h, and the flow rate at the outlet overflow of the recirculation cylinder is between 0.9x and 0.95x m ³ /h.

The underflow 41 of the second hydrocyclone can possibly be recycled if it contains for example residual activated carbon (used for the treatment of organic matter in the flocculation installation - weighted decantation) or carbonate ions if the flocculation installation - weighted decantation is used to remove hardness from water. 5. Experimental results

Tests were carried out with dirty overflow water coming from a hydrocyclone forming part of a ballasted flocculation-settling installation, this dirty overflow water presenting 0.91 g/L of suspended matter. The results of these tests are presented below. According to these results, the clarified water, leaving the recirculation cylinder, represents 90-95% of the water entering the DISP device. The tests also determined that at lower temperatures, performance was better due to the increased viscosity of the water.

For an internal diameter of 8 mm at the Venturi effect nozzle outlet: For an internal diameter of 10 mm at the Venturi effect nozzle outlet:

Claims

1. Method for treating dirty overflow water (100) to separate, on the one hand treated water, and on the other hand solid particles contained in the dirty overflow water, said solid particles comprising materials in suspension and/or an insoluble granular material heavier than water, process characterized in that it comprises the following steps: a step of degassing (1) said dirty overflow water (100); a step of supplying dirty degassed overflow water to a hydrocyclone (4) via a hydroejector (3) supplied by an accelerator pump (2), said accelerator pump (2) being able to adjust a flow rate and a pressure; a first step of separating the suspended matter and/or granular material present in said dirty degassed overflow water, by hydrocycloning within said hydrocyclone (4); a step of recovering an overflow (42) of said hydrocyclone (4), said overflow of the hydrocyclone (4) mainly comprising residual suspended matter; a second step of separating residual suspended matter from the water recovered from overflow of said hydrocyclone (4), within a recirculation cylinder (5); a step of recirculating towards said hydrocyclone (4) water comprising said residual suspended matter coming from said recirculation cylinder (5) by suction by said hydroejector (3); a step of extracting clarified water, at the outlet (51) of said recirculation cylinder; a step of continuous measurement of the flow rate at the inlet and outlet of said hydroejector (3), and at the outlet of said recirculation cylinder (5). 2. Method according to claim 1 characterized in that it comprises: a step of continuously measuring the flow rate of said accelerator pump (2); and a step of adjusting this flow rate continuously as a function of the measurement result recorded during said measurement step; said step of adjusting the flow rate of said accelerator pump being such that, for a flow rate at the inlet of said hydroejector of xm ³ /h, then the flow rate at the outlet of the hydroejector is between l.lx and l.5x m ³ /h , the flow rate at the underflow outlet of the hydrocyclone is less than 0.lx m ³ /h, the recirculation flow rate is between 0.lx and 0.5x m ³ /h, and the flow rate at the overflow outlet of the recirculation cylinder is between 0.9x and 0.95x m ³ /h. 3. Method according to claim 1 or 2, characterized in that it comprises a step of adding polymer to the water upstream of said hydroejector and/or at the inlet of said recirculation cylinder. 4. Method according to one of claims 2 or 3, characterized in that said flow rate at the hydroejector inlet (2) x is between 4 and 50 m ³ /h. 5. Method according to one of claims 1 to 4, characterized in that the hydraulic residence time is between 2 s and 4 s within said hydrocyclone (4) and said recirculation cylinder (5). 6. Method according to one of claims 1 to 5, characterized in that said hydrocyclone (4) has a cut-off threshold of between 10 and 25 pm, and said recirculation cylinder (5) has a cut-off threshold of between 5 and 3 p.m. 7. Method according to one of claims 1 to 6, characterized in that the pressure of the water leaving said accelerator pump is between 2 and 5 bars. 8. Method according to one of claims 1 to 7, characterized in that said hydroejector (3) comprises a nozzle (31) whose outlet diameter is between 1/5th and half of its inlet diameter. 9. Method according to one of claims 1 to 8, characterized in that said dirty overflow water comes from a hydrocyclone (4000) of a ballasted flocculation-settling installation. 10. Method according to one of claims 1 to 8, characterized in that the dirty overflow water comes from a filtration system, said filtration system belonging to the group comprising: sand filters; granular activated carbon filters; ultrafiltration membranes; biological filters using biomass fixed on a granular support; mechanical filters. 11. Device (DISP) for treating dirty overflow water with a view to separating suspended matter and/or insoluble granular material heavier than water for the implementation of a process according to one of claims 1 to 10 characterized in that it comprises: means (1) for degassing at the inlet of said installation; an accelerator pump (2) capable of adjusting a flow rate and a pressure; a hydroejector (3); a hydrocyclone (4) powered by water from the hydroejector; a recirculation cylinder (5) supplied by water coming from the overflow of the hydrocyclone; means (6) for measuring pressure and/or flow; said hydroejector (3) being capable of recirculating part of the water coming from the recirculation cylinder (5) towards the hydrocyclone (4). 12. Device according to claim 11 characterized in that it comprises means (7) for adding polymer provided upstream of said hydroejector, and/or at the inlet of said recirculation cylinder. 13. Device according to one of claims 11 or 12, characterized in that said hydrocyclone (4) has a cut-off threshold of between 10 and 25 pm, and said recirculation cylinder (5) has a cut-off threshold of between 5 and 3 p.m. 14. Device according to one of claims 11 to 13, characterized in that said hydroejector (3) comprises a nozzle (31) whose outlet diameter is between 1/5 and half of its inlet diameter. 15. Device according to one of claims 11 to 14 characterized in that the recirculation cylinder (5) is positioned horizontally. 16. Device according to one of claims 11 to 15 characterized in that said hydrocyclone (4) and said recirculation cylinder (5) each have a length of between 0.8 and 2m.