FR2899891A1 - Treatment of suspended material effluent having excess hydraulic content, comprises coagulating the effluents in a coagulation zone, flocculating the effluents in a flocculation zone, and decanting the effluents in a decantation zone - Google Patents

Abstract

The process of treating suspended material effluent having excess hydraulic content, comprises coagulating the effluents in a coagulation zone (2), flocculating the effluents in a flocculation zone (3), and decanting the effluents in a decantation zone during a part of sludge contained in the suspension material separated from the clear effluent. Decantation additive having granular material such as sand is introduced into a capture zone (4), which is situated between flocculation and decantation zones. The capture zone comprises an air bubbling device in its upper part. The process of treating suspended material effluent having excess hydraulic content, comprises coagulating the effluents in a coagulation zone (2), flocculating the effluents in a flocculation zone (3), and decanting the effluents in a decantation zone during a part of sludge contained in the suspension material separated from the clear effluent. Decantation additive having granular material such as sand is introduced into a capture zone (4), which is situated between flocculation and decantation zones. The capture zone comprises an air bubbling device in its upper part for creating an air mattress (16) to ensure a homogeneous distribution of decantation additive. A quantity of air injected in the capture zone to form the air mattress is 10-15 Nm3>/h by meter square. The sludge and the decantation additive are extracted from decantation zone in which the additive is separated from the sludge and reintroduced into the capture zone. The decantation additive is separated, washed and dried from the sludge using a sand separator (9) and then reintroduced into the capture zone in a form of pulverulent. An independent claim is included for an installation for treating a suspended material effluent having excess hydraulic content.

Description

The present invention relates to a method and a treatment device

  water using coagulation, flocculation and decantation techniques. This effluent treatment process is more particularly adapted to the treatment of rainwater which have rather low concentrations of suspended solids (MES) with high hydraulic flow rates. The field of this invention is that of physicochemical processes which aim at separating the particles in suspension in the effluent to be treated, by amalgamating them in the form of "flocs", which are then separated by decantation. The successive operations, which generally include the steps of coagulation, flocculation and settling, are carried out in different compartments. The compartments of the same apparatus called settling tank are most often used.

  Classically, the first of these compartments corresponds to an area in which a coagulation step is carried out. In this step, the electrical charges carried by the suspended particles are neutralized by the injection of iron or aluminum salts. The second compartment is the site of flocculation, where the previously neutralized particles meet and agglomerate to form the floc, thanks to the addition of a polymer. The last compartment corresponds to the settling stage, where the flocs and the water are separated. The set of decanted flocs is mud. The main improvement brought by the latest techniques is to increase the density of the flocs by inserting a dense mineral material, usually fine sand. Methods for treating slurry-laden effluents comprising the successive steps of coagulation-flocculation-quenching, wherein the slurry or settling aid is injected while the floc is mature, into a specific compartment which is a non-agitated zone, located after zone of flocculation and before the settling zone, have been the subject of the filing of patent applications FR 9816532 and FR 0008385 by the Applicant.

  These methods made it possible to obtain a high hydraulic load during decantation (the hydraulic load corresponding to the flow rate of the treated effluent per unit area of lamellae, the lamellae area considered being the horizontal section formed by all of said lamellae) , and had the advantages of extracting concentrated sludge, while maintaining quality treated water. In the context of such effluent-laden effluent treatment processes in which the settling aid is injected into a non-agitated zone called the capture zone, the Applicant has discovered an improvement in creating an air mattress in the upper part of the capture zone so as to improve the distribution of the settling aid in this zone. In this type of process, the introduction of ballast is an important technical issue to ensure the efficiency of the process. The new method which is the subject of the present invention is particularly advantageous since it makes it possible to distribute the agent as uniformly as possible over the entire surface of the non-agitated zone in order to optimize the capture rate between the ballast and the flocs formed in the the flocculation zone. Furthermore, the method according to the present invention is simple to implement, inexpensive, simplifies the implementation of the injection of the capping agent, and allows the improvement of the purification performance vis-à-vis the MES about 10%.

  The subject of the present invention is thus a process for treating an effluent charged with suspended matter with a high hydraulic load comprising: at least one coagulation step carried out in at least one coagulation zone, at least one flocculation step carried out in at least one flocculation zone (3), at least one settling step carried out in at least one settling zone (5), during which part of the sludge containing the suspended matter is separated from the clarified effluent, wherein a settling aid (15) is introduced at at least one tranquilized capture zone (4) situated between the flocculation zone and the settling zone, the capture zone (4). ) having in its upper part an air bubbling device so as to create an air mattress (16) to ensure a homogeneous distribution of the settling aid in this area. For the purposes of the present invention, the term "hydraulic load" means the flow rate of the effluent treated per unit area of lamellae, the lamellae area considered being the horizontal section formed by all of said lamellae. A high hydraulic load advantageously corresponds to a hydraulic load greater than 100 m / h. Advantageously according to the present invention, the settling aid is a granular material such as sand.

  In a particular embodiment of the present invention, the amount of air injected to form the air mattress (16) is between 10 and 15 Nm3 / h per square meter of area of the capture zone (4). Advantageously according to the present invention, the phase containing the sludge and the settling aid is extracted from the settling zone (5), the settling aid is separated from the sludge, and the settling aid (15) is reintroduced. in the capture zone (4). In a particular embodiment of the present invention, the settling aid is separated from the sludge, washed and dried by means of a sand separator (9), and is then reinjected in pulverulent form over the zone. capture (4). In another particular embodiment of the present invention, the settling aid is separated from the sludge and washed with a sand separator (9) and is then reinjected by a liquid route into the capture zone ( 4). According to a particular feature of the present invention, the settling step is carried out in a lamellar settling chamber comprising a plurality of inclined plates (6) parallel to each other and provided at the upper part of the settling zone (5). . The major advantage of lamellar settling with respect to conventional settling is the ability to treat a large amount of effluent in a small volume. This performance is expressed mainly through the hydraulic head at the base of the settling surface. At present, weighted flake lamellar settling tanks achieve maximum hydraulic loads of the order of 130 m / h (FR 2 758 812 A1).

  The present invention also relates to an installation for treating an effluent charged with suspended matter with a high hydraulic load comprising successively: - a coagulation zone (2) provided with a stirring means, comprising a means of introducing (1) the effluent to be treated, a means for introducing the coagulation adjuvant (12) and means for discharging the effluent from the coagulation zone; a flocculation zone (3) provided with a stirring means, comprising a means for introducing the effluent to be treated, a means for introducing the flocculation adjuvant (13) and an evacuation means; effluent from the flocculation zone; a tranquilized capture zone (4) situated between the flocculation zone (3) and the settling zone (5), said capture zone (4) comprising a means for introducing the effluent to be treated, a means for introducing introduction of the settling aid (15) and means (14) for evacuating the effluent from the capture zone, said capture zone (4) further comprising in its upper part a bubbling device of air ensuring the maintenance of an air mattress (16); and a settling zone (5) comprising means for introducing the effluent to be decanted, means for evacuating the clarified effluent (7) and means for discharging the settled sludge (8).

  This installation is suitable for carrying out the process according to the present invention. In a particular embodiment of the present invention, the air bubbling device (16) is immersed in the capture zone (4), advantageously at a depth of 40 to 80 cm below the free surface, more advantageously at a distance of depth of 40 to 60 cm below the free surface, more preferably to a depth of about 50 cm below the free surface. This arrangement of the air bubbling device (16) in the capture zone (4) makes it possible not to disturb the hydraulics of the main flow in the capture zone (4), to ensure the tranquilization of the lower part of the the capture zone (4), and to minimize the risk of entrainment of air bubbles towards the settling zone (5). In a particular embodiment according to the present invention, the hydraulic communication between the capture zone (4) and the settling zone (5) is carried out by means of a communication opening (14), advantageously located under the end upper lamellae. In this embodiment, the air bubbling device (16) is advantageously disposed above the upper edge of the communication opening (14) arranged between the capture zone (4) and the settling zone (5). ). Particularly advantageously, the air bubbling device (16) is arranged at least 20 cm above the upper edge of said communication opening (14). In another particular embodiment according to the present invention, the hydraulic communication between the capture zone (4) and the settling zone (5) is carried out by overflow above the wall common to these two zones. The capture zone (4) comprises in its upper part an air bubbling device (16) immersed, preferably above said common wall, advantageously at a depth of 40 to 60 cm, preferably at a depth of the order of 50 cm below the surface of the effluent.

  Advantageously according to the present invention, the air mattress (16) is distributed over the entire surface of the capture zone (4). Particularly advantageously according to the present invention, the air mattress (16) is formed by means of a network of air injection ducts (17) covering the entire surface of the capture zone. (4). According to a particular feature of the present invention, the plant contains, downstream of the settling zone (5), a sand separator (9) for separating the settling aid from the settled sludge (8) and washing the sludge. decantation aid, the settling aid (15) being then reintroduced directly or indirectly into the capture zone (4). In a particular embodiment of the present invention, the sand separator (9) is disposed above the capture zone (4) so that the settling aid (15) falls directly into the said zone. capture (4). In another particular embodiment of the present invention, the plant contains, downstream of the sand separator (9), a suspension tank (10) so that the settling aid from the sand separator (9) falls directly into said slurry tank (10), the sludge settling aid (15) being subsequently reintroduced into the capture zone (4).

  Various objects and advantages of the present invention will become apparent to those skilled in the art by reference to the following illustrative drawings. FIG. 1 corresponds to a particular embodiment of the present invention, in which the settling aid is recovered from the settling zone, washed, dried and then reintroduced into the dry capture zone. Figure 1 is a schematic view of an installation according to the present invention comprising in series: - a coagulation zone (2) provided with a stirring element, a conduit for introducing the effluent to be treated (1) and a conduit for introducing a coagulation adjuvant (12); a flocculation zone (3) provided with a stirring element and a conduit for introducing a flocculating adjuvant (13); - A capture chamber (4), tranquilized, provided with an introduction line of the effluent to be treated, a means for introducing the settling aid (15), and a means for evacuation (14) of the effluent from the capture zone. Said capture chamber (4) further comprises in its upper part a submerged air bubbling device ensuring the maintenance of an air mattress (16); - A settling zone (5), comprising in its upper part a trough for the recovery of treated effluents (7) and a lamellar element (6), and also comprising in its lower part a sludge withdrawal means (8); downstream of the settling zone (5), an inclined screw sand separator (9) for separating the settling aid from the settled sludge (8), washing and drying the settling aid, said sand separator (9) being disposed above the capture zone (4) so that the settling aid (15) falls directly in pulverulent form into said capture zone (4). The sand separator (9) further comprises a sludge discharge pipe (11). FIG. 2 corresponds to another particular embodiment of the present invention, wherein the settling aid is recovered from the settling zone, washed, and is reintroduced into the capture zone by a liquid route. FIG. 2 is a schematic view of an installation according to the present invention comprising in series: - a coagulation zone (2) provided with a stirring element, an effluent introduction line to be treated; (1) and a conduit for introducing a coagulation adjuvant (12); a flocculation zone (3) provided with a stirring element and a conduit for introducing a flocculating adjuvant (13); - A capture chamber (4), tranquilized, provided with an introduction line of the effluent to be treated, a means for introducing the settling aid (15), and a means for evacuation (14) of the effluent from the capture zone. Said capture chamber (4) further comprises in its upper part a submerged air bubbling device ensuring the maintenance of an air mattress (16); - A settling zone (5), comprising in its upper part a trough for the recovery of treated effluents (7) and a lamellar element (6), and also comprising in its lower part a sludge withdrawal means (8); downstream of the settling zone (5), an inclined screw sand separator (9) for separating the settling aid from the settled sludge (8), washing and drying the settling aid, the sand separator (9) further comprising a sludge discharge pipe (11); and - downstream of the sand separator (9), a slurry tank (10) so that the settling aid from the sand separator (9) falls directly into said slurry tank (10), settling aid (15) suspended in the slurry tank (10) is then reintroduced by liquid into the capture zone (4).

  FIG. 3 shows a particular embodiment of the capture zone 30 (4) (in enlarged view), showing an example of height dimensions (in mm) of the various elements present in this zone. The hydraulic communication between the capture zone (4) and the settling zone (5) is carried out in this particular embodiment by means of a "slot" (14) (communication opening) located under the upper end of the lamellae. The capture zone (4) further comprises in its upper part an immersed air bubbling system (16) at a depth of 50 cm below the surface of the effluent, said air bubbling system (16) being arranged well above the upper edge of the communication opening (14) arranged between the capture zone (4) and the settling zone (5). FIG. 4 shows another particular embodiment of the capture zone (4) (in enlarged view), in which the hydraulic communication between the capture zone and the settling zone is carried out by overflow over the common wall at these two areas. The capture zone (4) always has in its upper part, more precisely above said wall, an air bubbling system (16) immersed, preferably at a depth of 50 cm below the surface of the effluent. FIG. 5 is a diagrammatic bottom view of the device for implementing the air mattress (16), said mattress being formed by means of a network of air injection ducts (17) provided with nozzles (18) the mattress covering the entire surface of the capture area (4). Fig. 6 is a schematic representation of the operation of a global weight distribution device in the capture zone (4) according to an embodiment of the present invention. The ballast (15) is supplied by liquid, through a network of ducts added, above the surface of the volume of water and falls on the air mattress (16). This is generated by the bubbles (16) escaping from the nozzles (18) arranged along the ducts (17) for supplying air.

  The process according to the invention can be carried out as follows: an effluent to be treated (1) is introduced into the decanter at the so-called coagulation zone (2). In this zone (2), which is kept stirred, is introduced at least one coagulation aid (12) which is for example a metal salt: ferric chloride or aluminum sulfate. During the coagulation step, suspended solids are electrically neutralized. The effluent from the coagulation zone is then introduced at the level of the flocculation zone (3) of the decanter.

  In this flocculation zone (3), which is kept stirred, is introduced at least one flocculation adjuvant (13) which is for example a cationic polymer, anionic or neutral, natural or synthetic. During the flocculation step, the suspended matter in the effluent agglomerate with flocculation aid which allows the growth of the floc. Agitation of the coagulation zones (2) and flocculation zones (3) can be carried out by mechanical means (stirrers) or by an injection of air. With regard to the flocculation zone (3), an important parameter to consider is the speed gradient G, which is conventionally between 70 s and 300 s1. In the case of weighted flocs, G can be 10 times higher, as reported in patent FR 2 627 704 B1. Each of the coagulation zones (2) and flocculation zones (3) may comprise one or more tanks. Generally, each of these zones comprises one or two tanks, and preferably only one tank. After these floc maturation treatments, the effluent to be treated is then introduced at the capture zone (4), which is a non-agitated zone, comprising in its upper part an immersed air bubbling device ensuring the maintenance of an air mattress (16), the settling aid then being injected homogeneously into said capture zone (4). The Applicant has thus discovered a means for optimizing the distribution of the sand-type agent when it is introduced into the non-agitated zone (4). This homogeneous distribution of the ballast grains is obtained by creating and maintaining an air bubbling (16) which constitutes a "mattress", and through which the ballast grains are distributed randomly. This air mattress (16) is created only below the water level of the non-agitated zone (4) (approximately 50 cm below the surface of the water). This device (see Figure 6) creates local turbulence near the surface of the effluent to force the distribution of ballast grains after their introduction. This air mattress (16) being created near the free surface does not disturb the tranquilized regime of the capture zone (4). Thus, this superficial air mattress (16) does not change the overall hydraulics of the unstirred zone (4) which remains a tranquilized zone, in which the integrity of the flocks is preserved and in which the flow allows catch flocs / ballast grains.

  With this air bubbling device, the means for introducing the sand can be simplified. A coarse spatial distribution of the sand is advantageous, and makes it possible to maintain high purification performance in terms of elimination of suspended solids from the effluents to be treated.

  Advantageously according to the present invention, the air injection device consists of a network of ducts (17), typically of diameter 15 mm, covering the entire surface of the capture zone (4), and located from preferably at a depth of 40 to 80 cm, even more preferably at a depth of about 50 cm, below the free surface.

  The ducts are generally spaced between them from 100 to 150 mm. Each duct (17) is provided at its lower part with nozzles (18) (see Figure 5). The nozzles typically have a diameter of 2 mm, and are generally spaced 50 mm apart. The amount of air to be supplied to create and maintain the air mattress (16) is preferably from 10 to 15 Nm3 / h, per square meter of dispersion area of the capture zone (4). The air mattress (16) thus formed allows a better distribution of the sand in the capture zone (4) before it encounters the waves. The gain obtained vis-à-vis the global allowance in MES is about 10%. The capture zone (4), located between the flocculation zone (3) and the settling zone (5), has two main important functions. In this zone, it is first and foremost essential to maintain the integrity of the neo-formed floc and to ensure the effective meeting between the floc and the grains of the agent. For this purpose, it is therefore essential to size the capture area appropriately. In a particular embodiment tested, the capture zone (4) had particular dimensions, in particular with regard to its horizontal section, on which the different injection points of the ballast agent, such as sand, are homogeneously distributed. . For the implementation of a treatment corresponding to 150 m3 / h, the dimensions of this zone are for example the following ones: 30 - height: 4625 mm - width: 1500 mm - length: 500 mm It is also important to take into account counts the hydraulic speed within this capture zone (4). At the nominal flow rate of the treatment process, it is advantageous that this hydraulic speed does not exceed 200 m / h, so as not to destructure the flocs formed. This precautionary measure applies to singular passages, that is to say the passages between flocculation zone / capture zone and capture zone / settling zone. The results of the tests carried out on such an installation having a treatment capacity of 150 m 3 / h confirmed the optimal operation of the process according to the present invention. Indeed, more than 90% of the MES have been removed with a passage rate 10 in the settling zone of the order of 100 m / h. To achieve such a result, it is essential to obtain, in the flocculation zone, a floc generally relatively large and compact appearance.

  The settling zone (5), disposed downstream of the capture zone (4), may comprise one or more tanks (s). Generally, this zone comprises one or two tanks. Very dense flocs decant directly into the lower part of the capture zone (4). Also, the capture zone advantageously comprises, in its lower part, a sludge withdrawal pipe. These are then extracted regularly and mixed with the sludge withdrawn into the settling zone (5). At the outlet of the settling zone (5), two distinct phases will thus be obtained: a liquid phase corresponding to the clarified effluent (7) and a phase at least partly solid corresponding to the sludge. This sludge includes suspended solids, settling aids, coagulation and flocculation additives, and interstitial fluid. This partially solid phase is recovered mainly at the bottom of the settling zone (5). All decantation aids known to those skilled in the art such as sand may be used to carry out settling in the process according to the present invention. Generally, the settling aid has a particle size of between 30 micrometers (gin) and 600 micrometers (m). In general, a material which combines both a density greater than 2.5 kg / dm 3 and a total insolubility under the conventional conditions of water treatment will be preferred.

  These materials, used as a settling aid, make it possible to ballast the aggregates of particles initially present (flocs) and to accelerate their settling. According to a preferred variant of the process according to the present invention, the settling aid is extracted from the sludge, advantageously it is subjected to a treatment, for example a rinsing or washing, possibly a drying, and then it is recycled to the non-agitated zone. capture (4) of the treatment plant. In order to minimize the consumption of settling aid such as sand, the settling aid is separated from the extraction sludge before being reintroduced into the treatment plant. Advantageously, the settling aid must be recoverable to more than 99%, with a proportion of organic matter (MO) less than 3%. Separation of sludge and settling aid can be carried out by sieving, centrifugation, hydrocycloning, or any other suitable means known to those skilled in the art.

  Particularly advantageously according to the present invention, the settling aid is separated from the sludge by means of a sand separator (9) before being recycled to the capture zone (4). The sand separator (9) according to the invention has two essential functions: - separation by decantation of the settling aid, such as sand, and sludge, - at least partial washing of the settling aid by injection of water. The quality of the sludge / sand mixture, from the settling zone (5) of the water treatment plant, allows this mixture to be introduced directly into the sand separator (9) without any prior step. When the mixture is introduced into the separator, the sand is decanted and washed by a rising stream of wash water. The organic materials are discharged as sludge (11) through the overflow of the separator. The washed sand is regularly extracted at the bottom of the separator, then advantageously transferred by means of a screw to be recycled. In a particular embodiment of the present invention, the sand separator (9), thanks to its inclined screw system, makes it possible to at least partially dry the settling aid which has been previously separated from the sludge and washed.

  The sand separator (9) according to the invention is particularly advantageous since it makes it possible to recover the sand with a yield greater than 99% and a proportion of organic matter (MO) of less than 3%. Two preferred embodiments are possible in the recovery and reinjection of the settling aid as settable: the settling aid can be reinjected into the capture zone (4) either by the dry route or by liquid. In a first embodiment according to the invention, the sand is separated from the sludge by means of a sand separator (9) which ensures the separation of sludge / sand, partial washing and drying of the sand, and then ci is re-injected in pulverulent form over the unrestrained capture zone (4), as shown in FIG. 1. In this embodiment, the sand separator (9) is advantageously disposed above the structure water treatment, especially above the capture zone (4) (see Figure 1). In an exemplary embodiment, the washed sand from the inclined screw of the separator (9) falls directly into a hopper connected to a dispersion system of the fertilizer dispersion type. This configuration is advantageous since it allows the economy of a sand injection pump, a tank and an agitator.

  In a second embodiment according to the invention, the sand is separated from the sludge by means of a sand separator (9) which ensures the sludge / sand separation, the partial washing and the partial drying of the sand, then the washed sand, coming from the inclined screw, falls into a tank, called a suspension tank (10). In this tank (10), filled with water and sand, the sand is kept in homogeneous suspension by powerful stirring. A centrifugal pump draws the contents of the tank (10) and continuously feeds the sand dispersal system over the entire surface of the capture zone (4), as shown in FIG. realization, the sand is recovered as a suspension at the outlet of the suspension tank (10), before being reintroduced into the unstirred capture zone (4). In a particularly advantageous manner according to the invention, the sand is activated by a liquid polymer which is added to the suspension tank (10) before being reintroduced into the installation. The addition of polymer improves the contact and bonding between the grains of sand and the flocs.

  Sludge separated from the sand is evacuated via a discharge line (11) and can be stored in a storage tank. They are then advantageously sent to a sludge treatment circuit.

Claims (12)

  A method of treating an effluent charged with suspended matter with a high hydraulic load comprising: at least one coagulation step carried out in at least one coagulation zone (2), at least one flocculation step carried out in at least one a flocculation zone (3), - at least one settling step carried out in at least one settling zone (5), during which part of the sludge containing the suspended matter is separated from the clarified effluent, characterized in a settling aid (15) is introduced at at least one tranquilized capture zone (4) situated between the flocculation zone and the settling zone, the capture zone (4) comprising in its upper part an air bubbling device so as to create an air mattress (16) to ensure a homogeneous distribution of the settling aid in this area.   2. Method according to claim 1, characterized in that the settling aid is a granular material such as sand.   3. Method according to claim 1 or 2, characterized in that the amount of air injected to form the air mattress (16) is between 10 and 15 Nm3 / h per square meter of area of the capture zone ( 4).   4. Method according to any one of claims 1 to 3, characterized in that the phase containing the sludge and the settling aid is extracted from the settling zone (5), the settling aid is separated from the sludge, then the settling aid (15) is reintroduced into the capture zone (4).   5. Method according to claim 4, characterized in that the settling aid is separated from the sludge, washed and dried using a sand separator (9), and is reinjected in pulverulent form over the capture area (4).   6. Method according to claim 4, characterized in that the settling aid is separated from the sludge and washed with a sand separator (9), and is then reinjected by a liquid route into the capture zone (4). ).   7. Installation for treating an effluent containing suspended solids with a high hydraulic load comprising successively: a coagulation zone (2) provided with a stirring means, comprising means for introducing (1) effluent to be treated, means for introducing the coagulation adjuvant (12) and means for discharging the effluent from the coagulation zone; a flocculation zone (3) provided with a stirring means, comprising a means for introducing the effluent to be treated, a means for introducing the flocculation adjuvant (13) and an evacuation means; effluent from the flocculation zone; a tranquilized capture zone (4) situated between the flocculation zone (3) and the settling zone (5), said capture zone (4) comprising a means for introducing the effluent to be treated, a means for introducing introduction of the settling aid (15) and means (14) for evacuating the effluent from the capture zone, said capture zone (4) further comprising in its upper part a bubbling device of air ensuring the maintenance of an air mattress (16); and a settling zone (5) comprising means for introducing the effluent to be decanted, means for evacuating the clarified effluent (7) and means for discharging the settled sludge (8).   8. Installation according to claim 7, characterized in that the air bubbling device (16) is immersed in the capture zone (4), preferably at a depth of 40 to 80 cm below the free surface, even more advantageously at a depth of about 50 cm below the free surface.   9. Installation according to claim 7 or 8, characterized in that the air mattress (16) is distributed over the entire surface of the capture zone (4).   10. Installation according to any one of claims 7 to 9, characterized in that it contains, downstream of the settling zone (5), a sand separator (9) for separating the sludge settling aid decanted (8) and washing the settling aid, the settling aid (15) is then reintroduced directly or indirectly into the capture zone (4).   11. Installation according to claim 10, characterized in that the sand separator (9) is disposed above the capture zone (4) so that the settling aid (15) falls directly into said zone capture (4).   12. Installation according to claim 10, characterized in that it contains, downstream of the sand separator (9), a suspension tank (10) so that the settling aid from the sand separator (9) ) falls directly into said slurry tank (10), the sludge settling aid (15) being subsequently reintroduced into the capture zone (4).