FR2982255A1 - WATER TREATMENT PROCESS COMPRISING POWDER ACTIVATED CHARCOAL ADSORPTION (CAP), NON-MEMBRANE MECHANICAL FILTRATION AND CAP RECIRCULATION - Google Patents

Abstract

The invention relates to a method for treating water with a suspended solid concentration of less than 20 mg / l in order to reduce the content of dissolved organic pollutants, said process consisting in: - a step of introduction of said water in a stirred reactor (11) in which it is brought into contact with powdered activated carbon in a concentration of between 150 and 500 mg / l, said water flowing in said stirred reactor (11) at a speed of between and 50 m / h; a step of direct filtration of the water from said stirred reactor (11) for separating a filtrate from the powdered activated carbon in a non-membrane mechanical filtration unit (14) comprising filter media whose cut-off point is greater than or equal to at 1 micrometer; a step of recirculating in said stirred reactor (11) at least a portion of the powdered activated carbon separated from the water in said filtration step.

Description

Field of the invention The field of the invention is that of water treatment techniques with a view to their potabilization. or their purification. More specifically, the invention relates to water treatment techniques to reduce their content including organic matter, micropollutants, endocrine disruptors, chlorinated solvents, pesticides, drug residues and pharmaceuticals. 2. PRIOR ART AND ITS PROBLEMS Methods are commonly used on wastewater and natural waters in order to respectively ensure their purification or their potabilisation. These treatment processes are aimed in particular at removing from these waters, at least in part, organic matter, micro-pollutants, endocrine disruptors, chlorinated solvents, pesticides, drug residues, pharmaceuticals and others. adsorbable materials found there. Such processes generally employ an adsorbent, such as Activated Carbon Powder (CAP), on which these pollutants are adsorbed.

One of these methods consists in contacting a water to be treated with activated carbon powder. The resulting mixture is then filtered through filtration membranes. The powdered activated carbon separated from the water is at least partially recirculated upstream of the filtration membranes. The quantity of activated carbon used in this type of process must be important to enable the pollutants whose elimination is targeted to be removed satisfactorily. Nevertheless, the number of washings that must be used to separate the activated carbon powder from the filtration membranes is all the more important that the active carbon concentration of the filtered mixture is high. As a result, the concentration of active charcoal powder of the mixture of water and activated charcoal is generally limited to between 0.8 and 1 g / l above which, the duration of the periods of washing of the membranes, during which no water treated is produced, becomes too large compared to the duration of the treatment phases during which treated water is produced. In addition, the membranes are cleaned with a portion of the treated water produced which tends to reduce the total amount of treated water collected. Those skilled in the art consider that such a dose of CAP at a time is too small to ensure an effective water treatment, and high enough to cause, in addition to clogging the membranes, a mechanical wear of these which is most often manifested by an attrition of their surface and a loss of permeability up to a break. Such a technique is therefore finally not implemented very much. Another method consists in contacting a water to be treated with flocculants, coagulants, powdered activated carbon and optionally a ballast such as microsand. The mixture obtained is then introduced into a gravity separator whose overflows are successively filtered through a sand filter and filtration membranes. The implementation of such a method makes it possible to reduce target pollution satisfactorily and rapidly by limiting the damage to the filtration membranes by the use upstream thereof of a sand filter acting as a filter. kind of fuse, that is to say as a security element. However, coagulants and flocculants tend to agglomerate on the surface of activated carbon powder. The adsorption capacities of it are all the more reduced. In addition, the implementation of such a process is accompanied by losses of powdered activated carbon underflow of the separator. It is therefore necessary to inject new powdered activated carbon in order to compensate for these losses, which represents a significant cost item. Furthermore, this technique involves the use of a sand filter between the gravity separator and the filtration membranes to remove certain substances that are not separated from the water during the gravitational separation, in particular flocculant residues. The installations used to treat water according to such a process are therefore fairly compact and consuming chemical reagents. Another method consists of passing the water to be treated in a gravity separator and then bringing the clarified water obtained at the outlet of the separator into contact with powdered activated carbon. The mixture obtained is then filtered through filtration membranes, the powdered activated carbon separated from the water being at least partly recirculated upstream of the gravity separator. This technique is subject to the disadvantages inherent in the implementation of filtration membranes: clogging and loss of permeability involving the regular implementation of washing, wear or mechanical failure. It is also accompanied by the loss of activated carbon powder under the gravity separator. Finally, the facilities necessary for its implementation are also not very compact. In the end, there is currently no treatment technique involving adsorption on CAP for the removal of pesticides, endocrine disruptors, organic materials, chlorinated solvents, residues of drugs and pharmaceuticals and other adsorbable organic compounds present in drinking water in particular to be potabilized, which can be implemented economically in relatively compact facilities not including a membrane, and requiring no injection of coagulant and flocculant. 3. OBJECTIVES OF THE INVENTION The object of the invention is notably to overcome these disadvantages of the prior art.

More specifically, an object of the invention is to provide a water treatment technique involving adsorption on powdered activated carbon, in particular to reduce the content of pesticides and / or endocrine disruptors and / or organic matter and / or chlorinated solvents and / or residues of drugs and pharmaceuticals and / or other adsorbable organic compounds which can be implemented in at least one embodiment in a compact installation relative to the installations. of the prior art. Another objective of the invention is to implement such a technique which allows, in at least one embodiment, to optimize the exploitation of the CAP adsorption capacities. In particular, the invention aims to provide, in at least one embodiment, such a technique that allows to limit the consumption of reagents, including CAP. Another object of the invention is to provide, in at least one embodiment, such a technique, the implementation of which avoids the use of coagulant and flocculant. The invention also aims to provide, in at least one embodiment, such a technique that does not require or few maintenance campaigns.

An object of the invention is also to provide, in at least one embodiment, such a technique which is simple and / or reliable and / or economical to implement. 4. OBJECT OF THE INVENTION These objectives, as well as others which will appear subsequently, are achieved according to the invention by means of a water treatment process having a suspended solids concentration of less than 20 mg / l in order to reduce the content of dissolved organic pollutants, said method consisting in: a step of introducing said water into a stirred reactor in which it is brought into contact with powdered activated carbon in a concentration between 150 and 500 mg / l, said water flowing in said reactor stirred at a speed of between 5 and 50 m / h; a step of direct filtration of the water from said stirred reactor for separating a filtrate of powdered activated carbon in a non-membrane mechanical filtration unit comprising filter media whose cutoff threshold is greater than or equal to 1 micrometer; a recirculation step in said stirred reactor of at least a portion of the powdered activated carbon separated from the water in said filtration step. Thus, the invention is based on a completely original approach which consists of cutting down the content of certain pollutants dissolved in a weakly charged water, that is to say having a concentration of suspended solids (MES) of less than 20. mg / 1, bringing it into contact with CAP in a stirred tank or reactor at a relatively low concentration of between 150 and 500 mg / l and then filtering the mixture obtained directly through a filtration unit comprising filtering media mechanical non-membrane whose cutoff threshold is greater than or equal to 1 micrometer by providing to recirculate in the stirred tank at least partly the CAP separated from the treated water produced. Such an implementation makes it possible to satisfactorily reduce dissolved water pollution in a rapid manner by employing relatively little CAP in compact installations requiring few maintenance campaigns. The water to be treated circulates in the stirred reactor at a speed of between 5 and 50 m / h. This makes it possible to create a stirring movement inside the reactor which is suitable for suspending the powdered activated carbon in such a way that the CAP concentration of the mixture contained in the reactor is essentially the same everywhere in the reactor. this one. The contact between the CAP and the water to be treated is thus increased, which optimizes the use of the CAP adsorption capacities. As explained above, one skilled in the art believes that the concentration of CAP in a water to be treated in order to effectively reduce the adsorbable dissolved pollutants content must be important, and at least equal to 0.8 g / l. However, the inventors have found that a much lower CAP concentration of between 150 and 500 mg / l can satisfactorily kill most of the adsorbable pollutants dissolved in water. The CAP consumption is thus reduced. In addition, the losses in CAP are reduced because the technique according to the invention does not involve gravity separation. This also contributes to reducing the size of the installations implemented to ensure the treatment of water. The filter media whose cut-off point is greater than or equal to 1 micrometer makes it possible to retain almost all the CAP contained in the water.

The implementation of a recirculation in the stirred reactor of at least a portion of the CAP separated from the treated water then makes it possible to increase the average residence time of a particle of CAP in this reactor, and thus of optimize the use of its adsorption capacities. All this contributes to reducing CAP consumption.

In addition, a concentration of CAP between 150 and 500 mg / l is a relatively low concentration and therefore low clogging. The filter media used are also not subject to deep clogging since they are not membrane filtration. The frequency of washing operations and, more generally, filter media maintenance campaigns is thus reduced. The weakly charged water to be treated may or may not be from a prior clarification step. It may for example be drilling water that does not require prior clarification. Although a process according to the invention for the treatment of water whose concentration of suspended solids is less than 20 mg / l is effective, it is more particularly advantageous for waters having a MES concentration of less than 10 mg. / L. Thus, in an advantageous variant, the concentration of solids in suspension of the water to be treated is less than 10 mg / l.

In a preferred embodiment, said filtration is from the outside to the inside of said filter media. The low-loaded waters have a low sealing capacity of the fibers of the filtration means through which they are filtered. Their filtration does not induce a strong reduction in the permeability of these filtration means. These waters are usually filtered from the inside to the outside of the filtration means. The waters intended to be treated by the implementation of a process according to the invention are water with little charge. These should therefore be filtered from the inside to the outside. The inventors have, however, noticed that the fact of filtering them from the outside to the inside, which is contrary to the practice of the person skilled in the art, makes it possible to reduce or even to avoid the penetration of the activated charcoal powder in depth. in the filter media. These then clog more weakly and essentially in a non-irreversible way. These filter media are thus easy to clean which limits the number of washing operations to be performed which are also easier to implement. In this case, said filtration unit preferably comprises filter cassettes. Preferably, said filter cassettes each consist of a winding of a wire around a frame defining an internal cavity for the flow of said filtrate. This type of filter cassette has the advantages of allowing to retain almost all the CAP contained in the water to be filtered, to clog slowly and non-irreversibly, and therefore to be relatively easy to clean.

Advantageously, a method according to the invention further comprises washing steps of said filtration unit with a part of said filtrate. The implementation of a non-membrane mechanical filtration unit comprising filter media whose cut-off point is greater than or equal to 1 micrometer has the advantage of not requiring the use of chemicals to ensure their cleaning which can be only with treated water produced or with service water, that is, water delivered by water utilities. The quantity of chemicals to be used during the treatment of the water and the cost thereof are then limited. In an advantageous embodiment, a method according to the invention also consists of extraction steps of used powdered activated carbon and injection steps of new powdered activated carbon into said water to compensate said extraction. It is thus possible to keep the average age of the CAP in the reactor substantially constant while maximizing its adsorption capacities.

According to an advantageous variant, a process according to the invention also consists of at least one ozonation step of said water, said at least one ozonation stage comprising the injection of ozone upstream or into said stirred reactor. Ozonation can oxidize some of the adsorbable molecules and endocrine disruptors present in the water to be treated. Molecules of large sizes are thus broken down into smaller and more easily adsorbable molecules. Ozonation also makes it possible to oxidize non-adsorbable molecules present in the water to be treated. It also makes it possible to eliminate odorous molecules, algal toxins, etc. Finally, the implementation of an ozonation before or during the contacting of the water to be treated with the CAP makes it possible to improve the abatement. pollution especially by adsorption. The activated carbon powder preferably has a particle size of between 15 and 500 micrometers. According to an advantageous characteristic, a method according to the invention does not include injection of coagulant and / or flocculant in said stirred reactor or more generally in the water to be treated. This avoids the agglomeration of coagulant and / or flocculant around the particles of powdered activated carbon. This leads to optimize the adsorption capacity of the CAP, to reduce the consumption of CAP and other reagent because no injection of coagulant and / or flocculant is implemented, and to reduce accordingly the cost of implementation of water treatment while ensuring effective treatment of water. 5. List of Figures Other features and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given as a simple illustrative and nonlimiting example, and the accompanying drawings, among which: - Figure 1 illustrates a diagram of an example of an installation specially adapted to the implementation of a water treatment method according to the invention; FIG. 2 illustrates a perspective view of a filter cassette implemented in the installation shown in FIG. 1; - Figure 3 illustrates a perspective view of the assembly of filter cassettes shown in Figure 2 on a central collector portion. 6. Description of an embodiment of the invention 6.1. BACKGROUND OF THE GENERAL PRINCIPLE OF THE INVENTION The general principle of the invention rests on the fact of putting in contact a water to be treated with CAP in a stirred tank in a relatively low concentration of between 150 and 500 mg / l and a speed of circulating between 5 and 50 m / h, then directly filtering the mixture obtained through a filtration unit comprising non-membrane filter media whose cutoff threshold is greater than or equal to 1 micrometer by providing recirculation in the stirred tank at least partly the CAP separated from the treated water produced. It is thus possible to kill satisfactorily and quickly dissolved organic pollution in water, using relatively little CAP, in compact installations requiring few maintenance campaigns. 6.2. Example of a Water Treatment Plant According to the Invention With reference to FIGS. 1, 2 and 3, an embodiment of a treatment plant specially adapted for carrying out a water treatment process is presented. water treatment according to the invention. As shown in these figures, such an installation comprises a supply pipe 10 of water to be treated. This supply line 10 opens into a stirred reactor 11. In this embodiment, the supply line 10 opens near the bottom 110 of the reactor 11. In a variant, the reactor 11 may comprise a double bottom delimited by a floor traversed by a plurality of diffusion screens substantially uniformly distributed over the entire surface. The water to be treated can then be injected via the supply line under this floor and diffuse inside the reactor 11 through the strainers. In this case, the diameter of the holes of the strainers will be smaller than that of the CAP particles to prevent them from leaking through the floor. In this embodiment, the reactor 11 houses an internal recirculation pipe 27 whose inlet is placed close to the bottom 110 of the reactor 11, and whose outlet opens preferably into the upper half of the reactor 11. The inlet of this reactor 11 pipe 27 is connected to a pump 28. The installation comprises new CAP injection means 12 which open into the supply line 10 and into the agitated reactor 11. In a variant, these injection means may be the supply line 10 or in the stirred reactor 11. These injection means 12 comprise for example at least one injector which preferably allows to inject the new CAP near the bottom 110 of the reactor 10. The installation comprises a pipe extraction 13 of the mixture of water and CAP contained in the agitated reactor 11. The inlet of this pipe 13 is connected to a pump 29 placed near the bottom 110 of the reactor 11. The pump 29 may also be connected to a second internal recirculation pipe 27 '. The filtration unit 14 is a non-membrane mechanical filtration unit comprising filter media 141 whose cutoff threshold is greater than or equal to 1 micrometer. This filtration unit 14 therefore does not include a filtration membrane. In this embodiment, the filter media are filter cassettes 30, the implementation of which allows for a micro-fiber mechanical filtration. The basic element of the filter of the unit 14 are the filter cassettes 30 which are wound cassettes. A filter cassette 30 comprises a rigid fluted support 31 whose side walls are perforated. This support 31 forms a frame defining an interior cavity. It comprises a wire 32 which is wound around the rigid support 31 according to a specific voltage. The tension of the wire and its nature, in particular its diameter, may vary according to the fineness of filtration expected.

One end of the cassette 30 is traversed by filtered water flow passages 33 which protrude outside the support 31 and open into the interior cavity. These filter cassettes 30 are uniformly distributed around a tubular central manifold 142 with the interior of which each interior cavity communicates to form a filter. For this, the peripheral wall of this tubular manifold 142 is traversed by perforations 143 in which the passages 33 forming a projection can be introduced in a sealed manner so that the inside of the manifold 142 communicates with the internal cavity of the cassettes 30 via 33. In FIG. 3, only one row of filter cassettes 30 is shown around a portion of the collector 142. Of course, several rows of cassettes 30 may be juxtaposed along this collector 142. The installation comprises a filtrate extraction pipe 15 which communicates with the outlet of the central manifold 142. A filtrate recirculation pipe 16 is connected to one of the outlets of the filtrate extraction pipe 15. It opens into a storage tank 17. storage tank 17 comprises an outlet which is connected to a washing water injection pipe 18 on which is mounted a pump of Recirculation 19. The outlet of the injection pipe 18 is connected to washing water injection means, such as injectors, on the surface of the filter cassettes 30 of the filtration unit 14. extraction pipe 20 CAP of the filtration unit 14. In this embodiment, this extraction pipe 20 opens into a concentration tank 21.

The bottom of the concentration tank 21 is connected by a pipe 22 on which a pump 23 is mounted to a CAP 24 recirculation pipe which opens into the stirred reactor 11. In a variant, shown in dotted lines, the extraction pipe 20 is directly connected to the recirculation pipe 24 of CAP in the stirred reactor 11. The installation comprises CAP extraction means used. These means comprise a CAP extraction pipe 25 on which is mounted a pump (not shown). This extraction pipe 25 is connected to the pipe 22. In a variant, the CAP extraction means used include a pipe 26 connected to an outlet of the reactor 11 and located near the bottom 110. This pipe 26 is then connected to a withdrawal pump not shown. The installation further comprises means for controlling the CAP extraction means used and the new CAP injection means. These control means make it possible to control the injection and extraction means so as to maintain the average age of the CAP substantially constant inside the reactor 11 and to maintain therein the CAP concentration of between 150 and 500 mg / ml. L. Injection and extraction may take place simultaneously or successively. They may also take place alternately. In this case, an injection may be implemented in a filtration cycle and then an extraction in the next filtration cycle. The injection and extraction means may be implemented according to a predetermined frequency which may for example be defined in number of filtration cycle or time. A filtration cycle corresponds to the period during which water is filtered between two washes of the filtration unit. 6.3. Example of a Water Treatment Process According to the Invention A method of treating water according to the invention with a view to reducing the content of pesticides and / or endocrine disruptors and / or organic materials and / or Chlorinated solvents and / or residues of drugs and other pharmaceuticals and / or other adsorbable organic compounds will now be described. Such a process can be implemented to treat a water having a suspended solids concentration of less than 20 mg / l and advantageously less than 10 mg / l. Such a method comprises a step of introducing a water to be treated via the supply line 10 into the reactor 11. Powdered activated carbon is injected via the injection means 12 into the water to be treated at the level of the supply line 10 and / or the reactor 11. The CAP is injected in such a way that the concentration of CAP inside the reactor 11 is between 150 and 500 mg / l. The CAP concentration will be chosen according to the dissolved pollutants to be eliminated and their concentration. The CAP has a particle size of between 15 and 500 micrometers. The active surface of the particles of CAP will be all the more important that the particle size of CAP will be small. The size of the CAP particles will be chosen according to the dissolved pollutants to be eliminated and their concentration.

The pump 28 is implemented so as to create a recirculation movement of water inside the reactor via the pipe 27 so that the water to be treated circulates at a speed between 5 and 50 m / h. In a variant not implementing the pump 28 and the pipe 27, the water to be treated can be introduced into the reactor at such a speed by means of a pump provided for this purpose. Recirculation in the reactor 11 can also be obtained by means of the pump 29 and the recirculation pipe 27 '. A stirring movement is thus generated inside the reactor, which makes it possible to maximize the contact between the CAP and the water to be treated. The speed of circulation of the water to be treated will be determined empirically, in particular according to characteristics of the CAP, such as its particle size and its concentration in water. The CAP is kept in contact with the water in the reactor 11 in a contact time preferably between 3 and 25 minutes. The mixture of water to be treated and CAP is extracted from the reactor 11 via the pipe 13 and pumped by means of the pump 29 to the inlet of the filtration unit 14. The mixture of water to be treated and CAP is therefore pushed into the filtration unit 14. It is filtered during a filtration step from the outside to the inside of the filtration cassettes of the filtration unit according to a filtration flow preferably between 800 and 2000 Lmh. Specifically, the raw water in the filtration unit 14 outside the cassettes 30 passes through the multiple layers of wire. The water thus filtered flows through their perforations in their internal cavity to the grooves which channel it towards the passages 33 and then into the central collector 142, thus allowing external-internal filtration. The water is filtered through the coil that envelops each cassette, the latter retaining all the particles in suspension.

The CAP is thus separated from a filtrate within the filtration unit 14 inside which it undergoes a non-membrane mechanical filtration step at a cut-off point greater than or equal to 1 micrometer. The cutoff threshold of the filtering media will be chosen according to the particle size of CAP. The filtrate is extracted from the filtration unit 14 via the extraction pipe 15 connected to the central collector 142. It is partly conveyed into the storage tank 17 via the recirculation pipe 16. Washing steps of the filter cassettes of the filtration unit 14 are regularly implemented. They may, for example, be triggered after a given number of filter cycles, or at a fixed time interval, or as soon as the pressure drop across the filtration unit 14 reaches a predetermined threshold. These consist in injecting filtrate contained in the storage tank 17 on the surface of the filter cassettes 30 by means of the pump 19, the pipe 18 and the injectors provided for this purpose. Washing (only hydraulic) is therefore carried out by external rinsing of the cassettes after the body of the filter has previously been drained. The mixture of washing water and CAP is extracted from the filtration unit 14 via the extraction pipe 20 and conveyed into the concentration tank 21. This concentration tank 21 constitutes the recovery tank of the washing eluates filtration cassettes. The concentration of the CAP is therefore high, it is then at least partially recirculated in the contact tank or reactor 11. During a recirculation step, the CAP extracted from the filtration unit 14 is recirculated. in the reactor 11 via the pipe 22, the pump 23 and the pipe 24. Alternatively, it can be recirculated directly from the outlet of the filtration unit 14 into the reactor 11 via the recirculation pipe 24 without being routed in a concentration tank. The recirculation flow rate of the CAP may be determined according to the concentration of CAP in the basin, the CAP extraction modes of the basin, the fluxes acceptable by the mechanical filtration and unclogging times of the mechanical filter. The CAP can be recirculated continuously or at a frequency that is determined according to the washing of the cassettes. The washing frequency of the cassettes depends itself on the concentration of CAP in the contact tank. This frequency can vary between 10 minutes and 60 minutes. When the washing frequency of the cassettes is high, recirculation will be continuous.

Usual CAP is regularly extracted from the installation by means of the pipe 25 and a pump provided for this purpose during CAP extraction steps used. Alternatively, it can be extracted from the bottom 10 of the reactor 11 via the pipe 26 and a pump provided for this purpose. New CAP is regularly injected into the reactor 11 and / or into the supply line 10 via the injection means 12 during new CAP injection steps. The used CAP and the new CAP will respectively be extracted and injected at selected flow rates so as to maintain the average age of CAP in the reactor 11 between 1 day and 10 days. The frequency of extraction of CAP used and new CAP injection as well as the quantities extracted and injected will be determined according to the quality of the water to be treated, that is to say, its concentration of dissolved pollutants. eliminate. The quantities of coal injected into and extracted from the reactor 11 will be identical in order to maintain an equilibrium within the reactor 11. They will be determined so as to maintain within the reactor 11 a CAP concentration of between 150 and 500 mg / l. 6.4. Variant: Ozonation In a variant, an installation according to the invention comprises ozone injection means 40 in the supply line 10 or directly in the reactor 11.

In this case, a process according to the invention comprises a step of ozonation of the water to be treated circulating in the supply pipe 10 or the water circulating in the reactor 11. During these ozonation steps, the amount of ozone injected is such that the ozone concentration of the water is between 0.5 and 4 mg / L. 6.5. Advantages The CAP concentration in the water to be treated is between 150 and 500 mg / l, which corresponds to a low concentration. This has the advantage of being very little clogging, which reduces the frequency of washing filter cassettes.

The implementation of a non-membrane mechanical filtration on filter media having a cut-off threshold greater than or equal to 1 micrometer makes it possible to retain almost all the CAP injected into the water to be treated. This type of filtration is not subject to clogging, at least irreversible. The filtration means used can therefore be easily washed.

The CAP collected during the cleaning stages of the filter cassettes is recirculated in the reactor in which the water is brought into contact with the CAP. The CAP is then maintained during a significant contact time. The implementation of the invention therefore makes it possible to obtain a Ct parameter, which is equal to the product of the CAP concentration and the contact time of the water to be treated with CAP, although the concentration of CAP is low. Thus, the technique according to the invention makes it possible to satisfactorily reduce the organic pollution dissolved in water in a simple manner, in compact plants and by using relatively small quantities of activated carbon, and in particular of CAP, and no chemical reagent such as coagulant and flocculant.

Claims (9)

REVENDICATIONS1. A method of treating water having a suspended solids concentration of less than 20 mg / l to reduce the content of dissolved organic pollutants, said process comprising: - a step of introducing said water into a reactor agitated (11) in which it is brought into contact with powdered activated carbon in a concentration of between 150 and 500 mg / l, said water flowing in said stirred reactor (11) at a speed of between 5 and 50 m / h ; a step of direct filtration of the water coming from said stirred reactor (11) making it possible to separate a filtrate from the powdered active carbon in a non-membrane mechanical filtration unit (14) comprising filtering media whose cut-off point is greater or equal to 1 micrometer; - A step of recirculation in said stirred reactor (11) of at least a portion of the powdered activated carbon separated from the water in said filtration step. 2. Method according to claim 1, characterized in that the concentration of solids in suspension of said water is less than 10 mg / l. 3. Method according to claim 1 or 2, characterized in that said filtration is from the outside to the inside of said filter media. 4. Method according to claim 2 or 3, characterized in that said filtration unit (14) comprises filter cassettes each consisting of a winding of a wire around a frame defining an internal cavity for the flow of said filtrate . 5. Method according to any one of claims 1 to 4, characterized in that it further comprises steps of washing said filter unit (14) with a portion of said filtrate. 6. Method according to any one of claims 1 to 5, characterized in that it further comprises steps of extraction of spent activated carbon and injection steps of new powdered activated carbon into said water to compensate. said extraction. 7. Method according to any one of claims 1 to 6, characterized in that it further comprises at least one ozonation step of said water, said at least one ozonation step comprising the injection of ozone upstream or in said stirred reactor (11). 8. Method according to any one of claims 1 to 7, characterized in that said activated carbon powder has a particle size of between 15 and 500 micrometers. 9. Method according to any one of claims 1 to 8, characterized in that it does not include injection of coagulant and / or flocculant in said stirred reactor (11).