FR3081728A1 - REACTOR FOR THE DECANTATION AND FILTRATION OF WATER TO BE TREATED, CORRESPONDING TREATMENT AND WASHING METHOD AND INSTALLATION COMPRISING SAME - Google Patents

Abstract

The present invention relates in particular to a reactor (10) for the decantation and filtration of a water to be treated (200) comprising a means of injection (11) of the water to be treated (200) arranged in the lower part and a means recovery (13) of treated water (230) disposed in the upper part, in which: the interior volume is filled, at least in part, by at least five superimposed layers of media (20, 21, 22, 23, 24 ): a lowest layer (20), at least three intermediate layers (21, 22, 23) and a most upper layer (24); for each layer, the media of an upper layer have a strictly lower particle size and an apparent density less than or equal to the particle size and the apparent density of the media of the directly lower layer; and, the media of the lowest layer (20) have a particle size ranging from 50 to 200 millimeters.

Description

Reactor for decantation and filtration of water to be treated, corresponding treatment and washing process and installation comprising it

1. Field of the invention

The invention relates to the treatment of turbid and / or colored water, in particular for the production of drinking water.

More specifically, the present invention relates to a reactor for decanting and filtering water to be treated, an installation comprising it, a process for treating water in the reactor or in an installation comprising it and a process for washing the reactor.

2. The prior art today exists a large number of treatment processes intended to produce drinking water by technologies using coagulation-flocculation, settling and filtration. The decantation and filtration stages are always separated. The corresponding installations therefore have a strong footprint.

is known filtration on a single medium. For example, a sand layer filter on gravel support makes it possible to treat previously decanted water having a maximum turbidity of 5 NTU at a speed ranging from 6 to 15 m / h. In addition, an activated carbon layer filter removes organic matter and various micropollutants for previously decanted water with a maximum turbidity of 0.5 NTU.

It is known to those skilled in the art that multi-media filters represent a significant improvement over single-media filters. This is mainly due to the improvement of the action of the filter bed based on the use and selection of well differentiating filter media. Multi-media filtration allows the distribution of high quality filtered water compared to a conventional sand filter and allows higher floc or particle retention capacities. A bi-layer filter is particularly known in which sand is associated with anthracite. This two-layer filter makes it possible to treat previously decanted water with a maximum turbidity of 15 NTU. In practice, known multi-media filters are not very suitable for treating water with a turbidity greater than this value and additional clarification means must be implemented upstream to sufficiently reduce the turbidity of the water entering the multi filters. -media.

Thus, there is currently a real need to provide a device for decanting and filtering water, which is more compact on the one hand, and which allows the treatment of highly turbid water.

3. Objectives of the invention

The present invention aims to overcome at least some disadvantages of the prior art.

An objective of the invention is in particular to propose a reactor comprising several filtration media making it possible to ensure both decantation and filtration.

Another objective of the invention is to propose a reactor allowing the treatment of highly turbid and / or colored waters. In particular, at least according to certain embodiments, an objective of the invention is to provide a reactor allowing the treatment of water having a maximum turbidity ranging from 40 to 50 NTU and / or having a maximum color of 60 mg (Pt -Coj / L.

Another objective, at least according to certain embodiments, is to provide a mobile reactor which can be moved easily.

Another objective, at least according to certain embodiments, is to propose a reactor of simple design and having a low cost price.

Another objective, at least according to certain embodiments, is to propose a reactor which consumes little energy.

Another objective is to propose installations integrating the reactor.

Another objective is to propose methods for treating water in the reactor or in installations comprising it. One objective, at least according to certain embodiments, is to propose processes in which the addition of chemical reagents is optimized.

Finally, another objective, at least according to certain embodiments, is to propose a process for washing the reactor.

4. Statement of the invention

The invention relates to a reactor for decanting and filtering water to be treated. The reactor comprises a means for injecting the water to be treated arranged in the lower part and a means for recovering treated water arranged in the upper part. The interior volume of the reactor is filled, at least in part, by at least five superimposed layers of media: a bottom layer, at least three intermediate layers and a top layer. For each layer, the media of an upper layer have a strictly lower particle size and an apparent density less than or equal to the particle size and the apparent density of the media of the directly lower layer. The media of the lowest layer have a particle size ranging from 50 to 200 millimeters.

For the purposes of the present invention, the term "turbidity" means the reduction in transparency of a liquid due to the presence of undissolved material. Turbidity is usually caused by suspended matter and colloidal particles which absorb, scatter and / or reflect light. It is measured by various photometric methods well known to those skilled in the art. It is here expressed in Nephelometric Turbidity Units, the commonly used English abbreviation of which is NTU.

The color of the water can be mineral or organic. It is measured by comparison to standard reference solutions containing Platinum and Cobalt and expressed in mg (Pt-Co) / L or in Hazen degree.

"Bulk density" means the ratio of the density of a material, in the form of grains or in powder form, to the density of water. The apparent density of the same dry material in the form of grains or in pulverulent form can vary according to whether it is more or less packed or, on the contrary, according to whether it is more or less aerated, because the volume occupied by the material includes the void volume between the grains.

"Granulometry" means the statistical distribution of the sizes of a collection of solid particles. Particle size analysis is the set of operations used to determine the size distribution of the solid particles making up a collection of particles. The methods used to define the particle size of a collection of particles are:

- i) Screening: it consists in measuring the weight of dry matter which passes through the calibrated meshes of a screen cloth. It is particularly suitable for measuring particle sizes larger than about 0.1 millimeter. The material specifications are based on knowledge of the dlO and d60 values and the uniformity coefficient (CU). The uniformity coefficient characterizes the size distribution of the elements that make up a soil-type material. It is calculated using the formula:

Cu = d60 / dl0, in which: Cu is the uniformity coefficient, d60 is the mesh opening of the sieve corresponding to 60% of the cumulative weight of the pass, defined on the grain size curve and, dlO is the mesh opening of the sieve corresponding to 10% of the weight of cumulative pass, defined on the particle size curve.

- ii) Laser diffraction: it consists in measuring the diffraction by particles suspended in water of the light emitted by a laser beam. The spatial distribution of this light, depending on the size of the particles, is recorded by a set of photodiodes which indicates the proportion of each dimensional class. This method is suitable for measuring particle sizes between 0.05 and 3500 pm.

The reactor according to the invention is intended to allow the treatment of an upward flow of water to be treated passing through the different layers of media. Due to their granulometry, the media of the lower layer ia make it possible to ensure a settling of particles and / or flocs contained in the water to be treated entering the reactor. Depending on the particle size of the layers above the lowest media layer, some of these layers which are directly above the lowest layer can also provide, at least in part, decantation. In other words, the lowest layer of media, and in certain embodiments some of the layers which are directly above it, make it possible to block the particles and / or flocs which are usually removed by decantation in the processes of the prior art. The selection of decreasing (strict) particle sizes of the media layers ensures an upward refinement by progressive tightening of the cut-off threshold as the filtration of the water to be treated increases. The selection of decreasing (strict) grain sizes over at least five layers ensures satisfactory filtration of the water to be treated, even in cases where it is highly turbid. In advantageous embodiments, the reactor according to the invention makes it possible to treat water with a maximum turbidity of between 40 and 50 NTU to obtain treated water with turbidity strictly less than 1 NTU, preferably for obtaining treated water with turbidity close to 0.5 NTU. In advantageous embodiments, the reactor according to the invention makes it possible to treat water having a maximum color of 60 mg (Pt-Co) / L to obtain treated water having a color of less than 10 mg (Pt-Co) / L. The selection of decreasing apparent densities and strictly decreasing particle sizes also makes it possible to ensure that there is no transfer of media between the layers during the operation of the reactor.

The reactor according to the invention is radically different from a conventional sand filter.

The reactor according to the invention is intended to operate with upward flow while a conventional sand filter is with downward flow (gravity). In the reactor according to the invention, all the layers of media take part in the treatment of water rather than only the first centimeters of a sand filter operating in gravity. The decantation is ensured by the lowest layer, corresponding to the media of higher particle size. However, decantation can continue on layers directly above the lowest layer, in particular on layers with a particle size between 10 and 20 mm.

The reactor of the present invention can therefore be used in a water treatment process free from a preliminary tank decantation step. The water to be treated can also be highly turbid.

The media of the different layers can in particular be selected from: anthracite, sand, garnet, pumice, granular activated carbon, calcite, manganese dioxide and iron oxyhydroxide. These materials have the advantage of having different apparent densities.

Advantageously, the media of the uppermost layer have a particle size ranging from 0.4 to 0.9 millimeters. This ensures a cutoff threshold of the filter strictly below 10 micrometers. The cutoff threshold is the property of a physical barrier to physically stop all elements whose size exceeds a limit value.

The material used for the media of the uppermost layer can in particular be chosen from sand, grains of anthracite, grains of activated carbon, pumice, iron oxyhydroxide or grains of materials suitable for remineralization . The advantage of sand and / or grains of anthracite is that they have a low cost price. The material used for the uppermost media can also be grains of activated carbon. The advantage of activated carbon is that it adsorbs a certain number of organic compounds and / or micro-pollutants. Activated carbon makes it possible in particular to adsorb colored substances for waters having a maximum color of 60 mg (Pt-Co] / L in order to obtain treated waters having a color of less than 10 mg (Pt-Co) / L.

The uppermost media material can also be iron oxyhydroxide to remove certain metals such as arsenic. The material used for the media of the uppermost layer may finally be a material for the remineralization of the treated water. It is well known to a person skilled in the art of materials for introducing a certain number of minerals into water. For example, for water with a low carbonate and / or bicarbonate content, a material capable of releasing these species can be used in order to make this water little or not aggressive. In this case, a calcite type material, calcite, a natural and sparse material, is advantageously used. The grains of activated carbon, calcite, pumice or iron oxyhydroxide advantageously have a particle size between 0.4 - 0.9 mm.

If several of the materials described above want to be used in the same reactor to make cumulative use of their properties, they are superimposed in layers by decreasing (strict) particle size and decreasing apparent density.

Garnets and manganese dioxide grains can be used in an intermediate layer of media. Garnets have the advantage of having a low cost price. Manganese dioxide removes manganese and dissolved iron.

Advantageously, the particle size of the at least three intermediate layers of the reactor is selected as follows:

- the media of a first intermediate layer have a particle size between 10 to 20 millimeters;

- the media of a second intermediate layer have a particle size between 2 to 10 millimeters; and

- the media of a third intermediate layer have a particle size between 1.0 to 2.0 millimeters.

Such a particle size gradient allows a sufficient presence of voids between the materials and ensures a pressure drop which does not lead to too short filtration cycles. In the case where the granulometry of the media of the uppermost layer is from 0.4 to 0.9 millimeters, this makes it possible to obtain an outlet turbidity in accordance with drinking standards, in particular a turbidity strictly less than 1 NTU, preferably a turbidity close to 0.5 NTU.

The height of each layer is determined to prevent particles from passing through the topmost layer. Advantageously, the at least five layers are superimposed over a height ranging from 2.4 to 4.3 meters.

Preferably, the media of the lowest layer have an apparent density between 4 and 5 and at least one of the intermediate layers has an apparent density strictly less than 3.5. This ensures that at an upward flow rate allowing fluidization of media with a density strictly less than 3.5, the lowest layer of media is not fluidized. This is particularly useful with regard to washing the reactor.

In particular, the reactor may also comprise intermediate washing means, arranged directly below the intermediate layer having an apparent density strictly less than 3.5 and with the highest particle size. The washing means make it possible to inject water and / or air through the layers of media overhanging them. This can in particular be implemented by a water ramp, an air ramp or a single ramp allowing water and air to be injected simultaneously. The reactor then comprises means for recovering the washing water arranged in the lower part and means for recovering the washing water arranged in the upper part.

The reactor can also include a low washing means, arranged in the lower part, making it possible to inject air through the at least five layers of superimposed media. This can in particular be implemented by an air ramp.

In a particular embodiment, the reactor comprises five layers of media superimposed as follows:

The lowest layer consists of pebbles, pebbles or equivalent, with a particle size between 50 and 200 millimeters, and an apparent density between 4 and 5.

The first intermediate layer consists of gravel, with a particle size between 10 and 20 millimeters and an apparent density between 4 and 5.

The second intermediate layer consists of gravel, with a particle size between 2 and 10 millimeters and an apparent density between 4 and 5.

The third intermediate layer consists of coarse sand, with a particle size between 1.0 and 2.0 millimeters and an apparent density close to 1.5.

The upper layer consists of fine sand, with a particle size between 0.4 and 0.9 millimeters and an apparent density close to 1.5.

In this embodiment, the heights of the layers are advantageously chosen as follows:

The lowest layer has a height ranging from 0.5 to 0.8 meters.

The first intermediate layer has a height ranging from 0.3 to 0.5 meters.

The second intermediate layer has a height ranging from 0.4 to 0.7 meters.

The third intermediate layer has a height ranging from 0.4 to 0.8 meters.

The topmost layer has a height ranging from 0.8 to 1.5 meters.

In this embodiment, the lowest layer achieves a cutoff threshold close to 1000 micrometers. The first intermediate layer makes it possible to obtain a cutoff threshold of between 200 and 1000 micrometers. The second intermediate layer makes it possible to obtain a cutoff threshold of between 50 and 200 micrometers. The third intermediate layer makes it possible to obtain a cutoff threshold of between 25 and 50 micrometers. The upper layer provides a cutoff threshold strictly below 10 micrometers. Thus, the reactor makes it possible to remove particles of size less than 10 micrometers and to effectively treat turbidity water between 40 and 50 NTU to obtain treated water having a turbidity strictly less than 1 NTU, in particular for obtaining treated water having a turbidity close to 0.5 NTU.

In a particular embodiment, the reactor can be installed in a pressurized structure, in a closed enclosure: the internal pressure can then reach 5 bars.

The present invention also relates to an installation comprising a reactor according to the invention. The installation and / or the reactor may include:

- means for measuring the flow of water to be treated entering the reactor; and or

- means for measuring the flow of treated water leaving said reactor; and or

- means for measuring the turbidity of the water to be treated entering said reactor; and or

- means for measuring the turbidity of the treated water leaving said reactor. The means for measuring the flow of water may in particular be flow meters.

The means for measuring the turbidity of the water can in particular be continuous turbidity measuring devices.

The installation may further comprise, upstream of the reactor, coagulation means, in line or in a tank, and / or flocculation means, in line or in a tank. Coagulation and / or flocculation make it possible to destabilize the colloidal particles of the water to be treated.

“Upstream” means everything that is connected to the reactor and which makes it possible to pre-treat the water to be treated before it enters the reactor.

By "in line" is meant the fact that chemical substances are added directly to a water supply pipe by a static mixer or other equivalent device. By "in tank" is meant the fact that chemical substances are added and left to react for a certain period in a dedicated tank equipped with a mixer.

Water can be coagulated beforehand by adding coagulant such as those skilled in the art use it today on drinking water plants. The coagulant can in particular be chosen from an organic coagulant or an inorganic coagulant, in particular based on iron or aluminum. In the case where a coagulation tank is used, the coagulation time in the tank is generally between 1 and 4 minutes.

The possibly coagulated water can be flocculated by the addition of a flocculant such as those skilled in the art use it today on drinking water plants. The flocculant can in particular be chosen from synthetic organic flocculants, natural flocculants (of starch, alginate type, etc.), mineral flocculants of activated silica type or equivalent. In the case where a flocculation tank is used, the flocculation time in the tank is generally between 5 and 20 min. In the case where the flocculation is carried out online, the flocculation will be effective in the lowest media layer of the reactor.

The installation can also include, downstream of the reactor:

- membrane filtration means and / or

- means for disinfecting treated water.

By "downstream" is meant everything that is connected to the reactor and which makes it possible to post-treat the treated water after it leaves the reactor.

The membrane filtration means can be chosen from microfiltration, ultrafiltration or reverse osmosis means.

The means for disinfecting the treated water can be chosen from means for adding chemical disinfectants or means for ultraviolet radiation.

The invention also relates to a process for treating water in a reactor according to the invention or in an installation comprising it. This process includes:

- the injection of water to be treated into the reactor by the injection means in order to obtain an upward flow of water in the reactor having a speed that does not cause significant expansion of the media layers.

- recovery of treated water from the reactor by recovery means.

Indeed, in order to be able to ensure good filtration, the media must not be fluidized, or at the very least if the media of the uppermost layer are fluidized, the expansion rate of the media bed must remain strictly less than 20%, preferably strictly less than 10%.

Advantageously, the speed of the ascending water flow is between Im / h and 15 m / h. The reactor according to the invention, the installations comprising it and the corresponding water treatment methods make it possible to ensure unit flow rates of less than 1500 m ³ / day per unit, which is advantageous for supplying in a simple manner a population that can range up to 15,000 inhabitants (base 100 liters / inhabitant day).

The invention also relates to a washing process in a reactor having intermediate washing means as previously described. Washing the reactor is particularly necessary when a too large pressure drop is observed between the point of injection of water to be treated and the point of recovery of treated water, which means a reduction in water treatment capacities. The washing process includes:

- emptying and evacuation, at least in part, of the water contained in the reactor, by the means for recovering washing water arranged in the lower part; and at least one intermediate washing cycle successively comprising:

the sending of water, and optionally air, by the intermediate washing means in order to obtain an ascending flow of water having a speed making it possible to cause a significant expansion of the layers of media overhanging them and the recovery of this water by the means for recovering washing water disposed in the upper part; and,

- stopping the sending of water, and optionally air, by the intermediate washing means during a rest period.

During washing an upward flow of water and / or air is created. The term “significant expansion” is understood to mean the fact that the layers overhanging the intermediate washing means are fluidized up to an expansion rate greater than or equal to 20%, preferably greater than or equal to 10%.

The emptying allows evacuation of the sludge retained in the lower part of the reactor. The lowering of the water level makes it possible to evacuate the filtered particles and / or flocs retained in the upper media layers. This facilitates the washing conditions because it is easier to extract most of the filtered particles and / or flocs by gravity. The intermediate washing cycle is implemented co-current, which forces the filtered particles and / or flocs overhanging the intermediate washing means to pass through all the media layers up to the top of the reactor. So that the filtered particles and / or flocs can pass through the media layers up to the upper part of the reactor, the media layers overhanging the intermediate washing means have an expansion rate greater than or equal to 20%, preferably greater than or equal at 10%.

According to a preferred embodiment, the intermediate washing cycle successively comprises:

- the simultaneous sending of water and air by the intermediate washing means, the water flow going from 6 to 20 m / h, and the air flow going from 30 to 70 Nm ³ / (m ² .h];

- stopping the water supply and the air supply by the intermediate washing means during a first rest period;

- the sending of water by the intermediate washing means at a rate ranging from 25 to 50 m / h; and,

- stopping the sending of water by the intermediate washing means during a second rest period.

According to a preferred embodiment, the washing process also comprises the sending of air by the low washing means through the layers of media. This makes it possible to destabilize the particles and flocs accumulated in the different layers.

According to a particular embodiment, the washing process successively comprises:

- optionally, sending air through the low washing means in order to destabilize the particles and flocs for 1 to 3 minutes, at a variable flow rate between 30 and 70 Nm ³ of air / m ² / h;

- emptying and evacuation of large particles and large flocs accumulated at the bottom of the reactor for 1 to 5 minutes, by means of recovery of washing water arranged in the lower part.

- the lowering of the reactor water level in order to evacuate the particles and flocs accumulated in the different layers for 1 to 5 minutes.

- sending air and water for 1 to 10 minutes through an intermediate air ramp arranged above the second intermediate layer, with a variable air flow rate between 30 and 70 Nm ³ / m ² .h and concomitantly of washing water at a speed of 6 to 20 m / h through a water ramp also arranged above the second intermediate layer. As such, and according to an alternative, a single ramp can be used for the sending of air and water concomitantly.

- stopping the flow of air and water for 10 to 60 seconds to allow the media to settle.

- sending rinse water only through the same ramp at a speed of 25 to 50 m / h for 3-10 min.

- stopping the water supply for 10 to 60 seconds to allow the media to settle.

Once the washing process is complete, the water treatment process can be resumed.

5. List of figures

The invention, as well as the various advantages which it presents, will be more easily understood thanks to the description which follows of a nonlimiting embodiment of the latter, given with reference to the drawings, in which:

- Figure 1 schematically shows an installation according to the present invention comprising a reactor with five layers of media and comprising means for coagulation and in-line flocculation.

- Figure 2 schematically shows an installation according to the present invention comprising a five-layer reactor and comprising in-line coagulation means and flocculation means in the tank.

6. Detailed description of an embodiment

Referring to Figure 1, the installation 100 comprises a reactor 10 suitable for the decantation and filtration of water to be treated 200 and coagulation means 201 and flocculation 202 in line in a supply line 101.

The reactor 10 comprises a floor 15, through which the water can flow freely, on which are arranged five superimposed layers of media 20, 21, 22, 23, 24.

Water injection means 11, arranged at the bottom of the reactor, make it possible to create an upward flow of water to be treated in the reactor 10. Recovery means 13 for treated water 230 make it possible to recover the water treated in a pipe 103.

Intermediate washing means 30 are arranged between the second intermediate layer 22 and the third intermediate layer 23 and make it possible to inject washing water 210 and / or air 211 with an upward flow. Low washing means 31, arranged in the lower part of the reactor 10 under the floor 15, make it possible to inject air 212 with an upward flow. Low recovery means 12 for washing water 220, arranged in the lower part of the reactor make it possible to drain the reactor and recover washing water in a pipe 102. High means for recovering washing water, arranged in part high of the reactor, here combined with the means for recovering the treated water, make it possible to recover the washing water arriving at the top of the reactor 10. Valves 111 and 112 make it possible to control the rate of injection of water respectively at treat and the flow of water evacuated by draining.

FIG. 2 represents an installation 100 in which the flocculation is implemented in a tank 300.

Tests were carried out in a reactor whose column formed by the five media layers has a height of 3.5 meters and a section of 0.1 m ² . The parameters (material, bed height, particle size and apparent density) of the five layers of media used are presented in Table 1 below:

Materials Bed height (cm) Granulometry (Mm) Apparent density Silica sand 88 0.4-0.9 1.5 Coarse sand 40 1-2 1.5 Gravel 2 62 5-10 4-5 Gravel 1 32 10-20 4-5 Wide pebbles 50 80-100 4-5

Table 1

The water treatment process was implemented with:

- a dosage of ferric chloride (coagulant) of 60 ppm (mg / L);

- a dosage of polymer (flocculant) of 0 mg / L;

- an upward flow of water to be treated in the reactor of 8m / h

- contact time from a few seconds to 14 minutes: the contact time corresponds to the coagulation and flocculation reaction which, after the addition of coagulant and polymer, destabilizes the colloidal particles and creates agglomeration of the flocs.

The results obtained (see table 2 below) show that the turbidity is strictly less than 1 NTU and is rather around 0.5 NTU.

Raw water Treated water Turbidity (NTU) 15 0.53 Total iron (ppm) * 5.2 0.21 UV absorbance (m-1) 5.7 5 *: amount of iron from FeCh

Table 2

Claims (17)

1. Reactor (10) for the decantation and filtration of a water to be treated (200) comprising an injection means (11) of the water to be treated (200) arranged in the lower part and a recovery means (13 ) of treated water (230) disposed in the upper part, characterized in that: the interior volume of said reactor is filled, at least in part, by at least five superimposed layers of media (20, 21, 22, 23, 24): a lower layer (20), at least three intermediate layers (21, 22, 23) and an uppermost layer (24); for each layer, the media of an upper layer have a strictly lower particle size and an apparent density less than or equal to the particle size and the apparent density of the media of the directly lower layer; and, the media of the lower layer (20) have a particle size ranging from 50 to 200 millimeters. 2. Reactor (10) according to claim 1, in which the media of the uppermost layer (24) have a particle size ranging from 0.4 to 0.9 millimeters. 3. Reactor (10) according to any one of the preceding claims, in which the material for the media of at least five superimposed layers of media (20, 21, 22, 23, 24) are chosen from anthracite, sand , garnet, pumice, granular activated carbon, calcite, manganese dioxide and iron oxyhydroxide. 4. Reactor (10) according to any one of claims 1 or 2, in which the material for the media of the uppermost layer is chosen from sand, grains of anthracite, grains of activated carbon, stone pumice, iron oxyhydroxide or grains of materials suitable for remineralization. 5. Reactor (10) according to any one of the preceding claims, in which: - the media of a first intermediate layer (21) have a particle size between 10 to 20 millimeters; - the media of a second intermediate layer (22) have a particle size between 2 to 10 millimeters; and - the media of a third intermediate layer (23) have a particle size between 1.0 to 2.0 millimeters. 6. Reactor (10) according to any one of the preceding claims, characterized in that the at least five layers (20, 21, 22, 23, 24) are superimposed over a height ranging from 2.4 to 4.3 meters. 7. Reactor (10) according to any one of the preceding claims, in which: - the media of the lowest layer (20) have an apparent density between 4 and 5; and, - at least one of the intermediate layers (21, 22, 23) has an apparent density strictly less than 3.5. 8. Reactor (10) according to claim 7 comprising: - Intermediate washing means (30), arranged directly below the intermediate layer having an apparent density strictly less than 3.5 and with the highest particle size, making it possible to inject water (210) and / or air (211) through the media layers overhanging them (23, 24); - washing water recovery means arranged in the lower part; and, - washing water recovery means arranged in the upper part. 9. Reactor (10) according to claim 8 comprising a bottom washing means (31), disposed in the lower part of the reactor (10), making it possible to inject air (212) through said at least five layers of media superimposed (20, 21,22, 23, 24). 10. Installation (100) comprising a reactor (10) according to any one of claims 1 to 9, said installation comprising: - means for measuring the flow of water to be treated entering said reactor; and or - means for measuring the flow of treated water leaving said reactor; and or - means for measuring the turbidity of the water to be treated entering said reactor; and or - Means for measuring the turbidity of the treated water leaving said reactor. 11. Installation (100) comprising a reactor (10) according to any one of claims 1 to 9, said installation comprising upstream of said reactor (10) coagulation means (201), in line (101) or in a tank, and / or flocculation means (202), in line (101) or in tank (300). 12. Installation (100) comprising a reactor (10) according to any one of claims 1 to 9, said installation comprising downstream of said reactor (10): - membrane filtration means and / or - means for disinfecting treated water. 13. Process for treating water in a reactor according to any one of claims 1 to 9 or in an installation comprising it according to one of claims 10 to 12, said process comprising: - the injection of water to be treated into the reactor by the injection means in order to obtain an upward flow of water in the reactor having a speed that does not cause significant expansion of the media layers. - recovery of treated water from the reactor by recovery means. 14. The method of water treatment according to claim 13, wherein the speed of the upward flow of water is between lm / h and 15m / h. 15. A method of washing in a reactor according to any one of claims 8 or 9, said method comprising: - emptying and evacuation, at least in part, of the water contained in said reactor, by the means for recovering washing water arranged in the lower part; and at least one intermediate washing cycle successively comprising: - the sending of water, and optionally air, by the intermediate washing means in order to obtain an ascending flow of water having a speed making it possible to cause a significant expansion of the layers of media overhanging them and, the recovery of this water by the washing water recovery means arranged in the upper part; and, - stopping the sending of water, and optionally air, by the intermediate washing means during a rest period. 16. A washing method according to claim 15, said intermediate washing cycle successively comprising: - the simultaneous sending of water and air by the intermediate washing means, the water flow going from 6 to 20 m / h, and the air flow going from 30 to 70 Nm ³ / (m ² .h); - stopping the sending of water and the sending of air by the intermediate washing means during a first rest period; - the sending of water by the intermediate washing means at a rate ranging from 25 to 50 m / h; and, - stopping the sending of water by the intermediate washing means during a second rest period. 17. Washing method according to one of claims 15 or 16 in a reactor according to claim 9, said method comprising sending air by the low washing means through the media layers.