WO2011131523A1 - Method for treating water with a view to desalinating same including high-speed filtration, and corresponding facility - Google Patents

Abstract

The aims of the invention, as well as others that will appear later, are achieved by means of a method for treating water with a view to desalinating same, said process consisting of: a step (I) of coagulating said water; a step (ii) of flocculating said water from said coagulation step (i); a step (iii) of the granular filtration of the water directly from said flocculation step (ii) through at least one granular filter including a filtration medium consisting of at least one layer of at least one filtration material; a step (iv) of filtering the water from said granular filtration step (iii) to a cutoff threshold of between 10 nanometers and 10 micrometers; a step (v) of filtering, by inverse osmosis, the water from said filtration step (iv) to a cutoff threshold of between 10 nanometers and 10 micrometers; and a step (vi) of recovering at least partially desalinated water from said reverse-osmosis filtration step (iv); said granular filtration step (iii) consisting of causing water from said flocculation step (ii) to pass through said granular filter at a speed of between 15 and 25 m/h.

Description

 A method of treating water for desalination including high speed filtration, and a corresponding installation.

 1. Field of the invention

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

 More specifically, the invention relates to such a method which comprises the implementation of a filtration stage by reverse osmosis.

 2. Prior Art

 The desalination of water (seawater, estuarine waters, industrial water highly loaded with salts, groundwater contaminated with salts, brackish water ...) generally includes a filtration step on one or more osmosis membranes. reverse.

 The manufacturers of this type of filtration membranes do not contractually guarantee the lifetime of their membranes unless the waters passing through these membranes have a certain level of quality.

 Thus, in order to prevent too rapid clogging of the membranes, the water supplying these membranes must have a very good quality.

The SDI (Silt Density Index), which is a parameter representative of the clogging power of water, is generally used to characterize the quality level of a water to be treated through reverse osmosis membranes. SDI ₁₅ is the SDI value of a water measured according to ASTM D4189-95 (June 15, 2007).

Wherever SDI ₁₅ is performed as follows. Water is filtered at a constant pressure of 2.1 bar through a filter whose cut-off point is equal to 0.45 micrometers. The time T0 required for the filtration of 500 ml of water as well as the time T15 necessary for the filtration of 500 ml of water after water has been filtered continuously for 15 minutes through the filter are measured. The SDI 15 is then calculated according to the following formula:

SDIi5 = (100/15). (L- (T0 / T15)) The SDI ₁₅ index ranges between 0 and 6.67 provided that the higher its value, the greater the clogging power of the water it is characterizes high.

For water whose clogging power is very important, it is impossible to determine the SDI ₁₅ according to this standardized method. In this case, alternative indices can be calculated by measuring the time required for the filtration of 500 ml of water after water has been filtered continuously through the filter either for 15 minutes but for 3, 5 or 10 minutes depending on the nature of the water to be treated.

Reverse osmosis membrane manufacturers typically recommend that the water intended to be filtered through the reverse osmosis membranes have, in the manner specified above, an SDI ₁₅ whose value is between 3 and 3.5.

 Filtration on reverse osmosis membranes therefore requires the pretreatment of the feedwaters so as to give them the level of quality required by reverse osmosis membrane manufacturers.

In order to produce water with SDI ₁₅ value is as close as possible values within this interval, several techniques are currently implemented.

 A first technique consists in filtering a previously coagulated and flocculated water:

 in a granular filter comprising a filter mass of about 1.6 meters high consisting of an anthracite layer and a layer of sand, at a speed of the order of 7.5 m / h, then

 in a 5 micron filter cartridge.

The filter cartridges (for example those supplied by Pall, Sartorius, etc.) are used upstream of reverse osmosis membranes or nanofiltration membranes as protection against particles (with dimensions greater than 5 μιη) which could cause malfunctions in reverse osmosis membranes. The filter medium integrated in these cartridges comprises an organic membrane (polysulfone, po lyethersulfone, polypropylene ...). These cartridges also allow elimination of microorganisms that have dimensions greater than the cutting threshold of the cartridge.

The implementation of this technique only allows the production of a water whose SDI ₁₅ is between 3 and 3.5 only if the raw water to be treated has an SDI ₃ of between 15 and 20. The implementation of a treatment of this type therefore does not allow most of the time to produce a water whose quality is sufficient to be filtered optimally later through reverse osmosis membranes.

 A second technique consists in implementing a separation step, for example by decantation upstream of the granular filtration step.

 A third technique is to combine the first two by successively implementing coagulation, flocculation, separation eg decantation, granular filtration, filtration filter cartridge and reverse osmosis filtration.

 The implementation of these second and third techniques can produce a reverse osmosis filtration unit feed water of satisfactory quality.

 3. Disadvantages of prior art

The first of the techniques currently being implemented to produce reverse osmosis filtration membrane feedwater achieves a level of quality that is consistent with the recommendations of membrane manufacturers of this type only if SDI ₃ of the raw water to be treated is between 15 and 20.

Filtration on water reverse osmosis membranes that do not have the required level of quality is accompanied by a rapid increase in the pressure drop in the membranes. This is mainly due to bacterial growth, adsorption of organic matter, and accumulation of microorganic and organic substances on the membranes. This increase in pressure drop generates the need for frequent chemical cleaning of the membranes. Such cleaning is in practice carried out with aggressive reagents which have a negative impact on the life of the membranes. It is therefore necessary to replace the membranes regularly, which represents a significant cost item.

 The second and third of these techniques can achieve a level of quality that is consistent with the recommendations of membrane manufacturers of this type. They nevertheless have the disadvantage of being relatively complex and therefore expensive to implement.

 4. Objectives of the invention

 The invention particularly aims to overcome these disadvantages of the prior art.

 The invention aims to provide, in at least one embodiment of the invention, a water treatment technique for its desalination by reverse osmosis that allows to extend the life of the reverse osmosis membranes implemented for this purpose.

 The invention also aims, in at least one embodiment of the invention, to produce such a technique which leads to reduce the clogging time of the reverse osmosis membranes and consequently to reduce the frequency of their washing and their replacement.

An object of the invention is to implement, in at least one embodiment of the invention, such a technique that enables the production of a water having a SDI ₁₅ between 3 and 3.5 (measured according to ASTM method D4189-95) prior to filtration through reverse osmosis membranes.

The invention also aims, in at least one embodiment of the invention, to provide such a technique that is simple and inexpensive to implement, at least compared to the techniques of the prior art. Another objective of the invention is to provide, in at least one embodiment of the invention, such a technique which leads to reduce the area occupied by the facilities used for the desalination of water.

 The invention still has the objective, in at least one embodiment of the invention, of reducing the quantity of reagents necessary for the desalination of water.

 5. Presentation of the invention

 These objectives, as well as others which will appear later, are achieved by means of a water treatment process with a view to its desalination, said method consisting of:

 a step (i) of coagulating said water;

 a step (ii) of flocculation of water from said coagulation step (i);

 a step (iii) granular filtration of water directly from said step (ii) of flocculation through at least one granular filter comprising a filter mass consisting of at least one layer of at least one filter material;

 a filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers of water from said granular filtration step (iii);

 a step (v) for reverse osmosis filtration of the water from said filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers;

 a step (vi) of recovering at least partially desalinated water from said reverse osmosis filtration step (iv);

 said granular filtration step (iii) of passing water from said flocculation step (ii) through said granular filter at a rate between 15 and 25 m / h.

Thus, the invention is based on an original approach that consists of the combination of coagulation, flocculation, filtration through a granular filter at high speed, that is to say between 15 and 25 m / h, and a filtration at a cutoff threshold of between 10 nanometers and 10 micrometers of a water in order to produce a water of feeding of filtration by reverse osmosis.

 The filtration step at a cutoff threshold between 10 nanometers and 10 micrometers may implement one or more filter cartridges whose cutoff threshold may be between 1 and ΙΟμιη and will preferably be equal to 5μιη. It may alternatively implement an ultrafiltration filtration unit whose cut-off threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 b ar, or a microfiltration unit whose threshold cutoff will preferably be between 0.1 micrometers and 10 micrometers and whose driving force will be between 0.1 and 3 bar.

 This particular implementation makes it possible for the flocs present in the water to penetrate rapidly by deep diffusion inside the filtering mass of the granular filter and to fill, at least in part, the void gaps left between them. media grains filtering essentially over the entire height of the filter media.

As opposed to the techniques according to the prior art, the technique according to the invention makes it possible to proscribe the implementation of a separation for example by decantation or by flotation upstream of the granular filtration when the SDI ₃ of the raw water is is between 15 and 20.

Indeed, considering that the technique according to the invention is based on the implementation of a granular filtration at high speed, the flocs present in the water are retained over substantially the entire height of the filter mass. On the contrary, during the implementation of the techniques of the prior art, these flocs penetrate the filter mass essentially on a low height. A layer of flocs is then formed on the surface of the filtering mass. This clogging of the surface of the filtering mass is accompanied by a rapid increase in the pressure drop across the granular filter. This requires frequent washing of the filters and requires to increase the frequency of chemical cleanings of reverse osmosis membranes placed downstream.

 On the contrary, the implementation of the technique according to the invention prevents: the formation of a layer of flocs on the surface of the filtering mass, and - on the other hand allows the flocs to be absorbed over almost the entire height of the the filtering mass.

 The technique according to the invention therefore makes it possible to retain a large part of the SDI titrating microparticles present in the water and to limit the frequency of cleaning and replacement of reverse osmosis membranes. It is also noted that it makes it possible to retain a much larger share of these particles, given that these fill the empty interstices left between the grains constituting the filtering mass, which makes it possible to retain microparticles having a size smaller than the size of these interstices.

All these advantages allow the technique according to the invention to produce a feed water reverse osmosis membranes in which the SDI ₁₅ is between 3 and 3.5, that is to say that conforms to the recommendations of membrane manufacturers.

 The filtration step at a cutoff threshold of between 10 nanometers and 10 microns is implemented between the granular filtration step and the reverse osmosis filtration step. This filtration, which acts as a fuse, makes it possible, for example when the nature of the water to be treated varies greatly, to ensure that the feed water of reverse osmosis membranes placed downstream has a level of quality. enough to prevent their damage. This implementation makes it possible to extend the life of these reverse osmosis membranes, which are very expensive.

 It is recalled that in a water treatment structure in general, the solid liquid separation rate through a filter is equal to the volume of treated water per hour divided by the filter surface.

It is thus possible to determine the surface of a filter knowing the volume of water to be treated per hour and the filtration rate. The total volume of Filter material can then be calculated according to the following relationship: Volume of Filter Material = Filter Surface x Filter Height.

 In some cases, the height of a filter can be determined by knowing the filtration rate and the contact time required to perform a given chemical reaction (eg adsorption ...) by the following relation: Filter height = velocity of the filter filtration x time.

 Preferably, said granular filtration step (iii) uses a granular filter whose filtering mass has a decreasing particle size.

 This characteristic makes it possible to promote the penetration of the flocs present in the water over almost the entire height of the granular filter. Indeed, the largest size flocs are retained first letting the smaller size flocs gradually penetrate the core of the filter mass.

 A process according to the invention is also preferably constituted by a sieving step preceding said coagulation step (i).

 In this case, said sieving step is carried out at a cutoff threshold of between 50 and 500 micrometers.

 This step is implemented so as to retain the algae and / or the microparticles present in the water to be treated so as to prevent the formation of very large flocs that would seal the surface of the granular filter.

 The present technique also relates to an installation for implementing a method for treating water with a view to its desalination according to the invention, said installation consisting of:

- a coagulation zone of said water;

 a zone of flocculation of water from said coagulation zone;

 a granular filter comprising a filtering mass consisting of at least one layer of at least one filter material;

means for drawing off said water coming directly from said zone flocculation through said granular filter, said withdrawing means being adapted to convey said water from said flocculation zone through said granular filter at a speed of between 15 and 25 m / h, filtration means at a cutoff threshold between 10 nanometers and 10 micrometers of water from said granular filter (15), and

 a unit for reverse osmosis filtration of water from said filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers.

 The filters used are downflow filters or gravity filters.

 The means for withdrawing water from the flocculation zone through the granular filter at a speed of between 15 and 25 m / h may be natural means when the water flows by gravity through the filter or mechanical when water is pumped through the filter.

 The filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers act as a fuse to protect the membranes of the reverse osmosis filtration unit as has been indicated above. These filtration means may comprise a filter cartridge whose cutoff threshold may be between 1 and Ι Ο μιη and will preferably be equal to 5μιη. They may alternatively comprise an ultrafiltration filtration unit whose cut-off threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 bar, or a microfiltration unit whose cut-off point will be preferably between 0.1 micrometers and 10 micrometers and whose driving force will be between 0.1 and 3 bar.

 Preferably, said filtering mass has a total height of between 2.5 and 4 meters.

Such a filter mass height is sufficient to allow the production of water whose SDI ₁₅ complies with the recommendations mentioned upper.

 According to a particular embodiment, said first granular filter comprises a stack of two layers of a first filter material and a second filter material, said first and second filter material having decreasing particle sizes.

 The material at the top has a larger particle size and a lower density than the material at the bottom. This configuration is particularly interesting.

 During the filtration of the water, the fi jons it contains gradually accumulate within the layer of granular material located at the top of the filter. They fill the initially empty interstices that exist between the grains. This material thus makes it possible to store the fiocs present in the water, which themselves have trapped the clogging materials, as well as the suspended solids. The highest material acts as a tank of fiocs, a maturation filter, and allows the removal of SDI. This mechanism occurs after a few hours, that is, once the filter has begun to accumulate enough fiocs.

 The second layer of material located lower in the filter plays a role of refining and retains fiocs that could escape the first layer. This is the reason why the particle size of the material of the second layer is always smaller than that of the first layer.

 According to another advantageous characteristic, said first material has a particle size of between 0.8 and 2.5 mm, and in that said second material has a particle size of between 0.5 and 0.9 mm.

 In this case, the height of said first material advantageously represents between 50 and 80% of the total height of said filtering mass.

The implementation of this feature allows a large part of the fiocs to diffuse inside the layer of first material and thus avoid the formation of a cake on this layer. The clogging speed of the granular filter is consequently reduced, which makes it possible to reduce the frequency of backwashing thereof. The fact that these fiocs are diffusing inside the first layer, that is to say that they are trapped in the interstitial spaces left between the grains of the first layer of material, without clogging the filter also allows to retain other particles whose size is smaller than that of these interstices. Thus, this first layer of material makes it possible to retain most of the SDI titrating particles initially contained in the water to be treated while limiting the clogging of the granular filter.

 According to a preferred characteristic, said first material is anthracite and said second material consists of grains of sand or garnet.

 According to another preferred feature, said first material is pumice stone and said second material consists of grains of sand or garnet.

 According to another particular embodiment, said first filter comprises a stack of three layers of a first, a second and a third filtering material, said first, second and third filtering materials having decreasing particle sizes.

Further refinement can be achieved by implementing a third layer of even more dense and thinner material than the material of the second layer. Such refining increases the removal of suspended particles and to reduce the value CIU ₁₅ of treated water from 0.2 to 0.5.

 In this case, the height of said first, second and third materials respectively represent between 40 and 75%, between 7.5 and 40%> and 7.5 and 20%> of the total height of said filtering mass.

 According to a preferred characteristic, the grains of said materials (17, 18, 17 ', 18', 22) of said layers of said granular filter (15) have increasing densities from the upper layer to the lower layer of said filter.

The fact that the grain density of the materials constituting the different layers of the granular filter is greater from the lower layer to the upper layer allows, after the filter is washed, the various layers of the constituent to reform naturally. Indeed, after the agitation of the layers of material due to the filter backwash has ceased, the densest material grains constituting the lower layer are deposited first while the other grains are deposited in order of decreasing density.

 An installation according to the invention is furthermore optionally constituted by a sieving unit placed upstream of said coagulation zone.

 6. List of figures

 Other features and advantages of the invention will emerge more clearly on reading the following description of preferred embodiments, given as simple illustrative and non-limiting examples, and the appended drawings, among which:

 Figure 1 shows a diagram of a water treatment plant according to a first embodiment of the invention;

 Figure 2 shows a diagram of a water treatment plant according to a second embodiment of the invention;

Figure 3 is a curve illustrating the evolution of the SDI ₁₅ of a filtered water according to the prior art in a two-layer filter (anthracite-sand) of 1, 6 meters high at 7.5 m / h;

Figure 4 is a curve which illustrates the change of the filtered water ₁₅ of a SDI according to the prior art in a two-layer filter (anthracite-sand) of 1, 6 meters high at 9.5 m / h;

Figure 5 is a curve which illustrates the change CIU ₁₅ of a filtered water according to the invention in a two-layer filter (anthracite-sand) of 3 meters high and 15 m / h.

 7. Description of an embodiment of the invention

 7.1. General principle of the invention

The general principle of the invention is based on the combination of a coagulation, a flocculation, a filtration through a granular filter at high speed, that is to say between 15 and 25 m / h, and filtration at a cut-off point between 10 nanometers and 10 micrometers of water for the purpose of producing a feed water for reverse osmosis filtration. Given the high speed of water filtration, a significant portion of f ocs grading SDI ₁₅ present in the water is quickly absorbed into the filter material essentially over its entire height without observation of fouling of the surface the filtering mass.

The implementation of the technique according to the invention therefore makes it possible to produce water with the SDI ₁₅ is between 3 and 3.5. This water can then be optimally filtered through reverse osmosis membranes to be desalted.

 As opposed to the techniques according to the prior art, the technique according to the invention makes it possible to proscribe the implementation of a separation for example by decantation or by flotation upstream of the granular filtration when the SDI3 of the raw water is situated between 15 and 20.

 7.2. Example of a first embodiment of a treatment plant according to the invention

 In relation to FIG. 1, a first embodiment of a water treatment installation according to the invention is presented.

 As represented in this FIG. 1, an installation according to this first embodiment comprises a pipe 10 for supplying a water to be treated in a coagulation zone 11 inside which a coagulating agent is injected which this embodiment is ferric chloride (FeCls).

 The coagulation zone 11 is connected by a line 12 to a flocculation zone 13 inside which is injected a foculant agent which in this embodiment is the FLOPAM A 905 synthetic flocculating polymer. In a variant, a polymer natural flocculant can be implemented.

 The flocculation zone 13 is connected by a pipe 14 to a filter 15.

The filter 15 is an open granular filter through which the water to be treated previously coagulated and flocculated circulates under the effect of gravity. In a variant, this filter 15 may be a granular filter under pressure through which the water to be treated circulates under pressure by the implementation of withdrawal means such as a pump. In this embodiment, it comprises a filter mass 16 which is constituted by a stack of two layers 17, 18 of two granular filter materials. The layers 17 and 18 constitute respectively the upper layer and the lower layer of the filter. The materials constituting the layers 17, 18 having decreasing granulometries and increasing densities from the top layer 17 to the lower layer 18.

 The first layer 17 consists of anthracite whose particle size is between 0.8 and 2.5 millimeters.

 The second layer 18 consists of sand whose particle size is between 0.5 and 0.9 millimeters.

 In this embodiment, the height of each of the layers of material represents approximately 50% of the total height of the filtering mass 16. In variants, the height of the first 17 and second 18 layers of material may vary in such proportions that the height of the first layer 17 may reach up to about 80% of the total height of the filtering mass 16.

 In a variant, the anthracite may be replaced by pumice with a particle size of between 0.8 and 2.5 millimeters.

 In all cases, the sand, preferably rolled or crushed, may be replaced by grains of garnet or any other equivalent material.

 The total height of the filtering mass 16 can vary between 2.5 and 4 meters depending on the operating conditions.

 The filter 15 is connected, at its outlet, to the inlet of a filter cartridge 24 by means of a pipe 20. The reverse osmosis unit 19 has a treated water outlet 21. This filter cartridge 24 has a cutoff threshold equal to 5 micrometers. The outlet of this filter cartridge 24 is connected to the inlet of a reverse osmosis filtration unit 19 via a pipe 25.

7.3. Example of a Second Embodiment of a Treatment Plant According to the Invention FIG. 2 illustrates a second embodiment which differs from the first embodiment which has just been described with regard to the structure of the filter 15.

 In this second embodiment, the filter 15 comprises a filtering mass 16 'which consists of the stack of three layers 17', 18 'and 22 of three granular filter materials having decreasing particle sizes.

 The first layer 17 ', or top layer, consists of a thickness of 1 to 6 meters of anthracite whose particle size is between 1.0 and 2.5 mm.

 The second layer 18 ', or intermediate layer, consists of a thickness of 0.3 to 1 meter of sand whose particle size is between 0.6 and 0.9 mm.

 The third layer 22, or lower layer, consists of a thickness of 0.3 to 0.5 meters of garnet or sand whose particle size is between 0.3 and 0.55 mm.

 The materials constituting the layers 17 ', 18' and 22 having decreasing granulometries and increasing densities from the top layer 17 'to the lower layer 22.

 The height of the first layer 17 'of material represents between approximately 40 and 75% of the total height of the filtering mass 16'. The height of the second layer 18 'of material is between about 7.5 and 40% of the total height of the filter mass 16', and the height of the third layer 22 of material is about 7.5 to 20% of the total height of the filter mass 16 '.

 The total height of the filtering mass 16 'is between 2.5 and 4.5 meters.

 7.4. Variants of the first and second embodiments of a treatment plant according to the invention

In a variant of the first and second embodiments (not shown), an installation according to the invention will also consist of a sieving unit placed upstream of the coagulation zone 11. This sieving unit preferably include elements for retaining the algae and / or microparticles present in the water to be treated having a size greater than 500 microns.

 In another variant, the granular filter 15 may be composed of a single layer of sand having a particle size of between 0.5 and 1.5 mm.

 In other variants, the filter cartridge may be replaced by: an ultrafiltration filtration unit whose cutoff threshold will preferably be between 10 nanometers and 0.1 micrometers and whose driving force will be between 1 and 5 bar, or

a micro filtration unit whose cutoff threshold will preferably be between 0.1 micrometers and 10 micrometers and whose motive force will be between 0.1 and 3 bar.

 However, the use of a filter cartridge has the advantage of requiring a driving force and therefore a lower energy consumption than those required by filtration or microleakage.

 7.5. Example of a treatment method according to the invention

 In relation with FIG. 1, a water treatment process is presented with a view to its desalination according to the invention.

 Such a process consists in conveying raw water via line 10 into the coagulation zone 11 so that it undergoes a coagulation step therein.

 The coagulated water from the coagulation zone 11 is then introduced via the pipe 12 into the flocculation zone 13 so that it undergoes a flocculation step therein.

 The flocculated water coming from the flocculation zone 13 is then introduced via the pipe 14 into the granular filter 15.

 The coagulated and flocculated water then passes through the filtering mass 16 at a speed of between 15 and 25 m / h.

The flocs 23 present in this water then penetrate rapidly by diffusion in depth inside the filtering mass 16 and fill, at least in part, the void gaps left between the media grains filtering substantially over the entire height of the filter media. Under these circumstances, the granular filter matures much more quickly.

 Given that a portion of these flocs fills the empty interstices left between the grains constituting the filter mass, the microparticles having a size smaller than the size of these interstices are also retained in the filter mass.

 The combination according to the invention of coagulation, flocculation and granular filtration at high speed also makes it possible to prevent the formation of a layer of flocs on the surface of the filtering mass, which leads to a reduction in the frequency washing the granular filter.

The implementation of this granular filtering step results in the production of an effluent with the SDI ₁₅ is a value between 3 and 3.5. This effluent therefore has a level of quality that is in line with the recommendations of manufacturers of reverse osmosis membranes. This effluent can then constitute a feed water of the membranes of the reverse osmosis filtration unit 19.

Nevertheless, it can happen, so quite exceptional, this effluent has an SDI ₁₅ whose value is slightly higher than 3.5. This may for example be related to the fact that the quality of the raw water has deteriorated. For this reason, the effluent is directed towards the inlet of a filter cartridge 24. This filter cartridge 24 acts as a fuse which retains, if appropriate, the particles present in the effluent at the outlet of the granular filter so that the The feed water of the reverse osmosis unit 19 has in all circumstances a SDI ₁₅ whose value is between 3 and 3.5.

This effluent is then systematically directed to filtration means with a cutoff threshold of between 10 nanometers and 10 micrometers, which in this embodiment comprise the reverse osmosis filtration unit 19 so as to subtract it, at the very least in part, the salts it contains and to produce desalinated water. In variants, the filter cartridge may be replaced by a filtration unit by ultrafiltration or microfiltration.

 In a variant, the coagulated and flocculated water will be filtered through a tri-layer filter according to the second embodiment.

 In another variant, the raw water will undergo a sieving step before the coagulation step.

 7.6. Other advantages

 The implementation of a granular filter according to the techniques of the prior art is insufficient to produce a water having the required qualities in order to be subsequently filtered through reverse osmosis membranes. It is therefore necessary to multiply the equipment used to reach the required level of quality. This leads to an increase in the size of the facilities. On the contrary, the invention makes it possible to produce water of the required quality, in particular by means of a granular filter operating at high speed and upstream of which it is not necessary to use separation means such as, for example, a decanter or means of fiottation. Its implementation therefore reduces the overall size and especially the floor area occupied by a desalination plant.

 The technique according to the invention also makes it possible to reduce the costs associated with the desalination of water. On the one hand, the facilities required for desalination according to the invention are less complex, less bulky and therefore less expensive. On the other hand, the technique according to the invention makes it possible to reduce the washing and replacement frequencies of reverse osmosis membranes.

 Reducing the washing frequency of reverse osmosis membranes also makes it possible to reduce the water losses used for this purpose.

The implementation of the invention also contributes to reducing the volumes of reagents used for the desalination of water. Indeed, the progressive maturation of a filter according to the invention, which is characterized by the fact that it accumulates flocs within its filtering mass during filtration, makes it possible to maintain the adsorption kinetics of the clogging materials. (and therefore SDI) even if we reduce the dosage of coagulant initially injected. This gradual reduction in the amount of coagulant injected during a filtration cycle makes it possible to reduce the overall consumption of reagents.

 7.7. testing

 Tests have been carried out to demonstrate the effectiveness of a water treatment process according to the invention.

A first series of tests consisted in filtering, according to the prior art, a water in a filtration column containing a 0.8 meter layer of anthracite and a layer of 0.8 meter of sand, at a rate of 7.5 m / h then in a filter cartridge at 5 micrometers. Figure 3, which illustrates the results of these tests shows that the SDII ₅ of filtered water during the tests was generally greater than 3.5.

A second series of tests consisted in filtering, according to the prior art, the waters of a first station (St 1) and a second station (St 2) in a filtration column containing a layer of 1, 5 meters of anthracite and a layer of 1, 5 meters of sand, at a speed of 9.5 m / h and then in a filter cartridge at 5 micrometers. Figure 4, which illustrates the results of these tests reveals that the ₅ SDII filtered water during these trials was on average between 4 and 5.

A third series of tests consisted in filtering, according to the invention, a water in a filtration column containing a layer of 1.5 meters of anthracite and a layer of 1.5 meters of sand, at a speed of 15. m / h then in a filter cartridge at 5 micrometers. Figure 5 illustrates the results of these tests shows that the SDII ₅ filtered water during these tests was always less than 3.5.

Claims

 A method of treating water for desalting, said method comprising:  a step (i) of coagulating said water; - a step (ii) of flocculation of water from said step (i) of coagulation;  a step (iii) granular filtration of water directly from said step (ii) of flocculation through at least one granular filter comprising a filter mass consisting of at least one layer of at least one filter material;  a filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers of water from said granular filtration step (iii);  a step (v) for reverse osmosis filtration of the water from said filtration step (iv) at a cutoff threshold of between 10 nanometers and 10 micrometers;  a step (vi) of recovering at least partially desalted water from said reverse osmosis filtration step (v); said granular filtration step (iii) of passing water from said flocculation step (ii) through said granular filter at a rate between 15 and 25 m / h.  2. Method according to claim 1, characterized in that said step (iii) granular filtration implements a granular filter (15) whose filter mass (16) has a decreasing particle size. 3. Method according to claim 1 or 2, characterized in that it further comprises a sieving step preceding said step (i) of coagulation.  4. Method according to claim 3, characterized in that said sieving step is performed at a cutoff threshold of between 50 and 500 micrometers. 5. Method according to any one of claims 1 to 4, characterized in that said step (iv) filtration at a cutoff threshold between 10 nanometers and 10 micrometers uses a filter cartridge or an ultrafiltration unit or microfiltration unit.  6. Installation for the implementation of a water treatment process for desalination according to any one of claims 1 to 5, said installation being constituted:  a coagulation zone (11) of said water;  a flocculation zone (13) of water from said coagulation zone (11);  a granular filter (15) comprising a filter medium (16, 16 ') consisting of at least one layer of at least one filter material;  means for withdrawing said water coming directly from said flocculation zone (13) through said granular filter (15), said withdrawal means being able to convey said water coming from said flocculation zone (13) through said granular filter (15) at a speed of between 15 and 25 m / h;  filtration means at a cutoff threshold of between 10 nanometers and 10 micrometers of water from said granular filter (15), and  a reverse osmosis filtration unit (19) of water from said filtration means at a cut-off of between 10 nanometers and 10 micrometers.  7. Installation according to claim 6, characterized in that said granular filter (15) comprises a stack of two layers (17, 18) of a first filter material and a second filter material, said first and second filter material having decreasing grain size.  8. Installation according to claim 7, characterized in that the height of said first material (17) is between 50 and 80% of the total height of said filter mass (16, 16 '). 9. Installation according to claim 6, characterized in that said granular filter (15) comprises a stack of three layers of a first (17 '), a second (18 ') and a third (22) filter material, said first, second and third filter materials having decreasing particle sizes.  10. Installation according to claim 9, characterized in that the height of said first (16 '), second (IV) and third (22) materials respectively represent between 40 and 75%, between 7.5 and 40%> and 7, 5 and 20%> of the total height of said filtering mass (16 ').  11. Installation according to any one of claims 6 to 10, characterized in that the grains of said materials (17, 18, 17 ', 18', 22) of said layers of said granular filter (15) have increasing densities from the layer upper to the lower layer of said filter.  12. Installation according to any one of claims 6 to 1 1, characterized in that it further comprises a sieving unit placed upstream of said coagulation zone (11).  13. Installation according to any one of claims 6 to 12, characterized in that said filtering means with a cutoff threshold of between 10 nanometers and 10 micrometers belong to the group comprising:  the filter cartridges (24);  ultrafiltration filtration units;  filtration units by micro filtration.