AU2011244513A1

AU2011244513A1 - Method for treating water with a view to desalinating same including high-speed filtration, and corresponding facility

Info

Publication number: AU2011244513A1
Application number: AU2011244513A
Authority: AU
Inventors: Abdelkader Gaid
Original assignee: Veolia Water Solutions and Technologies Support SAS
Current assignee: Veolia Water Solutions and Technologies Support SAS
Priority date: 2010-04-19
Filing date: 2011-04-12
Publication date: 2012-12-06
Anticipated expiration: 2031-04-12
Also published as: WO2011131523A1; CN102985377B; FR2958929A1; IL222483B; CN102985377A; AU2011244513B2; IL222483A0; ZA201207767B; FR2958929B1

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

1 Process for treating water with a view to its desalination, including a high speed filtering, and corresponding installation. 1. Field of the invention the field of the invention is that of treating water with a view to its 5 desalination. More specifically, the invention pertains to a method of this kind comprising the implementation of a step for filtering by reverse osmosis. 2. Prior art The desalination of water (sea water; estuary water, industrial water highly 10 charged with salts, ground water contaminated with salts, brackish water etc) generally includes a step for filtration on one or more reverse osmosis membranes. Manufacturers of this type of filtration membranes can contractually guarantee the service life of their membranes only to the extent that the water 15 passing through these membranes is of a certain level of quality. Thus, in order to prevent an excessively rapid clogging of the membranes, the water supplying these membranes must be of very high quality. The SDI (Silt Density Index) which is a parameter representing the clogging potential of water, is generally used to characterize the level of quality of 20 a water to be treated through reverse osmosis membranes. The SDI, 5 is the SDI value of water measured according to the standardized ASTM D4189-95 method (15 June 2007). The SDI 5 is measured as follows. Water is filtered at constant pressure of 2.1 bars through a filter with a cut-off point is equal to 0.45 micrometers. First, 25 the time TO needed to filter 500 ml of water is measured. Then, after fifteen minutes of continuous filtering of the water through the filter, a second measurement is made to determine the time T15 needed to filter 500 ml of water. The SDI,, is thus computed according to the following formula:

SDI

15 =(l0O/15).(1-(TO/T15)) 2 The SDI, 5 varies between 0 and 6.67, given that the greater its value the higher the clogging potential of the water that it characterizes. For water of very great clogging potential, it is impossible to determine the

SDI

15 by this standardized method. In this case, alternative indices can be 5 computed by measuring the time needed to filter 500ml of water after the water has been filtered continuously through the filter not for 15 minutes but for 3, 5 or 10 minutes depending on the nature of the water to be treated. The manufacturers of reverse osmosis membranes generally recommend that the water to be filtered thorough reverse osmosis membranes should, 10 according to the method indicated here above, have an SDI,, ranging from 3 to 3.5. Filtration on reverse osmosis membranes therefore requires the implementation of a pre-treatment of the feed waters so as to endow them with the level of quality required by manufacturers of reverse osmosis membranes. 15 At present, several techniques are being implemented to produce water with an SDI,, as close as possible to the values included in this range. In a first technique, a pre-coagulated and pre-flocculated water is filtered: - in an granular filter comprising a filter mass about 1.6 meters high constituted by a layer of anthracite and a layer of sand, at a speed of the 20 order of 7.5 m/hr, then - in a 5-micrometer filter cartridge. The filter cartridges (for example those supplied by Pall, Sartorius etc) are used upstream to the reverse osmosis membranes or nanofiltration membranes as a protection against particles (with sizes greater than 5 pm) that could create 25 dysfunction within the reverse osmosis membranes. The filtering medium integrated into these cartridges includes an organic membrane (polysulfone, polyethersulfone, polypropylene or the like). These cartridges also enable elimination of the microorganisms having sizes greater than the cut-off point of the cartridge.

3 This technique can be used to produce water with an SDI of 3 to 3.5 only if the raw water to be treated has an SDI 3 ranging from 15 to 20. Implementing treatment of this type therefore, in most cases, cannot be used to produce water of a quality suited to subsequent optimal filtering through reverse osmosis 5 membranes. A second technique implements a step for separation, for example by decantation, upstream to the granular filtration step. A third technique combines the first two techniques by successively applying operations of coagulation, flocculation, separation, for example by 10 decantation, granular filtration, filtration on filter cartridge and then filtration by reverse osmosis. The implementation of these second and third techniques can be used to produce feed water of satisfactory quality for a reverse osmosis filtration unit. 3. Drawbacks of the prior art 15 The first of the techniques, currently used to produce feed water for reverse osmosis filtration membranes, gives a level of quality compliant with the recommendations of manufacturers of such membranes only if the SDI 3 of the raw water to be treated ranges from 15 to 20. The filtering of water that does not have the requisite level of quality on 20 reverse osmosis membranes is accompanied by a rapid increase in head loss in the membranes. This is essentially due to bacterial growth, an adsorption of organic matter and an accumulation of mineral and organic micro-substances on the membranes. This increase in head loss necessitates frequent operations of chemical 25 cleaning of the membranes. Such cleaning operations are implemented in practice with aggressive reagents that have a negative impact on the service life of the membranes. The membranes therefore need to be replaced regularly, and this amounts to a major cost item. The second and third of these techniques give a level of quality compliant 30 with the recommendations of manufacturers of such membranes. However, these 4 techniques have the drawback of being relatively complicated and therefore costly to implement. 4. Goals of the invention The invention is aimed especially at overcoming these drawbacks of the 5 prior art. It is an aim of the invention, in at least one embodiment, to provide a technique for treating water with a view to its desalination by reverse osmosis that prolongs the service life of the reverse osmosis membranes implemented for this purpose. 10 The invention is also aimed, in at least one embodiment of the invention, at producing a technique of this kind that lengthens the time taken to clog the reverse osmosis membranes and therefore reduces the frequency of washing and replacement of these membranes. It is an aim of the invention, in at least one embodiment, to implement a 15 technique of this kind for producing water with an SDI,, ranging from 3 to 3.5 (measured according to the ASTM D4189-95 method) prior to its filtration through reverse osmosis membranes. It is yet another aim of the invention, in at least one embodiment, to provide a technique of this kind that is simple and costs little to implement, at 20 least as compared with the techniques of the prior art. It is yet another goal of the invention, in at least one embodiment, to provide a technique of this kind that reduces the surface area occupied by the installations implemented for the desalination of water. The invention further pursues the goal, in at least one embodiment, of 25 reducing the quantity of reagents needed to desalinate water. 5. Summary of the invention These goals as well as other that shall appear here below are achieved by means of a method for treating water for its desalination, said method being constituted by: 30 - a step of coagulation (i) of said water; 5 - a step (ii) of flocculation of the water coming from said step of coagulation (i); - a step (iii) of granular filtration of the water coming directly from said step (ii) of flocculation through at least one granular filter comprising a filter 5 mass constituted at least by a layer of at least one filter material; a step (iv) for the filtering, with a cut-off point ranging from 10 nanometers to 10 micrometers, of the water coming from said granular filtration step (iii); - a step (v) for the filtering, by reverse osmosis, of the water coming from 10 said step (iv) for filtering with a cut-off point ranging from 10 nanometers to 10 micrometers; - a step (vi) for recovering at least partly desalinated water coming from said step (iv) of reverse osmosis filtration; said step (iii) of granular filtration consisting in conveying water coming 15 from said flocculation step (ii) via said granular filter at a speed ranging from 15 m/hr to 25 m/hr. Thus, the invention relies on an original approach which consists of the combination of a coagulation, a flocculation, a filtration through a granular filter at high-speed, i.e. a speed of 15 to 25 m/h, and a filtration of water with a cut-off 20 point of 10 nanometers to 10 micrometers in order to produce a feed water for reverse osmosis filtration. The step of filtration with a cut-off point of 10 nanometers to 10 micrometers could entail the use of one or more filter cartridges for which the cut off point could range from I jim to 10pm and will preferably be equal to 5p1m. It 25 could alternately implement a filtration unit for filtering by ultra-filtration for which the cut-off point will preferably range from 10 nanometers to 0.1 micrometers, with a motive pressure ranging from I to 5 bars or a microfiltration unit for which the cut-off point would preferably range from 0.1 micrometers to 10 micrometers with a motive pressure force ranging from 0.1 to 3 bars.

6 This special implementation enables the flocs present in the water to swiftly penetrate the filter mass of the granular filter in depth by diffusion and at least partly fill the vacant interstices left between the grains of filtering medium essentially throughout the height of the filter mass. 5 As opposed to the techniques of the prior art, the technique of the invention prevents the implementation of a separation, for example by decantation or by flotation upstream to the granular filtration when the SDI 3 of raw water is situated between 15 and 20. Indeed, given that the technique of the invention relies on the 10 implementation of granular filtration at high speed, the flocs present in the water are retained essentially throughout the height of the filter mass. On the contrary, when implementing prior art techniques, these floes penetrate the filter mass essentially on a small height. A layer of floes is then formed on the surface of the filter mass. This surface fouling of the filter mass is accompanied by a rapid 15 increase in the head loss through the granular filter. This necessitates frequent washings of the filters and makes it necessary to increase the frequency of chemical cleaning of the reverse osmosis membranes placed downstream. On the contrary, the implementation of the technique of the invention prevents: 20 - the formation of a layer of flocs on the surface of the filter mass, and - on the other hand enables the flocs to be absorbed almost on the entire height of the filter mass. The technique of the invention therefore makes it possible to hold back a major part of the micro-particles contributing to the SDI that are present in the 25 water and to limit the frequency.of cleansing and replacing of the reverse osmosis membranes. It can furthermore be noted that it enables the retention of an even greater pait of these particles given the fact that they fill the vacant interstices left between the grains constituting the filter mass, thus enabling the retention of micro-particles of a size smaller than the size of these interstices.

7 All these advantages enable the technique of the invention to produce a feed water for reverse osmosis membranes with an SDI,, of 3 to 3.5, i.e. a value compliant with the recommendations of the membrane manufacturers. The filtration step with a cut-off point of 10 nanometers to 10 micrometers 5 is implemented between the granular filtration step and the reverse osmosis filtration step. This filtration, which acts like a safety fuse, makes it possible for example when the nature of the water to be treated varies greatly to make sure that the level of quality of the feed water for the reverse osmosis membranes placed downstream is sufficient to prevent damage to these membranes. This 10 implementation extends the service life of these reverse osmosis membranes which are very costly. It may be recalled that, in a water treatment structure in general, the speed of liquid/solid separation through the filter is equal to the volume of water treated per hour divided by the surface area of the filter. 15 Thus, the surface area of a filter can be determined if we know the volume of water to be treated per hour and the filtration speed. The total volume of filter material can then be computed according to the following relationship: Volume of filter material = Surface area of filter x Height of filter. In certain cases, the height of a filter can be determined, knowing the 20 speed of filtration and the contact time needed to effect a given chemical reaction (for example adsorption or the like), by the following relationship: Height of the filter = Filtration speed x time. Preferably, said step (iii) of granular filtration implements a granular filter for which the filter mass has a decreasing grain size. 25 This characteristic favors the penetration of the flocs present in the water on almost the entire height of the granular filter. Indeed, the larger-sized flocs are retained first, allowing the smaller-sized flocs to gradually penetrate the core of the filter mass. A method according to the invention furthermore preferably consists of a 30 screening step prior to said step (i) of coagulation.

8 In this case, said screening step is performed with a cut-off point of 50 to 500 micrometres. This step is implemented so as to retain algae and/or the micro-particles present in the water to be treated so as to prevent the formation of very large-sized 5 flocs which would block the surface of the granular filter. The present invention also pertains to an installation for the implementation of a method for treating water for its desalination according to the invention, said installation consisting of: - a coagulation zone (11) for said water; 10 - a flocculation zone (13) for the water obtained from said coagulation zone (11); - a granular filter comprising a filter mass consisting of at least one layer of at least one filter material; - means for extracting said water coming directly from said flocculation 15 zone via said granular filter, said means for extracting being capable of conveying said water. coming from said flocculation zone through said granular filter at a speed of 15 m/hr to 25 m/hr; - means for the filtering, with a cut-off point of 10 nanometers to 10 micrometers, of the water coming from said granular filter, and 20 - a unit for the reverse osmosis filtering of the water obtained from said means for filtering with a cut-off point between 10 nanometers and 10 micrometers. The filters used are downflow filters or gravity filters. The means for extracting the water coming from the flocculation zone via 25 the granular filter at a speed of 15 m/hr to 25 m/hr may be natural means when the water flows by gravity through the filter or mechanical means when the water is pumped through the filter. The filtering means, with a cut-off point of 10 nanometers to 10 micrometers, act as a safety fuse to protect the membranes of the reverse osmosis 30 filtration unit as indicated further above. These filtering means may include a 9 filter cartridge with a cut-off point ranging from 1 pm to 10pm and preferably equal to 5pm. They could alternately include a filtration unit for filtering by ultra filtration with a cut-off point preferably ranging from 10 nanometers to 0.1 micrometers, and a motive pressure of 1 to 5 bars or a microfiltration unit with a 5 cut-off point preferably ranging from 0.1 micrometers to 10 micrometers and motive pressure of 0.1 to 3 bars Preferably, said filter mass has a total height ranging from 2.5 to 4 meters. Such a height of filter mass is sufficient to enable the production of water with an SDI, 5 compliant with the above-mentioned recommendations. 10 According to one 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 materials having decreasing grain sizes The material situated on top has a greater grain size greater and a lower density than the grain size and density of the material situated at the bottom. This 15 configuration is particularly worthwhile. During the filtering of water, the flocs that it contains gradually collect within the layer of material in the form of grains situated at the top of the filter. They fill the initially vacant interstices existing between the grains. This material therefore makes it possible to store the flocs present in the water which have 20 themselves trapped the clogging matter as well as the suspended matter. The material at the greatest height acts as a reservoir of flocs, a maturing filter, and enables the elimination of the SDI. This mechanism occurs after a few hours, i.e. once the filter has started accumulating a sufficient quantity of flocs. The second layer of material situated lower down in the filter fulfils a 25 refining role and enables the retention of flocs that might have escaped from the first layer. This is why the grain 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 grain size of 0.8 to 2.5 mm, and said second material has a grain size of 0,5 to 0,9 30 mm.

10 In this case, the height of said first material is advantageously 50 to 80% of the total height of said filter mass. The implementation of this characteristic enables a large portion of the flocs to spread within the layer of first material and thus prevents the formation of 5 a filter cake on this layer. The speed of clogging of the granular filter is therefore reduced, thus reducing the frequency of backwashing of this filter. The fact that these flocs get diffused within the first layer, i.e. the fact that they are trapped in the interstitial spaces left between the grains of the first layer of material without thereby clogging the filter also enables the retention of other particles whose size 10 is smaller than that of these interstices. Thus, this first layer of material enables the retention of the essential part of the particles contributing to the SDI initially contained in the water to be treated while at the same time limiting the clogging of the granular filter. According to a preferred characteristic, said first material is anthracite and 15 said second material is constituted by grains of sand or garnet. According to another preferred characteristic, said first material is pumice stone and said second material is constituted by grains of sand or garnet. According to another particular embodiment, said granular filter comprises a stack of three layers of a first, second and third filter material, said 20 first, second and third filter materials having decreasing grain-size. A more intensive refining can be obtained by using a third layer of material that is even more dense and finer than the material of the second layer. A refining operation of this kind increases the elimination of the particles in suspension and reduces the value of the SDI, of the treated water by 0.2 to 0.5. 25 In this case, the height of said first, second and third materials respectively amount to 40% to 75%, 7.5% to 40% and 7.5% to 20% of the total height of said filter mass. According to another preferred characteristic, the grains of said materials (17, 18, 17', 18', 22) of said layers of said granular filter (15) have 30 increasing densities from the top layer to the bottom layer of said filter.

The fact that the density of the grains of material constituting the different layers of the granular filter is greater starting from the bottom layer going towards the top layer makes it possible, after the filter has been washed, for the different layers forming it to reform in a natural way. Indeed, after the shaking of the layers 5 of materials by the backwashing of the filter has stopped, the grains of the densest materials constituting the bottom layer get deposited first while the other grains get deposited in decreasing order of density. An installation according to the invention is optionally constituted by a screening unit positioned upstream to said coagulation zone. 10 6. List of figures Other features and advantages of the invention shall appear more clearly from the following description of preferred embodiments, given by way of simple, illustrative and non-exhaustive examples, and. from the appended drawings, of which: 15 - Figure I is drawing of a water treatment installation according to a first embodiment of the invention; - Figure 2 is a drawing of a water treatment installation according to a second embodiment of the invention; - Figure 3 is a curve illustrating the progress of the SDI, 5 of a water filtered 20 according to the prior art in a two-layer (anthracite-sand) filter, 1.6 meters high, at 7.5m/hr; - Figure 4 is a curve illustrating the progress of the SDI,, of a water filtered according to the prior art in a two-layer (anthracite-sand) filter, 1.6 meters high, at 9.5m/hr; 25 - Figure 5 is a curve illustrating the progress of the SDI 5 of a water filtered according to the invention in a two-layer (anthracite-sand) filter, 3 meters high, at 15m/hr. 7. Description of one embodiment of the invention 7.1. General principle of the invention 12 The general principle of the invention relies on the combination of operations of coagulation, flocculation, filtration through a granular filter at high speed, i.e. between 15 and 25 m/hr, and filtration of water, with a cut-off point of 10 nanometers to 10 micrometers, in order to produce feed water for a reverse 5 osmosis filtration. Given the high speed of filtration of the water, a major part of the flocs contributing to the SDI, 5 that are present in the water are swiftly absorbed by the filter mass essentially throughout its height without any clogging being observed on the surface of the filter mass. 10 The implementation of the technique according to the invention therefore enables the production of water whose SDI,, ranges from 3 to 3.5. This water can then be filtered optimally through the reverse osmosis membranes in order to be desalinated. As opposed to the prior art techniques, the technique of the invention 15 makes it possible to rule out the use of separation, for example by decantation or flotation, upstream to the granular filtration when the SDI 3 of the raw water is situated between 15 and 20. 7.2. Example of a first embodiment of a treatment installation according to the invention 20 Referring to figure 1, we present a first embodiment of a water treatment installation according to the invention. As shown in figure 1, an installation according to this first embodiment comprises an inlet conduit 10 for bringing water to be treated to a coagulation zone 11 into which a coagulant is injected, this coagulant being in this 25 embodiment ferric chloride (FeCl 3 ). The coagulation zone I1 is linked by a conduit 12 to a flocculation zone 13 into which a flocculating agent is injected, this agent in this embodiment being the synthetic flocculent polymer FLOPAM AN905. In one variant, a natural flocculent polymer can be implemented. 30 The flocculation zone 13 is connected by a conduit 14 to a filter 15.

13 The filter 15 is an open granular filter through which the pre-coagulated and pre-flocculated water to be treated flows under the effect of gravity. In one variant, this filter 15 could be a granular filter under pressure through which the water to be treated flows under pressure by the use of extraction means such as a 5 pump. In this embodiment, it comprises a filter mass 16 constituted by a stack of two layers 17, 18 made of two granular filter materials. The layers 17 and 18 respectively constitute the top layer and bottom layer of the filter. The materials constituting the layers 17, 18 have decreasing grain sizes and increasing densities going from the top layer 17 to the bottom layer 18. 10 The first layer 17 is constituted by anthracite, the grain size of which ranges from 0.8 to 2.5 millimeters. The second layer 18 is constituted by sand, the grain size of which ranges from 0.5 to 0.9 millimeters. In this embodiment, the height of each of the layers of material represents 15 about 50% of the total height of the filter mass 16. In variants, the height of the first layer 17 and second layer 18 of material could vary in proportions such that the height of the first layer could go up to 80% of the total height of the filter mass 16. In one variant, the anthracite could be replaced by pumice stone with a 20 grain size of 0.8 to 2.5 millimeters. In every case, sand, advantageously rolled or crushed, could be replaced by grains of garnet or any other equivalent material. The total height of the filter mass 16 may vary from 2.5 to 4 meters depending on the conditions of use. 25 The filter 15 is connected at output to the inlet of a filter cartridge 24 by means of a conduit 20. This filter cartridge 24 has a cut-off point equal to 5 micrometers. The outlet of this filter cartridge 24 is connected to the inlet of a reverse osmosis filtering unit 19 by a conduit 25. The reverse osmosis unit 19 has a treated water outlet 21.

14 7.3. Example of a second embodiment of a treatment installation according to the invention Figure 2 illustrates a second embodiment which differs from the first embodiment that has just been described only in the structure of the filter 15. 5 In this second embodiment, the filter 15 has a filter mass 16 constituted by the stacking of three layers 17', 18' and 22 of three granular filter materials having decreasing grain sizes., The first layer 17', or top layer, is constituted by a thickness of 1.6 to 3 meters of anthracite with a grain size of 1.0 to 2.5 mm. 10 The second layer 18', or intermediate layer, is constituted by a thickness of 0.3 to 1 meter of sand with a grain size of 0.6 to 0.9 mm. The third layer 22, or bottom layer, is constituted by a thickness of 0.3 to 0.5 meters of garnet or sand with a grain size of 0.3 to 0.55 mm. The materials constituting the layers 17', 18' and 22 have decreasing grain 15 sizes and increasing densities from the top layer 17' going towards the bottom layer 22. The height of the first layer 17' of material amounts to about 40% to 75% of the total height of the filter mass 16'. The height of the second layer 18' of the material amounts to about 7.5% to 40% of the total height of the filter mass 16' 20 and the height of the third layer 22 of material amounts to about 7.5% to 20% of the total height of the filter mass 16'. The total height of the filter mass 16' ranges from 2.5 to 4.5 meters. 7.4. Variants of the first and second embodiments of a treatment installation according to the invention 25 In one variant of the first and second embodiments (not shown), an installation according to the invention will furthermore be constituted by a screen unit placed upstream to the coagulation zone 17. This screen unit will preferably include elements used to retain the algae and/or the micro-particles present in the water to be treated having a size of over 500 micrometers.

15 In another variant, the granular filter 15 could be formed by a single layer of sand with a grain size of 0.5 to 1.5 mm. In other variants, the filter cartridge could be replaced by: - a filtration unit for filtering by ultra-filtration with a cut-off point of 10 5 nanometers to 0.1 micrometer and a motive pressure of I to 5 bars, or - a microfiltration unit with a cut-off point of preferably 0.1 micrometers to 10 micrometers and a motive pressure of 0.1 to 3 bars. The implementation of a filter cartridge has however the advantage of requiring a motive pressure and therefore an energy consumption that is smaller 10 than that required by ultra-filtration or microfiltration. 7.5. Example of a treatment method according to the invention Referring to figure 1, a method is presented for the treatment of water with a view to desalinating it according to the invention. A method of this kind consists in conveying raw water through the conduit 15 10 into the coagulation zone 11 in such a way that it undergoes a coagulation step therein. The coagulated water coming from the coagulation zone 11 is then introduced through the conduit 12 into the flocculation zone 13 in such a way that it undergoes a flocculation step therein. 20 The flocculated water coming from the flocculation zone 13 is then introduced through the conduit 14 into the granular filter 15. The coagulated and flocculated water then passes through the filter mass 16 at a speed of 15 to 25 in/hr. The flocs 23 present in this water then swiftly penetrate the interior of the 25 filter mass 16 by diffusion and in depth and at least partly fill the vacant interstices left between the grains of filtering media essentially throughout all the height of the filter mass. Thus, the granular filter reaches maturation far more swiftly. Given the fact that a pail of these flocs fill the vacant interstices left 30 between the grains constituting the filter mass, the micro-particles of a size 16 smaller than the size of these interstices are also retained in the filter mass. The combination according to the invention of a coagulation, a flocculation and a granular filtration at high speed furthermore prevents the formation of a layer of flocs on the surface of the filter mass, thus diminishing the 5 frequency of washing of the granular filter. The implementation of this step of granular filtration leads to the production of an effluent for which the SDI, 5 has a value of 3 to 3.5. This effluent therefore has a level of quality that complies with the recommendations of the manufacturers of reverse osmosis membranes. This effluent can then be a feed 10 water for the membranes of the reverse osmosis filtration unit 19. However, it can happen, quite exceptionally, that this effluent has an SDI,, of a value slightly greater than 3.5. This may be related for example to the fact that the quality of the raw water has deteriorated. For this reason, the effluent is sent towards the interior of a filter cartridge 24. This filter cartridge 24 acts like a 15 safety fuse which, if necessary, retains the particles present in the effluent at .the outlet of the granular filter so that the feed water for the reverse osmosis unit 19 has, in all circumstances, an SDI, with a value of 3 to 3.5. This effluent is then systematically directed towards filtration means with a cut-off point of 10 nanometers to 10 micrometers. In this embodiment, these 20 filtration means comprise the reverse osmosis filtration unit 19 so as to remove from it at least a part of the salts that it contains and produce a desalinated water. In variants, the filter cartridge could be replaced by a filtration unit for filtering by ultra-filtration or microfiltration. In one variant, the coagulated and flocculated water will be filtered 25 through a three-layered filter according to the second embodiment. In another variant, the raw water will undergo a screening step before the coagulation step. 7.6. Other advantages The use of a granular filter according to the prior-art techniques does not 30 suffice to produce water with the requisite qualities by which it can be 17 subsequently filtered through reverse osmosis membranes. It is therefore necessary to greatly increase the number of apparatuses implemented so as to attain the required level of quality. This leads to increasing, the size of the installations. The invention on the contrary makes it possible to produce water of 5 the requisite quality especially by means of a high-speed granular filter that does not require the use of upstream separation means.such as for example a decanter or flotation means. Its implementation therefore reduces the total size and especially the ground surface area occupied by a desalination installation. The technique of the invention also enables a reduction in the costs related 10 to water desalination. Firstly, the installations needed for desalination according to the invention are less complex, more compact and therefore less costly. Secondly, the technique of the invention reduces the frequency of washing and replacing the reverse osmosis membranes. Reducing the washing frequency of reverse osmosis membranes further 15 reduces losses in water 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 gradual maturation of a filter according to the invention, which is characterized by the fact that it collects flocs within the filter mass during the filtration, maintains the 20 adsorption kinetics of the clogging matter (and therefore of the SDI) even if the initially injected dose of coagulant is reduced. This gradual reduction of the quantity of coagulant injected during a filtration cycle reduces the total consumption of reagents. 7.7. Trials 25 Trials were made in order to demonstrate the efficiency of a water treatment process according to the invention. A first series of trials consisted of the filtering, according to the prior art, of a water in a filtration column containing a layer of 0.8 meters of anthracite and a layer of 0.8 meters of sand at a speed of 7.5 m/hr and then in a 5-micrometer 18 filter cartridge. Figure 3 which illustrates the results of these trials shows that the SDI1 5 of the water filtered during this trial was generally greater than 3.5 A second series of trials consisted of the filtering, according to the prior art, of the water from a first station (St 1) and a second station (St 2) in a filtration 5 column containing a layer of 1.5 of meters of anthracite and a layer of 1.5 meters of sand at a speed of 9.5 m/hr and then in a 5-micrometer filter cartridge. Figure 4 which illustrates the results of these trials shows that the SDI, 5 of the water filtered during these trials was on average between 4 and 5. A third series of trials consisted of the filtering, according to the invention, 10 of 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/hr in a 5-micrometer filter cartridge. Figure 5 which illustrates the results of these trials shows that the SD 1 5 of the water filtered during these trials was always below 3.5.

Claims

1. Method for treating water for its desalination, said method consisting of: - a step of coagulation (i) of said water; a step (ii) of flocculation of the water coming from said step of coagulation (i); 5 - a step (iii) of granular filtration of the water coming directly from said step (ii) of flocculation through at least one granular filter comprising a filter mass constituted at least by a layer of at least one filter material; - a step (iv) for the filtering, with a cut-off point ranging from 10 nanometers to 10 micrometers, of the water coming from said granular 10 filtration step (iii); - a step (v) for the filtering, by reverse osmosis, of the water coming from said step (iv) for filtering with a cut-off point ranging from 10 nanometers to 10 micrometers; - a step (vi) for recovering at least partly desalinated water coming from said 15 step (iv) of reverse osmosis filtration; said step (iii) of granular filtration consisting in conveying water coming from said flocculation step (ii) via said granular filter at a speed ranging from 15 m/hr to 25 m/hr.

2. Method according to claim 1, characterised in that said step (iii) of 20 granular filtration implements a granular filter for which the filter mass has a decreasing grain size. 20

3. Method according to claim 1 or 2, characterised in that it further consisted of a screening step prior to said step (i) of coagulation.

4. Method according to claim 3, characterised in that said screening step is performed with a cut-off point between 50 and 500 micrometres.

5 5. Method according to any of claims I to 4, characterised in that said step (iv) for the filtering, with a cut-off point between 10 nanometres and 10 micrometres, uses a filter cartridge or an ultrafiltration unit or a microfiltration unit.

6. Installation for the implementation of a method for treating water for its 10 desalination according to any of claims 1 to 5, said installation consisting of: - a coagulation zone (11) for said water; - a flocculation zone (13) for the water obtained from said coagulation zone (11); - a granular filter (15) comprising a filter mass (16, 16') consisting of at 15 least one layer of at least one filter material; - means for extracting said water directly obtained from said flocculation zone (13) via said granular filter (15), said means for extracting being able to flow said water obtained from said flocculation zone (13) through said granular filter (15) at a speed between 15 and 25 m/hr; 20 - means for the filtering, with a cut-off point of 10 nanometres to 10 micrometres, the water obtained from said granular filter (15), and - a reverse osmosis filtration unit (19) for the water obtained from said means for filtering with a cut-off point between 10 nanometres to 10 micrometres.

7. Installation according to claim 6, characterised in that said granular 25 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 materials having decreasing grain-size.

8. Installation according to claim 7, characterised in that the height of said first material (17) is 50 to 80% of the total height of said filter mass (16, 16'). 30

9. Installation according to claim 6, characterised 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 grain-size.

10. Installation according to claim 9, characterised in that the height of 35 said first (16'), second (17') and third (22) materials represent between 40 and 21 75%, between 7.5 and 40% and 7.5 and 20% of the total height of said filter mass (16'), respectively.

11. Installation according to any of claims 6 to 10, characterised in that the grains of said materials (17, 18, 17', 18', 22) of said layers of said granular 5 filter (15) have increasing densities from the top layer to the bottom layer of said filter.

12. Installation according to any of claims 6 to 11, characterised in that it further consists of a screening unit positioned upstream from said coagulation zone (11). 10

13. Installation according to any of claims 6 to 12, characterised in that said filtration means with a cut-off point between 10 nanometres and 10 micrometres belong to the group comprising: - filter cartridges (24); - ultrafiltration filtration units; 15 - microfiltration filtration units.