Pre-treatment unit of a reverse-osmosis desalination plant and associated pre-treatment method

Abstract

The present invention relates to a unit (14) for pre-treatment of raw water (12), the pre-treatment unit (14) being intended to be fluidically connected to a reverse-osmosis desalination unit (16), the pre-treatment unit (14) comprising at least: - at least one biological filtration device (17) comprising a downflow biological filter (18) including a single bed of granular material (24) intended to filter the raw water (12) so as to produce biologically treated water (26); and - at least one first series (20) of self-cleaning filters (22) which are fluidically connected to the biological filtration device (17) downstream of the biological filtration device (17), and are intended to filter the biologically treated water (26) so as to produce pre-treated water (23), the first series (20) of self-cleaning filters (22) being intended to be fluidically connected upstream of the reverse-osmosis desalination unit (16).

Description

Pretreatment unit of a reverse osmosis desalination plant and associated pretreatment process

[0001] The present invention relates to a pretreatment unit of a reverse osmosis desalination plant for raw water.

[0002] Raw water is typically sea water.

[0003] Reverse osmosis membranes used for seawater desalination are particularly vulnerable to fouling caused by the presence of silts or colloids in the seawater.

[0004] It is known to pretreat seawater first by coagulation, for example with ferric chloride, and flocculation, with a coagulation aid, and then filter it with a granular filter comprising a filter bed comprising at least one layer of granular material. Before passing through the membranes, the seawater passes through safety filters which protect the membranes.

[0005] This type of pretreatment unit has a large footprint.

[0006] Furthermore, to be effective over time, this type of pretreatment requires the use of a large quantity of chemical reagent. This poses environmental problems linked to the polluting nature of these chemical reagents and, on the other hand, results in high operating costs for the desalination plant.

[0007] This type of pretreatment also requires regular replacement of cartridge filter cartridges, which also results in replacement, labor, and waste management costs over time.

[0008] One aim of the invention is to propose a pretreatment unit that is easy to implement, compact and has a reduced environmental footprint and operating and maintenance costs.

[0009] To this end, the invention relates to a unit for pre-treating raw water, said pre-treatment unit being intended to be fluidically connected to a reverse osmosis desalination unit, said pre-treatment unit comprising at least:

[0010] - at least one biological filtration device comprising a downflow biological filter comprising a single bed of granular material intended to filter raw water to produce biologically treated water, and

[0011] - at least a first series of self-cleaning filters fluidly connected to the biological filtration device, downstream of said biological filtration device, and intended to filter the biologically treated water to produce pre-treated water, the first series of self-cleaning filters being intended to be fluidly connected upstream of the reverse osmosis desalination unit.

[0012] Thus, the pretreatment unit according to the invention makes it possible to eliminate the consumption of chemical reagent and filter cartridges while allowing efficient pretreatment of raw water.

[0013] The self-cleaning filter plays a dual role: it refines the water that has passed through the biological filter, depending on the quality of the raw water, and protects the membranes. Thus, the footprint of filter sets with self-cleaning filters is reduced compared to that of a granular filter with cartridge filters used in state-of-the-art pretreatment.

[0014] Using a single-layer biological filter avoids potential reclassification issues. Reclassifying filter media types when backwashing a filter can lead to potential problems such as degradation of the filtration system's efficiency and capacity, uneven particle distribution affecting the filter's ability to retain particles uniformly, and complications in filter operation.

[0015] The pretreatment unit according to the invention may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

[0016] - the granular media bed has a height of between 1.5 m and 4 m;

[0017] - the granular material comprises expanded clay, activated carbon, ceramic or zeolite;

[0018] - the effective size of the granular material is between 1.5 mm and 5 mm, the porosity of the granular material being between 0.35 and 0.6;

[0019] - the self-cleaning filters of the first series of self-cleaning filters have the same first filtration threshold between 5 µm and 100 µm;

[0020] - the pretreatment unit further comprises at least a second series of self-cleaning filters fluidly connected to the first series of self-cleaning filters, upstream of said first series of self-cleaning filters, the self-cleaning filters of the second series of self-cleaning filters having the same second filtration threshold of between 50 µm and 300 µm, said second filtration threshold being greater than the first filtration threshold; and

[0021] - the pretreatment unit further comprises a pressurized downflow granular filter comprising a single bed of granular material, said granular material having an effective size smaller than the effective size of the granular material of the biological filter, said granular filter being arranged downstream of the biological filtration device and upstream of the first series of self-cleaning filters.

[0022] The invention also relates to a raw water desalination plant comprising:

[0023] - a pretreatment unit as described above,

[0024] - a reverse osmosis desalination unit fluidically connected downstream of the pretreatment unit.

[0025] The invention also relates to a method for pretreating raw water intended to be desalinated by reverse osmosis, said method comprising the following steps:

[0026] - a step of biological filtration of raw water with a biological filtration device comprising a downward flow biological filter comprising a bed of granular material to produce biologically filtered water,

[0027] - at least one stage of filtration of the biologically filtered water with at least a first series of self-cleaning filters to produce pre-treated water.

[0028] According to a particular embodiment, the method according to the invention may comprise the following characteristic:

[0029] - the method further comprises an intermediate filtration step with a pressurized downflow granular filter comprising a single bed of granular material, said granular material having an effective size smaller than the effective size of the granular material of the biological filter, the intermediate filtration step being carried out between the biological filtration step and the filtration step with the at least one first series of self-cleaning filters.

[0030] The invention will be better understood by reading the following description, given solely by way of example, and made with reference to the appended drawings in which:

[0031] -is a partial schematic view of a desalination installation according to a first embodiment of the invention;

[0032] -is a partial schematic view of a desalination plant according to a second embodiment of the invention; and

[0033] -is a partial schematic view of a desalination installation according to a third embodiment of the invention.

[0034] Illustrates a desalination installation 10 for raw water 12 according to a first embodiment of the invention.

[0035] The desalination plant 10 comprises a pretreatment unit 14 and a reverse osmosis desalination unit 16 fluidically connected downstream of the pretreatment unit 14.

[0036] In the remainder of the description, the terms “upstream” and “downstream” are used with reference to the direction of circulation of the raw water to be pretreated in the desalination plant 10, i.e. from its collection point, i.e. a direct intake from the sea or a coastal well, to the desalination unit 16.

[0037] The desalination unit 16 preferably comprises a plurality of reverse osmosis modules (not shown) arranged in racks. Each reverse osmosis module comprises a plurality of semi-permeable reverse osmosis membranes which make it possible to separate a salt-concentrated retentate from a salt-depleted permeate. This type of desalination unit 16 is known from the prior art and will not be described in detail in the present application.

[0038] According to the invention, the pretreatment unit 14 is intended to pretreat raw water 12, typically seawater, for example captured by direct intake from the sea or from a coastal well. The pretreatment unit 14 is fluidically connected upstream of the desalination unit 16.

[0039] The raw water 12 typically comprises suspended matter and colloidal particles which must be filtered with the pretreatment unit 14 to prevent fouling of the reverse osmosis membranes of the desalination unit 16.

[0040] According to the invention, the pretreatment unit 14 comprises at least one biological filtration device 17 comprising at least one biological filter 18. The pretreatment unit 14 further comprises at least a first series 20 of self-cleaning filters 22 fluidically connected downstream of the biological filtration device 17 to produce pre-treated water 23.

[0041] The biological filter 18 comprises a bed of granular material 24 for filtering the raw water 12 to produce biologically filtered water 26.

[0042] Biological filtration eliminates the need for chemical reagents. It involves using living organisms to capture and biologically degrade contaminants in raw water. 12

[0043] In particular, the biological filter 18 is capable of degrading the biodegradable fraction of the natural organic matter (“NOM” in English for “Natural Organic Matter”) by fixed cultures forming a biofilm on the granular material. This makes it possible to prevent bacterial development in the pipes of the desalination plant 10, downstream of the pretreatment unit 14. In addition, the biological filter 18 is also capable of retaining suspended matter by the circulation of the raw water 12 through the bed of granular material 24.

[0044] Advantageously, the biological filter 18 is configured to degrade the biodegradable fraction without injecting oxygen into the raw water 12 or into the bed of granular material 24. In other words, the biological filter 18 does not comprise an oxygen injection device capable of injecting oxygen into the bed of granular material 24 and / or into the raw water 12.

[0045] Indeed, the raw water 12, and in particular the sea water, comprises a quantity of oxygen in dissolved form sufficient to allow effective degradation of the biodegradable fraction present in the raw water 12.

[0046] According to the invention, the biological filter 18 is a downward flow filter, also called “downflow” in English, that is to say that the raw water 12 flows through the bed of granular material 24 from the top to the bottom. The terms “top” and “bottom” are defined in relation to a vertical direction.

[0047] In the example of the, the biological filter 18 is for example a gravity filter, that is to say that the raw water 12 flows through the bed of granular material 24 under the effect of gravity.

[0048] In this embodiment, the biological filtration device 17 further comprises a reservoir 25 for recovering the biologically filtered water 26, and a pump 27 configured to pressurize the biologically filtered water 26 to the first series 20 of self-cleaning filters 22.

[0049] The pump 27 is for example configured to apply an overpressure relative to atmospheric pressure of between 0.4 and 20 bar, in particular between 0.4 and 5 bar.

[0050] In filtration, the height of raw water above the bed of granular material 24 is for example between 1 m and 4 m.

[0051] Alternatively (not shown), the biological filter 18 is a pressure filter, i.e. the raw water 12 passes through the bed of granular material 24 under pressure. The overpressure applied relative to atmospheric pressure is generally between 0.4 and 20 bar, in particular between 0.4 and 5 bar. In such an embodiment, the biological filtration device 27 does not comprise a reservoir 25 or a pump 27 as described above.

[0052] According to the invention, the biological filter 18 comprises a single bed of granular material 24. In other words, the biological filter 18 is said to be single-layer.

[0053] This avoids problems related to the reclassification of granular materials from multi-layer filters which may occur at the end of each filter wash before re-filtration.

[0054] The height of the bed of granular material 24 is for example between 1.5 m and 4 m, preferably between 2.5 m and 4 m, advantageously between 3 m and 4 m.

[0055] A high height allows for a longer contact time between the raw water 12 and the biofilm formed on the bed of granular material 24, which improves the efficiency of the biological filter 18.

[0056] The granular material is advantageously chosen from a material comprising an expanded clay such as for example biolite® or filtralite®, a material comprising an activated carbon, a material comprising a ceramic such as for example macrolite® or a zeolite.

[0057] Advantageously, the porosity of the granular material is between 0.35 mm and 0.6 mm.

[0058] This allows to increase the accumulation of active biomass and the activity of the biomass, to retain more microbes on their surface and to increase the biological activity developed in the environment.

[0059] The effective grain size of the granular material is advantageously between 1.5 mm and 5 mm. The effective grain size corresponds to the mesh opening allowing 10% by weight of a sample subjected to analysis to pass through. In other words, 90% of the grains have a diameter greater than the effective size.

[0060] This makes it possible to limit the pressure drop of the biological filter 18 when it is in operation, also reducing the frequency of cleaning and consequently the consumption of cleaning water.

[0061] The filtration speed of the biological filter 18 is for example between 15 m / h and 50 m / h.

[0062] In the first embodiment, the pretreatment unit 14 comprises a single first series 20 of self-cleaning filters 22.

[0063] Preferably, the first series 20 of self-cleaning filters 22 comprises between 5 and 40 self-cleaning filters 22. The number of self-cleaning filters is chosen by a person skilled in the art depending on the capacity of the desalination plant.

[0064] The self-cleaning filters 22 are fluidically connected in parallel to the biological filter 18, downstream of said biological filter 18. Each self-cleaning filter 22 receives a portion of the biologically filtered water 26.

[0065] The quality of the biologically treated water 26 at the outlet of the biological filter 18 may vary over time, in particular because the effectiveness of the treatment depends on natural processes such as the production of extracellular polymeric substances by bacteria. In addition, the suspended matter content and the turbidity may also vary and be higher compared to a coagulation / flocculation treatment of the state of the art. Thus, the self-cleaning filters 22 make it possible to limit the impact of these variations in quality.

[0066] Preferably, the self-cleaning filters 22 of the first series 20 of self-cleaning filters 22 have the same first filtration threshold of between 5 µm and 100 µm.

[0067] Self-cleaning filters 22 are known to those skilled in the art. The article “SC Filter Cleaning Mechnaisms: A Review” by Shahane et al., 2019, International Journal of Research in Engineering, Science and Management, Vol. 2, Issue 3, describes examples of self-cleaning filters.

[0068] Each self-cleaning filter 22 is intended to remove particles, sediments and / or suspended solids from the biologically filtered water 26 continuously, without interruption of the flow.

[0069] The self-cleaning filter 22 is configured to clean itself automatically without the need for manual intervention.

[0070] The self-cleaning filter 22 comprises, for example, a first chamber, a second chamber, and a permeable filter element separating the first chamber and the second chamber.

[0071] The self-cleaning filter 22 further comprises a water inlet fluidly connected to the first chamber intended to supply the first chamber under pressure with water to be filtered, i.e. biologically filtered water 26 in the example of the, and a water outlet fluidly connected to the second chamber intended to extract the filtered water, i.e. the pre-treated water 23, from the second chamber.

[0072] Particles and suspended matter are filtered by the filter element and accumulate in the first chamber. The filter element is, for example, a sieve, such as a cylindrical sieve, filter discs or a filter candle.

[0073] The self-cleaning filter 22 further comprises a self-cleaning device. The self-cleaning device comprises, for example, a backwash system, a mechanical scraping system or a suction system.

[0074] A method for pretreating raw water 12 intended to be desalinated by reverse osmosis will now be described.

[0075] First, the method comprises a step of biological filtration of the raw water 12 with the biological filtration device 17 to produce biologically filtered water 26.

[0076] The biologically filtered water 26 is then filtered by at least a first series 20 of self-cleaning filters 22 arranged downstream of the biological filtration device 17 to produce pre-treated water 23.

[0077] The pre-treated water 23 is then intended to supply the desalination unit 16.

[0078] Illustrates a desalination plant 10 according to a second embodiment of the invention. This embodiment will be described by differences compared to the first embodiment.

[0079] In this embodiment, the pretreatment unit 10 further comprises at least one pressurized downflow granular filter 28 comprising a single bed of granular material 30. The granular filter 28 is typically a high-speed filter, i.e. the biologically treated water 26 is filtered at a speed greater than 12 m / h, preferably greater than or equal to 15 m / h.

[0080] The height of the granular material bed 30 is preferably between 0.2 m and 1 m, for example between 0.4 m and 1.0 m.

[0081] The overpressure relative to atmospheric pressure applied to the biologically treated water 26 in the granular filter 28 is generally between 0.4 bar and 20 bar, in particular between 0.4 bar and 5.0 bar.

[0082] The granular material 30 has an effective size less than the effective size of the granular material 24 of the biological filter 18, typically less than or equal to 1.0 mm, for example between 0.1 mm and 1.0 mm, preferably between 0.2 mm and 0.6 mm.

[0083] The granular material 30 comprises, for example, anthracite, pumice stone, expanded clay, activated carbon, zeolite, sand, glass beads, polymer beads or ceramic beads.

[0084] According to a particular embodiment, the granular filter 28 further comprises a support layer (not shown) arranged under the bed of granular material 30 which does not participate in the filtration. Such a support layer has the purpose of leveling the bottom of the filter 28, in particular by covering the piping. The materials used for the support layer generally have a particle size greater than that of the filter material, typically greater than 2 mm. For example, the material is gravel.

[0085] In this embodiment, the pretreatment method further comprises an intermediate filtration step with the granular filter 28. The intermediate filtration step is implemented between the filtration step and the filtration step(s) with the self-cleaning filters 22.

[0086] The use of the granular filter 28 depends in particular on the quality of the raw water 12 and in particular on its content of suspended matter and colloidal particles. In certain cases, the content of suspended matter and colloids may not be satisfactory at the outlet of the biological filter 18. The granular filter 28 then makes it possible to filter these suspended matters and colloids.

[0087] Illustrates a desalination plant 10 according to a third embodiment of the invention. This embodiment will be described by differences from the first embodiment.

[0088] In this embodiment, the pretreatment unit 14 comprises a second series 32 of self-cleaning filters 34. The second series 32 of self-cleaning filters 34 is fluidically connected to the biological filter 18, downstream of said biological filter 18, and connected to the first series 20 of self-cleaning filters 22, upstream of said first series 20 of self-cleaning filters 22.

[0089] Preferably, the self-cleaning filters 34 of the second series 32 of self-cleaning filters 34 have the same second filtration threshold of between 50 µm and 300 µm. The second filtration threshold is greater than the first filtration threshold.

[0090] The number of self-cleaning filters 34 of the second series 32 of self-cleaning filters 34 is chosen by a person skilled in the art depending on the capacity of the desalination plant. It is for example between 5 and 40 self-cleaning filters 34.

[0091] The pretreatment method then comprises a step of filtration of the biologically filtered water 26 with the self-cleaning filters 34 of the second series 32 of self-cleaning filters 34. This filtration step is carried out downstream of the biological filtration and upstream of the filtration step with the self-cleaning filters 22 of the first series 20 of self-cleaning filters 22.

[0092] Alternatively, the pretreatment unit 14 comprises a number of series 20, 32 of self-cleaning filters 22, 34 greater than two, for example three series 20, 32 of self-cleaning filters 22, 34. The series 22, 32 of self-cleaning filters 22, 34 are fluidically connected to each other, in series, downstream of the biological filter 18. The filtration thresholds of the self-cleaning filters 22, 34 of each of the series 20, 32 of filters are chosen so as to be decreasing from upstream to downstream.

[0093] As a further variant (not shown), the pretreatment unit 14 also comprises a granular filter 28 as described in the second embodiment.

Claims

Pretreatment unit (14) for raw water (12), said pretreatment unit (14) being intended to be fluidically connected to a reverse osmosis desalination unit (16), said pretreatment unit (14) comprising at least:- at least one biological filtration device (17) comprising a downflow biological filter (18) comprising a single bed of granular material (24) intended to filter the raw water (12) to produce biologically treated water (26), and- at least a first series (20) of self-cleaning filters (22) fluidically connected to the biological filtration device (17), downstream of said biological filtration device (17), and intended to filter the biologically treated water (26) to produce pre-treated water (23), the first series (20) of self-cleaning filters (22) being intended to be fluidically connected upstream of the reverse osmosis desalination unit (16). Pretreatment unit (14) according to claim 1, wherein the granular media bed (24) has a height of between 1.5 m and 4 m. A pretreatment unit (14) according to claim 1 or 2, wherein the granular material comprises an expanded clay, an activated carbon, a ceramic or a zeolite. Pretreatment unit (14) according to any one of claims 1 to 3, wherein the effective size of the granular material is between 1.5 mm and 5 mm, the porosity of the granular material being between 0.35 and 0.

6. Pretreatment unit (14) according to any one of claims 1 to 4, in which the self-cleaning filters of the first series of self-cleaning filters have the same first filtration threshold of between 5 µm and 100 µm. Pretreatment unit (14) according to claim 5, further comprising at least a second series (32) of self-cleaning filters (34) fluidly connected to the first series (20) of self-cleaning filters (22), upstream of said first series (20) of self-cleaning filters (22), the self-cleaning filters (34) of the second series (32) of self-cleaning filters (34) having the same second filtration threshold of between 50 µm and 300 µm, said second filtration threshold being greater than the first filtration threshold. A pretreatment unit (14) according to any one of claims 1 to 6, further comprising a pressurized downflow granular filter (28) comprising a single bed of granular material (30), said granular material having an effective size less than the effective size of the granular material of the biological filter (18), said granular filter being disposed downstream of the biological filtration device (17) and upstream of the first series (20) of self-cleaning filters (22). Desalination plant (10) for raw water (12) comprising:- a pretreatment unit (14) according to any one of claims 1 to 7,- a reverse osmosis desalination unit (16) fluidically connected downstream of the pretreatment unit (14). A method for pretreating raw water (12) intended to be desalinated by reverse osmosis, said method comprising the following steps:- a step of biological filtration of the raw water (12) with a biological filtration device (17) comprising a downward flow biological filter (18) comprising a bed of granular material (24) to produce biologically filtered water (26),- at least one step of filtration of the biologically filtered water (26) with at least a first series (20) of self-cleaning filters (22) to produce pre-treated water (23). The method of claim 9, further comprising an intermediate filtration step with a pressurized downflow granular filter (28) comprising a single bed of granular material (30), said granular material having an effective size smaller than the effective size of the granular material of the biological filter (18), the intermediate filtration step being carried out between the biological filtration step and the filtration step with the at least one first series (20) of self-cleaning filters (22).

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