Raw water treatment unit comprising a degassing device and a carbon dioxide recycling loop and associated treatment process

Abstract

Raw water treatment unit comprising a degassing device and a carbon dioxide recycling loop and associated treatment process. The present invention relates to a raw water (12) treatment unit (10) comprising: - at least one treatment device (14) for treating the raw water (12) to provide at least one first stream of treated water (16), - at least one degassing device (18) for degassing at least a portion of the carbon dioxide contained in the first stream of treated water (16), - at least one recycling loop (20) for the degassed carbon dioxide (22), said recycling loop (20) being configured to channel the degassed carbon dioxide (22) and to inject at least a portion of said degassed carbon dioxide (22) into the raw water (12) so as to acidify said raw water (12). Figure for the abstract: Figure 1

Description

Title of the invention: Raw water treatment unit comprising a degassing device and a carbon dioxide recycling loop and associated treatment process

[0001] The present invention relates to a raw water treatment unit.

[0002] Raw water is, for example, water to be made potable, urban or industrial effluent, or water to be desalinated such as seawater or brackish water.

[0003] Many water treatments, for example membrane device filtration or coagulation / flocculation treatment, require conditioning of the raw water, and in particular acidification of the raw water in order to adjust its pH to optimize the treatment.

[0004] The problem with advanced membrane treatments such as nanofiltration and reverse osmosis is that they concentrate mineral salts and are subject to significant fouling risks. The higher the conversion rate, i.e., the ratio between the volume / flow of water treated by filtration and the volume / flow of water feeding the membrane, the more concentrated the salts rejected by the membranes become, and the greater the risk of exceeding saturation limits and thus precipitating within the membrane module. To delay precipitation, inhibitors, called sequestrants or "antiscalants," are systematically injected into the raw water to be treated, but sometimes this must be accompanied by acidification of the water to be treated, particularly in the case of more highly mineralized waters and / or systems operating with high conversion rates.Similarly, organic substrates are also concentrated, thus promoting the development of biofilm: acidification then inhibits bacterial growth.

[0005] The acids commonly used are mineral acids such as hydrochloric acid and sulfuric acid.

[0006] Sulfuric acid is generally preferred because its commercial form is more concentrated and requires smaller storage tanks. However, the sulfate ions it carries also concentrate on the membrane and can lead to sulfate-based precipitation, such as calcium sulfate. Hydrochloric acid does not lead to precipitation risks due to the chloride ions it carries, although these are also concentrated at the membrane. Both hydrochloric and sulfuric acids remain corrosive, and the chloride and sulfate ions they carry increase the corrosivity of the concentrates with which they are discharged.

[0007] In coagulation / flocculation treatments followed by clarification, acidification of the raw water is often necessary for several reasons. The optimal pH for coagulation / flocculation varies depending on the reagents used. The addition of acid allows the pH to be regulated to enable optimal floc formation.

[0008] Some coagulation reagents, such as metal salts, require an acidic environment for maximum effectiveness.

[0009] Finally, the addition of acid also helps to stabilize the water to be treated, preventing the premature precipitation of the chemicals used in the treatment.

[0010] A strong acid such as hydrochloric acid or sulfuric acid is generally used because it allows the pH of the water to be treated to be adjusted effectively.

[0011] Mineral acids conventionally used in water treatment are very effective but generate inconveniences.

[0012] First of all, they constitute significant operating expenses.

[0013] In addition, the production of these acids and their transport to the processing sites are sources of greenhouse gas emissions.

[0014] The impact of the use of these acids on the environment can also be significant, particularly in the case of treatments by membrane devices which generate discharges highly concentrated in chlorides or sulfates.

[0015] The risks of corrosion in treatment facilities induced by the use of these acids are significant.

[0016] Finally, the handling of these acids by an operator on an installation is not without risk and requires high vigilance.

[0017] One object of the invention is to provide a raw water treatment unit which makes it possible to significantly reduce the impact on the environment, to reduce operating expenses related to the use of mineral acids, to limit the risks associated with the handling of these substances while maintaining operational efficiency.

[0018] To this end, the invention relates to a raw water treatment unit, said treatment unit comprising:

[0019] - at least one treatment device intended to treat raw water to supply the minus an initial flow of treated water,

[0020] - at least one degassing device intended to degas at least part of the carbon dioxide contained in the first treated water stream,

[0021] - at least one loop for recycling the degassed carbon dioxide, said loop of recycling being configured to channel the degassed carbon dioxide and to inject at least a portion of said degassed carbon dioxide into the raw water so as to acidify said raw water.

[0022] Thus, the treatment unit enables the in-situ production of a weak acid, carbon dioxide, as a replacement for conventional strong acids, by recovering at least part of it downstream of the treatment so that it can be reinjected upstream. This reduces the environmental impact, the risk of corrosion of the installations, and the risks associated with handling conventional strong acids.

[0023] The pretreatment unit according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0024] - the recycling loop includes a purification device configured for purify the outgassed carbon dioxide;

[0025] - the recycling loop includes a compression device configured for compress the degassed carbon dioxide;

[0026] - the degassing device includes a cascade or a degassing tower;

[0027] - the treatment device is a membrane device configured to filter water raw and provide a concentrate and a permeate, the first treated water stream being said concentrate or said permeate; and

[0028] - the treatment device is a coagulation-flocculation-clarification device configure to treat raw water and provide clarified water, the first stream of treated water being said clarified water.

[0029] The invention also relates to a process for treating raw water, said process comprising the following steps:

[0030] - treat the raw water to provide at least a first stream of treated water with a treatment device,

[0031] - to degas at least some of the carbon dioxide contained in the first stream treated water with a degassing device,

[0032] - channel the degassed carbon dioxide and inject at least a part of said carbon dioxide is degassed into the raw water in such a way as to acidify said raw water.

[0033] The method according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0034] - the process further comprises an additional step of injecting acid into raw water;

[0035] - the raw water treatment step includes a filtration step by a membrane device for providing a concentrate and a permeate, the first treated water stream being said concentrate or said permeate; and

[0036] - the raw water treatment step includes a treatment step by a coagulation-flocculation-clarification device to provide clarified water, the first stream of treated water being said clarified water.

[0037] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the accompanying drawings, among which: - [Fig.1] [Fig.1] is a schematic view of a raw water treatment unit according to a first embodiment of the invention; - [Fig. 2] [Fig. 2] is a partial schematic view of a raw water treatment unit according to a second embodiment of the invention; and - [Fig.3] [Fig.3] is a partial schematic view of a raw water treatment unit according to a third embodiment of the invention.

[0038] Fig. 1 schematically illustrates a raw water treatment unit 10 12 according to a first embodiment of the invention.

[0039] According to the invention, the treatment unit 10 comprises at least one treatment device 14 for treating the raw water 12 to provide at least one first stream of treated water 16, at least one degassing device 18 for degassing at least some of the carbon dioxide contained in the first stream of treated water, at least one recycling loop 20 for the degassed carbon dioxide 22.

[0040] In the example of [Fig.1], the treatment device 10 is a membrane device 24 configured to filter raw water 12 and provide a concentrate 26 and a permeate 28.

[0041] The membrane device 24 is, for example, a nanofiltration or reverse osmosis membrane device. These membrane devices 24 are known to those skilled in the art and will not be described in detail in this description.

[0042] Raw water 12 is for example water to be made potable, seawater or brackish water to be desalinated or water from a treatment plant intended for reuse.

[0043] In the example of [Fig.1], the first treated water flow 16 is formed by the permeate 28 produced by the membrane device 24.

[0044] The degassing device 18 is fluidically connected to the membrane device 24, downstream of said membrane device 24. The degassing device 18 is intended to degas the carbon dioxide contained in the permeate 28 and to provide a flow of degassed permeate 30.

[0045] The degassing device 18 includes a first inlet 32 ​​intended to receive the first stream of treated water 16, in particular the permeate 28 in the example of [Fig.1], a first outlet 34 intended for the extraction of the gas and in particular carbon dioxide, and a second outlet 36 intended for the extraction of a first stream of degassed treated water 38.

[0046] The degassing device 18 includes, for example, a cascade 40 known from the prior art, as schematically represented in [Fig. 1]. A cascade 40 allows for the aeration of the first treated water stream 16. A cascade 40 comprises a plurality of steps arranged in a stepped fashion. The steps are arranged so that the first treated water stream 16, the permeate 28 in the example of [Fig. 1], can flow continuously over each of them. The water to be degassed is introduced upstream of the cascade 40 and is then distributed uniformly over the first step, where it begins to descend. The falling water creates mechanical agitation, which promotes contact between the water and the air, thus increasing the transfer of carbon dioxide from the water to the air.

[0047] Alternatively, the degassing device 18 comprises a degassing column known from the prior art. The water to be degassed is introduced at the top of the degassing column and flows down through it. The degassing column is configured to promote a regular and uniform flow of water throughout its entire height. As the water descends the column, it is exposed to air. This exposure allows carbon dioxide to be released as the water passes through the column.

[0048] For example, the degassing device 18 allows between 40% and 80% of the carbon dioxide contained in the permeate 28 to be degassed.

[0049] The recycling loop 20 is configured to channel the degassed carbon dioxide and to inject at least a part of said degassed carbon dioxide 22 into the raw water 12 so as to acidify said raw water 12.

[0050] The recycling loop 20 is fluidly connected to the first outlet 34 of the degassing device 18 and to the upstream of the treatment device 14.

[0051] To inject all or part of the carbon dioxide into the raw water 12 to be treated, the recycling loop 20 preferably includes a carbon dioxide injection device 42.

[0052] Carbon dioxide is a gas that dissolves in water according to Henry's Law, and the amount of dissolved gas increases with the applied pressure. In water, dissolved carbon dioxide partially hydrates to form carbonic acid (H₂CO₃). Dissolved carbon dioxide and carbonic acid together form free carbon dioxide. Carbonic acid is an acid because it is capable of releasing protons (H⁺) by partially dissociating into bicarbonate ions (HCO₃⁻), which in turn can release a proton by partially dissociating into carbonates (CO₃²⁻). These dissociations are governed by carbonic equilibria.

[0053] Unlike mineral acids, carbon dioxide does not bring corrosive counter-ions (chlorides or sulfates) into the water and the bicarbonate and carbonate ions produced have, on the contrary, an inhibitory action with regard to the corrosive character of the water.

[0054] Unlike strong acids, which are completely dissociated in water in ionic form with H+ protons and chloride or sulfate counter-ions that are retained by the membrane and concentrated in the discharge, carbonic acid is a weak acid that is not completely dissociated, and only carbonate and bicarbonate ions are retained by the membrane. Dissolved carbon dioxide and carbonic acid are not retained by the membrane and are found in the permeate and concentrate at concentrations substantially equivalent to those of the water entering the membranes. The resulting permeate and concentrate therefore contain approximately as much free carbon dioxide as the acidified raw water.

[0055] For example, the device 42 includes a device for injecting carbon dioxide under pressure into the raw water. The recycling loop 20 then preferably includes a carbon dioxide compression system. The carbon dioxide injection device 42 includes, for example, one or more nozzles adapted for injecting carbon dioxide under pressure into all of the raw water 12 or a device for producing seltzer water from a fraction of the raw water or from clean water. The seltzer water is then mixed with the remaining raw water or with all of the raw water when clean water is used.

[0056] Alternatively, the injection device 42 includes a column reactor. The column reactor is a device designed to allow prolonged contact between carbon dioxide and the water to be treated. Carbon dioxide is injected at the base of the column, while the water to be treated is introduced at the top. As the water descends through the column, it is exposed to carbon dioxide, thus promoting the dissolution of carbon dioxide in the raw water 12.

[0057] Alternatively, the injection device 42 includes a membrane system configured to dissolve carbon dioxide in raw water 12. Carbon dioxide is injected on one side of the membrane, while raw water 12 flows on the other side. The membrane allows the carbon dioxide molecules to pass through and dissolve in the raw water 12.

[0058] Advantageously, the recycling loop 20 includes a purification device configured to purify the degassed carbon dioxide stream 22.

[0059] In the example of [Fig. 1], the treatment unit 10 further includes a supplementary acid injection device 44. The supplementary acid injection device is configured to inject acid into the raw water to be treated.

[0060] Advantageously, the acid is carbon dioxide.

[0061] Alternatively, the acid is a mineral acid such as hydrochloric acid or sulfuric acid.

[0062] The free carbon dioxide content in the acidified raw water 46 is for example between 10 mg / L and 100 mg / L.

[0063] Preferably, in the example of [Fig.1], the treatment unit 10 further includes a device for injecting sequestrant 48 into the raw water to be treated 12. The sequestrant is used to prevent the deposition of minerals on the membrane, which could reduce its effectiveness or damage it.

[0064] Alternatively (not shown), the treatment unit 10 includes a bypass loop (not shown) configured to inject at least some of the degassed carbon dioxide 22 downstream of the degassing device 18 to perform a treatment requiring acidification.

[0065] A process for treating raw water 12, associated with the first embodiment, will now be described.

[0066] The process first includes a raw water treatment step 12 to provide at least a first stream of treated water 16 with the treatment device 14. In the example of [Fig.1], the raw water treatment step 12 includes a filtration step by a membrane device 24 to provide a concentrate 26 and a permeate 28.

[0067] The process then includes a step of degassing at least some of the carbon dioxide contained in the first treated water stream, using the degassing device. The first treated water stream consists of the permeate.

[0068] The process includes a step of channeling the degassed carbon dioxide 22 and a step of injecting at least a part of said degassed carbon dioxide 22 into the raw water 12 so as to acidify the raw water 12.

[0069] According to a particular embodiment, the process further comprises an additional step of injecting acid into the raw water 12. The acid is advantageously carbon dioxide. Alternatively, the acid is a mineral acid, such as sulfuric acid or hydrochloric acid.

[0070] According to a particular embodiment, the process includes injecting a sequestrant into the water to be treated.

[0071] Alternatively, the process includes injecting at least some of the degassed carbon dioxide 22 downstream of the degassing device 18.

[0072] Figure 2 illustrates a second embodiment of the invention. This embodiment is identical to the first embodiment, except that the first treated water flow 16 is formed by the concentrate 26 produced by the membrane device 24.

[0073] Figure 3 illustrates a third embodiment according to the invention. This embodiment will be described by its differences from the first embodiment.

[0074] In the example of [Fig. 3], the treatment device 14 is a coagulation-flocculation-clarification device 50 configured to coagulate and flocculate and then clarify the raw water 12 and provide clarified water 52. It is understood that the same Device 50 may perform the various treatments, or it may comprise a plurality of sub-devices to perform all or part of one of the treatments. For example, the coagulation-flocculation-clarification device 50 is configured to inject a dose of coagulant and a dose of flocculant into the raw water to be treated, successively during the treatment process.

[0075] Alternatively, the coagulation-flocculation-clarification device 50 is configured to inject only a dose of coagulant into the raw water to be treated. Indeed, the addition of a dose of flocculant is not always necessary for flocculation to occur.

[0076] According to a particular embodiment, the clarification step includes at least one filtration step.

[0077] These treatments are known to those skilled in the art and will not be described in detail in this description.

[0078] Raw water 12 is, for example, water to be made potable, water from a treatment plant intended for reuse, or seawater to be pre-treated.

[0079] In the example of [Fig.3], the first treated water stream 16 is formed by the clarified water 52 produced by the coagulation-flocculation-clarification device 50.

[0080] The degassing device 18 is fluidly connected to the coagulation-flocculation-clarification device 50, downstream of said coagulation-flocculation-clarification device 50. The degassing device 18 is intended to degas the carbon dioxide contained in the clarified water 52 and to provide a flow of degassed clarified water.

[0081] As in the first and second embodiments, according to a particular embodiment, the treatment unit 10 further includes a complementary acid injection device 44. The complementary acid injection device 44 is configured to inject acid into the raw water 12 to be treated.

[0082] Advantageously, the acid is carbon dioxide.

[0083] Alternatively, the acid is a mineral acid such as hydrochloric acid or sulfuric acid.

[0084] The free carbon dioxide content in the acidified raw water 46 is for example between 10 mg / L and 80 mg / L.

[0085] In the embodiment of [Fig.3], the treatment unit 10 does not include a device for injecting sequestrant 48 into the raw water 12 to be treated.

[0086] Degassing carbon dioxide from clarified water 52 can be particularly advantageous because it allows limiting or even eliminating the injection of alkaline reagent needed downstream of the coagulation-flocculation-clarification device 50 to bring the clarified water 52 back to its equilibrium pH, i.e. to restore the calcium-carbonate balance.

[0087] A process for treating raw water 12, associated with the embodiment of [Fig.3], will now be described.

[0088] The process first comprises a raw water treatment step 12 to provide at least a first stream of treated water 16 with the treatment device 14. In the example of [Fig. 3], the raw water treatment step 12 comprises successive coagulation-flocculation-clarification substeps to provide a stream of clarified water 52. In particular, the coagulation step includes the injection of a coagulant into the raw water 12 to be treated. The flocculation substep optionally includes the injection of a flocculant into the raw water 12 to be treated.

[0089] According to a particular embodiment, the clarification substep includes at least one filtration step.

[0090] The process then includes a step of degassing at least some of the carbon dioxide contained in the first treated water stream 16, using the degassing device 18. The first treated water stream 16 is formed by the clarified water stream 52.

[0091] The process includes a step of channeling the degassed carbon dioxide 22 and a step of injecting at least a part of said degassed carbon dioxide 22 into the raw water 12 so as to acidify the raw water 12 to be treated.

[0092] According to a particular embodiment, the process further comprises an additional step of injecting acid into the raw water 12, preferably before the injection of the coagulant and any flocculant. The acid is advantageously carbon dioxide. Alternatively, the acid is a mineral acid, such as sulfuric acid or hydrochloric acid.

[0093] Alternatively, as shown in [Fig.3], the process includes injecting at least some of the degassed carbon dioxide 22 downstream of the degassing device 18, for example for downstream treatment 54 requiring acidification.

Claims

Demands

1. Raw water (12) treatment unit (10), said treatment unit (10) comprising: - at least one treatment device (14) for treating the raw water (12) to provide at least one first stream of treated water (16), - at least one degassing device (18) for degassing at least some of the carbon dioxide contained in the first stream of treated water (16), - at least one recycling loop (20) for the degassed carbon dioxide (22), said recycling loop (20) being configured to channel the degassed carbon dioxide (22) and to inject at least some of said degassed carbon dioxide (22) into the raw water (12) so as to acidify said raw water (12).

2. Processing unit (10) according to claim 1, wherein the recycling loop (20) includes a purification device configured to purify the outgassed carbon dioxide (22).

3. Processing unit (10) according to claim 1 or 2, wherein the recycling loop (20) includes a compression device configured to compress the degassed carbon dioxide (22).

4. Processing unit (10) according to any one of claims 1 to 3, wherein the degassing device (18) comprises a cascade (40) or a degassing tower.

5. Processing unit (10) according to any one of claims 1 to 4, wherein the processing device (14) is a membrane device (24) configured to filter raw water (12) and provide a concentrate (26) and a permeate (28), the first stream of treated water (16) being said concentrate (26) or said permeate (28).

6. Processing unit (10) according to any one of claims 1 to 4, wherein the processing device (14) is a coagulation-flocculation-clarification device (50) configured to treat raw water (12) and provide clarified water (52), the first stream of treated water (16) being said clarified water (52).

7. A process for treating raw water (12), said process comprising the following steps: - treating the raw water (12) to provide at least a first stream of treated water (16) with a treatment device (14), - degas at least part of the carbon dioxide contained in the first treated water stream (16) with a degassing device (18), - channel the degassed carbon dioxide (22) and inject at least part of said degassed carbon dioxide (22) into the raw water (12) so as to acidify said raw water (12).

8. A treatment process according to claim 7, further comprising a supplementary step of injecting acid into the raw water (12).

9. A treatment process according to claim 7 or 8, wherein the raw water treatment step (12) comprises a filtration step by a membrane device (24) to provide a concentrate (26) and a permeate (28), the first stream of treated water (16) being said concentrate (26) or said permeate (28).

10. A treatment process according to claim 7 or 8, wherein the raw water treatment step (12) comprises a treatment step by a coagulation-flocculation-clarification device (50) to provide clarified water (52), the first treated water stream (16) being said clarified water (52).

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