IL210718A

IL210718A - Reverse osmosis water treatment process that includes a decarbonation of a concentrate and a remineralization of a filtrate

Info

Publication number: IL210718A
Application number: IL210718A
Authority: IL
Original assignee: Veolia Water Solutions & Tech
Priority date: 2008-07-31
Filing date: 2011-01-18
Publication date: 2014-04-30
Also published as: EP2310327B1; AU2009275989A1; ES2398882T3; MA32584B1; FR2934584B1; WO2010012691A1; EP2310327A1; FR2934584A1; IL210718A0

Description

REVERSE OSMOSIS WATER TREATMENT PROCESS THAT INCLUDES A DECARBONATION OF A CONCENTRATE AND A RE MINERALIZATION OF A FILTRATE Veolia Water Solutions & Technologies Support C. 207435 REVERSE OSMOSIS WATER TREATMENT PROCESS THAT INCLUDES A DECARBONATION OF A CONCENTRATE AND A REMINERALI ZATION OF A FILTRATE Field of the invention The field of the invention is that of water treatment. More precisely, the invention relates to the treatment of water, particularly seawater, for the purpose of the desalination thereof.

Prior art The desalination of seawater is a practice currently implemented for the purpose of lowering the concentration of various salts dissolved in the water. To that end, it is known to filter salt-rich water through reverse osmosis-type membranes. This type of reverse osmosis filtration can be applied to any type of water containing salts,■ such as -seawater.

This type of reverse osmosis treatment proves to be particularly effective in that it enables the production of purified water (or a filtrate) the salt concentration of which is considerably reduced. This is clearly apparent in the columns of Table 1, which shows the results of treating seawater by reverse osmosis, with a conversion rate equal to 52.8%. In reverse osmosis or nanofiltration, the conversion rate can be defined as being equal to the ratio (in %) between the permeate flux rate and the feed rate of the filtration unit .

Table 1: results of treating seawater by reverse osmosis with a conversion rate of 52.8%.

It can indeed be observed that the concentrations of various salts initially present in the seawater are considerably reduced in the desalinated water obtained after treatment by reverse osmosis. Depending on the species in question, these concentrations can reach values close to zero.

However, although very effective, desalination by reverse osmosis is not devoid of disadvantages.

Disadvantages of the prior art In particular, the desalination of water by reverse osmosis is not a selective treatment in that the water produced is depleted of numerous salts. However, although the desalination of water does indeed aim to reduce the salt concentration of the water, it more particularly aims to reduce the concentrations of certain salts so as to soften the water. In particular, the desalination of water aims to limit the sodium chloride concentration of the water so as to eliminate the salty seawater taste thereof and to render same suitable for consumption, or more simply to render same usable .

In other words, the treated water derived from the desalination process is depleted of certain salts such as bicarbonates , whereas the presence thereof proves to be necessary-1.

In order to mitigate this disadvantage, it is common practice to remineralize treated water with a view to enriching same with certain salts. Several techniques have been developed for this purpose.

A first remineralization technique consists in injecting carbon dioxide (CO2) and lime (CaO) into the treated water, which results in the formation of calcium bicarbonates (Ca(HC03)2) in the treated water.

A second remineralization technique consists in passing the treated water over a calcareous material after having injected an acid therein. The calcareous material used for this purpose, for example, can have characteristics such as those which appear in Table 2. In particular, it may be of land origin, or formed naturally by evaporation of sea salts on coastal sites.

Apparent density > 1.02 Table 2: Exemplary characteristics of a conventional calcareous material The injection of carbon dioxide (C02) and the flow of water over a calcareous material is accompanied by the following reaction: CaC03 + C02 + H20 ? Ca(HC03)2 The injection of sulfuric acid (H2SO¾)and the flow of water over a calcareous material is accompanied by the following reaction: 2CaC03 + H2S04 ? Ca(HC03)2 + CaS04 The injection of hydrochloric acid (HC1) and the flow of water over a calcareous material is accompanied by the following reaction: 2CaC03 + 2HC1 ? Ca(HC03)2 + CaCl2 These remineralizataion techniques are particularly effective in that they enable the treated water to be enriched with calcium and bicarbonates . However, they have a few disadvantages.

Among these disadvantages is the fact that the calcareous material used according to the second technique for remineralizing treated water is consumed during the remineralization reaction. In this sense, it comprises a consumable material which must be continuously replenished. This requires the site on which the water is treated to be regularly supplied with this consumable material.

This constraint has significant repercussions of a logistic and economic nature in particular.

Besides the aspects related to the supply of consumables, the desalination of water by reverse osmosis is accompanied by the production and discharge of concentrates into a natural environment.

When derived from the implementation of both the first technique and the second technique, these concentrates have very high concentrations of salts initially present in the water being treated. This appears in the right-hand column of Table 1 in which the concentrations of the various salts present in these concentrates appear. In particular, it can be observed that these concentrates are particularly rich in calcium and magnesium ions and in bicarbonates .

Due to the high salt concentrations of these concentrates, the rejection of same into the natural environment is not impact neutral, but can, however, have a considerable impact on the environment.

In particular, during desalination of seawater, these concentrates are discarded into the sea. This brings about a sudden localized change in the salinity of the water. Such being the case, it has been observed that, even though certain animal or plant species are not affected by these sudden modifications in their environment, others are particular sensitive to same.

Objectives of the invention In particular, the objective of the invention is to mitigate these disadvantages of the prior art.

More precisely, one objective of the invention is to provide a water desalination technique which includes a phase for remineralizing the treated water, the implementation of which results in limiting the supplies of consumables required for the remineralization process.

The invention likewise pursues the objective of enabling a particularly effective remineralization of desalinated water.

Another objective of the invention is to implement such a desalination technique the implementation of which has only a limited impact on the environment, at the very least in comparison with the techniques of the prior art.

The invention also has the objective of providing such a technique which is simple to implement and which is effective and economical.

Disclosure of the invention These objectives as well as others, which will become apparent hereinbelow, are achieved by a process for treating water containing at least calcium and/or magnesium .salts through reverse osmosis-type membranes, said process comprising at least one step for collecting at least partially desalinated water, a step for collecting a concentrate coming from said membranes and containing bicarbonates , a step for injecting C02 or an acid into said at least partially desalinated water, and a step for remineralizing said at least partially desalinated water in a remineralization reactor, characterized in that said process includes: - a step for decarbonating said concentrate so as to form carbonates, and - a step for recycling at least some of said carbonates in said remineralization reactor.

The invention is based on the implementation of a step for decarbonating the concentrate derived from the reverse osmosis treatment, for the purpose of producing carbonates. These carbonates are then recycled inside the remineralization reactor in which they react with the treated water into which C02 or acid has been previously injected so as to enrich the treated water with bicarbonates .

Implementation of the invention consists therefore in directly producing on the water treatment site the calcareous material required for remineralization of the treated water. Therefore, implementing a water treatment process according to the invention makes it possible to avoid continuously supplying the production site with a consumable calcareous material.

In addition, the fact of subjecting the concentrate derived from the reverse osmosis water treatment to decarbonat ion results in the discharge of a bicarbonate-depleted supernatant into the natural environment. This aspect of the invention therefore makes it possible to reduce the environmental impact- of the desalination process.

According to one advantageous characteristic, said water consists of seawater.

Implementation of the invention enables seawater to be effectively desalinated and remineralized so as to render same suitable for consumption, or more generally to render same usable.

Said decarbonation step is preferably of the catalytic type.

Catalytic decarbonation is a process which makes it possible to , effectively form carbonates from the concentrate derived from reverse osmosis of the water being treated.

In this case, said carbonates formed are preferably in the form of balls.

Catalytic decarbonation actually enables the formation of carbonate balls encasing a grain of sand. These balls have the advantage of being easily reusable as a particularly effective remineralizing material.

According to one advantageous aspect of the invention, said carbonates formed include calcium carbonate (CaCOs) .

Their use as a remineralizing material therefore enables the demineralized water to be calcium-enriched.

According to another advantageous aspect of the invention, said carbonates formed include magnesium carbonate (MgC03) .

Their use as a remineralizing material therefore enables the demineralized water to be magnesium-enriched.

It is of course possible to anticipate for different carbonates to be produced, so as to enrich the treated water with other mineral species.

According to the invention, said decarbonation step is preferably preceded by a step for injecting soda or lime into said concentrate.

These reactants actively participate in the formation of the carbonates.

Said lime or said soda is preferably injected in stoichiometric amounts with respect to the amount of bicarbonates to be precipitated, with a margin of 20 to 50% .

Preferably, said C02 or said acid is injected in stoichiometric amounts with respect to the amount of carbonates to be formed, with a margin of 20 to 50%.

Such proportions enable good results to be obtained in terms of carbonate formation and in terms of lowering the salt concentration of the supernatant which is rejected into the natural environment.

A treatment process according to the invention advantageously includes a step for rejecting a supernatant coming from said decarbonation step into the natural environment, said supernatant being salt-depleted .

Implementation of the invention therefore makes it possible to limit the environmental impact of water desalination .

List of the figures Other characteristics and advantages of the invention will become more apparent from the following description of a preferred embodiment, which is given for purely non-limiting and illustrative purposes, and from the appended drawings, in which: - figure 1 is a schematic representation of an embodiment of a installation intended for implementing a water treatment process according to the invention; - figure 2 shows an installation implemented when conducting tests carried out as part of the development of the present technique.

Description of an embodiment of the invention Reiteration of the principle of the invention The general principle of the invention relates to a process for desalinating water by reverse osmosis, which includes a step for remineralizing the resulting desalinated water in a reactor.

The invention is based on the implementation of a step for decarbonating the concentrate derived from the reverse osmosis treatment for the purpose of producing carbonates. These carbonates are then recycled inside the remineralization reactor in which they react with the treated water into which C02 or acid has been previously injected so as to enrich the treated water with bicarbonates .

Implementation of the invention therefore consists in directly producing on the water treatment site the calcareous material required for remineralization of the treated water. Therefore, implementing a water treatment process according to the invention makes it possible to avoid continuously supplying the production site with a consumable calcareous material.

In- addition, the fact of subjecting the concentrate derived from the reverse osmosis water treatment to decarbonation results in the rejection of a bicarbonate-depleted supernatant into the natural environment. This aspect of the invention therefore makes it possible to reduce the environmental impact of the desalination process.

Exemplary embodiment of an installation for implementing a water treatment process according to the invention In relation with figure 1, an installation is introduced, which is intended for implementing a water treatment process according to the invention.

As shown in this figure 1, such an installation includes means for feeding 10 salt-rich water, such as a pipeline, into a reverse osmosis filtration unit 11.

This reverse osmosis treatment unit 11 is connected to means for discharging 12 treated water which is at least partially salt-depleted, i.e., of which the salt concentrations that it initially contained are reduced. These discharge means 12 can assume the form of pipeline elements.

The means for discharging treated water 12 have an outlet which discharges into a remineralization reactor 13, which is itself connected to a pipeline for discharging 14 remineralized desalinated water.

Reactant-injecting means 15, such as injectors, are placed on the treated water discharge means 12 upstream from the remineralization reactor 13.

The treatment unit 11 is likewise connected to means for discharging 16 a concentrate derived from the reverse osmosis treatment of the water being treated.

These means for discharging 16 a concentrate, which can assume the form of pipeline elements, have an outlet which discharges into a decarbonation reactor 17.

Reactant-injecting means 18 are placed on the means for discharging 16 the concentrate upstream from the decarbonation reactor 17.

The decarbonation reactor 17 has means for discharging 19 a supernatant. It further has means for discharging carbonates 20. These means for discharging carbonates 20 have an outlet which is connected to the remineralization reactor 13.

Example of a water treatment process according to the invention with a view to calcium enrichment of the desalinated water A water treatment process according to the invention, for example, can be implemented in order to enrich desalinated water with calcium.

Water which is rich in salts, in particular calcium, e.g., such as seawater or any other type of water, is conveyed towards and injected into the reverse osmosis treatment unit 11 via pipeline 10.

The salt-rich water is then filtered within this unit 11 so that treated water which is at least partially depleted of certain salts is extracted from the unit 11 and flows through the pipeline 12 in the direction of the remineralization reactor 13.

Before the water is injected into the remineralization reactor 13, the injected means 15 are implemented so as to inject therein carbon dioxide (C02) or an acid, e.g., such as sulfuric acid (H2SO4) or hydrochloric acid (HCl) .

The concentrate derived from the reverse osmosis treatment of the salt-rich water is discharged from unit 11 by means of pipeline 16. This concentrate is particularly rich in calcium bicarbonates (Ca(HC03)2).

Injection means 18 are implemented so as to inject lime (CaO) or soda (NaOH) into this concentrate.

The mixture of the concentrate and lime or soda is then poured into the decarbonation reactor 17 for the purpose of undergoing a preferably catalytic type of decarbonation.

The principle of decarbonation is known by those skilled in the art. In this case, in particular, the catalytic decarbonation process developed and marketed by the applicant under the name of ACTINA' may be implemented.

Either of the following reactions then occurs within the decarbonation reactor, depending on whether lime or soda has been injected into the concentrate: Ca(OH)2 + Ca2+ + 2HC03" ? 2CaC037 + 2H20 NaOH + Ca2+ + 2HC03" ? CaC03? + H20 + Na+ + HC03" It can thus be observed that decarbonation of the concentrate results in the production of calcium carbonates (CaC03) . The calcium carbonates thus produced constitute a calcareous material which is advantageously in the form of balls.

A supernatant produced during decarbonation of the concentrate is discharged from the decarbonation reactor 17 into the natural environment, via pipeline 19. Compared to the concentrate, this supernatant is bicarbonate-depleted, such that the discharge thereof into the natural environment has less impact than that of direct disposal of the concentrate, as is the case in the techniques of the prior art.

The calcium carbonates produced during decarbonation of the concentrate are directed towards the remineralization reactor 13 with a view to being injected therein by means of pipeline 20.

One of the following reactions then occurs within the remineralization reactor 13, depending on the type of reactant previously injected into the treated water.

The injection of carbon dioxide (C02) into the water produces the following reaction: CaC03 + C02 + H20 ? Ca(HC03)2 The injection of sulfuric acid (H2SO4) into the water produces the following reaction: 2CaC03 + H2S04 ? Ca{HC03)2 + CaS04 The injection of hydrochloric acid (HCl) into the water produces the following reaction: 2CaC03 + 2HC1 ? Ca{HC03)2 + CaCl2 Within the scope of this embodiment, implementation of the invention therefore enables the desalinated water to be calcium enriched, while at the same time preventing the purchase and supply of consumable calcareous material, insofar as the latter is produced directly at the production site by recycling and reprocessing the residues derived from desalination of the water.

Example of a water treatment process according to the invention with a view to magnesium enrichment of the desalinated water In the same way as just described, a water treatment process according to the invention, for example, can be implemented for the purpose of enriching desalinated water with magnesium.

To accomplish this, water which is rich in salts, particularly magnesium, e.g., such as seawater or any other type of water, is conveyed towards and injected into the reverse osmosis treatment unit 11 via pipeline 10.

Before the water is injected into the remineralization reactor 13, the injected means 15 are implemented so as to inject therein carbon dioxide (C02) or an acid, e.g., such as sulfuric acid (H2SO4) or hydrochloric acid (HC1) .

The concentrate derived from the reverse osmosis treatment of the salt-rich- water is discharged from unit 11 by means of pipeline 16. This concentrate is particularly rich in magnesium ( g(HC03)2) - Injection means 18 are implemented so as to inject lime (CaO) or soda (NaOH) into this concentrate.

The mixture of the concentrate and lime or soda is then poured into the decarbonation reactor 17 for the purpose of undergoing a preferably catalytic type of decarbonation. Either of the following reactions then occurs within the decarbonation reactor, depending on whether lime or soda has been injected into the concentrate : Mg(OH)2 + Mg2+ + 2HC03" ? 2MgC03? + 2H20 NaOH + Mg2+ + 2HC03" ? MgC03? + H20 + Na+ + HC03" It can thus be observed that decarbonation of the concentrate results in the production of magnesium carbonates (MgCC>3) . The magnesium carbonates thus produced constitute a calcareous material which is advantageously in the form of balls.

The magnesium carbonates produced during decarbonation of the concentrate are directed towards the 'remineralization reactor 13 with a view to being injected therein by means of pipeline 20.

The injection of carbon dioxide (C02) into the water produces the following reaction: MgC03 + C02 + H20 ? Mg(HC03)2 The injection of sulfuric acid (H2S04) into the water produces the following reaction: 2MgC03 + H2S04 ? Mg(HC03)2 + MgS04 The injection of hydrochloric acid (HC1) into the water produces the following reaction: 2MgC03 + 2HC1 ? g(HC03)2 + MgCl2 Within the scope of this embodiment, implementation of the invention therefore enables the desalinated water to be magnesium enriched, while at the same time preventing the purchase and supply of consumable calcareous material, insofar as the latter is produced directly at the production site by recycling and reprocessing the residues derived from desalination of the water.

Tests Several series of tests were conducted so as to verify the effectiveness of a water treatment process according to the invention, and, in particular, so as to compare the results of remineralization by means of conventional calcareous materials and by means of balls of calcareous material produced by implementing the invention .

The experimental protocol adopted during these tests is described with reference to figure 2, which shows an installation implemented for this purpose.

A first series of tests consisted in circulating an upward flow F of a sulfuric acid solution in a hollow cylindrical column 21 having a diameter of 3 cm and containing a bed of calcareous material 22, which was previously washed in order to eliminate the fine particles therefrom, resting on a sintered glass support 23.

The sulfuric acid solution used had a concentration equal to 200 mg/L and flowed at a rate of 6 ml/min.

The bed of calcareous material 22 had a height of 24 cm, i.e., a volume of 170 cm3. Three types of calcareous material were alternatively used: Israeli limestone, Pyrenean limestone, and balls of calcareous material produced by implementing the treatment process according to the invention.

For each of the alternatively used calcareous materials, 300 ml of solution were sampled after an operating time of 1 hour and 30 minutes, for the purpose of taking various measurements.

A second series of tests was carried out. These tests were identical to those carried out as part of the first series, except for the fact that the sulfuric acid solution used contained 1% seawater.

The results of the measurements taken as part of these first and second series of tests are listed in Tables 3 and 4, respectively.

Table 3: results of the first series of tests Table 4: results of the second series of tests These' tests make it possible to demonstrate the fact that the remineralization is more significant when using balls obtained by implementing a process according to the invention than by using Israeli limestone or Pyrenean limestone.

As a matter of fact, it appears that the total hardness (THTotal) of the solution sampled as part of the first series, after treatment with balls of calcareous material, was of the order of 50°F, whereas it was less than 30°F after treatment with a conventional calcareous material.

In the same way, it appears that the total hardness (THTotal) of the solution sampled as part of the second series of tests, after treatment with balls of calcareous material, was of the order of 65°F, whereas it was lower than 40°F after treatment with a conventional calcareous material.

It can likewise be observed that remineralization is more significant in the presence of seawater.

In conclusion, it can be noted that the balls of calcareous material obtained by implementing a process according to the invention have a high remineralizing capability than a conventional calcareous material.

Advantages The implementation of" a process according to the invention obtains numerous advantages.

In particular, the invention enables desalinated water to be produced directly on the production site. Implementing the invention therefore results in preventing the purchase of such a consumable and the continuous supplying of same, which has a direct positive impact on the cost of operating water desalination systems.

Furthermore, the calcareous material produced on site is obtained by treating and recycling the concentrate, i.e., by reusing the concentrate derived from desalination of the water by reverse osmosis. This calcareous material is therefore produced at a lower cost due to the fact that it is produced from residues from the desalination of salt water directly available on site.

It is likewise noted that the remineralizing capability of the calcareous material obtained by implementing the invention is greater than that of a conventional calcareous material. This invention thus also . has the advantage of enabling a more effective remineralization of desalinated water than the techniques of the pri'or art.

In addition, reprocessing of these concentrates results in the rejection into the natural environment of a supernatant which is carbonate-depleted and which therefore has lower salt concentrations than those of the concentrate itself. In other words, the rejection of same into the natural environment has a less significant impact than the rejection of the salt-rich concentrates produced according to the techniques of the prior art.

Claims

1. Process for treating water containing at least calcium and/or magnesium salts through reverse osmosis membranes, which process includes at least one step for collecting water that is at least partially desalinated, a step for collecting a concentrate coming from said membranes and containing bicarbonates , a step for injecting CO2 or an acid into said at least partially desalinated water, and a step for remineralizing said at least partially desalinated water in a remineralization reactor, characterized in that said process includes: - a step for decarbonating said concentrate so as to form carbonates, and - a step for recycling at least some of said carbonates in said remineralization reactor.

2. Treatment process according to claim 1, characterized in that said water is seawater.

3. Treatment process according to either one of claims 1 or 2, characterized in that said decarbonation step is of the catalytic type.

4. Treatment process according to any one of claims 1 to 3, characterized in that said carbonates formed are in the form of balls.

5. Treatment process according to any one of claims 1 to 4, characterized in that said carbonates formed include calcium carbonate (CaCC>3 ) .

6. Treatment process according to any one of claims 1 to 4, characterized in that said carbonates formed include magnesium carbonate (MgC03) .

7. Treatment process according to any one of claims 1 to 7r characterized in that said decarbonation step is preceded by a step of injecting soda or lime into said concentrate.

8. Treatment process according to claim 7, characterized in that said lime or said soda is injected in stoichiometric amounts with respect to the amount of bicarbonate to be precipitated with a margin of 20 to 50%.

9. Treatment process according to any one of claims 1 to 8, characterized in that said C02 or said acid are injected in stoichiometric proportions with respect to the amount of carbonate to be formed with a margin of 20 to 50%.

10. Treatment process according to any one of claims 1 to 9, characterized in that it includes a step of rejecting, in a natural environment, a supernatant resulting from said decarbonation step, which supernatant is salt-depleted. For the Applicants BEINH0LD COHN AND PARTNERS By 1