WO2021144042A1

WO2021144042A1 - Water treatment process

Info

Publication number: WO2021144042A1
Application number: PCT/EP2020/070441
Authority: WO
Inventors: Bernardus Cornelis Maria In 't Veen; Samy YASSIN; Lucas Petrus Simon Keyzer; Yiu Chung Cheng; Jan Hermen Hendrik Meurs; Arian Van Mourik; Anke Derking
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2020-01-15
Filing date: 2020-07-20
Publication date: 2021-07-22

Abstract

The invention relates to a process for the treatment of a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, comprising the steps of: a) subjecting the water stream to bipolar membrane electrodialysis, resulting in a gas stream comprising carbon dioxide and a treated water stream comprising water and acid equivalents of the one or more additional salts; and b) removing the acid equivalents of the one or more additional salts from at least a portion of the treated water stream resulting from step a).

Description

WATER TREATMENT PROCESS

Field of the invention

The present invention relates to a water treatment process, in specific to a process for the treatment of a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts.

Background of the invention

In a variety of industrial and non-industrial processes water streams are generated comprising both (i) a carbonate (C0₃ ² ) salt and/or a hydrogen carbonate (HC03) salt and (ii) one or more additional salts. In some cases, there is a need to remove such additional (non-carbonate) salts from the water stream, for example to comply with government regulations. For example, certain salts may need to be removed to below a certain concentration before discharging the water stream to the environment.

Examples of such additional salts which may need to be removed from such water streams are phosphate, nitrate, sulphate and borate salts. For example, a waste water stream originating from a process for producing propylene oxide, in specific a process for co-producing propylene oxide and styrene (so-called "SM/PO" process), may contain phosphate salts in an amount which may be relatively high and which may need to be reduced before discharging the water stream to the environment. Such phosphate salts may for example originate from the use of phosphoric acid or sodium pyrophosphate (Na4P2C>7) in the above-mentioned SM/PO process or in any other industrial production process. For example, US4950794 discloses that the molecular oxygen oxidation of ethylbenzene to form ethylbenzene hydroperoxide may be carried out in the presence of alkali such as sodium carbonate, sodium hydroxide, sodium pyrophosphate, and the like.

However, a problem with the co-existence of both (hydrogen) carbonate salts and additional (non-carbonate) salts in the same water stream, such as sodium (hydrogen) carbonate and sodium phosphate, is that it is difficult to selectively reduce the concentration of such additional salts to a lower level by a common salt removal method. An example of such salt removal method is precipitation wherein by adding a precipitating agent, such as calcium hydroxide (Ca(OH)₂₎, to the water stream, a precipitate is formed which can then be removed from the water stream by filtering or any other method. However, generally, such precipitating agent can also form a precipitate with (hydrogen) carbonate salts, not only with phosphate salts. Therefore, it is difficult to selectively reduce the concentration of such additional (non carbonate) salts, either by precipitation or any other salt removal method.

Further, a disadvantage of the above-mentioned precipitation process is that precipitated (hydrogen) carbonate salts are also produced that need to be further handled as a solid waste. Such further handling may involve incineration or off-site disposal of the solid precipitate. This is especially disadvantageous in a case where the amount of (hydrogen) carbonate salts in the water stream is relatively high as compared to the amount of the additional (non-carbonate) salts, as is generally the case. Thus, this results in a large amount of solid precipitate that need to be further handled.

Therefore, it is desired to provide a simple and efficient process for the treatment of a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, such as phosphate, nitrate, sulphate and borate salts, which process preferably does not have one or more of the above-mentioned disadvantages and in which process the concentration of these additional salts can be reduced to a relatively low level.

Summary of the invention

Surprisingly it was found that the above-mentioned process can be provided by first removing the (hydrogen) carbonate salts from the water stream by bipolar membrane electrodialysis, wherein said salts are removed as carbon dioxide gas, followed by a second step wherein the remaining acid equivalents of the one or more additional salts are removed, easily and to a great extent, from the water stream in the absence of or in the presence of a reduced amount of the (hydrogen) carbonate salts. Thus, in this way, not only a low level of additional salts may be achieved such that the treated water may be discharged to the environment, but also by first applying bipolar membrane electrodialysis the thus treated intermediate stream may contain substantially less carbonates, so that the remaining additional salts, which may include phosphates, can be dealt with using more traditional means at reduced costs, which may result in a more compact process which utilises less utilities, including for example less solids handling, resulting in reduced capital and operational expenditures.

Accordingly, the present invention relates to a process for the treatment of a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, comprising the steps of: a) subjecting the water stream to bipolar membrane electrodialysis, resulting in a gas stream comprising carbon dioxide and a treated water stream comprising water and acid equivalents of the one or more additional salts; and b) removing the acid equivalents of the one or more additional salts from at least a portion of the treated water stream resulting from step a).

Brief description of the drawing

Figure 1 shows an electrodialysis unit cell that may be used in step a) of the present process.

Detailed description of the invention

The process of the present invention comprises steps a) and b). Said process may comprise one or more intermediate steps between steps a) and b). Further, said process may comprise one or more additional steps preceding step a) and/or following step b).

While the process of the present invention and the stream or streams used in said process are described in terms of "comprising", "containing" or "including" one or more various described steps and components, respectively, they can also "consist essentially of" or "consist of" said one or more various described steps and components, respectively.".

In the context of the present invention, in a case where a stream comprises two or more components, these components are to be selected in an overall amount not to exceed 100%.

Further, where upper and lower limits are quoted for a property then a range of values defined by a combination of any of the upper limits with any of the lower limits is also implied.

In step a) of the present process, a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, is subjected to bipolar membrane electrodialysis. Thus, in addition to water, the water stream to be treated in step a) comprises a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, all of which salts are dissolved salts, that is to say dissolved in said water stream. Within the present specification, the phrase "(hydrogen) carbonate salts" means "carbonate and/or hydrogen carbonate salts".

Step a) of the present process is not limited to a specific bipolar membrane electrodialysis method. In bipolar membrane electrodialysis, a bipolar membrane is used which causes the dissociation of water into protons (H⁺) and hydroxide ions (OH-). In the present invention, these protons react with the carbonate (CC>₃ ² ) and/or hydrogen carbonate (HC0₃ ) anions from the water stream to be treated, resulting in an aqueous solution containing carbonic acid (H₂CO₃). Said carbonic acid dissociates into water (H₂O) and carbon dioxide (CO₂)· In the present invention, carbon dioxide dissolved in the aqueous solution is then released as a gas. Such release of carbon dioxide as a gas advantageously already takes place at atmospheric pressure.

Thus, step a) of the present process results in a gas stream comprising carbon dioxide and a treated water stream which is a liquid stream. In this way, advantageously, the (hydrogen) carbonate salts are removed, in a simple and efficient way, as a gas, from the liquid water stream to be treated. Such removal of the (hydrogen) carbonate salts in a first step, in step a), makes it possible to remove the acid equivalents of the one or more additional salts in a second treatment step, in step b), in an easy way and to a great extent because the (hydrogen) carbonate salts are then no longer present or only present in a reduced amount in the water stream to be further treated.

Alternatively, in a first step, (hydrogen) carbonate salts could be removed by adding an acid, such as sulphuric acid or hydrochloric acid, followed by above-mentioned second treatment step b), wherein the remaining acid equivalents of the one or more additional salts are removed. However, disadvantages of such process as compared to the present process involving bipolar membrane electrodialysis step a) and said step b) are that since generally the concentration of dissolved carbonate salts is relatively large, corresponding relatively large volumes of acid are required to initially neutralise the dissolved salts.

Step a) of the present process is performed in a stack of repeating electrodialysis unit cells, wherein each of said unit cells comprises a bipolar membrane and in addition a cation exchange membrane and/or an anion exchange membrane. The bipolar membrane comprises a cation exchange layer, an anion exchange layer and optionally a catalytic intermediate layer. When used in electrodialysis, a bipolar membrane causes the dissociation of water into protons and hydroxide ions under the influence of an applied electric potential difference. The protons migrate though the cation exchange layer and the hydroxide ions migrate through the anion exchange layer. Within the present specification, "cation exchange" has the same meaning as "cation selective", and similarly, "anion exchange" has the same meaning as "anion selective". "Cation exchange" or "cation selective" implies that the membrane or layer permits cations to pass through and is a barrier to anions. Similarly, "anion exchange" or "anion selective" implies that the membrane or layer permits anions to pass through and is a barrier to cations.

The nature of the bipolar membrane to be used in step a) is not particularly limited and may be any bipolar membrane. Examples of bipolar membranes which can be used include those described in US2829095, US4024043, US4116889, US4082835 and US4806219, the disclosures of which are herein incorporated by reference.

The nature of the cation exchange membrane and/or anion exchange membrane to be used in step a) is neither particularly limited and may be any cation exchange membrane and/or anion exchange membrane. Suitable cation exchange membranes may be moderately acidic (for example, phosphonic group-containing) or strongly acidic (for example, sulfonic group containing). Further, suitable examples of cation exchange membranes are DuPont's Nation® 110 and 324 cation membranes. Further, a suitable cation exchange membrane is the one described in US4738764, the disclosure of which is herein incorporated by reference. Suitable anion exchange membranes may include strongly, moderately, or weakly basic membranes. Suitable commercially available anion exchange membranes include those from Ionics, Inc., Watertown, Mass, (sold as Ionics 204-UZL-386 anion membrane), or from Asahi Glass Co. (sold under the trade name Selemion® AMV AAV, ASV anion permselective membranes).

The specific bipolar membrane electrodialysis technology that may be applied in step a) of the present process may be any known technology. Suitable examples are disclosed in the following publications, the disclosures of which are herein incorporated by reference: JP4065386; US5200046; US5972191; US5200046; US4592817; CN107215996; CN203393285; US9586181;

US8778156; CN105923853; US5888368; US5395497; US4238305; CN106518811; and Eisaman et al., "C02 separation using bipolar membrane electrodialysis", Energy Environ. Sci., 2011, 4, pages 1319-1328.

The protons originating from the bipolar membrane may react with anions, such as the above-mentioned carbonate (C0₃ ²-) and/or hydrogen carbonate (HCC>3) anions, but also with anions of the one or more additional salts from the above-mentioned water stream to be treated in step a). For example, reaction of these protons with a phosphate anion (PO4³-) may result in the formation of HPC>4² (monohydrogen phosphate), fUPCy (dihydrogen phosphate) and H3PO4 (phosphoric acid). Within the present specification, such compounds resulting from the reaction of protons with anions of the one or more additional salts are referred to as "acid equivalents" of such additional salts.

Further, the hydroxide ions originating from the bipolar membrane may form a salt with cations, for example sodium (Na⁺), of the above-mentioned carbonate and/or hydrogen carbonate salts and one or more additional salts, resulting for example in the formation of sodium hydroxide (NaOH).

Each unit cell in the above-mentioned stack of repeating electrodialysis unit cells comprises two or more compartments, such compartments being formed by the above- mentioned membranes. Further, as mentioned above, step a) of the present process is not limited to a specific bipolar membrane electrodialysis method. It is preferred that protons originating from the bipolar membrane migrate into a compartment wherein they can come into contact with anions of the above-mentioned carbonate and/or hydrogen carbonate salts and one or more additional salts. Similarly, it is preferred that hydroxide ions originating from the bipolar membrane migrate into a compartment wherein they can come into contact with cations of the above-mentioned carbonate and/or hydrogen carbonate salts and one or more additional salts.

Thus, in step a) of the present process, the above- mentioned water stream to be treated is sent as a feed stream to one of the compartments of the electrodialysis unit cell, and another water stream or other water streams not to be treated is or are sent as a feed stream or feed streams to the other compartment or compartments. Such other water stream may be a water stream comprising a base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH).

Further, step a) of the present process results in above- mentioned gas stream comprising carbon dioxide and at least two liquid effluent streams, the number of which corresponds with the number of compartments in the unit cell. One of these liquid effluent streams is a treated water stream comprising water and acid equivalents of the one or more additional salts. In the present invention, said treated water stream comprising water and acid equivalents of the one or more additional salts is a water stream originating from a compartment, comprised in a unit cell in a stack of repeating electrodialysis unit cells, in which compartment protons are migrated into from the bipolar membrane. Said compartment is preferably a compartment formed by the bipolar membrane and another membrane. Said other membrane may be a cation exchange membrane or an anion exchange membrane. Another liquid effluent stream is a water stream containing the salt resulting from contacting of hydroxide ions originating from the bipolar membrane with cations of the above-mentioned carbonate and/or hydrogen carbonate salts and one or more additional salts.

As mentioned above, the electrodialysis unit cells comprises two or more compartments, preferably two or three, such compartments being formed by the above-mentioned membranes, which membranes comprise a bipolar membrane and in addition a cation exchange membrane and/or an anion exchange membrane.

The configuration of the electrodialysis unit cell in step a) of the present process is not particularly limited.

In one embodiment, the electrodialysis unit cell comprises two compartments formed by a bipolar membrane and either a cation exchange membrane or an anion exchange membrane, preferably a cation exchange membrane, and wherein the water stream to be treated is preferably fed into the compartment wherein protons are migrated into from the bipolar membrane. In another embodiment, the electrodialysis unit cell comprises three compartments formed by a bipolar membrane, a cation exchange membrane and an anion exchange membrane, and wherein the water stream to be treated is fed into the compartment formed by the cation exchange membrane and the anion exchange membrane. An example of such electrodialysis unit cell is shown in Figure 3 of WO2016124646, the disclosure of which is herein incorporated by reference. In the process as disclosed in WO2016124646, bipolar membrane electrodialysis technology is applied in one of the process steps wherein metal oxalate is converted to oxalic acid and a metal hydroxide.

In yet another embodiment, the electrodialysis apparatus as claimed and disclosed in US5200046 may be used in step a) of the present process. The disclosure of US5200046 is incorporated herein by reference. The electrodialysis apparatus as claimed in US5200046 comprises at least one unit cell comprising: at least two adjacent, serially aligned electrodialytic means for splitting water, separated by an intermediate compartment disposed between said adjacent means for splitting water; at least one ion selective membrane serially aligned with and adjacent to at least one of the means for splitting water, there being a product compartment between the ion selective membrane and the means for splitting water; and an aqueous salt solution feed compartment serially aligned with and adjacent to the product compartment with the ion selective membrane therebetween. Thus, the unit cell as claimed in and as shown in Figure 2 of US5200046 contains 3 compartments. Further, said means for splitting water is preferably a bipolar membrane having an anion selective layer and a cation selective layer. Still further, said US5200046 discloses that typical salts which can be treated include NaCl, sodium formate, sodium acetate, sodium citrate, NH₄C1, Na₂S0₄, NaHCOs, NaN0₃, NH4NO3, Na₃P0₄, (NH₄₎₂S0₄, KF, trimethylammonium chloride and lysine hydrochloride, and that mixtures of salts can also be treated.

Figure 1 shows an electrodialysis unit cell that may be used in step a) of the present process, in specific an electrodialysis unit cell which comprises two compartments formed by a bipolar membrane and a cation exchange membrane, wherein the water stream to be treated is fed into the compartment wherein protons are migrated into from the bipolar membrane.

The electrodialysis unit cell "UC" shown in Figure 1 comprises a bipolar membrane 1 and a cation exchange membrane 2. Bipolar membrane 1, in Figure 1, comprises an anion exchange layer on the left side and a cation exchange layer on the right side.

In the process using a bipolar membrane electrodialysis configuration containing the electrodialysis unit cell "UC" shown in Figure 1, feed stream 3 is fed into the first compartment I formed by the cation exchange layer of the bipolar membrane 1 and the cation exchange membrane 2. Feed stream 3 is the water stream to be treated in accordance with step a) of the present process and comprises water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts. Further, a feed stream 4 is fed into the second compartment II formed by the cation exchange membrane 2 and the anion exchange layer of the bipolar membrane 1. Feed stream 4 is a water stream not to be treated and may be a water stream comprising a base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH).

Step a) of the present process is performed in a stack of repeating electrodialysis unit cells, of which Figure 1 only shows one cell. In Figure 1, there is an anode 5 on the left and a cathode 6 on the right. Through application of an electric potential difference between these electrodes 5 and 6 a direct current is produced. At the cathode, the following reaction takes place: 2e + 2H2O tkig) + 20th. At the anode, the following reaction takes place: H2O 2H⁺ + 0.50₂₍g) + 2e

. Such current causes the dissociation of water, within the space between the anion exchange layer and the cation exchange layer of the bipolar membrane 1, into protons (H⁺) and hydroxide ions (Ofh), followed by migration of the protons through the cation exchange layer into compartment I and migration of the hydroxide ions through the anion exchange layer into compartment II.

Further, the cations (denoted in Figure 1 as "M⁺") of the carbonate salt and/or hydrogen carbonate salt and the one or more additional salts originating from feed stream 3 migrate through the cation exchange membrane 2 into compartment II, where they form a salt with the hydroxide ions originating from the bipolar membrane. The effluent stream 8 from compartment II is a liquid water stream comprising such salt which is a base. For example, in case where feed stream 4 is a water stream comprising NaOH and said cations are sodium ions, effluent stream 8 is a water stream enriched in base (i.e. NaOH).

In compartment I, the protons originating from the bipolar membrane come into contact with the anions of the carbonate salt and/or hydrogen carbonate salt and the one or more additional salts. As described above, this results in the formation of carbonic acid (H2CO3), which dissociates into water (H2O) and carbon dioxide (CO2)· Further, this results in reaction of these protons with the anions of the one or more additional salts resulting in acid equivalents of such additional salts, as also described above. Thus, the effluent stream 7 from compartment I comprises water, carbonic acid and/or carbon dioxide (dissolved or gas), optionally a reduced amount of the carbonate salt and/or hydrogen carbonate salt, and the acid equivalents of the one or more additional salts. Said effluent stream 7 then separates into a gas stream 7a comprising carbon dioxide and a treated water stream 7b (effluent stream 7b) comprising water, optionally a reduced amount of the carbonate salt and/or hydrogen carbonate salt, and the acid equivalents of the one or more additional salts.

In the present invention, the one or more additional salts in the water stream to be treated in step a) are preferably no carbonate or hydrogen carbonate salts. The one or more additional salts may be any salt which by protonation can be converted into an acid (above-mentioned "acid equivalent"). For example, these additional salts can be nutrient pollutants, which may contain nitrogen or phosphorus. Discharging such nutrient pollutants to surface waters may cause eutrophication. In the present invention, this is advantageously prevented or minimized.

In particular, it is preferred that the one or more additional salts in the water stream to be treated in step a) are selected from the group consisting of phosphate, nitrate, sulphate and borate salts, more preferably phosphate and nitrate salts, most preferably phosphate salts.

Further, the cation in the above-mentioned one or more additional salts is preferably a monovalent cation, more preferably a monovalent metal cation. The cation in the one or more additional salts may be a monovalent metal cation selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, preferably sodium and potassium. An example of a suitable monovalent non-metal cation is ammonium. Still further, the cation in the above-mentioned carbonate salt and/or a hydrogen carbonate salt is preferably a monovalent cation, more preferably a monovalent metal cation. The cation in the carbonate salt and/or a hydrogen carbonate salt may be a monovalent metal cation selected from the group consisting of lithium, sodium, potassium, rubidium and cesium, preferably sodium and potassium. An example of a suitable monovalent non-metal cation is ammonium.

The above-mentioned phosphate salts may comprise one or more salts wherein the anion is derived from one or more phosphoric acid compounds selected from the group consisting of orthophosphoric acid (H₃PO₄₎ , polyphosphoric acid and polymetaphosphoric acid.

In particular, the above-mentioned phosphate salts may comprise one or more salts wherein the anion is derived from polyphosphoric acid which is represented by the following formula (2), wherein n is an integer and is of from 0 to 10, preferably 0 to 5:

Said polyphosphoric acid may be one or more of pyrophosphoric acid (H4P2O7) wherein n is 0, triphosphoric acid (H5P3O10) wherein n is 1, and tetraphosphoric acid (H6P4O13) wherein n is 2, preferably pyrophosphoric acid.

Further, in particular, the above-mentioned phosphate salts may comprise one or more salts wherein the anion is derived from polymetaphosphoric acid which is represented by the following formula (3) wherein m is an integer and may be of from 1 to 10, preferably 1 to 5:

Said polymetaphosphoric acid may be one or more of trimetaphosphoric acid (H3P3O9) wherein m is 1, and tetrametaphosphoric acid (H4P4O12) wherein m is 2.

The water stream to be treated in step a) of the present process may originate from any source. It may originate from a process for producing propylene oxide, preferably from a process for co-producing propylene oxide and styrene. Other sources where said water stream to be treated may originate from, are: phosphates containing detergent waste streams, agriculture (fertilizer) waste streams, streams resulting from overboarding of wells, and sewage or drainage streams.

Further, the amount of the carbonate salt and/or hydrogen carbonate salt in the water stream to be treated in step a) may vary within wide ranges. Said amount may be up to saturation level (i.e. maximum solubility). In specific, said amount may be of from 1 to 10 wt.%, suitably of from 3 to 6 wt.%. Further, the weight ratio of sodium carbonate to sodium hydrogen carbonate in said water stream may vary within wide ranges and may be of from 0:1 to 1:0, for example of from 0.1:1 to 10:1.

The amount of the one or more additional salts in the water stream to be treated in step a) may also vary within wide ranges and may be of from 1 part per million by weight (ppmw) to 10 wt.%, suitably of from 10 ppmw to 6 wt.%. For example, the latter amount may be of from 1 to 10 wt.%, suitably of from 3 to 6 wt.%, and it may also be of from 1 to 100 parts per million by weight (ppmw), suitably of from 10 to 30 ppmw.

Step b) of the present process comprises removing the acid equivalents of the one or more additional salts from at least a portion of the treated water stream resulting from step a). Thus, in said step b), at least a portion of the acid equivalents of the one or more additional salts is removed from at least a portion of the treated water stream resulting from step a). Any method suitable for such removal may be applied. In step b) one of the following methods may be applied: precipitation, reverse osmosis, electrodialysis without a bipolar membrane, absorption and biotreatment. Of these methods, precipitation and biotreatment are preferred. Thus, said removal method in step b) may be precipitation or biotreatment .

In a preferred embodiment of step b) of the present process, a precipitating agent is added to at least a portion of the treated water stream resulting from step a), wherein the precipitating agent forms a precipitate with the acid equivalents of the one or more additional salts which precipitate is then removed from the treated water stream. Said precipitate may be removed by filtering or any other method suitable for removing a precipitate, including settling and skimming.

Advantageously, since in step a) of the present process the carbonate salt and/or hydrogen carbonate salt from the water stream to be treated has already been removed, either completely or partially, only a relatively low amount of precipitating agent is needed to perform step b) of the present process. Consequently, this also advantageously results in a relatively low amount of solid precipitate that need to be further handled.

If a precipitating agent is used in step b) of the present process, it should be capable of forming a precipitate with the anion of the one or more additional salts, preferably selectively, that is to say to a greater extent than with the anion of any remaining carbonate salt and/or hydrogen carbonate salt from step a). The precipitating agent is preferably a metal salt wherein the metal cation is not monovalent but di- or trivalent. Suitable examples of such precipitating agents are calcium hydroxide (Ca(OH)2), calcium chloride (CaCl2), aluminum sulphate (Al_{2 (}SO4)3) and iron trichloride (FeCls)· For example, by adding calcium hydroxide to phosphoric acid (acid equivalent of sodium phosphate), a calcium phosphate precipitate and water are formed. Further, by adding calcium hydroxide to sodium phosphate, a calcium phosphate precipitate and sodium hydroxide are formed.

The invention is further illustrated by the following Examples .

Examples

1. Step a)

In the experiments of the Examples, step a) of the present process was illustrated by using the bipolar membrane electrodialysis configuration as shown in Figure 1 and as described above in general terms. The electrodialysis apparatus used in the Examples (commercially available at PCCell: Bench Scale Laboratory Electrodialysis system PCCell BED 1-4 with ED64004 stack) contained (a) 10 cell pairs, in specific 10 serially aligned electrodialysis unit cells consisting of a total of 9 cation exchange membranes (9x "PC MV", all of which are of the sulfonic acid type) and 10 bipolar membranes (lOx "PC BiP"), and (b) a cation exchange membrane near the anode and another one near the cathode (2x "PC MTE"). Figure 1 shows one of these unit cells. Each of the membranes had an active area of 8 cm x 8 cm ("active area" means area in contact with solution).

Feed stream 3 was a water stream comprising (initially) water, sodium carbonate (Na2CC>3), sodium hydrogen carbonate (NaHCCb) and sodium phosphate (NasPCh), wherein the weight ratio Na2CC>3:NaHCCg was 1.3:1. Feed stream 4 was a water stream comprising water and potassium hydroxide (KOH) in an initial concentration of 0.1 mole/1 of KOH. The electrolyte used, as fed into the compartment containing the anode and a cation exchange membrane (i.e. 1 of above "PC MTE") and into the compartment containing the cathode and a cation exchange membrane (i.e. 1 of above "PC MTE"), was an aqueous solution of 10 wt.% of potassium sulfate (K2SO4).

A direct current of 1.6 A (ampere), corresponding to a current density of 250 A/m², was maintained by adjusting the electric potential difference between electrodes 5 and 6, up to a limit of 36 V (volt). The temperature and pressure during the experiments were ambient. Effluent stream 7 was sent to a diluent reservoir, which contained a vent for releasing gas stream 7a comprising carbon dioxide, and the remaining treated water stream (effluent stream 7b) was recirculated as feed stream 3. Similarly, effluent stream 8 was sent to a concentrate reservoir and then recirculated as feed stream 4. The electrolyte stream was also recirculated, via an electrolyte reservoir wherein O2 and ¾ as produced at the anode and cathode, respectively, were diluted with nitrogen (N2).

The experiment was stopped when the CO2 flow rate (for stream 7a) was dropped to a negligible value. The compositions of feed streams 3 and 4 (before starting the experiment) and the compositions of effluent streams 7b and 8 (after ending the experiment) are shown in Table 1 below.

Table 1

"Phosphate" includes P0₄ ³ , HPC>4² , ¾Rq4 and H3PO4.

"Carbonate" includes CC>3² and HCC>3 . "Sulphate" includes SC>4² , HSC>4, H2SO4. n.d. = not detected, n.m. = not measured.

From the results in Table 1 it can be seen that at the end of the experiment, the treated water stream (effluent stream 7b) advantageously contained a substantially reduced amount of carbonate and sodium. From effluent stream 7 from compartment I a carbon dioxide gas stream 7b had been removed, resulting in said effluent stream 7b being depleted of carbon dioxide. Further, the phosphate amount in the treated water steam remained more or less the same. Further, advantageously, in effluent stream 8 from compartment II no phosphate was detected, so that no additional treatment of such stream to remove phosphates is required. 2. Step b)

In the experiments of the Examples, step b) of the present process was illustrated by adding (i) a calcium hydroxide (Ca(OH)2) solution containing the Ca(OH)2 in an amount of 21 mmol/1 (calculated as Ca²⁺), as a precipitating agent to (ii) an aqueous solution containing phosphate (NasPCy) and carbonate (Na2CC>3 and NaHCCq) in the amounts as shown in Table 2 below, wherein the weight ratio of (i):(ii) was 1:1.

Table 2 below also shows the amount of phosphate after precipitation and subsequent removal of the formed precipitate by filtration.

Table 2

n.d. = not determined.

From the results in Table 2 it can be seen that the treated aqueous solution advantageously contained a substantially reduced amount of phosphate.

Claims

C LA IM S

1. Process for the treatment of a water stream comprising water, a carbonate salt and/or a hydrogen carbonate salt, and one or more additional salts, comprising the steps of: a) subjecting the water stream to bipolar membrane electrodialysis, resulting in a gas stream comprising carbon dioxide and a treated water stream comprising water and acid equivalents of the one or more additional salts; and b) removing the acid equivalents of the one or more additional salts from at least a portion of the treated water stream resulting from step a).

2. Process according to claim 1, wherein the one or more additional salts are selected from the group consisting of phosphate, nitrate, sulphate and borate salts, preferably phosphate and nitrate salts, more preferably phosphate salts.

3. Process according to claim 2, wherein the cation in the one or more additional salts is a monovalent cation, preferably a monovalent metal cation.

4. Process according to any one of claims 1 to 3, wherein the cation in the carbonate salt and/or hydrogen carbonate salt is a monovalent cation, preferably a monovalent metal cation.

5. Process according to any one of claims 1 to 4, wherein the water stream to be treated in step a) originates from a process for producing propylene oxide, preferably from a process for co-producing propylene oxide and styrene.

6. Process according to any one of claims 1 to 5, wherein step a) is performed in a stack of repeating electrodialysis unit cells, each electrodialysis unit cell comprising two compartments formed by a bipolar membrane and either a cation exchange membrane or an anion exchange membrane, preferably a cation exchange membrane, and wherein the water stream is preferably fed into the compartment wherein protons are migrated into from the bipolar membrane.

7. Process according to any one of claims 1 to 5, wherein step a) is performed in a stack of repeating electrodialysis unit cells, each electrodialysis unit cell comprising three compartments formed by a bipolar membrane, a cation exchange membrane and an anion exchange membrane, and wherein the water stream is fed into the compartment formed by the cation exchange membrane and the anion exchange membrane.

8. Process according to any one of claims 1 to 7, wherein in step b) one of the following methods is applied: precipitation, reverse osmosis, electrodialysis without a bipolar membrane, absorption and biotreatment.

9. Process according to claim 8, wherein in step b) a precipitating agent is added to at least a portion of the treated water stream resulting from step a), wherein the precipitating agent forms a precipitate with the acid equivalents of the one or more additional salts which precipitate is then removed from the treated water stream.