METHOD FOR THE PRODUCTION OF DRINKING WATER

Abstract

The present invention relates to a method for the production of drinking water. In addition, the present invention also relates to the use of minerals extracted from a feed water stream by using a combination of a Donnan dialysis unit and a membrane unit as a source of minerals for the production of drinking water originating from said feed water stream.

Claims

1. A method for production of drinking water, wherein the method comprises the following steps: i) providing a feed water stream; ii) treating said feed water stream of i) in a Donnan dialysis unit thereby producing a feed water stream depleted from divalent cations and an effluent stream enriched with divalent cations; iii) treating said feed water stream depleted from divalent cations of i) in a membrane unit thereby producing a concentrate stream and a permeate stream; and iv) combining said permeate stream of iii) with said effluent stream enriched with divalent cations of ii) for the production of drinking water.

2. The method for the production of drinking water according to claim 1, wherein in ii) said effluent stream enriched with divalent cations is treated in a nano filtration unit (NF) for recovering said divalent cations, said nano filtration unit (NF) producing a concentrate stream enriched with divalent cations and a permeate, said concentrate stream enriched with divalent cations being used in step iv) as said effluent stream enriched with divalent cations, said permeate being used as a draw solution in said Donnan dialysis unit.

3. The method for the production of drinking water according to claim 1, wherein in ii) said effluent stream enriched with divalent cations is treated in a selective electrodialysis unit (S-ED) for recovering said divalent cations by removing monovalent cations, said selective electrodialysis unit (S-ED) producing a stream enriched with divalent cations and an S-ED effluent stream enriched with monovalent cations, said stream enriched with divalent cations being used in step iv) as said effluent stream enriched with divalent cations, said S-ED effluent stream being used as draw solution in said Donnan dialysis unit.

4. The method for the production of drinking water according to claim 1, wherein in ii) said effluent stream enriched with divalent cations is first treated in a nano filtration unit (NF) for recovering said divalent cations, said nano filtration unit (NF) producing a concentrate stream enriched with divalent cations and a retentate, said retentate being used as a draw solution in said Donnan dialysis unit, wherein said concentrate stream enriched with divalent cations is further treated in a selective electrodialysis unit (S-ED) for recovering said divalent cations, said selective electrodialysis unit (S-ED) producing a stream enriched with divalent cations and an S-ED effluent stream, said stream enriched with divalent cations being used in step iv) as said effluent stream enriched with divalent cations, said S-ED effluent stream being used as a draw solution in said Donnan dialysis unit.

5. The method for the production of drinking water according to claim 1, wherein a draw solution in said Donnan dialysis unit comprises a solution of monovalent cations having at least one of sodium salts, potassium salts, or a combination thereof.

6. The method for the production of drinking water according to claim 5, wherein said draw solution is a sodium chloride solution.

7. The method for the production of drinking water according to claim 1, wherein said membrane unit in iii) is at one of of a nanofiltration (NF) unit and or a reverse osmosis (RO) unit.

8. The method for the production of drinking water according to claim 1, wherein a concentration of divalent cations in the drinking water produced in iv) is in a range between 1.0 and 2.5 mM.

9. The method for the production of drinking water according to claim 1, wherein the maximum concentration of monovalent cations in the drinking water produced in iv) is 150 mg/L.

10. The method for the production of drinking water according to claim 1, wherein in step iv) said effluent stream enriched with divalent cations of step ii) is combined with said permeate stream of step iii) to obtain a desired amount of divalent cations in the drinking water.

11. The method for the production of drinking water according to claim 1, wherein in step ii) said Donnan dialysis unit comprises multiple stages of Donnan dialysis, namely several Donnan dialysis units placed in series.

12. The method for the production of drinking water according to claim 11, wherein said Donnan dialysis unit consists of a first stage Donnan dialysis unit for removing ammonium and a second stage Donnan dialysis unit for recovering hardness.

13. The method for the production of drinking water according to claim 1, further comprising: extracting minerals from the feed water stream by using a combination of the Donnan dialysis unit and the membrane unit; and using the minerals.

Description

[0043] For better understanding of the invention, reference should be made to the detailed description of preferred embodiments and process schemes.

[0044] FIG. 1 shows an embodiment according to the present invention.

[0045] FIG. 2 shows another embodiment according to the present invention.

[0046] FIG. 3 shows another embodiment according to the present invention.

[0047] FIG. 4 shows the results of a Donnan dialysis hardness removal of groundwater.

[0048] FIG. 5 shows the Donnan dialysis hardness removal of groundwater.

[0049] FIG. 1 shows a process where Donnan Dialysis (DD) is used to exchange divalent cations from feed water with monovalent cations. Nanofiltration (NF) is used to separate mono and divalent cations from the DD draw solution. The NF retentate is used for drinking water remineralization (combined with RO permeate). The NF permeate is reused as DD draw solution with additional monovalent salt.

[0050] According to the process scheme 10 shown in FIG. 1 a feed water stream 1 containing dissolved cations, such as calcium and magnesium, is treated in a Donnan dialysis unit 2 comprising a membrane 22. In the Donnan dialysis unit 2 a draw solution 11 is present as well. Due to the driving force the divalent cations, such as magnesium ions and calcium ions, are transferred to the draw solution 11 resulting in a feed water stream 5 depleted from divalent cations. The feed water stream 5 depleted from divalent cations is subsequently sent to a membrane unit 3, for example of the type reverse osmosis. The membrane unit 3 produces a concentrate stream 6 and a permeate stream 7. The concentrate stream 6 can be identified as a waste stream. The effluent 7 from the membrane unit is a permeate stream. The effluent stream 8 enriched with divalent cations from the Donnan dialysis unit 2 is further treated in another membrane unit 4, for example a nano filtration unit. The concentrate stream 9 produced in the nano filtration unit 2 now contains the cations originally present in the feed water stream 1 and is subsequently mixed with the permeate stream 7 from the reverse osmosis thereby producing drinking water 13. The permeate stream 14 produced in the nano filtration unit 4 is supplied as draw solution 11 to the Donnan dialysis unit 2. In the beginning of the process a solution 12 containing monovalent salts, preferably Na.sup.+ or K.sup.+ salts, is used as a draw solution.

[0051] FIG. 2 shows a process where Donnan Dialysis (DD) is used to exchange divalent cations from feed water with monovalent cations. Selective ED (S-ED) is used to separate mono and divalent cations from the DD draw solution. The S-ED is using monovalent selective cation exchange membranes to remove the monovalent salts from the stream containing the Ca/Mg. The Ca/Mg-containing stream can be reused for drinking water remineralization. The monovalent salts are reused for DD draw solution with additional monovalent salt.

[0052] According to the process scheme 20 shown in FIG. 2 a feed water 1 stream containing dissolved cations, such as calcium and magnesium, is treated in a Donnan dialysis unit 2 comprising a membrane 22. In the Donnan dialysis unit 2 a draw solution 11 is present as well. Due to the driving force the divalent cations, such as magnesium ions and calcium ions, are transferred to the draw solution resulting in a feed water stream 5 depleted from divalent cations. The feed water stream 5 depleted from divalent cations is subsequently sent to a membrane unit 3, for example of the type reverse osmosis. The membrane unit 3 produces a concentrate stream 6 and a permeate stream 7. The concentrate stream 6 can be identified as a waste stream. The effluent 7 from the membrane unit 3 is a permeate stream. The effluent stream 8 enriched with divalent cations from the Donnan dialysis unit 2 is further treated in a selective electrodialysis unit 21 (S-ED). In selective electrodialysis unit 21 (S-ED) a membrane 23 is present. A concentrate stream 24 produced in the S-ED 21 now contains the cations originally present in the feed water stream 1 and is subsequently mixed with the permeate stream 7 from the reverse osmosis 3 thereby producing drinking water 13. The retentate stream 25 produced in the S-ED 21 is supplied as draw solution to the Donnan dialysis unit. In the beginning of the process a solution 12 containing monovalent salts, preferably Na.sup.+ or K.sup.+ salts, is used as a draw solution.

[0053] FIG. 3 shows a process 30 where Donnan Dialysis (DD) is used to exchange divalent cations from feed water with monovalent cations. Nanofiltration (NF) is used to separate mono and divalent cations from the DD draw solution. The NF retentate is further treated by selective ED (S-ED) to remove monovalent ions, to meet the required quality of drinking water. The water with divalent cations is then used for drinking water remineralization (combined with RO permeate). The NF permeate is reused as DD draw solution with additional monovalent salt.

[0054] The process scheme 30 shown in FIG. 3 can be seen as a kind of a combination of the process scheme shown in both FIGS. 2 and 3. According to the process scheme shown in FIG. 3 a feed water stream 1 containing dissolved cations, such as calcium and magnesium, is treated in a Donnan dialysis unit 2 comprising a membrane 22. In the Donnan dialysis unit 2 a draw solution is present as well. Due to the driving force the divalent cations, such as magnesium ions and calcium ions, are transferred to the draw solution resulting in a feed water stream 5 depleted from divalent cations. The feed water stream 5 depleted from divalent cations is subsequently sent to a membrane unit 3, for example of the type reverse osmosis. The membrane unit 3 produces a concentrate stream 6 and a permeate stream 7. The concentrate stream 6 can be identified as a waste stream. The effluent 7 from the membrane unit 3 is a permeate stream. The effluent stream 8 enriched with divalent cations from the Donnan dialysis unit 2 is further treated in a nano filtration unit 31. The concentrate stream 35 produced in the nano filtration unit 31 now contains the cations originally present in the feed water stream 1 and is subsequently treated in a selective electrodialysis unit 32 (S-ED) to remove excess of monovalent salts. In selective electrodialysis unit 32 (S-ED) a membrane 33 is present. A concentrate stream 34 produced in the S-ED 32 now contains the cations originally present in the feed water stream 1 and is subsequently mixed with the permeate stream 7 from the reverse osmosis 3 thereby producing drinking water 13. The monovalent-salt enriched stream 36 produced in the S-ED 32 is supplied as a draw solution to the Donnan dialysis unit 2. The permeate stream 37 produced in the nano filtration unit 31 is supplied as a draw solution to the Donnan dialysis unit, too. In the beginning of the process a solution 12 containing monovalent salts, preferably Na.sup.+ or K.sup.+ salts, is used as a draw solution.

EXAMPLES

[0055] For a first set of tests, small diffusion cells were used. Hardness removal from groundwater over time with 0.1 M NaCl draw solution with two different membranes can be seen in FIG. 4. FIG. 4 shows a graph of feed water treated by in DD with 100 mM NaCl solution using CMV and CMX membranes over time, using lab-scale DD units, i.e. the results of a Donnan dialysis hardness removal of groundwater with 100 mM NaCl draw solution and two types of membranes, namely CMV or CMX membranes. The cations Mg.sup.2+ and Ca.sup.2+ are exchanged with (twice as much moles of) Na.sup.+ for a certain period of time. After approximately 60 m.sup.2 s/L (surface contact time) about 75% of the hardness is removed. This is sufficient for the reverse osmosis to run on a higher recovery (from 80 to 90 or 95%).

[0056] The present inventors did test with less NaCl for the draw solution. This may result in a lower NaCl concentration in the final drinking water, i.e. not to exceed 150 mg/L (or 4 mM). The inventors also tested with 40 and 20 mM NaCl draw solutions for Donnan Dialysis, as shown in FIG. 5. FIG. 5 shows a graph of feed water treated by in DD with 40 and 20 mM NaCl solution using CMV membranes over time, using lab-scale DD units, i.e. the Donnan dialysis hardness removal of groundwater with 40 and 20 mM NaCl draw solution with a CMV membrane. From FIG. 5 one can see that almost the same hardness removal has been achieved here. This means there is still sufficient driving force for ion exchange. In fact, with a relatively low concentration of approximately brackish water (20 mM NaCl), the inventors are able to soften groundwater. This is promising to be able to recover hardness from the draw solution and then to make the draw solution suitable for adding to the RO permeate with a single NF step.

[0057] On basis of the above the present inventors conclude that Donnan dialysis is easily scalable for hardness removal. Moreover, membranes with sufficiently high permselectivity (>95%) are able to perform the exchange without too much salt leakage. For remineralization a draw solution having a slightly higher salt concentration as the feed water will ensure sufficient driving force. For example, in an embodiment 20 mM of sodium is enough to exchange ˜30% of divalent cations for remineralization purposes. The salt can be in any anion form, i.e. chloride, bicarbonate, hydroxide or even sulfate. The present inventors found that ammonium in the feed water transports as well through the membranes of a Donnan dialysis unit. In that context, a staged Donnan dialysis unit may be used, where the first stage Donnan dialysis unit is used to remove ammonium to a large extent, and in the second stage Donnan dialysis unit hardness is recovered for the mineralization step.

[0058] For the recovery of the minerals using nanofiltration, open nanofiltration (NF) membranes can be used that have low (0˜5%) retention for monovalent cations (i.e. sodium and ammonium) and moderate (20-30%) retention for divalent cations (i.e. calcium and magnesium) with groundwater concentrations. In this embodiment dNF80 membranes manufactured by NX Filtration BV (NL) were used. Approximate membranes fluxes for this separation are between 25 to 50 liters of permeate per m.sup.2 membrane area per hour (LMH) at 6 bar transmembrane pressure.