MULTI-VALENT ION CONCENTRATION USING MULTI-STAGE NANOFILTRATION

Abstract

A system and method for producing from saline source water a product containing an increased ratio of multi-valent ions to mono-valent ions, which includes multiple nanofiltration units arranged to selectively remove mono-valent ions from the water fed into each nanofiltration stage in the nanofiltration permeate stream while retaining multi-valent ions in the nanofiltration reject stream. The rate at which the increase in the multi-valent ion to mono-valent ion ratio is obtained may be enhanced by introduction of lower salinity water into the nanofiltration reject between stages, and by recirculating a portion of downstream nanofiltration reject flow into an upstream nanofiltration unit. The enhanced multi-valent ion product is suitable for multiple uses, including irrigation of plants and remineralization of desalinated water. The relative concentrations of the multi-valent ions in the product may be adjusted, for example by selection of nanofiltration membrane technologies which have higher or lower rejection for specific multi-valent ions.

Claims

1.-18. (canceled)

19. A multi-valent ion concentration system comprising: a plurality of nanofiltration (NF) units arranged in series, wherein each NF unit comprises one or more feed water inlet, a membrane configured to reject multi-valent ions, one or more NF unit permeate stream outlet configured to output a NF permeate stream, and one or more NF unit reject stream outlet configured to output a NF reject stream, and piping configured to fluidically connect the plurality of NF units arranged in series, wherein a NF reject stream outlet of an upstream NF unit is fluidically connected to a feed water inlet of a downstream NF unit by piping between the upstream NF unit and the downstream NF unit and wherein a NF reject stream outlet of the downstream NF unit is fluidically connected to a feed water inlet of a further downstream NF unit by piping between the downstream NF unit and the further downstream NF unit, and wherein a first one of the plurality of NF units is configured to receive a saline source water at a feed water inlet, and wherein at least one of: a) piping between the upstream NF unit and the downstream NF unit and b) the downstream NF unit, and at least one of: a) piping between the downstream NF unit and the further downstream NF unit and b) the further downstream NF unit, are configured to individually receive a lower total dissolved salts (TDS) water stream that comprises a TDS content lower than a TDS content of the saline source water, to form a diluted NF reject stream at a volume ratio of 2:1 to 5:1 of the lower TDS water to the NF reject stream prior to contact with the membrane.

20. The multi-valent ion concentration system of claim 19, wherein a last one of the plurality of NF units in the series is configured to output an NF reject stream comprising a ratio of multi-valent ions to mono-valent ions that is increased by at least 200% as compared to a ratio of multi-valent ions to mono-valent ions in the saline source water.

21. The multi-valent ion concentration system of claim 19, wherein the system is configured to reduce a concentration of Na.sup.+ and Cl.sup.− in a NF reject stream to at least ⅙.sup.th a concentration of Na.sup.+ and Cl.sup.− in the saline source water.

22. The multi-valent ion concentration system of claim 19, wherein the system is configured to produce a diluted NF reject stream comprising a concentration of CaSO.sub.4 that is less than 250% of a CaSO.sub.4 saturation concentration.

23. The multi-valent ion concentration system of claim 19, further comprising recirculation piping configured to fluidically connect at least a portion of a NF reject stream from one or more NF units and provide the at least a portion of the NF reject stream to one or more feed water inlets of one or more NF units upstream in the series.

24. The multi-valent ion concentration system of claim 23, wherein the recirculation piping is configured to fluidically connect a feed water inlet of the first one of the plurality of NF units in the series with a NF reject stream of a NF unit downstream in the series.

25. The multi-valent ion concentration method of claim 23, wherein the recirculation piping is configured to fluidically connect at least a portion of the NF reject stream of a last NF unit of the series with a NF unit upstream in the series.

26. The multi-valent ion concentration method of claim 23, wherein the recirculation piping is configured to fluidically connect at least a portion of the NF reject stream of a penultimate NF unit of the series with a NF unit upstream in the series.

27. The multi-valent ion concentration system of claim 19, further comprising permeate recirculation piping configured to receive at least a portion of a NF permeate stream and provide the permeate stream to one or more feed water inlets of one or more NF units downstream in the series.

28. The multi-valent ion concentration system of claim 27, wherein the permeate recirculation piping is configured to fluidically connect at least a portion of the NF permeate stream of the first one of the NF units in the series with a feed water inlet of a NF unit downstream in the series.

29. The multi-valent ion concentration system of claim 27, wherein the permeate recirculation piping is configured to fluidically connect at least a portion of a NF permeate stream to a feed water inlet of a last one of the NF units in the series.

30. The multi-valent ion concentration system of claim 19, comprising at least four NF units in the series.

31. The multi-valent ion concentration system of claim 19, wherein the NF unit reject stream outlet from a last one of the NF units in the series is fluidically connected with a concentration unit configured to concentrate the NF reject stream of the last one of the NF units in the series.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a schematic illustration of an embodiment of a multi-stage nanofiltration multi-valent ion concentration system in accordance with the present invention.

[0018] FIGS. 2 to 5 are additional embodiments of a multi-stage nanofiltration multi-valent ion concentration system in accordance with the present invention.

DETAILED DESCRIPTION

[0019] FIG. 1 shows an example embodiment of a multi-valent ion concentration system with multi-stage nanofiltration in accordance with a first embodiment of the present invention. In this embodiment, a saline source water 100, such as seawater, is fed into a first nanofiltration (NF) unit 110 as the first NF unit feed. In the first NF unit 110, a membrane 111 selectively permits mono-valent ions such as sodium and chloride to pass to a permeate side 112, leaving multi-valent ions such as calcium and magnesium in a retentate side 113 of the first NF unit 110. The first NF permeate stream 114 which leaves the first NF unit 110 is transported downstream for further processing and/or use in other applications. The first NF unit reject stream 115, now higher multi-valent-to-mono-valent ratio than the saline source water 100, passes out of the first NF unit 110.

[0020] Between emerging from the first NF unit 110 and entry as feed water to a second NF unit 120, the first NF unit reject stream 115 is diluted by the addition of lower salinity water 101, which lowers the concentration of both multi-valent and mono-valent ions, and helps minimize scaling concerns by reducing the stream concentrations to below scaling risk limit concentrations. The lower salinity water has a lower concentration of minerals than the original saline source water. Although diluted, the ratio of multi-valent ions to mono-valent ions remains the same as when the first NF unit reject stream 115 emerges from the first NF unit 110, and the total quantity of the ions (milligram equivalents) in the reject stream is unchanged. The present invention is not limited to dilution of an upstream NF unit's NF reject before the NF reject enters a downstream NF unit. Alternatively, the lower salinity water may be received in the downstream NF unit's retentate side and mixed with the incoming NF reject stream from the upstream NF unit, as long as sufficient dilution occurs near the downstream NF unit's feed inlet to avoid undesirably large localized scale deposition from the incoming undiluted NF reject stream.

[0021] As in the first NF unit 110, the diluted first NF unit reject 115 is processed in the second NF unit 120 with mono-valent ions traversing the membrane 121. The mono-valent ions exit from the permeate side 122 in the second NF permeate 124, while the multi-valent ion-rich second NF unit reject stream 125 leaves the second NF unit's retentate side 123. With this second nanofiltration step, the ratio of the concentration of multi-valent ions to mono-valent ions in the second NF unit reject stream 125 is further increased.

[0022] After the second NF unit reject stream 125 leaves the second NF unit 120, in this embodiment the lower salinity water is also injected to dilute the stream 125 to lower concentrations and avoid scaling. Depending on the facility design and processing requirements, the addition of lower salinity water is not required between every pair of previous and next NF stages. For example, if at a next stage the scaling risk is already low, further dilution may not be required. Also, in this embodiment the source of the lower salinity water is shared between all of the stages, but the inter-stage lower salinity water injection may be provided at different locations with different lower salinity water sources, as long as these sources provide sufficient dilution to obtain the desired product. For example, a portion of the low salinity stream from a reverse osmosis (RO) unit may be used at one injection point, thereby making additional utilization of the output from the RO unit, while a different-concentration stream output from another desalination process or other industrial process is used between other NF stages.

[0023] There may be “n” number of stages in the present invention, with the number of stages typically determined by the level of purity desired in the final product from the “nth” NF unit reject stream. In the FIG. 1 embodiment, similar third and fourth NF units 130, 140, with respective corresponding membranes 131, 141, permeate sides 132, 142, retentate sides 133, 143, permeate streams 134, 144, and NF reject streams 135, 145 are illustrated. The final NF unit reject stream 145 has the highest ratio of multi-valent to mono-valent ions, i.e., high purity, with very low mono-valent ion concentrations. This product may be used in a liquid form as-is, may be diluted to suit a particular application, may be further concentrated by liquid removal, dried into a solid form, transported for further processing as a feed stock for another process, or otherwise handled to suit a downstream application.

[0024] A quantitative illustration of example system flows and concentrations is presented in Tables 1-4, below, in conjunction with a second embodiment of the present invention shown in FIG. 2.

[0025] In the embodiment shown in FIG. 2, a portion 202 of the fourth NF unit reject stream 245 is recirculated into the saline source water 200, thereby increasing the amount of multi-valent ions in the NF unit 210's feed and the ratio of multi-valent ions to mono-valent ions, and helping maintain the concentrations below their respective scaling risk limits. This embodiment shows recirculation of the fourth NF unit reject stream 245 into the saline source water 200 feed, but such recirculation may be directed between any pair of NF unit stages. The recirculation also does not have to be from the final NF unit stage, but may be from one or more of the intermediate NF stages.

[0026] Table 1 shows example the changes in concentrations of key ions and the increase in the ratio of the primary multi-valent to mono-valent ions of interest (Ca+Mg)l(Na+Cl) in the first stage of an embodiment such as in FIG. 2, in which the scaling risk is limited to a CaSO4 saturation of 250%, and the feed saline source water receives 30% recirculation from the final stage nanofiltration unit.

TABLE-US-00001 TABLE 1 NF#1 Feed (after mixing with 30% NF#1 (unit: ppm) Seawater recirculation) Reject TDS (total dissolved solids) 41,000 40,300 65,900 Cl− 22,400 21,200 26,300 Na+ 12,500 11,800 14,700 SO4−− 3,500 4,540 17,160 Mg++ 1,500 1,660  5,420 Ca++ 450 460  1,300 K+ 480 450   560 HCO3− 160 160   390 (Ca + Mg)/(Na + Cl) 5.6% 6.4% 16.4% CaSO.sub.4 saturation N/A N/A  250%

[0027] In this first stage the ratio of multi-valent ions to mono-valent ions has approximately tripled to 16.4%.

[0028] Table 2 shows changes in concentrations and the increase in the ratio of multi-valent to mono-valent ions (Ca+Mg)l(Na+Cl) in the second stage, where the scaling risk is again limited to a CaSO4 saturation of 250%, and the NF reject 215 from the first NF unit 210 receives lower salinity water 201 from a reverse osmosis process at a ratio of approximately 2.75 liters of lower salinity water added per liter of NF reject. At the end of this stage, the ratio of multi-valent ions to mono-valent ions has reached 41.6%.

TABLE-US-00002 TABLE 2 NF#1 NF#2 Feed (after mixing NF#2 (unit: ppm) Reject with RO product water) Reject TDS 65,900 17,400 36,700 Cl− 26,300 7,000 8,600 Na+ 14,700 3,900 4,900 SO4−− 17,160 4,520 17,100 Mg++  5,420 1,430 4,600 Ca++  1,300 340 970 K+   560 150 190 HCO3−   390 100 250 (Ca + Mg)/(Na + Cl) 16.4% 16.3% 41.6% CaSO.sub.4 saturation  250% N/A N/A

[0029] At the end of the second stage, the ratio of multi-valent ions to mono-valent ions has further increased 2.5 times from 16.3% to 41.6%.

[0030] Table 3 shows changes in concentrations and the increase in the ratio of multi-valent to mono-valent ions (Ca+Mg)/(Na+Cl) in the third stage, where the NF reject 225 from the second NF unit 220 receives lower salinity water 201 at a ratio of approximately 2.5 liters of lower salinity water added per liter of NF reject.

TABLE-US-00003 TABLE 3 NF#2 NF#3 Feed (after mixing NF#3 (unit: ppm) Reject with RO product water) Reject TDS 36,700  10,700 29,300  Cl− 8,600 2,500 3,200 Na+ 4,900 1,400 1,800 SO4−− 17,100  5,000 18,900  Mg++ 4,600 1,360 4,400 Ca++   970 280   800 K+   190 60   70 HCO3−   250 70   180 (Ca + Mg)/(Na + Cl) 41.6% 41.2% 105.4% CaSO.sub.4 saturation  250% N/A   250%

[0031] At the end of the third stage, the ratio of multi-valent ions to mono-valent ions has increased another 2.5 times to 105.4%.

[0032] Table 4 shows the results of the fourth and final nanofiltration stage in the FIG. 2 embodiment. Similar to the previous stages, the NF reject 235 from the third NF unit 230 receives lower salinity water 201 at a ratio of approximately 2.25 liters of lower salinity water added per liter of NF reject. Following the fourth stage nanofiltration process the increase in the ratio of multi-valent to mono-valent ions (Ca+Mg)/(Na+Cl) is again approximately 2.5 times, to 263.1%, while the original saline source water's Na+Cl concentration has been reduced nearly 20 times, from 34,900 ppm to 1,900 ppm. Throughout the nanofiltration stages in this embodiment, scaling risk has been managed to the CaSQ4 saturation limit of 250%.

TABLE-US-00004 TABLE 4 NF#3 NF#4 Feed (after mixing NF#4 (unit: ppm) Reject with RO product water) Reject TDS 29,300  9,000 29,000   Cl− 3,200 1,000 1,200   Na+ 1,800 600 700 SO4−− 18,900  5,800 22,000   Mg++ 4,400 1,400 4,400   Ca++   800 240 680 K+   70 20  30 HCO3−   180 60 130 (Ca + Mg)/(Na + Cl) 105.4% 102.5% 263.1% CaSO.sub.4 saturation   250% N/A   250%

[0033] In the FIG. 2 embodiment the targeted amount of multi-valent ion concentration increase was 200%, a level which is suitable for use of the final NF reject for, for example, re-mineralization of desalinated water or for agricultural irrigation of crops which have high demand of magnesium and calcium for enhanced yield, such as mangoes. In this example, the target increase was achieved in four NF stages. Additional, or fewer, nanofiltration stages may be used as needed to obtain a desired ratio of multi-valent ions to mono-valent ions. Appropriate adjustment (or between some stages, even elimination) of the amount of lower salinity water introduced into the intermediate NF reject streams and/or the original saline source water stream, as well as use of different nanofiltration membrane types may also be used to adjust process performance.

[0034] The amount of lower salinity water and the amount of recirculated NF reject may be varied as necessary to suit the desired product parameters, for example, by increasing or decreasing the amount of lower salinity water added between different stages. Preferably, the ratio or lower salinity water to NF reject is in the range of 2:1 to 5:1, however, the present invention is not limited to these ranges, and includes a ratio of 0:1, i.e., no lower salinity water addition between one or more stages. Similarly, the amount of recirculation may be increased or decreased, and/or the recirculation arrangements may be varied, for example, by supplying downstream NF reject to more than one upstream NF reject stream, and/or more than one downstream NF reject stream may be used as a source for recirculation to one or more upstream NF reject streams.

[0035] FIG. 3 illustrates another embodiment of the present invention. As compared to the FIG. 2 embodiment, in FIG. 3 lower salinity water 301 is only introduced into the NF reject streams 315, 325 from the first and second NF units 310, 320, respectively. At least a portion 302 of the intermediate third NF unit reject stream 335 is recirculated to the original saline source water being fed into the first NF unit 310. Alternatively, depending on the system needs and design, recirculated NF reject may originate from any of the downstream NF units, up to and including the “n-lth” NF unit at the end of an “n”-length NF unit system.

[0036] In addition to the above differences, in the FIG. 3 embodiment the feed stream into the last NF unit 340 (which may be the “nth” NF unit in an embodiment with more NF stages) is the previous NF unit's permeate stream 334, which is then processed in the NF unit 340 to obtain the desired product. Some of the benefits of this type of configuration is that the final NF unit's reject 345 is free from substances which are rejected at the previous NF unit 330's NF membrane 331. For example, if an anti-sealant is being added to the NF system to minimize scale deposition, but the presence of the anti-sealant is not desired in the final product, a configuration such as in FIG. 3 eliminates this concern. Similarly, because nanofiltration rejects undesired contaminants such as bacteria, colloids, viruses and color, a configuration such as in FIG. 3 permits the final product to be free from such contaminants that might be present in the “n-lth” NF unit's NF reject. This arrangement may also result in a lower total dissolved solids content in the final product, and final product might have desirably different ratio of multi-valent ions. This latter advantage is due to different NF membranes having different rejections for some same-valent ions (e.g., selective decreasing of the ratio of calcium to magnesium by using an NF membrane with higher rejection performance for magnesium as compared to calcium).

[0037] The use of NF permeate as a feed source is not limited to using the “n-lth” NF unit's permeate as the feed into the “nth” NF unit, but may be applied between any of the upstream nanofiltration stages. For example, FIG. 4 is an embodiment of the present invention in which the NF permeate 414 from the first NF unit 410 is the feed water, along with a recirculated portion 402 of NF reject 445, that is fed into the second NF unit 420.

[0038] In the FIG. 4 embodiment, in which the NF permeate stream 414 from the first NF unit 410 is used as the feed water for the second NF unit 420, along with a recirculated portion 402 of the NF reject 445 from the last NF unit 440. In this arrangement, the undesired contaminates are removed from the system at an earlier stage as compared to the FIG. 3 embodiment, minimizing contaminate content in the downstream nanofiltration units.

[0039] The present invention is not limited to embodiments in which only a single NF permeate stream is the feed water to a downstream NF unit, but includes embodiments in which more than one NF permeate stream is used as the feed water to one or more downstream NF units.

[0040] The embodiment shown in FIG. 5 is a variation of the present invention in which the final stage of the system includes a desalinator/concentrator 550. This unit is included in the process train after the target ratio of multi-valent ion to mono-valent ions has been achieved. The ion content of the product water is then concentrated by the desalinator/concentrator 550 to a desired concentration level. A desalinator/concentrator typically rejects all ions at similar rates, i.e., does not further selectively remove different ions. As a result the desalinator/concentrator 550 only concentrates the ions in the previous NF unit 530's NF reject 535, which already has a multi-valent ion to mono-valent ion ratio at or above the target ratio. Accordingly, the concentrated product 555 has approximately the same multi-valent ion to mono-valent ion ratio, in a lower product volume. A desired concentration of a target ratio-satisfying NF reject is not limited to the use of a desalinator/concentrator, but may be performed by any suitable process which reduces the volume of the final NF reject.

[0041] The present invention provides the capability to concentrate selected multi-valent ions relative to mono-valent ions in a saline source water, with the ratio of desired multi-valent ions and/or undesired mono-valent ions being able to be determined by variations in the system design and operating parameters, e.g., use of different nanofiltration membrane technologies, use of different scaling risk limits, use of different amounts of lower salinity water dilution and/or NF reject recirculation, rearrangement of dilution and/or recirculation injection locations.

[0042] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Because such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

TABLE-US-00005 Listing of reference labels: 100, 200, 300, 400, 500 saline source water 110, 120, 130, 140, 210, 220, 230, 240, nanofiltration unit 310, 320, 330, 340, 410, 420, 430, 440, 510, 520, 530 550 desalinator/concentrator 111, 121, 131, 141, 211, 221, 231, 241, nanofiltration membrane 311, 321, 331, 341, 411, 421, 431, 441, 511, 521, 531 112, 122, 132, 142, 212, 222, 232, 242, nanofiltration unit 312, 322, 332, 342, 412, 422, 432, 442, permeate side 512, 522, 532 113, 123, 133, 143, 213, 223, 233, 243, nanofiltration unit 313, 323, 333, 343, 413, 423, 433, 443, retentate side 513, 523, 533 114, 124, 134, 144, 214, 224, 234, 244, nanofiltration permeate 314, 324, 334, 344, 414, 424, 434, 444, 514, 524, 534 554 desalinator/concentrator - low concentration water 115, 125, 135, 145, 215, 225, 235, 245, nanofiltration reject 315, 325, 335, 345, 415, 425, 435, 445, 515, 525, 535 555 desalinator/concentrator - high concentration product