Nanofiltration systems and methods

Abstract

A multi-stage nanofiltration (NF) system for filtering a solute from a feed solution, where a downstream NF stage is more permissive to the solute than an upstream NF stage. In some examples, the nanofiltration system includes multiple nanofiltration stages in series, where each nanofiltration stage is more permissive to the solute than the nanofiltration stage immediately upstream.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of Chinese Application No. 202110772265.3, filed July 8, 2021, which is incorporated herein by reference.

[0002] The present disclosure relates to nanofiltration systems and methods. [Background technology]

[0003] The following paragraphs are not an admission that anything discussed therein is prior art or part of the knowledge of those skilled in the art.

[0004] Membrane separation processes concentrate solutes in an aqueous solution by applying positive pressure to one side of a filtration membrane. Membrane separation processes process a feed stream solution and produce a portion as a permeate and the remainder as a retentate. The concentration of solutes in the permeate is decreased compared to the concentration of solutes in the feed stream. The concentration of solutes in the retentate is increased compared to the concentration of solutes in the feed stream. The retentate is alternatively called a concentrate. Different separation membranes are capable of separating and concentrating different solutes.

[0005] Examples of such processes are reverse osmosis (RO), microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF). Nanofiltration uses membranes with pores of nanometer size. Nanofiltration membranes have pore sizes smaller than microfiltration and ultrafiltration membranes, but larger than reverse osmosis membranes. Nanofiltration membranes can have pores with pore sizes between 1 and 10 nanometers. Summary of the Invention

[0006] The following introduction is intended to orient the reader to the specification and is not intended to define any invention. One or more inventions may reside in combinations or subcombinations of system elements or method steps described below or elsewhere in this document. The inventors do not relinquish or waive rights to any one or more inventions disclosed herein by merely not claiming such other one or more inventions.

[0007] At high concentrations of solutes, the permeate recovery of a nanofiltration system with a single type of nanofiltration membrane may be undesirably low and / or may require undesirable operating conditions. For example, an aqueous feed stream with 25-27 wt. % H2SO4 and 25-35 g / L ferrous sulfate may result in a 40% recovery of sulfate by processing in a nanofiltration system with a single type of nanofiltration membrane at optimized operating conditions. In another example, a nanofiltration system with a single type of nanofiltration membrane for processing a sodium sulfate (Na2SO4) solution may need to be operated at undesirably high pressures, such as 120 bar, to achieve a concentration in the retentate of greater than 220 g / L, which is necessary to qualify as a zero liquid discharge (ZLD) process.

[0008] One or more of the described examples attempt to address or ameliorate one or more shortcomings associated with nanofiltration systems and methods that use a single type of nanofiltration membrane.

[0009] In one aspect, the present disclosure provides a multi-stage nanofiltration system for filtering a solute from a feed solution, where a downstream nanofiltration stage is more permissive to the solute than an upstream nanofiltration stage.

[0010] In some cases, the nanofiltration system includes multiple nanofiltration stages in series, with each nanofiltration stage being more permissive to a solute than the nanofiltration stage immediately upstream.

[0011] In some examples, the disclosure provides a nanofiltration system comprising a first nanofiltration stage producing a retentate and a permeate, and a second nanofiltration stage producing a retentate and a permeate, the second nanofiltration stage downstream of the first nanofiltration stage and receiving at least a portion of the retentate from the first nanofiltration stage, the second nanofiltration stage being more permeable to solutes than the first nanofiltration stage.

[0012] In another aspect, the disclosure provides a method of filtering a solute from a feed solution, the method comprising successive nanofiltration steps, each subsequent nanofiltration step being more permissive to the solute than the previous nanofiltration step.

[0013] In some examples, the disclosure provides a method of filtering a solute from a feed solution, the method including processing the feed solution in a first nanofiltration process to produce a first permeate and a first retentate, and processing at least a portion of the first retentate in a second nanofiltration process to produce a second permeate and a second retentate. The second nanofiltration process uses a nanofiltration member that is more permeable to the solute than the nanofiltration membrane used in the first nanofiltration process.

[0014] Nanofiltration systems and methods according to the present disclosure can be operated with NF stages at substantially the same pressures.

[0015] Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0016] [Figure 1] 1 is a schematic process flow diagram of a nanofiltration system according to the present disclosure. [Diagram 2] 1 is a schematic process flow diagram of another nanofiltration system according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Generally, the present disclosure provides a multi-stage nanofiltration (NF) system and method for filtering solutes from a feed solution.

[0018] In nanofiltration systems according to the present disclosure, downstream NF stages are more permissive to solutes than upstream NF stages, hi some examples, a nanofiltration system includes multiple nanofiltration stages in series, each nanofiltration stage being more permissive to solutes than the nanofiltration stage immediately upstream.

[0019] The tolerance of an NF stage can be quantified based on the solute rejection rate. A more permissive stage rejects less solute than a less permissive stage. The rejection rate can be determined by comparing the concentration of the solute in the permeate stream to the concentration of the solute in the feed stream. For example, if 5% of a solute from the feed stream passes through the stage into the permeate stream, the stage is 5% permissive to that solute. Alternatively, the stage may be said to have a solute rejection rate of 95%. A more permissive stage would be one that allows more than 5% of the solute from the feed stream to pass through the stage.

[0020] A nanofiltration stage may include multiple nanofiltration membranes, for example multiple nanofiltration membranes in a spiral wound membrane module. If all of the NF membranes in a NF stage are the same, the NF stage has the same tolerance as the membrane.

[0021] The tolerance of an NF stage depends on which solute is being evaluated. For example, one NF stage may tolerate 5% sulfate ions but 99% chloride ions. In the context of this disclosure, the tolerance of different NF stages is relative to a single solute of interest. For example, in a nanofiltration system for removing sulfate ions from a feed stream, an upstream NF stage may tolerate 5% sulfate ions and a downstream NF stage may tolerate 20% sulfate ions, regardless of their relative tolerance to other solutes such as chloride ions.

[0022] In the context of this disclosure, NF stages may be identified by a number, which corresponds to their relative placement from upstream to downstream. For example, a "second NF stage" (NF2) should be understood to be downstream of a "first NF stage" (NF1) without an intervening nanofiltration stage, and a "third NF stage" (NF3) should be understood to be downstream of a second NF stage, if present, without an intervening nanofiltration stage. Process equipment other than nanofiltration stages may be included between the nanofiltration stages. For example, pumps, tanks, online chemical dosing devices, filtration devices, or any combination thereof may be installed between the nanofiltration stages to achieve a desired pressure, temperature, pH, or turbidity, or to dose a desired chemical. In some exemplary systems, there is no process equipment between the nanofiltration stages. Operating such a system may result in energy savings and / or a simpler process.

[0023] In some cases, the tolerance of the first NF stage is 15% or less (i.e., the first NF stage may have a solute rejection rate of 85% or more). In some cases, the tolerance of the second NF stage is 5% or more (i.e., the second NF stage may have a solute rejection rate of 95% or less). In some cases, the tolerance of the third NF stage is 5% or more (i.e., the third NF stage may have a solute rejection rate of 95% or less). In some cases, the difference between the tolerance of the first NF stage and the tolerance of the second NF stage may be 5% to 70%.

[0024] It should be understood that, in the context of the present disclosure, any disclosure of a range of contemplated values ​​is also a disclosure of any value or subrange within the recited range, including the endpoints. For example, a contemplated percentage of "15% or less" is also a disclosure of, for example, 1%, 2.5%, 10%, between 2% and 15%, between 4% and 8%, and between 2% and 6%.

[0025] Notwithstanding the overlapping ranges noted above (e.g., NF1 tolerance may be 15% or less and NF2 tolerance may be 5% or more), it should be understood that downstream NF stages must still be more tolerant to solutes than the preceding NF stage. Thus, for example, if NF1 tolerance is 10%, then NF2 tolerance must be greater than 10%. If such an exemplary system includes NF3, then NF3 tolerance must be greater than NF2 tolerance.

[0026] In an example having two NF stages, the first NF stage may have a solute tolerance of 5% or less and the second NF stage may have a solute tolerance of 15% or more, e.g., about 30%, and the difference in tolerance between the first and second NF stages may be 10% to 70%. In a particular example, the first NF stage may have a solute tolerance of 3% and the second NF stage may have a solute tolerance of 50%, resulting in a tolerance difference of 47%.

[0027] In an example having three NF stages, the first NF stage may have a solute tolerance of 5% or less, the second NF stage may have a solute tolerance of 5% or more, e.g., 10% or more, and the third NF stage may have a solute tolerance of 5% or more, e.g., 20% or more. In an example having three NF stages, the first NF stage may have a solute tolerance of 1% to 5%, the second NF stage may have a solute tolerance of 5% to 15%, and the third NF stage may have a solute tolerance of 20% to 60%. In another example having three NF stages, the first NF stage may have a solute tolerance of 1% to 5%, the second NF stage may have a solute tolerance of 5% to 15%, and the third NF stage may have a solute tolerance of 40% to 60%.

[0028] One particular exemplary system according to the present disclosure is a two-stage nanofiltration system in which the solute is FeSO4, with NF1 having a tolerance of about 5% and NF2 having a tolerance of about 30%. Such a system can accept a feed solution containing 25 g / L Fe in H2SO4.

[0029] Another specific exemplary system according to the present disclosure is a three-stage nanofiltration system in which the solute is Na2SO4 or Li2SO4, with NF1 having a tolerance of about 1%, NF2 having a tolerance of about 10%, and NF3 having a tolerance of about 50%. Such a system can accept a feed solution containing 100 g / L Na2SO4, or 78 g / L Li2SO4.

[0030] It is understood that a NF stage immediately downstream of another NF stage is configured to receive at least a portion of the retentate from the immediately upstream NF stage. NF systems according to the present disclosure may be configured as a continuous high pressure centralized array. For example, a concentrated solution produced by an upstream NF stage can be fed directly to a subsequent downstream NF stage with substantially no pressure drop between the two NF stages. However, it should be understood that references to "immediately downstream" and "immediately upstream" do not exclude the possibility that process equipment other than a nanofiltration stage may be included between the nanofiltration stages. In certain instances, the process equipment does not provide a substantial pressure drop between the two NF stages.

[0031] NF systems according to the present disclosure can include two, three, or more NF stages. The NF system may include a recycle stream that returns at least a portion of the permeate from a downstream NF stage (e.g., from the second NF stage) to the inlet of the upstream NF stage (e.g., to the first NF stage). In a NF system with three NF stages, the system can include a recycle stream that returns at least a portion of the permeate from the third NF stage to the inlet of the first NF stage or to the inlet of the second NF stage.

[0032] The NF stage may include a thin film composite (TFC) NF membrane. The thin film composite NF membrane may be a polyamide or non-polyamide TFC membrane. The NF membrane may have a molecular weight cut-off (MWCO) of 150-3500 g / mol, e.g., 150-350 g / mol, 500-3500 g / mol, or 500-2500 g / mol. In some examples, the NF1 NF membrane has a MWCO of 150-350 g / mol and the NF2 NF membrane has a MWCO of greater than 350 g / mol, e.g., 500-3500 g / mol. The MWCO of NF3 may be higher than the MWCO of NF2, which may be higher than the MWCO of NF1.

[0033] A specific example of a suitable NF membrane is the Suez 1812 NF element, which is a thin film composite membrane with a MWCO of 150-300. Different stages can include membranes from the same product family with different solute tolerances. For example, one NF stage can include a Suez 1812 NF element with a solute tolerance of 3% and a downstream NF stage can include a Suez 1812 NF element with a solute tolerance of 45%. Two examples of Suez 1812 NF elements were used in the examples discussed below. On the one hand, the NF membrane was a polyamide thin film composite nanofiltration membrane with a sodium sulfate retention of 97% and a MgSO4 retention of more than 98% at 2000 ppm MgSO4, 110 psi, and 25°C. On the other hand, the NF membrane was a non-polyamide thin film composite nanofiltration membrane with 80%-95% sodium sulfate retention and 80%-95% MgSO4 retention at 2000 ppm MgSO4, 110 psi, and 25°C.

[0034] In some examples, the disclosure provides a nanofiltration system that includes a first nanofiltration stage that produces a retentate and a permeate, and a second nanofiltration stage that produces a retentate and a permeate. The second nanofiltration stage is downstream of the first nanofiltration stage and receives at least a portion of the retentate from the first nanofiltration stage. The second nanofiltration stage is more permeable to solutes than the first nanofiltration stage.

[0035] FIG. 1 illustrates an exemplary nanofiltration system according to the present disclosure. The nanofiltration system 10 includes a first nanofiltration stage 12 and a second nanofiltration stage 14. The first NF stage 12 receives a feed 16 and produces a retentate 18 and a permeate 20. The second NF stage 14 receives at least a portion of the retentate 18. The second NF stage 14 includes a nanofiltration membrane (not shown) that makes the second NF stage 14 more permeable to solutes than the first NF stage 12. The second NF stage 14 produces a retentate 22 and a permeate 24. The illustrated NF system 10 includes an optional recycle stream 26 that returns at least a portion of the permeate 24 to the inlet of the first NF stage 12.

[0036] FIG 2 illustrates an exemplary nanofiltration system according to the present disclosure. Nanofiltration system 30 includes the same features as nanofiltration system 10 illustrated in FIG 1, and therefore uses the same reference numbers for the common features. Nanofiltration system 30 further includes a third NF stage 32. Third NF stage 32 produces a retentate 34 and a permeate 36. Third NF stage 32 includes a nanofiltration membrane (not shown) that makes third NF stage 32 more permeable to solutes than first NF stage 12, and optionally more permeable to solutes than second NF stage 14.

[0037] The illustrated NF system 30 includes an optional recycle stream 38 that returns at least a portion of the permeate 36 to the inlet of the first NF stage 12. Such an optional recycle stream is suitable for systems operated to produce a permeate 36 having a solute concentration similar to the solute concentrations entering the first NF stage 12.

[0038] The illustrated NF system 30 includes an optional recycle stream 40 that returns at least a portion of the permeate 36 to the inlet of the second NF stage 14. Such an optional recycle stream is suitable for systems operated to produce a permeate 36 having a higher solute concentration than the solute concentration entering the first NF stage 12. Systems including the optional recycle stream 40 may further include a high pressure interstage pump 42.

[0039] In another aspect, the disclosure provides a method for filtering solutes from a feed solution. The method includes processing the feed solution in a first nanofiltration process to produce a first permeate and a first retentate, and processing at least a portion of the first retentate in a second nanofiltration process to produce a second permeate and a second retentate, the second nanofiltration process rejecting solutes at a lower rate than the first nanofiltration process. A portion of the second permeate can be recycled back to the first nanofiltration process. Optionally, the method may also include processing at least a portion of the second retentate in a third nanofiltration process, the third nanofiltration process rejecting solutes at a lower rate than the first nanofiltration process, and optionally at a lower rate than the second nanofiltration process.

[0040] In some exemplary methods, the first NF process may tolerate 15% or less of the solute (i.e., the first NF process may reject 85% or more of the solute). In some exemplary methods, the second NF process may tolerate 5% or more (i.e., the second NF process may reject 95% or less of the solute). In some examples, the third NF process may tolerate 5% or more (i.e., the third NF process may reject 95% or less of the solute). In some examples, the difference between the tolerance of the first NF process and the tolerance of the second NF process may be 5% to 70%.

[0041] As discussed above with respect to nanofiltration systems, it should be understood that, despite the overlap ranges noted above, the downstream NF process must still be more tolerant of the solute than the upstream NF process. Thus, for example, if a first NF process filters 90% of the solute (i.e., the process must tolerate 10% of the solute), then the second NF process must filter less than 90% of the solute (i.e., the process must tolerate more than 10% of the solute).

[0042] In an exemplary method having two NF processes, the first NF process may reject more than 95% of the solutes and the second NF process may reject 85% or less, e.g., about 70%, and the difference in rejection rate between the first and second NF processes may be 10% to 70%. In a particular example, the first NF process may reject 97% of the solutes and the second NF process may reject 50% of the solutes, resulting in a difference of 47%.

[0043] In an example having three NF processes, the first NF process may reject 95% or more, the second NF process may reject 95% or less, e.g., 90% or less, and the third NF stage may reject 95% or less, e.g., 80% or less. In an exemplary method having three NF processes, the first NF process may reject 99%-95% of the solutes, the second NF process may reject 95%-85% of the solutes, and the third NF process may reject 80%-40% of the solutes. In another exemplary method using three NF processes, the first NF process may reject 99%-95% of the solutes, the second NF process may reject 95%-85% of the solutes, and the third NF process may reject 60%-40% of the solutes.

[0044] One particular exemplary process according to the present disclosure is a two-stage nanofiltration process that receives a solution containing FeSO4 and processes the solution with a first NF process that retains about 95% of the solutes and a second NF process that retains about 70% of the solutes. The feed solution may contain 25 g / L Fe in H2SO4.

[0045] Another specific exemplary process according to the present disclosure is a three-stage nanofiltration process that receives a solution containing Na2SO4 or Li2SO4 and processes the solution in a first NF process that retains about 99% of the solutes, a second NF process that retains about 90% of the solutes, and a third NF process that retains about 50% of the solutes. The feed solution can contain 100 g / L Na2SO4, or 78 g / L Li2SO4.

[0046] The NF systems and methods according to the present disclosure may be configured to treat solutions containing solutes of interest, i.e., (i) sulfate salts in solutions containing sodium sulfate (NaSO), lithium sulfate (LiSO), aluminum sulfate (Al(SO)), ferrous sulfate (FeSO), and / or sulfuric acid (HSO); or (ii) soluble organic molecules, e.g., molecules present in landfill leachate bio-effluents, e.g., molecules having a molecular weight between 100 g / mol and 3500 g / mol.

[0047] When the solute of interest is sulfate, the feed solution may have a sulfate concentration of 5 g / L to 200 g / L. The feed solution may be, for example, a titanium dioxide waste stream, such as a waste stream containing 25-30 wt. % H2SO4 and at least 45 g / L Fe ions, an aluminum electroplating waste stream, such as a waste stream containing about 25 wt. % H2SO4 and at least 10 g / L Al ions, or a solution containing soluble sodium or lithium sulfates at a concentration of at least 100 g / L Na2SO4 or at least 80 g / L Li2SO4.

[0048] In the particle example, the initial feed solution may contain sodium sulfate at a concentration of 10-250 g / L and the system or method may be operated at a feed pressure of 10-250 bar. In a system or method having three NF stages or processes, the total dissolved solids of sodium sulfate may be (a) 100-200 g / L, e.g., 150-200 g / L, in the retentate of the first stage or process, and (b) 100-350 g / L, e.g., 150-300 g / L, in the retentate of the second and / or third stage or process.

[0049] When the solute of interest is a soluble organic compound, the solution can have an organic compound concentration of 3-10 g / L. EXAMPLES

[0050] An exemplary nanofiltration system according to the present disclosure was modeled. The table below illustrates the modeled flows and solute concentrations for the system depicted in Figure 1, where all of the NF1 retentate is transferred to NF2 and all of the NF2 permeate is recycled to NF1. The NF1 feed is a combination of the system feed and the NF2 permeate.

[0051] [Table 1]

[0052] In the modeled nanofiltration system of Table 1, the NF1 stage has a 45% recovery (i.e., 45% of the feed water becomes the permeate), rejects 97% of the Fe and -10% of the H2SO4, and the NF2 stage has a 50% recovery, rejects 50% of the Fe and rejects -10% of the H2SO4.

[0053] [Table 2]

[0054] In the modeled nanofiltration system of Table 2, the NF1 stage has a recovery of 45%, rejects 95% of the Al and -10% of the H2SO4, and the NF2 stage has a recovery of 50%, rejects 60% of the Al and -10% of the H2SO4.

[0055] [Table 3]

[0056] In the modeled nanofiltration system of Table 3, the NF1 stage has a recovery of 67% and rejects 99% of the total dissolved solids (TDS), and the NF2 stage has a recovery of 50% and rejects 60% of the TDS.

[0057] Two NF stages, corresponding to NF1 and NF2, were tested at 900 psi with different amounts of Na2SO4 total dissolved solids in the feed: NF1 stage used a polyamide thin film composite nanofiltration membrane with 97% sodium sulfate retention and >98% MgSO4 retention at 2000 ppm MgSO4, 110 psi, 25°C, and NF2 stage used a non-polyamide thin film composite nanofiltration membrane with 80%-95% sodium sulfate retention and 80%-95% MgSO4 retention at 2000 ppm MgSO4, 110 psi, 25°C.

[0058] When the feed was 100 g / L sodium sulfate, the NF1 stage produced 4.51 LMH (liters per m 2 It was determined that the NF2 stage could produce a permeate with an acceptable flux of 100 g / L sodium sulfate / hr and produce a retentate at 175 g / L sodium sulfate. Producing a retentate with a higher concentration of sodium sulfate resulted in an unacceptably high osmotic pressure difference and an unacceptably reduced permeate flux. The NF2 stage was able to produce a 175 g / L permeate with a flux of 14.5 LMH at the same feed concentration and pressure (100 g / L sodium sulfate and 900 psi), meaning that the NF2 stage was able to further concentrate the retentate produced by the NF1 stage at the same operating pressure.

[0059] In the foregoing description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. Thus, what has been described is merely illustrative of the application of the described embodiments, and numerous modifications and variations are possible in light of the above teachings.

[0060] The above description provides examples, and those skilled in the art will appreciate that modifications and variations can be made to the specific examples. Therefore, the claims should not be limited by the specific examples set forth herein, but should be interpreted consistent with the specification as a whole.

Claims

1. A nanofiltration system comprising: a first nanofiltration stage for producing a retentate and a permeate; a second nanofiltration stage for producing a retentate and a permeate, the second nanofiltration stage being downstream of the first nanofiltration stage and receiving at least a portion of the retentate from the first nanofiltration stage; and wherein the second nanofiltration stage is more permeable to solutes than the first nanofiltration stage.

2. The nanofiltration system according to claim 1, further comprising a recirculation flow that returns at least a portion of the permeate from the second nanofiltration stage to the inlet of the first nanofiltration stage.

3. wherein the solute is a sulfate, and the first nanofiltration stage includes an inlet for a sulfate-containing feed solution such as a sodium sulfate (Na ２ SO ４ ), aluminum sulfate (Al ２ (SO ４ ) ３ ), or iron sulfate (FeSO ４ ) feed solution, or wherein the solute is a soluble organic molecule, and the first nanofiltration stage includes an inlet for an organic molecule-containing feed solution. The nanofiltration system according to claim 1.

4. The nanofiltration system according to claim 3, wherein the sulfate-containing solution entering the system has a sulfate concentration of 5 g / L to 200 g / L.

5. The first nanofiltration stage has a solute rejection rate of at least 85%, for example, 95% to 99%, and the second nanofiltration stage has a solute rejection rate of up to 95%, for example, 85% to 95% or 50% to 85%. Optionally, the difference in rejection rate between the first NF stage and the second NF stage is 5% to 70%. For example, the first nanofiltration stage has a solute rejection rate of about 95%, and the second nanofiltration stage has a solute rejection rate of about 70%. The nanofiltration system according to any one of claims 1 to 4.

6. The first nanofiltration stage includes a polyamide thin film composite nanofiltration membrane having a molecular weight cut-off of 150 to 350 g / mol, and / or The second nanofiltration stage includes a non-polyamide thin film composite nanofiltration membrane having a molecular weight cut-off of 500 to 3500 g / mol. The nanofiltration system according to any one of claims 1 to 4.

7. Further includes a third nanofiltration stage for generating a retentate and a permeate. The third nanofiltration stage is downstream of the second nanofiltration stage, receives the retentate from the second nanofiltration stage, and the third nanofiltration stage is more permeable to the solute than the first nanofiltration stage. The nanofiltration system according to claim 1.

8. The third nanofiltration stage is more permeable to the solute than the second nanofiltration stage. The nanofiltration system according to claim 7.

9. The third nanofiltration stage has a solute rejection rate of at least 5%, for example, about 20% to about 80%, for example, about 40% to about 60%. For example, the first nanofiltration stage has a solute rejection rate of about 99%, the second nanofiltration stage has a solute rejection rate of about 90%, and the third nanofiltration stage has a solute rejection rate of about 50%. The nanofiltration system according to claim 7.

10. The third nanofiltration stage includes a non-polyamide thin film composite nanofiltration membrane having a molecular weight cut-off of 500 to 3500 g / mol. The nanofiltration system according to any one of claims 7 to 9. Claim 11 The nanofiltration system according to any one of claims 7 to 9, further comprising one or more recirculation flows that return at least a portion of the permeate from the third nanofiltration stage to the inlet of the first nanofiltration stage and / or the inlet of the second nanofiltration stage. Claim 12 A method for filtering a solute from a feed solution, comprising: processing the feed solution in a first nanofiltration process to produce a first permeate and a first retentate; processing at least a portion of the first retentate in a second nanofiltration process to produce a second permeate and a second retentate, wherein the second nanofiltration process uses a nanofiltration member that is more permeable to the solute than the nanofiltration membrane used in the first nanofiltration process. Claim 13 The method according to claim 12, further comprising recirculating at least a portion of the second permeate back to the first nanofiltration process. Claim 14 wherein the solute is a sulfate and the feed solution is a sulfate-containing feed solution, such as a sodium sulfate (Na ２ SO ４ ), aluminum sulfate (Al ２ (SO ４ ) ３ ), or iron sulfate (FeSO ４ ) feed solution, or wherein the solute is a soluble organic molecule and the feed solution is an organic molecule-containing feed solution, The method according to claim 12. Claim 15 The method according to claim 14, wherein the sulfate-containing solution entering the system has a sulfate concentration of 5 g / L to 200 g / L. Claim 16 The nanofiltration membrane used in the first nanofiltration process has a solute rejection rate of at least 85%, for example, about 95% to about 99%, and the nanofiltration membrane used in the second nanofiltration process has a solute rejection rate of up to 95%, for example, about 85% to about 95% or 50% to 85%. Optionally, the difference in rejection rates of the membranes used in the first and second NF processes is 5% to 70%. For example, optionally, to treat a solution containing FeSO ４ The method according to any one of claims 12 to 15, wherein the nanofiltration membrane used in the first nanofiltration process has a solute rejection rate of about 95%, and the nanofiltration membrane used in the second nanofiltration process has a solute rejection rate of about 70%.

17. Further comprising treating at least a portion of the second retentate in a third nanofiltration process to produce a third permeate and a third retentate, wherein the third nanofiltration process uses a nanofiltration membrane that is more permeable to the solute than the nanofiltration membrane used in the first nanofiltration process, and optionally further comprising returning at least a portion of the third permeate to the first nanofiltration process, the second nanofiltration process, or both for recirculation. The method according to claim 12.

18. The method according to claim 17, wherein the nanofiltration membrane used in the third nanofiltration stage is more permeable to the solute than the nanofiltration membrane used in the second nanofiltration process.

19. The nanofiltration membrane used in the third nanofiltration process has a solute rejection rate of at least 5%, for example, about 20% to about 80%, for example, about 40% to about 60%. For example, optionally, Na ２ SO ４ or Li ２ SO ４To treat a solution containing, the nanofiltration membrane used in the first nanofiltration process has a solute rejection rate of about 99%, the nanofiltration membrane used in the second nanofiltration process has a solute rejection rate of about 90%, and the nanofiltration membrane used in the third nanofiltration process has a solute rejection rate of about 50%. The method according to claim 17 or 18.

20. A method of filtering solutes from a feed solution, comprising successive nanofiltration steps, each subsequent nanofiltration step being more permissive of the solutes than the previous nanofiltration step.

21. The method according to any one of claims 12 to 15, 17, 18, 20, wherein the nanofiltration steps are at substantially the same pressure.

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