Pretreatment method and pretreatment system for effluent

Abstract

The present invention provides a pretreatment method for efficiently recovering solvent from a raw material mixture comprising water, solvent, and dissolved and / or dispersed polymer. The pretreatment method includes the step of subjecting the raw material mixture (12) to ultrafiltration (16, 18) to filter out the polymer and obtain a polymer-reduced filtrate (36).

Description

Technical Field

[0001] This invention relates to a pretreatment method for wastewater. More specifically, this invention relates to a pretreatment method for wastewater containing solid components. Specifically, this method can be implemented as a preparation for the easy recovery of useful components contained in the wastewater, for pretreatment of the wastewater. Background Technology

[0002] Various manufacturing processes generate wastewater, but besides unwanted components, most of this wastewater also contains useful components. From both environmental and product cost perspectives, ideally, these useful components should be recycled and reused.

[0003] For example, in the case of preparing a dope by dissolving a polymeric material (e.g., a polysulfone polymer) in a suitable solvent (e.g., dimethylacetamide and water), and using this dope to manufacture hollow fibers, the dope is expelled from a nozzle along with a core liquid (e.g., dimethylacetamide). The expelled dope is then immersed in a coagulation bath, where coagulation and phase separation occur, forming hollow fibers.

[0004] In the process of manufacturing hollow fibers in this way, a discharge containing solvent and core liquid is generated as a spinning discharge. Preferably, the solvent and / or core liquid contained in this discharge are recovered and reused in the form of useful components. For example, distillation is often used to recover useful components from the discharge in this way.

[0005] Distillation can recover useful components such as solvents and / or core fluids with high purity, making it a useful processing method for reusing the recovered useful components. However, distillation involves phase transformation, and therefore the energy required for phase transformation is usually large, which is not ideal from the perspectives of manufacturing cost and environmental protection.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2016-30233 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] As mentioned above, the processing of the drain liquid requires a significant amount of energy when using distillation, thus necessitating efforts to reduce this energy expenditure. Furthermore, when the drain liquid contains high-boiling-point components or solids such as dissolved and / or dispersed polymers in addition to the solvent and core fluid, distillation leaves these components as residue in the reboiler. This residue is likely to adhere to the reboiler's heat transfer tubes as fouling, negatively impacting heat transfer.

[0011] Therefore, it is not necessarily preferred to perform distillation on the drain as is, and it is desirable to provide a useful new method for recovering the useful components contained therein from such drains.

[0012] Solution for solving the problem

[0013] To address the aforementioned problems, the inventors have repeatedly studied new methods for recovering useful components (e.g., solvents used to prepare sizing agents) from wastewater containing useful components, such as wastewater generated during the spinning of hollow fibers. They discovered that by pretreatment of the wastewater by filtering it before applying a distillation operation, the useful components can be recovered efficiently.

[0014] In a first embodiment, the present invention provides a pretreatment method for a raw material mixture as a drain, the raw material mixture comprising water, a solvent, and dissolved and / or dispersed polymers. The pretreatment method includes applying ultrafiltration to the raw material mixture to obtain a filtrate and a concentrate with increased polymer content. The resulting filtrate is substantially free of polymers, and therefore can be subjected to distillation to recover the solvent, for example.

[0015] In a preferred embodiment, the pretreatment method further includes a step of applying reverse osmosis filtration to the filtrate obtained by ultrafiltration to obtain a residue with reduced moisture content. Similarly, the resulting residue can be subjected to distillation treatment to recover the solvent, for example.

[0016] Such a pretreatment method can be implemented, for example, when recovering the solvent from the spinning wastewater generated as a raw material mixture in the manufacture of hollow fibers. Therefore, the present invention provides a method for manufacturing hollow fibers, wherein the raw material mixture generated by the spinning process comprises water, solvent, and dissolved and / or dispersed polymer, the method comprising (1) applying ultrafiltration to the raw material mixture to filter out the polymer and obtain a polymer-reduced filtrate in the form of a filtrate mixture, and in a preferred embodiment, after step (1), comprising (2) applying reverse osmosis filtration to the obtained filtrate to obtain a residue as a concentrated mixture with reduced moisture content.

[0017] In a second embodiment, the present invention provides a pretreatment system for a raw material mixture as a drain, the raw material mixture comprising water, solvent, and dissolved and / or dispersed polymers, the drain pretreatment system comprising: an ultrafiltration device to filter out the polymers from the raw material mixture to obtain a filtrate substantially free of polymers; and, if necessary, a reverse osmosis filtration device to process the filtrate to obtain a residue with reduced moisture content.

[0018] Invention Effects

[0019] In this invention, it is preferable to subject the filtrate obtained by ultrafiltration and / or the residue obtained by reverse osmosis filtration to distillation. When the filtrate is distilled, it is substantially free of polymers compared to the original feed mixture, or at most present in very small amounts. Therefore, the amount of polymer-induced fouling in the reboiler heated during distillation is significantly reduced, thus suppressing the adverse effects of fouling on heat transfer. Furthermore, when the residue is distilled, the water content is significantly reduced compared to the original feed mixture or the filtrate from ultrafiltration, thus saving energy required for distillation. Attached Figure Description

[0020] Figure 1 This is a flowchart schematically illustrating a preprocessing system for implementing the preprocessing method of the present invention. Detailed Implementation

[0021] In this invention, "pretreatment" refers to the treatment of a raw material mixture after which, for the purpose of recovering specific useful components from the raw material mixture as a drain, other treatments (such as distillation) are performed to facilitate and / or efficiently carry out the other treatments.

[0022] In this invention, the raw material mixture includes water, a solvent, and a polymer. The solvent has a higher boiling point than water, while the polymer has virtually no boiling point. The polymer typically exists in the raw material mixture as a solid component, and decomposes upon heating instead of evaporating; sometimes, depending on the environment, it may oxidize or carbonize. With such a raw material mixture, as the solid content increases, its viscosity increases, making it more difficult to filter.

[0023] In this invention, the solvent may be, for example, dimethylformamide, N-methyl-2-pyrrolidone, dimethylacetamide, dimethyl sulfoxide, etc., but is not limited thereto.

[0024] In this invention, the polymer can be, for example, cellulose or its derivatives, such as cellulose acetate, polymethyl methacrylate, polyacrylonitrile, polysulfone, polyethersulfone, polyvinylpyrrolidone, etc., but is not limited thereto. Such a polymer can be in a dissolved state, or in a dispersed state, or both, in the raw material mixture.

[0025] In this invention, when performing filtration, regardless of the method used, the filtration performance typically decreases gradually over time due to membrane clogging and other reasons. In this case, it is necessary to temporarily stop the filtration process and perform maintenance to restore filtration performance (e.g., membrane cleaning, membrane replacement, etc.) before restarting the filtration process. If a mixture of raw materials to be treated is still generated during the period when the filtration process is stopped, the raw material mixture generated during this period needs to be treated after restarting.

[0026] In this situation, if the capacity of the retention tank for the raw material mixture and the filtration performance after restart are sufficient, a single-series filtration unit can be used to carry out the filtration process. Conversely, if the capacity of the retention tank and / or the filtration performance are insufficient, multiple-series filtration units can be used, switching to other series during maintenance of one series to continue filtration. Two or more series of filtration units can be used depending on the maintenance time, filtration capacity, and retention tank capacity.

[0027] In this invention, ultrafiltration is a process commonly referred to as UF filtration. The filter membrane used in ultrafiltration can be of any suitable form, including flat membranes and hollow fiber membranes. Furthermore, when using a flat membrane, the filter assembly with the flat membrane can be pleated or spiral. When using a hollow fiber membrane, the filter assembly with the hollow fiber membrane is typically tubular. In this invention, the ultrafiltration device has such a filter assembly.

[0028] The ultrafiltration process yields a filtrate as a permeate and a concentrate as a concentrated mixture, wherein the polymer content in the concentrate is higher than that in the feed mixture to be treated. In this ultrafiltration process, the polymer, as a solid component, does not substantially move to the filtrate side through the filter membrane; therefore, the filtrate contains solvent and water. The polymer content of the concentrate is preferably high, preferably at least 20 times, more preferably at least 40 times, particularly preferably at least 50 times, most preferably at least 60 times, and for example, 70 times, the polymer content of the feed mixture. By performing ultrafiltration in this manner to achieve a high concentration ratio, most of the solvent and water contained in the feed mixture can be obtained as a filtrate.

[0029] In a particularly preferred embodiment of the invention, ultrafiltration is performed using a dead-end flow method followed by a cross-flow method. That is, the dead-end flow method and the cross-flow method are combined in series to perform ultrafiltration. By employing this combination, it becomes easier to obtain the aforementioned concentrate with a high polymer content.

[0030] For example, through initial dead-end flow ultrafiltration, only the filtrate that has passed through the filter membrane is removed at a concentration ratio preferably at least 5 times, more preferably 10 times, for example 15 times or more. At a stage where the polymer content in the concentrate reaches such a concentration ratio, the process is switched to another dead-end flow filtration process to continue filtration, and the concentrate is removed. This concentrate is then subjected to cross-flow ultrafiltration, with a concentration ratio preferably at least 2 times, more preferably at least 4 times, and particularly preferably at least 5 times, to obtain the filtrate that has passed through the filter membrane and a concentrate with a high polymer content at such a concentration ratio.

[0031] In dead-end flow mode, the raw material mixture is continuously fed to the filtration device, while only the filtrate is continuously removed and the concentrate that has not permeated through the membrane is accumulated. When the polymer content of the concentrate reaches the concentration ratio described above, for example, switching to another series of ultrafiltration devices to perform the ultrafiltration process in the same way. Therefore, when performing ultrafiltration in this manner, it is preferable to use, for example, at least two series of ultrafiltration devices. For example, filtration is performed in one series of filtration devices, while maintenance such as removing the accumulated concentrate and cleaning the filter membrane is performed in another series of filtration devices.

[0032] While ultrafiltration can be performed continuously using multiple series of filtration units, a single series of ultrafiltration units can also be used as described above, provided that the capacity of the tank holding the feed mixture to be ultrafiltered is sufficient, and the subsequent filtration capacity is also sufficient. In this case, the ultrafiltration process can be stopped, the concentrate removed, and maintenance performed. During the period when the filtration process is stopped, the feed mixture can simply be stored in the tank holding the feed mixture.

[0033] In dead-end flow ultrafiltration, as described above, as the polymer content of the concentrate increases, the viscosity of the concentrate becomes higher, making it difficult to continue dead-end flow ultrafiltration to further increase the polymer content. Therefore, it is preferable to use cross-flow ultrafiltration to perform ultrafiltration substantially continuously. In cross-flow, both the permeate that has passed through the filter membrane and the retentate that has not passed through the filter membrane are continuously removed. Similar to dead-end flow, two series of cross-flow ultrafiltration units can be provided to consider the maintenance of the filtration device. If a sufficient capacity retention tank is provided between the dead-end flow ultrafiltration unit and the cross-flow ultrafiltration unit, the cross-flow ultrafiltration unit can also be a single series. For example, ultrafiltration is performed in dead-end flow until the polymer content of the first concentrate reaches, for example, 5-20% by mass, preferably 7-15% by mass, more preferably 8-12% by mass, for example, 10% by mass, and then ultrafiltration is applied to the first concentrate in cross-flow.

[0034] Thus, by performing ultrafiltration in two stages, a concentrate with a high polymer content as a solid component, for example at least 30% by weight, preferably 40% by weight, and more preferably 50% by weight, can be obtained. It should be noted that, as described above, when combining dead-end flow and cross-flow modes, compared to ultrafiltration performed only in dead-end flow mode, a higher polymer content in the concentrate can be obtained, and the membrane life of the ultrafiltration device can be extended. One possible reason for this is that membrane clogging is relatively suppressed in the cross-flow mode.

[0035] In this invention, reverse osmosis filtration is a process commonly referred to as RO filtration. The filter membrane used in reverse osmosis filtration can be of any suitable form, including flat membranes and hollow fiber membranes. Furthermore, when using flat membranes, the filter assembly with the flat membrane can be pleated or spiral. When using hollow fiber membranes, the filter assembly with the hollow fiber membrane is typically tubular. In this invention, the reverse osmosis filtration device includes such a filter assembly.

[0036] The filtrate generated by ultrafiltration is treated by reverse osmosis filtration to remove water from the filtrate. In terms of the amount of water removed, a substantial portion, preferably at least 40%, more preferably at least 60%, and for example 65%, of the water in the filtrate obtained by ultrafiltration is removed as permeate. This permeate may contain trace amounts of solvent, preferably less than 50 ppm, more preferably less than 20 ppm, and even more preferably less than 10 ppm. The reverse osmosis filtration produces a residue with reduced moisture content, which contains the remaining solvent and water.

[0037] The resulting residue can be further processed, such as by distillation, to recover the contained solvent in high purity, for example, as a high-boiling-point residue. This solvent can then be recycled or reused, for example, in the spinning process, if needed. Alternatively, in such distillation, water containing solvent at a low concentration can be recovered as a low-boiling-point distillate, which can then be subjected to reverse osmosis filtration, for example, together with the filtrate generated by ultrafiltration.

[0038] In a particularly preferred embodiment of the invention, reverse osmosis filtration is performed in a cross-flow manner. In this manner, the filtrate to be treated is continuously supplied to the filtration unit, and both the residual material that has not permeated the filter membrane and the permeate that has permeated the filter membrane are continuously removed. When the performance of the filter membrane deteriorates, it is preferable to switch to another series of reverse osmosis filtration units to perform the reverse osmosis filtration process in the same way in order to clean the filter membrane. Therefore, at least two series of reverse osmosis filtration units are required to perform reverse osmosis filtration in this manner. For example, filtration is performed using one series of filtration units, while maintenance such as membrane cleaning is performed in another series of filtration units during this period.

[0039] While reverse osmosis filtration can be performed continuously using multiple series of filtration units, as mentioned above, a single series of reverse osmosis filtration units can also be used if the capacity of the filtrate retention tank and the filtration capacity are sufficient. In this case, the reverse osmosis filtration process can be stopped, and then maintenance can be performed. During the period when the filtration process is stopped, the filtrate from the ultrafiltration process can be stored in the filtrate retention tank.

[0040] In a preferred embodiment of the invention, the raw material mixture comprising water, solvent, and polymer is wastewater generated during the spinning of hollow fibers. This wastewater comprises water as the main component (e.g., 95% or more), a solvent (e.g., polyacetamide) as the remainder (e.g., a few percent), and typically a small amount (e.g., 1% or less) of dissolved and / or dispersed polymer (e.g., polysulfone) in the raw material mixture. From both environmental and manufacturing cost perspectives, the solvent is desirable to be recycled and reused.

[0041] Next, the implementation scheme of the present invention will be described in detail with reference to the accompanying drawings. Figure 1The flowchart schematically illustrates a pretreatment system for a raw material mixture for implementing the pretreatment of the present invention. In the illustrated scheme, ultrafiltration is performed using two series of dead-end flow ultrafiltration units and one series of cross-flow ultrafiltration units connected in series therewith. The resulting filtrate is then subjected to reverse osmosis filtration using two series of reverse osmosis filtration units. That is, the raw material mixture is ultrafiltered in two stages to obtain filtrate, which is then subjected to reverse osmosis filtration. In the illustrated scheme, for ultrafiltration, the first stage of ultrafiltration is performed by alternately switching between two series of dead-end flow filtration units, and the resulting concentrate is fed to the cross-flow filtration unit for the second stage of ultrafiltration, resulting in a filtrate and a concentrated mixture.

[0042] The raw material mixture 12 generated in the spinning process 10 contains water, solvent, and polymer as a solid component, and is stored in the raw material mixture retention tank 14. This raw material mixture is pretreated using the pretreatment method of the present invention. The raw material mixture stored in the tank 14 is then fed to a series of ultrafiltration units 16 to perform dead-end ultrafiltration as a first stage of ultrafiltration. In the illustrated flowchart, another series of ultrafiltration units 18 is also provided, resulting in two series of ultrafiltration units. By switching between them, ultrafiltration can be performed in a dead-end flow manner.

[0043] More specifically, a dead-end flow ultrafiltration process is performed in a series 16 to obtain a filtrate. This filtration process yields a first concentrate 20, containing a large portion, preferably substantially, of the total amount, of the solvent, water, and polymers contained in the feed mixture. Additionally, a first filtrate 22, containing the solvent and water, is obtained, with the polymers removed. The polymer content of the first concentrate is considerably greater than the polymer content of the feed mixture (e.g., 0.5–1.0% by weight), for example, 15 times the polymer content. At points where the filtration capacity becomes insufficient with each filtration process, the process is switched to another series of filtration devices 18 to continue filtration, similarly yielding the first concentrate 20' and the first filtrate 22'. During this time, the concentrate is removed and the filter membrane is cleaned in the previous series of filtration devices 16, performing necessary maintenance and preparing for the next filtration process. By providing two series of filtration devices 16 and 18 in this way, the feed mixture can be continuously filtered.

[0044] Thus, the first concentrate 20 (or 20') generated by dead-end flow ultrafiltration is supplied to a cross-flow ultrafiltration apparatus 24, which serves as the ultrafiltration unit, and subjected to a second stage of ultrafiltration. This treatment yields a second filtrate 26 that permeates through the membrane, and a second concentrate 28 is obtained as a concentrated mixture. The second concentrate has a polymer content that is, for example, five times greater than that of the first concentrate. The amount obtained by subtracting the second concentrate 28 from the first concentrate 20 permeates through the membrane, generating the second filtrate 26, which contains water and a solvent. In the illustrated embodiment, both the first and second filtrates are stored in a filtrate retention tank 30.

[0045] The first and second filtrates obtained as described above contain water and solvent, and are substantially free of polymer. They are then subjected to reverse osmosis filtration as filtrates obtained by ultrafiltration. In the illustrated embodiment, the reverse osmosis filtration is performed in two series of filtration devices arranged in parallel and switchable as described above. During filtration in one series, necessary maintenance, such as cleaning of the filter membrane, can be performed in the other series. Two series of reverse osmosis filtration devices 32 and 34 are used in the reverse osmosis filtration. In the case of pretreatment of the above-described feed mixture, the polymer is substantially entirely moved to the concentrate side by ultrafiltration. Therefore, the filtrate 36 supplied to the reverse osmosis filtration device 32 (or 34), after reverse osmosis filtration, yields water, possibly containing trace amounts of solvent, as permeate 38, and a residue as permeate 40.

[0046] By performing reverse osmosis filtration, the water contained in the filtrate can be removed to the permeate 38, resulting in a residue 40 with an increased solvent concentration. For example, by performing reverse osmosis filtration on a filtrate 36 with a solvent concentration of 2-3% by mass, a residue 40 containing 5-10% by mass of solvent can be obtained. Due to the mechanism of ultrafiltration, the amount of polymer contained in the filtrate, even if present, is extremely small; therefore, the polymer contained in the residue obtained by reverse osmosis filtration is also small.

[0047] Therefore, when the generated permeate 40 is subjected to distillation treatment 42 to recover the solvent, low-boiling-point water and the accompanying solvent are generated as distillate 44, and a bottom residue 46 (e.g., bottom liquid of a distillation apparatus) containing a large amount of high-boiling-point solvent, preferably substantially anhydrous, is discharged. This residue is substantially free of polymers and is preferably reused in the spinning process 10. Furthermore, since polymers do not enter the distillation system, the problem of polymer-induced fouling in the reboiler used in distillation is substantially eliminated. It should be noted that the distillate 44 (e.g., overhead distillate of a distillation apparatus) contains solvent and water, which can be recycled, for example, in a reverse osmosis filtration process, and ultimately, the contained water can move to the permeate 38 side.

[0048] It should be noted that the filtrate obtained by ultrafiltration is essentially free of polymers, therefore reverse osmosis filtration is not necessary. Instead, distillation treatment 42 can be applied directly to the filtrate from the retention tank 30, as shown by the dashed line 50. In this case, water is not removed from the filtrate via reverse osmosis filtration, thus offering no energy advantage, but avoiding the problem of fouling in the reboiler used in distillation.

[0049] Furthermore, by increasing the polymer content through ultrafiltration, the concentration ratio is increased (e.g., 50 times). The resulting concentrate 28 has a small absolute amount and contains substantially all the polymer contained in the raw material mixture in solid form. Therefore, by feeding it to the solid-liquid separation unit 52 for solid-liquid separation, it can be separated into a solid component 54 containing the polymer and the remaining liquid component 56. If necessary, the liquid component 56 can be fed to the retention tank 30 as shown by the dashed line 58 and fed together with the filtrate 36 to a reverse osmosis filtration process for further treatment. The solid component 54 can be disposed of or incinerated as needed after appropriate treatment.

[0050] Example

[0051] The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to such embodiments.

[0052] The spinning waste liquor, which is a raw material mixture generated during the spinning of hollow fibers for use in a spinning dialyzer, is pretreated. In addition to water, the spinning waste liquor contains, for example, dimethylformamide (e.g., 3-5% by mass) as a solvent, and cellulose acetate (e.g., 0.5-1.2% by mass) as a solid component.

[0053] The raw material mixture 12 is fed to dead-end flow ultrafiltration units 16 and 18, yielding first concentrates 20 and 20' with a solid content of 10% by mass and first filtrates 22 and 22' that are substantially free of solids. The first concentrate is fed to cross-flow ultrafiltration unit 24, yielding a second concentrate 28 as a concentrated mixture with a solid content of 45% by mass and a second filtrate 26 that is substantially free of solids. The second concentrate is then fed to solid-liquid separation unit 52 to separate the solid and liquid components.

[0054] The first and second filtrates stored in the filtrate retention tank 30 are supplied as filtrate 36 to the reverse osmosis filtration units 32 and 34, resulting in a permeate 40 with a solvent concentration of 5-10% by mass. The permeate is recovered as permeate 38. The solvent contained in the permeate is in trace amounts, for example, less than 10 ppm.

[0055] Industrial availability

[0056] The pretreatment method and pretreatment apparatus of the present invention are effective methods that can be implemented before the actual solvent recovery process when recovering solvent from a mixture containing water, solvent and polymer, thereby facilitating and maximizing solvent recovery.

[0057] Explanation of reference numerals in the attached figures

[0058] 10: Spinning process; 12: Raw material mixture; 14: Raw material mixture retention tank; 16, 18: Dead-end ultrafiltration device; 20, 20': First concentrate; 22, 22': First filtrate; 24: Cross-flow ultrafiltration device; 26: Second filtrate; 28: Second concentrate as a concentrated mixture; 30: Filtrate retention tank; 32, 34: Reverse osmosis filtration device; 36: Filtrate; 38: Permeate; 40: Permeate residue; 42: Distillation process; 44: Distillate; 46: Kettle residue; 50: Pipeline for directly supplying filtrate to the distillation process; 52: Solid-liquid separation device; 54: Solid component; 56: Liquid component; 58: Pipeline for supplying liquid component from the solid-liquid separation device to the filtrate retention tank.

Claims

1. A pretreatment method for recovering solvent from a distillation process of a raw material mixture that is a discharge from a hollow fiber spinning process, the raw material mixture comprising water, a solvent, and dissolved and / or dispersed polymers. The pretreatment method includes the step of applying ultrafiltration to the raw material mixture to filter out the polymer and obtain a filtrate with reduced polymer content. Ultrafiltration is performed by combining a dead-end flow method with a concentration ratio of at least 5 times and a cross-flow method with a concentration ratio of at least 2 times in series. The ultrafiltration is performed by first filtering the raw material mixture through the dead-end flow method to obtain a first concentrate and a first filtrate, and then filtering the first concentrate through the cross-flow method to obtain a second concentrate and a second filtrate.

2. The preprocessing method according to claim 1, wherein, By using dead-end ultrafiltration, a concentrate with a solid content of 5-20% by mass is obtained.

3. The pretreatment method according to claim 1 or 2, wherein, It also includes the process of applying reverse osmosis filtration to the filtrate after ultrafiltration to obtain a residue with reduced moisture content.

4. A pretreatment system for recovering solvent from a distillation treatment apparatus comprising a raw material mixture as a discharge from a hollow fiber spinning process, the raw material mixture comprising water, a solvent, and dissolved and / or dispersed polymers, the system comprising: An ultrafiltration device filters out polymers from a mixture of raw materials to obtain a filtrate with reduced polymer content; as well as The reverse osmosis filtration system processes the filtrate, reducing the amount of water and residual substances. The ultrafiltration device performs ultrafiltration by combining a dead-end flow mode with a concentration ratio of at least 5 times and a cross-flow mode with a concentration ratio of at least 2 times in series. The ultrafiltration is performed by first filtering the raw material mixture through the dead-end flow mode to obtain a first concentrate and a first filtrate, and then filtering the first concentrate through the cross-flow mode to obtain a second concentrate and a second filtrate.

5. The preprocessing system according to claim 4, wherein, Implement the preprocessing method as described in claim 3.

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