Methods for treating textile wastewater

Abstract

This invention provides a method for treating textile wastewater that consumes less energy, achieves high Na2SO4 permeability and high dye blocking, and a method for recovering Na2SO4 using this method. [Solution] A method for treating textile wastewater containing Na2SO4 and anionic dye, comprising a separation step of treating the textile wastewater with an NF membrane to obtain a permeate in which the Na2SO4 has selectively permeated and a concentrate in which the anionic dye has been concentrated, wherein the NF membrane is an anionic charged NF membrane, and the feed liquid supplied to the NF membrane contains an anionic dye in an amount of 1.5% by mass or less relative to Na2SO4.

Description

Technical Field

[0001] The present invention relates to a method for treating textile wastewater containing Na2SO4 and an anionic dye with an NF membrane, a method for treating textile wastewater, and a method for recovering Na2SO4 using the same.

Background Art

[0002] In recent years, the zero liquid discharge (ZLD) of textile wastewater containing dyes and the like has been accelerating in China and India. Further, as a method for treating wastewater containing dyes, a method using various separation membranes is known (for example, Patent Documents 1 and 2).

[0003] Textile wastewater may contain a dye and NaCl or Na2SO4 used as a color fixing agent. In the conventional membrane treatment method, only water is recovered from textile wastewater using a reverse osmosis (RO) membrane with a relatively large energy consumption, and after concentrating NaCl or Na2SO4 and the dye, the concentrated water is solidified with an evaporator to form a mixed salt.

[0004] The mixed salt increases in accordance with the ZLD, and also imposes a large burden on the environment in storage and landfill. Therefore, in recent years, there has been a demand for a technology capable of recovering and reusing NaCl or Na2SO4, which is a color fixing agent, from textile wastewater. In the case of textile wastewater containing NaCl and a dye, if a piperazine-based nanofiltration (NF) membrane is used, NaCl and the dye can be separated.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

[0006] However, in the case of textile wastewater containing Na2SO4 and dyes, selective separation using piperazine-based NF membranes was not possible, making it difficult to recover and reuse the Na2SO4. In other words, the conventional method for separating Na2SO4 and dyes involved cooling concentrated water containing concentrated Na2SO4 and dyes using an RO membrane or piperazine-based NF membrane to precipitate the Na2SO4, and then recovering it by centrifugation. However, this method had the problem of high energy consumption due to cooling and centrifugation.

[0007] Therefore, the object of the present invention is to provide a method for treating textile wastewater that consumes less energy, has high Na2SO4 permeability and high dye blocking properties, and a method for recovering Na2SO4 using the same. [Means for solving the problem]

[0008] The present inventors, after diligent research to solve the aforementioned problems, have found that by treating textile wastewater containing Na2SO4 and anionic dyes using an anionic charged NF membrane, and performing membrane separation in such a way that the feed solution contains 1.5% by mass or less of anionic dye relative to Na2SO4, energy consumption is reduced, and high Na2SO4 permeability and high dye blocking properties can be obtained, thus completing the present invention. That is, the present invention includes the following embodiments.

[0009] [1] The process includes a separation step of treating textile wastewater containing Na2SO4 and anionic dyes with an NF membrane to obtain a permeate in which the Na2SO4 has selectively permeated and a concentrate in which the anionic dyes have been concentrated. The aforementioned NF film is an anionic charged NF film, A method for treating textile wastewater, wherein the feed liquid supplied to the NF membrane contains an anionic dye at a concentration of 1.5% by mass or less relative to Na2SO4.

[0010] According to the textile wastewater treatment method of the present invention, since an anionic charged NF membrane is used, it is possible to ensure the blocking of anionic dyes through charge repulsion while allowing Na2SO4 to easily permeate through a pore size, thereby reducing the operating pressure. The detailed reason why high Na2SO4 permeability and high dye blocking are obtained by using a feed solution containing 1.5% by mass or less of anionic dye relative to Na2SO4 is unclear, but it is thought to be as follows.

[0011] When textile wastewater contains Na2SO4 and anionic dyes, Na2SO4 functions as a dye fixative. As can be seen from the comparative experiments described later, when Na2SO4 is at conventional concentrations (low concentrations), the dye tends to fix (adsorb) to the film surface, which tends to hinder the permeability of Na2SO4. In contrast, when Na2SO4 is at a higher concentration than conventional concentrations, Na2SO4 preferentially localizes to the film surface, inhibiting the fixation (adsorption) of the dye to the film surface, and it is thought that the localization of Na2SO4 improves permeability (see Figure 4).

[0012] As a result, a method for treating textile wastewater can be provided that consumes less energy, has high Na2SO4 permeability, and exhibits high dye blocking properties.

[0013] [2] The method for treating textile wastewater according to [1], wherein the supply liquid contains 50,000 ppm or more of Na2SO4.

[0014] As mentioned above, it is thought that permeability is improved by the localization of Na2SO4, but by including 50,000 ppm or more of Na2SO4, not just the relative amount to the dye, a higher level of Na2SO4 permeability can be reliably obtained.

[0015] [3] The method for treating textile wastewater according to [1] or [2], wherein the operating pressure of the NF membrane in the separation step is 3 MPa or less.

[0016] As described above, by using an anionic charged NF membrane, it is possible to have a pore size that allows easy permeation of Na2SO4 while ensuring the blocking property of anionic dyes due to charge repulsion, and the operating pressure can be reduced. However, when the operating pressure is 3 MPa or less, the permeability of Na2SO4 becomes sufficiently high, and the energy consumption becomes smaller.

[0017] [4] The method for treating textile wastewater according to any one of [1] to [3], wherein the separation step is carried out while returning a part of the permeate to the feed liquid.

[0018] Since the permeate contains a higher concentration of Na2SO4 than the anionic dye, by carrying out the separation step while returning a part of it to the feed liquid, it is possible to efficiently recover Na2SO4 while reducing the amount of Na2SO4 introduced from outside the system.

[0019] [5] A method for recovering Na2SO4 by recovering Na2SO4 from the permeate obtained in the separation step of the method for treating textile wastewater according to any one of [1] to [4].

[0020] [[ID=1​​​​​​​​​​​​​​​​​ [Figure 2] It is a schematic configuration diagram showing another example of a method for treating textile wastewater. [Figure 3] It is a perspective view with a part cut away showing an example of a spiral type membrane element used in a method for treating textile wastewater. [Figure 4] It is an explanatory diagram schematically showing the state of Na2SO4 and an anionic dye existing near the membrane surface. The left side shows the case where the concentration of Na2SO4 is low, and the right side shows the case where the concentration of Na2SO4 is high.

Embodiments for Carrying Out the Invention

[0023] Hereinafter, embodiments of the present invention will be described.

[0024] The method for treating textile wastewater of the present invention includes a separation step of treating textile wastewater containing Na2SO4 and an anionic dye with an NF membrane to obtain a permeate in which the Na2SO4 selectively permeates and a concentrate in which the anionic dye is concentrated. By such a separation step, Na2SO4, which has been difficult to selectively separate conventionally, can be efficiently recovered.

[0025] (Textile wastewater) The textile wastewater to be treated may be any as long as it contains Na2SO4 and an anionic dye, and may be one from which impurities, colloidal suspensions, miscellaneous bacteria, etc., have been removed by a pretreatment step using a UF membrane (ultrafiltration membrane), MF membrane (microfiltration membrane), or the like.

[0026] Examples of the anionic dye include reactive dyes, direct dyes, acid dyes, metal-containing dyes, etc., but acid dyes having anionic groups such as sulfonic acid groups and carboxyl groups, and direct dyes in which the anionic group forms a salt with a counter ion are preferred.

[0027] For example, examples of the acid dye include, in addition to Cold Dye Hot Red, Acid Red 52, DC Violet 2, DC Red 33, DC Orange 4, DC Red 27, DC Yellow 10, etc.

[0028] The concentration of the anionic dye is, for example, 100 to 10,000 ppm, but a typical concentration is 500 to 2,000 ppm. Regarding the molecular weight of the anionic dye, from the viewpoint of selectively separating Na2SO4 by the NF membrane, 300 to 800 daltons is preferred, and 500 to 800 daltons is more preferred.

[0029] The concentration of Na2SO4 can range from, for example, 10,000 to 100,000 ppm, but a typical concentration is 30,000 to 40,000 ppm.

[0030] Textile wastewater may contain other components, such as surfactants, dyeing aids, and non-anionic dyes.

[0031] (separation process) The present invention relates to a method for treating textile wastewater, which includes a separation step of treating the textile wastewater as described above with an NF membrane to obtain a permeate in which Na2SO4 has selectively permeated and a concentrate in which the anionic dye has been concentrated. Such a wastewater treatment method can be carried out using a membrane separation apparatus such as the one shown in Figure 1 or Figure 2.

[0032] For example, the membrane separation apparatus shown in Figure 1 includes a separation membrane 1 which is an anionic charged NF membrane, a membrane module M1 which comprises a supply section for feed liquid 7, a discharge section for permeate 8, and a discharge section for concentrated liquid 9, and a chemical supply tank 16 for supplying an aqueous Na2SO4 solution 17 to the feed liquid 7 line. Furthermore, the membrane separation apparatus shown in Figure 2 includes a recirculation path 6 for returning a portion of the permeate 8, which has a relatively high concentration of Na2SO4, back to the feed liquid 7.

[0033] Such membrane separation devices are configured to operate under desired conditions, and may be equipped with other devices such as pumps, sensors, tanks, control valves, and control devices as needed. Furthermore, a line may be provided to circulate part or all of the concentrated liquid 9 back into the supply liquid 7.

[0034] For membrane separation equipment, general operating conditions for NF membranes can be used, such as operating pressure, permeate recovery rate, and operating temperature. However, from the viewpoint of stable continuous use, a recovery rate (permeate flow rate / supply liquid flow rate × 100) of 50-85% is preferable.

[0035] When returning a portion of the permeate 8 to the supply liquid 7 using the membrane separation apparatus shown in Figure 2, it is preferable to return 10-50% of the permeate 8 obtained in the separation process, from the viewpoint of increasing the recovery efficiency of the permeate 8 recovered outside the system.

[0036] Furthermore, since the present invention uses an anionic charged NF membrane with high Na2SO4 permeability, the operating pressure (pressure of the feed liquid) can be reduced. Therefore, from the viewpoint of further reducing energy consumption, the operating pressure is preferably 3 MPa or less, and more preferably 2 MPa or less.

[0037] (supply liquid) In the present invention, the feed liquid supplied to the NF membrane in the membrane separation step is characterized by containing an anionic dye in an amount of 1.5% by mass or less relative to Na2SO4. From the viewpoint of obtaining higher Na2SO4 permeability, the content of the anionic dye relative to Na2SO4 is preferably 1.3% by mass or less, more preferably 1.1% by mass or less, and even more preferably 0.9% by mass or less. Furthermore, from the viewpoint of operating pressure and Na2SO4 recovery, it is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more.

[0038] While Na2SO4 can be added as a powder, it is preferable to supply it as an aqueous solution because the supply amount is easier to adjust. The concentration of Na2SO4 in the aqueous solution should be as high as possible to facilitate Na2SO4 recovery.

[0039] When supplying Na2SO4 as an aqueous solution, it is preferable to continuously supply the Na2SO4 aqueous solution 17 stored in the chemical supply tank 16 so that the anionic dye content relative to the Na2SO4 is 1.5% by mass or less, as shown in Figure 1. It is also possible to supply the Na2SO4 aqueous solution 17 while measuring the concentration with a conductivity meter or similar device to adjust the concentration.

[0040] Alternatively, textile wastewater may be stored in a tank or the like, and Na2SO4 powder or aqueous solution may be added to it so that the supply liquid supplied to the NF membrane contains 1.5% by mass or less of anionic dye relative to Na2SO4.

[0041] Furthermore, as shown in Figure 2, the concentration of Na2SO4 in the supply solution 7 can be increased by returning a portion of the permeate 6, which has a relatively high concentration of Na2SO4, back to the supply solution 7. In this case, it becomes possible to reduce the amount of Na2SO4 aqueous solution 17 supplied from the chemical supply tank 16, or to supply it intermittently.

[0042] From the viewpoint of obtaining higher Na2SO4 permeability more reliably, the concentration of Na2SO4 in the feed solution supplied to the NF membrane is preferably 50,000 ppm or higher, more preferably 80,000 ppm or higher, and even more preferably 100,000 ppm or higher. Furthermore, from the viewpoint of operating pressure and Na2SO4 recovery, it is preferably 200,000 ppm or lower, more preferably 180,000 ppm or lower, and even more preferably 150,000 ppm or lower.

[0043] (NF membrane) An anionic charged NF membrane is used as the NF membrane (nanofiltration membrane). From the viewpoint of increasing the permeability of the NF membrane to Na2SO4 while increasing the inhibitory effect on anionic dyes, the molecular weight cutoff of the NF membrane is preferably 500 to 1000 daltons, and more preferably 800 to 1000 daltons.

[0044] Here, the molecular weight cutoff of the NF film is measured as follows: First, several polyethylene glycols with different average molecular weights and monodisperse molecular weight distributions are prepared. An aqueous solution containing one of the polyethylene glycols at a concentration of 5000 ppm is prepared at a temperature of 25°C and a pressure of 4 kg / cm². 2 The material is supplied to the surface of the NF film under these conditions. This allows for the measurement of the rejection rate of polyethylene glycol. The rejection rate of other polyethylene glycols is measured in a similar manner. A fractionation curve is created showing the relationship between the obtained rejection rates and the average molecular weight of polyethylene glycol. Based on the fractionation curve, the average molecular weight of polyethylene glycol at which the rejection rate is 90% is identified. The identified average molecular weight can be considered as the fractional molecular weight of the NF film.

[0045] The NF membrane may be a composite semipermeable membrane comprising, for example, a porous support membrane and a separation functional layer, wherein the separation functional layer is supported by the porous support membrane. The material and structure of the porous support membrane are not particularly limited. As the porous support membrane, for example, an ultrafiltration membrane is used in which a microporous layer having an average pore size of 0.01 to 0.4 μm is formed on a nonwoven fabric. Examples of materials for forming the microporous layer include polysulfone, polyaryl ethersulfone such as polyethersulfone, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0046] NF films can be classified into charged and uncharged types depending on the presence or absence of surface charge. In this invention, an anionic charged NF film, which is a negatively charged charged NF film, is used.

[0047] Examples of anionic charged NF films include those having a separation functional layer with anionic groups. Examples of anionic groups include sulfonic acid groups and carboxylic acid groups, but sulfonic acid groups, which are strong acidic groups, are preferred.

[0048] Furthermore, examples of resins constituting the separation functional layer include polysulfone resins, polyamides, cellulose acetate, and polyvinyl alcohol, but polysulfone resins are particularly preferred from the viewpoint of chemical, mechanical, and thermal stability. Examples of polysulfone resins include polysulfone, polyethersulfone, and polyphenylsulfone.

[0049] In other words, a preferred separation functional layer for an NF membrane is one that contains a polysulfone resin having sulfonic acid groups. In particular, NF membranes having such a separation functional layer exhibit higher durability against alkaline cleaning solutions and chlorine-based cleaning solutions.

[0050] As for NF films in which the separation functional layer is made of sulfonated polyethersulfone having a negative fixed charge, those described in Japanese Patent Publication No. 61-4505 and Japanese Patent Publication No. 61-4506 are particularly preferred.

[0051] Examples of polysulfone resins having sulfonic acid groups include those having the following repeating units (A) or (B).

[0052] [ka]

[0053] [ka]

[0054] A membrane module M1 using an NF membrane may consist of one or more membrane elements. Typically, the membrane elements are spiral-type membrane elements using an NF membrane. The membrane module M1 may also consist of a pressure vessel and one or more spiral-type membrane elements arranged inside the pressure vessel. However, the structure of the membrane elements containing the NF membrane is not limited to the spiral type, and may be other types such as hollow fiber type, tubular type, or frame and plate type.

[0055] (Spiral-type membrane element) A spiral membrane element, for example as shown in Figure 3, comprises a perforated central tube 5 and a wound body R containing a separation membrane 1 wound around the central tube 5. In the example shown in Figure 3, it comprises a plurality of membrane leaves L with a permeate-side flow channel material 3 interposed between opposing separation membranes 1, a supply-side flow channel material 2 interposed between the membrane leaves L, a perforated central tube 5 around which the membrane leaves L and the supply-side flow channel material 2 are wound, and a sealing portion 12 to prevent mixing of the supply-side flow channel and the permeate-side flow channel. In this case, the permeate-side flow channel within the membrane leaves L can be formed by the permeate-side flow channel material 3 (also called a permeate-side spacer).

[0056] Figure 3 shows an example in which the sealing portion includes both end sealing portions and an outer peripheral sealing portion 12. Of the sealing portions, the both end sealing portions are formed by sealing the two end surfaces on both sides of the membrane leaf L in the axial direction A1 with adhesive. The outer peripheral sealing portion 12 is formed by sealing the end of the outer peripheral tip of the membrane leaf L with adhesive. The region enclosed by the opposing separation membrane 1, the both end sealing portions, and the outer peripheral sealing portion 12 becomes the permeable channel, which is in communication with the opening 5a of the central tube 5. A first end member 10 having a function such as a seal carrier may be provided on the upstream side of the wound body R of the membrane element, and a second end member 20 having a function such as an anti-telescopic material may be provided on the downstream side.

[0057] When using the membrane element, it is housed in a pressure vessel, and the supply liquid 7 is supplied from one end face of the membrane element. The supplied supply liquid 7 flows along the supply-side flow channel material 2 in a direction parallel to the axial direction A1 of the central tube 5, and is discharged as concentrated liquid 9 from the other end face of the membrane element. In addition, the permeate 8 that has permeated the separation membrane 1 as the supply liquid 7 flows along the supply-side flow channel material 2 flows along the permeate-side flow channel material 3, then flows into the interior of the central tube 5 through the opening 5a, and is discharged from the end of the central tube 5.

[0058] [Pre-treatment process] In this invention, it is also possible to perform pretreatment steps, such as the removal of solids and bacteria, prior to the separation step using an NF membrane. For example, by separating the wastewater to be treated using a UF membrane (ultrafiltration membrane) or an MF membrane (microfiltration membrane), solids such as impurities, colloidal suspensions, bacteria, and microorganisms can be removed. Here, a UF membrane is a membrane with an average pore size of about 0.001 μm to 0.01 μm. An MF membrane is a membrane with an average pore size of about 0.01 μm to 10 μm.

[0059] The material of the UF film or MF film is not particularly limited, and polymer materials such as cellulose ester polymers such as cellulose acetate, polyethylene, polypropylene, polysulfone, polyvinylidene fluoride, and polyethersulfone can be used. From the viewpoint of durability and cleanability, polyvinylidene fluoride and polyethersulfone are preferred. The shape of the UF film or MF film is not particularly limited, and can be selected and used from flat film, hollow fiber film, pleated film, and tubular film shapes.

[0060] [Method for recovering Na2SO4] The method for recovering Na2SO4 according to the present invention is characterized by recovering Na2SO4 from the permeate obtained in the separation step of the textile wastewater treatment method of the present invention described above. Therefore, the method for recovering Na2SO4 according to the present invention utilizes the textile wastewater treatment method of the present invention, and the separation step, pretreatment step, etc. are as described above.

[0061] Regarding the method for recovering Na2SO4 from the permeate, it is sufficient if it can be recovered in solid or liquid form, and the permeate can be reused as an aqueous Na2SO4 solution, either as is or after concentration. RO membranes or piperazine-based NF membranes can be used to concentrate the permeate.

[0062] Furthermore, as a method for recovering Na2SO4 in a solid state, the permeate can be concentrated as needed, and then Na2SO4 can be separated from the Na2SO4 aqueous solution using solid-liquid separation. This can be done by combining concentration, crystallization, and solid-liquid separation processes.

[0063] For concentration, heating evaporation, vacuum evaporation, and membrane separation using RO membranes can be used; for crystallization, cooling crystallization, vacuum crystallization, and reaction crystallization can be used; and for solid-liquid separation, centrifugation and filter separation can be used. [Examples]

[0064] The present invention will be described below with reference to examples, but the present invention is not limited in any way by these examples. In the examples, the physical properties, etc., were measured or evaluated by the following methods. Specifically, the physical property values, etc., in the present invention are values ​​measured by the following methods.

[0065] (1) Concentration and rejection rate of Na2SO4 Ten minutes after the start of the membrane separation operation, the feed solution, permeate, and concentrate were analyzed for SO4 concentration using an ion chromatography device (Thermo Fisher Scientific, Dionex ICS-6000). 2- The concentration was measured and converted to Na2SO4 concentration (ppm). Based on these results, the rejection rate of Na2SO4 was calculated using the following formula.

[0066] Na2SO4 rejection rate (%) = (1 - (Na2SO4 concentration in permeate / ((Na2SO4 concentration in feed solution + Na2SO4 concentration in concentrate) / 2))) × 100

[0067] (2) Dye concentration and rejection rate Ten minutes after the start of the membrane separation operation, the dye concentration (ppm) of the feed solution, permeate, and concentrate was measured using a gas chromatography-based concentration analyzer (Shimadzu Corporation, Nexis GC-2030). The dye rejection rate was then determined from these results based on the following formula.

[0068] Dye rejection rate (%) = (1 - (Dye concentration in permeate / ((Dye concentration in supply solution + Dye concentration in concentrate solution) / 2))) × 100

[0069] (Example 1) Considering the typical composition of textile wastewater, a model wastewater containing 40,000 ppm of Na2SO4 and 1,100 ppm of anionic dye (Katsuraya Fine Goods Co., Ltd., Call Dye Hot Red) was prepared. Na2SO4 was then added to this to prepare a feed solution with the composition shown in Table 1.

[0070] Using a membrane separation apparatus as shown in Figure 1, the operating conditions shown in Table 1 were set so that the recovery rate (permeate flow rate / feed liquid flow rate × 100) was 50%. The above feed liquid (temperature 25°C) was supplied to an anionic charged NF membrane (Nitto Denko, HYDRACore7450), and the concentrate and permeate were discharged. After 10 minutes, samples were taken from the permeate and concentrate, and the concentration of each component was measured as described above to determine the rejection rate of each component. The results are shown in Table 1.

[0071] (Comparative Example 1) In Example 1, the feed solution was composed as shown in Table 1, and a low-pressure RO membrane (Nitto Denko, ESPA4) was used. Except for the same recovery rate as in Example 1, samples were taken from the permeate and concentrate, and the concentration of each component was measured as described above to determine the rejection rate of each component. The results are shown in Table 1.

[0072] (Comparative Example 2) In Example 1, the feed solution was composed as shown in Table 1, and a piperazine-based NF membrane (Nitto Denko, PRO-XS3) was used. Except for the same recovery rate as in Example 1, samples were taken from the permeate and concentrate under the same operating conditions, and the concentration of each component was measured as described above to determine the rejection rate of each component. The results are shown in Table 1.

[0073] [Table 1]

[0074] As shown in the results in Table 1, in Example 1, which used an anionic charged NF film, the energy consumption was lower due to the lower operating pressure (supply fluid pressure), and high Na2SO4 permeability and high dye blocking properties were obtained.

[0075] In contrast, in Comparative Example 1, which used a low-pressure RO membrane, the energy consumption was higher due to the higher operating pressure, and since the rejection rates for Na2SO4 and the dye were almost the same, selective separation of the two was not possible. Furthermore, in Comparative Example 2, which used a non-charged NF membrane, although the operating pressure was slightly lower, it was almost impossible to increase the permeability of Na2SO4, making it difficult to selectively separate Na2SO4 and the dye.

[0076] (Examples 2-6) In Example 1, samples were taken from the permeate and concentrate under the same operating conditions as in Example 1, except that the amount of Na2SO4 added was varied to create the feed solution with the composition shown in Table 2. The concentration of each component was measured as described above, and the rejection rate of each component was determined. The results are shown in Table 2.

[0077] (Examples 7-9) In Example 1, samples were taken from the permeate and concentrate under the same operating conditions as in Example 1, except that the concentrations of the dye and Na2SO4 were varied to create the feed solution composition shown in Table 2. The concentration of each component was measured as described above, and the rejection rate of each component was determined. The results are shown in Table 2.

[0078] (Comparative Examples 3-5) In Example 1, samples were taken from the permeate and concentrate under the same operating conditions as in Example 1, except that the concentrations of the dye and Na2SO4 were varied to create the feed solution composition shown in Table 2. The concentration of each component was measured as described above, and the rejection rate of each component was determined. The results are shown in Table 2.

[0079] [Table 2]

[0080] As shown in the results in Table 2, in Examples 2-9, which used an anionic charged NF membrane and a feed solution containing 1.5% by mass or less of anionic dye relative to Na2SO4, energy consumption was lower, and high Na2SO4 permeability and high dye inhibition were obtained. In particular, in Examples 4-7, which used a feed solution containing 1.0% by mass or less of anionic dye relative to Na2SO4, not only was the operating pressure (feed solution pressure) reduced, but even higher Na2SO4 permeability was obtained.

[0081] In contrast, in Comparative Examples 3-5, where the feed solution contained more than 1.5% by mass of anionic dye relative to Na2SO4, the permeability of Na2SO4 could not be sufficiently increased, making it difficult to selectively separate Na2SO4 from the dye.

[0082] (Comparative experiment) To investigate why high Na2SO4 permeability and high dye inhibition can be obtained by membrane separation using an anionic charged NF membrane, a comparative experiment was conducted using a flat membrane, by using a feed solution containing 1.5% by mass or less of anionic dye relative to Na2SO4.

[0083] Specifically, two feed solutions were prepared: feed solution A (anionic dye ratio 1.7% by mass) containing 62,000 ppm of Na2SO4 and 1,087 ppm of the anionic dye, and feed solution B (anionic dye ratio 0.68% by mass) containing 160,000 ppm of Na2SO4 and 1,087 ppm of the anionic dye.

[0084] Using a flat anionic charged NF membrane (HYDRACore7450, manufactured by Nitto Denko), membrane separation of feed solution A and feed solution B was performed, and the coloration of the permeate, the behavior of the Flux, the appearance of the membrane surface, and the changes in the zeta potential were investigated when the recovery rate was varied. The following results were obtained.

[0085] Regarding the coloration of the permeate, it remains almost transparent until the recovery rate reaches around 10%, after which it gradually becomes colored. The permeate from supply solution A is more colored than the permeate from supply solution B. Therefore, it can be said that the dye permeates more easily from supply solution A.

[0086] Regarding the behavior of the flux, in the case of supply solution A, the flux was initially high but then decreased rapidly. In contrast, in the case of supply solution B, the decrease in flux was relatively gradual. Therefore, it can be said that adsorption to the membrane surface is more likely to occur with supply solution A.

[0087] Regarding the appearance of the film surface, the film was well-stained with supply solution A, while staining was suppressed with supply solution B. Therefore, it can be said that supply solution A resulted in greater adsorption of dye to the film surface.

[0088] Regarding the change in zeta potential, in the case of supply solution A, the negative charge was amplified due to the effect of the dye. In the case of supply solution B, the amplification of the negative charge was smaller than in the case of supply solution A. Therefore, it can be said that supply solution A adsorbs a larger amount of dye onto the membrane surface.

[0089] From these results, it was found that by using a feed solution containing 1.5% by mass or less of anionic dye relative to Na2SO4, the fixation (adsorption) of the dye onto the anionic charged NF membrane was suppressed, as was the decrease in Flux. It is thought that the suppression of dye fixation (adsorption) and decrease in Flux is due to the presence of excess Na2SO4 near the membrane surface, which selectively allows Na2SO4 to permeate, and as a result, membrane separation using the anionic charged NF membrane yields high Na2SO4 permeability and high dye inhibition.

[0090] Figure 4 schematically illustrates this state. In other words, Figure 4 is an explanatory diagram schematically showing the state of Na2SO4 and anionic dyes present near the membrane surface, with the left side showing the case where Na2SO4 is at a low concentration and the right side showing the case where Na2SO4 is at a high concentration. [Industrial applicability]

[0091] According to the present invention, a method for treating textile wastewater can be provided that consumes less energy and achieves high Na2SO4 permeability and high dye blocking properties. Therefore, Na2SO4, which was previously difficult to separate, can be recovered and reused with less energy consumption. [Explanation of Symbols]

[0092] 1. Separation membrane (anionic charged NF membrane) 7 Feed liquid 8 Permeate 9 Concentrate 16. Chemical supply tank 17 Na2SO4 aqueous solution M1 Membrane Module

Claims

1. Na ２ SO ４ Furthermore, textile wastewater containing anionic dyes is treated with an NF membrane, and the Na ２ SO ４ The separation step includes obtaining a permeate through which the anionic dye has been selectively permeated and a concentrate in which the anionic dye has been concentrated. The aforementioned NF film is an anionic charged NF film, The supply liquid supplied to the NF membrane is Na ２ SO ４ A method for treating textile wastewater, comprising an anionic dye containing 1.5% by mass or less of the above.

2. The supply liquid contains the Na ２ SO ４ A method for treating textile wastewater according to claim 1, comprising 50,000 ppm or more of the above.

3. The method for treating textile wastewater according to claim 1, wherein the operating pressure of the NF membrane in the separation step is 3 MPa or less.

4. The method for treating textile wastewater according to claim 1, wherein the separation step is carried out while returning a portion of the permeate to the supply liquid.

5. Recovering Na from the permeate obtained in the separation step of the method for treating textile wastewater according to any one of claims 1 to 4 ２ SO ４ Thereby recovering Na ２ SO ４ A method for recovering SO

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