Textile wastewater treatment method

Abstract

The invention relates to a treatment method of textile wastewater. The invention provides a method for treating textile wastewater and a method for recovering Na2SO4 by using the method for treating textile wastewater, wherein the energy consumption is lower, and high Na2SO4 permeability and high dye retention can be obtained. The present invention relates to a method for treating textile wastewater, the method comprising a separation step for treating textile wastewater containing Na2SO4 and an anionic dye by using a nanofiltration membrane, the nanofiltration membrane is an anion-charged nanofiltration membrane, and the anionic dye is contained in an amount of 1.5 mass% or less relative to the Na2SO4 in the supply liquid supplied to the nanofiltration membrane, so that a permeated liquid in which the Na2SO4 selectively permeates and a concentrated liquid in which the anionic dye is concentrated are obtained.

Description

Technical Field

[0001] This invention relates to a method for treating textile wastewater containing Na2SO4 and anionic dyes using nanofiltration membranes, and a method for recovering Na2SO4 using the same method. Background Technology

[0002] In recent years, the implementation of zero liquid discharge (ZLD) for textile wastewater containing dyes has accelerated in China and India. Furthermore, various separation membrane methods are known as treatment methods for dye-containing wastewater (e.g., Patent Documents 1-2).

[0003] Textile wastewater sometimes contains dyes and NaCl or Na2SO4 used as fixing agents. In the past, reverse osmosis (RO) membranes, which consume a lot of energy, were used to recover only water from textile wastewater while concentrating NaCl or Na2SO4 and dyes. Then, the concentrated water was turned into a solid by an evaporator to make mixed salt.

[0004] Mixed salts increase with ZLD (Zero-Local Dioxide) formation, imposing a significant environmental burden even during storage and landfill. Therefore, in recent years, there has been a demand for technologies capable of recovering and reusing NaCl or Na2SO4, used as fixing agents, from textile wastewater.

[0005] In the case of textile wastewater containing NaCl and dyes, NaCl and dyes can be separated by using piperazine nanofiltration (NF) membranes.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 54-31969

[0009] Patent Document 2: Japanese Patent No. 6436483 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, in the case of textile wastewater containing Na2SO4 and dyes, piperazine-based NF membranes cannot be used for selective separation, resulting in difficulties in recovering and reusing Na2SO4. Specifically, existing methods for separating Na2SO4 and dyes involve cooling the concentrated water containing Na2SO4 and dyes after concentration using an RO membrane or piperazine-based NF membrane to precipitate Na2SO4, which is then recovered by centrifugation. However, this method suffers from high energy consumption due to cooling and centrifugation.

[0012] Therefore, the object of the present invention is to provide a method for treating textile wastewater with lower energy consumption and higher Na2SO4 permeability and dye retention, as well as a method for recovering Na2SO4 using the textile wastewater treatment method.

[0013] means for solving problems

[0014] To solve the aforementioned problems, the inventors conducted repeated and in-depth research, and discovered that by using anion-charged nanofiltration membranes to treat textile wastewater containing Na₂SO₄ and anion-type dyes, with the feed solution containing less than 1.5% by mass of anion-type dyes relative to Na₂SO₄, membrane separation results in lower energy consumption and achieves high Na₂SO₄ permeability and high dye retention, thus completing this invention. Specifically, this invention includes the following methods.

[0015] [1] A method for treating textile wastewater, wherein the method includes a separation step, wherein a nanofiltration membrane is used to treat textile wastewater containing Na2SO4 and anionic dyes to obtain a permeate in which Na2SO4 is selectively permeated and a concentrate in which the anionic dyes are concentrated.

[0016] The nanofiltration membrane is an anion-charged nanofiltration membrane.

[0017] The feed solution supplied to the nanofiltration membrane contains less than 1.5% by mass of anionic dye relative to Na2SO4.

[0018] According to the textile wastewater treatment method of the present invention, since an anion-charged nanofiltration membrane is used, the retention of anionic dyes is ensured by charge repulsion, while the pore size, which allows for easy permeation by Na2SO4, can be adopted, thereby reducing operating pressure. The details of why high Na2SO4 permeability and high dye retention can be obtained by adjusting the supply solution to contain less than 1.5% by mass of anionic dye relative to Na2SO4 are not yet clear, but it is believed to be as follows.

[0019] In textile wastewater containing Na₂SO₄ and anionic dyes, Na₂SO₄ acts as a fixing agent for the dyes. Therefore, as shown in the comparative experiments described later, when Na₂SO₄ is at its previous concentration (low concentration), there is a tendency for dyes to be easily fixed (adsorbed) onto the membrane surface, which hinders the permeability of Na₂SO₄. Conversely, it is believed that when the Na₂SO₄ concentration is higher than before, Na₂SO₄ preferentially exists locally on the membrane surface, thus hindering dye fixation (adsorption) on the membrane surface, and through the localized presence of Na₂SO₄, permeability is increased (see reference).Figure 4 ).

[0020] As a result, a method for treating textile wastewater with lower energy consumption and higher Na2SO4 permeability and dye retention has been developed.

[0021] [2] According to the textile wastewater treatment method described in [1], the supply liquid contains more than 50,000 ppm of the Na2SO4.

[0022] As mentioned above, it is believed that permeability is improved by the local presence of Na2SO4. By specifying not only the relative amount with respect to the dye, but also containing more than 50,000 ppm of Na2SO4, high Na2SO4 permeability can be obtained more reliably.

[0023] [3] According to the textile wastewater treatment method described in [1] or [2], the operating pressure of the nanofiltration membrane in the separation process is below 3 MPa.

[0024] As described above, by using anion-charged nanofiltration membranes, the retention of anion-type dyes is ensured through charge repulsion, while the pore size that allows Na2SO4 to pass through is adopted, which can reduce the operating pressure. When the operating pressure is below 3 MPa, the permeability of Na2SO4 is sufficiently improved, and the energy consumption is even lower.

[0025] [4] The method for treating textile wastewater according to any one of [1] to [3], wherein the separation process is performed while a portion of the permeate is returned to the supply liquid.

[0026] The permeate contains a higher concentration of Na2SO4 than anionic dyes. Therefore, by performing a separation process while returning a portion of the permeate to the feed solution, the amount of Na2SO4 introduced from outside the system can be reduced, and Na2SO4 can be recovered efficiently.

[0027] [5] A method for recovering Na2SO4, wherein Na2SO4 is recovered from the permeate obtained in the separation step of the method for treating textile wastewater as described in any one of [1] to [4].

[0028] According to the Na2SO4 recovery method of the present invention, Na2SO4 is recovered using the textile wastewater treatment method of the present invention. Therefore, the energy consumption is smaller, and high Na2SO4 permeability and high dye retention can be obtained. Thus, Na2SO4 can be recovered from textile wastewater efficiently.

[0029] Invention Effects

[0030] According to the present invention, a method for treating textile wastewater with lower energy consumption, higher Na2SO4 permeability and higher dye retention, and a method for recovering Na2SO4 using the textile wastewater treatment method are provided. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating an example of a method for treating textile wastewater.

[0032] Figure 2 This is a schematic diagram illustrating another example of a method for treating textile wastewater.

[0033] Figure 3 This is a perspective view showing a portion of a spiral membrane element used in a textile wastewater treatment method after it has been cut off.

[0034] Figure 4 This is a schematic diagram illustrating the state of Na2SO4 and anionic dyes present near the membrane surface. The left side represents the case where Na2SO4 is at a low concentration, and the right side represents the case where Na2SO4 is at a high concentration.

[0035] Label Explanation

[0036] 1. Separation membrane (anion-charged nanofiltration membrane)

[0037] 7. Supply fluid

[0038] 8. Permeable liquid

[0039] 9. Concentrate

[0040] 16 Chemical Liquid Supply Tanks

[0041] 17 Na2SO4 aqueous solution

[0042] M1 membrane module Detailed Implementation

[0043] The embodiments of the present invention will be described below.

[0044] The textile wastewater treatment method of the present invention includes a separation step in which textile wastewater containing Na2SO4 and anionic dyes is treated using a nanofiltration membrane to obtain a permeate in which Na2SO4 has selectively permeated and a concentrate in which the anionic dyes have been concentrated. This separation step enables the efficient recovery of Na2SO4, which was previously difficult to selectively separate.

[0045] (Textile wastewater)

[0046] Any textile wastewater containing Na2SO4 and anionic dyes can be treated. It can also be textile wastewater that has been pretreated by using UF membranes (ultrafiltration membranes) or MF membranes (microfiltration membranes) to remove impurities, colloidal suspended solids, bacteria, etc.

[0047] Examples of anionic dyes include reactive dyes, direct dyes, acid dyes, and metal-containing dyes. Among them, acid dyes with anionic groups such as sulfonic acid groups and carboxyl groups, and direct dyes in which anionic groups and counterions form salts are preferred.

[0048] For example, examples of acid dyes include: Cold Dye Hot Red, as well as Acid Red 52, DC Violet 2, DC Red 33, DC Orange 4, DC Red 27, DC Yellow 10, etc.

[0049] The concentration of the anionic dye is, for example, 100 ppm to 10,000 ppm, with a typical concentration of 500 ppm to 2,000 ppm. The molecular weight of the anionic dye, from the viewpoint of selectively separating Na₂SO₄ by nanofiltration, is preferably 300 Daltons to 800 Daltons, and more preferably 500 Daltons to 800 Daltons.

[0050] The concentration of Na2SO4 is, for example, 10,000 ppm to 100,000 ppm, with a typical concentration of 30,000 ppm to 40,000 ppm.

[0051] Textile wastewater may contain other components, such as surfactants, dyeing auxiliaries, and dyes other than anionic dyes.

[0052] (Separation process)

[0053] The textile wastewater treatment method of the present invention includes a separation step in which the textile wastewater described above is treated using a nanofiltration membrane to obtain a permeate in which Na2SO4 is selectively permeated and a concentrate in which the anionic dye is concentrated. Such a wastewater treatment method can, for example, use... Figure 1 or Figure 2 The membrane separation device shown is used to implement this.

[0054] For example, Figure 1 The membrane separation apparatus shown includes a membrane module M1, which comprises a separation membrane 1 (an anion-charged nanofiltration membrane), a supply section for feed liquid 7, a discharge section for permeate 8, and a discharge section for concentrate 9. A chemical liquid supply tank 16 for supplying an aqueous Na₂SO₄ solution 17 is provided on the supply liquid 7 pipeline. Additionally, Figure 2The membrane separation device shown also includes a reflux path 6, which is used to return a portion of the permeate 8, in which Na2SO4 is present at a relatively high concentration, to the feed solution 7.

[0055] Such membrane separation devices may be equipped with pumps, sensors, tanks, control valves, control devices, and other equipment as needed, configured to operate under desired conditions. Additionally, pipelines may be provided to circulate part or all of the concentrate 9 to the supply liquid 7.

[0056] Operating conditions such as operating pressure, permeate recovery rate, and operating temperature in a membrane separation device can be those commonly used for nanofiltration membranes. However, from the viewpoint of stable continuous use, the recovery rate (permeate flow rate / feed flow rate × 100) is preferably 50% to 85%.

[0057] In use Figure 2 In the case where the membrane separation device shown returns a portion of the permeate 8 to the supply liquid 7, from the viewpoint of improving the recovery efficiency of the permeate 8 that is recycled outside the system, it is preferable to return 10% to 50% of the permeate 8 obtained in the separation process.

[0058] Furthermore, in this invention, since anion-charged nanofiltration membranes with high Na2SO4 permeability are used, the operating pressure (pressure of the supply 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.

[0059] (Supply fluid)

[0060] The present invention is characterized in that, in the membrane separation process, the feed liquid supplied to the nanofiltration membrane contains 1.5% by mass or less of anionic dye relative to Na₂SO₄. From the viewpoint of obtaining higher Na₂SO₄ permeability, the content of 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, more preferably 0.5% by mass or more.

[0061] While Na₂SO₄ can be added in powder form, it is preferred to supply it as an aqueous solution because the supply rate is easier to adjust. From the perspective of Na₂SO₄ recovery, the concentration of Na₂SO₄ in the aqueous solution is preferably as high as possible.

[0062] When Na2SO4 is supplied in the form of an aqueous solution, it is preferable to, for example, Figure 1As shown, the Na2SO4 aqueous solution 17 stored in the chemical solution supply tank 16 is continuously supplied in such a manner that the content of the anionic dye relative to Na2SO4 is 1.5% by mass or less. Alternatively, the Na2SO4 aqueous solution 17 can be supplied while the concentration is measured using a conductivity meter or the like for concentration adjustment.

[0063] Alternatively, the following method can be used: storing textile wastewater in tanks or the like, and adding powdered or aqueous solution of Na2SO4 to it, so that the content of anionic dyes relative to Na2SO4 in the feed liquid supplied to the nanofiltration membrane is less than 1.5% by mass.

[0064] In addition, such as Figure 2 As shown, by returning a portion of the permeate 6, which has a relatively high Na2SO4 concentration, to the supply liquid 7, the concentration of Na2SO4 in the supply liquid 7 can be increased. In this case, the supply amount of the Na2SO4 aqueous solution 17 from the chemical liquid supply tank 16 can be reduced, or the supply can be made intermittently.

[0065] From the viewpoint of reliably obtaining high Na2SO4 permeability, the concentration of Na2SO4 in the feed solution supplied to the nanofiltration membrane is preferably 50,000 ppm or more, more preferably 80,000 ppm or more, and even more preferably 100,000 ppm or more. Furthermore, from the viewpoint of operating pressure and Na2SO4 recovery, it is preferably 200,000 ppm or less, more preferably 180,000 ppm or less, and even more preferably 150,000 ppm or less.

[0066] (Nanofiltration membrane)

[0067] Anion-charged nanofiltration membranes are used as nanofiltration membranes (NF membranes). From the viewpoint of improving the permeability of Na2SO4 to the nanofiltration membrane while improving the retention of anion-type dyes, the molecular weight cutoff of the nanofiltration membrane is preferably 500 Daltons to 1000 Daltons, and more preferably 800 Daltons to 1000 Daltons.

[0068] Here, the molecular weight cutoff of the nanofiltration membrane is determined 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 various polyethylene glycols at a concentration of 5000 ppm is heated at a temperature of 25°C and a pressure of 4 kg / cm². 2The polyethylene glycol (PEG) is supplied to the surface of the nanofiltration membrane under specific conditions. This allows for the determination of the PEG rejection rate. The rejection rates of other PEGs are determined using the same method. A rejection curve is constructed showing the relationship between the obtained rejection rate and the average molecular weight of the PEG. Based on the rejection curve, the average molecular weight of the PEG with a 90% rejection rate is determined. This determined average molecular weight can be considered as the molecular weight cutoff of the nanofiltration membrane.

[0069] Nanofiltration membranes can be, for example, composite semi-permeable membranes comprising a porous support membrane and a separation functional layer, wherein the separation functional layer is supported by the porous support membrane. There are no particular limitations on the material and structure of the porous support membrane. For example, ultrafiltration membranes with a microporous layer having an average pore size of 0.01 μm to 0.4 μm formed on a nonwoven fabric can be used as the porous support membrane. Examples of materials forming the microporous layer include polysulfone, polyethersulfone, polyarylene ethersulfone, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0070] Nanofiltration membranes can be classified as charged or uncharged depending on whether their surface is charged. In this invention, an anion-charged nanofiltration membrane, which is a charged nanofiltration membrane with a negative charge, is used.

[0071] Examples of anion-charged nanofiltration membranes include nanofiltration membranes having a separation functional layer with anionic groups. Examples of anionic groups include sulfonic acid groups, carboxylic acid groups, etc., with sulfonic acid groups being preferred as strong acid groups.

[0072] In addition, examples of resins constituting the separation functional layer include polysulfone resins, polyamides, cellulose acetate, and polyvinyl alcohol. Polysulfone resins are particularly preferred from the viewpoints of chemical stability, mechanical stability, and thermal stability. Examples of polysulfone resins include polysulfone, polyethersulfone, and polyphenylsulfone.

[0073] That is, the preferred separation functional layer of the nanofiltration membrane is a separation functional layer comprising a polysulfone resin having sulfonic acid groups. In particular, nanofiltration membranes with such a separation functional layer also exhibit higher durability against alkaline cleaning solutions and chlorine-containing cleaning solutions.

[0074] Nanofiltration membranes comprising sulfonated polyethersulfone with a negative fixed charge as the separation functional layer are particularly preferred, as described in Japanese Patent Application Publication Nos. 61-4505 and 61-4506.

[0075] Examples of polysulfone resins having sulfonic acid groups include polysulfone resins having repeating units (A) or (B) as described below.

[0076]

[0077]

[0078] The membrane module M1 using nanofiltration membranes can consist of one or more membrane elements. Typically, the membrane element is a spiral membrane element using nanofiltration membranes. The membrane module M1 can also consist of a pressure vessel and one or more spiral membrane elements disposed within the pressure vessel. However, the structure of the membrane element containing the nanofiltration membrane is not limited to spiral; it can also be other types such as hollow fiber, tubular, or plate-and-frame.

[0079] (Helical membrane element)

[0080] Spiral membrane elements, for example, Figure 3 The diagram shows a central tube 5 with holes and a wound body R wound around the central tube 5, containing a separation membrane 1. Figure 3 In the example shown, the spiral membrane element includes a plurality of membrane sheets L with a permeate flow path material 3 sandwiched between opposing separation membranes 1, a supply flow path material 2 sandwiched between the membrane sheets L, a perforated central tube 5 wound with the membrane sheets L and the supply flow path material 2, and a sealing portion 12 to prevent mixing of the supply flow path and the permeate flow path. In this case, the permeate flow path within the membrane sheet L can be formed by the permeate flow path material 3 (also called a permeate flow path spacer).

[0081] exist Figure 3 An example is shown where the sealing portion includes end sealing portions and an outer peripheral sealing portion 12. The end sealing portions are formed by sealing the two ends of the membrane L in the axial direction A1 using an adhesive. The outer peripheral sealing portion 12 is formed by sealing the outer peripheral front end of the membrane L using an adhesive. The structure is such that the area surrounded by the opposing separation membrane 1, the end sealing portions, and the outer peripheral sealing portion 12 forms a permeate flow path, and it communicates with the opening 5a of the central tube 5. A first end member 10 with functions such as a sealing carrier may be provided upstream of the membrane element winding body R, and a second end member 20 with functions such as an anti-stretching material may be provided downstream of the membrane element winding body R.

[0082] When using the membrane element, it is housed within a pressure vessel (container), 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 path material 2 in a direction parallel to the axial direction A1 of the central tube 5, and is discharged from the other end face of the membrane element as concentrate 9. Additionally, as the supply liquid 7 flows along the supply-side flow path material 2, the permeate 8 that has passed through the separation membrane 1 flows along the permeate-side flow path 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.

[0083] [Pretreatment process]

[0084] In this invention, pretreatment steps such as removing solid components and bacteria can be performed before the separation process using a nanofiltration membrane. For example, by using a UF membrane (ultrafiltration membrane) or an MF membrane (microfiltration membrane) to separate the wastewater being treated, solid components such as impurities, colloidal suspended solids, bacteria, or other contaminants can be removed. Here, a UF membrane refers to a membrane with an average pore size of about 0.001 μm to about 0.01 μm. An MF membrane refers to a membrane with an average pore size of about 0.01 μm to about 10 μm.

[0085] There are no particular restrictions on the material of UF or MF membranes. For example, 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. There are no particular restrictions on the shape of UF or MF membranes, and they can be selected from flat membranes, hollow fiber membranes, corrugated membranes, and tubular membranes.

[0086] [Methods for recovering Na2SO4]

[0087] The Na2SO4 recovery method of the present invention is characterized in that Na2SO4 is recovered from the permeate obtained in the separation step of the textile wastewater treatment method of the present invention described above. Therefore, the Na2SO4 recovery method of the present invention is a Na2SO4 recovery method utilizing the textile wastewater treatment method of the present invention, and the separation step, pretreatment step, etc., are as described above.

[0088] Regarding methods for recovering Na2SO4 from the permeate, any method that allows recovery in either solid or liquid form is acceptable. Alternatively, the permeate can be reused directly or after concentration as an aqueous Na2SO4 solution. Reverse osmosis membranes or piperazine nanofiltration membranes can be used for permeate concentration.

[0089] In addition, as a method for recovering Na2SO4 in solid state, it is only necessary to concentrate the permeate as needed and then separate the Na2SO4 solid from the Na2SO4 aqueous solution. This can be done through a combination of concentration treatment, crystallization treatment, and solid-liquid separation treatment.

[0090] Concentration processes can utilize methods such as heating evaporation, reduced pressure evaporation, and membrane separation using reverse osmosis membranes. Crystallization processes can utilize methods such as cooling crystallization, reduced pressure crystallization, and reactive crystallization. Solid-liquid separation processes can utilize methods such as centrifugal separation and filter separation.

[0091] Example

[0092] The following examples illustrate the present invention, but the invention is not limited to these examples. It should be noted that in the examples, etc., physical properties were measured or evaluated using the following methods. Specifically, the physical property values, etc., in the present invention are values ​​measured using the following methods.

[0093] (1) Concentration and rejection rate of Na2SO4

[0094] For the feed solution, permeate, and concentrate 10 minutes after the start of the membrane separation operation, SO4 was determined using an ion chromatography-based ion concentration measuring device (Thermo Fisher Scientific, Dionex ICS-6000). 2- The concentration was calculated and converted to Na2SO4 concentration (ppm). Based on this result, the Na2SO4 rejection rate was calculated using the following formula.

[0095] Na2SO4 rejection rate (%) = (1 - (Na2SO4 concentration in permeate / (Na2SO4 concentration in supply solution + Na2SO4 concentration in concentrate) / 2))) × 100

[0096] (2) Dye concentration and retention rate

[0097] For the feed solution, permeate, and concentrate 10 minutes after the start of the membrane separation operation, the dye concentration (ppm) was measured using a gas chromatography-based concentration measuring device (Shimadzu Corporation, Nexis GC-2030). Based on the results, the dye rejection rate was calculated using the following formula.

[0098] Dye retention rate (%) = (1 - (dye concentration in permeate / (dye concentration in supply solution + dye concentration in concentrate) / 2))) × 100

[0099] (Example 1)

[0100] Considering the general composition of textile wastewater, a model wastewater containing 40,000 ppm Na2SO4 and 1,100 ppm anionic dye (manufactured by Katsura Fine Goods Co., Ltd., COLD DYE HOT Red) was prepared by adding Na2SO4 to it, thereby preparing the supply solution with the composition shown in Table 1.

[0101] use Figure 1The membrane separation apparatus shown was set to the operating conditions shown in Table 1 with a recovery rate (permeate flow rate / feed flow rate × 100) of 50%. The feed solution (temperature 25°C) was supplied to the anion-charged nanofiltration membrane (manufactured by Nitto Denko Corporation, HYDRACore7450), and the concentrate and permeate were discharged. After 10 minutes, samples were collected from the permeate and concentrate, and the concentrations of each component were measured as described above, and the rejection rates of each component were calculated. The results are shown in Table 1.

[0102] (Comparative Example 1)

[0103] In Example 1, the feed solution was adjusted to the composition shown in Table 1, and a low-pressure reverse osmosis membrane (manufactured by Nitto Denko Corporation, ESPA4) was used. Otherwise, under operating conditions achieving the same recovery rate as in Example 1, samples were collected from the permeate and concentrate, and the concentrations of each component were determined as described above, and the rejection rates of each component were calculated. The results are shown in Table 1.

[0104] (Comparative Example 2)

[0105] In Example 1, the feed solution was adjusted to the composition shown in Table 1, and a piperazine nanofiltration membrane (manufactured by Nitto Denko Corporation, PRO-XS3) was used. Otherwise, under operating conditions achieving the same recovery rate as in Example 1, samples were collected from the permeate and concentrate, and the concentrations of each component were determined as described above, and the rejection rates of each component were calculated. The results are shown in Table 1.

[0106] [Table 1]

[0107]

[0108] As shown in Table 1, in Example 1 using anion-charged nanofiltration membranes, the operating pressure (supply liquid pressure) is low, resulting in lower energy consumption, and high Na2SO4 permeability and high dye rejection are achieved.

[0109] In contrast, in Comparative Example 1, which used a low-pressure reverse osmosis membrane, the operating pressure was high, resulting in greater energy consumption. Furthermore, the rejection rates of Na2SO4 and dye were essentially the same, making selective separation of the two impossible. Additionally, in Comparative Example 2, which used a non-charged nanofiltration membrane, although the operating pressure was slightly reduced, the permeability of Na2SO4 was barely improved, making selective separation of Na2SO4 and dye difficult.

[0110] (Examples 2 to 6)

[0111] In Example 1, the amount of Na2SO4 added was varied to adjust the composition of the feed solution as shown in Table 2. Otherwise, under operating conditions that achieved the same recovery rate as in Example 1, samples were collected from the permeate and concentrate, and the concentrations of each component were determined as described above, and the rejection rates of each component were calculated. The results are shown in Table 2.

[0112] (Examples 7 to 9)

[0113] In Example 1, the feed solution was adjusted to the composition shown in Table 2 by changing the concentrations of dye and Na2SO4. Otherwise, under operating conditions that achieved the same recovery rate as in Example 1, samples were collected from the permeate and concentrate, and the concentrations of each component were determined as described above, and the rejection rates of each component were calculated. The results are shown in Table 2.

[0114] (Comparative Examples 3 to 5)

[0115] In Example 1, the feed solution was adjusted to the composition shown in Table 2 by changing the concentrations of dye and Na2SO4. Otherwise, under operating conditions that achieved the same recovery rate as in Example 1, samples were collected from the permeate and concentrate, and the concentrations of each component were determined as described above, and the rejection rates of each component were calculated. The results are shown in Table 2.

[0116] [Table 2]

[0117]

[0118] As shown in Table 2, in Examples 2 to 9, where anionic charged nanofiltration membranes were used and the supply solution was adjusted to contain anionic dyes at a concentration of 1.5% by mass or less relative to Na2SO4, lower energy consumption was achieved, along with higher Na2SO4 permeability and higher dye retention. In particular, in Examples 4 to 7, where the supply solution was adjusted to contain anionic dyes at a concentration of 1.0% by mass or less relative to Na2SO4, not only was the operating pressure (supply solution pressure) reduced, but also higher Na2SO4 permeability was obtained.

[0119] In contrast, in Comparative Examples 3 to 5, where the supply solution was adjusted to contain more than 1.5% by mass of anionic dye relative to Na2SO4, the permeability of Na2SO4 could not be sufficiently improved, making it difficult to selectively separate Na2SO4 from the dye.

[0120] (Comparative Experiment)

[0121] To investigate the reasons why high Na2SO4 permeability and high dye retention can be obtained by using an anionic charged nanofiltration membrane to separate a feed solution containing less than 1.5% by mass relative to Na2SO4, a comparative experiment using a flat sheet membrane was conducted.

[0122] That is, supply solution A (anionic dye ratio of 1.7 wt%) containing 62,000 ppm Na2SO4 and 1,087 ppm of the above-mentioned anionic dye and supply solution B (anionic dye ratio of 0.68 wt%) containing 160,000 ppm Na2SO4 and 1,087 ppm of the above-mentioned anionic dye were prepared as supply solutions.

[0123] A sheet membrane of anion-charged nanofiltration membrane (manufactured by Nitto Denko Corporation, HYDRACore7450) was used for membrane separation of feed solution A and feed solution B. The coloration of the permeate, flux behavior, membrane surface appearance, and changes in zeta potential were investigated as the recovery rate changed. The following results were obtained.

[0124] Regarding the coloring of the permeate, it becomes essentially transparent until the recovery rate reaches approximately 10%, after which it gradually acquires color. The permeate from supply solution A shows further coloring compared to the permeate from supply solution B. Therefore, it can be said that the dye in supply solution A permeates more easily.

[0125] Regarding flux behavior, with feed solution A, the flux starts high but then decreases sharply. In contrast, with feed solution B, the flux decreases more slowly. Therefore, it can be said that feed solution A is more readily adsorbed onto the membrane surface.

[0126] Regarding the appearance of the membrane surface, the membrane was well stained with supply solution A, while staining was inhibited with supply solution B. Therefore, it can be said that supply solution A resulted in a greater amount of dye adsorbed on the membrane surface.

[0127] Regarding the change in zeta potential, with supply solution A, the negative charge increases due to the influence of the dye. With supply solution B, the increase in negative charge is less than that with supply solution A. Therefore, it can be said that supply solution A has a greater amount of dye adsorbed on the membrane surface.

[0128] The results above show that by adjusting the supply solution to contain less than 1.5% by mass of anionic dye relative to Na2SO4, the fixation (adsorption) of dye on the anionic charged nanofiltration membrane was suppressed, and the flux reduction was also suppressed. Furthermore, it is believed that the suppression of dye fixation (adsorption) and flux reduction is due to the presence of excess Na2SO4 near the membrane surface, which selectively allows Na2SO4 to permeate. As a result, membrane separation using anionic charged nanofiltration membranes can achieve high Na2SO4 permeability and high dye retention.

[0129] Figure 4 The diagram illustrates such a state. That is, Figure 4 This is a schematic diagram illustrating the state of Na2SO4 and anionic dyes present near the membrane surface. The left side represents the case where Na2SO4 is at a low concentration, and the right side represents the case where Na2SO4 is at a high concentration.

[0130] Industrial practicality

[0131] According to the present invention, a method for treating textile wastewater with lower energy consumption and higher Na2SO4 permeability and dye retention is provided. Therefore, Na2SO4, which was previously difficult to separate, can be recovered and reused with less energy consumption.

Claims

1. A method for treating textile wastewater, wherein, The method for treating textile wastewater includes a separation step, in which a nanofiltration membrane is used to treat the textile wastewater containing Na2SO4 and anionic dyes, thereby obtaining a permeate in which Na2SO4 has selectively permeated and a concentrate in which the anionic dyes have been concentrated. The nanofiltration membrane is an anion-charged nanofiltration membrane. The feed solution supplied to the nanofiltration membrane contains less than 1.5% by mass of anionic dye relative to Na2SO4.

2. The method for treating textile wastewater according to claim 1, wherein, The supply solution contains more than 50,000 ppm of Na2SO4.

3. The method for treating textile wastewater according to claim 1, wherein, The operating pressure of the nanofiltration membrane in the separation process is below 3 MPa.

4. The method for treating textile wastewater according to claim 1, wherein, The separation process is performed while a portion of the permeate is returned to the supply liquid.

5. A method for recovering Na2SO4, wherein, Na2SO4 is recovered from the permeate obtained in the separation step of the method for treating textile wastewater according to any one of claims 1 to 4.

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