Process for the treatment of waste water and recovery of low molecular weight valuable substances

Abstract

Disclosed is a wastewater treatment method capable of selectively separating and highly purifying a low-molecular-weight valuable substance. The present invention is a wastewater treatment method including a step of separating a wastewater containing a low-molecular-weight valuable substance having a molecular weight of less than 100 and a cation using a nanofiltration membrane, characterized in that the nanofiltration membrane has a SO4 2‑ rejection rate of 90% or more when a 2000 mg / L aqueous MgSO4 solution is treated at 25°C under an operating pressure of 0.76 MPa, and the wastewater treatment method includes a step of adding a sulfate ion component to the wastewater.

Description

Technical Field

[0001] This invention relates to a wastewater treatment method that uses nanofiltration membranes to separate wastewater containing low molecular weight valuable substances, and a method for recovering low molecular weight valuable substances using the wastewater treatment method. Background Technology

[0002] For example, wastewater generated in industries such as plating and metal processing sometimes contains low-molecular-weight valuable substances such as boron. Depending on the state of the wastewater, boron may also be present in the wastewater in the form of boric acid. Therefore, various methods for recovering / reusing elements such as boron and compounds containing boron have been studied among the low-molecular-weight valuable substances contained in wastewater. Known methods for such recovery / reuse include coagulation precipitation, resin adsorption, drying and concentration, and methods combining crystallization with drying and concentration. However, from the perspective of reducing the amount of chemicals used and energy consumption, more efficient treatment methods are desired.

[0003] As a method for treating wastewater with high energy efficiency, various methods for treating wastewater using separation membranes and methods for using separation membranes as pretreatment steps in various wastewater treatment processes are known. For example, Patent Document 1 discloses a wastewater treatment method for treating wastewater containing sulfate ions and boron, wherein the wastewater treatment method includes the following steps: a first step of removing sulfate ions from the wastewater; and a second step of contacting the wastewater from which sulfate ions have been removed with a layered double hydroxide to adsorb boron onto the layered double hydroxide and thereby remove it, wherein a reverse osmosis membrane or a nanofiltration membrane (NF membrane) is used when removing sulfate ions in the first step.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2015-144997 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] However, in the processing method of Patent Document 1, a reverse osmosis membrane or a nanofiltration membrane is used as the separation membrane for separating sulfate ions and boron, but no countermeasures are taken for the permeation of alkali metals when using a nanofiltration membrane. That is, there is no step to increase the concentration of sulfate ions in the wastewater, so when using a nanofiltration membrane, it is impossible to utilize sulfate ions to improve the selective permeability of low molecular weight valuable substances passing through the separation membrane.

[0009] Therefore, the object of the present invention is to provide a wastewater treatment method capable of selectively separating and purifying low molecular weight valuable substances, and a method for recovering low molecular weight valuable substances using the wastewater treatment method.

[0010] means for solving problems

[0011] To solve the aforementioned problems, the inventors conducted repeated and in-depth research, and discovered that when using nanofiltration membranes to separate wastewater containing low-molecular-weight valuable substances, increasing the concentration of sulfate ions in the feed solution can suppress the permeation of cations such as alkali metals, thereby improving the selective permeability of low-molecular-weight valuable substances. This led to the completion of the present invention. Specifically, the present invention comprises the following methods.

[0012] [1] A wastewater treatment method, the wastewater treatment method comprising a step of separating wastewater containing low molecular weight valuable substances with a molecular weight less than 100 and cations using a nanofiltration membrane, wherein, The nanofiltration membrane was used to treat a 2000 mg / L MgSO4 aqueous solution at an operating pressure of 0.76 MPa and a temperature of 25°C. The resulting SO4 content was... 2- The retention rate is over 90%. The wastewater treatment method includes a step of adding sulfate ions to the wastewater.

[0013] According to the wastewater treatment method of the present invention, since a nanofiltration membrane is used, low molecular weight valuable substances with a molecular weight of less than 100 can easily pass through. On the other hand, due to the SO42- of the nanofiltration membrane... 2- The rejection rate is over 90%, therefore it can remove SO4 2- Most of the SO42- is separated to the feed liquid side. It is believed that ions passing through the nanofiltration membrane tend to pass in an electrically neutral state, i.e., in the form of ion pairs. By adding sulfate ions, the SO42- on the feed side is reduced. 2- The increased cation density leads to the formation of ion pairs with cations such as alkali metals, resulting in a relative decrease in the number of cations passing through the nanofiltration membrane. Consequently, the rejection rate of cations such as alkali metals is increased. Normally, cations such as alkali metals easily pass through nanofiltration membranes; the increased rejection rate results in the wastewater treatment method of the present invention being able to selectively separate and purify low-molecular-weight valuable substances.

[0014] [2] According to the wastewater treatment method of [1], wherein the wastewater contains monovalent anions, and the equivalent of sulfate ions supplied to the nanofiltration membrane is greater than 30 relative to the equivalent of the monovalent anions.

[0015] As mentioned above, ions tend to pass through nanofiltration membranes in ion-pair form. As the amount of monovalent anions forming ion pairs with cations such as alkali metals increases, the amount of alkali metal cations passing through the nanofiltration membrane relatively increases, thus easily reducing the rejection rate of alkali metal cations. Therefore, in the wastewater supplied, the higher the equivalent ratio of sulfate ions to monovalent anions, the higher the rejection rate of alkali metal cations due to the balance between the two. With an equivalent ratio greater than 30, low-molecular-weight valuable substances can be further selectively separated and purified to a higher purity.

[0016] [3] The wastewater treatment method according to [1] or [2], wherein the low molecular weight valuable substance is selected from one or more of the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids and alcohols.

[0017] These compounds, with a molecular weight of less than 100, can easily pass through nanofiltration membranes. From the perspective of treating various wastewaters and recovering valuable low-molecular-weight substances, they are preferred as low-molecular-weight substances that can be purified to high purity.

[0018] [4] The wastewater treatment method according to any one of [1] to [3], wherein when adding sulfate ion components, an aqueous solution of sulfuric acid is added.

[0019] Aqueous solutions of sulfuric acid generate protons as hydrogen cations, thus increasing the number of protons. This increases the amount of monovalent anions that form ion pairs with alkali metal cations, thereby further reducing the rejection rate of alkali metal cations in nanofiltration membranes and improving the selective separation of low-molecular-weight valuable substances.

[0020] [5] A method for recovering a low molecular weight valuable substance, the method comprising the steps of separating wastewater containing a low molecular weight valuable substance with a molecular weight less than 100 and cations using a nanofiltration membrane, and recovering the low molecular weight valuable substance from the permeate, wherein, The nanofiltration membrane was used to treat a 2000 mg / L MgSO4 aqueous solution at an operating pressure of 0.76 MPa and a temperature of 25°C. The resulting SO4 content was... 2- The retention rate is over 90%. The method for recovering low molecular weight valuable substances includes a step of adding sulfate ions to the wastewater.

[0021] According to the method for recovering low molecular weight valuable substances of the present invention, wastewater treatment can be performed as described above, which selectively separates and purifies low molecular weight valuable substances. Therefore, high-purity low molecular weight valuable substances can be recovered in the form of permeate. Consequently, energy saving and process simplification can be achieved in subsequent processes.

[0022] [6] The method for recovering low molecular weight valuable substances according to [1], wherein the wastewater contains monovalent anions and the equivalent of sulfate ions in the wastewater supplied to the nanofiltration membrane is greater than 30 relative to the equivalent of the monovalent anions.

[0023] According to this recovery method, as described above, in the supplied wastewater, the greater the ratio of the equivalent of sulfate ions to the equivalent of monovalent anions, the higher the retention rate of cations such as alkali metals due to the balance between the two. With an equivalent ratio greater than 30, low molecular weight valuable substances can be further selectively separated and purified to a higher purity.

[0024] [7] The method for recovering low molecular weight valuable substances according to [5] or [6], wherein the process of recovering low molecular weight valuable substances includes a concentration process by evaporation of water.

[0025] In cases involving a concentration process through the evaporation of water, the present invention’s method for recovering low molecular weight valuable substances is effective, particularly from an energy-saving perspective, as it can selectively separate and highly purify low molecular weight valuable substances.

[0026] Invention Effects

[0027] According to the present invention, a method for treating wastewater that can selectively separate and purify low molecular weight valuable substances is provided, as well as a method for recovering low molecular weight valuable substances using the wastewater treatment method. Attached Figure Description

[0028] Figure 1 This is a schematic diagram illustrating an example of a wastewater treatment method.

[0029] Figure 2 This is a schematic diagram illustrating an example of a method for recovering valuable low-molecular-weight substances.

[0030] Figure 3 This is a perspective view showing a portion of an example of a spiral membrane element used in a wastewater treatment method. Detailed Implementation

[0031] The embodiments of the present invention will be described below.

[0032] Wastewater treatment methods

[0033] (Wastewater being treated)

[0034] The wastewater treatment method of the present invention includes a step of separating wastewater using a nanofiltration membrane. The wastewater to be treated contains low molecular weight valuable substances with a molecular weight less than 100 and alkali metals. The wastewater to be treated may also be wastewater for which impurities, silt, clay, bacteria, algae, and colloidal suspended solids have been removed through pretreatment steps using UF membranes (ultrafiltration membranes), MF membranes (microfiltration membranes), etc.

[0035] Low-molecular-weight valuable substances contained in wastewater include all elements or compounds containing such elements with a molecular weight less than 100 that can be recycled. These elements or compounds can dissolve in the wastewater as is or in ionic or other forms. Examples of such low-molecular-weight valuable substances include, for example, one or more selected from the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids, and alcohols.

[0036] Examples of organic acids include aliphatic carboxylic acids such as acetic acid, formic acid, and propionic acid; and sulfonic acids such as methanesulfonic acid. Examples of alcohols include ethanol, methanol, propanol, and butanol. Examples of boron-containing compounds include boric acids such as orthoboric acid and metaboric acid, metal salts of boric acids, and boron halides. Examples of deuterium-containing compounds include compounds in which one or more hydrogen atoms are replaced by deuterium. Since wastewater generated in industries such as plating and metal processing contains large amounts of boron, various recovery methods have been studied. Therefore, low-molecular-weight, valuable boron is particularly preferred.

[0037] The concentration of low molecular weight valuable substances contained in the wastewater being treated is, for example, about 100 mg / L to about 10,000 mg / L. Considering the concentration after membrane treatment and the recovery of valuable substances, a high concentration is preferred.

[0038] Examples of cations contained in wastewater include monovalent or divalent cations. However, in this invention, the retention performance of monovalent cations is particularly improved, making this invention especially effective for wastewater containing monovalent cations. Examples of monovalent cations include alkali metal ions, ammonium ions, and oxygen ions. Ions, etc. Examples of alkali metals include one or more of sodium, potassium, and lithium. Alkali metals exist in wastewater in the form of cations, but may also exist in particulate form. The concentration of cations or alkali metals in the wastewater being treated is, for example, 100 mg / L to 10000 mg / L. From the viewpoint of maintaining the alkali metal ion rejection rate of the nanofiltration membrane at a certain level or above, and from the viewpoint of membrane treatment osmotic pressure, it is preferably 50000 mg / L or less.

[0039] In addition, wastewater may contain monovalent anions. Examples of monovalent anions in wastewater include: anions of halogen atoms such as fluorine, chlorine, bromine, and iodine, and nitrate ions.

[0040] From the viewpoint of maximizing the effect of adding sulfate ions, the concentration of monovalent anions in the wastewater being treated is preferably below 1000 mg / L, more preferably below 100 mg / L. A lower concentration of monovalent anions is preferred, but a lower limit is, for example, 1 mg / L.

[0041] In addition to these, the wastewater being treated may also contain divalent metal cations, divalent anions, etc.

[0042] (Separation process)

[0043] The wastewater treatment method of the present invention includes a step of separating the wastewater as described above using a nanofiltration membrane, characterized in that it includes a step of adding sulfate ions to the wastewater. Such a wastewater treatment method can, for example, use... Figure 1 The membrane separation device shown is implemented.

[0044] For example, Figure 1 The membrane separation device shown includes a membrane module M1, which has a separation membrane 1 as a nanofiltration membrane, a supply section for a supply liquid 7, a discharge section for a permeate 8, and a discharge section for a concentrate 9. A chemical liquid supply tank 16 for supplying an aqueous sulfuric acid solution 17, which is a sulfate ion component, is provided on the pipeline of the supply liquid 7.

[0045] Such membrane separation devices are equipped with pumps, sensors, tanks, control valves, control devices, and other equipment as needed, and are 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.

[0046] As a sulfate ion component, it is as long as it contains SO4. 2- Any compound can be used, such as sulfuric acid, sulfates, and hydrogen sulfates. However, from the viewpoint of further reducing the rejection rate of alkali metal cations in the nanofiltration membrane and thus improving the selective separation of low molecular weight valuable substances, sulfuric acid is preferred. When adding sulfate ions, from an operability point of view, a liquid such as an aqueous solution is preferred.

[0047] From an operational point of view, the concentration of the added sulfate ion component when added to wastewater is preferably 1% to 90% by mass, and more preferably 15% to 75% by mass.

[0048] From the viewpoint of improving the selective separation of valuable low-molecular-weight substances, when adding sulfate ions to the feed wastewater of a nanofiltration membrane, it is preferable that the equivalent ratio of sulfate ions to monovalent anions is greater than 30, and more preferably greater than 40. Furthermore, even if this equivalent ratio becomes excessively large, the corresponding effect will decrease; therefore, if the equivalent ratio is approximately 100, sufficient separation performance can be obtained.

[0049] Considering this viewpoint and the concentration of monovalent anions in general wastewater, the concentration of sulfate ions in the wastewater supplied with sulfate ions is preferably 1000 mg / L to 50000 mg / L, more preferably 2000 mg / L to 10000 mg / L.

[0050] Operating conditions such as operating pressure, permeate recovery rate, and operating temperature in a membrane separation unit can be those typical for nanofiltration membranes. However, from the viewpoint of stable continuous operation, it is preferable to adjust the operating pressure to achieve a membrane flux of 5 LMH to 40 LMH (L·m³). -2 ·h -1 ).

[0051] (Nanofiltration membrane)

[0052] As a nanofiltration membrane (NF membrane), it is used to treat an aqueous solution of MgSO4 with a concentration of 2000 mg / L at an operating pressure of 0.76 MPa and a temperature of 25°C. 2- A nanofiltration membrane with a rejection rate of 90% or higher, preferably 95% or higher, more preferably 98% or higher, and most preferably 99% or higher. For example, SO4... 2- The higher the rejection rate, the more SO4 content in the permeate can be reduced. 2- Furthermore, it can increase the rejection rate of cations such as alkali metal ions. It should be noted that, specifically, SO4 2- The retention rate is a value determined by the method described in the examples.

[0053] On the other hand, from the viewpoint of improving the permeability of nanofiltration membranes to low molecular weight valuable substances and their recovery rate, the molecular weight cutoff of nanofiltration membranes is preferably 50 Daltons to 200 Daltons, and more preferably 80 Daltons to 120 Daltons.

[0054] 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. The average molecular weight of the PEG at which a 90% rejection rate is achieved is determined based on the rejection curve. This determined average molecular weight can be considered as the molecular weight cutoff of the nanofiltration membrane.

[0055] 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 for forming the microporous layer include: polysulfone, polyethersulfone, polyarylene ethersulfone, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0056] Nanofiltration membranes can be classified as charged or uncharged depending on whether their surface is charged, and both can be used in this invention. However, in cases where the wastewater contains low levels of organic compounds, a negatively charged nanofiltration membrane is preferred.

[0057] Examples of negatively 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.

[0058] In addition, examples of resins constituting the separation functional layer include: polysulfone resins, polyamides, cellulose acetate, and polyvinyl alcohol. Polysulfone resins are preferred, especially from the viewpoints of chemical stability, mechanical stability, and thermal stability. Examples of polysulfone resins include: polysulfone, polyethersulfone, and polyphenylsulfone.

[0059] 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.

[0060] As a nanofiltration membrane comprising a sulfonated polyethersulfone having a negative fixed charge as the separation functional layer, the nanofiltration membrane described in Japanese Patent Application Publication Nos. 61-4505 and 61-4506 is particularly preferred.

[0061] Examples of polysulfone resins having sulfonic acid groups include polysulfone resins having repeating units (A) or (B) as described below.

[0062]

[0063] The membrane module M1 using nanofiltration membranes can consist of one or more membrane elements. A typical 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.

[0064] (Helical membrane element)

[0065] 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).

[0066] 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 area surrounded by the opposing separation membrane 1, end sealing portions, and outer peripheral sealing portion 12 forms a permeate flow path, and this permeate flow path is connected to the opening 5a of the central tube 5. A first end member 10 with functions such as a sealing seat may also 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 also be provided downstream of the membrane element winding body R.

[0067] 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 permeates 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.

[0068] [Pre-processing steps]

[0069] In this invention, before the separation process using a nanofiltration membrane, pretreatment processes such as removal of solid components, reduction of monovalent anions, water softening, and removal of dissolved organic impurities that are not intended for recycling can be performed. For example, by using UF membranes (ultrafiltration membranes) or MF membranes (microfiltration membranes) to separate the wastewater being treated, solid components such as inclusions, colloidal suspended solids, monosaccharides, amino acids, and soluble polymers 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.

[0070] 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 perspective 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 sheet membranes, hollow fiber membranes, corrugated membranes, and tubular membranes.

[0071] In addition, when the concentration of monovalent anions in the wastewater being treated is high, it is sometimes effective to use an ion exchange device equipped with an ion exchange membrane, ion exchange resin, etc., to reduce the concentration of monovalent anions.

[0072] Examples of anion exchange resins include: strongly basic anion exchange resins with quaternary amines as functional groups, and weakly basic anion exchange resins with primary to tertiary amines. Anion exchange membranes with the same chemical structure can also be listed.

[0073] In particular, strongly basic anion exchange resins can be used over a wide pH range and can adsorb a variety of anions, such as SO42-. 2- >I - >NO3 - >Br - >Cl - The sequence exhibits ion exchange properties. Even when the wastewater contains sulfate ions, these ions are adsorbed by the anion exchange resin. However, this can be mitigated by adding sulfate ions to the supplied wastewater during the separation process to replenish the adsorbed / reduced sulfate ions.

[0074] Methods for recovering valuable low-molecular-weight substances

[0075] The method for recovering low molecular weight valuable substances of the present invention comprises the steps of separating wastewater containing low molecular weight valuable substances with a molecular weight less than 100 and alkali metals using a nanofiltration membrane, and recovering the low molecular weight valuable substances from the permeate. The method is characterized in that the nanofiltration membrane, when treating a 2000 mg / L MgSO4 aqueous solution at an operating pressure of 0.76 MPa and 25°C, contains SO4. 2- The retention rate is over 90%, and the method for recovering low molecular weight valuable substances includes a step of adding sulfate ions to the wastewater. The method for recovering low molecular weight valuable substances of the present invention utilizes the wastewater treatment method of the present invention, including separation steps, pretreatment steps, etc., as described above.

[0076] Methods for recovering low molecular weight valuable substances from permeate can be carried out by a combination of steps such as further concentrating the low molecular weight valuable substances in the permeate, crystallizing (or reacting) the low molecular weight valuable substances in the permeate, separating the solid low molecular weight valuable substances from the liquid, and drying the low molecular weight valuable substances.

[0077] Concentration processes can utilize methods such as heating evaporation, reduced pressure evaporation, and membrane separation using reverse osmosis (RO) 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.

[0078] It should be noted that when low molecular weight valuable substances are liquid at room temperature (such as alcohols), by combining absorption, dehydration, membrane separation (VP method) and other processes with a distillation apparatus, it is possible to recover low molecular weight valuable substances with high purity.

[0079] The method for recovering low molecular weight valuable substances of the present invention can, for example, use... Figure 2 The apparatus shown is implemented. In this apparatus, the permeate obtained by the wastewater treatment method of the present invention is further concentrated by heating to evaporate the water, and then cooled to crystallize the low molecular weight valuable substances in the permeate. The crystals of the low molecular weight valuable substances are separated from the liquid, and the obtained low molecular weight valuable substances are dried, thereby enabling the recovery of the low molecular weight valuable substances.

[0080] Example

[0081] The following examples illustrate the present invention, but the invention is not limited to these examples. It should be noted that in the examples, physical properties were measured or evaluated using the following methods. Specifically, the physical property values ​​in the present invention are values ​​measured using the following methods.

[0082] (1) SO4 2- retention rate

[0083] The determination was performed according to JIS K3805 (1990) by the following method: A 2000 mg / L aqueous solution of MgSO4 (pH 6.5–7) was passed through a nanofiltration membrane of a specified size at an operating pressure of 0.76 MPa and a temperature of 25°C. After a 30-minute preparation period, the SO4 concentration of the permeate and feed solution was measured using an ion chromatography-based ion concentration measuring apparatus (Dionex ICS-6000, Thermo Fisher Scientific). 2- Concentration (mg / L) determination. Based on the results, SO4 is calculated using the following formula. 2- The retention rate.

[0084] SO4 2- Retention rate (%) = (1 - (SO4 in the permeate) 2- Concentration of SO4 in the supply solution 2- Concentration) × 100

[0085] (2) Retention rate of low molecular weight valuable substances

[0086] Similar to (1), an aqueous solution of a low molecular weight valuable substance at a concentration of 2000 mg / L was used and passed through a nanofiltration membrane of a specified size (manufactured by Nitto Denko Corporation, PRO-XS3) at an operating pressure of 0.76 MPa and a temperature of 25°C. After a 30-minute preparation period, the concentrations (mg / L) of the low molecular weight valuable substance in the permeate and the feed solution were determined using a gas chromatography-based concentration measuring device (manufactured by Shimadzu Corporation, Nexis GC-2030). Based on the results, the rejection rate of the low molecular weight valuable substance was calculated using the following formula.

[0087] Retention rate (%) of low molecular weight valuable substances = (1 - (concentration of low molecular weight valuable substances in the permeate / concentration of low molecular weight valuable substances in the feed solution)) × 100

[0088] (3) The concentration of various ions contained in the wastewater.

[0089] Wastewater supplied to the nanofiltration membrane (without sulfuric acid and with sulfuric acid) was used as samples, and the concentrations (mg / L) of various ions were determined using an ion chromatography-based ion concentration measuring device (Dionex ICS-6000, Thermo Fisher Scientific). Additionally, for metal ions and boron, an ICP-MS (ICP-Mass Spectrometry) based device (ICPS-7510, Shimadzu Corporation) was used. It should be noted that the boric acid concentration was calculated from the measured boron concentration using the following formula.

[0090] Boric acid concentration = boron concentration ÷ 0.175

[0091] Therefore, even when the boron concentration is used directly in the calculation of the retention rate, the boron retention rate is the same as that of boric acid.

[0092] (4) Retention rates of various ions

[0093] Wastewater was supplied and permeated through a nanofiltration membrane at an operating pressure of 1.2 MPa and a temperature of 25 °C. After a 30-minute preparation period, the concentrations (mg / L) of various ions in the permeate were measured using the same measuring apparatus as in (3). Based on the results and those in (3), the rejection rates of various ions were calculated using the following formula.

[0094] Retention rate (%) of low molecular weight valuable substances = (1 - (ion concentration of permeate / ion concentration of feed wastewater)) × 100

[0095] (Experimental Example 1)

[0096] Using the compounds shown in Table 1 as low-molecular-weight valuable substances, the retention rates of low-molecular-weight valuable substances in nanofiltration membranes were determined. The results are presented in Table 1 along with the molecular weights.

[0097] [Table 1]

[0098] This result shows that even when using SO4 2- Even with nanofiltration membranes (manufactured by Nitto Denko Corporation, PRO-XS3) achieving a rejection rate of over 99%, valuable low-molecular-weight substances with a molecular weight less than 100 can permeate into the permeate with a high recovery rate (low rejection rate). Therefore, it is believed that using SO4... 2- Nanofiltration membranes with a rejection rate of over 90% can achieve high purity of low molecular weight valuable substances with a molecular weight of less than 100 with a high recovery rate.

[0099] (Example 1)

[0100] The PRO-XS3 nanofiltration membrane, manufactured by Nitto Denko Corporation, was used. The SO4 content of this nanofiltration membrane was measured using the method described above. 2- The retention rate, resulting in SO4 2- The rejection rate was 99.7%. It was used with a concentration of 1000 mg / L containing SO4. 2- The wastewater was used as the wastewater to be treated. This wastewater was a model of boric acid wastewater that could be generated in various processes, and had the composition shown in Table 5 (Comparative Example 1).

[0101] use Figure 1 The membrane separation device shown was prepared by adding 4 mL of industrial sulfuric acid (75% sulfuric acid aqueous solution) to 1 L of wastewater, as shown in Table 2, to prepare the wastewater supply. The wastewater was then supplied to the nanofiltration membrane under the same operating pressure and at 25°C, allowing the concentrate and permeate to be discharged. After 30 minutes, a sample was collected from the permeate, and the concentrations of each component in the permeate were determined as described above to calculate the rejection rate. The results, along with the difference in rejection rates and the anion equivalent ratio, are shown in Table 2.

[0102] [Table 2]

[0103] As shown in Table 2, by adding sulfate ions to the wastewater, boric acid water with low Na and K concentrations and high purity can be obtained.

[0104] Here, a negative iodine anion rejection rate indicates that the concentration of iodine anions in the permeate is higher than that in the feed solution. It is believed that ions passing through the nanofiltration membrane tend to pass through in an electrically neutral state, i.e., in the state of ion pairs. Due to the addition of sulfuric acid, the hydrogen ions on the feed side increase, thereby increasing the amount of iodine anions that form ion pairs with it. As a result, the concentration of iodine anions in the permeate increases.

[0105] Furthermore, it is believed that the molar amount of hydrogen ions that form ion pairs with iodide anions is related to Na + Compared to cations, Na increases + As the permeation of cations decreases, the cation rejection rate increases. That is, the greater the equivalent of sulfuric acid that generates hydrogen ions, and the smaller the equivalent of the monovalent anion that forms an ion pair with the cation, the more effectively the permeation of the cation is suppressed. Furthermore, the greater the ratio of the equivalent of sulfate ions to the equivalent of monovalent anions, the more effectively the Na+ ion rejection rate is increased. + The rejection rate of cations is increased, thereby improving the purity of low molecular weight valuable substances.

[0106] (Example 2)

[0107] Except for membrane separation using the feed wastewater adjusted to the composition shown in Table 3 by adding 75% sulfuric acid aqueous solution at a ratio of 10 mL to 1 L of wastewater in Example 1, samples were collected from the permeate under the same conditions as in Example 1, and the rejection rates of each component were determined. The results, along with the difference in rejection rates and the anion equivalent ratio, are shown in Table 3.

[0108] [Table 3]

[0109] As shown in Table 3, by adding sulfate ions to the wastewater, boric acid water with low Na and K concentrations and high purity can be obtained.

[0110] (Example 3)

[0111] Except for membrane separation using the feed wastewater adjusted to the composition shown in Table 4 by adding 2 mL of 75% sulfuric acid aqueous solution relative to 1 L of wastewater in Example 1, samples were collected from the permeate under the same conditions as in Example 1, and the rejection rates of each component were determined. The results, along with the difference in rejection rates and the anion equivalent ratio, are shown in Table 4.

[0112] [Table 4]

[0113] As shown in Table 4, adding sulfate ions to the wastewater resulted in boric acid water with lower Na and K concentrations and higher purity compared to the case without sulfate ions. However, it is evident that when the rejection rates of Na and K ions are adjusted to above 90%, the equivalent amount of sulfate ions relative to monovalent anions (e.g., I) remains relatively high. - A ratio of equivalence greater than 30 is valid.

[0114] (Comparative Example 1)

[0115] Except for membrane separation using wastewater with the composition shown in Table 5, which was used in Example 1 without the addition of sulfuric acid aqueous solution, samples were collected from the permeate under the same conditions as in Example 1, and the rejection rates of each component were determined. The results, along with the differences in rejection rates and the anion equivalent ratio, are shown in Table 5.

[0116] [Table 5]

[0117] As shown in Table 5, without the addition of sulfate ions to the wastewater, the retention rates of Na and K decrease, making it difficult to obtain high-purity boric acid water.

[0118] (Comparative Example 2)

[0119] The PRO-XS3 nanofiltration membrane, manufactured by Nitto Denko Corporation, was used. A solution containing SO4 at a concentration of 400 mg / L was prepared. 2- Containing Cl at a concentration of 2500 mg / L - Wastewater with the composition shown in Table 6 is used as the wastewater to be treated.

[0120] use Figure 1 The membrane separation device shown supplies wastewater to the nanofiltration membrane at 25°C, causing the concentrate and permeate to be discharged. After 30 minutes, a sample is collected from the permeate, and the concentration of each component in the permeate is determined as described above to calculate the rejection rate. The results, along with the difference in rejection rates and the anion equivalent ratio, are shown in Table 6.

[0121] [Table 6]

[0122] As shown in Table 6, when using products containing Cl - The wastewater contains boric acid, and without the addition of sulfate ions, the retention rates of Na and K are reduced, making it difficult to obtain high-purity boric acid water.

[0123] (Refer to Example 1)

[0124] The PRO-XS3 nanofiltration membrane, manufactured by Nitto Denko Corporation, was used. A solution containing SO4 at a concentration of 10400 mg / L was prepared. 2- Containing Cl at a concentration of 750 mg / L - Wastewater with the composition shown in Table 7 is used as the wastewater to be treated.

[0125] use Figure 1 The membrane separation device shown supplies wastewater to the nanofiltration membrane at 25°C, causing the concentrate and permeate to be discharged. After 30 minutes, a sample is collected from the permeate, and the concentration of each component in the permeate is determined as described above to calculate the rejection rate. The results, along with the difference in rejection rates and the anion equivalent ratio, are shown in Table 7.

[0126] [Table 7]

[0127] As shown in Table 7, when using products containing Cl - In wastewater containing sulfate ions, although the retention rates of Na and K increase, the equivalent of sulfate ions relative to monovalent anions (such as Cl-) remains low. - The equivalent ratio of Na and K is low, so the retention rates of Na and K do not reach more than 90%.

[0128] Industrial practicality

[0129] According to the present invention, a method for treating wastewater can be provided that selectively separates and purifies low-molecular-weight valuable substances such as boric acid. The high-purity low-molecular-weight valuable substances can be recovered in the form of permeate, thus enabling energy saving and process simplification in subsequent processes, making it particularly useful as a method for recovering low-molecular-weight valuable substances.

[0130] Label Explanation

[0131] 1. Separation membrane (nanofiltration membrane)

[0132] 7. Supply liquid (for wastewater)

[0133] 8. Permeable liquid

[0134] 9. Concentrate

[0135] 16 Chemical Liquid Supply Tanks

[0136] 17. Sulfuric acid aqueous solution (sulfate ion component)

[0137] M1 membrane module

Claims

1. A method for treating wastewater, the method comprising a step of separating wastewater containing low molecular weight valuable substances with a molecular weight less than 100 and cations using a nanofiltration membrane, wherein, The nanofiltration membrane, when treating a 2000 mg / L MgSO4 aqueous solution at an operating pressure of 0.76 MPa and 25°C, produces SO4 2- The retention rate is over 90%. The wastewater treatment method includes a step of adding sulfate ions to the wastewater.

2. The wastewater treatment method according to claim 1, wherein, The wastewater contains monovalent anions, and the ratio of the equivalent amount of sulfate ions in the wastewater supplied to the nanofiltration membrane to the equivalent amount of the monovalent anions is greater than 30.

3. The wastewater treatment method according to claim 1, wherein, The low molecular weight valuable substance is selected from one or more of the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids, and alcohols.

4. The wastewater treatment method according to claim 1, wherein, When adding sulfate ions, an aqueous solution of sulfuric acid is added.

5. A method for recovering a low molecular weight valuable substance, the method comprising the steps of separating wastewater containing a low molecular weight valuable substance with a molecular weight less than 100 and cations using a nanofiltration membrane, and recovering the low molecular weight valuable substance from the permeate, wherein... The nanofiltration membrane, when treating a 2000 mg / L MgSO4 aqueous solution at an operating pressure of 0.76 MPa and 25°C, produces SO4 2- The retention rate is over 90%. The method for recovering low molecular weight valuable substances includes a step of adding sulfate ions to the wastewater.

6. The method for recovering low molecular weight valuable substances according to claim 5, wherein, The wastewater contains monovalent anions, and the ratio of the equivalent amount of sulfate ions in the wastewater supplied to the nanofiltration membrane to the equivalent amount of the monovalent anions is greater than 30.

7. The method for recovering low molecular weight valuable substances according to claim 5, wherein, The process for recovering low-molecular-weight valuable substances includes a concentration process through the evaporation of water.

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