Water treatment method and water treatment system

Abstract

To further improve efficiency in a water treatment method including an osmosis treatment using a lower limit critical solution temperature (LCST) draw solute.SOLUTION: The water treatment method comprises: a forward osmosis step of bringing one surface of a semi-permeable membrane 10 into contact with a target solution including water and other components, and bringing the other surface of the semi-permeable membrane into contact with a draw solution including a draw solute reduced in solubility due to a temperature increase, and moving the water included in the target solution through the semi-permeable membrane into the draw solution; a phase separation step of separating the draw solution into a low-concentration draw solution and a high-concentration draw solution by increasing the temperature of the draw solution after the forward osmosis step; and a membrane separation step of separating the low-concentration draw solution into water and a concentrated liquid including a draw solute by using a separation membrane 20. The draw solute includes a copolymer represented by Formula (1).SELECTED DRAWING: Figure 1

Description

Technical Field

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[0006]

[0001] The present invention relates to a water treatment method and a water treatment system.

Background Art

[0002] Conventionally, in the field of water treatment, a method of desalination by a reverse osmosis (RO) process is widely known. On the other hand, the forward osmosis (FO) phenomenon is a phenomenon in which a solvent (such as water) on the low-concentration side moves toward a solution on the high-concentration side.

[0003] The membrane separation process is a process in which water is moved from a high-concentration target solution (such as seawater) to a low-concentration solution (such as water) side, contrary to forward osmosis, by artificially applying a high pressure to the high-concentration target solution. Thereby, for example, water can be produced from the target solution. Since the membrane separation process requires a high pressure, the energy consumption is extremely large and the energy efficiency is low. Therefore, in recent years, in order to improve the energy efficiency of water treatment, a desalination method by a forward osmosis process that does not require artificial pressure application has been studied.

[0004] [[ID=​​​​​​​​When using a Draw solute (a temperature-responsive polymer with a lower critical eutectic temperature (LCST)) whose solubility decreases with increasing temperature, it is possible to separate the Draw solution into a low-concentration fraction (low-concentration Draw solution) and a high-concentration fraction (high-concentration Draw solution) by increasing the temperature. However, in most cases, water and the Draw solute cannot be completely separated by simply increasing the temperature.

[0007] Therefore, as shown in Figure 1, the diluted draw solution is separated into a low-concentration draw solution and a high-concentration draw solution in the separation tank 3 by phase separation in the forward osmosis module 1 through heating or other means. Then, the low-concentration draw solution is subjected to membrane separation treatment in the separation membrane module 2, and water is ultimately recovered from the low-concentration draw solution. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Special Publication No. 2019-529077 [Patent Document 2] International Publication No. 2018 / 150690 [Patent Document 3] Japanese Patent Publication No. 2015-47541 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, in order to further improve the efficiency of such water treatment methods and water treatment systems, there was room for improvement in conventional lower critical eutectic temperature (LCST) type draw solutes. Specifically, it was desired to further improve the efficiency of water treatment methods and water treatment systems by using a draw solute that can impart sufficient osmotic pressure to the draw solution to separate water from the target solution in forward osmosis treatment, and that can be easily separated from the water recovered in the draw solution in the above-mentioned phase separation and membrane separation.

[0010] In view of the above issues, the object of the present invention is to further improve the efficiency of a water treatment method including a forward osmosis treatment using a lower critical temperature (LCST) type draw solute. [Means for solving the problem]

[0011] [1] A forward osmosis step is performed by bringing one side of a semipermeable membrane into contact with a target solution containing water and components other than water, and bringing the other side of the semipermeable membrane into contact with a draw solution containing a draw solute whose solubility decreases with increasing temperature, thereby moving the water contained in the target solution through the semipermeable membrane to the draw solution. A phase separation step is performed after the forward osmosis step, in which the temperature of the draw solution is increased to separate the draw solution into a low-concentration draw solution and a high-concentration draw solution. The process includes a membrane separation step, in which the low-concentration draw solution is separated into water and a concentrated solution containing the draw solute using a separation membrane. A water treatment method wherein the draw solute contains a copolymer represented by the following formula (1). [ka] (In equation (1), x, y, and z are all independent natural numbers.)

[0012] [2] The number average molecular weight of the Draw solute is 2000 to 3000, The LCST of a 1% by mass aqueous solution of the aforementioned Draw solute is 35-50°C. The water treatment method of claim 1, wherein the proportion of ethylene oxide groups in the draw solute is 35 to 50% by mass.

[0013] [3] The temperature of the forward osmosis process is less than 40°C. In the phase separation process, the temperature of the draw solution rises to 40°C or higher. The water treatment method according to [1] or [2], wherein the temperature of the membrane separation step is 40°C or higher.

[0014] [4] A forward osmosis treatment system used in the forward osmosis water treatment method according to any one of [1] to [3], comprising: A forward osmosis module including a semipermeable membrane, a first chamber provided to contact a target solution with one surface of the semipermeable membrane, and a second chamber provided to contact a draw solution with the other surface of the semipermeable membrane; A separation tank having a heating mechanism for separating the draw solution into a low-concentration draw solution and a high-concentration draw solution by raising the temperature of the draw solution; A separation membrane for separating the low-concentration draw solution into water and a concentrated solution containing a draw solute.

Advantages of the Invention

[0015] According to the present invention, in a water treatment method including forward osmosis treatment using a lower critical solution temperature (LCST) type draw solute, further efficiency improvement can be achieved.

Brief Description of the Drawings

[0016] [Figure 1] A schematic diagram showing an example of the water treatment system according to the present invention. [Figure 2] A graph showing the relationship between the concentration and osmotic pressure of an example of the draw solute (ProNip (registered trademark) 17R4) used in the present invention. [Figure 3] A graph showing the relationship between the concentration and osmotic pressure of an example of another draw solute (ProNip (registered trademark) 25R2). [Figure 4] A graph showing the relationship between the concentration and LCST of an example of the draw solute (ProNip (registered trademark) 17R4) used in the present invention. [Figure 5] A flowchart showing each step of the water treatment method of the present invention.

Embodiments for Carrying Out the Invention

[0017] <Water Treatment Method> The present invention relates to a water treatment method for separating water from a target solution (a liquid containing water and components other than water). Examples of target solutions include seawater, river water, lake water, and industrial wastewater.

[0018] Referring to Figure 5, the water treatment method of the present invention includes at least a forward osmosis step, a phase separation step, and a membrane separation step, which are described below.

[0019] [Forward osmosis process] In the forward osmosis process, one side of a semipermeable membrane is brought into contact with the target solution containing water and other components, while the other side of the semipermeable membrane is brought into contact with a Draw solution containing a Draw solute whose solubility decreases with increasing temperature. This causes the water contained in the target solution to be transferred to the Draw solution through the semipermeable membrane.

[0020] Referring to Figure 1, the target solution (FS: feed solution) is introduced into the first chamber 11, which is located in contact with one side of the semipermeable membrane 10, bringing the target solution into contact with one side of the semipermeable membrane 10. Simultaneously, the draw solution (DS) containing the draw solute is introduced into the second chamber 12, which is located in contact with the other side of the semipermeable membrane 10, bringing the draw solution into contact with the other side of the semipermeable membrane 10. In this way, water contained in the target solution moves from the first chamber 11 to the second chamber 12 through the semipermeable membrane 10 by forward osmosis.

[0021] The semipermeable membrane used in the forward osmosis process is not particularly limited, and various membranes that can be used for forward osmosis can be used.

[0022] Examples of semipermeable membranes include reverse osmosis membranes (RO membranes), forward osmosis membranes (FO membranes), nanofiltration membranes (NF membranes), and ultrafiltration membranes (UF membranes). The semipermeable membrane is preferably a reverse osmosis membrane, a forward osmosis membrane, or a nanofiltration membrane. When using a reverse osmosis membrane, a forward osmosis membrane, or a nanofiltration membrane as the semipermeable membrane, the pressure of the target solution in the first chamber is preferably 0.01 to 10 MPa.

[0023] Typically, the pore size of RO and FO membranes is about 2 nm or less, and the pore size of UF membranes is about 2 to 100 nm. NF membranes are RO membranes with relatively low ion and salt rejection rates, and typically, the pore size of NF membranes is about 1 to 2 nm. When RO membranes, FO membranes, or NF membranes are used as semipermeable membranes, the salt removal rate of the RO membrane, FO membrane, or NF membrane is preferably 90% or higher.

[0024] The materials constituting the semipermeable membrane are not particularly limited, but examples include cellulose resins, polysulfone resins, and polyamide resins. Preferably, the semipermeable membrane is composed of a material containing at least one of a cellulose resin and a polysulfone resin.

[0025] The cellulose resin is preferably a cellulose acetate resin. Cellulose acetate resins have the characteristic of being resistant to chlorine, a disinfectant, and can suppress the growth of microorganisms. The cellulose acetate resin is preferably cellulose acetate, and more preferably cellulose triacetate, from the viewpoint of durability. An example of a cellulose-based semipermeable membrane is CTA (manufactured by Toyobo Co., Ltd.).

[0026] The polysulfone resin is preferably a polyethersulfone resin. The polyethersulfone resin is preferably a sulfonated polyethersulfone.

[0027] The shape of the semipermeable membrane is not particularly limited, but examples include flat membranes, spiral membranes, or hollow fiber membranes. Hollow fiber membranes (hollow fiber semipermeable membranes) are advantageous compared to spiral semipermeable membranes, etc., because they have a smaller film thickness, and the membrane area per module can be increased, thereby improving penetration efficiency.

[0028] <Draw solute> The draw solute used in this embodiment (a solute whose solubility decreases with increasing temperature) includes a polymer compound represented by the following formula (1).

[0029] [ka] (In equation (1), x, y, and z are all independent natural numbers.)

[0030] As shown in formula (1), the draw solute of this embodiment is a copolymer, consisting of a polyethylene oxide (PEO) chain (hydrophilic segment) consisting of at least one ethylene oxide (EO) group (hydrophilic group) and a polypropylene oxide (PPO) chain (hydrophobic segment) consisting of at least one propylene oxide (PO) group (hydrophobic group) bonded to each end of the PEO chain. As shown in formula (1), hydrogen is added to both ends of the copolymer.

[0031] The number of EO groups (y) constituting the PEO chain is, for example, 10 to 30. The number of PO groups (x or z) constituting the PPO chain is, for example, 10 to 30. The ratio of x:z is preferably 1:2 to 2:1, and more preferably substantially 1:1.

[0032] Examples of the draw solute products used in this embodiment include Pluronic (registered trademark) 17R4, manufactured by ADEKA Corporation and sold as a nonionic surfactant.

[0033] The above-mentioned Draw solute functions as a so-called LCST-type temperature-responsive polymer. When the temperature of an aqueous solution of an LCST-type temperature-responsive polymer is increased, dehydration of the solute occurs when the temperature exceeds its intrinsic lower critical solution temperature (LCST). This leads to a temperature phase transition (liquid-liquid phase separation) due to the aggregation (micellation) of solute molecules, resulting in phase separation into a dilute phase (low-concentration Draw solution) and a concentrated phase (high-concentration Draw solution). Furthermore, when cooled to a temperature lower than the LCST, it exhibits reversible phase separation behavior, returning to the original solution.

[0034] (average molecular weight) The number-average molecular weight (Mn) of the Draw solute is preferably 2000 to 3000.

[0035] A lower average molecular weight of the Draw solute results in a higher molar concentration in the Draw solution when the same mass of Draw solute is added. This allows for a higher osmotic pressure in the Draw solution, enabling more efficient forward osmosis. For example, if the number-average molecular weight of the Draw solute is 3000 or less, a Draw solution with high osmotic pressure can be provided.

[0036] For example, Pluronic® 17R4 (Mn: 2650) has a smaller molecular weight than Pluronic® 25R2 (Mn: 3100), making it easier to increase the molar concentration of the draw solution. Furthermore, because Pluronic® 17R4 is less prone to association than Pluronic® 25R2, it can exist as a single polymer even when added in large quantities to the draw solution. Therefore, Pluronic® 17R4 can provide a draw solution with a higher osmotic pressure compared to Pluronic® 25R2.

[0037] For example, Figure 2 is a graph showing the relationship between the concentration and osmotic pressure of Pronic® 17R4. From Figure 2, for example, in a forward osmosis process using an 80% by mass aqueous solution of Pronic 17R4 as the DS and seawater as the target solution (FS), in a water treatment where the DS is diluted to 50% by mass, the DS will always have a higher osmotic pressure than the seawater (FS), and it is thought that the forward osmosis process can be carried out efficiently.

[0038] On the other hand, Figure 3 is a graph showing the relationship between the concentration and osmotic pressure of Pronic® 25R2. From Figure 3, it can be seen that Pronic 25R2 has a narrower concentration range for DS, which has a higher osmotic pressure than seawater (FS). Therefore, the amount of water that can be recovered by DS in the forward osmosis process is less than in the case of Pronic 17R4, and it is considered that the forward osmosis process cannot be carried out efficiently.

[0039] On the other hand, if the number-average molecular weight of the Draw solute is too small, it is considered that in the forward osmosis process, the Draw solute in the second chamber 12 is likely to permeate the semipermeable membrane 10 (FO membrane) and leak out to the FS side (first chamber 11 side). For this reason, it is preferable that the number-average molecular weight of the Draw solute be 2000 or more. For example, Pluronic 17R4 (Mn: 2650) also satisfies this condition.

[0040] (LCST) It is preferable that both the LCST of a 50% by mass aqueous solution of the Draw solute and the LCST of an 80% by mass aqueous solution are 45°C or higher.

[0041] Generally, the forward osmosis process is preferably carried out at room temperature or close to room temperature to avoid requiring extra energy. If the draw solution undergoes phase separation at such temperatures, the osmotic pressure may decrease, potentially reducing the efficiency of the forward osmosis process. Therefore, to prevent phase separation of the draw solution at room temperature or close to room temperature, it is preferable that the LCST of the draw solution be 45°C or higher. For example, if we assume that the draw solution is diluted from 80% by mass to 50% by mass in the forward osmosis process, it is considered preferable that both the LCST of the 50% by mass aqueous solution of the draw solute and the LCST of the 80% by mass aqueous solution be 45°C or higher.

[0042] Furthermore, the LCST of a 50% by mass aqueous solution of the Draw solute is preferably 65°C or lower, and more preferably 55°C or lower.

[0043] For example, if we assume that the draw solution is diluted from 80% by mass to 50% by mass in the forward osmosis step, then in the phase separation step, heating to a temperature higher than the LCST of the 50% by mass aqueous solution of the draw solute is necessary. A lower heating temperature is preferable in terms of energy efficiency of water treatment because it reduces the energy consumption of the phase separation step. Therefore, it is considered preferable that the LCST of the 50% by mass aqueous solution of the draw solute be, for example, 65°C or lower.

[0044] Figure 4 is a graph showing the relationship between the concentration of Pluronic® 17R4 and the response temperature (cloud point). As shown in Figure 4, the LCST of an aqueous solution of Pluronic 17R4 is 45°C or higher in the concentration range of 50-80% by mass, and the LCST of a 50% by mass aqueous solution of Pluronic 17R4 is 65°C or lower. It is preferable to use a Draw solute such as Pluronic 17R4.

[0045] (EO ratio) The proportion of ethylene oxide (EO) groups in the draw solute (EO ratio) is preferably 35 to 50% by mass.

[0046] The temperature response (phase separation behavior) of a Draw solute is influenced by its hydrophilicity. The hydrophilicity of a Draw solute is influenced by the ratio of EO groups (hydrophilic groups) contained in the Draw solute (polymer) molecule, that is, the balance between EO groups and propylene oxide (PO) groups (hydrophobic groups).

[0047] The inventors investigated various LCST-type temperature-responsive polymers and found that when the EO ratio is 35-50% by mass, both the LCST in a 50% by mass aqueous solution and the LCST in an 80% by mass aqueous solution reach 45°C or higher, while the LCST in a 50% by mass aqueous solution reaches 65°C or lower. Therefore, for the same reasons as those related to LCST, it is preferable to use an LCST-type temperature-responsive polymer with an EO ratio of 35-50% by mass as the draw solute.

[0048] (cloud point) The cloud point of a 1% by mass aqueous solution of Draw solute is preferably 35°C to 50°C, and more preferably 40°C to 50°C. The cloud point is the temperature at which precipitation (solid-liquid phase separation) of the Draw solute begins when the temperature of the Draw solution is increased. At temperatures above the cloud point, the Draw solute dissolved in micelles also separates from the liquid phase and precipitates. The cloud point is usually higher than the LCST (Liquid-Liquid Phase Separation) at which liquid-liquid phase separation begins.

[0049] If the cloud point is 35°C or higher, and the temperature of the membrane separation process is approximately 40°C, similar to the phase separation process, then the draw solute will be dissolved in the low-concentration DS (dilute phase) obtained by the phase separation process. In the membrane separation process, it is desirable to maintain this state of dissolution of the draw solute in the low-concentration DS.

[0050] On the other hand, if the cloud point is below 35°C, the stability of Unimar in water is low, making micelle formation more likely, and resulting in lower osmotic pressure in the medium to high concentration range. Therefore, the concentration range in which it can be used as a draw solution becomes narrower.

[0051] For example, the cloud point of a 1% by mass aqueous solution of Pluronic 17R4 is 38-44°C, which is above 35°C (around 40°C). Therefore, the monopolymer of Pluronic 17R4 has high stability in water. From this, as shown in Figure 2, the osmotic pressure of an aqueous solution of Pluronic 17R4 exceeds that of seawater at concentrations of 50% by mass or higher, meaning that it has a wide concentration range in which it can be used as a draw solution.

[0052] On the other hand, the cloud point of a 1% by mass aqueous solution of Pluronic 25R2 is 27-32°C, which is below 35°C. In this case, the stability of the Pluronic 25R2 monopolymer in water is low. Therefore, as shown in Figure 3, the osmotic pressure of an aqueous solution of Pluronic 25R2 exceeds that of seawater at concentrations of 70% by mass or higher, resulting in a narrow concentration range in which it can be used as a draw solution.

[0053] Furthermore, if the cloud point exceeds 50°C, the LCST of the draw solute will also become hot, requiring heating to a high temperature in the phase separation process, which may lead to increased energy costs. Therefore, it is preferable that the cloud point be 50°C or lower.

[0054] [Phase separation process] In the phase separation process, after the forward osmosis process, the temperature of the draw solution is increased to separate the draw solution into a low-concentration draw solution and a high-concentration draw solution (phase separation).

[0055] The temperature of the draw solution after the rise in the phase separation step is preferably 40°C to 90°C, more preferably 40°C to 85°C, and even more preferably 40°C to 80°C.

[0056] The draw solution containing water from the target solution is introduced into the separation tank 3, where its temperature is increased to separate it into a dilute phase (low-concentration draw solution) and a concentrated phase (high-concentration draw solution). For example, if the draw solute contains an LCST-type temperature-responsive polymer, the draw solution can be separated into a dilute phase (low-concentration draw solution) and a concentrated phase (high-concentration draw solution) by raising its temperature above LCST. In order to raise the temperature of the draw solution in this manner, it is preferable that the separation tank 3 has a heating mechanism.

[0057] [Membrane separation process] In the membrane separation step, a separation membrane is used to separate the low-concentration draw solution (dilute phase) separated in the phase separation step into a concentrated solution containing water and the draw solute. In the membrane separation step, membrane separation (filtration) is performed using a pressure difference, such as by pressurizing or depressurizing. Examples of separation membranes that can be used include UF membranes, NF membranes, and RO membranes.

[0058] The temperature of the membrane separation step is preferably higher than the temperature of the forward osmosis step. It is also preferably lower than the phase separation temperature. The temperature of the membrane separation step is more preferably 40°C to 90°C, even more preferably 40°C to 85°C, and even more preferably 40°C to 80°C.

[0059] Furthermore, in the low-concentration DS (dilute phase) obtained by the above phase separation step, the Draw solute (LCST-type temperature-responsive polymer) exists in a dissolved state. In the membrane separation step, it is desirable to maintain this state in which the Draw solute is dissolved in the low-concentration DS. Therefore, it is preferable to maintain the temperature of the membrane separation step at a temperature equal to or higher than the cloud point of the low-concentration DS (for example, a solution containing about 1% by mass of Draw solute).

[0060] Furthermore, by recovering the water separated by this membrane separation process, the target product of the water treatment method, water, can be obtained. The obtained water may be further treated to improve its quality.

[0061] The draw solute separated in the membrane separation step, and the concentrated liquid (high-concentration draw solution) containing the draw solute obtained in the phase separation step, are preferably reused as a draw solution in the forward osmosis step.

[0062] <Water Treatment System> Figure 1 is a schematic diagram showing an example of a water treatment system according to the present invention. Referring to Figure 1, the water treatment system used in the above water treatment method is: A forward osmosis module 1 includes a semipermeable membrane 10, a first chamber 11 provided for contacting the target solution with one side of the semipermeable membrane 10, and a second chamber 12 provided for contacting the draw solution with the other side of the semipermeable membrane. A separation tank 3 having a heating mechanism for separating the draw solution into a low-concentration draw solution and a high-concentration draw solution by raising the temperature of the draw solution, The system includes a separation membrane 20 (separation membrane module 2) for separating a low-concentration draw solution into water and the draw solute.

[0063] As shown in Figure 1, pumps 41-46 flow FS, DS, etc., in the direction of the arrows. The separation tank 3 has a heating mechanism. That is, hot water, such as waste hot water, is supplied around the separation tank 3 to raise the temperature of the diluted DS supplied to the separation tank 3 and cause phase separation. The temperature of the hot water should be adjusted to match the target temperature of the DS after it has risen in the phase separation process.

[0064] In separation tank 3, the draw solution (diluted DS) undergoes phase separation into low-concentration DS and high-concentration DS due to the rise in temperature. The high-concentration DS separated in separation tank 3 and the low-concentration DS concentrated in separation membrane module 2 are sent to tank 5 by pump 44, where they are temporarily stored and can be reused as DS in the forward osmosis process.

[0065] Examples of separation membranes 20 include semipermeable membranes such as ultrafiltration membranes (UF membranes), nanofiltration membranes (NF membranes), and reverse osmosis membranes (RO membranes).

[0066] The separation membrane module 2 preferably has heat resistance at high temperatures (for example, 40°C or higher). In this case, the membrane separation process can be efficiently carried out for the high-temperature, low-concentration DS discharged from the separation tank 3 without requiring a cooling process or the like.

[0067] Examples of materials for heat-resistant separation membranes include polyethersulfone (PES) resins, polyamide (PA) resins, and polyvinyl alcohol (PVA) resins. Furthermore, it is preferable that the separation membrane module has heat resistance not only in the separation membrane itself but also in other components, thus providing overall heat resistance. Examples of heat-resistant separation membrane module products include Thermoplus (manufactured by Nitto Denko Corporation), Duratherm (manufactured by GE Water Technologies), and the Romembra® TS series (manufactured by Toray Industries, Inc.).

[0068] Other materials for heat-resistant separation membranes include ceramics such as alumina and silica. Examples of silica for heat-resistant membranes include silica derived from bistolylethoxysilylethane (see Minoru Tsuru, "Development of Robust RO / NF Membranes that Can Handle Diverse Water Sources," Journal of the Japan Society on Water Environment, vol.36(A), No.1, pp.8-10, 2013). [Examples]

[0069] Various draw solutes (LCST-type temperature-responsive polymers) were evaluated according to the following criteria [1] to [3].

[0070] [1] Study on the sequence of copolymers (upper limit of LCST) Table 1 shows the LCSTs for each copolymer (Pluronic®, ADEKA Corporation, or BASF) in 50% by mass aqueous solutions and 80% by mass aqueous solutions of the Draw solute. An LCST of 65°C or lower was evaluated as "OK," and an LCST above 65°C was evaluated as "NG." The evaluation results are shown in Table 1. Note that Mn in Table 1 represents the number-average molecular weight.

[0071] [Table 1]

[0072] The results shown in Table 1 indicate that when copolymers having a PEO-PPO-PEO sequence different from that of chemical formula (1) above (Pluronic L64 and L43) are used as the draw solute, the LCST of the 50% by mass aqueous solution is high (exceeding 65°C), and the amount of heat required to carry out the phase separation process is large, resulting in poor energy efficiency when carrying out water treatment.

[0073] Furthermore, regarding osmotic pressure, a number-average molecular weight of the draw solute was evaluated as "OK" if it was between 2000 and 3000 or less, and as "NG" if it was greater than 3000. For the reasons mentioned above, a number-average molecular weight of 3000 or less was used as the evaluation criterion for obtaining sufficient osmotic pressure necessary for the forward osmosis process. The evaluation results are shown in Table 2.

[0074] [Table 2]

[0075] The results shown in Table 2 indicate that, from the perspective of efficiently carrying out the forward osmosis process, Pluronic 17R4 and Pronic 25R2 are more preferable copolymers for "LCST".

[0076] Furthermore, since the EO ratio is highly correlated with LCST, the results shown in Table 2 suggest that when the EO ratio is 35-50% by mass, as in Pluronic 17R4, both the LCST in a 50% by mass aqueous solution and the LCST in an 80% by mass aqueous solution will be 45°C or higher, while the LCST in a 50% by mass aqueous solution will be 65°C or lower.

[0077] Furthermore, regarding osmotic pressure, it was found that Pluronic 17R4, Pluronic 17R2, and Pronic 25R1 are more preferred copolymers.

[0078] Furthermore, it was found that the preferred copolymer in terms of both LCST and osmotic pressure is Pluronic 17R4.

[0079] [2] Examination of solute leakage in the forward osmosis process Each of the pronics shown in Table 3 was used as the draw solute, and a forward osmosis process (semipermeable membrane: hollow fiber membrane made of cellulose triacetate, DS concentration: 70% by mass, FS concentration: 0% by mass (RO water)) was performed. In this forward osmosis process, the amount of draw solute that permeated through the semipermeable membrane (FO membrane) and leaked out to the FS side (solute leakage) was measured by TOC (total organic carbon concentration). The measurement results are shown in Table 3.

[0080] [Table 3]

[0081] The results shown in Table 3 suggest that when the number-average molecular weight (Mn) of the draw solute is less than approximately 2000, there is a high probability that a large amount of the draw solute will permeate the semipermeable membrane (FO membrane) and leak out to the FS side during the forward osmosis process. [Explanation of symbols]

[0082] 1 forward osmosis module, 10 semipermeable membranes, 11 first chamber, 12 second chamber, 2 separation membrane modules, 20 separation membranes, 3 separation tanks, 41-46 pumps, 5 tanks.

Claims

1. A forward osmosis step is performed by bringing one side of a semipermeable membrane into contact with a target solution containing water and components other than water, and bringing the other side of the semipermeable membrane into contact with a draw solution containing a draw solute whose solubility decreases with increasing temperature, thereby moving the water contained in the target solution through the semipermeable membrane to the draw solution. A phase separation step is performed after the forward osmosis step, in which the temperature of the draw solution is increased to separate the draw solution into a low-concentration draw solution and a high-concentration draw solution. The process includes a membrane separation step, in which the low-concentration draw solution is separated into water and a concentrated solution containing the draw solute using a separation membrane. The proportion of ethylene oxide groups in the aforementioned draw solute is 35 to 50% by mass. A water treatment method wherein the draw solute contains a copolymer represented by the following formula (1). 【Chemistry 1】 (In equation (1), x, y, and z are all independent natural numbers.)

2. A forward osmosis step in which one side of a semipermeable membrane is brought into contact with a target solution containing water and components other than water, and the other side of the semipermeable membrane is brought into contact with a draw solution containing a draw solute whose solubility decreases with increasing temperature, thereby moving the water contained in the target solution through the semipermeable membrane to the draw solution, A phase separation step is performed after the forward osmosis step, in which the temperature of the draw solution is increased to separate the draw solution into a low-concentration draw solution and a high-concentration draw solution. The process includes a membrane separation step, in which the low-concentration draw solution is separated into water and a concentrated solution containing the draw solute using a separation membrane. The cloud point of the 1% by mass aqueous solution of the aforementioned Draw solute is 35 to 50°C. A water treatment method wherein the draw solute contains a copolymer represented by the following formula (1). 【Chemistry 2】 (In equation (1), x, y, and z are all independent natural numbers.)

3. The water treatment method according to claim 1 or 2, wherein the number-average molecular weight of the draw solute is 2000 to 3000.

4. The temperature during the forward osmosis process is less than 40°C. In the phase separation process, the temperature of the draw solution rises to 40°C or higher. The water treatment method according to claim 1 or 2, wherein the temperature of the membrane separation step is 40°C or higher.

5. A forward osmosis water treatment system used in the forward osmosis water treatment method described in claim 1 or 2, A forward osmosis module comprising a semipermeable membrane, a first chamber provided on one side of the semipermeable membrane for contacting the target solution, and a second chamber provided on the other side of the semipermeable membrane for contacting the draw solution, A separation tank having a heating mechanism for separating the draw solution into a low-concentration draw solution and a high-concentration draw solution by raising the temperature of the draw solution, A water treatment system comprising a separation membrane for separating the low-concentration draw solution into water and a concentrated liquid containing the draw solute.

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