WO2018150690A1

WO2018150690A1 - Water treatment method and water treatment system

Info

Publication number: WO2018150690A1
Application number: PCT/JP2017/043819
Authority: WO
Inventors: 秀人松山; 智輝高橋; 英治神尾; 政宏安川; 熊野　淳夫
Original assignee: 国立大学法人神戸大学; 東洋紡株式会社
Priority date: 2017-02-17
Filing date: 2017-12-06
Publication date: 2018-08-23
Also published as: JPWO2018150690A1; JP7117718B2

Abstract

A water treatment method characterized in: comprising a forward osmosis step for moving water contained in a solution to be treated that comprises water and components other than water through a forward osmosis membrane into a draw solution by bringing one surface of the forward osmosis membrane into contact with the solution to be treated and bringing the other surface of the forward osmosis membrane into contact with a draw solution comprising a draw solute, the solubility of which decreases with increasing temperature, a phase separation step after the forward osmosis step 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, and a membrane separation step for separating the low concentration draw solution into water and a concentrated draw solute-containing solution using a separation membrane; wherein the temperature of the membrane separation step is higher than the temperature of the forward osmosis step.

Description

Water treatment method and water treatment system

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

Conventionally, in the field of water treatment, a desalination method using 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 a low concentration side moves toward a solution on a high concentration side.

In the membrane separation process, artificially applying high pressure to the high concentration target solution moves water from the high concentration target solution (seawater, etc.) to the low concentration solution (water, etc.) as opposed to forward osmosis. It is a process to make. Thereby, for example, water can be produced from the target solution. Since the membrane separation process requires high pressure, the energy consumption is extremely high and the energy efficiency is low. Therefore, in recent years, in order to increase the energy efficiency of water treatment, a desalination method using a forward osmosis process that does not require artificially applying pressure has been studied.

Here, as a solute (draw solute) of an induction solution (draw solution) which is a hypertonic solution for forward osmosis, in recent years, it is easy to recover the draw solute from the draw solution and it can be reused. Draw solutes (temperature-responsive draw solutes) that reversibly change their compatibility with water are drawing attention.

Patent Document 1 (Japanese Patent Publication No. 2014-512951) discloses a water treatment method including a forward osmosis treatment using a draw solute whose solubility is reduced by heating (temperature increase). When using a draw solute (lower critical solution temperature (LCST) type draw solute) whose solubility decreases with increasing temperature, the draw solution can be separated from the low concentration fraction (low concentration draw solution) by increasing the temperature. Although it can be phase-separated into a concentration fraction (high-concentration draw solution), in many cases, it is not possible to completely separate water and the draw solute only by increasing the temperature.

For this reason, referring to FIG. 8, the diluted draw solution is phase-separated into a low-concentration draw solution and a high-concentration draw solution in the separation tank 3 by heating or the like by forward osmosis treatment in the forward osmosis module 1. Thereafter, the separation membrane module 2 performs membrane separation treatment on the low-concentration draw solution, thereby recovering water (separating the draw solute). In addition, although the thermal energy for raising the temperature of a draw solution is required, the influence on the cost side by consumption of a thermal energy is comparatively small by utilizing the waste heat of other facilities.

Special table 2014-512951 gazette

However, the low concentration draw solution often contains several to several tens of mass% draw solute. In order to separate this low concentration draw solution into water and draw solute by membrane separation, membrane separation is required. In this case, a high pressure needs to be applied, and a lot of energy for pressurization is required. For this reason, there was a problem that the energy efficiency of water treatment could not be sufficiently improved, and it was difficult to sufficiently reduce the cost of water treatment.

In view of the above problems, an object of the present invention is to provide a water treatment method and a water treatment system that can separate a draw solute from a draw solution with low energy and can further increase the energy efficiency of water treatment.

[1]
While bringing one surface of the forward osmosis membrane into contact with the target solution containing water and components other than water,
By bringing the other side of the forward osmosis membrane into contact with a draw solution containing a draw solute whose solubility decreases with increasing temperature,
Forward osmosis step of transferring water contained in the target solution to the draw solution through the forward osmosis membrane;
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;
Separating the low-concentration draw solution into a concentrated solution containing water and a draw solute using a separation membrane, and a membrane separation step,
The water treatment method is characterized in that the temperature of the membrane separation step is higher than the temperature of the forward osmosis step.

[2]
The water treatment method according to [1], wherein the draw solute has a lower osmotic pressure due to a temperature rise.

[3]
The water treatment method according to [1] or [2], wherein the temperature of the membrane separation step is 40 ° C. or higher.

[4]
The water treatment method according to any one of [1] to [3], wherein the temperature of the forward osmosis step is less than 40 ° C.

[5]
The water treatment method according to any one of [1] to [4], wherein in the phase separation step, the draw solution rises to a temperature of 40 ° C. or higher.

[6]
A forward osmosis water treatment system used in the forward osmosis water treatment method according to any one of [1] to [5],
A first osmosis membrane, a first chamber provided for bringing the target solution into contact with one surface of the forward osmosis membrane, and a second chamber provided for bringing the draw solution into contact with the other surface of the forward osmosis membrane Including forward osmosis module;
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 a low-concentration draw solution into water and a concentrate containing a draw solute.

According to the present invention, it is possible to provide a water treatment method and a water treatment system that can separate a draw solute from a draw solution with low energy and can further increase the energy efficiency of water treatment.

It is a schematic diagram which shows an example of the water treatment system which concerns on this invention. It is a schematic diagram which shows another example of the water treatment system which concerns on this invention. It is a figure which shows the phase-separation curve of an example (PPG400) of the draw solute used by this invention. It is a figure which shows the osmotic pressure characteristic of an example (PPG400) of the draw solute used by this invention. It is a figure which shows the phase-separation curve of another example ([ _N4444 ] [TMBS]) of the draw solute used by this invention. It is a figure which shows the osmotic pressure characteristic of another example ([ _N4444 ] [TMBS]) of the draw solute used by this invention. It is a flowchart which shows each process of the water treatment method of this invention. It is a schematic diagram which shows an example of the conventional water treatment method. When PPG400 is used by 20 mass% as DS, it is a graph which shows the relationship between the applied pressure at the time of applying a pressure to DS side and performing a reverse osmosis process, and a water-permeable flux.

<Water treatment method>
The water treatment method of the present invention is a method for separating water from a target solution (liquid containing water and components other than water). Examples of the target solution include seawater, river water, lake water, industrial wastewater, and the like.

Referring to FIG. 7, the water treatment method of the present invention includes at least a forward osmosis step, a phase separation step, and a membrane separation step described below.

[Forward osmosis process]
In the forward osmosis process, one side of the forward osmosis membrane is brought into contact with water and a target solution containing components other than water, and the other side of the forward osmosis membrane is brought into contact with a draw solute containing a draw solute that decreases in solubility due to temperature rise Contact the solution. Thereby, the water contained in the target solution is moved to the draw solution through the forward osmosis membrane.

The forward osmosis membrane (semipermeable membrane) is not particularly limited, and various membranes that can be used for forward osmosis can be used.

Examples of the semipermeable membrane include a reverse osmosis membrane (RO membrane: Reverse Osmosis Membrane), a forward osmosis membrane (FO membrane: Forward Osmosis Membrane), a nanofiltration membrane (NF membrane: Nanofiltration Membrane), and an ultrafiltration membrane (UF membrane). : Semipermeable membrane called Ultrafiltration Membrane). The semipermeable membrane is preferably a reverse osmosis membrane, a forward osmosis membrane, or a nanofiltration membrane. When a reverse osmosis membrane, a forward osmosis membrane, or a nanofiltration membrane is used as the semipermeable membrane, the pressure of the target solution in the first chamber is preferably 0.01 to 10 MPa.

Usually, the pore size of the RO membrane and the FO membrane is about 2 nm or less, and the pore size of the UF membrane is about 2 to 100 nm. The NF membrane has a relatively low blocking rate of ions and salts among the RO membrane, and the pore size of the NF membrane is usually about 1 to 2 nm. When an RO membrane, FO membrane, or NF membrane is used as the semipermeable membrane, the salt removal rate of the RO membrane, FO membrane, or NF membrane is preferably 90% or more.

The material constituting the semipermeable membrane is not particularly limited, and examples thereof include cellulose resins, polysulfone resins, and polyamide resins. The semipermeable membrane is preferably composed of a material containing at least one of a cellulose resin and a polysulfone resin.

The cellulose resin is preferably a cellulose acetate resin. Cellulose acetate resin is resistant to chlorine, which is a bactericidal agent, and has a feature that it can suppress the growth of microorganisms. The cellulose acetate resin is preferably cellulose acetate, and more preferably cellulose triacetate from the viewpoint of durability. Examples of the cellulose semipermeable membrane include CTA (manufactured by Toyobo Co., Ltd.).

The polysulfone resin is preferably a polyethersulfone resin. The polyethersulfone resin is preferably a sulfonated polyethersulfone.

The shape of the semipermeable membrane is not particularly limited, and examples thereof include a flat membrane, a spiral membrane, and a hollow fiber membrane. Hollow fiber membranes (hollow fiber type semipermeable membranes) are smaller in thickness than spiral type semipermeable membranes, and can increase the membrane area per module and increase the penetration efficiency. It is advantageous.

(Draw solute)
The draw solute used in the present invention is not particularly limited as long as the solubility of the draw solute decreases with increasing temperature. Note that such a decrease in solubility is reversible, and conversely, if the temperature decreases, it is preferable that the solubility increase.

Examples of such draw solutes include LCST (lower critical solution temperature) type phase change materials. In general, it is considered that an aqueous solution containing an LCST type phase change substance causes dehydration of a solute as the solution temperature rises, and causes a temperature phase transition (liquid-liquid phase separation) due to aggregation of solute molecules. In addition, since the polymolecular association is formed even in the temperature range below the phase transition temperature (homogeneous phase range below the LCST curve), it is considered that the osmotic pressure of the solution decreases due to the temperature rise even in the temperature range below the phase transition temperature. It is done.

Examples of the LCST type phase change material include LCST type temperature responsive polymers. LCST type temperature-responsive polymer dissolves in water at a relatively low temperature, and when the temperature exceeds a predetermined temperature (specific lower critical solution temperature: LCST), a dilute phase (low concentration draw solution) and a concentrated phase Phase separation into (high concentration draw solution).

The LCST type temperature-responsive polymer preferably has at least one hydrophilic group in at least some or all of the structural units (monomer units). Moreover, the temperature-responsive polymer may have a hydrophobic group in some structural units while having a hydrophilic group. In addition, it is considered that the balance between the hydrophilic group and the hydrophobic group contained in the molecule is important for the temperature responsive polymer to have temperature responsiveness.

Examples of the hydrophilic group include a hydroxyl group, a carboxyl group, an acetyl group, an aldehyde group, an ether bond, and an ester bond.

Specific temperature-responsive polymers include, for example, polyvinyl ether polymers, polyvinyl acetate polymers, (meth) acrylic acid polymers, and the like.

Examples of the polyvinyl ether polymer include polymethyl vinyl ether, vinyl ether having an oxyethylene chain, and polyhydroxybutyl vinyl ether.

Specific examples of polyvinyl ether polymers include the polymers shown in Table 1. In addition, the phase transition temperature shown in Table 1 is a lower critical solution temperature (LCST) in aqueous solution.

References 1 to 4 listed in Table 1 are as follows.
Reference 1: Journal of Japan Rubber Association, Vol. 63, No. 1, 29-39 (1990).
Reference 2: Journal of Polymer Science Part A: polymer Chemistry Vol. 30, 2407-2413 (1992).
Reference 3: Journal of Polymer Science Part A: polymer Chemistry Vol. 41, 3300-3312 (2003).
Reference 4: Macromolecules, 36, 8312-8319 (2003).

Examples of the polyvinyl acetate polymer include polyvinyl acetate part saponified products.

Examples of the (meth) acrylic acid polymer include poly (meth) acrylate having an oxyethylene chain, (meth) acrylic acid copolymer, and the like.

Examples of LCST type temperature-responsive polymers other than the above include polypropylene glycol (PPG), for example. The phase separation characteristics and osmotic pressure characteristics of PPG (molecular weight 400, diol type) are shown in FIGS. 3 and 4, respectively. FIG. 3 shows that PPG phase-separates into a low concentration phase of about 15% by mass and a high concentration phase of about 80% by mass in an aqueous solution at about 70 ° C., for example. Moreover, it can be seen from FIG. 4 that the osmotic pressure of PPG decreases as the temperature increases.

FIG. 9 shows a case where the temperature-responsive draw solution is a 20% by mass polypropylene glycol (molecular weight 400, PPG400) solution, and the values of the water permeability flux are different when reverse osmosis treatment is performed by applying pressure to the draw solution side. The temperature is shown (25 ° C. and 60 ° C.). In FIG. 9, the applied pressure at which the water permeation amount becomes 0 (intercept on the horizontal axis) is 19 bar when the temperature is 25 ° C. and 11 bar when the temperature is 60 ° C., and the concentration of the draw solution is the same. Nevertheless, the pressure at which the amount of water permeation becomes zero decreases due to an increase in temperature. This indicates that the osmotic pressure of PPG 400 varies with temperature. For example, at 25 ° C., the permeability flux obtained at a pressure of 25 bar is about ² L / (m ² · H), which corresponds to the permeability flux obtained at a pressure of about 12 bar at 60 ° C. This shows that the applied pressure at which the same permeable flux can be obtained can be greatly reduced.

Examples of LCST type temperature-responsive polymer products include Pluronic (manufactured by BASF), which is a block copolymer of polyoxypropylene and polyoxyethylene.

Examples of LCST type phase transition materials other than LCST type temperature responsive polymers include LCST type ionic liquid, glycol ether type LCST type phase change material, acid amide type LCST type phase change material, and amine type LCST type. Examples include phase change substances.

As the LCST type ionic liquid, for example,
Tetrabutylammonium-2,4,6-trimethylbenzenesulfonate (N ₄₄₄₄ -TMBS),
_{Tetrabutylammonium} -trifluoroacetate (N ₄₄₄₄ -CF ₃ COO),
Tetrabutylphosphonium-2,4,6-trimethylbenzenesulfonate (P ₄₄₄₄ -TMBS),
Tetrabutylphosphonium-trifluoroacetate (P ₄₄₄₄ -CF ₃ COO)
Etc.

Examples of glycol ether type LCST type phase transition materials include:
Diethylene glycol hexyl ether,
Propylene glycol butyl ether,
Dipropylene glycol propyl ether,
Ethylene glycol n-butyl ether,
Ethylene glycol n-pentyl ether,
Diethylene glycol n-pentyl ether,
Ethylene glycol 2-methyl-1-butyl ether,
Diethylene glycol 2-methyl-1-butyl ether,
Ethylene glycol n-hexyl ether,
Triethylene glycol n-hexyl ether,
Ethylene glycol n-heptyl ether,
Diethylene glycol n-heptyl ether,
Triethylene glycol n-heptyl ether,
Propylene glycol n-propyl ether,
Tripropylene glycol n-propyl ether,
Dipropylene glycol n-butyl ether,
And tripropylene glycol n-butyl ether.

Examples of acid amide LCST type phase transition materials include:
N, N-bis (2-butyramidoethyl) butyramide (nBu-DETA)
N- (2- (Diethylamino) ethyl) pentanamide (Val-DEEA)
N- (2- (Diethylamino) ethyl) isobutyramide (iBu-DEEA)
N, N ′-((Methylazanediyl) bis (propane-3,1-diyl)) dibutyramide (nBu-DAPMA)
N, N ′-((Methylazanediyl) bis (propane-3,1-diyl)) dipentanamide (Val-DAPMA)
N-Butylpropionamide (N-BPA)
N- (2- (diisopropylamino) ethyl) butyramide (nBu-DIPA)
N-Butylated polyethyleneimine (nBu-PEI)
Etc.

Examples of amine-based LCST type phase change materials include:
Triethylamine,
N-ethylpiperidine,
2-methylpiperidine,
N-methylpiperidine and the like.

The phase separation characteristics and osmotic pressure characteristics of N ₄₄₄₄ -TMBS are shown in FIGS. 5 and 6, respectively. In FIG. 5, N ₄₄₄₄ -TMBS ([N ₄₄₄₄ ] [TMBS]) is, for example, in an aqueous solution at about 70 ° C., a low concentration phase of about 12% by mass, a high concentration phase of about 65% by mass, Are shown to phase separate. Further, FIG. 6 shows that N ₄₄₄₄ -TMBS has a lower osmotic pressure as the temperature is higher. In FIG. 5, the osmotic pressure of the low-concentration phase (about 12% by mass) phase-separated at 70 ° C. is about 4.6 bar at room temperature (25 ° C.) from FIG. is there.

The present inventors constructed a method for measuring the actual osmotic pressure of the above-mentioned draw solute (LCST type phase change material), and from the measurement result, the osmotic pressure of the aqueous solution (draw solution) decreased due to temperature rise. I found out. The reason why the osmotic pressure of the aqueous solution (draw solution) decreases due to the temperature rise is estimated as follows.

The draw solute (DS) used in the present invention is solvated (hydrated) in a solvent (water) at a low temperature. When dehydration of DS molecules occurs as the temperature of the draw solution increases, the interaction between DS molecules works relatively strongly compared to the interaction between solvent molecules (water molecules) and DS molecules. Molecules are thought to form aggregates (aggregates).

As represented by the Vanthoff equation (π = cRT), the osmotic pressure π [bar] of the ideal solution is the solute molar concentration c [mol·L ⁻¹ ] and the gas constant R [bar · L · K ^−1]. -Proportional to mol ^-1 ] and absolute temperature T [K]. Since the solute molar concentration is the molar molar concentration of all solute particles, the calculated solute molar concentration decreases when an association of solute particles occurs. For this reason, it is thought that the osmotic pressure of a solution falls by formation of an aggregate.

The osmotic pressure of the draw solution (after raising the temperature) used for the membrane separation step is preferably 0.01 to 2 MPa, more preferably 0.01 to 1 MPa.

Referring to FIG. 1, a target solution (FS: feed solution) is caused to flow into a first chamber 11 provided in contact with one surface of the forward osmosis membrane 10, and the target solution is supplied to one side of the forward osmosis membrane 10. Touch the surface. At the same time, a draw solution (DS) containing a draw solute is caused to flow into the second chamber 12 provided in contact with the other surface of the forward osmosis membrane 10, and the draw solution is introduced into the other surface of the forward osmosis membrane 10. Make contact.

By maintaining such a state for a predetermined time, water contained in the target solution passes through the forward osmosis membrane 10 and moves from the first chamber 11 to the second chamber 12 due to the forward osmosis phenomenon.

[Phase separation process]
In the phase separation step, after the forward osmosis step, 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).

The temperature after the rise of the draw solution in the phase separation step is preferably 40 ° C. or higher and 90 ° C. or lower, more preferably 40 ° C. or higher and 85 ° C. or lower, and further preferably 40 ° C. or higher and 80 ° C. or lower.

The draw solution containing water in the target solution is allowed to flow into the separation tank 3 where the temperature of the draw solution is raised to separate the dilute phase (low concentration draw solution) and the concentrated phase (high concentration draw solution). Let For example, when the draw solute contains an LCST-type temperature-responsive polymer, the draw solution temperature is raised to LCST or higher to separate the dilute phase (low concentration draw solution) and the concentrated phase (high concentration draw solution). Can be made. Thus, in order to raise the temperature of a draw solution, it is preferable that the separation tank 3 has a heating mechanism.

[Membrane separation process]
In the membrane separation step, the low-concentration draw solution is separated into water and a concentrate containing a draw solute using a separation membrane as a driving force by a pressure difference due to pressurization, decompression, or the like. As the separation membrane, for example, a UF membrane, NF membrane, RO membrane or the like can be used.

The temperature of the membrane separation process is preferably higher than the temperature of the forward osmosis process. Further, a temperature lower than the phase separation temperature is preferable. The temperature in the membrane separation step is more preferably 40 ° C. or higher and 90 ° C. or lower, still more preferably 40 ° C. or higher and 85 ° C. or lower, and even more preferably 40 ° C. or higher and 80 ° C. or lower.

In addition, the water which is the target object of a water treatment method can be obtained by collect | recovering the water isolate | separated by this membrane separation process. The obtained water may be subjected to a treatment for further improving the quality of the water.

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.

<Water treatment system>
FIG. 1 is a schematic diagram showing an example of a water treatment system according to the present invention.

With reference to FIG. 1, the water treatment system used for said water treatment method is as follows.
The forward osmosis membrane 10, the first chamber 11 provided for contacting the target solution with one surface of the forward osmosis membrane 10, and the first chamber 11 provided for contacting the draw solution with the other surface of the forward osmosis membrane Forward osmosis module 1 comprising two chambers 12;
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 increasing the temperature of the draw solution;
A separation membrane 20 (separation membrane module 2) for separating the low-concentration draw solution into water and a draw solute.

As shown in FIG. 1, FS, DS, etc. are flowed in the direction of the arrow by the pumps 41-46. The separation tank 3 has a heating mechanism. That is, warm water such as waste hot water is supplied around the separation tank 3 in order to raise the temperature of the dilution DS supplied to the separation tank 3 and cause phase separation. What is necessary is just to adjust the temperature of warm water according to the target value of the temperature after DS raises in a phase-separation process.

In the separation tank 3, the draw solution (diluted DS) is phase-separated into a low concentration DS and a high concentration DS as the temperature rises. The high-concentration DS separated in the separation tank 3 and the low-concentration DS concentrated in the separation membrane module 2 are sent to the tank 5 by the pump 44 and temporarily stored. Can be reused as DS.

Examples of the separation membrane include a semipermeable membrane called an ultrafiltration membrane (UF membrane: Ultrafiltration Membrane), a nanofiltration membrane (NF membrane: Nanofiltration Membrane), and a reverse osmosis membrane (RO membrane: Reverse Osmosis Membrane).

The separation membrane module preferably has heat resistance at high temperatures (for example, 40 ° C. or higher). This is because the membrane separation process can be performed on the high-temperature low-concentration DS discharged from the separation tank 3.

Examples of the material for the heat-resistant separation membrane include polyethersulfone (PES) resin, polyamide (PA) resin, polyvinyl alcohol (PVA) resin, and the like. Further, in the separation membrane module, parts other than the separation membrane also have heat resistance, and preferably have heat resistance as a whole. Examples of heat-resistant separation membrane module products include Thermo Plus (manufactured by Nitto Denko Corporation), Durathermo (manufactured by GE Water Technologies), and Romenbra (registered trademark) TS series (manufactured by Toray Industries, Inc.). Can be mentioned.

Also, other materials for the heat-resistant separation membrane include ceramics such as alumina and silica. Examples of the silica for the heat resistant membrane include silica derived from bistryl ethoxysilyl ethane (Atsushi Tsuru, “Development of a Robust RO / NF Membrane That Can Correspond to Various Water Sources”, Journal of Water Environment Society, vol. 36 (A), No.1, pp.8-10, 2013).

FIG. 2 is a schematic view showing another example of the water treatment system according to the present invention.
The water treatment system shown in FIG. 2 includes

heat exchangers

61 and 62 in addition to the water treatment system shown in FIG. This reduces the heat energy required for the phase separation process by heating the diluted DS using the low-concentration DS and high-concentration DS waste heat discharged from the separation tank 3, and the energy efficiency of the water treatment Can be increased.

Here, in the conventional water treatment system shown in FIG. 8, since the separation membrane module 2 used for normal water treatment is often used at room temperature, the heat resistance at high temperature (for example, 40 ° C. or more). Often does not have. For this reason, the separation membrane module 2 is provided downstream from the heat exchanger 61, and in the separation membrane module 2, the membrane separation step is performed on the low concentration DS after the temperature is lowered.

On the other hand, in FIG. 2, the separation membrane module 2 (separation membrane 20) is arranged on the upstream side of the heat exchanger 61. Thereby, the membrane separation process is performed on the high-temperature low-concentration DS discharged from the separation tank 3. In this way, since the pressure of the pump required for the membrane separation process is reduced by performing the membrane separation process on the high-temperature DS having a low osmotic pressure, the present inventors reduce the energy required for the pump. It has been found that the energy efficiency of water treatment is improved.

The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 forward osmosis module, 10 forward osmosis membrane, 11 first chamber, 12 second chamber, 2 separation membrane module, 20 separation membrane, 3 separation tank, 41-48 pump, 5 tanks, 61, 62 heat exchanger.

Claims

While bringing one surface of the forward osmosis membrane into contact with the target solution containing water and components other than water,
By bringing the other surface of the forward osmosis membrane into contact with a draw solution containing a draw solute whose solubility decreases with increasing temperature,
Forward osmosis step of transferring water contained in the target solution to the draw solution through the forward osmosis membrane;
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;
Using a separation membrane, separating the low-concentration draw solution into water and a concentrate containing the draw solute, and a membrane separation step,
The water treatment method is characterized in that the temperature of the membrane separation step is higher than the temperature of the forward osmosis step.
The water treatment method according to claim 1, wherein the draw solute has a lower osmotic pressure as the temperature rises.
The water treatment method according to claim 1 or 2, wherein the temperature of the membrane separation step is 40 ° C or higher.
The water treatment method according to any one of claims 1 to 3, wherein the temperature of the forward osmosis step is less than 40 ° C.
The water treatment method according to any one of claims 1 to 4, wherein, in the phase separation step, the draw solution rises to a temperature of 40 ° C or higher.
A forward osmosis water treatment system used in the forward osmosis water treatment method according to any one of claims 1 to 5,
A forward osmosis membrane, a first chamber provided for bringing the target solution into contact with one surface of the forward osmosis membrane, and a contact between the draw solution and the other surface of the forward osmosis membrane A forward osmosis module including a second chamber;
A separation tank having a heating mechanism for separating the draw solution into the low-concentration draw solution and the high-concentration draw solution by increasing 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.