Concentration method and concentrator for water to be treated

Abstract

To provide a method for concentrating water to be treated and a concentrator capable of efficiently concentrating small-sized substances contained in water to be treated.SOLUTION: A concentrator 1 for concentrating water to be treated is equipped with a semipermeable membrane unit 3 that separates water to be treated having substances passing through a semipermeable membrane 3a into permeate water and non-permeate water by the semipermeable membrane 3a, and discharges the permeate water as concentrated water, and the concentrator 1 is provided with a polymer adding part 20 that adds a thermosensitive polymer to the water to be treated supplied to the semipermeable membrane unit 3.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for concentrating water to be treated. [Background technology]

[0002] A conventional method for concentrating water to be treated is known in which a draw solution consisting of an aqueous solution of a thermosensitive polymer is brought into contact with the water to be treated via a semipermeable membrane to concentrate the water to be treated (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-170954 A Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned conventional concentration methods concentrate the treated water by forward osmosis, taking advantage of the osmotic pressure difference between the treated water and the draw solution. Therefore, it is difficult to apply these methods to the concentration of low-molecular-weight substances that can pass through commercially available semipermeable membranes, such as lower alcohols.

[0005] SUMMARY OF THE PRESENT EMBODIMENTS Accordingly, an object of the present invention is to provide a method and apparatus for concentrating water to be treated, which can efficiently concentrate small-sized substances contained in the water to be treated. [Means for solving the problem]

[0006] The above-mentioned object of the present invention is achieved by a method for concentrating treated water, in which water to be treated containing a substance that passes through a semipermeable membrane is supplied to a semipermeable membrane unit equipped with the semipermeable membrane to separate the water to be treated into permeated water and non-permeated water, and the permeated water is discharged as concentrated water, the method comprising the steps of: a polymer addition step of adding a thermosensitive polymer that changes from hydrophilic to hydrophobic at a transition temperature to the water to be treated; a supply step of supplying the water to which the thermosensitive polymer has been added, to the semipermeable membrane unit while the thermosensitive polymer is in a hydrophilic state; and a recovery step of recovering the non-permeated water containing the thermosensitive polymer.

[0007] It is preferable that this method of concentrating the treated water further includes a polymer separation step in which the thermosensitive polymer contained in the non-permeated water recovered in the recovery step is rendered hydrophobic by a temperature change and separated from the non-permeated water, and the polymer addition step can add the thermosensitive polymer separated in the polymer separation step to the treated water.

[0008] The thermosensitive polymer used can have a lower critical solution temperature, and the supplying step can supply the water to be treated to which the thermosensitive polymer has been added to the semipermeable membrane unit at a temperature lower than the lower critical solution temperature of the thermosensitive polymer.

[0009] Furthermore, the above-mentioned object of the present invention can be achieved by an apparatus for concentrating treated water, which comprises a semipermeable membrane unit that separates treated water containing substances that pass through a semipermeable membrane into permeated water and non-permeated water by the semipermeable membrane, and discharges the permeated water as concentrated water, and which comprises a polymer addition section that adds a temperature-sensitive polymer to the treated water supplied to the semipermeable membrane unit. Effect of the Invention

[0010] According to the present invention, it is possible to provide a method and an apparatus for concentrating water to be treated, which are capable of efficiently concentrating small-sized substances contained in the water to be treated. [Brief description of the drawings]

[0011] [Figure 1]1 is a schematic diagram of a concentrating device for treated water according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a schematic diagram of a concentrating apparatus for water to be treated (hereinafter, simply referred to as "concentrating apparatus") according to an embodiment of the present invention. As shown in Fig. 1, the concentrating apparatus 1 includes a semipermeable membrane unit 3 and a supply line 30 that supplies the water to be treated to the semipermeable membrane unit 3.

[0013] The semipermeable membrane unit 3 is equipped with a semipermeable membrane 3a, and separates the supplied water to be treated into permeated water and non-permeated water by reverse osmosis. The pore size of the semipermeable membrane 3a is set so that the substances to be concentrated (molecules, ions, etc.) pass through together with the permeated water. The type of the semipermeable membrane 3a is not necessarily limited, but preferred examples include a nanofiltration membrane (NF membrane) and a reverse osmosis membrane (RO membrane).

[0014] The supply line 30 includes a polymer addition section 20 and a high-pressure pump 2, and the water to be treated that is supplied to the semipermeable membrane unit 3 is added with a thermosensitive polymer in the polymer addition section 20. The thermosensitive polymer is a polymer that changes from hydrophilic to hydrophobic at a transition temperature. The thermosensitive polymer used in this embodiment is a thermosensitive polymer having a lower critical solution temperature (LCST), which is hydrophilic at a temperature lower than the lower critical solution temperature and bonds with the water molecules to be treated, while it becomes hydrophobic at a temperature higher than the lower critical solution temperature and breaks the bond with the water molecules. The thermosensitive polymer may be of a size that does not pass through the pores of the semipermeable membrane 3a, and for example, a thermosensitive polymer used in the draw solution of a conventional forward osmosis water treatment method can be used. When the molecular weight of the thermosensitive polymer is large, the semipermeable membrane 3a may be a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), or the like. By using a microfiltration membrane or an ultrafiltration membrane as the semipermeable membrane 3a, the flux (membrane permeation flux) increases and the operating pressure decreases, thereby enabling energy savings.

[0015] The concentrating apparatus 1 further includes a heating device 4 for heating the non-permeated water recovered from the semipermeable membrane unit 3, a polymer separation tank 5 for separating the thermosensitive polymer from the heated non-permeated water, and a cooling device 7 for cooling the separated thermosensitive polymer.

[0016] The heating device 4 heats the non-permeated water to a temperature higher than the lower critical solution temperature by a heating medium such as hot water generated by utilizing waste heat from exhaust gas, etc. As a result, the thermosensitive polymer contained in the non-permeated water becomes hydrophobic and insoluble.

[0017] The polymer separation tank 5 separates and recovers the insolubilized thermosensitive polymer from the permeate. The recovered thermosensitive polymer is supplied to the polymer addition section 20 via the cooling device 7 by the operation of the recovery pump 6, and is added to the water to be treated in the polymer addition section 20.

[0018] Meanwhile, the separated water, in which the thermosensitive polymer has been separated from the non-permeated water in the polymer separation tank 5, is supplied to a low-pressure RO membrane unit 10 equipped with a low-pressure RO membrane 10a by the operation of a treatment pump 9. Since a small amount of thermosensitive polymer that was not separated in the polymer separation tank 5 may remain in the separated water, the recovery rate of the thermosensitive polymer in the polymer separation tank 5 can be increased by returning the separated water that does not pass through the low-pressure RO membrane 10a to the polymer separation tank 5. The separated water that has passed through the low-pressure RO membrane 10a is discharged as dilution water in which the concentrations of substances contained in the water to be treated are diluted, and can be used, for example, as washing water.

[0019] The cooling device 7 cools the thermosensitive polymer recovered in the polymer separation tank 5 with a cooling medium such as cooling water. This makes it possible to easily maintain the temperature of the water to be treated to which the thermosensitive polymer has been added in the polymer adding section 20 at a temperature lower than the lower critical solution temperature.

[0020] The concentrating apparatus 1 of the present embodiment includes a heat exchanger 8 that exchanges heat between the non-permeated water before being heated by the heating device 4 and the thermosensitive polymer before being cooled by the cooling device 7, thereby achieving energy saving. However, the concentrating apparatus 1 may be configured without including the heat exchanger 8.

[0021] Next, a method for concentrating the water to be treated using the concentrating device 1 having the above-mentioned configuration will be described. First, a polymer addition step is performed in which a temperature-sensitive polymer is added to the water to be treated, which is supplied to the supply line 30, in the polymer addition section 20. The water to be treated is preferably an aqueous solution of a lower alcohol such as methanol, ethanol, or propanol, but is not particularly limited as long as the substance to be concentrated passes through the semipermeable membrane 3a. For example, the water to be treated may be water in which a water-soluble gas such as carbon dioxide, hydrogen, ammonia, or hydrogen chloride is dissolved in water. When the water to be treated is an acidic solution such as a dilute hydrochloric acid solution, it is preferable that the temperature-sensitive polymer has acid resistance.

[0022] The thermosensitive polymer to be added to the water to be treated is a thermosensitive polymer having a lower critical solution temperature. The concentration of the thermosensitive polymer in the water to be treated is not particularly limited, and can be appropriately set depending on the type and concentration of the substance to be concentrated in the water to be treated.

[0023] Next, a supply step is performed in which the water to be treated to which the thermosensitive polymer has been added is supplied to the semipermeable membrane unit 3 at a temperature lower than the lower critical solution temperature of the thermosensitive polymer. Because the thermosensitive polymer becomes hydrophilic at a temperature lower than the lower critical solution temperature, the water molecules in the water to be treated bind to the thermosensitive polymer to become bound water, and the proportion of free water decreases.

[0024] Next, a recovery process is performed to recover the non-permeated water containing the thermosensitive polymer. Since the thermosensitive polymer is recovered together with the bound water, the amount of free water passing through the semipermeable membrane 3a is reduced, and the permeated water that has passed through the semipermeable membrane 3a is discharged to the outside as concentrated water in which the target substance has been concentrated. In this way, small-sized substances that are difficult to concentrate using conventional membrane treatments can be efficiently concentrated.

[0025] Thereafter, the non-permeated water recovered in the recovery step is heated in the heat exchanger 8 and the heating device 4 to a temperature higher than the lower critical solution temperature, thereby changing the thermosensitive polymer into a hydrophobic state, and then a polymer separation step is performed in which the thermosensitive polymer contained in the non-permeated water is separated in the polymer separation tank 5. The separated thermosensitive polymer is cooled in the heat exchanger 8 and the cooling device 7 for the cooling step, and then supplied to the polymer addition section 20 for use in the above-mentioned polymer addition step. In this way, the thermosensitive polymer can be repeatedly used for concentrating the water to be treated.

[0026] In addition, if the temperature of the treated water to which the thermosensitive polymer has been added can be maintained lower than the lower critical solution temperature by naturally cooling the thermosensitive polymer separated in the polymer separation process, the concentrating apparatus 1 may be configured not to include a cooling device 7.

[0027] The method for concentrating the water to be treated in this embodiment uses a thermosensitive polymer having a lower critical solution temperature, but a polymeric substance having an upper critical solution temperature (UCST) that becomes hydrophilic at a temperature higher than the transition temperature and becomes hydrophobic at a temperature lower than the transition temperature may be used. In this case, the above-mentioned supplying step can concentrate the water to be treated by supplying the water to which the thermosensitive polymer has been added to the semipermeable membrane unit 3 at a temperature higher than the upper critical solution temperature at which the thermosensitive polymer becomes hydrophilic. In this case, the concentrating device 1 shown in FIG. 1 is configured by switching the heating device 4 and the cooling device 7, so that the above-mentioned recovery step can make the thermosensitive polymer contained in the recovered non-permeated water hydrophobic by cooling in the cooling device and separate it from the non-permeated water, and the above-mentioned polymer adding step can add the separated thermosensitive polymer to the water to be treated after heating it in the heating device. [Explanation of symbols]

[0028] 1 Concentrator 3. Semipermeable membrane unit 3a Semi-permeable membrane 4 Heating device 5 Polymer separation tank 7 Cooling device 20 Polymer Addition Section

Claims

1. A method for concentrating water to be treated, comprising: supplying water to be treated, which contains a substance that passes through a semipermeable membrane, to a semipermeable membrane unit having the semipermeable membrane, separating the water into permeated water and non-permeated water, and discharging the permeated water as concentrated water, a polymer addition step of adding a thermosensitive polymer that changes from hydrophilic to hydrophobic at a transition temperature to the water to be treated; A supplying step of supplying the water to be treated to which the thermosensitive polymer has been added to the semipermeable membrane unit while the thermosensitive polymer is in a hydrophilic state; and recovering non-permeated water containing the temperature-sensitive polymer.

2. The method further includes a polymer separation step of making the thermosensitive polymer contained in the non-permeated water recovered in the recovery step hydrophobic by changing the temperature, and separating the thermosensitive polymer from the non-permeated water, 2. The method for concentrating water to be treated according to claim 1, wherein the polymer adding step adds the thermosensitive polymer separated in the polymer separating step to the water to be treated.

3. The temperature-sensitive polymer has a lower critical solution temperature, 3. The method for concentrating water to be treated according to claim 1, wherein the supplying step supplies the water to be treated, to which a thermosensitive polymer has been added, to the semipermeable membrane unit at a temperature lower than a lower critical solution temperature of the thermosensitive polymer.

4. 2. The method for concentrating water to be treated according to claim 1, wherein lower alcohol contained in the water to be treated is concentrated.

5. A water concentrating device for treated water, comprising a semipermeable membrane unit that separates water to be treated, which contains a substance that passes through a semipermeable membrane, into permeated water and non-permeated water by the semipermeable membrane, and discharges the permeated water as concentrated water, The apparatus for concentrating water to be treated includes a polymer adding section that adds a temperature-sensitive polymer to the water to be treated that is supplied to the semipermeable membrane unit.

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