WO2007037583A1

WO2007037583A1 - Apparatus and method for preparing oxygen water

Info

Publication number: WO2007037583A1
Application number: PCT/KR2006/001774
Authority: WO
Inventors: Heoug-Yoon Kweon; Ki-Young Lee
Original assignee: Chemicore Inc.
Priority date: 2005-07-19
Filing date: 2006-05-12
Publication date: 2007-04-05
Also published as: JP2007021472A; WO2007037583A8; DE112006001896T5; KR20070010498A; CN101218181A; KR100678489B1

Abstract

Disclosed are an apparatus and a method for preparing oxygen water. The apparatus includes an oxygen generator for separating oxygen from compressed air received therein, and a membrane contactor for dissolving oxygen in water by receiving the oxygen generated from the oxygen generator. The apparatus and method are capable of simply preparing oxygen water with high efficiency and high concentration without causing contamination while maintaining highly concentrated oxygen for a long period of time.

Description

APPARATUS AND METHOD FOR PREPARING OXYGEN

WATER

Technical Field

[1] The present invention relates to an apparatus and a method for preparing oxygen- dissolved water(i.e. Oxygen water). More particularly, the present invention relates to an apparatus capable of dissolving oxygen in water with high efficiency and high concentration without causing contamination while maintaining highly concentrated oxygen for a long period of time, and a method for dissolving oxygen in water with high efficiency and high concentration. Background Art

[2] As generally known in the art, drinking water for people includes tap water, purified water, natural water, boiled water and distilled water. Recently, as people pay attention to their health, there are various attempts to provide clean and healthy water. Accordingly, people's preference for purified water or natural water other than tap water has gradually increased. However, purified water presents problems in that it may lose useful mineral components and the content of dissolved oxygen is significantly reduced when water has passed through a filter. In the case of natural water, although people prefer natural water to tap water, the amount of use of natural water has become reduced due to environmental pollution. In addition, since distilled water or boiled water does not contain dissolved oxygen owing to evaporation of oxygen, it cannot supply oxygen to a human body.

[3] Oxygen dissolved in water may increase the oxygen blood level, and such an increase of the oxygen blood level can promote metabolism, which is good for a person's health. In particular, oxygen deficiency derived from the malfunction of a respiratory organ, lack of a respiratory function in the skin or excessive use of oxygen may cause the malfunction of cells in the human body, leading to various diseases or even death in extreme case. Disclosure of Invention Technical Problem

[4] For this reason, various attempts have been performed in order to increase the content of oxygen dissolved in water, and various conventional methods have been proposed for the purpose of obtaining oxygen-dissolved water. For instance, there have been proposed a blowing scheme, which improves the dissolution rate of oxygen by enlarging a contact area between gas and liquid through distributing gas using micro- bubbles and a blower, a pressing scheme for increasing pressure, and a dispersion scheme adopting an operational principle of an ejector. Among the above schemes, the pressing scheme can improve the dissolution rate of oxygen in view of super- saturation, but it may apply high-pressure to people. In addition, the blowing scheme and the dispersing scheme may lose oxygen because oxygen moves upward together with bubbles before it has been dissolved in water, so that the dissolution efficiency for oxygen may be lowered. Thus, the blowing scheme and the dispersing scheme are inadequate for providing highly-concentrated and super-saturated oxygen water.

[5] In the case of an adsorption scheme, which dissolves oxygen in water using an adsorbent, particles may be generated from the adsorbent. Such particles, which are bad for one's health, may be dissolved in water so that it is necessary to provide various filters or a storage container in order to remove such particles, causing a complicated system, which is disadvantageous in view of economics. Technical Solution

[6] Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an apparatus capable of preparing oxygen water with high concentration and high quality.

[7] Another object of the present invention is to provide a method for preparing oxygen water by using the above apparatus. Advantageous Effects

[8] The apparatus and method are capable of simply preparing oxygen water with high efficiency and high concentration without causing contamination while maintaining highly concentrated oxygen for a long period of time.

Brief Description of the Drawings

[9] The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: [10] FIG. 1 is a schematic view illustrating a system of an apparatus for preparing oxygen water according to one embodiment of the present invention; [11] FIG. 2 is a view illustrating a separation membrane and an operational principle of the gas separation membrane according to one embodiment of the present invention; [12] FIG. 3 is a schematic view illustrating a structure for realizing an adsorption scheme according to one embodiment of the present invention; and [13] FIG. 4 is a schematic view illustrating an operational principle of a membrane contactor according to one embodiment of the present invention.

Best Mode for Carrying Out the Invention [14] In order to accomplish the above objects, according to one aspect of the present invention, there is provided an apparatus for preparing oxygen water, the apparatus comprising: an oxygen generator for separating oxygen from compressed air received therein; and a membrane contactor for dissolving oxygen in water by receiving the oxygen generated from the oxygen generator.

[15] According to the exemplary embodiment of the present invention, the oxygen generator and the membrane contactor are made from a porous film or a non-porous film.

[16] Preferably, the oxygen generator and the membrane contactor are made from a hollow fiber or a flat film.

[17] The oxygen generator includes an adsorbent or a membrane.

[18] According to the exemplary embodiment of the present invention, the apparatus further comprises a two-head compressor, wherein the two-head compressor includes a first head receiving air so as to feed the air to the oxygen generator, and a second head applying a vacuum to the oxygen generator so as to increase purity of oxygen generated from the oxygen generator and to feed the purified oxygen to the membrane contactor under pressure.

[19] According to the exemplary embodiment of the present invention, the apparatus further comprises a compressor, which receives air so as to feed the air to the oxygen generator, and a vacuum pump, which applies a vacuum to the oxygen generator so as to increase purity of oxygen generated from the oxygen generator and to feed the purified oxygen to the membrane contactor under pressure.

[20] Preferably, the apparatus further comprises a container for storing oxygen water obtained through the membrane contactor. A flow meter and/or a dissolved oxygen meter can be installed at one side of the container.

[21] According to the exemplary embodiment of the present invention, the oxygen generator and the membrane contactor are subject to an antibiotic sterilization process.

[22] An ultraviolet lamp or antibiotic sponge can be installed at a front end portion or a rear end portion of a source water port so as to sterilize source water.

[23] According to another aspect of the present invention, there is provided a method for preparing oxygen water, the method comprising the steps of: supplying compressed air to an oxygen generator; feeding oxygen, which is separated from the compressed air by means of the oxygen generator, to a membrane contactor; and supplying water to the membrane contactor such that the oxygen is dissolved in the water. Mode for the Invention

[24] Hereinafter, the present invention will be described in detail with reference to accompanying drawings.

[25] FIG. 1 is a schematic view illustrating a system of an apparatus for preparing oxygen water according to one embodiment of the present invention.

[26] The apparatus basically includes an oxygen generator 2 for generating oxygen by receiving air, and a membrane contactor 1 for dissolving highly concentrated oxygen supplied from the oxygen generator 2 by allowing the highly concentrated oxygen to make contact with water.

[27] Various conventional oxygen generators, which are generally known in the art, can be used as the oxygen generator 2 according to the present invention. For instance, an adsorption scheme or a membrane scheme can be employed in the present invention in order to generate oxygen. Since the adsorption scheme uses an adsorbent, particles can be generated from the adsorbent, so that a device for removing the particles of the adsorbent is required. However, according to the present invention, the particles of the adsorbent are filtered by means of the membrane contactor 1 used for dissolving oxygen in water, so that it is not necessary to separately provide a device for removing the particles of the adsorbent. Thus, the present invention can employ the adsorption scheme without limitations.

[28] According to a basic principle of the adsorption scheme, as shown in FIG. 3, as highly compressed air passes through an adsorption tower filled with adsorbents including molecular sieves, the adsorption tower primarily adsorbs a component having a higher selectivity, such as nitrogen, and allows oxygen having a lower selectivity to be discharged to an exterior through the adsorption tower. Air compressed by a compressor can be cooled if it is necessary to increase the adsorption force or to remove moisture from the air. In addition, cooled air is introduced into one of two adsorption towers, and then, the absorption tower adsorbs nitrogen molecules contained in the cooled air prior to oxygen molecules because of the difference of the adsorption properties between nitrogen and oxygen molecules. When the adsorbent provided in the adsorption tower is saturated with the nitrogen molecules, an electronic control unit detects it and operates a solenoid valve, so that compressed air is introduced into the other adsorption tower. At the same time, the adsorption tower saturated with nitrogen gas is subject to the purging process, in which nitrogen molecules are released from the adsorbent and discharged to the exterior through a nitrogen adjustment valve 10, so that the adsorption tower can be reused. Thus, it is possible to continuously produce oxygen by repeating the above procedure. The above procedure may repeat with an interval of several seconds so that the adsorbent repeatedly adsorbs and releases the nitrogen molecules.

[29] According to one embodiment of the present invention, the membrane scheme can be used in addition to the adsorption scheme so as to generate oxygen. A membrane employed in the membrane scheme includes a porous membrane or a non-porous membrane and is classified into a flat-film type membrane and a hollow fiber type membrane.

[30] According to a basic principle of the membrane scheme, oxygen is generated based on a difference of permeability between components of a gaseous mixture passing through the membrane. As shown in FIG. 2, compressed air introduced into the membrane consists of various gases including oxygen and nitrogen. Such a gaseous mixture makes contact with one surface of the membrane so that at least one gaseous component of the gaseous mixture may selectively pass through the membrane. The gaseous component, which selectively passes through the membrane, must has a speed higher than that of at least one gaseous component of the gaseous mixture that has not yet passed through the membrane. In addition, gaseous components that have not yet passed through the membrane must pass through the membrane with a speed lower than that of at least one gaseous component of the gaseous mixture. Based on this principle, the gaseous mixture may discretely flow along two flow paths, in which one flow path mainly has the gaseous components that have passed through the membrane and the other flow path mainly has gaseous components that have not passed through the membrane. Accordingly, in order to properly separate the gaseous mixture, the membrane must be made from a material having a high permselectivity with respect to specific gaseous components.

[31] In order to selectively separate and concentrate the gaseous mixture, the membrane preferably has an asymmetrical structure including a membrane layer having a dense surface and a porous supporter provided at a lower portion of the membrane. The permselectivity with respect to the specific gaseous components of the gaseous mixture may vary depending on the quality of the dense surface of the membrane layer, and the flow rate of the gaseous components separated from the gaseous mixture may vary depending on a thickness of the membrane layer and porosity of the porous supporter, which forms a lower portion of the asymmetrical structure. In order to selectively separate the gaseous mixture with a high efficiency, defects must not be formed on the surface of the membrane layer and pores preferably have sizes of 5 A or less. In addition, in order to obtain higher gas permeability, the membrane layer must be fabricated with a thin thickness, if possible. This is because the gas permeability is inversely proportional to an effective film thickness. Furthermore, it is preferable for the lower portion of the asymmetrical structure to have a porous structure in order to minimize resistance against the flow of gas components that have selectively passed through the membrane layer. For instance, the lower portion of the asymmetrical structure can be fabricated using a hollow fiber or a flat- film.

[32] Oxygen generated by means of the oxygen generator 2, such as the membrane or the adsorbent, moves toward the membrane contactor, in which oxygen makes contact with water so that oxygen is dissolved in water. The structure of the membrane contactor 1 according to the present invention allows liquid to flow into or out of a tube by passing through the membrane contactor 1. At this time, concentrated oxygen is fed into the tube from a position opposite to the liquid so that the concentrated oxygen is dissolved in the liquid. Oxygen generated by the oxygen generator 2 is introduced through the membrane contactor 1 without forming bubbles. That is, oxygen that has passed through the membrane contactor 1 is dissolved in the liquid, such as water, due to diffusion of oxygen. The present invention simply increases the content of dissolved oxygen by dissolving oxygen in the form of a molecule in water through the membrane contactor 1 and maintains dissolved oxygen for a long period of time. In particular, the present invention does not require a conventional spray unit. In addition, it is not necessary to increase pressure in a container or to drop the temperature of the container.

[33] When taking a contact area and a fluid speed into consideration, it is preferable for liquid to flow into the tube through the membrane contactor 1, rather than to flow out of the tube through the membrane contactor 1. In addition, if the length of the membrane contactor 1 is too long, the efficiency of the membrane contactor 1 may be lowered, so it is preferable for the membrane contactor 1 to have a proper length. Preferably, the membrane contactor 1 has a corrugated structure so as to enlarge a surface area making contact with the liquid. In addition, the membrane contactor 1 has either a porous structure or a non-porous structure. Preferably, the membrane contactor 1 has the non-porous structure in order to properly induce diffusion of gas. The membrane contactor 1 can be fabricated using a hollow fiber or a flat film. In addition, the membrane contactor 1 is made of a material, which includes one selected from the group consisting of polysulfone, polysulfone composites, polypropylene, polyethylene, polyethersulfone, polyimide, Teflon, silicon, polytetrafluoroethylene, and a mixture thereof.

[34] According to one embodiment of the present invention, liquid, such as water, is introduced into one end portion of the membrane contactor 1. At this time, pressure of the liquid is measured by means of a pressure measurement unit 6 installed in a pipe. If the pressure of the liquid measured by the pressure measurement unit 6 is too high, the liquid is discharged through a bypass valve 5, so that the liquid having constant pressure is always introduced into the membrane contactor 1.

[35] According to one embodiment of the present invention, a compressor and a vacuum pump can be separately provided in order to feed compressed air into the oxygen generator 2. In this case, the compressor supplies air to the oxygen generator 2 by compressing the air, and the vacuum pump applies a vacuum to oxygen generated from the oxygen generator 2, so that highly purified oxygen is fed into the membrane contactor 1. [36] It is more preferred if a two-head compressor 3 is provided instead of the compressor and the vacuum pump. In this case, the two-head compressor 3 includes a first head, which compresses air received therein so as to supply compressed air, and a second head applying a vacuum to oxygen generated from the oxygen generator so as to supply highly purified oxygen to the membrane contactor under pressure. Since the two-head compressor 3 can simultaneously generate the pressure and vacuum, compressed air and/or concentrated oxygen can be simultaneously supplied to the oxygen generator 2 and the membrane contactor 1 while maximally inducing differential pressure using only one compressor under the low-pressure operation, so that highly concentrated oxygen can be generated from the oxygen generator 2. In addition, since the highly concentrated oxygen passes through the membrane contactor 1 under pressure, it is possible to generate oxygen having purity 10% higher than that of concentrated oxygen generated from the oxygen generator 2. Such highly purified oxygen in the form of a mono-molecule is dissolved in water. In addition, since one compressor is used only, power consumption and noise can be reduced during operation.

[37] According to one embodiment of the present invention, a pressure meter 8 is installed at an output port of the first head of the two-head compressor 3. The pressure meter 8 measures pressure of compressed air supplied into the oxygen generator 2 in such a manner that the compressed air can be maintained with constant pressure. An additional pressure meter 7 is installed at an output port of the second head of the two- head compressor 3. The pressure meter 7 measures pressure of concentrated oxygen generated from the oxygen generator 2 in such a manner that the concentrated oxygen can be maintained with constant pressure.

[38] According to one embodiment of the present invention, the apparatus for preparing oxygen water further includes a container 4 for storing oxygen water discharged through the membrane contactor 1. The container 4 includes a flow meter 11 for measuring the flow rate of the oxygen water and/or a measurement unit 12 for measuring the content of oxygen dissolved in water, so that the container 13 may control the operation of an oxygen water generation system. That is, if oxygen water is excessively supplied, the flow meter 11 detects it and stops the operation of the oxygen water generation system. If oxygen water is needed again, the flow meter 11 operates the oxygen water generation system. In particular, since the content of dissolved oxygen can be measured by means of the measurement unit 12, the oxygen water generation system can be properly controlled by means of an automatic control unit such that the content of oxygen may vary depending on applications.

[39] A typical oxygen sensor can be used as the measurement unit 12 without any limitations. For instance, a galvanic type oxygen sensor is used as the measurement unit 12. The galvanic type oxygen sensor is an electrochemical oxygen sensor, which is a fuel cell type sensor where a reduction reaction occurs at an electrode made from a precious metal and a reduction current is generated according to the concentration degree of oxygen making contact with the electrode. The galvanic type oxygen sensor is a representative oxygen sensor, which can be used for analyzing low-concentration oxygen of PPM unit in a normal temperature range.

[40] The above container 4 may be usefully applied to a water purifier. That is, after storing oxygen water in the container 4, the oxygen water stored in the container 4 is heated or cooled through a predetermined scheme, so that the oxygen water can be conveniently supplied to a user. Since the oxygen water is generally used for people, it is preferred to purify source water. That is, it is preferred to purify the source water before it makes contact with the membrane contactor 1 of the oxygen generator. It is not preferable if such purification is performed with respect to the oxygen water, rather than source water, because dissolved oxygen may be removed when the oxygen water passes through various filters.

[41] According to one embodiment of the present invention, an antibiotic treatment is preferably performed with respect to the apparatus for preparing oxygen water so as to prevent bacteria from penetrating into the oxygen water. That is, the oxygen generator 2, the membrane contactor 1, a housing, the container 4, and the pipe are treated with silver-nano particles or an optical catalyst, such as TiO , WO , ZnO, SiC, or CdS, thereby imparting antibiotic characteristics thereto. In addition, an ultraviolet sterilization process can be performed with respect to source water while the source water is being supplied, or the source water can be antibiotic-treated by allowing the source water to pass through antibiotic sponge. It is also possible to directly radiate an ultraviolet ray onto the container so as provide antibiotic treatment to the container. Such an antibiotic treatment may improve stability and safety of the oxygen water.

[42] Hereinafter, the procedure for preparing the oxygen water according to the present invention will be described in detail.

[43] First, air compressed through the first head of the two-head compressor 3 is fed into the adsorption tower or the membrane of the oxygen generator 2, thereby generating highly concentrated oxygen. Gaseous components except for oxygen, such as nitrogen, etc., are discharged to the exterior through the adjustment valve 10. Then, a vacuum is applied to the highly concentrated oxygen through the second head of the two-head compressor 3, so that the highly concentrated oxygen, which is separated from the compressed air by means of the oxygen generator 2, becomes highly purified oxygen. The highly purified oxygen is introduced into one end portion of the membrane contactor 1 under pressure. Then, the highly purified oxygen is diffused toward the other end portion of the membrane contactor 1 so that the highly purified oxygen is dissolved in water by making contact with the water, which is introduced into the other end portion of the membrane contactor 1. Thus, oxygen water, in which oxygen is dissolved with high concentration, is introduced into the container 4. At this time, the flow meter 11 attached to one side of the container 4 may properly control the circulation of the oxygen water. In addition, the measurement unit 12 attached to one side of the container 4 may properly control the content of oxygen dissolved in water.

[44] The oxygen water prepared through the above procedure is applied to a water purifier, so that people can drink the oxygen water. In addition, the oxygen water can be used as a source material for alcoholic drinks, ice creams, or beverages. In particular, since high-quality oxygen is dissolved in water with high concentration, oxygen can be easily circulated through the human body, which is very useful for one's health.

[45] Hereinafter, an embodiment of the present invention will be described in detail. However, the present invention is not limited to the embodiment described below.

[46] [47] Embodiment [48] The apparatus for preparing oxygen water has been fabricated as shown in FIG. 1. Then, compressed air of lkg/cm is supplied to the oxygen generator 2 (non-porous polysulfone composite membrane having a diameter 48mm, a length 182mm, and an effective film area 0.58m ) through the first head of the two-head compressor 3. In addition, the vacuum of about 400mmHg is applied to the concentrated oxygen, which is generated from the oxygen generator 2, through the second head of the two-head compressor 3, thereby creating highly purified oxygen having purity of about 42%. The highly purified oxygen is introduced into the membrane contactor 1 (non-porous polysulfone composite membrane having a diameter 70mm, a length 232mm, and an effective film area 1.58m ) under pressure. At this time, if hydraulic pressure applied to the membrane contactor 1 is 1.5kg/cm or more, the flow rate of the oxygen is adjusted by using the bypass valve.

[49] When the test temperature is 25°C, and the pressure of the concentrated oxygen is lkg/cm , the content of dissolve oxygen is measured as 34mg/L by using a dissolved oxygen meter available from Maxtec company. Table 1 shows variation of dissolved oxygen according to the pressure at the temperature of 25°C, and Table 2 shows variation dissolved oxygen according to time.

[50] Table 1

[51] Table 2

[52] As can be understood from Tables 1 and 2, the apparatus for preparing oxygen water according to the present invention dissolves oxygen in water in a molecule unit by using the membrane contactor so that the content of dissolved oxygen can be increased while maintaining the dissolved oxygen for a long period of time. Thus, it is possible to generate the oxygen water with high efficiency. In addition, although the conventional apparatus must apply a pressure of about 2kg/cm or more in order to generate the oxygen, the present invention can generate the oxygen by simultaneously applying a pressure of about lkg/cm and a vacuum using the two-head compressor, so that the differential pressure can be maximally induced by using only one compressor under the low-pressure operation. Thus, highly concentrated oxygen can be generated and introduced into the membrane contactor, thereby reducing the power consumption and noise generated during operation.

[53] In particular, tap water generally contains insufficient dissolved oxygen of 6 to

10mg/L under the normal temperature, and the convention method requires the spray unit and is necessary to increase pressure in the container and to drop the temperature of the container in order to increase the content of the dissolved oxygen. However, according to the present invention, it is possible to increase the content of the dissolved oxygen to a level of about 14 to 41 mg/L or more, without using a separate apparatus for generating dissolved oxygen. As a result, if the apparatus for preparing oxygen water according to the present invention is applied to facilities, such as a water purifier, the content of the dissolved oxygen can be increased and the dissolved oxygen can be maintained for a long period of time.

[54] In addition, the content of the dissolved oxygen can be further increased if the water temperature drops, and it is possible to provide an oxygen water generator, which can be conveniently used for the user, by enlarging the separation membrane and the membrane contactor according to the capacity as required by the user.

[55] Furthermore, according to the characteristic of the separation membrane, if the pressure of the compressed air is increased to a level of lkg/cm or more, or if the vacuum degree is increased, or if the oxygen purity is increased by PSA(pressure swing adsorption) or oxygen bomb, concentration of dissolved oxygen in water may also be increased. Thus, the content of the dissolved oxygen may be increased to a level of 70mg/L or more, so that people can drink water having highly concentrated oxygen. In addition, bacteria can be propagated in the conventional oxygen water purifier because some of water stays in the conventional oxygen water purifier. However, the present invention can solve the above problem, because the apparatus is equipped with the sterilization function.

[56] As described above, the apparatus for preparing oxygen water according to the present invention employs the membrane contactor so that particles of the adsorbent cannot be generated and the content of the dissolved oxygen can be increased while maintaining the dissolved oxygen for a long period of time. In addition, the present invention employs the two-head compressor, so that it is possible to generate oxygen water with high efficiency while reducing the power consumption and noise. In addition, the oxygen water obtained through the present invention can be applied to a water purifier, so that people can drink the oxygen water. Furthermore, the oxygen water can be used as a source material for alcoholic drinks, ice creams, or beverages.

[57] Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] An apparatus for preparing oxygen water, the apparatus comprising: an oxygen generator for separating oxygen from compressed air received therein; and a membrane contactor for dissolving oxygen in water by receiving the oxygen generated from the oxygen generator. [2] The apparatus as claimed in claim 1, wherein the oxygen generator and the membrane contactor are made from a porous film or a non-porous film. [3] The apparatus as claimed in claim 1, wherein the oxygen generator and the membrane contactor are made from a hollow fiber or a flat film. [4] The apparatus as claimed in claim 1, wherein the oxygen generator includes an adsorbent or a membrane. [5] The apparatus as claimed in claim 1, further comprising a two-head compressor, wherein the two-head compressor includes a first head receiving air so as to feed the air to the oxygen generator, and a second head applying a vacuum to the oxygen generator so as to increase purity of oxygen generated from the oxygen generator and to feed the purified oxygen to the membrane contactor under pressure. [6] The apparatus as claimed in claim 1, further comprising a compressor, which receives air so as to feed the air to the oxygen generator, and a vacuum pump, which applies a vacuum to the oxygen generator so as to increase purity of oxygen generated from the oxygen generator and to feed the purified oxygen to the membrane contactor under pressure. [7] The apparatus as claimed in claim 1, further comprising a container for storing oxygen water obtained through the membrane contactor. [8] The apparatus as claimed in claim 7, further comprising a flow meter and/or a dissolved oxygen meter installed at one side of the container. [9] A method for preparing oxygen water using an apparatus as claimed in any one of claims 1 to 8, the method comprising the steps of: supplying compressed air to an oxygen generator; feeding oxygen, which is separated from the compressed air by means of the oxygen generator, to a membrane contactor; and supplying water to the membrane contactor such that the oxygen is dissolved in the water.