JP7125850B2 - Gas-dissolved water supply system and gas-dissolved water supply method - Google Patents

Description

The present invention relates to a gas-dissolved water supply system and a gas-dissolved water supply method for supplying gas-dissolved water in which gas is dissolved in water to be treated.

2. Description of the Related Art In semiconductor and liquid crystal manufacturing processes, semiconductor wafers and glass substrates are cleaned using pure water or ultrapure water from which impurities have been highly removed. In particular, in order to suppress the generation of static electricity during the cleaning of semiconductor wafers, gas (carbon dioxide gas, ozone, hydrogen, etc.) is dissolved in the water to be treated (pure water or ultrapure water) to adjust the resistivity value to be low. Dissolved water (functional water) is used as washing water and rinsing water. In general, gas-dissolved water is supplied to the point of use after fine particles are removed by a filtration membrane device (filter). A microfiltration membrane device and an ultrafiltration membrane device are used as the filtration membrane device, and it is preferable to use the ultrafiltration membrane device in order to remove finer particles. Patent Literature 1 discloses a cleaning apparatus using carbon dioxide dissolved water, which includes a carbon dioxide dissolution membrane module and a fine particle removal membrane module (see FIG. 1 of Patent Literature 1). Patent Document 1 describes that it is preferable to use a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) as the particle removal membrane module (see paragraph [0038] of Patent Document 1). . Such a cleaning device can supply highly clean gas-dissolved water to a point of use.

JP 2017-175075 A

When using an ultrafiltration membrane device to remove fine particles contained in gas-dissolved water, there is a problem that the permeation flux of gas-dissolved water passing through the ultrafiltration membrane device gradually decreases. can occur.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a system for supplying water with dissolved gas and a method for supplying water with dissolved gas that can stably supply highly clean water with dissolved gas.

In order to achieve the above object, the gas-dissolved water supply system of the present invention supplies water to be treated in which gas is not dissolved and gas-dissolved water in which gas is dissolved in water to be treated. a heat exchanger for adjusting the temperature of the water to be treated in which the gas supplied from the supply path is not dissolved; a first ultrafiltration membrane device for removing fine particles in the water to be treated in which the gas in which the gas is adjusted is not dissolved ; A heat exchanger that is included in a gas-dissolved water supply path connected to a branch path and adjusts the temperature of the water to be treated that is supplied from the branch path, and a gas dissolver that dissolves gas in the water to be treated a second ultrafiltration membrane device connected to the gas dissolving device, the temperature of which is adjusted by the heat exchanger, and which removes fine particles in the water to be treated in which the gas is dissolved by the gas dissolving device; Of the water to be treated in which the gas is not dissolved and which has been treated by the first ultrafiltration membrane device, the water to be treated that is not used at the point of use is returned to a position on the upstream side of the connection to the branch path. and the water to be treated, which is not used at the point of use, out of the water to be treated in which the gas is dissolved, which has been treated by the second ultrafiltration membrane device, through the heat exchange of the path for supplying the gas-dissolved water. and another circulation path returning to a position upstream of the gas dissolving device, and at least the second ultrafiltration membrane device has a dense layer on at least the primary side.

According to the gas-dissolved water supply system and the gas-dissolved water supply method of the present invention, highly clean gas-dissolved water can be stably supplied.

1 is a block diagram schematically showing a gas-dissolved water supply system according to a first embodiment of the present invention; FIG. 2 is a cross-sectional view schematically showing an ultrafiltration membrane device of the gas-dissolved water supply system shown in FIG. 1. FIG. Fig. 3 is a cross-sectional view schematically showing a modification of the ultrafiltration membrane device shown in Fig. 2; FIG. 4 is a block diagram schematically showing a gas-dissolved water supply system according to a second embodiment of the present invention; 4 is a graph showing initial differential pressure maintenance rates of gas-dissolved water supply systems of first and second examples of the present invention and comparative examples. 5 is a graph showing the initial flux retention rate of the gas-dissolved water supply systems of the first and second examples of the present invention and a comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
The configuration of the gas-dissolved water supply system according to the first embodiment of the present invention will be described. FIG. 1 is a block diagram schematically showing the gas-dissolved water supply system of this embodiment. The gas-dissolved water supply system 1 of the present embodiment supplies gas-dissolved water (functional water) obtained by dissolving gas in water to be treated (pure water or ultrapure water) for cleaning semiconductor wafers, for example. is. This gas-dissolved water supply system 1 includes a storage tank 2 for storing primary pure water as raw material water, a heat exchanger 6, a degassing device 7, an ultraviolet irradiation device 8, an ion exchange device 3, and a gas dissolving device. 4 and an ultrafiltration membrane device 5 are arranged in this order and connected in series. The heat exchanger 6 is mainly a device that adjusts the temperature of the raw material water in advance so that the gas-dissolved water reaches a suitable temperature when used for desired processing (for example, cleaning of semiconductor wafers). The degassing device 7 is, for example, a device that removes gas (especially oxygen) in the raw water to prevent oxidation. A vacuum deaerator, a membrane deaerator, or the like is employed as the deaerator 7, which reduces dissolved oxygen (DO) to, for example, less than 50 ppb. The ultraviolet irradiation device 8 is a device that performs sterilization using ultraviolet rays.

In this way, the temperature of the raw water is adjusted, the dissolved oxygen content is adjusted, and sterilization is performed, and the ion exchange device 3 removes ionic impurities. The ion exchange device 3 removes ionic impurities contained in the raw water by ion exchange treatment, and has an ion exchange tower filled with an ion exchanger. The ion exchange device 3 is preferably a non-regenerative ion exchange device (cartridge polisher: CP) in order to maintain high processing performance. Specifically, it has an ion exchange tower filled with an anion exchange resin for removing anionic (anionic) impurities and a cation exchange resin for removing cationic (cationic) impurities. The packed form of the anion-exchange resin and the cation-exchange resin in the ion-exchange column is preferably a mixed-bed form in that it is possible to efficiently remove minute amounts of impurities derived from the ion-exchange resin and to increase the degree of cleanliness. preferable. That is, the ion exchange device 3 of the preferred embodiment is a non-regenerative mixed bed ion exchange device filled with a mixed resin of a cation (cation) exchange resin and an anion (anion) exchange resin.

The gas dissolving device 4 is a device for dissolving gas in the pure water or ultrapure water supplied from the ion exchange device 3 . By dissolving gas in pure water or ultrapure water, the specific resistance value is adjusted to a desired range, thereby suppressing the generation of static electricity. As the gas dissolving device 4, for example, a device that dissolves gas using a hollow fiber gas permeable membrane, a device that directly causes gas to be bubbled into a pipe, and a device that dissolves gas by a dispersing means such as a static mixer after injecting gas. , a device that supplies the gas to the upstream side of a pump that supplies pure water or ultrapure water to the gas dissolving tank and stirs the gas in the pump to dissolve the gas. In particular, an apparatus for dissolving gas using a gas-permeable membrane made of hollow fibers is preferable because it has a large membrane area (gas-liquid contact area) and can efficiently dissolve gas. Gas can be supplied to the gas dissolver 4 through a gas line (not shown). Gases to be dissolved include carbon dioxide gas, nitrogen gas, hydrogen gas, ozone gas, and the like.

The ultrafiltration membrane device 5 removes fine particles from gas-dissolved water (functional water) in which gas is dissolved in pure water or ultrapure water. The ultrafiltration membrane device 5 can remove very small fine particles having a diameter of about 0.001 μm to 0.01 μm, and enables the supply of highly clean gas-dissolved water to the point of use 12 . The ultrafiltration membrane (UF membrane) used in the ultrafiltration membrane device 5 is composed of a porous sponge layer and a dense layer (skin layer) having a pore size smaller than that of the sponge layer. have a so-called single-skin structure having a dense layer on the inner or outer surface of the membrane, and a so-called double-skin structure having dense layers on both the inner and outer surfaces of the membrane. The ultrafiltration membrane device 5 of this embodiment has a dense layer at least on the primary side (inflow side). For example, as schematically shown in FIG. 2, it is composed of a so-called single-skin ultrafiltration membrane having a dense layer (skin layer) 5a on the primary side (inflow side). The dense layer 5a is a layer provided with finer pores than the portion (sponge layer) 5b of the ultrafiltration membrane device 5 other than the dense layer 5a. Further, as shown in FIG. 3, it is possible to adopt a structure (double skin structure) in which dense layers 5a are provided on the primary side and the secondary side of the ultrafiltration membrane device 5, respectively. The present invention can be applied to both a configuration having an internal pressure type ultrafiltration membrane device and a configuration having an external pressure type ultrafiltration membrane device. The gas-dissolved water treated by the ultrafiltration membrane device 5 is selectively supplied from the supply unit 10 to all or some of the plurality of points of use 12 . At each point of use 12, the gas-dissolved water is used, for example, as cleaning water or rinsing water for cleaning semiconductor wafers (not shown). Also, the gas-dissolved water that is not used at the point of use is returned to the storage tank 2 through the circulation path 9 .

With such a configuration, it is possible to circulate the gas-dissolved water having a high degree of cleanliness and a desired quality along the circulation path 9, and to supply the required amount of the gas-dissolved water to the point of use 12 when necessary. can be done. That is, it is possible to stably supply gas-dissolved water with high cleanliness and desired quality to the point-of-use 12 . Further, according to the present embodiment, continuous operation of the gas-dissolved water supply system 1 is possible regardless of the usage status of the gas-dissolved water at the point of use 12 . By continuously operating the dissolved gas water supply system 1, only a small amount of gas dissolved water with insufficient cleanliness and water quality, which may be generated immediately after starting the dissolved gas water supply system 1, is generated. Useless consumption of dissolving water can be suppressed. In particular, according to the present embodiment, by circulating the gas-dissolved water, it is possible to further reduce impurities and obtain gas-dissolved water with a higher degree of cleanliness. When the amount of gas-dissolved water used at the point of use 12 is large and the amount of circulating gas-dissolved water is reduced, raw water may be supplied to the storage tank 2 from a supply line (not shown). The storage tank 2 has a gas introduction line (not shown) for introducing nitrogen gas and clean air, and a gas discharge line (not shown) for discharging nitrogen gas and clean air from the storage tank 2 to the outside. is connected.

Further, although not shown, the gas-dissolved water used as, for example, washing water or rinse water at the point of use 12 can be recovered and joined to the return path to the reservoir 2 for reuse. In this case, however, the recovered gas-dissolved water may be passed through a water quality adjusting means (for example, a filter) (not shown) in order to adjust the quality of the water before flowing into the reservoir 2 . Alternatively, the gas-dissolved water (see FIG. 1) used at the point of use 12 may be recovered, refined to the level of primary pure water, and then returned to the storage tank 2 . When the gas-dissolved water is reused in this way, it is possible to reduce the amount of replenishment of the raw water and the gas (for example, carbon dioxide gas) to be dissolved therein, and to further reduce the consumption of raw water and gas. In this way, a centralized supply system for supplying dissolved gas water to a plurality of points of use 12 from one dissolved gas water supply system is a circulation system (so-called In the ultrapure water subsystem), the present invention is particularly effective in suppressing the decrease in permeation flux of the ultrafiltration membrane device 5 over time.

The technical significance of the ultrafiltration membrane device 5 of this embodiment will be described below. As described above, by passing the gas-dissolved water through the ultrafiltration membrane device 5, highly clean gas-dissolved water can be obtained, which is effective for cleaning semiconductor wafers and the like. However, there has been a problem that the permeation flux (flux) in the ultrafiltration membrane device is lowered. In contrast, the permeation flux ( flux) can be suppressed. One possible reason for this is that the pores on the primary side surface of the ultrafiltration membrane device 5 are not blocked by fine bubbles present in the liquid.

If the primary side of the ultrafiltration membrane device does not have a dense layer and relatively large pores exist, bubbles in the liquid may enter the relatively large pores and be trapped. Bubbles trapped in this manner substantially block the pores and are thus impermeable to liquid. As the ultrafiltration membrane device continues to be used, the pores blocked by the bubbles gradually increase, and the amount of liquid flowing out through the ultrafiltration membrane device decreases. That is, the permeation flux (flux) in the ultrafiltration membrane device is lowered. Bubbles in the liquid are particularly likely to occur in the circulation path 9 as shown in FIG. That is, bubbles are likely to occur due to water flow into the reservoir 2 in the circulation path 9, entrainment of gas, cavitation at the time of suction by a pump (not shown), and various pressure changes and fluctuations. The bubbles generated in this manner include not only visible large bubbles but also invisible microbubbles and nanobubbles. Microbubbles and nanobubbles are said to exist stably in liquids. When microbubbles and nanobubbles that exist stably in a liquid enter and are trapped in the pores on the primary side of the ultrafiltration membrane device, the ultrafiltration membrane device locally dries (gas is partially clogged by ), the water flow differential pressure rises over time and the permeation flux decreases. If there is a dense layer on the secondary side of the ultrafiltration membrane device, it will be effective in filtering fine particles, but if the pores on the surface of the primary side are large, membrane clogging will occur due to microbubbles and nanobubbles. However, it does not solve the problem that the permeation flux tends to decrease due to the increase in the differential pressure of the water flow due to the difficulty of permeation of the liquid.

In contrast, in the present embodiment, an ultrafiltration membrane device 5 having a dense layer 5a in which extremely small pores are densely arranged is used on the primary side. Since the pores of the dense layer 5a are very small, almost no bubbles in the liquid can enter the pores of the dense layer 5a. In particular, microbubbles and nanobubbles, which are likely to be generated when the gas-dissolved water is circulated through the circulation path 9, are less likely to enter the minute pores of the dense layer 5a, and the pores on the primary side of the ultrafiltration membrane device 5 are blocked. Less likely to come off. As a result, a decrease in permeation flux in the ultrafiltration membrane device 5 can be suppressed, and an increase in differential pressure can be suppressed.

As described above, reducing the possibility that the pores on the surface of the primary side of the ultrafiltration membrane device 5 are clogged with bubbles suppresses a decrease in the permeation flux of the ultrafiltration membrane device 5 according to the present invention. It is considered to be one of the factors that However, it is possible that the present invention achieves the effect of suppressing the decrease in the permeation flux of the ultrafiltration membrane device 5 based on factors other than this.

(Second embodiment)
FIG. 4 shows a second embodiment of the present invention, in which treatment using water to be treated (pure water or ultrapure water) in which gas is not dissolved and treatment using gas-dissolved water can be performed in parallel. A form of gas dissolved water supply system 13 is shown. The same reference numerals are assigned to the same parts as in the dissolved water supply system 1 of the first embodiment. In the gas-dissolved water supply system 13 of the present embodiment, a heat exchanger 6, a degassing device 7, an ultraviolet irradiation device 8, an ion exchange device 3, a first limit The outer filtration membrane devices 14 are arranged in this order and connected in series. This path does not include a gas dissolver. That is, the supply path 17 for supplying the water to be treated in which gas is not dissolved is connected to the first ultrafiltration membrane device 14 , and the first ultrafiltration membrane device 14 is supplied from the supply path 17 . The released gas removes undissolved fine particles in the water to be treated. A part of the water (pure water or ultrapure water) treated by the first ultrafiltration membrane device 14 is supplied from the supply unit 10 to the point of use 15 (gas-free pure water or ultrapure water). supplied to the site where the treatment to be used is performed). The rest of the water (pure water or ultrapure water) treated by the first ultrafiltration membrane device 14 is returned to the reservoir 2 via the pressure control valve 11 . The ultrafiltration membrane used in the ultrafiltration membrane device 14 is not limited to having the dense layer on the primary side, and may be on the secondary side.

In this embodiment, a branch path 18 branching from the supply path 17 is provided on the upstream side in the flow direction of the water to be treated in which gas is not dissolved toward the first ultrafiltration membrane device 14 . This branch path 18 is connected via a replenishment amount control valve 16 to the reservoir 2 of the path for supplying dissolved gas water. Therefore, the water to be treated from which the ionic impurities have been removed by the ion exchange device 3 is not only sent to the first ultrafiltration membrane device 14, but is also sent to the branch path 18 via the replenishment amount control valve 16 to produce gas-dissolved water. It is also sent to reservoir 2 in the feed channel. This water to be treated is sent to the gas dissolving device 4 via the heat exchanger 6 , the ultraviolet irradiation device 8 , and the ion exchange device 3 from the reservoir 2 on the path for supplying the gas-dissolved water. That is, the branch path 18 branching from the supply path 17 on the upstream side of the first ultrafiltration membrane device 14 passes through the reservoir 2 , the heat exchanger 6 , the ultraviolet irradiation device 8 and the ion exchange device 3 to dissolve the gas. It is connected to device 4 . The gas dissolving device 4 dissolves the gas in the water to be treated in which the gas supplied from the branch path 18 is not dissolved. Then, the gas-dissolved water (functional water) in which gas has been dissolved by the gas dissolver 4 is sent to the second ultrafiltration membrane device 5 connected to the gas dissolver 4 . At least part of the gas-dissolved water from which fine particles have been removed by the second ultrafiltration membrane device 5 is supplied from the supply unit 10 to the point of use 12 . The remaining part of the gas-dissolved water from which fine particles have been removed is returned to the reservoir 2 via the pressure regulating valve 11 and circulated. The ultrafiltration membrane device (second ultrafiltration membrane device) 5 of the path for supplying the gas-dissolved water is similar to the ultrafiltration membrane device 5 (see FIGS. 2 and 3) of the first embodiment. It has a dense layer 5a with very fine pores at least on the primary side. This is to prevent gas bubbles dissolved in the water to be treated by the gas dissolving device 4 from entering pores on the primary side surface of the ultrafiltration membrane device 5 and being captured. As a result, as described above, it is possible to suppress a decrease in the permeation flux of gas-dissolved water and an increase in differential pressure. The secondary side of the ultrafiltration membrane device 5 may or may not be provided with the dense layer 5a. Note that the degassing device 7 is not provided in the gas-dissolved water supply path, but the heat exchanger 6, the ultraviolet irradiation device 8, and the ion exchange device 3 are provided. In the present embodiment, which devices are to be arranged in the gas-dissolved water supply path and which devices are to be omitted are determined by appropriately judging whether or not it is desirable to perform processing by each device in duplicate. Just do it. In the example shown in FIG. 4, heat exchange, ultraviolet irradiation, and ion exchange (removal of ionic impurities) are performed twice before introduction into the path for supplying dissolved gas water and within the path. there is

According to this configuration, gas-free pure water or ultrapure water and gas-dissolved water can be supplied to the points of use 12 and 15, respectively. A decrease in the permeation flux (flux) in the membrane device 5 can be kept small.

(Example)
In the system shown in FIG. 1, carbon dioxide gas is supplied by the gas dissolving device 4, and as shown in FIG. Example 1), and a configuration in which the ultrafiltration membrane device 5 has an external pressure double skin structure having dense layers 5a on both the primary side and the secondary side as shown in FIG. 3 (Second Example), The inlet pressure, permeation pressure, concentration pressure, and permeation flow rate of the ultrafiltration membrane device 5 in the configuration (comparative example not shown) in which the ultrafiltration membrane device has an external pressure type single skin structure having a dense layer only on the secondary side are measured. was measured to determine changes in differential pressure and permeation flux (flux) over time. The differential pressure is the difference in liquid pressure between the primary side and the secondary side of the ultrafiltration membrane device, and differential pressure=(inlet pressure+concentration pressure)/2-permeation pressure. In FIG. 5, the initial differential pressure is assumed to be 100%, and the change in the differential pressure over time is expressed as a percentage (initial differential pressure maintenance rate). Flux (permeation flux) is the amount of permeated water that permeates the ultrafiltration membrane device (the total amount of permeated water when the gas-dissolved water supply system is operated for one day (24 hours)). It is a value divided by the differential pressure, and the flux is the amount of permeated water per day/(membrane area×differential pressure). In FIG. 6, the flux in the initial state is assumed to be 100%, and the change in the flux over time is expressed as a percentage (initial flux retention rate).

5 and 6, in the comparative example, which has a dense layer only on the secondary side of the ultrafiltration membrane device (no dense layer on the primary side), the differential pressure increases over time and permeation It can be seen that the amount of water decreases. That is, even if a constant amount of raw material water is continuously supplied, the amount of gas-dissolved water obtained from this gas-dissolved water supply system decreases with time. It becomes difficult to obtain an amount of gas-dissolved water. For example, after about 1000 hours have passed since the start of operation of the gas-dissolved water supply system, only about half the amount of gas-dissolved water is obtained in the initial stage. By increasing the supply pressure of the raw material water, especially the supply pressure (UF supply pressure) to the ultrafiltration membrane device 5, a required amount of gas-dissolved water can be obtained. However, in that case, the differential pressure further increases. Since the ultrafiltration membrane has a permissible value for the supply pressure and the differential pressure, when the permissible value is reached, the ultrafiltration membrane needs to be replaced. If the rate of increase in differential pressure or decrease in flux is high, the life of the ultrafiltration membrane will be shortened, resulting in increased frequency of membrane replacement and higher costs. On the other hand, in the first embodiment, even after nearly 400 hours have elapsed, the differential pressure and flux can be maintained substantially equal to those in the initial stage. In the second embodiment, after about 400 hours from the start of operation of the gas-dissolved water supply system, similar to the first embodiment, almost the same differential pressure and flux as in the initial stage can be maintained. Even after the passage of time, the differential pressure is about 110% of the initial stage, and the flux is about 90% of the initial stage. Therefore, according to the first embodiment and the second embodiment, it is possible to easily obtain the amount of gas-dissolved water required for processing (for example, cleaning of semiconductor wafers) without the need to increase the UF supply pressure. The life of the ultrafiltration membrane can be lengthened by keeping the rise in pressure moderate. As a result, the frequency of replacement of the ultrafiltration membrane is reduced, and an increase in cost can be avoided. Thus, in the configuration (first embodiment and second embodiment) in which the dense layer 5a is provided on the primary side of the ultrafiltration membrane device 5, the increase in the differential pressure over time and the decrease in the permeation flux are suppressed. It turns out that it can be kept low. And it can be seen that this effect is not much affected by the presence or absence of the dense layer on the secondary side. However, in the configuration (comparative example) where there is a dense layer on the secondary side of the ultrafiltration membrane device but no dense layer on the primary side, it is not possible to suppress the increase in the differential pressure and the decrease in the amount of permeated water over time. .

1 gas solution water supply system 2 storage tank 3 ion exchange device 4 gas dissolution device 5 ultrafiltration membrane device (second ultrafiltration membrane device)
5a dense layer (skin layer)
6 heat exchanger 7 degassing device 8 ultraviolet irradiation device 9 circulation path 10 supply unit 11 pressure control valve 12 use point 13 gas dissolved water supply system 14 first ultrafiltration membrane device 15 use point 16 replenishment amount control valve 17 supply Route 18 branch route

Claims (7)

A gas-dissolved water supply system for supplying water to be treated in which gas is not dissolved and gas-dissolved water in which gas is dissolved in water to be treated,
a heat exchanger connected to a supply path that supplies water to be treated in which gas is not dissolved, and that adjusts the temperature of the water to be treated that is supplied from the supply path and in which gas is not dissolved; a first ultrafiltration membrane device for removing fine particles in the water to be treated in which the gas is not dissolved, the temperature of which is adjusted by the device;
Water to be treated which is included in a gas-dissolved water supply path connected to a branch path branched from the supply path on the upstream side of the first ultrafiltration membrane device and supplied from the branch path and a gas dissolver for dissolving gas in the water to be treated, and a heat exchanger for adjusting the temperature of the water to be treated, and a gas dissolver for dissolving gas in the water to be treated. a second ultrafiltration membrane device for removing fine particles in the water to be treated;
Of the water to be treated in which the gas is not dissolved and which has been treated by the first ultrafiltration membrane device, the water to be treated that is not used at the point of use is returned to a position on the upstream side of the connection to the branch path. a circulation path;
Of the water to be treated in which the gas is dissolved, which has been treated by the second ultrafiltration membrane device, the water to be treated that is not used at the point of use is treated by the heat exchanger and the gas in the path for supplying the gas-dissolved water. another circulation path returning to a position upstream of the dissolving device,
A gas-dissolved water supply system, wherein at least the second ultrafiltration membrane device has a dense layer on at least the primary side. An ion exchange device is further provided on the upstream side of the gas dissolving device in the gas-dissolved water supply path and downstream of the position where the water to be treated is supplied from the branch path . Item 2. The gas-dissolved water supply system according to item 1. 3. The gas-dissolved water supply system according to claim 1, wherein said gas is at least one of carbon dioxide gas, nitrogen gas, hydrogen gas and ozone gas. 4. The gas-dissolved water supply system according to any one of claims 1 to 3, wherein at least the surface on the primary side of the second ultrafiltration membrane device is denser than the surface on the secondary side. . 2. The primary and secondary surfaces of at least the second ultrafiltration membrane device are denser than an intermediate portion located between the primary and secondary sides. 4. The gas-dissolved water supply system according to any one of 3 above. 6. The dense layer according to any one of claims 1 to 5, wherein the dense layer is a layer provided with finer pores than at least portions of the second ultrafiltration membrane device other than the dense layer. The gas-dissolved water supply system according to item 1. A gas-dissolved water supply method for supplying water to be treated in which gas is not dissolved and gas-dissolved water in which gas is dissolved in water to be treated,
adjusting the temperature of the water to be treated supplied from the supply path in which the gas is not dissolved by a heat exchanger connected to the supply path for supplying the water to be treated in which the gas is not dissolved; removing fine particles in the water to be treated in which the gas supplied from the supply path and the temperature of which is adjusted by the heat exchanger is not dissolved, by the first ultrafiltration membrane device connected to the
a step of supplying water to be treated in which gas is not dissolved to a path for supplying gas-dissolved water via a branch path branching from the supply path on the upstream side of the first ultrafiltration membrane device;
A part of the water to be treated in which the gas that has been treated by the first ultrafiltration membrane device is not dissolved is supplied to a point of use, and the gas that has been treated by the first ultrafiltration membrane device is returning the remaining portion of the undissolved water to be treated to a position upstream of the connection to the branch path;
a step of adjusting the temperature of the water to be treated supplied from the branch path and dissolving the gas by means of a heat exchanger and a gas dissolver included in the path for supplying the gas-dissolved water;
A second ultrafiltration membrane device having a dense layer on at least the primary side of fine particles in the water to be treated, the temperature of which is adjusted in the step of adjusting the temperature of the water to be treated and dissolving the gas in the step of dissolving the gas. removing by
A portion of the water to be treated in which the gas treated by the second ultrafiltration membrane device is dissolved is supplied to a point of use, and the gas treated by the second ultrafiltration membrane device is dissolved. returning the remaining part of the treated water to a position upstream of the heat exchanger and the gas dissolver in the path for supplying the gas dissolved water. water supply method.

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