JP7715893B1 - Water treatment method and water treatment device - Google Patents

Abstract

An ultraviolet irradiation device efficiently decomposes organic matter when the water to be treated that is supplied to the device has a specific water quality.
[Solution] The water treatment method includes supplying water to be treated, which has a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at an inlet 35A of the ultraviolet irradiation device 35, to the ultraviolet irradiation device 35, and irradiating the water to be treated supplied from the inlet 35A to the ultraviolet irradiation device 35 with ultraviolet light from the ultraviolet irradiation device 35. The hydrogen peroxide concentration of the water to be treated at the inlet 35A is 30 μg/L or less.
[Selected Figure] Figure 1

Description

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

As demands for higher quality pure water become more apparent, various methods for decomposing and removing trace amounts of organic matter contained in pure water have been investigated in recent years. Patent Document 1 describes a water treatment method in which hydrogen peroxide is added to the water to be treated upstream of an ultraviolet irradiation device.

JP 2011-245380 A

Containing hydrogen peroxide in the water to be treated supplied to the ultraviolet irradiation device is advantageous in promoting the decomposition of organic matter by the ultraviolet irradiation device. However, the inventors of the present application have discovered that when the water to be treated supplied to the ultraviolet irradiation device has a specific water quality, excessive amounts of hydrogen peroxide can actually inhibit the decomposition of organic matter by the ultraviolet irradiation device.

The present invention aims to provide a water treatment method that can efficiently decompose organic matter when the water to be treated supplied to an ultraviolet irradiation device has a specific water quality.

The water treatment method of the present invention includes supplying water to be treated, the water having a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at an inlet of the ultraviolet irradiation device, to the ultraviolet irradiation device, and irradiating the water to be treated supplied from the inlet to the ultraviolet irradiation device with ultraviolet light from the ultraviolet irradiation device. The hydrogen peroxide concentration of the water to be treated at the inlet is 30 μg/L or less.
In one embodiment, the method further comprises reducing the dissolved oxygen concentration of the water to be treated at the inlet to less than 1000 μg/L by a deoxygenation device provided upstream of the ultraviolet irradiation device.
In another aspect, the method includes returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device to the upstream side of the ultraviolet irradiation device via a return pipe, and performing at least one of (A) adjusting the amount of ultraviolet rays irradiated from the ultraviolet irradiation device so that the hydrogen peroxide concentration of the treated water at the inlet is 30 μg/L or less, and (B) adjusting the dissolved oxygen concentration of the treated water at the inlet using a deoxygenation device provided upstream of the ultraviolet irradiation device.
In yet another aspect, the method further includes returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device to an area upstream of the ultraviolet irradiation device, measuring the hydrogen peroxide concentration of the water to be treated that is supplied to the ultraviolet irradiation device, and adjusting the amount of treated water that is returned upstream of the ultraviolet irradiation device so that the measured hydrogen peroxide concentration is 30 μg/L or less.

The present invention provides a water treatment method that can efficiently decompose organic matter when the water to be treated supplied to the ultraviolet irradiation device has a specific water quality.

1 is a schematic configuration diagram of a water treatment device according to a first embodiment of the present invention. FIG. 1 is a schematic diagram showing the relationship between the total organic carbon concentration, the dissolved oxygen concentration, the hydrogen peroxide concentration in the water to be treated, and the organic matter decomposition performance of the ultraviolet irradiation device. FIG. 10 is a schematic configuration diagram of a water treatment device according to a second embodiment of the present invention. FIG. 1 is a schematic diagram of a test device used in Examples 1 to 3. 1 is a graph showing the relationship between hydrogen peroxide concentration and TOC concentration reduction rate (ultraviolet irradiation amount 0.06 kWh/m ³ ). 1 is a graph showing the relationship between hydrogen peroxide concentration and TOC concentration reduction rate (ultraviolet irradiation amount 0.1 kWh/m ³ ). 1 is a graph showing the relationship between hydrogen peroxide concentration and TOC concentration reduction rate. 1 is a graph showing the relationship between the dissolved oxygen concentration and the TOC concentration reduction rate.

Embodiments of the water treatment method and water treatment device of the present invention will now be described with reference to the drawings. Figure 1 shows the schematic configuration of a water treatment device 1 according to a first embodiment of the present invention. The water treatment device 1 has an upstream pretreatment device 2 and a downstream pure water production device 3 (primary system). Together with a downstream subsystem (secondary system), the water treatment device 1 constitutes an ultrapure water production system. The raw water supplied to the pretreatment device 2 contains dissolved oxygen and organic matter. In the following description, upstream and downstream are defined with respect to the direction of water flow in the main pipe. Furthermore, "adjustment" includes both automatic control of various operating parameters (device on/off, flow rate, pressure, valve opening/closing, power consumption, etc.) by a control device and manual adjustment of these operating parameters by an operator. The present invention can be applied not only to the water treatment device 1 but also to subsystems.

The pretreatment device 2 includes a filter 21 for removing relatively large particles such as dust, and an activated carbon tower 22 for removing impurities such as high-molecular-weight organic matter and oxidizing agents. The filter 21 can be, for example, a sand filter. The pure water production system 3 includes an ion removal device 31, a reverse osmosis membrane device 32, an intermediate tank 33, a first deoxygenation device 34, an ultraviolet irradiation device 35, an ion exchanger packing device 36, and a second deoxygenation device 37. These devices and tanks are arranged in series in this order on the main pipe L1 from upstream to downstream in the flow direction D of the water to be treated. Although not shown, in addition to the intermediate tank 33, tanks for storing treated water from each device of the pretreatment device 2 and the pure water production system 3, such as the activated carbon tower 22, the ion removal device 31, and the reverse osmosis membrane device 32, may also be provided. Downstream of the second deoxygenation device 37, a return pipe L2 branches off from the main pipe L1 and merges with the intermediate tank 33. Although not shown in the figure, in addition to the return pipe L2, a pipe may be provided to return a portion of the treated water from one of the devices in the pure water production system 3 to an upstream tank or the like.

The ion removal device 31 comprises a cation tower (not shown) filled with cation exchange resin, a decarbonation tower (not shown), and an anion tower (not shown) filled with anion exchange resin, which are arranged in series from upstream to downstream. A decarbonation membrane may be provided instead of the decarbonation tower. Instead of the ion removal device 31, a softener that removes hardness components such as calcium and magnesium may be arranged upstream, and an electrodeionized water production device (EDI) may be arranged downstream in series. The EDI removes ionic components that inhibit the organic matter decomposition process in the ultraviolet irradiation device 35.

The reverse osmosis membrane device 32 removes impurities such as ions. In this embodiment, an ion removal device 31 is provided upstream of the reverse osmosis membrane device 32, so the reverse osmosis membrane device 32 mainly removes uncharged substances such as organic matter. The reverse osmosis membrane device 32 may be provided in multiple stages. If the total organic carbon concentration of the water to be treated is high, the efficiency of decomposing organic matter in the ultraviolet irradiation device 35 decreases. Removing organic matter with the reverse osmosis membrane device 32 reduces the load on the ultraviolet irradiation device 35 downstream. A pH adjustment mechanism may be provided upstream of the reverse osmosis membrane device 32. The treated water from the reverse osmosis membrane device 32 is stored in the intermediate tank 33.

The first deoxygenation device 34 removes oxygen from the water being treated, reducing the dissolved oxygen concentration in the water. The first deoxygenation device 34 can also simultaneously remove volatile organic compounds and carbon dioxide into the gas phase (secondary side) to reduce their concentrations in the water. Because the first deoxygenation device 34 is located upstream of the ultraviolet irradiation device 35, the water being treated with a reduced (adjusted) dissolved oxygen concentration is supplied to the ultraviolet irradiation device 35. The type of the first deoxygenation device 34 is not limited as long as it can remove dissolved oxygen; for example, a vacuum degassing device can be used. In general, a vacuum degassing device fills a degassing tower with a gas-liquid contact material to increase the surface area of the water, reduces the gas pressure in the degassing tower with a vacuum pump, and places the water being treated in a vacuum state to remove dissolved oxygen. The dissolved oxygen concentration can be adjusted by adjusting the degree of vacuum in the degassing tower using a vacuum pump. The degree of vacuum can be adjusted using an inverter connected to the vacuum pump. Furthermore, degassing performance can be improved by introducing nitrogen. In this case, the dissolved oxygen concentration can be adjusted by adjusting the degree of vacuum and the amount of nitrogen inflow (nitrogen partial pressure).

A degassing membrane device may be used as the first deoxygenation device 34. In this case, a vacuum pump is used, as in the vacuum degassing device, and the dissolved oxygen concentration can be adjusted by adjusting the degree of vacuum. The degree of vacuum can be adjusted using an inverter connected to the vacuum pump. To automatically adjust the degree of vacuum and nitrogen inflow rate in the first deoxygenation device 34, for example, the dissolved oxygen concentration in the water being treated in the ultraviolet irradiation device 35 can be measured by a measuring device (not shown), and a control device (not shown) can adjust the degree of vacuum and nitrogen inflow rate in the first deoxygenation device 34 based on the measured value. Alternatively, a platinum catalyst-loaded device carrying a platinum catalyst such as palladium (Pd) may be used as the first deoxygenation device 34. By contacting the water being treated with hydrogen-added water with the platinum catalyst, the dissolved oxygen concentration in the water can be reduced. The first deoxygenation device 34 described above may be a single-stage configuration or a multi-stage configuration in which multiple devices are connected in series.

The ultraviolet irradiation device 35 irradiates the water to be treated with ultraviolet light to decompose organic matter contained in the water. The ultraviolet light emitted from the ultraviolet irradiation device 35 reacts with the water to produce hydrogen peroxide. The ultraviolet irradiation device 35 can be, for example, an ultraviolet irradiation device (e.g., a low-pressure ultraviolet irradiation device) that generates ultraviolet light with at least one of wavelengths of 185 nm and 254 nm. The ultraviolet irradiation dose is defined as the irradiation energy applied per unit volume of water (unit: kWh/m ³ ). Therefore, the irradiation dose can be adjusted by adjusting the number of lamps and dimming of the ultraviolet irradiation device 35, as well as by changing the flow rate of the water to be treated. To automatically adjust the ultraviolet irradiation dose in the ultraviolet irradiation device 35, for example, a TOC meter (not shown) can be used to measure the total organic carbon concentration (hereinafter referred to as TOC concentration) of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35, and a control device (not shown) can adjust the number of lamps and dimming of the ultraviolet irradiation device 35 and the flow rate of the water to be treated based on the measured value.

The ion exchanger packing device 36 removes organic decomposition products generated in the treated water from the ultraviolet irradiation device 35 by irradiating it with ultraviolet light. The ion exchanger packing device 36 is packed with ion exchange resin, but it may also be packed with monolithic or fibrous ion exchangers. The ion exchanger packing device 36 may also be an EDI packed with ion exchange resin. Because EDI is a continuous regeneration type, there is no need for a regeneration process for the ion exchange resin.

The second deoxygenation device 37 is located downstream of the ion exchanger packing device 36 and can have a configuration similar to that of the first deoxygenation device 34. The second deoxygenation device 37 removes dissolved oxygen, carbon dioxide, etc. from the water to be treated.

The treated water from the second deoxygenation device 37 is sent to the subsystem. The return pipe L2, which branches off downstream of the second deoxygenation device 37, adjusts the flow rate of the pure water supplied to the subsystem. For this purpose, a flow rate adjustment valve V1 is provided in the return pipe L2. Since the return pipe L2 is preferably located at the most downstream position of the pure water production system 3, it branches off from the main pipe L1 downstream of the second deoxygenation device 37, but another return pipe may be provided that branches off from the main pipe L1 upstream of the second deoxygenation device 37.

As described above, the ultraviolet irradiation device 35 generates hydrogen peroxide from water, so the water flowing downstream of the ultraviolet irradiation device 35 contains hydrogen peroxide. Furthermore, the water containing hydrogen peroxide is returned to the intermediate tank 33 via the return pipe L2. However, because no hydrogen peroxide removal means is provided between the intermediate tank 33 and the ultraviolet irradiation device 35, the water flowing through the section between the intermediate tank 33 and the inlet of the subsystem contains hydrogen peroxide. In other words, the water to be treated supplied to the ultraviolet irradiation device 35 contains hydrogen peroxide. On the other hand, because the hydrogen peroxide contained in the raw water is almost entirely removed by the activated carbon tower 22, the water flowing through the section between the ion removal device 31 and the intermediate tank 33 contains virtually no hydrogen peroxide. Therefore, the hydrogen peroxide concentration in the water to be treated supplied to the ultraviolet irradiation device 35 is higher than the hydrogen peroxide concentration in the water flowing through the section between the ion removal device 31 and the intermediate tank 33. In this embodiment, no hydrogen peroxide removal means (such as the hydrogen peroxide removal device 38 of the second embodiment or a reducing agent addition device) is provided between the intermediate tank 33 and the inlet of the subsystem. Therefore, the hydrogen peroxide concentration in the water flowing between the outlet of the ultraviolet irradiation device 35 and the inlet of the subsystem and the water flowing through the return pipe L2 is higher than the hydrogen peroxide concentration in the water flowing in the section between the ion removal device 31 and the intermediate tank 33. Note that while the ion exchanger filling device 36 removes trace amounts of hydrogen peroxide, its removal efficiency is extremely low. The hydrogen peroxide removal means refers to a means that removes hydrogen peroxide more efficiently than the ion exchanger filling device 36.

(Water treatment method)
Next, a water treatment method using the water treatment device 1 will be described. As described above, in this embodiment, the water to be treated flows sequentially through the pretreatment device 2 and the various devices constituting the pure water production device 3, producing pure water. In particular, in this embodiment, the water to be treated having a specific water quality (hereinafter referred to as specific water quality) of a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at the inlet 35A of the ultraviolet irradiation device 35 is supplied to the ultraviolet irradiation device 35. The water to be treated supplied to the ultraviolet irradiation device 35 from the inlet 35A of the ultraviolet irradiation device 35 is irradiated with ultraviolet light from the ultraviolet irradiation device 35. Because a high ultraviolet irradiation dose increases energy costs, the ultraviolet irradiation dose is preferably 0.1 kWh/m ³ or less, more preferably 0.08 kWh/m ³ or less, and even more preferably 0.06 kWh/m ³ or less. Because a too low ultraviolet irradiation dose reduces the decomposition efficiency of organic matter, the ultraviolet irradiation dose is preferably 0.02 kWh/m ³ or more. If the total organic carbon concentration is low, the load on the downstream ion exchanger packing device 36 and the amount of ultraviolet radiation can be reduced, so the total organic carbon concentration is preferably 5 μg/L or less.

FIG. 2 shows a schematic diagram of the relationship between the TOC concentration, dissolved oxygen concentration (hereinafter referred to as DO concentration), and hydrogen peroxide concentration (hereinafter referred to as _H2O2 concentration) at the inlet 35A of the ultraviolet irradiation device 35 and _the organic matter decomposition performance of the ultraviolet irradiation device 35. Generally, hydrogen peroxide is an oxidation promoter, so adding hydrogen peroxide to the water being treated by the ultraviolet irradiation device 35 promotes the decomposition of organic matter. For example, when the DO concentration is less than 30 μg/L, the decomposition of organic matter is promoted regardless of the TOC concentration. Even if the DO concentration is 30 μg/L or higher, the decomposition of organic matter is promoted as long as the TOC concentration is greater than 10 μg/L. In contrast, in water with specific water quality, excessive hydrogen peroxide concentrations reduce the efficiency of organic matter decomposition by ultraviolet light. This is thought to be because hydrogen peroxide acts as an inhibitor of organic matter decomposition by ultraviolet light. Therefore, for water to be treated with a specific water quality, _a lower _H2O2 concentration promotes the decomposition of organic matter. _{The H2O2} concentration is preferably 30 μg/L or less, more preferably 20 μg/L or less, even more preferably less than 10 μg/L, and even more preferably 5 μg/L or less. However, for reasons described below, _{the H2O2} concentration is preferably greater than 1 μg/L, and even more preferably greater than 2 μg/L.

For water _not meeting the specified water quality standard, adding hydrogen peroxide to decompose organic matter (or not performing treatment to reduce the _H2O2 concentration) is effective. However, reducing the DO concentration to less than 30 μg/L may require increased costs for the first deoxidizer 34 and increased power costs for the vacuum pump. Therefore, it may be preferable to maintain the DO concentration at 30 μg/L or higher. It may also be reasonable to maintain a relatively high TOC concentration at the inlet 35A of the ultraviolet irradiation device 35 in order to reduce the costs and operating costs of the upstream ion removal device 31 and reverse osmosis membrane device 32. Thus, whether the water being treated by the ultraviolet irradiation device 35 meets the specified water quality standard depends on factors such as the design of the water treatment device 1, the design of the entire ultrapure water production system including its subsystems, and the quality of the raw water. This determination cannot be based solely on the organic matter decomposition performance of the ultraviolet irradiation device 35. However, when the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 has a specific water quality, it is preferable to operate the device to reduce the H ₂ O ₂ concentration.

(DO concentration)
The DO concentration of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 is not limited as long as it is 30 μg/L or more, but is preferably less than 1000 μg/L. As described in Example 3 below, a concentration of less than 1000 μg/L improves the decomposition performance of organic matter by ultraviolet light. The DO concentration of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 is more preferably 500 μg/L or less, and even more preferably 100 μg/L or less. The DO concentration of the water to be treated can be adjusted by the first deoxygenation device 34 provided upstream of the ultraviolet irradiation device 35. The DO concentration can be measured online, for example, using a DO concentration meter (not shown) provided between the first deoxygenation device 34 and the ultraviolet irradiation device 35.

To improve the decomposition efficiency of organic matter, the amount of ultraviolet light emitted from the ultraviolet irradiation device 35 can be adjusted (Method A). Furthermore, because dissolved oxygen has the property of absorbing ultraviolet light, a low DO concentration reduces the amount of ultraviolet light absorbed by dissolved oxygen, thereby improving the decomposition efficiency of organic matter. Therefore, to improve the decomposition efficiency of organic matter, the DO concentration of the treated water can also be adjusted using the first deoxygenation device 34 (Method B). Methods A and B may be performed in combination, or either method may be performed alone. However, it is preferable to perform these methods so that the _H2O2 concentration _of the treated water at the inlet 35A of the ultraviolet irradiation device 35 is 30 μg/L or less. A hydrogen peroxide sampling pipe L3 is installed at any position between the intermediate tank 33 and the ultraviolet irradiation device 35 (in this embodiment, between the first deoxygenation device 34 and the ultraviolet irradiation device 35). The _H2O2 concentration of the treated water at the inlet 35A of the ultraviolet irradiation device 35 can be measured from the water sampled through the sampling pipe _L3 . The _H2O2 concentration _may be measured online using an _H2O2 concentration meter (not shown) installed at any position between the intermediate tank 33 and the ultraviolet irradiation device 35 (for example, between the first deoxygenation device 34 and the ultraviolet irradiation device 35). When adjusting the _ultraviolet irradiation amount using method A, the ultraviolet irradiation amount is usually increased, but it may also be decreased to the extent that the decomposition efficiency of organic matter is not significantly reduced. When adjusting the DO concentration of the water to be treated using method B, the DO concentration of the water to be treated is usually decreased, but it may also be increased to the extent that the decomposition efficiency of organic matter is not significantly reduced.

Method A and Method B can be implemented depending on the TOC concentration in the water to be treated or the treated water from the ultraviolet irradiation device 35. Specifically, a TOC meter is placed on the inlet side of the ultraviolet irradiation device 35 (e.g., between the first deoxygenation device 34 and the ultraviolet irradiation device 35) and on the outlet side of the ultraviolet irradiation device 35 (e.g., at the outlet of the second deoxygenation device 37). Then, when the TOC concentration of the water to be treated from the ultraviolet irradiation device 35 exceeds a predetermined value, or when the TOC concentration of the treated water from the ultraviolet irradiation device 35 exceeds a predetermined value, at least one of Method A and Method B, or preferably both, can be implemented. Note that at least one of Method A and Method B can be implemented even if the TOC concentration of the water to be treated or the treated water from the ultraviolet irradiation device 35 does not exceed the predetermined value.

The first deoxygenation device 34 and ultraviolet irradiation device 35 also serve as means for adjusting the _H2O2 concentration of the water to 30 μg/L or less at the inlet 35A of the ultraviolet irradiation device _35. Because ultraviolet light generates hydrogen peroxide from water, increasing the amount of ultraviolet light irradiation also increases the amount of hydrogen peroxide generated. When the DO concentration is low, less ultraviolet light is absorbed by dissolved oxygen, resulting in the generation of more hydrogen peroxide even with the same amount of ultraviolet light irradiation. A portion of the generated hydrogen peroxide is returned upstream of the ultraviolet irradiation device 35 via the return pipe _L2 . Therefore, the _H2O2 concentration of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 can be adjusted by at least one of the first deoxygenation device 34 and the ultraviolet irradiation device 35. As described above, the _H2O2 concentration _of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 can be measured from water sampled via the sampling pipe L3. When the measured _H2O2 concentration exceeds a predetermined standard value (e.g., 25 μg/L) lower than 30 μg/L, the _H2O2 concentration _in the treated water can be adjusted by adjusting at least one of the ultraviolet irradiation amount and the _DO concentration.

As described above, hydrogen peroxide contained in the raw water is almost entirely removed in the activated carbon tower 22. Therefore, the hydrogen peroxide present at the inlet 35A of the ultraviolet irradiation device 35 is almost entirely limited to that generated in the ultraviolet irradiation device 35 and returned upstream of the ultraviolet irradiation device 35 via the return pipe L2. Therefore, the flow rate of the return water can be adjusted so that the _H2O2 concentration of the water to be treated is 30 _μg /L or less. Specifically, _{the H2O2 concentration} of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 is measured using the sampling pipe L3. When _the measured _H2O2 concentration exceeds a predetermined reference value (e.g., 25 μg/L) lower than 30 μg/L, the flow rate of the treated water returned from the return pipe L2 to the upstream of the ultraviolet irradiation device 35 is reduced. This allows _{the H2O2} concentration of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 to be adjusted to 30 μg/L or less. For example, increasing the amount _of ultraviolet radiation to improve the decomposition efficiency of organic matter increases the _H2O2 concentration in the treated water. In this case, the opening of valve V1 can be narrowed to reduce the flow rate of return water in order to suppress the increase in _{the H2O2} concentration in the water to be treated at the inlet 35A of the ultraviolet radiation device 35.

However, the return water flow rate is originally determined to accommodate fluctuations in the amount of ultrapure water used at the point of use and the resulting fluctuations in the amount of pure water supplied to the subsystem. Reducing the return water flow rate to lower _{the H2O2} concentration at the inlet 35A of the UV irradiation device 35 may make it difficult to accommodate fluctuations in the amount of ultrapure water used at the point of use. Therefore, to appropriately adjust the pure water supply flow rate to the subsystem, _{the H2O2} concentration at the inlet 35A of the UV irradiation device 35 is preferably greater than 1 μg/L, and more preferably 2 μg/L or greater. Reducing the UV irradiation dose is also effective in reducing _{the H2O2} concentration at the inlet 35A of the UV irradiation device 35, but this reduces the decomposition efficiency of organic matter in the UV irradiation device 35. Thus, reducing the _H2O2 concentration at the inlet 35A of the UV irradiation device ₃₅ to 1 μg/L or less may be disadvantageous from another perspective.

Second Embodiment
FIG. 3 shows a schematic configuration of a water treatment device 1 according to a second embodiment of the present invention. The water treatment device 1 of this embodiment includes a hydrogen peroxide removal device 38 located downstream of the ultraviolet irradiation device 35 and the ion exchanger filling device 36 and upstream of the second deoxidizer 37. The second embodiment is identical to the first embodiment except for this point. The hydrogen peroxide removal device 38 removes a portion of the hydrogen peroxide from the treated water irradiated with ultraviolet light from the ultraviolet irradiation device 35. The portion of the treated water from which the hydrogen peroxide has been removed is returned upstream of the ultraviolet irradiation device 35 through the return pipe L2. Therefore, the hydrogen peroxide removal device 38 and the return pipe L2 function as a means for adjusting _{the H2O2} concentration of the treated water at the inlet 35A of the ultraviolet irradiation device 35 to 30 μg/L or less. In this embodiment, since the return water passing through the return pipe L2 contains almost no hydrogen peroxide, the flow rate of the return water can be increased by increasing the aperture of the valve V1 to suppress an increase in _{the H2O2} concentration of the treated water at the inlet 35A of the ultraviolet irradiation device 35.

The hydrogen peroxide removal device 38 is a platinum group catalyst-loaded device in which a platinum group catalyst made of a platinum group metal is supported on an anion exchanger (e.g., resin). Examples of platinum group metals include platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), osmium (Os), and iridium (Ir). These metals may be used alone or in combination. Among these platinum group metals, Pt and Pd are preferred, with Pd being preferred from a cost perspective. Contacting the treated water with a platinum group catalyst can reduce the _H2O2 concentration in the treated water. A _hydrogen addition section (not shown) may be provided upstream of the platinum group catalyst-loaded device to reduce the DO concentration. Alternatively, an ion exchanger supported on a metal catalyst such as Pd may be loaded into the EDI. In this case, hydrogen generated at the cathode of the EDI can be used to contact the metal catalyst.

In the above-described embodiments, _{the H2O2} concentration is adjusted to 30 μg/L or less. However, if the hydrogen peroxide concentration of the water to be treated at the inlet 35A of the ultraviolet irradiation device 35 is 30 μg/L or less even without adjustment, adjustment of the hydrogen peroxide concentration is not necessary. As described above, in the first embodiment, the water flowing between the activated carbon tower 22 and the intermediate tank 33 contains almost no hydrogen peroxide. Therefore, the hydrogen peroxide present at the inlet 35A of the ultraviolet irradiation device 35 is almost exclusively hydrogen peroxide generated in the ultraviolet irradiation device 35 and returned upstream to the ultraviolet irradiation device 35 via the return pipe L2. Because the hydrogen peroxide concentration depends on the amount of ultraviolet radiation emitted by the ultraviolet irradiation device 35 and the flow rate of the return water passing through the return pipe L2, adjustment of the hydrogen peroxide concentration may not be necessary. In the second embodiment, the water flowing between the activated carbon tower 22 and the intermediate tank 33 contains almost no hydrogen peroxide. Furthermore, the provision of the hydrogen peroxide removal device 38 ensures that the water flowing through the return pipe L2 also contains almost no hydrogen peroxide. Therefore, regardless of the amount of ultraviolet radiation from the ultraviolet radiation device 35, the flow rate of the return water passing through the return pipe L2, etc., the hydrogen peroxide concentration of the water to be treated at the inlet 35A of the ultraviolet radiation device 35 is almost zero (30 μg/L or less). Therefore, in the second embodiment, adjustment of the hydrogen peroxide concentration is basically not required.

Example 1
The TOC concentration reduction rate was measured using the test equipment shown in Figure 4. The test equipment consisted of a deoxygenation device (deaeration membrane device) 41, an ultraviolet irradiation device 42, and an ion exchange resin column 43 arranged in this order along the flow direction D of the water to be treated. A portion of the water treated by the ultraviolet irradiation device 42 was supplied to the ion exchange resin column 43, and the remainder was discharged through the blow line. The TOC concentration (T1) of the water to be treated between the deoxygenation device 41 and the ultraviolet irradiation device 42 and the TOC concentration (T2) of the water treated by the ion exchange resin column 43 were each measured using a TOC meter. The definition of the TOC concentration reduction rate is shown in Figure 4.

Hydrogen peroxide was added upstream of the deoxygenation device 41 to raw water adjusted to a TOC concentration of 10 μg/L, a _DO concentration of 100 μg/L, and an _H2O2 concentration of 5 μg/L. The _H2O2 concentration of the water to be treated supplied to the deoxygenation device 41 was adjusted to 5 μg/L (no hydrogen peroxide added), 30 μg/L, 100 μg/L, or 200 μg/L. The _DO concentration of the water to be treated supplied to the ultraviolet irradiation device 42 was adjusted to 5 μg/L, 30 μg/L, or 100 μg/L (vacuum pump off) by adjusting the degree of vacuum inside the deoxygenation membrane using the vacuum pump of the deoxygenation device 41. The ultraviolet irradiation dose was adjusted to 0.06 kWh/ ^m3 or 0.1 kWh/ ^m3 by adjusting the flow rate of the water to be treated supplied to the ultraviolet irradiation device _42. The _H2O2 concentration was measured by absorbance. Other test conditions are shown below.
Ultraviolet irradiation device 42: Low-pressure ultraviolet irradiation device JPW (manufactured by Japan Photo Science Co., Ltd.)
Ion exchange resin column 43: A mixed bed of cation exchange resin AMBERJET 1024H type (manufactured by Organo Corporation) and anion exchange resin AMBERJET 4002OH type (manufactured by Organo Corporation), totaling 300 mL, packed in a volume ratio of 1:2. Ion exchange resin SV: 60 (/h)
・TOC meter...M500e (VEOLIA)
・Dissolved oxygen meter: Orbisphere 510 (manufactured by Hach)

Figure 5 shows the relationship between _H2O2 concentration and TOC concentration reduction rate at a UV irradiation dose of 0.06 _kWh / ^m3 . Figure 6 shows the relationship between _H2O2 concentration and TOC concentration reduction rate at a UV irradiation dose of 0.1 _kWh / ^m3 . Referring to Figure 5, at a DO concentration of 5 μg/L, the TOC concentration reduction rate improved as _{the H2O2} concentration increased, saturating at approximately 100 μg/L. At a DO concentration of 30 μg/L, the TOC concentration reduction rate remained almost constant up to _{an H2O2} concentration of 30 μg/L, but gradually decreased above 30 _μg /L. In particular, the TOC concentration reduction rate decreased significantly at an _H2O2 concentration of 200 μg/L. At a DO concentration of 100 μg/L, a similar trend was observed as at a DO concentration of 30 μg/L, and the TOC concentration reduction rate decreased significantly at _{an H2O2} concentration of 100 μg/L. From the above, it was found that when the DO concentration is 30 μg/L or higher, the decrease in organic matter decomposition performance due to ultraviolet light can be suppressed by adjusting _{the H2O2} concentration to 30 μg/L or lower. Furthermore, as shown in Figure 6, the trend remains the same even when the ultraviolet light irradiation dose is 0.1 kWh/ ^m3 , which means that organic matter can be efficiently decomposed by adjusting _{the H2O2} concentration to 30 μg/L or lower regardless of the ultraviolet light irradiation dose.

Example 2
Under the same conditions as in Example 1 (DO concentration: 30 μg/L, irradiation dose: 0.1 kWh/ ^m3 ), the TOC concentration was adjusted to 10 μg/L, 30 μg/L, and 50 μg/L. The _H2O2 concentration of the water to be treated supplied to the deoxygenation device ₄₁ was adjusted to 5 μg/L (no _hydrogen peroxide added), 30 μg/L, 100 μg/L, and 200 μg/L. Figure 7 shows the relationship between _H2O2 concentration and the TOC concentration reduction rate. At a TOC concentration of 10 μg/L, the TOC concentration reduction rate remained almost constant up to _{an H2O2} concentration of 30 μg/L, but gradually decreased once the H2O2 concentration exceeded 30 μg/L. At TOC concentrations of 30 μg/L and 50 μg/L, the TOC _{concentration} reduction rate improved as the _H2O2 concentration increased. From the above, it was found that when the TOC concentration is 10 μg/L or less, the decrease in organic matter decomposition performance due to ultraviolet light can be suppressed by adjusting the H ₂ O ₂ concentration to 30 μg/L or less.

Example 3
Under the same conditions as in Example 1 ( _H2O2 concentration: 30 μg/L, _UV irradiation dose: 0.1 kWh/ ^m3 ), the DO concentration was adjusted to 5 μg/L, 30 μg/L, 100 μg/L, 1000 μg/L, and 10,000 μg/L. Figure 8 shows the relationship between DO concentration and TOC concentration reduction rate. DO concentrations of 5 μg/L, 30 μg/L, and 100 μg/L are the same as in Example 1. Looking at the range of DO concentrations of 30 μg/L or more, the TOC concentration reduction rate was highest at a DO concentration of 30 μg/L, and decreased as the DO concentration increased. From the above, it was found that a DO concentration of 1000 μg/L or less is preferable.

REFERENCE SIGNS LIST 1 Water treatment device 2 Pretreatment device 3 Pure water production device 31 Ion removal device 32 Reverse osmosis membrane device 33 Intermediate tank 34 First deoxidation device 35 Ultraviolet irradiation device 36 Ion exchanger filling device 37 Second deoxidation device 38 Hydrogen peroxide removal device

Claims (9)

supplying water to be treated, the water having a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at an inlet of the ultraviolet irradiation device, to the ultraviolet irradiation device;
irradiating the water to be treated supplied from the inlet to the ultraviolet irradiation device with ultraviolet light from the ultraviolet irradiation device;
and reducing the dissolved oxygen concentration of the water to be treated at the inlet to less than 1000 μg/L by a deoxygenation device provided upstream of the ultraviolet irradiation device ,
A water treatment method, wherein the concentration of hydrogen peroxide in the water to be treated at the inlet is 30 μg/L or less. The water treatment method according to claim 1 , further comprising adjusting the hydrogen peroxide concentration of the water to be treated at the inlet to 30 μg/L or less. The water treatment method of claim 1, further comprising adjusting the dissolved oxygen concentration of the water to be treated at the inlet to less than 1000 μg/L. supplying water to be treated, the water having a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at an inlet of the ultraviolet irradiation device, to the ultraviolet irradiation device;
irradiating the water to be treated supplied from the inlet to the ultraviolet irradiation device with ultraviolet light from the ultraviolet irradiation device;
Returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device to an upstream side of the ultraviolet irradiation device through a return pipe;
The hydrogen peroxide concentration of the water to be treated at the inlet is in the range of 30 μg / L or less,
(A) adjusting the amount of ultraviolet light emitted from the ultraviolet irradiation device; and (B) adjusting the dissolved oxygen concentration of the water to be treated at the inlet portion using a deoxygenation device provided upstream of the ultraviolet irradiation device. supplying water to be treated, the water having a total organic carbon concentration of 10 μg/L or less and a dissolved oxygen concentration of 30 μg/L or more at an inlet of the ultraviolet irradiation device, to the ultraviolet irradiation device;
irradiating the water to be treated supplied from the inlet to the ultraviolet irradiation device with ultraviolet light from the ultraviolet irradiation device;
Returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device upstream of the ultraviolet irradiation device;
measuring a hydrogen peroxide concentration of the water to be treated supplied to the ultraviolet irradiation device;
adjusting the amount of the treated water returned upstream of the ultraviolet irradiation device so that the measured hydrogen peroxide concentration is 30 μg/L or less ;
A water treatment method , wherein the concentration of hydrogen peroxide in the water to be treated at the inlet is 30 μg/L or less . removing a portion of the hydrogen peroxide from the treated water irradiated with ultraviolet light from the ultraviolet irradiation device by a hydrogen peroxide removal device provided downstream of the ultraviolet irradiation device;
returning a portion of the treated water from which a portion of the hydrogen peroxide has been removed upstream of the ultraviolet irradiation device;
The water treatment method according to any one of claims 1 to 5 , comprising: an ultraviolet irradiation device;
a means for adjusting the hydrogen peroxide concentration of the water to be treated at an inlet of the ultraviolet irradiation device to 30 μg/L or less;
and
The ultraviolet irradiation device irradiates the water to be treated supplied from the inlet with ultraviolet rays,
The water quality of the water to be treated at the inlet is such that the total organic carbon concentration is 10 μg/L or less and the dissolved oxygen concentration is 30 μg/L or more ,
The water treatment device further comprises a deoxygenation device provided upstream of the ultraviolet irradiation device, the deoxygenation device adjusting the dissolved oxygen concentration of the water to be treated at the inlet portion to less than 1000 μg/L . an ultraviolet irradiation device;
and a means for adjusting the hydrogen peroxide concentration of the water to be treated at an inlet of the ultraviolet irradiation device to 30 μg/L or less;
The ultraviolet irradiation device irradiates the water to be treated supplied from the inlet with ultraviolet rays,
The water quality of the water to be treated at the inlet is such that the total organic carbon concentration is 10 μg/L or less and the dissolved oxygen concentration is 30 μg/L or more,
a return pipe for returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device to an upstream side of the ultraviolet irradiation device;
a deoxidation device provided upstream of the ultraviolet irradiation device,
The hydrogen peroxide concentration of the water to be treated at the inlet is in the range of 30 μg / L or less,
(A) the ultraviolet irradiation device adjusts the irradiation amount of ultraviolet light emitted from the ultraviolet irradiation device;
(B) adjusting the dissolved oxygen concentration of the water to be treated at the inlet portion by the deoxygenation device;
The water treatment device performs at least one of the above. an ultraviolet irradiation device;
and a means for adjusting the hydrogen peroxide concentration of the water to be treated at an inlet of the ultraviolet irradiation device to 30 μg/L or less;
The ultraviolet irradiation device irradiates the water to be treated supplied from the inlet with ultraviolet rays,
The water quality of the water to be treated at the inlet is such that the total organic carbon concentration is 10 μg/L or less and the dissolved oxygen concentration is 30 μg/L or more,
a return pipe for returning a portion of the treated water irradiated with ultraviolet rays from the ultraviolet irradiation device to an upstream side of the ultraviolet irradiation device;
a sampling pipe for measuring the hydrogen peroxide concentration of the water to be treated that is supplied to the ultraviolet irradiation device;
a flow rate adjusting valve provided in the return pipe for adjusting the amount of the treated water returned upstream of the ultraviolet irradiation device so that the measured hydrogen peroxide concentration is 30 μg/L or less.