Ultrapure water production device

Abstract

In the present invention, an ultrapure water production method comprises: pretreatment in which suspended matter in raw water including tap water is removed to obtain pretreated water; primary treatment in which all organic carbon components and ion components in the pretreated water are removed to produce primary pure water; and secondary treatment in which impurities in the primary pure water are removed to produce ultrapure water. Second raw water including sewage treatment water is caused to merge with raw water or the pretreated water that is on the upstream of the primary treatment.

Description

Ultrapure water production equipment

[0001] The present disclosure relates to an ultrapure water production system.

[0002] In an ultrapure water production apparatus that supplies ultrapure water to semiconductor manufacturing facilities, etc., a technology has been disclosed that uses tap water as raw water and wastewater (sometimes called recovered water) of ultrapure water used in the manufacturing of semiconductors, etc. (see JP-A-6-233997, JP-A-7-313994, JP-A-10-272465, JP-A-11-226569, JP-A-2000-189760, JP-A-2006-61779, and JP-A-2013-202587).

[0003] Generally, recovered water is originally ultrapure water mixed with substances used in semiconductor manufacturing, such as acids, alkalis, IPA (isopropyl alcohol), surfactants, CMP abrasives, etc. Therefore, the components are relatively clear, and it is relatively easy to treat it and mix it with raw water for use.

[0004] However, because a large amount of ultrapure water is used in the manufacture of semiconductors and other products, the amount of tap water used to produce ultrapure water is also increasing. Because the supply of tap water is limited, measures are needed to reduce the amount of tap water used in ultrapure water production so that tap water does not run out for other uses. Furthermore, there is a limit to the use of the above-mentioned recycled water, and although it is effective in reducing raw water use, it is not sufficient to accommodate the current increase in the size of semiconductor and other manufacturing factories. Furthermore, due to current environmental and climate change issues, the supply of tap water is becoming unstable, creating a need to secure new raw water.

[0005] Generally, sewage or treated sewage water has poorer water quality than drinking water, and contains particularly difficult-to-biodegrade substances such as urea, as well as a large amount and variety of other substances. Removal equipment for difficult-to-biodegrade substances such as urea tends to be large and difficult to put into practical use, and even small amounts can reach the end of a pure water system. Furthermore, the water quality fluctuates widely, and the amount of water is unstable. Therefore, it has not been common to use sewage or treated sewage water as raw water for ultrapure water for semiconductor manufacturing, etc.

[0006] The present disclosure reduces the proportion of tap water in the raw water used to produce ultrapure water.

[0007] The method for producing ultrapure water according to a first aspect includes a pretreatment step in which suspended solids are removed from raw water, including drinking water, to obtain pretreated water; a primary treatment step in which total organic carbon components and ionic components are removed from the pretreated water to produce primary pure water; and a secondary treatment step in which impurities are removed from the primary pure water to produce ultrapure water, and a second raw water, including treated sewage water, is merged with the raw water or the pretreated water upstream of the primary treatment step.

[0008] In this method for producing ultrapure water, in addition to raw water made from tap water, second raw water containing wastewater from a sewage treatment plant is used to produce ultrapure water, so the proportion of tap water in the raw water used to produce ultrapure water can be reduced.

[0009] In a second aspect, in the ultrapure water manufacturing method according to the first aspect, the amount of the second raw water to be mixed with the raw water or the pretreated water is set so that the weighted average of the urea concentration in the raw water or the pretreated water and the urea concentration in the second raw water is equal to or less than the urea concentration required for the ultrapure water.

[0010] In this method for producing ultrapure water, the amount of second raw water to be mixed with the raw water is set according to the urea concentration required for the ultrapure water, so that the urea concentration in the ultrapure water can be kept below the standard.

[0011] In a third aspect, in the method for producing ultrapure water according to the first or second aspect, when the urea concentration of the second raw water exceeds an allowable value determined based on the urea concentration required for the ultrapure water, the second raw water is passed through a urea decomposition device to reduce the urea concentration to below the allowable value.

[0012] In this method for producing ultrapure water, if the urea concentration of the second raw water exceeds an allowable value determined by the urea concentration required for the ultrapure water, the second raw water is passed through a urea decomposition device to reduce the urea concentration to or below the allowable value, and then the second raw water is merged with the raw water that is made from clean water. This allows for more effective use of sewage.

[0013] The ultrapure water manufacturing apparatus according to a fourth aspect comprises a pretreatment section that removes suspended matter from raw water, including drinking water, to obtain pretreated water; a primary treatment section that removes total organic carbon components and ionic components from the pretreated water to produce primary pure water; a secondary treatment section that removes impurities from the primary pure water to produce ultrapure water; and a urea meter that measures the urea concentration of second raw water, including sewage treatment plant water, and the second raw water, whose urea concentration is below an allowable value, is merged with the raw water or the pretreated water upstream of the primary treatment section.

[0014] In this ultrapure water production system, ultrapure water is produced using not only raw water made from tap water but also second raw water containing wastewater from a sewage treatment plant and having a urea concentration below the allowable value, thereby reducing the proportion of tap water in the raw water used to produce ultrapure water.

[0015] In a fifth aspect, the ultrapure water producing system according to the fourth aspect further comprises a urea decomposition device that decomposes urea in the second raw water, and a urea decomposition section that passes the second raw water through the urea decomposition device to reduce the urea concentration when the urea concentration exceeds an allowable value determined by the urea concentration required for the ultrapure water.

[0016] In this ultrapure water production system, when the urea concentration of the second raw water exceeds an allowable value determined by the urea concentration required for the ultrapure water, the second raw water is passed through a urea decomposition device to reduce the urea concentration to or below the allowable value, and then the second raw water is merged with the raw water made from clean water. Therefore, sewage can be used more effectively.

[0017] In a sixth aspect, in the ultrapure water manufacturing apparatus according to the fourth or fifth aspect, the amount of the second raw water to be mixed with the raw water or the pretreated water is set so that the weighted average of the urea concentration in the raw water or the pretreated water and the urea concentration in the second raw water is equal to or less than an allowable value determined by the urea concentration required for the ultrapure water.

[0018] In this ultrapure water production system, the amount of second raw water to be mixed with the raw water is set according to the urea concentration required for the ultrapure water, so the urea concentration in the ultrapure water can be kept below the standard.

[0019] According to the present disclosure, the proportion of clean water in the raw water used to produce ultrapure water can be reduced.

[0020] Fig. 1 is a block diagram showing an overview of an ultrapure water production apparatus according to an embodiment of the present invention. Fig. 2 is a block diagram showing an overview of an ultrapure water production apparatus according to a modified example of the embodiment. Fig. 3 is a block diagram showing an example of the configuration of a urea meter. Fig. 4 is a diagram showing a change in the urea concentration of second raw water over time and an example of the receiving conditions for the second raw water.

[0021] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. Components indicated by the same reference numerals in each drawing are the same or similar components. Note that duplicated explanations and reference numerals may be omitted in the embodiments described below. Furthermore, all drawings used in the following description are schematic, and the dimensional relationships, ratios, etc. of each element shown in the drawings do not necessarily match those in reality. Furthermore, the dimensional relationships, ratios, etc. of each element between multiple drawings do not necessarily match.

[0022] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0023] 1, an ultrapure water production system 100 according to this embodiment can produce ultrapure water by receiving raw water derived from natural sources, such as city water or industrial water, as well as second raw water derived from sewage. The ultrapure water production system 100 includes a pretreatment unit 10, a primary treatment unit 11, a secondary treatment unit 12, and a urea meter 30. The ultrapure water production system 100 may further include a urea decomposition unit 13.

[0024] The pretreatment unit 10 removes suspended solids from raw water (first raw water), including drinking water, to obtain pretreated water. For example, impurities such as residual chlorine and large debris are filtered from the raw water using a sand filter filled with media such as filter sand, a packed tank filter tower such as an MMF (multimedia filter tower), a membrane filter using filters such as an MF (microfilter) or an UF (ultrafiltration membrane), and / or a granular activated carbon tower. The raw water, including drinking water, is so-called tap water obtained by subjecting natural water 14, such as river water, lake water, or well water, to a coagulation and sedimentation process 17 and a filtration process 18 at a drinking water treatment plant 16. The raw water is stored, for example, in a raw water tank 20 and used to produce ultrapure water.

[0025] The primary treatment unit 11 is a device that produces primary pure water by removing total organic carbon (TOC) components and ionic components from the pretreated water. For example, the primary pure water is purified by combining a reverse osmosis membrane device that uses a semipermeable membrane to perform reverse osmosis on the pretreated water, an ion exchange resin device filled with ion exchange resin, an electrodeionization device (EDI), an ultraviolet irradiation device that decomposes organic matter, and a degassing membrane device that degasses dissolved gases. The primary treatment unit 11 may also be equipped with a urea decomposition device, such as a high-pressure reverse osmosis membrane device or an oxidation reaction tank that decomposes urea by adding hypobromous acid, as needed. The produced primary pure water is supplied to the pure water tank 22.

[0026] The secondary treatment unit 12 is a device that further removes impurities from the primary pure water to produce ultrapure water. For example, trace amounts of inorganic ions contained in the primary pure water are removed using a non-regenerative ion exchange tower (polisher). It also includes an ultraviolet irradiation unit, a hydrogen peroxide decomposition unit, a degassing unit, and other units. An ultrafiltration membrane is installed at the end of the secondary treatment unit 12 to remove fine particles. The produced ultrapure water is supplied to a POU (point of use) 19 for use in semiconductor manufacturing and the like, and a portion is circulated to a pure water tank 22 via a circulation line 28.

[0027] The second raw water is water containing treated sewage water obtained by subjecting sewage 24 to, for example, a biological treatment process 27 in a sewage treatment plant 26. The second raw water, whose urea concentration is equal to or lower than an allowable value determined based on the urea concentration required for ultrapure water, merges with raw water or pretreated water upstream of the primary treatment unit 11. In the example shown in FIG. 1 , the second raw water merges with the raw water and is stored in the raw water tank 20. In the modified example shown in FIG. 2 , the second raw water merges with the pretreated water between the pretreatment unit 10 and the primary treatment unit 11. Because the pretreatment unit 10 is not a device intended for removing urea and because the removal of suspended solids performed in the pretreatment unit 10 has already been performed in the sewage treatment plant, the raw water and the second raw water may be merged at the position of modified example 2.

[0028] The urea meter 30 measures the urea concentration of the second raw water, which includes treated sewage water, and for example, the one shown in FIG. 3 can be used. In this example, the urea meter 30 is made up of a pretreatment device 36 and a TOC meter 54. The pretreatment device 36 is made up of, for example, an ultraviolet irradiation device, a reverse osmosis membrane device, or an ion exchange device, and since the second raw water contains urea and TOC components other than urea, for example, the TOC meter 54 removes the TOC components other than urea. The TOC meter 54 is made up of a decarbonation device 56, an ultraviolet oxidation device 58, a CO ２ Detector 60, and CO ２ The TOC meter 54 is a device for measuring the concentration of urea in the second raw water from which TOC components other than urea have been removed, as TOC. The liquid to be measured is not limited to the second raw water, which is treated sewage water, but can also be the second raw water treated in the urea decomposition device 32, the ultrapure water obtained in the secondary treatment unit 12, or any liquid obtained in any intermediate process of the ultrapure water producing system 100.

[0029] The decarbonation device 56 of the TOC meter 54 removes carbon dioxide from the liquid to be measured by using, for example, a hollow fiber (vacuum degassing) module. Specifically, phosphoric acid is added to the liquid to be measured, and then the liquid is passed through a gas-permeable membrane with one side evacuated. Note that the liquid to be measured sent to the TOC meter 54 has a low concentration of carbon dioxide and a high concentration of CO ２ If the effect on the detection of carbon dioxide is small, removal of carbon dioxide by the carbon dioxide remover 56 may be omitted.

[0030] Furthermore, the TOC meter 54 may be configured without the decarbonation device 56. In this case, if it is necessary to remove carbon dioxide from the liquid to be measured, the decarbonation device 56 may be provided, for example, before (upstream of) the TOC meter 54. Examples of such a decarbonation device 56 include a degassing device, and possible examples include a vacuum degassing membrane, a degassing tower, and a vacuum degassing tower, but a vacuum degassing membrane is preferred.

[0031] Before supplying the liquid to be measured to the decarbonation device 56, the pH may be adjusted as necessary. When a degassing device is used for the decarbonation device 56, it is preferable to acidify the liquid to be measured (for example, pH 5 or less) because this increases the carbon dioxide removal rate. The pH can be adjusted, for example, by adding a known acid. Furthermore, if a reverse osmosis membrane (RO) or an ion exchange device is used for the aforementioned filtration device 50, hydroxide ions can be removed if present in the liquid to be measured, and so at least part of the pH adjustment can be performed.

[0032] Inside the TOC meter 54, the flow path of the liquid to be measured is branched downstream of the decarbonation device 56, and one of the flow paths is connected to an oxidation device 58 and a CO ２ A detection device 60 is provided, and a CO ２ A detection device 62 is provided.

[0033] The oxidizer 58 of the TOC meter 54 oxidizes the organic matter contained in the liquid to be measured, including urea, by, for example, irradiating the liquid to be measured with ultraviolet light. In essence, most of the organic matter contained in the liquid to be measured other than urea has been removed, so the organic matter oxidized by the oxidizer 58 is urea. That is, in the oxidizer 58, the urea in the liquid to be measured is oxidized and decomposed, and carbon dioxide is generated again at this stage.

[0034] Total TOC of 54 CO ２ The detectors 60 and 62 measure the amount of carbon dioxide in the liquid to be treated. The amount of carbon dioxide is proportional to the amount of carbon contained in the organic matter in the liquid to be measured that has been oxidatively decomposed.

[0035] where CO ２Upstream of the detector 60, organic matter in the liquid to be measured is oxidized by the oxidizer 58, and CO ２ The detector 60 detects the total carbon concentration of the liquid to be measured. ２ On the upstream side of the detector 62, the organic matter in the liquid to be measured is not oxidized by the oxidizer 58, and CO ２ The detector 62 detects the inorganic carbon concentration of the liquid to be measured. The TOC meter 54 calculates the CO ２ From the total carbon concentration detected by the detector 60, CO ２ The total organic carbon concentration of the liquid to be measured is obtained by subtracting the inorganic carbon concentration detected by the detector 62. Total organic carbon concentration = total carbon concentration - inorganic carbon concentration (1)

[0036] If the carbon dioxide removal rate of the decarbonation device 56 is high, some of the urea may be removed before it reaches the oxidation device 58 of the TOC meter 54. In such a case, by checking these urea removal rates in advance and correcting the urea concentration measurement results by the TOC meter 54 accordingly, the urea concentration can be determined with even greater accuracy.

[0037] Furthermore, the specific configuration of the TOC meter 54 is not limited to that described above, and various types of TOC meters can be used in this embodiment.

[0038] The liquid to be treated after the total organic carbon concentration has been measured by the TOC meter 54 may be discarded as waste liquid or may be returned to the second raw water line, for example.

[0039] The urea concentration measurement process in the TOC meter 54 may be a continuous type in which the measurement target liquid is continuously flowed, or a batch type in which the measurement target liquid is temporarily stored.

[0040] Even when the urea concentration measurement process in the TOC meter 54 is performed batchwise, the measurement target liquid may be supplied more frequently or continuously than in the measurement by the TOC meter 54. Also, the measurement target liquid may be supplied at a flow rate greater than that required for the urea concentration measurement process in the TOC meter 54. In this case, the measurement target liquid that is not supplied to the TOC meter 54 may be discharged from a drain pipe provided immediately before the TOC meter 54, for example.

[0041] Supplying the liquid to be measured into the pipe at a high flow rate and / or frequently or continuously can suppress the generation of bacteria and the adhesion of organic matter to the side walls of the pipe. The generated bacteria and the attached organic matter may detach from the side walls at unexpected times and flow into the liquid to be measured, reducing the accuracy of the urea concentration measurement. The urea meter 30 is not limited to the above-mentioned one, and a commercially available device may also be used.

[0042] The urea decomposition unit 13 includes a urea decomposition device 32 that decomposes urea in the second raw water. When the urea concentration in the second raw water exceeds an allowable value determined based on the urea concentration required for ultrapure water, the second raw water is passed through the urea decomposition device 32 to reduce the urea concentration. The urea decomposition unit 13 may include a urea meter 31, similar to the urea meter 30 described above, downstream of the urea decomposition device 32 to measure the urea concentration in the second raw water that has passed through the urea decomposition device 32.

[0043] As a means for decomposing urea in the urea decomposition unit 32, for example, the following methods can be used, and an appropriate method can be selected depending on the size of the pure water unit and the quality of the second raw water.

[0044] 1) Biological treatment 2) Biological activated carbon treatment 3) Oxidative decomposition treatment using oxidizing agents (bromic acid, hypobromous acid, etc.) 4) Urea decomposition treatment using reducing agents (sulfurous acid) 5) High-pressure reverse osmosis (RO) treatment 6) Ultraviolet irradiation treatment and treatment using ultraviolet irradiation and an oxidizing agent in combination

[0045] Methods 1 and 2 use living organisms, so stable treatment is an issue, but when combined with this method, ultrapure water production can continue even if the performance of the biological treatment deteriorates. Also, if a pit is installed before the biological treatment tank and the urea in the second raw water is gradually decomposed in the biological treatment tank, stable treatment can be expected.

[0046] In the third method, a large amount of oxidizing agent is generally added, and a large amount of oxidizing agent remains after the reaction, which must be removed, resulting in a large-scale device. However, in this embodiment, the urea decomposition device 32 is provided in the branch line of the second raw water, making it possible to reduce the size of the device. Furthermore, by providing a pit upstream of the oxidative decomposition device and decomposing the second raw water little by little, it is possible to reduce the size of the device.

[0047] Methods 4, 5, and 6 require large-scale equipment, and method 5 has issues with the removal rate. However, in this embodiment, the urea decomposition device 32 is provided in the branch line of the second raw water, making it possible to reduce the size of the equipment.

[0048] Method 4 is carried out, for example, using an ultraviolet irradiation device as follows. Oxidative decomposition is promoted by irradiating ultraviolet light onto water to be treated (second raw water) to which a reducing agent has been added. The wavelength of the ultraviolet light may be any wavelength that is capable of oxidizing the water to be treated. For example, ultraviolet light with a wavelength of approximately 185 nm, which is generally used for ultraviolet oxidation, or ultraviolet light with a wavelength of approximately 254 nm, which is used for sterilization, may be used.

[0049] It is believed that the reducing agent added to the water to be treated is converted into persulfate radicals or similar active species by the action of ultraviolet light, and the generated active species then decomposes urea.

[0050] As a method other than the above 1) to 6), it is also possible to use a urease or a catalyst carrying a urease.

[0051] The amount of second raw water to be mixed with the raw water or pretreated water, i.e., the amount of second raw water to be accepted for ultrapure water production, is set so that the weighted average of the urea concentrations in the raw water or pretreated water and the second raw water (i.e., the urea concentration after mixing) is equal to or less than the allowable value determined by the urea concentration required for ultrapure water (urea spec). The amount of second raw water to be accepted in the raw water tank 20 may be determined by back-calculating, taking into account the urea removal rate from the confluence point to the secondary treatment unit 12. In other words, the weighted average of the urea concentrations may be calculated by back-calculating the urea concentration at the confluence point of the raw water and the second raw water from the urea concentration required for ultrapure water, and the amount of second raw water to be accepted in the raw water tank 20 may be set. Note that if the urea concentration of the second raw water is equal to or less than the allowable value, it can be accepted for ultrapure water production without being mixed with the raw water. In other words, ultrapure water can be produced even if the second raw water is 100% pure.

[0052] Alternatively, second raw water having a urea concentration exceeding the allowable value may be stored in the second raw water pit 38, and the treated water from the urea decomposition device 32 may be returned to the second raw water pit 38 via a return line (not shown) to the second raw water pit 38 and circulated. This reduces the urea concentration of the second raw water in the second raw water pit 38, and once the value on the urea meter 30 has dropped sufficiently, the second raw water may be supplied to the raw water tank 20 or the primary treatment unit 11. If the urea concentration in the second raw water decreases during this circulation operation, the second raw water may be supplied to the raw water tank 20 or the primary treatment unit 11 via a bypass line (not shown) of the second raw water pit 38.

[0053] If the urea concentration in the second raw water is extremely high and the urea cannot be decomposed even by using the urea decomposition device 32, the water may be supplied to the cooling tower or the like 34 via the cooling tower supply line 40 and used as water for the cooling tower or the like. The cooling tower supply line 40 may be installed so as to branch off from the upstream stage of the second raw water pit 38.

[0054] Furthermore, the quality of raw water, including drinking water, may temporarily fluctuate significantly, for example, immediately after a typhoon. It is possible to respond to such a situation. In this case, for example, the amount of second raw water used can be increased. Alternatively, the supply line 42 for raw water, including drinking water, to the urea decomposition unit 32 can be used to decompose urea in the raw water, including drinking water, and use the decomposed water as raw water. In this case, it is possible to respond to a temporary deterioration in the quality of raw water, including drinking water, without installing any special equipment.

[0055] FIG. 4 shows an example of the change in urea concentration of sewage (second raw water) over time and the conditions for accepting the second raw water. Assuming that the urea removal rate of the ultrapure water production system 100 is 90%, the urea concentration of the raw water is 0.1 ppb, and the urea concentration required for ultrapure water (urea spec) is 0.5 ppb, the second raw water can be fully accepted for ultrapure water production as long as the urea concentration of the second raw water is 5 ppb or less. The "Total Acceptable" in FIG. 4 indicates this range. Furthermore, if the raw water and second raw water are mixed at a ratio of 9:1 and second raw water exceeding this ratio is not accepted, the urea concentration of the second raw water can be up to 49 ppb. The "Mixed Acceptable" in FIG. 4 indicates this range. In this case, 43% of the raw water required for ultrapure water production can be second raw water.

[0056] [Ultrapure Water Production Method] The ultrapure water production method according to this embodiment includes a pretreatment step in which suspended solids are removed from raw water, including drinking water, to obtain pretreated water; a primary treatment step in which total organic carbon (TOC) components and ionic components are removed from the pretreated water to produce primary pure water; and a secondary treatment step in which impurities are removed from the primary pure water to produce ultrapure water. Second raw water, which includes treated sewage water and has a urea concentration below an allowable value determined by the urea concentration required for ultrapure water, is merged with the raw water or pretreated water upstream of the primary treatment unit.

[0057] Here, if the urea concentration of the second raw water exceeds an allowable value determined by the urea concentration required for ultrapure water, the second raw water may be passed through a urea decomposition device to reduce the urea concentration to below the allowable value.

[0058] In addition, the amount of second raw water to be merged with the raw water or pretreated water may be set so that the weighted average of the urea concentration in the raw water or pretreated water and the urea concentration in the second raw water is equal to or lower than the urea concentration required for ultrapure water.

[0059] (Operation) This embodiment is configured as described above, and its operation will be described below. The ultrapure water production system 100 according to this embodiment produces ultrapure water using second raw water containing treated sewage water in addition to raw water made from tap water, so that the proportion of tap water in the raw water used for producing ultrapure water can be reduced.

[0060] Furthermore, by using the urea decomposition unit 13, when the urea concentration of the second raw water exceeds the allowable value required for ultrapure water, the second raw water is passed through the urea decomposition device 32 to reduce the urea concentration to the allowable value or below, and then the second raw water is merged with the raw water made from clean water. Therefore, sewage can be used more effectively.

[0061] Furthermore, the amount of second raw water to be mixed with the raw water is set according to the urea concentration required in the ultrapure water, so that the urea concentration in the ultrapure water can be kept below the standard.

[0062] Furthermore, when the urea decomposition device 32 is installed before mixing the raw water and the second raw water, it can be made smaller than when the urea decomposition device 32 is installed after mixing the raw water and the second raw water. In particular, when the urea decomposition device 32 is installed closer to the primary treatment unit 11 or pretreatment unit 10 than the confluence of the raw water and the second raw water, it must be designed so that the urea concentration of the ultrapure water meets specifications even when the urea concentration reaches its maximum value. This requires a very large urea decomposition device 32, which increases the installation area and running costs. On the other hand, in the embodiment shown in FIG. 1 , this can be achieved by using a minimal amount of equipment, taking into account the utilization rate of the second raw water. This ratio can be roughly 1 / 2 to 1 / 10, depending on the sewage water quality, water quality fluctuations, and the target sewage utilization rate.

[0063] Furthermore, even if the urea concentration of the second raw water becomes higher than designed, the second raw water can be used in the cooling tower, etc. 34, and the ultrapure water production system 100 can operate using only the first raw water, which has the advantage that the production of ultrapure water does not have to be stopped.

[0064] Furthermore, since the second raw water is used by mixing the raw water with the second raw water, it is possible to minimize the amount of new equipment that needs to be installed.

[0065] As described above, according to this embodiment, it is possible to reduce the proportion of clean water in the raw water used to produce ultrapure water. This also makes it possible to reduce the amount of clean water used in semiconductor manufacturing. In other words, it is possible to provide an improved method for producing ultrapure water.

[0066] In addition to urea, other persistent substances include urea derivatives and organic fluorine compounds such as perfluorooctasulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), and the present disclosure can also be suitably applied to cases where these are included.

[0067] [Other Embodiments] Although one example of an embodiment of the present disclosure has been described above, the embodiment of the present disclosure is not limited to the above, and it goes without saying that various modifications can be made without departing from the spirit of the present disclosure.

[0068] The disclosure of Japanese Patent Application No. 2023-166244, filed on September 27, 2023, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

[0069] 10 Pretreatment section 11 Primary treatment section 12 Secondary treatment section 13 Urea decomposition section 19 POU (point of use) 20 Raw water tank 30 Urea meter 32 Urea decomposition device 100 Ultrapure water production device

Claims

1. A method for producing ultrapure water comprising: a pretreatment step of removing suspended solids from raw water, including drinking water, to obtain pretreated water; a primary treatment step of removing total organic carbon components and ionic components from the pretreated water to produce primary pure water; and a secondary treatment step of removing impurities from the primary pure water to produce ultrapure water, in which a second raw water, including sewage treatment water, is merged with the raw water or the pretreated water upstream of the primary treatment step.

2. A method for producing ultrapure water as described in claim 1, wherein the amount of the second raw water to be mixed with the raw water or the pretreated water is set so that the weighted average of the urea concentration in the raw water or the pretreated water and the urea concentration in the second raw water is equal to or less than an allowable value determined by the urea concentration required for the ultrapure water.

3. A method for producing ultrapure water as described in claim 1 or claim 2, characterized in that when the urea concentration of the second raw water exceeds an allowable value determined by the urea concentration required for the ultrapure water, the second raw water is passed through a urea decomposition device.

4. An ultrapure water producing apparatus comprising: a pretreatment section for removing suspended matter from raw water, including drinking water, to obtain pretreated water; a primary treatment section for removing total organic carbon components and ionic components from the pretreated water to produce primary pure water; a secondary treatment section for removing impurities from the primary pure water to produce ultrapure water; and a urea meter for measuring the urea concentration of second raw water, including sewage treatment plant water, wherein the second raw water is merged with the raw water or the pretreated water upstream of the primary treatment section.

5. An ultrapure water producing system as described in claim 4, further comprising a urea decomposition unit which is provided with a urea decomposition unit for decomposing urea in the second raw water, and which, when the urea concentration exceeds an allowable value required by the urea concentration required for the ultrapure water, passes the second raw water through the urea decomposition unit to reduce the urea concentration.

6. The ultrapure water producing apparatus of claim 4, wherein the amount of the second raw water to be mixed with the raw water or the pretreated water is set so that the weighted average of the urea concentration in the raw water or the pretreated water and the urea concentration in the second raw water is below an allowable value determined by the urea concentration required for the ultrapure water.

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