JP2024063499A - Pure water production method, pure water production device, and ultrapure water production system - Google Patents

Abstract

[Problem] To provide a pure water production method and pure water production apparatus that can reduce the amount of chemicals used by reducing the processing load of three-stage reverse osmosis membrane devices on the front side, and can efficiently produce pure water from which the boron concentration has been removed. [Solution] A pure water production method in which raw water is treated with at least three stages of ultra-low pressure reverse osmosis membrane devices to remove boron from the raw water, in which the reverse osmosis membranes are all negatively charged membranes with a skin layer made of crosslinked aromatic polyamide, and the raw water contains 1 to 100 mg/L of carbon dioxide and 150 μg/L or less of boron, and the method includes the steps of treating the raw water with the first-stage reverse osmosis membrane device, obtaining alkaline water to be treated, treating the alkaline water to be treated with the second-stage reverse osmosis membrane device, and treating the permeate through the third-stage reverse osmosis membrane device to obtain pure water with a boron concentration of 3 to 20 μg/L and a conductivity of 0.3 to 40 μS/cm. [Selected Figure] Figure 4

Description

The present invention relates to a pure water production method and apparatus that can remove boron from raw water and significantly reduce the boron concentration, as well as an ultrapure water production system that can use these to obtain high-quality ultrapure water.

A method for producing pure water by treating raw water containing boron is known in which an alkali is added to the raw water to adjust the pH to 9.2 or more, and then reverse osmosis membrane processing is performed. In this method, a non-regenerative ion exchange device that does not perform chemical regeneration is placed in the latter stage, and in order to reduce the load on this non-regenerative ion exchange device, an acid is added to the permeate from which boron has been removed, and further, a special reverse osmosis membrane called a positive charge reverse osmosis membrane device is used to perform reverse osmosis membrane processing, thereby increasing the resistivity of the treated water to, for example, 5.0 MΩ·cm. In addition, in a reverse osmosis membrane device that treats alkaline treated water, scale blockage due to hardness is likely to progress, so scale blockage is suppressed by providing a hardness removal mechanism and a deaeration device with high decarbonation function in the upstream stage of the reverse osmosis membrane device (see, for example, Patent Documents 1 and 2).

In addition, a method has been proposed in which, without using acid for decarbonation, a membrane degassing device is combined with a three-stage reverse osmosis membrane device, a scale inhibitor and an alkali are added to the raw water before it is passed through the first-stage reverse osmosis membrane separation device, and a reverse osmosis membrane with a high salt rejection rate in the low salt concentration range is used as the reverse osmosis membrane of the third-stage reverse osmosis membrane separation device to produce pure water with a high purity of 15 MΩ cm (see, for example, Patent Document 3).

JP 11-128921 A JP 11-128922 A JP 2000-061465 A

However, in the conventional pure water production method described above, although the load of ion removal on the downstream side of the reverse osmosis membrane is reduced, the load of ion removal on the multiple reverse osmosis membranes on the upstream side is extremely high. For example, in a positively charged reverse osmosis membrane that treats acidic water to be treated, scale and fine particles due to silica are likely to adhere, so the higher the ion removal load, the more frequently backwashing, cleaning, installation of cleaning equipment, and membrane replacement are required, and the equipment must be stopped each time, resulting in a problem of reduced efficiency in pure water production. In addition, there is also the problem that the scale of the equipment becomes large and a large site is required for installation because a hardness removal mechanism, degassing device, and membrane degassing device are installed on the upstream side. Furthermore, there is a problem that a large amount of chemicals, such as acids added for degassing and ion component removal, and detergents for frequent cleaning and backwashing of the reverse osmosis membrane equipment, are used, resulting in a high environmental load.

In addition, the pure water produced by the conventional method using the three-stage reverse osmosis membrane is of insufficient quality to be used directly in the manufacture of semiconductors, liquid crystals, etc. Therefore, in order to obtain water quality usable in the manufacture of semiconductors, liquid crystals, etc., further deionization processing is required in the subsequent stage, for example, by a mixed bed ion exchange resin tower or an electric deionization device. In these deionization processing devices, processing can be performed as long as the required water quality in the device specifications is met, but the high water quality obtained by the conventional method is not essential in the processing in the previous stage of the device, so there is a problem that the equipment for the processing in the previous stage is excessive and wasteful. For example, if a degassing device (degassing tower) is provided in the previous stage of the three-stage reverse osmosis membrane to remove carbon dioxide, a pump is required to supply the treated water from the degassing tower to the subsequent stage, which makes the device even larger. In addition, it is possible to improve the water quality by using a high-pressure reverse osmosis membrane, but in this case, operation at high pressure is required, and the problem is that power consumption is high.

The present invention aims to provide a method and apparatus for producing pure water using an ultra-low pressure reverse osmosis membrane device, which can produce pure water that can be supplied to regenerative deionization devices such as mixed bed ion exchange resin devices and electrical deionization devices, thereby improving the efficiency of producing pure water from which boron has been removed.

In summary, the present invention has been made to solve the above-mentioned problems, and aims to provide a pure water production method, a pure water production apparatus, and an ultrapure water production system using the same, which can reduce the amount of chemicals used by reducing the processing load on the upstream reverse osmosis membrane device and efficiently produce pure water from which boron has been removed.

The pure water production method of the present invention is a method for treating raw water with at least three stages of ultra-low pressure reverse osmosis membrane devices to remove boron from the raw water, and the reverse osmosis membranes provided in the three stages of reverse osmosis membrane devices are all negatively charged membranes having a skin layer made of crosslinked aromatic polyamide, the raw water contains 1 mg/L to 100 mg/L of carbon dioxide and 150 μg/L or less of boron, and the method includes the steps of treating the raw water with a first stage reverse osmosis membrane device to obtain a first permeate, adjusting the first permeate to an alkaline state to obtain alkaline water to be treated, treating the alkaline water to be treated with a second stage reverse osmosis membrane device to obtain a second permeate, and treating the second permeate with a third stage reverse osmosis membrane device to obtain pure water with a boron concentration of 3 μg/L to 20 μg/L and a conductivity of 0.3 μS/cm to 40 μS/cm.

In the pure water production method of the present invention, the pH of the alkaline water to be treated is preferably 9.0 or more and 10.0 or less.

The pure water production method of the present invention further includes a step of adding an acid to the second permeate to obtain a second treated water, and it is preferable that the second treated water is treated in a third stage reverse osmosis membrane device.

In the pure water production method of the present invention, it is preferable that the raw water contains chlorine, and in the step of obtaining the second treated water, sulfamic acid is added to the second permeate, and the concentrated water from the third stage reverse osmosis membrane device is mixed with the raw water and treated in the first reverse osmosis membrane device.

In the pure water production method of the present invention, it is preferable that the pH of the second treated water is 5.5 or more and 7.5 or less.

In the pure water production method of the present invention, it is preferable that the pure water obtained as the permeate of the third stage reverse osmosis membrane device is further treated with an electrodeionization device.

The pure water production apparatus of the present invention has a first reverse osmosis membrane device, a second reverse osmosis membrane device, and a third reverse osmosis membrane device connected in series, and is a pure water production apparatus for removing boron, the first reverse osmosis membrane device, the second reverse osmosis membrane device, and the third reverse osmosis membrane device are ultra-low pressure reverse osmosis membrane devices, the reverse osmosis membranes provided in the first reverse osmosis membrane device, the second reverse osmosis membrane device, and the third reverse osmosis membrane device are negatively charged membranes having a skin layer made of crosslinked aromatic polyamide, and the pure water production apparatus removes boron from raw water containing 1 mg/L or more and 100 mg/L or less of carbon dioxide and 150 μg/L or less of boron. The system has a raw water supply mechanism that supplies water to a first reverse osmosis membrane device, a first supply pipe that sends the permeate of the first reverse osmosis membrane device to the second reverse osmosis membrane device, an alkali adjustment mechanism that is provided in the path of the first supply pipe and adjusts the alkalinity of the water to be treated flowing through the first supply pipe, and a second supply pipe that sends the permeate of the second reverse osmosis membrane device to the third reverse osmosis membrane device, and is characterized in that the permeate of the third reverse osmosis membrane device is pure water having a boron concentration of 3 μg/L to 20 μg/L and a conductivity of 0.3 μS/cm to 40 μS/cm.

The pure water production system of the present invention preferably has an electrical deionization device downstream of the third reverse osmosis membrane device.

The ultrapure water production system of the present invention is an ultrapure water production system comprising a primary pure water apparatus and a secondary pure water apparatus in this order, wherein the primary pure water apparatus comprises the pure water production apparatus described in claim 7 or 8 and an electrical deionization apparatus arranged downstream of the pure water production apparatus, and the secondary pure water apparatus comprises an ultraviolet oxidation apparatus, a non-regenerative polisher, a membrane degassing apparatus and an ultrafiltration apparatus in this order, and produces ultrapure water having a boron concentration of 0.1 μg/L or less.
In this specification, the symbol "~" indicates a range of values from the value to the left of the symbol to the value to the right of the symbol.

According to the pure water producing method and pure water producing apparatus of the present invention, while using three upstream reverse osmosis membrane devices, the processing load of these upstream reverse osmosis membrane devices can be reduced, thereby reducing the amount of chemicals used and efficiently producing pure water from which the boron concentration has been removed.
According to the ultrapure water producing system of the present invention, the amount of chemicals used in a pure water producing apparatus that uses a three-stage reverse osmosis membrane device on the upstream side can be reduced, so that ultrapure water of high quality can be produced efficiently.

1 is a flow diagram illustrating a method for producing pure water according to an embodiment of the present invention; FIG. 2 is a flow diagram illustrating a schematic diagram of a method for producing pure water, in which the pure water obtained by the method for producing pure water shown in FIG. 1 is further treated. FIG. 11 is a flow chart illustrating a modified example of a pure water producing method. 1 is a block diagram illustrating a pure water producing system according to an embodiment. FIG. 11 is a block diagram illustrating a pure water producing system according to another embodiment. 1 is a block diagram illustrating an ultrapure water producing system according to an embodiment. FIG. 1 is a block diagram showing a schematic diagram of a pure water producing system used in the examples. 1 is a graph showing the carbon dioxide concentrations in the feed water and treated water of each stage of the reverse osmosis membrane device in the methods of the examples and comparative examples. 1 is a graph showing the boron concentration in the feed water and treated water of each stage of the reverse osmosis membrane device in the methods of the examples and comparative examples. 1 is a graph showing the electrical conductivity of the feed water and treated water of each stage of the reverse osmosis membrane device in the methods of the examples and comparative examples.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a flow diagram that shows a schematic diagram of a pure water production method 100 according to an embodiment of the present invention. In the pure water production method 100 shown in FIG. 1, raw water is treated in at least three stages of ultra-low pressure reverse osmosis membrane devices. The pure water production method 100 is suitable for producing pure water to be supplied to, for example, an electric deionization device or a regenerative ion exchange resin device.

The pure water production method 100 includes a reverse osmosis membrane treatment process 101 in which raw water is treated with a first-stage reverse osmosis membrane device to obtain permeate W10 (first permeate), an alkali adjustment process 102 in which the permeate W10 is adjusted to be alkaline to obtain alkaline water to be treated W11, a reverse osmosis membrane treatment process 103 in which the alkaline water to be treated W11 is treated with a second-stage reverse osmosis membrane device to obtain permeate W20 (second permeate), and a reverse osmosis membrane treatment process 104 in which the permeate W20 is treated as water to be treated and subjected to a third-stage reverse osmosis membrane treatment to obtain permeate W30. The permeate W30 is the pure water produced by the pure water production method 100.

The raw water used in the pure water production method 100 of this embodiment is, for example, city water, well water, industrial water, etc. Also, the raw water may be used recovered water that has been used at a place where ultrapure water is used, recovered, and then subjected to chemical removal treatment or the like as necessary. The raw water (or pretreated water described later) provided to the reverse osmosis membrane treatment process 101 contains 1 mg/L to 100 mg/L of carbonic acid and 150 μg/L or less of boron. The boron concentration in the raw water is preferably 5 μg/L or more, and within this range, the effects of the present invention are easily obtained. The raw water may also contain, for example, hardness components such as calcium and magnesium, in a total amount of 10 mg/L to 300 mg/L in terms of calcium carbonate. The raw water may also contain, for example, silica (Si) of about 1 mg/L to 50 mg/L and chlorine of about 0.1 mg/L to 0.6 mg/L in terms of Cl. The pH of the raw water is, for example, about 5.0 to 7.5. Carbonic acid includes carbon dioxide, bicarbonate ions, and carbonate ions, and the carbonate concentration is a value obtained by converting the concentration of total carbonic acid (CO ₂ +HCO ₃ ⁻ +CO ₃ ²⁻ ) into a CO ₂ concentration.

Instead of or in addition to the raw water supplied to the reverse osmosis membrane treatment process 101, pretreated water may be used, which is obtained by pretreating the raw water before use in the pure water production method 100. Examples of pretreatment include coagulation and sedimentation treatment, pressure flotation treatment, sand filtration treatment, and precision filtration treatment, which remove turbid matters such as suspended matter and colloidal matter from the raw water. Activated carbon treatment may be performed in the pretreatment, which removes chlorine from the water. In the pretreatment, the water temperature may be adjusted to a range of 15°C to 30°C by a heat exchanger.

All three stages of reverse osmosis membrane devices used in the pure water production method 100 are ultra-low pressure reverse osmosis membrane devices. Ultra-low pressure reverse osmosis membranes have operating pressures of, for example, 0.4 MPa to 1.1 MPa, and preferably 0.6 MPa to 0.7 MPa. Note that the operating pressure of the reverse osmosis membrane device is the design pressure at the time of manufacturing each reverse osmosis membrane, and in reality, it may be operated at a pressure outside the above range.

In the pure water production method 100, the supply pressure to the first-stage reverse osmosis membrane device is adjusted so that the water to be treated is supplied to each of the first to third reverse osmosis membrane devices at a water supply pressure of preferably 0.4 MPa to 1.1 MPa, more preferably 0.6 MPa to 0.7 MPa. The water supply pressure is expressed as a differential pressure obtained by subtracting the permeate pressure from the supply side pressure of the reverse osmosis membrane device (the average of the supply water pressure and the concentrated water pressure). In the reverse osmosis membrane treatment process 101, the raw water is treated with a reverse osmosis membrane to remove hardness components such as calcium and magnesium in the raw water. The concentrated water of the first-stage reverse osmosis membrane device is discharged outside the system, and the permeate W10 is sent to the subsequent stage. In the reverse osmosis membrane treatment process 101, a hardness component removal rate of 99% to 99.9% can be obtained. The water quality of the permeate W10 can be, for example, 0.1 mg/L to 3 mg/L of hardness components such as calcium and magnesium in total calculated as calcium carbonate. The water quality of the permeated water W10 is, for example, a carbonate concentration of 0.5 mg/L to 50 mg/L, a boron concentration of 3 μg/L to 120 μg/L, and a conductivity of 2 μS/cm to 10 μS/cm.

In the alkali adjustment step 102, the permeated water W10 obtained in the reverse osmosis membrane treatment step 101 is adjusted to be alkaline to obtain alkaline treated water W11. For example, in the alkali adjustment step 102, a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution, preferably a sodium hydroxide aqueous solution, is added to the permeated water 10 to generate the treated water W11. The pH of the treated water W11 is preferably 9.0 to 10.0, and the conductivity is preferably 10 μS/cm to 50 μS/cm. The carbon dioxide concentration and boron concentration of the treated water W11 are the same as those of the permeated water 10. By making the pH of the treated water W11 9.0 or higher, the boron removal rate in the subsequent reverse osmosis membrane treatment step 103 can be improved. In addition, by making the pH of the treated water W11 10.0 or lower, ion components can be sufficiently removed in the pure water production method 100 while reducing the amount of chemicals used. In addition, by keeping the pH of the water to be treated W11 at 10.0 or less, the amount of chemicals (alkali) used can be reduced, which reduces sodium leakage to downstream stages (leaking of sodium into the permeate water W20).

In the reverse osmosis membrane treatment process 103, the water to be treated W11 is supplied to the second-stage reverse osmosis membrane device and treated with the reverse osmosis membrane. The concentrated water from the second-stage reverse osmosis membrane device is discharged outside the system or returned to the upstream side of the first-stage reverse osmosis membrane device. The permeated water W20 from the second-stage reverse osmosis membrane device is sent to the downstream stage.

In the reverse osmosis membrane treatment process 103, as described above, the water to be treated that has been adjusted to be alkaline is treated, so the boron removal rate can be improved, and the boron removal rate in the reverse osmosis membrane treatment process 103 can reach 50% to 90%. In addition, in the reverse osmosis membrane treatment process 103, the carbon dioxide removal rate can also be improved, and the carbon dioxide removal rate can reach 95% to 98%. The water quality of the permeated water W20 is, for example, a carbon dioxide concentration of 0.025 mg/L to 2.5 mg/L, a boron concentration of 3 μg/L to 40 μg/L, a conductivity of 1 μS/cm to 40 μS/cm, and a pH of about 8.5 to 10. The boron removal rate is calculated by [1 - (boron concentration in the permeated water W20 / boron concentration in the water to be treated W11)] x 100 (%). The carbon dioxide removal rate is calculated by [1 - (carbon dioxide concentration in the permeated water W20 / carbon dioxide concentration in the water to be treated W11)] x 100 (%).

In the reverse osmosis membrane treatment process 104, as described above, the permeated water W20 from which boron and carbon dioxide have been removed is supplied to the third-stage reverse osmosis membrane device as the water to be treated, and is treated with the reverse osmosis membrane. The concentrated water from the third-stage reverse osmosis membrane device is discharged outside the system or returned to the upstream side of the first-stage reverse osmosis membrane device. The permeated water W30 from the third-stage reverse osmosis membrane device is sent to the downstream side as pure water and treated, or is supplied directly to the point of use (POU).

In the reverse osmosis membrane treatment process 104, ionic components in the water to be treated (permeate W20) are removed, such as anionic components such as chloride ions, sulfate ions, nitrate ions, fluoride ions, and bicarbonate ions, cationic components such as sodium ions and potassium ions, and weak electrolytes such as boron and silica. The quality of the permeate (pure water) W30 obtained in the reverse osmosis membrane treatment process 104 is such that the boron concentration is 3 μg/L to 20 μg/L, preferably 5 μg/L to 10 μg/L, the carbonate concentration is, for example, 0.005 mg/L to 0.5 mg/L, and the conductivity is 0.3 μS/cm to 40 μS/cm, preferably 1 μS/cm to 20 μS/cm, and more preferably 1 μS/cm to 10 μS/cm.

In order to obtain pure water of the above water quality, it is preferable that the water recovery rate in the three-stage reverse osmosis membrane device is 50% to 80% in the reverse osmosis membrane treatment process 101, 70% to 90% in the reverse osmosis membrane process 102, and 80% to 95% in the reverse osmosis membrane process 104.

In addition, the total boron removal rate through the three-stage reverse osmosis membrane device (steps 101, 102, 104) can be achieved at 40% to 95%.

Although the above describes a method in which reverse osmosis membrane treatment is performed in only three stages, reverse osmosis membrane treatment may be performed in four or more stages. In this case, a method in which reverse osmosis membrane treatment is performed upstream of the first stage reverse osmosis membrane device, a method in which reverse osmosis membrane treatment is performed between the first and second stage reverse osmosis membrane devices, a method in which reverse osmosis membrane treatment is performed between the second and third stage reverse osmosis membrane devices, a method in which reverse osmosis membrane treatment is performed downstream of the third stage reverse osmosis membrane device, or any combination of these methods can be adopted. In the additional reverse osmosis membrane treatment, any of ultra-low pressure, low pressure, and high pressure reverse osmosis membrane devices may be used. Among them, a method in which reverse osmosis membrane treatment is performed in an ultra-low pressure between the second and third stage reverse osmosis membrane devices is preferable, which can further reduce the boron concentration.

The operating pressure of a low-pressure reverse osmosis membrane is, for example, greater than 0.8 MPa and less than 2.5 MPa, preferably 1 MPa to 1.6 MPa. The operating pressure of a low-pressure reverse osmosis membrane is, for example, greater than 0.8 MPa and less than 2.5 MPa. The operating pressure of a high-pressure reverse osmosis membrane is, for example, greater than 2.5 MPa and 8 MPa or less. The operating pressures of the ultra-low pressure, low pressure, and high pressure reverse osmosis membrane devices described above are the design pressures at the time of manufacture of each reverse osmosis membrane, and in reality, they may be operated at pressures outside the above ranges.

According to the embodiment of the pure water production method 100 described above, an excellent carbon dioxide removal rate is achieved in the reverse osmosis membrane treatment process 103 performed in the second-stage reverse osmosis membrane device, so that a hardness removal mechanism and a degassing device for carbon dioxide removal in the previous stage can be omitted. This reduces the amount of chemicals used and simplifies the device, making it possible to produce pure water efficiently at low cost. Furthermore, if the water supply pressure in the three-stage reverse osmosis membrane treatment process is set to an ultra-low pressure as described above, the number and output of water supply pumps can be reduced, leading to further simplification of the device, reduced pure water production costs, and improved production efficiency.

Next, a preferred method for treating the permeated water W30 will be described. FIG. 2 is a flow diagram showing a schematic of a pure water production method 110 according to an embodiment. The pure water production method 110 is an example of a method for treating the permeated water W30 to produce pure water of higher purity. The pure water production method 110 includes an electrodeionization process 111 in which the permeated water W30 is treated with an electrodeionization device, an ultraviolet oxidation process 112 in which the desalted water W40 obtained in the electrodeionization process 111 is treated as the water to be treated by ultraviolet oxidation, and a non-regenerative ion exchange (Primary/Polisher) process 113 in which ionic components in the treated water of the ultraviolet oxidation process 112 are removed.

In the electrodeionization process 111, the permeate W30 is supplied to an electrodeionization device (EDI) to remove ionic components from the permeate W30. The electrodeionization device has, for example, alternating desalting compartments separated by anion exchange membranes and cation exchange membranes, and concentrating compartments into which concentrated water containing the removed ionic components flows. The electrodeionization device further has a mixture of anion exchange resin and cation exchange resin filled in the desalting compartment, and electrodes for applying a DC voltage.

In the electrodeionization device, the water to be treated is passed through the anion exchange resin and the cation exchange resin with a direct current applied to the electrodes, so that the ionic components in the water to be treated are adsorbed onto the ion exchange resin. The adsorbed ionic components migrate to the ion exchange membrane surface by electrophoresis, are electrodialyzed in the ion exchange membrane, and are transferred to the concentration compartment and discharged into the concentrated water. In the desalting compartment, a dissociation reaction of water proceeds, producing H ⁺ and ^OH- , thereby continuously regenerating the ion exchange resin in the desalting compartment. The desalted water W40 collected in the desalting compartment is sent to the subsequent stage, and the concentrated water in the concentration compartment is discharged outside the system.

As a treatment condition in the electrodeionization treatment step 111, a current density in the deionization compartment is preferably 0.2 A/ ^dm2 to 1.0 A/ ^dm2 , which improves the regeneration efficiency of the ion exchange resin in the deionization compartment and improves the ion removal efficiency. The current density in the deionization compartment can be adjusted by the voltage between the anode and cathode of the electrodeionization device. The quality of the desalted water obtained in the electrodeionization treatment step is, for example, a resistivity of 15 MΩ·cm to 18 MΩ·cm.

Examples of electrodeionization devices (EDI) used in the electrodeionization process 111 include the VNX series (manufactured by EVOQUA) such as the model IP-VNX-MAX, the E-Cell series (manufactured by SUEZ) such as the model SUEZMK3-27EU, and the MDI series (manufactured by ECORBIT) such as the model UX5015.

In the pure water production method 110 of this embodiment, the water quality of the pure water obtained in the pure water production method 100 is acceptable for the electrodeionization process 111, so that the deionization process in the electrodeionization process 111 can be performed appropriately. Specific examples of the water quality acceptable for the electrodeionization process 111 include a boron concentration of 3 μg/L to 20 μg/L, a conductivity of 0.3 μS/cm to 40 μS/cm, and a carbonate concentration of 0.001 mg/L to 3 mg/L.

Next, the desalted water W40 obtained in the electrodeionization process 111 is supplied to an ultraviolet oxidation device in an ultraviolet oxidation process 112. The ultraviolet oxidation device has an ultraviolet lamp capable of irradiating ultraviolet rays having a wavelength of around 185 nm, and irradiates the water to be treated with ultraviolet rays from this ultraviolet lamp to oxidize and decompose the total organic carbon component (TOC) in the water to be treated. The ultraviolet lamp used in the ultraviolet oxidation device can be a lamp that generates ultraviolet rays with a wavelength of around 185 nm, or a low-pressure mercury lamp that emits ultraviolet rays with a wavelength of around 254 nm as well as ultraviolet rays with a wavelength of around 185 nm. The ultraviolet rays emitted by the ultraviolet oxidation device decompose water to generate OH radicals, and the organic matter in the water to be treated (the above-mentioned desalted water W40) is oxidized and decomposed into organic acids by these OH radicals. The amount of ultraviolet irradiation in the ultraviolet oxidation process 112 can be appropriately changed depending on the water quality of the water to be treated.

The treated water W41 produced in the ultraviolet oxidation treatment process 112 is then supplied to a non-regenerative ion exchange resin device (primary/polisher) in the non-regenerative ion exchange resin treatment process 113. The non-regenerative ion exchange resin device is made up of a resin tower filled with a mixture of strongly acidic cation exchange resin and strongly basic anion exchange resin, and removes ionic components from the treated water W41. The ionic components removed here are mainly trace amounts of organic acids produced by the decomposition of organic matter in the ultraviolet oxidation process 112.

The treated water W42 produced through the non-regenerative ion exchange resin treatment process 113 can have a resistivity of 18 MΩ·cm or more, and the TOC concentration can be reduced to, for example, 10 μg C/L or less. The non-regenerative ion exchange resin treatment process 113 does not have to be performed. In this case, the ultraviolet oxidation treatment process 112 can be performed before the electrical deionization process 111, and the permeated water W30 can be treated in this order through the ultraviolet oxidation treatment process 112 and the electrical deionization process 111.

According to the pure water production method 110 described above, since the electrical deionization process 111 is adopted, ionic components can be continuously removed without using any chemicals such as acids or alkalis that are normally used to regenerate ion exchange resins. This makes it possible to reduce the cost of producing pure water, downsize the equipment, reduce the environmental load, and improve production efficiency. In addition, since most of the ionic components can be removed in the electrical deionization process 111, the processing load of the three-stage reverse osmosis membrane device in the previous stage can be reduced, which makes it possible to reduce the cost of producing pure water, downsize the equipment, and reduce the environmental load.

Next, a pure water production method 120, which is a modified example of the pure water production method 100 of the embodiment, will be described with reference to Figure 3. This modified pure water production method differs from the pure water production method 100 shown in Figure 1 in that the pure water production method 100 includes an acid adjustment step in which an acid is added to the permeate water W20 to produce a second treated water having a lower pH than the alkaline treated water. That is, the modified pure water production method 120 includes a reverse osmosis membrane treatment process 101 in which raw water is treated with a first-stage reverse osmosis membrane device to obtain permeated water W10 (first permeated water), an alkali adjustment process 102 in which the permeated water W10 is adjusted to be alkaline to obtain alkaline water to be treated W11, a reverse osmosis membrane treatment process 103 in which the alkaline water to be treated W11 is treated with a second-stage reverse osmosis membrane device to obtain permeated water W20 (second permeated water), an acid adjustment process 121 in which an acid is added to the permeated water W20 to obtain second water to be treated W21, and a reverse osmosis membrane treatment process 122 in which the second water to be treated W21 is treated with a third-stage reverse osmosis membrane device to obtain permeated water W31. The permeated water W31 is the pure water produced by the pure water production method 120.

In the acid adjustment step 121, acid is added to the permeate W20 obtained in the reverse osmosis membrane treatment step 103 to obtain the second treated water W21. For example, in the acid adjustment step 121, hydrochloric acid, sulfuric acid, sulfamic acid aqueous solution, etc. are added to the permeate W20 to generate the treated water W21. From the viewpoint of reducing the environmental load, the liquid property of the treated water W21 is preferably weakly acidic to weakly alkaline, and specifically, the pH of the treated water W21 is preferably 5.5 to 7.5, and more preferably 6.5 to 7.5. By making the pH of the second treated water W21 5.5 or higher, the removal rate of cationic components in the subsequent reverse osmosis membrane treatment step 122 can be improved, and since excessive acid is not added, the treated water quality can be improved. The salts (ionic components) that pass through the reverse osmosis membrane treatment process 102 in minute amounts are present in the permeate W20 of the reverse osmosis membrane treatment process 102 in the form of hydroxide salts such as sodium hydroxide. By adjusting the pH of the water to be treated W21 to 7.5 or less, these hydroxide salts are converted into sulfamate salts such as sodium sulfamate, which makes them easier to remove in the reverse osmosis membrane treatment process 122. In addition, by adjusting the pH of the water to be treated W21 to 5.5 or more, the treatment load in the reverse osmosis membrane treatment process 122 can be reduced. In addition, it is preferable to use an aqueous sulfamic acid solution as the acid added in the acid adjustment process 121, in order to reduce the amount of chemicals used and to reduce the environmental load.

When sulfamic acid is used in the acid adjustment step 121, it is preferable that the raw water is not treated with activated carbon and the chlorine concentration in the raw water is 0.1 μg/L to 0.4 μg/L. By circulating the concentrated water generated in the reverse osmosis membrane treatment step 122 to the supply side of the first-stage reverse osmosis membrane device, the chlorine in the raw water and the sulfamic acid in the concentrated water react to generate a bactericidal power, and the effect of suppressing biofouling in the three-stage reverse osmosis membrane device is obtained. In particular, biofouling in the first-stage reverse osmosis membrane, where biofouling is most likely to occur, can be prevented. This allows the amount of bactericide added to the treated water for normal sterilization to be reduced, leading to a reduction in the environmental load. In addition, free chlorine in the treated water, which deteriorates the reverse osmosis membrane, can be converted to combined chlorine by adding sulfamic acid, which also has the effect of reducing the load on the reverse osmosis membrane. In this manner, in the pure water production method 120 of this modification, by adding sulfamic acid in the acid adjustment step 121, two effects can be obtained: improving the removal rate of weak electrolytes in the reverse osmosis membrane treatment step 122, and suppressing biofouling in the three-stage reverse osmosis membrane. Therefore, in the conventional pure water production method, it was necessary to add two types of acid at two locations, to the feed water for the first-stage reverse osmosis membrane and the feed water for the third-stage reverse osmosis membrane, but in the pure water production method 120 of this modification, it is possible to reduce this to adding only one type of acid at one location in the acid adjustment step 121.

When sulfamic acid is used in the acid adjustment process 121, the amount of sulfamic acid added is preferably within a range that allows the total amount of chemicals used in the pure water production method 120 to be reduced by balancing the amount of the bactericide reduction with the amount of sulfamic acid added, for example, 1 mg/L to 10 mg/L of the water to be treated.

Next, a pure water production apparatus 200 according to an embodiment of the present invention will be described. FIG. 4 is a block diagram showing a pure water production system 4 including the pure water production apparatus 200 according to the embodiment. The pure water production apparatus 200 has three stages of ultra-low pressure reverse osmosis membrane devices 201, 202, and 203.

The pretreatment device 210 is provided in the upstream of the pure water production system 200 as a raw water supply mechanism. The pretreatment device 210 is provided with an activated carbon device (AC) 211, a storage tank TK, a pump P1, a heat exchanger (HEX) 212, and a precision filtration device (PF) 213 in this order. The configuration of the pretreatment device 210 can be changed appropriately depending on the quality of the raw water. For example, the pretreatment device 210 does not need to be provided with the activated carbon device (AC) 211, the heat exchanger (HEX) 212, and the precision filtration device (PF) 213, and may be configured by any combination of a coagulation sedimentation device, a pressurized flotation device, a sand filtration device, etc. in addition to the above-mentioned activated carbon device (AC) 211, the storage tank TK, the heat exchanger (HEX) 212, and the precision filtration device (PF) 213.

Arranged downstream of the pure water production system 200 are an electrodeionization system (EDI) 221, an ultraviolet oxidation system (TOC-UV) 222, and a non-regenerative ion exchange system (Primary/Polisher) 223, in that order. The electrodeionization system (EDI) 221, the ultraviolet oxidation system (TOC-UV) 222, and the non-regenerative ion exchange system (Polisher) 223 may also be arranged appropriately depending on the desired water quality.

The pure water production system 200 has a supply pipe L1 (first supply pipe) that sends the permeate from the reverse osmosis membrane device 201 to the reverse osmosis membrane device 202, and a supply pipe L2 (second supply pipe) that sends the permeate from the reverse osmosis membrane device 202 to the reverse osmosis membrane device 203. In addition, discharge pipes L3, L4, and L5 are connected to the concentrated sides of the reverse osmosis membrane devices 201, 202, and 203, respectively. The pure water production system 200 has a circulation pipe L6. One end of the circulation pipe L6 is connected to the tank TK, and the other end is connected to the discharge pipes L4 and L5, so that the circulation pipe L6 circulates the concentrated water from the reverse osmosis membrane devices 202 and 203 to the tank TK.

The reverse osmosis membrane devices 201, 202, and 203 are each equipped with one or more reverse osmosis membrane modules that are configured by, for example, housing a reverse osmosis membrane and a flow path material for passing water to be treated through the reverse osmosis membrane in a casing. The reverse osmosis membranes provided in the reverse osmosis membrane devices 201, 202, and 203 are negatively charged membranes having a skin layer made of crosslinked aromatic polyamide. These reverse osmosis membranes are asymmetric membranes or composite membranes, and are preferably composite membranes made of polysulfone, polyarylethersulfone such as polyethersulfone, polyimide, polyvinylidene fluoride, etc., have a support layer having micropores, and the above-mentioned skin membrane is provided on the support layer. Note that a negatively charged membrane means that when the pH is 7, the membrane made of the skin layer shows a negative charge. The shape of the reverse osmosis membrane is a hollow fiber shape, a spiral shape, a flat plate shape, a tubular shape, etc. These reverse osmosis membranes are preferably spiral-shaped in terms of increasing pressure resistance and improving treatment efficiency.

The reverse osmosis membranes used in the reverse osmosis membrane devices 201, 202, and 203 preferably have a salt rejection rate (salt removal rate) of 99.0% or more, more preferably 99.2% or more, even more preferably 99.5% or more, and even more preferably 99.6% or more. The salt rejection rate is indicated by the sodium chloride removal rate when an aqueous sodium chloride solution with a pH of 7 and a concentration of 500 ppm or 1500 ppm is supplied at a water temperature of 25°C, a water recovery rate of 15%, and a supply pressure of 1.03 MPa or 0.69 to 0.7 MPa.

Examples of ultra-low pressure reverse osmosis membrane devices in which ultra-low pressure types are used as the reverse osmosis membrane devices 201, 202, and 203 include the ESPA series manufactured by Nitto Denko Corporation and the TBW series and TMHA series manufactured by Toray Industries, Inc.

The supply pipe L1 is provided with an alkali adjustment mechanism 204 that adjusts the alkalinity of the water to be treated flowing through the supply pipe L1. The alkali adjustment mechanism 204 is composed of a tank that stores an alkali adjuster such as an aqueous sodium hydroxide solution or potassium hydroxide, and a chemical injection pump that measures a predetermined amount of the alkali adjuster in the tank and adds it to the supply pipe L1.

The production of pure water using the pure water production system 200 shown in FIG. 4 is similar to the pure water production method 100 (FIG. 1) of the embodiment described above. First, raw water consisting of city water, well water, industrial water, used recycled water, etc. is supplied to the pretreatment device 210. The raw water passes through the pretreatment device, where turbidity and chlorine are removed from the water, and the water temperature is adjusted to 15-30°C to produce pretreated water.

The pretreated water obtained in this manner contains 1 mg/L to 100 mg/L of carbon dioxide and 150 μg/L or less of boron, with the boron concentration preferably being 5 μg/L or more. The pH of the pretreated water is, for example, about 5.0 to 7.5.

The pretreated water is pressurized by pump P1 and supplied to the reverse osmosis membrane device 201. The pretreated water is treated by reverse osmosis membrane in the reverse osmosis membrane device 201 to remove hardness components in the water and produce permeated water W10 (reverse osmosis membrane treatment step 101 in Figure 1). The concentrated water from the reverse osmosis membrane device 201 is discharged outside the system via discharge pipe L3. The permeated water W10 is sent to the subsequent stage via supply pipe L1.

As the permeate W10 flows through the supply pipe L1, it is adjusted to be alkaline by the alkali adjustment mechanism 204 (alkali adjustment step 102 in FIG. 1). Specifically, the alkali adjustment mechanism 204 quantitatively injects an alkali adjuster into the supply pipe L1, whereby the alkali adjuster is mixed with the permeate W10 to produce water to be treated W11 that has been adjusted to be alkaline. The pH of the water to be treated W11 is preferably 9.0 to 10.0.

The alkaline water to be treated W11 is then supplied to the reverse osmosis membrane device 202. The water to be treated W11 is treated by reverse osmosis membrane treatment in the reverse osmosis membrane device 202 to remove boron and carbon dioxide from the water, producing permeate water W20 (reverse osmosis membrane treatment step 103 in FIG. 1). Because the water to be treated W11 is alkaline as described above, the reverse osmosis membrane device 202 can achieve a boron removal rate of 50% to 90% and a carbon dioxide removal rate of 95% to 98%. The concentrated water from the reverse osmosis membrane device 202 is circulated to the tank TK via the discharge pipe L4 and the circulation pipe L6. The permeate water W20 is sent to the subsequent stage via the supply pipe L2.

The permeated water W20 is then supplied to the reverse osmosis membrane device 203. The water to be treated W11 is treated with a reverse osmosis membrane in the reverse osmosis membrane device 203 to remove ionic components in the water, and permeated water W30 is produced (reverse osmosis membrane treatment step 104 in FIG. 1). The concentrated water from the reverse osmosis membrane device 203 is circulated to the tank TK via the discharge pipe L5 and the circulation pipe L6. The permeated water (pure water) W30 is sent to the subsequent stage.

The water quality of the permeate (pure water) W30 obtained in the reverse osmosis membrane treatment process 104 is, for example, a boron concentration of 3 μg/L to 20 μg/L, preferably 5 μg/L to 10 μg/L, a carbonate concentration of 0.005 mg/L to 0.5 mg/L, and a conductivity of 0.3 μS/cm to 40 μS/cm, preferably 1 μS/cm to 20 μS/cm, and more preferably 1 μS/cm to 10 μS/cm.

In order to obtain pure water (permeate water W30) of the above water quality, it is preferable that the water recovery rate in the three-stage reverse osmosis membrane device is 50% to 80% for reverse osmosis membrane device 201, 70% to 90% for reverse osmosis membrane device 202, and 80% to 95% for reverse osmosis membrane device 203.

The permeate W30 is then supplied to an electrodeionization device (EDI) 221, an ultraviolet oxidation device 222, and a non-regenerative ion exchange device 223 in that order. This executes the pure water production method 110 shown in FIG. 2. First, the permeate W30 is supplied to the electrodeionization device 221, and the ionic components in the permeate W30 are removed (electrodeionization process 111 in FIG. 2). In the electrodeionization device 221, the water to be treated is passed through the anion exchange resin and the cation exchange resin with a direct current applied to the electrodes, so that the ionic components in the water to be treated are adsorbed to the ion exchange resin. The adsorbed ionic components migrate to the ion exchange membrane surface by electrophoresis, are electrodialyzed in the ion exchange membrane, and are transferred to the concentration chamber and discharged into the concentrated water. The desalted water W40 from which the ionic components have been removed is sent to the subsequent stage, and the concentrated water in the concentration chamber is discharged outside the system.

The desalted water W40 is supplied to the ultraviolet oxidation device 222, where the desalted water W40 is irradiated with ultraviolet light (ultraviolet oxidation process step 112 in FIG. 2). This causes the total organic carbon (TOC) in the desalted water W40 to be oxidized and decomposed. The ultraviolet light irradiated is preferably ultraviolet light having a wavelength of about 185 nm, and more preferably ultraviolet light having a wavelength of about 185 nm and ultraviolet light having a wavelength of about 254 nm.

The treated water W41 produced in the ultraviolet oxidation device 222 is then supplied to the non-regenerative ion exchange device 223, where the ionic components in the treated water W41 are removed. The treated water W42 produced through the regenerative ion exchange resin device 223 can have a resistivity of 18 MΩ·cm or more, and the TOC concentration is reduced to, for example, 10 μg C/L or less.

The non-regenerative ion exchange device (Primary/Polisher) 223 does not have to be provided. In this case, the ultraviolet oxidation device 222 can be placed before the electrodeionization device (EDI) 221, and the permeate water W30 can be treated in the ultraviolet oxidation device 222 and the electrodeionization device (EDI) 221 in that order.

According to the pure water production system 200 described above, an excellent carbon dioxide removal rate is achieved in the reverse osmosis membrane device 202, so that the hardness removal mechanism in the upstream stage and the deaeration device for carbon dioxide removal can be omitted. In addition, since the hardness removal mechanism and the deaeration device can be omitted and the processing load of the pure water production system 200 is reduced, the amount of chemicals used can be reduced and the system can be simplified. As a result, pure water can be produced efficiently at low cost. Furthermore, if the water supply pressure in the three-stage reverse osmosis membrane processing process is set to an ultra-low pressure as described above, the number and output of water supply pumps can be reduced, leading to further simplification of the system, reduced pure water production costs, and improved production efficiency.

Next, a pure water production apparatus 300, which is a modified example of the pure water production apparatus 200 of the embodiment, will be described. FIG. 5 is a block diagram showing a pure water production system 5 including the pure water production apparatus 300. The pure water production apparatus 300 is an apparatus for realizing the pure water production method 120 of the above-mentioned embodiment, and differs from the pure water production system 4 shown in FIG. 4 in that it includes an acid adjustment mechanism 205 in the path of the supply pipe L2. As a result, the processing mode in the third stage reverse osmosis membrane device is different, but the other configurations and effects are the same as those of the pure water production apparatus 200. Therefore, the same reference numerals are used for configurations that perform the same functions as the pure water production apparatus 200, and detailed descriptions are omitted.

The pure water production system 5 is equipped with a pretreatment device 310 arranged in front of the pure water production device 300. The pretreatment device 310 is equipped with a storage tank TK, a pump P1, a heat exchanger (HEX) 212, and a precision filtration device (PF) 213, in that order. In the rear of the pure water production device 300, an electric deionization device (EDI) 221, an ultraviolet oxidation device (TOC-UV) 222, and a non-regenerative ion exchange resin device 223 are arranged in that order.

The pure water production system 300 is equipped with three stages of reverse osmosis membrane devices 201, 202, and 303. The pure water production system 300 also has a supply pipe L1 (first supply pipe) that sends the permeate of the reverse osmosis membrane device 201 to the reverse osmosis membrane device 202, and a supply pipe L2 (second supply pipe) that sends the permeate of the reverse osmosis membrane device 202 to the reverse osmosis membrane device 203. Discharge pipes L3, L4, and L35 are connected to the concentrated sides of the reverse osmosis membrane devices 201, 202, and 303, respectively. The pure water production system 300 has a circulation pipe L6, one end of which is connected to a tank TK, and the other end of which is connected to discharge pipes L4 and L35, so that the circulation pipe L6 circulates the concentrated water of the reverse osmosis membrane devices 202 and 303 to the tank TK.

The configuration of the reverse osmosis membrane device 303 is similar to that of the reverse osmosis membrane devices 201 and 202 described above, and is an ultra-low pressure reverse osmosis membrane device.

The supply pipe L1 is provided with an alkali adjustment mechanism 204 that adjusts the alkalinity of the water to be treated flowing through the supply pipe L1. The alkali adjustment mechanism 204 is composed of a tank that stores an alkali adjuster such as an aqueous sodium hydroxide solution or potassium hydroxide, and a chemical injection pump that measures a predetermined amount of the alkali adjuster in the tank and adds it to the supply pipe L1.

The supply pipe L2 is provided with an acid adjustment mechanism 205 that adds acid to the water being treated flowing through the supply pipe L2. The acid adjustment mechanism 205 is composed of a tank that stores an acid adjuster such as hydrochloric acid, sulfuric acid, or sulfamic acid, and a chemical injection pump that measures a predetermined amount of the acid adjuster in the tank and adds it to the supply pipe L2. Of the above, the acid added by the acid adjustment mechanism 205 is preferably sulfamic acid.

The production of pure water using the pure water production system 300 shown in FIG. 5 is similar to the pure water production method 120 (FIG. 3) of the embodiment described above. First, raw water consisting of city water, well water, industrial water, used recycled water, etc. is supplied to the pretreatment device 310. The raw water passes through the pretreatment device 310, where turbid matter in the water is removed, and the water temperature is adjusted to 15°C to 30°C, producing pretreated water.

The pretreated water obtained in this manner contains 1 mg/L to 100 mg/L of carbon dioxide and 150 μg/L or less of boron. The pretreated water also contains, for example, about 0.1 mg/L to 0.4 mg/L of chlorine, calculated as Cl. The pH of the pretreated water is, for example, about 5.0 to 7.5.

The pretreated water is pressurized by pump P1 and supplied to the reverse osmosis membrane device 201, preferably at a water supply pressure of 0.4 MPa to 1.1 MPa, more preferably 0.6 MPa to 0.7 MPa, where hardness components in the water are removed and permeated water W10 is produced (reverse osmosis membrane treatment step 101 in Figure 3). The concentrated water from the reverse osmosis membrane device 201 is discharged outside the system via discharge pipe L3. The permeated water W10 is sent to the subsequent stage via supply pipe L1.

While passing through the supply pipe L1, the permeate water W10 is adjusted to be alkaline by the alkali adjustment mechanism 204 (alkali adjustment step 102 in FIG. 3). Specifically, the alkali adjustment mechanism 204 quantitatively injects an alkali adjuster into the supply pipe L1, whereby the alkali adjuster is mixed with the permeate water W10 to produce water to be treated W11 that has been adjusted to be alkaline. When sulfamic acid is used in the acid adjustment mechanism 205, the pH of the water to be treated W11 is preferably 9.0 to 10.0.

The alkaline water to be treated W11 is then supplied to the reverse osmosis membrane device 202. The water to be treated W11 is treated by the reverse osmosis membrane in the reverse osmosis membrane device 202 to remove boron and carbon dioxide from the water, and permeate water W20 is produced (reverse osmosis membrane treatment step 103 in FIG. 3). The concentrated water from the reverse osmosis membrane device 202 is circulated to the tank TK via the discharge pipe L4 and the circulation pipe L6. The permeate water W20 is sent to the subsequent stage via the supply pipe L2.

As the permeate W20 flows through the supply pipe L2, acid is added by the acid adjustment mechanism 205 (acid adjustment step 121 in FIG. 3). Specifically, the acid adjustment mechanism 205 quantitatively injects an acid adjuster into the supply pipe L2, mixing the permeate W20 with the acid adjuster, preferably sulfamic acid, to produce water to be treated W21 with a lower pH than the water to be treated W11. When sulfamic acid is used in the acid adjustment mechanism 205, the pH of the water to be treated W21 is preferably 5.5 to 7.5, and more preferably 6.5 to 7.5.

The water to be treated 21 is then supplied to the reverse osmosis membrane device 303, where ionic components in the water are removed to produce permeate water W30 (reverse osmosis membrane treatment step 122 in FIG. 3). The concentrated water from the reverse osmosis membrane device 303 is circulated to the tank TK via the discharge pipe L35 and the circulation pipe L6. The permeate water (pure water) W31 is sent to the subsequent stage. By adding an acid to the water to be treated 21, the removal rate of ionic components in the reverse osmosis membrane device 303 can be improved.

The quality of the permeate (pure water) W31 obtained by the reverse osmosis membrane device 303 is as follows: boron concentration is 3 μg/L to 20 μg/L, preferably 5 μg/L to 10 μg/L, carbonate concentration is, for example, 0.1 mg/L to 1 mg/L, preferably 0.2 mg/L to 0.5 mg/L, and conductivity is 0.3 μS/cm to 40 μS/cm, preferably 1 μS/cm to 20 μS/cm, more preferably 1 μS/cm to 10 μS/cm.

The permeate W31 is then supplied to an electrodeionization device (EDI) 221, an ultraviolet oxidation device 222, and a non-regenerative ion exchange device 223 in that order. In the electrodeionization device 221, ionic components in the permeate W31 are removed. The desalted water from which the ionic components have been removed is sent to the subsequent stage, and the concentrated water in the concentration chamber is discharged outside the system. In the ultraviolet oxidation device 222, the desalted water is irradiated with ultraviolet light as described above, and the total organic carbon (TOC) in the desalted water is oxidized and decomposed. The treated water produced by irradiation with ultraviolet light is then supplied to a non-regenerative ion exchange device 223, and the ionic components in the treated water are removed. The treated water produced through the regenerative ion exchange resin device 223 can have a resistivity of 18 MΩ·cm or more, and the TOC concentration is reduced to, for example, 10 μg C/L or less.

In this modified example, the non-regenerative ion exchange device (Primary/Polisher) 223 does not have to be provided. In this case, the ultraviolet oxidation device 222 can be placed before the electrodeionization device (EDI) 221, and the permeate water W30 can be treated in the ultraviolet oxidation device 222 and the electrodeionization device (EDI) 221 in that order.

When sulfamic acid is used in the acid adjustment mechanism 205, the concentrated water generated in the reverse osmosis membrane device 303 is circulated in the tank TK in the front stage of the reverse osmosis membrane device 201, whereby the chlorine in the raw water reacts with the sulfamic acid in the concentrated water to produce a bactericidal effect, and the effect of suppressing biofouling in the three-stage reverse osmosis membrane device is obtained. In particular, biofouling in the first stage reverse osmosis membrane, where biofouling is most likely to occur, can be prevented. This reduces the amount of bactericide added to the water to be treated for normal sterilization, leading to a reduction in the environmental load. In addition, free chlorine in the water to be treated, which deteriorates the reverse osmosis membrane, can be converted to combined chlorine by adding sulfamic acid, which also has the effect of reducing the load on the reverse osmosis membrane. By adding sulfamic acid with the acid adjustment mechanism 205, two effects can be obtained: improving the removal rate of weak electrolytes in the reverse osmosis membrane device 202 and suppressing biofouling in the three-stage reverse osmosis membrane device 303. Conventional pure water production methods require the addition of two types of acid at two locations, the supply water for the first-stage reverse osmosis membrane and the supply water for the third-stage reverse osmosis membrane. However, with the pure water production system 200, this can be reduced to adding only one type of acid at one location in the acid adjustment mechanism 205.

When sulfamic acid is used in the acid adjustment mechanism 205, it is preferable that the amount of sulfamic acid added is within a range that allows the total amount of chemicals used to be reduced by balancing the amount of the bactericide reduced and the amount of sulfamic acid added.

Next, an embodiment of an ultrapure water production system 6 using the above-mentioned pure water production device 200 will be described with reference to FIG. 6. FIG. 6 is a block diagram showing a schematic configuration of the ultrapure water production system 6.

As shown in FIG. 6, the ultrapure water production system 6 comprises a pretreatment device 60, a primary pure water device 61, and a secondary pure water device (subsystem) 62, in that order. The secondary pure water device 62 is connected to a point of use (POU) 63 by piping, so that the ultrapure water produced by the ultrapure water production system 6 is supplied to the POU 63.

The pretreatment device 60 performs processes such as coagulation, filtration, and membrane separation, and adjusts the temperature using a heat exchanger or the like as necessary to remove turbid matter such as suspended matter and colloidal matter contained in the water to be treated (raw water). Specifically, the pretreatment device 60 is equipped with an appropriate combination of an activated carbon device, a coagulation sedimentation device, a pressure flotation device, a sand filter device, a precision filter device, an ultrafilter device, a heat exchanger, and the like. The pretreatment device 60 may also have a configuration similar to the pretreatment device 210 (Figure 4) or pretreatment device 310 (Figure 5) described above. Note that if the quality of the raw water is sufficient to supply it to the primary pure water device 61, the pretreatment device 60 may be omitted.

The ultrapure water production system 6 is equipped with a tank TK1 downstream of the pretreatment device 60, and the water to be treated that has been pretreated by the pretreatment device 60 is introduced into the tank TK1 and temporarily stored there. The water to be treated in the tank TK1 is supplied to the primary pure water device 61 by a pump P2.

The primary pure water system 61 produces primary pure water by removing organic matter, ionic components and dissolved gases from the pretreated water. The primary pure water system 61 comprises, in this order, a pump P2, the pure water production system 200 of the above embodiment, an electric deionization device (EDI) 611, an ultraviolet oxidation device (TOC-UV) 612 and a non-regenerative ion exchange resin device (Primary/Polisher) 613. Note that the primary pure water system 61 may comprise the pure water production system 300 of the above embodiment or a variation thereof, instead of the pure water production system 200.

In the primary pure water system 61, first, the pure water production system 200 removes hardness components, carbon dioxide, and boron from the pretreated water.

The water to be treated is then supplied to an electrodeionization device (EDI) 611. The electrodeionization device 611 has, for example, anion exchange membranes and cation exchange membranes arranged alternately between an anode and a cathode, and alternates between desalting compartments separated by anion exchange membranes and cation exchange membranes, and concentrating compartments into which concentrated water containing the removed ionic components flows. The electrodeionization device has a mixture of anion exchange resin and cation exchange resin filled in the desalting compartment, and electrodes for applying a DC voltage.

In the electrodeionization device 611, for example, the water to be treated is supplied to the deionization chamber and the concentration chamber in parallel, and the mixture of anion exchange resin and cation exchange resin in the deionization chamber adsorbs the ionic components in the water to be treated. The adsorbed ionic components are transferred to the concentration chamber by the action of a direct current, and the concentrated water in the concentration chamber is discharged outside the system.

The electrodeionization device 611 can continuously remove ionic components without using any chemicals such as acids or alkalis to regenerate the ion exchange resin. This improves safety in ultrapure water production, reduces production costs, and allows for the miniaturization of equipment, leading to improved production efficiency.

Next, the desalted water generated by the electrodeionization device 611 is supplied to the ultraviolet oxidation device 612. The ultraviolet oxidation device 612 has an ultraviolet lamp capable of irradiating ultraviolet rays having a wavelength of, for example, about 185 nm, and irradiates the water to be treated with ultraviolet rays from this ultraviolet lamp to oxidize and decompose the total organic carbon component (TOC) in the water to be treated. The ultraviolet lamp used in the ultraviolet oxidation device 612 can be a lamp that generates ultraviolet rays with a wavelength of about 185 nm, or a low-pressure mercury lamp that emits ultraviolet rays with a wavelength of about 254 nm as well as ultraviolet rays with a wavelength of about 185 nm. The ultraviolet rays emitted by the ultraviolet oxidation device 612 decompose water to generate OH radicals, and the organic matter in the water to be treated is oxidized and decomposed into organic acids by these OH radicals. The amount of ultraviolet irradiation in the ultraviolet oxidation device 612 of the primary pure water device 61 can be appropriately changed depending on the water quality of the water to be treated.

The treated water from the ultraviolet oxidation device 612 is treated in a non-regenerative ion exchange resin device (primary/polisher) 613. The non-regenerative ion exchange resin device 613 mainly removes trace amounts of ionic components such as organic acids that are generated by the decomposition of organic matter in the ultraviolet oxidation device 612.

The primary pure water obtained in this manner has, for example, a resistivity of 18 MΩ·cm or more and a TOC concentration of 10 μg C/L or less.

The ultrapure water production system of this embodiment comprises, in this order, a primary pure water tank TK2 for storing primary pure water, a pump P3, and a secondary pure water system 62 downstream of a primary pure water system 61. The primary pure water produced by the primary pure water system is temporarily stored in the primary pure water tank TK2, and then sent to the secondary pure water system 62 by the pump P3. The secondary pure water system 62 comprises an ultraviolet oxidation system (TOC-UV) 621, a non-regenerative polisher 622, a membrane degassing system (MDG) 623, and an ultrafiltration system (UF) 624.

The ultraviolet oxidation device 621 in the secondary pure water system 62 has the same configuration as the ultraviolet oxidation device 612 in the primary pure water system 61. The non-regenerative polisher 622 is a mixed-bed ion exchange resin device in which a strong acid cation exchange resin and a strong base anion exchange resin are mixed and filled in a container such as a cylinder. The non-regenerative polisher 622 does not regenerate the ion exchange resin in the container, and is replaced with another one when the ion exchange capacity decreases. The non-regenerative polisher 622 adsorbs and removes ion components generated by the ultraviolet oxidation device 621 decomposing organic matter.

The membrane degassing device 623 removes dissolved gases through a degassing membrane. The membrane degassing device 623 removes trace amounts of dissolved oxygen from the primary pure water, reducing the dissolved oxygen concentration to, for example, about 1 μg/L or less. The ultrafiltration membrane device 624 performs a filtration process using an ultrafiltration membrane, removing trace amounts of eluted material and fine particle components from the upstream ion exchange resin, reducing the number of fine particles of 0.05 μm or more to, for example, about 250 Pcs./L or less.

In the ultrapure water production system 6 of this embodiment, an excellent carbon dioxide removal rate is achieved in the pure water production apparatus 200, so that the hardness removal mechanism in the upstream stage and the degassing device for carbon dioxide removal can be omitted. In addition, since the hardness removal mechanism and the degassing device can be omitted and the processing load of the pure water production apparatus 200 is reduced, the amount of chemicals used can be reduced and the apparatus can be simplified. As a result, pure water can be produced efficiently at low cost. Furthermore, if the water supply pressure in the three-stage reverse osmosis membrane treatment process in the pure water production apparatus 200 is set to an ultra-low pressure, the number and output of water supply pumps can be reduced, leading to further simplification of the apparatus, reduced pure water production costs, and improved production efficiency.

In each of the above-mentioned embodiments, the quality of the raw water, pretreated water, pure water, or ultrapure water can be measured by the following method or device.
pH: Electrode method Boron concentration: ICP (inductively coupled plasma) emission spectroscopy or ICP-MS (inductively coupled plasma mass spectrometry) method Hardness components: ICP-MS method Dissolved carbon dioxide (calcium carbonate equivalent): Sievers M9e manufactured by SUEZ Co., Ltd.
Silica (Si): Atomic absorption spectrophotometry/absorption spectrophotometry Chlorine (Cl equivalent): DPD (diethyl paraphenylenediamine) method Conductivity: Conductivity meter (HORIBA, Ltd. HE-960CW)
Resistivity (specific resistance): Resistivity meter (Horiba, Ltd., HE-960RW)
Total organic carbon (TOC) concentration: TOC meter (other than ultrapure water: SUEZ Sievers M9e, ultrapure water: BECKMAN COULTER Anatel A-1000XP)
Number of particles of 0.05 μm or more: Particle counter (UDI-50, manufactured by Particle Measuring Systems, Inc.)

Next, examples will be described. The present invention is not limited to the following examples.

Example 1
Pure water was produced using the pure water production system 7 shown in FIG. 7. The pure water production system 7 shown in FIG. 7 is composed of a combination of the pretreatment device 310 and the pure water production device 300 shown in FIG. 5. Atsugi city water was treated with activated carbon and then supplied to the pretreatment device 310. Pretreated water W1 (raw water with pH=7.2, boron concentration of 150 ppb, and carbon dioxide concentration of 30 ppm) produced by treatment in the pretreatment device 310 was treated with reverse osmosis membranes in order in the reverse osmosis membrane devices 201, 202, and 303. A sodium hydroxide aqueous solution was added to the supply water of the reverse osmosis membrane device 202 by the alkali adjustment mechanism 204 so that the pH of the water was 9.5. The concentrated water of the reverse osmosis membrane devices 202 and 303 was returned to the tank TK.

Example 2
Pure water was produced using the pure water producing system 7 shown in Fig. 7. Pretreated water (raw water with a pH of 7.2, a boron concentration of 50 ppb, and a carbon dioxide concentration of 15 ppm) was produced in the pretreatment device 310, and the obtained pretreated water was subjected to reverse osmosis membrane treatment in sequence in the reverse osmosis membrane devices 201, 202, and 303. Aqueous sodium hydroxide solution was added to the feed water of the reverse osmosis membrane device 202 by the alkali adjustment mechanism 204 so that the pH of the water was 9.5. The concentrated water of the reverse osmosis membrane devices 202 and 303 was returned to the tank TK.

Example 3
Pretreated water W1 was obtained in the same manner as in Example 1, and the pretreated water W1 was subjected to reverse osmosis membrane treatment in the reverse osmosis membrane devices 201, 202, and 303 in that order. Aqueous sodium hydroxide solution was added to the feed water of the reverse osmosis membrane device 202 by the alkali adjustment mechanism 204 so that the pH of the water was 9.0. Aqueous sulfamic acid solution was added to the feed water of the reverse osmosis membrane device 303 so that the pH of the water was 6.5. The concentrated water of the reverse osmosis membrane devices 202 and 303 was refluxed to the tank TK.

(Comparative Example 1)
Pretreated water W1 was obtained in the same manner as in Example 1, and the pretreated water W1 was subjected to reverse osmosis membrane treatment in two stages of reverse osmosis membrane devices (corresponding to the reverse osmosis membrane devices 201 and 202 in FIG. 7 ). Aqueous sodium hydroxide solution was added to the feed water of the second stage reverse osmosis membrane device so that the pH of the water was 9.5. The concentrated water of the second stage reverse osmosis membrane device was returned to the tank TK in the same manner as in Example 1.

(Comparative Example 2)
Pretreated water W1 was obtained in the same manner as in Example 1, and the pretreated water W1 was sequentially subjected to reverse osmosis membrane treatment in three stages of reverse osmosis membrane devices (corresponding to reverse osmosis membrane devices 201, 202, and 303 in FIG. 7). Sulfuric acid was added to the feed water of the first stage reverse osmosis membrane device so that the pH of the water was 6.0. A sodium hydroxide aqueous solution was added to the feed water of the second stage reverse osmosis membrane device so that the pH of the water was 9.0. A sulfamic acid aqueous solution was added to the feed water of the third stage reverse osmosis membrane device so that the pH of the water was 4.7. The concentrated water of the second and third stages of reverse osmosis membrane devices was returned to the tank TK in the same manner as in Example 1.

(Comparative Example 3)
Pretreated water W1 was obtained in the same manner as in Example 1, and after adding acid to the pretreated water W1, it was treated in a degassing tower (not shown) to obtain degassed water. The degassed water was used as raw water and sequentially treated with reverse osmosis membranes in three stages (corresponding to the reverse osmosis membrane devices 201, 202, and 303 in FIG. 7). Aqueous sodium hydroxide solution was added to the feed water of the second stage reverse osmosis membrane device so that the pH of the water was 9.0. Sulfuric acid was added to the feed water of the third stage reverse osmosis membrane device so that the pH of the water was 4.7. The concentrated water of the second and third stages reverse osmosis membrane devices was returned to the tank TK in the same manner as in Example 1.

In the above examples and comparative examples, the discharge pressure of pump P1 was 1.6 MPa, and the reverse osmosis membrane device used was a TBW-HR (manufactured by Toray Industries, Inc.), which is an ultra-low pressure reverse osmosis membrane.

(Comparative Example 4)
Pretreated water W1 was obtained in the same manner as in Example 1, and the pretreated water W1 was subjected to reverse osmosis membrane treatment using a high-pressure reverse osmosis membrane device. In the comparative example, the discharge pressure of the pump P1 was set to 1.6 MPa, and a high-pressure reverse osmosis membrane TM820K-400 (manufactured by Toray Industries, Inc.) was used as the reverse osmosis membrane device.

In the above-mentioned examples and comparative examples, the pretreated water quality, the feed water quality of the reverse osmosis membrane device 202 (second stage reverse osmosis membrane device), the feed water quality of the reverse osmosis membrane device 303 (third stage reverse osmosis membrane device), and the treated water quality (permeate water quality of the final stage reverse osmosis membrane device) in each example were measured. The results are shown in Table 1. In addition, the removal rate of each component and the total amount of chemicals (sodium hydroxide, sulfamic acid, sulfuric acid) used in the examples and comparative examples were measured. The amount of chemicals used was calculated by taking the amount used (g) in Example 1 as 1. Since the amount of chemicals used varies greatly depending on the operating conditions, it is shown as an estimate based on the results of 100 days of operation. The results are shown in Table 2. In Tables 1 and 2, "DG" means the degassing tower, "RO2" means the second stage reverse osmosis membrane device (corresponding to the reverse osmosis membrane device 202 in FIG. 7), and "RO3" means the third stage reverse osmosis membrane device (corresponding to the reverse osmosis membrane device 303 in FIG. 7).

In the examples and comparative examples, the water qualities were measured using the following devices and methods.
pH: Horiba HP-200
Boron concentration: ICP (inductively coupled plasma) emission spectroscopy Carbonate concentration: SUEZ Sievers M9e

As shown in Tables 1 and 2, the pure water production methods of Examples 1 to 3 provide pure water with sufficient boron concentration reduction and appropriate water quality. The method of Comparative Example 1 uses only two reverse osmosis membranes, so sufficient reduction in boron concentration is not achieved. The method of Comparative Example 2 provides high-quality pure water, but the conductivity is too low, making the water quality excessive, and a large amount of chemicals is used. In addition, the method of Comparative Example 2 does not perform degassing treatment in the previous stage, so the carbon dioxide concentration of the treated water is also somewhat high. The method of Comparative Example 3 also provides high-quality pure water, but the conductivity is too low, making the water quality excessive. In addition, the method of Comparative Example 3 performs treatment using a degassing tower in the previous stage, so there are problems with increased chemical usage and larger equipment. The method of Comparative Example 4 uses a high-pressure reverse osmosis membrane, and although the power consumption is the same as in Example 1, sufficient reduction in boron concentration is not achieved.

Figures 8 to 10 show the carbonate concentration, boron concentration, and electrical conductivity of the pretreated water, the feed water, and the treated water of each stage of the reverse osmosis membrane device in Example 1 and Comparative Example 3. Figure 8 shows the carbonate concentration in logarithm, Figure 9 shows the boron concentration, and Figure 10 shows the electrical conductivity. In Figures 8 to 10, "RO1" refers to the first stage reverse osmosis membrane device.

8, in the method of Example 1, carbon dioxide is removed in the first and second stage reverse osmosis membrane devices. In contrast, in the method of Comparative Example 3, carbon dioxide is removed in the deaeration tower, and the water to be treated (deaeration water) from which carbon dioxide has been removed is supplied to the first stage reverse osmosis membrane device, thereby obtaining treated water with high conductivity (conductivity 0.125 μS/cm).
9, in the method of Example 1, most of the boron is removed in the second stage reverse osmosis membrane device, and a portion of the boron is removed in the third stage reverse osmosis membrane device, whereas in the method of Comparative Example 3, most of the boron is removed in the second stage reverse osmosis membrane device.
10, in the method of Example 1, conductive components (such as ionic components) are removed through the first to third reverse osmosis membrane devices, whereas in the method of Comparative Example 3, most of the conductive components (such as ionic components) are removed in the degassing tower and the first reverse osmosis membrane device.

From the above, it can be seen that the method of Example 1 and the method of Comparative Example 3 share the same three-stage reverse osmosis membrane device and some of the chemicals used, but the effects of each reverse osmosis membrane device are different.

100, 110, 120... Pure water production method, 101, 103, 104, 122... Reverse osmosis membrane treatment process, 102... Alkaline adjustment process, 111... Electrical deionization treatment process, 112... Ultraviolet oxidation treatment process, 113... Non-regenerative ion exchange (Primary/Polisher) treatment process, 121... Acid adjustment process, W10, 20... Permeated water, W30, 31... Permeated water (pure water), W11, 12, 21... Water to be treated, W40... Desalted water, W41, W42... Treated water, 200, 300... Pure water production apparatus, 210, 310... Pretreatment apparatus, 2 11: Activated carbon device (AC), TK: Storage tank, 212: Heat exchanger (HEX), 213: Precision filtration device (PF), 221,611: Electrodeionization device (EDI), 222,612: Ultraviolet oxidation device (TOC-UV), 223,613: Non-regenerative ion exchange device (Primary/Polisher), 201,202,203,303: Reverse osmosis membrane device, 204: Alkaline adjustment mechanism, 205: Acid adjustment mechanism, 4,5,6: Pure water production system, L1,L2: Supply pipe, L3 to L5: Discharge pipe, L6: Circulation pipe

Claims (9)

A method for producing pure water, comprising treating raw water with at least three stages of ultra-low pressure reverse osmosis membrane devices to remove boron from the raw water,
Each of the reverse osmosis membranes in the three-stage reverse osmosis membrane device is a negatively charged membrane having a skin layer made of a crosslinked aromatic polyamide,
The raw water contains carbon dioxide of 1 mg/L or more and 100 mg/L or less and boron of 150 μg/L or less,
treating the raw water in a first stage reverse osmosis membrane device to obtain a first permeate;
adjusting the first permeate to an alkaline state to obtain alkaline water to be treated;
treating the alkaline water to be treated in a second stage reverse osmosis membrane device to obtain a second permeate;
treating the second permeate in a third stage reverse osmosis membrane device to obtain pure water having a boron concentration of 3 μg/L or more and 20 μg/L or less and a conductivity of 0.3 μS/cm or more and 40 μS/cm or less;
A method for producing pure water, comprising the steps of: The method for producing pure water according to claim 1, wherein the pH of the alkaline water to be treated is 9.0 or more and 10.0 or less. The method for producing pure water according to claim 1 or 2 further comprises a step of adding an acid to the second permeate to obtain a second treated water, and treating the second treated water in a third stage reverse osmosis membrane device. The raw water contains chlorine,
In the step of obtaining the second treated water, sulfamic acid is added to the second permeate,
4. The method for producing pure water according to claim 3, wherein the concentrated water from the third stage reverse osmosis membrane device is mixed with raw water and treated in the first reverse osmosis membrane device. The method for producing pure water according to claim 3, wherein the pH of the second treated water is 5.5 or more and 7.5 or less. The method for producing pure water according to claim 1 or 2, wherein the pure water obtained as the permeate of the third stage reverse osmosis membrane device is further treated in an electric deionization device. A pure water production apparatus for removing boron, comprising a first reverse osmosis membrane device, a second reverse osmosis membrane device, and a third reverse osmosis membrane device connected in series,
the first reverse osmosis membrane device, the second reverse osmosis membrane device, and the third reverse osmosis membrane device are ultra-low pressure reverse osmosis membrane devices,
the reverse osmosis membranes provided in the first reverse osmosis membrane device, the second reverse osmosis membrane device, and the third reverse osmosis membrane device are negatively charged membranes having a skin layer made of a crosslinked aromatic polyamide;
a raw water supply mechanism that supplies raw water containing 1 mg/L or more and 100 mg/L or less of carbon dioxide and 150 μg/L or less of boron to a first reverse osmosis membrane device;
a first supply pipe for supplying permeate from the first reverse osmosis membrane device to the second reverse osmosis membrane device;
an alkali adjustment mechanism provided in the path of the first supply pipe and configured to adjust the alkalinity of the water to be treated flowing through the first supply pipe;
a second supply pipe for sending the permeate from the second reverse osmosis membrane device to the third reverse osmosis membrane device, wherein the permeate from the third reverse osmosis membrane device is pure water having a boron concentration of 3 μg/L or more and 20 μg/L or less and a conductivity of 0.3 μS/cm or more and 40 μS/cm or less. The pure water production system according to claim 7, further comprising an electrodeionization device downstream of the third reverse osmosis membrane device. An ultrapure water production system including a primary water purifier and a secondary water purifier in this order,
The primary pure water system includes the pure water production system according to claim 7 or 8, and an electrodeionization device disposed downstream of the pure water production system,
The secondary water purification device includes an ultraviolet oxidation device, a non-regenerative polisher, a membrane degassing device, and an ultrafiltration device in this order;
Produce ultrapure water with a boron concentration of 0.1 μg/L or less;
Ultrapure water production system.

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