Pure water production system and pure water production method

Abstract

Provided are a pure water production system and a pure water production method where, during pure water production in which a reverse osmosis membrane treatment, an ion-exchange treatment using an ion-exchange resin, and a reverse osmosis membrane treatment are sequentially carried out, the concentration of TOC components in obtained pure water can be reduced while suppressing decreases in the water recovery rate of the entire system. A pure water production system (1) comprises: a first reverse osmosis membrane treatment device (16) that causes raw water to pass through a first reverse osmosis membrane; an ion-exchange treatment device (22) that subjects first reverse osmosis (RO) permeated water to an ion-exchange treatment by using an ion-exchange resin; a second reverse osmosis membrane treatment device (26) that causes the ion-exchange treated water to pass through a second reverse osmosis membrane; and a return means that returns second RO concentrated water to a stage preceding the first reverse osmosis membrane treatment device (16).

Description

Pure water production system and pure water production method

[0001] The present invention relates to a pure water production system and a pure water production method.

[0002] In pure water production systems, groundwater, industrial water, and municipal water are used as raw water. In recent years, in order to make effective use of water resources, recycled water, which is produced by treating and reusing sewage generated in daily life, is also being used as raw water. These raw waters may contain scale-forming components, such as hardness components and silica.

[0003] In pure water production systems, methods are sometimes employed to remove ionic components from raw water containing scale components using reverse osmosis membrane treatment and ion exchange resin treatment. In this method, the majority of the scale components are separated into concentrated water using reverse osmosis membrane treatment, and the components remaining in the permeate are removed by adsorption with ion exchange resin. The ion exchange resin is regenerated using regeneration chemicals, and the components adsorbed by the ion exchange resin are discharged from the system as regeneration wastewater.

[0004] In order to remove even trace amounts of TOC (Total Organic Carbon) components, such as organic matter, from ion-exchange treated water, a reverse osmosis membrane is sometimes installed after the ion-exchange resin treatment. The permeate from this subsequent reverse osmosis membrane treatment is used as primary pure water, and the concentrated water may be used as regeneration water or washing water for the ion-exchange resin.

[0005] As described above, when reverse osmosis membrane treatment is used before ion exchange resin treatment, concentrated water is generated, which presents a challenge in that the water recovery rate of the system is low.

[0006] For example, Patent Document 1 describes introducing concentrated water from the reverse osmosis membrane treatment, which follows the ion exchange resin treatment, into a wastewater recovery system for non-potable water or a wastewater treatment system in an ultrapure water production apparatus. If the concentrated water from the reverse osmosis membrane treatment, which follows the ion exchange resin treatment, is recovered as non-potable water, a decrease in the overall water recovery rate of the system can be suppressed. However, if there is no use for the non-potable water in other systems, the excess water must be discharged, and if the water quality does not meet discharge standards, additional treatment equipment may be required.

[0007] Even when reverse osmosis membrane treatment is used as a prerequisite for ion exchange resin treatment, the water used for regenerating the ion exchange resin and the washing water can be used as the concentrated water for the subsequent reverse osmosis membrane treatment. However, when reverse osmosis membrane treatment is used as a prerequisite for ion exchange resin treatment, the amount of water used for regenerating the ion exchange resin and washing water is small. As a result, the uses for the concentrated water from the subsequent reverse osmosis membrane treatment are limited, leading to a surplus. If the water quality does not meet discharge standards, it becomes necessary to set up separate treatment facilities to treat the water before discharging it.

[0008] Patent No. 3789619

[0009] The object of the present invention is to provide a pure water production system and a pure water production method that can reduce the concentration of TOC components in the obtained pure water while suppressing a decrease in the overall water recovery rate of the system, in a pure water production process that sequentially performs reverse osmosis membrane treatment, ion exchange treatment using ion exchange resin, and reverse osmosis membrane treatment.

[0010] The present invention relates to a pure water production system for producing pure water, comprising: a first reverse osmosis membrane treatment device that passes raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment device that performs ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water; a second reverse osmosis membrane treatment device that passes the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water; and a return means for returning the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device.

[0011] In the aforementioned pure water production system, it is preferable that the permeability coefficient of the first reverse osmosis membrane is smaller than that of the second reverse osmosis membrane.

[0012] In the aforementioned pure water production system, it is preferable to regenerate the ion exchange resin using recycled water and recover at least one of the acids and alkalis from the resulting recycled wastewater.

[0013] In the aforementioned pure water production system, it is preferable to further include at least one of an electrodialysis device and an electroregenerative desalination device for recovering at least one of the acids and alkalis from the regenerated wastewater.

[0014] In the aforementioned pure water production system, it is preferable to use water from multiple water sources as raw water, and to bypass the first reverse osmosis membrane treatment device for a portion of the raw water according to the flow rate ratio of the water from each of the water sources.

[0015] In the pure water production system described above, if the water from the multiple water sources consists of water that has permeated through the reverse osmosis membrane and water that has not permeated through the reverse osmosis membrane, and the flow rate ratio of the water that has not permeated through the reverse osmosis membrane is 50 vol% or less of the raw water, it is preferable to bypass the first reverse osmosis membrane treatment device for a portion of the raw water.

[0016] The present invention relates to a method for producing pure water, comprising: a first reverse osmosis membrane treatment step of passing raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment step of performing ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water; and a second reverse osmosis membrane treatment step of passing the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water, wherein the second RO concentrated water is returned to the stage preceding the first reverse osmosis membrane treatment step.

[0017] In the above-mentioned method for producing pure water, it is preferable that the permeability coefficient of the first reverse osmosis membrane is smaller than that of the second reverse osmosis membrane.

[0018] In the above-mentioned method for producing pure water, it is preferable to regenerate the ion exchange resin using recycled water and recover at least one of the acids and alkalis from the resulting recycled wastewater.

[0019] In the above-mentioned method for producing pure water, it is preferable to further include at least one of an electrodialysis step and an electroregenerative desalination step for recovering at least one of an acid and an alkali from the regenerated wastewater.

[0020] In the above-mentioned method for producing pure water, it is preferable to use water from multiple water sources as raw water, and to bypass the first reverse osmosis membrane treatment step for a portion of the raw water according to the flow rate ratio of the water from each of the water sources.

[0021] In the above-mentioned method for producing pure water, if the water from the multiple water sources consists of water that has permeated through a reverse osmosis membrane and water that has not permeated through a reverse osmosis membrane, and the flow rate ratio of the water that has not permeated through the reverse osmosis membrane is 50 vol% or less of the raw water, it is preferable to bypass the first reverse osmosis membrane treatment step for a portion of the raw water.

[0022] The present invention provides a pure water production system and method that can reduce the concentration of TOC components in the obtained pure water while suppressing a decrease in the overall water recovery rate of the system, in a pure water production process that sequentially performs reverse osmosis membrane treatment, ion exchange treatment using ion exchange resin, and reverse osmosis membrane treatment.

[0023] This is a schematic diagram showing an example of a pure water production system according to an embodiment of the present invention. This is a schematic diagram showing another example of a pure water production system according to an embodiment of the present invention. This is a schematic diagram showing another example of a pure water production system according to an embodiment of the present invention. This is a diagram showing the overall water recovery rate of the pure water production system of Comparative Example 1. This is a diagram showing the overall water recovery rate of the pure water production system of Example 1.

[0024] Embodiments of the present invention will be described below. This embodiment is just one example of how the present invention can be implemented, and the present invention is not limited to this embodiment.

[0025] Figure 1 shows a schematic of an example of a pure water production system according to an embodiment of the present invention, and its configuration will be described. The pure water production system 1 shown in Figure 1 is an example of a pure water production system that sequentially performs reverse osmosis membrane treatment, ion exchange treatment using ion exchange resin, and reverse osmosis membrane treatment.

[0026] The pure water production system 1 includes a first reverse osmosis membrane treatment device 16 that passes raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water, an ion exchange treatment device 22 that performs ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water, a second reverse osmosis membrane treatment device 26 that passes the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water, and a return pipe 64 as a return means for returning the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device 16.

[0027] The pure water production system 1 may include, in front of the first reverse osmosis membrane treatment device 16 (first reverse osmosis membrane treatment), a raw water tank 10 for storing raw water, a filtration device 12 as a filtration means for removing turbidity from the raw water, and a tank 14 for storing the turbidity-removed water obtained by the turbidity-removing treatment. The pure water production system 1 may also include, in front of the ion exchange treatment device 22, a tank 18 for storing the first RO permeate obtained by the first reverse osmosis membrane treatment, and an activated carbon treatment device 20 as an activated carbon treatment means for removing or reducing residual chlorine and organic matter from the first RO permeate. The pure water production system 1 may also include, in front of the second reverse osmosis membrane treatment device 26, a tank 24 for storing the ion-exchanged water obtained by the ion exchange treatment. The pure water production system 1 may include a tank 28 for storing the first RO concentrated water obtained by the first reverse osmosis membrane treatment, and a third reverse osmosis membrane treatment device 30 for passing the first RO concentrated water through a third reverse osmosis membrane to obtain third RO permeate and third RO concentrated water. The pure water production system 1 may also include a tank 32 for diluting a regenerating agent for regenerating the ion exchange resin of the ion exchange treatment device 22 with a diluent such as water to obtain regeneration water, and a tank 34 for storing regeneration wastewater generated during the regeneration of the ion exchange resin.

[0028] In the pure water production system 1 shown in Figure 1, a pipe 40 is connected to the raw water inlet of the raw water tank 10. The raw water outlet of the raw water tank 10 and the inlet of the filtration device 12 may be connected by a pipe (not shown). The turbidity-removed water outlet of the filtration device 12 and the turbidity-removed water inlet of the tank 14 are connected by a pipe 42. The turbidity-removed water outlet of the tank 14 and the turbidity-removed water inlet of the first reverse osmosis membrane treatment device 16 may be connected by a pipe (not shown). The first RO permeate outlet of the first reverse osmosis membrane treatment device 16 and the first RO permeate inlet of the tank 18 are connected by a pipe 44. The first RO permeate outlet of the tank 18 and the first RO permeate inlet of the activated carbon treatment device 20 may be connected by a pipe (not shown). The activated carbon treated water outlet of the activated carbon treatment device 20 and the activated carbon treated water inlet of the ion exchange treatment device 22 may be connected by a pipe (not shown). The ion-exchange treated water outlet of the ion-exchange treatment device 22 and the ion-exchange treated water inlet of the tank 24 are connected by piping 52. The ion-exchange treated water outlet of the tank 24 and the ion-exchange treated water inlet of the second reverse osmosis membrane treatment device 26 may also be connected by piping (not shown). Piping 62 is connected to the second RO permeate outlet of the second reverse osmosis membrane treatment device 26. The second RO concentrated water outlet of the second reverse osmosis membrane treatment device 26 and the second RO concentrated water inlet of the raw water tank 10 are connected by return piping 64. The regeneration water outlet of the tank 32 and the regeneration water inlet of the ion-exchange treatment device 22 are connected by piping 54. The regeneration wastewater outlet of the ion-exchange treatment device 22 and the regeneration wastewater inlet of the tank 34 are connected by piping 56. Piping 58 is connected to the regeneration wastewater outlet of the tank 34. Piping 66, which branches off from return piping 64, is connected to the second RO concentrated water inlet of the tank 32. The first RO concentrated water outlet of the first reverse osmosis membrane treatment device 16 and the first RO concentrated water inlet of the water tank 28 are connected by piping 46. The first RO concentrated water outlet of the water tank 28 and the first RO concentrated water inlet of the third reverse osmosis membrane treatment device 30 may also be connected by piping (not shown). Piping 48 is connected to the third RO permeate outlet of the third reverse osmosis membrane treatment device 30, and piping 50 is connected to the third RO concentrated water outlet.

[0029] The operation of the pure water production method and the pure water production system 1 according to this embodiment will be described.

[0030] Raw water is stored in the raw water tank 10 as needed through piping 40. The raw water is sent from the raw water tank 10 to the filtration device 12. The filtration device 12 performs turbidity removal treatment on the raw water as needed (turbidity removal treatment step). The turbidity-removed water is stored in the tank 14 as needed through piping 42. The turbidity-removed water is sent from the tank 14 to the first reverse osmosis membrane treatment device 16. In the first reverse osmosis membrane treatment device 16, the raw water (in this case, the turbidity-removed raw water, which is the turbidity-removed water) is passed through the first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water (first reverse osmosis membrane treatment step). The first RO permeate obtained in the first reverse osmosis membrane treatment is stored in the tank 18 as needed through piping 44. The first RO permeate is sent from the tank 18 to the activated carbon treatment device 20. If necessary, the first RO permeate is treated with activated carbon in the activated carbon treatment apparatus 20 to remove or reduce residual chlorine and organic matter in the first RO permeate (activated carbon treatment step). The activated carbon treated water obtained from the activated carbon treatment is sent from the activated carbon treatment apparatus 20 to the ion exchange treatment apparatus 22. In the ion exchange treatment apparatus 22, the first RO permeate (in this case, the activated carbon treated water, which is the first RO permeate treated with activated carbon) is subjected to ion exchange treatment with an ion exchange resin to obtain ion-exchange treated water (ion exchange treatment step). The ion-exchange treated water obtained in the ion exchange treatment step is stored in the water tank 24 as necessary through the piping 52. The ion-exchange treated water is sent from the water tank 24 to the second reverse osmosis membrane treatment apparatus 26. In the second reverse osmosis membrane treatment apparatus 26, the ion-exchange treated water is passed through the second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water (second reverse osmosis membrane treatment step). The second RO permeate obtained from the second reverse osmosis membrane treatment is discharged as pure water through piping 62. At least a portion of the second RO concentrated water obtained from the second reverse osmosis membrane treatment is returned through return piping 64 to the raw water tank 10, which is the upstream stage of the first reverse osmosis membrane treatment device 16 (return process).

[0031] The first RO concentrated water obtained in the first reverse osmosis membrane treatment is stored in the water tank 28 as needed through the piping 46. The first RO concentrated water is sent from the water tank 28 to the third reverse osmosis membrane treatment device 30. In the third reverse osmosis membrane treatment device 30, the first RO concentrated water may be passed through the third reverse osmosis membrane to obtain third RO permeate and third RO concentrated water (third reverse osmosis membrane treatment step). The third RO permeate obtained in the third reverse osmosis membrane treatment may be discharged through the piping 48, or sent to the raw water tank 10 through the piping 48 as needed. The third RO concentrated water obtained in the third reverse osmosis membrane treatment is discharged as wastewater through the piping 50.

[0032] A regenerating agent for regenerating the ion exchange resin is introduced into the water tank 32, for example, through piping 60. In the water tank 32, the regenerating agent is diluted with a diluent such as water to obtain regenerative water (regenerative water production process). At least a portion of the second RO concentrated water may be sent to the water tank 32 through piping 66 branched from the return piping 64 and used as a diluent to dilute the regenerating agent. When regeneration of the ion exchange resin of the ion exchange treatment device 22 becomes necessary, the regenerative water containing the regenerating agent is sent to the ion exchange treatment device 22 through piping 54. The ion exchange resin is regenerated in the ion exchange treatment device 22 using the regenerative water (regeneration process). The regeneration waste liquid generated during the regeneration of the ion exchange resin is stored in the water tank 34 as needed through piping 56. The regeneration waste liquid is discharged from the water tank 34 through piping 58.

[0033] The inventors have found that in a pure water production method in which reverse osmosis membrane treatment (first reverse osmosis membrane treatment), ion exchange treatment using ion exchange resin, and reverse osmosis membrane treatment (second reverse osmosis membrane treatment) are performed in sequence, the decrease in the overall water recovery rate of the system can be suppressed by returning the concentrated water from the second reverse osmosis membrane treatment (second RO concentrated water) to the stage before the reverse osmosis membrane treatment (first reverse osmosis membrane treatment) that precedes the ion exchange treatment (for example, the raw water tank 10). Furthermore, if the second RO concentrated water is returned to, for example, a tank that stores the water to be treated in the ion exchange treatment (for example, the water tank 18), another problem arises in which trace amounts of TOC components leached from the ion exchange resin are circulated and concentrated within the system, increasing the TOC concentration of the primary pure water. However, by returning the second RO concentrated water to the stage before the first reverse osmosis membrane treatment, the TOC components leached from the ion exchange resin can be removed in the first reverse osmosis membrane treatment, and their concentration within the system can be suppressed. Furthermore, because reverse osmosis membrane treatment (first reverse osmosis membrane treatment) is performed before the ion exchange treatment, even if activated carbon treatment is performed before the ion exchange treatment, it has the effect of extending the lifespan of the activated carbon.

[0034] In Patent Document 1, the concentrated water from the reverse osmosis membrane treatment after the ion exchange resin treatment in a pure water wastewater recovery system is recovered in a separate wastewater recovery system for general-purpose water or a wastewater treatment system. On the other hand, in the pure water production method and pure water production system according to this embodiment, the second RO concentrated water from the second reverse osmosis membrane treatment is returned to the stage before the first reverse osmosis membrane treatment in the same system and recovered. As a result, there is almost no excess recovered water, and water can be recovered, suppressing a decrease in the overall water recovery rate of the system. Furthermore, as will be described later, by making the permeability coefficient of the first reverse osmosis membrane of the first reverse osmosis membrane treatment device 16 smaller than that of the second reverse osmosis membrane of the subsequent second reverse osmosis membrane treatment device 26, the circulation and concentration of TOC components eluted from the ion exchange resin in the system can be suppressed, and the water can be efficiently discharged outside the system.

[0035] The raw water is not particularly limited, and for example, groundwater, industrial water, municipal water, etc. are used. The raw water is water containing components that can clog the reverse osmosis membrane, such as at least one of hardness components and silica. The raw water is stored in the raw water tank 10 as necessary. Water from multiple water sources may be used as the raw water. In some cases, water with reduced hardness components and silica, such as recycled water or recovered water that has been treated once with a reverse osmosis membrane, may be partially mixed with the raw water for use.

[0036] When the raw water contains hardness components, the concentration of the hardness components in the raw water is, for example, in the range of 1 to 100 mg / L. When the raw water contains silica, the concentration of silica in the raw water is, for example, in the range of 1 to 100 mg / L.

[0037] The raw water is sent to the raw water tank 10 by means such as a pump, and is subjected to turbidity removal treatment by a filtration device 12 such as sand filtration treatment or turbidity removal membrane treatment as necessary, and then introduced into the first reverse osmosis membrane treatment device 16. Before the first reverse osmosis membrane treatment device 16 (the first reverse osmosis membrane treatment step), a reducing agent, a bactericide (slime inhibitor), a dispersant (scale inhibitor), etc. may be added. Also, a water tank 28 for storing the first RO concentrated water of the first reverse osmosis membrane treatment device 16 and a pump, etc. may be provided, and a third reverse osmosis membrane treatment device 30 may be provided as a brine reverse osmosis membrane treatment device for further concentrating the first RO concentrated water. The third RO permeate water of the brine reverse osmosis membrane treatment (the third reverse osmosis membrane treatment) may be returned to the raw water tank 10, etc., or discharged outside the system.

[0038] The recovery rate (including brine reverse osmosis membrane treatment) represented by (first RO permeate water volume / inlet water volume) × 100 of the first reverse osmosis membrane treatment varies depending on the quality of the raw water, but is, for example, in the range of 30 to 95%, preferably in the range of 50 to 93%, and more preferably in the range of 60 to 90%. If the recovery rate of the first reverse osmosis membrane treatment is too high, the risk of scale precipitation may increase, and if it is too low, the amount of raw water used may increase.

[0039] When using a part of the water that has been treated once with a reverse osmosis membrane, such as recycled water, as the raw water, the recycled water, etc. may be sent to the water tank 18 for storing the treated water (the first RO permeate water) of the activated carbon treatment instead of the raw water tank 10.

[0040] The first RO permeate water obtained by the first reverse osmosis membrane treatment apparatus 16 (first reverse osmosis membrane treatment step) is sent to the subsequent stage, stored in the water tank 18 as necessary, and then subjected to activated carbon treatment as necessary, and then sent to the ion exchange treatment apparatus 22.

[0041] The ion exchange resin used in the ion exchange treatment apparatus 22 is at least one of cation exchange resins such as strongly acidic cation exchange resin and weakly acidic cation exchange resin, and anion exchange resins such as strongly basic anion exchange resin and weakly basic anion exchange resin. In the ion exchange treatment apparatus 22 (ion exchange treatment step), for example, a so-called two-bed three-column type (2B3T) including a cation exchange tower containing a cation exchange resin, a decarbonation tower for removing or reducing carbon dioxide by blowing air or the like, and an anion exchange tower containing an anion exchange resin, or a known method such as a mixed bed type in which a cation exchange resin and an anion exchange resin are housed in one tower is adopted. The ion exchange treated water obtained by the ion exchange treatment is sent to the subsequent second reverse osmosis membrane treatment apparatus 26.

[0042] The ion exchange resin is periodically regenerated with a regenerant such as an acid such as hydrochloric acid, sulfuric acid, and nitric acid, and an alkali such as sodium hydroxide and potassium hydroxide. The regenerant used for regenerating the ion exchange resin is, for example, diluted with a diluent such as water in the water tank 32 and then used.

[0043] The second RO permeate water obtained by the second reverse osmosis membrane treatment apparatus 26 (second reverse osmosis membrane treatment step) is used as primary pure water. A part of the second RO concentrated water may be used as a diluent for regenerating the ion exchange resin or as washing water for the ion exchange resin, and the other part is returned to the upstream of the first reverse osmosis membrane treatment apparatus 16 (first reverse osmosis membrane treatment step), such as the raw water tank 10. The second RO concentrated water may be returned to the pipes 40, 42, and the water tank 14 upstream of the first reverse osmosis membrane treatment apparatus 16 (first reverse osmosis membrane treatment step). At this time, if the second RO concentrated water is returned to, for example, the water tank 18 or the pipe 44 that stores the water to be treated (first RO permeate water) for activated carbon treatment, the TOC component eluted from the ion exchange resin may be circulated and concentrated in the system, and the TOC of the primary pure water may increase. Therefore, it is returned to the upstream of the first reverse osmosis membrane treatment apparatus 16.

[0044] The recovery rate of the second reverse osmosis membrane treatment, expressed as (second RO permeate volume / inlet volume) × 100, is, for example, in the range of 50 to 98%, preferably in the range of 60 to 96%, and more preferably in the range of 70 to 95%. If the recovery rate of the second reverse osmosis membrane treatment is too high, the risk of scale deposition may increase, and if it is too low, the amount of primary pure water obtained may be small.

[0045] The entire amount of the second RO concentrated water may be returned to the upstream stage of the first reverse osmosis membrane treatment device 16, or a portion may be used as a diluent for regenerating the ion exchange resin or as washing water for the ion exchange resin. In order to increase the water recovery rate of the entire pure water production system, it is preferable to return 80% or more, preferably 90% or more, of the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device 16.

[0046] As the second reverse osmosis membrane, a membrane is used with a permeability coefficient (a value calculated as permeate volume / membrane area / operating pressure when pure water is passed through at a temperature of 25°C and an operating pressure of 0.75 MPa) in the range of, for example, 0.2 m / d / MPa to 2.0 m / d / MPa, preferably in the range of 0.4 m / d / MPa to 1.8 m / d / MPa. In this case, the permeability coefficient of the first reverse osmosis membrane may be set to, for example, less than 1.0 m / d / MPa, and the permeability coefficient of the second reverse osmosis membrane may be set to, for example, 1.0 m / d / MPa or higher, so that the permeability coefficient of the first reverse osmosis membrane is smaller than that of the second reverse osmosis membrane. Generally, membranes with a high permeability coefficient have a low rejection rate of TOC components, and membranes with a low permeability coefficient have a high rejection rate of TOC components. Therefore, by making the permeability coefficient of the first reverse osmosis membrane lower than that of the second reverse osmosis membrane, even if the TOC components eluted from the ion exchange resin are concentrated by the second reverse osmosis membrane and returned to the previous stage as components contained in the concentrated water, they can be effectively removed by the first reverse osmosis membrane, thereby suppressing circulation and concentration within the system.

[0047] The pure water production method and pure water production system according to this embodiment can achieve a water recovery rate of, for example, 85% or more, preferably 87% or more, for the entire pure water production system.

[0048] Figure 2 shows another example of a pure water production method and pure water production system according to this embodiment, and is a diagram illustrating an example of a flow in which at least one of the acids and alkalis from the regeneration wastewater of an ion exchange resin is recovered using at least one of an electrodialysis machine and an electroregenerative desalination machine.

[0049] The pure water production system 3 shown in Figure 2 further includes a recovery device 36 that recovers at least one of the acids and alkalis from the recycled wastewater, in addition to the configuration shown in Figure 1.

[0050] In the pure water production system 3, the wastewater outlet of the water tank 34 and the wastewater inlet of the recovery device 36 are connected by a pipe 58. A pipe 68 is connected to the recovered acid / alkali outlet of the recovery device 36, and a pipe 70 is connected to the recovered liquid outlet.

[0051] The regeneration waste liquid generated during the regeneration of the ion exchange resin is stored in the water tank 34 as needed through piping 56. The regeneration waste liquid may be discharged from the water tank 34 through piping 58 as shown in Figure 1, or it may be recovered by separating the acid and alkali from the regeneration waste liquid using a recovery device 36 as shown in Figure 2 (recovery step). The acid and alkali recovered by the recovery device 36 are discharged through piping 68, and the recovered liquid is discharged through piping 70.

[0052] As the recovery device 36, methods such as electrodialysis (ED) and electroregenerative desalination (EDI) are used, employing equipment such as an electrodialysis (ED) device or an electroregenerative desalination (EDI) device. The recovered acid and alkali may be reused as a regenerating agent or used for other purposes. The recovered liquid from which the regenerating agent has been recovered can be drained.

[0053] An electrodialysis (ED) apparatus, for example, has cation exchange membranes and anion exchange membranes arranged alternately between a pair of electrodes to form a desalination chamber and a concentration chamber. In an electrodialysis apparatus, when regenerated wastewater is supplied to the desalination chamber and a direct current is passed through it, cations are attracted to the cathode side and move through the cation exchange membrane to the concentration chamber, while anions are attracted to the anode side and move through the anion exchange membrane to the concentration chamber. As a result, ionic components are removed or reduced from the regenerated wastewater in the desalination chamber to obtain a recovered liquid, and the ionic components are recovered in the concentration chamber.

[0054] An electro-regenerative desalination (EDI) apparatus is a device that combines electrophoresis and electrodialysis. The configuration of an EDI apparatus is, for example, one in which a desalination chamber, partitioned by a pair of ion exchange membranes, is placed between the anode and the cathode. In an electro-regenerative desalination apparatus, at least the desalination chamber is filled with ion exchange resin, and the regeneration waste liquid is passed through the desalination chamber while a DC voltage is applied between the anode and the cathode. As a result, desalination treatment is performed on the regeneration waste liquid in the desalination chamber, and the recovered liquid, from which ionic components have been removed or reduced, flows out of the desalination chamber.

[0055] In the pure water production method and pure water production system according to this embodiment, a first reverse osmosis membrane treatment device 16 (first reverse osmosis membrane treatment step) is provided before the ion exchange treatment device 22 (ion exchange treatment step) that uses an ion exchange resin. As a result, the load of scale components on the ion exchange resin is reduced, which reduces the risk of clogging of the electrodialysis device or electroregenerative desalination device with scale. This allows for efficient recovery of acids and alkalis and a system that reduces the salt concentration in the regenerated wastewater.

[0056] On the other hand, since the amount of water required for regeneration and washing in the ion exchange treatment device 22 is reduced, the use of the second RO concentrated water in the subsequent second reverse osmosis membrane treatment device 26 (second reverse osmosis membrane treatment process) is limited. Therefore, this method and system, which return the second RO concentrated water to the preceding first reverse osmosis membrane treatment device 16 (first reverse osmosis membrane treatment process), is effective.

[0057] While providing a first reverse osmosis membrane treatment device 16 before the ion exchange treatment device 22 presents the problem of reduced overall water recovery rate due to the generation of first RO concentrated water, as in the pure water production method and pure water production system according to this embodiment, by returning the second RO concentrated water from the second reverse osmosis membrane treatment device 26 downstream of the ion exchange treatment device 22 to the stage before the first reverse osmosis membrane treatment device 16, it is possible to reduce the salt concentration in the recycled wastewater discharged from the system while suppressing the reduction in overall water recovery rate.

[0058] Figure 3 shows another example of a pure water production method and pure water production system according to this embodiment, and is a diagram showing an example of the flow when water from multiple water sources flows into the raw water tank 10.

[0059] In the pure water production system 5 shown in Figure 3, one or more pipes 72 for supplying raw water from other water sources may be further connected to the raw water inlet of the raw water tank 10. In the pure water production system 5, the turbidity-removed water outlet of the tank 14 that stores turbidity-removed water and the turbidity-removed water inlet of the tank 18 that stores the first RO permeate may be connected by a bypass pipe 74.

[0060] When water from multiple sources is introduced as raw water into the raw water tank 10, the first reverse osmosis membrane treatment device 16 (first reverse osmosis membrane treatment process) may be bypassed for a portion of the raw water, depending on the flow rate ratio of each source. If a large proportion of the raw water is recycled water with a low scale risk that has already permeated the reverse osmosis membrane, the water recovery rate can be further increased by partially bypassing the first reverse osmosis membrane treatment device 16 and sending the water, for example, from tank 14 to tank 18. If a large proportion of water has a low scale risk, the recovery rate can be increased while sufficiently reducing the load on the ion exchange resin without passing the entire amount of water through the first reverse osmosis membrane. If the equipment supplying water that has permeated the reverse osmosis membrane experiences a water shortage or the amount of water supplied decreases due to some trouble, and the proportion of water that has permeated the reverse osmosis membrane supplied to this system decreases, the water recovery rate can be maintained at a high level while reducing the load on the ion exchange resin by reducing the amount of water bypassed and passing the water through the first reverse osmosis membrane.

[0061] In particular, when the water from multiple water sources consists of water that has permeated the reverse osmosis membrane and water that has not permeated the reverse osmosis membrane, and the flow rate ratio of water that has not permeated the reverse osmosis membrane is 50 vol% or less of the raw water, preferably 30 vol% or less, it is preferable to bypass the first reverse osmosis membrane treatment device 16 (first reverse osmosis membrane treatment step) for a portion of the raw water. Since water that has permeated the reverse osmosis membrane generally contains less scale, the water recovery rate can be increased by bypassing a portion of it.

[0062] The ratio of water flow rates from multiple water sources may be determined based on the measured flow rates of each raw water source, by installing flow rate measuring means such as flow meters in each of the pipes 40 and 72.

[0063] This specification includes the following embodiments: (1) A pure water production system for producing pure water, comprising: a first reverse osmosis membrane treatment device that passes raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment device that performs ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water; a second reverse osmosis membrane treatment device that passes the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water; and a return means for returning the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device.

[0064] (2) A pure water production system according to (1), wherein the permeability coefficient of the first reverse osmosis membrane is smaller than the permeability coefficient of the second reverse osmosis membrane.

[0065] (3) A pure water production system according to (1) or (2), wherein the ion exchange resin is regenerated using recycled water, and at least one of acids and alkalis is recovered from the resulting recycled wastewater.

[0066] (4) A pure water production system according to (3), further comprising at least one of an electrodialysis device and an electroregenerative desalination device for recovering at least one of an acid and an alkali from the regenerated wastewater.

[0067] (5) A pure water production system according to any one of (1) to (4), wherein water from multiple water sources is used as raw water, and a portion of the raw water is bypassed by the first reverse osmosis membrane treatment device according to the flow rate ratio of the water from each of the water sources.

[0068] (6) A pure water production system according to (5), wherein the water from the plurality of water sources consists of water that has permeated through a reverse osmosis membrane and water that has not permeated through a reverse osmosis membrane, and when the flow rate ratio of water that has not permeated through a reverse osmosis membrane is 50 vol% or less of the raw water, the first reverse osmosis membrane treatment device is bypassed for a portion of the raw water.

[0069] (7) A method for producing pure water, comprising: a first reverse osmosis membrane treatment step of passing raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment step of performing ion exchange treatment of the first RO permeate with an ion exchange resin to obtain ion-exchange treated water; and a second reverse osmosis membrane treatment step of passing the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water, wherein the second RO concentrated water is returned to the stage preceding the first reverse osmosis membrane treatment step.

[0070] (8) A method for producing pure water as described in (7), wherein the permeability coefficient of the first reverse osmosis membrane is smaller than the permeability coefficient of the second reverse osmosis membrane.

[0071] A method for producing pure water according to (9), (7), or (8), wherein the ion exchange resin is regenerated using recycled water, and at least one of the acids and alkalis is recovered from the resulting recycled wastewater.

[0072] A method for producing pure water according to (10)(9), further comprising at least one of an electrodialysis step and an electroregenerative desalination step for recovering at least one of an acid and an alkali from the regenerated wastewater.

[0073] A method for producing pure water according to any one of (11)(7) to (10), wherein water from multiple water sources is used as raw water, and the first reverse osmosis membrane treatment step is bypassed for a portion of the raw water according to the flow rate ratio of the water from each of the water sources.

[0074] A method for producing pure water according to (12)(11), wherein the water from the plurality of water sources consists of water that has permeated through a reverse osmosis membrane and water that has not permeated through a reverse osmosis membrane, and when the flow rate ratio of water that has not permeated through the reverse osmosis membrane is 50 vol% or less of the raw water, the first reverse osmosis membrane treatment step is bypassed for a portion of the raw water.

[0075] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0076] <Comparative Example 1> As Comparative Example 1, the water balance under the following treatment conditions is shown below in a flow system that sequentially includes a first reverse osmosis membrane treatment device as shown in Figure 4, an ion exchange treatment device 2B3T (cation exchange column (K column), decarbonation column (D column), anion exchange column (A column)), and a second reverse osmosis membrane treatment device.

[0077] (Processing conditions) - Flow rate of raw water: 12,000 m³ ３ Let / d. • In sand filtration treatment, sand filtration treated water is used as the backwash water for backwashing, and the backwash wastewater is 150 m ３ / d is discharged. • The recovery rate of the first reverse osmosis membrane treatment device is set at 90%, with 10% (1185m) being discharged. ３ / d) is discharged as first RO concentrated water. *Reverse osmosis membrane recovery rate (%) = Permeate volume / Inlet volume × 100 (11850 / 10665 × 100) *In the activated carbon tower, first RO permeate water etc. is used as backwash water for backwashing, and the backwash wastewater is 50m ３ / d is discharged. ・At Tower K and Tower A, 40m³ each is discharged as recycled wastewater. ３ / d is discharged. The recovery rate of the second reverse osmosis membrane treatment device is set to 95%, and 5% (531m) ３ / d) is discharged as second RO concentrated water, and a portion of the second RO concentrated water (40m ３(per day) is used as the recycled water for the K tower and the A tower, and the remainder (451 m ３ / d) is "discharged outside the system". As a result, the water recovery rate of the entire pure water production system is (10084 / 12000) × 100 = 84.0%.

[0078] <Example 1> As Example 1, the water balance under the following treatment conditions in the flow that sequentially includes a first reverse osmosis membrane treatment device, 2B3T (cation exchange tower (K tower), decarbonation tower (D tower), anion exchange tower (A tower)) as an ion exchange treatment device, and a second reverse osmosis membrane treatment device is shown below.

[0079] (Treatment conditions) - The flow rate of raw water is 12000 m ３ / d. - In the sand filtration treatment, sand filtration treated water or the like is used as the backwash water for backwashing, and the backwash drainage is 150 m ３ / d is discharged. - The recovery rate of the first reverse osmosis membrane treatment device is 90%, and 10% (1272 m ３ / d) is discharged as the first RO concentrated water. - In the activated carbon tower, the first RO permeated water or the like is used as the backwash water for backwashing, and the backwash drainage is 50 m ３ / d is discharged. - In the K tower and the A tower, 40 m each ３ / d is discharged as the regeneration waste liquid. - The recovery rate of the second reverse osmosis membrane treatment device is 95%, and 5% (552 m ３ / d) is discharged as the second RO concentrated water, and a part of the second RO concentrated water (40 m ３ / d each) is used as the recycled water, and the remainder (472 m ３ / d) is "circulated to the raw water tank". As a result, the water recovery rate of the entire pure water production system is (10488 / 12000) × 100 = 87.4%, and by returning the second RO concentrated water to the front stage of the first reverse osmosis membrane treatment device, the recovery rate can be made higher than that in Comparative Example 1.

[0080] Thus, in the production of pure water by sequentially performing reverse osmosis membrane treatment, ion exchange treatment using ion exchange resin, and reverse osmosis membrane treatment, by returning the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device as in the example, it was possible to suppress the decrease in the overall water recovery rate of the system. Furthermore, it was possible to reduce the concentration of TOC components in the obtained pure water.

[0081] 1, 3, 5 Pure water production system, 10 Raw water tank, 12 Filtration device, 14, 18, 24, 28, 32, 34 Tanks, 16 First reverse osmosis membrane treatment device, 20 Activated carbon treatment device, 22 Ion exchange treatment device, 26 Second reverse osmosis membrane treatment device, 30 Third reverse osmosis membrane treatment device, 36 Recovery device, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 66, 68, 70, 72 Piping, 64 Return piping, 74 Bypass piping.

Claims

1. A pure water production system for producing pure water, comprising: a first reverse osmosis membrane treatment device that passes raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment device that performs ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water; a second reverse osmosis membrane treatment device that passes the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water; and a return means for returning the second RO concentrated water to the upstream stage of the first reverse osmosis membrane treatment device.

2. A pure water production system according to claim 1, characterized in that the permeability coefficient of the first reverse osmosis membrane is smaller than the permeability coefficient of the second reverse osmosis membrane.

3. A pure water production system according to claim 1 or 2, characterized in that the ion exchange resin is regenerated using recycled water, and at least one of acids and alkalis is recovered from the resulting recycled wastewater.

4. A pure water production system according to claim 3, further comprising at least one of an electrodialysis device and an electroregenerative desalination device for recovering at least one of an acid and an alkali from the regenerated wastewater.

5. A pure water production system according to any one of claims 1 to 4, characterized in that water from a plurality of water sources is used as raw water, and a portion of the raw water is bypassed by the first reverse osmosis membrane treatment device according to the flow rate ratio of the water from each of the water sources.

6. A pure water production system according to claim 5, characterized in that the water from the plurality of water sources consists of water that has permeated through a reverse osmosis membrane and water that has not permeated through a reverse osmosis membrane, and when the flow rate ratio of water that has not permeated through the reverse osmosis membrane is 50 vol% or less of the raw water, the first reverse osmosis membrane treatment device is bypassed for a portion of the raw water.

7. A method for producing pure water, comprising: a first reverse osmosis membrane treatment step of passing raw water through a first reverse osmosis membrane to obtain first RO permeate and first RO concentrated water; an ion exchange treatment step of performing ion exchange treatment on the first RO permeate using an ion exchange resin to obtain ion-exchange treated water; and a second reverse osmosis membrane treatment step of passing the ion-exchange treated water through a second reverse osmosis membrane to obtain second RO permeate and second RO concentrated water, wherein the second RO concentrated water is returned to the stage preceding the first reverse osmosis membrane treatment step.

8. A method for producing pure water according to claim 7, characterized in that the permeability coefficient of the first reverse osmosis membrane is smaller than the permeability coefficient of the second reverse osmosis membrane.

9. A method for producing pure water according to claim 7 or 8, characterized in that the ion exchange resin is regenerated using recycled water, and at least one of acids and alkalis is recovered from the resulting recycled wastewater.

10. A method for producing pure water according to claim 9, further comprising at least one of an electrodialysis step and an electroregenerative desalination step for recovering at least one of an acid and an alkali from the regenerated wastewater.

11. A method for producing pure water according to any one of claims 7 to 10, characterized in that the raw water is water from a plurality of water sources, and the first reverse osmosis membrane treatment step is bypassed for a portion of the raw water according to the flow rate ratio of the water from each of the water sources.

12. A method for producing pure water according to claim 11, characterized in that the water from the plurality of water sources consists of water that has permeated through a reverse osmosis membrane and water that has not permeated through a reverse osmosis membrane, and when the flow rate ratio of water that has not permeated through the reverse osmosis membrane is 50 vol% or less of the raw water, the first reverse osmosis membrane treatment step is bypassed for a portion of the raw water.

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