Water treatment method and water treatment device

Abstract

A water treatment method according to the present invention comprises the steps of: mixing raw water and demineralized water, supplying the mixture to a reverse osmosis membrane as water to be treated, thereby separating same into permeated water and concentrated water; and adding a scale inhibitor to the water to be treated. The step of adding the scale inhibitor includes a step of adjusting the amount of the scale inhibitor added to the water to be treated on the basis of the mixing ratio of the raw water and the demineralized water in the water to be treated.

Description

Water treatment method and water treatment device

[0001] The present invention relates to a water treatment method and a water treatment device.

[0002] A water treatment system using a reverse osmosis (RO) membrane is known as a water treatment system for removing impurities from water to be treated. In this system, the water to be treated is supplied to the RO membrane at a predetermined supply pressure, and the RO membrane separates the water into permeate and concentrate. This allows the treated water (permeate) from which impurities have been removed to be obtained.

[0003] Water treatment devices with RO membranes are required to operate stably. To achieve this, it is important to prevent impurities in the water being treated from precipitating and forming scale on the surface of the RO membrane. A known countermeasure to prevent such scaling is to add a scale inhibitor to the water being treated. A commonly used method for this purpose is to add a fixed amount of the scale inhibitor (see, for example, Patent Document 1).

[0004] JP 2011-147899 A

[0005] From the viewpoint of running costs, it is preferable to add as little scale inhibitor as possible. However, with the above-mentioned fixed-quantity addition method, the amount of scale inhibitor must be determined assuming the situation in which the risk of scale formation is highest, which inevitably results in an excessive amount of scale inhibitor being added. The risk of scale formation changes when the concentration of scale components in the water to be treated fluctuates. Such concentration fluctuations can occur, for example, when the water to be treated is a mixture of raw water such as surface water or groundwater with recovered treated water from other systems, such as in relatively large pure water production systems installed in various plants.

[0006] Therefore, an object of the present invention is to provide a water treatment method and a water treatment apparatus that reduce running costs by optimizing the amount of scale inhibitor added.

[0007] In order to achieve the above-mentioned objectives, the water treatment method of the present invention includes a step of mixing raw water and desalinated treated water, supplying the mixed water to a reverse osmosis membrane as treated water, and separating the mixed water into permeate and concentrated water, and a step of adding a scale inhibitor to the treated water, wherein the step of adding the scale inhibitor includes a step of adjusting the amount of scale inhibitor added to the treated water based on the mixing ratio of the raw water and desalinated treated water in the treated water.

[0008] The water treatment device of the present invention also includes a tank for storing a mixture of raw water and desalinated water as the water to be treated, a reverse osmosis membrane device for separating the water to be treated supplied from the tank into permeate and concentrated water, an addition device for adding a scale inhibitor to the water to be treated supplied to the reverse osmosis membrane device, and a control device for adjusting the amount of scale inhibitor added to the water to be treated by the addition device based on the mixture ratio of raw water and desalinated water in the water to be treated.

[0009] According to this water treatment method and water treatment device, even if the concentration of scale components in the water to be treated fluctuates due to changes in the mixing ratio of raw water and desalinated water, it is possible to add the scale inhibitor to the water to be treated in the right amount accordingly.

[0010] According to the present invention, the amount of scale inhibitor added can be optimized to reduce running costs.

[0011] 1 is a schematic configuration diagram of a water treatment device according to an embodiment of the present invention.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification, an ultrapure water production system that produces ultrapure water by sequentially treating water to be treated (mixed water) that is a mixture of raw water and desalinated water is exemplified as a water treatment system of the present invention, but the present invention is not limited thereto. For example, the present invention may be applied to other water treatment systems as long as the water to be treated is a mixture of water with a high risk of scale generation and water with a low risk of scale generation.

[0013] 1 is a schematic diagram of a water treatment system according to one embodiment of the present invention. Note that the configuration of the ultrapure water production system shown in the figure is merely an example and is not intended to limit the present invention. Needless to say, the configuration can be modified as appropriate depending on the purpose, application, and required performance of the system.

[0014] The water treatment device 1 sequentially treats the treated water, which includes raw water such as surface water and groundwater, to produce ultrapure water, and supplies the ultrapure water to a use point 2. The water treatment device 1 has a primary pure water system (pure water production device) 10 and a secondary pure water system (subsystem) 20.

[0015] The pure water production system 10 produces pure water (primary pure water) by sequentially treating the water to be treated, and includes a raw water tank 11, a first reverse osmosis membrane (RO membrane) device 12, and an ion removal device 13.

[0016] The first RO membrane device 12 treats the water to be treated supplied from the raw water tank 11 and separates it into concentrated water containing impurities and permeated water from which the impurities have been removed. The first RO membrane device 12 is connected to a water supply line L1 that supplies raw water from the raw water tank 11 to the first RO membrane device 12, a permeated water line L2 that distributes permeated water from the first RO membrane device 12, and a concentrated water line L3 that distributes concentrated water from the first RO membrane device 12. The permeated water line L2 is connected downstream to an ion removal device 13, and the concentrated water line L3 is connected downstream to a concentrated water tank 16 (described below). A raw water line L4 is connected to the raw water tank 11, through which raw water is supplied as needed, as described below. A recovered treated water line L5 is also connected to the raw water tank 11, through which recovered treated water is continuously supplied, as described below. The recovered treated water is desalinated water recovered from another water treatment device, and hereinafter, the water stored in the raw water tank 11, including this recovered treated water, will be collectively referred to as the water to be treated.

[0017] The water supply line L1 is provided with a pressure pump 14 and a conductivity sensor 15, and the concentrated water line L3 is provided with a manual valve V1. The pressure pump 14 pressurizes the water to be treated in the raw water tank 11 and supplies it to the first RO membrane device 12. The pressure pump 14 also adjusts the supply pressure by controlling its rotation speed with an inverter (not shown). The conductivity sensor 15 detects the conductivity of the water to be treated supplied to the first RO membrane device 12 and is used to control the amount of scale inhibitor to be added, as described below. The manual valve V1 adjusts the flow rate of the concentrated water flowing through the concentrated water line L3.

[0018] The ion removal device 13 is a device that removes ionic components from the permeate from the first RO membrane device 12 to produce primary pure water. There are no particular limitations on the configuration of the ion removal device 13, and for example, an electrodeionized water production device or a non-regenerated or regenerated mixed-bed ion exchange resin tower can be used. A primary pure water line L6 that circulates the primary pure water from the ion removal device 13 and supplies it to the sub-tank 21 is connected to the ion removal device 13.

[0019] Furthermore, the pure water production system 10 is configured to treat the concentrated water from the first RO membrane device 12 with another RO membrane to separate it into permeate and concentrated water. Specifically, the pure water production system 10 has a concentrated water tank 16 that stores the concentrated water from the first RO membrane device 12 (hereinafter also referred to as "primary concentrated water"), and a second RO membrane device 17 that treats the primary concentrated water stored in the concentrated water tank 16. The second RO membrane device 17 is connected to a concentrated water supply line L7 that supplies the primary concentrated water from the concentrated water tank 16 to the second RO membrane device 17, a return line L8 that circulates the permeate from the second RO membrane device 17 and returns it to the raw water tank 11, and a drainage line L9 that discharges the concentrated water from the second RO membrane device 17 to the outside. The concentrated water supply line L7 is provided with a pressure pump 18 that pressurizes the primary concentrated water in the concentrated water tank 16 and supplies it to the second RO membrane device 17, and the discharge line L9 is provided with a manual valve V2 for adjusting the flow rate of the concentrated water flowing through the discharge line L9. Note that the permeated water from the second RO membrane device 17 is also returned to the raw water tank 11 and treated as water to be treated together with the raw water and recovered treated water. Hereinafter, the water to be treated other than the raw water will also be collectively referred to as desalinated treated water.

[0020] Subsystem 20 processes the primary pure water produced by the pure water production system 10 to produce ultrapure water, and includes, for example, an ultraviolet oxidation device, a non-regenerative mixed-bed ion exchange device (cartridge polisher), and an ultrafiltration (UF) membrane device. These are installed on a circulation line L10, both ends of which are connected to a subtank 21, and circulation line L10 is equipped with a circulation pump 22 that circulates the primary pure water in the subtank 21 through the circulation line L10. The circulation line L10 and point of use 2 are connected by a water supply line L11, and water supply line L11 is equipped with a flow rate sensor 23 that detects the flow rate of ultrapure water supplied to the point of use 2.

[0021] During operation of the water treatment device 1, raw water stored in the raw water tank 11 is treated in the first RO membrane device 12 by operating the pressure pump 14 and adjusting the aperture of the manual valve V1, and separated into permeate and concentrated water. The permeate separated in the first RO membrane device 12 is supplied to the ion removal device 13 via the permeate line L2, and the concentrated water (primary concentrated water) separated in the first RO membrane device 12 is supplied to the concentrated water tank 16 via the concentrated water line L3 and stored there. The permeate supplied to the ion removal device 13 has ionic components removed therefrom, and is then supplied as primary pure water to the sub-tank 21 via the primary pure water line L6 and stored there. The primary concentrated water stored in the concentrated water tank 16 is treated in the second RO membrane device 17 by operating the pressure pump 18 and adjusting the aperture of the manual valve V2, and separated into permeate and concentrated water. The permeated water separated by the second RO membrane device 17 is returned to the raw water tank 11 through a return line L8 and stored therein, and the concentrated water separated by the second RO membrane device 17 is discharged to the outside through a drainage line L9. Meanwhile, the primary pure water stored in the sub-tank 21 is supplied to the subsystem 20 by operation of a circulation pump 22 and treated therein. The ultrapure water thus obtained is returned to the sub-tank 21 through a circulation line L10, and a portion of it is supplied from the circulation line L10 to the point of use 2 through a water supply line L11 in response to a water sampling request from the point of use 2.

[0022] During operation of the water treatment device 1, as described above, the raw water tank 11 is continuously supplied with permeated water from the second RO membrane device 17 via the return line L8, and also continuously supplied with recovered treated water from another water treatment device via the recovered treated water line L5. Furthermore, raw water, such as surface water or groundwater, is intermittently supplied to the raw water tank 11 via the raw water line L4 depending on the water level in the raw water tank 11 detected by a water level sensor (not shown). Specifically, while the water level in the raw water tank 11 is below a predetermined upper water level, the on-off valve (not shown) of the raw water line L4 is opened, and raw water is replenished to the raw water tank 11. When the water level in the raw water tank 11 reaches the predetermined upper water level, the on-off valve (not shown) of the raw water line L4 is closed, and the replenishment of raw water is stopped. This allows the water level in the raw water tank 11 to be maintained below the predetermined upper water level.

[0023] The water treatment device 1 also has a chemical additive device 30 that adds a scale inhibitor to the water to be treated that is supplied to the first RO membrane device 12 in order to prevent impurities (particularly silica or calcium) from precipitating and forming scale on the surface of the RO membrane of the first RO membrane device 12. The water treatment device 1 also has a control device 40 that controls the operation of the water treatment device 1, including the addition of the scale inhibitor by the chemical additive device 30.

[0024] The chemical addition device 30 includes a chemical tank 31 that stores a scale inhibitor, and a chemical injection pump 32 that is connected to the water supply line L1 via a chemical supply line L12 and that injects the scale inhibitor stored in the chemical tank 31 into the water supply line L1. The location where the scale inhibitor is added is not limited to the location shown in the figure, as long as it is upstream of the first RO membrane device 12, and may be, for example, between the pressure pump 14 and the first RO membrane device 12.

[0025] The scale inhibitor is not limited to any particular substance as long as it is capable of inhibiting the deposition of scale components such as silica and calcium. Examples of the scale inhibitor include phosphonic acid compounds such as phosphonic acids and salts thereof, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, and nitrilotrimethylphosphonic acid; phosphoric acid compounds such as orthophosphates and polymerized phosphates; maleic acid compounds such as polymaleic acid and maleic acid copolymers; and acrylic acid polymers. Examples of acrylic acid-based polymers include copolymers such as poly(meth)acrylic acid, maleic acid / (meth)acrylic acid, (meth)acrylic acid / sulfonic acid, and (meth)acrylic acid / nonionic group-containing monomers; terpolymers of (meth)acrylic acid / sulfonic acid / nonionic group-containing monomers, (meth)acrylic acid / acrylamide-alkylsulfonic acid / substituted (meth)acrylamide, and (meth)acrylic acid / acrylamide-arylsulfonic acid / substituted (meth)acrylamide. Examples of (meth)acrylic acids constituting the terpolymer include methacrylic acid, acrylic acid, and (meth)acrylate salts thereof, such as sodium salts. Examples of acrylamide-alkylsulfonic acids constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and its salts. Examples of substituted (meth)acrylamides constituting the terpolymer include t-butylacrylamide, t-octylacrylamide, and dimethylacrylamide.

[0026] Among these, it is preferable to use one containing at least one of a phosphonic acid compound and an acrylic acid polymer. Furthermore, in order to simultaneously suppress scale derived from calcium and silica, it is particularly preferable to use a scale inhibitor comprising 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid / 2-acrylamido-2-methylpropanesulfonic acid / substituted (meth)acrylamide.

[0027] Commercially available scale inhibitors for RO membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon (registered trademark)" series manufactured by BWA Water Additives, the "PermaTreat (registered trademark)" series manufactured by Nalco, the "Hypersperse (registered trademark)" series manufactured by General Electric Company, and the "Kuriverter (registered trademark)" series manufactured by Kurita Water Industries Ltd.

[0028] During operation of the water treatment device 1, the control device 40 controls the pressure pump 14 to maintain a constant flow rate of the permeate flowing through the permeate line L2 (a predetermined set flow rate). For example, as the water temperature changes, the viscosity of the water changes, which in turn changes the flow rate of the permeate separated by the RO membrane. In response to this change, the control device 40 controls the rotation speed of the pressure pump 14 via an inverter. That is, as the water temperature decreases, the viscosity of the water increases, resulting in a decrease in the flow rate of the permeate separated by the RO membrane. Therefore, the control device 40 increases the rotation speed of the pressure pump 14 to compensate for this decrease, thereby increasing the supply pressure of the water to be treated. Furthermore, as the water temperature increases, the viscosity of the water decreases, resulting in an increase in the flow rate of the permeate separated by the RO membrane. Therefore, the control device 40 reduces the rotation speed of the pressure pump 14 to offset this increase, thereby reducing the supply pressure of the water to be treated. In this way, the rotation speed of the pressure pump 14, i.e., the supply pressure of the water to be treated, is adjusted, and thereby the flow rate of the permeated water flowing through the permeated water line L2 is adjusted to a set flow rate.

[0029] In addition, the control device 40 adjusts the amount of scale inhibitor added by the chemical agent adding device 30 based on the detection result of the conductivity sensor 15. The method of adjusting the amount of scale inhibitor added, which is executed by the control device 40, will be described below.

[0030] For example, when only raw water is supplied to the first RO membrane device 12 as the water to be treated, a method of adding a fixed amount of scale inhibitor to the water to be treated is commonly used. This is because, as long as the recovery rate of the first RO membrane device 12 (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrate flow rate) is constant, the risk of scale formation on the RO membrane surface remains unchanged unless the concentration of scale components in the raw water fluctuates. However, in this embodiment, as described above, desalinated water substantially free of scale components is preferentially supplied to the raw water tank 11. If this amount is insufficient, raw water containing scale components is replenished to the raw water tank 11. As a result, the mixing ratio of raw water to desalinated water in the water to be treated supplied to the first RO membrane device 12 fluctuates, and the concentration of scale components in the water to be treated also fluctuates accordingly. Therefore, when a fixed amount of scale inhibitor is added in this embodiment, the amount to be added must be determined assuming a situation in which the proportion of raw water in the water to be treated is highest, from a fail-safe perspective. As a result, an excessive amount of scale inhibitor is inevitably added, which raises concerns about an increase in running costs.

[0031] Therefore, in the method for adjusting the amount of scale inhibitor added according to this embodiment, first, the conductivity of the water to be treated supplied to the first RO membrane device 12 via the water supply line L1 is detected by the conductivity sensor 15 to determine the mixture ratio of raw water and desalinated water in the water to be treated. As a result, the lower the detected conductivity, the lower the proportion of raw water in the water to be treated is determined, and the higher the detected conductivity, the higher the proportion of raw water in the water to be treated is determined. Once the conductivity of the water to be treated is detected, the amount of scale inhibitor to be added to the water to be treated is determined based on the detected conductivity. Specifically, the amount of scale inhibitor to be added is set to be smaller as the detected conductivity decreases and to be larger as the detected conductivity increases. The amount of scale inhibitor added may be increased or decreased continuously or stepwise. Alternatively, the amount of scale inhibitor added may be set to a predetermined minimum amount when the detected conductivity is equal to or less than a predetermined lower limit, set to a predetermined maximum amount when the detected conductivity is equal to or greater than a predetermined upper limit, and increased or decreased continuously or stepwise depending on the increase or decrease in conductivity when the detected conductivity is between these values. Once the new amount to be added is determined in this manner, the chemical feed pump 32 is controlled to add the predetermined amount of scale inhibitor to the water to be treated flowing through the water supply line L1.

[0032] In this way, according to this embodiment, even if the mixing ratio of raw water and desalinated water changes, which causes the scale component concentration in the water to be treated to fluctuate, it is possible to add the appropriate amount of scale inhibitor to the water to be treated accordingly. As a result, the amount of scale inhibitor added can be optimized, and the running costs of the water treatment device 1 can be reduced.

[0033] To appropriately evaluate the risk of scale formation in the first RO membrane device 12, the concentration of scale components in the treated water may be directly detected. To achieve this, a sensor capable of detecting the scale components online may be used. However, when the raw water contains multiple scale components, such as silica and calcium, installing online sensors corresponding to each scale component is cost-inefficient and unrealistic. Furthermore, for raw water with stable water quality, such as surface water or groundwater, the mixing ratio of the raw water and desalinated water can be determined simply by detecting the conductivity of the treated water, without directly detecting the concentrations of each scale component. This allows the scale component concentration in the treated water to be substantially determined. Therefore, to determine the optimal amount of scale inhibitor to be added, it is preferable to detect the conductivity of the treated water, as in this embodiment. Note that the indicator for determining the mixing ratio of raw water and desalinated water is not limited to the conductivity of the treated water described above. For example, the cumulative flow rate of raw water and desalinated water flowing into the raw water tank 11 within a predetermined period may also be used.

[0034] As described above, during operation of the water treatment device 1, the pure water production system 10 performs flow control to adjust the flow rate of the permeate flowing through the permeate line L2 to a target flow rate. At this time, the target flow rate may be changed depending on factors such as the usage status of ultrapure water at the point of use 2. For example, if the amount of ultrapure water supplied to the point of use 2 through the water supply line L11 decreases, the amount of primary pure water required for the subtank 21 also decreases accordingly, and the target flow rate of the permeate may be changed to a lower value. In this case, the recovery rate of the first RO membrane device 12 changes, and the concentration of scale components in the concentrated water changes accordingly. Therefore, even if the quality of the water to be treated remains constant, the risk of scale formation in the first RO membrane device 12 changes. Therefore, by taking the flow rate of the permeate from the first RO membrane device 12 into consideration when determining the amount of scale inhibitor to be added, the amount of scale inhibitor to be added can be more optimized than if this amount were not taken into consideration. In other words, when the target flow rate of the permeate described above is changed depending on the flow rate of the ultrapure water supplied to the use point 2, the amount of scale inhibitor to be added determined based on the detection result of the conductivity sensor 15 may be further corrected based on the detection value of the flow rate sensor 23.

[0035] The desalinated water to be mixed with the raw water is not limited to the above-mentioned examples, i.e., recovered treated water from another water treatment device or permeated water from the second RO membrane device 17, as long as it is water with a low risk of scale formation, specifically, water with a conductivity of, for example, 50 μS / cm or less. For example, the permeated water from the first RO membrane device 12 is returned to the raw water tank 11 as needed, and the primary pure water from the ion removal device 13 is also returned to the raw water tank 11 as needed. Such permeated water and primary pure water may also be treated as desalinated water. Furthermore, if the ion removal device 13 is an electrodeionization water production device, concentrated water from the electrodeionization water production device is also returned to the raw water tank 11 during normal operation (pure water production) of the pure water production system 10. Such concentrated water may also be treated as desalinated water.

[0036] 1 Water treatment device 2 Point of use 10 Primary pure water system (pure water production device) 11 Raw water tank 12 First RO membrane device 13 Ion removal device 14, 18 Pressure pump 15 Conductivity sensor 16 Concentrated water tank 17 Second RO membrane device 20 Secondary pure water system (subsystem) 21 Sub-tank 22 Circulation pump 23 Flow rate sensor 30 Chemical addition device 31 Chemical tank 32 Chemical injection pump 40 Control device L1 Water supply line L2 Permeate line L3 Concentrated water line L4 Raw water line L5 Recovered treated water line L6 Primary pure water line L7 Concentrated water supply line L8 Reflux line L9 Drain line L10 Circulation line L11 Water supply line V1 to V2 Opening and closing valve

Claims

1. A water treatment method comprising: a step of mixing raw water and desalinated water, supplying the resulting mixture to a reverse osmosis membrane as water to be treated, and separating the resulting mixture into permeate and concentrated water; and a step of adding a scale inhibitor to the water to be treated, wherein the adding step includes a step of adjusting the amount of scale inhibitor added to the water to be treated based on the mixing ratio of the raw water and the desalinated water in the water to be treated.

2. The water treatment method according to claim 1, wherein the adding step includes a step of detecting the conductivity of the water to be treated as the mixing ratio, and the adjusting step includes adjusting the amount of scale inhibitor added based on the detected conductivity.

3. The water treatment method according to claim 2, further comprising a step of treating the permeated water from the reverse osmosis membrane to produce pure water and supplying the produced pure water to a point of use, wherein the separating step comprises detecting the flow rate of the pure water supplied to the point of use and adjusting the pressure of the water to be treated supplied to the reverse osmosis membrane based on the detected flow rate, and wherein the adjusting step comprises adjusting the amount of scale inhibitor to be added based on the detected conductivity and the detected flow rate.

4. The water treatment method according to any one of claims 1 to 3, wherein the raw water is surface water or groundwater containing a plurality of scale components.

5. The water treatment method according to any one of claims 1 to 3, further comprising a step of supplying the concentrated water from the reverse osmosis membrane to another reverse osmosis membrane to separate it into permeate and concentrated water, wherein the desalted water comprises the permeate from the other reverse osmosis membrane.

6. A water treatment device comprising: a tank for storing a mixture of raw water and desalinated water as water to be treated; a reverse osmosis membrane device for separating the water to be treated supplied from the tank into permeate and concentrated water; an addition device for adding a scale inhibitor to the water to be treated supplied to the reverse osmosis membrane device; and a control device for adjusting the amount of scale inhibitor added to the water to be treated by the addition device based on the mixture ratio of the raw water and the desalinated water in the water to be treated.

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