Water treatment method and water treatment device

Abstract

As a countermeasure against calcium fluoride scale on a reverse osmosis membrane, a water treatment method and apparatus are provided that enable the addition of a scale inhibitor in an appropriate amount. A water treatment method and water treatment apparatus that treat water containing fluorine and calcium by passing the water to be treated through at least a reverse osmosis membrane (11), wherein a pH adjustment unit (21) that adjusts the pH of the water to be treated is provided in a feed water line (1) that passes the water to be treated to the reverse osmosis membrane (11), a measurement unit (22) measures the fluoride ion concentration under conditions in which the pH of the water to be treated is 5.0 or higher, the amount of scale inhibitor added to the water to be treated is determined based on the measured fluoride ion concentration, the scale inhibitor is added from an addition line (23), and the water to be treated is then passed through the reverse osmosis membrane (11) to discharge permeate water from a permeate water line (2) and concentrate water from a concentrate water line (3).

Description

TECHNICAL FIELD

[0001] The present application relates to a water treatment method and a water treatment apparatus each of which includes a reverse osmosis membrane device and in which scale deposition is inhibited. BACKGROUND

[0002] In recent years, the use of reverse osmosis membranes has increased in water treatment. Various components are contained in the object water (referred to as treated water) of water treatment, and scale (referred to as scale) is deposited due to various reasons. Due to this, the reverse osmosis membrane is clogged, leading to a decrease in treatment efficiency. Scale deposition (fouling) is one of the important factors in the operation management of reverse osmosis membranes. In particular, fouling caused by calcium is the most common, and fouling caused by calcium carbonate and calcium fluoride is known.

[0003] In Singapore, Malaysia, Ireland, the United States, Australia, New Zealand, the United Kingdom, and the like overseas, fluorine is added to the water supply for the purpose of maintaining the health of the teeth. Also, the concentration of hardness components is high in surface water in overseas countries compared to Japan. Therefore, in the above-mentioned countries, there is a concern about scale deposition caused by calcium fluoride in pure water production.

[0004] In Patent Literature 1, as a countermeasure against scale caused by calcium carbonate, a method of lowering the pH of the treated water to 4 to 6 or the like is proposed. On the other hand, in Patent Literature 2 or Patent Literature 3, as a countermeasure against scale caused by calcium fluoride, the application of a scale inhibitor is proposed.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2018-153732

[0008] Patent Literature 2: Japanese Patent Application Publication No. 2014-184365

[0009] Patent Literature 3: Japanese Patent Application Publication No. 2002-186835 SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] In recent years, due to water shortage, water demand for treating and using various water has increased, thereby causing an increase in water recovery rate, and as a result, the concentration of scale components of concentrated water has increased. Therefore, the concentration of scale inhibitors has increased, and an increase in the operating cost of a water treatment device has become a problem. As a conventional scale countermeasure, a certain amount of scale inhibitor is quantitatively injected through a prior test, water quality analysis. However, in the case where the water quality greatly changes in the water treatment process, or in the case where the scale inhibitor is excessively added, there is a problem that the operating cost increases. In particular, the scale inhibitor is expensive, and the cost burden cannot be ignored. On the other hand, if the scale inhibitor is added too little, scale deposition on a reverse osmosis membrane cannot be sufficiently prevented, and the replacement period of the reverse osmosis membrane becomes short.

[0012] In addition, as shown in Patent Document 3, on the acidic side where the pH is low, the precipitation of calcium fluoride is small, and it is possible to use no scale inhibitor or to reduce the amount of scale inhibitor used. However, a state where the pH is low becomes a cause of corrosion of piping and the like, and an additional cost on a liner and the like for preventing corrosion increases.

[0013] Therefore, an object of the present application is to provide a water treatment method and a water treatment device that achieve optimization of the operating cost by adding an appropriate amount of scale inhibitor at a moderate pH.

[0014] Technical solution for solving the problem

[0015] The present application provides a water treatment method of treating water containing fluorine and calcium by passing the water through at least a reverse osmosis membrane, characterized in that,

[0016] The water treatment method comprises:

[0017] a step of measuring the concentration of fluoride ions in the water to be treated;

[0018] a step of adding a scale inhibitor that inhibits the precipitation of calcium fluoride; and

[0019] a reverse osmosis membrane treatment step of passing the water to be treated to which the scale inhibitor has been added through the reverse osmosis membrane to obtain permeated water and concentrated water,

[0020] The measurement of the concentration of fluoride ions in the water to be treated is performed after adjusting the pH to 5 or more, and the amount of the scale inhibitor is determined based on the measured concentration of fluoride ions.

[0021] In addition, the present application provides a water treatment device having:

[0022] a reverse osmosis membrane that passes water to be treated containing fluorine and calcium to obtain permeated water and concentrated water; and

[0023] a water supply line that supplies the water to be treated to the reverse osmosis membrane,

[0024] characterized in that

[0025] The supply water line is provided with:

[0026] a pH adjusting unit that adjusts the pH of the treated water;

[0027] a measurement unit that measures the concentration of fluoride ions in the treated water under conditions in which the pH is 5 or higher; and

[0028] an addition unit that adds an antiscalant that inhibits the precipitation of calcium fluoride to the treated water,

[0029] The water treatment device has an addition amount control device that determines the addition amount of the antiscalant added by the addition unit based on the measured concentration of fluoride ions and controls the addition amount of the antiscalant in the addition unit.

[0030] Effects of the Invention

[0031] According to the present invention, it is possible to provide a water treatment method and a water treatment device that can add an appropriate amount of an antiscalant at a moderate pH. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a schematic diagram showing the structure of a water treatment device according to the first embodiment of the present invention.

[0033] Figure 2 is a schematic diagram showing the structure of a water treatment device according to the second embodiment of the present invention.

[0034] Figure 3 is a graph showing the relationship between the detected amounts of calcium fluoride detected in the examples and the comparative example. DETAILED DESCRIPTION

[0035] In the present invention, a water treatment method and a water treatment device are provided that treat treated water containing fluoride and calcium with a reverse osmosis membrane. Here, the treated water can include surface water in rivers and lakes, tap water, and industrial water. The treated water treated in the present invention is used for ultra-pure water manufacturing applications and the like.

[0036] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0037] (First Embodiment)

[0038] Figure 1 is a schematic diagram showing the structure of a water treatment device according to the first embodiment of the present invention.

[0039] The water treatment device 100 of the present embodiment is a device that removes impurities (fluoride ions, calcium ions, etc.) contained in the treated water to produce treated water, and has a reverse osmosis membrane 11 that separates into concentrated water containing impurities and permeated water from which the impurities have been removed.

[0040] In addition, the water treatment device 100 has a plurality of lines connected to the reverse osmosis membrane 11, respectively. That is, there are a supply water line 1 that supplies the treated water to the reverse osmosis membrane, a permeated water line 2 that discharges the permeated water from the reverse osmosis membrane 11, and a concentrated water line 3 that discharges the concentrated water from the reverse osmosis membrane 11. Further, the supply water line 1 has a pH adjusting device 21 as a pH adjusting unit, a measurement unit (fluoride ion meter) 22 that performs measurement of the concentration of fluoride ions, and an addition line 23 as an addition unit of an antiscalant.

[0041] Although not illustrated in the present embodiment, in order to suppress calcium carbonate scale and improve the quality of the permeated water, a decarbonation process is sometimes provided before the pH adjusting device 21. In this case, the treated water of the process becomes pH < 5.0 in many cases. When pH < 5.0, the fluoride ions follow the back-off curve of hydrogen fluoride, and a part of them become hydrogen fluoride. For example, when pH = 5.0, the fluoride ions that become hydrogen fluoride are about 1%, which is an amount that can be almost ignored. However, when pH = 4.5, about 4.5% of the fluoride ions become hydrogen fluoride, when pH = 4.0, about 13% of the fluoride ions become hydrogen fluoride, and when pH = 3.5, about 32% of the fluoride ions become hydrogen fluoride. The fluoride ions that become hydrogen fluoride become fluoride ions by the equilibrium reaction between hydrogen fluoride and fluoride ions as the pH rises, and contribute to the generation of calcium fluoride. However, the fluoride ions that become hydrogen fluoride cannot be measured by the fluoride ion meter. Therefore, the pH at the time of measurement by the fluoride ion meter changes the concentration of the fluoride ions in the treated water, and the amount of the antiscalant calculated from the concentration of the fluoride ions at low pH is added too little to sufficiently suppress scale.

[0042] Therefore, the measurement of the concentration of the fluoride ions is performed under the condition that the pH becomes 5.0 or more. Specifically, the pH adjusting device 21 is provided with a pH adjustment process so that the treated water becomes pH ≥ 5.0. Thereby, it is possible to prevent a part of the fluoride ions from becoming hydrogen fluoride, and it is possible to accurately measure the concentration of the fluoride ions in the treated water in the supply water line 1 using the fluoride ion meter 22. In the pH adjusting device 21, a method of injecting a chemical solution that does not affect the fluoride ions in the treated water is selected, and an alkali, particularly a low-concentration sodium hydroxide aqueous solution, is added. In addition, in the pH adjusting device, there is a pH measurement unit such as a pH meter that measures the pH of the treated water before and after the addition of the alkali in addition to such an addition unit of the alkali.

[0043] In addition, the following describes the reason for measuring the fluoride ion concentration instead of the calcium ion concentration in the calcium fluoride scale precipitation. The calcium fluoride scale precipitation is determined by the product of the molar concentrations of the fluoride ion and the calcium ion (hereinafter, referred to as the ion product). The solubility product of calcium fluoride is 3.9 x 10 －11 (mol 3 / L 3 ). If the ion product exceeds the value of the solubility product, scale precipitates. Calcium fluoride is composed of calcium ions and fluoride ions at a molar ratio of 1 : 2. In addition, the ion product Kap of calcium fluoride is represented by the following formula (1).

[0044] Kap = [Ca + ]([F - ] 2 ) • • • Formula (1)

[0045] In formula (1), [Ca + ] is the calcium ion concentration, and [F - ] is the fluoride ion concentration.

[0046] The ion product Kap of calcium fluoride is calculated by the product of the square of the fluoride ion concentration and the calcium ion concentration. Thus, the fluoride ion concentration has a greater effect on the ion product than the calcium ion concentration. In view of the above, when the amount of scale inhibitor to be added is determined by the ion product according to the changed water quality, monitoring the fluoride ion is more effective than the calcium ion.

[0047] The lower limit of the measurement of the general fluoride ion meter is about 1 mg / L. When the fluoride ion concentration is 20 mg / L or less, the change in the ion product when the fluoride ion concentration changes by 1 mg / L exceeds about 10%. Thus, the present method is particularly effective in the region where the fluoride ion concentration is 20 mg / L or less. Furthermore, since the ion product is the solubility product of calcium fluoride or more, the calcium ion concentration at this time is 1.4 mg / L or more. In addition, when the fluoride ion concentration is 10 mg / L or less, the change in the ion product is 20% or more, and thus the present method is more effective. Furthermore, the calcium ion concentration at this time is 5.6 mg / L or more.

[0048] Based on the measurement result of the above fluoride ion concentration, and the recovery rate (sometimes also referred to as the concentration factor) calculated from the first flow rate and the second flow rate measured by the first and second flow rate sensors (not shown) connected to any two of the feed water line 1, the permeate water line 2, and the concentrated water line 3, the amount of the scale inhibitor is determined in real time. Thus, no time is required from the analysis to the determination of the amount of the scale inhibitor. Generally, if the flow rate of the feed water line 1 is set to 100, the total of the flow rates of the permeate water line 2 and the concentrated water line 3 is also 100. The recovery rate is the amount of the permeate water with respect to the treated water, and is set to 75% or the like as the performance of the reverse osmosis membrane, for example. However, in actual water treatment operations, variations in the amount of the permeate water and the amount of the concentrated water occur due to variations in the water quality and the water temperature of the treated water. Thus, by measuring the actual water amount, when the concentration of the fluoride ion is measured, the concentration of the fluoride ion can be measured accurately without being affected by variations in the amount of the permeate water and the amount of the concentrated water due to variations in the water quality and the water temperature, and the appropriate amount of the scale inhibitor is determined. Here, from the viewpoint of avoiding the addition of an expensive scale inhibitor in excess, the "appropriate amount of the scale inhibitor" in the present application means that the amount is preferably the minimum required amount.

[0049] Based on the determined amount of the scale inhibitor, the scale inhibitor is added through the scale inhibitor addition line 23.

[0050] The scale inhibitor is not limited to a specific substance as long as it is a substance capable of inhibiting the precipitation of scale components such as silicon dioxide and calcium, and is particularly preferably a scale inhibitor that inhibits the precipitation of calcium fluoride. As the kind thereof, for example, phosphonic acid and salt thereof such as 1-hydroxyethylidene-1, 1-diphosphonic acid, 2-phosphonobutane-1, 2, 4-tricarboxylic acid, ethylenediaminetetramethylene phosphonic acid, nitrilotrimethylene phosphonic acid, and the like; phosphoric acid-based compounds such as orthophosphate, polyphosphate, and the like; maleic acid-based compounds such as polymaleic acid, maleic acid copolymer, and the like; acrylic acid-based polymers such as poly(meth)acrylic acid, maleic acid / (meth)acrylic acid, (meth)acrylic acid / sulfonic acid, (meth)acrylic acid / monomer containing a nonionic group, and the like copolymer, (meth)acrylic acid / sulfonic acid / monomer containing a nonionic group, (meth)acrylic acid / acrylamide-alkyl sulfonic acid / substituted (meth)acrylamide, (meth)acrylic acid / acrylamide-aryl sulfonic acid / substituted (meth)acrylamide terpolymer, and the like can be given. As the (meth)acrylic acid constituting the terpolymer, for example, methacrylic acid and acrylic acid, and (meth)acrylic acid salts such as sodium salt thereof, and the like can be given. As the acrylamide-alkyl sulfonic acid constituting the terpolymer, for example, 2-acrylamido-2-methylpropane sulfonic acid and a salt thereof, and the like can be given. In addition, as the substituted (meth)acrylamide constituting the terpolymer, for example, t-butyl acrylamide, t-octyl acrylamide, dimethyl acrylamide, and the like can be given.

[0051] Among them, it is preferable to use a substance containing at least one of a phosphonic acid-based compound and an acrylic acid-based polymer. For example, a copolymer composed of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid is preferable. In addition, in order to simultaneously inhibit scale from calcium and silica, it is particularly preferable to use a scale inhibitor composed of a mixture of a terpolymer of 2-phosphonobutane-1, 2, 4-tricarboxylic acid, acrylic acid, and (meth)acrylic acid / 2-acrylamido-2-methylpropane sulfonic acid / substituted (meth)acrylamide.

[0052] Specifically, as a commercially available scale inhibitor for reverse osmosis membranes, "ORPERSION" series manufactured by Oga Co., Ltd., "Flocon (registered trademark)" series manufactured by BWA Water Additives Co., Ltd., "PermaTreat (registered trademark)" series manufactured by Nalco Co., Ltd., "Hypersperse (registered trademark)" series manufactured by General Electric Co., "Kuriverter (registered trademark)" series manufactured by Kureha Industrial Co., Ltd., and the like can be mentioned.

[0053] The addition of the scale inhibitor can be performed within the pH range in which calcium fluoride is precipitated. Generally, calcium fluoride starts to precipitate when the pH is 3.5 or higher. In Figure 1 In the illustrated embodiment, in order to measure the fluoride ion concentration between the pH adjusting device 21 and the reverse osmosis membrane 11, the pH of the treated water is adjusted to 5 or higher, and after the pH of the treated water is adjusted to 5 or higher, the measurement of the fluoride ion and the addition of the scale inhibitor are performed. Note that, in order to improve the water quality of the permeated water of the reverse osmosis membrane, the pH is preferably 4 or higher, and more preferably 5 or higher. Therefore, it is preferable to perform the pH adjustment in the vicinity (upstream) of the reverse osmosis membrane 11.

[0054] In addition, in Figure 1 In the illustrated embodiment, the measurement unit of the fluoride ion concentration is provided downstream of the pH adjusting unit, but is not limited thereto, and the measurement unit of the fluoride ion concentration can be provided upstream of the pH adjusting unit as long as the pH of the treated water is 5 or higher before the pH is lowered by the decarbonator or the like. In addition, it is also preferable to provide a pH meter or the like that confirms that the pH of the treated water at that time is 5 or higher.

[0055] (Second Embodiment)

[0056] Figure 2 is a schematic view showing the structure of a water treatment device 200 according to the second embodiment of the present application. Hereinafter, for the same structure as the first embodiment, the same reference numerals are assigned in the drawings and the description thereof is omitted, and only the different structure from the first embodiment is described.

[0057] As with the first embodiment, there is a supply water line 1 that supplies treated water and a reverse osmosis membrane 11. In the water treatment device 200 related to the second embodiment, there is a water supply tank 13 that stores treated water to be supplied in the supply water line 1, a pre-treatment (coagulation and filtration) device 31, a heat exchanger 32, an activated carbon column (activated carbon filter) 33, a decarbonation column 34, the reverse osmosis membrane 11, and a brine reverse osmosis membrane 12.

[0058] The storage amount of the water supply tank 13 is adjusted by a pressure pump PI (pressure adjustment unit) that adjusts the pressure of the treated water flowing in the supply water line. In addition, not only the treated water after permeation through the reverse osmosis membrane 11 but also the permeated water from the permeated water line 2 can be returned to the water supply tank 13 via the backflow line 2b. In addition, the permeated water after the filtered treatment of the concentrated water separated by the reverse osmosis membrane 11 by the brine reverse osmosis membrane 12 can also flow in through the circulation line 5. However, since there is also a case where the permeated water after permeation through the reverse osmosis membrane 11 is taken, in this case, it is not circulated to the water supply tank 13 but is taken via the water taking line 2a. The concentrated water in the brine reverse osmosis membrane 12 is discharged from the discharge line 4 and is discarded after the post-treatment as necessary.

[0059] As the pre-treatment device 31, devices that can perform coagulation treatment, sand filtration, and membrane filtration can be given. The coagulation treatment is a treatment that neutralizes the electric charge of fine particles in water having a negative electric charge using a coagulant having a positive electric charge to cause the fine particles to coagulate and thereby generate primary floes, and that causes the primary floes to be adsorbed using a coagulation aid such as a polymer to thereby generate coarse floes and make them easy to settle. The coagulant can be given aluminum sulfate, polyaluminum chloride, ferric chloride, ferrous sulfate, and the like. The sand filtration is a treatment that uses packed sand as a filter material and causes water to pass through the packed sand to be filtered. The membrane filtration is a treatment that filters water through a filter membrane. As the filter membrane, depending on the size of the object to be filtered and the driving force of the filtration, a microfiltration (MF) membrane, an ultrafiltration (UF) membrane, a nanofiltration (NF) membrane, an ion exchange membrane, and the like can be given.

[0060] The heat exchanger 32 is a device that heats the treated water supplied after the pre-treatment and is provided in order to generate hot water for heat sterilization.

[0061] The activated carbon column 33 is provided in order to remove chlorine from the treated water supplied by the heat exchanger.

[0062] The decarbonation column 34 is a device that reduces the pH by injecting an acid, thereby converting carbonate ions or bicarbonate ions into carbon dioxide, and removes carbonic acid in water by blowing air into a packed column, and is provided in order to suppress calcium carbonate scale and improve permeated water quality.

[0063] The pH adjustment device 21 has a pH meter 21a as a pH measurement unit, and a pH control device 21b that determines the amount of pH adjuster (alkali) to be added to the pH adjuster adding device 21c based on the pH value measured by the pH meter 21a. A given amount of pH adjuster (alkali) is added from the pH adjuster adding device 21c controlled by the pH control device 21b, and the pH of the treated water is adjusted to 5.0 or higher. At this time, the flow meter as explained in the first embodiment is arranged upstream of the pH adjustment device 21, and by inputting the measured flow rate and pH value to the pH control device 21b, the amount of pH adjuster to be added in real time can be set. Alternatively, a pH meter can also be arranged downstream of the pH adjustment device, so that the pH adjuster corresponding to the difference between the pH meters before and after is added. The treated water after the pH adjustment is collected, and the fluoride ion concentration is measured by the fluoride ion meter 22. The measured fluoride ion concentration is transmitted to the scale inhibitor addition amount control device 41, and the amount of scale inhibitor required as a minimum is calculated, and based on this information, the scale inhibitor is added from the scale inhibitor adding device 42 to the feed water line 1 via the scale inhibitor addition line 23. In this case, the scale inhibitor addition unit 40 includes the fluoride ion meter 22, the control device 41, the adding device 42, and the addition line 23.

[0064] Further, after the scale inhibitor is added from the scale inhibitor addition line 23, the pressure is adjusted by the pressure pump P2 (pressure adjustment unit) so that the water is supplied to the reverse osmosis membrane 11.

[0065] In this way, if the fluoride ion concentration is measured in the de-carbonator 34 while maintaining the state where the pH is temporarily lowered, when the scale inhibitor is added after the pH is subsequently increased, the amount of scale inhibitor added based on the measured fluoride ion concentration is too small, and the scale of the reverse osmosis membrane cannot be sufficiently prevented from adhering. Therefore, in the present embodiment, the fluoride ion concentration is measured by the fluoride ion meter 22 after the pH is increased to 5.0 or higher in the pH adjustment device 21, and based on this value, the amount of scale inhibitor added from the scale inhibitor addition line 23 is set to the amount required as a minimum, and the scale of the reverse osmosis membrane can also be sufficiently prevented from adhering. The position where the fluoride ion meter 22 is arranged is not limited to Figure 2The pH adjusting device 21 is provided downstream of the reverse osmosis membrane 11. The pH adjusting device 21 is provided at any position of the feed water line 1 in which the pH of the feed water is 5.0 or more. For example, the pH of the permeate water of the reverse osmosis membrane 11 or the permeate water of the brine reverse osmosis membrane 12 is increased to 5.0 or more, and the pH of the treated water in the feed water tank 13 into which the permeate water is circulated is also sometimes 5.0 or more. Therefore, a line fluoride ion meter (not shown) is provided in the feed water line 1 in the feed water tank 13 or upstream of the pre-treatment device, and the measured fluoride ion concentration is connected to the addition amount control device 41 to add the minimum necessary amount of scale inhibitor.

[0066] The permeate water after the water is supplied to the reverse osmosis membrane 11 is sometimes collected through the permeate water line 2, and sometimes circulated to the feed water tank 13 without being collected.

[0067] On the other hand, the concentrated water after the water is supplied to the reverse osmosis membrane 11 is supplied to the brine reverse osmosis membrane 12 through the concentrated water line 3, and the pressure is adjusted by the pressure pump P3 (pressure adjusting unit) provided in the concentrated water line 3. After being filtered by the brine reverse osmosis membrane 12, the water is separated into the drain water discharged through the drain line 4 and the circulation water circulated to the feed water tank 13.

[0068] Example

[0069] Next, a specific example is given to describe the effects of the present application.

[0070] (Example 1)

[0071] A 1 L beaker was used to prepare simulated water containing fluoride ions and calcium ions using pure water as raw water. The pH at the time of adjustment was 3.5. Hydrochloric acid or sodium hydroxide solution was used for pH adjustment. Sodium fluoride was used to adjust the fluoride ion concentration to 6.7 mg / L. Calcium chloride was used to adjust the calcium ion concentration to 200 mg / L. After the pH was adjusted to 5.0, the fluoride ion concentration was measured using a fluoride ion meter (F ion measurement value). The amount of scale inhibitor added was calculated from the measured fluoride ion concentration. As the scale inhibitor, a copolymer composed of acrylic acid, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid was used. In the measurement of the fluoride ion concentration, an ion electrode (model number: F-2021) manufactured by Toa DKK Corporation was used. After the scale inhibitor was added, the supernatant after the test was filtered using a 0.1 μm filter, and the pH was adjusted to 6.0 to 7.0, and the fluoride ion concentration was measured. The amount of scale precipitated was calculated from the measured results and the fluoride ion concentration before the test. The amount of calcium fluoride precipitated was calculated from the results.

[0072] (Example 2)

[0073] The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1, except that the simulated water was adjusted to pH 4.0.

[0074] (Example 3)

[0075] The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1, except that the simulated water was adjusted to pH 4.5.

[0076] (Example 4)

[0077] The simulated water was prepared at pH 5.0. The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1, except that the determination of the fluoride ion concentration was performed at this pH.

[0078] (Example 5)

[0079] The simulated water was prepared at pH 5.5. The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1, except that the determination of the fluoride ion concentration was performed at this pH.

[0080] (Comparative Example 1)

[0081] The simulated water prepared in the same manner as in Example 1 was used. Before the determination of the fluoride ion with a fluoride ion meter, the pH was not adjusted to 5.0, the fluoride ion concentration (F ion measured value) was determined while the pH was maintained at 3.5, and the scale inhibitor was added in an amount calculated from the determined fluoride ion concentration. Except for this, the amount of calcium fluoride precipitated was calculated in the same manner as in Example 1.

[0082] (Comparative Example 2)

[0083] The simulated water prepared in the same manner as in Example 2 was used. Before the determination of the fluoride ion with a fluoride ion meter, the pH was not adjusted to 5.0, the fluoride ion concentration (F ion measured value) was determined while the pH was maintained at 4.0, and the scale inhibitor was added in an amount calculated from the determined fluoride ion concentration. Except for this, the amount of calcium fluoride precipitated was calculated in the same manner as in Example 1.

[0084] (Comparative Example 3)

[0085] The simulated water prepared in the same manner as in Example 3 was used. Before the determination of the fluoride ion with a fluoride ion meter, the pH was not adjusted to 5.0, the fluoride ion concentration (F ion measured value) was determined while the pH was maintained at 4.5, and the scale inhibitor was added in an amount calculated from the determined fluoride ion concentration. Except for this, the amount of calcium fluoride precipitated was calculated in the same manner as in Example 1.

[0086] The detected amount ratio of calcium fluoride (comparative example / example) of each example to each comparative example is shown in Table 1. In Table 1, the fluoride ion is indicated as "F ion", the calcium ion is indicated as "Ca ion", and the calcium fluoride is indicated as "CaF2".

[0087] [Table 1]

[0088]

[0089] In addition, Figure 3 is a graph showing the relationship of the detected amount ratio (comparative example / example) of calcium fluoride detected in the examples and comparative examples. It is known that the antiscalant is added too little in the region where pH < 5.0. Note that the detected amount ratio of Examples 4 and 5 is set to 1.00. In the case where pH is low, the amount of CaF2precipitation becomes small, but if it is assumed that the antiscalant is added at pH ≥ 5 or more, it is predicted that the increase in scale corresponding to the insufficient amount indicated by the detected amount ratio will occur.

[0090] The present application has been described above with reference to the embodiment examples, but the present application is not limited to the above-described embodiment examples. The structure and details of the present application can be variously changed within the scope of the present application by those skilled in the art.

[0091] This application claims priority based on Japanese Patent Application No. 2021-144780 filed on September 6, 2021, the disclosure of which is incorporated herein in its entirety.

[0092] The present application includes the following method.

[0093] [Method 1]

[0094] A water treatment method is a water treatment method in which water to be treated containing fluorine and calcium is at least passed through a reverse osmosis membrane to be treated, characterized in that

[0095] The water treatment method comprises:

[0096] a step of measuring the concentration of fluoride ions in the water to be treated;

[0097] a step of adding an antiscalant that inhibits the precipitation of calcium fluoride; and

[0098] a reverse osmosis membrane treatment step of passing the water to be treated to which the antiscalant has been added through the reverse osmosis membrane to obtain permeate water and concentrated water;

[0099] The measurement of the concentration of fluoride ions in the water to be treated is performed after adjusting the pH to 5 or more, and the amount of addition of the antiscalant is determined based on the measured concentration of fluoride ions.

[0100] [Method 2]

[0101] The water treatment method according to [Method 1], wherein the addition of the scale inhibitor is performed after adjusting the pH of the treated water to 5 or more.

[0102] [Method 3]

[0103] The water treatment method according to [Method 1] or [Method 2], wherein the adjustment of the pH is performed by adding a pH adjuster to a feed water line in which the treated water is supplied to the reverse osmosis membrane,

[0104] The measurement of the fluoride ion concentration is performed in the feed water line after the addition of the pH adjuster.

[0105] [Method 4]

[0106] The water treatment method according to [Method 3], wherein a first flow rate and a second flow rate are measured in any two of the feed water line, a line of permeated water from the reverse osmosis membrane, and a line of concentrated water from the reverse osmosis membrane, and the amount of addition of the scale inhibitor is determined based on a recovery rate calculated from a comparison of the first flow rate and the second flow rate and the measured fluoride ion concentration.

[0107] [Method 5]

[0108] The water treatment method according to [Method 3] or [Method 4], wherein a decarbonation process for removing a carbonic acid component in the treated water is performed before the addition of the pH adjuster to the feed water line.

[0109] Further, the present application includes the following structure.

[0110] [Structure 1]

[0111] A water treatment device having:

[0112] a reverse osmosis membrane that allows a treated water containing fluorine and calcium to pass therethrough to obtain permeated water and concentrated water; and

[0113] a feed water line that supplies the treated water to the reverse osmosis membrane,

[0114] characterized in that

[0115] the feed water line is provided with:

[0116] a pH adjustment unit that adjusts the pH of the treated water;

[0117] a measurement unit that measures the fluoride ion concentration in the treated water under a condition where the pH is 5 or more; and

[0118] an addition unit of a scale inhibitor that suppresses precipitation of calcium fluoride in the treated water,

[0119] The water treatment device has an addition amount control device that decides an addition amount of the scale inhibitor added by the addition unit based on the measured concentration of the fluoride ion, and controls the addition amount of the scale inhibitor in the addition unit.

[0120] [Structure 2]

[0121] The water treatment device according to [Structure 1], wherein either of the supply water line, the permeated water from the reverse osmosis membrane, and the concentrated water from the reverse osmosis membrane is provided with a first flow rate sensor that measures a first flow rate and a second flow rate sensor that measures a second flow rate.

[0122] The addition amount control device decides the addition amount of the scale inhibitor based on a recovery rate calculated from a comparison between the first flow rate and the second flow rate, and the concentration of the fluoride ion measured by the measurement unit.

[0123] [Structure 3]

[0124] The water treatment device according to [Structure 1] or [Structure 2], wherein the supply water line is provided with a decarbonation tower that removes a carbonic acid component in the treated water upstream of the pH adjustment unit.

[0125] [Structure 4]

[0126] The water treatment device according to any one of [Structure 1] to [Structure 3], wherein the water treatment device further has: a brine reverse osmosis membrane that further treats the concentrated water of the reverse osmosis membrane; and a water supply tank that is located in the supply water line, mixes the permeated water of the brine reverse osmosis membrane and / or the permeated water of the reverse osmosis membrane with the treated water of the supply water line.

[0127]

[0128] [Structure 5]

[0129] The water treatment device according to any one of [Structure 1] to [Structure 4], wherein the measurement unit is disposed downstream of the pH adjustment unit.

[0130] Explanation of symbols

[0131] 1 supply water line

[0132] 2 permeated water line

[0133] 3 concentrated water line

[0134] 4 drain line

[0135] 5 circulation line

[0136] ​11 reverse osmosis membrane

[0137] 12 brine reverse osmosis membrane

[0138] 13 water supply tank

[0139] 21 pH adjusting device

[0140] 21a pH meter

[0141] 21b pH control device

[0142] 21c pH adjuster adding device

[0143] 22 fluoride ion meter

[0144] 23 scale inhibitor adding line

[0145] 31 pretreatment device (coagulation and filtration)

[0146] 32 heat exchanger

[0147] 33 activated carbon tower

[0148] 34 decarbonation tower

[0149] 40 scale inhibitor adding unit

[0150] 41 scale inhibitor adding amount control device

[0151] 42 scale inhibitor adding device

[0152] 100, 200 water treatment device

[0153] P1 first pump

[0154] P2 second pump

[0155] P3 third pump

Claims

1. A water treatment method of treating water containing fluorine and calcium by passing the water through at least a reverse osmosis membrane, characterized by comprising: a step of measuring a concentration of fluoride ions in the water to be treated; a step of adding an antiscalant that inhibits precipitation of calcium fluoride; and a reverse osmosis membrane treatment step of passing the water to which the antiscalant has been added through the reverse osmosis membrane to obtain permeated water and concentrated water, wherein the measurement of the concentration of the fluoride ions in the water to be treated is performed after adjusting the pH of the water to be treated to be greater than 5, and the amount of the antiscalant to be added is determined based on the measured concentration of the fluoride ions.

2. The water treatment method according to claim 1, wherein the addition of the antiscalant is performed after adjusting the pH of the water to be treated to be greater than 5.

3. The water treatment method according to claim 1 or 2, wherein the adjustment of the pH is performed by adding a pH adjuster in a feed water line through which the water to be treated is supplied to the reverse osmosis membrane, and the measurement of the concentration of the fluoride ions is performed in the feed water line after the addition of the pH adjuster.

4. The water treatment method according to any one of claims 1 to 3, wherein a first flow rate and a second flow rate are measured in any two of the feed water line, a line of the permeated water from the reverse osmosis membrane, and a line of the concentrated water from the reverse osmosis membrane, and the amount of the antiscalant to be added is determined based on a recovery rate calculated from a comparison between the first flow rate and the second flow rate and the measured concentration of the fluoride ions.

5. The water treatment method according to any one of claims 1 to 4, further comprising a decarbonation step of removing a carbonic acid component in the water to be treated before the addition of the pH adjuster to the feed water line.

6. A water treatment apparatus comprising: a reverse osmosis membrane that passes water containing fluorine and calcium through the reverse osmosis membrane to obtain permeated water and concentrated water; and a feed water line that supplies the water to be treated to the reverse osmosis membrane, characterized in that the feed water line comprises: a pH adjustment unit that adjusts the pH of the water to be treated; a measurement unit that measures the concentration of fluoride ions in the water to be treated under a condition where the pH is greater than 5; and an addition unit that adds an antiscalant that inhibits precipitation of calcium fluoride to the water to be treated, and the water treatment apparatus comprises an addition amount control device that determines the amount of the antiscalant to be added by the addition unit based on the measured concentration of the fluoride ions and controls the amount of the antiscalant to be added by the addition unit.

2. The water treatment method of claim 1, wherein, 7. The water treatment apparatus according to claim 6, wherein first flow rate sensors that measure a first flow rate and second flow rate sensors that measure a second flow rate are respectively provided in any two of the feed water line, a line of the permeated water from the reverse osmosis membrane, and a line of the concentrated water from the reverse osmosis membrane, and the addition amount control device determines the amount of the antiscalant to be added based on a recovery rate calculated from a comparison between the first flow rate and the second flow rate and the concentration of the fluoride ions measured by the measurement unit.

3. The water treatment method according to claim 1 or 2, wherein, 8. The water treatment apparatus according to claim 6, wherein the feed water line comprises a decarbonation tower that removes a carbonic acid component in the water to be treated upstream of the pH adjustment unit.

9. The water treatment apparatus according to claim 6, further comprising: a brine reverse osmosis membrane that further treats the concentrated water from the reverse osmosis membrane; and 4. The water treatment method of claim 3, wherein, ​ 5. The water treatment method of claim 3, wherein, ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ A water supply tank is located in the feed water line to mix the permeate water of the reverse osmosis membrane and / or the permeate water of the reverse osmosis membrane with the treated water of the feed water line.

10. The water treatment device of any one of claims 6 to 9, wherein, The measuring unit is arranged downstream of the pH adjusting unit.

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