Water treatment method and water treatment device

Abstract

To provide a water treatment method and apparatus capable of adding scale inhibitors in appropriate amounts to counter calcium fluoride scaling on reverse osmosis membranes.SOLUTION: The present invention provides a water treatment method and a water treatment apparatus for treating treated water containing fluorine and calcium by passing the treated water through at least a reverse osmosis membrane 11, in which pH adjustment means 21 for adjusting the pH of the treated water is provided in a supply water line 1 for passing to-be-treated water through the reverse osmosis membrane 11; measuring means 22 is provided to measure the fluoride ion concentration under the condition that the pH of the treated water is 5.0 or higher; the amount of scale inhibitor to be added to the treated water is determined based on the measured fluoride ion concentration; the scale inhibitor is added from an addition line 23; and then the treated water is discharged through the reverse osmosis membrane 11 with permeate water from permeate water line 2 and concentrated water from concentrated water line 3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a water treatment method and a water treatment device including a reverse osmosis membrane device in which scale deposition is suppressed. [Background technology]

[0002] In recent years, reverse osmosis membranes have been increasingly used in water treatment. The water to be treated (referred to as untreated water) contains a variety of components, which precipitate (called scale) due to various factors, clogging the reverse osmosis membrane and reducing treatment efficiency. Scale precipitation (scaling) is one of the important factors in the operational management of reverse osmosis membranes. Calcium scaling is particularly common, and scaling caused by calcium carbonate and calcium fluoride is known.

[0003] In countries such as Hong Kong, Singapore, Malaysia, Ireland, the United States, Australia, New Zealand, and the United Kingdom, fluoride is added to drinking water supplies to maintain dental health. Furthermore, surface water in these countries has a higher concentration of hardness components than in Japan. As a result, there are concerns about the deposition of calcium fluoride scale during the production of purified water in these countries.

[0004] Patent Document 1 proposes a method of lowering the pH of the water to be treated to, for example, 4 to 6 as a measure against calcium carbonate scale. On the other hand, Patent Document 2 and Patent Document 3 propose the use of a scale inhibitor as a measure against calcium fluoride scale. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-153732 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-184365 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-186835 Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, water shortages have led to increased demand for water treatment and reuse, resulting in increased water recovery rates, leading to an increase in the concentration of scale components in concentrated water. This has led to an increase in the concentration of scale inhibitors added, which has led to increased operating costs for water treatment equipment. Conventional scaling countermeasures involve the injection of a fixed amount of scale inhibitor determined through preliminary testing and water quality analysis. However, when water quality fluctuates significantly during the water treatment process, excessive addition of scale inhibitor can lead to increased running costs. In particular, scale inhibitors are expensive, and the cost burden cannot be ignored. On the other hand, adding too little scale inhibitor does not adequately prevent scale buildup on reverse osmosis membranes, shortening the replacement interval for the reverse osmosis membranes. Furthermore, as shown in Patent Document 3, calcium fluoride deposition is low on the acidic side with a low pH, and it is possible to use no scale inhibitor or reduce the amount of scale inhibitor used. However, a low pH state can cause corrosion of pipes and the like, resulting in additional costs for linings to prevent corrosion.

[0007] Therefore, an object of the present invention is to provide a water treatment method and a water treatment apparatus that optimizes running costs by adding an appropriate amount of scale inhibitor at an appropriate pH. [Means for solving the problem]

[0008] The present invention provides a water treatment method for treating water containing fluorine and calcium by passing the water through at least a reverse osmosis membrane, measuring the fluoride ion concentration in the water to be treated; adding a scale inhibitor that inhibits precipitation of calcium fluoride; a reverse osmosis membrane treatment step in which the water to be treated after adding the scale inhibitor is passed through the reverse osmosis membrane to obtain permeated water and concentrated water; Equipped with The present invention provides a water treatment method characterized in that the fluoride ion concentration in the water to be treated is measured after adjusting the pH to 5 or more, and the amount of scale inhibitor to be added is determined based on the measured fluoride ion concentration.

[0009] The present invention also provides a water treatment device having a reverse osmosis membrane through which water to be treated containing fluoride and calcium is passed to obtain permeate and concentrated water, and a supply water line for supplying the water to be treated to the reverse osmosis membrane, wherein the supply water line is provided with: a pH adjusting means for adjusting the pH of the water to be treated; a measuring means for measuring the fluoride ion concentration in the water to be treated under a condition of a pH of 5 or higher; and an adding means for adding a scale inhibitor to the water to be treated that inhibits precipitation of calcium fluoride; The water treatment device is characterized by having an addition amount control device that determines the amount of scale inhibitor to be added by the addition means based on the measured fluoride ion concentration and controls the amount of scale inhibitor to be added by the addition means. [Effects of the Invention]

[0010] 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 scale inhibitor at an appropriate pH. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing the configuration of a water treatment device according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a schematic diagram showing the configuration of a water treatment device according to a second embodiment of the present invention. [Figure 3] 1 is a graph showing the relationship between the ratio of the amounts of calcium fluoride detected in an example and a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides a water treatment method and a water treatment apparatus for treating water containing fluorine and calcium using a reverse osmosis membrane. The water to be treated may be surface water from rivers or lakes, tap water, or industrial water. The water treated by the present invention can be used for producing ultrapure water, etc. Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] (First embodiment) FIG. 1 is a schematic diagram showing the configuration of a water treatment device according to a first embodiment of the present invention. The water treatment device 100 of this embodiment is a device that removes impurities (fluoride ions, calcium ions, etc.) contained in the water to be treated to produce treated water, and has a reverse osmosis membrane 11 that separates the water into concentrated water containing impurities and permeate water from which the impurities have been removed.

[0014] The water treatment device 100 also has a plurality of lines each connected to the reverse osmosis membrane 11, namely, a feed water line 1 that supplies the water to be treated to the reverse osmosis membrane, a permeate line 2 that discharges the permeate from the reverse osmosis membrane 11, and a concentrated water line 3 that discharges the concentrated water from the reverse osmosis membrane 11. In addition, the feed water line 1 has a pH adjuster 21 as a pH adjuster, a measuring means (fluoride ion meter) 22 that measures the concentration of fluoride ions, and an addition line 23 as a scale inhibitor adding means.

[0015] Although not shown in the present embodiment, a decarbonation process may be provided before the pH adjustment device 21 to inhibit calcium carbonate scale formation and improve the quality of the permeated water. In this case, the treated water in this process often has a pH of less than 5.0. When the pH is less than 5.0, some fluoride ions become hydrogen fluoride according to the hydrogen fluoride dissociation curve. For example, at pH = 5.0, the amount of fluoride ions that become hydrogen fluoride is approximately 1%, which is almost negligible. However, at pH = 4.5, approximately 4.5% of fluoride ions become hydrogen fluoride, at pH = 4.0, approximately 13% of fluoride ions become hydrogen fluoride, and at pH = 3.5, approximately 32% of fluoride ions become hydrogen fluoride. The fluoride ions that have become hydrogen fluoride then become fluoride ions as the pH increases due to an equilibrium reaction between hydrogen fluoride and fluoride ions, contributing to the production of calcium fluoride. However, since the fluoride ions that have become hydrogen fluoride cannot be measured with a fluoride ion meter, the fluoride ion concentration in the water being treated varies depending on the pH when measured with a fluoride ion meter, and the amount of scale inhibitor calculated from the fluoride ion concentration at low pH will be too low, resulting in insufficient scale inhibition.

[0016] Therefore, the fluoride ion concentration is measured under conditions that result in a pH of 5.0 or higher. Specifically, a pH adjustment step is provided using a pH adjuster 21 to adjust the pH of the water to 5.0 or higher. This prevents some fluoride ions from converting to hydrogen fluoride, and the fluoride ion concentration of the water being treated in the supply water line 1 can be accurately measured using a fluoride ion meter 22. The pH adjuster 21 selects a chemical injection method that does not affect the fluoride ions in the water being treated, and adds an alkali, particularly a low-concentration aqueous solution of sodium hydroxide. In addition to the alkali addition means, the pH adjuster also has a pH measurement means, such as a pH meter, that measures the pH of the water being treated at least either before or after the addition of the alkali.

[0017] The reason why fluoride ion concentration is measured instead of calcium ion concentration in calcium fluoride scale deposition is explained below. Calcium fluoride scale deposition is determined by the product of the molar concentrations of fluoride ions and calcium ions (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 ionic product exceeds the value of this solubility product, scale will precipitate. Calcium fluoride is composed of calcium ions and fluoride ions in a molar ratio of 1:2. The ionic product Kap of calcium fluoride is expressed by the following formula: Kap=[Ca + ]([F - ] 2 ) In the formula, [Ca + ] is the calcium ion concentration, [F - ] is the fluoride ion concentration.

[0018] The ionic product Kap of calcium fluoride is calculated by multiplying the square of the fluoride ion concentration by the calcium ion concentration. This means that the fluoride ion concentration has a greater effect on the ionic product than the calcium ion concentration. For these reasons, when determining the amount of scale inhibitor to be added based on the ionic product in response to fluctuating water quality, it is more effective to monitor fluoride ions than calcium ions.

[0019] The lower limit of measurement for a typical fluoride ion meter is approximately 1 mg / L. When the fluoride ion concentration is 20 mg / L or less, the ion product fluctuates by more than 10% when the fluoride ion concentration fluctuates by 1 mg / L. This makes this method particularly effective in the range of fluoride ion concentrations below 20 mg / L. Furthermore, the calcium ion concentration at this point is 1.4 mg / L or more, since the ion product is greater than the solubility product of calcium fluoride. Furthermore, when the fluoride ion concentration is 10 mg / L or less, the ion product fluctuates by more than 20%, making this method even more effective. Furthermore, the calcium ion concentration at this point is 5.6 mg / L or more.

[0020] The amount of scale inhibitor to be added is determined in real time based on the measurement results of the fluoride ion concentration and the recovery rate (sometimes referred to as the concentration ratio) calculated from the first and second flow rates measured by first and second flow sensors (not shown) connected to any two of the feed water line 1, the permeate line 2, and the concentrate line 3. This eliminates the need for time from analysis to determining the amount of scale inhibitor to be added. Typically, if the flow rate of the feed water line 1 is 100, the sum of the flow rates of the permeate line 2 and the concentrate line 3 is also 100. The recovery rate is the amount of permeate relative to the water to be treated and is set, for example, to 75% based on the performance of the reverse osmosis membrane. However, in actual water treatment operations, fluctuations in the water quality and water temperature of the water to be treated cause fluctuations in the amount of permeate and concentrate. Therefore, by measuring the actual water volume, the fluoride ion concentration can be accurately measured without being affected by fluctuations in the amount of permeate and concentrate due to fluctuations in water quality and water temperature, and an appropriate amount of scale inhibitor to be added can be determined. Here, the "appropriate amount of scale inhibitor added" in the present invention is preferably the minimum amount necessary to avoid adding an excessive amount of expensive scale inhibitor.

[0021] Based on the determined amount of scale inhibitor to be added, the scale inhibitor is added through the scale inhibitor addition line 23 .

[0022] The scale inhibitor is not limited to a specific one as long as it is a substance that can inhibit the deposition of scale components such as silica and calcium, but a scale inhibitor that inhibits the deposition of calcium fluoride is particularly preferred. Examples of such compounds 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 the acrylic acid 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 monomer; (meth)acrylic acid / sulfonic acid / nonionic group-containing monomer; terpolymers of (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.

[0023] Among these, it is preferable to use one containing at least one of a phosphonic acid compound and an acrylic acid polymer. For example, a copolymer of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid is preferable. Furthermore, to simultaneously inhibit scale derived from calcium and silica, it is particularly preferable to use a scale inhibitor containing a mixture of 2-phosphonobutane-1,2,4-tricarboxylic acid and a terpolymer of acrylic acid, (meth)acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and a substituted (meth)acrylamide.

[0024] In particular, commercially available scale inhibitors for reverse osmosis membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon®" series manufactured by BWA Water Additives, the "PermaTreat®" series manufactured by Nalco, the "Hypersperse®" series manufactured by General Electric Company, and the "Kuriverter®" series manufactured by Kurita Water Industries Ltd.

[0025] The scale inhibitor can be added within a pH range in which calcium fluoride precipitates. Normally, calcium fluoride begins to precipitate at a pH of 3.5 or higher. However, in the embodiment shown in FIG. 1, the pH of the water to be treated is adjusted to 5 or higher in order to measure the fluoride ion concentration between the pH adjuster 21 and the reverse osmosis membrane 11. After adjusting the pH of the water to 5 or higher, the fluoride ions are measured and the scale inhibitor is added. Note that, to improve the quality of the water permeated through the reverse osmosis membrane, a pH of 4 or higher is preferable, and 5 or higher is even more preferable. Therefore, it is a preferred embodiment to adjust the pH before (upstream of) the reverse osmosis membrane 11. 1 shows an example in which the fluoride ion concentration measuring means is disposed downstream of the pH adjusting means, but is not limited thereto, and the fluoride ion concentration measuring means can be disposed upstream of the pH adjusting means if the pH of the water to be treated is 5 or higher before treatment to lower the pH value in a decarbonation tower or the like. It is also preferable to provide a pH meter or the like to confirm that the pH of the water to be treated is 5 or higher.

[0026] (Second embodiment) 2 is a schematic diagram showing the configuration of a water treatment device 200 according to a second embodiment of the present invention. Hereinafter, the same components as those in the first embodiment will be denoted by the same reference numerals in the drawings and their description will be omitted, and only the components different from those in the first embodiment will be described.

[0027] As in the first embodiment, the water treatment device 200 has a feed water line 1 for supplying water to be treated and a reverse osmosis membrane 11. In the water treatment device 200 according to the second embodiment, the feed water line 1 has a water supply tank 13 for storing the water to be treated that is passed through it, a pretreatment (coagulation / filtration) device 31, a heat exchanger 32, an activated carbon tower (activated carbon filter) 33, a decarbonation tower 34, the reverse osmosis membrane 11, and a brine reverse osmosis membrane 12.

[0028] The amount of water stored in the water supply tank 13 is adjusted by a pressure pump P1 (pressure adjustment means) that adjusts the pressure of the water to be treated flowing through the water supply line. In addition to the water supply line 1, permeated water after passing through the reverse osmosis membrane 11 can also be returned to the water supply tank 13 from the permeated water line 2 via the return line 2b. In addition, permeated water obtained by filtering the concentrated water separated by the reverse osmosis membrane 11 with the brine reverse osmosis membrane 12 can also be introduced into the water supply tank 13 via the circulation line 5. However, the permeated water after passing through the reverse osmosis membrane 11 may also be sampled. In such cases, the permeated water is not circulated to the water supply tank 13 but is sampled via the water sampling line 2a. The concentrated water from the brine reverse osmosis membrane 12 is discharged from the discharge line 4 and, if necessary, is post-treated and then discarded.

[0029] Examples of pretreatment devices 31 include devices capable of coagulation, sand filtration, and membrane filtration. Coagulation is a process in which a positively charged coagulant neutralizes the charge of negatively charged particles in water, causing them to coagulate and form basic flocs. A coagulation aid such as a polymer adsorbs the basic flocs to form coarse flocs that are more easily precipitated. Examples of coagulants include aluminum sulfate, polyaluminum chloride, ferric chloride, and ferrous sulfate. Sand filtration is a process in which water is filtered by passing it through sedimentary sand, using accumulated sand as a filter medium. Membrane filtration is a process in which water is filtered by passing it through a filtration membrane. Filtration membranes include microfiltration (MF) membranes, ultrafiltration (UF) membranes, nanofiltration (NF) membranes, ion exchange membranes, and other membranes, depending on the size of the substances to be filtered and the driving force of the filtration.

[0030] The heat exchanger 32 is a device that heats the water to be treated that is supplied after pretreatment, and is provided to generate hot water for thermal sterilization. The activated carbon tower 33 is provided to remove chlorine from the water to be treated that is supplied from the heat exchanger.

[0031] The decarbonation tower 34 is a device that converts carbonate ions and bicarbonate ions into carbon dioxide gas by lowering the pH by injecting acid, and removes carbon dioxide from the water by blowing air into the packed tower. It is installed to suppress calcium carbonate scale and improve the quality of the permeated water.

[0032] The components from the decarbonation tower 34 to the reverse osmosis membrane 11 are essentially the same as those in the first embodiment, and include a pH adjuster 21, a fluoride ion meter 22, and a scale inhibitor addition line 23. The pH adjuster 21 includes a pH meter 21a (a pH measurement means) and a pH control device 21b that determines the amount of pH adjuster (alkali) added by the pH adjuster adder 21c based on the pH value measured by the pH meter 21a. The pH adjuster adder 21c, controlled by the pH control device 21b, adds a predetermined amount of pH adjuster (alkali) to adjust the pH of the water to 5.0 or higher. A flow meter, as described in the first embodiment, is installed upstream of the pH adjuster 21, and the measured flow rate and pH value are input into the pH control device 21b, allowing the amount of pH adjuster to be added in real time to be set. A pH meter may also be installed downstream of the pH adjuster to add an amount of pH adjuster equivalent to the difference between the pH meter readings before and after. The water to be treated, whose pH has been adjusted in this manner, is sampled, and the fluoride ion concentration is measured by the fluoride ion meter 22. The measured fluoride ion concentration is transferred to a scale inhibitor addition amount control device 41, which calculates the minimum amount of scale inhibitor required, and based on this information, scale inhibitor is added from a scale inhibitor addition device 42 to the supply water line 1 via a scale inhibitor addition line 23. In this example, the scale inhibitor addition means 40 includes a fluoride ion meter 22, a control device 41, an addition device 42, and an addition line 23.

[0033] Furthermore, after adding a scale inhibitor through the scale inhibitor addition line 23, the pressure is adjusted by the pressure pump P2 (pressure adjusting means) and the water is passed through the reverse osmosis membrane 11.

[0034] If the fluoride ion concentration is measured while the pH is still lowered in the decarbonation tower 34, the amount of scale inhibitor added based on the measured fluoride ion concentration will be insufficient when the pH is subsequently increased and a scale inhibitor is added, failing to sufficiently prevent scale buildup on the reverse osmosis membrane. Therefore, in this embodiment, the pH is increased to 5.0 or higher in the pH adjustment device 21, and then the fluoride ion concentration is measured using the fluoride ion meter 22. Based on the measured fluoride ion concentration, the amount of scale inhibitor added via the scale inhibitor addition line 23 is reduced to a minimum, thereby sufficiently preventing scale buildup on the reverse osmosis membrane. The installation location of the fluoride ion meter 22 is not limited to downstream of the pH adjustment device 21 shown in FIG. 2 , but can be installed in any supply line where the pH is 5.0 or higher. For example, the permeate of the reverse osmosis membrane 11 and the brine reverse osmosis membrane 12 have pH values ​​increased to 5.0 or higher. The pH of the water being treated in the water supply tank 13, which is then circulated and mixed with surface water, may also be 5.0 or higher. Therefore, an online fluoride ion meter (not shown) can be installed in the water supply tank 13 or in the supply water line 1 before the downstream pretreatment device, and the measured fluoride ion concentration can be communicated to the addition amount control device 41, allowing the minimum required amount of scale inhibitor to be added.

[0035] The permeated water after passing through the reverse osmosis membrane 11 may be collected via the permeated water line 2 or may be circulated to the water supply tank 13 without being collected. On the other hand, the concentrated water after passing through the reverse osmosis membrane 11 is passed through a concentrated water line 3, and the pressure is adjusted by a pressure pump P3 (pressure adjustment means) in the concentrated water line 3, and the water is passed through a brine reverse osmosis membrane 12. After being filtered by the brine reverse osmosis membrane 12, the water is separated into wastewater that is discharged through a drainage line 4 and circulating water that is circulated to a water supply tank 13. [Example]

[0036] Next, the effects of the present invention will be described with reference to specific examples. Example 1 Model water containing fluoride ions and calcium ions was prepared in a 1-L beaker using pure water as the raw water. The pH was adjusted to 3.5 using hydrochloric acid or sodium hydroxide solution. The fluoride ion concentration was adjusted to 6.7 mg / L using sodium fluoride. The calcium ion concentration was adjusted to 200 mg / L using calcium chloride. After adjusting the pH to 5.0, fluoride ions were measured using a fluoride ion meter. A scale inhibitor was added according to the concentration calculated from the fluoride ion concentration. A copolymer of acrylic acid, acrylic acid, and 2-acrylamido-2-methylpropanesulfonic acid was used as the scale inhibitor. The fluoride ion concentration was measured using an ion electrode (model F-2021) manufactured by DKK-TOA Corporation. After adding the scale inhibitor, the solution was stirred for 24 hours using a magnetic stirrer, and the amount of calcium fluoride precipitated was calculated. The method for calculating the amount of calcium fluoride precipitation is as follows. After the test, the supernatant was filtered using a 0.1 μm filter, and the pH was adjusted to 6.0 to 7.0, after which the fluoride ion concentration was measured. From the measurement result and the fluoride ion concentration before the test, the concentration of fluoride ions consumed for the precipitation of scale was calculated, and from this result the amount of calcium fluoride precipitation was calculated.

[0037] Example 2 The pH of the simulated water was adjusted to 4.0 when it was prepared. Otherwise, the amount of precipitated calcium fluoride was calculated in the same manner as in Example 1.

[0038] Example 3 The pH of the simulated water was adjusted to 4.5 when it was prepared. The amount of precipitated calcium fluoride was calculated in the same manner as in Example 1 except for the above.

[0039] Example 4 The pH of the simulated water when it was prepared was adjusted to 5.0. The amount of precipitated calcium fluoride was calculated in the same manner as in Example 1, except that the fluoride ion concentration was measured at the same pH.

[0040] Example 5 The pH of the simulated water when it was prepared was adjusted to 5.5. The amount of precipitated calcium fluoride was calculated in the same manner as in Example 1, except that the fluoride ion concentration was measured at the same pH.

[0041] (Comparative Example 1) The adjustment was made in the same manner as in Example 1. Before measuring fluoride ions with a fluoride ion meter, the pH was not adjusted to 5.0, but the fluoride ions were measured at a pH of 3.5, and a scale inhibitor was added in accordance with the concentration calculated from the fluoride ion concentration. The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1 except for the above.

[0042] (Comparative Example 2) The adjustment was made in the same manner as in Example 2. Before measuring fluoride ions with a fluoride ion meter, the pH was not adjusted to 5.0, but the fluoride ions were measured with the pH at 4.0, and a scale inhibitor was added in accordance with the concentration calculated from the fluoride ion concentration. The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1 except for the above.

[0043] (Comparative Example 3) The adjustment was made in the same manner as in Example 3. Before measuring fluoride ions with a fluoride ion meter, the pH was not adjusted to 5.0, but the fluoride ions were measured at a pH of 4.5, and a scale inhibitor was added in accordance with the concentration calculated from the fluoride ion concentration. The amount of calcium fluoride precipitated was calculated in the same manner as in Example 1 except for the above.

[0044] The ratio of the amount of calcium fluoride detected between each Example and each Comparative Example (Comparative Example / Example) is shown in Table 1. In Table 1, fluoride ions are represented as "F ions," calcium ions as "Ca ions," and calcium fluoride as "CaF2."

[0045] [Table 1]

[0046] Furthermore, Figure 3 is a graph showing the relationship between the ratio of the detected amount of calcium fluoride (Comparative Example / Example) detected in the Example and Comparative Example. It can be seen that in the pH < 5.0 region, the scale inhibitor was added insufficiently. Note that the detection amount ratio for Examples 4 and 5 is 1.00. When the pH is low, the amount of CaF2 precipitated is small, but if it is assumed that the scale inhibitor is added at a pH of 5 or higher, it is possible to predict an increase in scale corresponding to the deficiency indicated by the detection amount ratio. [Explanation of symbols]

[0047] 1. Water supply line 2 Permeate line 3 Concentrated water line 4. Drainage line 5 Circulation Line 11 Reverse osmosis membrane 12 Brine reverse osmosis membrane 13 Water tank 21 pH adjustment device 21a pH meter 21b pH controller 21c pH adjuster adding device 22 Fluoride ion meter 23 Scale inhibitor addition line 31 Pretreatment equipment (coagulation and filtration) 32 Heat exchanger 33 Activated carbon tower 34 Decarboxylation tower 40 Means for adding scale inhibitor 41 Scale inhibitor addition amount control device 42 Scale inhibitor adding device 100,200 Water treatment equipment P1 First pump P2 Second pump P3 Third pump

Claims

1. A water treatment method for treating water containing fluorine and calcium by passing the water through at least a reverse osmosis membrane, measuring the fluoride ion concentration in the water to be treated; adding a scale inhibitor that inhibits precipitation of calcium fluoride; a reverse osmosis membrane treatment step of passing the water to be treated after adding the scale inhibitor through a reverse osmosis membrane to obtain permeated water and concentrated water; Equipped with The measurement of the fluoride ion concentration in the water to be treated is carried out after adjusting the pH of the water to 5 or more, and the amount of the scale inhibitor to be added is determined based on the measured fluoride ion concentration; The addition of the scale inhibitor is carried out after adjusting the pH of the water to be treated to 5 or more, The pH adjustment is performed by adding a pH adjuster to a supply water line that supplies the water to be treated to the reverse osmosis membrane; The water treatment method, wherein the measurement of the fluoride ion concentration is carried out on the water to be treated in the supply water line after the pH adjuster has been added.

2. 2. The water treatment method according to claim 1, wherein a first flow rate and a second flow rate are measured in any two of the supply water line, the line for permeated water from the reverse osmosis membrane, and the line for concentrated water from the reverse osmosis membrane, and the amount of scale inhibitor to be added is determined from a recovery rate obtained by comparing the first flow rate and the second flow rate and the measured fluoride ion concentration.

3. 3. The water treatment method according to claim 1, further comprising a decarbonation step of removing carbonate components from the water to be treated before adding the pH adjuster to the supply water line.

4. a reverse osmosis membrane through which water to be treated containing fluorine and calcium is passed to obtain permeated water and concentrated water; A water treatment device having a supply water line that supplies the water to be treated to the reverse osmosis membrane, The supply water line pH adjusting means for adjusting the pH of the water to be treated; a measuring means for measuring the fluoride ion concentration in the water to be treated under a condition of pH 5 or more; a means for adding a scale inhibitor to the water to be treated to inhibit precipitation of calcium fluoride; Equipped with an addition amount control device that determines the amount of the scale inhibitor to be added by the addition means based on the measured fluoride ion concentration and controls the amount of the scale inhibitor to be added by the addition means; and the pH adjusting means has an adding means for adding a pH adjusting agent to the supply water line, the measuring means is disposed downstream of the pH adjusting means so that the measurement of the fluoride ion concentration is performed on the water to be treated in the supply water line after the pH adjuster has been added; A water treatment device characterized in that the scale inhibitor addition means is arranged downstream of the pH adjustment means so that the addition of the scale inhibitor is carried out after the pH of the treated water is adjusted to 5 or more.

5. a first flow rate sensor for measuring a first flow rate and a second flow rate sensor for measuring a second flow rate in any two of the supply water line, the permeated water line from the reverse osmosis membrane, and the concentrated water line from the reverse osmosis membrane, 5. The water treatment device according to claim 4, wherein the addition amount control device determines the amount of the scale inhibitor to be added based on a recovery rate obtained by comparing the first flow rate with the second flow rate and the fluoride ion concentration measured by the measuring means.

6. The water treatment device according to claim 4 or 5, wherein the supply water line includes a decarbonation tower upstream of the pH adjusting means for removing carbonate components from the water to be treated.

7. a brine reverse osmosis membrane for further treating the retentate of the reverse osmosis membrane; a water supply tank in the supply water line for mixing the brine reverse osmosis membrane permeate and / or the reverse osmosis membrane permeate with the water to be treated in the supply water line; The water treatment device according to claim 4 , further comprising:

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