Water treatment method and silica-based scale inhibitor

Abstract

To provide a water treatment method capable of suppressing formation of silica-base scales in a reverse osmosis membrane under neutral condition even when silica content in a target water is high concentration, and a silica-base scale inhibitor used in the water treatment method.SOLUTION: A water treatment method includes: an addition step of adding a scale inhibitor to a silica-containing target water; and a reverse osmosis membrane treatment step of conducting the scale inhibitor-added target water to a reverse osmosis membrane for separation into permeated water and concentrated water. The scale inhibitor comprises a copolymer including a bisphenol S monomer unit having a specified structure and a phenolsulfonic acid monomer unit having a specified structure.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water treatment method using a reverse osmosis membrane treatment, and a silica-based scale inhibitor that can be used in the water treatment method. [Background technology]

[0002] In recent years, reverse osmosis membranes have been increasingly used for pure water production, water recovery, etc. Scaling is one of the most important operational management issues for reverse osmosis membranes. If scale forms on the surface of a reverse osmosis membrane, it causes an increase in differential pressure, which leads to a deterioration in the operational efficiency of the facility. Therefore, suppressing this scale formation is extremely important for stable operation of the facility.

[0003] Among various types of scale, silica-based scale is particularly difficult to prevent from forming. This is because silica scale formation is promoted by other ionic components, and silica scale is uncharged.

[0004] The main countermeasure against scaling is the addition of scale dispersants to the water being treated. Scale dispersants have structures such as acrylic acid, maleic acid, and phosphonic acid, and have the function of capturing cations electrically to inhibit scale deposition, and also obtain dispersing ability through steric hindrance. However, as mentioned above, because silica is an uncharged substance, there are few effective silica-based scale dispersants. Countermeasures against scaling are limited to indirect measures that physically and sterically inhibit the growth of silica scale, such as periodic membrane flushing and the use of hyperbranched polymers with structures such as tertiary butyl groups (see, for example, Patent Document 1).

[0005] Therefore, when taking measures against silica scale, reverse osmosis membranes have been operated by making the water to be treated acidic to control the silica precipitation time and thereby suppressing silica scaling (see, for example, Patent Document 2), or by making the water to be treated alkaline to increase the solubility of silica and thereby suppressing silica precipitation.

[0006] However, when the water to be treated is acidified, the rejection rate of the reverse osmosis membrane drops significantly, leading to a deterioration in the quality of the treated water.When the water to be treated is alkaline, the risk of hardness scale deposition increases significantly, making it necessary to add a dispersant for hardness components or install pretreatment equipment such as a softening treatment.

[0007] Furthermore, although the silica-based scale dispersants are effective against silica at relatively low concentrations, they are unable to exert a sufficient inhibitory effect at high concentrations (for example, 200 mg / L or more). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6512322 [Patent Document 2] Patent No. 3187629 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a water treatment method that can inhibit the formation of silica-based scale on a reverse osmosis membrane under neutral conditions even when the water to be treated contains a high concentration of silica, and a silica-based scale inhibitor that can be used in the water treatment method. [Means for solving the problem]

[0010] The present invention includes a process for adding a scale inhibitor to water to be treated that contains silica, and a reverse osmosis membrane treatment process for passing the water to be treated to which the scale inhibitor has been added through a reverse osmosis membrane to separate it into permeate and concentrate, wherein the scale inhibitor contains a copolymer containing bisphenol S monomer units represented by the following chemical formula (1) and phenolsulfonic acid monomer units represented by the following chemical formula (2): The pH of the water to be treated to which the scale inhibitor has been added is 6.6 or more and 11 or less. This is a water treatment method. [ka] (1) [ka] (2)

[0012] In the water treatment method, the silica concentration of the concentrated water is preferably 150 mg / L or more.

[0013] In the water treatment method, the molecular weight of the copolymer is preferably in the range of 1,000 to 100,000 in terms of weight average molecular weight.

[0014] In the water treatment method, the scale inhibitor is preferably added to the water to be treated at a concentration of 5 mg / L or more in terms of solid content.

[0015] The present invention relates to a copolymer containing a bisphenol S monomer unit represented by the following chemical formula (1) and a phenolsulfonic acid monomer unit represented by the following chemical formula (2): A silica-based scale inhibitor, wherein the pH of the water to be treated to which the scale inhibitor is added is 6.6 or more and 11 or less. , a silica-based scale inhibitor. [ka] (1) [ka] (2)

[0016] In the silica-based scale inhibitor, the molecular weight of the copolymer is preferably in the range of 1,000 to 100,000 in terms of weight average molecular weight. [Effects of the Invention]

[0017] The present invention provides a water treatment method that can inhibit the formation of silica-based scale in a reverse osmosis membrane under neutral conditions even when the silica content of the water to be treated is high, and a silica-based scale inhibitor that can be used in the water treatment method. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram illustrating an example of a water treatment device according to an embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram showing the configuration of a water treatment device used in the examples. [Figure 3] 1 is a graph showing the flux retention (%) versus water flow time (hr) in Example 1 and Comparative Example 1. [Figure 4] 1 is a graph showing the behavior of Fluxt=t / Fluxt=0(-) when the pH is changed in Example 1. [Figure 5] 1 is a graph showing the flux retention (%) at each addition concentration after 40 hours of water flow in Example 2. [Figure 6] 10 is a graph showing the results of evaluating the dispersibility of hardness components (Ca dispersion rate (%)) in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0020] <Water treatment method> The water treatment method according to this embodiment includes an addition step of adding a scale inhibitor to water to be treated that contains silica, and a reverse osmosis membrane treatment step of passing the water to be treated, to which the scale inhibitor has been added, through a reverse osmosis membrane to separate it into permeate and concentrate, wherein the scale inhibitor contains a copolymer including bisphenol S monomer units represented by the following chemical formula (1) and phenolsulfonic acid monomer units represented by the following chemical formula (2).

[0021] [ka] (1) [ka] (2)

[0022] An example of a water treatment apparatus for carrying out the water treatment method according to this embodiment is outlined in FIG. 1, and its configuration will be described.

[0023] 1 includes a reverse osmosis membrane treatment device 10 as a reverse osmosis membrane treatment means for passing water to be treated, to which a scale inhibitor has been added, through a reverse osmosis membrane to separate the water into permeate and concentrated water. The water treatment device 1 may also include a scale inhibitor addition pipe 18 as an addition means for adding a scale inhibitor to the water to be treated that contains silica.

[0024] In the water treatment device 1, a water-to-be-treated pipe 12 is connected to the inlet of a reverse osmosis membrane treatment device 10. A permeate pipe 14 is connected to the permeate outlet of the reverse osmosis membrane treatment device 10, and a concentrate pipe 16 is connected to the concentrate outlet. A scale inhibitor addition pipe 18 is connected to the water-to-be-treated pipe 12.

[0025] The water treatment method and the operation of the water treatment device 1 according to this embodiment will be described.

[0026] Water to be treated that contains silica (silica-containing water) is sent to the reverse osmosis membrane treatment device 10 through the water to be treated pipe 12. Here, a scale inhibitor is added to the water to be treated that contains silica in the water to be treated pipe 12 through the scale inhibitor addition pipe 18 (addition step). A water to be treated tank that stores the water to be treated may be installed upstream of the reverse osmosis membrane treatment device 10, and the scale inhibitor may be added in the water to be treated tank.

[0027] In the reverse osmosis membrane treatment device 10, the water to be treated, to which a scale inhibitor has been added, is passed through a reverse osmosis membrane and separated into permeated water and concentrated water (reverse osmosis membrane treatment step). The permeated water is discharged through a permeated water pipe 14, and the concentrated water is discharged through a concentrated water pipe 16.

[0028] The scale inhibitor used in this water treatment method is a scale inhibitor containing a copolymer including bisphenol S monomer units represented by the above chemical formula (1) and phenolsulfonic acid monomer units represented by the above chemical formula (2).

[0029] The present inventors have discovered that silica scaling in reverse osmosis membranes can be reduced by adding a sulfonic acid polymer to silica-containing water to be treated as a means of suppressing the formation of silica-based scale in reverse osmosis membranes under neutral conditions, even when the water to be treated contains a high concentration of silica.By using a scale inhibitor containing a copolymer containing bisphenol S monomer units and phenolsulfonic acid monomer units, silica scaling can be suppressed even under neutral conditions and when the silica content is high (e.g., silica content of 200 mg / L or more), and stable operation can be achieved.

[0030] Although silica is an uncharged substance, its surface is slightly anionically charged, and it is thought that the strong anion charge of the sulfonic acid groups in the sulfonic acid polymer can disperse the silica. If this sulfonic acid polymer also has a bisphenol S structure, the bisphenol S structure and the surface of the reverse osmosis membrane are attracted to each other through hydrogen bonding, causing the sulfonic acid polymer to remain near the membrane, which is thought to more effectively suppress silica scale deposition on the membrane surface.

[0031] The copolymer contained in the scale inhibitor is a copolymer containing bisphenol S monomer units represented by the above chemical formula (1) and phenolsulfonic acid monomer units represented by the above chemical formula (2). The copolymer contained in the scale inhibitor may be a copolymer represented by the following chemical formula (3), which is composed of bisphenol S monomer units represented by the above chemical formula (1) and phenolsulfonic acid monomer units represented by the above chemical formula (2). In the copolymer containing the monomer units represented by the above chemical formula (1) and the monomer units represented by the above chemical formula (2), or in the copolymer composed of the monomer units represented by the above chemical formula (1) and the monomer units represented by the above chemical formula (2), the molar ratio of the monomer units (m) represented by the above chemical formula (1) to the monomer units (n) represented by the above chemical formula (2) is not particularly limited, but may be, for example, in the range of m:n = 1:99 to 99:1, or may be in the range of 1:9 to 9:1, with m>n being preferred and m>>n being more preferred.

[0032] [ka] (3)

[0033] A copolymer containing the monomer units represented by the chemical formula (1) and the monomer units represented by the chemical formula (2) may contain monomer units other than the monomer units represented by the chemical formula (1) and the monomer units represented by the chemical formula (2). The monomers constituting the monomer units other than the monomer units represented by the chemical formula (1) and the monomer units represented by the chemical formula (2) are not particularly limited, and examples thereof include acrylic acid, maleic acid, methacrylic acid, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0034] The molecular weight of the copolymer is not particularly limited, but is preferably in the range of 1,000 to 100,000 in weight-average molecular weight, more preferably in the range of 1,000 to 25,000, and even more preferably in the range of 4,000 to 25,000. The weight-average molecular weight of the copolymer can be measured, for example, by gel permeation chromatography (GPC), a common measurement method. If the molecular weight of the copolymer is less than 1,000 in weight-average molecular weight, the scale inhibition effect may be insufficient, while if it exceeds 100,000, the reverse osmosis membrane may be clogged or the copolymer may be highly viscous, making it difficult to handle.

[0035] The pH of the water to be treated is not particularly limited, but is preferably 6.5 or higher, and more preferably 6.6 or higher. The upper limit of the pH is preferably 11 or lower from the viewpoint of membrane deterioration, and is more preferably less than 9 because silica solubility may increase at pH 9 or higher. If the pH of the water to be treated is less than 6.5, the permeation flux (flux) of the reverse osmosis membrane may decrease. Note that the pH of the water to be treated is the pH of the water to be treated after the scale inhibitor has been added.

[0036] The silica concentration of the concentrated water is not particularly limited, but is preferably 150 mg / L or more, and more preferably 200 mg / L or more. The upper limit of the silica concentration of the concentrated water is, for example, 400 mg / L, and preferably 300 mg / L. That is, a silica concentration in the concentrated water in the range of 200 to 300 mg / L is particularly effective. Note that the silica concentration here refers to the concentration of ionic silica. If the silica concentration of the concentrated water is less than 150 mg / L, the scale inhibitor will have advantages, but existing chemicals can still inhibit scale. If the silica concentration exceeds 400 mg / L, the scale inhibitor will have advantages, but long-term effects may not be observed.

[0037] The concentration of the scale inhibitor added to the water to be treated is not particularly limited, but is preferably in the range of 1 to 1000 mg / L in terms of solids concentration, more preferably in the range of 1 to 100 mg / L, and even more preferably in the range of 5 to 25 mg / L. If the concentration of the scale inhibitor added to the water to be treated is less than 1 mg / L in terms of solids concentration, a sufficient scale inhibition effect may not be obtained, and if it exceeds 1000 mg / L, the amount of agent added becomes enormous, which is economically problematic and not practical.

[0038] The above polymer may be used in combination with other "scale inhibitors," "bactericides," "anticorrosives," and the like.

[0039] Other scale inhibitors include polymer electrolytes and phosphonic acid compounds.

[0040] Examples of the polymer electrolyte include anionic polymers, amphoteric polymers, and cationic polymers.

[0041] Examples of anionic polymers include polyacrylic acid, polymaleic acid, phosphinic acid polymers, copolymers of acrylic acid and 2-hydroxy-3-allyloxypropanesulfonic acid, copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid, copolymers of acrylic acid and isoprene sulfonic acid, copolymers of acrylic acid and 2-hydroxyethyl methacrylate, copolymers of acrylic acid, 2-hydroxyethyl methacrylate, and isopropylenesulfonic acid, copolymers of acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and substituted acrylamide, copolymers of maleic acid and pentene, and alkali metal salts and alkaline earth metal salts of these anionic polymers. These anionic polymers and their salts may be used alone or in combination of two or more.

[0042] Examples of amphoteric polymers include copolymers of diallylamine hydrochloride and maleic acid, copolymers of diallylamine amide sulfate and maleic acid, copolymers of diallyldimethylammonium chloride and maleic acid, etc. These amphoteric polymers and salts thereof may be used alone or in combination of two or more.

[0043] Examples of the cationic polymer include polydiallylamine, polydiallyldimethylammonium chloride, etc. These cationic polymers and salts thereof may be used alone or in combination of two or more.

[0044] Examples of phosphonic acid compounds include 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoacetic acid, nitrilotrimethylenephosphonic acid, and salts of the above phosphonic acids. The phosphonic acid compounds may be used as free acids or as salts. Examples of phosphonic acid salts include alkali metal salts such as lithium salts, sodium salts, and potassium salts, and alkaline earth metal salts such as magnesium salts and calcium salts. The phosphonic acid salts may be either normal salts or acid salts. These phosphonic acids and their salts may be used alone or in combination of two or more.

[0045] Examples of disinfectants include halogen-based compounds such as hypochlorite, chloramine, chlorosulfamic acid, hypobromous acid, and stabilized hypobromous acid compositions, isothiazolone compounds, organic nitrogen-based compounds such as 2,2-dibromo-3-nitrilopropionamide (DBNPA) and 2,2-dibromo-2-nitriethanol (DBNE), and quaternary ammonium compounds. Examples of stabilized hypobromous acid compositions include stabilized hypobromous acid compositions containing a mixture of a "bromine-based oxidizing agent" and a "sulfamic acid compound," and stabilized hypobromous acid compositions containing a "reaction product of a bromine-based oxidizing agent and a sulfamic acid compound."

[0046] Examples of the anticorrosive agent include azole compounds such as benzotriazole and tolyltriazole, phosphates, molybdates, zinc salts, and nitrites.

[0047] Examples of applications of the reverse osmosis membrane treatment in the water treatment method according to this embodiment include pure water production, seawater desalination, and wastewater recovery.

[0048] Examples of the water to be treated include industrial water, well water, surface water, tap water, water discharged from abatement systems, water discharged from semiconductor manufacturing processes such as acid-alkali neutralization wastewater, cooling tower blowdown water, etc. The water to be treated may also be seawater or brackish water.

[0049] In the water treatment method and water treatment device according to this embodiment, a device is provided upstream of the reverse osmosis membrane treatment device 10, which performs at least one of biological, physical, or chemical pretreatments on the water to be treated, such as pH adjustment, biological treatment, coagulation treatment, coagulation sedimentation treatment, pressure flotation treatment, filtration treatment, membrane separation treatment, activated carbon treatment, ozone treatment, ultraviolet irradiation treatment, and decarbonation treatment, and the water to be treated in the reverse osmosis membrane treatment device 10 (reverse osmosis membrane treatment process) may be subjected to at least one of biological, physical, or chemical pretreatments, such as pH adjustment, biological treatment, coagulation treatment, coagulation sedimentation treatment, pressure flotation treatment, filtration treatment, membrane separation treatment, activated carbon treatment, ozone treatment, ultraviolet irradiation treatment, and decarbonation treatment.

[0050] Furthermore, in the water treatment method and water treatment device according to this embodiment, at least one device selected from the group consisting of a regenerative ion exchange treatment device, an electrical demineralization treatment device (EDI), a non-regenerative ion exchange resin device, a degassing membrane treatment device, a UV sterilization treatment device, a UV oxidation treatment device, a particulate removal treatment device, and a second reverse osmosis membrane treatment device may be provided downstream of the reverse osmosis membrane treatment device 10 to treat the permeate from the reverse osmosis membrane treatment device 10, and at least one device selected from the group consisting of a regenerative ion exchange treatment device, an electrical demineralization treatment device, a non-regenerative ion exchange resin treatment device, a degassing membrane treatment device, a UV sterilization treatment device, a UV oxidation treatment device, a particulate removal treatment, and a second reverse osmosis membrane treatment may be provided downstream of the reverse osmosis membrane treatment device 10 (reverse osmosis membrane treatment process).

[0051] <Silica-based scale inhibitor> The silica-based scale inhibitor according to this embodiment is a silica-based scale inhibitor for inhibiting the formation of silica-based scale in an aqueous system, which contains a copolymer including bisphenol S monomer units represented by the above chemical formula (1) and phenolsulfonic acid monomer units represented by the above chemical formula (2).

[0052] The copolymer is as explained above in the <Water Treatment Method>.

[0053] The silica-based scale inhibitor according to this embodiment may further contain, in addition to the polymer, other "scale inhibitors", "bactericides", "anticorrosives", and the like.

[0054] The silica-based scale inhibitor according to the present embodiment can suppress the formation of silica-based scale in an aqueous system, such as a water treatment system including a reverse osmosis membrane treatment process (reverse osmosis membrane treatment device), by being present in the aqueous system. In particular, even if the aqueous system contains a high concentration of silica, the silica-based scale inhibitor can suppress the formation of silica-based scale in the aqueous system under neutral conditions.

[0055] The water system to be treated is not particularly limited, but examples include cooling water systems and water systems such as membrane separation devices that use separation membranes such as reverse osmosis membranes (RO membranes), nanofiltration membranes (NF membranes), ultrafiltration membranes (UF membranes), and microfiltration membranes (MF membranes), and scale generation can be suppressed on the heat transfer surfaces of heat exchangers and the surfaces of separation membranes in membrane separation devices.

[0056] The pH of the aqueous system during treatment is not particularly limited, but is preferably 6.5 or higher, and more preferably 6.6 or higher. The upper limit of the pH is preferably 11 or lower from the viewpoint of membrane deterioration, and is more preferably less than 9 because silica solubility may increase at pH 9 or higher. If the pH of the aqueous system during treatment is less than 6.5, the permeation flux (flux) of the reverse osmosis membrane may decrease. Note that the pH of the aqueous system during treatment is the pH after the scale inhibitor has been added.

[0057] The silica concentration in the aqueous system is not particularly limited, but is preferably 150 mg / L or higher, and more preferably 200 mg / L or higher. The upper limit of the silica concentration in the aqueous system is, for example, 400 mg / L, and preferably 300 mg / L. That is, the silica concentration in the aqueous system is particularly effective in the range of 200 to 300 mg / L. Note that the silica concentration here refers to the concentration of ionic silica. If the silica concentration in the aqueous system is less than 150 mg / L, the scale inhibitor will have advantages, but existing chemicals can still inhibit scale. If the silica concentration exceeds 400 mg / L, the scale inhibitor will have advantages, but long-term effects may not be observed.

[0058] The concentration of the silica-based scale inhibitor added to the aqueous system is not particularly limited, but is preferably in the range of 1 to 1000 mg / L in terms of solids concentration, more preferably 1 to 100 mg / L, and even more preferably 5 to 25 mg / L. If the concentration of the silica-based scale inhibitor added to the aqueous system is less than 1 mg / L in terms of solids concentration, a sufficient scale inhibition effect may not be obtained, and if it exceeds 1000 mg / L, the amount of agent added becomes enormous, which is economically problematic and not practical. [Example]

[0059] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0060] Example 1 In the flow chart shown in Figure 2, the reverse osmosis membrane treatment was carried out using a polymer of the above chemical formula (3) with a weight average molecular weight of 15,200 (m:n = 1:9 to 9:1) as a silica-based scale inhibitor for the water to be treated. The reverse osmosis membrane treatment was carried out under the following test conditions, and the flux retention was measured. The flux retention is the ratio of the initial flux (permeation flow rate m 3 / d / (membrane area m 2 The values ​​shown are the percentages when the value (× transmembrane pressure difference MPa) × temperature correction) is taken as 100%. To stabilize the test system, the water flow time was set to 0 hours one hour after the start of water flow. Figure 3 shows the flux retention (%) versus water flow time (hours).

[0061] <Comparative Example 1> Reverse osmosis membrane treatment was carried out in the same manner as in Example 1, except that a conventional terpolymer of acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and substituted acrylamide (weight average molecular weight: 5000) was used as the silica-based scale inhibitor instead of the polymer of chemical formula (3), and flux retention was measured. Figure 3 shows the flux retention (%) versus water flow time (hr).

[0062] (Test conditions) Test water (untreated water): Pure water SiO2: 400mg / L Ca, Mg: 300 mg CaCO3 / L Al: 0.25 mg / L HCO3 - : 150mg CaCO3 / L ·Water temperature: 25℃ pH: 7.5 Reverse osmosis membrane: Nitto Denko ES20 flat membrane Silica scale inhibitor concentration: 25mg / L as solid

[0063] From FIG. 3, it can be seen that the polymer of the above chemical formula (3) in Example 1 has a higher flux retention rate and a higher silica scale dispersibility.

[0064] [Consideration of pH conditions] In the flow shown in Figure 2, to evaluate the change in behavior due to pH, the pH of the water to be treated was gradually lowered from pH 7, and reverse osmosis membrane treatment was carried out under the following test conditions, and the flux retention rate was confirmed. t=t / Flux t=0 The behavior of (-) is shown in Figure 4.

[0065] (Test conditions) Test water: Sagamihara well water sterilized with hypochlorous acid, sand filtered, and treated with activated carbon HCO3 - :8mgCaCO3 / L Nitrate ions: 10 mg / L Sulfate ions: 6 mg / L Chloride ions: 33 mg / L Na: 4 mg / L Ca: 13 mg / L Mg: 6 mg / L SiO2: 22 mg / L ·Water temperature: 25℃ pH: Starting at 7.0, it can go up to 6.9, 6.8, 6.7, 6.6, and 6.5 Silica scale inhibitor concentration: 30mg / L as product Reverse osmosis membrane: Nitto Denko ES20 element pH adjuster: Hydrochloric acid

[0066] 4, there is almost no decrease in the flux retention rate up to pH 6.6, but a slight decrease is confirmed at pH 6.5. Therefore, it can be seen that the preferable pH range is pH 6.6 or higher.

[0067] <Example 2> [Checking the appropriate concentration] The appropriate concentration of silica-based scale inhibitor for use in reverse osmosis membranes was evaluated. The flux retention (%) for each concentration after 40 hours of water flow is shown in Figure 5. The test conditions were the same as in Example 1.

[0068] Figure 5 shows that all samples have a higher flux retention rate than the blank. At additions of 5.25 to 15 mg / L, the performance is roughly equivalent, so it can be said that an addition of 5 mg / L will provide sufficient performance. Furthermore, because scale dispersibility is improved at additions of 20 mg / L or more, it can be said that additions of 20 mg / L or more are desirable when applying to water with a relatively high ionic load.

[0069] Example 3 [Formulation with 2-phosphonobutane-1,2,4-tricarboxylic acid] The polymer of formula (3) used in Example 1 was blended with 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) as a hardness scale dispersant, and the dispersibility of the hardness component (Ca dispersion rate (%)) was evaluated by a beaker test under the following test conditions. The results are shown in Figure 6.

[0070] (Test conditions) Test water: pure water Calcium: 400mg CaCO3 / L M-Alk: 400mg CaCO3 / L pH:8 Soaking time: 18 hours ·Water temperature: 70℃

[0071] Figure 6 shows that hardness dispersibility is low without PBTC. Silica-based scale inhibitors are usually blended with a polymer for hardness dispersion. When similar tests were conducted with PBTC added, high hardness dispersibility was obtained. Therefore, when used as a chemical for reverse osmosis membranes, it can be said that scaling can be further suppressed by blending a hardness scale dispersant such as PBTC.

[0072] As described above, by using the silica-based scale inhibitor of the example, it was possible to suppress the formation of silica-based scale on the reverse osmosis membrane under neutral conditions, even when the silica content of the water to be treated was high. [Explanation of symbols]

[0073] 1 water treatment device, 10 reverse osmosis membrane treatment device, 12 treated water piping, 14 permeate water piping, 16 concentrated water piping, 18 scale inhibitor addition piping.

Claims

1. an adding step of adding a scale inhibitor to the water to be treated containing silica; a reverse osmosis membrane treatment step in which the water to be treated to which the scale inhibitor has been added is passed through a reverse osmosis membrane to separate it into permeate and concentrated water; Including, The scale inhibitor contains a copolymer including a bisphenol S monomer unit represented by the following chemical formula (1) and a phenolsulfonic acid monomer unit represented by the following chemical formula (2), A water treatment method characterized in that the pH of the water to be treated to which the scale inhibitor has been added is 6.6 or more and 11 or less. 【Chemical 1】 （１） 【Chemistry 2】 （２）

2. The water treatment method according to claim 1, A water treatment method characterized in that the silica concentration of the concentrated water is 150 mg / L or more.

3. The water treatment method according to claim 1 or 2, The water treatment method is characterized in that the molecular weight of the copolymer is in the range of 1,000 to 100,000 in terms of weight average molecular weight.

4. The water treatment method according to any one of claims 1 to 3, A water treatment method characterized in that the concentration of the scale inhibitor added to the water to be treated is 5 mg / L or more in terms of solids concentration.

5. A silica-based scale inhibitor containing a copolymer including a bisphenol S monomer unit represented by the following chemical formula (1) and a phenolsulfonic acid monomer unit represented by the following chemical formula (2), A silica-based scale inhibitor characterized in that the pH of the water to be treated to which the scale inhibitor has been added is 6.6 or more and 11 or less. 【Chemistry 3】 （１） 【Chemistry 4】 （２）

6. The silica-based scale inhibitor according to claim 5, A silica-based scale inhibitor characterized in that the molecular weight of the copolymer is in the range of 1,000 to 100,000 in terms of weight average molecular weight.

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