Water treatment method and water treatment device

Abstract

In reverse osmosis membrane treatment of water to be treated that contains calcium ions and at least one of fluoride ions and sulfate ions, the decrease in water recovery rate is suppressed and scale formation is suppressed without using a scale dispersant. [Solution] This method includes a reverse osmosis membrane treatment step in which water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, is treated using a reverse osmosis membrane, wherein the calcium ion concentration in the concentrated water is 30 mg / L or less, or the fluoride ion concentration is 30 mg / L or less and the sulfate ion concentration is 100 mg / L or less, the ionic product of the calcium ions and fluoride ions in the concentrated water is equal to or greater than the solubility product of calcium fluoride, or the ionic product of the calcium ions and sulfate ions is equal to or greater than the solubility product of calcium sulfate, and the shear flux on the primary side of the reverse osmosis membrane is adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less.

Description

[Technical Field]

[0001] The present invention relates to a water treatment method and a water treatment device using a reverse osmosis membrane. [Background technology]

[0002] In recent years, reverse osmosis membranes have been increasingly used in water treatment processes such as pure water production and water recovery. Scaling, which occurs when solutes in the water being treated become concentrated during reverse osmosis membrane treatment and exceed the solubility of the solute, precipitates as scale, and clogs the membrane surface, is one of the most common problems encountered in water treatment using reverse osmosis membranes. Typical scale components include calcium, aluminum, and silica.

[0003] Among these, calcium is one of the most common scale components, and typical calcium scales include calcium carbonate, calcium sulfate, calcium phosphate, and calcium fluoride.

[0004] As a countermeasure against scaling, the recovery rate (concentration ratio) of the reverse osmosis membrane treatment equipment is determined so that the concentration of scale components in the concentrated water from the reverse osmosis membrane treatment does not exceed the solubility of each solute, as a rule. Alternatively, a scale dispersant that can suppress the deposition of scale is added.

[0005] Due to growing environmental awareness and water shortages, there is a demand for improved water recovery rates in reverse osmosis membrane treatment. There is also a trend to reduce the use of scale dispersants in order to reduce running costs and eliminate chemicals.

[0006] For example, Patent Document 1 discloses a water treatment method in which water to be treated containing fluoride ions and calcium ions is treated using a reverse osmosis membrane, in which the ionic product of fluoride ions and calcium ions in concentrated water is 1.6 × 10 -8 mol 3 / L 3The method is described as follows: the calcium ion concentration in the concentrated water is 1.0 mg / L or less, the fluoride ion concentration in the concentrated water is 1500 mg / L or more, and the ratio of the flow rate of the concentrated water to the flow rate of the permeated water is permeated water flow rate:concentrated water flow rate=1:4 or more.

[0007] However, in the method of Patent Document 1, the upper limit of the calcium ion concentration in the concentrated water is low, which is insufficient as a concentration range for inhibiting scale formation. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 7449107 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 and a water treatment device that can suppress a decrease in water recovery rate and suppress scale formation without using a scale dispersant in reverse osmosis membrane treatment of water to be treated that contains calcium ions and at least one of fluoride ions and sulfate ions. [Means for solving the problem]

[0010] The present invention provides a water treatment method comprising a reverse osmosis membrane treatment step of treating water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, with a reverse osmosis membrane to obtain a concentrate and a permeate, wherein the concentrate has a calcium ion concentration of 30 mg / L or less, or the concentrate has a fluoride ion concentration of 30 mg / L or less and a sulfate ion concentration of 100 mg / L or less, the calcium ion concentrations and the fluoride ion concentrations in the concentrate are equal to or greater than the solubility product of calcium fluoride, or the calcium ion concentrations and the sulfate ion concentrations in the concentrate are equal to or greater than the solubility product of calcium sulfate, and the shear flux on the primary side of the reverse osmosis membrane is adjusted to a range of 0.04 m / sec to 0.30 m / sec.

[0011] In the water treatment method, it is preferable that at least a portion of the concentrated water is circulated to the primary side of the reverse osmosis membrane.

[0012] The present invention provides a water treatment device comprising a reverse osmosis membrane treatment means for treating water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, using a reverse osmosis membrane to obtain a concentrate and a permeate, wherein the concentrate has a calcium ion concentration of 30 mg / L or less, or the concentrate has a fluoride ion concentration of 30 mg / L or less and a sulfate ion concentration of 100 mg / L or less, the ionic product of calcium ions and fluoride ions in the concentrate is equal to or greater than the solubility product of calcium fluoride, or the ionic product of calcium ions and sulfate ions in the concentrate is equal to or greater than the solubility product of calcium sulfate, and the water treatment device further comprises an adjustment means for adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less.

[0013] The water treatment device preferably further comprises a circulation unit that circulates at least a portion of the concentrated water to the primary side of the reverse osmosis membrane. [Effects of the Invention]

[0014] The present invention provides a water treatment method and a water treatment device that can suppress a decrease in water recovery rate and suppress scale formation without using a scale dispersant in reverse osmosis membrane treatment of water to be treated that contains calcium ions and at least one of fluoride ions and sulfate ions. [Brief explanation of the drawings]

[0015] [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. 10 is a schematic configuration diagram illustrating another example of a water treatment device according to an embodiment of the present invention. [Figure 3] 1 is a schematic diagram illustrating an example of a reverse osmosis membrane element used in a water treatment device according to an embodiment of the present invention. [Figure 4] 1 is a graph showing the flux retention (%) of the membrane versus the operation time (hr) in Example 1 and Comparative Example 1. [Figure 5] 1 is a graph showing the flux retention (%) of the membrane versus the operation time (hr) in Example 2 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0016] 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.

[0017] An example of a water treatment apparatus according to an embodiment of the present invention is outlined in FIG. 1, and its configuration will be described.

[0018] The water treatment device 1 includes a reverse osmosis membrane treatment device 12 as a reverse osmosis membrane treatment means for treating water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, with a reverse osmosis membrane to obtain concentrated water and permeate. The water treatment device 1 may also include a water tank 10 to store the water to be treated.

[0019] In the water treatment device 1 of FIG. 1 , a water-to-be-treated pipe 24 is connected to the water-to-be-treated inlet of the water tank 10. The water-to-be-treated outlet of the water tank 10 and the water-to-be-treated inlet of the reverse osmosis membrane treatment device 12 are connected by a water-to-be-treated supply pipe 26 via a pump 14. A permeate pipe 28 is connected to the permeate outlet of the reverse osmosis membrane treatment device 12, and a concentrated water pipe 30 is connected to the concentrated water outlet via valves 18 and 20. A circulation pipe 32 branching off from the concentrated water pipe 30 between the valves 18 and 20 may be connected to the concentrated circulating water inlet of the water tank 10 via a valve 22. A pH adjuster addition pipe 34 may be connected upstream of the pump 14 to the water-to-be-treated supply pipe 26 as a pH adjustment means for adjusting the pH of the concentrated water. A pH measuring device 16 may be installed in the concentrated water pipe 30 as a pH measuring means for measuring the pH of the concentrated water.

[0020] In the water treatment device 1 of Figure 1, the water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, is stored in the water to be treated tank 10 as needed through the water to be treated piping 24, and is sent by the pump 14 through the water to be treated supply piping 26 to the reverse osmosis membrane treatment device 12.

[0021] In the reverse osmosis membrane treatment device 12, the water to be treated is treated with a reverse osmosis membrane to obtain permeated water and concentrated water (reverse osmosis membrane treatment step). The permeated water is discharged through the permeated water pipe 28, and the concentrated water is discharged as concentrated blown water through the concentrated water pipe 30 when the valves 18 and 20 are open. A pH adjuster may be added to the water to be treated in the water to be treated supply pipe 26 to adjust the pH of the concentrated water. The pH adjuster may be added to the water to be treated in the water to be treated tank 10. The pH of the concentrated water may be measured in the concentrated water pipe 30.

[0022] In the water treatment method and water treatment device of this embodiment, the shear flux on the primary side of the reverse osmosis membrane is adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less (adjustment step) under the conditions that the calcium ion concentration in the concentrated water is 30 mg / L or less, or the fluoride ion concentration in the concentrated water is 30 mg / L or less and the sulfate ion concentration is 100 mg / L or less, and the ionic product of calcium ions and fluoride ions in the concentrated water is equal to or greater than the solubility product of calcium fluoride, or the ionic product of calcium ions and sulfate ions in the concentrated water is equal to or greater than the solubility product of calcium sulfate.

[0023] The inventors' studies have shown that even under conditions where calcium scale is supersaturated, i.e., when the calcium ion concentration in the concentrate is 30 mg / L or less, or when the fluoride ion concentration in the concentrate is 30 mg / L or less and the sulfate ion concentration in the concentrate is 100 mg / L or less, and the ionic product of calcium ions and fluoride ions in the concentrate is equal to or greater than the solubility product of calcium fluoride, or when the ionic product of calcium ions and sulfate ions in the concentrate is equal to or greater than the solubility product of calcium sulfate, a decrease in water recovery rate can be suppressed and scale formation can be suppressed without the use of a scale dispersant by adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec to 0.30 m / sec. This is thought to be because, by adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec to 0.30 m / sec, calcium scale formed on the primary side of the reverse osmosis membrane is flushed out of the system by the shear flux, suppressing scale formation and membrane fouling.

[0024] Flushing, often using clear feed water or permeate, is used to dissolve and disperse contaminants on the surface of a reverse osmosis membrane, but this results in a decrease in water recovery rate. The water treatment method and water treatment device according to this embodiment use concentrated water from the reverse osmosis membrane, thereby preventing a decrease in water recovery rate. Furthermore, the inventors' studies have shown that this method is largely ineffective in preventing scale formation for scale components other than calcium, such as silica, aluminum, and iron. This is thought to be due to differences in the adsorption strength of scale components to the reverse osmosis membrane.

[0025] Here, the solubility product of calcium fluoride is [Ca 2+ ][F - ] 2 =3.9×10 -11 and the solubility product of calcium sulfate is [Ca 2+ ][SO4 2- ]=4.5×10 -9 is.

[0026] Shear flux is the flow rate on the primary side of a reverse osmosis membrane (= concentrated water volume) divided by the cross-sectional area of ​​the reverse osmosis membrane element. Figure 3 shows a schematic diagram of an example of a reverse osmosis membrane element. The cross-sectional area of ​​the reverse osmosis membrane element used to calculate shear flux does not include the thickness of the reverse osmosis membrane leaf that makes up the reverse osmosis membrane element.

[0027] For example, for the amount of concentrated water per 8-inch reverse osmosis membrane vessel, a shear flux of 0.04 m / sec is approximately 2.1 to 2.3 m 3 / h, and a shear flux of 0.30 m / sec is approximately 15.7 to 17.2 m 3 / h. For example, for the amount of concentrated water per 4-inch reverse osmosis membrane vessel, a shear flux of 0.04 m / sec is approximately 0.7 m 3 / h, and a shear flux of 0.30 m / sec is approximately 3.6 m 3 / h.

[0028] The shear flux on the primary side of the reverse osmosis membrane can be adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less by, for example, -Adjust the valve opening of the concentrated water piping. A predetermined amount of concentrated water is circulated to the treated water tank or the treated water supply pipe. -Adjust the pump frequency for the reverse osmosis water supply, i.e., adjust the water supply pressure to the reverse osmosis membrane treatment device. Use a reverse osmosis membrane element with a thin water supply spacer (raw water spacer in Figure 3). The water supply spacer serves to ensure the flow path of the water to be treated. Examples include:

[0029] 1, for example, valves 18 and 20 of concentrated water pipe 30 are opened, and the apertures of valves 18 and 20 are adjusted to adjust the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less. By adjusting the apertures of valves 18 and 20, the flow rate on the primary side of the reverse osmosis membrane (=amount of concentrated water) can be adjusted.

[0030] 1 may include, for example, a circulation pipe 32 as a circulation means for circulating at least a portion of the concentrated water to the primary side of the reverse osmosis membrane. For example, valve 18 of concentrated water pipe 30 is opened, valve 20 is closed, and valve 22 of circulation pipe 32 is opened. The apertures of valves 18 and 22 are adjusted to circulate a predetermined amount of concentrated water to the primary side of the reverse osmosis membrane (circulation step), thereby adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less. The flow rate on the primary side of the reverse osmosis membrane (=the amount of concentrated water) can be adjusted by adjusting the apertures of valves 18, 20, and 22.

[0031] In the water treatment device 1 of Figure 1, for example, the shear flux on the primary side of the reverse osmosis membrane can be adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less by adjusting the frequency of the pump 14 for supplying water to the reverse osmosis membrane, i.e., by adjusting the water supply pressure to the reverse osmosis membrane treatment device 12. By manipulating the output of an inverter installed in the pump 14, the water supply pressure to the reverse osmosis membrane treatment device 12 can be adjusted, and the flow rate on the primary side of the reverse osmosis membrane (= amount of concentrated water) can be adjusted.

[0032] For example, in the reverse osmosis membrane element of Figure 3, by using a thin water supply spacer (raw water spacer in Figure 3) (for example, a thickness of 0.70 to 0.75 mm), the shear flux on the primary side of the reverse osmosis membrane can be adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less.

[0033] 2, the water treatment device 3 may include a control device 36 as an adjusting means for adjusting the shear flux on the primary side of the reverse osmosis membrane within the above range. The control device 36 and the pump 14 may be connected to each other so as to be able to communicate with each other via a wired or wireless electrical connection. Alternatively, the control device 36 and the valves 18, 20, and 22 may each be connected to each other so as to be able to communicate with each other via a wired or wireless electrical connection.

[0034] In the water treatment device 3 of Figure 2, the control device 36, for example, opens the valves 18 and 20 of the concentrated water pipe 30 and adjusts the opening of the valves 18 and 20 to adjust the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less.

[0035] In the water treatment device 3 of Figure 2, the control device 36, for example, opens valve 18 of the concentrated water pipe 30, closes valve 20, opens valve 22 of the circulation pipe 32, and adjusts the opening of valves 18 and 22 to circulate a predetermined amount of concentrated water to the primary side of the reverse osmosis membrane (circulation process), thereby adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less.

[0036] In the water treatment device 3 of FIG. 2, the control device 36 may adjust the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less, for example, by adjusting the frequency of the pump 14 for supplying water to the reverse osmosis membrane, i.e., by adjusting the water supply pressure to the reverse osmosis membrane treatment device 12.

[0037] In this way, at least one of the control device 36, valve 18, valve 20, valve 22, pump 14, etc. functions as an adjusting means for adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less.

[0038] In the water treatment method and water treatment device according to this embodiment, the shear flux on the primary side of the reverse osmosis membrane is adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less. If the shear flux on the primary side of the reverse osmosis membrane is less than 0.04 m / sec, it becomes difficult for the shear flux to sufficiently flush out the calcium scale formed on the primary side of the reverse osmosis membrane. If the shear flux on the primary side of the reverse osmosis membrane is greater than 0.30 m / sec, the pressure loss during water flow increases, which may lead to damage to the element. The shear flux on the primary side of the reverse osmosis membrane is preferably adjusted to a range of 0.04 m / sec or more and 0.30 m / sec or less, and more preferably adjusted to a range of 0.04 m / sec or more and 0.10 m / sec or less.

[0039] The water treatment method and water treatment device according to this embodiment can achieve a flux retention rate (see Examples) of 90% or more, preferably 95% or more, after 500 hours of operation.

[0040] The calcium ion concentration in the concentrated water is 30 mg / L or less, and preferably 25 mg / L or less. If the calcium ion concentration in the concentrated water exceeds 30 mg / L, the amount of scale deposition increases, making stable operation difficult.

[0041] It is preferable that the fluoride ion concentration in the concentrated water is 30 mg / L or less and the sulfate ion concentration is 100 mg / L or less, and that the fluoride ion concentration is 15 mg / L or less and the sulfate ion concentration is 72 mg / L or less. If the fluoride ion concentration in the concentrated water exceeds 30 mg / L or the sulfate ion concentration exceeds 100 mg / L, the amount of scale deposition increases, making stable operation difficult.

[0042] The ionic product of calcium ions and fluoride ions in the concentrated water is the solubility product of calcium fluoride [Ca 2+ ][F - ] 2 =3.9×10 -11 or more, or the ionic product of calcium ions and sulfate ions in the concentrated water is greater than the solubility product of calcium sulfate [Ca 2+ ][SO4 2- ]=4.5×10 -9 That's all. The water treatment method and water treatment device according to this embodiment can operate stably even if these solubility products are equal to or greater than the above values. Even more stable operation is possible if these solubility products are equal to or greater than the above values ​​but are lower.

[0043] The water to be treated contains calcium ions and at least one of fluoride ions and sulfate ions, and examples thereof include wastewater discharged from semiconductor manufacturing processes, steel mill wastewater, and heat exchanger cleaning wastewater. The calcium ion concentration in the water to be treated is, for example, in the range of 0.1 to 500 mg / L, the fluoride ion concentration is, for example, in the range of 1.0 to 600 mg / L, and the sulfate ion concentration is, for example, in the range of 0.1 to 800 mg / L. The water to be treated may contain other ions other than calcium ions, fluoride ions, and sulfate ions, such as magnesium ions and ammonium ions. The concentrations of other ions in the water to be treated are, for example, in the range of 0.1 to 200 mg / L.

[0044] There are no particular limitations on the pH of the water to be treated.

[0045] The pH of the concentrate is, for example, in the range of 4.0 to 9.0, and preferably in the range of 5.5 to 8.0. If the pH of the concentrate is less than 4.0, the ion rejection rate of the reverse osmosis membrane may be significantly reduced, and if it exceeds 9.0, calcium carbonate may precipitate and clog the reverse osmosis membrane.

[0046] To adjust the pH of the concentrated water, a pH adjuster such as an acid, for example, hydrochloric acid, sulfuric acid, or nitric acid, or an alkali, for example, an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide, may be used.

[0047] The reverse osmosis membranes included in the reverse osmosis membrane treatment device 12 include neutrally charged membranes, anion-charged membranes, and cation-charged membranes. A neutrally charged membrane refers to a membrane with a zeta potential of −15 to 5 mV at pH 7.0, as determined by the zeta potential measurement method described in the Examples below. A cation-charged membrane refers to a membrane with a zeta potential of more than 5 mV at pH 7.0. An anion-charged membrane refers to a membrane with a zeta potential of less than −15 mV at pH 7.0.

[0048] Examples of commercially available neutrally charged membranes include OFR-625 (manufactured by Organo Corporation), BW30XFR (manufactured by The Dow Chemical Company), LFC3 (manufactured by Nitto Denko Corporation), and TML20 (manufactured by Toray Industries, Inc.).

[0049] An example of a commercially available cation-charged membrane is ES10C (manufactured by Nitto Denko Corporation).

[0050] Examples of commercially available anion-charged membranes include ES15, ES20, and CPA3 (all manufactured by Nitto Denko Corporation), and RE-8040BLN (manufactured by Woongjin Co., Ltd.).

[0051] The filtration method used in the reverse osmosis membrane treatment device 12 is a cross-flow method in which the water to be treated flows in a direction approximately parallel to the surface of the reverse osmosis membrane.

[0052] A programmable logic controller (PLC), a personal computer (PC), etc. can be used as the control device 36. The control device 36 is composed of a microcomputer and electronic circuits, which are made up of, for example, a calculation means such as a CPU that calculates a program, and storage means such as a ROM and RAM that store the program and calculation results, and has the function of controlling the flow rate of the pump 14, the opening and closing degrees of the valves 18, 20, 22, etc.

[0053] In the water treatment method and water treatment device according to this embodiment, a scale dispersant may not be used, but may be used to further suppress scale formation. The scale dispersant may be added to the water to be treated, for example, in the water to be treated tank 10 or the water to be treated supply pipe 26.

[0054] The scale dispersant is not limited to any particular substance as long as it is capable of dispersing scaling substances such as calcium fluoride and calcium sulfate. Examples of the scale dispersant include phosphonic acid compounds such as phosphonic acids and their salts, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, and nitrilotrimethylphosphonic acid; phosphoric acid compounds such as orthophosphates and polymerized phosphates; maleic acid compounds such as polymaleic acid and maleic acid copolymers; and acrylic acid polymers. Examples of acrylic acid-based polymers include copolymers such as poly(meth)acrylic acid, maleic acid / (meth)acrylic acid, (meth)acrylic acid / sulfonic acid, and (meth)acrylic acid / nonionic group-containing monomers, as well as terpolymers of (meth)acrylic acid / sulfonic acid / nonionic group-containing monomers, (meth)acrylic acid / acrylamide-alkylsulfonic acid / substituted (meth)acrylamides, and (meth)acrylic acid / acrylamide-arylsulfonic acid / substituted (meth)acrylamides. Examples of (meth)acrylic acids constituting copolymers and terpolymers include methacrylic acid, acrylic acid, and (meth)acrylate salts such as their sodium salts. Examples of acrylamide-alkylsulfonic acids constituting terpolymers include 2-acrylamido-2-methylpropanesulfonic acid and its salts. Examples of substituted (meth)acrylamides constituting terpolymers include t-butylacrylamide, t-octylacrylamide, and dimethylacrylamide.

[0055] The weight average molecular weight of the scale dispersant is not particularly limited, but is preferably in the range of 500 to 100,000, and more preferably in the range of 1,000 to 50,000.

[0056] Among these, a copolymer of (meth)acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid is preferred because it has excellent dispersibility of calcium fluoride.

[0057] Commercially available scale dispersants 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, and the "Kuriverter®" series manufactured by Kurita Water Industries Ltd.

[0058] In the water treatment method and water treatment device according to this embodiment, in addition to the scale dispersant and pH adjuster, other agents such as a slime control agent and an anticorrosive agent such as a heavy metal, phosphate, or azole compound may be used. [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, Comparative Example 1> Calcium chloride and sodium fluoride were dissolved in pure water so that the ion concentrations in the concentrated water were as shown in Table 1, to prepare simulated wastewater containing calcium ions and fluoride ions, and a reverse osmosis membrane water flow test was conducted under the following conditions. A portion of the concentrated water was circulated to adjust the shear flux.

[0061] [Test conditions] Reverse osmosis membrane element: LFC3-4040 (Nitto Denko Corporation, fouling-resistant, neutrally charged membrane, zeta potential -1.3 mV) ·Concentrated water pH: 7.0 ·Permeated water amount: 140L / h ·Concentrated water volume: 720L / h Concentrated blow-off water volume: 40~720L / h ·Concentrated circulating water amount: 0~680L / h Shear flux: 0.04~0.10m / sec

[0062] [Measurement of zeta potential of reverse osmosis membrane] The zeta potential of the reverse osmosis membrane was determined using the ELSZ series zeta potential and particle size measurement system manufactured by Otsuka Electronics Co., Ltd. The zeta potential of the reverse osmosis membrane was calculated from the measured electroosmosis plot using the Mori-Okamoto equation and the Smoluchowski equation below.

[0063] (Mori-Okamoto formula) U obs (z)=AU0(z / b) 2 +ΔU0(z / b)+(1-A)U0+U p where: z: distance from the cell center position U obs (z): Apparent mobility at z position in the cell A:1 / [(2 / 3)-(0.420166 / K)] K=a / b: 2a and 2b are the horizontal and vertical lengths of the cell cross section, a>b U p : real mobility of particles U0: Average mobility on the top and bottom surfaces of the cell ΔU0: Difference in mobility between the top and bottom surfaces of the cell (Smoluchowski's formula) ζ=4πηU / ε where: U: Electrical mobility ε: Dielectric constant of the solvent η: viscosity of the solvent

[0064] The measurement solution was a 10 mM NaCl aqueous solution (pH approximately 5.4). Two pairs of this solution and sample were prepared for each sample, one with an acidic pH (pH 2, 3, 4, 5, 6, 7) and the other with an alkaline pH (pH 8, 9), and the zeta potential was measured at each pH. The physical properties of the solvent were those of pure water at 25°C (refractive index: 1.3328, viscosity: 0.8878, dielectric constant: 78.3).

[0065] The occurrence of blockage of the reverse osmosis membrane after 500 hours of operation is shown in Table 1. Fig. 4 shows the membrane flux retention (%) versus operation time (hr) in Example 1 and Comparative Example 1. The water recovery rate in Example 1 was 16%, and the water recovery rate in Comparative Example 1 was 16%.

[0066] In each test, if the flux retention rate after 500 hours of operation was 90% or higher, it was judged that stable operation was possible. The flux value was calculated by multiplying the permeate flow rate per membrane area and effective operating pressure by a temperature correction coefficient to obtain a corrected flux (Equation 1 below). The flux retention rate is the percentage of the corrected flux when the corrected flux at the start of operation is set to 100%.

[0067] Correction Flux(m / d / MPa at 25℃)=Permeate flow rate(m 3 / d) / membrane area(m 2 ) / (operating pressure (MPa) - osmotic pressure (MPa)) x temperature correction coefficient (Equation 1)

[0068] [Table 1]

[0069] <Example 2, Comparative Example 2> Calcium chloride and sodium sulfate were dissolved in pure water so that the ion concentrations in the concentrated water were as shown in Table 2, to prepare simulated wastewater containing calcium ions and sulfate ions, and a reverse osmosis membrane water flow test was conducted under the same conditions as in Example 1. A portion of the concentrated water was circulated to adjust the shear flux.

[0070] Table 2 shows whether or not the reverse osmosis membrane was clogged after 500 hours of operation. Fig. 5 shows the membrane flux retention (%) versus operation time (hr) in Example 2 and Comparative Example 2. The water recovery rate in Example 2 was 16%, and the water recovery rate in Comparative Example 2 was 16%.

[0071] [Table 2]

[0072] In Examples 1 and 2, the occurrence of blockage of the reverse osmosis membrane was suppressed even after 500 hours of operation, and stable operation was possible.

[0073] Thus, in the examples, in the reverse osmosis membrane treatment of water to be treated containing calcium ions and at least one of fluoride ions and sulfate ions, it was possible to suppress a decrease in water recovery rate and to suppress scale formation without using a scale dispersant. [Explanation of symbols]

[0074] 1, 3 water treatment device, 10 treated water tank, 12 reverse osmosis membrane treatment device, 14 pump, 16 pH measuring device, 18, 20, 22 valves, 24 treated water piping, 26 treated water supply piping, 28 permeate piping, 30 concentrated water piping, 32 circulation piping, 34 pH adjuster addition piping, 36 control device.

Claims

1. a reverse osmosis membrane treatment step of treating water containing calcium ions and at least one of fluoride ions and sulfate ions with a reverse osmosis membrane to obtain concentrated water and permeated water; The calcium ion concentration in the concentrated water is 30 mg / L or less, or the fluoride ion concentration in the concentrated water is 30 mg / L or less and the sulfate ion concentration is 100 mg / L or less, the ionic product of calcium ions and fluoride ions in the concentrated water is equal to or greater than the solubility product of calcium fluoride, or the ionic product of calcium ions and sulfate ions in the concentrated water is equal to or greater than the solubility product of calcium sulfate; A water treatment method characterized by adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec or more and 0.30 m / sec or less.

2. The water treatment method according to claim 1, A water treatment method comprising circulating at least a portion of the concentrated water to the primary side of the reverse osmosis membrane.

3. a reverse osmosis membrane treatment means for treating water to be treated, which contains calcium ions and at least one of fluoride ions and sulfate ions, with a reverse osmosis membrane to obtain concentrated water and permeated water; The calcium ion concentration in the concentrated water is 30 mg / L or less, or the fluoride ion concentration in the concentrated water is 30 mg / L or less and the sulfate ion concentration is 100 mg / L or less, the ionic product of calcium ions and fluoride ions in the concentrated water is equal to or greater than the solubility product of calcium fluoride, or the ionic product of calcium ions and sulfate ions in the concentrated water is equal to or greater than the solubility product of calcium sulfate; The water treatment device further comprises an adjusting means for adjusting the shear flux on the primary side of the reverse osmosis membrane to a range of 0.04 m / sec to 0.30 m / sec.

4. The water treatment device according to claim 3, The water treatment device further comprises a circulation means for circulating at least a portion of the concentrated water to the primary side of the reverse osmosis membrane.

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