WO2025013363A1 - Water treatment apparatus and water treatment method - Google Patents

Abstract

Provided are a water treatment apparatus and a water treatment method, with which it is possible to suppress the progress of clogging of a reverse osmosis membrane caused by scales in a reverse osmosis membrane treatment. A water treatment apparatus (1) comprises: a reverse osmosis membrane treatment device (12) for passing water to be treated through a reverse osmosis membrane so as to obtain permeated water and concentrated water, the water to be treated containing at least one of calcium and aluminum; and a scale inhibitor addition pipe (22) for adding a scale inhibitor to the water to be treated. The reverse osmosis membrane is a neutrally charged membrane.

Description

Water treatment device and water treatment method

The present invention relates to a water treatment device and a water treatment method that use a reverse osmosis membrane.

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 are concentrated by the reverse osmosis membrane process and exceed the solubility of the solute, precipitates as scale and clogs the membrane surface of the reverse osmosis membrane, is one of the most common problems with reverse osmosis membranes. Typical scale components include calcium, aluminum, and silica.

As a countermeasure against scaling, the recovery rate 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, and the Langelier index (LSI) is less than 0. Here, LSI (Langelier Saturation Index) is known as an index of the risk of scale including hardness components, and is calculated from the pH, calcium ion concentration, total alkalinity, and soluble substance concentration of the water. The larger this value is, the more likely it is that calcium carbonate will precipitate, and the more likely it is that other scales with similar properties, centered around calcium carbonate, will precipitate or co-precipitate. LSI can be calculated using formula (1) described below.

Generally, the higher the concentration of scale components, the higher the LSI value, so it is recommended to keep LSI < 0. Furthermore, as another measure against scaling, it is known to add a scale inhibitor to the feed water for reverse osmosis membrane treatment. For example, in the examples of Patent Document 1, it is described that an agent (scale inhibitor) that disperses scale is added to the feed water for reverse osmosis membrane treatment that uses an anion-charged membrane (ultra-low pressure RO membrane "ES20" manufactured by Nitto Denko).

However, lowering the recovery rate by setting LSI<0 leads to an increase in the number of reverse osmosis membranes and an increase in the size of the system, which increases initial and running costs. Furthermore, in recent years, the concentration of scale components in the water being treated by reverse osmosis membranes has been increasing due to localized water shortages and the spread of water recycling, making it difficult to suppress the progression of membrane blockage caused by scale by adding scale inhibitors alone. Therefore, there is a demand for effective means of suppressing scale, in addition to scale inhibitors.

Patent No. 6512322

The object of the present invention is to provide a water treatment device and a water treatment method that can suppress the progression of clogging of the reverse osmosis membrane caused by scale during reverse osmosis membrane treatment.

The present invention is a water treatment device that includes a reverse osmosis membrane treatment means for passing water to be treated that contains at least one of calcium and aluminum through a reverse osmosis membrane to obtain permeate water and concentrated water, and a scale inhibitor addition means for adding a scale inhibitor to the water to be treated, the reverse osmosis membrane being a neutrally charged membrane.

In the water treatment device, it is preferable that the water to be treated further contains silica.

In the water treatment device, the scale inhibitor preferably contains at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound.

In the water treatment device, it is preferable that the neutrally charged membrane is a neutrally charged membrane with a zeta potential on the membrane surface of -10 mV or more and +10 mV or less.

In the water treatment device, it is preferable that the Langelier index of the concentrated water exceeds 0.

In the water treatment device, it is preferable that the silica concentration in the concentrated water is 120 mg/L or more.

In the water treatment device, it is preferable that the aluminum concentration in the concentrated water is 0.25 mg/L or more.

In the water treatment device, it is preferable that the concentrated water has a Langelier index greater than 0, a silica concentration of 120 mg/L or more, and an aluminum concentration of 0.25 mg/L or more.

The present invention is a water treatment method that includes a reverse osmosis membrane treatment step in which water to be treated that contains at least one of calcium and aluminum is passed through a reverse osmosis membrane to obtain permeate water and concentrated water, and a scale inhibitor addition step in which a scale inhibitor is added to the water to be treated, the reverse osmosis membrane being a neutrally charged membrane.

In the water treatment method, it is preferable that the water to be treated further contains silica.

In the water treatment method, the scale inhibitor preferably contains at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound.

In the water treatment method, it is preferable that the neutrally charged membrane is a neutrally charged membrane whose membrane surface has a zeta potential of -10 mV or more and +10 mV or less.

In the water treatment method, it is preferable that the Langelier index of the concentrated water exceeds 0.

In the water treatment method, it is preferable that the silica concentration in the concentrated water is 120 mg/L or more.

In the water treatment method, it is preferable that the aluminum concentration in the concentrated water is 0.25 mg/L or more.

In the water treatment method, it is preferable that the concentrated water has a Langelier index greater than 0, a silica concentration of 120 mg/L or more, and an aluminum concentration of 0.25 mg/L or more.

The present invention provides a water treatment device and a water treatment method that can suppress the progression of blockage of the reverse osmosis membrane caused by scale during reverse osmosis membrane treatment.

1 is a schematic diagram illustrating an example of a water treatment device according to an embodiment of the present invention. 1 is a graph showing the change in flux retention (%) versus operation time (hr) in Example 1 and Comparative Example 1. 1 is a graph showing the change in flux retention (%) versus operation time (hr) in Example 2 and Comparative Example 2. 1 is a graph showing the change in flux retention (%) versus operation time (hr) in Example 3 and Comparative Example 3. 1 is a graph showing the change in flux retention (%) versus operation time (hr) in Example 4 and Comparative Example 4. 1 is a graph showing the change in flux retention (%) versus operation time (hr) in Example 5 and Comparative Example 5.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

The water treatment device according to an embodiment of the present invention is a water treatment device that includes a reverse osmosis membrane treatment means for passing water to be treated that contains at least one of calcium and aluminum through a reverse osmosis membrane to obtain permeate water and concentrated water, and a scale inhibitor addition means for adding a scale inhibitor to the water to be treated, and the reverse osmosis membrane is a neutrally charged membrane.

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

The water treatment device 1 includes a reverse osmosis membrane treatment device 12 as a reverse osmosis membrane treatment means for passing water to be treated, which contains at least one of calcium and aluminum, through a reverse osmosis membrane to obtain permeate water and concentrated water. The reverse osmosis membrane used in the reverse osmosis membrane treatment device 12 is a neutrally charged membrane. The water treatment device 1 may also include a water tank 10 for storing water to be treated, which contains at least one of calcium and aluminum.

In the water treatment device 1 of Figure 1, a treated water pipe 14 is connected to the treated water inlet of the treated water tank 10. The treated water outlet of the treated water tank 10 and the treated water inlet of the reverse osmosis membrane treatment device 12 are connected by a treated water pipe 16. A permeated water pipe 18 is connected to the permeated water outlet of the reverse osmosis membrane treatment device 12, and a concentrated water pipe 20 is connected to the concentrated water outlet. A scale inhibitor addition pipe 22 is connected to the chemical inlet of the treated water tank 10.

In the water treatment device 1 of FIG. 1, the water to be treated, which contains at least one of calcium and aluminum, is stored in the water to be treated tank 10 as necessary through the water to be treated piping 14. Here, in the water to be treated tank 10, a scale inhibitor is added to the water to be treated through the scale inhibitor addition piping 22 (scale inhibitor addition process). The water to be treated to which the scale inhibitor has been added is sent to the reverse osmosis membrane treatment device 12 through the water to be treated piping 16. The scale inhibitor may be added via a line in the water to be treated piping 14 or the water to be treated piping 16.

In the reverse osmosis membrane treatment device 12, reverse osmosis membrane treatment is performed in which the water to be treated is passed through a reverse osmosis membrane to obtain permeated water and concentrated water (reverse osmosis membrane treatment process). The permeated water is discharged through the permeated water pipe 18, and the concentrated water is discharged through the concentrated water pipe 20.

Until now, it has been believed that the deposition of scale during reverse osmosis membrane treatment is due to the solute concentration in the concentrated water, particularly near the surface of the reverse osmosis membrane, and that the type of reverse osmosis membrane has no effect. However, the present inventors have discovered that the type of reverse osmosis membrane affects the progression of scaling.

The inventors have found that when a neutrally charged membrane is used as the reverse osmosis membrane, clogging of the reverse osmosis membrane by scale is less likely to progress compared to when an anion-charged membrane is used. This is thought to be because cations such as calcium and aluminum, which are typical scale components, are attracted to the vicinity of the membrane by charge and concentrated to a high concentration in an anion-charged membrane, whereas this phenomenon is less likely to occur in a neutrally charged membrane. Although silica is anionic, it is known that precipitation is promoted by cations such as calcium and aluminum. Therefore, it is thought that using a neutrally charged membrane is also effective for the purpose of suppressing silica precipitation.

Neutral-charged membranes are mainly used in the wastewater treatment field. Normally, the surface of a reverse osmosis membrane is anion-charged, but by neutralizing the charge through surface treatment such as coating, it is possible to suppress membrane surface contamination, mainly caused by organic matter. On the other hand, due to the surface treatment, the amount of permeated water is lower than with an anion-charged membrane, and there is a disadvantage that the operating pressure of the pump and power consumption increase when trying to produce the same amount of water. Therefore, unless there is a special reason, it is common to use an anion-charged membrane. For example, in applications where pure water is produced from groundwater, anion-charged membranes are used together with scale inhibitors. For example, in applications where pure water is produced from groundwater and the concentration of scale components in the groundwater is high, using a neutral-charged membrane makes it less likely for the reverse osmosis membrane to become clogged, making it possible to reduce power consumption during long-term operation.

Reverse osmosis membranes include neutrally charged membranes, anionically charged membranes, and cationically charged membranes. In this specification, a membrane with a zeta potential at pH 7.0 determined by the zeta potential measurement method described in the Examples below in the range of -10 mV to 10 mV is defined as a neutrally charged membrane, a membrane with a zeta potential of more than 10 mV is defined as a cationically charged membrane, and a membrane with a zeta potential of less than -10 mV is defined as an anionically charged membrane. Organic substances that tend to contaminate reverse osmosis membranes are often anionic, and in order to suppress their adhesion, it is preferable that the zeta potential of a neutral membrane is in the range of -10 to 0 mV.

Commercially available neutrally charged membranes include, for example, BW30XFR (Dow Chemical Company), LFC3 (Nitto Denko Corporation), TML20 (Toray Industries, Inc.), and OFR625 (all manufactured by Organo Corporation).

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

Commercially available anion-charged membranes include, for example, ES15, ES20, CPA3, and CPA5 (all manufactured by Nitto Denko Corporation) and RE-8040BLN (manufactured by Woongjin Co., Ltd.).

The water to be treated contains at least one of calcium and aluminum. The water to be treated may further contain silica, magnesium, etc. The water to be treated containing at least one of calcium and aluminum may be water containing at least one of calcium and aluminum, and is not particularly limited, but examples include coagulation treated water, membrane filtration treated water, industrial water, etc.

The pH of the water to be treated is, for example, in the range of 4.0 to 10.0, and preferably in the range of 5.5 to 9.0. If the pH of the water to be treated is less than 4.0, the rejection rate of the reverse osmosis membrane may drop sharply and the quality of the permeated water may deteriorate, and if it exceeds 10.0, the reverse osmosis membrane may deteriorate due to hydrolysis.

To adjust the pH of the water to be treated, a pH adjuster such as an acid, e.g., hydrochloric acid, sulfuric acid, or nitric acid, or an alkali, e.g., an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide, may be used.

The calcium concentration in the treated water is, for example, in the range of 0.1 to 500 mg/L, the aluminum concentration is, for example, in the range of 0.25 to 5 mg/L, and the silica concentration is, for example, in the range of 120 to 400 mg/L.

The scale inhibitors used include copolymers containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as structural units, phosphonic acid compounds, and polymers containing tert-butyl groups. From the viewpoint of a large anionic charge, copolymers containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as structural units and phosphonic acid compounds are preferred.

Copolymers containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as structural units include copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid, terpolymers of acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and substituted acrylamide, and terpolymers of acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and a monomer having a tert-butyl group.

The weight average molecular weight of the copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units is not particularly limited, but is, for example, in the range of 2000 to 11000, and preferably in the range of 4000 to 5000. If the weight average molecular weight of this copolymer is less than 2000, the steric hindrance effect in suppressing scale may be insufficient, and if it exceeds 11000, the reverse osmosis membrane may be clogged. The molar ratio of acrylic acid to 2-acrylamido-2-methylpropanesulfonic acid in this copolymer is not particularly limited, but is, for example, in the range of 80:20 to 70:30, and preferably in the range of 78:22 to 72:28. If the molar ratio of acrylic acid to 2-acrylamido-2-methylpropanesulfonic acid in the copolymer is less than 70:30, the ability to chelate cations may be insufficient, and if the amount of 2-acrylamido-2-methylpropanesulfonic acid is less than 80:20, the dispersion force due to charge repulsion of the scale inhibitor polymer itself may be insufficient.

Phosphonic acid compounds include those that function primarily as scale dispersants, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, hydroxyphosphonoacetic acid, nitrilotrimethylenephosphonic acid, ethylenediamine-N,N,N',N'-tetramethylenephosphonic acid, or at least one compound selected from the group consisting of alkali metal salts and alkaline earth metal salts of these phosphonic acids.

One or more types of copolymers containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as constituent units may be used, one or more types of phosphonic acid compounds may be used, or one or more types of copolymers containing acrylic acid and AMPS as constituent units may be used in combination with one or more types of phosphonic acid compounds. When using in combination, for example, a copolymer containing a predetermined amount of acrylic acid and AMPS as constituent units and a predetermined amount of a phosphonic acid compound may be mixed and added to the water to be treated, or a copolymer containing a predetermined amount of acrylic acid and AMPS as constituent units and a predetermined amount of a phosphonic acid compound may be added separately to the water to be treated.

The amount of scale inhibitor in the water to be treated may be, for example, in the range of 0.1 to 50 mg/L relative to the amount of water to be treated, and preferably in the range of 1 to 20 mg/L. If the amount of scale inhibitor is less than 0.1 mg/L relative to the amount of water to be treated, the scale inhibition effect may not be obtained, and if it exceeds 50 mg/L, the scale inhibition effect relative to the chemical ratio may be small and running costs may increase.

The Langelier Saturation Index (LSI) of the concentrated water is preferably greater than 0, and more preferably 0.16 or greater. If the Langelier Saturation Index of the concentrated water is 0 or less, membrane blockage due to scale may not occur even if a neutrally charged membrane is not used for the reverse osmosis membrane. There is no particular upper limit for the Langelier Saturation Index of the concentrated water, but it is, for example, 3.0.

The LSI can be calculated by the following formula (1), and can be calculated, for example, by desk calculation or using calculation software provided by a separation membrane manufacturer or a scale inhibitor manufacturer.
LSI=pH-pHs+1.5× ^10-2 ×(T-25) Formula (1)
pHs=8.313-log(Ca ²⁺ )-log(A)+S
Here, pH is the pH of water, T is the water temperature [°C], (Ca ²⁺ ) is the milliequivalent concentration of Ca ²⁺ [meq/L], (A) is the milliequivalent concentration of total alkalinity [meq/L], S is the correction value (S = (2 × √μ)/(1 + √μ), μ: 2.5 × 10 ^-5 × sd), and sd is the soluble substance concentration [mg/L].

The calcium concentration in the concentrated water is preferably 0.1 mg/L or more, and more preferably 1 mg/L or more. If the calcium concentration in the concentrated water is less than 0.1 mg/L, membrane blockage due to scale may not occur even if a neutrally charged membrane is not used for the reverse osmosis membrane. There is no particular upper limit for the calcium concentration in the concentrated water, but it is, for example, 500 mg/L.

The silica concentration in the concentrated water is preferably 120 mg/L or more, and more preferably 150 mg/L or more. If the silica concentration in the concentrated water is less than 120 mg/L, membrane blockage due to scale may not occur even if a neutrally charged membrane is not used for the reverse osmosis membrane. There is no particular upper limit for the silica concentration in the concentrated water, but it is, for example, 400 mg/L.

The aluminum concentration in the concentrated water is preferably 0.25 mg/L or more, and more preferably 5 mg/L or more. If the aluminum concentration in the concentrated water is less than 0.25 mg/L, membrane blockage due to scale may not occur even if a neutrally charged membrane is not used for the reverse osmosis membrane. There is no particular upper limit for the aluminum concentration in the concentrated water, but it is, for example, 10 mg/L.

In the water treatment method according to this embodiment, in addition to the above-mentioned scale inhibitors and pH adjusters, other agents such as slime control agents and anticorrosive agents such as heavy metals, phosphates, and azole compounds may also be used.

This specification includes the following embodiments.
[1] A reverse osmosis membrane treatment means for passing water to be treated, which contains at least one of calcium and aluminum, through a reverse osmosis membrane to obtain a permeate and a concentrated water;
A scale inhibitor adding means for adding a scale inhibitor to the water to be treated,
The water treatment device, wherein the reverse osmosis membrane is a neutrally charged membrane.

[2] The water treatment device according to [1],
The water to be treated further contains silica.

[3] The water treatment device according to [1] or [2],
The scale inhibitor in the water treatment device includes at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound.

[4] The water treatment device according to any one of [1] to [3],
The water treatment device, wherein the neutrally charged membrane has a zeta potential of the membrane surface of -10 mV or more and +10 mV or less.

[5] The water treatment device according to any one of [1] to [4],
The water treatment device, wherein the concentrated water has a Langelier Index of greater than 0.

[6] The water treatment device according to any one of [1] to [5],
The water treatment device, wherein the silica concentration in the concentrated water is 120 mg/L or more.

[7] The water treatment device according to any one of [1] to [6],
The water treatment device, wherein the aluminum concentration in the concentrated water is 0.25 mg/L or more.

[8] The water treatment device according to any one of [1] to [4],
The water treatment device, wherein the concentrated water has a Langelier index exceeding 0, a silica concentration of 120 mg/L or more, and an aluminum concentration of 0.25 mg/L or more.

[9] A reverse osmosis membrane treatment step in which water to be treated containing at least one of calcium and aluminum is passed through a reverse osmosis membrane to obtain a permeate and a concentrated water;
A scale inhibitor adding step of adding a scale inhibitor to the water to be treated;
Including,
The method for treating water, wherein the reverse osmosis membrane is a neutrally charged membrane.

[10] The water treatment method according to [9],
The water treatment method, wherein the water to be treated further contains silica.

[11] The water treatment method according to [9] or [10],
The scale inhibitor comprises at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound.

[12] The water treatment method according to any one of [9] to [11],
The water treatment method, wherein the neutrally charged membrane has a zeta potential of the membrane surface of -10 mV or more and +10 mV or less.

[13] The water treatment method according to any one of [9] to [12],
The method for water treatment, wherein the Langelier Index in the concentrate is greater than 0.

[14] The water treatment method according to any one of [9] to [13],
The water treatment method, wherein the silica concentration in the concentrated water is 120 mg/L or more.

[15] The water treatment method according to any one of [9] to [14],
The water treatment method, wherein the aluminum concentration in the concentrated water is 0.25 mg/L or more.

[16] The water treatment method according to any one of [9] to [12],
The water treatment method, wherein the concentrated water has a Langelier index of more than 0, a silica concentration of 120 mg/L or more, and an aluminum concentration of 0.25 mg/L or more.

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

[RO flat membrane water flow test]
An RO flat membrane water passing test was carried out. A predetermined amount of each solute was dissolved in pure water to prepare raw water. The ionic silica concentration was adjusted to a predetermined concentration using sodium metasilicate, the aluminum ion concentration was adjusted to a predetermined concentration using aluminum chloride, the calcium ion concentration was adjusted to a predetermined concentration using calcium chloride, the magnesium ion concentration was adjusted to a predetermined concentration using magnesium chloride, and the bicarbonate ion was adjusted to a predetermined concentration using sodium bicarbonate. The concentration of the soluble substance was a predetermined concentration of 500 to 1000 [mg/L]. The pH of the raw water was adjusted to pH 7.5 using hydrochloric acid or sodium hydroxide. The water was passed through the flat membrane at a room temperature of 25±2°C. The raw water in this flat membrane water passing test was simulated as RO concentrated water when passing through a reverse osmosis membrane element.

In the RO flat membrane water flow test, raw water was used as the water to be treated, and was fed to the reverse osmosis membrane using a pump, and separated into permeate and concentrated water. The permeate was sampled on an electronic balance, and the concentrated water was returned to the raw water tank. The amount of permeate that was removed from the system was replenished with pure water. The operating pressure of the pump was 0.75 MPa or 1.22 MPa. The amount of concentrated water was 100 mL/min, ensuring a sufficient amount for the amount of permeate. After starting the water flow, it was waited for about 30 minutes to confirm that the entire system was almost uniform, and the flux at that point was taken as the initial value. Flux refers to the permeation flux, and is calculated by the amount of permeate water [g] / density of water [g/mL] / membrane area [m ² ]. Thereafter, the flux was measured over time, and the ratio [%] of the flux at each time to the initial flux was recorded as the flux retention rate [%].

In addition, when water is passed through a typical reverse osmosis membrane element, the treated water is separated into permeate water and concentrated water by the reverse osmosis membrane, with a recovery rate of, for example, around 50% to 90%, which translates to a concentration rate of around 2 to 10 times. On the other hand, the recovery rate in a flat membrane water flow test is, for example, around 0.01% to 0.05%, so the quality of the raw water is approximately equal to the quality of the concentrated water. This is because the flat membrane water flow test is a research method used at the laboratory level, and its purpose is to reduce the amount of sample water required to conduct the test. In a flat membrane water flow test, the risk of clogging is evaluated by passing water simulating concentrated water, which has the highest concentration of clogging substances, through the reverse osmosis membrane.

[Measurement of Zeta Potential of Reverse Osmosis Membrane]
The zeta potential of each reverse osmosis membrane was measured by plate electrophoresis using a 10 mM NaCl aqueous solution of pH 4 to 6 as the measurement solution, and a reverse osmosis membrane measuring 15 mm×33 mm (thickness 5 mm or less) was immersed in the measurement solution.

The zeta potential of the reverse osmosis membrane was determined using the ELSZ series zeta potential/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 formula and the Smoluchowski formula below.

(Mori-Okamoto method)
U _obs (z)=AU ₀ (z/b) ² +ΔU ₀ (z/b)+(1-A)U ₀ +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 : True mobility of the particle U ₀ : Average mobility on the upper and lower surfaces of the cell ΔU ₀ : Difference in mobility on the upper and lower surfaces of the cell

(Smoluchowski's formula)
ζ=4πηU/ε
Where:
U: Electric mobility ε: Dielectric constant of the solvent η: Viscosity of the solvent

The physical properties of the solvent used were those of pure water at 25°C (refractive index: 1.3328, viscosity: 0.8878, dielectric constant: 78.3).

<Example 1 and Comparative Example 1>
The raw water was adjusted so that the concentration of each solute was the value shown in Table 1. The LSI of the raw water of the water quality in Table 1 was calculated to be 0.17. The pump operating pressure was set to 0.75 MPa, and an RO flat membrane water flow test was performed. As the scale inhibitor, a binary copolymer polymer (weight average molecular weight: 4500) of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) mixed with 2-phosphonobutane-1,2,4-tricarboxylic acid as a phosphonic acid compound was used. The amount of the scale inhibitor in the treated water was added so that the binary copolymer polymer was 11.5 mg/L and the phosphonic acid compound was 4.3 mg/L relative to the amount of raw water. In Example 1, a neutral charged membrane (BW30XFR) manufactured by Film Tec was used as the reverse osmosis membrane. The zeta potential of BW30XFR was measured to be -7.9 mV. In Comparative Example 1, an anion-charged membrane (ES20) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. The zeta potential of ES20 was measured and found to be −35 mV.

The change in flux retention rate (%) versus operating time (hr) is shown in Figure 1. In Example 1, the flux retention rate was able to be maintained at a high level compared to Comparative Example 1.

<Example 2, Comparative Example 2>
The raw water was adjusted so that the concentration of each solute was the value shown in Table 2. The LSI of the raw water of the water quality in Table 2 was calculated to be 0.21. The pump operating pressure was set to 0.75 MPa, and an RO flat membrane water flow test was performed. The above-mentioned binary copolymer of acrylic acid and AMPS was used as the scale inhibitor. The amount of the scale inhibitor in the treated water was added so that the binary copolymer was 46 mg/L relative to the amount of raw water. In Example 2, a neutral charged membrane (LFC3) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. The zeta potential of LFC3 was measured to be -1.3 mV. In Comparative Example 2, an anion charged membrane (ES20) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane.

The change in flux retention rate (%) versus operating time (hr) is shown in Figure 2. In Example 2, the flux retention rate was able to be maintained at a high level compared to Comparative Example 2.

<Example 3, Comparative Example 3>
The raw water was adjusted so that the concentration of each solute was the value shown in Table 3. The LSI of the raw water of the water quality in Table 3 was calculated to be 0.49. The pump operating pressure was set to 0.75 MPa, and an RO flat membrane water flow test was performed. As the scale inhibitor, a mixture of the above-mentioned phosphonic acid compound and the binary copolymer of acrylic acid and AMPS was used. The amount of the scale inhibitor in the treated water was the same as that in Example 2. In Example 3, a neutral charged membrane (LFC3) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. In Comparative Example 3, an anion charged membrane (ES20) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane.

The change in flux retention rate (%) versus operating time (hr) is shown in Figure 3. In Example 3, the flux retention rate was able to be maintained at a high level compared to Comparative Example 3.

<Example 4, Comparative Example 4>
The raw water was adjusted so that the concentration of each solute was the value shown in Table 4. The LSI of the raw water of the water quality in Table 4 was calculated to be -0.84. The pump operating pressure was set to 0.75 MPa, and an RO flat membrane water flow test was performed. The above-mentioned binary copolymer of acrylic acid and AMPS was used as the scale inhibitor. The amount of the scale inhibitor in the water to be treated was added so that the binary copolymer was 13.8 mg/L relative to the amount of raw water. In Example 4, a neutral charged membrane (LFC3) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. In Comparative Example 4, an anion charged membrane (ES20) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane.

Figure 4 shows the change in flux retention rate (%) versus operating time (hr). Although the conditions were such that LSI < 0, a slight decrease in flux was observed. This is thought to be due to scale caused by silica or aluminum, rather than hardness components. Even in this case, Example 4 was able to maintain a higher flux retention rate than Comparative Example 4.

<Example 5, Comparative Example 5>
The raw water was adjusted so that the concentration of each solute was the value shown in Table 5. The LSI of the raw water of the water quality in Table 5 was calculated to be 0.17. As the scale inhibitor, a mixture of the binary copolymer of acrylic acid and AMPS and the phosphonic acid compound was used. The amount of the scale inhibitor in the treated water was added so that the binary copolymer was 100 mg/L relative to the amount of raw water. In Example 5, a neutral charged membrane (LFC3) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. In Comparative Example 5, an anion charged membrane (ES20) manufactured by Nitto Denko Corporation was used as the reverse osmosis membrane. The operating pressure of the pump was set to 1.22 MPa for LFC3 and 0.75 MPa for ES20. In Example 5 and Comparative Example 5, the permeation rates [g/min] of LFC3 and ES20 were made equal to each other, and an RO flat membrane water flow test was performed.

The change in flux retention rate (%) versus operating time (hr) is shown in Figure 5. In Example 5, the flux retention rate was maintained at a high level compared to Comparative Example 5.

In this way, the device and method of the embodiment were able to suppress the progression of blockage of the reverse osmosis membrane caused by scale during reverse osmosis membrane treatment.

1 Water treatment device, 10 Treated water tank, 12 Reverse osmosis membrane treatment device, 14, 16 Treated water piping, 18 Permeated water piping, 20 Concentrated water piping, 22 Scale inhibitor addition piping.

Claims (12)

a reverse osmosis membrane treatment means for passing water to be treated, which contains at least one of calcium and aluminum, through a reverse osmosis membrane to obtain permeated water and concentrated water;
A scale inhibitor adding means for adding a scale inhibitor to the water to be treated,
2. A water treatment device according to claim 1, wherein the reverse osmosis membrane is a neutrally charged membrane. The water treatment device according to claim 1,
The water treatment device according to claim 1, wherein the water to be treated further contains silica. The water treatment device according to claim 1 or 2,
The water treatment device is characterized in that the scale inhibitor contains at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound. The water treatment device according to any one of claims 1 to 3,
The water treatment device is characterized in that the neutrally charged membrane is a neutrally charged membrane having a zeta potential of the membrane surface of -10 mV or more and +10 mV or less. a reverse osmosis membrane treatment step of passing the water to be treated, which contains at least one of calcium and aluminum, through a reverse osmosis membrane to obtain a permeate and a concentrated water;
A scale inhibitor adding step of adding a scale inhibitor to the water to be treated;
Including,
A water treatment method, wherein the reverse osmosis membrane is a neutrally charged membrane. The water treatment method according to claim 5,
The water treatment method, wherein the water to be treated further contains silica. The water treatment method according to claim 5 or 6,
The water treatment method is characterized in that the scale inhibitor contains at least one of a copolymer containing acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid as constituent units, and a phosphonic acid compound. The water treatment method according to any one of claims 5 to 7,
The water treatment method is characterized in that the neutrally charged membrane is a neutrally charged membrane having a zeta potential of the membrane surface of -10 mV or more and +10 mV or less. The water treatment method according to any one of claims 5 to 8,
A water treatment method, characterized in that the concentrated water has a Langelier index of more than 0. The water treatment method according to any one of claims 5 to 9,
A water treatment method characterized in that the silica concentration in the concentrated water is 120 mg/L or more. The water treatment method according to any one of claims 5 to 10,
A water treatment method characterized in that the aluminum concentration in the concentrated water is 0.25 mg/L or more. The water treatment method according to any one of claims 5 to 8,
The water treatment method is characterized in that the concentrated water has a Langelier index of more than 0, a silica concentration of 120 mg/L or more, and an aluminum concentration of 0.25 mg/L or more.

Images