Multi-stage reverse osmosis membrane treatment system

Abstract

This multi-stage reverse osmosis membrane treatment system 1 comprises a storage tank 2 storing to-be-treated water W0 and water piping 3 connected to the storage tank 2, with a liquid pump 4, a first reverse osmosis membrane 5, and a second reverse osmosis membrane 6 being provided in this order in the water piping 3. An NaOH solution adding means 7 is connected upstream of the second reverse osmosis membrane 6, and the amount of an NaOH solution to be added is controllable such that to-be-treated water at the second reverse osmosis membrane 6 reaches the alkaline range according to the flow rate of the water piping 3 and the pH of the to-be-treated water W0. The first reverse osmosis membrane 5 has a permeate flux of at least 2.0 m3 / (m2·day) per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)). The second reverse osmosis membrane 6 has a permeate flux of at least 0.8 m3 / (m2·day) and less than 2.0 m3 / (m2·day) per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)). Such a multi-stage reverse osmosis membrane treatment system can be used to obtain high-quality treated water while keeping operating energy in check.

Description

Multi-stage reverse osmosis membrane treatment system

[0001] The present invention relates to a multi-stage reverse osmosis membrane treatment system, and more particularly to a multi-stage reverse osmosis membrane treatment system that can obtain treated water of a predetermined level of quality while suppressing operating energy.

[0002] Conventionally, pure water, such as ultrapure water, from which organic matter, ionic components, fine particles, bacteria, etc. have been highly removed, has been used as cleaning water in the manufacturing processes of semiconductor devices and liquid crystal displays. To produce such pure water, a multistage reverse osmosis membrane treatment system, in which two or more stages of reverse osmosis membranes (RO membranes) are connected in series, is used because it is highly versatile and can easily produce a desired water quality. However, reverse osmosis membranes consume a lot of power, so they are not necessarily suitable for meeting recent energy conservation demands.

[0003] These reverse osmosis membranes come in a variety of performances depending on the membrane surface effective pressure, permeation flux, etc., and attempts have been made to combine reverse osmosis membranes with different performances. For example, Patent Document 1 discloses a system comprising a first reverse osmosis membrane treatment means for passing water to be treated through a first reverse osmosis membrane to obtain a first permeate and a first concentrate, and a second reverse osmosis membrane treatment means for passing at least the first permeate through a second reverse osmosis membrane to obtain a second permeate and a second concentrate, wherein the permeation flux per 1 MPa of effective pressure of the second reverse osmosis membrane is lower than the permeation flux per 1 MPa of effective pressure of the first reverse osmosis membrane, and the permeation flux per 1 MPa of the second reverse osmosis membrane is 0.5 m ３ / m ２ A reverse osmosis membrane treatment system has been proposed in which the water content is 1 / d or less.

[0004] 3, Patent Document 2 discloses a pure water production system 11 including a storage tank 12 for storing raw water to be treated W0, and a water supply pipe 13 connected to the storage tank 12, the water supply pipe 13 being sequentially provided with a liquid supply pump 14, a first reverse osmosis membrane 15, a second reverse osmosis membrane 16, and an ion exchange device 17. In Patent Document 2, the first reverse osmosis membrane 15 has a permeation flux of 0.8 m per 1 MPa of effective membrane surface pressure (water temperature 25°C, pure water (RO permeate)). ３ / (m ２A reverse osmosis membrane having a permeation flux of 2.0 m per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)) is used as the second reverse osmosis membrane 16. ３ / (m ２ By using a reverse osmosis membrane of 1000 kJ / s or more, it is possible to reduce operating energy while maintaining water quality.

[0005] JP 2018-79451 A JP 2023-70396 A

[0006] However, the reverse osmosis membrane treatment system described in Patent Document 1 aims to improve water quality by removing IPA from the permeate water, and has the problem of being less effective in reducing operating energy. Therefore, it is conceivable to combine the system with a reverse osmosis membrane that requires less operating energy, but this creates the problem of making it difficult to obtain treated water from which weakly acidic ionic species such as boron and silica have been sufficiently removed.

[0007] Furthermore, the multi-stage reverse osmosis membrane treatment system described in Patent Document 2 is capable of removing weakly acidic ion species such as boron and silica while reducing operating energy, but there is still room for improvement in terms of water quality.

[0008] The present invention has been made in view of the above-mentioned problems, and has an object to provide a multi-stage reverse osmosis membrane treatment system that can obtain high-quality treated water while reducing operating energy.

[0009] In view of the above object, the present invention provides a multi-stage reverse osmosis membrane treatment system in which reverse osmosis membranes are arranged in series in two stages, and of the two stages of reverse osmosis membranes, the first reverse osmosis membrane in the front stage has a permeation flux of 2.0 m per an effective membrane surface pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)). ３ / (m ２ The second reverse osmosis membrane in the latter stage has a permeation flux of 0.8 m per membrane surface effective pressure of 1 MPa (water temperature 25 ° C, pure water (RO permeate)). ３ / (m ２ ・Day) or more, 2.0m ３ / (m ２ The present invention provides a multi-stage reverse osmosis membrane treatment system having a reverse osmosis membrane with a pH of less than 1 / 2 days, and having an alkali addition means for adjusting the pH of the water to be treated by the first reverse osmosis membrane to the alkaline side.

[0010] According to this invention (Invention 1), by using a reverse osmosis membrane (first reverse osmosis membrane) with a high permeation flux per 1 MPa of effective membrane surface pressure (low effective membrane surface pressure at the same permeation flux), it is possible to significantly reduce operating energy compared to a multi-stage reverse osmosis membrane treatment system configured solely with reverse osmosis membranes with lower permeation fluxes. Furthermore, by adjusting the pH of the feedwater (water to be treated) to the alkaline side for the subsequent reverse osmosis membrane (second reverse osmosis membrane) with a low permeation flux per 1 MPa of effective membrane surface pressure (high effective membrane surface pressure at the same permeation flux), boron and TOC can be maintained at or above predetermined levels. This allows for both reduced operating energy and water quality. In particular, the boron removal rate can be improved compared to when the order of the first and second reverse osmosis membranes is reversed. Furthermore, it is generally known that upstream reverse osmosis membranes have a higher water supply load and are more susceptible to membrane degradation than downstream reverse osmosis membranes. However, when multiple membranes with significantly different low-molecular-weight TOC removal rates are installed, as in Invention 1, the final water quality will depend on the membrane with the higher removal rate. Therefore, if the upstream reverse osmosis membrane is an ultra-low-pressure reverse osmosis membrane, its degradation will lead to a deterioration in the water quality of the entire system. However, if the upstream reverse osmosis membrane is an extremely ultra-low-pressure reverse osmosis membrane, as in Invention 1, the impact on the TOC removal rate of the entire system will be minimal, as long as the downstream ultra-low-pressure membrane is in good condition. Furthermore, the upstream reverse osmosis membrane, which has a large water volume, will be at a lower pressure, which also reduces power consumption.

[0011] In the above invention (Invention 1), the first reverse osmosis membrane has a Ca and Mg removal rate of 99% or more, and SiO ２ It is preferable that the removal rate is 95% or more (Invention 2).

[0012] According to this invention (Invention 2), by using such a first reverse osmosis membrane, it is possible to suppress scaling in the second reverse osmosis membrane when treatment is performed at a high pH in the subsequent second reverse osmosis membrane.

[0013] In the above invention (Invention 1), it is preferable to have a means for adding a scale inhibitor to the treated water of the second reverse osmosis membrane (Invention 3).

[0014] According to this invention (Invention 3), scaling in the second reverse osmosis membrane can be prevented when treatment is performed at a high pH in the second reverse osmosis membrane in the subsequent stage.

[0015] Furthermore, in the above inventions (Inventions 1 to 3), it is preferable that the water quality of the water to be treated in the multistage reverse osmosis membrane treatment system has a boron concentration of 1 to 500 μg / L (Invention 4).

[0016] According to this invention (Invention 4), by treating water having the above boron concentration with a multistage reverse osmosis membrane treatment system, it is possible to remove boron from the treated water at a high level of 95% or more.

[0017] The multi-stage reverse osmosis membrane treatment system of the present invention uses a reverse osmosis membrane in the first stage with a permeation flux per 1 MPa of effective membrane surface pressure greater than a predetermined value, and a reverse osmosis membrane in the second stage with a permeation flux per 1 MPa of effective membrane surface pressure less than a predetermined value, thereby adjusting the pH of the feedwater (water to be treated) to the alkaline side and treating it, thereby achieving both reduced operating energy consumption and good water quality. Furthermore, the impact of a decrease in the TOC removal rate of the entire system can be reduced.

[0018] 1 is a flow diagram showing a multistage reverse osmosis membrane treatment system according to an embodiment of the present invention. 2 is a flow diagram showing a multistage reverse osmosis membrane treatment system of Comparative Example 1. 3 is a flow diagram showing a pure water production apparatus using a conventional multistage reverse osmosis membrane treatment system.

[0019] Hereinafter, a multistage reverse osmosis membrane treatment system of the present invention will be described with reference to the accompanying drawings.

[0020] [Multistage reverse osmosis membrane treatment system] Figure 1 shows a multistage reverse osmosis membrane treatment system according to one embodiment of the present invention. In Figure 1, the multistage reverse osmosis membrane treatment system (two-stage reverse osmosis membrane treatment system) 1 includes a storage tank 2 for storing raw water to be treated (water WO), and a water supply pipe 3 connected to the storage tank 2. The water supply pipe 3 is sequentially provided with a liquid supply pump 4, a first reverse osmosis membrane 5, and a second reverse osmosis membrane 6. An NaOH aqueous solution supplying means 7 is connected upstream of the second reverse osmosis membrane 6 as an alkali supplying mechanism. A control mechanism (not shown) can control the amount of NaOH solution added to the second reverse osmosis membrane 6 based on the flow rate of the water supply pipe 3 and the pH of the water WO, so that the water reaches an alkaline region.

[0021] (Reverse osmosis membrane) In this specification, the first reverse osmosis membrane and the second reverse osmosis membrane are each defined as having the following performance: Note that although there are reverse osmosis membranes that have a smaller permeation flux per 1 MPa of effective membrane surface pressure than the first reverse osmosis membrane and the second reverse osmosis membrane, these are very general-purpose reverse osmosis membranes.

[0022] <First reverse osmosis membrane> Permeation flux (flux) of 0.6 m under the condition of an effective membrane surface pressure of 0.3 MPa (water temperature 25°C, pure water (RO permeate)) ３ / (m ２ ・ day) or more, permeation flux of 2.0 m per membrane effective pressure of 1 MPa (water temperature 25 ° C, pure water (RO permeate)) ３ / (m ２ - Salt removal rate: 95% or more (membrane surface effective pressure 0.3 MPa (water temperature 25°C, feed water 500 mg / L at NaCl) - IPA removal rate: 60% or more (membrane surface effective pressure 0.3 MPa (water temperature 25°C, feed water 500 mg / L at IPA)

[0023] In particular, it is preferable that the first reverse osmosis membrane has an effective membrane surface pressure of 0.3 MPa, a Ca and Mg removal rate of 99% or more, and a SiO2 removal rate of 95% or more.

[0024] <Second reverse osmosis membrane> Permeation flux (flux) of 0.6 m under the condition of a membrane surface effective pressure of 0.75 MPa (water temperature 25°C, pure water (RO permeate)) ３ / (m ２・ day) or more, permeation flux per membrane surface effective pressure 1 MPa (water temperature 25 ° C, pure water (RO permeate)) 0.8 to less than 2.0 m ３ / (m ２ - days) - Salt rejection rate: 98% or more (membrane surface effective pressure 0.75 MPa (water temperature 25°C, feed water 500 mg / L at NaCl) - IPA rejection rate: 80% or more (membrane surface effective pressure 0.75 MPa (water temperature 25°C, feed water 500 mg / L at IPA)

[0025] (Method of Operating a Multistage Reverse Osmosis Membrane Treatment System) The method of operating the above-described multistage reverse osmosis membrane treatment system will be described below. First, the liquid feed pump 4 is driven to supply the water to be treated W0 stored in the storage tank 2 to the first reverse osmosis membrane 5. For the treatment of this embodiment, the water to be treated W0 preferably has a boron concentration of 1 to 500 μg / L. Ionic impurities are removed to a certain extent by this first reverse osmosis membrane 5, yielding primary treated water W1. Note that the water supply pressure of the water to be treated W0 from the liquid feed pump 4 at this time may be set based on the configuration of the reverse osmosis membrane and the desired permeation flux.

[0026] Specifically, the first reverse osmosis membrane 5 has a permeation flux of 2.53 m per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)). ３ / (m ２ A reverse osmosis membrane of 1.00 m permeation flux per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)) was used as the second reverse osmosis membrane 6. ３ / (m ２ When a reverse osmosis membrane of 1.00 m 2 was used, the first reverse osmosis membrane 5 and the second reverse osmosis membrane 6 each had a permeation flux (flux) of 1.00 m 3. ３ / (m ２ When water is passed through the first reverse osmosis membrane 5 at a permeation flux (flux) of 1.00 m / s, the water supply pressure from the liquid feed pump 4 may be set to approximate the sum of the effective pressures on the membrane surface at that flux. ３ / (m ２ The effective membrane surface pressure of the second reverse osmosis membrane 6 is 0.39 MPa and the effective membrane surface pressure of the second reverse osmosis membrane 6 is 1.0 MPa. Therefore, the effective membrane surface pressure of the second reverse osmosis membrane 6 may be set to 1.4 (≈0.39 + 1.0) MPa.

[0027] Next, an NaOH aqueous solution is added to this primary treated water W1 by the NaOH aqueous solution adding means 7 to adjust the pH of the primary treated water W1 to the alkaline side before being treated by the second reverse osmosis membrane 6. Specifically, the pH of the primary treated water W1 is preferably adjusted to 8 to 11. By adjusting the pH of the primary treated water W1 to 8 to 11, the removal rate of weakly acidic ionic species, such as boron and silica, remaining in the primary treated water W1 can be improved. A scale inhibitor may also be added to the primary treated water W1. The scale inhibitor is not particularly limited and can be selected appropriately depending on the quality of the primary treated water W1. For example, phosphonic acids such as 2-phosphonobutane-1,2,4-tricarboxylic acid, copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid, polyacrylic acid, phosphonic acids such as 2-phosphonobutane-1,2,4-tricarboxylic acid, polyacrylic acid, etc. can be used. The amount of these scale inhibitors added is approximately 10 to 1000 mg / L. Then, by treating with this second reverse osmosis membrane 6, remaining ionic impurities, particularly boron and silica, are removed, and secondary treated water W2 is obtained.

[0028] The retention time of the water to be treated W0 in the treatment with the first reverse osmosis membrane 5 and the second reverse osmosis membrane 6 as described above is preferably 10 seconds or more, particularly 30 seconds or more. If the retention time is less than 30 seconds, particularly less than 10 seconds, boron, as well as Ca, Mg, and SiO ２ This is undesirable because it becomes impossible to maintain a sufficiently high removal rate.

[0029] Furthermore, this secondary treated water W2 can be treated with an ion exchange device or the like as needed to further remove remaining ionic impurities, thereby producing pure water.

[0030] According to the multistage reverse osmosis membrane treatment system 1 of this embodiment, a permeation flux of 2.0 m per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)) ３ / (m ２Because the first reverse osmosis membrane 5 has a flow rate of at least 100 sq m (days) and the pH of the primary treated water W1 treated by the second reverse osmosis membrane 6 is adjusted to the alkaline side, the boron removal rate of the resulting secondary treated water W2 can be maintained at a high level (e.g., 95% or higher). Furthermore, the operating energy of the multistage reverse osmosis membrane treatment system 1 is approximately proportional to the feed pressure of the water to be treated W0 from the liquid feed pump 4, i.e., the output of the liquid feed pump 4, and this can be used to compare the magnitude of operating energy. For example, under the same conditions as the setting of the feed pressure from the liquid feed pump 4 described above, if two second reverse osmosis membranes 6 are provided in series, the feed pressure of the liquid feed pump 4 is theoretically 2.0 (≒ 1.0 × 2) MPa, resulting in an operating energy reduction rate of approximately 30% ((2.0 - 1.4) / 2.0 × 100 ≒ 30).

[0031] While the present invention has been described above based on the above-mentioned embodiment, the present invention is not limited to the above-mentioned embodiment and can be embodied in various modifications. For example, in the above-mentioned embodiment, a case where a first reverse osmosis membrane and a second reverse osmosis membrane are connected in two stages in series has been described. However, in the present invention, in a multi-stage reverse osmosis membrane treatment system in which reverse osmosis membranes are connected in series in multiple stages, it is sufficient to use a first reverse osmosis membrane as the reverse osmosis membrane in the preceding stage and adjust the pH of the liquid to the alkaline side in the subsequent stage, and the system may be configured with three or more stages of reverse osmosis membranes.

[0032] The present invention will be described in more detail below based on specific examples, but the present invention is not limited to the following examples.

[0033] [Confirmation of performance of first reverse osmosis membrane 5 and second reverse osmosis membrane 6] In this example, reverse osmosis membranes having a permeation flow rate (flux) per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)) shown in Table 1 were used as the first reverse osmosis membrane 5 and the second reverse osmosis membrane 6. As shown in Figure 1, these reverse osmosis membranes were configured as an extremely low-pressure reverse osmosis membrane (first reverse osmosis membrane 5) in the first stage and an ultra-low-pressure reverse osmosis membrane (second reverse osmosis membrane 6) in the second stage. Treated water obtained by adding 500 μg / L of boron to pure water was treated with the treated water, and the pH of the treated water from the second-stage reverse osmosis membrane was adjusted. The boron concentration was measured, and the boron removal rate was calculated. The results are shown in Table 2. As shown in FIG. 2 , the first stage was an ultra-low pressure reverse osmosis membrane (second reverse osmosis membrane 6) and the second stage was an extremely low pressure reverse osmosis membrane (first reverse osmosis membrane 5). Pure water was treated with 500 μg / L of boron added, and the treated water was used to adjust the pH of the water treated by the second reverse osmosis membrane. The boron concentration was measured and the boron removal rate was calculated. The results are also shown in Table 2.

[0034] In a multi-stage reverse osmosis membrane treatment system 1 having a first-stage ultra-low pressure reverse osmosis membrane (second reverse osmosis membrane 6) and a second-stage extremely ultra-low pressure reverse osmosis membrane (first reverse osmosis membrane 5) as shown in Figure 2, the boron concentration was measured when pure water was treated with 500 μg / L of boron added and the pH was adjusted, and the boron removal rate was calculated. The results are shown in Table 2.

[0035]

[0036]

[0037] Examples 1 to 4 In a multi-stage reverse osmosis membrane treatment system 1 shown in FIG. 1 , the first stage is an extremely low-pressure reverse osmosis membrane (first reverse osmosis membrane 5) and the second stage is an ultra-low-pressure reverse osmosis membrane (second reverse osmosis membrane 6). The feedwater was pure water treated with 20 μg / L of boron (B). An aqueous NaOH solution was added via an aqueous NaOH solution adding means 7 to adjust the pH to 7, 9, 10, or 11. The boron concentrations of the treated water from the first and second reverse osmosis membranes were measured when the first and second reverse osmosis membranes were operated at a recovery rate of 85% and a recovery rate of 90%, respectively. The results are shown in Table 3, along with the boron concentration in the feedwater and the pH of the treated water from the second reverse osmosis membrane. The treatment performance of various components of the extremely low-pressure reverse osmosis membrane (first reverse osmosis membrane 5) used is shown in Table 4.

[0038] Comparative Examples 1 to 4 In a multi-stage reverse osmosis membrane treatment system 1 shown in Figure 2, in which the first stage is an ultra-low pressure reverse osmosis membrane (second reverse osmosis membrane 6) and the second stage is an extremely ultra-low pressure reverse osmosis membrane (first reverse osmosis membrane 5), pure water to be treated (W0) with 20 μg / L of boron (B) added was used as feedwater, and an aqueous NaOH solution was added from the aqueous NaOH solution adding means 7 to adjust the pH to 7, 9, 10, or 11, respectively. The boron concentrations of the water treated by the first-stage reverse osmosis membrane and the second-stage reverse osmosis membrane were measured when the recovery rate of the first-stage reverse osmosis membrane was 85% and the recovery rate of the second-stage reverse osmosis membrane was 90%. The results are shown in Table 1, along with the boron concentration of the feedwater and the pH of the water treated by the second-stage reverse osmosis membrane.

[0039]

[0040]

[0041] As is clear from Table 3, Example 1, in which the pH was not adjusted, had a higher boron concentration than Comparative Example 1, but Examples 2 to 4, in which the pH was adjusted, had a lower boron concentration than Comparative Examples 2 to 4, and it can be seen that the effect of reducing the boron concentration is greater as the pH increases, especially in the range of pH 9 to 11. Furthermore, Table 4 shows that the extremely low-pressure reverse osmosis membrane (first reverse osmosis membrane 5) used in this example does not have a high boron removal performance, but it has a high boron removal rate due to the presence of Ca, Mg, SiO ２The removal rate of urea is high, and it is expected to have the effect of suppressing scaling in the second-stage ultra-low pressure reverse osmosis membrane, an effect that cannot be obtained with NF membranes.

[0042] REFERENCE SIGNS LIST 1 Multistage reverse osmosis membrane treatment system (two-stage reverse osmosis membrane treatment system) 2 Storage tank 3 Water flow piping 4 Liquid feed pump 5 First reverse osmosis membrane 6 Second reverse osmosis membrane 7 NaOH aqueous solution adding means (alkali adding mechanism) W0 Water to be treated (raw water to be treated) W1 Primary treated water W2 Secondary treated water

Claims

1. A multi-stage reverse osmosis membrane treatment system in which reverse osmosis membranes are arranged in series in two stages, wherein the first reverse osmosis membrane of the two stages has a permeation flux of 2.0 m per 1 MPa effective membrane surface pressure (water temperature 25°C, pure water (RO permeate)). ３ / (m ２ The second reverse osmosis membrane in the latter stage has a permeation flux of 0.8 m per membrane surface effective pressure of 1 MPa (water temperature 25°C, pure water (RO permeate)). ３ / (m ２ ・Day) or more, 2.0m ３ / (m ２ a multi-stage reverse osmosis membrane treatment system having a reverse osmosis membrane of less than 1000 kJ / h (days), and an alkali addition means for adjusting the pH of the water to be treated by the first reverse osmosis membrane to the alkaline side.

2. The first reverse osmosis membrane has a Ca and Mg removal rate of 99% or more, and SiO ２ 2. The multi-stage reverse osmosis membrane treatment system according to claim 1, wherein the removal rate of 95% or more is 95% or more.

3. The multi-stage reverse osmosis membrane treatment system according to claim 1, further comprising means for adding a scale inhibitor to the treated water from the second reverse osmosis membrane.

4. A multi-stage reverse osmosis membrane treatment system according to any one of claims 1 to 3, wherein the water quality of the water to be treated in the multi-stage reverse osmosis membrane treatment system has a boron concentration of 1 to 500 μg / L.

Images