Reverse osmosis membrane water purification system

Abstract

Provided is a novel reverse osmosis membrane water purification system capable of stably removing heavy metals. A reverse osmosis membrane water purification system (100) generates purified water by performing a process of causing water to pass through a reverse osmosis membrane. The reverse osmosis membrane water purification system (100) comprises: a reverse osmosis membrane module (10) that has a reverse osmosis membrane (10A); and an addition unit (50) that adds water-soluble salt to water to be introduced into the reverse osmosis membrane module (10).

Description

Reverse osmosis membrane water purification system

[0001] The present disclosure relates to reverse osmosis water purification systems.

[0002] Patent Document 1 discloses a water purification system including a reverse osmosis membrane element and a heavy metal removal means located downstream of the reverse osmosis membrane element.

[0003] Chinese Utility Model No. 207243582

[0004] Reverse osmosis membranes are generally known as filters with the highest removal efficiency and are believed to be able to remove heavy metal ions as well. However, there is room for improvement in the heavy metal removal performance of reverse osmosis water purification systems.

[0005] An object of the present disclosure is to provide a novel reverse osmosis membrane water purification system that can stably remove heavy metals.

[0006] The present inventors have developed the technology of the present disclosure based on the new finding that the concentration of leaked heavy metals such as lead varies depending on the quality of water introduced into a reverse osmosis membrane module. The present disclosure can be realized in the following forms.

[0007] The reverse osmosis membrane water purification system according to the present disclosure is a reverse osmosis membrane water purification system that produces purified water by passing water through a reverse osmosis membrane, and includes a reverse osmosis membrane module having the reverse osmosis membrane, and an addition unit that adds water-soluble salt to the water introduced into the reverse osmosis membrane module.

[0008] 1 is a diagram schematically showing a reverse osmosis membrane water purification system according to embodiment 1. FIG. 2 is a diagram schematically showing a reverse osmosis membrane water purification system according to embodiment 2. FIG. 3 is a diagram schematically showing a reverse osmosis membrane water purification system according to embodiment 3. FIG. 4 is a diagram for explaining a presumed mechanism by which cation hydrates inhibit leakage of heavy metal ions in a reverse osmosis membrane. FIG. 5 is a graph showing the relationship between the cumulative permeate volume and the lead removal rate in experiment 1. FIG. 6 is a graph showing the relationship between the cumulative permeate volume and the lead removal rate in experiment 2. FIG. 7 is a graph showing the relationship between the cumulative permeate volume and the lead removal rate in experiment 3. FIG. 8 is a graph showing the relationship between the cumulative permeate volume and the lead concentration in the permeate in experiment 3.

[0009] A first embodiment of a reverse osmosis membrane water purification system according to the present disclosure will be described with reference to the drawings. The reverse osmosis membrane water purification system according to each of the following embodiments generates purified water by passing water through a reverse osmosis membrane. The water to be treated (hereinafter referred to as raw water) is tap water, which is water supplied from a water supply such as a municipal waterworks. In addition to tap water, other examples of raw water include well water and rainwater.

[0010] <Embodiment 1> As shown in FIG. 1, a reverse osmosis membrane water purification system 100 according to Embodiment 1 includes a reverse osmosis membrane module 10 and an addition unit 50. The reverse osmosis membrane module 10 includes a reverse osmosis membrane 10A. The reverse osmosis membrane module 10 filters raw water using a so-called crossflow system. The reverse osmosis membrane module 10 performs reverse osmosis membrane treatment on the introduced water. Each reverse osmosis membrane module 10 separates the water into permeate, which is water that has permeated the reverse osmosis membrane 10A, and non-permeate, which is water that has not permeated during the reverse osmosis membrane treatment. The material of the reverse osmosis membrane 10A is not particularly limited. The reverse osmosis membrane 10A can be selected from, for example, a composite membrane with a polyamide functional layer and a composite membrane with a cellulose acetate-based polymer functional layer.

[0011] The reverse osmosis membrane module 10 has an inlet 11, a first outlet 12, and a second outlet 13. Water is introduced into the reverse osmosis membrane module 10 through the inlet 11. The reverse osmosis membrane module 10 discharges permeated water through the first outlet 12 and non-permeated water through the second outlet 13. The permeated water discharged through the first outlet 12 has a lower total dissolved solids concentration (TDS concentration) than the water introduced through the inlet 11. The non-permeated water discharged through the second outlet 13 has a higher TDS concentration than the water introduced through the inlet 11. The TDS concentration indicates the total concentration of inorganic salts and the like dissolved in water and is usually expressed in ppm.

[0012] The addition unit 50 adds a water-soluble salt to the water introduced into the reverse osmosis membrane module 10. The water-soluble salt is not particularly limited. From the viewpoint of removing heavy metals, the water-soluble salt preferably includes a salt of one or more metals selected from Groups 1 and 2 of the periodic table. Examples of metals in Group 1 of the periodic table include lithium (Li), sodium (Na), potassium (K), and rubidium (Rb). Examples of metals in Group 2 of the periodic table include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). In addition to salts of metals in Groups 1 and 2 of the periodic table, the water-soluble salt may also be aluminum (Al), iron (Fe), zinc (Zn), manganese (Mn), or the like.

[0013] The cation of the metal salt is preferably one or more selected from sodium ion, magnesium ion, and calcium ion. The anion of the metal salt is preferably one or more selected from chloride ion, sulfate ion, carbonate ion, and bicarbonate ion. The metal salt may be NaCl (sodium chloride), CaCl ２ (Calcium chloride), MgSO ４ (Magnesium sulfate), Na ２ CO ３ (sodium carbonate), and NaHCO ３ (sodium bicarbonate), and NaCl is more preferred because it is a readily available general-purpose material and is inexpensive.

[0014] From the viewpoint of removing heavy metals, the water-soluble salt preferably contains a plurality of metal salts. More preferably, the water-soluble salt contains a salt of a first metal selected from Groups 1 and 2 of the periodic table, and a salt of a second metal different from the first metal selected from Groups 1 and 2 of the periodic table. The combination of the salt of the first metal and the salt of the second metal in the water-soluble salt is not particularly limited. Examples of the water-soluble salt include NaCl, CaCl ２ , MgSO ４ , and NaHCO ３ It is preferable that the composition contains two or more kinds of metal salts selected from the group consisting of the following, and it may contain three or more kinds of metal salts, or it may contain four kinds of metal salts.

[0015] The TDS concentration of the water introduced into the reverse osmosis membrane module 10 is not particularly limited. Hereinafter, the TDS concentration of the water introduced into the reverse osmosis membrane module 10 will also be simply referred to as the TDS concentration of the introduced water. The TDS concentration of the introduced water can be increased by adding the above-mentioned water-soluble salts to the water introduced into the reverse osmosis membrane module 10. Note that the TDS concentration of the introduced water is the concentration of the water immediately before the reverse osmosis membrane 10A.

[0016] The TDS concentration of the inlet water is preferably 10 ppm or more, more preferably 50 ppm or more, and from the viewpoint of sufficiently removing heavy metals, even more preferably 200 ppm or more, and may even be higher. On the other hand, the higher the TDS concentration of the inlet water, the higher the osmotic pressure, and therefore higher pressure is required to obtain a large amount of permeate water by reverse osmosis. From the viewpoint of pressure and flow rate, the TDS concentration of the inlet water is preferably 1000 ppm or less, more preferably 750 ppm or less. In summary, the TDS concentration of the inlet water is preferably 10 ppm or more and 1000 ppm or less, more preferably 50 ppm or more and 1000 ppm or less, and even more preferably 200 ppm or more and 750 ppm or less, and can be set to a range that appropriately combines the above upper and lower limits.

[0017] The addition unit 50 of this embodiment has a storage unit 51 that stores a water-soluble salt. The storage unit 51 of this embodiment stores an aqueous solution of water-soluble salt having a TDS concentration higher than that of the introduced water. Hereinafter, the aqueous solution of water-soluble salt stored in the storage unit 51 will also be simply referred to as a concentrated salt solution.

[0018] The reservoir 51 is connected to an addition water flow section 52. The addition water flow section 52 forms a flow path through which the concentrated salt solution flows. The addition water flow section 52 is provided with a flow rate adjustment valve 53. The flow rate adjustment valve 53 can freely adjust the flow rate of the concentrated salt solution flowing through the addition water flow section 52. The flow rate adjustment valve 53 is an electric valve driven by an actuator such as a solenoid or a motor. The flow rate adjustment valve 53 is driven and controlled by a control section 70, which will be described later. The downstream end of the addition water flow section 52 is connected to an addition port 21 of the introduction water flow section 20, which will be described later. The concentrated salt solution that has flowed through the addition water flow section 52 flows into the introduction water flow section 20 from the addition port 21.

[0019] The reverse osmosis membrane water purification system 100 includes an inlet water flow section 20, a permeate water flow section 30, a non-permeate water flow section 40, and a control section 70.

[0020] The inlet water passage 20 forms a flow path through which water introduced into the reverse osmosis membrane module 10 flows. The upstream side of the inlet water passage 20 is connected to a water supply source (not shown). The downstream end of the inlet water passage 20 is connected to the inlet 11 of the reverse osmosis membrane module 10. The inlet water passage 20 is provided with, in order from the upstream side of the flow path, an addition port 21, a concentration measurement unit 60, a prefilter 20A, and a pump 20B. The pump 20B applies a pressure equivalent to reverse osmosis pressure to the water introduced into the reverse osmosis membrane module 10. The pump 20B is driven and controlled by the control unit 70. The prefilter 20A is configured with a filter material such as activated carbon. The prefilter 20A removes turbid components such as particulate matter and free residual chlorine from the raw water before reverse osmosis membrane treatment. This prevents clogging and deterioration of the reverse osmosis membrane 10A in the reverse osmosis membrane water purification system 100.

[0021] The concentration measurement unit 60 measures the TDS concentration of water introduced into the reverse osmosis membrane module 10. The concentration measurement unit 60 has a measurement means, such as electrical conductivity, that measures a value that changes depending on the concentration of dissolved solids in the water. The concentration measurement unit 60 calculates the TDS concentration based on the measurement value of this measurement means. The concentration measurement unit 60 in this embodiment is located downstream of the inlet 21 for adding the concentrated salt solution. The concentration measurement unit 60 measures the TDS concentration of water after water-soluble salts have been added to the introduced water. The TDS concentration measured by the concentration measurement unit 60 is approximately the same as the TDS concentration of the introduced water.

[0022] The permeate flow section 30 forms a flow path through which permeate discharged from the reverse osmosis membrane module 10 flows. As shown in FIG. 1 , the upstream end of the permeate flow section 30 is connected to the first outlet 12 of the reverse osmosis membrane module 10. The downstream end of the permeate flow section 30 is connected to a water faucet (not shown). The permeate flow section 30 is provided with a post-filter 30A. Similar to the pre-filter 20A, the post-filter 30A is configured with a filter material such as activated carbon. The post-filter 30A can remove components such as VOCs (volatile organic compounds) and odors from the permeate. This allows the reverse osmosis membrane water purification system 100 to improve the taste of the permeate. The permeate that passes through the post-filter 30A is discharged as purified water from a faucet downstream of the permeate flow section 30.

[0023] The non-permeate water flow section 40 forms a flow path through which non-permeate water discharged from the reverse osmosis membrane module 10 flows. As shown in FIG. 1 , the upstream end of the non-permeate water flow section 40 is connected to the second outlet 13 of the reverse osmosis membrane module 10. The non-permeate water flow section 40 is provided with an on-off valve 41. The downstream end of the non-permeate water flow section 40 is open. The non-permeate water that has flowed through the non-permeate water flow section 40 is discharged as wastewater from the downstream end of the non-permeate water flow section 40.

[0024] The control unit 70 controls each component of the reverse osmosis membrane water purification system 100. The control unit 70 is primarily a microcontroller and includes a computing device such as a central processing unit (CPU), and storage means such as RAM and ROM. The control unit 70 is connected to the pump 20B, the on-off valve 41, the flow control valve 53, and the concentration measurement unit 60. The control unit 70 controls the addition unit 50 to add water-soluble salt when the total dissolved solids concentration measured by the concentration measurement unit 60 is equal to or lower than a predetermined value. The predetermined value of the total dissolved solids concentration may be, for example, the lower limit of the TDS concentration of the inlet water. Specifically, the control unit 70 compares the TDS concentration measured by the concentration measurement unit 60 with the lower limit of the TDS concentration of the inlet water stored in advance in a storage unit (not shown) to determine the amount of concentrated salt solution to be added. The control unit 70 controls the flow control valve 53 to adjust the flow rate of the concentrated salt solution.

[0025] As described above, the reverse osmosis membrane water purification system 100 according to the first embodiment includes the reverse osmosis membrane module 10 and the addition unit 50. The operation and effects of the reverse osmosis membrane water purification system 100 will be described, along with the background to the development of the technology of the present disclosure.

[0026] The present inventors conducted tests using reverse osmosis membrane elements to purify water of various qualities and discovered new findings that heavy metals, particularly lead, leak depending on the water quality. Specifically, in the heavy metal removal performance evaluation test according to the NSF / ANSI 58 standard, either water with a TDS concentration of 750 ppm added with NaCl or tap water with a TDS concentration of 200 ppm to 500 ppm is used as the base. While the base water is specified depending on the heavy metal being tested, either base water can be used for soluble lead, and the test water is prepared by adding 150 ppb of soluble lead to the base water. Furthermore, the JIS standard also provides a test condition for soluble lead removal performance, which uses ultrapure water, i.e., water with a TDS concentration of 0 ppm, as the base water, and adding 50 ppb of soluble lead alone. The NSF standard test water has a higher lead concentration than the JIS standard test water. However, it was found that lead leakage into the permeate water under the NSF test conditions was kept low for a longer period of time than lead leakage into the permeate water under the JIS test conditions. On the other hand, in the JIS test, lead leakage into the permeate water was observed about 60 minutes after the start of water flow, and the leakage concentration tended to increase with increasing water flow time. In other words, it was found that water quality that is prone to lead leakage is water with high purity in which only lead is dissolved, i.e., water with a low TDS concentration.

[0027] Based on the above test results, it can be said that water with a TDS concentration equal to or greater than a predetermined value is more likely to have heavy metals removed by a reverse osmosis membrane than water with a TDS concentration less than the predetermined value. The present inventors came up with a novel idea of ​​filtering water with a TDS concentration equal to or greater than a predetermined value through a reverse osmosis membrane in order to achieve stable heavy metal removal performance, and developed the reverse osmosis membrane water purification system 100. By adding water-soluble salt through the addition unit 50, the reverse osmosis membrane water purification system 100 can filter water with a TDS concentration equal to or greater than a predetermined value through the reverse osmosis membrane 10A, regardless of the quality of the raw water, or even if the quality of the raw water is unknown. As a result, the reverse osmosis membrane water purification system 100 can achieve stable heavy metal removal performance using the reverse osmosis membrane 10A.

[0028] As a method for removing heavy metals other than using reverse osmosis membranes, a technique using a removal material such as a heavy metal adsorbent by ion exchange is known. However, heavy metal adsorbents have the problem that they are unable to fully remove heavy metals when the TDS concentration of the raw water is high. In this respect, the reverse osmosis membrane water purification system 100 can be said to be a highly versatile technique that can fully remove heavy metals even when the TDS concentration of the raw water is high.

[0029] The reverse osmosis membrane water purification system 100 has a simple configuration and is therefore useful as a reverse osmosis membrane water purification system for home use. The reverse osmosis membrane water purification system 100 is also considered to be applicable to medium-scale reverse osmosis membrane water purification systems.

[0030] Although the reason why the reverse osmosis membrane 10A can exhibit stable heavy metal removal performance when the TDS concentration is equal to or higher than a predetermined value is unclear, a hypothesized mechanism will be explained with reference to FIG. 4 . Note that the present disclosure is not limited to this hypothesized mechanism. When the TDS concentration is equal to or higher than a predetermined value, hydrates formed by Na ions, a TDS component, are thought to concentrate on the surface of the reverse osmosis membrane 10A. It is then thought that Pb ions and Pb ion hydrates are hindered by the concentrated Na ion hydrates, making it difficult for them to pass through the reverse osmosis membrane 10A. It is thought that a similar effect is achieved when the TDS component contains cations of metal salts listed in paragraph

[0012] other than Na ions.

[0031] <Embodiment 2> A reverse osmosis membrane water purification system 200 according to Embodiment 2 differs from Embodiment 1 in that it includes an addition unit 250 instead of the addition unit 50 according to Embodiment 1. In the following description, the same components as those in Embodiment 1 are designated by the same reference numerals, and redundant description will be omitted.

[0032] As shown in FIG. 2 , the adding unit 250 adds water-soluble salt to the water in the raw water tank 254. The raw water tank 254 is a tank for storing raw water. The water inlet and outlet unit 20 of this embodiment is connected upstream to the raw water tank 254, which is a water supply source. The raw water tank 254 is provided with a concentration measuring unit 60. The concentration measuring unit 60 measures the TDS concentration of the water in the raw water tank 254. The configuration of the adding unit 250 for adding water-soluble salt to the water in the raw water tank 254 is not particularly limited, and may be, for example, a configuration including the storage unit 51, the water addition unit 52, and the flow rate adjustment valve 53 of the first embodiment.

[0033] The reverse osmosis membrane water purification system 200 of this embodiment can store raw water in the raw water tank 254 and stably remove heavy metals even in a water treatment system that is not connected to a water supply source such as a municipal water supply.

[0034] <Embodiment 3> A reverse osmosis membrane water purification system 300 according to Embodiment 3 differs from Embodiment 1 in that it includes an addition unit 350 instead of the addition unit 50 according to Embodiment 1. In the following description, the same components as those in Embodiment 1 are denoted by the same reference numerals, and redundant description will be omitted.

[0035] The reverse osmosis membrane water purification system 300 includes a return water passage 351 that forms a flow path for returning non-permeated water to the inlet water passage 20. As shown in FIG. 3 , the return water passage 351 is connected at its upstream end to a three-way valve 341 in the non-permeated water passage 40. The three-way valve 341 is an electrically operated valve that is driven and controlled by an actuator such as a solenoid or a motor. The three-way valve 341 can be switched to allow non-permeated water discharged to the non-permeated water passage 40 to flow into the return water passage 351. The return water passage 351 is connected at its downstream end to the addition port 21 in the inlet water passage 20. The non-permeated water that has flowed through the return water passage 351 flows into the inlet water passage 20 from the addition port 21.

[0036] The addition section 350 returns the non-permeated water from the return water flow section 351 and adds water-soluble salts contained in the non-permeated water to the water in the inlet water flow section 20. The non-permeated water has a higher concentration of water-soluble salts than the water supplied from the water supply source. This configuration makes it possible to increase the TDS concentration of the water in the inlet water flow section 20 depending on the inflow rate of non-permeated water, thereby making the TDS concentration of the introduced water equal to or higher than a predetermined value. Note that if the TDS concentration of the water supplied from the water supply source is low, the addition section 50 of embodiment 1 may be provided in addition to the return water flow section 351 to supplement the water-soluble salts contained in the non-permeated water.

[0037] The control unit 70 is connected to the pump 20B, the concentration measurement unit 60, and the three-way valve 341. The control unit 70 controls the addition unit 350 to add a water-soluble salt when the total dissolved solids concentration measured by the concentration measurement unit 60 is equal to or lower than a predetermined value. The predetermined value of the total dissolved solids concentration can be, for example, the lower limit of the TDS concentration of the inlet water. Specifically, the control unit 70 compares the lower limit of the TDS concentration of the inlet water stored in advance in a memory means (not shown) with the TDS concentration measured by the concentration measurement unit 60, and determines the amount of non-permeated water to be introduced into the return water flow unit 351. The control unit 70 controls the three-way valve 341 to adjust the flow rate of the non-permeated water.

[0038] According to this embodiment, water-soluble salts derived from water supplied from a water source or water-soluble salts added from a reservoir (not shown) can be concentrated by the reverse osmosis membrane 10A to a TDS concentration equal to or higher than a predetermined value.

[0039] The present disclosure is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included within the technical scope of the present disclosure.

[0040] In the reverse osmosis membrane water purification system according to the present disclosure, the reservoir may store a solid water-soluble salt, in which case the water-soluble salt may be appropriately measured and added to the water introduced into the reverse osmosis membrane module.

[0041] In the reverse osmosis membrane water purification system according to the present disclosure, water-soluble salts may be manually added to the water introduced into the reverse osmosis membrane module.

[0042] The reverse osmosis membrane water purification system according to the present disclosure does not necessarily have to include a concentration measurement unit. For example, the TDS concentration of water supplied from a water supply source may be measured in advance, and the amount of water-soluble salt to be added may be determined based on the amount of water passing through the water inlet. The concentration measurement unit may also measure the TDS concentration of raw water supplied from the water supply source, or may measure the TDS concentration of water that has passed through a prefilter.

[0043] The present invention will be explained in more detail below with reference to examples.

[0044] 1. Experiment 1: Test water was prepared by dissolving only lead nitrate in pure water in accordance with JIS S 3201:2019. Tests were conducted with soluble lead concentrations of 100 ppb and 150 ppb in addition to the JIS condition of 50 ppb. A DuPont BW60-1812-75 reverse osmosis membrane element was used to conduct a soluble lead removal performance test. The test results are shown in the graph in Figure 5. The horizontal axis represents the cumulative permeate volume (L), and the vertical axis represents the lead ion removal rate (%). The plots in Figure 5 are as follows: White triangles: soluble lead concentration 50 ppb, TDS concentration 0 ppm; White circles: soluble lead concentration 100 ppb, TDS concentration 0 ppm; White squares: soluble lead concentration 150 ppb, TDS concentration 0 ppm.

[0045] The results of Experiment 1 showed that when the TDS concentration of the test water was 0 ppm, a decrease in the lead ion removal rate was observed even when the cumulative permeate volume was 20 L or less. Furthermore, when the cumulative permeate volume was 20 L or more, the lead ion removal rate was 80% or less for all test waters. These results suggest that when the TDS concentration is 0 ppm, the reverse osmosis membrane may not be able to sufficiently remove heavy metals.

[0046] 2. Experiment 2 Test water was prepared by dissolving lead nitrate and NaCl in pure water. The soluble lead concentration of the test water was 150 ppb in accordance with NSF / ANSI 58. The TDS concentrations of the test water were 0 ppm, 10 ppm, 20 ppm, 50 ppm, 200 ppm, 500 ppm, and 750 ppm. A soluble lead removal performance test was conducted using a DuPont BW60-1812-75 reverse osmosis membrane element. The test results are shown in the graph in Figure 6. The horizontal axis represents the cumulative permeate volume (L), and the vertical axis represents the lead ion removal rate (%). The plots in Figure 6 are as follows: White star: Soluble lead concentration 150ppb, TDS concentration 0ppm White triangle: Soluble lead concentration 150ppb, TDS concentration 10ppm White circle: Soluble lead concentration 150ppb, TDS concentration 20ppm White square: Soluble lead concentration 150ppb, TDS concentration 50ppm Black triangle: Soluble lead concentration 150ppb, TDS concentration 200ppm Black circle: Soluble lead concentration 150ppb, TDS concentration 500ppm Black square: Soluble lead concentration 150ppb, TDS concentration 750ppm

[0047] The results of Experiment 2 showed that when the TDS concentration of the test water was 10 ppm or higher, the lead ion removal rate was 95% or higher, even when the cumulative permeate volume reached 60 L. Furthermore, the lead ion removal rate did not decrease significantly for any of the test waters with cumulative permeate volumes ranging from 0 L to 60 L. These results suggest that adding a water-soluble salt as a TDS component enables stable removal of heavy metals using a reverse osmosis membrane.

[0048] 3. Experiment 3: Test water was prepared by dissolving lead nitrate and the following water-soluble salts in pure water. The water-soluble salts were NaCl, CaCl ２ ・2H ２ O (calcium chloride dihydrate), MgSO ４ ・7H ２ O (magnesium sulfate heptahydrate), NaHCO ３(sodium bicarbonate) were mixed in a mass ratio of 1.00:3.18:2.66:3.36. The test water had a soluble lead concentration of 150 ppb. The test water had a TDS concentration of 350 ppm. A DuPont BW60-1812-75 reverse osmosis membrane element was used, and the soluble lead concentration of the test water was set to 150 ppb in accordance with NSF / ANSI 58. A soluble lead removal performance test was conducted. The test results are shown in the graphs of Figures 7 and 8. The horizontal axis of Figure 7 represents the cumulative permeate volume (L), and the vertical axis represents the lead ion removal rate (%). The horizontal axis of Figure 8 represents the cumulative permeate volume (L), and the vertical axis represents the lead ion concentration (ppb) in the permeate. The plots in Figures 7 and 8 are as follows. For comparison, the results of Experiment 2 are also shown in Figures 7 and 8. Cross mark: Soluble lead concentration 150ppb, TDS concentration 350ppm (composite)

[0049] From the results of Experiment 3, NaCl, CaCl ２ ・2H ２ O (calcium chloride dihydrate), MgSO ４ ・7H ２ O (magnesium sulfate heptahydrate), NaHCO ３ When sodium bicarbonate was used, the lead ion removal rate was 99% or more and the lead concentration in the permeate was 0.11 ppb or less when the cumulative permeate volume was 0 L to 100 L. These results suggest that using multiple types of metal salts as TDS components in the test water can improve the heavy metal removal performance compared to when a single type of metal salt is used.

[0050] 4. Experiment 4: A similar test to Experiment 2 was conducted using test water with a soluble lead concentration of 50 ppb and a TDS concentration of 20 ppm, and test water with a soluble lead concentration of 100 ppb and a TDS concentration of 20 ppm. As a result, for the test water with a soluble lead concentration of 50 ppb, the lead ion removal rate was 97% or more, even when the cumulative permeate volume reached 60 L. Furthermore, for the test water with a soluble lead concentration of 100 ppb, the lead ion removal rate was 96% or more, even when the cumulative permeate volume reached 60 L. The results of Experiment 4 suggest that adding a water-soluble salt as a TDS component can stably remove heavy metals using a reverse osmosis membrane, regardless of the soluble lead concentration.

[0051] 10... reverse osmosis membrane module, 10A... reverse osmosis membrane, 20... inlet water passage section, 30A... post filter, 40... non-permeate water passage section, 50, 250, 350... addition section, 51... storage section, 60... concentration measurement section, 100, 200, 300... reverse osmosis membrane water purification system, 254... raw water tank, 351... return water passage section

Claims

1. A reverse osmosis membrane water purification system that produces purified water by treating water to pass it through a reverse osmosis membrane, comprising: a reverse osmosis membrane module having the reverse osmosis membrane; and an addition unit that adds water-soluble salt to the water introduced into the reverse osmosis membrane module.

2. The reverse osmosis membrane water purification system according to claim 1, wherein the water-soluble salt comprises a salt of one or more metals selected from Groups 1 and 2 of the periodic table.

3. The reverse osmosis membrane water purification system according to any one of claims 1 to 2, wherein the water-soluble salt includes a plurality of types of metal salts.

4. A reverse osmosis membrane water purification system as described in any one of claims 1 to 3, wherein the addition section has a storage section for storing the water-soluble salt.

5. A reverse osmosis membrane water purification system according to any one of claims 1 to 4, wherein the addition section adds the water-soluble salt to the water in the raw water tank.

6. A reverse osmosis membrane water purification system as described in any one of claims 1 to 5, comprising: a concentration measuring unit that measures the total dissolved solids concentration of water introduced into the reverse osmosis membrane module; and a control unit that controls the addition unit to add the water-soluble salt when the total dissolved solids concentration measured by the concentration measuring unit is below a predetermined value.

7. A reverse osmosis membrane water purification system as described in any one of claims 1 to 6, comprising: an inlet water passage section forming a flow path through which water introduced into the reverse osmosis membrane module flows; a non-permeated water passage section forming a flow path through which non-permeated water discharged from the reverse osmosis membrane module flows without permeating the reverse osmosis membrane; and a return water passage section forming a flow path for returning the non-permeated water to the inlet water passage section, wherein the addition section returns the non-permeated water from the return water passage section and adds the water-soluble salt contained in the non-permeated water to the water in the inlet water passage section.

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