JP3536294B2 - Pure water production method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing pure water. More specifically, the present invention relates to a method for producing pure water by making raw water alkaline and passing it through a reverse osmosis membrane, wherein there is no risk of scale generation in the reverse osmosis membrane and an alkaline agent necessary for pH adjustment is used. The present invention relates to a method for producing pure water capable of obtaining pure water with improved water quality by reducing the amount of addition and reducing the salt concentration in the resulting pure water. [0002] In desalination using a reverse osmosis membrane, the desalination rate is substantially determined by the type of the reverse osmosis membrane. Therefore, the quality of the demineralized water obtained as permeate depends on the quality of the water supplied to the reverse osmosis membrane. It depends. When desalting using a reverse osmosis membrane under high pH conditions, boron, silica, organic acids and the like are easily dissociated as the pH increases, and ionized substances can be separated by the reverse osmosis membrane. For this reason, substances that cannot be removed under neutral conditions can be removed by the reverse osmosis membrane under high pH conditions. However, in order to increase the pH of the raw water, it is necessary to add a large amount of an alkaline agent, so that the salt concentration of the feed water to the reverse osmosis membrane increases, the salt concentration of the demineralized water also increases, and the water quality increases. This will lead to a decline. For example, Oil and Gas Journal, 199
In the September 20, 3rd issue, pp. 88-91, after treating raw water with a weakly acidic cation exchange resin, adjusting the pH to 10 and passing the water through a reverse osmosis membrane, boron and the like are effective. Has been reported to be removed. However, according to this method, there is a problem in that the amount of the alkali agent added when a high pH condition such as pH 10 is used is large, and these alkali components leak into the permeated water of the reverse osmosis membrane. In addition, the pretreatment with only the weakly acidic cation exchange resin has a problem that the removal of the calcium component becomes insufficient and causes scale generation in the reverse osmosis membrane. Therefore, there is no risk of scale generation in the reverse osmosis membrane, the amount of alkali agent required for pH adjustment is reduced, the salt concentration in the demineralized water is reduced, and pure water with improved water quality is obtained. There is a demand for a method for producing pure water that can produce water. SUMMARY OF THE INVENTION The present invention relates to a method for producing pure water by making raw water alkaline and passing it through a reverse osmosis membrane. The purpose of the present invention is to provide a method for producing pure water that can reduce the amount of an alkali agent required for the purpose of reducing the concentration of salts in the resulting pure water and obtain pure water with improved water quality. It was done. The present inventor has made intensive studies to solve the above-mentioned problems, and as a result, before adjusting the pH of raw water to alkaline, degassing was performed under acidic conditions. Pretreatment by the process and the ion exchange process of contacting with a strongly acidic cation exchange resin completely removes carbonic acid and calcium ions in the raw water to prevent the generation of scale, and reduces the alkali required for pH adjustment. The inventors have found that it is possible to produce high-quality pure water by reducing the amount of the agent, and have completed the present invention based on this finding. That is, the present invention relates to (1) adjusting the pH of raw water to 9.
In a method for producing pure water by adjusting the water to 2 or more and passing it through a reverse osmosis membrane, a step of deaeration under acidic conditions before adjusting the pH of raw water, and a step of contacting with a strongly acidic cation exchange resin The present invention provides a method for producing pure water, wherein raw water is pretreated by an ion exchange step. Further, as a preferred embodiment of the present invention,
(2) The degassing treatment by adjusting the pH of raw water to 6 or less (1)
(3) The method for producing pure water according to (3), wherein in the ion exchange step, contact with a weakly acidic cation exchange resin is used in combination.
(4) The method for producing pure water according to (1), wherein (4) the reverse osmosis membrane is provided in multiple stages, and the permeated water of the former reverse osmosis membrane is passed through the latter reverse osmosis membrane. Manufacturing method. DETAILED DESCRIPTION OF THE INVENTION The method of the present invention adjusts the pH of raw water to 9.2.
In the method for producing pure water by passing water through the reverse osmosis membrane adjusted as described above, before adjusting the pH of the raw water, a step of degassing under acidic conditions and contacting with a strongly acidic cation exchange resin The raw water is pretreated by the ion exchange step. The order of the degassing process and the ion exchange process is not particularly limited, and the degassing process can be followed by the ion exchange process, or conversely, the ion exchange process can be followed by the degassing process. In the method of the present invention,
When degassing raw water under acidic conditions, the pH of raw water
Is preferably 6 or less, and more preferably the pH is 5 or less. By degassing the raw water under acidic conditions, carbonate ions and bicarbonate ions contained in the raw water can be removed as carbon dioxide gas. If carbonate and bicarbonate ions are present in the raw water, the carbonate and bicarbonate ions have a large pH buffering action, so that the amount of the alkali agent required to adjust the pH of the raw water to 9.2 or more is large. This increases the amount of alkali leakage in the reverse osmosis membrane treatment. In the method of the present invention, since the raw water is degassed under acidic conditions to remove carbonic acid, the pH is adjusted to 9.2.
The amount of the alkali agent necessary for the above adjustment is small, and the amount of alkali leakage in the reverse osmosis membrane treatment is greatly reduced. There is no particular limitation on the method for making the raw water acidic. For example, the raw water can be made acidic by adding hydrochloric acid, sulfuric acid, or the like, or can be made acidic by contacting solid particles containing an acidic substance. Examples of the solid particles containing an acidic substance include a weakly acidic cation exchange resin and a strongly acidic cation exchange resin regenerated with hydrochloric acid or the like. In the method of contacting with an ion-exchange resin, the pH may not always be constant, for example, the pH may become too low during the initial stage of water flow to the ion-exchange resin tower, or the pH may gradually increase with the progress of water flow. Therefore, it is preferable that the pH is fixed by adding an acid and the pH is kept constant so that the removal rate of carbonic acid in the degassing treatment is kept constant. There is no particular limitation on the degassing device used,
For example, a decarbonation tower, a vacuum degassing tower, a nitrogen degassing tower, a membrane degassing apparatus and the like can be mentioned. [0006] In the method of the present invention, calcium ions, magnesium ions and the like in the raw water are removed by bringing the raw water into contact with a strongly acidic cation exchange resin. Calcium ions and magnesium ions in the raw water consume an alkaline agent when adjusting the pH of the raw water to 9.2 or more. Therefore, by removing calcium ions and magnesium ions, the pH is raised to 9.2 or more. The amount of alkali agent required for adjustment can be reduced, and the amount of leak of alkali in the reverse osmosis membrane treatment can be greatly reduced. Weakly acidic cation exchange resins have the advantage of high calcium adsorption and low regeneration frequency, but weakly acidic cation exchange resins have the ability to degrade alkaline salts such as calcium bicarbonate, but neutral salts such as calcium chloride. Has no resolution. Because of this, some of the calcium in the raw water is removed by the weakly acidic cation exchange resin, but not all of the calcium can be removed. In the method of the present invention, since a strongly acidic cation exchange resin is used, both alkaline salts and neutral salts can be decomposed, and calcium ions in raw water can be almost completely removed. In the method of the present invention, the strongly acidic cation exchange resin can be used in H form or in Na form. If there is a degassing process after the ion exchange process, since the degassing process is performed under acidic conditions,
When the strongly acidic cation exchange resin is used in the H form, hydrogen ions H ⁺ are released into water to become acidic, so that the amount of acid used to make acidic conditions in the degassing treatment can be reduced. Even when the strongly acidic cation exchange resin is used in the Na type, calcium can be removed because calcium is higher in selectivity than sodium in the strongly acidic cation exchange resin. Accordingly, water can be passed until the strongly acidic cation exchange resin is substantially saturated with calcium or magnesium, and the amount of treated water is greater than when used in the H form, which is preferable. The strong acid cation exchange resin is regenerated with a mineral acid in the case of the H form, and is regenerated with a saline solution or with a mineral acid in the case of the Na form. When regenerated with mineral acid, it becomes H form, but gradually changes from H form to Na form while passing water. In the method of the present invention, a weakly acidic cation exchange resin can be used together with a strongly acidic cation exchange resin. By removing the calcium of the alkaline salt with a weakly acidic cation exchange resin of H form and removing the calcium of the neutral salt with a strongly acidic cation exchange resin, taking advantage of the characteristics of the weakly acidic cation exchange resin and the strongly acidic cation exchange resin, The amount of calcium removed can be increased, and the frequency of regeneration can be reduced. In the method of the present invention, the pH of raw water after the deaeration step under acidic conditions and the ion exchange step by contact with a strongly acidic cation exchange resin is adjusted to 9.2 or more, and a reverse osmosis membrane is formed. Pass water. In the method of the present invention, the method for adjusting the pH of the raw water to 9.2 or more is not particularly limited,
For example, a method of adding an aqueous alkali solution such as sodium hydroxide, a method of contacting with a strongly basic anion exchange resin, and the like can be given. As a method of adding the aqueous alkali solution, for example, a pH adjusting tank with a stirrer may be provided, or an alkaline aqueous solution injection port may be provided in the water flow line, and a static mixer or the like may be provided downstream of the inlet. The pH of the raw water has an acid dissociation index pKa of boric acid of 9.2.
(25 ° C.) or more, more preferably 10 or more. Reverse osmosis membranes used in the method of the present invention
It is preferable to use an alkali-resistant reverse osmosis membrane that is not deteriorated even when it is 10 or more. In this case, since the pH of the concentrated water is higher than the pH of the supplied alkaline raw water, it is necessary to select an alkali-resistant reverse osmosis membrane in consideration of the pH of the concentrated water. Such alkali-resistant reverse osmosis membranes include, for example, FILMTEC type FT30, which is commercially available as a material having a long-term durability up to pH 11, and Nitto Denko Corporation, which is commercially available as a material having a long-term durability up to pH 10. Made of ES20, ES10, NTR7
59, a polyamide-based film such as SU700 manufactured by Toray Industries, Inc., and the like. In the method of the present invention, reverse osmosis membranes through which raw water whose pH has been adjusted to 9.2 or more are provided in multiple stages, and the permeated water of the previous stage reverse osmosis membrane may be used as the feed water of the subsequent stage reverse osmosis membrane. it can. When removing boron from raw water using a reverse osmosis membrane, it was known that the performance of reverse osmosis membrane for removing boron under high alkali conditions was higher, but unlike the performance of removing cation components such as sodium ions, It has been revealed as a new finding that there is little dependence and the degree of reduction in the removal rate is small even at a low concentration. For this reason, the boron removal rate can be increased by combining the reverse osmosis membranes in multiple stages and adjusting the pH of the feed water to 9.2 (25 ° C.) or more, which is the acid dissociation index pKa of boric acid. Hereinafter, embodiments of the method of the present invention will be described with reference to the drawings. FIG. 1 is a process flow chart of one embodiment of the method of the present invention. In this embodiment, the raw water is passed through the strongly acidic cation exchange resin tower 1 and then deaerated in the membrane deaerator 2. The water flowing out of the membrane deaerator is added with an alkali agent by an alkali addition device equipped with a pH sensor 3, a controller 4, an alkali storage tank 5, and a chemical injection pump 6, and the pH is reduced.
Adjusted to 9.2 or higher. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. FIG. 2 is a process flow chart of another embodiment of the method of the present invention. In the present embodiment, the raw water is deaerated in the membrane deaerator 2 after passing through the weakly acidic cation exchange resin tower 9 to remove the alkaline salt of calcium. The water flowing out of the membrane deaerator is passed through the strongly acidic cation exchange resin tower 1 to remove the remaining neutral salts of calcium, and then a pH sensor 3, a controller 4, an alkaline storage tank 5, and a chemical injection pump 6 Alkali agent is added by an alkali addition device equipped with
Adjusted to 9.2 or higher. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. FIG. 3 is a process flow chart of another embodiment of the method of the present invention. In this embodiment, the raw water is a pH sensor 10, a controller 11, and an acid storage tank 1.
The acid is added by an acid addition device equipped with a chemical pump 2 and a chemical injection pump 13, the pH is adjusted to 6 or less, and then deaeration is performed in the membrane deaerator 2. After the water flowing out of the membrane deaerator is passed through the strongly acidic cation exchange resin tower 1, the alkali agent is removed by an alkali addition device equipped with a pH sensor 3, a controller 4, an alkali storage tank 5, and a chemical injection pump 6. Added
Adjusted to pH 9.2 or higher. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. FIG. 4 is a process flow chart of another embodiment of the method of the present invention. In this embodiment, the raw water is passed through the weakly acidic cation exchange resin tower 9 to remove the alkaline salts of calcium, and then passed through the strongly acidic cation exchange resin tower 1 to remove the remaining neutral salts of calcium. Removed. The water flowing out of the strongly acidic cation exchange resin tower is then deaerated in the membrane deaerator 2. The water flowing out of the membrane deaerator is pH sensor 3,
An alkali agent is added by an alkali addition device equipped with a controller 4, an alkali storage tank 5, and a chemical injection pump 6, and the pH is adjusted to 9.
Adjusted to 2 or more. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. FIG. 5 is a flow chart of another embodiment of the method of the present invention. In this embodiment, the raw water is passed through the mixing tower 14 of the strongly acidic cation exchange resin and the weakly acidic cation exchange resin, and then deaerated in the membrane deaerator 2. The water flowing out of the membrane deaerator is adjusted to pH 9.2 or higher by adding an alkali agent by an alkali addition device having a pH sensor 3, a controller 4, an alkali storage tank 5, and a chemical injection pump 6. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. In this embodiment, the addition of an acid is unnecessary, the number of ion-exchange resin towers is only one, and a mixed tower of a weakly acidic cation exchange resin and a strongly acidic cation exchange resin is used. Therefore, the method for producing pure water of this embodiment is economically excellent. FIG. 6 is a process flow chart of another embodiment of the method of the present invention. In this embodiment, the raw water is supplied to the pH sensor 10, the controller 11, the acid storage tank 12, and the chemical pump 1.
An acid is added by an acid addition device provided with the pressure control device 3, the pH is adjusted to 6 or less, and then deaeration treatment is performed in the membrane deaeration device 2. Water flowing out of the membrane deaerator is passed through a mixing tower 14 of a strongly acidic cation exchange resin and a weakly acidic cation exchange resin, and then provided with a pH sensor 3, a controller 4, an alkaline storage tank 5, and a chemical pump 6. An alkali agent is added by an alkali addition device to adjust the pH to 9.2 or higher. The water whose pH has been adjusted to 9.2 or more is passed through the reverse osmosis membrane 7 in the former stage and the reverse osmosis membrane 8 in the latter stage to remove impurities and obtain pure water. According to the method of the present invention, the raw water is passed through a reverse osmosis membrane under alkaline conditions, and when producing pure water, calcium and carbonic acid in the raw water are sufficiently removed.
The amount of the alkali agent added to adjust the water content can be reduced, the concentration of the alkali component leaking from the reverse osmosis membrane can be reduced, and pure water with high purity can be obtained. In addition, the deposition of scale such as calcium carbonate on the reverse osmosis membrane can be prevented. The present invention will be described in more detail with reference to the following examples, which should not be construed as limiting the present invention. Example 1 Pure water was produced by a process having a two-stage reverse osmosis membrane device shown in FIG. Atsugi City Water was passed through a tower filled with 30 liters of activated carbon [Kuraray Co., Ltd., Kuraray Coal KW10 / 32] at 600 liters / hr, and water excluding residual chlorine was used as raw water. The calcium concentration in the raw water was 24 mg / liter, and the boron concentration was 30 μg / liter. The membrane deaerator 2 includes a deaeration membrane [Dainippon Ink Industries Co., Ltd.
SEPAREL EF-040P], the strongly acidic cation exchange resin tower 1 is filled with 30 liters of strongly acidic cation exchange resin [Bayer, Lewatit S100], and the reverse osmosis membrane devices 7 and 8 are alkali-resistant reverse osmosis. The membrane [FILMTEC type FT30] was mounted.
Sulfuric acid was added to the raw water to adjust the pH to 4.5, and water was passed at a rate of 600 liter / hr to perform deaeration in a membrane deaerator and ion exchange in a strongly acidic cation exchange resin tower. The calcium concentration of the water flowing out of the strongly acidic cation exchange resin tower was 0.01 mg / liter or less. After adjusting the pH to 10 by adding an aqueous sodium hydroxide solution to the water flowing out of the strongly acidic cation exchange resin tower,
The water was passed through the reverse osmosis membrane apparatus at the preceding stage under the conditions of supply water 600 liter / hr, permeated water 540 liter / hr, and concentrated water 60 liter / hr. Pure water was produced by passing water through the osmosis membrane apparatus under the conditions of 540 liter / hr of supplied water, 486 liter / hr of permeated water, and 54 liter / hr of concentrated water. The sodium concentration of pure water obtained as permeated water in the subsequent reverse osmosis membrane device was 380 μg / liter, and the boron concentration was 0.1 μg / liter. Comparative Example 1 Atsugi City Water was passed through an activated carbon packed tower, and the same raw water as in Example 1 except for residual chlorine was replaced with a weakly acidic cation exchange resin [Bayer, Lewatit CNP80] filled with 30 liters. 600 liters / hr in tower
Water was passed at the speed of The calcium concentration of the water flowing out of the weakly acidic cation exchange resin column was 0.2 mg / liter. To the water flowing out of this weakly acidic cation exchange resin tower,
After adjusting the pH to 10 by adding an aqueous sodium hydroxide solution, the same alkali-resistant reverse osmosis membrane [FILM] as in Example 1 was used.
[TEC type FT30]. The first-stage reverse osmosis membrane device was passed under the conditions of 600 liters / hr of feed water, 540 liters / hr of permeated water, and 60 liters / hr of concentrated water. Pure water was produced by passing water through the reverse osmosis membrane apparatus under the conditions of 540 liter / hr of supply water, 486 liter / hr of permeated water, and 54 liter / hr of concentrated water. The sodium concentration of the pure water obtained as the permeate of the reverse osmosis membrane device at the subsequent stage is 1,800 μg / liter, and the boron concentration is 0.1 μg.
g / liter. Table 1 shows the results of Example 1 and Comparative Example 1. [Table 1] As can be seen from Table 1, both the pure water obtained in Example 1 and the pure water obtained in Comparative Example 1 have a boron concentration of 0.1 μg / liter, and the boron concentration is the same. The removal effect has been obtained. However, while the pure water obtained in Example 1 had a sodium concentration of 380 μg / liter, the sodium concentration of pure water obtained in Comparative Example 1 was 1,800 μg / liter, which is four times that of Example 1. From the above, it can be seen that in Example 1, the leakage of sodium from the reverse osmosis membrane was small. This can be seen from the difference in the calcium concentration of the water supplied to the reverse osmosis membrane device in the preceding stage. In Example 1, since the raw water was brought into contact with the strongly acidic cation exchange resin, the calcium was completely removed. Since carbon dioxide is also removed by the deaeration treatment, the amount of sodium hydroxide added to adjust the pH of the water supplied to the reverse osmosis membrane to 10 is small, and as a result, the amount of sodium leaking from the reverse osmosis membrane It is considered that the number has also decreased. According to the method of the present invention, raw water is passed through a reverse osmosis membrane under alkaline conditions, and when producing pure water, calcium and carbon dioxide in the raw water are sufficiently removed. Therefore, the amount of the alkali agent added for adjusting the pH can be reduced, the concentration of the alkali leaking from the reverse osmosis membrane can be reduced, and pure water with high purity can be obtained. In addition, the deposition of scale such as calcium carbonate on the reverse osmosis membrane can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow diagram of one embodiment of the method of the present invention. FIG. 2 is a process flow chart of another embodiment of the method of the present invention. FIG. 3 is a process flow chart of another embodiment of the method of the present invention. FIG. 4 is a process flow chart of another embodiment of the method of the present invention. FIG. 5 is a process flow chart of another embodiment of the method of the present invention. FIG. 6 is a process flow chart of another embodiment of the method of the present invention. [Description of Signs] 1 Strongly acidic cation exchange resin tower 2 Membrane deaerator 3 pH sensor 4 Controller 5 Alkaline storage tank 6 Chemical injection pump 7 Reverse osmosis membrane in front stage 8 Reverse osmosis membrane in rear stage 9 Weakly acidic cation exchange resin tower 10 pH sensor 11 Controller 12 Acid storage tank 13 Chemical injection pump 14 Mixing tower of strong acid cation exchange resin and weak acid cation exchange resin

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification code FI C02F 1/44 C02F 1/44 J (58) Investigated field (Int.Cl. ⁷ , DB name) C02F 1/20 B01D 19 / 00 B01D 61/00

Claims (1)

(57) [Claims 1] In a method for producing pure water by adjusting the pH of raw water to 9.2 or more and passing it through a reverse osmosis membrane, before adjusting the pH of raw water. A method for producing pure water, wherein raw water is pretreated by a step of degassing under acidic conditions and an ion exchange step of bringing it into contact with a strongly acidic cation exchange resin.