JP6627943B2 - Pure water production method - Google Patents

Description

  The present invention relates to a method for producing pure water, and more particularly, to an economically advantageous method for producing pure water using a reverse osmosis membrane device (RO) and an electric deionization device (EDI) arranged at a subsequent stage.

  A pure water production system for producing high-purity pure water generally incorporates RO as a pretreatment device and EDI as a deionization device for highly treating the obtained RO permeated water ( For example, Patent Documents 1 to 3).

  By the way, EDI is basically composed of a plurality of sets of anion exchange membranes and cation exchange membranes formed alternately and sequentially formed between an anode chamber having an anode and a cathode chamber having a cathode. It comprises a salt room and a concentration room, and the desalination room contains a mixture of a cation exchanger and an anion exchanger.

  When a direct current is passed from the anode and the cathode, impurity ions in the water to be treated (RO permeated water) are trapped and removed by an anion exchange group and a cation exchange group in each desalting chamber, and pure water is produced. The trapped impurity ions are electrodialyzed by an anion exchange membrane and a cation exchange membrane which are also diaphragms of a desalting chamber, move to an adjacent concentration chamber, are concentrated and discharged.

  The flow direction of the EDI to each of the desalting chamber and the concentrating chamber is not particularly limited. However, from the viewpoint of preventing the entrapment of air, which is a concern when water flows from the top to the bottom, the desalting is performed. It is common for both the chamber and the concentrator to flow upward.

JP 2000-317457 A JP 2001-104959 A JP-A-2002-320971

  By the way, the water to be treated by EDI is liable to be affected by clogging of precipitated components (silica which is a weak electrolyte) on the ion-exchange membrane, so that a water quality limit value is specified. Therefore, a water quality limit value is also provided for the water to be treated by the RO disposed upstream of the EDI, and in order to maintain the water quality limit value of the EDI, the RO is divided into two stages, and the RO recovery rate is reduced. Although some countermeasures are taken, this leads to complication of the processing system and an increase in processing cost. Increasing the RO recovery rate as a measure to reduce processing costs may lead to an increase in the frequency of replacement due to the aging of ROs. Incidentally, the silica concentration of the RO permeated water supplied to the EDI as the water to be treated is usually 1.0 mg / L or less, preferably 1.0 mg / L or less.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to enhance the removal effect of silica in EDI to reduce the flow of RO treated water exceeding the specified water quality limit value for EDI to be treated. It is an object of the present invention to provide an economically advantageous pure water production method that is made possible.

  The present inventors have conducted intensive studies and, as a result, surprisingly, the effect of removing silica differs depending on the direction of water flow to each of the above-mentioned desalting chambers and concentration chambers of EDI. For example, it has been found that the effect of removing silica is significantly enhanced.

  The present invention has been completed based on the above findings, and the gist of the present invention is to provide a method for producing pure water using a reverse osmosis membrane device and an electric deionization device arranged at a subsequent stage, wherein The apparatus comprises a plurality of sets of a desalination chamber and a concentration chamber, which are sequentially formed by alternately arranging an anion exchange membrane and a cation exchange membrane between an anode chamber having an anode and a cathode chamber having a cathode. An electric deionization apparatus in which a mixture of a cation exchanger and an anion exchanger is accommodated in a desalination chamber and a concentration chamber, wherein the mixture of the cation exchanger and the anion exchanger is cation-exchanged. Using an electric deionization apparatus that is a mixture of an ion exchange resin and an anion exchange resin, the water flow direction of each of the above desalting chambers and the water flow direction to each of the above concentration chambers are countercurrent to each other. As water to be treated in a deionizer It resides the silica concentration of the permeate of the reverse osmosis unit to be fed to the pure water production method characterized by the range of 1.5~3.0mg / L.

  Advantageous Effects of Invention According to the present invention, it is possible to reduce the number of water softeners, reduce the number of introduced ROs or improve the RO water recovery rate, and reduce the frequency of replacement when RO is deteriorated. A method for producing pure water that can be operated continuously for a long time with a small amount is provided.

FIG. 1 is a schematic vertical front view of an example of an EDI.

  Hereinafter, the present invention will be described in detail. In the pure water production method of the present invention, RO and EDI arranged at the subsequent stage are used.

  RO is not particularly limited in its type and membrane type, and various types used in the field of water treatment can be used. The same applies to processing conditions such as operating pressure.

  The EDI is the same as the above, and is not particularly limited. For example, the EDI shown in FIG. 1 proposed by the present applicant in Japanese Patent Application Laid-Open No. 2001-137859 can be used.

  EDI (1) shown in FIG. 1 comprises an anion exchange membrane (61) and a cation exchange between an anode chamber (3) having an anode (2) and a cathode chamber (5) having a cathode (4). A plurality of sets of desalting chambers (81), (82)... And concentrating chambers (91), (92).

  That is, a desalting chamber (81) is formed between the anion exchange membrane (61) and the cation exchange membrane (71), and similarly, the anion exchange membrane (62) and the cation exchange membrane (72) To form a second desalting chamber (82). In this way, the anion exchange membrane (61) and the cation exchange membrane (71) are alternately arranged, and in the case of the illustrated apparatus, five desalting chambers are formed. On the other hand, a first enrichment chamber (91) is formed between the cation exchange membrane (71) and the anion exchange membrane (62). Similarly, the cation exchange membrane (72) and the anion exchange membrane (63) are formed. ) To form a second concentration chamber (92). Thus, in the case of the illustrated apparatus, four concentrating chambers are formed. The mixture (A) of the cation exchanger and the anion exchanger is housed in each of the five desalting chambers. Also, the mixture (a) of the cation exchanger and the anion exchanger is stored in each of the four concentrating chambers.

  The EDI (1) shown in FIG. 1 has a feature of being excellent in electrical stability because the mixture (a) of the cation exchanger and the anion exchanger is stored in the concentration chamber as described above.

As the ion exchange membrane for forming the desalting chamber and the concentrating chamber, those used in ordinary EDI are used. For example, trade names “Selemion (Asahi Glass Co., Ltd.)”, “Neosepta (Tokuyama)” And Aciplex (Asahi Kasei Corporation).

  As the ion exchanger filled in the above-mentioned desalting chamber, ion-exchange fibers processed into a nonwoven fabric of a predetermined thickness in addition to the ion-exchange resin used for the desalination treatment during the production of ordinary pure water are used. Can be used. The ion exchange resin is appropriately selected from ion exchange resins used in ordinary pure water production. For example, as the strongly acidic cation exchange resin, "Diaion (Mitsubishi Chemical Corporation) SK1B", "PK208" and the like, and as the strongly basic anion exchange resin, "Diaion SA10A", "PA316" And the like. Specific examples of the ion-exchange fiber include those obtained by introducing an ion-exchange group into a composite fiber of a polystyrene-based fiber and an auxiliary agent, those obtained by introducing an ion-exchange group into a polyvinyl alcohol fiber base, and radiation-irradiated polyolefin-based fibers. And commercially available products such as those into which an ion exchange group has been introduced using radiation graft polymerization. The above-mentioned cation exchanger and anion exchanger are generally used in such amounts that the exchange capacities of both are the same.

  Further, the ion exchanger may be used in any of a regeneration type and a salt type, but it is preferable to use a regeneration type in order to speed up the rise of water quality. In addition, as a regeneration type mixed resin of a cation exchange resin and an anion exchange resin, there is, for example, a product “SMT100L” manufactured by Mitsubishi Chemical Corporation.

  The greatest feature of the present invention is that by operating the EDI with the flow direction of each desalting chamber and the flow direction of each of the above-mentioned concentrating chambers as countercurrent directions, the EDI is treated as water to be treated. The point is that the silica concentration of the permeated water of the supplied RO is increased to the range of 1.5 to 3.0 mg / L (preferably 1.5 to 2.5 mg / L). By the above-mentioned countercurrent method, the effect of removing the silica in the EDI is remarkably enhanced (in other words, the silica concentration in the RO permeated water supplied to the EDI as the water to be treated is not severely restricted, and the silica concentration is high as described above. The reason may be in the range of concentration) is presumed as follows.

  In the case of counterflow, the inlet side of the concentrated water and the outlet side of the water to be treated are of course parallel, and the outlet side of the concentrated water and the inlet side of the water to be treated are parallel. At this time, the salt component of the to-be-treated water moves to the concentrated water side by electric desalination, and increases the salt concentration of the concentrated water. This upward tendency is greater on the inlet side of the water to be treated. As a result, in the case of the countercurrent method, even if the salt concentration of the concentrated water increases, the concentrated water can be immediately drained from the outlet, and the concentrated water flows into the concentration chamber (ion exchange membrane, particularly anion exchange membrane). The contact time is short, and it is possible to suppress the precipitation of salt components in the concentration chamber (a cause of an increase in voltage and a decrease in desalination efficiency).

  If the precipitation of the salt component in the concentration chamber can be suppressed, the flow of current is not hindered, the desalting efficiency is easily maintained, and the effect of removing the weak electrolyte (silica component) is improved. In other words, the effect of removing the weak electrolyte (silica component) is maintained by suppressing the precipitation of the salt component inside the concentration chamber, which is the cause of the increase in the voltage and the decrease in the desalination efficiency.

  In the EDI (1) used in the present invention, a mixture of a cation exchanger and an anion exchanger is also contained in the enrichment chamber. The water direction can be either upward flow or downward flow. Therefore, it is also possible to adopt a countercurrent system in which the direction of water flow in the desalting chamber is downward, and the direction of water flow in the enrichment chamber is upward.

  However, as shown in FIG. 1, a counterflow method in which the direction of water flow to the desalination chamber is an upward flow and the direction of water flow to the concentration chamber is a downward flow is recommended. This is because a more stable flow is required in the desalination chamber where ion exchange and ion flow occur, and it is necessary to prevent the opportunity of air entrapment more reliably by making the flow direction upward. This is because it is preferable.

  EDI (1) shown in FIG. 1 operates as follows. Water to be treated (RO permeated water) is supplied to the five desalting chambers in parallel from the desalting chamber side inlet pipe (131). The deionized water (pure water) flows out of the outflow pipe (132) on the desalination chamber side. Concentrated water is supplied to the four concentrating chambers in parallel from the concentrating chamber side inflow pipe (141). The concentrated water supplied to each of the concentration chambers is discharged from the concentration chamber-side outflow pipe (142) as concentrated water in which impurity ions are concentrated. Simultaneously with the supply of the concentrated water to the concentration chamber, water to be treated (electrode water) is supplied from the anode chamber side inflow pipe (121) to the anode chamber (3) and from the cathode chamber side inflow pipe (123) to the cathode chamber ( 5) and discharged from the anode chamber side outflow pipe (122) and the cathode chamber side outflow pipe (124), respectively.

  When a direct current is passed from the anode (2) and the cathode (4) while allowing water to flow through each of the above channels, impurity ions in the water to be treated are mixed in each desalting chamber with a mixture of a cation exchanger and an anion exchanger. Is trapped and removed by the anion exchange group and the cation exchange group of the compound to produce pure water, and the impurity ions trapped in the mixture of the cation exchanger and the anion exchanger are formed on the anion, which is also the diaphragm of the desalting chamber. It is electrodialyzed by the ion-exchange membrane and the cation-exchange membrane, moves to the adjacent concentration chamber, is concentrated, and is discharged from the outlet pipe (142) on the concentration chamber side.

  In the present invention, the RO and the EDI arranged in the subsequent stage are used. However, since the silica removal rate in the EDI is increased, the RO is not only sufficient in one-stage treatment, but also the quality of the water to be treated by the EDI is limited. It is not necessary to lower the RO recovery rate by considering the value regulation more than necessary. Therefore, in order to realize an economically advantageous pure water production method, the silica concentration of the RO permeated water (EDI-treated water) in the present invention is in a much higher range than the conventional 1.0 mg / L or less, ie, , 1.5 to 3.0 (preferably 1.5 to 2.5) mg / L.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples as long as the gist is not exceeded.

Example 1
For the RO, a module “SU-710” (manufactured by Toray Industries, Inc.) was used. The EDI having the structure shown in FIG. 1 and having the following specifications was used.

(EDI specifications)
As an anion exchange membrane, Selemion AMD (manufactured by Asahi Glass Co., Ltd., Selemion is a registered trademark of the company) was used, and as a cation exchange membrane, Selemion CMD (manufactured by Asahi Glass Co., Ltd.) was used. As the ion exchange resin to be filled in the desalting chamber and the concentration chamber, a regenerated mixed resin of a cation exchange resin and an anion exchange resin (a product "Diaion SMT100L" manufactured by Mitsubishi Chemical Corporation: water content of 50% by weight) It was used. The number of the desalination chambers is 45, the number of the concentration chambers is 44, and the number of the desalination chambers is 390 mm in length, 130 mm in width and 2 mm in thickness. The concentration chamber is 390 mm in length, 130 mm in width and 2 mm in thickness.

  Using Yokohama City Water as raw water, treated with activated carbon tower and then treated with RO to obtain permeated water with silica concentration: 1.5 mg / L, electric conductivity: 20 μS / cm, which was used as EDI water as treated water. It was supplied and treated to produce pure water.

  In the EDI operation, a countercurrent method was adopted in which the direction of water flow to the desalination chamber was an upward flow, and the direction of water flow to the concentration chamber was a downward flow. The flow rate of water to the desalting chamber and the concentrating chamber was set to LV 100 m / h, and a DC current of 4 A was applied to the electrode plates of both electrode chambers simultaneously with the flow of water. The water flow time was 700 hours, and the EDI water recovery rate was 90%.

  For comparison, in the operation of the EDI, pure water was produced by performing the same operation as described above, except that the direction of water flow to the concentration chamber was changed to an upward flow and a parallel flow method was employed.

  The silica in the obtained pure water was analyzed, and the silica removal rate in the above-described countercurrent method and that in the cocurrent method were compared. As a result, the silica removal rate in the countercurrent method was 4.9 compared to the cocurrent method. It was twice as high. The analysis of silica was performed by molybdenum blue absorption spectroscopy.

1: EDI
2: anode 3: anode compartment 4: cathode 5: cathode compartment 61: anion exchange membrane 71: cation exchange membrane 81: desalination compartment 91: enrichment compartment 121: anode compartment side inlet pipe 122: anode compartment side outlet pipe 123 : Cathode chamber side inflow pipe 124: cathode chamber side outflow pipe 131: desalination chamber side inflow pipe 132: desalination chamber side outflow pipe 141: enrichment chamber side inflow pipe 142: enrichment chamber side outflow pipe A: cation exchanger and Mixture of anion exchanger a: Mixture of cation exchanger and anion exchanger

Claims (4)

  In a pure water production method using a reverse osmosis membrane device and an electric deionization device arranged in a subsequent stage, as an electric deionization device, a cathode chamber having an anode and a cathode chamber having a cathode are provided. It consists of a plurality of sets of desalting and concentrating chambers which are sequentially formed by alternately arranging an anion exchange membrane and a cation exchange membrane. An electric deionization apparatus containing a mixture, wherein the mixture of the cation exchanger and the anion exchanger is a mixture of a cation exchange resin and an anion exchange resin. The direction of water flow in each of the desalting chambers and the direction of water flow to each of the concentrating chambers are set in countercurrent directions, and the permeated water of the reverse osmosis membrane device supplied to the electric deionization device as the water to be treated. Silica concentration 1.5-3.0mg Pure water production method characterized by the range of L. The method for producing pure water according to claim 1 , wherein the mixture of ion exchange resins is a mixture of a strongly acidic cation exchange resin and a strongly basic anion exchange resin. The pure water production method according to claim 1 or 2, wherein the mixture of the ion exchange resin is a regenerated type mixed resin of a cation exchange resin and an anion exchange resin. The flow direction of water in each of the desalting chambers is an upward flow, and the flow direction of water to each of the enrichment chambers is a downward flow, The method according to any one of claims 1 to 3, wherein Pure water production method.

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