JP2012217943A - Method for desalting and system for desalting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent slime formation on the surface of an ion-exchange membrane or in channels of concentrate brine and desalted water of an electrodialysis apparatus without using a specific sterilizing agent used conventionally in desalting treatment at a low concentration and a low salt rejection rate.SOLUTION: In desalting treatment raw water such as well water and drainage from an infrastructure by using an electrodialysis cell 1, the concentrate brine from the electrodialysis cell 1 is subjected to electrolysis in an apparatus 42 for generating water by electrolysis to produce electrlytic formed acid water, which is introduced into a desalted water circulation system and a concentrate brine circulation system of the electrodialysis cell 1 or a treatment raw water system. Because the electrolytic fromed acid water has a proper sterilizing power, a chemical agent for slime prevention and a supply system thereof are not necessary to be especially prepared.

Description

  The present invention relates to a desalination treatment method and a desalination treatment system.

  For example, when water is purified, there is an electrodialysis method as a technique for removing ionic substances dissolved in water. In this technology, the movement of ions moving to the opposite pole to the positive and negative charges held by voltage by alternately arranging cation exchange membranes and anion exchange membranes in the section of adjacent cells built in between the electrode plates. Is controlled, desalted from the target cell, and concentrated to the other.

  Regarding the desalination technology, there is a reverse osmosis membrane technology using an RO membrane or the like typified by a seawater desalination project. The electrodialysis method described above has a higher recovery rate than the reverse osmosis membrane technology. The RO salt desalting technique has a recovery rate of about 70%, while the electrodialysis method can recover 90% or more. Due to the superiority of the electrodialysis method, introduction into a region where drainage regulations are becoming legislated, particularly in the Middle East region, is being considered. The target treated water is the purification of unused water such as well water and the purification and reuse of infrastructure drainage such as cooling tower blow drainage, which targets water quality that is much lower than seawater salt concentration. Yes.

  In this way, the electrodialysis method can increase the recovery rate of desalted and purified water by increasing the concentration factor, but a substance with low solubility typified by calcium carbonate is deposited on the surface of the ion exchange membrane, and the desalting performance. May decrease. Therefore, operation management in which the pH of concentrated water is lowered using an acidic chemical is generally performed. Concentrated water is ultimately sodium hydroxide (caustic soda), etc., and these will be discarded after pH adjustment again, but the costs for initial, running, and maintenance related to the treatment are always in the desalination process. It is something that needs to be considered.

  On the other hand, microorganism-derived contaminants form a slime on the membrane surface, causing ion migration inhibition and deterioration of desalting performance due to a decrease in desalting flow rate. In order to avoid the performance deterioration due to the slime formation, the bactericide is introduced and the electrodialyzer is disassembled and washed. The chemical cost and maintenance cost for this treatment account for a high proportion of the electrodialysis treatment cost.

The problems of the electrodialysis method described above are listed as follows.
(1) Low pH control is necessary for concentrated water.
(2) The cost required for prevention of slime generated on the ion exchange membrane and removal of the slime is high.
(3) pH adjustment is required when discarding concentrated water.

  As means for solving the above problems, a technique (Patent Document 1) that effectively uses concentrated water discharged by a desalting process has been proposed. In this technology, RO membrane filtration concentrated wastewater discharged by desalination and desalination of seawater is desalted by electrodialysis, and sterilized with a strong oxidizing solution derived from chlorine generated between the electrodialysis anolyte and the electrode plate. Thus, the system aims to overcome the problems in the electrodialysis system. Furthermore, pH adjustment of concentrated water discharged by electrodialysis, hardness treatment with chelate, and electrolysis of the water into membranes produce acid and alkali solutions that are used as pH adjustment solutions in the desalting process It is.

JP-A-6-269777

The technology disclosed in Patent Document 1 has a very high hardness in the concentrated water from the raw water for the purpose of purifying the high-concentration raw water and the low-concentration water quality at the drinking level with a high recovery rate. There is a problem that a lot of equipment is introduced to remove hardness. In detail,
(1) High equipment cost The technology disclosed in Patent Document 1 is a desalination treatment from seawater (electric conductivity of about 5500 mS / m) to a drinking water level having an electric conductivity of about 15 to 30 mS / m. Concentrated water from electrodialysis has a salt concentration that is several times higher than the raw water depending on the recovery rate, and hardness that is a causative agent of scale failure such as performance deterioration and blockage of piping is also concentrated. In order to achieve this, it is essential to install a separate hardness processing device.
(2) Difficulty in controlling the concentration of the electrolyte produced using the anolyte The applied voltage varies depending on the number of cells designed depending on the treatment flow rate and the desalination rate, but the amount of chlorine produced between the electrode plate and the anode is It will change according to the voltage. Furthermore, the amount of chlorine necessary for slime prevention is determined by the concentration rate in RO membrane filtration and the concentration rate in electrodialysis, so the electrolysis conditions necessary for slime prevention and the design conditions for desalination, which is the main purpose, must be set. Matching is a cumbersome task.
(3) Degradation of anode film In the technique disclosed in Patent Document 1, chlorine generated at the anode of electrodialysis is used for sterilization. However, since chlorine has a strong oxidizing action, A chemical solution such as sulfuric acid or sodium sulfate is used in another system, and the reaction at the anode promotes the electrolysis of water, and the anodic membrane between the concentrated layer and the anode chamber is chlorinated using a cation exchange membrane. Adopting a method to reduce contamination from the concentrated layer of product ions as much as possible. It has been reported that anti-slime treatment in an electrodialysis tank can be effective by controlling the residual chlorine concentration to a low concentration that does not significantly affect the ion exchange membrane destruction of about 0.1 mg / l (special No. 2004-8851), a desalination system based on a chlorine production amount of about 0.1 mg / l between the electrode plate and the anode is unrealistic in terms of electric energy cost.

  The present invention has been made in view of the above points, and in performing desalination treatment at a low concentration and a low desalination rate, ion exchange of an electrodialysis apparatus is performed without using a conventional sterilizing agent. An object of the present invention is to prevent the occurrence of slime on the membrane surface and the flow path of concentrated water and demineralized water to solve the above problems.

  In order to achieve the purpose of the previous term, the desalinating method of the present invention uses an electrodialysis tank to electrolyze the concentrated water from the electrodialysis tank in a method of desalinating well water or infrastructure drainage as raw water for treatment. The electrolyzed acidic water generated by the electrolysis is introduced into the desalted water circulation system and the concentrated water circulation system or the treated raw water system of the electrodialysis tank. As used herein, infrastructure wastewater refers to building wastewater, plant wastewater, air conditioning, sanitary water wastewater, cooling tower blow water, kitchen wastewater, sewage or sewage treated in-site.

  According to the present invention, the concentrated water generated when the well water or the infrastructure drainage is electrodialyzed is electrolyzed, and the electrolytically generated acidic water generated at that time is converted into the desalted water circulation system and the concentration of the electrodialysis tank. Since it is introduced into the water circulation system or the treated raw water system, generation of slime can be prevented without using a dedicated sterilizing agent. In addition, since the treated raw water is well water or infrastructure drainage, the concentration of the concentrated water generated when electrodialyzing this is low. Therefore, even if the low concentration concentrated water is electrolyzed, hardness (water Substances that are easily precipitated by cations such as calcium and magnesium) and silica are difficult to precipitate, and hardness adjustment is not required. Therefore, stable electrolysis capability can be maintained with simple equipment as a whole, and desalting treatment can be stably performed over a long period of time.

  The electrolytically generated alkaline water generated by the electrolysis may be supplied to a concentrated waste water system to adjust the pH of the concentrated waste water.

  Moreover, as for the process raw | natural water used by this invention, it is desirable that the electrical conductivity is 40-300 mS / m.

  The desalination treatment system of the present invention is a system for carrying out the desalination treatment method, and an electrolyzed water generating device that electrolyzes a part of the concentrated water from the electrodialysis tank, and the electrolyzed water generation A pipe for introducing the electrolytically generated acidic water generated by the apparatus into the desalted water circulating system and the concentrated water circulating system of the electrodialysis tank, or the treated raw water introducing system, and the electrolytically generated alkaline water generated by the electrolytic water generating apparatus. And a pipe to be introduced into the concentrated waste water system.

  According to the present invention, it is possible to prevent slime from being generated on the surface of the ion exchange membrane of the electrodialysis apparatus and the flow path of concentrated water and demineralized water without using a dedicated sterilizing agent.

It is explanatory drawing which shows the outline of the system | strain of the desalination processing system concerning embodiment.

  FIG. 1: has shown the outline | summary of the system | strain of the desalination processing system concerning embodiment, The electrodialysis tank 1 used as the core of a system is an anion exchange membrane (anion membrane) A and a cation exchange membrane (cation). Film) K are arranged alternately, the cathode chamber 11 is formed at the cathode side end portion, and the anode chamber 21 is formed at the anode side end portion. Between the cathode chamber 11 and the anode chamber 21, the anion exchange membrane (anion membrane) A and the cation exchange membrane (cation membrane) K are alternately arranged. The raw water supplied to the anion exchange membrane (anion membrane) A and the cation exchange membrane (cation membrane) K is desalted by applying a DC voltage between the cathode 2 and the anode 3. Is done.

  The catholyte is circulated between the cathode chamber 11 and the catholyte tank 13 by a pump 12. On the other hand, the anolyte is circulated between the anolyte tank 21 and the anolyte tank 23 by the pump 22.

  The treated raw water is temporarily supplied to the concentrated water tank 14 and partly to the desalted water tank 24 through the pipe 31. Then, the treated raw water supplied to the concentrated water tank 14 is supplied to the electrodialysis tank 1 by the pump 25 together with the treated concentrated water flowing in, and is introduced into the concentrated water circulation system. The treated raw water supplied to the desalted water tank 24 is supplied to the electrodialysis tank 1 together with the treated desalted water flowing in, and is introduced into the desalted water circulation system.

  The concentrated water generated when the desalting treatment is performed in the electrodialysis tank 1 is returned to the concentrated water tank 14 through the pipe 32, and the desalted water generated by the desalting treatment is returned to the desalted water tank 24 through the pipe 33. It is.

  A part of the concentrated water in the concentrated water tank 14 is supplied to the electrolyzed water generating device 42 by the pump 41. The electrolyzed water generating device 42 is a diaphragm type electrolyzed water generating device having a diaphragm, and electrolyzes the concentrated water supplied from the concentrated water tank 14. A part of the electrolytically generated acidic water generated when electrolyzed by the electrolytic water generating device 42 can be returned to the concentrated water tank 14 through the pipes 43 and 44. The other part of the electrolytically generated acidic water can be supplied to the desalted water tank 24 through the pipes 43 and 45. The pipes 44 and 45 are provided with corresponding valves V1 and V2, respectively.

  The electrolyzed alkaline water generated by electrolysis in the electrolyzed water generating device 42 is sent to the pH adjusted water tank 47 through the pipe 46. Concentrated water exhausted from the concentrated water tank 14 is sent to the pH adjusted water tank 47 through a pipe 48. The pH-adjusting water tank 47 is supplied with the pH-adjusting alkaline chemical solution by the pump 52 from the pH-adjusting alkaline chemical solution tank 51, adjusted to pH, and then discharged.

  The concentrated water tank 14 is supplied with a pH adjusting acid chemical solution from a pH adjusting acid chemical solution tank 61 through a pipe 63 by a pump 62. Further, the pH-adjusting acid / chemical solution in the pH-adjusting acid / chemical solution tank 61 can be supplied to the pipe catholyte tank 13 through the pipe 65 by the pump 64.

  The main part of the desalination treatment system according to the embodiment has the above-described configuration. As raw water for treatment, well water and infrastructure wastewater having an electrical conductivity of 40 to 300 mS / m are removed at a recovery rate of about 90%. When electrodialysis is performed in the electrodialysis tank 1 to perform the salt treatment, the electric conductivity of the produced concentrated water is about 400 to 3000 mS / m. This concentrated water contains a sufficient amount of chloride ions necessary for the electrolytic generation of chlorine, and the hardness component possessed is a concentration that does not cause problems in precipitation and fouling as long as the pH is adjusted.

  Therefore, by directly introducing concentrated water to the electrolyzed water generating device 42 without requiring a hardness treatment as in the prior art, electrolyzed acidic water having a bactericidal action derived from chlorine can be generated. Moreover, the electrolyzed water production | generation apparatus 42 can set up control independent of the electrodialysis tank 1 which desalinate-controls. Furthermore, by adjusting the supply concentration by the voltage applied to the electrolyzed water generating device 42, the supply addition amount by the pump 41, and the control of the valves V1 and V2, the desalted circulating water circulated by the pump 25 and the pump 15 are circulated. It is possible to control the residual chlorine concentration of the concentrated circulating water.

  Electrolytically generated acidic water has a higher bactericidal action on the basis of residual chlorine concentration than sodium hypochlorite, which is often used as a bactericidal agent. The slime prevention effect of the ion exchange membrane can be expected. In addition, by adding the electrolytically generated acidic water generated by the electrolyzed water generating device 42 to the concentrated water in the electrodialysis process, it is possible to reduce the amount of acid solution originally added for low pH control. Become. Furthermore, due to the water quality regulation of the wastewater, the concentrated water is adjusted to pH = 5.0 to 9.0 or less (sewage discharge) with an alkaline solution when it is discarded, but is generated by the electrolyzed water generating device 42. The amount of the alkaline solution used can be reduced by adding the electrolytically generated alkaline chemical.

Below, the result of having conducted the experiment of the desalination process using the said system is shown. Performance verification was performed by taking continuous experimental data of 1 m ³ / day × 1 month with the system. The test was conducted as a simultaneous comparison experiment with a conventional desalting method without the electrolyzed water generating device 42, and the amount of pH adjusting reagent, bactericidal solution used and slime generation status were confirmed.

  As comparative system experiment conditions, the amount of sodium hypochlorite added at 200 mg / l was controlled so that the residual chlorine concentration in the desalted water tank 24 and the concentrated water tank 14 was maintained at 0.05 to 0.1 mg / l. In the test system, electrolyzed acid water having a residual salt concentration of 30 to 50 mg / l is generated by adjusting the applied voltage by the electrolyzed water generating device 42, and the residual salt concentrations in the desalted water tank 24 and the concentrated water tank 14 are each 0. The amount added was controlled to be 01 to 0.03 mg / l. This is a condition set by a material survey that strongly acidic electrolyzed water exhibits a sterilizing effect 10 to 20 times at the same concentration as sodium hypochlorite.

  After the experiment, when the electrodialysis was decomposed and the slime production state was visually confirmed, no slime was confirmed in the test system and the comparative system. Further, there was no decrease in the circulating flow rate of desalted water and concentrated water, and no decrease in desalting performance. In a separate experiment in which no bactericidal agent was added, a decrease in the desalination flow rate was observed two weeks after the start of the experiment, and it was visually confirmed that slime was generated in the electrodialysis tank 1 during decomposition and cleaning. ing.

  And while the amount of solution used with a residual chlorine concentration of 200 ppm used as a bactericide in the prior art was 3 kg / ton, electrodialysis was performed only under the condition that electrolytic water (electrolytic acid water, electrolytic alkaline water) was charged in the test system. Confirmed the continuation of steady operation of the electrodialysis tank 1 without generating slime on the surface of the ion exchange membrane (anion exchange membrane A, cation exchange membrane K) in the tank 1 or in the flow path of concentrated water and demineralized water. did. Moreover, about the usage-amount of pH adjustment solution, the reduction effect of about 10% was confirmed.

  The present invention is useful when performing a desalting treatment with a low desalting rate using an electrodialysis method.

DESCRIPTION OF SYMBOLS 1 Electrodialysis tank 11 Cathode chamber 12, 15, 22, 25, 41, 52, 62, 64 Pump 13 Catholyte tank 14 Concentrated water tank 21 Anode chamber 23 Anode solution tank 24 Desalted water tank 42 Electrolyzed water production apparatus 47 pH adjustment water tank 51 Alkaline chemical tank for pH adjustment 61 Acid chemical tank for pH adjustment A Anion exchange membrane (anion membrane)
K cation exchange membrane (cation membrane)

Claims (4)

In a method of desalinating well water or infrastructure drainage as raw water using an electrodialysis tank,
Electrolyze the concentrated water from the electrodialysis tank,
A desalinization treatment method, wherein the electrolytically generated acidic water generated by the electrolysis is introduced into a desalted water circulation system and a concentrated water circulation system or a treated raw water system of the electrodialysis tank. The desalinization processing method according to claim 1, wherein the electrolytically generated alkaline water generated by the electrolysis is supplied to a concentrated waste water system to adjust the pH of the concentrated waste water. The desalination treatment method according to claim 1 or 2, wherein the electric conductivity of the treated raw water is 40 to 300 mS / m. A system for carrying out the desalting method according to any one of claims 1 to 3,
An electrolyzed water generator for electrolyzing a part of the concentrated water from the electrodialysis tank;
Piping for introducing the electrolytically generated acidic water generated by the electrolyzed water generating device into the desalted water circulation system and the concentrated water circulation system of the electrodialysis tank, or the treated raw water introduction system;
Piping for introducing electrolytically generated alkaline water generated by the electrolytic water generator into the concentrated waste water system;
A desalinization treatment system comprising:

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