JPH1085755A - Desalting method using electrodialysis equipment - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To not only improve the recovery rate of desalinated water by avoiding scale trouble, but also to increase the limiting current density to improve the processing performance itself of electrodialysis. A method for desalination using an electrodialysis device is provided. An electrodialysis apparatus in which a cation exchange membrane (C) and an anion exchange membrane (A) are alternately arranged between electrodes (38, 40) to alternately form a desalting chamber (24) and a concentration chamber (34) is treated water other than seawater. In the desalination method using an electrodialysis apparatus for desalinating the salts in the water to be treated by supplying water, the water to be treated is passed only through the desalting chamber 24, and the concentrated water is passed through seawater or brine.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a desalination method using an electrodialysis apparatus, and more particularly to desalination of industrial wastewater from a thermal power plant or a factory located in a seaside area, for example, flue gas desulfurization wastewater. The present invention relates to a desalination method using an electrodialysis device for reusing water for use.

[0002]

2. Description of the Related Art Various factories are studying the collection and reuse of factory wastewater (hereinafter referred to as wastewater) in order to reduce the amount of industrial water used. Generally, the pollution degree of wastewater is low and the concentration of salts contained in the wastewater is 500
When the amount is less than about mg / l, the wastewater is separated and collected as a collection wastewater or a low-salt wastewater and reused as it is or after performing a simple wastewater treatment such as a turbidity treatment.

[0003] However, when the degree of pollution of the wastewater is high and the concentration of salts contained in the wastewater is about 500 mg / l or more, the wastewater is separated and recovered as non-collection wastewater or high-salt wastewater. The wastewater is treated so as to comply with the wastewater standards set forth in, the prefectural ordinances, and even the local agreement, before being released into rivers and seas.

[0004] The discharge rate of non-recovery system wastewater varies from factory to factory. For example, in the case of a thermal power plant in which the amount of factory wastewater generated is large, it differs depending on the power generation scale and the type of fuel, but is generally about 100 to 5000 m ³ per day. It is about. The quality of the effluent is determined by inorganic salts (NaCl, Na ₂ SO ₄ , Ca
Cl _2, CaSO ₄ other, a Mg compound, etc.) is the main component, an organic component is not contained little. And the total salt concentration in the waste water is approximately 2000 to 5000 mg / l. The amount of wastewater collected and reused in the collection system is
It is only 10-20% of the total drainage combined with non-recovery system,
Most of the wastewater is non-collection wastewater, and the recovery / reuse rate is extremely low.

[0005] Against this background, studies have been made to increase the rate of recovery and reuse of non-recovery wastewater. However, in order to reuse the wastewater, in addition to the water quality equivalent to the effluent water, the salt concentration and the industrial water level are reduced. Specifically, it is a necessary condition to secure about 500 mg / l or less, and desalting treatment is required. In addition, regardless of the desalination treatment from wastewater, desalination treatment is also required when, for example, groundwater containing salts is used as improvement water.

[0006] Desalination methods include an evaporation method, a reverse osmosis membrane method, an electrodialysis method, and an ion exchange method. When the salt concentration is low, the evaporation method requires a large amount of energy, so that the reverse osmosis method is used. The membrane method, the electrodialysis method, and the ion exchange method are advantageous in cost. Among them, the electrodialysis method has attracted attention because it has a small mass transfer amount and is advantageous in terms of energy.

FIG. 5 shows a desalination method using a conventional electrodialysis apparatus 1.
After being filtered, the water is sent to the desalting water tank 3, and is distributed and supplied from the desalting water tank 3 to the desalting chamber 4 and the concentrating chamber 5 formed by the ion exchange membrane. Dialysis (transfer) of the dissolved salts from the desalting chamber 4 to the enrichment chamber 5
Let it. Thereby, desalted demineralized water is generated in the desalination chamber 4, and concentrated water in which salts are concentrated is generated in the concentration chamber 5. Then, by recovering the desalinated water having a reduced salt concentration generated in the desalination chamber 4, it can be reused as industrial water. On the other hand, the concentrated water concentrated in the concentration chamber 5 is sent to the wastewater treatment device 7 via the concentrated water tank 6, and the concentrated wastewater regulated component, which has exceeded the wastewater standard value, is again reduced to the discharge standard or less by the wastewater treatment, and then discharged. Is done.

[0008]

However, the conventional desalting method using an electrodialysis apparatus has the following disadvantages. In the case where a large amount of scale components such as CaSO ₄ is contained, as in the case of treated water, particularly flue gas desulfurization wastewater generated in a large amount in a thermal power plant, if the concentration ratio in the concentration chamber is too high, scale will precipitate. In addition, the scale adheres to the ion exchange membrane and deteriorates the membrane performance. Therefore, in order to avoid this scale trouble, it is necessary to reduce the concentration ratio of the salts on the enrichment chamber side, and for that purpose, it is necessary to increase the amount of wastewater supplied to the enrichment chamber side. As a result, the supply amount of the wastewater supplied to the desalination chamber side is relatively reduced, so that the recovery rate of the desalinated water is reduced, and there is a disadvantage that the recovery rate is usually 50% or less.

On the other hand, the amount of concentrated water generated is increased to increase the amount of wastewater supplied to the enrichment chamber, and the concentration of wastewater-regulating components in the concentrated water is too high to be discharged as it is, so that a large amount of wastewater is discharged in the subsequent process. Wastewater treatment of concentrated water is required. As a result, there is a drawback that the wastewater treatment cost increases. In order to enhance the processing performance in electrodialysis, it is preferable that the current density be as high as possible within the range of the limit current density or less. However, since the limiting current density is proportional to the salt concentration and inversely proportional to the thickness of the diffusion layer, there is a disadvantage that when the thickness of the diffusion layer is constant, it is influenced by the salt concentration in the wastewater as the water to be treated.

The present invention has been made in view of such circumstances, and not only can the scale trouble be avoided and the recovery rate of desalinated water can be drastically improved, but also the critical current density can be increased. It is an object of the present invention to provide a desalination method using an electrodialysis apparatus that can enhance the processing performance itself of electrodialysis.

[0011]

According to the present invention, in order to achieve the above-mentioned object, a cation exchange membrane and an anion exchange membrane are alternately arranged between electrodes to alternately form a desalting chamber and a concentrating chamber. In a desalination method by an electrodialysis device that supplies water to be treated other than seawater to a dialysis device to desalinate salts in the water to be treated,
The treatment water is passed only through the desalting chamber, and seawater or brine is passed through the concentrating chamber.

According to the present invention, the water to be treated other than seawater passes only through the desalination chamber, and the seawater or brine which is salt-containing water different from the water to be treated passes through the enrichment chamber. As a result, substantially all of the water to be treated can be recovered as desalinated water. In this case, since seawater can be obtained abundantly at low cost, the seawater passing through the concentration chamber is reduced to such an extent that scale components dialyzed from the water to be treated in the desalination chamber into the seawater in the concentration chamber do not precipitate in the concentration chamber. If a large amount of is supplied, the scale trouble can be reliably prevented, and the running cost is hardly required. In addition, if the supply amount of concentrated seawater concentrated in the enrichment chamber, that is, seawater or brackish water, is controlled to be below the discharge standard, the seawater or brackish water is drained. There is no need to release.

Further, by passing seawater or brackish water having a high salt concentration through the concentration chamber, the critical current density can be increased, so that the processing performance of the electrodialysis apparatus can be improved.

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a desalting method using an electrodialysis apparatus according to the present invention. FIG. 1 is an overall configuration diagram illustrating the configuration of an electrodialysis apparatus to which an embodiment of the desalting method of the present invention is applied.

As shown in FIG. 1, the electrodialysis apparatus 10 comprises:
The desalinated water system 12 and the concentrated water system 14 are formed in a separate and independent manner, and the water to be treated as demineralized water is supplied to the desalinated water system 12 and the concentrated water system 14 is supplied to the concentrated water system 14. It is configured to supply seawater that is treated as. The desalination water passage 12 is provided with a filtration device 1 for filtering the water to be treated.
6, the desalinated water tank 18 and the water pump 20 to which the water to be treated filtered by the filtration device 16 is sent, and the water to be treated is circulated between the desalinated water tank 18 and the desalination chamber 24 in the dialysis tank 22. And a circulating pump 26 to be operated.

The concentrated water passage 14 includes a filtration device 28 for filtering seawater, a concentrated water tank 30 and a water pump 32 to which the filtered seawater is fed, and a concentrated water tank 30 and a concentration chamber 34 in the dialysis tank 22. And a circulation pump 36 for circulating seawater. As the filtration devices 16 and 28 used for filtering the water to be treated or seawater, the suspension of suspended matter (hereinafter, referred to as SS) contained in these waters may block the flow path of the electrodialysis device 10 or an ion exchange membrane described later. A filtration device capable of performing microfiltration so as not to cause a decrease in processing performance due to adhesion to the filter is appropriate. The SS concentration of the water to be treated or seawater filtered by the filtration devices 16 and 28 is preferably 0.5 mg / l or less.

As shown in FIG. 2, in the dialysis tank 22, a desalting chamber 24 and a concentrating chamber 34 are provided in an area in which cation exchange membranes C and anion exchange membranes A are alternately arranged. The water to be treated is passed alternately through the desalting chamber 24, and the seawater is passed through the concentration chamber 34. An anode 38 and a cathode 40 are arranged at both ends of the ion exchange membrane group in the dialysis tank 22, and a DC voltage is applied between the anode 38 and the cathode 40. Thereby, the salts in the water to be treated passed through the desalting chamber 24 permeate the cation exchange membrane C, such as Ca ²⁺ and Na ^+, and are dialyzed (moved) toward the concentration chamber 34 to remove Cl ^- ,
Anions such as SO ₄ ^2- permeate the anion exchange membrane A and are dialyzed (moved) toward the concentration chamber 34 to be desalted, while in the concentration chamber 34, the dialyzed salts are dissolved in seawater to form salts. The concentration increases.

Next, the desalination method of the present invention using the electrodialyzer 10 configured as described above will be described with an example in which desalination of flue gas desulfurization wastewater containing a large amount of scale components as water to be treated. . Only the flue gas desulfurization effluent filtered by the filtration device 16 is passed through the desalination water passage 12, and circulated between the desalination water tank 18 and the desalination chamber 24 by the circulation pump 26. On the other hand, the seawater filtered by the filtration device 28 is passed through the concentrated water system 14 and circulated between the concentrated water tank 30 and the concentration chamber 34 by the circulation pump 36. When a DC voltage is applied between the anode 38 and the cathode 40 in this state, the salts in the flue gas desulfurization effluent are dialyzed (moved) from the desalting chamber 24 to the concentration chamber 34 side. As a result, the salts in the flue gas desulfurization effluent passed through the desalination chamber 24 are desalted to produce desalinated water, while the salts are concentrated in the seawater passed through the concentration chamber 34 to form concentrated water. That is, seawater with increased salt concentration is produced. Then, a part of the desalted demineralized water is recovered from the desalination water tank 18, and fresh flue gas desulfurization wastewater is supplied to the desalination water tank 18 by the recovered amount.

As described above, in the desalination method using the electrodialysis apparatus of the present invention, the flue gas desulfurization effluent which is the water to be treated is supplied to the desalination chamber 2
4, the flue gas desulfurization effluent is not distributed between the desalting chamber 24 and the concentrating chamber 34 as in the prior art, so that substantially all of the water to be treated can be recovered as desalinated water. In this desalination treatment, a large amount of seawater is supplied to the enrichment chamber 34 so that scale components dialyzed from the flue gas desulfurization wastewater on the desalination chamber 24 side into the seawater on the enrichment chamber 34 do not precipitate in the enrichment chamber 34. Thus, a scale trouble can be reliably prevented. Specifically, it is assumed that all of the salts contained in the flue gas desulfurization effluent, which is the water to be treated, have moved to the seawater on the enrichment chamber 34 side, and the supply amount of the flue gas desulfurization effluent to be supplied to the desalination water passage 12 is the same. It is only necessary to supply a certain amount of seawater to the concentrated water system 14. Thus, a scale trouble can be avoided, and stable operation can be performed. In addition, since seawater can be easily obtained at a low cost with a large amount of seawater, the running cost is hardly required. Generally, thermal power plants and factories are located in the coastal area, so seawater is available at low cost and abundantly.

Further, if the supply amount of seawater supplied to the concentrated water system is controlled so that the concentration of the wastewater regulatory component in the seawater concentrated in the concentration chamber 34 does not exceed the discharge standard, the seawater can be discharged to the sea as it is. Can be. Therefore, it is not necessary to subject the concentrated seawater to wastewater treatment in a subsequent step, so that the running cost is greatly reduced. In addition, by passing seawater having a high salt concentration through the concentration chamber 34, the critical current density can be increased, so that the processing performance of the electrodialysis apparatus 10 can be improved. In this case, it is more preferable to use the used seawater for the cooling water which is used as the cooling water and which has been heated and used as the seawater passing through the concentration chamber 34. The reason for this is that, in addition to being able to effectively use the used seawater for cooling water that has been conventionally discarded, by using heated seawater, the electric resistance in the dialysis tank 22 can be reduced and the current efficiency can be improved. Therefore, the processing performance can be improved.

In this embodiment, the desalinated water system 12
Although the example of the flue gas desulfurization effluent is described as the water to be supplied to the tank, the present invention is not limited to this, and may be applied to any salt-containing water other than seawater. In particular, the desalination method of the present invention is most suitable for salt-containing water, such as flue gas desulfurization effluent, which contains a large amount of hardly soluble salts such as CaSO _{4 and} is likely to cause scale trouble.

The salt-containing water passing through the concentrated water passage 14 is not limited to seawater, but may be, for example, brine, which is a saline solution in which salt is not saturated. . In short, any electrically conductive aqueous solution can be used as long as it can be obtained abundantly at low cost.

[0023]

EXAMPLE Next, an example in which a desalting method using the electrodialysis apparatus of the present invention was tested using the above-described electrodialysis apparatus of FIG. 1 will be described. As the water to be treated, use the flue gas desulfurization effluent of the thermal power plant, and use the seawater collected from near the thermal power plant,
It carried out by the desalination method by the electrodialysis apparatus of the present invention in which the entire amount of wastewater was passed through the desalting chamber and the entire amount of seawater was passed through the concentration chamber.

The specifications of the experimental apparatus, the experimental conditions, and the effluent and seawater qualities are as shown in (Table 1) and (Table 2).

[0025]

[Table 1]

[0026]

[Table 2] As can be seen from Table 2, the wastewater contains 456 mg / l of calcium ion (Ca ²⁺ ) and 1000 mg / l of sulfate ion (SO ₄ ²⁻ ) as scale components.
In seawater, Ca ²⁺ contains 350 mg / l and SO ₄ ^2-
00 mg / l. The operation was performed with a salt concentration (TDS) in the wastewater of 3540 mg / l and a control target value of the salt concentration of the treated water as the desalinated water of 400 mg / l or less.

The operation was performed at a constant supply rate of 5 liters / hour for both the drainage and seawater supply rates, and a continuous operation was performed for 500 hours with a constant charge. As a comparative example,
Using the same wastewater as in the example, as shown in FIG. 5, the wastewater was distributed by a conventional desalting method using an electrodialysis apparatus in which the wastewater was distributed to a desalination chamber and a concentration chamber. Was.

As the measurement data for evaluating the experimental results, the current density, the current efficiency and the salt concentration of the demineralized water in both the examples and comparative examples can be grasped as changes over time in 500 hours of continuous operation. FIG. 3 shows an experimental result of an example performed by the desalting method of the present invention, and FIG. 4 shows an experimental result of a comparative example performed by the conventional desalting method.

In the case of this embodiment, as apparent from FIG. 3, the current density, the current efficiency, and the salt concentration of the deionized water are the same as those at the start of operation even after 500 hours, so that stable operation can be performed. Was completed. Further, the salt concentration of the obtained demineralized water was 400 mg / l or less, which was sufficiently satisfactory as industrial water (it can be used if the salt concentration is 500 mg / l or less).

The recovery rate of the demineralized water was calculated by measuring the flow rate per unit time by temporarily storing the generated demineralized water in a storage tank. As a result, a recovery rate of 97% or more was always maintained. This is a dramatic improvement since the conventional desalting method is about 50%. After the experiment was completed, the electrodialysis apparatus was disassembled, and the presence or absence of scale components in the anion exchange membrane was visually observed, but no scale deposition was observed.

On the other hand, in the case of the comparative example, as is clear from FIG. 4, the current density and the current efficiency decreased from about 30 hours after the start of the operation, and the salt concentration of the demineralized water gradually increased. A slight cloudiness began to occur in the concentrated water. After the experiment, the electrodialysis device was disassembled and the presence or absence of scale components in the anion exchange membrane was visually observed to confirm whether or not the cloudiness was a scale.The white crystals adhered to the ion exchange membrane. I was As a result of collecting and analyzing the crystals, it was found that the scale was a scale of gypsum dihydrate (CaSO ₄ .2H ₂ O).

[0032]

As described above, according to the desalination method using the electrodialysis apparatus of the present invention, the water to be treated is passed only through the desalination chamber, and the seawater or the brine is passed through the concentration chamber. Therefore, almost all of the water to be treated can be recovered as desalinated water. Therefore, the recovery rate of the desalinated water can be dramatically improved as compared with the desalination method using the conventional electrodialysis apparatus.

In this desalting treatment, a large amount of seawater or brine is introduced into the concentration chamber so that scale components dialyzed from the water to be treated in the desalination chamber into seawater or brine in the concentration chamber do not precipitate in the concentration chamber. With this supply, scale trouble can be reliably prevented. Accordingly, stable operation can be performed, and scale does not adhere to the ion exchange membrane, so that the life of the ion exchange membrane can be extended.

Further, if the supply amount of seawater or brackish water to be supplied to the enrichment chamber is controlled so that the concentration of the wastewater regulatory component in the concentrated seawater or brackish water does not exceed the discharge standard, the seawater is directly discharged to the sea. Can be. Therefore, it is not necessary to perform the wastewater treatment of the concentrated water unlike the related art, and the running cost can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing a configuration of an electrodialysis apparatus to which a desalting method of the present invention is applied.

FIG. 2 is an explanatory view illustrating the inside of a dialysis tank of the electrodialysis apparatus.

FIG. 3 is a graph showing the results of examples performed by the desalting method of the present invention.

FIG. 4 is a graph showing the results of a comparative example performed by a conventional desalting method.

FIG. 5 is an overall configuration diagram showing a configuration of an electrodialysis apparatus to which a conventional desalting method is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Electrodialysis apparatus 12 ... Demineralized water system path 14 ... Concentrated water system path 16, 28 ... Filtration apparatus 18 ... Demineralized water tank 22 ... Dialysis tank 24 ... Demineralization room 30 ... Concentrated water tank 34 ... Concentrated room 38 ... Anode 40 ... Cathode A: Anion exchange membrane C: Cation exchange membrane

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroshi Uchiyama 1-1-1 Uchikanda, Chiyoda-ku, Tokyo Inside Hitachi Plant Construction Co., Ltd.

Claims (4)

[Claims] The present invention relates to an electrodialysis apparatus in which a cation exchange membrane and an anion exchange membrane are alternately arranged between electrodes to alternately form a desalting chamber and a concentrating chamber. In a desalination method using an electrodialysis device for desalting salts in treated water, the treated water is passed only through the desalting chamber, and seawater or brine is passed through the concentrating chamber. Desalting method using an electrodialysis device. 2. The desalination method according to claim 1, wherein said seawater is used seawater for cooling water which has been used as cooling water and has been heated. 3. The desalination method using an electrodialysis apparatus according to claim 1, wherein the water to be treated contains a sparingly soluble salt such as calcium sulfate such as flue gas desulfurization wastewater. . 4. The method according to claim 1, wherein the amount of water for passing said treated water to a desalination chamber and the amount of water for passing said seawater to said concentration chamber are substantially equal. Desalting method using an electrodialysis device.