JP2003094063A - Electrodialysis method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology of electrodialysis capable of efficiently obtaining harmless fresh water. SOLUTION: In the electrodialysis method uses a plurality of electrodialytic chamber sets comprising a group of ion exchange membranes having a selective ion permeability, wherein for at least either of a group of electrodialytic chambers comprising ion (electrolyte) enriching sections or a group of electrodialytic chambers comprising ion (electrolyte) removing sections, current efficiency can be improved by allowing a current to flow in a serial direction to the plurality of electrolytic chamber sets having solution passage structures disposed in cascade so as to allow the conductivity (electric conductivity) of each electrolytic chamber comprising the group of electrolytic chambers to change step by step, thereby the harmless fresh water can be efficiently obtained. Furthermore, high-conductivity and harmless fresh water can be efficiently obtained by supplying low-conductivity water containing a water pollutant of high BOD, or the like, to the electrolytic chambers on the ion (electrolyte) enriching section sides and by supplying the high-conductivity water not containing water pollutants such as the sea water to the electrolytic chambers on the ion (electrolyte) removing section sides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique of electrodialysis method suitable for application to the field of wastewater treatment or utilization.

[0002]

2. Description of the Related Art An electrodialysis method in which ions are moved by passing an electric current between two liquids using an ion exchange membrane as a partition wall,
Up to now, it has been rarely applied to the water treatment field, and there was no other application to see except for salt production. The place where it comes near is due to an increase in the electric resistance value of low-concentration salt water. That is, the final water quality (salt concentration) of the treated water was insufficient compared with other methods. Technically speaking, in the conventional electrodialysis methods, salt water having different electric resistance values are arranged in parallel, and the electric current is mainly supplied to a high salt concentration portion having a small electric resistance value and is treated water. It was difficult to increase the salt removal rate of low salt water.

[0003]

One of the main problems to be solved by the present invention is to efficiently and inexpensively obtain low-polluted fresh water (low salt water) that can be reused.
Another challenge is to efficiently remove contaminated water (sometimes in combination with other water treatment technologies).

[0004]

Means for Solving the Problems In electrodialyzing conductive water having different electric conductivity, that is, electrolyte concentration, the present inventor arranges water of each (electrolyte concentration) conductivity in series (in series) to conduct electricity. By doing so, it was found that a large amount of current is applied to low electrolyte concentration water, the electrolyte removal rate is improved, and the above problems can be solved, and the present invention has been accomplished.

The invention of claim 1 of the present application has a plurality of sets of electrodialysis chambers constituted by a group of ion-exchange membranes having selective ion permeability, and by ion transfer by energization,
In a so-called electrodialysis method that causes a difference in ion (electrolyte) concentration between adjacent electrodialysis chambers across an ion exchange membrane, an electrodialysis chamber group or an ion (electrolyte) removal unit that constitutes an ion (electrolyte) concentration unit At least one of the electrodialysis room groups constituting the electrodialysis room group has a liquid passage structure arranged in cascade so that the electric conductivity (electrical conductivity) of each electrodialysis room constituting the electrodialysis room group changes stepwise. The electrodialysis method is characterized by energizing the plurality of sets of electrodialysis chambers in a serial direction.

According to the invention of claim 2 of the present application, in performing electrodialysis using an ion exchange membrane, a low conductivity water containing a nonionic water pollutant is contained in the electrodialysis chamber on the ion (electrolyte) concentrating section side. 2. The electrodialysis method according to claim 1, wherein the electrodialysis chamber on the side of the ion (electrolyte) removing unit is supplied with high-conductivity water containing no nonionic water pollutant.

In the invention of claim 3 of the present application, the low-conductivity water containing the nonionic water pollutant is wastewater, and the high-conductivity water containing no nonionic water pollutant is seawater. The electrodialysis method according to claim 2, wherein:

In the invention of claim 4 of the present application, the low conductivity water containing the nonionic water pollutant is treated water of the sewage, and the high conductivity water containing no nonionic water pollutant is seawater. The electrodialysis method according to claim 2, wherein

According to the invention of claim 5 of the present application, in performing electrodialysis using an ion exchange membrane, waste water containing both a water pollutant that is toxic to microorganisms and a water pollutant that can be decomposed by microorganisms and non-toxic water are used. The wastewater treatment operation is performed individually after separating the water pollutants that are microbially degradable and the water pollutants that are toxic to microorganisms by performing ion transfer between the two. It is a dialysis method.

The invention according to claim 6 of the present application is characterized in that, when performing electrodialysis using an ion exchange membrane, an ion exchange resin is present in at least one electrodialysis chamber liquid. It is an electrodialysis method.

The invention of claim 7 of the present application is the electrodialysis method according to claim 1, characterized in that the ion exchange resin present in the electrodialysis chamber liquid is an amphoteric ion exchange resin.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The first feature of the present invention is to energize each electrodialysis chamber having different conductivity in a serial direction. Due to this limitation, the amount of current passing through each electrodialysis chamber is the same. And the potential gradient in each chamber is proportional to the electrical resistance (reciprocal of conductivity). Then, the water to be treated that requires the removal of the electrolyte is passed through the ion removal section in a cascade form from the high concentration side to the low concentration side, so that the ions (electrolyte) in the treated water after passing through the final dialysis chamber Are efficiently removed. However, arranging all the dialysis chambers in series has a poor current efficiency (especially in the case of high electrolyte concentration), and the rate of decrease in the electrolyte concentration when passing through one dialysis chamber (electrolyte concentration of outflow water / inflow water) Electrolyte concentration) is kept at about 20-30%, 3-4
A practical method is to achieve a desired electrolyte concentration by carrying out staged electrodialysis. The membrane areas of the electrodialysis chambers arranged in series do not have to be the same, and it is preferable to increase the energization area (membrane area) in the portion where the ion concentration is low.

It is a particularly preferable apparatus shape to form an electrodialysis membrane having a pair of anion exchange membrane and cation exchange membrane into a spiral shape, and to supply electricity between the center portion and the outer peripheral portion of the spiral. As described above, in order to allow the water to be treated, which requires the removal of the electrolyte, to pass through the ion removing section in a cascade form from the high concentration side to the low concentration side, a structure in which each dialysis portion is spirally continuous is required. Most desirable. The ion exchange membranes that are currently generally used are in the form of flat plates because they are derivatives of cross-linked polystyrene and are not flexible, but other materials (flexible ion exchange resin) or It is not difficult to form a spiral shape by molding or the like. The spiral has a double spiral structure that has both an ion (electrolyte) concentration part and an ion (electrolyte) removal part. It is desirable that the high-salt-concentration water that requires removal of the electrolyte flows from the central portion toward the outer peripheral portion, and that the low-salt-concentration water portion has a larger current cross-sectional area.

If the ion transfer between the two liquids does not require a concentration operation, it is possible to transfer the ions from the high salt concentration water to the low salt concentration water even if an alternating current is applied, but the final desorption is performed. From the viewpoint of salt concentration, it is desirable that the energizing current be direct current (capable of ion concentration). It is also an embodiment of the present invention to dispose an ion exchange resin inside (in the liquid) of the dialysis chamber for electrolyte removal in order to enhance the efficiency of collecting ions. The shape of the ion-exchange resin in the liquid is not limited to a granular shape, and it is also preferable that the ion-exchange resin in the liquid has a net shape so as to improve the mixed state of the liquid while reducing the liquid passage resistance.
An amphoteric ion exchange resin having both an anionic group and a cationic group in the same molecule (for example, 2-acrylamido-2-
Crosslinkable copolymers of methylpropanesulfonic acid and dimethylaminopropylacrylamide, etc.) are particularly suitable for ion collection.

The main object of the present invention is to obtain useful fresh water by using waste water. To achieve this goal,
It is beneficial to separate various pollutants and obtain harmless water or water that is easily treated. Seawater is clear water except that it contains salt (that is, electrolyte), and can be easily converted into useful water such as drinking water and agricultural water by desalting. Waste water generated from human life mainly contains organic substances (non-electrolytes), and often has a low salt content. As for the treatment performed at the sewage treatment plant, simple sedimentation is used as the primary treatment and biological treatment is used as the secondary treatment. However, even when the secondary treatment is performed, the water quality is still unsuitable for water use and It has been released into public waters such as rivers and rivers. Even if these non-electrolyte-containing wastewaters that cannot be used as water are discharged into the sea before they are discharged, and the electrolyte in the seawater is transferred to the wastewater, it does not lead to pollution of the sea. .
On the other hand, desalinated seawater is reborn as fresh water without pollutants and can be used as water. When the degree of desalting by electrodialysis is insufficient as agricultural water or drinking water, a desired water quality can be obtained by using the reverse osmosis membrane method together. It is clear that this method of transferring electrolytes in seawater to low conductivity water is more energy efficient than desalting by electrodialysis of seawater alone.

The fifth limitation of the present invention is to subject the wastewater containing both the water pollutant having toxicity to microorganisms and the water pollutant capable of microbial decomposition to electrodialysis to separate both water pollutants, and then to separate them. It is characterized by performing a wastewater treatment operation. For example, nitric acid is used for various industrial purposes and is discharged as wastewater, but nitrate ions are subject to eutrophication regulation as a nitrogen source. Nitrate ions can be easily reduced and removed by a biological denitrification method, but when used in industrial applications, they usually contain harmful substances for microbial treatment. For example, when it is used for surface treatment of stainless steel or aluminum, it contains polyvalent metal ions and cannot be biologically treated as it is. It is known that cation exchange membranes for salt production do not pass polyvalent metal ions such as magnesium, and if polyvalent metal ion-containing nitric acid is applied to the above electrodialysis using such cation exchange membranes, nitric acid is harmless. It becomes a low conductivity water and becomes biologically denitrifying. Further, nitric acid from which polyvalent metal ions have been removed can be reused in the factory by adjusting the concentration. Even acids other than nitric acid can be reused by this method.
Further, in many cases, industrial wastewater (for example, semiconductor manufacturing wastewater) involving a chemical reaction contains a large amount of both salts and persistent organic substances. It is desirable that such wastewater be desalted by electrodialysis, then evaporate and concentrate the hardly decomposable organic matter, and finally be incinerated. If desalting is not performed, a large amount of ash is generated and it is considered difficult to process.

[0017]

The basic principle of the present invention is that when electrodialysis is performed, water having different conductivity is arranged in series and energized so that the current value flowing in each chamber is constant, in other words, high conductivity water is supplied. The potential gradient is low and the potential gradient of low conductivity water is automatically adjusted to be high. Since the water flow is also multi-stage in series, the amount of ion movement for a constant current increases in proportion to the number of stages, and high power consumption is possible even if a thin electric wire is used.

[0018]

EXAMPLES Next, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples unless it exceeds the gist.

(Example 1) Anion exchange membranes and cation exchange membranes are alternately arranged to form a dialysis chamber, and every other dialysis chamber is arranged so that liquid can be passed (electrolyte removing section is located next to electrolyte removing section). , The electrolyte concentrating section was connected to the adjacent electrolyte concentrating section) in series, and an electrodialyzer consisting of a pair of multistage dialysis chambers was used to electrodialyze the nitric acid waste liquid generated from the pickling step of stainless steel.
The waste liquid is passed through the electrolyte removal portion side of the device, and the nitric acid transfer water containing no heavy metal is passed through the electrolyte concentration portion side countercurrently with the waste liquid to utilize the selective permeability of the ion exchange membrane. The result of obtaining purified nitric acid containing no heavy metal is shown below. 1. Type of resin composition of ion exchange membrane: polystyrene type 2. Size of ion exchange membrane: width 5 cm x length 40 cm 3. Number of stages in dialysis room: 256 stages 4. Dialysis current: about 27 amps 5. Dialysis voltage: About 200 V DC 6. Nitrate ion concentration in waste liquid: 15% by weight 7. Heavy metal ion concentration in the waste liquid: 3% by weight 8. Nitric acid-transferred water: Tap water Electrodialysis was carried out under the above conditions to obtain 2 m3 of 17 wt% nitric acid in 24 hours, which was then reused.

(Example-2) The nitric acid waste liquid used in Example-1 was diluted 100 times with tap water, and denitrification treatment was carried out with nitric acid reducing bacteria while adding methanol and controlling PH. However, it could not be processed because of heavy metal ions. On the other hand, nitric acid obtained by the electrodialysis of Example-1 was diluted 100 times with tap water, and denitrification treatment was performed with nitrate-reducing bacteria while adding methanol and controlling PH. Nitric acid was successfully reduced to nitrogen gas without being disturbed.

(Embodiment 3) Anion exchange membranes and cation exchange membranes are alternately arranged to form a dialysis chamber, and every other dialysis chamber is provided so that liquid can be passed through (the electrolyte removing portion is placed in the adjacent electrolyte removing portion). , Electrolyte concentrating part is an electrodialyzer consisting of a multi-stage dialysis chamber pair connected in series to the adjacent electrolyte concentrating part), and treated water (water quality; dissolved ignition) Residual concentration: about 450 ppm, ammonium ion concentration: about 25 ppm, SS: 3 ppm
The following) is flowed countercurrently under the following conditions so that the flow rate of the electrolyte removal section side is 10 and the flow rate of the electrolyte concentration section side is 1
Ion transfer was performed. 1. Type of resin composition of ion exchange membrane: polystyrene type 2. Size of ion exchange membrane: width 25 cm x length 40 cm 3. Number of stages in dialysis room: 64 stages 4. Ion-exchange resin filling: Amphoteric resin in 3 chambers on the outlet side of the electrolyte removal unit. Dialysis current: about 27 amps 6. Dialysis voltage: About 200 V DC 7. Nitrogen content in electrolyte removal part outflow water: 1-2 ppm 8. Nitrogen content in electrolyte concentrate outflow water: 180 to 200 ppm Calcium hydroxide was added to the obtained electrolyte enrichment part outflow water to perform ammonia stripping, and most of the nitrogen content in the municipal wastewater could be efficiently removed.

Claims (7)

[Claims] 1. An electrodialysis chamber having a plurality of sets of electrodialysis chambers composed of a group of ion exchange membranes having selective ion permeability, and between the electrodialysis chambers that are adjacent to each other across the ion exchange membrane due to ion transfer by energization. In the so-called electrodialysis method that causes a difference in the concentration of ions (electrolytes),
Electrodialysis room group or ions (electrolytes) that make up the concentration unit
A liquid passage structure in which at least one of the electrodialysis chamber groups forming the removal unit is arranged in a cascade so that the electric conductivity (electric conductivity) of each electrodialysis chamber forming the electrodialysis room group changes stepwise. An electric dialysis method, wherein electricity is applied in series to the plurality of electrodialysis chambers having 2. When performing electrodialysis using an ion exchange membrane, low conductivity water containing a non-ionic water pollutant is supplied to the electrodialysis chamber on the side of the ion (electrolyte) concentrating section, and the ion (electrolyte) The electrodialysis method according to claim 1, wherein high electroconductivity water containing no nonionic water pollutant is supplied to the electrodialysis chamber on the side of the removing unit. 3. The low-conductivity water containing nonionic water pollutants is wastewater, and the high-conductivity water containing no nonionic water pollutants is seawater. The electrodialysis method according to 2. 4. The low-conductivity water containing non-ionic water pollutants is treated water, and the high-conductivity water containing no non-ionic water pollutants is seawater. The electrodialysis method according to claim 2. 5. When performing electrodialysis using an ion exchange membrane, ion transfer is performed between waste water containing both water pollutants that are toxic to microorganisms and water pollutants that can be decomposed by microorganisms and non-toxic water. The electrodialysis method according to claim 1, wherein the wastewater treatment operation is performed individually after separating the water pollutants that can be decomposed by microorganisms and the water pollutants that are toxic to microorganisms. 6. The electrodialysis method according to claim 1, wherein at the time of performing electrodialysis using an ion exchange membrane, an ion exchange resin is present in at least one electrodialysis chamber liquid. 7. The electrodialysis method according to claim 1, wherein the ion exchange resin present in the electrodialysis indoor liquid is an amphoteric ion exchange resin.