JP2723422B2 - Production method of domestic water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a reverse osmosis membrane method and an electrodialysis method using a specified anion exchange membrane to remove nitrate ions from an aqueous solution containing various anions.
Particularly, the present invention relates to a method for producing domestic water suitable for drinking water.

[0002]

2. Description of the Related Art Nitrate ion is a harmful ionic species contained in drinking water. Particularly in recent years, the problem of acid rain based on industrial exhaust gas and the increase of nitrate ions contained in groundwater based on agricultural fertilizer or livestock excreta in low-rain areas have become serious problems. This phenomenon is particularly noticeable in Europe where rainfall is low. In addition, industrial waste containing nitrate ions is increasing, and this is also an important problem from the viewpoint of environmental protection of the earth.

Conventionally, various methods have been known for removing these nitrate ions. For example, the nitrate ion N
A number of techniques have been developed, including a method of decomposing O ₃ ^- into N ₂ and O ₂ , a method of adsorbing and removing with an ion exchange resin, a method of decomposing by anaerobic fermentation, and many others. Examples of the method using a separation membrane include a reverse osmosis membrane method and an electrodialysis method using an ion-exchange membrane. These are important technologies that are properly used for each purpose and industrialized.

[0004]

However, removing nitrate ions present in a dilute salt solution such as groundwater by the above-mentioned prior art has problems to be solved. For example, adsorption and removal of nitrate ions by an ion exchange resin requires a regeneration operation of the resin, which is not only complicated, but also requires disposal of a waste liquid after regeneration. Also,
The membrane separation method is very useful for removing a small amount of nitrate ions in that it can be operated continuously without the need for regeneration, but it is extremely difficult to selectively allow only nitrate ions to permeate the membrane.

[0005] For example, in the reverse osmosis membrane method, dissolved ions are eliminated by a simple operation of simply applying pressure by a membrane, and energy consumption is small.
The resulting demineralized water has almost all ionic substances removed to varying degrees, and is often close to pure water. Therefore,
Such demineralized water is not preferable because it becomes water free of minerals when used for domestic use, especially for drinking.

On the other hand, an electrodialysis method using an ion exchange membrane (hereinafter simply referred to as an electrodialysis method) involves controlling the ion species passing through the membrane by selecting a cation exchange membrane type and an anion exchange membrane type. Is possible. For example, an anion exchange membrane that selectively permeates nitrate ions has already been disclosed in
It is disclosed in Japanese Patent Publication No. 298396. That is, if these anion exchange membranes are used, nitrate ions can be selectively removed through the membrane to obtain demineralized water. However, in the electrodialysis method, since the permeation of ions is carried out by an electric current, the operating current density is significantly reduced when treating water having an extremely low salt concentration. Therefore, when a certain amount of a dilute salt aqueous solution containing nitrate ions is treated by such an electrodialysis method, a large electrodialysis facility is required, resulting in an increase in facility costs.

[0007]

DISCLOSURE OF THE INVENTION In view of the above-mentioned problems, the present inventors have found that a beverage containing only a predetermined amount of nitrate ions from a dilute salt aqueous solution containing nitrate ions and containing a desired mineral component can be obtained. For the production of domestic water suitable for water,
We continued our research. As a result, the present invention has been completed by a simple method combining a reverse osmosis membrane method and an electrodialysis method using an ion exchange membrane.

That is, according to the present invention, a dilute salt aqueous solution containing nitrate ions is separately prepared by (1) desalting by a reverse osmosis membrane method and (2)
Subjected to a desalting by electrodialysis using an ion exchange membrane which is selectively permeable to nitric acid ions, the desalination by reverse osmosis
Demineralized water from which most of the ionic substances have been removed
Nitrate ion below 50ppm by dialysis desalination
Undiluted demineralized water is obtained, and then, in mixing each demineralized water, nitrate ions of the mixed water are reduced by 50 pp.
m and a conductivity of 50 μs (microsiemens) or more.

The dilute aqueous salt solution containing nitrate ions (hereinafter simply referred to as “salt water”) which is the object of the present invention generally has a total soluble salt (TDS) of 2000 p.
pm, especially water used for domestic water including beverages of 1000 ppm or less, nitrate ion is 50 ppm
The above is included.

[0010] In the reverse osmosis method (RO) of the present invention, a conventionally known reverse osmosis apparatus is used without any limitation. For example, a membrane module such as a flat membrane type using a cellulose acetate type asymmetric membrane, a spiral type, a tubular type, a hollow fiber type using a condensed polymer, etc. Alternatively, a charged reverse osmosis membrane having a negative charge, a loose RO membrane operated at a low pressure, and the like are preferably used, but an apparatus using a reverse osmosis membrane having a salt rejection as high as possible is preferable. It is preferable to use a reverse osmosis membrane having a salt rejection of 95% or more, particularly 98% or more, which is generally used in seawater desalination. Therefore, the desalinated water obtained by such a reverse osmosis method is almost pure water although it slightly varies depending on the composition and concentration of the raw water, the type of the reverse osmosis membrane, and the operating conditions.

On the other hand, in the electrodialysis method of the present invention, an anion exchange membrane which selectively permeates nitrate ions is used together with a cation exchange membrane. Generally, a strongly basic anion exchange group and a weakly basic anion exchange group coexist, the exchange capacity is relatively low, the crosslink density is high, and a hydrophobic alkyl chain having two or more carbon chains is bonded to the anion exchange group. An anion exchange membrane is preferably used. That is, such an anion exchange membrane hardly permeates sulfate ions and carbonate ions, and selectively permeates nitrate ions as compared with chloride ions. Therefore,
As anions in all the soluble salts of the raw water, only the nitrate ion mainly decreases, and the other ions hardly change. The cations vary depending on the cation exchange membrane to be used. In a normal cation exchange membrane, multivalent cations such as calcium ions and magnesium ions selectively pass. By using the membrane, monovalent cations are selectively removed through the membrane.

The mixing ratio of the demineralized water obtained by the reverse osmosis method and the electrodialysis method of the present invention is not unconditionally determined by the composition of the demineralized water. Demineralized water is generally mixed at a ratio of 0.5 to 5 times to maintain the nitrate ion of the mixture at 50 ppm or less and the conductivity at 50 μm or more.

According to the present invention, by simply mixing the desalted water obtained by the reverse osmosis method and the electrodialysis method, nitrate ions which are harmful to the human body as the drinking water are set as the reference value.
Domestic water having a necessary amount of minerals can be easily obtained by setting the electric conductivity to 0 ppm or less, preferably 25 ppm or less and the electric conductivity to 50 μs or more.

That is, in the reverse osmosis method of the present invention, demineralized water from which almost all ionic substances have been removed together with nitrate ions can be obtained without considering the loss of minerals. Energy consumption can be extremely reduced by the operation.

Further, in the electrodialysis process of the present invention, depending on the mixing ratio of demineralized water that by the reverse osmosis method, since the amount of limitation of dilute salt solution is processed, the need for large electrodialysis equipment Desalination can be carried out without any need . And, in the present invention,
In electrodialysis, the nitrate ion of deionized water should be 50 ppm or less.
Performed without lowering down. Such electrodialysis
Thus, the electric energy required for desalting at an electric conductivity of 50 μs or more can be reduced.

The outline of the manufacturing method (process) of the present invention will be described with reference to FIG. The water pumped from the well (1) is stored in a tank (2), and after a pretreatment step (3), a part is supplied to an electrodialysis tank (4) to remove water from which sulfate ions have been removed. It is stored in the mixing tank (5).
On the other hand, the desalinated water obtained in the reverse osmosis unit (6) is similarly supplied to the tank (5). Tank (5) Automatically operates the electrodialysis tank (4) or reverse osmosis device (6) so that the nitrate ion of the mixed water is monitored to be 50 ppm or less and the total salt concentration is monitored by a conductivity meter to be 50 μs or more. Then, while maintaining the nitrate ion concentration at 50 ppm or less, the operation is performed so as to obtain the maximum product water.

In order to control the production amount of the desalinated water in the reverse osmosis method as described above, it is generally sufficient to change the operating pressure. In the electrodialysis method, the production amount is changed by changing the current density of the operation. Can be changed. That is,
The operation of the process in the present invention is designed so that the nitrate ion concentration in the mixed production water is 50 ppm or less, and the total mineral amount is 50 μs or more as detected by conductivity,
When the raw water composition fluctuates, the operating conditions of the reverse osmosis method or the electrodialysis method, the production amount of demineralized water, and the like, in order to keep the nitrate ion concentration of the production water at 50 ppm or less and the total mineral content at 50 μs or more, Need to be controlled. For example, when the concentration of nitrate ions in the production water exceeds 50 ppm and the conductivity is 50 μs or more, the production amount of the reverse osmosis device is increased with respect to the production amount of the electrodialysis method to maintain the above-mentioned predetermined value. There is a need to. The nitrate ion concentration in the production water is 50 ppm or less, but the conductivity is 5 ppm.
If it is 0 μs or less, it is necessary to increase the amount of water produced from the electrodialysis method and decrease the amount of water produced from the reverse osmosis method to maintain the above-mentioned predetermined value. Further, the nitrate ion concentration in the production water exceeds 50 ppm, and the conductivity is 50 μs.
In the following cases, the purpose is achieved by lowering the current density in the electrodialysis method, reducing the amount of ions desalted per unit time, increasing the conductivity, and increasing the amount of water produced from the reverse osmosis method be able to.

The above operation is performed by detecting the conductivity of the produced water by a conductivity meter and detecting the nitrate ion concentration by an appropriate nitrate ion electrode , and automatically producing the product from the reverse osmosis device and the electrodialysis device. It is preferable to stop the amount of water, operate, or control operating conditions. Of course,
We regularly sample the production water and use chemical analysis to
The operation may be controlled by knowing the nitrate ion concentration and the conductivity by analysis by ion chromatography.

[0019]

According to the water treatment method of the present invention, a large amount of domestic water which can be used as drinking water from which nitrate ions containing appropriate minerals are removed can be produced inexpensively from water containing harmful nitrate ions. be able to.

[0020]

The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.

Example 1 Nitrate ion: 153 ppm, chlorine ion: 88.5 pp
m, sulfate ion 120 ppm, bicarbonate ion 152.
5 ppm, sodium 230 ppm (TDS744p
pm) is treated by the following reverse osmosis method (1) and electrodialysis method (2) according to the process of FIG. 1, respectively. (Desalinated water) was mixed to produce domestic water. The raw water was filtered beforehand with a sand filter to remove suspended substances.

(1) Reverse Osmosis Method A flat membrane having an asymmetric structure of a cellulose acetate system having a salt rejection of 98.5% was desalted using a spiral-shaped module having an effective area of 24 m ² . The TDS in the resulting product water is 6
ppm, and water from which minerals were almost completely removed was obtained. The production amount was 1 m ³ / hr.

(2) Electrodialysis method Cation exchange membrane (NEOSEPTA CMX) manufactured by Tokuyama Soda Co., Ltd. and anion exchange membrane (NEOSEPTA AN) selectively permeating nitrate ion
R-2) was desalted in pairs.

The electrodialysis layer used was one in which 10 pairs of membranes having an effective current-carrying area of 20 cm ² were incorporated. The desalination of the salt water (raw water) was performed at a current density of ² mA / cm ² while extracting the desalinated water by a partial circulation method. The nitrate ion in the obtained deionized water was 52 ppm, and the TDS was 540 ppm. Demineralized water production is 0.5m ³
/ Hr. The electric energy required for desalination was 0.3 KwH / m ³ .

(3) Production of Domestic Water Demineralized water produced by the reverse osmosis membrane method and the electrodialysis method was mixed to obtain 1.5 m ³ / hr of service water. In this mixture, a nitrate ion electrode is provided to detect nitrate ions,
Also, the amount of water produced from the monitor and the electrodialysis tank and the amount supplied from the reverse osmosis membrane device were controlled by a conductivity meter. as a result,
The nitrate ion concentration in the produced water was kept at 50 ppm or less, and produced water having an electric conductivity between 50 μs and 900 μs was obtained. As an example, the nitrate ion concentration is 17p
pm, conductivity is 450 μs, TDS is 185p
pm. In addition, when the concentration of all ion species including nitrate ion in the raw water rises, the electrodialysis tank is operated as it is, the operation pressure of the reverse osmosis membrane device is increased, and the water generated from the reverse osmosis membrane device is increased. The volume was increased and this was mixed with the water obtained from the electrodialysis cell.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating the process of the present invention.

[Explanation of symbols]

 1. Well 2. Tank 3. Pretreatment step 4. Electrodialysis tank 5. Mixing tank 6. Reverse osmosis device 7. Monitor device

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location C02F 9/00 502 C02F 9/00 503A 503 504E 504 1/46 103

Claims (1)

(57) [Claims] A dilute salt solution containing nitrate ions is separately desalted by (1) a reverse osmosis membrane method and (2) an electrodialysis method using an ion exchange membrane selectively passing nitrate ions. And most of the ions are desalted by reverse osmosis membrane method.
Demineralized water from which toxic substances have been removed
The nitrate ion does not drop below 50 ppm
A method for producing domestic water, comprising obtaining salt water and then mixing and mixing each of the demineralized water with monitoring and controlling nitrate ions of the mixed water to 50 ppm or less and conductivity to 50 μs or more.