JP2002001069A - Method for producing pure water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce pure water which is high in purity and stable in quality by improving the efficiency of CO2 elimination in a membrane deaerator, in a method for producing pure water by the use of R0 apparatuses and the membrane deaerator. SOLUTION: In the method for producing pure water, raw water, after being conditioned to have pH 4-7.5 and CO2 concentration of 10 mg/L or below, is passed through the R0 apparatuses 2 and 3 to be subjected to multi-stage R0 desalination treatment and passed through the membrane deaerator 4 to be deaerated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing pure water, and more particularly to a method for desalination using a reverse osmosis membrane desalination apparatus (hereinafter referred to as "RO apparatus") and deaeration processing using a membrane deaeration apparatus. The present invention relates to a method for producing pure water having high purity and stable water quality by increasing the CO ₂ removal efficiency in a membrane deaerator in an adopted pure water production method.

[0002]

2. Description of the Related Art Pure water used in a semiconductor manufacturing process or the like is
The following method has been conventionally proposed as a method for producing by reverse osmosis membrane desalination treatment (hereinafter referred to as “RO treatment”). A method in which acid is added to raw water such as city water to perform decarboxylation treatment, then alkali is added, and water is sequentially passed through RO devices arranged in two stages in series to perform two-stage RO treatment. After deaeration with a deaerator,
A method in which an alkali is added and water is sequentially passed through RO devices arranged in series in two stages to perform a two-stage RO treatment. That is, the final influence on the specific resistance of ultrapure water is mainly a carbonic acid (CO ₂ ) component ( (Carbonic acid, carbonate ion, bicarbonate ion), since it is important in the production of ultrapure water to effectively remove the carbonic acid component.
In the method (1), prior to the RO treatment, an acid is added to the raw water to convert the carbonic acid component in the raw water into CO ₂ , and the decarbonation is performed using a decarbonation tower or a membrane deaerator. Further, in any way, HCO ₃ the carbonate component in water and adding alkali ^- turned into, are removed by RO device.

[0003]

However, in the above-described conventional method for producing pure water, ordinary salts are sufficiently removed, but it is not always sufficient from the viewpoint of removing carbonic acid components. That is, in the method (2), carbon dioxide is removed by a decarbonation tower, but CO ₂ removal in the decarbonation tower may be insufficient. In the method ( ₂₎ , a membrane deaerator for CO ₂ removal is provided by an RO apparatus. In some cases, the deaeration film may be contaminated to cause performance degradation, and in any case, CO ₂ removal is not stable.

[0004] The present invention solves the above-mentioned conventional problems and provides an RO
In a method for producing pure water using an apparatus and a membrane deaerator, it is an object to provide a method for increasing pure CO ₂ efficiency in a membrane deaerator and producing pure water with high purity and stable water quality. I do.

[0005]

According to the method for producing pure water of the present invention, the water to be treated has a pH of 4 to 7.5 and a CO ₂ concentration of 10 mg / water.
After adjusting to L or less, water is sequentially passed through a reverse osmosis membrane desalination device arranged in series in two or more stages to perform desalination treatment, and then water is passed through a membrane deaeration device to perform deaeration treatment. Features.

[0006] In the present invention, the CO₂Concentration 10
mg / L or less ₂To remove
CO in the downstream membrane deaerator₂Efficient with reduced load
Na CO₂Can be removed. That is, CO in the membrane deaerator is
₂Removal is performed by HCO depending on pH conditions.₃ ⁻⇔CO₂+ H₂O
Depends on the dissociation equilibrium of ₂
The lower the concentration, the greater the CO₂The concentration is 10mg /
L₂By reducing
Efficient CO in membrane deaerator₂Can be removed.

In the present invention, a first-stage RO device (hereinafter referred to as a “first RO device”) in a multi-stage RO process preceding a membrane deaerator.
Device ". In the method (2), salts and TOC components are removed from the relatively low-pH water in which CO ₂ is thus reduced, and a part of ionized CO ₂ is removed. In addition, the second-stage RO device (hereinafter, “second RO device”)
It may be called. ) Or later RO devices,
In addition, the remaining trace amount of salts is removed to increase the purity.

As described above, the CO₂Reduce the concentration
Water from which most of the salts have been removed by multi-stage RO treatment of two or more stages
Is treated with a membrane deaerator to ensure stable and efficient
CO ₂Removal is possible, and CO₂Removes almost completely
be able to. In other words, the process in this membrane deaerator
Of the water to be treated₂Concentration reduced to 10mg / L or less
In addition, the pH of the water flowing into the membrane deaerator
Since the pH is suitable for degassing, CO 2₂Surely remove
You can leave. In addition, this membrane deaerator is dissolved
Extremely high purity and high specific resistance because oxygen can also be removed
Pure water with stable water quality can be obtained.

[0009] Since water whose purity has been increased by the multi-stage RO treatment flows into the membrane deaerator, the performance of the deaeration membrane is deteriorated due to membrane contamination as in the case of the membrane deaeration treatment prior to the multi-stage RO treatment. No problem. Also, since only the permeated water of the multi-stage RO is treated, the amount of treated water corresponding to the amount of RO concentrated water discharged to the outside is small, and the load on the membrane deaerator is smaller than when the entire treated water is subjected to the membrane deaeration treatment. And the processing performance can be stably maintained, reducing the number of degassing membranes,
The size of the membrane deaerator can be reduced.

According to the method of the present invention, although it depends on the quality of raw water, for example, by treating city water as raw water, raw water having a CO ₂ concentration of approximately 60 mg / L can be reduced to 10 mg / L or less in the CO ₂ adjusting step. 3m with permeated water of the first RO device
High purity water having a specific resistance of about 3 to 5 MΩ · cm can be obtained at a g / L and a CO ₂ concentration of 50 μg / L or less after membrane degassing.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a system diagram showing an embodiment of the pure water production method of the present invention.

In this embodiment, first, pH, CO ₂
The concentration adjusting means 1 adjusts the raw water to pH 4 to 7.5 and reduces the CO ₂ concentration to 10 mg / L or less.

The water to be treated in the present invention is municipal water, industrial water, well water, various process wastewaters, or a mixture thereof. These waters may be separated by a UF (ultrafiltration) membrane if necessary. The turbidity treatment is performed by a device or the like.

There are no particular restrictions on the means 1 for adjusting the pH and CO ₂ concentration of the raw water, but the following means can be mentioned. If necessary, the pH is adjusted, and CO ₂ is removed by countercurrent contact with air in a decarbonation tower. If necessary, adjust the pH and remove R
CO ₂ is removed with an O (may be a low desalting membrane) membrane. Adjust the pH as needed and dilute the CO ₂ concentration by mixing with low CO ₂ containing water. Here, low CO ₂ content water (CO
_As water containing almost no ₂ ), recovered water from a semiconductor manufacturing process can be used. In other words, the ultrapure water used in the semiconductor manufacturing process contains a small amount of cleaning chemicals, but is almost similar to the quality of pure water. Is done. This recovered water is CO
_Since almost no ₂ is contained, the concentration of CO ₂ can be reduced by mixing the recovered water with the raw water.

The pH adjusting means may be, for example, acid addition or water passing through an H-type cation exchange resin packed column. The pH value is adjusted so that the pH of the water supplied to the first RO device 2 becomes 4 to 7.5. Moreover, CO ₂ concentration is carried out to be equal to or less than 10 mg / L.

In the present invention, CO 2 is supplied to the membrane deaerator 4 in the subsequent stage.
_In this pH and CO ₂ concentration adjusting step, the CO ₂ concentration is 10 mg / L or less, for example,
It is sufficient that CO ₂ can be removed to about 6 to 10 mg / L, and it is not necessary to remove CO ₂ to an extremely low concentration.

Therefore, the pH does not need to be so low that all of the carbonate ions and bicarbonate ions are changed to carbonic acid, and may be such that the pH of the inflow water of the first RO device 2 becomes 4 to 7.5. Preferably, the pH is weakly acidic, about 6 to 7.

If CO ₂ remains more than 10 mg / L, the load on the subsequent membrane deaerator 4 increases,
Since CO ₂ removal may be insufficient, CO ₂ concentration is adjusted to below 10 mg / L.

The pH and CO ₂ concentration adjusting means 1 adjust the pH
The water adjusted to 7.5 and a CO ₂ concentration of 10 mg / L or less is then sequentially passed through the first RO device 2 and the second RO device 3 for two-stage RO treatment.

Of these, the first RO device 2 mainly removes salts and at the same time removes TOC components. Also, p
A part of the CO ₂ ionized by H-acid is removed. The RO film of the first RO device 2 is not particularly limited in material, processing capacity, and the like, and an RO film of a polyamide system, a cellulose acetate system, or the like can be used.

The second RO device 3 removes trace amounts of salts still remaining in the permeated water of the first RO device 2 to increase the purity. In the second RO device 3, salts, particularly N, which is easy to leak
To ensure removal of a ^+, it is preferred membrane surface zeta potential uses zero or more positive charged membrane as RO membrane, thereby, a high purity can be reliably prevented permeation of cations such as Na ⁺ Of treated water can be obtained. As such an RO film, "ES10C" manufactured by Nitto Denko Corporation, "SU-900" manufactured by Toray Industries, Inc., or the like can be used.

In the method shown in FIG. 1, the first RO device 2 and the second RO device
Although the RO device 3 performs a two-stage RO process, the RO process only needs to be performed in two or more stages.
Processing may be used. Generally, two-stage or three-stage RO processing is performed in consideration of the apparatus cost and the apparatus scale. When performing a three-stage RO process, it is preferable that the RO film in the final stage be the above-mentioned positively charged film.

Incidentally, the RO apparatus is generally constructed by connecting a plurality of membrane modules each having a built-in RO membrane element in order to increase the water recovery rate. For example, as in an RO device 14 used in an example described later, three membrane modules 14A
To 14C, the membrane modules 14A and 14B are arranged in parallel, and the concentrated water is processed by the membrane module 14C.
May be taken out as treated water, and the concentrated water of the membrane module 14C may be discharged to the outside of the system.
A and 15B are provided to constitute a second RO device, and the concentrated water of the membrane module 15A is treated by the membrane module 15B, and the permeated water of each of the membrane modules 15A and 15B is taken out as treated water, and the concentrated water of the membrane module 15B is removed. You may make it discharge | emit out of a system. Of course, the RO device in each stage may be constituted by only one membrane module.

The water subjected to the two-stage RO treatment in this manner is:
Next, deaeration is performed by the membrane deaerator 4. This membrane deaerator 4
There is no particular limitation as long as it has a means for performing gas-liquid separation on the surface of the degassing membrane and degassing the gas phase by applying a vacuum to the gas phase.

In this membrane deaerator 4, pH, CO
_The salt is almost removed by the two-stage RO treatment using the _two- concentration adjusting means 1 and the first and second RO devices 2 and 3, and the CO
₂ Water with a low concentration and moderate pH can be treated to remove CO ₂ almost completely.

In order to more surely remove CO ₂ in the membrane deaerator 4, the specific resistance of the outlet water of the membrane deaerator 4 is measured, and based on this value, the inlet of the second RO unit 1 (the The inlet of the 1RO device 2 may be used.
When the O device is provided, the O device may be provided at a stage preceding the RO device at the last stage. It is preferable to adjust the pH by adding a pH adjusting agent in the step ()). That is, the pH of the water injected into the membrane deaerator 4
Is preferably 5 to 6, and when the pH value exceeds 6, CO
_{2 The} removal rate decreases. Here, the pH adjuster is not particularly limited, and mineral acids such as HCl and H ₂ SO ₄ and NaO
An alkali such as H or KOH can be used. Since the added pH adjuster is removed before the RO device at the final stage, there is no problem of lowering the specific resistance due to the addition of the pH adjuster.

In order to improve the efficiency of removing CO ₂ in the membrane deaerator 4, an inert gas may be mixed into the water flowing into the membrane deaerator 4. The inert gas used is A
include r and _{N 2} but, _{N 2} is particularly preferable in terms of cost. If the amount of the inert gas blown is too small, the effect of improving the CO ₂ removal rate by blowing the inert gas is small, and if the amount is too large, the cost is increased. It is preferably set to 3 to 6%.

The water thus degassed is then treated in a non-regenerative or regenerative mixed-bed ion exchange resin tower 5 to further increase its purity, and then to a system such as a subsystem or a place of use. Water is sent outside. The ion exchange resin tower 5 is not necessarily required, and the treated water of the membrane deaerator 4 may be used as it is as the final treated water.

Further, in the present invention, in order to improve water quality, a sterilizing device,
An optional treatment device such as an oxidation device, an ion exchange device, an electric desalination device, and a membrane filtration device can be provided. In particular, when the method of the present invention is applied to the primary pure water production of the ultrapure water production process, an ultrapure water production system (generally, UV irradiation, The exchange and membrane separation treatments are provided in this order or in any order.).

[0031]

The present invention will be described more specifically below with reference to examples and comparative examples.

Examples 1 and 2, Comparative Example 1 City water (pH: 7.4, CO ₂ : 38 mg / L, electric conductivity: 160 μS / cm) was treated with the apparatus shown in FIG.
First, the hollow fiber UF (NTU305 manufactured by Nitto Denko Corporation)
0 ", molecular weight cut off: 20,000) After turbidity treatment with the membrane separation device 11, HCl was added to adjust the pH as shown in Table 1, and introduced into the decarbonation tower 12 with a water flow LV: 30 m / hr. Then, decarbonation treatment was carried out by countercurrent contact under the condition of air / water amount: 15 (flow ratio) to obtain water having a CO ₂ concentration shown in Table 1. This water was subjected to a two-stage RO treatment and then to a membrane deaerator.

The RO device of the two-stage RO uses a membrane module having a built-in RO membrane element "ES-20" (4 inches) manufactured by Nitto Denko Corporation for both the first RO device 14 and the second RO device 15. As shown in FIG. 2, the one RO device connects three RO membrane modules 14A, 14B, and 14C, and has an inflow amount of 1000 L / hr and a permeate amount of 700 L / h.
r, treatment was performed with a concentrated water amount of 300 L / hr discharged to the outside of the system. Further, the second RO device 15 connects the two RO membrane modules 15A and 15B as shown in FIG. The concentrated water in the second RO device 15 was returned to the water supply tank 13 on the inlet side of the first RO device 14 and circulated to the first RO device 14.

As the membrane deaerator 16, “Liqui-cell” (for 4 inches) manufactured by Hoechst Japan Co., Ltd. was used for each one.
The three stages are used, and the degree of vacuum is 30 to 35 Torr.
r, Sweep _{N 2} gas flow rate: 30 NL / hr was treated with (the degassing membrane per). The specific resistance of the obtained treated water (outlet water of the membrane deaerator) was examined, and the results are shown in Table 1.

Example 3 A process was performed in the same manner as in Example 2 except that the pH of the permeated water of the first RO apparatus was adjusted to 6.5 to 7 by adding NaOH. Investigation and the results are shown in Table 1.

Example 4, Comparative Example 2 In Example 1, except that NaOH was added to the effluent of the decarbonation tower to adjust the pH to 7.3 (Example 4) or 7.8 (Comparative Example 2). Was treated in the same manner, and the specific resistance of the obtained treated water was examined. The results are shown in Table 1.

[0037]

[Table 1]

Table 1 shows that according to the present invention, high-purity pure water having a large specific resistance can be produced.

[0039]

As described above in detail, according to the method for producing pure water of the present invention, the water to be treated is preliminarily adjusted for pH and CO ₂ concentration, then subjected to multi-stage RO treatment, and then to membrane deaeration treatment. CO ₂ removal in the membrane deaerator can be performed stably and efficiently, and high-purity pure water can be produced.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an embodiment of a pure water production method of the present invention.

FIG. 2 is a system diagram showing a processing apparatus used in an embodiment.

[Explanation of symbols]

1 pH, CO ₂ concentration adjusting means 2 first 1RO device 3 first 2RO device 4 film degassing device 11 UF membrane separator 12 decarbonation tower 13 water tank 14 first 1RO device 15 first 2RO device 16 membrane degasifier

Continued on the front page F term (reference) 4D006 GA03 GA32 KA12 KA52 KA54 KA55 KA56 KA67 KB11 KD11 KE12R KE15R KE19P MA01 MC18 MC54 PA02 PB02 PB05 PB08 PB26 PB64 PC03 4D037 AA03 AB11 BA23 BB05 BB07 CA03

Claims (1)

[Claims] 1. The water to be treated is adjusted to a pH of 4 to 7.5 and a CO ₂ concentration of 10 mg / L or less, and then successively passed through a reverse osmosis membrane desalination apparatus arranged in series in two or more stages to remove water. A method for producing pure water, comprising performing a salt treatment and then passing water through a membrane deaerator to perform a deaeration treatment.