JPH10272455A - Pure water production method - Google Patents

Abstract

(57) [Problem] To provide a method for producing pure water by decarbonating raw water and then deionizing the raw water, thereby eliminating the need for alkali addition after decarboxylation and reducing the amount of chemicals used. SOLUTION: Raw water is subjected to decarboxylation treatment under acidic conditions to obtain pH.
Of decarbonation-treated water, and contacted with activated carbon without adding an alkali to the decarbonation-treated water, followed by deionization. [Effect] Using decarbonation means with high CO ₂ removal efficiency, p
CO ₂ is highly removed so as to obtain decarbonated water with increased H. By obtaining decarbonated water having an increased pH, it is not necessary to add an alkali prior to deionization.
Activated carbon treatment of the decarbonated water promotes the ionization of a small amount of residual CO ₂ by the catalytic action of the activated carbon, so that efficient deionization can be performed without the addition of alkali.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for efficiently producing high-purity pure water.

[0002]

2. Description of the Related Art Conventionally, as a method for producing pure water from city water, well water, industrial water, recovered water, and other water, an acid is added after pretreatment (turbidity, chlorine removal) of the water. And then decarbonated by a degassing device, and sequentially pass water through a reverse osmosis (RO) membrane separation device in which decarbonated water is arranged in two stages (two-stage RO treatment)
In addition, there is a method of treating RO treated water with an ion exchange device.

In such a two-stage RO process,
In order to improve the quality of the treated water, an alkali such as sodium hydroxide (NaOH) is injected into the feed water of the RO membrane separation device,
Carbonic acid (CO) remaining in water supplied to the RO membrane separation device
₂ ) A method of ionizing (HCO ₃ ⁻ , CO ₃ ²⁻ ) and subjecting to RO treatment has been proposed.

For example, Japanese Patent Publication No. 8-29315 discloses that an acid is added to raw water to convert a carbonic acid component into CO ₂ gas, followed by deaeration by a membrane deaerator, and then, alkali water is added to the deaerated water. Is added to ionize the remaining CO ₂ , followed by a two-stage RO treatment. This Tokuho 8
No. 29315, pH 3 and CO ₂ , HCO ₃
^{As is} clear from the graph showing the relationship between ^- and CO ₃ ^2- distribution, acidic conditions of pH 4 or less are optimal for removing CO ₂ in the membrane deaerator.

In Japanese Patent Publication No. 6-49191,
It describes a method in which an acid is added to raw water to perform a decarboxylation treatment, an alkali is added to the decarbonation-treated water, and then water is sequentially passed through an RO membrane separation device arranged in two stages in series.

Japanese Patent Application Laid-Open No. 6-134446 discloses that the dissolved oxygen (DO) concentration is 50 ppb or less,
As a membrane deaerator capable of reducing the flow rate to ppb or less, an apparatus in which the flow path interval of raw water is set to 190 μm or less is described. There is no description about the combination with.

[0007]

In the conventional method, it is necessary to add an acid, decarbonate, add an alkali, and ionize the remaining CO ₂ before RO treatment.

[0008] For this reason, the raw water is added with an acid prior to the decarboxylation treatment and then with an alkali prior to the RO treatment, and thus has a drawback that a large amount of chemical is required. In particular, in the method described in Japanese Patent Publication No. 8-29315, the amount of alkali added prior to the RO treatment was large because the removal of CO ₂ was insufficient. Also, in the method described in Japanese Patent Publication No. 6-49191, after the decarboxylation treatment,
Although there is a slight increase in pH, it is still necessary to add an alkali prior to the RO treatment.

The present invention solves the above-mentioned conventional problems and provides a method for producing pure water by decarbonating raw water and then deionizing the same to eliminate the need for alkali addition after decarbonation.
It is an object of the present invention to provide a method for producing pure water that reduces the amount of drug used.

[0010]

According to the method for producing pure water of the present invention, raw water is decarbonated under acidic conditions to obtain decarbonated water having an increased pH, and an alkali is added to the decarbonated water. It is characterized in that it is subjected to a deionization treatment after being brought into contact with activated carbon without being used.

That is, in the present invention, decarbonation means having high CO ₂ removal efficiency is used, and CO ₂ is highly removed so as to obtain decarbonated water having an increased pH. By obtaining decarbonated water having a low CO ₂ and an increased pH, it becomes unnecessary to add an alkali prior to deionization.

The decarbonation means having high CO ₂ removal efficiency suitable for the present invention and the operation and effect thereof will be described below.

As described above, CO _{2 in} an ordinary membrane deaerator is used.
The removal efficiency varies depending on the pH conditions, and is considered to be best when the pH is 4 or less. On the other hand, removal of DO is not affected by pH or the like. In the present invention, by using a high-efficiency deaerator with a good DO removal rate, the CO ₂ removal rate is also increased,
Efficient removal of CO ₂ . As a result, the dissolved carbonic acid component in the water becomes H ⁺ + HCO ₃ ⁻ → H ₂ O + CO ₂ (gas), and during the gasification of CO ₂ , H ⁺ ions, that is, acids are consumed, and the pH of the water rises.

In the present invention, the pH is increased by removing CO ₂ using such a deaerator having a high CO ₂ removal efficiency.
Obtain alkaline decarbonated water. Moreover, in the present invention the decarbonated water by activated carbon treatment, to promote the ionization of CO ₂ that little residual by the catalytic action of activated carbon, enabling an efficient deionization treatment with an alkaline additive-free.

[0015]

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

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

First, acid is added to raw water obtained by subjecting city water, industrial water, well water, recovered water, and the like to pretreatment such as turbidity and dechlorination as required, and then deaerated by the deaerator 1. Care.

The deaerator 1 has a high DO removal rate capable of reducing the DO of the treated water to 2 ppb or less.
A high efficiency membrane deaerator with a high CO ₂ removal rate is preferred. As a type of the degassing membrane of such a membrane degassing device, a hollow fiber membrane which can obtain a large membrane area with a compact module is most preferable. When a hollow fiber membrane is used, either an internal reflux type or an external reflux type can be used. In the internal reflux type hollow fiber module, the inner diameter of the hollow fiber membrane is 190 μm or less, Inner diameter is 60 μm-1
It is preferably 90 μm. When the inner diameter is less than 60 μm, the pressure loss of water flowing inside the hollow fiber becomes extremely large, and it becomes difficult to treat a large amount of water.

The hollow fiber membrane to be used may be any membrane as long as it does not permeate water as a liquid but allows gas to be degassed to permeate sufficiently, and the overall permeation rate Q does not become the gas permeation rate control of the membrane itself. For example, when the oxygen transmission rate is 1 × 10 ⁻⁵ cm ³
/ Cm ² · sec · cmHg or more and a film having a water vapor transmission rate of 400 cm ³ (STP) / cm ² · sec · cmHg or less are preferable. The measurement of the gas permeation rate of the film itself is easily performed according to ASTM-D1434.
The measurement of the water vapor transmission rate of the membrane can be easily obtained by filling one side of the membrane with water, depressurizing the other side, capturing the permeated water in a cold trap, and measuring the amount. At this time, the pressure difference between the water vapor on both sides of the membrane is a value obtained by subtracting the degree of vacuum on the reduced pressure side from the saturated water vapor pressure of the water at the measurement temperature.

As the material of the degassing film, a material having high hydrophobicity is preferable, and examples thereof include polyolefin, polyethylene, polypropylene, polyvinylidene fluoride, silicone, fluororesin, and polymethylpentene. Hydrophilic membranes are not preferred in terms of water barrier properties and water vapor barrier properties.

The structure of the membrane may be a porous membrane, a homogeneous membrane, a heterogeneous membrane, a composite membrane, or any other type, and is not particularly limited. However, a heterogeneous membrane, particularly a heterogeneous membrane containing polymethylpentene as a main component, is Most preferred is a high gas permeation rate of oxygen, nitrogen, carbon dioxide, and the like, and a high barrier property of water vapor.

When such a high-efficiency membrane deaerator is used, the pH of the water introduced into the deaerator 1 does not need to be excessively low, and may be about 4.5 to 5.0. This p
When H is lower than 4.5, it is difficult to obtain decarbonated water whose pH has been increased by decarboxylation because a large amount of acid remains. If the pH is higher than 5.0, CO 2
The removal efficiency of ₂ decreases.

The acid added here is preferably sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCl) or the like.

In the present invention, the decarbonated water p
H is preferably about 6.0 to 6.8. This p
If H is lower than 6.0, the deionization treatment cannot be performed efficiently, and it is practically difficult to decarbonate to a pH higher than 6.8. In addition, since the temperature of water is large in removing CO ₂ , it is desirable to adjust the temperature with a heat exchanger at the entrance of the membrane deaerator.

The effluent from the deaerator 1, which has been adjusted to pH 6.0 to 6.8 by decarboxylation, is passed through the activated carbon tower 2 without adding alkali, and then the first stage RO membrane separator (Hereinafter referred to as “first RO membrane separation device”) 3. Water is sequentially passed through a second-stage RO membrane separation device (hereinafter referred to as “second RO membrane separation device”) 4 for deionization.

In order to sufficiently advance the ionization of CO ₂ under the catalytic action of the activated carbon, the residence time of the activated carbon tower 2 is 60 seconds or more, particularly 150 to 200.
It is preferable to set the speed to about seconds.

In the present invention, the effluent of the activated carbon tower 2 obtained by treating the alkaline decarbonated water with activated carbon is water in which CO _{2 in} the water is almost completely ionized.
Very high quality treated water can be obtained by the deionization treatment by the two-stage RO treatment.

Although it is not necessary to add an alkali to the activated carbon treated water, a small amount of an acid may be added in order to enhance the removal of ammonia ions and the like in the RO membrane separation apparatus.

In the present invention, the RO film used for the RO treatment is not particularly limited. In addition to a normal RO film, a charged film such as a positively charged film whose surface is cationically charged may be used.

Incidentally, in the method of FIG.
Although the RO process is performed in a three-stage RO process, a three-stage RO process may be performed by further providing an RO membrane separation device. After the two-stage RO treatment, an ion exchange treatment may be performed to increase the purity.

The method for producing pure water of the present invention is particularly effective for reducing the processing cost, improving the water quality, and improving the processing efficiency in the primary pure water system of an ultrapure water system such as a semiconductor factory.

[0032]

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

Example 1 Tap water was used as raw water, and acid (HCl) was added thereto to add p
After dewatering the water of H4.7 to 4.8 with a membrane deaerator, the water was passed through the activated carbon tower at SV = 20 hr ^-1 (residence time: 3 minutes), and then arranged in series in two stages. Water was sequentially passed through the RO membrane separation device.

The deaeration membrane used in the membrane deaerator was a heterogeneous hollow fiber membrane made of polymethylpentene (inner diameter of the hollow fiber membrane: 185 μm, oxygen transmission rate: 1.0 × 10 ⁻⁵ cm ³ / c).
m ² · sec · cmHg, water vapor transmission rate 400 cm ³
/ Cm ² · sec · cmHg). Also, the first RO
The RO membranes used in the membrane separator and the second RO membrane separator are as follows.

First RO membrane separation device: “ES-20” (4 inches) manufactured by Nitto Denko Second RO membrane separation device: “ES-20” (4 inches) manufactured by Nitto Denko DO concentration of water and IC (Inor) of the obtained treated water (permeated water of the second RO membrane separation device)
ganic Carbon: Total carbonic acid components (CO ₂ , HCO ₃ ^- and CO
_The concentration of ₃ ^2- ) in terms of carbon) was examined, and the results are shown in Table 1.

Comparative Example 1 The procedure of Example 1 was repeated, except that the membrane degassed water was directly subjected to two-stage RO treatment without passing water through the activated carbon tower. The results are shown in Table 1.

[0037]

[Table 1]

Table 1 shows that according to the present invention, high-purity pure water can be produced without adding an alkali prior to RO treatment.

[0039]

As described in detail above, according to the method for producing pure water of the present invention, an alkali is added to the decarbonated water when the raw water is decarbonated and then deionized to produce pure water. An efficient deionization process can be performed without performing. Therefore, the amount of drug used is reduced, and high-purity pure water can be efficiently produced at low cost.

[Brief description of the drawings]

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Deaerator 2 Activated carbon tower 3 1st RO membrane separation apparatus 4 2nd RO membrane separation apparatus

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C02F 1/44 C02F 1/44 H

Claims (1)

[Claims] 1. Raw water is decarboxylated under acidic conditions to obtain a pH
Pure water production method characterized by obtaining decarbonated water with increased water content, contacting the decarbonated water with activated carbon without adding an alkali, and then deionizing the water.