JPH10309574A - Pure water producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce high purity water by measuring the pH and the specific resistance of flowing water to a reverse osmotic membrane separation device and adjusting the pH of the flowing water based on a relational curve between the both measured values so as to increase the specific resistance. SOLUTION: A pH adjuster is added so as to become a prescribed pH by feed-back-controlling each of pH adjuster adding means 11, 13 and 15 linking the measured value of each of pH meters 12, 14 and 16. The measured value of each of the pH meters 12, 14 and 16, the measured value of specific resistance in the permeated water of a 3rd reverse osmotic membrane device 6 by a specific resistance meter 17 and the measured value of Na<1+> ion concentration in the permeated water by a Na meter 18 are inputted to a controller 10 and the relation among each of the pH value, the specific resistance value and the Na<1+> ion concentration is obtained based on the newest data. Then when the optimum pH is changed and the specific resistance is lowered, the specific pH adjuster adding means 11, 13 to 15 is selected and the set value of the pH is shifted up and down to keep the optimum specific resistance value. As the result, the high purity water is stably obtained.

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 in which raw water is decarbonated under acidic conditions and then deionized in a reverse osmosis (RO) membrane separation apparatus.

[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. Decarbonation treatment with a degassing device, and RO in which decarbonation-treated water is arranged in two stages in series
Water is sequentially passed through the membrane separation device (two-stage RO treatment), and
There is a method of treating O-treated water with an ion exchange device. There is also a method of performing a three-stage RO process using an RO membrane separation device instead of the ion exchange device.

Further, in such a two-stage or three-stage RO treatment, an alkali such as sodium hydroxide (NaOH) is injected into the feed water of the RO membrane separation device in order to improve the quality of the treated water. Carbonic acid (CO ₂ ) remaining in the water supplied to the reactor is ionized (HCO ₃ ⁻ , CO ₃ ²⁻ )
A method of performing O treatment has been proposed.

[0004] That is, when the pH is low, CO ₂ is in the form of CO ₂ gas, but when the pH is high, it is in the form of ions and cannot be removed by the deaerator, so that acid is added to the water supply to the deaerator. CO ₂ gas form, and alkali is added to the feed water of the RO membrane separation apparatus to remove it in the form of ions by RO treatment.

[0005] Japanese Patent Application Laid-Open No. 7-16565 discloses that, in a three-stage RO treatment in which such alkali addition is performed, NaOH is added to a second-stage RO membrane separation apparatus.
It describes that the pH of the concentrated water of the RO membrane separation device at the stage is measured, and NaOH is added so that the pH of the concentrated water becomes 7 to 8.

[0006] In the production of pure water by such RO treatment, the optimum pH of feed water varies depending on the type of RO membrane used, and the pH range in which the specific resistance of the produced water (permeate) becomes high is very narrow. It has been reported.

[0007]

As described above, in the RO membrane separation process, since the pH condition at which the specific resistance of the produced water becomes high is very narrow, the pH value of the feed water fluctuates even slightly due to poor calibration of the pH meter or the like. Then, the specific resistance of the obtained permeated water is greatly reduced.

Therefore, pH control is an extremely important requirement for improving water quality. However, as described in Japanese Patent Application Laid-Open No. 7-16565, concentrated water of a second-stage RO membrane separation device is used. In the method of performing pH adjustment with a pH value, a time delay corresponding to the residence time of the RO membrane separation device occurs with respect to the fluctuation in pH of raw water, and it is difficult to perform immediate pH adjustment. For this reason, it has been difficult to maintain such water quality continuously and stably even when product water having high specific resistance is obtained instantaneously.
Further, each time the type of the RO membrane of the RO membrane separation apparatus is changed, it is necessary to change the set pH, and the operation is complicated.

Further, the present inventor has determined that the pH in RO treatment
As a result of repeated investigations on the conditions and the specific resistance of the produced water, it was found that, even with the same type of RO membrane, the optimum pH condition that gives the maximum specific resistance changes when the quality of the feed water changes. It has also been found that a similar phenomenon occurs due to the temporal change of the RO film itself.

However, at present, no pH control technology has been proposed which can cope with such a change in the optimum pH condition.

[0011] The present invention has been made in view of the above-mentioned conventional situation.
In the method for producing pure water deionized by a membrane separation device, the supply water for RO treatment is adjusted to the ratio of production water in response to fluctuations in various water flow conditions such as the quality of raw water, the type of RO membrane, and the performance of a pH meter. It is an object of the present invention to provide a method for stably producing high-purity pure water by adjusting the pH to a pH condition at which the resistance becomes optimal.

[0012]

The pure water production method of the present invention is a method for producing pure water by decarbonating raw water under acidic conditions and then deionizing it with an RO membrane separator.
The pH of the inflow water flowing into the O membrane separator and the specific resistance of the permeated water of the RO membrane separator were measured, and based on the relationship curve between the measured pH value and the specific resistance, the specific resistance was increased. It is characterized in that the pH of the inflow water is adjusted so as to be as follows.

As for the RO membrane, even if the same RO membrane is used, the optimum pH conditions are different as shown by the conditions A and B in FIG. 2 due to, for example, a change in the quality of the raw water. Therefore, if the pH of the feedwater and the specific resistance of the produced water (permeated water) are measured only once, for example, in FIG.
When the specific resistance is M, it cannot be determined which condition is included.

However, in this case, if the specific resistance of the product water decreases as the pH of the feedwater increases, the condition A is satisfied. Conversely, if the specific pH of the product water increases, the specific resistance of the product water increases. It can be seen that the condition B is satisfied.

In the present invention, the state of the fluctuation is tracked by measuring the pH of the feed water and the specific resistance of the produced water, a relation curve is obtained, and the pH is adjusted based on this curve so as to increase the specific resistance. .

Therefore, even if there are fluctuations in water flow conditions such as fluctuations in water quality, the pH can be adjusted so as to always increase the specific resistance of the produced water.

According to the study by the present inventor, cations such as Na ions mainly produce cations such as Na ions when the feedwater pH is higher than the optimum pH value in the relationship between pH and specific resistance as shown in FIG. , The specific resistance decreases,
Conversely, it was found that when the pH of the feedwater was lower than the optimum pH value, the specific resistance decreased mainly due to an increase in anions such as carbonate ions in the production water.

[0018]

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 the illustrated method, deionization treatment is performed by sequentially passing water through RO membrane separators arranged in three stages in series, wherein 1 is a raw water tank, 2 is a deaerator, 3 is an activated carbon tower,
Is a first-stage RO membrane separation device (hereinafter, referred to as “first RO device”), and 5 is a second-stage RO membrane separation device (hereinafter, “second RO device”).
Device ". ) And 6 are third-stage RO membrane separation devices (hereinafter, referred to as “third RO device”). 10 is a control device,
11, 13, and 15 are means for adding a pH adjuster;
16 is a pH meter, 19 is a specific resistance meter, 18 is a Na ion monitor (hereinafter referred to as "Na meter"), and P ₁ and P ₂ are pumps.

First, the pH of raw water in the raw water tank 1, that is, water obtained by subjecting city water, industrial water, well water, recovered water, and the like to pretreatment such as turbidity and dechlorination as required, is adjusted. After the addition of the agent (acid), degassing is performed by the degassing device 2. As the deaerator 2, a decarbonation tower, a membrane deaerator or the like can be employed.

The supply water of the deaerator 2 has a pH of 4.5 to 4.5.
It is preferably 5.0. That is, as described above, in the deaerator 2, since the carbonate component is removed in the form of CO ₂ gas under a low pH condition, the pH of the feedwater is preferably lower in this point, but the pH is excessively lowered. And an ion load (for example, H ₂ SO ₄ ) by the pH adjuster is applied to the RO device in the subsequent stage, so that if the pH is excessively lowered, the specific resistance of the final treated water is undesirably lowered. Also, depending on the carbon content and the Ca concentration of the raw water, CaCO ₃
The pH is preferably set to 4.0 to 5.0 because of the problem of scale adhesion of CaF ₂ and CaF ₂ .

The effluent from the deaerator 2 is passed through the activated carbon tower 3, after which a pH adjuster (acid or alkali) is added, followed by passing through the first RO unit 4, and again a pH adjuster (acid or alkali). ) Is added, the second RO device 5, the third R
Water is sequentially passed through the O device 6 to be deionized.

In this pure water production method, the activated carbon tower 3
Is for catalysis by activated carbon and for promoting the ionization of CO ₂ to increase the removal efficiency in the RO device. For this ionization, the pH of the feed water of the activated carbon tower 3 is 6.0. It is preferable to perform the deaeration treatment in the deaerator 2 so that the pressure becomes about 6.8. In order to adjust the pH condition of the activated carbon tower 3, an alkali may be added to the effluent of the deaerator 2 if necessary. In this case, the alkali addition means is also controlled by a control device described later. PH
Preferably, an adjustment is made.

Here, as the acid as a pH adjuster,
Sulfuric acid (H ₂ SO ₄ ), hydrochloric acid (HCl) and the like are suitable,
It is preferable to use sodium hydroxide (NaOH) or the like as the alkali.

In this embodiment, each pH adjusting agent adding means 1
A pH adjuster is added so that a predetermined pH value is obtained by performing feedback control of 1, 13, and 15 in conjunction with the measurement results of the pH meters 12, 14, and 16, respectively.

The measured values of the pH meters 12, 14, 16 and the measured values of the specific resistance meter 17 for measuring the specific resistance of the final treated water (the permeated water of the third RO device 6) and the Na ion concentration of the final treated water are shown in FIG. The measured value of the Na meter 18 to be measured is input to the control device 10. The control device 10 is designed so that the relationship between each pH value, the specific resistance value and the Na ion concentration is obtained, and is constantly updated with the latest data. Therefore, when the optimum pH fluctuates and the specific resistance decreases due to fluctuations in water flow conditions, the pH set value is raised or lowered at a specific pH adjuster addition point, and p
The relationship between the H value and the specific resistance is examined, and a signal for changing the pH setting at the relevant location is output so as to obtain the optimum specific resistance. It is desirable to use artificial intelligence software such as a neural network for the control device 10.

In the apparatus shown in FIG. 1, a Na meter (for example, a Na meter manufactured by Toyo Medec) 18 is provided.
The increase in the Na ion concentration by the Na meter 18 can be used as a criterion for determining that the current pH is in a higher pH range than the optimum pH value. Also, a TOC meter (for example, a TOC meter manufactured by Seabass) for measuring carbonate ions is provided in place of the Na meter, and IC (Inorganic Carbon: total carbonic acid components (CO ₂ , HCO ₃ ⁻ and CO ₃ ²⁻ ) are converted to carbon dioxide. The value can be used as a criterion for pH adjustment.

In the three-stage RO process, the first RO device 4
The concentrated water of the second RO device 5 and the concentrated water of the third RO device 6 have already been purified by the RO device of the first RO device 4, and thus the water recovery rate Is returned to the raw water tank 1 to improve the quality.

As shown in FIG. 1, when the three-stage RO process is performed, the permeated water of the second RO device 5 flowing into the third RO device 6 has already undergone the two-stage RO process to be sufficiently deionized. The water quality is relatively high. Thus, the permeated water of the second RO device 5 having a low ion concentration is
As the RO film of the third RO device 6 to be processed, it is preferable to use an RO film having a high salt rejection in a low salt concentration region. With such an RO membrane, ions in the permeated water of the second RO device 5 whose ion concentration has already been considerably reduced by the two-stage RO treatment are removed to an extremely low concentration to obtain treated water of extremely high quality. be able to.

As the RO film used in the third RO device 6, an RO film having a salt rejection of 90% or more in a low salt concentration range such as a salt concentration of 0.1 to 2 ppm, for example,
Nitto Denko Corporation "NTR-719HF""ES10C"
(Both are 99% or more of NaCl rejection at NaCl concentration of 1 to 10 ppm).

The method shown in FIG. 1 is an example of an embodiment of the present invention, and the present invention is not limited to the illustrated method unless it exceeds the gist.

For example, the RO devices may be arranged in two stages, or may be arranged in four or more stages. Further, the alkali addition and the activated carbon treatment may be provided in a stage preceding the RO membrane separation device after the second RO device.

[0034]

The present invention will be described more specifically with reference to the following examples.

Example 1 Pure water was produced using the apparatus shown in FIG.

Acid (HCl) in tap water (300 L / hr)
Was added to pH 4.7 to 4.8, and water having a pH of 6.5 obtained by degassing the water with a membrane deaerator (EF040P degassing membrane manufactured by Dainippon Ink and Chemicals, Inc.) 2 was activated carbon. SV on the tower
= 20 hr ^-1 (residence time 3 minutes), and then sequentially passed through RO devices 4, 5, and 6 arranged in series in three stages.

The RO films used (both 4 inches R)
O film) is as follows.

First RO device 4: polyacrylamide film (“ES20” manufactured by Nitto Denko Corporation) Second RO device 5: polyacrylamide film (“ES20” manufactured by Nitto Denko Corporation) Third RO device 6: polyacrylamide film (Nitto Denko) (“ES10C” manufactured by Co., Ltd.) Further, at the inlet side of the first RO device 4, NaOH was added to adjust the pH of the feedwater in the range of 6.9 to 7.6.

In such a treatment, the pH of the feed water of the deaerator 2 is adjusted to 4.7 to 4.8, the pH of the feed water of the first RO device 4 is changed, and this pH and the production water (third RO) are changed.
When the relationship with the specific resistance of the permeated water of the apparatus 6 was determined, it was found that the optimum pH of the first RO apparatus 4 was about 7.1, as shown in FIG.

Therefore, when the pH of the feed water of the first RO device 4 was controlled to be 7.1, the specific resistance was 14 to 14.
Produced water of high quality of 15 MΩ · cm could be obtained stably.

In this apparatus, the recovered water was mixed into the raw water tank so that the raw water was converted from tap water to recovered water in the semiconductor manufacturing process: tap water = 7: 3.
At 7.1, the specific resistance had a tendency to decrease. Therefore, the first
When the water supply pH of the RO device 4 is raised by the control device 10, the specific resistance decreases, and when the water supply pH is lowered, the specific resistance increases. Therefore, the pH is gradually increased, and after one hour, the optimum pH is about 6.0.
Turned out to be. During this time, the specific resistance is 13-16 MΩ
・ Production water with high water quality of 1 cm was obtained. Therefore,
When the pH of the first RO device 4 was controlled so that the supply water became pH 6.0, product water having a specific resistance of 16 MΩ · cm was continuously obtained. Thereafter, the relationship between the pH and the specific resistance was examined, as shown in FIG. 3 (b), and it was confirmed that the optimum pH was about 6.0.

In the above embodiment, the pH control of the feed water of the first RO device is performed, but the pH control of the feed water of the second RO device can be similarly performed to stabilize the specific resistance at a high value.

[0043]

As described above in detail, according to the method for producing pure water of the present invention, it is possible to control the pH so that the specific resistance of the produced water becomes optimum in response to the fluctuation of various water flow conditions. And high-purity pure water can be stably 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 graph showing the relationship between the pH of feed water and the specific resistance of product water in an RO membrane separation device.

FIG. 3 shows the pH of the feed water of the first RO device obtained in Example 1.
4 is a graph showing the relationship between the product water and the specific resistance.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Deaerator 3 Activated carbon tower 4 1st RO device 5 2nd RO device 6 3rd RO device 10 Control device 11,13,15 pH adjuster addition means 12,14,16 pH meter 17 Resistivity meter 18Na meter

Claims (2)

[Claims] 1. A method for producing deionized water by subjecting raw water to decarbonation under acidic conditions, followed by deionization in a reverse osmosis membrane separator, wherein the pH of the influent water flowing into the reverse osmosis membrane separator; The specific resistance of the permeated water of the reverse osmosis membrane separation device is measured, and the pH of the inflow water is adjusted so that the specific resistance increases based on a relationship curve between the measured pH value and the specific resistance. A method for producing pure water, comprising: 2. The pure water production method according to claim 1, wherein the relationship curve is obtained by measuring the specific resistance while changing the pH value of the inflow water.