JPS6359387A - Method for making ultrapure water - Google Patents

Abstract

PURPOSE:To prevent the lowering in capacity of a semipermeable membrane, by supplying raw water after ion exchange treatment, the addition of a chlorine type sterilizing agent and pH adjustment to the semipermeable membrane composed of a cellulosic derivative. CONSTITUTION:After raw water is subjected to ion exchange treatment, a sterilizing agent such as sodium hypochlorite, hydrogen peroxide or sodium bisulfite is added to raw water in a range of 0.01-1ppm. Subsequently, raw water is adjusted to pH 4.0-6.5 and supplied to a semipermeable member formed using cellulose, cellulose acetate, cellulose butyrate or methyl cellulose as a polymer component to be treated. By this method, the lowering in the performance of the semipermeable membrane is suppressed to reduce trouble and the long-term stable operation of a process is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing ultrapure water using a semipermeable membrane made of a cellulose derivative.

[Conventional technology] In recent years, in advanced industries such as the electronics industry, pharmaceutical industry, and biomedical industry, water for washing integrated circuits and related products, water for intravenous fluids, water for drug manufacturing, water for washing medical products, water for supplying microorganisms and cell culture, etc. Purified water of high purity, that is, ultrapure water, is increasingly required for various applications. As a method for producing ultrapure water for such purposes, a process using demethods is gradually becoming the mainstream method.

Generally, in the dehydration method, after pretreatment such as coagulation, precipitation, and filtration of turbidity and colloidal substances in raw water, reverse osmosis membrane, ultrafiltration, etc.
By passing through semipermeable membranes such as filtration membranes and ion exchange membranes,
Ultrapure water is obtained by removing residual salts, dissolved organic substances, fine particles, viable bacteria, etc. In addition, a sterilizing agent is added during or after pretreatment in order to sterilize viable bacteria in raw water and maintain a sterile state in the process.

However, when this sterilizing agent is added, there is a problem in that the membrane performance rapidly decreases during operation because the semipermeable membrane made of a sterile polymer is generally susceptible to oxidation. Conventionally, semipermeable membranes made of cellulose acetate derivatives have been known to be the most durable semipermeable membranes against sterilization using oxidizing agents in ordinary water treatment processes, and are suitable for ultra-pure manufacturing processes. It has also been recommended for

[Problems to be solved by the invention] However, as the requirements for ultra-pure purity become stricter, pretreatment technology improves, and the purity of purified water supplied to membrane modules increases, semipermeable membranes made of cellulose derivatives Even when using sterilizers, there is often a decrease in membrane performance that is thought to be caused by the sterilizing agent, and as a result, membrane modules must be replaced extremely frequently in order to produce and supply ultrapure products of a specified high purity. This has led to the problem that the economic efficiency and maintainability, which are the advantages of circumventing the law, are being significantly impaired.

This is super pure! ! Advances in pretreatment technology in the water production process have enabled the amount of organic components dissolved in water (
Ding OC: Total Organic Content
) is likely to decrease. Such TOC materials are believed to react competitively with the sterilant, thereby increasing the rate at which the added sterilant degrades membrane performance. Therefore, in practice, there is currently no effective countermeasure other than reducing the amount of sterilizing agent used, such as by intermittent addition or lowering the sterilizing agent concentration, even if the sterilizing effect is sacrificed to some extent.

[Means for Solving the Problems] The present invention has the following configuration in order to eliminate the drawbacks of the prior art.

``A method for producing ultrapure water, which comprises subjecting raw water to the following treatments and then supplying the raw water to a semipermeable membrane made of a cellulose derivative.

(a) Addition of ion exchange treatment port, chlorine-based sterilizer (c) Adjustment to pH 4.0 to 6.5.''Here, the semipermeable membrane made of cellulose derivatives refers to cellulose, cellulose acetate, cellulose butyrate, methyl cellulose, It refers to a semi-permeable membrane formed using ethyl cellulose or the like as a main polymer component, and the method of forming the film is not limited. Semipermeable membranes used in the ultrapure water production process include reverse osmosis membranes, ultrafiltration membranes, and ion exchange membranes, and semipermeable membranes made of various membrane materials are used.

The present invention is particularly effective for semipermeable membranes made of cellulose-based derivatives as membrane materials, but also for semipermeable membranes made of polymers such as polysulfone and polyamide, and for interfacial polymerization on porous polysulfone semipermeable membranes. It is also effective for composite semipermeable membranes in which an ultra-thin film is formed by 1q.

Membrane forms include flat membranes, hollow fiber membranes, and annular membranes.
It is assembled into a structure corresponding to each membrane type, that is, a membrane element, and inserted into a pressure vessel to be used as a membrane module in an ultrapure water production process. Also suitable for those.

Sterilizing agents include sodium hypochlorite, hydrogen peroxide,
Examples include sodium bisulfite, peracetic acid, and formalin, but sodium hypochlorite is the most commonly used because it is inexpensive and has a high sterilization effect.

Strictly speaking, sterilization conditions vary depending on the quality of the water in the pretreatment process, and are difficult to define generally, but for example, when sodium subzinc chlorate is continuously injected, the effective chlorine concentration is 2 p.
I) m or less, preferably in the range of 0.01 to 1 ppm. If it exceeds 2 pl)m, the sterilizing effect will be great, but at the same time, the membrane performance will be greatly degraded by the sterilizing agent, which is not preferable. Moreover, below Olo'+ ppm, the sterilization effect becomes too small and deterioration of membrane performance due to proliferation of viable bacteria becomes a problem. In addition, for the same reason, in the case of intermittent injection, the concentration is 1 oppm or less, preferably 0.2 to 2 ppm.
It is preferable to do so. However, it goes without saying that the application of the present invention is not limited by the method of adding the sterilizing agent.

In the present invention, when supplying water containing the sterilizing agent to the semipermeable membrane, it is important to adjust the pH of the supplied water to a range of 4.0 to 6.5.

In the case of multiple stages, adjustment is required in front of each module. Of course, the decrease in membrane performance due to active chlorine is due to this PH
In particular, at PH near the boundary of the preferred PH range, performance deterioration progresses more quickly than under conditions in the middle range, so if the active chlorine temperature is high or other factors such as TOC described below Considering the influence of
It is preferable to adjust it to a range of 6.0.

Here, the conditions to be considered as ultrapure production conditions are that when water is treated with a semipermeable membrane, the water supplied and absorbed by the membrane module has been pretreated to remove suspended matter and colloidal substances, so it contains trace amounts of impurities, such as It is the amount of substance measured as TOC. Although this so-called TOC substance is a small amount, depending on its type and amount, it affects the deterioration of membrane performance caused by chlorine-based sterilizers.

For example, if the dissolved organic component has high reactivity to active chlorine, if even a trace amount exists, the membrane will be less likely to be attacked, and the deterioration in membrane performance will be suppressed. As mentioned above, the reactivity with active chlorine differs depending on the TOC substance, but the TOC of pretreated purified water is 1.
001) When the concentration is less than pb, the degree of attack on the membrane increases and membrane performance tends to deteriorate. Purified water treated with ion exchange usually contains almost no TOC (<
20 ppb) Furthermore, the membrane performance is likely to deteriorate. The fact that chlorine is consumed by TOC components means that the point in the line at which the sterilant is injected and the residence time before it contacts the semipermeable membrane also affect the deterioration of membrane performance. It shows. In other words, even if the TOC is the same, if the sterilant is injected into the feed water tank where the residence time is considerable, the sterilant will be consumed by organic components in the line, compared to when it is injected just before the semipermeable membrane. This is because the concentration of sterilizing agent in the water that comes into contact with the semipermeable membrane is thought to be decreasing.

The influence of the residence time of this sterilant addition is on the TOC concentration, TO
Although it varies depending on the type of C component, in general, IOC substances consume free M chlorine relatively quickly, so in the case of normal pretreatment, if the residence time is 10 minutes or more, preferably 30 minutes or more, the membrane can be removed. Attacking free chlorine can be reduced.

By the way, in the method of the present invention, the order of the steps of mouth and c is as follows:
Either one may come first.

[Example] The present invention will be explained in the following example. The amount of free chlorine was measured using a measurement method that utilizes the reduction reaction of free chlorine on a platinum electrode (measurement equipment used: portable residual chlorine meter).
del RC-3T (manufactured by Bionics Equipment Co., Ltd.)
. In the measurement of pH, the ion concentration in water is low under ultrapure water conditions, and it is difficult to accurately measure I)H as it is, so in the present invention, 50 ppm of KCI was added for measurement. The membrane performance was measured by pressurizing a saline solution containing 1500 ppm of salt at 30 -i/ci and flowing it over the membrane surface at a flow rate of 8 m/min.

In addition, for organic components in water (TOC), TOC meter 1-AS
TI'? 018001) I) Measured by bJ.

Example 1 A cellulose acetate-based reverse osmosis membrane (salt rejection rate 97.0%, water passage rate 0.9Trl!/Tr12·day) was set in an evaluation apparatus, and the following experiment was conducted.

Put ion exchange water (TOC: 15ppb) into the tank,
The desired I)H was adjusted with sulfuric acid and sodium hydroxide. This water was pressurized at 20 kq/cm and supplied to the membrane surface at a flow rate of 51/min. Further, in the middle of the piping, sodium subzinc oxide was added so that the free chlorine concentration in the water supplied to and absorbed by the membrane was 10 ppm. The residence time of sodium subzinc oxide from the injection point to the membrane surface was 45 seconds. The membrane was degraded by chlorine in this manner, and the membrane performance was then measured at regular intervals.

The figure shows the pH dependence of membrane performance (salt rejection rate) after 200 hours of chlorination. l) When H is 4 to 6.5, the salt rejection rate decreases by 5% or less, and in particular, in the range of 4.5 to 6.0, there is virtually no decrease in the salt rejection rate. - Method 1) H is 6.5
The decrease is 15% at 7.0 and 25% at pH 6.
．． It can be seen that when the value exceeds 5, the salt rejection rate decreases significantly.

Example 2 Using the same cellulose acetate-based reverse osmosis membrane as in Example 1 (salt rejection rate 97.0%, water permeation Wh 0.9 m''/m2·day), the following experiment was conducted with the same evaluation equipment.Ion Exchange water (TOC: 10ppb) and purified water (TOC: 1200pp)
b) to make various types of raw water (pH: 6.5)
was prepared and supplied to the membrane surface while adding sodium hypoalthite to a concentration of 10 ppm. The residence time from the injection point of sodium hypoalthite to the membrane surface was 45 seconds. The decrease in membrane performance (salt rejection rate) occurs when the TOC amount is 15
It is 5% at 0ppb, 2.5% or less at 3ooppb or more, and the decrease in membrane performance is small at 150pl)b or more. On the other hand, when TOCffi is 75 ppb, the chlorine rejection rate decreases by 13%, and when it is below 30 ppb, it decreases to 16%.
When the amount is less than ppb, the membrane performance deteriorates significantly.

Example 3 Cellulose acetate reverse osmosis membrane same as Example 1 (salt rejection rate 9
7.0%, water permeation ff 10.9 m'/m2·day), and the following experiment was conducted using the same evaluation apparatus.

Put ion exchange water (TOC: 10ppb) into the tank,
1) H was adjusted to 6.5 and supplied to the membrane surface under the same conditions as in Example 1. Furthermore, sodium hypoalthite was added in the middle of the piping so that the free chlorine concentration in the water was 10 ppm. At this time, by changing the injection point, the residence time from the injection point to the membrane surface was varied and the effects thereof were investigated.

If the residence time exceeds 15 minutes, the membrane performance (salt rejection rate decreases by 5
% or less, but it becomes 12% when the residence time is 7.5 minutes, and 15% when the residence time is 1 minute, indicating that the membrane performance is greatly degraded when the residence time is 10 minutes or less.

Example 4 As a result of conducting an experiment under the same conditions as in Example 1 on a composite membrane formed by interfacial polymerization of polyethyleneimine and toluene diisocyanate on a polysulfone base film, the p++ dependence of the decrease in membrane performance was found to be the same as in Example 1. It was the same.

[Effects of the Invention] The present invention provides raw water with ion exchange treatment,
By adding a chlorine-based sterilizing agent and controlling the pH within a specific range, deterioration in membrane performance is suppressed, troubles associated with this are eliminated, and long-term stable operation of the process is made possible.

[Brief explanation of drawings]

The drawing shows the relationship between the pH of water containing a chlorine-based sterilizing agent supplied to a semipermeable membrane made of cellulose acetate and the salt rejection rate.

Claims (4)

[Claims] (1) A method for producing ultrapure water, which comprises subjecting raw water to the following treatments and then supplying the raw water to a semipermeable membrane made of a cellulose derivative. B. Ion exchange treatment B. Addition of chlorine-based sterilizer C. Adjustment to pH 4.0 to 6.5 (2) An ultra-pure manufacturing method characterized by allowing a residence time of 10 minutes or more after adding a chlorine-based sterilizing agent. (3) The method for producing ultrapure according to claim (1), wherein the pH is adjusted to 4.5 to 6.0. (4) Ion exchange treatment reduces the amount of organic components to 100 ppb
The ultrapure manufacturing method according to claim (1), characterized in that the following treatment is performed.