KR101730402B1

KR101730402B1 - Manufacturing method for Ultrafilter Membrane through a Recycle of Polyamide Membrane

Info

Publication number: KR101730402B1
Application number: KR1020150042661A
Authority: KR
Inventors: 권영남; 김유경; 임지원
Original assignee: 울산과학기술원; 한남대학교 산학협력단
Priority date: 2015-03-26
Filing date: 2015-03-26
Publication date: 2017-05-11
Also published as: KR20160116194A

Abstract

The present invention relates to a process for producing an ultrafiltration membrane comprising removing a polyamide (PA) membrane by treating a separation membrane having a polyamide (PA) layer formed on its surface with chlorine dioxide (ClO ₂ ) of pH 11 or more. The present invention can recycle the polyamide separator membrane into an ultrafiltration membrane by treating the chlorine dioxide (ClO ₂ ) at specific pH conditions and removing the contaminated polyamide layer.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing an ultrafiltration membrane by recycling a polyamide membrane,

The present invention relates to a method for producing an ultrafiltration membrane in which a polyamide separation membrane is recycled. More particularly, the present invention relates to a method for producing an ultrafiltration membrane by recycling a polyamide separation membrane by treating a polyamide separation membrane with chlorine dioxide (ClO ₂ ) To a method of manufacturing an ultrafiltration membrane to be recycled.

Membrane separation processes are used in various water treatment fields such as water treatment, under-wastewater treatment, and seawater desalination. At this time, the membrane is the core of the process. Here, the separation membrane can be defined as a third phase that separates the raw water and the treated water, and pure water is permeated through the raw water and the separation is performed by excluding the solute. In this case, the excluded solutes accumulate on the surface of the membrane, resulting in a reduction in the performance during operation. The materials causing the performance degradation are largely composed of i) insoluble minerals, ii) soluble organic materials, iii) particles, iv) . In this case, the microorganisms in the raw water adhere to the surface of the membrane and grow various kinds of contaminants as nutrients, thereby forming biofilms while secreting extracellular polymeric substances (EPSs).

Since such exocrine secretory substances are strongly adherent and are generated on the surface of the separation membrane, they can not be easily removed by various cleaning methods, and they act as protective membranes of microorganisms and have been pointed out as main causes of membrane contamination by biofilm. Therefore, In order to reduce the membrane contamination by the biofilm, chlorine was injected at the stage before the separation membrane process to sterilize the microorganisms and to reduce the membrane contamination by secretion of secretory materials.

However, the chlorine used in the above technique has an advantage of reducing the activity of microorganisms, but it is fatal to the damage of the polyamide membrane widely used in water treatment. In the water treatment plant, chlorine permeation process and membrane separation process, There is a problem in that chlorine reacts with organic substances in the raw water to generate disinfection by-products known as carcinogens.

For example, Korean Patent Laid-Open Publication No. 2014-0031874 discloses the use of chlorine dioxide for cleaning a microfiltration membrane or an ultrafiltration membrane, A specific method for recycling the polyamide membrane into the ultrafiltration membrane without damaging the polyamide membrane has not been disclosed. Therefore, it is necessary to study a technique for recycling such a polyamide separator as an ultrafiltration membrane.

Korean Patent Publication No. 2014-0031874

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems occurring in the prior art,

It is an object of the present invention to provide a process for producing an ultrafiltration membrane which can be recycled as an ultrafiltration membrane without damaging the polyamide separation membrane while treating the contaminated polyamide separation membrane with chlorine dioxide.

According to an aspect of the present invention,

Treating the separation membrane having a polyamide (PA) layer formed on its surface with chlorine dioxide (ClO ₂ ) of pH 11 or more to remove the polyamide (PA) membrane.

The pH of the chlorine dioxide may be 12 or more.

The separation membrane in which the polyamide layer is formed on the surface may be a multilayer film of three or more layers including a polyamide (PA) layer, a polysulfone (PS) layer, and a nonwoven layer and a polyamide (PA) layer formed on the surface. Also, the polyamide layer of the separation membrane may include a copolymer of a TMC monomer and an MPD monomer, and a hydrophilic material may be further coated on the polyamide layer. The hydrophilic material may be a polyvinyl alcohol and a polyethylene It can be any of the glycols. In addition, the polyamide layer can be interfacially synthesized on the polysulfone layer.

The chlorine dioxide can be treated for 10 minutes to 3 hours on the polyamide separator.

The treatment concentration of the chlorine dioxide may be 10 to 1,000 ppm.

In the method of manufacturing the ultrafiltration membrane, after chlorine dioxide treatment, the step of removing chlorine dioxide from the surface of the separation membrane may be further included.

The separation membrane may be a reverse osmosis separation membrane or a nanofiltration membrane.

The ultrafiltration membrane may be used for filtration or cross flow filtration, and the normalized flux during filtration may be 6 or more or 8 or more. Further, in the case of cross flow filtration, the removal rate may be 20% or less.

In the case of using the method of producing an ultrafiltration membrane according to the present invention, it is possible to minimize the damage of the separation membrane and improve the performance of the separation membrane by treating the aqueous solution of chlorine dioxide to the contaminated polyamide separation membrane under optimal conditions.

1 is a view showing a structure of a separation membrane used in the present invention.
2 is a graph showing the normalized flux before and after exposure to chlorine dioxide according to the pH of the separation membrane used in the present invention.
3 is a graph showing the flux before exposure to chlorine dioxide according to the pH of another separation membrane used in the present invention.
FIG. 4 is a graph showing the flux after exposure to chlorine dioxide according to the pH of another separation membrane used in the present invention. FIG.
FIG. 5 is a graph showing the XPS spectrum of the separation membrane according to the present invention before and after exposure to chlorine dioxide. FIG.
6 is a SEM photograph (10,000 magnification) showing the pH of the separation membrane used in the present invention.
7 is a SEM photograph (35,000 magnification) showing the pH of the separation membrane used in the present invention.
FIG. 8 is a graph showing XPS spectral results before and after exposure to chlorine dioxide according to the pH of another separation membrane used in the present invention. FIG.
9 is a graph showing the XPS spectrum of another separation membrane according to the present invention.
10 is a photograph showing an SEM photograph (10,000 magnification) according to the pH of another separation membrane used in the present invention.
11 is a photograph showing an SEM photograph (35,000 magnification) according to the pH of another separation membrane used in the present invention.
12 is a graph showing real-time flux changes at the time of operation of the cross-flow type separation membrane after exposure to chlorine dioxide.
13 is a graph showing the change in real-time NaCl removal rate during the operation of the cross-flow type separation membrane after chlorine dioxide exposure.
FIG. 14 is a graph showing the flux according to time before exposure to chlorine dioxide at pH 4. FIG.
FIG. 15 is a graph showing flux with time after exposure to chlorine dioxide at pH 4. FIG.

Hereinafter, the present invention will be described in detail.

The method for producing an ultrafiltration membrane of the present invention includes treating a separation membrane having a polyamide (PA) layer formed on its surface with chlorine dioxide (ClO ₂ ) of pH 11 or more to remove the polyamide (PA) membrane.

In the present invention, the separation membrane having the polyamide (PA) layer formed on its surface is a separator membrane used for removing salts (or ions) in the raw water, and a polyamide membrane having a polyamide layer formed on the surface thereof However, it is possible to use a multi-layered film of three or more layers, preferably including a polyamide (PA) layer, a polysulfone (PS) layer and a nonwoven layer, and a polyamide (PA) layer formed on the surface thereof.

As shown in FIG. 1, the polyamide (PA) layer in the multi-layered film of three or more layers may include a copolymer of trimesoyl chloride (TMC) monomer and m-phenylenediamine (MPD) PA) layer is formed by interfacial synthesis on the polysulfone layer. The polyamide (PA) layer thus formed is capable of removing monovalent ions in the raw water. It is used for the process of making the nose water into fresh water, and when the additive is administered or the active layer is made dense and thick, it develops as a membrane which desalinates the sea water. Most of the reverse osmosis membranes used for seawater desalination are membranes using TMC and MPD monomers copolymerized.

Also, on the polyamide (PA) layer, a hydrophilic material may be additionally coated, as shown in Figure 1 (a). The hydrophilic material may be used without any particular limitation as long as it contains a hydrophilic group such as OH. Preferably, the hydrophilic material is selected from the group consisting of polyvinyl alcohol, polyethylene glycol, and the like. Can

As shown in FIG. 1, the polysulfone (PS) layer of the three or more multi-layered films is used as a support layer and functions as a separator having an ultrafiltration level.

The nonwoven fabric of the above three or more multilayer films may be used without any particular limitation as long as it is used in the related art. Preferably, the nonwoven fabric is one or more selected from the group consisting of polyester (PET), polyethylene (PE), polypropylene Can be used.

In the method of manufacturing an ultrafiltration membrane of the present invention, the polyamide (PA) membrane prepared as described above may be treated with chlorine dioxide (ClO ₂ ) of pH 11 or more to remove the polyamide (PA) The polyamide (PA) film can be removed by treating chlorine dioxide (ClO ₂ ). If the pH of the chlorine dioxide (ClO ₂ ) is lower than 11, there is a problem that the removal effect of the polyamide (PA) separation membrane is inadequate. If the pH is higher than 14.8, have.

Further, in the method for producing an ultrafiltration membrane of the present invention, chlorine dioxide (ClO ₂ ) is treated for 10 minutes to 3 hours in the polyamide (PA) separation membrane prepared as described above. If the treatment time is less than 10 minutes, the removal of the polyamide layer on the surface is insignificant and the effect of the treatment is insignificant. If the treatment time exceeds 3 hours, the advantage of the time increase effect is lost. Can sufficiently hydrate the membrane prior to treatment of the chlorine dioxide (ClO _2), preferably, it may process the chlorine dioxide (ClO ₂₎ after 48 hours or more hydrate.

It is preferable that the treating concentration of chlorine dioxide (ClO ₂ ) is 10 to 1,000 ppm. If the treatment concentration is less than 10 ppm, the removal of the polyamide layer on the surface is insufficient and the effect of the treatment is insignificant. If the treatment concentration exceeds 1,000 ppm, the advantage of the effect due to the increase in concentration is lost.

The present invention may further comprise the step of after processing the chlorine dioxide (ClO ₂₎ as described above, to remove the chlorine dioxide (ClO ₂₎ remaining on the membrane surface.

In the present invention, the separation membrane treated with chlorine dioxide may be a reverse osmosis separation membrane or a nanofiltration membrane.

In the present invention, the ultrafiltration membrane produced by treatment with chlorine dioxide may be used for a dead-end type or a cross-flow type.

The ultrafiltration membrane may have a normalized flux at the time of dead-end type of 6 or more, preferably 8 or more. The normalized flux is a value representing the degree of flux of the membrane after chlorine dioxide exposure based on the flux prior to chlorine dioxide exposure (flux of the membrane after exposure to chlorine dioxide / flux of chloride before exposure to chlorine dioxide) There is a problem that it is not enough to be used as an ultrafiltration membrane.

In addition, the ultrafiltration membrane may have a NaCl rejection of less than 20% at the time of cross-flow filtration.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the embodiments of the present invention described below are illustrative only and the scope of the present invention is not limited to these embodiments. The scope of the present invention is indicated in the claims, and moreover, includes all changes within the meaning and range of equivalency of the claims. In the following Examples and Comparative Examples, "%" and "part" representing the content are on a mass basis unless otherwise specified.

Example - Membranes treated with chlorine dioxide

In this experiment, two types of system were used, one for dead-end type and the other for cross-flow type.

(1) Sign language (48 hours)

(2) Filtration (flux & rejection measurement after operation until flow rate of permeated water is stabilized)

(3) Exposure of the membrane to chlorine dioxide (standard: 1hr, 100 ppm)

In the experiments of the present invention, chlorine dioxide was prepared by referring to 4500-ClO ₂ B as described in standard methods for examination of water & wastewater 21 ^st edition (by Arnold E. "Archie Greenberg).

(4) Cleaning (removing residual chlorine dioxide on the membrane surface using Milli-Q)

(5) Re-filtration (flux & rejection measurement after operation until flow rate of permeable water is stabilized)

Through the above procedure, the pH value of chlorine dioxide was varied as shown in Table 1 below.

pH value Membrane Example 1 12 A Example 2 12 B Example 3 11 A Comparative Example 1 2 A Comparative Example 2 3 A Comparative Example 3 4 A Comparative Example 4 5 A Comparative Example 5 6 A Comparative Example 6 8 A Comparative Example 7 9 A Comparative Example 8 10 A Comparative Example 9 4 B Comparative Example 10 7 B Comparative Example 11 10 B Comparative Example 12 7 A Comparative Example 13 - A Comparative Example 14 - B

A: NF90 (Dow-FilmTec?, Polyamide based TFC (Thin-Film Composite) film

B: BW30 (Dow-FilmTecⓒ, polyamide based TFC (Thin-Film Composite) film

Comparative Examples 13 and 15: No chlorine dioxide treatment

Experimental Example

Experimental Example One. pH According to the change Flux change ( Total volume filtration )

For the membranes treated in Examples 1 and 3 and Comparative Examples 1 to 9, the pure fluxes before and after exposure to the chlorine dioxide solution were measured by the dead-end type. The total volume filtered during operation, "Effect of chlorination condition and permeability of chlorine species on the chlorination of a polyamide membrane , Joung-Eun Gu . "Amicon cell device" was used, and the conditions were as follows.

(1) pressure; 4.0 kgf / cm2 (56.9 psi)

(2) feed; NaCl 0 ppm

(3) temperature; 25?

(4) Membrane area; 4.18 * 10-3m2

(5) Experimental process and time required;

compaction (7hr)

pure water flux measurement (1 hr measurement)

Exposure to ClO ₂ (pH dependent test for 1 hour)

re-compaction (7hr) and re-water flux measurement (1hr measurement)

The results measured in the above are shown in Table 2 and FIG. 2, respectively.

pH value 정화ized flux Example 1 12 8.57 Example 3 11 6.06 Comparative Example 1 2 0.75 Comparative Example 2 3 0.89 Comparative Example 3 4 0.90 Comparative Example 4 5 0.91 Comparative Example 5 6 1.01 Comparative Example 6 8 1.06 Comparative Example 7 9 1.12 Comparative Example 8 10 1.85

The normalized flux is defined as the degree of flux of the membrane after chlorine dioxide exposure based on the flux prior to chlorine dioxide exposure. If there is no change in the value before and after exposure to chlorine dioxide, it will have a value of 1. If the pure flux after exposure is less than the pre-exposure value, the normalized flux will have a value lower than 1. The normalized flux changes shown in Table 2 and FIG. 2 are shown in the order of pH. First, after chlorine dioxide exposure at pH 5 or lower, flux decreased rather than before exposure. Especially, at pH 2, the flux decreased by about 25% after chlorine dioxide exposure. Secondly, at pH 6 and above, the flux increased slightly after exposure to chlorine dioxide, but the flux increased rapidly after exposure to chlorine dioxide at pH 10 or higher. In particular, in Example 3 (pH 11), the flux after exposure was increased by 6 times or more compared with that before exposure, and increased by 8 times or more in Example 1 (pH 12).

The pure water fluxes before and after exposure to the chlorine dioxide solution were measured for the membranes treated in Example 2 and Comparative Examples 9 to 11, and the results are shown in FIG. 3 to FIG. 4, respectively.

Similar to FIGS. 1 and 2, the flux was noticeably increased in the case of exposure to chlorine dioxide at a pH of 12. This seems to be similar because the polyamide structure on the membrane surface is similar.

Experimental Example 2. pH Surface Analysis by Change ( Total volume filtration )

The separation membrane treated with chlorine dioxide (Comparative Example 13, virgin), Example 1 (pH 12) and Comparative Example 3 (pH 4), Comparative Example 12 (pH 7) and Comparative Example 8 (pH 10) And polysulfone layer (PS), the membrane was dried using a freeze dryer for surface analysis. After separating chlorine dioxide and flux, each membrane was washed once more with milli-Q and then dried for 24 hours using a freeze dryer.

In Comparative Example 14 (pH 12) and Comparative Example 9 (pH 4), Comparative Example 10 (pH 7) and Comparative Example 11 (pH 10), the separation membranes not treated with chlorine dioxide (Comparative Example 14, virgin) A separator and polysulfone layer (PS) were also dried in the same manner.

Using the dried separation membrane, FT-IR, XPS and SEM analyzes were performed as described below.

One) FT - IR analysis

The experimental conditions are as follows.

(1 hours; 64s

(2) the number of measurements; 8 to 10 measurements per sample

(3) accessories; Ge

(4) Resolution; 8

(5) Purging time; 24hr or longer, use N ₂

Under the above conditions, FT-IR spectrum analysis was performed. First, treatment in a chlorine dioxide-unseparated membrane (Comparative Example 13, virgin) in Example 1 (pH 12) and Comparative Example 3 (pH 4), Comparative Example 12 (pH 7) and Comparative Example 8 (pH 10) FT-IR spectrum analysis results for a separator and a polysulfone layer (PS) are shown in FIG. In the case of the membrane exposed to chlorine dioxide pH 12, it was found to be almost similar to the polysulfone layer graph. On the other hand, when exposed to chlorine dioxide at the rest of the pH, it was found to be similar to that of a pure membrane (NF90, virgin) before exposure. The result shows that when the separator is exposed to chlorine dioxide at pH 12, the polyamide layer, which is the surface layer of the three-layered separator, is peeled off and the polysulfone layer beneath it appears. As a result, it was confirmed that amide I and II among the peaks of the membrane were decreased before and after exposure to chlorine dioxide.

Next, the separation membrane without chlorine dioxide (Comparative Example 14, virgin), Example 2 (pH 12) and Comparative Example 9 (pH 4), Comparative Example 10 (pH 7) and Comparative Example 11 (pH 10) FT-IR spectrum analysis results of the treated separator and the polysulfone layer (PS) are shown in FIGS. 8 and 9.

Referring to the graphs of FIGS. 8 and 9, when exposed to chlorine dioxide at a pH of 12, the appearance of the polysulfone layer is similar to that of the graph. It was found that when exposed to chlorine dioxide at pH 12, the polyamide layer on the surface of the membrane was lost. In the case of pHs other than pH 12, it was found that they have a similar shape near the graph of Comparative Example 14 (virgin). Therefore, it can be seen that the FT-IR spectral results of Example 2 (BW30) and related comparative examples are similar to the FT-IR spectral results of Example 1 (NF90) and the related comparative examples.

2) XPS analysis

X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the composition of elements in the separator before and after exposure to chlorine dioxide. The results are shown in Table 3.

Comparative Example 13 Comparative Example 3 Comparative Example 12 Comparative Example 8 Example 1 Reference example NF90_ Virgin (%) NF90_pH4
(%) NF90_pH7 (%) NF90_pH10 (%) NF90_pH12
(%) NF90_PS (%) C ^1s 77.78 74.39 74.26 72.18 65.86 83.70 ^O1s 10.97 15.78 15.70 17.89 28.92 11.66 N ^1s 11.25 9.83 10.04 9.93 3.88 0.28 S ^2p 0 0 0 0 1.34 4.34 TOTAL 100 100 100 100 100 100

As shown in the above Table 3, it can be seen that as the pH of chlorine dioxide increases, the carbon element decreases, the oxygen element increases, and the nitrogen element decreases as compared with Comparative Example 13 (NF90, virgin) before chlorine dioxide exposure. In particular, when exposed to chlorine dioxide at a pH of 12, sulfur (S) was newly detected on the surface of the membrane. It appears that the polyamide active layer was stripped and sulfur present in the polysulfone below was detected.

3) SEM analysis

In order to confirm the surface structure of the membrane, changes in the surface and internal structure of the membrane were confirmed by taking each membrane before and after exposure to chlorine dioxide.

First, treatment in a chlorine dioxide-unseparated membrane (Comparative Example 13, virgin) in Example 1 (pH 12) and Comparative Example 3 (pH 4), Comparative Example 12 (pH 7) and Comparative Example 8 (pH 10) SEM analysis photographs of a separator and a polysulfone layer (PS) are shown in FIG. 6 (× 10,000 times) and FIG. 7 (× 35,000 times).

As shown in Fig. 6 and Fig. 7, almost similar surface appearance was observed except for the case of Example 1 (pH 12). In the case of Example 1 (pH 12), the branch-like surface disappeared and the surface filled with pores of various sizes could be identified. This shows that the polyamide layer of Example 1 is lost and the polysulfone layer supporting the polyamide layer immediately below is exposed.

Next, the separation membrane without chlorine dioxide (Comparative Example 14, virgin), Example 2 (pH 12) and Comparative Example 9 (pH 4), Comparative Example 10 (pH 7) and Comparative Example 11 (pH 10) SEM analysis photographs of the separated membranes and polysulfone layers (PS) were shown in Fig. 10 (x 10,000 times) and Fig. 11 (x 35,000 times).

As shown in Figs. 10 and 11, almost similar surface (polyamide) appearance was observed except when exposed to chlorine dioxide of Example 2 (pH 12). In Example 2 (pH 12), a surface with a twig shaped surface is left behind and a surface filled with pores of various sizes beneath. This shows that the polyamide layer of Example 2 (pH 12) is lost and the polysulfone layer supporting the polyamide layer immediately below is exposed.

Experimental Example 3. Time-dependent Flux change ( Total volume filtration )

For the separation membrane treated in Comparative Example 3 (pH 4), the pure water flux before and after exposure to the chlorine dioxide solution by time was measured by a dead-end type equation and shown in FIGS. 14 and 15 .

As can be seen from the comparison of FIGS. 14 and 15, the difference in the pure water permeability between before and after exposure to chlorine dioxide is low when the pH is low. Thus, exposure of chlorine dioxide at low pH It can not be changed.

Experimental Example 4. pH According to the change Flux change ( Cross-flow filtration )

Exposure to the chlorine dioxide solution to the separation membrane treated in the chlorine dioxide-untreated separation membrane (Comparative Example 13, virgin), Example 1 (pH 12) and Comparative Example 3 (pH 4) and Comparative Example 8 (pH 10) The pure water flux before and after was measured by cross-flow type. The cross-flow filtration during operation, "Journal of applied Polymer science 102 (2006), 5895-5902 "was used, and the conditions were as follows.

(1) pressure; 125 psi for BW30, 225 psi for BW30,

(2) feed; NaCl 2000 ppm (50 L)

(3) temperature; 25 ° C ± 0.1 ° C

(4) Membrane area; 6.49 * 10 ^-3 m ²

(5) flow rate; 1 L / min

(6) Experimental procedure and time required;

compaction, flux & rejection measurement (36hr)

ClO ₂ reaction (exposure for 1 hour per pH)

Re-compaction, flux & rejection measurement (48hr)

The results measured in the above are shown in Figs. 12 and 13, respectively.

As shown in FIG. 12 and FIG. 13, when the chlorine dioxide is exposed to the chlorine dioxide of Example 1 (pH 12), the flux after exposure increases sharply. On the other hand, it was confirmed that the flux slightly increased in Comparative Example 3 (pH 4).

Conversely, in Rejection, in Example 1 (pH 12), the rejection drops to almost zero, which is inversely proportional to the flux change. It was confirmed that when exposed to chlorine dioxide at pH 12, the polyamide present in the surface layer of the membrane was lost.

Claims

And removing the polyamide (PA) membrane by treating chlorine dioxide (ClO ₂ ) having a pH of 11 or more to the separation membrane having a polyamide (PA) layer formed on the surface thereof for 10 minutes to 3 hours.

The method according to claim 1,
Wherein the chlorine dioxide (ClO ₂ ) has a pH of 12 or higher.

The method according to claim 1,
(PA) layer, a polyamide (PA) layer, a polysulfone (PS) layer and a nonwoven fabric layer, and a polyamide (PA) A method for producing a filtration membrane.

The method of claim 3,
Wherein the polyamide (PA) layer comprises a copolymer of trimesoyl chloride (TMC) based monomer and m-phenylenediamine (MPD) based monomer.

5. The method of claim 4,
Wherein a hydrophilic material is further coated on the polyamide (PA) layer.

6. The method of claim 5,
Wherein the hydrophilic material is at least one of polyvinyl alcohol and polyethylene glycol.

The method of claim 3,
Wherein the polyamide (PA) layer is surface-synthesized on the polysulfone layer.

delete

The method according to claim 1,
Wherein the treatment concentration of the chlorine dioxide (ClO ₂ ) is 10 to 1000 ppm.

The method according to claim 1,
After treatment of the chlorine dioxide (ClO _2), method for producing a ultrafiltration membrane, further including the step of removing the chlorine dioxide (ClO ₂₎ of membrane surface.

The method according to claim 1,
Wherein the separation membrane is a reverse osmosis separation membrane or a nanofiltration membrane.

The method according to claim 1,
Wherein the ultrafiltration membrane is a dead-end type or a cross-flow type ultrafiltration membrane.

13. The method of claim 12,
Wherein the normalized flux at the time of dead-end type of the ultrafiltration membrane is at least 6, wherein the flux of the membrane before exposure to chlorine dioxide is 6 or more.

14. The method of claim 13,
Wherein the normalized flux is 8 or more.

13. The method of claim 12,
Wherein the ultrafiltration membrane has a rejection of 20% or less at the time of cross-flow type filtration.