JP2004057883A - Water cleaning method using external pressure type hollow fiber membrane module and apparatus therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water cleaning method using an external pressure type hollow fiber membrane module, which makes it unnecessary to wash using chemicals of high concentration because washing effect of hollow fiber membranes is high and a rise in the differential pressure on the surfaces of membranes is suppressed over a long period of time, and which enhances the recovery ratio of cleaned water because the rate of operation of equipment is enhanced, energy consumed in operation can be reduced and the consumption of cleaned water and formation of wastewater is reduced, and also to provide an apparatus therefor. <P>SOLUTION: This water cleaning method includes a water passing process for introducing raw water 1 from the outside of the hollow fiber membranes 22 of the external pressure type hollow fiber membrane module 20 to obtain the cleaned water 4 from the inside thereof, a first backwashing process continued to the water passing process to discharge the concentrated raw water 1 in the casing 24 of the module 20 and a membrane surface chemical washing process continued to the first backwashing process to allow a chemical liquid 5 to flow to the outside from the inside of the hollow fiber membranes 22 of the module 20 and discharging a waste liquid from the casing 24. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing water purification using an external pressure type hollow fiber membrane module and an apparatus therefor, and more particularly, to remove dirt adhering and accumulating on a membrane surface and a filtration layer of a hollow fiber to obtain purified water for a long period of time. The present invention relates to a method and an apparatus for producing purified water using an external pressure type hollow fiber membrane module capable of producing water.
[0002]
[Prior art]
As a method for treating raw water such as river water, brackish water, seawater and wastewater, conventionally, hollow fiber membranes such as hollow fiber type microfiltration membrane (MF), ultrafiltration membrane (UF), and nanofiltration membrane (NF) have been used. The methods used are known. Hollow fiber membranes have high particle removal performance and can almost completely remove particles in raw water, and their operation is simple. Widely used. Specifically, alternative treatment means for sand filtration in tap water treatment, pretreatment means for obtaining fresh water from brackish water or seawater by a reverse osmosis membrane method, means for removing insoluble solids from wastewater, etc. It is typical.
[0003]
However, when raw water is passed in a water treatment apparatus using such a hollow fiber membrane, suspended membranes and microorganisms contained in the raw water, the presence of microorganisms, soluble high-molecular organic substances, and the like cause the hollow fiber membrane surface and In addition, dirt adheres and accumulates in a space formed of bundles of hollow fibers (hereinafter, referred to as a “filtration layer”), and the filtration resistance increases. Therefore, in order to obtain a predetermined amount of filtered water, it is necessary to operate at a high pressure, and there is a problem that energy consumed for operating the water treatment device increases. Further, as this contamination progresses, there is a problem that continuation of filtration becomes impossible.
[0004]
Therefore, at regular intervals or when the apparatus exhibits a predetermined pressure rise, the membrane surface and the filtration layer of the hollow fiber are back-washed with (1) purified water subjected to filtration treatment, and (2) reversely washed with compressed air. (3) A method of cleaning a material membrane surface and a filtration layer, such as mixing and supplying compressed air to raw water to vibrate and separate attached matter, is performed alone or in combination (hereinafter, reverse). Washing).
[0005]
In addition, since the backwashing operation is usually performed using purified water, which is treated water, a large amount of purified water is consumed at the time of backwashing, thereby lowering the recovery rate of the purified water. There is a problem in that a large amount of wastewater containing the wastewater is generated, so that the backwash wastewater needs to be treated.
Further, when compressed air is used in the backwashing operation, air accumulates inside the pores of the hollow fiber membrane, and the processing performance is reduced. In order to prevent this, it is necessary to perform an air removal operation.However, performing the air removal operation increases the backwashing time, shortening the operation time for recovering purified water, and consequently reducing the equipment operation rate. is there.
[0006]
This backwashing operation is usually performed every tens of minutes to every several hours. However, there is a problem in that, due to the operation for a long time (several hundred hours to several thousand hours), substances that cannot be completely removed by backwashing are accumulated, and the resistance when flowing water increases.
For this reason, when the pressure difference on the membrane surface during water passage becomes large due to long-term use, the operation operation by repeating normal filtration and backwashing is stopped, and high-concentration chemicals such as acids, alkalis, and oxidizing agents are removed. It is necessary to carry out a method of removing the accumulated deposits by carrying out washing (hereinafter referred to as chemical washing or chemical washing; hereinafter, referred to as chemical washing in this specification).
[0007]
On the other hand, Japanese Patent Application Laid-Open No. Hei 7-275671 discloses a more effective method by combining a method of cleaning by mixing a medicine with a back pressure cleaning liquid that is regularly performed and a method of cleaning with raw water mixed with compressed air. It describes that the hollow fiber membrane is washed. However, in this case, a chemical is used at the time of back washing, so that the amount of chemical used increases. Since a large amount of raw water mixed with compressed air is used, there is a problem that backwash drainage increases. Further, depending on the properties of the raw water, the cleaning effect may be insufficient. Therefore, there is a problem that a chemical cleaning operation using a high-concentration chemical needs to be performed about every 1000 hours.
[0008]
[Problems to be solved by the invention]
In view of the above problems, the present invention has a high cleaning effect on a hollow fiber membrane and can suppress an increase in the differential pressure on the membrane surface for a long period of time. The external pressure type hollow fiber membrane module which can improve the purification rate of purified water can be improved because the energy consumption for operation can be reduced as well as the consumption of purified water and the amount of generated wastewater can be reduced. It is an object of the present invention to provide a method for producing purified water and an apparatus therefor.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, in the method for producing water purification using the external pressure type hollow fiber membrane module according to the present invention, raw water is introduced from outside the hollow fiber membrane of the external pressure type hollow fiber membrane module, and purified water is obtained from the inside. It is characterized by including a water passing step, and a first backwashing step of discharging the concentrated raw water in the casing of the module following the water passing step. Further, following the first backwashing step, a chemical solution flows from the inside to the outside of the hollow fiber membrane of the module, and then flows downward along the membrane surface, and the waste liquid is discharged from the casing. A step can also be included.
[0010]
As described above, since solids, colloidal substances, microorganisms, and the like are concentrated in the water remaining in the module in the water passing step, the accumulated solids and the like are removed from the casing by draining them in the first backwashing step. Can be removed from At this time, since the raw water containing a high concentration of solids and the like is discharged as it is without using purified water for washing, the recovery rate of purified water can be improved. Further, since the inside of the casing is almost empty and has no water, the chemical solution flowing in the membrane washing step is not diluted, and a high washing effect can be exerted on the membrane surface of the hollow fiber and the filtration layer. In addition, since the liquid medicine is not diluted, the used amount of the liquid medicine can be reduced.
[0011]
Further, as another form, the water purification method using the external pressure type hollow fiber membrane module according to the present invention is a method of introducing raw water from outside the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside. A first backwashing step of discharging the concentrated raw water in the casing of the module following the water step and the water passing step; and an inner side of the hollow fiber membrane of the module following the first backwashing step. And a second backwashing step of discharging the waste liquid from inside the casing after flowing the purified water out of the casing.
[0012]
As described above, since the raw water in which the solid content and the like are concentrated in the water passing step is drained as it is in the first backwashing step before introducing the purified water, the recovery rate of the purified water can be improved. Further, since the casing is almost empty without water in the first backwashing step, the hollow fiber is released from the water pressure at the time of the water passing step, the hollow fiber itself is easy to move, and the deposit due to physical shaking is formed. The dirt attached to the membrane surface and the filtration layer can be effectively removed even with a small amount of purified water.
[0013]
Furthermore, as another form, the water purification method using the external pressure type hollow fiber membrane module according to the present invention is a method for introducing raw water from outside the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside. A first backwashing step of discharging the concentrated raw water in the casing of the module following the water step and the water passing step; and an inner side of the hollow fiber membrane of the module following the first backwashing step. After flowing the chemical solution from the outside to the outside, the waste water is discharged from the inside of the casing, and following the membrane cleaning process, the purified water is flowed from the inside to the outside of the hollow fiber membrane of the module. Thereafter, a second backwashing step of discharging the waste liquid from the casing is included.
[0014]
In this manner, by performing the second backwashing step after the membrane surface washing step, the inside of the hollow fiber can be effectively used for washing.
[0015]
In addition, the method for producing purified water using the external pressure type hollow fiber membrane module according to the present invention is characterized in that, in a state where the raw water is introduced into the casing of the external pressure type hollow fiber membrane module, the raw water mixed with the compressed air in the casing. After the introduction, the air washing step of discharging the waste liquid from the inside of the casing may be further included.
[0016]
In this way, by supplying compressed air while the casing is filled with raw water, dirt attached to the membrane surface and the filtration layer, particularly, sticky deposits that are difficult to remove, are forcibly removed. Can be. In addition, since raw water is used instead of purified water, it can contribute to an improvement in the recovery rate of purified water.
[0017]
It is preferable to use air, especially compressed air, for discharging the raw water or waste liquid. Thereby, the raw water or the waste liquid can be discharged from the casing in a short time. Further, in the water purification method using the external pressure type hollow fiber membrane module according to the present invention, after repeating the water passing step and the first backwashing step two or more times, preferably 10 to 50 times, the first backwashing step is repeated. Subsequent to the step, the film surface chemical washing step or the second backwashing step can be performed.
[0018]
It is preferable to use two or more types of chemicals as the chemical. In addition, as the chemical, it is preferable to use an aqueous solution of sodium hypochlorite, sodium hydroxide, chlorine, chlorine dioxide, citric acid, hydrochloric acid, sulfuric acid, nitric acid, or a mixed aqueous solution thereof. Furthermore, it is preferable to use a microfiltration membrane, an ultrafiltration membrane, or a nanofiltration membrane as the hollow fiber membrane of the external pressure type hollow fiber membrane module. Furthermore, as the raw water, river water, brackish water, seawater or wastewater can be used.
[0019]
Further, as another aspect, the present invention relates to a water purification production device, comprising an external pressure type hollow fiber membrane module, a drainage system for discharging a liquid in a casing of the module, and a hollow fiber membrane of the module. A first air supply system for supplying compressed air to the inside.
[0020]
The water purification production device according to the present invention may further include a chemical solution supply unit for supplying a chemical solution inside the hollow fiber membrane of the module.
Further, the water purification apparatus according to the present invention includes a back pressure cleaning means for flowing purified water inside the hollow fiber membrane of the module, and a second air supply system for supplying compressed air to the back pressure cleaning means. Can be included.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram for explaining an embodiment of a water purification production method using an external pressure type hollow fiber membrane module according to the present invention. As shown in FIG. 1, a raw water tank 10 that stores raw water 1 that is river water, brackish water, seawater, or wastewater before filtration is connected to a hollow fiber membrane module 20 via a raw water pipe 12. The raw water pipe 12 is provided with a raw water pump 14 and a raw water valve 16. The raw water pipe 12 is connected between the raw water valve 16 and the hollow fiber membrane module 20 to a drain pipe 26 connected to a waste water treatment facility (not shown). The drain pipe 26 is provided with a drain valve 28.
[0022]
The hollow fiber membrane module 20 is connected to a backwash tank 30 via a backwash tube 32. The backwash pipe 32 is provided with a backwash valve 34. The backwash tank 30 is further connected to a water purification tank 40 via a water purification pipe 42. The water purification pipe 42 is provided with a water purification valve 44. Further, the backwash pipe 32 is connected between the hollow fiber membrane module 20 and the backwash valve 34 with a chemical pipe 52 leading to the chemical tank 50. The chemical pipe 52 is provided with a chemical pump 54 and a chemical valve 56. Further, as pipes from a compressed air supply facility (not shown), the first air pipe 61 is connected to the hollow fiber membrane module 20, the second air pipe 64 is connected to the backwash tank 30, and the third air pipe 67 is connected to the raw water pipe 12. It is connected to the. The first air pipe 61, the second air pipe 64, and the third air pipe 67 are provided with a first air valve 62, a second air valve 65, and a third air valve 68, respectively.
[0023]
Here, the hollow fiber membrane module 20 will be further described. FIG. 2 is a simplified cross-sectional view showing an example of the hollow fiber membrane module according to the present invention. As shown in FIG. 2, the hollow fiber membrane module 20 includes a cylindrical casing 24 and thousands of bundles of hollow fiber membranes 22 arranged in the center of the casing. As the hollow fiber membrane 22, it is preferable to use a microfiltration membrane (MF), an ultrafiltration membrane (UF), or a nanofiltration membrane (NF). As shown in FIG. 2, the raw water pipe 12 is connected to a flow path outside the hollow fiber membrane 22. On the other hand, the backwash pipe 32 and the first air pipe 61 are connected to a flow path inside the hollow fiber membrane 22.
[0024]
According to such a configuration, first, in order to start the water passing step, the raw water valve 16, the backwash valve 34, and the water purification valve 44 are opened, and the drain valve 28, the chemical liquid valve 56, the first to third air valves 62, 65 and 68 are closed. Then, the raw water pump 14 is started to start the water passage process. In the water passing step, the raw water 1 is introduced from the raw water tank 10 into the channel outside the hollow fiber membrane 22 of the hollow fiber membrane module 20. The raw water 1 introduced into the flow path outside the hollow fiber membrane 22 passes through the hollow fiber membrane 22 and flows into the flow path inside the hollow fiber membrane 22, whereby the raw water 1 is filtered to become purified water 4. The purified water 4 is introduced into the backwash tank 30 through the backwash pipe 32, and then sent to the purified water tank 40 through the clean water pipe 42. Thus, purified water 4 from which insoluble solids (including colloids) have been removed from raw water 1 can be produced.
[0025]
Next, in order to start the first backwashing step, the raw water pump 14 is stopped, and the raw water valve 16, the backwash valve 34, and the water purification valve 44 are closed. Then, the drain valve 28 and the first air valve 62 are opened to start the first backwashing step. In the first backwashing step, the raw water 1 remaining in the casing 24 of the hollow fiber membrane module 20 is discharged from the drain pipe 26 as drain water 2 by the compressed air 6 supplied from the first air pipe 61 together with gravity. Thus, solids and the like accumulated in the raw water 1 can be discharged from the hollow fiber membrane module 20, and an increase in the membrane surface pressure difference of the hollow fiber membrane 22 can be suppressed. The first backwashing step is periodically performed every time the water passing step is performed for 15 minutes to 2 hours, preferably 30 minutes to 1 hour.
[0026]
Here, the compressed air 6 is supplied from the first air pipe 61 to forcibly discharge the raw water 1, but the raw water 1 may be discharged only by gravity without supplying the compressed air 6. However, by using the compressed air 6, the raw water 1 can be discharged in a short time. Further, in FIG. 2, the compressed air 6 is supplied only to the inside of the hollow fiber membrane 22, but may be supplied to both the inside and the outside of the hollow fiber membrane 22. Thereby, the raw water 1 can be discharged in a shorter time. Further, the purified water 4 remaining in the hollow fiber membrane 22 may be discharged together with the raw water 1. In particular, in the case where a membrane cleaning step described later is performed, it is preferable to discharge substantially all of the raw water 1 and the purified water 4.
[0027]
Next, in order to start the membrane cleaning process, the drain valve 28 and the first air valve 62 are closed, and the chemical liquid valve 56 is opened. Then, the chemical pump 54 is started to start the membrane cleaning process. The chemical solution 5 stored in the chemical solution tank 50 is introduced into the hollow fiber membrane module 20 inside the hollow fiber membrane 22 through the chemical solution pipe 52 and the backwash pipe 32. Here, the raw water 1 is discharged in the first backwashing step, and since the inside of the casing 22 is almost empty, the chemical solution 5 passes through the hollow fiber membrane 22 without being diluted with the raw water 1 or the like. Smash the outside of the Therefore, even with a small amount of the chemical solution 5, a high cleaning effect can be exerted on the inside and outside surfaces of the pores of the hollow fiber membrane 22.
[0028]
When the cleaning of the hollow fiber membrane 22 is completed, the chemical solution pump 54 is stopped, and the chemical solution valve 56 is closed. Then, by opening the drain valve 28 and the first air valve 62, the cleaned chemical solution 5 collected in the lower portion of the casing 24 of the hollow fiber membrane module 20 is discharged from the drain pipe 26 as the drain 2 by gravity and compressed air 6. I do. By this membrane surface washing step, solids and the like adhering to the membrane surface and the filtration layer of the hollow fiber membrane 22 can be more completely removed, so that an increase in the membrane surface differential pressure can be effectively suppressed. .
[0029]
Generally, in the hollow fiber membrane module, the hollow fiber filling rate in the casing is about 35 to 50%, and the space is about 50 to 65%. Conventionally, when the hollow fiber membrane is washed with a chemical, it has been necessary to fill the space with a chemical. On the other hand, in the membrane surface chemical washing step according to the present invention, since the raw water or the like remaining in the casing of the hollow fiber membrane module is drained, the chemical solution is not diluted in the space, and only the inside of the hollow fiber membrane is filled with the chemical solution. Is good. Therefore, according to the present invention, the used amount of the chemical can be reduced to about 50% of the conventional amount.
[0030]
Also in this case, the compressed air 6 is supplied from the first air pipe 61 to forcibly discharge the chemical 5, but the chemical 5 may be discharged only by gravity without supplying the compressed air 6. However, by using the compressed air 6, the chemical solution 5 can be discharged in a short time. Further, in FIG. 2, the compressed air 6 is supplied only to the inside of the hollow fiber membrane 22, but may be supplied to both the inside and the outside of the hollow fiber membrane 22. Thereby, the chemical solution 5 can be discharged in a shorter time. The film surface chemical washing step is periodically performed at a rate of one time or more, preferably 2 to 10 times or more, more preferably 10 to 50 times of the first backwashing step.
[0031]
Next, the first air valve 62 is closed to start the second backwashing step. Then, the backwash valve 34 and the second air valve 65 are opened to start the second backwash process. Since the compressed air 6 is supplied to the backwash tank 30 through the second air pipe 64, the purified water 4 stored in the backwash tank 30 passes through the backwash pipe 32 and passes through the hollow fiber of the hollow fiber membrane module 20. It is introduced inside the membrane 22. Since the inside of the casing 22 of the hollow fiber membrane module 20 is almost empty due to the previous process, the hollow fiber itself is easy to move, and the effect of removing extraneous matter due to physical sway is added. A high cleaning effect can be exerted on the inside and outside surfaces of the pores. The purified water 4 that has passed through the hollow fiber membrane 22 falls on the outside of the hollow fiber membrane 22 and is discharged from the drain pipe 26. The second backwashing step is periodically performed once or more, preferably 2 to 10 times or more, more preferably 10 to 50 times, once per 1st backwashing step.
[0032]
Next, the drain valve 28, the backwash valve 34, and the second air valve 65 are closed to start the air cleaning process. Then, the raw water valve 16 is opened, the raw water pump 14 is started, and the air cleaning process is started. First, the raw water 1 is introduced through the raw water pipe 12 to the outside of the hollow fiber membrane 22 of the hollow fiber membrane module 20. Then, after the outside of the hollow fiber membrane 22 is filled with the raw water 1, when the third air valve 68 is opened, the compressed air 6 passes through the third air pipe 67 and the outside of the hollow fiber membrane 22 of the hollow fiber membrane module 20. Supplied to The compressed air 6 can forcibly remove dirt such as solids attached to the hollow fiber membrane 22, particularly, sticky substances that are difficult to remove, and can exhibit a high cleaning effect. When the cleaning of the hollow fiber membrane 22 is completed, the raw water tank 14 is stopped, the raw water valve 16 and the third air valve 68 are closed, the drain valve 28 and the first air valve 62 are opened, and the raw water 1 in the casing 24 is drained. The liquid is discharged to the pipe 26, and the inside of the casing 24 is emptied again. The air cleaning step is periodically performed once or more, preferably 2 to 10 times or more, more preferably 10 to 50 times, once per 1st backwashing step.
[0033]
Finally, the drain valve 28 and the first air valve 62 are closed, and the raw water valve 16, the backwash valve 34, and the water purification valve 44 are opened in order to start the water passing step again. Then, by starting the raw water pump 14, the water passing step is started. Since the adhering matter on the membrane surface of the hollow fiber membrane 22 and the filtration layer has been removed by the above-described first backwashing step, membrane washing step, second backwashing step, and air washing step, the membrane surface pressure difference And purified water can be produced with less energy consumption.
[0034]
Specifically, according to the present invention, since the amount of purified water used is reduced, the recovery ratio of purified water to raw water is increased to about 95 to 98%, and the generation rate of wastewater is increased to about 2 to 5%. be able to. On the other hand, in the conventional water purification equipment, the recovery ratio of purified water to raw water is usually about 90 to 95%, and the generation rate of wastewater is about 5 to 10%. Therefore, according to the present invention, the recovery rate of purified water can be increased by about 3 to 5%, and the amount of generated wastewater can be reduced to about 50 to 60% of the conventional amount.
[0035]
In the above description, the water-passing step, the first backwashing step, the membrane cleaning step, the second backwashing step, the air cleaning step, and the water-passing step are described in this order. Except for performing the steps, the order is not particularly limited, and many variations can be adopted. For example, after repeating the order of the water-passing step, the first back-washing step, and the water-passing step twice to 50 times, performing a membrane surface chemical washing step, a second back-washing step or an air washing step, or an air washing step After, to perform the membrane cleaning step or the second backwashing step, and after the second backwashing step, to perform the membrane cleaning step, or after the membrane cleaning step or the second backwashing step Performing a water-passing step can also be adopted.
[0036]
【The invention's effect】
As described above, according to the present invention, the cleaning effect of the hollow fiber membrane is high, and it is possible to suppress the rise in the differential pressure of the membrane surface for a long period of time. The external pressure type hollow fiber membrane module which can improve the purification rate of purified water can be improved because the energy consumption for operation can be reduced as well as the consumption of purified water and the amount of generated wastewater can be reduced. A method for producing purified water and an apparatus therefor can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic view showing one embodiment of a water purification method using an external pressure type hollow fiber membrane module according to the present invention.
FIG. 2 is a simplified cross-sectional view showing an example of an external pressure type hollow fiber membrane module according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Raw water 2 Drainage 3 Purified water 5 Chemical solution 6 Compressed air 10 Raw water tank 12 Raw water pipe 14 Raw water pump 16 Raw water valve 20 Hollow fiber membrane module 22 Hollow fiber membrane 24 Casing 26 Drain pipe 28 Drain valve 30 Backwash tank 32 Backwash pipe 34 Reverse Valve 40 Water purification tank 42 Water purification pipe 44 Water purification valve 50 Chemical liquid tank 52 Chemical liquid piping 54 Chemical liquid pump 56 Chemical liquid valve 61 First air pipe 62 First air valve 64 Second air pipe 65 Second air valve 67 Third air pipe 68 3 air valve

Claims (14)

A water passing step of introducing raw water from the outside of the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside,
A method for producing purified water using an external pressure type hollow fiber membrane module, comprising: a first backwashing step of discharging the concentrated raw water in the casing of the module following the water passing step. A water passing step of introducing raw water from the outside of the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside,
A first backwashing step of discharging the concentrated raw water in the casing of the module following the water passing step;
After the first backwashing step, after flowing a chemical solution from the inside to the outside of the hollow fiber membrane of the module, a membrane surface washing step of discharging the waste liquid from inside the casing. A water purification method using a hollow fiber membrane module. A water passing step of introducing raw water from the outside of the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside,
A first backwashing step of discharging the concentrated raw water in the casing of the module following the water passing step;
A second backwashing step of flowing the purified water from the inside to the outside of the hollow fiber membrane of the module after the first backwashing step, and then discharging the waste liquid from the inside of the casing. Water purification method using a hollow fiber membrane module. A water passing step of introducing raw water from the outside of the hollow fiber membrane of the external pressure type hollow fiber membrane module and obtaining purified water from the inside,
A first backwashing step of discharging the concentrated raw water in the casing of the module following the water passing step;
Following the first backwashing step, after flowing a chemical solution from the inside to the outside of the hollow fiber membrane of the module, a membrane surface washing step of discharging the waste liquid from the inside of the casing;
A second back washing step of flowing the purified water from the inside to the outside of the hollow fiber membrane of the module following the membrane washing step, and then discharging the waste liquid from the casing. Water purification method using a hollow fiber membrane module. The method for producing purified water using an external pressure type hollow fiber membrane module according to claim 2 or 4, wherein two or more types of chemicals are used as the chemical liquid. An aqueous solution of sodium hypochlorite, sodium hydroxide, chlorine, chlorine dioxide, citric acid, hydrochloric acid, sulfuric acid, nitric acid, or a mixed aqueous solution thereof, as the chemical solution. Water purification method using external pressure type hollow fiber membrane module. In a state where the raw water is introduced into the casing of the external pressure type hollow fiber membrane module, after introducing the raw water mixed with the compressed air into the casing, the method further includes an air washing step of discharging the waste liquid from the casing. A method for producing purified water using the external pressure type hollow fiber membrane module according to any one of claims 1 to 6. 3. The method according to claim 2, wherein after the water-passing step and the first backwashing step are repeated twice or more, the membrane backwashing step or the second backwashing step is performed following the first backwashing step. 4. A method for producing purified water using the external pressure type hollow fiber membrane module according to any one of claims 1 to 7. When discharging raw water or waste liquid in the casing, use the external pressure type hollow fiber membrane module according to any one of claims 1 to 8, wherein air is supplied into the casing to promote discharge. Water purification method. The external pressure type hollow fiber membrane module according to any one of claims 1 to 9, wherein the hollow fiber membrane of the external pressure type hollow fiber membrane module is a microfiltration membrane, an ultrafiltration membrane, or a nanofiltration membrane. Water purification method. The method for producing purified water using an external pressure type hollow fiber membrane module according to any one of claims 1 to 10, wherein the raw water is river water, brackish water, seawater, or wastewater. Including an external pressure type hollow fiber membrane module, a drainage system for discharging the liquid in the casing of the module, and a first air supply system for supplying compressed air to the inside of the hollow fiber membrane of the module A water purification apparatus characterized by the following. 13. The water purification apparatus according to claim 12, further comprising a chemical supply unit for supplying a chemical to the inside of the hollow fiber membrane of the module. 13. The method according to claim 12, further comprising: a back-pressure cleaning unit for flowing purified water inside the hollow fiber membrane of the module; and a second air supply system for supplying compressed air to the back-pressure cleaning unit. A water purification apparatus according to claim 13.

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