FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION [See section 10, Rule 13]
WATER TREATMENT SYSTEM;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANISED AND
EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3,
MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 100-8310, JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

DESCRIPTION
TECHNICAL FIELD
[0001] The present disclosure relates to a water treatment
system.
BACKGROUND ART
[0002] Membrane treatment in which a filtration membrane is used has been utilized as a method for separating a suspension substance from treatment-target water. For example, treatment of sewage water and drainage from factories involves: treating treatment-target water through an activated sludge process; and then, separating and removing a suspension substance by using a filtration membrane. As filtration membranes, cylindrical or sheet-shaped microfiltration membranes and ultrafiltration membranes are generally used. As filtration types, there are: an external pressure filtration type in which treatment-target water is flowed on the outer side of a cylindrical or sheet-shaped filtration membrane, and filtered water is flowed on the inner side of the filtration membrane; and an internal pressure filtration type in which treatment-target water is flowed on the inner side of a cylindrical filtration membrane, and filtered water is flowed on the outer side of

the filtration membrane. In such membrane treatment in which a filtration membrane is used, filtering performance of the filtration membrane decreases in accordance with continuous use thereof.
[0003] Specifically, in accordance with continuous use of a filtration membrane, clogging occurs owing to adhesion of contaminants in pores of the filtration membrane, or on a surface of the filtration membrane with which treatment-target water comes in contact (the outer surface in the case of the external pressure filtration type or the inner surface in the case of the internal pressure filtration type) or on a surface of the filtration membrane with which the filtered water comes into contact (the inner surface in the case of the external pressure filtration type or the outer surface in the case of the internal pressure filtration type). Consequently, the filtering performance of the filtration membrane gradually decreases. In particular, if clogging occurs in the filtration membrane, an increased pressure is required at the time of filtration, and thus a membrane filtration flux (a membrane-filtered water amount per unit time or unit membrane area) also decreases. Thus, it is required to periodically clean the filtration membrane in order to maintain the performance of the filtration membrane. [0004] In view of this, as a method for maintaining filtering performance, there is a method in which reverse

flow cleaning is performed on a filtration membrane from a secondary side of the filtration membrane opposite to a primary side thereof by using cleaning water that contains an oxidant such as sodium hypochlorite (also called “hypochlorite of soda”) or ozone so that contaminants that, by intermolecular force, have been chemically adhered in pores of the filtration membrane or on a surface of the filtration membrane with which membrane-filtered water comes into contact, are oxidized and decomposed in order to clean the filtration membrane.
[0005] In addition, there is a filtration membrane cleaning method in which a plurality of chemical solutions, e.g., alkaline substances such as sodium hypochlorite or acidic substances such as hydrochloric acid, sulfuric acid, and citric acid, are stored in separate chemical solution tanks, and a plurality of cleaning waters pass sequentially by means of a switching valve (see Patent Document 1). [0006] Further, there is also a method in which, in order to furthermore improve the cleaning effect of an oxidant, reverse flow cleaning is performed on a filtration membrane by using cleaning water that contains ozone after reverse flow cleaning is performed on the filtration membrane by using cleaning water that contains sodium hypochlorite (see Patent Document 2).

CITATION LIST
PATENT DOCUMENT
[0007] Patent Document 1: Japanese Laid-Open Patent
Publication No. 2005-193132
Patent Document 2: Japanese Patent No. 5933854
SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0008] In membrane treatment in which a filtration membrane is used, the filtration membrane might be fractured in association with continuous use of the filtration membrane so that a suspension substance and a dissolved organic substance in treatment-target water might flow into filtered water. If a chemical solution remaining in a pipe is removed by using the cleaning water into which the suspension substance and the dissolved organic substance in the treatment-target water have flowed, there is a problem in that the chemical solution and each of the suspension substance and the dissolved organic substance in the cleaning water react to each other so that the concentration of the chemical solution cannot be maintained.
[0009] The present disclosure has been made to solve the above problem, and an object of the present disclosure is to provide a water treatment system that enables a plurality of chemical solutions to be used for a filtration membrane while

maintaining the concentrations of the chemical solutions.
MEANS FOR SOLVING THE PROBLEMS
[0010] A water treatment system according to the present disclosure includes a treatment-target water tank accommodating treatment-target water; a membrane module provided in the treatment-target water tank and having a filtration membrane configured to perform filtration treatment of the treatment-target water; a plurality of chemical solution supply portions configured to supply a plurality of respective chemical solutions for cleaning the filtration membrane; a cleaning water tank accommodating filtered water obtained by the filtration treatment in the membrane module, the filtered water being cleaning water used for removing chemical solution remaining in pipes connecting the chemical solution supply portions and the treatment-target water tank; a gas supply device configured to supply a gas used for removing the chemical solution remaining in the pipes; and an integrating flowmeter provided in the pipe and configured to measure an integrated flow amount of the cleaning water or the gas used for removing the chemical solution. And when a value indicated by the integrating flowmeter is equal to or larger than a pipe internal capacity of the pipe, removal of the chemical solution is ended.

EFFECT OF THE INVENTION
[0011] The water treatment system according to the present disclosure enables a plurality of chemical solutions to be used for a filtration membrane while maintaining the concentrations of the chemical solutions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] [FIG. 1] FIG. 1 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 1.
[FIG. 2] FIG. 2 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 2.
[FIG. 3] FIG. 3 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 3.
[FIG. 4] FIG. 4 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 4.
EMBODIMENTS FOR CARRING OUT THE INVENTION [0013] Embodiment 1
The present embodiment relates to a water treatment system including a filtration membrane cleaning device used when a suspension substance is separated, by a filtration

membrane, from treatment-target water such as water from waterworks, water from a sewerage, industrial water, or any type of drainage.
In a method for cleaning a filtration membrane by using a plurality of chemical solutions, it is assumed that, if the different types of chemical solutions are mixed and reacted with one another in a pipe, the concentrations of the chemical solutions used for cleaning decreases and toxic gas is generated. Therefore, it is necessary to perform an operation so as to prevent mixing of these chemical solutions as much as possible. In particular, it is known that, if ozone and sodium hypochlorite are mixed with each other, the concentration of the ozone immediately decreases. Thus, in the method for cleaning the filtration membrane by using the plurality of chemical solutions, it is desirable to provide a mechanism for removing each chemical solution remaining in the pipe. Considering this, in the method for cleaning the filtration membrane by using the plurality of chemical solutions, the chemical solutions are supplied to the filtration membrane, subsequently cleaning water (filtered water) passes through the inside of the pipe, and then each chemical solution remaining in the pipe is removed. Consequently, the different types of chemical solutions are prevented from being mixed to each other in the pipe and concentrations are prevented from being decreased. The

present embodiment provides a water treatment system for preventing different types of chemical solutions from being mixed to each other in a pipe and for preventing concentrations from being decreased.
[0014] Hereinafter, a preferred embodiment of a chemical solution removal device according to the present embodiment will be described with reference to the drawing.
FIG. 1 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 1. In the drawing, the water treatment system includes a filtration device. The filtration device includes: a treatment-target water tank 2 accommodating treatment-target water 1; a membrane module 3 having a filtration membrane configured to perform filtration treatment of the treatment-target water 1; and a filtered water pipe 4 for discharging filtered water (to be used as cleaning water 17 described later) on which the filtration treatment is performed in the membrane module 3. The treatment-target water tank 2 is provided with a treatment-target water supply pipe 5 for supplying the treatment-target water 1. The filtered water pipe 4 is provided with, in order from the membrane module 3 side, an integrating flowmeter 13, a second reverse cleaning valve 36, a first reverse cleaning valve 32, a cleaning water supply valve 25, a pressure gauge 14, a filtration valve 15, and a filtration

pump 16. A sludge drawing pipe 6 and a sludge circulation pipe 7 are connected to the treatment-target water tank 2, and a diffuser 8 is located at a bottom portion of the treatment-target water tank 2. The sludge drawing pipe 6 is provided with a sludge drawing pump 9 for drawing sludge, and the sludge circulation pipe 7 is provided with a sludge circulation pump 10 for circulating the sludge in the treatment-target water tank 2. A membrane surface aeration blower 12 is connected via an air supply pipe 11 to the diffuser 8.
[0015] When filtration treatment is performed with respect to the treatment-target water 1 in the filtration device, the filtration valve 15 is opened and the filtration pump 16 is activated so that the treatment-target water 1 is filtered in the membrane module 3. Then, filtered water obtained by the filtering in the membrane module 3 is discharged via the filtered water pipe 4 to a cleaning water tank 18. If filtration treatment is continuously performed with respect to the treatment-target water 1 in the membrane module 3, clogging occurs in the filtration membrane in the membrane module 3 owing to contaminants, and thus it is required to clean the filtration membrane.
Considering this, the water treatment system further includes a filtration membrane cleaning device. The filtration membrane cleaning device performs: a method for

cleaning the filtration membrane by using, as a chemical solution, mixture water obtained by mixing a first chemical with the cleaning water 17; and a method for cleaning the filtration membrane by using, as a chemical solution, mixture water obtained by mixing a second chemical with the cleaning water 17.
[0016] Specifically, as shown in FIG. 1, the filtration membrane cleaning device includes: the cleaning water tank 18 accommodating the cleaning water 17; a first chemical storing tank 22 accommodating the first chemical; and a second chemical storing tank 23 accommodating the second chemical. The cleaning water tank 18 is provided with a turbidity meter 41 and a cleaning water pipe 24. The cleaning water pipe 24 is provided with a reverse cleaning pump 19 and a switching valve 20. The cleaning water pipe 24 branches into two cleaning water pipes. One of them, i.e., a cleaning water pipe 24A, is connected at the cleaning water supply valve 25 to the filtered water pipe 4. The other one of them, i.e., a cleaning water pipe 24B, is connected at a switching valve 21 to a first reverse cleaning pipe 26 and a second reverse cleaning pipe 27.
[0017] The first reverse cleaning pipe 26 is connected at a first air supply valve 31 to a first air supply pipe 39, and the second reverse cleaning pipe 27 is connected at a second air supply valve 35 to a second air supply pipe 40. A

cleaning blower 38 is provided in the first air supply pipe 39 and the second air supply pipe 40. The first air supply pipe 39, the second air supply pipe 40, and the cleaning blower 38 compose an air supply device for cleaning the filtration membrane. Limitation to air is not made, and other gas may be used. As described later, chemical solution remaining in the pipes can be removed by supplying the gas by the gas supply device.
[0018] The first chemical storing tank 22 is provided with a first chemical supply pipe 28, and the first chemical supply pipe 28 is connected to the first reverse cleaning pipe 26. The second chemical storing tank 23 is provided with a second chemical supply pipe 29, and the second chemical supply pipe 29 is connected to the second reverse cleaning pipe 27. The first chemical supply pipe 28 is provided with a first chemical supply valve 30 and a first chemical supply pump 33 (operated also as a suction device as described later), and the second chemical supply pipe 29 is provided with a second chemical supply valve 34 and a second chemical supply pump 37 (operated also as a suction device as described later). The first chemical storing tank 22, the first chemical supply pump 33, the first chemical supply pipe 28, and the first reverse cleaning pipe 26 compose a chemical supply portion. The second chemical storing tank 23, the second chemical supply pump 37, the second chemical supply

pipe 29, and the second reverse cleaning pipe 27 compose a chemical supply portion.
Although not shown, all the pumps, valves, and switching valves are connected to a control device, and the control device controls operations of all the pumps, valves, and switching valves.
[0019] When cleaning treatment is performed on the filtration membrane by using the filtration membrane cleaning device having the above configuration, the filtration pump 16 is stopped and the filtration valve 15 is closed first, and then cleaning treatment of the filtration membrane is started. Switching from the filtration treatment of the treatment-target water 1 to the cleaning treatment of the filtration membrane only has to be managed according to the time of the filtration treatment.
After the filtration treatment is ended, preliminary treatment may be performed with respect to the filtration membrane in the membrane module 3 before the cleaning treatment of the filtration membrane is started. For example, by exposing the filtration membrane in the membrane module 3 to air for a certain period, it becomes easy to remove contaminants adhering on a surface of the filtration membrane with which the treatment-target water 1 comes into contact. Alternatively, preliminary cleaning may be performed with respect to the filtration membrane by

opening the switching valve 20, the cleaning water supply valve 25, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24 and 24A and the filtered water pipe 4 to the membrane module 3. [0020] Alternatively, preliminary cleaning may be performed with respect to the filtration membrane by opening the switching valve 20, the switching valve 21, the first chemical supply valve 30, the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24 and 24B, the first reverse cleaning pipe 26, and the filtered water pipe 4 to the membrane module 3.
Further, preliminary cleaning may be performed with respect to the filtration membrane by opening the switching valve 20, the switching valve 21, the second chemical supply valve 34, the second air supply valve 35, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24 and 24B, the second reverse cleaning pipe 27, and the filtered water pipe 4 to the membrane module 3.

[0021] Alternatively, preliminary cleaning may be performed with respect to the filtration membrane by opening the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the cleaning blower 38 so that air is supplied via the first air supply pipe 39, the first reverse cleaning pipe 26, and the filtered water pipe 4 to the membrane module 3.
Further, preliminary cleaning may be performed with respect to the filtration membrane by opening the second air supply valve 35 and the second reverse cleaning valve 36 and activating the cleaning blower 38 so that air is supplied via the second air supply pipe 40, the second reverse cleaning pipe 27, and the filtered water pipe 4 to the membrane module 3.
In the case of performing any of these types of preliminary cleaning as well, by the preliminary cleaning, it becomes easy to remove contaminants adhering on the surface of the filtration membrane with which the treatment-target water 1 comes into contact.
[0022] In the cleaning treatment of the filtration membrane, the switching valve 20 is opened and the reverse cleaning pump 19 is activated first so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24B and the switching valve 21 to the first reverse cleaning pipe 26, and furthermore, the first

chemical supply valve 30 is opened and the first chemical supply pump 33 is activated so that the first chemical is supplied from the first chemical storing tank 22 via the first chemical supply pipe 28 to the first reverse cleaning pipe 26. Consequently, in the first reverse cleaning pipe 26, the cleaning water 17 and the first chemical are mixed to each other, whereby mixture water is generated. Although not shown, the first reverse cleaning pipe 26 may be provided with a device (for example, static mixer) for uniformly mixing the cleaning water 17 and the first chemical to each other.
[0023] Then, the mixture water containing the first chemical is supplied via the filtered water pipe 4 to the membrane module 3, whereby reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3. Alternatively, reverse flow cleaning may be performed with respect to the filtration membrane in the membrane module 3 by providing a tank that accommodates the cleaning water 17 in the first reverse cleaning pipe 26, supplying the first chemical into the tank, uniformly mixing the cleaning water 17 and the first chemical to each other, and supplying the resultant mixture water to the membrane module 3 by means of a pump. After the reverse flow cleaning, the mixture water containing the first chemical is discharged from the membrane module 3 into the treatment-

target water tank 2 and can be used as treatment-target water 1 used for filtration treatment. Alternatively, the mixture water containing the first chemical and discharged from the membrane module 3 after the reverse flow cleaning, may be separately collected as a treated liquid and disposed of. It is noted that mixture water after each reverse cleaning treatment described below, can also be treated in the same manner as above.
[0024] In a control method for chemical solution removal, an integrated flow amount during the chemical solution removal is measured by using the integrating flowmeter 13 interposed between the membrane module 3 and the second reverse cleaning valve 36, whereby control is performed. It is preferable that the chemical solution removal process is ended when a value indicated by the integrating flowmeter 13 interposed between the membrane module 3 and the second reverse cleaning valve 36 becomes equal to or larger than a pipe internal capacity at each of a location through which the mixture water obtained by mixing the first chemical with the cleaning water 17 passes as a chemical solution and a location through which mixture water obtained by mixing the second chemical with the cleaning water 17 passes as a chemical solution.
Here, the pipe internal capacity refers to the amount of a chemical solution that fills the inside of a

pipe. For example, if the diameter of a pipe at a location through which a chemical solution passes is 10 cm and the length (total length) of the pipe is 10 m, the pipe internal capacity is 0.05 m × 0.05 m × 3.14 × 10 m = 0.0785 m3. Thus, regarding the time during which chemical solution removal is performed, the cleaning water 17 or the gas has to be kept flowing until the amount thereof becomes equal to or larger than the pipe internal capacity. For example, in the case in which the cleaning water is flowed at 1 m3/min. until the amount thereof reaches the pipe internal capacity which is 0.0785 m3, the said time is 0.0785 m3 ÷ 1 m3/min. = 0.0785 min. Therefore, if the cleaning water 17 is kept flowing for at least 0.0785 min., the chemical solution remaining in the pipe can be replaced (removed).
If the viscosity of the chemical solution to be removed is 2 cP (CENTIPOISE) (mPa•s), it is preferable that the chemical solution removal process is ended at the time of obtaining an amount that is equal to or larger than 2 multiples of the pipe internal capacity. Meanwhile, if the viscosity is 3 cP (mPa•s), it is preferable that the chemical solution removal process is ended at the time of obtaining an amount that is equal to or larger than 3 multiples of the pipe internal capacity. The chemical solution removal method, the time for the removal, and the velocity of flow for the removal may be set to differ between the mixture

water obtained by mixing the first chemical with the cleaning water 17 and the mixture water obtained by mixing the second chemical with the cleaning water 17.
[0025] Next, if the turbidity of the cleaning water 17 measured by the turbidity meter 41 is lower than 1 NTU (nephelometric turbidity unit) (1 NTU is defined as a turbidity setting value), reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3 by opening the switching valve 20, the cleaning water supply valve 25, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24A and the filtered water pipe 4 to the membrane module 3.
Alternatively, reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3 by opening the switching valve 20, the switching valve 21, the first chemical supply valve 30, the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24B, the first reverse cleaning pipe 26, and the filtered water pipe 4 to the membrane module 3.

Alternatively, reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3 by opening the switching valve 20, the switching valve 21, the second chemical supply valve 34, the second air supply valve 35, and the second reverse cleaning valve 36 and activating the reverse cleaning pump 19 so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24B, the second reverse cleaning pipe 27, and the filtered water pipe 4 to the membrane module 3. [0026] Meanwhile, if the turbidity of the cleaning water 17 measured by the turbidity meter 41 is equal to or larger than 1 NTU, reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3 by opening the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 and activating the cleaning blower 38 so that air (gas) is supplied via the first air supply pipe 39, the first reverse cleaning pipe 26, and the filtered water pipe 4 to the membrane module 3.
Alternatively, reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3 by opening the second air supply valve 35 and the second reverse cleaning valve 36 and activating the cleaning blower 38 so that air (gas) is supplied via the second air supply pipe 40, the second reverse cleaning pipe 27, and the

filtered water pipe 4 to the membrane module 3. [0027] Alternatively, if the cleaning blower 38 is unusable, the cleaning water supply valve 25 is closed, and then the first reverse cleaning valve 32, the second reverse cleaning valve 36, the first air supply valve 31, and the first chemical supply valve 30 are opened and the first chemical supply pump 33 (suction device) is activated so that the mixture water that has been obtained by mixing the first chemical with the cleaning water 17 and that is located in each of the filtered water pipe 4 and the reverse cleaning pipe 26 is sucked and transferred via the filtered water pipe 4, the first reverse cleaning pipe 26, and the first chemical supply pipe 28 to the first chemical storing tank 22. [0028] By performing such chemical solution removal, the chemical solution remaining in the pipes can be efficiently removed, and furthermore, the chemical solution and the cleaning water used for the chemical solution removal can be prevented from reacting to each other. Therefore, while the concentration of the mixture water containing the second chemical solution is maintained, the mixture water can be used for the filtration membrane.
Specifics are as follows. The case in which only the cleaning water 17 is used as chemical solution removal means, will be contemplated. In the case of using the cleaning water 17 as chemical solution removal means, if the

quality of the cleaning water 17 is poor (a suspension substance or a dissolved organic substance is contained in the cleaning water 17), the suspension substance or the dissolved organic substance in the cleaning water 17 might react with the second chemical solution (which is supplied after the mixture water obtained by mixing the first chemical is removed). Unlike in a stage in which the cleaning water 17 and each chemical are mixed to each other in the corresponding reverse cleaning pipe so that mixture water is generated, it is necessary to keep the pipe clean as much as possible in a stage in which the chemical solution remaining in the pipe is removed. If the cleaning water 17 and the second chemical solution react to each other in the pipe, the concentration of the second chemical solution might decrease before arrival at the filtration membrane, and this might lead to decrease in the ability to clean the filtration membrane. In order to prevent such a phenomenon, cleaning by means of air is performed, and furthermore, suction and transference are performed.
[0029] After the chemical solution removal, the mixture water containing the first chemical is discharged from the membrane module 3 into the treatment-target water tank 2 and can be used as treatment-target water 1 used for filtration treatment. Alternatively, the mixture water containing the first chemical and discharged from the membrane module 3

after the reverse flow cleaning, may be separately collected as a treated liquid and disposed of.
[0030] Next, the cleaning water supply valve 25 and the first reverse cleaning valve 32 are closed. Thereafter, the second reverse cleaning valve 36 is opened and the second chemical supply pump 37 is activated so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24B and the switching valve 21 to the second reverse cleaning pipe 27. Then, the second chemical supply valve 34 is opened and the second chemical supply pump 37 is activated so that the second chemical is supplied from the second chemical storing tank 23 via the second chemical supply pipe 29 to the second reverse cleaning pipe 27. Consequently, the cleaning water 17 and the second chemical are mixed to each other in the second reverse cleaning pipe 27, and mixture water is made. It is noted that the second reverse cleaning pipe 27 may be provided with a device (for example, static mixer) for uniformly mixing the cleaning water 17 and the second chemical to each other, which is not shown in FIG.1. Then, the mixture water containing the second chemical is supplied via the filtered water pipe 4 to the membrane module 3 so that reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3. Alternatively, reverse flow cleaning may be performed with respect to the filtration membrane in the

membrane module 3 by providing a tank that accommodates the cleaning water 17 in the second reverse cleaning pipe 27, supplying the second chemical into the tank, uniformly mixing the cleaning water 17 and the second chemical to each other, and supplying the resultant mixture water to the membrane module 3 by means of a pump.
[0031] Although the case in which two types of chemical-containing mixture waters are used has been described regarding the above filtration membrane cleaning device, three or more types of chemical-containing mixture waters may be used. In this case, cleaning treatment can be performed by using the same method by increasing the number of chemical storing tanks.
When the cleaning treatment in which the filtration membrane cleaning device is used is ended, the second reverse cleaning valve 36, the first reverse cleaning valve 32, the cleaning water supply valve 25, and the filtration valve 15 are opened and the filtration pump 16 is activated so that filtration treatment of the treatment-target water 1 is performed again. Whereby, the filtration treatment of the treatment-target water 1 can be continuously and efficiently performed.
[0032] in the filtration membrane cleaning device having the above configuration, it is possible to, in a chemical solution removal method in the case of cleaning a filtration

membrane by using a plurality of chemical solutions, efficiently remove a chemical solution remaining in a pipe and use the plurality of chemical solutions for the filtration membrane while maintaining the concentrations of the plurality of chemical solutions. In addition, since a method that involves cleaning other than the cleaning by the cleaning water can be selected for chemical solution removal, the cleaning water can be saved.
In addition, each chemical solution in the corresponding pipes can be removed also by causing the gas and the cleaning water filtered by the filtration membrane to alternately pass. Such a method is a cleaning method which is so-called “pulse cleaning” in which a liquid and a gas are caused to alternately pass, whereby the ability to clean a filtration membrane or a pipe is more improved than in the case in which only the liquid or the gas is caused to pass. [0033] The chemical solution removal method according to the present embodiment is a method that, in order to avoid mixing of the different types of chemical solutions in any of the pipes, includes: cleaning the filtration membrane by using the mixture water obtained by mixing the first chemical with the cleaning water 17; then removing the mixture water that is obtained by mixing the first chemical with the cleaning water 17 and that remains in the corresponding pipes; further cleaning the filtration membrane by using the

mixture water obtained by mixing the second chemical with the cleaning water 17; and then removing the mixture water that is obtained by mixing the second chemical with the cleaning water 17 and that remains in the corresponding pipes. In this case, if the turbidity of the cleaning water is small, each chemical solution remaining in the corresponding pipes is removed by using the cleaning water. Meanwhile, if the cleaning water cannot be supplied, the chemical solution remaining in the pipes is removed by using a gas. Meanwhile, if the gas supply device is unusable, the chemical solution remaining in the pipes is removed by means of suction. [0034] The number of the mixture waters used in the filtration membrane cleaning method according to the present embodiment is not particularly limited as long as the number is more than one. However, if the number of the mixture waters is excessively large, the number of types of the chemicals used for the respective mixture waters has to be increased. Therefore, the number of the types of mixture waters used for the filtration membrane cleaning method according to the present embodiment is preferably two or three and more preferably two, from the viewpoint of decreasing cost for the cleaning. Although the filtration membrane cleaning method in which two types of mixture waters are used has been described above as an example, a filtration membrane cleaning method in which three or more types of

mixture waters are used may be utilized. [0035] In the filtration membrane cleaning method according to the present embodiment, after the cleaning water is caused to enter into the filtration membrane, the cleaning water may be retained in the membrane, or the cleaning water may be retained while the filtration membrane is immersed in a cleaning liquid.
The cleaning water used for the chemical solution removal according to the present embodiment has to be changed to chemical solution removal means other than the cleaning water in real time according to the amount of a suspension substance in the cleaning water. Therefore, regarding the cleaning water used for the chemical solution removal according to the present embodiment, the turbidity of the cleaning water has to be measured in real time by using an online turbidity meter. A cleaning water having a small turbidity is more preferable, and the turbidity is preferably smaller than 1 NTU and more preferably 0 NTU. [0036] As the gas used for the chemical solution removal according to the present embodiment, it is preferable that a gas having a low toxicity is used. However, if a gas that necessitates a bomb is used, cost required for chemical solution removal cannot be decreased. Thus, it is preferable that air generated by a blower is used as the gas used for the chemical solution removal method according to the present

embodiment.
The filtration membrane to be subjected to the filtration membrane cleaning method according to the present embodiment is a filtration membrane in a state in which contaminants have adhered on a surface thereof or in pores thereof as a result of performing filtration treatment with respect to treatment-target water such as water from waterworks, water from a sewerage, sewage secondary treated water, industrial drainage, seawater, or human waste. [0037] The material of the filtration membrane usable for the filtration membrane cleaning method according to the present embodiment is not particularly limited as long as the material is not degraded by the chemicals. Examples of the material of the filtration membrane include polyolefins such as polyethylene, polypropylene, and polybutene. Other examples of the material of the filtration membrane include fluorine-based resin compounds such as tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (fluorinated ethylene propylene (FEP)), tetrafluoroethylene-ethylene copolymer (ethylene tetrafluoroethylene (ETFE)), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ethylene chlorotrifluoroethylene (ECTFE)), polyvinylidene fluoride (polyvinylidene difluoride (PVDF)), and polytetrafluoroethylene (PTFE). Other examples

of the material of the filtration membrane include celluloses such as cellulose acetate and ethyl cellulose and further include ceramic. Among these materials, it is preferable that the material of the filtration membrane is a fluorine-based resin compound having excellent resistance to strong oxidants such as ozone. As the material of the filtration membrane, the above substances may be used singly, or two or more types of the substances may be used in combination. [0038] The type of the filtration membrane is not particularly limited, and any type of filtration membrane such as a microfiltration (MF) membrane or an ultrafiltration (UF) membrane can be used. Although the average pore diameter of the filtration membrane is not particularly limited, the average pore diameter is preferably 0.001 μm to 1 μm and more preferably 0.01 μm to 0.1 μm. If the filtration membrane has an average pore diameter that falls within this range, it is possible to, by the filtration membrane cleaning method according to the present embodiment, efficiently remove not only contaminants adhering on the surface of the filtration membrane with which the treatment-target water comes in contact, but also contaminants chemically adhering on the surface of the filtration membrane with which the cleaning water comes into contact or in the pores of the filtration membrane. The shape of the filtration membrane is not particularly limited and may be a

cylindrical shape or a flat shape. Out of these shapes, the shape of the filtration membrane is preferably a cylindrical shape. It is noted that the filtration membrane may be incorporated in the membrane module 3, and a membrane module of an immersion type, a casing type, or a monolith type may be used as the membrane module 3. As the filtration method of the filtration membrane, either a dead-end filtration method or a cross-flow filtration method may be used. [0039] The water passage type of the filtration membrane is not particularly limited and may be either: an external pressure filtration type in which treatment-target water is flowed on the outer side of a filtration membrane, and cleaning water is flowed on the inner side of the filtration membrane; or an internal pressure filtration type in which treatment-target water is flowed on the inner side of a filtration membrane, and cleaning water is flowed on the outer side of the filtration membrane.
The mixture water obtained by mixing the first chemical with the cleaning water 17 and the mixture water obtained by mixing the second chemical with the cleaning water 17 are not particularly limited as long as the mixture waters are substances that can decompose organic matter or inorganic matter. Examples of the chemical solutions that can decompose organic matter include sodium hypochlorite, hydrogen peroxide, sodium hydroxide, ozone water, and the

like. These chemical solutions may be used singly, or two or more types of these chemical solutions may be used in combination.
As the chemical solution obtained by mixing the first chemical, mixture water obtained by dissolving sodium hypochlorite or hydrogen peroxide in the cleaning water 17 can be used. As the chemical solution obtained by mixing the second chemical, mixture water obtained by dissolving ozone in the cleaning water 17 can be used. [0040] Meanwhile, as a substance that can decompose inorganic matter, an inorganic acid such as hydrochloric acid, sulfuric acid, or nitric acid, or an organic acid such as oxalic acid or citric acid, can be used. These substances may also be used singly, or two or more types of these substances may be used in combination. Further, two or more types of the substances that can decompose organic matter and two or more types of the substances that can decompose inorganic matter may also be used in combination. when two or more types of the substances that can decompose organic matter and two or more types of the substances that can decompose inorganic matter are used in combination, no particular limitation is imposed on which of the substances is to be used as the mixture water obtained by mixing the first chemical or which of the substances is to be used as the mixture water obtained by mixing the second chemical. If

a substance that can decompose organic matter is used as the mixture water obtained by mixing the first chemical, a substance that can decompose inorganic matter can be used as the mixture water obtained by mixing the second chemical. Meanwhile, if a substance that can decompose inorganic matter is used as the mixture water obtained by mixing the first chemical, a substance that can decompose organic matter can be used as the mixture water obtained by mixing the second chemical.
[0041] The chemical solution concentration of the cleaning water is not particularly limited. However, if a substance that can decompose organic matter is used, it is preferable that the chemical solution concentration of sodium hypochlorite (effective chlorine concentration) is equal to or larger than 1.0 g/L and equal to or smaller than 5.0 g/L, and it is preferable that the chemical solution concentration of sodium hydroxide is equal to or larger than 1.0 g/L and equal to or smaller than 4.0 g/L. it is preferable that the chemical solution concentration of ozone water is equal to or larger than 5 mg/L and equal to or small than 100 mg/L and more preferably equal to or larger than 20 mg/L and equal to or smaller than 30 mg/L. Meanwhile, if a substance that can decompose inorganic matter is used, it is preferable that the chemical solution concentration of each of hydrochloric acid, sulfuric acid, and nitric acid is equal to or larger than 1.0

g/L and equal to or smaller than 10.0 g/L, it is preferable that the chemical solution concentration of oxalic acid is equal to or larger than 1.0 g/L and equal to or smaller than 2.0 g/L, and it is preferable that the chemical solution concentration of citric acid is equal to or larger than 1 g/L and equal to or smaller than 10 g/L. If any of the chemical solution concentrations is smaller than a corresponding one of the above ranges, it takes time to decompose contaminants adhering to the filtration membrane. In addition, the capacity of the chemical storing tank is also increased in association with increase in the amount of the cleaning water that is necessary. Meanwhile, if any of the concentration of the mixture water obtained by mixing the first chemical or the concentration of the mixture water obtained by mixing the second chemical is larger than a corresponding one of the above ranges, the amount of the chemical solution that is consumed is increased, and thus cost required for the chemical solution is increased.
[0042] The cleaning time for the filtration membrane taken in using each of the mixture waters containing the chemical solutions is not particularly limited, and has to be appropriately set according to the amount of contaminants adhering to the filtration membrane. In general, it is preferable that the cleaning time is equal to or shorter than 90 minutes in the case of using sodium hypochlorite, and the

cleaning time is preferably 5 minutes to 7 minutes in the case of using oxalic acid or citric acid.
A shorter cleaning time is more preferable. If the cleaning time is elongated, the total cleaning time is elongated. Consequently, the time during which filtration treatment of the treatment-target water by the filtration membrane is interrupted, is also elongated, whereby the amount of water that is filtered by the filtration membrane decreases.
Specifics are as follows. In filtration treatment by a filtration membrane during sewage and drainage treatment, filtration of each treatment-target water is ordinarily performed all day long. Chemical solution cleaning of the filtration membrane is performed while filtration treatment of the treatment-target water is stopped, and thus a longer cleaning time leads to a shorter time for filtering the treatment-target water. That is, since the time for filtering the treatment-target water is shortened, the amount of water that can be treated by the filtration treatment (cleaning water amount) also decreases. [0043] The membrane-surface permeation flux (the amount of water transmitted through the membrane per area thereof) of the mixture water containing the chemical solution is not particularly limited. In general, a flux at which the filtration membrane can be filled to ends thereof has to be

ensured. In particular, in the case of using ozone water, a flux at which the concentration thereof can be maintained during transference has to be ensured. Specifically, in the case of using sodium hypochlorite, the membrane-surface permeation flux thereof is equal to or smaller than 6 LMH (L/m2/h). Meanwhile, in the case of using ozone water, the membrane-surface permeation flux thereof is equal to or smaller than 50 LMH (L/m2/h) and more preferably equal to or smaller than 30 LMH (L/m2/h). If the membrane-surface permeation flux is excessively large, cost required for the chemical increases in association with increase in the amount of the mixture water that is necessary. In addition, the capacity of the chemical storing tank also increases. Meanwhile, if the membrane-surface permeation flux is excessively small, the filtration membrane is not filled to the ends thereof by the mixture water, whereby contaminants adhering to the filtration membrane cannot be decomposed. In addition, in the case of using ozone water, the concentration thereof decreases during transference. [0044] As means used for chemical solution removal, cleaning water and gas are generally used, and suction means is further used. Among these means, it is preferable that gas is used, and cleaning water is more preferably used. The turbidity of the cleaning water is generally 0 NTU in a state in which the filtration membrane is not fractured, i.e., in a

state in which a suspension substance or a dissolved organic substance in the treatment-target water is not flowing into the cleaning water. Meanwhile, the turbidity of the cleaning water is generally equal to or larger than 1 NTU in a state in which the filtration membrane is fractured, i.e., in a state in which a suspension substance or a dissolved organic substance in the treatment-target water is flowing into the cleaning water.
[0045] If the cleaning water into which the suspension substance or the dissolved organic substance from the treatment-target water has flowed is used as means for removing each chemical solution remaining in the corresponding pipes, the chemical solution and the suspension substance or the dissolved organic substance in the cleaning water react to each other. Consequently, the concentration of the chemical solution cannot be maintained. Specifics are as follows. Unlike in a process in which the cleaning water 17 and each chemical are mixed to each other in the corresponding reverse cleaning pipe so that mixture water is generated, it is necessary to keep the pipe clean as much as possible in a process in which the chemical solution remaining in the pipe is removed. Thus, although cleaning water is preferentially selected in selection of chemical solution removal means, a gas is selected if the turbidity of the cleaning water is equal to or larger than 1 NTU.

Meanwhile, if it is difficult to supply gas, suction means is selected.
[0046] The timing for performing chemical solution removal is preferably any of: a timing that precedes supply of the mixture water obtained by mixing the first chemical; a timing between supply of the mixture water obtained by mixing the first chemical and supply of the mixture water obtained by mixing the second chemical; and a timing that is subsequent to supply of the mixture water obtained by mixing the second chemical. Among these timings, the timing between supply of the mixture water obtained by mixing the first chemical and supply of the mixture water obtained by mixing the second chemical is more preferable.
The direction of performing chemical solution removal may be either: the same flow direction as that of each of the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical; or a flow direction opposite to that of each of the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical. The same flow direction as that of each of the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical is preferable.
That is, there are two directions of performing chemical solution removal. One of them is a direction from

the secondary side to the primary side of the filtration membrane (the same direction as that of flow of each chemical solution), and the other one is a direction from the primary side to the secondary side of the filtration membrane (the same direction as the direction in which the treatment-target water 1 is filtered). Although chemical solution removal may be performed in either of the directions, the same direction as that of each chemical solution (the direction from the secondary side to the primary side of the filtration membrane) is preferable since the said direction enables complete removal of the chemical solution in the corresponding pipes and in the filtration membrane. [0047] The manner for disposing of each chemical solution removed by the chemical solution removal is not particularly limited. In the case of the same flow direction as that of each of the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical, it is preferable that the removed chemical solution is sent to the filtration membrane or the reaction tank (treatment-target water tank 2), and more preferably sent to the filtration membrane.
Meanwhile, in the case of the flow direction opposite to that of each of the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical, it is preferable that the removed

chemical solution is sent to the corresponding chemical storing tank or the treatment water tank (cleaning water tank 18), and more preferably sent to the chemical reserving tank.
In general, the time for chemical solution removal has to be a removal time that makes it possible to ensure a flow amount equal to or larger than the pipe internal capacity at each of the location through which the mixture water obtained by mixing the first chemical passes and the location through which the mixture water obtained by mixing the second chemical passes. If the removal time is short, the chemical solution remaining in the pipes cannot be completely removed. Consequently, when mixture water containing a chemical is supplied, the concentration of the chemical decreases. Meanwhile, if the removal time is too long, the total cleaning time is elongated. Consequently, the time during which filtration treatment of the treatment-target water by the filtration membrane is interrupted, is also elongated, whereby the amount of water that is treated by the filtration membrane decreases.
[0048] The velocity of flow in chemical solution removal has to be set appropriately according to the amount of the chemical solution remaining in the pipes. A higher velocity of flow for the removal is more preferable. If the velocity of flow for the removal decreases, a shearing stress that is based on the velocity of flow for the removal decreases,

whereby the chemical solution adhering on the pipes cannot be completely removed. Consequently, when mixture water containing a chemical is supplied, the concentration of the chemical solution decreases.
In a control method for chemical solution removal, the integrated flow amount of a fluid used for chemical solution removal is measured by using the integrating flowmeter 13 interposed between the filtration membrane and the second reverse cleaning valve 36, whereby control is performed. it is preferable that the chemical solution removal process is ended when a value indicated by the integrating flowmeter 13 interposed between the filtration membrane and the second reverse cleaning valve 36 becomes equal to or larger than the pipe internal capacity at each of the location through which the mixture water obtained by mixing the first chemical passes and the location through which the mixture water obtained by mixing the second chemical passes. If the viscosity of the chemical solution to be removed is 2 cP (mPa•s), it is more preferable that the chemical solution removal process is ended at the time of obtaining an amount that is equal to or larger than 2 multiples of the pipe internal capacity. Meanwhile, if the viscosity is 3 cP (mPa•s), it is more preferable that the chemical solution removal process is ended at the time of obtaining an amount that is equal to or larger than 3

multiples of the pipe internal capacity. The chemical solution removal method, the time for the removal, and the velocity of flow for the removal may be set so as to differ between the mixture water obtained by mixing the first chemical and the mixture water obtained by mixing the second chemical.
[0049] In the present embodiment, in order to avoid mixing of the different types of chemicals in any of the pipes when the filtration membrane is cleaned by using the plurality of chemicals, the chemical solution removal process is performed between cleaning of the filtration membrane by using the mixture water containing the first chemical and cleaning of the filtration membrane by using the mixture water containing the second chemical, whereby each chemical solution remaining in the corresponding pipes is removed. If the turbidity of the cleaning water 17 that passes through the filtration membrane is small, the chemical solution remaining in the pipes is removed by using the cleaning water 17. Meanwhile, if the turbidity of the cleaning water 17 is large, the chemical solution remaining in the pipes is removed by using a gas. Meanwhile, if gas cannot be supplied, the chemical solution remaining in the pipes is removed by means of suction.
Consequently, when the filtration membrane is cleaned by using the plurality of chemical solutions, it is

possible to: efficiently remove each chemical remaining in the corresponding pipes; prevent the chemical solutions from reacting each other; and use the plurality of chemical solutions for the filtration membrane while maintaining the concentrations of the plurality of chemical solutions. In addition, since a method that involves cleaning other than the cleaning by the cleaning water passing through the filtration membrane can be selected for chemical solution removal, the cleaning water can be saved. [0050] Embodiment 2
FIG. 2 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 2. It is noted that the water treatment system including the filtration membrane cleaning device according to the present embodiment has the same basic configuration as that of the water treatment system including the filtration membrane cleaning device according to embodiment 1, and thus only differences therebetween will be described. The same components as those of the water treatment system including the filtration membrane cleaning device according to embodiment 1 are denoted by the same reference characters.
[0051] In FIG. 2, the water treatment system including the filtration membrane cleaning device according to the present embodiment has a structure suitable for the case of using

ozone as the mixture water obtained by mixing the second chemical. Specifically, the water treatment system including the filtration membrane cleaning device according to the present embodiment differs from the water treatment system including the filtration membrane cleaning device according to embodiment 1 in the point that an ozone water generation tower 45 is connected to the second reverse cleaning pipe 27 branching at the second reverse cleaning valve 36. A diffuser 44 is located at a bottom portion of the ozone water generation tower 45, and an ozone generator 42 is connected via an ozone supply pipe 43 to the diffuser 44. An ozone material to be supplied to the ozone generator 42 is not particularly limited, and it is possible to use, for example, liquid oxygen or oxygen that is generated by pressure swing adsorption (PSA) or pressure vacuum swing adsorption (PVSA). [0052] In the water treatment system having such a structure, when reverse flow cleaning is performed by using mixture water obtained by mixing ozone (second chemical) with the cleaning water 17, the switching valve 20, the switching valve 21, and the second chemical supply valve 34 are opened and the reverse cleaning pump 19 is activated so that the cleaning water 17 is supplied from the cleaning water tank 18 via the cleaning water pipes 24,24B and the second chemical supply valve 34 to the ozone water generation tower 45. Further, ozone gas generated by the ozone generator 42 passes

through the ozone supply pipe 43 and is supplied from the diffuser 44 so that ozone water as the mixture water obtained by mixing the ozone with the cleaning water 17 is generated in the ozone water generation column 45. Then, the ozone water generated in the ozone water generation column 45 is supplied via the second reverse cleaning pipe 27 and the filtered water pipe 4 to the membrane module 3 so that reverse flow cleaning is performed with respect to the filtration membrane in the membrane module 3. The water treatment system according to the present embodiment further includes an ozone-discharging pipe 46, a discharged ozone treating facility 47, and a treated ozone pipe 48. [0053] As described above, by providing the ozone water generation column 45, the ozone water can be efficiently generated. Although the case of using the diffuser 44 as an ozone-gas-supply device has been presented as an example in FIG. 2, the ozone-gas-supply device is not particularly limited as long as the device enables generation of ozone water obtained by contact between ozone gas and the cleaning water 17. For example, an ozone-gas-supply device of an ejector type, a mechanical agitation type, a downward injection type, or the like, can be used. Although not shown, all the pumps, valves, and switching valves are connected to a control device, and the control device controls operations of all the pumps, valves, and switching

valves.
[0054] Although the case in which mixture water obtained by mixing ozone as the second chemical is used has been described regarding the filtration membrane cleaning device in the present embodiment, no particular limitation is imposed as to which of ozone and a chemical other than ozone has to be mixed as the first chemical to obtain mixture water or as to which of ozone and a chemical other than ozone has to be mixed as the second chemical to obtain mixture water. If mixture water obtained by mixing ozone as the first chemical is used, mixture water obtained by mixing a chemical other than ozone as the second chemical may be used. [0055] Embodiment 3
FIG. 3 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 3. It is noted that the water treatment system including the filtration membrane cleaning device according to the present embodiment has the same basic configuration as that of the water treatment system including the filtration membrane cleaning device according to embodiment 1, and thus only differences therebetween will be described. The same components as those of the water treatment system including the filtration membrane cleaning device according to embodiment 1 are denoted by the same reference characters.

[0056] In FIG. 3, the water treatment system including the filtration membrane cleaning device according to the present embodiment differs from the water treatment system including the filtration membrane cleaning device according to embodiment 1 in the point that a water supply pipe 49 is provided instead of the reverse cleaning pump 19. The water supply pipe 49 is connected via the cleaning water pipe 24B branching off at the switching valve 20, to the first reverse cleaning pipe 26 and the second reverse cleaning pipe 27. Water to be supplied from the water supply pipe 49 in the present embodiment is not particularly limited as long as the turbidity of the water is smaller than 1 NTU. Examples of the water include tap water, underground water, water from a reservoir, surface water, subsoil water, water from a lake/marsh, and seawater.
[0057] In the water treatment system having such a structure, when chemical solution removal is performed by the water from the water supply pipe 49, the switching valve 20, the cleaning water supply valve 25, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 are opened, and the water is supplied. Alternatively, the switching valve 20, the switching valve 21, the first chemical supply valve 30, the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 may be opened in order to supply the water.

Alternatively, the switching valve 20, the switching valve 21, the second chemical supply valve 34, the second air supply valve 35, and the second reverse cleaning valve 36 may be opened in order to supply the water. As described above, by providing the water supply pipe 49, chemical solution removal can be efficiently performed while the cleaning water is saved. [0058] Embodiment 4
FIG. 4 is a schematic view of a water treatment system including a filtration membrane cleaning device according to embodiment 4. It is noted that the water treatment system including the filtration membrane cleaning device according to the present embodiment has the same basic configuration as that of the water treatment system including the filtration membrane cleaning device according to embodiment 1, and thus only differences therebetween will be described. The same components as those of the water treatment system including the filtration membrane cleaning device according to embodiment 1 are denoted by the same reference characters.
[0059] In FIG. 4, the water treatment system including the filtration membrane cleaning device according to the present embodiment differs from the water treatment system including the filtration membrane cleaning device according to embodiment 1 in the point that a gas bomb 50 as a gas supply

device is provided instead of the cleaning blower 38. The gas bomb 50 is connected to the first air supply pipe 39 and the first reverse cleaning pipe 26 branching at the first air supply valve 31. In addition, the gas bomb 50 is connected to the second air supply pipe 40 and the second reverse cleaning pipe 27 branching at the second air supply valve 35.
Gas to be supplied from the gas bomb 50 in the present embodiment is not particularly limited, and a substance known in this technical field can be used, as long as the gas is not toxic to the human body. Examples of the gas include oxygen, nitrogen, carbon dioxide, and carbonic acid gas.
[0060] In the water treatment system having such a structure, when chemical solution removal is performed by using the gas bomb 50, the first air supply valve 31, the first reverse cleaning valve 32, and the second reverse cleaning valve 36 are opened, and the gas is supplied. Alternatively, the second air supply valve 35 and the second reverse cleaning valve 36 are opened, and the gas is supplied. As described above, by providing the gas bomb 50, chemical solution removal can be performed by using a gas other than air. Therefore, the chemical solution removal in which a gas is used can be performed even during membrane treatment in an anaerobic and oxygen-free environment in which neither air nor oxygen can be used. In the case of

operating the filtration membrane cleaning device according to the present embodiment in an anaerobic and oxygen-free environment, the diffuser 8, the air supply pipe 11, and the membrane surface aeration blower 12 for supplying oxygen are not provided.
[0061] In the case of using the gas bomb 50 for supplying the gas, a chemical solution removal method can be selected according to the pressure in the gas bomb 50. Further, in the case of using an air pump, a chemical solution removal method can be selected according to the pressure in the air pump. For example, if the residual pressure in the gas bomb 50 becomes small, the pressure indicated by the pressure gauge 14 decreases. In the case of the air pump as well, if a failure thereof or leakage from a pipe thereof occurs, the pressure indicated by the pressure gauge 14 decreases too. Even if such a malfunction occurs, the chemical solution removal is not hindered since another chemical solution removal method can be selected.
Specifically, if the pressure of the gas to be supplied from the gas supply device is smaller than a reference value, one method is selected from out of: a method in which each chemical solution remaining in the corresponding pipes is removed by using the cleaning water 17; or a method in which the chemical solution remaining in the pipes is sucked and transferred to the chemical storing

tank 22, 23 by the corresponding suction device. [0062] Although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but they can be applied, alone or in various combinations to one or more of the embodiments of the disclosure.
It is therefore understood that numerous modifications which have not been exemplified can be devised without departing from the scope of the present disclosure. For example, at least one of the constituent components may be modified, added, or eliminated. At least one of the constituent components mentioned in at least one of the preferred embodiments may be selected and combined with the constituent components mentioned in another preferred embodiment.
DESCRIPTION OF THE REFERENCE CHARACTERS [0063] 1 treatment-target water
2 treatment-target water tank
3 membrane module
13 integrating flowmeter 17 cleaning water

18 cleaning water tank
22 first chemical storing tank
23 second chemical storing tank
33 first chemical supply pump
37 second chemical supply pump
38 cleaning blower

We Claim :
[1] A water treatment system comprising:
a treatment-target water tank accommodating treatment-target water;
a membrane module provided in the treatment-target water tank and having a filtration membrane configured to perform filtration treatment of the treatment-target water;
a plurality of chemical solution supply portions configured to supply a plurality of respective chemical solutions for cleaning the filtration membrane;
a cleaning water tank accommodating filtered water obtained by the filtration treatment in the membrane module, the filtered water being cleaning water used for removing chemical solution remaining in pipes connecting the chemical solution supply portions and the treatment-target water tank;
a gas supply device configured to supply a gas used for removing the chemical solution remaining in the pipes; and
an integrating flowmeter provided in the pipe and configured to measure an integrated flow amount of the cleaning water or the gas used for removing the chemical solution, wherein
when a value indicated by the integrating flowmeter is equal to or larger than a pipe internal capacity of the pipe, removal of the chemical solution is ended.

[2] The water treatment system according to claim 1, further comprising a suction device configured to suck the chemical solution remaining in the pipe and transfer the chemical solution to the chemical solution supply portion.
[3] The water treatment system according to claim 2, wherein
when a turbidity of the cleaning water is smaller than a turbidity setting value, the chemical solution remaining in the pipes is removed by using the cleaning water, and
when the turbidity of the cleaning water is equal to or smaller than the turbidity setting value,
the gas is supplied from the gas supply device, or
when the gas supply device is unusable, the chemical solution remaining in the pipe is sucked and transferred to the chemical solution supply portion by the suction device.
[4] The water treatment system according to claim 1, wherein removal of the chemical solution remaining in the pipe performed by using the cleaning water, and removal of the chemical solution remaining in the pipe performed by

supplying the gas from the gas supply device, are alternately performed.
[5] The water treatment system according to claim 2, wherein
when the pressure of the gas supplied from the gas supply device is smaller than a reference value, one choice is selected from out of:
removal of the chemical solution remaining in the pipe performed by using the cleaning water, or
suction and transference of the chemical solution remaining in the pipe to the chemical solution supply portion performed by the suction device.
[6] The water treatment system according to any one of claims 1 to 5, wherein mixture water obtained by dissolving sodium hypochlorite in the cleaning water is used as the chemical solution.
[7] The water treatment system according to any one of claims 1 to 5, wherein mixture water obtained by dissolving hydrogen peroxide in the cleaning water is used as the chemical solution.
[8] The water treatment system according to any one of

claims 1 to 5, wherein mixture water obtained by dissolving ozone in the cleaning water is used as the chemical solution.