JP7270383B2 - MEMBRANE FILTRATION DEVICE AND MEMBRANE FILTRATION METHOD - Google Patents

Description

TECHNICAL FIELD The present invention relates to a membrane filtration device and a membrane filtration method using submerged membranes.

As a method for solid-liquid separation of suspended substances such as organic substances in the water to be treated, there is a method of performing solid-liquid separation by performing a membrane filtration treatment with an immersed membrane in which the membrane is immersed in a tank after adding a coagulant.

For example, in Patent Document 1, sewage is flowed into a coagulation reaction tank (coagulation treatment tank) and subjected to acidic coagulation treatment, and then the coagulation mixture is supplied to the upper part of a filtration separation tank in which a water-permeable filter is immersed, and A method for treating sewage is described in which a dynamic filtration layer of flocculated sludge is formed on the surface of an aqueous filter, and filtered water is obtained from the water-permeable filter by hydraulic head pressure.

In the method of performing solid-liquid separation using such an immersed membrane, washing of the immersed membrane is necessary in order to remove suspended matter adhering to the immersed membrane due to solid-liquid separation. As a method for cleaning the immersed membrane, backwash water is passed from the secondary side to the primary side of the membrane in the tank to wash deposits on the membrane surface, and alkaline chemicals such as sodium hypochlorite are used. Common methods include chemically enhanced backwashing in which backwashing is performed with added backwashing water.

When this chemical-strengthened backwashing is performed, even if the pH of the coagulation treatment tank is controlled, the pH in the immersion tank rises due to the alkaline chemical used during backwashing, and the chromaticity and TOC of the treated water increase. Sometimes. In addition, when the quality of the water to be treated suddenly changes, or when the pH control of the coagulation treatment tank is not effective, the pH in the immersion tank rises, and the chromaticity and TOC of the treated water may increase. . Therefore, it is required to stabilize the quality of treated water even in these cases.

Japanese Patent No. 3874635

An object of the present invention is to provide a membrane filtration apparatus and a membrane filtration method capable of stabilizing the water quality of treated water in membrane filtration using an immersed membrane for water to be treated containing organic matter and the like.

The present invention provides coagulation treatment means for adding a coagulant containing polyaluminum chloride to the water to be treated and performing coagulation treatment in a pH range of 6.2 to 7.5, and measuring the coagulation pH during the coagulation treatment. Aggregation pH measuring means, membrane filtration means for performing solid-liquid separation of the flocculation-treated water in a pH range of 6.2 to 7.5 by an immersion membrane installed in the immersion tank, and pH of the immersion tank. An immersion tank pH measuring means for measuring and an immersion tank pH adjuster adding means for adding a pH adjuster for adjusting the pH of the immersion tank, wherein the pH of the immersion tank is higher than the aggregation pH. In this membrane filtration device, the pH adjuster is added using the immersion tank pH adjuster adding means when the water is in the immersion tank, and the pH of the immersion tank is controlled to be equal to or lower than the flocculation pH.

Preferably, the membrane filtration device further comprises chemically enhanced backwashing means for backwashing the immersed membrane with chemical-containing backwash water containing a cleaning chemical.

In the membrane filtration device, the content of the cleaning chemical in the chemical-containing backwash water is preferably 10 ppm or more.

In the membrane filtration device, the immersion tank pH measuring means preferably measures the pH below the lower end of the immersion membrane in the immersion tank.

In addition, the present invention includes a flocculation treatment step of adding a flocculating agent containing polyaluminum chloride to the water to be treated and performing flocculation treatment in a pH range of 6.2 to 7.5, and a flocculation pH when performing the flocculation treatment. A coagulation pH measurement step to be measured, a membrane filtration step of performing solid-liquid separation of the coagulation-treated water in the pH range of 6.2 to 7.5 by an immersion membrane installed in the immersion tank, and the immersion tank. and a step of measuring the pH of the immersion bath, wherein when the pH of the immersion bath becomes higher than the aggregation pH, a pH adjuster is added to adjust the pH of the immersion bath. It is a membrane filtration method in which the pH of is controlled to be equal to or lower than the flocculation pH.

The membrane filtration method preferably further includes a chemically enhanced backwashing step of backwashing the submerged membrane with chemical-containing backwash water containing a cleaning chemical.

In the membrane filtration method, it is preferable that the content of the cleaning chemical in the chemical-containing backwash water is 10 ppm or more.

In the immersion tank pH measurement step in the membrane filtration method, it is preferable to measure the pH below the lower end of the immersion membrane in the immersion tank.

INDUSTRIAL APPLICABILITY In the present invention, water quality of treated water can be stabilized in membrane filtration using an immersed membrane for water to be treated containing organic substances and the like.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic block diagram which shows an example of the membrane filtration apparatus which concerns on embodiment of this invention.

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

An outline of an example of a membrane filtration device according to an embodiment of the present invention is shown in FIG. 1, and its configuration will be described.

The membrane filtration device 1 of FIG. 1 includes a coagulation treatment device 10 as coagulation treatment means for performing coagulation treatment by adding a coagulant to the water to be treated, and an coagulation pH measurement means for measuring the coagulation pH during the coagulation treatment. Membrane filtration device 20 as a membrane filtration means for performing solid-liquid separation of coagulation-treated water by means of a pH measurement device 28 and an immersion membrane 16 installed in the immersion tank 12, and an immersion tank for measuring the pH of the immersion tank 12. An immersion bath pH measuring device 30 as pH measuring means and a pH adjuster addition pipe 46 as immersion bath pH adjuster addition means for adding a pH adjuster for adjusting the pH of the immersion bath 12 are provided. The membrane filtration device 1 may include a treated water tank 14 that stores treated water.

In the membrane filtration device 1 of FIG. 1 , a water pipe 32 to be treated is connected to an inlet of the water to be treated of the coagulation treatment device 10 . The coagulated water outlet of the coagulation treatment apparatus 10 and the coagulated water inlet at the bottom of the immersion tank 12 are connected by a coagulated water pipe 34 . An immersion membrane 16 is installed inside the immersion tank 12 , and the secondary side of the immersion membrane 16 and the treated water inlet of the treated water tank 14 are connected by a treated water pipe 36 via a pump 22 . A backwash water outlet of the treated water tank 14 and the upstream side of the pump 22 in the treated water pipe 36 are connected by a backwash water pipe 38 via the pump 24 . A sludge discharge pipe 40 for discharging the sludge 52 in the immersion tank 12 is connected to the lower part of the immersion tank 12 . To the coagulation treatment apparatus 10, a coagulant addition pipe 42 as a coagulant addition means for adding a coagulant and a pH adjuster addition pipe 44 as a pH adjuster addition means for adding a pH adjuster are connected. A stirring device 26, which is a stirring means having a rotary drive means, stirring blades, etc., and an agglomeration pH measuring device 28 are installed. A pH adjusting agent addition pipe 46 is connected in the middle of the flocculated water pipe 34 . An immersion tank pH measuring device 30 is installed in the immersion tank 12 . An air diffuser 18 is installed in the lower part of the immersion tank 12 , and a gas supply pipe 50 is connected to the air diffuser 18 . A chemical addition pipe 48 may be connected to the backwash water pipe 38 downstream of the pump 24 as a chemical addition means for adding a chemical.

The operation of the membrane filtration method and the membrane filtration device 1 according to this embodiment will be described.

<Aggregation process>
The water to be treated containing suspended matter such as organic matters is sent to the coagulation treatment apparatus 10 through the water to be treated pipe 32 . In the tank of the flocculation treatment device 10, a flocculant is added to the water to be treated through the flocculant addition pipe 42 while stirring is performed by the agitating device 26 (flocculant addition step), and flocculation treatment is performed (flocculation treatment step ).

The water to be treated is, for example, water containing suspended matter such as organic matter, and examples thereof include river water, industrial water, groundwater, and waste water.

The SS of the water to be treated is, for example, in the range of 5 to 1000 mg/L, the TOC is, for example, in the range of 0.5 to 10 mg/L, and the chromaticity is, for example, in the range of 2 to 200 degrees. .

As the flocculant, for example, at least one of an inorganic flocculant and a polymer flocculant is used.

Examples of inorganic flocculants include iron-based inorganic flocculants such as ferric chloride and polyferric sulfate, and aluminum-based inorganic flocculants such as aluminum sulfate and polyaluminum chloride (PAC). As an inorganic coagulant, a polyaluminum chloride solution having a basicity of 60% or more and 70% or less may be used. Furthermore, an ultra-high basicity polyaluminum chloride solution having a basicity of about 70% may be used. A polyaluminum chloride solution with a basicity of 60% or more and 70% or less suppresses a decrease in filtration flux in the membrane filtration process than a normal polyaluminum chloride solution with a basicity of 40% or more and 50% or less, and is added in a small amount. However, it is possible to form flocks with good releasability from the immersion film. Therefore, it is possible to further reduce the cost of cleaning the membrane in the membrane treatment using the immersion membrane. The reason why flocs with good peelability are formed by using a polyaluminum chloride solution with a basicity of 60% or more and 70% or less is not clear, but polyaluminum chloride with a basicity of 60% or more and 70% or less The solution has a high charge-neutralizing power, and the flocs formed by the polyaluminum chloride solution with a basicity of 60% or more and 70% or less have progressed charge neutralization. It is presumed that this is due to a decrease in affinity with the membrane. In addition, the polyaluminum chloride solution with a basicity of 70% is characterized by low residual Al, excellent organic matter removal, and excellent flocculation, etc., and is a chemical that can reduce the pH adjustment reagent. .

A polyaluminum chloride solution having a basicity of 60% or more and 70% or less can be produced, for example, by the method described in JP-A-2009-203125. Specifically, the Al ₂ O ₃ concentration is 5 to 17% by mass, the Cl/Al ₂ O ₃ (molar ratio) is 1.80 to 3.60, and the SO ₄ /Al ₂ O ₃ (molar ratio) is 0 to To a basic aluminum chloride solution having a basicity of 0.35 and a basicity of 40-63% is added a compound of an alkali metal and/or an alkaline earth metal at a temperature of 85°C or less, and then at a temperature of 65-85°C. It can be produced by aging for 0.5 to 2 hours. The basicity of polyaluminum chloride solution can be measured by titration (JIS K-154: 2016).

The amount of inorganic flocculant added is, for example, in the range of 1 to 100 mg/L.

Examples of polymer flocculants include nonionic polymer flocculants, anionic polymer flocculants, cationic polymer flocculants, and the like, but are not particularly limited. Acrylate copolymer, sodium acrylamidopropanesulfonate, chitosan, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate and polyamidine. The polymer flocculants may be used singly or in combination of two or more.

The amount of polymer flocculant added is, for example, in the range of 0.1 to 2 mg/L.

pH adjustment may be performed in the aggregation step. Examples of pH adjusters include acids such as hydrochloric acid and sulfuric acid, and alkalis such as sodium hydroxide. For example, when polyaluminum chloride is used as a flocculating agent, the flocculation pH may be adjusted in the range of 6 to 8, preferably in the range of 6.2 to 7.5, more preferably 6.5. ~7.0. If the flocculation pH is less than 6 or exceeds 8, aluminum and the like derived from the flocculant may leak into the treated water.

The pH adjuster is added, for example, through the pH adjuster addition pipe 44 (aggregation pH adjuster addition step). Aggregation pH during the aggregation treatment is measured by the aggregation pH measurement device 28 installed in the aggregation treatment apparatus 10 (aggregation pH measurement step).

<Membrane filtration process>
The coagulated water that has undergone coagulation treatment is sent from the coagulation treatment apparatus 10 to the immersion tank 12 through the coagulation treatment water pipe 34 . Solid-liquid separation of the coagulated water is performed in the immersion tank 12 using the immersed membrane 16 immersed in the coagulated water (membrane filtration step). The coagulated water is sucked from the outside (primary side) to the inside (secondary side) of the immersion membrane 16, and suspended solids and the like contained in the coagulated water are filtered. The treated water that has undergone membrane filtration is sent to the treated water tank 14 through the treated water pipe 36 by the pump 22 . The naturally settled flocs and cakes peeled off from the surface of the submerged film 16 by washing the submerged film 16 to be described later deposit as sludge 52 in the lower part of the submerged tank 12 . The deposited sludge 52 is discharged through the sludge discharge pipe 40 at predetermined intervals. A predetermined amount of sludge may remain in the immersion tank 12 because the water recovery rate decreases when all the sludge is discharged.

The liquid temperature in the flocculation step and membrane filtration step is not particularly limited, and is, for example, in the range of 15 to 35°C. Since the separability changes depending on the viscosity and the like, it is desirable to adjust the liquid temperature so as to be as constant as possible.

The submerged membrane 16 used in the membrane filtration method according to the present embodiment is not particularly limited as long as it can filter suspended solids and the like in the coagulation treated water. As the immersion film 16, for example, an inorganic film such as ceramic, polyvinylidene fluoride (PVDF), polyethersulfone (PES), polytetrafluoroethylene (PTFE), cellulose acetate (CA), polyethylene ( A flat film such as an organic film made of PE) or the like can be used. Ceramic inorganic membranes are, for example, inorganic membranes such as alumina, silica, titania, zirconia, mullite, spinel, or mixtures thereof.

The pH of the immersion tank 12 is measured by an immersion tank pH measuring device 30 installed in the immersion tank 12 (immersion tank pH measurement step). In the membrane filtration method and the membrane filtration device 1 according to the present embodiment, when the pH of the immersion tank 12 becomes higher than the aggregation pH, the pH adjuster for adjusting the pH of the immersion tank 12 is added to the pH adjuster addition pipe. It is added to the coagulation treatment water in the coagulation treatment water pipe 34 through 46 (immersion tank pH adjuster addition step), and the pH of the immersion tank 12 is controlled to be below the aggregation pH (immersion tank pH adjustment step).

For example, when the water quality of the water to be treated suddenly fluctuates during a sudden rainfall, or when the pH control of the coagulation treatment device 10 is not effective due to a failure of the coagulation pH measurement device 28, etc., the pH in the immersion tank 12 increases, the pH of the immersion tank 12 becomes higher than the coagulation pH, and as a result, the chromaticity and TOC of the treated water may increase. It is considered that this is because the pH of the immersion tank 12 becomes higher than the coagulation pH, and coloring components such as organic matter taken into the sludge by coagulation are eluted. Here, since the eluted organic substances and the like are soluble, they cannot be removed by membrane filtration using the immersion membrane 16 . In the membrane filtration method and the membrane filtration device 1 according to the present embodiment, the pH of the immersion tank 12 is controlled to be below the flocculation pH, so even in such cases, coloring components such as organic substances taken into the sludge is suppressed, an increase in the chromaticity and TOC of the treated water is suppressed, and the water quality of the treated water is considered to be stabilized.

Examples of pH adjusters include acids such as hydrochloric acid and sulfuric acid, and alkalis such as sodium hydroxide.

The pH adjuster may be added so as to adjust the pH of the immersion tank 12. A pH adjuster addition pipe is connected to the immersion tank 12, and the pH adjuster is immersed through this pH adjuster addition pipe. It may also be added directly to tank 12 . In this case, the gas such as air used in the gas cleaning step described later is used, for example, the gas such as air is discharged from the air diffuser 18 through the gas supply pipe 50, and the pH in the immersion tank 12 is adjusted. It is preferable to stir so that the content becomes as uniform as possible.

The pH of the immersion bath may be adjusted, for example, within the range of 6.2 to 7.5, preferably within the range of 6.2 to 7.0. If the pH of the immersion bath is less than 6.2 or exceeds 7.5, aluminum derived from the flocculant, chromatic components, and the like may leak into the treated water.

<Washing process>
As the filtration progresses, suspended solids and the like accumulate on the surface of the immersion membrane 16, generating filtration resistance. In this case, the immersion film 16 may be washed (washing step). For example, the treated water is passed as backwash water from the treated water tank 14 by the pump 24 through the backwash water pipe 38 and the treated water pipe 36 from the secondary side to the primary side of the submerged membrane 16 to perform backwashing (backwashing). process). In this case, the pump 24, the backwash water pipe 38, and the treated water pipe 36 function as backwash means. In order to enhance the cleaning effect of backwashing, the immersed membrane 16 may be backwashed with chemical-containing backwash water containing a cleaning chemical (chemically enhanced backwashing process). For example, a predetermined amount of the cleaning chemical is added to the treated water through the chemical addition pipe 48 in the backwash water pipe 38, and the treated water added with the cleaning chemical is used as the backwash water by the pump 24 to the backwash water pipe 38, Water is passed through the treated water pipe 36 from the secondary side to the primary side of the submerged membrane 16 to perform backwashing (chemically enhanced backwashing process). In this case, the pump 24, the chemical addition pipe 48, the backwash water pipe 38, and the treated water pipe 36 function as chemically enhanced backwash means. After the washing process is finished, the process may be returned to the flocculation process or the membrane filtration process.

As the backwash water, water other than the treated water may be used. Cleaning agents used in chemically enhanced backwashing include alkaline agents such as sodium hypochlorite and sodium hydroxide. Sodium hypochlorite and sodium hydroxide may be used in combination to further enhance the cleaning effect of backwashing.

The content of the cleaning chemical in the chemical-containing backwash water is, for example, 10 ppm or more, preferably in the range of 10 to 500 ppm, more preferably in the range of 50 to 400 ppm, and more preferably in the range of 100 to 400 ppm. It is even more preferable to have If the content of the cleaning chemical in the chemical-containing backwash water is less than 10 ppm, the cleaning effect of the submerged membrane 16 may not be obtained. Economic efficiency may be impaired.

Chemically enhanced backwashing may be performed, for example, at predetermined time intervals or at predetermined filtered water volume intervals, and is not particularly limited. Chemically enhanced backwashing may be performed, for example, at a frequency of once/day to once/week.

Instead of backwashing (including chemically enhanced backwashing), or together with backwashing, air scrubbing is performed by supplying gas from below the submerged membrane 16 toward the primary side of the submerged membrane 16 by a gas supply means or the like. (gas washing process), high-pressure washing may be performed by spraying high-pressure water (injection pressure is, for example, 1 MPa to 10 MPa) to wash (high-pressure washing process), or vibration is applied to the film to remove it. It is good to encourage. Backwashing with backwash water may be used in combination with air scrubbing or high-pressure washing.

In the present embodiment, it is preferable to carry out the gas cleaning step together with the backwashing in order to further suppress the decrease in the filtration flux of the submerged membrane 16 during the membrane filtration step. For example, the compressor is turned on and gas such as air is supplied toward the submerged membrane 16 through the gas supply pipe 50 and the air diffuser 18 . The supplied gas disturbs the flow in the vicinity of the submerged membrane 16 , and the suspended solids and the like adhering to the submerged membrane 16 are peeled off.

The gas washing step may be performed, for example, continuously during the membrane filtration step, may be performed at predetermined time intervals, or may be performed at predetermined filtered water volume intervals. not to be

When the chemical-strengthened backwashing is performed, especially when the addition amount of the alkaline chemical such as sodium hypochlorite increases, even if the pH of the coagulation treatment apparatus 10 is controlled, the alkaline or the like used for the chemical-strengthened backwashing is reduced. If the pH in the immersion tank 12 is increased by the cleaning chemical, and the pH in the immersion tank 12 becomes higher than the flocculation pH, the chromaticity and TOC of the treated water may increase as a result. It is considered that this is because the pH of the immersion tank 12 becomes higher than the coagulation pH, and coloring components such as organic matter taken into the sludge by coagulation are eluted. In the membrane filtration method and the membrane filtration device 1 according to the present embodiment, the pH of the immersion tank 12 is controlled to be equal to or lower than the flocculation pH. Elution is suppressed, an increase in the chromaticity and TOC of the treated water is suppressed, and the water quality of the treated water is stabilized.

Although the pH of the immersion tank 12 may be measured by the immersion tank pH measuring device 30, it is preferable to measure the pH below the lower end of the immersion membrane 16 in the immersion tank 12. It is more preferable to measure the pH above the deposited sludge 52. By measuring the pH below the lower end of the immersion membrane 16 in the immersion tank 12, that is, by measuring the pH near the sludge 52 deposited in the lower part of the immersion tank 12, the measured pH of the immersion tank 12 is the aggregation pH. By controlling the pH of the immersion bath 12 to be equal to or lower than the flocculation pH when the pH becomes higher than , the increase in the chromaticity and TOC of the treated water is further suppressed.

The SS of the treated water obtained by the membrane filtration method and membrane filtration apparatus according to the present embodiment is, for example, less than 2 mg/L, the TOC is, for example, in the range of 0.5 to 5 mg/L, and the chromaticity is , for example, in the range of 1 to 100 degrees.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

<Example 1 and Comparative Example 1>
Using the membrane filtration device 1 of FIG. 1, suction membrane filtration was performed under the conditions of a flux of 0.8 m/d and a water recovery rate of 99%. A case where the pH of the immersion tank was controlled was set as Example 1, and a case where the pH of the immersion tank was not controlled was set as Comparative Example 1. In Example 1, when the pH of the immersion tank became higher than the coagulation pH, hydrochloric acid was added as a pH adjuster to control the pH of the immersion tank to be equal to or lower than the coagulation pH. Table 1 shows the water quality (pH, chromaticity (degree), TOC (mg/L)) of the treated water used and the obtained treated water. The chromaticity was measured using a colorimeter (WA6000, manufactured by Nippon Denshoku Industries Co., Ltd.), and the TOC was measured using a TOC meter (Sievers M9, manufactured by GE).

In Comparative Example 1, both chromaticity and TOC were higher than those of the water to be treated. This is thought to be due to the organic matter eluted from the sludge. In Example 1, both chromaticity and TOC were lower than those of the water to be treated and Comparative Example 1. This is probably because the elution of organic matter and the like from the sludge was suppressed.

As described above, according to the method of the example, the quality of the treated water could be stabilized in the membrane filtration treatment using the immersed membrane for the water to be treated containing organic substances and the like.

1 Membrane Filtration Device 10 Aggregation Treatment Device 12 Immersion Tank 14 Treatment Water Tank 16 Immersion Membrane 18 Air Diffuser 20 Membrane Filtration Device 22, 24 Pump 26 Stirrer 28 Aggregation pH Measuring Device 30 Immersion Tank pH measuring device 32 water to be treated pipe 34 coagulation treated water pipe 36 treated water pipe 38 backwash water pipe 40 sludge discharge pipe 42 coagulant addition pipe 44, 46 pH adjuster addition pipe 48 chemical addition piping, 50 gas supply piping, 52 sludge.

Claims (8)

a flocculation treatment means for adding a flocculating agent containing polyaluminum chloride to the water to be treated and performing flocculation treatment in a pH range of 6.2 to 7.5;
Aggregation pH measuring means for measuring aggregation pH when performing the aggregation treatment;
Membrane filtration means for performing solid-liquid separation of the flocculated water in a pH range of 6.2 to 7.5 by means of an immersion membrane installed in the immersion tank;
immersion tank pH measuring means for measuring the pH of the immersion tank;
an immersion tank pH adjuster adding means for adding a pH adjuster for adjusting the pH of the immersion tank;
with
When the pH of the immersion tank becomes higher than the flocculation pH, a pH adjuster is added using the immersion tank pH adjuster adding means, and the pH of the immersion tank is controlled to be equal to or lower than the flocculation pH. A membrane filtration device characterized by: The membrane filtration device according to claim 1,
A membrane filtration apparatus, further comprising chemically enhanced backwashing means for backwashing the immersed membrane with chemical-containing backwash water containing a cleaning chemical. The membrane filtration device according to claim 2,
The membrane filtration device, wherein the content of the cleaning chemical in the chemical-containing backwash water is 10 ppm or more. The membrane filtration device according to any one of claims 1 to 3,
The membrane filtration device, wherein the immersion tank pH measuring means measures the pH below the lower end of the immersion membrane in the immersion tank. A flocculation treatment step of adding a flocculating agent containing polyaluminum chloride to the water to be treated and performing flocculation treatment in a pH range of 6.2 to 7.5;
a flocculation pH measuring step of measuring flocculation pH during the flocculation treatment;
a membrane filtration step of performing solid-liquid separation of the flocculation-treated water in a pH range of 6.2 to 7.5 with an immersion membrane installed in an immersion tank;
an immersion bath pH measuring step of measuring the pH of the immersion bath;
including
When the pH of the immersion tank becomes higher than the coagulation pH, a pH adjuster is added to adjust the pH of the immersion tank, and the pH of the immersion tank is controlled to be equal to or lower than the aggregation pH. A membrane filtration method characterized by: The membrane filtration method according to claim 5,
A membrane filtration method, further comprising a chemically enhanced backwashing step of backwashing the submerged membrane with chemical-containing backwash water containing a cleaning chemical. The membrane filtration method according to claim 6,
A membrane filtration method, wherein the content of the cleaning chemical in the chemical-containing backwash water is 10 ppm or more. The membrane filtration method according to any one of claims 5 to 7,
A membrane filtration method, wherein, in the immersion bath pH measurement step, the pH is measured below a lower end portion of the immersion membrane in the immersion bath.

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