JP2024032980A - How to operate a reverse osmosis membrane - Google Patents

Abstract

An object of the present invention is to provide a method for operating a reverse osmosis membrane, which can suppress the precipitation of inorganic substances in water to be treated, while also suppressing the precipitation of organic substances, and prevent clogging of the reverse osmosis membrane. [Solution] A method for operating a reverse osmosis membrane that separates raw water into permeated water and concentrated water using a reverse osmosis membrane, in which the pH of the water to be treated supplied to reverse osmosis membrane modules 18, 18, 18 is 5.8 to 7.8, and after treating the water to be treated with the reverse osmosis membranes of the reverse osmosis membrane modules 18, 18, 18, the reverse osmosis membranes of the reverse osmosis membrane modules 18, 18, 18 are periodically How to operate a reverse osmosis membrane to release pressure. [Selection diagram] Figure 1

Description

The present invention relates to a method for operating a reverse osmosis membrane.

A water treatment method is known in which raw water is separated into permeated water and concentrated water using a reverse osmosis membrane. FIG. 4 is a schematic diagram for explaining an example of a conventional water treatment method using a reverse osmosis membrane. In the method using the treatment system 101 shown in FIG. 4, first, well water from the well 111 flows through the channel 121 as raw water and is supplied to the water tank 112 to be treated. In the water tank 112 to be treated, chlorine is added by the chlorine adding means 130, and sulfuric acid is added by the sulfuric acid adding means 131, and the raw water, chlorine, and sulfuric acid are mixed to become the water to be treated.
Next, the water to be treated flows through the channel 122, polyaluminum chloride is added by the addition means 132, and the water is sent to the sand filter tower 141 by the pump 114. In the sand filter tower 141, aggregates of organic matter, iron, and manganese in the water to be treated are removed.
Thereafter, the water to be treated is passed through activated carbon 142, chlorine is added by chlorine adding means 130, and then supplied to a membrane filtration device 143 equipped with an ultrafiltration membrane. In the filtration device 143, protozoa such as Cryptosporidium and other bacteria in the water to be treated are removed by an ultrafiltration membrane.

Next, the permeated water from the filtration device 143 flows through the channel 127 and is stored in the intermediate treatment tank 144 . A part of the permeated water in the intermediate treatment tank 144 is sent by the pump 114, flows through the flow path 127, and is stored in the treated water tank 119 as it is. On the other hand, the remainder of the permeated water in the intermediate treatment tank 144 is sent by the pump 114 and flows through the channel 128, and after being pressurized by the boost pump 116, is supplied to the reverse osmosis membrane filtration device 117 equipped with a reverse osmosis membrane. . In the reverse osmosis membrane filtration device 117, calcium and magnesium in the water to be treated are removed by the reverse osmosis membrane.
Thereafter, the permeated water from the reverse osmosis membrane filtration device 117 flows through the flow path 124 and is supplied to the treated water tank 119. On the other hand, concentrated water from the reverse osmosis membrane filtration device 117 is collected in the flow path 123 and discharged to the outside of the treatment system 101.
As described above, in the conventional method using the treatment system 101, treatments are performed to improve water quality in terms of total organic carbon (TOC), iron and manganese, protozoa and bacteria, and hardness. Therefore, there is a problem that the equipment configuration and treatment flow become complicated, and the cost of water quality treatment increases.

Therefore, as shown in the example shown in FIG. 5, a treatment method is used in which all of the treatments for improving the water quality of TOC, iron and manganese, protozoa and bacteria, and hardness are carried out in a reverse osmosis membrane filtration device 117. is being considered.
In the method using the treatment system 102 shown in FIG. 5, raw water first flows from the well 111 through the channel 121 and is supplied to the water tank 112 to be treated. Sulfuric acid is added into the treated water tank 112 by the sulfuric acid addition means 131, and the sulfuric acid and raw water are mixed to become treated water.
Next, the water to be treated is pumped by the pump 114 and flows through the channel 122, and after being passed through the pre-filter 115 and the booster pump 116, it is supplied to the reverse osmosis membrane filtration device 117. In the treatment system 102, a reverse osmosis membrane filtration device 117 removes organic matter, calcium, silica, inorganic matter such as iron and manganese, protozoa and various bacteria, calcium and magnesium in the water to be treated all at once.
Thereafter, the permeated water from the reverse osmosis membrane filtration device 117 flows through the channel 124, sulfuric acid and chlorine are added thereto, and the hardness is adjusted by the mineralizer 135. Thereafter, the permeated water is stored in the treated water tank 119, and chlorine is added thereto by the chlorine adding means 130 as needed.

As described above, according to the method using the treatment system 102, water quality in almost all items can be improved by the reverse osmosis membrane filtration device 117, which simplifies the device and treatment flow, reduces costs, and further saves the treatment system. You can create more space.
However, on the other hand, when multiple items of water quality improvement treatment are performed at once using the reverse osmosis membrane filtration device 117, as in the method using the treatment system 102, there is a problem that the reverse osmosis membrane frequently becomes clogged. arise.
Therefore, it has been proposed to treat water so that the pH of the water to be treated by a reverse osmosis membrane is 7.0 or less (for example, Patent Document 1).

JP 2019-107592 Publication

FIG. 6 is a schematic diagram for explaining an example of a method of treating water so that the pH of the water to be treated by the reverse osmosis membrane is 7.0 or less. In the treatment system 103 shown in FIG. 6, concentrated water from the reverse osmosis membrane filtration device 117 is collected in a channel 123, a part of the concentrated water in the channel 123 flows through a channel 125, and is returned to the channel 122. The water is re-supplied to the reverse osmosis membrane filtration device 117 as water to be treated. In the method using the treatment system 103, similarly to the method disclosed in Patent Document 1, the pH of the treated water supplied to the primary side of the reverse osmosis membrane of the reverse osmosis membrane filtration device 117 is 7.0 or less. The pH of the water to be treated is adjusted in the water tank 112 by the sulfuric acid addition means 131. Therefore, precipitation of inorganic substances in the water to be treated can be suppressed and deposition on the surface of the reverse osmosis membrane can be reduced.
However, when water is treated so that the pH of the water to be treated by the reverse osmosis membrane is 7.0 or less, organic substances tend to precipitate from the water to be treated, and the surface of the reverse osmosis membrane is clogged by the precipitated organic substances. There is a problem.
The present invention provides a method for operating a reverse osmosis membrane that can suppress the precipitation of organic substances while suppressing the precipitation of inorganic substances in water to be treated, and can prevent clogging of the reverse osmosis membrane.

The present invention has the following aspects.
[1] A method for operating a reverse osmosis membrane in which raw water is separated into permeated water and concentrated water by a reverse osmosis membrane, wherein the pH of the water to be treated supplied to the reverse osmosis membrane is 5.8 to 7. 8, the method for operating a reverse osmosis membrane, which comprises periodically releasing pressure on the reverse osmosis membrane after treating the water to be treated with the reverse osmosis membrane.
[2] The method for operating a reverse osmosis membrane according to [1], wherein the surface of the reverse osmosis membrane is washed by flushing when the pressure of the reverse osmosis membrane is released.
[3] The method for operating a reverse osmosis membrane according to [2], in which permeated water or washing water containing pure water is used during flushing.
[4] The method for operating a reverse osmosis membrane according to any one of [1] to [3], wherein the raw water is directly treated with the reverse osmosis membrane.
[5] The method for operating a reverse osmosis membrane according to any one of [1] to [4], wherein the raw water further contains at least one inorganic substance selected from the group consisting of silica, calcium, magnesium, iron, and manganese.
[6] The method for operating a reverse osmosis membrane according to any one of [1] to [5], wherein the M alkalinity of the raw water is 100 mg/L or more.
[7] The method for operating a reverse osmosis membrane according to any one of [1] to [6], wherein the total organic carbon of the water to be treated is 3.0 mg/L or more.

According to the method for operating a reverse osmosis membrane of the present invention, while suppressing the precipitation of inorganic substances in the water to be treated, the precipitation of organic substances can also be suppressed, and clogging of the reverse osmosis membrane can be prevented.

FIG. 1 is a schematic diagram of a treatment system used in a method of operating a reverse osmosis membrane according to an embodiment. FIG. 1 is a schematic diagram showing the configuration of a flat membrane testing device used in Examples. 4 is a schematic cross-sectional view of a closed container and a flat membrane cell included in the flat membrane testing apparatus of FIG. 3. FIG. 1 is a schematic diagram of a treatment system used in a conventional water treatment method. 1 is a schematic diagram of a treatment system used in a conventional water treatment method. 1 is a schematic diagram of a treatment system used in a conventional water treatment method.

"Total organic carbon" is defined in, for example, JIS K 0101 20. It can be measured based on organic carbon (TOC).
"M alkalinity" is defined in, for example, JIS K 0101 13. Can be measured according to acid consumption.
"~" indicating a numerical range means that the numerical values written before and after it are included as the lower limit and upper limit.

EMBODIMENT OF THE INVENTION Hereinafter, the operating method of the reverse osmosis membrane of this invention is demonstrated by showing the embodiment example. However, the present invention is not limited to the following embodiments.
FIG. 1 is a schematic diagram of a treatment system 1 used in a method for operating a reverse osmosis membrane according to an embodiment of the present invention. The treatment system 1 includes a well 11 , a treated water tank 12 , an acid addition means 31 , a reverse osmosis membrane filtration device 17 , and a treated water tank 19 .

The water tank 12 to be treated is a tank that stores water to be treated. A raw water flow path 21 , a water flow path 22 and a circulation flow path 25 are connected to the water tank 12 to be treated.
The raw water flow path 21 is a flow path for supplying raw water (well water) from the well 11 to the water tank 12 to be treated. The water to be treated channel 22 is a channel for supplying the water to be treated from the water tank 12 to the reverse osmosis membrane filtration device 17 . The circulation channel 25 is a channel for supplying concentrated water from the reverse osmosis membrane filtration device 17 to the water tank 12 to be treated. Therefore, the water to be treated in the water tank 12 includes concentrated water from the reverse osmosis membrane filtration device 17 and raw water (well water).

The acid addition means 31 adds an acid component to the water to be treated in the water tank 12 to be treated. The acid component is not particularly limited. Examples include hydrochloric acid and sulfuric acid.
The acid addition means 31 may be in the form of adding the acid component directly to the water to be treated in the liquid phase portion in the water tank 12 to be treated, or may be in the form of adding the acid component through the gas phase portion in the water tank 12 to be treated. .

A pump 14, a pre-filter 15, and a boost pump 16 are provided in the treated water flow path 22 in this order from the treated water tank 12 side.
The pump 14 is for sending and supplying the water to be treated to the reverse osmosis membrane filtration device 17 .
The pre-filter 15 removes large-sized impurities such as sand, silt, and clay, and protects the reverse osmosis membrane of the reverse osmosis membrane filtration device 17 from clogging, abrasion, and the like.
The boost pump 16 applies the pressure necessary for filtering the water to be treated with the reverse osmosis membrane filtration device 17 .

The reverse osmosis membrane filtration device 17 includes reverse osmosis membrane modules 18 , 18 , 18 . The reverse osmosis membrane modules 18, 18, 18 each have a reverse osmosis membrane therein. The reverse osmosis membrane filtration device 17 is a device that reverse osmoses the water to be treated through the reverse osmosis membranes in the reverse osmosis membrane modules 18, 18, 18, and separates the water to be treated into permeated water and concentrated water.
Each reverse osmosis membrane module 18 is connected to each end of a branched water flow path 22 to be treated. Therefore, the water to be treated in the water to be treated channel 22 is supplied to the primary side of the reverse osmosis membrane of each reverse osmosis membrane module 18 .

Furthermore, each reverse osmosis membrane module 18 is connected to each branched end of the concentrated water flow path 23 and the permeated water flow path 24.
The concentrated water channel 23 is a channel for collecting concentrated water separated by the reverse osmosis membranes in the reverse osmosis membrane modules 18, 18, 18. The concentrated water flow path 23 is connected to a circulation flow path 25 and a drainage flow path 26 at a connection point D. The circulation channel 25 is a channel for supplying a portion of the concentrated water from the concentrated water channel 23 to the water tank 12 to be treated. The drainage channel 26 is a channel for discharging the remainder of the concentrated water to the outside of the treatment system 1.
The permeated water channel 24 is connected to the treated water tank 19. The permeated water flow path 24 is a flow path for supplying the permeated water separated by the reverse osmosis membrane filtration device 17 to the treated water tank 19.
The treated water tank 19 is a tank that stores permeated water separated by the reverse osmosis membrane filtration device 17 as treated water.

Next, a method of operating a reverse osmosis membrane according to an embodiment of the present invention will be described.
The method of operating a reverse osmosis membrane according to this embodiment is a method of separating raw water into permeated water and concentrated water using a reverse osmosis membrane and treating the raw water.
Examples of raw water include groundwater such as well water and underground water; river water; and lake water. However, raw water is not limited to these examples.

In this embodiment, the raw water contains soluble organic matter. The organic matter in the raw water is not particularly limited, but examples include humic substances. Examples of humic substances in raw water include humic acid and fulvic acid.
The total organic carbon in the raw water is preferably 3.0 mg/L or more, more preferably 4.0 mg/L or more, from the viewpoint of suppressing the amount of acid components used and suppressing membrane clogging of the reverse osmosis membrane.5. Particularly preferred is 0 mg/L or more. In this embodiment, as will be described later, the lower limit of pH of the water to be treated is adjusted to a predetermined value or higher, so even if the raw water has total organic carbon equal to or higher than the lower limit, clogging of the reverse osmosis membrane can be prevented. As a result, even if the total organic carbon content in the raw water is high, the permeation capacity is less likely to decrease due to clogging of the reverse osmosis membrane, and permeated water can be obtained while maintaining treatment efficiency.
The upper limit of total organic carbon in raw water is not particularly limited. For example, from the viewpoint of treatment efficiency, the total organic carbon in raw water is preferably 100 mg/L or less, more preferably 50 mg/L or less.

The content of the organic compound having a molecular weight of 10,000 or more in the raw water may be 0.001 to 1 mg/L, 0.005 to 0.5 mg/L, or 0.01 to 0.1 mg/L. When the content of organic compounds having a molecular weight of 10,000 or more in the raw water is below the upper limit, precipitation of organic substances is less likely to occur, and clogging of the reverse osmosis membrane by the precipitated organic substances can be further suppressed. Even if the content of organic compounds with a molecular weight of 10,000 or more in the raw water is equal to or higher than the lower limit, in this embodiment, as described below, the lower limit of the pH of the water to be treated is adjusted to a predetermined value or higher. It is thought that organic matter is less likely to precipitate, it is possible to prevent blockage of the reverse osmosis membrane, and it is possible to obtain permeated water while maintaining treatment efficiency.

In addition to organic substances, the raw water may further contain at least one ionic inorganic substance selected from the group consisting of silica, calcium, magnesium, iron, and manganese. If the raw water further contains these inorganic substances, in this embodiment, the upper limit of the pH of the water to be treated is adjusted to a predetermined value or less as described later, so that the inorganic substances are less likely to precipitate and blockage of the reverse osmosis membrane is prevented. It is thought that it is possible to obtain permeated water while maintaining treatment efficiency.

The M alkalinity of the raw water is preferably 100 mg/L or more, more preferably 200 mg/L or more, particularly preferably 300 mg/L or more, from the viewpoint of suppressing the amount of acid components used and suppressing membrane clogging of the reverse osmosis membrane. . Even if the M alkalinity of the raw water is equal to or higher than the lower limit, in this embodiment, the lower limit of the pH of the water to be treated is adjusted to a predetermined value or higher, so the amount of acid components used is difficult to increase, and water treatment is improved. Cost reduction can be achieved.
The upper limit of the M alkalinity of raw water is not particularly limited. For example, from the viewpoint of treatment efficiency, the M alkalinity of raw water is preferably 1000 mg/L or less, more preferably 700 mg/L or less.

In the method for operating a reverse osmosis membrane according to the present embodiment, raw water is separated into permeated water and concentrated water by the reverse osmosis membrane. As shown in FIG. 1, raw water first flows from the well 11 through the raw water channel 21 and is supplied to the water tank 12 to be treated. In the water tank 12 to be treated, an acid component is added by the acid addition means 31, and the acid component and raw water are mixed. In addition, a dispersant may be further added to the raw water for the purpose of dispersing organic substances in the raw water. By adding a dispersant, it can be expected that the cleaning effect on the surface of the reverse osmosis membrane during flushing, which will be described later, will be improved.
Next, the water to be treated is pumped by the pump 14 and flows through the water to be treated channel 22 . Thereafter, the water to be treated is supplied to the reverse osmosis membrane of the reverse osmosis membrane module 18 in the reverse osmosis membrane filtration device 17 via the pre-filter 15 and the boost pump 16 in this order, and is treated by the reverse osmosis membrane.

As described above, in the reverse osmosis membrane operating method according to the present embodiment, it is preferable that raw water (well water) be directly treated with the reverse osmosis membrane of the reverse osmosis membrane module 18. "Directly treating raw water with a reverse osmosis membrane" means supplying raw water to the reverse osmosis membrane without pre-treating the raw water, such as groundwater or well water. Here, pretreatment includes, for example, treatment performed for the purpose of reducing total organic carbon, removing iron and manganese, removing protozoa and various bacteria, and the like.
In conventional water treatment methods using reverse osmosis membranes, it is recommended that the water to be treated be supplied to the reverse osmosis membrane after the total amount of impurities and the like in the water is reduced to a certain amount or less. This is to prevent a decrease in permeability due to clogging of the reverse osmosis membrane and damage to the reverse osmosis membrane caused by increasing the pressure difference to maintain permeability.
On the other hand, in the reverse osmosis membrane operating method according to the present embodiment, since the pH of the water to be treated by the reverse osmosis membrane is controlled within a predetermined range as described later, precipitates are generated from the water to be treated. is suppressed. Therefore, even if underground water (raw water) such as well water is directly supplied to the reverse osmosis membrane as water to be treated, permeated water can be continuously obtained through the reverse osmosis membrane. By directly and collectively treating raw water with the reverse osmosis membrane of the reverse osmosis membrane module 18, the water quality of each item such as total organic carbon (TOC), iron and manganese, protozoa and bacteria, and hardness can be improved by the reverse osmosis membrane. . Therefore, the apparatus and treatment flow can be simplified, the cost of water treatment can be reduced, and the space of the treatment system 1 can be saved.

Next, the water to be treated supplied to each reverse osmosis membrane module 18 is separated into permeated water and concentrated water by the reverse osmosis membrane of each reverse osmosis membrane module 18, respectively. Thereafter, each permeated water of each reverse osmosis membrane module 18 flows through the permeated water channel 24 and is stored in the treated water tank 19 as treated water.
On the other hand, each concentrated water of each reverse osmosis membrane module 18 is collected in the concentrated water channel 23. A portion of the concentrated water in the concentrated water channel 23 then flows through the circulation channel 25 and is again supplied to the water tank 12 to be treated. Therefore, a part of the concentrated water is mixed with the raw water from the well 11 in the water tank 12 to be treated, and is re-supplied to the reverse osmosis membrane filtration device 17 as water to be treated. Further, the remainder of the concentrated water in the concentrated water flow path 23 flows through the drainage flow path 26 and is discharged to the outside of the treatment system 1.

In the method for operating a reverse osmosis membrane according to the present embodiment, the pH of the water to be treated that is supplied to each reverse osmosis membrane included in each reverse osmosis membrane module 18 is adjusted to 5.8 to 7.8.
The pH of the water to be treated in each reverse osmosis membrane module 18 is within the range of 5.8 to 7.8, preferably within the range of 6.3 to 7.5, and within the range of 6.5 to 7.3. is more preferable, and a range of 6.7 to 7.2 is particularly preferable.
When the pH of the water to be treated is equal to or higher than the lower limit value, organic matter becomes difficult to precipitate on the primary side surface of the reverse osmosis membrane of each reverse osmosis membrane module 18, and blockage of the reverse osmosis membrane can be prevented. In addition, compared to the conventional operating method in which there is no particular restriction on the lower limit of the pH of the water to be treated, the amount of acid component used to lower the pH is reduced, making it possible to reduce costs. In addition, since the pH of the water to be treated is below the above-mentioned upper limit, oxides of metals such as iron and manganese are difficult to precipitate on the primary side surface of the reverse osmosis membrane of each reverse osmosis membrane module 18, and reverse osmosis Membrane blockage can be prevented.
The pH of the water to be treated can be adjusted, for example, by adding an acid component using the acid addition means 31.

The total organic carbon in the water to be treated supplied to each reverse osmosis membrane module 18 may be 3.0 mg/L or more, 5.0 mg/L or more, or 10.0 mg/L or more. In this embodiment, since the lower limit of the pH of the water to be treated is adjusted to be equal to or higher than a predetermined value, clogging of the reverse osmosis membrane is prevented even if the total organic carbon in the water to be treated is equal to or higher than the lower limit. This makes it possible to obtain permeated water while maintaining treatment efficiency, making it difficult for the permeation capacity to decrease due to clogging of the reverse osmosis membrane.
The upper limit of total organic carbon in the water to be treated is not particularly limited. For example, the total organic carbon in the water to be treated may be 100 mg/L or less, or 500 mg/L or less.

In the method for operating a reverse osmosis membrane according to the present embodiment, the water to be treated is filtered and treated by the reverse osmosis membrane of each reverse osmosis membrane module 18 for a predetermined period of time.
When filtering water to be treated with a reverse osmosis membrane, each reverse osmosis membrane module 18 is filled with the water to be treated, the water pressure inside each reverse osmosis membrane module 18 increases, and pressure is applied to the reverse osmosis membrane. To go. After that, when the water pressure applied to each reverse osmosis membrane becomes higher than the osmotic pressure, a reverse osmosis phenomenon occurs in the reverse osmosis membrane, and the solute concentration in the water to be treated locally increases on the primary side of each reverse osmosis membrane, and the permeate water increases. can get. As a result, concentrated water is generated on the primary side of each reverse osmosis membrane, and each concentrated water is separated from each permeated water on the secondary side of each reverse osmosis membrane. In this way, the water to be treated can be filtered and treated.

The time period for filtering the water to be treated using the reverse osmosis membrane is not particularly limited, and can be set as appropriate, for example, taking treatment efficiency into consideration. The time for filtering the water to be treated using the reverse osmosis membrane may be, for example, 5 to 1500 minutes, or 15 to 180 minutes. When the time for filtering the water to be treated is equal to or longer than the lower limit, the amount of permeated water obtained increases, and the treatment efficiency tends to be maintained sufficiently high for practical purposes. When the time for filtering the water to be treated is equal to or less than the upper limit value, clogging of the reverse osmosis membrane can be further prevented.

In the method for operating a reverse osmosis membrane according to the present embodiment, the risk of clogging of the reverse osmosis membrane increases after the water to be treated is filtered and treated by the reverse osmosis membrane for a predetermined period of time.
Therefore, in the method for operating a reverse osmosis membrane according to the present embodiment, after treating the water to be treated for a predetermined period of time, the pressure of the reverse osmosis membrane of each reverse osmosis membrane module 18 is periodically released. By periodically releasing pressure from the reverse osmosis membrane, the solute on the surface of the reverse osmosis membrane diffuses, thereby eliminating the concentration polarization state. Therefore, concentration polarization on the primary side of the reverse osmosis membrane can be periodically eliminated, and precipitation of inorganic substances and organic substances on the primary side surface of the reverse osmosis membrane can be prevented.
Here, "regularly releasing the pressure of the reverse osmosis membrane" means to release the reverse osmosis membrane from the water pressure of the water to be treated in each reverse osmosis membrane module 18 after filtering the water to be treated for a predetermined period. It means that. The term "predetermined period" as used herein basically means a certain period of time. However, the "predetermined period" does not necessarily have to be a strictly constant period, but may be a certain period of time (for example, a difference of about 1 to 10 minutes) as long as the effect of the present invention can be obtained. Long and short are acceptable.

When releasing the pressure of the reverse osmosis membrane, the pressure may be released on the secondary side (permeated water side) of the reverse osmosis membrane. By releasing pressure on the secondary side (permeated water side) of the reverse osmosis membrane, water on the secondary side of the reverse osmosis membrane moves to the primary side (to-be-treated water side) of the reverse osmosis membrane by forward osmosis. As a result, the solute concentration on the primary side of the reverse osmosis membrane can also be periodically reduced, and precipitation of inorganic and organic substances on the primary side surface of the reverse osmosis membrane can be reliably prevented.

The time for releasing the pressure of the reverse osmosis membrane is not particularly limited, and can be set as appropriate, for example, taking treatment efficiency into consideration. The time for releasing pressure from the reverse osmosis membrane may be, for example, 10 to 600 seconds, or 30 to 120 seconds. When the time for releasing pressure on the reverse osmosis membrane is equal to or longer than the lower limit, precipitation of inorganic substances and organic substances on the primary surface of the reverse osmosis membrane can be easily prevented. When the time for releasing the pressure of the reverse osmosis membrane is equal to or less than the above upper limit, there is a tendency that the treatment efficiency of permeated water can be maintained sufficiently high for practical purposes.

A specific method for releasing pressure from the reverse osmosis membrane is a method that can release the reverse osmosis membrane from the water pressure of the water to be treated in each reverse osmosis membrane module 18 and eliminate concentration polarization on the surface of the reverse osmosis membrane. However, there are no particular limitations. For example, the following methods may be mentioned, but the specific mode of pressure release is not limited to the following examples.
For example, the reverse osmosis membrane filtration device 17 includes an unillustrated extraction pipe for extracting the water to be treated supplied into the reverse osmosis membrane module 18, and an unillustrated extraction valve provided in the extraction pipe. In this case, the pressure on the reverse osmosis membrane can be released by loosening the extraction valve and lowering the pressure on the primary side of the reverse osmosis membrane.
Alternatively, the pressure of the reverse osmosis membrane may be released by stopping the boost pump 16 and reducing the pressure on the primary side of the reverse osmosis membrane.

In the method of operating a reverse osmosis membrane according to the present embodiment, even if components such as organic matter are precipitated from the water to be treated, or even if the precipitates are deposited on the primary side surface of the reverse osmosis membrane, the pressure is not released. At this time, by further performing flushing, deposits on the surface of the reverse osmosis membrane can be washed away. Therefore, blockage of the reverse osmosis membrane can be more effectively prevented.

A specific method for performing flushing is not particularly limited. For example, the following methods may be mentioned, but the specific aspects of flushing are not limited to the following examples.
For example, the flow rate of the water to be treated that is sent into each reverse osmosis membrane module 18 via the water flow path 22 by the pump 14 and the booster pump 16 may be made larger than the flow rate when filtering the water to be treated. Flushing can be performed with As a result, more water to be treated than the flow rate when filtering the water to be treated flows over the surface of the reverse osmosis membrane as washing water, and the precipitates accumulated on the surface of the membrane can be washed away. The effect of preventing blockage becomes even higher.
In addition, a back pressure valve may be installed in the concentrated water flow path 23, and the degree of opening of the back pressure valve may be increased to allow more water to be treated to flow onto the surface of the reverse osmosis membrane than the flow rate when filtering the water to be treated. You may also perform flushing.

During flushing, in addition to raw water and concentrated water, cleaning water that further contains permeated water or pure water in the water to be treated may be used. By using cleaning water containing permeated water or pure water, the reverse osmosis membrane can be cleaned with cleaner cleaning water, and deposits including organic matter deposited on the surface of the reverse osmosis membrane can be more effectively removed. The effect of preventing blockage becomes even higher.

The flow rate of the water to be treated during flushing is preferably 0.05 m/s or more, more preferably 0.2 m/s or more, and even more preferably 0.5 m/s or more. If the flow rate of the water to be treated during flushing is equal to or higher than the lower limit value, further improvement of the cleaning effect by flushing can be expected. Here, the flow rate of the water to be treated during flushing is measured as the flow rate of the water to be treated on the surface of the reverse osmosis membrane of the reverse osmosis membrane module 18.

Flushing is performed when the pressure of the reverse osmosis membrane is periodically released, but flushing itself may be performed periodically after a certain period of time each time the pressure of the reverse osmosis membrane is released; It may be done irregularly. However, in consideration of the cleaning effect of the reverse osmosis membrane and the recovery effect of the separation ability due to flushing, it is considered preferable to perform the flushing periodically as well as the pressure release.

(effect)
In the method of operating a reverse osmosis membrane according to the present embodiment described above, in order to adjust the pH of the water to be treated supplied to the reverse osmosis membrane module 18 to be 5.8 or higher, the primary side of the reverse osmosis membrane is Organic substances are less likely to precipitate on the surface, and clogging of the reverse osmosis membrane by organic substances can be suppressed.
In addition, in the reverse osmosis membrane operating method according to the present embodiment, in order to adjust the pH of the water to be treated supplied to the reverse osmosis membrane module 18 to be 7.8 or less, the reverse osmosis membrane of the reverse osmosis membrane module 18 is It is difficult for inorganic substances to precipitate on the primary side surface of the membrane, and it is also possible to suppress clogging of the reverse osmosis membrane by metal oxides.
In addition, in the method for operating a reverse osmosis membrane according to the present embodiment, after the water to be treated is treated with the reverse osmosis membrane, the pressure of the reverse osmosis membrane is periodically released. Therefore, the state of concentration polarization on the surface of the reverse osmosis membrane after filtering the water to be treated can be periodically eliminated. As a result, precipitation of inorganic and organic substances on the primary surface of the reverse osmosis membrane can be suppressed, and clogging of the reverse osmosis membrane by inorganic and organic substances can be prevented.
Therefore, according to the method for operating a reverse osmosis membrane according to the present embodiment, while suppressing the precipitation of inorganic substances in the water to be treated, the precipitation of organic substances can also be suppressed, and clogging of the reverse osmosis membrane can be prevented.
Furthermore, when releasing pressure on the secondary side of a reverse osmosis membrane, it is also possible for permeate to periodically move from the secondary side (permeated water side) to the primary side (to-be-treated water side) by forward osmosis. be. Therefore, in this case, the solute concentration of organic matter on the primary side of the reverse osmosis membrane can be periodically reduced. As a result, it is possible to reliably prevent the reverse osmosis membrane from being blocked by inorganic and organic substances on the primary surface of the reverse osmosis membrane.

Further, in the reverse osmosis membrane operating method according to the present embodiment, the pH of the water to be treated supplied to the reverse osmosis membrane module 18 is adjusted to be 5.8 or higher. Therefore, compared to the case where the pH is lowered to less than 5.8, the amount of acid component used is reduced, and the effect of reducing the cost of water treatment is also obtained.

In conventional operating methods in which reverse osmosis membranes are cleaned by flushing alone, the cleaning water after flushing may contain a large amount of deposits. Washing water after flushing that contains such a large amount of sediment cannot be reused as water to be treated for filtration with a reverse osmosis membrane, and may have to be disposed of as is. As described above, using treated water for flushing and then discarding it may cause a decrease in the recovery efficiency of the entire water treatment system that recovers treated water from treated water.
On the other hand, in the reverse osmosis membrane operating method according to the present embodiment, the number and frequency of flushing can be reduced because the reverse osmosis membrane can be prevented from clogging by periodically releasing the pressure of the reverse osmosis membrane. Can be reduced. In addition, the amount of deposits on the surface of the reverse osmosis membrane is reduced, and even if flushing is performed, it is thought that the amount of deposits mixed into the wash water after flushing will be relatively reduced. Therefore, it is fully possible to reuse the wash water after flushing as treated water for reverse osmosis membrane filtration, and even if treated water is used during flushing, the recovery efficiency of treated water will not decrease. Water treatment systems become more efficient.
As described above, according to the operating method of the reverse osmosis membrane according to the present embodiment, it is expected that the amount of treated water that can be recovered from the same amount of water to be treated and the recovery efficiency will be improved, and the cost of water treatment can be reduced. Can be done.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, the present invention is not limited to the examples, and various changes can be made without departing from the spirit of the present invention.

In both the present example and the comparative example, well water was used as raw water. The measured values of each water quality item of well water before concentration are listed below.
pH: 8.3
Total organic carbon: 2.0mg/L
Iron: 0.03mg/L
Manganese: less than 0.005mg/L Calcium: 16mg/L
Magnesium: 5mg/L
Silica: 52mg/L
M alkalinity: 233mg/L

In the present example and comparative example, a flat membrane testing device 40 shown in FIG. 2 was used. The flat membrane test device 40 includes a container 41, a flat membrane cell 42, a supply pipe 43, a permeated water pipe 44, a concentrated water pipe 45, a pump 46, a pressure gauge 47, a back pressure valve 48, a stirrer 49, and a stirrer 50.
FIG. 3 is a schematic cross-sectional view showing the container 41 and flat membrane cell 42 of the flat membrane testing apparatus 40 of FIG. A flat membrane cell 42 is fixed to the container 41, and the container 41 has a first case 41a below the flat membrane cell 42 and a second case 41b above the flat membrane cell 42. A flat membrane cell 42 is fixed via an O-ring 51 between the first case 41a and the second case 41b. A space surrounded by the first case 41a below the flat membrane cell 42 becomes a raw water chamber A. Further, a space surrounded by the second case 41b above the flat membrane cell 42 becomes a permeated water chamber B. The flat membrane cell 42 includes a reverse osmosis membrane 52 and a porous support plate 53 that supports the surface of the reverse osmosis membrane 52 on the permeated water chamber B side (permeated water side).

In the flat membrane test device 40, as shown in FIGS. 2 and 3, the water to be treated flows through the supply pipe 43, is pressurized by the pump 46, and is supplied to the lower raw water chamber A in the closed container 41. The water to be treated supplied into the raw water chamber A is stirred by a stirrer 50 rotated by a stirrer 49, filled into the raw water chamber A, pressurized, and treated by the reverse osmosis membrane 52. The permeated water of the reverse osmosis membrane 52 flows through the permeated water pipe 44 through the permeated water chamber B above the flat membrane cell 42 . Concentrated water from the reverse osmosis membrane 52 flows through concentrated water piping 43 . When the water to be treated is treated by the reverse osmosis membrane 52, the pressure in the raw water chamber A can be adjusted by a back pressure valve 48 provided in the concentrated water pipe 45, and is measured by a pressure gauge 47. In the examples and comparative examples, "ESPA2 (membrane area: 8 cm ² )" manufactured by Nitto Denko was used as the reverse osmosis membrane 52.

<Example 1>
First, sulfuric acid was added to adjust the pH of the well water to 7.0. Next, 5 L of well water whose pH was adjusted to 7.0 was concentrated 3.3 times using a flat membrane test device 40. During concentration, the opening degree of the back pressure valve 48 is adjusted to set the well water supply pressure to 0.5 MPa, and while the concentrated water of the reverse osmosis membrane 52 is supplied again to the supply pipe 43 and circulated, the reverse osmosis membrane 52 The well water was concentrated by discharging the permeated water out of the system. Thereafter, sulfuric acid was added to the concentrated well water to adjust the pH to 7.0 again, and the water was used as water to be treated.
Next, the water to be treated was treated with a reverse osmosis membrane for 59 minutes to separate it into permeated water and concentrated water. When treating the water to be treated, the opening degree of the back pressure valve 48 is adjusted so that the water pressure becomes 0.5 MPa, and each of the permeated water and concentrated water is supplied to the supply piping 44 again. The mixture was circulated while being mixed, and changes in the permeation flow rate of the reverse osmosis membrane 52 over time were observed.
Thereafter, the back pressure valve 48 was opened for 1 minute to release pressure from the reverse osmosis membrane. This process of separating the water to be treated for 59 minutes and then releasing the pressure of the reverse osmosis membrane for 1 minute was defined as one cycle for a total of 60 minutes, and this process was performed for a total of 100 cycles.
The ratio of the flow rate of permeated water after 100 cycles to the flow rate of permeated water at 0 minutes after the start of separation of the water to be treated was calculated as a relative flux. The results are shown in Table 1. Table 1 also shows the Langelier index (LSI) of the water to be treated whose pH was adjusted to 7.0 and the amount of sulfuric acid used to adjust the pH of the water to be treated.

<Example 2>
First, sulfuric acid was added to adjust the pH of the well water to 6.0. Next, 5 L of well water whose pH was adjusted to 6.0 was concentrated 3.3 times using the flat membrane test device 40 in the same manner as in Example 1. Thereafter, sulfuric acid was added to the concentrated well water to adjust the pH to 6.0 again, and the water was used as water to be treated.
Next, the water to be treated was treated with a reverse osmosis membrane for 60 minutes to separate it into permeated water and concentrated water. When treating the water to be treated, the opening degree of the back pressure valve 48 is adjusted so that the water pressure becomes 0.5 MPa, and each of the permeated water and concentrated water is supplied to the supply piping 44 again. The mixture was circulated while being mixed, and changes in the permeation flow rate of the reverse osmosis membrane 52 over time were observed.
Thereafter, the back pressure valve 48 was opened for 0.5 minutes (30 seconds) to release pressure from the reverse osmosis membrane. Separation of this water to be treated was performed for 60 minutes, and then the pressure of the reverse osmosis membrane was released for 0.5 minutes (30 seconds), a process for a total of 60.5 minutes, which was one cycle, and this process was repeated for a total of 100 cycles. .
The ratio of the flow rate of permeated water after 100 cycles to the flow rate of permeated water at 0 minutes after the start of separation of the water to be treated was calculated as a relative flux. The results are shown in Table 1. Table 1 also shows the Langelier index (LSI) of the water to be treated whose pH was adjusted to 6.0 and the amount of sulfuric acid used to adjust the pH of the water to be treated.

<Comparative example 1>
Adjust the pH of the water to be treated to 6.0, treat the water with a reverse osmosis membrane, and allow the water to pass through the reverse osmosis membrane for 100 hours without periodic pressure release or flushing. Treated water was obtained in the same manner as in Example 1 except that it was separated into water and concentrated water.

<Comparative example 2>
The treatment was carried out in the same manner as in Example 1, except that the raw water was treated with a reverse osmosis membrane without adding sulfuric acid as an acid component to the water to be treated in the water tank 12 to be treated, and the pH of the water to be treated remained at 8.3. Got water.

<Comparative example 3>
Treated water was obtained in the same manner as in Example 1, except that the pH of the water to be treated was adjusted to 5.5 and the water to be treated was treated with a reverse osmosis membrane.

As shown in Table 1, the amounts of sulfuric acid required for pH adjustment in Examples 1 and 2 were 57 mg/L and 171 mg/L, respectively. Moreover, the relative fluxes after 100 hours were 0.92 and 0.97, respectively.

On the other hand, in Comparative Example 1 in which periodic pressure release and flushing were not performed, the relative flux after 100 hours had decreased to 0.87. The reason for the decrease in relative flux in Comparative Example 1 was thought to be that organic matter was deposited on the surface of the reverse osmosis membrane.

In Comparative Example 2, in which the water to be treated was treated with a reverse osmosis membrane while the pH of the water to be treated remained at 8.3 without adding sulfuric acid as an acid component to the water to be treated, the relative flux after 100 hours was 0.49. It had declined to . The reason why the relative flux decreased in Comparative Example 2 is that the pH of the water to be treated exceeds the upper limit specified by the present invention, so oxides of metals such as iron and manganese in the water to be treated precipitate. This was thought to be due to inorganic substances deposited on the surface of the reverse osmosis membrane.

From the results of the Examples and Comparative Examples explained above, according to the operating method of the reverse osmosis membrane according to the present embodiment, it is possible to suppress the precipitation of organic substances while suppressing the precipitation of inorganic substances in the water to be treated. It was thought that blockage could be prevented.
In addition, in Comparative Example 3, the amount of sulfuric acid used to adjust the pH of the water to be treated to 5.5 was 220 mg/L, which was increased compared to the amount of sulfuric acid used in Examples 1 and 2. As described above, in Examples 1 and 2, the pH of the water to be treated could be adjusted within a range that could suppress clogging of the reverse osmosis membrane with a smaller amount of the acid component used. Therefore, it was considered that the method of operating a reverse osmosis membrane according to the present embodiment could reduce the cost of water treatment.

1 Water treatment system 11 Well 12 Water tank to be treated 14 Pump 15 Pre-filter 16 Boosting pump 17 Reverse osmosis membrane filtration device 18 Reverse osmosis membrane module 19 Treated water tank 21 Raw water channel 22 Water channel to be treated 23 Concentrated water channel 24 Permeated water channel 25 Circulation channel 26 Drainage channel 31 Acid addition means

Claims (7)

A method for operating a reverse osmosis membrane, which separates raw water into permeated water and concentrated water using a reverse osmosis membrane,
The pH of the water to be treated that is supplied to the reverse osmosis membrane via a booster pump is 6.0 to 7.8,
After treating the water to be treated with the reverse osmosis membrane, stop the boost pump and periodically release the pressure of the reverse osmosis membrane,
A method for operating a reverse osmosis membrane, comprising washing the surface of the reverse osmosis membrane by performing flushing when pressure is released from the reverse osmosis membrane. 2. The method for operating a reverse osmosis membrane according to claim 1, wherein when the pressure of the reverse osmosis membrane is released, pressure is also released on the secondary side of the reverse osmosis membrane. 3. The method for operating a reverse osmosis membrane according to claim 2, wherein during flushing, cleaning water containing permeated water or pure water is used. The method for operating a reverse osmosis membrane according to any one of claims 1 to 3, wherein the raw water is directly treated with the reverse osmosis membrane. The method for operating a reverse osmosis membrane according to any one of claims 1 to 4, wherein the raw water further contains at least one inorganic substance selected from the group consisting of silica, calcium, magnesium, iron, and manganese. The method for operating a reverse osmosis membrane according to any one of claims 1 to 5, wherein the raw water has an M alkalinity of 100 mg/L or more. The method for operating a reverse osmosis membrane according to any one of claims 1 to 6, wherein the total organic carbon of the water to be treated is 3.0 mg/L or more.

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