JP7645680B2 - Water treatment method and water treatment device - Google Patents

Description

The present invention relates to a water treatment method and a water treatment device.

Water treatment devices that have a reverse osmosis membrane (RO membrane) are known as devices that remove impurities from the water to be treated. In these devices, the water to be treated (raw water) is supplied to the RO membrane at a specified supply pressure, and the RO membrane separates the water to be treated (permeated water) and concentrated water. This makes it possible to obtain treated water (permeated water) from which impurities have been removed.

Water treatment equipment with RO membranes is required to continue stable operation, and for this purpose, it is important to prevent silica in the raw water from precipitating on the membrane surface of the RO membrane and causing scale formation. In response to this, methods for suppressing the formation of silica scale have been known, such as a method of adjusting the pH of the concentrated water to 6 or less to delay silica precipitation even if the silica concentration in the concentrated water is equal to or higher than the silica solubility (see, for example, Patent Document 1), and a method of increasing the silica solubility by adjusting the pH of the raw water to 9.5 or more (see, for example, Patent Document 2). In addition, a method of adding colloidal silica to the concentrated water and polymerizing the ionic silica in the concentrated water using the colloidal silica as a nucleus to reduce the silica concentration in the concentrated water (see, for example, Patent Document 3) is also known.

Patent No. 3187629 Patent No. 4496795 JP 2017-148746 A

However, in the method of adjusting the pH, the pH is adjusted by adding an acid or alkali to the raw water, which not only raises concerns about a decrease in the rejection rate of the RO membrane, but also increases the risk of calcium scale formation when the pH is shifted toward an alkaline side. In addition, in the method of adding colloidal silica, it is necessary to circulate the concentrated water at a flow rate significantly higher than the flow rate of the permeate, which results in huge energy costs and the cost of the colloidal silica itself.

The object of the present invention is to provide a water treatment method and water treatment device that suppresses clogging of the reverse osmosis membrane by silica scale and exhibits stable water treatment performance.

In order to achieve the above-mentioned objective, the water treatment method of the present invention is a water treatment method in which water to be treated stored in a tank is treated with at least one reverse osmosis membrane module to separate it into treated water, concentrated wastewater, and concentrated reflux water, and the treated water and concentrated wastewater are each discharged to the outside while the concentrated reflux water is returned to the tank , and includes a step of adjusting the flow rate of the concentrated wastewater to be discharged to the outside so that the residence time from when the water to be treated is supplied to the tank and when it is treated with at least one reverse osmosis membrane module and impurities contained in the water to be treated are discharged to the outside together with the concentrated wastewater is one hour or less.

In addition, the water treatment device of the present invention has a tank for storing water to be treated and at least one reverse osmosis membrane module, treats the water to be treated with at least one reverse osmosis membrane module to separate it into treated water, concentrated wastewater, and concentrated reflux water, discharges the treated water to the outside through a treated water line, discharges the concentrated wastewater to the outside through a drainage line , and returns the concentrated reflux water to the tank through a reflux water line , and has a flow rate adjustment means for adjusting the flow rate of concentrated wastewater flowing through the drainage line so that the residence time from when the water to be treated is supplied to the tank to when it is treated by one reverse osmosis membrane module and the impurities contained in the water to be treated are discharged to the outside together with the concentrated wastewater is one hour or less.

This water treatment method and water treatment device can suppress the formation of silica scale without adjusting the pH of the water to be treated (raw water) or adding colloidal silica to the concentrated water.

As described above, the present invention can suppress clogging of the reverse osmosis membrane due to silica scale, and provide stable water treatment performance.

1 is a schematic diagram showing a configuration of a water treatment device according to a first embodiment of the present invention. FIG. 5 is a schematic diagram showing a configuration of a water treatment device according to a second embodiment of the present invention. 1 is a graph showing changes in flux retention over time in Examples 1 and 2 and Comparative Examples 1 to 3.

The following describes an embodiment of the present invention with reference to the drawings.

First Embodiment
FIG. 1 is a schematic diagram showing the configuration of a water treatment device according to a first embodiment of the present invention.

The water treatment device 10 has a raw water tank 11 and a reverse osmosis membrane (RO membrane) device 12, and is a device that removes impurities from the raw water (water to be treated) stored in the raw water tank 11 by treating the raw water (water to be treated) with the RO membrane device 12 to produce treated water. The RO membrane device 12 separates the raw water supplied from the raw water tank 11 into concentrated water containing impurities and permeate water from which the impurities have been removed, and has an RO membrane. Specifically, the RO membrane device 12 is composed of an RO membrane module and has at least one RO membrane element housed in a cylindrical vessel (pressure vessel). The RO membrane module of the RO membrane device 12 may have a single vessel, or may be a unit formed by connecting multiple vessels in series. The RO membrane device 12 is connected to a water supply line L1 that supplies raw water to the RO membrane device 12, a treated water line L2 that circulates permeate (treated water) from the RO membrane device 12, and a concentrated water line L3 that circulates concentrated water from the RO membrane device 12. The concentrated water line L3 branches into two lines: a drain line L4 that discharges a portion of the concentrated water to the outside, and a return water line L5 that returns the remainder to the raw water tank 11. A raw water supply line L6 is connected to the raw water tank 11, and raw water is supplied as needed, as described below.

The water treatment device 10 also has a pressure pump 13 provided in the water supply line L1 and a water level sensor 14 provided in the raw water tank 11. The pressure pump 13 has a function of pressurizing the raw water supplied to the RO membrane device 12, that is, a function of adjusting the flow rate of the permeate flowing through the treated water line L2 by adjusting the water supply pressure (operating pressure) to the RO membrane device 12. The water level sensor 14 has a function of detecting the water level in the raw water tank 11. In addition, the drainage line L4 and the return water line L5 are provided with manual valves (flow rate adjustment means) V1 and V2 for adjusting the flow rate of the concentrated water flowing therethrough, and the raw water supply line L6 is provided with an opening/closing valve V3. The opening/closing valve V3 is configured to open and close according to the water level in the raw water tank 11 detected by the water level sensor 14. Specifically, while the water level in the raw water tank 11 is below a predetermined upper water level, the opening/closing valve V3 is opened, and raw water is supplied to the raw water tank 11 through the raw water supply line L6. When the water level in the raw water tank 11 reaches a predetermined upper limit, the on-off valve V3 is closed and the supply of raw water is stopped. This allows the water level in the raw water tank 11 to be maintained below the predetermined upper limit. The configuration of the water level sensor 14 is not particularly limited as long as it can detect the two water levels described above, and for example, a float-type level switch can be used. Also, instead of the water level sensor 14 and the on-off valve V3, a ball tap may be used to maintain the water level in the raw water tank 11 below the predetermined upper limit.

During operation of the water treatment device 10, the raw water stored in the raw water tank 11 is supplied to the RO membrane device 12, where it is treated and separated into permeate and concentrated water. The permeate is supplied (discharged) as treated water to the outside (e.g., a treated water tank or a use point, etc.) through the treated water line L2, a portion of the concentrated water is discharged to the outside as concentrated wastewater through the drainage line L4, and the remainder is returned to the raw water tank 11 as concentrated reflux water through the reflux water line L5. Note that the treatment of raw water by the RO membrane device 12 is stopped by stopping the pressurizing pump 13 when the water level in the raw water tank 11 falls below a predetermined lower water level, and is resumed by operating the pressurizing pump 13 when the water level in the raw water tank 11 reaches a predetermined upper water level due to the supply of raw water to the raw water tank 11. Hereinafter, the concentrated water flowing through the drainage line L4 and the reflux water line L5 will be referred to as "concentrated drainage water" and "concentrated reflux water", respectively, and the concentrated water flowing through the concentrated water line L3 will be referred to simply as "concentrated water".

In order to prevent silica from being precipitated on the membrane surface of the RO membrane and causing scale to adhere, the treatment of raw water by the RO membrane device 12 is preferably performed under conditions in which the silica concentration of the concentrated water does not exceed the silica solubility at a pre-measured water temperature. Specifically, it is preferable that a recovery rate (the ratio of the permeate flow rate to the sum of the permeate flow rate and the concentrated wastewater flow rate) is set based on the silica concentration of the raw water measured in advance so that the silica concentration of the concentrated water does not exceed the silica solubility at a pre-measured water temperature, and the flow rate of the concentrated wastewater is adjusted to achieve the set recovery rate. The flow rate of the concentrated wastewater is adjusted by a manual valve V1 provided on the drain line L4, and the set flow rate is determined based on the set value of the recovery rate and the flow rate of the permeate estimated from the set value of the operating pressure of the RO membrane device 12. It is known that the silica solubility S (mg/L) under neutral conditions is expressed by the following formula (1) when the water temperature is T (K).
logS=a+bT ^-1 +cT ² +dlogT (1)
Here, a = -3.697, b = -485.24, c = -2.268 x 10 ^-6 and d = 3.608.

However, the calculation of silica solubility should be done taking into account the interaction with various components (e.g., calcium, magnesium, aluminum, iron, etc.) that promote silica precipitation, but this is very complicated and generally not understood. Therefore, the actual silica solubility may be lower than the silica solubility calculated by the above formula (1), and as a result, the silica concentration of the concentrated water may be higher than the silica solubility at the water temperature measured in advance. In that case, there is a concern that silica may precipitate on the membrane surface of the RO membrane, which may cause the membrane surface of the RO membrane to become clogged. In addition, even if the water quality (pH) or water temperature of the raw water fluctuates, it is often not monitored in real time to adjust the operating conditions, and therefore, if the silica concentration of the concentrated water changes due to the fluctuation of the water quality (pH) or water temperature of the raw water, it may exceed the silica solubility calculated by the above formula (1).

In response to this, the inventors have focused on the time that the raw water remains in the water treatment device 10, and have found that by setting this time to a predetermined time or less, clogging of the membrane surface of the RO membrane due to the generation of silica scale can be suppressed even when the silica concentration of the concentrated water is equal to or greater than the silica solubility as described above. That is, in this embodiment, the raw water to be treated in the RO membrane device 12 is supplied to the raw water tank 11 and temporarily stored there, and then supplied to the RO membrane device 12, where it is separated into permeate and concentrated water. In addition, a portion of the separated concentrated water is returned to the raw water tank 11 as concentrated reflux water without being discharged to the outside. Therefore, a certain amount of retention time is inevitably generated from the time the raw water is supplied to the raw water tank 11 until it is treated in the RO membrane device 12 and discharged to the outside as concentrated wastewater, that is, until the impurities contained in the raw water are discharged to the outside together with the concentrated wastewater. The inventors have focused on this residence time and, as shown in the examples below, have found that by setting the residence time to 1 hour or less, preferably 15 minutes or less, it is possible to suppress the formation of silica scale even when the silica concentration in the concentrated water is equal to or greater than the silica solubility.

The term "retention time" here is defined as the sum of the volumes of raw water supplied to the water treatment device 10 that can be retained until it is discharged as concentrated wastewater, divided by the flow rate of concentrated wastewater flowing through the drain line L4. Specifically, it is defined as the sum of the volume equivalent to the maximum water volume of the raw water tank 11 (the water volume retained when the water level in the raw water tank 11 is at a predetermined upper water level), the volume of the primary side of the RO membrane module of the RO membrane device 12, and the total volume of the piping constituting each line L1, L3 to L5 excluding the treated water line L2 and the raw water supply line L6, divided by the flow rate of concentrated wastewater flowing through the drain line L4. However, the total volume of the piping is usually much smaller than the volume of the raw water tank 11, and water rarely retains in the piping, so it may be excluded from the calculation of the retention time. In addition, the volume of the primary side of the RO membrane module may be excluded from the calculation of the retention time when the volume of the vessel constituting it is smaller than the volume of the raw water tank 11. In other words, because the vessel contains an RO membrane element, the space through which the water passes is smaller than the actual volume, and shear forces are also generated, so in such cases the effect of retention when the raw water passes through the vessel can be ignored.

According to the above definition, the retention time can be adjusted by adjusting the flow rate of the concentrated wastewater flowing through the drain line L4, or by adjusting the maximum water capacity of the raw water tank 11 if the maximum water capacity is adjustable. The set flow rate (target flow rate) of the concentrated wastewater is determined based on the target retention time (1 hour or less, preferably 15 minutes or less), and the opening of the manual valve V1 is adjusted so that the flow rate of the concentrated wastewater becomes the target flow rate. However, if the set flow rate of the concentrated wastewater is smaller than the minimum concentrated water volume required for the RO membrane device 12 (the minimum flow rate of concentrated water to be flowed through the concentrated water line L3), the opening of the manual valve V2 can be adjusted to adjust the flow rate of the concentrated wastewater to ensure the minimum concentrated water volume. Once the set flow rate of the concentrated wastewater is determined in this manner, the set flow rate (target flow rate) of the treated water flowing through the treated water line L2 is determined based on the set flow rate and the target value of the recovery rate of the RO membrane device 12 calculated from the water quality of the raw water as described above, and the operating pressure of the RO membrane device 12 is adjusted by controlling the rotation speed of the pressure pump 13 so that the flow rate of the treated water becomes the target flow rate.

In this embodiment, in order to more reliably suppress the precipitation of scale components such as silica and calcium on the membrane surface of the RO membrane, a scale inhibitor may be added to the raw water by a chemical injection pump. The type of scale inhibitor is not limited to a specific one, and examples thereof include phosphonic acid compounds such as phosphonic acids and salts thereof, such as 1-hydroxyethylidene-1,1-diphosphonic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, ethylenediaminetetramethylenephosphonic acid, and nitrilotrimethylphosphonic acid; phosphoric acid compounds such as orthophosphates and polymerized phosphates; maleic acid compounds such as polymaleic acid and maleic acid copolymers; and acrylic acid polymers. Examples of the acrylic acid polymers include copolymers such as poly(meth)acrylic acid, maleic acid/(meth)acrylic acid, (meth)acrylic acid/sulfonic acid, and (meth)acrylic acid/nonionic group-containing monomers, (meth)acrylic acid/sulfonic acid/nonionic group-containing monomers, (meth)acrylic acid/acrylamide-alkylsulfonic acid/substituted (meth)acrylamide, and (meth)acrylic acid/acrylamide-arylsulfonic acid/substituted (meth)acrylamide terpolymers. Examples of (meth)acrylic acids constituting the terpolymer include methacrylic acid, acrylic acid, and (meth)acrylate salts such as their sodium salts. Examples of acrylamide-alkylsulfonic acids constituting the terpolymer include 2-acrylamido-2-methylpropanesulfonic acid and its salts. Examples of substituted (meth)acrylamides constituting the terpolymer include t-butylacrylamide, t-octylacrylamide, and dimethylacrylamide.

Among these, it is preferable to use one that contains at least one of a phosphonic acid compound and an acrylic acid polymer. In order to simultaneously suppress scale derived from calcium and silica, it is particularly preferable to use a scale inhibitor that is composed of 2-phosphonobutane-1,2,4-tricarboxylic acid and a mixture of acrylic acid and a terpolymer of (meth)acrylic acid/2-acrylamido-2-methylpropanesulfonic acid/substituted (meth)acrylamide.

Commercially available scale inhibitors for RO membranes include the "Orpersion" series manufactured by Organo Corporation, the "Flocon (registered trademark)" series manufactured by BWA Water Additives, the "PermaTreat (registered trademark)" series manufactured by Nalco, the "Hypersperse (registered trademark)" series manufactured by General Electric Company, and the "Kuriverter (registered trademark)" series manufactured by Kurita Water Industries Ltd.

Second Embodiment
2 is a schematic diagram showing the configuration of a water treatment device according to a second embodiment of the present invention. Hereinafter, the same components as those in the first embodiment will be denoted by the same reference numerals in the drawings and their description will be omitted, and only the components different from those in the first embodiment will be described.

This embodiment differs from the first embodiment in that the RO membrane device 12 is composed of three RO membrane modules 12a to 12c. That is, the RO membrane device 12 of this embodiment is composed of a first RO membrane module 12a corresponding to the RO membrane device 12 of the first embodiment, a second RO membrane module 12b that treats the permeate from the first RO membrane module 12a, and a third RO membrane module 12c that treats the concentrated water from the first RO membrane module 12a.

Accordingly, in this embodiment, some of the configurations in the first embodiment are changed, and some new configurations are added. That is, the water treatment device 10 of this embodiment has a permeate line L7 that circulates the permeate (hereinafter also referred to as "primary permeate") from the first RO membrane module 12a, a permeate tank 21 that stores the primary permeate, and a second water supply line L8 that supplies the primary permeate stored in the permeate tank 21 to the second RO membrane module 12b. The second water supply line L8 is provided with a pressure pump 22 that pressurizes the primary permeate in the permeate tank 21 and supplies it to the second RO membrane module 12b, and the permeate tank 21 is provided with a water level sensor 23 that detects the water level in the permeate tank 21. In this embodiment, the treated water line L2 and the reflux water line L5 are connected to the second RO membrane module 12b, and are configured to circulate the permeate and concentrated water separated in the second RO membrane module 12b, respectively.

Furthermore, the water treatment device 10 of this embodiment has a concentrated water tank 24 that stores concentrated water from the first RO membrane module 12a (hereinafter also referred to as "primary concentrated water"), a third water supply line L9 that supplies the primary concentrated water stored in the concentrated water tank 24 to the third RO membrane module 12c, and a permeate merging line L10 that circulates the permeate from the third RO membrane module 12c. The permeate merging line L10 is connected to the permeate tank 21 and has the function of merging the permeate from the third RO membrane module 12c with the primary permeate. Hereinafter, the permeate stored in the permeate tank 21, including the permeate from the third RO membrane module 12c, is referred to as "primary permeate". The third water supply line L9 is provided with a pressure pump 25 that pressurizes the primary concentrated water in the concentrated water tank 24 and supplies it to the third RO membrane module 12c, and the concentrated water tank 24 is provided with a water level sensor 26 that detects the water level in the concentrated water tank 24. Furthermore, the concentrated water line L3 is provided with a manual valve V4 for adjusting the flow rate of the concentrated water flowing through the concentrated water line L3. In this embodiment, the drainage line L4 is connected to the third RO membrane module 12c and allows the concentrated water from the third RO membrane module 12c to flow.

In this embodiment, when the water treatment device 10 is in operation, the raw water stored in the raw water tank 11 is treated in the first RO membrane module 12a by operating the pressure pump 13 and adjusting the opening of the manual valve V4, and separated into permeate and concentrated water. The permeate (primary permeate) separated in the first RO membrane module 12a is stored in the permeate tank 21 through the permeate line L7, and the concentrated water (primary concentrated water) separated in the first RO membrane module 12a is stored in the concentrated water tank 24 through the concentrated water line L3. The primary concentrated water stored in the concentrated water tank 24 is treated in the third RO membrane module 12c by operating the pressure pump 25 and adjusting the opening of the manual valve V1, and separated into permeate and concentrated water. The concentrated water separated by the third RO membrane module 12c is discharged to the outside through the drainage line L4, and the permeated water separated by the third RO membrane module 12c is stored in the permeated water tank 21 through the permeated water merging line L10. The primary permeated water stored in the permeated water tank 21 is treated in the second RO membrane module 12b by operating the pressure pump 22 and adjusting the opening of the manual valve V2, and separated into permeated water and concentrated water. The permeated water separated by the second RO membrane module 12b is supplied (discharged) to the outside through the treated water line L2, and the concentrated water separated by the second RO membrane module 12b is returned to the raw water tank 11 through the return water line L5.

In other words, the raw water stored in the raw water tank 11 is supplied to the RO membrane device 12, treated, and separated into treated water (permeate from the second RO membrane module 12b) that is supplied (discharged) to the outside through the treated water line L2, concentrated wastewater (concentrated water from the third RO membrane module 12c) that is discharged to the outside through the drainage line L4, and concentrated reflux water (concentrated water from the second RO membrane module 12b) that is returned to the raw water tank 11 through the reflux water line L5. Thus, in this embodiment, it is possible to improve the quality of the treated water while maintaining a high recovery rate of the RO membrane device 12 compared to the first embodiment.

In this embodiment, the treatment of raw water by the first RO membrane module 12a is stopped by stopping the pressurizing pump 13 when the water level in the raw water tank 11 falls below a predetermined lower limit, when the water level in the permeated water tank 21 reaches a predetermined upper limit, or when the water level in the concentration tank 24 reaches a predetermined upper limit, and is resumed by operating the pressurizing pump 13 when the water level in the raw water tank 11 reaches a predetermined upper limit. The treatment of the primary permeated water by the second RO membrane module 12b is stopped by stopping the pressurizing pump 22 when the water level in the permeated water tank 21 falls below a predetermined lower limit, or when the water level in the raw water tank 11 reaches a predetermined upper limit, and is resumed by operating the pressurizing pump 22 when the water level in the permeated water tank 21 reaches a predetermined upper limit. In addition, the treatment of the primary concentrated water by the third RO membrane module 12c is stopped by stopping the pressure pump 25 when the water level in the concentration tank 24 reaches a predetermined lower limit or when the water level in the permeate tank 21 reaches a predetermined upper limit, and is resumed by operating the pressure pump 25 when the water level in the concentration tank 24 reaches the predetermined upper limit.

By the way, the treatment of raw water by the first RO membrane module 12a is preferably performed under conditions in which the silica concentration of the primary concentrated water does not exceed the silica solubility at the water temperature measured in advance, but even in this embodiment, this condition may not be met. Therefore, the residence time during operation of the above-mentioned water treatment device 10 is preferably 1 hour or less, and more preferably 15 minutes or less. Note that, as in the first embodiment, the "residence time" in this embodiment is defined as the total volume in which raw water supplied to the water treatment device 10 can remain until it is discharged as concentrated wastewater, divided by the flow rate of the concentrated wastewater flowing through the drain line L4. Specifically, it is defined as the sum of the volume equivalent to the maximum water capacity of the raw water tank 11, the volume equivalent to the maximum water capacity of the concentration tank 24, the volume of the primary side of the first RO membrane module 12a, the volume of the primary side of the third RO membrane module 12c, and the total volume of the piping constituting the supply line L1, the concentrated water line L3, the drain line L4, and the third water supply line L9, divided by the flow rate of the concentrated drainage water flowing through the drain line L4. However, as in the first embodiment, the volume of the primary side of the RO membrane modules 12a and 12c and the total volume of the piping constituting each line L1, L3, L4, and L10 may be excluded from the calculation of the retention time in some cases.

As in the first embodiment, the retention time can be adjusted by adjusting the flow rate of the concentrated wastewater flowing through the drain line L4, or by adjusting the maximum water capacity of each tank 11, 21, 24 if the maximum water capacity is adjustable. The set flow rate (target flow rate) of the concentrated wastewater is determined based on the target retention time (1 hour or less, preferably 15 minutes or less), and the opening of the manual valve V1 is adjusted so that the flow rate of the concentrated wastewater becomes the target flow rate.

The third RO membrane module 12c (and its associated configuration) that treats the concentrated water from the first RO membrane module 12a may be omitted. In that case, as in the first embodiment, a portion of the concentrated water from the first RO membrane module 12a may be discharged to the outside as concentrated wastewater through the drain line L4, and the remainder may be returned to the raw water tank 11.

Next, we will explain the effects of the present invention by giving specific examples.

Example 1
In this example, a test device simulating the water treatment device shown in FIG. 1 was used for continuous operation, and the change over time in the flux (permeation flux) of the RO membrane was measured. The raw water tank had a maximum water capacity of 60 L in total, including the piping. The raw water was made by adding salt water to pure water to adjust the pH to 7.0 and adjusting the water temperature to 25° C. using a heat exchanger. Sodium metasilicate was added to the raw water so that the silica concentration of the concentrated water was 200 mg/L, which is equal to or higher than the silica solubility (116 mg/L from the above formula (1)) at a water temperature of 25° C., and calcium chloride and magnesium chloride were added so that the calcium concentration and magnesium concentration of the concentrated water were 60 mg/L, respectively. In addition, "Orpersion" (product number: G510) manufactured by Organo Corporation was used as a scale inhibitor, and the scale inhibitor was added to the raw water so that the concentration in the concentrated water was 25 mg/L.

The RO membrane module of the RO membrane device was a single vessel containing one RO membrane element, and the RO membrane element was an RO membrane element (product number: ES20) manufactured by Nitto Denko Corporation. The target flow rate of the permeate was set to 140 L/h, and the operating pressure of the RO membrane device was set to 0.75 MPa. The target flow rate of the concentrated wastewater was set to 60 L/h so that the retention time was 1 hour. Note that the volume of the primary side of the RO membrane module was not taken into consideration because the volume of the vessel constituting it was smaller than the volume of the raw water tank, and the retention time estimated from that volume was negligibly small compared to the overall retention time. In addition, the flow rate of the concentrated reflux water was adjusted to 640 L/h to ensure a minimum concentrated water volume of 700 L/h.

Example 2
Measurements were performed under the same conditions as in Example 1, except that the target flow rate of the concentrated wastewater was set to 240 L/h so that the residence time was 15 minutes, and the flow rate of the concentrated reflux water was adjusted to 460 L/h accordingly.

(Comparative Example 1)
Measurements were performed under the same conditions as in Example 1, except that the target flow rate of the concentrated wastewater was set to 30 L/h so that the residence time was 2 hours, and the flow rate of the concentrated reflux water was accordingly adjusted to 670 L/h.

(Comparative Example 2)
The measurement was performed under the same conditions as in Example 1, except that the target flow rate of the concentrated wastewater was set to 20 L/h so that the residence time was 3 hours, and the flow rate of the concentrated reflux water was adjusted to 680 L/h accordingly.

(Comparative Example 3)
Measurements were performed under the same conditions as in Example 1, except that the target flow rate of the concentrated wastewater was set to 2.5 L/h so that the residence time was 24 hours, and the flow rate of the concentrated reflux water was accordingly adjusted to 697.5 L/h.

Figure 3 is a graph showing the measurement results for Examples 1 and 2 and Comparative Examples 1 to 3. The flux retention rate shown on the vertical axis of the graph is a relative value when the flux at the start of operation is set to 100%.

As is clear from Figure 3, in all of Comparative Examples 1 to 3, a rapid drop of 50% or more in the flux retention was observed 24 hours after the start of operation. This is believed to be due to the effect of operation being performed under conditions where the silica concentration in the concentrated water was equal to or greater than the silica solubility at the water temperature measured in advance, causing silica to precipitate on the membrane surface of the RO membrane, which then clogged the membrane surface of the RO membrane. On the other hand, in Examples 1 and 2, even though the silica concentration in the concentrated water was equal to or greater than the silica solubility as in Comparative Examples 1 to 3, no such drop was observed, and in particular, in Example 2, it was confirmed that there was almost no change in the flux retention. This is believed to be due to the fact that, by shortening the residence time, impurities including silica in the raw water were replaced on the membrane surface of the RO membrane in a time shorter than the silica precipitation time, and were sufficiently discharged, suppressing clogging of the membrane surface due to the generation of silica scale.

REFERENCE SIGNS LIST 10 Water treatment device 11 Raw water tank 12 Reverse osmosis membrane (RO membrane) device 12a First reverse osmosis membrane (RO membrane) module 12b Second reverse osmosis membrane (RO membrane) module 12c Third reverse osmosis membrane (RO membrane) module 13, 22, 25 Pressure pump 14, 23, 26 Water level sensor L1 Water supply line (first water supply line)
L2 Treated water line L3 Concentrated water line L4 Drainage line L5 Circulation water line L6 Raw water supply line L7 Permeated water line L8 Second feed water line L9 Third feed water line L10 Permeated water merging line V1-V2, V4 Manual valve V3 Opening and closing valve

Claims (7)

A water treatment method comprising treating water to be treated stored in a tank with at least one reverse osmosis membrane module, separating the water into treated water, concentrated wastewater, and concentrated reflux water, discharging the treated water and the concentrated wastewater to the outside , and refluxing the concentrated reflux water to the tank ,
A water treatment method comprising a step of adjusting a flow rate of the concentrated wastewater to be discharged to the outside so that the residence time from when the treated water is supplied to the tank, to when the treated water is treated in the at least one reverse osmosis membrane module and impurities contained in the treated water are discharged to the outside together with the concentrated wastewater is one hour or less. The water treatment method according to claim 1, wherein the flow rate of the concentrated wastewater is adjusted so that the residence time is 15 minutes or less. separating the water to be treated into permeate and concentrated water by a reverse osmosis membrane module, and discharging the permeate from the reverse osmosis membrane module to the outside as the treated water;
The water treatment method according to claim 1 or 2, further comprising a step of discharging a portion of the concentrated water from the reverse osmosis membrane module to the outside as the concentrated wastewater and returning the remainder of the concentrated water to the tank as the concentrated return water. separating the water to be treated into permeate and concentrated water by a reverse osmosis membrane module, discharging a portion of the concentrated water from the reverse osmosis membrane module to the outside as the concentrated wastewater, and returning the remainder of the concentrated water to the tank as the concentrated reflux water;
3. The water treatment method according to claim 1, further comprising a step of separating the permeated water from the reverse osmosis membrane module into permeated water and concentrated water in another reverse osmosis membrane module, and discharging the permeated water from the other reverse osmosis membrane module to the outside as the treated water. a step of separating the water to be treated into permeate and concentrated water in a reverse osmosis membrane module, separating the concentrated water from the reverse osmosis membrane module into permeate and concentrated water in another reverse osmosis membrane module, and discharging the concentrated water from the other reverse osmosis membrane module to the outside as the concentrated wastewater;
3. The water treatment method according to claim 1, further comprising a step of combining the permeated water from the other reverse osmosis membrane module with the permeated water from the other reverse osmosis membrane module, separating the combined permeated water into permeated water and concentrated water in a further reverse osmosis membrane module, discharging the permeated water from the further reverse osmosis membrane module to the outside as the treated water, and returning the concentrated water from the further reverse osmosis membrane module to the tank as the concentrated return water. The water treatment method according to any one of claims 3 to 5, wherein the separation by the reverse osmosis membrane module is performed so that the silica concentration in the concentrated water from the reverse osmosis membrane module is equal to or greater than the silica solubility at a water temperature measured in advance. A water treatment device comprising a tank for storing water to be treated and at least one reverse osmosis membrane module, which treats the water to be treated with the at least one reverse osmosis membrane module to separate it into treated water, concentrated wastewater, and concentrated reflux water, and discharges the treated water to the outside through a treated water line, discharges the concentrated wastewater to the outside through a drain line, and refluxes the concentrated reflux water to the tank through a reflux water line,
A water treatment device having a flow rate control means for adjusting the flow rate of the concentrated wastewater flowing through the drainage line so that the retention time from when the treated water is supplied to the tank and when it is treated in at least one reverse osmosis membrane module and impurities contained in the treated water are discharged to the outside together with the concentrated wastewater is one hour or less.

Images