JP7621906B2 - Water purification system and method - Google Patents

Description

The present invention relates to a water purification system and method for obtaining purified water from water containing impurities or pollutants, such as seawater, river water, lake water, domestic wastewater, or industrial wastewater.

One method for obtaining purified water from water containing impurities or pollutants, such as seawater, river water, lake water, domestic wastewater, or industrial wastewater (hereinafter referred to as "water to be treated"). When the water to be treated is purified using a reverse osmosis membrane, organic matter contained in the water to be treated accumulates on the surface of the reverse osmosis membrane, causing fouling.

When fouling occurs in the reverse osmosis membrane, the purification speed of the water being treated decreases, so in order to maintain the purification speed, it is necessary to increase the pressure applied to the water being treated. In this case, the energy consumption of the purification process increases. Furthermore, if the amount of organic matter that accumulates increases, it becomes necessary to clean the reverse osmosis membrane, and the purification process will be stopped.

In order to suppress the decrease in efficiency of the purification process due to such reverse osmosis membrane fouling, for example, Patent Document 1 discloses a water purification system 101 having a configuration shown in FIG. 6. The water purification system 101 has a forward osmosis membrane device 102 and a reverse osmosis membrane device 103, and the secondary side 102c of the forward osmosis membrane device 102 and the primary side 103b of the reverse osmosis membrane device 103 are connected by a ring water channel 104 that circulates circulating water. In the water purification system 101, moisture is permeated from the treated water 105 received on the primary side 102b of the forward osmosis membrane device 102 to the circulating water on the secondary side 102c by the forward osmosis membrane 102a. Then, this circulating water is pressurized by a pump 107 and introduced into the primary side 103b of the reverse osmosis membrane device 103, and the moisture is permeated to the secondary side 103c by the reverse osmosis membrane 103a to obtain purified water 106. The circulating water discharged from the primary side 103b of the reverse osmosis membrane device 103 is introduced again into the secondary side 102c of the forward osmosis membrane device 102 through the annular water channel 104. The circulating water circulating through the annular water channel 104 contains solutes, and organic matter has been removed. Patent Document 1 describes that the water purification system configured as above prevents fouling of the reverse osmosis membrane, since fouling-causing substances such as organic matter do not come into direct contact with the reverse osmosis membrane.

Patent No. 5941629

However, in the water purification system described in Patent Document 1, although fouling in the reverse osmosis membrane device can be suppressed, there is a risk of fouling occurring in the forward osmosis membrane device. If fouling occurs in the forward osmosis membrane device, the water purification process must be stopped in order to clean the forward osmosis membrane, which poses the problem that the water purification process cannot be performed stably. In addition, if the forward osmosis membrane device breaks down, the water purification process must be stopped in order to repair it. If the forward osmosis membrane device breaks down, for example, the water to be treated may leak into the annular water channel, causing an increase in the concentration of suspended solids in the circulating water.

The present invention was made in consideration of these problems, and aims to provide a water purification system and water purification method that suppresses the occurrence of fouling in a reverse osmosis membrane device, and that can stably purify water without the need to stop the water purification process even if fouling or a malfunction occurs in the forward osmosis membrane device.

After extensive investigation, the inventors have discovered that the above object can be achieved by the following invention.

A water purification system according to one aspect of the present invention is a water purification system for obtaining purified water from water to be treated, the system comprising: an annular water channel for circulating circulating water having a higher solute concentration than the water to be treated and a lower organic matter concentration than the water to be treated; a first forward osmosis membrane device and a second forward osmosis membrane device provided in parallel to the annular water channel, the first forward osmosis membrane device and the second forward osmosis membrane device permeating moisture from the water to be treated into the circulating water; a first water tank arranged downstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the annular water channel in the direction in which the circulating water flows and for storing the circulating water; a reverse osmosis membrane device arranged downstream of the first water tank in the annular water channel in the direction in which the circulating water flows and for obtaining the purified water by permeating moisture from the circulating water; a pump for pressurizing the circulating water in the annular water channel toward the reverse osmosis membrane device; and a second state in which the introduction of the circulating water and the water to be treated to the first forward osmosis membrane device is stopped and the introduction of the circulating water and the water to be treated to the second forward osmosis membrane device is stopped and the circulating water and the water to be treated are introduced to the second forward osmosis membrane device; a first measuring device that is arranged in the annular water channel downstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the flow direction of the circulating water and upstream of the reverse osmosis membrane device, and measures the electrical conductivity and suspended solids concentration of the circulating water; and a first adjusting device that is arranged in the annular water channel downstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the flow direction of the circulating water and upstream of the reverse osmosis membrane device, and supplies at least one of a solute and water to the circulating water to adjust the solute concentration of the circulating water and reduce the suspended solids concentration .

In the above water purification system, circulating water with a lower organic matter concentration than the water to be treated is introduced into the reverse osmosis membrane device, so the occurrence of fouling in the reverse osmosis membrane device can be suppressed compared to when the water to be treated is introduced into the reverse osmosis membrane device.

In addition, in the above water purification system, the introduction state of the circulating water and the water to be treated into the first forward osmosis membrane device and the second forward osmosis membrane device can be switched between a first state and a second state using a switching mechanism. As a result, in the case where fouling occurs in the first forward osmosis membrane device in use in a water purification system operating in the first state, the first forward osmosis membrane device can be stopped and the second forward osmosis membrane device, which is stopped, can be started by switching from the first state to the second state using the switching mechanism. In addition, until switching is performed by the switching mechanism, circulating water can be supplied to the reverse osmosis membrane device from the first water tank.

Therefore, even if fouling occurs in the first forward osmosis membrane device, there is no need to stop the water purification process, and the water purification process can be performed stably. In addition, cleaning of the first forward osmosis membrane device in which fouling has occurred can be performed without stopping the water purification process. In addition, even if fouling occurs in the second forward osmosis membrane device, the water purification process can be performed stably in the same way.

Furthermore , even if fouling occurs in the first forward osmosis membrane device during use, if the electrical conductivity or suspended matter concentration of the circulating water measured by the first measuring device is slightly outside a predetermined range (for example, the first range of electrical conductivity and suspended matter concentration described below) and cleaning of the first forward osmosis membrane device is not necessary, the first adjusting device can be used to adjust the solute concentration of the circulating water or reduce the suspended matter concentration, thereby supplying circulating water of an appropriate electrical conductivity and suspended matter concentration to the reverse osmosis membrane device and enabling stable water purification processing.

In addition, if the circulating water cannot be adjusted to an appropriate electrical conductivity and suspended solids concentration even when the first adjustment device is used, and it becomes necessary to clean the first forward osmosis membrane device (for example, if the electrical conductivity or suspended solids concentration falls outside the second range described below), the switching mechanism can stop the first forward osmosis membrane device that is in use and start the use of the second forward osmosis membrane device that is stopped.

The above-mentioned water purification system may further include a second water tank arranged in the annular water channel downstream of the reverse osmosis membrane device in the direction in which the circulating water flows, for storing the circulating water; a second measuring device arranged in the annular water channel downstream of the reverse osmosis membrane device in the direction in which the circulating water flows and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device, for measuring the electrical conductivity of the circulating water; and a second adjusting device arranged in the annular water channel downstream of the reverse osmosis membrane device in the direction in which the circulating water flows and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device, for supplying at least one of a solute and water to the circulating water to adjust the solute concentration of the circulating water.

If the electrical conductivity of the circulating water measured by the second measuring device falls outside a specified range (e.g., the third range described below) due to reasons such as fouling occurring in the reverse osmosis membrane device, the difference in solute concentration between the circulating water and the water to be treated will fall outside the appropriate range, and the amount of water passing from the water to be treated to the circulating water by the first forward osmosis membrane device and the second forward osmosis membrane device may decrease.

However, in this embodiment, even if the electrical conductivity of the circulating water measured by the second measuring device falls outside the predetermined range, the second adjusting device can be used to adjust the solute concentration of the circulating water so that the electrical conductivity measured by the second measuring device falls within the predetermined range. Therefore, circulating water with an appropriate solute concentration can be supplied to the first forward osmosis membrane device and the second forward osmosis membrane device, and water purification processing can be performed more stably.

The water purification system further includes a control device that controls the first adjustment device and the switching mechanism, and the control device may control the first adjustment device when the electrical conductivity or suspended solids concentration measured by the first measurement device falls outside a first range, and may control the switching mechanism to switch the introduction state of the circulating water and the water to be treated when the electrical conductivity or suspended solids concentration measured by the first measurement device exceeds the first range and falls outside a second range.

In this embodiment, when the electrical conductivity or suspended solids concentration of the circulating water measured by the first measuring device is outside the first range but within the second range, the degree of fouling of the first forward osmosis membrane device tends to be low, so that the solute concentration of the circulating water can be adjusted or the suspended solids concentration can be reduced with the assistance of the first adjusting device without cleaning the first forward osmosis membrane device, etc.

In addition, if the electrical conductivity or suspended solids concentration of the circulating water measured by the first measuring device falls outside the second range, the first forward osmosis membrane device tends to be highly fouled, and it may not be possible to sufficiently adjust the solute concentration of the circulating water or reduce the suspended solids concentration even with the assistance of the first adjusting device. Therefore, since it is necessary to clean the first forward osmosis membrane device, the switching mechanism stops the first forward osmosis membrane device in use and starts the use of the second forward osmosis membrane device that is stopped.

As a result, the first forward osmosis membrane device can be used to its full potential, and water purification processing can be performed stably.

The water purification system further includes a control device that controls the first adjustment device, the switching mechanism, and the second adjustment device, and the control device controls the first adjustment device when the electrical conductivity or suspended solids concentration measured by the first measurement device falls outside a first range, controls the switching mechanism to switch the introduction state of the circulating water and the water to be treated when the electrical conductivity or suspended solids concentration measured by the first measurement device exceeds the first range and falls outside a second range, and may control the second adjustment device when the electrical conductivity measured by the second measurement device falls outside a third range.

In this aspect, not only can the first adjustment device and the switching mechanism be controlled based on the electrical conductivity or suspended solids concentration of the circulating water measured by the first measurement device as described above, but the second adjustment device can also be controlled based on the electrical conductivity or suspended solids concentration of the circulating water measured by the second measurement device. If the electrical conductivity of the circulating water measured by the second measurement device falls outside the third range, the second adjustment device can be used to adjust the solute concentration of the circulating water so that the electrical conductivity falls within the third range.

As a result, circulating water with an appropriate solute concentration can be supplied to the first forward osmosis membrane device and the second forward osmosis membrane device, allowing for more stable water purification processing.

A water purification method according to another aspect of the present invention is a water purification method for obtaining purified water from water to be treated using a water purification system, the water purification system including: an annular water channel for circulating circulating water having a higher solute concentration than the water to be treated and a lower organic matter concentration than the water to be treated; a first forward osmosis membrane device and a second forward osmosis membrane device arranged in parallel to the annular water channel, the first forward osmosis membrane device and the second forward osmosis membrane device permeating moisture from the water to be treated to the circulating water; and a first forward osmosis membrane device in the annular water channel in the direction in which the circulating water flows. a reverse osmosis membrane device disposed downstream of the first water tank in the flow direction of the circulating water in the annular water channel and configured to permeate moisture from the circulating water to obtain the purified water; a pump for pressurizing the circulating water in the annular water channel toward the reverse osmosis membrane device; and a pressure measuring device disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the flow direction of the circulating water in the annular water channel and configured to measure the electrical conductivity and suspended solids concentration of the circulating water. and a first adjusting device disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows, the first adjusting device being capable of adjusting the solute concentration of the circulating water and reducing the suspended solid concentration by supplying at least one of a solute and water to the circulating water. In this embodiment, the circulating water and the water to be treated are introduced into the first forward osmosis membrane device while the introduction of the circulating water and the water to be treated into the second forward osmosis membrane device is stopped, and the circulating water is measured by the first measuring device. The method measures the electrical conductivity and suspended solids concentration of the circulating water, and when the electrical conductivity or suspended solids concentration measured by the first measuring device falls outside a first range, operates the first adjusting device so that the electrical conductivity and suspended solids concentration of the circulating water fall within the first range, and when the electrical conductivity or suspended solids concentration measured by the first measuring device exceeds the first range and falls outside a second range, stops the introduction of the circulating water and the water to be treated to the first forward osmosis membrane device and introduces the circulating water and the water to be treated to the second forward osmosis membrane device.

In the above water purification method, if the electrical conductivity or suspended solids concentration of the circulating water measured by the first measuring device is outside the first range but within the second range, the degree of fouling of the first forward osmosis membrane device tends to be low, so that the solute concentration of the circulating water can be adjusted or the suspended solids concentration can be reduced with the assistance of the first adjusting device without cleaning the first forward osmosis membrane device, etc.

In addition, if the electrical conductivity or suspended solids concentration of the circulating water measured by the first measuring device falls outside the second range, the first forward osmosis membrane device tends to be highly fouled, and therefore, even with assistance from the first adjusting device, it may not be possible to sufficiently adjust the solute concentration of the circulating water or reduce the suspended solids concentration. Therefore, since it is necessary to clean the first forward osmosis membrane device, the first forward osmosis membrane device that is in use is stopped and the second forward osmosis membrane device that is stopped is started.

As a result, the first forward osmosis membrane device can be used to its full potential, and water purification processing can be performed stably.

In the above water purification method, the water purification system may further include a second water tank arranged downstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows, the second water tank storing the circulating water; a second measuring device arranged downstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device, the second measuring device measuring the electrical conductivity of the circulating water; and a second adjusting device arranged downstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device, the second adjusting device being capable of adjusting a solute concentration of the circulating water by supplying at least one of a solute and water to the circulating water, wherein the second measuring device measures the electrical conductivity of the circulating water, and when the electrical conductivity measured by the second measuring device is outside a third range, the second adjusting device may be operated so that the electrical conductivity of the circulating water falls within the third range.

In this embodiment, if the electrical conductivity of the circulating water measured by the second measuring device falls outside the third range due to reasons such as fouling occurring in the reverse osmosis membrane device, the solute concentration of the circulating water can be adjusted using the second adjusting device so that the electrical conductivity falls within the third range. This allows circulating water with an appropriate solute concentration to be supplied to the first forward osmosis membrane device and the second forward osmosis membrane device, and allows for more stable water purification processing.

The present invention provides a water purification system and a water purification method that can suppress the occurrence of fouling in a reverse osmosis membrane device, and can perform stable water purification without the need to stop the water purification process even if fouling occurs in a forward osmosis membrane device.

FIG. 1 is a schematic diagram showing the overall configuration of a water purification system according to this embodiment. FIG. 2 is a flowchart of the water purification method according to this embodiment. FIG. 3 is a partial schematic diagram showing a variation of the water purification system according to this embodiment. FIG. 4 is a partial schematic diagram showing a water purification system according to another embodiment. FIG. 5 is a partial schematic diagram showing a water purification system according to another embodiment. FIG. 6 is a schematic diagram showing the overall configuration of a conventional water purification system.

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

<Water purification system>
1 is a schematic diagram showing the overall configuration of a water purification system 1 according to an embodiment of the present invention. The water purification system 1 is a system for obtaining purified water W3 from water to be treated W1 via circulating water W2.

The water to be treated W1 is water containing impurities or pollutants, such as seawater, river water, lake water, domestic wastewater, or industrial wastewater. The circulating water W2 has a higher solute concentration than the water to be treated W1, and a lower organic matter concentration than the water to be treated W1. More specifically, the circulating water W2 is an aqueous solution in which salts such as sodium chloride, ammonium carbonate, or calcium carbonate are dissolved as solutes in clean water from which organic matter has been removed, and has a higher osmotic pressure than the water to be treated W1.

As shown in FIG. 1, the water purification system 1 includes a raw water channel 2 through which the water to be treated W1 flows, an annular water channel 12 through which the circulating water W2 flows, and a purified water channel 3 through which the purified water W3 flows. Hereinafter, the direction in which the circulating water W2 flows in the annular water channel 12 (the direction of the arrow on the annular water channel 12 in FIG. 1) is also referred to as the "circulation direction."

The water purification system 1 includes a first forward osmosis membrane device 13a and a second forward osmosis membrane device 13b arranged in parallel in the annular water channel 12. The water purification system 1 also includes a first water tank 14, a pump 15, a reverse osmosis membrane device 16, and a second water tank 17, which are arranged in this order from upstream to downstream in the circulation direction in the annular water channel 12 downstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b in the circulation direction.

The water purification system 1 further includes a pretreatment device 11 provided in the raw water channel 2, a first measuring device 21 provided in the first water tank 14, a second measuring device 22 provided in the second water tank 17, a first adjustment device 23 connected to the first water tank 14, a second adjustment device 24 connected to the second water tank 17, a switching mechanism 18 provided in the raw water channel 2 and the annular water channel 12, and a control device 20 that controls the first adjustment device 23, the switching mechanism 18, and the second adjustment device 24.

(First forward osmosis membrane device and second forward osmosis membrane device)
The first forward osmosis membrane device 13a includes a housing 13a1 and a forward osmosis membrane 13a2 disposed in the housing 13a1, and the space inside the housing 13a1 is divided into a primary side 13a3 and a secondary side 13a4 by the forward osmosis membrane 13a2. Similarly, the second forward osmosis membrane device 13b includes a housing 13b1 and a forward osmosis membrane 13b2 disposed in the housing 13b1, and the space inside the housing 13b1 is divided into a primary side 13b3 and a secondary side 13b4 by the forward osmosis membrane 13b2. For example, flat membranes or hollow fiber membranes can be used as the forward osmosis membranes 13a2 and 13b2, and hollow fiber membranes are preferred.

The primary side 13a3 of the first forward osmosis membrane device 13a and the primary side 13b3 of the second forward osmosis membrane device 13b are connected in parallel to the raw water channel 2. In addition, the secondary side 13a4 of the first forward osmosis membrane device 13a and the secondary side 13b4 of the second forward osmosis membrane device 13b are connected in parallel to the annular water channel 12.

In the first forward osmosis membrane device 13a, the water to be treated W1 filtered in the pretreatment device 11 is supplied to the primary side 13a3, and the circulating water W2 is supplied to the secondary side 13a4. When the water to be treated W1 and the circulating water W2 come into contact with each other via the forward osmosis membrane 13a2, a portion of the water contained in the water to be treated W1 permeates into the circulating water W2 due to the osmotic pressure difference between the water to be treated W1 and the circulating water W2. At this time, the permeation of impurities other than water contained in the water to be treated W1, such as suspended matter and salt, into the circulating water W2 is suppressed. The second forward osmosis membrane device 13b is similar to the first forward osmosis membrane device 13a.

(First water tank)
The first water tank 14 can store the circulating water W2 supplied from the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b, and acts as a so-called buffer in the annular water channel 12. That is, when the circulating water W2 supplied from the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b is insufficient, the stored circulating water W2 can be supplied from the first water tank 14 to the reverse osmosis membrane device 16 through the annular water channel 12. The first water tank 14 can also limit the supply of the circulating water W2 to the reverse osmosis membrane device 16 while receiving the circulating water W2 supplied from the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b.

(Reverse osmosis membrane device)
The reverse osmosis membrane device 16 obtains purified water W3 by permeating water from the circulating water W2. The reverse osmosis membrane device 16 includes a housing 16a and a reverse osmosis membrane 16b disposed within the housing 16a, and the space within the housing 16a is divided into a primary side 16c and a secondary side 16d by the reverse osmosis membrane 16b. The reverse osmosis membrane 16b may be, for example, a flat membrane or a hollow fiber membrane, with a hollow fiber membrane being preferred.

In the reverse osmosis membrane device 16, circulating water W2 pressurized by pump 15 is supplied to the primary side 16c. Due to the pressure applied by pump 15, some of the moisture in the circulating water W2 permeates from the primary side 16c to the secondary side 16d, and purified water W3 is obtained.

(Second water tank)
The second water tank 17 can store the circulating water W2 supplied from the reverse osmosis membrane device 16, and acts as a buffer in the annular water channel 12, similar to the first water tank 14. That is, when the circulating water W2 supplied from the reverse osmosis membrane device 16 is insufficient, the stored circulating water W2 can be supplied from the second water tank 17 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b through the annular water channel 12. The second water tank 17 can also limit the supply of the circulating water W2 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b by receiving the circulating water W2 supplied from the reverse osmosis membrane device 16 while reducing the discharge amount of the circulating water W2.

(Switching mechanism)
The switching mechanism 18 switches the introduction state of the circulating water W2 and the water to be treated W1 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b between a first state and a second state described below.

The switching mechanism 18 has a first switching mechanism 18a and a second switching mechanism 18b. The first switching mechanism 18a is provided in the raw water channel 2 so as to be located at the inlet side of the primary side 13a3 of the first forward osmosis membrane device 13a and the primary side 13b3 of the second forward osmosis membrane device 13b. The first switching mechanism 18a switches whether the treated water W1 supplied from the pretreatment device 11 is introduced into the primary side 13a3 of the first forward osmosis membrane device 13a or the primary side 13b3 of the second forward osmosis membrane device 13b.

The second switching mechanism 18b is provided in the annular water channel 12 so as to be positioned at the inlet side of the secondary side 13a4 of the first forward osmosis membrane device 13a and the secondary side 13b4 of the second forward osmosis membrane device 13b, and switches whether the circulating water W2 supplied from the second water tank 17 is introduced into the secondary side 13a4 of the first forward osmosis membrane device 13a or the secondary side 13b4 of the second forward osmosis membrane device 13b.

The first switching mechanism 18a and the second switching mechanism 18b may have a three-way valve, for example, as shown in FIG. 1.

In this embodiment, the first state is a state in which the circulating water W2 and the water to be treated W1 are introduced into the first forward osmosis membrane device 13a and the introduction of the circulating water W2 and the water to be treated W1 into the second forward osmosis membrane device 13b is stopped. The second state is a state in which the introduction of the circulating water W2 and the water to be treated W1 into the first forward osmosis membrane device 13a is stopped and the circulating water W2 and the water to be treated W1 are introduced into the second forward osmosis membrane device 13b.

(Pretreatment Device)
The pretreatment device 11 is provided in the raw water channel 2 and filters the water to be treated W1 to be supplied to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b. The pretreatment device 11 includes a microfiltration membrane or an ultrafiltration membrane, and reduces suspended solids and contaminants contained in the water to be treated W1.

(First measuring device, second measuring device)
The first measuring device 21 includes an ion concentration measuring device and a suspended solids concentration measuring device, measures the ion concentration and suspended solids concentration of the circulating water W2, and transmits a signal indicating the measured values to the control device 20. The second measuring device 22 includes an ion concentration measuring device, measures the ion concentration of the circulating water W2, and transmits a signal indicating the measured values to the control device 20.

(First Adjustment Device)
The first adjustment device 23 can supply clean water from which organic matter has been removed and solutes (salts) contained in the circulating water W2 to the circulating water W2 contained in the first water tank 14. The first adjustment device 23 can reduce the solute concentration and suspended matter concentration of the circulating water W2 contained in the first water tank 14 by supplying water, and can increase the solute concentration of the circulating water W2 by supplying a solute. In this way, the first adjustment device 23 can adjust the solute concentration and reduce the suspended matter concentration of the circulating water W2 contained in the first water tank 14.

(Second Adjustment Device)
The second adjustment device 24 can supply clean water from which organic matter has been removed and solutes (salts) contained in the circulating water W2 to the circulating water W2 contained in the second water tank 17. The second adjustment device 24 can reduce the solute concentration of the circulating water W2 contained in the second water tank 17 by supplying water, and can increase the solute concentration of the circulating water W2 by supplying a solute. In this way, the second adjustment device 24 can adjust the solute concentration of the circulating water W2 contained in the second water tank 17.

(Control device)
The control device 20 receives measurement results transmitted from the first measuring device 21 and the second measuring device 22, and controls the first adjustment device 23, the switching mechanism 18 and the second adjustment device 24 based on these measurement results.

<Water purification method>
A water purification method using the water purification system 1 according to this embodiment will be described.

Figure 2 is a flowchart of the water purification method according to this embodiment. The water purification method according to this embodiment will be described with reference to Figure 2.

First, we will explain the first and second ranges of electrical conductivity and suspended solids concentration, and the third range of electrical conductivity, used in the water purification method according to this embodiment.

The first range (a1-b1) and the second range (a2-b2) of electrical conductivity are ranges in which the osmotic pressure of the circulating water W2 is favorable for obtaining purified water W3 from the circulating water W2 in the reverse osmosis membrane device 16. It is more favorable for obtaining purified water W3 from the circulating water W2 in the reverse osmosis membrane device 16 if the electrical conductivity is within the first range than if it is within the second range. The first range of electrical conductivity is included in the second range. That is, the lower limit a1 of the first range is greater than the lower limit a2 of the second range, and the upper limit b1 of the first range is less than the upper limit b2 of the second range.

The first range (c1 or less) and the second range (c2 or less) of the suspended solids concentration are preferred ranges for suppressing the occurrence of fouling in the reverse osmosis membrane device 16. A suspended solids concentration in the first range is more preferred for suppressing the occurrence of fouling in the reverse osmosis membrane device 16 than a suspended solids concentration in the second range. Since the lower the suspended solids concentration, the better, there is no lower limit to the first and second ranges. The first range of suspended solids concentration is included in the second range. In other words, the upper limit c1 of the first range is smaller than the upper limit c2 of the second range.

The third range of electrical conductivity (d to e) is a range in which the osmotic pressure of the circulating water W2 is a difference in osmotic pressure between the circulating water W2 and the water to be treated W1 that is favorable for forward osmosis in the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b.

In the water purification method according to this embodiment, when the water purification system 1 starts operating (step S1), the introduction state of the water to be treated W1 and the circulating water W2 by the switching mechanism 18 is set to a first state. That is, the water to be treated W1 and the circulating water W2 are set to a state in which they flow into the first forward osmosis membrane device 13a. In this state, the introduction of the water to be treated W1 into the raw water channel 2 and the circulation flow of the circulating water W2 in the annular water channel 12 by the pump 15 are started.

Next, the first measuring device 21 measures the electrical conductivity and suspended solids concentration of the circulating water W2 stored in the first water tank 14, and the second measuring device 22 measures the electrical conductivity of the circulating water W2 stored in the second water tank 17 (step S2). The control device 20 receives these measurement results. Hereinafter, the electrical conductivity of the circulating water W2 measured by the first measuring device 21 is also referred to as "x", the suspended solids concentration as "y", and the electrical conductivity of the circulating water W2 measured by the second measuring device 22 as "z".

First, it is determined whether the electrical conductivity (x) and suspended solids concentration (y) measured by the first measuring device 21 are within a first range, i.e., whether a1≦x≦b1 and y≦c1 are satisfied (step S3).

If at least one of the electrical conductivity and the suspended solids concentration measured by the first measuring device 21 is outside the first range (NO in step S3), it is further determined whether the electrical conductivity and the suspended solids concentration are within a second range (step S4).

If the electrical conductivity and the suspended solids concentration are within the second range (YES in step S4), there is no need to switch the switching mechanism 18. However, since the electrical conductivity and the suspended solids concentration have already exceeded the first range, it is necessary to supply at least one of solute and water to the circulating water W2. Therefore, in this embodiment, in order to determine whether it is necessary to supply solute, it is first determined whether the electrical conductivity is smaller than the lower limit a1 of the first range (step S6).

If the electrical conductivity is not less than the lower limit a1 of the first range (NO in step S6), the electrical conductivity is equal to or greater than the lower limit a1, and there is no need to supply solute to the circulating water W2; only water needs to be supplied. Therefore, the control device 20 controls the first adjustment device 23 to supply an amount of water according to the measured value to the circulating water W2 (step S7). Thereafter, the first measurement device 21 and the second measurement device 22 continue to measure the electrical conductivity and suspended solids concentration of the circulating water W2 (step S2).

If the electrical conductivity is less than the lower limit a1 of the first range (YES in step S6), it is necessary to supply solute to the circulating water W2, but it is unclear whether water needs to be supplied. Therefore, to determine whether water needs to be supplied to the circulating water W2, it is next determined whether the suspended solids concentration is greater than the upper limit c1 of the first range (step S8).

If the suspended solids concentration is not greater than the upper limit c1 of the first range (NO in step S8), there is no need to supply water to the circulating water W2, and only solute needs to be supplied. Therefore, the control device 20 controls the first adjustment device 23 to supply an amount of solute according to the measured value to the circulating water W2 (step S9). After that, the first measurement device 21 and the second measurement device 22 continue to measure the electrical conductivity and suspended solids concentration of the circulating water W2 (step S2).

If the suspended solids concentration is greater than the upper limit c1 of the first range (YES in step S8), it is necessary to supply both solute and water to the circulating water W2. Therefore, the control device 20 controls the first adjustment device 23 to supply the amount of solute and water according to the measured values to the circulating water W2 (step S10). After that, the first measurement device 21 and the second measurement device 22 continue to measure the electrical conductivity and suspended solids concentration of the circulating water W2 (step S2).

If at least one of the electrical conductivity and the suspended solids concentration exceeds the first range and is outside the second range (NO in step S4), the control device 20 controls the switching mechanism 18 to switch the introduction state of the circulating water W2 and the water to be treated W1 (step S5). That is, the introduction state of the circulating water W2 and the water to be treated W1 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b is switched from the first state to the second state. This allows the use of the first forward osmosis membrane device 13a to be stopped and the use of the second forward osmosis membrane device 13b to be started, and the first forward osmosis membrane device 13a can be repaired, etc. Thereafter, the measurement of the electrical conductivity and suspended solids concentration of the circulating water W2 by the first measuring device 21 and the second measuring device 22 continues (step S2).

If the electrical conductivity and suspended solids concentration measured by the first measuring device 21 are within the first range (YES in step S3), the control device 20 does not need to control either the first adjusting device 23 or the switching mechanism 18. In this case, it is determined whether the electrical conductivity (z) measured by the second measuring device 22 is within the third range, i.e., whether d≦z≦e is satisfied (step S11).

If the electrical conductivity measured by the second measuring device 22 is outside the third range (NO in step S11), it is further determined whether the electrical conductivity is smaller than the lower limit d of the third range (step S12).

If the electrical conductivity is not less than the lower limit d of the third range (NO in step S12), it is found that the electrical conductivity is already outside the third range and is therefore greater than the upper limit e of the third range. Therefore, the control device 20 controls the second adjustment device 24 to supply an amount of water according to the measured value to the circulating water W2 (step S13). Thereafter, the first measurement device 21 and the second measurement device 22 continue to measure the electrical conductivity and suspended solids concentration of the circulating water W2 (step S2).

If the electrical conductivity is less than the lower limit d of the third range (YES in step S12), the control device 20 controls the second adjustment device 24 to supply an amount of solute corresponding to the measured value to the circulating water W2 (step S14). After that, the first measurement device 21 and the second measurement device 22 continue to measure the electrical conductivity and suspended solids concentration of the circulating water W2 (step S2).

If the electrical conductivity measured by the second measuring device 22 is within the third range (YES in step S11), and the operation of the water purification system 1 is to be continued (YES in step S15), the measurement of the electrical conductivity and suspended solids concentration of the circulating water W2 by the first measuring device 21 and the second measuring device 22 is continued (step S2). If the operation of the water purification system 1 is not to be continued (NO in step S15), the operation of the water purification system 1 is stopped (step S16).

(Action, Effect)
The operation and effects of the water purification system and water purification method according to this embodiment will be described.

In the water purification system 1 according to this embodiment, circulating water W2, which has a lower organic matter concentration than the water to be treated W1, is introduced into the reverse osmosis membrane device 16, so that the occurrence of fouling in the reverse osmosis membrane device 16 can be suppressed compared to when the water to be treated W1 is introduced into the reverse osmosis membrane device 16.

In addition, in the water purification system 1, the introduction state of the circulating water W2 and the water to be treated W1 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b can be switched between a first state and a second state using the switching mechanism 18. As a result, in the water purification system 1 operating in the first state, if fouling occurs in the first forward osmosis membrane device 13a in use, the first forward osmosis membrane device 13a can be stopped and the second forward osmosis membrane device 13b, which is stopped, can be started by switching from the first state to the second state using the switching mechanism 18. In addition, until switching is performed by the switching mechanism 18, the circulating water W2 can be supplied to the reverse osmosis membrane device 16 from the first water tank 14.

Therefore, even if fouling occurs in the first forward osmosis membrane device 13a, there is no need to stop the water purification process, and the water purification process can be performed stably. In addition, cleaning of the first forward osmosis membrane device 13a in which fouling has occurred can be performed without stopping the water purification process. Even if fouling occurs in the second forward osmosis membrane device 13b, the water purification process can be performed stably by switching from the second state to the first state using the switching mechanism 18.

Even if fouling occurs in the first forward osmosis membrane device 13a during use, if the electrical conductivity or suspended matter concentration of the circulating water W2 measured by the first measuring device 21 is slightly outside the first range but within the second range, and cleaning of the first forward osmosis membrane device 13a is not necessary, the first adjusting device 23 can be used to adjust the solute concentration of the circulating water W2 or reduce the suspended matter concentration, thereby supplying circulating water W2 with an appropriate electrical conductivity and suspended matter concentration to the reverse osmosis membrane device. This allows stable water purification processing.

In addition, if the electrical conductivity and suspended solids concentration of the circulating water W2 cannot be adjusted to within the first range even when the first adjustment device 23 is used, and, for example, the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measurement device 21 falls outside the second range, and cleaning of the first forward osmosis membrane device 13a or the like becomes necessary, the switching mechanism 18 can stop the first forward osmosis membrane device 13a that is in use and start the use of the second forward osmosis membrane device 13b that is stopped.

Furthermore, if the electrical conductivity of the circulating water W2 measured by the second measuring device 22 falls outside the third range due to reasons such as fouling occurring in the reverse osmosis membrane device 16, the solute concentration difference between the circulating water W2 and the water to be treated W1 will fall outside the appropriate range, and the amount of water passing from the water to be treated W1 to the circulating water W2 by the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b may decrease.

However, in the water purification system 1, even if the electrical conductivity of the circulating water W2 measured by the second measuring device 22 falls outside the third range, the second adjusting device 24 can be used to adjust the solute concentration of the circulating water W2 so that the electrical conductivity measured by the second measuring device 22 falls within a predetermined range. Therefore, circulating water W2 with an appropriate solute concentration can be supplied to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b, and water purification processing can be performed more stably.

In the water purification system 1, when the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measuring device 21 is outside the first range but within the second range, the degree of fouling of the first forward osmosis membrane device 13a tends to be low, so the solute concentration of the circulating water can be adjusted or the suspended solids concentration can be reduced with the assistance of the first adjusting device 23 without cleaning the first forward osmosis membrane device 13a, etc.

In addition, when the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measuring device 21 falls outside the second range, the degree of fouling of the first forward osmosis membrane device 13a tends to be high, and therefore the solute concentration of the circulating water W2 cannot be sufficiently adjusted or the suspended solids concentration reduced even with the assistance of the first adjusting device 23. Therefore, since it is necessary to clean the first forward osmosis membrane device 13a, the switching mechanism 18 stops the first forward osmosis membrane device 13a in use and starts the use of the second forward osmosis membrane device 13b, which is currently stopped.

As a result, the first forward osmosis membrane device 13a can be used sufficiently and the water purification process can be performed stably. The same is true for the second forward osmosis membrane device 13b.

After switching to the second state by the switching mechanism 18, the device may continue to be used in that state, or the device may be switched to the first state and used after cleaning the first forward osmosis membrane device 13a, etc. The switching mechanism 18 may be operated by the control device 20 or manually.

In addition, in the water purification system 1, not only can the first adjustment device 23 and the switching mechanism 18 be controlled based on the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measurement device 21, but the second adjustment device 24 can also be controlled based on the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the second measurement device 22. If the electrical conductivity of the circulating water W2 measured by the second measurement device 22 falls outside the third range, the second adjustment device can be used to adjust the solute concentration of the circulating water W2 so that the electrical conductivity falls within the third range.

As a result, circulating water W2 with an appropriate solute concentration can be supplied to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b, allowing for more stable water purification processing.

In the water purification method according to this embodiment, when the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measuring device 21 is outside the first range but within the second range, the degree of fouling of the first forward osmosis membrane device 13a tends to be low, and therefore the solute concentration of the circulating water W2 can be adjusted or the suspended solids concentration can be reduced with the assistance of the first adjusting device 23 without cleaning the first forward osmosis membrane device 13a, etc.

In addition, if the electrical conductivity or suspended solids concentration of the circulating water W2 measured by the first measuring device 21 falls outside the second range, the degree of fouling of the first forward osmosis membrane device 13a tends to be high, and it may not be possible to sufficiently adjust the solute concentration of the circulating water W2 or reduce the suspended solids concentration even with the assistance of the first adjusting device 23. Therefore, since it is necessary to clean the first forward osmosis membrane device 13a, the first forward osmosis membrane device 13a in use is stopped and the second forward osmosis membrane device 13b, which is stopped, is started.

As a result, the first forward osmosis membrane device 13a can be used sufficiently and the water purification process can be performed stably. The same applies to the second forward osmosis membrane device 13b.

In the water purification method according to this embodiment, if the electrical conductivity of the circulating water W2 measured by the second measuring device 22 falls outside the third range due to reasons such as fouling occurring in the reverse osmosis membrane device 16, the second adjusting device 24 can be used to adjust the solute concentration of the circulating water W2 contained in the second water tank 17 so that the electrical conductivity falls within the third range. This makes it possible to supply circulating water with an appropriate solute concentration to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b, and to perform the water purification process more stably.

<Example of variation>
When the solute concentration of the circulating water W2 is increased using the first adjusting device 23 and the second adjusting device 24, the solute is supplied to the circulating water W2 in the above embodiment (steps S7 and S11 in FIG. 2), but both water and the solute may be supplied. In this case, the water and the solute are mixed in a ratio according to the solute concentration of the circulating water W2. Also, an aqueous solution having a higher solute concentration than the circulating water W2 (hereinafter also referred to as a "high concentration solution") may be prepared in advance and this high concentration solution may be supplied.

In the water purification method of the above embodiment, after the introduction state of the circulating water W2 and the water to be treated W1 to the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b is switched from the first state to the second state by the switching mechanism 18 (step S2 in FIG. 2), the water purification system 1 continues to be used in the second state, but it may be switched to the first state immediately after cleaning the first forward osmosis membrane device 13a, for example.

Figure 3 shows a modified example of the switching mechanism 18. In the above embodiment, the first switching mechanism 18a is described as using a three-way valve, but a valve other than a three-way valve may be used. For example, as shown in Figure 3, a valve 18a1 provided on the inlet side of the primary side 13a3 of the first forward osmosis membrane device 13a of the raw water channel 2 and a valve 18a2 provided on the inlet side of the primary side 13b3 of the second forward osmosis membrane device 13b may be used. Similarly, for the second switching mechanism 18b, a valve 18a1 provided on the inlet side of the secondary side 13a4 of the first forward osmosis membrane device 13a of the raw water channel 2 and a valve 18b2 provided on the inlet side of the secondary side 13b4 of the second forward osmosis membrane device 13b may be used.

Figure 4 shows a modified arrangement of the first measuring device 21 and the second measuring device 22. In the above embodiment, the first measuring device 21 is provided in the first water tank 14, and the second measuring device 22 is provided in the second water tank 17. However, the first measuring device 21 may be provided in another position in the annular water channel 12, so long as it is downstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b in the circulation direction and upstream of the reverse osmosis membrane device 16. In addition, the second measuring device 22 may be provided in another position in the annular water channel 12, so long as it is downstream of the reverse osmosis membrane device 16 in the circulation direction and upstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b.

For example, as shown in FIG. 4, the first measuring device 21 may be composed of a first upstream measuring device 21a provided downstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b and upstream of the first water tank 14, and a first downstream measuring device 21b provided downstream of the first water tank 14 and upstream of the reverse osmosis membrane device 16.

In this case, the first upstream measuring device 21a measures the electrical conductivity and suspended solids concentration of the circulating water W2 before it is adjusted by the first adjusting device 23, and the first downstream measuring device 21b measures the electrical conductivity and suspended solids concentration of the circulating water W2 after it has been adjusted. The control device 20 receives the measurement results sent from the first upstream measuring device 21a and the first downstream measuring device 21b, and controls the amount of water and solute supplied from the first adjusting device 23 to the circulating water W2 based on these measurement results. The control device 20 can adjust the electrical conductivity and suspended solids concentration of the circulating water W2 more accurately by receiving feedback of the measurement results after adjustment from the first downstream measuring device 21b. Note that only one of the first upstream measuring device 21a and the first downstream measuring device 21b may be provided.

Similarly, the second measuring device 22 may be composed of a second upstream measuring device 22a provided downstream of the reverse osmosis membrane device 16 and upstream of the second water tank 17, and a second downstream measuring device 22b provided downstream of the second water tank 17 and upstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b. Also, only one of the second upstream measuring device 22a and the second downstream measuring device 22b may be provided.

Figure 5 shows a modified arrangement of the first adjustment device 23 and the second adjustment device 24. In the above embodiment, the first adjustment device 23 is connected to the first water tank 14, and the second adjustment device 24 is connected to the second water tank 17. However, the first adjustment device 23 may be provided at another position in the annular water channel 12, so long as it is downstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b in the circulation direction and upstream of the reverse osmosis membrane device 16. Also, the second adjustment device 24 may be provided at another position in the annular water channel 12, so long as it is downstream of the reverse osmosis membrane device 16 in the circulation direction and upstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b.

For example, as shown in FIG. 5, the first adjustment device 23 may be provided so as to be connected to the annular water channel 12 downstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b and upstream of the first water tank 14, or may be provided so as to be connected to the annular water channel 12 downstream of the first water tank 14 and upstream of the reverse osmosis membrane device 16, as shown by the two-dot chain line.

Similarly, the second adjustment device 24 may be provided so as to be connected to the annular water channel 12 downstream of the reverse osmosis membrane device 16 and upstream of the second water tank 17, or may be provided so as to be connected to the annular water channel 12 downstream of the second water tank 17 and upstream of the first forward osmosis membrane device 13a and the second forward osmosis membrane device 13b, as shown by the dashed line.

In the above embodiment, the water purification system 1 is described as including the first measuring device 21 and the first adjusting device 23, but the installation of these devices is optional and does not have to be provided. Similarly, the installation of the second water tank 17, the second measuring device 22, the second adjusting device 24, and the control device 20 are also optional and do not have to be provided.

REFERENCE SIGNS LIST 1 Water purification system 12 Annular water channel 13a First forward osmosis membrane device 13a1 Forward osmosis membrane 13b Second forward osmosis membrane device 13b1 Forward osmosis membrane 14 First water tank 15 Pump 16 Reverse osmosis membrane device 16a Reverse osmosis membrane 17 Second water tank 18 Switching mechanism 20 Control device 21 First measuring device 22 Second measuring device 23 First adjustment device 24 Second adjustment device W1 Water to be treated W2 Circulating water W3 Purified water

Claims (6)

A water purification system for obtaining purified water from water to be treated,
a circular water channel for circulating circulating water having a solute concentration higher than that of the water to be treated and a lower organic matter concentration than that of the water to be treated;
A first forward osmosis membrane device and a second forward osmosis membrane device are provided in parallel with the annular water channel, and the first forward osmosis membrane device and the second forward osmosis membrane device transmit water from the water to be treated to the circulating water;
A first water tank that is disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the flow direction of the circulating water in the annular water channel and that stores the circulating water;
a reverse osmosis membrane device disposed downstream of the first water tank in the annular water channel in the direction in which the circulating water flows, the reverse osmosis membrane device permeating moisture from the circulating water to obtain the purified water;
a pump that pressurizes the circulating water in the annular water channel toward the reverse osmosis membrane device;
a switching mechanism for switching between a first state in which the circulating water and the water to be treated are introduced into the first forward osmosis membrane device and the introduction of the circulating water and the water to be treated into the second forward osmosis membrane device is stopped, and a second state in which the introduction of the circulating water and the water to be treated into the first forward osmosis membrane device is stopped and the circulating water and the water to be treated are introduced into the second forward osmosis membrane device;
a first measuring device disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the flow direction of the circulating water in the annular water channel, the first measuring device measuring the electrical conductivity and the suspended solids concentration of the circulating water;
a first adjustment device that is disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the annular water channel in the flow direction of the circulating water, and that is capable of adjusting the solute concentration and reducing the suspended solid concentration of the circulating water by supplying at least one of a solute and water to the circulating water . a second water tank disposed downstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows, the second water tank storing the circulating water;
a second measuring device disposed downstream of the reverse osmosis membrane device and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the annular water channel in the flow direction of the circulating water, the second measuring device measuring the electrical conductivity of the circulating water;
2. The water purification system of claim 1, further comprising: a second adjustment device arranged downstream of the reverse osmosis membrane device and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the annular water channel in the flow direction of the circulating water, the second adjustment device being capable of adjusting a solute concentration of the circulating water by supplying at least one of a solute and water to the circulating water. A control device that controls the first adjustment device and the switching mechanism,
The control device controls the first adjustment device when the electrical conductivity or the suspended solids concentration measured by the first measurement device falls outside a first range,
The water purification system described in claim 1, wherein the switching mechanism is controlled so that the introduction state of the circulating water and the treated water is switched when the electrical conductivity or suspended solids concentration measured by the first measuring device exceeds the first range and falls outside the second range. A control device that controls the first adjustment device, the switching mechanism, and the second adjustment device,
The control device controls the first adjustment device when the electrical conductivity or the suspended solids concentration measured by the first measurement device falls outside a first range,
When the electrical conductivity or the concentration of suspended solids measured by the first measuring device exceeds the first range and falls outside the second range, the switching mechanism is controlled so that the introduction states of the circulating water and the water to be treated are switched;
The water purification system according to claim 2 , further comprising: a step of controlling the second adjusting device when the electrical conductivity measured by the second measuring device falls outside a third range. A water purification method for obtaining purified water from water to be treated using a water purification system,
The water purification system comprises:
a circular water channel for circulating circulating water having a solute concentration higher than that of the water to be treated and a lower organic matter concentration than that of the water to be treated;
A first forward osmosis membrane device and a second forward osmosis membrane device are provided in parallel with the annular water channel, and the first forward osmosis membrane device and the second forward osmosis membrane device transmit water from the water to be treated to the circulating water;
A first water tank that is disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the flow direction of the circulating water in the annular water channel and stores the circulating water;
a reverse osmosis membrane device disposed downstream of the first water tank in the annular water channel in the direction in which the circulating water flows, the reverse osmosis membrane device permeating moisture from the circulating water to obtain the purified water;
a pump that pressurizes the circulating water in the annular water channel toward the reverse osmosis membrane device;
a first measuring device disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the flow direction of the circulating water in the annular water channel, the first measuring device measuring the electrical conductivity and the suspended solids concentration of the circulating water;
a first adjusting device that is disposed downstream of the first forward osmosis membrane device and the second forward osmosis membrane device and upstream of the reverse osmosis membrane device in the annular water channel in the flow direction of the circulating water, and that is capable of adjusting the solute concentration and reducing the suspended solid concentration of the circulating water by supplying at least one of a solute and water to the circulating water;
The circulating water and the water to be treated are introduced into the first forward osmosis membrane device, and the introduction of the circulating water and the water to be treated into the second forward osmosis membrane device is stopped, while the first measuring device measures the electrical conductivity and the suspended solids concentration of the circulating water;
When the electrical conductivity or the concentration of suspended solids measured by the first measuring device is outside a first range, the first adjusting device is operated so that the electrical conductivity and the concentration of suspended solids of the circulating water are within the first range;
A water purification method, comprising: when the electrical conductivity or suspended solids concentration measured by the first measuring device exceeds the first range and falls outside the second range, stopping the introduction of the circulating water and the water to be treated into the first forward osmosis membrane device and introducing the circulating water and the water to be treated into the second forward osmosis membrane device. The water purification system further comprises:
a second water tank disposed downstream of the reverse osmosis membrane device in the annular water channel in the direction in which the circulating water flows, the second water tank storing the circulating water;
a second measuring device disposed downstream of the reverse osmosis membrane device and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the annular water channel in the flow direction of the circulating water, the second measuring device measuring the electrical conductivity of the circulating water;
a second adjusting device disposed downstream of the reverse osmosis membrane device and upstream of the first forward osmosis membrane device and the second forward osmosis membrane device in the annular water channel in the flow direction of the circulating water, the second adjusting device being capable of adjusting the solute concentration of the circulating water by supplying at least one of a solute and water to the circulating water;
The electrical conductivity of the circulating water is measured by the second measuring device;
The water purification method according to claim 5, wherein when the electrical conductivity measured by the second measuring device falls outside a third range, the second adjusting device is operated so that the electrical conductivity of the circulating water falls within the third range.

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