Forward osmosis concentration system

Abstract

To provide a forward osmosis concentration system having a high recovery rate of an object even when the object to be concentrated in an object liquid is a compound having a small ion radius such as a nitrogen compound and the composition of the object liquid varies.SOLUTION: A forward osmosis concentration system comprising a semipermeable membrane module for concentrating a target substance in a target liquid. The object includes a nitrogen compound. In a semipermeable membrane module having a semipermeable membrane, and a first chamber and a second chamber partitioned by the semipermeable membrane, by allowing an object liquid to flow into the first chamber and allowing a draw solution having a higher osmotic pressure to flow into the second chamber, water contained in the object liquid in the first chamber is transferred to the draw solution in the second chamber via the semipermeable membrane, and the object liquid is concentrated and discharged from the first chamber. The concentration of the concentrated liquid is continuously measured, and at least one of a first pressure, which is the pressure of the target liquid in the first chamber, the flow rate of the target liquid flowing into the first chamber, and a second pressure, which is the pressure of the draw solution flowing into the second chamber, is feedback-controlled so that the concentration reaches a target value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a forward osmosis concentration system. [Background technology]

[0002] The separation and concentration of liquid mixtures by membrane separation is an energy-saving method because it does not involve a phase change compared to conventional separation techniques such as distillation, and because it does not involve a change in the state of the substance, it is widely used in many fields, such as the food industry, where fruit juice is concentrated and beer yeast is separated, and the recovery of organic matter from industrial wastewater.Among membrane separation methods, forward osmosis (FO) is a more energy-efficient method than reverse osmosis and other methods.

[0003] Various forward osmosis concentration systems using such forward osmosis methods are known (see, for example, Patent Document 1 (JP 2014-100624 A), Patent Document 2 (JP 2020-199430 A), and Patent Document 3 (JP 2021-146297 A)). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-100624 [Patent Document 2] Japanese Patent Application Publication No. 2020-199430 [Patent Document 3] Japanese Patent Publication No. 2021-146297 Summary of the Invention [Problem to be solved by the invention]

[0005] When the target substance to be concentrated in the target liquid is a compound with a small ionic radius, such as a nitrogen compound, it easily permeates the semipermeable membrane, resulting in increased leakage of the target substance into the draw solution and a decrease in the recovery rate of the target substance. For this reason, various operating conditions are adjusted to minimize leakage of the target substance into the draw solution.

[0006] However, when the target liquid is a liquid such as domestic wastewater or industrial wastewater in which the content of nitrogen compounds can fluctuate, the amount of leakage of the target substance (nitrogen compounds, etc.) may be significantly affected by the fluctuation in composition. If the amount of leakage of the target substance fluctuates significantly, the recovery rate of the target substance (the concentration of the obtained concentrated liquid) may also fluctuate significantly.

[0007] The present invention provides a forward osmosis concentration system for concentrating a target substance in a target liquid, wherein the target substance to be concentrated in the target liquid is a compound with a small ionic radius, such as a nitrogen compound, and the forward osmosis concentration system has a high recovery rate of the target substance even when the composition of the target liquid fluctuates. [Means for solving the problem]

[0008] (1) A forward osmosis concentration system having a semipermeable membrane module for concentrating a target substance in a target liquid, the target object includes a nitrogen compound; The semipermeable membrane module has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane, In the semipermeable membrane module, the target liquid is flowed into the first chamber, and a draw solution having an osmotic pressure higher than that of the target liquid is flowed into the second chamber, whereby water contained in the target liquid in the first chamber is transferred to the draw solution in the second chamber through the semipermeable membrane, concentrating the target liquid, and the concentrated liquid, which is the concentrated target liquid, is discharged from the first chamber; The concentration of the concentrate is measured continuously or periodically; a forward osmosis concentration system in which feedback control is performed to control at least one of a first pressure that is the pressure of the target liquid in the first chamber, a target liquid flow rate that is the flow rate of the target liquid flowing into the first chamber, and a second pressure that is the pressure of the draw solution flowing into the second chamber, based on the concentration of the concentrated liquid, so that the concentration of the concentrated liquid becomes a target value.

[0009] (2) In the feedback control, increasing the first pressure when the concentration of the concentrate is less than the target value and the first pressure is less than a threshold value; When the concentration of the concentrated liquid is less than the target value, the first pressure is equal to or greater than a threshold value, and the target liquid flow rate exceeds a threshold value, the target liquid flow rate is reduced; increasing the second pressure when the concentration of the concentrated liquid is less than the target value, the first pressure is equal to or greater than a threshold, the target liquid flow rate is equal to or less than a threshold, and the second pressure is less than a threshold; (1) The forward osmosis concentration system according to (1).

[0010] (3) The forward osmosis concentration system according to (1) or (2), wherein an alarm is issued if the concentration of the concentrated solution is below the target value even after the feedback control is performed.

[0011] (4) The forward osmosis concentration system according to any one of (1) to (3), wherein the semipermeable membrane is a hollow fiber membrane.

[0012] (5) The forward osmosis concentration system according to (4), wherein the second chamber is inside the hollow fiber membrane and the first chamber is outside the hollow fiber membrane.

[0013] (6) The forward osmosis concentration system according to any one of (1) to (5), wherein the semipermeable membrane contains cellulose triacetate.

[0014] (7) The forward osmosis concentration system according to any one of (1) to (6), comprising a multiple module formed by connecting a plurality of the semipermeable membrane modules. [Effects of the Invention]

[0015] According to the present invention, there can be provided a forward osmosis concentration system for concentrating a target substance in a target liquid, which has a high recovery rate of the target substance even when the target substance to be concentrated in the target liquid is a compound with a small ionic radius, such as a nitrogen compound, and the composition of the target liquid fluctuates. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of a forward osmosis concentration system according to an embodiment. [Figure 2] FIG. 10 is a flow chart showing an example of the flow of feedback control of the forward osmosis concentration system of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] The forward osmosis concentration system of this embodiment will be described below with reference to the drawings. Note that the same reference numerals throughout the drawings represent the same or corresponding parts. Furthermore, dimensional relationships such as length, width, thickness, and depth have been appropriately changed for clarity and simplification of the drawings, and do not represent actual dimensional relationships.

[0018] <Forward osmosis concentration system> Referring to FIG. 1, the forward osmosis concentration system of this embodiment includes a semipermeable membrane module 1 for concentrating a target substance in a target liquid. The semipermeable membrane module 1 has a semipermeable membrane 10 and a first chamber 11 and a second chamber 12 separated by the semipermeable membrane.

[0019] In the semipermeable membrane module 1, a target liquid (feed solution (FS)) is passed through the first chamber 11, and a draw solution (DS) having a higher osmotic pressure than the target liquid is passed through the second chamber 12. As a result, water contained in the target liquid in the first chamber 11 migrates through the semipermeable membrane to the draw solution in the second chamber 12, concentrating the target liquid. The concentrated target liquid (concentrated liquid) is discharged from the first chamber 11. At the same time, the draw solution in the second chamber 12 is diluted, and the diluted draw solution (diluted liquid) is discharged from the second chamber 12.

[0020] In this embodiment, the draw solution is supplied to the first chamber 11 by a liquid supply pump 31. The target liquid (feed solution) is supplied to the second chamber 12 by a liquid supply pump 32.

[0021] In the semipermeable membrane module 1, the flow directions of the liquids on both sides of the semipermeable membrane 10 (the target liquid in the first chamber 11 and the draw solution in the second chamber 12) may be in any direction, and may be opposite directions (counterflow method) or parallel directions (parallel flow method).

[0022] (feedback control) In the concentration system of this embodiment, feedback control is performed based on the concentration of the concentrated liquid (concentrated target liquid discharged from the first chamber 11) so that the concentration of the concentrated liquid reaches a target value. The concentration of the concentrated liquid is the concentration of the target substance (nitrogen compound) contained in the concentrated liquid.

[0023] Specifically, the concentration of the target substance (nitrogen compound) contained in the concentrated liquid is continuously or periodically measured (monitored). For this purpose, for example, a concentration measuring device 41 (for example, a measuring device including an ammonium electrode or the like) for measuring the concentration of nitrogen compounds is provided on the outlet (downstream) side of the first chamber 11 of the semipermeable membrane module 1.

[0024] Based on the concentration of the concentrated liquid, at least one of the first pressure (target liquid pressure) which is the pressure of the target liquid in the first chamber 11, the target liquid flow rate which is the flow rate of the target liquid flowing into the first chamber 11, and the second pressure (draw solution pressure) which is the pressure of the draw solution flowing into the second chamber is controlled (feedback controlled) so that the concentration of the concentrated liquid becomes a (predetermined) target value.

[0025] The first pressure (target hydraulic pressure) can be controlled, for example, by controlling the opening of the valve 5 on the outlet side of the first chamber 11. That is, the first pressure can be increased by decreasing the opening of the valve 5 (reducing the opening area). The first pressure can be measured by a pressure gauge 42 provided on the outlet (downstream) side of the first chamber 11.

[0026] The flow rate of the target liquid can be controlled, for example, by controlling the frequency of the liquid feed pump 32. That is, by increasing the frequency of the liquid feed pump 32, the flow rate of the target liquid can be increased. The target liquid flow rate can be measured by a flow meter 43 provided on the inlet (upstream) side of the first chamber 11.

[0027] The second pressure (draw solution pressure) can be controlled, for example, by controlling the frequency of the liquid feed pump 31. That is, the second pressure can be increased by increasing the frequency of the liquid feed pump 31. Note that, by increasing the frequency of the liquid feed pump 31, the flow rate of the draw solution flowing into the second chamber 12 can also be increased. The second pressure can be measured by a pressure gauge 44 provided on the inlet (upstream) side of the second chamber 12.

[0028] The above feedback control is performed by the control device 2. The control device 2 can receive data on the measurements of the concentration measuring device 41, the pressure meters 42 and 44, and the flow meter 43, and can control the opening of the valve 5 and the frequencies of the liquid feed pumps 31 and 32.

[0029] In this embodiment, as described above, feedback control is performed to adjust the operating conditions of the forward osmosis concentration system in accordance with fluctuations in the composition of the target liquid, thereby eliminating the effect of fluctuations in the composition of the target liquid on the leakage amount of targets with small ionic radii (nitrogen compounds, etc.), and improving the recovery rate of targets with small ionic radii.

[0030] Using a semipermeable membrane with a low solute permeability coefficient (i.e., one through which the target substance is less likely to permeate) reduces leakage of the target substance into the draw solution. However, because the amount of permeate decreases, the concentration efficiency decreases, and the target solution may not be concentrated until the target concentration is reached. As such, there is a trade-off between concentration efficiency and the amount of target substance leakage into the draw solution. It is difficult to completely prevent leakage of the target substance into the draw solution, and it is necessary to maintain a balance between concentration efficiency and the amount of target substance leakage into the draw solution. When the composition of the target solution fluctuates, feedback control in accordance with the composition fluctuations of the target solution, as described above, is effective in maintaining a balance between concentration efficiency and the amount of target substance leakage into the draw solution, thereby maintaining or improving the desired concentration efficiency.

[0031] (An example of feedback control) The flow diagram of FIG. 2 shows an example of the flow of feedback control for adjusting the concentration of the concentrated liquid to a target value based on the concentration of the concentrated liquid. In the feedback control in this embodiment, it is sufficient to control at least one of the three pressures, namely, the first pressure (target liquid pressure), the target liquid flow rate, and the second pressure (draw solution pressure), based on the concentration of the concentrated liquid so that the concentration of the concentrated liquid becomes a target value. The priority of the three controls is not limited to the priority shown in FIG. 2, and only one or two of the three controls may be performed.

[0032] In the feedback control shown in FIG. 2, the following control is performed to make the concentration of the concentrated liquid reach the target value. (i) If the concentration of the concentrate is less than the target value and the first pressure is less than the threshold value, increase the first pressure. (ii) When the concentration of the concentrated liquid is less than the target value, the first pressure is equal to or greater than the threshold value, and the target liquid flow rate exceeds the threshold value, the target liquid flow rate is reduced. (iii) If the concentration of the concentrated liquid is less than the target value, the first pressure is greater than or equal to the threshold, the target liquid flow rate is less than or equal to the threshold, and the second pressure is less than the threshold, the second pressure is increased.

[0033] Specifically, in the feedback control shown in FIG. 2, the following steps 1 to 5 (S1 to S5) are carried out.

[0034] (Step 1) It is determined whether the concentration of the concentrated liquid is less than a target value (S1). (i) If NO If the concentration of the concentrate (concentrate concentration) is equal to or greater than the target value (NO), nothing is done, and step 1 is carried out again after a predetermined time (T1) has elapsed. (ii) If YES If the concentration of the concentrate is less than the target value (YES), proceed to step 2. This is because the following steps are required to control the concentration of the concentrate to the target value.

[0035] (Step 2) It is determined whether the first pressure (target hydraulic pressure) is equal to or greater than a threshold value (S21). (i) If NO If the first pressure is less than the threshold value (NO), the first pressure is increased (S22). By increasing the first pressure, the concentration of the concentrate can be increased to a target value. Then, after a predetermined time (T1) has elapsed, step 1 is performed again. This is because there are cases where the concentration of the concentrated liquid cannot be increased to the target value simply by increasing the first pressure, and cases where the concentration of the concentrated liquid once increases to the target value but then falls below the target value. (ii) If YES If the first pressure is equal to or greater than the threshold value (YES), proceed to step 3. This is because the threshold value of the first pressure is an upper limit value that takes into consideration the pressure resistance of the semipermeable membrane module, and when the first pressure is equal to or higher than the threshold value, it is not desirable to further increase the first pressure.

[0036] (Step 3) It is determined whether the target liquid flow rate is equal to or greater than a threshold value (S31). (i) If NO If the flow rate of the target liquid is less than the threshold (NO), the flow rate of the target liquid is increased (S32). By increasing the flow rate of the target liquid, the concentration of the concentrated liquid can be increased to the target value. Then, after a predetermined time (T1) has elapsed, step 1 is performed again. This is because there are cases where the concentration of the concentrated liquid cannot be increased to the target value simply by increasing the flow rate of the target liquid, and cases where the concentration of the concentrated liquid once increases to the target value but then falls below the target value. (ii) If YES If the target liquid flow rate is equal to or less than the threshold value (YES), proceed to step 4. The threshold value of the target liquid flow rate is a lower limit value that takes into consideration the occurrence of concentration polarization of the target liquid in the first chamber 11, and it is not desirable to further reduce the target liquid flow rate when the target liquid flow rate is below the threshold value.

[0037] (Step 4) It is determined whether the second pressure is equal to or greater than a threshold value (S41). (i) If NO If the second pressure is less than the threshold value (NO), the second pressure is increased (S42). By increasing the second pressure, the concentration of the concentrate can be increased to a target value. Then, after a predetermined time (T1) has elapsed, step 1 is performed again. This is because there are cases where the concentration of the concentrated liquid cannot be increased to the target value simply by increasing the second pressure, and there are also cases where the concentration of the concentrated liquid once increases to the target value but then falls below the target value. (ii) If YES If the second pressure is equal to or greater than the threshold value (YES), proceed to step 5. The threshold value of the second pressure is an upper limit value that takes into consideration the pressure loss of the draw solution in the second chamber 12 and the pressure resistance of the semipermeable membrane module, and it is not desirable to further increase the second pressure when the second pressure is above the threshold value.

[0038] In particular, when the semipermeable membrane 10 is a hollow fiber membrane and the second chamber 12 is the inside (hollow portion) of the hollow fiber membrane, increasing the second pressure is likely to result in large pressure loss, which may significantly increase energy consumption in the liquid feed pump 31, etc. Furthermore, because hollow fiber membranes are relatively weak against pressure from the inside, if the pressure in the hollow portion becomes too high, the hollow fiber membrane is likely to be damaged. For this reason, it is preferable to avoid increasing the second pressure (or the flow rate of the draw solution) as much as possible, and it is preferable to first increase the concentration of the concentrate by controlling the first pressure or the flow rate of the target liquid. The feedback control shown in Figure 2 takes these factors into consideration.

[0039] (Step 5) An alarm is issued (S5). That is, if the concentration of the concentrated liquid is below the target value even after the above feedback control (all of steps 1 to 4) is performed, an alarm is issued. The alarm may be generated by emitting an audible alarm, displaying a visual alarm on the interface screen, or by any other means that notifies the user of the system that the concentrate concentration is below the target value. After that, step 1 is performed again after a predetermined time (T1) has elapsed. The user or other person who recognizes the alarm can take some kind of action to increase the concentration of the concentrated liquid to the target value, but if the concentration of the concentrated liquid cannot be increased to the target value, control must be performed again.

[0040] (Target liquid) The target liquid is a liquid (such as an aqueous solution) containing a target substance.

[0041] The target substance includes at least one nitrogen compound, but may be only a nitrogen compound.

[0042] The target liquid is not particularly limited, but examples thereof include wastewater containing nitrogen compounds (domestic wastewater, industrial wastewater). When the target liquid is a liquid whose composition (nitrogen compound content) can fluctuate, such as wastewater, the amount of leakage of the target substance can fluctuate greatly, and the recovery rate of the target substance (concentration of the concentrated liquid) can also fluctuate greatly, so the forward osmosis concentration system of this embodiment is particularly useful.

[0043] The nitrogen compound is not particularly limited as long as it is a compound containing nitrogen, and examples thereof include nitrogen compounds such as ammonium ion, nitrate ion, nitrite ion, urea, and ammonium chloride. In addition, in the target liquid (solvent such as water), ammonium ions (NH4 + The ionic radius of ammonium chloride (NH4Cl) is 246 pm, and that of ammonium chloride (NH4Cl) is 564 pm. For reference, the ionic radius of NaCl hydrate (the sum of the hydrated ionic radii of the Na hydrate ion and the Cl hydrate ion) is 621 pm.

[0044] The target substance preferably contains a compound (nitrogen compound) having an ionic radius of 600 pm or less in the target liquid. The ionic radius of the target substance (nitrogen compound) in the target liquid is preferably 50 to 600 pm, more preferably 130 to 600 pm. The "ionic radius in the target liquid" of a target substance is the ionic radius if the target substance in the target liquid (such as an aqueous solution) is an ion, and is the radius of the hydrate ion if the target substance is a hydrate ion (hydrated ionic radius). Also, if the target substance exists in the target liquid dissociated into multiple ions (for example, anions and cations) or multiple hydrate ions, the sum of the ionic radii (or hydrated ionic radii) of each ion (or hydrate ion) is the "ionic radius of the target substance." When the ionic radius of the target substance in the target liquid is within this range, the amount of the target substance leaking into the draw solution increases (resulting in a lower recovery rate of the target substance), making the forward osmosis concentration system of this embodiment particularly useful.

[0045] (draw solution) The draw solution is not particularly limited as long as it has a higher osmotic pressure than the target liquid, and various known draw solutions used in forward osmosis treatment can be used. As the draw solution, for example, seawater, which is easily available, can be used.

[0046] (semi-permeable membrane) The semipermeable membrane 10 is not particularly limited, and various known semipermeable membranes that can be used for forward osmosis can be used.

[0047] The material constituting the semipermeable membrane is not particularly limited, but examples thereof include cellulose-based resins, polysulfone-based resins, polyamide-based resins, etc. The semipermeable membrane is preferably made of a material containing at least one of a cellulose-based resin and a polysulfone-based resin.

[0048] The cellulose-based resin is preferably a cellulose acetate-based resin. The cellulose acetate-based resin is resistant to chlorine, a disinfectant, and can inhibit the growth of microorganisms. The cellulose acetate-based resin is preferably cellulose acetate, and from the viewpoint of durability, more preferably cellulose triacetate.

[0049] The polysulfone-based resin is preferably a polyethersulfone-based resin. The polyethersulfone-based resin is preferably a sulfonated polyethersulfone.

[0050] The shape of the semipermeable membrane is not particularly limited, but examples thereof include a flat membrane, a spiral membrane, and a hollow fiber membrane. Although the semipermeable membrane 10 is depicted as a simplified flat membrane in the drawings, the shape is not limited thereto.

[0051] The semipermeable membrane is preferably a hollow fiber membrane. When the semipermeable membrane is a hollow fiber membrane, the water permeability per unit membrane area is not as large as that of a spiral membrane (flat membrane), but the membrane area per volume of the semipermeable membrane module can be increased. This allows for a larger water permeability for the entire module. In other words, this has the advantage of very high volumetric efficiency and excellent compactness. Furthermore, when both high-concentration and low-concentration solutions are supplied to the module and brought into contact through the semipermeable membrane, the module is less likely to experience drift, minimizing concentration polarization on the membrane surface. This allows for efficient forward osmosis concentration treatment.

[0052] When the semipermeable membrane is a hollow fiber membrane, it is preferable that the draw solution flows inside the hollow fiber membrane and the target liquid flows outside the hollow fiber membrane, i.e., the outside of the hollow fiber membrane is the first chamber and the inside of the hollow fiber membrane is the second chamber.

[0053] The form of the semipermeable membrane module 1 is not particularly limited, but when hollow fiber membranes are used, examples include a module in which multiple hollow fiber membranes are arranged in a straight line, a cross-wound module in which multiple hollow fiber membranes are wound around a core tube, etc. When flat membranes are used, examples include a stacked module in which multiple flat membranes are stacked, and a spiral module in which multiple flat membranes are wrapped around a core tube in the form of an envelope.

[0054] The outer diameter of the hollow fiber membrane is not particularly limited as long as it is suitable for use in forward osmosis treatment, etc., but is, for example, 150 to 250 μm. If the outer diameter is smaller than the above range, the inner diameter will inevitably be small as well, which can cause problems by increasing the pressure loss of the fluid flowing through the hollow portion of the hollow fiber membrane. On the other hand, if the outer diameter is larger than the above range, it will be impossible to increase the membrane area per unit volume in the module, and compactness, one of the advantages of hollow fiber membrane modules, will be lost.

[0055] The hollow fiber membrane may have any hollow ratio, as long as it is suitable for use in forward osmosis treatment, etc., but is, for example, 15 to 35%. If the hollow ratio is smaller than the above range, the pressure loss in the hollow portion increases, and the desired amount of permeated water may not be obtained. If the hollow ratio is larger than the above range, sufficient pressure resistance may not be ensured during osmosis treatment. The hollow ratio (%) is calculated using the following formula: Hollowness ratio (%) = (inner diameter / outer diameter) 2 ×100 It can be calculated by:

[0056] When the semipermeable membrane 10 is a hollow fiber membrane, it is preferable that the second chamber 12 is inside the hollow fiber membrane (the drawing liquid flows inside the hollow fiber membrane) and the first chamber 11 is outside the hollow fiber membrane (the target liquid flows outside the hollow fiber membrane).

[0057] Furthermore, the forward osmosis concentration system of this embodiment may include a multiple module formed by connecting a plurality of semipermeable membrane modules (see, for example, Patent Documents 1 to 3). In this case, the solute permeability coefficients of the semipermeable membranes of the plurality of semipermeable membrane modules may be the same or different. [Explanation of symbols]

[0058] 1 semipermeable membrane module, 10 semipermeable membrane, 11 first chamber, 12 second chamber, 2 control device, 31, 32 liquid transfer pump, 41 concentration measuring device, 42 pressure gauge, 43 flow meter, 44 pressure gauge, 5 valve.

Claims

1. A forward osmosis concentration system comprising a semipermeable membrane module for concentrating a target substance in a target liquid, the target object includes a nitrogen compound; The semipermeable membrane module has a semipermeable membrane and a first chamber and a second chamber separated by the semipermeable membrane, In the semipermeable membrane module, the target liquid is flowed into the first chamber, and a draw solution having an osmotic pressure higher than that of the target liquid is flowed into the second chamber, whereby water contained in the target liquid in the first chamber is transferred to the draw solution in the second chamber through the semipermeable membrane, concentrating the target liquid, and the concentrated liquid, which is the concentrated target liquid, is discharged from the first chamber; The concentration of the concentrate is measured continuously or periodically; a feedback control is performed to control at least one of a first pressure that is a pressure of the target liquid in the first chamber, a target liquid flow rate that is a flow rate of the target liquid flowed into the first chamber, and a second pressure that is a pressure of the draw solution flowed into the second chamber, based on the concentration of the concentrated liquid, so that the concentration of the concentrated liquid becomes a target value.

2. In the feedback control, increasing the first pressure when the concentration of the concentrate is less than the target value and the first pressure is less than a threshold value; When the concentration of the concentrated liquid is less than the target value, the first pressure is equal to or greater than a threshold value, and the target liquid flow rate exceeds a threshold value, the target liquid flow rate is reduced; increasing the second pressure when the concentration of the concentrated liquid is less than the target value, the first pressure is equal to or greater than a threshold, the target liquid flow rate is equal to or less than a threshold, and the second pressure is less than a threshold; 10. The forward osmosis concentration system of claim 1.

3. The forward osmosis concentration system according to claim 1 , wherein an alarm is issued if the concentration of the concentrate is below the target value even after the feedback control is performed.

4. 2. The forward osmosis concentration system of claim 1, wherein the semipermeable membrane is a hollow fiber membrane.

5. 5. The forward osmosis concentration system of claim 4, wherein the second chamber is inside the hollow fiber membrane and the first chamber is outside the hollow fiber membrane.

6. 10. The forward osmosis concentration system of claim 1, wherein the semipermeable membrane comprises cellulose triacetate.

7. The forward osmosis concentration system according to claim 1 , comprising a multiple module formed by connecting a plurality of the semipermeable membrane modules.

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