METHOD AND APPARATUS FOR MEMBRANE TREATMENT OF LIQUID TO BE TREATED

Abstract

To provide a membrane treatment apparatus for a liquid to be treated that is capable of efficiently concentrating to-be-treated liquid at a high concentration.SOLUTION: A membrane treatment apparatus 1 for a liquid to be treated, comprising: a first concentration device 10 comprising a reverse osmosis (RO) membrane unit 12 that produces a concentrated liquid not passed through a RO membrane by boosting water to be treated and feeding fresh water to the RO membrane; and a second concentration device 20 equipped with semipermeable membrane units 21-1, 21-2 in which the produced concentrated liquid is allowed to contact with a recovery liquid of a pressure lower than that of the condensed liquid through a semipermeable membrane, so that the concentrated liquid could be further concentrated, wherein at least a part of the concentrated liquid concentrated by the second concentration device 20 is used as the recovery liquid.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and apparatus for membrane treatment of a liquid to be treated, and more particularly to a method and apparatus for membrane treatment of a liquid to be treated that utilizes a reverse osmosis membrane. [Background technology]

[0002] As a method for treating a liquid to be treated using a reverse osmosis membrane, for example, a fresh water production method is disclosed in Patent Document 1. In the fresh water production method of Patent Document 1, seawater is pressurized and supplied to a reverse osmosis membrane module to separate fresh water, and then concentrated saltwater that does not permeate the reverse osmosis membrane is reduced in pressure by energy exchange with seawater and supplied to a forward osmosis membrane module, where the concentrated saltwater is diluted with water that permeates the forward osmosis membrane. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-97331 A Summary of the Invention [Problem to be solved by the invention]

[0004] The reverse osmosis membrane method requires the application of a pressure higher than the osmotic pressure of the liquid to be treated, and thus has limitations on how highly concentrated the liquid can be. On the other hand, the above-mentioned conventional freshwater production method requires diluting concentrated salt water with water in the forward osmosis membrane module, and therefore has room for improvement in terms of increasing the concentration rate of the liquid to be treated.

[0005] SUMMARY OF THE PRESENT EMBODIMENTS The present invention has an object to provide a membrane treatment method and apparatus for a liquid to be treated, which can efficiently concentrate the liquid to a high concentration. [Means for solving the problem]

[0006] The object of the present invention is achieved by a membrane treatment method for a liquid to be treated, comprising: a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; and a second concentration step in which the produced concentrated liquid is brought into contact with a recovery liquid at a lower pressure than the concentrated liquid via a semipermeable membrane, the second concentration step comprising a step of contacting the concentrated liquid with the recovery liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid being supplied in series to the semipermeable membrane units of each stage, the recovery liquid being supplied to the semipermeable membrane units of each stage such that at least two of the semipermeable membrane units are in parallel, at least a portion of the concentrated liquid concentrated in the second concentration step is used as the recovery liquid, and the second concentration step comprises a step of supplying the recovery liquid to the semipermeable membrane units of each stage to which the recovery liquid is supplied in parallel, without passing the recovery liquid through the semipermeable membrane units of the other stages.

[0007] Alternatively, the object of the present invention is achieved by a membrane treatment method for a liquid to be treated, comprising: a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; and a second concentration step in which the produced concentrated liquid is brought into contact, via a semipermeable membrane, with a recovery liquid at a lower pressure than the concentrated liquid, wherein the second concentration step comprises a step of contacting the concentrated liquid with the recovery liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied in series to the semipermeable membrane units of each stage, and the recovery liquid is supplied to the semipermeable membrane units of each stage so that at least two of the semipermeable membrane units are in parallel, and at least a portion of the concentrated liquid concentrated in the second concentration step is used as the recovery liquid, and the second concentration step comprises a step of merging a portion of the recovery liquid discharged from at least any of the semipermeable membrane units of each stage with the recovery liquid supplied to at least any of the semipermeable membrane units of each stage. In this membrane treatment method for the liquid to be treated, the first concentration step preferably includes a step of combining the remainder of the recovery liquid, which is combined with the recovery liquid supplied to the semipermeable membrane unit in the second concentration step, with the liquid to be treated to produce the concentrated liquid.

[0008] Alternatively, the object of the present invention can be achieved by a membrane treatment method for a liquid to be treated, comprising: a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; and a second concentration step in which the produced concentrated liquid is brought into contact with a recovery liquid at a lower pressure than the concentrated liquid via a semipermeable membrane, the second concentration step comprising a step of contacting the concentrated liquid with the recovery liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid being supplied in series to the semipermeable membrane units of each stage, the recovery liquid being supplied to the semipermeable membrane units of each stage so that at least two of the semipermeable membrane units are in parallel, at least a portion of the concentrated liquid concentrated in the second concentration step is used as the recovery liquid, and the second concentration step comprises a step of individually merging a portion of the recovery liquid discharged from the semipermeable membrane units of each stage after merging with the recovery liquid supplied to the semipermeable membrane units of each stage.

[0009] The object of the present invention is also achieved by a membrane treatment device for a liquid to be treated, which comprises a first concentrating device having an RO membrane unit that generates a concentrated liquid that is concentrated without permeating the reverse osmosis membrane by increasing the pressure of the liquid to be treated and passing it through a reverse osmosis membrane, and a second concentrating device that further concentrates the generated concentrated liquid by contacting it with a recovery liquid at a lower pressure than the concentrated liquid via a semipermeable membrane, the second concentrating device being configured so that contact between the concentrated liquid and the recovery liquid via the semipermeable membrane is performed in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied in series to the semipermeable membrane units of each stage, the recovery liquid is supplied to the semipermeable membrane units of each stage so that at least two of the semipermeable membrane units are in parallel, at least a portion of the concentrated liquid concentrated in the second concentrating device is used as the recovery liquid, and the second concentrating device supplies the recovery liquid to the semipermeable membrane units of each stage to which the recovery liquid is supplied in parallel, without passing the recovery liquid through the semipermeable membrane units of the other stages.

[0010] Alternatively, the object of the present invention can be achieved by a membrane treatment device for a liquid to be treated, comprising: a first concentrating device having an RO membrane unit which generates a concentrated liquid that is concentrated without permeating the reverse osmosis membrane by increasing the pressure of the liquid to be treated and passing it through a reverse osmosis membrane; and a second concentrating device which further concentrates the generated concentrated liquid by bringing it into contact with a recovery liquid at a lower pressure than the concentrated liquid via a semipermeable membrane, wherein the second concentrating device is configured so that the contact between the concentrated liquid and the recovery liquid via the semipermeable membrane is performed in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied in series to the semipermeable membrane units of each stage and the recovery liquid is supplied to the semipermeable membrane units of each stage so that at least two of the semipermeable membrane units are in parallel, and at least a portion of the concentrated liquid concentrated in the second concentrating device is used as the recovery liquid, and the second concentrating device merges a portion of the recovery liquid discharged from at least any of the semipermeable membrane units of each stage with the recovery liquid supplied to at least any of the semipermeable membrane units of each stage.

[0011] Alternatively, the object of the present invention can be achieved by a membrane treatment device for a liquid to be treated, comprising: a first concentrating device having an RO membrane unit that generates a concentrated liquid that is concentrated without permeating the reverse osmosis membrane by increasing the pressure of the liquid to be treated and passing it through a reverse osmosis membrane; and a second concentrating device that further concentrates the generated concentrated liquid by contacting it with a recovery liquid at a lower pressure than the concentrated liquid via a semipermeable membrane, wherein the second concentrating device is configured so that the contact between the concentrated liquid and the recovery liquid via the semipermeable membrane is performed in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied in series to the semipermeable membrane units of each stage and the recovery liquid is supplied to the semipermeable membrane units of each stage so that at least two of them are in parallel, and at least a portion of the concentrated liquid concentrated in the second concentrating device is used as the recovery liquid, and the second concentrating device individually combines a portion of the recovery liquid discharged from the semipermeable membrane units of each stage after being combined with the recovery liquid supplied to the semipermeable membrane unit of each stage.

[0012]

[0013] Effect of the Invention

[0014] According to the present invention, it is possible to provide a membrane treatment method and apparatus for a liquid to be treated, which can efficiently concentrate the liquid to a high concentration. [Brief description of the drawings]

[0015] [Figure 1] 1 is a schematic configuration diagram of a membrane treatment device for a liquid to be treated according to an embodiment of the present invention. [Diagram 2] FIG. 11 is a schematic configuration diagram of a membrane treatment device for a liquid to be treated according to another embodiment of the present invention. [Diagram 3] FIG. 13 is a schematic configuration diagram of a membrane treatment device for a liquid to be treated according to still another embodiment of the present invention. [Figure 4] FIG. 13 is a schematic configuration diagram of a membrane treatment device for a liquid to be treated according to still another embodiment of the present invention. [Diagram 5] FIG. 13 is a schematic configuration diagram of a membrane treatment device for a liquid to be treated according to still another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a schematic diagram of a membrane treatment device for a liquid to be treated (hereinafter, simply referred to as "membrane treatment device") according to an embodiment of the present invention. As shown in Fig. 1, the membrane treatment device 1 includes, as main components, a first concentrator 10 that concentrates the liquid to be treated to produce a concentrated liquid, a second concentrator 20 that further concentrates the concentrated liquid produced by the first concentrator 10, and an energy recovery device 30 that performs pressure exchange between the concentrated liquid supplied to the second concentrator 20 and the concentrated liquid discharged from the second concentrator 20.

[0017] The first concentrator 10 includes an RO membrane unit 12 configured by arranging an RO membrane (reverse osmosis membrane) module in a casing, and the liquid to be treated that does not permeate the RO membrane is discharged as a concentrated liquid. The first concentrator 10 may be configured with RO membrane units 12 arranged in multiple stages, and the liquid to be treated concentrated in the RO membrane unit 12 in the first stage is supplied to the RO membrane unit 12 in the second stage for further concentration to produce a concentrated liquid. The material and type of the RO membrane are not particularly limited, and examples thereof include a hollow fiber membrane type or a flat membrane type made of polyamide, cellulose acetate, or the like.

[0018] The second concentrator 20 includes two semipermeable membrane units 21-1 and 21-2. The inside of the casing of each of the semipermeable membrane units 21-1 and 21-2 is partitioned by a semipermeable membrane to form a high-pressure chamber 22 and a low-pressure chamber 23. The high-pressure chamber 22 and the low-pressure chamber 23 are configured such that the concentrated liquid and the recovered liquid are supplied from the inlet and come into contact with each other via the semipermeable membrane while being discharged from the outlet. The semipermeable membrane may be a hollow fiber membrane other than a flat membrane. The semipermeable membrane may be a FO membrane (forward osmosis membrane), but may be another semipermeable membrane such as an RO membrane. The concentrated liquid is supplied in series to the high-pressure chamber 22 of each of the semipermeable membrane units 21-1 and 21-2, while the recovered liquid is supplied in parallel to the low-pressure chamber 23 of each of the semipermeable membrane units 21-1 and 21-2.

[0019] The energy recovery device 30 is composed of a turbocharger, and rotates a turbine using the concentrated liquid discharged from the final-stage semipermeable membrane unit 21-2 of the second concentrator 20, and uses this power to pressurize the concentrated liquid supplied to the front-stage semipermeable membrane unit 21-1. The configuration of the energy recovery device 30 is not particularly limited as long as it is capable of recovering the energy of the concentrated liquid discharged from the second concentrator 20 and pressurizing the concentrated liquid supplied to the second concentrator 20, and for example, other energy recovery devices such as a rotor type or a piston type other than a turbine type may be used.

[0020] Next, a membrane treatment method for a liquid to be treated using the membrane treatment device 1 having the above-mentioned configuration will be described. First, a first concentration step is performed by pressurizing the liquid to be treated, such as seawater, with a high-pressure pump 2 and supplying it to a first concentration device 10. In the RO membrane unit 12, the liquid to be treated permeates the RO membrane to produce fresh water, and a concentrated liquid that is concentrated without permeating the RO membrane of the RO membrane unit 12 is produced.

[0021] Next, the concentrated liquid produced in the first concentration step is boosted by the boost pump 3 and supplied to the second concentrator 20 to perform the second concentration step. The high-pressure concentrated liquid that has been boosted by passing through the first concentrator 10 is supplied in series to the high-pressure chamber 22 of each semipermeable membrane unit 21-1, 21-2. On the other hand, a part of the concentrated liquid after passing through the high-pressure chamber 22 of the semipermeable membrane unit 21-1, 21-2 is supplied in parallel as a recovered liquid after passing through the filter 4 to the low-pressure chamber 23 of each semipermeable membrane unit 21-1, 21-2. This reduces the pressure loss of the low-pressure recovered liquid, and ensures that it is supplied to each semipermeable membrane unit 21-1, 21-2, thereby increasing the concentration rate of the liquid to be treated. The filter 4 is not an essential component, and the configuration may not include the filter 4. The number of stages of the semipermeable membrane units 21-1, 21-2 is not particularly limited as long as it is a plurality of stages, and may be three or more stages. The remainder of the concentrated liquid that is not used as the recovered liquid is discharged outside the system and can be used in other processes, such as forward osmosis power generation and desalination by evaporation and concentration for salt production.

[0022] The recovery liquid supplied to the low pressure chamber 23 is depressurized by passing through each semipermeable membrane unit 21-1, 21-2, and therefore has a lower pressure than the concentrated liquid supplied to the high pressure chamber 22. At the same time, the osmotic pressure difference between the recovered liquid and the concentrated liquid supplied to the high pressure chamber 22 is small, and therefore the pressure difference between the high pressure chamber 22 and the low pressure chamber 23 promotes the movement of water from the high pressure chamber 22 to the low pressure chamber 23 via the semipermeable membrane. Therefore, the concentrated liquid can be further concentrated without particularly increasing the performance or number of the high pressure pump 2, and can be concentrated to a high concentration with reduced energy. The concentrated liquid concentrated in the second concentrator 20 is depressurized by passing through the energy recovery device 30, and at the same time, the concentrated liquid supplied to the second concentrator 20 is pressurized, so that the pressure difference between the high pressure chamber 22 and the low pressure chamber 23 can be increased.

[0023] Of the recovered liquid obtained by recovering water from the concentrated liquid in the second concentrator 20, a portion of the recovered liquid discharged from the subsequent semipermeable membrane unit 21-2 is returned by the circulation pump 5 and is again supplied in parallel to the semipermeable membrane units 21-1, 21-2. The remaining recovered liquid is merged with the liquid to be treated upstream of the high-pressure pump 2 and is again supplied to the first concentrator 10. As a result, the efficiency of freshwater production in the first concentrator 10 can be improved, and the liquid to be treated can be diluted and its osmotic pressure reduced, allowing it to be supplied to the RO membrane unit 12 at a low pressure, thereby saving energy for the high-pressure pump 2.

[0024] Fig. 2 is a schematic diagram of a membrane treatment device according to another embodiment of the present invention. In the membrane treatment device 101 shown in Fig. 2, the first concentrator 10 in the membrane treatment device 1 shown in Fig. 1 is provided with an NF membrane unit 14 in addition to the RO membrane unit 12. In Fig. 2, the same components as those in Fig. 1 are denoted by the same reference numerals (the same applies to Figs. 3 to 5 described below).

[0025] The NF membrane unit 14 is configured by disposing an NF membrane (nanofiltration membrane) module in a casing, and the liquid to be treated that has permeated the NF membrane of the NF membrane unit 14 is supplied to the RO membrane unit 12. As with the RO membrane, the material and type of the NF membrane are not particularly limited. The NF membrane unit 14 may be configured in multiple stages instead of in one stage.

[0026] The second concentrator 20 includes three stages of semipermeable membrane units 21-1, 21-2, and 21-3. The configuration of each of the semipermeable membrane units 21-1, 21-2, and 21-3 is the same as that of the semipermeable membrane units 21-1 and 21-2 shown in Fig. 1, and the concentrated liquid is supplied in series from the RO membrane unit 12 to each high-pressure chamber 22, while a recovered liquid obtained by combining a part of the concentrated liquid that has passed through the high-pressure chamber 22 with the non-permeated liquid from the NF membrane unit 14 is supplied in parallel to each low-pressure chamber 23 by the operation of the feed pump 7. Since the pressure loss can be reduced by supplying the recovered liquid in parallel to each of the semipermeable membrane units 21-1, 21-2, and 21-3, the configuration may not include the feed pump 7. The flow rate of the recovery liquid supplied to the low-pressure chamber 23 of each semipermeable membrane unit 21-1, 21-2, 21-3 can be individually controlled by adjusting the opening of the flow control valves 40a, 40b, 40c, thereby allowing the recovery liquid to be supplied at an appropriate flow rate to each of the semipermeable membrane units 21-1, 21-2, 21-3.

[0027] Since the osmotic pressure of the non-permeated liquid produced in the NF membrane unit 14 increases due to an increase in the concentration of solutes, even if the non-permeated liquid is included in the recovered liquid, the osmotic pressure difference with the concentrated liquid can be maintained small in the second concentrator 20. The recovered liquid only needs to contain the concentrated liquid discharged from the second concentrator 20, and another liquid having a concentration close to that of the concentrated liquid discharged from the second concentrator 20 may be merged with the recovered liquid together with the non-permeated liquid of the NF membrane unit 14 or instead of the non-permeated liquid of the NF membrane unit 14.

[0028] In the energy recovery device 30, the concentrated liquid supplied to the semipermeable membrane unit 21-1 in the front stage is pressurized by the concentrated liquid discharged from the semipermeable membrane unit 21-3 in the last stage. This ensures that the concentrated liquid is reliably supplied to each of the semipermeable membrane units 21-1, 21-2, and 21-3, ensuring a pressure difference with the recovered liquid, and promoting concentration in the second concentrator 20.

[0029] The recovered liquid discharged from each of the semipermeable membrane units 21-1, 21-2, and 21-3 is joined and then a portion is circulated individually to each of the semipermeable membrane units 21-1, 21-2, and 21-3 by the circulation pump 5, thereby diluting the remaining recovered liquid discharged outside the system. The circulation flow rate to each of the semipermeable membrane units 21-1, 21-2, and 21-3 can be individually controlled by adjusting the opening of the flow control valves 40d, 40e, and 40f.

[0030] The configuration for circulating the recovered liquid by the circulation pump 5 is not limited to that of this embodiment, and may be any configuration in which the recovered liquid discharged from at least any of the semipermeable membrane units 21-1, 21-2, 21-3 in each stage is joined to the recovered liquid supplied to at least any of the semipermeable membrane units 21-1, 21-2, 21-3 in each stage. Furthermore, circulation of the recovered liquid by the circulation pump 5 is not essential to the present invention, and a configuration in which the recovered liquid is not circulated may be used.

[0031] The membrane treatment device 1, 101 shown in Figures 1 and 2 is configured to supply the recovery liquid in parallel to all of the semipermeable membrane units in the second concentration step, but it is sufficient that the recovery liquid is supplied to at least two semipermeable membrane units in each stage in parallel. If there is a risk that it will be difficult to control the flow rate of the recovery liquid supplied to each semipermeable membrane unit or to control the concentration difference between the recovery liquid and the concentrated liquid, the recovery liquid may be supplied in series between some of the semipermeable membrane units.

[0032] For example, as shown in FIG. 3, in a membrane treatment device 201 having three semipermeable membrane units 21-1, 21-2, and 21-3, the recovered liquid supplied in parallel to the two semipermeable membrane units 21-2 and 21-3 on the downstream side can be joined together and supplied to the semipermeable membrane unit 21-1 on the upstream side. The joined recovered liquid can be reliably supplied to the semipermeable membrane unit 21-1 by the operation of the booster pump 8, but depending on the pressure of the recovered liquid after joining, the booster pump 8 may not be provided. The number of semipermeable membrane units to which the recovered liquid is supplied in parallel on the downstream side may be three or more, and a semipermeable membrane unit to which the recovered liquid is supplied in series may be added to at least some of the semipermeable membrane units arranged in parallel. The number of semipermeable membrane units to which the recovered liquid is supplied in series on the upstream side may be two or more.

[0033] Alternatively, as shown in FIG. 4, in a membrane treatment device 301 equipped with three semipermeable membrane units 21-1, 21-2, and 21-3, the recovered liquid supplied to the semipermeable membrane unit 21-3 on the downstream side can be supplied in parallel to the two semipermeable membrane units 21-1 and 21-2 on the upstream side by operating a booster pump 8. The number of semipermeable membrane units to which the recovered liquid is supplied in parallel on the upstream side may be three or more, and a semipermeable membrane unit to which the recovered liquid is supplied in series may be added to at least a part of each semipermeable membrane unit arranged in parallel. The number of semipermeable membrane units to which the recovered liquid is supplied in series on the downstream side may be two or more. The membrane treatment device 301 shown in FIG. 4 may also be configured without including a booster pump 8.

[0034] Alternatively, as shown in Fig. 5, in a membrane treatment device 401 equipped with four semipermeable membrane units 21-1, 21-2, 21-3, and 21-4, the recovered liquid supplied in parallel to the two semipermeable membrane units 21-3 and 21-4 on the rear stage side can be joined together and supplied in parallel to the two semipermeable membrane units 21-1 and 21-2 on the front stage side by operation of the booster pump 8. The number of semipermeable membrane units to which the recovered liquid is supplied in parallel on the front stage side and / or rear stage side may be three or more, and a semipermeable membrane unit that supplies the recovered liquid in series may be added to at least some of the semipermeable membrane units arranged in parallel. The membrane treatment device 401 shown in Fig. 5 may also be configured without including the booster pump 8.

[0035] In this way, by appropriately combining parallel supply with serial supply for supplying the recovery liquid to each stage of the semipermeable membrane unit in the second concentration step, the concentration and flow rate of the recovery liquid supplied to each semipermeable membrane unit can be easily adjusted. As a result, it is possible to reduce the number of pumps required and improve pump efficiency by increasing the capacity, and it is also possible to equalize the membrane flux in each semipermeable membrane unit.

[0036] 3 to 5, as in the configuration shown in Fig. 2, a portion of the recovered liquid that has passed through and joined with each of the semipermeable membrane units 21-1, 21-2, and 21-3 can be circulated to each of the semipermeable membrane units 21-1, 21-2, and 21-3 individually, and the circulation flow rate to each of the semipermeable membrane units 21-1, 21-2, and 21-3 can be individually controlled by adjusting the opening of the flow control valve. The configuration for circulating the recovered liquid may be any configuration in which the recovered liquid discharged from at least any of the semipermeable membrane units 21-1, 21-2, and 21-3 in each stage is joined with the recovered liquid supplied to at least any of the semipermeable membrane units 21-1, 21-2, and 21-3 in each stage. [Explanation of symbols]

[0037] 1. Membrane treatment equipment 10 First concentrator 12 RO membrane unit 14 NF membrane unit 20 Second concentrator 21 Semipermeable membrane unit 30 Energy Recovery Device

Claims

1. a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; A second concentration step of further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane, The second concentration step includes a step of contacting the concentrated liquid with the recovered liquid through the semipermeable membrane in a plurality of semipermeable membrane units, supplying the concentrated liquid to the semipermeable membrane units in series in each stage, and supplying the recovered liquid to the semipermeable membrane units in each stage such that at least two of the semipermeable membrane units are in parallel, At least a part of the concentrated liquid concentrated in the second concentration step is used as the recovered liquid; The second concentration step is a membrane treatment method for a liquid to be treated, comprising a step of supplying the recovery liquid to the semipermeable membrane units of each stage to which the recovery liquid is supplied in parallel, without passing the recovery liquid through the semipermeable membrane units of other stages.

2. a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; A second concentration step of further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane, The second concentration step includes a step of contacting the concentrated liquid with the recovered liquid through the semipermeable membrane in a plurality of semipermeable membrane units, supplying the concentrated liquid to the semipermeable membrane units in series in each stage, and supplying the recovered liquid to the semipermeable membrane units in each stage such that at least two of the semipermeable membrane units are in parallel, At least a part of the concentrated liquid concentrated in the second concentration step is used as the recovered liquid; The second concentration step is a membrane treatment method for a liquid to be treated, comprising a step of merging a portion of the recovery liquid discharged from at least any of the semipermeable membrane units in each stage with the recovery liquid supplied to at least any of the semipermeable membrane units in each stage.

3. 3. The membrane treatment method for a liquid to be treated according to claim 2, wherein the first concentration step comprises a step of combining a remainder of the recovery liquid, which is combined with the recovery liquid supplied to the semipermeable membrane unit in the second concentration step, with the liquid to be treated to produce the concentrated liquid.

4. a first concentration step in which the liquid to be treated is pressurized and passed through a reverse osmosis membrane to produce a concentrated liquid that is concentrated without permeating the reverse osmosis membrane; A second concentration step of further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane, The second concentration step includes a step of contacting the concentrated liquid with the recovered liquid through the semipermeable membrane in a plurality of semipermeable membrane units, supplying the concentrated liquid to the semipermeable membrane units in series in each stage, and supplying the recovered liquid to the semipermeable membrane units in each stage such that at least two of the semipermeable membrane units are in parallel, At least a part of the concentrated liquid concentrated in the second concentration step is used as the recovered liquid; The second concentration step is a membrane treatment method for a liquid to be treated, comprising a step of individually merging a portion of the recovered liquid discharged from the semipermeable membrane units of each stage after the recovery liquid is merged with the recovered liquid to be supplied to the semipermeable membrane unit of each stage.

5. a first concentrating device having an RO membrane unit for generating a concentrated liquid by increasing the pressure of the liquid to be treated and passing the liquid through a reverse osmosis membrane without permeating the reverse osmosis membrane; a second concentrating device for further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane; The second concentrating device is configured to cause contact between the concentrated liquid and the recovered liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied to the semipermeable membrane units of each stage in series, and the recovered liquid is supplied to the semipermeable membrane units of each stage such that at least two of the semipermeable membrane units are in parallel, At least a portion of the concentrated liquid concentrated in the second concentrator is used as the recovered liquid; The second concentrator is a membrane treatment device for a liquid to be treated, which supplies the recovered liquid to the semipermeable membrane units in each stage to which the recovered liquid is supplied in parallel, without allowing the recovered liquid to pass through the semipermeable membrane units in the other stages.

6. a first concentrating device having an RO membrane unit for generating a concentrated liquid by increasing the pressure of the liquid to be treated and passing the liquid through a reverse osmosis membrane without permeating the reverse osmosis membrane; a second concentrating device for further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane; The second concentrating device is configured to cause contact between the concentrated liquid and the recovered liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied to the semipermeable membrane units of each stage in series, and the recovered liquid is supplied to the semipermeable membrane units of each stage such that at least two of the semipermeable membrane units are in parallel, At least a portion of the concentrated liquid concentrated in the second concentrator is used as the recovered liquid; The second concentrator is a membrane treatment device for a liquid to be treated that merges a portion of the recovered liquid discharged from at least any of the semipermeable membrane units in each stage with the recovered liquid supplied to at least any of the semipermeable membrane units in each stage.

7. a first concentrating device having an RO membrane unit for generating a concentrated liquid by increasing the pressure of the liquid to be treated and passing the liquid through a reverse osmosis membrane without permeating the reverse osmosis membrane; a second concentrating device for further concentrating the produced concentrated liquid by contacting the concentrated liquid with a recovery liquid having a lower pressure than the concentrated liquid through a semipermeable membrane; The second concentrating device is configured to cause contact between the concentrated liquid and the recovered liquid via the semipermeable membrane in a plurality of stages of semipermeable membrane units, the concentrated liquid is supplied to the semipermeable membrane units of each stage in series, and the recovered liquid is supplied to the semipermeable membrane units of each stage such that at least two of the semipermeable membrane units are in parallel, At least a portion of the concentrated liquid concentrated in the second concentrator is used as the recovered liquid; The second concentrator is a membrane treatment device for a liquid to be treated, which individually merges a portion of the recovered liquid discharged from the semipermeable membrane units of each stage into the recovered liquid to be supplied to the semipermeable membrane unit of each stage.

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