Sequential circulation process reverse osmosis desalination plant

Abstract

The present invention relates to a sequential circulation reverse osmosis desalination apparatus, and more particularly to a sequential circulation reverse osmosis desalination apparatus that employs closed-circuit reverse osmosis technology to ensure a fundamentally high recovery rate, and sequentially applies reverse reverse osmosis technology to reduce fouling, while performing highly efficient water treatment without a separate energy recovery device.

Description

[Technical field]

[0001] The present invention relates to a sequential circulation reverse osmosis desalination apparatus, and more particularly to a sequential circulation reverse osmosis desalination apparatus that employs closed-circuit reverse osmosis technology to ensure a fundamentally high recovery rate, and sequentially applies reverse reverse osmosis technology to reduce fouling, while performing highly efficient water treatment without a separate energy recovery device. [Background technology]

[0002] Water shortages refer to the difficulty in obtaining fresh water due to resource depletion, and water shortages are becoming more severe worldwide due to industrial advancement and abnormal weather phenomena.

[0003] However, water is an essential element in human life and in various industrial sectors, and the demand for water is constantly increasing. As one effective method for meeting this demand, the so-called desalination method, which involves the large-scale desalinization of seawater, has been proposed.

[0004] Seawater desalination refers to a series of water treatment processes in which dissolved substances, including salt, are removed from seawater that is difficult to directly use for domestic or industrial purposes, to obtain high-purity drinking water, domestic water, industrial water, and the like.

[0005] Currently, seawater desalination facilities use either the Multiple-Stage Flash Distillation process (MSF) or the reverse osmosis process. Of these, the reverse osmosis process applies pressure greater than the osmotic pressure to raw water or salt water to remove solutes such as ions and organic molecules, and move the pure water through a semi-permeable membrane to produce fresh water.

[0006] Meanwhile, there is a seawater desalination method called Closed-circuit Reverse Osmosis (CCRO) that is different from the general reverse osmosis process in that it mixes the concentrated water discharged from the reverse osmosis equipment with raw water or salt water and injects it back into the reverse osmosis equipment to achieve the target recovery rate. When the target concentration is reached, the concentrated water is discharged and the operation is continued.

[0007] In this regard, Korean Patent Registration No. 10-1052662 discloses a feature in which a plurality of closed circuit desalination apparatus (CCRO) are connected in parallel as an apparatus for continuous batch sequential desalination in a closed circuit of a saltwater solution by reverse osmosis.

[0008] However, with existing closed-circuit desalination equipment (CCRO) that uses reverse osmosis, the time during which the pressure of the concentrated water discharged from the reverse osmosis equipment increases decreases as the process progresses, which shortens the cleaning or replacement cycle of the reverse osmosis equipment. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been proposed to solve the above problems, and aims to provide a sequential circulation reverse osmosis desalination apparatus that employs closed-circuit reverse osmosis technology to basically guarantee a high recovery rate, sequentially applies reverse reverse osmosis technology to reduce fouling, and performs highly efficient water treatment without a separate energy recovery device.

[0010] Other objects of the present invention will become more apparent from the following examples. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the present invention proposes, as one embodiment, a sequential circulation process reverse osmosis desalination apparatus in which a plurality of forward reverse osmosis modules (forward RO modules) are connected in parallel with at least one reverse reverse osmosis module (backward RO module), and the concentrate discharged from the forward reverse osmosis modules flows into the reverse reverse osmosis modules, and the concentrate discharged from the reverse reverse osmosis modules flows into each of the forward reverse osmosis modules.

[0012] In one embodiment, the reverse osmosis desalination apparatus includes a feed pump for supplying raw water, a first pump connected to an output end of the feed pump, a second pump connected to an output end of the forward reverse osmosis module, a tertiary pump connected to an output end of the first pump, and a controller for controlling the feed pump and the first pump so that raw water is input to the forward reverse osmosis module in a first operation step, and for controlling the second pump so that concentrated water discharged from the forward reverse osmosis module is input to the reverse reverse osmosis module in a second operation step, and the tertiary pump inputs the concentrated water discharged from the reverse reverse osmosis module to the forward reverse osmosis module after it is mixed with the raw water in a third operation step.

[0013] A high pressure pump may be used as the primary pump and the secondary pump, and a jet pump may be used as the tertiary pump, which accelerates the raw water in the three stages of operation using a flow of concentrated water.

[0014] In addition, a 3-way control valve is arranged at the input and output ends of the forward reverse osmosis module and the reverse reverse osmosis module, respectively, and the controller controls the 3-way valve so that the reverse reverse osmosis module switches to the forward reverse osmosis module when a preset first condition is satisfied in the 4th step of operation, and continues to supply raw water to the input ends of all the reverse osmosis modules while discharging concentrated water from the output ends of all the reverse osmosis modules to the outside until a preset second condition is satisfied.

[0015] The first condition may be that at least one of a concentration sensor, a flow sensor, and a pressure sensor arranged in the concentrated water flow path reaches a preset standard, or that a preset time has elapsed after the three stages of operation.

[0016] Moreover, the second condition may be that the concentration of the concentrated water reaches a preset initial concentration.

[0017] The controller may execute a fifth step of operation (5th step) to control the three-way valve so that after one batch process from the first step to the fourth step is completed, at least one of the forward reverse osmosis modules is switched to a reverse reverse osmosis module and the existing reverse reverse osmosis module is switched to a forward reverse osmosis module.

[0018] In addition, the controller can execute the five stages of operation after executing the one batch process multiple times. Effect of the Invention

[0019] According to an embodiment of the present invention, before the closed circuit reverse osmosis (CCRO) process in which the concentrate and raw water are mixed, the concentrate discharged from the forward reverse osmosis module flows into the backward reverse osmosis module and is circulated in a more concentrated state once again, thereby significantly reducing energy consumption and chemical usage compared to the existing multi-stage reverse osmosis process.

[0020] In addition, according to an embodiment of the present invention, the injection direction of raw water is periodically reversed to slow down the rate at which biofouling and scale occur in the reverse osmosis module and remove some of the scale, thereby shortening the reduction in the time it takes for the concentrate pressure to increase and extending the CIP (Clean in Place) cycle.

[0021] The effects of the present invention are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art from the following description. [Brief description of the drawings]

[0022] [Figure 1] FIG. 2 is a schematic diagram of a sequential circulation reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0023] [Diagram 2] FIG. 1 is a block diagram showing the configuration of a sequential circulation reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0024] [Diagram 3] 2 is a flow chart showing the operation steps of a sequential circulation process reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0025] [Figure 4] FIG. 2 is a schematic diagram showing four stages of operation of a sequential circulation reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0026] [Diagram 5]FIG. 2 is a schematic diagram showing five stages of operation of a sequential circulation reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0027] [Figure 6] 1 is a graph showing the reduction in pressure increase time each time one batch process is repeated from operation stage 1 to operation stage 4 of a reverse osmosis desalination apparatus with a sequential circulation process according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention can be modified in various ways and can have various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description, but it is not intended to limit the present invention to the specific embodiments, and it should be understood that the present invention includes all modifications, equivalents, or alternatives within the spirit and technical scope of the present invention.

[0029] In the description of the present invention, if it is determined that a detailed description of related publicly known techniques may obscure the gist of the present invention, the detailed description will be omitted.

[0030] The terms used in this application are merely for describing certain embodiments and are not intended to limit the present invention. The singular term includes the plural term unless otherwise specified in the context. In this application, the terms "comprise" or "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood not to preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0031] Unless otherwise specified, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention pertains. Terms as defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning they have in the context of the relevant art, and should not be interpreted as having an ideal or overly formal meaning unless expressly defined in this application.

[0032] The term "module" in this specification means a unit that processes a specific function or operation, and can mean hardware, software, or a combination of hardware and software.

[0033] Furthermore, terms such as first, second, etc. may be used to describe various components, but these components should not be limited by these terms. These terms are used only to distinguish one component from another component.

[0034] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. When describing the embodiments with reference to the accompanying drawings, the same reference numerals will be used to refer to the same or corresponding elements regardless of the drawing reference numerals, and duplicate descriptions thereof will be omitted.

[0035] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A sequential circulation reverse osmosis desalination apparatus 1 according to an embodiment of the present invention will be described in detail below with reference to the drawings.

[0036] In the following description of the sequential circulation process reverse osmosis desalination apparatus 1 according to an embodiment of the present invention, the term "connected" should be interpreted to mean either directly connected to the reverse osmosis module or connected via components such as a pump, valve, controller, etc.

[0037] FIG. 1 is a schematic diagram of a sequential circulation process reverse osmosis desalination apparatus according to one embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of a sequential circulation process reverse osmosis desalination apparatus according to an embodiment of the present invention, and FIG. 3 is a flowchart showing the operating steps of the sequential circulation process reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0038] As shown in Figures 1 and 2, a sequential circulation process reverse osmosis desalination apparatus 1 according to one embodiment includes a plurality of reverse osmosis modules (RO modules) 10, 20, and 30, a feed pump FP, a first pump P1, a second pump P2, a tertiary pump P3, a plurality of 3-way valves, a concentrated water discharge valve (Brain valve) DV, and a controller C.

[0039] The reverse osmosis modules 10 , 20 , 30 include a plurality of forward reverse osmosis modules (forward RO modules) 10 , 20 and at least one backward reverse osmosis module (backward RO module) 30 .

[0040] In this specification, an embodiment including two forward reverse osmosis modules and one reverse reverse osmosis module is shown, but the present invention is not limited to this, and the reverse osmosis module may be configured with two or four or more modules depending on the volume of salt water to be treated. As a specific example, the reverse osmosis module may be configured with one forward reverse osmosis module and one reverse reverse osmosis module, or may be configured with three forward reverse osmosis modules and one reverse reverse osmosis module, or may be configured with three forward reverse osmosis modules and two reverse reverse osmosis modules.

[0041] For ease of explanation, the reverse osmosis modules (Ro modules) 10, 20, 30 will be referred to as a first reverse osmosis module 10, a second reverse osmosis module 20, and a third reverse osmosis module 30, respectively.

[0042] The first reverse osmosis module 10, the second reverse osmosis module 20, and the third reverse osmosis module 30 are connected in parallel to each other, and the concentrated water (Brine, B) discharged from the first reverse osmosis module 10 and the second reverse osmosis module 20 flows into the third reverse osmosis module 30, and the concentrated water B discharged from the third reverse osmosis module 30 flows again into the first reverse osmosis module 10 and the second reverse osmosis module 20, respectively, forming a circulation circuit.

[0043] The feed pump FP is installed to supply raw water R to the reverse osmosis module, and is capable of transporting the raw water R at a pressure of, for example, about 3 bar(g).

[0044] The primary pump P1 is connected to the output end of the feed pump FP, and is started when the flow rate of the raw water R supplied from the feed pump FP exceeds a preset standard.

[0045] The secondary pump P2 is connected to the output ends of the first reverse osmosis module 10 and the second reverse osmosis module 20, and moves the concentrated water B discharged from the first reverse osmosis module 10 and the second reverse osmosis module 20 to the input end of the third reverse osmosis module 30.

[0046] High pressure pumps may be used as the primary pump P1 and the secondary pump P2.

[0047] The concentrated water B discharged from the third reverse osmosis module 30 is merged with the raw water R supplied from the feed pump FP and flows into the tertiary pump P3. The tertiary pump P3 transfers the merged water to the input ends of the first reverse osmosis module 10 and the second reverse osmosis module 20.

[0048] As the tertiary pump P3, a jet pump that accelerates the flow of the raw water R by using the flow of the concentrated water B may be used.

[0049] That is, since the pressure of concentrated water B is higher than that of raw water R, a pressure drop must be added to smoothly merge and transfer raw water R and concentrated water B, but this requires an energy loss. Therefore, by using a jet pump as the tertiary pump P3, raw water R can be accelerated using the flow of concentrated water B without an additional pressure drop.

[0050] Three-way control valves V11, V12, V21, V22, V31, and V32 (collectively referred to as V) are arranged at the input and output ends of the first reverse osmosis module 10, the second reverse osmosis module 20, and the third reverse osmosis module 30, respectively, and are opened and closed in the horizontal flow direction or the vertical flow direction under the control of the controller C.

[0051] The concentrated water drain valve (Drain Valve) DV is disposed on the piping where the output terminals of the first reverse osmosis module 10, the second reverse osmosis module 20, and the third reverse osmosis module 30 join, and is opened and closed under the control of the controller C when the discharged water B meets predetermined conditions.

[0052] The controller C controls the start and operation level of each of the pumps FP, P1, and P2 according to the operation stages (S10 to S50) described later, and controls the opening and closing of the three-way valve V and the concentrated water discharge valve DV.

[0053] The operation steps controlled by the controller C include an operation step 1 (1st step) (S10) for supplying raw water R, an operation step 2 (2nd step) (S20) for supplying concentrated water B, an operation step 3 (3rd step) (S30) for circulating concentrated water B, an operation step 4 (4th step) (S40) for discharging concentrated water B, and an operation step 5 (5th step) (S50) for changing the operating direction of each reverse osmosis module.

[0054] Here, it is assumed that the first reverse osmosis module 10 and the second reverse osmosis module 20 are set in a forward direction, and the third reverse osmosis module 30 is set in a reverse direction, and each operation step will be described in detail.

[0055] In operation step 1 (S10), the controller C controls the three-way valves V11, V12, V21, and V22 arranged at the input and output ends of the first reverse osmosis module 10 and the second reverse osmosis module 20, respectively, to open in the horizontal flow direction, and controls the three-way valves V31 and V32 arranged at the input and output ends of the third reverse osmosis module 30 to open in the vertical flow direction.

[0056] Also, in the first operation step (S10), the controller C controls the feed pump FP and the primary pump P1 so that raw water R is input to the first reverse osmosis module 10 and the second reverse osmosis module 20. Specifically, the controller C drives the feed pump FP to supply raw water, and starts the primary pump P1 when the flow rate of the raw water R supplied from the feed pump FP exceeds a preset standard. At this time, the controller C controls the operation frequency Hz of the primary pump P1 based on a signal from a flow rate sensor (not shown) arranged in the supply line of the raw water R.

[0057] In the second operation step (S20), the controller C controls the secondary pump P2 so that the concentrate B discharged from the first reverse osmosis module 10 and the second reverse osmosis module 20 is input to the third reverse osmosis module 30. Specifically, the controller C starts the secondary pump P2 when the flow rate of the concentrate B exceeds a preset standard, and controls the operation Hz of the secondary pump P2 based on a signal from a flow rate sensor (not shown) disposed in the supply line of the concentrate B. Meanwhile, the fresh water permeated through the first reverse osmosis module 10 and the second reverse osmosis module 20 is discharged through a pipe.

[0058] In the third operation step (S30), the concentrated water B discharged from the third reverse osmosis module 30 flows into the tertiary pump P3 via a three-way valve V31 disposed at the output end of the third reverse osmosis module 30. Raw water R supplied by the feed pump FP and the primary pump P1 also flows into the tertiary pump P3 and merges with the discharged concentrated water B. Meanwhile, fresh water permeated through the third reverse osmosis module 10 is also discharged through a pipe.

[0059] The above-mentioned operation step 1 (S10) to operation step 3 (S30) are repeatedly performed until a predetermined sensor S detects that any one of the concentration, pressure, flow rate, and scale amount of the concentrated water B exceeds a preset threshold. Therefore, the predetermined sensor S may be any one of a concentration sensor, a flow sensor, a pressure sensor, and a scale sensor, and at least one of these sensors is disposed in the movement path of the concentrated water B.

[0060] The fourth operation stage (S40) is executed when any one of the concentration, pressure, flow rate, and amount of scale of the concentrated water B exceeds a preset threshold. Figure 4 is a schematic diagram showing four operation stages of a sequential circulation process reverse osmosis desalination apparatus according to one embodiment.

[0061] In the fourth operation stage (S40), when a first preset condition is met, the controller C controls the three-way valves V31, V32 so that the third reverse osmosis module 30 switches to a forward reverse osmosis module, and continues to supply raw water R to the input terminals of all the reverse osmosis modules 10, 20, 30 until a second preset condition is met, while discharging concentrated water B from the output terminals of all the reverse osmosis modules 10, 20, 30 to the outside.

[0062] Specifically, when the measured value of the sensor S reaches a preset threshold, the controller C opens all of the three-way valves V11 to V32 arranged at the input and output ends of all of the reverse osmosis modules 10, 20, 30 in the horizontal flow direction and opens the concentrated water discharge valve DV that was closed. At this time, the secondary pump P2 may stop operating, operate at the minimum Hz, or switch to operate in the opposite direction.

[0063] In operation stage 4 (S40), the first condition means that the measurement value of the sensor S arranged in the movement path of the concentrated water B reaches a predetermined standard (e.g., at least one threshold of concentration, salinity, flow rate, pressure, or scale amount) or a predetermined time has elapsed after operation stage 3 (S30).

[0064] In addition, in the fourth operation stage (S40), the second condition means that the concentration of the concentrated water B reaches a preset initial concentration.

[0065] The concentrated water discharge valve DV is in a blocked state in operation stages 1 (S10) to 3 (S30), but opens to discharge concentrated water B when the first condition is satisfied in operation stage 4 (S40), and then switches back to the blocked state when the second condition is satisfied.

[0066] The above-described operation stage 1 (S10) to operation stage 4 (S40) can be defined as one batch process, and is abbreviated to "one batch."

[0067] In one embodiment, the sequential circulation process reverse osmosis desalination apparatus resets the operational direction of the reverse osmosis modules after each batch is completed.

[0068] For example, if the first reverse osmosis pressure module 10, the second reverse osmosis pressure module 20, and the third reverse osmosis pressure module 30 are operated in the forward direction, forward direction, and reverse direction, respectively, in the first batch, the first reverse osmosis pressure module 10, the second reverse osmosis pressure module 20, and the third reverse osmosis pressure module 30 are operated in the forward direction, reverse direction, and forward direction, respectively, in the third batch, the first reverse osmosis pressure module 10, the second reverse osmosis pressure module 20, and the third reverse osmosis pressure module 30 are operated in the reverse direction, forward direction, and forward direction, respectively, and in the fourth batch, the first reverse osmosis pressure module 10, the second reverse osmosis pressure module 20, and the third reverse osmosis pressure module 30 may be operated in the forward direction, forward direction, and reverse direction, respectively, in the same manner as in the first batch.

[0069] However, this is merely an example, and the number of forward and reverse flow modules may vary depending on the number of reverse osmosis modules, and the order in which the operation directions are reset may also be modified in various ways.

[0070] That is, the controller C can determine in real time which reverse osmosis modules are to be operated in the reverse direction based on the measurement values ​​of the sensors S for each reverse osmosis module, and determine the operation direction of each reverse osmosis module based on the results of that determination.

[0071] For example, if the pressure of concentrated water B for each reverse osmosis pressure module is measured and the pressure of a specific reverse osmosis pressure module is found to be high for an unforeseen reason, then that specific reverse osmosis pressure module needs to be operated in the reverse direction to intentionally remove fouling or scale. Therefore, the schedule can be changed to operate the specific reverse osmosis pressure module in the reverse direction even though it is the time to operate other reverse osmosis pressure modules in the reverse direction according to a pre-set schedule.

[0072] The fifth step of operation (5th step) is a process of resetting the operating direction of the reverse osmosis module after one batch from the first operation step (S10) to the fourth operation step (S40) is completed, and FIG. 5 is a schematic diagram showing a reverse osmosis desalination apparatus of a sequential circulation process that performs the five operation steps.

[0073] In the fifth operation step (S50), the controller C controls the associated three-way valves (at least four of V11 to V32) so that one of the first reverse osmosis module 10 and the second reverse osmosis module 20, which were previously operated in the forward direction, is switched to a reverse reverse osmosis module, and the third reverse osmosis module 30, which was previously operated in the reverse direction, is switched to a forward reverse osmosis module.

[0074] When raw water R is injected into a reverse osmosis module, the concentration increases toward the rear, increasing the probability of scale formation, and as the amount of scale increases, the overall flux decreases and performance declines. Therefore, by periodically reversing the injection direction of raw water R, some of the scale can be removed by the injection pressure of raw water R, while slowing down the rate of biofouling and scale formation within the reverse osmosis module.

[0075] On the other hand, the controller C can execute the fifth operation step (S50) immediately after one batch is completed, or can execute the fifth operation step (S50) only after one batch is repeated multiple times and preset conditions are met.

[0076] FIG. 6 is a graph showing the reduction in pressure increase time each time one batch process is repeated from operation stage 1 to operation stage 4 of a sequential circulation reverse osmosis desalination apparatus according to an embodiment of the present invention.

[0077] Referring to FIG. 6, in a typical closed circuit reverse osmosis (CCRO), the time during which the pressure of the concentrated water B discharged while circulating through the reverse osmosis module in one batch process increases decreases with each batch. In the present invention (Circle-SEQ RO), however, before the closed circuit reverse osmosis (CCRO) process in which the concentrated water B and raw water R are mixed, the concentrated water B discharged from the forward reverse osmosis module flows into the backward reverse osmosis module, and the direction of the raw water injection is periodically reversed, thereby slowing down the rate at which biofouling and scale occur in the reverse osmosis module and removing some of the scale, thereby shortening the decrease in the time during which the concentrated water pressure increases and lengthening the CIP (Clean in Place) cycle.

[0078] Although the present invention has been described above with reference to several embodiments thereof, it will be understood by those skilled in the art that various modifications and variations of the present invention can be made without departing from the spirit and scope of the present invention as set forth in the following claims.

Claims

1. A sequential circulation process reverse osmosis desalination apparatus, in which a plurality of forward reverse osmosis modules are connected in parallel to at least one backward reverse osmosis module, and a concentrate discharged from the forward reverse osmosis module flows into the reverse reverse osmosis module, and a concentrate discharged from the reverse reverse osmosis module flows into each of the forward reverse osmosis modules.

2. A feed pump for supplying raw water; a primary pump connected to an output end of the feed pump; a second pump connected to an output end of the forward reverse osmosis module; a tertiary pump connected to an output end of the primary pump; a controller for controlling the feed pump and the primary pump so that raw water is input to the forward reverse osmosis module in a first operation step (1st step), and for controlling the secondary pump so that concentrated water discharged from the forward reverse osmosis module is input to the reverse reverse osmosis module in a second operation step (2nd step); 2. The reverse osmosis desalination apparatus according to claim 1, wherein the tertiary pump is driven without a separate control when the concentrate discharged from the reverse reverse osmosis module is mixed with the raw water and then input to the forward reverse osmosis module in the third operation step.

3. 3. The reverse osmosis desalination apparatus according to claim 2, wherein the primary pump and the secondary pump are high pressure pumps, and the tertiary pump is a jet pump that accelerates the raw water in the third operation stage using a flow of concentrated water.

4. A 3-way control valve is disposed at the input end and the output end of the forward reverse osmosis module and the reverse reverse osmosis module, respectively; The controller:

4. The reverse osmosis pressure desalination apparatus according to claim 3, wherein, in a fourth operation step, when a first preset condition is satisfied, the three-way valve is controlled so that the reverse reverse osmosis module is switched to a forward reverse osmosis module, and raw water is continuously supplied to the input terminals of all the reverse osmosis modules while concentrated water is discharged from the output terminals of all the reverse osmosis modules to the outside, until a second preset condition is satisfied.

5. 5. The reverse osmosis desalination apparatus according to claim 4, wherein the first condition is that at least one measurement value of a concentration sensor, a flow sensor, or a pressure sensor arranged in a flow path of the concentrated water reaches a preset standard, or that a preset time has elapsed after the three stages of operation.

6. 6. The sequential circulation reverse osmosis desalination apparatus according to claim 5, wherein the second condition is that the concentration of the concentrate reaches a preset initial concentration.

7. The controller:

6. The reverse osmosis desalination apparatus according to claim 5, wherein after one batch process from the operation step 1 to the operation step 4 is completed, a fifth step of operation is performed in which at least one of the forward reverse osmosis modules is switched to a reverse reverse osmosis module and the three-way valve is controlled so that an existing reverse reverse osmosis module is switched to a forward reverse osmosis module.

8. The controller:

8. The reverse osmosis desalination apparatus according to claim 7, wherein the five operation steps are carried out after the one batch process is carried out a plurality of times.

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