KR20240135346A - Operation method for reserve osmosis seawater desalination system that can reduce fouling of reserve osmosis membrane through nanobubble-injected concentrated water cleaning - Google Patents

Abstract

According to one aspect of the present invention, a method for operating a reverse osmosis seawater desalination system may be provided, including an operation step of operating a reverse osmosis seawater desalination system for a preset operation time to extract fresh water from seawater; a pause step of stopping the operation of the reverse osmosis seawater desalination system for a preset pause time after the operation step; and a cleaning step of supplying concentrated water injected with nanobubbles to a reverse osmosis membrane of the seawater desalination system after the lapse of the operation time to clean the reverse osmosis membrane.

Description

Operation method for reserve osmosis seawater desalination system that can reduce fouling of reserve osmosis membrane through nanobubble-injected concentrated water cleaning

The present invention relates to a method for operating a seawater desalination system, and more specifically, to a method for operating a reverse osmosis seawater desalination system capable of effectively reducing fouling of a reverse osmosis membrane by cleaning concentrated water injected with nanobubbles.

Water is not only an essential element for sustaining human life, but also accessibility to water has a significant impact on the quality of human life. In Korea, the average daily water usage per person increased by about 10% from 274 L in 2009 to 302 L in 2021, and the trend of increasing water usage in Korea and around the world is expected to accelerate further. This trend of increasing water usage is understood to be due to the nuclear family and improved living standards, and is expected to accelerate further due to the strengthening of personal hygiene resulting from the recent outbreak of the novel coronavirus infection (COVID-19).

While the amount of water required for human life is gradually increasing, the shortage of water for human life (fresh water) is gradually worsening due to factors such as environmental pollution and climate change caused by industrialization. Academic circles, research institutes, and governments around the world are actively conducting multifaceted research to solve this fresh water shortage.

In particular, while in urban and semi-urban areas, access to freshwater can be improved to a certain level by expanding infrastructure such as water supply and sewage systems, in island and mountainous areas, there are geographical, environmental, and economic difficulties in expanding infrastructure such as water supply and sewage systems to improve access to freshwater. Therefore, research and development on effective measures to improve access to freshwater in island and mountainous areas is necessary.

Korean Patent Publication No. 10-2116458 (Registered on May 22, 2020)

One aspect of the present invention is to provide a method for operating a reverse osmosis seawater desalination system that is environmentally friendly, economical, and effective by cleaning a reverse osmosis membrane using concentrated water without using chemicals.

Another aspect of the present invention is to provide a method for operating a reverse osmosis seawater desalination system capable of effectively reducing fouling of a reverse osmosis membrane by cleaning the reverse osmosis membrane using the concentrated water generated during the seawater desalination process during the downtime that inevitably occurs due to the characteristics of a marine mobile seawater desalination system.

Another aspect of the present invention is to provide a method for operating a reverse osmosis seawater desalination system capable of effectively reducing fouling of a reverse osmosis membrane by cleaning the reverse osmosis membrane using concentrated water injected with nanobubbles.

The subject matter of the present invention is not limited to the above-described content. Those skilled in the art to which the present invention pertains will not have difficulty easily understanding additional subjects of the present invention from the overall content of this specification.

A method for operating a reverse osmosis seawater desalination system according to one aspect of the present invention comprises: an operation step of operating the reverse osmosis seawater desalination system for a preset operation time to extract fresh water from seawater; a pause step of stopping the operation of the reverse osmosis seawater desalination system for a preset pause time after the operation step; and a cleaning step of supplying concentrated water to a reverse osmosis membrane of the seawater desalination system after the operation time has elapsed to clean the reverse osmosis membrane.

The above cleaning step may include a nanobubble injection step of injecting nanobubbles into a concentrated water tank in which the concentrated water is used; and a concentrated water supply step of supplying the concentrated water into which the nanobubbles have been injected to the reverse osmosis membrane after the injection of the nanobubbles is completed.

The above nanobubbles can be injected into the concentrated water at an injection rate of 0.3 L/min or more.

The above nanobubble-injected concentrated water may be a nanobubble-saturated concentrated water.

The above driving time is 4 to 12 hours, the above rest time is 8 to 24 hours, and the above driving time and the above rest time can be continuously and cross-repeatedly repeated.

Cleaning of the above reverse osmosis membrane can be performed immediately after the above operating time has elapsed.

In the above cleaning step, the reverse osmosis membrane can be cleaned by supplying the concentrated water for a cleaning time of 50 to 100 minutes.

In the above cleaning step, the reverse osmosis membrane can be cleaned by supplying the concentrated water at a cleaning flow rate of 2.0 L/min to 4.0 L/min.

In the above operation step, the fresh water can be extracted by applying a reverse osmosis pressure of 40 bar to 80 bar to the seawater.

The above reverse osmosis seawater desalination system can be provided in a mobile form that can move on the sea.

According to one aspect of the present invention, a method for operating a reverse osmosis seawater desalination system that is environmentally friendly, economical, and effective can be provided by cleaning a reverse osmosis membrane using concentrated water without using chemicals.

According to another aspect of the present invention, a method for operating a reverse osmosis seawater desalination system can be provided in which fouling of a reverse osmosis membrane is effectively reduced by cleaning the reverse osmosis membrane using the concentrated water generated during the seawater desalination process during a downtime that inevitably occurs due to the characteristics of a marine mobile seawater desalination system.

According to another aspect of the present invention, a method for operating a reverse osmosis seawater desalination system can be provided that effectively reduces fouling of a reverse osmosis membrane by cleaning the reverse osmosis membrane using concentrated water injected with nanobubbles.

The effects of the present invention are not limited to the matters described above, and may include matters that can be reasonably inferred from the following description by a person having ordinary knowledge in the technical field to which the present invention belongs.

Figure 1 is a drawing showing the production amount (Flux) per unit time measured for each NaCl concentration under continuous operation and intermittent operation conditions of Example 1.
Figure 2 is a drawing showing the production quantity (Flux) per unit time measured for each HA concentration under continuous operation and intermittent operation conditions of Example 2.
Figure 3 is a diagram showing the production quantity (Flux) measured per unit time under the intermittent operation conditions of Example 2 converted into a normalized production quantity (Normalized Flux) compared to the initial production quantity (Flux).
Figure 4 is a drawing showing the production quantity (Flux) measured per unit time for each cleaning time during intermittent operation of Example 3 converted into a normalized production quantity (Normalized Flux) compared to the initial production quantity (Flux).
Figure 5 is a drawing showing the production quantity (Flux) measured per unit time for each cleaning liquid and cleaning time during intermittent operation of Example 4 converted into a normalized production quantity (Normalized Flux) compared to the initial production quantity (Flux).
Figure 6 is a drawing showing the production quantity (Flux) per unit time measured at each cleaning flow rate during intermittent operation in Example 5.
Figure 7 is a diagram showing the measured production quantity (Flux) per unit time at each cleaning flow rate under the intermittent operation conditions of Example 5 converted into a normalized production quantity (Normalized Flux) compared to the initial production quantity (Flux).
Figure 8 is a drawing showing the measured production quantity per unit time (Flux) under the intermittent operation and continuous operation conditions of Example 6.
Figure 9 is a diagram showing the measured production amount (Flux) per unit time in simple intermittent operation, intermittent operation including concentrated water cleaning, and intermittent operation including concentrated water cleaning with nanobubbles injected in Example 7.

Hereinafter, preferred embodiments of a method for reducing fouling of a reverse osmosis membrane for desalination of seawater using concentrated water injected with nanobubbles according to one aspect of the present invention will be described in more detail with reference to the attached drawings. The embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to explain the present invention in more detail to those skilled in the art to which the present invention pertains. Therefore, the shape of each element shown in the drawings may be emphasized or exaggerated for a clearer explanation.

In many countries around the world, research is being conducted in various fields to solve the freshwater shortage, and among these, research on seawater desalination technology to realistically solve the freshwater shortage is actively being conducted. In particular, in the case of islands and mountainous areas, the freshwater shortage is serious due to geographical, environmental, and economic factors, but to date, no seawater desalination technology has been presented that can effectively solve the freshwater shortage in islands and mountainous areas.

Seawater desalination technology can be broadly categorized into distillation, freezing, and membrane filtration methods based on their basic principles. Distillation is a method of extracting fresh water from heated seawater using a heat source, freezing is a method of extracting fresh water by cooling seawater, and membrane filtration is a method of separating and extracting fresh water using a membrane as a filter medium.

Distillation is one of the seawater desalination methods proposed from the early stage of research, and it is advantageous for mass production and has high purity of produced water, but there are problems that a large amount of energy is consumed and a large amount of hot water is discharged. The freezing method has less corrosion problem and the material cost required for plant facilities is relatively low, but there are problems that the purity of produced water is low and a large amount of energy is consumed. In particular, distillation and freezing are seawater desalination methods that require large-scale facilities and are not suitable for seawater desalination plants in islands and mountainous areas with many geographical and environmental obstacles. On the other hand, membrane filtration is one of the seawater desalination methods that has been proposed relatively recently, and it is receiving attention as a method that can solve the freshwater shortage in islands and mountainous areas because the quality of produced water is good, energy consumption is low, and plants can be built with small facilities. As technology advances, membrane materials have diversified, the degree of freedom in the size of pores that can be formed in the membrane has increased, optimal operation methods have been introduced, and treatment facilities have become more compact, so membrane filtration is receiving significant attention as a global ESG (Environmental Social and Governance) technology. Membrane filtration seawater desalination plants that can be operated in islands and mountainous areas can be operated in various ways, and can be operated by being classified as a fixed plant that is fixedly installed and operated in the area, or a mobile plant that is moved and operated by a small vessel, such as in Patent Document 1.

Membrane filtration is a process that uses a membrane to separate substances. When seawater passes through the membrane, substances in the seawater that are larger than the pores of the membrane cannot pass through the membrane and may be adsorbed or attached to the surface of the membrane. Since seawater contains suspended solids such as fine particles, organic and inorganic pollutants, and microorganisms, fouling, which reduces the membrane treatment capacity due to suspended solids adsorbed or attached to the surface of the membrane, inevitably occurs during the seawater desalination process using a membrane. Membrane filtration seawater desalination plants operated in islands and mountainous areas also need to effectively reduce the fouling phenomenon to secure economic feasibility, productivity, and environmental friendliness. The inventor of the present invention has conducted in-depth research on a method for effectively reducing the fouling phenomenon of small-scale membrane filtration seawater desalination plants operated in islands and mountainous areas, and in particular, a method for effectively reducing the fouling phenomenon of mobile seawater desalination plants, and as a result, has derived the present invention.

Hereinafter, a method for operating a reverse osmosis seawater desalination system capable of reducing fouling of a reverse osmosis membrane through concentrated water cleaning according to one aspect of the present invention will be described in more detail.

The seawater desalination system used in the present invention may be a reverse osmosis seawater desalination system that extracts fresh water from seawater using a reverse osmosis membrane, and may be interpreted as a concept that includes a conventional reverse osmosis seawater desalination system used for seawater desalination. As a non-limiting example, in the reverse osmosis seawater desalination system according to one aspect of the present invention, seawater, which is an inlet raw water, may pass through the membrane by applying a reverse osmosis pressure of 40 bar to 80 bar. The seawater desalination system used in the present invention may be a fixed seawater desalination system with a plant facility installed on land, but a mobile seawater desalination system in which the plant facility moves on the sea and is operated by a ship or the like may be more suitable.

A method for operating a reverse osmosis seawater desalination system according to one aspect of the present invention comprises: an operation step of operating the reverse osmosis seawater desalination system for a preset operation time to extract fresh water from seawater; a pause step of stopping the operation of the reverse osmosis seawater desalination system for a preset pause time after the operation step; and a cleaning step of supplying concentrated water to a reverse osmosis membrane of the seawater desalination system after the operation time has elapsed to clean the reverse osmosis membrane.

According to the experimental results of the inventors of the present invention, when comparing the production quantity (flux) per unit time during intermittent operation in which fresh water is intermittently extracted by repeating operation and stop of a reverse osmosis seawater desalination system and the production quantity (flux) per unit time during continuous operation in which seawater is continuously supplied to a reverse osmosis membrane to continuously extract fresh water, it was confirmed that the production quantity (flux) per unit time during intermittent operation corresponds to or is relatively somewhat superior to the production quantity (flux) per unit time during continuous operation.

An operating method of a reverse osmosis seawater desalination system according to one aspect of the present invention includes an operating step of operating the reverse osmosis seawater desalination system for a preset operating time to extract fresh water from seawater, and a resting step of stopping the operation of the reverse osmosis seawater desalination system for a preset rest time after the operating step, so that fouling can be effectively prevented.

The operation method of the reverse osmosis seawater desalination system according to one aspect of the present invention includes an operation phase and a stop phase, and thus can be particularly suitably applied to a mobile seawater desalination system. For example, the system can be operated more efficiently by stopping the operation of the reverse osmosis seawater desalination system during the process of anchoring near a first area requiring fresh water supply, operating the reverse osmosis seawater desalination system to complete the supply of fresh water to the first area, and then anchoring near the second area to operate the reverse osmosis seawater desalination system again.

The operation stage and the pause stage form one set, and multiple cycles can be executed as a set. That is, the reverse osmosis seawater desalination system can be operated so that the operation time and the pause time form one time unit and are continuously repeated. The operation time of the operation stage and the pause time of the pause stage are not particularly limited, and the operation time and the pause time can be selected and applied by applying an appropriate time according to the operation environment. In terms of reducing the fouling phenomenon, the preferable operation time and the pause time can be 4 to 12 hours and 8 to 24 hours, and the more preferable operation time and the pause time can be 6 to 10 hours and 12 to 20 hours.

The operating method of the reverse osmosis seawater desalination system according to one aspect of the present invention may include a cleaning step of supplying cleaning water to the reverse osmosis membrane during a downtime after an operating time has elapsed to clean the surface of the membrane. By performing the cleaning operation on the membrane surface during the downtime, the fouling phenomenon reduction effect can be further maximized. The cleaning operation is preferably performed during the downtime, and considering the cleaning efficiency, it is preferable to perform the cleaning of the membrane immediately after the operating time.

The method for operating a reverse osmosis seawater desalination system according to one aspect of the present invention can clean a membrane using concentrated water. Since the London Convention prohibits the discharge of marine pollutants from a marine seawater desalination system, if a chemical agent is used to clean a membrane applied to a marine seawater desalination system, additional facilities and costs for the treatment and purification of the chemical agent are inevitably required. On the other hand, if the product water (fresh water) produced in the seawater desalination system is used as cleaning water, additional facilities and costs for the treatment and purification of the chemical agent are not incurred, but since the final product, the product water (fresh water), is applied to the cleaning of the membrane, it is not desirable in terms of productivity and economy. If the concentrated water generated during the operation of the seawater desalination system is used as cleaning water used to clean the membrane, the economic and environmental losses due to the use of chemicals and the product water (fresh water) can be minimized, while obtaining a membrane cleaning effect similar to that obtained when the product water (fresh water) is used as cleaning water.

Concentrated water is concentrated seawater supplied to the membrane without passing through the membrane, and has a higher salinity and contains a large amount of suspended matter compared to seawater and distilled water. The inventor of the present invention conducted an experiment to confirm the influence of salt and suspended matter contained in the concentrated water when cleaning a membrane using the concentrated water, and was able to confirm that there was a similar difference in the membrane cleaning effect between membrane cleaning using the concentrated water and membrane cleaning using distilled water. Although it has not been clearly identified theoretically, since the concentrated water contains a large amount of suspended matter and salt, it is expected that these components cause physical and chemical reactions with membrane contaminants on the surface of the membrane, thereby contributing to the improvement of the membrane cleaning effect.

Considering the membrane cleaning effect, the preferable concentrated water supply time may be 50 minutes or longer, and the more preferable concentrated water supply time may be 60 minutes or longer. However, if the concentrated water is supplied for a long period of time, there is a concern about membrane surface contamination by floating substances contained in the concentrated water, so the concentrated water supply time is preferably 100 minutes or shorter, and the more preferable concentrated water supply time may be 90 minutes or shorter. When cleaning the membrane using concentrated water, the membrane cleaning effect tends to improve as the supply flow rate (cleaning flow rate) of the concentrated water increases. However, as the cleaning flow rate increases, the energy consumption of the reverse osmosis seawater desalination system increases, and when the cleaning flow rate reaches a certain level, the membrane cleaning effect tends to converge even if the cleaning flow rate increases, so it is preferable to supply the concentrated water at an appropriate cleaning flow rate. In terms of the membrane cleaning effect and economic efficiency, the preferable cleaning flow rate may be 2.0 L/min to 4.0 L/min, and a cleaning flow rate of 2.5 L/min to 3.5 L/min may be more preferable.

According to another aspect of the present invention, a method for operating a reverse osmosis seawater desalination system can use concentrated water injected with nanobubbles as cleaning water. The inventor of the present invention has conducted in-depth research on a method for further enhancing the membrane cleaning effect while using concentrated water as cleaning water without using chemicals, and has experimentally confirmed that a superior fouling reduction effect can be secured when cleaning a separation membrane using concentrated water injected with nanobubbles.

As an example, a nanobubble injection device is provided on one side of a concentrated water tank containing concentrated water, and nanobubbles generated by the nanobubble injection device can be directly injected into the concentrated water contained in the concentrated water tank. The nanobubble injection device starts injecting nanobubbles into the concentrated water tank when the end of the operating time is imminent, and can complete the injection of nanobubbles at the start of the downtime or during the cleaning of the reverse osmosis membrane. Considering economic feasibility and cleaning efficiency, nanobubbles can be injected into the concentrated water at an injection rate of 0.3 L/min or more, and the concentrated water into which nanobubbles are injected can mean concentrated water into which nanobubbles are injected up to a saturation state. Since the nanobubble injection device can be modified in various ways, such as the nanobubble injection rate and nanobubble injection time, depending on the nozzle design, the scope of the present invention is not limited to the operating method of the nanobubble injection device described above, and it is preferable to interpret it as including all technical means that can be applied by a person skilled in the art to which the present invention belongs without special technical difficulty to inject nanobubbles.

When cleaning a membrane using concentrated water injected with nanobubbles, membrane contaminants attached to and adsorbed on the membrane surface can be more effectively removed by the simultaneous action of the nanobubbles and floating substances contained in the concentrated water.

Hereinafter, a method for reducing fouling of a reverse osmosis membrane for seawater desalination using concentrated water according to one aspect of the present invention will be described in more detail through examples. The following examples are provided to help explain the present invention, and it should be noted that the scope of the present invention is not limited to the following examples.

(Experimental Example 1)

In a seawater desalination process using a reverse osmosis membrane, the change in the production amount (Flux) according to salinity change during continuous and intermittent operation was evaluated in a laboratory scale. Since the marine mobile seawater desalination system moves to various regions to perform seawater desalination treatment, the concentration of seawater flowing into the seawater desalination system may be different. Therefore, in order to confirm the change in the production amount (Flux) according to salinity change, the change in the production amount (Flux) during continuous and intermittent operation at various NaCl concentrations was evaluated.

The reverse osmosis membrane used was FilmTec's SW20ULE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm. The influent was prepared by dissolving NaCl in pure water at concentrations of 5,000 mg/L, 15,000 mg/L, 25,000 mg/L, 35,000 mg/L, 45,000 mg/L, and 55,5000 mg/L, and NaCl (CAS No. 7647-14-5 of Samjeon Pure Chemical Industry Co., Ltd.) was used. The seawater desalination process using the reverse osmosis membrane was operated under continuous and intermittent operation conditions, as described in Table 1. The production amount (Flux) was measured under each operation condition, and the results are shown in Fig. 1. The production amount (Flux) was calculated by measuring the filtered amount according to the 'Installation Standards for Membrane Filtration Water Treatment Facilities' and converting the measured filtered amount into the production amount (Flux). Continuous operation was performed for 3 days at a cross flow rate of 1.0 L/min by continuously applying 60 bar of reverse osmosis pressure. Intermittent operation was performed for 3 days by repeating 8 hours of operation and 16 hours of rest. During intermittent operation, 60 bar of reverse osmosis pressure was applied as in continuous operation and a cross flow rate of 1.0 L/min was performed.

NaCl
density
change
test Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) Operating conditions NaCl concentration
(mg/L) 3 1.0 60 Continuous driving
(Continuous) 5,000 15,000 25,000 35,000 45,000 55,000 Intermittent operation
(Intermittent) 5,000 15,000 25,000 35,000 45,000 55,000

As illustrated in Fig. 1, when comparing the production volume (Flux) per operating hour of continuous operation and intermittent operation, it can be confirmed that intermittent operation with a 16-hour rest period secures a production volume (Flux) equivalent to or higher than continuous operation without a rest period. In addition, regardless of the NaCl concentration, intermittent operation secures a production volume (Flux) equivalent to or higher than continuous operation, and even at a NaCl concentration of 35,000 mg/L, which is similar to the average salinity of seawater, it can be confirmed that intermittent operation secures a production volume (Flux) equivalent to or higher than continuous operation. Therefore, it can be easily inferred that when intermittent operation is applied in actual desalination plant operation, a production volume (Flux) equivalent to or higher than when continuous operation is applied can be secured, and that such technical advantage can be secured regardless of the salinity of seawater.

(Experimental Example 2)

In a seawater desalination process using a reverse osmosis membrane, the change in production volume (Flux) according to the concentration of humic acid (HA) during continuous and intermittent operation was evaluated in a laboratory scale. The reverse osmosis membrane used was FilmTec's SW20ULE-400i (11,000 GPD) with a size of 14.1 cm × 9.1 cm. The influent was prepared by dissolving humic acid (HA) in pure water at concentrations of 1.0 mg/L, 3.0 mg/L, and 5.0 mg/L, and the humic acid (HA) used was CAS No. 7647-14-5 from Sigma-Aldrich. To examine in more detail the membrane fouling pattern according to the amount of organic matter in the influent, NaCl was additionally added to the influent containing humic acid (HA) so that each influent satisfied the same NaCl concentration of 35,000 mg/L. At this time, NaCl (CAS NO. 7647-14-5 of Samjeon Pure Chemical Industry Co., Ltd.) was used. The reverse osmosis membrane process was operated under continuous and intermittent operation conditions, as described in Table 2. The production amount (Flux) was measured under each operation condition, and the results are shown in Fig. 2. Fig. 3 shows the results of converting the production amount (Flux) per unit time under the intermittent operation condition into the normalized production amount (Normalized Flux) versus the initial production amount (Flux). Continuous operation was performed for 3 days at a cross flow rate of 1.0 L/min by continuously applying a reverse osmosis pressure of 60 bar. Intermittent operation was performed for 3 days by repeating 8-hour operation and 16-hour rest. During intermittent operation, reverse osmosis pressure of 60 bar was applied and flow rate of 1.0 L/min (cross flow rate) was performed as in continuous operation.

Humic acid
density
change
test Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) NaCl
density
(mg/L) Operating conditions HA concentration
(mg/L) 3 1.0 60 35,000 Continuous driving
(Continuous) 1.0 3.0 5.0 Intermittent operation
(Intermittent) 1.0 3.0 5.0

As illustrated in Fig. 2, it can be confirmed that in the case of intermittent operation applying a 16-hour rest period, a higher production amount (Flux) per operating hour is secured compared to continuous operation without applying a rest period. In addition, it can be confirmed that in the case of the same humic acid (HA) concentration, intermittent operation secures a production amount (Flux) per operating hour that is equivalent to or higher than continuous operation. Meanwhile, as illustrated in Fig. 3, the decrease in the production amount (Flux) in intermittent operation according to the concentration of humic acid (HA) shows a large difference as the operation and rest cycles are repeated, and therefore, it can be confirmed that the amount of organic matter in the influent has a greater effect on the production amount (Flux) when applying intermittent operation than the salt concentration in the influent.

(Experimental Example 3)

In order to verify the effect of performing flushing using concentrated water during the idle period during intermittent operation, the change in production volume (Flux) according to the flushing time during flushing using concentrated water was evaluated on a laboratory scale. The reverse osmosis membrane used was FilmTec's SW20ULE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm. Intermittent operation was repeated for 3 days, with actual seawater used for 8 hours of operation and 16 hours of stoppage (rest). During operation, a reverse osmosis pressure of 60 bar was applied and the system was operated at a cross flow rate of 1 L/min, and after the operation was terminated, the concentrated water was supplied for flushing times of 0, 10, 20, 30, 60, 90, and 120 minutes at a flushing flow rate of 3.0 L/min, as shown in Table 3, to perform membrane cleaning, and operation and rest were repeated. A cleaning time of 0 minutes means that no membrane cleaning was performed. The concentrated water used at this time had a salt concentration of 4.5% and was concentrated water maintained at a temperature of 25℃. The production amount (Flux) under each cleaning condition was measured, and Fig. 4 shows the result of converting the production amount (Flux) per unit time under each condition into the normalized production amount (Normalized Flux) compared to the initial production amount (Flux).

behavior
condition Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) Washing flow rate
(L/min) Cleaning time
(min) Intermittent operation
(intermittent)

8 hours of operation/
16 hours of rest 3 1.0 60 3.0 0 10 20 30 60 90 120

As shown in Fig. 4, it can be confirmed that the decrease in the production quantity (Flux) is smaller in the case where cleaning is applied using concentrated water compared to the case where cleaning is not applied. In addition, it can be confirmed that the decrease in the production quantity (Flux) becomes smaller as the cleaning time increases, and the best production quantity (Flux) is secured when cleaning times of 60 and 90 minutes are applied. However, when the cleaning time exceeds 90 minutes, it can be confirmed that the production quantity (Flux) is significantly reduced despite the long-term cleaning. In other words, when the membrane is cleaned for an appropriate amount of time using concentrated water during intermittent operation, the decrease in the production quantity (Flux) can be effectively alleviated, whereas when the membrane is cleaned for an excessively long period of time using concentrated water, the contaminants contained in the concentrated water are adsorbed to the membrane surface, which increases the decrease in the production quantity (Flux).

(Experimental Example 4)

In order to confirm the membrane cleaning effect according to the type of cleaning liquid in the membrane cleaning of the idle period during intermittent operation, the change in production amount (Flux) of membrane cleaning using concentrated water and membrane cleaning using distilled water was evaluated on a laboratory scale. The reverse osmosis membrane used was FilmTec's SW20ULE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm. Intermittent operation was repeated for 3 days, with actual seawater used for 8 hours of operation and 16 hours of stoppage (stop). During operation, a reverse osmosis pressure of 60 bar was applied and the process was conducted at a cross flow rate of 1 L/min, and after the operation was terminated, concentrated water and distilled water were supplied for 60 minutes and 90 minutes, respectively, as described in Table 4, at a flushing flow rate of 3.0 L/min to perform membrane cleaning, and operation and stoppage were repeated. The concentrated water used at this time had a salt concentration of 4.5%, and the concentrated water and distilled water were supplied under conditions where the temperature was maintained at 25℃. The production amount (Flux) under each cleaning condition was measured, and Fig. 5 shows the result of converting the production amount (Flux) per unit time under each condition into a normalized production amount (Normalized Flux) compared to the initial production amount (Flux).

behavior
condition Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) Washing flow rate
(L/min) Detergent Cleaning time
(min) Intermittent operation
(intermittent)
8 hours of operation/
16 hours of rest 3 1.0 60 3.0 Distilled water 60 90 Concentrated water 60 90

As shown in Fig. 5, the decrease in the production amount (Flux) on the initial day (Day 1) showed little difference between the two conditions of distilled water and concentrated water, but as time passed (Days 2 and 3), it could be confirmed that there was a gradual difference in the decrease trend of the production amount (Flux). When a cleaning time of 60 minutes was applied, cleaning using distilled water somewhat suppressed the decrease in the production amount (Flux) compared to cleaning using concentrated water, and when a cleaning time of 90 minutes was applied, it could be confirmed that cleaning using concentrated water suppressed the decrease in the production amount (Flux) more compared to cleaning using distilled water.

(Example 5)

In order to confirm the membrane cleaning effect according to the flushing flow rate in the membrane cleaning of the idle period using concentrated water during intermittent operation, the change in the production amount (Flux) according to the flushing flow rate during cleaning using concentrated water was evaluated on a laboratory scale. The reverse osmosis membrane used was FilmTec's SW20ULE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm. Intermittent operation, in which actual seawater was used for 8 hours of operation and then 16 hours of stoppage (idle), was repeated for 3 days, and continuous operation was also performed for 3 days using actual seawater for comparison. During intermittent operation, a reverse osmosis pressure of 60 bar was applied and the operation was performed at a cross flow rate of 1.0 L/min, as in continuous operation. During continuous operation, a reverse osmosis pressure of 60 bar was continuously applied and the operation was performed at a cross flow rate of 1.0 L/min. As shown in Table 5, membrane cleaning using concentrated water was performed for 60 minutes after the end of intermittent operation, and flushing flow rates of 0 L/min, 1.0 L/min, 3.0 L/min, and 5.0 L/min were applied, respectively. A flushing flow rate of 0 L/min indicates that concentrated water cleaning was not performed after the end of intermittent operation. The concentrated water used at this time had a salt concentration of 4.5%, and the concentrated water and distilled water were supplied under conditions maintained at a temperature of 25℃. The production amount (Flux) under each cleaning condition was measured, and the results are shown in Fig. 6. Figure 7 shows the result of converting the production quantity per unit time (Flux) under intermittent operation conditions into the normalized production quantity (Normalized Flux) compared to the initial production quantity (Flux).

behavior
condition Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) Cleaning time
(min) Detergent Washing flow rate
(L/min) Intermittent operation
(intermittent)
8 hours of operation/
16 hours of rest 3 1.0 60 60 Concentrated water 0 1.0 3.0 5.0 Continuous driving
(continuous) - - -

As shown in Fig. 6, it can be confirmed that the decrease in the production amount (Flux) under continuous operation conditions is greater than the decrease in the production amount (Flux) under intermittent operation conditions. In addition, as shown in Fig. 7, it can be confirmed that the decrease in the production amount (Flux) is lower in the case where concentrated water flushing is performed during intermittent operation compared to the case where concentrated water flushing is not performed, and it can be confirmed that the decrease in the production amount (Flux) is lower as the flushing flow rate increases. However, since the change in the decrease in the production amount (Flux) is minimal compared to the change in the flushing flow rate, it is expected that it is unnecessary to apply an excessively high flushing flow rate in terms of energy efficiency.

(Experimental Example 6)

To verify the effect of intermittent operation in an actual desalination plant, continuous and intermittent operations were performed on a plant scale under the conditions of Table 6. The reverse osmosis membrane used was FilmTec's SW30XLE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm, and the experiment was conducted using actual seawater. Continuous operation was performed at a cross flow rate of 1.0 L/min while continuously applying a reverse osmosis pressure of 60 bar. Intermittent operation was performed for 5 days by repeating 8-hour operation and 16-hour rest periods. During intermittent operation, a reverse osmosis pressure of 60 bar was applied as in continuous operation and a cross flow rate of 1.0 L/min was performed. Intermittent operation applied the conditions of performing concentrated water cleaning for 60 minutes immediately after operation and the conditions of not performing concentrated water cleaning. During concentrated water cleaning, concentrated water with a salt concentration of 4.5% and maintained at 25℃ was supplied at a flushing flow rate of 3.0 L/min to clean the membrane. The production amount (Flux) under each operating condition was measured and the results are shown in Fig. 8.

behavior
condition Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) Cleaning time
(min) Detergent Washing flow rate
(L/min) Intermittent operation
(intermittent)
8 hours of operation/
16 hours of rest 5 1.0 60 60 Concentrated water 3.0 - - Continuous driving
(continuous) 3 - - -

As shown in Fig. 8, even at the plant scale, intermittent operation showed a smaller tendency to decrease in the production amount (Flux) than continuous operation, and it can be confirmed that the tendency to decrease in the production amount (Flux) was even smaller in the case of performing concentrated water washing during intermittent operation than in the case of not performing concentrated water washing. During continuous operation, organic substances including particulate matter present in actual seawater form aggregates to form a gel-cake layer on the surface of the reverse osmosis membrane, which significantly decreases the production amount (Flux), whereas intermittent operation, especially performing concentrated water washing, effectively suppresses the formation of such gel-cake layer, thereby effectively preventing the decrease in the production amount (Flux).

(Experimental Example 7)

A laboratory-scale experiment was conducted to verify the effect of suppressing the decrease in production amount (Flux) when performing membrane cleaning using concentrated water injected with nanobubbles during intermittent operation. The reverse osmosis membrane used was FilmTec's SW30ULE-400i (11,000 GPD) with a size of 14.1 cm x 9.1 cm, and the influent used was seawater collected from Yeongjong Island, Incheon. The components of the seawater used in the experiment are listed in Table 7 below. As listed in Table 8, the reverse osmosis membrane process was operated under three conditions: simple intermittent operation, intermittent operation including concentrated water cleaning, and intermittent operation including nanobubble-injected concentrated water cleaning. The production amount (Flux) per unit time measured under each condition is shown in Fig. 9. Intermittent operation was performed for 9 days with 8-hour operation and 16-hour rest repeating. During intermittent operation, reverse osmosis pressure of 60 bar was applied and the separation membrane was cleaned at a cross flow rate of 1.0 L/min. Cleaning using concentrated water and membrane cleaning using concentrated water injected with nanobubbles were performed for 60 minutes after the end of operation by supplying concentrated water and concentrated water injected with nanobubbles at a flushing flow rate of 3.0 L/min. Nanobubbles were injected into the concentrated water tank containing the concentrated water for 5 minutes at an injection rate of 0.4 to 0.5 L/min to saturate the nanobubbles, and then the membrane was cleaned by supplying the concentrated water injected with nanobubbles to the separation membrane for 60 minutes.

pH EC
(ms/cm) TDS
(mg/L) SS
(mg/L) TOC
(mg/L) Turbidity
(NTU) 7.9 32,240 29,882 62 2.2 7.88

behavior
condition Driving period
(Day) Reverse osmosis flow rate
(L/min) Reverse osmosis
(bar) sejung
condition Cleaning time
(min) sejung
Flow rate
(L/min) Intermittent operation
(intermittent)
8 hours of operation/
16 hours of rest 9 1.0 60 X - - Concentrated water 60 3.0 Concentrated water
+Nanobubble

As shown in Figure 9, it can be confirmed that the tendency of decrease in the production quantity (Flux) decreases in the order of simple intermittent operation, intermittent operation with concentrated water cleaning, and intermittent operation with concentrated water cleaning injected with nanobubbles, and it can be confirmed that the difference in the decrease in the production quantity (Flux) also increases as the operation period is prolonged.

Although the present invention has been described in detail above through examples, other forms of examples are also possible. Therefore, the technical spirit and scope of the claims described below are not limited to the examples.

Claims (7)

An operating step of extracting fresh water from seawater by operating a reverse osmosis seawater desalination system for an operating time of 4 to 12 hours;
A rest stage in which the operation of the reverse osmosis seawater desalination system is stopped for a rest period of 8 to 24 hours after the above operation stage; and
Including a cleaning step of cleaning the reverse osmosis membrane by supplying concentrated water injected with nanobubbles to the reverse osmosis membrane of the seawater desalination system immediately after the above-mentioned operating time has elapsed.
An operating method of a reverse osmosis seawater desalination system, wherein the above-mentioned operating time and the above-mentioned rest time are continuously and cross-repeatedly repeated. In the first paragraph,
The above cleaning step is,
A nanobubble injection step for injecting nanobubbles into a concentrated water tank containing the concentrated water; and
An operating method of a reverse osmosis seawater desalination system, comprising a concentrated water supply step of supplying concentrated water injected with nanobubbles to the reverse osmosis membrane after the injection of the nanobubbles is completed. In the second paragraph,
A method for operating a reverse osmosis seawater desalination system, wherein the nanobubbles are injected into the concentrated water at an injection rate of 0.3 L/min or more. In the first paragraph,
A method for operating a reverse osmosis seawater desalination system, wherein the above nanobubble-injected concentrated water is a nanobubble-saturated concentrated water. In the first paragraph,
A method for operating a reverse osmosis seawater desalination system, wherein the reverse osmosis membrane is cleaned by supplying the concentrated water at a cleaning flow rate of 2.0 L/min to 4.0 L/min for a cleaning time of 50 to 100 minutes in the above cleaning step. In the first paragraph,
An operating method of a reverse osmosis seawater desalination system, wherein, in the above-mentioned operating step, a reverse osmosis pressure of 40 to 80 bar is applied to the seawater to extract the fresh water. In the first paragraph,
The above reverse osmosis seawater desalination system is provided as a mobile type that can move on the sea, and is a method for operating a reverse osmosis seawater desalination system.