An ultra-low pressure seawater desalination system and a seawater desalination method

Abstract

The application belongs to the field of seawater desalination and relates to an ultra-low pressure seawater desalination system and a seawater desalination method. The ultra-low pressure seawater desalination system comprises a rough desalination cabin, a rough desalination water conveying device and a low-pressure desalination cabin. The rough desalination cabin is arranged at a deep sea with a water depth of 150-200 m, and the rough desalination cabin comprises a desalination unit, and a reverse osmosis membrane group is arranged in the desalination unit. The desalination rate of the reverse osmosis membrane is 65-75%. The ratio of the water production of the desalination unit to the water inflow is less than 15%. The rough desalination water conveying device is used for conveying the rough desalination water obtained from the rough desalination cabin to the low-pressure desalination cabin, and the low-pressure desalination cabin is located above the sea surface, and a reverse osmosis membrane with a desalination rate greater than 99.5% is arranged in the low-pressure desalination cabin. The seawater desalination system of the application can significantly reduce the pressure resistance requirement and cost of the equipment, can make the concentration of the discharged concentrated water close to the original seawater, can directly and safely discharge the concentrated water, can reduce the chemical agent pollution, and can realize low-cost and environmentally-friendly seawater desalination.

Description

Technical Field

[0001] This invention relates to the field of seawater desalination technology, specifically to an ultra-low pressure seawater desalination system and a seawater desalination method. Background Technology

[0002] Seawater desalination technologies are mainly divided into two categories: thermal methods and membrane methods. Among them, membrane desalination has become the most widely used technology globally due to its relatively low energy consumption and high operating cost-effectiveness, playing an important role in solving the water shortage problem in coastal areas. This technology separates seawater from salt under high pressure using a reverse osmosis membrane, thereby obtaining usable freshwater.

[0003] With technological advancements, utilizing hydrostatic pressure in deep-sea or deep-well environments to drive reverse osmosis processes has become an important technological approach for reducing energy consumption. For example, patent application CN105236517A proposes a deep-well gravity reverse osmosis seawater desalination device. By placing the reverse osmosis equipment at a depth of approximately 534 meters, it utilizes deep-sea hydrostatic pressure to replace the high-pressure pump, thereby eliminating the need for a high-pressure pump and reducing energy consumption and operating costs. Similarly, patent application CN1261059A proposes a deep-sea seawater desalination system, which includes a seawater desalination and purification device and a freshwater storage and transportation control device. It utilizes the natural pressure difference between the deep sea and sea level as the driving force for reverse osmosis, achieving both seawater desalination and freshwater extraction. Furthermore, patent application CN109809581A provides a reverse osmosis seawater desalination device and method utilizing deep-sea hydrostatic energy. This also relies on high-pressure conditions in the deep-sea environment to drive the desalination process and utilize concentrated brine.

[0004] While the aforementioned technologies have made some progress in utilizing natural energy and reducing energy consumption, current seawater desalination systems still generally suffer from several problems. First, in membrane desalination processes, the discharge rate of concentrate is typically as high as 55-60%, with a total dissolved solids (TDS) concentration nearly twice that of the original seawater. Direct discharge into the ocean can easily cause a significant increase in salinity in localized areas, posing a potential threat to marine ecosystems and biodiversity. Second, existing technologies require the addition of flocculants, coagulants, and disinfectants during the pretreatment stage before starting the reverse osmosis process to remove suspended solids and microorganisms from the seawater. After entering normal operation, scale inhibitors and other agents to prevent membrane fouling must be continuously added, and regular maintenance with chemical cleaning agents is necessary. These chemicals, discharged into the ocean along with the concentrate, may cause secondary pollution, affecting marine environmental quality. Furthermore, existing deep-sea desalination technologies often require equipment to be deployed in deep-sea areas above 400 meters, placing extremely high demands on the pressure resistance of the equipment and increasing the difficulty and cost of equipment manufacturing and maintenance.

[0005] Therefore, how to reduce the dependence of membrane desalination on the high-pressure environment of the deep sea, reduce the use of chemical agents, and effectively control the impact of concentrated water discharge on the marine ecosystem has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] In view of the above-mentioned technical problems, in order to solve the problems of existing membrane seawater desalination relying on deep-sea high pressure, chemical pollution, and ecological impact of concentrated water discharge, this invention provides an ultra-low pressure seawater desalination system and seawater desalination method.

[0008] (II) Technical Solution

[0009] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0010] This invention provides an ultra-low pressure seawater desalination system, including a coarse desalination chamber, a coarse desalination water conveying device, and a low-pressure desalination chamber;

[0011] The coarse desalination chamber is located in the deep sea at a depth of 150-200m. The coarse desalination chamber includes a desalination unit, and the desalination unit is equipped with a reverse osmosis membrane module. The desalination rate of the reverse osmosis membrane module is 65-75%.

[0012] The ratio of the water production rate of the desalination unit to its influent rate is <15%;

[0013] The coarse desalination water conveying device is used to transport the coarse desalination water obtained from the coarse desalination chamber to the low-pressure desalination chamber. The low-pressure desalination chamber is located above the sea surface and is equipped with a reverse osmosis membrane with a desalination rate of more than 99.5% to desalinate the coarse desalination water and obtain fresh water.

[0014] In the ultra-low pressure seawater desalination system described above, preferably, the coarse desalination chamber further includes a filter unit connected to the desalination unit, wherein the filter unit has a filter pore size of 2-8 mm.

[0015] Deep-sea seawater is filtered through a filtration unit before entering the desalination unit.

[0016] The ultra-low pressure seawater desalination system described above preferably further includes a coarse freshwater collection device. The inlet of the coarse freshwater collection device is connected to the product water outlet of the coarse desalination chamber, and the outlet of the coarse freshwater collection device is connected to the inlet of the low-pressure desalination chamber through the coarse desalination water conveying device. The coarse freshwater collection device is set in the same deep-sea static pressure environment as the coarse desalination chamber.

[0017] The ultra-low pressure seawater desalination system described above, preferably, further includes a membrane shell and a main product water pipe in the desalination unit;

[0018] The membrane housing is provided with an inlet cap and a concentrate cap at both ends, and the inlet cap and the concentrate cap are provided with several holes with a diameter of 3-6mm.

[0019] The reverse osmosis membrane module is located inside the membrane shell. The reverse osmosis membrane module includes multiple reverse osmosis membranes connected in parallel. The permeate flux of the reverse osmosis membrane is 15-20 LMH.

[0020] The main product water pipe is located inside the membrane housing. The inlet end of the main product water pipe is connected to the product water collection pipe of each reverse osmosis membrane, and the outlet end of the main product water pipe is connected to the inlet of the coarse desalination water collection device.

[0021] In the ultra-low pressure seawater desalination system described above, preferably, the reverse osmosis membrane inside the desalination unit and the reverse osmosis membrane in the low-pressure desalination chamber are new reverse osmosis membranes or decommissioned and regenerated reverse osmosis membranes.

[0022] The preparation method of the new reverse osmosis membrane is as follows:

[0023] A1: 1,3,5-trimethylphloroglucinol and 2,5-disulfonic acid-p-phenylenediamine were added to methanol or acetic acid solvent at a mass ratio of 1:(1-2), and the mixture was refluxed at 80-90℃ for 9-12 h to obtain functionalized nanosheets.

[0024] A2: Functionalized nanosheets and m-phenylenediamine are added to water and ultrasonically dispersed to obtain an aqueous working solution containing nanosheets.

[0025] A3: Immerse the porous support base membrane in the aqueous working solution, remove it, and then immerse it in the oil working solution for interfacial polymerization. After the reaction is completed, wash and dry it to obtain a new reverse osmosis membrane.

[0026] In the ultra-low pressure seawater desalination system described above, preferably, in step A3, the oil phase working fluid is isooctane containing 0.1-0.15 wt% trimesoyl chloride.

[0027] In the ultra-low pressure seawater desalination system described above, preferably, the method for preparing the decommissioned and regenerated reverse osmosis membrane is as follows:

[0028] B1: Test the desalination rate of decommissioned seawater desalination reverse osmosis membranes, and then classify them according to different desalination rates to obtain Class I slightly fouled membranes, Class II moderately fouled membranes, Class III heavily fouled membranes, and Class IV severely fouled membranes.

[0029] B2: Clean the membrane obtained from step B1. First, wash it with alkali at pH 10-13 for 30-90 minutes, then soak it for 6-12 hours. Repeat the washing and soaking process 1-3 times, and then rinse it until neutral. Next, wash it with acid at pH 1-3 for 30-90 minutes, then soak it for 6-12 hours. Repeat the washing and soaking process 1-3 times.

[0030] B3: The four types of membranes after cleaning were oxidized using a mixed aqueous solution with pH 9-10 containing 0.8-1.2 wt% sodium hypochlorite and 0.8-1.2 wt% hydrogen peroxide;

[0031] B4: Immediately after oxidation treatment, reduce the membrane with an aqueous solution containing 0.5-1.5 wt% sodium bisulfite for 10-15 minutes. After rinsing with water, test the desalination rate of the membrane again. If the desalination rate is 65-75%, the decommissioned and regenerated reverse osmosis membrane is obtained. If the desalination rate is >75%, repeat steps B2-B4 until the desalination rate drops to 65-75%. If the desalination rate is <65%, the membrane is considered waste.

[0032] In the ultra-low pressure seawater desalination system described above, preferably, in step B1, a desalination rate ≥98% is classified as a Class I lightly fouled membrane, a desalination rate of 96-98% is classified as a Class II moderately fouled membrane, a desalination rate of 92-96% is classified as a Class III heavily fouled membrane, and a desalination rate ≤92% is classified as a Class IV severely fouled membrane.

[0033] In the ultra-low pressure seawater desalination system described above, preferably, during the oxidation process in step B3, the treatment time is as follows: 13-17 min for mildly fouled membranes, 18-22 min for moderately fouled membranes, 23-27 min for heavily fouled membranes, and 28-32 min for severely fouled membranes.

[0034] The present invention also provides a seawater desalination method using the above-mentioned ultra-low pressure seawater desalination system, comprising the following steps:

[0035] S1: The coarse desalination chamber is set up in the deep sea at a depth of 150-200m. The seawater is filtered through the filtration unit and then enters the desalination unit. The ratio of the desalination unit's water production to its water intake is kept <15%. The seawater is then coarsely desalinated through a reverse osmosis membrane with a desalination rate of 65-75% in the desalination unit to obtain coarsely desalinated water. At the same time, the concentrated water produced is directly discharged into the ocean.

[0036] S2: The crude desalination water conveyance device conveys the crude desalination water in the crude desalination water collection device to the low-pressure desalination tank located above the sea surface;

[0037] S3: In the low-pressure desalination chamber, a reverse osmosis membrane with a desalination rate of greater than 99.5% desalinates the crude desalinated water to obtain fresh water, while the concentrated water produced is directly discharged into the ocean.

[0038] (III) Beneficial Effects

[0039] First, this invention places the desalination chamber in a deep-sea area at a depth of 150-200m, utilizing the hydrostatic pressure at this depth as the driving force. Combined with an improved reverse osmosis membrane module with a desalination rate of 65-75%, preliminary desalination of seawater can be achieved without the need for an external high-pressure pump. Compared to existing systems that require deployment at depths of over 400m, this invention significantly reduces the pressure resistance requirements of the equipment, thereby reducing the cost and technical difficulty of equipment manufacturing, installation, and maintenance.

[0040] Secondly, unlike traditional technologies that operate at 5-6 MPa and produce concentrated seawater with significantly increased salt concentration, the land-based low-pressure desalination chamber in this invention operates at a pressure below 2.5 MPa, resulting in lower energy consumption and reduced maintenance costs compared to existing technologies. Furthermore, the total dissolved solids (TDS) concentration of the desalination unit's discharged concentrate is similar to that of the original seawater, avoiding the problem of significantly increased TDS concentration in the concentrate caused by traditional high-recovery-rate reverse osmosis processes. Therefore, the concentrate obtained by this invention can be safely and directly discharged back into the ocean without causing a dramatic increase in salinity in localized sea areas, effectively preventing potential threats to marine ecosystems and biodiversity.

[0041] Third, the recovery rate of the coarse desalination chamber in this invention is controlled below 15%, resulting in low concentration of pollutants on the reverse osmosis membrane surface and significantly reduced risk of scaling and fouling. Therefore, there is no need to add flocculants, coagulants, or disinfectants, nor is it necessary to continuously add scale inhibitors during operation. This not only simplifies the seawater desalination process and reduces the floor space required, but also avoids secondary pollution caused by the discharge of chemical agents with the concentrate, achieving a more environmentally friendly operating mode. Attached Figure Description

[0042] Figure 1 This is a process flow diagram of the seawater desalination method in this invention;

[0043] Figure 2 This is a schematic diagram of the overall structure of the ultra-low pressure seawater desalination system in this invention;

[0044] Figure 3 This is a schematic diagram of the overall structure of the desalination unit in this invention.

[0045] [Explanation of Labels in the Attached Image]

[0046] 1: Filtration unit; 2: Desalination unit inlet pump; 3: Desalination unit; 4: Coarse freshwater collection device; 5: Deep-sea submersible pump; 6: High-pressure pump; 7: Low-pressure desalination chamber; 8: Freshwater pool; 9: Reverse osmosis membrane; 10: Membrane housing; 11: Inlet of desalination unit; 12: Product water outlet of desalination unit; 13: Concentrate outlet of desalination unit; 14: Main product water pipe; 15: Product water collection pipe. Detailed Implementation

[0047] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] like Figures 1-2 As shown, this invention provides an ultra-low pressure seawater desalination system, including a coarse desalination chamber, a coarse desalination water conveying device, and a low-pressure desalination chamber 7. The coarse desalination chamber is located in deep sea at a depth of 150-200m and includes a desalination unit 3. The desalination unit 3 contains a reverse osmosis membrane assembly, in which the reverse osmosis membrane 9 has a desalination rate of 65-75%. The ratio of the produced water to the influent water of the desalination unit 3, i.e., the recovery rate, is <15%. The coarse desalination water conveying device is used to transport the coarse desalination water obtained from the coarse desalination chamber to the low-pressure desalination chamber 7. The low-pressure desalination chamber 7 is located above the sea surface and contains a reverse osmosis membrane with a desalination rate greater than 99.5% for desalinating the coarse desalination water to obtain fresh water.

[0049] This invention places the coarse desalination chamber in a deep-sea area at a depth of 150-200m, utilizing the hydrostatic pressure at this depth as the driving force. Combined with a reverse osmosis membrane module with a desalination rate of 65-75%, preliminary desalination of seawater can be achieved without the need for an external high-pressure pump. Compared to existing systems that require deployment at depths of over 400m, this invention significantly reduces the pressure resistance requirements of the equipment, thereby reducing the cost and technical difficulty of equipment manufacturing, installation, and maintenance. Furthermore, compared to standard seawater membranes with a desalination rate of over 99%, the low-to-medium desalination rate reverse osmosis membrane requires lower operating pressures and is better matched to the hydrostatic pressure at 150-200m.

[0050] This invention controls the permeate recovery rate of the desalination chamber to below 15%, ensuring that the total dissolved solids (TDS) concentration of the concentrate discharged from desalination unit 3 is similar to that of the original seawater. This avoids the problem of a significant increase in TDS concentration in the concentrate caused by traditional high-recovery-rate reverse osmosis processes. Therefore, the concentrate obtained by this invention can be safely and directly discharged back into the ocean without causing a sharp increase in salinity in local sea areas, effectively avoiding potential threats to marine ecosystems and biodiversity.

[0051] The desalination unit 3 of this invention operates at a low recovery rate. A low recovery rate means that most of the feed water is discharged as concentrate. The high flow velocity of the fluid on the reverse osmosis membrane surface allows the concentrated salt to be quickly carried away, preventing supersaturation and scaling of sparingly soluble salts. Therefore, the concentration of contaminants on the reverse osmosis membrane surface is low, significantly reducing the risk of scaling and fouling. Thus, the seawater desalination system of this invention does not require the addition of flocculants, coagulants, or disinfectants, nor does it require continuous addition of scale inhibitors during operation. This not only simplifies the seawater desalination process and reduces the floor space required, but also avoids secondary pollution caused by the discharge of chemical agents with the concentrate, achieving a more environmentally friendly operating mode.

[0052] In the deep sea, the concentrated seawater relies on automatic diffusion, and the recovery rate of the low-pressure desalination chamber 7 is about 66.7%, with the salinity of the concentrated water close to that of the original seawater.

[0053] Preferably, the coarse desalination chamber also includes a filter unit 1 connected to the desalination unit 3, the filter unit 1 having a filter pore size of 2-8mm. After being filtered by the filter unit 1, the deep-sea seawater can enter the desalination unit 3 through the desalination unit inlet pump 2. As a physical barrier, the filter unit 1 can intercept larger suspended particles, marine organisms such as zooplankton, algae, and other debris in the seawater, effectively preventing large particles from scratching, clogging, and contaminating the subsequent reverse osmosis membrane components, ensuring the stable operation and long lifespan of the desalination unit 3. The filter unit 1 can be a wedge-shaped grid, and the material of the filter unit 1 can be one or two of copper, copper-nickel alloy, and beryllium bronze.

[0054] Preferably, the ultra-low pressure seawater desalination system of the present invention further includes a crude desalination water collection device 4. The inlet of the crude desalination water collection device 4 is connected to the product water outlet of the crude desalination chamber, and the outlet of the crude desalination water collection device 4 is connected to the inlet of the low-pressure desalination chamber 7 through a crude desalination water conveying device. The crude desalination water collection device 4 is set in the same deep-sea static pressure environment as the crude desalination chamber. The crude desalination water collection device 4 is used to temporarily store the crude desalination water produced by the crude desalination chamber and transport it to the low-pressure desalination chamber 7. Specifically, the crude desalination water collection device 4 can be a crude desalination water tank.

[0055] The desalination water delivery system includes a deep-sea submersible pump 5 and a high-pressure pump 6.

[0056] The deep-sea submersible pump 5 can be installed in the coarse desalination tank. Its head is 200-250m, and it can be made of 316L stainless steel. The outlet of the deep-sea submersible pump 5 is connected to the inlet of the high-pressure pump 6. The high-pressure pump 6 has a head of 200-230m and is also made of 316L stainless steel. The outlet of the high-pressure pump 6 is connected to the low-pressure desalination chamber 7. The treated water from the low-pressure desalination chamber 7 can be stored in the freshwater pool 8.

[0057] The working principle of the deep-sea desalination chamber is as follows: After the deep-sea submersible pump 5 is turned on, the water level in the coarse desalination tank decreases, creating a negative pressure state. Under the conditions of deep-sea hydrostatic energy and negative pressure on the product water side, seawater enters the coarse desalination chamber and undergoes desalination through the reverse osmosis membrane 9. Product water enters the coarse desalination tank, while concentrated water diffuses freely in the ocean. If the deep-sea submersible pump 5 is turned off, the coarse desalination tank is full, and the coarse desalination chamber stops working.

[0058] Preferably, such as Figure 3 As shown, the desalination unit 3 also includes a membrane housing 10 and a main product water pipe 14. The membrane housing 10 has an inlet cap and a concentrate cap at both ends, with several holes of 3-6 mm in diameter. The membrane housing 10 is preferably made of fiberglass. The holes in the inlet cap and concentrate cap serve as channels for the inlet and concentrate, and their diameter determines the uniformity of the water flow distribution and prevents larger particles from entering the membrane housing 10. The membrane housing 10 also has an inlet 11, a product water inlet 12, and a concentrate inlet 13 for the desalination unit.

[0059] The reverse osmosis membrane module is located inside the membrane housing 10. The module comprises multiple reverse osmosis membranes 9 connected in parallel to ensure that each membrane 9 faces essentially the same feed water pressure and flow rate. The permeate flux of each membrane 9 is 15-20 LMH. The main permeate pipe 14 is located inside the membrane housing 10, with one end penetrating the housing and connecting to the permeate outlet 12 of the desalination unit. The section of the main permeate pipe 14 inside the housing 10 connects to the permeate collection pipe 15 of each reverse osmosis membrane 9 element for collecting the coarse desalination water. The section of the main permeate pipe 14 outside the housing 10 serves as the outlet and connects to the inlet of the coarse desalination tank.

[0060] Preferably, the reverse osmosis membrane inside the desalination unit 3 of the present invention and the reverse osmosis membrane in the low-pressure desalination chamber can be a new reverse osmosis membrane or a decommissioned and regenerated reverse osmosis membrane, and the size is preferably 8 inches.

[0061] The preparation method of the new reverse osmosis membrane is as follows:

[0062] A1: 1,3,5-Trimethylphloroglucinol (TP) and 2,5-disulfonic acid-based p-phenylenediamine (Pa-SO3H) were added to methanol or acetic acid solvent at a mass ratio of 1:(1-2), and the mixture was refluxed at 80-90℃ for 9-12 h to obtain functionalized nanosheets TpPa-SO3H. Step A1 prepares nanomaterials with sulfonic acid groups through organic synthesis. These nanosheets can serve as carriers and modifiers for aqueous monomers in subsequent interfacial polymerization.

[0063] A2: Add 0.3-0.7% (by weight of water) of functionalized nanosheets and 1-1.2% (by weight of water) of m-phenylenediamine (MPD) to water, and disperse by ultrasonication to obtain an aqueous working solution containing nanosheets. In step A2, the functionalized nanosheets are blended with the m-phenylenediamine monomer. By utilizing the interaction between the functional groups on the nanosheets and MPD, the diffusion rate and distribution of MPD to the oil phase interface are controlled.

[0064] A3: Immerse a porous supporting membrane, such as a polysulfone membrane or a polyethersulfone membrane, in the aqueous working solution for 2.5-3.5 minutes. After removal, immerse it in the oil-phase working solution for 3-5 seconds to carry out interfacial polymerization. After the reaction, wash with 40°C warm water and dry to obtain a new reverse osmosis membrane with a desalination rate of 65-75%. In step A3, the oil-phase working solution is isooctane containing 0.1-0.15 wt% trimesoyl chloride. MPD and trimesoyl chloride undergo a condensation reaction on the surface of the supporting membrane to form a polyamide desalination layer. The introduction of functionalized nanosheets can disrupt the compactness of the polyamide network, forming more water channels, while moderately increasing the network pore size and improving hydrophilicity. Thus, while maintaining a certain desalination rate, it can significantly increase water flux and reduce the required operating pressure, and can capture heavy metals, improving the quality of freshwater production.

[0065] Using the above method, the present invention can produce customized low-pressure, high-flux reverse osmosis membranes suitable for this ultra-low pressure system.

[0066] The desalination unit of this invention has relaxed requirements for desalination rate, which provides an opportunity to reuse decommissioned membranes that have degraded but are not damaged, greatly reducing the core material cost of the system and realizing resource recycling, which is in line with the green and sustainable environmental protection concept. Specifically, the preparation method of the decommissioned and regenerated reverse osmosis membrane is as follows:

[0067] B1: Test the desalination rate of the decommissioned seawater desalination reverse osmosis membrane, i.e., the desalination rate of the fouled membrane. Then, classify the membranes according to different desalination rates: a desalination rate ≥98% or a weight ≤15kg is classified as Class I lightly fouled membrane; a desalination rate of 96-98% or a weight of 15-16kg is classified as Class II moderately fouled membrane; a desalination rate of 92-96% or a weight of 16-17kg is classified as Class III heavily fouled membrane; and a desalination rate ≤92% or a weight ≥17kg is classified as Class IV severely fouled membrane. This step classifies the membranes according to the degree of fouling and performance degradation, providing a basis for subsequent regeneration intensity to achieve precise treatment.

[0068] B2: The membranes obtained in step B1 are cleaned. First, they are alkaline washed for 30-90 minutes at a pH of 10-13, then soaked for 6-12 hours. This washing and soaking process is repeated 1-3 times, followed by rinsing until neutral. Next, they are acid washed for 30-90 minutes at a pH of 1-3, then soaked for 6-12 hours. This washing and soaking process is repeated 1-3 times. It should be noted that in this invention, decommissioned membrane elements of the same type are cleaned in the same batch. In this step, alkaline washing mainly removes organic contaminants such as microbial slime and grease, while acid washing mainly removes inorganic scale such as calcium and magnesium carbonates and sulfates.

[0069] B3: The four types of membranes after cleaning are oxidized using a mixed aqueous solution with pH 9-10 containing 0.8-1.2 wt% sodium hypochlorite and 0.8-1.2 wt% hydrogen peroxide. The treatment time is 13-17 min for Class I lightly fouled membranes, 18-22 min for Class II moderately fouled membranes, 23-27 min for Class III heavily fouled membranes, and 28-32 min for Class IV severely fouled membranes. This step utilizes the synergistic oxidation effect of sodium hypochlorite and hydrogen peroxide to controllably and slightly degrade the cross-linked structure of the membranes, reducing their selectivity (i.e., decreasing the desalination rate) while maintaining their mechanical strength.

[0070] B4: Immediately after oxidation, reduce the membrane for 10-15 minutes with an aqueous solution containing 0.5-1.5 wt% sodium bisulfite. The sodium bisulfite neutralizes any remaining oxidant, terminating the reaction and preventing excessive oxidation of the membrane. Then, rinse with water and test the membrane's desalination rate again. If the desalination rate is 65-75%, the decommissioned and regenerated reverse osmosis membrane is obtained. If the desalination rate is >75%, repeat steps B2-B4 until the desalination rate drops to 65-75%. If the desalination rate is <65%, the membrane is considered waste.

[0071] like Figure 1 As shown, the present invention also provides a seawater desalination method using the above-mentioned ultra-low pressure seawater desalination system, comprising the following steps:

[0072] S1: The coarse desalination chamber is set up in the deep sea at a depth of 150-200m. The seawater is filtered through the filtration unit and then enters the desalination unit. The ratio of the desalination unit's production water to its influent water is kept <15%. The seawater is then coarsely desalinated through a reverse osmosis membrane with a desalination rate of 65-75% in the desalination unit to obtain coarsely desalinated water with TDS <10000mg / L. At the same time, the concentrated water produced is directly discharged into the ocean.

[0073] S2: The crude desalination water conveying device transports the crude desalination water from the crude desalination water collection device to the low-pressure desalination chamber located above the sea surface.

[0074] S3: In the low-pressure desalination chamber, a reverse osmosis membrane with a desalination rate greater than 99.5% desalinates the crude desalinated water to obtain fresh water with TDS < 150 mg / L, while the resulting concentrate is directly discharged into the ocean. The recovery rate of the low-pressure desalination chamber, i.e., the ratio of permeate to feed water, is greater than 60%. The operating pressure of the low-pressure desalination chamber is < 3 MPa, and BW membranes (brine reverse osmosis membranes) can be used.

[0075] The concentrated water produced in steps S1 and S3 above is similar in quality to the original seawater.

[0076] To further clarify the present invention and its technological advancements, the following description is provided in conjunction with specific embodiments and technical effects.

[0077] Example 1

[0078] This embodiment provides an ultra-low pressure seawater desalination system, including a coarse desalination chamber, a coarse desalination water tank, a deep-sea submersible pump, a high-pressure pump, and a low-pressure desalination chamber located above the sea surface.

[0079] The coarse desalination chamber includes a copper filtration unit and a desalination unit. The filtration unit has a pore size of 5mm. The desalination unit includes a fiberglass membrane housing, a reverse osmosis membrane module, and a main product water pipe. The membrane housing has inlet and concentrate end caps at both ends, each with several 4mm diameter holes. The reverse osmosis membrane module is located inside the membrane housing and consists of multiple reverse osmosis membranes connected in parallel. The permeate flux of the reverse osmosis membrane is 17 LMH. The main product water pipe is located inside the membrane housing, penetrating it. The section of the main product water pipe inside the membrane housing connects to the product water collection pipe of each reverse osmosis membrane element, while the section outside the membrane housing serves as the outlet and connects to the inlet of the coarse desalination tank. In the reverse osmosis membrane module, the desalination rate is 70%, and the recovery rate of the desalination unit is 12%.

[0080] The coarse desalination tank is set up in the same deep-sea hydrostatic environment as the coarse desalination chamber. The deep-sea submersible pump is installed in the coarse desalination tank. It has a head of 220m and is made of 316L stainless steel. The outlet of the deep-sea submersible pump is connected to the inlet of the high-pressure pump. The high-pressure pump has a head of 215m and is made of 316L stainless steel. The outlet of the high-pressure pump is connected to the low-pressure desalination chamber.

[0081] The low-pressure desalination chamber is equipped with a reverse osmosis membrane with a desalination rate of 99.8% and a recovery rate of 65%.

[0082] The ultra-low pressure seawater desalination system of this embodiment is used to desalinate seawater:

[0083] S1: The coarse desalination chamber is placed at a depth of 150m in the deep sea. Seawater is filtered through a filtration unit, and the filtered seawater enters the desalination unit, where it undergoes coarse desalination treatment using a reverse osmosis membrane module to obtain coarsely desalinated water. Simultaneously, the concentrated water produced is directly discharged into the ocean. Analysis of the deep-sea seawater in this step revealed the following composition: TDS: 22310 mg / L, Ca... 2+ 352.7 mg / L, Mg 2+ 787.6 mg / L, SO4 2- 1833.8 mg / L, Cl - 10659 mg / L, HCO3 - The concentration of nitrogen (TDS) was 154.1 mg / L, and the turbidity was less than 0.1 NTU. The composition of the crude desalinated water obtained in this step is as follows: TDS: 7808.5 mg / L, Ca... 2+ 123.5 mg / L, Mg 2+ 275.7 mg / L, SO4 2- 641.8 mg / L, Cl - 3730.65 mg / L, HCO3 - TDS: 154.1 mg / L, turbidity less than 0.1 NTU. The composition of the concentrate obtained in this step is as follows: TDS: 23235 mg / L, Ca 2+ 407 mg / L, Mg 2+ 907.8 mg / L, SO4 2- 2119 mg / L, Cl - 12141 mg / L, HCO3 - 491.5 mg / L, turbidity less than 0.1 NTU.

[0084] S2: The coarse desalination water in the coarse desalination tank is transported to the low-pressure desalination chamber by a deep-sea submersible pump and a high-pressure pump.

[0085] S3: The reverse osmosis membrane in the low-pressure desalination chamber desalinates the crude desalinated water to obtain fresh water, while the concentrated water produced is directly discharged into the ocean. Analysis of the fresh water obtained in this step revealed the following composition: TDS: 231 mg / L, Ca... 2+ 2.1 mg / L, Mg 2+ 4.8 mg / L, SO4 2- 8.1 mg / L, Cl - 128.3 mg / L, HCO3 - The concentration of the concentrate obtained in this step is 6.5 mg / L, and the turbidity is less than 0.1 NTU. The composition of the concentrate is as follows: TDS: 22986.26 mg / L, Ca... 2+ 366.66 mg / L, Mg 2+818.31 mg / L, SO4 2- 1911.1 mg / L, Cl - 10946.17 mg / L, HCO3 - 449.74 mg / L, turbidity less than 0.1 NTU.

[0086] In this embodiment, both the reverse osmosis membrane inside the desalination unit and the reverse osmosis membrane in the low-pressure desalination chamber are new reverse osmosis membranes, and both are 8 inches in size.

[0087] The preparation method of the new reverse osmosis membrane is as follows:

[0088] A1: 1,3,5-Trimethylphloroglucinol and 2,5-disulfonic acid-p-phenylenediamine were added to methanol or acetic acid solvent at a mass ratio of 1:1.5 and refluxed at 85°C for 10 h to obtain functionalized nanosheets.

[0089] A2: Add 0.3wt% functionalized nanosheets and 1.2wt% m-phenylenediamine to water, and disperse by ultrasonication to obtain an aqueous working solution containing nanosheets.

[0090] A3: Immerse the polysulfone membrane in the aqueous working solution for 3 minutes, then remove it and continue immersing it in the oil working solution for 3 seconds to carry out the interfacial polymerization reaction. After the reaction is completed, wash it with 40°C warm water and dry it to obtain a new reverse osmosis membrane.

[0091] Example 2

[0092] This embodiment provides an ultra-low pressure seawater desalination system, including a coarse desalination chamber, a coarse desalination water tank, a deep-sea submersible pump, a high-pressure pump, and a low-pressure desalination chamber located above the sea surface.

[0093] The coarse desalination chamber includes a copper-nickel alloy filtration unit and a desalination unit. The filtration unit has a pore size of 2mm. The desalination unit includes a fiberglass membrane housing, a reverse osmosis membrane module, and a main product water pipe. The membrane housing has inlet and concentrate end caps at both ends, each with several 3mm diameter holes. The reverse osmosis membrane module is located inside the membrane housing and consists of multiple reverse osmosis membranes connected in parallel. The permeate flux of the reverse osmosis membrane is 15 LMH. The main product water pipe is located inside the membrane housing, penetrating it. The section of the main product water pipe inside the membrane housing connects to the product water collection pipe of each reverse osmosis membrane element, while the section outside the membrane housing serves as the outlet, connecting to the inlet of the coarse desalination tank. In the reverse osmosis membrane module, the desalination rate is 65%, and the recovery rate of the desalination unit is 13%.

[0094] The coarse desalination tank is set up in the same deep-sea hydrostatic environment as the coarse desalination chamber. The deep-sea submersible pump is installed in the coarse desalination tank. It has a head of 200m and is made of 316L stainless steel. The outlet of the deep-sea submersible pump is connected to the inlet of the high-pressure pump. The high-pressure pump has a head of 200m and is made of 316L stainless steel. The outlet of the high-pressure pump is connected to the low-pressure desalination chamber.

[0095] The low-pressure desalination chamber is equipped with a reverse osmosis membrane with a desalination rate of 99.6% and a recovery rate of 68%.

[0096] The ultra-low pressure seawater desalination system of this embodiment is used to desalinate seawater:

[0097] S1: The coarse desalination chamber is placed at a depth of 160m in the deep sea. Seawater is filtered through a filtration unit, and the filtered seawater enters the desalination unit, where it undergoes coarse desalination treatment using a reverse osmosis membrane module to obtain coarsely desalinated water. Simultaneously, the concentrated water produced is directly discharged into the ocean. Analysis of the deep-sea seawater in this step revealed the following composition: TDS: 24081 mg / L, Ca... 2+ 368.5 mg / L, Mg 2+ 807.5 mg / L, SO4 2- 1897.6 mg / L, Cl - 11076 mg / L, HCO3 - The concentration of nitrogen (TDS) in the crude desalinated water obtained in this step is 159.7 mg / L, and the turbidity is less than 0.1 NTU. The composition of the water is as follows: TDS: 8049.2 mg / L, Ca... 2+ 133.4 mg / L, Mg 2+ 289.5 mg / L, SO4 2- 658.3 mg / L, Cl - 3865.2 mg / L, HCO3 - The concentration of the concentrate obtained in this step is 160.3 mg / L, and the turbidity is less than 0.1 NTU. The composition of the concentrate is as follows: TDS: 24862 mg / L, Ca... 2+ 436.8 mg / L, Mg 2+ 947 mg / L, SO4 2- 2268 mg / L, Cl - 12804 mg / L, HCO3 - 504.6 mg / L, turbidity less than 0.1 NTU.

[0098] S2: The coarse desalination water in the coarse desalination tank is transported to the low-pressure desalination chamber by a deep-sea submersible pump and a high-pressure pump.

[0099] S3: The reverse osmosis membrane in the low-pressure desalination chamber desalinates the crude desalinated water to obtain fresh water, while the concentrated water produced is directly discharged into the ocean. The composition of the fresh water obtained in this step is as follows: TDS: 246.3 mg / L, Ca... 2+ 2.4 mg / L, Mg 2+ 4.7 mg / L, SO4 2- 8.6 mg / L, Cl - 131.4 mg / L, HCO3 - The concentration of the concentrate obtained in this step is 6.2 mg / L, and the turbidity is less than 0.1 NTU. The composition of the concentrate is as follows: TDS: 23678.43 mg / L, Ca... 2+ 395.79 mg / L, Mg 2+ 859.96 mg / L, SO4 2- 1959.65 mg / L, Cl - 11344.01 mg / L, HCO3 - 468.92 mg / L, turbidity less than 0.1 NTU.

[0100] In this embodiment, both the reverse osmosis membrane inside the desalination unit and the reverse osmosis membrane in the low-pressure desalination chamber are new reverse osmosis membranes, and both are 8 inches in size.

[0101] The preparation method of the new reverse osmosis membrane is as follows:

[0102] A1: 1,3,5-Trimethylphloroglucinol and 2,5-disulfonic acid-p-phenylenediamine were added to methanol or acetic acid solvent at a mass ratio of 1:1 and refluxed at 80°C for 9 h to obtain functionalized nanosheets.

[0103] A2: Add 0.4 wt% functionalized nanosheets and 1 wt% m-phenylenediamine to water, and disperse by ultrasonication to obtain an aqueous working solution containing nanosheets.

[0104] A3: Immerse the polysulfone / polyethersulfone membrane in the aqueous working solution for 2.5 min, remove it and continue to immerse it in the oil working solution for 4 s to carry out the interfacial polymerization reaction. After the reaction is completed, wash it with 40℃ warm water and dry it to obtain a new reverse osmosis membrane.

[0105] Example 3

[0106] This embodiment provides an ultra-low pressure seawater desalination system, including a coarse desalination chamber, a coarse desalination water tank, a deep-sea submersible pump, a high-pressure pump, and a low-pressure desalination chamber located above the sea surface.

[0107] The coarse desalination chamber includes a beryllium bronze filter unit and a desalination unit. The filter unit has an 8mm pore size. The desalination unit includes a fiberglass membrane housing, a reverse osmosis membrane module, and a main product water pipe. The membrane housing has inlet and concentrate end caps at both ends, each with several 6mm diameter holes. The reverse osmosis membrane module is located inside the membrane housing and consists of multiple reverse osmosis membranes connected in parallel. The permeate flux of the reverse osmosis membrane is 20 LMH. The main product water pipe is located inside the membrane housing, penetrating it. The section of the main product water pipe inside the membrane housing connects to the product water collection pipe of each reverse osmosis membrane element, while the section outside the membrane housing serves as the outlet and connects to the inlet of the coarse desalination tank. In the reverse osmosis membrane module, the desalination rate is 75%, and the recovery rate of the desalination unit is 14%.

[0108] The coarse desalination tank is set up in the same deep-sea hydrostatic environment as the coarse desalination chamber. The deep-sea submersible pump is installed in the coarse desalination tank. It has a head of 250m and is made of 316L stainless steel. The outlet of the deep-sea submersible pump is connected to the inlet of the high-pressure pump. The high-pressure pump has a head of 230m and is made of 316L stainless steel. The outlet of the high-pressure pump is connected to the low-pressure desalination chamber.

[0109] The low-pressure desalination chamber is equipped with a reverse osmosis membrane with a desalination rate of 99.7% and a recovery rate of 70%.

[0110] The ultra-low pressure seawater desalination system of this embodiment is used to desalinate seawater:

[0111] S1: The coarse desalination chamber is placed at a depth of 200m in the deep sea. Seawater is filtered through a filtration unit, and the filtered seawater enters the desalination unit, where it undergoes coarse desalination treatment using a reverse osmosis membrane module to obtain coarsely desalinated water. Simultaneously, the concentrated water produced is directly discharged into the ocean. Analysis of the deep-sea seawater in this step revealed the following composition: TDS: 21652 mg / L, Ca... 2+ 338.7 mg / L, Mg 2+ 769.2 mg / L, SO4 2- 1947.6 mg / L, Cl - 10471 mg / L, HCO3 - The concentration of nitrogen (TDS) in the crude desalinated water obtained in this step is 163.7 mg / L, and the turbidity is less than 0.1 NTU. The composition of the water is as follows: TDS: 7538.6 mg / L, Ca... 2+ 118 mg / L, Mg 2+ 257.4 mg / L, SO4 2- 635.2 mg / L, Cl - 3687.1 mg / L, HCO3 -The concentration of the concentrate obtained in this step is 141.3 mg / L, and the turbidity is less than 0.1 NTU. The composition of the concentrate is as follows: TDS: 22964 mg / L, Ca... 2+ 416.2 mg / L, Mg 2+ 885 mg / L, SO4 2- : 2018.6 mg / L, Cl - 11706 mg / L, HCO3 - 496.3 mg / L, turbidity less than 0.1 NTU.

[0112] S2: The coarse desalination water in the coarse desalination tank is transported to the low-pressure desalination chamber by a deep-sea submersible pump and a high-pressure pump.

[0113] S3: The reverse osmosis membrane in the low-pressure desalination chamber desalinates the crude desalinated water to obtain fresh water, while the concentrated water produced is directly discharged into the ocean. Analysis of the fresh water obtained in this step revealed the following composition: TDS: 207 mg / L, Ca... 2+ 1.8 mg / L, Mg 2+ 4.2 mg / L, SO4 2- 6.5 mg / L, Cl - 134.6 mg / L, HCO3 - The concentration of the concentrate obtained in this step is 6.1 mg / L, and the turbidity is less than 0.1 NTU. The composition of the concentrate is as follows: TDS: 22223.8 mg / L, Ca... 2+ 350.75 mg / L, Mg 2+ 764.56 mg / L, SO4 2- 1894.49 mg / L, Cl - 10802.78 mg / L, HCO3 - 412.11 mg / L, turbidity less than 0.1 NTU.

[0114] In this embodiment, both the reverse osmosis membrane inside the desalination unit and the reverse osmosis membrane in the low-pressure desalination chamber are new reverse osmosis membranes, and both are 8 inches in size.

[0115] The preparation method of the new reverse osmosis membrane is as follows:

[0116] A1: 1,3,5-Trimethylphloroglucinol and 2,5-disulfonic acid-p-phenylenediamine were added to methanol or acetic acid solvent at a mass ratio of 1:2 and refluxed at 90°C for 12 h to obtain functionalized nanosheets.

[0117] A2: Add 0.7 wt% functionalized nanosheets and 1.1 wt% m-phenylenediamine to water, and disperse by ultrasonication to obtain an aqueous working solution containing nanosheets.

[0118] A3: Immerse the polysulfone / polyethersulfone membrane in the aqueous working solution for 3.5 min, remove it and continue to immerse it in the oil working solution for 5 s to carry out the interfacial polymerization reaction. After the reaction is completed, wash it with 40℃ warm water and dry it to obtain a new reverse osmosis membrane.

[0119] Example 4

[0120] This embodiment provides an ultra-low pressure seawater desalination system. The difference from Embodiment 1 is that the reverse osmosis membrane inside the desalination unit and the reverse osmosis membrane in the low-pressure desalination chamber are both decommissioned and regenerated reverse osmosis membranes.

[0121] The method for preparing the decommissioned and regenerated reverse osmosis membrane in this embodiment is as follows:

[0122] B1: Test the desalination rate of decommissioned seawater desalination reverse osmosis membranes, and then classify them according to different desalination rates. Desalination rates ≥98% are classified as Class I lightly fouled membranes, desalination rates 96-98% are classified as Class II moderately fouled membranes, desalination rates 92-96% are classified as Class III heavily fouled membranes, and desalination rates ≤92% are classified as Class IV severely fouled membranes.

[0123] B2: The membrane obtained in step B1 is cleaned. First, it is washed with an alkaline solution at pH 12 for 60 minutes, then soaked for 9 hours. This washing and soaking process is repeated twice, followed by rinsing until neutral. Then, it is washed with an acid solution at pH 2 for 60 minutes, then soaked for 9 hours. This washing and soaking process is repeated twice.

[0124] B3: The four types of membranes after cleaning were oxidized using a mixed aqueous solution with pH 9.5 containing 1 wt% sodium hypochlorite and 1 wt% hydrogen peroxide. The treatment time was 15 min for mildly fouled membranes, 20 min for moderately fouled membranes, 25 min for heavily fouled membranes, and 30 min for severely fouled membranes.

[0125] B4: Immediately after oxidation treatment, the membrane is reduced for 12 minutes using an aqueous solution containing 1 wt% sodium bisulfite, and then rinsed with water to obtain a decommissioned regenerated reverse osmosis membrane with a desalination rate of 74%.

[0126] The ultra-low pressure seawater desalination system of this embodiment was used to desalinate seawater. The composition of the deep-sea seawater in this embodiment was as follows: TDS: 21780 mg / L, Ca... 2+ 342.1 mg / L, Mg 2+ 786.4 mg / L, SO4 2- 1983 mg / L, Cl - 10863 mg / L, HCO3 - The total dissolved solids (TDS) concentration was 165.1 mg / L, and the turbidity was less than 0.1 NTU. The composition of the coarse desalination water was as follows: TDS: 7743.0 mg / L, Ca...2 + 128.7 mg / L, Mg 2+ 268.5 mg / L, SO4 2- 621.1 mg / L, Cl - 3895.2 mg / L, HCO3 - The concentration of the first batch of concentrate was 142 mg / L, and the turbidity was less than 0.1 NTU. The composition of the concentrate was as follows: TDS: 22573 mg / L, Ca... 2+ 428.4 mg / L, Mg 2+ 874.8 mg / L, SO4 2- 2040 mg / L, Cl - 10897 mg / L, HCO3 - 485.3 mg / L, turbidity less than 0.1 NTU.

[0127] The composition of the freshwater obtained in this embodiment is as follows: TDS: 214 mg / L, Ca 2+ 2.3 mg / L, Mg 2+ 3.6 mg / L, SO4 2- 6.9 mg / L, Cl - 141.8 mg / L, HCO3 - The concentration of the first batch of concentrate was 5.7 mg / L, and the turbidity was less than 0.1 NTU. The composition of the second batch of concentrate was as follows: TDS: 22837.63 mg / L, Ca... 2+ 382.88 mg / L, Mg 2+ 797.90 mg / L, SO4 2- 1852.15 mg / L, Cl - 11427.70 mg / L, HCO3 - 414.2 mg / L, turbidity less than 0.1 NTU.

[0128] Example 5

[0129] This embodiment provides an ultra-low pressure seawater desalination system. The difference from Embodiment 4 is that the method for preparing the decommissioned and regenerated reverse osmosis membrane in this embodiment is as follows:

[0130] B1: Test the desalination rate of decommissioned seawater desalination reverse osmosis membranes, and then classify them according to different desalination rates. Desalination rates ≥98% are classified as Class I lightly fouled membranes, desalination rates 96-98% are classified as Class II moderately fouled membranes, desalination rates 92-96% are classified as Class III heavily fouled membranes, and desalination rates ≤92% are classified as Class IV severely fouled membranes.

[0131] B2: Clean the membrane obtained from step B1. First, wash it with alkali at pH 10 for 30 minutes, then soak it for 6 hours. Repeat the washing and soaking once, and then rinse until neutral. Next, wash it with acid at pH 1 for 30 minutes, then soak it for 6 hours. Repeat the washing and soaking three times.

[0132] B3: The four types of membranes after cleaning were oxidized using a mixed aqueous solution with pH 9 containing 0.8 wt% sodium hypochlorite and 0.8 wt% hydrogen peroxide. The treatment time was 13 min for Class I slightly fouled membranes, 18 min for Class II moderately fouled membranes, 23 min for Class III heavily fouled membranes, and 28 min for Class IV severely fouled membranes.

[0133] B4: Immediately after oxidation treatment, the membrane is reduced for 10 minutes with an aqueous solution containing 0.5 wt% sodium bisulfite, and then rinsed with water to obtain a decommissioned regenerated reverse osmosis membrane with a desalination rate of 67%.

[0134] The ultra-low pressure seawater desalination system of this embodiment was used to desalinate seawater. The composition of the deep-sea seawater in this embodiment was as follows: TDS: 22528 mg / L, Ca... 2+ 352.6 mg / L, Mg 2+ 795.4 mg / L, SO4 2- : 2025.1 mg / L, Cl - 10883 mg / L, HCO3 - The total dissolved solids (TDS) concentration was 170.2 mg / L, and the turbidity was less than 0.1 NTU. The composition of the coarse desalination water was as follows: TDS: 7312.8 mg / L, Ca... 2 + 121.3 mg / L, Mg 2+ 247.2 mg / L, SO4 2- 654 mg / L, Cl - 3567.3 mg / L, HCO3 - The concentration of the first batch of concentrated water was 143.8 mg / L, and the turbidity was less than 0.1 NTU. The composition of the concentrated water was as follows: TDS: 23842 mg / L, Ca... 2+ 409.8 mg / L, Mg 2+ 907.1 mg / L, SO4 2- 1997.3 mg / L, Cl - 12071 mg / L, HCO3 - 511.8 mg / L, turbidity less than 0.1 NTU.

[0135] The composition of the freshwater obtained in this embodiment is as follows: TDS: 215.8 mg / L, Ca 2+ 1.9 mg / L, Mg2+ 4.7 mg / L, SO4 2- 6.8 mg / L, Cl - 137 mg / L, HCO3 - The concentration of the first batch of concentrate was 6.4 mg / L, and the turbidity was less than 0.1 NTU. The composition of the second batch of concentrate was as follows: TDS: 21545.74 mg / L, Ca... 2+ 360.66 mg / L, Mg 2+ 733.92 mg / L, SO4 2- 1950.94 mg / L, Cl - 10443.01 mg / L, HCO3 - 419.61 mg / L, turbidity less than 0.1 NTU.

[0136] Example 6

[0137] This embodiment provides an ultra-low pressure seawater desalination system. The difference from Embodiment 4 is that the method for preparing the decommissioned and regenerated reverse osmosis membrane in this embodiment is as follows:

[0138] B1: Test the desalination rate of decommissioned seawater desalination reverse osmosis membranes, and then classify them according to different desalination rates. Desalination rates ≥98% are classified as Class I lightly fouled membranes, desalination rates 96-98% are classified as Class II moderately fouled membranes, desalination rates 92-96% are classified as Class III heavily fouled membranes, and desalination rates ≤92% are classified as Class IV severely fouled membranes.

[0139] B2: Clean the membrane obtained from step B1. First, wash it with alkali at pH 13 for 90 min, then soak it for 12 h. Repeat the washing and soaking process 3 times, and then rinse it until neutral. Next, wash it with acid at pH 3 for 90 min, then soak it for 12 h. Repeat the washing and soaking process once.

[0140] B3: The four types of membranes after cleaning were oxidized using a mixed aqueous solution with pH 10 containing 1.2 wt% sodium hypochlorite and 1.2 wt% hydrogen peroxide. The treatment time was 17 min for Class I slightly fouled membranes, 22 min for Class II moderately fouled membranes, 27 min for Class III heavily fouled membranes, and 32 min for Class IV severely fouled membranes.

[0141] B4: Immediately after oxidation treatment, the membrane is reduced for 15 minutes with an aqueous solution containing 1.5 wt% sodium bisulfite, and then rinsed with water to obtain a decommissioned and regenerated reverse osmosis membrane with a desalination rate of 72%.

[0142] The ultra-low pressure seawater desalination system of this embodiment was used to desalinate seawater. The composition of the deep-sea seawater in this embodiment was as follows: TDS: 20786 mg / L, Ca... 2+343.6 mg / L, Mg 2+ 742.3 mg / L, SO4 2- 1986.4 mg / L, Cl - 10157 mg / L, HCO3 - The total dissolved solids (TDS) concentration was 168.6 mg / L, and the turbidity was less than 0.1 NTU. The composition of the coarse desalination water was as follows: TDS: 7747 mg / L, Ca... 2+ 120.4 mg / L, Mg 2+ 261 mg / L, SO4 2- 619.8 mg / L, Cl - 3780.4 mg / L, HCO3 - The concentration of the first batch of concentrated water was 144.7 mg / L, and the turbidity was less than 0.1 NTU. The composition of the concentrated water was as follows: TDS: 22348 mg / L, Ca... 2+ 428.4 mg / L, Mg 2+ 862.3 mg / L, SO4 2- 1988.9 mg / L, Cl - 11436 mg / L, HCO3 - 511.4 mg / L, turbidity less than 0.1 NTU.

[0143] The composition of the freshwater obtained in this embodiment is as follows: TDS: 205 mg / L, Ca 2+ 1.7 mg / L, Mg 2+ 4.4 mg / L, SO4 2- 6.8 mg / L, Cl - 138.2 mg / L, HCO3 - The concentration of the first batch of concentrated water was 5.9 mg / L, and the turbidity was less than 0.1 NTU. The composition of the second batch of concentrated water was as follows: TDS: 22853.65 mg / L, Ca... 2+ 358.16 mg / L, Mg 2+ 774.97 mg / L, SO4 2- 1847.64 mg / L, Cl - 11075.74 mg / L, HCO3 - 422.71 mg / L, turbidity less than 0.1 NTU.

[0144] Comparative Example 1

[0145] This comparative example provides a seawater desalination system, which differs from Example 1 in that the recovery rate of the desalination unit is 40%.

[0146] The seawater of the same quality as in Example 1 was desalinated using the seawater desalination system of this comparative example. The quality of the resulting crude desalinated water was as follows: TDS: 7812.3 mg / L, Ca... 2+ 123.7 mg / L, Mg 2+ 275.9 mg / L, SO4 2- 642.1 mg / L, Cl - 3780.4 mg / L, HCO3 - The concentration of the first concentrate was 144.7 mg / L, and the turbidity was less than 0.1 NTU. The water quality of the first concentrate was as follows: TDS: 32883.5 mg / L, Ca... 2+ 533.9 mg / L, Mg 2+ 1225.1 mg / L, SO4 2- 2619.7 mg / L, Cl - 16227.1 mg / L, HCO3 - The concentration was 840.3 mg / L, and the turbidity was less than 0.1 NTU. The quality of the permeate and concentrate in other stages was similar to that in Example 1.

[0147] In this comparative example, the TDS of the concentrate produced by the desalination unit was as high as 32883.5 mg / L, and the Ca... 2+ Mg 2+ The concentration has also increased significantly, far exceeding the original seawater quality range. Direct discharge would cause a sharp increase in salinity in local sea areas, damaging the habitat of marine life.

[0148] In addition, Ca in the concentrated water 2+ Mg 2+ Excessive concentration can easily form a dense scale layer on the surface of the reverse osmosis membrane, causing the membrane flux to drop in a short period of time, requiring frequent shutdowns for acid cleaning, increasing operation and maintenance costs and system energy consumption.

[0149] Comparative Example 2

[0150] This comparative example provides a seawater desalination system, which differs from Example 1 in that the desalination rate of the reverse osmosis membrane in the desalination unit is 45%.

[0151] The seawater desalination system of this comparative example was used to desalinate seawater of the same quality as that in Example 1. The quality of the crude desalinated water obtained was as follows: TDS: 12270.5 mg / L, Ca 2+ 193.9 mg / L, Mg 2+ 433.2 mg / L, SO4 2- 1008.6 mg / L, Cl - 5862.5 mg / L, HCO3 -The concentration of the desalination solution was 154.5 mg / L, the turbidity was less than 0.1 NTU, and the TDS of the obtained freshwater was 368.2 mg / L, which far exceeded the standard for drinking water (TDS < 150 mg / L) and could not meet the actual application requirements. This indicates that a desalination rate of 65-75% is the key range for balancing the coarse desalination effect and the low-pressure desalination load.

[0152] In this comparative example, the TDS of the desalinated water increased by approximately 57% compared to Example 1, and the Ca... 2+ Mg 2+ The concentrations of Cl⁻ also increased significantly, failing to achieve the effect of reducing the low-pressure desalination load through coarse desalination water pretreatment.

[0153] Comparative Example 3

[0154] This comparative example provides a seawater desalination system, which differs from Example 1 in that the desalination rate of the reverse osmosis membrane in the desalination unit is 85%.

[0155] Comparative Example 3, with its higher desalination rate, reduced TDS and ion content in the coarse desalination water. However, the reverse osmosis membrane required higher operating pressure, which was incompatible with the hydrostatic pressure of the deep sea. This resulted in a significant decrease in membrane flux and a substantial reduction in water production, failing to meet the demand for large-scale water supply.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An ultra-low pressure sea water desalination system, characterized in that, The crude desalination cabin, the crude desalination water conveying device and the low-pressure desalination cabin; The crude desalination cabin is arranged at a deep sea with a water depth of 150-200 m, and the crude desalination cabin comprises a desalination unit, and a reverse osmosis membrane group is arranged in the desalination unit, wherein the desalination rate of the reverse osmosis membrane in the reverse osmosis membrane group is 65-75%; The ratio of the water production of the desalination unit to the water inflow thereof is less than 15%; The crude desalination water conveying device is used for conveying the crude desalination water obtained by the crude desalination cabin to the low-pressure desalination cabin, the low-pressure desalination cabin is located above the sea surface, and a reverse osmosis membrane with a desalination rate greater than 99.5% is arranged in the low-pressure desalination cabin to desalinate the crude desalination water to obtain fresh water; The reverse osmosis membrane in the desalination unit and the reverse osmosis membrane in the low-pressure desalination cabin are new reverse osmosis membranes; The preparation method of the new reverse osmosis membrane is as follows: A1: 1,3,5-trimethylphloroglucinol and 2,5-disulfonic acid p-phenylenediamine are added to a methanol or acetic acid solvent in a mass ratio of 1:(1-2), and reflux reaction is carried out at 80-90 ℃ for 9-12 h to obtain functionalized nanosheets; A2: The functionalized nanosheets and m-phenylenediamine are added to water, and after ultrasonic dispersion, an aqueous phase working solution containing nanosheets is obtained; A3: A porous support membrane is immersed in the aqueous phase working solution, and after being taken out, it is continuously immersed in an oil phase working solution to carry out interfacial polymerization reaction, and after the reaction is completed, washing and drying treatment are carried out to obtain a new reverse osmosis membrane; the oil phase working solution is isooctane containing 0.1-0.15 wt% of trimesoyl chloride.

2. The ultra-low pressure sea water desalination system as claimed in claim 1, wherein, The crude desalination cabin further comprises a filtering unit connected with the desalination unit, and the filtering unit has a filtering pore size of 2-8 mm; After the deep sea water is filtered by the filtering unit, the filtered water enters the desalination unit.

3. The ultra-low pressure sea water desalination system of claim 2, wherein, The crude desalination cabin further comprises a crude fresh water collecting device, the water inlet of the crude fresh water collecting device is connected with the water outlet of the crude desalination cabin, the water outlet of the crude fresh water collecting device is connected with the water inlet of the low-pressure desalination cabin through the crude desalination water conveying device, and the crude fresh water collecting device is arranged in the same deep sea static pressure environment as the crude desalination cabin.

4. The ultra-low pressure sea water desalination system as claimed in claim 3, wherein, The desalination unit further comprises a membrane shell and a main water production pipe; The two ends of the membrane shell are respectively provided with a water inlet end cover and a concentrated water end cover, and a plurality of holes with a diameter of 3-6 mm are formed in the water inlet end cover and the concentrated water end cover; The reverse osmosis membrane group is located in the interior of the membrane shell, and the reverse osmosis membrane group comprises a plurality of reverse osmosis membranes connected in parallel, and the water production flux of the reverse osmosis membrane is 15-20 LMH; The main water production pipe is located in the interior of the membrane shell, the water inlet end of the main water production pipe is connected with the water production collecting pipe of each reverse osmosis membrane, and the water outlet end of the main water production pipe is connected with the water inlet of the crude fresh water collecting device.

5. A method of desalination of sea water using the ultra-low pressure sea water desalination system as claimed in any one of claims 1 to 4, wherein, The method comprises the following steps: S1: The crude desalination cabin is arranged at a deep sea with a water depth of 150-200 m, seawater is filtered by a filtering unit, the filtered seawater enters a desalination unit, the ratio of the water production of the desalination unit to the water inflow thereof is kept to be less than 15%, and then the seawater is subjected to crude desalination treatment by a reverse osmosis membrane with a desalination rate of 65-75% in the desalination unit to obtain crude desalination water, and the concentrated water generated is directly discharged into the sea; S2: the coarse desalination water conveying device conveys the coarse desalination water in the coarse desalination water collecting device to a low-pressure desalination cabin above the sea surface; S3: a reverse osmosis membrane with a desalination rate greater than 99.5% in the low-pressure desalination cabin desalinates the coarse desalination water to obtain fresh water, and the concentrated water generated is directly discharged into the sea.

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