Online forward osmosis-nanofiltration integrated membrane method and device for seawater desalination

Abstract

The application discloses a kind of online permeation cleaning forward osmosis-nanofiltration integrated membrane method seawater desalination method, comprising: a seawater, draw solution is respectively sent into the raw material liquid side and draw solution side of forward osmosis system, concentrated water is sent into raw material liquid tank, and product water is sent into draw solution tank;B after dilution draw solution is sent into nanofiltration system, concentrated water side after concentration product water is sent into concentrated water tank, and product water of clean water side is sent into product water tank;C when the product water rate of nanofiltration system clean water side drops to set product water rate, forward osmosis system and nanofiltration system are closed, and solution in nanofiltration system membrane assembly is discharged;D concentrated water in concentrated water tank is sent into permeation cleaning liquid tank as cleaning liquid, cleaning liquid in permeation cleaning liquid tank is sent into the concentrated water side of nanofiltration system, and product water in product water tank is sent into nanofiltration system clean water side, and nanofiltration membrane is carried out online permeation cleaning.The method of the application adopts the coupling mode of forward osmosis technology and nanofiltration technology, realizes low energy consumption seawater desalination, and realizes online cleaning of nanofiltration membrane simultaneously.

Description

Online Osmosis Cleaning Forward Osmosis-Nanofiltration Integrated Membrane Seawater Desalination Method and Apparatus Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to an online permeation cleaning method for seawater desalination using an integrated forward osmosis-nanofiltration membrane, and further relating to an online permeation cleaning device for seawater desalination using an integrated forward osmosis-nanofiltration membrane. Background Technology

[0002] The main seawater desalination technologies currently used are multi-effect evaporation, multi-stage flash evaporation, and reverse osmosis. However, these technologies have drawbacks such as high energy consumption, high processing costs, serious membrane fouling, or low water production rates, which restrict the development and utilization of seawater resources.

[0003] Forward osmosis technology, as a novel membrane separation technology with low energy consumption, minimal membrane fouling, easy cleaning, and high water production rate, has broad application prospects in fields such as seawater desalination and wastewater treatment.

[0004] Forward osmosis utilizes the osmotic pressure difference between the solutions on both sides of a membrane as the driving force, causing water to spontaneously pass from the feed solution (with lower osmotic pressure) through a selectively permeable membrane to the driving solution (with higher osmotic pressure). Forward osmosis uses a special solute to prepare the draw-drive solution, and high-concentration draw-drive solutions can be artificially controlled to achieve higher osmotic driving pressures and thus higher water recovery rates.

[0005] However, the current forward osmosis seawater desalination technology is complex and not conducive to cost reduction, so further in-depth research on seawater desalination technology is needed. Summary of the Invention

[0006] This invention is based on the inventors' discoveries and understanding of the following facts and problems: Forward osmosis faces two main problems in seawater desalination. First, the draw solution in forward osmosis is diluted after water permeates through, requiring concentration and recovery to maintain osmotic pressure. This recovery process must be combined with other treatment processes, such as evaporation, reverse osmosis, and membrane distillation. Second, there is the problem of nanofiltration membrane fouling in the draw solution regeneration process. Membrane fouling during seawater desalination increases the overall cost due to cleaning and replacement. Therefore, there is an urgent need to develop a simple and low-cost seawater desalination method.

[0007] This invention aims to at least partially solve one of the technical problems in related technologies. To this end, embodiments of this invention propose an online permeation cleaning method for seawater desalination using an integrated forward osmosis-nanofiltration membrane. This method couples nanofiltration with forward osmosis processes, enabling not only the recovery of the draw solution but also online permeation cleaning of the fouled nanofiltration membrane. This improves water recovery rates, effectively alleviates nanofiltration membrane fouling, and reduces membrane usage costs during seawater desalination.

[0008] The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method of this invention includes:

[0009] a) Seawater and draw liquid are fed into the feed liquid side and draw liquid side of the forward osmosis system, respectively. After forward osmosis treatment, the concentrate produced on the feed liquid side is sent into the feed liquid tank, and the permeate on the draw liquid side is sent into the draw liquid tank.

[0010] b. The draw solution diluted by the forward osmosis system in step a is sent to the concentrate side of the nanofiltration system. After being concentrated by nanofiltration, the permeate from the concentrate side is sent to the concentrate tank, and the permeate from the clear water side is sent to the permeate tank.

[0011] c. When the water production rate on the clear water side of the nanofiltration system drops to the set water production rate, shut down the forward osmosis system and the nanofiltration system, reduce the pressure in the nanofiltration system pipeline to zero, and discharge the solution from the nanofiltration system membrane module.

[0012] d. The concentrated water from the concentrate tank in step b, after being concentrated by nanofiltration, is sent as the cleaning solution to the permeate cleaning solution tank. The cleaning solution in the permeate cleaning solution tank is sent to the concentrate side of the nanofiltration system. The permeate from the permeate tank in step b is sent to the clean water side of the nanofiltration system. A permeate process, which is opposite to the nanofiltration permeate process, is carried out in the membrane module of the nanofiltration system to perform online permeate cleaning of the nanofiltration membrane.

[0013] The advantages and technical effects of the online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method of this invention are as follows: 1. In this embodiment of the invention, forward osmosis technology is used to desalinate seawater, and nanofiltration technology is used to concentrate and regenerate the draw solution, achieving low-energy seawater desalination and effectively reducing nanofiltration membrane fouling; 2. In the method of this embodiment of the invention, forward osmosis and nanofiltration technologies are integrated. The diluted draw solution generated by the forward osmosis system is concentrated by nanofiltration, and the concentrated water generated during the nanofiltration process is used as the cleaning solution for cleaning the nanofiltration membrane. The two membrane processes complement each other, reducing the use of external reagents and realizing online cleaning of the nanofiltration membrane; 3. In the method of this embodiment of the invention, the cleaning of the nanofiltration membrane does not require disassembly of the membrane components or external equipment, realizing online cleaning of the membrane module, reducing damage to the membrane module, extending the service life of the membrane module, and reducing the operating cost of the process.

[0014] In some embodiments, in step a, the extracting solution comprises a sodium sulfate solution, and / or the concentration of the extracting solution is 1.0 mol / L to 1.7 mol / L.

[0015] In some embodiments, the forward osmosis process in step a is continuous, and the nanofiltration process in step b is intermittent. When the permeate rate on the draw solution side of the forward osmosis process decreases by 20-30%, the nanofiltration process in step b is started. The draw solution is sent from the draw solution tank to the concentrate tank by a peristaltic pump. After being concentrated by nanofiltration, the draw solution in the concentrate tank is returned to the concentrate tank by a peristaltic pump. Then, the concentrated draw solution in the concentrate tank is sent back to the draw solution tank by a peristaltic pump.

[0016] In some embodiments, in step c, when the permeate rate of the nanofiltration system on the clean water side decreases by 20-30% during the nanofiltration process in step b, the forward osmosis system and the nanofiltration system are shut down, the pressure in the nanofiltration system pipeline is reduced to zero, the solution in the nanofiltration system membrane module is discharged, and the nanofiltration membrane is subjected to the online permeation cleaning described in step d.

[0017] In some embodiments, in step d, the online permeation cleaning time for the nanofiltration membrane is 60-600 seconds, and the flow rate of the cleaning solution during the cleaning process is 0.01-0.03 m / s.

[0018] This invention also provides an online permeation-cleaning integrated forward osmosis-nanofiltration membrane seawater desalination device, comprising a forward osmosis system, a nanofiltration system, a feed liquid tank, a draw liquid tank, a concentrate tank, a product water tank, and a permeation-cleaning liquid tank, wherein...

[0019] The forward osmosis system includes a feed liquid side and a draw liquid side. The inlet of the feed liquid side is connected to the outlet of the feed liquid tank, and the outlet of the feed liquid side is connected to the inlet of the feed liquid tank. The inlet of the draw liquid side is connected to the outlet of the draw liquid tank, and the outlet of the draw liquid side is connected to the inlet of the draw liquid tank.

[0020] The nanofiltration system includes a concentrate side and a clear water side. The inlet of the concentrate side is connected to the outlet of the concentrate tank via a plunger pump, and the inlet of the concentrate side is connected to the outlet of the permeation cleaning solution tank via a peristaltic pump. The outlet of the concentrate side is connected to the inlets of the concentrate tank and the permeation cleaning solution tank, respectively. The inlet of the clear water side is connected to the outlet of the product water tank, and the outlet of the clear water side is connected to the inlet of the product water tank.

[0021] The inlet of the extraction tank is connected to the outlet of the concentrate tank, and the outlet of the extraction tank is connected to the inlet of the concentrate tank.

[0022] The inlet of the permeation cleaning fluid tank is connected to the outlet of the concentrate tank.

[0023] The advantages and technical effects of the online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination device of this invention are as follows: 1. The device of this invention integrates a forward osmosis system and a nanofiltration system. The nanofiltration system concentrates the diluted draw solution produced by the forward osmosis system, and the concentrated water produced by the nanofiltration system is used as the cleaning solution for cleaning the nanofiltration membrane. The two membrane processes complement each other, reducing the use of external reagents; 2. The device of this invention does not require disassembly of the nanofiltration membrane component or external equipment during membrane cleaning, realizing online cleaning of the membrane module, reducing damage to the membrane module, extending the service life of the membrane module, and reducing the operating cost of the process.

[0024] In some embodiments, the nanofiltration membrane of the nanofiltration system is NF90.

[0025] In some embodiments, the membrane assembly of the nanofiltration system is a spiral wound membrane or a plate membrane.

[0026] In some embodiments, a stirrer and / or a temperature controller are provided in the feed liquid tank, the draw liquid tank, the concentrate tank and / or the permeation cleaning liquid tank.

[0027] In some embodiments, a conductivity meter probe is provided in the liquid extraction tank. Attached Figure Description

[0028] Figure 1 is a schematic diagram of an online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination device according to an embodiment of the present invention;

[0029] Figure 2 is a normalized flux curve of the nanofiltration process in the long-term operation of the online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination device according to an embodiment of the present invention.

[0030] Figure 3 is a graph showing the change in cleaning efficiency of the online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination device according to an embodiment of the present invention as a function of the number of cleaning cycles. Detailed Implementation

[0031] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0032] This invention provides an online permeation cleaning method for seawater desalination using an integrated forward osmosis-nanofiltration membrane, comprising:

[0033] a) Seawater and draw liquid are fed into the feed liquid side and draw liquid side of the forward osmosis system, respectively. After forward osmosis treatment, the concentrate produced on the feed liquid side is sent into the feed liquid tank, and the permeate on the draw liquid side is sent into the draw liquid tank.

[0034] b. The draw solution diluted by the forward osmosis system in step a is sent to the concentrate side of the nanofiltration system. After being concentrated by nanofiltration, the permeate from the concentrate side is sent to the concentrate tank, and the permeate from the clear water side is sent to the permeate tank.

[0035] c. When the water production rate on the clear water side of the nanofiltration system drops to the set water production rate, shut down the forward osmosis system and the nanofiltration system, reduce the pressure in the nanofiltration system pipeline to zero, and discharge the solution from the nanofiltration system membrane module.

[0036] d. The concentrated water from the concentrate tank in step b, after being concentrated by nanofiltration, is sent as the cleaning solution to the permeate cleaning solution tank. The cleaning solution in the permeate cleaning solution tank is sent to the concentrate side of the nanofiltration system. The permeate from the permeate tank in step b is sent to the clean water side of the nanofiltration system. A permeate process, which is opposite to the nanofiltration permeate process, is carried out in the membrane module of the nanofiltration system to perform online permeate cleaning of the nanofiltration membrane.

[0037] This invention discloses an online permeation cleaning method for seawater desalination using an integrated forward osmosis-nanofiltration membrane. The method employs forward osmosis for seawater desalination and nanofiltration for concentrating and regenerating the draw solution, achieving low-energy seawater desalination and effectively reducing nanofiltration membrane fouling. In this method, forward osmosis and nanofiltration are integrated. The diluted draw solution from the forward osmosis system is concentrated using nanofiltration, and the concentrated water produced during nanofiltration is used as the cleaning solution for the nanofiltration membrane. The two membrane processes complement each other, reducing the use of external reagents and enabling online cleaning of the nanofiltration membrane. Furthermore, the method eliminates the need to disassemble the membrane components or connect external equipment for cleaning the nanofiltration membrane, achieving online cleaning of the membrane module. This reduces damage to the membrane module, extends its service life, and lowers the operating costs of the process.

[0038] In some embodiments, in step a, the extracting solution comprises a sodium sulfate solution, and / or the concentration of the extracting solution is 1.0 mol / L to 1.7 mol / L.

[0039] In some embodiments, the forward osmosis process in step a is continuous, and the nanofiltration process in step b is intermittent. When the permeate rate on the draw solution side of the forward osmosis process decreases by 20-30%, the nanofiltration process in step b is started. The draw solution is sent from the draw solution tank to the concentrate tank by a peristaltic pump. After being concentrated by nanofiltration, the draw solution in the concentrate tank is returned to the concentrate tank by a peristaltic pump. Then, the concentrated draw solution in the concentrate tank is sent back to the draw solution tank by a peristaltic pump.

[0040] In some embodiments, in step c, when the permeate rate of the nanofiltration system on the clean water side decreases by 20-30% during the nanofiltration process in step b, the forward osmosis system and the nanofiltration system are shut down, the pressure in the nanofiltration system pipeline is reduced to zero, the solution in the nanofiltration system membrane module is discharged, and the nanofiltration membrane is subjected to the online permeation cleaning described in step d.

[0041] In some embodiments, in step d, the online permeation cleaning time for the nanofiltration membrane is 60-600 seconds, and the flow rate of the cleaning solution during the cleaning process is 0.01-0.03 m / s.

[0042] As shown in Figure 1, this embodiment of the invention also provides an online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination device, including a forward osmosis system 1, a nanofiltration system 2, a feed liquid tank 3, a draw liquid tank 4, a concentrate tank 5, a product water tank 6, and a permeation cleaning liquid tank 7.

[0043] The forward osmosis system 1 includes a feed liquid side and a draw liquid side. The inlet of the feed liquid side is connected to the outlet of the feed liquid tank 3, and the outlet of the feed liquid side is connected to the inlet of the feed liquid tank 3. The inlet of the draw liquid side is connected to the outlet of the draw liquid tank 4, and the outlet of the draw liquid side is connected to the inlet of the draw liquid tank 4.

[0044] The nanofiltration system includes a concentrate side and a clean water side. The inlet of the concentrate side is connected to the outlet of the concentrate tank 5 via a plunger pump 8, and the inlet of the concentrate side is connected to the outlet of the permeation cleaning fluid tank 7 via a peristaltic pump 9. The outlet of the concentrate side is connected to the inlets of the concentrate tank 5 and the permeation cleaning fluid tank 7, respectively. The inlet of the clean water side is connected to the outlet of the product water tank 6, and the outlet of the clean water side is connected to the inlet of the product water tank 6.

[0045] The inlet of the draw liquid tank 4 is connected to the outlet of the concentrate tank 5, and the outlet of the draw liquid tank 4 is connected to the inlet of the concentrate tank 5. The inlet of the permeate cleaning liquid tank 7 is connected to the outlet of the concentrate tank 5.

[0046] In some embodiments, the nanofiltration membrane used in nanofiltration system 2 is NF90.

[0047] In some embodiments, the membrane assembly of the nanofiltration system 2 is a spiral wound membrane or a plate membrane.

[0048] In some embodiments, a stirrer and a temperature controller (not shown in the figure) are provided in the raw material tank 3, the extraction tank 4, the concentrate tank 5, and the permeation cleaning tank 7.

[0049] In some embodiments, a conductivity meter probe is installed in the draw liquid tank 4. When the concentration of the draw liquid in the draw liquid tank is insufficient for the forward osmosis process, inorganic salts can be added to carry out the forward osmosis seawater desalination process.

[0050] Example 1

[0051] Seawater desalination is performed using the apparatus shown in Figure 1, an embodiment of the present invention.

[0052] Seawater is fed into the feed liquid side of the forward osmosis system 1 via feed liquid tank 3. A 1.2 mol / L sodium sulfate solution is used as the draw liquid and fed into the draw liquid side of the forward osmosis system 1 via draw liquid tank 4. After the seawater is treated by the forward osmosis system 1, the concentrate on the feed liquid side is returned to the feed liquid tank 3, and the permeate on the draw liquid side is returned to the draw liquid tank 4.

[0053] When the permeate rate on the draw solution side of the forward osmosis system 1 decreases by 25%, the nanofiltration system 2 is started. The nanofiltration membrane used in the membrane module of the nanofiltration system is an NF270 nanofiltration membrane. The draw solution in the draw solution tank 4 is pumped into the concentrate tank 5, and then enters the concentrate side of the nanofiltration system 2 through the plunger pump 8 via the outlet of the concentrate tank 5. After nanofiltration treatment, the concentrated draw solution is returned to the concentrate tank 5, and the permeate on the clear water side of the nanofiltration system 2 enters the permeate tank 6, completing the seawater desalination treatment.

[0054] When the permeate flow rate on the clear water side of nanofiltration system 2 decreases by 30%, close the inlet and outlet valves of forward osmosis system 1, shut off plunger pump 8, open the pressure regulating valve to bring the pressure in the nanofiltration system 2 pipeline to zero, discharge the solution from the membrane module of nanofiltration system 2, and close the valve connecting the concentrate side of nanofiltration system 2 to concentrate tank 5. Pump the concentrate in concentrate tank 5 into permeate cleaning solution tank 7 as permeate cleaning solution, and send the cleaning solution into the concentrate side of nanofiltration system 2 through peristaltic pump 9. Pump the permeate in permeate tank 6 into the clear water side of nanofiltration system 2. At this time, the membrane module of nanofiltration system 2 is undergoing a permeate process that is opposite to the permeate flow process of nanofiltration treatment, performing online cleaning of the nanofiltration membrane. The flow rate of the cleaning solution is 0.0213 m / s, and the cleaning time is 10 min. During the permeate cleaning, the permeate from the concentrate side of nanofiltration system 2 is sent into permeate cleaning solution tank 7.

[0055] In this embodiment, the cleaning efficiency of the nanofiltration membrane after cleaning is 93.6%.

[0056] The cleaning efficiency is calculated as follows:

[0057] Cleaning efficiency = J1 / J0

[0058] In the formula, J0 represents the normalized flux of the nanofiltration membrane at the start of the nanofiltration process, in L / (m²). 2 ·h);

[0059] J1 — Normalized flux of the nanofiltration membrane after the nanofiltration process is completed and the membrane has been thoroughly cleaned, L / (m²) 2 ·h).

[0060] The quality of the produced water in the desalination tank 6 of this embodiment is shown in Table 1.

[0061] Table 1

[0062]

[0063] As shown in Table 1, the concentrations of most ions meet the standards for drinking water. However, the concentration of Cl... - The content is relatively high, mainly because some ions from the feed solution permeate through the forward osmosis membrane into the draw solution during the forward osmosis process. The diluted draw solution, during nanofiltration desalination and concentration, exhibits reduced SO42- content due to the Donnan effect. 2- The retention effect is stronger than Cl - .

[0064] Comparative Example 1

[0065] Similar to the seawater desalination method used in Example 1, the difference is that when the water production rate on the clear water side of nanofiltration system 2 decreases by 30%, the nanofiltration concentrate used in Example 1 is not used as the cleaning solution for online permeation cleaning. Instead, the forward osmosis system and the nanofiltration system are shut down, the nanofiltration membrane module is disassembled, and a 1.2 mol / L sodium sulfate solution is used as the cleaning solution to clean the nanofiltration membrane.

[0066] After cleaning in this comparative example, the cleaning efficiency of the nanofiltration membrane was 84.3%.

[0067] Comparative Example 2

[0068] The method is the same as the seawater desalination method in Example 1, except that when the water production rate on the clean water side of nanofiltration system 2 decreases by 30%, the nanofiltration concentrate in Example 1 is not used as the cleaning solution for permeation cleaning. Instead, the forward osmosis system and the nanofiltration system are shut down, the nanofiltration membrane module is disassembled, and clean water is used as the cleaning solution to clean the nanofiltration membrane.

[0069] After cleaning in Comparative Example 2, the cleaning efficiency of the nanofiltration membrane was 81.6%.

[0070] The online penetration cleaning method of Embodiment 1 of the present invention combines the functions of physical rinsing, penetration backwashing and dissolution, which greatly improves the cleaning efficiency. In contrast, the salt washing used in Comparative Example 1 can only achieve physical rinsing and dissolution, while Comparative Example 2 can only achieve physical rinsing, and the cleaning effect is significantly reduced.

[0071] The online permeation cleaning method of forward osmosis-nanofiltration integrated membrane seawater desalination in Example 1 and the method of Comparative Example 2 were used to investigate the flux recovery of the nanofiltration membrane during long-term operation.

[0072] Test conditions: Temperature was 25±1℃, and the transmembrane pressure difference during normal operation of the nanofiltration system was 0.7MPa. During the nanofiltration membrane cleaning process, the concentration of the cleaning solution in the online permeation cleaning process of Example 1 was 1.2mol / L, the flow rate of the cleaning solution in Example 1 and Comparative Example 2 was 0.0213m / s, and the cleaning time for each nanofiltration membrane was 10min.

[0073] Figure 2 shows the normalized flux curves of the nanofiltration process during long-term operation of the online permeation cleaning method of Example 1 and the hydraulic cleaning method of Comparative Example 2. As can be seen from Figure 2, the normalized flux gradually decreases during long-term operation. After four cleaning cycles, both the online permeation cleaning method of Example 1 and the hydraulic cleaning method of Comparative Example 2 increase the normalized flux, which then continues to decrease. However, with the increase in the number of cleaning cycles, the online permeation cleaning method of Example 1 is more beneficial to improving the normalized flux of the nanofiltration membrane after cleaning compared to the hydraulic cleaning method of Comparative Example 2. Therefore, the permeation cleaning method of Example 1 is significantly superior to the hydraulic cleaning method of Comparative Example 1.

[0074] As shown in Figure 3, with the increase in the number of nanofiltration cycles, the cleaning efficiency of both Example 1 and Comparative Example 2 showed a decreasing trend to varying degrees. Using the online permeation cleaning method of Example 1, the cleaning efficiency decreased from 93.6% to 86.7% after four cleaning cycles. In contrast, using the hydraulic cleaning method of Comparative Example 2, the cleaning efficiency decreased significantly from 81.6% to 73.3% after four cleaning cycles. It is evident that after multiple cleaning cycles, the cleaning efficiency of the hydraulic cleaning method in Comparative Example 2 decreased at a significantly higher rate than that of the permeation cleaning method in Example 1. In Comparative Example 2, the cleaning efficiency decreased by 8.3% in the fourth cleaning cycle compared to the first cleaning cycle, while the online permeation cleaning method in Example 1 only decreased by 6.9%.

[0075] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," and "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0076] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for seawater desalination using an integrated forward osmosis-nanofiltration membrane with online permeation cleaning, characterized in that, include: Step a: Seawater and draw solution are fed into the feed liquid side and draw liquid side of the forward osmosis system, respectively. After forward osmosis treatment, the concentrate produced on the feed liquid side is sent to the feed liquid tank, and the permeate from the draw liquid side is sent to the draw liquid tank. Step b: The draw solution diluted by the forward osmosis system in step a is sent to the concentrate side of the nanofiltration system. After nanofiltration concentration, the concentrate from the concentrate side is sent to the concentrate tank, and the permeate from the clear water side is sent to the permeate tank. Step c: When the permeate rate on the clear water side of the nanofiltration system drops to the set permeate rate, the forward osmosis system is shut down. In step d, the pressure inside the nanofiltration system pipeline is reduced to zero, and the solution inside the nanofiltration system membrane module is discharged. The concentrated water from the concentrate tank in step b, after being concentrated by nanofiltration, is sent as a cleaning solution to the permeate cleaning solution tank. The cleaning solution in the permeate cleaning solution tank is sent to the concentrate side of the nanofiltration system, and the permeate from the permeate tank in step b is sent to the clean water side of the nanofiltration system. A permeate process, which is opposite to the nanofiltration permeate process, is carried out inside the membrane module of the nanofiltration system to perform online permeate cleaning of the nanofiltration membrane.

2. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 1, characterized in that, In step a, the extracting solution includes a sodium sulfate solution, and / or the concentration of the extracting solution is 1.0 mol / L to 1.7 mol / L.

3. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 1, characterized in that, The forward osmosis process in step a is continuous, while the nanofiltration process in step b is intermittent. When the permeate rate on the draw liquid side of the forward osmosis process decreases by 20-30%, the nanofiltration process in step b is started. The draw liquid is sent from the draw liquid tank to the concentrate tank by a peristaltic pump. The draw liquid in the concentrate tank is concentrated by nanofiltration and returned to the concentrate tank. Then, the concentrated draw liquid in the concentrate tank is sent back to the draw liquid tank by a peristaltic pump.

4. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 1, characterized in that, In step c, when the water production rate of the nanofiltration system on the clean water side decreases by 20-30% during the nanofiltration process in step b, the forward osmosis system and the nanofiltration system are shut down, the pressure in the nanofiltration system pipeline is reduced to zero, the solution in the nanofiltration system membrane module is discharged, and the nanofiltration membrane is subjected to the online permeation cleaning described in step d.

5. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 1, characterized in that, In step d, the online permeation cleaning time for the nanofiltration membrane is 60-600 seconds, and the flow rate of the cleaning solution during the cleaning process is 0.01-0.03 m / s.

6. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 1, characterized in that, The online permeate-cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method employs an apparatus including a forward osmosis system, a nanofiltration system, a feed liquid tank, a draw liquid tank, a concentrate tank, a product water tank, and a permeate-cleaning liquid tank. The forward osmosis system includes a feed liquid side and a draw liquid side. The inlet of the feed liquid side is connected to the outlet of the feed liquid tank, and the outlet of the feed liquid side is connected to the inlet of the feed liquid tank. The inlet of the draw liquid side is connected to the outlet of the draw liquid tank, and the outlet of the draw liquid side is connected to the inlet of the draw liquid tank. The nanofiltration system includes a concentrate side and a purified water side. The inlet on the concentrate side is connected to the outlet of the concentrate tank via a plunger pump, and the inlet on the concentrate side is connected to the outlet of the permeation cleaning solution tank via a peristaltic pump. The outlet on the concentrate side is connected to the inlets of both the concentrate tank and the permeation cleaning solution tank. The inlet on the clear water side is connected to the outlet of the product water tank, and the outlet on the clear water side is connected to the inlet of the product water tank. The inlet of the draw liquid tank is connected to the outlet of the concentrate tank, and the outlet of the draw liquid tank is connected to the inlet of the concentrate tank. The inlet of the permeation cleaning solution tank is connected to the outlet of the concentrate tank.

7. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 6, characterized in that, The nanofiltration membrane of the nanofiltration system is NF90.

8. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 6 or 7, characterized in that, The membrane module of the nanofiltration system is a spiral wound membrane or a plate membrane.

9. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 6, characterized in that, A stirrer and / or a temperature controller are installed in the raw material tank, the extraction tank, the concentrate tank, and / or the permeation cleaning tank.

10. The online permeation cleaning forward osmosis-nanofiltration integrated membrane seawater desalination method according to claim 6, characterized in that, A conductivity meter probe is installed in the liquid extraction tank.

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