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Research Trends in Reverse Osmosis Desalination Technology The latest statistics from the International Desalination Association (IDA) show that by the beginning of 2002, the total freshwater output of desalination devices around the world had reached 32.4 million t/d. And it will increase at an annual growth rate of 10% to 30%. Seawater desalination technology is playing an increasingly important role in solving the worldwide freshwater shortage problem. Among the main seawater desalination methods currently used in the world (including multi-stage flash evaporation, multi-effect evaporation, and RO), reverse osmosis (RO) technology has the advantages of low investment, easy operation, short construction period, and fastest development speed. RO technology was initially used for brackish water (BW) desalination. Later, with the advancement and development of RO membrane technology, it was first used to treat seawater by Loeb and Sourirajan in 1960. At present, seawater reverse osmosis (SWRO) desalination technology has been widely used around the world and is relatively mature technically. However, the cost of water production is still higher than the price of traditional municipal water supply, and the negative impact of its process operations on the environment is becoming increasingly obvious and prominent. In the 21st century, energy and environmental issues are increasingly affected by various countries.* * Nowadays, the above two aspects have become the focus of SwRO desalination technology research in the future. 1 Research on energy recovery device for SWRO desalination In the SWRO desalination system, the power consumption of the high-pressure pump accounts for about 35% of the system operating costs and is one of the main factors affecting the cost of product water. Usually the operating pressure of SWRO is 5.5~8.0 MPa, and the residual pressure of the concentrated brine discharged from the membrane device is also as high as 5.0~6.0 MPa. Calculated based on a 40% recovery rate, about 60% of the feed pressure energy in the concentrated brine has huge recovery potential and significance. Energy recovery devices used in SwRO desalination systems currently on the market are mainly divided into two categories based on their working principles.: Positive displacement principle and hydraulic turbine principle. The four types of energy recoverers, Px, DWEER, PES and Aqua-lyng, which use the principle of positive displacement, only need to go through the one-step conversion process of "pressure energy to pressure energy" when recovering concentrated brine energy. The energy recovery efficiency is high (91%--96%) and can be used in systems of different sizes. The economic benefits are more obvious when used in large systems. ； In the process of recovering concentrated brine energy, the hydraulic turbine energy recovery device needs to go through a two-step conversion process of "pressure energy - mechanical energy - pressure energy". The process recovery efficiency is relatively low (50% to 70%). It is still used in small-scale SwRO desalination systems. The use of new high-efficiency energy recovery devices has significantly reduced the cost of SWRO desalinated water ； Using Px and Aquatyng energy recovery devices, the power consumption of the SWRO desalination process can be lower than 2.0 kW·h/m3. Qingdao, Changdao in Shandong Province and Shengsi SWRO desalination plants in Zhejiang Province use PX energy recovery devices. Our country has also listed the "research and development of new high-efficiency residual pressure recovery devices" as a key research project in the "Tenth Five-Year Plan". Currently, experimental research on related aspects of this project is in progress. 2 Research on the new SWRO desalination process Among the operating parameters of the SWRO desalination process, the recovery rate has the greatest impact on system investment and operating costs14. Normally, the recovery rate of a desalination system is 40% to 45%. However, the BCS (brine conversion system) system developed by Japan's Toray Company uses SWRO's first-stage concentrated water as feed material to produce product water under extreme operating conditions (operating pressure 8.0-10.0 mPa, feed salt content 5.8%-8.7%). The system's recovery rate can reach 60% r50j. The successful development of the new high-pressure and anti-fouling membrane unit SU-820BCM and the application of the new high-efficiency energy recovery device made the B(§ system possible. In March 1999, the BCS system was first commercialized in the MasPalomas 4 500 t/d device in Spain. There are currently 12 BCS systems in operation around the world. 3 Research on New SWRO Membranes At present, the main seawater desalination modules in the world are the "Pem~sep B-10" aromatic polyamide hollow fiber module of the American Du Pont Company, the "Hollosep HM-9255" CTA hollow fiber module of Japan's Toyobo, and the seawater desalination roll components of Filmtec, Fluid System, Hydranautics, Toray and Desal. Seawater reverse osmosis membrane and its components are the core of SWRO desalination technology. There are two development trends in SWRO membrane research: Research on low-pressure SWRO membrane and research on high-pressure SWRO membrane|9J． As the efficiency of energy recovery devices continues to improve, high-pressure membranes are relatively more used in the seawater desalination process, while low-pressure membranes are mainly used in the brackish water desalination process. In terms of research on high-pressure membranes, the high-performance hollow fiber membrane developed by Japan's Toyobo Company is manufactured using high-concentration polymer hollow fiber spinning technology and microporous technology, and is heat treated at high temperature. The membrane has a larger water flux. The membrane device composed of this hollow fiber is arranged in staggered layers and is resistant to high pressure (8.4 MPa) and pollution. In practical applications, the membrane device is used for single-stage operation, that is,_uI achieves a large recovery rate (60%), and the consumption of chemicals is low. Using this membrane device l port I can reduce the total cost of process water production by about 20%. In addition, Japan's Toray Public Foundation has developed a U'FC-80BCM membrane that is resistant to high pressure and can operate at large concentrations, and has been successfully used in BCS systems. Filmtec developed by Dow Filmtec SW-30HR-320 seawater desalination RO membrane element adopts a special structural design, which can increase the effective area of the membrane and the turbulence performance of the feed water without increasing the external dimensions of the membrane. Therefore, it is particularly suitable for highly polluted seawater sources (such as when the incoming water is surface seawater), and can save 25% of process energy consumption and reduce chemical cleaning costs of the membrane by 4%. In terms of low-pressure SWRO membrane research, TM710, TM720-400, and FM720 430 produced by Japan's Foray Company belong to the low-pressure membrane series. ； In addition, the company's 'FMC-10, 'I" MG20-400 and TMG20-430 energy-saving ultra-low pressure reverse osmosis membrane series have a minimum operating pressure of 0.75 MPa. 4 Development of environmentally friendly processes The impact of SWRO desalination on the environment is often ignored. With the proposal and implementation of the sustainable development strategy in the 21st century, the development of environmentally friendly seawater desalination processes is increasingly favored and valued. In SWRO desalination operations, due to the characteristics of seawater itself: High hardness and TDS (total dissolved solids) values, turbidity and unstable bacterial content often bring problems such as scaling, membrane fouling and high energy consumption to process operations. Therefore, good pretreatment is crucial to the success of the SWRO desalination process. The traditional seawater pretreatment method, that is, the method of adding chemicals, is not only complicated in process, but also has high pretreatment cost. Especially when a variety of chemicals added in the process are discharged with concentrated brine, it will have a greater negative impact on the environment of the discharge site. In addition, the waste liquid from membrane cleaning contains strong acid or alkali, and its discharge has also become part of the environmental problems in the SWRO desalination process. In this case, the use of full-membrane pretreatment instead of traditional chemical pretreatment has become an inevitable development trend. SWRO desalination process membrane pretreatment mainly includes filtration (MF), ultrafiltration (UF) and nanofiltration (NF)_Two kinds. MF has an obvious effect in removing suspended solids and reducing SDI (siltation density index) t, and the cost is less than that of beachwell, which is half of the cost of traditional methods. UF can not only intercept suspended solids and large bacteria, but also large molecules, colloids and small bacteria. NF is a membrane process between UF and RO. Its selective permeability for monovalent and divalent ions has a special role in preventing structure and reducing feed TDS. In recent years, MF, UF and NF technologies have all achieved considerable development, but in comparison, the development and application of UF and NF are more prominent. The new UF pretreatment is based on a large-diameter capillary membrane with a diameter of 0.7 to 0.9 mm. This component has two major advantages: Capable of automatically flushing the capillary membrane frequently and in short cycles, with stable permeate water flux ； Can operate with low flow rate, cross flow or dead end. Compared with traditional pretreatment methods, there is no need to continuously add chemicals and less manpower is required. Capillary membrane technology as pretreatment of SWRO can enable the process to have a high water production rate and reduce system water costs by 10%. As a pretreatment, NF is different from MF and UF. NF pretreatment is not only related to the water quality of the pretreatment product, but also related to the SWRO desalination process itself. The first person who used NF for pretreatment and conducted research in this area was SWCC (Saline Water ConversionCorlx)ration). The research results show that using NF as SWRO pretreatment can (1) remove scaling ； (2) Remove S042-up to 98% and total hardness up to 93% to prevent SWRO scaling ； (3) Reduce the TDS of the feed water (30% to 60%) to reduce the process operating pressure. As SWRO pretreatment, NF has the above three advantages, which can increase the system recovery rate from 35% to 50% to 70%, increase the SWRO output by 50% to 100%, and reduce the cost of SWRO water production by about 30%. ． The currently more popular full-membrane pretreatment method better combines the advantages of the three pretreatment methods of MF, UF and NF. While effectively reducing the amount of chemicals added, it can also reduce the number of cleanings of the membrane components used in the SWRO desalination process, making the operation process more environmentally friendly, and the process operation cost is also significantly reduced. 5 NF-SWRO-MSF integrated process The application and development of NF technology has created favorable conditions for the NF-SWRO-MSF (multi-stage flash evaporation) process integration. Not only the hardness of the feed seawater treated by NF is greatly reduced, but the TI)S is also reduced to a certain extent._l cited. In this way, the concentrated brine discharged by SWRO can be used as the feed material for the MSF process, which can improve the recovery rate of the entire system. At the same time, SWRO's product water can be mixed with the distilled water produced by MSF to produce drinking water, saving surface water and groundwater. While improving water quality, it can also reduce post-processing costs. In addition, the seawater preheated by the MSF heat rejection section is pretreated by NF and used as feed material for the SWRO desalination process, which can reduce the impact of seasonal water temperature changes on the process.