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Membrane separation technology, represented by polymer separation membranes, is a novel and highly efficient fluid separation unit operation technology. Over the past thirty years, it has achieved remarkable rapid development and has been widely applied in various sectors of the national economy. This report will introduce the current application status, latest progress, and development trends of several major liquid separation membranes.
Among various membrane separation technologies, reverse osmosis technology has been the most successful, fastest-growing, and most widely adopted in China in recent years. It is estimated that since 1995, the usage of reverse osmosis membranes has increased by an average of 20% annually. According to conservative estimates, the market supply of industrial reverse osmosis membrane elements in 1999 was 16,000 8-inch membranes and 26,000 4-inch membranes. The market was even stronger in 2000. According to incomplete statistics from nearly 300 water treatment engineering companies in China, in the first quarter of this year, the daily water production capacity of reverse osmosis projects under contract in China reached 120,000 tons. It is expected that the annual membrane usage will increase significantly compared to last year. It is estimated that the application of reverse osmosis technology has driven the water treatment industry's annual output value to over one billion RMB.
The largest application area for reverse osmosis membranes in China remains large-scale boiler feedwater, followed by various industrial pure water and drinking water markets. Membrane applications in industries such as electronics, semiconductors, pharmaceuticals, medical, food, beverages, alcohol, chemicals, environmental protection, metallurgy, and textiles have also reached a certain scale. Potential future applications include wastewater treatment for power plant cooling circulating water, large-scale seawater desalination, brackish water desalination, and large-scale municipal and industrial wastewater treatment.
Ultra-Low Pressure Membranes
Due to advantages such as energy savings and reduced material costs due to lower pressure ratings of related mechanical components, the application of ultra-low pressure membranes has been increasing since 1996. This is most prominent in small-scale plants primarily using 4-inch membranes. For example, Hydranautics, currently the largest reverse osmosis membrane supplier in China, has sold 80% of its 4-inch membranes in the past two years as ultra-low pressure membranes. The application of ultra-low pressure membranes in large-scale plants is also showing a significant upward trend. Currently, the largest plant in China using ultra-low pressure membranes has a production capacity of 650 tons/hour. 2.2 Low-Fouling Membranes
Membrane fouling is the greatest hazard in reverse osmosis applications. It not only shortens membrane lifespan and increases operating costs, but also directly affects the efficient and continuous operation of the membrane system. Currently, several low-fouling membranes with strong anti-fouling properties, long service life, low cleaning frequency, and easy cleaning have been developed. Hydranautics' LFC1 membrane, due to its electrically neutral and hydrophilic surface, stands out as a leader in the low-fouling family and is the only low-fouling membrane used in large-scale reverse osmosis plants in China. It is currently operating successfully in several large power plants, automobile factories, electronic materials factories, and tea concentration plants in China.
Positively Charged Reverse Osmosis Membranes
Currently, widely used low-pressure and ultra-low-pressure composite membranes are made of aromatic polyamide, whose membrane surfaces carry a negative charge. Some membrane manufacturers have developed low-pressure composite membranes with positively charged surfaces. These membranes are mainly used in systems producing high-resistivity, high-purity water. In Japan, Nitto Denko's positively charged ES10C membrane has achieved high-purity water with a resistivity of 10-15 megohms in a three-stage reverse osmosis system in the semiconductor industry. In South Korea, Hyundai Electronics' three-stage reverse osmosis system, with a combined final water production of 800 tons/hour, has a water resistivity of 8-9 megohms. A 170-ton/hour three-stage reverse osmosis system in a Shanghai semiconductor plant in China has also achieved the above indicators. Furthermore, some pharmaceutical plants in China have achieved reverse osmosis water resistivity of 1-4 megohms in two-stage reverse osmosis systems with a capacity of 5-20 tons/hour. In 2000, the domestic pharmaceutical industry was expected to put 20 such reverse osmosis systems into operation.
High-Temperature Resistant, Food-Grade, and Sanitary Reverse Osmosis Membranes
Ordinary reverse osmosis membranes for water treatment operate at temperatures ranging from 0 to 45℃. However, in special applications requiring sterilization at 90℃, high-temperature resistant and chemical-resistant reverse osmosis membranes can be used. Furthermore, various food-grade or sanitary-grade reverse osmosis membranes with special membrane element structures are beginning to be used domestically.
Many reverse osmosis seawater desalination plants with a daily production capacity of over 100,000 tons are already in operation abroad. Currently, the single-unit capacity of large-scale spiral wound membrane seawater desalination plants in operation is mostly 6,000 tons per day. Domestically, the scale of reverse osmosis seawater desalination plants that have been built or are under construction ranges from 350 to 2,500 tons per day. The highest water utilization rate of single-stage reverse osmosis seawater desalination abroad is 45%, while domestically it is currently mostly 35%. Additionally, the reverse osmosis seawater desalination machines mounted on domestic fishing vessels mostly use small membrane elements with a diameter of 2.5 inches. Currently, there are fewer than 10 companies in China with experience in mass production of seawater desalination equipment. The "Yahai Water" desalination plant, currently under construction in Hebei Province with a daily production capacity of 18,000 tons, is the largest reverse osmosis system in China using seawater desalination membranes. The application of seawater desalination membranes in China will enter a new era. In the near future, my country will also build seawater desalination plants with a daily production capacity of tens of thousands of tons. Furthermore, commercial production of seawater desalination reverse osmosis membrane elements has already begun domestically.
Three membrane companies in Japan have developed seawater desalination membranes capable of withstanding pressures exceeding 9.0 MPa. Using these membranes, Japanese water treatment engineering companies have built a two-stage reverse osmosis seawater desalination plant in Spain with a water recovery rate as high as 60%. Currently, a high-recovery-rate reverse osmosis seawater desalination plant with a daily production capacity of 58,000 tons is under construction in Fukuoka, Japan. Hydranautics, Inc. of the United States has begun supplying seawater desalination membranes with high boron ion removal rates for large-scale drinking water projects.
The development and application of nanofiltration membranes began approximately twenty years later than reverse osmosis membranes. However, research and development of nanofiltration membranes have been very active in the last decade, with a research and application boom starting about five years ago in my country. To date, our understanding of the mechanism and characteristics of nanofiltration membranes is far from complete. The academic consensus is that nanofiltration membranes fall between reverse osmosis and ultrafiltration membranes, and their surface separation layer may have a nanoscale microporous structure. In contrast, reverse osmosis membranes typically achieve NaCl removal rates above 95%, and membranes with NaCl removal rates below 90% are generally considered nanofiltration membranes. While reverse osmosis membranes have high removal rates for almost all solutes, nanofiltration membranes only remove specific solutes. Nanofiltration membranes primarily remove solute particles around one nanometer in size, with a molecular weight cutoff typically between 100 and 2000. Unlike reverse osmosis membranes, which are almost entirely made of polyamide, nanofiltration membranes use a wider range of materials. Some nanofiltration membranes form polymer electrolytes on their surface, often resulting in strong charge. For example, the well-known NTR-7400 series nanofiltration membranes are made of sulfonated polysulfone, and their surface has a strong negative charge.
Overseas, the largest application market for nanofiltration membranes is in the drinking water sector, primarily for removing trihalomethane intermediates (THMs), odors, color, pesticides, synthetic detergents, soluble organic matter, and hardness components such as calcium and magnesium. Currently, large-scale urban drinking water treatment plants abroad have a daily production capacity of 100,000 tons. However, in China, there are currently only experimental plants with a daily production capacity of several hundred tons, but it is believed that significant breakthroughs will gradually be achieved in this field in the future. Currently, polyamide nanofiltration membranes, made of the same material as reverse osmosis membranes, are mainly used in the drinking water sector. Among the limiting factors affecting the large-scale use of nanofiltration membranes in the drinking water sector are insufficient application technology and a lack of public awareness. Currently, some systems in China producing "high-quality drinking water" use a mixture of reverse osmosis and ultrafiltration product water. Another promising application area for nanofiltration membranes is environmental protection and wastewater treatment. Domestic efforts in this area are still insufficient. On the one hand, companies familiar with traditional wastewater treatment are unfamiliar with membrane technology; on the other hand, companies accustomed to membrane-based pure water treatment are unwilling to invest significant resources in developing and applying membrane separation technology to complex wastewater treatment applications.
Nanofiltration membranes are a popular area for application development in the separation, purification, or concentration of aqueous solutions from various pharmaceutical, biochemical, food, and chemical materials. There are already many successful examples, but unfortunately, due to commercial confidentiality reasons, many companies often find it difficult to promote and popularize them in a timely manner.
Ultrafiltration is one of the most widely used membrane technologies and also the membrane type with the highest domestic production rate in my country. However, the limited variety and low quality of products still affect its widespread promotion and popularization. Furthermore, domestic manufacturers are too small, too numerous, and produce generic membranes; simple imitation is still prevalent, and there is a lack of large-scale enterprises with significant production capacity and development capabilities. It is believed that this situation will change in the near future.
The current development trends of ultrafiltration include the following:
Larger and more efficient modules: High-flux ultrafiltration modules (6 inches and above), bundled, and modular designs have emerged.
Ultrafiltration membranes with large pore sizes and coarse hollow fibers are becoming increasingly common. For example, Hydracap ultrafiltration membrane modules from Hydranautics in the US are increasingly used not only independently but also in the pretreatment process of reverse osmosis, forming what is known as an integrated membrane treatment system (IMS). Replacing traditional sand filters, activated carbon, and microfiltration with ultrafiltration or ultramicrofiltration membranes is a new trend.
New developments in spiral-wound ultrafiltration. For instance, Nitto Denko in Japan launched the RS series spiral-wound ultrafiltration membrane two years ago. This membrane has a molecular weight cutoff of 750,000, performs full filtration during use, does not discharge filtered water, and can be backwashed under pressure from the product water side during cleaning. This membrane has been used in the pretreatment of seawater desalination plants in the Middle East, with a processing capacity of thirteen tons per day. 5.4 Specialized performance: For applications such as semiconductor ultrapure water and pharmaceutical injection water, there are already ultrafiltration membrane modules that can withstand high temperatures of 95°C or steam sterilization of 121°C, achieving complete sterility and pyrogen-free operation, such as the ST-V6 ultrafiltration membrane module from Nitto Denko Corporation of Japan.
