Introduction of water treatment technology: ultrafiltration membrane

The basic function of ultrafiltration technology is to filter and sieving, which cannot remove soluble substances. For this problem, the industry tends to adopt a solution combining ultrafiltration technology and traditional technology.

Using ultrafiltration, such as ammonia nitrogen cannot be removed. The best way to remove ammonia nitrogen is biological. If we combine ultrafiltration membranes with biological oxidation, we can increase the removal rate of ammonia nitrogen and increase the COD , BOD removal rate.” Similarly, for the problem of poor removal of medium and small molecular organics in water by ultrafiltration, especially trace organic pollutants, the setting of the coagulation sedimentation unit will greatly expand the effect of ultrafiltration on the turbidity of raw water. The scope of application can improve the service life of the membrane. “The turbidity of the ultrafiltration water is generally about 0.1NTU, and is basically independent of the turbidity of the water before ultrafiltration, so the water can directly enter the ultrafiltration. However, when the turbidity of the raw water is high, the flushing water volume increases, which will cause the membrane filtration The cycle is shortened, so it is advisable to add a coagulation sedimentation unit in front of the membrane.” “When the raw water is polluted, it is necessary to add a processing unit to remove organic matter in front of the membrane, such as activated carbon. In granular activated carbon and biological powder activated carbon, the latter is due to physical adsorption The function is more durable, so the effect is better. After the ultrafiltration is placed on the activated carbon, the microorganisms and carbon particles in the effluent can be intercepted”

Ultrafiltration generally removes microorganisms almost completely. The disinfection of effluent is not mainly to inactivate pathogenic microorganisms in the water, but to make the water have long-lasting disinfection ability…Chlorine, chloramine and chlorine dioxide all have continuous disinfection ability. Chloramine is a better choice.

The coagulation-precipitation, filtration, and chlorine disinfection water purification process developed in the early 20th century can be called the first-generation urban drinking water purification process. Faced with the disadvantage that the first-generation water and drinking water treatment process cannot control harmless substances, the second-generation urban drinking water purification process has emerged. The second generation of drinking water treatment is a process in which ozone and granular activated carbon are added after the first generation process. At present, my country’s water supply plants generally use this process. However, the second-generation drinking water treatment process has gradually revealed many problems: one is that the bromide-containing water source water is prone to produce carcinogenic bromate after being oxidized by ozone; the second is that with the intensification of water pollution and the improvement of detection technology As more and more bacteria and microorganisms have been found in the effluent of the second-generation drinking water treatment process, the biological safety of the water has been challenged.

Membrane technology is simply a kind of filtration technology. The reverse osmosis membrane, which originated from seawater desalination in the 1960s, is collectively referred to as the six new technologies in the 21st century industrial field together with superconductivity, optical fiber, carbon fiber, and nanotechnology. Then the membrane technology has been developed very quickly and is widely used in more and more fields. Following desalination and reverse osmosis, a series of looser membranes were developed, including nanofiltration, ultrafiltration, and microfiltration.

Data shows that among the existing membranes of various pore sizes, nanofiltration and ultrafiltration are the most effective methods to remove microorganisms in water. The size of pathogenic microorganisms in water is 20 nm to hundreds of nm for viruses, hundreds of nm to several μm for bacteria, several μm to tens of μm for protozoa, and several μm to hundreds of μm for algae. Among all kinds of membranes, the pore size of nanofiltration membranes is about -1nm, and that of ultrafiltration membranes is a few nm. But from the perspective of technical economics, Academician Li affirmed the dominant position of ultrafiltration membranes in the future third-generation processing technology. “Nanofiltration membranes still need to be imported in my country, and the cost is very high. Ultrafiltration membranes have formed a large-scale production capacity in my country, which can provide membrane materials for water plants with a scale of tens of thousands of tons per day, and the price has dropped to an acceptable level. The pore size of the microfiltration membrane is several hundred nanometers, which cannot fully intercept and remove viruses. It is more feasible for my country to choose ultrafiltration membranes to improve the biological safety of water.

Internationally, the use of ultrafiltration technology as a treatment process in water plants has gradually become the mainstream. In 1996, the total water treatment capacity of the ultrafiltration water plant was 200,000 m3/d, and in 2006, the treatment water capacity was over 8 million m3/d. There are 250 ultrafiltration and microfiltration water plants in North America, with a total water treatment capacity of 3 million m3/d; in Europe, there are 33 ultrafiltration water plants over 10,000 m3/d, and there are more than 100 cities in the UK The water plant has a processing capacity of 1.1 million m3/d; in Asia, the membrane water plant in Japan has a water output of 4 million m3/d, and Singapore has built an ultrafiltration capacity of 275,000 m3/d. In China, with the development of my country’s membrane industry, the price problem that hindered the application of ultrafiltration membranes is no longer obvious. At present, the price of hollow fiber ultrafiltration membranes on the market is 150 yuan per m2 of filtration area. Calculated by 1m2 ultrafiltration membrane filtering 0.1m3 of water per hour, the price of 1m3/d ultrafiltration membrane is 60 yuan. The ultrafiltration membrane is calculated based on 3 years of use, and it only costs 0.057 yuan per m3 of water to replace the membrane. Take the 10,000 m3/d ultrafiltration water purification plant built in Suzhou as an example. The construction cost is about 300 yuan/m3/d and the operating cost is 0.0782 yuan/m3, which is roughly the same as the original traditional technology of the water plant.

Academician Li is very optimistic about the prospects of ultrafiltration membrane technology in China. He said: “Taiwan has built a membrane water filtration plant of 300,000 m3/d. In recent years, many medium and small ultrafiltration water plants have been constructed one after another, of which the water production capacity is the largest. It is 20,000 m3/d. The construction of a large ultrafiltration plant is just around the corner.”

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