As a new and efficient pollution-free purification and separation technology in the water treatment industry - membrane separation technology, it has been widely used in chemical, electronics, medical, food, environmental protection and other fields. The separation mechanism of hollow fiber micro-pore membrane is screening. Under pressure drive, molecules or particles with a smaller size than the separation pore diameter of the membrane can pass through the fiber wall, while molecules or particles with a larger size are retained by the fiber wall, thus achieving the separation of different sizes of particles. PVDF membrane is a new type of membrane material, which is superior to PE, PP, PVC, etc. in terms of resistance to acid and alkali and other harsh environmental conditions, high mechanical strength, good chemical stability, high separation precision, and high efficiency.
1. Physical and Chemical Properties and Structure Character of PVDF hollow fiber membrane
Polyvinylidene fluoride (PVDF), with a crystallinity of 60%-80% and a fluorine content of 59%, has a density of 1.75-1.78 g/cm3, a glass transition temperature of -39°C, a brittle temperature of below -62°C, a crystalline melting point of about 170°C, a thermal decomposition temperature of over 316°C, and a long-term service temperature of -40°C to 150°C. It has high mechanical strength and self-extinguishing properties. PVDF has excellent rigidity, hardness, resistance to creep, wear, and cutting. Its chemical stability is good, and it can resist corrosion and swelling of oxidizing agents, acids, bases, salts, halogens, aromatic hydrocarbons, fats, and chlorinated solvents. PVDF has excellent resistance to ultraviolet light and aging, and its film remains flexible and intact when placed outdoors for a long time. Only dimethylacetamide and dimethylimidazole, etc., strong polar organic solvents, can dissolve it into a colloidal solution.
PVDF hollow fiber membrane module has a high unit volume packing density, and the module has a large water output; the separation pore size is 0.05-0.22 microns, with high filtration accuracy; dynamic filtration, strong resistance to blockage; no phase change, no need to add flocculant in water, no pollution to the filtration system.
2. Application in water treatment
2.1 Application in the field of household water purification
2.2 A household water purifier is a device that purifies tap water for the second time, using cylindrical fiber cotton, activated carbon (in the form of granules or powder), activated carbon fiber, microporous ceramic, reverse osmosis membrane, microfiltration or ultrafiltration membrane, ion exchange resin, and ultraviolet disinfection device as filtering and purification media. It removes impurities and contaminants from water through the methods of isolation, adsorption, and killing bacteria and viruses. Among the many materials used for drinking water purification, membrane materials, as a new type of material, have the advantages of good filtration effect and high cost-effectiveness, and therefore represent the trend of future development of household water purifiers. PVDF hollow fiber UF membrane has the advantages of no energy consumption, green environmental protection, high filtration accuracy, and can filter out all bacteria, viruses, turbidity, endospores, Giardia lamblia, rust, etc., while retaining the trace elements essential to human body. The combination of UF membrane and activated carbon fiber is a new direction. Using the activated carbon fiber in front to adsorb organic molecules such as THMs and heavy metal ions, odors, etc., and using the UF membrane as the final filtration, can prevent the leakage of bacteria from the activated carbon fiber, and has a very high water purification efficiency.
3.2 Application in the field of wastewater resourceization
3.2.1 Brief introduction of continuous membrane filtration technology
The continuous membrane filtration technology (CMF) based on PVDF hollow fiber membrane meets the requirements of membrane technology integration and high automation at present. CMF technology includes continuous microfiltration and continuous ultrafiltration technologies. The entire CMF system includes the host, compressed air system, backwash system, automatic detergent washing system, and human-machine interface control system, etc. It adopts modular design and can be combined according to needs. Each module has its own control system and can be sampled for water quality testing and complete integrity testing of the membrane. Even if the membrane component is damaged, it can be repaired in time without affecting the normal operation of the system.
3.2.2 Application in reclaimed water reuse
Water recycling using membrane technology is being used in many countries around the world, with many successful examples. The CMF/RO/UV (disinfection) system was adopted in the Tuas Water Reclamation Demonstration Project in Singapore in 2000. Directly using CMF to treat the secondary treated wastewater is better than the water quality after the traditional three-stage treatment, which fully meets the requirements for general landscaping, flushing toilets, etc. The 25000t/d reclaimed water reuse project in Tianjin Development Area is the largest municipal sewage treatment project in China. The project uses the integrated membrane system (IMS) technology, uses the secondary biochemical treatment water of the sewage treatment plant in the development zone as raw water, and uses the CMF technology with PVDF hollow fiber membrane as the core for preliminary in-depth treatment, and then enters the RO for in-depth treatment. It can meet the water quality standards of greening, industrial production, groundwater recharge and boiler replenishment, and provide an important water source for the development zone. CMF produced water quality is high, turbidity ≦0.1NTU, SDI≦3, suspended matter <5mg/L. The water recovery rate is higher than 95%. It has been running well since construction began.
3.2.3 Application in oil field reinjection water purification treatment
With the investment and development of many low permeability oil fields in China, there is an urgent need for high-precision purification and filtration equipment. Due to the poor retention effect of PE and PVC sintered pipe filter, it can only reach about 2 inches, and the retention diameter of polysulfone hollow fiber membrane can reach 1 μm, but the water production of a single membrane component is low. After deep purification treatment of reinjection water in Dagang Oilfield by PVDF hollow fiber membrane CMF technology, the particle size of suspended solids in the purified water is <0.8 μm, the amount of suspended solids is <0.5mg/L, the single membrane component is 89mm×1000mm, and the purification capacity is >1.5t/h. The initial purification treatment of oil-bearing wastewater produced in the oilfield is carried out. The oil content is less than 3mg/L, which fully meets the requirements of reinjection water purification treatment in oil fields.
3.3 Applications in the field of seawater desalination
As an important way to solve the water resources crisis, seawater desalination technology is increasingly showing its unique advantages and good prospects. Most coastal cities have per capita water resources below 500m3, with Tianjin, Shanghai, Dalian, Qingdao, and Lianyungang having per capita water resources below 200m3. To solve the water crisis problem, Tianjin is actively promoting the application and promotion of seawater desalination technology. A 1000t/d seawater desalination base has been built in the industrial zone in Dagang near the port. The base uses double-membrane technology for seawater desalination, using CMF to replace the traditional coagulation, mechanical filtration, and fine filtration processes as the pretreatment system for reverse osmosis, greatly reducing equipment space, producing high-quality water with stable quality, and extending the service life of the reverse osmosis system; the system has high automation control level, which can reduce labor intensity and labor costs and operating costs, and is the next-generation RO pretreatment system.
Since its operation at the beginning of 2004, the system has operated stably, and by monitoring the CMF outlet water quality indicators such as turbidity and SDI for a long time, it has been found that the CMF outlet water quality indicators can meet the RO pretreatment requirements regardless of how the pretreatment water quality fluctuates. The average turbidity is 0.064NTU, and the SDI is stable between 1-2, and the water output is stable.
3. Conclusion
As the water crisis becomes increasingly severe, the resourceization of urban sewage and industrial wastewater, as well as the rational development of marine resources, has become the trend of the world today. PVDF hollow fiber membrane with excellent performance will gradually become the mainstream in the market, and the continuous membrane filtration technology (CMF) based on PVDF hollow fiber membrane will have huge market application prospects in urban sewage treatment and reuse, as well as seawater desalination.
