Wastewater Treatment Glossary: Ultrafiltration Membranes
Ultrafiltration membranes are used extremely widely in industry and have become one of the newer chemical unit operations. They serve in the separation, concentration and purification of biological products, pharmaceuticals and foods, and are also used in blood treatment, wastewater treatment and as terminal units in ultrapure water production. In China they have been applied successfully to concentrate and purify traditional Chinese medicine extracts. As the technology advances, the sieving capability of ultrafiltration membranes will keep improving and their contribution to society will keep growing.
Ultrafiltration membranes are either symmetric or asymmetric in structure. The former are isotropic, with no skin layer and pores of the same size in every direction — that is, depth filtration. The latter have a denser surface layer over a substrate dominated by finger-like structures; the surface layer is 0.1 micron thick or less with orderly micropores, while the substrate is 200–250 microns thick — that is, surface filtration. Industrial ultrafiltration membranes are generally asymmetric. Membrane materials mainly include cellulose and its derivatives, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, polysulfonamide, sulfonated polysulfone, cross-linked polyvinyl alcohol and modified acrylic polymers.
Ultrafiltration is a sieving process driven by the pressure difference across the membrane, with the ultrafiltration membrane acting as the filter medium. Under a given pressure, as the feed flows across the membrane surface, the dense array of tiny pores allows only water and small molecules through as permeate, while substances larger than the surface pore size are retained on the feed side as concentrate — thereby purifying, separating and concentrating the feed. Each metre of ultrafiltration hollow-fibre wall contains roughly six billion pores of 0.01 micron, sized to let water molecules and beneficial minerals and trace elements pass, while the smallest bacteria, at 0.02 microns and above, are retained — as are colloids, rust, suspended solids, silt and macromolecular organics, all far larger than bacteria. This is how purification is achieved.
Building on this, industry experts have also carried out a great deal of research and refinement.
Polyacrylonitrile, abbreviated PAN, is obtained by free-radical polymerisation of acrylonitrile monomer, with the acrylonitrile units in the macromolecular chain linked head-to-tail. It appears as a white powder with a density of 1.14–1.15 g/cm³, and softens and decomposes when heated to 220–300°C.
Polyacrylonitrile is used mainly to make synthetic fibres such as acrylic. The fibre is spun from a high polymer copolymerised from more than 85% acrylonitrile with second and third monomers. Acrylic is the Chinese trade name for polyacrylonitrile fibre, colloquially known as artificial wool. DuPont of the United States developed pure polyacrylonitrile fibre (trade name Orlon) in the 1940s, but difficulties with dyeing and a tendency to fibrillate kept it out of industrial production. Only after the polymer's spinnability and the fibre's dyeability were improved did industrial acrylic production become possible. Different countries use different trade names: Orlon, Acrilan, Creslan and Zefran in the United States, Courtelle in the UK, and Cashmilon, Exlan, Vonnel and Beslon in Japan. Acrylic generally has a density of 1.16–1.18 g/cm³ and a standard moisture regain of 1.0%–2.5%. The fibre is bulky and warm with a soft hand, and offers good weather resistance along with mildew and moth resistance. It is used chiefly as artificial fibre — commonly called artificial wool — for knitting yarn, knitted fabrics (pure or blended with wool) and woven fabrics, and suits furnishing fabrics such as curtains particularly well. In materials science, polyacrylonitrile is often used as a matrix for synthesising porous materials, for example PAN-based activated carbon.
Many materials can be used to make ultrafiltration membranes, including polyvinylidene fluoride (PVDF), polyethersulfone (PES), polypropylene (PP), polyethylene (PE), polysulfone (PS), polyacrylonitrile (PAN) and polyvinyl chloride (PVC). Polyethersulfone found commercial application in the early 1990s, and by the late 1990s the better-performing polyvinylidene fluoride ultrafiltration membrane was being widely adopted in the water treatment industry. PVDF and PES are therefore the most widely used ultrafiltration membrane materials today.
Beyond this, a range of other factors must also be weighed in practical engineering applications.
1) The housing uses impact-resistant ABS with a pressure rating above 16 kg and a wall thickened by 1 mm, fully able to withstand the various pressure shocks that may arrive with the feed water. It will not rupture under shock pressure, and it prevents the long-term compression of the membrane in service that causes material creep and leakage.
2) Each HUF90 membrane is packed with 1,400 fibres, lengthened by 100 mm to increase membrane area by 15% — an effective membrane area greater than any comparable domestic product of the same specification, which raises water output.
3) The end cap uses a hemispherical convex structure that distributes feed water more evenly across the fibres at the end face than a traditional flat end face, and its wall is thickened by 1 mm to ensure it will not rupture under shock pressure.
In addition, this technology is widely applied and practised in related fields.
Among inorganic membranes, ceramic ultrafiltration membranes see considerable use in household water purifiers. Ceramic membranes last a long time and resist corrosion, but the water they produce can carry an earthy taste that affects the drinking experience. They also foul easily and are not easy to clean. Hollow-fibre ultrafiltration membranes, with their high packing density, large effective membrane area, high pure-water flux and simple, easy cleaning, are widely used in the domestic water purification industry.
It is worth noting that technologies and standards in this field continue to develop and improve.
A single ultrafiltration membrane module consists of hundreds to thousands of fine hollow fibres. Membranes with hollow-fibre inner diameters between 0.6 and 6 mm are generally called capillary ultrafiltration membranes; their larger bore makes them less prone to blockage by large particles.
Taking the Yingge water purifiers on the market as an example, the ultrafiltration membrane undergoes a special hydrophilic treatment that gives the fibres lasting hydrophilicity, reducing the hydrolysis contact angle from 79–90 degrees before modification to 30–35 degrees. This yields high water throughput at a lower transmembrane pressure while improving the fibres' resistance to fouling.
Yingge hollow-fibre ultrafiltration membranes have uniform pores smaller than 0.1 micron, removing microorganisms, colloids, diatoms and other substances that cause turbidity.
The mechanical strength of an ultrafiltration membrane reflects the fibres' resistance to breakage. Broken fibres destroy the membrane's separation performance, which makes strength an important indicator of membrane quality.
Yingge uses high-quality PVDF as its raw material, giving its ultrafiltration hollow-fibre membranes good resistance to chemical corrosion, oxidation and photo-ageing. They can therefore be cleaned repeatedly by various methods to remove foulants and restore flux.
The work starts with raw material supply, manufacturing process and product testing: stable raw material quality, consistent and precise control throughout manufacture, and 100% product inspection together ensure stable product performance.
Membrane filtration that uses an ultrafiltration membrane driven by a pressure difference is called ultrafiltration membrane filtration. Most ultrafiltration membranes are made from cellulose acetate or polymers with similar properties. The technique suits the separation and concentration of solutes in solution and is often used for separating colloidal suspensions that other separation techniques struggle with; its field of application keeps expanding.
Pressure-driven membrane filtration divides into three categories: ultrafiltration, microfiltration and reverse osmosis. They are distinguished by the smallest particle size or molecular weight the membrane can retain. Using nominal pore size ranges as the criterion, microfiltration membranes (MF) span 0.02–10 μm, ultrafiltration membranes (UF) 0.001–0.02 μm, and reverse osmosis membranes (RO) 0.0001–0.001 μm. Ultrafiltration is therefore best suited to separating and concentrating solutes in solution, or to separating colloidal suspensions that other techniques cannot handle. Membrane fabrication technology — obtaining pores of the intended size with a narrow distribution — is critical. Many factors control pore formation: the type and concentration of the casting solution, evaporation and coagulation conditions and so on all yield membranes with different pore sizes and distributions. Ultrafiltration membranes are generally polymer separation membranes, with the main polymer materials being cellulose derivatives, polysulfone, polyacrylonitrile, polyamide and polycarbonate.
Ultrafiltration membranes are used very widely — in the food industry, pharmaceutical manufacturing and elsewhere — for concentrating and purifying drugs, fruit juices and dairy products, and for purifying pure water and mineral water. Ultrafiltration systems offer good filtration performance, high output and strong stability.
1. The ultrafiltration membrane elements come from world-renowned membrane manufacturers, guaranteeing customers the finest organic membrane elements available today and thus assuring both rejection performance and membrane flux.
3. The process involves no phase change and has no adverse effect on the composition of the material; separation, purification and concentration all take place at ambient temperature. This makes it especially suitable for heat-sensitive substances, completely avoiding the destruction of bioactive substances by high temperature and effectively preserving the bioactive substances and nutrients in the original material system.
4. System energy consumption is low and production cycles are short. Compared with conventional process equipment, operating costs are lower, which effectively reduces production costs and improves the enterprise's economic returns.
Beyond this, a range of other factors must also be weighed in practical engineering applications.
7. The control system can be tailored to the user's specific requirements. Combined with Shijia's advanced control software, it provides centralised on-line monitoring of key process parameters on site, avoiding manual operating errors and safeguarding long-term stable operation from multiple angles.
Ultrafiltration is the process of using membranes with pore sizes of 1 to 20 nm to filter solutions containing macromolecules or fine particles, thereby separating those macromolecules or particles from the solution. Driven by the pressure difference across the membrane and using the ultrafiltration membrane as the filter medium, it lets small solute molecules and solvent pass through pores of a given size under a certain pressure while macromolecular solutes cannot, remaining on one side of the membrane — thus purifying, separating and concentrating the solution. It is used chiefly to retain macromolecular solutes; for example, when treating feed liquids free of solids, solutes of lower relative molecular mass pass through the membrane with the water while those of higher relative molecular mass are retained.
Physical methods use mechanical force to remove foulants from the membrane surface with no chemical reaction involved, which makes them simple, convenient and safe. Several practical approaches are outlined below.
Beyond this, a range of other factors must also be weighed in practical engineering applications.
Before use, ultrafiltration membranes are generally immersed in a protective solution and sealed, to stop the wet membrane dehydrating and shrinking, which would reduce pore size, damage the membrane structure and lower water flux.
Short-term storage: if an ultrafiltration membrane is taken out of service for less than 10 days, it should be given one sterilising backwash with 15 ppm (ml/L) of HY-240 biocide added to the backwash water, after which the feed, drain and control valves are closed to keep the membrane sealed and sterile.
Long-term storage: if the membrane is out of service for more than 10 days, first give it one sterilising backwash, then fill it with HY-310 protective solution (ideally made up with RO water) and seal it for storage.
Ultrafiltration equipment is built around the ultrafiltration membrane, using the interception capability of porous materials to remove impurity particles above a certain size from water by physical rejection. Under pressure, small-sized substances in solution such as water, low-molecular organics and inorganic ions pass through the micropores in the fibre wall to the other side of the membrane, while large-sized substances such as bacteria, colloids, particulates and macromolecular organics are retained, thereby sieving the different components of the solution.
Ultrafiltration equipment is a pressure-driven physical sieving process that separates liquids using ultrafiltration membranes of different pore sizes. Molecular weight cut-off (MWCO) generally runs from 6,000 to 500,000, with pore sizes around 100 nm. The membranes used are asymmetric, with an active surface separation layer of about 10–200 average pore size, able to retain macromolecules and colloidal particles with molecular weights above 500, and operating pressure differentials of 0.1–0.5 MPa.
Ultrafiltration membranes have an extremely broad range of applications — essentially any industry involving filtration can use them. Typical fields include: pretreatment ahead of reverse osmosis and terminal treatment in pure and ultrapure water production; separation of bacteria, pyrogens, colloids, suspended impurities and macromolecular organics in industrial water; purification of drinking water and mineral water; concentration, purification and clarification in fermentation, enzyme preparation and pharmaceutical manufacturing; concentration and separation of fruit juices; separation, concentration and clarification in the soybean, dairy, sugar, liquor, tea extract and vinegar industries; purification and recovery of industrial and domestic wastewater; and recovery of electrophoretic paint.
Ultrafiltration membrane separation can replace conventional steps such as natural sedimentation, plate-and-frame filtration, vacuum drum filtration, centrifugal separation, solvent extraction, resin purification and activated carbon decolourisation. The process runs at ambient temperature with no phase change and produces no secondary pollution.
Overseas, ultrafiltration is used mainly in drinking water treatment, whereas in China it is used mainly for industrial wastewater reuse as pretreatment ahead of reverse osmosis. In China's water industry market, ultrafiltration has already seen considerable application in power, steel, chemical and other industrial wastewater treatment.
As the economy and society develop, large-scale wastewater treatment projects will multiply, opening broad market space for ultrafiltration membrane technology. Abroad, many waterworks already use ultrafiltration to produce tap water; in China, funding and other issues have so far limited its adoption. But as national and local drinking water standards are revised and new codes issued, more and more waterworks are bound to take up ultrafiltration. According to the Ministry of Water Resources' analysis in "China's Water Supply and Demand in the 21st Century", China would enter a period of severe water shortage after 2010, while water pollution has gradually become the biggest obstacle to safe urban water supply. Urban sewage treatment and reclaimed water reuse will be one of the effective routes to resolving the future urban water resource crisis. Ultrafiltration membranes therefore have broad market space in the municipal wastewater treatment market to come.
At the same time, the supporting processes and equipment continue to be optimised and upgraded.
Removal rate for particles >0.2 μm: 100% (at a molecular weight cut-off of 80,000 daltons)
At the same time, the supporting processes and equipment continue to be optimised and upgraded.
As China's water treatment technology has grown more sophisticated, the filtration performance of ultrafiltration membranes has improved greatly. Today, virtually any industry that involves filtration equipment and processes can make use of them.
Almost every industry in China involves water treatment — for example, separating bacteria, pyrogens, colloids, suspended impurities and macromolecular organics in industrial water; terminal treatment in pure and ultrapure water production; purification of drinking water and mineral water; concentration, purification and clarification in fermentation, enzyme preparation and pharmaceutical manufacturing; fruit juice concentration and separation; separation, concentration and clarification in the soybean, dairy, sugar, liquor, tea extract and vinegar industries; purification and recovery of industrial and domestic wastewater; and recovery of electrophoretic paint. Clearly the role of ultrafiltration membranes is a wide one.
Membranes must be cleaned regularly to maintain flux and extend service life. The cleaning method is generally determined by the nature of the membrane and of the feed being treated. It usually resembles reverse osmosis practice: hydraulic cleaning first, then different chemical detergents as circumstances require — for example ionic solubilisers for electro-coating materials, or "bridging" solvents for water-soluble organic coatings. Protein deposits in the food industry can be handled with proteolytic enzyme solutions or alkaline detergents based on phosphates or silicates. Inorganic salt scale on the membrane surface can be dissolved with chelating agents such as EDTA or with acids and alkalis. Different module types call for different cleaning methods: tubular modules can be cleaned mechanically with sponge balls, while hollow-fibre modules can be backwashed. Membranes used in the food industry also require disinfection (with NaOH, H2O2 and similar).
As membrane fabrication technology has advanced and production has scaled up, the performance of GE ultrafiltration and RO membranes has grown more stable and membrane costs have fallen substantially. Ultrafiltration membranes are now widely used in drinking water purification, industrial water treatment, beverages, biotechnology, food, pharmaceuticals, environmental protection and many other fields, and have won consistent praise across the industry. [1]
In addition, this technology is widely applied and practised in related fields.
Swimming pools use UF ultrafiltration membranes, effectively solving the problem of pool water purification and providing technical support for a better recreational environment.
