What Actually Governs Ultrafiltration Membrane Performance: Pores, Polymer and Cleaning

Sieving, not absorption: what the membrane really does

Ultrafiltration is a sieving process driven by the pressure difference across the membrane, with the membrane itself acting as the filter medium. Under a given pressure, feed flows across the membrane surface and the dense array of tiny pores lets only water and small molecules through as permeate, while anything larger than the surface pore size stays on the feed side as concentrate. That is the whole mechanism, and it is why ultrafiltration purifies, separates and concentrates without a phase change.

Scale helps. Each metre of ultrafiltration hollow-fibre wall carries roughly six billion pores of 0.01 micron. That dimension passes water molecules, beneficial minerals and trace elements, while the smallest bacteria at 0.02 microns and above are retained, along with colloids, rust, suspended solids, silt and macromolecular organics that are far larger still. Put another way, ultrafiltration uses membranes with pore sizes of 1 to 20 nm to strip macromolecules or fine particles out of solution.

Symmetric versus asymmetric: two ways to build a wall

Symmetric membranes are isotropic, with no skin layer and pores of the same size in every direction, which makes them depth filters. Asymmetric membranes carry 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 runs 200-250 microns thick. That is surface filtration, and industrial ultrafiltration membranes are generally asymmetric.

Fabrication is the hard part. Getting pores of the intended size with a narrow distribution depends on the type and concentration of the casting solution, on evaporation conditions and on coagulation conditions. Change any of them and pore size and distribution shift.

Material choice sets the chemical envelope

Candidate materials are broad: cellulose and its derivatives, polycarbonate, polyvinyl chloride, polyvinylidene fluoride, polysulfone, polyacrylonitrile, polyamide, polysulfonamide, sulfonated polysulfone, cross-linked polyvinyl alcohol and modified acrylic polymers. In practice the short list for membranes is 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.

Polyacrylonitrile, abbreviated PAN, is obtained by free-radical polymerisation of acrylonitrile monomer with the acrylonitrile units linked head-to-tail. It is a white powder with a density of 1.14-1.15 g/cm3, and it softens and decomposes when heated to 220-300 °C. It is used mainly to make synthetic fibres such as acrylic, spun from a high polymer copolymerised from more than 85% acrylonitrile with second and third monomers. DuPont of the United States developed pure polyacrylonitrile fibre under the trade name Orlon in the 1940s, but dyeing difficulties and a tendency to fibrillate kept it out of industrial production until spinnability and dyeability were improved. Trade names differ by country: 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%. In materials science, polyacrylonitrile also serves as a matrix for synthesising porous materials such as PAN-based activated carbon.

Among inorganic membranes, ceramic ultrafiltration membranes see considerable use in household water purifiers. They last a long time and resist corrosion, but the water they produce can carry an earthy taste, and they foul easily and are not easy to clean. Hollow-fibre ultrafiltration membranes, with high packing density, large effective membrane area, high pure-water flux and simple cleaning, dominate the domestic water purification industry instead.

Pore-size bands and where UF belongs

Pressure-driven membrane filtration divides into ultrafiltration, microfiltration and reverse osmosis, distinguished by the smallest particle size or molecular weight retained. On nominal pore size, microfiltration membranes span 0.02-10 μm, ultrafiltration membranes 0.001-0.02 μm, and reverse osmosis membranes 0.0001-0.001 μm. Molecular weight cut-off (MWCO) for ultrafiltration generally runs from 6,000 to 500,000, with pore sizes around 100 nm. The active surface separation layer carries an average pore size of about 10-200, retaining macromolecules and colloidal particles with molecular weights above 500, and operating pressure differentials sit at 0.1-0.5 MPa. Removal rate for particles above 0.2 μm reaches 100% at a molecular weight cut-off of 80,000 daltons.

Hardware details that decide service life

Mechanical detail matters as much as polymer chemistry. The housing uses impact-resistant ABS with a pressure rating above 16 kg and a wall thickened by 1 mm, which withstands the pressure shocks arriving with the feed water and prevents the long-term compression that causes material creep and leakage. Each HUF90 membrane is packed with 1,400 fibres, lengthened by 100 mm to increase membrane area by 15%, raising water output. The end cap uses a hemispherical convex structure that distributes feed water more evenly across the fibres than a flat end face, and its wall is thickened by 1 mm so it will not rupture under shock pressure.

Membranes with hollow-fibre inner diameters between 0.6 and 6 mm are generally called capillary ultrafiltration membranes, and their larger bore makes them less prone to blockage by large particles. Hydrophilic treatment also changes behaviour: on Yingge units the hydrolysis contact angle drops from 79-90 degrees before modification to 30-35 degrees, giving high throughput at lower transmembrane pressure and better fouling resistance. Yingge hollow-fibre membranes have uniform pores smaller than 0.1 micron and use high-quality PVDF as raw material, so they resist chemical corrosion, oxidation and photo-ageing and can be cleaned repeatedly. Mechanical strength deserves attention too, because broken fibres destroy separation performance outright.

Cleaning, disinfection and storage

Flux has to be defended. Physical methods use mechanical force to remove foulants without chemical reaction, which is simple and safe. Chemical cleaning follows the foulant: ionic solubilisers for electro-coating materials, bridging solvents for water-soluble organic coatings, proteolytic enzyme solutions or alkaline detergents based on phosphates or silicates for protein deposits, and chelating agents such as EDTA or acids and alkalis for inorganic salt scale. Tubular modules can be cleaned mechanically with sponge balls while hollow-fibre modules are backwashed, and membranes in food service also need disinfection with NaOH and H2O2.

Storage is where many systems are spoiled. Before use, membranes are immersed in a protective solution and sealed to stop the wet membrane dehydrating and shrinking, which would reduce pore size and lower flux. If a membrane is out of service for less than 10 days, it gets one sterilising backwash with 15 ppm (ml/L) of HY-240 biocide added to the backwash water, then the feed, drain and control valves are closed to keep it sealed and sterile. Beyond 10 days, it gets a sterilising backwash and is filled with HY-310 protective solution, ideally made up with RO water, then sealed.

Where the market actually puts it

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, with considerable application already in power, steel and chemical wastewater. The application list is long: pretreatment ahead of reverse osmosis and terminal treatment in pure and ultrapure water; removal of bacteria, pyrogens, colloids, suspended impurities and macromolecular organics; drinking water and mineral water purification; concentration and clarification in fermentation, enzyme preparation and pharmaceutical manufacture; juice, soybean, dairy, sugar, liquor, tea extract and vinegar streams; industrial and domestic wastewater recovery; and electrophoretic paint recovery. Ultrafiltration can replace natural sedimentation, plate-and-frame filtration, vacuum drum filtration, centrifugal separation, solvent extraction, resin purification and activated carbon decolourisation, running at ambient temperature with no secondary pollution. Swimming pools use it too.