Ultrafiltration Membrane Selection: Pore Size, Material and the Cleaning Regime Behind Service Life

What ultrafiltration actually separates

Ultrafiltration is the process of filtering a solution containing macromolecules or fine particles using a membrane with a pore size of 1 to 20 nm, so that the macromolecules or fine particles are separated from the solution. It uses the pressure difference across the membrane as the driving force and the membrane as the filtration medium. Under pressure, small-molecule solutes and solvent pass through, while macromolecular solutes cannot and remain on one side. That is the entire principle, and it is why the process is used mainly to retain macromolecular solutes, concentrating, purifying or clarifying a stream, or separating colloidal suspensions that other techniques handle badly.

Set it against its neighbours, which are distinguished by the minimum particle size or molecular weight the membrane layer retains. By rated pore-size range, the microfiltration membrane (MF) runs 0.02-10 mum, the ultrafiltration membrane (UF) 0.001-0.02 mum, and the reverse osmosis membrane (RO) 0.0001-0.001 mum. Molecular weight cut-off (MWCO) for ultrafiltration is generally 6,000 to 500,000 at a pore size of 100 nm. The membrane is an asymmetric membrane whose active separation layer averages about 10-200 angstrom and retains macromolecules and colloidal particles with molecular weight above 500. Operating pressure difference is 0.1-0.5 MPa. Particle removal above 0.2 um is quoted at 100% under a molecular weight cut-off of 80,000 Daltons.

Structure matters as much as pore size. Symmetric membranes are isotropic, with no skin layer and the same pores in all directions, and behave as depth filters. Asymmetric membranes, which is what industry uses, carry a denser top layer no more than 0.1 micron thick with orderly arranged micropores over a bottom layer 200-250 microns thick of mainly finger-like structure, and behave as surface filters.

Picking a material

Membrane materials span 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 field has narrowed considerably. Polyethersulfone (PES) found commercial application in the early 1990s, and the better-performing PVDF ultrafiltration began to be widely used in the water treatment industry in the late 1990s. PVDF and PES are now the most widely used ultrafiltration membrane materials, with polypropylene (PP), polyethylene (PE), polysulfone (PS), polyacrylonitrile (PAN) and PVC making up the rest of the common list.

Polyacrylonitrile, abbreviation PAN, is worth knowing for other reasons. It is obtained by free-radical polymerization of the monomer acrylonitrile, with acrylonitrile units linked head-to-tail in the polymer chain. 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 degrees C. DuPont developed pure polyacrylonitrile fibre under the trade name Orlon in the 1940s, but it was never put into industrial production because dyeing was difficult and it fibrillated easily. Later work on spinnability and dyeability produced acrylic as it is now known, sold as Orlon, Acrilan, Creslan and Zefran in the United States and Courtelle in the United Kingdom among others. Acrylic density is generally 1.16-1.18 g/cm3 with a standard moisture regain of 1.0%-2.5%, and the fibre is valued for bulkiness, warmth retention, soft handle, weather resistance and resistance to mildew and moths. In materials science PAN is also used as a matrix for synthesising porous materials, for example PAN-based activated carbon.

Among inorganic membranes, ceramic ultrafiltration membranes are used more in household water purifiers. They have a long life and resist corrosion, but the effluent carries an earthy taste that affects mouthfeel, and they clog easily and do not clean easily. Hollow-fibre ultrafiltration membranes, with high packing density, large effective membrane area, high pure-water flux, simple operation and easy cleaning, are widely used in the household water purification industry instead.

Construction details that change field performance

Look closely at a commercial hollow-fibre element and the differences are all in the details. The shell is made of impact-resistant ABS material with a pressure-bearing capacity above 16 kg and wall thickness increased by 1 mm, fully able to withstand the pressure shocks that occur in feed water, ensuring no rupture under shock water pressure and avoiding the long-term creep that causes leakage. Each HUF90 membrane is loaded with 1400 membrane fibres with the length extended by 100 mm, increasing membrane area by 15% and giving a higher effective membrane area than any domestic product of the same specification, which raises water yield. The end cap uses a hemispherical convex structure rather than the traditional flat end face, making feed water distribution across the end-face fibres more uniform, with wall thickness again increased by 1 mm for shock resistance.

A typical element consists of hundreds to thousands of tiny hollow fibres. Generally, a membrane with hollow-fibre inner diameter between 0.6-6 mm is called a capillary ultrafiltration membrane, and because the bore is larger this type is not easily clogged by large particles. Hydrophilicity is the other lever. Fibres given special hydrophilic treatment keep long-term hydrophilicity, with the water contact angle dropping from 79-90 degrees before modification to 30-35 degrees, which yields high flux at lower transmembrane pressure while improving fouling resistance. Mechanical strength matters as much: broken fibres make the membrane lose separation performance outright, so the ability to resist fibre breakage is an important indicator in evaluating any ultrafiltration membrane.

Using high-quality PVDF as raw material gives hollow-fibre ultrafiltration membranes good chemical corrosion resistance, oxidation resistance and light-aging resistance, which means they can be cleaned repeatedly by various methods to remove contaminants and restore flux. That recoverability is worth more in practice than a slightly higher initial flux figure, and it is the main reason PVDF displaced earlier materials.

Keeping the membrane alive

The membrane must be cleaned regularly to maintain permeate flux and extend service life, and the cleaning method follows the nature of the membrane and the material being treated. The sequence usually mirrors reverse osmosis practice: hydraulic cleaning first, then chemical detergents as the situation requires. An ionic solubiliser suits electrodeposition materials, a “bridge-bond” type solvent suits water-soluble organic coatings, protein deposits in the food industry come off with a protease solvent or an alkaline detergent based on phosphate or silicate, and inorganic salt precipitates on the membrane surface dissolve with chelating agents such as EDTA or with acid and alkali. Module type dictates the physical options: tubular modules can be cleaned mechanically with sponge balls, hollow-fibre modules by backwashing. Food-industry membranes also require disinfection, with NaOH, H2O2 and similar agents.

Storage is a separate discipline and it is where a lot of avoidable damage happens. Before use, ultrafiltration membranes are immersed in a protective solution and sealed, preventing the wet membrane from shrinking after dehydration, which would reduce pore size, destroy structure and lower flux. For short-term storage, if the membrane is suspended for less than 10 days, sterilise and backwash once, add 15 ppm (ml/L) of HY-240 bactericide to the backwash water, then close the inlet, drain and regulating valves to maintain sealing and sterilisation. For long-term storage beyond 10 days, sterilise and backwash once, then inject HY-310 protective solution, preferably made up with RO water, and store sealed.

Where ultrafiltration earns its place

The application range is extremely wide. It serves as reverse osmosis pretreatment and ultrapure water terminal treatment in pure and ultrapure water preparation; removes bacteria, pyrogens, colloids, suspended impurities and macromolecular organics from industrial water; purifies drinking water and mineral water; concentrates, purifies and clarifies in the fermentation, enzyme preparation and pharmaceutical industries; concentrates and separates fruit juice; separates, concentrates and clarifies soybean, dairy, sugar, liquor, tea juice and vinegar streams; purifies and recovers industrial and domestic sewage; and recovers electrodeposition paint. Ultrafiltration membrane separation can replace natural sedimentation, plate-and-frame filtration, vacuum drum, centrifugal separation, solvent extraction, resin purification and activated carbon decolourisation in traditional processes.

Two properties explain most of the appeal. The treatment process has no phase change and no adverse effect on material components, and separation, purification and concentration all run at room temperature, which suits heat-sensitive substances and completely avoids the drawback of high temperature destroying biologically active substances. And the system consumes little energy with a short production cycle, so operating cost undercuts traditional process equipment.

Market direction differs by region. Abroad, ultrafiltration is mainly applied to drinking water treatment, and many waterworks already use it to produce tap water. In China it is mainly used for wastewater reuse in the industrial field, as reverse osmosis pretreatment, and it has been widely applied in power, steel and chemical industrial wastewater treatment. Chinese waterworks have been slower to adopt it, largely for funding reasons, but with the revision of national and local drinking water standards and the introduction of new norms, ultrafiltration will be adopted by more of them. The Ministry of Water Resources’ analysis in “China’s Water Supply and Demand in the 21st Century” projected that China would begin to enter a severe water-shortage period after 2010, with water pollution gradually becoming the biggest obstacle to safe urban water supply, making municipal sewage treatment and reclaimed water reuse one of the effective ways out of the urban water crisis. Ultrafiltration membranes therefore have broad market space ahead of them, in municipal sewage treatment as much as anywhere else.