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The removal of impurities and contamination from water supplies or industrial chemicals is a process existing all over the world to varying degrees of sophistication. Filtration systems vary enormously according to location, the type of purification required and funding available.
For example, drinking water is filtered to different degrees to remove microorganisms such as Cryptosporidium and Giardia, to remove nitrates or hardness, or to remove color to make it more attractive and palatable. There are many examples of the effluent treatment process using reverse osmosis and ultrafiltration prior to discharge.
However, the drive for improved efficiency throughout manufacturing industries, coupled with the introduction of stricter effluent discharge legislation, means that more chemical and industrial processes can benefit from the installation of reliable membrane filtration equipment.
Membrane filtration relies on pressure to pass a liquid through a membrane and separate both suspended and dissolved materials from the incoming feed or concentrate, which is the desired end product. The size of particle allowed to permeate the membrane can be accurately controlled according to the membrane type, and the pressure or driving force can be applied in several different ways, according to the application.
The Filtration Spectrum
The four main types of pressure driven membrane filtration processes can be classified according to the size of particle retained by the membrane. The finest degree of separation is provided by reverse osmosis (RO). Next comes the "filtration spectrum" which consists of nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Between them, these processes separate particles differing by anything from a few angstroms up to a few microns in size.
Membrane filtration technology has developed both in the way membranes are packaged and in the type of material used. The result is a wide range of module configurations and membrane geometries suited to a variety of applications. Membranes can be configured in tubular, spiral, flat sheet or hollow fibre arrangements.
Tubular membranes have the advantage of being able to handle viscous liquids with high levels of suspended solids and can be chemically or mechanically cleaned. Tubular polymeric membranes are housed in modules of stainless steel or plastic designed to allow flow conditions which optimize membrane performance.
Spirals, as the name suggests, consist of tightly packed filter material
sandwiched between mesh spacers. The high packing density significantly increases the surface area compared with tubular membranes. Spiral membranes require careful pre-filtration to avoid blocking if suspended solids are present. However, developments in mesh spacer sizes and designs are helping to increase the number of applications to which spirals are suited.
Polymeric spiral membranes are generally used when a high throughput is required, while polymeric tubular membranes, which can often be mechanically cleaned, are more suited for low maintenance operations or where highly viscous products are being processed.
Hostile environments, high levels of solvents, wide pH range and other process considerations may dictate the use of ceramic membranes. This technology, normally adopted for UF and MF, typically uses an alumina or zirconia coating which is applied to the inside surface of the ceramic support. While the use of ceramic membranes is sometimes the only viable proposition, the capital cost is much higher than conventional polymeric membrane, but in most cases a longer operational lifetime can be expected.
In addition to the membrane configuration, each application has an appropriate membrane type around which the plant is designed, and the resolution of a filtration system can be classified according to a spectrum of sizes. The four basic membrane filtration processes have a number of features in common, however operating conditions, material of construction and performance vary greatly with the application.
Dewatering and chemical concentrating processes often utilize RO, the finest filtration achievable. RO membranes can separate particles as small as a few angstroms, such as metal ions and aqueous salts, and are characterized by their ability to retain sodium chloride. The systems are operated at high barometric pressures, typically up to 60 bar.
RO operates on the principle that a pressure exceeding the osmotic pressure of a solution is applied across a semi-permeable membrane. For aqueous solutions this causes water to flow from the retentate (the liquid or solids held back by a membrane) to the permeate (the liquid which passes through a membrane).
A number of factors affect the passage of solute including the concentration of ions in the permeate, concentration at the membrane surface and the pressure difference across the membrane. RO is used widely in water purification-for the production of deionized water-and in separating and concentrating metal ions and aqueous salts.
Moving up the filtration spectrum, nanofiltration is sometimes referred to as "open RO." It operates efficiently at the molecular level, where particles are between one nanometer (nm) and two nm in size. Osmotic pressure is still the major resistance to solvent flow through the membrane, but osmotic pressure in the permeate is often significant, thus reducing the effective resistance to flow compared with RO. Consequently, operating pressures are normally considerably lower. Inorganic ions can pass through the membrane along with the permeate, but larger ions and organic molecules are retained by the membrane.
Unlike RO, NF is suitable for desalting, concentration and demineralization and operates typically at between 20 and 40 bar. A number of NF plants have been installed for the production of dyestuffs in the chemical industry. For example, raw dye is usually introduced as a liquor or slurry with the concentration of sodium chloride (NaCl) typically between eight percent and 10 percent. Other salts account for up to two percent of the solids and the dye itself is between six percent and eight percent. After filtration, the de-salted dye accounts for up to 33 percent of the concentrate and the concentration of NaCl is reduced to less than 0.5 percent. The use of NF in this application can reduce overall operating costs and increases product quality.
Moving further up the filtration spectrum, ultrafiltration is used to separate particles typically up to a few tenths of a micron in diameter with different molecular weights. Ultrafiltration differs from RO and NF because the membrane itself is slightly porous. Operating pressures are typically between 5 and 15 bar, less than those required for NF. As the porosity of the membrane increases, the further up the filtration spectrum one operates, so the process flux-a measure of the permeate and the size of components -which can be filtered increases.
Separation of species according to molecular size means that UF is used widely in chemical industries such as pharmaceutical manufacturing and fermentation. For instance, broths containing antibiotic, proteins and cell debris can be filtered to produce clarified or purified antibiotic. Organic acids also can be successfully purified using UF, although its performance is influenced by polarized molecules.
Microfiltration achieves separation on the basis of particle size. MF membranes have a surface porosity which is 50 times greater than UF membranes and can successfully filter out particles in the macromolecular range, from between 0.1µm and 1µm and larger. The technique is used to separate paint pigments, small bacteria and viruses.
Although pore size is uniform across any membrane and filters are characterized by their pore diameter, it sometimes is better to use a pore size which is significantly smaller than the particles being filtered so that particles (which are much larger than the pores) roll over the pore itself rather than sitting in the mouth of the pore.
Application of Membrane Processes
Several processes can benefit from membrane technology. Often a combination of membrane processes can be used. For example, in the treatment of landfill leachate in several plants in Europe and the Far East, UF is used to concentrate biomass as part of the biological treatment process, and RO is used to concentrate the leachate, leaving the bulk of the leachate suitable for discharge.
Although waste minimization is the priority in many cases, membrane filtration also fulfills a vital role in reducing the release of harmful and environmentally-damaging material, for example in the treatment of low level radioactive waste. In the US, water used for washing down contaminated machinery and equipment is passed prior to discharge through RO and UF plants which are at the center of the treatment process. Any radioactive contaminants are contained by the membrane and subsequently dried and disposed of safely.
Because of the wide range of separations possible using membranes, their ability to treat waste water and recover valuable by products in a variety of different environments, this form of filtration technology now is accepted widely in many areas of industry and looks set to become increasingly important.