There are eight major material categories for filtration media: ceramics, filter aids, monofilament nonwoven fabrics, membranes, fabricated metals, porous plastics and wet-laid fibers. Moreover, there are other types and many variants of these eight major categories.
There are eight major material categories for filtration media: ceramics, filter aids, monofilament nonwoven fabrics, membranes, fabricated metals, porous plastics and wet-laid fibers. Moreover, there are other types and many variants of these eight major categories. The list is virtually endless, including new entrants in recent years like nanofiber webs, which many might consider a sub-category of nonwoven fabrics.
No two filter types are made on the same production equipment. For example, nonwoven fabrics cannot be produced on membrane manufacturing lines. Filter aids are manufactured using an entirely different category of materials and process than woven media, such as monofilament fabrics, and so on. Filter media companies tend to specialize in one or two distinct types of filter media and rarely supply three or more types. The majority tend to concentrate on one category of media, whether monofilament woven fabric, membranes, metals such as woven wire cloth, or filter aids, to cite a few. The cost of the production equipment to produce each medium is expensive and the variations of media constructions are so numerous, companies specialize to capture enough volume to fully utilize their equipment.
A somewhat similar situation exists with manufacturers of filtration devices and equipment. Filter manufacturers tend to offer a limited number of filter types, for example, RO/UF membrane filter module manufacturers rarely make and sell microporous membrane cartridges and porous plastic producers do not make metal filters from wire cloth. Companies who produce wet-laid media from cellulose or synthetic fiber rarely provide other media. If they do, it’s most often from another division at a different location.
Each media has different functional capabilities and properties. Filters made from metal or ceramic are rigid and used in high-pressure applications, such as for polymer filtration or where a corrosive fluid is present. An example would be diesel exhaust filters or high-temperature environments. They are almost always capable of being cleaned and reused. With a few exceptions nonwoven fabric media are used at moderate temperatures, have some depth to capture and hold solid particles, and are typically used a single time in applications where the filter is subsequently disposed. Certain exceptions are baghouse filters and dust collection cartridges, which are designed to be part of large systems, allowing for in-situ back-pulse cleaning. Microporous membranes are some of the most costly filter media and always found where extremely fine filtration is required, such as in the medical, pharmaceutical or semiconductor industries. Filters from microporous membranes are generally used to remove bacteria whereas reverse osmosis membranes can remove salt and monovalent ions from seawater. Ultrafiltration membranes are widely used to purify solutions in food processing.
Wet-laid cellulosic media have been the mainstay in transportation filters for lube oil and engine air-intake filters for many years. Cellulose is capable of withstanding the environment under the hood of automobiles and trucks and is relatively inexpensive. Fiberglass wet-laid media is found in high-pressure hydraulic filtration, because the glass fibers are non-compressible and provide excellent dirt-holding capacity. Fiberglass fiber can be made quite fine, even sub-micron in diameter, and is the material of choice for HEPA filters for clean rooms, coalescing media and certain laboratory filters.
Monofilament woven fabrics (think: filaments like fishing line or “weed-whacker” cord) are fabricated for use as surface filters, as is wire cloth. Both are woven from very fine diameter strands or yarns from 20 microns up to 1,000 microns in diameter to satisfy a very wide range of applications from high-end blood and ophthalmic fluid pleated filters to sludge dewatering belts found in many municipal treatment facilities. Porous plastic filtration media consist of fine plastic powders molded into porous rigid shapes, often using hydrophobic polymers. These filters are used in medical devices, automotive lamp assemblies and battery vents. In recent years porous plastics are molded into membrane tube supports for use in waste water or pleated pool and spa filters.
Obviously, one porous medium does not fit all needs when it comes to filtration.