Common Filter-Media Products And Their Uses
Filtration is a process used widely for industrial and domestic use. When first applied for such purposes, it was essentially a means for removing any undissolved solids that might be present in a liquid. One of the earliest methods used was to pass water through linen or similar fabrics which served to trap any solids present and render it clear. Since those earliest efforts, the range of filter-media products available has grown considerably and now even includes materials that are capable of removing the molecules of soluble compounds from a solution. While the purification of water remains one of the most important applications for these new materials, their uses now include the clarification of many other liquids, as well as the cleansing of air and various gases.
While a relatively crude filtration device, a strainer consisting of perforated metal or wire mesh is often used to remove coarser particles that might compromise the efficiency of any finer filter-media products that might be used subsequently. In a typical water-treatment plant, coarse straining is often necessary to remove leaves, branches, and discarded refuse before surface water can be passed through a sand bed to remove smaller particles, after which more stringent treatments must be applied to remove organic material and harmful bacteria. Physical filtration relies on the use of porous materials to manufacture filter-media products and so their efficiency will be determined by the thickness of their material and the diameter of their pores. While, in the latter respect, charcoal is more effective than sand, only man-made materials are able to produce the minute pore sizes required to retain the tiniest particles. These include materials such as sintered glass and ceramics.
In order to create a compact filtration system, it is possible to combine a selection of different materials, arranged in successively smaller pore size, and house them in a single casing. Because the pores of the last medium included are so tiny, water must be passed through the device under pressure. This type of device is often attached to a domestic tap. Among the more advanced filter-media products in common use today are ion-exchange resins. Unlike the materials discussed so far, they do not rely on physical properties such as porosity but, instead, make use of their chemical properties. Furthermore, they are not designed to retain undissolved solids, which, if present, must first be removed by physical filtration. In practice, ion-exchange resins act to remove the dissolved chemicals present in a solution and the clue to how they achieve this is to be be found in their name. These filter media products are available in two types. They can be either anionic or cationic. Consisting of small beads of an organic polymer, their surfaces possess functional groups which, in the case of cationic resins, carry a negative charge, while those of the anionic beads are positively charged.
When the beads come into contact with ionised compounds in solution, they attract oppositely charged ions, surrendering similarly charged but harmless ions of their own in exchange. Thus cation resins can attract sodium, calcium, and magnesium ions to replace them with hydrogen, while the anion resins are able to retain chloride and sulphate ions in exchange for hydroxyl groups. By using both of these filter-media products either simultaneously or in succession, unwanted ions are removed and replaced with hydrogen and hydroxyl ions that combine to form more water. Ion-exchange systems may employ a variety of different anionic and cationic resins, each with its own particular application, such as the removal of CO2.
The filtration technique that has had the greatest impact on industrial and commercial applications, however, is reverse osmosis. Once again, the clue to the operation of this technology lies in its name. The technique employs a new breed of synthetic filter-media products to emulate the semi-permeable structure of a plant’s cell wall. Made from polymers like cellulose acetate, thin membranes are used in systems that apply external pressure to overcome the osmotic pressure that would otherwise tend to equalise the solute concentrations on either side of the membranes, thus separating them completely from the solvent. The technology has been widely adopted, especially to replace the costly distillation process used by desalination plants.