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First to be considered in this review of filter media are three types of material whose prime function is not the separation of solid particles from fluids, but in whose use such separation does occur. They operate by different physical processes
from filtration (and from each other), and are used for different purposes.

Absorbent media

An absorbent material is akin to a sponge in that it can draw fluid into itself and retain this fluid within its structure, and the fluid can only then desorb by a change in phase (such as evaporation or drying). In this sense, it can act as a filter by removing liquid droplets, for example, provided that they wet the absorbent. Thus, an untreated paper, being an absorbent material, could filter out water droplets from an air stream.

However, its usefulness as a practical filter in such a case would be very limited, both as regards to retention of moisture, as it would soon achieve the maximum possible absorption capacity, and to mechanical strength, as it will be weakened by the
moisture content (which may also act as a solvent for any binder in the paper).

Increased wet strength can be achieved for papers by impregnation with substances like neoprene, or phenol formaldehyde and other synthetic resins. However, the impregnation lowers the absorption capacity, and it is more often the case that
papers for use as filters are specially treated to make them non-absorbent, to provide maximum mechanical strength and resistance to solvent action.

Natural fibres, such as wool and cotton, and the felts made from them, are also absorbent materials, but the use of such absorbent media purely as absorbent filters is very limited. These materials are widely used as filter media, with their absorptive properties of secondary or negligible significance.

It has been stated in Section 1C that the fundamental method of entrapment of a particle from a fluid by a filter medium is an adsorptive process, but it is a different adsorption system that is being referred to here, namely the removal from a fluid
(liquid or gas) of a fluid component that is intimately mixed with it. The adsorption process occurs at the surface of a solid material, whose surface area should therefore be as large as possible, and happens because of electrical attraction at the
molecular level. Specific types of material, normally in the form of finely divided granular or powdered solids, can exhibit high adsorptive properties when in contact with vapours and non-solid contaminants present in fluids.

If such granules can be suitably contained (for example, in a packed bed or column), they will also act as a mechanical depth filter, trapping the suspended particles within the bed. Adsorbent filters can also be made by the addition of adsorbent
solids to a normal piece of filter medium, either as a layer sandwiched between two sheets of medium, or by embedding particles of adsorbent in the material of the filter medium, to make what is called a combination medium.

The chief adsorptive medium used is granular or powdered carbon that has been activated by steam or chemical treatment to create a porous structure with very high surface area per unit weight of carbon. Fuller ’ s earth, a naturally active clay, is also used as an adsorbent. Adsorbent media are widely used for the removal of odours, smoke, fumes etc., in a wide range of applications from domestic (e.g. kitchen cooker-hood filters) through to air-conditioning plants and to industrial fume removal. Activated carbon is also the filter medium normally used for water purifying or clarifying on small-scale applications such as drinking water supplies in caravans or boats. Industrially, adsorbent filters are used for the removal of odours from oils, and of odours and tastes from foodstuffs and beverages. The adsorbent properties of the medium
means that it removes dissolved as well as undissolved contaminants, the medium being chosen accordingly.

An important development in activated carbon formats is the production of charcoal cloth as a quite distinct form of the material, first developed in 1977. It has a high adsorption capacity and has the distinct advantage of being strong and flexible, with good resistance to shock and vibration, and hence it can provide a self supporting filter element. It is manufactured from pretreated woven cellulose fibre cloth, reduced to 100% carbon in a controlled atmosphere furnace, maintained so as to ensure the desired strength and adsorptive capacity. The process reduces the cellulose cloth to a quarter of its original weight, yielding an activated charcoal cloth with high porosity and high surface area. Although expensive to produce, charcoal cloth has special applications such as respirators and gas masks.

Biological filters

There is another group of equipment to which the word ‘ filter’ is attached in their name that have a prime function that is not the act of filtration, but in which filtration does play a part. These are the biological or trickle filters much used in the
processing of water and wastewater. They consist of a packed bed of coarse granules or pieces of plastic over which the water to be treated is allowed to run. The purification takes place in a layer of biological material that forms on the surface
of the packing. A certain amount of filtration of suspended solids does take place by adhesion to the packing, but the digestion process also produces solids that are swept out in the effluent stream and so these types of equipment do not clarify the
feed from suspended solids so much as remove dissolved organic material from solution in the liquid.

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