About Filter

The primary purpose of using a filter is to remove impurities from fluids or air. It is a device which usually contains a membrane or layer which is designed in such a manner so that it obstructs the impurities from passing through. The membrane or layers used in the filter are semi-permeable barriers placed perpendicular to the liquid or air-flow. In laboratories the commonly used material for membrane is the filter paper.

Filtration is the process of filtering. It is done by passing the mixture through the filter. The liquid obtained after filtration is known as the filtrate, while the solid left behind in the filter is called the residue, retentate or filtrand. The methods of filtration are various; however, all of the methods endeavor separation of substances. The substance which is to be passed through a filter has to be necessarily a fluid, i.e. a liquid or gas. The methods are different dependent upon whether the material to be filtered is in the fluid phase or not.

Filter is classified in two types - surface filter and depth filter. A surface filter consists of a solid sieve with or without a filter paper which impedes the solid substances from passing through. Examples of surface filter are B¨¹chner funnel, Belt press, Rotary vacuum-drum filter, Crossflow filters. A depth filter on the other hand consists of a bed of granular material which hinders the passage of the solid particles. The former allows the residue to be collected intact while the latter does not.

Filtration systems are used in various industries. To name a few, they are used in Chemical & Petrochemical Processing, Pharmaceutical, Cosmetic and Biotechnology Processing, Water Treatment, Purification & Reclamation, Food, Beverage & Fermentation, etc. The scope and purpose of the filtration systems vary in accordance to the substance which is to be filtered. Oil filtration systems are usually used in automobiles and hydraulic machinery. Air filtration systems are used majorly to clean polluted air in automobiles, buildings as well as aircrafts. Water filtration systems are majorly used to purify water for drinking or other purposes. Filtration systems are also used for waste management.

Filtration is a majorly used process. For example, a coffee filter is used to separate the coffee from the grounds, HEPA filters present in air-conditioning remove particles from air, belt filters separate precious metals in mining, B¨¹chner funnels along with filter papers used in chemical laboratories.

A filter where the substance to be filtered and to be freed from impurities is water is a water filter. The basic methods of filtration are applicable to water filter along with other chemical and biological processes. Filters are used to purify the water to different extents for irrigation, drinking water, etc. Water filters use techniques such as sieving, adsorption, ion exchanges. The various types of water filters include media filters, screen filters, disk filters, slow sand filter beds, rapid sand filters and cloth filters. For home use point-of-use filters are used. They include granular activated carbon filters (GAC) which is used for carbon filtering, metallic alloy filters, microporous ceramic filters, carbon block resin (CBR) and ultra filtration membranes. Portable water filters are also available. A water filter can be made from local materials like grass, charcoal and sand. But these do not purge the water of micro-organisms and pathogens.

Air filters are of two types - particulate air filter and chemical air filter. The former constitutes of fibrous substances which remove solid particulates such as dust, pollen, mold, and bacteria from the air, while the latter cleanses the air of volatile compounds or gases with the help of an absorbent or catalyst. Air filters are generally used in building ventilation systems and in engines where air quality is important. Usually foam, pleated paper, or spun fiberglass elements are used in air filters. A more sophisticated method is that of using air ionisers which use substances with a static electricity charge on them which attract dust particles.

Press filters are used to separate out solid and liquid. They are industrial machines. This solid/liquid separation is obtained through filtering cloths and plates.

Oils like engine oil, transmission oil, lubricating oil contain many impurities. Oil filter is used to filter these impurities. These are mainly used in hydraulic machinery.