Middle English filtre, from Medieval Latin filtrum piece of felt used as a filter, of Germanic origin; akin to Old High German filz felt
- 1a : a porous article or mass (as of paper or sand) through which a gas or liquid is passed to separate out matter in suspension
- b : an apparatus containing a filter medium
- 2a : a device or material for suppressing or minimizing waves or oscillations of certain frequencies (as of electricity, light, or sound)
- b : a transparent material (as colored glass) that absorbs light of certain wavelengths or colors selectively and is used for modifying light that reaches a sensitized photographic material —called also color filter
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Oversize solids in the fluid are retained, but the separation is not complete; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size and filter thickness). Filtration is also used to describe some biological processes, especially in water treatment and sewage treatment in which undesirable constituents are removed by absorption into a biological film grown on or in the filter medium as in slow sand filtration.
Examples of filtration include
- The coffee filter to keep the coffee separate from the grounds.
- HEPA filters in air conditioning to remove particles from air.
- Belt filters to extract precious metals in mining.
- Horizontal plate filter, also known as Sparkler filter.
- Furnaces use filtration to prevent the furnace elements from fouling with particulates.
- Pneumatic conveying systems often employ filtration to stop or slow the flow of material that is transported, through the use of a baghouse.
- In the laboratory, a Büchner funnel is often used, with a filter paper serving as the porous barrier.
An experiment to prove the existence of microscopic organisms involves the comparison of water passed through unglazed porcelain and unfiltered water. When left in sealed containers the filtered water takes longer to go foul, demonstrating that very small items (such as bacteria) can be removed from fluids by filtration.