Filter (chemistry)

A selection of laboratory filters. Clockwise from top left: glass fibre, in-line polypropylene, glass frit, paper and PTFE syringe filters.

In chemistry and common usage, a filter is a device (usually a membrane or layer) that is designed to physically block certain objects or substances while letting others through. Filters are often used to remove solid substances suspended in fluids, for example to remove air pollution, to make water drinkable, and to prepare coffee. Some devices that are called filters may also carry out other processes, such as waste treatment, (e.g. biofilter). Several types of filters are used in chemistry in order to facilitate separation, thereby purifying a liquid (or gas). Many filters use gravity, or gravity enhanced by vacuum (suction) in order to create this separation, often through a funnel-shaped device. Filter efficiency can be improved in a number of ways, such as with the use of fluted filter paper. Other types of materials may be used to effect separations based on size, similar to filters, such as molecular sieves.

The process of passing a mixture through a filter is called filtration. The liquid produced after filtering a suspension of a solid in a liquid is called filtrate, while the solid remaining in the filter is called retentate, residue, or filtrand.

Filter media

There are two types of filter media — a surface filter, a solid sieve which traps the solid particles, with or without the aid of filter paper (e.g. Büchner funnel, Belt press, Rotary vacuum-drum filter, Crossflow filters), and a depth filter, a bed of granular material which retains the solid particles as it passes (e.g. sand filter). The first type allows the solid particles, i.e. the residue, to be collected intact; the second type does not permit this. However, depth filters are less prone to clogging due to the greater surface area where the particles can be trapped. Also, when the solid particles are very fine, it is often cheaper and easier to discard the contaminated granules than to clean the solid sieve.

Filter media can be cleaned by rinsing with solvents or detergents. Alternatively, in engineering applications, such as swimming pool water treatment plants, they may be cleaned by backwashing.

Alternatives

For certain situations, such as separating an aqueous substance from a solid one, filtering may be replaced with decanting or centrifugation. Magnets can be used to remove magnetic contaminants from lubrication oils etc. Separation based on differences in physical and/or chemical properties can be achieved through a variety of techniques, including chromatography.

See also

• Tubular fluid, the renal glomerulus filtrate

Filters for various substances:

• Water filter
• Air filter
• Oil filter

Filters for specific applications:

• Pneumatic filter
• Coffee filter
• Filter mask

Filters by various methods:

• Disk filter
• Paper filter
• Media filter
• Membrane filter
• Cross-flow filtration
• Sand separator
• Screen filter
• Sand filter

Related devices and concepts:

• Activated carbon
• Artificial membrane
• Non-Filter (trade name).
• Gas mask
• Sieve
• Colander
• Reverse osmosis
• Filter feeder
• Filter binding assay