Approach to the Theory of Filtration . Practical advice for the correct selection of filter media.

The leak could be defined as the physical- mechanical separation of two phases that could fall into four main groups:

  • introduccion_1solid-gas separación . Solids retention air
  • Solid-liquid separation . Retention of particles from a fluid
  • Liquid-liquid separation . Retention of a fluid present in other liquid
  • Gas -liquid separation . Retention of fluid in air - gas médium


Phase separation that occurs during the filtration process requires an expenditure of energy is effected by gravity or by applying a pressure or a vacuum.

In the laboratory generally uses gravity filtration using filter paper or folded flat.

Latter accelerates the filtration rate to better exploit the increased filtration area.

In industrial processes where liquid volumes are higher filter press filters are often used in which we can use many different fabrics and types of filter papers according to the type of liquid to be filtered.


The performance of a leak is determined to balance the liquid filter and the type of paper selected.

Keep in mind that the efficiency of the paper decreases with the pollutant concentration of the liquid to be filtered and the filtration rate required.

Selecting a filter paper should be performed in response to three factors:

  • Retention efficiency in terms of particle size ( micron rating)
  • required filtration rate ( liquid to be filtered per minute , hour, day ... )
  • retention capacity ( amount of particles is capable of retaining a role within its porous structure while maintaining the flow rate and the filtration pressure loss within the practical limits of work)


The units used to measure the retention of filter paper are used for determining a qualitative rating between them.

introduccion_2One of the systems used to establish the efficacy of different filter papers involves controlling retention capacity in percentage of water dispersions of very defined precipitates ( ferric hydroxide , lead sulfate , calcium oxalate and barium sulfate ).

In the case of liquid filtration efficiency also provides a filter paper depending on the particle size retained . Expressed in microns is known as Micron Rating. Retention represents 98% of uniformly sized particles dispersed in the liquid to be filtered ( BS 6410 ) . Thus being 2 microns Micron Rating indicates that it is able to retain approx. 98% of particles larger than 2 microns which are in suspension in a fluid.

In phase separations (solid - gas) efficiency is fixed in accordance with BS 4400.