Membrane Filtration

Membrane filtration through a very thin filter medium is also known as 'surface filtration'. The solid particles to be separated are usually large compared to the pore size characteristic of the membrane. The pores on the surface are of irregular shapes. The rejection of particles is dependent on several factors affecting the transport through these pores into the tortuous channels. The separation is based on exclusion discrimination by physical size, charge or affinity or a combination of these properties. Large particles are rejected on the surface and do not accumulate on the surface and do not get a chance to enter into the interior of the filter.

Other types of membrane filters are screen filters and here the pores do not lead into tortuous capillary paths. The pore size is uniform but the distribution of the pores is random on the filter surface. The filter is made by bombarding a thin polycarbonate film with neutrons in a reactor. The film is then placed in a bath of etching solution which preferentially attacks the polymer along the track of the neutrons. The pore size is regulated by selecting the appropriate reagent, exposure time and temperature.

Membrane filtration can be dead end or cross-flow. In dead-end filtration all the solution is forced through the membrane. Retained particles collect on the membrane surface and in the filter greatly reducing flow. A current application of dead-end filtration is in bacterial testing where the liquid to be tested is passed through the filter retaining all bacteria on the surface. Most chromatographic filtration applications are of this type. In cross-flow membrane filtration, the feed liquid flows tangentially to the membrane surface, which prevents the build up of cake on the membrane. Both types of filtration use similar membranes.

By convention, membrane filtration or microfiltration is limited to membranes used to remove particles larger than 0.1 |im in diameter. Membranes able to remove smaller particles are called 'ultrafiltration membranes' and microsolutes can be removed by reverse osmosis. Ultrafiltration and reverse osmosis are discussed elsewhere. This article is limited to the process of microfiltration.

The filtration thresholds of common membranefiltration processes are shown in Table 1.

Table 1 Filtration threshold of common membrane-filtration processes

Type offiltration

Impermeability ofmembrane

Reverse osmosis

<0.001 |im

Ultrafiltration

0.001-0.1 |im

Microfiltration

0.1-10 |im

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