Filtration

From Wikipedia, the free encyclopedia

This article is about the separation process. For the mathematical concept see filtration (abstract algebra). For the noun see filter.

In chemistry, alchemy and water treatment, filtration is the process of using a filter to mechanically separate a mixture.

1 Methods of filtration
2 Filter media
3 See also
4 External links

[edit] Methods of filtration

There are many different methods of filtration; all aim to attain the separation of two or more substances. This is achieved by some form of interaction between the substance or objects to be removed and the filter. In addition the substance that is to pass through the filter must be a fluid, i.e. a liquid or gas.

The simplest method of filtration is to pass a solution of a solid and fluid through a porous interface so that the solid is trapped, while the fluid passes through. This principle relies upon the size difference between the particles making up the fluid, and the particles making up the solid. In the laboratory, a Büchner funnel is often used, with a filter paper serving as the porous barrier.

For example 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 filtrated water takes longer to go foul, showing that very small items (such as bacteria) can be removed from fluids by filtration.

A further disadvantage with the physical barrier method of filtration is that the substance being filtered from the fluid will clog the channels through the filter over time. Thus the filter becomes less and less efficient over time (for example, a vacuum cleaner bag). Thus methods have been developed to prevent this from happening. Most such methods involve replacing the filter. However, if the filter is needed for a continuous process, this is highly problematic, and complex scraping and in-situ cleaning mechanisms have to be used.

For separation where there is a very small size difference, chemical filters may be used. These will use a filter that has properties so that undesirable items are attracted and retained by the filter, and the fluid from which it is to be separated is not. Filters of this kind most often take the form of electrostatic attractions. These form of filters again have the problem of either becoming clogged, or the active sites on the filter all become used by the undesirable. However, most chemical filters are designed so that the filter can be flushed with a chemical that will remove the undesirables and allow the filter to be re-used.

Filtration is a more efficient method for the separation of mixtures than decantation, but is much more time consuming. If very small amounts of solution are involved, most of the solution may be soaked up by the filter medium.