Filtration

Water filters removes impurities from the water using physical barriers of varying types such as screens, sand or man-made membranes, chemical or biological processes. Physical barrier filtration is typically employed to remove suspended organic or inorganic matter in the water whereas chemical and biological filters, which can also be employed to remove this matter, are generally used to filter out impurities in the water that are dissolved in solution. Examples of these filters are the ion exchange resin bead filters used to soften water by removing calcium and manganese and magnesium ions in solution.

Sometimes multi-barrier combination filters are used, such as granular activated carbon filters (GAC) which make use of physical separation of suspended impurities as well as adsorption of dissolved contaminants onto the surface of the activated carbon. These types of filters are particularly useful as portable drinking water filters as they are low maintenance and very easy to use. Sand filters remove particles in water in two main zones: on the surface and throughout the porous body of the filter and are usually used to separate small amounts (<10 ppm) of fine solids (<100 microns) from aqueous solutions with this solids material then discarded. Sand bed filters work by providing the particulate solids with many opportunities to be captured on the surface of a sand grain. As fluid flows through the porous sand of the filter, whether under gravity or under pressure, the particulates come close to sand grains where they are captured by one of several mechanisms:

1. Direct entrapment
2. Van der Waals forces
3. Surface charge attraction
4. Diffusion