Swelling, filter media

The resultant porous sheet can be used as a filter medium, but if the untreated paper gets wet then the fibres absorb moisture, with two significant consequences the fibres swell, so that the spaces between them reduce and the paper improves in filtration efficiency, but the mechanical strength drops sharply, making the paper less useful as a filter. To be used as a filter, then, the paper must be fully supported for use in wet filtration (as, for example, in the filter papers used in the laboratory filter funnel) or it must be restricted to dry filtration (such as in building air cleaning). 

Other factors to consider in the selection of cartridge filters are the type of filter medium and its characteristics. As an example, polypropylene cartridges are a better selection than cotton for water service, since cotton swells. The compatibihty of filter membranes and binders with chemical additives or impurities in the water should be checked. The designer should contact specific manufacturers for detailed information. 

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