Filtration operates entirely on particle or droplet size (and, to some extent, shape), such that particles below a certain size will pass through the barrier, while larger particles are retained on or in the barrier for lateral removal. The separating size is a characteristic of Filter medium. The wide range of Filter Designs is very largely a consequence of the need to handle the accumulated solids that collect in the filter, although the need to pack as much filter medium area into a given equipment floor space (or volume) can be another design decider.
The operation of a filter usually needs a pressure difference across the filter medium, and this can be effected by means of fluid pressure upstream of the medium (pressure filters) or suction downstream (vacuum filters).
The particle sizes covered by filtration range from the large pebbles of the mineral sector’s screens to the ultrafine particles and large molecules of the membrane ultrafiltration systems. Most systems involving contaminant removal are concerned with fine particles – fine enough for example, to have stayed suspended in atmospheric air for long periods of time.
The size of the separated particle is used to delineate the terms used for various filtration processes. Thus, filtration ( or more specifically, now a days), ‘macrofiltration’ is used for separating particles in the approx range of 1mm down to 5µ. From 5µ down to about 0.1µ the process is termed ‘microfiltration’, while below that termed as ‘ultrafiltration’. Ultrafiltration covers the finest distinct particles (such as colloids), but its lower limit is usually set in molecular weight terms, measured in Daltons.