The separation of solid particles from fluids - either gas or liquid - is widely accomplished by filtration technique. The dispersed solid particles together with the fluid are forced to move towards a filter medium that represents some flow resistance but at the same time most, if not all, of the particles can be separated while the continuous fluid passes through this porous medium. The collection of the particles may occur inside the filter medium by so called depth filtration mechanism, and/or on the fluid approach surface through surface and subsequent cake filtration.

Especially since beginning of 1990ies with the development of more reliable and efficient filter media filters have become so called "best available technology" (BAT) for particle removal from the gas phase which is, though only in part, also true for the liquid phase. Of course, also other technologies, mostly based on sedimentation, electrical or magnetic principle, are used for good reason sometimes as pre-separators or for particularly harsh environments. Clearly, filter development is closely linked with fiber development and also material development. While steel, glass or ceramic filters have been available for a long time for high temperature applications, other cheaper and more easy-to-use filter materials have been developed to withstand higher temperatures. Still a temperature beyond around 260°C is critical for most standard filter media. Recently, also so called nano-fibers have been developed and are incorporated in coarse filter structures with even better particle removal efficiencies but at the same time remarkably low flow resistance, i.e., pressure difference.

Fine particles with an unsuitable filter medium pairing will result in problems with low particle removal efficiency. Generally particles in the size range of 0.4 to 0.9µm are particularly difficult to remove from a fluid stream since the effect of Brownian motion becomes lower with increasing particle size but particles are still too small that inertia would lead to high collection efficiency.

Sticky, liquid droplets or reactive materials are still a reason why filtration technique is not applicable. Through the use of auxiliary equipment as well as rigorous control of the operation it is sometimes possible to avoid condensation in or on the filter medium that will trigger the formation of permanent scales with solid particles. Moreover fibrous particles can represent a problem for filtration when they form strong bonds among themselves and even more critical with the filter medium. Clearly, particles that tend to enter the filter media can hardly be removed during operation.

Except for so called cross-flow filtration, filter operation is intrinsically not continuous but it is a collection of particles (in or on the surface) on the filter medium followed by either a filter cleaning step or filter medium replacement, i.e., often the filter is discarded. But apparent continuous operation can be achieved if filter cleaning is initiated on-line, e.g., in many gas/solid separation baghouse filters. But also off-line cleaning can support continuous operation with either main continuous flow being redirected (e.g. bag filter compartments are sealed off for sequentially cleaning) or the loaded filter is continuously removed from the main flow (e.g. disk or drum filters.) Anyhow, cleaning is critical and if not accomplished thoroughly filter operation will deteriorate. Besides filter cake liberation it is mostly the challenge to not fully disperse the liberated filter cake but to allow the detached solids to settle and be removed from the filter. Many parameters are important, i.e., fluid, particle and filter medium properties as well as operating, particularly cleaning conditions. Thus performance prediction and optimization are difficult.