Basics of Filtration

Mechanical retention occurs when a particle is mechanically restricted from passing through the filter medium. Direct interception, sieving, and bridging are mechanisms of capture that facilitate mechanical retention. Of these three methods of capture, sieving is the most dependable under normal forward flow conditions. If a particle is too large to move through a pore, unless the actual physical structure of the filter medium of particle is altered, the particle cannot be pushed through the pore. Particles captured by both bridging and direct interception are mechanically retained, but are more condition dependent than sieving. Pulsing or surging will dislodge a filter cake and/or small particles directly intercepted by media obstructions, and, hence, release the mechanically retained particles. However, if operating conditions ate stable, particles held by mechanical retention should not be released.