How is filtration achieved? Filtration is usually a combination of physical and chemical processes. Mechanical straining removes some particles by trapping them between the grains of the filter medium(such as sand). Adhesion is an equally important process nly by which suspended particles stick to the surface of filter grains or previously deposited of material. Biological processes are also impor tant in slow sand filters. These filters form a filter skin containing microorganisms that trap and break down algae, bacteria, and other y be organic matter before the water reaches the filter medium itself.