Filtration belongs to the oldest technical treatment. It is just a separation of solid and liquid substances in a suspension and is the last process of the beer production. The yeast, protein and carbohydrate particles, which must be removed from beer to achieve the necessary degree of clarity, are of small size and are compressible. Any attempt to remove this material using a single filter medium, for example a filter sheet, would result in a rapid blockage of the filter and make this method impracticable. The high resistance to flow offered by the accumulation of material on the filter means that it is described as having a low 'Permeability'. These problems may be overcome by injecting a suitable material into the beer stream, referred to as the 'Filter-Aid'. This forms, together with the yeast and other suspended solids, an almost incompressible mass on the filter medium and referred to as the 'Filter-Cake'. This is capable of maintaining the flow of beer and is described as having a much higher permeability. Filter-aid added into the flow of beer this manner is referred to as 'Body-Feed'. But at first filter aids (very coarse ones) have to form an intermediate layer bridging the gaps in the sieves, leaves or cartridges. This so called first precoat layer acts as a support for the subsequent finer precoat or second precoat layer and later on the body feed.

Filter aids Kieselguhr, Perlite, Cellulose etc.