Filtration is the process of removing solids from liquid by passage across or through a porous medium. The effectiveness of filtration depends on particle size and filter medium porosity. Materials suspended in liquid will remain on or in the filter medium if they are larger than the pore opening. Filtration is widely used throughout brewing, and most beers sold today are filtered to some extent. The range of particle size removal is enormous, from the molecular level (1.0 μm) in water filtration to whole hop removal (1.0 cm) after the kettle boil.
There are two basic types of filtration: depth and surface. Depth filtration, also called powder filtration, uses a convoluted labyrinth of channels in the filter media to trap particles. The media can be diatomaceous earth (DE), Perlite, or other porous media. Depth filtration is often considered rough or primary filtration, though in many small breweries, this is the only filtration performed. Examples are plate and frame, screen, and candle filters. Surface filtration uses a thin film material with pores smaller than the particles to be removed. Particles remain on the surface of the filter while clarified liquid flows through. If the pores are of a defined size (for instance, up to 5 μm), filtration is said to be "absolute" to the pore size. Membrane and cross-flow filtration are examples.
