Bead filters are closed pressure vessels filled with millions of plastic beads. Because these beads float, they provide the mechanical filtration, and because of their large bonding surface they also provide biological filtration. Mechanical filtration: the beads float in a closed vessel and are therefore very close to each other. The water is pushed through them by the pressure of the pump, while the dirt particles become trapped among the beads. As the filter is used for a longer period of time, the biofilm around the beads becomes thicker and thicker, allowing increasingly fine dirt to be collected (down to 10 microns!). Biological filtration: the plastic beads have an adhesion surface area of about 1600 m²/m³. The biofilm layer on the beads therefore provides a huge amount of surface area for the bacteria to break down ammonium, nitrite and nitrate.