Bead filters are closed (pressure) vessels filled with thousands of plastic beads. These beads are very close together and form a layer through which the water is forced by a pump. As a result, the dirt particles stick in the beads. If the filters are used for some time, the biofilm (the sticky layer) around the beads becomes thicker, so that finer and finer dirt is caught.
Both mechanical filtration and biological filtration take place in a bead filter. This is because useful bacteria stick to the beads and break down harmful substances such as ammonium, nitrite and nitrate. A bead filter can easily be operated manually using valves. The valves can be used to set various functions, such as cleaning the filter.