Bead filters are closed (pressure) vessels filled with thousands of plastic beads inside. 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. After the filters are used for some time, the sticky layer or Biofiilm around the beads gets thicker, enabling finer dirt particles to be captured. A bead filter uses mechanical filtration and biological filtration. Useful bacteria stick to the beads and break down harmful substances such as ammonium, nitrite and nitrate.