<p>A biological filter includes a drum which rotates around a horizontal axle. The walls of the drum are designed as an Archimedean spiral which picks up a certain amount of the liquid to pass through its filling of a nutrient medium for bacteria. At the wheel hub, the water runs off purified into a tray for discharge or recycling. This ensures a filter path long enough to give a satisfactory purification. It takes up less space than biological filter towers.</p>
