Typically, settled sewage flow enters at a high level and flows through the primary settlement tank. The supernatant from the tank flows into a dosing device, often a tipping bucket which delivers flow to the arms of the filter. The flush of water flows through the arms and exits through a series of holes pointing at an angle downwards. This propels the arms around distributing the liquid evenly over the surface of the filter media.
Most are uncovered (unlike the accompanying diagram) and are freely ventilated to the atmosphere.
The removal of pollutants from the waste water stream involves both absorption and adsorption of organic compounds and some inorganic species (such as nitrite and nitrate ions) by the layer of microbial biofilm. The filter media is typically chosen to provide a very high surface-to-volume ratio. Typical materials are often porous and have considerable internal surface area, in addition to the external surface of the medium. Passage of the wastewater over the media provides dissolved oxygen, which the biofilm layer requires for the biochemical oxidation of the organic compounds and releases carbon dioxide gas, water and other oxidized end products. As the biofilm layer thickens, it eventually sloughs off into the liquid flow and subsequently forms part of the secondary  sludge. Typically, a trickling filter is followed by a clarifier or sedimentation tank for the separation and removal of the sloughed film.  Filters utilizing higher-density media, such as sand, foam and peat moss do not produce a sludge that must be removed, but may require forced air blowers, backwashing, and/or an enclosed anaerobic environment.