A trickling filter consists of a fixe bed of rocks, gravel, slag, polyurethane foam, sphagnum peat moss, or plastic media over which sewage or other wastewater flows downward and is contacted with a layer or film of microbial slime covering the bed media. Aerobic conditions are maintained either by forced air flowing through the bed or natural convection of air if the filter medium is porous. The process mechanism involves adsorption of organic compounds in the sewage or other wastewater by the layer of microbial slime, diffusion of air into the slime layer to furnish the oxygen required for the biochemical oxidation of the organic compounds to release carbon dioxide gas, water and other oxidized end products. As the slime layer thickens, it becomes more difficult for air to penetrate the layer and an inner anaerobic layer is probably formed. For some plastic-mesh material filters this slime layer will build and eventually slough off the smooth plastic walls into the treated effluent as a sludge that requires subsequent removal and disposal. Other filters utilizing higher-density media such as sand, foam and peat moss do not produce a sludge that must be removed.