Biofilter

apparatus for the biological treatment of sewage. It is a round or rectangular tank with a double bottom filled with a filtering material (boiler slag, granite chips, gravel, porous clay filler, and so on). The bottoms are at least 0.4 m apart. The filtering layer is 1.5–2 m high; the grainsize of the filtering layer is 30–50 mm, and of the lower underlying layer, 60–100 mm. When sewage passes through the filtering material, a biological film forms on its surface. The film consists of accumulations of bacteria and fungi that oxidize and mineralize the organic matter in the sewage. The oxidation capacity of a biofilter is determined experimentally.