Aerated filters have the dual function of biological oxidation and filtration and are expected to replace rapid sand filters in tertiary treatment of sewage. Tertiary-treated sewage can have a high nitrogenous biochemical oxygen demand if there is inadequate nitrification, and it is important to keep nitrification levels in the treatment system high throughout the year. In an effort to develop an aerated filter with high nitrification capabilities, we formulated a new filter with two media. This filter has 2 layers. The upper layer is filled with a plastic medium and the lower with anthracite. We tested the filter with a pilot-scale apparatus on secondary-treated sewage. In the initial experiment, we filled the upper layer with a medium that had a small specific surface area (Type A, specific surface area about 500 m2/m3), and the medium was fixed with perforated plates. In second experiment, we filled the upper layer with a medium that had a large specific surface area (Type B, specific surface area about 800m2/m3), and the medium was fluidized by aeration. Higher nitrification performance was achieved in the second experiment the nitrification rate was 0.87 kgN/m3 per whole bed volume per day, at a water temperature of 20°C. A comparison between this second experiment and the initial experiment suggests that this improvement was due to an increase in the specific surface area of the medium.
