Aerobic denitrification occuring in the biofilms attached to a partially submerged RBC, was investigated. Denitrification using polyvinyl alcohol (PVA) as a organic carbon source, was well proceeded by aerobic RBC systems at 25 °C. At an influent C/N ratio of around 1.2, the maximum net-denitrification efficiency was about 78% at a TOC loading of 2g/m2/d. In a chemostat experiment, aerobic denitrification was well proceeded under the dissolved oxygen concentration of 3 to 6 mg/L. The PVA-decomposing bacteria, nitrifiers, and denitrifiers co-existed in the biofilm, but the population of PVA-decomposing bacteria and denitrifiers in the surface layer was 1 to 2 orders of magnitude higher than those in the middle and bottom layers. It may indicate that the surface layer had a higher denitrifying activity. The nitrogen mass balance obtained using the experimental data clearly indicates a reasoning for aerobic denitrification.

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