Mathematical modeling of autotrophic denitrification in a nitrifying biofilm of a rotating biological contactor

The mechanism of significant nitrogen loss in an aerobic rotating biologica l contactor (RBC) treating ammonium-rich waste water could be identified wi th the aid of batch experiments, mathematical modeling and microbial analys is. It turned out that the aerobic ammonium and nitrite-oxidizing organisms (nitrosomonas and nitrobacter clusters) are dominant in the upper aerobic 100-200 mu m of the biofilm, it is assumed that nitrite, appearing as an in termediate product close to the surface of the aerobic biofilm layer, diffu ses into deeper anoxic layers of the biofilm. It is reduced anaerobically t here in parallel with ammonium oxidized as described in the Anammox (anaero bic ammonium oxidation) process. This was confirmed in an anoxic batch test with suspended biofilm removed from the RBC. Unknown micro-organisms were detected, some of them similar to the Anammox organisms. A mathematical mod el which describes the kinetics of the aerobic ammonium-oxidizing, aerobic nitrite-oxidizing and anoxic ammonium-oxidizing micro-organisms was formula ted and fitted into the AQUASIM biofilm compartment. The distribution of th e populations within the biofilm as well as the ammonium, nitrite and nitra te degradation along the RBC in a steady state and during short-term experi ments were modeled and compared with the experimental data.