The dynamics of population and biofilm structure of nitrifying and heterotrophic biomass in biofilms on small suspended particles in an airlift reactor were measured during shifts from purely nitrification to a heterotrophic medium and back to nitrification. Biofilms from a full scale reactor with predominantly heterotrophic activity were used as start material. In the first twenty days of the nitrification period ammonia was oxidized to nitrite. Hereafter the oxidation was mainly to nitrate. A conversion of nearly 5 kgN/(m3 d) was reached in fifty days. Following the change to heterotrophic medium the nitrifying biofilm served as carrier for the development of a heterotrophic biofilm layer. The nitrification capacity of the biofilms dropped to 1 kgN/(m3 d) due to oxygen diffusion limitation in the heterotrophic layer. After the switch back to nitrification the heterotrophic biofilm layer was sheared off very rapidly, while the nitrification activity increased very fast to the level at the end of the first nitrification period due to decreased diffusion limitation.

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