A biofilm system with Kaldnes biofilm carrier was used in these studies to cultivate bacteria responsible for both partial nitritation and Anammox processes. Due to co-existence of oxygen and oxygen-free zones within the biofilm depth, both processes can occur in a single reactor. Oxygen that inhibits the Anammox process is consumed in the outer layer of the biofilm and in this way Anammox bacteria are protected from oxygen. The impact of oxygen concentration on nitrogen removal rates was investigated in the pilot plant (2.1 m3), supplied with reject water from the Himmerfjärden Waste Water Treatment Plant. The results of batch tests showed that the highest nitrogen removal rates were obtained for a dissolved oxygen (DO) concentration around 3 g O2 m−3. At a DO concentration of 4 g O2 m−3, an increase of nitrite and nitrate nitrogen concentrations in the batch reactor were observed. The average nitrogen removal rate in the pilot plant during a whole operating period oscillated around 1.3 g N m−2d−1 (0.3 ± 0.1 kg N m−3d−1) at the average dissolved oxygen concentration of 2.3 g O2 m−3. The maximum value of a nitrogen removal rate amounted to 1.9 g N m−2d−1 (0.47 kg N m−3d−1) and was observed for a DO concentration equal to 2.5 g O2 m−3. It was observed that increase of biofilm thickness during the operational period, had no influence on nitrogen removal rates in the pilot plant.

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