Abstract:A two-stage deammonification pilot plant with two different second-stage reactors, namely a sequencing batch reactor (SBR) with suspended sludge and a moving bed biofilm reactor (MBBR) with biofilm carriers, was investigated over a 1.5-year period to compare reactor performances. Additionally, dissolved nitrous oxide (N 2 O) was measured to determine the reactors' N 2 O formation potential. Although the nitritation performance was moderate (NO 2 -N/NH 4 -N effluent ratio of 0.32 ± 0.15 in combination with SBR and 0.25 ± 0.14 with MBBR), nitrogen turnover and degradation rates exceeding 500 g N/(m 3 ·day) and 80%, respectively, were achieved in both second stages, yet requiring additional aeration. The SBR's average nitrogen removal was 19% higher than the MBBR's; however, the SBR's nitrite influent concentration was comparably elevated. Concerning N 2 O formation, the nitritation reactor exhibited the lowest N 2 O concentrations, while the buffer tank, interconnecting the first and second stages, exhibited the highest N 2 O concentrations of all reactors. Given these high concentrations, a transfer of N 2 O into the second stage was observed, where anoxic phases enabled N 2 O reduction. Frequent biomass removal and a decreased hydraulic retention time in the buffer tank would likely minimize N 2 O formation. For the second stage, enabling anoxic periods in the intermittent aeration cycles right after feeding to support N 2 O reduction and thus minimize the stripping effects or the implementation of a complete anoxic ammonium oxidation will mitigate N 2 O emissions.