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Abstract
The direct and indirect influence of temporary oxygen limitation on the nitrification of reject water was investigated using a continuous stirred tank bioreactor operated for 330 d at laboratory temperature (23 ± 1°C). A decrease in dissolved oxygen (DO) concentration from 3 to 0.7 mg L−1 lasted for 38 d and led to effective nitrite accumulation—more than 95% of total oxidized nitrogen was present as nitrite. The drop of DO concentration, at the same time, caused a decrease in the nitrogen oxidation rate from 1.36 to 0.73 kg N m−3 d−1. After a subsequent DO concentration increase to 3 mg L−1, the nitrite accumulation remained stable for another 90 d. This development was caused mainly by the indirect effects of DO limitation, consisting especially in the change of nitrogen species represented. A significant increase in free nitrous acid concentration induced by temporary DO limitation seems to be the key factor in this respect. The results of Fluorescence in situ hybridization analysis confirmed a washout of nitrite-oxidizing bacteria (NOB) during the period with high nitrite accumulation. The representation of NOB in total biomass decreased from 6.4% to less than 1% as a consequence of temporary DO limitation.