Direct and indirect effects of oxygen limitation on nitrification process applied to reject water treatment

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 330d at laboratory temperature (23 +/- 1 degrees C). A decrease in dissolved oxygen (DO) concentration from 3 to 0.7mgL(-1) lasted for 38d and led to effective nitrite accumulationmore 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 0.73kgNm(-3)d(-1). After a subsequent DO concentration increase to 3mgL(-1), the nitrite accumulation remained stable for another 90d. 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.