Insight into how high dissolved oxygen favors the startup of nitritation with aerobic granules

To elucidate how high dissolved oxygen (DO) favors the startup of nitritation with aerobic granular sludge, two granular reactors were operated under low (1-2 mg O₂·L^-1) and high DO (3-5 mg O₂·L^-1) conditions with similar effluent ammonium concentrations (>20 mg N·L^-1). The results showed that though nitritation with an average nitrite accumulation ratio of above 95% was finally achieved in both reactors, a five-fold start-up time (eleven weeks) was required for the low DO reactor compared to the high DO reactor. Moreover, the nitritation performance was positively correlated with the extent of nitrifiers stratification in granules. The faster startup of nitritation under high DO conditions mainly resulted from the faster formation of well-stratified nitrifiers, with ammonium oxidizing bacteria (AOB) dominating granule surface. High DO operation combined with sufficient ammonium supply ensured the faster growth of AOB, which should provide a competitive advantage to AOB in competing for habitable space (i.e., granule surface). Besides, the lower porosity, larger size, and more active extracellular polymeric substances (particularly proteins) production of granules was observed under the high DO condition. Overall, these findings supported the proposition that the switch from mixed to stratified distribution of nitrifiers in granule was primarily driven by their competition for habitable space rather than by oxygen-limitation.