Characterization of acid-tolerant ammonia-oxidizing Nitrosospira bacteria suggests wide implementation of the acidic nitritation process

• Nitrosospira tolerates acidity and free nitrous acid better than Nitrosomonas. • Nitrosospira grows slower than Nitrosomonas. • Nitrosospira outcompetes acidophilic AOB in neutral or oxygen-limited conditions. • Nitrosospira tolerates high salinity and low temperatures better than acidophilic AOB. Acidic activated sludge process has been recently developed for the suppression of nitrite-oxidizing bacteria, addressing the bottleneck in achieving robust partial nitritation (PN) for low-strength ammonia wastewater treatment. In the acidic process, three types of ammonia-oxidizing bacteria (AOB) have been detected, including acid-sensitive neutrophilic Nitrosomonas , acid-tolerant neutrophilic Nitrosospira , and acidophilic Candidatus ( Ca. ) Nitrosoglobus . Among them, Nitrosospira is much less studied. This study successfully enriched Nitrosospira in a membrane bioreactor at a pH of around 5.3. Stoichiometric and kinetic characterizations of the enriched Nitrosospira culture were determined, including yield, maximum growth and decay rates, affinity for oxygen and total ammonia, and susceptibility to common influencing factors such as pH, nitrite, free nitrous acid (FNA), salinity, and temperature. The results indicated that, compared to Nitrosomonas, Nitrosospira has a higher tolerance to acidity and FNA but a lower maximum growth rate. Nitrosospira has a competitive advantage over acidophilic AOB under neutral, oxygen-depleted, high-salinity or low-temperature conditions. These findings advance our understanding of utilising Nitrosospira for the acidic PN process, indicating a wider implementation of the process under these conditions.