Free Nitrous Acid-Guided Microbial Restructuring Enables Selective Enrichment of Candidatus Nitrosoglobus in Acidic Partial Nitritation Process

Selective enrichment of the acid-tolerant ammonium-oxidizing bacteria (AOB) Candidatus (Ca.) Nitrosoglobus is critical for achieving stable partial nitritation (PN) under acidic conditions. This study explored the ecological mechanisms and operational strategies that enable Ca. Nitrosoglobus dominance in low-pH PN systems. Two parallel air-lift reactors were established using the same activated sludge, with one receiving free nitrous acid (FNA) pretreatment (FNA-PR) and the other operating without any pretreatment (wFNA-PR). The FNA-PR reactor exhibited robust nitrite accumulation and complete suppression of nitrite-oxidizing bacteria (NOB) while facilitating the replacement of Nitrosomonas by Ca. Nitrosoglobus. In contrast, the wFNA-PR reactor failed to enrich Ca. Nitrosoglobus, despite maintaining pH levels favorable for its growth. Batch activity assays and 16S rRNA analysis revealed that long-term FNA accumulation, rather than low pH alone, is the most important key driver of niche selection and microbial succession to Ca. Nitrosoglobus. Logistic modeling showed significant enhancement of AOB activity under acidic-FNA conditions (specific ammonium oxidation activity: 135.9 mg N g–1 VSS d–1) driven by Ca. Nitrosoglobus, while pH-driven systems led to the dominance of Nitrosospira and persistence of Nitrospira. These findings underscore the importance of sustained FNA accumulation and NOB suppression for targeted enrichment of Ca. Nitrosoglobus and offer practical guidance for engineering stable, low-pH PN systems capable of supporting downstream processes.