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

High Resolution Image Download MS PowerPoint Slide 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.