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

Selective enrichment of the acid-tolerant ammonium-oxidizing bacteria (AOB) <i>Candidatus</i> (<i>Ca.</i>) <i>Nitrosoglobus</i> is critical for achieving stable partial nitritation (PN) under acidic conditions. This study explored the ecological mechanisms and operational strategies that enable <i>Ca. Nitrosoglobus</i> 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 <i>Nitrosomonas</i> by <i>Ca. Nitrosoglobus</i>. In contrast, the wFNA-PR reactor failed to enrich <i>Ca. Nitrosoglobus</i>, 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 <i>Ca. Nitrosoglobus</i>. 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 <i>Ca. Nitrosoglobus</i>, while pH-driven systems led to the dominance of <i>Nitrosospira</i> and persistence of <i>Nitrospira</i>. These findings underscore the importance of sustained FNA accumulation and NOB suppression for targeted enrichment of <i>Ca. Nitrosoglobus</i> and offer practical guidance for engineering stable, low-pH PN systems capable of supporting downstream processes.