Controlled transition from anammox to partial denitrification-anammox system enhanced nitrogen removal: Microbial...

Integrated partial denitrification-anammox (PDA) offers a sustainable strategy for mainstream wastewater treatment. However, the dynamic transitions and microbial mechanisms during the shift from anammox to coupled PDA remain inadequately characterized. In this study, a PDA system was systematically established by the gradual replacement of nitrite with nitrate and controlled increases in acetate concentrations. Subsequently, acetate was replaced with sludge fermentation liquor (SFL) as the organic carbon source. The process achieved progressive enhancement in nitrogen removal, which stabilized at 93.9%-96.1%. The contribution of anammox in nitrogen removal accounted for > 74% of influent total nitrogen. Concurrently, the mean particle size increased from 85.6 μm to 387.5 μm, and this granulation process significantly improved the stability of the PDA system. 16S rRNA sequencing revealed a marked enrichment of Candidatus Brocadia (0.3% to 4.6%) and Thauera (5.8% to 17.3%). Furthermore, metagenomic analysis confirmed the high abundance of anammox-related genes (hdh, hzs) and higher abundance of the genes encoding nitrate reductase (narG/H/I, napA/B) compared to nitrite reductase genes (nirS/K). This metabolic bias reinforced the PD ecological niche, ensuring stable PDA functionality when SFL was used as the carbon source. Notably, enhanced activity of polysaccharide and protein hydrolase highlighted the critical roles of hydrolysis and acidogenesis in sustaining non-competitive PD performance, particularly under SFL conditions. This study provides a potentially reproducible strategy for the cultivation of PDA communities from anammox inoculum, elucidating microbial dynamics and functional stability during process transitions. These findings provide valuable insights for efficient wastewater treatment by replacing external chemical carbon sources to improve the recovery and utilization of sludge resources.