Metabolic responses and microbial community dynamics of denitrifying phosphorus removal sludge under nitrite stress using two types of volatile fatty acids.

This study investigated the metabolic responses and microbial community dynamics of denitrifying phosphorus removal (DPR) sludge enriched with sodium propionate (R1) or sodium acetate (R2) as the sole carbon source under various nitrite stress conditions. A stepwise acclimation strategy was employed by gradually increasing NO-N concentrations, adjusting the nitrite addition modes, and extending the anoxic durations. The results showed that the critical NO tolerance thresholds for phosphorus removal were 20 and 30 mg/L for the R1 and R2 systems, respectively, beyond which performance declined significantly. The sodium propionate-fed system exhibited higher DPAO activity under low NO-N concentrations, whereas the sodium acetate-fed system demonstrated better buffering capacity under high nitrite stress. At 50 mg/L NO-N, the R1 system retained partial DPAO activity, likely attributable to the energy and reducing power advantages of propionate metabolism and a slower yet stable poly-β-hydroxyalkanoate (PHA) degradation rate. By contrast, the R2 system relied on denitrifying glycogen accumulating organisms (DGAOs) to rapidly mobilize PHA for cell maintenance and nitrite detoxification via endogenous denitrification, followed by DPAO-mediated phosphorus removal. This metabolic division of labor was corroborated by high-throughput sequencing, which identified Flavobacterium and Candidatus Competibacter as the dominant DPAO and DGAO in the R1 and R2 systems, respectively. Their relative abundances were 24.71 % and 9.86 % in R1, and 4.28 % and 15.61 % in R2, respectively. These findings indicate that different VFA types guided microbial community evolution and shaped the respective metabolic pathways under nitrite stress.