Comparative study on microbial mechanisms of denitrification failure in AAO process and air-lift internal circulation...

Denitrification failure under low-carbon conditions limits biological nitrogen removal. This study compared the recovery performance of a conventional anaerobic-anoxic-oxic (AAO) process and an air-lift internal circulation reactor (AICR) after carbon supplementation, following long-term operation at ultra-low C/N (≈1). Batch experiments showed that the AICR initiated denitrification more rapidly than the AAO. At C/N = 7, the AICR removed 62% of nitrate nitrogen (NO₃^--N) within 4.5  h, while the AAO removed only 43%. After 10  h, removal efficiencies reached 78% in the AICR and 69% in the AAO. The AICR also had higher adenosine triphosphate (ATP) levels and greater denitrification enzyme activities. Quantitative polymerase chain reaction (qPCR) confirmed that nirS and nosZ copy numbers in the AICR were approximately twice those in the AAO. Metagenomic analysis revealed distinct genomic architectures in the two systems. The AAO relied on a limited number of versatile strains carrying complete denitrification gene clusters. In contrast, the AICR harbored a distributed network where denitrification steps were partitioned among phylogenetically diverse and functionally specialized strains. These differences in community architecture were linked to the distinct flow regimes of the two reactors and corresponded to enhanced functional modularity and metabolic redundancy in the AICR, which contributed to its faster recovery under carbon fluctuations.