This study evaluates the changes in the organic and nitrogen removal performance, nitrogen transformation rate, microbial enzymatic activity, oxidative stress, antioxidant response and microbial community of sequencing batch biofilm reactor (SBBR) under different ofloxacin concentrations. When the ofloxacin concentration increases from 0.2 to 20 mg/L, the organic and ammonia nitrogen removal efficiencies, nitrogen transformation rate and their corresponding enzymatic activities of SBBR gradually decrease. The presence of ofloxacin can increase the concentration of reactive oxygen species, activate antioxidant system and induce the increase of oxidative stress. These phenomena are more obvious with the increase of ofloxacin concentration in the inlet. The ofloxacin stress varies the microbial community structure, decrease the relative abundance of nitrifying genera Nitrosomonas and Nitrospira as well as denitrifying genus Denitratisoma, enrich some drug-resistant heterotrophic bacteria in SBBR, thereby weakening the ability of nitrogen removal. This study analyzes the impact of ofloxacin on the denitrification performance of SBBR from multiple perspectives, providing a theoretical basis for evaluating the ecological risks of antibiotics on wastewater biological treatment systems.

The effects of dissolved oxygen (DO) concentration at aerobic phase on nitrogen removal performance, enzymatic activities, microbial community and nitrogen metabolism functional genes were investigated in a sequencing batch reactor (SBR). With the DO concentration at aerobic phase decreased from 5~6 to 2~3 mg·L^-1, the nitrification rates and nitrifying enzymatic activities of SBR activated sludge system gradually decreased, while the denitrification rates and denitrifying enzymatic activities gradually increased. Metagenomic analysis revealed that the microbial community of the activated sludge system changed significantly with the decrease of DO concentration at aerobic phase. The relative abundance of typical nitrifying bacteria decreased gradually, while that of multiple denitrifying bacteria increased gradually. In the process of nitrogen metabolism, the decrease of DO concentration at aerobic stage inhibited the relative abundance of functional genes and key enzymes related to the nitrification process, but promoted the relative abundance of functional genes and key enzymes related to the denitrification process, which explained the reasons for the decreased nitrification performance and the increased denitrification performance of SBR from the perspective of genes. According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) database analysis, the decrease of DO concentration at aerobic phase significantly inhibited the carbon metabolism process of activated sludge system and promoted the process of energy generation and electron transfer to some extent. This study further explored the effect of DO concentration at aerobic phase on biological nitrogen removal process of SBR, and provided a theoretical basis for the DO parameter setting of SBR.

This study investigated the effects of varying aerobic/hypoxia alternation number on organic removal performance in a sequencing batch reactor (SBR) and elucidated the underlying mechanisms through nitrogen transformation rates, enzyme activities, microbial community structure, and functional gene abundance. As the alternation frequency increased from 1 to 4, nitrite accumulation in the effluent gradually intensified, accompanied by declines in nitrite oxidation rates, nitrate reduction rates, and significant reductions in nitrite oxidoreductase and nitrate reductase activities. Metagenomic analysis revealed that increased alternation frequencies universally reduced the relative abundances of nitrite-oxidizing bacteria (e. g., Nitrospira) and denitrifiers (e. g., Hyphomicrobium and Defluviicoccus), while diminishing the abundances of nitrogen-transforming enzymes and their encoding functional genes (e. g., nxrAB, nirK, and nirS). These findings collectively explain the mechanisms of nitrite accumulation and impaired nitrogen transformation efficiency from microbial, enzymatic, and genetic perspectives. This work provides theoretical and technical insights for evaluating the impacts of aerobic/hypoxia alternation number on nitrogen removal performance, microbial community dynamics, and functional gene regulation in SBR systems.