Impacts of Combined Fluoroquinolone Antibiotics on Performance, Microbial Community, and Metabolic Adaptation of Biological Nitrification: A Comprehensive Study

Fluoroquinolone antibiotics (FQs) are a class of antibiotics frequently detected in wastewater treatment plants. However, the response of nitrifying systems to combined FQs exposure remains unclear. This study investigated the effects of combined exposure to ciprofloxacin, norfloxacin, and enrofloxacin on a biological nitrification system, with a focus on the long-term impact of different FQs concentrations (0.12 and 1.2 mg/L). The results showed that low-concentration FQs caused slight inhibition of nitrification, followed by a rapid recovery of nitrification performance. In contrast, high-concentration FQs significantly inhibited nitrite oxidation activity, leading to the accumulation of effluent NO2−-N at 22.79 mg/L. This system maintained activated sludge stability primarily by secreting protein components and optimizing microbial community structure. High-concentration FQs disrupted the nitrification process mainly by suppressing the growth and metabolism of Nitrospira. KEGG metabolic pathway analysis revealed that FQs exposure downregulated carbon metabolism in the system, while promoting oxidative phosphorylation pathways involved in energy generation and electron transfer, which played a critical role in maintaining system stability and resisting FQs stress. This study provided a mechanistic understanding of the metabolic adaptation of biological nitrification under different stresses, which was significant for its application in actual FQ-contaminated wastewater treatment.