Effects of norfloxacin on the stability of aerobic granular sludge systems

Abstract

With the widespread use of antibiotics in medicine and aquaculture, their environmental residues have become a global issue, disrupting aquatic ecosystems, fostering antibiotic-resistant bacteria and genes, and impairing biological wastewater treatment efficiency. Norfloxacin (NOR), a broad-spectrum antibiotic, enters wastewater treatment systems via incomplete metabolism, exerting chronic adverse effects on functional microbes, pollutant removal efficiency and microbial community stability of traditional biological treatment systems. Aerobic granular sludge (AGS), regarded as an innovative direction in wastewater biological treatment technology owing to its high degradation capacity and resistance to shock loads, has an unclear response mechanism under NOR stress. In this study, 7 mg L⁻¹ NOR was added to sequencing batch reactor (SBR) systems, C–C (graphite-graphite dual-electrode modified SBR) and C–Fe (iron-graphite electrode modified SBR) systems, where the single-point NOR degradation rates reached 92.3%, 87.6% and 95.8% respectively. The dynamic effects of NOR on the morphological structure, biomass, structural-settling stability, extracellular polymeric substances (EPS), and microbial communities of AGS were investigated individually over the long term. After 40 days of NOR exposure, the average AGS particle size rose to 4.03 mm in the SBR system, while stably remaining at 3.19 mm (C–C) and 2.84 mm (C–Fe). The PN/PS ratio of EPS in the C–Fe system was stably maintained at 2–3, and the system exhibited the highest EPS secretion. This research reveals the differential impacts of NOR on AGS stability in different systems, providing a theoretical basis for constructing wastewater treatment systems with high resistance to antibiotics and preventing antibiotic-related environmental risks.