Iron speciation and microbial functional responses shape sludge dewatering and phase-specific biological risks during Fe2+/ peracetic acid treatment.

The disposal of waste activated sludge (WAS) is challenged by poor dewaterability and environmental risks associated with antibiotic resistance genes (ARGs). Although advanced oxidation is promising for sludge conditioning, whether prolonged oxidation continuously benefits both dewatering and biological risk control remains unclear. Here, Fe2+-activated peracetic acid (PAA) was applied to WAS to evaluate the time-dependent responses of sludge dewaterability, ARGs, potential pathogens, extracellular polymeric substances, iron speciation and microbial functions. The Fe2+/PAA process exhibited different temporal changes between sludge dewatering and phase-specific biological risks rather than continuous improvement with prolonged oxidation. At 40 min, sludge dewaterability was markedly improved, accompanied by the greatest reduction in the total abundance of ARGs and mobile genetic elements (MGEs) in solid phase. Meanwhile, potential pathogens harboring multiple ARGs were also suppressed, suggesting reduced potentials for horizontal and vertical ARGs dissemination. Extending the reaction to 60 min achieved the highest removal efficiency for liquid phase ARGs and MGEs, while these ARGs-carrying pathogens remained inhibited; but accompanied by sludge dewaterability deterioration and a rebound in solid-associated ARGs and MGEs abundance. These changes were attributed to late-stage iron precipitations, which increased filtration resistance and caused the retention of genetic materials in the solid phase. Moreover, the recovery of stress-adaptive microbial functions may provide favorable conditions for ARGs transfer. Overall, this study reveals new insights into the time-dependent mechanisms governing dewatering performance and phase-specific risk control, providing a basis for tailoring treatment according to downstream management priorities.