Hormesis effects of sulfadiazine on aerobic granular sludge at decreasing temperature: Granulation, performance, microbial community and antibiotic resistance

Abstract

Achieving stable granulation and operation at decreasing temperature has become a challenge for application of aerobic granular sludge (AGS). Antibiotic occurrence in wastewater may exacerbate the challenge, but relevant information is still lacking. Herein, we investigated whether and how the antibiotic sulfadiazine (SDZ) at different concentrations (0, 10, and 1000 μg/L in reactors RC, RL and RH, respectively) influencing AGS systems at seasonal temperature decrease from 25 to 10 °C. Results showed that all reactors experienced processes of granule formation, disintegration and recovery, and the SDZ had hormesis effects on AGS. Among the reactors, RL showed the fastest granulation and possessed the most stable granule structure, whereas RH obtained the slowest granule formation and had the severest granule disintegration. The SDZ exhibited low-level stimulation and high-level suppression on granule settleability, density and biomass, as well as nitrogen removal performance. Lower amount of extracellular polymeric substances (EPS) during initial start-up stage in RH could be related to the delay in granulation. While lower abundance of functional genera Nitrosomonas and Ca. Nitrotoga in RH could explain the deterioration in nitrogen removal. Metagenomic analysis further revealed higher abundances of resistome and mobilome in RH, mainly attributed to the enrichment of SDZ-corresponding resistance genes (sul1 and sul2) and transposases (tnpA), respectively. The abundance of sulfonamide resistance genes was mainly contributed by acquired antibiotic resistance genes (ARGs), indicating high potential for ARGs exchange in AGS microbes. This study provides novel insights into the effects of antibiotics on AGS at decreasing temperature.