Biodegradation mechanism of typical fluoroquinolones in sewage-sludge composts

The biodegradation mechanism of fluoroquinolones (FQs) during composting is the key information to accurately assess and effectively control the environmental risk caused by FQs in sewage sludge. This study investigated the biodegradation of two FQs (norfloxacin [NOR] and ofloxacin [OFL]) using a series of sludge composts over a 56-d incubation period. Sludge composts were collected at the mesophilic, thermophilic, and curing phases from the matrix of a pilot-scale sewage sludge composter; spiked with NOR and OFL (each 5 mg/kg); and incubated at 35 (mesophilic), 55 (thermophilic), and 40 degrees C (curing). Compost samples were evaluated by determining FQ concentration, intermediate metabolites, enzymatic activity (laccase, manganese peroxidase, and lignin peroxidase), and bacterial and fungal communities. Results showed that NOR was likely to be predominantly biodegraded via the acetylation route and the routes for OFL in each phase varied, while the main metabolites from both NOR and OFL retained the FQ core structure related to the antibacterial activity. Only manganese peroxidase had a significant correlation with FQ residue during the mesophilic phase. Proteobacteria and Ascomycota were the dominant bacterial and fungal phyla in the incubated sludge compost, respectively, and Pseudomonas was the most involved bacterial genus for FQ biodegradation in the sludge composts with Exophiala, Dissoconium, Talaromyces as the fungal genera.