Responses of antibiotics, antibiotic resistance genes, and mobile genetic elements in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion conditions

Sewage sludge from wastewater treatment plants (WWTPs) harbours large amounts of antibiotics, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs), the variation and fate of these emerging pollutants during sludge treatment processes must be thoroughly studied to reduce their potential risks to human health. In this study, 7 pilot-scale CSTR anaerobic digesters were established with the same seed sludge and fed with the same thermal hydrolysis pre-treated sewage sludge, while operating under different conditions. High-throughput quantitative PCR, UPLC-MS/MS and Illumina Hiseq-sequencing were used to systematically evaluate the responses of antibiotics, ARGs, and MGEs in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion (AD) conditions. The results showed that thermal hydrolysis effectively reduced the abundance (>94%) of almost all subtypes of ARGs and MGEs, and it was a powerful technology for reducing tetracyclines, macrolides, and lincosamides. Besides, the abundance of ARGs and MGEs in thermophilic digesters was lower than that in mesophilic digesters, suggesting that thermophilic digesters could be used to avoid the ARGs rebounding. In addition, the thermophilic system further reduced the concentrations of quinolones. For the digesters operated under the mesophilic conditions, a longer hydraulic retention time (HRT) facilitated the removal of antibiotics, ARGs, and MGEs. Furthermore, the microbial community and MGEs had important effects on the persistence and proliferation of ARGs in AD process. The findings of this study provide effective clues for controlling the spread of antibiotic resistance and suggest the optimal operating conditions of digesters.