Thermophilic anaerobic digestion reduces ARGs in excess sludge even under high oxytetracycline concentrations

The feasibility of thermophilic anaerobic digestion (AD) for the attenuation of antibiotic resistance genes (ARGs) in biomass wastes under high antibiotic concentrations remains unclear. In this study, a thermophilic completely stirred digester (55 °C) was fed with municipal excess sludge spiked with increasing concentrations of oxytetracycline (OTC) (0-1000 mg/L) over a period of 280 days. Results showed that thermophilic AD could maintain stable methane production (338.40 ± 26.26 mL/d/gVS) even at an OTC dose of 1000 mg/L with the sludge phase OTC concentration reaching around 24,000 mg/kg. More important, the abundance of resistome detected by high-throughput quantitative PCR in the substrate was reduced (p < 0.01) by 55.54%-86.27% by thermophilic AD over the whole period. Partial canonical correspondence and network analyses showed that the reduction of ARGs was achieved mainly through two ways: eliminating the original hosts of ARGs in the substrate (from 41.74% ± 2.60% in the substrate to 12.08% ± 1.02% in digested sludge), and blocking the horizontal proliferation of ARGs in the digested sludge by reducing the abundance of mobile genetic elements and restricting their horizontal exchange within a small number of thermophilic genera. This study showed that thermophilic AD is feasible for the attenuation of ARGs in biomass even containing high level of OTC.