Variations of antibiotic resistome in swine wastewater during full-scale anaerobic digestion treatment

Anaerobic digesters have been widely used to treat wastewaters in livestock farms. With the increasing risk of antibiotic resistance originated from livestock husbandry, removal of antibiotics and antibiotic resistance genes (ARGs) via anaerobic digesters deserved more attention. Here we investigated the removal of antibiotics and ARGs in swine wastewater by three on-farm full-scale anaerobic digesters, including buried biogas digester (BBD), up-flow anaerobic sludge blanket (UASB) and high density polyethylene covered biogas digester (HDPE-BD). Variations of antibiotic resistome in swine wastewater were further revealed by metagenomic sequencing. Results showed the removal efficiencies for antibiotics, ARGs and mobile genetic elements (MGEs) varied in the three digesters, ranging from 65.1% to 98.1%, 3.5%-71.0% and 26.9%-77.2%, respectively. In general, UASB and HDPE-BD showed better removal efficiencies than BBD. However, enrichment of metal resistance genes (MRGs) was noted in UASB. Pathogens could not be effectively removed by all of the three digesters. What's more, accumulation of pathogens was found in UASB (removal efficiencies: −8.5%-13.6%). Bacterial community succession, horizontal genetic transfer and biocide and metal resistance genes (BMRGs) profiles jointly structured the variation of antibiotic resistome during anaerobic digestion. A total of 334 high-quality bins were identified from swine wastewater, 96 of which belonged to phylum of Firmicutes, Bacteroidetes and Proteobacteria carried ARGs. Proteobacteria was the dominant multi-drug resistant flora. Meanwhile, ARG-carrying pathogens (Bacteroides and Mycolicibacter) were found in the swine wastewater, suggesting a potential threat to human and animal health. The findings from this study showed that HDPE-BD is the most eco-friendly and effective anaerobic digester in controlling risks from antibiotic resistance determinants in swine wastewater.