Fates of intracellular and extracellular antibiotic resistance genes and microbial community structures in typical swine wastewater treatment processes

Swine wastewater is an important reservoir of spread antibiotic resistance to the environment. Intra- and extracellular antibiotic resistance genes (iARGs and eARGs) were quantified during two typical swine wastewater treatment processes including a sequencing membrane bioreactor (SMBR) at pilot-scale and anaerobic-anoxicoxic (A(2)O) at full-scale. The concentrations of iARGs and eARGs in raw wastewater were 3.42E + 09 and 3.79E + 07 copies/mL, respectively. The compositions were different between iARGs and eARGs. SMBR showed 0.63 log higher removals in the concentrations of iARG than A(2)O, while similar removal effects (3.01-3.44 log copies/mL) of eARGs were performed by the two processes. It suggested that membrane separation had advantages in the concentration removals of iARG rather than eARG. sul1 took the dominance in eARGs in effluent and had positive correlations with intl1, which indicated the risk of horizontal gene transfer of eARGs after wastewater discharge. Microbial community structures were estimated by 16S rRNA gene sequencing with both intra- and extracellular DNA (iDNA and eDNA). Compared between the effluent samples of the two treatment processes, microbial community structures estimated by iDNA had great differences, however which were similar for eDNA. Microbial community and water-quality parameters were the major influencing factors on ARG occurrences during swine wastewater treatment.