Microplastics deteriorate the removal efficiency of antibiotic resistance genes during aerobic sludge digestion

Sludge from wastewater treatment plants (WWTPs) is considered to be reservoirs of antibiotic resistance genes (ARGs), which can be efficiently removed by sludge treatment processes, e.g., aerobic sludge digestion. However, recent studies report microplastics, which also accumulate in sludge, may serve as carriers for ARGs. In the presence of microplastics, whether ARGs can still be efficiently destroyed by aerobic sludge digestion remains to be urgently investigated. In this study, the fate of ARGs during aerobic digestion was investigated with and without the addition of three prevalent categories of (i.e., polyvinyl chloride (PVC), polyethylene (PE), and polyethylene terephthalate (PET)). Nine ARGs and class 1 integron-integrase gene (intI1) that represents the horizontal transfer potential of ARGs were tested in this study. Compared with the control, the ARGs removal efficiency decreased by 129.6%, 137.0%, and 227.6% with the presence of PVC, PE, and PET, respectively, although a negligible difference was observed with their solids reduction efficiencies. The abundance of potential bacterial hosts of ARGs and intI1 increased in the reactors with the addition of microplastics, suggesting that microplastics potentially selectively enriched bacterial hosts and promoted the horizontal transfer of ARGs during aerobic sludge digestion. These may have contributed to the deteriorated ARGs removal efficiency. This study demonstrated that microplastics in sludge would decrease the ARGs removal efficiency in aerobic digestion process, potentially leading to more ARGs entering the local environment during sludge disposal or utilization.