The fate of antibiotic resistance genes during treatment and disposal of residual municipal wastewater solids

Abstract

The development of resistance to antibiotics among pathogens is a global public health dilemma with significant consequences for the length and quality of human life. As a result, the bacterial antibiotic resistance genes (ARGs) that confer resistance are increasingly regarded as environmental contaminants. A significant body of knowledge has been generated that catalogues the occurrence of ARGs in numerous environmental reservoirs, among which residual municipal wastewater solids are one of the largest. Only a handful of studies, however, make the critical paradigm shift to considering treatment technologies and management strategies intended to reduce the quantities of ARGs in those reservoirs. The objective of the work presented here was to evaluate various treatment technologies and management strategies for reducing the quantity of ARGs discharged from the municipal wastewater treatment process during treatment and disposal of residual solids. Aerobic digestion, air drying, and hyperthermophilic (¡Ý 60¡ãC) anaerobic digestion were evaluated for their abilities to reduce ARG quantities in residual municipal wastewater solids using laboratory-scale treatment units. The technologies were compared among each other and to mesophilic (40¡ãC) and thermophilic (56¡ãC) anaerobic digestion on the basis of the kinetics of ARG removal from residual solids. While all technologies were effective, hyperthermophilic anaerobic digestion tended to exhibit the fastest kinetics. In addition, class 1 integrons were identified as a candidate design gene, and batch or semi-batch flow configurations were demonstrated to be a potential means of optimizing the removal of ARGs from residual solids during aerobic digestion, thermophilic anaerobic digestion, and hyperthermophilic anaerobic digestion. The fate of ARGs in soil following simulated disposal was also investigated by applying treated residual solids from full-scale treatment facilities and from numerous laboratory-scale treatment units to soil microcosms. ARGs from residual solids treated at typical full-scale treatment facilities persisted in soil at high concentrations for relatively long periods of time, with half-lives on the order of months. Alkali stabilization, thermophilic anaerobic digestion, hyperthermophilic anaerobic digestion, and pasteurization, however, caused drastic decreases in ARG quantities in soil within one month. The results presented here can be used to optimize and design the residual municipal wastewater solids treatment and disposal process to remove ARGs.