The effect of different treatment technologies on the fate of antibiotic resistance genes and class 1 integrons after the application of residual municipal wastewater solids to soil

Authors: Burch, Tucker; SADOWSKY, MICHAEL - University Of Minnesota; LAPARA, TIMOTHY - University Of Minnesota

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2017
Publication Date: 11/17/2017
Citation: Burch, T.R., Sadowsky, M.J., Lapara, T.M. 2017. The effect of different treatment technologies on the fate of antibiotic resistance genes and class 1 integrons after the application of residual municipal wastewater solids to soil. Journal of Environmental Science and Technology. 51:14225-14232.

Interpretive Summary: Antibiotic resistance genes (ARGs) are increasingly regarded as environmental pollutants because they form the genetic basis for antibiotic resistance in human pathogens and can spread among pathogens in the natural environment. Land-application of biosolids – including livestock manure and residual solids from human sewage treatment – is an important source of ARGs to the environment. Treatment technologies exist that can reduce ARG levels in the biosolids during normal municipal wastewater treatment operations, but the effect that these treatment technologies have on ARG levels in soil following subsequent land-application is unknown. Therefore, we studied the fate of several ARGs from residual sewage solids in laboratory soil microcosms following biosolids treatment. The ARGs we studied occur naturally in bacteria from sewage solids. ARGs were measured using quantitative real-time PCR, and they included resistance genes for erythromycin, sulfonamides, and tetracyclines. Several biosolids treatment technologies were used to treat sewage solids prior to addition to soil microcosms; these included air drying, aerobic digestion, mesophilic anaerobic digestion, thermophilic anaerobic digestion, pasteurization, and alkaline stabilization. We found that the fate of ARGs in soil depended on the type of technology used to treat biosolids prior to land-application. Technologies that aggressively kill bacteria in biosolids resulted in faster and more extensive degradation of ARGs in soil than those that did not. These aggressive technologies included pasteurization, alkaline stabilization, and thermophilic anaerobic digestion. This phenomena can be exploited to optimize treatment of residual sewage solids for ARG removal. Our results also suggest that treatment of livestock manure with these technologies could help mitigate the discharge of ARGs from livestock facilities to the environment.

Technical Abstract: Land-application of residual wastewater solids is an important environmental source of antibiotic resistance genes (ARGs). Treatment technologies exist that can reduce ARG levels in residual solids prior to land-application, but the effect of these technologies on ARG levels in soil following land-application is unknown. Therefore, we studied the fate of ARGs from residual solids in laboratory soil microcosms following treatment with several different technologies. Treated residual solids from laboratory-scale treatment units were applied to sets of triplicate soil microcosms. The treatment technologies included air drying, aerobic digestion, mesophilic anaerobic digestion, thermophilic anaerobic digestion (at three different thermophilic temperatures), pasteurization, alkaline stabilization, and no-treatment controls. Six ARGs (erm(B), qnrA, sul1, tet(A), tet(W), and tet(X)), the integrase gene of class 1 integrons (intI1), and 16S rRNA genes were quantified using real-time PCR. ARG and intI1 quantities decreased in most microcosms, but the nature of the decrease varied depending on treatment technology. Observable rates of decrease tended to be fastest in microcosms that received untreated residual solids, with half-lives of 41 to 75 days. However, for many combinations of genes and treatment technologies, decay coefficients could not be computed because gene quantities declined below the detection limits of the assays early in the experiment – presumably due to either initially low ARG levels in the treated residual solids or due to rapid decay of ARGs in soil. This effect was most pronounced for residual solids undergoing pasteurization, alkaline stabilization, and thermophilic anaerobic digestion. These results demonstrate that the selection of residual solids treatment technology can be used to decrease the persistence of ARGs and intI1 in soil following land-application of treated residual municipal wastewater solids.