Ozonation and UV_{254 nm} radiation for the removal of microorganisms and antibiotic resistance genes from urban wastewater

Conventional wastewater treatment has a limited capacity to reduce antibiotic resistant bacteria and genes (ARB&ARG). Tertiary treatment processes are promising solutions, although the transitory inactivation of bacteria may select ARB&ARG. This study aimed at assessing the potential of ozonation and UV_{254 nm} radiation to inactivate cultivable fungal and bacterial populations, and the selected genes 16S rRNA (common to all bacteria), intI1 (common in Gram-negative bacteria) and the ARG vanA, bla_TEM, sul1 and qnrS. The abundance of the different microbiological parameters per volume of wastewater was reduced by ∼ 2 log units for cultivable fungi and 16S rRNA and intI1 genes, by ∼3–4 log units, for total heterotrophs, enterobacteria and enterococci, and to values close or below the limits of quantification for ARG, for both processes, after a contact time of 30 min. Yet, most of the cultivable populations, the 16S rRNA and intI1 genes as well as the ARG, except qnrS after ozonation, reached pre-treatment levels after 3 days storage, suggesting a transitory rather than permanent microbial inactivation. Noticeably, normalization per 16S rRNA gene evidenced an increase of the ARG and intI1 prevalence, mainly after UV_{254 nm} treatment. The results suggest that these tertiary treatments may be selecting for ARB&ARG populations.