Ozonation and UV254nm radiation for the removal of microorganisms and antibiotic resistance genes from urban wastewater

•Ozonation and UV can remove up to 4 log units of microorganisms from wastewater.•UV radiation and mainly ozonation can remove some ARGs to values close to LOQ.•Re-growth to pre-treatment levels may occur after ozonation or UV radiation.•Enterococci is less prone to re-grow than enterobacteria, heterotrophs or fungi.•Ozonation and UV radiation may lead to an increase of ARG prevalence. 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 UV254nm 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, blaTEM, 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 30min. 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 3days 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 UV254nm treatment. The results suggest that these tertiary treatments may be selecting for ARB&ARG populations.