## 3 Results  
### Study Selection and Study Characteristics  
The search for literature published between March 2015 and January 2021 yielded 1019 references across the four electronic databases (Table 2). The initial search in ProQuest returned 2544 references, a number that was considered excessive and difficult to manage. Therefore, a more specific analysis was conducted to search for the keywords only within the title and abstract; this enabled a reduction to 618 of the references. More than 65% of the literature returned was published after 2015. This is likely to indicate that the problem related to the presence of PRs responsible for AMR discharged in WWTPs has come only recently to researchers' attention. On the other hand, 35% of the literature was excluded from this review, aware of studying a recent and ever-changing problem (the first paper relevant to the question is dated 1992). Of the 1019 references collected during the search phase, the duplicates finder of RefWorks identified 246 copies to be removed (724 remainings); a further manual screening enabled to exclude another 227 references (546 remainings). The 546 remaining references were subjected to title screening. Following the selection criteria set out in Table S1, 448 references were excluded:  
* 17 did not meet the criteria for the key element PRs since dealing with non-pharmaceutical compounds or PRs non-relevant to AMR (Supplementary Material, Data_extraction.xlsx, Sheet PRs List). * 29 analysed surface water, drinking water or sludge samples and therefore did not satisfy the requirements for WW. * 256 were off-topic since they investigated specifically resistant organisms or removal treatments. * 17 were overviews, reviews, global perspectives or insights. * 2 studies written in Portuguese, a language not included in the list (Table S1). * 127 were considered not relevant for other reasons not covered in the pre-set criteria: these dealt with metagenomic or metatranscriptomic analyses, bacterial population dynamics, disinfection systems, phages or viruses in WW, bacteria in the aerosol or indicators of pollution. \begin{table}
\begin{tabular}{l l l} \hline \hline \multirow{2}{*}{**Database**} & **No. of References after 2015** & **No. of References before 2015** \\ \cline{2-3}  & **(Percentage)** & **(Percentage), Year of the First Publication** \\ \hline Web of Science & 170 (68.0\%) & 80 (32.0\%), 2000 \\ PubMed/Medline & 195 (62.3\%) & 118 (37.7\%), 2001 \\ ProQuest 1 st search & 2544 (66.5\%) & 1280 (33.5\%), 1992 \\ ProQuest 2 st search & 618 (68.4\%) & 286 (31.6\%), 1992 \\ BASE & 36 (66.7\%) & 18 (33.3\%), 2009 \\
**Total** & **1019** (mean 67.0\%) & 502 (mean 33.0\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the string search in four databases. Ninety-eight references passed the title screening. Of these, 68 were rejected following the abstract inclusion and exclusion criteria:  
* 7 focused on one PR only (triclosan, amoxicillin, vancomycin) and therefore did not meet the requirements for the key element PH. * 5 investigated the PRs concentrations in surface water and not in WW. * 1 analysed the samples deriving from the WW of a nursing home and not from a municipal or hospital WWTPs. * 27 did not comply with the criteria defined for the topic, as they investigated resistant bacteria or genes, or new removal treatments in small conditions. * 13 were reviews or similar. Moreover,
* 1 was written in Slovenian, a language not included in the list (Table S1). * 1 was unavailable. * 13 were excluded based on criteria set during the screening phase (metagenomic or metatranscriptomic, the dynamic of bacterial populations, disinfection systems, phages or virus in WW, bacteria in the aerosol, and indicators of pollutions).