## 4 Discussion  
### Concentrations of Pharmaceuticals in Influents and Effluents Wastewater  
Most authors employed high-throughput techniques for the extraction and detection stages during the analytical phase of the research. Indeed, all the analyses made use of SPE for the isolation of the compounds. Out of eighteen studies, three used Online-SPE [5; 16; 39], two employed automated extraction systems [12; 35], and one proposed a simplified SPE procedure [37]. For what concerns the identification and quantification of PRs, all the studies, except for two [16; 48], combined liquid chromatography techniques (twelve HPLC and four UPLC) with mass spectrometry. Among the 218 PRs ascertained responsible for the development of AMR (Supplementary Material, Data_extraction.xlsx, Sheet PRs List), 45 were detected within the eighteen studies in 88 different WWTPs. The PRs belong to 16 different classes of antibiotics, antifungals, antiprotozoals and antimalarials. Among them, quinolones, sulfonamides, macrolides, tetracyclines and azoles represent 69% of all the PRs reported by the authors  
Figure 3: The twenty-five countries covered by the literature (**a**) and detail on Europe (**b**). (Source: eSpatial). (Figure 4). Seven of those, namely ciprofloxacin, clarithromycin, erythromycinomycin, metronidazole-zole, ofloxacin, sulfamethoxazole and trimethoprim, occurred in more than 50% of studies (nine). In particular, ciprofloxacin, sulfamethoxazole and trimethoprim were detected in more than 75% of papers (fourteen). Azithromycin, clindamycin, norfloxacin, oxytetracycline, roxithromycin, sulfadiazine and tetracycline occurred within five and nine studies (25-50%). The influent concentration of the most common PRs occurring in 46 WWTPs was investigated (Table 4). Only compounds shared in more than four studies (out of fourteen) were considered in this analysis. It should be pointed out that a constraint in this comparative study lies in that the studies dealt with different limits of quantification. \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Pharmaceutical**} & \multicolumn{2}{l}{**Max. Average Conc. (ng/L)**} & \multirow{2}{*}{**City (Country)**} & \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Mean across Studies (ng/L)**} \\  & **Detected in Influents** & & & \\ \hline Azithromycin & 115,413 & Sanya City (China) & [17] & 12,856 \\ Ciprofloxacin & 88,012 & Durban (South Africa) & [39] & 8706 \\ Clarithromycin & 6917 & Sanya City (China) & [17] & 732 \\ Clindamycin & 134 & Warsaw (Poland) & [13] & 53 \\ Erythromycin & 1193 & Choutarina (Tunisia) & [36] & 295 \\ Metronidazoleazole & 20,656 & Durban (South Africa) & [39] & 2002 \\ Norfloxacin & 2800 & Kangemi (Kenya) & [15] & 454 \\ Ofloxacin & 5742 & Durban (South Africa) & [39] & 764 \\ Oxytetracycline & 1531 & Sanya City (China) & [17] & 241 \\ Roxithromycin & 19,135 & Sanya City (China) & [17] & 2045 \\ Sulfadiazine & 574 & Xinjiang (China) & [48], H & 179 \\ Sulfamethoxazole & 49,300 & Machakos (Kenya) & [15] & 4434 \\ Tetracycline & 374 & Xinjiang (China) & [48], H & 115 \\ Trimethoprim & 8430 & Kampala (Uganda) & [6] & 982 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Max average concentrations and means (ng/L) of the PRs detected in more than a quarter of the studies in the influent of 46 WWTPs. Figure 4: Sixteen different classes of PRs detected among studies. Within the fourteen studies that assessed influent WW, authors reported worryingly high levels for several compounds. The highest concentrations were reported in Durban (South Africa) and Machakos (Kenya) for ciprofloxacin and sulfamethoxazole, respectively, at 88,012 and 49,300 ng/L [15; 39]. [PERSON] et al. reported an alarming high level of 100,000 ng/L of azithromycin in a WWTP in Sanya City (China) during the high tourist seasons [17]. The highest concentrations of trimethoprim (8430 ng/L), norfloxacin (2800 ng/L) anderythromycin (1193 ng/L) were detected in Choutrana (Tunisia), Kangemi (Kenya) and Kampala (Uganda), respectively [6; 15; 36].