## 4 Discussion
### Removal Efficiencies of Wastewater Treatment Plants  
The removal performances of 23 municipal WWTPs investigated in six studies were assessed. Removal capacities of WWTPs depend mainly on the typologies of treatments carried out in WWTPs. Besides the precipitation step common for most facilities, in thirteen of them, WW treatment was represented only by standard biological treatment (e.g., activated sludge) [5; 36; 37; 50]. On the other hand, eight WWTPs employed oxidation ditches or anaerobic/anoxic/aerobic processes [5; 17]. In detail, among the six studies, the process of activated sludge was widely shared. In addition to this step, five WWTPs employed biological active filter [39] or membrane bioreactor [5] during the secondary treatments. Regarding tertiary treatment, one of the WWTPs used the highly effective process of reverse osmosis [37], whereas four employed chlorination [39]. Considering the variability of the typologies of treatments across studies, it is inconceivable to produce a consistent and credible comparison between studies; moreover, it goes beyond the scope of this review to report the variation within the single study. Therefore, no correlation between the kind of treatment and the removal performance of the WWTPs was examined. Seven compounds, namely ciprofloxacin, clarithromycin, erythromycinromycin, metronidazole, ofloxacin, sulfamethoxazole and trimethoprim, were the most representative PRs (Table 7). The average removal efficiencies for each PR varied significantly and were highly influenced by negative removal levels, reported when concentrations in effluents were higher than those in influents. With the exception of ciprofloxacin, which seemed efficiently removed by all the WWTPs in which it was detected, all the PRs exhibited an erratic removal pattern. In detail, sulfamethoxazole, erythromycinromycin and metronidazole are demonstrated the most resistance to degradation. Although low removal performances for the latter two compounds were previously described [5; 36; 39; 50], the negative values reported by [PERSON] et al. in two WWTPs in Gran Canaria (Spain) [37] strongly influenced the average reduction for sulfamethoxazole. Negative values were also detected for trimethoprim; [PERSON] and colleagues registered \(-23\%\) removal of this antibiotic in a treatment facility in Sanya City [17]. Contrastingly, [PERSON] et al. reported a complete removal of ciprofloxacin and clarithromycin in seven Tunisian WWTPs [36]. These unpredictable outcomes are the result of the treatment facility characteristics, some of which were mentioned above. However, conclusions about the efficacy of the treatment should be drawn with caution, considering that the absence of antimicrobial substances in the effluent does not always correspond to high efficiencies of removal. In particular, [PERSON] et al. correlated percentages of reduction for fluoroquinolones higher than 90% to the chemical properties of these compounds being absorbed by treatment sludge [39]. The chemical properties of PRs are relative to the success of the sorption process and in the PR removal performances of WWTPs, and these further explain most of the negative percentage removal values. For instance, N4-acetylsulfamethoxazole, the conjugate form of sulfamethoxazole, might transform back into the parent compound, leading to an increase in the concentration of sulfamethoxazole in the effluents [15; 36; 39]. Moreover, [PERSON] and colleagues reported negative removal efficiencies of this compound potentially due to water flow variations and evapotranspiration [37].