## 3 Results and Discussion
### Removal Efficiencies of PhACs by WWTPs  
The removal efficiencies of all five WWTPs combined as well as the individual removal efficiencies for each WWTP are presented in Figure 4.  
The global removal efficiency ranged between \(-90\%\) and \(100\%\) with a medium and an average of \(98\%\) and \(74\%\), respectively. Two PhACs, DCF and CBZ, showed several negative values of removal efficiencies across all WWTPs. The compounds with the lowest removal efficiencies were CBZ, DCF, ERT, AZM, and PPNL with median removal efficiencies of \(2.7\%\), \(49.2\%\), \(50\%\), \(59.6\%\), and \(2.7\%\), respectively. The remaining PhACs showed median removal efficiencies between \(85.6\%\) (SMX) and \(100\%\) (APAPAP, MTPL, CTS, TTE, IBUP, and CLR). The dispersal of obtained values was high for all compounds in all five WWTPs, except for APAP. This discrepancy is due to the differences in removal efficiencies of each WWTP. Except for CBZ and DCF, Evora WWTP had consistently high removal efficiencies with the least dispersal, median values of \(94\%\), and an average of \(72\%\). This behavior was similar for Reguengos de Monsaraz WWTP, which showed median removal efficiencies of \(99\%\) and an average of \(78\%\); however, the only exception in this case was CBZ. Borba WWTP had lower removal efficiencies than the first two WWTPs, with a median and an average of \(67\%\) and \(55\%\), respectively. As for Redondo WWTP, CBZ was again the compound with the lowest removal efficiency. However, this plant registered a median and an average removal efficiency of \(100\%\) and \(85\%\), respectively. Lastly, for Portel, despite being the smallest WWTP, this facility achieved median and mean removal efficiencies of \(98\%\) and \(77\%\), respectively. Moreover, DCF had a median removal efficiency of \(81\%\), which is the highest of all five WWTPs. Globally, CBZ was the least removed compound with efficiencies ranging between \(-2.7\%\) (Portel WWTP) and \(37.2\%\) (Borba WWTP). This tendency is also noted for DCF,  
Figure 4: Removal efficiencies of target PhACs in WWTPs of the Évora district: removal efficiency of all WWTPs and the removal efficiency of each WWTP. which registered removal efficiencies between \(-13.2\%\) (Borba WWTP) and \(81.1\%\) (Portel WWTP). These negative removal efficiencies can be explained because for certain PhACs, parent compounds may be released during treatment, probably caused by the simultaneous presence of deconjugable substances, that is, human metabolites, of these compounds in the raw influent. Therefore, the negative removal efficiencies observed for certain PhACs, namely, carbamazepine and diclofenac, can be explained by deconjugation of glucuronidated or sulphated PhACs and desorption from particles or hydrolysis. These effects have been observed in several PhACs of different therapeutic classes, in particular diclofenac, ibuprofen, carbamazepine, estrone, and iopromide [3; 38; 39; 23]. Due to the recalcitrant properties of organic compounds such as diclofenac, they are slowly biodegradable or non-biodegradable in UWWTP, remaining unaffected, and as such, persist in the effluent after treatment [40; 41; 42]. The occurrence of these organic contaminants in wastewater may be either in solution or adsorbed onto solids. The hydrophobic or lipophilic nature of many organic contaminants result into their adsorption on solid particles during wastewater treatment, eventually resulting in their accumulation in the sludge solids [43; 44]. Diclofenac has a log \(K_{ow}\) = 4.51 and it was expected to adsorb significantly to the organic matter of sludge solids. The \(K_{ow}\) is also dependent on the \(pH\) of the aqueous system and therefore the \(pKa\) of the analyte [45]. An ionized PhAC is more hydrophilic (and therefore less lipophilic) than the neutral compound. This is reflected in a lower log \(K_{ow}\) value at a \(pH\) where the compound is ionized.