**4. Conclusions**

In order to provide a safe and valuable resource of water in arid and semi-arid locations by using reclaimed water in agriculture, it is necessary to ensure its quality. One of the reluctances of farmers and legislators is related to the presence of some emerging pollutants, such as pharmaceutical residues, in treated waters. Although the concentration of these compounds in such waters are usually at trace levels, a deep knowledge about their presence and elimination is needed.

Due to the singular characteristics of each family of target pharmaceutical compounds, their removal rates in the studied conventional purification treatments were very variable, but grea<sup>t</sup> efficiencies (up to 99.8%) were achieved for some of them. Satisfactory removal values were also provided by the studied natural treatment system, with comparable results for stimulants such as nicotine or ca ffeine and other drugs like atenolol and naproxen. However, for other compounds, namely ibuprofen, diclofenac, and gemfibrozil, natural treatments were not so e ffective as conventional ones. In addition, remarkable di fferences were also observed among tertiary technologies. Reverse osmosis revealed itself as a grea<sup>t</sup> option for the elimination of emerging pollutants. In this sense, the combination of secondary treatments and reverse osmosis provided very satisfactory removal e fficiencies, over 95% for most compounds under study. Regarding other tertiary technologies, electrodialysis reversal also showed moderate removals for some pharmaceuticals, but in all cases, they were significantly lower than reverse osmosis.

Since all the target pharmaceutical compounds are still present in the studied treated water after using both conventional and natural systems (with concentration levels from ng·L−<sup>1</sup> to <sup>μ</sup>g·L−1), further studies are demanded in order to improve the purification systems. Special interest must be paid to some recalcitrant compounds, like carbamazepine, for which very low removal rates were achieved.

The inclusion of emerging compounds, such as pharmaceuticals, in national and European contexts, is also mandatory in order to carry out adequate monitoring programs and to establish reliable control of these pollutants.

**Supplementary Materials:** The following are available online, Table S1: Gradient used for the chromatographic separation of target pharmaceuticals, title, Table S2: Parent and fragmentation ions and collision conditions for the mass spectrometry detection of target pharmaceuticals.

**Author Contributions:** Conceptualization, R.G.-A., S.M.-E., Z.S.-F., and J.J.S.-R.; funding acquisition, Z.S.-F. and J.J.S.-R.; sampling, R.G.-A., J.P.-J., and S.M.-E.; methodology, R.G.-A. and J.P.-J.; data analysis, R.G.-A. and J.P.-J.; project administration, R.G.-A., S.M.-E., Z.S.-F., and J.J.S.-R.; supervision, Z.S.-F. and J.J.S.-R.; writing—original draft, R.G.-A. and S.M.-E.; writing—review and editing, R.G.-A., S.M.-E., Z.S.-F., and J.J.S.-R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by Project ADAPTaRES (MAC/3.5b/102) co-funded by the European Union program INTERREG MAC (2014-2020), by means of the European Regional Development Fund.

**Acknowledgments:** Sarah Montesdeoca-Esponda would like to thank Universidad de Las Palmas de Gran Canaria for her postdoctoral fellowship. The authors would also like to thank Mancomunidad del Sureste de Gran Canaria and Consejo Insular de Aguas de Gran Canaria for their help and support in sampling activities.

**Conflicts of Interest:** The authors declare no conflict of interest.
