**4. Conclusions**

A hybrid system, Open SolWat, was developed that uses only solar energy to purify water and generate electricity. The system consists of a PV module through which a thin film of water (1 mm) circulates, allowing cooling of the module and solar disinfection of the water, from the top of the module until it is collected in an open water tank, which is exposed to solar radiation and thus also acts as a solar water disinfection reactor. The experiments took place in spring and summer during sunny days, on the rooftop at the E.P.S. of Linares (Jaén, España), with a duration of 4 h under real sunshine. Water samples from the secondary effluent of a WWTP were used. In comparison to the SolWat technologies studied so far, Open SolWat was able to obtain better quality reclaimed water and generate energy simultaneously, but this time it also productively improved its energy efficiency (15–21%). Finally, this technology showed a potential that should be studied in more detail to fulfil the final objective of being implemented as s tertiary treatment in a WWTP (the

system would have water availability, and three batches per day of 4 h each could be carried out in sunny conditions), with the use of larger photovoltaic modules and solving the problems mentioned above. With the use of energy-efficient pumps and thanks to the electrical energy generated in the prototype, the energy obtained would be able to be used for the consumption of the pumping system and for other energy costs in a WWTP facility. Furthermore, other parameters stipulated in the Spanish regulations for the use of reclaimed water (e.g., intestinal nematodes, *Salmonella* and *Legionella*) that have not been taken into account in this study should be analysed.

**Author Contributions:** Conceptualization, J.T.L., M.V.G. and M.F.C.; methodology, J.T.L., M.V.G. and M.F.C.; software, J.T.L., M.V.G. and M.F.C.; validation, J.T.L., M.V.G. and M.F.C.; formal analysis, J.T.L. and A.M.P.V.; investigation, J.T.L., M.V.G., M.F.C. and A.M.P.V.; resources, J.T.L., M.V.G. and M.F.C.; data curation, J.T.L. and M.V.G.; writing—original draft preparation, J.T.L.; writing—review and editing, J.T.L. and M.V.G.; visualization, J.T.L., M.V.G. and M.F.C.; supervision, M.V.G. and M.F.C.; project administration, M.V.G.; funding acquisition, M.V.G. and J.T.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Innovation, Program "Research Challenges 2018 R+D+i Projects" (grant number RTI2018-099308-A-I00) and "Predoctoral Training Contracts" (contract number PRE2019-089349). In addition, thanks go to M.J. Rodrigo and his organization (Aqualia Group, Aguas de Linares "Linaqua") for their collaboration with the project. M. Vivar recognizes the funding of the Ministry of Economy and Competitiveness, "Ramón y Cajal" Program (grant number RYC-2015-17306).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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