*Experimental Set-Up*

Six experiments were carried out during the spring and summer of 2022, on the rooftop facilities of the E.P.S. de Linares (Spain), at the University of Jaén. Linares has a temperate climate. The tests were carried out outdoors, under sunny weather conditions.

The Linares WWTP provided the wastewater samples of the wastewater effluent obtained directly after secondary treatment in different seasons of the year and with varying microbiological loads. The experiments always started around 11:00–12:00 p.m. local time (2–3 h before solar noon). The total exposure of the water samples was carried out under sunlight (SODIS treatment) for 4 h, as this provided an adequate treatment that could be carried out during 3 shifts over the course of a day. Microbiological analyses of the initial raw water and after 4 h of SODIS treatment were performed, which included *E. coli*, *E. faecalis* and *C. perfringens* (including spores) as microbiological indicators, to assess the microbiological quality of the water according to Spanish Royal Decree 1620/2007 and Regulation (EU) 2020/741 for water reuse. Similarly, the basic physicochemical parameters of the water samples were analysed. The climatic conditions were monitored and the electrical parameters of this experiment were measured in the same way and under the same methodology previously explained in Vivar et al. (2021) [7] as a first stage of research to obtain reclaimed water and simultaneous energy production.

### **3. Results and Discussion**

Table 1 shows the results obtained from the meteorological conditions in Open SolWat, together with the temperature of the water sample, the initial and final sample volumes obtained, the evaporation percentage and the leakage of the water sheet after 4 h of SODIS treatment.

Water temperature is one of the most important parameters of water quality, as it affects water chemistry and the functions of water microorganisms. The water samples used showed increases between 8.1–10.7 ◦C, during the experimental tests, in their temperature, influenced by: solar radiation, ambient temperature, heat transfer of the SolWat PV module and the volumes used experimentally (4.2 and 6.2 L). The water losses were mainly due to the evaporation of the water sample (with full exposure to the environment during the experimental treatment), which was related to the heat loss in the PV module cooling, and to the leakage of the water sheet in the PV module, which reached 37.1–52.4%. These water losses varied with the solar intensity, PV module temperature, water sample temperature and water volume used. On the other hand, the turbidity of the water samples (experimental studies carried out with wastewater and Milli-Q) and the thin water film did not show significant evidence with regards to the final energy production.

Under similar experimental conditions, it was observed that, when using a larger sample volume, bacterial inactivation became slightly more difficult. At no time was total bacterial inactivation (*E. coli*, *E. faecalis* or *C. perfringens*) achieved in the treated water samples, but optimal disinfection results were obtained for *E. coli* (99.79–99.98%) and *E. faecalis* (92.65–97.75%), as shown in Figure 2. *C. perfringens* was shown to be the most resistant of the bacteria tested, with low inactivation percentages.



**Figure 2.** Concentration of *E. coli*, *E. faecalis* and *C. perfringens* in the experimental water samples vs. UV dose for the Open SolWat. Tests carried out in spring, (**a**) 9 May 2022 and (**b**) 10 May 2022, and in summer, (**c**) 7 July 2022 and (**d**) 8 July 2022.

However, due to the exposure of the water sample to the environment there was a tendency for certain physicochemical parameters to increase in concentration as a consequence of climatic conditions and water losses during the SODIS treatment. Moreover, together with the incorporation of other particles in suspension in the water sample, this caused

an increase in the most important parameters, turbidity and TSS, which were detrimental to the possible uses for reclaimed water according to Spanish and European regulations. Even so, this problem could be solved with the use of filters in the system, which could also help in the microbiological disinfection of the experimental water. The quality of the reclaimed water obtained during the experimental tests was good and covered the possible uses shown in Table 2.

**Table 2.** Summary of the possible uses of the reclaimed water obtained experimentally thanks to the Open SolWat, according to the maximum admissible values (bacteriological and physicochemical limits (TSS, turbidity, BOD5 and other established criteria) established in RD 1620/2007 [12] and in RD (EU) 2020/741 [13].


\* No limit is set. \*\* The problem must be solved for the possible use of reclaimed water. \*\*\* OTHER POLLUTANTS (Annex II of RD 849/1986 of 11 April) contained in the wastewater discharge authorization → limit entry into the environment. In the case of dangerous substances (Annex IV of RD 907/2007, of 6 July), respect for the NCA must be ensured [12].

Table 3 shows the electrical results from Open SolWat and the reference photovoltaic module, after 4 h of solar exposure. Open SolWat allowed the front surface of the SolWat photovoltaic module to be cooled by a thin sheet of water that constantly flowed from the top of the module, thanks to a pumping system. The influence of the water cooling and the effect of the evaporation of the water sample allowed the high temperatures reached in the PV module during the spring and summer tests to be reduced, with a decrease of 16.2–30.6 ◦C with respect to the reference photovoltaic module, which resulted in increased electrical performance. As a consequence, energy efficiency was simply and effectively improved by 15–21%, which at the same time guarantees a longer service life. Thus, the system managed to generate an average of 18.6 ± 1.8% more energy compared to the individual photovoltaic module in the experimental studies. In addition, cooling with water allowed cleaning of the module surface.

**Table 3.** Main electrical results obtained from the photovoltaic module integrated in the Open SolWat system and the reference photovoltaic module, after 4 h of experimental treatment under real sun, on the roof of the E.P.S. of Linares. Avg.: average, Max.: maximum.

