**4. Conclusions**

Although it is generally perceived that the addition of ZnO as a photocatalyst increases the effectiveness of UV treatments for degrading pharmaceutical micro-pollutants, our work revealed that there are various outcomes that require careful investigation. We found that IBP was not as efficiently degraded when ZnO was used in the solution during the UVC treatment (the maximum degradation potential was 86.6%) as it was without ZnO (94.4%). Despite the ability of ZnO to degrade the IBP faster, this advantage was superseded by its inability to degrade the IBP by-product. In other cases, such as that of CTZ, a high degradation efficiency with and without ZnO was observed. The use of ZnO for degrading CTZ appears to be better than that without, with maximum degradation potentials of 99.9% and 92%, respectively. The optimal ZnO concentration was determined to be ~1 g/L, above which screening effects due to increased turbidity started to dominate the system, which lowered the photodegradation effectiveness. Overall, it is clear that UVC without ZnO was sufficiently effective in degrading IBP and CTZ, with a maximum degradation potential

of more than 90% in both cases. Furthermore, the by-product of IBP was also shown to be more effectively degraded without ZnO. Thus, we hope that our results may further encourage the adoption of a simple UVC batch-stirred treatment system to treat wastewater in households, hospitals, and the pharmaceutical industry.

**Author Contributions:** Conceptualization, S.P. and E.S.; formal analysis, S.P., E.J. and E.S.; investigation, S.P., E.J. and E.S.; writing, S.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by SPH Applied Science Academy Program at Sekolah Pelita Harapan Lippo Village, period 2021–2024.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We thank Sekolah Pelita Harapan Lippo Village and Universitas Pelita Harapan for the generous lab access. Special thanks to the school's and university's leadership in supporting our research through the SPH Applied Science Academy Program.

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