**4. Conclusions**

Analyzing the number of publications on wastewater and advanced oxidation from 1990 to 2018, a great increase in the evolution is shown, demonstrating the growing scientific interest in this research area. The evolution in the number of publications is especially outstanding from the year 2002 onwards. This growing interest is likely due to the growing need to solve the problem of water purification in all points of the planet. However, the trend regarding the number of publications was not the only item studied. Other variables also received our attention. Thus, it was shown how China and Spain are the countries that publish the largest number of scientific publications in this area, and how five of their institutions (two Chinese and three Spanish) lead the scientific production on wastewater and advanced oxidation, with the CIEMAT-Solar Platform of Almeria leading the ranking. The analysis of the keywords allowed us to establish different scientific communities, focusing each of them on different AOP treatments, type of wastewater, and the target pollutant to remove. The most important scientific communities in terms of the number of published articles are those related to the elimination of pollutants of biological origin, such as bacteria, and of industrial nature, such as pesticides or pharmaceutical products. The treatments mostly present in these articles are those related to ozone/UV, heterogeneous photocatalysis, and H2O2/UV. The following points are worthy of special attention for future research in the advanced oxidation field:

(1) The current research in wastewater treatments with AOPs is mainly focused at a pilot scale in batch mode. The evaluation in real conditions in continuous flow mode is mandatory for large-scale implementation.


**Author Contributions:** Conceptualization, J.A.G.-C. and F.M.-A.; methodology, J.A.G.-C., J.A.S.-P., A.A. and B.E-G.; formal analysis, J.A.G.-C. and F.M.-A.; investigation, J.A.G.-C., and B.E.-G.; writing—original draft preparation, J.A.G.-C. and B.E.-G.; writing—review and editing, F.M.-A., J.A.S.-P. and A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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