*3.1. Progression of Scientific Output*

The search returned 3208 documents. More than 90% of these documents are written in English, but there are also articles written in other languages, such as Spanish, French, Russian, or Chinese. No document from Scopus was excluded. Figure 1 shows the evolution of the number of documents on wastewater and advanced oxidation since 1990. As can be seen, the topic is of recent interest, because only in the last 30 years has the advanced oxidation in wastewater treatment been used.

**Figure 1.** Trend of the number of publications per year in wastewater and advanced oxidation from the years 1990–2018.

There has been continuous growth since the first year analyzed, but this occurs at two speeds. In the period 1990–2002, there is a slow growth in the number of annual publications, while in the period 2002–2018, the growth is much greater with a publication intensity greater than 25 articles per year, reaching the absolute maximum in the year 2018 with 450 articles published.
