*3.2. Publication Distribution by Countries and Institutions*

Figure 2 shows, in a funnel chart, the distribution by country of the scientific production on wastewater and advanced oxidation. Thirteen countries have published at least 100 articles on this subject in the period under review. Of these, China stands out with 508 articles. It is followed, in this order, by Spain, the USA, and India, with 200–400 publications. The rest of the countries have published between 100 and 200 documents. China's water pollution has increased in the last 40 years due to population and industrialization growth. Compared with the European Union and the United States standards, China's wastewater discharge standards still have shortcomings, and there exist many challenges in this field. For this reason, the Chinese government is putting more effort into exploring new wastewater treatments [66]. In the United States, the potential use of municipal reclaimed water in the power sector has led to an increase in the number of publications about wastewater reclamation

treatments. The implementation of technologies capable of supplying reclaimed water for electric power plants is a major challenge for this developed country [67]. As it is presented in Figure 2, Spain holds second place in the ranking with 371 publications related to the studies about advanced oxidation treatments applied to wastewater. In recent years, the changes of legislation at the European level have been a point of departure for increasing the investigation in this area, especially in countries with a huge water scarcity, such as Spain.

**Figure 2.** Representation of the countries with the highest number of publications on wastewater and advanced oxidation.

A total of 87 countries have published at least one article in this scientific field. However, the 13 most important countries account for almost 85% of all publications. In Figure 3, the countries with at least one publication are represented in a color-coded world map.

**Figure 3.** World map representing the scientific production by countries.

Figure 4 shows an analysis of the relations established between the different countries based on articles published by several countries. In the analysis of communities, each node represents a country, and from this, there are as many lines of union as there are relations at the level of publications with other countries. The size of each node refers to the number of publications carried out in that country, and the thickness of each union line refers to the number of collaborations carried out with another country. It can be observed that, in the analysis, there are three clusters, each identified with a color. The largest cluster is represented in red and is dominated by China. The USA also plays a very important role in this cluster. The rest of the countries belonging to the cluster are Australia, Canada, India, South Korea, Taiwan, Malaysia, and Iran. The second cluster, represented in green, is formed by European and Latin American countries. The central role is occupied by Spain, and the rest of the countries are the United Kingdom, Switzerland, Poland, Germany, Portugal, Brazil, and Mexico. The third cluster is made up of France, Italy, Turkey, and Japan and is represented in blue.

**Figure 4.** Graph of the analysis of communities by country representing the relations established with other countries.

Figure 5 shows the institutions with more than 25 publications on wastewater and advanced oxidation. Of these, five are Spanish (CIEMAT-Plataforma Solar de Almería, Universitat de Barcelona, Universidad de Almeria, Universidad de Extremadura, and Universidad de Granada), four are Chinese (Ministry of Education China, Tsinghua University, Chinese Academy of Sciences, and Harbin Institute of Technology), three are Turkish (Istanbul Teknik Üniversitesi, Middle East Technical University METU, and Bogaziçi Üniversitesi), and there is an institution of Brazil (Universidade de Sao Paulo), Italy (Università di Salerno), Portugal (Universidade do Porto), Switzerland (Swiss Federal Institute of Technology EPFL), India (Institute of Chemical Technology), France (CNRS Centre National de la Recherche Scientifique), and Poland (Lodz University of Technology). It is striking that, of the 20 institutions with the highest number of publications, none are American, despite the fact that the USA is the third country, after China and Spain, with the highest number of publications in wastewater and advanced oxidation. This is because, while in other countries such as Switzerland, the majority of research in this field is carried out in a single institution, in the USA, the research is not led by a single center. There are many institutions spread all over the country that publish in this scientific field without highlighting, as far as the number of publications is concerned, any of them.

**Figure 5.** Main institutions related to scientific production in wastewater and advanced oxidation.
