*2.1. Bibliometric Analysis*

The article performs a bibliometric analysis of wastewater treatment costs. This method makes it possible to identify, organize and evaluate the constituent elements of a specific area of study, and is, thus, an important tool for literature reviews [52–54]. Bibliometric analysis involves various different types of indicators relating to quantity (productivity), quality (impact of publications) and structure (analysing connections) [55]. This article uses the WoS database from Thomson Reuters to conduct the bibliometric analysis. WoS has high visibility in the different areas of knowledge, a selection filter for prestigious publications, and is also widely used to carry out bibliometric studies [56,57]. The bibliometric analysis technique has been used to study areas such as the use of water or wastewater [58,59], wastewater treatment by advanced oxidation processes [17], infectious diseases and microbiology [60], or renewable energies, sustainability and the environment [61].

#### *2.2. Data Selection and Processing*

The sample of documents analysed in this study was obtained by conducting a search of the entire WoS database with the term "wastewater treatment cost" in the option "topic":


Bibliometric studies distinguish between three types of indicators [55]: quantity indicators, which refer to productivity; quality indicators, which refer to the impact of publications; and structural indicators, which measure the connections established between the different agents. In this study, quality and quantity indicators were analysed. In addition to the measure of productivity of the countries and institutions, the following indicators were used to evaluate the quality of the journals in which the documents were published:


After selecting the sample, the information available in the WoS database was downloaded and prepared for analysis by eliminating duplications, correcting mistakes and adding non-complete information [54]. First, the evolution of the field over the period 1950–2020 was analysed. Secondly, the main areas of study in wastewater research classified by WoS were identified, before reviewing the leading countries in this research. In the next step, the main journals were identified along with their SJR, JIF and total citations index for the year 2019 (to evaluate the impact of the journals). The main institutions are shown below. Lastly, the results of the search for research on the costs of UV and chlorination treatments were studied.
