**3. Results**

#### *3.1. Evolution of the Research on Wastewater Treatment Costs*

The scientific community has shown very little interest in analysing wastewater treatment costs, although there has been some growth in the theoretical and applied literature on the subject over the last decade. Figure 1 presents the evolution of articles published in this field from 1950 to 2020, revealed by the bibliometric analysis using WoS (2020) as the main database. The search for "wastewater treatment cost" returns 22,788 results, a very small number compared to those returned by the search "wastewater treatment", with 184,697 articles published. In percentage terms, wastewater treatment cost research comprises only 12.34% of the total for wastewater treatment. The research on wastewater treatment costs shows an increasing trend between 1950 and 2020, with a marked rise from 2010, albeit with some fluctuations. It can be seen that, in both cases, the majority of articles are concentrated from 2010 onwards (16,204 studies, 71.11% of the total for "wastewater treatment cost" and 125,235 articles, 67.81% of the total for "wastewater treatment"). The period between 1950 and 1980 yields very few results, although the number grows over time. Looking at the results since 1980, 20 articles were published for "wastewater treatment cost" and 235 for "wastewater treatment" in 1980, while the corresponding figures for 2020 were 2528 and 16,708, respectively. The growth in the research on wastewater treatment costs is slower than that for wastewater treatment: although both show a rising trend during the period 1950–2020, research on "wastewater treatment" grows at a much faster rate, especially after the year 2000.

**Figure 1.** Trends in the research on wastewater treatment costs and wastewater treatment (number of articles). Source: own elaboration from WoS (2021).

#### *3.2. Main Areas of Study in Wastewater Treatment Costs Research*

The results of the bibliometric analysis make it possible to distinguish between the different disciplines to which the analysed scientific articles belong. It should be noted that an article can belong to more than one category; for this reason, the results are analysed in percentages. Figure 2 shows the main areas of study in wastewater treatment costs in the period under study. Among the many areas of research, the most important are Environmental Sciences and Ecology, accounting for 14% of the total, Engineering (11%) and Water Resources (10%). These areas are followed by Public Environmental Occupational Health (8%), Energy and Fuels (6%), Business Economics (6%) and Chemistry (6%). The item labelled "other" (39% of the total) includes a wide and diverse range of areas, none of them with a percentage higher than 5%, such as Materials Science, Toxicology, Biochemistry Molecular Biology, Mathematics, Biodiversity Conservation, Physics, Marine Freshwater Biology, Plant Sciences, Microbiology, Mathematical Computational Biology, Computer Science, Food Science Technology, Polymer Science, Meteorology Atmospheric Sciences, Science Technology (other topics), Biotechnology Applied Microbiology, Instruments Instrumentation and Agriculture.

**Figure 2.** Main areas of study in wastewater treatment costs (percentage). Source: own elaboration from WoS (2021).

#### *3.3. Relevant Countries in Wastewater Treatment Costs Research*

The articles published on wastewater treatment costs in the period 1950–2020 come from a total of 162 countries. Figure 3 shows the map of the countries with results on wastewater treatment cost research, although the majority of studies in this field come from a relatively small number of main countries. In order of quantity of results, they are China, the USA, India, Spain, the UK, Australia, Brazil, Canada, Turkey and Iran. Altogether, there are countries from almost all continents, and with a wide diversity of economic and sociocultural characteristics.

**Figure 3.** Map of the research on wastewater treatment costs (number of articles). Source: own elaboration from WoS (2021).

Figure 4 presents the top countries in the field. China is the most prolific country, representing 28.84% of the total, with 6571 results. The second place is occupied by the USA (12.05% of the total, 2745 articles). Third is India, with 1975 results and 8.67%. These countries are followed by Spain (1116 results, 4.90%), the UK (1112 results, 4.88%), Australia, Brazil, Canada, Turkey, Iran, Italy, Germany, Malaysia, South Korea, France, Japan, Egypt and the Netherlands.

**Figure 4.** Top countries in wastewater treatment cost research (percentage). Source: own elaboration from WoS (2021).

#### *3.4. Journals in Wastewater Treatment Cost Research*

This part presents the most relevant journals publishing articles on wastewater treatment cost research and analyses their main indexes (Table 1). The main journals by number of articles are Water Science and Technology (1471 articles), Chemical Engineering Journal (709) and Desalination and Water Treatment (651). Taken together, these three journals published 12.42% of the total papers on this research subject. Among the list of journals, there are diverse nationalities, with the United Kingdom, the Netherlands, Switzerland and the USA being the most prolific countries.

**Table 1.** Journals and relevant indexes in wastewater treatment cost research. Source: own elaboration based on WoS (2021).


It is important to note that there is a broad range of journals publishing articles on wastewater treatment costs. The journal with the largest number of articles is Water Science and Technology, with 6.46% of the total sample. This journal has an SJR in 2019 of 0.47 (quartile Q2), a JIF of 1.638 and a total of 20,937 citations in 2019. The second is Chemical Engineering Journal, with 3.11% of the total sample. The SJR in 2019 for this journal is 2.32 (Q1), with a JIF of 10.652 in 2019 and 129,806 total citations in 2019. In third place is Desalination and Water Treatment, with 2.86% of the total sample and an SJR (2019) of 0.33 (Q2), a JIF index of 0.854 and 14,535 total citations. Below these three results, there is a wide variety of journals, almost all of which present high index scores—most are in the first and second quartiles. These 20 journals published 36.15% of the articles, most of them included in the first two quartiles of the SJR. They include Journal of Hazardous Materials, Water Research, Journal of Cleaner Production, Bioresource Technology, Journal of Environmental Management, Environmental Science and Pollution Research, Desalination, Environmental Technology, Journal of Environmental Chemical Engineering, Science of the Total Environment, Chemosphere, Environmental Science & Technology, Journal of Chemical Technology and Biotechnology, Water, International

Journal of Environmental Science and Technology, Water Environment Research, and Water, Air and Soil Pollution.

#### *3.5. Leader Institutions in Wastewater Treatment Cost Research*

Figure 5 presents the main institutions focusing on wastewater treatment costs research. This large number of institutions (62) accounts for 31.10% of the total results, which indicates a low concentration index at the institutional level. The leading institution in number of related articles is the Chinese Academy of Sciences (China), with 561 articles (2.46% of the total), followed by the Indian Institute of Technology System (India), with 445 articles (1.95% of the total). Institutions from China, India and the USA predominate, with those from the USA having a lower concentration by institution. It is worth noting the wide dispersion of articles among different institutions.

#### *3.6. Differences in Scientific Research on Treatments for Wastewater Intended for Reuse: UV and Chlorination*

This section analyses the results on disinfection treatments for wastewater intended for reuse; namely, UV and chlorination. These are the most commonly applied treatments for wastewater disinfection for various economic and environmental reasons [63]. The classification as UV or chlorination reveals a difference, with 1118 articles for UV and 140 for chlorination, representing 4.91% and 0.61%, respectively, of the total research on wastewater treatment costs. This is a very small number of results considering the importance of these treatments for the reuse of wastewater, and the fact that they are the most commonly used options. The analysis of the number of publications over the period 1950–2020 shows a growing amount of research on wastewater treatment costs relating to UV and chlorination over the years. Figure 6 shows the evolution of the studies published on these two treatments throughout the period analysed, clearly depicting the scant research up until the year 2000. From then on, we see a rising trend in both cases, with a particularly notable increase in studies on UV, especially from 2014 onwards. In the last 10 years, we see a rise in publications on UV from 29 in 2010 to 147 in 2020, representing a growth rate of 36.99%. In parallel, those addressing chlorination register an increase of 27.27% in the last decade, with two results in 2010 and eight in 2020.

Figure 7 shows the leading countries in research on wastewater treatment costs related to UV and chlorination: China is the leader on UV treatment, with 256 results (22.90% of total results for UV), and the USA for chlorination treatment, with 31 articles (22.14% of total results for chlorination). In the research on the costs of tertiary treatment with UV, China is followed by India, the USA, Spain, Iran, Brazil, Italy, Canada, Germany and Egypt, in that order. In the case of chlorination cost research, after the USA come China, Brazil, India, Australia, Canada, France, Italy, Spain and the United Kingdom. It can be seen that 8 of the 10 leading countries are the same for the two treatments, although they hold different positions according to their numbers of results.


**Figure 5.** Main institutions in wastewater treatment cost research. Source: own elaboration from WoS (2021).

**Figure 6.** Evolution of articles on UV and chlorination treatments for wastewater reuse from 1950 to 2020 (number of articles). Source: own elaboration from WoS (2021).

**Figure 7.** Leading countries in the research on wastewater treatment costs related to UV and chlorination (number of articles). Source: own elaboration from WoS (2021).

## **4. Discussion and Conclusions**

## *4.1. Summary of Findings*

The objective of this work was to show the current status and evolution of research on the wastewater treatment costs during the period 1950–2020. To achieve this goal, the main drivers of the subject, the main lines of research, the trends over several years, and the gaps in research were analyzed in depth. In addition, two disinfection treatments for water reuse have been considered: chlorination and UV. Bibliometric techniques have been used to carry out this study, with the WoS database and statistical processing tools. Globally, research on wastewater treatment costs has yielded 22,788 results: an increasing trend of wastewater treatment costs research was observed. Nevertheless, this topic remains largely understudied compared to the results for the search "wastewater treatments", representing only 12.34% of the total output for research on wastewater treatments. For the sake of easy understanding, the findings of this study are summarized as follows:

(1) The evolution of research suggests that the study of wastewater treatment costs has experienced a steady increase from 2010. This changing trend is related to the growing social concern for the environment and its resources. Reflecting the rise in "wastewater treatment" research, there has been a rise in research on wastewater treatment costs since 1950, showing strong growth since 2010, with 71.11% of the articles being published from this date onwards.

(2) The main areas of research on this topic are very diverse, with the most important being Environmental Sciences and Ecology (14% of the total), Engineering (11%) and Water Resources (10%).

(3) As for the countries of origin of the scientific output in this field during the period 1950–2020, it is worth noting the high dispersion; all together, 162 countries were responsible for the results in wastewater treatment costs research. The main countries are China (28.84% of the total), the USA (12.05%), India (8.67%), Spain (4.90%) and the UK (4.88%).

(4) Concerning to the leader institutions, there is grea<sup>t</sup> diversity from which the research on this subject comes, as reflected in the Low Concentration Index at the institutional level; some that stand out are the Chinese Academy of Sciences (China), with 2.46% of the total, and the Indian Institute of Technology System (India), with 1.95% of the total.

(5) Journal analysis revealed that the articles relating to the analysed subject are published in international journals with a high impact factor. There is a wide range of journals that publish articles on the costs of wastewater treatment. Researchers' preferred journals are Water Science and Technology, Chemical Engineering Journal, and Desalination and Water Treatment, with high impact factors of 1.638, 10.652 and 0.854 (JIF), respectively. Taken together with the number of publications, these are an indicator of the high level of institutional scientific quality. These three journals together have published 12.42% of the total research in this field.

(6) Regarding to UV and chlorination costs, very few results were returned in the search for articles (4.91% for UV and 0.61% for chlorination) in comparison with the total results for wastewater treatment costs. It is remarkable that these topics have registered an increase over the period analysed, with a particularly notable rise from the year 2000. The leading countries in this research are the following:

