*4.2. Implications and Limitations*

The bibliometric analysis conducted in this study shows the trends in wastewater treatment costs research. It is very scarce in comparison with the research on wastewater treatment, highlighting the existence of a research field ye<sup>t</sup> to be explored. The findings of this study provide valuable information for researchers and institutions, helping them explore much-needed managemen<sup>t</sup> pathways. In practical terms, this study focuses on the importance of the improvements in the managemen<sup>t</sup> of wastewater treatment.

Despite its contributions, this study has limitations. It is relevant to remark the fact that the findings of this study might not fully reflect the complete research on wastewater treatment costs, given that the information is derived only from the WoS database. There are other databases, such as Scopus, with quality publications, that are not taken into account.

## *4.3. Future Research Opportunities*

Using a bibliometric analysis and the WoS, this study identifies the contributions to the topic of wastewater treatment costs, with China, the USA, India, Spain and the UK being the leading countries in this regard. From 2010, we see greater concern for sustainability, which has been promoted by the UN since 2000 when it declared its Millennium Development Goals. Indeed, one of those goals was to ensure environmental sustainability, which was followed by the inclusion of water in the Sustainable Development Goals in 2015. Nevertheless, the bibliometric analysis carried out reveals a topic that calls for further research, given the scarcity of scientific publications in the area of wastewater treatment costs. In addition, the few results obtained for UV and chlorination costs contrast to the prominence of these two treatments in the disinfection and reuse of wastewater; indeed, they are the most commonly used options for this purpose. The scientific contribution in this field is minimal compared to that on wastewater treatments, revealing a segmen<sup>t</sup> that has ye<sup>t</sup> to be explored. This study serves as a guide for researchers, pointing to new trends, and also informs the scientific community and institutions, while the growing interest in the subject enables improvements in the managemen<sup>t</sup> of wastewater treatment. In addition, society should be more aware of the need to fund wastewater treatments given the relevant role they play in securing additional water resources.

Wastewater reclamation has the disadvantage of entailing high costs, above the average of those associated with naturally-occurring resources. The study of wastewater treatment costs is essential for the proper managemen<sup>t</sup> of water resources. The decisionmaking process regarding the planning and managemen<sup>t</sup> of water resources requires the constant generation of information to achieve high levels of efficiency. Increasingly, the use of reclaimed water has strategic value in that it takes the pressure off water resources, while minimizing health risks for downstream users and helping to maintain the quality of ecosystems. Wastewater treatment and reclamation processes are an essential element of the efficient managemen<sup>t</sup> of the water cycle, becoming hugely important in countries with high levels of water stress.

**Author Contributions:** Conceptualization, L.G.-V., E.M.-P. and I.M.R.-S.; methodology, L.G.-V. and I.M.R.-S.; validation, E.M.-P. and I.M.R.-S.; formal analysis, L.G.-V.; investigation, L.G.-V.; resources, L.G.-V.; data curation, L.G.-V.; writing—original draft preparation, L.G.-V.; writing—review and editing, E.M.-P. and I.M.R.-S.; visualization, L.G.-V.; supervision, E.M.-P.; project administration, E.M.-P. All authors have read and agreed to the published version of the manuscript.

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

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

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