## 2 Materials and Methods
### Screening and Selection of the Studies  
The initial selection of articles was based on reading the titles and abstracts, followed by reading the full text of the selected articles. Two independent reviewers conducted the title/abstract and full-text screening using Covidence [43]. Studies without unanimous agreement between reviewers were excluded. Disagreements were resolved by a third reviewer to ensure methodological rigor (PRISMA Item 8). Relevant data were extracted from these selected studies, including data on the optimization of RSCSW derived from solid waste, their applications in the circular economy through MCDA and AI, and the associated environmental and socio-economic impacts. The following outlines the step-by-step procedure: Process of selection: Two reviewers conducted the screening process. In cases of disagreement, a third reviewer was consulted to reach a consensus. This approach ensured objectivity and minimized bias in the selection process. Data collection process: Data extraction was performed independently by the two reviewers using a standardized form. To ensure accuracy, the reviewers contacted the authors of the studies whenever additional information or clarifications were needed. The extracted data included information on the study context, methodology, results, and conclusions. Assessment of research study quality: The quality of the included studies was assessed using the Critical Appraisal Skills Programme (CASP) [44] checklist. Two independent reviewers evaluated each study, focusing on criteria, such as clarity of the research question, appropriateness of study design, population selection, exposure and outcome measurement, control of confounders, and statistical analysis. Discrepancies between reviewers were resolved through discussion. Preparation of data for synthesis: The data were prepared for synthesis through the normalization of metrics and the identification of key themes. Heterogeneity among the studies was assessed qualitatively, considering differences in methodologies and contexts. Although a quantitative meta-analysis was not performed due to the heterogeneity of the studies, a narrative synthesis was conducted. Formal sensitivity analyses (e.g., subgroup analyses or exclusion of lower-quality studies) were not performed due to the small final sample size (\(n=22\)), but the unanimous reviewer consensus and uniformly high quality of included studies (CASP 10/10) [44] ensure the robustness of findings (PRISMA Item 13f). Risk of bias assessment: Reviewers assessed the risk of bias in the included studiesusing the CASP checklist [44]. The evaluation focused on potential sources of bias, such as study design, data collection methods, and reporting of results. Discrepancies between reviewers were resolved through discussion. For the synthesis of results, a qualitative analysis was conducted, which included the normalization of metrics and the identification of key themes. The heterogeneity among the studies was assessed qualitatively, and the findings were integrated into a coherent narrative. Table 2 presents a summary of the quality assessment of the included studies, conducted using the CASP [44]. All studies were classified as high quality as they met the criteria for clarity of research question, appropriateness of study design, population selection, exposure and outcome measurement, control of confounders, statistical analysis, presentation of results, discussion, and conclusion. To mitigate potential publication bias, comprehensive searches across multiple databases were conducted and included relevant gray literature, such as technical reports and theses, where applicable. Funnel plot analysis and Egger's test were considered to assess the presence of publication bias but were not applied due to the heterogeneity of the included studies. A detailed CASP [44] assessment, including individual scores for each criterion, is available in the Supplementary Material (Table S1).