This study investigates the impact of separate waste collection on the composition of mixed municipal solid waste (MSW) in Italy. With increasing environmental concerns and regulatory pressure, Italian municipalities have adopted various strategies to improve waste separation at the source. By analyzing data from multiple regions and timeframes, this research examines how the expansion and efficiency of separate waste collection influence the residual composition of mixed MSW. Findings reveal that higher rates of separate collection significantly reduce the proportion of recyclable and organic materials in mixed waste, indicating improved sorting behavior among residents. Additionally, regional disparities in infrastructure, public awareness, and policy enforcement are shown to affect collection efficiency. The study highlights the importance of optimizing collection systems and public engagement to enhance the quality of waste streams and support circular economy objectives.