Microplastics in solid waste streams: Research needs and gaps in research methodology

Over the last decade, the presence of microplastics in municipal waste streams has become increasingly recognized as a significant environmental issue. While most studies have focused on liquid waste, specifically wastewater treatment plants, as a primary source of microplastic contamination, emerging research has begun to highlight solid waste systems. This review evaluates forty-six peer-reviewed studies reporting microplastic concentrations in landfill leachate, digestate from anaerobic digesters, composting facilities, and recycling operations. The concentrations of microplastics in these systems remain sparsely explored, and among available studies, methodological inconsistencies limit reproducibility and synthesis. The inconsistencies identified in this review included omitting essential sample processing steps, minimal reporting of detection limits for analytical instruments, and limited documentation of tactics for reducing inevitable laboratory contamination. These gaps highlight the importance of developing standardized methodologies and transparent reporting practices that account for sampling design, analytical sensitivities, and contamination control. This review synthesizes recent methodological trends, identifies key information to report, and proposes foundational elements for a more unified approach to microplastic analysis in solid waste streams.