Microplastics in Central Asian Wastewater Systems: Analytical Workflows, Quality Assurance, Quality Control, Uncertainties, and Research Priorities

Microplastics (MPs) are increasingly detected in wastewater treatment systems, where treatment plants act simultaneously as interception nodes and point sources via treated effluents and sludge management. This contribution synthesizes a PRISMA-guided critical review focused on Kazakhstan and Central Asia, benchmarking the region against a harmonized global dataset while explicitly interrogating how methodological choices drive inter-study variability.

A structured evidence map (2010–Sept 2025) was compiled and curated into a comparable database of 63 wastewater-treatment studies worldwide, yielding 402 matrix–stage observations across influent, effluent, and sludge streams. Observation-level descriptive statistics show that global raw influent concentrations cluster around 100 particles/L (median ≈65 particles/L), whereas final/tertiary effluents are typically 1 particles/L (median ≈2.2 particles/L). Overall MP removal increases from secondary treatment (median ≈85.5%) to tertiary/advanced trains (median ≈95.0%), while sludge acts as the dominant sink, retaining MP burdens on the order of 1000–100.000 particles/kg dry weight. Across matrices, fibers dominate the reported morphologies and polymer signatures are consistently led by PET/PES, PP, and PE, consistent with textile and packaging sources.

Central Asian plant-level evidence remains extremely limited (two eligible wastewater treatment plants case studies, both in Kazakhstan), but when comparisons are restricted to like-for-like analytical windows, influent levels align with the global interquartile range. In contrast, secondary-only configurations tend to place effluent concentrations in the upper half of the global envelope, supporting the inference that the presence/absence of post-secondary barriers (filtration, DAF/BAF, membranes/MBR) is the primary determinant of regional performance relative to international benchmarks. The review identifies three dominant uncertainty drivers—sampling representativeness (grab vs. composite), minimum size cut-offs (especially <100 µm), and incomplete quality assurance and quality control (QA/QC) reporting—and proposes an actionable 2025–2030 agenda: ISO-aligned protocol harmonization with explicit QA/QC, expansion of monitoring to tertiary/advanced trains, coordinated interlaboratory ring trials and reference-library development, and integration of monitoring with hydrodynamic/fate models to translate plant upgrades into reach-scale benefits in arid, episodic-flow receiving waters.