Microplastics in Wastewater Systems of Kazakhstan and Central Asia: A Critical Review of Analytical Methods, Uncertainties, and Research Gaps

Microplastics are increasingly recognized as contaminants of emerging concern in wastewater systems, where treatment plants act both as sinks and as point sources. However, Central Asian wastewater infrastructures are under-represented in the literature, and global syntheses are hindered by strong methodological heterogeneity (sampling regimes, size cut-offs, QA/QC). This PRISMA-guided critical review compiles and harmonizes data from 63 WWTP studies worldwide (402 matrix-stage observations), including the few available case studies from Kazakhstan and neighboring countries, to benchmark Central Asian plants against a global envelope and identify methodological and infrastructure gaps. Globally, influent concentrations cluster around a median ≈65 particles/L, while final/tertiary effluents show a median ≈2.2 particles/L. Median removal efficiency is 85.5% for secondary and 95.0% for tertiary/advanced trains, with ≈103–105 particles/kg DW typically retained in sludge. Across influent, effluent and sludge, fibers and fragments of PE, PP and PET dominate polymer morphology patterns, with similar PET/PE/PP signatures also reported in downstream river water. Central Asian influents fall within global interquartile ranges, but secondary-only facilities tend to yield effluents in the upper half of the global distribution. Overall, the review provides a first integrated, methodologically explicit assessment of WWTP microplastics in Central Asia and underscores the need for protocol harmonization, longitudinal monitoring, and targeted upgrades of polishing steps and sludge management in arid hydrosystems.