High-density microplastics in wastewater treatment plants and aquatic environment: review, challenges, and future prospects

The pervasive discharge of high-density microplastics (HDMPs) from wastewater treatment plants (WWTPs) into aquatic ecosystems represents a critical environmental and engineering challenge. While conventional WWTPs achieve moderate removal efficiencies (60%–99%), their performance is highly variable for larger, denser particles (e.g., polyvinyl chloride, polyethylene terephthalate) due to complex interactions between particle characteristics (e.g., size distribution, shape anisotropy, and biofilm colonization), fluid dynamics (e.g., turbulent kinetic energy, and shear stresses), and process design parameters (e.g., retention times, and tank geometries). This review systematically evaluates the fate and transport dynamics of HDMPs in WWTPs, highlighting the underexplored role of dimensionless parameters (e.g., Shields parameter θ, particle Reynolds number Re p , Stokes number St, Densimetric Froude number Fr′, particle shape factors ψ, Rouse number Z, and biofilm buoyancy number B f ) in predicting their behavior across treatment stages and post-discharge environments. The study critically assesses knowledge gaps in existing removal technologies, ranging from membrane bioreactors (fouling risks) to emerging techniques such as electrocoagulation (energy costs), and identifies methodological shortcomings in sampling, analysis, and reporting that hinder global comparability. Key findings reveal that particle–fluid interactions (e.g., resuspension thresholds, and turbulent mixing) are poorly quantified for HDMPs, necessitating the development of revised hydrodynamic models that incorporate shape factors and biofilm effects. Additionally, synergistic stressors (e.g., temperature, and co-pollutants) are found to exacerbate ecological risks but remain understudied. Meanwhile, standardised protocols for HDMP characterisation and dimensionless parameter frameworks are urgently needed to unify research and policy efforts. The study proposes three priority research directions: (i) development of cost-effective, scalable hybrid systems (e.g., coagulation-membrane bioreactors); (ii) integration of dimensionless parameters into WWTP design and outfall modelling; and (iii) long-term studies on HDMPs’ ecological impacts under realistic exposure scenarios. By bridging fundamental hydrodynamics with applied engineering, this review provides a roadmap to mitigate HDMP pollution through science-informed policy and innovation.