From capture to destruction: A critical review of microplastic separation and degradation in water and wastewater treatment.

Microplastics (MPs) have emerged as contaminants of increasing concern in water and wastewater treatment systems because of their persistence, mobility, and potential risks to ecosystems and human health. This review critically examines two major technological routes for MP control: separation-based removal, including coagulation, filtration, membrane separation, and adsorption, and degradation-based transformation, including advanced oxidation processes and biological degradation. Under optimized conditions, separation technologies can achieve over 95% retention, while degradation technologies can induce polymer-chain scission, surface oxidation, and mass loss. This review reveals a fundamental distinction between these two routes. Separation-based technologies mainly transfer MPs from the aqueous phase to flocs, sludge, membrane retentates, or spent adsorbents, thereby concentrating particles rather than destroying the polymer structure. In contrast, biodegradation and transformation technologies aim to transform polymer chains into lower-molecular-weight products, but complete mineralization is rarely achieved under realistic treatment conditions. Incomplete degradation may generate oligomers, organic acids, aldehydes, ketones, and released additives, whose environmental fate and ecotoxicity remain insufficiently understood. Therefore, the commonly reported removal rate is insufficient for evaluating the actual environmental benefit of MP treatment, as it may represent phase transfer rather than risk elimination, or partial transformation rather than complete degradation. Overall, this review concludes that meaningful evaluation of MP treatment requires a shift from single-parameter efficiency metrics to mass-balance-based and toxicity-informed frameworks that account for the fate of both separation residues and degradation products. Future research should move beyond single efficiency metrics and establish integrated assessment frameworks based on mass balance, life cycle analysis, product identification, toxicity evolution, and residual management. Coupling efficient separation with controlled deep degradation offers a promising direction for advancing MP treatment from efficient interception towards verifiable risk reduction and sustainable control.