Hybrid wastewater treatment: efficiency and challenges in microplastic removal

Hybrid treatment trains, integrating complementary physical, biological, and chemical barriers, deliver more robust control of microplastics (MPs) in wastewater treatment plants than conventional single-unit operations. However, fibers and small fragments (<50–100 μm) often persist. Across laboratory, pilot, and full-scale evidence, conventional baselines are size- and site-dependent: rapid sand filtration achieves ∼75% removal at full scale for coarse classes; dissolved air flotation ∼95% for >20 μm; disc filtration ∼98%; and conventional activated sludge ∼64–98%. These removals are reported on a count basis above study-specific lower size cutoffs (typically ≥ 20 μm). However, hybrid treatments report removals ∼100%, though values vary with detection limits and MP size classes analyzed. Membrane-centric trains (e.g., membrane bioreactor with ultrafiltration) reach nearly complete removal. Chemical–physical hybrid treatments (coagulation–flocculation with membranes; electrocoagulation–electroflotation plus ultrafiltration) can reach ∼100% removal. Ozonation coupled with granular activated carbon sustains ∼92% removal at pilot scale and up to 98% at full scale. Bubble-enhanced flotation exceeds 98% removal for 20–100 μm. Adsorbent-assisted hybrids capture ∼99% of smaller MPs. Despite these gains, fibers and <100 μm MPs remain the dominant challenge. This review consolidates laboratory, pilot, and full-scale evidence to guide influent-tuned hybrid designs that deliver robust MP removal approaching complete removal.