Micro-Nanobubble Integrated Dissolved Air Flotation: A High-Efficiency Strategy for Microplastic Mitigation in Wastewater

Microplastics (MPs; <5 mm) are emerging contaminants of global concern in aquatic environments, with wastewater treatment plants (WWTPs) recognized as major pathways for their release into natural waters. While conventional WWTPs can remove a substantial proportion of MPs, smaller particles, particularly nanoplastics (NPs; <1 μm), often persist in both effluent and sludge. This study investigates an optimized dissolved air flotation (DAF) process enhanced with microbubbles (MBs) and nanobubbles (NBs) for the removal of 100 μm polyethylene (PE) and polystyrene (PS) MPs from synthetic wastewater. MBs (105 ± 15 μm) generated at 5 bar with a 5 min saturation time achieved removal efficiencies of 87% (PE) and 91% (PS) via buoyant flotation. NBs (820 ± 78 nm) generated at 5 bar with a 20 min saturation time further improved removal to 92% (PE) and 95% (PS) by enhancing bubble-particle interactions and promoting aggregation. A combined micronanobubble (MNB) DAF configuration outperformed both individual systems, achieving the highest removal efficiencies of 95% (PE) and 97% (PS). The presence of dissolved organic matter and fats, oils, and grease did not diminish removal; rather, in the presence of coagulants, these constituents increased MPs’ hydrophobicity and promoted the formation of larger, more buoyant flocs. These findings demonstrate the strong potential of MNB-assisted DAF for efficient MPs/NPs removal and provide practical insights for upgrading WWTP primary treatment to mitigate microplastic pollution.