Process-dependent removal of fine microplastics in pilot- and full-scale sewage treatment plants

This study presents the first comparative multi-technology assessment of fine (2.5–50 μm) microplastic (MP) removal across pilot- and full-scale sewage treatment plants (STPs) in Saudi Arabia. Influent and effluent samples were collected from two STPs employing different treatment configurations: oxidation ditch (OD) and aerobic granular sludge (AGS) at STP-A, and a full-scale membrane bioreactor (MBR) alongside pilot-scale moving bed biofilm reactor (p-MBBR) and p-AGS at STP-B. Influent MP concentrations exceeded 10 ³ MP L ⁻¹, while effluent concentrations ranged from 2.4 to 6.1 × 10 ² MP L ⁻¹, corresponding to removal efficiencies of 80–93%. At STP-A, OD and AGS achieved comparable removal, whereas at STP-B the MBR showed the highest performance, consistent with membrane-based size exclusion (0.4 μm nominal pore size). Fragments dominated all samples (∼80–85%), with ultrafine MPs (< 2.5 μm) being most abundant. A comparative assessment indicated that 1.4–1.8 × 10 ¹³ MPs yr ⁻¹ entered the two plants, with ∼2.2 × 10 ⁹ particles discharged daily in treated effluents. Raman spectroscopy identified 13 polymers with process-specific profiles. The persistence of fine MPs highlights the need for post-treatment polishing and size-resolved monitoring to support sustainable wastewater reuse under water-scarce conditions.