Microplastics separation in reject water from wastewater sludge treatment line using micro and ultrafiltration membranes

Microplastics (MPs) and other microparticles (MPr) in wastewater sludge streams represent a challenge in wastewater treatment plants (WWTPs). Some of these MPr accumulate when reject clarified streams are recycled to the plant inlet. This study evaluates the performance of ultrafiltration (UF) and microfiltration (MF) membranes for MPr removal and nutrient preservation from two sludge-derived streams: Primary Sludge Liquor (PSL) and Digestate Centrate (DC). Three membranes (UH050, V6, and V0.2) were tested under total recycling and variable concentration modes. For PSL, the UH050 membrane showed superior overall performance by combining stable operating permeate flux (15 L·m −2 ·h −1 ), efficient MPr retention, and negligible NH 4 + -N and PO 4 3- -P retention. Moreover, it showed excellent cleanability, with > 90% permeability recovery after alkaline cleaning. For DC, all membranes achieved high MPr retention in the permeate, though no significant accumulation was observed in the retentate. Again, UH050 provided the lowest rejection of NH 4 + -N (15%) and PO₄³ ⁻-P (almost insignificant), with good cleaning efficiency. Overall, UH050 was identified as the most reliable option for PSL, and DC combining MPr removal, nutrient selectivity, hydraulic stability, and cleanability. These findings support the implementation of membrane-based solutions for MPs control and nutrient recovery in sludge lines, contributing to resource efficiency and circular economy in WWTP.