Transfer of microplastics in sludge upon Fe(II)-persulfate conditioning and mechanical dewatering

Sewage treatment plants act as both sinks and sources of microplastics with elevated concentrations of microplastics accumulating in the sludge. Consequently, the effects of sludge conditioning and dewatering processes on the fate of microplastics need to be clarified. Microplastic characteristics in sludge, before and after advanced oxidation Fe(II)-activated persulfate conditioning were studied using a microplastics dynamic flotation separator (MDFS). In the unconditioned sludge (no dewatering), white and transparent microplastics dominated and seven types of plastic polymer were detected with polyethylene (30.3%) and polypropylene (23.9%) being the main ones. Pellet microplastics were found to be the dominant morphology, accounting for 67.0% of the total number of microplastics. The abundance of microplastics extracted using the MDFS device from the unconditioned (no dewatering) sludge was 320 ± 3 particles g^-1 dried sludge, which was greater by 37% than extracted using microplastics static flotation separation. Due to the release of the adsorbed microplastics from the destroyed sludge flocs after conditioning, the abundance of extractable microplastics increased by 19 ± 2% as compared to the unconditioned sludge (both with no dewatering). After filter presses (plate-frame filter, vacuum filter) and centrifuge dewatering, 81-90% of the microplastics were present in the filter cake, of which microplastics <500 μm accounted for more than 80% of the total number. The abundance of microplastics per unit volume of filtrate after filter press dewatering was significantly smaller than after centrifuge dewatering (3.2-4.4 × 10³ cf 13.0 × 10³ particles L^-1, respectively). The difference increments in relative abundance of <10 μm microplastics in the centrifuge filtrate was about twice that of the filter presses. The surface morphology of the microplastics did not change in the conditioning process. This study highlights the need to assess the application of advanced oxidation conditioning which has significant influence on the microplastics distribution via the subsequent sludge dewatering.