Effect of microplastic size on biofouling in membrane bioreactors

Wastewater treatment plants are gaining recognition as key sources of aquatic microplastic pollution, emphasizing the need for effective microplastic removal during the wastewater treatment process. Although membrane bioreactors (MBRs) have been reported to show superior microplastic removal efficiency compared to other technologies, there is a lack of fundamental research on the impact of microplastics on MBR operation. We investigated how different microplastic particle diameters (~0.25, ~31.6, and ~225 μm) impact the biofouling of MBRs and examined changes in membrane resistance post-cleaning. More severe biofouling was observed when smaller microplastics were added. Physical backwashing followed by chemical backwashing recovered approximately 98 % of the membrane resistance regardless of microplastic size. Notably, irreversible fouling was most pronounced when filtering 0.25 μm microplastics, which are of a similar size to the membrane pore diameter (0.1 μm). Scanning electron microscopy confirmed the presence of 0.25 μm microplastics in the porous structure of the membrane even post-cleaning. This indicates that microplastics of similar size to the membrane pores can block or narrow the pores, thereby accelerating fouling and reducing the membrane lifespan. These findings deepen our understanding of how the microplastic size influences membrane biofouling, thereby contributing to the wider applicability of MBRs for wastewater treatment.