Effect of microplastics on the physical structure of cake layer for pre-coagulated gravity-driven membrane filtration

Microplastics have been frequently detected in surface water and have recently emerged as a threatening global pollutant. Although gravity-driven membrane (GDM) filtration is a promising technique for drinking water treatment, the effect of microplastics on the GDM filtration has rarely been studied. In this study, to explore how to optimize GDM filtration process when microplastics are present in surface water, different systems (pre-coagulated and noncoagulated systems; with and without microplastics; AlCl₃ or PACl as coagulant) are examined. The results show that the flux of noncoagulation systems is low (2.67 L/m² h) due to the pore blocking caused by low molecular weight of HA, while the microplastics have a negligible contribution to the membrane fouling. Further, pre-coagulation significantly alleviates the membrane fouling and increases the fluxes (13.68–20.93 L/m²h). By employing optical coherence tomography and ultra-depth three-dimensional microscopy to observe the physical structure of the cake layer and then using the quasi-uniform cake filtration model for analysis, we clarify that the presence of microplastics in the PACl pre-coagulated GDM system is beneficial to form more uniform cake layer and achieve the physical stabilization of flux. Moreover, we propose that the fractal dimension of the particles (D_f) of small flocs (<6 μm) can be used to predict the surface roughness of cake layer as well as the flux of pre-coagulated GDM in advance, providing a valuable guidance for the optimization of GDM filtration process.