Treatment process of pre-coagulated waters involving polyethylene (PE) microplastics by ultrafiltration membranes coupled without or with pre-deposited aggregate-based layer

Abstract

In recent years, environmental and ecological issues caused by microplastics pollution have received widespread attention. The coexistence of polyethylene (PE) microplastics with other soluble and particulate contaminants in water environments is considered to pose a new challenge for water treatment engineering. This study first focused on the combined effects of microplastic size (13 and 150 μm) and concentration (0, 20 and 100 mg/L) of floc properties in the coagulation-flocculation process (CF). The results showed that the involvement of PE microplastics hindered larger floc formation but enhanced the floc strength. The presence of either smaller microplastic particles at a fixed concentration (100 mg/L), or a higher concentration of 13 μm microplastic particles resulted in smaller and more highly-branched floc aggregates. During ultrafiltration of pre-coagulated water (CF-UF), the “13 μm + 100 mg/L” case exhibited less severe membrane fouling than the other cases (“without PE”, “13 μm + 20 mg/L” and “150 μm + 100 mg/L”). Comparing the CF-UF process in the “without PE” and “13 μm + 100 mg/L” cases, the pre-deposited aggregate-based layer could give a lower degree of membrane flux decline and mitigate membrane fouling during ultrafiltration of pre-coagulated water (CF-UFPL). Moreover, both the involvement of PE microplastics and the properties (thickness and compactness) of the pre-deposited layer affected membrane filterability and fouling degree. Lastly, the influencing mechanisms of coexisting microplastics in UF, CF-UF and CF-UFPL processes were explored by comparatively analyzing the correlation of pre-coagulated floc physicochemical characteristics with membrane filterability, water purification efficiency and membrane fouling degree in both the absence and presence of microplastic particles.