[Effects of Microplastics on Membrane Fouling During a Shortened Ultrafiltration Membrane Process]

Microplastics have garnered much attention worldwide as a new emerging pollutant. As they are gradually detected in freshwaters, understanding how microplastics will behave during current drinking water treatment processes is urgently needed. In recent years, the shortened process with an ultrafiltration (UF) membrane has shown excellent performance because of its low land use and high water purification efficiency. In this work, the membrane performance induced by microplastics was investigated with a shortened UF membrane process. The results showed that membrane fouling was always induced by the cake layer before and after coagulating with microplastics. Owing to the small UF membrane pore size (d<0.1 μm), slight membrane fouling was caused by microplastics (d<5 mm) alone. However, although the loose cake layer was formed because of the existence of flocs, the cyberspace formed by flocs was easily entered by small microplastics with increasing coagulant dosage. As a result, server membrane fouling was induced because of the formation of a dense cake layer. It was shown that the specific membrane flux induced by flocs alone was 0.82 and 0.76 in the presence of 0.1 mmol·L^-1 and 0.9 mmol·L^-1 FeCl₃·6H₂O, respectively. However, after coagulation the specific membrane fouling induced by the 0.1 g small microplastics (d<0.5 mm) was 0.76 and 0.62 with 0.1 mmol·L^-1 and 0.9 mmol·L^-1 FeCl₃·6H₂O, respectively. In addition, microplastics were always negatively charged in water. In comparison with alkaline conditions, Fe-based flocs were positively charged under acidic conditions, which were also much smaller. Therefore, microplastics were more easily adsorbed by Fe-based flocs under acidic conditions, leading to severe membrane fouling because of the dense cake layer formed. After coagulating with 0.3 mmol·L^-1 FeCl₃·6H₂O, the specific membrane flux induced by 0.1 g small microplastics (d<0.5 mm) was 0.55 and 0.79 at pH 6.0 and 8.0, respectively.