Characteristics of microplastic removal via coagulation and ultrafiltration during drinking water treatment

Microplastics have garnered much attention worldwide as a new emerging pollutant, especially because of their eco-toxicological effects in marine environments. As they are gradually detected in freshwaters, understanding how microplastics, with their small particle size and low density, will behave during current drinking water treatment processes is urgently needed. In this study, Al- and Fe-based salts were used in the presence of polyethylene (PE), which is suspended/floats easily in water and is the main constituent of microplastics. Results showed that Al-based salts performed better in PE removal efficiency than Fe-based salts. The smaller the PE particle size, the higher the removal efficiency. However, a low removal efficiency was observed, even with a high Al-based salt dosage of 15 mM (below 40%). Additionally, water conditions, such as ionic strength, turbidity level, barely influenced the removal efficiency. In comparison to pH, polyacrylamide (PAM) addition played an important role in removing PE; especially anionic PAM addition, because of the positively charged Al-based flocs it generates under neutral conditions. For ultrafiltration, although PE particles can be completely rejected, slight membrane fouling was induced after coagulation with conventional Al-based salts. With increasing dosage, membrane fouling was gradually aggravated owing to the thick cake layer formed. However, the larger the PE particles, the greater the roughness of the Al-based floc cake layer, leading to less severe membrane fouling. Based on this investigation, the microplastic removal behaviors exhibited during coagulation and ultrafiltration processes have potential application in drinking water treatment.