Removal of polystyrene microplastic spheres by alum-based coagulation-flocculation-sedimentation (CFS) treatment of surface waters

Abstract

• Real river and lake water with carboxylated PS microplastics were treated via CFS. • Higher alum doses generally benefited the removal of PS microplastics ≤ 90 μm. • Smaller PS microplastics were better removed by alum CFS treatment. • Water source affected the removal efficiency of PS microplastics. • PS microplastics did not impact the removal of turbidity, NOM fractions, or metals. Microplastics are an emerging issue in water systems, and an understanding of their occurrence and behavior is complicated by a high diversity in physical (i.e., size and shape) and chemical characteristics. To date, there have been limited studies on microplastics in drinking water treatment processes. As the coagulation-flocculation-sedimentation (CFS) process is the first step of the particle/colloid removal at many plants, its ability to remove microplastics requires further investigation. By performing alum-CFS bench tests, this study examined the removal of carboxylated polystyrene (PS) microspheres in a wide size range (3, 6, 25, 45, and 90 μm) in two types of real surface waters (Grand River and Lake Erie water) that are sources for full-scale drinking water treatment plants. Based on experiments using water from the Grand River with a variety of alum doses (10, 20, 30, 40, and 50 mg/L), a higher alum dose generally led to better removal of microspheres smaller than 90 μm, while an alum dose > 30 mg/L did not substantially improve the removal of particles between 3 and 25 μm. Further experiments indicated that smaller PS microspheres were more effectively removed in both river (Grand River_Dec 06) and lake water (Lake Erie_Nov 29). For example, with an alum dose of 30 mg/L, 75.6 and 85.2% of the 6-μm PS microspheres were removed from Grand River and Lake Erie water, respectively. Moreover, the water source significantly (p = 0.0021) affected the removal of PS microspheres, which may have been primarily related to the difference in turbidity. No impacts of PS microspheres were observed on the removal efficiency for turbidity, natural organic matter fractions, or major metal elements by alum-CFS treatment.