Microplastics are detected in many surface waters including those used for drinking water production. Although the impact of microplastics on human health is not well understood yet, it is prudent to be proactive and to establish how well currently used drinking water treatment processes remove these microplastics. Drinking water treatment plants are comprised of a series of treatment steps and coagulation/flocculation/sedimentation (CFS) is a key process designed to remove particles from surface water.
This study examined the removal of microplastics by said CFS process using commercially available microspheres to represent microplastics encountered in surface waters. Aluminum sulphate (alum) is often used as a coagulant, and this research investigated different alum doses applied to several microsphere sizes and source waters. Specifically, the study looked at the removal of carboxylated polystyrene (PS) microspheres in a wide size range (3, 6, 25, 45, and 90 μm) in two different sources of surface water (Grand River and Lake Erie water) that supply full-scale drinking water treatment plants. The alum-based CFS tests were performed at bench scale in the laboratory.
The objectives were to 1) evaluate the performance of CFS in removing spherical PS microplastics from natural waters; 2) investigate the influence of alum dose on the removal of PS microspheres; 3) examine the effect of particle size on the removal of PS microplastic spheres in alum-based CFS treatment; and 4) explore the impact of water source (quality) on the removal of PS microplastics by alum-based CFS.
The results suggested that alum dose, microsphere size, and water source substantially influenced the removal of PS microspheres in alum-based CFS treatment. The findings indicate that small microplastics (≤ 25 µm) can be effectively removed with alum-CFS treatment, whereas bigger microplastics (≥ 45 µm) would require additional treatment steps such as granular media or membrane filtration.
There are limited studies on microplastics in drinking water treatment processes, although it is a current topic of investigation and the findings of this study will improve the current understanding of the behavior and mitigation of spherical and similarly shaped microplastics by CFS treatment of drinking water.
As part of the microplastics fingerprinting project, researchers at the NSERC Chair in water treatment are expanding on the work above by considering approaches to minimize microplastics in drinking water should regulatory interventions be deemed necessary to protect public health. These approaches could include for example monitoring and treatment goals.
The full results of the study can be found in the article: Xue, J., Peldszus, S., Van Dyke, M. I., & Huck, P. M. 2021. Chemical Engineering Journal, 422, 1-11. Please contact water-chair@uwaterloo.ca to obtain a copy of the article.