Conventional and biological treatment for the removal of microplastics from drinking water

This study examines the removal of microplastics and other anthropogenic particles (>10 mu m) from surface water by a full-scale conventional drinking water treatment plant. The treatment process is composed of coagulation with aluminum hydroxide, flocculation, anthracite-sand filtration, and chlorination. Samples were also collected from pilot-scale biological filters consisting of anthracite-sand or granular activated carbon (GAC) media operated with or without pre-ozonation and at a range of different empty-bed contact times (EBCTs). Particles in 10 L water samples collected in duplicate using a fully enclosed sampling apparatus were separated using sieves with 500 mu m, 300 mu m, 125 mu m, and 45 mu m openings followed by filtration through 10 mu m polycarbonate filters. Particles were counted using stereomicroscopy and characterized using mu-Raman spectroscopy. Full-scale conventional treatment removed 52 % of anthropogenic particles when comparing raw (42 +/- 18 particles/L) and finished water (20 +/- 8 particles/L). Coagulation, flocculation, and sedimentation accounted for the highest removal (70 %) of any individual unit process. Overall removal was reduced to 52 %, the difference being attributed to airborne particle deposition that occurred while water was detained in a clearwell (exposed to atmosphere via ventilation) that was used to achieve the required contact time for disinfection. The majority of the particles (>80 %) were identified as fibers 10-45 mu m; microplastics were predominantly composed of polyester while the non-plastic anthropogenic particles were primarily cellulose. None of the pilot filter configurations examined resulted in significantly fewer microplastics when compared to full-scale conventional filtration. This study illustrates that the removal efficiency of conventional treatment may be limited when considering microfibers <45 mu m in size.