Factors affecting microplastic retention and emission by a wastewater treatment plant on the southern coast of Caspian Sea

Understanding how wastewater treatment plants (WWTPs) process microplastics (MPs) will help informing management practices to reduce MP emissions to the environment. We show that composite 24 h samples taken at three replications from the outflow of the grit chamber, primary settling tank and clarifier of the WWTP of Sari City, on the southern coast of the Caspian Sea, contained 12667 ± 668, 3514 ± 543 and 423 ± 44.9 MP/m³, respectively. Fibers accounted for 94.9%, 89.9% and 77.5% of the total number of MPs, respectively. The MP removal efficiency was 96.7%. MP shape (fiber, particle), size and structure were the most important factors determining their removal in different steps of the wastewater treatment process. The structure of microfibers (polyester, acrylic and nylon) and the consequent higher density than water explained their high removal (72.3%) in the primary settling tank. However, size was more important in microparticle removal with particles ≥500 μm being removed in the primary settling tank and <500 μm in the clarifier unit. The smallest particles (37–300 μm) showed the lowest removal efficiency. The predominant types of fibers and particles were polyester and polyethylene, respectively, which are likely to originate from the washing of synthetic textiles and from microbeads in toothpaste and cosmetics. Despite the efficiency of the Sari WWTP in removing MPs, it remains a major emission source of MPs to the Caspian Sea due to its high daily discharge load.