No seasonal differences in the emission of microplastics from an urban wastewater treatment plant on the Southern Coast of the Caspian Sea

This paper is the first to report on the role of a wastewater treatment plant (WWTP) in Sari, as a source of microplastics (MPs) in the Caspian Sea. Composite 270-liter/24-hour samples were taken the treated effluent of the WWTP in winter and spring, two seasons with different levels of human activity. The effluent contained 380±52.5 and 423±44.9 MPs/m³ in winter and spring, respectively, with the total numbers of MPs/m³ not differing between the two seasons. The dominant type of MPs in the effluent was microfibers with 237±68.7 and 328±33.4 per m³ in winter and spring, respectively. In both seasons, fiber sizes of <300 μm were the most abundant in comparison to larger sizes. For the microparticles, sizes of <300 and ≥500 μm were the most abundant in spring and winter, respectively. The predominant types of fibers and particles were polyester and polyethylene, respectively, likely originating from the washing of synthetic clothing and from microbeads in toothpaste and cosmetics. The results of this study show that the Sari wastewater treatment plant is an important source of MP release to the aquatic environment and the difference in sampling time has no significant effect on the amount of microplastics released.