The abundance, removal efficiency, and characteristics of microplastics in three urban wastewater treatment plants (WWTPs) on the southern coast of the Caspian Sea

Wastewater treatment plants (WWTPs) are one of the major collection points of microplastics (MPs). The MPs in influents and effluents of WWTPs were assessed for three cities on the southern coast of the Caspian Sea in the winter and spring seasons. The MP removal rate of WWTPs ranged between 71.12 and 88.13% depending on the season and treatment methods. The higher removal efficiency of MPs in the city of Sari compared to that in the other two cities might be attributed to the implementation of more effective treatment techniques at the Sari WWTP. Our findings showed that the presence of MPs in influents was greater in the spring than in the winter, attributed to the increased usage of personal care and washing products during the COVID-19 lockdown period. Conversely, in effluents, the abundance of MPs was higher in the winter than in the spring, likely due to precipitation and inadequate management of sewage overflows. On average, more than 50% of identified MPs had black/grey and white/transparent colours. The majority of the identified MPs that were found in both the influents and effluents of all three investigated WWTPs were smaller than 500 µm and had a fibre shape. Our findings indicate that removing plastics with a size smaller than 100 µm is more challenging compared to larger-sized plastics in WWTPs. Moreover, a strong correlation was observed between TSS and the abundance of MPs in influents. The predominant types of MP that were identified with spectroscopic analysis in most samples were polyamide (PA), acrylic, polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), and polypropylene (PP). Based on the results of our study, it can be estimated that a total of 132 × 10⁶ and 223 × 10⁶ MPs are discharged per day from the three investigated WWTPs into aquatic environments that ultimately reach the Caspian Sea in the spring and winter, respectively. Our findings enhance our knowledge about the load of MPs from WWTPs into the Caspian Sea, and it provides a foundation for future discussions on management strategies and the reuse of wastewater.