Wastewater treatment plants as a pathway for the release of microplastics into the environment: Investigation of sludge and treated effluent of Sari wastewater treatment plant

The widespread presence and problems of microplastics in the environment have attracted the attention of many researchers. Urban wastewater treatment plants have been one of their important releasing sources. In this study, Sari municipal wastewater treatment plant was investigated as one of the potential sources of contaminating aquatic ecosystems (Tajan River and the Caspian Sea) and terrestrial. The final effluent and sludge samples were taken in 3 replications, in May and June 2018. The effluent samples were passed through 500, 300 and 37 μm steel sieve. Organic matter in the sludge and effluent samples were digested using hydrogen peroxide and then the microplastics were extracted by density-based separation using sodium iodide salt and examined by stereomicroscope and micro-Raman. The average number of microplastics in the effluent was 423/4 per m3, of which more than 77% were fiber and the predominant size of the microplastics was 37-300 μm. Also, sewage sludge had an average 128/7 microplastic per g(d.w), and fiber (87/5%) was the most common shape of microplastics in it. Furthermore, the dominant types of fibers and particles in the samples were polyester (effluent 40% and sludge 59%) and polyethylene (effluent 73% and sludge 68%), respectively, which are mainly obtained from the wastewater from washing clothes and microbeads used in cosmetics. Therefore, considering the amount of microplastics present in the effluent and sludge, Sari wastewater treatment plant can be considered as one of the most important sources of microplastics in the aquatic and terrestrial ecosystem in the north of the country.