Wastewater treatment plants elevating microplastic abundances, ecological risks, and loads in Japanese rivers: a source-to-sink perspective.

Little were certain about how wastewater treatment plants (WWTPs) affected the Japanese riverine microplastic contamination. This study explored the influences of WWTPs on microplastic pollution, assessed ecological risks, and looked at the sources-to-sinks phenomenon (WWTPs-to-rivers-to-marine) in riverine settings in Japan's Yamaguchi prefecture. Fifty surface water samples from the five selected rivers (Koya, Saba, Shimaji, and Fushino, Nishiki) and 11 effluent samples from WWTPs in the rivers' catchment were examined. Microplastics were analyzed using filtration, wet-peroxidation, density-separation, and attenuated reflectance-Fourier transform infrared spectroscopy techniques. Results suggested that the less populated and rural river (Nishiki) was less contaminated compared to the WWTPs and urban areas affected rivers (Koya, Saba, Shimaji, and Fushino). The WWTPs increased microplastic abundance twofold in the downstream regions compared to upstream stations. Microplastic characterization showed that the smaller microplastics < 500 µm, fiber-shaped, transparent, blue, and green color particles were major. Polymer identification demonstrated that the polyethylene, polypropylene, and polyethylene terephthalate were prevalent both for the rivers and WWTP effluents. There was a significant emission of microplastics from WWTPs to rivers (4.671 billion pieces per day; 71.8 kg per day) and rivers to Seto Inland Sea (0.13/billion pieces per day/km; 7.1 kg per day). The per capita MP emissions to the rivers via WWTPs ranged from 0.02 to 6.49 g per day, which was approximately 2% of per capita single-use plastic wastes in Japan. An assessment of ecological risks showed that the WWTPs posed high ecological risks to rivers, and built up the pollution hotspots to their downstream areas by releasing higher number of microplastics and highly toxic polymers. Overall, the WWTPs influenced the rivers through both abundances and characteristics (shapes-size-color-polymers), increased the complexity of microplastic compositions as well as elevated ecological risks in the rivers. This study contributed to bridging the knowledge gaps about microplastic sources-to-sinks, ecological risks, and pollution management in Japan and beyond.