A Comprehensive Modeling of Microplastic Emission from Wastewater Treatment Plants to the Sea via Rivers in China

Wastewater treatment plants (WWTPs) are significant sources of microplastic (MP) emissions. In order to quantify the potential MP emission from WWTPs, a database of more than 10,000 WWTPs in China with an estimated MP emission rate was built. The MP riverine retention after emission was also estimated based on Stokes' law for both fragments and fibers. The results show that the mass of MPs discharged from WWTPs is (4.48 ± 7.48) × 107 kg, yet the amount of MPs released from untreated sewage is (4.55 ± 3.00) × 107 kg, which is comparable to that of treated wastewater, indicating the capacity of wastewater treatment should be further strengthened. The flux of MPs into the sea after river interception is between 4.08 × 107 kg and 8.20 × 107 kg with 14–27% being directly discharged to the sea within 20 km from the shore.