Towards a Better Understanding of the Contribution of Wastewater Treatment Plants to the Microplastic Pollution in Receiving Waterways.

Microplastics (MPs; 1 µm - 5 mm), are ubiquitous in daily-use products and regularly end up in the wastewater. The main part of the wastewater is treated in wastewater treatment plants (WWTPs) which allow for at least partial removal of MP. The research aimed to understand the contribution of domestic wastewater on MP pollution in Flanders (Belgium) via two main discharge routes of MPs: (1) the effluent and (2) removed fractions. Furthermore the effect of effluent discharge on the microplastic contamination in the waterway was studied in both surface water and sediment samples of upstream and downstream locations of a discharge of three WWTPs. On average, 12.64 ± 20.20 MP per L enter a WWTP (10 µm - 5 mm). The effluent contained on average 0.41 ± 0.91 MP per L, resulting in an average removal efficiency of 97.46 ± 2.33 %, which is comparable with various (non-)European countries. Removal efficiencies are both polymer- and size-specific and data suggest that smaller particles are less efficiently removed from the wastewater, which also causes an increased input of smaller particles in the environment. The sludge is the most efficient treatment process to remove MPs. Despite the high removal efficiencies, still 1.11 x 10 7 ± 3.07 x 10 7 MPs end up in the nearby waterway daily. Nonetheless, based on the results gathered in this study, this does not seem to impact the MP concentration in the waterway significantly. In summary, this research offers a holistic approach in the research on the impact of wastewater on the microplastic pollution in the ecosystem, integrating different discharge routes and measuring the impact on environmental microplastic pollution. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2022;00:0-0. © 2022 SETAC.