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

Microplastics (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 microplastics. The present study aimed to understand the contribution of domestic wastewater to microplastic pollution in Flanders (Belgium) via two main discharge routes of microplastics: (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 the discharge from three WWTPs. On average, 12.64 ± 20.20 microplastic/L entered a WWTP (10 µm-5 mm). The effluent contained on average 0.41 ± 0.91 microplastic/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 to the environment. The sludge is the most efficient treatment process to remove microplastics. Despite the high removal efficiencies, still 1.11 × 10⁷ ± 3.07 × 10⁷ microplastics end up in the nearby waterway daily. Nonetheless, based on the results gathered in the present study, this does not seem to impact the microplastic concentration in the waterway significantly. In summary, the present study offers a holistic approach in the research on the impact of wastewater on microplastic pollution in the ecosystem, integrating different discharge routes and measuring the impact on environmental microplastic pollution. Environ Toxicol Chem 2023;42:642-654. © 2022 SETAC.