Comparative Long-Term Monitoring of Microplastics in the Effluent of Three Different Wastewater Treatment Plants with Two, Three, and Four Treatment Stages

Wastewater treatment plants (WWTPs) are important point sources for microplastics (MPs) in the environment. For effective mitigation measures and regulations, it is important to monitor their release into the environment and understand the level of MPs in the WWTP effluents based on different treatment technologies. In this study, we compare the MP levels in the effluents of three different municipal WWTPs which each use a different treatment concept: a conventional three-stage WWTP, one with an additional fourth cleaning stage using powdered activated carbon, and a two-stage WWTP utilizing a membrane bioreactor (MBR). Long-term monitoring was performed on the WWTP effluents using the same standardized methods for sample collection, preparation, and detection, based on fluorescent staining. Despite the various advanced treatment processes, there are no significant differences in the resulting MP contamination in the investigation of WWTP effluents. The average MP concentrations in the effluents were 21.8 MPs/L for the conventional three-stage WWTP, 15.1 MPs/L for the four-stage WWTP, and 15.1 MPs/L for the MBR. Further, the MP contamination in all effluents shows a strong fluctuation over time. These findings highlight the need for standard MP monitoring at WWTPs, to gain a better understanding of the MP emission in different treatment processes. Further, it highlights the need for a fourth treatment stage that specifically targets MP removal to effectively prevent the MP release from WWTPs into the environment.