Microplastic and microbial community loading from a wastewater treatment plant to an estuary

Microplastics (MPs) enter rivers and estuaries directly from urban water discharges, yet their type, distribution, residence, and role as potential vectors for harmful microbial communities remain poorly understood. This study characterises the presence of different MP types and identifies the potential associated microbial communities originating from a wastewater treatment plant (WWTP) and wider catchment sources in the Conwy estuary, UK, over a 15-month sampling period. Advanced laser direct infrared (LDIR) spectroscopy was used to classify MPs by size, shape, and polymer type, while molecular techniques identified microbial communities in associated water samples. MPs were present consistently in both the effluent and the river, with concentrations highest at the WWTP effluent, where daily loads peaked at nearly 1 kg. Across the campaign, 9,199 MPs were detected at the effluent pipe and 2,008 downstream, spanning 14 polymer types—polyurethane being the most abundant. Seasonal variations in MP abundance overall appeared not to be driven directly by changes in river flow rates, water level, nor rainfall. Microbial analysis of the water samples revealed diverse communities, including human pathogens (Moraxellaceae spp.) and antibiotic resistance genes (ARG). Potential markers of sewage contamination, such as Zoogloea and Prevotella, were also identified. This study highlights the challenges in effectively removing MPs during wastewater treatment and discharge into aquatic environments. It emphasises the need for standardised methods to quantify MPs accurately. The research provides valuable insights into MP contamination, environmental fate, and the potential for MPs to harbour harmful microbial communities. These findings are essential for developing strategies to mitigate the impacts of MPs on aquatic ecosystems and public health.