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

Microplastics (MPs) enter rivers directly from urban wastewater discharges, however, their residence and distribution and their role as potential vectors for harmful microbial communities remains poorly understood. This study characterises the presence of a range of MPs types and their associated microbial communities originating from a wastewater treatment plant (WWTP) and riverine sources in the macrotidal Conwy estuary, UK. Monthly water samples were collected over a period of fifteen months from the WWTP effluent pipe and a downstream river site. Samples were analysed using advanced laser direct infrared (LDIR) spectroscopy to characterise MPs based on their size, shape, and polymer type. Molecular techniques were used to identify microbial communities present in the water samples that may become associated with MPs. MPs were present at both sampling locations ranging in size between 0-500 µm and a total of 14 polymer types were detected throughout the survey, but with generally a higher concentration by the WWTP (total of 9199 MPs were identified across the campaign at the effluent pipe compared with 2056 MPs downstream). Seasonal variations in MPs abundance overall were not influenced by an increase in river and effluent flow rate, river depth nor rainfall. The associated water samples were found to have diverse microbial communities, including a range of human pathogens and antibiotic-resistant genes. The study highlights the challenges in effectively removing MPs during wastewater treatment and the subsequent discharge into riverine and estuarine environments. It emphasises the need for standardised methods to quantify MPs accurately. It provides valuable insights into the extent of MPs contamination, their environmental fate, and their role as potential vectors for harmful microbial communities. This knowledge is essential for developing informed water management strategies to mitigate the impacts of MPs on marine ecosystems and public health.