Seasonal variation in microplastic abundance, morphology, polymers, and adsorbed contaminants in wastewater treatment sludges and biosolids

This study investigated the seasonal impact on microplastic (MP) characteristics and concentration in the sludge streams (primary, secondary (waste activated) and digested) at a municipal wastewater treatment plant (WWTP) in Ontario. MPs were recovered after digestion and density separation of primary, secondary and digested sludge samples and were quantified and characterized (shape, colour, polymer type, and chemical contaminants) using microscopy and FTIR. The primary sludge had the highest number of MP particles among the three sludges, regardless of the season. The results show that sludge samples collected during the spring season had the highest MP concentrations (1.5–2.5 times higher), along with increased levels of tire and road wear particles (TRWPs), attributed to snowmelt and stormwater. Black and blue were the frequently recorded MP colours, with no discernible seasonal trends among different sludge types. Fibres remained the predominant MP type irrespective of the season and the sludge type. FTIR analysis of the 20–30% subset of MPs also revealed that PET was the most common polymer, with significant amounts of PA, PE, and PP recorded. There were no significant seasonal changes in the polymer type of the MPs. The spectral analysis further revealed the presence of a range of pharmaceutical drugs, industrial chemicals, and emerging contaminants on MPs, often obscuring the polymer identification while highlighting the role of MPs as carriers of environmental contaminants. Seasonal dynamics play a crucial role in assessing the emission of MPs into aquatic and terrestrial ecosystems. Understanding these fluctuations enables the development of more targeted and effective intervention strategies that are responsive to periods of heightened risk and that mitigate their environmental impacts.