Microplastics in Sewage Sludge: A Known but Underrated Pathway in Wastewater Treatment Plants

Interest in the presence of microplastics (MPs) in wastewater has grown significantly in recent years. In fact, wastewater treatment plants (WWTPs) represent the last barrier before the discharge of MPs into an aquatic ecosystem. The research has highlighted how MPs are in part effectively removed from the waters and accumulated inside the sewage sludge (SeS) produced by the WWTP, being a cause for concern, especially in the case of agricultural reuse. This work aims to analyze the existing literature on the (i) methodical procedure for MPs analysis (thermal, spectroscopic, optical analyses), (ii) qualitative and quantitative presence of MPs in SeS, (iii) effect on sludge properties, and (iv) the possible accumulation in amended soils. Based on the results already obtained in the literature, this work aims to provide critical insights to stimulate interest in the topic and direct future research on aspects that should be deepened. In particular, it emerges that there is a clear need for standardization of the collection methods and the analytical techniques for identifying and quantifying MPs, since their physico-chemical characterization and the study on aging and on the response towards acid or basic pre-treatments are fundamental for the understanding of microplastics ecotoxicological potential.