Microplastics characterization in wastewater sludge treatment line optimizing the pretreatment and the examination steps

In this study, we present a novel methodology to reduce the interference of organic matter in the analysis, counting, and characterization of microplastics (MPs) in the sludge line of a wastewater treatment plant (WWTP). By tracking easily monitored laboratory variables, it was determined that colorimetry effectively monitored the oxidation state of organic matter in samples, a necessary pretreatment step for subsequent sample handling. A 3D-printed piece, adapted for a filtration system, was utilized for the first time, facilitating subsequent visual scanning and Raman spectroscopy. The proposed methodology, including the oxidation time required to decrease the colour index of the sample to 1, was applied to six samples from various points in the WWTP sludge treatment line. This colour tracking allowed us to determine if the organic matter was sufficiently oxidized so that it did not interfere with MPs analysis. Key findings include a high presence of polyethylene in all analysed samples and a significant presence of polypropylene. The study also noted a majority presence of light MPs, although this percentage decreases as the process progresses along the sludge line. Additionally, a distribution of smaller MPs (less than 200 µm) was observed in the primary sludge liquor stream, with particles smaller than 100 µm prevalent in concentrated mixed sludge, indicating fractionation. In conclusion, this methodology accelerates the characterization of MPs in wastewater sludge samples without compromising accuracy, contributing to the future standardization of this essential process in the growing field of MP research.