Integrated Assessment of Digestion Methods, Analytical Techniques, and Sampling Strategies for Microplastics Characterization in Municipal Wastewater Influent

High Resolution Image Download MS PowerPoint Slide Microplastics entering wastewater treatment plants (WWTPs) are challenging to quantify due to the presence of complex organic matter. This study evaluates microplastic extraction and quantification workflows (particle size: 25–1000 μm) based on their performance, limitations, types of information, and potential biases. Among the tested extraction methods, enzymatic digestion coupled with sequential wet peroxidation provided the best balance between sample cleanliness and particle recovery (47%, for 45 μm polystyrene (PS) microspheres). This method was used for extracting microplastics from municipal wastewater influent (MWWI) samples collected from Helsinki. The extracted microplastics were quantified using Fourier transform infrared microspectroscopy (μFTIR) and pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The μFTIR quantified nonfiber particles, revealing an abundance of small-sized microplastics (<350 μm) with broadly comparable microplastic concentration in composites and grab samples. Prevalent polymer types were alkyd resin/polyester, polyethylene (PE), and polypropylene (PP). Microplastic concentration variability was observed across sequential sampling days, sampling strategies, and replicates. Py-GC/MS analysis indicated the most abundant polymer mass concentrations were not the same as the most abundant particle number concentrations as identified by μFTIR, highlighting the complementary nature of these methods. Overall, this study demonstrates complementary analytical techniques are needed for comprehensiveness microplastic characterization in wastewater samples.