Microplastics in an anaerobic digester treating sewage sludge: Occurrence and factors affecting their identification with Raman spectroscopy

Significant share of microplastics entering wastewater treatment is accumulated in sewage sludge, which is commonly treated by anaerobic digestion. The digestate is typically separated to solid and liquid fractions, and the solid fraction is often land-applied. However, the division of microplastics between these fractions is poorly understood. Thus, we investigated the occurrence of microplastics (>20 μm) at an anaerobic digester and the subsequent solid-liquid separation. Sewage sludge and digestate exhibited 7600 ± 6800 and 7200 ± 1100 microplastics per g dry weight, respectively, indicating that anaerobic digestion can decrease the fluctuation in microplastics' occurrence in sewage sludge. The microplastic flow was predominantly directed to the solid fraction, while the liquid fraction carried about 1 % of the microplastics, mostly polymers with low density (polyethylene and polypropylene). We also investigated factors affecting microplastics' identification by Raman spectroscopy. Microplastic degradation and presence of organic matter after oxidative digestion in the sample preparation, and anaerobic digestion of sewage sludge affected the spectra of different microplastics. For polyethylene and polypropylene, these changes did not interfere with recognizing their characteristic peaks, allowing high-certainty identification. In contrast, polyamide identification was notably compromised as signal-to-noise ratio can be deteriorated and natural organic matter can be misidentified as polyamide.