Digestate from Biowaste and Sewage Sludge as Carriers of Microplastic into the Environment: Case Study of a Thermophilic Biogas Plant in Ostrobothnia, Finland

Microplastic contamination in wastewater treatment plants (WWTPs) is well documented, but only a few studies have investigated plastic pollutants in waste fractions from biogas plants. We have analysed microplastic contaminants in reject water and digestate from biowaste digestion (at three time points) and sewage sludge digestion (at two time points). Samples were collected from a thermophilic biogas plant in Ostrobothnia in the western part of Finland. Plastic particles in the samples were identified with µ-FTIR (Fourier transform infrared) spectroscopy or attenuated total reflection (ATR)-FTIR, and the grain sizes of the investigated particles were > 40 µm and < 5 mm. All investigated sample matrices contained microplastic particles, but the numbers differed significantly at each sampling time. In reject water from biowaste digestion, the numbers of microplastic particles were between 80 and 3320/L. In digestate from biowaste digestion, the number of microplastic particles was between 2290 and 7200/L. Only 20–30 microplastic particles per litre were found in reject water from sewage sludge digestion, but in the digestate, the quantity of microplastic particles was between 1870 and 36,800/L. Seventeen different polymer types were identified from the samples all together. Our results show that digestate from biowaste and sewage sludge digestion can both be carriers of microplastic into the environment and can contain higher numbers of microplastics at specific time.