Mass quantification of microplastic at wastewater treatment plants by pyrolysis-gas chromatography-mass spectrometry

Abstract

Municipal wastewater treatment plants (WWTPs) are a central point of collection of plastic particles from households and industry and for their re-distribution into the environment. Existing studies evaluating levels of plastics in WWTPs, and their removal rates have reported and used data on polymer type, size, shape, colour, and number of plastic particles, while the total mass concentration of plastic particles (especially >1 μm) remains unclear and unknown. To address this knowledge gap, raw influent, effluent, and reference water samples from three WWTPs in Australia were collected to analyse the mass concentrations and removal rates of seven common plastics (>1 μm in size) across the treatment schemes. Quantitative analysis was performed by pressurized liquid extraction followed by pyrolysis coupled to gas chromatography mass spectrometry. Results showed that the total plastic content in the WWTPs raw influent samples was between 840 and 3116 μg/L, resulting in an inflow of between about 2.1 and 196.4 kg/day of the total measured plastics. Overall, >99 % by mass of the plastics entering the three WWTPs from the raw influent was removed during the pre-treatment stages, presumably ending up in the sewage sludge, which means emissions (via treated effluent) from the treatment plants are low. Compared with the raw influent, the plastic mass concentrations in the treated effluents (i.e., Class C, A, and final effluent) from the three WWTPs, as well as the reference water samples within their catchments were below the limits of reporting. Of the five quantified plastic types, polyethylene (PE, 76.4 %), and polyvinylchloride (PVC, 21 %) dominated by mass, while polyethylene terephthalate (PET, 1.9 %), polypropylene (PP, 0.4 %) and polymethyl methacrylate (PMMA, 0.3 %) accounted for a small proportion of the total. Overall, this study investigated the mass concentrations of plastic particles above 1 μm in wastewater and their removal, which provided valuable information regarding the pollution level and distribution characteristics of plastic polymers in Australian WWTPs.

Keywords: Microplastic mass concentrations; Plastics; Pyr-GC/MS; Removal efficiency; Wastewater treatment.