Wastewater treatment plants as a source of microplastics to the environment in New Zealand

, Ruffell Helena, Gaw Sally, Pantos Olga, Northcott Grant.

Microplastics are ubiquitous in the environment, due to the intensification in global commercial demand for plastics since the 1960s. The detection of microplastics in remote locations and in a range of aquatic organisms has raised questions about the sources of entry into the environment. Wastewater treatment plants (WWTPs) are thought to be a major source of microplastics, particularly microfibres sourced from washing machine effluent, into aquatic and terrestrial environments. WWTPs are not designed to remove microplastics from sewage, and microplastics are retained in sewage sludge or released with effluent. There is currently a lack of data in New Zealand on the amounts and types of microplastics entering and being discharged from WWTPs, and the risk they pose to the environment. This study is the first of its kind to characterise the contribution of microplastics to coastal ecosystems from different WWTPs in Canterbury, New Zealand. A field study of four tertiary WWTPs was undertaken in the Canterbury region across the month of June 2018. Representative influent and effluent samples were collected from each WWTP, comparing weekdays to weekends. Microplastics were extracted from the sewage by wet sieving, chemical digest, and vacuum filtration and identified using microscopy and Fourier transform infrared spectroscopy (FTIR). Microplastic concentrations ranged from 0.9 – 4.8 particles/L and 0.4 – 2.0 particles/L in influent and effluent respectively, fragments the dominant morphotype and polyester the most frequently detected polymer. Glitter and sponge particles were among the microplastics isolated. An additional field study focusing on microplastic differences in the effluent from three WWTPs was undertaken bi-monthly from June – December 2018 to assess temporal trends. Microplastic concentration in temporal effluent ranged from 0.2 – 2.1 particles/L. The findings of this study suggest the need for greater regulation of plastic consumer products to mitigate the risk of microplastics to the environment.

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