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Wastewater treatment plants as a source of microplastics to the environment.
Summary
This study was the first to characterize the contribution of a wastewater treatment plant to microplastic pollution in a receiving water body, confirming that plants are significant sources of microplastics despite removing the majority of particles. The findings highlight the need to upgrade treatment processes or implement new technologies specifically targeting microplastic removal.
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 to the environment, but their degree of contribution worldwide as a source is unknown. WWTPs are not designed to remove microplastics from sewage. This study is the first of its kind to characterise the contribution of microplastics to coastal ecosystems from different types of WWTPs in Canterbury. A method was developed to extract and identify microplastics present in sewage influent and effluent. A field study of four WWTPs was undertaken in the Canterbury region. 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. Potential microplastics were visually identified, and their polymer type classified by Fourier transform infrared spectroscopy (FTIR). Microplastics were characterized from each of these WWTPs in terms of abundance, morphotype, size, and polymer type. Microplastics were detected in the incoming influent and treated effluent and were more abundant in the influent (67%) than effluent (33%) which was consistent with the literature. Low removal efficiencies from each WWTP were detected in comparison to the literature. An additional temporal field study focusing on microplastic differences in the effluent from three WWTPs did not find a consistent trend. It can be concluded that microplastics are present in WWTPs and are a significant source of microplastics through effluent discharged to coastal environments in Canterbury. Further work will be required to understand the environmental fate of the discharged microplastics to the Canterbury coastline and environmental impacts.