Microplastics in wastewater treatment systems and receiving waters

Blair Espinoza, Reina Maricela (2019) Microplastics in wastewater treatment systems and receiving waters. PhD thesis, University of Glasgow.

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Plastic pollution is a problem of global scale and will increase as synthetic polymers continue to be produced, used, and discarded. Microplastic (MPs, <5 mm in size) pollution is of increasing concern, because this is estimated to account for more than 92% of global plastic counts and expected to present risks to aquatic fauna and humans. Often, MPs are too small to be seen and are unevenly distributed in the environment due to differences in shape, size, and density, rendering them difficult to find and quantify in environmental samples. Hence, adequate quantitative and qualitative assessment of these materials remains scarce, particularly in freshwaters and wastewaters that remain largely unexplored. However, both systems receive and transport different types of anthropogenic waste, including MPs, so warrant further attention for identification of mitigation strategies.

The purpose of this research was to fill gaps in knowledge of the role of freshwater rivers and
wastewater treatment plants (WWTPs) as transport vectors of MPs to the environment, and generate incisive understanding of the distribution and behaviour of MPs in these systems. These research findings are expected to be relevant to stakeholders and regulators as they can aid in the identification of priority areas for further research, monitoring, and regulation of MP pollution. Therefore, this research focussed on the abundance and distribution of MPs (60-2800 μm) in urban fresh- and wastewater systems in a river whose catchment contains a large city: the River Clyde catchment (4000 km2) in the city of Glasgow, Scotland. First, liquid fractions were sampled at eight treatment stage points within a tertiary WWTP with 184,500 population equivalents and receiving a mix of household and trade effluent. Then, sediment and water samples were collected in the recipient river, the River Clyde, upstream and downstream from the WWTP. In addition, sediment samples were collected from another nearby freshwater river, the River Kelvin, which also drains through Glasgow and the Clyde at its estuary. The overall aim of this research was to assess the extent of MP pollution in these systems and the distribution, transport, and possible fate of primary and secondary types of
MPs. Microplastics were separated from their environmental matrix using the widespread protocols of density separation, hydrogen peroxide oxidation of labile organics, and filtration. Particles were identified by visual sorting followed by chemical confirmation of plastics.

Microplastics were ubiquitous and present in all water bodies in varying quantities: 161-432 MPs kg-1 in the River Kelvin tributary, <1-13 MPs L-1 in the WWTP, 1-26 MPs kg-1 in River Clyde sediment, and 0-4 MPs 24 L-1 in River Clyde water. The WWTP displayed high efficiency, removing 96% of incoming pieces, with the majority removed by the primary treatment stage. However, at least one fibre was observed in treated effluent and this may represent daily discharges of at least 12 million particles to the River Clyde from this WWTP. Total MP concentrations in sediment and water samples of the recipient river were higher in the most downstream site compared to the upstream point furthest from the effluent pipe. Fibre concentrations were higher in downstream sediment samples that may indicate some retention in rivers by sedimentation processes – this is supported by the high abundance of fibres in River Kelvin sediments. Fragments were abundant in the main river sediments in similar concentrations across upstream and downstream sites, suggesting these are more
likely to be introduced from diffuse sources via surface runoff and in-stream transport. The
comparable concentrations observed across sampling events for each of these systems suggest a continuous input of MPs from their source to the environment.

For further insight into the relative distribution of primary and secondary MPs and their potential sources, it is necessary to confirm material composition of these particles. A subset of specimens extracted from wastewater (5%), Clyde sediment (15%), and Clyde water (56%) were analysed by Fourier transform infrared spectroscopy (FTIR) for this assessment. Secondary MPs especially fibres were predominant, while primary MPs that have received the most media and public attention and prompted plastic and MP strategies, were lowest in concentration. Polypropylene (PP) was the most detected polymer across all analysed particles and was mainly present in the form of fibres and fragments. Polyester and nylon fibres that may be expected in high abundances in wastewater appeared absent in the WWTP in this study, although this was concluded mainly due to size limitations of the characterisation method. However, the PP fibres in wastewater could originate from
sanitary products, medical applications, thermal clothing, and construction materials. This is
important as fibres are often linked to washing machine effluent and currently little information from alternate sources for this type of MPs exists. In River Clyde sediment, fibres identified as polyethylene terephthalate were observed and concluded to originate mainly from fishing gear, based on combined assessment of chemical and visual properties.

Understanding the causes and significance of MP pollution is a new but expanding area of water research. It was important to share these research findings with the community and so this research was published when possible. This thesis is thus constructed from a series of published and unpublished papers.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Supported by funding from the Hydro Nation Scholarship Programme.
Keywords: Microplastics, wastewater, freshwater, rivers, fibres, FTIR.
Subjects: Q Science > Q Science (General)
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Waldron, Professor Susan and Gauchotte-Lindsay, Dr. Caroline
Date of Award: 2019
Depositing User: Dr. Reina Blair
Unique ID: glathesis:2019-76781
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 11:06
Last Modified: 31 May 2022 09:54
Thesis DOI: 10.5525/gla.thesis.76781
URI: https://theses.gla.ac.uk/id/eprint/76781
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