| dc.description.abstract | Wastewater treatment works have been identified as an important pathway for the release of microplastic pollution into the environment. The discharge of treated wastewater (effluent) can lead to the point source release of microplastic pollution into aquatic environments, while biosolid application to agricultural soils can lead to the diffuse distribution of microplastics into terrestrial environments. The release of microplastics via water industry operations has been identified as a key focus area for future investigation, with further understanding of sources, pathways and sinks of microplastics in wastewater and sludge required. Further research on the fate and behaviour of microplastics across the sludge treatment process is needed, and there is a lack of evidence on how microplastics alter in abundance and characteristics in wastewater treatment works. This thesis investigated the occurrence, fate, and behaviour of microplastics in wastewater treatment works and identified pollution pathways and microplastic loads released into the environment. The abundance of microplastics in the wastewater influent and effluent was established for one site in Devon, Southwest England, along with the removal efficiency and microplastic load released via effluent discharge. The fate and distribution of microplastics across the whole sludge treatment stream after different treatment processes, and in two biosolid products produced at the same site, located in Devon, was investigated. The microplastic load released to agricultural soils via biosolid application from two sites was determined. In this thesis, an average of 27.8 microplastics (MPs)/L were detected in wastewater influent, and while 98% of microplates were removed following the tertiary treatment process when considering the average daily flow rate, 19 million microplastics were still discharged daily into receiving waters. A preliminary investigation on microplastics in sludge and biosolids identified variations in microplastic abundances after different treatment processes with concentrations ranging from 27.7 – 286.5 MPs/g (dry weight of sludge) (dw). Microplastic concentrations of 37.7 MPs/g (dw) and 97.2 MPs/g (dw) were detected in the lime-stabilised and anaerobically digested biosolid products respectively. A significant difference in microplastic abundance was further detected between the anaerobically digested biosolid (429 MPs/g (dw)) and lime-stabilised biosolid (38.4 MPs/g) in a more comprehensive investigation. The site produced a monthly average of 1000 tonnes of sludge, resulting in 69 billion microplastics entering the biosolids each month, which then enter the terrestrial environment when the biosolids are recycled to agricultural soils as fertiliser. The data identified peaks in microplastic abundances after different treatment processes, with microplastics accumulating after the anaerobic digestion and secondary digestion process, while a loss of microplastics was detected following the dewatering of the digested sludge. For a second wastewater treatment works, also located in Devon about 15 miles from the first site, a mean concentration of 50.6 MPs/g (dw) was identified in the lime-stabilised biosolids, leading to 19 billion microplastics entering the biosolids every month. A systematic review established the variation in microplastic abundance and characteristics in literature across the world with a huge variation in microplastic concentrations reported between studies. The extent to which biosolid application pollutes the terrestrial environment on a national scale was established for sixteen countries, with between 8.2 ×1010 and 1.29 ×1015 MPs/year estimated to reach fields via biosolid application. The presence of microplastics in biosolids puts a valuable source of nutrients and organic matter at risk. The environmental benefits of recycling biosolids to land are now brought into question with the increasing evidence of high concentrations of microplastics in the product, and the lack of current technology for the separation of these pollutants from sludge and biosolids | en_GB |
