A wealth of studies in microplastic research has been centred on marine habitats due to concerns arising from the potential eco-toxicological implications that this new category of emerging pollutants could have on aquatic species. In comparison, the body of knowledge in terrestrial and freshwater ecosystems is limited. Wastewater treatment plants have received attention because they have been identified as conduits of microplastics in the environment. To date, limited work has been undertaken worldwide with respect to microplastic transport in different wastewater treatments.
This thesis had the overall aim to assess whether wastewater treatment plants act as a point and/or diffuse source of microplastics via final effluent and sludge, which they do. This was achieved by investigating and comparing microplastics removal and fate in three sites featuring different technologies: a trickling filter, an activated sludge process and a membrane bioreactor. Removal efficiencies were above 90% and the ultrafiltration system was found to outperform the other technologies with a removal close to 100%.
This study estimated that a daily average of 48.1 billion microplastics entered the three sewer networks investigated of which, depending on treatment type, between 0.1 and 8.8% were detected in the final effluent. Therefore, the majority of particles were transferred to the sludge; in the UK, 80% of all sludge produced is used as fertiliser and applied on agricultural soils. Another knowledge gap addressed was the investigation of seasonality and its impacts on particle counts, sizes and polymers.
One of the main contributions to knowledge of this thesis was in the analysis of
seasonality by accounting for flow rate; this is of critical importance when microplastic loading emissions are to be estimated. This study estimated that an average of 5.6 million microplastics were emitted to the environment annually per person from the sewers investigated. With respect to polymers, polypropylene, polyamide, nitrile rubber and the acrylate-polyurethane-varnish cluster dominated the influents; whereas the final effluent profile slightly varied based on treatment type, but it mainly featured polypropylene, polyamide, ethylene propylene diene monomer rubber and the acrylate-polyurethane-varnish cluster.
Finally, the lack of standardised methodologies in this novel field of research hampers the comparison across studies and limits its advancement. A method optimisation for organic matter removal has been presented for the detection of smaller-sized microplastics and which was shown to be inexpensive, reliable and suitable for monitoring purposes. The results of this research have demonstrated that wastewater treatment plants represent a pathway for the dispersion of microplastics in the aquatic and terrestrial environments through effluent outfalls and biosolid applications, respectively. The quantification of microplastics in the environment is a high priority to enable the risks to be assessed and better target mitigation strategies and policy interventions.
|Date of Award||9 Jan 2023|
|Supervisor||Fay Couceiro (Supervisor), John Williams (Supervisor) & Michelle Hale (Supervisor)|