Titolo della tesi: Microplastics in the urban water cycle: focus on analytical determination methods and presence in drinking water treatment plants effluents
The European Union has assessed in the new Drinking Water Directive the need for the definition of an analytical protocol for microplastics (MPs) monitoring in water for human consumption by 2024. This specification stems for the awareness of the public and the authorities of MPs as a pervasive and persistent pollutant, that needs to be understood in its presence and fate to assess the associated health risk. MPs in drinking water however cannot be thought and studied without consideration of the whole urban water cycle, due to the strong interconnections between its elements.
The work presented in this thesis addresses therefore the topic of MPs presence in the urban water cycle focusing on their determination (analytical methods and data reporting), having as specific object the MPs in the effluents of drinking water treatment plants (DWTPs).
The research is also framed in the context of the integrated water management and the topic is observed considering the point of view of the water utilities too. In this context, the aim is to offer an overview according to the state of the art on the presence of MPs in the urban water cycle and the main influencing parameters, as well as to develop a method for the routine analysis of MPs in DWTPs effluents, balancing the time-cost-reliability triad.
These goals were achieved by extensive literature research on MPs presence and analytical methods. Moreover, laboratory activities were run at the facilities of Acea Elabori, society of engineering and services of Acea S.p.A, the main water utility of central Italy. Aim of the laboratory activities was the full understanding of potential and limitation of the µ-Raman technique for the routine analysis of MPs in the effluent of DWTPs, and the definition of the analytical method.
The application of a spectroscopic technique coupled to a microscope has however inner limitations, such as the lack of MPs volume and mass among the analysis outputs. These metrics are of utmost relevance for proper management of the urban water cycle by water utilities (e.g. for mass balances, source apportionment, risk assessment). To overcome these limitations, the experience abroad, carried out at the Aquatic Ecology and Water Quality Management Group of the Wageningen University and Research(WUR) (Netherlands), was devoted to the study and evaluation of methods for the estimation of MPs volume from the available outputs of spectroscopic techniques.
The findings of the research allow the consideration that MPs are continuously fed and recycled within the water cycle. The influencing factors on MPs abundance need however to be better framed at various time-space scales. Analytical methods are improving not only from a technological point of view, but also on the quality of provided data: procedures for quality assessment and control are being developed by the scientific community to frame the reliability and uncertainties of the methods, and propose data that can be critically read and evaluated on solid bases.
The µ-Raman is a strong method for MPs analysis, especially for routine analysis, due to the high level of automatization that can be reached. However, the potential and analytical limits of the µ-Raman should be considered also as a function of such an automatization: for example, the precision of the microscope stage might increase the minimum size analysable and the chemical identification of the particles should be critically evaluated.
The cost for MPs analysis with the method developed appears from preliminary evaluations to be comparable to that of other Contaminants of Emerging Concern (CECs).
The time required for the analysis depends however on the efficiency of the pre-treatments applied and the volume analysed. These two aspects need to be balanced to accommodate the need of the laboratory organization in the context of routine analysis.
The results from the application of the studied method to the effluent of a DWTP confirm the data on the presence of MPs with size >20µm reported in the scientific literature. However, their quantification was possible only in one sample of one litre over five samples total. This testifies on one hand the presence of MPs >20µm in the effluents from DWTPs, but also their low concentration, especially when evaluated per polymer group (<1 MPs/L). The method applied is considered suitable for the evaluation of the analytical procedure and the screening of DWTPs effluent. However, it needs to be modified if the goal is the complete characterization of the MPs pollution in the DWTPs effluent: samples of higher volume are in this case to be taken, and the whole procedure slightly modified. The core aspects of the analysis are however the same and the main observations and points of attention still valid. The innovation of the research lies in the definition of a screening method for MPs >20µm in effluent from drinking water plants, considering its cost and taking into account the perspective of water utilities. Further innovation lies in the approach used for the research conducted at WUR to estimate volume from data obtainable from micro-spectroscopic techniques.