Water treatment plants are responsible for producing and supplying quality drinking water. To achieve this, various treatments are combined to eliminate contaminants and dissolved organic matter and deactivate pathogens that may compromise human health. A critical operation in the water treatment process is the final disinfection, as it must ensure that water is free from microbiological hazards from the moment it leaves the plant’s reservoirs until it reaches the taps. This is typically done using chlorine-derived oxidants (hypochlorite, chloramines, or chlorine dioxide). However, the addition of these disinfectants can lead to unintended consequences. Despite their contribution to controlling microbiological risks, excessive dosing reacts with any remaining dissolved organic matter (DOM) and forms disinfection by-products (DBPs), an extensive family of contaminants with varying degrees of toxicity.

To this day, the main challenge for plant managers and operators is to strike a balance between ensuring microbiological safety and avoiding the toxicity derived from disinfection by-products. To effectively monitor water quality throughout the treatment process, it is necessary to have markers to track the removal of organic matter, one of the main precursors of DBPs. However, the high complexity of the water treated (heterogeneous composition, subject to seasonal changes, and site-specificity of the water source) means that it acts as a black box, typically monitored using surrogate parameters reflecting some of its general physicochemical characteristics such as total organic carbon (TOC) content or absorbance. In order to optimize organic matter removal processes in water treatment and thus minimize DBP generation, it is crucial to establish specific markers for the reactivity of organic matter towards DBP formation. This requires a detailed study of the characteristics of the organic matter present in the waters to be treated.

This work proposes, as a first step, the development of an advanced characterization method based on high-performance size exclusion chromatography (HPSEC) that allows the separation of the main compounds in a complex mixture such as DOM based on their apparent molecular weight. This separation is combined with a diode array detector (DAD) and a dissolved organic carbon detector (OCD) to determine the spectroscopic absorbance signature of the compounds across the UV-VIS range, relative to their organic carbon content. The thesis’ methodology included literature review, experimental work (water sampling and analysis at four different drinking water treatment plants in northeastern Catalonia), and the application of several statistical techniques (from Pearson correlation analysis to multiple linear regression models).

Main results are:

• The establishment of a small database to analyze in depth the characteristics of the main DOM fractions.
• The proposal of various specific markers of organic matter attributes, including DOC content and various spectroscopic parameters (wavelengths, spectroscopic slopes, and differential absorbance) to study their relationship with the formation of regulated organic disinfection by-products: trihalomethanes and haloacetic acids. Based on the results obtained through the statistical analysis, this thesis advocates for the development of spectroscopic methodologies, as they can serve to close the gap between advanced knowledge generated from exhaustive offline analysis and simple and economical online monitoring.
• The contrast of information obtained from characterization using the HPSEC-DAD-OCD with other advanced characterization techniques such as fluorescence and high-resolution mass spectrometry (HRMS).
• Evaluation of the efficiency of the main conventional treatment processes and advanced treatments through a literature review and integration of knowledge derived from case studies.
• Proposal of application guidelines for implementation in water treatment plants to help mitigate and minimize DBP formation in accordance with current legislation.

These findings and developments contribute to United Nations Sustainable Development Goal 6, which aims to ensure the availability and sustainable management of water and sanitation for all). Meritxell Valentí’s doctoral thesis was supervised by Dr Maria Martín (University of Girona, Laboratory of Chemical and Environmental Engineering), Dr Pepus Daunis (University of Girona, research group in Statistics and Compositional Data) and Dr. Maria José Farré (Catalan Institute of Water Research, Water Quality area).

Publications:

• NOM fractionation by HPSEC-DAD-OCD for predicting trihalomethane disinfection by-product formation potential in full-scale drinking water treatment plants. Meritxell Valenti-Quiroga, Pepus Daunis-i-Estadella, Pere Emiliano, FernandoValero, Maria J.Martin, Water Research, Volume 227, 1 December 2022, 119314. https://doi.org/10.1016/j.watres.2022.119314
• Upgrading water treatment trains to comply with the disinfection by-products standards introduced by the Directive (EU) 2020/2184. Meritxell Valenti-Quiroga, Maria José Farré, Paolo Roccaro. Current Opinion in Environmental Science & Health, Volume 39, June 2024, 100547. https://doi.org/10.1016/j.coesh.2024.100547