Tracking changes in dissolved natural organic matter composition

During the last decades, the abundance of dissolved organic matter (DOM), a complex heterogeneous mixture of organic compounds, has increased in many surface waters in Northern Europe and North America. Surface waters are widely used as raw water sources for drinking water production. Increasing DOM is problematic for water treatment plants (WTPs) due to e.g. increased coagulant demand and because its constituents act as precursors for potentially harmful disinfection by-products. Many WTPs employing conventional treatment are currently struggling to maintain sufficient DOM removal, and are considering additional novel techniques. In this thesis, changes in the DOM composition were investigated during i) conventional water treatment and disinfection practices, ii) pilot scale studies of novel processes (nanofiltration and ion exchange), and iii) transport in a large Swedish boreal lake. Analytical techniques used include common spectroscopic methods (absorbance and fluorescence), and more advanced molecular level techniques (ultra-high resolution mass spectrometry). Results show that coagulation was highly selective towards oxidized DOM components, and that the removed fraction resembled terrestrial endmembers. Meanwhile, DOM with microbial endmember characteristics, which correlated with chemically reduced DOM components, was especially reactive during slow sand filtration. After disinfection, 499 different chlorine-containing by-products were identified, of which many had not been reported in the literature earlier. Both nanofiltration and ion exchange removed more DOM than coagulation, and components with a wider range of chemical properties. Both techniques were, however, most efficiently removing terrestrial DOM. In the studied large boreal lake, terrestrial, aromatic compounds were lost over time, while no significant net in-lake production occurred. Decreasing components overlapped largely with those removed during coagulation treatment at WTPs, indicating that coagulation had contributed significantly to DOM removal within the lake, especially considering a concurrent loss of dissolved iron.