Molecular level seasonality of dissolved organic matter in freshwater and its impact on drinking water treatment†

Improved characterization of dissolved organic matter (DOM) in source waters used for drinking water treatment is necessary to optimize treatment processes and obtain high drinking water quality. In this study, seasonal differences in freshwater DOM composition and associated treatment-induced changes, were investigated at four drinking water treatment plants (DWTPs) in Sweden, during all seasons and a full-year. The objective was to understand how effectively DWTPs can adapt to seasonal changes and compare how optical and mass spectrometry methods detected these changes. In addition to bulk DOM analysis, this work focused on excitation–emission matrix (EEM) fluorescence including parallel factor (PARAFAC) analysis, and molecular level non-target analysis by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Overall, seasonal variability of raw water DOM composition was small and explained primarily by changes in the contributions of DOM with aromatic and phenolic moieties, which were more prevalent during spring in two surface water sources as indicated by absorbance measurements at 254 nm, computed specific ultraviolet absorbance (SUVA) and phenol concentrations. These changes could be balanced by coagulation, resulting in seasonally stable DOM characteristics of treated water. While EEM fluorescence and PARAFAC modelling effectively revealed DOM fingerprints of the different water sources, FT-ICR MS provided new insights into treatment selectivity on DOM composition at the molecular level. Future DOM monitoring of surface waters should target more specific seasonal DOM changes, such as features with a known impact on certain treatment processes or target certain events, like algal or cyanobacterial blooms.