BOOK OF ABSTRACTS 127 IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 Thursday, 27 August 2026 / Hall C NOM and Aquatic Systems SL61 Drinking Water Biostability: Connecting Microbial Regrowth with Bioavailable Dissolved Organic Matter Composition Sofija Djukanovic1, Elin Lavonen1,2, Ilkka Miettinen1,3, Olli Varis1 1 Department of Built Environment, School of Engineering, Aalto University, P.O. Box 15200, FI-00076 Aalto, Finland, sofija.djukanovic@aalto.fi 2 Norrvatten, P.O. Box 2093, SE-16902, Solna, Sweden 3 Finnish Institute for Health and Welfare (THL), P.O. Box 95, FI-70701 Kuopio, Finland Biostability is one of the objectives in drinking water production, as microbial regrowth in the distribution network can negatively affect drinking water quality. Nutrients, such as those present in bioavailable organic matter (BOM)—especially assimilable organic carbon (AOC) as a key component—can promote microbial regrowth, making it critical to monitor their presence in raw waters and their potential formation during water treatment [1, 2]. However, quantifying BOM abundance and composition remains challenging, as BOM may constitute only a small fraction of dissolved organic matter (DOM), particularly when assessed using bulk parameters such as UV absorbance and dissolved organic carbon (DOC). The DOM itself is omnipresent in all natural waters. It consists of a complex, heterogeneous mixture of thousands of different organic molecules. In Finland, the main source of drinking water is surface water [3]. This study investigated how different treatment processes influence BOM concentration and composition across three surface water treatment plants (WTPs). Samples were collected from the sources – lakes, and at multiple treatment steps within each WTP. To examine the bioavailable fraction, we conducted 21-day incubation experiments in the dark at approximately 20 °C to simulate conditions favourable for microbial regrowth. Microbiological and chemical parameters were measured before and after incubation. Microbial regrowth was assessed over time using traditional plate counting and flow cytometry, while changes in the DOM amount and composition were characterised using assimilable organic carbon (AOC) measurements and ultrahighresolution mass spectrometry. Potential phosphorus limitation was investigated at one of the treatment plants by adding phosphate to selected samples to compare growth capacity with and without phosphorus. Incubation experiments revealed distinct differences in growth potential and changes in the DOM composition among treatment plants. Principal component analysis (PCA) of molecular composition showed the most pronounced compositional
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